Seismic and acoustic studies of Lo`ihi Volcano and southeast Hawai`i

The growth patterns and internal structure of Lo`ihi submarine volcano have been investigated in two studies: an evaluation of the 1996 Lo`ihi earthquake swarm and a 1998 multi-channel seismic reflection survey. A velocity model constructed from the earthquake swarm and from refraction data collected in 1998 indicates that Lo`ihi's edifice has seismic velocities between 4–6 km/s while the shallow summit and flanks have velocities near 2 km/s. Earthquake relocations using the new model show that the 1996 swarm consisted of an early, tectonic phase in which a magma chamber drained, followed by the formation of a pit crater on Lo`ihi's summit. Use of an ocean bottom seismometer during the 1996 swarm suggested that instruments must be positioned on Lo`ihi to properly evaluate its behavior. More data were collected on Lo`ihi when the Hawai`i Undersea Geo-Observatory (HUGO) was deployed on the volcano in 1997. This real-time seafloor observatory contained a high-rate hydrophone on which 3 months of nearly continuous data were recorded. Data recorded by HUGO include local and teleseismic earthquakes, and Pacific-wide T-phases. Locations of offshore Hawai`i island earthquakes improve dramatically with data from HUGO. The majority of signals recorded on the HUGO hydrophone have the Kilauea ocean entry as their source. Many signals are impulsive events believed to be hydrovolcanic explosions. Other events, designated “roars”, are composed of a low-frequency rumble, accompanied by a prolonged broadband hiss. We interpret these events as submarine landslides because the largest of these events correlate with observed collapses of the Kilauea ocean entry. All of these collapses and some of the smaller landslides were also detected by the autonomous hydrophone array operated by the Pacific Marine Environmental Laboratory, >5000 km from Kilauea. These data represent the first confirmed hydroacoustic recordings of submarine landslides and could be a useful component in tsunami monitoring efforts. That landsliding is a fundamental process in the growth of a Hawaiian volcano was further made clear both by the MCS data which show that Lo`ihi's Ranks have experienced mass wasting throughout its growth.