Multisensor, Microseismic Observations of a Hurricane Transit Near the ALOHA Cabled Observatory

Abstract

The generation of microseisms is investigated at the ALOHA Cabled Observatory (ACO) north of Oahu during the close passage of Hurricane Lester in September 2016. Sensors include a seafloor ALOHA pressure gauge at ACO, KIP seismic data on Oahu, and nearby wave buoys. Examination of frequency‐direction spectra from wave buoys and numerical wave model outputs confirms two separate microseism generation processes: At frequencies <0.225 Hz, the microseisms are generated by swells from Hurricane Lester and Typhoon LionRock traveling in opposite directions in the vicinity of ALOHA. At higher frequencies >0.225 Hz, microseisms are dominated by waves originating from Hurricane Lester. The cross‐over frequency (0.225 Hz) occurs where the ocean wave group velocity matches the Hurricane storm track speed. Correcting for impedance, the spectrogram for energy at ALOHA closely correlates with KIP. When opposing swells meet at a distance from the Hurricane Lester and ACO, the resulting microseisms also spread geometrically in propagation to ALOHA and KIP, effectively equivalent to 1/R2. At the microseism peak, 4 September, the dominant motions of KIP are observed with retrograde particle motion characteristic of Rayleigh modes, in both the radial‐vertical and transverse‐vertical sagittal planes at distances of ≲400 km from the eye. Otherwise, the energy on the transverse component is comparable to the radial component. We hypothesize that the observed transverse energy arises locally: (1) from the extended microseism source region near ACO and (2) and from scattering by dipping structure and anisotropy embedded in the crust during emplacement at the Pacific‐Farallon ridge.

SMART Cables for Observing the Global Ocean: Science and Implementation

“…The larger, but termed "secondary, " microseism generation mechanism (Longuet-Higgins, 1950) in the deep ocean is from opposing trains of ocean waves, which interact and generate a pressure signal at the seafloor with half the period of the interacting waves. This pressure signal generates seismic waves in the seafloor that can be observed and used for analysis even at the farthest reaches from the oceans in central Asia (Bromirski et al, 2005;Ardhuin et al, 2011;Chen et al, 2015;Butler and Aucan, 2018). These observations improve our understanding of both seismic waves in the seafloor and waves at the ocean surface.…”

SMART Subsea Cables for Observing the Earth and Ocean, Mitigating Environmental Hazards, and Supporting the Blue Economy

“…With few exceptions, the occurrence of opposing wavetrains is limited to near shore regions due to wave reflection at the coast or shallows. Nevertheless, the excited Rayleigh waves have a long range and can be observed even at the farthest reaches from the oceans in central Asia (Bromirski et al, 2005;Ardhuin et al, 2011;Chen et al, 2015;Butler and Aucan, 2018) as well as in the centers of large ocean basins (Dahm et al, 2006).…”

Observing Sea States

“…Although there is a clear correlation between wave heights and microseism amplitude, the relation between the two varies because microseism amplitudes are the product of the amplitude of the wave trains traveling in opposing directions. When, the opposing waves are generated by coastal reflection, this gives one particular relation, but when the opposing waves are due to two uncorrelated wave systems, this typically gives a very strong noise, with a very weak correlation to the wave height (e.g., Obrebski et al, 2012;Butler and Aucan, 2018). One may use a wave model with or without wave reflection at the shoreline to probe this effect, and clearly, all the outliers in Figure 11B are caused by events unrelated to shoreline reflection.…”