Quantifying near‐field and off‐fault deformation patterns of the 1992 M_w 7.3 <scp>L</scp>anders earthquake

TLDR

Coseismic surface deformation in large earthquakes is typically measured with field mapping and geodetic methods, but these approaches miss near‑field patterns or lack pre‑event data, leaving off‑fault deformation characteristics poorly understood. We develop a standardized method to fully measure the surface, near‑field, coseismic deformation patterns at high resolution using the COSI‑Corr program by correlating pairs of aerial photographs taken before and after the 1992 M w 7.3 Landers earthquake. COSI‑Corr measures displacement across the entire zone of surface deformation and over a wider aperture than field geologists can access. Measured displacements are systematically larger than field measurements, revealing that 46 % of total surface displacement is off‑fault over a mean width of 154 m, with magnitude and width controlled mainly by fault system structural complexity and largest in stepovers, bends, and the southern termination, while slip distribution is self‑affine fractal with dimension 1.56.

Abstract

Abstract Coseismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near‐field coseismic surface deformation or lack preevent data. Consequently, the characteristics of off‐fault deformation and the parameters that control it remain poorly understood. We develop a standardized method to fully measure the surface, near‐field, coseismic deformation patterns at high resolution using the COSI‐Corr program by correlating pairs of aerial photographs taken before and after the 1992 M w 7.3 Landers earthquake. COSI‐Corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists. For the Landers earthquake, our measured displacements are systematically larger than the field measurements, indicating the presence of off‐fault deformation. We show that 46% of the total surface displacement occurred as off‐fault deformation, over a mean deformation width of 154 m. The magnitude and width of off‐fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system, with a weak correlation with the type of near‐surface materials through which the rupture propagated. Both the magnitude and width of distributed deformation are largest in stepovers, bends, and at the southern termination of the surface rupture. We find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self‐affine fractal with dimension of 1.56.