## Accuracy of the Explicit Planar Free-Surface Boundary Condition Implemented in a Fourth-Order Staggered-Grid Velocity-Stress Finite-Difference Scheme

Ellen Gottschaemmer, & Kim B. OlsenPublished June 2001, SCEC Contribution #555

We compute the accuracy of two implementations of the explicit planar free-surface boundary condition for 3-D fourth-order velocity-stress staggered-grid finite-differences, 1/2 grid apart vertically, in a uniform halfspace. Due to the staggered grid, the closest distance between the free surface and some wavefield components for both implementations is 1/2 grid spacing. Overall, the differences in accuracy of the two implementations are small. When compared to a reflectivity solution computed at the staggered positions closest to the surface, the total misfit for all three components of the wavefield is generally found to be larger for the free surface co-located with the normal stresses, compared to that for the free-surface co-located with the xz and yz stresses. However, this trend is reversed when compared to the reflectivity solution exactly at the free surface (the misfit encountered in staggered-grid modeling). When the wavefield is averaged across the free surface, thereby centering the staggered wavefield exactly on the free surface, the free surface condition co-located with the xz and yz stresses generates the smallest total misfit for increasing epicentral distance. For an epicentral distance/hypocentral depth of 10 the total misfit of this condition is about 15% smaller than that for the condition co-located with the normal stresses, mainly controlled by the misfit on the Rayleigh wave.

**Key Words**

surface waves, fourth order, three-dimensional models, guided waves, finite difference analysis, stress, statistical analysis, half-space, propogation, elastic waves, seismic waves, Rayleigh waves

**Citation**

Gottschaemmer, E., & Olsen, K. B. (2001). Accuracy of the Explicit Planar Free-Surface Boundary Condition Implemented in a Fourth-Order Staggered-Grid Velocity-Stress Finite-Difference Scheme. *Bulletin of the Seismological Society of America*, 91(3), 617-623. doi: 10.1785/0120000244.