Characteristics of Microearthquakes Accompanying Hydraulic Fracturing as Determined from Studies of Spectra of Seismic Waveforms

Abstract

A study of the spectral properties of the waveforms recorded during hydraulic fracturing earthquakes has been carried out to obtain information about the physical dimensions of the earthquakes. We find two types of events. The first type has waveforms with clear P and S arrivals and spectra that are very similar to earthquakes occurring in tectonic regions. These events are interpreted as being due to shear slip along fault planes. The second type of event has waveforms that are similar in many ways to long period earthquakes observed at volcanoes and is called long period. Many waveforms of these events represent repeated activation of a given source. We propose that the source of 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 to or more peaks to appear in the spectra, each peak being associated with one physical dimension of the crack. From the frequencies at which spectral peaks occur, we estimate crack dimensions of between 3 and 22 m.