1997 KRONOTSKY EARTHQUAKE AND TSUNAMI AND THEIR PREDECESSORS, KAMCHATKA, RUSSIA--LOCATIONS AND SEGMENTATION(S)

The northern segment of the Kamchatka subduction zone (KSZ) experienced three tsunamigenic earthquakes in the 20^th century (Feb 1923, April 1923, Dec 1997), events that help elucidate the behavior of this segment. A particular focus of this study is the nature and location of the 5 December 1997 Kronotsky (Peninsula) rupture via tsunami deposits and runup. Some prior studies have characterized the subduction zone off Kronotsky--where the Emperor Seamount Chain meets Kamchatka--as less seismogenic (as indicated by gravity-anomaly analyses), and have placed the 1997 rupture south of the promontory. However, 1997 tsunami-deposit elevations north of the peninsula require the rupture to extend farther north. For the two 1923 tsunamis, we cannot distinguish their deposits in southern Kamchatsky Bay, but they are in sum more extensive than the 1997 deposit. A reevaluation of the April 1923 earthquake (and its tsunami) suggests that its moment magnitude should be revised upward to Mw ~8. This revision makes the two 1923 events more like a pair, with the 1997 earthquake filling a gap between them. Deeper in time, the 1700-year prehistoric record of tsunamis in southern Kamchatsky Bay indicates that during this interval, no local events were significantly larger than the 20^th century earthquakes. Together, the historic and prehistoric record suggests a more northerly location of the 1997 rupture, a revision of the size of the April 1923 earthquake, and agreement with previous work suggesting the northern KSZ ruptures in smaller sections than the southern KSZ. The latter conclusion requires caution, however, as we continue to learn that our historic and even pre-historic records of earthquakes and tsunamis is limited, in particular as applied to hazard analysis. This study illustrates the challenges of specifically assigning deposits to multiple events closely spaced in time and geography, as well as of accurately locating earthquake rupture area and rupture distribution. Our work also contributes to continued efforts to understand tectonic behavior around the northern Pacific and in subduction zones, in general.