南北地震带南段远震P波走时层析成像研究

南北地震带南段位于青藏高原东南缘,是青藏高原与扬子克拉通的过渡地带.本文收集了该区域内90个固定台站和356个流动台站的远震波形数据,采用波形互相关方法拾取了88691个P波走时残差数据,应用FMTT(Fast Marching Teleseismic Tomography)层析成像方法获取了南北地震带南段深部的三维P波速度结构.结果显示了研究区深部的结构具有显著的不均匀性:腾冲火山地区深部400 km以浅的深度内分布着明显的低速异常;四川盆地西南部下方300 km内具有较强的高速异常;在上地幔顶部,沿川滇菱形块体周边的大型断裂带及川滇菱形块体南端分布着显著的低速异常,这些低速异常为青藏高原物质向东南方向挤出提供了必要的通道;保山地块下方存在一东倾的高速异常带,该高速异常带为印度板块岩石圈向东俯冲的体现.

The teleseismic tomography study by P-wave traveltime data beneath the southern South-north Seismic Zone

The southern South-north Seismic Zone is situated in the southeastern margin of the Tibetan plateau, where is a tectonic transitional zone between the Tibetan plateau and the Yangtze craton. We use the teleseismic tomography method to image the three dimension velocity structures beneath the southern South-north Seismic Zone, which has the scientific implication for revealing the uplift mechanism of the Tibetan plateau and the evolution of the plate tectonics. We collected the teleseismic waveform data recorded by the 90 permanent broadband seismic stations and 356 portable broadband seismic stations. The 88691 P-wave traveltime residuals were picked with the waveform correlation method. We obtained the three dimension P-wave velocity structures beneath the southern South-north Seismic Zone by the FMTT (Fast Marching Teleseismic Tomography). With the checkboard tests, we found that the scale of anomalies with 1°×1°×100 km could be recovered perfectly in most study area. The tomographic results with good resolution illuminate that the three dimension velocity structures are robust and reliable. The three dimension P-wave velocities reveal that the deep structures of the study area present significant inhomogeneities that the Tengchong volcanic region indicates the obvious low P-wave velocities, but the southwest of the Sichuan basin and the deep Baoshan block suggest the remarkable high P-wave velocities. Through analyzing our results determined by the teleseismic tomography, we draw several conclusions: 1) There is a low velocity anomaly above 400 km beneath the Tengchong volcanic region; 2) A high velocity anomaly down to 300 km beneath the southwestern Sichuan basin; 3) The obviously low velocity anomalies exist along the fault belts around the Chuandian diamond block and the south of the Chuandian diamond block, which provide a necessary channel for the lateral extrusion of the Tibetan plateau materials toward to the southeast; 4) The eastward-dipping high velocity anomaly belt beneath the Baoshan block imply the subducted eastwardly lithosphere of Indian plate.