I came across this interesting article by Dr. A. C. Johnston in a
scientific magazine called Nature. This may be of interest to the
PSN community. Gathering data on such sightings would be a good
project to do if your in an active seismic area.

Sorry about the length of the document but i think its worth it.

Arie
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Article Nature Vol 354  (5 December 1991) page 361.
By Arch C. Johnston


Light from seismic waves

Sir - Sonoluminescence (SL) - the production of light by
the action of sound waves in liquid - has been observed
and studied in the laboratory for more than 50 years.
But I believe it has been observed in nature for
centuries as earthquake lights (EQLs). EQLs are largely
a coseismic occurrence, second only to pre-seismic
abnormal behaviour for difficulty of reliable
documentation and lack of a verifiable explanatory
mechanism.


Many explanations for the generation of EQLs exist (see
refs 1-3). Most proposed mechanisms require assumptions
such the presence of special minerals, gases or
organisms, or unverified physical conditions in the
fault zone. All have  difficulty explaining the
persistent reports of EQLs at distances of up to
hundreds of kilometres from the earthquake, and at sea
or in association with large bodies of both fresh and
salt water or wetlands.

I propose that at least some EQLs are not the product of
high strain accumulation or shear rupture dynamics in
fault zones, but rather result from molecular reactions
in water that has been strongly shaken by the
compressional (P) waves produced by the earthquake. A
seismic wave is simply an earthquake generated sound
wave in a solid or liquid; hence if a P wave induces
light emission from liquid, it is a situation entirely
analogous to SL as generated in the chemist's
laboratory.

SL is a remarkable consequence of acoustic cavitation in
liquids irradiated by sound waves(4,5).  For it to
occur, a cavity or bubble must be created in the liquid
continuum, then rapidly compressed. The process
adiabatically heats the trapped gas or vapour
sufficiently to dissociate molecules. On recombination
or return to the ground state, photons are emitted. Once
a cavity or bubble is formed, two types of SL are
possible: "stable", in which the bubble resonates and
incrementally grows, usually in a standing wave field;
and 'transient', in Which the bubble expands and
implodes all within one cycle of a standing or
travelling sound wave. A travelling P wave should be an
efficient stimulus of transient cavitation, although
long trains or strong-motion P waves may induce Stable
cavitation SL as well.

The observed SL spectrum in water has a peak at 310 nm
(in the ultraviolet), arising from the return to the
ground state of the  excited hydroxyl radical (OH);
there is also a poorly understood continuum throughout
the visible waveband. A pure water SL spectrum will
appear blue to  bluish-white, but the but the presence
of dissolved salts or other impurities can appreciably
alter the basic aqueous spectrum(6), so that yellow or
red may predominate.

SL has been generated in the laboratory with ultrasonic
pressure amplitudes of 1-2 bar (0. 1-0.2 MPa)',
corresponding roughly to an energy density in the
ambient fluid of 10-20 erg cm-3. Whether SL is a viable
mechanism for EQLs hinges on the question of whether it
is reasonable that P waves of sub audible frequency can
supply this energy density in water.

The density of the kinetic energy, e (per unit volume),
induced in the transmission medium by one cycle of an
advancing seismic wavefront is a standard result in
seismology(7) and is given by

              2           2
   e = 2(Pi) * p *(Ao/to)

where "p" is the density of the medium and "Ao" and "to"
are the displacement amplitude and period respectively
of the seismic wave. Conservatively estimated values for
"Ao" of 1-10 cm and "to" of 0.1-1.0 s may be obtained
from the strong ground motion recordings of
earthquakes. This yields P-wave energy densities in
water of roughly 500-2,000 erg cm-3 at 10-1 Hz and
pressure differentials of 1.3-2.7 bar. Thus, seismic P
waves are capable of supplying pressure changes and
energy densities that exceed the laboratory values that
induce SL. Within the water volume irradiated by P
waves, EQLs would arise as the integrated light flux
from many SL cavitation bursts, all loosely synchronised
by the P-wave dilational half-cycles.

In the laboratory, SL produces an illuminance of ~10-(8)
lumen CM-(2) (ref.8), which is visible to the dark-
adapted eye. Thus, to reproduce an illuminance
equivalent to moonlight of 10-(4) lumen cm-(2), as
reported for EQLs in Japan(9), ~10(4) ultrasonic SL
bursts are required in the laboratory. For the much
larger P-wave cavitation events (with bubble micrometre
range), the same illuminance could arise from a single
event. To illuminate a landscape to moonlight brightness
from at least several kilometres distance, 10(2) -10(4)
individual SL P-wave bursts would be sufficient. A P-
wave with a dilational half-cycle wavelength of ~ 1 km
is certainly capable of spawning such an SL field.

Reports of EQLs for large nocturnal earthquakes are not
ubiquitous, but neither are they extremely rare.
Selected characteristics relevant to the SL hypothesis
that appear in the EQL literature are: (1) distinct blue
to bluish-white  EQLs reported from coastal Japan(10)
and  Hawaii(2); (2) extensive EQLs from an onshore
alluvial setting(11); (3) numerous accounts of EQLs
sighted offshore from California(1), Mexico(11) and
other coastal zones; and (4) well-defined, blue and
yellow spherical lights in tsunami wavecrests .
(Luminescent organisms may be another source of
luminescence in tsunami wavecrests(13), but I do not
believe that they would create well defined spheres of
light.)

Natural SL is perhaps not the only mechanism that
produces EQLS, but it is able to explain a wide range of
the existing reports. The hypothesis predicts that (1)
EQLs should not be confined to the immediate fault
rupture zone; (2) bodies of water must be present,
although it is possible that saturated soil can sustain
SL; (3) EOLs are essentially a coseismic phenomenon,
which in the absence of strong foreshocks or aftershocks
should not be observed before or more than several
minutes after the earthquake; and (4) the EQL spectrum
should contain a prominent hydroxyl  peak at 310 nm (and
for sea water(6) a sodium peak at 589 rim). Hence the
SL-EQL hypothesis can be tested by means of
spectrographic analysis although obtaining an EQL
spectrum will not be a trivial undertaking.

ARCH C. JOHNSTON
Center for Earthquake Research,
Memphis State University,
Memphis, Tennessee 38152, USA


 1. Derr, J. Bull. seis. Soc. Am. 63, 2177-2187 (1973).
 2. Lockner, D. A., Johnston, M. J. S. & Byerlee, J. D.
     Nature   302, 28-33 (1983).
 3. Brady, B. T. & Rowell. G. A. Nature 321, 488-492
     1986).
 4. Barber, B. P. & Putterman, S. J. Nature 352, 318-320
     (1991).
 5. Suslick, K. s. science 247, 1439-1445 (1990).
 6. Suslick, K. S. & Flint, E. B. J. Phys. Chem. 95,
     1484 (1990).
 7. Kasahara. K. Earthquake Mechanics (Cambridge
     University Press, (1981).
 8. Walton. A. J. & Reynolds, G. T. Adv. Phys. 33,
     595-660 (1984).
 9. Musya. K. Bull. Earthquake Res. inst. Tokyo univ. 9,
     214-215 1931).
10. Terada, T. Bull. Earthquake Res. Inst. Tokyo Univ.
     9, 225-255 (1931).
11. Fuller, M. 1. U.S. Geol. Sutv. Bull. 494, 120
     (1912).
12. Musya, K. Bull. Earthquake Res. Inst. Tokyo Univ.
      10, 666-673 (1932).
13. Terada, T. Bull. Earthquake Res. Inst. Tokyo Univ.
      1,  25-35
     (1934).



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