Multiple Event Analysis of the 2008 Mw 7.9 Wenchuan Earthquake: Implications for Variations in Radiated Seismic Energy During Faulting

Abstract

A forward modeling of P-waves for the 2008 Wenchuan earthquake revealed at least seven sub-events that occurred during faulting with the largest event (i.e., the third sub-event) located at a position ~50 km northeast of the epicenter. Simulations of P-waves showed that it would be more appropriate to model the P-waves using thrust faulting for the first three sub-events and using strike-slip faulting for the last four. In other words, the faulting for the 2008 Wenchuan earthquake was composed substantially of two mechanisms; the former was a thrust faulting and the latter was a strike-slip rupture. The mechanical transition was near the town of Beichuan, ~100 km northeast of the epicenter. Variations in radiated seismic energy (E S ) showed the largest E S released from the fourth sub-event. Results also indicated remarkable distinctions between E S and E S0 (called the available energy). On the whole, the total E S , which was higher than E S0 estimated from static stress drop, suggested that the earthquake should be interrupted by a stress model of abrupt-locking. Further, the former thrust faulting released a relatively lower amount of E S than the latter strike-slip event. Orowan's stress model, i.e., E S ≈ E S0 , can specify former thrust ruptures implying a high rupture velocity. Because E S > E S0 for latter strike-slip ruptures, a stress model of abrupt-locking, implying higher dynamic stress drop and lower friction during an earthquake, can account for the feature of the latter ruptures. This might suggest that the 2008 Wenchuan earthquake should have a high rupture velocity, perhaps approaching the crustal Swave velocity or even higher.

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

“…The total E S of 1.76 × 10 14 Nm obtained in this study was approximately three times larger than that estimated from the integral of velocity seismograms and reported by the IRIS (5.0 × 10 13 Nm; http://ds.iris.edu/spud/eqenergy/16344247). Our higher E S might indicate that highfrequency energy can be retrieved from the multiple-event analysis (e.g., Kikuchi and Fukao 1988;Hwang 2013). The largest E S was in the third sub-event (Table 1); however, the fourth sub-event had the highest E S /M 0 ratio.…”

“…2. In this study, we produced the synthetic teleseismic P-waves without source duration at the location of mainshock as the EGF instead of the reference earthquakes (aftershocks or small-sized earthquakes around the mainshock; Hartzell 1978) (e.g., Ruff and Kanamori 1983;Bilek et al 2004;Ammon et al 2006;Chu et al 2009;Hwang 2013;Benz and Herrmann 2014). The time-domain deconvolution can be regarded as a waveform inversion, so being controlled by large amplitudes in observations (Ruff and Kanamori 1983;Bilek et al 2004).…”

“…A result of P-wave back-pro-jection, obtained by Jian et al (2019), also displayed a slow rupture velocity, 1.85 -2.0 km s -1 . In addition to the aforementioned methods, the source time function (STF) would also be a useful approach for investigating the complex ruptures of the source (e.g., Ruff and Kanamori 1983;Hwang 2013). The STF, which describes the ruptures of an earthquake from its initiation to its end, is a crucial source parameter in understanding an earthquake's rupture time history (e.g., Tanioka and Ruff 1997).…”

“…The EGF could be also a synthetic wave, created in the position of the mainshock, to avoid inconsistency of the focal mechanism between the mainshock and EGF (e.g., Ruff and Kanamori 1983;Bilek et al 2004;Ammon et al 2006;Chu et al 2009;Hwang 2013;Benz and Herrmann 2014). Through the STF, not only can the source rupture history be understood, but the source duration, seismic moment, radiated seismic energy, multiple sources, and static stress drop can be estimated (Bilek et al 2004;Ammon et al 2006;Hwang 2013;Vallée Included also in this diagram are the focal mechanisms reported by the CWB, AutoBATS, and GCMT. The insert in the lower right denotes the rupture velocity (Vr) variation during the Hualien earthquake.…”

Rupture Directivity of the 2019 ML 6.3 Xiulin (Taiwan) Earthquake Estimated by Near-Field Seismograms: Implications for Source Scaling During Faulting

“…Through multiple-event analysis of the STF (cf. Hwang 2013;Hwang et al 2019), the mainshock comprised at least three subevents, each of which had its own isosceles triangle and source duration (Table 1 and Fig. 2).…”