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2017 Mw 5.4 Pohang earthquake, South Korea and poroelastic stress change associated with fluid injection

YoungHee Kim, Hobin Lim, Kai Deng, Jin-Han Ree, & Teh-Ru Song

Published August 14, 2018, SCEC Contribution #8499, 2018 SCEC Annual Meeting Poster #091

We compute the spatiotemporal change in Coulomb stress using a fault failure model under various conditions to better understand the nature of 2017 Mw 5.4 earthquake (2017-11-15, 05:29) that occurred near the geothermal power plant in Pohang, South Korea. This Mw 5.4 earthquake is the second largest earthquake in South Korea during the instrumental period, and the space-time variation of seismicity before this event is well correlated with fluid injection history from January 2016 (Kim et al., 2018). We compute the stress and pore pressure perturbed by fluid injection at the geothermal site in the framework of linear poroelasticity. Shear modulus, drained Poisson’s ratio, Skempton’s coefficient, the viscosity of the fluid, and frictional coefficient are assumed to be 30 GPa, 0.25, 0.75, 10-16 m2, 0.4×10-3 Pa·s, and 0.6. respectively, and these values are previously used in the modeling for understanding earthquakes induced by hydraulic fracturing in Central Alberta, Canada (Deng et al., 2016). We define our fault geometry with a strike of N36°E, a dip of N65°W, and a rake of 90° (i.e. thrust) based on the relocated aftershocks for 169 days and/or focal mechanism of the mainshock. We also test a range of the fault geometries and sense of fault motions to account for the uncertainty in the focal mechanism. The first-order pattern of the Coulomb stress change shows positive in the southeast and negative in the northwest relative to the location of the injection well at depths of 3 and 3.5 km. The polarity of the Coulomb stress is reversed at depths of 4.5 and 5.0 km. At a depth of the injecting fluid (4.127 km), the Coulomb stress shows positive in all direction. By comparing observed spatiotemporal stress change to the relocated seismicity, we find that most of the aftershocks including the M4.3 aftershock (2017-11-15, 07:49) occur in the region where the Coulomb stress is positive. Our modeling results indicate that fluid generates significant stress changes at earthquake depths.

Key Words
2017 M5.4 Pohang earthquake; EGS, enhanced geothermal system; fluid injection; poroelastic modelling

Kim, Y., Lim, H., Deng, K., Ree, J., & Song, T. (2018, 08). 2017 Mw 5.4 Pohang earthquake, South Korea and poroelastic stress change associated with fluid injection. Poster Presentation at 2018 SCEC Annual Meeting.

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