Wednesday, August 26, 2015

Earthquake Primers - What kills you - Part 1

Barring a great big wall of water (tsunami), what will kill you is something falling on your head.  The odds of getting killed in an earthquake are probably below that of getting hit by a bus, while drinking coffee in Starbucks.

Any given structure is designed to get live loads, such as wind, people dancing, etc.  Buildings collapse all the time without earthquakes, from corrosion or foundation problems.  Ground shaking will induce dynamic strains, and it is a question of whether the building has enough margin to take it.  I would call this the 'seismic capacity'.

A house of cards has no seismic capacity.  You can build the equivalent by piling up rocks in arches, such as the old Italian buildings.  It is human nature that all building styles seem to settle for a 1 in 500 chance per year of being destroyed by an earthquake.  In areas of frequent earthquakes they build stronger because everybody remembers the last disaster.

Our lovely stone building will collapse one day because of ground motion, and I only use peak ground velocity (PGV) to define it.  You'll find that other crazy people use peak ground acceleration.  This is the motion defined right at the surface of the dirt where you are building.  It is best measured directly by a good instrument, or in third world countries like Oklahoma, derived by looking at the damage.

Most people are generally confused by what the dying newspapers report about the earthquake.  This is always the magnitude and the location, which are the cheapest things to wring out of recorded data.  Thus, one would think a larger magnitude is worse, but not necessarily.  It all depends on the type of earthquake, and how far it is away from buildings.  A larger earthquake far away produces the exact same PGV as a smaller earthquake right under you.  

So, when you are calculating the odds of a brick falling on your head, you must think of the seismic capacity, and the chance of the PGV exceeding that.  This is the seismic risk of death.  There is one more thing, that I think is the most important of all -- your foundation.

-to be continued.

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