The structure of the Earth’s crust and uppermost mantle along the Baikal rift system based on teleseismic data

Abstract

Detailed data on the velocity structure of the Earth's crust and uppermost mantle are very impor� tant to study the complex structure of the Baikal rift system. Such data can be obtained not only from deep seismic sounding (DSS) but also on the basis of the teleseismic method of Pwave receiver function (PtoS). The analysis of the wave forms in this method is based on the approach of seismic prospecting using the records of strong distant earthquakes (1, 2). Owing to the sensitivity of converted shear seismic waves to velocity inhomogeneities at different depths, this method gave good results in the study of the velocity structure based on the records of temporary stations in the south of Siberia and in Mongolia (3, 4). In the past decade, after the recording at the sta� tionary Baikal seismic network was reconstructed and became digital, a large amount of data was accumu� lated in the Baikal region and Transbaikalia, which is applicable to study the deep structure in the regions around seismic stations (Fig. 1). Longterm observa� tions at stationary stations can compete with short� term seismic experiments in the amount of records of strong remote earthquakes providing greater reliability of the velocity models obtained on their basis. In this work, we used these data and applied the method of receiver function to determine onedimensional mod� els of Swave propagation velocities up to a depth of 70 km. Twodimensional models ( VStransects) were constructed on the basis of onedimensional models (VS(h)) in the regions of dense locations of stations along the lengthy Baikal Rift system. We separated the receiver functions of longperiod waves formed by converted SVwaves in the observa� tion region from the records of earthquakes with a magnitude greater than 6.0 that arrive from distances of 3000-9000 km. Taking into account the extremely complex geomorphological structure of the region and possible existence of seismic azimuthal anisotropy not only in the mantle but also in the Earth's crust (5), we used the records of remote earthquakes from the northeastern and southwestern directions. Thus, in this work, we analyzed only the earthquakes records that make possible sounding of the velocity structure approximately under the lines of stations, i.e., along the rift zone (Fig. 1). Summing of the receiver func� tions from two opposite directions evens the influence of irregular inhomogeneities amplifying the contribu� tion of the main structural peculiarities in the forma� tion of the wave forms. Using a program of receiver function inversion developed by G.L. Kosarev (2), we restored one� dimensional velocity vertical sections of shear seismic waves, VS(h), from the separated received functions. The detailed structure of velocity models is deter� mined by their parameterization, which we developed on the basis of numerical modeling (3). The experi� ments with numerical models demonstrated that divi� sion of the studied structure within the Earth's crust into elementary layers not thicker than 1 km is opti� mal. In order to determine the depth of the crust- mantle boundary and its character (sharp or gradient) more accurately within the upper 5-10 km of the mantle the thickness of the elementary layers should be close to 1 km. Deeper in the mantle, the depth step can increase to 5 km or more. The smoothed velocity function V0(h) calculated from the mean results of investigations based on the DSS method in the SayanyBaikal mountain region (6) was used as the first approximation in all cases of the inversion procedure. According to the DSS, the mean velocity of longperiod waves in the Earth's crust is equal to 6.4 km/s in the first approximation, while the velocity of the shear seismic waves is 3.7 km/s. The velocity of Pwaves in the mantle under the crust was assumed 8.0 km/s, and the ratio was V P /V S = 1.80. Using a spline based on the method of linear interpo� lation, we constructed twodimensional models of the velocity structure (Fig. 2) from the restored one� dimensional stationary models of V S (h). The models