Shallow 3-D velocity structure and its implications for seismogenic structure in the Xunwu (Jiangxi) region and its adjacent areas
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摘要:
寻乌地区位于邵武—河源断裂带中段, 发生过多次破坏性地震, 其中包括1806年M6地震以及1941年M~5.75地震和1987年M~5.5地震群, 是邵武—河源断裂带地震活跃的区域之一, 也是江西省防震减灾重点关注区域之一.获取该地区的浅层速度结构有助于提高地震定位以及震动图计算的精度.然而, 该地区固定台网稀疏, 加之研究区NW以及NE向断层交错发育, 增加了地震构造环境研究的难度.本文基于寻乌地区布设的密集台阵, 获得了持续一个多月的连续波形数据, 计算了台站对噪声互相关, 并提取了瑞利面波相速度(0.5~6 s)信息, 进而采用面波层析成像方法反演了寻乌以及邻区的地壳浅部6 km速度结构.相速度以及S波速度反演结果均很好地展示了断层的浅部空间特征、断陷盆地以及岩体的分布.约3 km深的浅层低速带揭示了会昌和寻乌等断陷盆地的分布, 也与邵武—河源断裂带在周田附近"S"型急转弯有很好的对应关系, 进一步说明区域断裂带对盆地发育的控制作用.区域典型地震集中在高低速结构过渡区, S波速度高低速变化显示了NW走向断层在深度上的空间展布情况, 推测NW向断裂主导了研究区域的地震.
Abstract:Situated within the Shaowu-Heyuan fault zone, the Xunwu area has experienced several moderate earthquakes, notably the M6 earthquake in 1806, the M~5.75 earthquake in 1941, and a swarm of M~5.5 earthquakes in 1987. It is one of the seismically active regions along the Shaowu-Heyuan fault zone, and also one of the key areas for earthquake prevention and mitigation in Jiangxi province. The shallow subsurface velocity structures in this area help improve the accuracy of earthquake location and seismogram calculation. However, the sparse seismic network in the region, coupled with the complex development of NW and NE-trending faults, has posed significant challenges to studying the seismogenic environment. In this study, based on continuous waveforms observed by a dense array across Xunwu and its neighboring areas, we computed noise cross-correlation between station pairs and extracted Rayleigh wave (0.5~6 s).The surface wave tomography method was applied to invert the velocity structure up to 6 km depth of the study area. Both phase velocity and S-wave velocity reveal the shallow spatial characteristics of faults, graben basins, and lithological structure. The shallow low-velocity zone approximately 3 km deep reveals the distribution of graben basins such as Huichang and Xunwu and exhibits a strong correspondence with the 'S'-shaped sharp turn of the Shaowu-Heyuan fault zone near Zhoutian, which highlights the influence of regional fault zones on the evolution of the basin. Earthquakes in the region predominantly occur in the transition zone between high and low-velocity structures. Variations of S-wave velocity demonstrate the spatial distribution of NW-trending faults, suggesting that NW-oriented faults are the primary contributors to the seismicity in the study area.
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图 1
研究区域地质构造以及密集台阵分布
Figure 1.
Studies the regional geological structure and dense array part of the map
图 3
(a) 面波群速度观测数据；(b) 面波相速度观测数据, 黑色横线代表 2倍标准差; (c) 不同周期频散曲线数目
Figure 3.
Observed group velocity (a) and phase velocity (b) data; the black horizontal line represents 2 times the standard deviation; (c) the number of dispersion curves of different periods
图 4
周期在1.0~6.0 s，网格大小为0.04°×0.04°的相速度的检测板分辨率恢复测试
Figure 4.
Checker board resolution tests on a 0.04°×0.04° grid for phase velocity of 1.0~6.0 s periods
图 5
不同周期(1.0 s；2.0 s；3.0 s；4.0 s；5.0 s；6.0 s)射线路径覆盖图, 灰线为射线路径，白色三角为台站
Figure 5.
Ray-path coverage for different periods: 1.0 s, 2.0 s, 3.0 s, 4.0 s, 5.0 s, 6.0 s. The grey lines represent the ray-path, and the white triangles represent the stations
图 6
(a) 阻尼参数选取时的L曲线；(b) 平滑因子选取时的L曲线(其中红色圆点代表选取的最合适参数)
Figure 6.
L-curve of damping parameter (a) and smoothing factor (b)(the red dots represent the most appropriate parameters selected)
图 9
(a) 寻乌及邻区地形和剖面位置(蓝线); (b)—(f) 分别为AA′-EE′剖面的S波速度结构
Figure 9.
The topography and profile position (blue line) of Xunwu and adjacent areas (a), and the S-wave velocity structure of AA′-EE′ profiles (b—f), respectively
图 10
(a) 相速度成像前的走时残差分布；(b) 相速度成像后的走时残差分布; (c) S波速度反演之后的数据拟合误差
Figure 10.
Frequency histograms showing fit to observed traveling data of (a) initial model, (b) inverted model; (c) Misfit errors of S-wave velocity inversion
图 13
(a) 寻乌及邻近区域6 km的主要地震分布，包括1806年会昌M6.0地震(红色五角)、1983年寻乌M4.5地震(红色五角)、1941年寻乌北M~5.75地震(红色五角)以及1987年M~5.5群震(红色五角和黑色圆点)和剖面位置(蓝线); (b) FF′剖面的S波速度结构以及断层位置，F3：鸡笼嶂—寻乌—八尺圩断层；F1-1：黄岗—笔架山断层
Figure 13.
(a) The primary seismic distribution, including the Huichang M6.0 earthquake in 1806(red pentagon), Xunwu M4.5 earthquake in 1983 (red pentagon), Xunwu North M~5.75 earthquake in 1941 (red pentagon), a swarm of M~5.5 earthquakes in 1987 (red pentagon and black dot) and profile position (blue line) of Xunwu and adjacent areas 6 km; (b) The S-wave velocity structure of FF′ profiles and fault location. F3: Jilongzhang—Xunwu—Bachixu fault; F1-1: Huanggang—Bijiashan fault
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