Volcano Watch — Earthquakes rock uplifting area in Central Oregon Cascades

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A series of small earthquakes struck the Three Sisters volcanic center in the central Oregon Cascades on March 23-25. At the same time, staff members of the U.S. Geological Survey's Cascades Volcano Observatory (CVO) were meeting with public officials in Bend, Oregon, only 50 km (30 miles) away.

The meeting was organized to develop an emergency-coordination plan for future eruptions in central Oregon. The earthquakes were coincidental but not unrelated.

In May 2001, volcanologists announced the discovery of a broad uplifted area about 15 to 20 kilometers (9 to 12 miles) across west of South Sister volcano. The uplift was discovered using a satellite-based method (called InSAR for short) for measuring ups and downs of the ground. Between 1997 and 2000, the area rose as much as 10 cm (4 inches). Now the area is higher than it was in 1997 by as much as 25 cm (10 inches), and it is still rising.

Volcanologists conclude that the uplift is most likely caused by magma rising to a depth of about 7 km (4.3 miles) below the ground surface. This process is called intrusion. Water samples from springs in the uplifted area contain isotopes of carbon and helium that geochemists find are telltale signs for the intrusion of magma. The total volume of intruded magma is probably about 40 million cubic meters (50 million cubic yards). By comparison, Kīlauea volcano erupts about 120 million cubic meters (157 million cubic yards) each year.

The uplifted area is within the Three Sisters Wilderness Area, under jurisdiction of the Willamette and Deschutes National Forests. Since discovery of the uplift, CVO scientists have been working with representatives from the national forests to develop an effective, but low-impact, real-time monitoring capability using ground-based instruments in the wilderness area.

A seismometer and GPS station were installed near the center of uplift in 2001. The seismometer is intended to detect small earthquakes that volcanologists expect to occur if the area continues to rise.

The March earthquake swarm consisted of more than 300 small earthquakes, the largest of 1.9. This is the first such swarm to be detected in the central Oregon Cascades.

Unfortunately, the locations and depths of the earthquakes are poorly known. Of the two seismometers closest to the swarm area, one was not operating during the swarm, and signals from the other were intermittent, owing to an above-average snow pack in the Cascades this winter. Out of 350 earthquakes, 63 were located, and only 8 had precise locations. None had well-constrained depths.

Local, State, and Federal officials and agencies agreed to develop a coordination plan for the Central Oregon Cascades after the area of uplift was discovered in 2001 and then found to be continuing. The eruption of Mount St. Helens in 1980 is still fresh in the memory of many residents and officials in the Pacific Northwest. If magma were indeed rising, and the activity continued for years, perhaps the magma would eventually break the surface and erupt.

The most recent eruptions at Three Sisters occurred during two closely spaced eruptive sequences at South Sister volcano about 2,200-2,000 years ago and near North Sister volcano between 1,600 and 1,200 years ago. The eruptions varied from explosive activity that generated volcanic ash and pumice to lava flows similar to those displayed by the ongoing eruption of Kīlauea.

The earthquake swarm last March prompted scientists to install two additional seismic stations where they could find suitable snow-free ground. Two more seismometers and a second, continuously operating GPS station will be installed this summer, with permission from the Forest Service, to improve the ability of the small network to locate future earthquakes accurately.

The improved monitoring, and the new coordination plan, for the central Oregon Cascades means that volcanologists and the public should be prepared for additional activity, if it occurs.

Volcano Activity Update

Eruptive activity at Pu`u `O`o continues. Most lava flows are breaking out from the Mother's Day lava tube downslope from the rootless shield complex south of Pu`u `O`o. Lava has been visible all week high on the pali above the end of the Chain of Craters Road. No lava is near the ocean. No eruptive activity has taken place in the crater of Pu`u `O`o during the past week.

One earthquake was felt on the island, in Leilani Estates, during the week ending April 22. The magnitude-2 earthquake at 10:01 p.m. April 15 was located 2 km (1 mile) northwest of `Opihikao and had a very shallow depth, about 1 km (six tenths of a mile).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains very low, with no earthquakes located in the summit area during the past week.