The Geometric And Kinematics Of The Foreland Active Thrust Faults: Insight From Geodetic Deformation Observations

This paper is concerned with the deformation and growth of continental mountain fronts.In places that are actively deforming today,abrupt increases in topography adjacent to foreland basins are created by thrust faulting that can produce major earthquakes.Many such places are also densely populated,and have been settled for a long time,as they are often associated with water supply,favourable conditions for agriculture,and strategic control of historic trade routes.As a result,especially in Asia,populations have concentrated into megacities at locations on mountain fronts that are vulnerable,many of which experienced destructive earthquakes in their past,when their populations were much smaller.There is,therefore,much human,as well as tectonic,interest in understanding how such range fronts develop.Precise observations of surface deformation from space-borne geodetic measurement(i.e.In SAR and GNSS)have led to major advances in our fundamental understanding of earthquake cycles and tectonic processes.We therefore mainly employ the geodetic measurements,in combination of multiple observations from teleseismic waveform,P and S arrival time from local array,seismic reflection and geomorphology investigate,to examine kinematics of the faulting on selected foreland thrust belts,i.e the Lenglongling segment of the northern Qilian,the South Qilian thrust belt and the Kepingtagh foreland fold and thrust belt in Southwest Tian Shan.The major conclusions are listed as below,(1)The ambiguous fault geometry derived from In SAR measurements of buried thrust earthquakes: a synthetic data based studyThe challenge of ruling out potential rupture nodal planes with opposite dip orientations during In SAR-based kinematic inversions has been widely reported.Typically,slip on two or more different fault planes can match the surface deformation measurements equally well.The ambiguous choice of the nodal plane for the In SARbased models was thought to be caused by In SAR's one-dimensional measurement and polar orbiting direction,leading to its poor sensitivity to north-south crustal motion.Through synthetic experiments and simulations,this paper quantitatively demonstrates the main reason of the ambiguous In SAR-based models,which confuse researchers in the small-to-moderate thrust earthquake cases investigation.We propose the inherent one-dimensional measurement is not the principle cause of the fault plane ambiguity,since models derived from the same In SAR data predict similar,but not identical,3-D deformation patterns.They key to differentiating between these different models is to be able to resolve the small asymmetry in the surface deformation pattern,which may be smaller in amplitude than the typical noise levels in In SAR measurements.We investigate the fault geometry resolvability when using In SAR data with different noise levels through “R” value.We find that the resolvability does not only rely on the In SAR noise,but also on the fault geometry itself(i.e.depth,dips angle,strike).Our result shows that it is impossible to uniquely determine the dip orientation of thrust earthquakes with Mw <6.0 and depth >5.0 km with In SAR data at a noise level that is typical for mountain belts.This inference is independent from the specific dataset(i.e.interferogram or time series)and allows one to assess if one can expect to be able to resolve the correct fault plane at all.(2)The depth segmented faulting in the foreland thrust belt: insights from the 1986 ? 2016 Mw 6.0 Menyuan earthquake and the 2003-2009 Mw 6.3 Qaidam earthquake sequenceWe have observed that some places could be ruptured for twice or more in a time interval far smaller than one earthquake cycle.The representatives are the Lenglonging segment of northern Qilian Shan(1986 and 2016 Mw 6.0 earthquake)and the South Qilian thrust belt(2003-2009 Mw 6.3 Haixi earthquake sequence).In combination of In SAR and afthershock relocation,we infer the reverse slip of both Menyuan earthquake was distributed on the same thrust fault,where the 2016 event ruptured the deep segment and the 1986 event ruptured the shallow segment.Together with earthquake seismology,In SAR/GNSS,and subsurface information from hydrocarbon exploration,the style of faulting in South Qilian thrust belt can be resolved,and is seen to involve numerous,elongate,sub-parallel ‘pop-up' ranges,bounded by thrust faults dipping inwards on both flanks,which are all active over an across-strike width of at least 50 km.This near-surface deformation pattern is superimposed on a general asymmetry that involves the basement of the Qaidam foreland under-thrusting the Qilian Shan to the north,most of which is below the seismogenic depth and is accommodated aseismically.Slip vectors on the shallow thrusts are nearly normal to the mountain front,whereas the overall convergence is more oblique,suggesting that the slip is partitioned into its thrust and strike-slip components,with the strike-slip component being accommodated further north,probably on the strike-slip faults in the north Qilian Shan.(3)The friction properties and the vertical variations in strain accommodation in the foreland thrust beltThe strength estimates based on force equilibrium assumption between Qilian Shan and Qaidam Basin suggest(1)the Qaidam crust is roughly as strong as than other places like Himalayas and Andes,where the faults can resist tens of MPa,and(2)the effective coefficient of friction on south Qilian thrust belt is < 0.1,contrasting to the laboratory and borehole results of 0.6-0.8.The weak faults are inferred to be due to the northward under-thrusting of the Qaidam crust or the sediments heating on top.In Kepingtag fold and thrust belt,the vertical separation between In SAR-derived slip model and seismology-based centroid depth and aftershock indicate an unusual vertical variation in strain accommodation between sediments and underlying basement.We therefore infer the surface deformation observed by In SAR is not caused by the coseismic rupture,but the rapid aseismic slip following the mainshock,probably triggered by the basement rupture.The dynamitic friction parameter of a-b,constrained with In SAR time series,is close to velocity neutral,which may allow the fault plane to transform between rate-strengthening and rate-weakening state.We suggest the characteristics of the strain accommodation and dynamic friction are strongly correlated to the weak layers(i.e.salt)in the sediment cover.In summary,the new constrains from multiple observations show us a new visions of the foreland thrust belt,indicating the depth segmented of the faults and the “popup” structures occur more frequently than expected.That will subsequently change the fault behaviors and regional seismicity in the foreland thrust belt.The effective frictions on the thrust is unexpected lower than the laboratory result.The contrasting rock properties in sediment covers and underlying basement will significantly affect the strain accommodations and fault behaviors,therefore the earthquake risks.