Abstract

This study assesses a 60 km north-northeast–south-southwest transect along the San Gabriel River for shallow shear velocities, in San Gabriel Valley and the Los Angeles Basin of southern California. We assessed a total of 214 sites, 199 along the transect at 300-m spacing, during a one-week field campaign with the refraction microtremor (ReMi) technique. The transect's maximum 30-m shear velocity (Vs30) occurs in coarse alluvium of San Gabriel Valley where the San Gabriel River exits the San Gabriel Mountains, at 730 m/sec, upper National Earthquake Hazards Reduction Program (nehrp) site class C. Much of the northeast section of the transect (in San Gabriel Valley) is also nehrp class C, or near the CD class boundary. The section of the transect south from Whittier Narrows to Seal Beach shows nehrp-D velocities in active alluvium. The transect's lowest Vs30, 230 m/ sec at the Alamitos Bay estuary, is also classed as nehrp-D. An increase toward the nehrp CD class boundary occurs at the shoreline beach outside Alamitos Bay, confirmed by additional measurements on Seal Beach. Our measured Vs30 values generally show good correlation with published site-classification maps and existing borehole data sets. There is no evidence in our data for an increase in velocity predicted by Wills et al. (2000) at their CD to BC site classification boundary at the San Gabriel Mountains front, nor for any decrease at their D to DE class boundary at Alamitos Bay. Very large Vs30 variations exist in soil and geologic units sampled by our survey. The Vs30 variations we measured are smaller than Vs30 variations of 30% or more we found between closely spaced (<0.5 km) downhole measurements in the Los Angeles Basin, which are not uncommon within a community data set we examined showing hundreds of boreholes. We find the San Gabriel River's hydraulic gradient to be a good predictor of minimum Vs30, based on the expected effect of the hydraulic gradient on the grain size of sediments deposited by a river. The Vs30 data show a fractal spatial dependence, which appears at distances greater than 700 m. The unprecedented number of shear-velocity measurements we have made suggests that large measurement populations may be necessary to properly characterize Vs30 trends within any surficial geological unit.

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