November 27, 1996
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Geologic hazards on Maui
Much has been said in past columns about the volcanic and seismic hazards on the Big Island, but what about the hazards on the other islands? The only active volcano in Hawai`i outside the Big Island is Haleakala on Maui. The last eruption of Haleakala occurred in the late 1700s, possibly as late as 1790, on the lower southwest rift zone. The vents for these flows are just upslope of La Perouse Bay.
Recent geologic mapping by Eric Bergmanis and John Sinton of the University of Hawai`i at Manoa suggests that this rift zone may have erupted as many as five times in the last 900 years! Unfortunately, the summit and east rift zone of Haleakala have not been dated and eruption frequencies for those areas are not available. How hazardous does this make Haleakala volcano?
On the island of Hawai`i, lava-flow hazards are rated on a scale of one through nine with one being the zone of highest hazard and nine being the zone of lowest hazard. For example, the summits and rift zones of Kilauea and Mauna Loa volcanoes are rated Hazard Zone 1.
Using this same scale, preliminary estimates of lava-flow hazard zones on Maui made in 1983 by the U.S. Geological Survey rated the summit and southwest rift zone of Haleakala as Hazard Zone 3. The steep, downslope areas of Kanaio and Kahikinui ahupua`a and the area north of Hana are rated as Hazard Zone 4. Other areas of Haleakala are rated comparable to the lava-flow hazards of Mauna Kea and Kohala (Hazard Zones 7 through 9).
These surprisingly high hazard estimates for Haleakala are based on the frequency of its eruptions. By way of comparison, both Mauna Loa and Kilauea have erupted more than a dozen times each in the last 90 years. Hualalai, the volcano rising up behind Kailua, has a more comparable eruption rate having erupted three times in the last 900 years. All of Hualalai is rated as Hazard Zone 4.
Does this mean that Haleakala is a more hazardous volcano than Hualalai? Frequency of eruption of a volcano is only one of the criteria on which hazards are based. The other important criterion is the lava flow coverage rate. If we use the preliminary dates for Haleakala flows, only 8.7 square miles of lava flows have been emplaced in the last 900 years. In comparison, approximately 43 square miles of Hualalai are covered with flows 900 years old or younger and approximately 104 square miles on Kilauea and 85 square miles on Mauna Loa are covered by lavas less than 200 years old.
In this preliminary comparison, Haleakala is a distant fourth in coverage rates, compared to the active volcanoes on the Big Island, while being third in eruption frequency. Over the next few years, the U.S. Geological Survey's Hawaiian Volcano Observatory will be expanding our study of Haleakala. Along with gathering baseline seismic and geodetic data, we plan to reevaluate coverage rates and eruption frequency at Haleakala as we have for Kilauea, Mauna Loa, and Hualalai.
Nor is Maui immune to the effects of large earthquakes. Since the arrival of Captain Cook, two magnitude 7 (M7) earthquakes have occurred offshore of Maui in 1871 and 1938. Studies by the University of Hawai`i suggest that Maui County can expect an M3 to M5 earthquake every 2 to 5 years. A M7 earthquake might happen every 250 years. By comparison, the southern coast of the Big Island should expect a M7 earthquake every 30 to 40 years and a M8 earthquake every 120 to 190 years.
Most of the island felt the M4.3 earthquake centered 7 miles southeast of Kilauea summit at about 4:39 p.m. on Saturday, November 23. The eruption continues without significant change; lava is entering the ocean at the Lae`apuki bench.
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