A PRIMER ON PALEOLIQUEFACTION INVERSE-ANALYSIS AND ITS RESEARCH POTENTIAL IN CASCADIA

The Cascadia Subduction Zone, New Madrid Seismic Zone, South Carolina Coastal Plain, and other regions of U.S. seismicity remain enigmas within the geoscience and geoengineering professions. In these regions of infrequent moderate-to-large earthquakes, historic records are insufficient to provide precise inputs to seismic hazard analyses, significantly impacting engineering practice and public safety. The uncertainties of these inputs (i.e., the magnitudes, locations, and recurrence-rates of fault ruptures, and the characteristics of resultant ground-motions) are more than an academic curiosity – these uncertainties influence computed seismic hazards, which are adopted into U.S. building code and used by engineers and policy-makers. In addition to being affected by this uncertainty, many U.S. seismic zones have another commonality: pre-historic and/or pre-instrumental earthquakes induced soil liquefaction (referred to as “paleoliquefaction” in such cases) that was later used to interpret the seismic parameters of the causative earthquakes. In fact, relic liquefaction is in some cases the only evidence of paleoseismicity. However, paleoliquefaction research has largely focused on field investigations, with less effort focused on advancing the techniques for inverse-analyzing field evidence. In this regard, increased multidisciplinary collaboration is needed to fully exploit the results of field studies. Towards this end, the objectives of this presentation are threefold, as follows: (1) to summarize the process of, and techniques inherent to, studying regional paleoliquefaction evidence, with emphasis on state-of-the-art inverse-analysis methods; (2) to demonstrate the application of these methods for probabilistically back-calculating the locations and magnitude distributions of fault-ruptures; and (3) to outline a path by which future paleoliquefaction research can help to decode the Cascadia enigma.