The 2011 Tohoku-oki earthquake was a tremendous and tragic ... 11 March 2011, at 14:46:23 Japan Standard Time (+9 UTC) and the rupture area, assumed to ...
Overview of the 2011 Tohoku Earthquake Tsunami Damage and Its Relation to Coastal Protection along the Sanriku Coast Nobuhito Mori,a) Daniel T. Cox,b) Tomohiro Yasuda,a) and Hajime Masea)
At 14:46 local time on 11 March 2011, a magnitude 9.0 earthquake occurred off the coast of northeast Japan. The very large local tsunami heights and damage to coastal structures and other civil infrastructure were strongly dependent on the location. To verify the effectiveness of several types of tsunami protection strategies, we analyzed three typical areas from the Sanriku ria coast. Detailed survey data are shown with photographs and are discussed with regard to severity of damage and tsunami event. The roles of offshore tsunami breakwaters and coastal forests are discussed for severe tsunami, which exceeds 10 m near the coast. The survey observations and results from numerical calculations clearly indicate the reduction of tsunami damage due to an offshore barrier in Kamaishi Bay. [DOI: 10.1193/1.4000118]
INTRODUCTION The 2011 Tohoku-oki earthquake was a tremendous and tragic earthquake-tsunami disaster for Japan. An earthquake of magnitude 9.0 occurred off the Pacific coast of Tohoku, Japan, on 11 March 2011, at 14:46:23 Japan Standard Time (+9 UTC) and the rupture area, assumed to be approximately 450 km × 200 km (Stewart et al. 2013), generated a tsunami that struck Japan from Hokkaido to Kyushu as well as various locations around the Pacific Ocean. The tsunami first reached the Japanese mainland about 20 minutes after the earthquake and ultimately affected a 2,000 km stretch of Japan’s Pacific coast (The 2011 Tohoku Earthquake Tsunami Joint Survey Group 2011). The Tohoku region consists of several prefectures ranging from north to south: Aomori Prefecture, Iwate Prefecture, Miyagi Prefecture, and Fukushima Prefecture, which border the Pacific Ocean. Sendai is the largest city in the region. The southern part of Tohoku is relatively flat, especially the Sendai plain. The coastal geomorphology of northern Tohoku features ria coasts, which are steep, narrow bays. The northeastern part of Pacific side of Tohoku is known as the Sanriku region, starting from North Miyagi Prefecture to South Aomori Prefecture. As of 8 August 2012, official fatalities were 15,867 with an additional 2,903 missing (National Police Agency of Japan). The major cause of death was the tsunami, and most fatalities occurred in Tohoku: 58% in Miyagi Prefecture, 33% in Iwate Prefecture, and 9% in Fukushima Prefecture. There were 128,530 totally damaged, 230,332 partially damaged buildings, along with more than 200 bridges
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Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011, Japan School of Civil and Construction Engineering, Oregon State University, 220 Owen Hall, Corvallis, OR 97331-3212 127
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(Akiyama et al. 2013), and an estimated ¥16.9 trillion (US$211 billion) in direct damage (e.g., Kajitani et al. 2013). Before this event, the risk of earthquakes and tsunamis off the Tohoku coast was believed to be high. The Japanese government reported that magnitude 7.5 and 7.7 earthquakes along a 200 km fault offshore of Sendai in southern Sanriku-oki were expected to occur with 99% probability and 70–80% probability within 30 years, respectively (The Headquarters for Earthquake Research Promotion 2005). The 1896 Meiji Sanriku earthquake (Mw 8.2–8.5) and tsunami caused 21,915 deaths, the 1933 Showa Sanriku earthquake (Mw 8.1) and tsunami caused 3,064 deaths, and smaller tsunamis have occurred roughly every 10 to 50 years. Thus, earthquake and tsunami disaster countermeasures—seawalls, gates, and offshore tsunami breakwaters, planted trees as a natural tsunami barrier, vertical evacuation buildings, and periodic evacuation training—were implemented and practiced in these areas. Therefore, we emphasize that Tohoku was an area highly prepared for tsunamis. Nevertheless, the tsunami disaster countermeasures were insufficient against the 2011 event. Tsunami barriers (onshore and offshore breakwaters and natural tsunami barriers) were severely damaged, some reinforced concrete buildings were totally destroyed (Chock et al. 2013), and the extent of inundation was underestimated in several areas. This event is important for future tsunami preparation to classify various modern counter measures against mega-tsunami. Therefore, field surveys are important for understanding the event and for planning future tsunami-disaster reduction. First, this paper briefly summarizes the results of a post-event field survey of the 2011 Tohoku-oki earthquake and tsunami. Second, the tsunami inundation heights and run-up heights are discussed for regional and bay scale analyses. Satellite images before and after the tsunami are discussed along with local photos from the survey. Also, a numerical simulation of tsunami heights is carried out to study the effects of an offshore breakwater. Finally, the details of failures of coastal structures at several locations are discussed in detail. OUTLINE OF POST-EVENT TSUNAMI SURVEY Tsunami surveys were conducted with the participation of 299 tsunami, coastal, seismology, and geology researchers from 64 universities and institutes throughout Japan (The 2011 Tohoku Earthquake Tsunami Joint Survey Group 2011). The 2011 Tohoku Earthquake and Tsunami Joint Survey Group (hereinafter, the survey group) is an autonomous survey organization, consisting of members from different fields of the natural sciences, tsunami engineering, coastal engineering, and tsunami-related research. The survey group was managed by researchers at the Faculty of Safety Science of Kansai University and the Disaster Prevention Research Institute of Kyoto University (The 2011 Tohoku Earthquake Tsunami Joint Survey Group 2011). Surveys began within two days after the earthquake in Ibaraki Prefecture, Chiba Prefecture, and other prefectures outside Tohoku that were not severely affected, and surveys in the severely affected Tohoku region began on 25 March 2011, after the completion of major search and rescue operations. Until the middle of April, teams were assigned to survey locations by the survey secretariat to ensure efficiency and to avoid interference with relief operations. Survey groups measured local tsunami heights along the coast, stretching 2,000 km from Hokkaido in the north to Okinawa in the south (Mori et al. 2011, Mori
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et al. 2012). The purpose of the surveys was to collect tsunami inundation heights throughout the regions and run-up heights at the maximum shoreward extent of the inundation. Inundation heights and run-up heights are defined relative to the astronomical tidal level at the time of arrival of the maximum tsunami height at the shoreline. Inundation and run-up were measured with an accuracy of a few centimeters from watermarks on buildings, trees, and walls using laser range finders, real-time kinematic (RTK) GPS receivers with cellular transmitters, and total stations. Run-up height was determined from the maximum landward extent of debris and seawater marks. As of the end of August 2012, the total number of survey locations was 5,285. CHARACTERISTICS OF INUNDATION AND RUN-UP DISTRIBUTION This tsunami was remarkable not only for the magnitude of the event, but also for the wide variety of inundation characteristics—from urban cities to rural coastal towns and agricultural lands. There were modern coastal defenses combining seawalls, breakwaters, planted trees, horizontal evacuation plans, and vertical evacuation buildings. Local coastal geomorphology also differed substantially across the affected region. For example, the urban center of Sendai (population 1 million) is situated on a fluvial lowland, and smaller cities and towns to the north are situated on a ria coast with steep terrain. Figure 1 shows the bathymetry of the
Figure 1. Bathymetry of the Tohoku region (meters).
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Tohoku region. A narrow continental shelf is located offshore from most of the Sanriku region, although a broader and shallower continental shelf is located offshore of the Sendai plain (refer to location names in Figure 3). From about 50 km to 200 km north of the Sendai plain, the local geomorphology of the Sanriku region is characterized by ria coast, steep terrain, and shallow, narrow bays. These features focused tsunami waves, generating the largest run-ups and resulting in the catastrophic destruction of towns and cities, including Taro, Miyako, and Rikuzen-Takata in Iwate Prefecture. Figure 2 shows the inundation heights and run-up heights with historical tsunami data from the 1896 Meiji Sanriku tsunami (Mw 8.2–8.5) and 1933 Showa Sanriku tsunami (Mw 8.4). Local topography amplified the tsunami height in many bays, and this amplification due to trapped edge waves was also observed along planar beaches. The maximum inundation height in Hokkaido was 6.78 m along planar beaches. The maximum inundation height in the Tokyo Bay area was 2.8 m and is located about 390 km to the southwest of the epicenter. These maximum inundation heights were observed in locations where the local geometry amplified the tsunami wave at the end of bays. The Sendai plain is the most populous area in Tohoku and consists of fluvial lowlands and a flat coastal plain formed by the Abukuma, Natori, and Nanakita Rivers, where the tsunami bore propagated a maximum of 5 km inland (Suppasri et al. 2012) and inundated the entire plain (Liu et al. 2013). The maximum measured inundation height was 19.50 m due to local amplification, and the mean inundation height along the Sendai coast shoreline was about 10 m.
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In the 2011 Tohoku tsunami, the maximum run-up height was 40.1 m at Ofunato, and in this ria coastal region, there was catastrophic destruction of towns and cities (latitude 38° to 40°). The historical records of maximum run-up are 38.2 m for the 1896 Meiji Sanriku tsunami and 28.7 m for the 1933 Showa Sanriku tsunami (Figure 2). The maximum run-up height for the 2011 event is similar to that for the Meiji Sanriku tsunami, but the extent of the affected coastline is several times larger for the Tohoku tsunami. In 2011, the areas where the maximum run-up height exceeded 30 m extend from Onagawa to Noda, which covers more than 180 km of the Sanriku coast. Compared with the northern Sendai plain, the southern Sendai plain has a steeper and narrower continental shelf, which amplified the tsunami height and caused severe damage in this region, which includes the Fukushima Daiichi plant. LOCAL TSUNAMI HEIGHT AND DAMAGE The damage to coastal structures, ports, houses, buildings, bridges, and other infrastructure was strongly dependent on location. Much information has been reported in various damage surveys from different areas (e.g., Fritz et al. 2011, Kakinuma et al. 2012, Ogasawara et al. 2012, Mase et al. 2013). Here we selected three different bays—Rikuzen-Takata, Otsuchi, and Kamaishi—along the Sanriku ria coast for analysis based on satellite images and photographs, as shown in Figure 3. These three locations have varied physical geometries, and each area
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prepared differently with tsunami countermeasures. The city of Rikuzen-Takata had natural coastal pine trees with a relatively lower onshore breakwater. The village of Otsuchi was protected by 6.4 m high onshore breakwater. And the city of Kamaishi was protected by an offshore breakwater, which was constructed at a water depth of 63 m at the bay mouth. These three locations show large differences, as discussed in this section. SANRIKU AREA: RIKUZEN-TAKATA (RIA COAST WITH NATURAL BEACH)
There were severe tsunami impacts along the ria coasts of northern Miyagi and Iwate Prefectures. A ria is a coastal inlet formed by the partial submergence of river valley and is characterized by steep, narrow bays. Due to these complex geographical effects, the tsunami heights became extremely large and showed extremely high run-up of up to 40.1 m at Ofunato (Mori et al. 2012). The inundation heights were roughly two times higher than that of the Sendai plain. Figure 4 shows satellite images before and after 11 March at Rikuzen-Takata in Iwate Prefecture. Rikuzen-Takata had a natural sandy beach with 80,000 pine trees as a part of a coastal protection plan. Because of the reliance on the pine trees and sandy beach as a natural buffer, the coastline of Rikuzen-Takata was protected by a breakwater of relatively low height (5 m) compared to other regions. This natural protection shown in Figure 4b was completely destroyed except for one pine tree. There was more than 100 m of shoreline loss due to a combination of subsidence and erosion from the tsunami. In addition, the tsunami propagated more than 5 km from the river mouth in the Kesen River. This area (upstream of the Kesen River) is located behind hills and suffered some effects of the tsunami, although the residents could not see the coast. The Rikuzen-Takata area is located in a small basin at the river mouth of Kesen River, and is a quite unusual setting for the Sanriku ria coast. The geometry is similar to the Sendai plain. However, Rikuzen-Takata is located between the Karakuwa peninsula and Hirota peninsula, with sudden changes of the offshore bathymetry. As a result, the tsunami was amplified in Rikuzen-Takata; the mean inundation height was about 13 m, and maximum run-up was more than 20 m. These were nearly twice the heights in the Sendai plain. Most of the houses were destroyed, and only a few reinforced concrete buildings remained in the surveyed area, as shown in the photo in Figure 5. The percentage of causalities in Rikuzen-Takata was more than 10%, and the number of causalities and refugees was about 80% of the total residents (Ogasawara et al. 2012). The role of coastal forests for protection has been extensively discussed in Japan for various regions. The coastal pine trees in Rikuzen-Takata protected houses well against the Showa Sanriku tsunami in 1933 and the Chilean tsunami in 1960. The similar roles of natural tree protection against tsunamis were reported for other events and areas (e.g., the Nihonkai-Chubu earthquake and tsunami in 1986). Even for the 2011 event, the coastal forests and dunes worked relatively well in areas of lower tsunami heights, such as Ibaraki and Aomori. However, the tsunami impact was too strong in the Sanriku region, and the coastal forests did not significantly reduce the tsunami damage (see Hosino 2012). Rikuzen-Takata was one of the most severely damaged communities, and the role of the combined effect of the natural and manmade barriers will be examined in future work on reducing the impact of massive tsunamis.
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Figure 4. Satellite images of Rikuzen-Takata city in Iwate Prefecture (solid line: area of pine trees; Google Earth): (a) 23 July 2010; (b) 14 March 2011. SANRIKU AREA: OTSUCHI VILLAGE (RIA COAST WITH COASTAL BREAKWATER)
Otsuchi village in Iwate Prefecture is located 50 km north of Rikuzen-Takata in the Sanriku ria coast area. The geometry of Otsuchi village is typical of the ria coast, with steep and narrow bays. The tsunami severely damaged this area as shown in Figure 6, comparing satellite images before and after 11 March at Otsuchi village in Iwate Prefecture.
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Figure 5. Snapshot of the city of Rikuzen-Takata in Iwate Prefecture, April 2011.
The coastal area was completely destroyed, as shown in Figure 6 and the local photograph of Figure 7. The tsunami destroyed the breakwater and propagated inland along the Otsuchi and Kotsuchi Rivers. No houses or buildings remained near the coastal area. The percentage of casualties in Otsuchi was about 10%, and the number of causalities and refugees was about 50% of the total residents (Ogasawara et al. 2012). Figure 8 shows the post-event survey data around Otsuchi Bay. The inundation/run-up height outside of the bay was initially 17 m, and the tsunami directly entered Otsuchi Bay without major changes in energy. The inundation height was more than 10 m near Otsuchi port, and the maximum run-up height was 18.1 m at Otsuchi. Figure 9 shows detailed mapping of coastal and river structures around Otsuchi port with an overview satellite image and several inset photographs (a to j) indicating some of the major damage in this area. Inset Figure 9a shows metal debris with an orange hue from a large fire that broke out shortly after the tsunami. Insets Figure 9b–c from the port area show examples of large scour holes, some large enough that several cars can fit inside. Insets Figure 9d–g near the port waterfront area show complete building damage, more scour, and subsidence of the port infrastructure. Insets Figure 9h–j show damage to hydraulic control structures and seawalls that were completely overtopped during the inundation. SANRIKU AREA: KAMAISHI CITY (RIA COAST WITH OFFSHORE TSUNAMI BREAKWATER)
Kamaishi city in Iwate Prefecture is located just south of Otsuchi in the Sanriku ria coast area. The water depth in the middle of Kamaishi Bay is similar to that of Otsuchi, so that the previous Showa Sanriku tsunami in 1933 and expected tsunami heights were similar for
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Figure 6. Satellite images of Otsuchi village in Iwate Prefecture: (a) 27 April 2005; (b) 1 April 2011 (solid circle = damaged area; Google Earth).
the two locations. For example, the measured inundation heights from the Showa Sanriku tsunami at Otsuchi Bay and Kamaishi Bay were 5.4 m and 6.0 m, respectively. Construction of an offshore tsunami breakwater began in 1978 at the mouth of Kamaishi Bay. The offshore breakwaters with lengths of 990 m and 670 m were finally completed in 2006 in a water depth of 63 m, making it the deepest breakwater in the world.
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Figure 7. Photo of Otsuchi village in Iwate Prefecture, April 2011.
Figure 10 indicates the post-event survey data around Kamaishi Bay. The run-up height outside of the bay was more than 30 m, but the tsunami height was reduced significantly in Kamaishi Bay. The right upper panel in Figure 10 indicates the distribution of inundation/ run-up height from the bay mouth to the coastline. The breakwater is located 2.2 km from the bay mouth and 2.3 km from shoreline, as defined in Figure 10. The run-up height of Kamaishi Bay is initially 22 m at the bay mouth, drops to 10 m near the offshore breakwater, and remains roughly constant at 10 m to the shoreline. This is significantly smaller than Otsuchi Bay. The differences in the tsunami between these two geometrically similar bays of Otsuchi Bay and Kamaishi Bay can be seen in Figures 8 and 10. A comparison of the tsunami at Otsuchi and Kamaishi shows that the offshore breakwater reduced the tsunami height by about 25% to 40% and significantly lessened the damage at Kamaishi. A numerical simulation was conducted to estimate the influence of offshore breakwaters on the tsunami inundation heights. The calculations were done using the quasi-3-D Euler equation with curvilinear-sigma coordinates (Yoneyama et al. 2012). Turbulence mixing was considered using a vertical k-ε model and the Smagorinsky model for horizontal propagation. Bathymetry for both Kamaishi Bay (the southern part) and Ryoishi Bay (the northern part) are provided at a resolution of 50 m by the Cabinet Office, Government of Japan. Due to the coarse bathymetry information, land areas are described with a uniform height. The computation was carried out at Δt ¼ 0.1 s time intervals from the time of the earthquake until two hours later. The time series of the measured tsunami by the Kamaishi GPS buoy (Kawai et al. 2011) was used as a lateral boundary condition for the offshore side. The local inundation heights were validated with the measured inundation
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Figure 8. Post-event survey data around Otsuchi Bay. Clockwise from the top-left corner: Location of study area; tsunami height as a function of distance from bay mouth; spatial distribution of tsunami heights; and histogram of height measurements. Inundation and run-up data are shown in red and blue, respectively.
and run-up heights (Yoneyama et al. 2012). The astronomical tide was not included in the computation. Figure 11 shows the maximum water surface elevation for computations with and without an offshore breakwater. For the case without an offshore breakwater, the maximum surface elevation is 12–15 m within Kamaishi Bay and not different from Ryoishi Bay. The amplification of the tsunami can be seen for several steep valleys in both bays. The influence of the offshore tsunami breakwater can be seen clearly in Figure 11b. Within the bay, the maximum surface elevation is reduced from about 12–15 m to 9–11 m depending on the location. This corresponds to a 20% to 40% reduction of inundation height along the shores of Kamaishi Bay. These numerical results agree with the observed survey observations within 10% accuracy. The reduced tsunami energy in Kamaishi Bay gives different damage characteristics compared to Otsuchi Bay, as described in the previous section.
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Figure 9. Details of damage around the Otsuchi port in Iwate Prefecture.
Figure 12 shows an overview satellite image shortly after the inundation event with several photographs taken from the area. Inset Figure 12a, taken from a nearby high-rise building, shows that although the damage to Kamaishi was severe, it was not at the level of destruction observed at Otsuchi, reinforcing the conclusion that the offshore breakwater significantly reduced the extent of damage in this area. Inset Figures 12b–d show damage to the tsunami defense systems along the wharf, subsidence of the wharf, and damage to buildings. Unlike the near-collapse of the steel moment frame seen in Figure 12d, the similarly constructed building in Figure 12d suffered damage due to the nonstructural siding, but the steel frame was relatively undamaged. Figure 12e shows that the fuel storage tanks did not suffer significant damage in this area, although in other areas similar tanks were affected. On the opposite side of the port, Figures 12f–g show moderate damage and a large ship displaced by the tsunami. Relative to other areas, such as Kesenuma, the damage to ships was relatively minor. On the basis of analysis of survey data and numerical calculations, it can be seen that the influence of offshore tsunami breakwaters significantly reduced the tsunami impact on onshore damage, in comparison with other similar areas such as Otsuchi. Not considering
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Figure 10. Post-event survey data around Kamaishi Bay. Clockwise from the top-left corner: Location of study area; tsunami height as a function of distance from bay mouth; spatial distribution of tsunami heights; and histogram of tsunami height measurements. Inundation and run-up data are shown in red and blue, respectively.
the cost of construction, effectiveness of tsunami mitigation through the use of breakwaters was verified for the first time by the experience of the Tohoku Earthquake tsunami. CONCLUSIONS The 2011 Tohoku-oki earthquake and tsunami was the first case where modern, welldeveloped tsunami countermeasures were put to the test for such an extreme event. One of the most important issues in the natural sciences, engineering, and social sciences is to understand the relationships between inputs (local tsunami occurrence) and outputs (local damage). For future improvement of tsunami disaster countermeasures, much can be learned from this catastrophic event. A high-quality, high-density survey dataset was collected by about 300 researchers since 11 March 2011, providing detailed information on various aspects of the tsunami behavior for different geometries and conditions, on the local scale of bays and larger. This study deals with the local tsunami impact to the Sanriku ria coast based on the analysis of the survey data set.
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Figure 11. Numerical results of maximum water surface elevation showing the effects of an offshore breakwater in Kamaishi Bay. Northern area is Ryoishi Bay and southern area is Kamaishi Bay. (a) Without offshore breakwater; (b) with offshore breakwater.
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Figure 12. Detail of damage around Kamaishi port in Iwate Prefecture.
The damage to coastal structures, ports, houses, buildings, bridges, and other infrastructure was strongly dependent on location. Here we selected three typical areas in the ria coastal region of Sanriku for study: (1) Rikuzen-Takata, (2) Otsuchi, and (3) Kamaishi. The three locations had different tsunami protection strategies, such as the natural planted trees, onshore tsunami barriers, and offshore breakwaters, respectively. The natural pine trees and onshore tsunami breakwater were not very useful against such unexpectedly large tsunamis. However, offshore tsunami breakwaters that were partially destroyed were still effective in mitigating the level of destruction in the Kamaishi port area. Our analyses of the survey results and numerical calculations indicate that such structural protection may have resulted in lower overall inundation heights. In this study, we qualitatively discussed the relation between different types of tsunami countermeasures and damage from an overview of survey results. Further quantitative analysis of the relation between tsunami fluid forces and structural damage is required. It is possible to clarify damage-fluid relations using a wide variety of measurements and survey data from this event (e.g., velocity information; Foytong et al. 2013). From important lessons learned from the 2011 Tohoku tsunami, we have the opportunity to further vital preparation against tsunamis in future.
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ACKNOWLEDGMENTS This report used the survey results of the tsunami joint survey group. We gratefully acknowledge their efforts on the survey. This study is dedicated to all who have been affected by the earthquake in Japan on 11 March 2011. This is a summary of the nationwide, postevent survey results for the 2011 Tohoku-oki earthquake and tsunami in Japan.
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OVERVIEW OF THE 2011 TOHOKU EARTHQUAKE TSUNAMI DAMAGE AND COASTAL PROTECTION
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