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Stratigraphic Constraints of Rupture Kinematics and Scenarios of Paleoearthquakes on the Southern Yangsan Fault, SE Korea
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Some parts of the Yangsan Fault, a prominent mature intraplate fault on the Korean Peninsula, are still active. However, structural and paleoseismic investigations are limited because a large portion of the fault zone is covered by Quaternary sediments. To characterize the northern Yangsan Fault (NYF) and its paleoseismic features, we conducted topographic analyses, geological mapping, electrical resistivity surveys, borehole drilling, SHRIMP U–Pb age dating, a trench survey, and optically-stimulated-luminescence age dating (OSL). This multidisciplinary approach shows that the NYF is expressed as a nearly straight incised valley and a ridge-disrupting topographic lineament with anastomosing multiple core zones at outcrops. The NYF in Yeonghae area exhibits a NNE-striking eastern strand and a NNW- to N–S-striking western strand. Between these, a Jurassic granite (> 300-m-wide) is distributed as an enclosed lens, bounded by Precambrian metamorphic rocks to the east and Cretaceous sedimentary rocks to the west. The eastern strand likely passes S–N through the offshore area (East Sea) to onshore Pyeonghae area. A trench survey identified faults transecting Quaternary strata, providing the first paleoseismic record along the NYF. Stratigraphic features and OSL ages show that the most recent rupture occurred after 97 ± 7 ka, with the rupture along the western boundary of the mature fault core. Although older structures are prominent, paleoseismic records are few—a limitation for our onshore investigations. To reveal YF neotectonic activities under the East Sea, we need further information about off-fault damage (landslide, turbidite, tsunami records) within marine deposits as well as on-fault damage.
The spatial distribution of large earthquakes in slowly deforming continental regions (SDCR) is poorly documented and, thus, has often been deemed to be random. Unlike in high strain regions, where seismic activity concentrates along major active faults, earthquakes in SDCR may seem to occur more erratically in space and time. This questions classical fault behavior models, posing paramount issues for seismic hazard assessment. Here, we investigate the M7, 1967, Mogod earthquake in Mongolia, a region recognized as a SDCR. Despite the absence of visible cumulative deformation at the ground surface, we found evidence for at least 3 surface rupturing earthquakes during the last 50,000 years, associated with a slip-rate of 0.06 ± 0.01 mm/year. These results show that in SDCR, like in faster deforming regions, deformation localizes on specific structures. However, the excessive length of return time for large earthquakes along these structures makes it more difficult to recognize earthquake series, and could conversely lead to the misconception that in SDCR earthquakes would be randomly located. Thus, our result emphasizes the need for systematic appraisal of the potential seismogenic structures in SDCR in order to lower the uncertainties associated with the seismogenic sources in seismic hazard models.
Quaternary fault research has mainly been done with the focus of the outcrop and trench to identify the sense and displacement of the fault. Because the result from the outcrop and trench is obtained from exposed sections, the interpretation of fault information is likely to be overestimated or underestimated. Quaternary faulting affected the current topography and/or Quaternary sedimentary deposit. In order to clarify the history of high-magnitude earthquake, study using geomorphic indicator of vertical and/or horizontal displacement must also be carried out simultaneously. The present study conducted a topographical analysis using aerial photographs and aviation LiDAR-based DEM data in the central area of Yangsan Fault in the southeastern part of Korea. Field survey was also conducted to estimate the timing of the depositions produced by Quaternary faulting. The topographic analysis indicated that the study area has experienced a horizontal displacement with a range of 250-500 m. The OSL dating result indicated that the fault-inducing sediment was deposited about 80 ka. The study demonstrates that it is potential to obtain information such as sense and displacement of fault through topographic analysis, and the average displacement rate from numerical dating results, and a long-term evolution of tectonically induced landforms which clearly exist along Yangsan Fault system.
Deriving paleoseismological fault parameters of active faults is essential for earthquake disaster provision. The purpose of this study is to document the fault characteristics of Wonwonsa fault, which was recently reported in the 2nd fault outcrop on the eastern side of the Ulsan fault, and to provide fault parameters, such as the timing of activity. The fault slipped along the boundary between biotite granite and andesitic dike, and cuts Quaternary fluvial deposits, which deposited above basement rock. Vertical separation of about 42 cm is recognized based on the unconformity surface between the bedrock and the Quaternary fluvial deposit, but net-slip using fault geometry is calculated to be 46 cm. The burial ages of the fluvial deposits are ca. 6 ka (hanging wall part) and ca. 3 ka (footwall part) based on the optically stimulated luminescence (OSL) dating. The boulder's exposure age, which is located on the terrace surface using ¹⁰Be, was ca. 9,000 years. Although there is a difference between two dating methods, it is interpreted that this fault slipped at least once during the Holocene. Because we cannot rule out the possibility of the inheritance although it is minimal considering the steep mountain stream, the ¹⁰Be exposure had better be taken as a maximum. So, the OSL age may be near to the real age. This fault seems to have been activated after 3,600 years ago, implying that an earthquake with the surface rupture occurred during the Holocene. This result indicates the youngest active faulting age among the Quaternary faults reported along the Ulsan fault.
The NNE-SSW-striking Yangsan Fault, which can be traced through Busan, Yangsan, Gyeongju, Pohang, and Yeongdeok, is a mature fault with a multi-stage deformation history that has been active since at least the Late Cretaceous. The fault is also associated with Quaternary deformation features at the surface. In the last several decades, academic and socioeconomic interest in the fault has motivated various studies focused on its movement history, wider tectonic implications, and the possibility of future earthquakes. A better understanding of the mechanical, seismological, and hydrological behavior of the fault requires a comprehensive description of the fault zone internal structure, fault rock materials, and geometric characteristics. Thus, we synthesize previously reported and newly acquired data regarding the fault core, and then use a standardized template to present detailed descriptions of the fault core where it is exposed at four recently identified localities. Based on the available data, we arrive at conclusions regarding the (1) distribution and orientation, (2) internal structure, (3) kinematics, and (4) Quaternary characteristics of the Yangsan Fault core. Understanding of behavioral patterns along each fault segment and/or within the fault core-damage zone could be improved by developing a fault data management system, and by further research on structural segmentation and paleoseismology within intraplate regions.
In this study, we conduct structural characterization and Anisotropy of Magnetic Susceptibility
(AMS) analysis of the Quaternary fault, and the OSL/IRSL age dating for the Quaternary sediments to identify
the characteristics and timing of the Quaternary faulting events for which the fault is found through trench survey
at Dangu-ri, Gyeongju-si, SE Korea. Based on these, the study of paloeseismology, including the calculation of
earthquake magnitude, was conducted to identify the characteristics and timing of the Quaternary faulting event.
The trench site is located 1 km north from the Byeokgye site, where Quaternary slip has been reported by previous
studies. We selected trench site location based on results of the geomorphic analysis through LiDAR image and
resistivity survey. In the analysis, N-S-trending geomorphic relief estimated to fault line scarp, and low resistivity
anomaly were found. In the trench section, seven fault surfaces with dextral reverse slip sense observed, and the
cross-cutting relationship between the Quaternary sediments and fault surfaces indicates that at least three surface
faulting events occurred during the Quaternary. The Quaternary sediments are subdivided into 9 unit layers, unit A~I from the top, based on grain size, type, content, roundness of gravel, degree of sorting, and color. In particular, soft-sediment deformation structure (SSDS) interpreted as a result of liquefaction and fluidization due to seismic
ground motion are clearly observed in the unconsolidated layers (unit E, G) composed of fine-medium sand. AMS
analysis reveals that the fault core 1 in contact with the Quaternary layer shows magnetic fabrics indicating a dextral
reverse slip sense, while fault core 2 shows magnetic fabrics indicating a dextral strike-slip sense. The paleostress
reconstruction from slickenline on fault surfaces and the magnetic fabrics of fault gouge correspond with the
maximum horizontal stress (σHmax) in the ENE-WSW directions, which is consistent with the current stress field
on the Korean Peninsula. The slickenline and vertical separation of the most recent event indicate 0.64~1.81 m
of a true displacement. The estimated moment earthquake magnitude (MW) using the empirical equation of
maximum displacement–moment earthquake magnitude is 6.7~7.0. To determine the timing of the faulting event,
OSL/IRSL dating was carried out on the Quaternary deposits cut by the fault. Based on the OSL ages of unit B,
it is concluded that the timing of the most recent event is younger than 3.2±0.2 ka.
In general, earthquakes larger than magnitude 6 involve surface ruptures. Therefore, geomorphic and stratigraphic offsets recorded in the surface provide clues to interpret history of moderate to major paleo-earthquakes. In this study, we carried out topographical analysis of the Quaternary fluvial terraces along the Yangsan Fault, one of the major geological structures in Korea. Our investigation focused on 4 sites: Yongjang-ri, Gyodong-ri, Sangcheon-ri and Chosan-ri, where river system flows across the central-southern part of the Yangsan Fault. We used aerial photographs (1968) and airborne LiDAR-based DEM data (2017) for precise topographical analysis. For each area, terrace tread and riser were used as geomorphic offset indicators. The horizontal offsets were statistically measured using the root-mean-square-error method for the sections where the straightness of the geomorphic offset indicator was secured. The vertical offsets were measured by comparing topographic progiles of fluvial terraces on both sides of the fault. As a result, various horizontal offsets over at least about 10 m (Yongjang-ri site: 19.09 ± 2.58 m, 21.92 ± 2.69 m, Gyodong-ri site: 195.43 ± 23.54 m, Sangcheon-ri site: 37.53 ± 6.56 m, Chosan-ri site: 9.68 ± 2.73 m, 34.44 ± 4.40 m) were identified. All these offsets indicate a right-lateral sense of slip. On the other hand, the vertical offsets were less than about 10% of the horizontal offsets at all sites. This implies that dextral deformation were dominated during the surface ruptures associated with paleo-earthquakes along the central-southern Yangsan Fault. Our results show that it is possible to acquire information on paleo earthquakes, such as fault trace distribution, slip sense, offset, through accurate topographical analysis. These results can be used to estimate long-term slip-rate along with depositional age of fluvial sediments.
Radiocarbon (C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.
A highly stressed area where eventual ruptures have often been observed to nucleate is characterized by low b values of earthquake frequency‐size distribution. Crustal deformation due to the occurrence of large earthquakes causes stress perturbation in nearby regions, so an investigation into spatiotemporal b values can play a crucial role in the distribution of postseismic hazards after the 2016 Kumamoto earthquake sequence along the Futagawa‐Hinagu fault zone, which culminated in the magnitude 7.3 mainshock. Together with an analysis of aftershock decay p value that can be used to infer stressing history, a highly stressed area with a characteristic dimension of 10 km at the southern end of the causative faults was found. Our observation is explained by postseismic deformation due to an afterslip on the causative faults and viscoelastic relaxation model. Similar to the Kumamoto mainshock rupture, which started at a low‐b‐value area, the observed highly stressed area shows a high likelihood of future earthquake ruptures.
In 1905, 14 days apart, two M ~ 8 continental strike-slip earthquakes: the Tsetserleg and Bulnay earthquakes, occurred on the Bulnay fault system, in Mongolia. Together they ruptured four individual faults, with a total length of ~ 676 km. Using sub-metric optical satellite images ‘Pleiades’ with ground resolution of 0.5 m, complemented by field observation, we mapped in detail the entire surface rupture associated with this earthquake sequence. Surface rupture along the main Bulnay fault is ~ 388 km in length, striking nearly E-W. The rupture is formed by a series of fault segments that are 29 km-long on average, separated by geometric discontinuities. Although there is a difference of about 2 m in the average slip between the western and eastern parts of the Bulnay rupture, along-fault slip variations are overall limited, resulting in a smooth slip distribution, except for local slip deficit at segment boundaries. We show that damage, including short branches and secondary faulting, associated with the rupture propagation, occurred significantly more often along the western part of the Bulnay rupture, while the eastern part of the rupture appears more localized and, thus possibly structurally simpler. Eventually, the difference of slip between the western and eastern parts of the rupture is attributed to this difference of rupture localization, associated at first order with a lateral change in the local geology. Damage associated to rupture branching appears to be located asymmetrically along the extensional side of the strike-slip rupture and shows a strong dependence on structural geologic inheritance.
To improve the empirical basis for estimating the likely length of future earthquake ruptures on mapped active faults, we measure map-scale complexities including fault bends, discontinuous rupture, overlaps, and fault-to-fault rupture from 67 historical ruptures and analyze the measurements for statistical relationships relevant to seismic hazard analysis. We observe that angles of bends at the ends of surface ruptures on strike-slip faults are systematically larger than interior bends (IBs), whereas corresponding interior and ending populations are similar for dip-slip events. The probability of a strike-slip rupture passing a bend decreases systematically with increasing bend angle roughly as PR = 3:1 − 0:083 × A, in which PR is the passing ratio and A is the bend angle, with values ranging between 5° and 30°. The regression shows the likelihood of a strike-slip rupture propagating through a bend of 25° is about 50%. The maximum IB angles through which ruptures propagate, and the net orientation differences of fault segments at the end of ruptures, may be explained to first order by changes in frictional resistance due to changes in fault strike in a locally constant orientation of regional stress. The average curvature of a fault rupture is defined by dividing the sum of absolute values of bends in the rupture by rupture length. Median and 95% curvatures of strike-slip ruptures are 0:5°=km and 1:5°=km, respectively; corresponding values for dip-slip ruptures are 1:6°=km and 5:6°=km, respectively. We find that most fault-to-fault rupture connections jump to a fault of like mechanism, such as strike slip to strike slip. Only two strike-slip ruptures out of a total of 42 jump to reverse structures and continue for a significant distance. Results here provide empirical data to support study of the dynamics of fault rupture and to improve rupture-length estimates for use in seismic hazard assessment.
Significance
U-Th dating of coseismic calcite veins in the Loma Blanca fault, New Mexico, quantifies an earthquake history spanning more than 400,000 years, the longest paleoseismic record ever documented for any fault. Data show that earthquakes on the Loma Blanca fault generally occurred at regular intervals, rather than aperiodically as previously hypothesized for faults in similar tectonic environments. However, periodic earthquake slip was interrupted by a relatively brief interval of increased earthquake frequency. Stable isotope data indicative of rapid CO 2 degassing suggest this interval was associated with elevated pore-fluid pressure. The Loma Blanca fault thus provides a record of “naturally induced” seismicity during which fault-valve behavior is inferred to have reduced earthquake recurrence intervals.
In order to characterize the Neotectonic crustal deformation and current stress field in and
around the Korean Peninsula and to interpret their tectonic implications, this paper synthetically analyzes
the previous Quaternary fault and focal mechanism solution data and recent geotechnical in-situ stress data
and examines the characteristics of crustal deformations and tectonic settings in and around East Asia after
the Miocene. Most of the Quaternary fault outcrops in SE Korea occur along major inherited fault zones
and show a NS-striking top-to-the-west thrust geometry, indicating that the faults were produced by local
reactivation of appropriately oriented preexisting weaknesses under EW-trending pure compressional stress
field. The focal mechanism solutions in and around the Korean Peninsula disclose that strike-slip faulting
containing some reverse-slip component and reverse-slip faulting are significantly dominant on land and in
sea area, respectively. The P-axes are horizontally clustered in ENE-WSW direction, whereas the T-axes
are girdle-distributed in NNW direction. The geotechnical in-situ stress data in South Korea also indicate the ENE-trending maximum horizontal stress. The current crustal deformation in the Korean Peninsula is
thus characterized by crustal contraction under regional ENE-WSW or E-W compression stress field.
Based on the regional stress trajectories in and around East Asia, the current stress regime is interpreted
to have resulted from the cooperation of westward shallow subduction of the Pacific Plate and collision
of Indian and Eurasian continents, whereas the Philippine Sea plate have not a decisive effect on the
stress-regime in the Korean Peninsula due to its high-angle subduction that resulted in dominant crust
extension of the back-arc region. It is also interpreted that the Neotectonic crustal deformation and presentday
tectonic setting of East Asia commenced with the change of the Pacific Plate motion during 5~3.2Ma.
Large earthquakes within stable continental regions (SCR) show that significant amounts of elastic strain can be released on geological structures far from plate boundary faults, where the vast majority of the Earth's seismic activity takes place. SCR earthquakes show spatial and temporal patterns that differ from those at plate boundaries and occur in regions where tectonic loading rates are negligible. However, in the absence of a more appropriate model, they are traditionally viewed as analogous to their plate boundary counterparts, occuring when the accrual of tectonic stress localized at long-lived active faults reaches failure threshold. Here we argue that SCR earthquakes are better explained by transient perturbations of local stress or fault strength that release elastic energy from a pre-stressed lithosphere. As a result, SCR earthquakes can occur in regions with no previous seismicity and no surface evidence for strain accumulation. They need not repeat, since the tectonic loading rate is close to zero. Therefore, concepts of recurrence time or fault slip rate do not apply. As a consequence, seismic hazard in SCRs is likely more spatially distributed than indicated by paleoearthquakes, current seismicity, or geodetic strain rates.
We analyze a set of 76 mapped surface ruptures for relationships between geometrical discontinuities in fault traces and earthquake rupture extent. The combined set includes 46 strike-slip, 16 normal, and 14 reverse mechanism events. The survey shows ∼90% of ruptures have at least one end at a mappable discontinuity, either a fault end or a step of 1 km or greater. Dip-slip ruptures cross larger steps than strike-slip earthquakes, with maxima of ∼12 versus ∼5 km, respectively. Large steps inside strike-slip ruptures are rare; only 8% (5 of 62) are ≥4 km. A geometric probability distribution model of steps as “challenges” to rupture propagation predicts that steps of 1 km or greater will be effective in stopping rupture about 46% of the time. The rate is similar for dip-slip earthquakes, but, within this set, steps are relatively more effective in stopping reverse ruptures and less effective in stopping normal ruptures. By comparing steps at rupture terminations to the set of steps broken in rupture, we can estimate the importance of step size for stopping rupture. We define the passing ratio for a given step size as the fraction of steps broken divided by the corresponding fraction that stop rupture. A linear model for steps from 1 to 6 km in strike-slip ruptures leads to the passing ratio 1:89–0:31× step width. Steps of ∼3 km are equally likely to be broken or to terminate rupture, and steps ≥6 km should almost always stop rupture. A similar comparison suggests that extensional steps are somewhat more effective than compressional steps in stopping ruptures. We also compiled the incidence of gaps of 1 km and longer in surface ruptures. Gaps occur in ∼43% of ruptures and occur more frequently in dip-slip than strike-slip ruptures.
The question of whether structural segment boundaries along multi-segment normal faults such as the Wasatch fault zone (WFZ) act as persistent barriers to rupture is critical to seismic-hazard analyses. We synthesized late Holocene paleoseismic data from 20 trench sites along the central WFZ to evaluate earthquake rupture length and fault segmentation. For the youngest (<3-ka) and best-constrained earthquakes, differences in earthquake timing across prominent primary segment boundaries, especially for the most-recent earthquakes on the north-central WFZ, are consistent with segment-controlled ruptures. However, broadly constrained earthquake times, dissimilar event times along the segments, the presence of smaller-scale (subsegment) boundaries, and areas of complex faulting permit partial- and multi-segment (e.g., spillover) ruptures that are shorter (~20–40 km) or longer (~60–100 km) than the primary segment lengths (35–59 km). We report a segmented WFZ model that includes 24 earthquakes since ~7 ka and yields mean estimates of recurrence (1.1–1.3 kyr) and vertical slip rate (1.3–2.0 mm/yr) for the segments. However, additional rupture scenarios that include segment-boundary spatial uncertainties, floating earthquakes, and multi-segment ruptures are necessary to fully address epistemic uncertainties in rupture length. We compare the central WFZ to paleoseismic and historical surface ruptures in the Basin and Range Province and central Italian Apennines and conclude that displacement profiles have limited value for assessing the persistence of segment boundaries, but can aid in interpreting prehistoric spillover ruptures. Our comparison also suggests that the probabilities of shorter and longer ruptures on the WFZ need to be investigated.
Structure from Motion (SfM) generates high-resolution topography and coregistered texture (color) from an unstructured set of overlapping photographs taken from vary- ing viewpoints, overcoming many of the cost, time, and logistical limitations of Light Detection and Ranging (LiDAR) and other topographic surveying methods. This paper provides the first investigation of SfM as a tool for mapping fault zone topography in areas of sparse or low-lying vegetation. First, we present a simple, affordable SfM workflow, based on an unmanned helium balloon or motorized glider, an inexpensive camera, and semiautomated software. Second, we illustrate the system at two sites on southern California faults covered by exist- ing airborne or terrestrial LiDAR, enabling a comparative assessment of SfM topography resolution and precision. At the first site, an ~0.1 km2 alluvial fan on the San Andreas fault, a colored point cloud of density mostly >700 points/m2 and a 3 cm digital elevation model (DEM) and orthophoto were produced from 233 photos collected ~50 m above ground level. When a few global positioning system ground control points are incorporated, closest point vertical distances to the much sparser (~4 points/m2) airborne LiDAR point cloud are mostly <3 cm. The second site spans an ~1 km section of the 1992 Landers earthquake scarp. A colored point cloud of density mostly >530 points/m2 and a 2 cm DEM and orthophoto were produced from 450 photos taken from ~60 m above ground level. Closest point vertical distances to exist- ing terrestrial LiDAR data of comparable density are mostly <6 cm. Each SfM survey took ~2 h to complete and several hours to generate the scene topography and texture. SfM greatly facilitates the imaging of subtle geomorphic offsets related to past earth- quakes as well as rapid response mapping or long-term monitoring of faulted landscapes.
In order to assess the fault behavior by the geometric analysis of fault slip, the study area between Yangsan city and Shinkwang-myon, Pohang city along the strike of the Yangsan fault is divided into 5 domains( domains) based on the strike change of main fault, the type of fault termination, the cyclic variation of fault zone width, deformation pattern of fault rocks and angular deviation of secondary shears. And, we would apply the relationship between the mode of fault sliding and the resultant deformation texture obtained from previous several experimental studies of simulated fault gouge to the study of the Yangsan fault. To understand sliding behavior of the fault we measured the data of fault attitude and fault slip, and analyzed relationships between the main fault and secondary Riedel shear along the Yangsan fault. The sliding behavioral patterns in each section were analyzed as followings; the straight sections of A, D and E domains were analyzed as the creeping section of stably sliding. In contrast, the curved section of B domain was analyzed as the locked section of stick-slip movement.
The propagation of the rupture of the Mw7.9 Denali fault
earthquake from the central Denali fault onto the Totschunda fault has
provided a basis for dynamic models of fault branching in which the
angle of the regional or local prestress relative to the orientation of
the main fault and branch plays a principal role in determining which
fault branch is taken. GeoEarthScope LiDAR and paleoseismic data allow
us to map the structure of the Denali-Totschunda fault intersection and
evaluate controls of fault branching from a geological perspective.
LiDAR data reveal the Denali-Totschunda fault intersection is
structurally simple with the two faults directly connected. At the
branch point, 227.2 km east of the 2002 epicenter, the 2002 rupture
diverges southeast to become the Totschunda fault. We use paleoseismic
data to propose that differences in the accumulated strain on each fault
segment, which express differences in the elapsed time since the most
recent event, was one important control of the branching direction. We
suggest that data on event history, slip rate, paleo offsets, fault
geometry and structure, and connectivity, especially on high slip
rate-short recurrence interval faults, can be used to assess the
likelihood of branching and its direction. Analysis of the
Denali-Totschunda fault intersection has implications for evaluating the
potential for a rupture to propagate across other types of fault
intersections and for characterizing sources of future large
earthquakes.
The Japanese Islands were separated from the Eurasian plate due to continental rifting during the Oligocene to mid-Miocene, which caused the opening of the East Sea (Sea of Japan). Such tectonic evolution in the East Sea is important for understanding the evolution of back-arc regions with active convergent margins. To understand the evolution of the paleo-rifted back-arc region, we investigate seismicity, crustal seismic anisotropy, focal mechanism solutions and ambient stress field around the Korean Peninsula. The Korean Peninsula displays diffused seismicity with small and moderate earthquakes. Shallow earthquakes rarely occur in the central East Sea. The crustal fast shear-wave polarization directions in the Korean Peninsula are observed to vary in azimuth between 40° and 90°. The focal mechanism solutions are calculated by long period waveform inversions. The ambient stress field is calculated from the focal mechanism solutions. The compressional stress field in the Korean Peninsula is observed to be in ENE-WSW, which is consistent with the fast shear-wave polarization directions. The compressional stress directions in the East Sea progressively change from ENE to SE with increasing longitude. The rapid change of compressional directions in the central East Sea prohibits accumulation of stress, causing rare shallow seismicity. High seismicity of reverse faulting events is observed at the fringes of the East Sea, in particular, around the east coast of the Korean Peninsula and the west coast of Japanese Islands, which correspond to paleo-rifted margins where compressional stresses are accumulated. The compressional stress field and active thrustal events suggest reverse activation of paleo-normal faults that were developed during the opening of the East Sea.
Recent revisions to the geomagnetic time scale indicate that global plate motion model NUVEL-1 should be modified for comparison with other rates of motion including those estimated from space geodetic measurements. The optimal recalibration, which is a compromise among slightly different calibrations appropriate for slow, medium, and fast rates of seafloor spreading, is to multiply NUVEL-1 angular velocities by a constant, α, of 0.9562. We refer to this simply recalibrated plate motion model as NUVEL-1A, and give correspondingly revised tables of angular velocities and uncertainties. Published work indicates that space geodetic rates are slower on average than those calculated from NUVEL-1 by 6±1%. This average discrepancy is reduced to less than 2% when space geodetic rates are instead compared with NUVEL-1A.
We re-evaluate the stress state across the Korean Peninsula by analyzing the stress tensor inversion results, strike–rake relationships, and faulting-type indicator–magnitude relationships of published earthquake focal mechanism data. Previous studies have shown that the stress state is characterized by a strike-slip faulting stress regime with ENE–WSW maximum compression, NNW–SSE minimum compression, and vertical intermediate stress. The differential stress magnitudes are estimated to be on the order of 100 MPa under the assumption of the strong-fault earthquake with a high Coulomb friction coefficient of 0.85. Conversely, other studies have shown that the weak-fault model is applicable across the Japanese Archipelago, where the differential stress level is only a few tens of MPa. We, therefore, review previous studies, evaluate the focal mechanism data across the Korean Peninsula, and infer the absolute stress level. Our comparison of the geological and tectonic histories of the Korean Peninsula and Japanese Archipelago suggests that the NNW–SSE tensional stresses in the Korean Peninsula are caused by slab rollback of the Philippine Sea Plate at around 5 Ma, west of Kyushu Island, southwestern Japan. We also find that the earthquake faulting types across the Korean Peninsula are closely related to their fault strikes, confirming a spatially uniform distribution of the strike-slip stress regime with nearly equal tensional and compressional tectonic stresses. However, this situation leads to the apparent paradox of strike-dependent fault strength. We eliminate this paradox by proposing a model whereby reverse- and normal-faulting events are caused by local stress heterogeneities around the edges of strike-slip faults under a uniform low-strength strike-slip stress state for all faulting types. We also highlight the usefulness of strike–rake and faulting-type indicator–magnitude diagrams in considering stress states.
The NNE–SSW-striking Yangsan Fault in southeastern Korea has been regarded as one of the most prominent seismogenic structures in the Korean Peninsula on the basis of instrumental and historical seismicity, and paleoseismic records along the fault zone. Its seismic behavior is, however, still uncertain due to long recurrence intervals of strong earthquakes and insufficient historical and geologic records. We conducted a detailed paleoseismic investigation, including an 8 m-deep excavation, in order to understand recent earthquake-faulting events along the Southern Yangsan Fault. Our observations indicate that at least two and possibly four dominantly strike-slip surface-faulting events have occurred along a subsidiary fault in the eastern boundary of the fault valley since the late Pleistocene. Stratigraphic features and OSL ages of the stratigraphy exposed in the trenches indicate that the most recent rupture(s) occurred after 29.2 ± 1.4 ka and the timing of earlier ruptures are constrained to between 70.0 ± 3.7 ka and 29.2 ± 1.4 ka. Using a contact of the oldest dated unit that has at least 7.5 m of vertical separation, we calculate a minimum vertical slip rate during the late Quaternary of 0.11 mm/yr. The minimum horizontal slip rate is presumed to be two times that of the vertical slip rate based on striations observed on clasts next to the fault core. We also propose that the late Quaternary earthquake-faulting kinematics along the Yangsan Fault, expressed as contractional dextral slip with east-side-up geometry, is strongly dependent on a pre-existing fault zone architecture with a strike of N10–20°E that dips to the east, and the direction of neotectonic maximum horizontal stress (ENE–WSW to E–W).
We describe the structural characteristics and tectonic evolution of the NNE–SSW-striking Yangsan Fault, Korea.
The surface trace of the fault extends for over 170 km on land, displaying ∼20–30 km of dextral offset. It
transects mainly Mesozoic–Cenozoic sedimentary and igneous rocks. Our field observations suggest that the style
of deformation within the fault core is controlled by the ductility of the protoliths. Where sedimentary rocks
have been faulted, fault cores tend to be relatively wide, having hosted multiple events. Internal fault core
structures are characterized by networks of multiple anastomosing strands of fault gouge and breccia, which
enclose lenses of fractured protolith. However, where igneous (crystalline) rocks have been faulted, slip tends to
be much more localized within narrow fault cores comprising cataclastic rocks. Combining our new data with
previous research results, we infer that four main movement phases have occurred on the fault: (1) Late
Cretaceous sinistral slip faulting with a component of extensional deformation, under a regime of NW–SE
compression; (2) late Paleogene intense dextral slip faulting, which occurred under a regional NE–SW compressional
regime; (3) middle Miocene weak sinistral slip faulting, during NNW–SSE compression; and (4) local
Quaternary dextral slip faulting with a reverse component, under ENE–WSW to E-W compression. The most
intense phase of deformation appears to have occurred during the late Paleogene due to a major reorganization
of plate motion in and around the NW Pacific area. Our findings highlight that the fault has been a preferred
zone of weakness for episodic reactivations having significant implications for the changing tectonic environments
of East Asia from Late Cretaceous to Quaternary, and that its segments underwent distinguishable deformations
at each phase probably due to their geometrical features and fault rock properties.
Restraining double-bends along strike-slip faults inhibit or permit throughgoing ruptures depending on bend angle, length, and prior rupture history. Modeling predicts that for mature strike-slip faults in a regional stress regime characterized by simple shear, a restraining bend of >18° and >4 km length impedes propagating rupture. Indeed, natural evidence shows that the most recent rupture(s) of the Xorkoli section (90°-93°E) of the eastern Altyn Tagh fault (ATF) ended at large restraining bends. However, when multiple seismic cycles are considered in numerical dynamic rupture modeling, heterogeneous residual stresses enable some ruptures to propagate further, modulating whether the bends persistently serve as barriers. These models remain to be tested using observations of the cumulative effects of multiple earthquake ruptures. Here we investigate whether a large restraining double-bend on the ATF serves consistently as a barrier to rupture by measuring long-term slip rates around the terminus of its most recent surface rupture at the Aksay bend. Our results show a W-E decline in slip as the SATF enters the bend, as would be predicted from repeated rupture terminations there. Prior work demonstrated no Holocene slip on the central, most misoriented portion of the bend, while 19-79 m offsets suggest that multiple ruptures have occurred on the west side of the bend during the Holocene. Thus we conclude the gradient in the SATF's slip rate results from the repeated termination of earthquake ruptures there. However, a finite slip rate east of the bend represents the transmission of some slip, suggesting that a small fraction of ruptures may fully traverse or jump the double-bend. This agreement between natural observations of slip accumulation and multi-cycle models of fault rupture enables us to translate observed slip rates into insight about the dynamic rupture process of individual earthquakes as they encounter geometric complexities along faults.
Two earthquakes (Mw 5.1 and 5.5) ruptured branches of the Yangsan fault system in Gyeongju, South Korea, on 12 September 2016. Aftershocks, including a notable Mw 4.3 earthquake on 19 September 2016, were clustered around the epicenters of the first two events. The Mw 5.5 earthquake is considered the largest earthquake in South Korea to have occurred during the modern instrumental recording period since 1978. Although there is no apparent surface rupture, these earthquakes have greatly shaken South Korea, leaving both physical and societal impacts. In this study, we determine the source mechanism and rupture directivity using regional seismic-waveform data to understand the earthquake source processes. Based on the waveform inversion, we report that the mainshock (Mw 5.5 event) is a strike-slip event with two nodal planes 117°/84°/21° and 24°/69°/173° at a depth of 14 km. The inversion also demonstrates that the mainshock event ruptured against the 24° seismogenic fault plane to the south-southwest, with a rupture length of ∼4:3 km. This rupture propagation direction agrees well with the spatial distribution of relocated aftershock events and reported seismic intensities.
Bayesian models have proved very powerful in analyzing large datasets of radiocarbon ( ¹⁴ C) measurements from specific sites and in regional cultural or political models. These models require the prior for the underlying processes that are being described to be defined, including the distribution of underlying events. Chronological information is also incorporated into Bayesian models used in DNA research, with the use of Skyline plots to show demographic trends. Despite these advances, there remain difficulties in assessing whether data conform to the assumed underlying models, and in dealing with the type of artifacts seen in Sum plots. In addition, existing methods are not applicable for situations where it is not possible to quantify the underlying process, or where sample selection is thought to have filtered the data in a way that masks the original event distribution. In this paper three different approaches are compared: “Sum” distributions, postulated undated events, and kernel density approaches. Their implementation in the OxCal program is described and their suitability for visualizing the results from chronological and geographic analyses considered for cases with and without useful prior information. The conclusion is that kernel density analysis is a powerful method that could be much more widely applied in a wide range of dating applications.
If radiocarbon measurements are to be used at all for chronological purposes, we have to use statistical methods for calibration. The most widely used method of calibration can be seen as a simple application of Bayesian statistics, which uses both the information from the new measurement and information from the 14 C calibration curve. In most dating applications, however, we have larger numbers of 14 C measurements and we wish to relate those to events in the past. Bayesian statistics provides a coherent framework in which such analysis can be performed and is becoming a core element in many 14 C dating projects. This article gives an overview of the main model components used in chronological analysis, their mathematical formulation, and examples of how such analyses can be performed using the latest version of the OxCal software (v4). Many such models can be put together, in a modular fashion, from simple elements, with defined constraints and groupings. In other cases, the commonly used “uniform phase” models might not be appropriate, and ramped, exponential, or normal distributions of events might be more useful. When considering analyses of these kinds, it is useful to be able run simulations on synthetic data. Methods for performing such tests are discussed here along with other methods of diagnosing possible problems with statistical models of this kind.
Many distinct lineaments have been recognized by Landsat images in Korean Peninsula. The Yangsan fault system situated in the southeastern part of Korea is especially linear, continuously traceable for a long distance (about 200km), and particularly remarkable among these lineaments. The topographic expression of the Yangsan fault system is derived from the straightly stretching fault valley with wide shattered zones in the direction of NNE-SSW. This fault system extends for about 200km from the mouth of the Nagdong River west of Busan in the south to Yeondong in the north, and geologically separates Korean Peninsula from the Japan Sea. The amount of horizontal displacement may reach 30km. It is recognized as one of the most important faults in Korean Peninsula.From the interpretations of aerial photographs, and field surveys along the central part of the Yangsan fault system, the main results are summarized as follows:1. The Yangsan fault system has repeatedly moved in the late Quaternary. The lower to higher river terrace surfaces on this system show cumulative vertical offsets.2. The vertical component is upthrown on the east side from considering the terrace offset and the distribution of the mountainous lands. This vertical movement is reverse to the topographical situation on the meso-scale.3. The fault trace is extremely straight. The fault plane is almost vertical. The shatteredzone exceeds tens of meters in width with a remarkable fault gouge.4. The longer axis of flat clasts within the gravel observed in excavated the exploratory trench showed the re-arrangement along the fault. The predominantly right-lateral movements were recognized as the elongation of clayey parts and breccias in the fault gouge.5. From these characteristics, the Yangsan fault was clarified to be active with predominantly right-lateral movement. Estimated ages of terraces and its deposits give average rates of vertical and right slip on the Yangsan fault system at about 0.02-0.03mm/y, and at least 0.05-0.1mm/y, respectively.6. The fault topography is not found on the lower and lowest terraces. As the surface of the terrace has widely been cultivated as paddy fields for long historical time, lower fault scarplets less than a few meters high might have been modified or destroyed by the human actions. Therefore, we cannot mention the existence of the younger movement on the lower and lowest terraces.
Plate-tectonic theory explains earthquakes at plate boundaries but not those in continental interiors, where large earthquakes often occur in unexpected places. We illustrate this difference using a 2000-year record from North China, which shows migration of large earthquakes between fault systems spread over a large region such that no large earthquakes rupture the same fault segment twice. However, the spatial migration of these earthquakes is not entirely random, because the seismic energy releases between fault systems are complementary, indicating that these systems are mechanically coupled. We propose a simple conceptual model for intracontinental earthquakes, in which slow tectonic loading in midcontinents is accommodated collectively by a complex system of interacting faults, each of which can be active for a short period after long dormancy. The resulting large earthquakes are episodic and spatially migrating, in contrast to the more regular spatiotemporal patterns of interplate earthquakes.
Tectonic geomorphology is the study of the interplay between tectonic and surface processes that shape the landscape in regions of active deformation and at time scales ranging from days to millions of years. Over the past decade, recent advances in the quantification of both rates and the physical basis of tectonic and surface processes have underpinned an explosion of new research in the field of tectonic geomorphology. Modern tectonic geomorphology is an exceptionally integrative field that utilizes techniques and data derived from studies of geomorphology, seismology, geochronology, structure, geodesy, stratigraphy, meteorology and Quaternary science. While integrating new insights and highlighting controversies from the ten years of research since the 1st edition, this 2nd edition of Tectonic Geomorphology reviews the fundamentals of the subject, including the nature of faulting and folding, the creation and use of geomorphic markers for tracing deformation, chronological techniques that are used to date events and quantify rates, geodetic techniques for defining recent deformation, and paleoseismologic approaches to calibrate past deformation. Overall, this book focuses on the current understanding of the dynamic interplay between surface processes and active tectonics. As it ranges from the timescales of individual earthquakes to the growth and decay of mountain belts, this book provides a timely synthesis of modern research for upper-level undergraduate and graduate earth science students and for practicing geologists. Additional resources for this book can be found at: http://www.wileycom/go/burbank/geomorphology.
The Ulsan fault system situated in the southeastern part of Korean Peninsula marks the western range front of the Mt. Tohan (745.1 m) to the Sandae Peak (629.1 m) with the remarkable Ulsan fault scarp. This system extends with a N-S trend for a distance of more than 40 km. The NNE-SSW trending 200 km-long Yangsan fault and this Ulsan fault are recognized as the most important active faults on the Korean Peninsula.From interpretations of aerial photographs and detailed topographic maps, and field surveys along the central part of the Ulsan fault system, the major results are summarized as follows : 1. The Ulsan fault system extends in the direction of NNW-SSW to N-S and the trace is slightly sinuous (Fig. 1). The middle to higher river terrace surfaces on this system are displaced up thrown to the east side (Fig. 2). The streams and the ridges across this fault trace are not accompanied by systematic lateral displacement. The Ulsan fault system is a typical example of reverse faulting, considered from the behavior of fault outcrops (Figs. 3, 4, 5).2. The vertical displacements are about 5 m on the middle terrace, and about 15 m on the higher terrace from measurements of topographical cross-section across the fault scarplets. The cumulative vertical displacement is recognized during and after the formation of the terraces. The sense of vertical displacement of fluvial terraces coincides with the magnificent fault scarp. This mode of mountain-building movement has continued repeatedly at least since the middle of Quaternary.3. The terrace surfaces originated from the dissected fan surfaces in the late Quaternary. From geomorphic correlation, facies analysis, development of soil covering the terraces, and results of dating of humic material, etc., the ages of terrace surfaces are roughly estimated and the average slip rates of vertical displacement are measured to be about 0.1-0.08 mm/y.4. Fault outcrops are clearly observed along the Ulsan fault in this area. The shattered granite rocks thrust over the gravel deposits composing the middle terrace at the north bank of Sagok Pond, Malbang-ri, Wedong-eop, Kyeongju-gun, Kyeongsang-bukdo (Fig. 3-A, B). The fault strikes in the direction of N-S and dips eastward at an angle of 20-30°E. The shattered granite is in contact with the slightly weathered and deformed gravel at the northeast of Kaegok-ri settlement (Fig. 5).5. Liquefaction phenomena are observed within the upper horizon of the middle terrace deposits at the northwest of the Sagok Pond (Fig. 6).6. As the NNW-SSE (or N-S) trending Ulsan fault system is predominantly a reverse fault and NNE-SSW trending Yangsan fault system has been dislocated with a predominant right-lateral slip in the late Quaternary, it is estimated that this area is situated under the maximum horizontal stress field in the ENE-WSW (or E-W) direction.
Large earthquakes are infrequent along a single fault, and therefore historic, well-characterized earthquakes exert a strong influence on fault behavior models. This is true of the 1857 Fort Tejon earthquake (estimated M7.7-7.9) on the southern San Andreas Fault (SSAF), but an outstanding question is whether the 330-km long rupture was typical. New paleoseismic data for 6-7 ground-rupturing earthquakes on the Big Bend of the SSAF restrict the pattern of possible ruptures on the 1857 stretch of the fault. In conjunction with existing sites, we show that over the last ~650 years, at least 75% of the surface ruptures are shorter than the 1857 earthquake, with estimated rupture lengths of 100 to <300 km. These results suggest the 1857 rupture was unusual, perhaps leading to the long open interval, and that a return to pre-1857 behavior would increase the rate of M7.3-M7.7 earthquakes.
The paleoseismological study in Korea has begun along the Yangsan fault zone (YFZ) and Ulsan fault zone (UFZ) since 1994. Some evidences related to late Quaternary movement are found at only some part of the YFZ, such as Pyonghae, Yuge, and Eonyang-Tongdosa areas. However, it is found along the most of the UFZ except the northen and southern ends of the fault. The dominant time span of faulting events along the YFZ and UFZ are quite different, and 500 ka to 200 ka and 300 ka to recent time, respectively. The dominant faulting senses of the YFZ and UFZ are right-lateral strike slip and reverse, respectively. These senses correspond well with the focal mechanism of recent occurring earthquakes along these two fault zones. If we evaluate the intensity of the activity of the YFZ from the average slip rate, which is 0.1~0.04 m/ka, it is comparable with the faults of higher C class in Japan. The slip rate of UFZ, which is 0.2~0.06 m/ka, is comparable with the faults of lower B to higher C class. Based on the relationship between maximum displacement and magnitude, the maximum earthquake magnitude is evaluated to be 6.8 and 7.0 in the YFZ and UFZ, respectively. An intensive studies are needed to clarify the problems such as segmentation of faults, return period, and geological evidences related to historical earthquakes.
Focal mechanism solutions in the central and western areas of the Korean Peninsula (36-37.8, 126-128) were obtained from the analysis of the recent seventeen earthquakes (M2.2) which occurred from January, 2005 to May, 2010. The spatial differences between the epicenters recalculated by this study and those announced by the Korea Meteorological Administration are less than , indicating a small deviation. Focal mechanism solutions were obtained from the analysis of P wave polarities, SH wave polarities and SH/P amplitude ratios. The focal mechanism solutions show dominant strike-slip faulting or oblique slip faulting with strike-slip components. The P-axes trends are mainly ENE-WSW or E-W directions. The direction of fault plane and auxillary fault plane with NNE-SSW and WNW-ESE are almost parallel to the general trends of lineaments in the study area. The results also show that focal mechanism solutions and the main axis of stress field in the Kyonggi massif and Okchon belt are almost same.
We analyzed fault plane solutions of the recent twenty-two earthquakes which occurred from 2004 to 2006 in the central part of the Korean Peninsula by using P- and S-wave polarities along with SH/P amplitude ratios. The fault plane solution shows that strike-slip fault is dominant here, especially for the events with local magnitude equal to or greater than 3.0. However, some events with local magnitude less than 3.0 show normal fault or strike-slip fault with normal components. In the case of strike-slip fault, its orientation is almost in the direction of NNE-SSW to NE-SW almost parallel to the general trend of faults, while the compressional axis of the stress field trends ENE to E-W. The result is almost consistent with the stress field in and around the Korean peninsula, as reported previously. We cannot give any appropriate explanations to the normal faulting events along the western offshore and inland areas whether it is related to the local stress changes or tectonically unidentified extensional structures. Thus, an extension of investigations is desirable to clarify the cause of such phenomena.
The time-dependent surface deformation due to thrust faulting in an elastic plate overlying a viscoelastic half space is examined and compared with geodetic measurements of earthquake-related crustal movements from Japan. The model fault is two-dimensional, and the half-space rheology is that of a Maxwell solid whose instantaneous response to applied loads is purely elastic but which subsequently flows to relieve imposed shear stresses. A characteristic feature of shallow-angle underthrusting shown by these computations is that buried slip produces predominantly land uplift, while asthenospheric relaxation due to surface faulting results principally in surface downwarping. These distinctive patterns, also reflected in the measured deformation, are particularly useful in formulating a viable model for the entire earthquake cycle. For the model constructed in this way, coseismic faulting in the upper part of the plate transfers a load to the lower lithosphere and asthenosphere, inducing postseismic slip downdip of the seismic rupture and relaxation in the asthenosphere, these transients eventually merging into the steady buried slip and smoother asthenospheric, flow that characterize the interseismic phase of the deformation cycle. Aseismic slip and asthenospheric relaxation cause the load supported by the plate-bounding fault to be gradually transferred back to the shallow locked segment of the fault, effecting the strain buildup for a subsequent earthquake, and the cycle is repeated. No explicit account of plate-driving forces is made; their net effect is presumed to be steady buried slip that persists throughout the cycle and provides the energy that drives it. Characteristic features of the observed deformation in southwest Japan, site of the 1946 Nankaido earthquake, and in the South Kanto district, where the great 1923 earthquake occurred, are matched by this model using conventional values of lithospheric thickness (60 km) and asthenospheric viscosity (1021 P, or 1020 N s/m2), although the model is not strongly tied to these exact values. Postseismic movements are adequately explained by episodic slip that occurs below the coseismic rupture, is 10-30% of the seismic slip, and acts in the same sense. Superimposed on these movements are the lesser effects of asthenospheric relaxation, which in the South Kanto district contribute significantly to the postseismic vertical level changes because post-1923 buried slip is comparatively small and involves largely strike slip movements. Interseismic subsidence in both regions is explained well by asthenospheric relaxation and less significant deformation effects due to steady aseismic slip on the lower part of the plate boundary.
There has been a lack of seismic data in the Korean Peninsula mainly because it is in a seismically stable area within the Eurasian plate (or Amurian microplate) and because a network of seismic stations has been poor until recently. Consequently, first motion studies on the peninsula showed a large uncertainty or covered only local areas. Also, a tectonic province map constructed based on pre-Cenozoic tectonic events in Korea has been used for a seismic zonation. To solve these problems, we made focal mechanism solutions for 71 earthquakes (ML = 1.9 to 5.2) occurred in and around the peninsula from 1999 to 2004 and collected by a new dense seismic network established since 1995. For this, we relocated the hypocenters and obtained fault plane solutions with errors of fault parameter less than 15° from the data set of 1,270 clear P-wave polarities and from 46 SH/P amplitude ratios. The focal mechanism solutions show that subhorizontal ENE P- and subhorizontal NNW T-axes are predominant, representing the common direction of P- and T-axes within the Amurian plate. The faulting mechanisms are mostly strike-slip faulting or strike-slip-dominant-oblique-slip faulting with a reverse-slip component, although normal-slip-dominant-oblique-slip faultings occur locally probably due to a local reorientation of stress. These results incorporated with those from the kinematic studies of the Quaternary faults imply that NNE-striking faults (dextral strike-slip or oblique-slip with a reverse-slip component) are highly likely to generate earthquakes in South Korea. The spatial distribution of the maximum horizontal stress direction and faulting types does not correlate with the preexisting tectonic province map of Korea, and a new construction of seismic zonation map is required for a better seismic evaluation.
The potential for earthquakes along the plate boundaries has been mapped with reasonable success. Our attention should now focus on the threat posed by unanticipated quakes located in the continental interiors.