Microseismic activity in the last five months before the m (sub w) 7.9 Wenchuan earthquake

Several recent studies have shown that large crustal earthquakes are often preceded by microseismic activities that accelerate or migrate in space and time. However, it is not clear whether such patterns are universal for other large earthquakes that do not have obvious foreshocks. Here, we apply a matched-filter technique to detect missing earthquakes around the rupture zone of the 2008 M (sub w) 7.9 Wenchuan earthquake from 1 January to 12 May 2008. Using continuous waveforms recorded by the Zipingpu reservoir seismic network, we detect approximately 4 times more earthquakes than listed in a local catalog. The newly detected events did not show any systematic spatiotemporal migration before the Wenchuan earthquake. The seismicity rate is rather stable during the entire study period, except following an M (sub L) 3.7 earthquake on 14 February 2008 that likely occurred on the extension of the blind-thrust fault beneath the Sichuan basin. We also inspect 2-hr-long seismograms right before the Wenchuan earthquake and confirm that there was no immediate foreshock. Finally, we do not find any correlation between the local seismicity and the water level of the Zipingpu reservoir in the last few months before the Wenchuan earthquake. Electronic Supplement: Figures of seismicity, satellite map, focal mechanisms with depth resolution, and static Coulomb stress changes.