Description: 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 ( t ) of S coda for the 2004 sequences fluctuate around zero and are sometimes negative as a function of lapse time, whereas ( 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 (– V S ) is about two times larger than that of P coda (– V P ); average velocity reduction of Rayleigh waves (– V LR ) is 3–4 times larger than that of Love waves (– V LQ ); and (3)  V S and V LR display temporal velocity recovery with calendar time, especially for the 2005 sequences. The form of temporal velocity recovery of V S 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.

Description: Deep-focus earthquakes in the Tonga–Fiji subduction zone make up greater than 66% of the globally recorded deep seismicity. The high number of deep-focus seismicity in this active subduction zone allows us to search for deep-focus similar earthquake pairs and repeating earthquakes. We compile a waveform dataset for deep earthquakes with magnitude above 4.7 occurring in the Tonga–Fiji–Kermadec subduction zone recorded teleseismically between 1990 and 2009. We identify 8 similar clusters and 18 similar doublets with an average cross-correlation coefficient greater than 0.8 among more than 45,000 potential earthquake pairs. These similar doublets and clusters are located in the central part of the Tonga–Fiji slab at the depth range of 480–650 km. A master event relocation algorithm is used to determine the precise relative location and depth among these similar earthquake pairs. We estimate and superpose circular fault areas for these similar clusters and doublets and find that one similar doublet appears to be a deep repeating earthquake pair. This deep doublet has a small separation less than 0.4 km and overlapping rupture areas, indicating that the same fault appears to slip. Other deep similar earthquake pairs are spatially offset or do not exhibit overlapping rupture areas. Time separation is on the order of years for the majority of similar earthquake pairs. Thermal (plastic) shear instability is more likely to explain these deep repeating earthquakes and similar earthquake pairs.

Description: This paper examines moderate shallow-focus repeating earthquakes along the Tonga–Kermadec–Vanuatu subduction zones by cross-correlating teleseismically recorded waveforms. A total of 23 clusters and 31 doublets are identified with an average cross-correlation coefficient of 〉0.8. A master-event algorithm is used to determine the precise relative locations. I then estimate and superpose the source areas among these event pairs. This analysis reveals that most of these correlated pairs are repeating earthquakes, which have similar seismic moments ( M 0 ) and completely overlapping source areas. Most of the moderate repeating earthquakes were quasi-periodic with a recurrence interval ( T r ) on the order of years, and occurred at the plate interface. The repeating earthquakes are used to study spatial–temporal variations in fault-slip rate ( ) and interplate coupling. Apart from spatial variations in , a temporal acceleration in is observed, associated with large interplate earthquakes in the Vanuatu region. Interplate coupling is weak for most of the study areas, except the northern section 15°–19° S of the Tonga arc. Strong coupling in the northern Tonga interplate region appears to be at odds with the decoupling expected of a region associated with active back-arc extension. Repeating earthquakes are also used to examine the scaling relationship between M 0 and T r derived from the San Andreas fault (SAF). The T r – M 0 scaling relationship derived from the SAF can adequately account for the normalized T r for the Kermadec, Vanuatu, and Tonga interplate regions where it is decoupled, suggesting that the convergence rate is the predominant influence on the recurrence interval in a repeating earthquake sequence.