Volcano Watch — The 25th Anniversary of M7.2 1975 Kalapana Earthquake

Wednesday, November 29, 2000, will mark the 25th anniversary of the magnitude 7.2 Kalapana earthquake that struck the Puna and Ka'u districts of Hawai'i County. The 1975 earthquake is the second largest ever documented in Hawai'i, overshadowed only by the 1868 great Ka'u earthquake, which has been estimated to be of magnitude 7.9.

The 1975 earthquake was felt throughout the entire state, but its greatest effects were registered in Hawai'i County. Over $4 million in damage was reported, mostly from Hilo. Had the earthquake occurred closer to more heavily developed areas rather than along the coast in Hawai'i Volcanoes National Park, the cost would have been greater. Two lives were lost as the tsunami generated by the earthquake inundated a low-lying beach campground at Halape.

If the M7.2 Kalapana earthquake were to occur today, the financial losses would no doubt exceed those incurred in 1975. With greater public awareness and understanding of natural hazards, we might be able to minimize the cost and avoid loss of life.

In Hawai'i and elsewhere, earthquakes of magnitude 7 and larger are geologically important events. In 1975, the ground surface shifted horizontally by as much as 8 m (26 feet). The earthquake produced cracks in the ground surface along 25 km (15 miles) of the Hilina fault system, on the south flank of Kīlauea. A 50-kilometer-long (30-mile-long) portion of the southeast coastline of the island subsided, with as much as 3.5 m (11.5 feet) recorded at Halape, where the two campers drowned. A short-lived eruption in Kīlauea caldera began half an hour after the earthquake.

We were able to see the effects of the earthquake, but could not directly observe the actual earthquake process itself. We therefore have to depend on data and techniques that allow us to "see" into the Earth to draw inferences and develop models of how the volcanoes grow and change. We operate our HVO geodetic and seismographic networks to provide important data baselines from which these models are built.

Perhaps the most important observations relating to the cause of the 1975 earthquake were made from repeated occupations of the geodetic networks, which we use to measure the displacements of the ground surface. These data suggested that magma was being forcefully injected into the rift zones of Kīlauea and was pushing against the flanks of the volcano. Intermittently, strain is accumulated from uplift and movement of the south flank of Kīlauea. This strain is subsequently released by earthquakes, faulting, and subsidence of the south flank.

HVO scientists noted a build-up of strain within the Hilina fault system through much of the 20th century prior to the 1975 earthquake. In their report, they described an expected subsidence event along the south flank of Kīlauea. As the HVO report awaited publication, the November 1975 earthquake confirmed the expectations of the scientists.

We have learned much about large earthquakes in Hawai'i, partly as a result of the 1975 earthquake. For example, it is widely viewed that the largest of Hawaiian earthquakes are the result of movements along a nearly horizontal fault. This fault is thought to be the boundary between the top of the ancient sea floor and the bottom of the volcanoes. Earthquakes represent the release of strain accumulated within the volcanoes.

A great deal remains to be learned about the details of processes related to earthquakes in Hawai'i. At some point, we hope that we will be able to identify and recognize behaviors that are precursory to large earthquakes and possibly forecast future magnitude 7 or larger events.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing southeast through a tube system down to the flats below Pulama pali and beyond to the ocean. Lava is entering the ocean at Kamokuna located 1.6 km (1 mi) west-southwest of Waha`ula. The bench has increased in area and is nearly the same size as it was prior to the partial collapse reported last week.

The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

Many residents of Maui and one on O`ahu felt an earthquake at 6:58 p.m. on November 16. The magnitude-3.8 earthquake was located 27 km (16 mi) north of Ke`anae on Maui at a depth of 27.1 km (16.2 mi). We thank the Maui residents who completed and filed earthquake felt report forms on our web site (http://hvo.wr.usgs.gov/earthquakes/felt/).