Abstract. Long-period (100 to 260 s) Love and Rayleigh waves excited by the eruption of Mount St. Helens on , and recorded by IDA, SRO, and ASRO stations were analyzed to determine the mechanism of the eruption.The amplitude radiation patterns of both Rayleigh and Love waves are two lobed with a nodal direction in E5°S for Rayleigh waves and in N5°E for Love waves. These radiation patterns preclude any double-couple mechanism. The radiation pattern, the initial phase, the relatively large amplitude ratio of Love to Rayleigh waves and the existence of clear nodes in the radiation patterns of fundamental mode and higher-mode Rayleigh waves suggest that the source is represented by an almost horizontal (less than 15° from the horizontal) single force pointed toward s5°W.The surface wave spectra fall off very rapidly at periods shorter than 75 s suggesting a very slow source process.Although the details of the source time history could not be determined, a smooth bell-shaped time function: f s(t) = (l/2)f (1-cos(~TI)) for 0 < t < 2T and o O T --f 0 s(t) = 0 for t ~ 2T, with T = 75 s is considered appropriate on the basis of comparison between synthetic and observed seismograms and of the shape of the source spectrum. The peak value of the force f 0 is about 10 18 dynes. The tailing end of the source time function could not be resolved, and some overshoot may be added. The magnitude and the time history of the force can be explained by a northward landslide followed by a lateral blast observed at the time of the eruption.Two distinct events about 110 s apart can be identified on body wave and short-period surface wave records. The first event may correspond to the earthquake which triggered the landslide and the lateral blast. The second event appears to correspond to a second large earthquake and explosion which took place about 2 minutes after the first earthquake.

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Body wave, su~ace wave, and .norm~! mode data are used to place constraints on the frequency depe_n.dence of Q m the mantle. With a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range. The quality factor Qs(w) is proportional to w" in the band and to ~ and w-1 at higher and lower frequencies, respectively, as appropriate for a relaxation mech~msm with a spectrum of relaxation times. The parameters of the band are Q(min) = 80, a= 0.15, and w1dt~, 5 decades. The center of the band varies from 10 1 seconds in the upper mantle, to 1.6 x 10 3 seconds m the lower mantle. The shift of the band with depth is consistent with the expected effects of temper~ture, pressure and stress. High Qs regions of the mantle are attributed to a shift of the absorptiOn band to longer periods. To satisfy the gravest fundamental spheroidal modes and the ScS data, the absorption band must shift back into the short-period seismic band at the base of the mantle This may be .due to a ~ig~ te.mp~rature gradient or high shear stresses. A preliminary attempt is als~ made to s~ec1fy bulk dissipation m the mantle and core. Specific features of the absorption band model are lo": Q m the body wave ~mnd at both the top and the base of the mantle, low Q for long-period body wave~ m the outer core, an mner.core ~s that increases with period, and low QP/Qs at short periods in the middle mantle. The short-penod Qs mcreases rapidly at 400 km and is relatively constant from this depth to 2400 km. The deformational Q of the earth at a period of 14 months is predicted to be 463.

Long-and short-period WWSSN seismograms from nuclear explosions in the Union of Soviet SocialistRepublics are incorporated with apparent velocity observations to derive an upper mantle model for northwest Eurasia. The compressional waves from these explosions have several distinctive features that provide important new information about the character of the upper mantle in the region. The seismograms from 9 ø to 13 ø exhibit impulsive first arrivals P,, implying a smooth, positive velocity gradient between depths of 60 and 150 km. There is a consistent pulse arriving about 2 s after P,, at the ranges from 13 ø to 17% and at larger ranges there are distinct reflections from the two major discontinuities of the mantle. Synthetic seismograms displaying these features indicate a model that correlates with other velocity models from around the world, with a distinctive lid and low-velocity zone. The arrival following P,, is modeled by positioning a low-velocity zone between 150 and 200 km. The model is relatively smooth from a depth of 200 km down to 420 km, where a 5% jump in velocity produces a triplication from 15 ø to 23 ø. The observations from 21ø to 26 ø clearly show another discontinuity at a depth of 675 km with a 4% change in velocity. These results suggest that stable continental regions may have a shadow zone that extends to beyond 17 ø. Below 250 km depth there is no distinguishable difference between the model proposed for northwest Eurasia and models derived for the United States.