Study On The Sedimentary And Crustal Structures Of The Binchuan Area Using Dense Arrary

Binchuan Basin,the second largest basin in Northwest Yunnan,located at the intersection of the Chenghai-Binchuan Fault(CBF)and the Red-River Fault(RRF).Minor faults are widely distributed in the Binchuan Basin,which caused the high seismicity in this area.The low-velocity sedimentary layers in the basin can amplify seismic waves and increase the destructive power of earthquakes.Meanwhile the Binchuan Basin has Binchuan,Binju and other cities with a large population.Therefore,it is important to study the structure of the Binchuan Basin for assessing local seismic harards and making timely preparations for earthquake hazrard mitigation.A series of studies in this area have been carried out since the 1980s.However,due to the sparseness of the observation system in previous studies,there is no detailed structural study of the Binchuan Basin,CBF and the northern section of the RRF.The existing research on underground structures in Binchuan area does not match the disaster prevent needs.In order to study the crust and basin structures,we deployed a dense array with short-period seismometer around the Binchuan basin in 2017.The experimental observation system consisted of a dense array and a profile.Totally 510 3-component short-period seismic stations were used.The dense array covered the Binchuan Basin with 381 stations.The profile,which collocate to the 2012 wide-angle reflection-refraction profile,extended from Ninglang to Yunxian with 129 stations.Using the experimental data,we studied the sedimentary structure of the Binchuan Basin and the crustal structure below Ninglang to Yunxian.It draws the following conclusions:1.Using the probability density function method,we evaluated the background noise level of the Z component of the array stations.The result shows,in the frequency band of 0.1 s-2 Hz,the noise level is lower than New Hight Nosie Modle(NHNM).However,in the 2-10 Hz band,it shows a peak that is supposed to be related basin amplification.The peak frequency of amplification in Z component is?3.5 Hz.2.By using the H/V method and teleseismic P-wave arrival time method,we studied the sedimentary structure of the Binchuan Basin.It is found that the sediments in the Binchuan Basin are concentrated in Binchuan and Binju depression.The thickness of the sedimentary layers in the Binchuan sag and the Binju sag is about 600 m and 900 m.which is consistent with the geological survey.3.Using the receiver function method,we studied the crustal structure below the Ninglang-Yunxian profile.The results show that(1)The depth of the Moho along the Ninglang-Yunxian profile is strongly different.The overall trend is that the Moho depth is deeper in the north and shallower in the south.The thickness of the crust below Yongshens is?50 km,the thickness of the crust below Binchuan is?43 km and the thickness of the crust below Yunxian is?40 km.(2)The velocity ratio of the two sides of RRF is obviously different.The velocity ratio is?1.75 in the north of the RRF and gradually decreases to?1.63 in the southern part.It further proves that RRF is a tectonic boundary.(3)Consistent with the results of DSS study in Ninglang-Yunxian,we did not observe clear depth variation of Moho crossing the RRF.There is no direct evidence demonstrate that RRF in this region cut trough the Moho.This may be due to the intersection of the profile and the RRF is the northern end of the RRF.This result proves that there are great differences in different sections of the RRF.