Microsoft Word - PSHA_Primer_v2

The goal of probabilistic seismic hazard analysis (PSHA) is to quantify the rate (or probability) of exceeding various ground-motion levels at a site (or a map of sites) given all possible earthquakes. The numerical/analytical approach to PSHA was first formalized by Cornell (1968). The most comprehensive treatment to date is the SSHAC (1997) report, which covers many important procedural issues that will not be discussed here (such as the use of “expert opinion”, the meaning of “consensus”, and how to document results). Except where otherwise noted, the SSHAC report represents the best source of additional information (that I know of). It’s a must-read for anyone conducting PSHA. Traditionally, peak acceleration (PGA) has been used to quantify ground motion in PSHA (it’s used to define lateral forces and shear stresses in the equivalent-static-force procedures of some building codes, and in liquefaction analyses). Today the preferred parameter is Response Spectral Acceleration (SA), which gives the maximum acceleration experienced by a damped, single-degree-of-freedom oscillator (a crude representation of building response). The oscillator period is chosen in accordance with the natural period of the structure (roughly number_of_stories/10), and damping values are typically set at 5% of critical (see Figure 1).