got a source for that? the reason i ask is because a mechanical engineer friend of mine mentioned the twin towers were only designed for a 10% safety margin for a 100 year earthquake. i tried to look up what a 100 year earthquake is, depending on location its on the order of .3 to .4 G forces.
If they really expect 1-2 G force accelerations.. very few modern buildings are going to survive.
United States Geological Survey and the Oregon State Seismic Safety Policy Advisory Commission. (Added in edit: also Oregon Department of Geology and Mineral Industries.) Published earthquake hazard maps for Oregon. These have the full-on expected acceleration spectra, along with a bunch of numeric parameters that I find impenetrable. (I'm an engineer, but not a geophysical engineer, and this is their bread-and-meat.) Best distillations I can find for the locally applicable magic numbers works out to 1-2G accelerations. Again, soils are crappy in my location.
Location, location, location. The twin towers are not situated close to a major tectonic plate subduction zone with geologically and archeologically proven history of massive (magnitude NINE PLUS, which is ten times more powerful than an insanely distructive magnitude 8) earthquakes. The Juan de Fuca plate is shoving underneath the North American plate and we are about due (based on that history I mentioned) for another rupture any time now. Mean time between major ruptures seems to be about 550 years, sometimes 300, sometimes 700. We actually know to within a few
hours the time of the last rupture (26 January 1700), because the resulting tsunami was recorded in Japan and it was note-worthy there because no one there felt any accompanying earthquake. (There is also corroborating evidence based on tree ring counts and isotope analysis from buried forests. The Japanese records just narrow the date down from +-50 years to a specific date and time of day.)
There is decent evidence that the west edge of the NA plate is being pushed east and up due to the friction lockup of the two plates. They've looked at century-old railroad and highway surveys, and it appears the whole area is tilting toward the east slightly. When the rupture takes place, the Pacific coastline is going to move, probably tens of yards, in the opposite direction and it's going to do it in a matter of minutes. Many low-lying coastal locations will end up under sea level simply because the NA plate dropped, not considering the tsunami.
Structure survival? Yeah, it's going to be pretty grim. Most older residential neighborhoods were built when you didn't even have to tie your house frame to the foundation. Downtown Portland still has a bunch of office buildings with
cast iron frames and brick infill. DOT went around about 10-15 years ago and tied most bridge and overpass spans to the columns with steel cables to (try to) prevent pancaking, but that's just going to reduce loss of life, not retain functionality.
They really,
really are not kidding here when they say you should have several weeks' supply of food and water for your family. Imagine the most intense devastation of the recent Hurricane Michael, but applied to a swath 400-700 miles long and 100-150 miles wide.
About the only good news for my location is that I won't have to worry about the resulting tsunami. The tsunamis are how they discovered this thing repeats every few centuries, because they move enough beach and bury enough hunks of forest to be readily found once you know what you are looking at. But I don't live where a tsunami can reach, even pushing back up the rivers.
Anyway, getting back to the original discussion: Should I be reconsidering my resigned apathy and bolting down my machines?
Added in edit: Here's a lovely quote from a
2015 article in The New Yorker
Kenneth Murphy, who directs FEMA’s Region X, the division responsible for Oregon, Washington, Idaho, and Alaska, says, “Our operating assumption is that everything west of Interstate 5 will be toast.”
In Oregon, I5 is typically 40 miles inland.