Chinese Journal of Geophysics- Chinese Edition

error degree variance and degree variance of different gravity model 
We calculate the admittance signatures from the ratio of the gravity and topography data in spectral domain for the four lunar mare basin (Smythii, Crisium, Imbrium, Orientale), which of them have different age. A multitaper method is used to compute the admittance, then compared the observed admittance with the theory values from the model of the lithosphere flexure. on the basis of this method, we obtained the elastic thickness for the four mare basin. the thickness of them is averaged about 8km to support the observed topography since the time of the loading. the upper crust thickness becoming thick when the evolution wheel of the moon goes ahead. these results are consistent with thermal history of this planets
Focal mechanism solutions and depths for 312 aftershocks of the 2008 M8.0 Wenchuan earthquake have been obtained by CAP method from broadband waveform data by Sichuan regional seismic network. The results indicate that depths of aftershocks vary in different sections along the fault zone, therefore our aim is to derive the spatial and temporal variations of aftershock faulting behaviors from focal mechanisms. For about 330 km-long dense aftershock zone, we divided it into 3 segments based on differential focal mechanisms and depths. Relatively shallow aftershocks (depth <10km) mainly occurred south of Wenchuan on the southern segment and east of Qingchuan on the northern segment. On the southern segment, thrust and strike-slip faulting are dominated, with a few normal faulting aftershocks, thrust aftershocks are more than those in strike-slip along the Longmenshan main faults before 31 Aug, 2008, but slightly less than strike-slip ones afterwards, showing significant increasing of strike-slip aftershocks. In addition, our present study reveals predominant NW strike-slip faulting near Lixian, implying the existence of a secondary NW-striking fault. On the central segment, thrust slip faulting is dominant. And further along the northern segment, right-lateral strike-slip aftershocks are almost twice of thrust ones with strike-slip component in the early stage, and approximately equal to the later afterwards, again indicating that thrust aftershocks were increasing. In fact, aftershocks after 24 July, 2008 near the northern end, are relocated further northwestwards and they are deviated from previous ones. We argue that the change in locations and faulting sense may mark the end of strong aftershock activity. Further, our finding that the adjustment from coseismic and short-term postseismic strike-slip to later thrust movement, is agreeable with our recognition that slip magnitude will decrease towards the tips of the strike-slip fault and the strain will be transformed and absorbed by the terminal thrust movement. Rose diagrams of azimuths and dips for P axes show dominant NWW orientations on the southern segment and NEE on the central segment with sub-horizontal dips, respectively; however, on the northern segment, dominant orientations are significantly different in two stages, NWW and NEE are both dominant orientations in the early stage, but the orientations are much more diffusive in the late stage. The above finding demonstrates the aftershock activity are both influenced by regional stress field and local stress field by the Wenchuan mainshock. From the focal mechanism solutions, we also determined the mean dip of fault plane varies little within the range of 50°~60° on the northern segment, implying the fault plane is relatively steep and simple and more favorable for strike-slip faulting. But on the southern segment, dip angle decreases from 70° to 40° from 10km to 20km as a typical listric fault plane. On the central segment, aftershocks are concentrated between 7~19km, variation of dip angle is different from those on the other segments, mean dip is relatively steady at depth more than 10km for about 50°, and obviously increases upwards. Our present results indicate the geometry of the Longmenshan fault plane is very complicated.
The New Zealand earthquake of February 21, 2011, M(w)6.1 occurred in the South Island, New Zealand with the epicenter at longitude 172.70 degrees E and latitude 43.58 degrees S, and with depth of 5 km. The M(w)6.1 earthquake occurred on an unknown blind fault involving oblique-thrust faulting, and this fault is 9 km south of Christchurch, which is the third largest city of New Zealand, with a striking direction from east toward west. At least 163 were killed and lots of structures were destroyed in Christchurch city. The Peak Value of Acceleration (PGA) observed at station Heathcote Valley Primary School (HVSC), which is 1 km away from the epicenter, is up to almost 2.0g. The ground-motion observation suggests that the New Zealand earthquake generated higher than normal near-fault ground motion. In this study, we analyzed the source spectral parameters based on the observations, and estimated the near-field ground motion based on the Brune's circle model. The results indicate that the larger ground motion may be produced by higher effective stress drop (dynamic stress drop, Delta sigma(d)) in this buried earthquake. A dynamical composite source model (DCSM) has been developed to simulate the near-fault strong ground motion with associated fault rupture properties from a kinematic point of view. For comparing purpose, we conduct broadband ground motion predictions for the station of HVSC; the synthetic seismogram produced for this station has good agreement with the observations in time histories, waveforms, peak values and frequency contents, which indicate that one of the key factors in ground-motion amplification was the higher effective stress drop. It should be pointed out that, in addition to the study of large earthquakes, the small and moderate earthquakes near the cities should be monitored in order to prevent the devastating destroy of the cities once earthquake occurs.
The 2001 MS 8.1 Kunlunshan earthquake and the 2008 MS 8.0 Wenchuan earthquake occured in the same tectonic unit. There are significant differences in spatial-temporal distribution, number of aftershocks and time duration for the aftershock sequence followed by these two main shocks. As we all know, aftershocks could be triggered by the regional seismicity change derived from the main shock, which was caused by the Coulomb stress perturbation. Based on the rate- and state- dependent friction law, we quantitative estimated the possible aftershock time duration with a combination of seismicity data, and compared the results from different approaches. The results indicate that, aftershock time durations from the Wenchuan main shock is about 20 times of that from the Kunlunshan main shock. This can be explained by the significant relationship between aftershock time duration and earthquake nucleation history, normal stress σN and shear stress loading rate on the fault. In fact, the obvious differences of shear stress loading rate from these two main shocks result in an observable seismicity rate change after the same Coulomb Failure Stress perturbation ΔCFS and the distinction of number and time duration of aftershocks sequence. It is necessary to point out that, from our current study, the quantified understanding and interpretation of spatial-temporal distribution and time duration for the aftershock sequence can be used in the Probabilistic Seismic Hazard Analysis (PSHA) in order to avoid overestimating of the seismic hazard levels, and provide reasonable and helpful physical parameters in earthquake disaster mitigation.
Although the planetary waves propagation has been studied a lot, most of them were based on the zonal symmetric basic flow in which planetary waves are trapped by the zero line of zonal wind. For this reason, the characteristics of stationary and non-stationary wave propagation in the horizontal non-uniform basic flow are researched separately in theory. Some interesting results are as follows. Stationary waves can propagate through the easterlies with a weak meridional wind, so the interactions between the easterlies and westerlies can be turned out in the view of planetary waves. Besides, the variation of the amplitude also depends on the meridional wind and the latitude. The periodic characteristics of non-stationary waves are discussed based on the necessary and sufficient condition of three and one real solutions of the dispersion relationship respectively. In the case of three real solutions, phases tend to propagate westward (eastward) in the easterlies (westerlies). Given the zonal basic flow, the differences in the phase speed and period of these three propagating waves may be larger with the strengthened meridional basic flow. Three propagating waves are low-frequency over both the Australian and Asian monsoon regions in 850 hPa during boreal summertime.
The Longmen Shan fault zone consists of three main Longmen Shan faults and the blind fault in the Chengdu Basin. Along the range front of the middle segment of the Longmen Shan, there is the lithological border in published geological maps. The existence and the latest active time of the range-front fault along the mid-segment of the Longmen Shan thrust belts are controversial for a long period. Petroleum seismic reflection and high-resolution shallow seismic reflection profile discovered the existence of the range-front fault and the fault offset the Quaternary strata. Based on detailed field observation, we found that there is an obvious linear feature along the mid-segment of the Longmen Shan front and the range-front fault displaced the late Quaternary fluvial terrace. Trench log indicates that a surface-rupture event occurred before ~1500a along the range-front fault. Differential GPS surveying and dating of fluvial terrace show that the range-front fault during late Quaternary underwent a vertical slip rate of bigger than 0.36mm/a, approximately equivalent to that along the main faults of the longmen Shan thrust belts, which demonstrates that the range-front fault also took an important role in accommodating the deformation of the Longmen Shan thrust zone. This study not only provides the fundamental data for seismic hazard assessment of the Chengdu Plain, but is helpful for the overall understanding of uplift mechanism of east Tibet.
Source displacement spectra of about 2573 events from 2001 to 2010, occurred in main seismic active regions of mainland China, are restored after the influences of instrument, noise, geometry spreading, inelastic attenuation, as well as the site contributions were corrected from SH components of the observed seismic waves,on the basis of both the frequency dependent Q(f) model for 13 regions and the site response for 348 sites were inversed firstly. The source parameters of 2573 events are further estimated assuming a Brune-type source model. Our results demonstrate that the estimated stress drops range from 0.1MPa to 20MPa with most less than 10MPa. The stress drops of events occurred in the middle and east part of Tianshan,Xinjiang, China, the east-northern boundary of Tibet,and the east-northern segment of Longmenshan fault zone, are relatively larger, implying that they may experience relatively stronger effective shear stress. We find that stress drops increase with earthquake magnitudes, satisfying a relationship of,not a constant model. The dependence of stress drop on focal depth are not found within the depth region of less than 20km, while there is a minor increasing trend when focal depth deeper than 20km. The dependence of stress drop on focal mechanism is not found as well. Through this work, we realized the near real time calculation of source parameters for an individual event, and our results provide a large amount of samples for studying the scaling relationship in the mainland. The obtained attenuation model and site effects will help in determining the calibration function and site corrections for an individual station, which are the two important factors for the purpose of precisely estimating the local ML magnitude.
The crustal thickness and the Poisson ratio of the North China Craton (NCC) are two important parameters for understanding the Phanerozoic tectonic reactivation and destruction of NCC. We collected waveforms of 93 teleseismic events, which occurred during August 2007 to March 2009 with magnitudes range from M6.5 to M8.5. These seismograms are recorded by 323 broadband stations deployed in and surround the NCC region. Based on these data, we calculated the P wave receiver functions with the iterative, time-domain deconvolution approach for each of the seismic stations, then used the H-k stacking method to obtain the crustal thickness and Poisson ratio images of NCC with fine azimuthal coverage. The results show that the crustal thickness decreases gradually from west to east. In the west part of the NCC, the crustal thickness is about 55km in Qilian Mountains, and decreases to 38 ~ 45 km in Ordos block, and in the further east, the crustal thickness is only around 30km. Different with the gradually change pattern of crustal thickness, the Poisson ratio distribution pattern are very complicated in the studied area. There are four basical characteristics of the crustal thickness and the Poisson ratio: 1) Crustal thinning not only occurs in the east part of NCC, but also reaches to the North South Gravity Linearment. 2) Crustal thinning also occurs in the rifts and basins surrounding the Ordos block, and the Poisson ratio in the surrounding rifts is higher than that inside of the Ordos block. In some rifts the Poisson ratio is as high as 0.3. 3) the Poisson ratio in Mountains area are lower than 0.25 except North Taihang Mountain, whose Poisson ratio is even more than 0.3. 4) The crustal thickness of NCC is negative correlated to the topography, which is in consistence with Airy isostasy, which may imply that the NCC crust is experiencing a slow process of destructing and isostasy. While the isostasy degree is much lower in the northeast and northwest NCC.
Physical model for regular fault containing a columnar barrier has been designed and conceptually been used to simulate the barrier or strong area concerning earthquake preparation. Biaxial compression experiment with a middle-size rock sample of this model has been done and AE spatio-temporal developing characteristics have been studied. Results show that (1) Due to complex action between the hanging wall and lower wall, as well as the barrier, alternate AE activities in different areas become the most remarkable feature during deformation process of the sample. (2) The pre-condition for instability of regular fault containing a barrier is that the barrier should fail firstly with the increase of loading. The AE spatio-temporal distribution features are very different before and after the barrier failure. (3) The total deformation amount of this sample is large and its cumulated AE frequency increases exponentially. During the weakening period, there are only a few AE events distributed in barrier and surrounding area. Abrupt instability happens under such a relative quiet background. (4) b values of G-R relationship for different pre-setting tectonic patterns have some obvious differences, the regional differences of b value interrelated to different tectonics are remarkable.
Using seismic data from Chinese and ISC stations between 1980~2004, we inverted Pn wave velocity structure and anisotropy of the northeastern South China Sea and adjacent region. The velocity variations at uppermost mantle reflect the features of regional geology and tectonics. Southeastern China has fast velocities, correspondent to the lithospheric mantle in tectonically stable regions. Slow velocities appear near the Binhai fault zone along the coast of southeastern China, which indicates a possible penetration of the fault zone into the uppermost mantle. Similar to southeastern China, fast velocities throughout the northern South China Sea and Taiwan Strait reveal a property of the lithospheric mantle in continental margin, while fast velocities in the Xisha Trough reflect a southward extension of continental shelf and a mantle upwarp produced by Cenozoic rifting. There is no evidence of large-scale mantle heat flows in the northern South China Sea. However, a very slow velocity anomaly is observed in the eastern sub-basin of the South China Sea, where is an extinct mantle upwelling center with high heat flows, indicating a thinning or removal of lithospheric mantle. Along the eastern Taiwan, Luzon and northern Philippine, slow velocities are closely related to seismic and volcanic activity and magmatism in the arc zone, while fast velocities in the eastern South China Sea and western Philippine Sea reflect the character of oceanic lithospheric mantle. Pn velocity anisotropy also reveals the stress state in regional tectonics and the history of deformation of lithospheric mantle. Southeastern China has small anisotropy in response to less deformation there. Anisotropy is observed in the northern South China Sea, with fast directions of anisotropy aligned with shallow structures in crust, which reflect the evidence of the Mesozoic-Cenozoic rifting and shear deformation in lithospheric mantle. Strong anisotropy is also found along the Ryukyu-Taiwan-Luzon arc zone, with fast directions parallel to trenches, which indicates that strong deformation of lithospheric mantle in the leading edge of the Philippine Sea plate and Eurasian continent. The change of the fast directions of anisotropy near the eastern Taiwan is probably caused by a conversion of collision mechanisms between the Eurasian continent and the Philippine Sea plate, and the tear of the lithosphere.
This paper integrates the results of seismic tomography as well as gravity and magnetic data inversion in a region within 26°N~36°N and 120°E~130°E to study the deep structure of the Yellow Sea and East China sea and discuss its relation with surrounding geologic units. Density structure inversion is accomplished along two profiles. The result indicates that the crustal thickness in the East China Sea continental shelf is not obviously different from the land area. The crust thins remarkably in the Okinawa Trough where the middle crust disappears. Beneath the Ryukyu island arc the thickness of crust, especially the upper crust, increases again, which is probably caused by subduction and the difference of motion rates between the plate and the upper mantle near the subduction zone. The longitudinal belts of geophysical anomalies are manifestation of crustal structure change from west to east in the Yellow Sea and East China Sea. However, with the increase of depth, a lithospheric structure pattern dominantly in nearly-EW direction appears in the study region. It is inferred that the deep nearly-EW anomalies were caused by collision and compression between northern and southern plates in the Triassic, whereas the shallow NE-trending anomaly belts were the tectonic effect of shallow lithospheric structures in the later stage. There is an inconsistency between the shallow and deep structural trends of the lithosphere in the Yellow Sea and East China Sea. The result of seismic tomography confirms the existence of the East Margin Fault of the southern Yellow Sea, and also delineates the Shaoxin-Shiwandashan collision zone appearing as a high velocity zone dipping at about 40° towards NW. Another high velocity zone with similar dip is the reflection of the boundary between the northern and southern Yangtze blocks at depth.
Electrical conductivity of two-pyroxene granulite, collected from the north margin of the North China craton, was measured at 1.0~2.0 GPa and 523~1173 K by using three different methods at the same time. The three methods are an impedance spectroscopy for a broad frequency range of 0.05 to 10−6 Hz, a single lower frequency (0.1 Hz) and the DC-method. The experiments were carried out with a measurement system of electrical conductivity, which consisted of an YJ-3000t multi-anvil apparatus, a Solartron IS-1260 Impedance/Gain-Phase analyzer, an Agilent 34401A Multimeter and a computer. The experimental results indicate that within the range of experimental temperature and pressure, the electrical conductivity of two-pyroxene granulite changes from 2.66×10−5 S·m−1 to 0.056 S·m−1, and hardly has a dependence on the given pressure. With increasing temperature, the electrical conductivity increases. The conductivity-temperature relation follows an Arrhenius behavior. The pre-exponential factor and activation energy of the Arrhenius formula are 8.95~17.9 S·m−1 and 0.569~0.605 eV, respectively. The comparison of the results obtained by the three methods indicates that the result measured by the impedance spectroscopy is always higher than that by 0.1 Hz single frequency and the result determined by the DC-method is always the lowest. Nevertheless, the difference among the data points is less than 0.20 lg(S/m) other than two low-temperature data points. Based on the experimental results, the regional crust model and the geothermal gradient, we constructed the profiles of conductivity versus depth. The comparison of the calculated results with the electrical structure of this area reveals that the electrical conductivity of twopyroxene granulite intersects the range of the electrical conductivity of the lower middle crust and the lowermost crust beneath this area. Combining with the conclusion of the in situ seismic velocity measurement and the seismic refraction profile, we can conclude that two-pyroxene granulite could be a constituent of the lower crust in the northern margin of the North China craton.
With the development of exploration in the mature exploration areas, the subtle trap is playing a dominant role. So, joint migration of multicomponent in anisotropic media is the key technology. This paper developed a 2-D full elastic wave prestack reverse-time depth migration algorithm for multicomponent data in Vertical Transverse Isotropic (VTI) media. Extrapolation used the anisotropic velocity-stress equation which is discreted by a high order staggered grid finite-difference scheme in space and a second-order finite-difference scheme in time. A new imaging condition was proposed by combining the Excitation-time Imaging Condition with the Poynting vector to remove low-frequency artifacts. The algorithm was applied to synthetic records. The results show that this approach produces clear images and can remove low-frequency noises effectively.
Based on 38 000 P-wave arrival times recorded during the period of 1980 to 1997 in the northeast margin area of Sino-Korean platform (117°00'–126°00' E, 36°00'–44°00' N) the 3D velocity structures of the crust and upper mantle have been reconstructed by using the orthogonal projection inversion method. The tomography results have been proved reliable by detailed analysis and comparing the results to deep seismic prospecting profiles. It has been showed that there exists obvious lateral inhomogeneity in the crust and upper mantle which is still distinct even at the depth of 120km in the northeast margin area of Sino-Korean platform. It is clearly reflected in the velocity image of the upper crust that the velocity distribution corresponds to that of basement rocks of different nature in this area. Several low velocity anomaly regions (i.e., regions with reversed velocity increment downwards) have been discovered in the research area from the velocity images of upper and middle crust, such as Haicheng, Chaoyang, Yixian, south of Dandong, Tangshan etc., among which the low velocity layers in Haicheng, Tangshan, Chaoyang have also been proved by deep seismic sounding data. In the research area several strong continental earthquakes all happened in brittle medium with higher velocity above the layers with lower velocity in the crust, whereas in Bohai sea all the strong earthquakes happened at places where the velocity changes abruptly and clearly in horizontal direction. At different depths, Pulandian-Shanhaiguan low velocity anomaly zone across Bohai sea in NW direction has been found. By comparing the velocity images obtained from seismic tomography and that gotten from deep seismic sounding, we consider it is possible to gain similar effect with deep seismic sounding by taking advantage of seismic tomography under certain condition in this area.
According to analysis results of channel waves recorded in a 0.9m-thin coal seam when using blasting as seismic source, the result of energy spectrum shows two major frequency bands in those channel waves. One is centered at 2000 Hz, and the other is at 490 Hz. Furthermore, the two frequency bands are not continuous and the difference of peak values is not obvious between these two bands. It is apparent that these two frequency bands are triggered at the same time in the time frequency distribution graphics of channels wave, moreover, the distribution of wavelet coefficients in these two frequency bands is similar to some degree. This is totally different from channel waves that were recorded in a 2.1m-thick coal seam. The Airy phase frequency and speed observed in the testing field is about 550 Hz and 824. 5 m/s, respectively, which are close to theoretical ones of Airy phase with 0. 9m thickness. Because seismic boreholes were deployed in the middle of coal seam, the energy of dynamite would be distributed symmetrically on the face of coal seam. It would be very important for generating the symmetrical wave shape mode. Similarly, since the thickness of coal seam is only 0. 9 meters, the energy of dynamite would be enough to vibrate the roof and floor by the seismic source. The vibration plays a key role for generating the second higher wave mode of channel wave. Therefore, the speed of channel wave is close to the speed of S wave in rocks, which is positive to identify the speed of S wave in rocks from channel waves recorded in thin coal seams. Furthermore, the analysis method of combining low with high frequency region would be recommended to reinforce the effectiveness of detecting by channel waves.
NRLMSISE-00 model is widely used in orbit determination and prediction of low-Earth satellites. However, this model often exhibits large density errors compared with observations, especially in short-term prediction of density. The extensive density database from GRACE (Gravity Recovery And Climate Experiment) and CHAMP (Challenging Mini-Satellite Payload) satellites and lots of new observation data are used to build a calibration of density of NRLMSISE-00 model with the aim of increasing the accuracy of density prediction. Firstly, latest observation data in selected orbits is used to obtain the scale factors of calibration. These scale factors of calibration are then used to modify the density output of NRLMSISE-00 model. Several cases in GRACE and CHAMP orbits have been tested using this correction method. In high solar activity during 2003, the density RMS errors at CHAMP and GRACE orbits were slightly reduced with calibration. Under low solar activity during 2007-2008, the RMS density errors along their tracks were reduced from 59. 21% to 27. 85% at GRACE orbit and from 50. 70% to 21. 51% at CHAMP orbit after calibration.
The characteristic of Chang'E-1 orbital tracking data is presented in this paper, and the principle and solution strategy of lunar gravity field recovery based on dynamical precise orbit determination are summarized. Under the condition of limited accuracy and coverage of tracking data of Chang'E-1, a lunar gravity field model CEGM-01 up to degree and order 50 is solved independently using six months tracking data. The accuracy of the model CEGM-01 is assessed by multi-means, such as gravity power spectrum, observational residuals after precision orbit determination, lunar surface gravity anomaly character, admittance/coherence with topography model, and the corresponding physical feature and effect are analyzed. The results indicate the effectiveness and reasonability of CEGM-01 solution process. On the basis of current work the prospect of future development of our national lunar gravity field detection is presented.
There are two reasons make it essential to explore the deep, one is the need for deep mineral exploration, and the other better understanding the deep dynamic processes that control the formation and distribution of major ore deposits, particularly, in the intra-continental setting. Financed by the SinoProbe (China's largest national collaborative, multidisciplinary Earth science research project), National Nature Sciences foundation and Chinese Geological Survey, the authors have conducted multi-scale and integrated deep exploration across middle and lower reaches of Yangtze Metallogenic Belt (YMB) and its major ore districts in Eastern China. These data range in scale from terrane and belt, district to camp-scale. The methods included broadband seismic, deep reflection seismic, wide-angle reflection/refraction, magnetotelluric sounding and gravity and magnetic modelling. The results provide first-order insights into physical and structural properties of the lithosphere and upper mantle beneath the YMB. These insights provide geodynamic clues and constraints as to why YMB is so well endowed in metals. The results also provide first-order constraints for the upper crustal structure, composition and fault distribution of major ore districts. Based on these information at depth, the three dimension geological model was constructed, which provides knowledge of the depth extent of subsurface or ore-controlling geologic units (e.g., faults, strata, and intrusions) that thus leads to a new targeting for deep mineral. Some typical exploration models for “porphyry” iron, porphyry copper and hydrothermal polymetal deposits were summarized through the 3D integrated geophysical methods test, and deep potential for exploration was evaluated around the deposits. In term of technical progresses, a series of measures were taken from acquisition parameter design, shooting to receiving. New methods from weak seismic signal extraction, noise suppression, static to pre-stack migration were proposed and tested. Mathematical morphology filtering and other denoising techniques, and 2D/3D forward and inversion methods considering the influence of the topography and anisotropy were proposed in the magnetotelluric sounding (MT) data processing.
Along with the development of China's economy and the industrialization process of all countries in the world, the demand for oil and gas is growing quickly. In light of growing requirements for oil and gas resources abroad and the fact that China's internal supply is not enough or even deficient at the present time, we suggest that we must use the resources through different ways; at the same time, determine to have a foothold in national resources and to build up the strategic reserve base of gas and oil resources for safety which can ensure reliable and stable supply. At present, during the processes of the exploration and exploitation of oil- and gas-fields, there still exist large quantity of the oil and gas, which are buried more than or much greater than that defined by the depth lines and stable temperature. Furthermore, the extensive discovered deep hydrocarbon reservoirs have raised strong challenges to traditional oil and gas generating theory. Based on studies of sedimentary construction, crystalline basement, deep medium and physical environments (mainly referring to temperature and pressure) for formation and accumulation of hydrocarbon resources, the following four problems are discussed: (1)There exist marine sediments beneath the continental sediments widely, i.e., dual-phase sedimentary basins(continental phase and marine phase), and the basement is not only the bottom of Mesozoic and Cenozoic basement, but also the old metamorphic rocks basement; (2)The physical and chemical properties, material sources, and reservoir space of the medium in the deep crust supply the material for the formation and accumulation of oil and gas resources; (3)We put forward the new notion of mixing origin of oil and gas(organic and inorganic origins), dominated by organic origins; (4)Oil and gas exploration and exploitation in the second deep space(5000~10000 m) should be the inevitable trajectory for the further studies and development of the deep potential large-super large hydrocarbon reservoirs.
We develop an analysis method of empirical mode decomposition (EMD), applicable to nonlinear and unstable data. Our purpose is to study the periodical change and the response of the climate system to solar radiation on a obit scale in Pleistocene time. Our result shows that eccentricity band signal is much larger than previously estimated, i. e. 1% on solar radiations, and comparable with the precession and obliquity band signals. We should reconsider the role of solar radiations in the formation and maintenance of quaternary ice sheet cycles.
Distribution, saturation and hydrate indicator of free gas beneath gas hydrate stability zone have been the difficulty in the research of gas hydrate. Amplitude versus offset (AVO) and rock physics can be used to quantitatively interpret free gas saturation. In this paper, seismic data, acquired at K-G basin, India, have been processed to preserve true amplitude information. Based on the logging data and horizon calibration, free gas saturation of NGHP01-10A is assessed with effective medium theory and AVO modeling. It is found that free gas saturation is related to its distribution patterns. Assessments of free gas saturation in homogeneous and patchy distribution are 0.3%~0.4% and 3%~4%, respectively. The results show good agreement with the crossplot of Poisson's ratio which are calculated by the measurement at NGHP01-10D. Free gas is further deduced to be homogeneous distribution on the base of dry matrix Poisson's ratio with the saturation of 0.3%~0.4%.
Gas hydrate filled in fractures was the important occurrence morphology in fine-grained sediments from deep basin of India. Gas hydrate existed as solid nodules and subhorizontal veins filled in high angle fractures. Gas hydrate was formed along the tectonic principal stress. Gas hydrate filled in fractured reservoir shows anisotropic properties. Gas hydrate saturation estimated from NGHP01-10D was about 40% of the pore space using isotropic velocity model. However, gas hydrate saturation estimated from pressure cores was about 20% of the pore space. To reconcile the difference, an anisotropic model based on laminated media was used to estimate gas hydrate saturation in fractured reservoir. In vertical borehole, the incident angles of wave propagation have a relation with fracture dip angles. Gas hydrate saturations and fracture dip angles can be estimated from both P-wave and S-wave velocities obtained from the logging data. Gas hydrate saturations estimated from NGHP01-10D vary from 15% to 25% of the pore space using anisotropic velocities for laminated media. The fractures are high angle fractures and the dip angles vary from 60 to 90 degrees. The saturations estimated using the new method fit the results estimated from pressure core gassing well.
For the Mw9.0 earthquake occurred in the Japan trench on March 11, 2011, this paper used the constraints from the inter-seismic, co-seismic, and post-seismic deformation observations, and analyzed the relationship between the strong earthquake in the Japan trench and the fault movement according to the spatial distribution characteristics of the fault movement at different times. Based on crustal deformation observation, we study the 3D spatial distribution characteristics of inter-seismic, co-seismic, and post-seismic fault movement and provide the pattern diagram of fault motion at the trench areas of Japan. We invert the coseismic displacements of the fault based on uniform fault model (oblique fault model, the dip from the seabed to 50km depth changes from 5 degrees to 20 degrees, the unified strike 198) and 3D fault model (seismic parameters of the 3D fault model are given by fitting). The results show that, as for the 3D fault model, the coseismic dislocations better fit the data, and more agree with fault properties. Therefore, in carrying out the work of similar inversion accuracy of fault geometry, the reliability of the results is very important. The inversion results show that there are two coseismic slip extreme areas. The maximum coseismic dislocation is about 50 m, located in the shallower coseismic slip extreme area. The dislocation amount of the coseismic rupture is greater than 10m near the epicenter in a range of 400 km; in the South Japan Trench the coseismic displacement is relatively small, the earthquake is a typical Japan trench thrust type earthquakes. According to the spatial distribution characteristics of the magnitude 9.0 earthquake and the previous strong earthquake ruptures in the area, the magnitude 9 earthquake rupture bore the characteristics of recurrence of strong earthquake, and was the strong earthquake rupture to fill in the gaps. In inter-seismic stage in the Japan trench, the depth of the locking line is about 60km, moreover, the fault movement, from deep to shallow above the locking line, in order, is strong locking, aseismic slip, weak locking. From coseismic dislocation inversion, there are two coseismic slip extreme areas. The shallow one (coseismic displacement 10~50 m, depth less than 30 km) is weak locking before this earthquake; and the deep one (coseismic displacement 10~20 m, depth about 40 km) is strong locking before this earthquake; and the coseismic dislocation in transition zone between with extreme coseismic slip areas is relatively small and there is aseismic slip before this eartqhuake. In the initial period after the earthquake, afterslip is mainly in the deeper coseismic extreme slip areas. On the basis of the coseismic and interseismic fault motion inversion results, the seismic strong locked segment will take 100 years to accumulate 10m coseismic dislocation, and the regional history has 7 earthquake recurrence periods of earthquake activity; the weak locked segment will take about 300~600 years to accumulate 30~50 m coseismic dislocation, which is consistent with the recurrence period of magnitude 9.0 earthquake in Japan Trench given by relevant research. As to determining the actual seismogenic ability, because the different sections of one fault will form more combinations in the earthquake process, fault earthquake capacity determination result is more uncertain. But the research on regional deformation observation data is very important and of practical significance for judging fault strong earthquake ability.
Using wavelet analysis method, the cosmic ray intensity data observed at the Guangzhou muon station during the period from Jan. 7 to Jan. 11, 1997 is investigated. The results show that signal with periods of 16 similar to 32h has obvious variations, of which the period feature of 24 similar to 32h has not been reported before. The spectrum of the cosmic ray intensity in vertical direction has obvious variations 48h ahead of the geomagnetic storm which has more time ahead of the geomagnetic storm than the result obtained by the anisotropy method. The strongest signal in each direction is also studied and discussed briefly.
Based on the 1999-2009 IONEX total electron content (TEC) data over 120 degrees E from IGS, we studied annual and seasonal variations of the TEC at the daily northern anomaly crest (Inc) and southern anomaly crest (Isc) as well as solar and geomagnetic effects on the equatorial ionization anomaly (EIA). The variation of Inc and Isc is correlated well with solar P index but poorly correlated with geomagnetic Dst, Kp and Ap index. It is found that the Inc and Isc are almost highest in equinoctial season on northern hemisphere and the values in winter are greater than the summer values which showed the semiannual anomaly and winter anomaly in Inc. We attributed this to the semi-annual variation of the eastward electrojet-associated electric field. The method of support vector regression based on support vector machines (SVM) is used to establish the prediction model of ETA index. The results of the forecasting experiments with the model showed that this model can describe the variation of EIA index Inc and Isc and the mean relative error of Inc and Isc is about 10. 2% and 22. 96% respectively. The prediction effect of Inc is better than the Inc. This method provides a new approach and idea for the prediction of characteristic index of EIA and shows a good application perspective.
The L2A SABER/TIMED temperatures taken during 2002 - 2006 were used to estimate the mean temperatures in the MLT region in the meridian at 120°E. Good agreements were seen in the comparison between the seasonal mean temperatures in the meridian and that across the entire zonal cycle. In contrast, very large discrepancies were observed when comparing the temperatures with that derived from the CIRA-86 temperatures; and the discrepancies were attributed to the differences in the critical structures in the two temperatures. Daily temperatures were used to investigate the altitude and temperature of the mesopause in the meridian by applying a scheme that extracts the mesopause information through diagnosing the lapse rate in each temperature profile. Estimation results show that in summer season, the mesopause with the same height (83 km) as that of the polar mesopause is observed through the latitudes from 48°N to the polar area. In the meantime the equatorial and tropical mesopause are maintained at 97 km, which is critical in shaping the characteristic global "two-level mesopause". Moreover, daily analysis disclosed the extreme complexity in the appearance of the mid-latitude mesopause. It was observed that there are two kinds of mesopause, one locates at 83 km height with extremely low temperatures, the other locates at about 100 km. Although the theory proposed for interpreting the "two mode" mesopause in polar area was successfully used to explain the current estimation results, further detailed investigation is necessary to understand the complex situation.
The seasonal, temporal and spatial variations of the scintillation index along the longitude 120 degrees E at the E regional altitude (70 similar to 140 km) are analyzed using the GPS radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) during 2007-2013. The results show that the intense scintillation within +/- 30 magnetic latitude has the greatest value in summer, followed by winter, autumn and spring. The strong scintillation peak is in close connection with solar radiation intensity and sunshine duration. However, the scintillation index in the northern hemisphere is greater than in the southern hemisphere in summer/winter and that in the autumn hemisphere is slightly greater than in the spring hemisphere. The large S4max index at high latitudes is found to start at post-sunset (1800 LT), often persists till pre-sunrise and reaches maximum values at around midnight. However, the S4max index at the middle/low latitudes can be observed to start at around post-sunrise, persists till midnight or even dawn, and reaches its peak before afternoon/dusk. The S4max index in the equatorial regions can be observed to start at the post-sunrise and end before midnight, and reaches its peak at appoximate 1800 LT. The scintillation peaks in the middle and low latitudes often occur at an altitude of appoximate 110 km and yet the peaks in the high latitudes are below an altitude of 110 km. It is also found that enhanced solar or geomagnetic activities can suppress the ionospheric E-layer scintillation. The results can be useful for studying the ionospheric irregularities at the E region altitude, and provide valuable information for constructing a regional scintillation model.
Measuring the nighttime OI 135.6 nm airglow emissions for the study of ionosphere changes has been used as one of the most common ways to detect ionosphere at present. The sensor of measuring the nighttime OI 135.6 nm airglow emissions for obtaining the total electron content and peak electron density will be on board the FY-3 meteorological satellite of China. Because of few far-ultraviolet airglow radiative research in our country, the associated airglow emitting models and the retrieval algorithm study are imperative. Based on the nighttime OI 135.6 nm airglow excitation mechanism research, considering the radiative scattering of the atmospheric oxygen atoms and absorption of oxygen molecules in the band at the radiative transfer process, the iterative method was used to solve the radiative transfer equation including multiple scattering and the atmospheric absorption attenuation. The volume emissive rates at the band could be obtained.Furthermore the radiative intensity was calculated by the path integral calculation of radiative transfer at 135.6 nm band. Analysis of the results shows that the airglow model could be used to describe the volume emissive rates with height distribution well. The 135.6 nm radiative intensities calculated at different spatial temporal distribution and solar activity conditions have a good consistency with NmF2 and TEC. Under the same conditions of spatial temporal and solar activity input conditions, the radiative intensity calculated by the model was compared with similar foreign model and the mean deviation was 3%, and further confirms the correctness of the model. This paper finally introduced the retrieval algorithm of NmF2 and TEC by measuring the nighttime OI 135.6 nm airglow emissions.
Acceleration field of the Yushu earthquake was predicted by the stochastic method of ground motion simulation with the dynamic corner frequency based on the finite fault source. Firstly, the earthquake source model was established based on the method of modeling the finite fault source. Secondly, acceleration time histories of 191 near-fault nodes were predicted based on the above-mentioned ground motion simulation method. Finally, the acceleration field of the earthquake was drawn based on 191 nodes' PGAs (Peak Ground Accelerations). The results are as follows. (1) The near-fault acceleration field is significantly affected by both source rupture process and slip distribution on fault plane. The maximal PGA area on ground surface corresponds to the maximal asperity area, and this means that both buildings and other engineering structures on ground surface corresponding to the maximal asperity area on the earthquake fault may suffer severe damage. (2) For strike slip earthquake, not all sites in the vicinity of fault line are subjected to the forward directivity effect, and the occurrence of forward directivity effect is related to the location of asperities on the fault plane.
We present the rupture details of Yushu earthquake from the back projection of teleseismic P waves. By applying ARF we get 29 out of 206 stations with distance range of 30 to 95 degree, which have a good array response function and high waveform cross-correlation coefficient, in the final back projecting process. We apply 4th root stack method and moving window with 2 sec length and J sec step to J00 sec long waveform starting 20 sec before P arrival. The back projection result shows that the total rupture time of the Yushu earthquake is 20 sec, and the rupture length is 60 km. The energy release of the Yushu earthquake is mainly concentrated at two time-space points. One is about 6 sec after the origin time and close to the epicenter. The second energy burst is J2 sec after the origin time and located southeast of the epicenter closing to the Jiegu county. This energy burst is the biggest energy burst in the whole source rupture process. The biggest energy burst is close to the heavily damaged area. The source rupture model from the back projection can aid the earthquake rapid response and rescue and can be a complement to ShakeMap.
We study the substorm dipolarization occurring from 1730 to 1930 UT on Sep. 14, 2004 based on DSP/TC1, Cluster and Polar data, combined with the aurora and geosynchronous energetic particle measurements and ground-based magnetic field observations. During this time period the IMF was continuously southward. Substorm onset started at about 1823 UT. Two minute later, energetic electron injection was observed by LANL-02A in the pre-midnight sector at geostationary orbit. At ∼ 1827 UT TC1, which was located at (- 10, - 2, 0) RE (GSE) in the near-Earth tail, saw a sharp decrease of magnetic BX component, accompanied by sudden increases in both the ion temperature and thermal pressure. About 23 minutes later at ∼ 1850 UT, the same signatures as observed by TO were observed by Cluster at (- 16, 1, 3) RE (GSE) in the mid-tail. Meanwhile, Polar, which was located much higher and more close to the Earth at (- 7.5, 3.5, - 4.0) RE (GSE), saw a sudden change in the elevation of the magnetic field at about 1855 UT. Aforementioned time sequence of substorm activities indicates that the expansion onset took place at about 1823 UT and TC1 and Cluster observed dipolarization (tail expansion) 4 and 27 minutes later, respectively. This means that tail expansion was propagated from source region in the near-Earth tail to the mid-tail. Detailed calculations imply that the source of the dipolarization was situated at X = - 7.7 RE to - 8.6 RE and the propagation speed was about 70 km · s-1. A substorm paradigm is proposed to explain the observations in this substorm event: Earthward flow produced by lobe reconnection in the mid-tail creates favorable conditions for substorm initiation in the near-Earth; after the expansion onset the dipolarization front propagates down to the tail.
Taking five major, M≥7 earthquakes occurred in the northern section of South-north seismic zone, an inter-region between northeast Tibetan Plateau and North China block, during 360a from 1561 to 1920 as examples, the Coulomb failure stress changes (ΔCFS's) caused by the previous earthquakes are calculated on the rakes of fault surfaces of the later earthquakes to study earthquake triggering by building seismotectonic models, based on our knowledge about the seismogenic faults. The results suggest that, after the 1561 Luoshan M 71/4 earthquake happened, the next four events would occur in the stress triggering area (Δ CFS>0). △CFS's related to the Yunwushan fault, the Helanshan fault and the Haiyuan fault are larger than 0. 2 × 10 5 Pa, surpassing the triggering threshold of 0. 1 × 10 5 Pa. It means that the distribution of ΔCFS has a significant indication on the location of earthquakes in the future. According to ΔCFS and tectonic stress accumulation rate (δτ), the periods reduced by ΔCFS are calculated, which can be as large as 160a for the corresponding earthquakes. The triggering caused by the previous earthquake to the next shortens the accumulation time of earthquake re-occurrence, which makes the earthquake happen ahead. It can cause that earthquake activity shows temporally the feature of quasi-cycle for a seismogenic fault and spatially the phenomenon of clustering for different seismogenic faults. This study contributes to our understandings about fault interaction, relationship among strong earthquakes even at different seismotectonic provinces and seismic risky areas in the future.
We used broadband teleseismic data of Chifeng station to calculate the Receiver Functions, then obtained a velocity model with H-K stacking method in combination with CRUST2.0 and other previous work. With this velocity model and broadband records from 5 CDSN stations, we inverted focal mechanisms of Chifeng earthquake on August 16th, 2003 with the "Cut and Paste" (CAP) method. Then we confirmed focal depth and the source mechanisms by comparing synthetic body waves at teleseismic distance with broadband records of 9 IRIS stations. Our result shows that the best double couple solution of this Mw5.2 event is 315°, 64° and 19° for strike, dip and slip angles respectively, the second nodal plane solution is 216°, 74°, and 152°. The focal depth is 25 ± 2 km, suggesting that this quake occurred in the lower crust which is much deeper than most continental earthquakes. This lower crust earthquake requires that the rock should be colder than expected. We proposed generation mechanism of this deep earthquake and its implications in rock strength and thermal state.
The electrical conductivity of ( Mg0.9 Fe0.1 ) SiO3 enstatite has been measured at pressures of 10 -20 GPa and temperatures of 750-1600 K using a Kawai-type multianvil high-pressure apparatus. The experimental results demonstrate that there are two conduction mechanisms, small polaron is the dominant mechanism in the high temperature regions, while proton is in charge of the low temperature regions as water is determined in the recovered sample in this study. On the other hand, we observe a pressure induced phase transition from enstatite to ringwoodite under pressure of 20 GPa by X-ray diffraction. The conductivity of ringwoodite with water at 20 GPa is in good agreement with the available measurements of hydrous ringwoodite.
We have studied the earthquake occurred in vicinity of Luojiabu, southwest of Tianshui city, Gansu Province, where the M = 8 Lixian earthquake occurred on July 21, 1654. In geotectonic setting, this earthquake happened in the middle-northern segment of South-North Seismic Belt, which generates numerous strong earthquakes and consequently makes this area one of the most seismically hazardous regions in China. However, causative structure and mechanism of Lixian earthquake is still in debate. Combined with active fault mapping, deep geophysical data, globe position system vector field and the distribution of landslides along the fault, we discuss that the mechanism of Lixian earthquake and try to estimate the seismically hazardous in this region. Fault descriptions are based on field observations aided by satellite image from Google earth collected along the fault zone. Our primary purpose is to document features bearing on the recency and kinematics of Quaternary movement along the fault. The observations are largely synthesized on a set of maps. The maps show active fault traces and are annotated to indicate the location of features indicative of the sense, style and amount of offset of geomorphic features. We also map the distribution of Quaternary deposits in the vicinity of fault. Combined with the electrical structure profile derived by 2D inversion of the MT data, we analyzed seismogenic structures of Lixian 1654 earthquake. On July 21, 1654, the Lixian M=8 earthquake occurred at the east part of Lixian County, Gansu Province, China, in the middle-northern segment of the South-North Seismic Belt. In this region the tectonic activities and deformation are closely related to the north-eastward growth of Tibetan Plateau. Their complicated structural geometries were controlled by the Eastern Kunlun fault, Northern Marginal fault of West-Qinling, and a series of north-east trending faults. The Lixian-Luojiabu fault is a northeast trending sinistral strike-slip active fault and is 150 km long consisted by three sub-segments aligned in right-step-echelon. The fault cuts the T-1, terrace (lowest terrace in this area) deposits which contain the red ceramic fragments marking the Yangshao layer. On the terrace surface, the height of fault scarp is about 1. 5 m. Along the Lixian-Luojiabu fault, the sinistral displacement of gullies about 3 similar to 10 m and the height of fault scarps about 4. 5 similar to 8 m are found in a number of locations. And the Knickpoints about 3. 5 similar to 3. 9 m high are found 16 m away from the fault scarps. According to knickpoint recession rates at the same environment, these knickpoints may be formed as a 1654 earthquake in Lixian. A large number of landslides are distributed in a belt along Lixian-Yanguan-Luojiabu-Tianshui towns line, and its major axis direction is consistent with the trending of the Lixian-Luojiabu fault. Rear walls and side walls of the landslides are steep, with the outcrops of Late Quaternary loess and little vegetation cover. The Lixian-Luojiabu active fault is the seismogenic fault of the Lixian M=8 earthquake in 1654. The south-eastward sliding of the Lixian Sub-block, which is surrounded by the Northern Marginal fault of West-Qinling, Lixian-Luojiabu fault, and the Minxian-Dangchang fault, led to the occurring of the Lixian M=8 earthquake in 1654. Differences in electrical structure for the crust on the two sides of Lixian Luojiabu fault reveals the structural condition for strong earthquake generation. Lixian-Luojiabu fault is a sinistral strike-slip active fault, which generated M=8 Lixian earthquake on July 21, 1654. This earthquake produced 7 similar to 10 m horizontal offset and with 3 similar to 4 m fault scarps along the Lixian-Luojiabu fault. Tectonically, it took place on the middle-northern segment of the South-North Seismic Belt where a series of M> 5 had already occurred. Based on analysis of activity of other faults, GPS vector field and deep structural condition, the Lixian-Luojiabu fault activities and deformation are closely related to the north-eastward growth of Tibetan Plateau.
Frequency-domain modeling is the basis of frequency-domain full waveform inversion. The classical optimal 9-point scheme is only of second-order accuracy, and does not meet the need of high-accuracy seismic imaging. This paper deals with two fourth-order schemes, namely fourth-order 9-point scheme and optimized 17-point scheme. The 17-point scheme reduces the number of grid points required by the shortest wavelength to 2. 56. We also perform numerical tests on a simple model and the Overthrust model, and the modeling results with the three schemes are compared. The results demonstrate the superiority of the 17-point scheme over the fourth-order 9-point scheme and optimal 9-point scheme in terms of reducing dispersion errors. Numerical tests on complex model further confirm the feasibility of the 17-point scheme.
Dividing the world into 15 study areas and using the earthquake catalogue of Mw≥7.0 during 1900∼2009, we make a statistical analysis of the relationship between seismicity of each area and the period of lunar nodal motion. The result shows that among the 15 areas, 10 areas exhibit an 18.6-year cycle of great earthquakes in statistical sense, which consists of a 12.4-year active section and a 6.2-year quiet section. In the eastern, northern, southern and western areas of the Circum-Pacific seismic belt, such a periodicity is present and their active sections have certain regularity. In terms of seismic events during the sixth cycle (1991-2009) , this study has tested the stability and applicability of the 18.6-year periodicity derived from earlier five cycles (1900 ∼ 1990). It is found that most of periodicities are stable in the regions with such characters. The possible mechanisms of the 18.6-year periodicity are as follows: (1) Earthquakes are modulated by the 18.6-year tide via day and half-day tides. (2) Effect of fluid tide in upper mantle (tide within the Earth). And (3) earthquakes are dictated by 18.6-year tide through variation of Earth rotation.
On January 19, 2011, a magnitude M(L)4.8 earthquake occurred in the region between Anqing and Huaining in Anhui province, which is a relatively strong earthquake and can be felt in Anqing and surrounding areas. To better understand the seismotectonics of this event, we try to relocate it and further study its focal mechanism based on the local broadband waveform data of available stations in Anhui and adjacent province regional seismic networks. In this work, the earthquake was relocated with absolute relocating method Hypo2000 at first, and then the focal mechanism and focal depth were determined by the "Cut and Paste" (CAP) method. In order to better constrain the focal depth, we further refined it with depth phases such as P, sP, pP and sPmP. Finally, the results of focal mechanism and focal depth were confirmed by comparing the observed records and the synthetic waveforms which were computed by F-K method. The results indicate that the Anqing earthquake is a thrust event with a small part of strike-slip component. The best double couple solution is 131 degrees, 30 degrees and 29 degrees for strike, dip and slip angles respectively for one nodal plane, and 15 degrees, 75 degrees and 116 degrees for another. The moment magnitude is 4. 3, and the estimated focal depth is 4 similar to 5 km, which suggests that: this event is a shallow earthquake and capable of heavy hazard to the surrounding areas. According to the epicenter and the focal mechanism, Anqing earthquake likely occurred on the Susong-Zongyang Fault.
Southwestern China, located in the conjoined areas of the Chuandian, Bayankala and Huanan active blocks on the eastern margin of the Qinghai-Tibetan Plateau, is one of the regions with highest seismicity in China. The source parameters of historical major earthquakes in this region are missing or inaccurate. This work focuses on the following three aspects. (1) Based on the geological data and tectonic stress field, we infer focal mechanisms of historical major earthquakes (M >= 6.0) from 1900 to 1970 in southwestern China, and analyze uncertainties of the results to conclude that this indirectly method might not be applicable in all areas of southwestern China. (2) We relocate 41 events by using the Shide Circulars, International Seismological Summary and EHB Bulletin instrumental records within the time span and obtain 32 reliable results. (3) We calculate the focal mechanisms of 29 events by first motion records of P waveforms of ISS and obtain 14 reliable results. Consequently we enrich source parameter information of historical major earthquakes in southwestern China from 1900 to 1970.
Based on the seventh generation of International Geomagnetic Referance Held, the nondipole magnetic fields for 1900-2000 (interval is 5 years.) were calculated and drawn, the locations and intensities of each magnetic anomaly centre at different epoch were computed and analysed, too. The distribution rangs are very large, and the intensities are very strong for the following five magnetic anomalies: Eastern Asian positive anomaly, Oceanian negative anomaly, Southern Atlantic positive anomaly, African negative anomaly and Northern American positive anomaly. The intensity of Southern Atlantic positive anomaly is the biggest. The intensity of Eastern Asian positive anomaly is the second before 1990. The intensity of African negative anomaly exceeds the Eastern Asian anomaly and becomes the second since 1990. The increasing rate of magnetic anomaly intensity (absolute value) of African negative anomaly is the largest (average annual variation rate is 67.1 nT / a), that of Southern Atlantic positive anomaly is the second (that is 64.8 nT / a), that of Oceanian negative anomaly is the third (that is 49.6 nT / a). The location changes of magnetic anomaly centre of African and Southern Atlantic anomaly are the fastest in the all of the magnetic anomalies.
The location parameters of the Zayü, Tibet M8.6 earthquake sequence in 1950 in the published catalogues are poor, which cannot help further study of this great event. In order to provide more precise location results and some features of their spatial-temporal distributions, and to reveal the stress field in the corresponding region, we attempted to relocate this earthquake sequence and to determine the focal mechanisms. The data of global seismic stations from International Seismological Summary (ISS) and the Catalogue of China Earthquakes (BC 1831─AD 1969) were collected. The 16 events with M≥6 of the Zayü, Tibet M8.6 sequence were relocated by P arrivals at 239 worldwide stations using the routine location method and model of Chinese National Seismographic Network: Improved Geiger and the JB time tables. The corresponding fault plane solutions and composite fault plane solutions were determined by first motions of P waves based on the new relocations. (1) The relocations of 16 strong earthquakes of the Zayü M8.6 sequence have been completed. It is shown that the epicenters in different periods appeared in different sub-regions: A foreshock which occurred on 23 Feb 1950 appeared in the 1st sub-region, the top of the great bend of the Yarlung Zangbo River, which lies at the north of Motuo. The mainshock and the following 7 aftershocks in 3 days which occurred from 15 Aug 1950 to 18 Aug 1950 were located in the 2st sub-region near Zayü and distributed along the north-west belt. The 3 aftershocks in period 3 which occurred from 22 Aug 1950 to 13 Sep 1950, were located in the 3st sub-region extending to the south, India and Myanmar. The 3 aftershocks in period 4 which occurred from 30 Sep 1950 to 15 Apr 1951 appeared in the 4st sub-region at Motuo and Cuona, west of the aftershock epicenter area. Another aftershock occurred on the 110th day after the mainshock, the epicenter of which returned to the place near the mainshock. (2) The focal mechanism solutions determined based on the relocations indicate that the NWW strike of a nodal plane of the mainshock is consistent with the NWW direction of the major axis of the aftershock epicenter area in the 2st sub-region. All compressive axes P and tensional axes T of the earthquake sequence are nearly horizontal, of which most dip angles are less than 20°. The compressive axes of the mainshock and the aftershocks in the 2st sub-region are nearly north-south direction, and tension axes are nearly east-west direction. But in the 3st and 4st sub-region, the compressive axes of aftershocks are nearly east-west direction, and tensional axes are nearly north-south direction. The 16 events of the Zayü, Tibet M8.6 earthquake sequence in 1950 have been relocated. The new results are quite different from the parameters in the previous catalogues of which the locations of many aftershocks were exactly the same as the mainshock, i.e. only one point. The relocations indicate that the epicenters in different periods appeared in 4 different sub-regions rather than distributing along the determined rupture. The obvious correlation of these 4 sub-regions is clockwise rotation movement. The focal mechanism solutions determined based on the relocations display that the differences of focal mechanisms among the aftershocks of the sequence are large.
Located in the front margin of the northeastern Tibetan Plateau, the Longshoushan fault zone is one of the boundary faults between the Hexi corridor and the Alxa block. Although the resolution of observation is limited, the 1954 M7(1/4) earthquake on the Longshoushan fault zone is a rare big event in this zone with modern seismological observation and documentation. During this earthquake, one continuous dextral-normal rupture occurred with a length of only 7 km along a transfer fault connecting two secondary faults of the Longshoushan fault zone. However, the dextral-normal sense of this surface rupture is contrary to the tectonic kinematics in this area. Based on recent field investigation and mapping, two phenomena were discovered. Firstly, except the one appeared on the transform fault, no surface rupture zone is found on the branches of the Longshoushn fault zone. Secondly, the main damage caused by earthquake is distributed on the region between the northern and the southern branches of the Longshoushan fault zone. These two findings indicate that the 1954 earthquake was caused by the main faults rather than the transfer fault of the Longshoushan fault zone, and the co-seismic surface rupture on the transfer fault is only a form of stress release at local portion. According to calculation of static co-seismic Coulomb stress change, the surface rupture could be triggered by an earthquake on the southern branch of the Longshoushan fault zone. Furthermore, in consideration of the possibility of dynamic triggering along the northern branch of the Longshoushan fault zone, the surface rupture should be the co-seismic response to the earthquake occurred on the main branches of the Longshoushan fault zone. Relocation of the small earthquakes also shows that a narrow inverted north-extending triangle about 10 km wide was formed between the northern and the southern faults of the Longshoushan fault zone.
Chinese geophysicists carried on a series of geophysics research on the crust and upper mantle of Qinghai-Tibet plateau since 1958 and the total investigation length is about 45000km. In order to express the achievements in all aspects, the authors collect materials extensively, summarize the geophysical work on the crust and upper mantle of Qinghai-Tibet plateau, and draw a series of work degree figures according to different methods. This article gives a systematic account of the crustal structure, lateral heterogeneity of the upper mantle, resistivity of the lithosphere, uplifting mechanism of the Qinghai-Tibet plateau, geodynamic model and numerical simulations on lithosphere tectonic stress field. The available data shows that Moho is comparatively deep in this area. From south to north, Moho has different depths and features undulation beneath several main sutures. The results show that there is generally a discontinuous layer of low velocity and high conductivity in the crust of Tibetan plateau, which exists in 20 ± 5km depth generally with a velocity of 5.6-5.8km/s, a low resistivity of 1-10Ω·m and a thickness of 5 - 10km. The low velocity layer and high conductivity layer are similar but not same in the depth and thickness. The natural earthquake research achievements show similar seismic anisotropy direction in every block on Qinghai-Tibet plateau, but there is obvious change at the boundary of blocks. Though there are different opinions regarding the uplifting history of Qinghai-Tibet plateau, it has been generally acknowledged that the plateau has undergone multi-phase uplift under multiple action mechanisms. On the basis of the results of comprehensive geophysical survey and interpretation and geological information, a geodynamic model was built to demonstrate the deformation mode of the plateau lithosphere.
In this paper, the patterns of Coulomb stress change induced by a sequence of 4 strong earthquakes occurred in Songpan County, Sichuan province in 1973 and 1976 are calculated and studied. We analyze the near-field stress changes induced by each event in the sequence and their relationship to the occurrence of the succeeding events and to the aftershock distribution. Moreover, we evaluate the far-field effect of stress change that was also induced by this sequence on the regional seismicity of moderate-strong earthquakes occurred in the following 25 years. Our results suggest, firstly, that the MS6. 5 Huanglong, Songpan earthquake of Aug. 11, 1973 raised Coulomb stress on the mid-segment of the Huya fault and then triggered the MS7. 2 event of Aug. 16, 1976, and further successively triggered the MS6. 7 event of Aug. 22, 1976 and the MS7. 2 event of Aug. 23, 1976 southward along the fault. Secondly, we find that most aftershocks of the sequence occurred in areas where Coulomb stress has been increased along the near-field zones of the seismogenic fault-segments of the mainshocks. Thirdly, in the 25 years following the sequence and within an about 200 km radius area centered at the mid-segment of the seismogenic Huya fault of the sequence, 6 events with magnitudes 5. 0 to 6. 6 all occurred in those far-field zones with very small amount of increase in Coulomb stress induced by the sequence.
Using 3D finite difference code developed by Dr. Day in SCEC, we simulated fault dynamic rupture process and associated near-fault strong ground motion for the 1976 Ms7.8 Tangshan earthquake based on a simplified bi-lateral rupture model and slip-weakening frictional law. The fault length, width, and depth of the model are 200 km, 140 km, and 40 km, respectively. The discretized space and time steps are 200m and 0. 012 s, respectively, which make the total number of node points up to 0. 15 billion. In the implementation level, a parallel computational algorithm has been developed in DELL work station, and a computer visualization technique has been used in the numerical simulation in order to do data analysis. Furthermore, a regional 3D velocity model also has been embedded into the model to simulate seismic wave propagation and associated ground motion. According to the numerical results, we discussed the characteristics of the ground motions produced from the 3D rupture model with associated 3D velocity structure, including PGA and PGV distributions around fault. A physics based explanation related to the rupture directivity is also proposed to show that the radiated SH-type particle motion (fault-normal component) from ruptured fault has a significant influence on the near fault ground motion along the fault strike direction. Based on the radiated SH wave motion and propagation caused by directivity effect, we proposed that, for the 1976 Ms7. 1 Luanxian and Ms6. 9 Ninghe earthquakes, a dynamic triggering mechanism related to the temporal stress variation could play a significant role to trigger these two events. The result shows that the dynamic stress change could reach 2 ~ 3 MPa in strike direction and 0. 1 - 0. 2 MPa in thrust direction.
In this paper, it is analyzed the statistical characteristics of inter-annual change of floods/droughts in floods season (JJA) and annual precipitation amount in the southern parts of Yangtze River in China for the average of 16 stations during 1982 similar to 2005, and the significance relationship between floods/droughts (JJA) and annual precipitation and northernmost declination of the moon in summer that were discovered. The correlation coefficient (r) and the correlation probability (P) of anomalous between precipitation in summer (JJA) for average of 16 stations in the southern parts of Yangtze River and northernmost declination of the moon in summer during 1982 similar to 2005 that is very high, r = 0.812, P = 100% (24/24), the significance level is over 0.001. This obvious correlation result proves that inter-annual variations of northernmost declination of the moon in summer have important influence on rainfall in southern parts of China. We have also investigated the causality, it shows differential at the chart of summer (JJA) mean 500 hPa for 11 years (1992 similar to 2002) of rainy and floods with northernmost declination of the moon in summer is in southern 25-.5 degrees N, the west Pacific subtropical high (WPSH) is partial wester ten longitude than for 11 years average of 8 years (1984 - 1991) and 3 years (2003 similar to 2005) of dry and droughts with northernmost declination of the moon in summer is in northern 25.5 degrees N, the westerly circulation of first chart is southern than second chart, the longitudinal wave at Eurasia of first chart is stronger than second chart. The circulation pattern of first chart is of benefit to rainfall of the southern parts of China. Contrary, atmospheric circulation situation of second chart without the benefit of precipitation in the southern parts of China. It illuminated that inter-change of northernmost declination of the moon in summer inference to precipitation by atmospheric circulation.
An natural orthogonal component model of secular variation of the geomagnetic field in China was established for the period of 1985-1997 by means of natural orthogonal component methods (NOC's) on the basis of the data of 20 geomagnetic observatories in China and 3 geomagnetic observatories in Japan. The root mean square (RMS) of geomagnetic field components of 23 observatories were calculated for NOC truncation levels 1-9, the contour maps of orthogonal model values for k(NOC) = 1-3 were constructed for 1995 epoch. The results show that the contours of secular variation of X, Y and Z components are very smooth and uniform in distribution and have obvious regional feature. In some areas, the secular variations are very large, reflecting local feature of geomagnetic field and the related geophysics phenomena.
The morphology and its variations of an ionospheric storm related with a very strong magnetic storm on 12(th) similar to 16(th) March, 1989 are examined, using the ionosonde data obtained at stations in European and Asian areas. The strong ionospheric disturbances caused by the significant magnetic storm appeared on the whole world. Since the storm heating intensities at the lower thermosphere resulting from energetic particles precipitated from the magnetosphere are different from area to area, the features of the ionospheric storm are also discrepant in different longitudinal chain regions. As compared with deduction from the theory, there was no "positive" storm period of NmaxF2 (or f(0)F(2)) in Europe, while the developing tendency of the "negative" storm from high-to low-latitudes was consistent well with the result deduced from the thermospheric circulation. Moreover, at the mid-and low-latitude areas there was an evident wavelike process. On the other hand, at the high latitude of the Asian meridian there was a "positive" storm lasted similar to 10 h on March 13(th) in the early stage of the magnetic storm. As to the feature of the "negative" storm, it was similar to that in Europe, but not as distinct as there. Besides, there was no wavelike phenomenon at the Asian area. During the magnetic storm in the mid- and low-latitude areas along a same meridian one can also see the nighttime simultaneous sudden enhancement of h'F. It is proved once more that this is a universal phenomenon during a magnetic storm.
Geomagnetic pulsations at Great Wall Station and Zhongshan Station of Antarctica during magnetic storm on March 24, 1991 are analysed. In this paper the beginning time and characteristics of various pulsations are given and comparison analysis are made for two stations. All of them are helpful for understanding different behaviour of various pulsations at dayside and nightside or at high-latitude and mid-latitude and for realizing the relationship between pulsation morphology and storm process during a magnetic storm in Antarctica.
Variations of sea level are an important issue associated with global climate change, especially global warming. The sea level rising may lead to serious impact on the social-economic development of a country. Global oceans are continuously observed by altimetry satellite missions for more than 20 years. We use altimeter data of TOPEX/Poseidon (T/P), Jason-1 and Jason-2 from 1993 to 2012 to study the sea level change over China seas. China seas located in the western Pacific Ocean are selected as the study area. T/P MGDR data of 11 to 364 cycles span January 1993 to August 2002. Jason-1 GDR data of 1 to 259 cycles span January 2002 to January 2009. Jason-2 GDR data of 1 to 165 cycles span July 2008 to December 2012. Southern oscillation index (SOI) time series span January 1993 to December 2012. Altimetry data in the tandem stages are used to calibrate biases of sea level anomalies (SLAs) from T/P, Jason-1 and Jason-2 point by point. The spatial distribution of China seas' level change is studied with the continuous tension spline method. Temporal variations of China seas' level are analyzed with the linear fitting method and the wavelet analysis. Relationships between the El Niño-Southern Oscillation (ENSO) and sea level changes of the South China Sea and the East China Sea are studied with the correlation analysis. By unifying altimetry data point by point in tandem stages, the mean differences of sea level heights for Jason-1 vs T/P and Jason-2 vs Jason-1 are 0.21 cm and 0.03 cm, respectively. SLA time series are constructed after corrections of sea level biases from 1993 to 2012. In general, the mean SLA is positive over China seas. SLAs are higher in the south than those in the north, and lower in the west than those in the east. SLAs are negative over the western Bohai Sea, the northern Yellow Sea, the northern Taiwan Strait and the North Bay. SLAs rise with the increasing longitude and decrease with the growing latitude. This spatial distribution of SLAs over China seas is related to the water flux, sea surface wind stress, oceanic dynamics, monsoon, Kuroshio and ENSO. The mean rising rate of sea level for the 20 years is 4.64 mm·a-1 over the whole study area. Sea levels are rising in the Bohai Sea, Yellow Sea, East China Sea and South China Sea with rising rates 4.44 mm·a-1, 2.37 mm·a-1, 3.02 mm·a-1 and 4.25 mm·a-1, respectively. The annual sea level variation is obvious in the study area. The sea level is higher in summer and autumn than in winter and spring. The main cycles of sea level change include one year and 9 years over the whole seas. There are also minor cycles of 0.5 years, 1.5 years, 2 years and 4 years. Cyclical changes of sea level are related to the geographical position, climate, oceanic dynamics and submarine topography. The correlation coefficients between SOIs and SLAs of the South China Sea and the East China Sea are 0.39 and 0.02, respectively. The correlation coefficient is 0.44 after the two-month delay for the South China Sea. The correlation coefficient is 0.17 after the four-month delay for the East China Sea. This indicates that the sea level change over the South China Sea may be largely affected by ENSO. Altimetry data of T/P, Jason-1 and Jason-2 are processed to study the sea level change over China seas from 1993 to 2012. We used the altimetry data in tandem stages to calibrate the sea level biases of these three missions to achieve the seamless SLAs point by point. The spatial distribution of sea level change is given with the continuous tension spline method. The sea level is generally rising at the mean rate of 4.64 mm·a-1 over China seas in these 20 years. The sea level variations change with different seas and seasons. The mean rising rates are 4.44 mm·a-1, 2.34 mm·a-1, 3.02 mm·a-1 and 4.25 mm·a-1 over Bohai Sea, Yellow Sea, East China Sea and South China Sea, respectively. Temporal-spatial change of sea level over the study area is related to oceanic dynamics, submarine topography, monsoon, Kuroshio and ENSO. The main cycles of sea level change are one year and 9 years as derived from the wavelet analysis. El Niño and La Niña events make stronger effects on sea level variations over the South China Sea than the other seas.
Top-cited authors
Wu Jia
  • 上海立信会计金融学院
Xue-Ze Wen
  • Institute of Geology, China Earthquake Agency
Xiwei Xu
  • China University of Geosciences (Beijing)
Shengbiao HU
  • Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing,China
Zhang Peina Zhang
  • Capital Normal University