IDENTIFICATION AND STUDY OF BURIED SCARPS ASSOCIATED WITH CO-SEISMIC COASTAL SUBSIDENCE: EXAMPLES FROM KAMCHATKA, RUSSIA

Stratigraphically distinctive records of great subduction-zone earthquakes are generated following onshore co-seismic subsidence. The best studied records are buried soils such as marsh peats overlain by tidal muds. Here we feature buried scarps -- erosional beach scarps that are subsequently abandoned and buried. For example, Meyers et al. (1996) attributed eight buried scarps on the SW coast of WA State to prehistoric Cascadia-subduction-zone events. Buried scarps are formed after coastal co-seismic subsidence results in sea-level rise and erosional shoreline retreat. Erosion typically produces a steep to undercut upper face that commonly generates soil-block colluvium. When progradation resumes, a (buried) scarp separates the older soil profile landward from a younger soil profile seaward. Scarps may be associated with topographic ridges, because of the overlying scarp-retreat sand, but this is not always the case. Because beach scarps can be associated with other processes, criteria for identifying a co-seismic-associated scarp include coastwise continuity for some distance, coincidence with a tsunami deposit, and correlation with a buried soil.

Along the Kamchatka subduction zone, we have located and studied such buried scarps with ground penetrating radar (GPR) and trenching. On Kamchatka, marker tephra can be abundant enough to assign ages to these scarps and to correlate specific tsunami deposits with them. The exact location of a buried scarp may require continuous or closely spaced excavations; GPR can be used to choose a site to trench. A buried scarp on a radargram is typically steeply dipping and thus does not generate its own reflection but is expressed by the termination of normal, prograding stratification. Often, on the lower, more gently dipping part of this erosional-retreat boundary, there is an accumulation of heavy minerals, which amplifies the reflective signal on the radargram. In this study, from radargrams, we located possible buried scarps, hand dug trenches to expose the scarps, and studied their structure and stratigraphy. In our studies, we have developed methods to quantify co-seismic subsidence, tsunami size and erosional retreat associated with these earthquake/scarp events