Observations of long period earthquakes accompanying hydraulic fracturing

Waveforms of most seismic events accompanying hydraulic fracturing have been reported to contain clear P and S waves and have fault plane solutions consistent with shear displacement across a fault. This observation is surprising since classical hydraulic fracturing theory predicts the creation of a tensile opening of a cavity in response to fluid pressure. Very small long period events, similar to long period earthquakes observed at volcanoes, were found to occur during four hydraulic fracturing experiments carried out at Fenton Hill, New Mexico. Since the long period earthquakes occur in the same region as the shear type events, it is concluded that the unusual character of the long period earthquake waveforms is due to a source effect and not a path effect. The occurrence of long period earthquakes during hydraulic fracturing could indicate tensile fracturing. Many waveforms of these events are identical, which implies that these events represent repeated activation of a given source. A proposed source for these long period events is the sudden opening of a channel that connects two cracks filled with fluid at different pressures. The sizes of the two cracks differ, which causes two or more peaks to appear in the spectra, each peak being associated with one physical dimension of each crack. From the frequencies at which spectral peaks occur, crack lengths are estimated to be between 3 and 20m.