The Physics of the Super-Shear Earthquake

Long time readers will know that I've gone on and on about super-shear or high PGV earthquakes.  I have also called them 'Fist of God' or 'punching' earthquakes.  There have been a lot under cities which were smashed by the ground motions.  Armenia, Kobe, and Haiti may be some.

The amount of seismic damage from a fault rupture depends totally on the velocity of the rupture front, which in turn depends on other things.  I was quite amazed to find a great variety in this.  You can have extremely slow ruptures, such as happens yearly (or so) under BC.  You can have 'tsunami quakes' where the rupture front picks up a bit, enough to cause a tsunami, but no felt motions.  This can be quite deadly!

Your average earthquake has a rupture front that zooms along at the speed of the S wave.  As we recall from our Wikipedia reading, there are the two main seismic waves - S and P.  The Primary wave arrives first because it is a straight compressive wave, like sound waves.  When I felt my only earthquake, the day before I got married, the P waves made the brother-in-law's huge collection of glass airline bottles buzz like an angry rattlesnake!  That's the thing about P waves, the amplitude and induced strain (damage) is very small.  Then came the S waves which shook the whole house!  These are always larger amplitude shear waves, they have high PGV and are responsible for 90% of the damage in an earthquake.

I still get my electronic copy of the BSSA, and will spill an important paper which is hiding behind the paywall, even though Hilary might get mad at me!  :)

-to be continued.