Southeastern Korea Research Articles

Inter-comparison of DSDs between Jincheon and Miryang at South Korea

This study explores the characteristics of drop size distribution (DSD) using PARticle SIze and VELocity (Parsivel) disdrometer at Jincheon (JC; 36.9 °N, 127.5 °E) and Miryang (MY; 35.5 °N, 128.6 °E) sites, during the period June 2015 to March 2016. Twelve cases were selected from Parsivel disdrometer data to compare the characteristics of DSDs between central and southeastern Korea. The average DSD at both sites shows a similar pattern; there is a slightly larger magnitude of small drops at JC. A comparison was undertaken on average raindrop spectra at JC and MY during three different rainfall systems; namely the Changma (CS), Localized low pressure (LS), and Typhoon (TS) system. Rainfall at JC tends to have more raindrops than MY especially over small diameter drops (0.31−0.81 mm). The differences in JC and MY may be attributed to the errors related to use of the Parsivel disdrometer, differences in the amounts of rainfall, or differences in the prevailing rainfall systems (CS, LS, and TS). Total monthly rainfall at JC and MY were 783.10 (726.92) and 781.80 (701.17) mm, respectively based on the rain gauge and Automatic Weather Stations (AWS). However, there was good agreement (within 7%−11%) between Parsivel disdrometer data and data obtained from the rain gauges or AWS. The average raindrop spectra at JC tends to have more raindrops than MY in different rainfall systems especially over small drops. During CS, the rainfall at JC is higher than MY across all the diameters on July 23, 2015. During LS at both sites there is a similar pattern, but with slightly larger diameter at JC as measured on August 16, 2015 (D> 6.5 mm). The rainfall at JC during TS tends to have more small drops compared to MY on July 12, 2015. The DSD characteristics are different across rainfall systems and climatic regions.

Spatial distribution of vegetation along the environmental gradient on the coastal cliff and plateau of Janggi peninsula (Homigot), southeastern Korea

BackgroundCliffs are a major plant habitat around the coastal area, but in contrast to sand dunes and salt marshes, they have been little investigated in Korea. There are simple descriptions of cliff vegetation in studies on island vegetation, but there is no published paper, which addressed sea cliff vegetation synthetically. Furthermore, the coastal area where this study was carried out was designated as a conservation reserve. Even though, this area is exposed to intense recreational use such as trekking in these days.This study aims to clarify spatial distribution and structure of vegetation along the environmental gradient on coastal cliff and plateau in the Janggi peninsula (Homigot) located on southeastern Korea. Further, this study has also another objective to prepare a restoration plan to protect this conservation reserve from intense human disturbance.ResultsLandscape elements were arranged in the order of sea cliff risen directly on the sea, seashore, coastal cliff, and plateau covered with relatively deep soil in a coastal area of the Janggi peninsula (Homigot), southeastern Korea. Vegetation was sampled at 59 plots arranged from the sea cliff through the seashore and coastal cliff to plateau. The sea cliff, seashore, and coastal cliff, which compose the coastal landscape, were dominated by the seashore spatulate aster (Aster spathulifolius Maxim.) community, dwarf sand sedge (Carex pumila Thunb.) community, and seashore spatulate aster (Aster spathulifolius Maxim.) community. On the plateau corresponding to the ridge of the coastal cliff, black pine (Pinus thunbergii Parl.) community, golden rain tree (Koelreuteria paniculata Laxmann) community, east Asian hackberry (Celtis sinensis Pers.) community, sawleaf zelkova (Zelkova serrata Makino) community, and Korean oak (Quercus dentata Thunb.) community were established in the mentioned order along distance from the sea. Stand ordination showed a vegetation sequence from the seashore through the cliff to the plateau, consistent in its overall pattern among sites. This was dominated by topography. There is evidence for the importance also of salinity, drought and of soil depth.ConclusionThe lack of scientific interest in cliffs to date is in striking contrast to the commonness of cliffs around the whole national territory and to the attraction cliffs have had for humans throughout history. Cliffs provide a unique habitat, rarely investigated from an ecological viewpoint. Cliffs may represent an invaluable type of ecosystem, consisting of some of the least disturbed habitats on earth and contributing more to the biodiversity of a region than their surface coverage would indicate. Although this coastal area where this study was carried out was designated as a conservation reserve, this area is in danger of severe disturbance due to excessive recreational use. We recommended a restoration plan to protect this area from such a disturbance.

Palynological study of paleoclimate and paleoceanographic changes in the Eastern South Korea Plateau, East Sea, during the Plio-Pleistocene climate transition

A palynological study was performed using the core obtained from the Integrated Ocean Drilling Program (IODP) Expedition 346 Site U1430 in the Eastern South Korea Plateau in the East Sea, to reveal variability of paleoclimate and paleoceanography during the late Pliocene to early Pleistocene climate transition period (56.0–68.0 meters below sea floor [mbsf]). Two pollen and dinocyst zones are established with a boundary at ca. 3.0 Ma. Prior to ca. 3.0 Ma (first pollen and dinocyst zone), long-term oscillation of the East-Asian winter monsoon (EAWM) is indicated by fluctuations in the abundance of pollen taxa, representing cold–dry and cool–wet climates. After ca. 3.0 Ma (second pollen and dinocyst zone), an increase in cold–dry conifer pollen indicates an enhanced and stable EAWM. In addition, the prevalence of the xerophytic herb, Artemisia, increases markedly after 3.0 Ma, suggesting open grassland on the exposed South Yellow Sea shelf during the sea level lowstand under cold–dry climate conditions. Artemisia pollen are interpreted as having been delivered to the coast through fluvial channels in this grassland environment, and subsequently transported to the East Sea by inflow of a weak Tsushima Warm Current (TWC) through the narrow Korea Strait. The TWC intrusion signal is represented by an influx of Tuberculodinium vancampoae, a warm water dinoflagellate species, in the western Pacific region. The high abundances of cold–dry conifer pollen and cold water dinocyst elements after ca. 2.8 Ma suggest an intensified EAWM. Meanwhile, a short-term increase in moisture-loving Tsuga pollen after ca. 2.5 Ma was likely due to soil moisture derived from snowfall caused by evaporation of heated seawater from the weak TWC in the East Sea under cold climate conditions. The palynological studies reported here help verify the linkages between the EAWM, TWC, precipitation and vegetation responses in/around the East Sea region.

Seismic Fragility Analysis of Steel Liquid Storage Tanks Using Earthquake Ground Motions Recorded in Korea

Liquid‐containing storage tanks are important structures in industrial complexes. Because earthquake damages to liquid storage tanks can cause structural collapse, fires, and hazardous material leaks, there have been continuous efforts to mitigate earthquake damages using seismic fragility analysis. In this regard, this study focuses on the seismic responses and fragility of liquid storage tanks. First, the characteristics of earthquake ground motions are a critical factor influencing the seismic fragility of structures; thus, this study employs real earthquake records observed in the target area, southeastern Korea, with the earthquake characteristics estimated based on the ratio of peak ground acceleration to peak ground velocity. When a liquid storage tank oscillates during an earthquake, additional forces can impact the tank wall owing to hydrodynamic pressures. Therefore, this study presents a sophisticated finite element (FE) model that reflects the hydrodynamic effect of an oscillating liquid. Another advantage of such an FE model is that detailed structural responses of the entire wall shells can be estimated; this is not possible in simplified lumped mass or surrogate models. Lastly, probabilistic seismic demand models are derived for three critical limit states: elastic buckling, elephant’s foot buckling, and steel yielding. Using the real earthquake ground motion records, constructed FE model, and limit states, a seismic fragility analysis is performed for a typical anchored steel liquid storage tank in Korea. In addition, for comparison purposes, a ring‐stiffened model is investigated to derive a seismic fragility curve. The results of the seismic fragility assessment show that elastic buckling is the most vulnerable damage state. In contrast, elephant’s foot buckling and steel yielding indicate relatively severe damage levels. Furthermore, it is observed that ring stiffeners decrease the elastic buckling damage, although there is no practical effect on elephant’s foot buckling and steel yielding in all ground motion intensities.

Clinical features and outcomes of abdominal tuberculosis in southeastern Korea: 12\u2009years of experience

BackgroundAbdominal tuberculosis (TB) is an uncommon form of infection with Mycobacterium tuberculosis in Korea. In this study, we aimed to highlight the clinical features, diagnostic methods, and outcomes of abdominal TB over 12 years in Southeastern Korea.MethodsA total of 139 patients diagnosed as having abdominal TB who received anti-TB medication from January 2005 to June 2016 were reviewed. Among them, 69 patients (49.6%) had luminal TB, 28 (20.1%) had peritoneal TB, 7 (5.0%) had nodal TB, 23 (16.5%) had visceral TB, and 12 (8.6%) had mixed TB.ResultsThe most frequent symptoms were abdominal pain (34.5%) and abdominal distension (21.0%). Diagnosis of abdominal TB was confirmed using microbiologic and/or histologic methods in 76 patients (confirmed diagnosis), while the remaining 63 patients were diagnosed based on clinical presentation and radiologic imaging (clinical diagnosis). According to diagnostic method, frequency of clinical diagnosis was highest in patients with luminal (50.7%) or peritoneal (64.3%) TB, while frequency of microscopic diagnosis was highest in patients with visceral TB (68.2%), and frequency of histologic diagnosis was highest in patients with nodal TB (85.2%). Interestingly, most patients, except those with nodal TB, showed a good response to anti-TB agents, with 84.2% showing a complete response. The mortality rate was only 1.4% in the present study.ConclusionsMost patients responded very well to anti-TB therapy, and surgery was required in only a minority of cases of suspected abdominal TB.

Assessment of sudden sediment source areas incurred by extreme rainfall in a mountainous environment: Approach using a subsurface hydrologic concept

In mechanism studies on sudden sediment source areas (SSAs), such as those of shallow landslides, area-specific soil strength information at the soil layer between saturated and unsaturated soil is an important factor. Although various soil-testing methods have been developed, it is difficult to measure soil strength directly due to local variation. Therefore, the purpose of this study is to estimate SSAs of shallow landslides by considering the soil strength attributes reflected in a simple subsurface hydrologic concept (SHC). The selected study area is a granitic region in Jinbu, eastern South Korea, where shallow landslides occurred at more than 1200 locations in 2006. To estimate SSAs, soil strength was generated using several methods (i.e., direct shear tests [DSTs], Monte Carlo-simulated direct shear tests [MSDTs], and triaxial compression test [TCTs]). In addition, to investigate the effects of soil attributes in saturated/unsaturated soil on the SHC, soil depth was surveyed at a small hillslope site within the large study area. A topographically driven physical shallow landslide stability model (SHALSTAB) was utilized to investigate the performance based on the various soil strength attributes, and the results were analyzed using receiver operating characteristic (ROC) analyses. In the evaluation using the three soil strength measures (Case I), the accuracy assessment of the estimated SSAs was relatively low (range of 0.45–0.71). Using the soil strength attribute reflecting SHC (Case II), the results showed a reasonable range of accuracy (0.84–0.85). In addition, using the effects of soil strength reflected by SHC and average soil depth (Case III) also showed good predictive accuracy (0.81–0.82), but it was lower than that of Case II. This indicates that combining the hydraulic properties related to soil strength facilitates more accurate identification of SSAs causing geomorphic surface changes.

Current status of newborn hearing screening in low-income families in the southeastern region of Korea.

OBJECTIVESThe aim of this study was to analyze the current status and problems of hearing screening tests for newborns in low-income families in the southeastern Korea.METHODSThis study analyzed data from the Ministry of Health and Welfare’s project on the early detection of hearing loss in newborns in low-income families in the southeastern Korea (2011-2015).RESULTSThe referral rate was 1.33, 1.69, and 1.27% in Daegu, Gyeongbuk, and Ulsan, respectively. The confirmatory test rate was 36.09, 23.38, and 52.94% in Daegu, Gyeongbuk, and Ulsan, respectively. The incidence of hearing loss (adjusted) was 0.41, 0.62, and 0.41% in Daegu, Gyeongbuk, and Ulsan, respectively. After confirming hearing loss, newborns with hearing handicaps were mostly lost to follow-up, and rehabilitation methods, such as hearing aids or cochlear implants, were not used. The screening tests were performed within 1 month of birth, and the confirmatory tests were generally performed within 3 months of birth. However, more than 3 months passed before the confirmatory tests were performed in infants with risk factors for hearing loss in Gyeongbuk and Ulsan.CONCLUSIONSHearing screening tests were conducted in newborns from low-income families in southeastern Korea who received a coupon for free testing, but the newborns that were referred after the screening tests were not promptly linked to the hospitals where confirmatory tests were performed. Furthermore, hearing rehabilitation was not consistently performed after hearing loss was confirmed. To successful early hearing loss detection and intervention, a systematic tracking system of hearing loss children is needed.

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.

Moriphila furva gen. and sp. nov. (Hemiptera: Psylloidea: Homotomidae), a new jumping plant-louse from Korea associated with Morus australis (Moraceae).

Moriphila furva Burckhardt Cho, gen. and sp. nov., is described from the mountain region in north eastern South Korea. Adults were collected on Morus australis which is a likely host. We provide morphological evidence that the new monotypic genus constitutes the probable sister group of the afrotropical Phytolyma whose species develop on Milicia and Morus (Moraceae). Differences between the two genera are detailed and the phylogenetic relationships to other members of Homotomidae: Macrohomotominae, to which the new genus belongs, are discussed. The host relationships of Psylloidea associated with Moraceae are reviewed. The latter constitutes the fifth most important host taxon of Psylloidea even though it is only a moderately large family of angiosperms in terms of constituent species. Moraceae have been colonised by psyllids at least five times independently. Following new combinations are proposed: Homotoma brevis (Li, 1993), comb. nov. and Homotoma microphyllae (Li Yang, 1991), comb. nov. (both from Caenohomotoma Yang Li, 1981).

Mid-Holocene forcing of the Tsushima Warm Current to the coastal environments in southwestern Japan with a view to foraminiferal faunas

We investigated fossil benthic and planktonic foraminifera in sediment cores FV10-05 and FV10-06-2, off Fukuoka, southwestern Japan to evaluate the impact of the Tsushima Warm Current on coastal environments around the strait between southeastern Korea and southwestern Japan during the early to middle Holocene. A cluster analysis of benthic foraminifera in these cores established four sample clusters (A, B, C, and D), which are interpreted as indicators of relatively high energy condition due to wave/current processes. Clusters A and C, characterized by attached taxa, became dominant at ∼6.6 ka. Such faunal changes are consistent with the current-influenced sedimentation, confirming that the Tsushima Warm Current reached its present-day distribution at that time. A hiatus at ∼8–7 ka has been reported in cores from deeper water off Fukuoka, whereas our shallow core shows deposition under high energy conditions was continuous even at ∼8–7 ka near the coast off river mouth. Across the strait, the maximum landward extent of benthic foraminifera occurred in the Nakdong River delta (southeastern Korea) in ∼8–7 ka, and an outer bay fauna of benthic foraminifera with seagrass beds occurred transiently in the restricted bay of the San-in district (southwestern Japan) at that time. The evolution of the Tsushima Warm Current during the early to middle Holocene thus appears to play an important role in coastal environments not only in the strait areas between southeastern Korea and southwestern Japan, but also in the San-in district.

The Incidences of Dermatophytosis and Cutaneous Candidiasis Infection in Southeastern Korea between 2013 and 2016

The Incidences of Dermatophytosis and Cutaneous Candidiasis Infection in Southeastern Korea between 2013 and 2016

The Effects of the Educational Resources on Recruitment Rates of the Universities in South-Eastern Korea

The Effects of the Educational Resources on Recruitment Rates of the Universities in South-Eastern Korea

Variations in carbon and nitrogen stable isotopes and in heavy metal contents of mariculture kelp Undaria pinnatifida in Gijang, southeastern Korea

Variations in carbon and nitrogen stable isotopes and in heavy metal contents of mariculture kelp Undaria pinnatifida in Gijang, southeastern Korea

Deep Fault Plane Revealed by High‐Precision Locations of Early Aftershocks Following the 12 September 2016 ML 5.8 Gyeongju, Korea, Earthquake

An M-L 5.8 earthquake, which is large for a stable continental region, occurred in southeastern Korea on 12 September 2016. Ten days of data from a temporary seismic network deployed immediately after the mainshock are combined with data from permanent seismic stations to determine high-precision locations of early aftershocks to reveal the geometry of the causative structure at depth. Well-constrained relative earthquake hypocenters and focal mechanisms are used to define the subsurface fault plane with a strike of similar to N28 degrees E and dip of similar to 78 degrees to the east-southeast. This fault plane extends from 12 to 15 km depth and may have been responsible for most of the early earthquakes in the Gyeongju earthquake sequence. A pre-existing weak zone in a strike-slip duplex that formed from subsidiary Riedel shears beneath the Yangsan fault system may have been reactivated to nucleate the mainshock and aftershocks.

Classification of geological and engineering properties in weak rock: a case study of a tunnel in a fault zone in southeastern Korea

Given that the rock mass classifications of weak rocks observed in tunnels are evaluated only as ‘poor rock mass’ or grades IV–V using existing rock mass classification methods, a new scheme is needed that would better distinguish the various geological properties of weak rocks. In this study, geological and engineering properties of weak rocks were classified based on the analysis results of a total of 55 faces in a tunnel that passes through a fault zone in southeastern South Korea. Geological observations, point load tests, and Schmidt hammer tests were conducted to analyze the properties of the weak rocks. Through this analysis, the weak rocks were classified into two weathering types: ‘disintegrated rocks’ and ‘decomposed rocks’. This division is based on the tectonic mechanisms and parameters necessary to determine the engineering geological properties of individual rock-weathering types according to their geological properties. The ‘disintegrated rocks’ have been physically weathered by brittle deformation and are classified into three subtypes using the rock quality designation (RQD) and joint set number (Jn), which can characterize the discontinuity properties of the rocks. Point load tests were conducted on these individual subtypes to calculate the point load index (Is(50)). The resultant values are clearly related to the assigned three subtypes. The ‘decomposed rocks’ have been chemically weathered by ductile deformation and are classified into five subtypes based on the results of Schmidt hammer tests conducted on faces and Is(50) values calculated using the average of the Schmidt hammer values. The geological properties of the individual subtypes of the ‘disintegrated rocks’ and ‘decomposed rocks’ are clearly related to their engineering properties. Based on these results, rock mass classification diagrams are proposed that can be simply and easily applied to weak rocks.

The First Stage of an Earthquake Early Warning System in South Korea

ABSTRACT The first stage of the Korean Earthquake Early Warning System (KEEWS) began operations at the Korea Meteorological Administration in January 2015. The goal at this stage is to issue a warning within 50 s of detecting an earthquake of magnitude 5.0 or more. Since operations began, all local earthquakes of magnitude 3.0 or greater have been detected with sufficient accuracy. In the two years since operations started, three warnings have been issued, including for an offshore earthquake of magnitude 5.0 that occurred on 5 July 2016 and for two successive magnitude 5.1 and 5.8 earthquakes that occurred in the southeastern Korean Peninsula on 12 September 2016. In this article, we introduce the KEEWS and review its performance from January 2015 to July 2016. We also report on its responses for recent offshore and inland earthquakes.

Integrated In Situ Stress Estimation by Hydraulic Fracturing, Borehole Observations and Numerical Analysis at the EXP-1 Borehole in Pohang, Korea

It is desirable to combine the stress measurement data produced by different methods to obtain a more reliable estimation of in situ stress. We present a regional case study of integrated in situ stress estimation by hydraulic fracturing, observations of borehole breakouts and drilling-induced fractures, and numerical modeling of a 1 km-deep borehole (EXP-1) in Pohang, South Korea. Prior to measuring the stress, World Stress Map (WSM) and modern field data in the Korean Peninsula are used to construct a best estimate stress model in this area. Then, new stress data from hydraulic fracturing and borehole observations is added to determine magnitude and orientation of horizontal stresses. Minimum horizontal principal stress is estimated from the shut-in pressure of the hydraulic fracturing measurement at a depth of about 700 m. The horizontal stress ratios (S Hmax/S hmin) derived from hydraulic fracturing, borehole breakout, and drilling-induced fractures are 1.4, 1.2, and 1.1–1.4, respectively, and the average orientations of the maximum horizontal stresses derived by field methods are N138°E, N122°E, and N136°E, respectively. The results of hydraulic fracturing and borehole observations are integrated with a result of numerical modeling to produce a final rock stress model. The results of the integration give in situ stress ratios of 1.3/1.0/0.8 (S Hmax/S V/S hmin) with an average azimuth of S Hmax in the orientation range of N130°E–N136°E. It is found that the orientation of S Hmax is deviated by more than 40° clockwise compared to directions reported for the WSM in southeastern Korean peninsula.

Sedimentary and structural evolution of the Eastern South Korea Plateau (ESKP), East Sea (Japan Sea)

The East Sea (Japan Sea) is a semi-enclosed back-arc basin that is thought to preserve a significant record of tectonic evolution and paleo-climatic changes of Eastern Asia during the Neogene. We use here 2-D regional multi-channel seismic reflection profiles and borehole data from Expedition 346 of the Integrated Ocean Drilling Program (IODP) to provide new constraints on the geological history of the Eastern South Korea Plateau (ESKP). The ESKP represents a structurally-complex basement high in the southwestern East Sea which formed during rifting of the back-arc basin. Our new observations show that the ESKP is composed of numerous horsts and grabens controlled by NE-trending normal faults. The acoustic basement is blanketed by Oligocene to recent sediments that have preferentially accumulated in topographic lows (up to 1.5 km thick) and have been cored during Expedition 346 at Site U1430 close to the southern margin of the ESKP. Seismic profiles in the ESKP reveal three units separated by regional unconformities. These seismic units closely correspond to IODP lithostratigraphic units defined at Site U1430, where biostratigraphic data can be used to constrain the timing of three main evolutionary stages of the ESKP. Stage 1 was related to rifting in the late Oligocene and middle Miocene, terminated by a regional uplift leading to an erosional phase in the middle Miocene. Stage 2 was associated with subsidence in the middle and late Miocene and uplift and accompanying erosion or non-deposition in the latest late Miocene. Stage 3 (Pliocene to present) recorded overall uniform hemipelagic-pelagic subsidence of the ESKP with short-lived tectonically-induced uplifts in the late middle Miocene and latest Miocene-early Pliocene. The three stages of evolution of the ESKP closely correlate to sedimentary changes since the Oligocene and suggest a direct control of regional/local tectonics on sedimentation patterns in the southwestern East Sea, with secondary influence of regional climatic and paleo-oceanographic processes.

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

AbstractThe seismicity in the Korean Peninsula has increased since the 2011Mw9.0 Tohoku‐Oki megathrust earthquake. Two strike‐slip earthquakes with magnitudes ofML5.1 and 5.8 occurred in the southeastern Korean Peninsula on 12 September 2016. The two events occurred within 48 min. TheML5.8 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 2 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 twoML5.1 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.

Crustal evolution in the Gyeongsang Arc, southeastern Korea: Geochronological, geochemical and Sr-Nd-Hf isotopic constraints from granitoid rocks

A loosely assembled patchwork of Cretaceous to Paleogene granitoid plutons comprise the platform of the Gyeongsang Arc in southeastern Korea and represent a high-flux pulse of magmatism in Northeast Asia. The present study uses new and published zircon U-Pb ages, whole-rock geochemical and Sr-Nd isotopic data, and zircon Hf isotopic data of the plutons to decipher the crustal evolution in the Gyeongsang Arc and compare it with adjacent accretionary terranes. The ion microprobe zircon U-Pb ages range from 91 to 27 Ma, revealing the selective occurrence of Paleogene plutons in the eastern coastal area, to the east of the NNE-trending Yangsan fault system. Paleoproterozoic to Paleogene xenocrystic cores are occasionally observed in the zircon grains. The plutons are composed of dominantly magnesian, high- to medium-K calc-alkaline granites-granodiorites exhibiting typical geochemical characteristics of subduction zone magma, and less abundant ferroan alkaline granites. Their trace element patterns require residual amphibole and feldspar, but not garnet in the source. The whole-rock Sr-Nd and zircon Hf isotopic compositions of the granitoids (initial 87Sr/86Sr = 0.7043 to 0.7069; eNd(t) = –4.8 to +2.9; eHf(t) = –5.0 to +19.7) are distinctly more primitive than those of inland granitoids distributed outside of the Gyeongsang backarc basin. An important role of relatively young, most likely Paleozoic juvenile crust in the formation of the Late Cretaceous granitoids is suggested by the time-eHf trend of high-eHf zircons that converges toward data points of the Late Permian Yeongdeok adakite composing the arc basement. The asthenospheric mantle input is highlighted by significantly high (>+17) eHf(t) values of some zircons from the early Eocene alkaline plutons. Subsequent reworking of the rejuvenated crust yielded granitoid plutons possessing slightly but recognizably higher eNd(t) and eHf(t) values than the older plutons. These isotopic features demonstrate that the Cretaceous-Paleogene calc-alkaline granitoids in the Gyeongsang Arc are not “juvenile” (sensu stricto) despite their mostly positive epsilon Nd and Hf values, but are basically a product of crustal reworking. The Sr-Nd-Hf isotopic compositions of Late Cretaceous granitoids in the Gyeongsang Arc are comparatively more primitive than those in adjacent accretionary terranes such as Southwest Japan and Fujian province in southeastern China, reflecting differences in the formation age of the basement on which the arc system was built.