(Excerpts from a Survey conducted in November - December, 1975 and from subsequent reports)
On Wednesday, November 29, 1975, the largest, local earthquake to strike the Hawaiian Islands since 1868, subsequently designated as the Kalapana Earthquake of 1975, generated the most destructive local tsunami in Hawaii in the 20th Century.
Actually, two earthquakes occurred. The first earthquake, a foreshock, occurred earlier at 13:35 GMT (3:35 a.m local time). It had a magnitude of 5.7 and its epicenter was near Lae'apuki on Kilauea's south coast on the island of Hawaii.
The second and larger earthquake occurred a little over an hour later at 14:48 GMT (4:48 a.m. local time). This second earthquake was considerably larger with a magnitude of 7.2. Its epicenter was offshore at 19.3° N, 155.0° W, near Kamoamoa, just a few miles east but closer to the shoreline than the earlier foreshock. Its focal depth was only 8 km below the surface, near the magmatic chambers of the Puna Volcanic rift zone. The earthquake was induced by magmatic movements.
At the time the first earthquake struck, there were thirty-two (32) people camping for the night at the Halape beach park near the base of a cliff. According to campers' accounts, trembling of the earth and sounds of rocks falling from the cliff, awakened them. A few of the campers moved to the coconut grove adjacent to the beach, believing it was safer from falling rocks.
The second earthquake, a little over an hour later, caused larger boulders to start falling down the cliff, thus forcing the rest of the remaining campers to flee toward the sea. Subsequently, these same campers were forced back to the cliffs when campers at the coconut grove were seen fleeing the rising ocean.
Although the earthquake damage was relatively limited, a rather large tsunami was generated by the second earthquake. The tsunami was destructive along the southern coast of the island with lesser damage observed along the eastern and western parts.
The first tsunami wave observed by the campers at Halape was only 1.5 meters (almost 5 feet). However, the second wave was 7.9 meters (about 26 feet). This wave carried them inland toward the base of the cliff where they remained until the tsunami waves subsided. Nineteen (19) of these campers suffered injuries. Two (2) of them died.
At Hanape, the maximum tsunami runup reached 7.2 meters (about 23.5 feet). At the small bay of Punalu'u, the first tsunami wave arrived only 84 seconds after the quake was felt. Maximum run-up was 7.6 meters (about 25 feet). The tsunami waves were particularly destructive. Houses were swept off their foundations and properties were extensively damaged.
At the Keauhou Landing, the tsunami waves completely destroyed the pier. Maximum tsunami runup of 14.3 meters (almost 47 feet) was measured. Elsewhere in the islands the tsunami was small.
Post-earthquake surveys of the south coast of the island of Hawaii showed that a large crustal block had slid horizontally towards the ocean and had subsided. Maximum horizontal displacement of approximately 7.9 meters (26 feet) and vertical subsidence of approximately 3.5 meters (11.5 feet), occurred near Keauhou Landing. The displacements decreased to the east and west from this area. In fact, subsidence rapidly decreased to the west. At Punalu'u, the shoreline actually uplifted by about 10 centimeters (4 inches). Subsequent surveys determined a subsidence of about 3 meters (9.8 feet) at Halape Park to the east. A large coconut grove area adjacent to the beach subsided by as much as 3.0 and 3.5 meters (10-11.5 feet). Further to the east the subsidence decreased to 1.1 meters (3.6 feet) at Kamoamoa, 0.8 meters (2.6 feet) at Kaimu, o.4 meters (1.3 feet) at Pohoiki, and 0.25 meters (0.8 feet) at Kapoho.
The coastline was not the only area that was altered as a result of the earthquake. According to the Volcano Observatory of the U.S. Geological Survey, the summit of Kilauea subsided about 1.2 meters (3.9 feet) and moved towards the ocean by about the same amount. A small, short-lived eruption took place inside Kilauea's caldera.
Inspection of tide gauge records showed the initial wave motion to be upwards at all stations. The significance of this observation is that the offshore crustal displacement was an uplift as the onshore section subsided and moved outward. The dimensions of the sea floor affected by these crustal movements was approximately 70 km long, and 30 km wide with the long axis of the displaced block being parallel to the coast. This entire offshore region rose approximately 1.2 meters (3.9 feet).
Although the second Kalapana earthquake had a larger surface-wave magnitude of 7.2 , relatively minor damage occurred within 10 miles of the epicenter to approximately a dozen residences. However, at Hilo, about 45 miles from the epicenter, earthquake damage was heavy to Hilo Hospital and several other large buildings.
The death toll was remarkably low. Only two people lost their lives. 28 other people were injured. Both deaths, all the injuries, and approximately a third of all the property losses, were caused by the tsunami. Total property damage losses from both the earthquake and tsunami were estimated at about $4.1 million. Of the property losses, about $2.1 million was to private property and about $2 million to public property.
A local Tsunami Warning was issued by Hawaii's Civil Defense Agency. However, because of the short interval between earthquake occurrence and the arrival of the tsunami, the warning was issued after the first tsunami wave had arrived on the southern part of the island. As mentioned earlier, travel time to Punaluu was only 84 seconds.
Satellite Photo Composite (8 photos)
Subsequent inspection of the offshore bathymetry of the southern coastal region of the island of Hawaii where the earthquake occurred showed a similar pattern of topographical features suggestive of a long history of large crustal block movements and subsidences in the past. Contouring the bathymetry revealed the existence of numerous "horsts" and 'grabbens", geomorphological features characteristic of large crustal block movements and subsidences in distant past. These were aligned in parallel to the displacements of the 1975 Kalapana earthquake, indicating a pattern of similar activity.
Apparently, volcanically-induced earthquakes have occurred with regularity in the same area. These earthquake are induced by magmatic movements near the magmatic chambers of the Puna Volcanic rift zone of Kilauea volcano Earthquakes along the southern coast of Hawaii must have generated destructive tsunamis in prehistoric times and as recently as 1823(?), 1868, and 1975. Even as recently as 1989, this region has generated also smaller but yet damaging earthquakes (with no tsunami generation)
Since the population on the southern coast of Hawaii has more than doubled since 1975, if a similar earthquake and tsunami occurred presently, it would cause far greater casualties and property losses.
Pararas-Carayannis, George. The Earthquake and Tsunami of 29 November 1975 in the Hawaiian Islands, ITIC Report, 1976.
Pararas-Carayannis, George, International Tsunami Information Center - A Progress Report For 1974-1976. Fifth Session of the International Coordination Group for the Tsunami Warning System in the Pacific, Lima, Peru, 23-27 Feb. 1976
Pararas-Carayannis, George and Calebaugh P.J., Catalog of Tsunamis in Hawaii, Revised and Updated , World Data Center A for Solid Earth Geophysics, NOAA, 78 p., March 1977.
U.S. Geological Survey, Hawaii Volcano Observatory, Survey
Tsunami destruction at Punalu'u
Photo: National Park Service