Spatial Modeling of Tsunami Vortices in the Coastal Ocean

It is important to plan for potential tsunamis during the marine spatial planning process so that land uses may be modified or defensive infrastructure may be erected. Tsunami vortices had been observed during the occurrence and propagation of tsunami waves. Actual observations during the March 2011 Japan tsunami and the Indian Ocean tsunami of December 2004 showed the formation of vortices which lasted for several hours. The Palu tsunami of September 2018 in Indonesia also showed the formation of a tsunami vortex whose centre was photographed by a pilot and appeared as a deep hole in the ocean. Several vortices with various sizes lasted for several hours after the quake and they also generated a loud roar as the giant waves inundated low-lying coastal areas. This essay attempts to describe the development of a model that can explain the formation of tsunami vortices.

Global Tsunami Risk Assessment: Collaboration Between Industry and Academia in the Willis Research Network (WRN)

We present outcomes of our collaborative research between tsunami engineering laboratory, Tohoku University and the Willis Research Network (WRN) on global tsunami risk assessment since 2010. First we assessed tsunami hazards in Indian Ocean and west Pacific from major earthquakes based on historical records. After the 2011 Japan tsunami, various kind of fragility functions were developed for human casualty, buildings, marine vessels, etc based on the actual data. Especially, detailed tsunami hazard assessments were performed in many areas using fine bathymetry and topography data all over Japan including hazards from the worst case tsunamigenic earthquakes provided by central government and local governments in Hokkaido, Japan Sea and Nankai Trough. These results from the detailed hazard and vulnerability assessment were used for detailed tsunami risk in Japan. The Willis’s Japan tsunami model was then first released in December 2014. The model have been updating based on the updated or revised tsunami sources model and fragility functions. Detailed tsunami hazards from potential tsunami events in the Bay of Bengal, South China Sea and some parts of Indonesia were also performed in 2014. In October 2016, our contribution on the historical and future tsunami hazard assessment in global scale based on historical records over the last 400 years was conducted as an activity to increase tsunami awareness as part of World Tsunami Awareness Day. The current activities are to extend the target areas in Japan to Okinawa and assessing disaster risk reduction based on the present and planned tsunami countermeasures. We present the outcomes of the collaborative research done since 2010 by the Tsunami Engineering Laboratory of Tohoku University and the Willis Research Network (WRN) on global tsunami risk assessment. First, we assessed, based on historical records, the tsunami hazards in the Indian Ocean and western Pacific from major earthquakes. Since the 2011 Japan tsunami, various kinds of fragility functions have been developed for human casualties, buildings, marine vessels, etc., based on the actual data. Detailed tsunami hazard assessments have been performed in many areas of Japan using fine bathymetry and topography data from all over Japan, including data on hazards from the worst-case tsunamigenic earthquakes. These data have been provided by the Cabinet Office, Japan. The results from the detailed hazard and vulnerability assessments were used for detailed tsunami risk assessments in Japan. The Willis Japan tsunami model was then released in December 2014. The model has been updated based on the updated or revised tsunami source model and fragility functions. Detailed tsunami hazards from potential tsunami events in the Bay of Bengal, South China Sea, and some parts of Indonesia were also performed in 2014. In October 2016, our contribution to the historical and future tsunami hazard assessment on a global scale based on historical records over the last 400 years was conducted as an activity to increase tsunami awareness as part of World Tsunami Awareness Day. The current activities are to extend the target areas in Japan to Okinawa and to assess the disaster risk reduction based on the present and planned tsunami countermeasures.

Comparison of Pre-Event VHR Optical Data and Post-Event PolSAR Data to Investigate Damage Caused by the 2011 Japan Tsunami in Built-Up Areas

Combining pre-disaster optical and post-disaster synthetic aperture radar (SAR) satellite data is essential for the timely damage investigation because the availability of data in a disaster area is usually limited. This article proposes a novel method to assess damage in urban areas by analyzing combined pre-disaster very high resolution (VHR) optical data and post-disaster polarimetric SAR (PolSAR) data, which has rarely been used in previous research because the two data have extremely different characteristics. To overcome these differences and effectively compare VHR optical data and PolSAR data, a technique to simulate polarization orientation angles (POAs) in built-up areas was developed using building orientations extracted from VHR optical data. The POA is an intrinsic parameter of PolSAR data and has a physical relationship with building orientation. A damage level indicator was also proposed, based on the consideration of diminished homogeneity of POA values by damaged buildings. The indicator is the difference between directional dispersions of the pre and post-disaster POA values. Damage assessment in urban areas was conducted by using the indicator calculated with the simulated pre-disaster POAs from VHR optical data and the derived post-disaster PolSAR POAs. The proposed method was validated on the case study of the 2011 tsunami in Japan using pre-disaster KOMPSAT-2 data and post-disaster ALOS/PALSAR-1 data. The experimental results demonstrated that the proposed method accurately simulated the POAs with a root mean square error (RMSE) value of 2.761° and successfully measured the level of damage in built-up areas. The proposed method can facilitate efficient and fast damage assessment in built-up areas by comparing pre-disaster VHR optical data and post-disaster PolSAR data.

Vulnerability Characteristics of Tsunamis in Indonesia: Analysis of the Global Centre for Disaster Statistics Database

Regional disaster data are important for understanding the characteristics of disasters and for identifying potential mitigation measures. However, many countries have no official disaster database that includes information such as numbers of deaths or damaged buildings for each disaster event. The Global Centre for Disaster Statistics (GCDS) was established to assist countries and organizations in the collection of disaster data. At present, a significant amount of tsunami disaster data are available from Indonesia, which will be used to demonstrate its application for analyzing vulnerability characteristics of historical tsunamis. There are 53 data points covering 13 tsunami events between the year 1861 and 2014. Based on data availability, five tsunami events, namely the 1977 Sumba, the 2004 Indian Ocean, the 2006 Java, the 2010 Mentawai, and the 2011 Great East Japan, were selected. Numbers of deaths and damaged buildings were used in combination with hazard data to estimate vulnerability, defined as the ratio between maximum flow depth against death and building damage ratios. Numbers of evacuees were initially used to estimate actual numbers of exposed population but it was later discovered that this parameter overestimated the exposed population in certain cases. As a result, this study presents the vulnerability characteristics of people and buildings in Indonesia, exposed to unusual or extreme tsunamis, mostly in a condition without or with limited access to official warnings. In brief, a maximum flow depth of 5 m caused an approximate 100% death ratio in the majority of Indonesian tsunamis in this study. On the other hand, death ratio in the 2011 Japan tsunami was limited to 10% because of the early warning and high disaster awareness. Effective disaster risk reduction activities such as official warnings, evacuations, and tsunami education were observed for certain locations. Lastly, adding hazard and population data at the village level is recommended for improving the collection of future tsunami disaster data for the GCDS database.

Developing fragility functions for aquaculture rafts and eelgrass in the case of the 2011 Great East Japan tsunami

Since the two devastating tsunamis in 2004 (Indian Ocean) and 2011 (Great East Japan), new findings have emerged on the relationship between tsunami characteristics and damage in terms of fragility functions. Human loss and damage to buildings and infrastructures are the primary target of recovery and reconstruction; thus, such relationships for offshore properties and marine ecosystems remain unclear. To overcome this lack of knowledge, this study used the available data from two possible target areas (Mangokuura Lake and Matsushima Bay) from the 2011 Japan tsunami. This study has three main components: (1) reproduction of the 2011 tsunami, (2) damage investigation, and (3) fragility function development. First, the source models of the 2011 tsunami were verified and adjusted to reproduce the tsunami characteristics in the target areas. Second, the damage ratio (complete damage) of the aquaculture raft and eelgrass was investigated using satellite images taken before and after the 2011 tsunami through visual inspection and binarization. Third, the tsunami fragility functions were developed using the relationship between the simulated tsunami characteristics and the estimated damage ratio. Based on the statistical analysis results, fragility functions were developed for Mangokuura Lake, and the flow velocity was the main contributor to the damage instead of the wave amplitude. For example, the damage ratio above 0.9 was found to be equal to the maximum flow velocities of 1.3 m s−1 (aquaculture raft) and 3.0 m s−1 (eelgrass). This finding is consistent with the previously proposed damage criterion of 1 m s−1 for the aquaculture raft. This study is the first step in the development of damage assessment and planning for marine products and environmental factors to mitigate the effects of future tsunamis.

Developing fragility functions for aquaculture rafts and eelgrass in the case of the 2011 Great East Japan tsunami

Since the two devastating tsunamis in 2004 (Indian Ocean) and 2011 (Great East Japan), new findings have emerged on the relationship between tsunami characteristics and damage in terms of fragility functions. Human loss and damage to buildings and infrastructures are the primary target of recovery and reconstruction; thus, such relationships for offshore properties and marine ecosystems remain unclear. To overcome this lack of knowledge, this study used the available data from two possible target areas (Mangokuura Lake and Matsushima Bay) from the 2011 Japan tsunami. This study has three main components: 1) reproduction of the 2011 tsunami, 2) damage investigation and 3) fragility function development. First, the source models of the 2011 tsunami were verified and adjusted to reproduce the tsunami characteristics in the target areas. Second, the damage ratio of the aquaculture raft and eelgrass was investigated using satellite images taken before and after the 2011 tsunami through visual inspection and binarization. Third, the tsunami fragility functions were developed using the relationship between the simulated tsunami characteristics and the estimated damage ratio. Based on the statistical analysis results, fragility functions were developed for Mangokuura Lake, and the flow velocity was the main contributor to the damage instead of the wave amplitude. For example, the damage ratio above 0.9 was found to be equal to the maximum flow velocities of 1.3 m/s (aquaculture raft) and 3.0 m/s (eelgrass) This finding is consistent with the previously proposed damage criterion of 1 m/s for the aquaculture raft. This study is the first step in the development of damage assessment and planning for marine products and environmental factors to mitigate the effects of future tsunamis.