The 2016 Nine Mile Ranch Earthquakes: Hazard and Tectonic Implications of Orthogonal Conjugate Faulting in the Walker Lane
The 2016 Nine Mile Ranch Earthquakes: Hazard and Tectonic Implications of Orthogonal Conjugate Faulting in the Walker Lane
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2016 年九英里牧场地震：沃克巷正交共轭断层的危害和构造影响
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发表时间：
2022
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通讯作者：
I. Pierce
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作者：
Rachel Hatch;R. Abercrombie;C. Ruhl;Kenneth D. Smith;W. Hammond;I. Pierce
九英里牧场 (NMR) 序列始于 2016 年 12 月内华达州弗莱彻谷偏远地区在一小时内发生的三场 5.4-5.6 级地震；第一个和第二个事件仅相隔 4 分钟。我们分析了沃克巷这一复杂的地震序列，以确定其几何形状和驱动机制，并加深对该地区变形和地震危害的了解。现场勘察发现，这些地震对九英里牧场房屋造成了严重损坏，但没有出现地表破裂。我们精确地重新定位了 6000 多次地震，以揭示地表未绘制的激活平面结构，包括两个大型正交共轭断层。力矩张量解、震源机制和重定位表明，这两个共轭断层是一条垂直的、东北向的左旋走滑断层和一条向东北倾斜~60°的西北走向的右旋走滑断层。三个主要事件均位于两条断层的交汇处，但位置和方向与破裂左侧东北向断层面的第一次（Mw 5.6）和第三次（Mw 5.5）事件最为一致；第二次事件（Mw 5.4）使右侧西北向断层面破裂。计算出的静态应力变化支持了这一解释。较小的事件和构造主要表现为走滑和正断层。我们使用全球定位系统数据计算局部震间应变率张量和同震位移，以确定附近的火山中心是否在造成断层几何形状中发挥了作用。我们的结果结合层序的时空发展和矩张量解，表明区域尺度构造力是该层序的主导驱动因素。核磁共振序列增加了沃克巷内地震序列和群地震时空模式和驱动机制的多样性记录，为该活动区域的地震灾害提供了进一步的信息和限制。
The Nine Mile Ranch (NMR) sequence began with three Mw 5.4–5.6 earthquakes within one hour of each other in December 2016 in the remote area of Fletcher Valley, Nevada; only 4 min separated the first and second events. We analyze this complex earthquake sequence in the Walker Lane to determine the geometry and driving mechanism(s), and to improve understanding of deformation and seismic hazard in this region. Field reconnaissance found that these earthquakes caused significant damage to the Nine Mile ranch house but no surface rupture. We precisely relocate 6000+ earthquakes to reveal activated planar structures, unmapped at the surface, including two large, orthogonal, conjugate faults. Moment tensor solutions, focal mechanisms, and relocations show the two conjugate faults to be a vertical, northeast-trending left-lateral strike-slip fault, and a northwest-trending right-lateral strike-slip fault that dips ∼60° to the northeast. The three main events lie at the intersection of both the faults, but the locations and orientations are most consistent with the first (Mw 5.6) and third (Mw 5.5) events rupturing the left-lateral northeast-trending fault plane; the second event (Mw 5.4) ruptured the right-lateral northwest-trending fault plane. Calculated static stress changes support this interpretation. Smaller events and structures show predominantly strike-slip and normal faulting. We calculate the local interseismic strain rate tensor and coseismic displacements using Global Positioning System data to determine whether nearby volcanic centers played a role in causing the fault geometry. Our results, combined with the spatiotemporal development of the sequence and the moment tensor solutions, indicate that regional scale tectonic forces are the dominant driving factors of this sequence. The NMR sequence adds to the documented variety of spatiotemporal patterns and driving mechanisms of earthquake sequences and swarms within the Walker Lane, providing further information and constraints on seismic hazard in this active region.