Hi Steve:
The Ricter scale was developed based on pendulum seismometers and that
the "P" wave arrives first then the "S". The time delay between the P
and "S" wave gives the distance to the epicenter (time nuts connection).
The magnitude of the "S" wave (adjusted for the distance) gives a Ricter
magnitude. But this system does not relate to the energy contained in
the quake.
http://www.prc68.com/I/Seismometer.shtml
At the time of the Loma Prieta quake I was standing in front of the
company where I worked and watched the windows "oil can" with the
magnitude of the in-out motion increasing with each cycle. If the quake
had lasted a few seconds longer they would have exploded, either sending
glass into or out of the building. We were all starting to lay flat on
the ground in case the glass was coming out. Then the quake ended.
The subject locations are all on or adjacent to the Pacific plate as is
California.
Have Fun,
Brooke Clarke
http://www.PRC68.com
Steve Rooke wrote:
On 8 September 2010 12:49, jimlux<[email protected]> 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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--
Have Fun,
Brooke Clarke
http://www.PRC68.com
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