Detailed seismic imaging and monitoring of the subsurface material around the San Jacinto fault zone

本研究开发并使用了先进的地震学技术和南加州圣哈辛托断裂带（SJFZ）周围空间密集断裂带阵列记录的连续波形数据。对这些数据的分析提供了浅层地壳地震速度和衰减系数的详细特征（可能与时间有关）。为了为新技术提供基准，并了解地震性质如何随位置和深度变化，研究人员使用了安装在断层带内和附近的浅孔地震仪记录的数据，以及区域地震网络的记录。这些结果补充了层析成像研究模型，这些模型通常不能解决浅层地壳的地震特性，并允许在SJFZ周围开发地震结构模型，基本上可以达到地表。浅层地壳地震特性的知识与观测到的地面运动和破坏过程特别相关。这些结果可能与许多其他应用相关，包括地壳水文学，地热能，地下水库和储存，以及基础设施的地震安全。本研究所建立的分析方法可用于其他地点地下材料的地震特性研究。该研究使用最先进的数据和新的地震成像和监测技术来弥补关键的观测空白。这些研究解决了以下关键问题：(1)在SJFZ的各个部分和断层外的位置，地壳顶部几百米的地震速度和衰减系数是什么？(2)在何种应力/应变幅值下，浅层岩石在不同的加载机制和周期（例如，局地和远地地震、潮汐和季节变化）以及相对于断裂带、场地条件参数和深度的位置上具有非线性响应和属性的时间变化？(3)不同荷载作用下地震性质的时间变化幅度和恢复时间尺度随断裂带、场地条件和深度的变化规律如何？(4)是否存在断层段和/或断层外部位对断层的易感性更高（表现为随时间变化的地震性质），如果发现了这些断层段和/或断层外部位，是否可以识别出这些区域的异常性质，从而解释易感性的升高？这些研究可以显著地增加现有的地震速度和衰减系数、非线性响应和地壳顶部几百米性质的时间变化的原位表征。研究结果有望提供基础信息，从而提高人们对地震地面运动（以及由此产生的震动危害）的理解，以及其他应用。

This study develops and uses advanced seismological techniques and continuous waveform data recorded by spatially-dense fault zone arrays around the San Jacinto Fault Zone (SJFZ) in southern California. Analyses of these data provide detailed characterization of (potentially time-dependent) seismic velocities and attenuation coefficients of the shallow crust. To provide benchmarks for the new techniques and understand how seismic properties change with location and depth, the investigators use data recorded by shallow borehole seismometers installed within and adjacent to the fault zone, as well as recordings from regional seismic networks. These results complement models of tomographic studies that typically do not resolve seismic properties in the shallow crust, and allow development of seismic structural models around the SJFZ essentially up to the surface. Knowledge of seismic properties of the shallow crust is particularly relevant to observed ground motion and failure processes. These results may be relevant to numerous other applications including crustal hydrology, geothermal energy, subsurface reservoirs and storage, as well as seismic safety of infrastructure facilities. The analysis methods develop by this research can be used in studies of seismic properties of the subsurface material in other locations. The research uses state-of-the-art data and novel seismic imaging and monitoring techniques to close critical observational gaps. The studies address the following key questions: (1) What are the seismic velocities and attenuation coefficients in the top few hundred meters of the crust at various sections of the SJFZ and at sites that are off the fault? (2) At what stress/strain amplitudes do shallow rocks have nonlinear response and temporal changes of properties in relation to different loading mechanisms and periods (e.g., local and remote earthquakes, tides, and seasonal variations) and location with respect to the fault zone, site condition parameters, and depth? (3) How do amplitudes of temporal changes of seismic properties and recovery timescales produced by various loadings vary in relation to the fault zone, site conditions and depth? (4) Are there fault sections and/or off-fault sites with higher susceptibly to failures (manifested by time-dependent seismic properties), and if they are found, can anomalous properties in these regions be identified that may explain the elevated susceptibility? The studies can augment significantly the available in situ characterization of seismic velocity and attenuation coefficients, nonlinear response and temporal changes of properties in the top few hundred meters of the crust. The results are expected to provide fundamental information that may lead to improved understanding of seismic ground motion (and hence shaking hazard) as well as other applications.