RAPID: Collaborative Research: Nepal Array Measuring Aftershock Seismicity Trailing Earthquake

印度和亚洲的碰撞在过去的 5700 万年里创造了世界上最高的山脉喜马拉雅山。 今天，两个构造板块之间的聚合仍在继续，每年聚合 4 厘米（1 ½ 英寸），使喜马拉雅山的地壳变形，并引发大地震。 最近尼泊尔发生的毁灭性地震（2015年4月25日里氏7.8级；2015年5月12日里氏7.3级）就是这种活动的例子，尽管科学家认为过去发生过更大的地震，高达8.8级，并且在未来某个时候将不可避免地再次发生。 其中最严重的地震可能导致印度北部和尼泊尔多达 100 万人死亡。 这些大地震使大片区域的断层破裂，沿喜马拉雅山东西向可能有 100 至 500 公里。 最近发生尼泊尔地震的最大断层被称为喜马拉雅主断层。 科学家不知道为什么会发生7.8级“主震”？地震发生的地点位于加德满都西北 100 公里（60 英里）处；也不知道为什么地震断层停止向东南偏东移动约 160 公里（100 英里）。 科学家认为，地震在断层面改变几何形状的位置（称为“粗糙度”）开始和停止，也许是断层变得更陡或更陡的地方。 如果这些凹凸不平的地方相距很远，地震的强度可能会非常大。但如果凹凸不平的地方靠得很近，那么地震可能会较小。因此，为了量化喜马拉雅山的地震危险，科学家需要首先了解喜马拉雅主冲断层的几何形状。 在 NAMASTE 项目期间，美国科学家将与尼泊尔地震学家和学生一起了解断层几何形状，同时建设尼泊尔的科学能力。 为了应对 2015 年 4 月 25 日尼泊尔喜马拉雅主冲断层发生的 7.8 级地震，UTEP 和斯坦福大学的科学家正在尼泊尔东部紧急部署约 20 个宽带和短周期地震仪，以面阵方式覆盖最大余震地区。 历史上，喜马拉雅大地震的余震既发生在主俯冲带逆冲断层（喜马拉雅主逆冲断层）上，也发生在主中央逆冲断层、主边界逆冲断层和主锋面逆冲断层等张开逆冲断层上。余震地震活动的详细定位将为这些断层的几何形状提供前所未有的地下分辨率，目前这些断层几乎完全通过地表测绘得知。 知道目前哪些断层活跃吗？一个持续争论的话题？将更好地描述印度-亚洲碰撞的运动学。 知道下倾、斜坡和平坦吗？喜马拉雅主冲断层的几何形状，特别是是否存在以及在何处存在沿走向的横向坡道，将有助于更好地了解喜马拉雅大地震的历史记录以及未来潜在的破裂带尺寸。 德克萨斯大学埃尔帕索分校/斯坦福大学的 20 个站阵列将补充俄勒冈州立大学/加州大学河滨分校类似大小和面积尺寸的阵列，共同获取整个余震区的综合图像，该余震区在各个方向上延伸到初始破裂区域之外。 两个阵列将保留大约六个月。 该数据集将尽快提交给 IRIS 数据管理中心，供所有感兴趣的地震学家进行分析。

The collision of the India and Asia has created the Himalaya, the highest mountains in the world, over the last 57 million years. Convergence between the two tectonic plates continues today, at 4cm (1 ½ inches) per year, deforming the earth?s crust in the Himalaya, and creating great earthquakes. The recent devastating earthquakes in Nepal (April 25, 2015 magnitude 7.8; and May 12, 2015 magnitude 7.3) are examples of this activity, though scientists believe that far larger earthquakes have happened in the past, up to magnitude 8.8, and will inevitably occur again sometime in the future. The biggest of these earthquakes could kill as many as 1 million people in northern India and Nepal. These large earthquakes rupture faults over very large areas, perhaps 100 to 500 km West-East along the Himalaya. The biggest fault, on which the recent Nepal earthquakes occurred, is called the Main Himalayan Thrust. Scientists do not know why the magnitude 7.8 ?main shock? earthquake initiated exactly where it did, 100 km (60 miles) northwest of Kathmandu; nor why the earthquake fault stopped moving about 160 km (100 miles) to the east-south-east. Scientists believe earthquakes start and stop at locations (called ?asperities?) where the fault-plane changes geometry, perhaps where it becomes steeper or less steep. If these asperities are a long way apart, the earthquake can be devastatingly large; but if the asperities are close together, then the earthquakes are likely to be smaller. Hence, in order to quantify seismic hazard in the Himalaya, scientists need to first understand the geometry of the Main Himalayan Thrust. During Project NAMASTE, US scientists will work alongside Nepali seismologists and students to understand this fault geometry, while at the same time building the Nepali scientific capacity.In response to the April 25, 2015 M=7.8 earthquake on the Main Himalayan Thrust in Nepal, scientists from UTEP and Stanford are urgently deploying ~20 broadband and short-period seismometers in an areal array across eastern Nepal, spanning the region of the largest aftershocks. Historically, aftershocks of large Himalayan earthquakes occur on both the principal subduction-zone thrust (the Main Himalayan Thrust), and also on splay thrust faults such as the Main Central Thrust, Main Boundary Thrust and Main Frontal Thrust. Detailed location of the aftershock seismicity will provide unprecedented sub-surface resolution of the geometry of these faults that at present are known almost entirely from surface mapping. Knowing which faults are active at the present day ? a subject of ongoing controversy ? will lead to better kinematic descriptions of the India-Asia collision. Knowing the down-dip ?ramp-and-flat? geometry of the Main Himalayan Thrust, and particularly whether and where along-strike lateral ramps exist, will lead to better understanding of the historical record of great Himalayan earthquakes, and potential future rupture zone dimensions. The 20-station University of Texas at El Paso/Stanford University array will complement an Oregon State/University of California Riverside array of similar size and areal dimension to together acquire a comprehensive image of the entire aftershock zone that extends somewhat in all directions beyond the initial rupture area. Both arrays will remain in place for about six months. This dataset will be submitted as rapidly as possible to the IRIS Data Management Center for analysis by all interested seismologists.