Deep structure beneath the southwestern section of the Longmenshan fault zone and seimogenetic context of the 4.20 Lushan M_S7.0 earthquake
|
摘要
Magnetotelluric measurements were carried out along two profiles across the middle and southwestern sections of the Longmenshan fault zone(LMSf)from 2009 to 2011,after the 2008 Wenchuan MW7.9 earthquake.The former profile crosses the Wenchuan event epicenter and the latter one crosses 2013 Lushan MS7.0 event epicenter.The data were analyzed using advanced processing techniques,including phase tensor and two-dimensional inversion methods,in order to obtain reliable 2-D profiles of the electrical structure in the vicinity of the two earthquakes.A comparison of the two profiles indicates both similarities and differences in the deep crustal structure of the LMSf.West of the southwestern section,a crustal high conductivity layer(HCL)is present at about 10 km depth below the Songpan-Garzêblock;this is about 10 km shallower than that under the middle section of the LMSf.A high resistivity body(HRB)is observed beneath the southwestern section,extending from the near surface to the top of upper mantle.It has a smaller size than the HRB observed below the middle section.In the middle section,there is a local area of decreased resistivity within the HRB but there is absence of this area.The 2013 Lushan earthquake occurred close to the eastern boundary of HRB and the Shuangshi-Dachuan fault,of which the seismogenic context has both common and different features in comparison with the 2008 Wenchuan event.On a large scale,the 2013 Lushan earthquake is associated with the HCL and deformation in the crust including HCL of the eastern Tibetan Plateau.In order to assess seismic risk,it is important to consider both the stress state and the detailed crustal structure in different parts of the LMSf.
Magnetotelluric measurements were carried out along two profiles across the middle and southwestern sections of the Longmenshan fault zone(LMSf) from 2009 to 2011, after the 2008 Wenchuan MW7.9 earthquake. The former profile crosses the Wenchuan event epicenter and the latter one crosses 2013 Lushan MS7.0 event epicenter. The data were analyzed using advanced processing techniques, including phase tensor and two-dimensional inversion methods, in order to obtain reliable 2-D profiles of the electrical structure in the vicinity of the two earthquakes. A comparison of the two profiles indicates both similarities and differences in the deep crustal structure of the LMSf. West of the southwestern section, a crustal high conductivity layer(HCL) is present at about 10 km depth below the Songpan-Garzê block; this is about 10 km shallower than that under the middle section of the LMSf.A high resistivity body(HRB) is observed beneath the southwestern section, extending from the near surface to the top of upper mantle. It has a smaller size than the HRB observed below the middle section. In the middle section, there is a local area of decreased resistivity within the HRB but there is absence of this area. The 2013 Lushan earthquake occurred close to the eastern boundary of HRB and the Shuangshi-Dachuan fault, of which the seismogenic context has both common and different features in comparison with the 2008 Wenchuan event. On a large scale, the 2013 Lushan earthquake is associated with the HCL and deformation in the crust including HCL of the eastern Tibetan Plateau. In order to assess seismic risk, it is important to consider both the stress state and the detailed crustal structure in different parts of the LMSf.
Magnetotelluric measurements were carried out along two profiles across the middle and southwestern sections of the Longmenshan fault zone(LMSf) from 2009 to 2011, after the 2008 Wenchuan MW7.9 earthquake. The former profile crosses the Wenchuan event epicenter and the latter one crosses 2013 Lushan MS7.0 event epicenter. The data were analyzed using advanced processing techniques, including phase tensor and two-dimensional inversion methods, in order to obtain reliable 2-D profiles of the electrical structure in the vicinity of the two earthquakes. A comparison of the two profiles indicates both similarities and differences in the deep crustal structure of the LMSf. West of the southwestern section, a crustal high conductivity layer(HCL) is present at about 10 km depth below the Songpan-Garzê block; this is about 10 km shallower than that under the middle section of the LMSf.A high resistivity body(HRB) is observed beneath the southwestern section, extending from the near surface to the top of upper mantle. It has a smaller size than the HRB observed below the middle section. In the middle section, there is a local area of decreased resistivity within the HRB but there is absence of this area. The 2013 Lushan earthquake occurred close to the eastern boundary of HRB and the Shuangshi-Dachuan fault, of which the seismogenic context has both common and different features in comparison with the 2008 Wenchuan event. On a large scale, the 2013 Lushan earthquake is associated with the HCL and deformation in the crust including HCL of the eastern Tibetan Plateau. In order to assess seismic risk, it is important to consider both the stress state and the detailed crustal structure in different parts of the LMSf.
引文
1 Xu X,Wen X,Yu G,et al.Coseismic reverse and oblique-slip surface faulting generated by the 2008 MW7 .9 Wenchuan earthquake,China.Geology,2009,37:515–518
2 Huang Y,Wu J P,Zhang T Z,et a1.Relocation of the M8.0 Wenchuan earthquake and its aftershock sequence.Sci China Ser D-Earth Sci,2008,51:1703–1711
3 Zhang Y,Feng W P,Xu L S,et a1.Spatiotemporal rupture process of the 2008 great Wenchuan earthquake.Sci China Ser D-Earth Sci,2009,52:145–154
4 Zhang P Z,Wen X Z,Xu X W,et al.Tectonic model of the great Wenchuan earthquake of May 12,2008,Sichuan,China.Chin Sci Bull,2009,54:944–953
5 Shen Z K,Lu J,Wang M,et al.Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau.J Geophys Res,2005,110:1–17
6 Zhang Y X,Wang H M,Wang L Y,et al.Northeast Area Earthquake Prediction Research in 2009.See:China Earthquake Networks Center,China Earthquake Prediction Research.Beijing:Seismological Press,2008.326–366
7 Zhang Y Q,Li H L.Southwest section of the Longmen Shan fault zone Late Quaternary investigation and analysis technique(in Chinese).Quat Res,2010,30:699–710
8 Zhang H P,Zhang P Z,Kirby E,et al.Along-strike topographic variation of the Longmen Shan and its significance for landscape evolution along the eastern Tibetan Plateau.J Asian Earth Sci,2011,40:855 –864
9 Yi G X,Wen X Z,Xin H,et al.The southern section of Longmen Shan fault zone stress state and earthquake hazard studies(in Chi-nese).Chin J Geophys,2013,56:1112–1120
10 Qin X H,Chen Q C,Tan C X,et al.Longmen Shan fault zone southwest section of the present-day state of stress and seismic hazard analysis(in Chinese).Rock Mech Eng,2013,32:2870–2876
11 Liu J,Yi G X,Zhang Z W,et al.2013 April 20 M7.0 earthquake in Sichuan Lushan introduction(in Chinese).Chin J Geophys,2013,56:1404–1407
12 Xu X W,Han Z J,Li C Y,et al.Lushan MS7 .0 earthquake:A blind reserve-fault earthquake.Chin Sci Bull,2013,58:3437–3443
13 Wang E,Meng Q.Mesozoic and Cenozoic tectonic evolution of the Longmenshan fault belt.Sci China Ser D-Earth Sci,2009,52:579–592
14 Yang X P,Jiang B,Song F M,et al.The evidence of the south Longmenshan fault cutting late quaternary stratum(in Chinese).Seismol Geol,1999,21:341–345
15 Xu Z Q,Li H Q,Hou L W,et al.Uplift of the Longmen-Jinping orogenic belt along the eastern margin of the Qinghai-Tibtet Plateau:Large-scale detachment faulting and extrusion mechanism.Geol Bull Chin,2007,26:1262–1276
16 Zhao G,Chen X,Wang L,et al.Evidence of crustal‘channel flow’in eastern margin of Tibet Plateau from MT measurements.Chin Sci Bull,2008,53:1887–1893
17 Zhao G Z,Martyn J U,Yan Z,et al.Crustal structure and rheology of the Longmenshan and Wenchuan MW7 .9 earthquake epicentral area from magnetotelluric data.Geology,2012,40:1139–1142
18 Tapponnier P,Peltzer G,Armijo R.On the mechanics of the collision between India and Asia.Geol Soc Lond,1986,19:113–157
19 Deng Q,Zhang P Z,Ran Y K,et al.Basic characteristics of active tectonics of China.Sci China Ser D-Earth Sci,2003,46:356–372
20 Roger F,Malavieille J,Leloup Ph H,et al.Timing of granite emplacement and cooling in the Songpan-GarzêFold Belt(eastern Tibetan Plateau)with tectonic implications.J Asian Earth Sci,2004,22:465 –481
21 Wen X,Yi G.Re-zoning of statistic units of seismicity in SichuanYunnan region.J Seismol Res,2003,26:1–9
22 Wang Y Z,Wang E N,Shen Z K.GPS-constrained inversion of present-day slip rates along major faults of the Sichuan-Yunnan region China.Sci China Ser D-Earth Sci,2008,38:582–597
23 Zhu A,Xu X,Diao G L,et al.Relocation of the MW8 .0 Wenchuan earthquake sequence in part:Preliminary seismotectonic analysis(in Chinese).Seismol Geol,2008,30:759–767
24 Fan L H,Wu J P,Wang W L,et al.Relocation of main shock and aftershoke sequences of MS7 .0 Sichuan Lushan earthquake.Chin Sci Bull,2013,58:3451–3459
25 Cai J T,Chen X B,Zhao G Z.Refined techniques for data processing and two-dimensional inversion in magnetotelleric 1:Tensor decomposition and dimensionality analysis(in Chinese).Chin J Geophys,2010,53:516–526
26 Caldwell G T,Bibby M H,Brown C.The magnetotelluric phase tensor.Geophys J Inter,2004,158:457–469
27 Booker J R.Magnetotellutic phase tensor evolution.21st Electromagnetic(EM)Induction Workshop,Darwin,Australia,2012
28 Rodi W,Mackie R L.Nonlinear conjugate gradients algorithm for2 -D magnetotelluric inversion.Geophysics,2001,66:174–187
29 Chen X B,Zhao G Z,Tang J,et al.Magnetotelluric adaptive regularized uoze algorithm(in Chinese).Chin J Geophys,2005,48:937–946
30 Xiao Q,Zhao G,Dong Z.Electrical resistivity structure at the northern margin of the Tibetan Plateau and tectonic implications.J Geophys Res,2011,116:1–14
31 Yu G H,Xu X W,Klinger Y,et al.Fault-scarp features and cascading-rupture model for the Wenchuan earthquake(MW7 .9),eastern Tibetan Plateau,China.Bull Seismol Soc Am,2010,100:2590–2614
32 Zhang Y,Xu L S,Chen Y T.Lushan 4.20 hazard characteristics of earthquake rupture process and its preliminary analysis(in Chinese).Chin J Geophys,2013,56:1408–1411
33 Wang W M,Han K M,Yao Z X.2013 year April 20 Sichuan earthquake rupture process inversion Lushan preliminary results(in Chinese).Chin J Geophys,2013,56:1412–1417
34 Zeng X F,Luo Y,Han L B,et al.April 20,2013 earthquake in Si-chuan Lushan MS7 .0:A high-angle thrust earthquakes(in Chinese).Chin J Geophys,2013,56:1418–1420
35 Royden L H,Burchfiel B C,Robert W,et al.Surface deformation and lower crustal flow in eastern Tibet.Science,1997,276:788–790
36 Clark M K,Royden L H.Topographic ooze:Building the eastern margin of Tibet by lower crustal flow.Geology,2000,28:703–706
37 Wei W,Unsworth M J,Jones A G,et al.Detection of wide spread fluids in the Tibetan crust by magnetotelluric studies.Science,2001,292 :716–718
38 Unsworth M J,Jones A G,Wei W,et al.Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluric data.Nature,2005,433:78–81
39 Zhang Z J,Wang Y H,Chen Y,et al.Crustal structure across Longmenshan fault belt from passive source seismic profiling.Geophys Res Lett,2010,292:254–264
40 Bai D,Unsworth J M,Meju A M,et al.Crustal deformation of the eastern Tibetan Plateau revealed by magnetotelluric imaging.Nat Geosci,2010,3:358–362
41 Robert A,Pubellier M,Sigoyer J,et al.Structural and thermal characters of the Longmen Shan(Sichuan,China).Tectonophyics,2010,491 :165–173
42 Zhao G Z,Tang J,Zhan Y,et al.Relation between electricity structure of the crust and deformation of crustal blocks on the northeastern margin of Qinghai-Tibet Plateau.Sci China Ser D-Earth Sci,2005,48:1613–1626
43 Wang X B,Zhu Y T,Zhao X K,et al.Deep conductivity characteristics of the Longmen Shan,Eastern Qinghai-Tibet Plateau(in Chinese).Chin J Geophys,2009,52:564–571
44 Duvall A R,Clark M K.Dissipation of fast strike-slip faulting within and beyond northeastern Tibet.Geology,2010,38:223–226
45 Zhang P Z.Beware of slowly slipping faults.Nat Geosci,2013,6:323 –324
2 Huang Y,Wu J P,Zhang T Z,et a1.Relocation of the M8.0 Wenchuan earthquake and its aftershock sequence.Sci China Ser D-Earth Sci,2008,51:1703–1711
3 Zhang Y,Feng W P,Xu L S,et a1.Spatiotemporal rupture process of the 2008 great Wenchuan earthquake.Sci China Ser D-Earth Sci,2009,52:145–154
4 Zhang P Z,Wen X Z,Xu X W,et al.Tectonic model of the great Wenchuan earthquake of May 12,2008,Sichuan,China.Chin Sci Bull,2009,54:944–953
5 Shen Z K,Lu J,Wang M,et al.Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau.J Geophys Res,2005,110:1–17
6 Zhang Y X,Wang H M,Wang L Y,et al.Northeast Area Earthquake Prediction Research in 2009.See:China Earthquake Networks Center,China Earthquake Prediction Research.Beijing:Seismological Press,2008.326–366
7 Zhang Y Q,Li H L.Southwest section of the Longmen Shan fault zone Late Quaternary investigation and analysis technique(in Chinese).Quat Res,2010,30:699–710
8 Zhang H P,Zhang P Z,Kirby E,et al.Along-strike topographic variation of the Longmen Shan and its significance for landscape evolution along the eastern Tibetan Plateau.J Asian Earth Sci,2011,40:855 –864
9 Yi G X,Wen X Z,Xin H,et al.The southern section of Longmen Shan fault zone stress state and earthquake hazard studies(in Chi-nese).Chin J Geophys,2013,56:1112–1120
10 Qin X H,Chen Q C,Tan C X,et al.Longmen Shan fault zone southwest section of the present-day state of stress and seismic hazard analysis(in Chinese).Rock Mech Eng,2013,32:2870–2876
11 Liu J,Yi G X,Zhang Z W,et al.2013 April 20 M7.0 earthquake in Sichuan Lushan introduction(in Chinese).Chin J Geophys,2013,56:1404–1407
12 Xu X W,Han Z J,Li C Y,et al.Lushan MS7 .0 earthquake:A blind reserve-fault earthquake.Chin Sci Bull,2013,58:3437–3443
13 Wang E,Meng Q.Mesozoic and Cenozoic tectonic evolution of the Longmenshan fault belt.Sci China Ser D-Earth Sci,2009,52:579–592
14 Yang X P,Jiang B,Song F M,et al.The evidence of the south Longmenshan fault cutting late quaternary stratum(in Chinese).Seismol Geol,1999,21:341–345
15 Xu Z Q,Li H Q,Hou L W,et al.Uplift of the Longmen-Jinping orogenic belt along the eastern margin of the Qinghai-Tibtet Plateau:Large-scale detachment faulting and extrusion mechanism.Geol Bull Chin,2007,26:1262–1276
16 Zhao G,Chen X,Wang L,et al.Evidence of crustal‘channel flow’in eastern margin of Tibet Plateau from MT measurements.Chin Sci Bull,2008,53:1887–1893
17 Zhao G Z,Martyn J U,Yan Z,et al.Crustal structure and rheology of the Longmenshan and Wenchuan MW7 .9 earthquake epicentral area from magnetotelluric data.Geology,2012,40:1139–1142
18 Tapponnier P,Peltzer G,Armijo R.On the mechanics of the collision between India and Asia.Geol Soc Lond,1986,19:113–157
19 Deng Q,Zhang P Z,Ran Y K,et al.Basic characteristics of active tectonics of China.Sci China Ser D-Earth Sci,2003,46:356–372
20 Roger F,Malavieille J,Leloup Ph H,et al.Timing of granite emplacement and cooling in the Songpan-GarzêFold Belt(eastern Tibetan Plateau)with tectonic implications.J Asian Earth Sci,2004,22:465 –481
21 Wen X,Yi G.Re-zoning of statistic units of seismicity in SichuanYunnan region.J Seismol Res,2003,26:1–9
22 Wang Y Z,Wang E N,Shen Z K.GPS-constrained inversion of present-day slip rates along major faults of the Sichuan-Yunnan region China.Sci China Ser D-Earth Sci,2008,38:582–597
23 Zhu A,Xu X,Diao G L,et al.Relocation of the MW8 .0 Wenchuan earthquake sequence in part:Preliminary seismotectonic analysis(in Chinese).Seismol Geol,2008,30:759–767
24 Fan L H,Wu J P,Wang W L,et al.Relocation of main shock and aftershoke sequences of MS7 .0 Sichuan Lushan earthquake.Chin Sci Bull,2013,58:3451–3459
25 Cai J T,Chen X B,Zhao G Z.Refined techniques for data processing and two-dimensional inversion in magnetotelleric 1:Tensor decomposition and dimensionality analysis(in Chinese).Chin J Geophys,2010,53:516–526
26 Caldwell G T,Bibby M H,Brown C.The magnetotelluric phase tensor.Geophys J Inter,2004,158:457–469
27 Booker J R.Magnetotellutic phase tensor evolution.21st Electromagnetic(EM)Induction Workshop,Darwin,Australia,2012
28 Rodi W,Mackie R L.Nonlinear conjugate gradients algorithm for2 -D magnetotelluric inversion.Geophysics,2001,66:174–187
29 Chen X B,Zhao G Z,Tang J,et al.Magnetotelluric adaptive regularized uoze algorithm(in Chinese).Chin J Geophys,2005,48:937–946
30 Xiao Q,Zhao G,Dong Z.Electrical resistivity structure at the northern margin of the Tibetan Plateau and tectonic implications.J Geophys Res,2011,116:1–14
31 Yu G H,Xu X W,Klinger Y,et al.Fault-scarp features and cascading-rupture model for the Wenchuan earthquake(MW7 .9),eastern Tibetan Plateau,China.Bull Seismol Soc Am,2010,100:2590–2614
32 Zhang Y,Xu L S,Chen Y T.Lushan 4.20 hazard characteristics of earthquake rupture process and its preliminary analysis(in Chinese).Chin J Geophys,2013,56:1408–1411
33 Wang W M,Han K M,Yao Z X.2013 year April 20 Sichuan earthquake rupture process inversion Lushan preliminary results(in Chinese).Chin J Geophys,2013,56:1412–1417
34 Zeng X F,Luo Y,Han L B,et al.April 20,2013 earthquake in Si-chuan Lushan MS7 .0:A high-angle thrust earthquakes(in Chinese).Chin J Geophys,2013,56:1418–1420
35 Royden L H,Burchfiel B C,Robert W,et al.Surface deformation and lower crustal flow in eastern Tibet.Science,1997,276:788–790
36 Clark M K,Royden L H.Topographic ooze:Building the eastern margin of Tibet by lower crustal flow.Geology,2000,28:703–706
37 Wei W,Unsworth M J,Jones A G,et al.Detection of wide spread fluids in the Tibetan crust by magnetotelluric studies.Science,2001,292 :716–718
38 Unsworth M J,Jones A G,Wei W,et al.Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluric data.Nature,2005,433:78–81
39 Zhang Z J,Wang Y H,Chen Y,et al.Crustal structure across Longmenshan fault belt from passive source seismic profiling.Geophys Res Lett,2010,292:254–264
40 Bai D,Unsworth J M,Meju A M,et al.Crustal deformation of the eastern Tibetan Plateau revealed by magnetotelluric imaging.Nat Geosci,2010,3:358–362
41 Robert A,Pubellier M,Sigoyer J,et al.Structural and thermal characters of the Longmen Shan(Sichuan,China).Tectonophyics,2010,491 :165–173
42 Zhao G Z,Tang J,Zhan Y,et al.Relation between electricity structure of the crust and deformation of crustal blocks on the northeastern margin of Qinghai-Tibet Plateau.Sci China Ser D-Earth Sci,2005,48:1613–1626
43 Wang X B,Zhu Y T,Zhao X K,et al.Deep conductivity characteristics of the Longmen Shan,Eastern Qinghai-Tibet Plateau(in Chinese).Chin J Geophys,2009,52:564–571
44 Duvall A R,Clark M K.Dissipation of fast strike-slip faulting within and beyond northeastern Tibet.Geology,2010,38:223–226
45 Zhang P Z.Beware of slowly slipping faults.Nat Geosci,2013,6:323 –324