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

大断层的历史和史前复发间隔和滑移率的知识对于地震危险性评估和理解地震力学的基本原理是重要的。长期记录至关重要，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)。古地震研究和滑动速率测量被用来评估活动断层或断层网络的重复间隔和地震可能性。这些研究表明，地震可能会在空间和时间上聚集，滑移率可能会随着时间的推移发生显著变化，一些断层网络显示出协调的行为，但支持这些观点的明确研究很少。先前对加利福尼亚州南部加洛克断层的古地震和滑动速率研究表明，它可能经历了与地震群相关的快速滑动事件，而地震群交替发生的慢滑动事件或地震间歇与应变累积速度降低相关。这项研究将对史前地震发生和断层滑动的记录进行明确的分析，以确定这种行为是否在过去15,000年来的加洛克断层中是典型的。这一结果将帮助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，以便有可能进行更有见地的地震预报。其他期望的社会成果包括妇女和代表性不足的少数民族充分参与STEM，通过教师暑期研究计划改善STEM教育者的发展，以及通过培训本科生和研究生发展具有全球竞争力的STEM劳动力。越来越多的证据表明，在单一断层和断层系统上发生大地震不是一个随机过程。例如，地震通常在空间和时间上聚集。越来越多的人认识到，地震的发生在时间和空间上可能是高度不规则的，最终将需要一种新的、更复杂的地震危险性分析方法，将这种行为考虑在内。然而，在开发这样的新方法之前，有必要了解地震丛集等现象的性质和发生的条件。该项目将为Garlock断层开发一份详细的史前断层滑动随时间变化的记录，该断层已知曾产生地震，并随着时间的推移具有明显的聚集性。例如，加洛克断层在过去2000年中发生了4次大地震，在2000到5000年前没有发生过地震，在5000到7000年前发生了两次地震。该项目测试了在断层上的一个位置观察到的这种聚集性是否也可以在断层上的另外两个位置重现，并将史前地震的记录延长到更早的时间，以证明该断层是否在任何其他千年中一直处于地震静止状态。为此，将通过以下方式从加洛克断层中央获取古地震和增量滑动速率数据：(A)挖掘三个地点的古地震战壕，其中两个地点以前由于缺乏可测年碳而无法更新；(B)记录两个地点的增量断层滑移率，偏移量从30-80m不等；(C)根据对几个地点的GeoEarthScope激光雷达数据的分析，确定额外的小(3-18m)偏移量，以更好地限制过去几次地震期间的日期滑动路径；(D)将这些数据和以前的数据汇编成Garlock断层增量滑动和古地震年龄的全面公布记录，便于与其他主要断层的类似数据进行比较。利用新的后IR-IRSL225单颗粒发光测年方法，研究小组将能够确定以前无法确定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.