Testing the Hypothesis that Detachment Faults are Active and Generate Microearthquakes

自从20世纪70年代S认识到低角度正断层或拆离断层及其在适应大规模伸展中的作用以来，这些神秘的构造引起了构造和构造地质学家的广泛关注。有关滑脱断层的突出问题包括假定在地表及其附近的缓倾正断层上滑动的力学可能性，以及如果一些滑脱断层是活动的，为什么没有观察到地震活动？断层是无震性滑动的，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10个站的便携式临时地震仪网络，为期6至9个月。现场记录的数据将被分析，以得出地震震中的位置。地震仪将安装在死亡谷西侧的冲积扇上，比深度滑脱断层的预计位置高出约8公里。在黑山脚下死亡谷的东侧，西倾的、地质上的、可能还有地震活动的滑脱断层暴露得很好，因为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山脉前缘陡坎也很容易暴露。实验将检验以下假设：(1)缓倾滑脱断层是孕震的，(2)孕震深度的破裂和滑移成核产生微震。一个积极的结果将挑战这样的假设，即在滑脱上滑移要么是机械上不可能的，要么是无地震的。低角度正断层或滑脱断层是重要的构造元素，因为它们适应了伸展制度下的大规模伸展，如美国西部的盆地和山脉。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，而且地震很少观察到，尽管地质证据表明分离是活跃的。这是首次针对北美西部的滑脱断层进行的此类实验，将探索这些悖论。对微震的观测将挑战普遍持有的观点，即断层在断层上的运动要么在机械上是不可能的，要么是无地震的。

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.