The Pohang earthquake under DESTRESS’s scrutiny

Since the last DESTRESS meeting in Glasgow in April 2018, great progress has been made by the different teams of Work Package 5 in the areas of seismic data analysis, the evaluation of the dominant stress field in the Pohang area, and the modelling of the possible effect of stimulation activities on triggering the Pohang earthquake. In addition, potential effects on fault stability due to hydrogeochemical changes after water injection were discussed.

All teams focused on the question whether stimulation activities could have triggered the Pohang magnitude 5.4 earthquake. None of the works by these teams focused on the question whether the earthquake could have been natural, although there are several forms of evidence that the Pohang area is naturally more seismically active than was believed in 2010 when the project was started by the Korean Pohang project consortium.

Currently, the locations of seismic events during stimulation in the PX-1 and PX-2 boreholes differ between the involved DESTRESS teams due to different velocity models and data treatment. However, the overall picture of the spatial distribution of seismic events during the different stimulation periods are in relatively good agreement. Although the results are still preliminary and need to be verified, a tendency of clustering of micro-seismicity for each stimulation period in various regions has been observed, probably indicating that varying structures have been activated by each stimulation treatment, at least during the first three stimulation campaigns.

Because data from fewer stations are available, the locations of the foreshocks, the magnitude 5.4 mainshock, and the aftershocks are less precise compared to those determined for the DESTRESS stimulation campaigns. One group has located the mainshock close to or within the areas previously stimulated by injections in well PX-2 at high pressures. However, the uncertainties are relatively large. This is also reflected by work conducted outside of the DESTRESS project, which has been published within the last months. Some of these publications locate the foreshocks, the mainshock, and the aftershock between the wells PX-1 and PX-2 at depths of around 4 km, others report depths of the main shock of around 6 km and locate the aftershocks outside the areas directly influenced by the stimulations. Work is ongoing by the DESTRESS teams to increase the precision of the localizations and to evaluate the relation of the stimulations with the earthquake using independent methods such as waveform similarities and focal mechanisms.

The stress field in the Pohang area has been studied by several DESTRESS teams. All teams come independently to the conclusion that the azimuth of the maximum principal stress of ~70° commonly observed in South Korea may be valid only at great depths in the Pohang area. But at the ~4 km depth of the boreholes the azimuth is at ~100-115° and at shallow depth of several hundred of metres at ~130°.

First results of at least six different conceptual and numerical modelling approaches have been presented at the workshop to reproduce the hydraulic and geomechanical conditions during the stimulations and to assess their impact on a conjectured fault network intersected by wells PX-1 and PX-2. None of the numerical studies has been finalized yet and we expect most of the modelling work to be continued during the next half a year. However, we expect some teams to provide some pertinent information earlier.

The following next steps are planned: A next internal workshop for DESTRESS work package 5 members will be held after the next DESTRESS General Assembly in Strasbourg at the end of February 2019. After the national Korean commission for the investigation of the Pohang earthquake has presented their conclusions in spring 2019, the DESTRESS project will communicate a summary of advanced and verified own results. It is also planned to publish DESTRESS results in academic journals as special issues once technical analysis is completed.