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

Rob Westaway,Neil M Burnside

doi:10.1155/2019/1280721

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.

Highlights

Anthropogenic seismicity associated with geoengineering projects is emerging as a significant issue for society
Having noted that most of the water produced by flowback, following the injection experiment in August 2017, was originally groundwater, and that much of the injected surface water remained in the subsurface; we have investigated the possibility that hydrochemical “corrosion” of the granite by the injected surface water contributed to the earthquake
We have derived a theory linking the postinjection time delay of the seismicity to the temperature at depth by considering the kinetics of dissolution of quartz within the granite and linking the volume of surface water that remains in the subsurface to the seismic moment of the resulting earthquake

Summary

Introduction

Anthropogenic seismicity associated with geoengineering projects is emerging as a significant issue for society. Considerations of proximity between the two wells and the Namsong Fault indicate the potential importance of injection into well PX-1: top of the open-hole section of well PX-2 is ~400 m below the Namsong Fault, whereas if this fault dips at only 43° [5], it passes barely 100 m below the bottom of well PX-1, the aforementioned limited downward propagation of the seismicity accompanying this injection [17] suggesting that the injected fluid, and the fracture network it created, reached this fault Such considerations raise the possibility that significant causal factors of the 15 November 2017 Pohang earthquake have been overlooked in the work reported so far, including [5]. We analyze the geomechanics of the site, including calculations of the effect of hydrochemical “corrosion” on the state of stress

Conditions at the Pohang EGS Site

Water Sampling and Analysis

Findings

Discussion

Conclusions