5.6: Earthquakes in the Crust that are Closer to Home

• 5.6.1: Introduction

  We have heard the bad news regarding the next great subduction-zone earthquake, but there is one small bit of good news. Based on the edge of the ETS zone, west of which the subduction zone is currently locked, the epicenter is likely to be offshore or close to the coast: more than 50 miles from Portland and a bit farther from Vancouver. This means that due to attenuation, seismic waves will be smaller when they reach the major population centers than when they strike the coast.

• 5.6.2: Drowned Forests, Raised Shorelines, and the Seattle Fault

  When the level of Lake Washington was lowered in 1916 to accommodate the Lake Washington Ship Canal, boaters noticed something strange beneath the surface of the lake. Dead trees! In a growth position, underwater, like silent phantoms. In 1919, more than 175 of them, primarily Douglas fir, were removed as navigational hazards. But there were still enough of them left that in 1991 salvage logging was attempted, using a barge and crane to raise the tree trunks from the floor of the lake.

• 5.6.3: Other Active Faults in the Puget Sound Region

  The high crustal seismicity of the Puget Sound region is a clue that there should be additional active faults. The search for these faults faces two problems. First, much of the region is covered by dense forest and underbrush so that from the air, one cannot see small landforms like fault scarps as one could in semi-arid eastern Washington. Second, the region was buried by glacial ice similar to that found today in Greenland as recently as 14,000 years ago.

• 5.6.4: Earthquakes and Cascade Volcanoes

  The reawakening of Mt. St. Helens began on March 20, 1980, with an earthquake of magnitude 4.2 followed by a crescendo of earthquakes that rose to a peak on March 27, then decreased in number as the time of the climactic eruption approached. Many of these earthquakes were due to the passage of magma far beneath the surface and not the rupture of faults. (A comparison would be a growling stomach versus a stick breaking.)

• 5.6.5: The Portland Hills Fault

  Sixteen million years ago, great floods of basaltic lava issued from crustal fractures in easternmost Washington and Oregon and western Idaho and poured across the Columbia Plateau in a broad front, hemmed in only by the Cascades, through which the lava burst in a fiery flood more than twenty miles across to enter the northern Willamette Valley and finally to flow into the sea.

• 5.6.6: Earthquakes at the End of the Oregon Trail (Willamette Valley)

  Fifty million years ago, northwest Oregon was a low coastal plain, with the shoreline close to the western edge of the present Willamette Valley, extending northwestward toward Astoria into what would one day become the Coast Range. East of the shoreline, rivers deposited clean sand, and in their floodplains were broad swamps and marshes, like the tropical Pacific coast of Guatemala today.

• 5.6.7: Southwest British Columbia

  Northern Vancouver Island just doesn’t seem like the Earthquake Country. The highway north of Victoria runs past small towns along the east coast of the island; it is lined with firs, with breathtaking views of the Georgia Strait, the Gulf Islands, and on a clear day, the snowy peaks of the Coast Mountains. The road passes through Courtenay to Campbell River, past fishing villages and logging camps.

• 5.6.8: Eastern Washington and Northeastern Oregon

  John McBride and his partner, Jack Ingram, were in trouble with the law. Things had started out well; they had set up the first trading post in Wenatchee. But they were caught selling liquor to the Indians, and this got them arrested in Yakima. This resulted in them selling the trading post and began living in a cabin west of the Columbia River. In the early morning hours of December 15, 1872, they were awakened by a loud noise and, soon, they realized that they were experiencing an earthquake.

• 5.6.9: The Pasco Basin- Nuclear Wastes and Earthquakes

  The military aircraft droned over the bleak December landscape of eastern Washington, and its lone passenger took note of what he saw through the window. As he gazed down at the sagebrush-covered Hanford Reach, with the broad ribbon of the Columbia River curving away in the distance, Lt. Col. Franklin Matthias knew that he had the site he wanted: raw desert, virtually unpopulated, but with a dependable water source, the Columbia River, close at hand.

• 5.6.10: Basin and Range (The Klamath Falls Earthquakes of 1993)

  Vacations in their native Oregon were a tradition with Ken and Phyllis Campbell. They came at a time when they could avoid the hottest part of the summer at their home in Phoenix, Arizona. Their 1993 excursion had been a grand trip, visiting old high-school friends and taking a cruise ship up the Inside Passage to Alaska. But it was getting late, and Phyllis was anxious to reach their destination, a bed and breakfast in Klamath Falls, a city where she had gone to first grade.

• 5.6.11: Pacific Coast and Offshore

  The Northwest coastline is struck on occasion by winter storms of great ferocity, among the most violent in the world. The ocean waves that crash against the rocky headlands are agents of geologic change. They grind down rocky platforms and tide pools and eat into the base of the sea cliffs, occasionally causing beachfront homes and condos built on top of the cliffs to topple into the sea. The boundary between the rocky platform and the sea cliff is called the shoreline angle.

• 5.6.12: Summary

  n estimating the seismic hazard from crustal earthquakes, we study three lines of evidence: geology, seismology, and geodetic evidence using GPS. In the Puget Sound region, we have all three: Holocene active faults and folds, high instrumental seismicity, and GPS evidence of shortening. In northern California, we also have geological and seismological evidence of earthquake hazard, including damaging historical earthquakes that have caused fatalities.