A spectrum recorded on a global network of instruments for weeks after a great…
PHYSICS · Multi-part · Fermi University Physics · Mechanics & Dynamical Systems · Earth, Climate & Energy Physics · Seismology, tectonics and geodynamics
The Solid Earth, Ch Surface waves, free oscillations and seismic tomography
Problem
A spectrum recorded on a global network of instruments for weeks after a great earthquake shows three narrow, long-period peaks; the first two are split into small clusters of closely spaced lines by the Earth's rotation:
((a)) A peak at T=53.9min, split into five singlets, recorded on both horizontal and vertical seismometers and on gravimeters.
((b)) A peak at T=43.9min, split into five singlets, recorded on horizontal seismometers but essentially absent from vertical seismometers and gravimeters.
((c)) A single, unsplit peak at T=20.5min, recorded strongly on gravimeters as a nearly uniform radial pulsation of the whole planet. For each peak, name the mode using the nXl notation (stating the type, angular order l and overtone number n), and describe the character of the ground motion (horizontal shear with no volume change, or radial motion with a volume change) that explains which instruments detect it.