Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Factors Controlling Shallow Co-Seismic Deformation: Quantifying Distributed Co-Seismic Deformation Of The 1992 Landers Earthquake

Abstract

We use COSI-Corr, a sub-pixel optical image correlation program, to co-register, ortho-rectify, and correlate pairs of high-resolution NAPP aerial photos taken before and after the 1992 Landers (Mw 7.3) earthquake. These analyses reveal patterns of along- and across-strike near-field co-seismic deformation during this event. Specifically, using this technique we are able to measure the detailed horizontal displacement field produced by this earthquake. COSI-Corr offers the advantage of measuring displacement over a far wider aperture than that available to field geologists, who are often restricted to a relatively small number of discrete offset markers. Thus, where field measurements underestimate those determined by optical image correlation, we assume this difference to result from distributed deformation that was not mapped during field surveys. We extract stacked displacement profiles from the COSI-Corr-derived displacement maps, taken every 138 m along the entire length of the Landers rupture. The magnitude of off-fault displacement is calculated by taking the difference between 1051 COSI-corr measurements and 863 published co-seismic displacement data measured by field geologists. Furthermore, using displacements measured by COSI-Corr we can quantify the width of the deformation zone across the fault by measuring the distance between the minimum and maximum displacements on either side of the fault. We find the fault zone width (FZW) to vary from a few 10s of meters to as much 250 m. Additionally, we investigate the control of near-surface materials on the degree of distributed deformation by comparing the calculated value of off-fault deformation to published geological maps of Landers rupture. Furthermore, we apply a detailed fan analysis to investigate whether different ages of alluvial material have a control on the magnitude of off-fault deformation. Knowledge of the location, width and magnitude of distributed deformation for the Landers rupture has implications for insight into mechanics of fault zones and the potential for underestimation of fault slip rates derived from surface measurements, and thus seismic hazard. Understanding the width, variation and controls on fault zones has implications for reservoir potential along fault systems.