Abstract: 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. The Japanese government estimates that reoccurrence of any of these shocks today would cost about a trillion dollars, exceeding the Japanese annual budget, of which less than 10% is insured. Fueled by greater Tokyo’s rich seismic data trove but challenged by its magnificent complexity, our joint Japanese-U.S group carried out a new study of the capital’s earthquake hazards.

We used the prehistoric record of great earthquakes preserved by uplifted marine terraces and tsunami deposits (17 M~8 shocks in the past 7,000 years), a newly digitized dataset of historical shaking (10,000 intensity observations in the past 400 years), the dense modern seismic network (300,000 earthquakes in the past 30 years), and the world’s best geodetic array (150 GPS vectors spanning the past 10 years) to reinterpret the tectonic structure, identify major active faults and their slip rates, and estimate their earthquake frequency. We propose that a dislodged block of the Pacific plate is jammed between the Pacific, Philippine Sea and Eurasian plates beneath Tokyo. We suggest that the block controls much of Tokyo’s seismic behavior for large (M≤7.5) shocks, including the damaging 1855 M~7.3 Ansei-Edo earthquake. On the basis of frequency-magnitude curves for the seismicity beneath greater Tokyo, earthquakes similar to the Ansei-Edo event should be quite frequent, with a ~26% likelihood in an average 30-yr period, and so such events dominate the combined probabilities. In contrast, our renewal (timedependent) probability for the great M ≥7.9 plate boundary shocks such as struck in 1923 is 0.5% for the next 30 yr, with a time-averaged 30-yr probability of ~9%. The resulting net likelihood for severe shaking (0.75-1.0g) in Tokyo, Kawasaki, and Yokohama for the next 30 years is ~33%.

But how can this—or any—such forecast be validated? The long historical record in Kanto affords a rare opportunity to calculate the probability of shaking in an alternative manner, based almost exclusively on 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 for an average 30-yr period is ~35% in Tokyo, Kawasaki, and Yokohama, and ~10% in Chiba, in good agreement with our independent estimate, and thus bolstering our view that Tokyo’s hazard looms large.

Figure caption: Peak intensities observed during the past 400 years. Observed intensity distribution for (b) 1923 M=7.9 Kanto (c) 1855 M~7.4 Ansei-Edo, and (d) 1703 M~8.2 Genroku shocks [Bozkurt, et al., in prep.], together with our inferred seismic sources for these three earthquakes.