Rupture process of the 1948 Fukui earthquake (M 7.1) from the joint inversion of seismic waveform and geodetic data

Abstract

We used teleseismic body waves, regional low‐gain strong motion seismograms, and horizontal geodetic displacements from triangulation surveys to invert for the slip distribution of the 1948 Fukui, Japan, earthquake using the multiple time window method. The earthquake ruptured unilaterally from a depth of 10 km to the north with very little slip (0–0.5 m) evenly distributed across a fault of 24 by 18 km over a period of about 10 s. At 9 s after the initiation of rupture, an adjacent fault along the same strike ruptured to the south from the hypocenter to about 20–30 km toward Fukui city mostly along a shallow asperity between 0 and 12 km depth. The large amount of shallow slip (1–1.5 m) below and within the sedimentary basin and strong rupture directivity likely contributed to the high amount of damage in Fukui city. We obtain a total seismic moment of 1.6–2.2 × 1026 dyn cm (Mw 6.74–6.84) which is similar to results by Kanamori (1973), Sagiya (1999), and Kikuchi et al. (1999) using portions of the same regional strong motion and geodetic data sets. We needed to use very low rupture velocities (1.5 km/s) and a hypocenter 10–20 km north of the original location to fit the combined geodetic and seismic data sets with a single fault model. We developed an alternative fault model with two segments allowing for different rupture dynamics for each segment. This resulted in a slightly better fit to the data and allows for a more reasonable rupture velocity up to 2.4 km/s.