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December 10, 1998

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


How vulnerable are the Kulani prison sites to future lava flows?

At a recent public meeting, the State of Hawai`i outlined a proposal to locate a new, approximately 2,300-bed medium-security correctional facility 10 km (6 mi) downslope from the existing, approximately 100-bed minimum-security correctional facility. Both are located on the northeast rift zone of Mauna Loa, and many kama`aina remember that Kulani prison was in the path of the 1984 lava flows. The events of the 1984 eruption demonstrate the reality of lava flow hazards in this area and underscore the importance of hazard awareness for future construction projects, such as a new prison.

Mauna Loa's most recent eruption began in its summit caldera at 1:30 a.m. on March 25, 1984. In the next several hours, the eruption migrated into the southwest and northeast rift zones up to 10 km (6 mi) from the summit. At 4:41 p.m., a new eruptive fissure opened 19 km (12 mi) from the summit and only 15 km (9 mi) upslope from the Kulani correctional facility. The initial flow from this vent headed east toward Kulani, and prison officials were placed on alert. By daybreak the following day, four flows had moved up to 9 km (6 mi) east and northeast. By mid-morning, the flows moving eastward toward the prison had slowed, and officials decided against evacuation. The prison remained on "stand-by alert" for a few days, but the flows that had posed the most direct threat had stagnated by March 28. The bulk of erupted lava continued to flow to the northeast away from the prison and toward Hilo. The eruption ended on April 15, 1984, sparing both Kulani prison and Hilo. Only portions of a secondary road and power poles that supplied electricity to NOAA's meteorological observatory, and several communications relay stations, were destroyed.

How frequently can we expect these sites to be threatened with lava flows in the future? The best way to estimate that is to see how many times lava flows overran these areas in the past. For many years, geologists at the U.S. Geological Survey's Hawaiian Volcano Observatory have been mapping and dating lava flows all over Hawaii's active volcanoes, including Mauna Loa, for just this purpose. To appreciate the magnitude of this job, you should know that Mauna Loa is covered by more than 600 individual lava flows!

By tabulating all flows found within the boundaries of these sites and adjusting that number for buried flows, we found that the existing prison site was affected an average of once every 400-440 years and the proposed prison site once every 2,000 to 4,000 years. The probabilities of at least one lava flow entering each site within a 50-year period are 11-12% for the existing prison site and 1-2% for proposed site. Put another way, there is an 88-89% chance that flows will not enter the existing site within 50 years, and a 98-99% chance that the proposed site will be spared in the same period.

Lava flows are more likely to enter the current prison site primarily because it is nearly 50 times larger in area. That's the same thing as saying that it is easier to hit a large target than a small target at the same distance with a rock.

The decision to evacuate the prison will have to be made well before it can be determined whether the site is directly in the path of an oncoming lava flow or not. Even a lava flow that passes well outside the prison sites may still necessitate evacuation if the flow threatens access roads and utility lines. If another eruption occurs near the 1984 vents, lava flows could reach the current prison site within 9 to 24 hours. Locating the new prison farther downslope increases the possible warning time by about 48 hours. In either case, the probability of having to evacuate the prison is significantly higher than the probability of lava actually covering the site.

Eruption and Earthquake Update

There is no change in the eruptive activity of Kilauea Volcano. Lava continues to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the sea. No surface flows from breakouts of the tube system were observed on the coastal flats. Lava is entering the ocean near Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

A magnitude-2.8 earthquake was reported felt by residents of Ka`u at 3:14 a.m. on Wednesday, December 9. The earthquake was located 3 km (2 mi) northwest of Pahala at a depth of 5.8 km (3.5 mi).

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Updated: 18 Dec 1998