Heterogeneous in situ stress magnitudes due to the presence of weak natural discontinuities in granitic rocks

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.

P- and S-wave velocity model along crustal scale refraction and wide-angle reflection profile in the southern Korean peninsula

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.

Permo-Triassic changes in bulk crustal shortening direction during deformation and metamorphism of the Taebaeksan Basin, South Korea using foliation intersection/inflection axes: Implications for tectonic movement at the eastern margin of Eurasia during

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.

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.

Quantification of the spatial distribution of mineral phases and grains in rock using a 2-D multiple-area density map technique

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.

Back-arc rifting in the Korea Plateau in the East Sea (Japan Sea) and the separation of the southwestern Japan Arc from the Korean margin

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>

Interaction between regional stress state and faults: Complementary analysis of borehole in situ stress and earthquake focal mechanism in southeastern Korea

Chang, C.,Lee, J.B.,Kang, T.S. Elsevier Scientific Publishing Co 2010 Tectonophysics Vol.485 No.1

We characterize the present-day stress tensor in southeastern Korean Peninsula using two different sets of data (geotechnical in situ stress data and earthquake focal mechanism solutions), to understand the regional contemporary stress state and its relationship to the population of faults. Both sets of data show a comparable result of ENE-WSW maximum compression direction, which is in accord with the first order pattern of tectonic stress direction in the eastern Eurasian plate. More rigorous analyses of in situ stress as well as the inversion of focal mechanism show that the current stress field exhibits a systematic heterogeneity in its orientations and magnitudes, possibly caused by the influence of faults. The minimum and maximum horizontal principal stresses normalized by vertical stress at the shallow depths where stress measurements were conducted vary spatially. It turns out that the magnitude of stress field appears to be inversely correlated with the density of regional scale faults. This suggests that a stress release due to faulting may be one of the major factors that contribute to the low stress regime in the region. As a way to confirm the inference, we examine the attitudes of recently activated Quaternary faults with respect to the current stress field. A majority of the faults are oriented in the optimal directions for slip, as indicated by the overall high ratios of shear to normal stress acting on fault planes for the given stress condition, which implies that they might sustain the current stress field. The contemporary earthquake distribution indicates that the lower stressed region has a denser population of seismic activities, suggesting that fault strength in the corresponding region may be at frictional limit with the contemporary stress state. This may imply that the heterogeneity of the regional stress state is a result of the heterogeneity of the strength of faults.

Seismogenic structures of the 1999 M_w 7.6 Chi-Chi, Taiwan, earthquake and its aftershocks

Kim, K.H.,Chen, K.C.,Wang, J.H.,Chiu, J.M. Elsevier Scientific Publishing Co 2010 Tectonophysics Vol.489 No.1

High-resolution 3-D V<SUB>P</SUB> and V<SUB>S</SUB> velocity models around the source region of the M<SUB>w</SUB>7.6 Chi-Chi, Taiwan, earthquake show significant lateral and vertical variations. The mainshock occurred within a narrow low velocity zone along the Chelungpu fault. A sudden increase of velocity and seismicity took place across the Shuilikeng fault to the east. Most aftershocks were located in areas of high V<SUB>P</SUB> and V<SUB>S</SUB>. A sharp east-dipping zone extending from the Shuilikeng fault on the surface to a depth of about 15km separates the low velocity region in the west from the high velocity region in the east. An apparent high-angle west-dipping seismic zone lies at depths of 15-30km beneath the western Central Range. Numerous aftershocks with normal faulting were located west of the middle segment of the Chelungpu fault.

Seismicity and fault geometry of the San Andreas fault around Parkfield, California and their implications

Kim, W.,Hong, T.K.,Lee, J.,Taira, T. Elsevier Scientific Publishing Co 2016 Tectonophysics Vol.677 No.-

<P>Fault geometry is a consequence of tectonic evolution, and it provides important information on potential seismic hazards. We investigated fault geometry and its properties in Parkfield, California on the basis of local seismicity and seismic velocity residuals refined by an adaptive-velocity, hypocentral-parameter inversion method. The station correction terms from the hypocentral-parameter inversion present characteristic seismic velocity changes around the fault, suggesting low seismic velocities in the region east of the fault and high seismic velocities in the region to the west. Large seismic velocity anomalies are observed at shallow depths along the whole fault zone. At depths of 3-8 km, seismic velocity anomalies are small in the central fault zone, but are large in the northern and southern fault zones. At depths >8 km, low seismic velocities are observed in the northern fault zone. High seismicity is observed in the Southwest Fracture Zone, which has developed beside the creeping segment of the San Andreas fault. The vertical distribution of seismicity suggests that the fault has spiral geometry, dipping NE in the northern region, nearly vertical in the central region, and SW in the southern region. The rapid twisting of the fault plane occurs in a short distance of approximately 50 km. The seismic velocity anomalies and fault geometry suggest location-dependent piecewise faulting, which may cause the periodic M6 events in the Parkfield region. (C) 2016 Elsevier B.V. All rights reserved.</P>

3D crustal velocity structure beneath the broadband seismic array in the Gyeongju area of Korea by receiver function analyses

Lee, D.H.,Lee, J.M.,Cho, H.M.,Kang, T.S. Elsevier Scientific Publishing Co 2016 Tectonophysics Vol.689 No.-

<P>A temporary seismic array was in operation between October 2010 and March 2013 in the Gyeongju area of Korea. Teleseismic records of the seismic array appropriate for receiver function analysis were collected, and selected seismograms were split into five groups based on epicenters-the Banda-Molucca, Sumatra, Iran, Aleutian, and Vanuatu groups. ID velocity structures beneath each seismic station were estimated by inverting the stacked receiver functions for possible groups. The inversion was done by applying a genetic algorithm, whereas surface wave dispersion data were used as constraints to avoid non-uniqueness in the inversion. The composite velocity structure was constructed by averaging the velocity structures weighted by the number of receiver functions used in stacking. The uncertainty analysis for the velocity structures showed that the average of 95% confidence intervals was +/- 0.1 km/s. The 3D velocity structure was modeled through interpolation of 1D composite velocity structures. Moho depths were determined in each composite velocity structure based on the AK135-F S-wave velocity model, and the depths were similar to the H-kappa analysis results. The deepest Moho depth in the study area was found to be 31.9 km, and the shallowest, was 25.9 km. The Moho discontinuity dips in a southwestward direction beneath the area. A low velocity layer was also detected between 4 and 14 km depth. Adaldtic intrusions and/or a high geothermal gradient appear to be the causes of this low velocity layer. The 3D velocity structure can be used to reliably assess seismic hazards in this area. (C) 2016 Elsevier B.V. All rights reserved.</P>