The fault that moved during the 2017 Mw 5.4 Pohang earthquake in SE Korea;　implications for the EGS project and the generation of the Pohang earthquake

Abstract

<p>     Korean Government Commission concluded that the stimulation during the Pohang EGS project triggered the 2017 Mw 5.5 Pohang earthquake in SE Korea which propagated far beyond the stimulated zone of ~ 1 km in size in granitic basement where induced earthquakes had occurred (Ellsworth et al., 2019, SRL).  Distributions of aftershocks of the Pohang earthquakes (Kim et al., 2018, Science) and of the induced earthquakes both indicate that the Pohang earthquake fault (i.e., fault that moved during the Pohang earthquake) cuts PX-2 well at a depth of 3,800 m.  We found abundant fault gouge and fine fault breccia in forms of mud balls (round-shaped fragile fragments of fault rocks coated by thin drilling mud) in the PX-2 borehole cuttings at the depths of 3790-3816 m (26 m interval).  The fault rocks in mud balls are very similar to fault gouge and fine fault breccia constituting the fault cores of the Yangsan and Yeongdeok faults, major faults in SE Korea.  The average content of the fault rocks in the coarse cuttings is 48% and this corresponds to a fault zone of several meters in width, even wider than the fault cores (~ a few meters) of the major faults.  Thus the Pohang earthquake fault is a large fault in SE Korea.  Big mud loss occurred at depths of 3815~3850 m during the PX-2 drilling, causing induced seismicity, and the mud loss is likely to have occurred in fractured host rock in the damage zone.  The PX-2 drilling record reports 10~40 % of gouge at 3790~3805 m depths, nearly consistent with our result, but we could not find fault rocks at many other depths where the drilling record reports gouge.</p><p>     It was so unlucky for the EGS project to have a large-scale fault between PX-1 and PX-2 wells.  First, water injection during the EGS stimulation became nearly direct injection into the faut zone, causing the Pohang earthquake with a small amount of injected water (5,841 m^3; injected water minus flow back).  Second, an impermeable gouge zone could have shut down hydraulic connection between the two wells to inhibit water circulation in the EGS project.  On the other hand, the Pohang earthquake can be a prototype earthquake for studying the mechanisms of induced/triggered earthquake because the induced earthquakes occurred within about 1 km in rather homogeneous granitic rocks (simple geology) and fault-rock samples only several hundred meters away from the epicenter of the Pohang earthquake are available for physical property measurements.  Preliminary experiments on the fault rocks at a temperature of 200 degC, pore water pressure of 30 MPa, and effective normal stresses of 10, 20 and 30 MPa revealed friction coefficients of 0.55 to 0.7 with slight velocity weakening.  The frictional properties are distinctly different from those of the surface fault gouge from the Yangsan and Yeongdeok fault zones.  It is important to reproduce the Pohang earthquake by modeling with known injection history and with measured frictional and transport properties.</p>