Open AccessTeleseismic P n arrivals: influence of mantle velocity gradient and crustal scattering
Author(s) -
Nielsen L.,
Thybo H.,
Morozov I. B.,
Smithson S. B.,
Solodilov L.
Publication year - 2003
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1046/j.1365-246x.2003.01873.x
Subject(s) - coda , geology , seismogram , mantle (geology) , seismology , crust , amplitude , scattering , velocity gradient , low velocity zone , seismic array , geophysics , geodesy , lithosphere , optics , physics , tectonics , quantum mechanics
SUMMARY Teleseismic P n phases with a long, high‐amplitude coda, are observed to offsets >3000 km along the Peaceful Nuclear Explosion seismic profiles Quartz and Ruby. For the high‐frequency (>5 Hz) part of the Quartz data, which is shown here, coda lengths of more than 10 s can be visually observed in the seismic sections. We interpret the teleseismic P n arrivals as multiple sub‐Moho refractions (whispering‐gallery phases), which travel over large distances due to a positive upper‐mantle velocity gradient. The coda of these arrivals is explained by lower crustal scattering of the whispering‐gallery phases from the mantle. The modelling is based on calculation of reflectivity and 2‐D visco‐elastic finite‐difference synthetic seismograms. Our models are in agreement with other deep seismic data from continental areas, which typically show a reflective lower crust and an almost transparent uppermost mantle. The lower crustal heterogeneity is described by von Karman distribution with a standard deviation of 0.5 km s −1 for the P ‐wave velocity and correlation lengths of 2.4 km (horizontally) and 0.6 km (vertically). The positive vertical upper‐mantle velocity gradient of ∼0.004 km s −1 km −1 , which is characteristic for profile Quartz, forms a waveguide which gives rise to the teleseismic P n . It is not necessary to include upper‐mantle heterogeneity in order to explain the characteristics of this arrival.