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进行了现场地震实验，以获取加利福尼亚州帕克菲尔德圣安德烈亚斯断层（SAF）微震和爆炸产生的断裂带导波（FZGW）。2002年部署的55台三分量地震仪密集地震阵列记录的2-5 Hz捕获波的观测和初步建模显示，断层带内的事件沿断层轨迹沿500-1000米长的线路显示，主SAF上的核心区受到破坏。在地表至深度至少4 ~ 5 km的范围内，存在一个宽约150 m的低速波导，其中Q值为10 ~ 50，剪切速度比围岩速度降低30 ~ 40%，其中浅层速度降低幅度最大。这一明显的低速带被解释为帕克菲尔德主断层面上的M6事件和历史上的大地震造成的重复破坏的残余。在目前的FZGW实验中，在靠近SAFOD （San Andreas Fault Observatory at Depth）钻井现场进行的实验中，沿着和跨断层密集阵列记录了更多有价值的数据，包括USGS引爆的一系列射击和折射实验，以及发生在不同深度的微地震，包括SAFOD在~4km深度的“目标”事件和阵列下~11km深度的深层事件。对断裂带圈闭波数据进行定量分析和三维有限差分建模，以高分辨率描绘钻井现场附近的SAF的破坏程度和程度，并绘制断裂带内部结构和物质性质的深度依赖和横向变化。利用体波到达时间反演得到准三维局部速度结构，以改进导波的数值模拟，使之以断层深度和断层距离为函数的精细结构模型。pi计划在2004-2005年重新占用地震台站并在断层带内重复拍摄。来自重复实验的数据将用于检测帕克菲尔德即将发生的M6地震之前断裂带物质属性是否存在时间变化。地震资料的横波分裂（SWS）分析应提供近断层应力的信息。基于圈闭波圈定断裂带结构和SWS应力分布的三维有限元动态破裂模拟有助于更好地理解断裂带结构与动态破裂的关系。通过本项目的数据收集和分析工作，pi应描述Parkfield受损区域对SAF的尺寸，大小和时间依赖性。与周围基岩相比，断层弱点的空间范围，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以及估计未来地震损失的关键因素。本研究项目是对EarthScope各组成部分中主要项目SAFOD的补充。pi负责断裂带捕获波研究，而该补充项目与SAFOD计划中地球物理现场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.