Very precise measurements of ground motion are being used to study the distribution and magnitude of slow creep along faults of the San Andreas Fault (SAF) system. Measuring and modelling fault creep is improving our understanding of both earthquake hazard and earthquake physics. Much of the creep signal is close to the fault and therefore, best observed by a space-based technique called Interferometric Synthetic Aperture Radar (InSAR) because of its complete 50-m spatial resolution. The new aspect of this research is the use of L-band InSAR data from the recently launched ALOS spacecraft (JAXA). L-band interferogram remain coherent over longer time intervals than the previously available C-band interferograms. These new data are especially important in central and northern California where C-band permanent scattering methods are only partially successful in measuring slow crustal deformation. The three-year investigation includes the following tasks: (1) Participate in the Western North America InSAR consortium (WInSAR) to assemble the data needed for this investigation as well as to prepare for future seismic events in California. (2) Analyze InSAR data from ERS-1/2, ENVISAT, and ALOS to recover slow inter-seismic creep signals along the SAF system. The investigators are refining methods for stacking ERS and ENVISAT interferograms, as well developing new software for the analysis of L-band ALOS data. (3) Use the measurements of fault creep to improve our understanding of physical models for fault friction at shallow depth. In particular the analysis is revealing the relationship between creep rate, shallow stress accumulation rate, and major earthquake recurrence intervals. These proposed research activities are contributing to the objectives of NSF?s EarthScope Initiative by providing insight into fault mechanics and earthquake hazard. Funding will primarily be used to support graduate student research and training. This research has significant international collaboration with scientists at the Japanese and European Space Agencies. The researchers are also collaborating with the Scripps Institution of Oceanography (SIO) Visualization Center in cataloging and distributing images, animations, and 3-D visualizations. Educational activities include development of graduate and undergraduate courses at UCSD and presentations to local school groups on earthquake topics.
Is California prepared for the next big earthquake? (Fig 1.) Estimates of earthquake potential along major faults, such as the San Andreas Fault System, are used for developing scenario earthquakes, for setting regional building codes, and for setting earthquake insurance rates. While the timing of a major earthquake cannot be accurately predicted, the moment magnitude can be accurately estimated from geodetic measurements of present-day crustal deformation. The current array of 700 continuously operating GPS stations in western North America does not completely resolve the crustal deformation gradients (strain) along the major faults because the average station spacing is too large. During our 3-year investigation, we developed methods for processing synthetic aperture radar data collected by a Japanese Spacecraft (ALOS) to recover very fine spatial scale (200 m) crustal deformation between the sparse GPS stations. This involved the development of new software called GMTSAR that is now freely available at http://topex.ucsd.edu/gmtsar. We investigated the ability to form radar interferograms over vegetated areas not analyzed previously using other shorter wavelength satellite radars. Our analysis provided the first fine resolution crustal deformation map of the entire San Andreas Fault system that is now being used by the USGS to refine the earthquake hazard maps in California (Fig. 2). Funding from this award was used to train two graduate students in the highly technical field of satellite geodesy. Precise geodesy forms the basic spatial infrastructure for our modern GPS-based society and military operations so it is important to train students in this area. While most of our investigation was focused on the San Andreas Fault system, we also used the newly developed L-band radar interferometry methods to investigate three major earthquakes in remote areas of the Earth. 1) The 2008 M 7.9 Wenchuan China Earthquake devastated the Sichuan province killing 69,000 people and left 4.8 million people homeless. This thrust-type earthquake occurred in an area of moderate tectonic activity similar to the Ventura basin of California so it serves as an analog for a future California rupture. We used the new ScanSAR capabilities of the ALOS satellite radar to develop a surface displacement map for a 400 km by 400 km area containing the 240 km long surface rupture. Our model suggests that most of the moment release was limited to the shallow part of the crust (depth less than 10 km) and this shallow slip was responsible for the extensive destruction from the earthquake. 2) The 2010 M 8.8 Maule Chile earthquake caused extensive damage to the coastal regions of Chile killing 550 people. Again we used the new ScanSAR capabilities of the ALOS satellite to provide a complete 2-dimensional map for the surface deformation from the rupture and used this to estimate the slip distribution over the 600 km by 200 km rupture plane. 3) Closer to home, the 2010 M 7.2 El Major-Cucupah, Baja California earthquake was responsible for only 2 deaths but caused extensive damage to the agricultural areas of the Colorado River Delta where 200 square kilometers of irrigation systems had to be shut down, and ~35,000 people were displaced from their damaged homes. A similar type of damage will occur when the next large earthquake strikes in the Imperial valley region of the US so studying this event will help us prepare for the next big one.
