Aftershock Sequence and Statistics of the 2017 Mw 5.5 Pohang, South Korea, Earthquake: Implications of Fault Heterogeneity and Postseismic Relaxation

ABSTRACT The sequence of foreshocks, mainshock, and aftershocks associated with a fault rupture is the result of interactions of complex fault systems, the tectonic stress field, and fluid movement. Analysis of shock sequences can aid our understanding of the spatial distribution and magnitude of these factors, as well as provide seismic hazard assessment. The 2017 Mw 5.5 Pohang earthquake sequence occurred following fluid-induced seismic activity at a nearby enhanced geothermal system site and is an example of reactivation of a critically stressed fault system in the Pohang basin, South Korea. We created an earthquake catalog based on unsupervised data mining and measuring the energy ratio between short- and long-window seismograms recorded by a temporary seismic network. The spatial distribution of approximately 4000 relocated aftershocks revealed four fault segments striking southwestward. We also determined that the three largest earthquakes (ML>4) were located at the boundary of two fault segments. We infer that locally concentrated stress at the junctions of the faults caused such large earthquakes and that their ruptures on multiple segments can explain the high proportion of non-double-couple components. The area affected by aftershocks continues to expand to the southwest and northeast by 0.5 and 1 km decade−1, respectively, which may result from postseismic deformation or sequentially transferred static coulomb stress. The b-values of the Gutenberg–Richter relationship temporarily increased for the first three days of the aftershock sequence, suggesting that the stress field was perturbed. The b-values were generally low (<1) and locally variable throughout the aftershock area, which may be due to the complex fault structures and material properties. Furthermore, the mapped p-values of the Omori law vary along strike, which may indicate anisotropic expansion speeds in the aftershock region.