Abstract
AbstractLarge low‐velocity provinces (LLVPs) are hypothesized to be purely thermal features or possess some chemical heterogeneity but which exactly remains ambiguous. Regional seismology studies typically use travel time residuals and multipathing identification in the waveforms to infer properties of LLVPs. These studies have not fully analyzed all available information such as measuring the direction and inclination of the arrivals. These measurements would provide more constraints of LLVP properties such as the boundary velocity gradient and help determine their nature. Here, we use array seismology to measure backazimuth (direction) and horizontal slowness (inclination) of arriving waves to identify structures causing multipathing and wavefield perturbation. Following this, we use full‐wavefield forward modeling to estimate the gradients required to produce the observed multipathing. We use SKS and SKKS data from 83 events sampling the African LLVP, which has been extensively studied providing a good comparison to our observations. We find evidence for structures at heights of up to 600 km above the core‐mantle boundary causing multipathing and wavefield perturbation. Forward modeling shows gradients of up to 0.7% δVs per 100 km (0.0005 km s−1 km−1) can produce multipathing with similar backazimuth and horizontal slowness to our observations. This is an order of magnitude lower than the previous strongest estimates of −3% δVs per 50 km (0.0044 km s−1 km−1). As this is lower than that predicted for both thermal and thermochemical structures, lateral velocity gradients capable of producing multipathing are not necessarily evidence for a thermochemical nature.
Highlights
Forward modeling shows gradients of up to 0.7% δVs per 100 km (0.0005 km s−1 km−1) can produce multipathing with similar backazimuth and horizontal slowness to our observations
Gradients up to 0.7% δVs per 100 km (0.00050 km s−1 km−1) can produce multipathing for the 25 May 1997 event which is an order of magnitude lower than the strongest estimated gradients of −3% δVs per 50 km (0.044 km s−1 km−1) (Ni et al, 2002), though similar to that found by Ritsema et al (1998) −2% δVs per 300 km (0.00048 km s−1 km−1)
Through measuring the backazimuth and horizontal slowness of SKS and SKKS data sampling the lower mantle beneath Africa, we identify clear multipathing in ≈16% of our whole array observations and 8.0% of our subarray observations
Summary
Current observations have not been sufficient to constrain LLVP properties and their composition, origin, and influence remain ambiguous Both purely thermal and thermochemical structures can replicate properties such as the strong gradients, velocity reduction, morphology, and anticorrelation between S wave velocity and bulk sound speed (Davies et al, 2012; McNamara et al, 2010; McNamara & Zhong, 2004, 2005; Schuberth et al, 2009; Tackley, 1998). This study uses array seismology to measure the backazimuth (direction) and horizontal slowness (a proxy for inclination) to identify multipathing and regions of diffraction and refraction This is applied to data sampling the lower mantle beneath Africa, where several studies have identified multipathing and sharp travel time residuals (e.g., Ni et al, 2002; Sun et al, 2009; Wen et al, 2001). We estimate the gradients capable of producing multipathing with similar backazimuth and horizontal slowness deviations as our observations
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