The composition of P codas using magnetic tape seismograms

Abstract

abstract The short-period P codas of 41 seismic events recorded on the plains of western Alberta are examined for compressional and shear phases. A non-linear “P-Detection” polarization filter is applied to 25 seconds of the records following the P onset. Numerous compressional wave signal pulses are detected amid the codas; these include PcP and PKP phases for events at appropriate distances and the common depth phases pP and sP. The phase pP is detected on the records of 25 events, 16 of which have reported depths shallower than 40 km. For events with pP visible at two stations, pP-P times are accurate to about ±1 second, allowing focal depth assignment to an accuracy of about ±15 km. Locally-generated shear phases are studied using particle-motion plots in the vertical (vertical-radial) and horizontal planes. There are large variations in SV/P amplitude ratios for individual events at stations separated by 60 to 160 km. Anomalous SV and SH-type motions are attributed to a complex Precambrian basement.