This project is a space-geodetic study of the Ballenas Transform Fault in the Gulf of California, which is part of the North America-Pacific plate boundary. The research team will acquire campaign GPS and interferometric synthetic aperture radar data and use finite element modeling methods to study the accumulation and release of tectonic strain to test for crustal strength variations along the fault. Strength variations are expected because the heat flow varies with distance from two incipient mid-ocean ridge basins in the north and south, and because of significant variations in crustal structure on both sides of the fault (undeformed Baja California versus the Gulf of California extensional province). The fault, situated in a oceanic channel, affords a special opportunity to study strength variation for because: 1) the transform fault has a very high slip rate of approximately 46 mm/yr; 2) the fault accommodates the entire North America-Pacific plate relative motion without any other strike-slip faults in its vicinity and therefore no overlapping strain patterns; 3) the August 2009 magnitude 6.9 earthquake and its regular earthquake repeat time (every 55 years) allows for a clear definition of the state of the fault within the earthquake cycle which is very important for models of the strain accumulation.
This study has important implications for seismic hazard estimates for all major transform faults such as the San Andreas Fault system in California further to the north. Seismic hazard estimates are commonly based on models that ignore strength variations. This study will provide new data on how crustal strength variations affect strain accumulation.
In this project we used satellite data to study strain accumulation along the Ballenas transform fault, Mexico. The Ballenas transform fault is part of the oceanic ridge-transform fault system in the Gulf of California. The fault is located between the Baja California Peninsula and the Isla Angel de la Guarda and connects two mid-ocean ridges to the north and south. With an estimated slip rate of about 45 mm/yr, the Ballenas transform fault is one of the fasted moving fault systems on Earth. Two earthquakes with magnitude larger than 6 in 2009 and 2012 provide an opportunity to study the entire seismic cycle, i.e. deformation before, during and after large earthquakes. We used GPS surveys together with a technique called Interferometric Synthetic Aperture Radar (InSAR) to investigate deformation along this fault. In the latter technique we combined a time series of 30 SAR images to obtain precise estimates about ground deformation over time. Our study uncovered evidence that the Ballenas transform fault is in-fact moving at depth at a rate of 40-50 mm/yr. Prior to the earthquake sequence the fault was locked in the shallow, seismogenic portion of the fault (from the surface to 15 km depth) and freely slipping at depth. Ongoing work aims at entangling in more detail which portions of the fault moved during the different periods (before and during the first earthquake, and before and during the second earthquake), and what causes the differences with time. This project used satellite imagery from the Envisat satellite of the European Space Agency (ESA), which was provided by the Western U.S. InSAR consortium (WInSAR).
