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

0106534HoleSAFOD，圣安德烈亚斯断层深度观测站，打算钻透圣安德烈亚斯断层，研究控制地震的内部断层特性。 位于加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活跃的部分。 有趣的观测结果包括高电导率、低地震速度、高泊松比和断层带内的地震各向异性。 然而，除了由断层带导波识别的薄低速带之外，地震数据仅约束2km及更大尺度的地震速度结构。 地震在相对意义上是很好的定位，但有高达1公里的绝对误差，并不符合活动的地表断层痕迹。 更好地确定场地特征可以改进钻探计划，并提高其准确定位断层上重复地震的特定区域的能力，该项目将对SAFOD场地进行高分辨率二维地震剖面，具体目的是对断层内和附近的深度陡倾结构进行成像。 首席研究员先前的工作产生了有史以来第一个来自近垂直圣安德烈亚斯断层的地震反射迁移图像，允许在约1公里的深度直接成像断层痕迹。 该项目旨在重复这一成功，超过4公里的深度，钻探将穿透断层。 首席研究员计划在钻井现场的圣安德烈亚斯断层上建立一条50公里长的地震反射和折射线。 非常密集的炮点和接收器间距以及非常长的记录偏移距将使调查能够获得详细的地震速度结构，水平分辨率约为0.5公里，垂直分辨率约为0.25公里，深度约为5公里。这些图像将更好地限制断层痕迹和断层带在深度上的位置，它与异常断层带特性的关系，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.***