Generation of Reflections and PKP Precursors From a Scattering Layer in D″. Issue 4 (17th February 2022)

• Record Type: Journal Article
• Authors: Hiemer, Vanessa; Thomas, Christine
• Is Part Of: Geophysical research letters. Volume 49:Issue 4(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-17
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL096900
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: The D″ region consists of many different structures on many length‐scales and here we test whether an inhomogeneous scattering region could potentially explain two of these seismic observables, namely PKP precursors, which are generated by scattering near the core‐mantle boundary, and reflections of a seismic discontinuity in the lowermost mantle. The focus of this study is on modeling PKP precursors and lower mantle reflections. Testing different heterogeneity models with a range of correlation lengths and velocity perturbations for D″, we find that some of our models can produce both waves. Comparing our synthetic seismograms to real data for precursors in adjacent locations beneath the mid‐Atlantic near South America we find the best fitting model with correlation length of 10 km and velocity perturbation of 5% with a gradual increase of scattering defined by a taper from 200 to 400 km above the core‐mantle boundary. Plain Language Summary: The lowest 300 km of the Earth's mantle (called D″ layer) consists of many different structures that range from small‐scale features such as scatterers to large‐scale structures such as large regions with lower seismic velocity than the surrounding mantle. The structures are visible with a range of seismic waves but many studies are restricted to only one wavetype. In this study, we focus on two seismic waves that are usually associated with different structures. First, PKP precursors that are P waves scattered at small‐scale features in the lowermost mantle and second the PdP wave that reflects off the D″ boundary which is associated with a sharp velocity change. We compute synthetic seismic data assuming a scattering region in D″, testing models of different scatterer sizes and strength and find that some of these models can in fact produce both wavetypes. Additionally, we find a best fitting model by comparing the synthetics with real earthquake data imaging the lower mantle beneath the mid‐Atlantic near South America. We thereby show that perhaps some structures in the lowermost mantle, previously interpreted as different features, are in fact related. This may help to improve our knowledge of the geodynamical processes in the lower mantle. Key Points: We show that PKP precursors and PdP reflections can be produced by the same heterogeneity structure in the lowermost mantle (D″ layer) Our heterogeneity models consist of scatterers of different correlation lengths (10, 50 km) and velocity perturbations (1%, 3%, 5%) Best fitting model for PKP precursors that also generates PdP reflections has a correlation length of 10 km and velocity changes of 5%