Co‐ and postseismic slip of the great earthquakes can 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 and/or 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. This study analyzes lag times of high‐frequency (0.5–2.0 Hz) P‐ and S‐coda waves as well as long‐period (0.03–0.1 Hz) surface waves from repeating aftershock sequences of the great 2004 Sumatra–Andaman and 2005 Nias–Simeulue earthquakes. The observed lag times reveal several major characteristics: (1) lag‐time series τ(t ) of S coda for the 2004 sequences fluctuate around zero and are sometimes negative as a function of lapse time, whereas τ(t ) of S coda for the 2005 sequences exhibit a monotonic increase as a function of lapse time; (2) average velocity reduction of S coda (−δVS) is about two times larger than that of P coda (−δVP); average velocity reduction of Rayleigh waves (−δVLR) is 3–4 times larger than that of Love waves (−δVLQ); and (3) δVS and δVLR display temporal velocity recovery with calendar time, especially for the 2005 sequences. The form of temporal velocity recovery of δVS of the 2005 sequences is similar to available displacement time series of the nearby geodetic station. Whereas we discuss potential artifacts, such as source separation, temporal changes of the noise field, and instrument response, observations collectively point to nonuniform temporal velocity reduction in the crust modulated by co‐ or/and postseismic slip of the 2004/2005 great earthquakes.
Online Material: Table of source parameters and figure showing tests on lag‐time measurements of long‐period Rayleigh waves.