Earth's Free Oscillations Excited by the 26 December 2004 Sumatra-Andaman Earthquake
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Harvard CMT solutions can be accessed at www.seismology.harvard.edu/CMTsearch.html.
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More information is available online at www.fdsn.org.
More information is available online at www.iris.edu.
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More information is available online at http://geoscope.ipgp.jussieu.fr.
The Grotta Gigante pendulum tiltmeter recorded both the 1960 Chile and the 2004 Sumatra-Andaman earthquakes.
Free oscillations may be resolvable if GPS data from a global network are stacked and corrections for second-order ionospheric effects remove biases that are spatially broad; see ( 58 ).
To calibrate gravimeter and seismometer spectra the free-oscillation spectral peaks must be corrected for the motion-induced gravitational forces on the seismic sensor ( 16 ). The breathing mode 0 S 0 involves only motion parallel to Earth's radius and the free-air correction to local gravity magnifies its apparent motion by 12%. For data from the Sumatra-Andaman earthquake an apparent initial amplitude of 55 μm for 0 S 0 shrinks to 49 μm after this correction. Similar corrections must be made for all modal amplitudes at frequencies f = 1000 s) including corrections for motion-induced accelerations associated with tilt and redistribution of Earth's mass. For instance the true vertical motion of the football mode 0 S 2 is 81.2% that computed from the seismometer response alone.
The instrumental self-noise of the broadband seismometers used in global seismic networks increases at low frequencies. Earth's gravest free oscillations have therefore been studied primarily with superconducting gravimeter data not seismometer records after correction for fluctuations in atmospheric pressure ( 28 59 – 61 ). The signal-to-noise ratio for seismometer records of the 23 June 2001 Peru event ( M w = 8.4) was not high enough to allow the observation of hybridized toroidal modes on the vertical seismic components but they are evident for the 2004 Sumatra-Andaman earthquake.
S. Stein M. Wysession An Introduction to Seismology Earthquakes and Earth Structure (Blackwell Scientific Malden MA 2003).
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Synthetics are computed according to normal-mode perturbation theory ( 16 ). Perturbations from a spherical nonrotating Earth model such as rotation ellipticity and lateral heterogeneity are included. We adopt mantle model S20RTS ( 53 ) and crustal model crust2.0 ( 54 ) to account for lateral heterogeneity. The latest estimates of spheroidal and toroidal eigenfrequencies and quality factors are used in the calculation ( 55 ). Modal splitting and coupling due to rotation ellipticity and lateral heterogeneity are accounted for on the basis of a group coupling scheme using 36 subgroups of 117 modes below 3 mHz ( 56 ).
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This work was supported in part by NSF. Seismic waveform data from the Global Seismographic Network (funded by NSF and U.S. Geological Survey) were obtained from the Incorporated Research Institutions for Seismology (IRIS) Data Management System. Seismic waveform data was also obtained from the Geoscope Program (IPGP France) and the Southern California Earthquake Center.