as you might imagine I was a bit busy on and around Dec 21 with some mod duties (
) but now it's quietened down again it's good to be able to
relax a bit, catch up and add a few comments.
Great to see you have all been adding your observations and notes.
As generally expected among us here, nothing of any special significance
happened on Dec 21. In the quakes dept, the biggest was a mag 6.8 near Vanuatu, but as it was a little over 200km deep the effects were not great. The
locals are quite used to such shaking anyway so they probably weren't too bothered by it. I've checked some news sources and there were no reports of
damage or casualties. In fact, USGS is quoted as saying:
There were no immediate reports of damage or casualties following the earthquake, but the USGS estimated some 1,000 people on nearby islands may
have perceived moderate shaking. The agency said as many as 145,000 others may have felt light shaking, according to computer models which showed
serious damage or casualties are unlikely.
From wireupdate [dot] com
I'm mentioning the above for two reasons. The first is to reiterate something I mentioned quite a while back -- namely that with quakes, it's not just
the magnitude that matters, it's the location. And (all other things being equal), with "location" there are two primary aspects: the epicenter and
the hypocenter. I'm sure you're all familiar with "epicenter" -- the location on the earth's surface directly above where the quake happens. The
hypocenter, however, is also very important because it's the true
location of the quake, meaning its latitude and longitude and
location (depth) within the earth.
A magnitude 6.8 quake at a depth of (say) 10 km is far more serious than one with exactly the same epicenter, but with a hypocenter 200km down.
Generally speaking, the deeper the quake, the less its effects are felt at the surface. Putting it simply, it's because the energy released has a
greater distance of rock to travel through and so it gets better dissipated before it reaches the surface.
Remember that quake energy radiates in all directions. Not always equally -- as that depends on the type of faulting that occurs -- but it doesn't
just go "up" towards the surface. It spreads out all around, and the way it does can be measured by the various seismos and allow seismologists to
work out what type of quake it was and how damaging it may be.
But never mind all the technicalities. The general rule of thumb is: shallow quake -- bad. Deeper quake -- not so bad. Really deep quake -- probably
not too bad at all.
So, when you see a quake reported, have a look at the "depth" given in the data!
The Haiti quake on Jan 12, 2010 was a magnitude 7.0, so while it was "only" 0.2 magnitude bigger than the Vanuatu quake of 2 days ago, it was
incredibly destructive because it occurred at a depth of only 13 km (about 8 miles) and very close to that nation's capital city.
That 0.2 magnitude doesn't sound like much, but every full mag bigger means the shaking is 10 x worse and the energy released is around 32 x greater.
So, a magnitude 7.0 is about 1.5 x bigger in terms of shaking than a 6.8, and about 2 x stronger in energy released.
Fractions of magnitudes matter a lot, especially when we get into the higher numbers. But the Haiti quake was also very shallow and that's why it
caused such terrible destruction -- especially because the buildings were often masonry, quite old and not reinforced.
quake been 200 km deep like the Vanuatu one, the destruction would have been far, far less. Yes, some buildings might have been
damaged or even collapsed, but not thousands of them as actually happened.
The second point was mentioned in the quote: scientists use computer modelling to estimate the effects of quakes. They set parameters for such things
as location (epicenter), hypocentral depth, the type of faulting involved, the population distribution in the region, and the most common local
building types. For many regions, they have all this information pre-loaded so when an event occurs they can "run" it on computers and see what the
likely casualties and damage may be.
This is incredibly helpful in advance of quakes, for advising authorities on building codes and the like, and also for better coordinating emergency
services. However, computer models have their limitations, which is why it's also extremely valuable for agencies like USGS to get the "felt" reports
that people send in after a quake. So if you feel a quake -- even a small one -- then send in a "felt" report. It helps the experts, and that
could wind up helping you
if there's ever a bigger one!
For example, here's the USGS page for felt reports
for "unknown" events. (Meaning
they didn't get onto the maps.)
edit on 23/12/12 by JustMike because: added link for USGS felt reports page