Fault “Corrosion” by Fluid Injection: A Potential Cause of the November 2017 MW 5.5 Korean Earthquake

Abstract

The November 2017 MW 5.5 Pohang earthquake is one of the largest and most damaging seismic events to have occurred in the Korean peninsula over the last century. Its close proximity to an Enhanced Geothermal System (EGS) site, where hydraulic injection into granite had taken place over the previous two years, has raised the possibility that it was anthropogenic; if so, it was by far the largest earthquake caused by any EGS project worldwide. However, a potential argument that this earthquake was independent of anthropogenic activity considers the delay of two or three months before its occurrence, following the most recent injection into each of the wells. A better understanding of the physical and chemical processes that occur following fluid injection into granite is thus warranted. We show that hydrochemical changes occurring while surface water, injected into granite, reequilibrates chemically with its subsurface environment, can account for time delays for earthquake occurrence of such duration, provided the seismogenic fault was already critically stressed, or very close to the condition for slip. This candidate causal mechanism counters the potential argument that the time delay militates against an anthropogenic cause of the Pohang earthquake and can account for its relatively large magnitude as a consequence of a relatively small-volume injection. The resulting analysis places bounds on combinations of physical and chemical properties of rocks, injected volume, and potential postinjection time delays for significant anthropogenic seismicity during future EGS projects in granite.

Constraining maximum event magnitude during injection-triggered seismicity

“…Known mechanisms for such earthquakes include: (1) elevated pore pressures that reduce effective normal stress, thus decreasing the fault strength; and (2) local encapsulated stress halos that exceed far-field stresses, causing shear failure without changing the fault strength 3 . Moreover, the maximum magnitudes of fluid injection associated earthquakes have been substantial, such as the 2011 Mw 5.7 Prague earthquake 6 , the 2016 Mw 5.1 Fairview earthquake 10 , the Mw 5.0 Cushing earthquake 11 , the Mw 5.8 Pawnee earthquake 12 , 13 , and the 2017 Mw 5.5 Pohang earthquake 14 , 15 .…”

The Potential for Low‐Grade Metamorphism to Facilitate Fault Instability in a Geothermal Reservoir

“…The Cl‐Ep assemblages are likely to be related to the geological history of the Pohang geothermal site. During the Cretaceous period, low‐angle subduction of hot oceanic lithosphere resulted in regional hydrothermal mineralization (Westaway & Burnside, 2019) and the deposition of chlorite and epidote in natural granite fractures (Sarkar et al., 2020; Wehrens et al., 2016). Outcrop samples of granite obtained from the Gonghe geothermal site in China also show the presence of Ep and Cl as shown in Figure 1c.…”

Fault Instability in a Geothermal Reservoir Facilitated by Low-grade Metamorphism

“…The proportions might transform under favorable hydrothermal conditions as chlorite + calcite ↔ epidote + dolomite (Franz & Liebscher, 2004), however, the time scale of this transformation may be beyond the engineering scale of a few months or years. Thus, even if calcite in fault rocks (e.g., granite and granodiorite) or injected water (Westaway & Burnside, 2019) can promote this transformation from chlorite to epidote, which increases the possibility of fault instability and increases the seismic potential, whether this mechanism can explain the recent earthquake in Pohang merits further investigation.…”