59 Estimation of strong seismic ground motions

Abstract

This chapter discusses recording, interpretations, and prediction of strong motions in large earthquakes. The approach is largely historical and concentrates on the direct use of recordings of strong ground motion for estimation of seismic ground shaking for engineering purposes. The chapter addresses the modern success in acquiring and analyzing strong ground motion by seismologists, particularly those working on seismic hazard maps and geological site response. The emphasis is on the presently available strong-motion recordings and a description of ground-motion estimation methods. The ground-motion characteristics depend on the seismic source, wave propagation (attenuation), and site response. The most commonly used method for characterizing the ground motion is through attenuation relations, but more complex numerical simulations are also used. Near-fault ground motions often contain large long-period pulses in the ground motion. There are two causes of long-period pulses in near-fault ground motions. One is constructive interference of the dynamic shaking because of rupture directivity effects. The other is owing to the movement of the ground associated with the permanent offset of ground. Because of the lack of a complete library of strong ground motions and spectra from appropriate earthquakes, extrapolation from the available records is needed for estimation of large and unrepresented seismic motions. The construction of strong-motion seismograms is now an essential part of the definition of hazard for the design and testing of critical structures. There are two main approaches used to develop design ground motions: (1) scaling ground motions and (2) adjusting ground motions to match a design spectrum.