Woohan Kim

The 12 September 2016 ML5.8 midcrustal earthquake in the Korean Peninsula and its seismic implications

The seismicity in the Korean Peninsula has increased since the 2011 Mw9.0 Tohoku-Oki megathrust earthquake. Two strike-slip earthquakes with magnitudes of ML5.1 and 5.8 occurred in the southeastern Korean Peninsula on September 12, 2016. The two events occurred within 48 minutes. The ML5.8 earthquake was the largest event in the Korean Peninsula since 1978 when national seismic monitoring began. Both events produced strong high-frequency ground motions. More than 500 aftershocks with local magnitudes greater than or equal to 1.5 followed the events for two months. An unreported subsurface strike-slip fault with a dip of 65∘ to the east and a strike of N27∘ E was responsible for the earthquakes. The fault ruptured at depths of 11-16 km, resulting in a rupture plane of ∼26 km2. The aftershock distribution displayed horizontal streaks at a depth of ∼14 km, which was consistent with the focal mechanism solutions from long-period waveform inversion. The number of aftershocks decreased exponentially with time. The two ML5.1 and 5.8 earthquakes produced regional Coulomb stress changes of -4.9 to 2.5 bar. The spatial distribution of the aftershocks correlated with the Coulomb stress changes. The peak dynamic stress induced by strong ground motions reached 14.2 bar. The groundwater levels changed coseismically in some regions of decreased static stresses. The earthquakes on previously unidentified faults raised attention for the potential seismic hazards by earthquakes with long recurrence intervals.

Relocation of earthquakes beneath the East Sea of Korea: uncertainty of hypocentral parameters caused by refracted waves

In addition to direct waves, refracted (head) waves such as Pn and Sn have been used to determine hypocentral locations, especially when the number of direct waves is inadequate to determine the hypocentral parameters of local earthquakes. However, refracted waves tend to increase errors and the non-uniqueness problems of hypocentral parameters compared with direct waves, if the true velocity structure is not known. Beneath the East Sea of Korea (Japan Sea), conditions for determining hypocentral parameters of earthquakes are unfavorable. To estimate these parameters, this study applied the computational algorithm VELHYPO, which uses the best-fitting velocity model obtained from the results of grid searching methods. The accuracy of hypocentral parameters obtained from direct and different combinations of refracted waves was examined using a synthetic data set. The accuracy tests indicated that hypocentral parameters determined by direct waves alone tend to be more accurate and reliable than those determined by direct and refracted waves combined. The accuracy of hypocentral parameters increases as the grade of the weighting factor for refracted waves decreases when compared with that for direct waves. We relocated 72 earthquakes that occurred beneath the East Sea by applying VELHYPO and giving a lower weighting factor grade to refracted waves than to direct waves. The relocated epicenter distribution shows a close relationship with the regional fault distribution. We compared the hypocentral parameters determined by this study and the Korea Institute of Geoscience and Mineral Resources.

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

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.

Detailed one-dimensional seismic velocity profiles beneath the Himalayan collision zone: evidence for a double Moho?

Seismicity in the Himalaya indicates that relatively deep earthquakes (focal depth 40∼100 km) occur in specific regions beneath the High Himalaya and Nepal. This study focuses on these specific regions to estimate the detailed velocity structure of the lower crust and upper mantle. We selected 202 earthquakes from the Himalaya Nepal Tibet Seismic Experiment (HIMNT) and relocated these earthquakes accurately by applying a genetic algorithm locator, GA-MHYPO (Kim et al., 2006). The detailed onedimensional P-velocity structure is estimated based on travel-time inversion using hypocentral parameters determined by GA-MHYPO. Computational results show two velocity anomalies exhibiting upper-mantle velocities at depth of about 45–50 and 60 km, respectively, beneath the High Himalayan region, whereas a single Moho exists at about 55 km depth beneath Nepal. To validate the stability of these results, a bootstrapping method was used for the inversion.