Modelling Ground-to-Air Coupling for the Shallow ML 4.3 Folkestone, United Kingdom, Earthquake of 28 April 2007

Abstract

Abstract An earthquake that occurred with a local magnitude of 4.3 and at a depth of 3 km beneath the coastal town of Folkestone, United Kingdom, generated atmospheric acoustic waves in the 2–5 Hz bandwidth (infrasound). These were recorded at the FLERS microbarometer array in France at a range of 284 km. Earthquake-generated infrasound is often associated with large earthquakes close to large mountain ranges; the shaking of prominent topographic points act as infrasound sources, coupling seismic energy into the atmosphere. In this example there is little prominent topography in the source region apart from the coastal cliffs, which have an average height of 75 m. We explore the possibility that the seismic-to-infrasound coupling occurs at the cliffs by modelling a 23 km length of coast as a series of 305 pistons independently generating acoustic waves. Synthetic seismograms modelling the motion of the cliffs are constrained using recordings from a three-component accelerometer located 4 km from the epicenter. Meteorological data, combined with array processing of the microbarometer records, suggest that the acoustic energy propagated to FLERS within the troposphere. The synthetic microbarograms show close agreement with the observations; the modelled arrival time is 5 sec earlier than that observed, and the peak-to-peak amplitude and signal duration are within a factor of 2 of those recorded. This study shows that moderately sized, shallow seismic disturbances can generate observable infrasound if the topographic conditions close to the epicenter are favorable.