Permanent Displacement From Surface-Rupturing Earthquakes: Insights From Dynamic Rupture of Mw7.6 1999 Chi-Chi Earthquake

Abstract eng:
Large permanent displacements, sometimes named “fling”, resulted from surface-rupturing faulting have been observed in some earthquakes, such as the 1999 Kocaeli, 1999 Chi-Chi, 2002 Denali, reaching amplitudes over 8 m. This type of nearsource ground motion is different from ordinary ground motion often evaluated by engineers and seismologist, for example to derive traditional ground motion prediction equations or to conduct seismic hazard studies. These permanent displacements are formed from coherent long period velocity pulses caused mainly by the offset of the ground surface when fault-rupture extends to the earth surface. To further investigate this type of ground motion, we developed dynamic rupture simulations of the Mw7.6 1999 Chi-Chi, Taiwan earthquake using asperity models in which the shallow layer (SL) of the first 2km depth is assumed to operate during rupture with enhanced energy absorption mechanism, as such it is parameterized with negative stress drop. Our physics-based dynamic rupture models show that the velocity ground motion causing permanent displacement near the source is dominated by the energy carried by the slip velocity pulse at the shallow layer fault of the first 2km depth, though this shallow zone is characterized with negative or zero stress drop. The asperities are the main driver elements to break the shallow layer and free-surface, but if surface rupture is not allowed, the rupture is inhibited to extend along strike, consequently the velocity pulse causing permanent displacement is reduced significantly. This study suggests that the slip velocities at the shallow fault zone, in which zero or negative stress drop operates, are the main source causing strong velocity pulses causing permanent displacement near the source. Therefore, when using simplified models, such as kinematic models, to study near source ground motion of surface-rupturing earthquakes, these slip velocity functions at the shallow zone have to be carefully considered.

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