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A busy 24 Hours in seismology

  1. Sep 10, 2018 #1

    davenn

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  2. jcsd
  3. Sep 11, 2018 #2

    jim mcnamara

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    @davenn Just a point of clarification for us ignorant types, please. Suppose two unrelated seismic events are concurrent, i.e., overlapping in time. Using the seismic graph, how can you differentiate them?
     
  4. Sep 11, 2018 #3

    davenn

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    OK this does happen from time to time, actually reasonably regularly.
    Mainly with a large event with one or more aftershocks buried in the larger events' signal. I personally haven't been able to see,
    say a, M5.5 aftershock that is buried in the M7.2 mainshock if they have occurred within a minute of each other.

    here's a recent example ….
    Main shock
    M 8.2 - 286km NNE of Ndoi Island, Fiji
    2018-08-19 00:19:40 (UTC)

    Aftershock
    M 6.3 - 268km NNE of Ndoi Island, Fiji
    2018-08-19 00:23:06 (UTC)

    On my recording, I couldn't tell that there were 2 events separated by ~ 4 minutes. But the experts could :smile:

    upload_2018-9-12_13-18-4.png


    How they do it, I am not sure. Maybe I need to send some emails and see if they will tell me. :biggrin:

    Now for the same or closely times events that are separated by some distance, then it becomes easy to differentiate the events using a seismograph network.
    as there will always be one or more sensors in the network that are closer to one of those 2 events that what the other sensors are.
    It then just becomes a matter of the timing of the P wave arrivals at the various network sensors from which they can then produce locations for the events.


    hope that helps

    Dave
     
    Last edited: Sep 12, 2018
  5. Sep 11, 2018 #4

    davenn

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    OK with a bit of playing around with filtering. Mainly doing high pass filtering so I could cut out a large portion of the
    lower frequency signals. This helps the P wave arrivals from both events to stand out clearer.
    I did a 6 pole hi-pass filtered from 1Hz

    P arrivals.JPG
     
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