Bridging seismology and geomorphology: investigations into the 2006 and 2007 Kuril Islands earthquakes and tsunamis
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Numerous geophysical and geological observations of the 15 November 2006 and 13 January 2007 Kuril Island earthquake doublet and associated tsunamis help link earthquakes’ seismological characteristics with tsunamis’ coastal effects. Expression of the tsunamis in the central Kurils remained unknown until post-tsunami surveys in summers of 2007 and 2008. Surveyed runup in 192 locations over a distance of 600 km averaged ~10 m, maximum ~20 m. Higher runup generally occurred along steep, protruding headlands, and longer inundation distances on lower, flatter coastal plains. As often observed but rarely measured in other cases, the Kuril tsunamis were dominantly erosional, while also leaving deposits. Pre- and post-tsunami surveys, including reoccupied topographic profiles, provide confidence to attribute changes to tsunami processes, in some cases to quantify these changes. Areas with low runup (<8 m) experienced limited geomorphic change, near the shore; regions with high runup (>15 m) experienced massive erosion. Where sandy beaches existed, sheetlike tsunami deposits reached ~90% of tsunami runup and inundation. The volume of eroded sediment far outweighed the amount deposited on land in all cases studied. The tsunamis eroded the beach landward, stripped vegetation, created scours and trim lines, cut through ridges, and plucked rocks from the soil. The effects were dominantly erosive because high-relief topography accelerated tsunami outflow. Post-tsunami surveys primarily found and measured only one tsunami wrackline, indicative of the largest onshore wave, with few clues as to which tsunami formed it. Simulations of tsunamis based on published slip distributions of both earthquakes using the numerical MOST model (Method of Splitting Tsunamis) help untangle the events. These simulations suggest that the larger tsunami in most places was 2006, but that 2007 was larger on Matua and parts of Rasshua islands. The effect of slip distribution on nearfield tsunami runup was investigated using a diversity of slip-distribution inversions. The slip distribution in outer-rise earthquakes like 2007 causes less variation in runup patterns than is the case in subduction-zone earthquakes like 2006. Differences in length and width of inversions’ subfaults also affect runup patterns when these differences affect up-dip or down-dip distribution of slip.