Weston Albert Thelen

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

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 transect9s 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 transect9s 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 (

An Idaho-Nevada-California Refraction Experiment: Utilizing Large Mine Blasts For Long-range Profiles

Utilizing commercial mine blasts and local earthquakes, as well as a dense array of portable seismographs, we have achieved high-resolution crustal refraction profiles across northern Nevada and the central Sierra Nevada Mountains. Using a dense spacing of 411 portable seismographs and 4.5 Hz geophones, the instruments were able to record events ranging from large mine blasts, small local earthquakes (approximately magnitude 2), as well as two larger earthquakes (magnitudes 2.8 and 3.8). Our instruments sensed blast first arrivals out to a distance of approximately 400 km. We have obtained 99% data recovery and clear refractions across the Sierra Nevada and the northern Great Basin regions.