科研之友
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摘要
We have compiled all previous ultralow velocity zone (ULVZ) studies, and digitized their core-mantle boundary (CMB) sampling locations. For studies that presented sampling locations based on infinite frequency ray theory, we approximated Fresnel zones onto a 0.5 degrees x 0.5 degrees grid. Results for these studies were separated according to wave type: (1) core-reflected phases, which have a single location of ULVZ sampling (ScS, ScP, PcP), (2) core waves that can sample ULVZs at the core entrance and exit locations of the wave (e.g., SPdKS, PKKP, and PKP), and (3) waves which have uncertainties of ULVZ location due to long CMB sampling paths, e.g., diffracted energy sampling over a broad region (P-diff, S-diff). For studies that presented specific modeled ULVZ geographical shapes or PKP scatter probability maps, we digitized the regions. We present summary maps of the ULVZ coverage, as well as published locations arguing against ULVZ presence. A key finding is that there is not a simple mapping between lowermost mantle reduced tomographic velocities and observed ULVZ locations, especially given the presence of ULVZs outside of lowermost mantle large low velocity provinces (LLVPs). Significant location uncertainty exists for some of the ULVZ imaging wave types. Nonetheless, this compilation supports a compositionally distinct origin for at least some ULVZs. ULVZs are more likely to be found near LLVP boundaries, however, their relationship to overlying surface locations of hot spots are less obvious. The new digital ULVZ database is freely available for download.
Plain Language Summary Nearly half way to the center of Earth, small and thin regions of extremely anomalous mantle rock sit on top of Earth's fluid core. The speeds of seismic waves are reduced by tens of percent in these tiny zones, and for over 20 years have been interpreted as being partially molten. Here, we summarize all the past studies and show that the geographical distribution of the sluggish patches is consistent with a requirement that they be compositionally distinct from the surrounding mantle. Their composition remains unknown. Dubbed ultra-low velocity zones, they remain enigmatic - less than 20% of Earth's core mantle boundary has been explored in past investigations. However, this summary suggests they have a preference of being located near the margins of two much larger anomalies, continental-sized lowermost mantle low seismic wave speed provinces.
- 出版日期2018-2
