6:15 PM - 7:30 PM
[SCG57-P30] Holocene vertical movement history in northern Sanriku coast, NE Japan, related to megaquake cycle
Keywords:Holocene, Emerged coastal topography, Vertical movement history, After-slip movement, northern Sanriku coast, megaquake
Tephrochronologically age-determined MIS 5e marine terrace height of 30 m shows average uplift rate of 0.2 mm/yr in geological long term. Holocene emerged coastal topographies and dates present three times of rapid uplift events; about 1,000 years ago just before B-Tm ash fall, before 3,300 years ago and before 4,800 years ago. Records of Hachinohe tide-gauge station in the past 60 years indicate the mean subsidence rate of 2 mm/yr. We built the vertical movement diagram synthetically explaining the above results.
Emergences of coastal topographies suggest three sudden uplift events which are named E1, E2 and E3 in chronological order. Assuming that the geodetic subsidence (2 mm/yr) continued during 1,000 years between E3 and the present, E3 was associated with 6-7m uplift. Setting that the gradient of straight line connecting heights of emerged topographies attained just after uplift events satisfies the long-term uplift rate of 0.2 mm/yr, E2 was accompanied by 5-6 m uplift 3,800 years ago, and E1 by 4-5 m 6,200 years ago. These abrupt uplift phenomena is likely generated by an near-shore faulting (Miyauchi, 2012), which is not sufficiently testified by geophysical exploration yet. Correlating these uplift events with the ongoing after-slip uplift event after 2011 EQ, plate-boundary megaquakes associated with distinct coseismic subsidence necessarily occurred off the northern Sanriku coast, namely from the northern Japan Trench to the southern Kuril Trench, just before those uplift events. Such megaquakes are estimated to occur at least three times in Holocene.