GPS Measurements of Plate Interactions and Intra-Plate Deformation in the Western Mediterranean

在过去三年中，麻省理工学院、康奈尔大学及其摩洛哥合作伙伴开发并观测了一个网络，该网络由摩洛哥的3个连续（CGPS）和22个测量模式（SGPS）GPS站点组成，以更精确地确定非洲板块运动、西地中海非洲板块和欧亚板块的相互作用以及与非洲板块西北部（摩洛哥）现今构造有关的变形。这项工作得到了国际空间研究所在西班牙和葡萄牙的合作伙伴所进行的补充性全球定位系统研究的推动，这些合作伙伴正在AMIGO项目（大西洋和地中海跨学科全球定位系统观测）的框架内与这些合作伙伴开展合作。他们在摩洛哥的工作提供了关于板块运动和相互作用的新的运动学信息，有望限制大陆碰撞过程以及板块内变形的动力学模型。在该提案中，他们要求支持继续监测摩洛哥GPS网络3年，并增加台站覆盖范围，以确定最活跃变形区域的更好应变模式。从2年期间（10/99 - 10/01）的三次调查中得出的初步GPS站点速度已被解析为~2 mm/yr（1 sigma，或5 mm/yr 95%）。这些不确定性相当于整个非洲-欧亚大陆的会聚速率（在摩洛哥约为4 - 5 mm/年），尚不足以对非洲板块运动或板块内部变形提供可靠的约束。然而，目前的速度场确实表明，位于里夫山脉以南的地点相对于欧亚大陆的整体WNW运动约为6 +或- 2 mm/年（1 sigma），与我们根据整个非洲板块（~ 4.7 mm/yr WNW）的可用GPS数据得出的非洲板块运动的新GPS估计值在不确定性范围内一致。相比之下，位于沿着西部里夫山脉在西北摩洛哥的网站表示明显缓慢的向北运动，并可能向南运动，这表明在阿尔沃兰海的扩展。这两个观测结果与非洲板块的持续西北运动、阿尔沃兰海（地中海最西部摩洛哥和西班牙之间）的活跃延伸以及里夫西部和东部之间的左旋走滑变形相一致。这种整体的一阶模式与地震和新构造场研究估计的变形一致。PI的请求继续支持3年，以：1）通过2002年、2003年、2004年和2005年的调查，在现有的SGPS站点上解决更好的速度（所有实地费用由摩洛哥一方承担），并将摩洛哥的速度场与来自伊比利亚的全球定位系统观测结果结合起来，2）在里夫山脉建立10个新的SGPS站点，以更好地约束明显的晚内变形（缩短和左行，走滑），3）开发和应用弹性块体和有限元模型，用于地中海西部的活动变形，以及，4）进一步发展摩洛哥国内在全球定位系统地球动力学应用方面的能力。这最后一项任务将使摩洛哥合作伙伴（保留我们联合项目提供的设备）能够在请求的赠款期之后，在摩洛哥的支持下，自行进行所有未来的GPS观测。我们预计，如果需要，未来对NSF支持这项工作的请求将仅限于数据集成、建模和解释。

For the past three years, MIT, Cornell University, and their Moroccan partners have developed and observed a network comprising 3 continuous (CGPS) and 22 survey-mode (SGPS) GPS sites in Morocco to determine more precisely Africa plate motion, the interaction of the African and Eurasian plates in the western Mediterranean, and deformation associated with present-day tectonics of the NW part of the African plate (Morocco). This work is being facilitated by complementary GPS studies being pursuedby the PI's partners in Spain and Portugal with whom they are collaborating within the framework of the AMIGO Project (Atlantic and Mediterranean Interdisciplinary GPS Observations). Their work in Morocco is providing new kinematic information on plate motions and interactions that promise to constrain dynamic models for continental collision processes as well as intra-plate deformation. In this proposal, they request support to continue to monitor the Morocco GPS network for an additional 3 years, and to densify station coverage to determine better strain patterns in the most actively deforming regions.Preliminary GPS site velocities derived from three surveys over a 2 year period (10/99 - 10/01) have been resolved to ~2 mm/yr (1 sigma, or 5 mm/yr 95%). These uncertainties are on the order of the entire Africa-Eurasia convergence rate (~4 - 5 mm/yr in Morocco), and are not yet sufficient to provide reliable constraints on Africa plate motion or intra- plate deformation. However, the present velocity field does indicate overall WNW motion relative to Eurasia at about 6 +or- 2 mm/yr (1 sigma) for sites located south of the Rif Mountains, consistent within uncertainties with our new GPS estimates of Africa plate motion derived from available GPS data for the entire African plate (~ 4.7 mm/yr WNW). In contrast, sites located along the western Rif Mountains in NW Morocco indicate significantly slower northward motion, and possibly southward motion, suggesting present-day extension in the Alboran Sea. These two observations are consistent with the continuing NW motion of the African plate, active extension of the Alboran Sea (westernmost part of the Mediterranean between Morocco and Spain), and left-lateral, strike slip deformation between the western and eastern Rif. This overall, first-order pattern is consistent with deformation as estimated from earthquakes and neotectonic field studies. The PI's request continued support for 3 years to: 1) resolve better velocities on existing SGPS sites with surveys in 2002, 2003, 2004, and 2005 (all field costs borne by the Morocco side), and integrate the Morocco velocity field with GPS observations from Iberia, 2) establish 10 new SGPS sites in the Rif Mountains to constrain better apparent intra- late deformation (shortening and left lateral, strike-slip), 3) develop and apply elastic block, and finite element models for active deformation in the western Mediterranean, and, 4) further develop Moroccan in-country capabilities in GPS-geodynamic applications. This last task will allow the Moroccan partners (retaining the equipment provided by our joint project) to carry out all future GPS observations on their own and with Moroccan support beyond the requested grant period. We expect that future requests for support from NSF for this work, if required, will be restricted to data integration, modeling, and interpretation.