Reflection seismic profiles of the core‐mantle boundary

Abstract

[1] High amplitude, high frequency, and laterally coherent seismic phases from the coremantle boundary (CMB) are observed on data from Peaceful Nuclear Explosions in Siberia. These arrivals are observed at 2600-4000 km offset with travel times and moveouts consistent with CMB reflections (PcP) and are readily correlated because of the small station interval of 10 km. The duration and complexity of the arrivals are inconsistent with a simple reflection from a single CMB interface. They require the combination of PcP and a complex precursor phase (PdP) from the top of an ultralowvelocity zone (ULVZ) despite the fact that earthquake recordings indicate that there is no ULVZ present beneath this part of Siberia. Lateral variations in the waveform imply that the thickness and physical properties of the ULVZ change with a wavelength of 150-200 km and an amplitude of up to 8 km. We find at least one location where there is no ULVZ present. At another well-sampled location, the precursor has higher frequency content than the primary arrival. Reflectivity modeling of these PcP/PdP phases shows that kilometer-scale layering may be required in the ULVZ to explain the seismic waveform and the frequency split. The modeling shows very low velocity in the ULVZ with reductions of 25 and 40% for P and S waves, respectively. This indicates higher percentages of partial melt in the ULVZ than hitherto interpreted. Our observations indicate that the ULVZ may be present over wide areas of the Earth with a thickness below the resolution limit of seismological data with a lower frequency content. INDEX TERMS:7207 Seismology: Core and mantle; 7219 Seismology: Nuclear explosion seismology; 7203 Seismology: Body wave propagation; KEYWORDS: core-mantle boundary, ultralow velocity zone, nuclear explosion Citation: Ross, A. R., H. Thybo, and L. N. Solidilov (2004), Reflection seismic profiles of the core-mantle boundary,

Ultralow Velocity Zone Locations: A Global Assessment

“…ULVZs located beneath the surface locations of hot spots (e.g., Cottaar & Romanowicz, 2012;Rost et al, 2005;Yuan & Romanowicz, 2017) may point to a thermal origin (Figure 1a), though this does not preclude compositionally distinct material having been advected to plume root locations. Nonetheless, their existence around the edge of LLVPs (Figure 1b) combined with a proposed density elevation (Havens & Revenaugh, 2001;Ross et al, 2004; appears compatible with a compositional difference between ULVZs and the surrounding mantle McNamara et al, 2010). Also, we may expect to see more ULVZs in the center of LLVPs if their origin is related to the hottest mantle temperatures.…”

“…The modeled 3:1 ratio in S wave to P wave velocity reduction can be explained by 5-30% partial melting of the deepest mantle material (Berryman, 2000;Williams & Garnero, 1996), with the amount of melt depending on actual melt geometry (e.g., Williams & Garnero, 1996). The partial melt explanation faces difficulties in explaining ULVZs detected around the edge, or away from presumably hotter lowermost mantle regions (i.e., the large low velocity provinces, LLVPs) (Luo et al, 2001;Ni & Helmberger, 2001b;Rondenay & Fischer, 2003;Ross et al, 2004;Xu & Koper, 2009). ULVZs located beneath the surface locations of hot spots (e.g., Cottaar & Romanowicz, 2012;Rost et al, 2005;Yuan & Romanowicz, 2017) may point to a thermal origin (Figure 1a), though this does not preclude compositionally distinct material having been advected to plume root locations.…”

Seismic and gravitational studies of melting in the mantle's thermal boundary layers

“…There are several proposed explanations for low velocities in ULVZs. Probably the most often-quoted hypothesis is the presence of partial melt [Williams and Garnero, 1996;Revenaugh and Meyer , 1997;Helmberger et al, 1998;Vidale and Hedlin, 1998;Williams et al, 1998;Zerr et al, 1998;Berryman, 2000;Wen, 2000;Ross et al, 2004]. However, variations in chemical composition on the mantle side of the CMB [Manga and Jeanloz , 1996;Stutzmann et al, 2000], "sediments" of finite rigidity collecting on the top of the outer core [Buffett et al, 2000;Rost and Revenaugh, 2001], and a gradient in the mantle-core transition (rather than the traditional sharp CMB) [Garnero and Jeanloz , 2000a, b] have also been proposed, singly and in combination.…”

“…They include precursors and postcursors in stacks of short period and broad band ScP [Vidale and Benz , 1992;Garnero and Vidale, 1999;Castle and van der Hilst, 2000;Reasoner and Revenaugh, 2000;Persh et al, 2001;Revenaugh, 2001, 2003], ScS [Avants et al, 2006], and PcP [Mori and Helmberger , 1995;Kohler et al, 1997;Revenaugh and Meyer , 1997;Havens and Revenaugh, 2001;Persh et al, 2001;Ross et al, 2004]; scattered precursors to PKP [Vidale and Hedlin, 1998;Wen and Helmberger , 1998a;Thomas et al, 1999;Wen, 2000;Ni and Helmberger , 2001;Niu and Wen, 2001], and SKS [Stutzmann et al, 2000]; and travel time and waveform anomalies in PKPdf and…”

Some dynamical consequences of partial melting in Earth’s deep mantle

“…However, other studies have argued for the existence of a very thin (ca. 1 km) ubiquitous global layer of partial melt away from ULVZ based on the analysis of high-frequency seismic waves (Revenaugh and Meyer, 1997;Ross et al, 2004). Studies with somewhat lower effective resolutions (≈3 km), on the other hand, failed to find a similar ubiquitous signal along the CMB (Persh et al, 2001).…”