Abstract
The dense AlpArray network allows studying seismic wave propagation with high spatial resolution. Here we introduce an array approach to measure arrival angles of teleseismic Rayleigh waves. The approach combines the advantages of phase correlation as in the two-station method with array beamforming to obtain the phase-velocity vector. 20 earthquakes from the first two years of the AlpArray project are selected, and spatial patterns of arrival-angle deviations across the AlpArray are shown in maps, depending on period and earthquake location. The cause of these intriguing spatial patterns is discussed. A simple wave-propagation modelling example using an isolated anomaly and a Gaussian beam solution suggests that much of the complexity can be explained as a result of wave interference after passing a structural anomaly along the wave paths. This indicates that arrival-angle information constitutes useful additional information on the Earth structure, beyond what is currently used in inversions.
Highlights
1.1 Importance of arrival anglesIt has been observed frequently that seismic phases do not always arrive from the direction that is expected if the Earth had a simple 1-D structure
∗ www.alparray.ethz.ch (Cotte et al 2000, 2002; Pedersen et al 2003; Maupin 2011; Foster et al 2014a; Kolınskyet al. 2014; Pedersen et al 2015; Chen et al 2018). It has been suggested, based on a linearization of ray-theoretical equations (e.g. Woodhouse & Wong 1986), that the deviation from the great-circle azimuth is proportional to the transverse derivative of phase velocity
Finite-frequency effects were incorporated by Ritzwoller et al (2002) by introducing sensitivity kernels to global ‘diffraction’ tomography taking into account the scattering over the first Fresnel zone
Summary
It has been observed frequently that seismic phases do not always arrive from the direction that is expected if the Earth had a simple 1-D structure. Arrival-angle deviations are pronounced for surface waves; they are usually found to be up to ±15◦ (Levshin et al 1994; Laske et al 1999) or ±10◦ with extremes up to ±30◦ (Laske 1995; Cotte et al 2000; Maupin 2011) These deviations are not mere curiosities, but they can be important for constraining the 3-D structure of the Earth. Woodhouse & Wong 1986), that the deviation from the great-circle azimuth is proportional to the transverse derivative of phase velocity (and that amplitude anomalies are sensitive to the second lateral derivative) This approach was used by Laske (1995) and Yoshizawa et al (1999) for inversion of polarization measurements into phase maps. The approach was generalized by Larson et al (1998) for anisotropic structures
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