A comparison of absolute velocity response spectra (1- 10 s) at S-net ocean bottom sites in the Japan Trench area and land sites in northeast Japan for two strong earthquakes

Abstract:

The Seafloor observation network for earthquake and tsunamis along the Japan Trench, named as S-net, was established in 2016. The network spans over a wide area off the coast of Honshu and Hokkaido islands in northeast Japan and consists of 150 seafloor observatories equipped with seismometers and pressure gauges. The largest magnitude earthquake recorded by S-net after the start of its operation was the Nov 22, 2016, Mw 7.0 event that occurred off the Fukushima Prefecture. The second-largest earthquake that occurred nearby the network was Sep 6, 2018, Mw 6.6, Hokkaido eastern Iburi earthquake, an inland earthquake. Dhakal et al. (2015) constructed ground motion prediction equation (GMPEs) for absolute velocity response spectra (AVRS) in the period range of 1 to 10 s using 12,401 strong-motion records from 36 events of Mw >= 6.5 that were recorded by K-NET and KiK-net stations. In the present study, we compared the AVRS in the period range of 1 to 10 s between the land sites of K-NET and KiK-net and ocean bottom sites of S-net using the GMPEs of Dhakal et al. (2015) for the abovementioned two earthquakes. One of the crucial factors that strongly affect the observed values in the above period range is the deep sediments above the seismic bedrock. Dhakal et al. (2015) derived site coefficients using the depths of Vs 1.4 km/s layer from Japan Seismic Hazard Information Station (J-SHIS). The Vs 1.4 km/s layer was selected because it produced the smallest standard deviation compared to those using the depth of other low or high-velocity layers. The J-SHIS model revealed that the S-net sites are underlain by thick sediments distributed widely above the sub-oceanic seismic bedrock. In contrast, the similar thick deposits are present at limited basins on land. As a result, we found that the observed values at the S-net sites were, on average, larger than those at the land sites in the analyzed period range. However, when the comparison was limited to the sites such as those underlain by greater than about 1 km thick sediments above the Vs layer of 1.4 km/s, the observed values at the land and ocean bottom sites were comparable. An improvement of the velocity model in the offshore region helps better understand the strong-motions in the offshore region for earthquake early warning.