# Volcano Watch — Looking into the volcano

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As volcano watchers in Hawaii, we are often rewarded with spectacular images of eruptions taking place in our own back yard.

As volcano watchers in Hawaii, we are often rewarded with spectacular images of eruptions taking place in our own back yard. As "professional" volcano watchers at the U.S. Geological Survey's Hawaiian Volcano Observatory, we are fortunate to be able to view the process in great detail. One of our main purposes, of course, is to make the observations that will help us piece together a picture of how our volcanoes work. This knowledge will improve our ability to forecast eruptions and to minimize loss of life and property both here and in the shadows of other active volcanoes.

Like much of the geosciences, most of our observations of the volcanic processes are restricted to the surface of the Earth where we either see the activity or make our instrumental measurements. An important part of the puzzle, however, is where the magma originates and accumulates prior to erupting. This takes place deep beneath our feet, well out of accessible viewing range. It is often difficult to make these observations and determine what lies within.

Much of our instrumental program is dedicated to geodetic and seismographic monitoring. With the information derived from our networks, we try to determine the locations and extent of faulting sources: earthquakes or magma bodies. Tracking this activity will help improve our mechanical models of the volcanoes.

Other important information is contained in the signals produced by the thousands of earthquakes that we record each year. The waves generated by the earthquakes and recorded at the stations in our seismographic network follow well-determined paths. The properties of Earth at each point along a path affect how the wave passes through most directly in time from the earthquake to the station.

We have recently begun to apply a technique call seismic travel-time tomography to view the insides of Kīlauea and Mauna Loa. This technique is like taking X-rays of the volcano. By measuring the time it takes for the waves to travel from earthquake to station, we are able to see how seismic wave propagation speeds are distributed within the volcanoes,.

Our tomographic images show large magma bodies beneath the summits and rift zones of Kīlauea and Mauna Loa. These parts of the volcanoes appear as “fast” features, and they are interpreted as olivine-rich rock mass accumulated as part of the repeated intrusive and eruptive activity in these regions. The magma chamber beneath Kīlauea's summit caldera is also capped with "fast" material, and, as in our catalog of earthquake locations, there appear to be two active regions beneath Kīlauea summit. Elsewhere, the Kaoiki fault zone between Kīlauea and Mauna Loa summits and the Hilina fault system lining the Kau-Puna coastline both appear to extend deep into the volcanic edifice. These fault systems possibly cut through to the entire volcanic flank and meet the ancient ocean bottom.

These results will help focus other monitoring efforts and add detail to our theoretical models of volcanic processes. Together with topical studies like the National Science Foundation-sponsored University of Hawaii research cruise to investigate possible submarine landslide structures offshore or the NSF Hilo deep drill hole, we continue to try to improve our views of how volcanoes work.

### Volcano Activity Update

Kīlauea's east rift zone eruption has nearly recovered from the surge of magma that occurred on January 14. Puu Oo erupts steadily, but lava there is confined to the crater. The tube system has been transporting lava since Tuesday, January 20. Steam plumes at the coast were intermittently active for a few days but resumed continuous discharge on Tuesday evening. Numerous breakouts of fresh lava from weak points along the tube, including the area between the ocean and the major coastal pali, an area accessible to tourists. Caution is advised owing to the high hazard near these flows and at the benches where lava enters the sea.

There were no earthquakes reported felt during the past week.