Download Subduction Zone Lab

Justin Tan
9/30/2011
Subduction Zone Lab
1. Q: What is the relation between the depth of earthquakes and their distance from the Tonga
Trench?
A: What I realized was that the further you went away from the Tonga Trench, the deeper the
earthquake focus point was. Basically, I can tell that the depth and distance are acting as if they
are moving together. The main thing that I have observed was that the foci points are getting
further and deeper at the same time. It looks like a steady slide, and the relation is that the depth
has less value than the distance, but when it gets to 448, the value of the depth over powers the
distance. I also see that the exact focus point where the depth and distance is the same is 448 km
deep and far.
2. Q: Is there a distinct pattern in the earthquake locations on your map? If so, describe and
draw al line on your graph that best represents this pattern.
A: Yes, there is a distinct pattern that is noticeable in the graph. I can see from my line of best fit,
the points are pretty much in a nice diagonal line. Like I have mentioned above, the foci points are
steady, and the focus points don’t jump all over the place. As you can see on the graph, the line
goes straight through the middle, so lucky for me, it is not a confusing line, but is a line that I can
work with. Amongst these foci points, I see there is one huge gap in the middle. I see that there is
a bunch of foci points on the top half, and then there is a break in between. On the other side of
the gap is where the second set of foci points continues on. One last thing that I noticed was that
the points of these foci’s look like a diagram for a subduction zone. As a conclusion to this
question, I would say that by looking at this graph, I could see that there are two separate groups
of foci points located. (All foci points are close to the line that I drew on the graph)
3. Q: What is your explanation for the distribution of the earthquake foci?
A: Well, as we all know, the Tonga Trench is located in the South Pacific Ocean, and the trench is
very close to a super active subduction zone. At this zone, the Pacific Plate sinks under the Tonga
Plate and the Indo-Australian Plate. This trench becomes a transform fault zone when turning west
north towards the Tonga Plate. I say that since the trench becomes a transform fault zone, I think
that the brushing of the plates causes the earthquakes to happen at the specified locations. At the
beginning, there are 4 foci points at the top. That’s when the Tonga Plate or a Plate is moving
slowly away from the trench while it was still afloat. Than, the points go down, like the
subduction zone, so my guess is that these points represents the movement of the plates going
through the subduction zone process, while banging against the transform fault zones. This is the
“jerk” that the plate makes. These “jerks” are the main earthquake causes. And I say as a
conclusion that maybe these earthquake foci points are located here because of the process of
subduction and the “jerks.”
4. Q: Does the shape of the earthquake focus pattern support the theory of plate tectonics?
Explain your answer.
A: If my theory about the whole subduction zone and foci points relation is true, than I think that
the pattern shape does support the theory of plate tectonics. This is because in my theory in
question 3, I mentioned that the foci points were allocated there because of the subduction process
with the plate. In the theory of plate tectonics, it studies the movement of the Earth’s plates. So if
it is true that the foci points were allocated there because of the plate’s subduction motion, than it
would show that the plates moved amongst each other. There is no way that these foci points
could have been assigned in the areas if the plates had not moved.
5. Q: As the tectonic plate dives down into the mantle it begins to melt. At what depth is the
plate no longer rigid enough to produce earthquakes? Explain.
A: In this graph, I see that the earthquake does not go past 670 km in depth. In general, the
earthquakes cannot form past this depth because as you go further down, we all know that it gets
hotter. Below the asthenosphere, the temperature is too high, and the rocks temperature at that
point in time prevents earthquakes to occur. Earthquakes need that special force or friction, and so
when there is a big jerk in the rocks, the earthquakes happen, but when the rocks are hot, they can
be easily moved, so there is no force or friction. The reason earthquakes can hang cool for a long
time is because the oceanic crust. The oceanic crust is cool, but close to 700 km deep is when the
crust gets hot. So as a conclusion to this question, I would say that subduction zones have the
advantage of having deeper earthquakes because regular earthquakes at regular places cannot go
that deep.
6. Q: What surface feature might result from the melting of the plates? Explain.
A: After melting the plate, the result is magma. This magma from the plates forms volcanoes at
the surfaces because the magma escapes through the fractures that are located in the crust. So
basically, the melting of the plates will cause volcanoes to arise. These volcanoes may do at most
critical harm to human beings, and at minimum blast, will do no harm. In the end, some crust may
be melted from the intense heat. This is the after math of the melting of the plates.
Earthquake of the Tonga Trench Area -­‐200 -­‐100 0 Depth (km) 100 200 300 Earthquake Focus 400 Linear (Earthquake Focus) 500 600 700 800 600 500 400 300 Distance (km) 200 100 0