OCEAN/ESS 410

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Lab 13 Focal Mechanisms and Collision Zone Tectonics In this exercise you are going to look at focal mechanism in two regions of plate collision (a) Figure 1 shows lower-hemisphere focal mechanisms for Tibet. (i) Identify a 2 strike-slip fault, 2 normal fault, and 2 thrust-fault mechanisms on the map. Circle them and number them on a copy of the map. Write below the approximate strikes and dips of the two alternate fault planes. Remember, either the strike should be chosen so that the fault plane dips to the right as you look in the direction of the strike or you should give the direction of the dip For strike slip faults, state whether the motion on each fault is left-lateral (looking across the fault, the other side moves to your left) or right-lateral. Event 1: strike-slip strike:____________ dip:_______________ ______-lateral strike:____________ dip:_______________ ______-lateral Event 2: strike-slip strike:____________ dip:_______________ ______-lateral strike:____________ dip:_______________ ______-lateral Event 3: normal strike:____________ dip:_______________ strike:____________ dip:_______________ Event 4: normal strike:____________ dip:_______________ strike:____________ dip:_______________ Event 5: thrust strike:____________ dip:_______________ strike:____________ dip:_______________ Event 6: thrust strike:____________ dip:_______________ strike:____________ dip:_______________ (ii) What type of fault-mechanism dominates in the Himalayas? Is this what you would expect? Explain why or why not?

OCEAN/ESS 410

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(b) Figure 2 and 3 shows a map of earthquakes and lower-hemisphere focal mechanisms in the Chilean subduction zone. The earthquakes are color-coded and change from cyan-blue (0-33 km) to green (33-70 km) to olive (70-150 km) to yellow (150-300 km) to magenta (> 300 km) as depth increase. Figure 4 shows a cross section of earthquake depths (with different color coding) (i) Identify a focal mechanism near the subduction zone that is consistent with motion on the plate boundary In Figures 2 and 3, there are clusters of deep earthquakes with focal mechanisms at longitudes of -69°, -65 to 66°, and -63° (negative longitudes are west) all of which are located in the subducting slab. These roughly coincide with the bright red, purple and green earthquakes in Figure 4. The figure below illustrates the expected orientation and sense of motion on fault planes for a slab dipping at 45° under compression and tension. If the slab were dipping at a different angle the fault planes would be likewise rotated.

(ii) Are the deepest earthquakes at 63°W consistent with tension or compression in the slab? Explain your reasoning. Can you think of a geological explanation for the state of stress in the slab at the location? (iii) Are the shallower earthquakes at 69°W consistent with tension or compression in the slab? Explain your reasoning. Can you think of a geological explanation for the state of stress in the slab at the location? (iv) The mechanisms at 65-66°W are quite variable. They occur at a location where the slab bends from shallow dipping to dipping at ~15° to dipping at ~50°. Can you explain why the mechanisms would vary at a location where the slab bends (HINT: Not all the earthquakes are at the same depth in the slab).

Figure 1

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