DOI: 10.25549/uscfac-m27
title: Correction for misalignment and cross axis sensitivity of strong earthquake motion recorded by sma-1 accelerographs, 1995
title: Civil Engineering Technical Report Series - Strong Motion Group Contributions CE 95-06

description: Two procedures are presented, one for correction of strong earthquake accelerations recorded on film for transducers' cross-axis sensitivity and misalignment, and another one for determining the misalignment angles from field tests. Misalignment refers to imperfections in the alignment of the penduli of the three transducers, and is typically of the order of few degrees. It results in the transducers recording motion in directions other than along the nominal sensitivity axis. Cross-axis sensitivity is the phenomenon of a transducer recording components of motion in directions other than its sensitivity axis, even when the transducer pendulum is perfectly aligned. This effect is pronounced for large amplitude recordings (peak acceleration close to 1g). The two effects are coupled. Correction for these effects was presented in the mid 1970's, and applied to the Pacoima dam accelerogram (San Fernando, 1971 California earthquake). Not until the 1994 Northridge, California, earthquake, so many large accelerations were recorded during one strong motion event.; The correction for these two effects, after the misalignment angles have been determined, is performed by expressing the motion along the ideal position of the sensitivity axes in terms of the motions in the actual directions of the penduli. This involves several successive coordinate transformations. The misalignment angles are determined by solving a generalized inverse problem. Namely, the deflection of the trace on the film caused by known forces (a component of the weight of the pendulum) is measured very accurately, and then an overdetermined system of equations is solved by a least squares procedure. The misalignment angles and the sensitivities are the only unknowns. A special tilt is used in the field, and the film record with static deflections of the traces is digitized by a scanner and a PC, to get the input for the inverse problem.; Distribution of teh values of the misalignment angles is presented for the 80 stations of the Los Angeles Strong Motion Network. These angels are typically of the order of 1° to 1.5°, but in few cases are as large as 3° to 4°. Also, results are presented illustrating the significance of these two effects for several recordings of the Northridge, 1994, earthquake (e.g., the record at the Rinaldi Receiving Station of the Department of Water and Power, City of Los Angeles). The results show that, for zero misalignment angles, the cross-axis sensitivity does not influence the record significantly. However, when coupled with misalignment, the effects are significant. The peak accelerations change by up to 3% for 1.5°, and by up to 7% for 3° misalignment angles. The misalignment and cross-axis sensitivity affect not only the peak values, but also the Fourier and Response spectra of the record, and their phase.; The largest systematic errors are associated with the limitation of the simple tilt test procedure. We show that the sensitivities specified by the manufacturer are larger than the estimates by our algorithm by 3% to 5% on average, and in most cases less than 10%. Coupled with the effects of cross-axis sensitivity and misalignment, this will result in errors in the computed accelerations which may approach about 15%.; The study concludes that correction of strong motion recordings by SMA-1 accelerographs for misalignment and cross-axis sensitivity effects is significant for large recorded accelerations, especially for some applications such as source inversion from displacements of recorded ground motion, and structural system identification procedures.