Temporal velocity changes in the crust associated with the 2007 South Sumatra great earthquake doublet

Co- and post- seismic slip of great earthquakes could give rise to temporal changes in the medium either due to strong ground motion damaging near-surface sediment layer or stress perturbations modulating crack density or/and fluid movement at depth. Such time-varying crustal properties can result in fractional change in seismic velocity that can be probed from cross-correlating waveforms and measuring their time lags within a repeating earthquake sequence. We examine lag-times of high frequency (0.5 - 2.0 Hz) coda waves and long period (0.03 - 0.1 Hz) surface waves from repeating earthquake sequences occurred in the Sunda megathrust subduction zones. Repeating earthquakes with high correlation coefficients occurred near afterslip zones of the Mw 9.2 2004 Sumatra, the Mw 8.6 2005 Nias, and the Mw 8.4 and Mw 7.9 2007 South Sumatra earthquakes, which cover a 12-year time window since the Mw 9.2 December 2004 Sumatra great earthquake. The observed lag-times reveal several characteristics: (1) for repeating earthquakes of the 2004 and 2005 earthquakes, lag-times of high frequency S coda waves exhibit a monotonic increase as a function of lapse time, and velocity exhibits a temporal recovery over time, with δV_S of -0.1% gradually increasing to the baseline. The detected temporal changes of δV_S could be explained by stress perturbations at depth induced by afterslip of the 2004 and 2005 earthquakes; (2) lag-times of Rayleigh waves exhibit negative values of -0.5 s before the 2007 event, followed by slightly positive values of 0.1 s after the 2007 South Sumatra event. In other words, the 2007 South Sumatra event would disrupt temporal velocity recovery process of the near-surface structures and produce an additional velocity reduction with δV_LR of -0.6%; and (3) for repeaters of the 2007 South Sumatra event, lag-times of Rayleigh waves exhibit a value of 0.5 s, corresponding to -0.4 % velocity reduction over time, followed by a velocity recovery. The δV_LR detected associated with the 2004, 2005, and 2007 main shocks are mainly due to the effect of strong ground motion induced by earthquakes that damage near-surface structures. In particular, for the repeaters of the 2004 and 2005 earthquakes, the stronger velocity reduction detected after 2007 reflects the re-damaged effects due to the 2007 South Sumatra earthquake.