Peak Ground AccelerationWhen you look at a seismogram, a classic one, what are the units on the vert

Peak Ground AccelerationWhen you look at a seismogram, a classic one, what are the units on the vertical axis? That’s the earthquake, right? But what are we actually measuring?A classic seismogram is built based on a pen, held in place by a mass, suspended over a turning drum. During an earthquake, that drum vibrates side to side and the pen records the shaking of the earthquake on it, right?While this simple model does record some details on an earthquake, the real world is much more complicated. Even in this simple case, the pen would be held in place by a mass and its own inertia, right? But if the whole room is shaking, will the mass really stay fixed in position over a long time?Although this isn’t a perfect example, picture the classic “Newton’s cradle” experiment, of balls on strings bouncing back and forth. If you start the balls swinging and they bounce off seemingly elastically, eventually smaller effects build and the balls that were previously stationary will swing from side to side. Inertia only gets you so far if there are lots of different things going on.Trying to record an earthquake is a very complicated problem. If the mass holding our pen in place moves at all, then suddenly the mark on the page doesn’t just show horizontal movement, it is a combination of the horizontal movement of the drum and whatever movement happens to the pen. Even in the case of the best classic seismographs this will be a problem – your pen is never truly fixed, just like the Newton’s cradle is not truly elastic, so the final record doesn’t only show the motion of the ground, it’s instead records a summation of the different motions.This problem actually can get even worse. Earthquakes aren’t just a simple back and forth shaking; they’re a summation of different waves with different frequencies all piled on top of each other. If you try to make your seismograph work so that the pen is fixed for some frequencies, it might shake at others, so the final recorded seismogram depends on which frequencies you measure. It’s kind of like audio speakers – you put low frequencies through a subwoofer and you put other frequencies through standard speakers to produce a more complete sound. If you put two seismographs right next to each other but design them differently, they will produce different records of the same earthquake!Thankfully, we don’t use pen and paper any more to record earthquakes; modern seismographs are electronic. They measure how masses move by converting those motions into electronic signals, which means that we can pull apart these problems if we build an instrument correctly.The instrument needs to respond to different frequencies and getting seismometers to do this has been complicated. It took decades of work by electronic engineers to build seismic instruments that could overcome all of those complexities to produce a standard, regularized signal. During an earthquake, what is actually being measured is the change in electric current associated with motion of a mass inside a seismograph. The instrument will always have some limitations; some frequencies will be filtered or undetectable due to noise within the instrument, but with electronics you can design different systems to capture most of the motion at a variety of different frequencies.That whole discussion finally lets me answer the question I posed at the beginning; what a seismogram is actually showing.A modern seismogram starts off as an electronic response to motion on a mass. In other words, it is directly related to the force the mass experiences. The force on a mass can be directly converted to “acceleration” thanks to Isaac Newton, so for modern seismic instruments, you often see the units presented as “Acceleration”. Plotting acceleration recorded by a seismic instrument versus time gives you a seismogram like this one, recorded during the 2011 Tohoku Earthquake in Japan.Ground acceleration is a useful parameter to measure because it directly relates to the force on an object. During an earthquake there will be lots of different waves with different frequencies passing, the ground will shake in all 3 directions and the shaking will change during the quake due to all the different, complex features of the Earth. However, if you measure the peak ground acceleration, you’re getting a measurement of the maximum force felt by any object during that quake. The peak acceleration on this graph is the point where the absolute value is the largest - where the line gets the farthest away from the center.Peak ground acceleration is a way to take all the complexities of an earthquake, the changes in frequency and intensity, and put a single number on it. In fact, that number can be expressed as a fraction of the acceleration due to gravity a percent of “g” since gravity is roughly constant. If you can get a single number out of an earthquake, that can be really helpful for the public because you can use that as your basic standard for building earthquake-safe buildings.Peak Ground Acceleration is therefore one of the parameters used to develop building safety standards in this country. If you’ve got a building hooked directly to the ground, the Peak Ground Acceleration is the strongest force it will feel during the quake. If you build your building strong enough to survive that moment, then the building could be in good shape.Those are the numbers the U.S. government puts out. The US Geological Survey put out updated seismic hazard maps in 2014 and the most commonly displayed maps showed the probability of feeling an earthquake with a peak acceleration that was a certain fraction of the acceleration of gravity.However, since an earthquake is a complex mixture of waves, designing an earthquake-safe building isn’t quite that simple. Taller buildings sway with different frequencies, so the response of the building is important as well. A building that hits its own resonant frequency during an earthquake could suffer heavy damage even if it survives the peak ground acceleration. The USGS therefore also put out maps of the spectral acceleration for certain frequencies – that’s the maximum acceleration which would be experienced by a structure with a natural vibration frequency in that range.These parameters can be directly related to damage during an earthquake, but that’s not the whole story. Ready for some calculus? We can also integrate acceleration to get “velocity” and plot up “peak ground velocity”. That’s another commonly shown parameter on seismic instruments.If you try to come up with an expression for damage risk during an earthquake, you’ll find that every building has its own properties. Some buildings are most likely damaged during the peak acceleration; some buildings actually are damaged most at the peak velocity. And you know what? It can be even more complicated than that. Some buildings show increases in damage during longer earthquakes even when the peak acceleration or peak velocity doesn’t change – somehow the damage works its way into the structure with multiple passes.To develop a truly quake-safe building code therefore you need to know both the way your structure behaves and the way the Earth behaves. An earthquake is a complicated mix of shaking at different frequencies and over different times. A seismograph can record that in a certain way, but interpreting that data requires a lot of math involving frequencies, wavelengths, travel times, etc. We can convert those complicated functions into single numbers useful in design, but in doing so we lose some constraints on the behavior of the system.Developing earthquake safe buildings therefore isn’t easy. Engineers are constantly testing building designs using large shaketables to see how they hold up for exactly these reasons (http://tmblr.co/Zyv2Js1NJfTeC); the ground can shake in many different ways and some buildings will be strong against one way of shaking and weak against another. These are the kinds of tasks that seismic engineers are faced with in quake-prone areas every day, and they’re not easy puzzles to solve.-JBBImage credit:http://geophysics.eas.gatech.edu/people/zpeng/Japan_20110311/http://eandt.theiet.org/…/2011/04/basics-of-seismology.cfmRead more:http://www.ipgp.fr/~brunet/Seismometers.pdf -- source link

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