2014 Vol. 39, No. 4
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2014, 39(4): 375-386.
doi: 10.3799/dqkx.2014.036
Abstract:
Through field investigation, microscopic analyses of oriented samples and zircon LA-MC-ICPMS U-Pb dating, granitic mylonites of the ducticle shear belt in Hanwula area have been studied, for their kinematic characteristics and ages. In these rocks, main rock-forming minerals (feldspar and quartz) all show the typical characteristics of plastic deformation and dynamic crystallization, with typical fabrics such as "σ", "δ" and "S-C" structures. Averages of 34 coupled attitudes of mineral lineation and shearing foliation are 338°∠24° and 303°∠17°. All the above indicate the left-lateral slip in this belt. The zircon U-Pb dating data suggest that these granitic mylonites are formed in Early Cretaceous with detail ages of 135.9±1.2 Ma to 130.0±0.5 Ma and the ductile shear belt is covered by glutenite of the Lower Cretaceous Damoguaihe Group (Valanginian-Hauterivian). Thus, we consider that Dong Ujimqin area had experienced regional left-lateral extensional structure movement in Early Cretaceous. This study filled the blank of petrology and geochronology study of the ductile shear belt in Hanwula and enhanced the understanding of the Early Cretaceous crust tectonic evolution in Dong Ujimqin.
Through field investigation, microscopic analyses of oriented samples and zircon LA-MC-ICPMS U-Pb dating, granitic mylonites of the ducticle shear belt in Hanwula area have been studied, for their kinematic characteristics and ages. In these rocks, main rock-forming minerals (feldspar and quartz) all show the typical characteristics of plastic deformation and dynamic crystallization, with typical fabrics such as "σ", "δ" and "S-C" structures. Averages of 34 coupled attitudes of mineral lineation and shearing foliation are 338°∠24° and 303°∠17°. All the above indicate the left-lateral slip in this belt. The zircon U-Pb dating data suggest that these granitic mylonites are formed in Early Cretaceous with detail ages of 135.9±1.2 Ma to 130.0±0.5 Ma and the ductile shear belt is covered by glutenite of the Lower Cretaceous Damoguaihe Group (Valanginian-Hauterivian). Thus, we consider that Dong Ujimqin area had experienced regional left-lateral extensional structure movement in Early Cretaceous. This study filled the blank of petrology and geochronology study of the ductile shear belt in Hanwula and enhanced the understanding of the Early Cretaceous crust tectonic evolution in Dong Ujimqin.
Detrital Zircon Age of Stratigraphic Sinian-Cambrian in East Guizhou and Its Geological Significance
2014, 39(4): 387-398.
doi: 10.3799/dqkx.2014.037
Abstract:
Lithology, ore-bearing potential and contact relation in Zhuxi stratigraphic section are studied in detail. 84 groups of effective ages concentrated in 864-742 Ma are obtained from U-Pb dating of detrital zircon by LA-ICP-MS in feldspar lithic sandstone in the black shales, which indicates the presence of large-scale magmatic activity of the East Guizhou at the Neoproterozoic. All zircon U-Pb ages indicate that sediments within Lower Cambrian black shales in East Guizhou are mainly from the Jiangnan orogen. Metal elements such as vanadium are derived from the basic-ultrabasic rocks. The Xuefeng movement peeled the top of ore-bearing rock off the basic-ultrabasic rocks, pouring a great amount of metal into the ocean. The anoxic submarine caused the death of algae whose bodies were rich of metallogenic elements, which introduced vanadium into the sediments along with the organic matter. The mineralization occurred in the Liuchapo and Jiumenchong formations that were favorable to sedimentation of metals. 536 Ma is the final depositional time of the Jiumenchong Formation, and maximum depositional age of the Liuchapo Formation. The boundary between Sinian and Cambrian is located in the upper siliceous rocks of the Liuchapo Formation. Lithology deformation, metamorphism, and unconformity with other strata of the Qingshuijiang Formation are the geological responses of the Xuefeng movement.
Lithology, ore-bearing potential and contact relation in Zhuxi stratigraphic section are studied in detail. 84 groups of effective ages concentrated in 864-742 Ma are obtained from U-Pb dating of detrital zircon by LA-ICP-MS in feldspar lithic sandstone in the black shales, which indicates the presence of large-scale magmatic activity of the East Guizhou at the Neoproterozoic. All zircon U-Pb ages indicate that sediments within Lower Cambrian black shales in East Guizhou are mainly from the Jiangnan orogen. Metal elements such as vanadium are derived from the basic-ultrabasic rocks. The Xuefeng movement peeled the top of ore-bearing rock off the basic-ultrabasic rocks, pouring a great amount of metal into the ocean. The anoxic submarine caused the death of algae whose bodies were rich of metallogenic elements, which introduced vanadium into the sediments along with the organic matter. The mineralization occurred in the Liuchapo and Jiumenchong formations that were favorable to sedimentation of metals. 536 Ma is the final depositional time of the Jiumenchong Formation, and maximum depositional age of the Liuchapo Formation. The boundary between Sinian and Cambrian is located in the upper siliceous rocks of the Liuchapo Formation. Lithology deformation, metamorphism, and unconformity with other strata of the Qingshuijiang Formation are the geological responses of the Xuefeng movement.
2014, 39(4): 399-410.
doi: 10.3799/dqkx.2014.038
Abstract:
In order to confirm the intensity of alongslope current in lacustrine basin and its effect on the sedimentation of delta front, the modern sedimentation of fault-depression lacustrine basin (Daihai Lake) in Inner Mongolia, North China has been explored in this study. Based on analysis of satellite images, on-site wave observation and floating ball measurement, trench excavation, sedimentary characteristics description, sample analysis, and sandbodies comparison at the framework section, it can be concluded that the steep side of Daihai lacustrine basin was dominated by the process of downslope current, where distributary channels and river mouth bar were developed with good connectivity of sandbodies in vertical, forming large set of massive sandbodies; the interaction between downslope and alongslope currents can be found in the delta front of the gentle side, where the wave-genetic lithofacies was well-developed with sandy sediments sheeted seriously, coarsening upward rhythm was not obvious in vertical sequence, sandbodies in the river mouth were distributed asymmetrically. Sandbodies in the upstream direction of alongslope drifting were distributed continuously and superposed of multi-layers, with good connectivity in lateral; in the downstream direction, muddy intercalations were well-developed, sandbodies were distributed isolatedly with strong heterogeneity.
In order to confirm the intensity of alongslope current in lacustrine basin and its effect on the sedimentation of delta front, the modern sedimentation of fault-depression lacustrine basin (Daihai Lake) in Inner Mongolia, North China has been explored in this study. Based on analysis of satellite images, on-site wave observation and floating ball measurement, trench excavation, sedimentary characteristics description, sample analysis, and sandbodies comparison at the framework section, it can be concluded that the steep side of Daihai lacustrine basin was dominated by the process of downslope current, where distributary channels and river mouth bar were developed with good connectivity of sandbodies in vertical, forming large set of massive sandbodies; the interaction between downslope and alongslope currents can be found in the delta front of the gentle side, where the wave-genetic lithofacies was well-developed with sandy sediments sheeted seriously, coarsening upward rhythm was not obvious in vertical sequence, sandbodies in the river mouth were distributed asymmetrically. Sandbodies in the upstream direction of alongslope drifting were distributed continuously and superposed of multi-layers, with good connectivity in lateral; in the downstream direction, muddy intercalations were well-developed, sandbodies were distributed isolatedly with strong heterogeneity.
2014, 39(4): 411-420.
doi: 10.3799/dqkx.2014.039
Abstract:
In order to analyze the genetic relationship between diagenesis and reservoir porosity evolution process and the formation process of physical property difference in different diagenetic facies, diagenesis types and facies of Chang 8 formation are studied in Huachi, Ordos basin. Furthermore, porosity evolutions of different diagenesis facies are simulated quantitatively, and their pathways are analyzed. According to the diagenesis characteristics, the reservoirs can be divided into four diagenesis facies, namely, grain-coating chlorite cementation, corrosion of unstable components, intense compaction with packing and dense carbonate cementation. COPL (compactional porosity loss) of them are 17.6%, 20.5%, 25.8% and 11.4% respectively; CEPL (cementational porosity loss) by early quartz overgrowth, grain-coating chlorite, and carbonate are 4.5%, 4.9%, 5.6% and 24.9% respectively; CRPI (corrosional porosity increase) are 1.4%, 2.3%, 0.2% and 0 respectively; CEPL by late pore-filling chlorite, kaolinite, illite, ferrocalcite and ferrodolomite are 7.8%, 9.7%, 3.2% and 0 respectively. The porosity evolution pathways of different diagenesis facies show that the differences of OP (original porosity) caused by the sediment components and structures are tiny, but the diagenesis types and grades are controlled by sedimentary processes to a great extent. It is concluded that the reservoirs with diagenesis facies have different diagenetic changes and porosity evolution pathways, which results in differences of components, structures and physical properties.
In order to analyze the genetic relationship between diagenesis and reservoir porosity evolution process and the formation process of physical property difference in different diagenetic facies, diagenesis types and facies of Chang 8 formation are studied in Huachi, Ordos basin. Furthermore, porosity evolutions of different diagenesis facies are simulated quantitatively, and their pathways are analyzed. According to the diagenesis characteristics, the reservoirs can be divided into four diagenesis facies, namely, grain-coating chlorite cementation, corrosion of unstable components, intense compaction with packing and dense carbonate cementation. COPL (compactional porosity loss) of them are 17.6%, 20.5%, 25.8% and 11.4% respectively; CEPL (cementational porosity loss) by early quartz overgrowth, grain-coating chlorite, and carbonate are 4.5%, 4.9%, 5.6% and 24.9% respectively; CRPI (corrosional porosity increase) are 1.4%, 2.3%, 0.2% and 0 respectively; CEPL by late pore-filling chlorite, kaolinite, illite, ferrocalcite and ferrodolomite are 7.8%, 9.7%, 3.2% and 0 respectively. The porosity evolution pathways of different diagenesis facies show that the differences of OP (original porosity) caused by the sediment components and structures are tiny, but the diagenesis types and grades are controlled by sedimentary processes to a great extent. It is concluded that the reservoirs with diagenesis facies have different diagenetic changes and porosity evolution pathways, which results in differences of components, structures and physical properties.
2014, 39(4): 421-430.
doi: 10.3799/dqkx.2014.040
Abstract:
Relationships of the vegetation and active layer are the foci of global change study since the interaction between the vegetation and hydrothermal processes of the active layer are important part of Qinghai-Tibet Plateau permafrost ecosystems, and they are liable to react on the global warming and anthropogenic activities. Based on the analysis of soil temperature and unfrozen water content of the active layer, surveys of vegetation, the influences of vegetation on the variations of hydrothermal processes of the active layer in the source area of the Yellow River (SAYR) are studied. Preliminary results show that variations of unfrozen water content and soil temperature of the active layer are significantly affected by vegetation cover, above-ground biomass, and dominant species. The distribution unfrozen water content is consistent with the vegetation coverage. The offsets between the ground surface temperature and air temperature, which are affected by the vegetation cover, are higher in winter-time and lower in summer-time. The beginning of freezing is latter, the finishing of freezing is earlier, and the duration of freezing is longer for those sites with higher vegetation cover. With lowering of vegetation cover, zones with higher content of unfrozen water shift towards the bottom of the active layer. This study reveals the effects of variations of vegetation on soil temperature and unfrozen water content of the active layer for warm, and high-altitude permafrost in the SAYR, and will also facilitate the research and protection of cold eco-environments in the SAYR.
Relationships of the vegetation and active layer are the foci of global change study since the interaction between the vegetation and hydrothermal processes of the active layer are important part of Qinghai-Tibet Plateau permafrost ecosystems, and they are liable to react on the global warming and anthropogenic activities. Based on the analysis of soil temperature and unfrozen water content of the active layer, surveys of vegetation, the influences of vegetation on the variations of hydrothermal processes of the active layer in the source area of the Yellow River (SAYR) are studied. Preliminary results show that variations of unfrozen water content and soil temperature of the active layer are significantly affected by vegetation cover, above-ground biomass, and dominant species. The distribution unfrozen water content is consistent with the vegetation coverage. The offsets between the ground surface temperature and air temperature, which are affected by the vegetation cover, are higher in winter-time and lower in summer-time. The beginning of freezing is latter, the finishing of freezing is earlier, and the duration of freezing is longer for those sites with higher vegetation cover. With lowering of vegetation cover, zones with higher content of unfrozen water shift towards the bottom of the active layer. This study reveals the effects of variations of vegetation on soil temperature and unfrozen water content of the active layer for warm, and high-altitude permafrost in the SAYR, and will also facilitate the research and protection of cold eco-environments in the SAYR.
2014, 39(4): 431-442.
doi: 10.3799/dqkx.2014.041
Abstract:
The Emeishan basalts are typical rocks of the Yangtze River basin because of their unique origin, the broad distribution and simplicity of identification, which can be used as an indicator of tracing the source of the upstream Yangtze sediments after cutting of the Three Gorges. The gravel and debris (between 1-2 mm) of basalts in the sediments of the middle and upper reaches of the Yangtze terraces(above Fuling City) and the Quaternary sediments of the Zhoulao core in the Jianghan Plain are mainly studied in the article. The gravels of the lithic basalts are abundant in the floodplain sediments and the terrace deposits along the upper reach of the river (upstream of Fuling City), and are rare along the downstream of the Three Gorges. The basalt debris are very common on the terraces of the upper reach of the Yangtze River, and are also evaluated by the loose sediments of the Zhoulao core. These lithic debris were identified as the Emeishan basalts by LA-ICP-MS and appear at the core depth of 156 m (up). This line of change (156 m depth) dated of about 1.7 Ma attests a provenance change. Meanwhile, a wide drainage basin similar as today's dimension of the Yangtze River had formed and the Three Gorges had formed around 1.7 Ma.
The Emeishan basalts are typical rocks of the Yangtze River basin because of their unique origin, the broad distribution and simplicity of identification, which can be used as an indicator of tracing the source of the upstream Yangtze sediments after cutting of the Three Gorges. The gravel and debris (between 1-2 mm) of basalts in the sediments of the middle and upper reaches of the Yangtze terraces(above Fuling City) and the Quaternary sediments of the Zhoulao core in the Jianghan Plain are mainly studied in the article. The gravels of the lithic basalts are abundant in the floodplain sediments and the terrace deposits along the upper reach of the river (upstream of Fuling City), and are rare along the downstream of the Three Gorges. The basalt debris are very common on the terraces of the upper reach of the Yangtze River, and are also evaluated by the loose sediments of the Zhoulao core. These lithic debris were identified as the Emeishan basalts by LA-ICP-MS and appear at the core depth of 156 m (up). This line of change (156 m depth) dated of about 1.7 Ma attests a provenance change. Meanwhile, a wide drainage basin similar as today's dimension of the Yangtze River had formed and the Three Gorges had formed around 1.7 Ma.
2014, 39(4): 443-450.
doi: 10.3799/dqkx.2014.042
Abstract:
Thermoluminescence from marine carbonate is found to be sensitively responding to the earth climate change of glacial-interglacial stage, which implies a novel proxy for paleoceanography. Yet it is to be confirmed by studies as to whether it is suitable for the terrestrial sediments. In this study, thermoluminescence and oxygen isotopic composition have been carried out on stalagmite CX-1 from Chuanxin cave, Qingjiang valley, Hubei Province, China, to determine the physical links between the climate and thermoluminescence intensity. It is found that thermoluminescence glow in air environment is mainly contributed by chemiluminescence derived from oxidation of the organic matter from stalagmite carbonates. Therefore, thermoluminescence is closely related to organic matter content of stalagmite derived from the degradation of organic matter in soil and the degradation rate is related to soil temperature controlled by the insolation. The variations of TL (total luminescence intensity) and δ18O show similar features which can reasonably be associated with climate change. Therefore, stalagmite thermoluminescence can act as a new proxy for terrestrial paleoenvironment.
Thermoluminescence from marine carbonate is found to be sensitively responding to the earth climate change of glacial-interglacial stage, which implies a novel proxy for paleoceanography. Yet it is to be confirmed by studies as to whether it is suitable for the terrestrial sediments. In this study, thermoluminescence and oxygen isotopic composition have been carried out on stalagmite CX-1 from Chuanxin cave, Qingjiang valley, Hubei Province, China, to determine the physical links between the climate and thermoluminescence intensity. It is found that thermoluminescence glow in air environment is mainly contributed by chemiluminescence derived from oxidation of the organic matter from stalagmite carbonates. Therefore, thermoluminescence is closely related to organic matter content of stalagmite derived from the degradation of organic matter in soil and the degradation rate is related to soil temperature controlled by the insolation. The variations of TL (total luminescence intensity) and δ18O show similar features which can reasonably be associated with climate change. Therefore, stalagmite thermoluminescence can act as a new proxy for terrestrial paleoenvironment.
2014, 39(4): 451-461.
doi: 10.3799/dqkx.2014.043
Abstract:
In order to study the carbon burial rate of various paleo-environments of Holocene strata in the Yellow River delta (YRD), one 30.3-meter-sediment core, obtained from YRD in 2007, is analyzed for sedimentary characteristics, water content, organic carbon, total carbon and nutritional components in this paper. By analyzing the strata sequence, the Holocene strata are divided into eight kinds of sedimentary environments. Comprehensive analysis method of historical geography and sedimentary geology is used to determine the precise depositional age of the modern Yellow River delta (MYRD) while pre-MYRD age is deduced. The carbon burial rates of various sedimentary environments are calculated based on the certain depositional age. The results show that the nutrient elements are well related to total carbon and organic carbon, and sediment rate is the main controlling factor of the accretion rate of organic carbon and total carbon. Although the carbon concentration is remarkably lower in the sediments, the average carbon accretion rate reaches 1 331 g/(m2·a) because of high sedimentation rate, which is significantly higher than that of the wetlands with high carbon concentration. It is concluded that the MYRD is likely the major sink of carbon.
In order to study the carbon burial rate of various paleo-environments of Holocene strata in the Yellow River delta (YRD), one 30.3-meter-sediment core, obtained from YRD in 2007, is analyzed for sedimentary characteristics, water content, organic carbon, total carbon and nutritional components in this paper. By analyzing the strata sequence, the Holocene strata are divided into eight kinds of sedimentary environments. Comprehensive analysis method of historical geography and sedimentary geology is used to determine the precise depositional age of the modern Yellow River delta (MYRD) while pre-MYRD age is deduced. The carbon burial rates of various sedimentary environments are calculated based on the certain depositional age. The results show that the nutrient elements are well related to total carbon and organic carbon, and sediment rate is the main controlling factor of the accretion rate of organic carbon and total carbon. Although the carbon concentration is remarkably lower in the sediments, the average carbon accretion rate reaches 1 331 g/(m2·a) because of high sedimentation rate, which is significantly higher than that of the wetlands with high carbon concentration. It is concluded that the MYRD is likely the major sink of carbon.
2014, 39(4): 462-472.
doi: 10.3799/dqkx.2014.044
Abstract:
In order to understand the geochemical processes of key minerals in the supercritical CO2 -water-rock system, we establish a one-dimensional vertical model to study the effect of chlorite contents in cap rock of 3%, 9% and 15% on CO2 -water-rock interaction by numerical simulation software of TOUGHREACT based on the basic geological conditions of deep saline aquifer in Ordos basin, China. It is found that the mineral composition and permeability of the caprock change greatly after CO2 broke into the caprock. It is not favorable for CO2 storage when chlorite volume fraction is 3% because of the increasing permeability in 5 000 years. The permeability of caprock increases first and then decreases when chlorite volume fraction is 9% and 15%, resulting in self-sealing which facilitates the sequestration of CO2. The results show that the dissolution of chlorite provides Mg2+, Fe2+ and AlO2- for precipitation of calcium montmorillonite, ankerite, dawsonite, and magnesite. The higher the chlorite content, the greater the amount of CO2 mineralization capture, and the more obvious the effect of caprock self-sealing with the maximum caprock permeability decrease of 10%. This study provides a theoretical basis for long-term geological storage of CO2 and its stability evaluation.
In order to understand the geochemical processes of key minerals in the supercritical CO2 -water-rock system, we establish a one-dimensional vertical model to study the effect of chlorite contents in cap rock of 3%, 9% and 15% on CO2 -water-rock interaction by numerical simulation software of TOUGHREACT based on the basic geological conditions of deep saline aquifer in Ordos basin, China. It is found that the mineral composition and permeability of the caprock change greatly after CO2 broke into the caprock. It is not favorable for CO2 storage when chlorite volume fraction is 3% because of the increasing permeability in 5 000 years. The permeability of caprock increases first and then decreases when chlorite volume fraction is 9% and 15%, resulting in self-sealing which facilitates the sequestration of CO2. The results show that the dissolution of chlorite provides Mg2+, Fe2+ and AlO2- for precipitation of calcium montmorillonite, ankerite, dawsonite, and magnesite. The higher the chlorite content, the greater the amount of CO2 mineralization capture, and the more obvious the effect of caprock self-sealing with the maximum caprock permeability decrease of 10%. This study provides a theoretical basis for long-term geological storage of CO2 and its stability evaluation.
2014, 39(4): 473-480.
doi: 10.3799/dqkx.2014.045
Abstract:
For the issue that borehole camera technology only is applied to the vertical borehole currently, the calculation process for discontinuity orientation in inclined borehole is presented in this paper. With the circle centre of drill orifice as the origin, the left-handed Cartesian coordinate system is constructed and the precise analytic formulas for discontinuity orientation in inclined borehole are obtained. On this basis, the IDOIB software is developed using C# programming language. The validity of analytic formulas and program are verified by the principle of borehole camera technology. In addition, the PVC pipe experiments are conducted to examine the reliability of borehole camera technology for measurement of discontinuity orientation. Results show that: (1) For vertical holes, absolute errors of dip directions range from -3° to 4°, and those of dips range from -1.5° to 1.0°. The technology is reliable for measurement of discontinuity orientation in vertical boreholes, and it can satisfy the needs of rock engineering. (2) For inclined PVC pipes with trend of 270° and plunge of 25°, the reliability degree of discontinuity orientation is 0.10; while inclined PVC pipes with trend of 176° and plunge of 60°, the reliability degree of discontinuity orientation is 0.67. The difference of pipe plunge results in the reliability difference of discontinuity orientation. (3) The reliability of discontinuity orientation should be high (the reliability degree is no less than 0.8) for general rock engineering, so the plunge of inclined borehole should be no less than 71°.
For the issue that borehole camera technology only is applied to the vertical borehole currently, the calculation process for discontinuity orientation in inclined borehole is presented in this paper. With the circle centre of drill orifice as the origin, the left-handed Cartesian coordinate system is constructed and the precise analytic formulas for discontinuity orientation in inclined borehole are obtained. On this basis, the IDOIB software is developed using C# programming language. The validity of analytic formulas and program are verified by the principle of borehole camera technology. In addition, the PVC pipe experiments are conducted to examine the reliability of borehole camera technology for measurement of discontinuity orientation. Results show that: (1) For vertical holes, absolute errors of dip directions range from -3° to 4°, and those of dips range from -1.5° to 1.0°. The technology is reliable for measurement of discontinuity orientation in vertical boreholes, and it can satisfy the needs of rock engineering. (2) For inclined PVC pipes with trend of 270° and plunge of 25°, the reliability degree of discontinuity orientation is 0.10; while inclined PVC pipes with trend of 176° and plunge of 60°, the reliability degree of discontinuity orientation is 0.67. The difference of pipe plunge results in the reliability difference of discontinuity orientation. (3) The reliability of discontinuity orientation should be high (the reliability degree is no less than 0.8) for general rock engineering, so the plunge of inclined borehole should be no less than 71°.
2014, 39(4): 481-491.
doi: 10.3799/dqkx.2014.046
Abstract:
The geodynamic mechanism of the great April 11, 2012, Mw8.6 earthquake off the west coast of the northern Sumatra, Indonesia is the difference of the crust northward movements in the west of Sunda trench. Furthermore, the results of static stress calculation show that the shock was triggered by the 2004 devastating Mw9.1 shock and the 2005 event of Mw8.6. There was no earthquake with Mw≥7.0 occurred in the seismogenic fault of the 2012 Sumatra shock from 1900 to 2011. In addition, a seismic gap with Mw≥5.0 was formed around the focal region and the gap was disturbed after the 2004 Sumatra earthquake occurred. Six years, four and a half years and three months before the great 2012 event, there were three groups of foreshocks respectively, and the Mw7.2 event, the direct pre-earthquake, occurred three months prior to the Mw8.6 shock, is the most significant one in the three groups of foreshocks.
The geodynamic mechanism of the great April 11, 2012, Mw8.6 earthquake off the west coast of the northern Sumatra, Indonesia is the difference of the crust northward movements in the west of Sunda trench. Furthermore, the results of static stress calculation show that the shock was triggered by the 2004 devastating Mw9.1 shock and the 2005 event of Mw8.6. There was no earthquake with Mw≥7.0 occurred in the seismogenic fault of the 2012 Sumatra shock from 1900 to 2011. In addition, a seismic gap with Mw≥5.0 was formed around the focal region and the gap was disturbed after the 2004 Sumatra earthquake occurred. Six years, four and a half years and three months before the great 2012 event, there were three groups of foreshocks respectively, and the Mw7.2 event, the direct pre-earthquake, occurred three months prior to the Mw8.6 shock, is the most significant one in the three groups of foreshocks.
2014, 39(4): 492-498.
doi: 10.3799/dqkx.2014.047
Abstract:
It is difficult to estimate precise magnetotelluric (MT) impedance tensor in high correlated noise areas. In order to approximate MT impedance tensor, we use remote sites which contain relatively lower correlated noise to compute separation tensor by using Robust least square method. The MT data in three sites which are contaminated by higher and lower correlated noises are processed using signal-noise separation method. The results show that it can separate higher correlated noise and get much smoother apparent resistivity and phase curves than those obtained by using Robust least square method and remote reference method. It is concluded that signal-noise separation method is efficient for much more precise MT impedance tensor.
It is difficult to estimate precise magnetotelluric (MT) impedance tensor in high correlated noise areas. In order to approximate MT impedance tensor, we use remote sites which contain relatively lower correlated noise to compute separation tensor by using Robust least square method. The MT data in three sites which are contaminated by higher and lower correlated noises are processed using signal-noise separation method. The results show that it can separate higher correlated noise and get much smoother apparent resistivity and phase curves than those obtained by using Robust least square method and remote reference method. It is concluded that signal-noise separation method is efficient for much more precise MT impedance tensor.
