The average interval of time between occurrences of such earthquakes in the Cascade Mountains is uncertain because they have occurred infrequently. However, the 1872 North Cascade earthquake and earthquake activity in the southern Cascades are reminders that Puget Sound is not the only region in Washington having significant earthquake hazards. A zone of deep earthquakes near the probable boundary between the Juan de Fuca plate and the North America plate . 6.5 Seattle-Tacoma earthquake occurred within this deep zone. In sum, The subduction of the Juan de Fuca plate beneath the North America plate is believed to directly or indirectly cause most of the earthquakes and young geologic features in Washington and Oregon.
Earthquakes occur when rocks break suddenly in response to various geologic forces.
Magma moving in the Earth's crust may create sufficient force to produce volcanic earthquakes. More common, however, are tectonic earthquakes, which periodically strike parts of Oregon. These earthquakes, the result of fault movements driven by regional crustal stresses, typically have no direct connection to magma movement. Tectonic earthquakes occur periodically in south-central and southeast Oregon , and they are capable of exceeding the magnitude of volcanic earthquakes. Aftershocks as large as magnitude 5.1 continued to disturb residents for as much as six months.