Volcano Watch — April has been the month for serious tsunami activiy in Hilo and the Hawaiian Islands

In historical times, two tsunamis occurred during the first week of April. The first of these occurred on April 2, 1868; it resulted from the great earthquake that took place that day near Pahala. Based on the extent and type of damage, the 1868 earthquake is estimated to have had a magnitude of about 8.0. Reports indicate that 46 people were killed and several entire Hawaiian villages were destroyed by the tsunami generated from the earthquake. Eyewitnesses estimated that the wave was 25 to 30 feet high.

More recently, on April 1, 1946, a tsunami generated from a large earthquake in the Aleutian Islands caused severe damage in Hawai`i. There is usually a succession of waves during a tsunami, with each crest flooding the shore. Between the crests, which are commonly 12-20 minutes apart, the water level drops and exposes the shallow sea floor. Often, the first indication of the arrival of a tsunami is a sudden withdrawal of water from the shore. In 1946, each of the first eight wave crests was largest at one place or another; thus, the first wave is not necessarily the largest. One should not return to evacuated low-lying coastal areas until the entire wave series has arrived. The waves reached a maximum height above sea level of 55 feet near the mouth of Pololu valley and of 26 feet at Hilo. On Kaua`i, the maximum height of the wave was 45 feet; on O`ahu, 31 feet; on Moloka`i, 54 feet; and on Maui, 33 feet. In all cases, the water reached its maximum height on the north shore. On Hawai`i, 124 people were killed and almost 600 homes destroyed or damaged. Elsewhere in the islands, 38 additional people were killed and about 800 homes were destroyed or damaged.

Another damaging tsunami hit the islands on May 23, 1960. It was generated by an earthquake along the coast of Chile. This tsunami caused little damage elsewhere in the islands but the Hilo Bay area was hard hit. Sixty-one people lost their lives and about 540 homes and businesses were destroyed or severely damaged. The wave heights in Hilo Bay reached 35 feet compared to only 3-17 feet elsewhere.

As these examples demonstrate, there are two classes of tsunami that have caused damage here: those generated by large, distant earthquakes and those generated by local earthquakes. Today, tsunami generated by distant earthquakes are tracked by the Pacific Tsunami Warning Center in Ewa Beach on O`ahu. The minimum elapsed time between the earthquake and the arrival of the tsunami in Hawai`i is about 4.5 hours for earthquakes in the central Aleutian Islands. Tsunami generated by earthquakes elsewhere around the rim of the Pacific Ocean have elapsed times of as long as 15 hours (for those from South America). These times are adequate to issue warnings and evacuate low-lying areas on the islands. However, tsunami generated by local earthquakes may have extremely short time periods between the earthquake and the tsunami. Although only a few local earthquakes have been large enough to generate tsunami during historical times, those in 1868 (magnitude 8.0) and in 1975 (magnitude 7.2) produced tsunami that were large enough to kill people.

With the increasing population along the coastlines of Hawai`i, any future locally-generated tsunami pose an even greater threat to life and property. Because of the short time period (as little as a few minutes if you are near the earthquake epicenter) between a local earthquake and a tsunami it could generate, it is unlikely that adequate warnings of orderly evacuation can occur. Your best precaution if you feel a strong earthquake is to immediately move to higher ground if you are near the coast at low elevation. The Civil Defense pages in the phone book include maps of coastal areas around the island showing the areas where tsunami inundation can occur and the evacuation routes to use.

Volcano Activity Update

The eruption at the episode 51 and 53 vents near Pu`u `O`o on the East Rift Zone of Kīlauea Volcano continues with little change from last week. Active lava flows enter the ocean east and west of Kamoamoa and at Lae`apuki. Visitors may see the lava near the end of the Chain of Craters Road in Hawai`i Volcanoes National Park.