|Title||Geodetic constraints on frictional properties and earthquake hazard in the Imperial Valley, Southern California|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Lindsey EO, Fialko Y|
|Journal||Journal of Geophysical Research-Solid Earth|
|Type of Article||Article|
|Keywords||blind fault; creep; deformation; dependent friction; earthquake hazard; elastic heterogeneity; GPS; Imperial Fault; InSAR; interseismic; postseismic; san-andreas fault; slip rates; stick-slip; surface rupture; w 7.2 earthquake|
We analyze a suite of geodetic observations across the Imperial Fault in southern California that span all parts of the earthquake cycle. Coseismic and postseismic surface slips due to the 1979 M 6.6 Imperial Valley earthquake were recorded with trilateration and alignment surveys by Harsh (1982) and Crook et al. (1982), and interseismic deformation is measured using a combination of multiple interferometric synthetic aperture radar (InSAR)-viewing geometries and continuous and survey-mode GPS. In particular, we combine more than 100 survey-mode GPS velocities with InSAR data from Envisat descending tracks 84 and 356 and ascending tracks 77 and 306 (149 total acquisitions), processed using a persistent scatterers method. The result is a dense map of interseismic velocities across the Imperial Fault and surrounding areas that allows us to evaluate the rate of interseismic loading and along-strike variations in surface creep. We compare available geodetic data to models of the earthquake cycle with rate- and state-dependent friction and find that a complete record of the earthquake cycle is required to constrain key fault properties including the rate-dependence parameter (a - b) as a function of depth, the extent of shallow creep, and the recurrence interval of large events. We find that the data are inconsistent with a high (>30mm/yr) slip rate on the Imperial Fault and investigate the possibility that an extension of the San Jacinto-Superstition Hills Fault system through the town of El Centro may accommodate a significant portion of the slip previously attributed to the Imperial Fault. Models including this additional fault are in better agreement with the available observations, suggesting that the long-term slip rate of the Imperial Fault is lower than previously suggested and that there may be a significant unmapped hazard in the western Imperial Valley.
We have used geodetic observations to infer in situ frictional properties on the Imperial Fault. We find that robust constraints on these properties require observations from all parts of the earthquake cycle. Our results are in good agreement with laboratory-derived values of the slip rate-dependence parameter (a − b), for a reasonable range of assumptions. The inferred depth extent of shallow creep is ∼4 km, in close agreement with the seismically inferred depth of sediments, suggesting a compositional or pore fluid pressure control on the occurrence of stable creep.
Our results also suggest that the Imperial Fault is not the only active plate boundary structure at the latitude of the U.S.-Mexico border. Geodetic evidence suggests that significant strain is accommodated by a subparallel fault located 10–20 km west of the Imperial Fault, which slips at a long-term rate comparable to that of the San Jacinto Fault to the north. If so, this fault represents a significant unmapped hazard to the U.S. and Mexican communities of El Centro, Calexico, Heber, and Mexicali.