Earthquakes
Like volcanoes, earthquakes are associated with plate tectonics, and the most powerful recorded earthquakes have occurred at plate boundaries. As plates move, the friction the develops as they grind against each other leads to unreleased tension. Eventually, this tension overcomes the friction force, and the plates spring free and move anywhere from less than a centimeter to several feet. There are three primary types of earthquake events associated with different plate interactions:
1) Tension faults
2) Compression faults
3) Strike-slip or transverse faults
Let's look at each of these in more detail to better understand where they occur, what forces create earthquakes on these faults, and some examples of recent earthquakes.
1) Tension (Normal) faulting
Tension faulting, also known as normal faulting, occurs when the two sides of a plate are being pulled in opposite directions, stretching them out. A good example of tension faulting is found in the Great Basin, where the valley blocks (called grabens) fall and the mountain blocks (called horsts) rise on either side of a fault line. Lake Tahoe, sitting on the border of California and Nevada, exists because of normal faulting.
1) Tension faults
2) Compression faults
3) Strike-slip or transverse faults
Let's look at each of these in more detail to better understand where they occur, what forces create earthquakes on these faults, and some examples of recent earthquakes.
1) Tension (Normal) faulting
Tension faulting, also known as normal faulting, occurs when the two sides of a plate are being pulled in opposite directions, stretching them out. A good example of tension faulting is found in the Great Basin, where the valley blocks (called grabens) fall and the mountain blocks (called horsts) rise on either side of a fault line. Lake Tahoe, sitting on the border of California and Nevada, exists because of normal faulting.
2) Compression (Reverse) faulting
In compression faulting, instead of two parts of a tectonic plate being pulled apart, they are being pushed together. As they are pushed together, the more buoyant side is pushed up, while the less buoyant plate is pushed down and underneath. Sound familiar? The most common form of compression faulting occurs along subduction zones. Compression faults are also found where two continental plates collide, pushing both side up into high mountains. There is compression faulting going on underneath the Himalayas, the highest mountains in the world.
3) Transform (strike-slip) faulting
Transform faults are found where two plates are sliding past each other along their sides. Because their edges are jagged, they don't slide past easily, and instead get caught (the "strike") and then, when enough pressure builds up, they finally release and move (the "slip"). The most famous and active transform fault in the world is the San Andreas fault in California. Watch the short video below to help you answer the question: Why is the San Andreas fault so famous?
In compression faulting, instead of two parts of a tectonic plate being pulled apart, they are being pushed together. As they are pushed together, the more buoyant side is pushed up, while the less buoyant plate is pushed down and underneath. Sound familiar? The most common form of compression faulting occurs along subduction zones. Compression faults are also found where two continental plates collide, pushing both side up into high mountains. There is compression faulting going on underneath the Himalayas, the highest mountains in the world.
3) Transform (strike-slip) faulting
Transform faults are found where two plates are sliding past each other along their sides. Because their edges are jagged, they don't slide past easily, and instead get caught (the "strike") and then, when enough pressure builds up, they finally release and move (the "slip"). The most famous and active transform fault in the world is the San Andreas fault in California. Watch the short video below to help you answer the question: Why is the San Andreas fault so famous?
Measuring Earthquakes
How do we describe and measure earthquakes? What information is being used?
First, we need to understand what is happening during an earthquake, when the pressure that has been building finally releases. That pressure is stored energy, and an earthquake is simply the release of energy. It is released in two seismic waves: the primary seismic wave, and the secondary seismic wave.
The primary wave (or p-wave) is a compressional wave that moves rapidly and will arrive first at a measuring station. The secondary wave (or s-wave) is a shear wave that moves more slowly and will arrive second at a measuring station. There are hundreds of measuring stations around the world; they contain equipment that converts the seismic energy into a movement of a pen on a drum to create seismographs. One of the best ways to visualize the difference between a p-wave and an s-wave, and understand why the p-wave travels faster, is to re-create these waves with a Slinky toy, like in the video below.
How do we describe and measure earthquakes? What information is being used?
First, we need to understand what is happening during an earthquake, when the pressure that has been building finally releases. That pressure is stored energy, and an earthquake is simply the release of energy. It is released in two seismic waves: the primary seismic wave, and the secondary seismic wave.
The primary wave (or p-wave) is a compressional wave that moves rapidly and will arrive first at a measuring station. The secondary wave (or s-wave) is a shear wave that moves more slowly and will arrive second at a measuring station. There are hundreds of measuring stations around the world; they contain equipment that converts the seismic energy into a movement of a pen on a drum to create seismographs. One of the best ways to visualize the difference between a p-wave and an s-wave, and understand why the p-wave travels faster, is to re-create these waves with a Slinky toy, like in the video below.
There have been vast improvements in our ability to mitigate earthquake hazards in the US. Across the country, building codes require the use of certain materials that are more "flexible" -- meaning they will give and move a little bit during an earthquake. Bridges are built to be able to move and allow energy to pass through without destroying the infrastructure, particularly in states with a high probability of earthquakes (such as Washington, Oregon, California, and Alaska). When a strong earthquake strikes in places where these mitigation measures are in place, there are fewer buildings and roads that collapse, as compared to places where mitigation measures are not implemented. A good example of this comparison is Japan versus Haiti.
In 2010, a magnitude 7.0 earthquake hit the island country of Haiti, centered close enough to the capital of Port-au-Prince to destroy much of the city. Estimates placed the death toll at between 100,000 and 150,000 people, in part because Haiti has no mitigation measures or building codes for earthquakes. The picture below at left shows how entire hillsides of homes were flattened by the quake.
In March 2011, a magnitude 9.0 earthquake struck just off the coast of Japan. Despite being 1,000 times stronger than the Haiti quake, the Japan earthquake resulted in far fewer fatalities and infrastructure destroyed (less than 20,000), and most of the destruction associated with the Japan quake resulted from a tsunami (more about tsunamis on the next page of the module).
In 2010, a magnitude 7.0 earthquake hit the island country of Haiti, centered close enough to the capital of Port-au-Prince to destroy much of the city. Estimates placed the death toll at between 100,000 and 150,000 people, in part because Haiti has no mitigation measures or building codes for earthquakes. The picture below at left shows how entire hillsides of homes were flattened by the quake.
In March 2011, a magnitude 9.0 earthquake struck just off the coast of Japan. Despite being 1,000 times stronger than the Haiti quake, the Japan earthquake resulted in far fewer fatalities and infrastructure destroyed (less than 20,000), and most of the destruction associated with the Japan quake resulted from a tsunami (more about tsunamis on the next page of the module).
One of the most impressive earthquake mitigation efforts in modern history occurred in a very remote place: Alaska. When engineers were building the Trans-Alaska pipeline to carry oil from the North Slope of Alaska for 800 miles to the City of Valdez port on the Pacific Ocean, they needed to mitigate for the likelihood of a strong earthquake shaking the pipeline. Alaska is shaped by a subduction zone along its southern half, and experienced the second-strongest earthquake in recorded history in 1964, when a magnitude 9.2 quake struck Prince William Sound, where Valdez is located.
To mitigate for earthquakes, engineers devised a system where the pipeline both zigzags across the landscape (providing more 'give') and sits above-ground on slider beam supports. Each support structure holds the pipeline up on a platform called a 'teflon shoe' that can slide laterally up to 20 feet and move vertically up to 5 feet during an earthquake.
This system was tested on November 2002 when a magnitude 7.9 earthquake shook the area. The pipeline moved 14 feet laterally and 2.5 feet vertically during this event! Despite suffering some minor structural damage, the pipeline did not rupture or leak, so the mitigation efforts were successful.
Log in to BbLearn and complete Assignment 7.2: Earthquakes.
To mitigate for earthquakes, engineers devised a system where the pipeline both zigzags across the landscape (providing more 'give') and sits above-ground on slider beam supports. Each support structure holds the pipeline up on a platform called a 'teflon shoe' that can slide laterally up to 20 feet and move vertically up to 5 feet during an earthquake.
This system was tested on November 2002 when a magnitude 7.9 earthquake shook the area. The pipeline moved 14 feet laterally and 2.5 feet vertically during this event! Despite suffering some minor structural damage, the pipeline did not rupture or leak, so the mitigation efforts were successful.
Log in to BbLearn and complete Assignment 7.2: Earthquakes.