Theoretical modeling and numerical simulation of seismic motions at seafloor

This paper proposes a modeling and simulation method of seafloor seismic motions on offshore sites, which are composed of the base rock, the porous soil layers and the seawater layer, based on the fundamental hydrodynamics equations and one-dimensional wave propagation theory. The base rock motions are assumed to consist of P- and S-waves and are modeled by the seismological model in southwest of Western Australia (SWWA). The transfer functions of the offshore site are calculated by incorporating the derived dynamic-stiffness matrix of seawater layer into the total stiffness matrix. The effect of water saturation on the P-wave velocity and Poisson's ratio of subsea soil layers are considered in the model. Both onshore and seafloor seismic motions are stochastically simulated. The comparison results show that the seafloor vertical motions are significantly suppressed near the P-wave resonant frequencies of the upper seawater layer, which makes their intensities much lower than the onshore vertical motions. The simulated seafloor motions are in compliance with the characteristics of available seafloor earthquake recordings and can be used as inputs in the seismic analyses of offshore structures.