The Physical Basis of mb:Ms and Variable Frequency Magnitude Methods for Earthquake/Explosion Discrimination
Abstract
We analyze the theoretical basis for the body wave magnitude: surface wave magnitude (mb:Ms) and Variable Frequency Magnitude (VFM) earthquake/ explosion discriminants in light of recent results from numerical modeling of earthquake and explosion dynamics. We examine the effects of source mechanism (double-couple versus point dilatation), source depth, source spectra, and source region elastic structure. We use six three-dimensional finite difference simulations of earthquakes for the earthquake source models, and four empirical (Mueller-Murphy) and four numerical models for explosions in different materials. We estimate that source mechanisms (i.e., quadrupole versus monopole) contributes about .35 magnitude on the average to the separation between earthquakes and explosions on a mb:Ms diagram. VFM discrimination depends primarily on spectral differences between earthquakes and explosions. Theoretical separation of the earthquake and explosion populations is very clear for explosions in granite at frequencies of 2 Hz or greater with mb 4.0. Smaller events may be discriminated using higher frequencies. Separation is not as good for explosions in low velocity materials such as tuff, especially at low yields. For small events, the mb:Ms and VFM discrimination methods work (or fail) for quite different reasons.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 01, 1982
- Accession Number
- ADA121617
Entities
People
- J. L. Stevens
- S. M. Day
Organizations
- Utility Systems Science and Software (United States)