This research project is focused on understanding the distribution of fault slip near the Earth's surface during earthquakes through the use of a powerful new technology (COSI-corr sub-pixel image correlation), which facilitates tracking of the location and amount of surface deformation along and around faults in much more detail than was possible using previously available methods. Previous studies suggest that the slip that occurs along faults during earthquakes is larger at depth than near the surface, leading to what has been termed a surface slip deficit. Determining the cause(s) of this surface slip deficit provides the basic rationale for this research. Using the COSI-corr image correlation technique, scientists from the University of Southern California and Caltech will examine the distribution of slip near the surface along more than a dozen large-magnitude earthquakes that occurred during the past few decades. The results will facilitate the mapping of on-fault versus off-fault deformation patterns in earthquakes, allowing correlation of these observations with basic features of the fault zone, such as the amount of total slip a fault has accommodated in its lifetime, which may prove to be good predictors of the percentage of slip that will occur on faults during future earthquakes.
The results of this research have basic implications for the understanding of the seismic hazards posed by major fault systems. Currently, forecasts of the probability of the future occurrence of earthquakes are based largely on the slip rate of the fault, that is, the rate at which the blocks on either side of the fault slide past each other over the course of many earthquakes. Such rates are mainly derived from the analysis of offset surface features, and usually do not include any deformation that might be distributed around the main fault trace. Thus, significant off-fault deformation may yield underestimates of the rate of movements on faults, and thus on seismic hazard. In addition, these data will allow the researcher to test models of possible preferred propagation direction of seismic ruptures, which will be of profound importance for determining in advance the regional distribution of strong ground motions. The anticipated results will also allow assessment of models for the likelihood that seismic ruptures will (or will not) propagate through structural complexities along the fault, an issue of basic importance for estimating in advance the sizes of future earthquakes. The results will thus provide the basis for the development of statistical measures of on-fault versus off-fault slip and rupture behavior that will be used in next-generation hazard assessment strategies to provide more accurate estimates of future seismic hazard.
