Analysis of long-period seismic waves excited by the May 18, 1980, eruption of Mount St. Helens - a terrestrial monopole?

Long-period (100 to 260 s) Love and Rayleigh waves excited by the eruption of Mount St. Helens on May 18, 1980, 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_(o)s(t) = (1/2)f_o(l-cos((t/τ)π)) for 0 ≤ t ≤ 2τ and f_(o)s(t) = 0 for t ≥ 2τ, with τ = 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

Lamb pulse observed in nature

Seismograms observed at Longmire, Washington, for four eruptions of Mt. St. Helens (May 18, June 13, August 7, and August 8, 1980), can be interpreted as Lamb pulses excited by a nearly vertical single force that represents the counter force of the eruption. These data provide reliable estimates of the impulse of the force K (time integral of the force) from which the total momentum and the kinetic energy, E, of the ejecta associated with the eruption can be estimated. The values of K are estimated to be 1.4 × 10^(19), 1.4 × 10^(16), 3.7 × 10^(15), and 2.8 × 10^(15) dyne·s for the four eruptions (chronological order), respectively. The corresponding values of E are estimated to range from 0.70 to 2.6 × 10^(23), 0.70 to 2.6 × 10^(20), 1.9 to 6.9 × 10^(19), and 1.4 to 5.3 × 10^(19) ergs using values of ejecta velocity ranging from 100 to 375 m/s. The ratio of K to the amplitude of the air wave excited by the eruption is 20 to 40 times larger for the main event on May 18 than for the other events suggesting a significant difference in the eruptive mechanism. Our results demonstrate that a digital seismograph in the vicinity of volcanoes provides a simple means for quantification of the explosive power of a volcanic eruption

Absorption Band Q Model for the Earth

Body wave, surface wave, and normal mode data are used to place constraints on the frequency dependence of Q in 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 Q_s (ω) is proportional to ω^α in the band and to ω and ω^(−1) at higher and lower frequencies, respectively, as appropriate for a relaxation mechanism with a spectrum of relaxation times. The parameters of the band are Q(min) = 80, α = 0.15, and width, 5 decades. The center of the band varies from 10^1 seconds in the upper mantle, to 1.6×10^3 seconds in the lower mantle. The shift of the band with depth is consistent with the expected effects of temperature, pressure and stress. High Q_s 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 high temperature gradient or high shear stresses. A preliminary attempt is also made to specify bulk dissipation in the mantle and core. Specific features of the absorption band model are low Q in the body wave band at both the top and the base of the mantle, low Q for long-period body waves in the outer core, an inner core Q_s that increases with period, and low Q_P/Q_S at short periods in the middle mantle. The short-period Q_s increases 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

Analysis of seismic body waves excited by the Mount St. Helens eruption of May 18, 1980

Seismic body waves which were excited by the May 18, 1980, eruption of Mount St. Helens and recorded by the Global Digital Seismographic Network stations are analyzed to determine the nature and the time sequence of the events associated with the eruption. The polarity of teleseismic p waves (period ∼20s) is identical at six stations, which are distributed over a wide azimuthal range. This observation, together with a very small S to P amplitude ratio (at 20 s), suggests that the source is a nearly vertical single force. A simple model shows that for seismic radiation a volcanic eruption can be represented by a single force applied in the direction opposite to the blast direction. The time history of the vertical force suggests two distinct groups of events, about 2 min apart, each consisting of several subevents with a duration of about 25 s. The magnitude of the force is approximately 2.6×10^(12) N. This vertical force is in contrast with the long-period (∼150 s) southward horizontal single force which has been determined by a previous study and interpreted to be due to the massive landslide. An M_s = 5.2 earthquake initiated the eruption sequence. The direction of the P wave first motion of this event observed at two nearby stations is consistent with the radiation pattern expected for the landslide and suggests that it represents the onset of the landslide. The ground motions observed at station LON (Δ = 67 km) are dominated by Rayleigh waves (i.e., Lamb pulse) and provide tight constraints on the time sequence of the events

Long-period mechanism of the 8 November 1980 Eureka, California, earthquake

The seismic moment and source orientation of the 8 November 1980 Eureka, California, earthquake (M_s = 7.2) are determined using long-period surface and body wave data obtained from the SRO, ASRO, and IDA networks. The favorable azimuthal distribution of the recording stations allows a well-constrained mechanism to be determined by a simultaneous moment tensor inversion of the Love and Rayleigh wave observations. The shallow depth of the event precludes determination of the full moment tensor, but constraining M_(zx) = M_(zy) = 0 and using a point source at 16-km depth gives a major double couple for period T = 256 sec with scalar moment M_(0) = 1.1 · 10^(27) dyne-cm and a left-lateral vertical strike-slip orientation trending N48.2°E. The choice of fault planes is made on the basis of the aftershock distribution. This solution is insensitive to the depth of the point source for depths less than 33 km. Using the moment tensor solution as a starting model, the Rayleigh and Love wave amplitude data alone are inverted in order to fine-tune the solution. This results in a slightly larger scalar moment of 1.28 · 10^(27) dyne-cm, but insignificant (<5°) changes in strike and dip. The rake is not well enough resolved to indicate significant variation from the pure strike-slip solution. Additional amplitude inversions of the surface waves at periods ranging from 75 to 512 sec yield a moment estimate of 1.3 ± 0.2 · 10^(27) dyne-cm, and a similar strike-slip fault orientation. The long-period P and SH waves recorded at SRO and ASRO stations are utilized to determine the seismic moment for 15- to 30-sec periods. A deconvolution algorithm developed by Kikuchi and Kanamori (1982) is used to determine the time function for the first 180 sec of the P and SH signals. The SH data are more stable and indicate a complex bilateral rupture with at least four subevents. The dominant first subevent has a moment of 6.4 · 10^(26) dyne-cm. Summing the moment of this and the next three subevents, all of which occur in the first 80 sec of rupture, yields a moment of 1.3 · 10^(27) dyne-cm. Thus, when the multiple source character of the body waves is taken into account, the seismic moment for the Eureka event throughout the period range 15 to 500 sec is 1.3 ± 0.2 · 10^(27) dyne-cm

Teleseismic analysis of the 1980 Mammoth Lakes earthquake sequence

The source mechanisms of the three largest events of the 1980 Mammoth Lakes earthquake sequence have been determined using surface waves recorded on the global digital seismograph network and the long-period body waves recorded on the WWSSN network. Although the fault-plane solutions from local data (Cramer and Toppozada, 1980; Ryall and Ryall, 1981) suggest nearly pure left-lateral strike-slip on north-south planes, the teleseismic waveforms require a mechanism with oblique slip. The first event (25 May 1980, 16^h 33^m 44^s) has a mechanism with a strike of N12°E, dip of 50°E, and a rake of −35°. The second event (27 May 19^h 44^m 51^s) has a mechanism with a strike of N15°E, dip of 50°, and a slip of −11°. The third event (27 May, 14^h 50^m 57^s) has a mechanism with a strike of N22°E, dip of 50°, and a rake of −28°. The first event is the largest and has a moment of 2.9 × 10^(25) dyne-cm. The second and third events have moments of 1.3 and 1.1 × 10^(25) dyne-cm, respectively. The body- and surface-wave moments for the first and third events agree closely while for the second event the body-wave moment (approximately 0.6 × 10^(25) dyne-cm) is almost a factor of 3 smaller than the surface-wave moment. The principal axes of extension of all three events is in the approximate direction of N65°E which agrees with the structural trends apparent along the eastern front of the Sierra Nevada

A discrepancy between long- and short-period mechanisms of earthquakes near the Long Valley caldera

The largest events in the 1980 Mammoth Lakes earthquake sequence show a discrepancy between fault mechanisms which are determined on the basis of the local short-period first motions and those determined by modeling of long-period regional and teleseismic waveforms. The short-period solutions are left-lateral strike-slip on north-striking, near vertical planes. The long-periods invariably require a much more moderately dipping fault plane with a significant dip-slip (normal) component. Persistence of disagreements between short- and long-period polarities to teleseismic distances suggests that the source-time functions are complicated and may be responsible for at least part of the discrepancy. In addition, there seems to be a systematic difference between local short-period polarities and teleseismic long-period polarities that is related to travel paths across portions of Long Valley Caldera. It is possible that a low velocity zone related to recent magmatic activity is causing the deflection of local seismic rays, thus distorting the fault plane projection