Direct Observation of Co-seismic Deformation at 1.5km Depth: The EGS Collab Fracture Stimulation Test
Abstract
A series of in-situ water injection experiments at 1480 m depth at the Sanford Underground Research Facility were performed for the U.S. Department of Energy's EGS Collab project. In a heavily instrumented testbed consisting of an injection borehole, a production borehole, and six monitoring boreholes, a series of hydraulic fractures were created in a metamorphic rock mass. Here we combined passive seismic monitoring and the SIMFIP borehole displacement sensor, sensitive to sub-µm deformation, to monitor the initiation and propagation of a hydraulic fracture. During the pressurization stage, and before the onset of hydraulic fracturing, we observed a single shear slip event. The injection pressure at the time of shearing was 13.4 MPa, which is substantially below the minimal principal stress of about 21 MPa. This interval pressure is consistent with the reactivation of one of the preexisting contact planes between the phyllitic rock mass and a quartz-rich inclusion.
Using the SIMFIP in-situ displacement probe, we were able to get a unique record of all phases of the rupture in shear, including pre-slip, co-seismic slip and after-slip. The pre-slip increased linearly over about 0.4 s, accumulating a total of 7 μm of shear displacement, preceding a 5.5 μm slip event associated with a microearthquake, lasting about 0.022 s. Deformation then exponentially decelerated over about 3 s, which we interpret as afterslip. At the end of the slip event a total of 22 μm of slip accumulated. Using passive seismic monitoring we located the event at the pressurized interval, and concluded that the event could be on the ruptured plane given the location uncertainty. No aftershocks of this event were observed. We think that this is the first time that the acceleration of the elastic strain rate to a seismic event and its postseismic arrest was directly observed under in-situ conditions.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S21F0574S
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7215 Earthquake source observations;
- SEISMOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS