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

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.

Coastal staircase sequences reflecting sea-level oscillations and tectonic uplift during the Quaternary and Neogene

“…The locus of slip release at depth with respect to the coast dictates the sense of coseismic land-level change. Earthquakes that tend to rupture the shallow part of the megathrust, such as the M9 2011 Tohoku event, result in coastal subsidence (Sawai et al, 2012). In turn, events that rupture deeper, such as the M8.8 2010 Maule earthquake, also lead to coastal uplift (Melnick et al, 2012).…”

“…This leads to the notion that earthquakes might cluster in supercycles . Indeed, variable recurrence intervals for earthquakes of different magnitudes have been deduced from paleoseismic and historical data in Chile (Cisternas et al, 2005), Sumatra (Jankaew et al, 2008) and Japan (Sawai et al, 2012), and apparently constitute a hallmark of subduction zones worldwide (e.g. : Satake and Atwater, 2007).…”

The damage is done: Low fault friction recorded in the damage zone of the shallow Japan Trench décollement

“…The margin has historically hosted high rates of seismic activity, notably 18 M w 7 or larger earthquakes in the past 400 years [e.g., Kanamori et al, 2006;Hashimoto et al, 2009], not including the 2011 M w 9.0 Tohoku-oki event and its aftershocks. Records of tsunamis suggest that great earthquakes recur at the Japan Trench in~500 [Sawai et al, 2012] to~1100 year [Minoura et al, 2001] intervals.…”

Population structure and persistence of Pacific herring following the Great Tohoku earthquake

“…As megathrust earthquakes occur at intervals of 500-800 years in northeastern Japan (Sawai et al 2012), Japanese herring in the Tohoku Pacific coast have likely experienced repeated disturbances by tsunamis. We speculate that the higher level of genetic differentiation of the Tohoku populations and their lower genetic diversity (Table 3) may be a result of recurring megathrust earthquakes.…”