I heard on Fox News that the airliners crashing into the WTC were equivilent to a 5.5 earthquake.
Not an earthquake guy is that a major quake? I recall the Northridge one was a 7 point something and the SF World Series one was a 8.
Is that why there aren’t a lot of tall buildings in SF and LA?
There are some so what do they do differently from Chicago when they build them?
Also the major earthquake in the USA was in New Madrid Fault in the 1800s if a quake happened there again of similar strength what would that do to Chicago’s skyscrapers?
First of all the Richter Scale doesn’t measure the force of explosions, it measures the force of an earthquake determined by the motion of the quake I believe (I’m not a seismologist, but I live in Southern California.)
A 5.5 earthquake is moderate.
The Northridge Earthquake in 1994 was only 6.7. The Loma Prieta earthquake (1989 World Series) was 7.1.
There are a lot of tall buildings in Los Angeles. They aren’t as big as the WTC were or the ESB or the Chrysler Building, but there are several buildings over 50 floors. There are tall buildings in San Francisco also.
I do believe that seismic concerns factored in to the size of skyscrapers in California, but so did the fact that Western cities tend not to be as centralized as cities like New York and Chicago.
(Again I ask, the tallest building west of the Mississippi is in L.A.? Anyone know the name of it?)
The New York Times has a graphic somewhere on its site that shows the seismograph activity for 9/11 and it’s quite different from an earthquake.
The most powerful earthquakes in recorded U.S. history predated the Richter Scale but they were believed to be in the 8.3-8.5 range.
An 8.3 earthquake on the San Andreas Fault near either San Francisco or L.A. will make NYC’s problems seem quite small unfortunately.
Fox News is mistaken. The USGS site has some interesting seismological info about the attack and its aftermath. The highest seismic disturbance - when the second tower collapsed - registered a 2.3 on the Richter scale.
And speaking as someone who lives in an active seismic area, a 2.3 magnitude earthquake is almost impossible to feel unless you are right on top of the activity.
They probably meant that the shock was comparable to what one gets from a 5.5 earthquake. I believe the Richter scale measures the total energy released in the earthquake, not the strength of the shock you measure on the ground.
It could well be that at, or near, Ground Zero, the resulting shock was close to that of a 5.5 earthquake (of course, there are no nearby seismometers that I’m aware of, so it’s pretty much just guesswork). Because of the differing nature of the shocks (explosion or collapse vs. earthquake), the WTC activity might have dissipated more rapidly than that of a real earthquake, resulting in the lower Richter numbers.
That would be Library Tower, the tallest building west of Chicago.
Tall buildings on the west coast
Los Angeles
Library Tower 1018’
First Interstate 858’
Two California Plaza 749’
So. Cal. Gas Center 749’
333 South Hope Building 743’
San Francisco
Transamerica 853’
Bank America 779’
Seattle
Columbia Seafirst Center 943’
Two Union Square 743’
Don’t know where they got their info, but none of the events at the WTC (first and second plane crashes, first and second collapses) had a seismic magnitude greater than 2.3. The seismologists down the hall have put together some web pages summarizing the seismic info and illustrating the seismograph traces.
If you want to get a feel for how the WTC events compare to some other recent earthquakes in the region, go to this page and scroll down to the “Recent Earthquakes” and “Earthquake Information Archive.” BobT is correct in saying that the waveform for the WTC events is different from that of a natural seismic event, and that a magnitude 2.3 is actually hard to feel unless you’re in close proximity to the event.
The magnitude (“Richter scale”) of an earthquake is a quantitative measure of energy release. The Mercalli scale is a more qualitative scale of intensity. Perhaps Fox News was trying to refer to the Mercalli scale and got discombobulated (although a V on the Mercalli scale doesn’t seem quite right either). They should have checked the links on this page before they went on the air.
From a seismic hazard perspective, constraints on the height of a building are in large part a function of the ground they’re built on; hard rock is a much better substrate for tall buildings than unconsolidated sediments are. That’s because sediments, when shaken during an earthquake, can begin behaving as if they are liquid rather than solid (a process called liquefaction). You can imagine that having the ground underneath you starting to act like quicksand is not a very good thing for maintaining building stability, especially if the building is tall and thus can sway.
IIRC, a lot of areas around L.A. and San Francisco have either thick natural sediment sequences or modern landfill lying beneath them, so these areas are not good candidates for very tall buildings. In Manhattan, you can get a rough idea of where bedrock comes close to the surface just by looking at the distribution of skyscrapers (e.g., Midtown and the southern tip of Manhattan, but not Greenwich Village).
The estimated magnitude of the New Madrid, Missouri earthquakes (1811-1812) has recently been revised downward a bit to 7.6 to 7.9 (see http://gldss7.cr.usgs.gov/neis/eqlists/10maps_usa.html ). If such quakes happened today, you might well feel it in Chicago, but I don’t think there would be any significant damage there. On the other hand, you could probably kiss the Arch in St. Louis goodbye.
The comparison to an earthquake would seem rather spurious. Even if you say the people in the building felt the initial impact in the same way they would feel a magnitude 5.5 earthquake, it doesn’t really relate to the damage done to the structure. Earthquakes are not usually accompanied by enormous quantities of burning jet fuel, and the fire was a major component in the WTC collapse.
micco, the seismic magnitude of the events relates specifically to the mechanical energy released by the impacts of the planes and of the buildings collapsing to the ground, as detectable via motion sensors elsewhere (i.e., the seismograph stations that picked up the vibrations caused by the energy release). In those terms, we can compare the energy release of the WTC events to the energy release of earthquakes, and use the same scale to measure them. Seismographs can’t measure things like weakening structural supports in the middle of a fire, because they aren’t designed for things like that. But while the average person doesn’t have a good feel for what a 1500 deg. F fire can do, they are familiar with different levels of earthquake damage; the media is just focusing on what’s familiar.
I didn’t see the Fox News report, so I don’t know exactly what they were trying to say. Maybe they were trying to convey the idea that the force of the plane impacts made the buildings shake, and the only “shaking” comparison they could come up with was an earthquake of a certain magnitude. Still sounds to me like a WAG on their part.
I think Fox News was trying to say that if they had a 5.5 earthquake that would have been enuff to collapse the tower.
That is what I got out of it anyway.
Also part two of my question. How do they build skyscrapers differently in SF or LA. Then say Chicago or NYC.
Is it simply a matter of the ground type. I was in NYC this summer and I read that when they build a skyscraper in Manhattan it can’t add to the weight of the island.
Or is a matter of having more support on the inside and out or what?
I think it was given as an explanation of why the neighboring buildings collapsed. The collapse of the WTC towers created quite a shock, causing damage to nearby buildings.
