Identification of seismogenic fault network using earthquake focal mechanisms and stress constraints: A case of the 2016 Gyeongju earthquake sequence, South Korea

Abstract

We present a method to identify fault planes from earthquake focal mechanisms using stress field constraints to determine subsurface seismogenic fault geometry. Fault-plane ambiguity in focal mechanisms is resolved by applying two-step stress constraints. First, fault planes are inferred from the two nodal planes in each case by selecting those with the higher fault instability parameter, a function defined by plane orientations and stress state, using a commonly employed iterative linear stress inversion method. Second, the inferred fault planes are further screened by extracting those with a sufficiently high fault instability relative to the respective corresponding auxiliary planes, which is quantified by the instability ratio (IR) between the fault and auxiliary planes. Synthetic tests show that the threshold IR value, above which the inferred faults are all actual faults, varies with the degree of dispersion in fault instability. We apply the fault plane identification method to the 2016 Gyeongju earthquake sequence, which includes the largest instrumentally recorded event (ML 5.8) on the Korean Peninsula. For the Gyeongju earthquake sequence, faults having IR values greater than either ∼1.2 or ∼1.3, depending on the variability in stress state, are considered actual faults. The orientations and locations of individual faults provide better constraints for modeling the fault network than using hypocentral locations only. The constructed fault network consists of several fault structures that display four distinct orientations and constitute two conjugate fault systems. Our method can contribute to fault modeling at depth by providing independent clues for seismogenic fault geometry.