Posted: Tuesday, September 25, 2007 12:00 am
Chances are you didn't notice when, from 13.8 miles below a spot about five miles northwest of Brooks, the earth shook Sunday night at 11:20 p.m. We are too far south, although people as far south as Salem and as far north as Portland felt the 3.6-magnitude earth "shiver."
What's mysterious, according to the U.S. Geological Survey, is that this quake occurred nowhere near a known fault line. But then, many of us may have a faulty understanding of what lies beneath the dirt under our feet. It's more like gelatin than bedrock; a combination of sand and gravel and boulders and some rock shelves that are still shifting against one another - sometimes noticeably.
For instance, we doubt too many people in the vicinity of Sisters or in Maupin felt the earthquakes recorded there last week by the Pacific Northwest Seismograph Network. It began Sept. 18 at 6:25 a.m. A 1.1-magnitude quake barely moved the ground 17 miles southwest of Sisters.
On Sept. 19, 20 and 24, three quakes - 1.2, 1.7 and 1.4, respectively, originated east south east of Maupin. They rumbled up from about 13 miles underground. We doubt even prairie dogs knew what happened.
In fact, you can check every day to see how many earthquakes shake our region of the world at the USGS Web site, http://earthquake.usgs.gov/eqcenter/recenteqsus/ and click on Oregon links. You also can view an archive of Northwest Oregon quakes at http://pasadena.wr.usgs.gov/shake/pnw/
archives.html.
We're not sure whether it will be more comforting, but we have many earthquakes that pass unnoticed. More disturbing is that it's clear just by the dots on the current world view of active earthquakes that there are so many in the Pacific Northwest that it's hard to see them all for the overlapping circles where the earth shook in recent days.
That may come as a surprise to our California friends, who have experienced more serious earthquakes in the past 18 years. But even considering the earthquake and fire that flattened San Francisco to rubble in 1906, ours is the region where the worst West Coast earthquake in 300 years was recorded in rock and wood.
According to USGS records, on Jan. 16, 1700, a magnitude-9 earthquake struck the land that's now British Columbia to northern California. It set off tsunamis that swamped Japan. It sank the coastal forests of Oregon near Tillamook.
Compared to that, we'll take the occasional 3.6 earth ripple - and keep our fingers crossed.