Imaging the Mechanics of Shallow Subduction: the Oaxaca GPS Array

瓦哈卡GPS研究是为了对中美洲海沟350公里长的瓦哈卡部分的俯冲机制进行详细的大地测量学研究而设计的，在那里，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在距海岸75-100公里范围内产生高弹性应变梯度。因此，通过大地测量技术测量的地表位移对附近俯冲界面的载荷分布和/或滑动非常敏感。这使得瓦哈卡的沿海和近海岸地区成为研究俯冲界面上部发震区的俯冲力学的极好和可能无与伦比的地区。对瓦哈卡地区40个站点的GPS阵列的空间分辨率进行正演和反演模拟，结果表明其分辨率是日本密集GPS阵列的5-12倍。在瓦哈卡的位移测量将允许解决许多问题，至少包括以下问题：(1)地震间耦合沿断层是否相对均匀，如果不是，非均匀性的空间尺度是多少？(2)大地测量耦合系数与该海沟段估计的0.5-0.75地震耦合系数相比如何，两者之间的差异对可能容纳俯冲界面滑动的地震过程意味着什么？(3)耦合的程度和/或模式是否随时间而演变？(4)是否像Suarez和Sanchez（1996）提出的那样，锁定带的下倾角边缘位于20 km的深度，以解释沿cocos -北美俯冲界面普遍没有大地震（8级）？(5)是否有证据表明缓慢传播的破裂（慢震）有助于减轻震间载荷？考虑到沿中美洲海沟的大部分地区，大地震往往每隔几十年就会发生一次，在震间阶段的测量也将最终使我们能够解决有关俯冲力学的许多其他问题，特别是震间载荷和同震滑动释放的比较模式。这项工作的结果将为限制浅层俯冲力学概念模型的仍然有限的一组观察结果增加新的和有用的观察结果，并且还将对速率和状态变量摩擦定律在俯冲力学中的适用性产生影响。这项研究由CONACYT、NSF和威斯康星大学麦迪逊分校的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博士和威斯康星大学麦迪逊分校的Charles DeMets博士之间的合作关系。

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.