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

Abstract

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.

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