The Excitation of Core Modes by Earthquakes
- D. E. Smylie and
- Raymond Hide
Published Online: 29 MAR 2013
Copyright 1988 by the American Geophysical Union
Structure and Dynamics of Earth's Deep Interior
How to Cite
Crossley, D. J. (1988) The Excitation of Core Modes by Earthquakes, in Structure and Dynamics of Earth's Deep Interior (eds D. E. Smylie and R. Hide), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM046p0041
- Published Online: 29 MAR 2013
- Published Print: 1 JAN 1988
Print ISBN: 9780875904504
Online ISBN: 9781118666562
Prediction of the theoretical spectrum of the internal motions of a fluid-filled rotating spherical shell, with application to the Earth's core, has recently become a pressing issue in physics of the Earth's deep interior. The recent claim that peaks in the observational record from the Brussels superconducting gravimeter are associated with inertial waves in the Earth's core raises many interesting questions concerning the calculation, identification, excitation and damping of core modes.
To date in core dynamics the concern has been with the free-mode eigenspectrum, which generally has to be solved as a preliminary to any forced problem. Considerable progress is being made on the calculations for certain models of density stratification and for certain period ranges of wave motions in the core, though results for an arbitrary core stratification at all periods are still incomplete. Nevertheless, the forced excitation problem can be solved approximately using standard normal mode excitation theory. The goal is to determine whether a large earthquake can excite core modes that generate perturbations in gravity potential at the Earth's surface with the approximate amplitudes of the spectral peaks observed at Brussels.
This process of summing a large number of spheroidal modes throughout the Earth is computationally prohibitive unless simplifying assumptions are made. It is reasonable to ignore Coriolis coupling (and also ellipticity, at least to this order of approximation) in the shell and inner core due to the high bulk rigidity of these regions. In the fluid core it is important to retain Coriolis coupling, though necessary to truncate the spherical-harmonic expansion after a number of terms. Results are presented for a sample earthquake source in a non-rotating Earth model and indicate that the excitation of a single mode is well below the current observational threshold. However in future calculations with an extended number of modes some enhancement of the response may be expected.