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

Abstract

We 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.

Discriminating Between Causes of D″ Anisotropy Using Reflections and Splitting Measurements for a Single Path

“…The stacking procedure used in producing vespagrams usually assumes that the energy travels along the great circle path. Because of 3D structures and reflector topography, however, the energy can also travel out of plane (e.g., Schumacher et al, ; Schumacher & Thomas, ). Therefore, we also performed slowness‐back azimuth analyses that allow the detection of out‐of‐plane reflections (e.g., Rost & Thomas, ).…”

Mapping lithological boundaries in mines with array seismology and in situ acoustic emission monitoring

“…These small residuals lie within the average standard deviation σ, which is representing the uncertainties in measuring the observed slowness (σ=0.0075 s/km) and backazimuth (σ=4°) values of the direct P-waves. To measure these uncertainties and estimate standard deviation, we consider all points that have a variation of up to 10% in beam power with respect to the maximum (white contour line in the polar plot of Figure 4c) as performed also by Schumacher et al (2018).…”

“…This approach has been used in global seismology for the detection and location of heterogeneities in the Earth mantle that cause scattering and/or reflection of seismic waves (e.g. Kaneshima, 2009Kaneshima, , 2016Kaneshima & Helffrich, 2003;Rost et al, 2008;Schumacher & Thomas, 2016;Schumacher et al, 2018;Weber and Wicks, 1996;Weber et al, 2015;Wicks and Weber, 1996;Wright, 1972;Wright & Muirhead, 1969).…”

D" Reflection Polarities Inform Lowermost Mantle Mineralogy

“…Ballmer et al. (2016) model the Pacific LLSVP with a combination of MORB and primordial material, which generates a velocity discontinuity at depths consistent with the D" reflector within an LLSVP‐like region (Schumacher et al., 2018). Furthermore, Deschamps et al.…”

“…Instead, our thermochemical modeling shows that an increase in bm or a decrease in MgO (and to a smaller degree FeO) would generate large R-values. Ballmer et al (2016) model the Pacific LLSVP with a combination of MORB and primordial material, which generates a velocity discontinuity at depths consistent with the D" reflector within an LLSVP-like region (Schumacher et al, 2018). Furthermore, Deschamps et al (2012) and Vilella et al (2021) show that an increase of bm with an increase of Fe has been suggested to be responsible for the LLSVPs.…”