[time-nuts] New Zealand, Iceland, Haiti

jimlux jimlux at earthlink.net
Wed Sep 8 00:49:00 UTC 2010

Kiwi Geoff wrote:
> Brooke Clarke wrote:
>> When you look at the time difference between the recent events on a geologic
>> scale you could say they all happened at the same time.
> Hi Brooke, I'm writing from my home in Christchurch, New Zealand.
> "Local Time" of the event  is an important variable.
> Last Saturday we had a 7.1 Richter magnitude event here, which was
> higher than that of Haiti (where there were 230,000 deaths). We had no
> loss of life in Christchurch mainly because it happened at 4:35 am
> local time, and because of our building code, as per:
> http://www.stuff.co.nz/the-press/news/4096880/Why-we-re-not-Haiti
> We are still experiencing magnitude 5 aftershocks here as I speak, and
> for those who like graphs, here is a live feed of the seismograph from
> Christchurch.
> http://www.geonet.org.nz/earthquake/drums/mqz-drum.html
> I now know my home can withstand a magnitude 7.1 earthquake, but it is
> a test that many houses in Christchurch have failed.

What you really want to know is the surface motion *at your house* 
during the quake.

The Northridge earthquake (6.7) was striking because of the radical 
difference in damage from houses that were close together.  Subsurface 
geology had a big effect. I'm about 15 km from the epicenter, and we had 
essentially no damage or even permanent effects (although it certainly 
woke us all up).  A friend who also lives 15 km away, but in a different 
direction, lost all their dishes and glassware when they were launched 
across the room ( as were he and his girlfriend).  The difference was 
that I had a strong motion of less than 0.1 g and he had >1 g.. peak 
surface acceleration (1.7g) was some 7km from the epicenter.

For the most part, the damage level was continuous (e.g. adjacent houses 
were damaged about the same amount) but there were some striking 
anomalies that could not be explained by construction technique, etc. 
It's theorized that there were reflections and refractions in the 
subsurface structures that resulted in some places with peaks and nulls.

That's aside from things like subsidence and liquefaction, which have 
big effects on damage.

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