M 7.7 Kalapana earthquake was 50 years ago

(BIVN) – A volcano monitoring article was written for the USGS Hawaiian Volcano Observatory this week by HVO geologist Katie Mulliken:

Fifty years ago this month, the largest earthquake in Hawaii of the twentieth and twenty-first centuries occurred. Seismic shaking, as well as ground subsidence and a local tsunami, contributed to a catastrophic series of events on November 29, 1975.

Named the “Kalapana Earthquake” after its epicenter near the town of Kalapana on the southeast side of Hawaii Island, the quake occurred about 5 miles (9 km) below the Earth's surface. It was preceded by small foreshocks, many in the 4 to 5 magnitude range, that began a month before the main aftershock. The largest forearm was a 5.7 magnitude earthquake at 3:36 a.m. GMT on November 29, 1975.

Just over an hour later, at 4:48 a.m., a magnitude 7.7 earthquake struck. The shaking lasted more than 30 seconds, and was felt by residents all over the island and even Maui and Oahu. The earthquake caused an estimated $2.7 million in damage, mostly in Hilo, and included cracks in roads, water pipes and walls, upturned chimneys, collapsed water tanks, rock walls, fences, broken windows, and items on shelves falling and breaking.

The greatest damage from the earthquake was the deadly tsunami that swept the island's shores shortly afterwards. Campers at Halapi, on the south coast of Hawaii Volcanoes National Park, woke up to shaking and falling rocks from the Pali above caused by the shaking.

As recorded in a USGS report detailing survivors' accounts, the campers knew that a tsunami could follow the earthquake. They saw the ocean rise significantly and quickly fled to higher ground, but the water overcame them. Most of the campers were swept into a crevasse, with one describing the numerous tsunami waves as if they were inside a washing machine. The water receded about 10 minutes after the earthquake. One wagon was swept away and was never found. Another drowned or was beaten to death by the waves; Seven others were subsequently taken to hospital.

USGS: “The submerged coconut grove at Halapi, in Hawaii Volcanoes National Park, represents a former shoreline that was inundated during ground motion associated with the November 29, 1975, magnitude 7.7 earthquake.” (USGS photo)

The tsunami covered Hawaii Island, with waves damaging docks and docks, stranding and sinking boats, demolishing homes and businesses in Punalua, and flooding Ali Drive in Kona. The highest wave reached an estimated 48 feet (14.6 m) above the shoreline east of Halape, leaving a line of plants, rocks and other debris. In total, the tsunami caused an estimated $1.4 million in damage, for a total of $4.1 million in damage from the earthquake and tsunami combined.

The tsunami reached Hilo just 20 minutes after the quake. This is much faster than tsunamis generated by distant earthquakes, such as on July 29, 2025, when an 8.8-magnitude earthquake struck Russia's Kamchatka Peninsula. The Pacific Tsunami Warning Center expected tsunami waves from the Kamchatka earthquake to reach Hawaii more than five hours after the earthquake, allowing time to evacuate coastal areas.

In Hawaii, when strong tremors are felt, it is a warning of a possible tsunami and people near the coast should immediately head to higher ground. If a local tsunami is generated, it will arrive quickly, perhaps before any sirens sound or emergency messages are issued.

Whether or not a local tsunami will be generated depends on whether there is significant submarine vertical ground motion associated with the earthquake. In 1975, areas along the southern coast of Hawaii Island, from Honouapu to Kaimu, moved as much as 26 feet (8 m) horizontally and retreated as much as 11.5 feet (3.5 m), submerging vegetation and inundating the former coastline. The earthquake also produced extensive fractures along a 15-mile (25 km) stretch of the Helena fault system, with up to 5 feet (1.5 m) of vertical displacement.

Kīlauea was at a high level of amplification in November 1975 and the earthquake also triggered a small-scale, short-lived eruption within Kīlauea, the summit caldera of Kīlauea. The eruption began less than an hour after the earthquake and ended within 24 hours.

While lava flows and fountains often change the topography of parts of Hawaii Island, large earthquakes and less frequent ground movements have the potential to reshape the coastline and cause damage over larger areas. For more information about earthquake preparedness, see (shakeout.org/hawaii). In next week's article, we'll learn how our understanding and monitoring of Kilauea's southern flank and the risks associated with its movement have changed since 1975. On Saturday, November 29, the Pacific Tsunami Museum in Hilo will host a series of special events to remember the events of that day 50 years ago; Learn more here: (tsunami.org/events).

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