G. G. Euler

Lithospheric instability and the source of the Cameroon Volcanic Line: Evidence from Rayleigh wave phase velocity tomography

The Cameroon Volcanic Line (CVL) is a 1800 km long volcanic chain, extending SW-NE from the Gulf of Guinea into Central Africa, that lacks the typical age progression exhibited by hot spot-related volcanic tracks. This study investigates the upper mantle seismic structure beneath the CVL and surrounding regions to constrain the origin of volcanic lines that are poorly described by the classic plume model. Rayleigh wave phase velocities are measured at periods from 20 to 182 s following the two-plane wave methodology, using data from the Cameroon Seismic Experiment, which consists of 32 broadband stations deployed between 2005 and 2007. These phase velocities are then inverted to build a model of shear wave velocity structure in the upper mantle beneath the CVL. Results show that phase velocities beneath the CVL are reduced at all periods, with average velocities beneath the CVL deviating more than –2% from the regional average and +4% beneath the Congo Craton. This distinction is observed for all periods but is less pronounced for the longest periods measured. Inversion for shear wave velocity structure indicates a tabular low velocity anomaly directly beneath the CVL at depths of 50 to at least 200 km and a sharp vertical boundary with faster velocities beneath the Congo Craton. These observations demonstrate widespread infiltration or erosion of the continental lithosphere beneath the CVL, most likely caused by mantle upwelling associated with edge-flow convection driven by the Congo Craton or by lithospheric instabilities that develop due to the nearby edge of the African continent.

Location of high‐frequency P wave microseismic noise in the Pacific Ocean using multiple small aperture arrays

We investigate source locations of P wave microseisms within a narrow frequency band (0.67–1.33 Hz) that is significantly higher than the classic microseism band (~0.05–0.3 Hz). Employing a backprojection method, we analyze data recorded during January 2010 from five International Monitoring System arrays that border the Pacific Ocean. We develop a ranking scheme that allows us to combine beam power from multiple arrays to obtain robust locations of the microseisms. Some individual arrays exhibit a strong regional component, but results from the combination of all arrays show high-frequency P wave energy emanating from the North Pacific basin, in general agreement with previous observations in the double-frequency (DF) microseism band (~0.1–0.3 Hz). This suggests that the North Pacific source of ambient P noise covers a broad range of frequencies and that the wave-wave interaction model is likely valid at shorter periods.

Geographic variations in lowermost mantle structure from the ray parameters and decay constants of core‐diffracted waves

We introduce an array-based approach for constraining seismic velocity structure in the lowermost mantle by measuring the frequency dependence of the ray parameter p and decay constant γ for core-diffracted waves (Pdiff & SHdiff). The approach uses an iterative multi-channel cross-correlation algorithm that solves for relative arrival times and amplitudes of core-diffracted waveforms from multiple peaks in normalized correlograms from pairs of co-azimuthal stations. The approach is applied to 60 mb ≥ 5.8 earthquakes for Pdiff and 36 for SHdiff during 2001-2002, with nearly 50,000 unique profile station pairs with epicentral-distance differences of ∆d ≥ 10° and azimuth differences of ∆φ ≤ 10°, sampling a significant portion of the lowermost mantle. Cap-averaging the resulting ray parameter estimates produces geographic variations that are largely consistent with the distribution of lowermost mantle large low-velocity provinces and seismically fast slab-accumulation regions seen in seismic tomography models, whose locations also strongly influence the geographic distribution of heat flow out of the core. Geographic variations in station-pair diffracted-wave decay constants differ from those of the ray parameters, suggesting that variations in the decay of core-diffracted waves are more linked to lowermost mantle seismic velocity gradients than absolute values of seismic velocity. The ray parameters and decay constants of core-diffracted waves are strongly frequency-dependent and these frequency variations also vary significantly with geographic location. Combining lateral and vertical seismic-velocity variations with mineral-physics data on elasticity and conductivity of lowermost mantle species can provide constraints on D” composition and CMB heat flux.

Analysis of explosive sources using a distributed network of infrasound arrays

Analysis of signatures observed on a distributed network requires a model describing the propagation of the energy between network nodes as well as a statistical framework to quantify confidence in analysis conclusions. For the case of infrasound analysis, the propagation model must account for the inhomogeneous, dynamic nature of the atmosphere and the resulting temporal and spatial variations of propagation effects as well as the anisotropic nature of infrasonic propagation due to the dependence on the directionality of the winds. Association and localization methods have been developed utilizing a Bayesian framework to identify explosive sources from distant, infrasonic observations with possible applications to other infrasonic sources. The confidence of the analysis conclusions is dependent on the accuracy and realism of the likelihood definitions utilized in the method, which are therefore an area of ongoing research. An overview of the association and localization methods will be presented along wi...