A statistical analysis on the Wenchuan aftershock activity triggered by tidal forces is systematically studied based on Schusters test, including earthquakes triggered by tidal force, tidal stress and tidal coulomb fa...A statistical analysis on the Wenchuan aftershock activity triggered by tidal forces is systematically studied based on Schusters test, including earthquakes triggered by tidal force, tidal stress and tidal coulomb failure stress. The results show that a group of strong aftershocks which occurred at the end of July to early August in 2008 at the north of Wenchuan were obviously triggered by earth tide, the same conclusion is drawn by Schusters smooth test of the tidal force, tidal stress and tidal coulomb failure stress. In addition, the Wenchuan aftershock activity is obviously triggered by fortnight tide. In the north, the aftershocks happened more frequently in the first and last quarters of the moon, and in the south, the aftershocks happened more frequently in the first and last quarters of the moon and during the full moon.展开更多
We use the slowness-azimuth station correction( SASC) method to improve the location accuracy of the Wenchuan aftershocks recorded by the Nagqu and Hotan seismic arrays.The results show that the standard deviations of...We use the slowness-azimuth station correction( SASC) method to improve the location accuracy of the Wenchuan aftershocks recorded by the Nagqu and Hotan seismic arrays.The results show that the standard deviations of back-azimuth and slowness errors of Wenchuan aftershocks recorded by the Nagqu array decreased by 32% and 58%respectively after correction. The decrease is 38% and 71% for the Hotan array. After the correction,the location accuracy of all Wenchuan aftershocks recorded by the Nagqu array is improved. For the Hotan array,the accuracy is improved in the slowness estimation for 78% of aftershocks and in back-azimuth estimation for all aftershocks.展开更多
Local site conditions play an important role in the effective application of strong motion recordings.In the China National Strong Motion Observation Network System（NSMONS）,some of the stations do not provide boreho...Local site conditions play an important role in the effective application of strong motion recordings.In the China National Strong Motion Observation Network System（NSMONS）,some of the stations do not provide borehole information,and correspondingly,do not assign the site classes yet.In this paper,site classification methodologies for free-field strong motion stations are reviewed and the limitations and uncertainties of the horizontal-to-vertical spectral ratio（HVSR） methods are discussed.Then,a new method for site classification based on the entropy weight theory is proposed.The proposed method avoids the head or tail joggle phenomenon by providing the objective and subjective weights.The method was applied to aftershock recordings from the 2008 Wenchuan earthquake,and 54 free-field NSMONS stations were selected for site classification and the mean HVSRs were calculated.The results show that the improved HVSR method proposed in this paper has a higher success rate and could be adopted in NSMONS.展开更多
Based on Gutenberg-Richter＇s relation,Bath＇s law,Omori＇s law and Well＇s relation of rupture scale,this paper forecasts the temporal decay,total number,possible area and greatest magnitude of strong aftershocks（gr...Based on Gutenberg-Richter＇s relation,Bath＇s law,Omori＇s law and Well＇s relation of rupture scale,this paper forecasts the temporal decay,total number,possible area and greatest magnitude of strong aftershocks（greater than or equal to M6.0） of the MS8.0 Wenchuan earthquake by using the magnitude and statistical parameters of earthquakes in California area of USA.The number of strong aftershocks,the parameters of Gutenberg-Richter＇s relation and the modified form of Omori＇s law are validated based on the relocation data of aftershock sequence of the MS8.0 Wenchuan earthquake.Moreover,the spatio-temporal characteristics and wave energy release of the strong aftershocks（M≥6.0） are analyzed.The result shows that strong aftershocks may occur at the end of local drop and sharp drop on the wave energy release curve.展开更多
The aftershock activity of Wenchuan Ms8.0 earthquake showed different spatial and temporal distri- butions along two different segments of the Longmenshan fault. This difference was likely the result of segmentation o...The aftershock activity of Wenchuan Ms8.0 earthquake showed different spatial and temporal distri- butions along two different segments of the Longmenshan fault. This difference was likely the result of segmentation of the earthquake rupture process, which in turn may be the result of the fault＇ s segmentation in its long-term geotectonic condition.展开更多
Because of the confining compression of the Pacific, Eurasian and Indian plates, the Chinese mainland is frequently stricken with earthquakes, especially in the Qinghai Tibet Plateau and surrounding areas and along th...Because of the confining compression of the Pacific, Eurasian and Indian plates, the Chinese mainland is frequently stricken with earthquakes, especially in the Qinghai Tibet Plateau and surrounding areas and along the NS-trending tectonic zone from Yinchuan to Lanzhou, Chengdu, and finally to the Kunming tectonic belt （Fig. 1）. Historical records show that there are 14 earthquakes of 〉Ms 8 occurring in the two regions, eight of which occurred along the latter tectonic belt. The 2008 Wenchuan earthquake located on both the NS tectonic zone and the Longmenshan fault zone along the eastern margin of the Qinghai-Tibet Plateau.展开更多
Micro-aftershocks with magnitude range of 1.5--4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value ...Micro-aftershocks with magnitude range of 1.5--4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aflershocks of M1.5-4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks ofM3.0-5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of 〈5 km, the b-values are very small （-0.4）, possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of-5-11 km, where most aftershocks concentrated, the b-values become as high as -0.9-1.0. At the depth below -11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.展开更多
New Vp and Vs models of the aftershock gap between the 2013 Lushan and 2008 Wenchuan earthquakes were obtained using 7190 events recorded by 102 stations in the southeastern margin of the Tibetan Plateau. The new velo...New Vp and Vs models of the aftershock gap between the 2013 Lushan and 2008 Wenchuan earthquakes were obtained using 7190 events recorded by 102 stations in the southeastern margin of the Tibetan Plateau. The new velocity models and event locations were inverted by the regional-scale version of double-difference tomography algorithm. The new velocity structure shows that the aftershock gap is an obviously low-velocity zone in the upper crust,whereas the aftershock regions of the Lushan and Wenchuan earthquakes show high-velocity anomalies. The low-velocity anomaly of the aftershock gap in the upper crust may indicate that the materials of the gap are weak and ductile,which is not prone to accumulate stress and cause big earthquakes. As a result,it is less likely that the big earthquakes will occur in the gap in the future.展开更多
By the aftershock frequency estimation method based on the calculation of coseismic static Coulomb stress changes and rate-and state-dependent fault constitutive law,we calculate the frequency of "direct "af...By the aftershock frequency estimation method based on the calculation of coseismic static Coulomb stress changes and rate-and state-dependent fault constitutive law,we calculate the frequency of "direct "aftershocks of the Wenchuan earthquake related to coseismic static Coulomb stress changes in its aftershock zone and the areas nearby.It shows that the frequency is significantly lower than the truth in the main rupture zone,especially in the southern rupture zone,due to the decrease of stress level on the rupture plane of the main shock resulting from coseismic Coulomb stress change.The study also shows that the effect of the Coulomb stress change on the duration of aftershock activity is associated with the lower limit magnitude.The duration is about 15-16 months for aftershocks above ML4.0,and close to 60 months for aftershocks above ML3.5.In this period,the ratio of the"direct"aftershocks caused by coseismic Coulomb stress change ranges between 44.7% to48.6%,which suggests that,even in the "effective"period of coseismic Coulomb stress changes,about half of the aftershocks on the main shock rupture plane are independent of coseismic Coulomb stress changes.It is pointed out that those aftershocks may be related to the afterslip or the viscoelastic relaxation,which are time dependent cases.展开更多
Based on abundant aftershock sequence data of the Wenchuan Ms8.0 earthquake on May 12, 2008, we studied the spatio-temporal variation process and segmentation rupture characteristic. Dense aftershocks distribute along...Based on abundant aftershock sequence data of the Wenchuan Ms8.0 earthquake on May 12, 2008, we studied the spatio-temporal variation process and segmentation rupture characteristic. Dense aftershocks distribute along Longmenshan central fault zone of NE direction and form a narrow strip with the length of 325 krn and the depth between several and 40 km. The depth profile （section of NW direction） vertical to the strike of aftershock zone （NE direction） shows anisomerous wedgy distribution characteristic of afiershock concentrated regions; it is related to the force form of the Longmenshan nappe tectonic belt. The stronger aftershocks could be divided into northern segment and southern segment apparently and the focal depths of strong aftershocks in the 50 km area between northern segment and southern segment are shallower. It seems like ＇to be going to rupture＇ segment. We also study focal mechanisms and segmentation of strong aftershocks. The principal compressive stress azimuth of aftershock area is WNW direction and the faulting types of aftershocks at southern and northern segment have the same proportion. Because afiershocks distribute on different secondary faults, their focal mechanisms present complex local tectonic stress field. The faulting of seven strong earthquakes on the Longmenshan central fault is mainly characterized by thrust with the component of right-lateral strike-slip. Meantime six strong aftershocks on the Longmenshan back-range fault and Qingchuan fault present strike-slip faulting. At last we discuss the complex segmentation rupture mechanism of the Wenchuan earthquake.展开更多
基金sponsored by the National Key Technology R&D Program,China(2008BAC38B03)
文摘A statistical analysis on the Wenchuan aftershock activity triggered by tidal forces is systematically studied based on Schusters test, including earthquakes triggered by tidal force, tidal stress and tidal coulomb failure stress. The results show that a group of strong aftershocks which occurred at the end of July to early August in 2008 at the north of Wenchuan were obviously triggered by earth tide, the same conclusion is drawn by Schusters smooth test of the tidal force, tidal stress and tidal coulomb failure stress. In addition, the Wenchuan aftershock activity is obviously triggered by fortnight tide. In the north, the aftershocks happened more frequently in the first and last quarters of the moon, and in the south, the aftershocks happened more frequently in the first and last quarters of the moon and during the full moon.
基金sponsored by the Basic Scientific Research Special Program of Institute of Geophysics,China Earthquake Administration(DQJB08819)
文摘We use the slowness-azimuth station correction( SASC) method to improve the location accuracy of the Wenchuan aftershocks recorded by the Nagqu and Hotan seismic arrays.The results show that the standard deviations of back-azimuth and slowness errors of Wenchuan aftershocks recorded by the Nagqu array decreased by 32% and 58%respectively after correction. The decrease is 38% and 71% for the Hotan array. After the correction,the location accuracy of all Wenchuan aftershocks recorded by the Nagqu array is improved. For the Hotan array,the accuracy is improved in the slowness estimation for 78% of aftershocks and in back-azimuth estimation for all aftershocks.
基金National Key Technology R&D Program Under Grant No.2009BAK55B05Nonprofit Industry Research Project of CEA Under Grant No.201108003Science Foundation of Institute of Engineering Mechanics,CEA Under Grant No.2010C01
文摘Local site conditions play an important role in the effective application of strong motion recordings.In the China National Strong Motion Observation Network System（NSMONS）,some of the stations do not provide borehole information,and correspondingly,do not assign the site classes yet.In this paper,site classification methodologies for free-field strong motion stations are reviewed and the limitations and uncertainties of the horizontal-to-vertical spectral ratio（HVSR） methods are discussed.Then,a new method for site classification based on the entropy weight theory is proposed.The proposed method avoids the head or tail joggle phenomenon by providing the objective and subjective weights.The method was applied to aftershock recordings from the 2008 Wenchuan earthquake,and 54 free-field NSMONS stations were selected for site classification and the mean HVSRs were calculated.The results show that the improved HVSR method proposed in this paper has a higher success rate and could be adopted in NSMONS.
基金supported by Basic Science and Research Fund for Chinese Commonweal Institutes under grant No. 2008B07the National Natural Science Foundation of China under grant No. 90715042+1 种基金Special Research Project of Earth-quake Engineering under grant No. 200808008National Science and Technology Support Plan under grant No. 2006BAC13B02
文摘Based on Gutenberg-Richter＇s relation,Bath＇s law,Omori＇s law and Well＇s relation of rupture scale,this paper forecasts the temporal decay,total number,possible area and greatest magnitude of strong aftershocks（greater than or equal to M6.0） of the MS8.0 Wenchuan earthquake by using the magnitude and statistical parameters of earthquakes in California area of USA.The number of strong aftershocks,the parameters of Gutenberg-Richter＇s relation and the modified form of Omori＇s law are validated based on the relocation data of aftershock sequence of the MS8.0 Wenchuan earthquake.Moreover,the spatio-temporal characteristics and wave energy release of the strong aftershocks（M≥6.0） are analyzed.The result shows that strong aftershocks may occur at the end of local drop and sharp drop on the wave energy release curve.
基金supported by the Earthguake Science Joint Foundation( A07007 ) The Project of China Eanthguake Administiton( 200708026)
文摘The aftershock activity of Wenchuan Ms8.0 earthquake showed different spatial and temporal distri- butions along two different segments of the Longmenshan fault. This difference was likely the result of segmentation of the earthquake rupture process, which in turn may be the result of the fault＇ s segmentation in its long-term geotectonic condition.
文摘Because of the confining compression of the Pacific, Eurasian and Indian plates, the Chinese mainland is frequently stricken with earthquakes, especially in the Qinghai Tibet Plateau and surrounding areas and along the NS-trending tectonic zone from Yinchuan to Lanzhou, Chengdu, and finally to the Kunming tectonic belt （Fig. 1）. Historical records show that there are 14 earthquakes of 〉Ms 8 occurring in the two regions, eight of which occurred along the latter tectonic belt. The 2008 Wenchuan earthquake located on both the NS tectonic zone and the Longmenshan fault zone along the eastern margin of the Qinghai-Tibet Plateau.
基金supported bythe Basic Research Foundation of the Institute of Ge-omechanics,Chinese Academy of Geological Sciences(No.DZLXJK200707)the National Natural Science Foundation of China(No.40674058)
文摘Micro-aftershocks with magnitude range of 1.5--4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aflershocks of M1.5-4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks ofM3.0-5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of 〈5 km, the b-values are very small （-0.4）, possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of-5-11 km, where most aftershocks concentrated, the b-values become as high as -0.9-1.0. At the depth below -11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.
文摘New Vp and Vs models of the aftershock gap between the 2013 Lushan and 2008 Wenchuan earthquakes were obtained using 7190 events recorded by 102 stations in the southeastern margin of the Tibetan Plateau. The new velocity models and event locations were inverted by the regional-scale version of double-difference tomography algorithm. The new velocity structure shows that the aftershock gap is an obviously low-velocity zone in the upper crust,whereas the aftershock regions of the Lushan and Wenchuan earthquakes show high-velocity anomalies. The low-velocity anomaly of the aftershock gap in the upper crust may indicate that the materials of the gap are weak and ductile,which is not prone to accumulate stress and cause big earthquakes. As a result,it is less likely that the big earthquakes will occur in the gap in the future.
基金funded by the special project of the National Science&Technology Pillar Program during the Twelfth "Five-year Plan" Period(2012BAK19B02-05-02)
文摘By the aftershock frequency estimation method based on the calculation of coseismic static Coulomb stress changes and rate-and state-dependent fault constitutive law,we calculate the frequency of "direct "aftershocks of the Wenchuan earthquake related to coseismic static Coulomb stress changes in its aftershock zone and the areas nearby.It shows that the frequency is significantly lower than the truth in the main rupture zone,especially in the southern rupture zone,due to the decrease of stress level on the rupture plane of the main shock resulting from coseismic Coulomb stress change.The study also shows that the effect of the Coulomb stress change on the duration of aftershock activity is associated with the lower limit magnitude.The duration is about 15-16 months for aftershocks above ML4.0,and close to 60 months for aftershocks above ML3.5.In this period,the ratio of the"direct"aftershocks caused by coseismic Coulomb stress change ranges between 44.7% to48.6%,which suggests that,even in the "effective"period of coseismic Coulomb stress changes,about half of the aftershocks on the main shock rupture plane are independent of coseismic Coulomb stress changes.It is pointed out that those aftershocks may be related to the afterslip or the viscoelastic relaxation,which are time dependent cases.
基金supported by National Key Basic Research 973bNational Scientific Technology Support Plan (2006BAC01B02-01-01).
文摘Based on abundant aftershock sequence data of the Wenchuan Ms8.0 earthquake on May 12, 2008, we studied the spatio-temporal variation process and segmentation rupture characteristic. Dense aftershocks distribute along Longmenshan central fault zone of NE direction and form a narrow strip with the length of 325 krn and the depth between several and 40 km. The depth profile （section of NW direction） vertical to the strike of aftershock zone （NE direction） shows anisomerous wedgy distribution characteristic of afiershock concentrated regions; it is related to the force form of the Longmenshan nappe tectonic belt. The stronger aftershocks could be divided into northern segment and southern segment apparently and the focal depths of strong aftershocks in the 50 km area between northern segment and southern segment are shallower. It seems like ＇to be going to rupture＇ segment. We also study focal mechanisms and segmentation of strong aftershocks. The principal compressive stress azimuth of aftershock area is WNW direction and the faulting types of aftershocks at southern and northern segment have the same proportion. Because afiershocks distribute on different secondary faults, their focal mechanisms present complex local tectonic stress field. The faulting of seven strong earthquakes on the Longmenshan central fault is mainly characterized by thrust with the component of right-lateral strike-slip. Meantime six strong aftershocks on the Longmenshan back-range fault and Qingchuan fault present strike-slip faulting. At last we discuss the complex segmentation rupture mechanism of the Wenchuan earthquake.
