This article is to review results from scientific drilling and fault-zone trapped waves （FZTWs） at the south Longman-Shan fault （LSF） zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan,China.I...This article is to review results from scientific drilling and fault-zone trapped waves （FZTWs） at the south Longman-Shan fault （LSF） zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan,China.Immediately after the mainshock,two Wenchuan Fault Scientific Drilling （WFSD） boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault （YBF）,the middle fault strand of the south LSF zone.Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m,respectively.The slip is accompanied with a 100-200-m-wide zone consisting of fault gouge,breccia,cataclasite and fractures.Close to WFSD-1 site,the nearly-vertical slip of ～4.3-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake.A dense linear seismic array was deployed across the surface rupture at this venue to record FZTWs generated by aftershocks.Observations and 3-D finite-difference simulations of FZTWs recorded at this cross-fault array and network stations close to the YBF show a distinct low-velocity zone composed by severely damaged rocks along the south LSF at seismogenic depths.The zone is several hundred meters wide along the principal slip,within which seismic velocities are reduced by ～30-55％ from wall-rock velocities and with the maximum velocity reduction in the ～200-m-wide rupture core zone at shallow depth.The FZTW-inferred geometry and physical properties of the south LSF rupture zone at shallow depth are in general consistent with the results from petrological and structural analyses of cores and well log at WFSD boreholes.We interpret this remarkable low-velocity zone as being a break-down zone during dynamic rupture in the 2008 M8 earthquake.We examined the FZTWS generated by similar earthquakes before and after the 2008 mainshock and observed that seismic velocities within fault core zone was reduced by ～10％ due to severe damage of fault rocks during the M8 mainshock.Scientific drilling and locations of aftershocks generating prominent FZTWs also indicate rupture bifurcation along the YBF and the Anxian-Guangxian fault （AGF）,two strands of the south LSF at shallow depth.A combination of seismic,petrologic and geologic study at the south LSF leads to further understand the relationship between the fault-zone structure and rupture dynamics,and the amplification of ground shaking strength along the low-velocity fault zone due to its waveguide effect.展开更多
Trapped waves in the Qingchuan fault zone were observed at Muyu near the northeastern end of the fractured zone of the Wenchuan Ms8. 0 earthquake. The results indicate a fault-zone width of about 200 m and a great dif...Trapped waves in the Qingchuan fault zone were observed at Muyu near the northeastern end of the fractured zone of the Wenchuan Ms8. 0 earthquake. The results indicate a fault-zone width of about 200 m and a great difference in physical property of the crust on different sides of the fault. The inferred location of crustal changes is consistent with land-form boundary on the surface展开更多
In this article,we review our previous research for spatial and temporal characterizations of the San Andreas Fault(SAF)at Parkfield,using the fault-zone trapped wave(FZTW)since the middle 1980s.Parkfield,California h...In this article,we review our previous research for spatial and temporal characterizations of the San Andreas Fault(SAF)at Parkfield,using the fault-zone trapped wave(FZTW)since the middle 1980s.Parkfield,California has been taken as a scientific seismic experimental site in the USA since the 1970s,and the SAF is the target fault to investigate earthquake physics and forecasting.More than ten types of field experiments(including seismic,geophysical,geochemical,geodetic and so on)have been carried out at this experimental site since then.In the fall of 2003,a pair of scientific wells were drilled at the San Andreas Fault Observatory at Depth(SAFOD)site;the main-hole(MH)passed a~200-m-wide low-velocity zone(LVZ)with highly fractured rocks of the SAF at a depth of~3.2 km below the wellhead on the ground level(Hickman et al.,2005;Zoback,2007;Lockner et al.,2011).Borehole seismographs were installed in the SAFOD MH in 2004,which were located within the LVZ of the fault at~3-km depth to probe the internal structure and physical properties of the SAF.On September 282004,a M6 earthquake occurred~15 km southeast of the town of Parkfield.The data recorded in the field experiments before and after the 2004 M6 earthquake provided a unique opportunity to monitor the co-mainshock damage and post-seismic heal of the SAF associated with this strong earthquake.This retrospective review of the results from a sequence of our previous experiments at the Parkfield SAF,California,will be valuable for other researchers who are carrying out seismic experiments at the active faults to develop the community seismic wave velocity models,the fault models and the earthquake forecasting models in global seismogenic regions.展开更多
Pingtong Town is located on the fractured zone of the Wenchuan 8.0 earthquake, and is seriously damaged by the earthquake. Our observation line is centered at an earthquake exploration trench across the fractured zone...Pingtong Town is located on the fractured zone of the Wenchuan 8.0 earthquake, and is seriously damaged by the earthquake. Our observation line is centered at an earthquake exploration trench across the fractured zone in the NW-SE direction, and is about 400 m long. The results reveal trapped waves in the rup- tured fault zone of the earthquake, and indicate a great difference in physical property between the media inside and outside the fault zone. The predominant frequency of the fault-zone trapped waves is about 3 -4 Hz. The wave amplitudes are larger near the exploration trench. The width of the fault zone in the crust at this location is estimated to be 200 m. In some records, the waveforms and the arrival times of S waves are quite different between the two sides of the trench. The place of change coincides with the boundary of uplift at the surface.展开更多
The observation of the fault-zone trapped waves was conducted using a seismic line with dense receivers across surface rupture zone of the M=8.1 Kunlun Mountain earthquake. The fault zone trapped waves were separated ...The observation of the fault-zone trapped waves was conducted using a seismic line with dense receivers across surface rupture zone of the M=8.1 Kunlun Mountain earthquake. The fault zone trapped waves were separated from seismograms by numerical filtering and spectral analyzing. The results show that: a) Both explosion and earthquake sources can excite fault-zone trapped waves, as long as they locate in or near the fault zone; b) Most energy of the fault-zone trapped waves concentrates in the fault zone and the amplitudes strongly decay with the distance from observation point to the fault zone; c) Dominant frequencies of the fault-zone trapped waves are related to the width of the fault zone and the velocity of the media in it. The wider the fault zone or the lower the velocity is, the lower the dominant frequencies are; d) For fault zone trapped waves, there exist dispersions; e) Based on the fault zone trapped waves observed in Kunlun Mountain Pass region, the width of the rupture plane is deduced to be about 300 m and is greater than that on the surface.展开更多
本文基于地面测量布格异常和EIGEN-6C4(European Improved Gravity model of the Earth by New techniques version 6C4)重力资料,以USTClitho2.0(Unified Seismic Tomography models for continental China lithosphere version 2.0)...本文基于地面测量布格异常和EIGEN-6C4(European Improved Gravity model of the Earth by New techniques version 6C4)重力资料,以USTClitho2.0(Unified Seismic Tomography models for continental China lithosphere version 2.0)岩石圈结构模型为约束,利用顾及地壳密度横向变化的布格改正、小波多尺度分解、均衡改正、Moho面重力反演和岩石圈有效弹性厚度(T_(e))反演等方法,计算了郯庐断裂带中南段布格异常及其多尺度分解结果、深部均衡异常、Moho面深度、岩石圈有效弹性厚度及荷载比,结合地震活动性和地球物理资料,综合分析了该地区重力异常构造背景及其与地震活动性的关系.结果表明,沿郯庐断裂带的重力异常在嘉山以南段、嘉山-新沂段和新沂-安丘段呈显著差异性3段式分布,且与地震活动性分段特征高度统一.地震活动性强的嘉山以南段和新沂-安丘段具有浅部布格异常线性条带、深部布格异常高梯度带、Moho面陡变带或上隆带、地壳重力不均衡、低岩石圈有效弹性厚度和高荷载比特征,表明其深部结构破坏程度高,岩石圈力学强度低,且存在均衡调整作用,有利于应力及时释放,因而地震活动频繁;地震活动性弱的嘉山-新沂段浅部布格异常散乱,深部布格异常高值区和低值正均衡异常区穿过郯庐断裂带,Moho面变化平缓,岩石圈有效弹性厚度值高,荷载比低,对应扬子地块俯冲于华北断块区之下的遗留深部构造,其深部结构破坏程度较低,岩石圈力学强度大,且均衡调整作用不强,岩石圈不易发生破裂,因而地震频率很低.郯城8.5级地震位于横穿郯庐断裂带的深部构造北缘附近,是最易于积累应力的部位,深部构造和岩石圈有效弹性厚度剧烈变化导致应力分布不均匀以及较强的均衡调整作用可能是地震的触发因素.嘉山-新沂段具备有利于应力长期积累的深部构造条件,应持续关注其应力积累状态和强震风险.展开更多
基金supported by the "Wenchuan Earthquake Fault Scientific Drilling" of the National Science Foundation of China
文摘This article is to review results from scientific drilling and fault-zone trapped waves （FZTWs） at the south Longman-Shan fault （LSF） zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan,China.Immediately after the mainshock,two Wenchuan Fault Scientific Drilling （WFSD） boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault （YBF）,the middle fault strand of the south LSF zone.Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m,respectively.The slip is accompanied with a 100-200-m-wide zone consisting of fault gouge,breccia,cataclasite and fractures.Close to WFSD-1 site,the nearly-vertical slip of ～4.3-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake.A dense linear seismic array was deployed across the surface rupture at this venue to record FZTWs generated by aftershocks.Observations and 3-D finite-difference simulations of FZTWs recorded at this cross-fault array and network stations close to the YBF show a distinct low-velocity zone composed by severely damaged rocks along the south LSF at seismogenic depths.The zone is several hundred meters wide along the principal slip,within which seismic velocities are reduced by ～30-55％ from wall-rock velocities and with the maximum velocity reduction in the ～200-m-wide rupture core zone at shallow depth.The FZTW-inferred geometry and physical properties of the south LSF rupture zone at shallow depth are in general consistent with the results from petrological and structural analyses of cores and well log at WFSD boreholes.We interpret this remarkable low-velocity zone as being a break-down zone during dynamic rupture in the 2008 M8 earthquake.We examined the FZTWS generated by similar earthquakes before and after the 2008 mainshock and observed that seismic velocities within fault core zone was reduced by ～10％ due to severe damage of fault rocks during the M8 mainshock.Scientific drilling and locations of aftershocks generating prominent FZTWs also indicate rupture bifurcation along the YBF and the Anxian-Guangxian fault （AGF）,two strands of the south LSF at shallow depth.A combination of seismic,petrologic and geologic study at the south LSF leads to further understand the relationship between the fault-zone structure and rupture dynamics,and the amplification of ground shaking strength along the low-velocity fault zone due to its waveguide effect.
基金supported by the National Natural Science Foundation ofChina(41074069,40974053,90814001)RRCEG201103
文摘Trapped waves in the Qingchuan fault zone were observed at Muyu near the northeastern end of the fractured zone of the Wenchuan Ms8. 0 earthquake. The results indicate a fault-zone width of about 200 m and a great difference in physical property of the crust on different sides of the fault. The inferred location of crustal changes is consistent with land-form boundary on the surface
文摘In this article,we review our previous research for spatial and temporal characterizations of the San Andreas Fault(SAF)at Parkfield,using the fault-zone trapped wave(FZTW)since the middle 1980s.Parkfield,California has been taken as a scientific seismic experimental site in the USA since the 1970s,and the SAF is the target fault to investigate earthquake physics and forecasting.More than ten types of field experiments(including seismic,geophysical,geochemical,geodetic and so on)have been carried out at this experimental site since then.In the fall of 2003,a pair of scientific wells were drilled at the San Andreas Fault Observatory at Depth(SAFOD)site;the main-hole(MH)passed a~200-m-wide low-velocity zone(LVZ)with highly fractured rocks of the SAF at a depth of~3.2 km below the wellhead on the ground level(Hickman et al.,2005;Zoback,2007;Lockner et al.,2011).Borehole seismographs were installed in the SAFOD MH in 2004,which were located within the LVZ of the fault at~3-km depth to probe the internal structure and physical properties of the SAF.On September 282004,a M6 earthquake occurred~15 km southeast of the town of Parkfield.The data recorded in the field experiments before and after the 2004 M6 earthquake provided a unique opportunity to monitor the co-mainshock damage and post-seismic heal of the SAF associated with this strong earthquake.This retrospective review of the results from a sequence of our previous experiments at the Parkfield SAF,California,will be valuable for other researchers who are carrying out seismic experiments at the active faults to develop the community seismic wave velocity models,the fault models and the earthquake forecasting models in global seismogenic regions.
基金supported by the Natural Science Fundation of China(40774043,40674043,90814001)
文摘Pingtong Town is located on the fractured zone of the Wenchuan 8.0 earthquake, and is seriously damaged by the earthquake. Our observation line is centered at an earthquake exploration trench across the fractured zone in the NW-SE direction, and is about 400 m long. The results reveal trapped waves in the rup- tured fault zone of the earthquake, and indicate a great difference in physical property between the media inside and outside the fault zone. The predominant frequency of the fault-zone trapped waves is about 3 -4 Hz. The wave amplitudes are larger near the exploration trench. The width of the fault zone in the crust at this location is estimated to be 200 m. In some records, the waveforms and the arrival times of S waves are quite different between the two sides of the trench. The place of change coincides with the boundary of uplift at the surface.
基金Joint Earthquake Science Foundation of China (201001).
文摘The observation of the fault-zone trapped waves was conducted using a seismic line with dense receivers across surface rupture zone of the M=8.1 Kunlun Mountain earthquake. The fault zone trapped waves were separated from seismograms by numerical filtering and spectral analyzing. The results show that: a) Both explosion and earthquake sources can excite fault-zone trapped waves, as long as they locate in or near the fault zone; b) Most energy of the fault-zone trapped waves concentrates in the fault zone and the amplitudes strongly decay with the distance from observation point to the fault zone; c) Dominant frequencies of the fault-zone trapped waves are related to the width of the fault zone and the velocity of the media in it. The wider the fault zone or the lower the velocity is, the lower the dominant frequencies are; d) For fault zone trapped waves, there exist dispersions; e) Based on the fault zone trapped waves observed in Kunlun Mountain Pass region, the width of the rupture plane is deduced to be about 300 m and is greater than that on the surface.
文摘本文基于地面测量布格异常和EIGEN-6C4(European Improved Gravity model of the Earth by New techniques version 6C4)重力资料,以USTClitho2.0(Unified Seismic Tomography models for continental China lithosphere version 2.0)岩石圈结构模型为约束,利用顾及地壳密度横向变化的布格改正、小波多尺度分解、均衡改正、Moho面重力反演和岩石圈有效弹性厚度(T_(e))反演等方法,计算了郯庐断裂带中南段布格异常及其多尺度分解结果、深部均衡异常、Moho面深度、岩石圈有效弹性厚度及荷载比,结合地震活动性和地球物理资料,综合分析了该地区重力异常构造背景及其与地震活动性的关系.结果表明,沿郯庐断裂带的重力异常在嘉山以南段、嘉山-新沂段和新沂-安丘段呈显著差异性3段式分布,且与地震活动性分段特征高度统一.地震活动性强的嘉山以南段和新沂-安丘段具有浅部布格异常线性条带、深部布格异常高梯度带、Moho面陡变带或上隆带、地壳重力不均衡、低岩石圈有效弹性厚度和高荷载比特征,表明其深部结构破坏程度高,岩石圈力学强度低,且存在均衡调整作用,有利于应力及时释放,因而地震活动频繁;地震活动性弱的嘉山-新沂段浅部布格异常散乱,深部布格异常高值区和低值正均衡异常区穿过郯庐断裂带,Moho面变化平缓,岩石圈有效弹性厚度值高,荷载比低,对应扬子地块俯冲于华北断块区之下的遗留深部构造,其深部结构破坏程度较低,岩石圈力学强度大,且均衡调整作用不强,岩石圈不易发生破裂,因而地震频率很低.郯城8.5级地震位于横穿郯庐断裂带的深部构造北缘附近,是最易于积累应力的部位,深部构造和岩石圈有效弹性厚度剧烈变化导致应力分布不均匀以及较强的均衡调整作用可能是地震的触发因素.嘉山-新沂段具备有利于应力长期积累的深部构造条件,应持续关注其应力积累状态和强震风险.
