GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L21309, 6 PP., 2012
doi:10.1029/2012GL053692
doi:10.1029/2012GL053692
Challenges of anticipating the 2011 Tohoku earthquake and tsunami using coastal geology
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology,
Tsukuba, Japan
Tsukuba, Japan
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology,
Tsukuba, Japan
Tsukuba, Japan
Earthquake Research Institute, University of Tokyo,
Tokyo, Japan
Tokyo, Japan
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology,
Tsukuba, Japan
Tsukuba, Japan
Abstract
[1] 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.
Received 27 August 2012; revised 11 October 2012; accepted 14 October 2012; published 9 November 2012.
Keywords: 2011 Tohoku earthquake, Jogan earthquake, diatom, tsunami deposit.
Index Terms: 4564 Oceanography: Physical: Tsunamis and storm surges (4302, 4304); 7240 Seismology: Subduction zones (1207, 1219, 1240).
1. Introduction
[2] In the region of the giant (magnitude-9) 2011 Tohoku
earthquake and tsunami in northcentral Japan, stratigraphic studies of
past tsunamis began nearly a quarter-century ago with the discovery of a
sand sheet linked to the Jogan earthquake and tsunami of July AD 869 [Abe et al., 1990; Minoura and Nakaya, 1991; Minoura et al., 2001; Sugawara et al., 2011] (Figures 1 and 2).
Historical documents and the distribution of the sand sheet on the
Sendai plain both showed that the Jogan tsunami ran inland kilometers
farther than did any later tsunamis–until 2011 (Figures S1 and S2 and
Table S1 in Text S1 in the auxiliary material).
However, a Japanese national seismic hazard map dated May 2010 showed
no hazard to the Sendai area from subduction earthquakes along the Japan
Trench larger than those of the past 400 years [The Headquarters for Earthquake Research Promotion, 2010].
[3] This apparent contradiction in recognized hazard
reflects uncertainty, which persists today, about the size of the Jogan
earthquake and the intervals at which such earthquakes recur. We sought
to address these unknowns through coastal geological studies and
geophysical modeling that began in 2004. Our results, first reported in
preliminary Japanese-language publications in 2007–2010 [Namegaya et al., 2010; Satake et al., 2008; Sawai et al., 2008b;Shishikura et al., 2007],
are presented here in English for the first time. Also reported here
are findings from Odaka and Juo that have not been published previously
in any language.
2. Methods
[4] Previous studies had identified Jogan tsunami deposits in the outskirts of Sendai (Figure 2a).
To test the size of past earthquakes and tsunamis, we searched beneath
coastal lowlands north and south of Sendai for the Jogan and other sandy
tsunami deposits and for stratigraphic signs of coseismic subsidence.
Such signs of long-lasting subsidence commonly record the greatest
earthquakes at other subduction zones [Satake and Atwater, 2007].
We studied sediment samples with a handheld gouge corer and
15–40-cm-wide, rectangular geoslicer at 399 locations along seven
leveled transects in Sendai plain and nine transects in other multiple
coastal lowlands facing the Japan Trench (Table S2 in Text S1). We used diatom assemblages to infer changes in environment and elevation following methods described by Sawai [2001] (Figure 3 and Figure S10 in Text S1). A widely mapped ash bed deposited in AD 915, identified in the field and through chemical analysis (To-a (Towada) ash of Aoki and Machida [2006]) (Table S3 in Text S1),
helped us trace tsunami deposits. To date tsunamis and subsidence we
analyzed 159 materials (plant macrofossils, insects, and charcoals),
mostly from peaty beds, with routine AMS 14C methods (Table S4 in Text S1).
[5] We further assessed earthquake size by modeling tsunamis that could explain the mapped sand sheets, following the method of Satake et al. [2008](Table S5 in Text S1).
Nonlinear shallow-water equations were solved with a finite-difference
method applied to bathymetry and topography. We used various data
sources to model present-day bathymetry and topography, then
reconstructed the Jogan shoreline in AD 869. Details on sites,
stratigraphy, data sources, and methods are in the auxiliary material.
3. Evidence for Historical and Prehistoric Tsunamis
[6] We mapped five extensive sand deposits in sediments
spanning 3,000 years along seven transects perpendicular to the
shoreline in the Sendai area (Figures 2–4 and Figures S3–S7 in Text S1).
From old to young the deposits are labeled A, B (Jogan), C, D, and K
(Keicho 1611). All sand deposits were bounded by peaty deposits that
accumulated in a freshwater swamp or marsh. The deposits show single or
multiple vertical upward-fining beds and are laterally continuous for
more than 0.5–1.5 km.
[7] The three sand deposits 1,000–3,000 years old are
widely preserved along a 100-km-long stretch of coast that includes the
Sendai plain. A sheltered lowland in Odaka, 70 km south of Sendai,
retains three sand deposits interbedded with peat and mud (Figures 3a–3c and Figure S8 in Text S1).
As on the Sendai plain, the deposits fine upward, commonly in multiple
beds or laminations. We could not trace sand B more than 1.8 km from the
present shoreline. Using 130 radiocarbon ages, we correlate the Odaka
sand deposits with sands B, C, and D in the Sendai plain (Figure 4 and Figure S11 and Table S4 inText S1).
[8] Lithology, sedimentary structures, and paleoecology
suggest that the sand deposits were laid down during rapid marine
incursions. Most deposits have abrupt lower contacts and taper landward
over hundreds of meters. Internal structures of the deposits include
multiple graded beds, parallel laminae, rip-up clasts, and flame
structures, all features consistent with deposition by tsunamis [Morton et al., 2007].
We ruled out fluvial deposition as a possible origin for the deposits
because they contain many marine and brackish diatoms (Figure S10 in Text S1).
[9] Three of the five sand deposits can be matched with
tsunamis known from written records. We correlated sand B with the Jogan
tsunami by means of radiocarbon ages just below and above sand B and by
its position just below the Towada ash of AD 915 (To-a, Figure 2c and Table S3 in Text S1).
Beneath seaward parts of the Sendai plain we found evidence for two
tsunamis younger than Jogan: sand A, which may correlate with an
earthquake in 1454 (Figures 2c and 4b);
and sand K, which may represent a tsunami in 1611 that had its greatest
reported effects farther north, on the Sanriku coast (location, Figure 1b) [Imamura, 1934].
Radiocarbon ages suggest time intervals of about 800 years between
sands D and C, and about 500 years between sands C and B—shorter than
the 1,000 year average recurrence interval inferred previously [Minoura et al., 2001] for unusually large tsunamis near Sendai.
[10] In Juo, near the southern Tohoku, we were not able
to find correlatives of the tsunami deposits identified in Sendai plain
and Odaka (Figure 4 and Figure S9 in Text S1).
Radiocarbon ages show that three sand deposits within freshwater peaty
sediment (sands H, J, and M) are younger than the Jogan tsunami. One
sand may record the tsunami of 1677 (Figure 4) [Takeuchi et al., 2007].
4. Evidence for Coseismic Subsidence
[11] At Odaka, we found diatom evidence for coseismic
subsidence coincident with sand deposition that we correlate by
stratigraphic sequence and radiocarbon age with sands C and B (Figures 3 and 4).
In each case the diatom assemblage below a sand deposit contains more
freshwater taxa, and fewer marine taxa, than does the assemblage above
it (Figure 3d).
[12] The inferred subsidence helps define the tsunami
sources in two ways. First, subsidence coincident with a sand deposit
implicates a nearby Japanese earthquake, not a large storm or a tsunami
from a distant source. Second, if at least partly tectonic, not just a
result of shaking-induced compaction, coseismic subsidence can help
constrain the fault-rupture model of the earthquake. As we show in the
next section, the inferred coastal subsidence shows that a fault rupture
on the subduction plate boundary was located directly offshore from
sand deposits, as illustrated by the coastal subsidence that accompanied
the 2011 earthquake (Figure 1b).
5. Modeling the Jogan Rupture
[13] We used tsunami simulation models, constrained in
part by evidence for coseismic subsidence, to reproduce inundation by
the Jogan tsunami of AD 869. Of the eight tsunami deposits identified in
this study, we knew best the lateral and inland distribution of the AD
869 Jogan deposit and the position of its contemporaneous shoreline. To
estimate parameters of the Jogan rupture, we computed tsunami inundation
for 14 different fault models, including plate-boundary ruptures with
various lengths and widths, and some with slip on outer-rise normal
faults like an earthquake along Japan Trench in 1933. We then compared
our inundation results with the inundation areas inferred from mapping
the distribution of the Jogan tsunami deposit in the Sendai plain and at
Ishinomaki and Odaka (Figures S12–S14 in Text S1).
[14] To help define inundation limits for modeling, we
first needed to estimate changes in the shoreline of the Sendai plain
during the past few thousand years. The estimates are most confident for
the Jogan shoreline because the AD 915 ash covered the lowland shortly
after the tsunami. The ash tends to be preserved best in swales between
the lowland's beach ridges. We thus drew the Jogan shoreline between the
most seaward swale that contains both the ash and the Jogan tsunami
deposit, sand B (swale X in Figure 2b), and the most landward swale containing only the ash (swale Y in Figure 2b).
We then estimated the minimum inundation distance by mapping sand B to
its landward limit. The Jogan tsunami probably ran beyond this limit by
analogy with the 2011 tsunami, which outran its sand deposit on the
Sendai plain (Figure 2b and Table S6 in Text S1).
[15] We found that a plate-boundary rupture at least 100
km wide is needed to yield the long tsunami wavelength implied by the
inland tsunami penetration minimally estimated from the distribution of
sand B. The best model is a fault rupture 200 km long and 100 km wide,
with an average slip of 7 m. The earthquake magnitude inferred from this
model, Mw 8.4, thus incorporates assumptions about tsunami inundation
that tend to minimize the estimated magnitude. Tsunamis produced by
large slip on a narrow fault rupture near the trench axis have shorter
wavelengths than our best model, and a correspondingly narrower zone of
coastal inundation [Satake et al., 2008]. A 100-km-wide rupture also produces a wider zone of subsidence that helps explain the coseismic subsidence at Odaka.
[16] All the models used estimates of fault-rupture
length that are probably minimums for the Jogan earthquake. Few of the
models extend more than 50 km south of Odaka. This southern limit
sufficed to account for the tsunami inundation and coseismic subsidence
that we inferred from stratigraphy and diatoms near Odaka (Figures 3 and 4),
but we do not know how much farther south the inundation and subsidence
extended. In the northward direction, the modeled ruptures extend to an
area offshore of Ishinomaki. This latitude is close to the northern
limit, as of 2010, of documented tsunami deposits that have been
correlated with the Jogan tsunami on the basis of stratigraphic position
beneath the AD 915 ash.
6. Discussion
[17] It is an open question whether Earth science could
have forewarned of the enormity of the 2011 earthquake and tsunami. Even
with the advantage of post-2011 hindsight, the magnitude of the Jogan
earthquake remains unknown [Sugawara et al., 2012].
To have shown that it attained magnitude 9, coastal geologists would
have needed to correlate evidence for tsunami inundation and coseismic
subsidence southward past Juo, northward along the Sanriku coast, or
both. The search in both directions would have been impeded by the
eastern limit of the AD 915 ash (Figure 1b).
Moreover, as in our radiocarbon-aided correlations between the Sendai
plain and Odaka, geologic dating rarely has the precision to distinguish
between a single long fault rupture and a swift series of shorter ones [Nelson et al., 1995]. It would also have been difficult to estimate, in the manner of Figure 2b,
the inland limits of tsunamis on parts of the mountainous Sanriku coast
where tsunamis of many sizes must have filled narrow valleys wall to
wall. Finally, the huge slip near the Japan Trench axis, which
contributed to the enormous size of the 2011 earthquake, cannot be
resolved from tsunami inundation modeling. The 2011 tsunami inundation
areas can be reproduced without such huge offshore fault slip [Satake et al., 2012].
[18] Still, tsunami geology had the potential to provide
estimates of the recurrence of past great Tohoku earthquakes and the
minimum inland extent of their accompanying tsunamis [Satake and Atwater, 2007],
with consideration of the uncertainties mentioned above. It might also
have reduced the 2011 casualties through tsunami awareness. Our mapping
and dating of tsunami deposits as described here (inundation maps in auxiliary material) could have focused emergency planning, improved evacuation maps, and informed public-awareness campaigns.
Acknowledgments
[19] This work was supported from 2005 to 2010 by The
Focused Survey and Observation on the Miyagi-ken-oki Earthquakes (MEXT,
Japan). J. Komatsubara, T. T. Aung, Y. Fujii, D. Matsumoto, S. Fujino,
T. Matsu'ura, H. Kimura, K. Kagohara, K. Tanigawa, O. Fujiwara, and N.
Sato helped with the fieldwork. The geoslicer was operated by K. Takada,
H. Kinoshita, and T. Ikeda. A. Furusawa and T. Matsu'ura helped
identify ashes and R. Nakashima identified bivalves. S. Yamaki helped
tsunami simulation. Brian Atwater and Alan Nelson suggested ways to
improve the manuscript, which was also reviewed by Yue-Gau Chen,
Benjamin Horton, Brian McAdoo, and Harvey Kelsey. Y.S., M.S., Y.O. did
the fieldwork. Y.S. prepared the text and figures. M.S. led the
fieldwork in Ishinomaki and Y.S. led at other sites. Y.S. analyzed
diatoms. K.S. provided preliminary designs for the fault models. K.S.
and Y.N. ran the simulated tsunami inundation models. Authors are listed
alphabetically except for Y.S.
[20] The Editor thanks Brian McAdoo and Harvey Kelsey for their assistance in evaluating this paper.
Citation: Sawai, Y., Y. Namegaya, Y. Okamura, K. Satake, and M. Shishikura (2012), Challenges of anticipating the 2011 Tohoku earthquake and tsunami using coastal geology,Geophys. Res. Lett., 39, L21309, doi:10.1029/2012GL053692.
©2012. American Geophysical Union
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