Foreshocks of the 2010 M_w 6.7 Yushu, China Earthquake Occurred Near an Extensional Step-Over

We conduct a detailed study of the foreshock sequence preceding the 2010 M-w 6.7 Yushu, Qinghai earthquake in the Tibetan plateau by examining continuous waveforms recorded at a seismic station near the mainshock rupture zone. By using a deep learning phase picker-EQTransformer and a matched-filter technique, we identify 120 foreshocks with magnitude ranging from -0.7 to 1.6, starting with a M-w 4.6 foreshock approximately 2 hr before the M-w 6.7 Yushu mainshock. Our analyses show that the foreshock sequence follows a typical Omori's law decay with a p-value of 0.73 and the Gutenberg-Richer frequency-magnitude b-value of 0.66. We do not find any evidence of accelerating events leading up to the Yushu mainshock. Hence, they could be considered as aftershocks of the M-w 4.6 earthquake. We further invert for the focal mechanisms and rupture directions for both the largest foreshock and the mainshock. The M-w 4.6 foreshock likely occurred on a NE-SW trending fault conjugating to the NW-SE trending fault of the mainshock. Coulomb stress analysis suggests the M-w 4.6 foreshock induces negative stress on the mainshock source area. These observations do not support either the pre-slip or the cascade triggering model for foreshock generation. The occurrence of the foreshock, mainshock and large aftershocks appear to be modulated by the Earth's tidal forces, likely reflecting the role of high pore-fluid pressures. Our observations, together with other recent studies, suggest that extensional step-overs and conjugate faults along major strike-slip faults play an important role in generating short-term foreshock sequences. We use both deep learning and matched-filter methods to detect foreshocks that occur 2 hr before the magnitude 6.7 Yushu, Qinghai earthquake in Western China on 13 April 2010. The foreshock sequence begins with a magnitude 4.6 foreshock and the activity gradually decays with time. We do not observe an increase of magnitude or number of the events right before the mainshock. Therefore we interpret this as a typical aftershock sequence of the magnitude 4.6 earthquake. We also resolve the fault geometry and rupture direction. The results suggest the magnitude 4.6 earthquake ruptures along a fault that is nearly perpendicular to the fault broken during the magnitude 6.7 mainshock. Due to the fault geometry and relative location, the magnitude 4.6 foreshock might have inhibited the occurrence of the mainshock by reducing the stress on the fault. These observations do not support either the pre-slip or cascade triggering models for foreshock generation. We calculate the theoretical tidal stress and strain in the study area and observe a correlation of earthquake activity and the tidal stress change. This correlation might provide evidence for fluid driven process. In conclusion our research shows complex geometry along major strike-slip faults can influence foreshock behaviors.