NUMERICAL MODELS OF FLUID-PRESSURE CHANGES RESULTING FROM THE 1999 CHI-CHI EARTHQUAKE, TAIWAN

A new 3D time-dependent pore-pressure diffusion model PFLOW is developed to investigate the response of pore fluids to the crustal deformation generated by strong earthquakes in heterogeneous geologic media. Using a carefully calibrated finite fault-rupture model (Ma et al., 2005), we calculate the coseismic pore pressure changes and diffusion induced by volumetric strain associated with the 1999 Chi-Chi earthquake (M_w = 7.6) in Taiwan. The Chi-Chi earthquake provides an unique opportunity to investigate the spatial and time dependent poroelastic response of near-field rocks and sediments because extensive field data of water level changes and crustal deformation are well documented and readily available. The integrated model allows us to explore the various mechanisms responsible for hydrologic anomalies observed in Taiwan’s western foothills and the Choshui River alluvial plain.

Of special interest is identifying which of the observed hydrologic changes can be explained by a coseismic strain hypothesis and whether the pore-pressure diffusion model can account for observed recovery (dissipation) rates of seismically induced water-level changes in the alluvial fan. Preliminary results show a strong correlation between areas of coseismic dilatational strain and water-level decline in regions where consolidated rocks are present in the foothills. However, in the Choshui River alluvial fan, water-level rises are observed in regions of dilatational strain, suggesting that seismic shaking and compaction or faulting-enhanced gravity flow may be responsible for hydrologic changes. Assuming pre-seismic hydraulic conductivity values, our modeling results also show that water-level recovery rates cannot be explained by simple diffusion processes, suggesting that seismic loading may have caused significant re-arrangement and compaction of sediments in the alluvial plain.