Location East or West of Trench
200 km West
20 km
220 km East
B Notice line B-B' on the map in part A and the fact that shallow, intermediate, and deep earthquakes occur along it. (Each earthquake begins at a point beneath the surface called the focus (plural, foci).] Using data that were provided by the US. Geological Survey in Fig. A2.7.2, plot the locations of earthquake foci on the cross-section (Fig. A2.7.3). Volcanoes also occur at Earth's surface along line B-B. Plot the locations of the volcanoes listed in the table by drawing a small triangle for each on the surface along the zero-depth line.
Location East
or West of
Trench
Depth of Earthquake (or volcano location)
Depth of Earthquake (or volcano location)
Depth of Earthquake (or volcano location)
150 km
Location East or West of Trench
160 km West
25 km
250 km East
30 km volcano
410 km East
450 km East
60 km West
10 km
260 km East
120 km
450 km East
30
km West
25 km
300 km East
volcano
470 km East
50 km 150 km 180 km
O (trench)
20 km
300 km East
110 km
500 km East
30 km
10 km East
40 km
330 km East
volcano
500 km East
160 km
20 km East
30 km
330 km East
40 km
500 km East
180 km
50 km East
60 km
330 km East
120 km
540 km East
30 km
51 km East
10 km
350 km East
volcano
590 km East
20 km
55 km East
30 km
390 km East
volcano
640 km East
10 km
60 km East
20 km
390 km East
40 km
710 km East
30 km
80 km East
70 km
390 km East
140 km
780 km East
530 km
100 km East
10 km
410 km East
volcano
800 km East
560 km
120 km East
80 km
200 km East
110 km
410 km East 410 km East
25 km
820 km East
610 km
110 km
880 km East
620 km
Figure A2.7.2
B
West
200
100 Trench 100 200
Distance East or West of Trench (km) 300
East
B
400 500 600 700 800 900
Depth of Earthquake (km)
0
100
15
200
300
400
500
600
700
Figure A2.7.3
1. What kind of plate boundary is shown in your cross-section?.
2. Draw a curve in the cross-section to show your interpretation of where the top surface of the subducting plate is located. 3. Label the part of your cross-section that probably represents earthquakes in the subducting slab. Then label the part of your cross-section that probably represents earthquakes in the South American Plate above the subducting Nazca Plate. 4. At what depth does magma probably originate below the volcanoes, just above the subducting plate:
km. How can you tell?
5. REFLECT & DISCUSS What is the deepest earthquake plotted on your cross-section? Why do you think that earthquakes occur at hundreds of kilometers depth along subducting slabs but not elsewhere in the mantle at that same depth?
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