Volcano Watch — Seismology at the Hawaiian Volcano Observatory

Seismology, the science of earthquakes and the mechanical properties of the Earth, is the primary method used to monitor the volcanoes of Hawaii and elsewhere. Seismographs were first installed in Hawaii in 1912, and the seismographic network operated by the Hawaiian Volcano Observatory (HVO) one of the oldest continuously running seismographic networks in the world.

Instrumentation and equipment developed at HVO by Jerry Eaton in the late 1950s and early 1960s led to the building (by Eaton and others) of the much larger regional seismographic networks in California under the U.S. Geological Survey's Earthquake Hazards Reduction Program.

Over many years of seismic monitoring, through the frequent eruptions and constant changes occurring within and on Kīlauea and Mauna Loa, HVO has recorded a range of earthquake behavior. Our observations have led to the identification of routinely active earthquake zones beneath Hawaii.

Earthquake locations, also called hypocenters, are determined by precisely measuring and analyzing the arrival times of seismic waves that are registered on seismographs around the island. Among the many thousands of earthquakes that are recorded each year, we are able to precisely locate about 10,000 or more.

In addition, we recognize different earthquake families or types based on the frequency-content, or shape, of the seismic signals. Along with important geologic and geodetic findings, the patterns of earthquake locations and earthquake types allow us to assess the levels of volcanic unrest. In Hawaii and elsewhere, the earthquake indicators reflect changes beneath a volcano, often long before an eruption or a geodetically measurable inflation or deflation of a magma storage chamber.

The present center of Hawaiian volcanic activity is Kīlauea. Beneath the summit caldera near Hawaii Volcanoes National Park headquarters lies Kīlauea's principal shallow magma storage chamber. The existence and extent of this storage chamber was indicated by the earthquake distribution—or, more precisely, by the absence of earthquake hypocenters—between 2 and 6 km beneath the caldera floor. Its existence has since been further highlighted by detailed studies of the way in which seismic waves travel beneath the caldera. This implied chamber coincides with a region where the seismic waves pass more slowly than they do through the surrounding crustal layers.

Depending on the nature of the volcanic activity, different types of volcanic earthquakes are recorded beneath the caldera. The sustained eruption of Pu`u 'O`o in the east rift zone has provided HVO with an unprecedented opportunity to improve our earthquake classification procedures.

Since the start of the eruption in 1983, we have identified four distinct families of earthquakes that occur in the Kīlauea summit region. The different earthquake types are easily recognized by their appearance on the visible, or analog, recording systems, such as the seismograph drum recorders on display at the Thomas A. Jaggar Museum at Hawaii Volcanoes National Park.

Our detailed computer procedures allow us to further sort the earthquakes of different appearance into different earthquake source zones, defined according to hypocentral depth below the caldera floor.

When magma moves rapidly into the summit magma chamber or from place to place within other parts of the volcano, swarms of short-period earthquakes are recorded. These earthquakes are characterized by sharp arrivals of the seismic waves, which clearly decay after the earthquake. The surrounding rocks break to make room for the volume of magma moving within the volcano.

By tracking the intrusive swarms preceding the current eruption, which began in January 1983, HVO scientists were able to go to the east rift zone to see the beginning of the eruption.

More recently, blessed by the relatively steady activity, with lava flowing into the ocean, we are more routinely able to record two other types of Kīlauea caldera earthquakes, both of which are long-period, or harmonic, earthquakes. These earthquakes involve the movement of a mixed gas and liquid magma through a network of cracks and fissures. Unlike the short-period earthquakes, they usually begin with gradual, or emergent, seismic wave arrivals with resonance, or driven oscillation.

Long-period earthquakes occurring deeper than 5 km—designated at HVO as LPC-C—are generally associated with magma gradually entering the shallow summit storage complex. Long-period earthquakes at shallower depths—designated at HVO as LPC-A—have been associated with withdrawl of magma from the summit region.

In late August, we began to register increased numbers of the deeper long-period earthquakes. On September 5, nearly 150 of these LPC-C events were recorded, a number significantly greater than the earlier average of less than 10 per day. The peak of this activity occurred on October 6, with 434 LPC-C earthquakes. Since late August, we have averaged more than 70 LPC-C earthquakes per day. During this same period, the average number of LPC-A earthquakes has remained relatively steady at approximately 40-50 per day.

Could there be a slightly increased rate of magma supply into the summit caldera of Kīlauea? The changes are not yet large enough to view, either as strong geodetically measured changes or as clearly established increases in lava flow activity.

Although we have come a long way in our geologic, geodetic, and seismologic monitoring practices towards recognizing imminent changes beneath Kīlauea and Mauna Loa, a great deal of research and funding support are needed to establish a truly comprehensive volcano monitoring program. While we have been observing Kīlauea and Mauna Loa for many years, we have only begun to apply modern analytic tools and techniques to the study of the deeper, inner processes of the Hawaiian volcanoes.