Are ENA Potentially Induced Earthquakes Different from Natural Earthquakes?

English Abstract:

The question of whether induced earthquakes have significantly different source characteristics than naturally occurring earthquakes (NOE) is important if such a difference exists. I review the Brune stress parameter (stress drop) estimates from some recent potentially induced earthquakes (PIE). Transverse velocity Fourier spectra are examined for a corner-frequency peak, taking into account the possible presence of a neighboring spectral peak due to the strong surface waves from shallow events. Brune stress parameters for an assumed geometrical spreading of R^-1 are estimated from observed magnitude and corner-frequency for each event. (An assumed geometrical spreading of R^-1.3 would yield a proportionally higher stress parameter as stated by Boore et al., 2010.) Atkinson and Boore (2006) found that the average ENA Brune stress parameter of this type to be 14 MPa. PIEs in Arkansas, Oklahoma, Texas, Kansas, and Ohio have Brune stress parameters ranging from 5 to 10 MPa. Shallow NOEs (depth less than 5 km) appear to have a similar range of Brune stress parameters. Ground motions for a given earthquake can be influenced by stress parameter and by crustal attenuation (Q). PIEs in Arkansas, Oklahoma, Texas, and Kansas are near a Q boundary at about 35N latitude between ENA mid-continental and Gulf Coast Q, which influences ground motions (and intensities) observed to the north differently than to the south. Justin Hollenback for the NGA East project at the PEER Center also shows that event terms from a mixed effects analysis for the NGA East database are similar for PIEs and shallow NOEs and lower than deeper NOEs. Thus both a Brune stress parameter analysis and an event term analysis (allowing for regional Q) show that ENA PIEs are similar to shallow NOEs and lower for deeper NOEs and hence may not be distinct from shallow NOEs.