Volcano Watch — What are Halema`uma`u's smoke and other signals telling us?

Though nearly all of the seismic data processing, analysis, and presentation at the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) have long been computer-based, we continue to operate drum seismographs that produce hard-copy records by pens or heated needles that write across papers wrapped around the drum.

12-hour seismic record from HVO station NPT, near Halemaumau crater, from 7:30AM to 7:30PM HST on August 20. Visible on the record are microearthquakes, volcanic tremor, and tremor bursts. Highlighted at the middle of the figure is the very long-period (VLP) tremor, preceded by a possible rockfall or small earthquake at 3:40PM HST, associated with visible changes in the plume issuing from the Halemaumau vent.

While increasingly difficult to maintain, these decades-old paper recording systems are kept operational and are still quite useful to our monitoring and analysis. They also give our lobby a "working volcano observatory/laboratory feeling."

It's 3:40 p.m., Wednesday, August 20, 2008: The pen on one of the HVO lobby drums that shows the seismic trace from a Kīlauea caldera seismic station begins to shake quickly and gives off a brushing or swishing sound as the pen writes its squiggly record?.

Earthquake! It appears to be about magnitude 3 at first glance. Looking at the other drums and a nearby bank of computer monitors, we deduce that the shaking originated south of Kīlauea's summit. A few minutes later, automatic earthquake location software locates the quake 8 km (5 miles) south of the summit.

Minutes later, an HVO staff member notes a dense, brown plume ascending into white clouds. The brown plume continues to issue from the 2008 Halemaumau vent for another several minutes, before slowly changing back to white. The white plume is principally steam; the brown plume contains more rock dust.

The array of cameras in HVO's observation tower and other instruments that we've trained on the vent are constantly tracking Halemaumau's activity. Kīlauea is fulfilling its role as the active laboratory volcano that Professor Jaggar envisioned nearly 100 years ago.

Along with yesterday's earthquake, we recorded what we've been calling VLP (very long period) tremor whose oscillations occur over periods of tens of seconds. Since March, with our increased attention on Halemaumau's continuing plume, we've been challenged by a diverse collection of seismic signatures whose patterns we're still trying to organize and understand.

This year's explosions and brown plume emissions from Halemaumau seem to correlate with VLP signals. We've also observed small earthquakes, possible rockfalls, and higher-frequency tremor bursts. A great deal of effort will be required to simply organize and catalog the activity and to understand the patterns of behavior and relationships among the diverse observations.

USGS colleagues Bernard Chouet and Phil Dawson have documented and analyzed VLP tremor signals from volcanoes around the world, from Italy to Japan. They have developed an impressive collection of analytical and modeling tools to apply to volcanic seismic signals and interpret the signals according to dynamic excitation of crack- and pipe-shaped elements in magma chambers and conduits. Our plan is to replicate their modeling environment at HVO and explore a wider range of data, in addition to the VLP signals.

Their analysis of the Halemaumau VLP record shows that it has been originating from rather consistent source locations, clustering at sea level, or roughly 1 km below the caldera floor, but also ranging over several hundred meters in depths, at the northeast edge of Halemaumau. This is a bit north of the vent, but it is beneath older eruptive fissures, where we have located VLP sources over the past decade. Among possible interpretations, the VLP seismic signals could represent the location where magma is feeding a nearly vertical pipe. Further improving any interpretations will require sorting through the source locations.

New location techniques are being developed and applied to the higher-frequency tremor bursts. These bursts appear to be clustered beneath the Halemaumau vent that opened on March 19, and they are considerably shallower - 100 to 200 m (330-650 ft) below the caldera floor - than the VLP source. While undoubtedly connected to the VLP excitation, these shallower tremor bursts will also need to be studied closely before specific mechanisms can be attached to our observations.

As for the brown plume episodes, including yesterday's, HVO's best thinking at this point suggests that they are triggered by rocks and debris falling on top of the degassing lava column beneath the vent. The VLP could signal pressure adjustments within the conduit as the falling material interacts with the degassing lava.

A quote from one of Dawson's emails can sum up much of our experience at HVO since March. "I am not able to fully convey the amazement that I feel each day as I watch this sequence unfold." A great deal of work and excitement lie ahead.

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Volcano Activity Update

Kīlauea Volcano continues to be active. A vent in Halemaumau Crater is erupting elevated amounts of sulfur dioxide gas and very small amounts of ash. Resulting high concentrations of sulfur dioxide in downwind air have closed the southern part of Kīlauea caldera and produced occasional air quality alerts in more distant areas, such as Pahala and communities adjacent to Hawai`i Volcanoes National Park, during kona wind periods. There have been several small ash-emission events, lasting only minutes, in the last week. These are preceded by small seismic events, and are probably caused by tiny rockfalls within the vent.

Puu Ōō continues to produce sulfur dioxide at even higher rates than the vent in Halemaumau Crater. Trade winds tend to pool these emissions along the West Hawaii coast, while Kona winds blow these emissions into communities to the north, such as Mountain View, Volcano, and Hilo. Incandescence continues to be observed at night inside Puu Ō`ō and suggests minor activity from vents within the crater.

Lava continues to erupt from fissure D of the July 21, 2007, eruption and flows toward the ocean through a well-established lava tube. No breakouts have been observed in the past week anywhere on the flow field. Lava continues to flow into the ocean at Waikupanaha, and over the last week has produced moderate littoral explosions.

Be aware that lava deltas could collapse at any time, potentially generating large explosions. This may be especially true during times of rapidly changing lava supply conditions, as have been seen lately. Do not venture onto the lava deltas. Even the intervening beaches are susceptible to large waves generated during delta collapse; avoid these beaches. In addition, steam plumes rising from ocean entries are highly acidic and laced with glass particles. Check Civil Defense Web site or call 961-8093 for viewing hours.

Mauna Loa is not erupting. No earthquakes were located beneath the summit this past week. Continuing extension between locations spanning the summit indicates slow inflation of the volcano.

One earthquake beneath Hawai`i Island was reported felt within the past week. A magnitude-2.0 earthquake occurred at 4:21 a.m., H.s.t., on Saturday, August 16, 2008, and was located 3 km (2 miles) south of Kīlauea summit, at a depth of 3 km (2 miles).

Visit our Web site for daily Kīlauea eruption updates, a summary of volcanic events over the past year, and nearly real-time Hawai`i earthquake information. Kīlauea daily update summaries are also available by phone at (808) 967-8862. Questions can be emailed to askHVO@usgs.gov.

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