High‐Frequency (⁠>100  Hz⁠) Earthquakes North of Moloka‘i Detected on the Seafloor at the Aloha Cabled Observatory

I describe the analysis of 10 earthquakes (⁠$ML$ 2.7–4.7) that occurred between 2012 and 2017, at depths of 10–33 km, north of Moloka‘i, Hawaiian Islands. Observed on the seafloor Aloha Cabled Observatory (ACO) 24 kHz hydrophone, the events show extraordinary high frequencies up to 165 Hz at distances between 130 and 213 km. The data show a low‐frequency spectral decay rate of $ω−2$ that steepens to $ω−4$ beyond 50 Hz. I interpret this change in slope to be the small wavelength manifestation of a smoothly accelerating and decelerating dynamic crack source with larger‐scale, variable‐rupture velocity. Nearby KIP data from O‘ahu are also considered. Although limited to 45 Hz in bandwidth, comparable spectral slopes are observed. Applying corner frequency analysis to the KIP data, the apparent moments and moment magnitudes of the earthquakes compare well with Hawaiian Volcanos Observatory (HVO) $ML$⁠. Characteristic fault dimensions are 0.4–0.9 km, and stress drops $Δσ$ are 1–9 MPa.