Seismic tomographic inversion of Russian PNE data along profile Kraton

Abstract

Abstract. Tomographic inversion of Russian peaceful nuclear explosion (PNE) data collected along the 3500 km east-west striking profile Kraton has provided new information about the two-dimensional P-wave velocity structure of the upper mantle beneath the Siberian platform. The 8 ø discontinuity is clearly identified as the top of a zone in the 100-200 km depth range. This zone appears as a low-velocity zone in large parts of the profile and has zero velocity gradient along the profile. The 400 km discontinuity is imaged as an increase in velocity of at least 0.4 km/s at -380-420 km depth. BackgroundMajor upper mantle P-wave velocity discontinuities can be identified on a global scale. Global discontinuities that should be expected in continental mantle include the 8 ø discontinuity [Thybo and Perchurl, 1997] at about 100 km depth, the 400 km discontinuity [Jeffreys, 1936] , 1997]. In high-density long-range seismic sections the existence of the 8 ø discontinuity and the heterogeneous material below it are indicated by strongly scattered and delayed first arriving P-waves with a strong coda observed in the 700-1200 km offset range. The depth to the Lehmann Discontinuity depends on the thermal state of the mantle; it is deep in hot areas and shallow in cold areas. The other discontinuities may represent phase changes of the mantle rocks.Upper mantle velocity models are often based on long period body-wave and surface-wave earthquake data. Here we use seismic data with nuclear explosions as the controlled sources. The advantage is that the exact position in space and time of the source is known, and the dense average receiver spacing of 20 km along the 3500 km long profile Kraton provides a dense sampling of upper mantle phases. Other scientists have previously used other high-density PNE data sections for mapping 2D velocity variations in the lithosphere-asthenosphere First arrival travel time picksFour peaceful nuclear explosions (PNEs) were detonated along profile Kraton (Fig. 1) at intervals between 673 and 1168 km. In the 0 to -125 km offset range the first arriving P-waves are crustal waves (Fig. 2). From 125 to about 800 km offset the first arrivals are from the sub-Moho uppermost mantle. Strong scattering of the first arrivals in the 800 to -1300 km offset range is interpreted to be the effect of scatterers in the low-velocity interval below the 8 ø discontinuity. Strong reflections from this reflective low-velocity interval form a more than 5 s long coda of secondary arrivals in this offset range. The low-amplitude Lehmann refraction from below the low velocity interval is the first arrival between -1300 and 2000 km. In the 2100 to 2500 km offset range the apparent velocity of the first arrivals increases as typical for the 400 km discontinuity. From -1800 to -2500 km prominent reflections from the 400 km discontinuity are clear secondary arrivals. Beyond -2500 km offset the first arriving Pwaves originate from below the 660 km discontinuity.Observations similar to the ones mentioned above are made in ...

Deep Europe today: geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga

“…According to travel-time inversion of seismic data along the PNE profiles Quartz and Kraton, this feature extends eastwards as a continuous layer for at least 3000 km into the West Siberian Basin and the Siberian Shield (Nielsen et al 1999). Similar reduced-velocity layers have been reported earlier for other cratonic regions of the world (Grand & Helmberger 1984;LeFevre & Helmberger 1989;Pavlenkova et al 1996;Darbyshire 2005) and suggest that it may be a global characteristic of Precambrian lithosphere (Thybo & Perchuc 1997;Thybo 2006).…”

“…8c). This highly reflective layer with reduced seismic velocities extends for 3000 km further eastwards (Thybo & Perchuc 1997) and is underlain by a high-velocity layer at c. 200-250km depth (Nielsen et al 1999;Kuzin 2001). Seismic reflection profiling of the Southern Urals (Knapp et al 1996) revealed mantle reflections at depths of c. 80 km and 175 km; the lower reflector was interpreted as possibly imaging the base of the lithosphere.…”

Deep Earth Structure - Transition Zone and Mantle Discontinuities

“…A low-velocity zone in the cratonic upper mantle near 100 km depth has frequently been observed in controlled source (including nuclear explosions) seismic data (Nielsen et al, 1999;Thybo, 2006;Thybo and Perchuc, 1997). The Svekalapko Seismic Tomography Working Group (2004) found the LAB at 100-130 km depth below Finland in wideangle data.…”

Deep Earth Structure – Transition Zone and Mantle Discontinuities

“…Parker et al (1984) observed below the Rio Grande Rift an upwarp of the asthenosphere from P residuals. Nielsen et al (1999) located the 8 discontinuity at 100-200 km depth from nuclear explosion data on the Siberian platform (8 discontinuity is frequently also interpreted as LAB; Thybo (2006)). Nielsen et al (1999) located the 8 discontinuity at 100-200 km depth from nuclear explosion data on the Siberian platform (8 discontinuity is frequently also interpreted as LAB; Thybo (2006)).…”