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Volcanowatch

April 19, 2001

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


April is Earthquake and Tsunami Awareness Month

damaged home 7 km north of Hilo overturned boat in Hilo, Hawaii
November 29, 1975 earthquake and tsunami damages. Left: The magnitude 7.2 earthquake crumbles the stairway of a house located about 7 km north of Hilo. | med | large | Right: The tsunami overturns a boat in Hilo, Hawai`i. | med | large |

Early Monday morning, residents of the northern half of the island were awakened by a magnitude-3.9 earthquake. This was a gentle reminder that we live in one of most seismically active areas in the United States. The month of April is observed as earthquake awareness month in the State of California and as tsunami awareness month in the State of Hawaii. Both natural hazards have struck this island with disastrous results, so it is especially important that kama`ainas are reminded, and malihinis are made aware, of these hazards.

April could also be considered earthquake awareness month here in Hawai`i because the largest historical earthquake occurred on April 3, 1868, in the Ka`u district. The estimated magnitude of this event was 7.9, larger than the magnitude-7.8 San Francisco earthquake of April 18, 1906. The Ka`u earthquake killed a number of people, mainly by a huge landslide near Kapapala and by a tsunami that swept the coastline.

We observe tsunami awareness month during April because of the destructive waves that killed 159 people in Hawai`i on April 1, 1946. The tsunami was generated by a magnitude-7.8 earthquake in the Aleutian trench south of Unimak Island. The first wave took 4.9 hours to reach Hilo and caught many residents by surprise. Thereafter, the Seismic Sea Wave Warning System, later called the Pacific Tsunami Warning System, was established to provide residents with an early notification if a tsunami is triggered by a distant Pacific basin earthquake.

The warning system is fine for tsunamis generated by distant earthquakes, because it takes several hours for the waves to reach Hawai`i. Large local earthquakes can also cause a tsunami, and the warning system may not be activated in time for some people to evacuate. The great Ka`u earthquake and the 1975 Kalapana earthquake (magnitude 7.2) both produced waves that killed people.

The greatest danger from a locally generated tsunami is the suddenness with which the waves can materialize. The only warning for such an event is the strong ground shaking. If you are near the ocean when a large earthquake occurs, head for high ground immediately; this reaction may save your life. Campers at Halape, near the epicenter of the 1975 earthquake, had only seconds after the shaking stopped before they were engulfed and swept away by a series of waves. The tsunami reached Hilo in 20 minutes and Kailua-Kona in 27 minutes. Many fishing boats at both locations were severely damaged or destroyed.

Hawai`i is a geologically dynamic island with two of the most active volcanoes in the world. Directly or indirectly, the volcanoes are the cause of the high seismicity of the island. Monday morning's earthquake belongs to a family of earthquakes generated by the slow crustal adjustment to the load or weight of the volcanoes. The most recent large earthquake from this family was the magnitude-6.2 Honomu earthquake in 1973.

When magma enters a volcano, the edifice has to make room for it, so the flanks move outward to accommodate the new magma. This flank movement has caused a number of large earthquakes. Recent earthquakes of this mechanism include the magnitude-6.7 Ka`oiki fault earthquake in 1983, the 1975 Kalapana earthquake, and the magnitude-6.9 Kealakekua fault earthquake in 1951.

Awareness of the earthquake hazard in Hawai`i is necessary for everyone because of such a high frequency of large, destructive local earthquakes. The hazard will not disappear or diminish, but residents can reduce the effects of the shaking and also be prepared for the event. Information on ways to mitigate the earthquake hazard can be obtained from the Hawai`i County Civil Defense office (935-0031) or from the Center for the Study of Active Volcanoes (CSAV) at the University of Hawai`i at Hilo (974-7631). CSAV also has a web site with the necessary information (http://www.uhh.hawaii.edu/%7Enat_haz/earthmit/eq.html).

Just as the rain in Hilo doesn't fall only in April, awareness of natural hazards should be a year-round practice.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week and provided visitors with an occasional glimpse of surface flow activity. Breakouts occur above and on Pulama pali, in addition to the coastal flats. Lobes of two flows are slowly advancing to the seacoast at Kamokuna and at Kupapa`u. The flow front closest to the ocean is within 130 m (425 ft) of the shoreline in the old Kupapa`u area near the eastern boundary of Hawai`i Volcanoes National Park.

The Monday morning earthquake was reported felt by residents of Hilo, Papa`ikou, Pa`auilo, Kapulena, Kalopa, Waimea, Hawi, Kawaihae, Waikoloa, Kailua-Kona, and Keauhou. The 4:18 a.m. earthquake was located near the Pohakuloa Training Area at a depth of 31.2 km (18.7 mi) and had a magnitude of 3.9. This was the only event reported felt during the week ending on April 19.


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Updated: April 23, 2001 (pnf)