Study On The Fine Shallow Structure And Moho Depth Of The Anninghe Fault Zone

The Anninghe fault forms the northeast boundary of the Sichuan-Yunnan rhomboidal block in Southwest China.As a major left-lateral strike-slip fault,it intersects with the Xianshuihe fault in the north and the Zemuhe fault in the south,spanning almost 200 kilometers.The seismic activity of the Anninghe fault is very intense and there have been a number of significant earthquakes in history.However,recent studies indicate that there is a clear seismic gap between the Liziba(north of Mianning)to Xichang segment of the Anninghe fault zone.It is concluded that the highest magnitude of a probable earthquake would be Mw7.4.Previous models lack the understanding of the shallow fine structure in the region,while the overall characteristics of the trans-rupture are not well understood.Conducting subsurface structure exploration in this region can provide an important reference for earthquake prevention and disaster mitigation,and further enrich the knowledge of the formation and evolution of the Anninghe fault zone.The sedimentary structure of the Anninghe fault zone was further investigated using the Nakamura method to obtain the H/V spectral ratio with the peak frequency at each station site.The results indicate that the peak frequency has a high correlation with lithology and is influenced by topography.In regions with magmatic intrusion or foothills,there is only one peak at around 2～3 Hz in the HVSR.In Anninghe valleys and fracture zones with abundant Quaternary sedimentation,another low-frequency peak around 0.4 Hz is also present.Using the empirical relationship,the thickness of the sedimentary layer around the fault fracture zone is estimated to range from 300～600 meters.Additionally,the sedimentary interface exhibits a downward dip to the east,likely affected by the east-west extrusion stress.Considering the resonance effect,it is vital to give extra attention to buildings with 6～9 stories in the valley area of the Anninghe for earthquake hazard prevention.For the deeper part,this study exploited ambient noise interferometry and receiver function to better constrain the crustal interface,particularly the Moho.Instead of using recordings of all time,we used beamforming to identify the time primarily dominated by body waves and removed times with weak body wave energy to improve Signal Noise Ratio(SNR).ACF/CCF profiles of two lines revealed some stable continuous signals.Based on previous velocity models and forwarding tests,we estimated the Moho depth to be around 60 km.Additionally,we discovered another interface within the crust with a depth of around 28 km that exhibited spatial variations along the lines.This model was supported by receiver functions,and the signals around 4.5 s and 9 s are likely to be the Ps-converted phase generated from the two interfaces mentioned above.The H-κ stacking and simulations indicated that the depth of the interfaces was around 32 km and 65 km which is close to the noise interferometry results.The differences could arise from the uncertainty of the velocity models and the position of the reflection points.In this thesis,various techniques and data processing programs are employed to establish the sedimentary structure model of the Anninghe fault zone and crustal interfaces,which contribute to better understanding the seismic gap.The improved fault zone model adds a new dimension to the understanding of the Anninghe fault,and is valuable for future studies.