Abrupt along-strike change in tectonic style: San Andreas fault zone
Abrupt along-strike change in tectonic style: San Andreas fault zone
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构造风格的突变沿走向变化：圣安德烈亚斯断层带
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
J. Olson
中科院分区：
文献类型：
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作者：
M. Zoback;R. Jachens;J. Olson
利用地震活动性和高分辨率航磁数据，在旧金山半岛的圣安德烈亚斯断层和金门附近10至15公里宽的区域内，确定了从挤压构造到伸展构造的突变。圣安德烈亚斯断层的这段100公里长，包括1906年旧金山7.8级地震的震源，以及沿着-470公里长的断裂处持续微震活动的最高水平。我们根据联合一维速度/震源反演和重新确定震源机制，利用1969年后所有地震的良好约束重新定位，沿着这段断层定义了两个不同的变形带。南区的特征是逆冲和逆断层震源机制，其P轴呈NE向，表明在圣安德烈亚斯断层两侧7至10公里宽的变形带中存在“断层-正向”压缩。在圣安德烈亚斯地表下有一个1到2公里宽的垂直带，其特点是几乎完全没有地震活动。挤压变形与圣克鲁斯山脉/海岸山脉年轻、高的地形是一致的，因为圣安德烈亚斯断层在最南端半岛形成了一个广泛的抑制左弯曲（-10度）。一条长达15公里的地震静止带将这个挤压带与位于最北端半岛和离岸金门台地的正断层和走滑断层联合震源机制带（包括1957年的一次ml = 5.3地震）分隔开来。根据航磁资料计算的线性伪重力梯度和地震反射资料表明，圣安德烈亚斯断层在离海岸不到5公里的地方形成了一个突然的-3公里右阶跃。在近平行的圣格雷戈里奥断层中也观察到类似的右步进（扩张）几何形状。持续的地震活动和伸展构造活动发生在圣安地列斯右台阶区域内，在东南和西北方向沿走向至少15公里。1906年的旧金山地震可能在圣安德烈亚斯右台阶内形成核，这可能有助于解释这次地震破裂的双边性质。我们的分析表明，除了与圣安德烈亚斯断层相关的7 -8级走滑地震的危险外，旧金山半岛还存在两种地震危险：旧金山以西5-8公里处可能发生6级正断层地震，半岛南部可能发生6级逆冲断层事件。
Seismicity and high-resolution aeromagnetic data are used to define an abrupt change from compressional to extensional tectonism within a 10- to 15-km-wide zone along the San Andreas fault on the San Francisco Peninsula and offshore from the Golden Gate. This 100-km-long section of the San Andreas fault includes the hypocenter of the Mw - 7.8 1906 San Francisco earthquake as well as the highest level of persistent microseismicity along that -470-km-long rupture. We define two distinct zones of deformation along this stretch of the fault using well-constrained relocations of all post- 1969 earthquakes based a joint one-dimensional velocity/hypocenter inversion and a redetermination of focal mechanisms. The southern zone is characterized by thrust- and reverse-faulting focal mechanisms with NE trending P axes that indicate "fault-normal" compression in 7- to 10-km-wide zones of deformation on both sides of the San Andreas fault. A 1- to 2-km-wide vertical zone beneath the surface trace of the San Andreas is characterized by its almost complete lack of seismicity. The compressional deformation is consistent with the young, high topography of the Santa Cruz Mountains/Coast Ranges as the San Andreas fault makes a broad restraining left bend (-10 o) through the southernmost peninsula. A zone of seismic quiescence -15 km long separates this compressional zone to the south from a zone of combined normal-faulting and strike-slip- faulting focal mechanisms (including a M L = 5.3 earthquake in 1957) on the northernmost peninsula and offshore on the Golden Gate platform. Both linear pseudo- gravity gradients, calculated from the aeromagnetic data, and seismic reflection data indicate that the San Andreas fault makes an abrupt -3-km right step less than 5 km offshore in this northern zone. A similar right-stepping (dilatational) geometry is also observed for the subparallel San Gregorio fault offshore. Persistent seismicity and extensional tectonism occur within the San Andreas right stepover region and at least 15 km along-strike both to the SE and NW. The 1906 San Francisco earthquake may have nucleated within the San Andreas right stepover, which may help explain the bilateral nature of rupture of this event. Our analysis suggests two seismic hazards for the San Francisco Peninsula in addition to the hazard associated with a M - 7 to 8 strike-slip earthquake along the San Andreas fault: the potential for a M 6 normal-faulting earthquake just 5-8 km west of San Francisco and a M -- 6 + thrust faulting event in the southern peninsula.