Time-advanced occurrence of moderate-size earthquakes in a stable intraplate region after a megathrust earthquake and their seismic properties

Abstract

The distance-dependent coseismic and postseismic displacements produced by the 2011 MW9.0 Tohoku-Oki megathrust earthquake caused medium weakening and stress perturbation in the crust around the Korean Peninsula, increasing the seismicity with successive ML5-level earthquakes at the outskirts of high seismicity regions. The average ML5-level occurrence rate prior to the megathrust earthquake was 0.15 yr−1 (0.05–0.35 yr−1 at a 95% confidence level), and the rate has increased to 0.71 yr−1 (0.23–1.67 yr−1 at a 95% confidence level) since the megathrust earthquake. The 2016 ML5-level midcrustal earthquakes additionally changed the stress field in adjacent regions, inducing the 15 November 2017 ML5.4 earthquake. The successive 2016 and 2017 moderate-size earthquakes built complex stress fields in the southeastern Korean Peninsula, increasing the seismic hazard risks in the regions of long-term stress accumulation. The increased seismic risks may continue until the medium properties and stress field are recovered.

The 2017 Mw 5.5 Pohang Earthquake, South Korea, and Poroelastic Stress Changes Associated With Fluid Injection

“…The pdf of the focal depth for the Pohang earthquake is 5.0 ± 0.8 (2σ) km, ranging from~4 to 6 km ( Figure 3c). The previous estimates of 4.27 km (KGC, 2019) and 6.2 km (Hong et al, 2018) are within our determined range, which corresponds to the depth range of granodiorite basement (Lee et al, 2015).…”

“…The direction of the maximum horizontal principal stress (S Hmax ) is indicated by black arrows (Soh et al, 2018). The red and blue “X” symbols indicate the locations of the Pohang earthquake according to Hong et al (2018) and Woo et al (2019), respectively. (b) Epicentral pdf s of the Pohang earthquake and aftershocks.…”

“…Herein, the term “induced” is used to include earthquakes triggered by anthropogenic causes that release tectonic stress, as well as earthquakes that release the stresses created by industrial activities (Doglioni, 2018; Ellsworth, 2013). In regions where the crust is critically stressed (e.g., Hong et al, 2018), small changes in stress can trigger fault slip, causing earthquakes (Reasenberg & Simpson, 1992; Stein, 1999).…”

Operational and geological controls of coupled poroelastic stressing and pore-pressure accumulation along faults: Induced earthquakes in Pohang, South Korea

“…This substantial discrepancy highlights that observed larger magnitude earthquakes than expected should involve additional physical mechanisms enhancing accumulation and subsequent release of elastic energy. Previous studies of basin-scale Coulomb stress modeling indicated that successive earthquakes driven by tectonics could attribute to the stored strain energy along nearby faults (e.g., stress transfer and subsequent accumulation due to 2016 M w 5.5 Gyeongju earthquake 25,38 ). These studies suggest that even a slight increase in static stress may enhance the potential of earthquake nucleation along critically stressed faults located in the area under subsurface energy activities.…”

Partitioned Fault Movement and Aftershock Triggering: Evidence for Fault Interactions During the 2017 Mw 5.4 Pohang Earthquake, South Korea

“…The orientation of the most compressional stress axis changed from near N-S to ENE-WSW due to the change in the tectonic regime (Soh et al, 2018). Grigoli et al (2018) and Kim et al (2018) suggested the possibility that the 2017 Pohang earthquake was induced by the activities of an EGS located near (<1 km to the east) the epicenter (Figure 2), although some controversy remains (Hong et al, 2018). Several researchers suggested that the Pohang earthquake was triggered by accumulated seismicity related to an unknown unstable fault (Ellsworth et al, 2019;Geological Society of Korea, 2019;Lee et al, 2019).…”