Volcano Watch — The Volcanic Hazards Program at HVO

The staff and associates of the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) extend a warm and hearty holiday greeting 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. As 1995 draws to a close, we can look back and reflect on the busy year that HVO has experienced.

Five related disciplines are involved in accomplishing the mission of the Volcanic Hazards Program at HVO. They are seismology, geodesy, gas geochemistry, geophysics, and geology.

Of these, the seismic program at HVO is the primary method used to monitor the volcanoes of Hawai'i. It is important because earthquakes occur as magma enters a volcano, and the detection of these tremors provide an early warning of possible activity.

Volcanic earthquakes are usually low in magnitude but can be felt because of their shallow depth. Earthquakesin Hawai'i are also caused by the weight of the islands, commonly called tectonic adjustment, and are only indirectly related to volcanic activity. Seismologists doing analyses of earthquake locations and waveforms can usually differentiate between volcanic and tectonic origins of an earthquake. The two largest earthquakes this year were of tectonic origin.

Since January 1, 1995, a total of 57,563 earthquakes were recorded by the 54-station seismic network operated by HVO on the island of Hawai'i. Of this total, 5,829 earthquakes were large enough to be located, and 47 were reported felt. Last year 67,145 earthquakes were detected, and 36 were felt.

The largest earthquakes in 1995 occurred on March 19 and on May 11. Both earthquakes registered 4.5 magnitude on the Richter scale and were felt from Hawai'i island to O'ahu. The first earthquake was located 39 miles west of Kawaihae at a depth of 30 miles, and the second was located 3 miles northeast of Pa'auilo at a depth of 14 miles.

The measurement of surface deformation or geodesy is another major volcano monitoring program at HVO. The surface of the ground around a volcano can be deformed by the change in volume or pressure of a magma body and by gravitational loading.

This year, two continuously monitoring Global Positioning System (GPS) stations were installed in the summit area of Kīlauea to detect changes in the ground surface caused by changes in the summit magma chamber. This system was able to detect small horizontal movements correlated to magma chamber changes that affected the eruptive activity at Pu'u 'O'o.

Intermittently occupied GPS stations in the south flank block of Kīlauea had significant horizontal movement of up to four inches toward the sea in 1995.

The gas geochemistry group measures changes in the volume of sulfur dioxide emitted by the volcanoes and in the ratio of carbon dioxide to sulfur dioxide gases at fumaroles.

Changes in gas volumes and ratios are caused by changes in the magma body from which these gases emanate. The recent increase of vog in the Hilo area, however, is the result of more frequent winds from the southerly direction and not from a change in the magma system.

The geophysics program monitors changes in the magnetic, gravitational, and geoelectric fields of the Earth. These geophysical changes are caused by stress or mass changes related to bodies of magma or water.

The geophysics group also monitors the volume of lava flowing through the tube system from Pu'u 'O'o to the coast. This is calculated by measuring the cross-sectional area of the tube and by measuring the velocity of the flowing lava with a radar gun.

The geology group, in addition to preparing geologic hazard maps and studying volcanic processes, monitors the actual eruption and lava flows. The information they derive from their on-site work is passed on to the National Park Service and to the Hawaii County Civil Defense agency.

The monitoring groups are supported by electronics, computer, library, and clerical staff, who keep the programs running. All of the employees at the U.S. Geological Survey's Hawaiian Volcano Observatory are very essential in our mission of providing unbiased Earth science information to the public. We join you in celebrating a safe and joyous holiday season and invite you to our next "open house" to be held on Earth Day in 1996.