Poster #065, Earthquake Geology

Pre and post rupture Fault Mapping Comparisons applications to Probabilistic Fault Displacement Hazard

Rachel N. Adam, Joseph Powell, John Ford, Chelsea P. Scott, & Ramon Arrowsmith
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Poster Presentation

2021 SCEC Annual Meeting, Poster #065, SCEC Contribution #11355 VIEW PDF
Pre- and post- earthquake fault maps inform Probabilistic Fault Displacement Hazard Assessment (PFDHA) to anticipate locations and other characteristics of coseismic ruptures. Our goal is to create pre-rupture fault maps for a series of recent ground rupturing earthquakes: Parkfield,California (2004, M6.0); Kaikoura, New Zealand (2016, M7.8); Napa, California (2014, M6.0); Iwaki/Fukushima-Hamadori, Japan (2011, M6.6); Borah Peak, Idaho (1983, M6.9); El Mayor Cucapah, Mexico (2010, M7.2); Kumamoto, Japan (2016, M7.0). Pre- rupture fault maps are made by utilizing geomorphic indicators of faulting from topography and imagery collected prior to the earthquakes using datasets consisting of airbo...rne and satellite- based optical imagery and structure-from-motion, lidar and radar topography. Post-rupture fault maps are made following the ground rupturing earthquake and only map the surface rupture. The pre and post rupture fault maps are compared and inform Probabilistic Fault Displacement Hazard Assessment (PFDHA) to anticipate locations and other characteristics of coseismic ruptures.

To standardize the pre-rupture mapping, we developed the Geomorphic Indicator Ranking (GIR) system as a quantitative and repeatable approach to place a certainty level (e.g., strong, distinct, weak, concealed) on the mapped fault segments based on the geomorphic indicators along the fault traces. Certain features give a better indication of active faulting than others. For example, an offset drainage is a better indication of faulting than a lineation in vegetation. With this system, we hope to standardize and improve understanding of uncertainties in pre-rupture fault trace mapping.

Further work includes a qualitative and quantitative comparison between the pre and post earthquake rupture. Common differences include the continuity of traces where the pre rupture mapping tends to have longer and more continuous traces and the post-rupture mapping has smaller traces, indicative of the earthquake surface rupture. When using geomorphic landforms as fault indicators, the proximity of the landform to the fault trace is variable. For example, an indicator like a landslide or sag pond can inform the mapper to a nearby fault location, but the proximity to the landform varies between the pre and post rupture maps. Along range fronts, the pre-rupture often shows the fault trace at the base of the front, while the earthquake often ruptured either upslope or downhill of the base.