Constraints on upper mantle discontinuities from observations of long-period reflected and converted phases


  • Peter M. Shearer


Stacked images combining over 5 years of long-period Global Digital Seismograph Network data reveal many phases associated with reflections and conversions from upper mantle discontinuities. These images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking. Results obtained for P, S, SS, and PP reference arrivals resolve numerous phases from discontinuities near 410 and 660 km, while some of the stacks also show evidence for a weaker discontinuity near 520 km. Phases of particular interest include P and SH multiples resulting from topside reflections, precursors to SS from underside reflections, and P-to-SV converted phases. These phases can be clearly seen both in the waveform stacks and in cross-correlation analysis of individual seismograms. Travel times for these arrivals are converted to discontinuity depths relative to velocities in the Preliminary Reference Earth Model, and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle. Average apparent depths to the discontinuities for the different phases agree to within ±3 km for the 410-km discontinuity, to within ±4 km for the 520-km discontinuity, and to within ±8 km for the 660-km discontinuity. The best global averages are obtained from the SS precursor data which indicate discontinuities at 415, 519, and 659 km. Discontinuity depths obtained from the P-to-SV converted phases at over 35 individual seismic stations exhibit variations of less than ±20 km. Apparent depths to the 660-km discontinuity consistently show greater variability than depths to the 410-km discontinuity, supporting recent laboratory results which indicate that the Clapeyron slope for the 660-km discontinuity is significantly larger in magnitude than the slope for the 410-km discontinuity. Precursors to SS (seen between 110° and 180°) are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point Apparent discontinuity depths computed from SS precursors for different tectonic regions agree to within about ±5 km. The SS precursors have especially good coverage near the subducting slabs in the northwest Pacific. Analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000- to 1500-km-wide region near the slab in which the 660-km discontinuity is depressed by about 20 km. Measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking. Relative amplitude analysis suggests that the P and S wave impedance changes at 410 km are about 0.8–1.1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0.3 and 0.6 of the changes at 410 km.