This 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.
