Volcano Watch — Five earthquakes shook Big Island in two weeks

Hawai`i had five felt earthquakes in the last two weeks. The activity included a magnitude 3.7 shock at 12:29 a.m. on August 12. This earthquake was located offshore from Kawaihae at a depth of nearly 34 miles. On the same day, a magnitude 4.0 earthquake struck north of Wai`ohinu at 3:01 p.m. This earthquake was 5.6 miles deep. On August 16, two more earthquakes occurred. The first, at 1:13 p.m., had a magnitude of 3.5 and was located 4.7 miles beneath Hilina Pali. The second, at 4:52 p.m., had a magnitude of 3.1 and was located just west of Pahala at a depth of 4.6 miles. The following day, at 4:34 a.m., another earthquake, one with a magnitude of 3.3, occurred in the same place as the Hilina Pali earthquake of the previous day.

Most of us think of Kīlauea as the region of the island that is seismically active, but the activity over the last few weeks shows the larger earthquakes can and do occur beneath nearly all parts of the island. In fact, several of the largest earthquakes in Hawai`i have occurred, not beneath Kīlauea Volcano, but beneath the Kona coast. The third largest earthquake in historical times struck the central Kona coast at 12:57 a.m. on August 21, 1951. This earthquake was located several miles offshore from Kealakekua Bay at a depth of about 5 miles. The magnitude was 6.9, and the earthquake was strongly felt everywhere on Hawai`i and weakly felt on Maui, Moloka`i, and O`ahu. Many smaller aftershocks followed the main shock. Observers near the epicenter reported that the ground shook nearly continuously for an hour after the main shock.

Damage in Kona was widespread, particularly from Kealakekua to Ho`okena. Nearly 200 water tanks collapsed, and several houses, a church, and a school building were badly damaged. Miles of stone walls collapsed, and landslides blocked some roads. Water lines were severed, and electrical and phone service was disrupted. Two small fires broke out, but only two people were injured by broken glass. As far away as Kīlauea, road pavement was cracked, and many landslides were triggered.

The main shock disabled all the seismograph stations then in operation on the island, so much of the data concerning the main shock was not recorded. In addition, the initial sequence of aftershocks was not recorded, as the first station to be repaired, that at Kīlauea, did not get back on line until about a half hour after the main shock. The station closest to the epicenter, located at Konawaena School, was not repaired for slightly more than a day-and-a-half, so the most intense part of the aftershock sequence was not recorded. The 33 aftershocks that were large enough to be located were scattered along the Kealakekua fault and then south along the highway to Kealia.

Many rockslides occurred along the Kealakekua fault near Napo`opo`o, and residents fled, fearing a tsunami. A small tsunami did occur, although its amplitude of about two feet was not large enough to cause damage.

This earthquake was apparently caused by seaward sliding of a large part of the western flank of Mauna Loa Volcano along a near-horizontal fault plane, called a decollement. The main shock triggered an aftershock sequence on steeply dipping faults that define the headwall of this large landslide. This type of movement is well known from Kīlauea's south flank, where the entire southern half of Kīlauea slides towards the sea. The two historic earthquakes larger than the 1951 Kona earthquake occurred in 1868 and 1975 and were both similar in style to the Kona earthquake, except that they occurred beneath Kīlauea's south flank.

At least some of these larger earthquakes that occur beneath the sliding flanks of the volcanoes may be caused by loading the ground surface with large lava flows. The 1951 Kona earthquake followed the emplacement, on the west flank of the island, of the voluminous Mauna Loa lava flows in June 1950. Similarly, the November 1975 Kalapana earthquake followed the emplacement of the voluminous Mauna Ula lava flows on the south flank of Kīlauea which ended in July 1974. Another Kalapana earthquake (magnitude 6.1) took place in June 1989 after six-and-a-half years of emplacement of the Pu`u `O`o-Kupaianaha lava flows onto the south flank of the island.

The continued emplacement of flows from the Pu`u `O`o vent area towards the coast suggests that another large earthquake could strike the south flank of Kīlauea within the next year or so, despite the short time elapsed since the last large earthquake in this region. Future large eruptions of Mauna Loa, particularly those where the flows accumulate on the steep west flank, could also trigger large earthquakes beneath the Kona coast.