HURRICANE MATTHEW'S MICROSEISMIC IMPRESSION ON CHATTANOOGA

Matthew approached south Florida as a Category 4 hurricane on October 6, 2016. During the next 3 days, the storm moved up the Atlantic coast to Cape Hatteras, NC, where it went to sea as a post-tropical cyclone. Secondary microseism generated by the storm during this period was measured in Chattanooga, TN, using a Guralp CMG-6TD broadband seismometer. The signal, at a frequency of 0.15 to 0.4 Hz (period of 2.5 to 6.7 s), peaked at 0600 on the 8th (UTC), prior to the storm's passage over Cape Romain, SC. The signal subsided briefly at 0100 on the 9th, while the storm was 70 km offshore and centered in the cusp-shaped Onslow Bay. The signal reintensified 5 hours later, when the storm approached the coast to within 30 km, in Raleigh Bay. This is expressed as a distinctive "pinch" in both the seismogram and the spectrogram.

Particle motion was analyzed for 10-second periods of relatively large-amplitude sinusoidal traces on all 3 axes. Results (hodograms) indicate elliptical paths in near-vertical planes. During the 3-day period, the plane of particle motion rotated counterclockwise about a vertical axis, characteristic of Rayleigh waves radiating from a northward-moving source east of Chattanooga. Bingham statistics applied to particle velocity vectors confirm the planar geometry and serve to better define the plane of particle motion, the strike of which is interpreted to point to the source. During October 7th, as the storm moved toward Chattanooga along an azimuth of 330°, the plane of particle motion was nearly steady with strikes of 134° to 131°. This suggests a source to the east of the storm's track. On the 8th, strikes varied from 128° to 120°, suggesting that the source had moved northward, but not in pace with the storm, which swept through a full 30° of azimuth during the same period, relative to Chattanooga. On the 9th, when the storm was east of Chattanooga, the plane of particle motion dipped consistently 67° to 72° to the north, though the strike oscillated 25° about 090°.

These observations suggest that the source of the microseism changed from the 8th to the 9th, so as to cause observed changes in the strike and dip of the plane of particle motion. Further, the "pinch" in both the seismogram and the spectrogram, early on the 9th, may relate to the storm's position near the center of curvature in the coastline of Onslow Bay.