Tremor Tracking by the PNSN

A semi-automated system has been developed to track the general occurence of when and where deep tremor takes place within the PNSN region. This covers from northern Puget Sound to southern Oregon. However, because of less complete station coverage in parts of Oregon we feel that our tremor catalog for this area is less complete. The technique we use is to select a few dozen station from the more than 200 operated by the PNSN and save continuous waveform files in hourly segments. The stations selected are those known to be sensitive to deep tremor from past experience and are usually ones from a strip inland from the coast, over the tremor source area and those at particularly remote and quiet locations. Some stations from along the west flank of the Cascade mountains are used because are particularly quiet. These waveforms are filtered in the pass band of 1-6Hz, rectified, smoothed and the plotted in hourly plots.

This plot shows an example of such an hourly envelope plot with a period of tremor from far northern Washington outlined by the green line. Note that at this time the traces for several stations rise and fall in roughly the same pattern indicating that seismic signals on those station changed amplitude in the same way, most likely due to a common seismic source; ie tremor. A different version of this sort of plot is being automatically produced using a different averaging and smoothing proceedure on a subset of stations. Such RMS or RSAM plots are updated about once an hour and are about two hours behind realtime.

Such plots are reviewed by hand about once a day and a tabulation of how much of each hour shows tremor is kept with an indication of the general region in which it occurs.

This is a plot of the number of hours for which tremor was tabulated using the technique mentioned above for the year 2007. While the regions are kept track of in more detail than indicated here they are separated into only Washington and Oregon in these plots.

We are working on ways to farther automate this process and include crude tremor locations rather than just hour summaries.