Influence of Moho topography on seismic beamforming: implications for mantle scattering

Authors

  • Shubham Agrawal School of the Earth, Ocean, and Environment, University of South Carolina, Columbia, USA https://orcid.org/0000-0001-8511-4901
  • Dan Frost School of the Earth, Ocean, and Environment, University of South Carolina, Columbia, USA https://orcid.org/0000-0001-7882-5166
  • Philip Crotwell School of the Earth, Ocean, and Environment, University of South Carolina, Columbia, USA

DOI:

https://doi.org/10.26443/seismica.v5i2.2693

Keywords:

Moho topography, Seismic array, Backazimuth offsets, Mantle scattering, beamforming

Abstract

Measurements of scattered seismic phases are used to infer the distribution of small-scale heterogeneities in the mantle, which in turn inform our understanding of mantle dynamics. Directionality of the wavefront can be measured at seismic arrays, and accuracy is essential, as even small deviations can significantly misplace the inferred location of these scatterers. However, near-surface structures under seismic stations may distort these directionality measurements. In this study, we assess how variations in crustal thickness affect array measurements across Alaska. We observe backazimuth offsets mostly within 5° of the great-circle path between source and array, with offsets varying systematically between sub-arrays for each event, indicating a receiver-side origin. We test the hypothesis that offsets are caused by P waves interacting with dipping Moho interfaces, and reproduce the observed offsets. We quantify how even modest tilts in the Moho (~6°) can deflect the wavefront and alter apparent arrival direction by observable amounts. These offsets can shift inferred scatterer locations by hundreds of kilometers, illustrating the need to correct for shallow structure in deep Earth imaging studies.

Author Biographies

Shubham Agrawal, School of the Earth, Ocean, and Environment, University of South Carolina, Columbia, USA

I'm a seismology post-doctoral researcher at the University of South Carolina, working with Dan Frost. I completed my PhD in Geophysics under the supervision of Caroline Eakin at the Australian National University.

Dan Frost, School of the Earth, Ocean, and Environment, University of South Carolina, Columbia, USA

My work focuses on the use of high frequency seismic waves to understand the detailed structure of Earth’s interior, throughout the core and mantle. While the rough velocity structure of the mantle is generally agreed upon, the smaller-scale structure is more enigmatic. By knowing the Earth’s structure across length-scales we can begin to understand the past and on-going physical processes. I employ arrays of seismometers to amplify small signals relative to the background noise, and to determine the incoming direction of seismic energy. I use local arrays to observe scattered seismic waves caused by kilometre-scale heterogeneities in the mantle, and regional arrays to determine waveform variations caused by broader mantle and core structure. Through waveform analysis and full waveform inversion, I extract more information from the wavefield to sharpen the resolution of our models of the Earth.

Philip Crotwell, School of the Earth, Ocean, and Environment, University of South Carolina, Columbia, USA

Research Associate at the Department of Earth Ocean and Environment.

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2026-07-19

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Agrawal, S., Frost, D., & Crotwell, P. (2026). Influence of Moho topography on seismic beamforming: implications for mantle scattering. Seismica, 5(2). https://doi.org/10.26443/seismica.v5i2.2693

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