Numerical simulation of hydro-mechanical constraints on the geometry of a critically tapered accretionary wedge

송인선,고희재 한국지질과학협의회 2020 Geosciences Journal Vol.24 No.3

A critically tapered active accretionary wedge was simulated using a numerical analysis of plastic slip-line theory to understand the mechanics of morphologic evolution. The concept of critical state soil mechanics was applied to describe the entire wedge area overlying a basal décollement fault. Presuming a condition of two-dimensional plane strain along the compressional direction, we obtained the numerical solution of conjugate plastic slip lines at a critical state of stress defined by the Coulomb yield criterion. The velocity vectors were obtained by applying the associate flow rule with the boundary conditions at the upper surface of the wedge. Finally, the detachment was determined from the effective stress condition inside the wedge and the sliding friction coefficient along the fault. Our numerical simulations demonstrate that the morphology of a critically tapered wedge is dependent on the frictional strengths of both the wedge materials and the basal fault. The critical taper angle decreases with increasing internal friction angle and decreasing basal friction coefficient. The results also revealed that the pore pressure controls the morphology of the accretionary wedge for cohesive sediments but not for non-cohesive materials. The effect of pore pressure on the morphology of a critically tapered accretionary wedge becomes more significant as the cohesion increases. Assuming that the cohesion is very low, we could infer the ranges of strengths that most observed wedge geometry data have 0.3–0.6 for the basal friction coefficient and ~35–45° for the internal friction angle of the wedge materials.

Trace element variation in olivine in the Skaergaard intrusion: petrologic implications

박병준,장윤득,이인성,김정진,추창오 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

Olivine from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard intrusion was analyzed to examine trace element variations. In general, olivine from all three series shows similar trends in trace elements with differentiation. Ba, Cs, Rb, and Sr, acted as excluded trace elements and the transition elements, Co, Cr, Cu, Ni, Sc, Zn, and V, are included in Skaergaard olivine. Owing to the high DNi in olivine, Ni decreases with differentiation in both olivine and the calculated Skaergaard magma. The low concentration of Cr in olivine is attributable to early depletion of Cr by pyroxene crystallization. Decreases in the abundances of V and Co are related to the onset of magnetite crystallization and the separation of immiscible Co-rich sulphide. In general, calculated partition coefficients (Di) for Skaergaard olivine show similar trends in the LS, UBS, and MBS. Calculated Di’s for Sc, Cr, and Ga in olivine, progressively increase, and Di’s for Zr, Nb, Hf, Cs, Ta, Th, and U progressively decrease, with differentiation. The systematic variation in trace element contents of Skaergaard olivine during differentiation would appear to preclude the possibility of any significant injection of new magma into the chamber during differentiation. The Ni content in Skaergaard olivine systematically decreases with differentiation as would be expected if the intrusion remained a closed system. In fact, no abrupt increases are observed in the abundances of any of the transition metal in olivine during differentiation.

Salinity effects on chlorpyrifos degradation and phosphorus fractionation in reclaimed coastal tideland soils

Eui-Yong Yun,노희명,Goon-Taek Lee,최우정 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

We examined the effect of soil salinity on the degradation of chlorpyrifos and the residual effect of chlorpyrifos and its metabolites on soil P fractionation during 60-day aerobic incubation. A sandy loam soil (Typic Psammaquents) was collected from the Daeho reclaimed tideland and two-thirds of the soil was applied with Na salt to get three different soil salinity levels: 4.6 (low, EL), 9.7 (medium, EM), and 14.4 (high, EH) dS m–1. Estimated half-lives for chlorpyrifos degradation were 7.1 in EL, 10.0 in EM and 16.9 days in EH soils. During the degradation of chlorpyrifos in soil, microbial activity decreased by increasing soil salinity and its inhibitory effect increased with time. In contrast, the addition of chlorpyrifos did not inhibit soil alkaline phosphatase (SAP) activity, which was higher in EH than in control soils. Chlorpyrifos added at a rate of 5.0 mg a.i. kg–1 dry soil did not affect the distribution pattern of P fractions in control soils. Both an increase in soil salinity and soil sterilization increased the Ca-bound P fraction and decreased the occluded Fe + Al-bound P fraction with a significant interaction between soil salinity and sterilization. With time, the Ca-bound P fraction increased and organic- and occluded Fe + Al-bound P fractions decreased, while total-P, available-P, and non-occluded + adsorbed P fraction remained unchanged. Particularly, organic-P was mineralized more in EH than in control soils and the Ca-bound P fraction contained the highest inorganic P released. Mineralization of organic P and partitioning of released P in the recalcitrant Ca-bound P fraction increased by increasing soil salinity, while available P fraction remained unchanged, suggesting that the addition of chlorpyrifos at the currently recommended dosage level did not seem to considerably affect the available P fraction with low P leaching potential to waterways.

Towards a new potential field theory of fractal objects

Mostafa E. Mostafa 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

The Potential Field Anomaly (PFA) data of the self similar Fractal Objects (FOs) include gravity and magnetic fields and potentials along with the related derivatives. These elements are calculated on grids due to buried FOs at different fractal orders. The objects have variable physical property distributions; while in magnetic, the orientation and magnitude of polarization or earth magnetic field is arbitrary. Using the structural index as Universal Fractal Order Invariant Measure, one of the contributions of this work is expressing the elements of the PFA data at any measuring point on a grid as geometric sequences in terms of the fractal order. We found that the common ratio of the sequences is equal to the Fractal Mass Ratio (FMR), a physical quantity characterizing the object. Therefore, we can interpolate the PFA data backward or forward from one fractal order to the other. This in turn allows us to directly calculate PFA data of FOs from the zero order objects equivalent to the solid sources or initiators. We conclude that the patterns of PFA data due to a self similar FO are scale-invariant and reflect the nature of this object. We express the FMR of a FO in a new equation describing the difference between the topological and fractal dimensions in terms of a linear scale.

Fracture imaging using Image Point transform and midpoint imaging of RVSP data

이창현,박권규,Toshiyuki Matsuoka,Toshifumi Matsuoka 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

An Image Point (IP) transform integrates along a hyperbolic path and converts data to a point in the IP domain so that the reflection events are accumulated to the image point for given sources in the IP domain. In addition, the image point by definition is uniquely determined and the midpoints between sources and the image points always lie on the reflector surface. Exploiting these two facts, the image point transform technique for Vertical Seismic Profile (VSP) to reverse VSP (RVSP) data is expanded to suggest a new method called the midpoint imaging method for imaging reflectors such as fractures. Applying the transform with synthetic data for Two-dimensional (2-D) models with single reflector and two reflectors, we confirm and demonstrate that the reflection events are clearly identifiable in the IP domain. The midpoint imaging method can successfully image the reflector surfaces and the dip information even with only two shot records in a simple case.

Grain size dependent rheology on the mobility of debris flows

Sueng Won Jeong 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

When dealing with natural geo-hazards it is important to understand the influence of flow characteristics on the submarine landslides, because the flow characteristics of fine-grained soils are most likely governed by the rheological properties. It may further relate to the mobility analysis of debris flow. Geotechnical and rheological investigation on the clay-rich materials were compared with those of silt-rich materials. For the examined fine-grained soils, the flow behavior depends on the volumetric concentration of solid and surface phenomenon, particularly for non-swelling materials. Assuming that the debris flow materials behave as the Bingham fluid, a possible relationship between geotechnical and rheological properties are proposed. From an empirical relationship between yield stress and plastic viscosity, a comparison emphasizes the grain size effect on the rheology of geomaterials encountered in various environments. The larger the particle size involved in the matrix, the lower the value of viscosity which is in the order of 10 times. Debris flow mobility with respect to rheology concerning the large particle size is emphasized.

Lessons from practice in the assessment and remediation of contaminated ground water

Franklin W. Schwartz,Eung Seok Lee,Yongje Kim 한국지질과학협의회 2008 Geosciences Journal Vol.12 No.2

The famous American humorist Mark Twain once wrote “don’t let school interfere with your education”. This paper builds on this theme by examining important lessons that come from work on practical problems of ground-water contamination and remediation in Canada and the United States. We draw lessons from the interesting features of the studies, and mistakes in execution, which have had important consequences. The first case study from Edmonton, Alberta Canada examines a problem of ground-water contamination due to leakage of water contaminated by 2,4-D and other contaminants from a small storage pond. This study highlights the problems related to an inadequate geologic understanding of a site, misunderstandings concerning the advantages and limitations of key tools for site investigation, and how projects can benefit from early and ongoing peer reviews. The second case study examines a problem of sewage contamination related to a deep tunnel system in Milwaukee Wisconsin. This system is designed to store surface-water overflows from an old, combined sewer system. This case study highlights the difficulties in working on unique problems without an effective conceptual hydrogeologic model, the need to always be concerned about the quality of chemical data, and the necessity of being alert to behaviors beyond typical experience. The lessons coming from these case studies have important implications for remedial work being undertaken in Korea and regulatory agencies with oversight of the projects.

Regional moment tensor determination in the southern Korean Peninsula

이준기,김성룡 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

The Korean Peninsula is a quiet region in terms of seismic activity. Although there are historical records of many large earthquakes that caused severe damage, such major events have not been frequent in recent times, with just a few small- and moderate-sized earthquakes (M≥3) occurring each year. Therefore, the precise analysis of small- to moderate-sized earthquakes is important to understand the characteristics of earthquakes in this region. Broadband seismometers have been deployed in South Korea since 1998 in continually increasing numbers. The current broadband seismic network in Korea is quite dense, and its spatial distribution is nearly homogeneous. This network provides high-quality waveform data and makes moment tensor determination of moderate-sized earthquakes possible. We have inverted regional three-component broadband seismic data for the source parameters of 26 earthquakes ranging in size from Mw 3.2 to 5.0. The moment tensor solutions obtained from time domain waveform inversion show that most of these events have strike-slip faulting mechanisms and that their pseudo P-axis directions are predominantly ENE-WSW, with an average azimuth of 72.77°. The source parameters and maximum horizontal shear stress directions obtained in this study can provide basic information that can act as a foundation for detailed seismotectonic studies and seismic hazard analyses.

Seasonal variations of particle fluxes in the northeastern equatorial Pacific during normal and weak El Niño periods

Hyung Jeek Kim,Kiseong Hyeong,Chan Min Yoo,Sang-Bum Chi,김부근,Dongseon Kim 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

A moored time-series sediment trap was deployed at the Korea Ocean Research and Development Institute Long-term Monitoring (KOMO) Station in the northeastern equatorial Pacific from July 2003 to June 2005. A weak El Ni–o event was recorded from June 2004 to February 2005, and normal conditions were observed in the remaining periods. The normal period was divided into two seasons based on the cycles of environmental properties in the surface ocean: cold (December–May) and warm (June–November) season. During the normal period, the total mass flux was 1.7 times higher in the cold season than in the warm season. Particularly, the CaCO3 flux was nearly three times higher in the cold season. The enhanced CaCO3 flux in the cold season was attributed to an increased foraminiferal flux, which may have influenced the seasonal variability of the total mass flux at the KOMO station. The enhanced foraminiferal flux during the cold season may have been caused by the environmental changes of the surface ocean in response to wind-driven mixing resulting in supply of subsurface nutrient-enriched water. Particle fluxes during the weak El Ni–o period were lower by 30% than those during the normal period, which was consistent with previous findings in the central and eastern equatorial Pacific.

Cenozoic tectonic evolution in the western Qaidam Basin inferred from subsurface data

Yadong Wang,Junsheng Nie,Tao Zhang,Guoqiang Sun,Xin Yang,Yuhu Liu,Xingwang Liu 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

It is an agreement that collision of Indian and Asian plates causes uplift of the Tibet Plateau. However, great controversy exists about uplifting history and mechanisms of the Tibetan Plateau. Uplift history of the Tibetan Plateau has been well recorded in the Qaidam Basin, located in the northern margin of the Tibet Plateau. In this paper, we recognize and classify faults recorded by subsurface seismic data in the western Qaidam Basin. In addition, we reconstruct the Cenozoic deformation history of the Qaidam Basin based on balanced section of 5 seismic profiles. The results indicates that 1) Faults in the western Qaidam Basin can be classified as growth faults and non-growth faults, and the growth faults could be divided into three subcategories. 2) According to timing and manner of fault activities, faults and strata in the western Qaidam Basin could be divided into two structural layers: the lower (Lulehe Fm-Xia Youshashan Fm) and the upper (Shang Youshashan Fm-Qigequan Fm) layer. 3) The western Qaidam Basin has experienced two intensive tectonic deformations: the first phase occurs at 43.8−22 Ma (Middle Eocene−Early Miocene), which reached peak at 31.5 Ma (Early Oligocene); the second phase occurred between 14.9 and 0 Ma (Middle Miocene−Present), and the second phase is stronger than the first phase. Recognizing early fault activities confirm previous results that northern Tibet has sensed collision between the India and the Asia shortly after the collision. However, our results here emphasize that the northern Tibet has experienced another phase of shortening and uplift in the late Neogene. It was the two-stage tectonic activities that work together to produce the current Tibetan Plateau.