The Distance Dependence of Regional Phase Discriminants
Abstract
A number of proposed discriminants for distinguishing the character of seismic sources use the amplitude of the phase Pn as a reference. The merit of Pn is that it constitutes the onset of regional seismograms, but the behaviour of the arrivals at an individual station can be quite complex. Results from long-range refraction experiments in Eurasia suggest that the complexity arises from the superimposition of a number of sub-phases returned from fine- scale structure in the uppermost mantle. The behaviour is consistent with fine scale horizontal variations superimposed on a gentle increase in seismic velocity with depth, so that equivalent one-dimensional models will show multiple low velocity zones. Long range refraction data for Sn is much less common but similar trends can be discerned. As frequency increases such complexity is likely to become more important. Sn is often observed at distance beyond 300 km but emerges from Sg (Lg) much less clearly in general than Pn emerges from Pg. However some refraction profiles in the Finnish Shield areas show very clear Sn arrivals at short distances. The differences in character can be associated with differing velocity gradients for S in the uppermost mantle. The variability in Sn behaviour means that it may prove difficult to generate discriminants based on amplitudes which can be readily transportable between different regions.
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 04, 1991
- Accession Number
- ADA247546
Entities
People
- Brian L. Kennett
Organizations
- Australian National University