Observations of transverse energy for P waves recorded on a deep-ocean borehole seismometer located in the northwest Pacific

Abstract

In the classical radially stratified earth, P waves are confined to the sagittal plane, implying that there should be no energy observed in the transverse direction. However, a transverse fraction of the energy is observed in the initial part of the P -wave train and can be measured by separating the P -wave energy into its radial (lying in the sagittal plane) and transverse vectors. We refer to this fraction as the T/R energy ratio. The data consist of P -wave signals recorded on a borehole seismometer and explosion-source data recorded on the North American continent. The borehole instrument is the Hawaii Institute of Geophysics' three-component Ocean Sub-bottom Seismometer (OSS IV) located in the northwest Pacific in DSDP Hole 581C. We compare transverse energy observations of initial P waves from regional earthquakes and teleseismic events recorded in the deep ocean with continental records at similar distances. Envelope maxima and integrated energy on three components of motion, as well as T/R energy ratios, were measured for a range of frequency bands: 1 to 12 Hz for regional events and 1 to 3 Hz for teleseismic event. The T/R energy ratios obtained from envelope maxima for earthquakes recorded in the northwest Pacific are comparable to the ratios observed at Long-Range Seismic Monitoring stations on the North American continent: approximately 0.1 for events at teleseismic distances and 0.2 for regional events. We speculate that the two important constituents to the T/R energy ratios observed are near-source and near-receiver scattering.