The earthquake threat to Tokyo

Tokyo and its outlying cities are home to one-quarter of Japan’s 127 million people. Highly destructive earthquakes struck the capital in 1703, 1855 and 1923, the last of which took 105,000 lives. Today, the population of greater Tokyo is six times larger than it was in 1923, so what are the consequences of such a quake if it were to strike now? The Cabinet Office of the Japanese government recently considered the impact of an M=7.2 earthquake beneath Tokyo, similar to the 1855 Ansei-Edo shock. If the earthquake struck during rush hour on a windy evening, the government estimates 11,000 deaths, 210,000 wounded, 840,000 destroyed buildings and 100 million tons of wreckage. More than half of the deaths and three-quarters of the housing collapses would be caused by fire. The projected cost the earthquake is $1.0 trillion (US), which is 130% of the Japanese annual budget. Only about 5% of this loss is insured, and so the burden would be borne principally by home and business owners and the government. Global financial markets would not be unaffected. For example, Japan is the largest foreign owner of U.S. Treasuries, holding 17% or $700 billion. The effect of a large sudden Treasury withdrawal on the US economy is unknown.

The Project goals

Given the enormous human, cultural, and financial cost of such a catastrophe, understanding its likelihood becomes imperative. Thus, our objective is an earthquake hazard understanding and forecast capable of explaining the salient features of the historical earthquake occurrence and land deformation. Our model will therefore be rigorously tested against the 400-year-long record of large earthquakes in and around Tokyo; the distribution and rate of contemporary seismicity; and GPS displacements at the earth’s surface. We will then use the model to develop time-dependent probabilistic forecasts for earthquake occurrence during the next years to decades. This work will be fully vetted by scientists in Japan, the U.S., and Europe.

The USGS-AFRC-Swiss Re collaboration

Swiss Re insures primary insurance companies throughout the world against earthquake losses that exceed the capacity of the primary carriers. The USGS, a federal science agency that assesses the impact of natural hazards and resources on society, has developed new tools to gauge the likelihood of future earthquakes. The Active Fault Research Center of Japan’s Advanced Institute of Science and Technology possesses a comprehensive knowledge of Tokyo’s seismic setting. Under a Cooperative Research & Development Agreement, the three partners launched an international collaboration to develop a comprehensive description and understanding of earthquake occurrence in greater Tokyo. All results are in the public domain; nothing is proprietary. Ross Stein (USGS) and Shinji Toda (AFRC) are the Principal Investigators. Martin Bertogg is the principal from Swiss Re; Swiss Re scientists from Munich, New York, and Tokyo were also deeply involved. We were also joined by young scientists from several of Japan’s leading government science agencies and universities. Collectively, we call ourselves ‘Team Tokyo.’

The Project results

Our findings were presented at scientific conferences and universities in Tokyo, San Francisco, New York, Zurich, and London, and at a bilingual seminar for Swiss Re’s Japanese clients in June 2005. Team Tokyo now has fifteen peer-reviewed studies submitted, in press, or published in international scientific journals of the highest caliber.

We used the prehistoric record of great earthquakes preserved by uplifted marine terraces and tsunami deposits (including seventeen M~8 shocks in the past 7,000 years), a newly digitized dataset of historical shaking (10,000 shaking observations in the past 400 years), the dense modern seismic network (300,000 earthquakes recorded in the past 30 years), and Japan’s GeoNet array of 150 GPS stations recording over the past 10 years) to reinterpret the tectonic structure, identify active faults and their slip rates, and estimate their earthquake frequency.

We propose that a dislodged fragment of the Pacific plate is jammed between the Pacific plate, the Philippine Sea plate and Eurasian plate beneath the Kanto plain on which Tokyo sits. We suggest that the Kanto fragment controls much of Tokyo’s seismic behavior for large earthquakes, including the damaging 1855 M~7.3 Ansei-Edo shock. On the basis of the frequency of earthquakes beneath greater Tokyo, events with magnitude and location similar to the M~7.3 Ansei-Edo event have a ~20% likelihood in an average 30-year period.

In contrast, our renewal or time-dependent probability for the great M>=7.9 shocks such as struck in 1923 and 1703 is 0.5% for the next 30 years, with a time-averaged 30-year probability of ~10%. The resulting net likelihood for severe shaking (~0.9 g peak ground acceleration) in Tokyo, Kawasaki, and Yokohama for the next 30 years is ~30%.

The long historical record in Kanto also affords a rare opportunity to calculate the probability of shaking in an alternative manner exclusively from intensity observations. This approach permits robust estimates for the spatial distribution of expected shaking, even for sites with few observations. The resulting probability of severe shaking is ~35% in Tokyo, Kawasaki, and Yokohama, and ~10% in Chiba for an average 30-year period, in good agreement with our independent estimate, and thus bolstering our view that Tokyo’s hazard looms large. Given the $1 trillion estimate for the cost of an M~7.3 shock beneath Tokyo, our probability implies a $13 billion annual probable loss.

Contributions of Team Tokyo to advance earthquake science

• New plate tectonic source model
• New Kanto seismic fault slip model
• New catalog of historic M>6.7 quakes
• New Kanto intensity attenuation model
• New source models for 1923 & 1703 quakes
• Vastly augmented geodetic data for 1923 shock
• New compilation of 10,000 intensities since 1600
• New method to measure quake frequency & periodicity