Following the July 2019 Ridgecrest, California earthquakes, multiple field investigators noted that pebble- to boulder-sized rocks had been displaced from their position in the desert pavement within a stepover along the right-lateral strike-slip M7.1 rupture trace, without evidence of dragging or shearing. This implies localized ground motions in excess of 1 g, in contrast to the instrumentally recorded peak of 0.57 g. Similar rock displacement occurred in a stepover in the predominantly strike-slip 2010 M7.2 El Mayor-Cucapah earthquake. Together, these examples suggest that some aspect of how earthquake rupture negotiates a strike-slip fault stepover produces extremely localized strong ground acceleration. Here, we use the 3D finite element method to investigate how rupture through a variety of strike-slip stepover geometries influences strong ground acceleration. For subshear ruptures, we find that the presence of a stepover in general matters more than its dimensions; the strongest ground acceleration always occurs at the end of the first fault. For supershear ruptures, the stepover is effectively irrelevant, since the strongest particle acceleration occurs at the point of the supershear transition on the first fault. Our model subshear and supershear ruptures alike do produce horizontal particle acceleration above 1 g, but over a region so close to the fault (< 1 km) that a seismic network may not catch it. We suggest that the physics of rupture through a fault stepover could have been responsible for the displaced rocks in the Ridgecrest and El Mayor-Cucapah earthquakes, and that stepover regions may have particularly high ground motion hazard. Our study suggests that ground motion predictions and local hazard assessments should account for much stronger accelerations in the immediate near field of active faults, especially around stepovers and other geometrical discontinuities.

中文翻译：

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对断层跨距中强地震动 （> 1 g） 的破裂动力学进行建模


在 2019 年 7 月加利福尼亚州里奇克莱斯特地震之后，多名现场调查人员指出，鹅卵石到巨石大小的岩石在沿着右侧走滑 M7.1 破裂轨迹的跨步范围内从沙漠路面的位置移位，没有拖拽或剪切的证据。这意味着局部地震动超过 1 g，而仪器记录的峰值为 0.57 g。在 2010 年以走滑为主的 M7.2 El Mayor-Cucapah 地震中，类似的岩石位移也发生在跨步中。总之，这些例子表明，地震破裂如何协商走滑断层跨步的某些方面会产生极其局部的强地面加速度。在这里，我们使用 3D 有限元方法来研究通过各种走滑跨距几何形状的破裂如何影响强大的地面加速度。对于亚剪切断裂，我们发现跨距的存在通常比其尺寸更重要;最强的地面加速度总是发生在第一个断层的末端。对于超剪切破裂，跨距实际上是无关紧要的，因为最强的粒子加速度发生在第一个断层的超剪切过渡点。我们的模型亚剪切破裂和超剪切破裂确实会产生 1 g 以上的水平粒子加速度，但在离断层如此之近的区域（< 1 km）上，地震网络可能无法捕捉到它。我们认为，断层跨距破裂的物理学可能是导致 Ridgecrest 和 El Mayor-Cucapah 地震中岩石移位的原因，并且跨距区域可能具有特别高的地震动危险。 我们的研究表明，地震动预测和局部灾害评估应该解释活动断层的直接近场中更强的加速度，尤其是在跨距和其他几何不连续性周围。