512 ENGINEERING GEOLOGY/Seismology Figure 16 Velocity time series obtained from integration of horizontal accelerograms recorded at Lucerne and Joshua Tree stations during the 1992 Landers earthquake in California The fault rupture propagated towards Lucerne and away from Joshua Tree, creating forward directivity effects (short duration shaking consisting of a concentrated high energy pulse of motion) at the former, and backward directivity effects (long duration shaking of several small pulses of motion) at the latter Reprinted with permission from: Somerville PG, Smith NF, Graves RW, and Abrahamson NA (1997) Modification of empirical strong ground motion attenuation relations to include the amplitude and dur ation effects of rupture directivity Seismological Research Letters 68(1): 199 222 ß Seismological Society of America postulated future events on past activity The catalogue of instrumentally recorded earthquakes extends back at the very most to 1898, which is a very short period of observation for events for which recurrence intervals can extend to hundreds or even thousands of years For this reason, great value is to be obtained from extending the catalogue through the careful interpretation of historical records, making use of empirical relationships between magnitude and intensity referred to previously The seismic record can also be extended through palaeoseismology, by quantifying and dating coseismic displacements on geological faults Although the distinction masks many equally marked differences among the various methods that fall within each camp, a basic division exists between deterministic and probabilistic approaches to seismic hazard assessment In the deterministic approach, only a few earthquake scenarios are considered, and sometimes just one is selected to represent an approximation to the worst case The controlling earthquake will generally correspond to an event with the nominal maximum credible magnitude, located at the Figure 17 Theoretical Fourier spectra for earthquakes of mag nitudes and at 10 km from a rock site, assuming a stress parameter of 70 bars; f0 is the corner frequency, which is inversely proportional to the duration of rupture The lower part of the figure shows stochastic acceleration time series generated from these spectra, from which the influence of magnitude on amplitude and duration (or number of cycles) can be clearly observed From the spectra in the upper part of the figure, it can be appreciated that scaling with magnitude is frequency dependent, with larger magnitude events generating proportionally more long period radiation location closest to the site within the seismogenic source; the ground motion is generally calculated as the 50- or 84-percentile value from the prediction equation In the probabilistic approach, all possible earthquake scenarios are considered, including events of every magnitude, from the minimum considered to be of engineering significance ($4) up to the maximum credible, occurring at every possible location within the source zones, and for each magnitude–distance combination various percentiles of the motion are considered to reflect the scatter in the ground-motion prediction equations Alternative options for the input parameters may be considered by using a logic-tree formulation, in which weights are assigned to different options that reflect the