霍山震群区三维地壳速度结构与孕震环境

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

霍山震群可能是反映华东地区乃至青藏高原地震活动的重要窗口,其孕震环境与发震机制备受关注。文中手动拾取获得了1987个地震的7706条P波、394条PmP波、9263条S波和557条SmS波共计17920条震相到时数据,采用多震相体波层析成像方法反演获得了霍山震群区下方深至30km的地壳三维P波速度(V_(P))、S波速度(V_(S))及纵横波速度比(V_(P)/V_(S))模型。结果显示,霍山震群区附近存在弱高V_(P)/V_(S)结构特征,但在震群区下方存在强高V_(P)/V_(S)结构特征,且该结构特征沿晓天-磨子潭断裂向深延伸至30km深度,推测霍山震群区下方存在流体作用。该流体作用可能降低断层面有效正应力,从而导致在晓天-磨子潭断裂与落儿岭-土地岭断裂交会的薄弱区发生霍山震群活动。这些流体的来源与太平洋板块在地幔转换带滞留脱水所形成的“大地幔楔”结构中的地幔热物质上涌有关,但还可能与青藏高原壳幔物质沿大别山造山带E向挤出有关,共同影响了霍山震群活动特征。文中成果也为深入理解霍山震群区的构造活动与华东地区及青藏高原构造活动的关联性提供了地震学证据。

The Huoshan earthquake swarm is tectonically 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 M≥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 China's Mainland.In this study,a total of 17920 seismic arrival-time data,including 7706 P,394 PmP,9263 S and 557 SmS arrivals,are hand-picked from 1987 local earthquakes to obtain three-dimensional crustal P-wave velocity(V_(P)),S-wave velocity(V_(S))and V_(P)/V_(S)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°×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 V_(P)/V_(S)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 V_(P)/V_(S)anomalies.At 30km depth,the areas with high V_(P)/V_(S)are reduced and concentrated on both sides of the Luoerling-Tudiling Fault.There are significant high V_(P)/V_(S)characteristics around the Huoshan earthquake swarm.The high V_(P)/V_(S)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.