[time-nuts] New Zealand, Iceland, Haiti

[Steve Rooke]

On 8 September 2010 12:49, jimlux <jimlux at earthlink.net> wrote:
> 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.

One point to understand is that the original 7.1 quake was just 10km
below the surface but we have been experiencing a lot of after-shocks
that are up to 5.4 which have been closer the the surface and closer
to Christchurch as well. This morning at around 8am was a 5.1 that was
just across the other side from the harbour and just 6km deep. These
after-shocks seem to be doing more damage than the initial quake as
more and more buildings and roads are affected. Believe me, even
though it's only a 5.1, when it's that close and shallow, it feels
like a massive shake. There have been about 150 after-shocks so far
and each days max quake is well over 5. The official estimate of the
damage has now doubled to $4bn and it looks like it will take more
like years to put everything right here.

Certainly the building codes here have saved peoples lives but the
fact that we reside on a gravel bed has still rendered a lot of
building damage, even to new properties, due to the liquefaction and
uneven subsidence. But watch this space as many people fear another
big one may occur, as there are new quakes occurring which are not on
the same fault line as the initial ones. Anyone got a spare room :)

Steve

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-- 
Steve Rooke - ZL3TUV & G8KVD
The only reason for time is so that everything doesn't happen at once.
- Einstein