Determining Crustal Upper Mantle Structure Of Sichuan-Yunnan Region Using Receiver Function

The Sichuan-Yunnan region is located at the southeastern margin of the Tibet Plateau,which adjacent to the stable South China Craton(SCC).The west side is uplifted by the Tibet Plateau and the eastward escape of the lower crust for the collision of the Indian plate and the Eurasian plate.The crustal material in these area exhibits strongly exchange characteristics,deep and large faults developed,multi-block convergence and strong earthquakes occur frequently as a result of the complicated form of tectonic stress fields,different regions and different earth circle layers may be controlled by different stress fields.Therefore,the Sichuan-Yunnan region is also considered to be a"natural laboratory" for studying land-continent collision orogeny and its evolution.To obtain the crustal upper mantle structure and the existence of the crust low-velocity layer in the Sichuan-Yunnan region,we extracted the P-wave receiver function with Gaussian parameter of 2.5 based on 1309 events greater than Ms5.5 with epicentral distance in the range of 30 degree to 90 degree which recorded by 313 broadband mobile stations from ChinaArry I,which are then used to estimate the crustal thickness and wave velocity ratio of studying area in the methods of traditional H-k,resonance filtering receiver function and time corrected H-k.Furthermore,According to the seismic catalogue data recorded by China Earthquake Network Cente(CENC)from 1985 to 2016,the improved Bayesian method is used to calculate the spatiotemporal distribution of b-values in Sichuan-Yunnan region,combined with regional tectonic background and active faults characteristics.The spatial difference of the b-value spatial distribution along the fault zones in the Sichuan-Yunnan region is analyzed to reflect the difference in seismic hazard.Results shows:(1)The depth of the Moho interface in the Sichuan-Yunnan region vary significantly which are characteristic of northern area thicker than southern area.The thickness varies from 30 to 64 km.There is a maximum thickness difference of about 30 km from the south to north.The Jinsha-Red river fault is an obvious boundary variations of lateral crustal thickness in Yunnan,which the crustal thickness in the western side is generally lower than 40km with also contour densely distributed implying prominently change of thickness.On the contrary,the eastern regional thickness is more than 40km.(2)The average wave velocity ratio of the crust below the station in the study area has obvious lateral block characteristics.The Xiaojiang fault serves as a geological structure boundary,and its Poisson's ratio on the east and west sides is significantly different.In the vicinity of the Tengchong-Baoshan and the southern part of the Simao block,the crustal wave velocity ratio is between 1.74 and 1.82(Poisson's ratio is between 0.25 and 0.29),and the northwestern region of the Sichuan-Yunnan diamond block(near Panzhihua area)has a high crust average wave velocity ratio of 1.76 to 1.86(Poisson's ratio is in the range of 0.26 to 0.30),suggesting that the crustal partial melting or mafic content increases in these regions.Combining the characteristics of GPS velocity field variation and the previous geothermal research results,we inferred that the lower crust low velocity anomalies in these several study areas may be due to the plastic flow of the lower crust material in the southeastern margin of the Tibet Plateau,after encountering the hard Sichuan Basin rotating counter-clockwise,which then a high-heat lower crust flow is formed near the area.(3)Testing using both synthetic and real data suggests that the effectiveness of the resonance filtering method to remove the deposition effect.The results show that time corrected H-k method can use the filtered receiver function to search for more accurate crustal thickness and average wave velocity ratio,which provides another theoretical basis for the promotion and application of the method.(4)The b-value of studying area varies greatly between 0.2371 to 1.8125.It can be seen that the apparent low b-value is consistent with the multi-fracture zone according to the distribution of earthquakes which more than Ms6.0 from 1985 to 2016.Before the occurrence of Ms 6.5 event in 2000,the b-value experienced a prominently drop up to 0.6 in 2 years,we consider that the stress level in the area is very high if an originally lower b-value dips in a short time.It is possible that the fracture strength of the fault has been reached or approached,and the possibility of earthquake occurrence is high,which provides some guiding significance for seismic hazard analysis.(5)The comparison of the distribution of Poisson's ratio in the Sichuan-Yunnan region to the b-value distribution shows that the low b value near the south of the Red-river fault and the Xiaojiang fault(about 0.72?1.0)corresponds to the low Poisson's ratio of the corresponding region(about 0.21?0.23),and the frequency of the medium-strong earthquake is relatively high;The high b value in the northwestern part of the Sichuan-Yunnan-rhombic block is consistent with the high Poisson's ratio in the corresponding area(about 0.29?0.31),and there is little occurrence of moderate-strong earthquakes,indicating that the b-value is consistent with the Poisson's ratio distribution in some areas of Sichuan and Yunnan(about 1.6?1.8).