Hawai'ian Volcanoes: Earthquake Hazards: Definition of Hazards Zones

Definition of Hazard Zones

Defining hazard zones for the effects of quakes is more difficult than for eruptions and most
hazard maps of Hawai'i are almost indecypherably complex. For the most part, quakes on
Hawaii are concentrated beneath Kilauea and Mauna Loa, particularly beneath the south flanks
of both volcanoes, and in the Kaoiki region between them. The likelihood of a damaging quake
on Kilauea or Mauna Loa increases with long-lived activity of the rift zones, but its precise time and magnitude are impossible to predict.

Small, non-damaging earthquakes will be felt more frequently by people living on the slopes of these volcanoes. The effects of a large quake under Kilauea or Mauna Loa, however, will not
be limited to the immediate area and may cause damage over much of the island. Similarly, deep earthquakes, such as the 1973 Honomu earthquake, can take place under any part of the island and can poduce damaging effects over a wide area.

One problem in assigning seismic hazard zones to the island is that the ground shaking during a strong quake may vary within a small area. This variation is because the effects of quakes are closely related to the nature of the underlying ground; for example, whether it is mainly lava bedrock or soil. Two homes in the same neighborhood may suffer different degrees of damage depending on the properties of the ground upon which they are built. In addition, local
topography strongly affects earthquake hazards. Steep slopes composed of loose material
may produce large landslides during a quake.

The risk from living in a seismically active area, unlike that of living in an area prone to being covered by lava, also depends to a large degree on the type of construction used in a given home. Quake shaking may damage certain types of houses, while leaving other types of construction unscathed. For all these reasons, earthquake hazards are highly localized, and it is difficult to define broad zones with the same relative degree of hazard.

The 1951 Kealakekua Earthquake

The third largest earthquake in historical times struck the central Kona coast at 12;57 a.m. on August 21, 1951. This quake 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 the other islands. 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 Kilauea, 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 (Kilauea) did not get back
on-line until about half an 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. 33 major aftershocks were located, however, and followed the Kealakekua fault and then moved south along the highway to Kealia.

Many rockslides occurred along the Kealakekua fault, and residents of Napoopoo fled,
fearing a tsunami. A tsunami did occur, although its amplitude of about 2 feet was not large enough to cause damage.

This quake 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 Kilauea's
south flank.

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