ON THE COHERENCE OF GROUND MOTION IN THE SAN-FERNANDO VALLEY

We present im analysis of the coherence of seismic ground motion recor ded on alluvial sediments in the San Fernando Valley, California. Usin g aftershocks of the 17 January 1994 M(w)6.7 earthquake recorded at a quasi-dense array of portable stations, we analyze the coherence of th ree well-recorded, magnitude 3.7 to 4.0 events over the frequency rang e 0.5 to 15 Hz and a distance range of 0.5 to 5.3 km. All stations are located at sites with broadly similar near-site geology, characterize d by medium to fine-grain Quaternary alluvial sediments. On average, r elatively high values of coherence are observed for distances up to 3 to 4 km and frequencies up to 2 to 3 Hz; coherence drops sharply at fr equencies near and above 3 Hz. Although average coherence functions ar e described reasonably well by a log-linear relationship with frequenc y, the curves at all distances exhibit a flattening at low frequencies that is not consistent with previous observations of coherence at har d-rock sites. The distance decay of coherence is also markedly less st rong, with high coherence values observed over station separations cor responding to multiple wavelengths, This may reflect fundamental diffe rences in shallow-wave propagation in the two environments, with high- frequency scattering relatively more dominant in regions of hard-rock near-surface geology, Within a sedimentary basin of valley, the sire r esponse itself generally reflects a resonance phenomenon that may tend to give rise to more uniform ground motions, However, previous studie s have demonstrated the existence of pathological focusing and amplifi cation effects within complex sedimentary basin environments such as t he greater Los Angeles region; our results undoubtedly do not quantify the full range of ground-motion variability at all sites, but rather represent tile level of that variability that fan be expected, and qua ntified, for typical source/receiver paths.