Premium
A technique to obtain a Q(f) model from microearthquake swarms
Author(s) -
Carpenter P. J.,
Sanford A. R.,
Ake J. P.
Publication year - 1987
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl014i008p00828
Subject(s) - microearthquake , spectral line , physics , seismology , confidence interval , range (aeronautics) , geology , computational physics , mathematics , statistics , induced seismicity , materials science , astronomy , composite material
The frequency dependence of the seismic quality factor, Q, is important in interpreting seismic wave spectra for nuclear test discrimination, earthquake damage studies, and in exploration. In this study, P and S wave spectra from 18 events recorded during three 1977 microearthquake swarms are used to compute Q(f) (frequency range = 3‐40 Hz) for a seismically active geothermal region of disturbed upper crust in the central Rio Grande rift near Socorro, New Mexico. Numerically differentiated spectra are fit with a model of the form Q(f) = Q o f η and standard linear regression techniques used to compute Q o and η, along with their uncertainties. Only two out of 18 events in these swarms exhibit significant Q p or Q s frequency dependence at the 95% confidence level. This is primarily due to large uncertainties in η generated by upper‐crustal reverberations. Averaging Q p and Q s over events in each swarm shows that for two swarms, a significant frequency dependent Q p exists with η ranging from 0.29 to 0.67 and Q op ranging from 5 to 17 (95% confidence intervals). A strong decrease in Q p or Q s with frequency (η < −0.2) appears to be ruled out. Likewise, the frequency dependence of Q p and Q s does not appear to change with event distance over the range 15.4‐16.9 km. Smoothed spectra produce less frequency dependence and higher Q o values than unsmoothed spectra (the only significant Q(f) is Q os = 123 and η = 0.33 for one swarm). This suggests that much of the apparent frequency dependence may be due to ripple in the spectra (caused by scattered phases in the time window, or the process of windowing itself) and that this technique should only be used with "smooth" spectra.