Coseismic and Postseismic Deformation from the 1999 Chi-Chi, Taiwan Earthquake

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Segall 0106695 The Mw = 7.5, September 21, 1999 Chi-Chi, Taiwan earthquake occurred in the center of a dense GPS network operated by the Taiwanese. Coseismic displacements were measured at 128 stations, with magnitudes of up to 12 meters. A remarkable set of transient postseismic deformation signals were also recorded. Several continuously recording GPS stations measured transient signals with amplitudes of more than 10 cm in both the vertical and horizontal components. The dense spatial coverage and extraordinary signal to noise ratio make the coseismic and postseismic deformation fields of the Chichi earthquake arguably the best ever recorded. Preliminary analysis of the coseismic displacements demonstrate that the earthquake was caused by an east-dipping thrust, consistent with field and seismological observations. However, the GPS data can not be fit with slip on a single fault surface. The surface rupture undergoes a 90 degree bend at its northern end, with a ~15 km long east-west trending break. Aftershock focal mechanisms suggest strike slip motion on this E-W striking branch. The GPS data, however, are best fit with a curved dipping fault with oblique slip on both the N-S trending and E-W trending segments. The preferred model explains 98% of the variance in the data. The residuals, however, are far too large, and spatially coherent to be explained by measurement error. The Chelungpu fault, is part of a fold thrust system, which emplaced older and stiffer rocks over sediments and the corresponding spatial variations in elastic properties may bias the modeling. The investigators and their Taiwanese collaborators propose an exhaustive analysis of the coseismic deformation accounting for vertical and lateral variations in elastic properties, non-planar fault geometry, spatial variations in slip, and possibly effects of irregular surface topography. As exciting as the coseismic displacements are, the postseismic deformations are even more significant. Accelerated post earthquake deformation has been known for decades, however the physics of the phenomenon are poorly understood. Afterslip, viscous flow of the lower crust and upper mantle, and poroelastic relaxation have all been proposed to explain transient postseismic deformation. Theoretical studies abound; the problem has been a lack of defining data sets. The data from the Chichi earthquake is of such high signal to noise ratio that we hope to finally answer these long standing problems. Specifically, the investigators propose an intensive modeling effort to compare predictions from viscoelastic, poroelastic, and afterslip models to the data. Preliminary results suggest that afterslip provides a reasonable fit to the data from the first 100 days after the mainshock, although much more analysis is needed. Inversion of the GPS data will reveal the spatiotemporal evolution of postseismic slip. These results will have important implications for the tectonics of collision zones. Numerous models of fold thrust belts show faults merging into a master decollement beneath Taiwan. By determining the geometry of the Chelungpu fault at seismogenic and subseismogenic depths the investigators will provide important insights into the structure of this classic collisional orogen. The investigators plan to develop models that both fit interseismic geodetic data and yield geologically reasonable displacements when integrated over multiple earthquake cycles. The proposed research will support tow Ph.D projects in an extensive collaboration with Taiwanese colleagues at the Institute of Earth Sciences, Academica Sinica Taiwan.