Fault reactivation and propagation during the 2017 Pohang earthquake sequence.

• The M L 5.4 2017 Pohang earthquake triggered by fluid injection generated a complex geometry of optimally oriented rupture planes. • For a moderate-size earthquake, the earthquake sequence is unique because rupture planes propagated for more than 11 months after the mainshock. • The injection of fluid directly into optimally oriented faults can induce earthquakes that are larger than those predicted by theory. A commission of the Korean government on the 2017 Mw 5.5 Pohang earthquake concluded that seismic activity was triggered by fluid injection from the nearby Pohang enhanced geothermal system. The temporal and spatial distribution of hypocenters (depth range of 3–6 km) was recorded by our local seismic array for 345 days. It included, in addition to the mainshock, 10 foreshocks and 3090 aftershocks, indicating a complex geometry of propagating rupture segments, including the main segment and four subsidiary segments (SS1 to SS4). The fault kinematics of the earthquake sequence were dominated by strike and reverse slips. The aftershock sequence showed a decrease in frequency during the first 83 days after the mainshock, in accordance with Omori's law. This was followed by a sudden surge in aftershock frequency associated with the M L 4.6 event (11 February 2018), and another decay following Omori's law. The aftershock surge of the M L 4.6 event corresponds to the development of SS4. A calculation of the Coulomb stress change caused by the mainshock shows that a significant increase in stress occurred around SS4, thus suggesting that rupture along this segment was triggered by the mainshock. With respect to the current stress field, all rupture planes apart from that of SS4 are optimally oriented for reactivation, highlighting the potential risk associated with fluid injection into this fault. [ABSTRACT FROM AUTHOR]