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We develop a procedure to improve estimates of relative earthquake locations using Rayleigh and Love wave arrivals for multiple earthquakes recorded at common stations. We fit predicted differential traveltimes to those measured using a cross-correlation technique, and correct the phases of the cross-correlation functions for phase delays that result from the surface-wave radiation patterns. We develop an empirical measure of uncertainty that provides realistic estimates of the errors in the earthquake locations. We investigate the effectiveness of the relocation procedure by first applying it to two suites of synthetic earthquakes. We then relocate real earthquakes in three separate regions: two ridge-transform systems and one subduction zone. We demonstrate that the inclusion of source corrections in the relocation procedure results in improved location estimates compared to relocations without source corrections. The source correction also allows for automated application of the relocation procedure, even in regions with a wide range of earthquake focal mechanisms. [ABSTRACT FROM AUTHOR]

Many large calving events at Greenland's marine‐terminating glaciers generate globally detectable glacial earthquakes. We perform a cross‐correlation analysis using regional seismic data to identify events below the teleseismic detection threshold, focusing on the 24 hr surrounding known glacial earthquakes at Greenland's three largest glaciers. We detect additional seismic events in the minutes prior to more than half of the glacial earthquakes we study and following one third of them. Waveform modeling shows source mechanisms like those of previously known glacial earthquakes, a result consistent with available imagery. The seismic events thus do not represent a failure of the high subaerial ice cliff like that expected to trigger large‐scale calving and a marine ice‐cliff instability but, rather, rotational, buoyancy‐driven calving events, likely of the full glacier thickness. A limited investigation of the prevalence of smaller seismic events at times outside glacial‐earthquake windows identifies several additional events. However, we find that calving at the three glaciers we study—Jakobshavn Isbræ, Helheim Glacier, and Kangerdlugssuaq Glacier—often occurs as sequences of discrete buoyancy‐driven events in which multiple icebergs ranging in size over as much as three orders of magnitude are all lost within ∼30 min. We demonstrate a correlation between glacial‐earthquake magnitude and iceberg size for events with well‐constrained iceberg‐area estimates. Our results suggest that at least 10–30% more dynamic mass loss occurs through buoyancy‐driven calving at Greenland's glaciers than previously appreciated. Key Points: Seismic data show ice motion prior to half of large‐scale calving events we study and following one third of eventsSeismogenic ice motion prior to large calving events represents not subaerial ice‐cliff failure but rotational, buoyancy‐driven calvingWe present the first empirical demonstration of a relationship between the seismic magnitude of a glacial earthquake and iceberg size [ABSTRACT FROM AUTHOR]

Glacial earthquakes are caused by large iceberg calving events, which are an important mechanism for mass loss from the Greenland ice sheet. The number of glacial earthquakes in Greenland has increased sixfold over the past two decades. We use teleseismic surface waves to analyze the 145 glacial earthquakes that occurred in Greenland from 2011 through 2013, and successfully determine source parameters for 139 events at 13 marine-terminating glaciers. Our analysis increases the number of events in the glacial-earthquake catalog by nearly 50% and extends it to 21 years. The period 2011-13 was the most prolific 3-year period of glacial earthquakes on record, with most of the increase over earlier years occurring at glaciers on Greenland's west coast. We investigate changes in earthquake productivity and geometry at several individual glaciers and link patterns in glacial-earthquake production and cessation to the absence or presence of a floating ice tongue. We attribute changes in earthquake force orientations to changes in calving-front geometry, some of which occur on timescales of days to months. Our results illustrate the utility of glacial earthquakes as a remote-sensing tool to identify the type of calving event, the grounded state of a glacier, and the orientation of an active calving front. [ABSTRACT FROM AUTHOR]

Glacial earthquakes are slow earthquakes of magnitude M~5 associated with major calving events at near-grounded marine-terminating glaciers. These globally detectable earthquakes provide information on the grounding state of outlet glaciers and the timing of large calving events. Seismic source modeling of glacial earthquakes provides information on the size and orientation of forces associated with calving events. We compare force orientations estimated using a centroid-single-force technique with the calving-front orientations of the source glaciers at or near the time of earthquake occurrence. We consider earthquakes recorded at four glaciers in Greenland – Kangerdlugssuaq Glacier, Helheim Glacier, Kong Oscar Glacier, and Jakobshavn Isbræ – between 1999 and 2010. We find that the estimated earthquake force orientations accurately represent the orientation of the calving front at the time of the earthquake, and that seismogenic calving events are produced by a preferred section of the calving front, which may change with time. We also find that estimated earthquake locations vary in a manner consistent with changes in calving-front position, though with large scatter. We conclude that changes in glacial-earthquake source parameters reflect true changes in the geometry of the source glaciers, providing a means for identifying changes in glacier geometry and dynamics that complements traditional remote-sensing techniques. [ABSTRACT FROM AUTHOR]

We examine the effect of overtone interference on fundamental-mode Love-wave phase measurements made using single-station and array-based techniques at 25-100 s periods. For single-station teleseismic measurements on USArray Transportable Array data, the contamination effects are small, less than 1% of the path-averaged phase velocity, consistent with previous studies. Single-station amplitude measurements provide complementary constraints on the interference pattern. For array-based measurements on the same data set, contamination effects are much larger: up to ~10% of the phase velocity for two-station measurements and up to ~20% for mini-array measurements. The interference pattern for single-station measurements from shallow earthquakes can largely be explained by interactions between only two modes, the fundamental mode and the first higher mode. This inteipretation is confirmed using measurements on both mode-summation synthetic waveforms for a 1D Earth model and synthetic waveforms calculated using SPECFEM3D Globe and a 3D Earth model. Array-based phase measurements are calculated from differences of the single-station phase delays, and we demonstrate that the overtone interference pattern for array-based measurements can be approximated using gradients of the single-station interference pattern with distance. This relationship can lead to an overall bias to higher phase velocities when combined with common quality selection and data-reduction procedures for array measurements. Our results indicate that arraybased Love-wave phase measurements must be carefully scrutinized for overtone contamination and suggest the possibility of new approaches for measuring overtone phase velocities. [ABSTRACT FROM AUTHOR]

Glacial earthquakes are a new class of seismic events, first discovered as signals in long-period seismograms recorded on the Global Seismographic Network. Most of these events occur along the coasts of Greenland, where they are spatially related to large outlet glaciers. Glacial earthquakes show a strong seasonality, with most events occurring during the late summer. The rate of glacial-earthquake occurrence increased between 2000 and 2005, with a stabilization of earthquake frequency at 2003-2004 levels in 2006-2008. Recent observations establish a strong temporal correlation between the distinct seismic signals of glacial earthquakes and large ice-loss events in which icebergs of cubic-kilometer scale collapse against the calving face, linking the seismogenic process to the force exerted by these icebergs on the glacier and the underlying solid earth. A sudden change in glacier speed results from these glacial-earthquake calving events. Seasonal and interannual variations in glacier-terminus position account for general characteristics of the temporal variation in earthquake occurrence. Glacial earthquakes in Antarctica are less well studied, but they exhibit several characteristics similar to glacial earthquakes in Greenland. [ABSTRACT FROM AUTHOR]

The 1975 Kalapana, Hawaii, earthquake occurred under the highly mobile south flank of Kilauea Volcano. It has been interpreted variously as a normal-faulting earthquake, a thrust-faulting earthquake, and a landslide. Primary evidence for the landslide model has been the failure of previous faulting models to explain the observed Love-wave radiation pattern and the tsunami amplitudes generated by the Kalapana event. Here, we present a reanalysis of the long-period, digital seismic data for this event. Centroid-moment-tensor analysis shows that the seismic radiation pattern can be explained well by thrust faulting on a plane dipping shallowly landward. The seismic moment of 3.8 x 1020 N m (Mw 7.7) that we determine is approximately twice as large as earlier estimates. The geometry and seismic moment of the focal mechanism determined here are consistent with the observed tsunami amplitudes. Inversion of long-period body-wave waveforms shows that the earthquake source duration (∼72 sec) is unusually long for an earthquake of this size. [ABSTRACT FROM AUTHOR]

We study the spectral amplitudes of the first two Earth radial modes, 0 S 0 and 1 S 0 , excited by the Sea of Okhotsk earthquake of 24 May 2013, the largest deep event ever recorded, in the search for an isotropic component to its source. In contrast to the case of the 1994 Bolivian earthquake, we detect an implosive component M I = - 1.1 × 10 27  dyn*cm, equivalent to 3% of the full scalar moment, but 14% of the lone deviatoric component exciting the Earth’s radial modes. An independent moment tensor inversion, using the standard GlobalCMT algorithm but after relaxing its zero-trace constraint, similarly yields an implosive isotropic component, albeit with a larger amplitude, while it fails to document one in the case of the 1994 Bolivian deep earthquake. An implosive component to the source is expected in the model of transformational faulting in which deep earthquake rupture nucleates and grows upon transformation of metastable olivine to ringwoodite in the cold subducting slab. This interpretation is supported by quantitative estimates (0.9–4 m) of the thickness of the transformed shear zone, which scale favorably, relative to earthquake fault length, with the upper end of the range of laboratory results reported for ices, germanates and silicates. The resulting extent of the transformation in the metastable wedge is consistent with the local geometry of the deep slab, as recently determined by rupture modeling and aftershock distribution. Our results are in contrast to those for the two runner-up largest deep earthquakes, the 1994 Bolivian and 1970 Colombian shocks, for which a similar isotropic component could not be detected. We attribute this difference to variability in the ratio of isotropic to deviatoric components, which combined with the smaller size of the 1970 and 1994 events, would make any putative implosive component fall below detection levels, especially in the case of the 1970 Colombian earthquake for which only analog narrow-band records were available. [ABSTRACT FROM AUTHOR]

This article presents an abstract of the response of the authors to the comments made by scholars S. Neetu, I. Suresh, R. Shankar, S.S.C. Shenoi on the paper "The Great Sumatra-Andaman Earthquake of 26 December 2004" that was published in the May 20, 2005 issue of the periodical "Science." The authors support the revised estimate of tsunami source length obtained by Neetu and others. Sea-level monitoring with a high sampling rate, good azimuthal coverage, and real-time access, along with detailed bathymetry data around the stations, would improve source region estimation from tsunami arrival times.