March 29, 1996

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

Tsunami

On Monday, April 1, 1996, scientists, historians, and public officials from Japan and the United States will convene in Hilo for a symposium to commemorate the 50th and 100th anniversaries of the disastrous tsunami earthquakes in the Aleutian Islands and in Sanriku, Japan, respectively.

The April 1, 1946, Aleutian Islands earthquake generated the tsunami that destroyed much of the Hilo bayfront. This 1946 event initiated research that led to the establishment of the Pacific Tsunami Warning System.

Tsunami is a Japanese word meaning "long harbor wave." Tsunami are perhaps most often associated with earthquakes occurring in coastal or submarine regions. However, tsunami can result from abrupt subsidence or uplift of the ocean bottom. While earthquake faulting can generate such changes on the ocean bottom, it is perhaps more common that earthquakes trigger landslides that lead to these changes and generate the tsunami.

In the open ocean, tsunami wavelengths - the distance between adjacent wave crests - can exceed 100 km (60 miles). The wave heights are relatively small and often difficult to detect onboard ships in the open ocean.

To assist with the tracking of tsunami generated from distant earthquakes, deep-water pressure gauges are used. Tsunami warning capabilities for distant earthquakes are built on earthquake monitoring around the Pacific Rim, on pressure gauges, and on tide gauges.

Many Hawaii residents will also recall that local earthquakes in Hawaii, including the great Ka`u earthquake of 1868 or the M7.2 Kalapana earthquake of 1975, also pose real tsunami hazards.

Because of the short times elapsing between the earthquake and possibly ensuing tsunami, warning systems are of limited effectiveness. If one feels a strong earthquake while at the beach or in a low-lying coastal region, the best course of action is to quickly move away from the water toward higher ground.

Volcano Update

The current eruption of Kilauea continues unabated, with flows entering the ocean at four sites near Kamokuna. Intermittent spattering at the coastal entries continued throughout the week, and visitors to the end of the Chain of Craters road were pleasantly surprised by the occasional good viewing.

For a brief period on Sunday morning, March 24, Kilauea Volcano tried to erupt in the summit caldera. Tiltmeter measurements indicated that the summit region inflated rapidly for slightly over an hour. Small, shallow earthquakes and a short burst of tremor accompanied the uplift. The crisis came to an end when the rapid inflation stopped, and the summit started to slowly subside. Magma moved downrift to the Pu'u 'O'o vent, and an increase in lava production was perceived by new breakouts from the lava tube system and a bright glow from the lava pond.

One earthquake was felt in Kona during the past week. A magnitude 3.0 located 4 miles east of Honaunau at a depth of 7 miles, occurred at 10:24 p.m. on March 23. No damage was reported. Several residents of Hilo reported feeling tremors on Thursday morning, but no earthquakes were recorded by our seismic network. Our network did register sonic waves from Pohakuloa that may have been the source of the shaking in Hilo.