Characteristics of Earthquake Ground Motion on the Seafloor

Abstract

The differences between onshore and offshore ground motions were studied based on the records from the K-net and SEMS. The horizontal response spectrum and vertical-to-horizontal response spectral ratio for offshore and adjacent onshore ground motions were analyzed. The results indicate that the characteristic period of the response spectra for offshore ground motions is obviously larger. The epicentral distance can influence the horizontal response spectra for offshore ground motions in a moderate earthquake, while the influence of water depth is not noticeable. Moreover, the vertical-to-horizontal response spectral ratio for offshore ground motions is significantly smaller for periods less than 1 s.

A prediction model for vertical-to-horizontal spectral ratios of ground motions on the seafloor for moderate magnitude events for the Sagami Bay region in Japan

“…The response spectra of the vertical offshore motion are much lower than those of the horizontal offshore motion mainly due to the effect of water layer (Boore and Smith 1999;Chen et al 2015Chen et al , 2017Diao et al 2014). The behaviors of median V/ H ratios as functions of independent variables are investigated in Fig.…”

“…Earthquake ground motions on the seafloor differ from onshore ground motions in vertical components and vertical-to-horizontal (V/H) response spectral ratios (Chen et al 2015). The characteristics of offshore ground motion have been investigated by some investigators, and vertical offshore ground motion values have been found to be much lower than those of horizontal offshore ground motion (e.g., Boore and Smith 1999;Chen et al 2017;Diao et al 2014;Li et al 2015); thus, the vertical-to-horizontal peak ground acceleration (PGA) and spectral ratios of offshore ground motions are considerably different from those of onshore ground motions.…”

“…Li et al (2015) proposed a method for modeling and simulating the offshore ground motion, and their results showed that the vertical ground motions on the seafloor are suppressed near the P-wave resonant frequencies. Chen et al (2015Chen et al ( , 2017 selected eight earthquakes recorded by the Kyoshin network (K-NET) to analyze horizontal normalized response spectra and V/H response spectral ratios using 36 offshore records from six offshore stations throughout the Sagami Bay and 31 onshore records from eight onshore stations adjacent to those offshore stations. Their results revealed that the normalized response spectra and V/H response spectral ratios of the offshore ground motions were different from those of the onshore motions.…”

Modeling and Simulation of Spatially Correlated Ground Motions at Multiple Onshore and Offshore Sites

“…The vertical-to-horizontal response spectral ratio (V/H RS ratio) is commonly employed to assess the difference between onshore and seafloor motions by researchers [Boore and Smith, 1999;Chen et al, 2015]. This is because it can eliminate the various factors on the seismic motions, such as earthquake magnitude, faulting mode, distance to epicenter and wave propagation path, except the effect of local geology of onshore and offshore sites [Boore and Smith, 1999].…”

“…They found that the seafloor vertical response can be significantly reduced near the resonant frequencies of P-waves in the water layer. More straightforward investigations were carried out by researchers through the statistical analyses of seafloor seismic data recorded by subsea earthquake stations, e.g., the Seafloor Earthquake Measurement System (SEMS) off the coast of Southern California, USA [Sleefe, 1990;Boore and Smith, 1999;Diao et al, 2014] and the strong-motion network (K-net) in Japan [Aoi et al, 2004;Okada et al, 2004;Chen et al, 2015]. It is revealed that the vertical component of seafloor motions are much lower than those of the onshore motions, especially at short periods, owing to the substantial reduction of the seafloor vertical motions near the P-wave resonant frequencies of the overlying seawater layer.…”