ALBERT

Description: Earthquake moment tensors reflecting seven years of global seismic activity (2004–2010) are presented. The results are the product of the global centroid-moment-tensor (GCMT) project, which maintains and extends a catalog of global seismic moment tensors beginning with earthquakes in 1976. Starting with earthquakes in 2004, the GCMT analysis takes advantage of advances in the mapping of propagation characteristics of intermediate-period surface waves, and includes these waves in the moment-tensor inversions. This modification of the CMT algorithm makes possible the globally uniform determination of moment tensors for earthquakes as small as MW = 5.0. For the period 2004–2010, 13,017 new centroid-moment tensors are reported.

Description: A large number of fundamental-mode Love and Rayleigh wave dispersion curves were determined from seismograms for 3330 earthquakes recorded on 258 globally distributed seismographic stations. The dispersion curves were sampled at periods between 25 and 250 s to determine propagation-phase anomalies with respect to a reference earth model. The data set of phase anomalies was first used to construct global isotropic phase-velocity maps at specific frequencies using spherical-spline basis functions with a nominal uniform resolution of 650 km. Azimuthal anisotropy was then included in the parametrization, and its importance for explaining the data explored. Only the addition of 2ζ azimuthal variations for Rayleigh waves was found to be resolved by the data. In the final stage of the analysis, the entire phase-anomaly data set was inverted to determine a global dispersion model for Love and Rayleigh waves parametrized horizontally using a spherical-spline basis, and with a set of B-splines to describe the slowness variations with respect to frequency. The new dispersion model, GDM52, can be used to calculate internally consistent global maps of phase and group velocity, as well as local and path-specific dispersion curves, between 25 and 250 s.

Description: Centroid moment tensor solutions are recomputed for 190 earthquakes from 1976 to 2010 along the Heezen, Tharp and Hollister transform faults of the Eltanin system using a 3-D seismic velocity model. The total length of the three en echelon faults is nearly 1000 km; each is characterized by fast long-term rates of displacement of about 80 mm yr-1. Strike-slip faulting with moment magnitudes Mw up to 6.4 characterizes most of these events. The few involving normal faulting are located up to 40 km on either side of the transforms and involve extension nearly normal to the transforms. This partitioning of slip likely results from changes during the last few million years in the Euler pole for relative motion between the Antarctic and Pacific plates. Some parts of the Heezen and Tharp transforms exhibit strong seismic coupling but others were aseismic at the resolution of our study, Mw 〉 5.0-5.5. Earthquakes were not found along nearby fast spreading ridges at that resolution. We calculate downdip widths of seismic coupling of about 5 km for four strongly coupled segments from observed moment rates and lengths along strike assuming earthquake activity accounts for the entire plate motion. Major differences in seismic coupling along strike are not in accord with common thermal models of plate cooling but instead are attributed to varying degrees of metamorphism, rock type and effective normal stress and possibly to the presence of short intratransform spreading centres. One 30-42-km-long segment of the Heezen transform that appears to be an isolated well-coupled asperity has ruptured in eight earthquakes of Mw 5.9-6.1 quasi-periodically with a coefficient of variation of 0.26 every 4.0 ± 1.0 yr. Other well-coupled fault segments, which were sites with earthquakes up to Mw 6.39 and fewer events since 1976, have average repeat times of about 7-24 yr. The fast rate of plate motion, maximum size of events and relatively short repeat times make these fault segments a good laboratory for research on quasi-periodic behaviour and earthquake prediction.