My new BSSA is in, and it's a big thick volume all about rotational seismology. And what do you know? I think it's all a crock! For hundreds of years people have looked at tombstones after an earthquake, and have seen them rotated. From that, they assume there is a strong rotational component to the seismic motion. I thought that was all debunked when they realized these things just walk that way through rocking, but no, this journal is full of those pictures!
Now, when it comes to building dynamics, rocking is very important, but this can be all figured out with multiple translational sensors, and some decent dynamic modeling. But here they make a big deal out of it. I'm just amazed.
11 comments:
But...
I assume this rotational stuff is related to motion of a point on the ground during an earthquake is in 3 dimensions and if plotted over time would trace an ellipse or some what round shape if view from the correct orientation.
You got pictures?
They got a website.
OK no what I said above is 3D ground motion which I guess is old hat.
This abstract says they need highly accurate million dollar laser ring gyros to measure the rotation. They also mention that it had been disregarded in the past because the effects so slight and difficult to measure.
So in their best example how many degrees (milli-radians?) of rotation has been measured?
Ok! So when my brain starts to work again, I'll actually read the journal! and put in some pictures.
You need to update yourself on the technology. They have been trying for years to derive rotational motion from translational sensors. If the sensors are not perfectly matched including all mechanical and electrical components that are an impossible feat, the math errors are greater than the results. The R-1 is immune to translational motion. It only senses rotational motion. Prof Trifunac and Dr. Graizer helped us develop the world's first field rotational seismometer that has the resolution and low noise required for monitoring rotational motion. This motion is smaller in amplitude than translational motion but can cause more damage since structures are designed to withstand translational motion and not twists and tilts. Also measuring rotational and translational motion one can compute permanent displacement, the "Holy Grail". In a wave equation published by Dr. Graizer years back has a rotational component. All current math models ignored this factor because there was no way to measure rotations until now. Ring gyros are huge and expensive lab toys. Not field instruments. Now there is a sensor that the field guys can use. That is why they are publishing the special edition on rotational seismology. This is an "emerging" branch of the science. Do your research and you will find that this is not “bunk
Yeah! Thanks for the comment. Life is always a learning experience.
So Bob is this somehow different than 3 dimensional ground motion during a quake? Or is it a 4th dimension of motion ie; rotation of the horizontal plane about the vertical axis?
Interestingly if one googles 'rotational seismology' this post is the 3rd hit.
Wow, hot topic! Makes me want to actually know something about it!
M'kay lets dig deeper. (almost geo pun?)
For reference Prof. Mihailo D. Trifunac's cv from USC with links to papers.
And Some papers from V. Graizer at the California Geological Survey.
A quick text search finds no mention of rotation on either page but more research is indicated.
OMG! Bob would be so proud!
Since this is the #2 hit in Google, I have written 2 follow-up articles, which can be searched for.
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