Short Timescale Analysis of Microseisms and Application to Array Calibration

Primary and secondary microseisms are analyzed in this study using a novel matched field processing approach that allows for analysis of features with temporal scales of the order of seconds. The majority of previous microseism research employs time averaging; hence, very little is currently known about the properties of the wavefield on such short timescales. We aim to better understand the nature of the microseismic wavefield through applying our novel matched field processing approach to example data from two seismic arrays (USArray and Rio Grande Rift-Flex Array) in the United States. We find that surface and body wave microseisms on short timescales are observed as pulses of coherent energy, which may be separated in time, embedded in the continuous signal. The pulses display a much larger coherence value in comparison to the commonly employed time averaging approaches, given that they can be separated in time. This allows us to study the spatial correlation of the wavefield and gives an insight into the source and path propagation effects of surface and body waves. We find that the correlation of the short timescale surface wavefield between two stations is dependent on the distance between them and is strongly dependent on their geometric position with respect to the source. Correlations on the PKP wavefield show a decrease with increasing source-station distance and a mild decrease for azimuthally distributed stations at the same source-station distance. Finally, we demonstrate how the pulse wavefield can be used for array calibration purposes.