安徽霍山窗中小地震活动与精定位研究

Precise Relocation of Small Medium Earthquakes in Anhui“Huoshan Seismic Window”

采用双差地震定位方法对安徽省霍山地区2009年1月1日至2014年6月13日发生的2679个ML≥0．0地震进行重新定位研究。对同一地震事件定位前后位置进行比较，得到各个地震在E-W、N-S及U-D三个方向的偏差平均值分别为0．05 km、0．06 km及0．07 km。重新定位后，原震源位置的定位精度大大提高，由定位误差所引起的原震源位置呈现的“网格状”假象分布现象得到明显改善，震中在空间上分布更加紧凑，丛集性明显，且呈现出3条NE向分布的地震密集带，与落儿岭—土地岭断裂方向大致一致。同时震源深度的分布更加精确，且在4～9 km处存在明显的优势分布现象，占总数的88％以上。霍山地区浅源地震发育，表明该地区地壳中上部地震波速度较高。

The frequency and intensity of small medium earthquakes in the Luoerling—Zhufoan area,west of Huoshan county,are particularly prominent in Anhui province and its neighboring areas.The earthquakes in this area are characterized as moderately strong and concentrating into a band,with small ones clustering at shallow focal depth and forming a swarm.Based on the above, it is of critical and urgent importance to strengthen earthquake monitoring and improve location accuracy in the region.The double-difference （DD）location algorithm,as a relative positioning method,has unique advantages in improving the accuracy of earthquake location.From January 1, 2009 to June 13,2014,2 679 earthquakes of ML≥ 0.0 occurred in the Huoshan area,Anhui prov-ince,and we relocated them in this study using the DD method.The average relative errors in the focal depths were 0.05,0.06,and 0.07 km in the E-W,N-S,and U-D directions,respectively,as compared with the original values after relocation.The positioning accuracy of the original source location was greatly increased and the“grid”illusion distribution phenomenon of the original source location,caused by positioning errors,was significantly improved after relocation.The spa-tial distribution of the epicenters is more compact and concentrated than previously determined. The relocation results show that there are three dense seismic zones distributed along the NE di-rection,consistent with the direction of the Luoerling—Tudiling fault.The distribution of the fo-cal depths is more precise and there is a predominant depth range from 4 to 9 km that accounts for over 88% of the total.More shallow earthquakes in the Huoshan area indicate that the upper crust may be in a period of higher seismic wave velocity.