The seismicity associated with the collapse of a volcanic caldera in the Galapagos Islands during June of 1968 has been studied in detail. The rate of seismic energy release, inferred from some 638 assigned surface wave magnitudes, was constant over a 9‐day period and appears to be consistent with removal of magmatic support at a constant rate. Early in the earthquake sequence, the energy release was periodic. Large earthquakes occurred, only at 6‐hour intervals coinciding with extremes in the ocean and earth tides; however, the mechanism of triggering, if any, is not clear. Alternative periodic mechanisms based on passive withdrawal or expulsion of supporting magma are suggested. Values of energy, moment, and stress drop are computed from the known source geometry and are found to compare favorably with those estimated from seismic data. Assuming a rather low rigidity, the seismic data are consistent with a cylindrical block of 2 × 1016 grams dropping some 300 meters in approximately 75 dislocation stages, each averaging about 4 meters. In support of this model, a cumulative frequency versus magnitude curve suggests that some 75 earthquakes larger than about Ms = 4.5 were of a different genre than those smaller. Other volcanic phenomena, including eruptions and large explosions, preceded the caldera collapse and were accompanied by minor seismicity. However, major seismicity accompanied only the collapse, which apparently was the only volcanic event that included major ground deformation through shear.

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Measurements of ground motion taken near large underground explosions, when they are compared with teleseismi½ recordings of the same event, allow a direct estimation of the earth's short-period attenuation. Spectra from ground motion measurements made within 15 km of the explosion Boxcar in Nevada are compared with those computed from recordings from Norway at A ___ 72 ø. The effective quality parameter Q• is estimated over the band 0.6-3.0 Hz. From three near-source stations, Q• is found to range between 1400 and 2300 with a mean value of 1700. From time-domain calculations a comparable value of Q• is obtained by scaling up Haskell's explosion source model for granite and finding the attenuation operator that produces the recorded P wave at Norway. A similar experiment by Trembly and Berg (1968) yielded a Q• ~ 450 km from the Nevada Test Site to Mould Bay, Canada, for A _• 37 ø. These two values of Q• verify that under western North America the upper mantle attenuates seismic waves much more severely than the lower mantle.

The observed regional dependence of surface‐wave (Ms) versus body‐wave (mb) magnitudes of underground explosions is due to differences in attenuation of body‐waves beneath different source and station sites. The yield of an underground explosion is more easily and accurately estimated from Ms than from mb.