Seismicity Patterns and High-Resolution Fault Interface Properties Associated with the 2010 Mw6.9 Yushu, Qinghai Earthquake

The 2010/04/13 Mw6.9 Yushu earthquake ruptured the Yushu segment of the left-lateral Ganzi-Yushu fault in eastern Tibet. This event was preceded by an Mw4.9 foreshock about two hours before the mainshock, and was followed by numerous aftershocks with the largest magnitude of 6.1. While the foreshocks and mainshock were only recorded by sparse permanent stations, its aftershocks were recorded by many temporary seismic stations deployed right after the mainshock, providing a rich data set for better understanding of the sequence and detailed fault zone structures. We first conduct a detailed study of the foreshock sequence by examining continuous waveforms recorded at station YUS near the mainshock rupture zone. We manually identify foreshocks with double peaks in the 5-Hz high-pass-filtered seismograms, and use waveforms of 12 earthquakes listed in a local catalog as templates to scan through the continuous waveforms for new detections. We have identified up to ~200 events in the last 9000 s before the Yushu mainshock. The waveforms of the newly detected events are very similar, suggesting that they occurred in a relatively compacted region. The entire sequence could be largely considered as aftershocks of the Mw4.9 foreshock, with statistical parameters (b and p values) not much different with aftershock sequences elsewhere. Our results suggest that the accelerated nucleation process of a large earthquake, as inferred by laboratory experiments and seismic studies, may not be reliably observed. In the second study, we examine fault zone head waves (FZHW) that refracted along the bi-material interface and recorded at stations on the slower side of the fault from the dense temporary stations. Our preliminary results have shown that several stations on the SE side have recorded low-amplitude FZHW-like phases with opposite polarities before the sharp P wave arrivals, suggesting that the SE side of the fault has lower seismic velocities on the NW side. Our next step is to systematically identify those FZHW phases and use them to better image deep fault interface properties along the rupture zone of the Yushu earthquake.