Multiple-event analysis of the 2018 ML 6.2 Hualien earthquake using source time functions

Abstract

Through forward multiple-event analysis of teleseismic P-waves using source time functions (STFs), derived by non-negative time-domain deconvolution, we inferred the rupture features of the 2018 Hualien earthquake. At least six sub-events composed the Hualien earthquake, with the largest one (corresponding to M_w = 6.3) occurring 4.8 s later than the initiation of rupture. The total seismic moment (M₀) of 6.48 × 10¹⁸ Nm (M_w = 6.5) and radiated seismic energy (E_S) of 1.76 × 10¹⁴ Nm led to the E_S/M₀ ratio ~2.72 × 10^-5. A static stress drop (Δσ_S) of 5.03 MPa was also derived for the earthquake. On average, the rupture parameters of the 2018 Hualien earthquake from this study were similar to globally average values. From M₀ and source duration (10.9 s), this implied an average rupture velocity (Vr) less than 2.0 km s^-1. The forward multiple-event modeling showed that Δσ_S varied with the sub-events and increased with E_S/M₀ to imply the frictional strength being heterogeneous along the fault. From the highest STF peak (6.9 s after the initiation) near the land-sea interface, we suggested that the Hualien earthquake be divided into two rupture processes. One with low Δσ_S, low E_S/M₀, and high Vr occurred at sea; the other with high Δσ_S, high E_S/M₀, and low Vr occurred on land. Both seawater and local velocity structures probably played crucial factors behind these rupture discrepancies during the 2018 Hualien earthquake.

Key points

• Source time function (STF) is derived by a non-negative time-domain deconvolution

• The STF is decomposed to obtain six sub-events constituting the Hualien earthquake

• Variation in ES/M0 implies that frictional strength is heterogeneous during faulting