ALBERT

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: We investigate the relationship between seismic moment M 0 and source duration t w of microearthquakes by using high-quality seismic data recorded with a vertical borehole array installed in central Taiwan. We apply a waveform cross-correlation method to the three-component records and identify several event clusters with high waveform similarity, with event magnitudes ranging from 0.3 to 2.0. Three clusters—Clusters A, B and C—contain 11, 8 and 6 events with similar waveforms, respectively. To determine how M 0 scales with t w , we remove path effects by using a path-averaged Q . The results indicate a nearly constant t w for events within each cluster, regardless of M 0 , with mean values of t w being 0.058, 0.056 and 0.034 s for Clusters A, B and C, respectively. Constant t w , independent of M 0 , violates the commonly used scaling relation ${t_w} \propto M_0^{1/3}$ . This constant duration may arise either because all events in a cluster are hosted on the same isolated seismogenic patch, or because the events are driven by external factors of constant duration, such as fluid injections into the fault zone. It may also be related to the earthquake nucleation size.

Description: The 2005 M (sub w) 8.6 Nias-Simeulue earthquake was caused by rupture of a portion of the Sunda megathrust offshore northern Sumatra. This event occurred within an array of continuous Global Positioning System (GPS) stations and produced measurable vertical displacement of the fringing coral reefs above the fault rupture. Thus, this earthquake provides a unique opportunity to assess the source characteristics of a megathrust event from the joint analysis of seismic data and near-field static co-seismic displacements. Based on the excitation of the normal mode data and geodetic data we put relatively tight constraints on the seismic moment and the fault dip, where the dip is determined to be 8 degrees to 10 degrees with corresponding moments of 1.24X10 (super 22) to 1.00X10 (super 22) N m, respectively. The geodetic constraints on slip distribution help to eliminate the trade-off between rupture velocity and slip kinematics. Source models obtained from the inversion of various combinations of the teleseismic body waves and geodetic data are evaluated by comparing predicted and observed long-period seismic waveforms (100-500 sec). Our results indicate a relatively slow average rupture velocity of 1.5 to 2.5 km/sec and long average rise time of up to 20 sec. The earthquake nucleated between two separate slip patches, one beneath Nias and the other beneath Simeulue Island. The gap between the two patches and the hypocentral location appears to be coincident with a local geological disruption of the forearc. Coseismic slip clearly tapers to zero before it reaches the trench probably because the rupture propagation was inhibited when it reached the accretionary prism. Using the models from joint inversions, we estimate the peak ground velocity on Nias Island to be about 30 cm/sec, an order of magnitude slower than for thrust events in continental areas. This study emphasizes the importance of utilizing multiple datasets in imaging seismic ruptures.

Description: We determine coseismic and the first-month postseismic deformation associated with the Sumatra-Andaman earthquake of 26 December 2004 from near-field Global Positioning System (GPS) surveys in northwestern Sumatra and along the Nicobar-Andaman islands, continuous and campaign GPS measurements from Thailand and Malaysia, and in situ and remotely sensed observations of the vertical motion of coral reefs. The coseismic model shows that the Sunda subduction megathrust ruptured over a distance of about 1500 km and a width of less than 150 km, releasing a total moment of 6.7-7.0X10 (super 22) N m, equivalent to a magnitude M (sub w) approximately 9.15. The latitudinal distribution of released moment in our model has three distinct peaks at about 4 degrees N, 7 degrees N, and 9 degrees N, which compares well to the latitudinal variations seen in the seismic inversion and of the analysis of radiated T waves. Our coseismic model is also consistent with interpretation of normal modes and with the amplitude of very-long-period surface waves. The tsunami predicted from this model fits relatively well the altimetric measurements made by the JASON and TOPEX satellites. Neither slow nor delayed slip is needed to explain the normal modes and the tsunami wave. The near-field geodetic data that encompass both coseismic deformation and up to 40 days of postseismic deformation require that slip must have continued on the plate interface after the 500-sec-long seismic rupture. The postseismic geodetic moment of about 2.4X10 (super 22) N m (M (sub w) approximately 8.8) is equal to about 30+ or -5% of the coseismic moment release. Evolution of postseismic deformation is consistent with rate-strengthening frictional afterslip.

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