February 28, 1997

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

Hazards of Mauna Loa

The Island of Hawai`i is the fastest-growing region in the State of Hawai`i, with over 100,000 residents and a population that continues to grow at a rate of 3% per annum. The Governor has referred to the Big Island as the crown jewel of the State. With such high expectations and affordable land, developmental pressures mean that more and more construction will occur on the flanks of active volcanoes.

Mauna Loa, one of the world's most active volcanoes, is also the world's largest volcano, at 80,000 km³ (19,193 cubic miles), encompassing 5,100 km² (1969 square miles) of the island of Hawai`i--slightly more than half of the surface area of the island. Mauna Loa has a summit caldera and two rift zones, one extending northeast from the caldera and the other to the southwest.

The rift zones and summit have been the source of the vast majority of Mauna Loa's eruptions. Most eruptions of Mauna Loa commence with a line of fountains in the summit region. This summit activity may be followed by a migration in the eruption to either rift zone. A few historical and prehistorical eruptions have issued from radial vents that ascend to the surface outside the defined rift and summit regions. Radial vents occur on the north and west flanks of the volcano and have erupted from the 3,355 m (11,000 ft.) elevation to below sea level (e.g., 1877 Kealakekua Bay submarine eruption).

In the last 150 years, Mauna Loa has erupted 33 times. Most of these eruptions were restricted to the summit area, but 17 "flank eruptions" (15 from rift zones and 2 from radial vents) produced lava flows that covered broad areas on the lower slopes of the volcano and reached the ocean five times along the west coast of Hawai`i. In 1855-56 and 1880-81, flows covered land that is now within the city limits of Hilo.

The population on the slopes of the volcano is growing rapidly and includes several multi-million dollar resorts in Waikoloa, in the city of Hilo, and in Hawaiian Ocean View Estates, the nation's largest development. Since the last eruption of Mauna Loa in 1984, approximately $2.3 billion dollars have been invested in new construction on the flanks of the volcano.

The greatest direct hazard associated with shield volcanoes is lava flows. Since more eruptions are inevitable on the flanks of Mauna Loa, the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO), as a public service organization, has an obligation to increase the public's awareness of the hazards posed by future eruptions and to improve the designation of hazard zones as more information becomes available.

Our recent and ongoing research shows that in the past 150 years, 14% of Mauna Loa's surface has been covered by lava flows; in the past 1,000 years, 35 to 40% of the volcano (an area equal to O`ahu or Maui) has been so covered. Our work indicates that eruption rates of 12 million cubic meters (15.7 million cubic yards) per day on slopes steeper than 10 degrees constitute an especially significant risk for the population of south Kona.

An indirect volcanic hazard on Mauna Loa is the ever-present threat of large earthquakes. As magma enters and inflates Mauna Loa, the volcano becomes unstable, setting the stage for large earthquakes. Extremely destructive earthquakes have occurred beneath Hawai`i during times of inflation -- notably the magnitude-8 earthquake of April 2, 1868, on Mauna Loa and, most recently, the magnitude-7.2 earthquake of November 29, 1975, on Kilauea. Such large earthquakes will occur in the future and will certainly cause great damage. While earthquakes cannot be predicted, we can warn people to be prepared through sound construction of buildings and by protecting valuables from damage.

So -- what is the current eruptive status of Mauna Loa? Our recent deformation measurements show that Mauna Loa is steadily inflating but that seismicity remains at a low level. If the pre-eruptive patterns of 1975 and 1984 are any indication of what we can expect, we should note not only an increase in the number of earthquakes recorded, but also in their intensity over a 1.5 to 2-year period prior to any outbreak. This increase has not yet begun.