Most of the events are associated with deep Benioff zone earthquake activity that effects the Pacific Northwest Region. The 1995 and 1996 events were shallow crustal events. Click Here for a drawing of the three different source zones for earthquake activity in the Pacific Northwest. Additionally, Portland has been damaged by earthquakes that occurred in the Puget Sound region, such as the 1949 magnitude 7.1 event near Olympia, WA, and the 1965 magnitude 6.5 event located between Seattle and Tacoma. Historical data and theory suggests that earthquakes only provoke other shocks within a limited area around the fault rupture.
Distant earthquakes have no direct effect on Washington and Oregon.
Earthquakes in California, Alaska and Japan are caused by the interaction of different plates than the earthquakes in the Pacific Northwest. However, the 1992 Landers earthquake in southern California caused an increase in tiny earthquakes in geothermal areas as far away as The Geysers in northern California. Yes, even modern bridges have sustained damage during earthquakes, leaving them unsafe for use. More rarely, some bridges have failed completely due to strong ground motion. Several collapsed in the Northridge earthquake in January 1994, even though they had been strengthened.