The P-wave upper mantle structure beneath an active spreading centre: the Gulf of California

Abstract

Detailed analysis of short-period travel-time, dT/dA and waveform data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km. More than 1400 digital seismograms from earthquakes in Mexico and central America recorded at SCARLET yield 1753 travel times and 58 direct measurements of dT/dA as well as high-quality, stable waveforms. The 29 events combine to form a continuous record section from 9" to 40" with an average station spacing of less than 5 km. First the travel times are inverted using the tau method of Bessonova et al.; the resultant model is adjusted to agree with the experimental p -A values. Further constraints arise from the observed relative amplitudes of mantle phases, which are modelled by trial and error using WKBJ synthetic seismograms (Chapman; Wiggins). Model GCA, which is appropriate for western Mexico north of 20" latitude, is similar to existing upper mantle models for shield, tectoniccontinental, and arc-trench regimes below 400 km, but differs significantly in the upper 350 km. GCA velocities are very low in this depth range; the model 'catches up' with the others with a very large velocity gradient from 225 to 390 km. This well-resolved feature is consistent with the shear-wave model TNA for western North America of Grand & Helmberger. The abundant data from 20" to 30" control the detailed shape of the 660-km discontinuity. Very large velocity gradients lie both above (620-660 km) and below (661-680km) a 2.8 per cent velocity change.

Measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy: 3. Inversion

“…Body waves have been used to look for displacements of the upper mantle seismic discontinuities (e.g., Grand and Helmberger, 1984;Walck, 1984). It will be necessary to have more data in this field if heterogeneities obtained from surface wave studies are to be related to the geoid (Hager, 1984).…”

P‐V‐Vp‐Vs‐T measurements on wadsleyite to 7 GPa and 873 K: Implications for the 410‐km seismic discontinuity

“…Chemical interactions between olivine and coexisting mantle phases through element partitioning (Fe, A1, and Ca) and latent heat associated with phase transformations have been taken into account using previous experimental data [e.g., Akaogi The results for P and S waves velocities of our pyrolite model are compared with regional seismic velocity profiles for continental shield (S25, SNA) and active tectonic regions (GCA, TNA) in Figure 7a [Walck, 1984;Grand and Helmburger, 1984;Le Fevre and Helmberger, 1989]. For depths in the upper mantle >200 km, the pyrolite model velocities agree with the seismic data both in absolute values and gradients.…”

Mantle discontinuities under southern Africa from precursors to P′ P′df

“…The "410" is likely even broader than 10 km under Africa, given that high resolution regional P-wave studies [Walck, 1984; Melbourne and Helmberger, 1997] find the velocity contrast across the "410" up to 65% greater than that of ak135. Our "410" thickness estimate is almost doubled if the ak135 P-wave velocity and density contrast are scaled by this amount.…”