Small-aperture-array translational and rotational seismograms from distant sources – An example of the Jan Mayen Mw 6.8 of 30 August 2012 earthquake

Abstract

We present the seismic rotation rate due to the earthquake of Mw 6.7 at the Jan Mayen island, obtained from broad-band seismograms at a distance of about 2740km. The order of magnitude of the rotation rate amplitude is only 10−9rad/s in this case. It is studied with a focus on rotation-to-translation relations. A joint analysis of the rotational and translational data allowed us to determine the true backazimuth and phase velocity of S- and Rayleigh waves. For the surface waves, we studied the frequency dependence of both the backazimuth and phase velocity (wave dispersion). The results are independently confirmed by a method based on time delays between translational records within a small-aperture array. Both methods revealed an unusual velocity drop in the dispersion curve between the periods of 18 and 22s. This feature may be an indication of a low-velocity zone in the lower crust.