We began this module in Section 1 by defining earthquakes, a task made confusing by the fact that two definitions, related but distinct, can be given for this single term. An earthquake is the sudden slip of one part of the Earth's crust relative to another along a fault surface; it is also the shaking we feel, caused by seismic waves that were generated during that sudden slip.
As representations of slip created from analysis of seismic waves, fault plane solutions are a perfect example of the synthesis of these two definitions made possible with modern seismological investigations. The two separate concepts -- shaking and fault slip -- merge into one model while each part retains its own charcteristics, much like the light and dark quadrants of the focal mechanism symbol are each a distinct but necessary part of the whole.
If you've worked through this entire module, you've been introduced to many different investigational methods and the results they can produce. Hopefully this material has given you the insight to connect these mulitple views into a broader understanding of the phenomenon we all know as an "earthquake".
In some ways obvious and expected, in other ways subtle and unpredictable, earthquakes remain an undeniable fact of life for millions of people around the world. The result of massive forces within our ever-changing planet, we can't expect to prevent earthquakes. It's even possible we'll never be able to predict their onset reliably. But in learning about these movements within our Earth, we can also learn how to live with them.