Pii: S0267-7261(98)00046-3

This article explores the possibility to measure deformations of building foundations from measurements of ambient noise and strong motion recordings. The case under study is a seven-storey hotel building in Van Nuys, California. It has been instrumented by strong motion accelerographs, and has recorded several earthquakes, including the 1971 San Fernando (ML ˆ 6.6, R ˆ 22 km), 1987 Whittier–Narrows (MLˆ 5.9, Rˆ 41 km), 1992 Landers (MLˆ 7.5, Rˆ 186 km), 1992 Big Bear (MLˆ 6.5, Rˆ 149 km), and 1994 Northridge (MLˆ 6.4, Rˆ 1.5 km) earthquake and its aftershocks (20 March: ML ˆ 5.2, R ˆ 1.2 km; 6 December, 1994: ML ˆ 4.3, R ˆ 11 km). It suffered minor structural damage in 1971 earthquake and extensive damage in 1994. Two detailed ambient vibration tests were performed following the Northridge earthquake, one before and the other one after the 20 March aftershock. These included measurements at a grid of points on the ground floor and in the parking lot surrounding the building, presented and analyzed in this article. The analysis shows that the foundation system, consisting of grade beams on friction piles, does not act as a ‘‘rigid body’’ but deforms during the passage of microtremor and therefore earthquake waves. For this geometrically and by design essentially symmetric building, the center of stiffness of the foundation system appears to have large eccentricity (this is seen both from the microtremor measurements and from the earthquake recordings). This eccentricity may have contributed to strong coupling of transverse and torsional responses, and to larger than expected torsional response, contributing to damage during the 1994 Northridge, earthquake. q 1999 Elsevier Science Ltd. All rights reserved.