As Kryon as said on many occasions, our Mother Earth is a "living, breathing" entity. Here's an interesting tid-bit of info about how scientists are now able to record subtle vibrations of the Earth herself, irrespective of larg-scale earthquakes and other major vibrational events -- sort of a "background activity" if you will:
SEISMOLOGY:
Enhanced: Shaking Without Quaking
Hiroo Kanamori[HN1]
After a large earthquake, seismic waves travel around
the planet many times [HN2] and eventually set up oscillations with typical
periods of 3 to 54 min (see figure). Recently, several reports, including
that by Suda et al. (1) on page 2089 of this issue, show that Earth appears
to be oscillating all the time, even without earthquakes.
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Ringing changes. (Top) Seismogram showing the ground motion
acceleration excited by the 1994 Bolivian earthquake (Mw = 8.3), recorded
at Pasadena 7500 km away. R indicates surface Rayleigh wave trains; R1
is the direct wave, and R2 is the wave propagating backwards from Bolivia
along the major arc. When R1 and R2 make another round trip, they become
R3 and R4, which in turn become R5 and R6, and so on. (Bottom) These waves,
after circling around Earth many times,
produce oscillatory motions, as shown schematically.
Successive deformation patterns during one cycle of oscillation of the
fundamental mode are shown from left to right. The waves in the top panel
would actually produce a more complex pattern.
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The idea that Earth can ring like a bell was suggested
more than 80 years ago (2), but when seismologists actually "saw" its oscillations
after the 1960 Chilean earthquake [HN3] (magnitude Mw = 9.5), the largest
earthquake to occur in this century, they were really excited. Bullen,
[HN4] who attended the 1960 meeting of the International Union of Geodesy
and Geophysics [HN5] held in Helsinki, where the first observations of
Earth's free oscillations [HN6] were presented by several
groups of investigators, wrote "there occurred one of
the most dramatic scientific sessions this author has witnessed" [(3),
p. 260].
Do such oscillations occur without earthquakes? In 1959, Benioff et al. (4) searched for such oscillations over a period range longer than 10 min, but found none. Now, nearly 40 years later, Nawa et al. (5) have detected, from the record of a superconducting gravity meter [HN7] at Showa station, Antarctica, almost continuous oscillations of Earth over a period range of 4 min to 1 hour. The successful detection was probably a result of a combination of a good performance of the superconducting gravity meter, a seismically quiet Antarctic site, a long, uninterrupted recording, and especially, a modern data analysis technique that made use of graphic spectral analysis.
Suda et al. (1) and Kobayashi and Nishida (6) investigated
the records of gravity meters and seismometers, respectively, at several
stations around the world and found clear evidence of continuous oscillations
over a period range of 3 to 8 min. These peaks are also evident in the
similar spectra presented by Tanimoto (7). The amplitudes of these oscillations
are small. For the 54-min spheroid-shaped oscillation excited by the Mw
= 9.5 Chilean earthquake, the vertical amplitude on
Earth's surface was about 1 cm, or about 3 mgal (1 Galileo
= 102 m s2) in acceleration. With the improvement in the signal-to-noise
ratio and digital recording system of seismic and gravity instruments over
the last two decades, we can now detect free oscillations for earthquakes
with Mw = 8 with acceleration amplitudes of about 30 ngal. The amplitudes
of the background oscillations recently detected in the absence of earthquakes
were on the order of 1 ngal. This measurement
reflects the notable improvement of signal detection
capability, including stable digital recording systems and data stacking
and analysis techniques, over the past decades.
Possible source mechanisms for the oscillations include
(i) atmospheric disturbance, (ii) variations in loading pressure on the
sea floor resulting from ocean tides and currents, and (iii) slow deformation
in Earth's interior, including the fluid core. Most of the authors of the
recent reports favor an atmospheric source for these background oscillations
(6, 7). The situation may be somewhat similar to that of solar oscillations
[HN8] (8), although the detailed mechanism is still under investigation.
Tanimoto (7) showed that turbulent convective motion in the atmosphere
with an average velocity of about 6.5 m s1 can explain the observed
background signal at periods longer than 6 min. Kobayashi and Nishida
(6) showed that dynamic pressure caused by atmospheric disturbances can
excite oscillations with an amplitude of 1 ngal over a period range of
2 to 5 min. The record of the
Antarctic gravity meter indicates enhanced excitation
during the winter seasons and at periods of about 4 min, which coincide
with those of atmospheric acoustic oscillations (5). These observations
support the conclusion that the main cause of the observed oscillations
is the atmospheric perturbations. However, other causes are not excluded.
It would be exciting if these observations lead to the discovery of some
slow, deep processes (for example, episodic plate motion, slow movement
associated with shallow and deep earthquakes, large-scale magmatic processes,
or slow processes associated with Earth's core). In fact, several spectral
peaks at periods longer than 15 min observed in the Antarctica records
cannot be attributed to the oscillations that can be excited by sources
near Earth's surface, such as the atmosphere and oceans (5). Deep sources
could excite oscillations that cannot be excited by shallow sources. Although
these spectral peaks at very long periods could be caused by some instrumental
effects, further studies are needed.
These findings may have several implications. Kobayashi
and Nishida (6) and Fukao et al. (9) suggest that atmospheric excitations
can be used to explore the internal structure of terrestrial planets with
atmospheres, that is, Mars and Venus. Kobayashi and Nishida (6) estimated
that the atmospheres of Mars and Venus can produce oscillations with an
amplitude of several nanogals. Such oscillations, if detected from just
a single instrument deployed on these planets, could provide
information on the radial variations of elastic properties
in the planets.
These observations underscore the importance of energy
coupling among the lithosphere, hydrosphere, and atmosphere. In most traditional
seismological studies, waves in the solid Earth (seismic waves), the ocean
(tsunami, or long-wavelength ocean waves), and the atmosphere (acoustic-gravity
waves) are treated separately. However, several studies demonstrated significant
energy coupling. For example, atmospheric waves excited by the 1991 Pinatubo
eruption [HN9], [HN10] were coupled to the solid Earth, and detailed studies
of these coupled waves with the global seismic network provided a means
for estimating the total thermal energy emitted by the eruption (10). The
deformation associated with the 1994 Northridge earthquake [HN11], [HN12]
caused significant perturbation to the ionosphere (11). The tsunami excited
by the 1968 Tokachi-Oki, Japan, earthquake caused ionospheric disturbances,
which suggest the use of ionospheric measurements for mapping the tsunami
wave field in the ocean, which in turn could be used for
tsunami warning purposes (12).
The observations reported in these recent papers encourage
enhanced efforts toward understanding the geophysical processes involving
both the atmosphere and solid Earth. Reports on disturbances in the ionosphere
before large earthquakes are numerous (13), but the physics is poorly understood
and skepticism prevails. A better understanding of the physics of the lithosphere-atmosphere
energy coupling will be a key to resolving these mysterious observations.
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References
1.N. Suda et al., Science 279, 2089 (1998).
2.A. E. H. Love, Some Problems of Geodynamics (Dover,
New York, 1911). [HN13]
3.K. E. Bullen, An Introduction to the Theory of Seismology
(Cambridge Univ. Press, Cambridge, 1963). [HN14]
4.H. Benioff, J. C. Harrison, L. LaCoste, W. H. Munk,
L. B. Slichter, J. Geophys. Res. 64, 1334 (1959).
5.K. Nawa et al., Earth Planets Space 50, 3 (1998).
6.N. Kobayashi, and K. Nishida, in preparation.
7.T. Tanimoto, Geophys. J. Int., in press.
8.P. Goldreich and D. A. Keeley, Astrophys. J. 212, 243
(1977) [ADS].
9.Y. Fukao et al., in New Images of the Earth's Interior
Through Long-Term Ocean-Floor Observations, Y. Fukao et al., Eds. (Ocean
Hemisphere Project, Chiba, 1997), pp. 184-185.
10.H. Kanamori and H. Mori, Geophys. Res. Lett. 19, 721
(1992) [GEOREF]; R. Widmer and W. Zurn, ibid., p. 765 [GEOREF]; S. Watada,
thesis, California Institute of Technology (1995), p. 118; P. Lognonne,
E. Clevede, H. Kanamori, Geophys. J. Int., in press.
11.E. Calais and J. B. Minster, Geophys. Res. Lett. 22,
1045 (1995) [GEOREF]. 12.K. Najita, P. F. Weaver, P. C. Yuen, Proc. IEEE
62, 563 (1974).
13.M. Gokhberg et al., Earthquake Prediction: Seismo-Electromagnetic
Phenomena
(Gordon and Breach, Amsterdam, 1995).
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The author is in the Seismological Laboratory, California
Institute of Technology, Pasadena, CA 91125, USA. E-mail: hiroo@gps.caltech.edu[HN15]