Present-day Crustal Deformation In The Himalayan Orogenic Belt:GPS Observation,Modeling And Interpretation

The Himalayan orogenic belt is located on the southern margin of the Tibeteau Plateau.The entire orogenic belt is about 2,500 km long from east to west and 400 km wide from north to south.Along the orogenic belt,there are a series of peaks with an altitude of over 8000 m,such as the highest peak in the world,Mount Everest.Therefore,the Himalayan orogenic belt is undoubtedly one of the most spectacular subduction structural belts on the boundary of the lithospheric plate of continent.The overall size of the Himalayan orogenic belt is large,the fault distribution is diverse in the region,the activity deformation is strong,and the driving mechanism is complex.It is an ideal example for studying land-continent collision and orogeny,and plays an important role in the study of tectonic deformation related to the orogenic process.At present,many scholars have proposed various structural models to try to explain the current convergent deformation and tectonic uplift of the Himalayan orogenic belt,such as the typical"radial expansion"and"oblique convergence"models.Different structural models support different driving mechanisms.The surface deformation data can provide quantitative numerical boundary conditions for testing different structural models,which is helpful for understanding the subduction and convergence deformation modes of the continental plate boundary zone.The current convergent deformation of the Himalayan orogenic belt is mainly absorbed by the sliding on the main thrust fracture?MHT?,and the shallow part of the MHT is basically in a locked state.Long-term fault locking causes a large amount of strain to accumulate on the surface of the fault and is finally released by earthquake.The Himalayan orogenic belt has experienced more than 8 Mw 7.5 earthquakes in the past500 years,causing serious damage to the lives and property of local people.For the seismogenic fault,the maximum magnitude and repetition period that can be triggered are controlled by the mechanical properties of the fault surface.The fault coupling coefficient is an important indicator to assess the stress accumulation state of the fault surface and can be used to evaluate the seismic hazard of active faults.In ocean subduction zones,such as Chile,Sumatra,and Japan,geodetic data such as GPS have been used to study the coupling mode of the subduction boundary zone.It has been confirmed that the resolution of the fault coupling model can be continuously optimized by enriching the observation data.In the Himalayas,the research on fault coupling models has only begun in recent years,and revealed the general characteristics of the MHT coupling distribution.However,in previous studies,most of the GPS observations used were in the frontal Himalayas.The number of sites in the Higher Himalaya and southern Tibet is very limited,so the fault coupling state under the Higher Himalaya cannot be reliably constrained.The earthquake events in the Himalayas are the"lights"that indicate fault activity and orogenic evolution in the Himalayan boundary zone,and are important for determining the seismic period,energy release and assessment of future seismic hazards in the Himalayan seismic zone.Does the coseismic and post-seismic deformation of the Himalayan earthquake have an impact on the current convergence of the boundary zone?Is post-earthquake deformation the main controlling factor affecting the MHT coupling state?Is the current coupling state of the Himalaya static or dynamic?One of the core scientific questions surrounding the above scientific issues is:How does the seismic period deformation control the current convergence deformation and strain distribution in the Himalayan boundary zone?The plate edge earthquake is a flexible rebound process for the deformation between plates.It provides a good opportunity to study the control of seismic period deformation on the current strain accumulation of the Himalayan orogenic belt.In the past,the tectonic deformation and seismic activities in the Himalayas were mainly studied by geological surveys.The development of spatial geodetic techniques represented by GPS provided a new way to understand the structural deformation modes of the Himalayan orogenic belt.Following the above research ideas,in this paper,the main work carried out and the main conclusions obtained are as follows:1.GPS Observvations in the Himalaya and Muti-Source GPS Data FusionGPS observations in the Himalayan orogenic zone can be geographically divided into two parts:the frontal Himalayas and the southern Tibet.In the southern Tibet,the Crustal Movement Observation Network of China?CMONOC?project?I and II?has built nearly90 GPS continuous stations and campaign stations since the 1990s.These stations have undergone multiple observations so far.In addition,the research team and the Seismological Research Institute of the China Earthquake Administration began GPS intensive observation work in southern Tibet since the 1990s,installing about 120 GPS stations,and each station observed at least three times.The internationally popular GAMIT/GLOBK software is used to jointly process the CMONOC data and our GPS data.We adopted the advanced data processing strategy to obtain the time series and velocities of GPS stations in southern Tibet under the stable Eurasian reference frame.In the Frontal Himalaya,there are a large number of GPS data.Since the original data cannot be obtained,only the velocities can be obtained.The multi-source GPS velocity field is converted to a unified Eurasian reference frame through the Euler rotation of the common stations.The most abundant and complete GPS velocity field of the Himalayan orogenic belt in this paper now provides an important data foundation for subsequent modeling analysis.2.Quantitative Analysis of Convergence Deformation and MHT Coupling Mode in Himalayan Subduction Zone?1?The current convergence rate of each segments of the Himalayan boundary zone is calculated using the two-dimensional elastic dislocation model.The results show that from west to east,the convergence rate increases from¹7 mm/yr in the west to²3mm/yr in the east,showing a gradual increase,basically consistent with the geological results.?2?For the first time,the post-earthquake viscoelastic deformation effects of the three major earthquakes in the Himalayas in the past century?M⁸.4 Bihar earthquake in 1934,M⁸.6 Chayu earthquake in 1950 and Mw7.6 Kashmir earthquake in 2005?were quantitatively evaluated.The differences between the Indian plate and the lithospheric rheological structure in southern Tibet are considered in the modeling.The results show that the post-earthquake viscoelastic effect of the Kashmir earthquake in 2005 was very weak.The post-earthquake viscoelastic relaxation of the Bihar earthquake in 1934showed the characteristics of contractional deformation,which increases long-term convergence rate across the eastern Nepal.According to the simulation results in this paper,the long-term convergence rate will be enlarged 2³ mm/yr.In contrast,the viscoelastic relaxation effect of the earthquake was obvious.The viscoelastic deformation rate in the southern Tibet reaches 6⁷ mm/yr,showing a consistent direction with plate convergence.The estimated results are not sensitive to source models.?3?After removing the influence of post-earthquake viscoelastic relaxation of three major earthquakes,the elastic negative dislocation model is used to invert the inter-seismic coupling distribution on the MHT.The post-earthquake deformation of the 1950-year-old earthquake has a great influence on the fault coupling model.After removing the post-earthquake deformation caused by the 1950 earthquake,the width of the fully locked region increases from 100 km to 140-180 km.The optimal coupling model indicates that the shallow part of MHT is in a completely locked state,and the overall distribution is relatively homogeneous.However,in the Bhutan segment,the depth of MHT is deeper than other parts of the Himalayas,indicating that the area has more stress accumulation.Considering the lower historical earthquake levels,the future seismic hazard in this region is worthy of attention.?4?The active block model of the Himalaya-Tibet region was constructed,and the block motion and the uniform strain inside the block were constrained by GPS data.The results show that there is obvious uniform strain inside the active block in the Himalayas,and the extension deformation in the southern Tibet is not uniform.The extension rate of the Yadong-Gulu fault is the largest,about 6 mm/yr.3.Seismic Activity Analysis of the Himalayan Orogenic Belt:A Case Study of the2015 Mw 7.9 Nepal Earthquake?1?The three-dimensional coseismic vertical displacement field is established by integrating the coseismic GPS horizontal displacement and the InSAR line of sight displacement.The results show that the earthquake caused the Kathmandu area to rise by about 0.95 m,and the Everest peak has been affected by the earthquake.The settlement of the main peak is 2³ cm,the main peak of the Shishapangma subsided²0 cm.?2?The"double ramp"geometrical model of the main Himalayan fault was constructed by using triangular dislocation elements,and GPS and InSAR data were used to invert the coseismic slip and afterslip of the 2015 Nepal earthquake.The result shows that the maximum slip of the mainshock is up to⁷.8 m at a depth of 15 km,near to the intersection between the upper flat and mid-crust ramp.The maximum slip could be underestimated if we ignore the mid-crust ramp.The inferred afterslip primarily concentrates on the downdip of the coseismic rupture.The total released moment by the afterslip is estimated to be 1.02×10²⁰ N·m,equivalent to a Mw 7.3 earthquake,approximately 12%of the coseismic moment.Both the coseismic static Coulomb stress change and the interseismic fault locking pattern suggest that the southern part to the rupture zone of the Nepal earthquake with a width of⁶0 km is likely to rupture in the near future.?3?The main shock ruptured the lower boundary of the locked MHT.Majority of the released energy is confined to the transition zone from complete locking to free creeping.The transition zone has a good correspondence with the distribution of the background earthquake,showing a strong stress accumulation rate?¹0 kPa/yr?during the interseismic period.The afterslip mainly occurred in the northern part of the coseismic rupture zone.This area has obvious creeping during interseismic period,and the fault plane shows the characteristics of rate-strengthening.?4?The Nepal earthquake represents a typical thrust-type earthquake in the Himalayan orogenic belt.Its rupture characteristics and energy release have a good enlightenment for studying the seismic activity of the Himalayas.First,the earthquake rupture in Nepal did not reach the surface,and its rupture zone may be restricted by the MHT geometry and the frictional properties of the fault plane.Secondly,through the statistics of released energy,the energy released by the Nepal earthquake is significantly greater than the accumulated strain in the region since the 1833 earthquake,indicating that the 1833earthquake only released a portion of the strain accumulated in this region before this event.The above characteristics suggest that the scale of earthquake rupture and recycle of large Himalayan earthquake are difficult to estimate effectively.From the perspective of the coupled distribution of MHT,large earthquakes may occur anywhere in the Himalayan border zone at any time.