Continuous coastal subsidence during the Holocene along a source region of the 2011 great Tohoku-oki, Japan, earthquake revealed from new paleo-geodetic data

Abstract:

Time-dependent inconsistency of crustal movement is suggested in the Sanriku coast, northeast Japan. In northern Sanriku, coseismic subsidence up to 0.4 m of the 2011 M=9.0 Tohoku-oki earthquake and a century-long subsidence apparently contradict to long-term uplift estimated from Pleistocene marine terraces. To explain the inconsistency, the following hypotheses have been proposed: 1) the 2011 event was typical, contributing long-term (10⁴ yr) subsidence, 2) a significant interseismic contribution to long-term uplift, or 3) unknown huge events may have occurred to uplift the coastal region. However, in the southern Sanriku, closer to the 2011 source, poorly distributed marine terraces do not allow us to confirm the long-term uplift. We instead focus on the coastal plains that preserve sediments recording the tectonic history with better age constraints.

We extracted 40 m-long samples of the Holocene sediments in the Rikuzen-takata and Kesennuma-Okawa plains, along the southern Sanriku. The two plains share sedimentary facies of river, estuary, and delta. At both sites, relative sea-level (RSL) from 10 to 9.0 ka estimated from altitude of intertidal deposits is significantly lower than the theoretical none-tectonic RSL. We interpret that this discrepancy is attributed to Holocene tectonic subsidence.

Observed Holocene subsidence is consistent with 2011 coseismic and a century-long subsidence. This long-term subsidence possibly suggests that the southern Sanriku has been submerged by both the 2011 type coseismic and interseismic deformation on the 10⁴ yr time-scale. A significant difference is their rates: long-term subsidence rate of ~1 mm/yr is slower than a century-long rate of 1–10 mm/yr. However, it may be accommodated with the recovery from the ongoing postseismic and continuous potential interseismic uplift. We thus argue that there is no reason to include tectonic contribution from unknown, different type megathrust earthquakes to form the southern Sanriku.