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

在过去的5700万年里，印度和亚洲的碰撞造就了喜马拉雅山脉，这是世界上最高的山脉。今天，两个构造板块之间的会聚仍在继续，每年以4厘米(1.5英寸)的速度变形，使喜马拉雅地区的S地壳变形，并引发大地震。最近在尼泊尔发生的破坏性地震(2015年4月25日7.8级；2015年5月12日7.3级)就是这种活动的例子，尽管科学家认为过去曾发生过规模大得多的地震，最高可达8.8级，未来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导致印度北部和尼泊尔多达100万人死亡。这些大地震在沿喜马拉雅山东西向西100至500公里的非常大的区域内破裂断层。最近发生尼泊尔地震的最大断层被称为喜马拉雅主冲断带。科学家不知道为什么会发生7.8级？主震？地震发生在加德满都西北100公里(60英里)的地方；也不知道为什么地震断层停止向东南偏东方向移动约160公里(100英里)。科学家们认为地震在某些地方开始和停止(称为？凹凸处？)断层面在哪里改变了几何形状，可能在哪里变得更陡峭或更不陡峭。如果这些凹凸体相隔很远，地震可能会非常大；但如果凹凸体靠得很近，那么地震可能会小一些。因此，为了量化喜马拉雅地区的地震风险，科学家们需要首先了解喜马拉雅主要冲断带的几何形状。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这条断层的几何形状，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带发生的7.8级地震，来自UTEP和斯坦福大学的科学家正在尼泊尔东部地区紧急部署约20台宽带和短周期地震仪，覆盖最大余震地区。从历史上看，喜马拉雅大地震的余震既发生在主俯冲带逆冲断层(喜马拉雅主冲断)上，也发生在中央主冲断、主边界冲断、主锋冲断层等张性逆冲断层上。余震活动的详细位置将提供这些断层几何形状的史无前例的地下分辨率，目前这些断层的几何形状几乎完全通过地表测绘得知。知道哪些断层在当今处于活动状态吗？一个持续存在争议的话题？将导致对印度-亚洲碰撞的更好的运动学描述。知道向下倾斜吗？坡道和平地？主要喜马拉雅逆冲断层的几何学，特别是是否以及在哪里存在沿走向的侧向斜坡，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。由20个台站组成的德克萨斯大学埃尔帕索/斯坦福大学阵列将补充类似大小和面积尺寸的俄勒冈州立大学/加州大学河滨阵列，共同获得整个余震区的全面图像，该图像在初始破裂区域之外的所有方向都有所延伸。这两个阵列将保留约六个月。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.