This project is focusing on ongoing crustal motions in northern Baja Califorrnia, Mexico, associated with the 4 April 2010 Mw7.2 Sierra El Mayor-Cucapah earthquake. The project is utilizing new continuous GPS (CGPS) stations located in northern Baja California to measure transient crustal motions following the earthquake. The new 6-station network complements and enhances the EarthScope PBO network, providing needed data for studies of lithospheric rheology and for monitoring earthquake hazards in northern Baja and southern California. A model for pre-earthquake crustal velocities at the six new CGPS stations is being developed for direct comparison with post-earthquake motions, using existing campaign GPS measurements collected over the past two decades.
New data from the continuous GPS stations and precise estimates for pre-earthquake crustal velocities are forming the basis for an evaluation of perturbations to the velocity field caused by the earthquake, including determination of realistic uncertainties in the earthquake-related velocity change estimates. These geodetic results are enabling a robust study of crust and mantle rheology and the role that postseismic processes play in the evolution of crustal stresses in southern California and northern Mexico, with implications for earthquake hazards assessment. This project is helping to support one PhD student at The University of Arizona, and is strengthening scientific ties with our Mexican collaborators at CICESE, who are our natural partners in earthquake science and hazard research associated with the southern San Andreas fault system.
The main focus of this project was to rapidly capture transient crustal motions following a major earthquake that occurred just south of the US border. Studying the effects of this earthquake is critical, because large earthquakes south of the border may influence large earthquakes elsewhere including the densely populated southern California region. One important outcome of this project was the installation of a new network of instruments (PBO GPS stations) in northern Baja California. These stations will allow the scientific community to study crustal motion associated with the El Mayor-Cucapah earthquake, as well as general studies of northern Baja California tectonics. The new instruments extend and enhance the the United States PBO GPS NSF facility, providing the first PBO stations across the international border into Mexico. The new data complement PBO and other networks in southern California, where large earthquakes are frequent, damaging, and costly. Data from the new northern Baja California instruments enhances our ability to study numerous active geologic processes and their associated hazards, especially hazards associated with the greater San Andreas fault system. To date, the six new instruments have been installed and they valuable producing data. All instruments conform to PBO performance standards (monumentation, power, communications, and GPS hardware standards). The data are flowing through the PBO data stream managed by the UNVCO Facility, and are being analyzed by the PBO Analysis Centers. Four of the six stations have been instrumented with Meteorological instrument packs for studies of atmospheric water vapor (important for monitoring weather, independent of earthquake hazards). Preliminary scientific investigations that have resulted from the new Baja California instruments include an assessment of ground displacements associated with the April 4 2010 M7.2 El Mayor-Cucapah earthquake, as well as displacements associated with the largest aftershock of the El Mayor-Cucapah earthquake, a M5.7 event located in southern California which occurred on June 15 2010. As part of this project, we have also acquired and begun processing data from all existing GPS data from northern Baja California. These existing data constrain pre-earthquake motions, which will be useful for gauging long- term changes in crustal motion associated with the El Mayor-Cucapah mainshock. The major scientific findings from our work so far are that transient signals following the El Mayor-Cucapah earthquake, which are important for understanding the nature of the Earth's outer layers where earthquakes occur, were short lived. This result has implications for how stress is transferred from one region to another in the crust following major earthquakes in the region. Understanding this stress transfer is important because as the stress is transferred, earthquake hazards potential is modified promoting earthquakes in some regions, and reducing risk in other regions. Preliminary analyses also reveal spatial variations in decay rate that may signify vertical stratification of material properties in the crust and upper mantle, or they may signify lateral variations across the Salton Trough region of southern California. Ongoing studies of earthquake induced motions associated with both the 2010 M7.2 El Mayor-Cucapah and 1999 M7.1 Hector Mine earthquakes will help to resolve these variations in material properties of the southern California crust, enhancing our ability to understand crustal motion and earthquake behavior in the future. This project has provided an opportunity for a PhD student to gain experience with models for crustal motion and earthquake mechanics, as well as a general appreciation for seismic, geologic, geodetic data sets relevant to earthquake studies. The project has also provided an opportunity for US scientists to collaborate with scientists at Mexican universities.
