Large angle reflection from a dipping structure recorded as a PKIKP precursor: Evidence for a low velocity zone at the core–mantle boundary beneath the Gulf of Mexico

We observed a clear phase-like arrival prior to the PKIKP wave at a broadband seismic array in eastern Tibet from an intermediate depth earthquake occurring in Guatemala. The measured incident angle and back azimuth of this phase indicate that it originates from scattering near the core–mantle boundary (CMB) of the source side. This phase, however, was not observed from another earthquake that is only 60 km away, suggesting that scattering depends strongly on the angles of the incident waves. Ray tracing and diffraction migration indicates that the precursor is a large-angle reflection from a dipping structure in the lowermost mantle east of Mexico. The seismic reflector dips northward by ∼ 52º and is centered at ∼ 93.31ºW and 23.89ºN with an east–west extension of ∼ 100 km. A decrease of P-wave velocity by a few to 10% is required to explain the amplitude and polarity of the phase. It is unlikely to explain the large P-wave velocity contrast and the large dipping feature with the post-perovskite phase transition. The reflector is located in a region where the lowermost mantle is marked by a high velocity anomaly related to the subducted Farallon slab. Previous numerical modeling suggested that a substantial amount of hot mantle could be trapped beneath a slab over long periods of time, leading to the formation of a mega-plume. Thus, the observed sharp dipping boundary here might correspond to the edge of a low velocity zone below the subducted Farallon slab. Mixed results on the presence of an ultra low velocity zone in the region have been reported by previous seismic studies. We speculated that the low velocity zone below the cold slab was developed as a consequence of long-term heating from core.