Volcano Watch — Calibrating earthquake counts beneath Mauna Loa

At the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO), many types of measurements are taken to monitor our active volcanoes and to have a better understanding of their behavior. Among these is the use of seismometers to detect earth movements. HVO operates a network of about 55 station sites on Hawai`i Island. Three sites are in the immediate vicinity of Mauna Loa's summit caldera, Moku`aweoweo. Five other station sites are within a 16-km (10-mile) radius.

To keep our readers abreast of the activity beneath Mauna Loa, we began publishing the number of earthquakes located beneath the summit region each week at the end of each "Volcano Watch" article. With this number, we assign a relative level to the week's activity. Presently, any weekly total under 20 is considered low; 21-40, moderate; 41-60, moderately high; and over 60, high.

One might ask, "What kind of numbers can we expect to see before a Mauna Loa eruption?" We have two eruptions serving as benchmarks - the short-lived eruption on July 5, 1975, and the three-week-long eruption beginning March 25, 1984. Previous eruptions, the last occurring in 1950, predate HVO's modern seismographic network and recording capabilities.

In the year prior to the 1975 eruption, HVO operated just the three summit station sites in a 16-km (10-mile) radius of Moku`aweoweo. This resulted in a greater proportion of poorly located, and fewer small, earthquakes catalogued compared to those recorded today. From statistical analysis, the earthquake catalog for the Mauna Loa summit region for the period 1974-1975 appears to be largely complete down to about magnitude 2.2. Below this magnitude, the catalog contains far fewer earthquakes than would be expected.

Prior to the 1984 eruption, seven of the eight existing station sites within 16 km (10 miles) of Moku`aweoweo were operational. While the coverage greatly improved earthquake locations, the catalog's completeness improved only marginally. The many aftershocks following the magnitude-7.2 Kalapana earthquake in 1975 placed a huge burden on HVO's seismic analysts. To eliminate the backlog, our staff catalogued only the largest of the earthquakes for many years after November 29, 1975. As a result, the catalog can be considered complete only for earthquakes of about magnitude 2.1 and larger during the 1983-1984 period for the Mauna Loa summit region. The same completeness level was also estimated for the inactive period between 1976 and 1981.

After 1984, our cataloging became complete to progressively smaller earthquakes as modern digital analysis tools were introduced. During the past two years, it appears that nearly all earthquakes larger than magnitude 1.3 have been catalogued in the Mauna Loa summit region.

Using this information, the catalog can be adjusted to correct for differences in completeness over time. Then we can estimate how frequently weekly earthquake activity preceding the 1975 or 1984 eruptions would have been considered moderate or higher by today's scale.

During the quiet period immediately after the 1975 eruption to a year prior to the 1984 eruption, we would have reported moderate or moderately high seismicity for only 16 of 402 weeks (4 percent of the time). Equally low percentages apply to other quiet periods. In contrast, we would have reported moderate to high seismicity 40 percent of the time during the year preceding the March 1984 eruption, and 83 percent of the time during the year before the July 1975 eruption.

In late April of this year, a deep earthquake swarm preceded the onset of extension across Moku`aweoweo. We would have reported that 13 and 17 earthquakes were located beneath Mauna Loa for the weeks ending April 24 and May 1, respectively. While such activity would have prompted additional discussion, its level would still have been considered low.

From this discussion, we can conclude that an isolated week of moderate or moderately high seismicity should not be alarming. Besides earthquake counts, unusual characteristics of the earthquake activity, such as intense clustering, migration to progressively shallower depths, and the occurrence of earthquakes or tremor directly related to magma movement are also important signs of unrest. Expect to see such discussion in "Volcano Watch" articles should Mauna Loa begin to exhibit unusual seismic behavior.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. A large surface flow was visible on Pulama pali all week, though it was slackening by October 24. Only scattered small breakouts occurred on the coastal flat, becoming less abundant as the week progressed. Lava continues to enter the ocean from the Wilipe`a and West Highcastle lava deltas. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. Do not venture onto the lava deltas and benches.

There were no earthquakes reported felt throughout Hawai`i in the week ending on October 24.

Mauna Loa is not erupting. The summit region continues to inflate. Earthquake activity is low; only five earthquakes were located in the summit area during the last seven days.