Testing the Hypothesis that Detachment Faults are Active and Generate Microearthquakes

检验滑脱断层活跃并产生微地震的假设

• 批准号: 1143693
• 负责人: Darrel Cowan
• 金额: $ 5.8万
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-09-15 至 2016-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1143693&HistoricalAwards=false
• 关键词: Testing; Hypothesis; Detachment; Faults; Active

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.

自从 20 世纪 70 年代人们认识到低角度正断层或脱离断层及其在容纳大规模伸展方面的作用以来，这些神秘的结构引起了结构和构造地质学家的广泛关注。关于滑脱断层作用的突出问题包括，在地球表面及其附近的平缓倾斜的正断层上假定机械上不可能发生滑动，如果某些滑脱断层是活动的，为什么没有观察到地震活动？断层是否发生抗震滑动，或者大地震的复发间隔是否比历史记录更长？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由十个站组成的便携式临时地震仪网络，为期六到九个月。将分析现场记录的数据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，距离滑脱断层预计深度位置上方约 8 公里。沿黑山脚的死亡谷东侧充分暴露出西倾的地质和可能地震活跃的滑脱断层，以及由于第四纪晚期或全新世地震期间滑动而形成的正断层的山脉前缘陡坡。该实验将检验以下假设：（1）平缓倾斜的滑脱断层是孕震的，（2）孕震深度的破裂和滑移成核会产生微地震。积极的结果将挑战分离滑动在力学上是不可能的或抗震的假设。低角度正断层或分离断层是重要的构造元素，因为它们适应伸展区域的大规模拉伸，例如盆地和山脉美国西部。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，并且很少观察到地震，即使地质证据表明分离是活跃的。这项实验是第一个针对北美西部滑脱断层的实验，将探索这些悖论。对微地震的观测将挑战人们普遍持有的观点，即分离体上的断层运动要么在机械上是不可能的，要么是抗震的。