PRELIMINARY ANALYSIS OF HURRICANE DORIAN'S MICROSEISMS FROM THE LITTLE BAHAMA BANK THROUGH ONSLOW BAY, NORTH CAROLINA, WITH COMPARISONS TO HURRICANES MATTHEW AND FLORENCE

Dorian approached Abaco and Grand Bahama islands from the east on Sept. 1, 2019, and stalled for 2 days on Grand Bahama, as it weakened from a category 5 to a category 3 hurricane. From Sept. 3 through Sept. 6, the storm further weakened to a category 1 hurricane as it moved up the southern Atlantic coast, passed through Onslow Bay, NC, and made brief landfall at Cape Hatteras. After Cape Hatteras, the storm sped toward Nova Scotia, where it made landfall as a post-tropical cyclone on Sept. 7.

Secondary (double-frequency) microseisms (SM) generated by Dorian were measured in Chattanooga, TN, using a Guralp CMG-6TD broadband seismometer. As measured on the vertical axis, Dorian's SM ranged from 0.12 to 0.50 Hz (2.0 to 8.3 s period) with peak power at 0.21 to 0.26 Hz (3.8 to 4.8 s dominant period). The storm's SM (vertical axis) intensified from -120 dB (relative to velocity) to a maximum of -105 dB, while it moved from Grand Bahama to a point 100 to 130 km off the coast of Georgia and South Carolina. The storm's SM diminished to -113 dB while it occupied Onslow Bay, increased slightly to -109 dB upon exiting the Bay, and waned completely as it approached Nova Scotia.

Diminishment of Dorian's SM while the storm occupied Onslow Bay closely coincides with a radical change in wave direction, from southeast to northwest, measured at Station (buoy) 41110, 10 km offshore, near Wrightsville Beach. At this time, significant wave height peaked at nearly 7 meters near the center of the bay, measured at Station (buoy) 41159.

Particle motion analyses based on 10-second periods with sinusoidal traces on all 3 axes indicate elliptical paths in near-vertical planes. Planes of polarization for Dorian's SM are consistent with Rayleigh waves that radiated from a source southeast, east, and northeast of Chattanooga, but not perfectly in keeping with the storm's track.

Remarkably, tracks of Matthew (Oct., 2016), Florence (Sept., 2018), and Dorian cross near the geometric center of Onslow Bay, at which point their SMs diminished similarly. This 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 SM and points to the potential influence of coastal geometry.