Comparing the anomalies of different phases and for different frequency bands, it is found that, within the measurement limits, all slowness and azimuth anomalies and all plane wave-front deviations of P signals registered at the Norwegian Seismic Array (NORSAR) have their origin in the upper mantle and crust beneath the array. Assuming that the anomalies are caused by structures as close to the receiver as possible, we as an order of magnitude find that a maximum of 70 per cent of these are produced within the last 40 km, and, in order to include the remaining 30 per cent, we have to go down to a depth of at least 60 km. P-wave slowness and azimuth anomalies at NORSAR are critically dependent on the array configuration, indicating that (dT/dΔ, ϕ) anomalies and plane wave-front deviations must have a common origin and therefore should not be logically separated. Both configurations giving anomalies inconsistent with and configurations giving anomalies consistent with the existence of mantle plumes under Iceland have been found, showing that the cause of these (dT/dΔ, ϕ) anomalies must be local.

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