Fault Rupture Model of the 2016 Gyeongju, South Korea, Earthquake and Its Implication for the Underground Fault System

Abstract

The 2016 Gyeongju earthquake (ML 5.8) was the largest instrumentally recorded inland event in South Korea. It occurred in the southeast of the Korean Peninsula and was preceded by a large ML 5.1 foreshock. The aftershock seismicity data indicate that these earthquakes occurred on two closely collocated parallel faults that are oblique to the surface trace of the Yangsan fault. We investigate the rupture properties of these earthquakes using finite‐fault slip inversion analyses. The obtained models indicate that the ruptures propagated NNE‐ward and SSW‐ward for the main shock and the large foreshock, respectively. This indicates that these earthquakes occurred on right‐step faults and were initiated around a fault jog. The stress drops were up to 62 and 43 MPa for the main shock and the largest foreshock, respectively. These high stress drops imply high strength excess, which may be overcome by the stress concentration around the fault jog.

The 2016 Gyeongju earthquake sequence revisited: aftershock interactions within a complex fault system

“…The asymmetric aftershock distributions with respect to hypocentres may suggest that E1 and E2 ruptured towards the southwest and northeast, respectively (Mendoza & Hatzell 1988). This is consistent with the rupture directivity inferred from the finite fault slip models of E1 and E2 (Uchide & Song 2018).…”

“…Following the scheme of Hong et al (2017), the apparent frictional coefficient μ', Young's modulus and Poisson's ratio were set to 0.4, 80 GPa and 0.25, respectively. The finite rupture models of E1 and E2 from Uchide & Song (2018) were used to configure the slip amounts on the faults. Fig.…”

“…Therefore, a more complicated structure than the simple Riedel shears is needed to explain minor faults. The hypothesis proposed by Uchide & Song (2018) suggested that the fault rupture models for E1 and E2 occurred in a fault jog of extensional oversteps and the pull-apart stress between them would produce normal faults in the aftershock area. However, we did not observe normal faulting events; we suspect that the faulting types of the Gyeongju aftershocks are more likely to be affected by local stress fields and pre-existing faults, rather than by the directivity of rupture propagations.…”

“…From the distribution of the hypocentres and inverted moment tensors of the three events, it has been demonstrated that these earthquakes occurred on a deep-seated fault system at a depth range of 10-18 km (Kim et al 2016a;Hong et al 2017;Son et al 2017;Kim et al 2017a,b;Lee et al 2018). In particular, Son et al (2017) delineated two distinct parallel dextral faults striking to the NNE-SSW direction from relocated aftershocks, and Uchide & Song (2018) observed that the inverted finite fault slips of E1 and E2 propagated towards SSW and NNE directions, respectively. A possible correlation between the Yangsan Fault and the Gyeongju earthquakes has been raised because the epicentres are located close to the fault, with 30 km of dextral displacement (Kyung 2003;Kim et al 2017b,c;Lee et al 2018).…”

“…The three moment tensor solutions of E1, E2 and E3 are distinguished as three different colours of compressional quadrants: green (E1), red (E2) and blue (E3). The blue dots in (a), (c) and (d) represent one hour of seismic activities following E3, and the estimated rupture propagation directions of E1 and E2 byUchide & Song (2018) are denoted as green and red arrows, respectively. Fault geometries of F1, F2a and F2b are illustrated as green, yellow and red lines, respectively.…”