Anomalous Love wave phase velocities

Abstract

Anomalously high Love wave phase velocities in the period range 60–110 sec have been measured between the station pair La Paz, Bolivia (LPB), and Arequipa, Peru (ARE), of the World Wide Standard Station Network for an earthquake in the Republic of the Congo. The implied upper mantle anisotropy necessary to produce agreement between Love and Rayleigh wave phase velocities is of the order of 15%. Examination of the LPB and ARE Gl seismograms shows clearly that the first and possibly also the second higher Love modes have been excited and that the first higher mode may be traveling with nearly the same group velocity as the fundamental mode at periods greater than 60 sec. G3 phase velocities for the same earthquake were measured in the period range 60–110 sec and exhibit ‘normal’ behavior. The normal G3 phase velocities, together with the clear indication of higher mode contamination of the Gl wave train, provide unambiguous evidence that in this instance anomalous Love wave phase velocities result from higher mode interference.