Surface Wave Diffraction Pattern Recorded on AlpArray: Cameroon Volcanic Line Case Study.

Petr Kolínský,Felix M Schneider,Götz Bokelmann

doi:10.1029/2019jb019102

Abstract

Stripe‐like patterns of surface wave arrival angle deviations have been observed by several seismological studies around the world, but this phenomenon has not been explained so far. Here we test the hypothesis that systematic arrival angle deviations observed at the AlpArray broadband seismic network in Europe are interference patterns caused by diffraction of surface waves at single small‐scaled velocity anomalies. We use the observed pattern of Rayleigh waves from two earthquakes under the Southern Atlantic Ocean, and we fit this pattern with theoretical arrival angles derived by a simple modeling approach describing the interaction of a seismic wavefield with small anomalies. A grid search inversion scheme is implemented, which indicates that the anomaly is located in Central Africa, with its head under Cameroon. Moreover, the inversion enables the characterization of the anomaly: The anomaly is inferred to be between 320 and 420 km wide, matching in length the 2,500 km long upper mantle low‐velocity region under the volcano‐capped swells of the Cameroon volcanic line. We show that this approach can be generally used for studying the upper mantle anomalies worldwide.

Highlights

Peculiar patterns of amplitudes and arrival angle deviations of surface waves had been observed by Pollitz (2008) (“band‐like patterns” of amplitudes), Liang and Langston (2009) (“belts of negative and positive azimuth variations”), Lin et al (2012) (“striping pattern” of amplitudes), Foster et al (2014) (“banded appearance,” “banded pattern” of arrival angle deviations) and Liu and Holt (2015) (“banding pattern” of various gradiometry parameters) for the USArray data, and by Chen et al (2018) (“belt‐like pattern” of arrival angle anomalies) for NECESSArray (Northeast China)
The inversion enables the characterization of the anomaly: The anomaly is inferred to be between 320 and 420 km wide, matching in length the 2,500 km long upper mantle low‐velocity region under the volcano‐capped swells of the Cameroon volcanic line
We show that diffraction pattern of surface waves can be used to study the anomalies in the upper mantle

Summary

Introduction

Peculiar patterns of amplitudes and arrival angle deviations of surface waves had been observed by Pollitz (2008) (“band‐like patterns” of amplitudes), Liang and Langston (2009) (“belts of negative and positive azimuth variations”), Lin et al (2012) (“striping pattern” of amplitudes), Foster et al (2014) (“banded appearance,” “banded pattern” of arrival angle deviations) and Liu and Holt (2015) (“banding pattern” of various gradiometry parameters) for the USArray data, and by Chen et al (2018) (“belt‐like pattern” of arrival angle anomalies) for NECESSArray (Northeast China) Even though all these studies gave hints on the causes of these observations, a general explanation is missing. Using the modeling approach by Nolet and Dahlen (2000) (referred as N&D2000 on), we show that interference of waves diffracted at a single anomaly can cause stripe‐like arrival angle patterns as the ones observed

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