A Transect of 200 Shallow Shear-Velocity Profiles across the Los Angeles Basin

Abstract

This study assesses a 60 km NNE-SSW 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-meter shear velocity (V s 30) occurs in coarse alluvium of San Gabriel Valley where the San Gabriel River exits the San Gabriel Mts.; at 730 m/s, upper 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 V s 30 , 230 m/s 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 V s 30 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 V s 30 variations exist in soil and geologic units sampled by our survey. The V s 30 variations we measured are smaller than V s 30 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 V s 30 , based on the expected effect of the hydraulic gradient on the grain size of sediments deposited by a river. The V s 30 data show a fractal spatial dependence, which appears at distances greater than 700 m. The unprecedented number of shearvelocity measurements we have made suggests that large measurement populations may be necessary to properly characterize V s 30 trends within any surficial geological unit.

Seismic velocities and geotechnical data applied to the soil microzoning of western Algarve, Portugal

“…This result agrees with those of others (Romero and Rix, 2001;Scott et al, 2004;Thelen et al, 2006) that conclude that surficial geology is not a good corollary for determining soil amplification factors (Lenz and Baise, 2007).…”

Empirical Site Response and Comparison with Measured Site Conditions at ANSS Sites in the Vicinity of Reno, Nevada

“…Consequently a wide range of observed V S30 measurements are often obtained within a single geological unit. This is demonstrated through recent work correlating direct measurements of shallow V S with geological classifications, including within the Reno-area basin (e.g., Scott et al, 2004Scott et al, , 2006Thelen et al, 2006;Pancha et al, 2007). In the Reno area, the variations in average velocity between the studied geological units were found to be much smaller than the standard deviations of velocities within the units.…”

“…2005; Thelen et al, 2006), with resolution increasing with depth (Pei et al, 2005(Pei et al, , 2007Pancha et al, 2008).…”

Refraction microtremors: Data analysis and diagnostics of key hypotheses

“…In spite of these shortcomings, ReMi has been applied increasingly (e.g., Thelen et al, 2006;Pancha et al, 2007Pancha et al, , 2008PerezGracia et al, 2009). Studies have been performed to compare the results of ReMi with other techniques (Stephenson et al, 2005;Jaume, 2006;Anderson et al, 2007;Chavez-Garcia et al, 2007;Richwalski et al, 2007;Castellaro and Mulargia, 2009;Rosenblad andLi, 2009a, 2009b), reaching the general conclusion that ReMi can yield accurate results if the key assumption of isotropic source distribution is satisfied and if care is taken in picking the dispersion curve.…”