Growth of South Sister bulge slows down in recent months

The bulge on the volcanic South Sister is still growing — but not nearly as quickly as it was just a couple of years ago.

Geologists are reporting that the rate of growth of a bulge on the west slope of the South Sister has slowed down to about half of its former annual rate of uplift. Instruments monitoring the activity on the ground show that the bulge now is only about a half-inch a year.

Geologists at the Cascades Volcano Observatory in Vancouver, Washington, have been monitoring the bulge since 2001 when measurements taken by satellite showed that a large area west of the mountain had risen four inches in the past four years.

Scientists emphasize that this level of activity poses no immediate threat to the public. More severe activity over a period of time would be likely before any major problems would be created.

“Technology to measure these small quakes has been available on the site only since 2001,” said Anthony Qamar, research associate professor at the University of Washington and Washington State Seismologist.

Qamar also is a principal investigator with the Pacific Northwest Seismograph Network at Seattle. These very small quakes may have been happening for a long time undetected, he added.

Over a dozen minor earthquakes were detected in 2004, created, geologists believe, by magma about four or five miles below ground slowly being forced to the surface. However, this year, only seven minor earthquakes have been recorded, the last a 0.6 magnitude on October 4. A single quake with a magnitude of 1.5 was recorded on January 11, four much smaller ones were recorded on February 6 ranging from 0.1 to 0.2 in magnitude, and a 0.2 quake was detected on March 30. All quakes were recorded just west of the South Sister.

Geologists have determined that the 10-mile affected area of the bulge extends from the McKenzie Pass on the north to the Cascade Lakes Highway to the south. The South Sister is located about 15 miles southwest of Sisters, 22 miles west of Bend, and 60 miles east of Eugene.

It rises to an elevation of 10,358 feet, Oregon’s third highest mountain behind Mt. Hood and Mt. Jefferson.

The network has monitored earthquake activity in the Cascade Mountains for many years, from Mount Baker in northern Washington to Crater Lake in southern Oregon. The U.S. Geological Survey, the Department of Energy, and the State of Washington fund the network.

The Richter scale measures the magnitude of earthquakes using graded steps. Each step is about ten times greater than the preceding step.

That means the four smaller quakes in February were about 50 times less ermagnitude than the January one, Qamar explained.

Quakes are reported by seismometers placed near the South Sister by scientists with the U. S. Geological Survey (USGS).

An instrument is located south of Broken Top, one near The Husband, one near The Wife, and one west of the South Sister.

A Web site provides information on Pacific Northwest earthquake activity and hazards.

View the latest information on earthquakes in the South Sister area at http://www.pnsn.org/SISTERS/sistersrec.