Subject: Mapping magma chambers at Kilauea volcano

How can seismology be used to detect and map the presence of magma chambers beneath Kilauea Volcano?

    The Hawaiian Volcano Observatory operates an array of seismometers over the surface of Kilauea which allows them to locate earthquakes beneath and within the body of the volcano. Earthquakes define a conduit which rises from 60 km depth to about 5 km below the volcano's summit where there is a large diffuse area with only very small earthquakes. This diffuse zone is inferred to be the magma chamber. Earthquakes also define dike intrusion zones which extend down the southwest and east rifts of the volcano.
    With a lot of earthquakes and a lot of stations, it is possible also to determine the rough three-dimensional seismic velocity structure of the volcano. As you might expect, the feeder conduit, the magma chamber, and the axes of the rift zones all show lower velocities, indicating the presence of magma.
    Active seismology (setting off explosive charges and listening) could be used to refine our models of Kilauea's structure, but little such work has been done for two reasons. First, the area is environmentally sensitive (it is a National Park); second, the attenuation is so high that large charge size is needed to see more than about ten kilometers. Natural sources (earthquakes) are abundant and work almost as well.

Dr. Gerard Fryer
Hawaii Inst. of Geophysics & Planetology
University of Hawaii, Honolulu, HI 96822

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