| dc.description.abstract | Before the occurrence of the September 21, 1999, Chi-Chi Taiwan earthquake, the previous studies of probabilistic seismic hazard analysis (PSHA) by different agencies and authors totally underestimated the seismic hazard in Central Taiwan. The importance to incorporate fault sources and closest distance to fault in developing the ground-motion attenuation relationships and in PSHA. Using logic tree to model parameters at nodes and weightings at branches in PSHA could obtain the uncertainty of PSHA influenced by parameters. Further, we could understand the characteristic of sources in vicinities by deaggregating hazard contributed from different magnitude and distance bins.
We adopt 3-D plate source to model fault planes and subduction zone plates except regional sources. Truncated-Exponential model developed by mainshock catalog in MW from 1900 to 1999 was used to describe the magnitude distribution of regional sources; Characteristic-Earthquake model developed by fault slip rate was used to describe the magnitude distribution of active fault and subduction interface sources. The ground-motion level of subduction plate sources are always higher than crustal sources. Hence, it is in need of adopting suitable attenuation relationship for each source in PSHA, especially the attenuation relationship of curustal source including the ground-motion data of Chi-Chi earthquake sequence, and which reveals the hanging-wall effect and site condition for specific site.
According to the iso-seismic hazard map of PGA, 0.2sec and 1.0sec spectral acceleration, there are obviously correlation between the hazard level and the activity of fault, especially on the center of the hanging wall. the highest hazard level can be found in eastern longitudinal valley and western foothills to coast plain, separated by the central range in low hazard level. Furthermore, the hazard level considering faults activity divided by regional sources shows that the prominent ratio always distribute on hanging wall, especially on the low background seismicity region in the past, such as TaiChung and HsinChu-MiaLi region. The iso-seismic hazard map could be referenced by the Taiwan Building Code in the future.
For specific site, the major parameters that influence uncertainty of PSHA are different because of various combinations of source parameters in vicinities. After uncertainty analysis by logic tree, we could understand the variation of parameter and how it influences the PSHA result. The most important one is the slip rate of fault, the second one is mu; the b-value of Characteristic- Earthquake model influence the uncertainty unapparent, but Truncated-Exponential model of regional sources which far away from active fault influence the uncertainty conspicuous. The variation of fault dip also influence uncertainty of PSHA obviously at the end of down-dip fault plane projection surface. The result of PSHA uncertainty analysis could help geo-scientists and earthquake engineers to estimate and judge the parameters more efficiently.
From the deaggregation of PSHA in several metropolis of Taiwan, results show the hazard contributed from each distance and magnitude bin by different return period. The deaggregation could provide information for hazard mitigation while choosing scenario earthquakes. After considering fault activity in PSHA, the PGA level of near-field always exceed 0.4g in 475yr return period, however, it is impossible to reach the proper hazard level without fault source in PSHA especially in long return period. The northern Taiwan is threatened not only an active fault in vicinity, but also subduction plate sources more than what we have known before. Therefore we should pay more attention on subduction zone sources in hazard mitigation of northern Taiwan in the future. | en_US |