Standardized power spectral density (PSD) computations performed for the large volumes of data flowing into the IRIS DMC have supported a wide range of applications. PSD computations have been used for many years in seismology as a standard tool. By computing the seismic PSD for many segments in time for a single data stream, the results can be combined into probability density functions (PDFs) that are a rich source of information about the seismic energy recorded at a given station location. McNamara and Buland (2004, Ambient noise levels in the continental United States, BSSA, doi:10.1785/012003001) demonstrated the utility of the PDF approach for evaluating noise characteristics at seismic stations. The DMC implemented a version of the McNamara and Buland software to compute PSDs for every hour of every day for every station that transmits its data to the DMC in real time. A web-based interface enables users to customize the display of the resultant PDFs for different periods of time. The PDF results, while originally intended as a station quality control tool, have been used for many different purposes, such as detecting annual and seasonal shifts in seismic energy, evaluating different station installation techniques, and to detect very subtle changes in station behavior that might signal the onset or resolution of operational problems or the time evolution of sensor vault conditions.</p>


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