# Volcano Watch — What's happening at Mauna Loa?

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Mauna Loa has gone 18.5 years without eruption--the second longest dry spell since detailed records begin in 1843. The longest period without eruption lasted 25 years, between 1950 and 1975. Clearly the past 52 years have been much less active than the previous 107.

Following its latest eruption, in March-April 1984, the volcano continued to inflate, quickly at first but then more slowly. By the early 1990s, the level of inflation had nearly reached that before 1984. This alone is an unreliable measure of the readiness for any volcano to erupt, because volcanoes change internally over time so that the basis of comparison also changes. The continued inflation, however, did make people wonder if the next eruption was to be sooner rather than later.

By 1994, however, the inflation had ceased, and the summit actually began to deflate. The rate of deflation was small but steady, continuing right up to May of this year. The pattern of slow deflation changed rather abruptly in mid-May. In fact, our best estimate is that the pattern changed on Mother's Day, the same day that Kilauea's currently active lava flow started.

Since then, the summit area of Mauna Loa has been slowly swelling and stretching. Distances across the summit caldera are lengthening at a rate of 5-6 cm (2-2.5 inches) per year. That means that, as of today, the caldera has widened about 2 cm (0.8 inches) since May 12. This is small stuff, indeed, but it does mark a noticeable, perhaps notable, change from the pattern of the preceding 9 years. The GPS measurements also show that the summit area is getting slightly higher, consistent with swelling.

These measurements are made with sophisticated GPS equipment that uses satellite orbits and signals to locate receivers on earth. This is acknowledged as the best means to track small changes in shape of the earth's surface. Could there be some error in this complicated method that we are overlooking, something that happened with the satellite orbits, for example?

To test that possibility, we have used a completely independent means to measure ground deformation-an old-fashioned, unsophisticated, non-electronic way to measure ground tilt. This method--called dry tilt in Hawaii and tilt-leveling, spirit-level tilt, or single-setup leveling elsewhere-uses standard surveying techniques to measure elevations of bench marks in a small area. If the elevations change from one survey to the next, the ground has tilted.

To make a long story short, the dry tilt measurements at the summit of Mauna Loa confirm the GPS results, though with less precision. The summit area is indeed swelling, slowly but measurably.

We then extended the measurement of existing GPS stations farther out on the flanks of the volcano to see if those parts of the volcano are also moving. The measurements show that the swelling is affecting more of the volcano than just the summit. In particular, the upper part of the southeast flank is showing outward movement.

You might think that this slow, slight swelling would be accompanied by increased seismicity. Well, that is not the case. Rocks bend before they break. That is an oversimplified way to say that slow swelling will likely not be accompanied by an increase in number or size of earthquakes.

Before the latest two eruptions, there were large increases in both numbers of earthquakes and the amount of energy released by these earthquakes. Though we must be cautious in saying that such an increase will definitely precede the next eruption, that is a reasonable expectation. On that basis alone, we see no reason to say that an eruption will take place any time soon-that is, in the next few weeks.

The small changes indicated by GPS measurements could have gone unseen in the past, when the instrumentation was less precise and the data were acquired infrequently rather than daily. There could have been several such spurts of swelling that we were unable to measure long before the 1975 and 1984 eruptions. And, such spurts may even be routine.

The overall story is a bit muddy because of what has happened since the 1984 eruption. The changed pattern--from swelling for the first 9 years to slight deflation for the next 9 years to very slight inflation now--is more difficult to interpret than one steady inflation.

With the help of Stanford University, HVO has already added one new GPS station on Mauna Loa and plans to install more GPS and electronic borehole tilt stations in the next few months. The seismic coverage is good and able to detect any increase in seismicity that might take place. We will report any significant changes as they take place via both the media and our web site (hvo.wr.usgs.gov); updates will be on the web site by the time you read this article.

### Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Puu Oo vent during the past week. Molten lava is flowing near the end of the Chain of Craters road, and the National Park Service is allowing visitors to get up close to the action where it is safe. The new ocean entry at Middle Highcastle between the older West Highcastle and Highcastle entries has developed a delta that measures 570 m (1,870 ft) along the coastline and extends 50 m (165 ft) beyond the old shoreline. We have received a disturbing report from a late night viewer of stupid people going beyond the boundary of the safe viewing area and on to the unstable bench of the active Wilipe`a ocean entry. Shortly after leaving the bench, the area collapsed into the sea!

No earthquakes were reported felt during the week ending on September 26.