URBAN SEISMOLOGY - NORTHRIDGE AFTERSHOCKS RECORDED BY MULTISCALE ARRAYS OF PORTABLE DIGITAL SEISMOGRAPHS

We deployed portable digital seismographs in the San Fernando Valley ( SFV), the Los Angeles basin (LAB), and surrounding hills to record aft ershocks of the 17 January 1994 Northridge California earthquake. The purpose of the deployment was to investigate factors relevant to seism ic zonation in urban areas, such as site amplification, sedimentary ba sin effects, and the variability of ground motion over short baselines . We placed seismographs at 47 sites (not all concurrently) and record ed about 290 earthquakes with magnitudes up to 5.1 at five stations or more. We deployed widely spaced stations for profiles across the San Fernando Valley, as well as five dense arrays (apertures of 200 to 500 m) in areas of high damage, such as the collapsed Interstate 10 overp ass, Sherman Oaks, and the collapsed parking garage at CalState Northr idge. Aftershock data analysis indicates a correlation of site amplifi cation with mainshock damage. We found several cases where the site am plification depended on the azimuth of the aftershock, possibly indica ting focusing from basin structures. For the parking garage array, we found large ground-motion variabilities (a factor of 2) over 200-m dis tances for sites on the same mapped soil unit. Array analysis of the a ftershock seismograms demonstrates that sizable arrivals after the dir ect S waves consist of surface waves traveling from the same azimuth a s that of the epicenter. These surface waves increase the duration of motions and can have frequencies as high as about 4 Hz. For the events studied here, we do not observe large arrivals reflected from the sou thern edge of the San Fernando Valley.