FACTORS CONTROLLING NEAR-FIELD COSEISMIC DEFORMATION PATTERNS: QUANTIFYING DISTRIBUTED DEFORMATION OF THE 1992 LANDERS AND 1999 HECTOR MINE EARTHQUAKES

Coseismic surface deformation is typically measured in the field by geologists and by a range of geophysical methods such as InSAR, LiDAR and GPS. Current methods, however, either fail to capture the near-field coseismic surface deformation pattern where vital information is needed, or lack pre-event data. There is also little understanding of the behavior of off-fault deformation and the parameters that control it. We develop a standardized and reproducible methodology to fully constrain the surface, near-field, coseismic deformation pattern in high resolution using aerial photography. We apply our methodology using the program COSI-corr to cross-correlate pairs of aerial, optical imagery before and after the 1992, Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes. This technique allows measurement of the surface deformation pattern with sub-pixel precision and can be applied to recent and historic earthquakes in a cost-effective manner. COSI-corr offers the advantage of measuring displacement across the entire fault zone and over a far wider aperture than that available to field geologists. For both earthquakes we find our displacement measurements derived from cross-correlation are systematically larger than the field displacement measurements, indicating the presence of off-fault deformation. Here we show the Landers and Hector Mine earthquake accommodated 46% and 38% of displacement away from the main primary rupture as off-fault deformation, over a mean deformation shear width of 183 m and 133 m, respectively. The magnitude and width of off-fault deformation across these structurally immature faults are observed to be primarily controlled by the macroscopic structural complexity with a weak correlation with the near-surface materials and is largest in stepovers, bends and terminations of the surface ruptures. We also utilize synthetic tests robustly constrain the measurement precision and accuracy when subjectively estimating the displacement and width of deformation. Knowledge of the behavior of distributed deformation gives important insight into mechanics and kinematics of fault zones, and has implications for the use of both fault slip rates and paleo-earthquake magnitudes that are derived from surface offset measurements for probabilistic seismic hazard analysis.