A STUDY ON SITE-SPECIFIC UNIFORM HAZARD RESPONSE SPECTRUM IN PROBABILITY SEISMIC HAZARD ANALYSIS

The uniform hazard response spectrum (UHRS) is determined based on the spectral accelerations corresponding to a given probability of exceedance in the probability seismic hazard curves. Generally, the seismic hazard curve for a specific site represents the relationship between occurrence frequencies considering various earthquake characteristics and spectral accelerations using an attenuation law for the hard site within a defined range around the site. The accuracy of the estimation for spectral accelerations at a specific site strongly depends on the geologic condition underlying the site. Therefore, the local site effect should be taken into account for a defensible seismic hazard analysis. In this study, the local site effect is considered using the site-specific attenuation law and local site amplification factor, that both are obtained from the earthquake database around the site, in probabilistic seismic hazard analysis. Through the aforementioned processes, the site-specific uniform hazard response spectrum corresponding to a given probability of exceedance can be determined. Finally, the damage earthquake data near the specific site are used to refine the uniform hazard response spectrum at several periods of the seismic characteristic of the site. Introduction On September 21, 1999 (Taiwan local time), a disastrous earthquake of magnitude ML=7.3 struck the central part of Taiwan. The epicenter was located near the town of Chi-Chi, Nautou County. Along the Chelungpu fault, a surface rupture of more than 90 km was observed. A horizontal offset about 10 meters was observed at another site. As a direct lost of this earthquake, 2403 people were killed, over 10,000 buildings collapsed or were severely damaged (Loh and Lee 2000). This is Taiwan’s worst disaster since the 1935 ShinChu-Taichung earthquake, where 3325 people were lost in a magnitude ML=7.1 earthquake. The devastating earthquakes in Taiwan occurred mostly with ground ruptures or resulted entirely from the reactivation of faults. Earthquake hazard mitigation is an important issue in Taiwan. The activity 1 Assistant Research, National Center for Research on Earthquake Engineering, Taipei, Taiwan 2 Researcher, National Center for Research on Earthquake Engineering, Taipei, Taiwan 3 Professor, Dept. of Civil Engineering, National Taiwan University, Taipei, Taiwan 4 Researcher, National Center for Research on Earthquake Engineering, Taipei, Taiwan Proceedings of the 9th U.S. National and 10th Canadian Conference on Earthquake Engineering Compte Rendu de la 9ième Conférence Nationale Américaine et 10ième Conférence Canadienne de Génie Parasismique July 25-29, 2010, Toronto, Ontario, Canada • Paper No 1017 of the faults must be determined to provide information for seismic hazard analysis and earthquake mitigation. The principal objective of seismic hazard analysis is to estimate the likelihood that different levels of ground shaking intensity will be experienced at a site. The analysis must account for the spatial and temporal randomness of earthquake occurrences, and the uncertainty in ground motion predictions for events of different magnitudes at random distances. The methodologies for performing a probabilistic seismic hazard analysis (PSHA), as initial described by Cornell (1968), McGuire (1976), Der Kiureghian & Ang (1977), and others, are well established in engineering practice. Actually, it is worthy of noting that there are two major sources of probabilistic uncertainties in the probabilistic seismic hazard analysis (McGuire, 2004): (1) aleatory uncertainties that is inherent in a random phenomenon and cannot be reduced by acquiring additional data or information; and (2) epistemic uncertainties that results from lack of some model or parameter, also been called statistical uncertainties. In this paper, the general methodology of PSHA and the basis of inputs to the PSHA are summarized. Although the characteristic earthquake model was adopted in the hazard analysis, the current seismic zonings of Taiwan’s building code utilized time-independent estimates of recurrence rate of earthquakes (Youngs and Coppermith 1985; Loh and Huang 2002). The timedependence of characteristic earthquake should be considered for some of the major faults that have semi-periodic behavior and for which sufficient information on previous earthquakes is available. To make the time varying probability calculations, one needs the information on the date a fault last ruptured, the mean inter-event time of large events, and its standard deviation. Based on the paleoseismology investigations conducted by the Central Geological Survey, Taiwan, the time-predictable characteristic earthquake model was adopted to evaluate the seismic hazard for the major active faults. Earthquake recurrence intervals for characteristic events from the active faults were analyzed and modeled by the lognormal probability distribution function (Chang et al. 2007). The time-predictable characteristic earthquakes model are used in the hazard analysis for the Type I active faults. For site effect, the Taiwan Strong Motion Instrumentation Program (TSMIP), which operated by the Central Weather Bureau (CWB) for the populated areas with dense digital strong-motion networks, composed of more than 700 stations widely deployed in Taiwan area. On the other hand, in 2000, NCREE and CWB collaborated to perform the site investigation to obtain basic soil properties and wave velocity of the stratum. This geological database is a good reference for understanding the characteristic of seismic data. Accordingly, a large amount of the high quality earthquake data collected by the network of TSMIP can be used in this study to more deeply investigate the site amplification effect on the par in response to different levels of excitation. Background Seismicity Monitoring of regional seismicity in Taiwan has been significantly improved since 1990 after the CWB installed dense networks of strong motion accelerographs and real-time seismic stations with high-gain velocity sensors. The history of earthquake activity in the Taiwan region can be dated back to the seventeenth century. The sources of earthquake catalogue include historical records and instrumented earthquake data. To review historical seismicity, the catalog of pre-instrumented earthquakes had been compiled and modified by Cheng and Yeh (1989). This catalog provides a list of historical earthquakes dating back as early as 1604. Additional, due to the high seismic activities in Taiwan, the Central Weather Bureau (CWB) launched a seismic instrumentation program using digital strong-motion seismograph since 1990. Fig. 1 shows a map of earthquake epicenters in Taiwan area from 1900 to 2007. In the figure the size of dots represents magnitude, whereas the color represents focal depth (h): red for h < 20 km, green for 50>h>20 km, and blue for h>50 km. It is apparently related to the subduction of the Philippine Sea plate under the Eurasian plate in northeastern Taiwan. There are significant earthquake activities under Taiwan Island due to collision of the two plates. Analysis Model Parameters for PSHA The PSHA provides a framework to address the uncertainties associated with the identification and characterization of seismic sources by incorporating multiple interpretations of seismological parameters. The role of geological, seismological, and geophysical investigations is to develop geosciences information about the site for the detailed design analysis of the facility, as well as to ensure that the seismic hazard analysis can include the up-to-date information. Besides, a logic tree approach was used to incorporate credible alternatives for seismic source interpretations, seismic source parameters, and ground motion attenuation models. Earthquake catalog In the last catalog, which collected earthquake data from 1900 to July 1989, was used to calculate the hazard for plant site in the previous PSHA study. In recent years, many efforts were performed to reexamine the source parameters (magnitude, epicenter, and focus depth) of the earthquake catalog by many researchers. Some double counted earthquakes were removed from last catalog, and, the earthquakes occurred in the last decade were added into the catalog. The instrumentally determined earthquakes in Taiwan from 1898 to 2000 had been recompiled (Cheng et al., 2003). Earthquake magnitudes used in different period were reviewed, and the relationships among them were derived. According the empirical formula, the currently used local magnitude, ML (Shin, 1993), was used as a uniform scale to describe the size of earthquake. Depending on the number of seismic station and the performance of seismograph, the history of earthquake observation was divided into 4 stages: initial stage (1898-1935); intermediate stage (1936-1972); TTSN stage (1973-1991.2), and CWBSN stage (1991.3-), to describe the quality of events. The data compiled in this catalog include the origin time, epicenter, focal depth, magnitude, information sources, and damaged earthquakes. Fig. 2 showed the occurrence time of earthquake events of magnitude greater than 5.0 for different instrumental period of the old catalog and the new one for selected sub-zones, BS02. It is noted that the earthquake catalog adopted by old catalog showed a large change in the occurrence rate for different instrumental period, especially, for the period from 1936 to 1973. The new compiled earthquake catalog, which is more reasonable then the old one, was used in this study. Time-predictable characteristic earthquake model The study work of paleoearthquakes investigation performed by Chen (Chen et al. 2004) showed that the rupture of the Type I active faults in Taiwan followed the occurrence-timepredictable earthquake model. Based on the paleoseismology investigations conducted by the Central Geological Survey, Taiwan, the time-predictable characteristic earthquake model wa