It is important to identify areas that are prone to earthquakes in order to prepare for the possibility that they may occur. This preparation takes the form of disaster planning and community planning, but most importantly involves infrastructure.
If there is an area that rarely receives earthquakes of any significant size, there is no reason to construct every bridge, building, etc, to be earthquake resistant. Building an earthquake-resistant structure is much more expensive than building one that will crumble in the event of a quake and so, just as people in the Arctic don't need tornado-proof homes or shelters built into them, people in geologically stable areas don't need to waste money on unnecessary infrastructure upgrades. Very often, however, the opposite situation is the case: we learn that an area is prone to earthquakes that has been built up without structures being engineered for resistance to shaking. In that case, some major structures will need to be upgraded (and what those upgrades would be are up to an engineer- I haven't the foggiest idea) and building codes will be adjusted so that future designs take earthquake resistance into account.
