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Lab 13. Earthquake Focal Mechanisms Review Exercise Key things to understand

1. Ray take-off direction (azimuth and inclination) – Lab 11

2. Fault planes – strike and dip – Lecture 11

3. Different styles of faulting – strike slip, normal, reverse (thrust) and their relationship to different plate boundaries. – Lecture 11

4. How P-wave first motions are related to the fault plane and slip directions and why two alternate fault

planes (focal planes) at 90° can fit a set of first motions – Lecture 11

5. How to identify the polarity of a P-wave from a seismogram (compressional, dilatational, nodal) – Lab 11

6. How to represent P-wave first motions (take-off directions) and fault planes on a stereogram and how to

find a fault plane solution – Lab 11

7. The use of upper and lower hemisphere projections – how they are related. – Lecture 11

8. How to interpret beach ball plots in terms of fault strike and dip, style of faulting. – Lab 12 9. Reading  approximate  strike  and  dip  from  a  focal  mechanism  plot  –  Lab  12  

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Exercise The table below gives the take-off azimuth, inclination (angles downwards relative to horizontal) and P-wave first motion polarity of arrivals recorded for a teleseismic (distant) earthquake in the ocean basins.

Station Ray Azimuth, ° Ray inclination, ° Polarity 1 038 00 C 2 213 01 C 3 062 04 D 4 051 05 D 5 228 07 C 6 351 08 D 7 280 09 D 8 069 09 D 9 119 13 D 10 192 16 D 11 005 18 C 12 328 20 N 13 242 24 C 14 054 25 D 15 300 27 C 16 029 34 C 17 188 38 N 18 148 45 D 19 195 54 C 20 099 60 D 21 028 72 C

(a) Using tracing paper and a push pin on the attached stereonet (Figure 3), plot the first motions on a lower hemisphere projection (b) Draw the location of the two focal planes that fit the first motions (c) Label each plane with the strike and dip. (d) Figure 1 shows a sketch of plate boundaries around a small oceanic plate (single lines show transforms, double lines spreading centers and hatched lines subduction zones. Is the mechanism you found above most compatible with an earthquake at location A, B, C, or D? Explain your answer. (e) Which one of the two focal planes in the mechanism you found above most likely represents the fault plane? Justify your answer. (f) On Figure 1 draw the focal mechanism (“beachball”) plots showing the focal mechanism you would expect at of the other three labeled locations. (g) On Figure 2, label each plane on the lower hemisphere focal mechanisms with the approximate strike and dip. Label the faulting style

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Figure 1

A

C

DE

B

100 km

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Figure 2

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Figure 3