Preliminary estimation of source parameters and site amplifications for the 2018 Mw 5.5 north Osaka earthquake in southwest Japan

The 2018 Mw 5.5 north Osaka earthquake occurred on 18 June, 07:58 local time. The JMA magnitude was 6.1 and focal depth13 km. Strong-motions were recorded at 121 stations within 100 km by K-NET and KiK-net. The large data set provided an opportunity to compare with the values from ground motion prediction equations (GMPEs) at short distances. The maximum horizontal acceleration reached approximately 0.8 g (794 cm/s/s) at the nearest station, and caused loss of human lives and significant damages to properties. A comparison of the PGAs, PGVs, and response spectra at 0.1 to 10 s with the GMPEs showed that the observed values were systematically larger than those from the GMPEs except at periods larger than about 4-5 s. We found that the PGAs and PGVs from the earthquake were very similar to those of the 13 April, 2013 Mw 5.8, focal depth 15 km, Awaji island area crustal earthquake, and were comparable to those of the relatively deeper intraslab event of 31 October, 2000 (focal depth 43 km, Mw 5.5) in southern Mie prefecture. Previous studies showed that the PGAs and PGVs of intraslab earthquakes had been significantly larger than those of the crustal and interplate events, and higher stress drop is attributed for the larger short period ground motions. Previous studies of crustal events also revealed that the deeper asperities generate stronger ground motions compared with the shallow asperities. Considering the focal depth and size of similar earthquakes in the past, the recorded larger peak ground motions of this event may be attributed to the higher stress drop due to the deeper asperity. We performed spectral inversion of the Fourier spectral amplitudes of recordings within about 110 km using constraints of no site amplifications at five F-net stations installed at tunnels excavated at rocks surrounding the epicenter. We compared the spectral ratios at the KiK-net sites with the inversion results and found that the results are reasonable. The estimated Mw varies between 5.4 to 5.8, and the Brune stress drop between 7 to 15 MPa depending on the constraints. The estimated crack radius is about 1 to 2 km, and corresponds well to the observed distribution of aftershocks within an area of about 4 km x 4km. We concluded that the both site and source caused large strong motions for this earthquake.