SEA99: A revised ground motion prediction relation for use in extensional tectonic regimes

We present SEA99, a revised predictive relation for geometric mean horizont al peak ground acceleration and 5%-damped pseudovelocity response spectrum, appropriate for estimating earthquake ground motions in extensional tecton ic regimes, which we demonstrate to have lower ground motions than other te ctonic regimes. SEA99 replaces SEA96, a relation originally derived by Spud ich cr al. (1996, 1997). The data set used to develop SEA99 is larger than that for SEA96, and minor errors in the SEA96 data set have been corrected. In addition, a one-step regression method described by Joyner and Boore (1 993, 1994) was used rather than the two-step method of Joyner and Boore (19 81). SEA99 has motions that are as much as 20% higher than those of SEA96 a t short distances (5-30 km), and SEA99's motions are about 20% lower than S EA96 at longer periods (1.0-2.0 sec) and larger distance (40-100 km). SEA99 dispersions are significantly less than those of SEA96. SEA99 rock motions are on the average 20% lower than motions predicted by Boore et al. (1994) except for short distances at periods around 1.0 sec, where SEA99 motions exceed those predicted by Boore et al. (1994) by as much as 10%. Comparison of ground motions from normal-faulting and strike-slip events in our data set indicates that normal-faulting horizontal ground motions are not signif icantly different from extensional regime strike-slip ground motions.