No abstract

The two largest earthquakes of the past 40 years ruptured a 1600-kilometer-long portion of the fault boundary between the Indo-Australian and southeastern Eurasian plates on 26 December 2004 [seismic moment magnitude ( M w ) = 9.1 to 9.3] and 28 March 2005 ( M w = 8.6). The first event generated a tsunami that caused more than 283,000 deaths. Fault slip of up to 15 meters occurred near Banda Aceh, Sumatra, but to the north, along the Nicobar and Andaman Islands, rapid slip was much smaller. Tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer.

[1] A surface wave dispersion data set of unprecedented size is used to obtain a variableresolution model of the radially anisotropic shear wave velocity structure of the upper mantle beneath North America and globally. Love and Rayleigh wave phase velocities for periods in the range 35-150 s constrain a three-dimensional model of velocity variations on a length scale of a few hundred kilometers within the North American continent and a few thousand kilometers globally. The short-and long-wavelength models are determined simultaneously. Long-period surface wave phase velocities (200-350 s) are used to help constrain longer-wavelength and transition zone structure. Laterally varying velocity sensitivity kernels are used to account for the dependence of the velocity sensitivity on lateral variations in crust and mantle velocity structure. The sensitivity kernels are updated in several iterations to avoid nonlinearities associated with the inverse problem for the determination of mantle structure. Variations in isotropic velocity in the uppermost several hundred kilometers of the mantle are found to correlate well with surface tectonic features. Within the North American craton, the locations of strongest radial anisotropy generally correlate with the locations of fastest isotropic velocity. Variations in radial anisotropy show a clear continent-ocean signature. Strong anisotropy occurs at shallow depths (<100 km) under the continents, with a secondary peak found at a depth of $200 km. Maximum anisotropy under the oceans occurs at a depth of $125 km, with no secondary maximum. Combined interpretation of isotropic and anisotropic continent-ocean differences suggests a different role for the low-velocity zone under continental and oceanic regions.

[1] We used satellite images to examine the calving behavior of Helheim and Kangerdlugssuaq Glaciers, Greenland, from 2001 to 2006, a period in which they retreated and sped up. These data show that many large iceberg-calving episodes coincided with teleseismically detected glacial earthquakes, suggesting that calving-related processes are the source of the seismicity. For each of several events for which we have observations, the ice front calved back to a large, pre-existing rift. These rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high, which agrees well with earlier theoretical predictions. In addition to the recent retreat in a period of higher temperatures, analysis of several images shows that Helheim retreated in the 20th Century during a warmer period and then re-advanced during a subsequent cooler period. This apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water. The cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in Greenland can advance rapidly. Greenland's larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance.

We have detected dozens of previously unknown, moderate earthquakes beneath large glaciers. The seismic radiation from these earthquakes is depleted at high frequencies, explaining their nondetection by traditional methods. Inverse modeling of the long-period seismic waveforms from the best-recorded earthquake, in southern Alaska, shows that the seismic source is well represented by stick-slip, downhill sliding of a glacial ice mass. The duration of sliding in the Alaska earthquake is 30 to 60 seconds, about 15 to 30 times longer than for a regular tectonic earthquake of similar magnitude.

[1] While it is agreed that the great Sumatra earthquake of December 26, 2004 was among the largest earthquakes of the past century, there has been disagreement on how large it was, which part of the fault ruptured, and how the rupture took place. We present a centroid-moment-tensor (CMT) analysis of the earthquake in which multiple point sources are used in the inversion to mimic a propagating slip pulse. The final model consists of five point sources, with the southernmost sources accounting for the majority of the moment release. The presumed fault planes of the southern sources strike northwest, while those in the north strike northeast, consistent with the geometry of the subduction trench. Slip on the fault is found to be more oblique in the north than in the south. The inversion with five sources leads to a moment magnitude for the Sumatra earthquake of M W = 9.3, consistent with estimates from long-period normal-mode amplitudes. Citation: Tsai, V. C., M. Nettles, G. Ekström, and A. M. , Multiple CMT source

No abstract

[1] Glacial earthquakes are anomalous earthquakes associated with large ice-loss events occurring at marine-terminating glaciers, primarily in Greenland. They are detectable teleseismically, and a proper understanding of the source mechanism may provide a remote-sensing tool to complement glaciological observations of these large outlet glaciers. We model teleseismic surface-wave waveforms to obtain locations and centroid-singleforce source parameters for 121 glacial earthquakes occurring in Greenland during the period 2006-2010. We combine these results with those obtained by previous workers to analyze spatial and temporal trends in glacial-earthquake occurrence over the 18-year period from 1993-2010. We also examine earthquake occurrence at six individual glaciers, comparing the earthquake record to independently obtained observations of glacier change. Our findings confirm the inference that glacial-earthquake seismogenesis occurs through the capsize of large, newly calved icebergs. We find a close correspondence between episodes of glacier retreat, thinning, and acceleration and the timing of glacial earthquakes, and document the northward progression of glacial earthquakes on Greenland's west coast over the 18-year observing period. Our results also show that glacial earthquakes occur when the termini of the source glaciers are very close to the glacier grounding line, i.e., when the glaciers are grounded or nearly grounded.