Collaborative Research: Using GPS to Unravel the Long-Term Kinematics and Dynamics of the American Southwest from an Ever-Changing Deformation Field

利用GPS从不断变化的形变场中揭示美国西南部的长期运动学和动力学本项目旨在通过开发一个解释大地测量和其他地球物理数据的动态模型，提高我们对美国西南部长期地壳运动学和动力学以及潜在地球结构的理解。美国西南部（这里定义为亚利桑那州，犹他州南部和内华达州南部）似乎大部分构造不活跃。然而，有证据表明，在最近的地质时代，这里发生过大地震。我们探索了这个地区的地壳变形量（因此可以产生（大）地震）随时间变化的可能性，并受到南加州和加利福尼亚湾大地震的调制。通过使用GPS对2010年El Mayor-Cucapah地震（7.2级）前后的变形模式的观测，我们已经看到了这一点的证据。我们将结合正在进行的GPS测量和不同的建模工具来描述和建模时变和长期变形的来源，我们的目标是分离它们。时变变形可以理解为大（远场）地震后岩石圈的粘弹性变形。考虑太平洋板块运动引起的远场应力与研究区地壳厚度和密度横向变化引起的应力的适当比例，可以模拟长期形变。这是我们目前在科罗拉多高原地区的EarthScope项目的后续提案。在过去的五年中，GPS测量的长期运动学被2010年El Mayor-Cucapah地震的震后变形所掩盖，地震发生在我们安装新的34个站点连续网络的三个月前。因此，重力势能（GPE）变化在驱动变形中的作用、亚利桑那州南部活动断层和地震活动缺乏的原因以及板块运动应力的范围等重要问题仍未得到解答。为了解决这些问题，同时进一步加深我们对震后过程的理解，我们提出了一个三层方法：1)扩展现有的GPS时间序列，以改善瞬态变形的表征；2)利用这些数据探索与GPS时间序列数据匹配所需的地壳和地幔粘度；3)创建长期预期变形的物理模型，包括最新的GPE估计，以解释震后效应校正后的残余运动。我们还将利用其他NSF资助项目的GPS数据，以及USArray获得的地震数据产品，这些数据有助于约束地壳和岩石圈的厚度结构和应力方向。大地测量应变率有可能作为区域地震危险性的独立约束条件，但前提是我们对所有震后影响进行了校正，并评估了地壳运动的长期驱动因素。我们将雇用一名联合国大学的女研究生参与这个项目。我们还将在第二年的夏天聘请RESESS本科生实习生。RESESS计划的目标是增加地球科学工作人员的多样性。我们将从UA残疾资源中心（DRC）招募一名学生进行一个与整个项目的大目标相关的小型研究项目。我们还将带领在DRC注册的学生到大峡谷进行实地考察，这是我们在2015年9月开发和实施的一个项目。这些活动的目标是提请注意由美国国家科学基金会和其他国家数据档案中的大量数据提供的众多研究机会，这些数据可供广泛的个人使用。最后，我们计划在该地区的公园和纪念碑进行外展讲座。

Using GPS to Unravel the Long-Term Kinematics and Dynamics of the American Southwest from an Ever-Changing Deformation FieldThis project aims to improve our understanding of the long-term crustal kinematics and dynamics of the American Southwest, and the underlying Earth structure, by developing a dynamic model that explains both geodetic and other geophysical data. The American Southwest (here defined as Arizona, southern Utah, and southern Nevada) appears to be mostly inactive tectonically. However, there is evidence that large earthquakes have occurred here in recent geologic times. We explore the possibility that the amount of crustal deformation in this area (and thus can generate (large) earthquakes) varies with time and is modulated by large earthquakes in southern California and Gulf of California. We have seen evidence for this by using GPS observations of the deformation pattern before and after the 2010 El Mayor-Cucapah earthquake (M7.2). We will combine ongoing GPS measurements with different modeling tools to characterize and model the source of the time-variable and long-term deformation, which we aim to separate. The time-variable deformation can be understood as visco-elastic deformation in the lithosphere following large (far-field) earthquakes. The long-term deformation can likely be modeled by considering the proper ratio of the far-field stresses imposed by Pacific plate motion and stresses due to lateral variation in crustal thickness and density in our study area. This is a follow-on proposal that extends our current EarthScope project on the Colorado Plateau area. Over the last five years, the long-term kinematics as measured by GPS are obscured by postseismic deformation from the 2010 El Mayor-Cucapah earthquake, which occurred 3 months before we installed a new 34-station continuous network. As a result, important questions on the role of Gravitational Potential Energy (GPE) variations in driving deformation, the reasons for the dearth of active faults and seismicity in southern Arizona, and the reach of plate motion stresses remain unanswered. In order to address these questions, and simultaneously further our understanding of the postseismic process, we propose a three-tiered approach: 1) extend existing GPS time-series to improve characterization of transient deformation, 2) use those data to explore the crustal and mantle viscosities required to match the GPS time-series data, and 3) create physical models of the long-term expected deformation, that includes the latest GPE estimates, to explain the residual motions after correction for postseismic effects. We will also leverage GPS data from other NSF funded projects, as well as seismic data products obtained by USArray that have helped to constrain crustal and lithospheric thickness structure and stress orientations. Geodetic strain rates could potentially be used as an independent constraint on the regional seismic hazard, but only after we have corrected it for all post-seismic effects, and assessed the long-term drivers of crustal motion. We will employ a female graduate student at UNR to work on the project. We will also engage a RESESS undergraduate intern during the summer of the second year. The goal of the RESESS program is to increase diversity in the Geosciences work force. We will recruit one student from the UA Disability Resource Center (DRC) to conduct a small research project associated with the larger goals of the overall project. We will also lead an accessible field trip to the Grand Canyon for students registered at the DRC in a program that we developed and implemented in September 2015. The goal of these activities is to draw attention to the numerous research opportunities made available by the large volume of data in NSF and other national data archives, available to a wide range of individuals. Finally, we plan to give outreach talks at the area's parks and monuments.