Collaborative research: A better understanding of seismic hazard in Tehuantepec, Mexico, using amphibious MT studies

墨西哥西南近海的俯冲带被特万特佩克湾划分为两个截然不同的地质区域。在海湾的北部，科科斯构造板块很浅，因为它俯冲到北美板块之下，地震较少，海沟离海岸很近。在特万特佩克，海岸线弯曲形成了海湾，海湾的南部板块更陡峭，地震更频繁，海沟离海岸很远。位于两者之间的特万特佩克湾被认为是一个地震间隙，因为没有记录到与构造板块相互滑动有关的大地震。这可能有两个原因。第一种解释是，与科科斯板块一起俯冲的特万特佩克山脊（Tehuantepec Ridge）的岩石可能会释放出润滑断层带的水，使板块之间缓慢滑动，而不会产生大地震。第二种解释是，山脊的地形正在机械地将两个板块锁在一起，在这种情况下，当应力最终积聚到断裂点时，一场非常大的地震是不可避免的。2017年，一场大型正常地震在下行的科科斯板块上爆发，这种地震在俯冲带通常不常见，这可能会增加应力和另一场大地震的可能性。这项研究将进行陆上和海上的地球物理测量，这些测量对俯冲带深处的水的存在很敏感，以了解这次地震发生的原因。这项工作将推进博士后研究者的职业生涯，并加强美国和墨西哥地球物理社区之间的联系。在该项目中，两栖大地电磁测深结合地震构造分析将：(1)确定墨西哥特万特佩克地震间隙及其周围耦合/滑动机制的差异；(2)在俯冲板块内对特万特佩克脊进行成像，确定其在耦合中的作用，以及是否有助于跨墨西哥火山带弧系与恰帕斯弧的断裂；(3)对特万特佩克断层中流体渗透的证据进行成像。MT探测已经在墨西哥成功地用于确定俯冲带内粘滑和瞬变滑动之间的变化以及下板块的几何形状。这项新的研究将分析特万特佩克异常大的Mw8.2正断层地震，这是一个没有地震记录的板块界面的下倾。关于这次地震发生的原因，人们提出的理由是，它是由于在强耦合界面下的板块拉动，或者是由于板块沿着一个重新激活的、可能充满流体的、耦合不良的俯冲外隆升断层弯曲。这项工作将代表唯一的物理测量，可以直接确定哪一个是正确的，并确定与未来可能发生的地震的联系。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The subduction zone offshore southwest Mexico is divided into two very different geological regions by the Gulf of Tehuantepec. North of the gulf, the Cocos tectonic plate is shallow as it subducts beneath the North American plate, earthquakes are less frequent, and the ocean trench is close to shore. At Tehuantepec the coastline bends to form the gulf, and south of the gulf the plate is steeper, earthquakes are more frequent, and the trench is far offshore. Located in between, the Gulf of Tehuantepec is considered a seismic gap, in that there have been no recorded large earthquakes associated with the tectonic plates slipping past each other. There are two possible reasons for this. The first is that the rocks of the Tehuantepec Ridge, which is being subducted with the Cocos plate, may be releasing water that lubricates the fault zone, allowing the plates to slowly slide past each other without producing large earthquakes. The second is that the topography of the ridge is mechanically locking the two plates together, in which case a very large earthquake is inevitable when the stress finally builds up to breaking point. In 2017 a large normal earthquake broke across the down-going Cocos plate, a type of earthquake not normally seen in subduction zones, and which could increase the stress and the chances of another large earthquake. This study will make geophysical measurements both onshore and offshore that are sensitive to the presence of water deep in the subduction zone to understand why this earthquake occurred. This work will advance the career of a postdoctoral investigator and strengthen ties between the US and Mexican geophysical communities.In this project an amphibious magnetotelluric (MT) sounding survey coupled with seismotectonic analysis will: (1) Determine differences in coupling/slip regimes in and around the Mexican Tehuantepec seismic gap; (2) Image the Tehuantepec Ridge within the subducted plate to determine its role in the coupling and if it contributes to the break between the Trans-Mexican Volcanic Belt arc system and the Chiapas arc; (3) Image any evidence of fluid infiltration in the Tehuantepec fault. MT soundings have already been successfully used in Mexico to identify changes between stick slip and transient slip within the subduction zone and the geometry of the lower plate. This new study will analyze the unusually large Mw8.2 normal fault Tehuantepec earthquake which is downdip of a slab interface that has no recorded earthquakes. The proposed reasons for why this earthquake occurred are that it was due to slab pull below a strongly coupled interface or that it was due to slab bending along a reactivated, possibly fluid-filled, and poorly coupled, subducted outer rise fault. This work would represent the only physical measurement to directly determine which is correct and determine the link to possible future earthquakes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.