ALBERT

Description: A new approach to estimation of strong-motion attenuation relations with multiple variance components is proposed. In the terminology of Abrahamson and Youngs (1992), the attenuation relationship considered is a "mixed" model, where the regression coefficients are treated as the "fixed effects" and the components of deviations are treated as the "random effects." Following Dempster et al. (1981), an MLR procedure is adopted, which performs the maximum likelihood (ML) estimation of mixed models where the fixed effects are treated as random (R) effects with infinite variance. Both the fixed and random effects are estimated in a unified framework via an expectation-maximization (EM) algorithm. A modification of the MLR procedure is performed to accommodate nonlinear attenuation models. Compared with other methods, our proposal requires no additional regression or searching procedures and hence is neat and simple. Explicit formulae are provided for the variance estimates of the estimated model parameters. Two types of the strong-motion prediction are discussed: the unconditional prediction without accounting for the site-specific deviation, and the conditional prediction that further incorporates that deviation. A simulation study shows that the proposed procedure yields estimates with smallest biases and least computation time, compared with the EM procedure in Brillinger and Preisler (1985) and with the two-stage method in Joyner and Boore (1993). The new method is applied to a dataset of Taiwan's ground motions for illustration. This application reveals that the site-to-site variability in this dataset is remarkable; hence, the ergodic assumption ignoring "the spatial variability of ground motions" (Anderson and Brune, 1999) may not be suitable for probabilistic seismic hazard analyses in Taiwan. Also, in this application the conditional prediction is shown to be much more accurate than the unconditional prediction.

Description: A quasi-static numerical method was used to simulate the failure of a strong stuck asperity on an otherwise creeping fault plane. The numerical model produced the same slip distribution as analytical asperity models, which, for constant loading rate, produced repeating events having a period T that scales with moment M (sub 0) as T varies as M (sub 0) (super 1/6) , the scaling relation observed by Nadeau and Johnson (1998) at Parkfield. When the asperity is a composite of smaller hard unit asperities, we still find T varies as M (sub 0) (super 1/6) , but the constant of proportionality depends on the density of unit asperities within the cluster. Since only clusters with a fixed asperity density follow the observed T varies as M (sub 0) (super 1/6) scaling, this result rules out the fractal spatial distribution (Cantor dust) of unit asperities at Parkfield suggested by Sammis et al. (1999). For solid asperities, the average stress drop on the asperity is on the order of 100 MPa, but the average stress drop over the entire rupture area is significantly lower, equivalent to that estimated from spectral analysis. However, the stress drop for asperity models is not independent of seismic moment but decreases with earthquake size. The energy release rate for asperity events is estimated to be g〉 or =10 (super 7) J/m (super 2) , near the upper limit of estimates by Li (1987) using parameters from large events on the San Andreas Fault.

Description: The 1999 Chi-Chi earthquake was caused by rupture of the Chelungpu fault, one of the most prominent active thrust faults of Taiwan. This largest of Taiwan's historical fault ruptures broke the surface for over 90 km at the western base of the rugged mountain range. A short right-lateral tear extended southwestward from the southern end of the Chelungpu fault, and a complex assemblage of shallow folds and faults ran northeastward from the northern end. Vertical offsets averaged about 2 m along the southern half of the Chelungpu fault and about 4 m along the northern half, and offsets of 5 to 7 m were typical along the northern part of the major thrust. The sinuous nature of the surface trace is consistent with seismographic data that indicate a dip of about 30 degrees . The 1999 rupture draws attention to the fact that this active fault system is highly segmented and that this segmentation influences the characteristics of seismic ruptures. Active faults to the south, north, and west of the Chelungpu fault have distinctly different characteristics. Faults to the south and north broke the surface during earthquakes in 1906 and 1935. The active Changhua fault to the west, a blind thrust similar in length to the Chelungpu, has not ruptured in the historical period and should be considered a prime candidate for generating a future earthquake.