Rise time and source duration of the 2008 M W 7.9 Wenchuan (China) earthquake as revealed by Rayleigh waves

Abstract

The fault parameters of the 2008 Wenchuan earthquake were studied in a rupture directivity analysis by simultaneously inverting the period of the rst Fourier spectral-node and the 100-s phase-delay time of the Rayleigh wave. The results show that the earthquake is a unilateral event with an optimal rupture azimuth of N59• E, consistent with the distribution of aftershocks. They also indicate that the fault plane strike is in the NE-SW direction, corresponding to the fault plane strike of 238• and NW-dipping (reported by the USGS). The inversion shows the source duration (including the rise time and rupture time) and rise time are 70±0.8 s and 9.3±0.6 s, respectively. The rupture velocity estimated only from the rupture time exhibits relatively higher value, 3.45±0.10 km/s, close to or larger than the S-wave velocity in the crust. One possible cause is that the rupture mechanism transferred from the thrust faulting in the southwestern portion of the fault to the strike-slip faulting in the northeastern one. The rise time offers an estimate of the dynamic stress drop (37.8±2.3 bars), from which through a macroscopic view the radiated seismic energy of (5.93±0.4) × 10 16 N m is calculated. Although the estimated rupture length (∼210 km) and source duration are shorter than several source rupture models, the current analyses show the rst-order rupture feature of the 2008 Wenchuan earthquake rupturing the Longmenshan fault zone.

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

“…2b were signals released from the main shock, this study only estimates the source duration to be 71 sec. Although the estimated rupture length and source duration are shorter than those derived from several source rupture models (e.g., Ji and Hayes 2008;Nishimura and Yagi 2008;Hao et al 2009;Shen et al 2009;Hashimoto et al 2010;Nakamura et al 2010;Wen et al 2012) and field surveys of surface ruptures (Lin et al 2009;Liu-Zeng et al 2009;Xu et al 2009a;Zhou et al 2010), this study is comparable to those from rupture directivity analysis of surface waves (Hwang et al 2011) and a source rupture model of Sladen (2008). An empirical relationship between the seismic moment and source duration (Furumoto and Nakanishi 1983) suggests that the 2008 Wenchuan earthquake should have a source duration of ~67 sec from M 0 = 7.448 × 10 20 Nm (this study), ~71 sec from M 0 = 8.97 × 10 20 Nm (GCMT), and ~73 -76 sec from M 0 = 1.0 -1.1 × 10 21 Nm (Wen et al 2012;Harztell et al 2013).…”

“…1). An analysis of surface waves indicated the optimal rupture direction of N59°E which agrees with the distribution of aftershocks (Hwang et al 2011). Here we assumed that all sub-events occurred at a focal depth of 12 km and successively ruptured the Longmenshan fault zone from the epicenter toward N59°E.…”

“…The transition in mechanism from thrust to strike-slip faulting occurs ~34.0 sec later after the onset and is ~100 km NE of the epicenter using an average rupture velocity of 3.0 km s -1 (Hwang et al 2011;Hartzell et al 2013). The location of mechanism transformation is near the town of Beichuan (Fig.…”

“…By adopting an average rupture velocity ~3.0 km s -1 derived from the rupture directivity analysis of surface waves (Hwang et al 2011) or the source rupture model used by Hartzell et al (2013), we derived a rupture length of approximately 210 km for the 2008 Wenchuan earthquake. The rupture length derived from this study is the so-called effective length which was created by the main energy release during earthquake rupture.…”

“…1). Analysis of rupture directivity from surface waves also showed a northeastward rupture with an azimuth of N59°E (Hwang et al 2011). Field surveys identified an obvious surface rupture terminating near the town of Shikan (also see Fig.…”