Testing the Hypothesis that Detachment Faults are Active and Generate Microearthquakes

自20世纪70年代认识到低角度正断层或拆离断层及其在容纳大规模伸展中的作用以来，这些神秘的构造引起了广泛的构造和构造地质学家的注意。关于拆离断层的悬而未决的问题包括假定的机械不可能在地球表面和附近的缓倾斜正断层上滑动，如果一些拆离断层是活跃的，为什么没有观察到地震活动？断层是地震滑动，还是大地震的复发间隔比历史记录更长？为了解决这些问题，华盛顿大学的研究人员将在死亡谷安装一个为期六到九个月的十站便携式临时地震仪网络。将对现场记录的数据进行分析，以确定地震震源的位置。地震仪将安装在死亡谷西侧的冲积扇上，距离滑脱断层的预计位置约8公里。在死亡谷的东侧，沿沿着黑山山脚，地质上和可能的地震活动性很强的西倾滑脱断层，以及在第四纪晚期或全新世地震期间由于滑动而形成的正断层的范围前沿陡坎。实验将检验以下假设：（1）缓倾拆离断层是孕震断层，（2）孕震深度的破裂和滑动成核产生微震。一个积极的结果将挑战的假设，滑动detentralis.Low角度正常，或拆离断层是重要的构造元素，因为它们容纳大规模的拉伸伸展体制，如盆地和山脉在美国西部。然而，这些断层是自相矛盾的，因为力学分析表明运动不应该发生在如此低的角度和地震很少观察到，即使地质证据表明determinants是活跃的。这个实验是第一个针对北美西部拆离断层的实验，将探索这些悖论。微地震的观测将挑战广泛持有的观点，即断层运动要么在机械上是不可能的，要么是不可能的。

Since low-angle normal, or detachment, faults and their role in accommodating large-scale extension were recognized in the 1970's, these enigmatic structures have captured the attention of a wide community of structural and tectonic geologists. Outstanding questions concerning detachment faulting include the putative mechanical impossibility of slip on gently dipping normal faults at and near the Earth's surface and if some detachment faults are active, why has not seismicity been observed? Are the faults slipping aseismically, or are recurrence intervals of large earthquakes longer than historic records? To address these questions, University of Washington researchers will install a ten-station, portable temporary network of seismometers in Death Valley for a period of six to nine months. Data recorded in the field will be analyzed to yield the locations of earthquake hypocenters. The seismometers will be situated on alluvial fans on the west side of Death Valley about 8 km above the projected positions of detachment faults at depth. West-dipping, geologically and possibly seismically active detachment faults are well exposed on the east side of Death Valley along the foot of the Black Mountains, as are range-front scarps of normal faults that formed due to slip during late Quaternary or Holocene earthquakes. The experiment will test the hypotheses that: (1) the gently dipping detachment faults are seismogenic, and (2) that rupture and slip nucleating at seismogenic depths produces microearthquakes. A positive result would challenge the hypothesis that slip on detachments is either mechanically impossible or aseismic.Low-angle normal, or detachment, faults are important tectonic elements in that they accommodate large-scale stretching in extensional regimes, such as the Basin and Range in western United States. However, these faults are paradoxical because mechanical analysis suggests movement should not occur at such low angles and earthquakes are rarely observed even though geological evidence suggests the detachments are active. This experiment, the first of its kind directed toward detachment faults in western North America, will explore these paradoxes. Observation of microearthquakes would challenge the widely-held idea that fault movement on detachments is either mechanically impossible or aseismic.