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A decrease in Q values within the hanging wall of the ruptured Chelungpu fault two years following the 1999 Chi-Chi earthquake was revealed by Q tomography images and an analysis of single-path Q. The synthetic and sensitivity tests of the Q determination were carried out accordingly to justify the temporal variation. A Q value within the hanging wall above the hypocenter was determined to be 157 ± 18 two years following the Chi-Chi earthquake, which is significantly lower than the Q tomography values of 238 ± 17 and 289 ± 13 prior to and two years after the main shock, respectively. Similar values using a signal-path Q analysis from events within the ruptured fault zone to stations along the fault were obtained. The corresponding Q values were 247 ± 85 prior to the Chi-Chi earthquake. After the earthquake we obtained Q values of 158 ± 75 and 318 ± 80 for 2 years following and 2 years after the earthquake, respectively. Considering the two independent methods in determination of Q, the reduction in Q by 89 two years following the Chi-Chi earthquake in both methods is significant. Along with 1 % V reduction revealed by the analysis of repeating earthquakes our studies suggested possible reduction both in V and Q values within the fault zone after the Chi-Chi earthquake. Temporal changes in Q after the Chi-Chi earthquake imply variations of pore-fluid saturation in the ruptured fault zone. The reduction in Q two years following the Chi-Chi earthquake indicates high pore-fluid saturation within the fractured fault zone due to the postseismic fluid redistribution. [ABSTRACT FROM AUTHOR]

The duration of strong shaking is particularly important for assessing building performance, potential landslides and liquefaction hazards. The results of this investigation can potentially help reduce related fatalities and economic losses. In this study, we analyzed the acceleration seismograms of the Taiwan strong motion network to characterize the strong shaking duration associated with earthquake sources, propagation paths and site effects. This study proposes a new definition for the strong shaking duration called 'effective shaking duration' (ESD), which considers the amplitude and radiation energy decays. We first consider the window of a time series during which the amplitude is ≥0.01 g, and we then defined the ESD as the length of the interval of the dissipated energy within 5-95 % of the total energy during this time frame. We calculated the strong shaking duration for 495 inter-plate events with magnitudes of M > 5.0 and focal depths <50 km in the Taiwan region from 1994 to 2012. Using a nonlinear regression procedure, we thus obtained an empirical equation for strong shaking durations. The equation is a function of earthquake magnitude, distance and site conditions, which are defined by the V30 value (the S-wave velocity structure of the top 30 m of the site). The results indicate that the shaking durations significantly increase with magnitude and also decrease with distance and V30. Compared with empirical equations from global datasets, our empirical equation is applicable to earthquakes in other regions and will produce smaller but more applicable duration values for smaller earthquakes. However, for larger events, our ESD values are comparable with those derived from other definitions (e.g., significant duration). Although the empirical relationship is mainly based on Taiwanese events, in view of the massive dataset, this empirical equation could provide important information to the global community regarding the ground shaking duration estimation in the ground motion prediction of future earthquakes. [ABSTRACT FROM AUTHOR]