Volcano Watch — Recent Big Island earthquakes

Hawaii is seismically very active. During a typical month, there are invariably several earthquakes that are large enough to be felt. This past month was no exception, as there were nine earthquakes having magnitude greater than 3.0. All but one of these was felt and reported to us, and one additional smaller earthquake was also felt. These earthquakes are not randomly distributed beneath the island, but instead occur in well-defined zones of recurrent activity.

The most active seismic zone on the island is located beneath the south flank of Kīlauea Volcano. Five of the felt earthquakes this past month occurred in this zone, including three on May 3, another on May 9, and a final one on May 22. All five events were located between 8.6 and 10.4 kilometers below the surface. The first three events were located near the hairpin turn in Chain of Craters Road, whereas the final two were located about halfway between the Pu'u 'O'o vent and the coast.

This class of earthquakes is caused by movement of the south flank of Kīlauea towards the ocean along a nearly-horizontal fault plane located near the base of the volcano. The flank is pushed seaward by pressure from the magma system inside Kīlauea. Although all of this type of earthquake this past month were barely felt, it includes the largest earthquakes to have occurred in Hawaii, including one with a magnitude of 7.2 in 1975 and another with an estimated magnitude of 7.9 in 1868.

On May 30, an earthquake with a magnitude of 2.4 occurred near Kealakekua Bay. Although this earthquake was caused by seaward slip of the flank of an active volcano, in this case Mauna Loa Volcano, it was a little deeper than those on Kīlauea. The Kealakekua region has also been the site of previous large earthquakes. In 1951, this area was rocked by an earthquake with an estimated magnitude of 6.9 and the following year another earthquake occurred in the same area that had an estimated magnitude of 6.0.

Also on May 30, a magnitude 3.4 earthquake occurred in the Kaioiki fault zone located between Kīlauea and Mauna Loa Volcanoes. The Kaioiki fault zone, like the Kealakekua Bay area, is a zone that is being pushed seaward by magma pressure inside Mauna Loa Volcano. The Kaioiki earthquake was about 10.5 kilometers deep, roughly along the same plane on which Kīlauea slides towards the ocean. The Kaioiki is also an area of repetitive seismic activity. Large earthquakes occurred here in 1941 (magnitude 6.0), 1962 (magnitude 6.2), and again in 1983 (magnitude 6.7)

On June 3, two earthquakes rattled the Kīlauea summit region. The first, at 8:21 p.m., had a magnitude of 3.6 and was followed an hour-and-twenty-minutes later by an aftershock with a magnitude of 3.0. These earthquakes were sharply felt in the Volcano Village area because of their shallow depth. Both were located about 3.5-4 kilometers deep near Keanakakoi Crater. These shallow earthquakes are adjustments caused by magma movement inside Kīlauea Volcano. A single large earthquake of this type occurred in 1951; it had a magnitude of 6.3.

The most interesting, and the largest, earthquake of the month occurred at 3:49 a.m. on May 11. It was located at least 16 kilometers beneath the Hamakua coastline. Unfortunately, its depth is poorly constrained by the few seismometers we have deployed in the northern half of the island. This earthquake had a magnitude of 4.5. Despite this rather modest magnitude, it was felt over the entire Island of Hawaii, as well as on Maui, Lana'i, Moloka'i, and O'ahu.

We received damage reports from various locations in the northern half of Hawaii. These reports included a small landslide on Kohala Mountain Road, cracked tiles in Honoka'a, and a house that shifted about an inch on its foundation in Pa'auhau, as well as a wall clock that fell and glassware that fell off shelves. Such damage from such a small magnitude earthquake is surprising, but emphasizes the damage to be expected when the next large earthquake hits this region. This earthquake also belongs to a familiar class of earthquakes caused by downward flexing of the nearly-rigid outer layer of the Earth due to the great load of the growing volcanoes. The largest earthquake of this type was a magnitude-6.2 one in 1973 that occurred beneath Honomu.

The felt earthquakes during this past month all occurred in places that have repetitive seismic activity and that have been the sites of large earthquakes in the past. Each is a not-so-subtle reminder of what occurred in these areas in the past and is likely to occur again in the future.