Wednesday, December 30, 2020

More on Croatia earthquake

 Central Croatia is located in a monster valley, located in compressive tectonics.  The whole Mediterranean basin is closing, creating the Alps and surrounding mountain ranges.  The Croatia mountains are moving with a sliding thrust, which has both strike-slip and compression.

Each monster zone around the world has about a one in 500 chance to light up with an M8 or 9.  That means that each year, Fate rolls a 500 sided die for each zone.  One side is painted red.  If the red comes up, then KABOOM!  

If there is something that looks like a foreshock, as with the recent earthquake, then the die is 100 sided for most areas.  If the Fish calls it, then the die might go to 20 sided.  All of this is fairly irrelevant for day-to-day life, but is good for planning purposes.

If the Big One rolls up, then we are looking at a PGV of 1-2 m/s on the valley soil and hanging wall.  This is enough to throw rocks as with Kern County 1952.  No building can take this, yet if you are on the footwall rock, then it is only a few centimetres per second.

If you want to build something that will survive, then you need tremendously deep foundations.  I doubt that anybody has done that.  Might not help with the giant boulders rolling down the mountains.

ps.  I define monster zones as that which needs large earthquakes to maintain its form.  Thus, we have very smooth subduction zones and then we have sharply defined mountain valleys.  All these zones would fill in and become ragged, if not for huge earthquakes.  Once the earthquakes stop, then everything becomes smooth and boring.

pps.  and as a great Christmas present from the fish, the die goes back to 500 sides in a few months.


CB said...

"...then you need tremendously deep foundations." Is that because the foundations have to be into the foot wall, or can they sit in overburden/loose material, as long as they are long enough?

Harold Asmis said...

The whole valley is the hanging wall. It's busted up rock. You are at the worst with loose soil on the top. Then you have a factor of 100 amplification over the footwall. A deep foundation can reach harder sediments, so you might only have a factor of 10, which can be handled. All of the fancy California tech companies are using deep foundations. So I would build a house with deep foundations, with concrete and steel roofing. Have it designed like a ship, with everything secured, and I would sleep comfortably.

CB said...

Thanks. Rereading my comment today I took 'deep foundations' to mean 'piling'. What's a deep foundation for a house in this sort of case? Is it just 'as deep as you need to get to some sort of competent material'?

Harold Asmis said...

Yes, for my dream house in the worst earthquake zone in the world, I would first do a simple soil auger drill. The first 3 m would have an N value of 3. This value is how many drops of a standard weight will drive the sampler a foot. At 3, you have liquefiable sediments. Have all the water and sewer connections flexible. Have a well, and compost toilet available.

I would find a depth where the N value is 20 or more (probably 10-20 m). Then I would send down screw piles. I would beam them, and provide a foundation for the walls. The basement slab could be free. Then I would use 'form in place' concrete with extra mesh, just like a concrete boat. The roofing would be a steel structure, with steel shingles, or that large-sheet composite. I would locate away from the mountains, unless it is the foot-wall. These earthquakes are notorious for house-sized boulders rolling down. In '52 Kern County, adobe cabins stood on the foot-wall. I would finish the outside with that adhesive stucco. No chimneys, no brick.

CB said...

Thanks for the detailed answer. One more question if I may; is the 'safe' N value you look for ( you have 20) independent of what the ground conditions are? Is building at a place like Woodbine and Danforth with a lot of old beach sand (let's say it's a uniform 30 metres of the stuff), the same as building in Croatia, which isn't old beach sand exactly ( assuming we could get the same max 1/2 m/s PGV in both places)?

I am not building anything at Woodbine and Danforth, or anywhere else :) I'm just curious if the N value is all you need to consider when thinking about piling depth.

Harold Asmis said...

The beaches area, and parts of High Park will have a very low n value. Up a bit, like my place, we are on compacted till. The n value is 40 or 50. For loose soil, the seismic hazard is the same throughout the world because the amplification is so huge. My general rule is that if you have trouble setting fence poles, then you are fine. Toronto probably has the same general 1 in 500 hazard of a large earthquake like Croatia. It's just that a blind thrust can produce such a huge ground motion over hundreds of klicks. In Toronto, a frame house on compacted till is fine.