Long-Period Ground Motions from Digital Acceleration Recordings: A New Era in Engineering Seismology

Abstract

Digital strong-motion instruments allow routine recovery of ground motions at periods much longer than possible with older analog instruments, and under favorable conditions it may be possible to recover the residual displacements near earthquakes, particularly if both the rotational and translational components of motion are recorded (currently only the translational components are recorded). In practice, however, digital recordings are commonly plagued by drifts in the velocity and displacement traces obtained by integrating the recorded acceleration traces. Various baseline-correction schemes can be designed that give the appearance of removing these drifts. Although comparisons with independent measures of residual displacements, such as from GPS or InSAR measurements, show that such schemes call work, in general the sources of the drifts are such as to prevent routine corrections for the baseline problems; removal of low-frequencies by filtering is then required for many recordings. That is the bad news. The good news is that the filter corners can be so low that little of engineering interest is lost. Recent data from several earthquakes (e.g., 1999 Hector Mine and 2002 Denali) shows that for displacement response spectra, the transition from increasing to constant spectral levels occurs at significantly longer periods than in Eurocode 8; the transition period is in good agreement, however, with the recent 2003 NEHRP code.