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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]

The southeastern Korean Peninsula (SeKP) has experienced intense deformation owing to subduction and backarc extension at the eastern continental margin of the Eurasian Plate, leading to the formation of complex tectonic structures. Abnormally high surface heat flux, Cenozoic volcanism, signatures of mantle degassing and hydrothermal alteration, and several active fault systems with extensional sedimentary basins have been identified; however, the major driving forces that promote local seismic events and hydrothermal activities remain enigmatic. Here, we constructed 3-D P -wave velocity of the crust and upper mantle in the SeKP for the first time using a teleseismic traveltime tomography method and an extensive data set obtained from a dense seismic network. Our model revealed three distinct velocity patterns at different depths: (1) in the upper crust (depth ∼0–10 km), a low-velocity anomaly beneath the Cenozoic sedimentary basin exhibiting a prominent lateral velocity contrasts with higher velocities in the Cretaceous sedimentary and plutonic rocks; (2) a N–S trending low-velocity anomaly extending from the lower crust to the uppermost mantle (depth ∼20–35 km) beneath the major active fault systems interpreted as a thermally or mechanically weakened structure that could transfer high surface heat flux and transport mantle-driven gases and (3) a low-velocity anomaly adjacent to the Cenozoic basin in the upper mantle at depths of 35–55 km interpreted as the higher temperature upper mantle. Via a series of geodynamic simulations, we demonstrated that the extensional deformation at the eastern continental margin during the Early to Middle Miocene locally enhanced the temperature of the crust and upper mantle beneath the SeKP. We propose that a hydrothermal system, resulting from the thermally modified lithosphere of the continental margin, has contributed to the enhanced local seismicity and geothermal activities observed in the SeKP region. [ABSTRACT FROM AUTHOR]

The coexistence of pseudotachylyte, a brittle fault rock representing seismic slip, and mylonite, a plastic or semiplastic fault rock associated with aseismic slip, has been debated for its origin in earthquake-generating faults. Deformation experiments on halite gouge at seismic slip rates show that mylonite forms simultaneously with frictional melt (pseudotachylyte) in the brittle regime, equivalent to upper crustal conditions. The plastic deformation of halite, resulting in mylonite, is facilitated by the conduction of heat from a slip-concentrated molten layer formed by frictional heating at seismic slip rates. Thus mylonites, which are (1) kinematically consistent and (2) just at the contact with pseudotachylyte, might have formed together with the pseudotachylyte in the brittle regime during seismic slip. doi: 10.1130/G31593.1

[1] NE trending granitoid batholiths in South Korea are part of the huge Phanerozoic granitoid belt in east Asia. The crystallization ages of granitoid plutons show three episodes of Mesozoic magmatism in South Korea: Triassic (248-210 Ma), Jurassic (197-158 Ma), and Late Cretaceous-early Tertiary (110-50 Ma), with a Late Jurassic-Early Cretaceous (~50 m.y.) hiatus. The granitoid ages and kinematic analyses of the Mesozoic structures in South Korea suggest that the Middle Jurassic (180-170 Ma) dextral strike-slip ductile shear zones formed by the northwest (orthogonal) subduction of the Izanagi plate, whereas the Cretaceous sinistral faults formed by the northward (oblique) subduction of the same plate, which generally agree with proposed reconstruction of plate motions, albeit uncertain, around east Asia. Among the models proposed to explain magmatic lull in a continental arc, shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in South Korea and Japan. The huge width (~500 km) of the east Asian granitoid belt may be explained by oceanward retreat of subduction zone in the Cretaceous in association with change in the movement direction of the subducting plate. doi: 10.1029/2004TC001720.

The Taean Formation on the central western margin of the Korean Peninsula contains layered metapelite, quartzo‐feldspathic schist, amphibolite, calcsilicate, and impure marble. The layering was formed by tectonic transposition of the original bedding. The formation has lithologic and isotopic age signatures similar to those of metasedimentary rocks of the Imjingang Belt, which is the eastern extension of the Qinling–Dabie–Sulu Belt, and the depositional age of the formation can be constrained to the Late Devonian. The metamorphic pressure–temperature (P–T) path for the formation is characterized by a high ΔP/ΔT prograde part and subsequent near‐isothermal decompression with peak P–T conditions of 600–620°C and 9.0–10.5 kbar, based on observations of mineral assemblages and phase equilibria modelling. Strong compressional deformation (Dn) produced a N–S‐trending Sn foliation that transposed the earlier Sn–1 and isoclinal Fn folds and was coeval with an episode of medium‐ to high‐pressure amphibolite‐facies metamorphism. The NNE–SSW‐trending left‐lateral Ryeseonggang Fault played a key role in producing the N–S distribution of the metamorphosed forearc deposits along the western coast of the Korean Peninsula. The peak metamorphism and deformation occurred during the early Permian to Early Triassic (276–250 Ma), with retrogression taking place during the Late Triassic (c. 230 Ma), as inferred from new U–Pb zircon age data and previous titanite and muscovite ages for the formation. The clockwise P–T–t (time)–d (deformation) path for the Taean Formation indicates rapid burial during Dn followed by rapid exhumation during the Permian–Triassic collision between the Qinling microcontinent and the North China Craton. [ABSTRACT FROM AUTHOR]

Structural, petrological, and geochronological data from the middle Korean peninsula indicate that the Qinling-Dabie-Sulu collisional belt of east-central China crosses the Yellow Sea and extends into the Imjingang belt. The Yeoncheon complex, first identified as the western Imjingang belt, comprises primarily north-dipping metamorphic sequences: (1) the northern Jingok unit, consisting of Barrovian-type metapelites, and (2) the southern Samgot unit, consisting of calc-silicate and amphibolitic rocks. South-vergent structures with reverse-sense shearing are dominant in the Jingok unit, whereas late normal-sense shearing is pervasive in the Samgot unit and the deformed granitoid to the south. These structural patterns are interpreted to correspond to extensional deformation associated with uplift following compression in a collisional belt. Pressure-temperature (P-T) estimates from the amphibolites suggest a high-P amphibolite-facies metamorphism (8-13 kbar and 630-790 [degrees] C), possibly evolving from eclogite facies conditions along a clockwise P-T path. Sm-Nd and Rb-Sr geochronological data suggest that the amphibolites emplaced in Late Proterozoic time were metamorphosed during Permian-Triassic time.

We analyzed 91 focal mechanism data with relocated hypocenters to estimate the 3‐D pore fluid pressure field in the source region of the 2017 Pohang earthquake in South Korea using the method of focal mechanism tomography. The upper limit of pore fluid pressure was 8 ± 3 MPa in a location northwest of the enhanced geothermal system. The overpressurized fluids were localized in the footwall of the causative fault. It was directly connected to the open hole section of PX2, and fluid injection into this hole could have triggered the mainshock. Plain Language Summary: The 2017 Pohang earthquake (Mw 5.5) raised a serious social debate as to whether it was triggered by the fluid injections at the Pohang enhanced geothermal system (EGS). In March 2019, the Overseas Research Advisory Committee organized by the Korean government authoritatively concluded that the Pohang event was triggered by high‐pressure fluid injection at the EGS. However, the level of the pore fluid pressure driving the mainshock and the vigorous aftershocks remains unclear. In this study, we estimated the 3‐D pore fluid pressure field in the Pohang EGS from 91 earthquake focal mechanism data, using an inversion method based on the Bayesian statistical inference and Akaike's Bayesian Information Criterion. We found that the pore fluid pressure was enhanced in regions with horizontal and vertical length scales of 2–3 and 1–2 km, respectively, around the hypocenter of the mainshock in the Pohang EGS. The maximum excess pore fluid pressure above hydrostatic was at most 8 ± 3 MPa. This overpressurized fluid reservoir was directly connected to the open hole section of one of the injection wells. This study provided with new evidence for involvement of fluids in the Pohang earthquake. Key Points: Three‐dimensional pore fluid pressures were estimated from earthquake focal mechanisms in the source region of the 2017 Pohang earthquakeOverpressurized fluids concentrated in the footwall of the mainshock fault were directly connected to the Pohang EGSThe upper limit of pore fluid pressure was 8 ± 3 MPa in a location northwest of the EGS [ABSTRACT FROM AUTHOR]

Abstract: Although the 2011 Tohoku‐Oki earthquake (Mw 9.0, 11 March 2011) occurred more than 1,000 km from South Korea, it significantly changed the magnitudes and orientations of Global Positioning System (GPS) velocity vectors in the country, which took two years to return to preearthquake values. Then, areas with relatively thick crust show contractional surface deformation, whereas areas with thinner crust show dilatation, suggesting that the deformation pattern is long‐term. Although there is no one‐to‐one correlation between earthquakes and surface deformation, earthquakes tend to be concentrated in areas with higher shear strain rates. In South Korea, the stress field inferred from earthquake data is inconsistent with surface deformation calculated from GPS data. This may arise because stress orientations at earthquake focal depths are different from those at the surface, or surface deformation measured by GPS may represent interseismic elastic deformation leading to future large earthquakes, while recent earthquakes without surface ruptures might not contribute to the surface deformation. Some effects of crustal thickness on surface deformation and inconsistency between surface deformation from GPS data and kinematics from earthquake data may be a common feature of intraplate regions. [ABSTRACT FROM AUTHOR]

Abstract: The geological relationship between the Okcheon and Taebaeksan basins of the Okcheon belt on the Korean peninsula is a key issue in reconstructing the tectonic evolution of the peninsula. The boundary between the two basin sequences has been variously interpreted as a conformable, unconformable, or thrust contact, without clear evidence being provided for any of these hypotheses. Detailed examination of structures and microfabrics of deformed rocks adjacent to the contact in the Bonghwajae area suggests that the boundary between the two basin sequences is a thrust. Based on the U–Pb ages of detrital zircons from metasedimentary rocks and pre‐existing geologic data from the Okcheon belt, the thrust is a relay structure between two segments of a continental transform fault along which the Okcheon Basin was juxtaposed against the Taebaeksan Basin during the Permian–Triassic suturing of the North and South China Cratons. [ABSTRACT FROM AUTHOR]

Fabric transition by a switch in the dominant slip system of minerals in the plastic regime can be induced by changes in temperature, strain rate, or water content. We propose here this fabric transition by frictional heating in seismogenic fault zones in the brittle regime. The Garam Thrust in the Taebaeksan Basin of South Korea has a hanging wall of Cambrian dolostone juxtaposed against a footwall of Ordovician limestone and records a minimum displacement of ~ 120 m. In a 10 cm thick plastically deformed layer adjacent to the principal slip layer of the fault zone, the lattice preferred orientation of calcite grains suggests that the dominant slip system changes, approaching the principal slip layer, from r 〈02–21〉 and e-twinning, through r 〈02–21〉 and basal 〈a〉, to basal 〈a〉. This fabric transition requires a high temperature-gradient of 40 °C/cm, which we infer to result from frictional heating of the seismic fault zone. We suggest that fabric transition within a thin plastically deformed layer adjacent to the principal slip layer of a fault zone indicates an unusually steep temperature gradient and provides strong evidence of seismic slip. [ABSTRACT FROM AUTHOR]

We review paleontologic evidence from the Upper Ordovician to Devonian strata in the Korean Peninsula and discuss their tectonostratigraphic origin. The Upper Ordovician-Devonian fossil-bearing strata are largely distributed in North Korea, and tectonostratigraphically in the southern margin of the Pyeongnam Basin and in the northern part of the Imjingang Belt. The fossils have been regarded as evidence that the 'great hiatus' of the middle Paleozoic is not a prevalent phenomenon across the Sino-Korean Block. Examination of selected fossils with stratigraphic and paleogeographic significance reveals that the fossils from the Sangsori, Koksan and Wolyangri series and the Rimjin System are of the Late Ordovician to Devonian and display affinity to those of the coeval strata of South China. In addition, the fossils included within clasts of the Songrim Conglomerate, the basal unit of the Jurassic Taedong System, are of the Silurian to Devonian, which also display affinity to South China. The faunal and floral affinity suggests that the Upper Ordovician to Devonian strata in North Korea most likely formed in a basin(s) of or peripheral to the South China Block, which indicates that the strata are allochthonous, contrary to the traditional interpretation of their autochthonous origin by North Korean geologists. The Permo-Triassic collision between the two Chinese cratons which resulted in the amalgamation of three massifs of the Korean Peninsula is considered to be responsible for the accretion and juxtaposition of the Upper Ordovician to Devonian strata onto the Sino-Korean Block. The autochthonous origin of the strata suggests the absence of the 'great hiatus' at least in North Korea, whereas the allochthonous origin its presence across the Sino-Korean Block. [ABSTRACT FROM AUTHOR]

The moment magnitude (Mw) 5.4 Pohang earthquake, the most damaging event in South Korea since instrumental seismic observation began in 1905, occurred beneath the Pohang geothermal power plant in 2017. Geological and geophysical data suggest that the Pohang earthquake was induced by fluid from an enhanced geothermal system (EGS) site, which was injected directly into a near-critically stressed subsurface fault zone. The magnitude of the mainshock makes it the largest known induced earthquake at an EGS site. [ABSTRACT FROM AUTHOR]

It has been possible to develop a new tectonic model in which the main accretion of South Korean tectonic blocks was finished during the Middle Triassic Songrim orogeny with a north-east to north-north-east vergent thrust tectonics. It is suggested that the right lateral Cheongsan shear zone is a significant boundary between the Okchon zone and the Yongnam massif with Paleozoic cover rocks.

Abstract: The Permo-Triassic Songrim (Indosinian) orogeny in South Korea was a major tectonic event involving complicated continental collisions at the eastern margin of Eurasia. Previous studies have examined the structural and metamorphic features of the Songrim orogeny in each of the Paleozoic terranes of the orogenic belt (i.e., the Taebaeksan Basin, the Okcheon Basin, and the Imjingang Belt), but correlations of these features among the terranes remain uncertain. The aim of this paper is to reveal deformation history including bulk crustal shortening directions in the Taebaeksan Basin, and to correlate the tectono-metamorphic evolution of the Taebaeksan Basin with other Phanerozoic mobile belts in eastern Asia based on a combined analysis of foliation intersection/inflection axes (FIA) trends and metamorphic P–T and T–t (time) paths. The orientations and relative timing of FIA preserved as inclusion trails within porphyroblasts of andalusite, chloritoid, garnet, and staurolite reveal two age groups of inclusion trails in the Pyeongan Supergroup at the northeastern margin of the Taebaeksan Basin. These microstructures indicate the development of early NNW–NNE-trending structures and fabrics, followed by later E–W-trending ones. These observations suggest a change in the orientation of bulk crustal shortening from E–W to N–S during the Songrim orogeny. Based on the similar microstructures and temperature–time paths of the three Paleozoic terranes, we interpret that the E–W bulk crustal shortening influenced the eastern part of the Korean Peninsula during the early stages of the Songrim orogeny, presumably related to amalgamation between the proto-Japan terrane and the eastern margin of Eurasia, whereas the N–S bulk crustal shortening was stronger in the western part of the peninsula during the later stages of the orogeny, related to collision between the South and North China blocks. [Copyright &y& Elsevier]

Abstract: The most widely used method for quantitative analyses of the spatial distribution of grains in a rock body is the nearest-neighbor method (a position-based method), which recognizes the grain center as the position of the grain without consideration of grain size or shape. However, the spatial distribution of grains is influenced by their size and shape, as well as their position. Here, we propose a multiple-area density map (MADM) method to quantify the spatial distribution of phases or grains. The method is based on image analysis and simultaneously considers the position, size, shape, and proportion of grains. The MADM constructed by overlaying density maps of grain areas with a set of template size produces a normalized standard deviation (NSD) value of phase area density, which represents the degree to which it is spatially clustered. The NSD value is used to quantify the distribution of the phase, without considering individual grains. To identify the spatial distribution of individual grains, the method employs the clustering index (CI), which is calculated from the ratio of NSD to the reciprocal of the number of grains in a log–log plot. [Copyright &y& Elsevier]

The question of whether millennial-scale geological slip rates are consistent with decade-scale geodetic slip rates is of great importance in evaluating the nature of continental deformation within the Tibetan Plateau. We determined the time-averaged slip rate of the Sulu He segment of the Altyn Tagh Fault, near Changma in Gansu Province, China, based on geomorphic analysis, remote sensing data, and cosmogenic Be surface-exposure age dating. Quaternary alluvial fan deposits in the study area (Qf1, Qf2, Qf3) are displaced by left-lateral movement along the Altyn Tagh Fault. Because of the large accumulated displacement of these fans, some of them have become disconnected from the fan apexes that are directly linked to the debris-source areas in the piedmont of the Qilian Shan to the south. The total minimum offsets are estimated to be about 429 ± 41 m for Qf1, about 130 ± 10 m for Qf2, and 32 ± 1 m for Qf3. The Be surface-exposure ages obtained for Qf1 and Qf2 are 100-112 ka and 31-43 ka, respectively. Accordingly, the slip rate since the period of Qf1 and Qf2 depositions is calculated to have been about 3.7 mm/yr. [ABSTRACT FROM AUTHOR]

Abstract: Metasedimentary rocks of the Pyeongan Supergroup in the northeastern flank of the Taebaeksan Basin, South Korea, commonly contain two Al2SiO5 polymorphs (And+Ky and And+Sil), or more rarely, three coexisting Al2SiO5 polymorphs. Lower-grade rocks preserve chloritoid+andalusite±kyanite in muscovite–chlorite phyllite, and higher-grade rocks contain andalusite+sillimanite (±kyanite)+staurolite±garnet in mica schists. Kyanite and sillimanite occur as partial paramorphs after andalusite. Textural relations and relative metamorphic P–T conditions of the Al2SiO5-bearing mineral assemblages suggest a crystallization sequence of andalusite→kyanite→sillimanite, with the growth of staurolite and garnet in the kyanite and sillimanite stability fields. Metamorphic P–T conditions determined from geothermobarometry, compositional isopleths of chloritoid and garnet, and petrogenetic grids in the MnO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnKFMASH), MnKFMASH–T(TiO2)–O(Fe2O3), and Mn–N(Na2O)–C(CaO)–KFMASH systems could be divided into two regimes temporally: a clockwise path at lower temperature (LT)/medium pressure (MP) conditions, and higher temperature (HT)/low pressure (LP) conditions. The peak metamorphic P–T conditions for regional LT/MP metamorphism are ca. 5.0–6.0kbar at 550–580°C, and those for HT/LP metamorphism are ca. 3.0–4.8kbar and 590–610°C. Kyanite paramorphs after andalusite may result from a LT/MP clockwise P–T path associated with a rapid contractional deformation during the early to middle Mesozoic, and then sillimanite paramorphs after andalusite may have been formed by magmatic heating due to the later intrusion of leucocratic granite. [Copyright &y& Elsevier]

Reconstruction of the Neoproterozoic to Early Cambrian paleogeography of northeast Asia proposed by Lee et al. (2016, Palaeogeography, Palaeoclimatology, Palaeoecology, 441, 770–786) is based on miscalculated statistical values and/or some radiometric data ignored without rationale. The Gondwana linkage between the Korean Peninsula and the Sino-Korean (North China) Block in their model should be reconsidered. [ABSTRACT FROM AUTHOR]

Abstract: Feldspar grain-size reduction occurred due to the fracturing of plagioclase and K-feldspar, myrmekite formation and neocrystallization of albitic plagioclase along shear fractures of K-feldspar porphyroclasts in the leucocratic granitic rocks from the Yecheon shear zone of South Korea that was deformed under a middle greenschist-facies condition. The neocrystallization of albitic plagioclase was induced by strain energy adjacent to the shear fractures and by chemical free energy due to the compositional disequilibrium between infiltrating Na-rich fluid and host K-feldspar. With increasing deformation from protomylonite to mylonite, alternating layers of feldspar, quartz and muscovite developed. The fine-grained feldspar-rich layers were deformed dominantly by granular flow, while quartz ribbons were deformed by dislocation creep. With layer development and a more distributed strain in the mylonite, lower stresses in the quartz-rich layers resulted in a larger size of dynamically recrystallized quartz grains than that of the protomylonite. [Copyright &y& Elsevier]

Abstract Two groups of Quaternary faults occur in the southeastern Korean Peninsula. The first group is north-northeast-striking, high-angle dextral strike–slip faults. The second group is north-northeast-striking, low-angle reverse faults that represent the reactivation of the pre-existing normal faults. Optically stimulated luminescence dating of Quaternary sediments cut by one of the reverse faults constrains the faulting age to post-32 Ka. These faults seem to be capable of further slip under the current tectonic stress regime, as determined by recent earthquake events in northeast Asia. Therefore, the traditional concept that the southeastern Korean Peninsula is seismically stable should be reappraised. [ABSTRACT FROM AUTHOR]

• Shearing accompanied metamorphism intensified at the boundary of the Gyeonggi massif. • Deformation-induced plagioclase zoning can cause misleading metamorphic history. • Metamorphism and multiple shearing resulted from multiple amalgamations. • Amalgamation closely related with the late Paleozoic collisional orogeny in Korea. Crustal-scale shear zones developed on the boundaries of the Precambrian Gyeonggi Massif (GM) can play key role on understanding collisional orogeny during the Late Paleozoic to Early Mesozoic in the Korean Peninsula. Garnet schist formed on the boundary between the GM and Okcheon Metamorphic Belt (OMB) experienced contractional (D n) and extensional (D n+1) shearing combined with a clockwise pressure–temperature–deformation (P–T–d) path. Two distinct mineral assemblages were found in the schists, (1) garnet (pre- and/or syn-D n) + staurolite (syn-D n and/or inter D n and D n+1 , and syn-D n+1) + andalusite (syn-D n+1)-bearing and (2) garnet (pre- and/or syn-D n) + epidote-bearing assemblages, depending on bulk-rock compositions. Various compositions (An 10–40 in the former assemblage and An 45–70 in the latter one) and texture of plagioclase resulted from bulk-rock compositions, heterogeneous strain and dissolution and precipitation during the multiple shearing. The P–T conditions estimated for the early contractional shearing (top-to-the-north reverse shear) are 560 °C–600 °C and 5.5–7.0 kbar based on the intersections between compositional isopleths for pre- and/or syn-D n garnet and plagioclase (An 30–40 & An 45–55) calculated in the MnO–Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O and MnO–Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –Fe 2 O 3 systems. Subsequent extensional shearing (top-to-the-south normal shear) was characterized by recrystallization of fine-grained syn-D n+1 staurolite with growth of syn-D n+1 andalusite porphyroblasts during cooling and decompression to 560 °C and 3.5 kbar. The contractional and extensional shearing probably occurred at the Permian and the Late Triassic to Early Jurassic, respectively, based on the previous geochronological data. Consequently, inferred P–T–t–d paths on the GM–OMB boundary probably suggest not only crustal thickening followed by thinning, but also multiple amalgamations of several terranes. [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]