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Abstract
The June 17, 2019, MS 6.0 Changning earthquake is the largest recorded event in the Sichuan basin, spatiotemporal variations of stress field may shed light on the seismogenic mechanism of the earthquake. We determined the focal mechanism solutions (FMSs) of 124 earthquakes with MS ≥ 3.0 occurring in the Changning area from April 1, 2007, to February 29, 2020, and analyzed changes of FMSs and stress field before and after Changning earthquake. The Changning aftershocks were predominantly thrust fault earthquakes, followed by strike slip. The P-axis azimuths of the aftershock FMSs were oriented predominantly in the NEE direction, notably differing from the NWW-oriented P-axis azimuths of pre-earthquake FMSs; it shows the rotation of local stress field before and after the Changning earthquake, it is speculated that the change of stress field in Changning area may be caused by long-term water injection and salt mining activities. From the southeast to the northwest of the aftershock zone, the azimuths of principal compressive stress (S1) change from NEE to near-EW in both horizontal and vertical planes. Significant changes occurred in the FMS types and stress field of the aftershock zone following the Changning earthquake, the FMSs became diverse, the S1 azimuth of the Changning area changed from NWW to NEE, and then EW, the plunge and stress tensor variances increased, it reflects that the stress field of the Changning area adjusts continually with time.
Highlights
The Changning Ms 6.0 earthquake is the largest recorded earthquake in the Sichuan Basin
Based on the one-dimensional velocity model in Changning area (Yi et al, 2019), the focal mechanism solutions (FMSs) and focal centroid depth of MS≥3.0 events in the Changning earthquake sequence were inverted by selecting high signal-to-noise ratio and broadband waveform records of stations within 250 km from the epicenter (Figure 1)
The fitting errors of the FMSs at different focal depths are shown in Figures 2A,C, the fitting errors are minimized at a focal depth of 3 and 8 km, the corresponding FMSs are optimal solutions
Summary
The Changning Ms 6.0 earthquake is the largest recorded earthquake in the Sichuan Basin. It occurred at 22:55:43.3 on June 17, 2019 in the Changning County of Yibin City in Sichuan Province according to the China Earthquake Networks Center. The epicenter was located at 28.34°N 104.90°E, with a focal depth of 16 km. The FMSs of the main shock show a set of nodal planes with the strike of (102°–135°) which is similar to that of the axis of the Changning anticline. The azimuth/dip angles of P-axis are 70°–76° and 7°–10°, respectively. The dislocation type of the main shock is thrust and strike-slip, and the focal depths are relatively shallow (only 3 km). The reported moment magnitude is of 5.68–5.79 (Lei et al, 2019; Yi et al, 2019; Liang et al, 2020; Liu & Zahradník, 2020; Hu et al, 2021)
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