What is Significance of 33Hz in Seismic?

[m_ridzon]

Folks, in many seismic discussions I have had over the years, I have frequently heard reference to 33Hz. In terms of seismic, what is the significance of that frequency? Is there some geological reason that seismic vibrations around/above/below that frequency may not matter? Is that a magical seismic frequency of some sorts? Why do I often hear reference to that frequency?

By the way, I'm asking in terms of the continental USA.

 

[GregLocock]

It's a rule of thumb for the upper limit of signal from an earthquake. Like most rules of thumb it works until it doesn't. On hard rock you can see 100 Hz.

So on this graph 33 Hz is a period of 0.030 s, so yes there's nothing much going on between 0 and 0.030, but it seems a pretty arbitrary cutoff.

 

[m_ridzon]

It's a rule of thumb for the upper limit of signal from an earthquake. Like most rules of thumb it works until it doesn't. On hard rock you can see 100 Hz.

So on this graph 33 Hz is a period of 0.030 s, so yes there's nothing much going on between 0 and 0.030, but it seems a pretty arbitrary cutoff.

Thank you for this feedback. Please help me understand your chart. It appears that most of the interesting activity occurs above 0.030s. Is that why you referred to it as an arbitrary cutoff? Did I understand correctly?

 

[GregLocock]

Yes, I can't see why on the basis of that data you'd choose 0.030. However, if buildings are designed to 33 Hz and don't tend to fall down then I suppose you could argue it is sufficient. You'd probably get a better response on the structural engineering forum where people design earthquake proof buildings.

 

[IRstuff]

The seismic activity cutoff would only be half of the equation; the other half is building response to the seismic activity.

Buildings likely don't have much seismic response above 33 Hz.

 

[m_ridzon]

Buildings likely don't have much seismic response above 33 Hz.

Is there some basis to justify 33Hz as the cutoff? Some theoretical data? Empirical data? I understand a building will likely no longer respond as the frequency gets higher. But what person/organization ever decided 33Hz was that magical number?

 

[IRstuff]

Given that it's a rule of thumb, you'll likely not find anything rigorous; see https://www.researchgate.net/post/W...r-dynamic-analysis-is-33-Hz-for-Seismic-loads