| As a very important geophysical method,the magnetotelluric sounding(MT)has been widely used in many applications such as deep structure exploration research,natural resource prospecting,geological engineering and environmental geophysics,geologic hazard research,etc.,however,limited by the inversion theory and computer technology,the earlier MT exploration and research dataset were almost inverted and interpreted by one-dimensional or two-dimensional inversion method to recover the true resistivity distribution under the surface.Many modeling data inversion test and verification of real data interpretation have proved that these dimensional-decreaseapproximation scheme could not get precise result to accurately render the complex deep geological structure.Because of many practical reasons such as violent surface topography,complex tectonic deformation effect,intrusion of deep materials and volcanic activities,the real exploration process are often facing a three-dimensional geo-electrical model,thus,to be accomplished with,the three-dimensional MT data acquisition and three-dimensional modeling and inversion research have been being the most important aspect of EM exploration method at recent time.Meanwhile,the application of three dimensional inversion in the complex structure regions has become the development direction of electrical structure research applications.On 14 April,2010,an Ms7.1 earthquake occurred in Yushu,Qinghai province,China.The epicenter located at 33.2°N,96.6°E,and the focal depth was about 14 km.The earthquake caused great casualties and property losses.The Yushu earthquake occurred at the boundary fault called Ganzi-Yushu fault between the Bayan Har block and the Qiangtang block,its strike was about NWW-NW,left-lateral strike-slip,with a total length of about 500 km.Many earthquakes happened on this fault and the seismic activity was strong.There are few studies on the electrical structure of the Yushu earthquake zone,and its unique surface segmental rupture characteristics,the temporal and spatial distribution of aftershock,and the deep dynamic background of the differential movement of the adjacent blocks in the northern and southern of the GanziYushu fault remains unclear,etc.,the 3D electrical structure research in Yushu earthquake area can provide a certain basis for interpretation,even for the interpretation of the seimogenesis in and around the Tibet plateau and large suture with reasonable electrical structure feature.The basic research contents and achievements of actual data are summarized as follows:1.2D inversion and field data reanalysisBased on the 2D inversion results of 4 survey lines in the earthquake area,we analysis the phase tensor decomposition and magnetic induction vector,which shows that the MT measurement data in Yushu research area express 2D or even 1D electrical structure characteristics in high frequency.However,from intermediate to lower frequency,the skew is increasing,especially the sites near the main earthquake epicenter.And the real magnetic induction vector are also cluttered,indicating the depth corresponding the frequency shows strong 3D characteristics.At the same time,3D inversion can fully and rationally use all the data and strengthen the data constrain ability.Therefore,the study area use 3D inversion to interpretation is more reasonable.2.Selection and verification of 3D inversion algorithmIn view of the complex terrain of the Yushu area,large difference of the elevation,and the distribution of the survey points is extremely uneven.Before to do 3D inversion of the field data,we compared the NLCG inversion code Mod EM and AR-QN inversion code Geolex in two step to test the recover ability for the synthesize data.Firstly,we synthesized a simple model response through a uniform regular sparse survey points,by added noise of different intensities,the comparison experiments show that the Geolex program is less rely on the initial model and the noise level than the Mod EM program.The inversion when used Geolex is more stable,and there is no more a large number of redundant structures caused by the points sparseness.Secondly,we test a complex model of the ‘#' type line with more sparse points,use the Geolex program to inverse different combinations of the magnetotelluric response parameters,which show that the Geolex program is more suitable for this dataset,that the survey sites is sparse and research area with complex structure,and could not select a suitable initial model.Moreover,through the result of seriously analysis of the two verification above,and the real data of the research area,we choose to use the sub-diagonal impedance(Z2)inversion to obtain a smooth low RMS model as the initial model to inverse the sub-diagonal impedance plus the dipper(Z2T2)to get the final model,which maybe more reliable and with high resolution.3.3D inversion of real measured data with terrain and electrical anomalies verificationFirstly,the Z2 inversion with terrain is carried out on the real measured data,the inversion result is used as the initial model to inverse the Z2T2 data with terrain.These inversion models can improve the resolution under the premise of ensuring more constraints.Comparing the spatial correspondence of the high and low resistance between the two-and three-dimensional inversion results,we divided the research area into four electrical anomalies(R1/R2/Rs/C1)related to seismogenic and rupture on surface.All the anomalies are verified by semi-quantitative forward modeling,which show that the high-resistance anomalies R1 and R2 exist in reality,and the possibility of existence of Rs cannot be ruled out though the survey line cannot be cover the anomaly areas.The verification of the high-conductivity anomaly(C1)confirms the existence of high-conductivity layer in the crust of Qiangtang block,and its bottom boundary could not be higher than the altitude of-30 km.4.Analysis of the genesis of high-conductivity layers in the research areaBased on the electrical structural model obtained by three-dimensional inversion,with reference to the results of rock conductivity experiment,the conductivity of different types of two-phase models is analyzed under different conditions of aqueous(fluid)or molten conditions.The interpretation results will be different if using different types of two-phase models.Reference to the results of the underground isotherm distribution in the study area,and using the empirical formulas of the conductance and rock component conductivity of different rocks in the laboratory,the possible water content or melting of the resistivity model obtained from the Yushu inversion is calculated.The results show that the conductance anomaly in the Yushu area of the upper crust may be caused by water,while the middle and lower crust the partially melted may be occurs.5.Spatial and temporal distribution of aftershocks and rupture characteristics of coseismicThe aftershocks of the Yushu earthquake are divided into two stages by the second largest Ms5.9 earthquake on May 29,2010.The first stage aftershocks are mainly distributed along the Ganzi-Yushu fault,and the second stage aftershocks are deviated southward from the Ganzi-Yushu fault,presented as a conjugate relationship with the aftershocks of the first stage,and the focal mechanism solutions of the two stages of earthquakes are also different.Comparing the electrical structural features,we speculated that there is a concealed fault intersecting the Ganzi-Yushu fault,and its strike may extend to an unknown fault perpendicular to the Zayun-Chumarhe fault.There is no obvious rupture near the main shock,a rupture depletion region over the epicenter.From the resistivity model profile along the Ganzi-Yushu fault,the main reason of this feature is that there exist a relatively low-resistance soft layer whose thickness is nearly 5km near the surface of the main shock,and the source depth is relatively large,the weak layer slows down the earthquake rupture from the epicenter to the surface.Due to the directional effect of the rupture,the rupture continues to expand to the southeast along the surface after penetrates to the surface,and terminated until the stress is reduced to insufficient to produce cracks or encounter high-intensity,high-resistance bodies.6.Explanation of seismogenic environmentFrom the view of the electrical structure,with the Ganzi-Yushu fault zone as the boundary,the northern Qiangtang block on the south side has obvious high-conductivity characteristics below the upper crust,while the north side is the highresistance which represent high intensity materials.The structure is consistent with the “Southern Fast North Slow” feature between the Qiangtang and Bayan Har block displayed by GPS.The high-resistance characteristics in the source depth is also consistent with the result of Pg wave.From the vertical profile of the electrical structure along the Ganzi-Yushu fault,we can see the main shock and the largest aftershock are both occurred in the high-resistance bodies which can be defined as asperities.It is estimated that the occurrence of the Yushu earthquake is the process of the material in the middle-low crust of the Tibet Plateau being extruded eastward along the “pipeline layer” in the northern Qiangtang,as the northern boundary of the “pipeline layer”,the left-lateral strike-slip of the Ganzi-Yushu fault makes the stress exceeds the critical value of the asperity under the fault,then the Yushu earthquake happened.The second major aftershock and its related earthquakes in the northwest of the main shock show different focal mechanism solutions.It is speculated that there may be exist a conjugated fault in the northwest direction.After the main shock occurs for a period of time,the local stress accumulation exceeds the critical value,the Ms5.9 aftershock and the corresponding aftershock cluster occurred,the strike of the cluster showing to the northwest. |
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