Lateral heterogeneity imaged by small‐aperture ScS retrieval from the ambient seismic field

Abstract

Interpreting core‐related body wave traveltimes is challenging for seismologists because Earth's heterogeneities are averaged over thousands of kilometers and the sparsity of earthquake measurements makes these heterogeneities difficult to localize. We show how the ambient seismic wave field can be used to overcome these limitations. We present a regional‐scale analysis of core‐mantle boundary reflections (ScS) under Mexico. We show that body wave arrivals (i.e., P and ScS) are retrieved from higher‐order cross correlations (C3), a technique that provides a more uniform and controlled source distribution using the scattered waves of the coda of classical ambient field cross correlations (C1). Then, we extract ScS traveltimes along a dense linear array in Mexico and find that lithospheric lateral heterogeneity, such as the subducting Cocos slab beneath Mexico, may have a strong impact on ScS traveltimes. In parallel, we show that lateral heterogeneity such as a possible ultralow velocity zone (ULVZ) near the core‐mantle boundary might also affect, although to a lesser extent, the traveltime anomalies. Our results and interpretation are supported through numerical simulations that account for slab and ULVZ properties.

Observation and explanation of spurious seismic signals emerging in teleseismic noise correlations

“…Using ray theory and stationary-phase arguments, Phm et al (2018) interpreted the spurious phases in global coda correlations as a result of the interferences between specific ray paths. Tkalčić and Phm (2018) inferred the shear properties of the Earth's inner core by modelling a spurious phase related to the J waves.…”

A Compositional Component to the Samoa Ultralow‐Velocity Zone Revealed Through 2‐ and 3‐D Waveform Modeling of SKS and SKKS Differential Travel‐Times and Amplitudes

“…Structure on the receiver-side of the path can also affect the waveforms we observe here. A large ULVZ is known to exist on the receiver-side beneath northern Mexico (Havens & Revenaugh, 2001;Spica et al, 2017;Thorne et al, 2019), but none of the Pd paths used in this study cross this ULVZ. However, we cannot rule out possible contamination from unknown ULVZs beneath N. America.…”

Empirical Green's Function Retrieval Using Ambient Noise Source‐Receiver Interferometry

“…In our implementation of C 2 , we focus on the nondiffuse part of the energy instead of the scattered wavefield of the EGFs, which is different from C 3 that utilizes the diffuse coda wave energy. The application of C 3 typically succeeds when stable coda waves are available from a dense seismic network (e.g., Froment et al, 2011;Ma & Beroza, 2012;Sheng et al, 2018;Spica et al, 2016Spica et al, , 2017Stehly et al, 2008;Zhang & Yang, 2013). In such cases, the uniformity and diffusivity of the source illuminations can be enhanced through the presence of (near receiver) scatters (Boschi & Weemstra, 2015), which produce a sufficiently diffuse scattering wavefield that is critical for the cancellation of the cross-terms in the correlation functions (Snieder et al, 2008).…”