Volcano Watch — Updated volcano maps incorporate new hazard zones

A few weeks ago, several members of the Hawaiian Volcano Observatory staff presented information about volcanic hazards on Hawaii to the Hawaiian Beaches/Shores Community Association. Several questions arose during the discussion concerning the history of the hazard zone maps and what new information led to changes in the hazard zone boundaries between the time of publication of the first hazard zone map in 1974 and the revised map of 1987. In order to address these questions more fully, I have reviewed the two maps and their history.

In 1971, the U.S. Department of Housing and Urban Development (HUD) established a policy which responded to volcanic hazards on Hawaii by identifying certain areas as too hazardous and therefore unacceptable for HUD Programs. The hazardous areas identified included the Puna area exposed to Kīlauea's east rift zone and portions of South Kona and Kau exposed to Mauna Loa's southwest rift zone. The primary geological data used to identify these areas was the distribution of historic flows, generally taken to mean flows less than 200 years old.

In 1974, HUD requested that the U.S. Geological Survey prepare more detailed maps delineating the distribution of hazards for lava flows, ashfall from explosive eruptions, gas emissions, subsidence, ground ruptures, earthquake shaking, and tsunami inundation. The purpose of the study was to insure that HUD's policy and program decisions were supported by the best scientific and technical information. In a 1991 document outlining HUD's volcanic hazards policy, HUD noted that the volcanic hazard policy was only rarely used during the 1970s and 1980s because urbanization and HUD-assisted projects were not locating in high hazard zones. More recently, as land development activities expanded, HUD/FHA has increasingly received inquiries about program assistance in non-participation high-hazard areas.

The 1974 map ranked lava flow hazard from zone A to zone F, with zone A the least hazardous and zone F the most hazardous. The criteria used for each volcano to establish the different zones of relative hazard were the number of eruptions, the proportion of area covered, and the causes of distribution of flows. Each of these criteria were evaluated for three different time periods: the historic period (the last 200 years), the recent prehistoric period covering the last few thousands of years, and an older interval covering the past 10,000 to 15,000 years. These broad age categories were utilized because the ages of prehistoric flows were only poorly known at the time. The extent of the different zones were based on these criteria, modified to account for topographic features that protect certain areas from lava inundation.

Kohala is in the lowest hazard zone, and the rift zones and summits of Kīlauea and Mauna Loa Volcanoes comprise the highest hazard zone. On the 1974 map, all of the remaining flanks of Mauna Loa and Kīlauea Volcanoes were lumped together in the second most hazardous zone E, with the exception of two areas - - one located southeast of Mauna Loa's summit and the other extending to the east-northeast from Kīlauea's summit - - that were deemed protected by topography. The surface flows in these two areas were fed from eruptions that overflowed the summit calderas and hence, could not recur until the present calderas fill and overflow.

The authors of the 1974 report noted that most zones did not have uniform hazard throughout their extent. The second most hazardous zone (flanks of Mauna Loa and Kīlauea) included areas with an enormous range of hazard, but the hazard universally decreased with increasing distance to the active rift zones. In 1974, the distribution of flows of different prehistoric ages was poorly known, and it was not possible to divide the zone into higher and lower risk portions. The 1974 hazard zone map - - a single map that combined the effects of all types of volcanic hazards - - was included in a small brochure entitled "Natural hazards on the Island of Hawaii" that was distributed for several years by the U.S. Geological Survey starting in 1976.

In the middle 1980s, the U.S. Geological Survey updated the hazard zone maps because of a wealth of new information that included new field mapping of Kīlauea, Mauna Loa, and Hualālai Volcanoes and new ages of many of the prehistoric lava flows. The new age data became available because of the application of carbon-14 dating to the charcoal that formed as lava flows covered vegetation. By the mid-1980s, perhaps 250 prehistoric flows on Kīlauea, Mauna Loa, and Hualālai had been dating using carbon-14 ages of charcoal located beneath the flows. The updated hazard map was published in 1987, and minor revisions were incorporated in a larger-scale (1:250,000) map published in 1991 (see Figure). At about the same time, the earlier brochure covering all types of geological hazards on Hawaii was updated and expanded as a general interest publication of the U.S. Geological Survey. The new booklet, entitled "Volcanic and Seismic Hazards on the Island of Hawaii," is still available from the Hawaiian Volcano Observatory.

The hazard zone boundaries were modified and limited to lava-flow hazards and, to preclude confusion with the earlier map, were numbered from zone 1 to zone 9. Zones 9, 8, and 7 corresponded directly with the earlier least hazardous zones A, B, and C on Kohala and Mauna Kea. Hualālai was assigned zone 4 in place of the old zone DE (between D and E), because it was now recognized that most of the surface was younger than 10,000 years old and that the eruption recurrence interval was on the order of a few hundred years. In addition, the steep slopes on Hualālai increases the hazard once an eruption begins by causing rapid advance of the flows.

The largest changes occurred within the old zone E, the flanks of Mauna Loa and Kīlauea Volcanoes, which was divided into more and less hazardous zones. The division was based on lava coverage during historic time and during the last 750 years. In addition, the distance to the vents within the summit and rift zones was taken into account. The more hazardous zone 2 could be covered by lava from even short-lived eruptions, whereas the less hazardous zone 3 would only be threatened by long-lived eruptions that occur much less frequently. It was also recognized that the zone on the southeast flank of Mauna Loa was protected from lava inundation by the high caldera bounding scarp on the southeast side of Mokuaweoweo; this area was designated zone 6. Likewise, a new area on the south side of Kīlauea caldera was also shielded from lava flow inundation and was designated zone 5. An additional topographically shielded region identified near Naalehu was added to zone 6.

We have recently completed flow-by-flow mapping of nearly all of Kīlauea Volcano and about half of Mauna Loa Volcano. The data are entered into a computerized Geographic Information System (GIS) that allows us to quantify the percent of the surface covered by lava flows in any time period. The Table shows the results for the parts of the volcanoes with detailed data available. The enormous difference in coverage between zones 2 and 3 during the last 150 years supports the subdivision, made in 1987, of the flanks of Kīlauea and Mauna Loa into zones 2 and 3. Hazard zones 1 and 2 have large percentage areas covered by lava in the last 150 years (greater than 25% for both Kīlauea and Mauna Loa), whereas hazard zone 3 has only 1.6-2.2% coverage during the same time period.

The current lava-flow hazard zone map is the the U.S. Geological Survey's best estimate of the relative hazards from lava inundation of the different parts of the island. As new mapping is completed and additional ages are established for prehistoric flows, our knowledge of the hazard posed by lava flows will continue to improve.
 

Lava Flow Coverage During Historic Time

Kīlauea Volcano (all)

Mauna Loa (northeast rift only)