Volcano Watch — The Kalapana earthquake of 1975

Wednesday, November 29, marks the 20th anniversary of the 1975 Kalapana earthquake, the largest earthquake to strike Hawaii since 1868. Residents were awakened at 3:35 a.m. by a magnitude-5.7 earthquake located a few miles inland of Lae'apuki on Kīlauea's south coast. A little more than one hour later, at 4:47 a.m., the main shock struck with a magnitude of 7.2. The epicenter was located at Kamoamoa, just a few miles east, and closer to the shoreline, from the foreshock.



Much of the south coast of Hawaii Island slid horizontally towards the ocean and subsided. The maximum horizontal displacement, near Keauhou Landing several kilometers east of Halape, was about 26 feet. The displacements decreased to the east and west from this area.



Similarly, subsidence of the coast was greatest in this same area and amounted to about 11.5 feet. The amount of subsidence rapidly decreased to the west, and, at Punalu'u, the shoreline actually uplifted by about 4 inches. To the east, subsidence was about 9.8 feet at Halape, 3.6 feet at Kamoamoa, 2.6 feet at Kaimu, 1.3 feet at Pohoiki, and 0.8 feet at Kapoho. At Halape, the subsidence resulted in submergence of a large stand of palm trees.



The coastline was not the only area that was altered as a result of the earthquake. The summit of Kīlauea subsided about 3.9 feet and moved towards the ocean about the same amount. A small, short-lived eruption broke out inside the caldera. The eruption was probably triggered by the shaking, which causes gas bubbles to form in the magma. The effect is similar to opening a shaken can of carbonated soda. This change in gas pressure, coupled with newly formed ground cracks, allowed a small amount of magma to escape to the surface.



The earthquake also caused a moderate-sized tsunami. Tide gauge records indicate that the initial motion was upwards at all stations. This observation suggests that the sea floor offshore uplifted as the onshore section of the volcano down-dropped and moved outward. The area of sea floor inferred to have uplifted is about 70 kilometers along the coast and 30 kilometers perpendicular to the coast. This entire offshore region rose about 3.9 feet. The tsunami, with a maximum height of 20 feet at Halape, arrived at Punalu'u a mere 84 seconds after a seismic sensor located there recorded the strong ground motion of the earthquake.



Losses from the earthquake and tsunami were 2 deaths, injuries to 28 people, and about  \$4.1 million property damage. Both deaths, all the injuries, and roughly a third of the property losses, were attributed to the tsunami. Of the property losses, about \$2.1 million was to private property and $2 million to public property (a combination of County, State, and Federal facilities and infrastructure). As there were only a few dozen residences located within 10 miles of the epicenter, the structural damage caused by shaking in the general area of the epicenter was surprisingly light. Damage in Hilo, about 45 miles from the epicenter, was fairly heavy and included damage to Hilo Hospital and several other large engineered buildings.



We learned much about the structure of Kīlauea Volcano from this earthquake and we are now able to forecast future activity more accurately than we could before. For example, we now know that the large earthquakes along Hawaii's south flank occur on a nearly flat-lying fault plane and that the pressures from within the magma system at Kīlauea, and probably from within Mauna Loa, as well, provide the push on the flank. We know that such large events occur more frequently than every hundred years, on average - - they have occurred in 1823(?), 1868, and 1975 - -, and that smaller (but still damaging) earthquakes occur in this region even more frequently (the last in 1989).



Today, we are measuring the rates of movement of the south flank using satellite technology. These measurements show us that energy is accumulating in the south flank. The accumulated and stored energy will eventually be released during the next large earthquake in this region.



In planning for future earthquakes, it is worth remembering that the population of the island in 1975 was less than half of what it is today, and that the population close to the epicenter and along the south coast of Hawaii, where the effects of ground subsidence and tsunami were large, has grown enormously.



A similar earthquake and tsunami today will cause far greater casualties and property losses than occurred in 1975. Such an event will occur again. Will we have planned our community growth and infrastructure to minimize life and property losses? Will we have built buildings that will withstand the ground shaking? Will we each have prepared for the next earthquake by maintaining emergency supplies in our homes?