Lower crustal relaxation beneath the Tibetan Plateau and Qaidam Basin following the 2001 Kokoxili earthquake

SUMMARY In 2001 November a magnitude 7.8 earthquake ruptured a 400 km long portion of the Kunlun fault, northeastern Tibet. In this study, we analyse over five years of post-seismic geodetic data and interpret the observed surface deformation in terms of stress relaxation in the thick Tibetan lower crust. We model GPS time-series (first year) and InSAR line of sight measurements (years two to five) and infer that the most likely mechanism of post-seismic stress relaxation is time-dependent distributed creep of viscoelastic material in the lower crust. Since a single relaxation time is not sufficient to model the observed deformation, viscous flow is modelled by a lower crustal Burgers rheology, which has two material relaxation times. The optimum model has a transient viscosity 9 × 1017 Pa s, steady-state viscosity 1 × 1019 Pa s and a ratio of long term to Maxwell shear modulus of 2:3. This model gives a good fit to GPS stations south of the Kunlun Fault, while displacements at stations north of the fault are over-predicted. We attribute this asymmetry in the GPS residual to lateral heterogeneity in rheological structure across the southern margin of the Qaidam Basin, with thinner crust/higher viscosities beneath the basin than beneath the Tibetan Plateau. Deep afterslip localized in a shear zone beneath the fault rupture gives a reasonable match to the observed InSAR data, but the slip model does not fit the earlier GPS data well. We conclude that while some localized afterslip likely occurred during the early post-seismic phase, the bulk of the observed deformation signal is due to viscous flow in the lower crust. To investigate regional variability in rheological structure, we also analyse post-seismic displacements following the 1997 Manyi earthquake that occurred 250 km west of the Kokoxili rupture. We find that viscoelastic properties are the same as for the Kokoxili area except for the transient viscosity, which is 5 × 1017 Pa s. The viscosities estimated for the Manyi and Kokoxili areas are consistent with constraints obtained from other earthquakes in the northwest and south central parts of the Tibetan Plateau.

[1]  F. Pollitz.  Post-seismic relaxation theory on a laterally heterogeneous viscoelastic model , 2003 .

[2]  George E. Hilley,et al.  Earthquake-cycle deformation and fault slip rates in northern Tibet , 2009 .

[3]  B. Burchfiel,et al.  Large-scale crustal deformation of the Tibetan Plateau , 2001 .

[4]  Y. Klinger,et al.  Slip rate on the Kunlun fault at Hongshui Gou, and recurrence time of great events comparable to the 14/11/2001, Mw∼7.9 Kokoxili earthquake , 2005 .

[5]  M. Unsworth.  Magnetotelluric Studies of Active Continent–Continent Collisions , 2010 .

[6]  T. Wright,et al.  InSAR Observations of Low Slip Rates on the Major Faults of Western Tibet , 2004, Science.

[7]  F. Pollitz.  Transient Rheology of the uppermost mantle beneath the Mojave Desert , 2003 .

[8]  David P. Robinson,et al.  The Mw 7.8, 2001 Kunlunshan earthquake: Extreme rupture speed variability and effect of fault geometry , 2006 .

[9]  Guangwei Fan,et al.  Strong Lg Attenuation in the Tibetan Plateau , 2003 .

[10]  Peizhen Zhang,et al.  Continuous deformation of the Tibetan Plateau from global positioning system data , 2004 .

[11]  Zhong Lu,et al.  The postseismic response to the 2002 M 7.9 Denali Fault earthquake: Constraints from InSAR 2003-2005 , 2009 .

[12]  P. Molnar,et al.  Thinning and Flow of Tibetan Crust Constrained by Seismic Anisotropy , 2004, Science.

[13]  Jeffrey T. Freymueller,et al.  Coupled afterslip and viscoelastic flow following the 2002 Denali Fault, Alaska earthquake , 2009 .

[14]  J. C. Savage,et al.  Geodetic determination of relative plate motion in central California , 1973 .

[15]  W. Gan,et al.  Postseismic deformation modeling of the 2001 Kokoxili earthquake, western China , 2003 .

[16]  Arthur J. Rodgers,et al.  Lithospheric structure of the Qiangtang Terrane, northern Tibetan Plateau, from complete regional waveform modeling: Evidence for partial melt , 1998 .

[17]  Xiwei Xu,et al.  Synthetic normal faulting of the 9 January 2008 Nima (Tibet) earthquake from conventional and along‐track SAR interferometry , 2008 .

[18]  A. Rodgers,et al.  Crustal structure of northern and southern Tibet from surface wave dispersion analysis , 2003 .

[19]  Yehuda Bock,et al.  Parkfield earthquake: Stress-driven creep on a fault with spatially variable rate-and-state friction parameters , 2009 .

[20]  Max A. Meju,et al.  Crustal deformation of the eastern Tibetan plateau revealed by magnetotelluric imaging , 2010 .

[21]  L. Ding,et al.  Cenozoic Volcanism in Tibet: Evidence for a Transition from Oceanic to Continental Subduction , 2003 .

[22]  Thomas J. Owens,et al.  Implications of crustal property variations for models of Tibetan plateau evolution , 1997, nature.

[23]  David T. Sandwell,et al.  The 1999 (Mw 7.1) Hector Mine, California, Earthquake: Near-Field Postseismic Deformation from ERS Interferometry , 2002 .

[24]  Peter Molnar,et al.  Active deformation of Asia : From kinematics to dynamics , 1997 .

[25]  M. Caffee,et al.  Holocene left-slip rate determined by cosmogenic surface dating on the Xidatan segment of the Kunlun fault (Qinghai, China) , 1998 .

[26]  J. Saul,et al.  The seismological structure of the Tibetan Plateau crust and mantle down to 700 km depth , 2011 .

[27]  M. Barazangi,et al.  High-frequency seismic wave propagation beneath the Indian Shield, Himalayan Arc, Tibetan Plateau and surrounding regions: high uppermost mantle velocities and efficient Sn propagation beneath Tibet , 1983 .

[28]  Semih Ergintav,et al.  Time-Dependent Distributed Afterslip on and Deep below the İzmit Earthquake Rupture , 2002 .

[29]  Roland Bürgmann,et al.  Evidence of power-law flow in the Mojave desert mantle , 2004, Nature.

[30]  P. Rosen,et al.  Updated repeat orbit interferometry package released , 2004 .

[31]  Leigh H. Royden,et al.  Topographic ooze: Building the eastern margin of Tibet by lower crustal flow , 2000 .

[32]  P. Tapponnier,et al.  Seismic evidence for stepwise thickening of the crust across the NE Tibetan plateau , 2002 .

[33]  Brendan J. Meade,et al.  Present-day kinematics at the India-Asia collision zone , 2007 .

[34]  Rongjiang Wang,et al.  Stress transfer and its implication for earthquake hazard on the Kunlun Fault, Tibet , 2010 .

[35]  Paul Tapponnier,et al.  Kinematic model of active deformation in central Asia , 1993 .

[36]  Zhu,et al.  Moho offset across the northern margin of the tibetan plateau , 1998, Science.

[37]  Yann Klinger,et al.  Coseismic deformation of the 2001 Mw = 7.8 Kokoxili earthquake in Tibet, measured by synthetic aperture radar interferometry , 2005 .

[38]  M. Handy,et al.  Experimental deformation of partially melted granite revisited: implications for the continental crust , 2005 .

[39]  Roland Bürgmann,et al.  Tandem afterslip on connected fault planes following the 2008 Nima‐Gaize (Tibet) earthquake , 2010 .

[40]  L. Ratschbacher,et al.  Hot and dry deep crustal xenoliths from tibet , 2000, Science.

[41]  F. Pollitz,et al.  Post-seismic relaxation following the great 2004 Sumatra-Andaman earthquake on a compressible self-gravitating Earth , 2006 .

[42]  Georg Dresen,et al.  Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics, Geodesy, and Field Observations , 2008 .

[43]  M. Unsworth,et al.  Quantifying crustal flow in Tibet with magnetotelluric data , 2010 .

[44]  Wang,et al.  Surface Deformation and Lower Crustal Flow in Eastern Tibet , 1997, Science.

[45]  Wenjin Zhao,et al.  Seismic evidence for a Moho offset and south-directed thrust at the easternmost Qaidam-Kunlun boundary in the Northeast Tibetan plateau , 2009 .

[46]  S. Klemperer.  Crustal flow in Tibet: geophysical evidence for the physical state of Tibetan lithosphere, and inferred patterns of active flow , 2006, Geological Society, London, Special Publications.

[47]  R. Bürgmann,et al.  Stress-dependent power-law flow in the upper mantle following the 2002 Denali, Alaska, earthquake , 2006 .

[48]  Philip England,et al.  Crustal thickening versus lateral expulsion in the Indian‐Asian continental collision , 1993 .

[49]  R. Bürgmann,et al.  Dynamics of Izmit Earthquake Postseismic Deformation and Loading of the Duzce Earthquake Hypocenter , 2002 .

[50]  F. Pollitz.  Transient rheology of the upper mantle beneath central Alaska inferred from the crustal velocity field following the 2002 Denali earthquake , 2005 .

[51]  Semih Ergintav,et al.  Izmit earthquake postseismic deformation and dynamics of the North Anatolian Fault Zone , 2009 .

[52]  P. Chopra.  High-temperature transient creep in olivine rocks , 1997 .

[53]  M. Caffee,et al.  Uniform postglacial slip-rate along the central 600 km of the Kunlun Fault (Tibet), from 26Al, 10Be, and 14C dating of riser offsets, and climatic origin of the regional morphology , 2002 .

[54]  Deng Ming,et al.  Crustal and upper mantle structure of northern Tibet imaged with magnetotelluric data , 2004 .

[55]  T. J. Owens,et al.  Upper mantle velocity structure beneath the Tibetan Plateau from Pn travel time tomography , 1997 .

[56]  Fred F. Pollitz,et al.  Postseismic relaxation theory on the spherical earth , 1992 .

[57]  P. Bedrosian,et al.  Detection of Widespread Fluids in the Tibetan Crust by Magnetotelluric Studies , 2001, Science.

[58]  Tim J. Wright,et al.  Post-seismic motion following the 1997 Manyi (Tibet) earthquake: InSAR observations and modelling , 2007 .

[59]  J. C. Savage.  Equivalent strike‐slip earthquake cycles in half‐space and lithosphere‐asthenosphere earth models , 1990 .