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Can EARTHQUAKES
by low frequency r
The 2010 Christchurch earthquake. (Photo: http://rebuildingchristchurch.wordpress.com/2010/09/07/rebuilding-christchurch/)
Do low frequency radio signal variations provide a clue to the onset
of an earthquake? There are no ready answers but it’s tempting to
investigate. This article gives some of the background and suggests
how you can monitor low frequency radio signals using a simple
preamp circuit feeding the sound-card input on your computer.
A
lthough seemingly a modern
nightmare, earthquakes have
always been a fact of life. Every
year, several deadly 8-8.9, scores of
7-7.9, hundreds of 6-6.9 and thousands of 5-5.9 magnitude quakes strike
around the globe.
The Richter scales are logarithmic,
so a magnitude 7 quake has a shaking
amplitude 10 times greater than a 6.
Further damage and deaths often result
from the quake’s aftermath.
Most fatalities in the offshore Indian
Ocean Boxing Day 2004 earthquake
(magnitude 9.2) were not caused by the
earthquake itself but were Tsunamirelated. Deaths can run to hundreds
of thousands but damage and casualties vary enormously, often relating
14 Silicon Chip
to population densities, building
techniques, terrain and soils – or sheer
luck and timing.
In 2010 the devastating January 12th
Haitian and Canterbury’s (NZ) early
morning 4th September quakes were of
similar (7.1) magnitude but casualties
of some 250,000 in Haiti contrasted
with none in Christchurch.
However on February 22nd 2011,
a close and shallow 6.3 “aftershock”
struck Christchurch at lunch time, killing hundreds and causing devastating
damage.
Precursors
It’s only in recent times that it’s
been realised earthquakes arise from
the earth’s tectonic plates pushing and
sliding against each other. Although
their cause may be now known, and
seismic monitoring well established, a
recent TV Horizon documentary titled
“ Why Can’t We Predict Earthquakes”
(broadcast on SBS on January 24th
2011), lamented that earthquake prediction remains agonisingly elusive.
Folklore links likely earthquakes
to birds showing strange behaviour,
well levels suddenly changing, flashes
in the night sky, rainbow clouds,
lunar-induced seismic tides, sunspots, ”earthquake weather”, gravity
waves, radon gas seepage, the onset of
headaches and even (predictably) conspiracy theories and electro-magnetic
weapons.
The danger that false alarms, arising
siliconchip.com.au
be predicted
radio?
By Stan Swan
This seismogram, recorded by the McQueen’s Valley seismograph on Banks Peninsula (courtesy www.geonet.org.nz), displays
the September 4 magnitude 7.1 Christchurch earthquake and some of its early aftershocks. The seismogram is coloured red
if it is clipped, ie, the largest parts of the signal are not shown. If this was not done, then a large earthquake would obscure
much of the seismogram from view – so if the signal is red, the real size is larger than shown. Lots of aftershocks can be seen
on this image, with some of the biggest ones appearing out of the coda (dying away of shaking) of the main shock.
from such incorrect quake alerts, may
eventually desensitise people has to be
considered as well.
Studies have shown the pre-earthquake wandering of domestic animals,
often dismissed as an urban myth, may
indeed be a valid indicator. An recent
Italian project showed striking toad
breeding behavioural changes in the
days before their (6.3) L’Aquila event
of 6th April 2009. (At last – a possible
use for cane toads?)
What about radio signals?
Radio propagation has long been
related to solar activity and atmospherics, with the earth’s charged ionosphere an especially significant factor.
In 1989 however submarine comsiliconchip.com.au
munication monitoring first
indicated seismic contributions as well, as significant
signal surges were noted
prior to California’s October
17th Loma Prieta earthquake that year.
More recently, the French
DEMETER (Detection of
Electro-Magnetic Emissions Transmitted from
Earthquake Regions) microsatellite (shown at right)
detected ionospheric perturbations while passing
over the September 29, 2009
Samoan and the January 12,
2010 Haitian earthquakes.
DEMETER findings inApril 2011 15
dicate that shallow earthquakes, of
magnitude 4.8 and larger occuring at
night, show an associated decrease in
natural EM radiation at around 3kHz.
No changes were observed for deep
quakes or for those that occur in the
daytime.
Although the disturbances were
extremely small and only revealed statistically, this study demonstrated that
seismic activity may also influence the
ionosphere, with the effect perceptible
at even the Low Earth Orbital (LEO)
satellite altitude of some 700km before
some earthquakes occur.
P-wave earthquake alerts
Fig.1 (above): the initial P-wave
shock, akin to a longitudinal impulse
along a slinky spring, travels at some
8 km/second but the more destructive
sideways S-waves travel at only half
this speed.
Fig.2 (below): this sample seismogram
reveals a ~38 second delay between
them (relating to quite a distant
earthquake) – perhaps just enough
time to scramble to safety?
Aside from EM alert possibilities,
it’s actually already feasible to have
very short advance warning of an
earthquake by detecting the initial
non-destructive P-wave (P = primary,
push or pressure) compressions.
These initial seismic waves travel
more quickly through the Earth’s crust
than the destructive “S” (secondary,
shear or shake) transverse waves and
subsequent rolling Rayleigh surface
waves.
P-waves, which are typically felt
by humans as a bang or thump, travel
at some 8km a second in dense earth
(about twice the speed of S-waves), so
advance warnings of perhaps seconds
(for local rumbles) or up to about
a minute (for deep or more distant
quakes) may be possible.
The effect is rather akin to seeing a
lightning flash and hearing thunder
some time later.
A smoke alarm-sized P-wave seismic alert device already is widely
marketed, especially in California.
(See www.earthquakealert.com).
As radio waves travel near instantly,
it may be feasible to use such a device
to “beat the P-wave” and radio ahead
an alert by cell phone.
Although time would be very precious indeed, even a few seconds
warning may be enough to “Drop,
Cover and Hold On”.
Chile, which is on the Pacific’s Ring
Of Fire, experienced a catastrophic
8.8 quake on 27th February 2010 and
is considering such a country-wide
alert system.
The effectiveness of a prediction
device relates to reliable P-wave detection, an acceptance that false alarms
(arising from normal local ground
vibrations) may occur and sufficient
time to react. An alert of minutes
would be much more valuable (even if
less reliable), which is where very low
frequency radio monitoring may assist.
Natural radio and earthquakes
The electromagnetic spectrum is full
of transient natural RF signals, many
often arising from the sun’s activity
and the scientific jury remains out on
just which, if any, are most related to
earthquakes.
The basis for the possible connection plausibly relates to stressed
sub-surface rock layers generating
voltages and signals in the manner of
piezo crystals.
Numerous accounts regarding the
monitoring of different frequencies for
broadband noise and static changes
have been made, especially in such
seismic regions as Italy – see www.nat-
Spectran displays of useful VLF signals – at left is the 15.625 kHz horizontal scan of a colour TV while at right is the
received signal from the Harold E Holt North West Cape (NWC) submarine communications station at Exmouth,WA. The
latter is so powerful that it can usually be received throughout Australia even without a FET preamp!
16 Silicon Chip
siliconchip.com.au
Fig.3: the circuit
of a wideband
preamplifier
suitable for use
with a PC sound
card. Because
the frequencies
of interest are so
low, virtually any
antenna you can
put on this will
theoretically be
too short – so just
use what you can!
A good earth would
also help greatly.
About this feature:
Fig.4: a suitable
breadboard
layout for the
above preamp,
drawn using
"PEBBLE" (see
SILICON CHIP,
September 2009).
hazards-earth-syst-sci.net/1/99/2001/
nhess-1-99-2001.pdf
A review of the literature indicates
that the most promising approaches for
earthquake precursors may be:
(1). Sub-Hertz and ELF magnetic
transients in the .01 to 10Hz region,
especially around 3Hz;
(2). VLF electromagnetic transients
around 10kHz;
(3). VLF-LF broadband noise measurements in the 10 to 100kHz band;
(4). LF-MF noise and propagation in
the 150kHz to 2000kHz region; and
(5). HF noise and propagation studies
over 2MHz.
For our purposes, monitoring the
bands between 10kHz and 100kHz
New Zealander Stan Swan is no
stranger to SILICON CHIP readers, having written numerous articles over the
years and is credited with introducing
Australian and New Zealand readers to
the PICAXE microcontroller.
Although not a resident of Christchurch and therefore not directly affected, Stan contacted us not long
after the September 2010 earthquake
talking about some of the work he
was doing in ultra-low-frequency radio
earthquake “detection”.
This article is the result, written and
in fact in production as the news came
through about the February 21 shake.
And as we were about to go to
press, on March 2 came the news
that New Zealand had suffered yet
another earthquake, fortunately (at 4.5)
significantly lower in magnitude than
Christchurch but this time located near
the NZ capital city, Wellington – just
across the harbour from Stan’s home
in Eastbourne!
Incidentally, in 1848 and 1855 Wellington suffered magnitude 7.1 and 8.0
earthquakes, the latter the largest ever
recorded in New Zealand and causing
considerable damage.
are the most practical and initial investigation requires little more than
a Windows PC with a working sound
card – even an old clunker XP laptop
with a 16-bit 48kHz sampling-rate
soundcard will do nicely.
But hang on – sound cards hearing
radio? Yes – quite correct! Low Radio
Frequencies (RF) produce a small
signal that a sound card treats just
like an equivalent audio signal from
a microphone. And although sound
ITU Abbrev.
Designation
Frequency
Wavelength
Typical EM signals
0 Sub-Hz
1
ELF
2
SLF
3
ULF
4
VLF
5
LF
6
MF
7
HF
8
VHF
9
UHF
10
SHF
11
EHF
Sub Hertz
Extremely Low Frequency
Super Low Frequency
Ultra Low Frequency
Very Low Frequency
Low Frequency
Medium Frequency
High Frequency
Very High Frequency
Ultra High frequency
Super High Frequency
Extremely High Frequency
<3Hz
3Hz - 30Hz
30Hz - 300Hz
300Hz - 3kHz
3kHz - 30kHz
30kHz - 300kHz
300kHz - 3000kHz
3 MHz - 30MHz
30 MHz - 300MHz
300MHz - 3000MHz
3GHz - 30GHz
30GHz - 300GHz
>100,000km
100,000km to 10,000km
10,000km to 1,000km
1,000km to 100km
100km to 10km
10km to 1km
1km to 100m
100m to 10m
10m to 1m
1m to 10cm
10cm to 1cm
1cm to 1mm
Natural earth, ionosphere, space
Deeply submerged submarines
Sub. communication, mains grids
Earth mode comms. – mine radio
Near-surface sub. & cave radio.
Long Wave radio, aircraft beacons
Medium Wave AM broadcasting
Short Wave radio, maritime, amateur
FM radio, TV, aircraft & marine
TV, cell phones, 2-way, WiFi, GPS
Radar, satellite TV, microwave comms.
Radio astronomy, microwave links
The electromagnetic spectrum from "DC to Daylight" (well, almost). The bands/frequencies above 300kHz are pretty much
understood but it's those below – and far below – which we are interested in here.
siliconchip.com.au
April 2011 17
The Richter Scale: not any more, it’s now the Moment Magnitude Scale
The Richter magnitude scale, also known as the local magnitude
(ML) scale, assigns a single number to quantify the amount of
seismic energy released by an earthquake.
It is a base-10 logarithmic scale obtained by calculating the
logarithm of the combined horizontal amplitude (shaking amplitude) of the largest displacement from zero on a particular type of
seismometer (Wood–Anderson torsion).
So, for example, an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that
measures 4.0. The effective upper limit of measurement for local
magnitude ML is just below 9 for local magnitudes and just below
10 for moment magnitude when applied to large earthquakes.
The Richter scale has been superseded by the moment magnitude scale, which is calibrated to give generally similar values for
medium-sized earthquakes (magnitudes between 3 and 7).
Unlike the Richter scale, the moment magnitude scale reports a
fundamental property of the earthquake derived from instrument
data, rather than reporting instrument data which is not always
comparable across earthquakes, and does not saturate in the
high-magnitude range.
Since the Moment Magnitude scale generally yields very similar
results to the Richter scale, magnitudes of earthquakes reported
in the mass media are usually reported without indicating which
scale is being used.
The energy release of an earthquake, which closely correlates
to its destructive power, scales with the 3⁄2 power of the shaking
amplitude. Thus, a difference in magnitude of 1.0 is equivalent to a
factor of 31.6 ( = (101.0)(3 / 2)) in the energy released; a difference
in magnitude of 2.0 is equivalent to a factor of 1000 ( = (102.0)(3 /
2)) in the energy released.
Richter
Description
Earthquake effects
magnitudes
Less than 2.0 Micro
2.0–2.9
Minor
3.0–3.9
Minor
4.0–4.9
Light
5.0–5.9
Moderate
6.0–6.9
Strong
7.0–7.9
Major
8.0–8.9
Great
9.0–9.9
Great
10.0+
Epic
Frequency
of occurrence
Micro earthquakes, not felt.
About 8,000 per day
Generally not felt, but recorded.
About 1,000 per day
Often felt, but rarely causes damage.
49,000 per year (est.)
Noticeable shaking of indoor items, rattling noises.
6,200 per year (est.)
Significant damage unlikely.
Can cause major damage to poorly constructed buildings over
800 per year
small regions. At most slight damage to well-designed buildings.
Can be destructive in areas up to about 160km (100 miles)
120 per year
across in populated areas.
Can cause serious damage over larger areas.
18 per year
Can cause serious damage in areas several hundred miles across.
1 per year
Devastating in areas several thousand miles across.
1 per 20 years
Never recorded in human history.
Extremely rare (unknown)
Courtesy Wikipedia – http://en.wikipedia.org/wiki/Richter_magnitude_scale
cards typically handle signals up to
around 24kHz this is quite adequate
for this purpose.
Just to clear up a confusion which
often occurs: why can't you hear lowfrequency radio signals?
Even though they may be in the audio frequency range, you can not hear
low-frequency radio waves as they're
an electrical rather than acoustic
phenomena. Your ears cannot "detect"
radio signals.
Software
Many specialised and complicated
panoramic display sound card Windows programs are freely available,
but it’s recommended you start with
simple ones to get a feel for things.
The tiny SAQrx (https://sites.google.
com/site/sm6lkm/saqrx) should cope
well, although Spectran (www.weaksignals.com) is better suited for more
demanding work.
18 Silicon Chip
Once installed verify operation by
whistling into the computer’s mike
to observe the resulting spectrogram.
A valuable waterfall display option
(plotting frequency versus time) is
included in Spectran – it can be set
to scroll sideways with a left mouse
button click.
This waterfall can be a visual goldmine when following transient signals,
as they remain on screen long after
they’ve ceased. It can be fascinating
to “see” the spectrum of such everyday sounds as music, speech and
bird calls!
An averaging option further allows
masking out of random noise to better show weak transmissions, and
recordings can be saved to hard disk.
More professional soundcard display
offerings, especially the Spectrum
Lab (http://dl4yhf.ssl7.com/spectra1.
html), may suit once you are familiar
with the panoramic technology.
Apart from the PC, the only other
hardware required to initially “hear”
the nearby EM spectrum is a suitable
3.5mm phone plug (usually mono) and
a short length of wire! Wavelengths
at VLF are so long almost any handy
length of insulated wire will do. Run
the wire vertically if possible, and
ensure it doesn’t snag or short to anything lively or your computer sound
card may be damaged.
Wind it in during any likely thunderstorms as well, at it could present
a hazard to you and your computer.
Performance
In a typical built-up area mains
noises (50Hz and harmonics) will
promptly show themselves to indicate
“receiver” operation but a more useful
beacon can be the 15.625kHz horizontal scan oscillator of a PAL colour CRT
TV set. This can usually be detected
from many metres away. Assorted spusiliconchip.com.au
rious signals may also be seen arising
from normal PC operation. Removing
the sound card input plug will verify
the true nature of such “ghosts”.
My urban location here in remote
NZ meant other VLF transmissions
were initially only weakly detected,
although these were revealed better in
quieter areas using a battery powered
laptop and long wire antenna positioned well away from mains wiring.
Readers living closer to powerful VLF
submarine stations may find even a
short hookup wire antenna will do!
Other low frequency “noise”
Aside from man-made noise, VLF
reception may be further complicated
by day/night variations and extensive
tropical storms.
Across the globe, lightning strikes
almost continually (refer the World
Wide Lightning Locator Network at
http://webflash.ess.washington.edu)
and it’s long been known that the
violent electrical discharge may also
even propagate upwards from storm
clouds and influence the ionosphere.
So-called “Schumann resonances”
may then arise, caused by a lightning
excited ~8Hz resonance in the waveguide cavity formed by the earth’s
surface and the ionosphere.
The bouncing EM pulses associated with such powerful lightning
pulses and the resulting atmospherics
(“spherics”) may propagate globally
on low RF frequencies, to be heard
as static crashes and even gliding frequency whistles and chirps.
An auroral display may also produce
such effects – listening to such atmospheric music can be part of the fun!
Enhancement
Given the very low frequency nature
of the signals almost any simple preamplifier may be used to boost the
input to the sound card.
A complete receiver (such as the
well known BBB-4 – http://www.auroralchorus.com/bbb4rx3.htm) could
even be constructed for standalone
listening, but this would not lack the
panoramic display and recording features that PC monitoring allows.
As broad-band low frequency boosting is needed, tuned circuits are not
even utilised, although a suitable low
pass filter may be needed to block any
nearby powerful AM radio stations.
After considering various low-noise
opamps, a simple general-purpose
siliconchip.com.au
A typical published account
Title: Geomagnetic precursors of intensive earthquakes in the
1-0.-2Hz frequency range of geomagnetic pulsations – Abstract Only
Corporate Source: Joint Publications Research Service, Arlington, VA.
During intensive geo-tectonic processes such as earthquakes, pulsations
are observed in the geomagnetic field at a frequency of 0.02 to 1Hz with
anomalously high amplitudes. These pulsations usually appear as beat
phenomena lasting from several minutes to several hours. It has been
found that the pulsations are excited only in magnetic components of
the terrestrial electromagnetic field. The periods and amplitudes of the
pulsations are nonlinearly related to the intensity of the earthquakes.
Pulsations of this type are not observed when earthquakes do not occur.
Additional analysis shows that frequently the pulsations precede intensive
earthquakes by 10 to 200 minutes, then drop for about 1 hour, then appear
once again during the actual earthquake. Oscillograms of such pulsations
are presented. The periods and amplitudes of the geomagnetic pulsations
preceding earthquakes are found to be linearly related to the magnitude of
the earthquakes. A regression equation relating earthquake magnitude to
pulsation characteristics is presented.
Author: GOGATISHVILI, Y. M. CASI Accession No. 85N23178 Published:
February 1985
(referenced at www.manuka.orcon.net.nz/eradio.htm)
MPF102 N-channel J-FET was eventually used – see Fig.3. Layout and
component values are not critical.
This setup, powered by a 9V battery performed very well, revealing
signals that were previously buried
in the noise.
The two back-to-back 3.3V zener
diodes ensure any larger voltages on
the antenna will be shorted to earth.
The circuit draws around 4mA and
can be easily assembled and enclosed
in a small metal case to give shielding. Leads to the sound card should
be shielded to reduce mains pick-up.
Suggested monitoring
approach
Launching VLF monitoring satellites or erecting gigantic antenna
farms, such as the military use, is
naturally a tad daunting.
Powerful VLF submarine communication transmitters conveniently
already blanket the world so it’s suggested that initial monitoring merely
follows the approach of simply checking their VLF signal strengths over an
extended period.
However, sudden variations may
well arise due to solar storms (see
www.swpc.noaa.gov) or the VLF site’s
transmitting activity. Northwest Cape
wasn’t set up just for your listening
pleasure!
Once the sound card-based equipment is organised at your location,
use a stable set up and antenna so any
on-screen changes will be noticeable.
To help gain initial experience
perhaps refer to your displays (and
recordings?) when an earthquake has
occurred somewhere, to see if unusual
VLF activity was associated with transmitters near it.
Details of the world’s latest earthquakes are soberingly shown at http://
earthquake.usgs.gov/earthquakes/
recenteqsww/
Conclusion
Don’t expect instant answers in
the VLF monitoring quest, as display
checking may be akin to watching
paint dry.
Opinions may differ and findings
are uncertain but seismic scientists
worldwide earnestly scrutinise such
displays in attempts to see if seismic
and low-frequency radio signals act as
possible earthquake precursors.
It may well be a false quest, with
no more pre rumble significance than
the birds going quiet, your dog hiding under the bed – or the cane toads
pausing their advances.
But there just may be something in
it – and your simple setup could help
provide a valuable key or stimulate
further investigations!
References, extensions, published
scientific studies and quoted web sites
are conveniently linked via a resource
site at www.manuka.orcon.net.nz/
eqradio.htm
SC
April 2011 19
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