Rupture Process of the 2017 Mw 5.5 Pohang, South Korea, Earthquake via an Empirical Green’s Function Method

ABSTRACT The Mw 5.5 earthquake occurred in Pohang, South Korea on 15 November 2017, which is known as a “runaway earthquake,” and was triggered from a critical-state fault as a result of fluid injection. As such earthquakes rarely occur, spatiotemporal slip distributions were investigated via the finite-fault inversion based on the empirical Green’s function in this study. The rupture process can be divided into three steps: first, slip initiated and propagated only to the southwest from the hypocenter during the initial 0.6 s; in the second step from 0.6 to 2.4 s, the slip occurred to the southwest and northeast parts, in which the maximum seismic moment was released; in the third step from 2.4 to 6.0 s, slip occurred around the edge of the fault plane farther from the hypocenter, particularly in the deep part in the northeast direction. In each step, the seismic moment was released as approximately 6%, 59%, and 35%, respectively. The first step can be interpreted not as a part of the rupture process of the mainshock but as the immediate and distinct foreshock. Overall, most of the slip distributed southwest is consistent with the results of the directivity analysis using apparent source time functions. Although the average stress drop (~1 MPa) of the Pohang earthquake is considerably lower than that (~20 MPa) of the Mw 5.5 Gyeongju earthquake that naturally occurred in the vicinity of the Pohang, it is difficult to attribute it only to the fluid injection effect. Through this study, we improve our comprehension of the seismic source physics and mechanisms of the Pohang earthquake by analyzing the spatiotemporal slip history, the directivity of rupture process, and the spatial distribution of the stress drop on the fault plane.