Originally Posted By: FL_Rob
Originally Posted By: andyd
Originally Posted By: Stu_Rock
Tempest and Drew, I feel like I should weigh in on the debate here, since it is my line of work, but I'm not sure what you guys are specifically trying to argue for/against.
please school me. I felt the floor move for about 15 seconds. I didnt regard the time, but what I felt was more toward 1:30 ish EDT. Are tremors like sound in that they travel at a certain rate away from the epicenter?
Yes, they move in waves, akin to ocean waves..the distinct difference is in the fact that the waves travel through hard rock ..therefore the frequency is much sharper for lack of a better term, and quicker as well.The movement of the waves through rock is tens of thousands of times the effect through any other substance.Thats' why the Richter scale is gradient to the tenth of a degree(5.8 is a thousand times less than a 5.9),simply because of the stratifcation of rock.The propensity of waves dissipating to the surface is less.
edit:I'll explain ...if that earthquakes' hypocenter were to be at, or near, the surface...It would be like like the Northridge quake in Cali.The very fact that the Atlantic plate at the interface of the North-American plate is
very deep is the sole reason the earthquake wasn't so harmfull to modern society.
Need to make some corrections here.
The waves are elastic body and surface waves. Compressional body waves (the same type that carries sound through air, but much more slowly) are the fastest--probably about 8 km/s in the eastern US. Shear waves (which don't occur in viscous material like air) are slower--maybe 3.5 km/s--and earthquakes are more efficient at radiating that type of energy. Body wave energy decays with the inverse of distance squared. Surface waves are even slower, and far away from the earthquake they will have much more amplitude, since their energy only decays with inverse distance.
The Richter scale is no longer in use. Rather, we look at the total moment of slip on the fault (think of it as energy, with units of force*distance) and apply a logarithmic scale to report a moment magnitude (Mw). There's a scale factor of 2/3, so that means that two magnitude units indicates a factor of 1000 in energy difference. Thus, this Mw 5.8 earthquake is 1/23 as powerful as the 1994 Mw=6.7 Northridge earthquake, and 1/63000 as powerful as the 2011 Mw=9.0 Japan earthquake.
The estimated depth of the Virgina earthquake is 6 km, which is somewhat toward the shallow end of the typical range of destructive earthquakes. The Northridge earthquake, by comparison, had a hypocenter at 19 km. It was still very destructive because of the presence of basins filled with soft sedimentary material, which lead to waveguide effects that trap seismic energy and locally intensify it.
The tectonic regime of the Virginia earthquake is intraplate, and the plate is the North American Plate. The NA plate extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge.
