THE EFFECT OF WEATHERING ON SHEAR WAVE VELOCITIES WITHIN A BEDROCK RIDGE AND ITS POSSIBLE INFLUENCE ON TOPOGRAPHIC AMPLIFICATION OF SURFACE ACCELERATIONS DURING EARTHQUAKE SHAKING

The topographic amplification of seismic ground motion intensity was not widely recognized until the 1950's (Richter, 1958). Early observers described a phenomena in which the crests of topographic highs exhibited evidence of having been subjected to much higher seismic accelerations than was known to have occurred elsewhere in the given region (Hadley, 1964). This phenomena came to be known by seismologists as the "Shattered Ridge Effect" (Nason, 1971). The interaction of topography and seismic ground motion has been studied by numerous workers since the 1960's. Pertinent works include: Davis and West, 1973; Boore, 1973; Bard and Tucker, 1985; and Sanchez-Sesma and others, 1986. These previous studies considered the effects of the surface geometry, wave type and various propagation models. Variations in near surface geology and/or weathering characteristics within these previous studies was ignored. To study the effects of near-surface weathering, shear wave velocities were measured, via ASTM D-2845-95, from samples obtained at varying depths within two pairs of borings. For each pair, one boring was located along a ridge-top and the other in the adjacent canyon-bottom. To minimize lithologic variation, both sites were located along the northern flank of the Santa Monica Mountains and within the Miocene age Modelo Formation, which locally consists of deeply weathered shales. Shear wave velocities within the first ridge-top boring varied from 2624 ft/sec at 68 feet, to 1347 ft/sec at 5 feet. Shear wave velocities within the adjacent canyon-bottom boring varied from 2490 ft/sec at 52 feet, to 1823 ft/sec at 5 feet. Similarly, shear wave velocities within the second ridge-top boring varied from 2374 ft/sec at 75 feet, to 1458 ft/sec at 5 feet. Shear wave velocities within the adjacent canyon-bottom boring varied from 2433 ft/sec at 55 feet, to 1737 ft/sec at 5 feet. These results indicate a relative increase in the theoretical surface particle acceleration, between the canyon-bottom and the ridge-top, of approximately 25 percent and 10 percent, respectively.