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Localized analysis of polar geomagnetic jerks
Kim, H.R.,von Frese, R.R.B. Elsevier Scientific Publishing Co 2013 Tectonophysics Vol.585 No.-
A new method is introduced to find the relatively sudden temporal changes or jerks of the geomagnetic field over the polar regions. Geomagnetic jerk events during the 20th century have mostly been identified from direct measurements at mid-latitude geomagnetic observatories. Recently, however, global magnetic spherical harmonic models like CM4 and CHAOS have been found effective in identifying the Antarctic events of 1969, 1978, 1985, 1991 and 2003 that were followed by jerks in the Arctic region with time delays of one to three years. The present study extends these events into earlier decades from 1900 to 1985 using the global harmonic model gufm1 with localized spherical coefficients or Slepian functions for the polar regions. Power spectral minima for the localized coefficients correlate with and thus help identify the abrupt changes in geomagnetic secular acceleration models. The time delay between the Antarctic and Arctic geomagnetic jerks was also confirmed.
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Cho, H.M.,Baag, C.E.,Lee, J.M.,Moon, W.M.,Jung, H.,Kim, K.Y. Elsevier Scientific Publishing Co 2013 Tectonophysics Vol.582 No.-
The onshore seismic experiment, KCRT2004, explored the crustal velocity structure of the southern Korean peninsula. We present an interpretation of seismic data along this 340km long NNW-SSE trending profile. The crust was found to consist of three layers: the upper, middle, and lower crust with P- and S-wave velocities ranging 5.50 to 6.95km/s and 2.82 to 3.91km/s, respectively. The average P-wave velocity (6.26km/s) and Pn velocity (7.82-7.88km/s) are lower than the worldwide average of continental crust. Moho depths are 29.0-34.9km, gradually thickening toward south. The Vp/Vs ratio of crustal material is estimated to be 1.73 (σ=0.249) for the upper and middle crust and the ratio increases with the depth in the lower crust. The Gyeonggi massif in the north of the profile has a lower Vp/Vs ratio than other tectonic units. The average crustal Vp/Vs ratio of 1.74 (σ=0.253) is remarkably lower than the average value 1.78 (σ=0.27) for the bulk continental crust. The low average crustal Vp/Vs ratio is similar to that measured in eastern China. The empirical analysis using both P-wave velocity and Vp/Vs ratio shows that the upper and middle crust is dominantly felsic and the lower crust is intermediate in composition. The absence of the mafic material in the lower crust that is also found in eastern China contrasts with the generally accepted global model of the mafic lower crust.
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Kim, S.,Ree, J.H. Elsevier Scientific Publishing Co 2012 Tectonophysics Vol.522 No.-
The most widely used method for quantitative analyses of the spatial distribution of grains in a rock body is the nearest-neighbor method (a position-based method), which recognizes the grain center as the position of the grain without consideration of grain size or shape. However, the spatial distribution of grains is influenced by their size and shape, as well as their position. Here, we propose a multiple-area density map (MADM) method to quantify the spatial distribution of phases or grains. The method is based on image analysis and simultaneously considers the position, size, shape, and proportion of grains. The MADM constructed by overlaying density maps of grain areas with a set of template size produces a normalized standard deviation (NSD) value of phase area density, which represents the degree to which it is spatially clustered. The NSD value is used to quantify the distribution of the phase, without considering individual grains. To identify the spatial distribution of individual grains, the method employs the clustering index (CI), which is calculated from the ratio of NSD to the reciprocal of the number of grains in a log-log plot.
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Kim, H.S.,Ree, J.H. Elsevier Scientific Publishing Co 2013 Tectonophysics Vol.587 No.-
The Permo-Triassic Songrim (Indosinian) orogeny in South Korea was a major tectonic event involving complicated continental collisions at the eastern margin of Eurasia. Previous studies have examined the structural and metamorphic features of the Songrim orogeny in each of the Paleozoic terranes of the orogenic belt (i.e., the Taebaeksan Basin, the Okcheon Basin, and the Imjingang Belt), but correlations of these features among the terranes remain uncertain. The aim of this paper is to reveal deformation history including bulk crustal shortening directions in the Taebaeksan Basin, and to correlate the tectono-metamorphic evolution of the Taebaeksan Basin with other Phanerozoic mobile belts in eastern Asia based on a combined analysis of foliation intersection/inflection axes (FIA) trends and metamorphic P-T and T-t (time) paths. The orientations and relative timing of FIA preserved as inclusion trails within porphyroblasts of andalusite, chloritoid, garnet, and staurolite reveal two age groups of inclusion trails in the Pyeongan Supergroup at the northeastern margin of the Taebaeksan Basin. These microstructures indicate the development of early NNW-NNE-trending structures and fabrics, followed by later E-W-trending ones. These observations suggest a change in the orientation of bulk crustal shortening from E-W to N-S during the Songrim orogeny. Based on the similar microstructures and temperature-time paths of the three Paleozoic terranes, we interpret that the E-W bulk crustal shortening influenced the eastern part of the Korean Peninsula during the early stages of the Songrim orogeny, presumably related to amalgamation between the proto-Japan terrane and the eastern margin of Eurasia, whereas the N-S bulk crustal shortening was stronger in the western part of the peninsula during the later stages of the orogeny, related to collision between the South and North China blocks.
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Chang, C.,Jo, Y. Elsevier Scientific Publishing Co 2015 Tectonophysics Vol.664 No.-
Two field examples of hydraulic fracturing stress measurements are reported, in which the determined stress magnitudes exhibit severe variations with depth. The stress measurements were conducted in vertical boreholes drilled in granites in two different locations in South Korea. Several isolated intervals of intact rocks in the boreholes were vertically fractured by injecting water. The magnitudes of the minimum horizontal principal compressive stress (S<SUB>hmin</SUB>) were determined from shut-in pressures. The magnitudes of the maximum horizontal principal compressive stress (S<SUB>Hmax</SUB>) were estimated based on the Kirsch equation using tensile strengths determined from hollow cylinder tests and Brazilian tests, in which pressurization-rate effects on tensile strength were taken into account. The stress states in both locations are in reverse-faulting stress regimes. The magnitudes of S<SUB>Hmax</SUB> are generally within a stress range defined by frictional limits of favorably oriented fractures having frictional coefficients of 0.6 and 1.0. However, S<SUB>Hmax</SUB> magnitudes do not increase linearly with depth, but rather scatter quite severely. It is noted that near the depths where the measured stresses are relatively low, natural discontinuities with wide apertures containing weak filling material exist, whereas near the depths of high stress, such wide discontinuities are scarce. Wide aperture discontinuities are predominantly oriented such that their slip tendency is high under the given stress conditions, meaning that if excessive shear stress is exerted, the weak discontinuities would slip to release the excessive stress. Such local processes would restrict S<SUB>Hmax</SUB> magnitudes within values that can only be sustained by the shear strengths of the discontinuities, leading to severe variations of S<SUB>Hmax</SUB> with depth. This result suggests that stress magnitudes are controlled quite locally by the frictional property of natural discontinuities, and that the stress state in granitic rock might be inherently heterogeneous because of the heterogeneous distribution of natural discontinuities having various frictional properties.
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Shear-wave splitting beneath southern Korea and its tectonic implication
Kang, T.S.,Shin, J.S. Elsevier Scientific Publishing Co 2009 Tectonophysics Vol.471 No.3
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.
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Kim, H.J.,Lee, G.H.,Choi, D.L.,Jou, H.T.,Li, Z.,Zheng, Y.,Kim, G.Y.,Lee, S.H.,Kwon, Y.K. Elsevier Scientific Publishing Co 2015 Tectonophysics Vol.638 No.-
<P>The South Korea Plateau is a remnant of continental crust in the middle of the eastern Korean margin that was deformed by extension in association with back-arc rifting and separation of the southwestern Japan Arc in the Cenozoic. Multichannel seismic profiles show that the South Korea Plateau preserves structure of rift basins flanked by uplifted footwall blocks. Rift basins in the plateau are filled with distinct syn- and post-rift sequences separated by a prominent breakup unconformity resulting from uplift. The crustal and upper mantle structures computed by ambient noise tomography indicate that the South Korea Plateau defines the seaward limit of rifted continental crust; in addition, rifting occurred with vigorous asthenospheric upwelling induced along the Korean margin and depth-dependent stretching. Therefore, the South Korea Plateau underwent the entire sequence of tectonic events typical of a passive continental margin encompassing rifting with subsidence, uplift, erosion, and breakup prior to the separation of the southwestern Japan Arc. Breakup at the Korean margin may substantiate successive episodes of back-arc spreading which migrated toward the arc in response to trench retreat. We suggest that the southwestern Japan Arc moved to its present location from the southern part of the Korean margin with a significant amount of clockwise rotation during back-arc spreading. (C) 2014 Elsevier B.V. All rights reserved.</P>
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