THREE-DIMENSIONAL CRUSTAL VELOCITY STRUCTURE AND SEISMOGENIC ENVIRONMENT AROUND THE HUOSHAN EARTHQUAKE SWARM

The Huoshan earthquake swarm is tectonieally located at the junction among the North China plate, Yangtze plate, and North Dabie orogenic belt. The geological environment in the region is complex, including the Feixi-Hanbaidu Fault, Meishan-longhekou Fault, Xiaotian-Mozitan Fault and Luoerling-Tudiling Fault, as well as the North Dabie tectonic belt, North Huaiyang tectonic belt and Liu'an basin. In the study region, seismicity is intense, and 9 earthquakes with 11^5.0 occurred along the Luoerling-Tudiling Fault in the history. In recent decades, small-to-moderate earthquakes were frequent, mainly gathering at the intersection of the Xiaotian-Mozitan Fault and Luoerling-Tudiling Fault. Furthermore, the frequency of small earthquakes in the Huoshan region has a significant correspondence to the moderate-to-strong earthquakes in East China and even eastern Tibet, so studying the deep structure can shed new light on the relationship between the Huoshan earthquake swarm and moderate-to-strong earthquakes in mainland China.In this study, a total of 17 920 seismic arrival-time data, including 7 706 P, 394 PmP, 9 263 S and 557 SmS arrivals, are hand-picked from 1987 local earthquakes to obtain three-dimensional crustal P-wave velocity (Vy), S-wave velocity (Vs) and Ve/Vs ratio models down to 30km depth beneath the Huoshan swarm area. The checkerboard resolution test results show that the imaging spatial resolution in most parts of the regions can reach 0. 33° X 0. 33°, and the North Huaiyang tectonic belt near the Huoshan earthquake swarm has good recovery in the entire crust, and the North Dabie tectonic belt and Lu'an basin also have good recovery at 8_30km depths. Due to the addition of PmP/SmS arrivals, the spatial resolution at 18 - 30km depths is significantly improved, and the pattern and amplitude of velocity anomalies are better recovered.Our tomography results show that a vertical continuous high Vr/Vs anomaly is observed around the intersection of the Xiaotian-Mozitan Fault and the Luoerling-Tudiling Fault, especially at 18km depth appear broad low-velocity and high Vp/Vs anomalies. At 30km depth, the areas with high Vp/Vs are reduced and concentrated on both sides of the Luoerling-Tudiling Fault. There are significant high Vp/Vs characteristics around the Huoshan earthquake swarm. The high Vp/Vs anomalies extend to 18 ~ 30km depths below the Xiaotian-Mozitan Fault, suggesting that fluids could have migrated upward along the fault to reduce the effective normal stress of the fault planes, triggering the activity of the Huoshan earthquake swarm at the weak intersection between the Xiaotian-Mozitan Fault and the Luoerling-Tudiling Fault.Combined with the low-velocity anomalies of the upper mantle revealed by the previous tomographic results, we propose that there may be a channel for upwelling of the wet and hot upper-mantle materials with fluids to the crust along the Xiaotian-Mozitan Fault. The upwelling of the wet and hot materials may be related to the dynamics of the big mantle wedge formed due to the deep subduction of the stagnant Pacific slab down to the mantle transition zone and the eastward extrusion of materials in the upper mantle from eastern Xizang along the Dabie orogenic belt. These factors may jointly affect the seismicity characteristics of the Huoshan earthquake swarm. Our results providea new piece of seismological evidence for the interactions among the tectonic activities in the Huoshan region, Tibetan plateau and East China. © 2024 State Seismology Administration. All rights reserved.