S U M M A R YObserved coseismic data as well as 97 days of post-seismic GPS data for the Chi-Chi earthquake are used as constraints in the modelling of crustal evolution using the 3-D finite-element method. First, the coseismic GPS data are used to justify the use of the elastic earth model and the source rupture model. Subsequently, the most likely rheological model is determined by analysing several modelled time-dependent displacements for various viscosity structures. The range of viscosities of the lower crust in central Taiwan is determined in advance from laboratory measurements and the long-term strain rate. The estimated viscosity of 5.0 × 10 17 Pa s seems to be very low and a relaxation time of 116 days seems very short, but the latter approximates the GPS measurement of 86 days. Since earlier studies have indicated that both the viscoelastic response model and the afterslip model may affect post-seismic deformation, we compare theoretical surface displacements for each of the two models that we evaluate. The results reveal that there is little doubt that while neither of these models alone is able to predict the GPS measurements well in a 97-day period, the combination of the two models improves the predictions considerably. We conclude that the afterslip mainly dominated Chi-Chi post-seismic deformation in the rupture area while the viscoelastic model did so elsewhere.

Ground roll noise on land data sets overwhelms the desired reflection seismic signal unless special steps are taken in data acquisition and processing to control it. This is usually done in the field by the design of group arrays for data acquisition. On the other hand, if multicomponent data are acquired, it is possible to remove ground roll during processing using polarization analysis. Even though this processing is computation-intensive, the potential exists for obtaining results similar to conventional data acquisition, but with deployment of fewer sensors in the field with minimal group array effects. It also has potential for deriving new information.We describe a two-dimensional polarization-filter analysis for use with vertical and in-line sensors. A time-domain spectral matrix technique is developed since the recorded seismic signal is the superposition of multiple signals in the time domain, each with different frequency content and time-varying polarization. This technique is implemented by decomposing the signal into individual frequency components using narrow band-pass filters and defining the polarization characteristic using sliding time windows. We show that both incoherent noise and specific linearly polarized constituents can be successfully filtered.

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Abstract. Various types of damage caused by the devastating 1999 Chi-Chi, Taiwan earthquake were not only experienced in the hanging wall and footwall but also in the southern and northern parts of the Chelungpu fault. In this paper, we determine the coseismic displacements and determine stress relaxation using a rupture model and compare the values with surface damage. The patterns of damage can be clearly correlated with the calculated coseismic displacement and stress relaxation caused by the event. It is found that the surface damage was predominately caused by coseismic deformation and that large-scale landslides were mainly a result of steep coseismic displacements.