Seismologic characteristics of the 2017, <italic>M</italic><sub>s</sub>7.0 Jiuzhaigou earthquake, Sichuan, China

Abstract

The M s7.0 Jiuzhaigou earthquake with the hypocenter depth of 20 km occurred at the NNW extension line of the Huya fault located next to the eastern boundary of the Bayan Har block. The focal mechanism solution of the mainshock based on the W-phase source inversion showed a sinistral slip and the moment magnitude was M w 6.5. Over 3000 aftershocks were recorded within six days, among which the largest was M 4.8 and three others were larger than M 4.0. The value of the aftershock frequency attenuation coefficient P was 1.03, and the b value of the magnitude-frequency relationship was 0.68. The aftershock activities were relatively inactive but the attenuation was normal. Relocation results of 603 aftershocks were obtained by using the double-difference earthquake location algorithm and the location errors of the EW, NS and the vertical directions were 0.81, 0.84 and 1.59 km respectively. The mainshock occurred in the center of the aftershock region and ruptured on both sides along the strike direction. The aftershocks were distributed linearly along a NNW direction, and the fault plane was nearly vertical with a length over 30 km and a width of about 15 km. Aftershocks were more concentrated in the southeast than in the northwest. The focal mechanism solutions of most aftershocks in the central and southeastern regions were consistent with the mainshock, while those in the northwest were thrust-types, which might be related to the Minjiang fault. Meanwhile, the regional stress was released more in the central and southeastern regions than in the northwest. The maximum intensity investigated in the damage zone was up to IX. The maximum intensity region was located in scenic areas, therefore few people were present when the earthquake occurred at night. The peak acceleration was highest in the NS records, followed by the EW and vertical records. The design spectra of intensity for Jiuzhaigou was VIII, which is higher than the acceleration response spectra obtained from the Jiuzhaibaihe station, the nearest station which had strong motion recordings. These aforementioned factors may have contributed to the minor casualties resulting from this earthquake. The aftershock regions of the Jiuzhaigou earthquake, the 1973 M s6.5 Songpan northeast earthquake, and the 1976 M s7.2 Songpan earthquake are connected along the Huya fault which may extend to the northwest in the deep earth. Considering the rupture displacement distribution of the mainshock, the focal mechanism of aftershocks, and the stress drop, strong aftershocks are unlikely to occur in the central and southeastern Jiuzhaigou aftershock regions; however, the development of seismicity in the northwest and the Minjiang River fault should be noted. Static Coulomb stress triggering of other 4 earthquakes larger than M 6.5 occurred near the Ganqingchuan border were calculated using the Coulomb 3.3 software. The results indicated that the Jiuzhaigou earthquake was triggered by the Wenchuan earthquake and related to the stress adjustment in the study area.