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

使用断层带导波对加利福尼亚州帕克菲尔德圣安德烈亚斯断层核心结构进行地震表征

• 批准号: 0342277
• 负责人: Yong-Gang Li
• 金额: $ 20.71万
• 依托单位: University of Southern California
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-02-15 至 2006-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0342277&HistoricalAwards=false
• 关键词: Seismic; Characterization; Core; Structure; San

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.

在该研究项目中，PI 进行了现场地震实验，以获取加利福尼亚州圣安德烈亚斯断层 (SAF) 帕克菲尔德微地震和爆炸产生的断层带导波 (FZGW)。对 2002 年部署在 500-1000 米长线上的 55 个三分量地震仪的密集地震阵列记录的 2-5 Hz 俘获波的观测和初步模拟主 SAF 的核心区域受损。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%，浅深度处的最大速度降低。主 SAF 上的这个明显的低速区被解释为由于 M6 事件和历史上帕克菲尔德主断层上的大地震而造成的重复破坏的残余。目前在靠近SAFOD（圣安德烈亚斯断层深度观测站）钻探现场进行的FZGW实验中，在沿断层和跨断层的密集阵列中记录了更有价值的数据，这些数据是由USGS和折射实验引爆的一系列炮弹以及发生的微地震不同深度的 SAFOD“目标”事件以及阵列下方约 11 公里深度的深层事件。对断层带陷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 SAF 的损坏程度和范围，并绘制与深度相关的断层带内部结构和材料特性的横向变化。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，以根据作为沿断层深度和距离的函数的精细结构模型来改进导波的数值建模。 PI 计划在 2004 年至 2005 年期间重新占领地震台站并在断层带内重复进行测试。重复实验的数据将用于检测帕克菲尔德即将发生的 M6 地震之前断层带材料特性是否存在时间变化。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。基于陷波描绘的断层带结构和SWS应力模式的3D有限元动态破裂模拟将有助于更好地理解断层带结构与动态破裂之间的关系。 通过本项目的数据收集和分析工作，PI 应描述帕克菲尔德 SAF 受损区域的规模、强度和时间依赖性。与周围基岩相比，断层弱点的空间范围，以及整个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EarthScope各组成部分中主程序SAFOD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现场工作的安排之间的总体协调将在主项目下完成。