Empirical Relationships Between Ground Motion Intensities and JMA Seismic Intensity Scale

Patrick Ball, Toby Sansom

There is a growing appetite for parametric Japanese earthquake insurance products that use Japanese Meteorological Agency seismic intensity, IJMA, triggers. However, most earthquake risk models used to price these contracts do not compute IJMA directly, instead relying on conversions that introduce additional basis risk. Here, we present empirical parameterisations of IJMA as a function of all combinations of moment magnitude, hypocentral distance, peak ground acceleration (PGA), and spectral acceleration (Sa) at 0.2, 0.3, 0.6, 1.0, 2.0, and 3.0 s, derived from earthquake risk model outputs. Parameterisations are fitted using ridge regression applied to 43,002 ground motion recordings from 111 Japanese earthquakes spanning January 2000 to April 2026. A parsimonious two-parameter model — combining PGA and Sa(1.0 s) — outperforms previously published parameterisations, substantially reducing both uncertainty and bias. When applied to a synthetic parametric insurance portfolio exposed to 25 years of Japanese seismicity, this parameterisation yields a measurable reduction in basis risk. These results support more reliable and transparent pricing of parametric earthquake insurance products.