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

在这项研究项目中，PI进行现场地震实验，以获取加州圣安德烈亚斯断层(SAF)帕克菲尔德微震和爆炸产生的断层带导波(FZGW)。对2002年部署在500-1000米长线路上的55台三分量地震仪的密集地震阵列记录的2-5赫兹捕获波的观测和初步模拟表明，位于断裂带内的事件在主SAF上有损坏的核心区。地表至至少4~5公里处有一条约150米宽的低速波导带，其中Q值为10~50，剪切速度比围岩速度降低30~40%，浅层减速最大。主SAF上的这一明显的低速带被解释为是由于6级地震和帕克菲尔德主断层面上历史上的大地震而造成的反复破坏的残余。在目前进行的更靠近SAFOD(圣安德烈亚斯断层观测站深度)钻探现场的FZGW实验中，在沿断层和跨断层的密集阵列上记录了更有价值的数据，其中包括由USGS引爆的一串炮击和折射实验，以及在不同深度发生的微震，包括SAFOD在~4公里深度的“目标”事件和阵列下~11公里深度的深地震。对断层圈闭波资料进行了定量分析和三维有限差分模拟，高分辨率地圈定了井点附近SAF的破坏程度和程度，绘制了断裂带内部结构和材料性质随深度和横向的变化规律。所有炮点的体波到达时间反演将被用来获得准三维局部速度结构，以改进导波数值模拟的精细结构模型，该模型是沿断层深度和距离的函数。PIS计划在2004-2005年重新使用地震台，并在断裂带内重复激发。重复实验的数据将被用来检测在帕克菲尔德即将发生的6级地震之前，断裂带物质性质是否有时间上的变化。地震数据的剪切波分裂(SWS)分析应提供有关近断层应力的信息。基于俘获波圈定的断裂带结构和SWS应力模式的三维有限元动态破裂模拟将有助于更好地了解断裂带结构与动态破裂的关系。通过本项目中的数据收集和分析工作，PI应描述Parkfield SAF受损区域的尺寸、大小和时间相关性。与周围基岩相比，断层软弱的空间范围，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。这项研究项目是对主程序SAFOD在EarthScope的各个组成部分中的补充。PIS负责断层带陷波研究，而这一补充项目与SAFOD项目中地球物理场地特征野外工作的调度将在主项目下进行全面协调。

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.