Attenuation and Detection Capability of Regional Phases Recorded at NORESS
Semi-Annual rept. no. 3, 1 Mar-31 Aug 1987
SCIENCE APPLICATIONS INTERNATIONAL CORP SAN DIEGO CA
Pagination or Media Count:
The attenuation and detectability of regional Pn and Lg phases recorded at the NORESS array in Norway are estimated. We determine the frequency-dependent attenuation by inverting Pn and Lg single-channel spectra recorded at NORESS from 186 events at ranges between 200-1400 km with magnitudes between 1.1 and 4.8. The Pn spectra are inverted between 1-15 Hz and the Lg spectra between 1-7 Hz for both seismic moment and apparent attenuation. To resolve parameter trade-offs, we require that the separate Pn and Lg attenuation models produce an internally consistent set of source parameters. We find that regional Pn spectra are accurately described by 1r to the 1.3 power geometric spreading and Qf 325f to the 0.48 power. Lg spectra computed for a group velocity window of 3.6-3.0 kms are consistent with cylindrical spreading and Qf 560f to the 0.26 power. We determine the sensitivity of our estimates to the window length used in Fourier analysis, geometric spreading and source assumptions, and the inversion bandwidth. The final result is a simple parameterization of the source and range dependence of regional Pn and Lg spectra. Since signal detection is based on time domain amplitudes measured on filtered beams, a parameterization of single-channel spectra is not sufficient to determine detection capability, we determine a relationship between the spectral amplitudes used in the inversion and the time domain amplitudes used in signal detection.
- Seismic Detection and Detectors