Volcano Watch — New instrumentation on Mauna Loa's southwest rift zone

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Sources/Usage: Public Domain. View Media Details

Their objective was not to see a bright red Mars or the brilliant trails of Perseids -- although they did see both -- but rather to install two electronic borehole tiltmeters. These instruments have been described in this column before, but a brief refresher certainly won't hurt.

Tiltmeters measure the tilt caused by the intrusion or pressurization of magma within a volcano. They are extremely sensitive instruments capable of discerning tilts of about 0.1 microradians (1 microradian equals 0.00006 degrees). Sliding a dime under the end of a bar 1 km (0.6 miles) tilts the bar 1 microradian. Potentially alarming volcanic tilts can be as small as half a microradian. Tilts of several hundred microradians have been measured at Kīlauea.

These tiltmeters must be very sensitive, but their sensitivity can be a nuisance. Heavy rain or snow, or even just the warming and cooling as the sun moves through the sky, can wreak havoc on a tiltmeter. The weight of a person standing next to a tiltmeter is sufficient to cause a measurable signal. To insulate the instruments from such effects, they are placed at the bottom of boreholes, typically drilled about 3-4 m (10-13 feet) deep.

Drilling a hole 10 cm (4 inches) across to a depth of 3 or 4 m (10 to 13 feet) through solid rock requires specialized equipment. The drill itself doesn't actually bore through the rock like an auger but rather cuts a circular ring around a central core that must then be extracted. A generator runs the drill and a powerful vacuum cleaner, which removes the detritus left in the hole after the core is removed. Taken together, all of these items weight hundreds of kilograms (pounds).

Indeed, by the time work was completed, more than 1,500 kg (3500 lbs) of gear and water (required in large quantities to lubricate and cool the drill) were ferried to and from the tiltmeter sites.

These two instruments bring the total number of tiltmeters monitoring Mauna Loa to 6 ? the two new instruments on the southwest rift zone, one right at the summit, one at the NOAA weather observatory, one at Hokukano Ranch in Kona, and one along the Mauna Loa Strip Road. HVO plans to install additional instruments next year, aimed at monitoring Mauna Loa's northeast rift zone, from which eruptions that threaten Hilo can emanate.

Also monitoring Mauna Loa are half a dozen continuous GPS receivers and about double that number of seismometers. This trio of instrumentation -- tiltmeters, GPS receivers, and seismometers -- gives geologists at HVO a comprehensive, though by no means complete, view of volcanic processes occurring at Mauna Loa. Each instrument radios data to HVO, so that we can watch the volcano in real time.

Tiltmeters are especially good at detecting fast, rapidly evolving magma movements. GPS receivers, on the other hand, excel at tracking slow, long-term processes that occur on the scale of years. Both of these instruments can detect magma movement that is not breaking rock and that, therefore, is not causing earthquakes. But, magma almost always has to break rock to reach the surface and erupt. This is when the earthquake-detecting seismometers become especially useful.

As most Hawai`i residents are well aware, Mauna Loa began to reawaken in May 2002. The gradual swelling has continued for more than a year. Last week's installation of the two tiltmeters is part of a long-term plan for better monitoring of Mauna Loa, a plan that has been accelerated significantly in the wake of Mauna Loa's stirrings. So far, the plan is far from complete, but it is on track and proceeding well. The success to date stems from the efforts of many people, not just those from HVO. For example, Yeehop Ranch permitted helicopter and vehicle access to its land. And, the National Park Service has gone out of its way to work with HVO scientists to site instruments at locations that are useful for monitoring, while remaining consistent with the wilderness and cultural goals of Hawai`i Volcanoes National Park.

Volcano Activity Update

Eruptive activity at the Pu`u `O`o vent of Kīlauea Volcano continued unabated during the past week. Surface activity is mainly visible in the westernmost section of the pali flow field. The Kohola arm of the Mother's Day flow has many breakouts on the coastal flat. The breakouts are distributed along a broad front one both slowly widening and traveling seaward. All breakouts are small and sluggish, but there are plenty of them. The east-side lobe of the main Mother's Day flow also remains visible as a series of incandescent patches from the top of Pulama pali out onto the gentle slope below. Lava stopped entering the ocean at the Highcastle delta, and there is no ocean entry with pillows being formed at this time.

Two earthquakes were felt in the Kapoho-`Opihikao area on the evening of August 18. The first, of magnitude 3.2, was at 7:51 p.m. and located about 4 km (2 miles) east of Pu`ulena Crater at a depth of about 4.5 km (2.8 miles). The second occurred only 26 minutes later, at 8:17 p.m., had a magnitude of 2.6, and came from the same location and depth as the first earthquake.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with 2earthquakes located in the summit area during the last seven days.