2008年MS6.1攀枝花地震震源区多参数结构成像及发震机制研究

通过联合反演123,053个P、Pn、Pg震相和100,176个S、Sn、Sg震相数据,获得了2008年M_S6.1攀枝花地震震源及其周边区域的高分辨率三维纵、横波速度(V_P,V_S)和泊松比(σ)图像.结合研究区域地壳应力数据综合分析发现,攀枝花地震发生在高-低纵、横波速度转换带,并且在震源下方存在一显著的低V_S和高σ异常体延伸至下地壳.本研究认为,该构造特征主要是由于西侧坚硬的川滇菱形块体对来自深部流体或熔融物质具有一定的阻挡作用,绝大部分流体或熔融物质通过断裂带向东南侧的块体内部迁移,造成断裂带两侧块体的岩石物理属性差异较大所致.研究结果表明,攀枝花地震发生在剪切应力较强的元谋—绿汁江断裂带上,震源下方的流体或部分熔融物质被挤入至震源的断层或裂隙中,增加了震源区岩石的流体应力,降低了横波速度(V_S)、增加了岩石的泊松比(σ).我们推测,流体侵入在攀枝花地震形成上扮演了重要角色,来自于青藏高原下地壳的大量的流体或部分熔融物质被挤入震源区岩体的断层或裂缝中,这一过程增加了震源区的孔隙流体压力、减弱岩石的机械强度,同时岩石的静摩擦力增加,导致容易引起岩体脆性形变,从而诱发地震.

Multi-parameter imaging and generating mechanism of the 2008 MS6.1 Panzhihua earthquake

High-resolution three-dimensional (3-D) seismic velocity (V_P,V_S) and Poisson's ratio (σ) structures were determined using 123,053 P, Pg, Pn phases and 100,176 S, Sg, Sn phases in the source area of the 2008 M_S6.1 Panzhihua earthquake and its surrounding regions. Our seismic tomographic results indicate that the mainshock was located at a boundary zone with high σ anomalies and high-to-low V_P and V_S variations. A prominent low-V_S and high-σ body extended well down into the lower crust is imaged clearly under the northwest region of the source epicenter, which might reflect the presence of fluid penetration from the lower crust. We consider that the anomalous feature is mainly caused by that most of the fluids and/or partial melting materials from the lower crust were pushed against the Chuandian fragment to change the migration direction into the southeastern blocks. Such a process leads to dramatical difference of the geophysical parameters beside the fault zone. We hypothesize that fluid penetrating into the source area might be a key factor in controlling the occurrence of the 2008 Panzhihua earthquake. This is because that widespread existence of lower crustal flow (fluids or partial melting) is revealed under the eastern Tibetan Plateau by numerous of previous studies. Once fluids suffuse the weakened crust in the source area, it is likely to change of hydrological properties that may lead to high pore pressure accumulation, in turn, reduce the mechanical strength of rocks and facilitates seismogenesis. In other words, the upward migration of deep-seated fluids into the brittle source rocks results in a brittle deformation and consquently triggered the earthquake.