Search

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.

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.

Centroid–moment-tensor solutions are presented for 1034 earthquakes that occurred during 2002. The solutions are obtained using the method of Dziewonski et al. [Dziewonski, A.M., Chou, T.-A., Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852] and applying corrections for aspherical Earth structure represented by the whole-mantle shear-velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., Acoustic imaging at the planetary scale, in: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, vol. 19. Plenum Press, New York, 1992, pp. 785–797]. The model of anelastic attenuation of Durek and Ekstrom [Durek, J.J., Ekstrom, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation. Bull. Seism. Soc. Am. 86, 144–158] is used to predict the decay of the waveforms. The focal mechanisms of the largest, or otherwise significant, earthquakes of 2002 are reviewed.

Centroid–moment-tensor (CMT) solutions are presented for 1087 earthquakes that occurred during 2003. The solutions are obtained using the method of Dziewonski et al. [Dziewonski, A.M., Chou, T.-A., Woodward, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852] and applying corrections for aspherical Earth structure as represented by the whole-mantle shear-velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, vol. 19. Plenum Press, New York, pp. 785–797]. The model of inelastic attenuation of Durek and Ekstrom [Durek, J.J., Ekstrom, G., 1996. A radial model of inelasticity consistent with long-period surface wave attenuation. Bull. Seism. Soc. Am. 86, 144–158] is used to predict the decay of the waveforms. The focal mechanisms of the largest, or otherwise significant, earthquakes of 2003 are reviewed.

Centroid-moment tensor (CMT) solutions are presented for 961 earthquakes occurring during 2001. The solutions are obtained using corrections for aspherical Earth structure represented by the whole-mantle shear-velocity model SH8/U4L8 of Dziewonski and Woodward [A.M. Dziewonski, R.L. Woodward, Acoustic imaging at the planetary scale, in: H. Emert, H.-P. Harjes (Eds.), Acoustical Imaging, vol. 19, Plenum Press, New York, 1992, pp. 785–797]. The model of anelastic attenuation of Durek and Ekstrom [Bull. Seism. Soc. Am. 86 (1996) 144] is used to predict the decay of the waveforms.

Centroid moment tensor solutions are presented for 76 intermediate-depth earthquakes (with reported depths between 130 and 300 km) covering the years 1962–1975. These solutions are obtained by applying the algorithm used for modern events to restricted datasets of analog (WWSSN) and digital (HGLP) seismograms.

A methodology is developed for determining accurate instrument response functions for seismographs of the High-Gain Long-Period (HGLP) Network. Recordings of daily transient calibration pulses provide a detailed history of the time-varying responses of the HGLP stations. When the instrument displacement transfer function is available, knowledge of the seismometer mass, calibration coil constant, and calibration current allows for the prediction of the shape and amplitude of the calibration signal by simple integration of the transfer function. In theory the inverse of this statement is also true, but in practice noise in the recordings contaminates the instrument response function recovered through direct differentiation of the calibration pulse. The unknown transfer function of the seismograph system is, instead, first described by the product of the nominal responses of its mechanical and electromagnetic components. Specific values of the characteristic parameters for each seismograph component are subsequently determined by iterative least-squares minimization of the misfit between observed and model transient calibration pulses. By application of this method to daily calibration pulses recorded on the ten stations of the HGLP network, refined instrument response functions for 1976 and part of 1975 are determined. The results agree well with single frequency amplitude calibration data recovered from station maintenance records, and produce synthetic waveforms which are consistent with data from other well-calibrated stations. We use the recalibrated HGLP data together with long-period data from early International Deployment of Accelerometers (IDA) and Seismic Research Observatory (SRO) digital stations to determine 108 centroid-moment tensor (CMT) solutions for moderate and large earthquakes of 1976. The new CMT results for several major earthquakes, including the Tangshan, Guatemala, and Friuli events, are compared with those of previous studies.