February 21,1997

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

Kilauea Update

Kilauea Volcano continues to be in a state of repose with no eruptive activity since January 31. Although no molten rock can be seen on the surface of the volcano, seismic and geodetic monitors indicate that magma is moving within.

Over 55,000 microearthquakes in the summit and the upper east rift zone regions have been recorded in the last three weeks. Daily counts have lessened with time. Nearly 6,000 events per day were counted during the first days of February, and now the daily totals have decreased to about a thousand. These earthquakes are thought to be related to pressure changes in the magma storage system caused by the withdrawal of magma during episode 54.

International seismologists conducting a seismic experiment at the summit of Kilauea collected a wealth of data during their two weeks of recording in early February. These data will help in defining the conduit system of the volcano and in recognizing precursory signals to an eruption. Small earthquakes that shake very slowly were recorded for the first time and were identified to be associated with rapid tilt changes in the summit region.

Continuous geodetic measurements by the permanent GPS network and an electronic tiltmeter show an irregular reinflation of the summit region. Periods of inflation and extension are interrupted by periods of minor deflation and contraction or no change. The net inflation since the end of eruptive activity is nearly 25% of the deflation that took place with episode 54.

Measurements of sulfur dioxide emissions from the volcano indicate an 80% to 90% reduction in volume of the gases being emitted. Sulfur dioxide is the principle component of vog, and this reduction in emission has greatly improved the air quality around the island. Residents of Kona can once again see blue skies, white clouds, and beautiful sunsets. People with respiratory ailments can breathe easier.

Chemical analyses of rock samples from episode 54 indicate that two separate and distinct magma bodies near Napau Crater supplied lava for the eruption. Both are also different from the lavas that were being erupted at Pu`u O`o. The Napau -Makaopuhi crater area has long been recognized as a place to store magma in the east rift zone. Large pit craters such as these are located above magma bodies and form when the surface collapses after magma is erupted.

The longest period of repose during the Pu`u O`o eruption was 65 days between episodes 3 and 4. Activity resumed after the summit tilt regained the level attained prior to episode 3. Time will tell if Kilauea will repeat this pattern.

There were three earthquakes felt since the start of the month. Residents in Ka`u, Kona, Kohala, and Hamakua felt two earthquakes at 6:55 p.m. and 11:56 p.m., on the night of February 2. The earthquakes were located in the Kona region and had magnitudes of 3.5 and 3.7, respectively. On February 6 at 10:28 p.m., a small earthquake of magnitude 2.4 located in lower Puna was felt by a resident of Leilani Estates.