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Strong Motions on Land and Ocean Bottoms in and Around Kanto Basin From Two Nearby Moderate Earthquakes East Off the Chiba Prefecture

Session: Amphibious Seismic Studies of Plate Boundary Structure and Processes [Poster]

Type: Poster

Date: 4/20/2021

Presentation Time: 04:15 PM Pacific

Description: 

We analyzed strong motions and their durations from two moderate magnitude earthquakes that occurred off the Chiba prefecture in 2018 and 2020, respectively. The two earthquakes occurred in an extremely complicated tectonic area north of the Sagami trough (a triple junction) where the Philippine Sea Plate overrides the Pacific Plate and the both plates subduct beneath the Kanto basin. The earthquakes had Mw values of 5.9 estimated by F-net, NIED. The JMA focal depths of the former and latter events were 56 and 36 km, respectively. The ground motion records on the Kanto basin were taken from the dense MeSO-net, while the ground motions on the ocean bottom were taken from newly established ocean bottom network, S-net. The seismographs of MeSO-net are set up at the depth of 20 m in a borehole. The S-net seismographs are housed inside inline-type cylindrical vessels, which are buried to the depth of about one meter in the shallow water region (< 1500 m) while the vessels are laid freely on the ocean bottom in the deeper region. PGAs, PGVs, and significant durations based on Arias intensity were compared between the two events on the Kanto basin and offshore area. To make a meaningful comparison between the data sets from the two networks, the waveforms at the MeSO-net stations were compared with the waveforms at the nearby K-NET and KiK-net stations. It was found that the waveforms at the 20 m depth were very similar to the waveforms at the nearby surface stations within one kilometer at frequencies lower than about 0.5 to 1.0 Hz. Our comparison at applicable frequencies showed that the PGAs were very similar between the two events at the MeSO-net sites despite the difference of the focal depths, but the PGVs were larger for the shallow event. The PGAs and PGVs were, on average, larger at the ocean bottom sites than those at the land sites for the deeper event. The horizontal component PGAs and PGVs were larger at the ocean bottom sites for the deeper event, while the values for the vertical component were similar. The significant durations between the two earthquakes and the two networks were comparable.

Presenting Author: Yadab P. Dhakal

Student Presenter: No

Authors