"Nature Guide Journal"

8 March 2001

(Some of the information in this article is repeated from the article published 27 July 2000.)

Last week's earth-rumblings remind us again how active our planet is. Although we always seem surprised by an earthquake, earthquakes are actually very common in our region. The First People in the Pacific Northwest had myths and stories that included earthquakes; but since Euroamericans have been taking notes for a very short time here, it may seem like news.

The surface of our planet is made up of plates that jostle with one another as they travel slowly about, drifting on the currents of the molten rock beneath. The North American plate is moving west 1½ to 2 inches a year, overrunning the adjacent ocean plates. This is not a smooth journey; we experience small and medium sized earthquakes as the move.

More interestingly, our continental land mass tends to snag on the ocean plates as they grind underneath. Where the continent is caught, it bulges upward, rather as a table cloth bows up as you try to push it across a table. Our current rise is about 8 inches per century—a little faster than the rising sea level.

But the snag doesn't last long. Eventually, it gives way and slips, suddenly dropping the land and causing one heck of an earthquake. Geologic records show that these very large magnitude earthquakes occur in this region about every 300 to 600 years.

Geologic and human records indicate that the last such earthquake off the Oregon coast occurred on the night of January 26^th, 1700. That quake dropped some coastal areas several feet and generated a seismic sea wave ("tsunami") that scoured the shoreline and decimated coastal communities from here to as far away as Japan.

Today delicate instruments (seismographs) measure the size (amplitude) of the seismic land wave generated by earthquakes. Readings from several seismographs are usually used together to arrive at the amplitude of the wave at the quake's epicenter. That amplitude is generally expressed using the Richter Scale.

Developed in the 1930's by Californian Charles F. Richter, the Richter Scale employs a logarithmic progression. That is, each whole number step actually represents a ten-fold increase in seismic wave amplitude. With the Richter Scale, an earthquake of a magnitude 7 is ten times stronger than one of a magnitude 6: An earthquake of magnitude 8 is a hundred times stronger than one of a magnitude 6.

Using physical evidence of ground movement, the regional slip-and-drop earthquake of 1700 has been estimated to have been nearly a magnitude 9 on the Richter Scale—about a hundred times more powerful than the 6.8 quake that hit Puget Sound last week.

Eight or 9 on the Richter Scale is the estimated size of the other slip-and-drop earthquakes that have periodically struck our region. And it is the predicted size of our next "very large magnitude" quake.