Sudden movement along shallow, brittle faults causes earthquakes. Such sudden breaks set off ground vibrations.
Faulting causes earthquakes (not the other way around). And faulting is common at every plate tectonic boundary type.
Stick Example (to give basic idea of what causes earthquakes)
rocks deform elastically on either side of a fault in response to a stress (stress inputs energy)
The resultant change in shape due to the stress is called strain (stores the energy input – this stored energy is called strain energy)
The strain continues to build up in the rocks until the fault suddenly breaks. When this happens all of that stored energy is released in two ways: as motion on the fault and as seismic waves (or sound waves in the stick example). The seismic waves travel through the earth until they reach the surface where you feel them as an earthquake (seismic shaking).
NOTE that in the stick example, small sticks break more easily, store less strain energy, and consequently produce less sound when they break than big sticks – remember, big breaks = big quakes.
Click here for a link to an animation showing how a tsunami is generated at a subduction zone: http://serc.carleton.edu/resources/12331.html
As you watch the video, notice that it is NOT the seismic wave activity (the earthquake) that causes the tsunami. Rather, the water is displaced when the fault moves.
It is simple - in order to make a big wave, you have to move a whole lot of water all of a sudden. To do that you need a sudden up/down change in the seafloor. One way that this can be accomplished is through sudden undersea faulting - when faults move the seafloor, they also move the water that lies above the seafloor, thus initiating the wave. Higher magnitude earthquakes signal larger fault breaks - the bigger the fault break the more it can move the water around. So, when an under water fault breaks with sudden vertical motion (like at subduction zones) it produces and earthquake AND, if the movement is large enough, it may also produce a tsunami.
Fault-related tsunami are all found at subduction zones because:
Fault related tsunami do not form at collision zones because collision zones are all on land (no water to make a wave with)
Fault related tsunami do not form at transforms because most of the faults in a transform move horizontally as the plates slide by each other - you must have vertical motion in order to displace the ocean water.
Fault-related tsunami do not form at spreading centers (divergent boundaries) because, although the motion is vertical, the fault motion is generally much smaller than at a subduction zone (so there isn't enough vertical motion to move the water substantially). This is because stress on the rocks at a spreading center is relieved quickly - strain doesn't build up as much there because the rocks are warm and the plate is thin, so it doesn't take much to get the fault to break. In a subduction zone, the rocks are cold and thick, so there is much more resistance to breaking such that a lot more stress (energy) must be put into the rock (and stored as strain) before it finally breaks in a big way.
Other ways to make a sudden vertical change in the seafloor (and displace water all at once) include having a huge asteroid fall into the ocean (as happened 65 million years ago when the dinosaurs went extinct), having a massive landslide into the sea (as happened in Lituya Bay in Alaska back in the 1950's), or by having a tremendous volcanic eruption into the ocean (as happened when Krakatoa erupted and collapsed into the sea).
This applies another of Paige Keeley's formative assessment techniques (called "Card Sort") to review what you know already about Plate tectonics and to introduce you to the three types of plate tectonic boundaries. We may or may not have time to do this activity.
This acitivity should help you to visualize the general characteristics of the three types of plate tectonic boundaries (divergent, convergent, and transform). Be sure to review the key concepts on p.138 and the background information on p.139-142.
A diagram showing age patterns of the seafloor due to seafloor spreading: http://dtc.pima.edu/blc/183/08_183/figures/Fig%203-spreading%20w_stripes.jpg
A map showing the ages of the seafloor (this is the map I had hanging up during class): http://commons.wikimedia.org/wiki/File:Earth_seafloor_crust_age_1996_-_2.png
A general guide to Plate Tectonics: http://www.pbs.org/wgbh/aso/tryit/tectonics/intro.html
Understanding Plate Motions: http://pubs.usgs.gov/publications/text/understanding.html
Geology of the San Joaquin Valley (also read p.140-142 of your lab manual): http://www.sjvgeology.org/geology/index.html
Read theselab notes to find out what causes earthquakes and tsunami, then read the material in your manual for each activity with care so that you understand what you missed, talk to a classmate to find out more. Dr. Munn has a plate tectonic machine in her office if you want to try any of the activities. Contact Dr. Munn to arrange to make up the quiz that you missed (this must be done sometime before the next class). Click here for the syllabus information about missing class. Worksheets collected for this lab will be listed on the Lab Schedule page and must be turned within a week. Here are more specifics about the worksheets that were collected:
Students also filled out the chart on p.137 - this was not collected but is a good study aid, so you might want to ask a classmate if you can look at their chart.
