In early April, 2008, over 600 ocean earthquakes were detected by hydrophones left over from Cold War submarine monitoring. The quakes occurred in the Pacific Ocean off the coast of Oregon, in a basin where the Juan de Fuca plate – a region of the earth’s crust – separates from the Pacific plate. In the Pacific Northwest, the Juan de Fuca plate subducts or moves under the North American plate (according to the United States Geological Survey).

At least 3 of the quakes were of substantial magnitude, over 5.0 on the Richter scale. Ocean earthquakes in rapid succession often indicate the impending eruption of a nearby volcano on the land, but in this case there was no nearby land volcano. There was, however, the Juan de Fuca Ridge, a submarine mountain chain consisting of developing volcanoes and erupting magma where the plates are moving apart. A volcanic eruption or a large scale earthquake in this area could create a sizable North American tsunami.

In fact, tsunamis associated with the Juan de Fuca plate have occurred as recently as three centuries ago. Researchers from the U.S. Geological Survey (USGS) discovered Native American ruins in the Pacific Northwest buried under tsunami sand (oceanic sand carried by the waters of the tsunami), dating this sand to the same time as a Japanese tsunami that hit Honshu in 1700.

Earthquakes are caused by the movement of portions of the earth’s crust (called plates) as they lie on top of the semi-liquid or molten magma beneath the surface. When two plates become “stuck” and tension builds, the sudden release of that tension produces a violent movement of the plates that we call an earthquake. The study of plates and their movement is called plate tectonics.

According to plate tectonics, there are three types of plate boundaries: divergent boundaries, convergent boundaries, and transform fault boundaries. A divergent boundary occurs when two plates move away from each other, as in the movements of the Pacific plate and the Juan de Fuca plate. However, the boundary between the North American plate and the Juan de Fuca plate is a convergent boundary – the two plates are moving towards each other. Convergence can have two effects: if an oceanic plate is involved, one plate will slide under another (subduct); if both are continental plates, the crust buckles and may be pushed upward or sideways. This collision is the cause of most non-volcanic mountain ranges. Since the Juan de Fuca plate is an oceanic plate, it is sliding under the North American plate and gradually becoming molten magma.

A transform fault boundary occurs when two plates slide against one another. This is commonly known as a fault line. Most transform faults are in the ocean, but some are on land. One of the best-known land faults is the San Andreas fault, where the Pacific plate (moving northwest) and the North American plate (moving southeast) grind against one another, causing periodic earthquakes. This fault line is the connection between the East Pacific Rise, a divergent boundary between Baja California and Mexico, and the South Gorda – Juan de Fuca – Explorer Ridge off the Pacific Northwest.

Although geologists have been able to determine ways to measure the effects of land and ocean earthquakes, volcanoes, and tsunamis, and can even detect signs that one of these events is imminent, geological prediction is still an inexact science at best. Anyone living near a fault line or volcano (even a dormant one), or on the Pacific coast, should be prepared to evacuate at any time in the event of a volcanic eruption or an approaching tsunami. Also, you should know the guidelines for safety in these potential disasters. Make sure you and your family are prepared!




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