A quick check on the data this morning shows another nice record from a M3.6 event in Cherokee, OK. However looking at our comparison seismic station at Wichita Mountains we notice that station also picked up an event in Mexico.
This poses a couple of questions.
1) why do both earthquakes register the same size on the Wichita Mountains station ?
2) why do we only see the local event on Ben's geophone ?
Answers
1) the larger event is further away, the "ripples" from the earthquake have to spread out over a much larger radius so become smaller. This is best explained by Father Teds small or far away clip
2) Harder to explain... it is all to do with frequency and attenuation. Earthquakes generate seismic waves with a range of different frequencies (ie different numbers of vibrations per second), the larger the earthquake the lower frequency signals it can generate. A small earthquake (M3) can only generate frequencies of vibrations above a few Hertz (cycles per second). A large earthquake (M5) can generate much lower frequencies (maybe a tenth of a hertz or so) . Really large earthquakes (M7+) can generate vibrations that last for minutes between each cycle. It is all to do with how long it takes the actual earthquake rupture to happen, samll earthquakes happen quickly, large earthquakes take longer.
So a large earthquake like the one in Mexico creates seismic waves with a range of frequencies from a few tenths of Hertz to a few tens of Hertz. If you are close to the event you will record all of these vibrations but as you go further away you start to see the higher frequencies disappearing and just the lower frequencies getting through. This is because the earth itself acts like a filter, attenuating (fancy word for reducing) higher frequencies more than lower frequencies. We are all familiar with this effect with sound, if you are listening to a loud concert from a long way away all you hear is the thud-thud-thud of the bass beat, all the high frequency treble signals are lost. The same thing happens with seismic waves in the earth.
Seismic Sensors come in different sizes and are sensitive to different ranges of frequencies. Ben's geophone is sensitive to seismic signals from 4Hz-100Hz, the sensor at Wichita Mountain is sensitive to signals from 0.01Hz to 100Hz (we call these broadband seismometers)
Note the geophone costs less than $100 and the broadband seismometer costs over $10,000 (one for the Christmas list maybe ?)
Two earthquakes this morning, A M3.6 in Oklahoma and a M5.1 in Mexico
This poses a couple of questions.
1) why do both earthquakes register the same size on the Wichita Mountains station ?
2) why do we only see the local event on Ben's geophone ?
Answers
1) the larger event is further away, the "ripples" from the earthquake have to spread out over a much larger radius so become smaller. This is best explained by Father Teds small or far away clip
2) Harder to explain... it is all to do with frequency and attenuation. Earthquakes generate seismic waves with a range of different frequencies (ie different numbers of vibrations per second), the larger the earthquake the lower frequency signals it can generate. A small earthquake (M3) can only generate frequencies of vibrations above a few Hertz (cycles per second). A large earthquake (M5) can generate much lower frequencies (maybe a tenth of a hertz or so) . Really large earthquakes (M7+) can generate vibrations that last for minutes between each cycle. It is all to do with how long it takes the actual earthquake rupture to happen, samll earthquakes happen quickly, large earthquakes take longer.
So a large earthquake like the one in Mexico creates seismic waves with a range of frequencies from a few tenths of Hertz to a few tens of Hertz. If you are close to the event you will record all of these vibrations but as you go further away you start to see the higher frequencies disappearing and just the lower frequencies getting through. This is because the earth itself acts like a filter, attenuating (fancy word for reducing) higher frequencies more than lower frequencies. We are all familiar with this effect with sound, if you are listening to a loud concert from a long way away all you hear is the thud-thud-thud of the bass beat, all the high frequency treble signals are lost. The same thing happens with seismic waves in the earth.
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| Local M3.6 quake (top) has high frequencies (wiggles close together), distant M5.1 quake (bottom) has lower frequencies (wiggles spaced further apart), amplitudes are similar. |
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| A spectrogram plots energy at each frequency as a colour against time, here most energy is at very low frequencies (less than 1Hz) |
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| For the local quake the spectrogram shows most energy is at high frequencies (2-10Hz) |
Seismic Sensors come in different sizes and are sensitive to different ranges of frequencies. Ben's geophone is sensitive to seismic signals from 4Hz-100Hz, the sensor at Wichita Mountain is sensitive to signals from 0.01Hz to 100Hz (we call these broadband seismometers)
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| Inside a modern "broadband" seismic sensor |
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| Inside a geophone |
Can Ben get a bigger more expensive seismometer :-) ... can give postal address. Its my(Ben) birthday on Sunday so a good gift! Father Ted is very clear.
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