Leonid N. Solodilov

Heterogeneity of the uppermost mantle beneath Russian Eurasia from the ultra-long-range profile QUARTZ

The 3850-km long Deep Seismic Sounding profile QUARTZ crosses six major geologic provinces in Eurasia and is sourced by 3 nuclear and 48 chemical explosions. We present the first interpretation of the entire data set, using two dimensional (2-D) ray tracing and inversion, resolution analysis, and 1-D amplitude modeling. Our interpretation shows a 42-km-thick, high-velocity crust under the Baltic Shield, a 29-km-thick crust and high-velocity upper mantle under the Mezenskaya depression, 52-km-thick crust with high-velocity lower crust and uppermost mantle under the Urals, and 40-km-thick crust under the West Siberian basin deepening to 45 km under the Altay-Sayan fold belt. High-velocity (8.4 km/s) uppermost mantle is found under the Mezenskaya depression and under the east flank of the Urals. One almost continuous upper mantle boundary occurs at 65- to 80-km depth, and another with an approximately 40-km-thick LVZ occurs at 120- to 140-km depth. The shallow upper mantle blocks and the two extensive interfaces indicate strong upper mantle heterogeneity. Resolution analysis based on direct multivariate model perturbations, artificial neural network and principal component analysis, indicate the depth uncertainty of the 410-km discontinuity within ±6 km, and also its trade-off with dip and velocities above and below the discontinuity. Decreased near-critical amplitudes of reflections from the 410-km and 660-km discontinuities indicate that these boundaries are most likely represented by gradient zones about 15-20 km thick. Lithosphere thins, asthenospheric velocity decreases, and the 410-km discontinuity dips to the SE approaching the Himalayan orogenic belt.

3D First-Arrival Regional Calibration Model of Northern Eurasia

Seismological monitoring of the Comprehensive Test Ban Treaty (CTBT) requires detailed knowledge of travel-time characteristics of seismic phases across large areas. We use first-arrival travel times from several of Russian Deep Seismic Sounding (dss) profiles, primarily those from Peaceful Nuclear Explosions (pnes), to construct a 3D regional travel-time model of northern Eurasia. The method used, which can be viewed as a generalization of the existing regionalization techniques, is based on apparent-velocity-based spatial interpolation of the travel times picked from dss records. The resulting travel-time field is described in terms of an apparent velocity model in the 3D space of geographic coordinates and ray parameter. In the travel-time calibration, this model could be used to construct approximate source-specific station correction surfaces for any location within the region and also as a region-specific reference model to be used as a background for further calibration effort. This approach allows incremental refinement of the model as additional travel-time data become available. By converting the travel-time model into depth, a 3D regional velocity model is obtained, providing a description of the general features of the upper mantle in northern Eurasia.

Imaging Crustal Structure along Refraction Profiles Using Multicomponent Recordings of First-Arrival Coda

In a way similar to receiver function imaging, coda of the first arrivals can be used to constrain crustal structure along controlled-source refraction profiles with multicomponent recording. Stacked cross-correlations of the radial and vertical components of recordings from three peaceful nuclear explosions of the 3850-km long profile QUARTZ (Russia) exhibit good correlation with the depth to the basement and provide a horizontal resolution level close to recording station spacing (10-15 km). The results also suggest high (∼0.35-0.4) average Poissons ratios within the sediments. When applied to other multicomponent long-range refraction profiles, this approach could provide a simple and inexpensive way to constrain the structure of the upper crust that is required for interpretation of the deeper structures and that cannot be constrained by other means. Most importantly, reverberations within the sedimentary column appear to account for much of the observed complexity of the first-arrival waveforms, and therefore, such reverberations should be taken into account in the interpretations of seismic scattering from within the mantle. Manuscript received 24 January 2002.