Collaborative Research: Spatial and Temporal Evolution of an Active Blind-Thrust Fault from Inception to the Most Recent Earthquake

这项研究的重点是通过对逆冲断裂上方发育的褶皱进行地质和地球物理调查，破译逆冲断裂系统的古地震记录。在盲冲断层的情况下，这些与断层相关的褶皱提供了下伏断层上唯一可识别的古地震活动记录，而地表褶皱陡峭本身就构成了潜在的重大地表变形危险。来自哈佛大学和南加州大学的科学家们正在研究加州洛杉矶盆地活跃的普恩特山盲冲断层上方的一系列埋藏的褶皱陡坎，以确定它们如何约束过去的地震历史。该方法包括分析石油工业反射数据，获取多尺度、高分辨率的地震反射图像，以及挖掘详细的钻孔横断面，以记录和直接采样最年轻的褶皱地层。地震反射和钻孔数据将收集在该断层系统的所有三个部分之上，从而有助于全面分析断层系统的行为。此外，为了阐明过去大约10万年来褶皱增长和断层滑动速率的相对恒定，将在普恩特山盲冲断层的洛杉矶西段上方直接钻入一个深(约220米深)的连续取心钻孔。总而言之，这些数据将有助于确定地震中发生的褶皱的哪些成分，盲冲系统的不同部分在许多破裂周期中如何相互作用，以及这些断层上的滑动速率如何在不同的时间尺度上变化，从地震周期到数百万年不等。这项研究的动机是了解盲冲断层和褶皱的基本机制，以及开发更好的区域地震危险性评估方法。普恩特山盲冲位于洛杉矶大都市的中心地带，由于其大小和位置，它是美国最严重的确定性地震危险之一。这项研究将有助于确定该断层未来可能发生的地震的震级和频率，从而提高对区域地震灾害的认识。此外，通过这项研究开发的方法将被证明是容易输出的，从而成为一种工具，以了解更多关于世界各地潜在危险的断层。

This research project is focused on deciphering paleo-earthquake records for thrust faults systems through geological and geophysical investigations of folds that develop above them. In the case of blind-thrust faults, these fault-related folds provide the only discernable record of paleo-earthquake activity on the underlying faults, and surface fold scarps in their own right present a potentially significant surface deformation hazard. Scientists from Harvard University and University of Southern California are examining a series of buried folds scarps above the active Puente Hills blind-thrust fault in the Los Angeles basin, California, to determine how they constrain past earthquake histories. The approach involves analysis of petroleum-industry reflection data, acquisition of multi-scale, high-resolution seismic reflection imagery, and excavation of detailed borehole transects to document and directly sample the youngest folded strata. Seismic reflection and borehole data will be collected above all three segments of this fault system, thus facilitating a comprehensive analysis of fault system behavior. Moreover, to elucidate the relative constancy of fold growth and fault slip rate over the past approximately 100,000 years, a deep (about 220 meters deep) continuously cored borehole will be drilled directly into the well-defined growth strata above the western, Los Angeles segment of the Puente Hills blind-thrust fault. Collectively, these data will help define what components of folding occur seismically, how various segments of a blind-thrust system interact over many rupture cycles, and how slip rates on these faults vary over different time scales, ranging from seismic cycles to millions of years.The motivations for this study are to understand the fundamental mechanism of blind-thrust faulting and folding, as well as to develop better methods of assessing regional earthquake hazards. The Puente Hills blind thrust underlies the heart of metropolitan Los Angeles and, because of its size and location, represents one of the worst deterministic seismic hazards in the United States. This study will help define the likely magnitude and frequency of future earthquakes on this fault, thereby improving the understanding of regional earthquake hazards. Moreover, the methodology developed through this study will prove readily exportable and thus serve as a tool to learn more about potentially dangerous faults around the world.