Teleseismic long-period P waves recorded at the World-Wide Standard Seismograph Network station LON (Longmire, Washington) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. In particular, a large and azimuthally smoothly varying tangential component is observed after vector rotation of horizontal P waves into the ray direction and after application of a deconvolution technique which equalizes effective source time functions and removes the instrument response. These tangential waves attain amplitudes comparable to the radial component and demonstrate wave form antisymmetry about a NNE azimuth. A model which contains a single high-contrast interface dipping toward the NNE at a depth of 15–20 km can explain most of the characteristics of the long-period P wave data, provided dips are greater than about 10° and only the interference of P and Ps generated at the interface is considered. The model breaks down for later arrivals which are presumably multiples or scattered waves. Examination of long-period S waves from several deep teleseisms shows a prominent Sp arrival 18 s before S. The timing of this phase conversion suggests an interface at about 145-km depth, and its sense of polarity suggests that the velocity contrast is from higher to lower velocities as depth decreases. This interface may correspond to the bottom of the upper mantle low-velocity zone in the area.

Structure under Mount Rainier, Washington, inferred from teleseismic body waves

Shear-coupled teleseismic P waves sampling the interior of the Tibetan plateau provide evidence of systematic variations in crustal structure. The crust thins by up to 20 km from south to north with a concomitant increase in Poisson's ratio from normal values in the south to unusually high values in the north. This suggests that the crust of the northern plateau is partially melted due to high temperatures. These changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained.

Implications of crustal property variations for models of Tibetan plateau evolution

Structure under Corvallis, Oregon, was examined using long-period Ps and Sp conversions and P reverberations from teleseismic events as recorded at the WWSSN station COR. A distinct low-velocity zone in the uppermost mantle is inferred by modeling these phases in the time domain using a data set composed of six deep and intermediate-depth earthquakes. The lower boundary occurs at 45-km depth and has S and P velocity contrasts of 1.3 and 1.4 km/sec, respectively. The material comprising the low-velocity zone has a Poisson ratio of at least 0.33 and is constrained by the average P and S travel times determined from the converted phases. The top of the earth model conforms to previously published refraction results.

/pdf/corvallis-oregon-crustal-and-upper-mantle-receiver-structure-a07zauyekz.pdf

Corvallis, Oregon, crustal and upper mantle receiver structure from teleseismic P and S waves

SUMMARY Recently, the S receiver function method has been successfully developed to identify upper mantle interfaces. S receiver functions have the advantage of being free of S-wave multiple reflections and can be more suitable than P receiver functions for studying mantle lithosphere. However, because of specific ray geometry and interference of diverse phases, the S receiver function method has some technical difficulties and limitations. We use synthetic seismograms to demonstrate the feasibility and limitations of S receiver functions for studying mantle structures. Full-wavefield seismograms were calculated using the reflectivity method and processed to generate synthetic S receiver functions for S, SKS and ScS waves. Results show that S receiver functions can be obtained from waveforms of S, SKS and ScS waves. The synthetic S receiver functions for these incident waves show S-to-P converted phases at all discontinuities in the crust and upper mantle. Useful ranges of epicentral distances for calculation of S receiver functions are: 55 ◦ ‐85 ◦ for S, >85 ◦ for SKS and 50 ◦ ‐75 ◦ for ScS waves. We apply both the S and P receiver function methods to data recorded at broadband station YKW3 in Northwest Canada. The study shows that there is significant agreement among different receiver function methods, and demonstrates the usefulness of S receiver functions for imaging the mantle lithosphere.

/pdf/the-s-receiver-functions-synthetics-and-data-example-3rkbjiduly.pdf

The S receiver functions: synthetics and data example

Precursors to S resulting from S to P conversions at discontinuities beneath a receiver (Sp phases) can provide useful constraints on crustal and upper mantle structure. In particular, Sp/Sv amplitude ratios are sensitive to the shear velocity contrast across the discontinuity at which the conversion takes place. We have observed large-amplitude M-converted Sp phases on long-period seismograms from seven stations in eastern Canada following a deep-focus event in the Peru-Brazil region of South America. From the S-Sp time differences we determine that the average crustal shear velocity beneath the stations is 3.65 ± 0.15 km/s. To satisfy the average observed Sp/Sv amplitude ratios, the shear velocity contrast across the Mohorovicic (M) discontinuity must be at least 1.25 km/s. These constraints, together with the observed apparent velocity of Sn waves, require the existence of a thick low-velocity channel for shear waves at the base of the crust in eastern Canada. From average P velocities observed in refraction studies, it appears that the value of Poisson's ratio within this channel must be greater than 0.33. The high values of Poisson's ratio obtained for the lower crustal low-velocity channel indicate the presence of water or another volatile component. We suggest that the hydration state of the lower crust beneath eastern Canada exceeds that of the intermediate crust. We have also observed Sp phases converted at discontinuities as deep as 200 km.

Crustal and upper mantle structure from Sp phases

Tlie long-period seismic records at Umea (Sweden) of an intermediate-depth earthquake in Burma 011 27 February, 1964, exhibit an exceptionally clear case of 8-P conversion at the Mohorovicie discontinuity. The records are examined from various points of view (arrival times, amplitude ratios, particle motions) and comparisons are made for each test between observations and theory. General aspects of mode conversions at the base of the crust are discussed, both their importance and why they are so seldom observed.

/pdf/s-p-conversion-at-the-base-of-the-crust-3o7h7krape.pdf

S-P conversion at the base of the crust

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S-P conversion at the base of the crust