Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Michael Rader
Subduction Zone Earthquakes
In recent lighting of what has happened in the world subduction zone earthquakes are becoming a phenomenon that needs to be known about. What is the Theory of Plate Tectonics? What is a subduction zone? What is an earthquake? What natural occurrences are needed for one to occur? What are subduction zone earthquakes? The better educated and individual is the more they understand the dangers.
In order to understand an earthquake, one must know what the Theory of Plate Tectonics is. . The Theory of Plate Tectonics states that each plate shifts a certain distance through time. Each plate slides in a different direction at a different speed. It is thought that each plate can travel from 2-10 centimeters a year. As the plates collide they create massive amounts of friction causing energy to be released to the earth’s surface. One explanation behind plate tectonics is asteroid impacts. Some scientists believe impacts by many asteroids caused the earth’s crust to melt and re-cast over time. When it re-casted it formed the different plates and started plate tectonics.
zone is where two lithospheric plates meet. Lithospheric plates otherwise known
as Tectonic Plates are massive portions of the earth’s crust. Subduction only
occurs when an oceanic plate is overtaken by a continental plate causing it to
melt into the mantle. As this happens earth’s crust is formed and new land is
created. There are approximately 20 total plates with 7 major plates. The seven
major plates are as followed: the
Indo-Australian Plate, North
American Plate, Plate, and
the South American Plate. The major secondary plates are the:
Arabian Plate, Plate, Cocos
de Fuca Plate, Nazca
Plate, and the Philippine Plate
Each plate meets at what we call a plate boundary, which is where subduction occurs. Below is an illustration of the lithospheric plates. The dark lines represent the borders of each.
When these plates move they crash into one another causing the subduction zone. A good way to illustrate this movement is to make two fists and squeeze them together until one overpowers the other and winds on top. Subduction zones have become relatively easy for geologists to identify. There are three main indicators of a subduction zone. These three indicators are mountain formations, volcanic activity, and deep sea trenches. One of the biggest subduction zones is the Cascadia subduction zone. The Cascadia subduction zone is a long fault that stretches from Vancouver, Washington all the way down to southern California. Scientists are scared that much like the super eruption we are long overdue for another major earthquake along Cascadia. If this were to happen a massive tidal wave would be created that would cover much of the western seaboard. Along this subduction zone where the oceanic and continental plates meet there is a large fracture known as the Mendocino Fracture Zone. This fracture is responsible for the world famous San Andreas Fault in California. Below is a picture of the San Andreas Fault Line and an illustration of a subduction zone.
The San Andreas Fault line is a plate boundary. At this boundary the plates
don’t pull apart or crash together, they sideswipe. Every so often the plates
shift and rub together. The common points where the plates touch one another is
where the earthquake starts or the focus.
As seen in the picture above, subduction zones are on the borders of each tectonic plate. As they crash together one plate moves below the other which causes subduction zone earthquakes.
Earthquakes are one of the biggest natural disasters known to man. Earthquakes occur when there is a sudden release of energy below the earth’s crust at the focus. Earthquakes are not felt below the surface but once the energy hits the epicenter directly above the focus at the earth’s crust the movement is felt. Earthquakes have two waves of damage. There are two types of seismic waves and they are the p-wave and the s-wave. The first is the p-wave or primary wave. The p-wave pushes and pulls rock and escapes to the surface in no time. This wave hits the surface first causing the initial damage. The s- wave or secondary wave reaches the surface last because it travels at ninety degree angles causing it to be much slower. The s-wave moves particles and rocks up and down and side to side. As seen below the p-wave only moves one direction where as the s-wave moves in multiple directions.
The picture above shows the focus of the earthquake with the epicenter located directly above at the earth’s surface. When earthquakes occur normally they leave behind a fault line. Earthquakes are measure in two different ways. The first measurement is by the Richter Scale. The Richter Scale is a series of seismographs. Seismographs look like zig-zag lines that rapidly move up and down. The seismographs measure the amount of energy released during the earthquake which gives it a magnitude from 1-10. The second form of measurement is the Mercalli Scale. The Mercalli scale measures the amount of damage done at the surface. Its measured by things as common as people waking up from a sleep because they feel the movement. Unlike the Richter Scale which uses the numerals 1-10 the Mercalli Scale is measured in roman numerals. Below are pictures of both the Richter Scale and Mercalli Scale.
Subduction zone earthquakes are an unavoidable disaster. They are caused by natural shifts in the earth’s crust. The Theory of Plate Tectonics states that each plate moves at its own pace in different directions. Because of the collisions and pulling apart, earthquakes will always happen. Tectonics shows why it is a natural occurrence.
Subduction zone earthquakes can be devastating in many ways. They are caused by a shift in the earth’s crust, mainly between oceanic and continental plates. When an oceanic plate slides under a continental plate it melts into the earth’s mantle and not only causes an earthquake below the surface but because there is so much energy escaping from the ocean floor, tidal waves are caused as well. Japan for instance has two foci, one is at the top of the tectonic plate and the other partially down into the plate. The movement of the plates causes the ocean floor to move upwards as the continental plate rides over the oceanic plate as illustrated in the picture below.