Volcano Watch — Kīlauea seismicity survey

The staff of the Hawaiian Volcano Observatory (HVO) joined an international team of 25 scientists and technicians from the United States, Japan, and Italy to make the most detailed seismic recordings on Kīlauea Volcano, using the largest total number of seismic instruments ever installed on a volcano. One hundred-sixteen portable seismographs were installed on January 11 in Kīlauea Caldera in Hawaii Volcanoes National Park as a joint Japan-United States research project to record volcanic earthquakes and tremor. On Friday, February 2, the last of these temporary seismographs were removed.

This project was initiated under a Japan/United States Science and Technology (JUST) agreement for natural hazards reduction or mitigation. In early 1993, the JUST panel on volcano hazards assembled a working group on volcano seismology and chose Hawaii, with its ongoing eruption of Kīlauea, for the first study employing such a large number of instruments and scientists.

The Japanese seismologists were from Tokyo, Nagoya, and Kyushu Universities; the American scientists came from the University of Alaska, the U. S. Geological Survey in Menlo Park, California, and HVO. A total of $1.3 million in seismographic equipment was shipped from Japan to the Big Island for the survey.

The equipment that was installed on Kīlauea included state-of-the-art digital data recorders, or data loggers, from Japan. These data recorders, about the size of a telephone book and powered by a small battery, were easily carried into position and installed to record up to three seismic data channels at locations throughout the crater and the surrounding volcano summit region. The data were recorded into computer flash memory and then retrieved by visiting the instrument sites with a notebook computer.

The principal goal for the data loggers was to record volcanic tremor and volcanic earthquakes. Like the sparser HVO permanent network, the portable stations were arranged to optimally determine the locations of earthquakes that occur beneath the summit caldera. A number of stations were also arranged into seismic antennas whose collected signals can be tuned to look at the passage of seismic waves across the network. Precise instrument positions were determined by Global Positioning System (GPS).

The network of portable data loggers was nested within the network of seismic stations which transmit their signals via FM radio for analysis and recording at HVO. Over the past few years, HVO has gradually upgraded its instrumentation and data transmission and acquisition schemes. With the 123 channels of seismic data presently transmitted to HVO, the data loggers brought the total number of seismic data channels recorded here in January to 389. Over 40 billion bytes (40 gigabytes) of data were collected during this time.

Over the following months and years, seismologists will analyze the data to learn about the causes of volcanic earthquakes and the nature of seismic wave propagation in volcanic settings. A wide range of volcanic seismicity was recorded while these instruments were in place. We were fortunate to be in place during the intense swarm of earthquakes and elevated levels of volcanic tremor in the summit region of Kīlauea during the volcanic alert on February 1. At other times, we recorded earthquakes originating from other parts of the Big Island as well as from Kīlauea, weaker episodes of volcanic tremor, and possibly even the nuclear test conducted in the western Pacific. We will be able to use the data from the data loggers to greatly enhance the studies that we normally conduct with data coming from the permanent HVO seismographic network.

The seismologists will apply a wide range of analytical and computational tools to their data, drawing from electrical engineering and signal processing, as well as from seismology.

Among the planned data analysis projects is the tomographic modeling of the seismic structure of the volcano. This can be viewed as taking X-rays of the volcano using the waves generated by earthquakes. With the precise station positions afforded by GPS, some of the seismologists on this team will study the effects of surface topography and buried geologic structures on seismic wave propagation and scattering. The results of these studies will be crucial to understanding volcanic earthquakes and volcanic tremor.

While many tools are already available, we will be able to take advantage of procedures developed in the future to analyze this unprecedented collection of data. Besides studying the data collected in this 1996 study, we are planning another seismic survey in Hawaii next year, followed by a similar survey in Japan. With these and similarly detailed seismologic investigations on volcanoes, we aim to build a greatly improved understanding of how volcanoes work and how volcanic disasters might be averted through improved monitoring practice.

The final day of this temporary deployment was the day of the large seismic swarm at Kīlauea's summit that we reported on last week. The high eruptive volume at Pu`u `O`o that began as the seismic swarm diminished about noon on February 1 continued until the night of February 4, when the eruption stopped. This activity provided a large variety of seismic signals that were recorded by the HVO seismic network and the temporary network.

This pause, like the preceding activity, was unusual in that harmonic tremor continued beneath the summit for most of this past week, although its amplitude has declined each day. In addition, the summit has been repressurizing very slowly. As the summit continues to repressurize and inflate, we expect the eruption to start up once again, probably within the next week. Because the tube has cooled down, it is unlikely that the lavawill reoccupy it for more than a short distance from Pu'u 'O'o. Most likely, new surface flows will break out of the tube and construct a new tube system to the coast.