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Surface-Wave Diffraction Stripes: Measurement, Observables, Explanation, Modeling and Inversion

Description: 

Peculiar stripe-like patterns of amplitude variations and arrival angle deviations of teleseismic surface waves have been observed thanks to large dense networks of broadband seismic stations, such as USArray in the United States, NECESSArray in China and recently AlpArray (2015-2019) in Europe. There are several properties of the stripes, which show that the pattern is not caused by the structure beneath the observation location – the large dense network, but rather imported with the wavefield to the network from outside. One of the most striking observables is the lateral shift of the stripes with varying periods of surface waves. We proposed and tested a hypothesis that these patterns are caused by diffraction and consequential interference after the wavefield has passed a single small-scale scatterer. The wavefield carries the imprint of the scatterer for thousands of kilometers and allows for localizing the position of the scatterer, its size and strength.

We summarize the observation of stripe-like patterns for several earthquakes recorded by the AlpArray seismic network, which was distributed over the greater Alpine region in Europe and consisted of 290 temporary and more than 300 permanent broadband stations. We show how for different earthquakes the interference pattern moves across the network when the period of surface wave changes. We give examples of patterns in group velocity measured at every station individually, in arrival angles (slowness vector, phase velocity) measured by sub-array beamforming and we show how the pattern emerges when we calculate the gradients of phase wavefronts similar to eikonal tomography. We observe the pattern also in the polarization of surface waves and their amplitudes. We discuss how all these observables match together and how the modeling explains them. We show that it is possible to invert the observed interference pattern to locate the scatterer which had caused it, giving examples of particular upper-mantle low-velocity bodies and plumes. We also discuss how the phase and group travel times distorted by interference can affect global and regional tomography.

Session: Earth’s Structure from the Crust to the Core - II

Type: Oral

Date: 5/1/2024

Presentation Time: 11:30 AM (local time)

Presenting Author: Petr

Student Presenter: No

Invited Presentation: 

Authors