The
differences in the coastal geomorphology along the Pacific coast of Tohoku in part
controlled the impact of the Tohoku-oki tsunami. a. The steep sided valleys of the Sanriku coastline
focussed the tsunami waves causing a run up height of
approximately 40 m above sea level. b.
In contrast along the low lying Sendai
Plain, the wave height was much less but it
travelled much further inland, with a maximum inundation from the shoreline of
5.4 km. Image courtesy of Kazuhisa Goto (Chiba Institute)
A couple of years ago, I wrote something of a rant on the topic of listening
to geologists in order to save lives. The post, Ignoring
tsunami records: hubris, complacency, or just human nature? was in the
aftermath of the catastrophe in Japan and reflected on what could –
should – have been learned from historical and geological testaments.
Now, thanks to a research report in Geology Today on the papers
published last year in a special issue of Sedimentary Geology, we have
a view of what has now been learned, geologically at least, in the
aftermath of the events of 2011.
The lead review paper in the journal, titled The future of tsunami
research following the 2011 Tohoku-oki event, is by Kazuhisa Goto of
Japan’s Planetary Exploration Research Center, Chiba Institute of Technology,
and the International Research Institute of Disaster Science, Tohoku University,
together with colleagues from Australia and the US. Their opening and closing
comments are worthy of attention:
The 2011 Tohoku-oki tsunami was the first example of a large, low-frequency
event occurring where historical and pre-historical tsunamis were already known
to have occurred through historical… and geological evidence... Magnitudes of
some of the historical earthquakes and their associated tsunamis had also been
estimated based on the known geological record and numerical modelling... This
point was highlighted by media soon after the 2011 event because such
information had not been taken into account in the tsunami disaster prevention
plan for the Pacific coast of Tohoku….
It is indeed true that geology is vital to gaining a better understanding of
past tsunamis along any coastline and to interpreting the hydrodynamic features
of paleotsunamis. However, the 2011 Tohoku-oki tsunami event tells us that there
is still much to be learnt from tsunami deposits if we are to produce reliable
tsunami risk assessments. Following the 2011 Tohoku-oki tsunami, we must return
to the key question – why were the results of geological studies of the AD869
Jōgan event not incorporated into disaster prevention plans in Japan? Although most
geological studies of the AD869 Jōgan tsunami had not been published in
mainstream, peer-reviewed international journals, the results were nonetheless
high quality and well disseminated. The AD869 Jōgan tsunami was indeed one of
the best studied events in the world. However, Goto et al. (2012) candidly admit
that tsunami geology is not a mature discipline and that prior to the 2011
earthquake it had not reached a sufficient level of recognition in Japan where
researchers and disaster prevention experts from various fields were interacting
effectively. Furthermore, in general terms, people continue to be unable to
comprehend the significance and relevance of extreme events that occur on
timescales spanning several 100s-1,000s of years that far exceed the human (or
building) lifespan. This is where much of the challenge lies for tsunami
scientists in education outreach to the general public.
The results that they report are fascinating, startling, and sobering – Geology Today provides a useful summary:
Future research following the 2011 Tohokuoki eathquake and tsunami
On 11 March 2011 a magnitude Mw 9.0 earthquake occurred off the Pacific coast
of Tohoku District Japan, generating a major tsunami which caused widespread
damage along the east coast of Japan and coastal areas around the whole Pacific
basin. This event, referred to as the Tohoku-oki (oki means offshore in
Japanese) generated a huge tsunami, with a run up height of up to 40 m,
resulting in 15,868 dead and 2,848 missing, along with substantial damage,
including that at the Fukushima Daiichi nuclear power plant. The damage and
death-toll was despite the fact that the Pacific coast of Tohoku was one of the
best prepared for a tsunami in the country. However, the event has also allowed
researchers to look at the effects of this very well documented tsunami and to
enable them to better interpret the historical sedimentary record to re-evaluate
the magnitude of previous events. Such work is very important when developing
tsunami disaster prevention plans.
Recently a special issue of the journal Sedimentary Geology has been devoted
to the ‘2011 Tohoku-oki tsunami’. This special issue comprising 15 papers based
on surveys and numerical modelling in Japan, along with one paper on the effects
of the tsunami in the USA is prefaced by a review article by Kazuhisa Goto and
colleagues looking at the sedimentological effects of the tsunami, future
research arising from this event and also the social relevance of this research
in the aftermath of the tsunami (Sedimentary Geology, 2012, v.282, pp.1–13).
The Pacific coast of Tohoku is divided into the Sanriku ria coast in the
north and coastal plain areas such as Sendai in the south. The Sanriku coast is
characterised by narrow drowned valleys, which have been damaged by tsunamis
every few tens to hundreds of years [header image]. In contrast the Sendai
plains are an alluvial lowland, with a beach, coastal dune ridges up to several
metres high covered by pine forests, and low lying former wetlands and rice
paddies. In this area there is no historical record of large tsunamis over the
last thousand years, except for one possible event in 1611, although small
tsunamis are recorded. These differences in the coastal geomorphology had marked
effects on the tsunami wave. On the Sanriku coast, its steep and narrow valleys
focussed the wave, generating the largest run ups, with a maximum rise of 40.4 m
above sea level, although the inundation distance inland was relatively short,
being generally up to 2 km. In contrast on the Sendai Plain, the tsunami
travelled up to 5.4 km inland, but the wave height reached a maximum of only
about 19 m. In the offshore area, about 1 km from the coastline, video footage
has allowed a flow speed of 14 m/s to be calculated, although near Miyako
sequential photographs taken from high ground suggest that the incoming wave
velocity may have reached 32 m/s. In contrast, on land the flow speeds are
variable as a result of the surface conditions, with estimates ranging between 3
and 8 m/s.
Sediment
deposits from the tsunami included deposits of sand and mud across the Sendai
Plain, along with gravel deposits and locally extremely large boulder deposits.
Image courtesy of Kazuhisa Goto
Following the tsunami, rapid geological surveys were conducted to gain an
understanding of the types of sedimentary deposits resulting from an event of
this magnitude. On the Sendai Plain a sand layer extended some 3 km inland which
continued as a mudstone layer almost to the maximum inundation limit of 5.4 km
[image above]. This unit reached a maximum thickness of 30 cm and generally
thinned inland. This bed was typically parallel laminated or structureless sands
and silts with fragments of wood, glass and ripped up mud clasts overlying an
erosional base. Based on an analysis of its heavy minerals, microfossils and
isotope chemistry, it appears that most of the deposited sediment on the Sendai
Plains was derived from beach, sand dune, lagoon and inland soils with only a
minor contribution from offshore sediments. Importantly, it was recognized that
a lot of the sediment resulted from liquefaction, with sand being vented from
beneath the soils in the rice paddies. In contrast, on the Sanriku coast, the
deposited sediment comprised both the eroded beach sands but also marine
sediment from both the inner bay and even more pelagic areas.
In addition to the deposition of sands and muds, granule to boulder sized
clasts were also observed. In some cases gravel deposits were up to 1 m thick
and thinned landward. Boulder deposits were also observed, including large
reworked fragments of concrete from the coastal defences, with the largest
observed natural boulder being 6.5 × 2.5 × 2.4 m in size.
Offshore, within coastal bays and harbours there was evidence for both
erosion, but also sediment deposition with the formation of large-scale
submarine dunes. Further offshore at water depths between 300 and 5940 m the
seafloor was covered by muddy sediments, in areas that prior to the earthquake
had sandy or gravel surfaces. These sediments are interpreted as the deposits of
turbidites, resulting from the tsunami backwash. Other turbidite deposits
between 1 and 25 cm thick were observed in cores recovered offshore from the
Sendai and Sanriku coastline, and caesium-134 and caesium-137 released from the
Fukushima-Daiichi nuclear plant was detected on the top of these deposits.
Such sedimentological studies as these are critical for us to correctly
interpret the ancient record of past tsunami events, which in turn help us
understand the scale and frequency of past events. This is indispensable in
developing tsunami risk assessments, and the improvement of disaster prevention
measures.
[I have to admit that I refrained from commenting that Kazuhisa Goto was the
guy they should have gone to when planning tsunami defences, but it seemed – and
probably still is – inappropriate. His 2012 article is online in
English. The complete list of all the papers in the journal can be found at
http://www.sciencedirect.com/science/journal/00370738/282.
The full articles are behind the pay wall, but I will be happy to supply a PDF
for personal use if any reader would be interested.]
