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 is originated 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 ~50° and is centered at ~95.6° W and 25.3° N with an east-west extension of ~40km. A decrease of P-wave velocity by a few to ten percent 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 of the core-mantle region that is marked by a high velocity anomaly related to the subducted Farallon slab. Numerical modeling suggested that a substantial amount of hot mantle can be trapped beneath a slab over long periods of time, leading to formation of a mega-plume. Thus, the observed sharp dipping boundary here might be corresponding to the edge of a low velocity zone that has been interpreted as evidence for the presence of trapped mantle in the core mantle boundary.