Major influencing factors for the nucleation of the 15 November 2017 Mw 5.5 Pohang earthquake

The occurrence mechanism and key factors to induce the 15 November 2017 M_W5.5 Pohang earthquake are investigated through combined interpretation of active fault zone, stress field evolution, earthquake source properties, fault structure, and spatiotemporal seismicity change. Analysis of historical earthquakes suggests the presence of active faults spawning moderate-size earthquakes around the Pohang Enhanced Geothermal System. The instrumental seismicity suggests that the faults were kept loaded until the occurrence of the Pohang earthquake. The Coulomb stress changes induced by the 2016 M_L 5.8 (M_W5.4) Gyeongju earthquake and the 2011 M_W9.0 Tohoku-Oki earthquake suggest additional stress loading on the faults. The spatial distribution and focal mechanism solutions of mainshock and aftershocks illuminate a complex segmented fault structure. The fault geometry suggests that the fluid injection was made at the northeastern fault-segment conjunction. Numerical modeling of stress transfer and rupture dimension suggests that a M_L3.1 (M_W3.3) earthquake 7 months before the mainshock loaded the stress of tens of kilopascals around the hypocenter of mainshock. A series of the seismic activities including the Tohoku-Oki earthquake and local moderate-size earthquakes as well as high-pressure fluid injection contributed to the occurrence of the Pohang earthquake.