A 1000 YEAR GEOLOGIC RECORD OF TSUNAMIS FROM THE JAPAN TRENCH: EVIDENCE FROM THE KUJUKURI BEACH RIDGE PLAIN, CHIBA PREFECTURE

Along the northern segment of the Japan Trench, the subduction of the Pacific plate under the Eurasia plate has frequently generated tsunamigenic-earthquakes up to ~M 8.0. In contrast, the middle and southern segments of the Japan Trench were considered relatively inactive until the 2011 Tohoku-oki event generated a M 9.0 multisegment earthquake and one of the largest tsunamis in recorded history. Seismic models indicate that the Tohoku-oki earthquake may have transferred stress southwards along the fault causing an anomalous increase in stress on the potentially locked southern segment of the Japan Trench. This scenario could produce an earthquake in the near future that would be comparable in magnitude to the Tohoku-oki event. Reconstructing the history of individual great earthquakes and accompanying tsunamis over many earthquake cycles provides the full assessment of the seismic hazard.

We have found two anomalous marine sand layers preserved in low-energy freshwater environments where they would not normally occur. The medium to coarse sand layers range in thickness from 3 - 6 cm, are intercalated with muddy peat, and can be traced 3.8 km inland and 5.5 km along the Kujukuri coastline. Both sand layers have sedimentary, microfossil and geochemical features consistent with tsunami deposits found elsewhere in Japan, such as a distinct erosional base, rip-up clasts, normal grading, offshore foraminifera, mud drapes, and a marine geochemical signature. Radiocarbon dating of seeds, charcoal and insect cuticles constrain the age of the upper sand to C.E. 1676 - 1714. Possible candidates for the upper sand are the Genroku tsunami of C.E. 1703 and the Empo Boso tsunami of C.E. 1677. The age of the lower sand is C.E. 1034 - 1110; marking an event for which there is no record.