Download Subduction Zone Earthquakes graph lab

Earthquakes and Subduction Zones
Name __________________________
Date _____________
Earthquakes occur when the brittle rock of the lithosphere moves in response to stress. The
worldwide distribution and depths of earthquakes are important evidence for plate tectonics. They
help geologist delineate active plate boundaries and infer locations of ancient plate boundaries.
Most earthquakes have shallow focus depths (0 to 70 km below the surface), which makes sense
since earthquakes do not occur in the hot, plastic rock of the asthenosphere or deeper in Earth’s
interior. Intermediate (70-350 km) and deep (350-700 km) focus earthquakes do occur, but only
near subduction zones. This indicates that solid, rigid material is being forced into the mantle in
these areas. Plots of earthquake focus depths along subduction zones provide geologists with
profiles of subducted plates and enhance our understanding of what happens at subduction zones.
The attached map shows an area in the western Pacific Ocean where several deep ocean trenches
mark the locations of subduction zones. Different symbols on the map represent different
earthquake focus depths. The table below summarizes the focus depths of 24 earthquakes near the
Izu Trench in the western Pacific and the distance from the focus of each earthquake to a seismic
station on Bonin Island. Plot a graph of earthquake focus depths on the graph paper on the back of
the map. The graph paper is already labeled. Select colors to represent shallow, intermediate and
deep focus earthquakes and make a key on the graph. Give the graph a title.
Earthquake
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Focus Depth
(kilometers)
390
32
295
190
540
54
90
215
32
630
230
150
13
680
335
460
70
10
400
530
110
55
58
605
Use the graph and map to answer the questions on the back.
Relative Distance
(km X 100)
400
120
350
290
530
200
220
340
150
520
310
240
80
570
390
460
190
50
440
480
260
165
190
520
Questions
1.
What is the approximate angle of descent of the subducted plate at the Izu Trench?
_______________
2. Identify the plates that are involved in this subduction zone.
_______________________________________________________________________
Which plate is being subducted? _________________________
What is the direction of subduction? ______________________________
There is oceanic crust on both sides of this plate boundary. What determines which of the two
plates is subducted? _______________________________________________________
_______________________________________________________________________
3. Compare the earthquake distribution at the Izu Trench (A-A- on the map) with the earthquake
distributions at the Mariana (B-B’) and Japan (C-C’) Trenches. The steepness of the angle of
plate descent can be inferred from the spacing of the earthquake locations. The closer
together they are, the steeper the angle of descent and vice versa. Using these relationships,
analyze the distribution of earthquakes along B-B- and C-C’ compared to A-A’ (the ones you just
graphed).
At which of the three trenches is the angle of plate descent steepest? _________________
Where is the angle of descent shallowest? _________________________
4. Why don’t earthquakes occur at depths greater than approximately 700 km? What happens to
the subducted plate at greater depths? _________________________________________
_______________________________________________________________________
_______________________________________________________________________
5. Examine the map of tectonic plates. Identify three other places around the world where you
would expect to find the same distribution of earthquakes as you see near Japan.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
6. Places on the east coast of the United States occasionally experience earthquakes along faults
that are left over from ancient plate activity. Do you think that these are likely to be shallow
or intermediate and deep focus earthquakes? EXPLAIN your reasoning.
_______________________________________________________________________
_______________________________________________________________________
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