Association of Rayleigh Waves Using Backazimuth Measurements: Application to Test Ban Verification

The defining characteristic of a fundamental mode Rayleigh wave in a spherical, isotropic Earth is its elliptical polarization in the plane of propagation. Measurement of this polarization is thus critical both in identification and in determining the backazimuth of a Rayleigh wave from receiver to source and hence the association with a particular seismic disturbance. Currently, the prototype International Data Center (pIDC) and the International Data Center (IDC) measure backazimuth but do not utilize this in association because it is believed that measurement of backazimuth is unreliable. In this article, we demonstrate that an accurate measurement of backazimuth can be obtained for the large majority of Rayleigh waves reported in the bulletins of the pIDC/IDC. A reliable backazimuth measurement confirms the association of a waveform with a seismic disturbance, whereas the inclusion of large backazimuth errors in the bulletin can cast doubt on the association. Because the object of event screening is to positively identify earthquakes, it is vital that surface-wave magnitudes are only measured from positively identified and associated Rayleigh waves.

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