Fundamental Aspects of Engineering Seismology for PSHA
This chapter discusses the fundamental aspects of engineering seismology, as far as they are relevant for understanding the basis of PSHA. The focus in engineering seismology is considerably different from the topics commonly referred to as classical earthquake seismology, which were covered in chapter 3. In contrast to classical earthquake seismologists, engineering seismologists focus on strong ground motion of engineering relevance and care less about weak motion. For this reason, engineering seismology is sometimes also referred to as strong-motion seismology. As a consequence, ground motion in engineering seismology is commonly recorded with accelerometers, which in contrast to classical velocity- or displacement transducers, are not subject to saturation once the shaking, e. g. close to a strong earthquake source, gets really strong. In weak-motion seismology, a seismogram can usually be seen as output of a linear system, which e. g. for strong motion recorded on soft sediments is no longer guaranteed. Engineering seismology also becomes non-linear when it comes to spectral concepts. In contrast to classical (linear-system based) seismology, where Fourier spectra are the basis of nearly all spectral concepts (e. g. the theoretical models for source spectra), in the context of seismic hazard analysis, ground motion is usually thought of in terms of the response spectral values, which describe the maximum response amplitudes of SDOF oscillators as a function of their oscillator frequencies under seismic loading. The reason for this is simply that in contrast to Fourier spectra, response spectra are better predictors of the response of buildings than Fourier spectra. Seismic response spectra, which describe the maximum response amplitudes of SDOF oscillators as a function of their oscillator frequencies, are actually the most common engineering tool to describe the action of buildings under seismic loading. Response spectra are non-linear systems and their theoretical relationship to Fourier spectra (which contains some rather counterintuitive aspects) will be discussed in detail in the second part of this book. Here, where the purpose is simply to understand the principles behind PSHA, it suffices to introduce the different types of response spectra used in PSHA and discuss what the calculation of these requires in terms of ground-motion observation and seismogram processing.