Collaborative Research: Slip-per-event Rupture History of the San Andreas Fault in the Carrizo Plain: Was the 1857 Earthquake Characteristic?

1857年1月9日，里氏7.8级的特琼堡地震使圣安德烈亚斯断层南部一段400公里长的断层破裂，其中包括卡里佐断层。圣安德烈亚斯断层的Carrizo段被提议仅在最大地震期间破裂，从而控制圣安德烈亚斯断层南部具有数百年复发时间的大地震在Carrizo平原上的发生。最近的发现表明，地震具有更复杂的破裂模式，复发时间显著缩短，可能出现群集性地震。据报道，过去5次或6次地震的每次滑动测量，与新的地震日期相结合，产生的滑动速率高达50毫米/年，远高于广泛接受的约35毫米/年的值。这种不匹配表明，要么释放的应变与破裂之间的时间不成正比，从而使人对时间可预测和滑动可预测的弹性应变模型产生怀疑，要么一些已报道的逐事件滑动测量代表了多个事件的累积滑动。这个难题的解决应该会导致对断层力学的基本洞察，以及预测未来大地震的更有效的方法。该项目的目标是测量卡里佐平原圣安德烈亚斯断层最近几次准确测定的断裂的滑动。挑战在于将滑动测量与地震日期联系起来。将应用几种方法，包括对Bidart扇区进行三维古地震挖掘，高精度放射性碳测年，使用高分辨率机载激光条带测绘数据分析地貌偏移量，以及汇编未发表研究的档案现场数据。这项研究的结果可能会对大地震的防震减灾和规划产生重大影响。圣安德烈亚斯断层是人口稠密、经济重要的加利福尼亚州地震危险和风险的主要来源，其已被证实的引发大地震的潜力使其对国家构成重大威胁。这项研究的数据将有助于评估加州南部发生下一次破坏性圣安德烈亚斯地震的可能性，并预测其位置和震级。

On January 9, 1857 the magnitude 7.8 Fort Tejon earthquake ruptured a 400 km long section of the southern San Andreas Fault including the Carrizo section. The Carrizo section of the San Andreas Fault has been proposed to rupture only during the largest earthquakes, and thereby to control the occurrence of great characteristic earthquakes on the southern San Andreas Fault with centuries-long recurrence times in the Carrizo Plain. Recent findings suggest a more complex rupture pattern with significantly shorter recurrence times and possibly clustered earthquakes. Reported measurements of slip-per-event for the last 5 or 6 earthquakes, when combined with new dates of earthquakes, yield slip rates up to 50 mm/yr, well above widely accepted values of approximately 35 mm/yr. The mismatch suggests that either the strain released is not directly proportional to the time between ruptures, thereby casting doubt on time-predictable and slip-predictable elastic strain models, or some of the reported slip-per-event measurements represent cumulative slip for multiple events. Resolution of this conundrum should lead to fundamental insights into fault mechanics, as well as more effective methods for forecasting future large earthquakes. The goal of this project is to measure the slip from several recent well-dated ruptures of the San Andreas Fault in the Carrizo Plain. The challenge is in linking slip measurements with dates of earthquakes. Several methods will be applied, including three-dimensional paleoseismic excavation of the Bidart fan site, high precision radiocarbon dating, analysis of geomorphic offsets using high resolution airborne laser swath mapping data, and compilation of archival field data from unpublished studies. Results of this research could have significant impact on seismic hazard mitigation and planning for major earthquakes. The San Andreas Fault is the major source of seismic hazard and risk in the densely populated and economically important state of California, and its proven potential to generate a large earthquake makes it a significant threat to the nation. Data from this research will help to assess the probability, and forecast the location and magnitude of the next damaging San Andreas earthquake in southern California.