Hydraulic fracturing is a well-established completion technique that enables the economic extraction of natural gas, natural gas liquids, and oil from unconventional reservoirs such as shale, which would otherwise behave as low permeability formations. Diffusion of water outward from the newly created hydraulic fractures into the reservoir helps reactivate the pre-existing faults and fractures and initiates shear failure on complex networks of pre-existing planes of weaknesses. Although the energy released by shear failure during hydraulic fracturing is minor when compared to the energy released by tectonic shear events and does not pose any risk of structural damage, surface seismic monitoring can be an important diagnostic tool for evaluating the effectiveness of reservoir stimulation. We carried out surface seismic monitoring during the hydraulic fracturing of horizontal Marcellus Shale wells in Greene County, Pennsylvania and in Monongalia County, West Virginia. We used a single broadband seismometer within the footprint of six lateral wells for surface monitoring in Greene County. In Monongalia County, we used an array of five broadband seismometers to monitor the hydraulic fracturing of two horizontal wells. Common field observations related to microseismic fracture-mapping indicates preferential growth along the maximum horizontal stress. Recent findings on long period, long duration tremors suggest that "slow slip emission" along weaknesses that are unfavorably oriented in the ambient stress field is likely the dominant mechanism of deformation and plays a vital role in reservoir stimulation. We identified a total of 117 high-amplitude, impulsive events and 473 long period, long duration (LPLD) events from the combined dataset of Greene and Monongalia Counties. The timing and location of the majority of the impulsive events does not favor any causal relationship with the hydraulic fracturing in the study area and are likely associated with natural/background seismicity or blasts associated with civil construction and mining activities. LPLD events identified in this study show a low-frequency, low-amplitude precursor followed by a relatively high-frequency, high-amplitude primary S-wave signal and are similar to other long duration events identified in previous studies. Spectral analysis of LPLD events reveals a concentration of energy at low frequencies (1–30 Hz). During various stages of hydraulic fracturing, LPLD events were found to occur most frequently when the pumping pressure and rate were at maximum values.

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