Earthquakes don't always equal eruption
The sight of stately Mauna Loa rising over 3,960 m (13,000 feet) above sea level is a familiar and welcome view to Hawai`i Island residents and visitors. The massive mountain is also a big celebrity in the world of volcanoes. As the largest and one of the most active volcanoes on earth, it has recently been featured on the front page of local newspapers, on TV news, and in the international press. As with much celebrity coverage, exaggeration has ruled the day.
The recent attention was spurred by an increase in the number of deep, long-period earthquakes recorded beneath the summit area since early July 2004. These small earthquakes could signify magma movement deep beneath Mauna Loa but do not indicate that an eruption is imminent. Slow inflation of the volcano has also been observed since May 2002, after nearly ten years of deflation. The rate of inflation has waxed and waned but has been fairly steady during 2004. The inflation could indicate swelling of the magma reservoir within the volcano, but again, these small changes do not signal an imminent eruption.
The most recent eruption of Mauna Loa occurred in 1984 and lasted for just over three weeks. After less than one day of summit activity, the eruption migrated down the northeast rift zone of the volcano to around the 2,850 m-elevation (9,350 ft), eventually sending flows to within 6.5 km (4 miles) of Hilo and 3.2 km (2 miles) of the Kulani correctional facility. Increases in seismic activity preceded the 1984 eruption, including a swarm of intermediate-depth earthquakes and a gradual increase in the number of shallow earthquakes as the time of eruption approached. There is no guarantee that the same signs and symptoms will herald the next eruption, but it is very likely that shallow, more intense earthquake activity, as well as increased ground deformation, will occur as the volcano draws closer to an eruption. None of this is currently happening.
In addition to the gradual changes in deformation and seismic activity, a change in gas signature was detected four months prior to the 1984 eruption. A sensor recorded increased emissions of magmatic gases from Moku`aweoweo, the summit caldera, until the monitoring site was buried by lava. Since trapped gas drives magma to the earthâ€™s surface, changes in gas emissions are often observed preceding an eruption. As magma and the gas it contains move toward the surface of the earth, the pressure decreases and gas can escape — much as the gas in your soda escapes when you decrease the pressure by opening the can. No change in gas output was noted in recent measurements above Mauna Loaâ€™s summit.
As many island residents can attest, the gases released from Kilauea contribute to decreased visibility, burned agricultural crops, and irritation of the eyes and respiratory systems of exposed individuals. The large amount of gas that would likely result from a future Mauna Loa eruption might be of concern to individuals affected by volcanic pollution. However, Mauna Loaâ€™s great height makes it likely that gases released from the upper slopes and summit of the volcano will be well above the trade-wind inversion. This layer of warm air, which persistently overlies cooler air, is at an average altitude of around 2 km (6,600 feet). That would keep volcanic emissions above population centers.
In contrast, Kilauea emissions are released from the summit caldera, at an elevation of 1100 m (3,600 feet), and from the east rift zone, at around 800 m (2,800 feet). Both of these sources are well below the temperature inversion, which is also where human activity is concentrated.
An eruption of Mauna Loa does not appear imminent. The Hawaiian Volcano Observatory continues to look closely at any signs and symptoms of restlessness. Those interested can follow daily Mauna Loa updates at our website.
Eruptive activity at Pu`u `O`o continues at a weak level. Lava is visible at times in the Banana flow, which breaks out of activity is lessening, however. Small breakouts are active along the PKK flow east of the Banana flow, but they are not visible from the Chain of Craters Road. The eruptive activity in Pu`u `O`oâ€™s crater is weak, with sporadic minor spattering.
Three shallow earthquakes were reported felt on the island during the week ending September 8.All three took place between 4 and 5 a.m. on September 11 in lower Puna and were felt in Leilani Estates, Pahoa, and near Puna Geothermal Venture. The first, at 4:12 a.m., was located 4 km (3 miles) south of Kapoho at a depth of 1 km (0.5 miles) and had a magnitude of 2.0. The second, of magnitude 2.8, took place 30 minutes later and was centered 2 km (1 mile) west-northwest of Pu`ulena Crater at a depth of 3 km (2 miles). The third felt earthquake had a magnitude of 2.0 and was located 1 km (0.5 miles) west of Pu`ulena Crater at a depth of 2 km (1 mile).
Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity was notably high for the 8th week in a row, with 154 small earthquakes recorded in the summit area. Nearly all of the earthquakes of this ongoing activity are of long-period type and deep, 40 km (23 miles) or more.
This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by HawaiiNews.com with permission.
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