Volcano Watch — 67,145 quakes recorded at HVO so far this year

The staff and associates of the U.S. Geological Survey's Hawaiian Volcano Observatory extend a warm and cheerful holiday greeting to all of the faithful readers of our column. Judging from comments that we receive, this column has proven to be an excellent way to keep you abreast of volcanic and seismic events and to inform you of geologic hazards. As 1994 draws to a close, we can look back and reflect on the busy year that HVO experienced.

Since January 1, 1994, a total of 67,145 earthquakes were recorded by the 52-station seismic network operated by the U.S. Geological Survey on the island of Hawai‘i. Of this total, 6,772 earthquakes were large enough to be located, and 36 were reportedly felt. The largest earthquake in 1994 occurred on February 1 and had a magnitude of 5.2. The origin of this large temblor was 10 miles south of Halema'uma'u at a depth of 21 miles. It was felt throughout the state with reports from as far away as Kaua‘i.

The earthquake detection program at HVO is the primary method used to monitor the volcanoes of Hawai‘i. As magma enters the volcano, rocks break to make room for the added volume, and the breaking of rocks is an earthquake. Volcanic earthquakes are usually low in magnitude but can be felt because of their shallow depth. Earthquakes in Hawai‘i are also caused by the weight of the islands, commonly called crustal or tectonic adjustment to the increased gravitational load. Seismologists doing analyses of earthquake locations and waveforms can usually differentiate between volcanic and tectonic origins of an earthquake. The vertical first motions of an earthquake recorded by a network of stations provide seismologists with the focal mechanism or relative directional movement of the ground slippage and the orientation of the fault plane. Harmonic tremor is a special type of seismic signal and is indicative of magma movement. Several key stations on Kīlauea and Mauna Loa are hooked up to an alarm system that is triggered when harmonic tremor is recorded.

Another major volcano monitoring program at HVO is geodesy, or the measurement of surface deformation. The surface of the ground around a volcano can be deformed by the change in volume of a magma body or by gravitational loading. HVO has a network of nearly a hundred spirit-level tilt stations on Hawai‘i, and changes in the slope of the volcanoes can be detected at these sites. There are also over a hundred GPS (Global Positioning System) locations on the island, where both horizontal and vertical changes in the ground surface can be measured. When people in lower Puna reported feeling a number of earthquakes in late October, the HVO tilt stations in the area were remeasured, and no significant tilt changes were detected. From these observations, we were able to conclude that magma was not intruding into the lower East Rift Zone of Kīlauea. An area where we do see movement is the south flank block of Kīlauea. GPS stations in the region had changes in horizontal position of up to four inches toward the sea during the last year.

Other monitoring programs at HVO include gas geochemistry, geophysics, and geology. The gas geochemistry group measures changes in the volume of sulphur dioxide emitted by the volcanoes and changes in the ratio of carbon dioxide to sulphur dioxide gases at fumaroles. Changes in gas volumes and ratios are caused by changes in the magma body from which these gases emanate. The geophysics program monitors changes in the gravitational, magnetic, and geoelectric fields of the Earth. These geophysical changes are caused by stress or mass changes related to magma bodies. The geophysics group also monitors the volume of lava flowing through the tube system from Pu‘u ‘O‘o to the coast. This is done by measuring the deflection of very low-frequency radio waves as they pass through the heated ground. In addition to preparing geologic hazard maps and studying volcanic processes, the geology group does the actual eruption monitoring, and in this column last week, they forecasted that the eastern lobe of the current flow near the coast would probably enter the ocean. The eastern flow started to enter the ocean on Tuesday of this week as the eruption continued to provide visitors with a great show at the end of the Chain of Craters Road.

All of the monitoring groups at HVO are supported by electronics, computer, library, and clerical staffers who keep the programs running. We join you in celebrating a safe and joyous holiday season.