LAUSR

Abstract The 2017 ML 5.4 Pohang Earthquake in southeastern Korea, which was induced by fluid injection from an enhanced geothermal system, and its foreshock–aftershock sequence has been recorded by a dense and portable temporary seismic array. The hypocentral distribution of the earthquake sequence reveals the reactivation of a complex subsurface fault system that was previously unmapped. Aftershock seismicity propagated to the southwest and revealed previously unknown fault segments. The aftershocks tend to concentrate along the tips of the main rupture, with a lack of aftershocks around the mainshock hypocenter. This pattern suggests that the stress in the mainshock area was almost completely released during the mainshock and early aftershock phase, and that the recurrence of moderate to large earthquakes in the hypocentral area is unlikely compared with the marginal areas. A new rupture occurred along the southwestern margin of the main rupture after the largest aftershock (ML 4.6).