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，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南加州圣安德烈亚斯断层的莫哈韦段(从西北部的弗雷泽尔山到东南部的卡琼溪)的断层滑动历史。该项目的主要目标是检验这一假设，即圣安德烈亚斯断层莫哈韦段的平均运动速度在1100年至4500年和6500年至8000年前的快速滑动时期(约45毫米/年)和0至1100年、4500年至6500年和8000年至10000年前的较慢运动时期(约16毫米/年)之间交替。为此，研究小组将通过测量10个不同断层水道的年龄和偏移量来确定这段圣安德烈亚斯断层在过去1万年中的断层运动历史。因为每个通道形成的时间不同，所以每个通道记录不同的断层运动周期。特别是，每个航道经历了不同数量的古代地震，因此积累了不同的断层滑移量，最年轻的航道的偏移量从26(+4/-3)m到最古老的航道的261(+46/-26)m不等。为了准确地测量每一条河道的总年龄和总偏移量，从而测量自河道形成以来的滑移率，研究小组将使用仔细的野外测绘、详细的地形测量分析和在每个地点的挖掘。然后将分析这些年龄和偏移量数据，以确定过去一万年的平均滑动速率以及在该运动周期内存在统计上显著的更快和更慢的滑动脉冲的程度。一般而言，活动断裂保持锁定，直到它们突然破裂并滑动以产生地震。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，地表地物，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，反复的地震会导致沿断层的地表位移和时间平均滑移率的累积。由于移动更快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。洛杉矶盆地和加利福尼亚州东部剪切带古代地震的地质记录似乎表明，断层活动在圣安德烈亚斯断层的莫哈韦段和加利福尼亚州东部剪切带以及南加州的其他断层系统之间迅速切换。该模型对地震危险性有重要影响，如果正确，将极大地修正对主要大陆断裂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.