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Comment by quartesixte | Hacker News Reader

Earthquake codes in California mitigate a lot of the intensity of a shake and its damage.

Anecdotally, I heard to level Downtown Los Angeles completely it will have to be hit with some absurd Earthquake like a 9.0

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The strike slip fault system in most of California can produce earthquakes up to about 8.0. Subduction zone earthquakes that occur in many places, like Northern California up to Alaska, Chile, Japan, etc, can produce 9+ earthquakes. 10 and above only occur during large impact events. So California can have buildings "earthquake proofed" for much lower cost than say Seattle and they come at higher frequency. That may be one of the reasons that California's infrastructure is pretty well prepared for coming earthquakes while Seattle will have major damage when the next 9.0 hits in 0-400 years.

The most dangerous fault line in Seattle, both in terms of earthquake and tsunami risk, is the eponymous system of shallow thrust faults[0] that run through the middle of the city and historically have produced ~7.0 earthquakes.

Seattle buildings built in the last 3 decades are explicitly designed to survive extreme earthquakes. Most of the risk is in the 1970s and earlier infrastructure, quite a lot of which has been torn down to make room for the massive growth of Seattle.

[0] https://en.wikipedia.org/wiki/Seattle_Fault

After living in an apartment constructed in 2019 in the Seattle area...see I want to believe you but I just can't.

There’s also a healthy local trade in earthquake retrofitting. After the nisqually quake the city got pretty serious, a lot of old buildings downtown were severely damaged

> designed to survive extreme earthquakes

What magnitude is that? (The highest they're designed to survive)

The modern building codes are derived from a 9.0 subduction zone model. I've heard people say that building standards derived from this model should work for just about any type of earthquake up to around 8.0-8.5 even if not specifically modeled; I have no idea how true that is but it seems plausible. Building codes are not defined just by the strength of the earthquake but also the earthquake type, depth, soil etc. It isn't the strength that is the problem so much as the kinds of loads different earthquakes put on structures.

Seattle has a couple ~7.0 earthquakes every century, so the city has experience with strong earthquakes and everything old that is still standing has at least some ability to resist earthquakes (survivor bias). The last big earthquake was in 2001 (magnitude 6.8).

Thanks!

> isn't the strength that is the problem so much as the kinds of loads different earthquakes put on structures.

Interesting, didn't know.

a number of years ago, i worked in one of the downtown LA office towers when a 5.x earthquake hit. it was a jolt and rumble, and looking out the window, we saw the other towers nearby swaying back and forth. then we realized we were also swaying back and forth. it was unnerving to say the least. the little engineer in me eventually kicked in to point out that that engineered-in flexibility was what kept us from breaking apart and crashing into the ground.

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