SURPRISINGLY DECENT RESULTS AND LIKELY PALEOSEISMOLOGICAL HYPOTHESES (RALPHS) FROM SOME LESS THAN IDEAL SITES IN OREGON AND PUERTO RICO

Site selection is critical when undertaking paleoseismic trenching studies. A Venn Diagram showing overlap of geologically suitable sites with access and various permit compliance can result in a discouraging small slice of Venn pie, made even smaller when studying long recurrence faults where preservation of faulting can be particularly elusive. Faced with a choice between abandoning trenching or trying less than ideal trench sites, Ralph Klinger was a staunch advocate of going forward with excavations, assuming we would more likely than not learn something helpful. This gambling approach yielded some surprisingly decent results.

Paleoseismic trenches excavated across potentially active faults in Puerto Rico and Oregon demonstrated both faults were Holocene-active. In Puerto Rico, a trench was excavated across a single, broad topographic scarp within a wider zone of deformation where access and permissions were immediately obtainable. Exposed stratigraphy and numerical ages enabled our group to provide evidence of active faulting in southern Puerto Rico. Our work has provided the basis for other scientists who continue to study this fault and will refine our initial interpretations.

Paleoseismic trenches excavated in unconventional settings, topographic scarps across bedrock saddles and fairly steep hillslopes, across the Gales Creek fault in northwestern Oregon demonstrated repeated surface rupturing earthquakes in the Holocene/latest Pleistocene. We considered these early results and interpretations tenuous and were hesitant to assert the paleoseismic record was accurate. Subsequent trenches excavated in more traditional and suitable paleoseismic sites surprised us. Multiple trenches excavated across sag ponds and uphill-facing topographic scarps, yielded the same earthquake history. The stratigraphic record in these locations was more clear, stratigraphic packages were thicker, and disruptions and discontinuities were more comfortably interpreted as tectonic.

Regardless of how the results were obtained, detailed mapping of stratigraphic units on hillslopes and ridgetops proved accurate and that knowledge might be helpful when directing future paleoseismic studies. This experience shows that Ralph was correct (aka you were right, RaLPH!). That is a sentence he did not hear from us too often, but he should have!