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.
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.]