Out‐of‐Plane Seismic Reflections Beneath the Pacific and Their Geophysical Implications

Abstract

AbstractWe detect seismic P wave arrivals that reach the surface from a different horizontal direction than the theoretical back azimuth of the earthquake. Slowness, back azimuth, and traveltime of observed out‐of‐plane signals are measured with array methods in relation to the main phases that travel along the great circle path. This directivity information is used to back trace the wave through a 1‐D velocity model to its scattering or reflection location. The focus of this study lies on out‐of‐plane signals reflected once beneath the Pacific at a depth greater than 800 km. Data analysis is carried out for a broad frequency range (band‐pass filter with corner periods of 0.5–5 s up to 5–50 s) to enable the detection of different structures and heterogeneities. In addition to mapping seismic heterogeneities in the lower mantle, we also qualitatively analyze waveforms and polarities of these signals to understand the nature of the structure. The observed 21 reflections with a reflection depth between 800 and 2,200 km illuminate heterogeneities in the mid‐ and lower mantle. Back‐traced locations show shallowest depths around Hawaii and increase in depth to the north and southwest. Analysis of the polarities indicates low velocities for the imaged structure, and complexity of waveforms argues for a likely thermochemical origin. Additional 11 deep reflections/scatterers with depth larger than 2,200 km suggest internal heterogeneities or a presence of the D'' reflector.