Volcano Watch — Large quakes have rocked isle before

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Earthquakes are a frequent occurrence on the Island of Hawaii. Rarely a week goes by that we do not experience at least one earthquake that is large enough to be felt.

Earthquakes are a frequent occurrence on the Island of Hawaii. Rarely a week goes by that we do not experience at least one earthquake that is large enough to be felt. Most of these earthquakes occur beneath the southern part of the island and are associated with incremental movement of the flank of Kīlauea Volcano towards the sea. However, large earthquakes can and do occur beneath nearly all parts of the island. Several of the largest earthquakes in Hawaii 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. Its magnitude was 6.9, and it was strongly felt everywhere on Hawaii and weakly felt on Maui, Molokai, and Oahu. 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 Hookena. 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 later 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 Napoopoo, and residents fled, fearing a tsunami. A small tsunami did occur, although its amplitude of about 2 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 in 1975 and were both similar in style to the Kona earthquake, except that they occurred beneath the south flank of the island.

This anniversary summary of the 1951 Kona earthquake should serve as a reminder to residents on the Kona coast that they should evaluate their homes for structural integrity and should maintain emergency supplies so that they do not become victims during the next large Kona earthquake. A difficulty in emergency planning is that most natural disasters occur infrequently, and people begin to think they will not occur again. Don't let the 43 years since the Kona earthquake lull you into a false sense of security.