ALBERT

Notes: Analysis of global traveltime data has been formulated in terms of the stochastic properties of the Earth's heterogeneity pattern and random errors in the data. the formalism relates the coherency of traveltime residuals within bundles of rays (summary rays) of varying size to the spherical harmonic power spectrum of the slowness field of the medium. It has been applied to mantle P-wave data from the ISC catalogue. the measure of coherency is the variance within summary rays. It is estimated within bins in source depth, epicentral distance and the scale size of the area defining a summary ray. the variance at infinitesimal scale length represents the incoherent component of the data (random errors). the variation of the variance with scale length contains information about the autocorrelation function or power spectrum of slowness perturbations within the Earth. the variation with epicentral distance reflects the depth variation of the spectrum. the formalism accounts for the uneven distribution (clustering) of stations and events.We find that estimates of random errors correlate well with complexities on the traveltime curve of P-waves. the variance peaks at 1.0–2.0 s2 at δ20°, where triplications occur on the traveltime curve, drops to 0.15–0.8s2 at teleseismic distances, and rises to 0.4–1.3 s2 approaching the core shadow, where the traveltime curves of P-waves and PcP-waves merge. These estimates should be considered upper bounds for the random error variance of the data. the signal to random noise ratio in the teleseismic ISC P-wave data is about S/N= 2.Inversion of the scale-dependent structural signal in the data yields models that concentrate heterogeneity strongly in the upper mantle. the product of correlation length and power drops by about two orders of magnitude from the surface of the Earth to the lower mantle. About half of this quantity in the upper mantle is due to small-scale features (〈300km). the lower mantle is devoid of small-scale structure. It contains 0.1 per cent velocity variations at a characteristic scale of about 1000km. This corresponds to a spectral band-width of l= 7. the D″ layer at the bottom 100–200 km of the mantle shows up as a distinct layer in our results. It has 0.3 per cent velocity variations at a characteristic scale of 350km. the top of the lower mantle contains 0.3 per cent velocity variations on a scale of 500km and also contains some small-scale power.These results are compatible with previous deterministic lower mantle studies, although some details differ. the strength of heterogeneity in the upper mantle may obscure attempts to model the Earth's deep interior.

Notes: Results of a 2-D, seismic undershooting experiment on the Katla central volcano in south Iceland are reported. Large localized traveltime anomalies (0.4s) are observed on an array within the Katla caldera. the traveltimes are forward modelled using a wavefront tracker developed in Appendix A. Thus, non-linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity. an extreme variation in compressional velocity is required to extend over a significant volume in order to model the data. the resulting model is not unique, but constraints on the allowable range of velocities (2.5-6.0 kms−1) render the basic features well constrained. A clear S-wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly. Low-amplitude P waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly. the model is interpreted in terms of a magma chamber containing extensively molten rock. the magma chamber is shallow, with a bottom at a depth of about 1.5km below sea-level (3.0 km below surface), and measures about 5 km across. the depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust. Owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved. On the other hand, the bottom of the chamber is well resolved. the chamber is underlain by rocks of average or high velocity for that depth. the magma chamber is a persistent feature, big enough (10km3) to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels. the chamber is fed by magma fracturing from below. the model agrees with phenomenological models of magma chambers in Iceland based on geological observations and provides a quantification of those models in terms of depth and size. On the other hand, it is fundamentally different from recent models of magma chambers at mid-ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath Iceland. This underlines the important effect of the Icelandic hotspot on tectonics and volcanism in Iceland and implies a substantially different crustal and thermal structure in Iceland from that of ‘normal’ mid-ocean ridges.