Imaging the Mechanics of Shallow Subduction: the Oaxaca GPS Array

0104299瓦哈卡全球定位系统研究旨在对沿中美海沟350公里长的瓦哈卡海沟的俯冲机制进行详细的大地测量研究，那里的海沟比地球上任何其他大陆块都更靠近海岸(50-65公里)。该地区闭锁俯冲界面的地震间载荷导致海岸75-100公里范围内到处都有高的弹性应变梯度。因此，通过大地测量技术测量的地表位移对沿附近俯冲界面的载荷和/或滑动的分布很敏感。这使得瓦哈卡的沿海和近海地区成为研究俯冲界面上部地震孕育区俯冲机制的绝佳和可能无与伦比的地区。对瓦哈卡40个站点的GPS阵列的预期空间分辨率的正反向建模表明，它的分辨率水平将比密集的日本GPS阵列高5-12倍。测量瓦哈卡的场地位移将解决许多问题，至少包括以下问题：(1)沿断层的地震间耦合是否相对均匀，如果不是，非均质性的空间尺度是什么？(2)大地测量耦合系数与该海沟段估计的地震耦合系数0.5-0.75相比如何，两者之间关于可能容纳俯冲界面滑动的无地震过程有什么不同？(3)耦合的程度和/或模式是否随时间演变？(4)锁定带的下倾边缘位于20公里深，正如Suarez和Sanchez(1996)提出的解释沿Cocos-北美俯冲界面一般没有大(8级)地震的原因？(5)是否有证据表明缓慢传播的破裂(慢地震)有助于减轻地震间的负荷？考虑到大地震往往每隔几十年在中美洲海沟的大部分地段重复发生，在地震间期的测量最终也将使我们能够解决许多关于俯冲机制的额外问题，特别是地震间加载和同震滑动释放的比较模式。这项工作的结果将为仍然有限的一组观测增加新的有用的观测，这些观测限制了浅俯冲力学的概念模型，也应该与速率和状态变量摩擦定律对俯冲力学的适用性有关。这项研究由CONACYT、NSF和UW-Madison的Vilas基金会共同资助，代表了UNAM的Enrique Cabral博士、瓜达拉哈拉大学的Bertha Marquez博士和UW-Madison的Charles Demets博士之间的合作伙伴关系。

DeMets 0104299 The Oaxaca GPS study is designed for a detailed geodetic study of the mechanics of subduction along the 350-km-long Oaxaca segment of the Middle America trench, where the trench lies closer to the coast (50-65 km) than for any other continental land mass on Earth. Interseismic loading of the locked subduction interface in this region gives rise to high elastic strain gradients everywhere within 75-100 km of the coast. Surface displacements measured via geodetic techniques are thus sensitive to the distribution of loading and/or slip along the nearby subduction interface. This makes the coastal and near-coastal areas of Oaxaca a superb and possibly unsurpassed region for studying the mechanics of subduction in the upper, seismogenic area of the subduction interface. Forward and inverse modeling of the expected spatial resolution of a 40-site GPS array in Oaxaca indicates it would have a level of resolution 5-12 times better than that of the dense Japanese GPS array. Measurements of site displacements in Oaxaca will allow many questions to be addressed, including at least the following: (1) Is interseismic coupling relatively homogeneous along the fault and if not, what is the spatial scale of the heterogeneity? (2) How does the geodetic coupling coefficient compare to the seismic coupling coefficient of 0.5-0.75 estimated for this trench segment and what does any difference between the two imply about aseismic processes that may accommodate slip along the subduction interface? (3) Does the degree and/or pattern of coupling evolve through time? (4) Is the downdip edge of the locked zone located at a depth of 20 km, as proposed by Suarez and Sanchez (1996) to explain the general absence of large (M 8) earthquakes along the Cocos-North America subduction interface? (5) Is there any evidence for slowly propagating ruptures (slow earthquakes) that help to relieve interseismic loading? Given that large earthquakes tend to recur every few decades along most segments of the Middle America trench, measurements during the interseismic phase will also eventually enable us to attack many additional questions about subduction mechanics, including in particular the comparative patterns of interseismic loading and coseismic slip release. The results of this work will add new and useful observations to the still-modest set of observations that constrain conceptual models for the mechanics of shallow subduction, and should also bear on the applicability of rate- and state-variable friction laws to subduction mechanics. The study, which is co-funded by CONACYT, NSF, and the Vilas foundation of UW-Madison, represents a partnership between Dr. Enrique Cabral at UNAM, Dr. Bertha Marquez at the University of Guadalajara, and Dr. Charles DeMets at UW-Madison.