关键词: 维西-乔后断层, 双差层析成像, 地震定位, 速度结构

Abstract:

The occurrence of strong earthquake is closely related to the distribution of crustal velocity anomalies. Some studies have shown that strong earthquakes occur in the transition zone between high-velocity anomalies and low-velocity anomalies in the middle and upper crust or inside the low-velocity anomaly zone. Thus, high-resolution imaging of the velocity structure in the seismic source area and accurate earthquake location can assist the evaluation of seismogenic settings of strong earthquakes. On May 21, 2021, an M_S6.4 earthquake occurred in Yangbi, Yunnan with casualties and property losses. The epicenter region of the Yangbi earthquake is in the western Yunnan area of the Sichuan-Yunnan block, which is located on the southeastern edge of the Qinghai-Tibet Plateau and characterized with intensive tectonic activity. Previous studies in this area are mostly on regional scales, and lacking on the three-dimensional fine crustal velocity structure in the Yangbi earthquakes area. To investigate the seismogenic environment and source characteristics of the 2021 Yangbi M_S6.4 sequence in Yunnan, we used the P-wave and S-wave arrival data of 12 652 earthquakes recorded by both the Yunnan regional digital network and the mobile observation arrays over a 10-year period(May 1, 2011, to May 31, 2021) and obtained the average V_P/V_S ratio of 1.79 via fitting the P-wave and S-wave arrival-time curves with the Wadati method. The magnitude ranges from M_S0.0 to M_S6.4, and the original focal depth ranges from 0 to 35km. To ensure the reliability of the calculation results, at least 4 stations records are required, and the maximum station azimuth gap allowed is 120°. Furthermore, the event-station distance is restricted to 400km and only earthquakes with travel time residuals<0.5s are retained. Our final velocity model is further refined via gridding(i.e., nodes)with an optimal horizontal grid of 0.25°×0.25° and a range between 0~65km vertically. A checkerboard test is also conduced to validate our inversion results. The test results showed that the recovery degree is high except for the depths of 0 and 65km, which were impacted by the uneven seismic distribution and rays. The high degree of recovery of 5~45km suggests high-resolution and robust imaging at these depths. Finally, the double-difference tomography method(TomoDD)was used to invert the three-dimensional P-wave and S-wave velocity structures in the Yangbi and its surrounding areas(24.5°~26.5°N, 99°~101°E). According to the result of precise location, the M_S6.4 main shock is located at 99.89°E, 25.70°N with a focal depth of 7.9km. The Yangbi M_S6.4 earthquake sequence is mainly distributed along the NW direction. Least-squares fitting prefers a~20km long axis with a strike of 312°, and the hypocenter depths are 5~20km. In general, the studied sequence is shallow and located within the upper crust, consistent with the depth distribution characteristics of historical earthquakes in this area. According to the spatio-temporal evolution characteristics of the aftershock sequence, the aftershocks of the M_S6.4 earthquake mainly spread unilaterally toward SE direction. Thus, we speculate that the overall medium in the NW of the mainshock is rigid and hinders aftershocks evolution. On the north side of the M_S6.4 mainshock epicenter, a group of earthquakes spread along the NNE direction and extended to the Weixi-Qiaohou Fault that hosted the M_S4.1 earthquake on May 27, 2021. Considering the geological and structural background, we believe this earthquake occurred on a parallel but unmapped fault on the SE side of the Weixi-Qiaohou Fault. In contrast, the earthquakes spreading in the NNE direction on the north side of the main shock maybe occurred on an unknown fault in the NNE direction. Therefore, the two faults form a conjugate structure. From the imaging results, the upper crustal velocity structure in the study area is consistent with the geological structure changes and the active faults, where the velocities are low. At 0km depth, the extremely low P-wave and S-wave velocities may reflect impacts from surface sediments. A velocity contrast is observed at a depth of 5km near the mainshock. In addition, a high-velocity anomaly was observed to the southeast side of the mainshock at 10-km depth, with a length of about 0.6°(EW)and a width of about 0.2°(SN). Within the depth range of 10~20km, the distribution of earthquakes near the mainshock shows a clear strip-like distribution, delineating the geometry of the fault. The velocity structure and seismic relocation results at 10-km depth suggest that majority of the events locate around the high-velocity anomaly on the west side of the Weixi-Qiaohou Fault. From the AA' profile, both P- and S-wave velocities suggest high-velocity anomalies in the SE direction of the mainshock. Combining with the distribution characteristics of aftershocks, the non-uniform variations of velocity structure are probably the major factor controlling the distribution of aftershocks, leading to the aftershock distribution extending along the SE direction.

Key words: Weixi-Qiaohou Fault, double difference tomography, earthquake locating, velocity structure