The present study is the first report of a Miocene mafic dyke from the Dahuichang, in the Western Hills of Beijing. The dyke cuts the fossil-dated Changxindian Formation of Eocene sequences and yields K-Ar ages of 14-...The present study is the first report of a Miocene mafic dyke from the Dahuichang, in the Western Hills of Beijing. The dyke cuts the fossil-dated Changxindian Formation of Eocene sequences and yields K-Ar ages of 14-15 Ma. The dyke is fine-grained diabase and has 49.84%-50.81% SiO2 and 3.56-3.97% Na2O＋K2O, high TiO2 （1.65%-1.93%） and MgO （7.36%-9.85%）, and low K2O （〈1.22%） contents, with Na2O〉K2O and slightly varied magnesium numbers （Mg#=55.54-62.74）. In trace elements geochemistry, the dyke is very similar to the Miocene basalts from Jining and Hanuoba. The enrichment of light rare earth elements （[La/Yb]N=5.03-6.12） and large ion lithophile elements （LILEs）, no negative Eu anomalies, relatively high Cr （265-326 ppm） and Ni （155-262 ppm）, and almost constant V concentrations （194-213 ppm） reveal that the composition close to the primary basaltic magma from an enriched-mantle source, with little crustal contamination and fractional crystallization. The basaltic magma was possibly derived from the upwelling asthenosphere mantle beneath eastern China during the Miocene lithospheric thinning.展开更多
Cross-well electromagnetic tomography has been used to monitor the changes of the abso rption coefficient of electromagnetic waves before and after earthquakes for 5 years at Dahuichang fault since 1992.It has been fo...Cross-well electromagnetic tomography has been used to monitor the changes of the abso rption coefficient of electromagnetic waves before and after earthquakes for 5 years at Dahuichang fault since 1992.It has been found that 3 months before the occurrence of moderately strong earthquakes(ML≥4.0,△【210 km),several special changes in tomogram appeared as follows:The mean-value of the absorption coefficient dropped about 10% compared with the annually decreasing tendency; the mean difference of pixels suddenly decreased or increased in different directions; and the entropy of the paragenic matrix in a tomogram decreased and the number of low absorption pixels increased.It is inferred that such changes in electromagnetic tomograms are attributed to the variation of micro-cracks and porosity in rocks.Electromagnetic tomography can be studied as a new technique to detect earthquake precursors.展开更多
Ground deformation may be caused by crustal movement and non-crustal movement.The non-crustal movements include those caused by temperature,rainfall,ground water,etc.Deformation caused by crustal movement includes sei...Ground deformation may be caused by crustal movement and non-crustal movement.The non-crustal movements include those caused by temperature,rainfall,ground water,etc.Deformation caused by crustal movement includes seismogenic deformation(that appearing in the process of earthquake preparation and that directly caused by the earthquake)and general tectonic activities.The key point in the study of the relationship between ground deformation and earthquakes is how to eliminate information of deformation caused by non-crustal movement and permanent(or slow)tectonic deformation in observed data.In this paper an analysis is made on the short leveling data recorded at Dahuichang Deformation-Observing Station,Beijing,for 22 years(1970-1991)by using the LMS algorithm method after lateral adaptive filtering of the modern digital signal processing technique.Eliminating the approximate annual variation in deformation caused by non-crustal movement such as temperature,rainfall,ground water,etc.,a unified standard展开更多
基金funded by the National NaturalScience Foundation of China(40672130)the Instrumental Analysis Fund of Peking University
文摘The present study is the first report of a Miocene mafic dyke from the Dahuichang, in the Western Hills of Beijing. The dyke cuts the fossil-dated Changxindian Formation of Eocene sequences and yields K-Ar ages of 14-15 Ma. The dyke is fine-grained diabase and has 49.84%-50.81% SiO2 and 3.56-3.97% Na2O＋K2O, high TiO2 （1.65%-1.93%） and MgO （7.36%-9.85%）, and low K2O （〈1.22%） contents, with Na2O〉K2O and slightly varied magnesium numbers （Mg#=55.54-62.74）. In trace elements geochemistry, the dyke is very similar to the Miocene basalts from Jining and Hanuoba. The enrichment of light rare earth elements （[La/Yb]N=5.03-6.12） and large ion lithophile elements （LILEs）, no negative Eu anomalies, relatively high Cr （265-326 ppm） and Ni （155-262 ppm）, and almost constant V concentrations （194-213 ppm） reveal that the composition close to the primary basaltic magma from an enriched-mantle source, with little crustal contamination and fractional crystallization. The basaltic magma was possibly derived from the upwelling asthenosphere mantle beneath eastern China during the Miocene lithospheric thinning.
基金Contribution No.97A0058 Institute of Geophysics, SSB, China
文摘Cross-well electromagnetic tomography has been used to monitor the changes of the abso rption coefficient of electromagnetic waves before and after earthquakes for 5 years at Dahuichang fault since 1992.It has been found that 3 months before the occurrence of moderately strong earthquakes(ML≥4.0,△【210 km),several special changes in tomogram appeared as follows:The mean-value of the absorption coefficient dropped about 10% compared with the annually decreasing tendency; the mean difference of pixels suddenly decreased or increased in different directions; and the entropy of the paragenic matrix in a tomogram decreased and the number of low absorption pixels increased.It is inferred that such changes in electromagnetic tomograms are attributed to the variation of micro-cracks and porosity in rocks.Electromagnetic tomography can be studied as a new technique to detect earthquake precursors.
文摘Ground deformation may be caused by crustal movement and non-crustal movement.The non-crustal movements include those caused by temperature,rainfall,ground water,etc.Deformation caused by crustal movement includes seismogenic deformation(that appearing in the process of earthquake preparation and that directly caused by the earthquake)and general tectonic activities.The key point in the study of the relationship between ground deformation and earthquakes is how to eliminate information of deformation caused by non-crustal movement and permanent(or slow)tectonic deformation in observed data.In this paper an analysis is made on the short leveling data recorded at Dahuichang Deformation-Observing Station,Beijing,for 22 years(1970-1991)by using the LMS algorithm method after lateral adaptive filtering of the modern digital signal processing technique.Eliminating the approximate annual variation in deformation caused by non-crustal movement such as temperature,rainfall,ground water,etc.,a unified standard
