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Solid Earth, 5, 389408, 2014 www.solid-earth.net/5/389/2014/ doi:10.5194/se-5-389-2014 Author(s) 2014. CC Attribution 3.0 License.

On the complexity of surface ruptures during normal faulting earthquakes: excerpts from the 6 April 2009 LAquila (central Italy) earthquake (Mw 6.3)

L. Bonini1, D. Di Bucci2, G. Toscani1, S. Seno1, and G. Valensise3

1Dipartimento di Scienze della Terra e dellAmbiente, Universit di Pavia, Pavia, Italy

2Dipartimento della Protezione Civile, Rome, Italy

3Istituto Nazionale di Geosica e Vulcanologia, Rome, Italy

Correspondence to: D. Di Bucci ([email protected])

Received: 13 September 2013 Published in Solid Earth Discuss.: 14 November 2013 Revised: 11 April 2014 Accepted: 15 April 2014 Published: 28 May 2014

Abstract. Over the past few years the assessment of the earthquake potential of large continental faults has increasingly relied on eld investigations. State-of-the-art seismic hazard models are progressively complementing the information derived from earthquake catalogs with geological observations of active faulting. Using these observations, however, requires full understanding of the relationships between seismogenic slip at depth and surface deformation, such that the evidence indicating the presence of a large, potentially seismogenic fault can be singled out effectively and unambiguously.

We used observations and models of the 6 April 2009, Mw6.3, LAquila, normal faulting earthquake to explore the relationships between the activity of a large fault at seismogenic depth and its surface evidence. This very well-documented earthquake is representative of mid-size yet damaging earthquakes that are frequent around the Mediterranean basin, and was chosen as a paradigm of the nature of the associated geological evidence, along with observational difculties and ambiguities.

Thanks to the available high-resolution geologic, geodetic and seismological data aided by analog modeling, we reconstructed the full geometry of the seismogenic source in relation to surface and sub-surface faults. We maintain that the earthquake was caused by seismogenic slip in the range 310 km depth, and that the slip distribution was strongly controlled by inherited discontinuities. We also contend that faulting was expressed at the surface by pseudo-primary

breaks resulting from coseismic crustal bending and by sympathetic slip on secondary faults.

Based on our results we propose a scheme of normal fault hierarchization through which all surface occurrences related to faulting at various depths can be interpreted in the framework of a single, mechanically coherent model. We stress that appreciating such complexity is crucial to avoiding severe over- or under-estimation of the local seismogenic potential.

1 Introduction

Starting in the 1980s many workers have attempted to infer the earthquake potential of large continental faults from their surface length and displacement (see Kim and Sanderson, 2005 for a review). As a result, over the past two decades several empirical relationships between earthquake magnitude and the extent of surface ruptures have been developed (e.g., Wells and Coppersmith, 1994; Wesnousky, 2008) and have since become the most popular analytical tool in earthquake geology.

On the one hand, these relationships have certainly helped in quantifying eld geological observations, ultimately allowing them to be incorporated in analytical earthquake and seismic hazard assessment models (e.g., Stirling et al., 2013).In fact, the assessment of the hazard posed by large continental faults is increasingly relying on eld investigations on all scales, and even a worldwide initiative for seismic hazard

Published by Copernicus Publications on behalf of the European Geosciences Union.

390 L. Bonini et al.: A complex hierarchy of active normal faults

assessment such as the Global Earthquake Model (GEM) has Gathering all knowledge on active faults worldwide as one of its founding pillars (http://www.globalquakemodel.org/what/seismic-hazard/active-faults-database/

Web End =http://www.globalquakemodel.org/ http://www.globalquakemodel.org/what/seismic-hazard/active-faults-database/

Web End =what/seismic-hazard/active-faults-database/ http://www.globalquakemodel.org/what/seismic-hazard/active-faults-database/

Web End = ). On the other hand, however, these empirical relationships have also contributed to put in the background the complexity of the relationships between the seismogenic source at depth and its surface expression. As a reminder to geologists, nature has recently spawned a number of damaging earthquakes that turned out to have been generated by blind, hidden, or otherwise hard-to-identify faults (e.g., Jan 2010, Haiti, Mw 7.0;

Sep 2010Feb 2011, DareldChristchurch, New Zealand, Mw 7.16.3; Oct 2011, Van, eastern Turkey, Mw 7.1; July

August 2013).

The Mw 6.3, 6 April 2009 LAquila (Abruzzi, central Italy) earthquake belongs to this category. Although the LAquila region had long been known for its high seismicity level along with most of Abruzzi, the 2009 earthquake challenged the standard approach for surface active fault identication because (1) clearly visible surface faults, that prior to 2009 were presumed to be active, showed no genetic relationships with the seismogenic source, and only some showed negligible reactivation; and (2) post-earthquake eld analyses documented only limited and discontinuous coseismic surface ruptures.

The 2009 event is currently one of the best documented continental extensional earthquakes worldwide, and hence it makes a rst-rate case for exploring the relationships between the activity of a seismogenic normal fault and its surface evidence in a structurally complex area such as the Apennines.

Notwithstanding the high quality of available data, the nearly 200 papers published to date about the 2009 LAquila earthquake have produced widely divergent seismotectonic interpretations and models (see Table 5 in Vannoli et al., 2012, for a summary).

We used a wealth of high-resolution geological, geodetic and seismological data combined with analog modeling to reconstruct the geometry of the seismogenic rupture in relation to sub-surface and surface faults. We aimed at devising a scheme for active fault hierarchization that explains all surface outcomes of shallow crustal seismogenic faulting in the framework of a single, mechanically coherent interpretative model. Proper appreciation of such complexity forms the basis for a correct assessment of the local earthquake potential.

2 Tectonic setting

The Italian Apennines exhibit a remarkably complex structure resulting from the overprinting of a number of subsequent tectonic phases (Fig. 1). In the early Mesozoic the region was part of the African passive margin of the Tethys Ocean; it hosted large carbonate platforms and intervening pelagic basins that were subsequently broken up by EW

TriassicLower Jurassic extension (Calamita et al., 2011; Di Domenica et al., 2014). Since the Cretaceous, the region evolved within the framework of the convergent motion between the African and European plates; east- to northeast-verging thrusts along with their associated foredeep/thrust-top basins progressed toward the Adriatic foreland up to the Middle Pleistocene (Patacca and Scandone, 1989) and were subsequently dissected by strike-slip and normal faulting.

Following a major geodynamic change at 800 ka, SW

NE extension became the dominant tectonic style over the core of the Apennines (e.g., Hyppolite et al., 1994; Galadini, 1999), as demonstrated also by breakout, seismicity and crustal strain data (e.g., Montone et al., 2012; Carafa and Barba, 2013). Extension is denitely a youthful process in the Apennines, however, and proceeds at the relatively slow rate of 23 mm yr1 (DAgostino et al., 2011).

In contrast, the core of the Apennines is undergoing vigorous regional-scale uplift at 12 mm yr1 (DAnastasio et al., 2006); this process induces fast exhumation and widespread differential erosion of the MesoCenozoic rocks comprising it, being by far more effective than extension in building and modifying the landscape. As a result of these competing processes, the Apennines landscape is largely dominated by the older compressional structures (Fig. 1), which tend to be emphasized by erosion despite their being inactive: some even simulate the typical basin-and-range landforms associated with the action of a mature normal fault, according to a process of geomorphological convergence referred to as mimicking (Valensise and Pantosti, 2001). In addition to that, and due to the combined effect of tectonic stress and gravity, extended terrains often exhibit a level of complexity that makes the correct hierarchization of active normal faults or even their mere identication extremely challenging.On the outcrop scale, a large normal fault of crustal signicance, a shallow reactivated normal fault on the backlimb of older thrust sheets, or an even shallower sackung scarp may appear equally evident and similarly convincing as to the existence of an underlying major seismogenic source (Gori et al., 2014).

3 The 6 April 2009 earthquake

The 6 April 2009 LAquila earthquake struck a seismically very active portion of the Apennines chain. It came as the culmination of a long foreshock/aftershock sequence recorded by permanent and temporary INGV seismometers (Chiarabba et al., 2009; Chiaraluce et al., 2011). The earthquake caused intensity up to IX-X MCS effects in a small number of villages located southeast of LAquila, but the largest number of collapsed buildings and casualties was reported in LAquila itself.

Extensive albeit limited coseismic surface breaks were reported by several workers over a 100 km2 region elongated in the NWSE direction between LAquila and Monticchio,

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L. Bonini et al.: A complex hierarchy of active normal faults 391

Ancona

Teramo

0 10 km

ADRIATIC

SEA

OLEVANO-ANTRODOCO

Mt. SIBILLINI

13E

Figure 1. (a) Tectonic map of the Abruzzi region (modied from Vezzani et al., 2009). The present morphology of the axial portion of the Apennines is still dominated by the folding and thrusting through which the chain was built up over a long time interval between the upper Miocene and the middle Pleistocene. Subsequent extension and intervening sedimentation in intermontane basins has so far only slightly modied the compressional architecture of the Apennines. A dashed box indicates the exact location of the study area. 1, 2, 3, and 4 indicate cross sections. (b) Digital elevation model of the central Apennines, showing the trend of the main thrust fronts. A number of secondary fronts are also clearly delineated by the topography.

about 15 km to the southeast (see detailed descriptions in

Boncio et al., 2010; the Emergeo Working Group, 2010, and Vittori et al., 2011). The most continuous surface breaks were extensional cracks, some with a maximum net throw of a few centimeters, often seen at the top of a 10 m-high

scarp formed by Quaternary continental deposits and running above the village of Paganica (Fig. 3).

DInSAR measurements based on extensive Envisat and

COSMO-SkyMed data sets constrained by scattered GPS observations revealed sizable coseismic crustal deformation resulting in bowl-shaped, gently asymmetric surface subsidence peaking at 1520 cm (Atzori et al., 2009; DAgostino et al., 2012; Cheloni et al., 2014; Fig. 4). All published co-seismic slip models agree that most coseismic slip up to1.0 m occurred between 910 and 34 km in depth, whereas slip in the shallowest crust was found to be 0.1 m or less over most of the fault length (e.g., Atzori et al., 2009; Cirella et al., 2009; Cheloni et al., 2010; DAgostino et al., 2012; Cheloni et al., 2014). Shallower slip was documented mainly near the northern end of the fault (DAgostino et al., 2012), about 3 km to the northeast of LAquila (Fig. 4).

P and S wave analyses pointed out that the main rupture corresponded to an asperity located in a high-velocity layer (Vp > 6 km s1 and Vs 4.2 km s1; from Bianchi et

al., 2010 and Di Stefano et al., 2011, respectively). This

high-velocity body has been interpreted as being composed of partially hydrated mac rocks between 34 and 10 km in depth (Fig. 4c), although in nearby areas the observed Vp and Vs values correspond to the characteristic velocities of the MesoCenozoic limestones and terrigenous rocks (Chiarabba et al., 2010; Di Stefano et al., 2011).

The sequence is well described by a detailed double-difference catalog of relocated events (3000 events with M

1.9 from Chiaraluce et al., 2011; 64 000 events of all mag

nitudes from Valoroso et al., 2013). When coupled with moment tensor solutions for the largest shocks (Mw 2.7; Her

rmann et al., 2011), these data allow imaging of individual faults activated during this complex sequence in great detail (Fig. 4). In the depth interval 34 to 10 km the aftershocks align rather regularly along a 9 km wide, single planar sur

face dipping 4550 to the SW over the entire fault length and extending for 16 km in the NWSE direction. Some

shallower aftershocks (i.e., above 34 km in depth) are seen around the northern end of the fault; even though they are rather scattered, their locations highlight the activation of minor high-angle faults. In contrast, no shallow aftershocks are seen along the central and southern portions of the fault.

While the models based on seismological and geodetic data agree with fault parameters at depth (for both fault geometry and coseismic slip distribution), those based on

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Thrust System

4232'N

4210'N

Campotosto

Perugia

Terni

43N

42N

Pescara

LAquila

Pescara

LAquila

Rome

Chieti

12E

14E

13E

15E

Extensional faults

(b)

Fig. 2

Thrust faults

Quaternary continental deposits

Middle Pliocene-Pleistocene

Lower Pliocene

Carbonate successions

Slope-to-basin carbonate succession

(Triassic-Miocene)

Carbonate platform succession

(Triassic-Miocene)

Siliciclastic deposits

FUCINO

BASIN

Messinian

14E

(a)

392 L. Bonini et al.: A complex hierarchy of active normal faults

(a)

13 20'E 13 30'E

north

Mw 4.9 06042009 (2:37 UTC)

Paganica

Mw 5.4 07042009 (17:47 UTC)

42 20'N

42 15'N

Mw 6.3 06042009 (1:32 UTC)

LAquila

-14

-10

-6

-2

0 5 10

Mw 4.9 07042009 (9:26 UTC)

(b)

13 20'E 13 30'E

Mt. Stabiata fault

Aragno fault

13 20'E 13 30'E

north

Pizzoli fault

-2

-4

Colle Enzano fractures

-2

Paganica fault

-4

San Gregorio f. (buried)

42 20'N

Mt. Pettino fault

-3

Bazzano fault

42 15'N

Monticchio Fossa fault

0 5 10

(c)

LAquila

Paganica

42 20'N

42 15'N

0 5 10

north

Figure 2. (a) Map view of the LAquila area showing the location (red stars) and focal mechanisms of the largest events of the 2009 sequence (Scognamiglio et al., 2010). Dashed lines are contours of elevation changes observed between 4 and 12 April 2009 (DAgostino et al., 2012). Red dots indicate relocated aftershocks (Chiaraluce et al., 2011). (b) Presumed active faults, shown by red lines. S1 and S2 indicate the traces of cross sections 1 and 2. Dashed lines are contours of the elevation changes observed between 12 April and 5 October 2009 (DAgostino et al., 2012). (c) Faults reported as activated during the 2009 earthquake. Those shown in red are discontinuous fractures mapped close to the Paganica fault; black lines are presumed active faults and brown segments represent the portion of the faults reported as partially reactivated in 2009 (Emergeo Working Group, 2010).

coseismic surface ruptures showed widely divergent results, especially regarding the net throw and the lateral extent of surface ruptures, respectively, ranging from none to about 10 cm and from 2.6 to 19 km (see Fig. 14 in Vittori et al., 2011, and Table 5 in Vannoli et al., 2012).

4 Buried tectonic structures in the LAquila area from geological and seismological data

To investigate the buried tectonic setting of the Apennines in the LAquila area we combined (1) surface geological data from various sources (e.g., Vezzani et al., 2009; see Fig. 1),(2) a three-dimensional distribution of geological units at depths derived from seismic tomography (Di Stefano et al., 2011), and (3) a reconstruction of the extensional active fault systems obtained from aftershock locations and moment tensor solutions (Herrmann et al., 2011; Valoroso et al., 2013).

Cross- and down-dip sections of the LAquila region (Fig. 5) show its intricate structural framework. Such complexity is not surprising in view of the overall structure of the Abruzzi Apennines, which were built by progressive stacking of heterogeneous thrust sheets. Strong lateral variations occur within each tectonic unit, mainly due to the architecture inherited from the Mesozoic extensional tectonics, and most tectonic structures of this part of the Apennines thrust belt are not cylindrical but exhibit a rather complex shape. These circumstances result in a complex lateral distribution of rock volumes that may have signicant geometric and rheological implications for contemporary deformation.

The extensional faults activated during the 2009 seismic sequence lie in the hanging wall of the large Gran Sasso thrust. The upper tip of the main fault surface (at 3

4 km) coincides with a distinct lithological change. Aftershock alignments, focal mechanisms, and receiver function analyses highlighted the presence of a discontinuity striking N 334 and dipping 20 towards SW near the upper tip of the fault. When coupled with the distribution of geological units, such a discontinuity can be interpreted as a thrust plane that appears to play a primary role in the development of the newly formed extensional faults. In the northwestern part of the study area just above this plane there is a change in fault geometry, whereas in the central (cross sections 2 and 3 in Fig. 5) and southern parts (cross section 1) of the study area the aftershocks are conned below it. In our view this observation alone prevents the coseismic ruptures mapped at the surface (e.g., the Paganica ruptures) from being simply connected with the seismogenic fault identied at depth.

To shed light on the genetic relationships between the master fault at depth and surface structures we simulated the evolution of a normal fault using analog models.

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L. Bonini et al.: A complex hierarchy of active normal faults 393

Mt. Stabiata fault

Paganica fault

13 20'E

Figure 3. Landscape view of the faults in the LAquila area (2x vertical exaggeration). On the right the Paganica fault (central image) with two details of the surface ruptures (upper and lower panels) recognized after the 2009 earthquake (Emergeo Working Group, 2010).

5 Deep and surface fault patterns: an analog experiment perspective

To analyze the mid- to long-term evolution of an upward propagating blind normal fault and the associated secondary structures related to the bending of the overlying rocks we constructed a series of analog models. We chose wet clay and dry sand as our preferred analog materials; this allowed us to reproduce different aspects of the faulting process and to make the most out of the characteristics of both these materials. Recent studies compared results obtained using wet clay and dry sand (e.g., Eisenstadt and Sims, 2005; With-jack and Schlische, 2006; Withjack et al., 2007), highlighting differences and similarities. On the one hand, wet clay has been used extensively to analyze brittle deformation related to folding (e.g., Cloos, 1968; Withjack and Jamison, 1986; Withjack and Schlische, 2006; Henza et al., 2010; Miller and Mitra, 2011), and its effectiveness as analog material has been stressed recently by new rheological tests (Cooke and van der Elst, 2012). On the other hand, dry sand is the most commonly used material for modeling tectonic deformation (see Graveleau et al., 2012 for a review). Using these two different materials allowed us to explore if and how the rhe-

ological differences between the two materials, and primarily the cohesion, affect our observations. All the experiments took place in a normal gravity eld.

5.1 Pre-existing mechanical discontinuities and fault evolution: wet kaolin experiments

Our rst goal was to understand if and how low-angle mechanical discontinuities affect the upward propagation of a normal fault. As a clay type we used kaolin with 65 % water content. Wet kaolin has a shear strength in the range 40 to 100 Pa (Eisenstadt and Sims, 2005; Cooke and van der Elst, 2012: Cooke et al., 2013) and its frictional coefcient is about 0.6. Following well-established scaling laws (e.g., Hubbert, 1937; Schellart, 2000), these parameters imply that 1 cm in the experiment represents about 1 km in nature.

The deformation device is composed of two plates (Fig. 6); one remains xed throughout the experiment, thus representing the footwall, whereas the other one is pulled by a stepper-motor simulating hanging-wall subsidence. The displacement rate was xed at 0.005 mm s1, a strain rate that is considered appropriate for wet clay experiments (e.g.,

Cooke and van der Elst, 2012). The apparatus has the top and both sides free so as to prevent undesired boundary effects.

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Mt. Pettino fault

Bazzano fault

Paganica fault

maximum ground lowering from InSAR data (15-20 cm)

Paganica fault

Monticchio Fossa fault

394 L. Bonini et al.: A complex hierarchy of active normal faults

CROSS SECTION 2

(a) (b)

CROSS SECTION 3

POST- AND COSEISMIC DEFORMATION PROFILE

Bazzano fault

POST- AND COSEISMIC DEFORMATION PROFILE

Mt. Pettino fault

0 cm

10 cm

POST-SEISMIC DEFORMATION PROFILE

0 cm

10 cm

0 cm

10 cm

COSEISMIC DEFORMATION PROFILE

0 cm

10 cm

0 cm

10 cm

Quaternary continental deposits

Paganica fault San Gregorio fault

SW NE

Aragno fault

Mt. Stabiata fault

10 km

(c)

5 10 15 20 km 5 10 15 20 km

DOWN-DIP SECTION DOWN-DIP SECTION

(d)

0 NNW

Paganica fault

SSE

post-seismic slip

low-angle structure

>6,5 km/s

dominantly coseismic slip

600

>6,5 km/s

400

200

15km 10 20 km

CS 2

SSE

CS 3

CS 1 CS 1

15km 10 20 km

CS 3

CS 2

5.0 5.5 6.0

Vp (km/s) 4.5< >6.5

Figure 4. (a) and (b): cross sections, showing dominantly coseismic (4 April 200912 April 2009; DAgostino et al., 2012), post-seismic(12 April5 October 2009; DAgostino et al., 2012) and cumulative elevation changes measured along the section trace. In all gures red dots indicate relocated aftershocks (Valoroso et al., 2013). Thick black lines represent the fault plane derived by geodetic inversions with coseismic slip (Atzori et al., 2009). Section traces are shown in Figs. 1 and 2. (c) and (d): down-dip sections along the LAquila master fault (strike N135E; dip 50 to the WSW); (c) shows Vp values imaged along the fault plane (from Di Stefano et al., 2011); (d) shows the mainshock location (star). Red dots are aftershocks located within 0.5 km of the fault plane (Chiaraluce et al., 2011). Coseismic slip pattern, shown by red lines (contour interval is 200 mm; Atzori et al., 2009), and location of the inferred inherited thrust faults (orange thick lines). Red areas mark portions of the fault that experienced post-seismic slip (modied from DAgostino et al., 2012).

An experiment is terminated when the upward-propagating faults reach the surface of the model. To monitor and quantify the deformation we took lateral photographs every 1 mm of displacement. To this end we used displacement image correlation (DIC) analyses, a non-destructive, non-invasive, high-resolution optical technique that has the capability to detect any subtle motion of the model material during the experiments.

The clay experiments were carried out based on two experimental congurations: the rst, Wet Kaolin #1 (WK1), representing a buried fault growing in a homogeneous setting (Fig. 6a); the second, Wet Kaolin #2 (WK2), similar to the previous one but with a low-angle discontinuity placed along the presumed propagating trajectory of the master fault (Fig. 6b). Such a pre-existing discontinuity has been reproduced by sliding an electried blade through the analog ma-

terial. This innovative procedure, which has been successfully tested by Cooke et al. (2013), allowed us to introduce in the model very thin discontinuities that simulate pre-existing fault systems. Figures 7 and 8 show signicant steps selected from the two experiments, though both experiments are photographed at every 1 mm of displacement. The initial thickness of the WK models is 5 cm, representing the shallower 5 km of a generic continental crust.

At the beginning of both experiments (0.1 mm), the displacement elds showed a triangular deformation zone (Figs. 7a and 8a), corresponding to the trishear zone described and modeled by several investigators (e.g., Erslev, 1991; Hardy and Ford, 1997; Jin and Groshong, 2006). The displacement eld showed by the two experiments during the very rst deformation step is very similar, conrming that the

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L. Bonini et al.: A complex hierarchy of active normal faults 395

CROSS-SECTION 1

Il Morrone Mt. Orsello Mt. dOcre Fossa fault Mt. Prena

SW NE

CROSS-SECTION 2 CROSS-SECTION 3

Paganica fault San Gregorio fault

SW NE

SW Mt. Pettino fault

SSE

Figure 5. Geological sections across the LAquila area. (a) Regional geological section showing the main tectonic features of the area (modied after Satolli and Calamita, 2008 and Vezzani et al., 2009). (b) and (c): geological sections across the central (b) and northern(c) parts of the LAquila seismogenic source. (d) Down-dip section (strike N135E; dip 50 to the WSW). In all sections red dots indicate relocated aftershocks (Valoroso et al., 2013). In all gures thick black lines represent fault segments from Valoroso et al. (2013) based on the analysis of the aftershock locations and focal mechanisms. Red star is the hypocenter of the April 6 mainshock.

mechanical properties of the analog material are the same in both wet kaolin experiments.

As displacement proceeds, brittle and ductile deformation taking place in the experiments begins to be visible. A monoclinal structure developed during both experiments after a displacement of 10 mm (Figs. 7b and 8b). In WK1, three synthetic faults and one antithetic extensional fault are formed at the tip of the master fault with a Mode II mechanism (Scholz, 1990). As a result of bending, two crestal fractures formed with a Mode I mechanism at the surface of the model (Fig. 7b), where tensile stress is maximal. A strain distribution analysis corroborates our observations showing two high-strain areas, a lower one and an upper one, spatially corresponding to the previously described brittle structures. At

the same displacement level (10 mm) WK2 showed a different development of brittle structures (Fig. 8b). An individual upward-propagating fault stemming from the buried tip of the master fault appeared more developed than those seen in WK1 (Fig. 9a) and with a steeper dip angle. When this fault encountered the pre-existing discontinuity, it stopped its propagation, splitting into two minor elements. Meanwhile, minor faults were seen to displace the pre-existing low-angle plane. Also in this experiment, some downward-propagating faults related to the bending developed at the surface of the model (Fig. 8b). Strain analyses conrmed the previous observations highlighting a narrow zone where strain is highest; signicantly, there is a sudden decrease in the strain value

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Gran Sasso thrust

10 km

(a)

Aragno fault

Mt. Stabiata fault

Bazzano fault

DOWNDIP SECTION

10 km

(b) (c)

NNW

Limestone and Foreeep Terrigenous Deposits(Jur.-L. Messinian)

Carbonate rocks and Evaporites (Triassic)

Basement

15km 10 20 km

CS 1

CS 3

CS 2

(d)

396 L. Bonini et al.: A complex hierarchy of active normal faults

(a) (b)

Experiment Wet Kaoline #1 (WK1) Experiment Wet Kaoline #2 (WK2)

region of interest

5 cm 5 cm

wet kaolin

wet kaolin pre-existing thrust

region of interest

hanging-wall (mobile plate)

foot-wall (fixed plate)

hanging-wall (mobile plate)

foot-wall (fixed plate)

STEPPER

MOTOR

STEPPER

MOTOR

0 5 10 cm

Figure 6. Experimental setup used to model (a) a fault evolving into a medium conguration and (b) with a low-angle discontinuity.

WK1 - total displacement 1.0 mm (D.I.C. from 0 to 0.1 mm)

D.I.C. area

0 5 10 cm

WK1 - total displacement 10.0 mm (D.I.C. from 9 to 10 mm)

D.I.C. area

0 5 10 cm

WK1 - total displacement 15.0 mm (D.I.C. from 14 to 15 mm)

0 5 10 cm

0.1 cm

(a)

strain distribution0.01 0.04 0.07 0.10

0 5 10 cm

(b)

strain distribution0.01 0.04 0.07 0.10

0.1 cm

0 5 10 cm

D.I.C. area

0 5 10 cm

(c)

strain distribution0.01 0.04 0.07 0.10

0.1 cm

Figure 7. Progressive evolution of the fault patterns in the WK1 wet clay experiment (left panels). For each step the brittle and ductile deformations have been analyzed using the DIC technique, which returned the displacement eld (right panels) and the strain distribution (central panels). Dashed lines indicate precursor faults (see text).

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L. Bonini et al.: A complex hierarchy of active normal faults 397

WK2 - total displacement 0.1 mm (D.I.C. from 0 to 0.1 mm)

D.I.C. area

0 5 10 cm

(a)

strain distribution0.01 0.04 0.07 0.10

0.1 cm

WK2 - total displacement 10.0 mm (D.I.C. from 9 to 10 mm)

0 5 10 cm

D.I.C. area

0 5 10 cm

(b)

strain distribution0.01 0.04 0.07 0.10

0.1 cm

WK2 - total displacement 15.0 mm (D.I.C. from 14 to 15 mm)

D.I.C. area

0 5 10 cm

WK2 - total displacement 25.0 mm (D.I.C. from 24 to 25 mm)

0 5 10 cm

(c)

strain distribution0.01 0.04 0.07 0.10

0.1 cm

D.I.C. area

0 5 10 cm

(d)

strain distribution0.01 0.04 0.07 0.10

0.1 cm

Figure 8. Progressive evolution of the fault patterns in the WK2 wet clay experiment (left panels). For each step the brittle and ductile deformations have been analyzed using the DIC technique, which returned the displacement eld (right panels) and the strain distribution (central panels). Dashed lines indicate precursor faults.

where the upward-propagating fault intersects the low-angle plane.

After 15 mm of displacement, in WK1 the upward-propagating fault reached the surface, connecting with one of the downward-propagating faults and showing an overall listric geometry (Fig. 7c); the connection of the two structures is well predicted by the strain distribution analysis. In

contrast, in WK2 the downward- and upward-propagating faults do not appear to have connected at the same displacement step (Fig. 8c). This different behavior must necessarily be related to the role played by the low-angle discontinuity; a circumstance conrmed by the strain analysis, which shows that most of the deformation remains conned below the discontinuity. The rate of upward propagation of the

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398 L. Bonini et al.: A complex hierarchy of active normal faults

A comparison of the two models at this nal development stage, i.e., when the upward- and downward-propagating faults are connected and the displacement produced by the two rigid blocks has reached the surface, shows signicant differences. Regarding fault geometry, we observed a listric prole in WK1 and a change in dip in WK2 coincident with the pre-existing low-angle discontinuity. Such a ramp-at-ramp prole in WK2 further stresses the role of the fault-bend folding mechanism connected to the growth of the upward-propagating fault; as a result, the bending-moment faults are more developed in this experiment. Despite the rigidity of the blocks that simulate the hanging wall and the footwall, the faulting evolution in WK1 and WK2 produced subsiding basins having different shapes (Figs. 9b andc); while in WK1 the basin depocenter falls close to the surface-breaking fault, the depocenter of the bowl-shaped basin that developed in WK2 lies 40 mm away from the surface-breaking fault.

In summary, the results of the experiments show how strongly the mechanical discontinuity cut in the uniform material of the model affected the growth of the simulated master blind fault, delaying the upward propagation of the synthetic faults that developed from its tip (Fig. 9). Both experiments displayed that sub-vertical bending-moment faults are the rst structures to form at the surface, as shown also by other investigators (e.g., van Gent et al., 2010). In our wet kaolin experiments, such secondary structures mainly developed with a Mode I mechanism showing an excessive width (see for example the nal step of WK2; Fig. 5g), probably due to the quite high cohesion of the wet clay itself.

5.2 Quartz sand experiments

Although wet kaolin is very effective as an analog material, its high cohesion may result in some undesirable modeling effects. To overcome this limitation we reproduced some key features of the models made with clay using dry sand.

We performed two different experiments. The rst one, Quartz Sand #1 (QS1), used the same experimental apparatus of the wet kaolin experiments, formed by two rigid blocks, one of which is mobile and acts as a subsiding hanging-wall fault block (Fig. 10a). This experiment was conceived to evaluate how much the rheological differences between wet kaolin and dry sand affect deformation patterns in an isotropic material. In the second experiment, Quartz Sand #2 (QS2), the hanging-wall fault block was replaced by a exing plate connected to the stepper motor. The aim of this experiment was strictly to reproduce a bending of the hanging wall caused by a slip on a blind fault (Fig. 10b). This conguration is reminiscent of the plateau phase of the upward propagation in the WK2 experiment, namely the phase when the deformation in the block on top of the low-angle discontinuity was dominated by bending rather than simple shearing (Fig. 9a). The low cohesion of the quartz sand was expected

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(a)

height above master fault (cm)

1.02.03.04.05.0

0.00.10.00.1 mm0.00.10.00.1 mm

0.00.10.00.1 mm

1.0 1.5 2.0 2.5

displacement on the master fault (cm)

from 0.0 to 0.1 cm

from 0.9 to 1.0 cm

from 1.4 to 1.5 cm

(b)

(c)

Figure 9. (a) Diagram showing the vertical location of the upper tip of the upward-propagating faults against the amount of displacement on the master fault. Thick black and dashed lines represent results from WK1 and WK2, respectively. Orange lines represent the location of the pre-existing discontinuity in WK2. (b) Topography proles of the WK1 (thick black lines) and WK2 (dashed lines) experiments at different intervals. (c) Final topographic prole of the wet clay experiments, showing the location of the surface-breaking faults (red lines). Data in diagrams (b) and (c) come from the DIC analyses shown in Figs. 7 and 8 (right panels).

fault in WK2 is much slower than in WK1, and downward-propagating faults in WK2 are more developed than in WK1 (Fig. 9a).

Notice that in WK2 a direct link between upward- and downward-propagating faults was reached only after 25 mm of total displacement (Fig. 8d), 10 mm more than was needed for WK1 (Fig. 7c), and with a peculiar ramp-at-ramp conguration.

L. Bonini et al.: A complex hierarchy of active normal faults 399

(a) (b)

Experiment Quartz Sand #1 (QS1)

0 5 10 cm

Experiment Quartz Sand #2 (QS2)

granular material (quartz sand)

6 cm

hanging-wall(mobile plate) foot-wall

(fixed plate)

granular material (quartz sand)

hanging-wall (flexing plate)

foot-wall (fixed plate)

STEPPER MOTOR

theoretical position of the master fault

0 5 10 cm

(d) Experiment Quartz Sand #2 (QS2)

precursor faults

(c)

Experiment Quartz Sand #1 (QS1)

0 5 10 cm

synthetic normal faults

crestal normal faults

STEPPER MOTOR

crestal normal faults

depocentre

hanging-wall (mobile plate)

foot-wall (fixed plate)

flexing plate

foot-wall (fixed plate)

STEPPER MOTOR

theoretical position of the master fault

STEPPER

MOTOR 0 5 10 cm

Figure 10. Analog modeling setup and results of dry sand experiments QS1 and QS2. (a) and (b) are sketches of the experimental modeling apparatus. (c) and (d) show the results of the experiment. Red lines represent newly formed faults and dashed black lines indicate precursor faults.

to prevent the unnatural generation of open cracks that we observed in WK2.

The experiment used quartz sand having the following mechanical properties: cohesion 30 Pa, coefcient of internal friction 0.88, angle of internal friction 41 . Using standard scaling rules (Hubbert, 1937), under our experimental conditions 1 cm represents 0.5 km in nature, implying that our model aimed at reproducing the shallowest 3 km of a continental crust. In both experiments, two lateral glass walls conned the quartz sand. To prevent undesired boundary effects, we reduced the friction by polishing the glass walls with graphite powder.

The results of QS1 did not show remarkable differences with respect to WK1; the upward-propagating fault nucleated at the upper tip of the simulated master fault and rapidly reached the surface (Fig. 10c), as already observed in several previous sand box models (e.g., Bonini et al., 2011, and references therein). The basin associated with slip on the master fault was quite similar to its equivalent in WK1.

In QS2, a forced fold developed above the tip of the hypothetical fault as a result of the imposed bending (Fig. 10d).A series of crestal fractures, some showing small but measurable vertical throw due to low cohesion of the dry sand, formed where the tensile stress was maximum, that is to say along the hypothetical up-dip prolongation of the master fault. Overall, these structures are reminiscent of the bending-moment faults seen in the wet kaolin experiments. Additionally, some upward-convex faults formed at the boundary between rigid and exing blocks. Such structures had also been recognized in the WK models (dashed lines in Figs. 7 and 8); they are known as precursor faults (Mandl, 2000) and have been described as one of the rst steps of brittle deformation connected to a buried fault, both in analog models and in natural examples (Mandl, 2000).

As observed in the WK models, the QS experiments also show that the relative position between the initial tip point of the master fault at depth and the surface-breaking faults (both bending-moment faults and secondary synthetic faults) is not much different. Bending-moment faults, however, are

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400 L. Bonini et al.: A complex hierarchy of active normal faults

located in a more external position, i.e., towards the footwall, so that they appear to be aligned with the deep master fault plane even though they are not connected with it.

6 Discussion

Our analog models, especially WK2 and QS2, show the mechanical feasibility of a long-lived buried extensional master fault, accompanied by disconnected secondary normal faults lying at the surface roughly along its projection. Model QS2 also demonstrates that bending-moment faults developed in a low-cohesion material may display some vertical displacement, mimicking a genuine upward-propagating normal fault, and that to some extent the size and the shape of the basin related to the growth of upward-propagating faults depends on the growth rate of the faults (Fig. 9).

Based on the available data about the LAquila sequence and on our modeling results, we conclude that the low-angle discontinuity that exists around 3 km depth played an active role in preventing coseismic slip from propagating to the surface, ultimately affecting the mid- to long-term evolution of the shallow portion of the entire fault system.

We believe that our results cast doubts on the nature of the surface fractures detected after the 2009 LAquila earthquake, in particular on the interpretation of the Paganica fault as the primary expression of seismogenic faulting at depth.In the following we discuss this point analyzing the available seismological and geological evidence in the light of our modeling results. We will address in detail what is seen in different portions of the fault system.

6.1 The Paganica fault: upward or downward propagating?

To address this question we move from the evidence supplied by the aftershock pattern. In this respect we wish to recall that, although very shallow aftershocks were imaged between 1 and 3 km depth near the northern end of the seismogenic source, no aftershocks where observed between the surface and 23 km depth in its central portion, i.e., near Paganica (Chiarabba et al., 2009; Chiaraluce et al., 2011; Chiaraluce, 2012; Valoroso et al., 2013). This could be well explained by the velocity strengthening behavior of faults at shallow crustal depth, that is to say, in the upper stability transition (UST) zone dened by Scholz (1988), but two additional explanations are equally likely: (1) the fault normal stress, which controls the effective coefcient of friction of the rupture, might be especially low in the shallowest portion of the fault plane, thus generating stable sliding (e.g., Brace and Byerlee, 1970); or, more simply, (2) there is no fault plane continuity in the shallowest 23 km of the crust in the Paganica area. It is well known that the UST zone is not always found in seismogenic areas; for instance, Marone and Scholz (1988) investigated continental faults and concluded

that the UST occurs only in mature fault zones. As a result, young faults and faults with long recurrence intervals or negligible gouge zones do not exhibit a UST zone.

Unfortunately, neither eld nor trenching observations allow the nature of the Paganica fault gouge to be assessed, as the only available information consists of extensional and shear joints nearby the village of Paganica. Paleoseismological investigations across the Paganica fault used standard trenches dug in loose Quaternary deposits up to 4 m in depth (e.g., Cinti et al., 2011) that did not reach the bedrock (i.e., the pre-Quaternary carbonate rocks). A seismic refraction and resistivity survey run perpendicular to the Aterno Valley south of LAquila (Improta et al., 2012) did constrain the shape of the basin at depth; the characteristics and effective penetration depth (around 300 m) of the selected methodology, however, do not allow one to draw rm conclusions on whether or how the surface ruptures extend at depth, or to understand the mechanical properties of the hypothetical fault gouge.

In summary, there is no surface exposure of the Paganica fault that could be used to state whether or not this fault is well developed at depth. Moreover, even if we considered the Paganica extensional and shear joints as secondary branches of the seismogenic source reaching the surface, we would expect afterslip along these features to result from velocity strengthening hours or days after the mainshock (e.g., Perfettini and Ampuero, 2008). Luckily, we know exactly the timing of the surface breakage near Paganica, as the PaganicaTempera aqueduct high-pressure pipe, which crosses the Paganica fault not far from the village center, was reported broken during the mainshock (Vittori et al., 2011). Hence this effect is incompatible with afterslip along a secondary branch of the main seismogenic fault.

As stated earlier, our experiments demonstrated the mechanical feasibility of a long-lived buried extensional master fault with disconnected secondary normal faults lying roughly along the same hypothetical plane: a conguration that reproduces satisfactorily all information available on the current setting and on the presumed evolution of the Pagan-ica surface rupture. We hence interpret the Paganica breaks as tensional and shear joints that accommodate the deformation within the surface fold induced by the buried master fault.

6.2 Northern end of the LAquila fault system

With regard to the northern end of the fault system activated in 2009, the aftershock distribution indeed appears to delineate a master fault plane and its associated shallow secondary structures (Fig. 3a). These may resemble a set of secondary splays of a master fault approaching the surface. As shown by the results of the WK2 experiment, however, this would imply that this portion of the fault system is more developed than the central portion.

To test this hypothesis we compared the central and northern portions of the LAquila fault system (Figs. 5b and c) in

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L. Bonini et al.: A complex hierarchy of active normal faults 401

the light of distinct subsequent steps of the wet clay experiments; one could visually relate the central portion to the step shown in Fig. 3d, and the northern portion to the step of Fig. 3g. Several observations, however, make this view questionable.

Seismological data, observations of surface co- and post-seismic deformation, and inferences on the morphotectonic evolution of the Aterno River Valley, all indicate that the LAquila fault system generated the maximum displacement in its central portion, as seen along most normal fault systems worldwide (Fig. 4). As a consequence, the secondary structures directly generated by the master fault are also expected to be more developed in its central portion, in contrast to all available observations.

A plausible explanation for this ambiguous conguration involves the reactivation of inherited extensional fault systems located close to the buried upper tip of the seismogenic master fault. As we mentioned describing the local geological setting, the Abruzzi Apennines are the result of three subsequent deformation phases: Mesozoic extension, Cenozoic compression and shortening, and PlioQuaternary extension.The high-angle faults that we see today at the surface may be equally well related to any of these deformation events: for instance, they could be Mesozoic faults incorporated and translated within the Cenozoic thrust sheets, or normal faults that developed in the backlimbs of thrust-related anticlines during Cenozoic compression (see Scisciani et al., 2002 for a review on this topic).

We have shown that a discontinuity specically a thrust plane occurs at 3 km depth in the crustal volume above the master fault; therefore, some of the surface faults that slipped during the 2009 sequence and that are in the hanging wall of that thrust may also be related to previous deformation phases. A feasible hypothesis for the nature of the fault system seen near the northern end of the LAquila fault system is the reactivation of inherited extensional faults fortuitously located near the upper tip of the seismogenic blind master fault.

We conclude by suggesting that, although most coseismic slip was conned below 3 km depth during the mainshock, it did trigger sympathetic slip on inherited extensional faults such as the buried portion of the Mt. Stabiata and Aragno faults. It is well known that blind earthquake ruptures impart stress on the overlying crust, possibly triggering preexisting faults (e.g., Lin and Stein, 2004). This hypothesis is supported by the post-seismic strain recovery observed just above this shallow structure (DAgostino et al., 2012).

6.3 A down-dip segmentation scheme of the seismogenic source

Based on the evidence discussed above we propose (a) that the causative rupture of the 2009 LAquila earthquake was conned at a depth > 3 km, and (b) that the complex interaction with inherited low-angle faults (thrusts) not only pre-

vented coseismic slip from propagating upwards, but also reduced the natural mid- to long-term tendency of the master fault to reach the surface. Accordingly, we interpret the Paganica breaks as tensional and shear joints that accommodate the deformation within the fold induced by the normal fault propagation. This outcome explains both the hidden nature of the earthquake causative fault prior to 2009 and the wide scatter of rupture models described in the literature (see Vittori et al., 2011, and Vannoli et al., 2012, for a review). We also suggest that in the depth interval 3 10 km the causative fault of the 2009 LAquila earthquake was laterally segmented due to the interaction with inherited structures. We believe that these limitations, which are due to permanent characteristics of the local geological setting, contributed to limit the fault size and hence the magnitude of the 2009 earthquake, and we expect it to have done so also through previous seismic cycles.

6.4 Categorization of active faults

Faulting is a complex phenomenon on all spatial and temporal scales. In the Abruzzi Apennines this complexity arises largely from (a) the superposition of a young (< 1 My) extensional tectonic regime over the outcomes of a compressional regime that built the chain by progressive thrusting over a timescale of a few million years, (b) the similarity between surface tectonic features that represent the primary outcome of the ongoing extensional regime and secondary, inherited extensional features that are commonly associated with thrusting, and (c) the progressive rejuvenation of land-forms by regional uplift, a fast landscape-building process that largely outpaces the ongoing regional extension and that is also responsible for generating fault-like gravity features.An extremely well-documented earthquake such as the 2009 event, occurring in a relatively isolated intermontane depression such as the middle Aterno Valley, affords the rare possibility to investigate the interplay between these competing processes without being tricked by appearances or even more importantly without being ruled by dogmas.

Based on previous observations we devised a hierarchization scheme of active normal faults in the LAquila area (Fig. 11). Our scheme includes four categories:

I. Seismogenic fault system (i.e., the LAquila master fault system): fault segments that generated the mainshock.They are the main players in the assessment of ground shaking hazards.

II. Inherited subsurface faults (i.e., the low-angle thrusts located above the main planar seismogenic plane) are faults generated during previous deformation phases and in this case acting passively as segment boundaries, both horizontally and vertically, effectively limiting the size and hence the magnitude of the earthquake generated by the master fault (mainshock). As such they

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402 L. Bonini et al.: A complex hierarchy of active normal faults

CROSS-SECTION 1

CROSS-SECTION 2 CROSS-SECTION 3

Gran Sasso thrust

Il Morrone Mt. Orsello Mt. dOcre Fossa fault Mt. Prena

Inherited Reactivated Faults (Category #4)

SW NE

Inherited Thrust Fault (Category #2)

10 km

Seismogenic Fault (Category #1)

(a)

Aragno fault

SW NE

SW Mt. Pettino fault

Inherited Reactivated Faults(Category #4) Inherited Reactivated Faults

(Category #4)

Paganica fault San Gregorio fault

Secondary Bending-Moment Faults

(Category #3)

Mt. Stabiata fault

Bazzano fault

10 km

Limestone and Foreeep Terrigenous Deposits(Jur.-L. Messinian)

Carbonate rocks and Evaporites (Triassic)

Basement

Inherited Thrust Fault (Category #2)

Seismogenic Fault (Category #1)

(b) (c)

Figure 11. Geological cross sections showing different fault categories recognized in the LAquila area. Section traces are shown in Fig. 1.

10 km

IRPINIA 1980

Castelgrande

Mt. Marzano

Platano River

10 km

Apennines and Apulia Carbonate rocks

Carbonate rocks and Evaporites (Triassic)

Basement

Pelagic basin deposits and related foredeep deposits

Apulia Apulia

Mw 6.2

Mw 6.9

(a)

COLFIORITO 1997

CAMPOTOSTO 2009 (Cross-Section 4)

Mt. Faeto Mt. Pennino

Mt. Giano thrust

Gran Sasso thrust

Gorzano fault

10 km

Acquasanta-Gorzano thrust

Teramo thrust

Mw 6.0

Limestone and Foreeep Terrigenous Deposits(Jur.-L. Messinian)

Carbonate rocks and Evaporites (Triassic)

Basement

Mw 5.4

(b) (c)

Figure 12. Geological cross sections across the causative faults of the 1980 Irpinia (modied from Mazzotti et al., 2007; faults from Pantosti and Valensise, 1990), 1997 Colorito (modied from Chiaraluce et al., 2005), and 2009 Campotosto (from Bigi et al., 2013) earthquakes. Thick red lines represent the seismogenic sources (Category I).

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L. Bonini et al.: A complex hierarchy of active normal faults 403

play an indirect role in the assessment of ground shaking hazards.

III. Bending-moment secondary surface faults (i.e., Pagan-ica ruptures): they represent breaks that are generated by the crustal bending taking place above the uppermost portion of the master fault. As such they are expected to(a) occur near the upward prolongation of the deeper master fault, thus simulating primary surface faulting, and (b) be restricted to the middle of the master fault, where slip is usually larger and bending is consequently tightest. Bending moment faults are also expected to nucleate at the surface and to extend downward up to a depth controlled by the bending geometry. They may cause sizable and somehow unpredictable surface faulting hazards.

IV. Inherited surface faults (e.g., the Bazzano and Pettino faults): they are faults formed during previous deformation phases and correspond mostly to faults bounding piggy-back basins, or more in general accompanying the progression of thrusting during the construction of the chain. They are usually very evident in the eld and may or may not be reactivated, depending on their location and geometry relative to the coseismic strain pattern imposed by the master fault. They may be relevant to the assessment of ground shaking hazard due to fault-trapped waves (Calderoni et al., 2012), whereas due to their clear visibility they pose a limited surface faulting hazard.

6.5 Seismogenic sources in the Italian Apennines: the challenge of youthful normal faulting

The complex interaction of the recent to currently active extension with the inherited tectonic setting of the LAquila area once again suggests that relating the seismogenic source to its surface expressions may be a very challenging task. For the same reasons, anticipating the location of major seismogenic sources based on their presumed surface expression is a difcult and potentially misleading exercise that may have crucial implications for the assessment of local seismic hazards.

With proper consideration of local geological and tectonic peculiarities, the causative faults of other normal faulting earthquakes that struck the Apennines during the past century can be revisited following our observations on the LAquila seismotectonic scenario.

The 23 November 1980, Irpinia, Southern Apennines earthquake (Mw 6.9) was generated by a seismogenic fault (Category I) that clearly reached the surface over a distance of nearly 40 km (Pantosti and Valensise, 1990: Fig. 12a). This causative fault is directly visible, but it is worth recalling that the observed ruptures did not bound any intermontane basin, but rather crossed the landscape, showing a tendency to reverse the existing topographic setting. In the same geolog-

ical context, the presumed causative fault of a large main-shock subevent that occurred 40 s after nucleation with an estimated Mw 6.2 was a blind antithetic fault conned in the

Apulia carbonate rocks (Pantosti and Valensise, 1990). The observed lateral fragmentation of the 1980 earthquake source can be interpreted as related to the widespread occurrence of preexisting tectonic boundaries interacting with the master fault at depth. These features can be assigned to our Category II (Inherited subsurface faults).

In contrast, the 26 September 1997 UmbriaMarche complex sequence in the centralnorthern Apennines (Mw 6.0)

did not generate sizable surface breaks (e.g., Basili et al., 1998), and the available knowledge on the location and geometry of the seismogenic sources shows that they do not connect with any of the known range-bounding faults (e.g., Barba and Basili, 2000). The surface faults that displayed some rejuvenation of the bedrock fault plane should hence be assigned to our Category IV (Inherited surface faults).

The catastrophic 13 January 1915 Avezzano earthquake (Mw 6.8) in the Abruzzi Apennines caused large coseismic surface breaks within the large Fucino intramontane basin.Some of the observed ruptures (e.g., the San Benedetto fault scarp, Michetti et al., 2004), however, occurred away from the basin boundaries, suggesting that some of the rejuvenated bedrock faults should be assigned to our Category IV (Inherited surface faults).

Coming back to the region of the 2009 LAquila earthquake, it is known that the northernmost portion of the aftershock sequence, located near Lake Campotosto, activated a further seismogenic fault system that released a Mw 5.4 event on 9 April 2009 (Bigi et al., 2013). This earthquake and its aftershocks occurred in a crustal volume conned both at the bottom and at the top by thrust faults inherited from the extinct compressional tectonic regime, and which for this reason should be assigned to our Category II (Inherited subsur-face faults: Fig. 12c). This further strengthens the view that inherited thrust faults played a signicant role as well in the LAquila earthquake.

In summary, seismogenic master faults may or may not reach the surface and cause primary surface faulting, depending on their size and depth, the amount of slip and its distribution on the fault plane, and the presence of favorably oriented, inherited discontinuities in the surrounding crustal volume (see Category II). Such down-dip segmentation may also be caused by velocity strengthening zones along the fault plane, or by the fault interaction with generic mechanical discontinuities within the host rocks (e.g., weak rock layers).

The geological expression of the Italian earthquakes leads to a sort of reversed tectonic hierarchy, such that the main seismogenic fault (Category I) may be expressed at the surface only by rather subdued bending-moment faults (Category III), may be constrained in length and width by preexisting faults and discontinuities (Category II), and may lie hidden beneath a blanket of clearly visible yet substantially harmless faults (Category IV).

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404 L. Bonini et al.: A complex hierarchy of active normal faults

CASE 1

CASE 2A

CASE 2B

CASE 3

INITIALSETTING

Crestal faults

Thrust

Extensional Faults

Thrust

Inherited Reactivated Fault (Category #4)

Inherited Thrust (Category #2)

Seismogenic Fault (Category #1)

Inherited Reactivated Faults (Category #4)

Seismogenic Fault (Category #1)

Bending-Moment Fault (Category #3)

Inherited Thrust (Category #2)

Seismogenic Fault (Category #1)

TIME 1

TIME 2

TIME 3

Seismogenic Fault (Category #1)

Bending-Moment Fault (Category #3)

Inherited Thrust (Category #2)

Seismogenic Fault (Category #1)

Inherited Reactivated Fault (Category #4) Bending-Moment Fault

(Category #3)

Inherited Thrust (Category #2)

Seismogenic Fault (Category #1)

Figure 13. Images showing an inferred sequential development of a seismogenic master fault (Category #1) and its ancillary structures (Category #2, Category #3, and Category #4) in four different tectonic scenarios. Gray and black lines represent inherited structures and active faults, respectively. The panels labeled TIME 1, TIME 2 and TIME 3 show successive steps of fault evolution. In CASE 1 the initial setting does not exhibit signicant inherited mechanical discontinuities; the seismogenic fault easily reaches the surface. CASE 2 (A and B) comprises two scenarios of geodynamic change, i.e., from compression to extension. In CASE 2A the seismogenic master fault (Category #1) is an inherited thrust ramp (Category #2) reactivated since the early stages of extension (TIME 1); crestal faults that developed along the extrados of the thrust-related fold may be reactivated (Category #4), mimicking a surface-breaking master fault. In CASE 2B an inherited thrust plays a segmentation role (Category #2) and slows down the upward propagation of the master fault, causing extreme bending of the surface rather than faulting and promoting the creation of bending-moment faults (Category #3). CASE 3 is the most complex, as it shows a scenario of both positive and negative inversion. The seismogenic fault interacts with an inherited thrust at shallower crustal levels, generating bending-moment faults (Category #3) and remobilizing an inherited extensional fault plane that simulates a fully primary surface rupture.

7 Conclusions

Our analysis of seismogenic faulting in the LAquila area reveals an unprecedented complexity in the interaction between coseismic slip and inherited structural features. In particular, it suggests that the main surface coseismic rupture related to the 2009 LAquila earthquake the Paganica fault can be better explained as a result of surface bending rather than as the direct prolongation of the seismogenic master fault. Under these circumstances, the length of the faulted zone and the extent of surface slip would be controlled more by the rheology of the shallow rocks than by the fault slip

at depth. This would ultimately prevent the surface rupture parameters from being used to obtain the earthquake magnitude using empirical relationships (e.g., Wells and Copper-smith, 1994), both for the considered event and for previous earthquakes detected through paleoseismological trenching.

We believe that the 2009 LAquila earthquake illustrates well the nature of the interaction between the seismogenic fault and other inherited structures at depth (e.g., buried thrust planes), the partial reactivation of inherited surface faults (e.g., the Bazzano, Mt. Stabiata and Pettino faults), and the occurrence of pseudo-primary surface ruptures (the Paganica fault). We contend that the 6 April 2009 LAquila

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L. Bonini et al.: A complex hierarchy of active normal faults 405

earthquake in fact illustrates a common style of complex tectonic deformation, implying that the number of unrecognized, blind or hidden seismogenic faults in Italy and probably elsewhere could be larger than previously thought.Driven by the current tectonic regime such faults break through a highly complex upper crust, interacting in various ways with the existing structural fabric. This may result in limitations in their size, reactivation of inherited faults and generation of new surface breaks (Fig. 13). What is absolutely crucial for the geological reconnaissance work is that in the eld these highly diverse faults may exhibit a reversed hierarchy, the most obvious being the least relevant in a fault-based seismic hazard assessment and vice versa. Proper appreciation of such complexity forms the basis for a correct assessment of the local earthquake potential.

From this perspective, the work and the ideas we presented have obvious and important implications in the context of seismic hazard assessment. The . . . ability to distinguish between tectonically induced primary and secondary faulting, faulting induced by strong ground motions, and faulting induced by nontectonic phenomena . . . has been seen by Hanson et al. (1999) as a fundamental pre-requisite for devising appropriate regulatory criteria in the siting of nuclear power plants and other critical facilties. McCalpin (2000), among several others, discussed strategies for the analysis of secondary features associated with the distributed expression of reverse faulting, including bending-moment faults, exural-slip faults and folds, and placed them in a hierarchical classication. Much of their work, however, deals with secondary faulting induced by blind faults in compressional environments, because this is the dominating tectonic style in the countries where these studies were initiated. So far there has been little appreciation for the fact that the same features and interpretative problems may also be seen in extensional environments: our study aims to ll this gap using the unique evidence from the 2009 earthquake.

Italy no longer has nuclear facilities in operation, but it is a highly seismic country that hosts a large fraction of the worldwide cultural heritage. Not all areas are equally at risk, hence any building retrotting strategy requires priorities to be set based on careful examination of the true seismogenic potential of each individual seismogenic area. Much work has already been carried out on the potential of Italian faults, and several quiescent areas are being watched closely following the identication of presumed active tectonic features.The results of our analysis suggest that an objective evaluation of the hazard posed by any recognized active fault requires full and proper appreciation of its true hierarchy level.This is to be achieved by blending surface, subsurface, geomorphic, structural and seismological data and, more importantly, by avoiding preconceptions and overly simplied models.

Acknowledgements. This work was nancially supported by Regione Lombardia, by the Italian Ministry of Research and Education (MIUR) in the framework of INGVs Progetto Abruzzo, and by Presidenza del Consiglio dei Ministri Dipartimento della Protezione Civile (DPC). The views and conclusions contained in this paper are those of the authors and should not be interpreted as necessarily representing ofcial opinions and policies, either expressed or implied, of the Italian Government. The editor F. Storti and the referee K. Decker are gratefully acknowledged for the review and the constructive criticisms that improved this paper.

Edited by: F. Storti

406 L. Bonini et al.: A complex hierarchy of active normal faults

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Over the past few years the assessment of the earthquake potential of large continental faults has increasingly relied on field investigations. State-of-the-art seismic hazard models are progressively complementing the information derived from earthquake catalogs with geological observations of active faulting. Using these observations, however, requires full understanding of the relationships between seismogenic slip at depth and surface deformation, such that the evidence indicating the presence of a large, potentially seismogenic fault can be singled out effectively and unambiguously. We used observations and models of the 6 April 2009, Mw 6.3, L'Aquila, normal faulting earthquake to explore the relationships between the activity of a large fault at seismogenic depth and its surface evidence. This very well-documented earthquake is representative of mid-size yet damaging earthquakes that are frequent around the Mediterranean basin, and was chosen as a paradigm of the nature of the associated geological evidence, along with observational difficulties and ambiguities. Thanks to the available high-resolution geologic, geodetic and seismological data aided by analog modeling, we reconstructed the full geometry of the seismogenic source in relation to surface and sub-surface faults. We maintain that the earthquake was caused by seismogenic slip in the range 3-10 km depth, and that the slip distribution was strongly controlled by inherited discontinuities. We also contend that faulting was expressed at the surface by pseudo-primary breaks resulting from coseismic crustal bending and by sympathetic slip on secondary faults. Based on our results we propose a scheme of normal fault hierarchization through which all surface occurrences related to faulting at various depths can be interpreted in the framework of a single, mechanically coherent model. We stress that appreciating such complexity is crucial to avoiding severe over- or under-estimation of the local seismogenic potential.

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Title

On the complexity of surface ruptures during normal faulting earthquakes: excerpts from the 6 April 2009 L'Aquila (central Italy) earthquake (M_w 6.3)

Author

Bonini, L; D Di Bucci; Toscani, G; Seno, S; Valensise, G

Pages

389-408

Publication year

2014

Publication date

2014

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Copernicus GmbH

ISSN

18699510

e-ISSN

18699529

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Scholarly Journal

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English

DOI

https://doi.org/10.5194/se-5-389-2014

ProQuest document ID

1757046744

On the complexity of surface ruptures during normal faulting earthquakes: excerpts from the 6 April 2009 L'Aquila (central Italy) earthquake (M_w 6.3)

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On the complexity of surface ruptures during normal faulting earthquakes: excerpts from the 6 April 2009 L'Aquila (central Italy) earthquake (Mw 6.3)

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On the complexity of surface ruptures during normal faulting earthquakes: excerpts from the 6 April 2009 L'Aquila (central Italy) earthquake (M_w 6.3)