Body Waves Retrieved From Noise Cross‐Correlation Reveal Lower Mantle Scatterers Beneath the Northwest Pacific Subduction Zone

Abstract

Seismological studies have revealed heterogeneities at scales of 10-1,000 km in the lower mantle. For the first time, we demonstrate here that seismic interferometry of ambient noise can be used to detect faint scattered waves. We locate scattering structures at depths of~900-1,000 km from P-toP scattered wave signals in stacks of vertical cross-correlograms from dense seismic arrays in northeast China. Independently, we show that this scattering structure produces S-toP converted signals in the recordings of a deep earthquake. We constrain the thickness (H), the contrasts of the shear wave speed (δVs), and density (δρ) of the anomaly using 1-D synthetics. Our modeling of both the reflected and converted signals indicates that H = 10-20 km, δVs = −7.2%, and δρ = 0.6%, which are similar to values reported previously and consistent with the interpretation that the scattering structure is a fragment of the basaltic oceanic crust associated with the subducted Izanagi plate. Plain Language Summary Analysis of surface waves in ambient noise has been widely applied to image the structure of the crust and mantle. However, it is more difficult to extract the signals of body waves in the deep mantle from ambient noise. We explore signals by cross-correlation using ground motion recordings from dense seismic arrays in northeast China. We identify a scattering signal at time~200 s. By analyzing and comparing the results of conversion waves generated by deep earthquakes, we confirm that this scattering signal represents a kind of body wave reflected by a small-scale structure in the mid-mantle at depths of~900-1,000 km. The successful extraction of the weak body waves generated by scatterers in the lower mantle provides a potential method for the global detection of fine-scale heterogeneities in the mantle, which will provide a better understanding of the composition and structure of the mantle, and the geodynamic processes in the deep earth.

Thermal Equations of State of Magnesite: Implication for the Complex Mid‐Lower Mantle Seismic Scatterers

“…The variations in these factors provide an explanation for the observed small-scale scatterers, featuring varying magnitudes of low V S anomalies within the mid-lower mantle. We recognized that the low V S anomalies greater than 7% detected in the regions such as northeast China and South America at depths of 950-1,750 km in the lower mantle cannot be explained by our models (Haugland et al, 2017;Zhang et al, 2020). Since experimental constraints on the influence of Al 2 O 3 on the density and velocity of SiO 2 are lacking, whether the formation of these mid-lower mantle low-velocity scatterers is associated with a variation in the Al 2 O 3 content within SiO 2 in the subducted oceanic crust or caused by other factors requires further investigation in future studies.…”

“…Color vertical bars are the observed seismic low V S anomalies in the mid‐lower mantle. Purple: northeast China (Zhang et al., 2020); yellow: Japan Sea (Niu, 2014); orange: Izu‐Bonin (Niu et al., 2003); blue: Mariana (Kaneshima & Helffrich, 1999); green: South America (Haugland et al., 2017).…”

“…Seismological studies have observed the existence of numerous scatterers in the Earth's lower mantle at depths ranging from 900 to 1,900 km (e.g., Haugland et al., 2017; Kaneshima & Helffrich, 1999; Niu, 2014; Niu et al., 2003; Zhang et al., 2020). These scatterers exhibit complex seismological structures, with most of them exhibiting a V S that is 2%–6% lower than that of the normal mantle (e.g., Kaneshima & Helffrich, 1999; Niu, 2014; Niu et al., 2003).…”

“…In certain regions, such as South America, the magnitude of the low V S anomaly can even exceed 10% (Haugland et al., 2017). In contrast, the V P anomalies of these seismic scatterers are less than 1%, and the density anomalies vary between 0.6%–9% (e.g., Kaneshima & Helffrich, 1999; Niu, 2014; Niu et al., 2003; Zhang et al., 2020). Previous studies have proposed that these low‐velocity scatterers should be associated with structural phase transitions of SiO 2 in the lower mantle which can produce a significant softening in the V S (Hirose et al., 2005; Sun et al., 2019; Zhang et al., 2021, 2022).…”

Retrieving PmP Travel Times From a Persistent Localized Microseismic Source

“…Similar efforts have been devoted to extracting body waves from ambient noise, especially the reflection body waves from various interfaces within the Earth, which can be utilized to constrain structural interfaces that cannot be constrained by surface waves (e.g., Feng et al., 2017; Zhan et al., 2010). However, due to the fact that the energy of body waves in microseisms is much weaker than that of surface waves and sources of ambient noise are mostly located on the surface of the Earth, retrievals of body waves from ambient noise have been less successful and are only limited to some specific areas (e.g., Poli, Campillo, et al., 2012; Poli, Pedersen, & Campillo, 2012; Zhan et al., 2010; Zhang et al., 2020). Such obstacles limit the applications of body waves from ambient noise in imaging the internal structures of the Earth.…”