The upper-mantle discontinuities underneath the GRF array from P-to-S converted phases

A data-processing method is applied which includes a rotation of the three components, normalization and delay-and-sum of broadband records of earthquakes from a large distance and azimuth distribution, recorded at a single station (or an array). Clear P-to-S converted phases at the mantle discontinuities are observed in the Grafenberg records, after this data processing. Theoretical seismograms are computed for the PREM model and processed in the same way as the observed data. A comparison with the data shows that the depth interval between the two discontinuities in the mantle transition zone (those at 400 and 670 km depth in PREM) is around 240 km. The 670-km discontinuity is sharper than the 400-km discontinuity and is comparable in sharpness with the crust-mantle transition, as far as it is possible to judge from the broadband data used. There are indications of pronounced lateral heterogeneity of the 400-km transition, underneath GRF. We have also observed converted and multiply reflected shear waves in the crust, which set sensitive limits to the average crustal model underneath the array. These data suggest that the velocity jump at the Moho is smaller than indicated by refraction studies.           ARK: https://n2t.net/ark:/88439/y067070 Permalink: https://geophysicsjournal.com/article/135 &nbsp

Seismic boundaries in the mantle beneath Iceland: a new constraint on temperature

To study the deep structure of Iceland, we conducted S receiver function analysis for almost 60 local broad-band seismograph stations of the Hotspot, ICEMELT and SIL networks. The structure was investigated separately for the central region of Iceland containing the neovolcanic zone and two peripheral regions to the east and west. S-to-P converted phases from upper-mantle discontinuities were detected by stacking recordings of several tens of teleseismic events. The analysis reveals previously unknown details. Magnitude and depth extent of the low S velocity anomaly in the upper mantle beneath Iceland are much larger than reported in earlier studies. Clear S-to-P converted phases are obtained from the discontinuity at a depth of 80 ± 5 km, separating the high-velocity mantle lid from the underlying low S velocity layer. This discontinuity can be interpreted as a chemical boundary between dry harzburgite in the upper layer and wet peridotite underneath. Beneath peripheral parts of Iceland, we detect a boundary at a depth of 135 ± 5 km with S velocity increasing downwards. This boundary may correspond to the onset of melting in wet peridotite at a potential temperature of around 1400 °C. Models of melting induced by CO2 are not incompatible with our observations. The seismic data demonstrate effects that may be caused by azimuthal anisotropy in the upper mantle. There are indications of a second low S velocity layer to the NNE of Iceland, with the top near 480 km depth, similar to one recently detected beneath the Afro-Arabian hotspot

Evidence from P-to-S mantle converted waves for a flat “660-km” discontinuity beneath Iceland

Iceland is the type example of a ridge-centered hotspot. It is controversial whether the seismic anomaly beneath it originates in the lower mantle or the upper mantle. Some recent studies reported that the 660-km discontinuity beneath central Iceland is shallow relative to peripheral regions and this was interpreted as an effect of elevated temperature at that depth. We investigate topography of the major upper mantle discontinuities by separating the effects of the topography and volumetric velocity heterogeneity in P receiver functions from 55 seismograph stations. Our analysis demonstrates that a significant (at least 10-km) shallowing of the 660-km discontinuity is only possible in the case of improbably low seismic velocities in the mantle transition zone beneath central Iceland. If, as in previous studies, lateral velocity variations in the mantle transition zone are neglected, the data require a depressed rather than an uplifted 660-km discontinuity. For a reasonable S-wave velocity anomaly in the mantle transition zone (around − 3%) no topography on the 660-km discontinuity is required. This can be explained by the lack of temperature anomaly or an effect of two phase transitions with opposite Clapeyron slopes