NUMERICAL MODELING OF CO-SEISMIC COASTAL SUBSIDENCE AND TSUNAMIS ASSOCIATED WITH UNUSUALLY WIDE, PREHISTORIC EARTHQUAKES, KAMCHATKA SUBDUCTION ZONE

The prograding strand plain of Avachinsky Bay, Kamchatka, Russia, exhibits geological evidence for more than 30 tsunamis in the last 4 ka, yet only 3 instances of detectable coseismic subsidence (in the form of buried erosional scarps) with the most recent from ~A.D. 800. All 3 events retain evidence of ~0.4-1.2 m of coseismic subsidence, coastal erosion and shoreline retreat, and tsunamis amongst the largest of >30 events in the strand plain in the last 4 ka. This same coast has a historical record (since A.D. 1737) of five subduction-zone earthquakes with large tsunami runup (>5 m), the last of which was the M_w 9 1952 Kamchatka earthquake, which did not generate measurable co-seismic subsidence at this locale. This rarity of buried scarps relative to large earthquakes contrasts with the Cascadia strand plain in SW Washington State, where most or all large events have been shown to be represented by buried scarps. The Avachinsky Bay coastline is 180-200 km from the trench, with ~25-degree slab dip, implying needing unusually wide ruptures to generate significant onshore coseismic subsidence.

Using numerical modeling methods, we assessed possible earthquake and tsunami characteristics that distinguish these three events from the other ~30 tsunamis in the last 4.0 ka. We evaluated surface deformation calculations using the Okada (1985) equations of slip on a subduction-zone interface defined by the Slab2.0 dataset, divided into a suite of 176 subfaults with a possible locked region from the trench to a possible down-dip width of 176 km. Synthetic earthquakes with magnitudes between M_w 8.8–9.2 were generated using the spatial random field technique of Mai & Beroza (2002). The suite of M_w 8.8-9.2 earthquakes resulted in subsidence/uplift at Avachinsky field sites from - 2.5 m to + 2.2 m. Results show that earthquakes with slip concentrated only in areas overlapping with modern-day strongly coupled zones of the subduction interface, which near Avachinsky are primarily near the trench (Bürgmann et al., 2005), do not produce significant subsidence in Avachinsky Bay. Higher values of subsidence require slip offshore of Avachinsky Bay to extend to ~ 100-125 km downdip of the trench. Tsunami modeling comparing the earthquakes in this study is ongoing.