focal mechanism analyses for virginia earthquakes 1978 1984 abstract focal mechanisms are presented for 11 earthquakes from the giles county virginia seismic zone and for 12 earthquakes from the central virginia seismic zone these earthquakes 0 m 4 were monitored by local networks between january 1978 and october 1984 in giles county six single event focal mechanisms and five composite event focal mechanisms were determined in central virginia eleven single event focal mechanisms and four composite event focal mechanisms were obtained a computer program focmec that systematically searches the focal sphere for valid mechanism solutions based on both p wave polarities and sv p z amplitude ratios was used to determine the focal mechanism solutions the results for the giles county seismic zone show predominately strike slip mechanisms on steeply dipping 73 16 nne right lateral motion and ese left lateral motion trending nodal planes however some 4 11 of the solutions show similar movement on nodal planes rotated 45 counterclockwise the means and standard deviations for the trends and plunges of the p axes are n46 e 24 and 13 5 20 respectively as computed from the six single event focal mechanisms and two composite event focal mechanisms focal mechanisms from central virginia exhibit much more scatter in mechanism types and nodal plane orientations than observed in giles county the p axes in central virginia are generally northeast trending for shallow earthquakes 8 km the focal mechanisms exhibit a mixture of reverse and strike slip faulting on planes that dip 62 16 the two virginia seismic zones are separated by only 200 km yet their seismogenic characteristics are very different in giles county the earthquakes occur beneath the appalachian decollement and the faulting and inferred stress orientations are uniform in central virginia however the seismicity is occurring at and above the decollement and the associated fault planes and stress orientations are quite variable

japan sea a pull apart basin abstract recent field work in the hokkaido central belt and marine geology studies along the eastern margin of japan sea in addition to previously published data lead us to propose a new model of opening of the japan sea the synthesis of both on land and offshore structural data gives new constraints about the structural evolution of the system the rhombohedral shape of the japan basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early eo oligocene rifting of a pull apart basin accommodated along two large right lateral shear zones east of korea and west of northeast japan and sakhalin it is followed during upper oligocene lower miocene by the main opening of the japan basin as a mega pull apart then a back arc spreading probably related to the subduction process induced the creation of the yamato and tsushima basins at the end of lower miocene and in middle miocene clockwise rotation of southwest japan larger than 20 or major bending of honshu mainland deduced from paleomagnetic studies is unlikely at this time since 1 or 2 my b p to present compression prevails along the eastern margin of the japan sea the generation of marginal basins as pull apart basins along intracontinental strike slip faults is a mechanism which has been proposed by other authors concerning the south china sea the question then is whether the fragmentation of the asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one

seismic evidence for reverse activation of a paleo rifting system in the east sea sea of japan abstract 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

shear wave splitting beneath southern korea and its tectonic implication abstract seismic anisotropy beneath the southern korean peninsula skp was investigated based on the scs waves of deep focused earthquakes recorded at 35 broadband stations the most striking feature of the observed splitting patterns is the preference of the fast directions nw se that are nearly parallel to the direction of the absolute plate motion in the region however the splitting patterns over the region show significant variation in splitting parameters indicating a complex anisotropic structure variations of the splitting directions and dissimilarity in the source domains of basaltic volcanisms suggest that the asthenospheric mantle flow since at least the late cenozoic cannot explain the seismic anisotropy beneath the region comparison to shear wave splitting measurements from eastern china revealed that the nw se fast direction of splitting measurements in the skp is close to that in the north china block ncb while the ne sw fast direction might be related to that in the south china block scb the shallow mantle lithosphere beneath the skp retains the fossil anisotropy amalgamated prior to the late paleozoic before the collision between the ncb and scb and the anisotropic structure was not completely realigned by the major orogenic events during the late paleozoic to mesozoic eras

seismogenic stress field beneath the tatun volcano group northern taiwan article i nfo the tatun volcano group tvg represents the main volcanic center in northern taiwan and based on various observations it is considered a potentially active volcano tvg has been monitored since 2003 by a seismic network that consists of eight stations equipped with three component short period and broadband seismometers in this study we use waveform data of high frequency earthquakes in order to investigate the stress field orientation beneath the tvg area the focal mechanisms of 35 selected events have been derived using p wave polarities and amplitude ratios assuming a double couple source these fault plane solutions and various subsamples derived from them were subsequently inverted for the best fitting stress tensor using a linear inversion method the results show stress homogeneity beneath tvg for depths larger than 3 km while the stress tensor is characterized by a subvertical 1 and a subhorizontal nw se trending 3 axis consistent with the regional stress field in northern taiwan on the other hand chihsinshan which is an area of vigorous hydrothermal activity exhibits a localized stress field with horizontal nw se trending 3 axis and ne sw trending horizontal 1 such an axes orientation is likely to be causing opening of microcracks and thus favour the ascent and circulation of fluids in the upper crust shear wave splitting measurements seem to confirm these results showing fast polarization directions along nne ne subparallel to the main fracture system in tvg shear wave anisotropy averages about 2 9 and is probably caused by an anisotropic volume of fluid saturated cracks within the upper 2 5 km of the crust

identification of a suspected quaternary fault in eastern korea proposal for a paleoseismic research procedure for the mapping of active faults in korea abstract in terms of seismic hazard in the korean peninsula the occurrence of destructive earthquakes has long been overlooked because of no serious modern instrumental seismicity and lack of historical earthquake records of large earthquakes in recent years although the importance and uses of paleoseismological studies has significantly increased in seismic hazard assessments of critical facilities there is still a huge lack of paleoseismological data to improve our knowledge for korean active tectonics and relevant seismic hazards in this study we identified and characterized a suspected quaternary fault in the eastern part of the korea peninsula based on lineament analyses field observations and trench surveys we focus on a prominent lineament around the northern part of the previously reported but not sufficiently studied maeup fault firstly a potential fault lineament is recognized by geomorphic expressions in a mountainous area from satellite and digital elevation model dem generated images then fault related geomorphic and hydrologic features such as disconnected drainages sag ponds and asymmetric water leaking out on an artificial ditch were observed along the lineament in the field studies finally the existence of the fault is confirmed by an exposure of a 2 5 m wide fault core in trench surveys the internal structures of the fault core i e slip surfaces slickenlines and s c fabrics indicate that the fault evolved via a series of slip events under a variety of tectonic settings probably leading up to the current stress regime sustained during quaternary thus our results suggest that the newly identified fault could be a quaternary fault and hence it is necessary to perform more paleoseismic investigations around the study region this case study emphasizes the importance of the careful interpretation of tectonic landforms and fault zone structures in the study of active faults and proposes a suitable research procedure for the identification of active faults in korea

source parameter determination of local earthquakes in korea using moment tensor inversion of single station data abstract the purpose of this investigation is to determine source parameters such as focal mechanism seismic moment moment magnitude and source depth from recent small earthquakes in the korcan peninsula using broadband records of three component single station it is very important and worthwhile to use a three component single station in korea because for most korean earthquakes it is not possible to read enough first motions of p wave arrivals because of the poor coverage of the seismic network and the small size m l 5 0 or less of the events furthermore the recent installation of the very broadband seismic stations in korea and use of a 3d tomography technique can enhance moment tensor inversion to determine the source parameters of small earthquakes m l 5 0 or less that occur at near regional distances 500 km the focal solution for the youngwol earthquake of 13 december 1996 is found to be a right lateral strike slip event with a ne strike and the kyongju earthquake of 25 june 1997 is found to be an oblique reverse fault with a slight component of left lateral slip in the se direction

elevations and ages of marine terraces and late quaternary rock uplift in southeastern korea 1 from a detailed gps survey of marine terraces and their former shoreline elevations along the southeastern coast of the korean peninsula we determined the pattern of late quaternary uplift in a part of the far eastern intraplate region of the eurasian plate using a paleosea level of 6 m for the 117 132 ka sea level highstand the average long term uplift rates are 0 3 m ka in the weolsung and 0 2 m ka in the daebo gori regions persistent uplift that generated marine terraces is mostly the result of local major faulting reflecting an ene wnw maximum compression stress the differences in uplift rates suggest a local triangular crustal block undergoing structural movement controlled by subregional tectonic processes notable local major faulting in an intraplate region plays an important role in raising the emergent flight of marine terraces generated by earthquakes

geomorphic disequilibrium in the eastern korean peninsula possible evidence for reactivation of a rift flank margin abstract tectonically inactive since the middle miocene the korean peninsula is generally considered a typical passive continental margin with asymmetric relief with only low levels of seismic activity recent reports of uplift rates as high as 150 350 mm ka 1 from geomorphic markers using numerical dating methods e g radiocarbon optically stimulated luminescence osl and cosmogenic nuclides however cast doubt on the tectonic quiescence of korea thus we evaluate the geomorphic status of eastern korea using denudation rates at geomorphic timescales of 103 106 years which are equivalent to the rates previously reported at local points to infer regional denudation rates we measured catchment wide denudation rates cwdr in 13 watersheds using in situ cosmogenic 10be and 14c analyses of riverine sediment samples in eastern korea these cwdr data suggest that the mean denudation rate during the past 5 14 ka centers around 74 7 25 4 mm ka 1 which is consistent with the long term geologic exhumation rate we derive of 74 10 1 mm ka 1 since the middle miocene however our cwdr data are 2 3 times lower than rates of coastal uplift and river incision of 150 350 mm ka 1 derived from data on marine and fluvial terraces respectively in addition to isotopic analyses we quantified topographic indices such as elevation relief and slope and then compared these indices with cwdrs to examine morphological changes that might be related to tectonic disturbances since the beginning of late pleistocene the relationship between cwdrs and topographic indices indicates that most of the study area rests in a near steady state condition however in contrast to the smooth concave longitudinal profile expected for a steady state landscape topographic analyses reveal the presence of major knickzones confirmed by field reconnaissance these knickzones may have been produced by changes in base level caused by tectonic events lithologic variations or sea level changes our 1 d modeling of knickpoint propagation suggests that tectonic disturbance is the factor most likely to produce knickzones in the study area raising the possibility that erosional signals caused by late pleistocene tectonic disturbance affected channel gradients but that the knickpoints have not yet propagated through the entire catchment area our results suggest that the eastern part of the korean peninsula is approaching but has not completely reached a geomorphic steady state and that transient conditions have been induced by reactivation of the rift flank margin no later than the late pleistocene causing tilting of the central korean peninsula to the west at a rate of 7 0 10 10 m m per year over the past 125 ka

morphometry of scoria cones located on a volcano flank a case study from mt etna italy based on high resolution lidar data abstract by using new high resolution 2 m digital elevation model derived from the 2005 lidar survey of mt etna volcano italy our study measured the classical morphometrical parameters for scoria cones i e w co cone width w cr crater diameter h cone height as well as volume inclination of cone slope and substrate and a number of other parameters for 135 scoria cones of mt etna volume and age distribution of cones shows that there is no direct structural control on their emplacement in terms of etna s rift zones the cones are progressively smaller in size toward summit which can be explained by the large volcano s feeding system and progressively frequent lava burial toward top a careful analysis of h w co ratio determined as 0 18 for other volcanic fields worldwide shows that this ratio strongly depends on 1 the calculation method of h and 2 lava burial of cone for etnean cones applying an improved method for calculating h relative to the dipping substrate results in a significantly lowered standard h w co ratio 0 137 which in turn questions the validity of the classical value of 0 18 in the case of large central volcanoes the reduction of the ratio is not only due to methodology but also to the common lava burial this can be expressed even better if h mean is used instead of h max h mean w co 0 098 using this measure at etna well formed cones have higher ratios than structurally deformed e g double or rifted cones furthermore although the sampled scoria cones at etna have formed in a relatively narrow time interval h w co corresponding to erosional changes detected globally h w co 0 143 0 135 and 0 115 for three age classes of etna s scoria cones corresponding to average slopes of 26 6 23 9 and 23 7 because the morphometrical effect of position on a dipping substrate as well as lava burial exceeds the effect of erosion we call attention to use caution in simply using the h w co ratio of scoria cones for detecting age especially on large active volcanoes

pretectonic and posttectonic emplacements of the granitoids in the south central okchon belt south korea implications for the timing of strike slip shearing and thrusting structural analyses of three granitic plutons in the south central okchon belt of the korean peninsula reveal that the baegnok and cheongsan plutons are pretectonic with respect to right lateral strike slip ductile shearing along the cheongsan shear zone and later top to the ese thrusting whereas the boeun pluton is posttectonic with respect to these two deformation events u pb sphene age data from the three plutons indicate that the intrusion ages of the baegnok cheongsan and boeun plutons are 222 7 2 1 ma 216 9 2 2 ma and 171 7 1 4 ma respectively microstructural evidence preserved in deformed rocks from the ne striking cheongsan shear zone and adjacent nne striking thrusts suggests that the right lateral ductile shearing event along the shear zone occurred before the ese directed thrusting since these thrusts crosscut the late triassic early jurassic taedong group we estimate that the thrusts developed at 180 ma during the daebo tectonic event just prior to the intrusion of the boeun pluton we also constrain the age of the cheongsan shear zone at middle late triassic time corresponding to the songrim orogeny so that the shear zone is not related to deformation along the middle jurassic honam shear zone 176 ma the honam shear zone is not a simple tectonic boundary between the okchon belt and the yongnam massif but an anastomosing ductile shear zone not only affecting the okchon belt and the yongnam massif but also crosscutting the boundary between them at a low angle we suggest that the right lateral cheongsan shear zone is a major boundary between the okchon zone and the yongnam massif with paleozoic cover rocks the kyonggi massif and the okchon zone south china block were juxtaposed against the yongnam massif and taebaeksan basin north china block along the cheongsan shear zone during a late stage of the middle triassic songrim orogeny probably related to the late paleozoic early mesozoic collision between the north and south china blocks

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

body wave analysis for shallow intraplate earthquakes in the korean peninsula and yellow sea abstract four medium size shallow intraplate earthquakes from the korean peninsula and yellow sea have been studied in order to find their source mechanisms focal depths source time functions and seismic moments focal depths obtained by matching short period synthetic seismograms to the observations are found to be very shallow between 6 and 9 km to determine the source mechanisms source time functions and seismic moments we inverted teleseismic long period p and sh waves from gdsn data although the inversions are performed on only a few teleseismic body waves the radiation patterns of the moment tensors are consistent with most of the p wave first motion polarities observed at regional and teleseismic distances the overall agreement in relative amplitudes and waveforms between observations and synthetics indicates that the earthquake source parameters are reasonably well determined for two earthquakes from the yellow sea and one from the north korean peninsula the mechanisms are predominantly of strike slip style while the earthquake in the central korean peninsula shows a dip slip faulting consistent ene wsw trending compression p axis and nnw sse tension t axis are found for all studied earthquakes

focal mechanisms of recent earthquakes in the southern korean peninsula summary we evaluate the stress field in and around the southern korean peninsula with focal mechanism solutions using the data collected from 71 earthquakes ml 1 9 5 2 between 1999 and 2004 for this the hypocentres were relocated and well constrained fault plane solutions were obtained from the data set of 1270 clear p wave polarities and 46 sh p amplitude ratios the focal mechanism solutions indicate that the prevailing faulting types in south korea are strike slip dominant oblique slip faultings with minor reverse slip component the maximum principal stresses 1 estimated from fault slip inversion analysis of the focal mechanism solutions show a similar orientation with e w trend 269 275 and low angle plunge 10 25 for all tectonic provinces in south korea consistent with the e w trending maximum horizontal stress hmax of the amurian microplate reported from in situ stress measurements and earthquake focal mechanisms the directions of the intermediate 2 and minimum 3 principal stresses of the gyeongsang basin are however about 90 deg off from those of the other tectonic provinces on a common 2 3 plane suggesting a permutation of 2 and 3 our results incorporated with those from the kinematic studies of the quaternary faults imply that nnw to ne striking faults dextral strike slip or oblique slip with a reverse slip component are highly likely to generate earthquakes in south korea

estimation of possible maximum earthquake magnitudes of quaternary faults in the southern korean peninsula the korean peninsula is located in the intraplate regime of the eurasian plate and numerous historical and instrumental earthquakes have been recorded among these the largest earthquakes were recorded in and around the gyeongju and ulsan areas in the southeastern part of the korean peninsula we recently reexamined faults in jinhyun and jintee part of the ulsan fault swarm to clarify the average slip rate and the maximum potential magnitude of future earthquakes the jinhyun fault extends to the tabgol fault j t fault and the jintee fault extends to the singye s j fault the faults cut through alluvial fans and are covered by unconsolidated granite washes on tertiary granite all of these faults show reverse sense of motion such that the tertiary granite has been moved upward with a high angle relative to the quaternary sediments most sediment samples from the jinhyun and jintee faults showed quartz osl ages of ca 40 60 ka the calculated vertical slip rate for the jinhyun and jintee faults is in the range of 0 18 0 28 mm y based on maximum earthquake magnitudes calculated from geological survey results as well as historical and instrumental earthquake information the maximum potential magnitudes of future earthquakes in the gyeongju area are estimated to range from 4 6 to 5 6 this suggests that the potential maximum magnitude in the southern korean peninsula is presumably not more than 6

the 2017 ml 5 4 pohang earthquake sequence korea recorded by a dense seismic network abstract the 2017 ml 5 4 pohang earthquake in southeastern korea which was induced by fluid injection from an enhanced geothermal system and its foreshock aftershock sequence has been recorded by a dense and portable temporary seismic array the hypocentral distribution of the earthquake sequence reveals the reactivation of a complex subsurface fault system that was previously unmapped aftershock seismicity propagated to the southwest and revealed previously unknown fault segments the aftershocks tend to concentrate along the tips of the main rupture with a lack of aftershocks around the mainshock hypocenter this pattern suggests that the stress in the mainshock area was almost completely released during the mainshock and early aftershock phase and that the recurrence of moderate to large earthquakes in the hypocentral area is unlikely compared with the marginal areas a new rupture occurred along the southwestern margin of the main rupture after the largest aftershock ml 4 6

focal mechanisms of earthquakes in southern quebec southeastern ontario and northeastern new york with implications for regional seismotectonics and stress field characteristics focal mechanisms have been derived for 30 earthquakes in the adirondack mountains western quebec seismic zone from a modified version of the computer program focmec using first motions of pn and pg waves and sv p amplitude ratios measured from sg and pg phases reverse faulting on medium to steeply dipping planes is prevalent focal depth and velocity structure can vary within realistic limits without much influence on the focal mechanism solution deviatoric compressive stress axes have a strong horizontal component but the trend varies most align approximately northeast southwest in accordance with both the orientation predicted by models based on plate tectonics and that derived from most other studied earthquakes in the region however for several earthquakes mechanisms have been obtained that deviate from this pattern notably for events near 45 5 n to 46 n 75 w and the north gower main shock aftershock series of october 1983 possible explanations for these anomalies in terms of local structural disturbances or reactivation of preexisting zones of weakness with different structural orientation than those created during the current tectonic regime are presented different hypotheses for the seismotectonic evolution and processes of the area are presented and discussed in addition to orientations of nodal planes and deviatoric stress axes there are other indicators such as the seismicity distribution and patterns of earthquake sequences of inhomogeneity in the proposed adirondack mountains western quebec seismic zone

tectonic stress field of china inferred from a large number of small earthquakes mean principal tress axes were inferred for china using 9621 p wave first motion polarity readings from 5054 small earthquakes 1 ml 5 the area studied was divided into 76 subregions the mean p compressive b intermediate and t relatively tensional axes corresponding to composite focal mechanism solutions of multiple earthquakes for each subregion were determined by a grid test to all possible orientations of the p b and t axes with a step of 5 or 10 in order to get a relatively homogeneous sampling in space we have avoided using the readings from spatially and temporally clustered earthquakes we have rechecked most of the polarity readings by inspection of original seismograms the focal mechanism solutions of large m 6 individual earthquakes are also presented for comparison the results indicate the existence of an overall radial pattern of the present day maximum horizontal stress orientations throughout the continental area of china this pattern is thought to be closely related to indentor models of continental collision between the indian ocean and the eurasian plate

the 20 january 2007 odaesan korea earthquake sequence reactivation of a buried strike slip fault the m w 4 6 odaesan korea earthquake of 20 january 2007 11 hr 56 min had nine locatable foreshocks in the previous 67 hours and nine aftershocks during the 11 hours after the main event we obtained accurate locations of this sequence by employing the double difference earthquake relocation method with differential travel time measurements using waveform cross correlation we found that 19 accurately relocated foreshocks mainshock and aftershocks aligned along a 1 2 km long lineation with an azimuth of west northwest east southeast 115 which coincides with a nodal plane of the mainshock focal mechanism that strikes 114 regional waveform modeling of the mainshock constrained a focal depth of 11 1 km and the focal mechanism is a pure strike slip faulting along the vertical nodal planes with a horizontal p axis plunge 1 trending east northeast 69 the pulse rise time of the mainshock source time function estimated from an empirical green s function analysis ranges from 0 1 to 0 16 sec which yields a source radius of 0 6 km and is consistent with the spatial distribution of the foreshock and aftershock sequences the stress drop of graphic 1 is obtained for the mainshock using a radius of 0 6 km for a circular rupture and seismic moment graphic 2 the 20 january 2007 odaesan earthquake may be the first significant earthquake in southern korea to date where the fault plane is constrained by accurate locations of the foreshock and aftershock sequences its source mechanism vertical strike slip fault at depth suggests that the event represents reactivation of a buried high angle fault in the precambrian basement by the contemporary east east northeast trending regional horizontal compressive stress 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif

retrodeformation analysis of the quaternary fault in the southeastern korean peninsula abstract the suryum fault f1 with subsidiary branches f2 3 varies in strike from n40 e to n14 e and in dip from

determination of slip rate by optical dating of fluvial deposits from the wangsan fault se korea abstract the time integrated slip rate in fault zones can be determined if the deformed deposits are reliably dated here we report optically stimulated luminescence osl ages of late pleistocene fluvial deposits cut by the wangsan fault southeastern korea which displaces a hanging wall block of about 28 m five sandy samples of the deformed quaternary deposits were dated by quartz osl using the single aliquot regenerative dose sar protocol three samples taken from the footwall block show stratigraphically consistent osl ages of 54 7 76 5 and 90 6 ka from top to bottom two samples collected from the same layer in the hanging wall block show reproducible osl ages of 81 5 and 82 5 ka which are also in good agreement with the stratigraphic relationships our osl ages yield an average sedimentation rate of the quaternary deposits as around 0 04 mm a 1 and a minimum value of time integrated slip rate as 0 52 mm a 1 this minimum slip rate is considerably higher than those reported earlier for quaternary faults in southeastern korea the youngest osl age 54 7 ka constrains the maximum value of the recurrence interval of the fault movement

long term evolution of intraplate seismicity in stress shadows after a megathrust abstract megathrusts produce large permanent lithospheric displacements as well as strong transient ground shaking up to regional distances the influence of the 2011 m9 0 tohoku oki earthquake on the seismicity in stable intraplate regions around the korean peninsula is investigated the differential lateral displacements by the megathrust build transient radial tension field over the backarc lithosphere constructing stress shadows for all types of faults in optimal orientation over the wide backarc region including the southern korean peninsula a characteristic seismic velocity decrease after the megathrust supports the medium relaxation the number of earthquakes with magnitudes greater than or equal to 2 5 was increased by 61 after the megathrust the earthquakes occurred episodically a series of unusual earthquake swarms and a temporal cluster of moderate size earthquakes were observed in the yellow sea region the significant seismicity increase since the megathrust may be due to the fluid diffusion during the transient tension field and the pore fluid pressure increase during the ambient compressional stress field recovery by tectonic loading the induced seismicity may continue until the ambient stress field is fully recovered

the 20 january 2007 ml 4 8 odaesan earthquake and its implications for regional tectonics in korea a moderate sized earthquake m l 4 8 occurred in the mideast korea peninsula on 20 january 2007 it was the largest inland earthquake to occur there since the inception of a modern seismic observation system although only four aftershocks were noticed in previous studies a careful review of continuous data revealed that the main event was accompanied by at least 74 micro foreshocks and aftershocks a subset of 25 events was selected for further analysis to determine precise earthquake locations focal mechanism solutions and the current status of regional tectonic stress as well as to answer questions raised about the sequence earthquake hypocenters were seen to be more clustered after the hypodd relocation a source radius of 1 km for the main event was estimated based on the distribution of precisely determined aftershock locations focal mechanism solutions of larger events in the sequence suggest either a left lateral strike slip fault trending west northwest east southeast or a right lateral strike slip fault trending north northeast south southwest as the responsible structure although the woljeongsa fault striking north northeast south southwest in the local geological map matches one of the proposed trends precise earthquake relocation results gave a contradictory result showing that a previously unknown west northwest east southeast striking fault was responsible for the earthquake sequence we also observed an unusual lack of large magnitude aftershocks a relatively large stress drop during the main event and no previous earthquake record in the region observations made in the study consistently indicate the sequence nucleated along a less developed fault focal mechanism solutions suggest the current status of tectonic stress governing earthquake generation in korea is east northeast west southwest compression and north northwest south southeast extension

fault slip analysis of quaternary faults in southeastern korea abstract the quaternary stress field has been reconstructed for southeast korea using sets of fault data the subhorizontal direction of the maximum principal stress 1 trended ene and the direction of the minimum principal stress 3 was nearly vertical the stress ratio 2 3 1 3 value was 0 65 two possible interpretations for the stress field can be made in the framework of eastern asian tectonics 1 the hmax trajectory for southeast korea fits well with the fan shaped radial pattern of maximum principal stress induced by the india eurasia collision thus we suggest that the main source for this recent stress field in southeast korea is related to the remote india eurasia continental collision 2 the stress field in korea shows a pattern similar to that in southwestern japan the origin for the e w trending hmax in japan is known to be related to the mantle upwelling in the east china sea thus it is possible that quaternary stress field in korea has evolved synchronously with that in japan we suggest further studies gps and in situ stress measurement to test these hypotheses

geodynamics of the baikal rift zone and plate tectonics of asia the baikal rift zone stretches from the southern termination of lake baikal as far as 118 e and includes the baikal basin and the barguzin upper angara and muysk grabens that form its easterly continuation coincident with the rift zone is a narrow seismic belt which extends further east along the stanovoy range to the okhotsk sea this belt can be considered as the boundary between the eurasian and amurian lithospheric plates earthquake focal mechanism solutions that show a consistent orientation of extensional axes across the strike of the rift zone indicate the behaviour of the eurasian and amurian plates as rigid bodies from 83 focal mechanism solutions the pole of rotation of the eurasian and amurian plates was computed to lie at the point 56 95n 117 45e the angular rate estimated from the amount of baikal opening is about 1 10 7 deg year the position of the pole at the eastern termination of the rift zone accounts for the dying out of the rift zone and for the appearance east of the pole of compressive structures in the stanovoy range to the southwest the baikal rift zone is replaced by the mosaic of microplates in central asia that occupy the space between the eurasian and indian lithospheric plates prominent are the mongolian dzungarian alashan tarim tibetan and pamir microplates earthquakes in central asia are dispersed although higher concentrations of earthquakes associated with microplate boundaries are prominent the tectonics of central asia are considered to be due to the interaction of rigid microplates assuming that plastic deformations occur within the relatively wide zones along the microplate boundaries by considering the strike slip faults to define the trajectories of microplate motions the parameters of relative movements of the microplates have been computed the microplate mosaic of central asia is moving northeasterly as a result of the collision and continued convergence of india and eurasia the mosaic acts as a wedge splitting off the amurian plate from eurasia causing it to rotate counterclockwise so opening the baikal rift

earthquake source mechanism and rupture directivity of the 12 september 2016 mw 5 5 gyeongju south korea earthquakeshort note

seismological constraints on the collision belt between the north and south china blocks in the yellow sea abstract the korean peninsula eastern china and the yellow sea comprise the eastern eurasian plate and are believed to share considerable tectonic evolution history the tectonic structures in the yellow sea are poorly understood raising difficulty in reconstruction of tectonic evolution history in the eastern eurasian plate the tectonic structures in the yellow sea are constrained by seismicity and fault plane solutions of earthquakes the fault plane solutions are determined by waveform inversions and seismic phase polarity analyses the ambient stress fields are deduced from the fault plane solutions the primary stress field around the yellow sea is composed of ene wsw directional compression and nnw sse directional tension normal faulting earthquakes with ene wsw directional strikes are observed in the central yellow sea between the shandong peninsula and the central korean peninsula the normal faulting region is interpreted to be a northern margin of collision belt between the north and south china blocks the normal faulting system suggests post collisional lithospheric delamination causing reverse activation of paleo thrustal faults that were developed by the collision between the north and south china blocks in the early jurassic period

source process and tectonic implication of the january 20 2007 odaesan earthquake south korea abstract the source process for the 20th of january 2007 mw 4 5 odaesan earthquake in south korea is investigated in the low and high frequency bands using velocity and acceleration waveform data recorded by the korea meteorological administration seismographic network at distances less than 70 km from the epicenter synthetic green functions are adopted for the low frequency band of 0 1 0 3 hz by using the wave number integration technique and the one dimensional velocity model beneath the epicentral area an iterative technique was performed by a grid search across the strike dip rake and focal depth of rupture nucleation parameters to find the best fit double couple mechanism to resolve the nodal plane ambiguity the spatiotemporal slip distribution on the fault surface was recovered using a non negative least square algorithm for each set of the grid searched parameters the focal depth of 10 km was determined through the grid search for depths in the range of 6 14 km the best fit double couple mechanism obtained from the finite source model indicates a vertical strike slip faulting mechanism the nw faulting plane gives comparatively smaller root mean squares rms error than its auxiliary plane slip pattern event provides simple source process due to the effect of low frequency that acted as a point source model three empirical green functions are adopted to investigate the source process in the high frequency band a set of slip models was recovered on both nodal planes of the focal mechanism with various rupture velocities in the range of 2 0 4 0 km s although there is a small difference between the rms errors produced by the two orthogonal nodal planes the sw dipping plane gives a smaller rms error than its auxiliary plane the slip distribution is relatively assessable by the oblique pattern recovered around the hypocenter in the high frequency analysis indicating a complex rupture scenario for such moderate sized earthquake similar to those reported for large earthquakes

assessing whether the 2017 mw 5 4 pohang earthquake in south korea was an induced event the moment magnitude m w 5 4 pohang earthquake the most damaging event in south korea since instrumental seismic observation began in 1905 occurred beneath the pohang geothermal power plant in 2017 geological and geophysical data suggest that the pohang earthquake was induced by fluid from an enhanced geothermal system egs site which was injected directly into a near critically stressed subsurface fault zone the magnitude of the mainshock makes it the largest known induced earthquake at an egs site

focal mechanism analyses for eastern tennessee earthquakes 1981 1983 to investigate the neotectonic processes in the southern appalachians 10 single event focal mechanisms and 6 composite focal mechanisms were determined from 37 events that occurred in eastern tennessee between september 1981 and july 1983 both p wave polarities and sv p z amplitude ratios are input to a computer program that systematically searches the focal sphere for solutions acceptable within preset error limits hypocenter locations azimuth and departure angles are taken from locations obtained by the tennessee earthquake information center with a four layer velocity model a gradient velocity model developed to improve focal depth estimates and to provide continuously varying rather than discrete departure angles is used to relocate the seismic events for which single event focal mechanisms and composite focal mechanisms solutions are obtained the two different velocity models each produced focal mechanisms with similar nodal plane and p axis orientations indicating stable and reliable mechanism solutions the differences between average strike dip and rake angles of the two data sets range from 2 to 11 both single event focal mechanisms and composite focal mechanisms solutions exhibit predominantly strike slip motion along nearly vertical northerly right lateral or easterly left lateral oriented nodal planes standard deviations about the average strike dip and rake angles are generally less than 20 p axis trends average about n50 e with a nearly horizontal average plunge and both trend and plunge estimates have standard deviations of 25 or less

relocation of earthquakes in the new madrid seismic zone estimation of 1d velocity structure and geometry of a seismogenic fault abstract determination of reliable hypocenters of earthquakes is crucial to earthquake seismology and to evaluate hazards associated with earthquakes there are many associated computer codes for this purpose however most of the location algorithms are designed to determine hypocentral parameters based on previously determined velocity models in contrast we employed a location method that is independent of the initial velocity model using a genetic algorithm ga to determine an optimal 1d velocity model and the locations of earthquakes using this ga we relocated earthquakes that occurred in the new madrid seismic zone nmsz in the central united states between october 1989 and august 1992 the goal of this work was to delineate the possible fault planes by reliable relocation of those earthquakes and to determine a 1d velocity structure for the nmsz a total of 502 earthquakes recorded by 37 portable array for numerical data acquisition panda stations were used in the relocation study in the relocation process the root mean square travel time residuals were reduced by 35 corresponding to an average of 2 3 km deeper in depth 0 7 km shift in latitude and 0 8 km shift in longitude compared with those in the initial catalog locations the hypocenters of the earthquakes can be subdivided into four groups based on their spatial distributions the group that corresponds to the cottonwood grove fault cgf in the southwestern nmsz represents a very steep plane whereas the other three groups fall into reelfoot fault rf we inverted p and s wave travel times from the new hypocentral parameters to determine 1d velocity models the resulting eight layered velocity models consist of a 2 km thick surface layer followed by seven 2 km thick layers with v p ranges from 5 36 to 6 74 km s and v s ranges from 2 83 to 3 90 km s for both cgf and rf regions online material interactive visualizations of hypocentral distributions

contemporary deformation and strain analysis in south korea based on long term 2000 2018 gnss measurements south korea forms the korean peninsula and is considered to be tectonically more stable than the other regions of eastern asia global navigation satellite system gnss sites installed in the country contribute to geoscientific studies on contemporary deformation strain pattern and current kinematics of tectonic blocks in the current study we used long term gnss measurements from 1 january 2000 to 31 december 2018 to study block wise geodetic deformation and strain pattern of the country including the co seismic effect of the 2011 tohoku oki to earthquake measured velocities of gnss sites are modelled via exponential smoothing method to analyse the gnss time series and obtain the secular tectonic crustal deformation the analysis of strain pattern and vertical displacements indicates that extensional deformation followed compressional deformation from the northern korean blocks to the southern korean blocks finally we examined the angular velocity and pole of rotation of the south korean blocks using pre seismic and post seismic displacements of the gnss sites the results indicate that the angular velocity of the northern blocks was lower while the southern block velocity is higher after the to earthquake we believe that the integrated approaches applied in the study would provide better insights into the active deformation of south korea

implications of a newly dated ca 1000 ma rhyolitic tuff in the indravati basin bastar craton india abstracta rhyolitic tuff with a minimum thickness of about 2 5 m occurs at the top of the sedimentary succession of the indravati group in south central india the tuff is a high k low na rhyolite with dominantly k feldspar phenocrysts cathodoluminescence color of the quartz phenocrysts is blue u pb isotopic analyses la mc icpms of magmatic zircons separated from the tuff give a weighted mean average 207pb 206pb age of ma that we interpret as the age of crystallization we conclude that the closure age of the indravati succession is ca 1000 ma very similar to that of the chhattisgarh and vindhyan successions the age of this tuff is identical within analytical errors to those of the sukhda and dhamda tuffs in the chhattisgarh basin some 350 km away we suggest that these three tuffs represent a ca 1000 ma rhyolitic flare up possibly related to the assembly of rodinia and docking of india and east antarctica