Collaborative Research: Paired paleoseismic and slip rate analysis of the central Garlock fault: Towards a true dated path of incremental slip on a major strike-slip fault

了解历史和史前大断层的复发间隔和滑动率对于地震危险性评估以及了解地震力学的基本原理非常重要。长期记录是必要的，因为大地震的复发间隔通常超过现代仪器记录（如东北、卡斯卡迪亚等）。古地震研究和滑动率测量用于评估活动断层或断层网的复发间隔和地震潜力。这些研究表明，地震可能在空间上和时间上都聚集在一起，滑动率可能随着时间的推移而显著变化，一些断层网络表现出协调的行为，但支持这些观点的明确研究很少。先前对南加州加洛克断层进行的古地震和滑动率研究表明，该断层可能经历过快速滑动事件，这与地震群交替发生的慢滑动事件或地震间歇有关，这与应变积累率下降有关。这项研究将为地震发生和断层滑动的史前记录提供一个明确的分析，以确定这种行为在过去的15000年里是否是加洛克断层的典型行为。这些结果将有助于科学界更好地了解是什么控制了断层系统上大地震的发生，从而使更有根据的地震预报成为可能。其他期望的社会成果包括女性和代表性不足的少数民族充分参与STEM，通过教师暑期研究项目改善STEM教育工作者的发展，以及通过培训本科生和研究生培养具有全球竞争力的STEM劳动力。越来越多的证据表明，无论是单断层还是断层系统，大地震的发生都不是一个随机过程。例如，地震通常在空间和时间上都聚集在一起。人们越来越认识到，地震的发生在时间和空间上可能是高度不规则的，这最终将需要一种新的、更复杂的地震危险性分析方法，把这种行为考虑在内。然而，在开发这种新方法之前，有必要了解地震群集等现象的性质及其发生的条件。这个项目将开发一个详细的史前断层滑动记录，作为Garlock断层的时间函数，这个断层已知会产生随着时间的推移而明显聚集的地震。例如，加洛克断层在过去的2000年里产生了四次大地震，在2000年到5000万年前没有地震，在5000年到7000万年前发生了两次地震。这个项目测试在断层的一个地方观察到的这种聚集是否也可以在断层的其他两个地方重现，并将史前地震的记录延长到更早的时间，以记录断层是否在其他任何千年里都是地震静止的。为此，将通过以下方式获取Garlock断层中部的古地震和增量滑动速率数据：(a)在三个地点挖掘古地震沟，其中两个地点以前由于缺乏可测碳而无法确定数据；(b)记录两个地点在30-80米偏移范围内的断层滑动增量速率；(c)基于对几个地点GeoEarthScope激光雷达数据的分析，对额外的小偏移（3-18 m）进行定年，以更好地约束过去几次地震的定年滑动路径；(d)将这些数据和以前的数据汇编成Garlock断层的增量滑动和古地震年龄的综合出版记录，以便与其他主要断层的类似数据进行比较。利用新的后ir - irsl225单粒发光测年方法将使研究小组能够确定以前无法确定年代的地层和地貌的年龄。

Knowledge of historic and prehistoric recurrence intervals and slip rates for large faults is important for seismic hazard assessment as well as understanding the fundamentals of earthquake mechanics. Long-term records are essential since recurrence intervals for large earthquakes typically exceed modern instrumental records (e.g. Tohoku, Cascadia, etc.). Paleoseismic studies and slip rate measurements are used to assess recurrence intervals and earthquake potential for active faults or fault networks. These studies suggest earthquakes may cluster both spatially and temporally, slip rates may change significantly over time, and some fault networks demonstrate coordinated behavior, but definitive studies in support of these ideas are sparse. Previous paleoseismic and slip rate studies on the Garlock fault, southern California, suggest that it may have experienced fast slip events that correlate with earthquake clusters that alternated with slow slip events, or seismic lulls, that correlate with decreased rates of strain accumulation. This study would provide a definitive analysis of the prehistoric record of earthquake occurrence and fault slip in order to determine if this behavior is typical for the Garlock fault over the past 15,000 years. The results will help the scientific community better understand what controls the occurrence of large earthquakes on systems of faults so that more informed earthquake forecasts are possible. Other desired societal outcomes include full participation of women and underrepresented minorities in STEM, improved STEM educator development through a teacher summer research program, and development of a globally competitive STEM workforce through training of undergraduate and graduate students.There is mounting evidence that the occurrence of large earthquakes on both single faults and fault systems is not a random process. For example, earthquakes commonly cluster in both space and time. The growing recognition that earthquake occurrence can be highly irregular in time and space will eventually call for a new and more sophisticated method of seismic hazard analysis that takes this behavior into account. Before such new methods can be developed, however, it is necessary to understand the nature of phenomena such as earthquake clustering and the conditions under which they occur. This project will develop a detailed prehistoric record of fault slip as a function of time for the Garlock fault, a fault known to have produced earthquakes with pronounced clustering over time. For example, the Garlock fault produced four large earthquakes in the past 2,000 years, no earthquakes between 2,000 and 5,000 thousand years ago, and two earthquakes between 5,000 and 7,000 thousand years ago. This project tests whether this clustering, which has been observed at one location on the fault, is also reproducible at two other locations along the fault and extends the record of prehistoric earthquakes farther back in time to document whether the fault has been seismically quiescent for any other millennia. To do so, paleoseismic and incremental slip rate data will be acquired from the central Garlock fault through: (a) excavation of paleoseismic trenches at three sites, two of which were previously un-datable because of the dearth of datable carbon; (b) documentation of incremental fault slip rates at two sites for offsets ranging from 30-80m; (c) dating of additional small (3-18 m) offsets based on analysis of GeoEarthScope lidar data at several sites to better constrain the dated slip-path during the past few earthquakes; (d) compilation of these and previous data into a comprehensive, published record of incremental slip and paleo-earthquake ages for the Garlock fault, facilitating comparison with similar data from other major faults. Utilization of the new post-IR-IRSL225 single-grain luminescence dating method will allow the research team to determine the ages of previously un-datable strata and landforms.