Analysis of the Stress Field Around Concealed Active Fault From Minor Faults-Slip Data Collected by Geological Survey: An Example in the 1984 Western Nagano Earthquake Region

Abstract

Earthquakes with magnitudes of 6–7 have been reported even in various active tectonic settings where fault deformation topography have not been detected. Therefore, delineating concealed active faults generating such earthquakes is necessary to reduce earthquake damage; however, few studies exist to provide its clues regarding such faults. The 1984 Western Nagano Earthquake in Japan was a main shock with a magnitude of Mj 6.8 and a depth of 2 km at the source. Solid bedrocks are well-exposed in the earthquake source region; however, no surface rupture have been identified, and the active fault is known to be concealed. In this study, we collected data on striations observed in fractures by geological survey around the source area of the 1984 Western Nagano Earthquake. Using the collected data, the multiple inverse method was used to estimate the stresses that affected the striation formation. Consequently, stresses similar to acting faults in this area were detected in minor faults around the known concealed active fault. This suggests that the minor faults might be part of the damage zone that has been developed around the concealed active fault. Some minor faults were recognized in Quaternary volcanic rocks, confirming that they experienced displacements recently. This study indicates the possibility of detecting concealed active faults in the bedrock by geological survey.

Introduction

Active faults have conventionally been determined using ground surface approaches through aerial photo-decipherment, reading detailed topographic maps, and topographic identification of fault displacement based on field surveys (Imaizumi et al., 2018; Research Group for Active Faults of Japan, 1991). However, magnitude 6–7 earthquakes frequently occur in various tectonic settings where active faults have not been identified (e.g., the 1983 Mw 6.4 Coalinga earthquake (Stein & Ekström, 1992), the 1994 Mw 6.7 Northridge earthquake (Carena & Suppe, 2002), the 1998 M 6.2 Zhangbei-Shangyi earthquake (Peng et al., 2022), the 2010 Mw 7.1 Darfield earthquake (Barrell et al., 2011) and the 2010 Mw 7.2 El Mayor-Cucapah earthquake (Gold et al., 2013)). Faults that generate these earthquakes do not appear on the ground surface but exist as concealed active faults. It is challenging to identify such concealed active faults without historical earthquake records or earthquake observation data. Furthermore, the concealed active faults might grow to a scale where they appear at the surface in the future. If so, they could cause physical damage to underground facilities in geological disposal and serve as migration pathways for radionuclides to the surface (Chapman et al., 2009; Geological Disposal Technology WG, 2017; McEwen et al., 2013; SKB, 2011).

An effective method for understanding concealed active faults is estimating them based on geodetic and topographic analysis, seismic reflection, and stratigraphic data (e.g., Gold et al., 2013; Stein & Ekström, 1992). However, effective survey methods are yet to be demonstrated for areas where solid bedrock not covered by sedimentary layers with known ages are exposed on the ground surface. Therefore, it is necessary to establish an evaluation method to identify concealed active faults.

Japan is located in the plate subduction zone, where numerous earthquakes have occurred, and much data has been obtained on these earthquakes. Earthquakes occurred in Japan with unknown relationships to active faults include the 1984 Western Nagano Earthquake (Mj 6.8), the 1997 Northern Yamaguchi Earthquake (Mj 6.6), the 1997 Northwestern Kagoshima Earthquake (Mj 6.6, 6.4), the 2000 Western Tottori Earthquake (Mj 7.3), and the 2016 Central Tottori Earthquake (Mj 6.6) (K. Kato et al., 2004; Toda, 2013, 2016; Iio et al., 2021). K. Kato et al. (2004) showed that a total of 10 inland earthquakes with magnitudes of Mj 6 or greater that occurred between 1953 and 2004 in Japan; among them, there were four cases where the active fault was not known in advance. Toda (2013, 2016) obtained the probability of occurrence of surface rupture according to the magnitude based on large inland crustal earthquake data from 1923 to 2009 and found that the probability is only 20% for Mj ≥ 6.5 and 44% for Mj ≥ 7.0.

Several examples exist of earthquakes with unknown relationships to active faults within regions known as strain concentration or shear zones. Global Navigation Satellite System (GNSS) observations define these regions as areas of concentrated strain velocity, and various strain concentration zones have been recognized (Miura et al., 2004; Nishimura & Takada, 2017; Sagiya et al., 2000). In these areas, strain is released at rates exceeding a few mm/y without distinct displacement topography (Nishimura & Takada, 2017; Ohzono et al., 2011; Watanabe & Asamori, 2019). In such regions, small-scale faults that do not appear on the surface exist as concealed active faults, and their activity could generate earthquakes. Among the earthquakes mentioned, the earthquake occurred within the Southern Kyushu High-strain Zone (the 1997 Northwest Kagoshima Earthquake, the earthquake occurred within the San-in Shear Zone (the 2000 Western Tottori Earthquake, the 2016 Central Tottori Earthquake), and the strain accumulation and release processes in the strain concentration zones have attracted attention (Sueoka et al., 2021; Tamura et al., 2020).

However, some earthquakes have occurred where no active faults have been identified and no strain concentration zones exist, such as the 1984 Western Nagano Earthquake (Mj 6.8). Yoshida and Koketsu (1990) used inversion analysis of seismic and geodetic data to determine the concealed source fault that generated the main shock. In this area, active faults have not been mapped (Imaizumi et al., 2018; Research Group for Active Faults of Japan, 1991) and major surface rupture have not appeared even after the 1984 earthquake (Iio, 1990; Yamashina et al., 1985). Furthermore, because it is not a strain concentration zone (Sagiya et al., 2000; Tamura et al., 2020), it is difficult to determine it as an active area and assume that a concealed active fault is distributed there based on GNSS observations in advance. The existence of concealed active faults cannot be determined without historical earthquake records in these areas.

Generally, the internal structure of a fault comprises single or multiple fault cores and the damage zones that develop around them. Damage zones are densely developed by minor faults and extend over an area larger than the fault core (Chester & Logan, 1987; Mitchell & Faulkner, 2009; Vermilye & Scholz, 1998). Considering this, the damage zone around it could reach the surface even if an active fault is concealed. Furthermore, minor faults developing in such damage zones are likely to have been activated by the same stresses as the fault core. Therefore, if minor faults that develop in damage zones appear at the surface, the stress that affects their activity can be detected, and it could be possible to estimate a concealed active fault.

The multiple inverse method detects and isolates the stresses that affect the fault activity from the slip data of many meso-scale faults (Yamaji, 2000a; Sato et al., 2017). Here, the term “meso-scale fault” is defined as faults whose displacement can be determined in a single outcrop (Yamaji, 2000a). This method has been used primarily to recover stresses during fault activity, such as on geological faults that are thought to have been active in older times (Abe & Sato, 2021; Haji & Yamaji, 2021; Otsubo et al., 2009; Yamaji, 2000b). However, the multiple inverse method results for recently active minor faults are consistent with the current stresses in the area (Niwa et al., 2024; Tamura et al., 2020). Tamura et al. (2020) used this method to show that the activity of geological faults distributed within the Niigata–Kobe Tectonic Zone (NKTZ) is consistent with the current stresses. Niwa et al. (2024) also used this method in the Southern Kyushu High-strain shear Zone, including minor faults, and obtained similar results. These results indicate that geological surveys can detect the distribution of active minor faults, making it a promising method for detecting recently formed active areas using tools other than GNSS observations.

In this study, slip data (strike and dip of the fault plane, strike rake angle of striation, and sense of displacement) of minor faults distributed on the ground surface were collected for the source area of the 1984 Western Nagano Earthquake, where a concealed active fault is known. We used a multiple inverse method using the collected minor fault-slip data to detect the stresses that affected the activity of a group of minor faults and determine the spatial distribution of the stresses at the ground surface around the concealed active faults. The detected stresses were compared with the current regional stresses in this study area to extract areas considered to be recently active and examine whether the damage zones of the concealed active fault can be identified from the minor fault-slip data collected from the geological survey.

Background Tectonic Background

The convergence of the Eurasian Plate, Philippine Sea Plate (PSP), and North American Plate (NAP) generated a dynamic stress field that has evolved over geological timescales (10⁶–10⁷ years) due to interactions among these plates in central Japan (Koyama, 1993; Hirata et al., 2010; Hoshi, 2018a, 2018b; Hall et al., 1995; Sdrolias et al., 2004; Kimura et al., 2014). Recently, a PSP subduction direction shift event occurred between 6 and 1.5 Ma, which expected to have considerably changed the stress field in southwest Japan (Kimura et al., 2014; Nakajima, 2018; Yamaji et al., 2003). Since this shift, no evidence has been obtained that indicates that central Japan's regional stress field has substantially changed. Based on their stress field analysis, Tsutsumi et al. (2012) proposed that the stress field in central Japan remained stable and uniform since 0.1 Ma.

The NKTZ, oriented NE–SW, extends 50–100 km wide in central Japan. It is a geodetically identified high-strain-rate zone (Figure 1; Sagiya et al., 2000) and contracts in the WNW–ESE direction at a rate of approximately 10⁷ year⁻¹. Within the NKTZ, strain release occurs through major fault activities, such as the Atotsugawa and Ushikubi faults (Imaizumi et al., 2018), minor faults within their damage zones, and the activities of other mesoscale faults outside these damage zones (Niwa et al., 2011; Oohashi & Kobayashi, 2008; Tamura et al., 2020). Conversely, mesoscale faults distributed outside the NKTZ exhibit stress states inconsistent with the current regional stress field, indicating that they do not contribute to relieving the accumulated strain of the NKTZ (Tamura et al., 2020).

Figure 1. (a) An index map showing the plate assemblage around the Japan arc. The black line showing the plate boundary between the Amur Plate (AP), North American Plate (NAP), Philippine Sea Plate (PSP), and Pacific Plate (PP). (b) The topographic map of the central Japan shows the location of the study area and the Niigata–Kobe Tectonic Zone (NKTZ) (Sagiya et al., 2000). Here, black line corresponds to the plate boundaries displayed in (a), blue bold line shows the Median Tectonic Line (MTL), red line denotes the distribution map of active faults (Research Group for Active Faults of Japan, 1991), and star indicates the source of the Western Nagano Earthquake. The study area is at the southern foot of Mt. Ontake and falls outside the NKTZ to the southeast.

The 1984 Western Nagano Earthquake region is southeast of the geodetically recognized range of the NKTZ. The earthquake's epicenter was at 35.822°N, 137.560°E, with a depth of 2 km, and the main shock's seismogenic mechanism involved a right lateral displacement (Japan Meteorological Agency, 1984). Due to its shallow epicenter and subsequent aftershock activity near the surface, this site holds significant seismological importance. Consequently, high-density seismographs have been installed and observed to investigate the rupture process of microseisms (Iio, 2009; Iio et al., 1999) and the spatiotemporal stress variation (Iio et al., 2017; Yukutake et al., 2010). Although the earthquake's epicenter was near the ground surface, no major surface fault was identified by post-earthquake field survey (K. Kato et al., 2004; Yamashina et al., 1985; Iio, 1990). Through inversion analysis using seismic and geodetic data, Yoshida and Koketsu (1990) estimated a buried active fault with a N20°E strike, 74°N dip, 15 km length, and 10 km width. This study conducted stress inversion of mesoscale faults for the area surrounding this buried active fault (Figure 1).

Geological Background

In the study area, the basement rocks comprise Middle to Late Jurassic accretionary complexes of the Mino Belt, and Late Cretaceous granites and rhyolites of the Naegi–Agematsu Granite and the Nohi Rhyolite. In the northern part, the Quaternary lava of the Ontake Volcano overlays the basement rocks, and in the southern part, Quaternary intrusive rocks of Ueno basalts are distributed (Figure 2).

Figure 2. Distribution of minor fault data obtained from geological surveys and the location of each area used in the multiple inverse method; Geologic and topographical relief data are from National Institute of Advanced Industrial Science and Technology and Geospatial Information Authority of Japan, respectively. The location of the concealed active fault is referred to Yoshida and Koketsu (1990). The red star indicates the location of the source of the 1984 Western Nagano Earthquake. The results of the multiple inverse method using minor fault data in the subregions a–m are shown in Figure 5.

The accretionary complexes of the Mino Belt in this area primarily comprise sandstone and mudstone, with minor occurrences of chert and siliceous mudstone. The sedimentary rocks are well-bedded, with a few mixed rocks exhibiting a block-in-matrix structure. Based on radiolarian biostratigraphy, the accretion age is estimated to be Middle Triassic to Late Jurassic (H. Kato et al., 2022; Shuto & Otsuka, 2004).

The Nohi Rhyolite is subdivided into NOHI-1–6 stages based on its active stage and constituent units (Koido, 1991, 1993). The intrusion ages are ca. 85 Ma for NOHI-1, ca. 81 Ma for NOHI-2, ca. 75 Ma for NOHI-3, and ca. 70 Ma for NOHI-4, as indicated by their whole-rock Rb–Sr and monazite Chemical U–Th total Pb Isochron Method (CHIME) ages (Shirahase, 2005; Suzuki et al., 1998; Yamada & Koido, 2005). However, Hoshi et al. (2016) propose that the NOHI-1–4 activity occurred between 72 and 70 Ma based on zircon U–Pb ages. The NOHI-5 and NOHI-6 in this study area are estimated to be ca. 70 Ma or earlier and 68–70 Ma, respectively, based on their intrusive relationships with the surrounding granitoids (Yamada & Koido, 2005). The Naegi–Agematsu Granite comprises biotite granite and exhibits a fine–medium-grained, isogranular to porphyritic structure. Suzuki and Adachi (1998) reported the monazite CHIME age of 67.2 ± 3.2 Ma in the Naegi district south of the study area.

In the northern part of the study area, the Quaternary lava of the Ontake Volcano widely covers the basement rocks. Particularly near the epicenter of the 1984 Western Nagano Earthquake, hornblende dacite lava with well-developed plates and columnar joints, known as the Kurakoshi Lava, is extensively exposed, forming a plateau. The eruption age of ca. 0.7 Ma is determined by the whole-rock K–Ar age (Kioka et al., 1998).

Methods Criteria for Recognizing Bedding Plane Slips at Outcrops

In this study, we measured the strike and dip of the fault surface, slip orientation, and shear direction of a mesoscale fault plane. Even fractured surfaces without gouges were measured as mesoscale fault-slip data if striations were observed to collect as much data as possible. Since the measurement of the fault-slip data was based on striations on the fractured surfaces, great care was taken not to confuse the outcrops of lava or extrusive tuff with flow or plume structures. Striations were selected for measurement by carefully observing whether the striation and fracture surface were straight. The direction of shear was determined by the fracture step formed by R1 shear (Petit, 1987; Figure 3a) or by the shape of the fracture surface, such as asymmetric ridges (Figure 3b) or grooves (Figure 3c) formed by displaced crusts. Several people performed the measurements, and only fault-slip data that correlated with the striation identification and shear direction were obtained.

Figure 3. An image of the shape of striations formed by fault slip and examples of striations: (a) fracture step; the photograph shows that the front side slipped from the bottom right to the top left (large white arrow), (b) asymmetric ridges; the photograph shows that a dextral-slip sense where the front side slipped from the right to the left (large white arrow). The small white arrows indicate the position of the top of the ridge. (c) Asymmetric grooves; the photograph shows a sinistral-slip sense where the front side slipped from the left to the right (large white arrow). The small white arrows indicate the bottom of the grooves.

Some mesoscale faults in the study area are accompanied by quartz and calcite veins. Although the shear sense can be determined for these faults based on the slickenside kinematic indicator (Duruev & Rausav, 1973), it is improbable that those with hydrothermal activity formed near the surface. Moreover, the striations along the bedding plane and on the cleavage planes of the mixed rocks were most likely formed during accretionary processes. Due to their nature, these fault-slip data are unsuitable for detecting concealed active faults and were thus excluded from the stress inversion data set.

Determining whether the minor fault-slip data acquired are of tectonic or non-tectonic origin is challenging because the outcrops are frequently fragmented. This study, collected data by avoiding areas with well-defined landslide topography to exclude data of non-tectonic origin as much as possible.

Stress Tensor Inversion

Stress determination was conducted employing the multiple inverse method (Yamaji, 2000a) based on the collected fault-slip data in the field. The multiple inverse method inversely determines the stress states that explain the motion of faults from the fault-slip data based on the Wallace–Bott hypothesis (Bott, 1959; Wallace, 1951), which assumes a fault slip along the resolved shear stress on the fault surface (Sato et al., 2017; Yamaji, 2000a). The characteristic stresses in each subregion were detected by dividing the study area into 13 subregions (a–m: Figure 2) to make the area as small as possible while collecting for a large number of minor fault-slip data. Specifically, each subregion included a minimum of 20 minor faults-slip data, and the location of the minor faults were confined within a maximum distance of 3 km within each subregion. The multiple inverse method estimated the stress state parameters (mean stress, azimuth and plunge of σ₁ and σ₃, and stress ratio) for each subregion. The stress ratio (R) is defined by the formula described by Yamaji and Sato (2006): [Image Omitted. See PDF]

The mean angular stress distance (Θ ̅), spanning from 0° to 90°, depicts the spread of the stress state within the subregion. For Θ ̅ = 0°, the data should have no noise and be explained by a single stress state (Yamaji & Sato, 2006).

The stresses determined in each subregion are compared to the current regional stresses in the study area. As mentioned in Section 2.1, the regional stress field in this region is thought to have changed over geological time, indicating that the mesoscale formed under the current stress state is likely to have been recently active. The current stress state in this study area is deduced as σ₁ (291.33°, 2.11°), σ₃ (199.34°, 43.34°), and R = 0.080, estimated from the source mechanism solutions of microearthquakes shallower than 20 km (Uchide et al., 2022a, 2022b). The angular stress distance (Θ) quantified the similarity between the average stress in each subregion and the regional stress (Yamaji & Sato, 2006). When confronted with two equivalent stress states, the Θ value is 0°, signifying fault movements in the same direction. Conversely, when Θ = 180°, two stresses induce a fault to move in opposite directions regardless of the fault's attitude. The parameters above were calculated using the multiple inverse method software package (Yamaji et al., 2004).

Results Fault Occurrence

This study collected 344 minor fault-slip data by geological survey. The surface lengths in the collected data range from large fracture surfaces that penetrate outcrops as high as 5 m to fracture surfaces of approximately 10 cm. The minor fault planes are both closed and open. Fracture surfaces where striations can be identified are often flat. Striations on non-flat fracture surfaces were not collected in this study to avoid collecting non-faulting data (e.g., plumose structures, sedimentary structure, flow structures, etc.). Most slip data were collected from striations developed on the bedrock surfaces, and only four data were obtained from minor fault planes of 5–30 cm breccia width in the minor fault plane and two data with clay sandwiched within them.

Minor fault-slip data were collected from various geological units, including Jurassic accretionary complexes, Cretaceous igneous rocks, and Quaternary lava. Ten minor fault-slip data points were in Quaternary volcanic rocks (Figure 4). However, trends in the attitude of the minor fault planes were not characterized for each geological unit.

Figure 4. Example of striations in Quaternary volcanic rocks. Striations can be seen from the upper left to the lower right of the photograph, which were determined to be minor faults with a sinistral-slip sense based on the asymmetric grooves. The small arrows indicate the bottom of the grooves.

Stress State

Figure 5 shows the subregion classification of the minor fault data used for the multiple inverse method and its results. The detected stress attitude and ratios differ significantly for each subregion. However, in subregions b, e, and k, in the surface trace of the concealed active fault extension (Yoshida & Koketsu, 1990), the orientation of the σ₁ axis is typically distributed WNW–ESE to NNW–SSE. The data with low-stress ratios are concentrated, especially in the NW–SE to NNW–SSE direction.

Figure 5. Results of multiple inverse method using minor fault data from geological surveys (pared stereonets: σ1, σ3). The location of subregions a–m shown in Figure 2. The lower right shows the stress axis for this study area calculated from focal mechanism data by Uchide et al. (2022a, 2022b).

Table 1 presents data on the stress states detected in each subregion. The mean angular stress distance, representing the variation of the stress state detected in this study, ranged from 17.934° to 59.380°. The analysis results for subregions a, b, c, h, and i showed that the mean angular stress distance was less than 45°. In particular, subregions h has the smallest mean angular stress distance in this study (17.934°), and the detected stresses have σ₁ from north to south (Figure 5). However, the mean angular stress distance for subregions d, e, f, g, j, k, l, and m exceeds 45°.

Table 1 Average Stress Axis Data Detected From the Multiple Inverse Method in Subregions a–m

[Table omitted. See PDF]

The angular stress distance between the stresses detected in each subregion based on the regional stresses of Uchide et al. (2022a, 2022b) (Table 1) yielded moderately small angular stress distance data in subregions b, c, e, and k. In Yamaji and Sato (2019), the angular stress distance where two stress states can be regarded as similar ranged from 0° to 59.26°. The angular stress distances in subregions b, c, e, and k were lower than 0°–59.26°, where the two stress states are similar.

Discussion Areal Dependence of Stress State

Some minor fault data collected in this study were found in Quaternary lava. This indicates that activity after 0.7 Ma (Kioka et al., 1998), the age of the lava, is also recorded in the minor faults. This result indicates that fault activity has occurred in this area very recently, after 0.7 Ma.

Here, we compare the current stresses in this study area with those obtained by the multiple inverse method. Stresses acting on this area have been analyzed using the main shock and subsequent aftershocks of the 1984 Western Nagano Earthquake (Iio et al., 2017; Yukutake et al., 2010). Yukutake et al. (2010) obtained analytical results with the σ₁ axis in the WNW–ESE direction as the “whole region” stress. This “whole region” is almost the same area as the whole this study area, and its stresses are important as reference data. In addition, the stress of Terakawa and Matsu'Ura (2010) and Uchide et al. (2022a, 2022b), which were obtained from the focal mechanism data, are examples of current regional stress studies. According to these studies, the current regional stress in this area is generally the stress with the σ₁ axis in the WNW–ESE direction similar to the stress direction suggested by Yukutake et al. (2010).

In this study, we used the data of Uchide et al. (2022a, 2022b), because numerical stress data have been published. The angular stress distances for each subregion based on the stresses from Uchide et al. (2022a, 2022b) (Table 1) were smaller in subregions b, c, e, and k than 59.26° (Yamaji & Sato, 2019). Of these, b, e, and k are in the surface trace of the extension of the concealed active fault (Yoshida & Koketsu, 1990) and are the closest to this fault. The only region c has smaller angular stress distance than 59.26° and is close to the surface trace of the concealed active fault (Yoshida & Koketsu, 1990). This result indicates that a group of minor faults that might have been active under the current stress is distributed near the concealed active fault.

A closer look at b, c, e, and k reveals a systematic deviation of σ₁ axis approximately 20° clockwise from the current regional stress in this study area (Uchide et al., 2022a, 2022b). There are some cases that the σ₁ axis close to the faults show systematic deviations compared to the surrounding stresses. For example, the σ₁ axis of stresses close to San Andreas fault (dextral) shows a 20–40° clockwise from that of surrounding area (e.g., Hardebeck & Hauksson, 2001; Townend & Zoback, 2004; Zoback et al., 1987). Hardebeck and Okada (2018) have also shown through modeling that the σ₁ axis close to faults rotates after earthquakes. Niwa et al. (2024) used the same method as in this study to obtain stresses, and found a systematic deviation of σ₁ axis approximately 20° anticlockwise from the current regional stress in the Southern Kyushu High-strain shear Zone (synistral). The concealed active fault does not reach the surface, to reflect local stress rotation due to complex fracture development at the fault edge (e.g., Wing cracks, Splaying, Horsetailing, Antithetic shear fractures; Fossen, 2016). Therefore, it is necessary to increase data density and collecting data outside the strike edge of the concealed active fault to examine the effects of complex local stresses at fault edges, as described above.

However, the stresses detected in subregions a, d, f, g, h, i, j, and m were very different from the current stress in this study area or were not settled in one distinct stress state, as indicated by the large mean angular stress distance. The angular stress distance with current stress in this study area (Uchide et al., 2022a, 2022b) in these subregions exceed the 59.26° (Yamaji & Sato, 2019), making it unlikely that they are a group of minor faults activated by the same stresses as currently. The stresses in subregions d, f, g, and j (Figure 5) indicate that stresses with σ₁ in the NNW–SE to NW–SE directions were also detected. However, the mean angular stress distance, which shows the degree of variation of the stresses in these subregions, is large in this study, at more than 45° for all, showing that they are not settled in one distinct stress state. Therefore, the effect of a single stress cannot explain the minor fault data collected in this study (Yamaji & Sato, 2006). These subregions are at different locations from the surface trace of the extension of the concealed active fault. Based on the above, minor fault groups that might have been active under the effect of stresses similar to the current could be concentrated near the concealed active fault.

The mean angular stress distance of the stresses in this study takes 38°–48° when looking at b, e, and k directly above the fault (Table 1). This result is interpreted as a scattered analysis with a high noise level (Yamaji & Sato, 2006). It is possible that the minor faults recognized in the field include those with complex activities that do not depend on the Wallace–Bott hypothesis, such as those caused by interacting faults and faults with significant block rotations (Nieto-Samaniego & Alaniz-Alvarez, 1997; Twiss & Unruh, 1998; Yamaji, 2003). In addition, most data in this study relied only on striations observed on the fractured surfaces, and only a few minor faults could be confirmed by outcrop observations of fault displacement and fracture continuity. Therefore, it is possible that the activities that formed the striations include not only tectonic activities, but also nontectonic ones influenced by the topography of the site. For example, the minor fault-slip data of subregions d, g, j, m were acquired along a mountainside road that has high relative height from the bottom of the valley. The mean angular stress distances in these subregions are 45–60°, which are larger than those of the subregions near the bottom of the valley. Therefore, it is possible that the topographic conditions at the sites the data were acquired may have influenced the stresses. The above factors might have contributed to the high mean angular stress distance value in the analysis.

The regional stress in central Japan stress field has been transitioned by various tectonic events up to the shift toward subduction of the PSP between 6 and 1.5 Ma (Hall et al., 1995; Hirata et al., 2010; Hoshi, 2018a, 2018b; Kimura et al., 2014; Koyama, 1993; Nakajima, 2018; Sdrolias et al., 2004; Yamaji et al., 2003). After the PSP subduction direction shift, it is considered to have been placed in a stress state similar to that of the current. Therefore, subregions b, c, e, and k, where similar stresses to the current are detected, are thought to be dominated by minor faults recently active from 6 to 1.5 Ma onward.

As described above, these minor faults are distributed in the area closest to the concealed active fault (Yoshida & Koketsu, 1990). The displacement of the concealed active fault is concentrates on the fault core at the main strand of the fault, whereas, it is dispersed on fanning secondary faults at tip of the fault (Matsuda, 2000; Preuss et al., 2020). Cappa et al. (2014) show that even at large distances from the main slip plane of a fault, the effects of faulting activity extend to several 10% of the fault length. The concealed active fault is 15 km long and 10 km wide and is approximately 1 km from the top of the concealed active fault to the ground surface (Yoshida & Koketsu, 1990). Therefore, the damage zone of the concealed active fault might appear at the ground surface, indicating that the group of minor faults, for which analysis results are consistent with the current stress near the concealed active fault, are part of the damage zone of the fault (Figure 6).

Figure 6. A sketch showing the distribution of minor faults around the concealed active fault in the study area. The minor fault groups that form damage zones that develop around the concealed active fault might be visible at the surface.

Possibility of Detecting Concealed Active Fault in Unclear Active Fault Topography Areas

As described in 5.1, the results of multiple inverse method of minor faults in this study shows the possibility of confirming the damage zone areas that develop around the concealed active fault. This study area is one of the few sites where the effects of the concealed active fault activity on the ground surface can be observed because the concealed active fault is close to the ground surface (Yoshida & Koketsu, 1990). This study's results are critical data showing that capturing the effects of the concealed active fault activity by geological surveys is possible.

However, no displacement topography due to fault activity has been observed on the ground surface. JAEA & CRIEPI (2021) conducted lineament interpretation in this area but did not detect topographically distinct and well-continuous lineaments. The geological survey in this study also revealed a few minor faults with intrafault material, such as gouges. These findings indicate that the study area is without significant displacement at the surface.

Since the results obtained in this study differed from the current stress in areas other than directly above the concealed active fault, it could affect detecting a zone of possible damage directly above the fault. The minor faults found in the area other than directly above the fault are thought to record activity before the current stress environment. This result indicates that the stress environment in this area has changed in the past and could have allowed us to detect a group of minor faults formed by recent activity. Thus, detecting different stresses in the area directly above the concealed active fault and other areas is considered crucial for recognizing a concealed active fault.

As described above, the survey method in this study shows potential for detecting concealed active faults, although specific conditions are necessary. The results demonstrate the usefulness of geological surveys for detecting concealed active faults.

Conclusions

In this study, we collected data on striations in fractures through a geological survey around the source area of the 1984 Western Nagano Earthquake, where a concealed active fault is known, and conducted the multiple inverse method. Consequently, the following findings were obtained.

• Minor faults were observed in the 0.7 Ma lava in the study area, and we could obtain data on their recent activities. This minor fault data in lava are crucial because it shows that collecting data on recently active minor faults through geological surveys is possible.

• The multiple inverse method results in each subregion show that stresses consistent with the present stresses were detected near the surface trace of the extension of the concealed active fault. Therefore, the group of recently active minor faults is distributed in the subregion closest to the concealed active fault, showing the possibility of detecting recently active areas by geological surveys. These subregions include the lava (0.7 Ma) distribution, consistent with the distribution of recently active minor faults.

• No displacement topography and faults with gouge were observed in this area, indicating that displacement at the surface is insignificant. The results show that even in such an area, it might be possible to determine currently concealed active faults by examining striations recorded in fractures through geological surveys.

Acknowledgments

We thank Takanori Ishihara of JAEA and Yuki Hiura of JAEA (currently JGI Inc.) for their assistance with field work. We appreciate the valuable comments by Dr. Shigeru Sueoka, Dr. Noriaki Abe, and Dr. Tomonori Tamura of JAEA. We thank Yuri Kato of JAEA for his support to aggregate field data. We thank the members of the Setogawa Forest Office for their support in field work. This study was funded by the Ministry of Economy, Trade and Industry, Japan as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (JPJ007597) (Fiscal Years 2020–2022).”

Data Availability Statement

The minor faults slip data used for multiple inverse method in this study are available at Nishiyama (2023).