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The 2016 magnitude (MW) 5.5 Gyeongju earthquake, which occurred in Korea near assumed epicenters of several substantial historical earthquakes and Quaternary fault segments, underscores the importance of seismic hazard assessment in the region. However, uncertainties about potential seismic sources make evaluating the potential for a moderate-to-large earthquake challenging. Microearthquake monitoring through a dense seismic network can provide crucial insights into the regional seismic characteristics. An extensive temporary seismic array known as the Gyeongju Hi-density Broadband Seismic Network (GHBSN) was established to investigate microearthquake activity in the southeastern Korean Peninsula. This included the zone of aftershocks from the 2016 Gyeongju earthquake sequence. The GHBSN comprises 200 broadband stations located at approximately 4.5 km intervals in an area of approximately 60 × 60 km2 around the epicenter of the mainshock. A total of 4,773 events were detected from November 2017 to December 2021, including 3,935 events within the GHBSN. The detected events were categorized into five seismic regions excluding quarry blasting sites, that is, the 2016 Gyeongju earthquake region, eastern part of the Ulsan Fault, 2017 Pohang earthquake region, eastern offshore Gyeongju, and western part of the Miryang Fault. A local magnitude scale was developed for the southeastern Korean Peninsula using events detected through the GHBSN. This reflects the distance attenuation and site conditions of the GHBSN stations for earthquakes. An event catalog was created using two automatic detection methods based on the measurement of the energy ratio. This provided high-resolution hypocenter parameters at a completeness magnitude (MC) of 0.0 despite the seismic environment of the network being exposed to high cultural noise. The Gutenberg-Richter b-value was estimated as 0.82 ± 0.02 for all events and 1.01 ± 0.02 for those inside GHBSN. This implies that the seismicity reflects a representative intraplate seismic environment. Testing the obtainability of the focal mechanism solutions showed that the GHBSN outperformed the regional network. Depending on the relationship between the magnitude and frequency of earthquakes, a relatively large number of small earthquakes can provide detailed information on the geometric properties of the causative faults and the state of the acting stress. High-precision microearthquake observation and analysis through GHBSN could provide an unprecedented opportunity with seismic datasets to understand the seismogenesis of the southeastern Korean Peninsula, including the zone of aftershocks of the 2016 Gyeongju earthquake. [ABSTRACT FROM AUTHOR]

On 2016 September 12, a moderate earthquake (M L 5.8) occurred in Gyeongju, South Korea, located hundreds of kilometres away from the nearest plate boundaries. The earthquake, the largest instrumentally recorded event in South Korea, occurred in a sequence of thousands of earthquakes, including a M L 5.1 event 50 min before the main quake and a M L 4.5 event a week later. As a case study, we analyse the source parameters of the 2016 Gyeongju earthquake sequence: precise relocations, fault structures, focal mechanisms, stress tensor analysis and Coulomb stress changes. To determine high-resolution hypocentres and focal mechanisms, we employ our temporary seismic network for aftershock monitoring as well as regional permanent seismic networks. The spatio-temporal distribution of events and inverted moment tensors indicate that the M L 5.1 event and the M L 5.8 event occurred on two parallel dextral faults striking NNE–SSW at a depth of 11–16 km, and the M L 4.5 event occurred on their conjugate fault with sinistral displacements. Seismicity on the fault for the M L 5.1 event abruptly decreased as soon as the M L 5.8 event occurred. This is not solely explained by the Coulomb stress change and requires more complex processes to explain it. The tectonic stress field obtained from inverted focal mechanisms suggests that the heterogeneity between the intermediate and minimum principal stresses exists along the NNE–SSW and vertical directions. The Coulomb stress changes imparted from the M L 5.1 event and the M L 5.8 event are matched with the off-fault seismicity, including that from the M L 4.5 event. Multifaceted observations, such as Coulomb stress interactions between parallel or conjugate faults and the heterogeneity of the tectonic stress field in the aftershock area, may reflect the reactivation processes of a complex fault system. This study offers a distinctive case study to understand the general characteristics of intraplate earthquakes in multifault systems. [ABSTRACT FROM AUTHOR]

Two earthquakes ( M 5.1 and 5.8) ruptured branches of the Yangsan Fault System in Gyeongju, S. Korea on September 12, 2016. After the M 5.8 earthquake, aftershock earthquakes continued to occur, including two notable earthquakes ( M 4.3 and 4.5) on September 12 and 19, 2016. This paper details the early reports of the Yangsan Fault System in the Gyeongsang Basin from various geological and geophysical/seismological perspectives. Based on a review and an initial seismological analysis of the results of the three earthquakes ( M 5.1, 5.8, and 4.5), we present and discuss the following topics: (1) the tectonic setting and the geophysical/seismic structure of the Yangsan Fault System, (2) historical seismicity and inferred seismic hazard from historical (literature) data, and (3) source mechanisms of the three earthquakes. In the end, we highlight some of the outstanding issues with regard to earthquakes and future research topics. [ABSTRACT FROM AUTHOR]

The M 5.8 earthquake in Gyeongju, southeastern Korea, on September 12, 2016 11:32:54 (UTC) was the largest earthquake on the Korean Peninsula since instrumental monitoring began in 1978. It was preceded by an M 5.1 foreshock and is being followed by numerous aftershocks. Within an hour of the mainshock, the first temporary seismic station to monitor aftershocks was installed at about 1.5 km east of the announced epicenter. The current temporary seismic network consists of 27 stations equipped with broadband sensors covering an area of ∼38 × 32 km in the mainshock region. This is the first high-density aftershock monitoring array in the Korean Peninsula. Initial results, using data from both the regional seismic networks and the aftershock monitoring array, indicate that earthquakes during the first 10 days following the mainshock are related to the Yangsan Fault System. Establishment of an official rapid-response team to monitor aftershocks of major earthquakes is advised. [ABSTRACT FROM AUTHOR]

• The M L 5.4 2017 Pohang earthquake triggered by fluid injection generated a complex geometry of optimally oriented rupture planes. • For a moderate-size earthquake, the earthquake sequence is unique because rupture planes propagated for more than 11 months after the mainshock. • The injection of fluid directly into optimally oriented faults can induce earthquakes that are larger than those predicted by theory. A commission of the Korean government on the 2017 Mw 5.5 Pohang earthquake concluded that seismic activity was triggered by fluid injection from the nearby Pohang enhanced geothermal system. The temporal and spatial distribution of hypocenters (depth range of 3–6 km) was recorded by our local seismic array for 345 days. It included, in addition to the mainshock, 10 foreshocks and 3090 aftershocks, indicating a complex geometry of propagating rupture segments, including the main segment and four subsidiary segments (SS1 to SS4). The fault kinematics of the earthquake sequence were dominated by strike and reverse slips. The aftershock sequence showed a decrease in frequency during the first 83 days after the mainshock, in accordance with Omori's law. This was followed by a sudden surge in aftershock frequency associated with the M L 4.6 event (11 February 2018), and another decay following Omori's law. The aftershock surge of the M L 4.6 event corresponds to the development of SS4. A calculation of the Coulomb stress change caused by the mainshock shows that a significant increase in stress occurred around SS4, thus suggesting that rupture along this segment was triggered by the mainshock. With respect to the current stress field, all rupture planes apart from that of SS4 are optimally oriented for reactivation, highlighting the potential risk associated with fluid injection into this fault. [ABSTRACT FROM AUTHOR]

The 2017 Mw 5.4 Pohang earthquake has been suggested to be induced by fluid injection from the nearby Pohang enhanced geothermal system (EGS). The temporal and spatial distribution of hypocenters (depth range: 2.5−7 km) including 6 foreshocks and 3,240 aftershocks as well as the mainshock recorded by our local seismic array for 340 days suggests a complex geometry and evolution of rupture segments. The initial rupture, based on 3-hour seismic data after the mainshock, consists of a main segment (N30E (strike)/51NW (dip), 3.5 km long; MS) and a subsidiary one (N10E/47NW, 1.5 km long; SS1) extending from the northeastern tip of MS. The fault slip of MS and SS1 is dominated by reverse and strike slips, respectively. The second subsidiary segment (N8E/58NW, 2 km long; SS2) has propagated from the northern tip of SS1. Toward southwest, the main segment (MS) has propagated further (~ 1.4 km more) from the first 3-hour configuration. Then, the third subsidiary segment (N67W/83SW, 1 km long; SS3) has developed at the southwestern tip of MS, almost perpendicular to MS. The fourth subsidiary segment (N13E/55NW, 2 km long; SS4), subparallel to MS, crosscuts SS3.The aftershock sequence shows that the frequency decreases with the reciprocal of time, following Omori's law, until 83 days after the mainshock and then there was a sudden surge of aftershock frequency with an ML 4.6 event (2018.02.11), followed by another Omori's-law decay. The timing of SS3 development corresponds to the aftershock surge with the ML 4.6 event. The calculation of Coulomb stress change caused by the mainshock shows that there is a significant increase in stress (~ 1 bar) around SS3. All these suggest that SS3 is an independent rupture triggered by the mainshock.Focal mechanism solutions of the Pohang earthquake sequence show strike-, reverse-, or oblique-slip faultings without any normal-slip component. Stress inversion of these focal mechanism solutions generates nearly horizontal σ1 (109(trend)/8(plunge)) and σ2 (18/6) with subvertical σ3 (252/81), indicating a compressional regime. The stress ratio, R = (σ1 - σ2)/(σ1 - σ3) = 0.89, is rather high, suggesting a permutation of σ2 and σ3 is highly likely. Using orientations of these principal stresses and rupture segments, and assuming a frictional coefficient of 0.85 (based on Byerlee's law for σn<200 MPa since the lithostatic pressure at the hypocentral depth of the mainshock is ca. 113 MPa), Sibson's (1985, https://doi.org/10.1016/0191-8141(85)90150-6) reactivation criterion shows that the angle between σ1 and rupture segments on σ1σ3 plane (55-67°) is all higher than the frictional lockup angle (~50°). This requires an abnormaly low static friction or high fluid pressure for slip reactivation of the rupture planes of the Pohang earthquake. The development of ubiquitous sand blows with fluid discharge around the epicenter during the mainshock and its hypocenter in the immediate vicinity of the injection well suggest that overpressurized fluid should have played an important role in the 2017 Pohang earthquake event. [ABSTRACT FROM AUTHOR]