Volcano Watch — 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 Hawaii outside the Big Island is Haleakalā on Maui. The last eruption of Haleakalā 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 Haleakalā have not been dated and eruption frequencies for those areas are not available. How hazardous does this make Haleakalā 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 Kīlauea 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 Haleakalā 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 Haleakalā are rated comparable to the lava-flow hazards of Mauna Kea and Kohala (Hazard Zones 7 through 9).

These surprisingly high hazard estimates for Haleakalā are based on the frequency of its eruptions. By way of comparison, both Mauna Loa and Kīlauea have erupted more than a dozen times each in the last 90 years. Hualālai, the volcano rising up behind Kailua, has a more comparable eruption rate having erupted three times in the last 900 years. All of Hualālai is rated as Hazard Zone 4.

Does this mean that Haleakalā is a more hazardous volcano than Hualālai? 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 Haleakalā flows, only 8.7 square miles of lava flows have been emplaced in the last 900 years. In comparison, approximately 43 square miles of Hualālai are covered with flows 900 years old or younger and approximately 104 square miles on Kīlauea and 85 square miles on Mauna Loa are covered by lavas less than 200 years old.

In this preliminary comparison, Haleakalā 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 Haleakalā. Along with gathering baseline seismic and geodetic data, we plan to reevaluate coverage rates and eruption frequency at Haleakalā as we have for Kīlauea, Mauna Loa, and Hualālai.

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.

Volcano Activity Update

Most of the island felt the M4.3 earthquake centered 7 miles southeast of Kīlauea 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.