Tectonic subduction and what it means for you.

I will preface this post with a very brief overview of a fascinating geological process. Lets start with a cross section of the earth.

Beneath our feet, as most of us know, are several large layers of rock, magma, and core materials. (see picture above)...

Our planet, on in its surface, is broken up into large pieces called tectonic plates. Over the course of millions of years, these plates have shifted around to create the continents as we now currently know them.


These processes still continue today, and for the most part are a large cause of many of the earthquakes and volcanoes that we experience today. Along the boundaries of these plates there are different movements that they make. At some locations, plates collide, at others they get pushed apart, and at others they grind past each other. The collisional boundary is called a subduction zone when one plate gets pushed beneath another and eventually is forced back into the mantle and is re-melted. The boundary where the plates get pushed apart is called a divergent plate boundary (and where this occurs in the ocean, it called a mid-ocean ridge).
Here is a diagram explaining these processes in a greater detail.

Hopefully that made sense. Lets take that bit of knowledge and apply it to the really cool part.

Off the coast of Washington, Oregon, and Northern California is a small tectonic plate called the Juan De Fuca Plate. This plate has been colliding with the North American Plate for the past several millions of years (however, it used to be called the Farralon plate). As the plate is getting thrust down into the earth, it goes deeper into the mantle. Obviously this heats it up quite a bit and re-melts some of the crust. That melted crust is then forced upward and becomes the volcanoes that we have in that part of the country.

I like this diagram a lot better...

That is why there are all of the volcanoes in the Pacific Northwest. The magma being erupted is re-melted crust that was initially made out at sea. Isn't it amazing how mother nature recycles herself???

Ok, so essentially you have an area that makes oceanic crust (a mid ocean ridge) and another that consumes it (a subduction zone). I wonder what would happen if the two were to collide? The answer to that is rather simple arithmetic. 1 + (-1) = 0. Such an event DID happen, and that is why we have the San Andreas Fault.

Read on...
"EVOLUTION OF THE SAN ANDREAS FAULT.

This series of block diagrams shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault from 30 million years ago to the present. Starting at 30 million years ago, the westward- moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the West Coast. (Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.) The change in plate configuration as the North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendicino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendicino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca Plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between Baja California (BC) and Manzanillo, Mexico (MZ). Evidence of the migration of the Mendicino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay range from only few million to as little as 10,000 years old. Both of these volcanic areas and older volcanic rocks in the region are offset by the modern regional fault system."

"Map of the modern San Andreas Fault in relation to the greater plate-tectonic setting of western North America and the northeastern Pacific Ocean basin. The San Andreas Fault represents a great transform-fault boundary between the North American Plate and the Pacific Plate. The San Andreas Fault system connects between spreading centers in the East Pacific Rise (to the south) and the Juan de Fuca Ridge and Mendicino fracture zone system (to the north). The San Andreas Fault system has gradually evolved since middle Tertiary time (beginning about 28 million years ago). The right-lateral offset that has occurred on the fault system since that time is about 282 miles (470 km); however, the fault system consists of many strands that have experienced different amounts of offset."


So that brings us up to today. What will happen in the next few million years? Well, if everything keeps going at the rate its going, the JdF Plate will eventually be consumed and the San Andreas fault will continue unabated north up to Alaska. Of course all of the continent on the pacific plate (ie. Western Cali, Oregon, and Washington) will be making a slow journey north, detaching themselves from the rest of the Continent)

So, to answer Dad and Ethan's question...I'm of the opinion that the long term earthquakes that the last post is in reference to is earthquakes caused by the subduction of the Juan deFuca Plate under Washington and Oregon. These long-term quakes don't make the area any more safe than they would be otherwise. You live on the top of a huge conveyor belt that is always moving, eventually it will snag. Not necessarily all at once, but parts here and there will get stuck and then you'll have your large earthquakes.. Here is a diagram as evidence for my theory:

What this shows is a plot of earthquake location and depth. It is a cross section, with the top being the surface. That nice pattern of dots the arcs gracefully off to the lower left is the actual plate being forced down. this subduction is not a stop-and-go procedure, but a slow, ever-moving event.