Observations of a hydrofracture induced earthquake sequence in Harrison County Ohio in 2014

Abstract:

On October 7, 2014, a Mw 1.9 earthquake was detected and located using the IRIS Earthscope Transportable Array stations in Ohio. The earthquake was located at a depth of ~3 km near the interface of the Paleozoic sedimentary rocks with the crystalline Precambrian basement. The location is within a few kilometers laterally of a 2013 earthquake sequence that was linked to hydraulic fracturing (HF) operations on three wells in Harrison county (Friberg et al, 2014). Using the Mw 1.9 event as a template in a multi-component cross correlation detector on station O53A, over 1000 matching detections were revealed between September 26 - October 17, 2014. These detections were all coincident in time with HF operations on 3 nearby (< 1km away) horizontally drilled wells (Tarbert 1H, 3H, and 5H) in the Utica formation (~2.4 km depth). The HF operations at two of the wells (1H and 5H) were coincident with the majority of the detected events. The final well (3H) stimulated in the series, produced only about 20 identified events. In addition to the coincident timing with nearby HF operations, the time clustered nature of the detections were similar to the 2013 sequence and two other Ohio HF induced sequences in 2014 (Skoumal et al, 2015). All of the other HF induced earthquake sequences in Ohio were related to operations in the Utica formation.

Interestingly, this sequence of earthquakes did not follow a simple Gutenberg-Richter magnitude frequency relationship and was deficient in positive magnitude events; the magnitude 1.9 was preceded by a magnitude 1.7, and only a ½ dozen events slightly above magnitude 0.0. The majority of the events detected were below magnitude 0.0, with some as low as magnitude -2.0. While the majority of detections are too small to locate, high similarity in waveform character indicate they are spatially near to the magnitude 1.9 event. Furthermore, gradual shifts in P phase arrival relative to S phases indicate events are moving away from the station with progressive HF stages. Given the orientation of the wells relative to the station, the migration of events away from the station with progressive stages is also supportive of this being an induced sequence.