Page - 1105 - in Book of Full Papers - Symposium Hydro Engineering

[1]). The main output obtained from this program is the probability of a ground motion parameter (PGA or spectral acceleration) not being exceeded during a fixed period of time at the site. For estimating the seismic potential (maximum magnitude) of a fault the Wells & Coppersmith relationship was used which is based on worldwide data [11]. Calculations were carried out for return periods between 100 and 2500 years. In order to obtain a weighted average of the results calculated with the three attenuation laws, a logic tree approach with three branches was applied. Selected results are shown in “Table 1”. The values obtained from the line source model were considerably higher than those derived from the point source model. Table 1 Values of PGA obtained from PSHA using line source model Design level Return period (year) Dam site Peak ground acceleration (g) horizontal vertical OBL (84th percentile) 200 0.30 0.18 DBL (84th percentile) 500 0.394 0.245 MDL (84th percentile) 2500 0.455 0.290 MCL (84th percentile) Deterministic 0.64 0.53 3.5. DETERMINISTIC SEISMIC HAZARD ANALYSIS (DSHA) The purpose of the DSHA is to find the worst possible scenario among all the possible seismic sources related to the studied site. The analysis comprises four steps: (1) Identification of the active faults closest to the site, (2) determining the maximum earthquake that could be generated by these faults, (3) selection of appropriate attenuation laws, and (4) determination of the hazard at the site. The maximum values of PGA were calculated for twelve faults or fault segments affecting the dam site using the same attenuation laws as for the PSHA. The distance to the seismic source was taken as the closest distance to the vertical projection of the rupture for Campbell & Bozorgnia [3], Ambraseys and Douglas [4], Boore et al. [5] and Mahdavian [6] laws. A weighted average was calculated using a logic tree approach. The results are given in “Table 2”. 4. ESTIMATION OF RESPONSE SPECTRA For design and analysis of structures a convenient way to express ground motions is the response spectrum which gives the maximum response 1105