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Chu G.,CAS Institute of Geology and Geophysics | Xie M.,CAS Institute of Geology and Geophysics | Li S.,Institute of Earthquake Science | Wang X.,CAS Institute of Geology and Geophysics | And 2 more authors.
Journal of Geophysical Research: Biogeosciences | Year: 2011

Glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in surface sediments from 24 lakes in China and Nepal were analyzed. The result shows that branched GDGT lipids occur widely in the studied lake sediments. The different distribution of branched GDGTs between the Chinese soils from Weijers et al. (2007b) and the Chinese lake sediments indicates that the branched GDGTs in the lacustrine sediment are the mixture of allochthonous and autochthonous branched GDGTs. Our result supports the suggestion that temperature is the principal factor affecting the distribution of GDGTs. Seasonality, sources, and precipitation increase the uncertainty of the correlation between the methylation index of branched tetraethers (MBT)/cyclization ratio of branched tetraethers (CBT) proxy and the temperature. The correlations of the MBT/CBT index with mean annual air temperature (T) for the global lakes are T = 6.803 - 7.062 × CBT + 37.090 × MBT (n = 139, p < 0.0001, R2 = 0.62, and RMSE = 5.24°C); for the global lakes with pH < 8.5, T = 3.949 - 5.593 × CBT + 38.213 × MBT (n = 100, p < 0.0001, R2 = 0.73, and RMSE = 4.27°C). The significant correlation suggests that the MBT/CBT proxy might be used as a potential paleotemperature proxy in the limnic environment. The MBT/CBT proxy provides an independent way to evaluate and verify the sensitivities of the different proxies for paleotemperature reconstruction from lacustrine sediments. Copyright 2011 by the American Geophysical Union.

Wu Y.,Institute of Geology | Wu Y.,Institute of Earthquake Science | Jiang Z.,Institute of Earthquake Science | Yang G.,First Crust Deformation Monitoring and Application Center | And 3 more authors.
Geophysical Journal International | Year: 2011

Using modelled and simulated data for comparison of several methods to compute GPS strain rate fields in terms of their precision and robustness reveals that least-squares collocation is superior. Large scale (75°E-135°E and 20°N-50°N) analyses of 1° grid sampling data and decimated 50 per cent data by resampling (then erasing data in two 5°× 10° region) reveal that the Delaunay method has poor performance and that the other three methods show high accuracy. The correlation coefficients between theoretical results and calculated results obtained with different errors in input data show that the order in terms of robustness, from good to bad, is least-squares collocation, spherical harmonics, multisurface function and the Delaunay method. The influence of data sparseness on different methods shows that least-squares collocation is better than spherical harmonics and multisurface function when sample data are distributed from a 2° grid to a 1° grid. Analysis to medium scale (90°E-120°E, 25°N-40°N) in 1°-0.5° grid sampling data reveals that least-squares collocation is superior to other methods in terms of robustness and sensitivity to data sparseness, but their difference is slight. Strain rate results obtained for the Chinese mainland using GPS data from 1999 to 2004 show that the spherical harmonics method has edge effects and that its value and range increase concomitantly with increased sparseness. The multisurface function method shows non-steady-state characteristics; the errors of results increase concomitantly with increased sparseness. The least-squares collocation method shows steady characteristics. The errors of results show no significant increase even though 50 per cent of input data are decimated by resampling. The spherical harmonics and multisurface function methods are affected by the geometric distribution of input data, but the least-squares collocation method is not. © 2011 The Authors Geophysical Journal International © 2011 RAS.

Shao Z.-G.,Institute of Earthquake Science | Zhou L.-Q.,China Earthquake Networks Center | Jiang C.-S.,Institute of Geophysics | Ma H.-S.,Institute of Earthquake Science | Zhang L.-P.,Institute of Earthquake Science
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2010

In order to analyze the effect of Wenchuan Ms8.0 earthquake on the seismic activity of surrounding faults, we calculate the dynamic changes of Coulomb stress caused by the Wenchuan Ms8.0 earthquake with a Burgers viscous relaxation model. Simulation result shows significant increase of Coulomb stress on four majors fault segments, which are Daofu-Kangding section in the Xianshuihe fault, Maqu section in the East Kunlun fault, the southern section of the Longmen Shan fault, and the Qingchuan fault respectively. Furthermore, the Coulomb stress change caused by the post-seismic relaxation during 4 years after Wenchuan Ms8.0 earthquake is so strong that it almost matches to that caused by the co-seismic rupture. That is to say, the effect of the postseismic relaxation is nearly equal to that caused by the Wenchuan earthquake itself. This is a strong evidence that the postseismic relaxation is critically important so it should not be neglected on evaluating the effects of strong earthquake. Taking the postseismic effect into consideration, the probability of seismicity is evaluated by the Dieterich (1994) model based on the dynamic evolution of Coulomb stress and the background seismic rate. Combining the relevant structural geology, historical earthquake, aftershocks activities and some other minor factors, a comprehensive analysis shows that the seismic hazards on the above four fault segments are ranked from high to low as Daofu-Kangding section in Xianshuihe fault, the southern section of the Longmen Shan fault, the Maqu section in East Kunlun fault and the Qingchuan fault.

Jiang C.-S.,Institute of Geophysics | Zhang L.-P.,Institute of Earthquake Science
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2011

In recent years, critical slowing down phenomenon have shown great potentials on the aspects of disclosing whether or not a complex dynamic system is tend to critical cataclysm. Based on the concepts of critical slowing down, the observed data of radon concentrations in water were processed in this article. Take radon concentration observation data at Longmen Mountain fault zone and nearby regions before the Wenchuan Ms8. 0 earthquake on May 12, 2008 as an example, the autocorrelation coefficients and variances which can characterize critical slowing down have been calculated separately. The result indicated that radon data at different station had obviously critical slowing down phenomenon before the Wenchuan Ms8. 0 earthquake, which indicated that critical slowing down phenomenon is a possible early warning signal for strong earthquakes. The introduction and study on critical slowing down theory into precursory data processing have practical significances and important scientific values to thoroughly understand precursor mechanism, to determine the stage of precursory anomalies and to enhance earthquake prediction level.

Based on spatial distribution and characteristics of seismic migration after the earthquakes in the northwest region of the the Tibet plateau, we discussed the main active tectonic structures and seismic activity, which related to seismic migration. By analyzing the relationship between active tectonics and seismic activity, the following conclusion-sareobtained: (1) Migration earthquakes are unevenly distributed in the border of active blocks, controlled by the large active tectonic zones; (2) There are three migration paths, dominated by the second path; (3) Within the next five years in the west area of the Altun fault zone, there will be one or two earthquakes of magnitude 6.5; (4) Earthquakes happened after 1900 are mainly distributed along the empty areas; (5) Within the next three to six months, strong migration earthquakes may occur in seismic empty areas, such as the south section of Longmenshan.

Wang Q.,Institute of Earthquake Science | Gao Y.,Institute of Earthquake Science
Earthquake | Year: 2012

Ambient noise tomography is a rapidly emerging field of seismological research. Applying ambient noise to earth structure research can avoid dependence of earthquake signals effectively, meanwhile, ambient noise method can obtain upper crust structure with higher resolution as well as the structure beneath urban area because ambient noise contains abundant short period information. This paper aims to analysis advantages and disadvantages using ambient noise tomography by showing its present state, process method and progress. Anisotropy inverted by a combination of ambient noise data, surface and body waves can accurately reveal crust-mantle structure and geodynamic characteristics of deep structure. Combining noise analysis and other active signal analysis techniques can further study variations of seismological parameter and physical properties before and after earthquake.

Nilupar T.,Institute of Earthquake Science | Zhang Y.-X.,China Earthquake Networks Center
Earthquake | Year: 2012

In this paper, we review recent advances and latest developments on the study of ionospheric disturbances prior to earthquakes. We also introduce DEMETER satellite and some statistical studies and earthquake case studies based on DEMETER satellite data. The common steps for studying ionospheric phenomena before strong earthquakes are expounded: select the earthquake examples-choose the parameters of the ionosphere-data analysis, extract and identify disturbance information-validate pre-earthquake ionospheric responses. The electromagnetic effects and correlated phenomena generated in seismo-electromagnetic process, spread to space by ways of chemistry, acoustics, and electromagnetism. The research results show clearly that ionospheric disturbances appearing several months, days or hours prior to earthquakes can be observed. Whether the precursors could be used as a tool in earthquake prediction need more research.

Gu G.-H.,Institute of Earthquake Science
Earthquake | Year: 2012

Progress in observation techniques, data processing techniques and software for crustal deformation observations and studies on earthquake precursors in the last decades is reviewed. Earthquake predictions in China have obtained rare successes but suffered more tragic failures since the Xintai earthquake in 1966, and have developed with twists and turns under the influence of the assertion that earthquakes are unpredictable and etc.. Though the 2008 M8.0 Wenchuan earthquake and the 2011 M9.0 East Japan earthquake were failed to predict, GPS observations before and after these two events and other events have shown that there were precursors to these events and large earthquakes are predictable. Features of different observation techniques, data processing methods are compared and some recent studies on precursory crustal deformations are summarized, so various advantages of GPS technique are emphasized. The facts show that anomalies or precursors detected from GPS observations before the great Wenchuan earthquake have been the most remarkable results of explorations in crustal movements and earthquake precursors in China.

Jiang Z.-S.,Institute of Earthquake Science | Wu Y.-Q.,Institute of Earthquake Science
Earthquake | Year: 2012

Based on the analysis of the preparation characteristics of the Kunlun 8.1 earthquake and the Wenchuan 8.0 earthquake revealed from GPS data, we review the main research developments of earthquake mechanism and forecast using crustal deformation data in the past decades, and discuss the similarities and differences of scientific approaches between Chinese and foreign scholars. Deformation characteristics of earthquake preparation associated with slip faults and dip-slip faults are analyzed. It is emphasized that, in order to understand the model of relationship between the crustal deformation and earthquake preparation, we should expand the attentions from fault scale to regional scale, and recognize the dynamic deformation characteristics associated with earthquake preparation in multi-scale, spatial-temporal process, and so to obtain the criteria of strong earthquake forecast.

Gu G.-H.,Institute of Earthquake Science | Wang W.-X.,Institute of Earthquake Science
Earthquake | Year: 2011

Five campaigns for non-continuous observations were carried out in the years of 1999, 2001, 2004, 2007 and 2009 from the regional network of 1000 GPS stations of CMONOC(Crustal Movement Observation Network of China). The great Wenchuan M8.0 earthquake of May 12, 2008 occurred in an area with dense GPS observation stations in the regional network. GPS observations in the regional network carried out in many campaigns for a long time can mitigate the effects of periodic annual changes of vertical displacements on long term trend and are favorable for detecting trend variations in vertical displacements. In this paper, the accuracy of vertical displacements obtained from GPS observations is briefly discussed. Ground subsidence, the major contamination in vertical displacements, particularly, large ground subsidence caused by ground water withdraw in large amount, is analyzed. In order to get vertical displacements before the Wenchuan earthquake, at first the results with large subsidence caused by ground water withdraw in large amount must be deleted. Spatial distributions on China mainland from trend surface mappings show 3 areas of significant ground subsidence. In the subsidence area close to the Wenchuan earthquake region there are dense GPS stations that are free from contaminations of ground water withdraw. The vertical displacements from 1999 to 2007 before the Wenchuan earthquake show significant subsidence in the area south east to the earthquake and the northern segment of the Longmenshan fault had been locked as shown in vertical displacements. This subsidence area showed different changes both in time and space from the changes of other 2 subsidence areas. This subsidence area and the area with anomalous strain accumulations appeared at the same time, but they were not overlapping. Large coseismic vertical displacements were located along the northern segment of the Longmenshan fault which was locked before the event. All these facts show that the subsidence area near the epicenter, detected by GPS observations before the earthquake, and the locked segment of the fault shown by vertical displacements were closely related with the occurrence of the Wenchuan earthquake.

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