Abstract
AbstractHydraulic injection by the Pohang enhanced geothermal systems (EGSs) has been suspected to trigger the 2017 moment magnitude (MW) 5.5 Pohang earthquake in South Korea. The last stimulation experiment in the EGS was conducted only 2 months before the disaster, which has led to this suspicion. In this study, we conducted a seismic analysis on the earthquakes that have occurred around the EGS site in the past 10 years. The study included the construction of a velocity model, earthquake detection, the determination of hypocenters, magnitudes, focal mechanisms, and stress inversion, and a clustering analysis. No seismic activity was detected near the study area until November 2015 when there was a loss of a large quantity of heavy drilling mud. For three stimulations of a geothermal well, earthquakes sequentially migrated to the southwest along a fault plane, leading to the location of the mainshock. The delineated fault plane crossed the injection well at approximately 3,800 m, which corresponds to the borehole interval of not only the mud loss but also the breakage of the well's casing due to the mainshock rupture. These findings can be treated as empirical evidence for the hypothesis that the 2017 MW 5.5 Pohang earthquake was initiated on a critically stressed fault zone by the anthropogenic activity of fluid injection, consequentially releasing accumulated strain energy via tectonic loading.
Highlights
The 2017 Pohang earthquake of moment magnitude (MW) 5.5 struck Pohang, South Korea, on 15 November 2017, at 05:29:31 UTC (Grigoli et al, 2018; Kim et al, 2018; Lee et al, 2019)
We were able to prove that there is a clear causal link between the origin of the 2017 MW 5.5 Pohang earthquake and injected fluids injected as part of an enhanced geothermal systems (EGSs) project through spatiotemporal relations
The relocated seismicity indicates that the earthquakes during the PX‐1 stimulation periods represented an ellipsoidal distribution to the NW‐SE, whereas seismicity during the PX‐2 stimulations had the shape of an NE‐SW fault plane dipping to the SE
Summary
The 2017 Pohang earthquake of moment magnitude (MW) 5.5 struck Pohang, South Korea, on 15 November 2017, at 05:29:31 UTC (hereinafter, the Pohang earthquake) (Grigoli et al, 2018; Kim et al, 2018; Lee et al, 2019). EGS projects, such as in the Pohang case, require fluid injection processes at high pressures through deep boreholes of a few kilometers (Zang et al, 2014) Such high pressure exerted on the surrounding rock during fluid injection causes cracks that increase the permeability of the rock mass, making it easy to extract heated fluid from deeper areas according to the geothermal gradient. This crack‐forming process can induce seismicity (Ellsworth, 2013; Maxwell, 2014).
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