Stress State and Fault Slip Susceptibility in the Pohang Enhanced Geothermal System, South Korea: Geomechanical Processes Affecting Injection-Induced Microseismicity and the MW 5.5 Pohang Earthquake
Abstract
The 2017 MW 5.5 Pohang earthquake occurred as a result of hydraulic stimulation associated with an enhanced geothermal system (EGS) project. Here we investigate the stress state prevailing prior to the earthquake at a depth of ~4.2 km, the target depth for the EGS hydraulic stimulation, in order to understand the geomechanical process that induced seismicity. We constrain the stress state using several different types of data acquired prior to and during stimulation: earthquake focal mechanism solutions, a dipole shear anisotropy log, and fluid pressure and injected water volume data recorded during hydraulic stimulation. The stress state at ~4.2 km depth is characterized by an ENE-WSW-trending subhorizontal maximum principal stress axis and a reverse-faulting stress regime (vertical stress =106 MPa, minimum horizontal principal stress = 115-121 MPa, and maximum horizontal principal stress = 243 MPa). The Pohang EGS project used two deep boreholes (PX-1 and PX-2, ~600 m apart at the injection depths) for hydraulic stimulation. Numerous microearthquakes of magnitudes <3.3 occurred in association with water injection, the hypocenters of which were distributed in spatially distinct clusters associated with the respective boreholes. Earthquakes associated with PX-2 injection lie along a plane (214°/43°), which is similar to the MW 5.5 mainshocks focal plane (214°/51°). This fault was well oriented for slip under the stress state that existed at the time of stimulation, and was susceptible to frictional failure even before the hydraulic stimulation began. The hypocenters of earthquakes associated with PX-1 injection exhibit an overall NW trend in their distribution, and a greater diversity of focal mechanisms than associated with PX-2. The faults presumed to have been activated by injection into PX-1 were not well oriented for slip under the prevailing stress state and required higher fluid pressures to induce frictional failure. Elevated fluid pressure associated with PX-2 injection induced progressive slip and microseismicity on a fault already susceptible to slip, and eventually triggered runaway rupture.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S14C..06T
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7223 Earthquake interaction;
- forecasting;
- and prediction;
- SEISMOLOGY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8168 Stresses: general;
- TECTONOPHYSICS