ACOustic keyhole

Abstract

The Aloha Cabled Observatory (ACO) is located on the seafloor about 100 km north of O‘ahu at 4800 m depth. Seismoacoustic T-waves generated by earthquakes in the South Pacific from Tonga-Kermadec to the Solomon Islands travel 40-70°Δ, passing through an acoustic keyhole—the Ka'ie'ie Waho channel—between O‘ahu and Kaua‘i before reaching ACO. In contrast to ACO and the Pacific seafloor at depths greater than 4500 m, the channel shallows to 1 and 3 km in depth, blocking deeper SOFAR ray/mode paths. T-waves from >50 earthquakes (Mw 6.1 to 8) are reviewed, correcting apparent velocities for near-source P-wave path effects. We examine T-waves crossing the Ka'ie'ie Waho channel and descending to ACO, comparing and contrasting with T-waves propagating unhindered across the North Pacific from events in offshore Japan (Tohoku aftershocks), Kuril Islands, Aleutian Islands, and offshore Alaska. Preliminary review indicates that the T-waves exhibit seismoacoustic modal-coupling to the seafloor, exhibiting a structure observed previously* at the Hawai‘i-2 Observatory (H2O) at 5 km depth between Hawai‘i and California. *R. Butler (2006), J. Acoust. Soc. Am. 120(6), 3599-3606. *R. Butler and C. Lomnitz (2002), Geophys. Res. Lett. 29(10), 57 1-4.