The disastrous Sumatra-Andaman earthquake of 26 December 2004 was one of the largest ever recorded. The damage potential of such earthquakes depends on the extent and magnitude of fault slip. The first reliable moment magnitude estimate of 9.0 was obtained several hours after the Sumatra-Andaman earthquake, but more recent, longer-period, normal-mode analyses have indicated that it had a moment magnitude of 9.3, about 2.5 times larger. Here we introduce a method for directly imaging earthquake rupture that uses the first-arriving compressional wave and is potentially able to produce detailed images within 30 min of rupture initiation. We used the Hi-Net seismic array in Japan as an antenna to map the progression of slip by monitoring the direction of high-frequency radiation. We find that the rupture spread over the entire 1,300-km-long aftershock zone by propagating northward at roughly 2.8 km s(-1) for approximately 8 minutes. Comparisons with the aftershock areas of other great earthquakes indicate that the Sumatra-Andaman earthquake did indeed have a moment magnitude of approximately 9.3. Its rupture, in both duration and extent, is the longest ever recorded.

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[1] Nonvolcanic tremor is a recently discovered weak seismic signal associated with slow slip on a fault plane and has potential to answer many questions about how faults move. Its spatiotemporal distribution, however, is complex and varies over different time scales, and the causal physical mechanisms remain unclear. Here we use a beam backprojection method to show rapid, continuous, slip-parallel streaking of tremor over time scales of several minutes to an hour during the May 2008 episodic tremor and slip event in the Cascadia subduction zone. The streaks propagate across distances up to 65 km, primarily parallel to the slip direction of the subduction zone, both updip and downdip at velocities ranging from 30 to 200 km/h. We explore mainly two models that may explain such continuous tremor streaking. The first involves interaction of slowly migrating creep front with slip-parallel linear structures on the fault. The second is pressure-driven fluid flow through structurally controlled conduits on the fault. Both can be consistent with the observed propagation velocities and geometries, although the second one requires unlikely condition. In addition, we put this new observation in the context of the overall variability of tremor behavior observed over different time scales.

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Abstract. Source time functions of 255 moderate to great earthquakes obtained from inversions of teleseismic body waves by Tanioka and Ruff [ 1997] and coworkers were compared in a systematic way. They were scaled to remove the effect of moment and to allow the direct comparison and averaging of time function shape as well as duration. Time function durations picked by Tanioka and Ruff [ 1997] are proportional to the cube root of seismic moment if moments from the Harvard centroid moment tensor catalog are used. The average duration of scaled time functions is shorter and the average shape has a more abrupt termination for deeper events than shallower ones, with a distinct change occurring at-40 km depth. The complexity of the time functions, as quantified by the number of subevents, appears to decrease below-40 km depth. Furthermore, among events shallower than 40 km, the average duration of scaled time functions is shorter, and their average shape has a more abrupt termination (1) for events with strike-slip focal mechanisms compared to thrust events and (2) for the few thrust events associated with an intraplate setting compared to the majority associated with an interplate (subduction) boundary. In each of these cases, events in more tectonically and seismically active settings have a longer duration and a more gradual termination. This can be interpreted in terms of lower stress drops and/or slower rupture velocities at active plate boundaries, suggesting that fault rheology depends on slip rate and may evolve as total fault slip accumulates. Furthermore, differences in average time function shape and duration associated with different subduction zones suggest that differences exist in the rheology on the plate boundaries at the various subduction zones.

Abstract. To constrain dynamic source properties of deep earthquakes, we have systematically constructed broadband time functions of deep earthquakes by stacking and scaling teleseismic P waves from U.S. National Seismic Network, TERRAscope, and Berkeley Digital Seismic Network broadband stations. We examined 42 earthquakes with depths from 100 to 660 km that occurred between July 1, 1992 and July 31, 1995. To directly compare time functions, or to group them by size, depth, or region, it is essential to scale them to remove the effect of moment, which varies by more than 3 orders of magnitude for these events. For each event we also computed short-period stacks of P waves recorded by west coast regional arrays. The comparison of broadband with short-period stacks yields a considerable advantage, enabling more reliable measurement of event duration. A more accurate estimate of the duration better constrains the scaling procedure to remove the effect of moment, producing scaled time functions with both correct timing and amplitude. We find only subtle differences in the broadband time-function shape with moment, indicating successful scaling and minimal effects of attenuation at the periods considered here. The average shape of the envelopes of the short-period stacks is very similar to the average broadband time function. The main variations seen with depth are (1) a mild decrease in duration with increasing depth, (2) greater asymmetry in the time functions of intermediate events compared to deep ones, and (3) unexpected complexity and late moment release for events between 350 and 550 km, with seven of the eight events in that depth interval displaying markedly more complicated time functions with more moment release late in the rupture than most events above or below. The first two results are broadly consistent with our previous studies, while the third is reported here for the first time. The greater complexity between 350 and 550 km suggests greater heterogeneity in the failure process in that depth range.

[1] Seismograms from a dense, high-quality seismic network in Japan are used to investigate the characteristics of the 26 December 2004 Sumatra-Andaman and the 28 March 2005 Sumatran earthquakes. The onset of the P waveforms are aligned through cross correlation, and a simple concept of back-projecting seismic energy to a grid of potential source locations is applied. The waveform alignment removes the effects due to lateral variations in wave speed between the hypocenter and each station. To better approximate the effects of three-dimensional heterogeneity for paths originating from grid points away from the hypocenter, cross-correlation results of the P waveforms from aftershocks are introduced. This additional information leads to improved resolution of smaller-scale features near many of the aftershocks by reducing wavefront distortion. The back-projection analysis provides a quick assessment of the spatiotemporal extent and variability of relative high-frequency energy release, which can be translated into an estimate of the moment magnitude, as well as an unparalleled view of high-frequency rupture propagation. The results are, in general, consistent with those obtained from more involved source inversion methods. The 2004 Sumatra-Andaman earthquake released most energy in a region northwest of the Sumatra island and the rupture extended to the northern Andaman islands, about 1300 km from the epicenter. This northern portion of the rupture radiated a considerable amount of energy, but there is little evidence of slow slip. The 2005 event is imaged to have bilateral rupture with northwestern slip occurring for about 50 s before it moved to the southeast of the epicenter.