Infrasonic tremor wavefield of the Pu`u `Ō`ō crater complex and lava tube system, Hawaii, in April 2007

Abstract

Long‐lived effusive volcanism at the Pu`u `Ō`ō crater complex, Kilauea Volcano, Hawaii produces persistent infrasonic tremor that has been recorded almost continuously for months to years. Previous studies showed that this infrasonic tremor wavefield can be recorded at a range of >10 km. However, the low signal power of this tremor relative to ambient noise levels results in significant propagation effects on signal detectability at this range. In April 2007, we supplemented a broadband infrasound array at ∼12.5 km from Pu`u `Ō`ō (MENE) with a similar array at ∼2.4 km from the source (KIPU). The additional closer‐range data enable further evaluation of tropospheric propagation effects and provide higher signal‐to‐noise ratios for studying volcanic source processes. The infrasonic tremor source appears to consist of at least two separate physical processes. We suggest that bubble cloud oscillation in a roiling magma conduit beneath the crater complex may produce a broadband component of the tremor. Low‐frequency sound sourced in a shallow magma conduit may radiate infrasound efficiently into the atmosphere due to the anomalous transparency of the magma‐air interface. We further propose that more sharply peaked tones with complex temporal evolution may result from oscillatory interactions of a low‐velocity gas jet with solid vent boundaries in a process analogous to the hole tone or whistler nozzle. The infrasonic tremor arrives with a median azimuth of ∼67° at KIPU. Additional infrasonic signals and audible sounds originating from the extended lava tube system to the south of the crater complex (median azimuth ∼77°) coincided with turbulent degassing activity at a new lava tube skylight. Our observations indicate that acoustic studies may aid in understanding persistent continuous degassing and unsteady flow dynamics at Kilauea Volcano.