# Volcano Watch — HVO: 1996 in review

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As 1996 draws to a close, we can look back and reflect on the tumultuous year that the U.S. Geological Survey's Hawaiian Volcano Observatory experienced. The year started with the Federal government in a partial shutdown, and HVO operated with a reduced staff for three weeks.

As 1996 draws to a close, we can look back and reflect on the tumultuous year that the U.S. Geological Survey's Hawaiian Volcano Observatory experienced. The year started with the Federal government in a partial shutdown, and HVO operated with a reduced staff for three weeks. Even with such an ominous start, there were many highlights throughout the year. The shutdown ended in time for our seismic staff to participate in a month-long experiment involving 25 visiting international seismologists.

The principal goal of the experiment was to record volcanic tremor and volcanic earthquakes with a dense, 116-station array of portable seismographs in the Kīlauea Volcano summit region. On the morning of February 1, a swarm of small, shallow earthquakes and rapid inflation of the caldera floor heralded the possibility of a summit eruption. After four hours of inflation, the summit started to subside, coincident with a three-fold increase of lava output from the Puu Oo vent. This surge in lava production lasted late into the night and led to the first of four pauses in activity during 1996.

Kīlauea was chosen as the site of the experiment because of the complementary modern seismic network operated by HVO and because of the high incidence of earthquakes. Since January 1, 1996, a total of 43,302 earthquakes were recorded by the 54-station HVO seismic network on the island of Hawai'i. Of this total, 4,950 earthquakes were large enough to be located, and 59 were reported felt.

The largest earthquakes in 1996 occurred in July during the Loihi Volcano earthquake swarm. A magnitude of 4.9 was registered by three of the 4,519 earthquakes recorded from Loihi. A total of 96 earthquakes from Loihi had magnitudes 4.0 or larger. Scientists from the University of Hawaii discovered a new pit crater in the summit of Lo`ihi.

HVO hosted a week-long geodesy conference involving forty participants who discussed various techniques in monitoring surface deformation and presented innovative methods of modeling and analyzing geodetic data. The HVO surface deformation group collaborated with investigators from the University of Hawaii and Stanford University in establishing an array of thirteen permanent Global Positioning System (GPS) stations on the south flank of the Big Island. The annual measurement of Mauna Loa Volcano revealed a retardation in the rate of inflation.

The geochemistry group deployed a network of continuous gas monitoring instruments developed at HVO to measure the flux of carbon dioxide and sulfur dioxide gases at fumaroles around Kīlauea. Changes in gas volumes and ratios are caused by changes in the magma body from which these gases emanate. The February 1 event caused a major increase in carbon dioxide emissions in the summit region.

The geophysics group continued to monitor the volume of lava flowing through the tube system from Pu'u 'O'o to the coast. The average output of lava is about 400,000 cubic meters per day, but there was no net change in the size of the island.

The static size of the island is the result of frequent collapses of the coastal lava delta (bench). Mapping by the geology group determined that a 25-acre block slumped beneath the waves on December 2. This was the largest bench-collapse in the 14 years of the eruption. Fortunately, National Park officials, on the advice of HVO, restricted visitor access to this hazardous area and no one was killed or injured.

The monitoring groups are supported by electronics, computer, library, and administrative staff, who keep the programs running. The staff and associates of the U.S. Geological Survey's Hawaiian Volcano Observatory wish a 'Happy New Year' to all of the faithful readers of this column. Your interest in keeping abreast of volcanic and seismic events and in being informed of geologic hazards and ways to mitigate those hazards has sustained our efforts in producing this column. We appreciate your support and will continue our mission of providing unbiased Earth science information to the public.