Anomalously large seismic amplifications in the seafloor area off the Kii peninsula

Abstract

Seismic wave amplifications were investigated using strong-motion data obtained from the ground's surface (K-net) on the Kii peninsula (southwestern Japan) and from the network of twenty seismic stations on the seafloor (DONET) located off the peninsula near the Nankai trough. Observed seismograms show that seismic signals at DONET stations are significantly larger than those at K-net stations, independent of epicentral distances. In order to investigate the cause of such amplifications, seismic wavefields for local events were simulated using the finitedifference method, in which a realistic 3D velocity structure in and around the peninsula was incorporated. Our simulation results demonstrate that seismic waves are significantly amplified at DONET stations in relation to the presence of underlying low-velocity sediment layers with a total thickness of up to 10 km. Our simulations also show considerable variations in the degree of amplification among DONET stations, which is attributed to differences in the thickness of the sediment layers. The degree of amplification is relatively low at stations above thin sediment layers near the trough axis, but seismic signals are much more amplified at stations closer to the Kii peninsula, where sediment layers are thicker than those at the trough axis. Simulation results are consistent with observations. This study, based on seafloor observations and simulations, indicates that because seismic signals are amplified due to the ocean-specific structures, the magnitude of earthquakes would be overestimated if procedures applied to data observed at land stations are used without corrections.

Long-period ocean-bottom motions in the source areas of large subduction earthquakes

“…Our simulation results for an offshore event also indicate the same features for the lateral variation at and near the ocean-bottom stations ( Figure S12a and Supplementary Movie 2 ). Recent studies 28 29 have shown that amplification at ocean-bottom stations can result in overestimation of displacement-amplitude magnitudes of earthquakes by ~0.5–0.6, compared with catalogued magnitudes or those estimated from land station data. Because in larger earthquakes, longer-period seismic waves are radiated and are dominantly observed at near-field stations, the amplified long-period motions at ocean-bottom stations in large subduction earthquakes could cause a larger overestimation for magnitude.…”

Scaled Energy Estimation for Shallow Slow Earthquakes

“…In contrast to deep slow earthquakes, the scaled energy of shallow slow earthquakes has not yet been evaluated. Because seismic waves observed by ocean bottom seismometers are significantly affected by soft sediment on the ocean floor (Kubo et al, ; Nakamura et al, , ), careful evaluation must include corrections for site amplification to accurately estimate the seismic energy.…”

Observations of Seismicity and Ground Motion in the Northeast U.S. Atlantic Margin from Ocean‐Bottom Seismometer Data

“…Although there have been studies such as the site amplification near the Nankai trough (Nakamura et al, 2014) and Scholte wave dispersion and waveform modeling at the Ninetyeast ridge (Nguyen et al, 2009), we are not aware of similar studies done at the continental margin. The clustering of 11 instruments within 1-2 km of the seafloor during DEPL1 (Fig.…”