Collapse of eyewall of a hurricane observed by seismic data

Abstract:

Decay of an eyewall of a hurricane after its landfall can be monitored by seismic data. We report our analysis of Hurricane Isaac in August, 2012, based on data from a dense seismological array by Earthscope.

We examined seismic amplitudes for frequencies between 0.01 Hz and 0.02 Hz, plotted as a function of distance from the center of the hurricane eye. Selection of this frequency range is critical as some seismic waves are excited by ocean waves, thus not representing the processes near the eye. For this frequency range, we analyzed distance-amplitude plots for every 6-hour interval for which the hurricane location was published.

At the time of the landfall, seismic amplitudes showed a sharp peak at about 75 km from the center of the eye. The maximum seismic amplitudes decreased over the next 12 hours but the distance from the center did not change. In the following 18 hours, the peak gradually moved to a distance of about 200-300 km with diminishing amplitudes. We inverted these seismic data for the time evolution of surface pressure that could explain the observed seismic amplitudes.

The surface-pressure solutions showed that pressure decayed in half during the first 12 hours after the landfall but the pressure peak remained at the same radius 75 km. In the next 18 hours, the peak moved to a distance of 200-300 km with additional halving of amplitude and broadening.

We postulate the mechanism as follows: the focused ascending flow in the eyewall generated intense pressure fluctuation on the Earth’s surface that in turn excited ground motions in the Earth. For Hurricane Isaac, the ascending flow in the eyewall existed at radius of about 75 km from the center during the first 12 hours. The ascending flow weakened during this period, indicated by halving of surface pressure. In the next 18 hours, the eyewall moved outward to distances of about 200-300 km, associated with weakening and broadening of the flow, suggesting a collapse of the eyewall over this period.