Challenges of anticipating the 2011 Tohoku earthquake and tsunami using coastal geology

TLDR

Attempts to estimate the size of the AD 869 Jogan earthquake, whose tsunami affected the same coast as the 2011 Tohoku tsunami, provide an opportunity to address the fundamental question of assessing earthquake and tsunami hazards at subduction zones. The study asks whether the magnitude of a giant earthquake can be estimated from paleoseismological data alone. The authors examined stratigraphy at 399 sites along 180 km of coast south of Sendai between 2004 and 2010, finding a 1.5 km inland tsunami deposit from the AD 869 Jogan earthquake and subsidence in one lowland, and they limited their analysis to this rupture length because coastal landforms and lack of volcanic ash make Jogan layers hard to identify along the Sanriku coast. Radiocarbon dating of pre‑Jogan tsunami deposits indicates recurrence intervals of 500–800 years, and modeling of inundation and subsidence estimates the Jogan earthquake at moment magnitude 8.4 or larger with a 200 km rupture, while the resulting tsunami geology could have helped reduce casualties by improving evacuation maps and public‑awareness campaigns.

Abstract

Can the magnitude of a giant earthquake be estimated from paleoseismological data alone? Attempts to estimate the size of the Jogan earthquake of AD 869, whose tsunami affected much of the same coast as the 2011 Tohoku tsunami, offers an excellent opportunity to address this question, which is fundamental to assessing earthquake and tsunami hazards at subduction zones. Between 2004 and 2010, examining stratigraphy at 399 locations beneath paddy fields along 180 km of coast mainly south of Sendai, we learned that a tsunami deposit associated with the AD 869 Jogan earthquake had run inland at least 1.5 km across multiple coastal lowlands, and that one of the lowlands had subsided during the Jogan earthquake and an earlier earthquake as well. Radiocarbon ages just below/above sand deposits left by the pre‐Jogan tsunamis suggested recurrence intervals in the range of 500 to 800 years. Modeling inundation and subsidence, we estimated size of the Jogan earthquake as moment magnitude 8.4 or larger and a fault rupture area 200 km long. We did not consider a longer rupture, like the one in 2011, because coastal landform and absence of a volcanic ash layer make any Jogan layer difficult to identify along the Sanriku coast. Still, Sendai tsunami geology might have reduced casualties by improving evacuation maps and informing public‐awareness campaigns.