Interseismic And Postseismic Deformation In The Mojave Desert, Southern California, USA

The June 28,1992 Mw 7.3 Landers and October 17,1999 Mw 7.1 Hector Mine earthquakes are two strike-slip events, which occurred in the Mojave section of Eastern California Shear Zone (Mojave ECSZ), Southern California, United States. Studies of the two earthquakes have become classic cases and advanced research in seismology and physics of the Earth interior significantly.The space geodesy techniques such as the Global Positioning System (GPS) have been used successfully to measure coseismic and postseismic deformation of the two Mojave earthquakes. Many previous studies achieved important findings by using the geodetic observations to constrain regional background deformation, coseismic deformation, and postseismic deformation associated with these two Mojave earthquakes. However, over the past two decades, it has been an open question whether several published GPS velocity solutions accurately depicted the regional background deformation field. The reason is that most of the GPS observations in the Mojave region were made after the Landers earthquake, and it is difficult to effectively separate the correlated interseismic and postseismic signals of the two events. The interseismic velocities derived from the post-Landers GPS observations may be inaccurate, and so is the representation of the postseismic deformation field based on its separation from the interseismic velocity contribution. Some recent studies recognized the existence and importance of this issue, but have not come up with adequate solutions. Therefore, it is necessary to examine whether the GPS velocity solutions in the Mojave region are valid representations of the secular deformation field, and if not, a more reasonable regional GPS velocity solution is needed. Only based on a more accurate background velocity field, can we obtain more reasonable data constraints for modeling of postseismic deformation.This study makes a thorough collection of pre-Landers historical triangulation and trilateration observations spanning the Mojave ECSZ, and compares the pre-Landers deformation field depicted by the data with the deformation field represented by the GPS velocities of the Southern California Earthquake Center (SCEC) Crustal Motion Map Version 4 (CMM4) solution. It is found that the CMM4 GPS velocities are indeed biased in the near field of the Landers and Hector Mine earthquakes, probably due to un-modeled postseismic transients.This study devises a series of fault-block models based on cluster analysis of the CMM4 GPS velocities and the regional tectonics in the Mojave region. The fault-blocks models are constrained by strain rates from triangulation networks, rates of line-length change from trilateration networks, and far-field CMM4 GPS velocities. The surface deformation field forward predicted by the preferred fault-block model is used as a surrogate of the background/secular deformation field. Modeling results show that the CMM4 GPS velocities in the near field of the two Mojave earthquakes contain systematic bias in the form of 2-3 mm/yr right-lateral shear across the fault rupture. Considering the focal mechanisms of the Landers and Hector Mine earthquakes and the GPS velocity derivation algorithm, the bias in the form of excess right-lateral shear arises from perturbations to the GPS velocity solution from long-term postseismic transients. Such bias also exists in GPS velocity solutions published by other research groups. In comparison with previous GPS velocity solutions, the secular GPS velocities derived in this study are to date the most accurate in the Mojave region, especially in the near field of the Landers and Hector Mine earthquakes. The cumulative deformation rate across the Mojave ECSZ is 13.2-14.4 mm/yr, at least twice the geologic rate averaged since the late Pleistocene (≤6.2±1.9mm/yr).One can use postseismic geodetic observations to study regionally lithospheric rheological structure and mechanical properties of fault zones, which help us to understand regional tectonic evolution. Several previous studies suggested that the rates of near-field GPS stations may have returned to the pre-seismic rates 5-6 years after the Mojave earthquakes. Postseismic GPS time series based on the secular velocity field result of this thesis, however, show significant postseismic motion in the near-field GPS stations 10 years after the Mojave earthquakes. Moreover, the intermediate-and far-field postseismic deformation field derived in this study is not as evident (active) as those used in previous studies. Preliminary postseismic modeling shows that neither afterslip on the fault plane nor distributed viscoelastic relaxation alone is capable of explaining the observations and that both mechanisms are involved in the physical processes. It also shows that the viscosities in the lower crust and upper mantle are ～3×1019Pa s and ～4×1019Pa s, respectively, which are inconsistent with the Creme Brulee model of the regional rheological structure inferred by previous studies.