N.Z. Version, 1998
Here is a very brief explanation of the basic concepts of plate tectonics. Read the section on plate tectonics, seismicity and subduction zones in the Learning Library or other reference Earth Science books you can find.
The surface of the Earth is composed of tectonic plates that slide past each other. The plates are either referred to as oceanic or continental, depending on the rock composition of the plate. The outlines of the plates (i.e. the plate boundaries) are usually characterized by high levels of seismicity. There are basically three types of plate boundaries:
At a convergent boundary two (or more) plates are colliding. If the plates are two oceanic plates, or one oceanic and one continental plate, a subduction zone is formed, where one plate subducts beneath the other. An example of a subduction zone is along New Zealand, west coast of South America and Alaska. If the two plates are both continental, neither wants to subduct, rather they pile up at the boundary, creating high mountain belts (e.g. Himalayas). At a divergent plate boundary, two plates are moving away from each other. This only occurs when both plates are oceanic, and we call the plate boundary a mid ocean ridge. An example of an ocean ridge is the Mid Atlantic Ridge. At a transform plate boundary the plates slide parallel, past each other. An example of a transform plate boundary is the San Andreas fault in California.
Figure 1
Figure 1 shows a topographic map of the world. The oceans are shown in blue (dark blue is deep ocean, light blue is shallow). The land areas are shown with green (low elevation) to brown (high elevation), highest elevation is shown with white (e.g. the Himalayas). You can compare this figure with the map you see in MAPPER when you select "view" and "flat world". Can you see some correlation between surface elevation and location of plate boundaries?
Use Mapper to find tectonic plate boundaries on the globe (select "plates"
in Mapper). Select an area on the globe that encompasses a whole plate
and search the hypocenter catalogue for seismicity of the area. Start with
a short time interval and expand your search if you don't find much seismic
activity. Also select a range of depths and magnitudes and plot your results
using Mapper. Here are some questions to think about:
Are there as many earthquakes at the plate boundaries as in the center
of the plate?
Are the earthquakes at a plate boundary generally shallower or deeper than
in the center?
Do the earthquakes at the plate boundary have larger or smaller magnitudes
than in the center?
Plot locations of active volcanoes and plate boundaries with Mapper. Select an area that has a plate boundary and active volcanism (for example Alaska) and search the hypocenter catalog. What is the deepest earthquake in the area you can find? What is the largest earthquake in the area you can find? Is there a relationship between the locations of the volcanoes and depth distribution of the earthquakes? Try drawing a cross section perpendicular to the plate boundary across your search and plot location of volcanoes and earthquakes.
Now repeat the search in an area that shows active volcanoes that are NOT close to a plate boundary (for example Hawaii). Again, identify the deepest and largest earthquakes you found. Compare your results for the two regions.
Select a plate boundary that separates:
(a) two continental plates at a convergent boundary (e.g. the Himalayas);
(b) two oceanic plates at a divergent boundary (e.g. the Mid Atlantic Ridge);
(c) an oceanic and a continental plate at a convergent boundary (e.g. New Zealand);
(d) an oceanic and a continental plate at a transcurrent boundary (e.g.
the San Andreas fault in California).
For each area use the query tool in Mapper to search the earthquake hypocenter catalog. Do the search for the same time period, depth and magnitudes for each type of plate boundary and plot in Mapper.
Plot histograms of the number of earthquakes vs. depth, for the convergent boundaries where two continental plates are colliding and a convergent boundary where an oceanic and a continental plate are colliding. How are the distributions of earthquakes with depth different in the two areas? Can you think of an explanation for the difference.
Do the same exercise comparing a plate boundary where an oceanic and a continental plate are colliding (subduction zone) and a divergent plate boundary with two oceanic plates. In addition to the depth distribution of earthquakes, also look at the earthquake magnitudes in the two regions. Print a map of the search results from Mapper. Does the areal extent of the zone of seismicity at a convergent boundary differ from the zone of seismicity at a divergent boundary?
Finally compare a plate boundary where an oceanic and a continental plate are colliding (subduction zone) and a transcurrent boundary. Is there a difference in depth distribution of seismicity between the regions? What about the areal extent of the seismicity in the two regions?
Look at the map in Figure 1 and find areas of high elevation (shown with brown to white colors) in the continental regions. Do a hypocenter search in three or more regions of high elevation. Can you find seismicity in all the areas you tried? If not, can you explain why?
Find other examples of the different types of plate boundaries than suggested above and compare to the previous results.
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