Abstract We have developed a new hybrid numerical method for 3-D viscoelastic modelling of seismic wave propagation and earthquake motion in heterogeneous media. The method is based on a combination of the fourth-order velocity–stress staggered-grid finite-difference (FD) scheme, that covers a major part of a computational domain, with the second-order finite-element (FE) method which can be applied to one or several relatively small subdomains. The FD and FE parts causally communicate at each time level in the FD–FE transition zone consisting of the FE Dirichlet boundary, FD–FE averaging zone and FD Dirichlet zone. The implemented FE formulation makes use of the concept of the global restoring-force vector which significantly reduces memory requirements compared to the standard formulation based on the global stiffness matrix. The realistic attenuation in the whole medium is incorporated using the rheology of the generalized Maxwell body in a definition equivalent to the generalized Zener body. The FE subdomains can comprise extended kinematic or dynamic models of the earthquake source or the free-surface topography. The kinematic source can be simulated using the body-force term in the equation of motion. The traction-at-split-node method is implemented in the FE method for simulation of the spontaneous rupture propagation. The hybrid method can be applied to a variety of problems related to the numerical modelling of earthquake ground motion in structurally complex media and source dynamics.

Abstract Numerical modelling of seismic motion in sedimentary basins often has to account for P -wave to S -wave speed ratios as large as five and even larger, mainly in sediments below groundwater level. Therefore,we analyse seven schemes for their behaviour with a varying P -wave to S -wave speed ratio. Four finite-difference (FD) schemes include (1) displacement conventional-grid, (2) displacement-stress partly-staggeredgrid, (3) displacement-stress staggered-grid and (4) velocity–stress staggered-grid schemes. Three displacement finite-element schemes differ in integration: (1) Lobatto four-point, (2) Gauss four-point and (3) Gauss one-point. To compare schemes at the most fundamental level, and identify basic aspects responsible for their behaviours with the varying speed ratio, we analyse 2-D second-order schemes assuming an elastic homogeneous isotropic medium and a uniform grid. We compare structures of the schemes and applied FD approximations. We define (full) local errors in amplitude and polarization in one time step, and normalize them for a unit time. We present results of extensive numerical calculations for wide ranges of values of the speed ratio and a spatial sampling ratio, and the entire range of directions of propagation with respect to the spatial grid. The application of some schemes to real sedimentary basins in general requires considerably finer spatial sampling than usually applied. Consistency in approximating first spatial derivatives appears to be the key factor for the behaviour of a scheme with respect to the P -wave to S -wave speed ratio.

Abstract We present an algorithm of the spatial discontinuous grid for the 3-D fourth-order velocity–stress staggered-grid finite-difference modelling of seismic wave propagation and earthquake motion. The ratio between the grid spacing of the coarser and finer grids can be an arbitrary odd number. The algorithm allows for large numbers of time levels without inaccuracy and eventual instability due to numerical noise inevitably generated at the contact of two grids with different spatial grid spacings. The key feature of the algorithm is the application of the Lanczos downsampling filter. The algorithm of the discontinuous grid is directly applicable also to the displacement-stress staggered-grid finite-difference scheme.