Geomorphic Evidence of Coseismic Coastline Changes in Southern Miura Peninsula Associated with the Recent Kanto Earthquakes: Analysis of the LIDAR Data, air Photos and Topo Maps

In order to study geomorphic evidence related to the past Kanto earthquakes, we analyzed LIDAR data, air photos and topographical maps, and traced uplifted marine terraces during the recent earthquakes including the 1923 and 1703 earthquakes. Tokyo Metropolitan Area's well-documented earthquake history is dominated by the 1703 and 1923 great Kanto earthquakes, that were resulted from the subducting Philippine Sea plate. Around the source region of the past Kanto earthquakes, Miura and Boso Peninsulas are located facing the Sagami Bay. The average recurrence interval of Kanto earthquake has been estimated on basis of the seismological, geodetic, geological and gemorophological data. The Earthquake Research Committee [2004] proposed that there are types of earthquakes with the recurrence intervals of 200-400 years, and about 2300 years. They produced different amounts of uplift at Boso Peninsula, but the uplifts of Miura Peninsula are similar. The uplift amounts of Miura Peninsula have been estimated about 1.5 m in 1923 and 1703, from the wave-cut-benches, -notches and the distribution of fossil remains along the coast [Matsuda et al. (1978), Shishikura et al. (2007)]. The coastline just before the 1923 earthquakes can be restored from the old topographical map. By using it, the coseismic uplifts associated with the 1923 and 1703 earthquakes may be more accurately estimated. The air photos we used are by 1946 U.S. forces photography and 1963/1966 Geographical Survey Institute photography; the topographical maps are 1:25,000 topographical maps measured in 1921 and 1:20,000 topographical maps of the Meiji period. In addition, we made a high-density (50 cm mesh) digital elevations map by aerial measurements of the Light Detection and Ranging (LIDAR). In Miura Peninsula, three additional steps of marine terrace surface are formed at 7 to 20 m above MSL, at ~5200,~3300 and ~1500 cal. BC, and these are called Nobi 1, 2 and 3 in order from top [Kumaki, 1985; 14C Age was calibrated]. These terraces are inferred to have been formed with coseismic uplift. However, geomorphic evidence of more recent earthquakes with the 200-400 year interval is scarce in Miura Peninsula. Three to five steps of marine terrace surface are identified, between the Nobi 3 surface (7 - 10 m above MSL) and the present coastline, in lower land of the alluvial valley of Miura Peninsula by an analysis of LIDAR data. Those are edged in a small cliff of the height of 1-2 m. In addition, LIDAR data indicate flights of wave-cut-bench on rocky coast (8 m above MSL) in Jogashima, southernmost tip of Miura. These marine terrace surfaces may indicate additional evidence of the uplift associated with the Kanto earthquakes.