| Popis: |
An exponential relation, log N = a - bM where N is the annual number of earthquakes of magnitude M, is generally accepted to exist between the number of earthquakes occurring in a given time and the earthquake magnitude. A linear least-squares solution of this equation indicates that a small earthquake (2 M 3) occurs every 30 sec. The only difference between these small P waves and an event is one of magnitude; therefore, it is reasonable to expect that the P-wave noise would contain discrete P-wave arrivals which travel across the array as plane waves. The purpose of this study is to identify and describe (statistically) such discrete arrivals. Two techniques were used to identify discrete arrivals. One method, Fisher analysis of variance, provides an output time function which measures the likelihood of an arrival in a specific segment of noise. The other technique calculates a moving power spectra and looks for spectral characteristics which are sufficiently deviant from the expected values of a Gaussian stationary process. Using an 8-min noise sample, the methods were able to detect no discrete arrivals propagating across the array as plane waves. Within the resolving power of these tests, no small P wavelets appeared to be in the mantle P-wave noise. |
