Evidence for azimuthal and temporal variations of the rupture propagation of the 1999 Chi-Chi, Taiwan earthquake from dense seismic array observations

Data recorded by the SMART-2 array and its nearby accelerometers in eastern Taiwan were used to make direct measurements of the rupture propagation of the 1999, M-W=7.6 Chi-Chi, Taiwan earthquake. The N-th root stack method w as employed to window and stack seismic waveforms, and it revealed that coh erent seismic energy showed clear temporal changes with regard to azimuth. The results of this study elicited unambiguous observations as to rupture p ropagation direction, rupture duration time and spatial distribution of the major asperities on the fault plane. The rupture propagation of the Chi-Ch i earthquake was reconstructed and demonstrated that the source of the earl y arrived coherent energy was close to the epicenter and that the subsequen t major energy moved southward. Twelve seconds after the initial rupture, c lear arrival energy was resolved and then propagated northward. The horizon tal apparent rupture velocity is estimated at 2.1 km/sec for its northward propagation. The largest source of radiated energy was identified and its l ocation was near the northern end of the earthquake fault. The strong shaki ng was continuous for about 24 seconds. The analysed results of this study provide direct observations of the rupture properties of this earthquake. T hese properties may serve as valuable constraints for other Chi-Chi earthqu ake rupture propagation studies based on other indirect measurement methods .