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Aspen Opinion | 1 ASPEN OPINION TSUNAMI RISK: THE COASTAL REGION OF THE CHINA SEA KIRSTY STYLES - EARTHQUAKE RISK SCIENTIST Kirsty Styles PhD welcomes the advances made in probabilistic tsunami hazard assessment since the Tohoku quake but admits earth scientists still have many challenges to overcome. The largest earthquakes occur at subduction zones which, by their submarine nature, are difficult to monitor and analyze. Understanding the incidence of tsunamis is even more complex but is vital to progress given the significant threat posed to areas including the Chinese coastal cities. View this article online at www.aspen.co Where in the world? After a big quake, especially one that caused loss of life and damage to critical infrastructure, it is natural to ask where else in the world this could happen. The Tohoku quake and tsunami in 2011 emphasized the relevance of this question and forced earth scientists to look at subduction zones in a new light. The earthquake and tsunami hazard posed by the Japan Trench, the subduction zone that ruptured during the Tohoku earthquake, had been significantly underestimated by earth scientists. The region was not thought to be capable of producing a megathrust earthquake with a magnitude exceeding 8.4 given the age and type of subduction zone. The event highlighted the serious consequences that can occur from a scarcity of data and not understanding the long-term earthquake potential of subduction zones. Estimated direct damage costs from the Tohoku disaster total ¥16.9 trillion (US$211 billion) including ¥2.9 trillion in insurance payouts (Kajitani et al., 2013). In addition, response and recovery costs are estimated at ¥17.7 trillion.1 The Tohoku quake is the costliest natural disaster in economic terms on record and earth scientists do not want to make the same underestimation again. Under the sea Most tsunamis are generated by earthquakes occurring at subduction zones. A subduction zone or trench is a boundary between two tectonic plates where a heavier oceanic plate plunges beneath a lighter continental plate. The largest 1 2 Kajitani, Y., S, E. Chang, and H. Tatano (2013) http://walrus.wr.usgs.gov/tsunami/workshop/questions.html earthquakes occur at subduction zones. As the plates converge, stress builds up and eventually they slip. The overriding plate springs up, abruptly deforming the seafloor and displaces the overlying water, generating a tsunami. There are 55,000 kilometers of subduction margins on Earth (Beck et al., 2014), but it is not fully understood how many of them move or what hazards they pose. This is because subduction zones are complex systems that cannot be observed directly. Data has to be collected for a greater understanding of these plate movements but this is dependent on the installation of an extensive system of seismometers and other geophysical instruments which are often not financially or practically (given the geography) possible. A US geological survey tsunami source workshop held in 2006 identified subduction zones that had perhaps been under-characterized or overlooked.2 Four such subduction zones (see Figure 1) in the western Pacific were identified (Kirby et al., 2006): the Manila Trench facing the South China Sea and the North Sulawesi subduction zone in the Celebes Sea; the Manus subduction zone north east of New Guinea; the southern Ryukyu subduction zone near Taiwan; and the western Aleutian subduction zone which runs along the southern coastline of Alaska and the Aleutian islands. Of these four, the Manila and Ryukyu trenches are of particular interest to the (re)insurance industry because of the significant tsunami threat posed to Chinese coastal cities such as Shanghai, Hong Kong and Macao (Liu et al., 2009). Aspen Opinion | 2 Figure 1: Asia Pacific Ocean Trenches Source: Catastrophe Risk Management, Aspen Re Data dearth The length of historic earthquake and tsunami records is one of the main obstacles in estimating seismic and tsunami hazard. These are often too short. For example, no earthquake greater than magnitude 7.6 has been observed along the Manila Trench in the past 100 years. This does not mean that a quake of greater magnitude cannot happen in this area, but the size and number are difficult to predict with such a short historic record. The common method used for estimating both these occurrences in a particular region is based on the Gutenberg-Richter relationship (Gutenberg and Richter, 1944) which expresses the total number of earthquakes of a particular magnitude or larger that will occur in a given region and time period. This relationship is more reliable the longer the earthquake record. An earlier study of tsunami hazard in the South China Sea (Liu et al., 2009) extrapolated the Gutenberg-Richter relationship and suggested that the possibility of a magnitude 8.0 or higher earthquake occurring in the Manila Trench is very low. Jing et al. (2013) revisited the problem in light of the Tohoku earthquake and found from analyzing other subduction zones around the world that without a long historic earthquake record, extrapolation of the GutenbergRichter relationship is uncertain and unreliable. They found that the return periods of huge subduction zone earthquakes may be shorter than previously estimated. A further uncertainty is whether or not a particular earthquake is likely to generate a tsunami. The occurrence of a tsunami is based on the physical shape of the seafloor that deforms as well as the magnitude of the earthquake. In other words, the size of the tsunami is not directly related to the magnitude of the earthquake. This is a problem facing many vendor catastrophe modelers as they begin to develop and release their first ever tsunami risk models. Probabilistic tsunami hazard assessment is a young science and needs further development. Worst case One way of assessing potential tsunami hazard in a region is to look at worst case scenarios. Jing et al. (2013) simulated several hypothetical earthquake and tsunami scenarios along the Manila and Ryukyu trenches. For example, they estimated how much water would be displaced by a magnitude 8.8 earthquake occurring at 24km depth and rupturing a region of seafloor that measures 300km by 100km. They modelled how the ensuing wave would travel across the South China and East China Seas and how it would inundate China’s coast. They found that the potential surge height could reach a couple of meters. Based on their simulations of a magnitude 8.8 quake in the Manila Trench, the highest tsunami wave would reach 5.1m when it hits Zhenjiang and Guangdong provinces. A magnitude 8.8 quake in the Ryukyu Trench could create a wave height of 6.2m on arrival in Shanghai. Model benefits This group of scientists is planning to extend this scenario work using different locations and magnitudes. The results will be of particular interest to the (re)insurance industry as both vendor catastrophe models and proprietary underwriter models continue to embrace the complicated science of probabilistic tsunami hazard assessment. Aspen’s R&D team has developed tsunami models for the Cascadia subduction zone and Japan which have been instrumental in structuring pricing and contracts. Further developments in tsunami modelling will bring additional benefits to underwriters and the architects of aid and disaster response initiatives. References Beck, S., A. Rietbrock, F. Tilmann, S. Barrientos, A. Meltzer, O. Oncken, K. Bataille, S. Roecker, J.-P. Vilotte and R. M. Russo (2014), Advancing Subduction Zone Science After a Big Quake, Eos Trans. AGU, 95(23), 193. Gutenberg, R., and C.F. Richter, (1944). Frequency of earthquakes in California, Bulletin of the Seismological Society of America, 34, 185-188. Jing, H. H., H. Zhang, D. A. Yuen, and Y. Shi (2013) A Revised Evaluation of Tsunami Hazards along the Chinese Coast in View of the Tohoku-Oki Earthquake, Pure Appl. Geophys. 170 (2013), 129–138, DOI 10.1007/s00024-012-0474-8 Kajitani, Y., S, E. Chang, and H. Tatano (2013) Economic Impacts of the 2011 Tohoku-Oki Earthquake and Tsunami. Earthquake Spectra: March 2013, Vol. 29, No. S1, pp. S457-S478. Kirby, S., E. Geist, W. H. K. Lee, D. Scholl and R. Blakely (2006), Tsunami Source Characterization for Western Pacific Subduction Zones, USGS Tsunami Subduction Source Working Group, USGS Tsunami Sources Workshop 2006: Great earthquake tsunami sources: Empiricism & Beyond, April 21-22, 2006. Liu, P. L. F., X. Wang and A. J. Salisbury (2009). Tsunami hazard and early warning system in South China Sea. Journal of Asian Earth Sciences 36(1): 2-12, doi:10.1016/j.jseaes.2008.12.010.