The northern part of the watershed is underlain by a network of faults capable of producing earthquakes at any time , and many small quakes have been recorded in the basin since the mid-19th century . In 1993 , the Scotts Mills earthquake — the largest recent earthquake in the valley , measuring 5.6 on the Richter scale — was centered near Scotts Mills , about south of Portland . It caused $ 30 million in damage , including harm to the Oregon State Capitol in Salem . Evidence suggests that massive quakes of 8 or more on the Richter scale have occurred historically in the Cascadia subduction zone off the Oregon coast , most recently in 1700 CE , and that others as strong as 9 on the Richter scale occur every 500 to 800 years . The basin 's high population density , its nearness to this subduction zone , and its loose soils , which tend to amplify shaking , make the Willamette Valley especially vulnerable to damage from strong earthquakes
Scotts Mills earthquake
1993 Scotts Mills earthquake
Willamette River