# Volcano Watch — A Finer Look Inside Kīlauea

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In our "Volcano Watch" series and in our scientific reports and presentations, we present and discuss findings relating to volcanoes and earthquakes.

In our "Volcano Watch" series and in our scientific reports and presentations, we present and discuss findings relating to volcanoes and earthquakes. We always hope to arrive at definitive results showing what we have learned, but it is equally, if not more, productive to learn and identify what we do not know and what requires further observation and study. For example, it is important to know what the insides of the volcanoes look like, to understand how, what, where, and when they erupt. We have working models that we base our interpretations on, but these are always subject to refinement and revision.

Recently, between mid-November 1999 and late-June 2000 a team of seismologists from the University of Wisconsin (UW) at Madison conducted a field study of earthquakes beneath the seismically active south flank of Kīlauea. Using data collected from the Hawaiian Volcano Observatory (HVO) seismographic network with data collected from their instruments, the UW group is tackling several different scientific problems. A study such as this requires much cooperation and assistance. We are greatly indebted to the National Park Service and those private landowners for allowing access and operation of the seismic networks.

One goal of the project is to look for evidence and information related to the presence of magma within Kīlauea's east rift zone. We use the data derived from the combined HVO and Wisconsin networks in a procedure referred to as seismic tomography. Through a series of calculations, applying rules governing seismic wave propagation from earthquake sources to seismometers, we can map distributions of seismic wave speeds below the ground surface to get a 3-D picture of the insides of Kīlauea.

There have been a number of earlier applications of such techniques. The HVO network is a network for monitoring earthquake activity regionally, while the Wisconsin equipment was deployed at much finer station spacings and in areas, away from the Kīlauea summit caldera and rift zones, where our regional network offers less resolving power. Combining the networks, we are able to infer finer detail and focus on specific areas of interest with greater confidence than with either network alone.

Happily, the preliminary results from the recently acquired data are quite consistent with earlier results. The Hilina fault system, which lines the south coast of Hawai'i Island, is related to a large and relatively abrupt change in seismic wave speeds, extending to depths approaching 10 km below the ground surface. Hilina Pali, then, is part of a system that possibly extends to the base of the volcanic edifice.

Another very interesting feature in our 3-D picture of Kīlauea is a volume characterized by relatively low seismic wave speeds at a depth of 9 km below Kīlauea's east rift zone. In other words, waves passing through this part of Kīlauea slow down. Immediately overlying this feature are materials with higher wave speeds. Together, these possibly represent consolidated concentrations of the mineral, olivine, capping a deeper, lower velocity magma body in the rift zone. This deep magma body was postulated a number of years ago on the basis of patterns of ground surface deformation. It has been difficult until now to clearly resolve seismic expressions of it and, independently of the geodetically monitored surface deformation, to infer its shape and extent.

There are many aspects of data from this recently completed study that need to be resolved, and we need to subject these data to other seismic analysis techniques to see how far we can carry our interpretations. We have a great deal of work remaining to answer the questions raised by this recent look inside Kīlauea.

### Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Puu Oo and flowing through a network of tubes toward the coast near the eastern boundary of Hawaii Volcanoes National Park. Multiple areas of the flow field in the coastal flats are inflating and forming tumuli and pillow-like lobes. Lava is entering the ocean mainly at two locations: the coast near the site of the buried Wahaula heiau and 1.2 km (0.7 mi) to the west of Wahaula at Kamokuna. Several small entries, located to the east of Waha`ula, are weak and ephemeral. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

A resident of Hilo felt an earthquake at 10:28 a.m. on July 21. The magnitude-2.8 earthquake originated from the south flank of Kīlauea Volcano and was located 11 km (6.6 mi) southeast of Kīlauea summit at a depth of 6.1 km (3.7 mi).