Volcano Watch — Quake rattles Mauna Kea

On May 22 at 7:50 in the morning, the northern part of Hawaii Island was rattled by an earthquake with an estimated magnitude of 4.2. The earthquake was located about 15 miles deep beneath the northwestern flank of Mauna Kea Volcano, and a few miles southeast of Waimea. This type of earthquake is caused by bending of the lithosphere, the outer rigid layer of the Earth, due to the enormous weight of the islands. A week later, on May 30, another small earthquake with the same cause occurred south of Molokai. This earthquake was felt on Oahu and on Molokai.

Mauna Kea Volcano is presently a dormant volcano, having last erupted about 4,500 years ago. However, this does not mean that Mauna Kea will not erupt again, but that the quiescent periods between eruptions are long compared to the active volcanoes Hualālai (which erupts every few hundred years), Mauna Loa (which erupts every few years to few tens of years) and Kīlauea (which erupts every few years).

The subaerial history of Mauna Kea consists of the Hamakua Volcanics, a sequence of mainly basaltic lavas, erupted between about 200 and 65 thousand years ago. The Hamakua Volcanics were followed by eruption of the Laupahoehoe Volcanics, a sequence of cooler, and therefore thicker, pastier flows, from 65 to 4.5 thousand years ago.

Since 4.5 thousand years ago, Mauna Kea has been volcanically quiet, although from time to time we record shallow earthquakes that appear to be volcanic in origin. These earthquakes generally occur beneath the northeast flank of the volcano. Needless to say, we pay particular attention to the locations and depths of earthquakes beneath Mauna Kea, Haleakalā, and Hualālai because earthquakes will almost surely be the first sign that these volcanoes might be coming back to life. Because none of these volcanoes has erupted in historic time, we do not know for how long earthquakes might precede an eruption. The only clue is provided by Hualālai, where an earthquake swarm in 1929 heralded a possible eruption for several weeks. The eruption never occurred, although magma apparently moved up into the volcano, where it stagnated and solidified. A swarm of earthquakes beneath Hualālai, Haleakalā, or Mauna Kea would signal that an eruption could occur within a short time, but it would not indicate that an eruption would definitely take place.

The earthquake on May 22 beneath Mauna Kea was not caused by magma movement; so we watch our instruments and wait, and wonder when Mauna Kea, Haleakalā, or Hualālai will become active once again.