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Resonant e+e- pair production by an electron in a magnetic field near the process threshold has been analytically studied. Using the Nikishov's theorem, an estimation of the number of events has been made in the magnetic field equivalent to laser wave in the SLAC experiment, [D. Burke et al, Phys. Rev. Lett. 79, 1626 (1997)]. The obtained estimation is in reasonable agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The Chyulu Hills, a 1.4 Ma B.P. to Holocene volcanic field located about 150 km to the east of the Kenya rift, is one of the few locations on Earth for which detailed geochemical (volcanic rocks), thermobarometric (xenoliths), seismological and gravity data are available. This paper combines these data to achieve an integrated seismic-petrological model for the deep structure of this volcanic field. Results of a wide-angle reflection and refraction experiment reveal an average crustal thickness of 40 km and a thickness of 20 km for the lower crust. Beneath the volcanic field, the crust thickens to about 44 km. In this region a low-velocity body (LVZ) is modelled which extends from about 30 +/- 5 km depth to the Moho. The LVZ is characterised by an increased v"P/v"S-ratio ranging from 1.81 to 1.93 depending on the possible extents of this body. This is in contrast to the surrounding crust where a ratio of only about 1.76 is observed. In the same area, the results of a teleseismic tomography study show a P-wave low-velocity anomaly of -3%. The seismic data can be explained by either an anorthositic body directly above the Moho in the region of the Chyulu Hills or by the presence of partial melt. Directly beneath the Chyulu Hills, a P-wave velocity of 7.9 km/s is determined for the uppermost mantle; this velocity is 0.2-0.3 km/s lower than that of the surrounding mantle region. The teleseismic tomography model suggests a P-wave low-velocity anomaly of -2.5 to -3.5% in the uppermost mantle (<70 km depth). Widespread garnet-bearing pyroxenitic and lherzolitic mantle xenoliths are mostly well equilibrated and suggest an apparent lithospheric thickness of about 105 km. Most garnet-free spinel harzburgitic xenoliths and some garnet pyroxenitic xenoliths were significantly heated before they were sampled and erupted by the host magmas. Heating events lasted for less than 210 ka as indicated by chemical diffusion profiles observed in orthopyroxene grains. It is suggested that heating was caused by stagnating magmas in the uppermost lithospheric mantle. At the same depth P-wave velocity perturbations of the tomographic model show a low-velocity zone directly underneath the youngest part (SE) of the volcanic field. At depths greater than about 70 km, this low-velocity zone is shifted towards the east, away from the volcanic field.

The 420-km-long northwest-southeast KRISP (Kenya Rift International Seismic Project) flank line F, part of the KRISP 94 experiment, extends from Athi River (30 km southeast of Nairobi) to the Indian Ocean near Mombasa, Kenya. Line F crosses the Chyulu Hills area, a young Quaternary volcanic field, surrounded by the basement of the Mozambique belt. The basement is at the surface almost along the entire profile with two exceptions, one in the Chyulu Hills area due to the presence of volcanic pyroclastics, the other next to the Indian Ocean where sediments reach a thickness of about 8 km. Below the basement the crust can be divided into three layers. The upper layer extends to about 10 km depth with P-wave velocities of roughly 6.25-6.4 km/s. The mid-crustal layer reaches a depth of about 20 km where P-wave velocities range from 6.55 km/s to 6.7 km/s. The lower crust in the area of the Chyulu Hills is unexpectedly thick (>20 km) and generates strong refracted phases with velocities of about 7.0 km/s. Crustal thickness is about 40 km but thickens in the area of the Chyulu Hills and thins towards the Indian Ocean to 22 km. P"n-phases, refracted waves travelling through the uppermost mantle, are identified on four record sections and give good control over the upper mantle velocities which are slightly reduced from 8.1-8.2 km/s to 7.9 km/s underneath the Chyulu Hills area. The upper and lower crust in the region of the Chyulu Hills are significantly heterogeneous, producing strong signal-induced noise which masks secondary arrivals in the corresponding distance range. Underneath the Chyulu Hills P"MP-reflections are hard to identify indicating that the crust-mantle boundary is a transition zone, rather than a first-order discontinuity.