Earthquake Damage: Newsletter Challenge (12/04/07)

You two are so full of BS!

There you go with your merry little thread about structural integrity and all that but the real problem is that even the most structurallly sound house that slips on its foundation is not going to survive. Hard rock= shaken, but not stirred. Soft soil= shake, rattle, and roll!

Look at post #99 for a short take on the REAL world!

Get a clue!

Ignorant (both senses) as ever. I fear the "good answer" ratings must be going to your head.

Here am I trying to explain some (not all) of the reasons why buildings are usually more sensitive on soils and aggregates, and all you come up with is infantile invective. Perhaps you don't ever care about the "how and whys", just about what happened when you did something last time?

It is perfectly possible to build highly earthquake resistant structures on soils. In fact, the hotel I was staying in was on soil, not rock. The reason it survived is that it was designed to float on the soil, and was strong enough to be self-supporting. It was still a thoroughly frightening experience - once what was happening had time to sink in (which the hotel didn't).

P.S. Is that creature who needs sunglasses to shield his eyes from his own brilliance by any chance your own good self?

"Ignorant (both senses) as ever. I fear the "good answer" ratings must be going to your head."

I represent that remark, both of them!

"It is perfectly possible to build highly earthquake resistant structures on soils. In fact, the hotel I was staying in was on soil, not rock. The reason it survived is that it was designed to float on the soil, and was strong enough to be self-supporting. It was still a thoroughly frightening experience - once what was happening had time to sink in (which the hotel didn't)."

Ah, the tail wags the dog!

Perhaps the reason your hotel HAD to be built so it would float and strong enough to be self-supporting was that it WAS built on soil, not rock. But I do appreciate that a specially built structure can survive on soft soil. Would not this same structure have done even better on top of a rock foundation, which would merely slip back and forth underneath it? Perhaps it would have even been a little less frightening! (Hope you didn't soil your pants or get seasick!)

It is possible to over-think some of these silly "challenge" questions, though, isn't it?

On the other hand, many answers are definitely, too simplistic and just plain wrong. I especially like the answers that say, "soil is soft, rock is hard, so the soil absorbs vibration, like rubber feet on a piece of machinery, and the house on softer soil does much better". Talk about comparing apples to oranges!

Well caught sir. Nearly everything you say in this post is both true and well reasoned - the sole exception being "overthinking" - so far as I'm concerned, the sole reason for attempting these somewhat open-ended challenges is that it provides scope to analyse the situations and understand the limitations of our capability. Also, it's a great (and relatively low-risk) forum to ensure that powers of explanation don't atrophy completely

No, I didn't soil my pants - at least not on that occasion - it takes something far smaller than an earthquake to cause that sort of thing. Yes, I've been in earthquakes on rock (both in LA and SF). The movement was indeed much smaller; but you'd certainly notice a 7.4 quake if you were near the epicentre, whatever the soils.

Perhaps somebody could test this. Pour some sand on your desk, a slab of jelly as well. Pile 3 dice on sand, 3 on jelly, and 3 on desk. Next hit the side of the desk and see which pile falls over. I'd say the jelly pile falls first (it will wobble for longer), but don't have any at hand to test this. Using butter as 'mortar' for the mini bricks may be useful to complete the modle. Foundations are optional, depending on whether you want to drill holes in your desk.

I'll remember not to take you up on a dinner invite.

I'd remove the plates, candelabra etc first. Mother in laws feet would be nail gunned to the floof so there shouldn't be a problem.

I feel that the house which is located in the softer ground will suffer less damage than the one on the harder surface, as the pressure gets absorbed much in the softer ground than the one on the harder surface.

Am I right?

No

The house fixed rigidly to the ground will suffer the most ground vibration. standing on bed rock it will get the most vibration and probably sustain the most damage.

However a rock avalanche could occur and bury the house in the valley.

GF

Good motto - so hopefully when (not if) you take a look at the history of earthquake damage or at the variations of insurance premiums versus substrate in earthquake-prone regions you will change your mind.

I say, folks that please remember it is the Season of Peace and Goodwill to all men.

As it happens,all 3 houses are constructed identically, same colour and all.

There were previously unknown faults in the Hard Rock.

By special arrangement with The Authorities, I managed to get short Video clips of what happened to the 3 houses when the Earthquake actually struck, and have permission to display those short video clips here in this very Topic Thread at CR4.

House # 1 - Rim of Valley

House # 2 - Part way down

House #3 - Valley Floor

So now you all know.

Happy Christmas, from far away..... .....

The house just near the rim will likely be wiped out by mudslides travelling at considerable velocity, so it will suffer the most damage. The next most likely candidate will be the house in the soft soil in the valley. The house sustaining the least damage will be the one built on solid rock above the rim of the valley.

"I can't stand it no longer". Crikey didn't the question say that the house in the valley was on soft earth? To all those who put "liquefaction" or "soil Liquefaction" wrong, wrong, wrong. Soil liquefaction only occurs in presence of water saturated soils. It has to do with water pressure. Nothing to do with how "soft" a soil is.

The house on the Rim (hard rock) = some damage due to P-wave and s-wave . - low surface wave intervention. P wave would probably have most effect.

The house on the Rim (soft ground)= greater damage due to P-wave , S-wave and surface wave intervention. Combination of all three.

The house in the valley would probably have the most damage by the seismic event because it would be struck by the Primary wave (compressional), the Secondary wave (Shear_up-down - side to side) and by the surface wave (like the s-wave) at once. Biggest effect - of all waves combined together at once. Soft soil would slow P-wave down so S-wave could hit at same time.

No soil liquefaction unless there was a high water table or river nearby.

"I can't stand it no longer". Crikey didn't the question say that the house in the valley was on soft earth? To all those who put "liquefaction" or "soil Liquefaction" wrong, wrong, wrong. Soil liquefaction only occurs in presence of water saturated soils. It has to do with water pressure. Nothing to do with how "soft" a soil is.

SNIP

No soil liquefaction unless there was a high water table or river nearby."

You make several good points, and maybe we ARE making an assumption here, but aren't most valleys formed over time by stream erosion and the soft soil at the bottom of these valleys by periodic flooding leaving alluvial* deposits from said stream? Therefore one might assume that, unless the water table was extremely low at that moment, the house "down in the valley" IS built on water saturated soils, being adjacent to a stream or at least in its flood plain, and therefore would be affected by liquefaction, as opposed to the house built on soft soil, but higher up near the rim, which would certainly be above the water table and unaffected, except by the energy waves as you described.

* Merriam-Webster's Online Dictionary, alluvium: clay, silt, sand, gravel, or similar detrital material deposited by running water (Sounds like "soft soil" to me!)

This is fun.

The a definition of a "soil" is a unconsolidated material on the surface that is capable of supporting life. Alluvium is Alluvium. Alluvium is not soil.

But back to your post. Yeah, I get your meaning. Sometimes we have to assume certain things. But for a bunch of engineers, physicists, chemists, electricians, geologists and the other assorted professions that are attracted to CR4 it is hard to "assume" anything.

At the moment I'm working in the mining industry. And I'm sure, a lot like other work places that if you assume something you can either kill yourself or somebody else.

As with the others, not enough information is there to make an accurate guestimate. Example: Which if any of the houses were constructed using Simpson Strong-Tie connectors? The structure of the building makes a very large differance in its ability to take punishment prior to failure.

Vincent R. Brown

Simpson Strong-Tie Employee

Well, all the discussions are fascinating, but the most basic part is missing. When the rock deep and under tension ruptures, all the stored Energy is released and travels away from the rupture location. The energy at any spot can be expressed as:

tension times amplitude

Strong, stiff rocks will show high tension, hence low amplitude.

Soils, having not much strenght will have large amplitudes, sometimes even liquifying.

Manmade structures are insignificant at the energies released, in the sense of withstanding them. Coping with them and riding them out is the rational way.

So, interestingly the house on bedrock will barely notice an event, that destroys

the valley houses. Additionally a houseboat on the lake will not notice much, because it is not coupled to the soil (short of a large wave swamping it).

For simplicity I ignored the energy reflection at the interface rock / soil etc.

The San Francisco earthquake is case in point. Quite a few years ago Scientific American had an excellent article of the 1900(?) quake that destroyed the city.

Can you cite examples of this from the real world? I think real world examples would support a different conclusion. This blog had a large number of responses without agreement among them.

Unless the question setter has taken this from a real example I doubt proof could be given. There are so many variables in the situation as described that I think it's impossible to give a definite answer. I'm inclined to the view that houses on the hard rock rim would suffer most, but we simply don't have enough data. The question setter has however set in train a fair bit of debate - few of these questions seem to have an absolute answer. Half the fun is arguing over various interpretations etc. Due to the half-life of challenge questions I fear we may not get much further input. There are some various threads on CR4 that discuss the issue of earthquakes. None are currently active , but you may enjoy the read if you search them.

fault movement, frequency travels farther through a soild low frequencies appear to develop in longer cycles while riding on mid frequency cycles the mid cycles become low cycles as they loose energy producing extended lower cycles. ultra high end frequencies give energy to these cycles at the point of movement. If the solid rock was releaved at the fault with, a braided teflon kevlar sleeve with sand, sea water around a felxable tube. Maybe used for running Fiber optic cable, maybe even fill the tube with mineral free water. This could prevent collapse of the tube, allow for some movement. If the fiber optic cable was damaged the data stream could bridge the gap because the water filled tube could act as a back up.