One consequence of plate tectonics is that in continental plate boundary zones, such as eastern Asia and western United States, tectonic deformation (and the associated earthquake activity) can be distributed over large areas. There are essentially two satellite-based observation methods with which this deformation can be accurately monitored. With the Global Positioning System (GPS) we can precisely measure the Earth's surface motion at the locations of GPS monuments. With the help of continuous GPS (CGPS) measurements, we can now track these motions (horizontal and vertical) on a daily basis. The other methodology is called InSAR, which uses radar images to map detailed changes in the surface of a large swath of land between two (infrequent) satellite passes. A limitation of InSAR is that it is most sensitive to vertical ground motions, which often have anthropogenic instead of tectonic origins.
This project aims to bridge the gap between the ability of GPS and InSAR to capture fundamental deformation processes at high spatial and temporal resolution. InSAR-detected crustal motions are limited by being sampled intermittently and only in the satellite's line-of-sight (LOS). On the other hand, GPS-derived descriptions of the deformation field fail to achieve the same spatial resolution that is obtained with InSAR. The goal of project is to significantly improve the spatial resolution of existing secular strain rate tensor models using horizontal GPS velocities, and create additional models of time-variable deformation using continuous GPS (CGPS) data. This project will provide useful baseline estimates of secular and time-dependent horizontal deformation that could aid the interpretation of InSAR results, while also allowing new advances in studies of the geodynamics and seismic hazard of areas undergoing continental deformation.
The work is focussed on the Pacific-North America (PA-NA) plate boundary zone, the Mediterranean and Middle East, and central and eastern Asia. There, all publicly available CGPS data will be analyzed and combined with estimates of the secular motions from published campaign-style GPS measurements. The horizontal velocities will be converted to continuous strain rate tensor models of secular deformation. Where data coverage is limited, other kinematic indicators (e.g,, fault slip rates/vectors, earthquake focal mechanisms) will be included for additional constraints on the strain style and localization. Improvements on the temporal variation of the strain rate field (most notably due to transients and postseismic deformation) will be limited to the PA-NA plate boundary zone, where CGPS sites are abundant. Reliable time-variable strain rate models (and associated changes in dilatational and shear strain) can likely be created for 1-4 week time windows, sufficient to capture most first order time-variable processes. Both static and dynamic deformation results will be converted to LOS equivalents for 1st order comparison with InSAR results.
Estimates of the secular motions of CGPS sites as well as strain rate results (tensor parameters, maps, and movies) will be made available on a dedicated web portal, as a valuable tool for scientists and educators. Secular strain rates correlate closely with expected seismic hazard and can contribute directly to improving hazard maps. The time-dependent strain rate models may contribute to investigations in stress transfer, earthquake triggering and, ultimately, time-dependent seismic hazard.
Through the project, a graduate student will be trained in the management of GPS data, the characterization of CGPS time-series, and crustal deformation modeling. The results (data and models) obtained from this project will be made available on a dedicated web portal, as a valuable tool for scientists and educators.
This project focused on the improvement of models that quantify the deformation of the Earth's crust mostly based on GPS measurements of horizontal crustal velocities. These models specify the strain rate the crust is undergoing, mostly as the result of the deformation that occurs in between rigidly-moving plates. These strain rates, and their spatial variation, tell us something about the strength of the crust and about seismic hazard. The strain rate reflect the accumulation stress due to relative crustal motion that typically will be released in future earthquakes. We focused on creating a high-resolution global model of strain rates as well as a strain rate model for the western United States. Both models were based on the latest set of GPS velocities created by the PI and supplemented with published values. These models are not unique and are particularly sensitive to bad data points or data points for which the uncertainty is overly optimistic. This can result in spurious results, and the goal is therefore to match all data equally well and to not fit outliers. Because the modeling technique used in this project requires a priori estimates of the uncertainty in the model (which is analogous to the expected strength), the goal of the project comes down to how to most effectively come up with an initial model that ensures that the final model fits the data well without causing spurious results. For the global model (see figure), we created an initial model by setting the 'strength' to the same low value everywhere. This then creates a very smooth model that already captures the first-order variation between high and low straining areas. This model is used as a priori estimate for creating the final model. For the model in the western U.S., we created an independent model based on a new technique that we developed here first. The results of the global model are being used to constrain seismic hazard estimates. In particular, they are now being converted to estimates of earthquake productivity.
