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April 29, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Let us not forget

As we enter the month of May, we are remiss in not remembering that April was "Tsunami Awareness Month". April was chosen as "Tsunami Awareness Month" because the deadliest tsunami to strike the Hawaiian Islands occurred on April 1, 1946. What is often overlooked is that the largest and deadliest locally generated tsunami also occurred in the month of April.

In the afternoon of April 2, 1868, an earthquake with a magnitude approaching 8 shook the entire chain of Hawaiian Islands. The earthquake was centered in the Ka`u district on the island of Hawai`i and generated a tsunami that swept through the islands. Entire villages along the Ka`u-Puna coast were destroyed, and 75 fatalities were attributed to the tsunami.

Wave heights recorded around the island were 13.7 m (44.9 ft) at Apua Point, 6.1 m (20.0 ft) at both Punalu`u and Honuapo, 3.4 m (11.2 ft) at Kailua-Kona, and 3.0 m (9.8 ft) at Hilo. These wave heights are higher than those recorded for the locally generated tsunami from the November 1975 magnitude-7.2 Kalapana earthquake. That was the last fatal tsunami to strike Hawai`i. Two campers in Hawai`i Volcanoes National Park near the epicenter of the earthquake were killed by the wave action.

The greatest danger from locally generated tsunami is the suddenness with which the waves can strike. When a tsunami is triggered by an earthquake far from Hawai`i as in 1946 and in 1960, the travel time for the waves to reach Hawai`i is from 4 to 12 hours, so there is ample warning. However, a tsunami generated by a local earthquake can be upon you in minutes.

In 1975 at Halape, near the epicenter of the earthquake, the sea started rising a few seconds after the ground stopped shaking. Within 12 minutes, the restaurant at Punalu`u was inundated, and 20 minutes after the earthquake, boats were destroyed in Wailoa River. The tsunami took 27 minutes to reach Kailua-Kona, where several boats were damaged or destroyed.

The only way to protect yourself from a locally generated tsunami is to immediately head for higher ground when you are near the ocean and an earthquake shakes you so violently that it is difficult to stand. In other words, if you are playing soccer at the Bayfront fields, and a large earthquake occurs, you must proceed up to Kino`ole Street or higher without hesitation. Do not wait for a warning siren to get moving, for there may not be an alarm sounded in time. Let this information save your life.

Eruption Update

Lava continues to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the sea near Kamokuna, in Hawai`i Volcanoes National Park. The sluggish lava flow on the coastal plain remains active and has advanced only a few meters closer to the coastline since last week. Lava flows are also commonly visible at the bench, where lava enters the sea. Explosive activity there has been less frequent than in previous weeks. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of new land. The steam clouds are highly acidic and laced with glass particles.

There were six earthquakes reported felt for the week ending on April 29. Two aftershocks of the April 16 magnitude-5.6 earthquake were felt in Pahala on April 23 at 2:50 a.m. (mag.-3.1) and at 2:17 p.m. (mag.-2.9). A shallow earthquake near Pu`ulena Crater was felt in lower Puna on April 23 at 12:44 p.m. (mag.-3.4). A resident of Ahualoa felt an earthquake on April 25 at 4:50 in the afternoon. The magnitude-3.4 temblor was located 10 km (6 mi) southwest of Honoka`a at a depth of 16.5 km (9.9 mi). On the morning of April 28, a resident of Volcano reported feeling two earthquakes. One at 5:23 a.m. (mag.-1.2) was located near Kipuka Nene in Hawai`i Volcanoes National Park at a depth of 33.1 km (19.9 mi). The other at 5:31 a.m. (mag.-1.8) was located 15 km (9 mi) east of the summit of Mauna Loa at a depth of 8.3 km (5 mi).

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Updated: 7 May 1999