South Sister jolted by tiny earthquakes

Probably no one but seismologists noticed it, but minor earthquakes returned to the South Sisters area in January and early February.

The tremors came after two months of no activity and nearly a year after a series of more than 100 minor quakes were recorded in March, 2004.

A single quake with a magnitude of 1.5 was recorded on January 11 at 3:59 p.m. PST and four much smaller ones were recorded on February 6 from 12:18 p.m. PST to 12:22 p.m. PST, ranging from 0.1 to 0.2 in magnitude. All quakes were recorded just west of the South Sister.

“The January 11 activity was the largest recording since November 8, 2004, but still a minor one,” 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. The network has monitored earthquake activity in the Cascade Mountains of Oregon and Washington for many years.

The Richter scale measures the magnitude of earthquakes using graded steps. Each step is about 10 times greater than the preceding step. That means the four smaller quakes in February were about 50 times lesser magnitude than the January one, Qamar explained.

The quakes were reported by seismometers placed near the South Sister by scientists with the U. S. Geological Survey (USGS). Instruments are located south of Broken Top, one near The Husband, one near The Wife, and one west of the South Sister.

Beginning in May 2001, USGS scientists detected a slight bulge just three miles west of the South Sister. The mountain 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.

Using new technology with data from the European Space Agency’s satellite to make radar images, scientists discovered an uplift of the ground covering an area of 9-by-12 miles. This uplift had occurred between 1996 and 2000 and measured about four inches, too small to be detected on the ground.

The bulge has averaged about an inch increase a year and has now uplifted about 10 inches.

Scientists are now reporting the estimated depth of each activity. The recent ones range from about four miles to eight miles below thesurface.

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 developed.

Technology to measure these small quakes has been available on the site only since 2001, Qamar explained. These very small quakes may have been happening for a long time but went undetected, he added.

Last year, the USGS reported that the cause of the uplift appeared to be the continuing intrusion of magma or molten rock. The magma appeared to be accumulating at a depth of about four miles below the ground surface.

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