2011/08/25

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Filed under: Uncategorized — WmX @ 00:10


Questions to the USGS
I was seated outside on a concrete slab during the seismic event Tuesday. I live in Charlottesville, VA. ( 38.023873°, -78.461908°)
I am disappointed in my lack of ability to recount accurately three things.

1- How long the event lasted
2- How much movement was there of the surface. If I had a “fixed point” a foot in the air above my concrete slab, did the earth move up and down two inches relative to my fixed point? Back and forth two inches? At what frequency?
3- The noise. How much noise did the SE make? Would it have been as noisy if I’d been out in an open field? Away from structures.

Answers from the USGS
1.  Your first inquiry is about earthquake duration of the Virginia earthquake of August 23, 2011.  The short answer to your question is that the duration of the earthquake was about 2 seconds.  However, allow me to provide a little description of what (our) meaning of duration is.

Duration can be technically defined as the time from the onset of an earthquake (the nucleation) to the cessation of shaking (when the moving fault finally runs out of steam).  This is called “source duration” and has a unique value.  However, the time over which observers close to the earthquake will feel the shaking of an earthquake can vary considerably depending on one’s distance and azimuth from the epicenter.  Reasons include:  (1) directivity; (2) partition of energy (between P, S and surface waves); and (3) local geology.  Earthquake directivity, analogous to the Doppler effect in acoustics and optics, is the variation in duration due to one’s position relative to the direction of rupture .  Complex rupturing, which is not uncommon for very large earthquakes, can intensify variations due to directivity.  This probably was not a strong factor for the Virginia earthquake. Also, P and S waves, as well as surface waves, travel with different velocities within the earth and carry different amounts of energy at different azimuths.  Observers may feel one or all of these waves with varying intensity and at different time intervals.  Finally, the time over which a seismic wave can be felt can be extended if the observer is situated over certain geologic structures, such as a sedimentary basement, which can trap energy and keep it reverberating for a while.  So the short answer is that you probably felt the earthquake for two long seconds.  However, depending on the geology of location and your distance from the epicenter, you might have felt shaking for many more seconds.

2.  I have seen one seismogram at a station located 200 kms from the epicenter showing a peak displacement of 2500 microns.  That converts to 2.5 mm. or about 0.1 in.  If you are closer the amplitude may be larger.  Note that this displacement is not the permanent rupture at the fault; it is elastic response (shaking) of the earth.

3.  Earthquake sounds have rarely been reported.   It is feasible that an earthquake can be heard.  The range of human hearing is approximately from 20-20,000 Hz.  The peak energy (corner frequency) of earthquakes large enough to be felt ranges from roughly 0.10-10 Hz.  So there could be an overlap.  Although reports of earthquake noise are not common, we do get them.  They are usually described as a “low rumbling”, which is consistent with the overlap range of frequencies I just mentioned.  Higher frequency sounds you may have heard could have been indirect effects of shaking on structures or items within your house.

1 Comment »

  1. This intrigues me.
    Some background – I’m a retired (18 years now)”computer guy”. A lifetime musician, I get a few bucks here ‘n there as a piano tuner – and a dang good one, I must add.
    I’ve Always been very sensitive to, well, “tones”.
    When the 5.8 hit, I was in my basement, which has a 4-6″ thick slab (house built in 1936)in Arlington VA.
    I sensed, more than felt or heard, when it began (and when the resident feline freaked out), and there was a VERY distinctive “tone” involved.
    You say .01 – 10 hz. This, at least the upper end, would fit.
    The tone did NOT change in the least throughout the, well, “experience”. It was dead steady, though the – I guess you’d call it – amplitude went up and down slightly in the 30 seconds or so it was, well, “sensible”.

    BUT

    Though I admit I do have ‘perfect pitch’ (by which I mean I can tell you what musical note, at least in the audible spectrum, is being sounded), I was a bit too busy at the time to try to discern exactly what “note” was being sounded.

    Again, it was absolutely unchanging for the duration of the event. I guarantee it. But in retrospect, I can’t tell you what it was.

    I suspect the old saw that humans are only capable of hearing sounds in a certain range (500-10,000 or 1,200-18,000 – whatever – it certainly varies significantly from person to person; I can no longer even ‘sense’ higher frequencies that I KNOW I could hear when younger, for example)doesn’t take into account the amplitude, or just plain VOLUME of the sound, ’cause I know for absolutely certain that I HEARD a certain, dead-solid, unvarying tone.

    Finally, the question(s): Can you (or some other source) tell me the Hz of the VA earthquake, do earthquakes vary in vibration frequency depending on distance from the epicenter, and, well, does it ‘matter’, as far as prediction etc. is concerned ?

    Ken Wild
    Arlington,VA

    Comment by Ken Wild — 2011/08/28 @ 03:10

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