Frequency-Dependent Moment Tensors of Induced Microearthquakes

Analysis of 984 induced microearthquakes from The Geysers geothermal reservoir in California reveals that the retrieved moment tensors depend on the frequency band of the inverted waveforms. The observed dependence is more significant for the percentages of the double-couple, compensated linear vector dipole, and isotropic (ISO) components than for the focal mechanisms. The average root-mean-square of the moment tensors obtained in different frequency bands is correlated with spectra of ambient noise. The percentages of double-couple and ISO components tend to decrease and increase with the upper cutoff frequency (f(u)), respectively. This suggests that shear rupture radiates energy preferentially in a lower frequency band and tensile rupture in a higher frequency band. Events displaying a strong increase of the ISO with f(u) are confined within the same depth interval as the injection points. This might be related to the strong thermoelastic effects in the vicinity of injection points that promote opening of small cracks adjacent to the main fractures.