Geomorphic Effects of Tsunamis in Asacha and Mutnaya Bays, Southern Kamchatka Peninsula, Russia

As graphically demonstrated in Sumatra last year, tsunamis erode and redeposit sediment, thereby altering the geomorphology of a coastline. Some alterations are transitory, but many effects are apparent decades to centuries later. Recognition of more permanent alterations is a useful tool for evaluating the long-term tsunami record of a coastline. Effects of the November 1952 Kamchatka tsunami generated by a Mw 9.0 subduction-zone earthquake are still apparent in Asacha and Mutnaya bays on the southeastern coast of the Kamchatka Peninsula, Russia. In these bays we used field mapping and stratigraphy (August 2005) as well as analysis of aerial photographs and satellite images of the beach-ridge plains to interpret pre- and post-tsunami geomorphology and geomorphological changes. Differences in the morphology of the tsunami's impact between the two bays are due, in part, to the level of soil development at each coastline. Along the shoreline of Asacha Bay, the tsunami eroded scour pockets tens of meters in diameter and several meters deep. After 50 years, there is still relief greater than one meter along the edges of some of these scours. In Mutnaya Bay, the tsunami appears to have widened the Mutnaya River channel on its north side. The tsunami also produced scour pockets, similar to those in Asacha Bay, along the southern riverbank during outflow from a wide trough between old beach ridges. Most noticeably, along the shoreline of Mutnaya Bay, the tsunami broke through the first prominent beach ridge, created channels and disrupted the ridge's continuity in many places. Older beach ridges in this bay show similar features, which may prove useful indicators of paleotsunamis.