Preparing for the next "big one"

[mumbojumbo 0]

I just read this about San Fransisco and the good chance of another big quake there whenever

 

http://news.bbc.co.uk/2/hi/science/nature/4906678.stm

 

"The 20th Century was a period of anomalously low earthquake activity; and it was during this period that the population developed and all the infrastructure was put in."

 

Some of those things in there they wrote. Just why, when they know that the place is prone to big quakes, do they redevelop there? Same with Tokyo and the like.

[7-11 2]

Humans are obviously just silly.

[xxx 2]

Such a lovely city as well....

[BagOfCrisps 24]

It's a very reasonable question. It does seem silly to develop on land that is so prone to quakes.

[soubriquet 0]

It's mostly population pressure which causes people to live in hazardous areas, and it's not just earthquakes. I wouldn't like to live in New Orleans or on the Ganges Delta.

 

The theory of plate tectonics was fully developed in the 1960s. Before that we had no reliable to method to predict the location of earthquakes. They were treated as acts of god. Now we know what is going on, but it's a bit late as all the areas were already developed.

[BagOfCrisps 24]

As late as that? Thanks for the info, interesting.

[soubriquet 0]

The theory of continental drift has been around for centuries. Since we have had decent maps of South America and Africa, people have been fitting them together. Early last century, Alfred Wegener made most of the connections. The geology matched across the South Atlantic, and the palaeontologists could match up the fauna and flora, then describe the evolutionary diversification. He also knew that continental crust is bouyant. The weight of ice sheets sink it, and it comes back up when the ice is gone. The problem was that there was no mechanism to drive it. Without a mechanism, a beautiful theory remains pure speculation.

 

The geological community was then divided into two camps: roughly, the up-and-downers (US) and the side-to-siders (Europe). After WWII, The NATO and Warsaw pact countries were very interested in who was setting off nuclear explosions, and where, so they set up world-wide nets of seismometers. This had the side effect of allowing us to map the location and depths of earthquakes.

 

At the same time, the World's navies were mapping the topography of the ocean floors. The mid-ocean ridges and the deep trenches were discovered. Other groups were working recording the magnetic reversals preserved as stripes in oceanic crust.

 

By the late 1950s enough work had been published on a diverse variety of phenomena for all the specialists to bring them together and assemble them into the big picture. Plate tectonics is one of the true great paradigms of science. It is a unifying theory which brings together an amazingly diverse range of observations and explains them in one. It was a revolution waiting to happen, and it was the undertanding of sea-floor spreading that was the key. Prof Fred Vine, my geophysics lecturer at UEA had a big hand in it. He was the one who realised that the magnetic striping was caused by the sea floor moving apart at mid-Ocean ridges.

[lin 0]

Just read another article about it. Scary things.

[rach 1]

You are well up on this kind of thing soubriquet. omoshiroi!

 

I was watching a bit of CNN last night when it was 100 years to the minute that it happened. They said that 50% of the people who would die in a new quake there would die in 5% of the old buildings.

[bobby12 0]

Just how earthquake proof are new houses in Japan? Has there been any big-ish quakes recently to test out the post-Kobe designs?

[bushpig 0]

The new ones (and some of the more expensive ones) are supposedly very earthquake resistant. One big problem will always be the stuff in the houses falling down in a real big one. Some of the new designs supposedly go a long way to damping down actual shaking within the house. Interesting to see just how well they would go in an earthquake like Kobe.

 

Bobby12, did you hear the news about the architect who got busted for falsifying the building standards, re dimensions required by law for earthquake resistance, of a number of high rise apartment buildings? Apparently they could come down in a 6+ earthquake. Doesn't inspire a lot of confidence in the building industry here regardless of what laws are in place...

[joshnii 2]

bobby12, didn't you hear about the powerful M6.8 one we had in October 04?

 

http://www.snowjapanforums.com/cgi-bin/ultimatebb.cgi/topic/8/3608.html

 

Lots of buildings destroyed/damaged.

The town of Kawaguchi near Nagaoka had a "for the first time ever" recording of shindo 7 shaking. I went through the town about 6 months after the quake and it looked like a bomb had been dropped on it.... hardly any building standing. Very creepy it was.

 

It seems the newer types of buildings came off much better than the older ones, esp. the type with concrete 1st floor.

[soubriquet 0]

Round here bobby12, I'd say the earthquake resistance of new houses is pretty good. Typically, they are lightweight timber construction on a reinforced concrete foundation. Roofs are timber clad with steel sheet, so there are no heavy tiles. Timber is very nice because it is flexible. Our house also has a steel frame, but that is not common.

 

[SnowJapan#Andrew 6]

I live in a house that was built about 10 years ago with a reinforced concrete foundation.

 

I remember on the night of the Chuetsu earthquake there were 3 main quakes within a period of about 45 minutes. By the time we had the third one, which was the strongest of the three where I live (shindo 6), I was stood outside looking towards my house. I remember quite clearly standing there with my legs wide apart trying to keep some sort of balance and watching my house wobble wildly like a huge jelly. I don't think I'll ever forget that.

[Yuki's Passion 1]

 

Were there any buildings or other such materials that could fall and hit you outside your house SJ#1? That mustve been a freaky experience for sure!!

[SnowJapan#Andrew 6]

Not really CB, luckily. I live in a very quiet area, lots of rice fields and vegetable patches nearby so not much to fall.

 

From what I saw in my area and just to the north, it seems that shindo 6 kyo is when the real structural damage starts to happen.

 

The unique thing about that one (so I hear) is that there were 3 main quakes that night - 2 within 10 minutes or so of each other around 6pm, and then the third one at about 6:30pm which was to the south of the others - and so closer to where I live. By the time that third one struck everyone was outside their houses thinking that the worst of it was over!

 

It was very freaky. It actually took me a long time to stop 'being scared'.

[Yuki's Passion 1]

>It was very freaky. It actually took me a long time to stop 'being scared'.

 

I woulda switched to diapers had I been up there!!

Seemed like everybody who lives up there on these forums was in the same boat. Its over for now...

[SnowJapan#Andrew 6]

Lets hope it stays that way!

[muikabochi 208]

I'll second that!

 

Even a slight rumble now - like the small one yesterday morning - sees a big hearbeat reaction now! I can't see me ever getting used to them.

[Amos in Utsunomia 0]

Just had an earthquake here. It felt pretty big to me - the most I've experienced. Very strange feeling.

[bobby12 0]

So what do you reckon is the safest place to live in? Big 15 storey mansion, 3 storey apaato, or a little old wooden shack?

 

After reading this when I get my place down nr Kobe Ill be sure to get something built in the last 5 years or so at least. Hopefully the Kobe area will be safe from big ones for the next 50 years or so at least anyway.

[joshnii 2]

I want to live in a new place and not high up.

[soubriquet 0]

After Kobe the building regulations were revised, so I'd start with something modern. I don't like high rise anyway, but being whipped around several floors up together with the TV and kitchen dresser doesn't appeal.

 

Avoid low flat and marshy areas. Find somewhere on a hill, but not on a ridge, a steep slope or at the base of a steep hill.

 

Try to keep a sense of proportion. Kobe was a disaster, but it happened right in the city. In comparison, last year's Pakistan earthquake occurred in a rural area, but there were 20-times the number of deaths. Structures here are probably better engineered than anywhere else.

 

http://www.seismo.unr.edu/ftp/pub/louie/class/100/effects-kobe.html

[brit-gob 9]

Why on a hill soubriquet?

[soubriquet 0]

It is because hills are generally rock or consolidfated material. Basins and valley floors filled with sediments, and are prone to liquifaction and amplification. Liquifaction can swallow buildings. Amplification is apparently caused by constructive interference of the seismic waves. Constructive interference can be seen at the beach, where swash and backwash meet, the height of one wave is added to the other.

 

Look here:

 

http://www.snowjapanforums.com/cgi-bin/ultimatebb.cgi/topic/8/4289/16.html?

 

"These are two seismograms from different sites in Kobe, from the 1995 earthquake. The site for the left hand seismogram was on solid rock, and right hand seismogram from a site in a sedimentary basin.

 

 

Clearly in a basin of saturated sediment, the intensity and duration of the event is much greater." This is referred to as "amplification".

 

Tsondaboy replied:

 

"(amplification) is highly depended on the frequency of the wave and the elastic constants of the formations. For frequency lower than 0.4 sec the amplification of a wave pashing through a rock formation is higher than that of passing through a sedimentary formation. Generally speaking though, rock formations have higher shear rigidity strength than sedimentary formation, which results in the amplification of the wave when passing through a rock layer to a sedimentary layer."

 

I posted:

"Some time googling turned up some stuff.

 

From here: http://earthquake.usgs.gov/regional/pacnw/ships/pdfs/SnelsonBSSA2005.pdf

 

"Amplification of strong ground motions around the edges of basins has been interpreted as resulting from interference patterns along crustal fault zones and thinning basins (Kawase, 1996; Graves et al., 1998). Finally, surface waves generated within these basins are thought to be responsible for much of the increased amplitude and duration of shaking during earthquakes (Frankel et al., 1999, 2002; Pratt et al., 2003a; Barberopoulou et al., 2004)"

 

And from here:

http://peer.berkeley.edu/year7/yr7_projects/ta4/2212003.html

 

 

"..show results of vertical wave propagation in soft and stiff soils. The plotted lines are the time histories of displacements from the depth of 30 meters to the surface. The seismic amplification in soft soil (clay) is apparent!""