Earthquakes swarm on South Sister

South Sisters and the last volcanic activity of about 1,000 years ago. photo by Jim Anderson

At approximately 10 a.m. on Tuesday, March 23, while Willie Scott, geologist with US Geological Survey (USGS) was giving a talk in Bend on the South Sister "bulge," a swarm of small earthquakes began shaking things up about four miles beneath the dormant South Sister volcano.

USGS scientists were quick to say that the activity posed no immediate threat to the public.

Larry Chitwood, geologist with the Deschutes National Forest (DNF) headquarters in Bend was so impressed with Scott's timing that he wrote him a note: "We're impressed; come to town more often!"

Later that same day, the regional seismic network had detected approximately 100 earthquakes ranging in magnitude up to about 1.5. Quakes of this extent are small by geological standards, and generally not felt by humans.

The rate of earthquakes peaked late Tuesday and appeared to be declining slowly by Wednesday, and then shut down at 1:30 a.m., Thursday, March 25.

According to USGS geologists, the earthquakes occurred about three miles west of South Sister volcano, under what is now known as "The Bulge."

In late 1997, European Space Agency's (ESA) satellite equipment detected a four-inch bulge, which has now uplifted to about 10 inches.

When asked about the six-inch growth of "The Bulge" Chitwood said, "According to what my geologist friends in Alaska tell me about the bulges in the Aleutian Chain, we shouldn't see any (volcanic) activity until the bulge near South Sister gets to be about eye-ball level."

On the basis of immediate studies of quake activity and additional geological evidence, scientists at the USGS center in Vancouver, Washington, infer that the cause of the uplift is the continuing intrusion of a modest volume of magma (molten rock) under the Three Sisters volcanic highlands.

Study of the gases found in the springs at Separation Creek Meadow near Collier Cone (six miles north of South Sister), indicate that the magma chamber beneath the bulge is in all probability basalt, not explosive rhyolite or andesite, the stuff that sent Mt. Helen's into a massive eruption.

The magma chamber appears to be accumulating at a depth of about four miles below the ground surface and further investigation now suggests there are about 50 million cubic yards in volume slowly rising toward the surface.

"Fifty million cubic yards and growing..." amounts to a volume similar to a lake one mile in diameter, 55 feet deep and getting nine feet deeper each year.

USGS scientists believe the processes that have been causing the uplift over the past six years could eventually lead to shallower intrusion of magma or even to a volcanic eruption; however, both are unlikely without significantly more intense preliminary earthquake activity. Scott pointed out that an eminent eruption would be heralded by thousands of small, harmonic quakes that would get everyone's attention.

An eruption of basalt would be the best possible scenario and would cause little damage to life and personal property. Collier Cone and Lava Butte are good examples of a possible eruption scenario.

Chitwood said an eruption of basalt might be very much like those in the Hawaiian volcanic hot spots where fire fountains and floods of pah-hoe-hoe lava quietly flows over the landscape.

"Yes," he said, "we might be able to sit on the north side of Mt. Bachelor in our lawn chairs and watch it go." He then added, "Even if an eruption should go to a cinder cone the best we could look for in ash fall would be about a mile downwind from the vent."

USGS geologists feel the most likely cause of the earthquakes is small amounts of slippage and cracking on faults as the Earth's crust adjusts to the slow ground deformation of the past seven years. This further suggests that heat and gases related to the magma intrusion have probably caused increases in fluid pressure deep underground, which also helps to trigger minor faulting events.

As of last week, USGS geologists were not absolutely sure of the exact location of the earthquakes due to the heavy snow pack of this winter that has caused problems to the existing seismographs located near the "The Bulge."

To make certain any future earthquake activity will be reported accurately, USGS scientists are deploying another seismometer in the immediate area. With the assistance of the Willamette and Deschutes National Forests, additional fieldwork over the next few weeks will fix problems with field instruments damaged by the heavy winter snow-pack.

A plan is being developed called "Cooperative Effort Among Government Agencies in the Event of Volcanic Unrest in the Central Cascades, Oregon" designed to deal with all possible incidents should "The Bulge" evolve into a real volcanic eruption.