|Early Phase of Trinity Test||Seismic Recording of Trinity at Tucson|
Recordings of the last Chinese nuclear test at three IRIS GSN stations, AAK (10 degrees), ABKT (23 degrees) and FFC (83 degrees). The three seismograms were all passed through a 0.5 hz high pass filter and are all plotted with the same time window of 20 minutes. All of the traces have been approximately time aligned to the first P arrival. The seismic P, S and Lg arrival times are indicated by the arrows. The time range for expected Rayleigh surface waves are also indicated.
The last Chinese test was relatively small with an estimated yield of 10 KT. This figure shows the importance of regional distance recordings for discrimination of nuclear tests with yields of tens of KTs, a likely scenario for an emerging nuclear program testing a first generation nuclear device. The traditional discriminant used for many years is based upon teleseismic recordings of the P wave and surface wave expressed as a ratio of body wave magnitude, Mb, to surface wave magnitude, Ms. It is clear that for this last test we cannot see the surface wave in either of the teleseismic recordings shown in this figure. This is a common problem with the Mb/Ms teleseismic discriminant for smaller events and the lack of observable teleseismic surface waves for small events limits the usefulness of teleseismic monitoring. The regional distance station AAK shows a strong Lg arrival. This arrival is commonly attributed to a crustal waveguide effect and is normally seen in the distance range of 3 to 15 degrees. Discriminants based upon P to Lg ratios have been shown to be effective for Chinese tests recorded at AAK. There is evidence that this regional distance discriminant can be effective in a wide range of geologic settings throughout the world. - Danny Harvey, University of Colorado
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