Inversion of the rupture process and high-frequency radiation of the 2017 Jiuzhaigou earthquake, northeastern Tibetan Plateau

Abstract

We studied the low- and high-frequency source processes of the 2017 Jiuzhaigou earthquake, and focused on when and where the high-frequency (2.0–10.0 Hz) seismic energy was radiated in relation to the low-frequency (0.02–0.5 Hz) seismic energy. The low-frequency source processes were jointly inverted using teleseismic P and SH broadband waves combined with near-field strong motion data. A three-dimensional model considering real topography was used to compute the near-field Green’s function. The high-frequency wave radiation was inversed using the envelope of acceleration seismograms of near-field records and the empirical Green’s function by the differential evolution algorithm. The results showed the 2017 Jiuzhaigou earthquake was a typical strike-slip earthquake with a seismic moment of about 6.9 × 1018 N m and maximum slip of about 1.5 m. During the 2 s following the initial rupture, the earthquake radiated large high-frequency waves but small low-frequency waves. Then, the rupture broke the high stress drop area in the up-dip direction of the hypocenter, generating an obvious asperity located just above the hypocenter. About 60% of the seismic moment of the earthquake was released during about 2–6 s after initiation of the rupture. The high-frequency radiation areas were mostly located on the lower periphery of the asperity, in accordance with the complementary relationship between the distribution of high-frequency waves and zones of large slip, which might be attributable to both the initiation of the rupture and the breaking of the high stress drop area. Parts of the high-frequency radiation areas overlapped the lower part of the asperity, where the high-frequency radiation occurred about 2 s earlier than the low-frequency radiation. The stress drop of the 2017 Jiuzhaigou earthquake was considerably lower than average for global intraplate earthquakes, but similar to that of the 2013 Lushan earthquake.