Seismic noise can be estimated from recordings on days without significant earthquakes. We have computed noise power spectra for all days in 2003 which did not have a magnitude greater than 5.5. This means that only 330 days could be used for the analysis in 2003. To focus on the seismic noise bounds, the one-sided acceleration power spectral density estimates at a given frequency were sorted and only the 5% and 95% bounds were used to eliminate outliers.
Site noise is defined as that part of the recording which is NOT due to earthquakes. At high frequencies (short periods), the noise is related to nearby traffic, automobiles or trains. At very long periods, the noise is related to atmospheric disturbances. The noise at 6 seconds, is related to ocean wave generated signals. Finally there is a noise component due to the instrumentation. Horizontal noise at long periods is very susceptible to atmospheric caused tilts which are interpreted as ground motion noise. varies with time of day, week and season. It is hoped that these 5% and 95% bounds will permit some appreciation of the detection and analysis capabilities of the current network.
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Acceleration Power Spectra |
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FVM |
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Z |
N |
E |
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SLM |
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Z |
N |
E |
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BLO |
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Z |
N |
E |
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USIN |
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Z |
N |
E |
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PVMO |
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Z |
N |
E |
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SIUC |
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Z |
N |
E |
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UTMT |
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Z |
N |
E |
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UALR |
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Z |
N |
E |
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PLAL |
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Z |
N |
E |
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MPH |
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Z |
N |
E |
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We also compute the site H/Z ratio for the PSD as 0.5*PSDN + 0.5*PSDE - PSDZ, where PSDN, PSDE and PSDZ are the power spectral densities for the N, E and Z components. The local H/Z ratio is a function of shallow structure at the site. The high frequency signal is excited by surface or atmospherics sources, while the long period, e.g., 6 second, is due to signals propagating upward to the station from within the Earth. Of the stations shown below MPH shows a profound peak near a 4 second period, which is related to the 1 second travel time of shear waves in the shallow low velocity sediments.
For each station two figures are shown. One has a logarithmic period axis and the other a linear frequency axis.
The ratio plots are color coded according to the day of the year: Red for Day 001 -> Dark Blue for Day 365.
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H/Z RATIO |
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FVM |
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SLM |
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BLO |
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USIN |
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PVMO |
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SIUC |
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UTMT |
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UALR |
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PLAL |
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MPH |
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