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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
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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.