Upper mantle structure beneath the Fiji Plateau: seismic observations of second P arrivals from the olivine-spinel phase transition zone

Summary Large and impulsive second P arrivals (4–12 s after the initial P arrivals) from intermediate depth earthquakes located in the New Hebrides island arc are observed at distances of 7–14° on the records produced by short-period seismographs in Fiji. These second P arrivals are caused by refraction from a zone of rapidly increasing P velocity at about 370 km depth beneath the Fiji plateau, presumably caused by the olivine–spinel phase transition in the upper mantle. This conclusion is consistent with: (1) the observed onset distance of the travel-time cusp; (2) the change with distance of δt (the time difference between first and second P arrivals); (3) the change of δt with the depth of the events; (4) the slowness of the first and second P arrivals; and finally, (5) comparisons with the predicted arrival times frop other published velocity models which contain a rapid P velocity increase in the upper mantle. The range of velocity models which fit our data is shown by giving examples of models with the maximum and minimum values for the depth, thickness and amount of velocity increase in the transition zone. These include: (1) a model with a discontinuous increase in velocity of 0.81 km/s occurring at 340 km depth (shallowest zone, thinnest zone, and smallest velocity increase); (2) a model with an increase of 1.55 km/s occurring over a thickness of 47 km and centred at 346 km depth (thickest zone and largest velocity increase); and (3) a model with an increase of 1.13 km/s occurring over a thickness of 21 km and centred at 393 km depth (deepest zone). However, a consideration of the effects of mantle self-compression, temperature, and Birch's law restricts further the family of acceptable velocity models that fit our data. Models that meet these restrictions have an increase in velocity greater than 1.0 km/s occurring over a thickness less than 30 km and centred between 360 km and 390 km depth. For sources in the New Hebrides, where the inclined seismic zone has a dip of 70–80°, the descending lithospheric slab does not seriously affect the pattern of second P arrivals. Applying Snell's law to a simple model of the slab shows that the slab decreases δt by less than one second, and decreases the onset distance of the travel-time cusp by less than 0.5 degrees. However, no second p arrivals at comparable distances are observed at Fiji from intermediate depth earthquakes occurring in the Tonga–Kermadec island arc. The simple model of the slab shows that the dip of the Tonga–Kermadec slab is too shallow to allow rays to leave the slab at the proper angle to become second P arrivals. The observed predominant frequency of the second P arrivals is only about 25 per cent higher than that of the first arrivals. This suggests that the average Qp for the first P arrival rays, which traverse only the upper 200 km of the mantle beneath the Fiji plateau, cannot differ greatly from the average Qp for the second P arrival rays, which penetrate to the phase transition zone. A survey of other studies of upper mantle velocity structures confirms that differences in the thickness and in the depth of the phase transition zone do exist throughout the world, although the comparison is complicated by differences in the type and uniformity of the data sets and in methods of analysis. Our results from the Fiji plateau (a recently generated oceanic lithosphere) differ significantly from the results obtained in northeastern Australia, but are remarkably similar to the results obtained in the western United States. Summary and conclusions (1) The presence of large second P arrivals at distances beyond 9O produced by intermediate depth earthquakes located in the New Hebrides island arc and recorded at Fiji almost certainly confirms the existence of a zone of rapidly increasing velocity beneath the Fiji plateau, probably associated with the olivine–spinel phase transition in the upper mantle. Extreme velocity models which fit these data can be constructed with the depth of the zone as shallow as 340 km or as deep as 395 km, with the thickness as great as 47 km, and with the velocity increase as small as 0.81 km/s or as large as 1.55 km/s. However, for more realistic models, which are consistent with the effects of mantle self-compression, mantle temperature, and Birch's law, and which agree more closely with dT/dδ measurements, the depth to the centre of the zone is about 370 km, the thickness is less than 30 km, and the amount of increase in velocity is greater than 1.0 km/s. (2) Because the descending lithospheric slab beneath the New Hebrides island arc is dipping steeply, at an angle near 70°, the slab does not affect significantly the pattern of second P arrivals that is observed at Fiji. A simple Snell's law analysis of the effect of the slab indicates that the slab should cause the difference in travel times between first and second P arrivals to decrease by about 1.0 s at most, and the distance where the onset of the cusp occurs would be decreased by less than 0.5 degree. This model may be used to place bounds on the properties of the lithospheric slab. In the New Hebrides near 12° S, if we assume the dip angle of the slab between 100 and 200 km depth is 70°, then the ratio Vslab/Vmantle must certainly be smaller than 1.10. If we assume that Vslab/Vmantle is 1.07, then the dip angle must certainly be larger than 65°. (3) For intermediate depth earthquakes occurring in the Tonga–Kermadec arc and recorded at Fiji at comparable distances to the New Hebrides events, no second P arrivals are observed. This fact is consistent with a simple Snell's analysis of the effect of the descending slab. This model predicts that for dip angles of 50 to 55°, the dip angle of the Tonga–Kermadec seismic zone, no rays can leave the slab at the proper take-off angle to become second P arrivals at the Fijian stations. (4) Apparent complexity in the pattern of second P arrivals from New Hebrides earthquakes beneath 200km probably reflects complexity either in the slab or in the mantle beneath this depth. It is not clear, however, whether this is due to elevation of the olivine–spinel phase boundary within the slab, lateral inhomogeneities in the slab, lateral inhomogeneities in the mantle, or topography on the olivine–spinel phase boundary several hundred kilometres away from the slab. (5) The average Qp for ray paths from New Hebrides to Fiji has been determined from the predominant frequency content of frrst and second P arrivals. The primary result is that (Qp) for ray paths penetrating to depths of less than 200 km is roughly the same as (Qp) for ray paths that penetrate to the phase boundary near 370 km. If the small differences in frequency content between first and second P arrivals (about 25 per cent) can be attributed to attenuation along the ray path, then Qp= 200 for first arrival paths, and Qp= 350 for second-arrival paths. (6) A survey of the most reliable seismic studies of the zone of rapidly increasing velocity indicates that the observed range of depths of the centre of the transition (between 360 and 400 km) is consistent with the variation expected due to temperature variations in the upper mantle. The depth of the transition is consistent with the results of high-pressure laboratory experiments. It appears that the zone is thinner (30 km or less) in the tectonically active areas (Fiji plateau and western United States) and thicker (50km or more) in the less active areas (northeastern Australia, and possibly the eastern United States). It is not likely that this variation could be due solely to differences in the temperature distribution of the mantle; differences in mantle composition would provide a more reasonable explanation. For example, where the mantle has not been depleted of its basalatic fraction, the presence of Al2O3 may allow pyroxenes to undergo a phase transformation between 300 and 350 km depth. In this case, the pyroxene and olivine transformations would occur in a zoiie more than 50 km thick. Where the mantle is completely depleted of its baslatic fraction, however, pyroxenes could not undergo a phase transformation, and only the olivine is left to undergo a phase transformation with a thickness of 30 km or less.