Earthquakes associated with underground nuclear explosions

The results of a search of records of the University of Nevada seismographic station network for grossly observable effects of underground nuclear tests on regional seismicity are presented. Most of the data were obtained at the Tonopah station, 100–150 km from the explosions studied. Periods of time before and after 21 explosions were examined. In all cases of explosions with magnitude mb; ≥ 5.0, an increase in seismicity was observed for at least one day following the test. The decay of post-shot activity and its relationships to the equivalent magnitude of the explosion provide confirmation of the determinative role of the nuclear event. For the most part, this activity was confined to the test site, and was probably located within 20km of the shotpoint. In one case, however, earthquake activity following the Faultless test in January 1968 may have extended to 40 km from the site of the explosion. One or two of the earlier tests studied may have influenced seismicity at larger distances. Most of the events studied had very small magnitudes, and all earthquakes related to underground tests had magnitudes lower by at least one magnitude unit, than those of the associated blasts. Attempts to determine the extent to which underground tests influence seismicity in active areas distant from the test site have indicated that, if such effects exist, they are probably minor compared with normal variations in seismicity.