A TALE OF TWO HURRICANES: THE PECULIAR MICROSEISMS OF MATTHEW AND FLORENCE IN ONSLOW BAY, NORTH CAROLINA

Secondary (double-frequency) microseisms generated by hurricanes Matthew and Florence were measured in Chattanooga, TN, using a Guralp CMG-6TD broadband seismometer. Spectra for both signals, measured on vertical axes, range from 0.15 to 0.40 Hz (2.5 to 6.7 s period) with peak power at 0.19 to 0.25 Hz (4.0 to 5.2 s dominant period).

Matthew passed over Cape Romain, SC, on October 8, 2016, before continuing up the North Carolina coast to Cape Hatteras and going out to sea as a post-tropical cyclone on the 9th. Curiously, its microseism (vertical axis) subsided from -105 Db (relative to velocity) to -110 Db while the storm traversed the 150-km length of Onslow Bay, southwest to northeast, from Cape Fear to Cape Lookout. The signal reintensified briefly, to -105 Db, before ultimately dying out on the 10th. This conjures questions as to the cause of the diminished signal and implications thereof for the generation of secondary microseism.

Florence approached Onslow Bay from the southeast, perpendicular to the coastline, on September 13, 2018. Once in the bay, the storm slowed, turned to the west, and made landfall along Wrightsville Beach on the 14th. Remarkably, Florence's track crossed that of Matthew very near the center of Onslow Bay. Despite the difference in its track, Florence's microseism also subsided slightly at this point, by only a few decibels and only briefly, just enough to further tease our curiosity.

Diminishment of secondary microseisms generated by Matthew and Florence while the storms occupied Onslow Bay may relate to the cusp shape of the bay and its relation to storm circulation, resulting in reduced reflected wave energy and reduced double-frequency standing-wave pressure oscillations. This is consistent with well-established models for generation of secondary microseism and points to the potential influence of coastal geometry. We further speculate that the more subtle expression of this in Florence’s microseism may relate to the storm's relatively short traverse along the minor axis of the semi-ellipse that approximates the geometry of the bay, rather than the long-axis traverse of Matthew.