Strong motion from surface waves in deep sedimentary basins

It is widely recognized that long-period surface waves generated by convers ion of body waves at the boundaries of deep sedimentary basins make an impo rtant contribution to strong ground motion. The factors controlling the amp litude of such motion, however, are not widely understood. A study of pseud ovelocity response spectra of strong-motion records from the Los Angeles Ba sin shows that late-arriving surface waves with group velocities of about I km/sec dominate the ground motion for periods of 3 sec and longer. The rat e of amplitude decay for these waves is less than for the body waves and de pends significantly on period, with smaller decay for longer periods. The a mplitude can be modeled by the equation log y = f(M, R-E) + C + bR(B) where y is the pseudovelocity response, f(M, R-E) is an attenuation relatio n based on a general strong-motion data set, M is moment magnitude, R-E is the distance from the source to the edge of the basin, R-E is the distance from the edge of the basin to the recording site, and b and c are parameter s fit to the data. The equation gives values larger by as much as a factor of 3 than given by the attenuation relationships based on general strong-mo tion data sets for the same source-site distance. It is clear that surface waves need to be taken into account in the design of long-period structures in deep sedimentary basins. The ground-motion levels specified by the eart hquake provisions of current building codes, in California at least, accomm odate the long-period ground motions from basin-edge-generated surface wave s for periods of 5 sec and less and earthquakes with moment magnitudes of 7 .5 or less located more than 20 km outside the basin. There may be problems at longer periods and for earthquakes located closer to the basin edge. Th e results of this study suggest that anelastic attenuation may need to be i ncluded in attempts to model long-period motion in deep sedimentary basins. To obtain better data on surface waves in the future, operators of strong- motion networks should take special care for the faithful recording of the long-period components of ground motion. It will also be necessary to insur e that at least some selected recorders, once triggered, continue to operat e for a time sufficient for the surface waves to traverse the basin. With v elocities of about I km/sec, that time will be as long as 100 sec for a bas in the size of the Los Angeles Basin.