江淮地区地震精定位及b值随深度的变化研究

使用双差地震定位法,利用安徽及邻省50个地震台站记录的1976年到2008年的551次M_L≥2.3地震,其中初始定位有205个无深度数据,包括数字化资料和传统的模拟资料,共5464条P波走时资料。经重新定位后得到其中468次地震的基本参数。重新定位结果显示了本地区较精细的震中分布图像和震源深度剖面图像,震源深度优势分布在6—10km,平均深度为10km,部分震中位置与震源深度变化较大的地震向断裂带靠近。基于地震精确定位结果,系统地计算了不同深度段的b值,发现研究区b值随震源深度的增加具有系统减小的趋势,且在地壳10km左右的减小趋势最为突出。表明江淮地区的地壳分层结构相对明显,在地壳浅部(0—10km)以小震为主,大地震较少,故b值高;而在深处(10—25km),大地震相对较多,b值减小。这一现象的背后物理机制可以从地壳介质复杂程度与应力状态的变化得以解释,破裂易于在地壳介质相对均匀、岩石静压力较高的地壳深处成核形成大地震。推测江淮地区未来强震多发生在10km以下的地壳深部。

Study on earthquakes precise relocation and b-value changes with depth in Jianghuai region

Using the 5 464 P wave travel-time data, digital or analogous, observed at 50 seismic sta- tions in Anhui and adjacent province, 551 earthquakes-with M≥2. 3 from 1976 to 2008, among them 205 without initial locating depth, are relocated with the double-difference earthquake location algorithm. The Basic parameters of 468 earthquakes have been obtained after location. The result shows a more precise image of epicenter distribution and a cross- section with convergent focus depths. The predominant focus depth is in 6--10 km, and the average depth is 10 km. Some earthquakes with divers epicenter and focus depth are close faults. Based on the results of precise relocation, the b values in different depth section are calculated systematically. A trend is found that the b value decreases with the increasing of focus depth and the trend is more obvious at 10 km depth of crust. It implies that the layered characteristics of crustal structure in Jianghuai region is relative remarkable. In the shallow part of crust （0--10 km）, there are more small earthquakes and a few large earthquakes, so the b value higher. While in the deep （10--25 km）, there are more large earthquake and few small ones, so the b value decreases. The physical mechanism of this phenomenon could be interpreted as the variation of stress state and the complex of crustal material. The rupture easily occurs at the relatively uniform medium and the higher static pressure crustal, because of the nucleation formed at there. The strong shock will occur below the earth＇s crust 10km deep at Jianghuai region.