Focal mechanisms for small magnitude earthquakes (M ∼ 1.3–4.1) in the mid-Atlantic region of the United States have been determined using a double-couple moment tensor inversion procedure. The 26 new focal mechanisms obtained, when combined with previously published mechanisms, show a pattern of reverse faulting in the easternmost portion of the study area and strike-slip faulting in the west, consistent with previous studies. The change in focal mechanisms from east to west helps to constrain the geographic location of the east-west transition in the stress regime to a NE-SW area within central Pennsylvania within proximity of the Allegheny Front. Stress inversions performed to constrain variations in the stress state across the region show that the maximum compressive stress varies only slightly, but that the near-vertical stress is the minimum compressive stress in the east and transitions to the intermediate compressive stress in the west, as expected for an east-west transition in reverse to strike-slip faulting. Analysis of driving forces causing the stress change suggests that tectonic terrane structure, glacial isostatic adjustment, and changes in gravitational potential energy have little effect on the stress field in this region, leaving the interaction of sublithospheric mantle flow with the eastern edge of the Laurentian cratonic lithosphere beneath central Pennsylvania as a primary explanation. The cratonic lithospheric keel may cause a deflection in mantle flow, thereby changing the stress field enough so that the magnitude of the vertical stress in relation to the minimum horizontal stress results in strike-slip as opposed to reverse faulting.

中文翻译：

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美国大西洋中部小地震和应力状态的矩张量


美国大西洋中部地区小震级地震 （M ∼ 1.3–4.1） 的震源机制已使用双耦合矩张量反演程序确定。获得的 26 个新的震源机制，当与以前发表的机制相结合时，显示研究区域最东端的反向断层和西部的走滑断层模式，与以前的研究一致。焦点机制从东到西的变化有助于将应力状态中东西过渡的地理位置限制在宾夕法尼亚州中部靠近阿勒格尼锋线的 NE-SW 区域。为约束整个区域应力状态的变化而进行的应力反演表明，最大压应力仅略有变化，但近垂直应力是东部的最小压应力，在西部过渡到中等压应力，正如预期的那样，东西向过渡与走滑断层相反。对引起应力变化的驱动力的分析表明，构造地表结构、冰川等静压调整和重力势能的变化对该地区的应力场影响不大，将岩石下层地幔流与宾夕法尼亚州中部下方劳伦森克拉通岩石圈东部边缘的相互作用作为主要解释。克拉通岩石圈龙骨可能导致地幔流发生偏转，从而改变应力场，从而使垂直应力的大小与最小水平应力相关，从而导致走滑，而不是反向断层。