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October 15, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Mapping and sampling the submarine slopes of Hawai`i a job for modern research tools

The major part of each Hawaiian volcano lies below sea level, which creates a logistical nightmare: How does one study the submarine slopes? You can't just lace up your boots and grab a map and compass. Perhaps scuba equipment works in the nearshore environment, but the depths accessible by such dives are trifling in comparison to the depth of the seafloor: 5 km (3 mi) along the south shore of the Big Island and commonly as deep as 4 km (2.5 mi) or more elsewhere along the island chain.

During late August and early September, the research vessel Kairei, operated and funded by the Japan Marine Science and Technology Center (JAMSTEC), was studying the giant landslide deposits north of O`ahu and Moloka`i, the faulted south flank of Kilauea, and the slopes of Lo`ihi, the state's youngest but entirely submarine volcano. These investigations have been a collaborative effort that includes researchers from the University of Hawai`i and the U.S. Geological Survey including HVO. The R/V Kairei is state of the art, with sophisticated computers, precise positioning systems, and sonar for mapping the hills and valleys of the sea floor. The data gathered by this equipment are plotted automatically so that scientists onboard can monitor the mapping as it progresses and change plans as new opportunities arise. Such a survey of the Puna Ridge was described in last week's Volcano Watch.

The crowning glory of the JAMSTEC survey, however, is an unmanned submersible that can reach depths of 11 km (6.5 mi). Named the Kaiko, this submersible is lowered to the sea floor by a winch from the rear deck of the ship. No small vessel could maneuver with so much cable to drag through the water, so the Kaiko consists of two parts: an unpowered hub or launcher connected directly to the cable, and a remotely operated vehicle, or ROV, that can then detach and wander as far as 200 m (1,650 ft) from the launcher. The ship pulls the launcher, but the ROV is maneuvered independently by several propellers positioned around it.

A typical traverse is best described by an actual example from our work in September along the submarine flank of the Big Island. The Kaiko is lowered to just above the ocean floor, the depth of which was determined by sonar mapping on the previous day. The ROV then detaches from the launcher and, tethered only by a lightweight cable, descends to the ocean floor. Four video cameras on the ROV transmit images directly to the surface through fiber-optic lines in the cables. As the traverse progresses, the ship slowly takes up cable to keep the launcher moving with the ROV. The ROV, meanwhile, motors up the slope, stopping to collect samples with its robotic arms. The key phrase is "remotely operated"; the command of the ROV is in the control room of the mother ship, where a skilled pilot maneuvers the ROV by way of foot pedals, joystick, and an instrument panel that rivals the cockpit of a 747 airliner. The robotic arms are controlled by hand-operated devices, a scene straight from modernistic science-fiction movies.

The ability to sample from submarine outcrops while videotaping the geologic setting allows us to understand sea-floor geology in ways not possible by conventional dredging. The cost of this modern equipment is staggering, but the collaborative efforts of earth scientists worldwide allows us to advance our knowledge in the face of formidable technical, fiscal, and logistical obstacles.

Eruption and Earthquake Update

There was no change in the eruptive activity at the Pu`u `O`o vent during the past week. Lava continues to flow through a network of tubes from the vent to the seacoast where it enters the ocean in two locations near Kamokuna. Occasional breakouts from the tube system produce surface flows - mainly near the base of the pali and on the coastal flats. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam clouds are highly acidic and laced with glass particles.

Last week's article reported that there were no felt earthquakes during the past week. Between the time that the article was submitted on Thursday afternoon and distributed on Sunday morning, a large earthquake was reported felt. The magnitude-4.3 earthquake occurred at 6:50 p.m. on the night of October 8 and was felt throughout the island. The earthquake was located 5 km (3 mi) southeast of the summit of Kilauea Volcano at a depth of 28 km (17 mi).

There were no earthquakes reported felt from October 9 to the late afternoon of October 15.

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Updated: 27 Oct 1998