Research On Electrical Structure And Seismogenic Structure In The Wulong Earthquake Area Of Chongqing

At 17:43 on November 23,2017,a Ms 5.0 earthquake?referred to as the Wulong earthquake?occurred in Wulong District?29.40°N,107.94°E?in Chongqing,with a focal depth of 10 kilometers?according to China Seismological Network?.The 11,000people in Wulong District and Pengshui Miao and Tujia Autonomous County were affected.The direct economic loss was more than 40 million yuan.It has a great impact on social and economic construction.It is the earthquake that has suffered a great deal of damage to the Wulong area since modern times.In this paper,magnetotelluric sounding method was used to study the underground electrical structure of Changshou Wulong earthquake area in Southeast Chongqing,and some results are obtained.A total of 24 measurement points were obtained with a profile of 155 kilometers.The acquisition time of each measuring station is more than 20 hours.The magnetotelluric sounding response functions as the 160 Hz?1370 s band is obtained after processing.The average point distance is 7km.The observation points are densified in the concentrated area of Wulong earthquake,and the point distance is 2km.The data was processed and analyzed.The phase tensor analysis showed that the two-dimensional declination value of the overall ellipse is smaller in the high-frequency part,and the shallow part shows the characteristics close to two-dimensional.At low frequencies,the data exhibits three-dimensionality,and the direction of the main axis of the tensor ellipse is complex.In order to reflect the deep features in detail,3D inversion is used.After a lot of calculations,we finally set the impedance error to5%?Z_{x y}?Z_yx ^1/2.In the inversion,the initial value?is 100.After 81 iterations,a three-dimensional electrical structure with an initial model of 100 ohm.m and RMS=3 was obtained.This paper conducted a sensitivity test to verify that the entire system of data frequency and measurement points meets the resolution required for this study.Furthermore,the reliability of the inversion result model is confirmed.The Wulong area of Chongqing is located at the intersection of two secondary tectonic units of the Sichuan Platform and the Upper Yangtze Platform in the Yangtze Plateau,east of the middle section of the South-North seismic belt in China.The shallow part of the area south of Wulong has high resistance,and the deep electrical structure has the characteristics of low resistance and thin skin structure.There is a low-resistance interlayer in the shallow high-resistance layer of the Pengshui structural unit,which has a typical sandwich electrical structure.In the deep structure,the high-resistance bedrock of Jiaoshiba was cut by a deep ductile shear zone that penetrated the crust.The Wulong crust-mantle ductile shear zone in the deep southeast of Qiyao mountain has a good correspondence with the epicenter of the Qiyao mountain fault zone in the shallow part.The crust in central and eastern Chongqing east to Two Lake Plain has a low wave velocity and is in a relatively unique stress environment.This indicates that the crust here is more deformable than the crust in the Sichuan Basin and the Two Lake Plains.The deformation prone area is also the area with strong tectonic activity.This should be the reason why the earthquake occurred in the mountainous area between Chongqing and the Two Lake Plains.The Qiyaoshan fault zone in central and eastern Chongqing is affected by the North-South tectonic stress field.The fault type is positive,which is obviously different from the stress environment in west of Chongqing.The Wulong Ms5.0 earthquake is a NE-direction normal fault tensile fracture event,which is inconsistent with the regional compressive stress environment.The unique environment in central and eastern Chongqing is different from the surrounding stress and tectonic activities.The results of multiple sets of focal mechanism solutions are all strike-slip,controlled by the Qiyaoshan fault.The stress in Wulong area is different from the surrounding area.The possible reason is the influence of the strong earthquakes around.The 2008 Wenchuan Ms8.0,2013 Lushan Ms7.0 and 2017Jiuzhaigou Ms7.0 earthquakes may disturb the stress fields in southeastern Chongqing on the eastern border of the Sichuan Basin.The Qiyingshan fault may be affected by the surrounding strong earthquakes.And the Wulong earthquake may be a significant event due to local stress adjustment due to multiple strong earthquakes in the region.