Precursors

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As precursor phase in the are seismology certain seismic waves designated by the lower-side reflection of seismic discontinuities are generated.

Sketch of the beam paths of a PP or SS phase with the associated precursor phases.

Precursory phases are usually considered in connection with waves that were reflected once on the earth's surface before they were registered by a seismometer . Depending on the wave type , these are referred to as PP or SS phase. The corresponding precursor phases are divided into PP or SS precursor phases. They are caused by reflection at layer boundaries in the interior of the earth, which are characterized by a sudden change in the speed of propagation of seismic waves. When a wave hits, a small part of the energy is reflected back at the respective discontinuity, as the illustration shows.

The reflected wave component thus covers a shorter distance to the measuring station and therefore reaches it faster than the corresponding PP or SS phase. As a result, the use of such a wave in the seismogram precedes the main phase, which gave this part of the wave field its name. The time difference depends on the depth of the reflective layer boundary.

Precursory phases are used in particular to investigate the depth of the most important mantle discontinuities, such as The 410 km discontinuity or the 660 km discontinuity is used. The depth of the respective discontinuity can be inferred from the maturity differences. However, the transit time can also be influenced by changes in seismic velocities. However, since the routes of the precursor and the main phase are largely identical, effects on the transit time can only come from the area near the reflection point (also: bounce point ).

literature

  • C. B. Estabrook & R. Kind (1996): The Nature of the 660-Kilometer Upper-Mantle Seismic Discontinuity from Precursors to the PP Phase. In: Science. 274., 1179-1182
  • M. P. Flanagan & P. ​​M. Shearer (1998): Global Mapping of topography on the 410-km discontinuity from PP precursors. In: Geophysical Research Letters. 26, 549-552
  • J. Schäfer (2007): Global stacking of broadband seismograms for the analysis of jacket discontinuities. Diploma thesis at the University of Frankfurt am Main