Does the Holocene Slip History for the Mojave Section of the San Andreas Fault Indicate Secular Variation in Slip and a Discrepancy with Geodetic Rates?

在这个项目中，来自加州大学戴维斯分校的一组科学家与美国地质调查局的研究人员合作，将利用地质观测来测量加州南部圣安德烈亚斯断层莫哈韦段（从西北部的弗雷泽山到东南部的卡洪溪）沿着断层滑动历史。该项目的主要目标是检验假设，即沿着圣安德烈亚斯断层莫哈韦段的平均运动速率在快速滑动期间交替在1100-4500年和6500-8000年前，（约16毫米/年）在0-1100，4500-6500，和8000-10000年前。为了做到这一点，研究小组将通过测量十个不同断层河道的年龄和偏移量，来确定过去一万年来圣安德烈斯断层沿着这一部分的断层运动历史。由于每条通道形成于不同的时间，每条通道记录了不同时期的断层运动。特别是，每个通道经历了不同数量的古代地震，因此积累了不同数量的断层滑动，偏移量从最年轻的通道的26（+4/-3）m到最古老的通道的261（+46/-26）m不等。为了精确测量这些通道的总年龄和总偏移量，从而测量通道形成以来的滑动速率，研究小组将使用仔细的实地测绘，详细的地形测量分析和每个地点的挖掘。这些年龄和偏移数据将被分析，以确定过去一万年的平均滑动速率，以及在这段运动期间，在统计学上有意义的更快和更慢的滑动脉冲的程度。这种运动大致类似于汽车在走走停停的交通中的运动。在地震破裂过程中，地表特征，如河道或道路，经常被快速的断层运动切割和移位。因此，随着时间的推移，重复的地震会导致地表位移沿断层沿着的累积和时间平均滑动速率。由于快速移动的断层会产生更频繁的地震，因此确定断层滑动的平均速率对于评估其地震危险性至关重要。洛杉矶盆地和东加州剪切带的古地震地质记录似乎表明，断层活动在圣安德烈亚斯断层的莫哈韦段和东加州剪切带之间以及南加州的其他断层系统之间迅速切换。这个模型对地震危险性有重要意义，如果正确的话，将大大改变对主要大陆断层系统力学行为的理解。

In this project a team of scientists from the University of California, Davis, in collaboration with researchers at the United States Geological Survey, will use geologic observations to measure the history of fault slip along the Mojave section of the San Andreas fault in Southern California (from Frazier Mountain in the northwest to Cajon Creek in the southeast). The primary goal of the project is to test the hypothesis that the average rate of motion along the Mojave section of the San Andreas fault alternated between periods of fast slip (approximately 45 mm/yr) at 1100-4500 and 6500-8000 yr ago and periods of slower motion (approximately 16 mm/yr) at 0-1100, 4500-6500, and 8000-10000 years ago. To do so, the research team will determine the history of fault motion along this section of the San Andreas fault over the last ten thousand years by measuring the ages and offsets of ten different faulted stream channels. Because each channel formed at a different time, each channel records a different period of fault motion. In particular, each channel experienced a different number of ancient earthquakes and thus accumulated a different amount of fault slip, with offsets ranging from 26 (+4/-3) m for the youngest channel to 261 (+46/-26) m for the oldest. To precisely measure the total age and total offset for each of these channels, and thus measure the slip rate since the channel formed, the research team will use careful field mapping, analysis of detailed topographic measurements, and excavations at each site. These age and offset data will then be analyzed to determine both the slip rate as averaged over the last ten thousand years and the extent to which there were statistically significant faster and slower pulses of slip within that period of motion.In general, active faults remain locked until they suddenly break and slip to produce an earthquake. Such motion is roughly analogous to that of a car in stop-and-go traffic. During earthquake ruptures, surface features such as stream channels or roads are often cut and displaced by the rapid fault motion. Thus, repeated earthquakes over time can result in both an accumulation of surface displacements along the fault and a time-averaged slip rate. Because faster-moving faults will produce more frequent earthquakes, determining the average rates at which faults slip is of fundamental importance for evaluating their earthquake hazards. Geological records of ancient earthquakes in the Los Angeles Basin and Eastern California Shear Zone appear to indicate that fault activity has rapidly switched between the Mojave section of the San Andreas fault and the Eastern California Shear zone, as well as other fault systems in southern California. This model has important implications regarding seismic hazards and if correct, would significantly revise understanding of the mechanical behavior of major continental fault systems.