Continuing the topic of earthquack, but on a different note, what exactly is Richter scale, which is used to measure the earthquacks?
And before you jump to Google to cut-paste the longest explanation for Richter, my question is founded on something much more practical and bewildering (atleast to me).
Two weeks ago, we in California experienced an earthquack. The hypocenter was in Central Cali and the tremors were felt from LA to SF. Two people died and one building collapsed. I am sure there were plenty more people, than the two who died, living in the town which was at the centre of the 'quack. Next day, the news papers proclaimed that the quack was of a magnitude of 6.5 on the Richter scale.
Then a few days ago, another earthquack hit Bam in Iran. Again we read that it was of 6.5 on the Richter scale. Thousands upon thousands of homes were destroyed and the death estimate currently stands at 50,000 people dead. This is tragic, and perhaps mind-boggling.
Why is there so much disparity in the damage toll for two earthquacks which scored identical in the Richter scale? I always thought that Richter scale is a measurement of the movement in earth surface which should co-relate rather closely in how much damage will be done in the earthquack. Apparently that is a very simplistic and perhaps inaccurate perception.
Faisal, I'm guessing the distinction here would be between "Magnitude" and "Intensity"... whereas the Richter scale may indicate the Magnitude of the earthquake, but perhaps not so much the Intensity.
I once read something about another popular scale called the "Mercalli" which is often used and tends to be more subjective. Don't have any ad lib details but someone with more knowledge of the subject may be able to comment.
There's the Mercali scale that measures actual damage. Without a knowledge of the local strata and the focal depth of the tremor it's difficult to compare and contrast Richter numbers with respect to damage.
Also the actual duration of the tremor and intensity and number of aftershocks is important and type of wave (P, S and L) and resulting motion. Shallow focussed earthquakes are usually more destructive but it depends on the above additional criteria.
We had a 4.8 in Cairo last year which lasted about 30 seconds but was centred close to the city and shallow (<5km) and so felt pretty rough. If it had gone on longer it would have probably increased in magnitude and caused more damage than it did, like the one back in 1990.
So yes the Richter scale is a good way to get an approximation for the layman, but when working with such phenomenon one has to know the ranges of certain other criteria (see above) to get a full picture of the earthquake in it’s entirety.
Just Luck. The Cali quakes epicenter wasn't a town but in an area of nothingness, if the epicenter had been downtown LA they would have been mass loss. Even with the new building codes, LA would be a mess as LA was built in the 50's and 60's before such rigorous building codes were put in place.
Now Bam was the epicenter and its 2000-year-old city wasn't designed to handle such a jolt (meaning that no earthquake of such magnitude had hit Bam in at least 2000 years). Soil type also effects the damage a quake does, softer soil = more shaking = more damage.
Well bedrock yep, i.e. type of foundation solid rock or loose sandy soil.
See a Magnitude 5 quake hitting a city built on Miocene sandy deposits similar to Cairo would cause untold damage as the process of liquefaction would come into play (if you shake loose solids quickly enough and at the right resonance they behave like liquids).
But the same magnitude 5 quake hitting Islamabad for example which is built on a mixture of older hard Palaeocene and Eocene bedrock the effect would not be as devastating.
The wave form is also important and the focal depth and not just the location of the epicentre, the effects of many quakes can be displaced 100km's away from the epicentre depending on the fault geometry in the intervening strata.
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*Originally posted by underthedome: *
Just Luck. The Cali quakes epicenter wasn't a town but in an area of nothingness, if the epicenter had been downtown LA they would have been mass loss. Even with the new building codes, LA would be a mess as LA was built in the 50's and 60's before such rigorous building codes were put in place.
Now Bam was the epicenter and its 2000-year-old city wasn't designed to handle such a jolt (meaning that no earthquake of such magnitude had hit Bam in at least 2000 years). Soil type also effects the damage a quake does, softer soil = more shaking = more damage.
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I don't think that is right. I asked myself the same question as Faisal in starting this thread and did some quick checking of magnitude quakes in California. In 1992 there was a 6.7 centered around Northridge (LA)california. The 1987 quake during the World Series in San Francisco was 7.1 . While soil and some of these other things play a role in the amount of damage caused, the reason BAM was wiped out had most to do with construction. LA and SF have been building "earthquake proof" buildings for quite some time. The 1992 LA quake and the 1987 SF quake brought down some bridges and overpasses and cracked a lot of foundations on older buildings. But the newer highrises and other buildings erected over the last 30 years or so were remarkably undamaged. They are designed to sway and diffuse energy caused by the earth shifting so that they survive very major quakes.
All California buildings in earthquake regins have to follow a code of practice which emphasizes buildings that can resist structural damage during earthquakes. Hence most of the new construction is fairly safe in CA for most areass.
Mud houses are fairly easily damaged by shaking cause they only have a very limited resistance in a side to side motion (as against vertical motion resistance), hence the huge damage in BAM. Most of the deaths occur due to fractures (which are always emphasized in the movies) but rather by things falling on people and people getting crushed under falling objects.
I've been thru earthquacks measuring 8.1 on da richter scale in pak, back in 1991-92...! There was no casualty Xcept cracked walls...!
Luck plays a role but...da foundation where a country lies is crucial. If an earthquack comes back there is a lot of destruction. There is a common sense 2 it....
Shake a bowl full of water - da water flips here & there. Now place a solid which is less dense than water, lets say thermocol. Keep a few solid object representing a building...The objects dont fall off...this is becoz da water absorbs da waves of destruction. Ships never feel any earthquacks...! Pakistan lies on a very similar bedrock surrounded by long mountain ranges
An earthquake hit da mountains of bannu(I'm not Xactly sure of da city), during benazirs tenure...da entire city was devastated....da gov of Pak was unaware of da destruction, until China told them of da destruction. There were lots of casualties, & western states helped a lot. This earthquack backtracked & caused a lot of destruction.
Earthquacks measuring 6 r a regular occurrence every year in Pak, maybe da Almighty loves some of da ppl living there or giving us a wake up call be4 a massive armaggeddon.