The southern part of the Big Island sustained massive ground movements and some smaller tsunamis.
A magnitude 7.2 earthquake, the largest experienced in the state of Hawaii since 1868, struck the island of Hawaii on November 29, 1975, killing two people, injuring several, and inflicting property damage estimated at $4 million. The epicenter was two miles offshore from Kalapana. Structural and nonstructural damage in Hilo, thirty miles north of the epicenter, was slight to moderate but was more extensive than elsewhere on the island.
Structural damage on the southeast part of the island included minor cracks, slight floor to-wall separations, and bending of walls at a hospital, at schools, and at libraries. While most of the buildings in the epicentral area sustained little or no structural damage from ground shaking, there were massive ground movements, hundreds of aftershocks, and a brief, small-volume volcanic eruption. The earthquake was felt throughout the island of Hawaii and on the islands of Lanai, Molokai, and Oahu.
The exceptional feature of this earthquake was the damaging tsunami that followed it. Hawaii is a frequent recipient of tsunamis from earthquakes around the Pacific Rim, but rarely does it experience tsunamis from the numerous earthquakes generated within the islands of the State of Hawaii. This particular tsunami consisted of five or more distinct waves. It was the only locally generated destructive tsunami in Hawaii in the twentieth century. It killed two people at Halape, where it reached a height of twenty-five feet and caused property damage.
Tsunami damage was also severe at Honuapo, with twenty-foot waves. At one site a mile east of Halape a height of forty-five feet was recorded. Farther away, across the Pacific, the tsunami impacted Alaska, California, Japan, Okinawa, Samoa, as well as the islands of Johnston and Wake. A one-foot wave struck Oahu thirty minutes after the earthquake. This same wave height was recorded at Los Angeles more than six hours after Oahu, representing a tsunami speed of 350 mph. In Japan, eight hours after the earthquake, a wave height of two feet was registered.
Questions were raised by geologists in the wake of the earthquake concerning the origin of the tsunami. Why was there a tsunami following this particular earthquake and not after many others? Kalapana’s predecessor, the 1868 quake, was investigated again in search of an answer. Its epicenter had been near that of the 1975 earthquake. They found that it had triggered a landslide on the slopes of Mauna Loa, close to the places affected by the 1975 event, that it had destroyed a village, killing many people and animals, and that it had triggered a tsunami. The final confirming data came from measurements of a slump that was known to exist on the south flank of Kilauea Volcano, an area close to the epicenters of both the 1868 and 1975 events.
The slump was found to move regularly in a southerly direction at a rate of four inches annually. From time to time it had an acceleration in its movement, perhaps triggered by an earthquake, and masses of rock rushed down into the sea, creating a tsunami. This was what happened in 1968 and 1975, just as had occurred in the Grand Banks, Nova Scotia, earthquake and landslide on November 18, 1929. Since the events of 1975, geologists have concluded that these recurring slumps are essential stages in the life of volcanic islands. They apply therefore to the entire landscape of the state of Hawaii.
The impact of the combined earthquake and slump on Kilauea Volcano was to distort its southern flank. Both vertical and horizontal displacements occurred, some by as much as ten feet. There were numerous ground cracks and new fault lines were exposed. Roads in the Hawaii Volcanoes National Park were damaged by extensive ground cracking. Landslides occurred on Coast Road. Some displacements of faults caused substantial subsidence, as much as eleven feet in the vicinity of Halape. Farther inland an almost continuous zone of ground was observed cracking and faulting, with five-foot vertical offsets in places, over an area of fifteen square miles.
A large part of the coastal area between Cape Kumukahi and Punaluu subsided during or soon after the earthquake. A leveling survey of the bench marks near the Keauhou tidal gauge indicated that the coast had subsided by about ten feet. At the location where two lives were lost there had been a group of thirty-two campers spending the night at the base of a large cliff near Halape. They woke up when a small earthquake shook their place shortly before the main one and decided to move closer to the ocean. As the main quake hit, nineteen of the thirty-six suffered injuries tying to escape both the rocks falling down and the tsunami waves from the ocean.