High-quality lowest-frequency normal mode strain observations at the Black Forest Observatory (SW-Germany) and comparison with horizontal broad-band seismometer data and synthetics

We present spectra concentrating on the lowest-frequency normal modes of the Earth obtained from records of the invar-wire strainmeters and STS-1 broad-band seismometers located in the Black Forest Observatory, Germany after the disastrous earthquakes off the NW coast of Sumatra in 2004 and off the coast near Tohoku, Japan in 2011. We compare the spectra to ones obtained from synthetic seismograms computed using a mode summation technique for an anelastic, elliptical, rotating, spherically symmetric Earth model. The synthetics include strain–strain-coupling effects by using coupling coefficients obtained from comparisons between Earth tide signals recorded by the strainmeters and synthetic tidal records. We show that for the low-frequency toroidal and spheroidal modes up to 1 mHz, the strainmeters produce better signal-to-noise ratios than the broad-band horizontal seismometers. Overall, the comparison with the synthetics is satisfactory but not as good as for vertical accelerations. In particular, we demonstrate the high quality of the strainmeter data by showing the Coriolis splitting of toroidal modes for the first time in individual records, the first clear observation of the singlet 2S0/1 and the detection of the fundamental radial mode 0S0 with good signal-to-noise ratio and with a strain amplitude of 10^−11. We also identify the latter mode in a record of the Isabella strainmeter after the great Chilean quake in 1960, the detection of which was missed by the original studies.

[1]  Site Correction for Long Period Seismometers, Tiltmeters and Strainmeters , 1976 .

[2]  D. Agnew,et al.  Detection of Seismic Signals Using Seismometers and Strainmeters , 2012 .

[3]  Guy Masters,et al.  Observation of low‐order toroidal modes from the 1989 Macquarie Rise event , 1992 .

[4]  Nicolas d'Oreye,et al.  Comparison of fluid tiltmeter data with long-period seismograms: Surface waves and Earth's free oscillations , 2006 .

[5]  J. Harrison.  Cavity and topographic effects in tilt and strain measurement , 1976 .

[6]  L. Crescentini,et al.  Long-period toroidal earth free oscillations from the great Sumatra–Andaman earthquake observed by paired laser extensometers in Gran Sasso, Italy , 2008 .

[7]  Severine Rosat,et al.  Geographical variations of the 0S0 normal mode amplitude: predictions and observations after the Sumatra-Andaman earthquake , 2007 .

[8]  John H. Woodhouse,et al.  Determination of earthquake source parameters from waveform data for studies of global and regional seismicity , 1981 .

[9]  S. Rosat,et al.  New determinations of Q quality factors and eigenfrequencies for the whole set of singlets of the Earth's normal modes 0S0, 0S2, 0S3 and 2S1 using superconducting gravimeter data from the GGP network , 2006 .

[10]  R. Widmer-Schnidrig,et al.  Spatiotemporal features of the Earth's background oscillations observed in central Europe , 2006 .

[11]  F. Press,et al.  Excitation of the free oscillations of the Earth by earthquakes , 1961 .

[12]  C. Beaumont,et al.  An analysis of tidal strain observations from the United States of America II. The inhomogeneous tide , 1976, Bulletin of the Seismological Society of America.

[13]  Thomas H. Jordan,et al.  Aspherical Earth structure from fundamental spheroidal-mode data , 1982, Nature.

[14]  R. Widmer-Schnidrig,et al.  The horizontal hum of the Earth: A global background of spheroidal and toroidal modes , 2008 .

[15]  Eugene M. Lavely,et al.  Three‐dimensional seismic models of the Earth's mantle , 1995 .

[16]  J. Tromp,et al.  Theoretical Global Seismology , 1998 .

[17]  S. Rosat,et al.  High‐resolution analysis of the gravest seismic normal modes after the 2004 Mw = 9 Sumatra earthquake using superconducting gravimeter data , 2005 .

[18]  A. Dziewoński,et al.  Multiple CMT source analysis of the 2004 Sumatra earthquake , 2005 .

[19]  Takashi Uchiyama,et al.  Analyses of far-field coseismic crustal deformation observed by a new laser distance measurement system , 2010 .

[20]  R. Widmer-Schnidrig,et al.  Signature of 3-D density structure in spectra of the spheroidal free oscillation 0S2 , 2013 .

[21]  I. M. Longman.  Formulas for Computing the Tidal Accelerations Due to the Moon and the Sun , 1959 .

[22]  Miaki Ishii,et al.  An Assessment of the Accuracy of GSN Sensor Response Information , 2005 .

[23]  C. Ji,et al.  An approach to detect afterslips in giant earthquakes in the normal-mode frequency band , 2012 .

[24]  Thomas Jahr,et al.  On reduction of long-period horizontal seismic noise using local barometric pressure , 2007 .

[25]  M. Vallée,et al.  Assessment of SCARDEC source parameters of global large (Mw ≥ 7.5) subduction earthquakes , 2013 .

[26]  Thomas Forbriger,et al.  Reducing magnetic field induced noise in broad-band seismic recordings , 2007 .

[27]  D. Agnew,et al.  Dynamic Strains at Regional and Teleseismic Distances , 2014 .

[28]  Takashi Uchiyama,et al.  A 100 m laser strainmeter system installed in a 1 km deep tunnel at Kamioka, Gifu, Japan , 2004 .

[29]  Duncan Carr Agnew,et al.  Strainmeters and tiltmeters , 1986 .

[30]  L. Rivera,et al.  Vertical and horizontal seismometric observations of tides , 2006 .

[31]  Emile A. Okal,et al.  Seismology: Speed and size of the Sumatra earthquake , 2005, Nature.

[32]  Rudolf Widmer-Schnidrig,et al.  What Can Superconducting Gravimeters Contribute to Normal-Mode Seismology? , 2003 .

[33]  F. A. Dahlen,et al.  The Effect of A General Aspherical Perturbation on the Free Oscillations of the Earth , 1978 .

[34]  E. Okal.  From 3-Hz P Waves to 0S2: No Evidence of A Slow Component to the Source of the 2011 Tohoku Earthquake , 2013, Pure and Applied Geophysics.

[35]  P. Lognonné,et al.  10 – Normal Modes of the Earth and Planets , 2002 .

[36]  Jeroen Tromp,et al.  Earth's Free Oscillations Excited by the 26 December 2004 Sumatra-Andaman Earthquake , 2005, Science.

[37]  W. Farrell.  Deformation of the Earth by surface loads , 1972 .

[38]  Jonathan Berger,et al.  Ambient Earth noise: A survey of the Global Seismographic Network , 2004 .

[39]  G. Laske,et al.  Theory and Observations – Normal Modes and Surface Wave Measurements , 2007 .

[40]  H. Benioff.  FUSED-QUARTZ EXTENSOMETER FOR SECULAR, TIDAL, AND SEISMIC STRAINS , 1959 .

[41]  P. Bogiatzis,et al.  Constraints on the Moment Tensor of the 2011 Tohoku‐Oki Earthquake from Earth’s Free Oscillations , 2014 .

[42]  D. L. Anderson,et al.  Preliminary reference earth model , 1981 .

[43]  E. Wielandt,et al.  On the minimum of vertical seismic noise near 3 mHz , 2007 .

[44]  E. Clévédé,et al.  Source models of great earthquakes from ultra low-frequency normal mode data , 2014 .

[45]  Noise Levels on Plate Boundary Observatory Borehole Strainmeters in Southern California , 2011 .

[46]  F. Gilbert,et al.  An application of normal mode theory to the retrieval of structural parameters and source mechanisms from seismic spectra , 1975, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[47]  Jeroen Tromp,et al.  The global seismographic network surpasses its design goal , 2004 .

[48]  C. Braitenberg,et al.  Comparative Analysis of the Free Oscillations Generated by the Sumatra- Andaman Islands 2004 and the Chile 1960 Earthquakes , 2007 .

[49]  J. Derr.  Free oscillation observations through 1968 , 1969 .

[50]  P. Lognonné,et al.  Fréchet derivatives of coupled seismograms with respect to an anelastic rotating earth , 1996 .

[51]  Stewart w. Smith.  Free oscillations excited by the Alaskan earthquake , 1966 .

[52]  F. Gilbert,et al.  Observation of Coriolis coupled modes below 1 mHz , 2000 .

[53]  L. Rivera,et al.  Rupture length and duration of the 2004 Aceh‐Sumatra earthquake from the phases of the Earth's gravest free oscillations , 2006 .

[54]  F. Klopping,et al.  From Chandler wobble to free oscillations: comparison of cryogenic gravimeters and other instruments in a wide period range , 1995 .