<!doctype html public "-//W3C//DTD W3 HTML//EN"> <html><head><style type="text/css"></style><title>Re: [Geysers] FW: Earthquake swarm - Question</title></head><body> <div>This is a follow-up to two previous responses.</div> <div><br></div> <div>The size of an earthquake is a question that requires some physics to understand.  The most important point derives from the fact that essentially all earthquakes happen under ground, usually many kilometers under ground.  As a consequence, people (and our stuff, including seismometers) are relatively far away from the place where the earthquake happened.  At those distances, it's as if the earthquake happened at a single point that we call the earthquake focus or a point source.  You already know what happens to energy emitted from a point source: as it moves away from the source, it gets progressively more and more spread out in space.  This results in the energy getting weaker and weaker, just as you experience moving away from a light bulb or from someone clapping their hands.</div> <div><br></div> <div>One of the key tricks in the history of seismology was learning to account for this geometric spreading of seismic energy.  The various magnitude scales were developed to see past this difference between what a seismometer records (at some distance from an earthquake) and the quake itself, and their goal is to quantify the source, at the source.</div> <div><br></div> <div>The Mercalli intensity scale of earthquakes (described in a previous posting) is different, in that it is based on what happens to things (e.g., houses and walls) sitting on the surface of the Earth at variable distance from the earthquake.  Of course, what generally is seen is that consequences of an earthquake are most severe on the ground above the earthquake focus, near the epicenter (the point on the ground directly above the focus), and diminish with distance from the source.  A general idea of the size of an earthquake can be gained by systematically mapping the pattern of damage.  This is a particularly important technique for studying pre-instrumental earthquakes, which occurred before seismometers were deployed all over the world.  But a better job can be done using seismograms, the records made by seismometers, so that is what seismologists mostly do.</div> <div><br></div> <div>In more direct answer to the question originally posed, it is possible to make analogy between the energy released by an earthquake and other punctuated energy releases, such as the impulse of an 18-wheeler falling on the ground (see http://en.wikipedia.org/wiki/Richter_magnitude_scale).  The intuition gained, however, must be put into the context of how far you are from an earthquake or its analogous energy source (as nicely done by another previous posting), also necessary to relate earthquake magnitude to the shaking at ground level.</div> <div><br></div> <div>In passing, it's worth pointing out that ground shaking depends not only on the earthquake, but on the character of the ground.  Bedrock, soil, and unconsolidated water-saturated mud (remember the Nimitz freeway after the 1989 earthquake in the Bay Area?) all respond differently to the same seismic energy, so people in different places can feel entirely different shaking from the same earthquake.  --Randy</div> <div><br></div> <div><br></div> <blockquote type="cite" cite><b>From:</b> Goh83642@wmconnect.com [mailto:Goh83642@wmconnect.com]<br> <b>Sent:</b> Monday, January 19, 2009 11:00 PM<br> <b>To:</b> geysers-bounces@lists.wallawalla.edu<br> <b>Subject:</b> Earthquake swarm - Question</blockquote> <blockquote type="cite" cite> </blockquote> <blockquote type="cite" cite>Gazers,<br> <br>   I have a question about the recent earthquake swarm in the north end of Yellowstone lake.  My question isn't so much about the swarm as it is about a relative analogy of how much an earthquake intensity is.  In other words, if an earthquake has a magitude of 1.0, how can I compare that to something I can see on a real time example: such as: dropping an 18 wheeler tractor from 100 feet will cause an equivalent of a 1.0 magitude earthquake.  <br>   Since there were hundreds of earthquakes in magitudes of 1.0 - 3.9 in the swarm, it would sure help to visualize how much shaking it really did to the surface of the earth.  <br>   If there are any specialists on earthquakes out there, please give this request some thought and let us all know the answer.</blockquote> <blockquote type="cite" cite><br>   Regards,  Gary Henderson</blockquote> <blockquote type="cite" cite >           <span ></span>      Meridian, Idaho</blockquote> </body> </html>