Collaborative Research: The Nepal Earthquake and Limits on Moment, Fault Geometry and Time Dependent Stress Changes

2015年4月25日发生在尼泊尔的7.8级地震是喜马拉雅地区近年来发生的唯一地震，因为它未能完全释放自1833年类似地震以来积累的上覆应力。 相反，压力从首都加德满都以北40公里处转移到以南8公里处，在此过程中，加德满都山谷的200万人口向西南方向移动了约1.3米。 主要关注的是，这种应力现在处于平衡状态，要么作为缓慢滑动良性扩散，要么在未来的Mw#8804;7.5地震中剧烈释放。 美国驻加德满都大使馆官员担心，在首都郊区发生的这种规模的大余震将对该市已经削弱的建筑物造成灾难性后果，并将极大地削弱对2015年4月25日地震摧毁的数千个村庄的救援行动。在这个合作的快速项目中，调查人员将在加德满都以南的地区安装仪器，以确定这种压力到底发生了什么。希望是类似于发生在加州圣安德烈亚斯断层的过程占上风，即应力正在缓慢释放，在这种情况下，可以推断未来不太可能发生大地震。 研究人员还认识到，2015年的地震可能为加德满都附近未来的地震提供线索，因为通过重新评估1833年至1866年期间加德满都附近发生的历史地震，可能会深入了解接下来可能发生的事情。 研究人员将与特里布万大学的尼泊尔学生和教师合作进行这些研究。 研究人员计划在加德满都和喜马拉雅山脉前冲断层之间的稳定纪念碑上安装两组GPS装置--35公里宽的距离，仪器间距近达5公里。 狭窄的间距将使他们能够探测到喜马拉雅山滑脱的滑动，如果它发生的话，并将使他们能够区分各种形式的空间应变扩散，或者局部锁定的凸体，如果这些存在的话。每个仪器的数据将遥测到博尔德，数据将每天处理并存档，便于公众访问。 还将安装一些运动全球定位系统站点，由特里布万大学的学生每隔六个月（或更频繁）测量一次。 研究人员预计将部署多达11个额外的GPS仪器。调查人员还计划从震中地区收集时间敏感的强度数据，这些数据可能用于校准2015年地震中地表加速度的衰减。他们将使用这些数据来校准1833年，1835年，1866年，1934年和1936年尼泊尔地震的历史强度数据，这些数据将用于限制这些以前事件的破裂区域，以及它们与2015年地震的关系。

The M7.8 earthquake occurred on 25 April 2015 in Nepal is unique among recent Himalayan earthquakes, as it failed to release entirely the pent up stress accumulated since a previous similar earthquake there in 1833. Instead, stress was transferred from 40 km north of the capital, Kathmandu, to 8 km to the south, shifting the 2 million population of the Kathmandu Valley about 1.3 m to the southwest in the process. Of major concern is that this stress is now poised, either to diffuse benignly as slow slip, or to be released violently in a future Mw≤7.5 earthquake. US Embassy officials in Kathmandu are concerned that a major aftershock of this size occurring on the outskirts of the capital would have catastrophic consequences to the already weakened buildings of the city, and would greatly impair the rescue operation to the thousands of villages wiped out by the April 25, 2015 earthquake. In this collaborative rapid project, the investigators will install instruments in the region south of Kathmandu to identify what is exactly happening to this stress. The hope is that processes similar to those occurring on the San Andreas fault in California prevail, namely that stress are being released slowly, in which case it would be possible to infer that a future major earthquake is unlikely. The investigators also recognize that the 2015 earthquake may provide clues to future earthquakes near Kathmandu, in that it may be possible to gain insights into what may happen next by re-evaluating historical earthquakes occurred near Kathmandu in the period from 1833 to 1866. The investigators will be collaborating with Nepali students and faculty in Tribhuvan University to undertake these studies. The investigators plan to install two arrays of GPS units on stable monuments between Kathmandu and the frontal thrusts of the Himalaya- a 35 km wide distance with instrument spacing as close as 5 km. The narrow spacing will permit them to detect slip of the Himalayan décollement should it be occurring, and will permit them to distinguish various forms of spatial strain diffusion, or locally locked asperities should these exist. Data from each instrument will be telemetered to Boulder and the data will be processed daily and archived with easy public access. A number of campaign GPS sites will be also installed that will be measured at six-month intervals (or more frequently) by students from Tribhuvan University. The investigators envisage that up to eleven additional GPS instruments will be deployed. The investigators plan also to gather time-sensitive intensity data from the epicentral region that may be used to calibrate the attenuation of surface acceleration in the 2015 earthquake. They will use these data to calibrate historical intensity data from the 1833, 1835, 1866,1934 and 1936 Nepal earthquakes, that will used to constrain the rupture areas of these former events, and how they relate to the 2015 earthquake.