Preface

The island of Hawai'i 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 Hawai'i'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 quakes endanger people and property by shaking structures and by causing ground
cracks, ground settling, and landslides. Strong quakes in Hawai'i'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 a quake, resulting in mudflows or landslides. An indirect hazard produced by some quakes is a tsunami, a large sea wave that can be far more damaging than any of the direct seismic hazards.

Volcanoes and Earthquakes

The quakes directly associated with the movement of magma are concentrated beneath the
island's active volcanoes, Kilauea and Mauna Loa. Very shallow quakes frequently precede or accompany an eruption. Hundreds of such quakes 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 quakes 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 quakes usually diminish.

Other quakes 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.
Sincethe 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 the pressure, causing quakes. Most such quakes are small, but a few are large and cause damage.

In 1975, Kilauea's south flank was the site of the magnitude 7.2 Kalapana earthquake,
the highest magnitude earthquake event in this century. The Kalapana coast subsided as much
as 11 feet, generating a huge tsunami that claimed two lives in the Hawai'i Volcanoes National Park, destroyed houses in Panalulu, sank fishing boats in Keauhou Bay, and damaged boats
and piers in Hilo. The most recent 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.

Earthquakes occur for similar reasons beneath Mauna Loa's southwest and southeast flanks.
The Kealakekua fault zone on Hawai'i's 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 Hawai'ian 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 quake-triggered mudflows killed 31 people and coastal subsidence produced
a tsunami that destroyed several villages. At least 79 people perished during this quake;
most of these casualties resulted from the landslide and tsunami.

Accounts of the Great Ka'u earthquake;

"Between 9 and 10 o'clock, a slight tremble, soon another, and another at short intervals. Bella tried to keep a record of them, but soon gave it up, when they went into the hundreds during the day - some of them harder, and continued thro the night . . . with more earthquakes, increasing in violence. On Saturday, just after lunch, there was a hard one, peculiar, it seemed as if we moved backwards and forwards, 2 or 3 feet each time, for several seconds - it made the small children seasick - and it threw down some of our stone walls . . . but the earthquake kept on too - every few minutes, often we could heard it coming from the south, then give us a good smart shake and pass on towards Kilauea, North East from us - at night it made the house rock and creak like a ship in a heavy sea, and we could not sleep . . ." A letter by Frederick Lyman, March 27, 1868

"For four days this state of things continued, until at 4 p.m. on the 2nd of April, 1868, an event occurred which defies description. Such a convulsion has no parallel in the memory, the history, or the traditions of the Hawaiian Islands. The shock was awful. The crust of the earth rose and sunk like the sea in a storm. The rending of rocks, the shattering of buildings, the crash of furniture, glass, and earthenware, the falling of walls and chimneys, the swaying of trees, the trembling of shrubs, the fright of men and animals, made throughout the southern half of Hawai'i such scenes of terror as had never been witnessed before. The streams ran mud, the earth was rent in thousands of places; and the very streets of Hilo cracked open. Horses and their riders were thrown to the ground; and multitudes of people were prostrated by the shocks. In the district of Ka'u more than three hundred shocks were counted upon this terrible day; people were made seasick by their frequency. By the culminating shock, nearly every stone wall and house in Ka'u was demolished in an instant." T.M. Coan, an article for Scribner's Weekly, 1871

Kaoiki

Quakes 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, quakes with magnitudes from 5.5 to 6.6 have shaken the Kaoiki region about once every ten years. The latest large quake 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 seven million dollars. Fortunately, there were only minor injuries because the
quake struck early in the morning when most people were still in bed.

Quakes Unrelated to Volcanic Activity

Large quakes unrelated to volcanic activity also occur at irregular intervals on the island of Hawai'i. In 1973, a 6.2 quake located 25 miles beneath Honomu, north of Hilo, injured 11
people and caused $5.6 million worth of damage. Such quakes have no known recurrence
interval and are difficult to predict.