Electronic Resource
Oxford, UK
:
Blackwell Publishing Ltd
Geophysical journal international
99 (1989), S. 0
ISSN:
1365-246X
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
Observations of underside P-wave reflections from discontinuities deep within the upper mantle which precede the P'P’ phase have provided important constraints on the existence and physical properties of these dynamically critical features. In this study, we extend earlier work by examining for the first time, broad-band recordings of those earthquakes for which one would expect to observe P'P’ and its associated precursors P'dP’. An exhaustive search of 20 yr of Carnegie data and 11 yr of Gräfenberg data uncovered a number of fine P‘P’ observations, but in only one or perhaps two instances are there evidence of a precursor from near 670 km depth. These null results are consistent with what has been reported in earlier studies and provide us with an opportunity to estimate the variability of P velocity near 670 km or, alternatively, the scale length of discontinuity topography which acts to defocus the precursor. This has been done by using the reflectivity and Gaussian beam methods to compute synthetic seismograms for a suite of 1-D and 2-D earth models which vary slightly from PREM. We find that the null data may be satisfied either by (i) reducing the PREM impedance jump at 670 km from 8.5 to 4-5 per cent; (ii) smoothing this 8.5 per cent jump linearly over a zone 15-30 km thick or (iii) imposing deformations in the 670 km discontinuity as small as 10 km in amplitude and 300 km in wavelength. For the one case of an observed precursor, the relative size of the underside reflection and main P‘P’ phase suggests that, in this instance, the precursor's amplitude is correctly predicted by PREM.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-246X.1989.tb02044.x
Permalink
|
Location |
Call Number |
Expected |
Availability |
