Seismic Characterization of Core Structure of the San Andreas Fault at Parkfield, California Using Fault-Zone Guided Waves

在本研究项目中，PI进行现场地震实验，以获取由加州的圣安德烈亚斯断层（SAF）帕克菲尔德的微震和爆炸产生的断层带导波（FZGW）。2002年，在500-1000 m长的线路上，沿沿着和穿过断层带内事件的断层迹线，部署了55个三分量地震仪，在密集地震阵列上记录了2-5 Hz陷波，观测和初步建模显示，主SAF上的核心区受损。从地表到至少4-5 km深度的区域以约150 m宽的低速波导为标志，其中Q为10-50，剪切速度比围岩速度降低30-40%，在浅深度处速度降低最大。主SAF上的这种明显的低速带被解释为帕克菲尔德主断层面上的M6事件和历史大地震造成的重复破坏的再现。在目前进行的FZGW实验更接近SAFOD（圣安德烈亚斯断层观测站在深度）钻井现场，更有价值的数据记录在沿着和跨断层密集阵列的一堆炮引爆美国地质调查局和折射实验，和微震发生在不同的深度，包括SAFOD“目标”事件在~ 4公里深处和深的事件到11公里深处的阵列之下。通过对断层陷波资料的定量分析和三维有限差分模拟，高分辨率地描绘出钻井附近SAF的破坏程度和范围，并绘制出断层内部结构和材料性质随深度和横向的变化。所有炮点的体波到达时间反演将用于获得准3D局部速度结构，以根据作为深度和沿断层沿着距离的函数的精细结构模型改进导波的数值模拟。PI计划在2004-2005年重新使用地震台站并在断层带内重复拍摄。重复实验的数据将被用来检测是否有断层带的物质性质的时间变化之前，在帕克菲尔德的M6地震即将发生。地震数据的剪切波分裂（SWS）分析应提供近断层应力的信息。基于地震陷波所描绘的断裂带结构和地震慢波应力图像的三维有限元动态破裂模拟将有助于更好地理解断裂带结构与动态破裂之间的关系。 通过本项目中的数据收集和分析工作，PI应表征帕克菲尔德SAF上受损区域的尺寸、幅度和时间依赖性。与周围基岩相比，断层薄弱的空间范围以及地震周期中强度的损失和恢复是理解断层力学和物理学以及估计未来地震损失的关键因素。该研究项目是对EarthScope各组成部分中SAFOD主程序的补充。PI负责断层带陷波研究，而该补充项目与SAFOD计划中地球物理场地表征的现场工作安排之间的总体协调将在主项目下完成。

In this research project, the PIs carry out field seismic experiments to acquire fault-zone guided waves (FZGW) generated by microearthquakes and explosions at the San Andreas Fault (SAF) Parkfield, California. Observation and preliminary modeling of 2-5 Hz trapped waves recorded at dense seismic arrays of 55 three-component seismometers deployed in 2002 on 500-1000-m long lines along and across the fault traces for events located within the fault zone shown evidence of a damaged core zone on the main SAF. The zone from the surface to at least 4-5 km depth is marked by a low-velocity waveguide ~150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-40% from wall-rock velocities, with the greatest velocity reduction at shallow depth. This distinct low-velocity zone on the main SAF is interpreted as being a remanent of repeated damage due to M6 episode and historical large earthquakes on the principal fault plane at Parkfield. In the current FZGW experiment conducted closer to the SAFOD (San Andreas Fault Observatory at Depth) drilling site, more valuable data are recorded at along- and across-fault dense arrays for a bunch of shots detonated by USGS and refraction experiments, and microearthquakes occurring at various depths, including SAFOD "target" events at ~4km depths and deep events to the ~11km depths beneath arrays. The quantitative analysis and 3-D finite-difference modeling of fault-zone trapped wave data are to be carried out for a high-resolution delineation of the magnitude and extent of damage on the SAF nearby the drilling site, and draw the depth-dependent and lateral variations in internal structure and material properties of the fault zone. The body wave arrival times inversion for all shots will be used to obtain a quasi-3D local velocity structure for improvement of the numerical modeling of guided waves in terms of a refined structural model as a function of depth and distance along the fault. PIs plan to reoccupy seismic stations and repeat shots within the fault zone in 2004-2005. The data from the repeated experiment will be used to detect if there are the temporal changes in fault-zone material property before a pending M6 earthquake at Parkfield. The shear-wave splitting (SWS) analysis for earthquake data shall provide information on near-fault stresses. The 3-D finite-element dynamic rupture simulations based on the fault-zone structure delineated by trapped waves and the stress pattern from SWS will help better understand the relationship between the fault zone structure and dynamic rupture. Through data collection and analysis efforts in this project, PIs shall characterize the dimensions, magnitude and time dependence of the damaged zone on the SAF at Parkfield. The spatial extent of fault weakness compared to surrounding bedrock, and the loss and recouping of strength across the earthquake cycle are critical ingredients in understanding of fault mechanics and physics, as well as in estimation of losses from future earthquakes. This research project is a supplement to the main program SAFOD among various components of EarthScope. The PIs are responsible on fault-zone trapped wave study while overall coordination between this supplementary project and scheduling of the field work for geophysical site characterization in SAFOD program will be done under the main project.