Shunichi Koshimura

DEVELOPING FRAGILITY FUNCTIONS FOR TSUNAMI DAMAGE ESTIMATION USING NUMERICAL MODEL AND POST-TSUNAMI DATA FROM BANDA ACEH, INDONESIA

Fragility functions, as new measures for estimating structural damage and casualties due to tsunami attack, are developed by an integrated approach using numerical modeling of tsunami inundation and GIS analysis of post-tsunami survey data of the 2004 Sumatra-Andaman earthquake tsunami disaster, obtained from Banda Aceh, Indonesia. The fragility functions are expressed as the damage probabilities of structures or death ratio with regard to the hydrodynamic features of tsunami inundation flow, such as inundation depth, current velocity and hydrodynamic force. They lead to the new understandings of the relationship between local vulnerability and tsunami hazard in a quantitative manner.

DAMAGE CHARACTERISTIC AND FIELD SURVEY OF THE 2011 GREAT EAST JAPAN TSUNAMI IN MIYAGI PREFECTURE

On March 11th, 2011, the Pacific coast of Japan was hit by a tsunami generated by the largest earthquake (M9.0) in the history of the country and causing a wide range of devastating damage. Using preliminary reported data from many sources, some topics such as tsunami fatality ratio and tsunami fragility curves for structural damage are discussed and compared with other countries. This paper aims to discuss the damage characteristics of this tsunami as well as its mechanism, as observed through field surveys conducted over the 4 months following the tsunami. The field survey covers 13 areas in the Miyagi prefecture from Kesennuma city in the northernmost region to Yamamoto town in the southernmost region. The arrival time of the first tsunami along the coastal areas in the Miyagi prefecture was confirmed by stopped clocks found during the survey. The damage mechanism of coastal structures such as breakwaters, seawalls, tsunami gates, and evacuation buildings was investigated and discussed. Damage characteristics for each area, i.e., urban areas, port, coastal structures, fisheries, and agricultural areas, were also summarized. The conclusions drawn from the data analysis suggest that experience and education (soft countermeasures) are important to reduce the loss of life, as shown for example in the Sanriku area. The field surveys indicate that wood and reinforced-concrete (RC) structures should be balanced to survive both earthquake and tsunami forces, and the structural design for buildings should be reconsidered after the example in Onagawa town. In addition, coastal structures for tsunami countermeasures (hard countermeasures) should be more properly designed for survival instead of becoming floating debris upon being overturned by a tsunami. The combination of both hard and soft measures is especially necessary for optimizing the outcomes following a great disaster. These recommendations should be taken into consideration in the reconstruction efforts for better tsunami countermeasures in the future.

DEVELOPING TSUNAMI FRAGILITY CURVES FROM THE SURVEYED DATA OF THE 2011 GREAT EAST JAPAN TSUNAMI IN SENDAI AND ISHINOMAKI PLAINS

Japan was hit by the tsunami generated by the greatest earthquake in the history of Japan. The authors conducted the post-tsunami field survey in Miyagi prefecture immediately after the event to measure the inundation depth and investigate damaged buildings. Most of the buildings surveyed were wooden houses and could be classified into 4 damage levels. The primary data of inundation depth and damage levels obtained from the field survey were used to create the tsunami fragility curves, which described the degree of structural damage as a function of the hydrodynamic characteristics of tsunami inundation. The developed fragility curves show that damage with more than a 50% chance of occurring is classified as minor damage, moderate damage, major damage and complete damage when the tsunami inundation depth is between 2.5-3.0 m, 3.0-4.0 m and 4.0-4.5 m and greater than 4.5 m, respectively. By comparing the developed tsunami fragility curves, wooden houses in the studied area have higher structural performance than those obtained from historical events in other areas. The new curves suggest that wooden houses will be severely damaged if the inundation depth is greater than 3 m and collapse if the depth is greater than 4 m while wooden houses from historical data can resist only 1–2 m. Moreover, wooden walls in Japan reduce the overall failure of a structural member because a wooden wall is easier to destroy (compared to brick walls in Thailand and Indonesia) and allows the tsunami to pass through, which reduces the pressure at the attacking front of the tsunami. The fragility curves are very important for the loss estimation and reconstruction plans of the city; they are also crucial for reducing the disaster damage from a future tsunami.

MAPPING OF BUILDING DAMAGE OF THE 2011 TOHOKU EARTHQUAKE TSUNAMI IN MIYAGI PREFECTURE

The authors visually inspected the building damage caused by the 2011 Tohoku earthquake tsunami, using the pre and post-event aerial photos. First, we prepared the mosaic of post-tsunami aerial photos acquired by Geospatial Information Authority of Japan (GSI), and conducted the visual inspection of buildings damage to classify the damage. The damage classification results are compiled with building shape files on GIS for mapping the structural vulnerability in the tsunami inundation zone. Finally, we discussed the structural vulnerability in the tsunami affected area based on mapping results of building damage.

Tsunami due to the 2004 September 5th off the Kii peninsula earthquake, Japan, recorded by a new GPS buoy

A tsunami due to the 2004 M7.4 September 5th earthquake off of the Kii peninsula, Japan, was recorded at the GPS buoy newly designed and established in April 2004, about 13 km off of the Muroto Promontory, southwestern Japan. The tsunami arrived at the buoy about 8 minutes before its arrival to the nearest tide gauge station at the tip of the Muroto Promontory. The predicted tsunami record based on preliminary source model shows excellent agreement (rms 2.7 cm) with the observed GPS record. This demonstrates that GPS buoy observations can be used not only for a tsunami warning system, but also for precise determination of the tsunami source.

Tsunami source of the 2004 off the Kii Peninsula earthquakes inferred from offshore tsunami and coastal tide gauges

Tsunamis from the 2004 off the Kii Peninsula earthquakes (M 7.1 and 7.4) were recorded on offshore tsunami gauges, a GPS tsunami gauge and eight bottom-pressure gauges, as well as coastal tide gauges located south of Honshu and Shikoku. The maximum amplitudes on the GPS and bottom-pressure gauges were several to ten cm, while those on tide gauges were up to 0.9 m. We first computed tsunami waveforms from the earthquake source models proposed Yamanaka (2004) and Yagi (2004) from seismic waveform analysis, and compared them with the observed waveforms. For the first event (foreshock), both models produce similar waveforms with the observations. For the second event (mainshock), the waveforms computed from the Yamanaka model is closer to the observed waveforms, but there are still discrepancies between the observed and computed waveforms. We then performed tsunami waveform inversions to estimate the water height distributions in the source area. The foreshock source is ≈1600 km2 with the maximum water height of 0.2 m. The estimated tsunami source area for the mainshock, ≈3600 km2 with the maximum of 0.6 m, extends ≈60 km toward northwest and ≈40 km southwest from the epicenter along the aftershock distribution, suggesting that multiple faulting was involved in the mainshock.

Investigation of Tsunami-Induced Damage and Fragility of Buildings in Thailand after the December 2004 Indian Ocean Tsunami

The December 2004 Indian Ocean tsunami caused human loss and devastating damage to civil engineering structures along the west coast of southern Thailand. This damage was investigated to evaluate the vulnerability of structures. Criteria for post-tsunami investigation were developed, and a database system was established to manage and present the data. The database of reinforced concrete (RC) buildings was then analyzed to find the relationship between the damage level and (a) the distance from the shoreline or (b) the inundation height. A representative RC columns capacity to resist lateral forces was measured by a full-scale loading test to gain insight into the relationship between the observed damage and the actual structural performance.

Tsunami run-up heights of the 2003 Tokachi-oki earthquake

Tsunami height survey was conducted immediately after the 2003 Tokachi-oki earthquake. Results of the survey show that the largest tsunami height was 4 m to the east of Cape Erimo, around Bansei-onsen, and locally at Mabiro. The results also show that the tsunami height distribution of the 2003 Tokachi-oki earthquake is clearly different from that of the 1952 Tokachi-oki earthquake, suggesting the different source areas of the 1952 and 2003 Tokachioki earthquakes. Numerical simulation of tsunami is carried out using the slip distribution estimated by Yamanaka and Kikuchi (2003). The overall pattern of the observed tsunami height distribution along the coast is explained by the computed ones although the observed tsunami heights are slightly smaller. Large later phase observed at the tide gauge in Urakawa is the edge wave propagating from Cape Erimo along the west coast of the Hidaka area.

Extraction of Tsunami-Flooded Areas and Damaged Buildings in the 2011 Tohoku-Oki Earthquake from TerraSAR-X Intensity Images

The Tohoku earthquake of 11 March 2011 caused very large tsunamis and widespread devastation. Various high-resolution satellites captured details of affected areas and were utilized in emergency response. In this study, high-resolution pre- and post-event TerraSAR-X intensity images were used to identify tsunami-flooded areas and damaged buildings. Since water surface generally shows very little backscatter, flooded areas could be extracted by the difference of backscattering coefficients between the pre- and post-event images. Impacted buildings were detected by calculating the difference and correlation coefficient within the outline of each building. The damage estimates were compared with visual interpretation results, which suggest that the overall accuracy of the proposed method for flooded areas was 80%, and for damaged buildings was 94%. Since the proposed half-automated method takes less processing time and is applicable to various cases, it is expected to provide quick and useful information in emergency management.

EFFECTS OF THE RUPTURE VELOCITY OF FAULT MOTION, OCEAN CURRENT AND INITIAL SEA LEVEL ON THE TRANSOCEANIC PROPAGATION OF TSUNAMI

Numerical simulation of tsunami is an effective method to reproduce what has occurred in the past and to predict future events for many tsunami-related research issues including warning systems. However, some real phenomena have not been fully integrated into numerical simulations for transoceanic tsunamis such as fault dynamics of rupture velocity, ocean currents, and the initial sea level. Considering the 2004 Indian Ocean tsunami event, this study evaluates the consequences of rupture velocity. Subsequently, numerical experiments were conducted to normalize the effects as represented by non-dimensional parameters. The rupture velocity, ocean current, and initial sea level were simplified to be uniform and common among simulations. Results of the experiment show that the sea depth along the propagation direction, distance from the tsunami source, rupture velocity, and initial sea level impart some considerable effects on a tsunamis arrival time and wave height. Nevertheless, ocean currents have almost no importance for the arrival time or wave height of oceanic propagation of tsunamis.