Global Observations of Mantle Discontinuities Using SS and PP Precursors

Abstract

SS and PP precursors are currently the only body wave data types that have significant coverage in both oceanic and continental regions to study the existence and characteristics of mantle discontinuities on a global scale. Here, the techniques used by global seismologists to observe SS and PP precursors are reviewed. Seismograms, aligned on SS or PP, are stacked using normal move out (NMO) techniques to obtain common depth point gathers. Bootstrap methods are employed to determine 95% confidence levels of the stacks and robustness of the observations. With these relatively simple techniques, a range of discontinuities has been found in the mantle up to 1,200 km depth. The stacks are dominated by the transition zone discontinuities at 410, 520 and 660 km depth, but additional discontinuities at 220, 300-350, 800-900 and 1,100-1,200 km depth are also seen in certain regions. An overview is given of the most recent observational results with a discussion of their mineral physical interpretation and geodynamical significance. Both seismology and mineral physics agree on the level of complexity at the transition discontinuities: a simple 410, a more complicated 520 and a highly complicated 660-km discontinuity are consistently found in both disciplines.

Sharp mantle transition from cratons to Cordillera in southwestern Canada

“…The thickness of the MTZ remains roughly constant, which is interpreted as evidence of a flat MTZ [Thompson et al, 2011] beneath the cratonic interior well northeast of our study region. In comparison, mantle layering west of our study region is considerably more complex, as evidence from long-period analyses of global SS precursors [Flanagan and Shearer, 1998;Gu et al, 1998Gu et al, , 2003], PP precursors [Chambers et al, 2005;Deuss, 2009], and intermediate-period receiver functions [Lawrence and Shearer, 2006] generally indicates reduced MTZ thicknesses in response to a hot upper mantle beneath the Pacific Ocean basin. The interplay between the broadly averaged low-velocity zone and more localized thermal structures, e.g., a thin low-velocity layer above the 410 and high velocities [Schaeffer and Bostock, 2010] within the descending Juan de Fuca Plate, highlights the complex seismic structure and layering west of the Cordillera.…”

“…Among the various seismic phases, long-period reflected (e.g., SS/PP precursors; [Shearer, 1991], see Deuss [2009] for review) and short-period converted (S-to-P or P-to-S) [Vinnik, 1977;Langston, 1979;Rondenay, 2009] waves have been most commonly adopted. In continental regions, the latter phase group is often the method of choice due to its high frequency nature.…”

Density structure of the mantle transition zone and the dynamic geoid

“…Study of precursors to the SS and PP waves are widely used to determine global variations of the TZ boundaries (e.g. Shearer, 2000;Deuss, 2009). The 660 discontinuity is the most important for a global dynamic modelling.…”

“…The 660 discontinuity is the most important for a global dynamic modelling. However, existing studies based on available date often provided controversial results, which could not be explained using a simple mineralogical model of 660 discontinuity as described in PREM (Deuss et al, 2006;Deuss, 2009). Normally, the 660 discontinuity is globally detected by the longperiod S660S precursors, but the long-term P660P precursors are available only for some places (e.g.…”

Triplicated P-wave measurements for waveform tomography of the mantle transition zone

“…Their migration from the time domain to depth depends on a velocity structure model, which either must have been obtained independently or is neglected, with corresponding systematic errors in the result. PP and SS precursors, i.e., body waves reflected at the undersides of MTZ discontinuities, have also been used in structural studies, (Shearer and Masters, 1992;Thomas and Billen, 2009;Deuss et al, 2006;Deuss, 2009). Their SNR is low, so that only the rare strong earthquakes that are recorded on seismic arrays can be used.…”