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 (ES) showed the largest ES released from the fourth sub-event. Results also indicated remarkable distinctions between ES and ES0 (called the available energy). On the whole, the total ES, which was higher than ES0 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 ES than the latter strike-slip event. Orowan¡¦s stress model, i.e., ES ≈ ES0, can specify former thrust ruptures implying a high rupture velocity. Because ES > ES0 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 S-wave velocity or even higher.

Keywords

• Multiple event analysis
• Radiated seismic energy
• Available energy
• Orowan’s stress model
• Rupture velocity
• Dynamic stress drop