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

墨西哥西南部近海的俯冲带被特万特佩克湾分成了两个截然不同的地质区。在墨西哥湾以北，科科斯构造板块很浅，因为它俯冲到北美板块之下，地震较少发生，而且海沟靠近海岸。在特万特佩克，海岸线弯曲形成了海湾，海湾以南的板块更陡峭，地震更频繁，海沟离海岸很远。特万特佩克湾位于两者之间，被认为是一个地震空区，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。首先，科科斯板块正在俯冲的特万特佩克山脊的岩石可能正在释放出润滑断裂带的水，使板块缓慢地相互滑动，而不会产生大地震。第二，山脊的地形将两个板块机械地锁在一起，在这种情况下，当应力最终聚集到断裂点时，不可避免地会发生非常大的地震。2017年，一场正常的大地震横跨下行的科科斯板块，这是一种在俯冲带中常见的地震，可能会增加应力和再次发生大地震的可能性。这项研究将对俯冲带深处的水的存在敏感的陆上和近海进行地球物理测量，以了解这次地震发生的原因。这项工作将推进一名博士后研究人员的职业生涯，并加强美国和墨西哥地球物理界之间的联系。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Tehuantepec地震空区及其周围耦合/滑动制度的差异；(2)对俯冲板块内的Tehuantepec山脊进行成像，以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之间的断裂；(3)成像Tehuantepec断层中流体渗透的任何证据。在墨西哥，MT测深已经成功地用于识别俯冲带内的粘滑和瞬时滑移之间的变化以及下部板块的几何形状。这项新的研究将分析异常大的Mw8.2正断层Tehuantepec地震，这是一个没有记录到地震的板块界面的下倾。这次地震发生的原因是由于强耦合界面下方的板块拉力，或者是板块沿着重新激活的、可能充满流体的、耦合较差的俯冲外部隆起断层的弯曲所致。这项工作将是唯一直接确定哪一项是正确的，并确定与未来可能发生的地震的联系的物理测量。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.