Seismic reflectors in the mid-lower mantle beneath central Pacific: The relationship with the Pacific LLSVP

Abstract

Seismic signals in P coda originating from deep mantle heterogeneity have not yet been investigated extensively, except for the observations of the waves reflected presumably at the top of the D″ layer. We show in this study that array processing of seismograms of deep earthquakes at Tonga-Fiji and Solomon Islands recorded by seismograph networks at southern California reveals strong off-great circle arrivals in P coda 5 to 10 s after direct P waves. We also show that the large arrivals observed for the Tonga-Fiji events are P-to-P reflected waves at a dipping interface in the mid-lower mantle beneath central Pacific. The reflector is located south-southeast of Hawaii around 2000 km depth and dips down to southeast by nearly 35°. The observed amplitude and polarity of the reflected waves could be explained if the Vp of the underlying side of the reflector is 1 to 2 % faster than the overlying side. The small Vp anomaly may not necessarily contradict the absence of a noticeable Vp anomaly in previous seismic tomography models at the site of the reflector. We also find that the reflected wave is approximately concomitant with a weaker arrival from a mid-mantle scattering object located nearly 1000 km closer to the hypocenters. The heterogeneous object causing the anomalous arrivals for the Solomon Islands events, although the properties of the object are less well constrained than the Tonga-Fiji reflector, also likely represents another dipping reflector at 2400 km located approximately below the Hawaiian hotspot. As in the Tonga-Fiji case the signals are occasionally followed by a weaker signal from a mid-mantle scattering object located nearer to the hypocenters. The mid-mantle reflection/scattering objects do not indicate the presence of a global discontinuity but must represent localized strong heterogeneities. It is notable that the localized heterogeneities are all located near the edges of a large low Vs body (the Pacific LLSVP) resolved by global tomography, up to 500 km above the LLSVP itself. The relation between the locations of the reflector/scatterers and the large scale Vp structure is unclear, probably reflecting poorer tomography images of Vp structure associated with the LLSVP. We discuss possible tectonic implications of these mid-mantle heterogeneities on the structure and evolution of the LLSVP.