FACTORS CONTROLLING NEAR-FIELD COSEISMIC DEFORMATION PATTERNS: A DIRECT COMPARISON OF DISTRIBUTED DEFORMATION BETWEEN THE 1992 LANDERS AND 1999 HECTOR MINE EARTHQUAKES USING CROSS-CORRELATION OF HIGH-RESOLUTION AIR PHOTOS

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 lack spatial coverage or the necessary pre-event data to completely constrain the complex near-field, coseismic surface deformation pattern where vital information is needed for accurate understanding of fault-zone mechanics and fault-slip kinematics. Thus the behavior of coseismic off-fault deformation and the factors that control it remain poorly understood. In this study we use the program COSI-Corr to cross-correlate pairs of pre- and post-event high-resolution (1 m) air photos to constrain the complete near-field, coseismic surface deformation pattern of the 1992 M_w 7.3 Landers and 1999 M_w 7.1 Hector Mine earthquakes. This technique also facilitates a direct comparison of the properties of near-field deformation between the events, because for both earthquakes, we use similar 1 m, air photo data and apply the same correlation technique. Furthermore, our technique 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 (n = 1541) are systematically larger than the field displacement measurements, indicating the presence of significant co-seismic off-fault deformation. Here we show the Landers and Hector Mine earthquakes 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 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 suggest that the overall lower magnitude and narrower width of distributed deformation for the Hector Mine surface rupture relative to the Landers event may be due to larger cumulative displacements and resulting slightly higher structural maturity of the Hector Mine fault system, reinforcing the importance of fault system evolution on constitutive properties of fault zone deformation and fault slip.