Engineering Misconceptions, Part 3 - Earthquake Magnitude and Intensity

Wrong: I designed your building to withstand an earthquake with a magnitude of 7.0 and intensity 10.

Right: I designed your building according to the minimum safety requirements of the building code which aims to protect life and limb from harm due to a 475-year return period earthquake. I also designed the foundations as appropriate. The intensity depends on the expected level of damage. If all buildings were designed properly, the intensity scale (expected level of damage) would go down for the area around your building.

Facts:

Your building may be permanently damaged after a 475-year return period earthquake. No one can say exactly what earthquake magnitude will that be. No one can say exactly up to what earthquake magnitude your building will not be damaged or up to what earthquake magnitude your building will stay standing. If anyone can say exactly what will happen, either he is God, or he is playing God.

However, if you are sure that the maximum magnitude that can be generated by the nearest fault line to your building is, say, magnitude 7.0, and let us say your building was designed properly (e.g. performance-based seismic design was used), at magnitude 7.0, it COULD have permanent, irreparable damage, its occupants COULD get injured or die, but it MIGHT still be generally standing; at magnitude around 6.6, it COULD have irreparable damage but its occupants MAY be able to evacuate without harm; at magnitude around 6.3, it COULD still have some repairable damage but occupants MAY be able to immediately come back inside after any strong aftershocks.

Even if the building itself was properly designed but the foundations or the soil/geological conditions at and around the building were not given as much attention as they should, the building MAY still not be safe.

Again, we design to certain levels of earthquakes based on statistical analysis of available data and data quality. Actually, THERE IS NO CERTAINTY. There are things that we still do not know. Engineering is still advancing and new discoveries are still being made each year.

Also, your building and its foundations may be well designed, but if an adjacent building was not, your building is still not safe.

A magnitude 7.0 earthquake 400 kilometers away MAY have little effect on your building. After such an event, there may be an intensity 10 near the earthquake epicenter, but it may just be an intensity 5 or 6 around your building.

Again, earthquake intensities describe the expected level of damage but if all buildings were designed properly, the intensity scales would go down.