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.

Development of a streamflow-dominated alluvial-fan system in the southwestern margin of Gyeongsang Basin (Lower Cretaceous): implications for initial basin-fill history

이정현,조성권,조형래 한국지질과학협의회 2019 Geosciences Journal Vol.23 No.2

This paper focuses on the initial basin-fill processes in an extensional nonmarine back-arc basin. Sedimentary facies analysis in the southwestern part of the Gyeongsang Basin (Jinju Subbasin), Hapcheon area reveals that the succession consists of four facies assemblages. The assemblage I mainly consists of disorganized conglomerate deposited by flash floods in a streamflowdominated alluvial fan. The assemblage II is dominated by stratified conglomerates and massive, stratified, and cross-stratified sandstones, showing architecture of stacked channels and bars. The assemblage III largely comprises massive, stratified to laminated, and trough cross-stratified sandstones, which are organized into channel-fill and bar-accretion structures. It most likely formed in sandy sinuous rivers. The assemblage IV comprises gray mudstone interlayered with fine sandstone, representing water-logged floodplains. Conglomerate and sandstone bodies formed in the fluvial systems appear to be randomly distributed within floodplain fines, displaying a great lateral and vertical lithologic variation. This study helps understand the complexity of basin-fill history, i.e., switching of fluvial networks in time and space, in contrast to the simple notion of margin-parallel stacking of lithostratigraphic units.

A preliminary geophysical analysis of gravity time-variation in Daejeon

나성호,하규철,문상호,윤희성,이봉주,황세호,신영홍,임무택,박영수 한국지질과학협의회 2019 Geosciences Journal Vol.23 No.2

A gravity time series slightly longer than a month acquired by using a Burris gravimeter at Daejeon, Korea has been analyzed. After removal of linear drift of the gravimeter, the observed data was found to closely match with the body tide prediction except; (i) small phase differences of each main tidal constituents and (ii) existence of long period component in the observed data. The r.m.s. observed gravity tide variation was 62.4 μGal. The overall g-factor was found as 1.165. The values of g-factor for the four main tidal constituents; M2, S2, O1, K1, were found as 1.166, 1.172, 1.168, and 1.142 respectively. The r.m.s. value of ocean tidal loading effect on gravity at Daejeon was 0.95 μGal during the time span. Both the rainfall and the atmospheric pressure change have induced gravity variations of amplitude of about 10 to 1 μGal. The long period component, which is clearly found in the observed spectrum, while not existent in the prediction, is possibly caused by the local ground water variation. It is desirable to have a few number of permanent gravity stations operating in Korean peninsula.

Effect of Tween surfactant components for remediation of toluene-contaminated groundwater

Dal-Heui Lee,Eun-Sik Kim,Ho-Wan Chang 한국지질과학협의회 2005 Geosciences Journal Vol.9 No.3

The objectives of this study were to select potentiallysuitable surfactants that solubilize toluene present as a contami-nant and to determine the effectiveness of toluene removal fromgroundwater by the selected surfactants. Four different surfac-tants of Tween series were chosen based on surfactant types, tox-icity, HLB (Hydrophilic-Lipophilic Balance), CMC (Critical Micellewas not able to solubilize at least 1 mL of toluene and was con-sidered inefective. Tween 40, Tween 60 and Tween 80 were rela-tively good toluene solubilizers. The highest recovery (98%) of thetoluene was obtained using a nonionic surfactant (Twen 60) in thebatch experiments. In Tween series surfactant, the trend of ionicstrength magnitude in sampled groundwater was closely relatedwith that of recovery rates. The ionic strength of aqueous phasehad a strong efect in aqueous activity. The aqueous activity wasdecrease when ionic strength was incremployed may be useful for rapid selection of surfactants and areessential for reducing cost in surfactant-assisted remediation. Also,these selected surfactants are expected to be in practical use forremediation of toluene-contaminated groundwater.

Deep electrical structure and dynamic mechanism of the Yinchuan Graben on the western margin of the Ordos Block

Gang Min,Bingxi Yin,Jinchao Chen,Yong Yang,Huiling Yuan,Yan Wang 한국지질과학협의회 2022 Geosciences Journal Vol.26 No.1

The Yinchuan Graben (YG) is situated in the transitional area between the Tibetan Plateau, Ordos Block (OB) and Alxa Block (AB). The dynamic mechanism of the YG is controversial because of its unique tectonic environment. This study used a broadband magnetotelluric profile constrained by high-density point spacing to reliably obtain the deep electrical structure across the Helanshan Orogenic Belt and YG and to discuss the dynamic mechanism of the YG. The main research results are as follows. 1) Our profile and previous seismic profiles show a low-resistivity and low-velocity body in the deep YG, which extends upward and cuts through the bottom boundary of the upper crust, and all results indicate that this body may be the result of partial melting due to upwelling mantle material. 2) Our profile also shows a high-resistivity anomaly in the middle–lower crust of the Helanshan orogenic belt, which can be interpreted as intrusive rocks formed by the emplacement of partially melted materials. 3) The Helanshan east piedmont fault dips eastward and intersects with the Yellow River fault at the depth of 20 km, and the typical structure of the intersection may be the seismogenic structure of the Pingluo Ms8.0 earthquake. 4) The evolution of the YG may have been controlled by various dynamic mechanisms, and the initial formation of the YG may have resulted from the upwelling mantle material beneath the northern OB; the evolution of the YG since 10 Ma may also have been affected by lateral mantle flow from the Tibetan Plateau to the gap between the OB and AB.

Study on pore evolution characteristics of gas adsorption and desorption in coal under the action of liquid nitrogen

Shuaiqi Dong,Dong Zhao,Lei Li,Xiaowei Li,Tao Chen 한국지질과학협의회 2023 Geosciences Journal Vol.27 No.1

CBM (Coalbed methane) belongs to unconventional natural gas mineral resources. The development and utilization of CBM can greatly reduce the occurrence of mine gas accidents, and how to improve the desorption capacity of CBM has always been a hot issue for researchers. Liquid nitrogen has obvious effect on coal seam cracking and permeability improvement. The purpose of this paper is to study the cracking mechanism of liquid nitrogen on coal, and provide a theoretical basis for using liquid nitrogen freezing and thawing to treat coal so as to improve the production of coalbed methane. Coal samples from Yangquan County, Shanxi Province were pretreated. The pore structure test, microscopic observation test and methane isothermal adsorption test were carried out after the coal samples with different particle sizes were treated with different liquid nitrogen freeze-thaw cycles. According to the experimental data, the evolution law of pores of coal before and after freezing and thawing with liquid nitrogen, the cracking mechanism of coal treated with freezing and thawing with liquid nitrogen, and the optimal number of freezing and thawing cycles with liquid nitrogen are analyzed. The results show that: 1) the evolution characteristics of pore structure of lower coal after liquid nitrogen freezing and thawing are that micropores in coal are promoted to develop into transition pores and mesopores, and mesopores and transition pores develop into macropores and fractures. 2) Through the observation of a metallographic microscope, the expansion degree of coal surface crack is positively correlated with the times of liquid nitrogen freeze-thaw. 3) Liquid nitrogen freeze-thaw treatment has an optimal number of cycles for the cracking degree of coal. With the increase of freeze-thaw cycles, the final adsorption capacity of coal pillar increases first and then decreases; The adsorption capacity of granular coal first increases and then decreases. After reaching freeze-thaw saturation, continued freeze-thaw is not conducive to the increase of adsorption capacity. These research results provide theoretical support for the implementation of liquid nitrogen fracturing and permeability enhancement technology in coal reservoirs.

Assessment of the completeness of earthquake catalogs

노명현 한국지질과학협의회 2019 Geosciences Journal Vol.23 No.2

Since earthquake catalogs are not complete over the magnitude range covered, one should check the completeness of catalogs before any use. To this end, I propose a new method based on the Chi-square test to assess the completeness of earthquake catalogs. The performance of the method was tested using simulated earthquake catalogs. The performance test revealed that the method slightly underestimates the magnitude of catalog completeness, mc at the significance level lower than about 20%. Even in this condition, however, the maximum earthquake, mmax and the Richter-b can still be estimated with acceptable accuracies unless the catalog size is not too small. It is not recommended to increase the significance level for a better estimation of mc because the estimation of mc itself is not an ultimate goal but just a preliminary step to the other studies such as estimation of Richter-b or mmax. The application of the method to the Korean earthquakes showed that an optimal significance level can be chosen from the investigation of the behavior of estimates over a range of the significance level. The performance of the method was also compared with those of the two existing methods using the sub-catalogs of the Japanese earthquakes and the Switzerland earthquakes: the present method gives the largest estimates of mc. The application to real catalogs suggests that an optimal significance level is specific to each catalog. Therefore, it is empathized that one should investigate the behaviors of estimates over a range of the significant level to confirm the stability of estimates.

Controls on sediment storage in wide mountain valleys − a case study from Srinagar (Garhwal) valley, NW Himalaya

Rahul Devrani,Vimal Singh,Hari S. Saini,Syed Ali I. Mujtaba 한국지질과학협의회 2023 Geosciences Journal Vol.27 No.1

The valley-fill deposits in a tectonically active setting play a key role in understanding a river’s response to climatic/tectonic shifts. These deposits are a result of the interaction between the regional and the local processes, which in turn are controlled by tectonics and climate. In this study, valley-fill deposits in the widest reach of the Alaknanda River (a headwater tributary of the Ganga River)—the Srinagar valley—in the northwest Himalaya is investigated for the interaction between, 1) the axial channel (representing regional processes) and local tributary (representing local processes) and 2) axial channel and hillslope (representing local processes). We identified eight levels of terraces and five levels of debris flow surfaces in the Srinagar valley. The terraces are composed of axial river deposits and local stream deposits, whereas debris flow surfaces mainly consist of hillslope deposits. Strath terraces are also present towards the western part of the valley. The 1D resistivity survey and field data show that thick sedimentary cover caps the bedrock in the eastern part of the Srinagar valley. Geochronology data based on optically stimulated luminescence (OSL) datingsuggests that some of the sediments in the Alaknanda valley are older than 80 ± 5 ka. Integration of results shows that the width of the Srinagar valley is controlled structurally by the presence of the North Almora Thrust (NAT), a regional structure present in the Alaknanda River basin. The valley width increases around the central part of the Srinagar valley; this increase in width has provided accommodation space and resulted in thick sedimentation. Major filling and evacuation of the Srinagar valley occurred between 80 ± 5 ka and 8 ± 1 ka. The study demonstrates that the long-term interaction of the axial river, tributaries, and the hillslope processes can often be masked by the younger sediments to give a deceptive morphology and evolution history, if not thoroughly investigated.

Comparison of landslide susceptibility maps using random forest and multivariate adaptive regression spline models in combination with catchment map units

Lei Chu,Liang-Jie Wang,Jiang Jiang,Xia Liu,Kazuhide Sawada,Jinchi Zhang 한국지질과학협의회 2019 Geosciences Journal Vol.23 No.2

Landslide susceptibility mapping (LSM) is a critical tool for mitigating the damages caused by geologic disasters. The selection of map units and mathematical models greatly affects the efficiency of LSM. To obtain the most appropriate combination of map units and mathematical models, four scales of catchment map units (CMUs) were analyzed and random forest (RF) and multivariate adaptive regression spline (MARSpline) models were applied in Gero City, Japan. The percentage of correctly identified landslides and the areas under the relative operating characteristic (ROC) curve were used to evaluate the model performances. The results indicate that the RF model had higher prediction accuracy than the MARSpline model, especially when the size of the CMU was 0.09 km2. A relatively high percentage of landslides fell into the high and very high landslide susceptibility classes (73%) and the lowest percentage of landslides fell into the very low landslide susceptibility classes (0.82%). The prediction-area (P-A) plots indicated that the prediction rates were higher for the RF model than the MARSpline model. The results of this study also suggest that the model accuracy can be increased if the appropriate CMU size is used. Therefore, the potential benefits of using the RF model in combination with the appropriate CMU size should be further explored using additional landslide-conditioning factors and other models.

On the discovery and study of anthraxolite in Triassic plagiorhyolite on the border of the Ural and western Siberia

Yuriy V. Erokhin,Kirill S. Ivanov 한국지질과학협의회 2019 Geosciences Journal Vol.23 No.2

This paper describes the discovery of the first solid bitumen in the plagiorhyolites of Sinara River (Kataysky District, Kurgan Oblast), which are part of the Early Triassic Turin volcanic-sedimentary series on the border of the Ural and Western Siberia. According to data provided by complex analytical studies, they were classified as low anthraxolites. It was found that the formation of solid bitumen in acidic volcanic rocks of Sinara River occurred during the late stage of establishment of the plagiorhyolite body during the formation of chalcedonic (agate) amygdales and secretions. Solidification and sedimentation of anthraxolite from hydrothermal solutions apparently took place under a radical prevalence of the carbonate ion, i.e., in the amygdales during the crystallization of calcite, while in the edge zones of brecciation of plagiorhyolite – of another carbonate, siderite. The source of the substance for the formation of anthraxolite in plagiorhyolites of Sinara River could be both the enclosing sedimentary rocks and deep (mantle) carbohydrate fluids.