The big aftershock last week was unprecedented. The latest info suggests that it may have been the most violent earthquake to have ever hit a built up area anywhere in the world. The magnitude was not especially large but several factors combined to exaggerate its power. First, it was very close to the city and very shallow. Second, the liquefaction that occurred enhanced the shaking. Third, the port hills (the NW edge of banks peninsula that mark the southern boundary of the city and are mostly dense basalt) acted as a lens, focusing the seismic energy onto the central city.
This is borne out by the data. At the epicentre of the much larger September quake, the peak ground acceleration (PGA) was 1.26g. In the Haiti earthquake that killed a quarter of a million people the PGA was 0.5g. During the earthquake that hit us last week, most of the city experienced a PGA of 2.2g and the ground was shaking side to side by a metre. The port hills now stand 50cm higher and 50cm westward than before.
The New Zealand building code is one of the strictest in the world. Buildings must be able to withstand a PGA of 0.7g. We experienced over three times that and it is no surprise that so much of the city has been destroyed. The only factor that worked in our favour is that the magnitude of the aftershock was only 6.3. This resulted in a shorter quake that lasted about 20 seconds whereas the September quake lasted 50 seconds.
No comments:
Post a Comment