Steep-dip seismic imaging at the SAFOD (San Andreas Fault Observatory at Depth) site

深部的圣安德烈亚斯断层观测站HoleSAFOD打算钻探圣安德烈亚斯断层，以研究控制地震的内部断层性质。位于加利福尼亚州中部的帕克菲尔德被选为钻探地点，因为它是断层中地震活动最典型的部分。有趣的观测包括高电导率、低地震速度、高泊松比和断裂带内的地震各向异性。然而，除了由断层导波识别的薄低速带外，地震资料仅约束了2公里或更大尺度的地震速度结构。地震在相对意义上定位良好，但绝对误差可达1公里，与地表活动断层轨迹不一致。更好的场地特征可以改进钻探计划，提高其准确定位断层上特定重复地震地块的能力。该项目将对SAFOD场地进行高分辨率的二维地震剖面，具体目的是对断层内和附近的陡峭构造进行深度成像。首席调查员之前的工作产生了有史以来第一张来自近垂直的圣安德烈亚斯断层的地震反射的迁移图像，使得能够在大约1公里深的地方直接成像断层踪迹。该项目的目标是在钻探将穿过断层的4公里深度之外重现这一成功。首席调查员计划在钻探现场铺设一条50公里长的地震反射和折射组合线，横跨圣安德烈亚斯断层。非常密集的激发和接收间隔以及非常长的记录炮检距将使勘探能够获得详细的地震速度结构，水平分辨率约0.5公里，垂直分辨率约0.25公里，深度约5公里。这些图像将更好地约束断层遗迹和断裂带在深部的位置，它与异常断裂带属性的关系，以及周围地质单元和相关断层的整体结构。

0106534HoleSAFOD, the San Andreas Fault Observatory at Depth, intends to drill through the San Andreas fault to study internal fault properties that control earthquakes. Parkfield, in central California, was chosen as the drill site because it is the best characterized seismically active segment of the fault. Interesting observations include high electrical conductivity, low seismic velocity, high Poisson's ratio, and seismic anisotropy within the fault zone. However, other than a thin low velocity zone identified by fault-zone guided waves, the earthquake data only constrain seismic velocity structure on the scale of 2 km and larger. Earthquakes are well located in a relative sense, but have up to 1 km of absolute error and do not line up with the active surface fault trace. Better site characterization can improve the drilling plan and its ability to accurately target a specific patch of repeating earthquakes on the fault.This project will perform a high resolution, 2-D seismic profile of the SAFOD site with the specific aim of imaging steeply dipping structures in and near the fault at depth. Previous work by the Principal Investigator produced the first-ever migrated image of seismic reflections from the near-vertical San Andreas fault, allowing direct imaging of the fault trace at a depth of about 1 km. This project aims to repeat this success beyond the 4 km depth at which drilling will penetrate the fault. The Principal Investigator plans a 50-km combined seismic reflection and refraction line across the San Andreas fault at the drill site. Very dense shot and receiver spacings and very long recording offsets will allow the survey to obtain detailed seismic velocity structure at a resolution of ~0.5 km horizontally and ~0.25 km vertically down to about 5 km depth. These images will better constrain the location of the fault trace and fault zone at depth, its relation to anomalous fault-zone properties, and the overall structure of surrounding geologic units and related faults.***