Volcano Watch — New map defines lava flow hazard zones on the Big Island

The U.S Geological Survey has recently published a new, full-sized map which defines the boundaries of nine lava flow hazard zones on the island. This map is an updated version of the page-sized map included in the USGS booklet Volcanic and Seismic Hazards on the Island of Hawaii published in 1990. We have discussed the lava flow hazards on Kīlauea and Hualālai Volcanoes in previous "Volcano Watch" columns, and in the future, we will detail the lava flow hazards for the remaining volcanoes.

The updated, more detailed map was produced in cooperation with the Hawai`i Office of State Planning, which digitized hazard zone boundaries to produce the nine-color map. The staff at HVO and the USGS Branch of Technical Reports added an underlay showing roads and towns. The map was published at a scale of 1:250,000, meaning that one inch on the map equals about four miles on the ground. It can be overlain on the USGS single-sheet topographic map of the Big Island, which was published at the same scale.

The designation of nine lava flow hazard zones, with number one used for the most hazardous areas, is based solely on geologic criteria, including frequency of past lava flows and coverage, distance from eruptive ventsand topography that currently protects certain areas from lava inundation. The areas designated as the most hazardous zones are the summits and rift zones of Kīlauea and Mauna Loa Volcanoes and the least hazardous zone is all of Kohala Volcano.

Boundaries between volcanoes are depicted with lines equivalent to a mile in width, to reflect the overlapping of flows from adjacent volcanoes along their common boundaries. Boundaries between hazard zones are drawn with a line equivalent to one-quarter to one-half mile in width to reflect uncertainties in the geological placement of boundaries. These boundaries are approximate and gradational and are not specific enough to determine the absolute degree of danger at any particular site.

The map was designed to be used for general planning purposes only, according to Thomas Wright, former Scientist-in-Charge at the Hawaiian Volcano Observatory and the map's first author. The lava flow hazard is one factor that should be considered in land-use decisions, but it is not the only factor. In order to use this map effectively in the development or preservation of any land parcel, a risk analysis should be made to balance positive economic and social benefits against risks posed by lava flows.

The new lava flow hazard map will be displayed at public libraries on the Big Island starting this week and at the Jaggar Museum in Hawai`i Volcanoes National Park, at the summit of Kīlauea Volcano. Copies of the map can be purchased by mail from Map Sales, U.S. Geological Survey, Box 25286, Federal Center, Denver, CO 80225. The cost of the map is $2.75, plus $1.00 for shipping and handling. Orders must include the name of the map ("Map showing lava flow hazards zones, Island of Hawaii"), its order number (MF-2193), and a check or money order payable to the Department of the Interior. State and County offices and community groups may obtain a free copy by calling the Hawaiian Volcano Observatory at 967-7328.

The booklet Volcanic and Seismic Hazards on the Island of Hawaii contains the original lava flow hazards maps for each of the five volcanoes on the island, an earthquake hazards map, volcanic eruptions and damage resulting from them and from earthquakes. Lava flows are only one of the geologic hazards in Hawai`i, which include earthquakes, explosive eruptions at the summit of Kīlauea, and ground cracking and subsidence along the rift zones and earthquake faults associated with the volcanoes. The booklet may be obtained at no cost from the map sales center in Denver at the address listed above. We distributed copies of the booklet and displayed the new map at the recent County Fair in Hilo. The Observatory maintains a small supply of the booklet for local distribution.

Volcano Activity Update

The eruption on the East Rift Zone of Kīlauea Volcano continues at the episode 51 vents, located on the west side of the Pu`u `O`o cone. Slowly advancing pahoehoe flows are heading toward the south and are slowly consuming the forest at about the 2,100-foot elevation. There were two earthquakes strong enough to be felt this past week. The first, which occurred September 19 at 7:56 a.m., was located beneath the south flank of Kīlauea Volcano and had a magnitude of 3.5. The second, which was recorded on September 22 at 4:23 a.m., was located northwest of Pahala and had a magnitude of 3.3.