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

在过去的5700万年里，印度和亚洲的碰撞创造了喜马拉雅山脉，世界上最高的山脉。两个构造板块之间的融合至今仍在继续，以每年4厘米（1.5英寸）的速度，使地球变形。它在喜马拉雅山脉的地壳中产生了巨大的地震。最近发生在尼泊尔的破坏性地震（2015年4月25日的7.8级和2015年5月12日的7.3级）就是这种活动的例子，尽管科学家们认为过去发生过更大的地震，达到8.8级，并且在未来的某个时候将不可避免地再次发生。其中最大的一次地震可能会在印度北部和尼泊尔造成多达100万人死亡。这些大地震在很大范围内破坏断层，沿喜马拉雅山脉从西到东可能有100到500公里。最近尼泊尔地震发生的最大断层被称为喜马拉雅主逆冲断层。科学家们不知道7.8级的原因是什么？主要冲击?地震就在它发生的地方，加德满都西北100公里（60英里）处开始；也不知道为什么地震断层停止向东-东南方向移动160公里（100英里）。科学家们认为，地震发生和停止的位置（称为“陡坡”），在那里断层平面的几何形状发生了变化，也许在那里它变得更陡或不那么陡。如果这些凸起相距很远，那么地震可能是毁灭性的；但如果这些凸起靠得很近，那么地震可能会更小。因此，为了量化喜马拉雅地区的地震危险性，科学家们需要首先了解喜马拉雅主逆冲的几何形状。在NAMASTE项目期间，美国科学家将与尼泊尔地震学家和学生合作，了解这一断层的几何形状，同时建设尼泊尔的科学能力。2015年4月25日尼泊尔喜马拉雅主断层发生7.8级地震后，UTEP和斯坦福大学的科学家们在尼泊尔东部地区紧急部署了约20台宽带和短周期地震仪，跨越了余震最大的地区。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逆冲断层上。余震地震活动的详细位置将为这些断层的几何结构提供前所未有的地下分辨率，而目前这些断层几乎完全是通过地表测绘来了解的。知道今天哪些断层是活跃的吗？一个持续争议的话题？将有助于对印亚碰撞进行更好的运动学描述。知道下坡吗？喜马拉雅主逆冲构造的几何结构，特别是沿走向的横向斜坡是否存在以及在哪里存在，将有助于更好地理解喜马拉雅大地震的历史记录，以及未来潜在的断裂带尺寸。位于埃尔帕索的德克萨斯大学/斯坦福大学的20个测点阵列将与俄勒冈州立大学/加州大学河滨分校的相似大小和面积的阵列相辅相成，共同获得整个余震区的综合图像，该图像在一定程度上向各个方向延伸，超出了最初的破裂区域。这两个阵列将保留约6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.