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Volcanowatch

December 15, 2004

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


New instruments on Mauna Loa detect changing inflation

In early December every year, thousands of earth scientists gather in San Francisco to present and discuss their latest findings. This meeting is organized by the American Geophysical Union (AGU). It has become the defacto forum for sharing early results of US volcanological research.

This year, the dominant topic for the volcanological community will be the ongoing eruption of Mount St. Helens. The lava dome offers a rare, close-up look at a very different kind of lava extrusion than we have here on Kilauea.

Despite the focus on Mount St. Helens, Hawai`i volcanoes will also get some attention. Asta Miklius, Stuart Koyanagi, and Maurice Sako of HVO along with HVO alum Peter Cervelli will present the latest data on the state of inflation of Mauna Loa.

As readers of Volcano Watch are aware, Mauna Loa has been inflating since May 2002. In addition, more than 1400 long-period (LP) earthquakes have been recorded deep beneath Mauna Loa's summit since July 2004. Both are signs of Mauna Loa unrest that we are watching very closely.

The AGU presentation will show Mauna Loa's recent changes in shape with unprecedented detail. Six new continuous GPS sites were installed on Mauna Loa this past year in order to watch these processes more closely. Those additional installations are now paying off, as they show that the inflation that started in 2002 has accelerated in the past five months.

The apparent increase is not due to inflation sources becoming shallower but rather to increases in pressurization. The best models so far show several sources for this inflation over the last two years, the shallowest at depths around 4-5 km (2.4-3 mi) beneath the summit.

Both the accelerated inflation and the deep LP earthquakes beneath Mauna Loa summit started in July 2004. The earthquakes are occurring at depths greater than 30 km (18 mi). The clearest pressure sources are much shallower at depths of around 4 km (2.4 mi). It is likely that these are related, but exactly what the connection is remains unclear.

We do have some inflation measurements preceding the 1975 and the 1984 eruptions with which to compare our current data. Even though these measurements are primitive by today's standards, they give us an idea of what we might see prior to the next Mauna Loa eruption.

Before GPS, inflation of Mauna Loa was monitored by repeat electronic distance measurements (EDM) between two points on opposite sides of Moku`aweoweo, the summit caldera. Measurements were made only a few times per year then. GPS measurements are made continuously now.

EDM measurements showed no discernible increase in inflation at Moku`aweoweo for the few years preceding the 1984 eruption. There was an accelerated change in the distance across Moku`aweoweo sometime during the year preceding the 1975 eruption. That accelerated rate, about 8 cm/yr (3.1 inches/yr), is significantly more than the average of about 3 cm/yr (1.2 inches/yr) we've observed since 2002 at the summit. The largest movements today are on the east and west flanks of Mauna Loa where there are no previous measurements.

There is no question that Mauna Loa will erupt again. After the last eruption in 1984, we knew that Mauna Loa was already moving toward the next eruption - that's what active volcanoes do. With the increased number and sensitivity of our instruments, we see these movements with ever increasing detail. The accelerated inflation in the last five months is another sign of magmatic processes operating within Mauna Loa.

The challenge for us at HVO is to figure out which of these signs is simply that of a volcano that is active but not erupting and which is indicative of an imminent eruption. We look back at the two previous eruptions of Mauna Loa for clues and see that both were preceded by months of elevated seismicity at moderate-to-shallow depths. We have not yet seen this.

Only one of the two previous eruptions was preceded by measurable, accelerated inflation. The rates of inflation observed then have not yet been reached.

Neither was preceded by deep LP earthquakes.

Therefore, we remain vigilant and will continue to scrutinize every symptom Mauna Loa displays. But we have not yet seen the particular ones we are looking for. Mauna Loa is showing more activity than at any time since the last eruption, but we do not yet see the signs that indicate to us that an eruption is imminent.

Activity update

Eruptive activity at Pu`u `O`o continues. The PKK flow continues to host scattered breakouts from near the top of Pulama pali to the coastal plain. Renewed supply to the coastline brought the ocean entry at Lae`apuki back to life on December 10. As of December 14, there were scattered surface flows on the coast plain, mainly in the interior of the PKK flow. The ocean entry is 3.2 km (2 mi) from the end of the pavement on Chain of Craters Road in Hawai`i Volcanoes National Park.

Expect a 2-hour walk each way and remember to bring lots of water. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs. The eruptive activity in the crater of Pu`u `O`o remains weak, with several spatter cones glowing but not doing much else.

During the week ending December 14, there was one earthquake felt island wide at 2:59 pm on December 12. The quake was a M3.8 located 6 km (4 mi) southeast of Kilauea summit at 32 km (20 mi) depth.

Mauna Loa is not erupting. The summit region continues to inflate. Since July 2004, the rate of inflation and number of deep earthquakes has increased. Weekly earthquake counts have varied from 5 to over 150. During the week ending December 14, 44 earthquakes were recorded beneath the summit area. This is a modest increase over the past week. Nearly all are 30 km (18 mi) or more deep and are the long-period type with magnitudes less than 3.

Visit our web site (hvo.wr.usgs.gov) for daily volcano updates and nearly real-time earthquake information.

1975
"That eruption was preceded by an increase in both shallow and intermediate-depth earthquakes, and by extension of EDM-survey lines across the caldera." Maximum rates of line-length change were MOLP-MLSP were 12.8-14.3 cm/yr from Aug83 to Jun84 across caldera.

1984
"Shallow earthquakes in the summit region continued unchanged in rate until a marked acceleration several weeks before the eruption. The number of larger earthquakes (M1.5) rose persistently as the time of eruption approached. The daily frequency of smaller earthquakes (less than M1.5) showed a more episodic increase and, in fact, decreased to below average on the day prior to the eruption."

"Continuously recording borehole tiltmeters and field surveys after this time showed that the inflation continued at a steady but nonincreasing rate almost to the 1984 outbreak and gave no cause for alarm." skip past bottom navigational bar


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Updated: December 21, 2004 (pnf)