ABSTRACT We develop a model to predict the amplification of ground motion for 5% damped pseudospectral accelerations for periods ranging from 0.01 to 0.5 s while considering the effects of varying topography. We use 487 ground motions recorded at densely deployed 118 temporary stations and one regular seismic station in Pohang, South Korea. The proposed model depends on the magnitude of detected motions and the distance from the epicenter. In addition to the relative elevation, we newly introduce the azimuth angle from the station to the epicenter relative to slope aspect as an important topography parameter, which we denote as α. We observe that ground motions are deamplified when the relative elevation is lower than −18 m, and the angle α is smaller than 90° (the slope faces the epicenter). When relative elevation is higher than 14 m, ground motions are amplified for α<90° and deamplified for α>90° (the slope is facing away from the epicenter). The ground motions are not affected by the angle α when the relative elevation ranges from −18 to 14 m. Finally, we propose a predictive model as a function of earthquake magnitude, epicentral distance, relative elevation, and α.

Gangwon Province, located in the central part of the Korean Peninsula, features northeast-southwest faults and tectonic structures formed by plutonic intrusions. Despite decades of geological investigations from near-surface to the upper crust in Gangwon Province, the lithospheric structure of this region remains poorly understood. The primary objective of this study is to identify velocity anomalies potentially associated with plutonic intrusions and to elucidate the formation processes and mechanisms governing the crustal and upper mantle structures in this region. We employed Helmholtz tomography to generate phase-velocity maps for periods of 10-40 s using a dense seismic network of 101 stations. These maps were subsequently inverted to obtain an S-wave velocity model from the upper crust to the uppermost mantle. Our results reveal northeast-southwest-trending low-velocity anomalies along major faults in central to northern Gangwon Province (i.e. eastern Gyeonggi Massif), extending to depths of approximately 25-30 km. These low-velocity anomalies align with the orientations of Jurassic granitoid intrusions formed through partial melting processes. Additionally, we identified other low-velocity anomalies, likely formed by Late Cretaceous intrusions, which are oriented perpendicular to the major faults. In contrast, the southeastern Gangwon Province (i.e. Taebaeksan Basin) exhibits a distinctly different velocity structure, lacking features indicative of granitic intrusions and showing low-velocity anomalies confined to shallow depths. The pronounced low-velocity anomalies observed at depths of 5-10 km in Taebaeksan Basin are attributed to a complex fault zone influenced by Permo-Triassic collisional orogeny.

An ML7.2 (Mw7.3) earthquake occurred near Hualien in eastern Taiwan on April 2 (April 3, local time), 2024. Numerous aftershocks immediately followed the mainshock. The spatial distribution and time variation of the mainshock and its aftershocks from April 2 to December 31, 2024, were investigated in this study. Most aftershocks were located around the Lingding fault and Meilun fault in northern Hualien. The primary aftershock sequence occurred from April 2 to April 21, and the secondary one happened on April 22 in the southern part of the narrow and long aftershock zone. The length of the fault inferred from aftershock activity is estimated to be about 80 km. The depth distribution of the number of events shows that the number first increases with depth and peaks at 8 km, then decreases with increasing depth. This depth distribution exhibits a frictional rupture/quasi-plastic (FR/QP) transition. The temporal variations of aftershocks in terms of latitude and focal depth within the aftershock area are also investigated. The b value of the Gutenberg-Richter frequency-magnitude relationship for the Hualien aftershock sequence is 1.05 using the least-squares method and 0.75 using the maximum likelihood method. The former is a better fit to the observations than the latter. The inferred Omori-Utsu law, i.e., n(t) = k/(t + c)p, fits the time variation of aftershocks well for magnitudes ML≥Mc=3.6. Results show p = 1.05 and c = 0.11 for primary aftershocks and p = 2.70 and c = 1.94 for secondary aftershocks. The p-value for primary aftershocks is lower than that for secondary ones, thus indicating a faster decay of events for the latter than for the former. To explain the frequency-magnitude relationship, the Omori-Utsu law, and the CWA data of the 2024 Hualien earthquake sequence To analyze the statistical characteristics of primary and secondary aftershock sequences Results show that the p-value for primary aftershocks is lower than that for secondary ones, thus indicating a faster decay of events for the latter than for the former

Located at the easternmost passive margin of the Eurasian Plate, the southeastern Korean Peninsula shows geological signatures consistent with Miocene backarc opening associated with the Pacific Plate subduction. The region comprises two contrasting crustal blocks-the Early Cretaceous Gyeongsang Basin and Miocene Yeonil Basin (YB)-and hosts multiple fault systems that record both extensional and contractional deformation, providing an ideal setting to investigate crustal evolution along a passive margin. Motivated by this complex setting, we performed high-resolution P-receiver function imaging using a dense broad-band seismic network. Our results reveal two Moho offsets: a western offset from 33 to 28 km, and an eastern offset from 28 to 26 km, coinciding with major fault zones and likely reflecting localized crustal thinning and subsequent reactivation. Crustal anisotropy, inferred from changes in fast-axis orientations, varies spatially, with Miocene fossil anisotropy in the GB and both fossil and present-day stress-induced anisotropy in the YB. Variations in P-to-S velocity ratio (VP/VS) reflect compositional heterogeneity and fault-related fracturing. Large earthquakes (M >= 4) occurred in low-VP/VS zones associated with relatively rigid and possibly locked crustal segments, while high-VP/VS regions coincide with zones of crustal weakening and microseismicity. Our findings suggest that extension-related deformation and inherited structural heterogeneity are preserved within the crust of this fossil backarc system, linking past tectonic processes to present-day structure and seismicity.

Ambient seismic noise from human activities, site conditions, meteorology, and instrument self-noise limits earthquake monitoring. We characterize noise in the Pohang Community Seismograph Network (PCSN), deployed after the 2017 M-w 5.5 Pohang earthquake, using power spectral density (PSD) estimates. The PCSN stations were categorized into four groups based on their location and depth: rural, urban, coastal, and borehole array. We compared PSDs across different period ranges. At very short periods (<0.03 s), we could occasionally identify elevated noise levels caused by everyday weather conditions. The seismic signature of rainfall was clearly recorded at surface stations, but barely recorded at 500 m depth. At short periods (0.02-1.00 s), anthropogenic noise was predominant. The noise levels were highest in urban areas, followed by coastal areas, and lowest in rural areas. At long periods (1-10 s), we investigated the effect of typhoons and ocean waves. As typhoons Hinnamnor and Nanmadol approached the Korean Peninsula, the background noise level increased and reached a peak as it passed the Pohang region. The geological setting affected the ambient seismic noise: the PSDs of the horizontal component are higher than those of the vertical component in areas with thick sediments. The detection thresholds map in the Pohang region shows the smallest magnitude in the middle of the network in regions surrounded by stations with low-ambient noise levels. The recently compiled PCSN earthquake catalog demonstrates the effect of background noise on earthquake detection capability. The number of detected earthquakes under the magnitude of completeness (0.34) was 53 at night and 5 during the day. These findings underscore the importance of reducing ambient noise levels by relocating or installing seismic stations at depth on bedrock for effective microearthquake observations in a densely populated metropolitan area.

Oceanic intraplate earthquakes are closely associated with seafloor geologic features, such as knolls, fracture zones, and seamounts. However, this inference is often challenged by significant earthquake location uncertainties and difficulties in detecting small earthquakes, largely owing to limited station coverage. In this study, we used ocean-bottom seismometer data from the Oldest-1 array consisting of 12 sites, which is part of the Pacific array, to construct a one-year earthquake catalog for intraplate events on the oldest Pacific plate (similar to 170-180 Ma), similar to 900 km east of the Mariana trench. This catalog includes 17 small intraplate earthquakes of magnitude m(b)(Sn) 0.9-3.7 that occur within the array (<700 km from the array center). Although their small magnitudes and sparse station coverage, with an average interstation distance of similar to 393 km did not permit the estimation of focal depths and mechanisms, (re)located events with horizontal location errors of 2-8 km highlight a clear zone of deformation within the plate. Of the 17 earthquakes, 13 occurred in close proximity to seafloor geologic features. We found that five events with m(b)(Sn) 0.9-1.6, located near knolls, exhibited high waveform similarity (correlation coefficient exceeding 0.92) and colocation within an 80x80 m(2) area, suggesting that they could be repeating earthquakes. The occurrence of five other events with m(b)(Sn) 1.3-2.2 at/near an old fracture zone (similar to 160 Ma) may indicate that the fracture zone may be seismically active regardless of its age. Conversely, three events with m(b)(Sn) 2.9-3.7, which show clear T-phase signals are located near seamounts, suggesting the reactivation of pre-existing faults. Our results provide a tighter spatial correlation between epicenters and zones of existing weakness on the seafloor as well as an avenue for detailing waveform characteristics in future experiments.

Passive margins offer valuable insights into the interplay between tectonic processes and surface elevation, as topographic signals are often preserved under long-term tectonic quiescence. Gangwon Province, located in the mid-eastern Korean Peninsula, exemplifies such a setting, shaped by back-arc opening of the East Sea (Sea of Japan). Here, we present Vp/Vs ratios, residual topography, and high-resolution S-wave crustal velocity models from joint inversion of teleseismic receiver functions and Rayleigh-wave dispersion curves. Our results reveal low S-wave velocities and high Vp/Vs ratios throughout the crust beneath the coastal region, suggesting shallow-reaching thermal effects from mantle upwelling. In contrast, west of the Taebaek Mountain Range, low velocities are confined to the lower crust, and Vp/Vs ratios are relatively low, consistent with thick, felsic crust inhibiting upward thermal propagation. These findings demonstrate how crustal properties modulate the surface and thermal expressions of mantle upwelling, likely caused by edge-driven convection in passive margin settings.

High-density seismic networks enable precise monitoring of seismic activity in specific regions by facilitating the detection of microearthquakes, accurate hypocenter determination, and detailed fault structure analysis. The importance and application of temporary dense arrays have steadily increased in recent years. This study examines how environmental factors, and instrumentation at installation sites affect the performance of temporary seismic networks. Following the ML 4.1 earthquake that occurred in Goesan, Korea, on October 28, 2022 (UTC) (October 29, 2022, KST), a temporary seismic array comprising one broadband and eight short-period stations was deployed within an similar to 8 km radius of the epicenter. The network was operated to monitor aftershock activity and detect smaller events. Preliminary results fromtwo weeks of manually reviewed data revealed 329 earthquakes, approximately 11 times more than those reported by the Korea Meteorological Administration. Power spectral density analysis was used to quantify background noise levels at each station and to quantitatively assess both site conditions and instrument performance. The located aftershocks were mostly concentrated within 1 km of the mainshock and at depths of 12-15km, with the majority occurring within four days of the main event. Combined a composite focal mechanism solution, derived from the first-motion polarities of the ML 4.1 mainshock and ML 0.8 aftershock, with the spatial distribution of earthquake hypocenters, the analysis indicates left-lateral strike-slip motion on a near-vertical fault striking WNW-ESE, consistent with the previously proposed fault geometry and sense of slip. Taken together, these findings demonstrate that dense seismic networks can effectively complement permanent arrays by enhancing the precision of earthquake location, subsurface structural analysis, and seismic hazard assessment. The results presented here are based on preliminary analyses, and more refined and comprehensive conclusions are expected through future automated and in-depth investigations.

The South Yellow Sea and its environs are pivotal for unraveling the complexities of crustal dynamics and continental collision processes. A holistic assessment of deep structural variations from northern China to the Korean Peninsula is essential for a comprehensive and accurate determination of the tectonic affinity of the Korean Peninsula.Thus, we deployed a pioneering active-source seismic profile (Line2016) spanning the South Yellow Sea and the eastern onshore region of the Korean Peninsula, provides crucial insights into the collision dynamics between the Sino-Korean Block and the Yangtze Block. Our innovative approach, incorporating forward modeling, tomography, and finite-difference wavefield modeling, yielded a high-resolution crustal P-wave velocity model, addressing a significant knowledge gap in understanding the geological intricacies between northern China and the Korean Peninsula. The results confirmed and precisely located the West Marginal Fault of the Korean Peninsula, a significant crustalscale tectonic structure, likely representing the eastern boundary between the Yangtze Block and the Sino-Korean Block. The study advocates for classifying the Korean Peninsula as part of the Sino-Korean Block, presenting evidence for the one-part affinity hypothesis. This collision resulted in the creation of two distinct suture zones-an orogenic belt in the northern part and a significant strike-slip fault zone in the eastern part of the South Yellow Sea. The study emphasizes the pivotal role of block morphology in regulating plate convergence, providing valuable insights for understanding similar phenomena in other collision zones. (c) 2024 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

The 2016 magnitude (M W ) 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 km 2 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 (M C ) 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.

Five sequences of deep fluid injections at the Pohang Enhanced Geothermal System (EGS) triggered an ML L 5.4 earthquake on November 15, 2017. The foreshock-mainshock-aftershock sequence was monitored using dense seismic networks. Between November 14, 2017, and May 31, 2023, this study detected 5,169 earthquakes and determined the relative locations of 4,902 earthquakes, including seven foreshocks. A heterogeneous subsurface fault model is proposed, in which the fault is reactivated by induced and triggered earthquakes. The earthquake frequency decreased after the mainshock, with a temporary increase after the ML L 4.6 event on February 10, 2018. The magnitude-frequency b-values are significantly lower than those for the background seismicity in the Korean Peninsula and those for the 2016 Gyeongju earthquake sequence. The aftershock decay rate p-values are within the range of typical values, regardless of decreasing over time. The earthquake focal mechanisms exhibit a predominance of strike-slip components, whereas the slip tendency indicates a higher value in reverse faulting geometry, implying stress redistribution after the mainshock. The seismic landscape with ongoing aftershock activity after the 2017 Pohang earthquake underscores the importance of sustained, long-term seismic monitoring to comprehensively grasp the implications of the new seismic environment for seismic hazards in the area.

Foreshocks and aftershocks occurred before and after the ML6.8 (Mw7.0) earthquake in eastern Taiwan on 18 September 2022. We explore the epicentral distribution and temporal variations for the mainshock, foreshocks, and aftershocks. Most of the events were located in the area around the Longitudinal Valley. Most foreshocks occurred around the mainshock, while the aftershocks happened outwards from the foreshock area. The temporal variations in seismic-wave energy show that the largest foreshock and the mainshock were responsible for releasing most of the energy during the earthquake sequence. In addition, the b values of the Gutenberg-Richter frequency-magnitude law were 0.62 for foreshocks, 0.87 for aftershocks, and 0.71 for the whole seismic activity by using the least squares method and 0.52 for foreshocks, 0.84 for aftershocks, and 0.65 for the whole seismic activity by using the maximum likelihood method. The b values increase from foreshocks to aftershocks, suggesting the possibility that the fluid pressure of faults during foreshocks is higher than that of the faults during aftershocks due to the outward migration of water. The p-value of the Omori-Utsu law for the aftershock sequence was estimated to be 0.92 for all aftershocks in the study, 1.39 for the aftershocks occurred in the first 6 days, and 1.30 for the aftershocks occurred in the first 12 days. The foreshock sequence could not be described by the inverse Omori law.

The 2017 Pohang earthquake, with a moment magnitude (M) of 5.5, caused severe building damage and widespread liquefaction. In this study, we evaluate the applicability of ground response and liquefaction triggering analyses for the Pohang earthquake using deep shear wave velocity (VS) profiles. The VS profiles are obtained at Handong University and the Songdo Pine Forest by inverting the Rayleigh wave dispersion curves based on microtremor array measurements (MAM) and multi-channel analysis of surface waves (MASW). In one-dimensional effective stress analyses for the two sites, we consider the uncertainty of the nonlinear soil properties for three cases and use 118 rock outcrop motions. At Handong University, the spectral accelerations of surface ground motions are larger than those of the current Korean design spectra with a return period of 500 years at the natural period of the damaged buildings. At the Songdo Pine Forest, for the Case 2, numerous ground motions result in the maximum pore water pressure ratio of 1 (i.e., liquefaction occurrence). Furthermore, we calculate the liquefaction potential index (LPI) values using the VS-based simplified method. To compute the cyclic stress ratio for depths, we utilize the peak ground accelerations estimated by ground response analyses and estimated by stress reduction factor (rd), respectively. The LPI values, based on the ground response analyses, range from 0 to 4, indicating minor or no damage, while the LPI value using the rd is zero. The results of the ground response and liquefaction triggering analyses are similar to the actual damage cases.

We present the results of seismic profiling aimed to characterize the structure of shallow formation in the Geoncheon Valley (GCV), an area we interpreted as the north tip of the Miryang Fault This profile was deployed in the GCV, near the City of Gyeongju, Republic of Korea. The Miryang Fault is one of the major faults of the Yangsan Fault System in the Cretaceous Gyeongsang Basin, the southeastern Korean Peninsula. Despite local earthquake records showing scattered micro-seismicity along the trace of Miryang Fault, other geophysical and geological information is rare. The location of the GCV is tectonically critical: it is on the major stratigraphic boundary in the Cretaceous Gyeongsang Basin: the Hayang Group (dominantly sedimentary rocks) to the north and Yucheon Group (volcanic and volcanoclastic rocks) to the south. The surface expression of the Miryang Fault in this area is difficult to define by geomorphology since the surface is covered by highly altered cultivated farmlands. We tackle this problem by conducting seismic profiling. On a 1,200-m long linear profile, we deployed a dense array using 3-component seismic sensors and acquired both passive and active seismic data. The passive seismic data acquisition lasted about 12 days. At the end of the deployment period directly before withdrawing the array, an active survey using a sledgehammer source was also conducted. The integrated seismic cross-section displays significant segmentation in the texture of the seismic image, the sediment-bedrock interface is between less than 10 meters to slightly greater than 20 meters with a general trend of dipping to the southeast. The preliminary seismic results suggest that it is likely the Miryang Fault terminates inside the GCV, and the type of tip damage zone appears to be a group of horsetail splays.

&lt;p&gt;The M&lt;sub&gt;L&lt;/sub&gt; 5.2 earthquake occurred in Mt.Sokri (September 16, 1978), the center of South Korea. It was the fourth largest earthquake in South Korea since the modern seismic observation began. The Korea Meteorological Administration (KMA), the United States Geological Survey (USGS), and the International Seismological Centre (ISC) announced the location of the hypocenter respectively, but they were different. In this study, we analyzed the subsurface fault structure using current micro-earthquakes. We have used data collected by temporary seismic stations installed in the Mt.Sokri area by Pusan National University since May 2019 and the permanent seismic stations installed by KMA since 1978. KMA reported 188 earthquakes from 2007 to 2021 in the study area. We detected additional 280 micro-earthquakes using STA/LTA and template matching methods. The initial result of earthquake locations using HYPOELLIPSE was scattered across the study area. To obtain reliable locations, we relocated earthquakes using HypoDD. As a result, 468 earthquakes were relocated, about twice as many as those reported by KMA. We recognized earthquakes have occurred along WNW-ESE subsurface faults at the depth of 14 to 18km. We determined the focal mechanisms of 15 earthquakes with magnitudes greater than 2. The location of the Mt.Sokri earthquake was reviewed by comparing these results with the locations announced by the three institutions. Joint analysis of the focal mechanisms, distribution of earthquakes, and geological setting, the WNW-ESE plane was interpreted as the major fault plane. Apparently, the micro-seismicity locations in this study better correlated with the epicenter announced by USGS. However, it is difficult to confidently specify the location of the 1978 earthquake only to the current earthquakes.&lt;/p&gt;

Many deep faults do not reach the earth’s surface and thus are not recognized. Such faults are rarely mapped by standard surface geological mapping. This seriously hinders seismic risk mitigation efforts. In this study, we applied the horizontal-to-vertical spectral ratio (HVSR) method to identify blind faults invisible at the surface. Despite its simplicity and low-cost implementation, we noticed that HVSR results were unstable using data collected by exposed seismometers or under higher wind speeds. Therefore, three-component seismic sensors for ambient noise observations were buried at different depths to examine the effects of ground coupling, wind speeds, and precipitations. Results from a series of field tests under diverse conditions guided us to establish data selection criteria. The first required condition is that seismic sensors should be buried (>0.3 meters) to secure ground coupling and to avoid any direct exposure to wind or precipitations. The other is that data should be collected at low wind speeds (< 3 m/s). The requirements were applied to ambient noise data along two profiles traversing unnamed and inferred faults in Pohang, Korea. We initially estimated the resonance frequencies for each site, which varied from 0.41 to 2.52 Hz. They were then converted to bedrock depths using an empirical relationship between the resonance frequency and depth to bedrock observed at boreholes in the area. The estimated depths to bedrock along profiles ranged from 8.0 to -472.0 meters. The resulting depth profiles show significant lateral variations in the bedrock depth, including the one near the Gokgang fault at which the thickness to the major impedance contrasts decreased from 196 to 20 meters. Sudden variations were also observed at unexpected locations along the profile. We examined the details, especially for sites of apparent changes in bedrock depth, and compared their characteristics with other geophysical studies, including Vs30, MASW, Bouguer gravity anomaly, and adjacent stations correlation. Their results are all well correlated to each other and indicate rapid changes in bedrock depth. We attribute the rapid changes to vertical displacements by ancient faulting activity.

The recent micro-earthquake activity in the Mt. Songnisan area was analyzed to identify the causative subsurface fault structure. We have used seismic data between April 2019 and April 2021 collected from a temporary network deployed in the Mt. Songnisan area by Pusan National University and between January 2007 and May 2021 from the regional seismic network by the Korea Meteorological Administration (KMA). Template matching was performed with 119 earthquakes announced by the KMA in the study area from 2007 to 2021, and as a result, 274 events were detected, and these events with the templates in a total of 393 events were analyzed. The initial result of the was determined using HYPOELLIPSE, and the relative earthquake location was performed using HypoDD for precise analysis. We determined the focal mechanisms of 18 earthquakes with magnitudes greater than 1.3. As a result of analyzing the focal mechanisms of 18 earthquakes with a magnitude of 1.3 or higher, all were horizontally moving faults with strikes of WNW-ESE or NNE-SSW. A comparison of the earthquake distribution and focal mechanism solutions with the mapped faults shows that the previously unknown subsurface fault in the WNW-ESE trend is the main fault responsible for the micro-earthquake activity of the area. The ML 5.2 Songnisan earthquakeoccurred on September 16, 1978, near the area where this underground fault exists. The KMA, U.S. National Earthquake Information Center (NEIC), and International Seismological Centre (ISC) recorded this earthquake, but each epicenter differed. In comparing the epicenters of each organization and the results of this study, it was confirmed that micro-earthquakes have continued to occur around the epicenters announced by NEIC and ISC. However, there are limitations in specifying the exact location of the 1978 Songnisan earthquake based solely on the current status of micro-earthquakes.

SUMMARY 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.

Geomorphic indices are a useful tool for rapid tectonic activity assessment over large areas. We assessed the tectonic activities along the Yangsan Fault where many Quaternary faults are observed, the Ulsan Fault where Quaternary faults and micro-earthquakes frequently occur, and the Miryang Fault where only micro-earthquakes are reported, using the geomorphic indices. The indices used in this study are Hypsometric Integral (HI), Hypsometric Curve (HC), and Basin Shape ratio (BS), which indicate the maturity of the drainage basins, Asymmetry Factor (AF), which implies the degree of asymmetry due to tilting of the basins, and Stream Length-gradient (SL), which represents the change in slope of the stream. These results were combined to evaluate Relative Tectonic Activity (RTA) of each basin. Drainage basins along the Yangsan and Ulsan faults dominantly show 'High' and 'Very High' RTAs and are also characterized with the asymmetry of RTA distribution of much higher tectonic activity in the eastern block of the faults. However, the drainage basins along the Miryang Fault dominantly show 'High' and 'Moderate' RTAs with their symmetric distribution. SL values generally depend on the lithology of bedrocks, but some SL anomalies in the Yangsan and Ulsan faults are nearly identical to the location of the Quaternary surface ruptures. These features indicate that the analysis results of geomorphic indices supported by geological interpretation can be useful for finding the Quaternary faults.