EARTHQUAKE HAZARDS |
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| Recent earthquakes on and near the Island of Hawaii, 1962-1985. |
The Island of Hawaii experiences thousands of earthquakes each year; most are so small that they can only be detected by instruments, but some are strong enough to be felt, and a few cause minor-to-moderate damage. Most of Hawaii's earthquakes are directly related to volcanic activity and are caused by magma moving beneath the earth's surface. Earthquakes may occur before or during an eruption, or they may result from the underground movement of magma that comes close to the surface but does not erupt. A few of the island's earthquakes are less directly related to volcanism; these earthquakes originate in zones of structural weakness at the base of the volcanoes or deep within the earth beneath the island.
Strong earthquakes endanger people and property by shaking structures and by causing ground cracks, ground settling, and landslides. Strong earthquakes in Hawaii's past have destroyed buildings, water tanks, and bridges, and have disrupted water, sewer, and utility lines. Locally, such damage can be intensified where soft, water-saturated soils amplify earthquake ground motions. On steep slopes, such soils may fail during an earthquake, resulting in mudflows or landslides. An indirect hazard produced by some earthquakes is a tsunami, a large sea wave that can be far more damaging than any of the direct seismic hazards.
The size of an earthquake is commonly expressed by its magnitude on the Richter scale, which is a measure of the relative size of the earthquake wave recorded on seismographs. An increase of one whole number on the Richter scale represents a tenfold increase in the amplitude of the seismograph recording. Earthquakes greater than about magnitude 3 usually can be felt by people near the source area, those greater than magnitude 5 are potentially damaging, and any earthquake of magnitude 7 or greater that occurs near populated areas is certain to cause widespread property damage.
The earthquakes directly associated with the movement of magma are concentrated beneath the island's active volcanoes, Kilauea and Mauna Loa. Very shallow earthquakes frequently precede or accompany an eruption. Hundreds of such earthquakes make up swarms that commonly occur over a period of several hours or days before an eruption as magma forces its way into a new area. These earthquakes are seldom large enough to cause widespread damage, but they may produce extensive ground fracturing close to the potential eruption site. Once an eruption begins, the earthquakes usually diminish.
Other earthquakes beneath the active volcanoes are generated by the pressures exerted by magma that never reaches the surface. Kilauea's east rift zone is continually being wedged apart by the injection of new magma, much of which is stored underground deep within the rift zone. Since the north flank of Kilauea is immobilized by the adjacent mass of Mauna Loa, the south flank, which faces the ocean, must move outward to make room for the additional magma. Periodically, Kilauea's south flank abruptly shifts seaward in response to this pressure, causing earthquakes. Most such earthquakes are small, but a few are large and can cause damage.
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In 1975, Kilauea's south flank was the site of the magnitude 7.2 Kalapana earthquake, the highest magnitude event in this century. The Kalapana coast subsided as much as 11 feet, generating a huge tsunami that claimed two lives in Hawaii Volcanoes National Park, destroyed houses in Punaluu, sank fishing boats in Keauhou Bay, and damaged boats and piers in Hilo. The most recent large earthquake on Kilauea's south flank occurred in June 1989. With a magnitude of 6.1, this quake caused much less damage than the 1975 event.
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| House destroyed in June 25, 1989, earthquake in Kalapana. (Photograph by J.D. Griggs, USGS) |
Earthquakes occur for similar reasons beneath Mauna Loa's southwest and southeast flanks. The Kealakekua fault zone on Hawaii's Kona coast was the site of an earthquake of about magnitude 6.9 in 1951 that may have been related to the 1950 eruption of Mauna Loa's southwest rift zone. The largest Hawaiian earthquake in recorded history occurred in 1868 beneath the Ka'u district on the southeast flank of Mauna Loa; it had an estimated magnitude of between 7.5 and 8.1. The 1868 earthquake caused damage across the entire island and was felt as far away as the Island of Kauai. The devastation was greatest in the Ka'u district, where an earthquake-triggered mudflow killed 31 people and coastal subsidence produced a tsunami that destroyed several villages. At least 79 people perished during this earthquake; most of these casualties resulted from the landslide and tsunami.
Earthquakes in the Kaoiki region, centered between Kilauea and Mauna Loa, are also thought to be related to stresses in the earth's crust that are produced by the activity of the two volcanoes. In the last half century, earthquakes with magnitudes from 5.5 to 6.6 have shaken the Kaoiki region about once every 10 years. The latest large earthquake in this area had a magnitude of 6.6 and occurred in 1983. This event caused substantial damage to structures in Ka'u, Puna, and North and South Hilo districts. Ground cracking and settling led to temporary road closures, and landslides occurred on steep slopes. The financial losses caused by the earthquake were estimated at $7 million. Fortunately, there were only minor injuries because the earthquake struck early in the morning when most people were still in bed.
Large earthquakes unrelated to volcanic activity also occur at irregular intervals on the Island of Hawaii. In 1973, a magnitude 6.2 earthquake located 25 miles beneath Honomu, north of Hilo, injured 11 people and caused $5.6 million worth of damage. Such earthquakes have no known recurrence interval and are difficult to predict.
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| Hilo building damaged by the November 1983 Kaoiki earthquake. (Photograph by J.M. Buchanan-Banks, USGS) |
Defining hazard zones for the effects of earthquakes is more difficult than for eruptions and has not been attempted for the Island of Hawaii. For the most part, earthquakes 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 earthquake on Kilauea or Mauna Loa probably 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 earthquake 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 produce damaging effects over a wide area.
One problem in assigning seismic hazard zones to the island is that the ground shaking during a strong earthquake may vary within a small area. This variation is because the effects of earthquakes 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 an earthquake.
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.
Earthquake shaking may damage certain types of houses, while leaving other types
of construction unscathed. For all of these reasons, earthquake hazards are
highly localized, and it is difficult to define broad zones with the same
relative degree of hazard.
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