Seismic array constraints on the D″ discontinuity beneath Central America

Abstract

We analyzed 16,150 transverse component seismic recordings from 54 deep‐focus earthquakes in the South American and Caribbean regions recorded at broadband stations in North America between 2005 and 2012. We treated subgroups of seismic stations within 3° radius geographical bins as seismic arrays and performed vespagram analysis. We focused on the S, ScS, and Scd arrivals and collected data in the epicentral distance range from 55° to 90°. In particular, we searched for D″ discontinuity presence in the vespagrams in a 25° by 35° (or 1520 by 2130 km) area beneath Central America. Analysis of these data showed 125 clear Scd observations, 180 Scd observations of lesser quality, and 343 nonobservations. We produced a new map of the discontinuity height beneath Central America. Our map shows an average discontinuity height of 286 ± 6 km (σ = 76 km). The region is punctuated by a large topographic high centered at approximately 10°N and 90°W with a maximum height of 380 km. Two smaller topographic highs are located at approximately 4°N and 81°W (discontinuity height of 320 km) and at 4°N and 70°W (height of 315 km). The observation of multiple Scd arrivals collocated with the strongest gradients in inferred topography provides evidence for topographic variation on the discontinuity rather than multiple discontinuities. The regions where the discontinuity has the greatest height can be explained by localized enrichment of mid‐ocean ridge basalt from the subducted Farallon slab impinging on the core‐mantle boundary.

Seismic Evidence for a Paleoslab in the D" Layer Residing Adjacent to the Southeastern Edge of the Perm Anomaly

“…Our observations appear to support the notion that subducting slabs, at least in part, more likely accumulate eventually at the base of the mantle (e.g., Hutko et al, ). Our detection of pv‐ppv transition is also consistent with the majority of observations that the D" discontinuity is often seen at regions with a velocity faster than average lower mantle seismic wave speeds; for example, the discontinuity is observable beneath Alaska (e.g., Sun et al, ) and Central America (e.g., Whittaker et al, ), whereas the occurrence of the discontinuity beneath slow regions in D" is primarily confined to the central Pacific region (e.g., Avants et al, ). It has been suggested that slow region D" discontinuities may have an origin distinct from those in fast regions (Hernlund & McNamara, ).…”

Deformation in the lowermost mantle beneath Australia from observations and models of seismic anisotropy

“…The D″ layer beneath the Antarctic Ocean and New Zealand is approximately~350 km thick based on waveform modeling and traveltime analysis [Usui et al, 2005]. The global average D″ thickness is approximately 250 to 300 km, based on studies of various regions including Alaska [Sun et al, 2016], southeastern Asia [Chaloner et al, 2009], Russia [Shen et al, 2014], Central America [Lay and Helmberger, 1983;Whittaker et al, 2015], and the Indian Ocean [Young and Lay, 1987]. Usui et al [2005] attribute the thicker than average D″ beneath our study region due to the presence of paleo-slab material, in agreement with the inferences of Steinberger [2000b].…”

Lateral heterogeneity imaged by small‐aperture ScS retrieval from the ambient seismic field

“…Both an ULVZ [Havens and Revenaugh, 2001] or a strong discontinuity in D'' [Lay et al, 2004;Lay, 2008;Sun et al, 2006;Kito et al, 2007;Whittaker et al, 2015] could affect our measurements. Considering the long period of the ScS waveforms used in this study (i.e., on the order of 700 km), there are two documented structures in the lower mantle beneath Mexico that are likely to have lateral variations on a small scale.…”

“…Under the Cocos Plate, several studies suggest that the sharpness of the D'' discontinuity and its topography have notable influence on phase arrivals [Lay et al, 2004;Lay, 2008;Sun et al, 2006;Kito et al, 2007;Whittaker et al, 2015]. Under the Cocos Plate, several studies suggest that the sharpness of the D'' discontinuity and its topography have notable influence on phase arrivals [Lay et al, 2004;Lay, 2008;Sun et al, 2006;Kito et al, 2007;Whittaker et al, 2015].…”

“…Small-scale variability of ScS time delays are often interpreted as the result of sharp contrasts in CMB topography, in the D'' seismic velocity discontinuity or due to lateral variations in the lower mantle. Under the Cocos Plate, several studies suggest that the sharpness of the D'' discontinuity and its topography have notable influence on phase arrivals [Lay et al, 2004;Lay, 2008;Sun et al, 2006;Kito et al, 2007;Whittaker et al, 2015]. The presence of an ULVZ in the vicinity of the CMB is another potential source of traveltime variation in the region [Havens and Revenaugh, 2001]; however, these studies are based on teleseismic data and their inference applies for large reflection angles, typically from 25 to 65 ∘ .…”