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dijelaskan secara ilmiah:
A light or glow in the sky sometimes heralds a big earthquake. On 17
January
1995, for example, there were 23 reported sightings in Kobe, Japan, of a
white, blue, or orange light extending some 200 meters in the air and
spreading 1 to 8 kilometers across the ground. Hours later a
6.9-magnitudeearthquake killed more than 5500 people. Sky watchers and
geologists have
documented similar lights before earthquakes elsewhere in Japan since the
1960s and in Canada in 1988.
Earthquake Alarm
By: Tom Bleier and Friedemann Freund
http://www.spectrum.ieee.org/dec05/2367
Impending earthquakes have been sending us warning signals—and people are
starting to listen
Deep under Pakistan-administered Kashmir, rocks broke, faults slipped, and
the earth shook with such violence on 8 October that more than 70 000
people
died and more than 3 million were left homeless [see photo,
"Devastated<http://www.spectrum.ieee.org/dec05/2367/eqf1>"].
But what happened in the weeks and days and hours leading up to that
horrible event? Were there any signs that such devastation was coming? We
think there were, but owing to a satellite malfunction we can't say for
sure.
How many lives could have been saved in that one event alone if we'd known
of the earthquake 10 minutes in advance? An hour? A day?
Currently, predictions are vague at best. By studying historical
earthquake
records, monitoring the motion of the earth's crust by satellite, and
measuring with strain monitors below the earth's surface, researchers can
project a high probability of an earthquake in a certain area within about
30 years. But short-term earthquake forecasting just hasn't worked.
Accurate short-term forecasts would save lives and enable businesses to
recover sooner. With just a 10-minute warning, trains could move out of
tunnels, and people could move to safer parts of buildings or flee unsafe
buildings. With an hour's warning, people could shut off the water and gas
lines coming into their homes and move to safety. In industry, workers
could
shut down dangerous processes and back up critical data; those in
potentially dangerous positions, such as refinery employees and high-rise
construction workers, could evacuate. Local government officials could
alert
emergency-response personnel and move critical equipment and vehicles
outdoors. With a day's warning, people could collect their families and
congregate in a safe location, bringing food, water, and fuel with them.
Local and state governments could place emergency teams and equipment
strategically and evacuate bridges and tunnels.
It seems that earthquakes should be predictable. After all, we can predict
hurricanes and floods using detailed satellite imagery and sophisticated
computer models. Using advanced Doppler radar, we can even tell minutes
ahead of time that a tornado will form.
Accurate earthquake warnings are, at last, within reach. They will come
not
from the mechanical phenomena—measurements of the movement of the earth's
crust—that have been the focus of decades of study, but, rather, from
electromagnetic phenomena. And, remarkably, these predictions will come
from
signals gathered not only at the earth's surface but also far above it, in
the ionosphere.
For decades, researchers have detected strange phenomena in the form of
odd
radio noise and eerie lights in the sky in the weeks, hours, and days
preceding earthquakes. But only recently have experts started
systematically
monitoring those phenomena and correlating them to earthquakes.
A light or glow in the sky sometimes heralds a big earthquake. On 17
January
1995, for example, there were 23 reported sightings in Kobe, Japan, of a
white, blue, or orange light extending some 200 meters in the air and
spreading 1 to 8 kilometers across the ground. Hours later a
6.9-magnitudeearthquake killed more than 5500 people. Sky watchers and
geologists have
documented similar lights before earthquakes elsewhere in Japan since the
1960s and in Canada in 1988.
Another sign of an impending quake is a disturbance in the ultralow
frequency (ULF) radio band—1 hertz and below—noticed in the weeks and more
dramatically in the hours before an earthquake. Researchers at Stanford
University, in California, documented such signals before the 1989 Loma
Prieta quake, which devastated the San Francisco Bay Area, demolishing
houses, fracturing freeways, and killing 63 people.
Both the lights and the radio waves appear to be electromagnetic
disturbances that happen when crystalline rocks are deformed—or even
broken—by the slow grinding of the earth that occurs just before the
dramatic slip that is an earthquake. Although a rock in its normal state
is,
of course, an insulator, this cracking creates tremendous electric
currents
in the ground, which travel to the surface and into the air.
The details of how the current is generated remain something of a mystery.
One theory is that the deformation of the rock destabilizes its atoms,
freeing a flood of electrons from their atomic bonds, and creating
positively charged electron deficiencies, or holes.
One of us, Freund, working at NASA Ames Research Center in Mountain View,
Calif., demonstrated through laboratory rock-crushing experiments that the
sundering of oxygen-to-oxygen bonds in the minerals of a fracturing rock
could produce holes. These holes manage to propagate through rock up
toward
the surface, while the electrons flow down into Earth's hot mantle. The
movement of these charges, measured at 300 meters per second in the lab,
causes changes in the rock's magnetic field that propagate to the surface.
Another theory is that the fracture of rock allows ionized groundwater
thousands of meters below the surface to move into the cracks. The flow of
this ionized water lowers the resistance of the rock, creating an
efficient
pathway for an electric current. However, some researchers doubt that
water
can migrate quickly enough into the rock to create large enough currents;
for this theory to be correct, the water would have to move hundreds of
meters per second.
Whatever the cause, the currents generated alter the magnetic field
surrounding the earthquake zone. Because the frequencies of these magnetic
field changes are so low—with wavelengths of about 30 000 kilometers—they
can easily penetrate kilometers of solid rock and be detected at the
surface. Signals at frequencies above a few hertz, by contrast, would
rapidly be attenuated by the ground and lost.
We can detect such electromagnetic effects in a number of ways [see
illustration, "Signs of Quakes to
Come<http://www.spectrum.ieee.org/dec05/2367/eqf3>"].
Earthquake forecasters can use ground-based sensors to monitor changes in
the low-frequency magnetic field. They can also use these instruments to
measure changes in the conductivity of air at the earth's surface as
charge
congregates on rock outcroppings and ionizes the air.
Using satellites, forecasters can monitor noise levels at extremely low
frequency (ELF)—below 300 Hz. They can also observe the infrared light
that
some researchers suspect is emitted when the positive holes migrate to the
surface and then recombine with electrons.
Scientists around the world are looking at all of these phenomena and
their
potential to predict earthquakes accurately and reliably. One group is at
QuakeFinder, a Palo Alto, Calif.based company cofounded by one of us,
Bleier, in 2000. QuakeFinder researchers have begun directly monitoring
magnetic field changes through a network of ground-based stations, 60 so
far, in California [see photo, "Earthquake
Investigator<http://www.spectrum.ieee.org/dec05/2367/eqf2>"].
In 2003, the company joined forces with Stanford and Lockheed Martin
Corp.'s
Sunnyvale, Calif., center to launch an experimental satellite designed to
remotely monitor magnetic changes. A larger, more sensitive satellite is
in
the design stages. QuakeFinder hopes to develop an operational earthquake
warning system within the next decade.
The 1989 Loma Prieta earthquake near San Francisco sent out strong signals
of magnetic disturbances fully two weeks before the 7.1-magnitude quake
occurred. The idea that such signals existed was still a new one then,
certainly not well enough accepted to justify a decision to issue a public
warning.
We happen to have excellent data from that quake. Stanford professor
Anthony
C. Fraser-Smith had buried a device called a single-axis search-coil
magnetometer to monitor the natural background ULF magnetic-field strength
at about 7 km from what turned out to be the center of that quake. He
selected this spot simply because it was in a quiet area, away from the
rumblings of the Bay Area Rapid Transit trains and other man-made ULF
noise.
He monitored a range of frequencies from 0.01 to 10 Hz, essentially, the
ULF
band and the lower part of the ELF band.
On 3 October, two weeks before the quake, Fraser-Smith's sensors
registered
a huge jump in the ULF magnetic field at the 0.01-Hz frequency—about 20
times that of normal background noise at that frequency. Three hours
before
the quake, the 0.01-Hz signal jumped to 60 times normal. Elevated ULF
signals continued for several months after the quake, a period rife with
aftershocks, and then they disappeared.
The Loma Prieta quake was a stunning confirmation of the value of ULF
signals in predicting earthquakes. This validation of the theory prompted
Bleier to establish a network of earthquake sensors in the Bay Area, an
effort that grew into QuakeFinder.
Other researchers around the world who monitored changes in the magnetic
field at ULF frequencies had noticed similar, but not as extreme, changes
prior to other events. These observations occurred shortly before a
6.9-magnitude quake in Spitak, Armenia, in December 1988 and before a
devastating 8.0-magnitude earthquake in Guam in August 1993.
Author Bleier recorded spikes of activity, four to five times normal size,
in the 0.2- to 0.9-Hz range for 9 hours before a 6.0-magnitude earthquake
in
Parkfield, Calif., on 28 September 2003. Solar storms sometimes cause
ripples in the magnetic field at those frequencies, but there had been no
appreciable solar activity for six days prior to the quake.
In Taiwan, sensors that continuously monitor Earth's normal magnetic field
registered unusually large disturbances in a normally quiet signal pattern
shortly before the 21 September 1999 Chi-Chi, Taiwan, earthquake, which
measured 7.7. Using data from two sensors, one close to the epicenter, and
one many kilometers away, researchers were able to screen out the
background
noise by subtracting one signal from the other, leaving only the magnetic
field noise created by the imminent earthquake. Two teams, one in Taiwan
and
one in the United States, calculated that the currents required to
generate
those magnetic-field disturbances were between 1 million and 100 million
amperes.
Besides detecting magnetic-field disturbances, ground-based sensors can
record changes in the conductivity of the air over the quake zone caused
by
current welling up from the ground. These sensors can vary in form, but
those we use are made from two 15-centimeter by 15-cm steel plates locked
into position about 1 cm apart. A 50-volt dc battery charges one plate;
the
other is grounded. A resistor and voltmeter between the battery and the
first plate senses any flow of current.
Normally, the air gap between the plates acts as an insulator, and no
current flows. If, however, there are charged particles in the air, a
current begins to flow, creating a voltage drop across the resistor that
registers with the voltmeter. The currents created in this way are not
large—on the order of millivolts—but are detectable.
Last year QuakeFinder installed 25 ELF detectors with such air-
conductivity
sensors in California's Mojave Desert to determine if increased air
conductivity actually precedes earthquakes and contributes to the
formation
of the so-called earthquake lights [see photo "Mysterious
Lights<http://www.spectrum.ieee.org/dec05/2367/eqf4>"].
But to date, no large earthquakes have struck near these sensors, so no
data
are available yet.
Ground-based sensors are not the only mechanisms for monitoring the
signals
given off by impending earthquakes. Above the ground, satellite-based
instruments are picking up interesting patterns in low-frequency signals
and
detecting other oddities.
In 1989, after the devastating earthquake in Armenia, a Soviet Cosmos
satellite observed ELF-frequency disturbances whenever it passed over a
region slightly south of the epicenter. The activity persisted up to a
month
after the quake. Unfortunately, no data were gathered just prior to the
initial quake. In 2003, the U.S. satellite QuakeSat detected a series of
ELF
bursts two months before and several weeks after a 22 December,
6.5-magnitude earthquake in San Simeon, Calif.
In June 2004, a multinational consortium lead by the French government
launched a new earthquake detection satellite called DEMETER (for
Detection
of Electro-Magnetic Emissions Transmitted from Earthquake Regions).
DEMETER,
much more sensitive than earlier satellites, has already detected some
unusual increases in ion density and ELF disturbances above large quakes
around the world. Unfortunately, the satellite was malfunctioning in the
days before October's temblor in Kashmir. Because the project is so new,
researchers are still working on the tools for processing DEMETER's data.
Its backers are expecting more detailed analyses to be available this
month.
Infrared radiation detected by satellites may also prove to be a warning
sign of earthquakes to come. Researchers in China reported several
instances
during the past two decades of satellite-based instruments registering an
infrared signature consistent with a jump of 4 to 5 oC before some
earthquakes. Sensors in NASA's Terra Earth Observing System satellite
registered what NASA called a "thermal anomaly" on 21 January 2001 in
Gujarat, India, just five days before a 7.7-magnitude quake there; the
anomaly was gone a few days after the quake [see satellite images, "Warm
Before the Storm <http://www.spectrum.ieee.org/dec05/2367/eqf5>"]. In both
cases, researchers believe, these sensors may have detected an infrared
luminescence generated by the recombination of electrons and holes, not a
real temperature increase.
Even the existing Global Positioning System may serve as part of an
earthquake warning system. Sometimes the charged particles generated under
the ground in the days and weeks before an earthquake change the total
electron content of the ionosphere—a region of the atmosphere above about
70
km, containing charged particles. If the ground is full of positively
charged holes, it would attract electrons from the ionosphere, decreasing
the airborne electron concentration over an area as much as 100 km in
diameter and pulling the ionosphere closer to Earth. This change in
electron
content can be detected by alterations in the behavior of GPS navigation
and
other radio signals. Each GPS satellite transmits two signals. The
relative
phase difference between the two signals when they reach a receiver
changes,
depending on the electron content of the ionosphere, so tracking these
phase
changes at a stationary receiver allows researchers to monitor changes in
the ionosphere.
Researchers in Taiwan monitored 144 earthquakes between 1997 and 1999, and
they found that for those registering 6.0 and higher the electron content
of
the ionosphere changed significantly one to six days before the
earthquakes.
Earthquake forecasters can also watch for changes in the ionosphere by
monitoring very-low-frequency (3- to 30-kilohertz) and high-frequency (3-
to
30-megahertz) radio transmissions. The strength of a radio signal at a
receiver station changes with the diurnal cycle: it is greater at night
than
in daylight, as anyone who listens to late-night radio from far-off
stations
knows. The altitude of the ionosphere, which moves lower as the positive
holes migrate to the surface, also has an effect on radio signals; the
lower
the ionosphere, the stronger the signals. So at dawn on an earthquake day,
a
curve drawn to represent the drop-off in radio signal strength will appear
markedly different from the normal curve for that signal at that location.
The connection between large earthquakes and electromagnetic phenomena in
the ground and in the ionosphere is becoming increasingly solid.
Researchers
in many countries, including China, France, Greece, Italy, Japan, Taiwan,
and the United States, are now contributing to the data by monitoring
known
earthquake zones.
Using these phenomena for earthquake prediction will take a combination of
satellite and ground-based sensors. Satellites can cover most of the
planet,
but at ELF frequencies signal sources are hard to pinpoint. Ground-based
monitors have smaller detection ranges, up to 50 km, depending on the
sensitivity of the magnetometer and the size of the quake, but are far
more
precise. With a network of such sensors, forecasters looking at the
amplitude of signals received at each sensor might be able to locate a
quake
within 10 to 20 km. This means that, for an area as large as California,
accurate earthquake detection might require that forecasters distribute
200
to 300 magnetic-field and air-conductivity sensors on the ground.
QuakeFinder and other groups are trying to get funding to integrate space-
and ground-based sensors to detect all these precursor
signals—electronically detected ELF and ULF magnetic-field changes,
ionospheric changes, infrared luminescence, and air-conductivity
changes—along with traditional mechanical and GPS monitoring of movements
of
the earth's crust. With such a broad range of phenomena being monitored,
spikes registered by different monitors detecting different types of
signals
would make forecasts more reliable. Forecasters may then be able to issue
graduated warnings within weeks, days, and hours, declaring increasing
threat levels as the evidence from different sensors begins pointing in
the
same direction.
Useful as such an earthquake warning system would be, we're not ready to
deploy one yet. For one thing, the scientific underpinnings of the
phenomena
need to be better understood before public officials and others have
confidence in the data. On this front, author Freund has been
investigating
the theory that currents are generated by breaking oxygen-to-oxygen bonds
in
rocks under stress. He has experimented with various rock samples,
demonstrating at the laboratory scale that cracking rock can produce
positive charges, which, on a geophysical scale, could form significant
ground currents and infrared emissions. Other rock-crushing experiments
are
under way in Japan and Russia. In Mexico, meanwhile, researchers are
focusing on understanding the related changes in the ionosphere.
A working prediction system won't come cheaply, but it's nothing compared
with the loss of life and the billions of dollars in damage that
earthquakes
can cause. The 200 to 300 ground-based sensors necessary to blanket
California alone will cost $5 million to $10 million. A dedicated
satellite
with magnetic, infrared, and other sensors would cost $10 million to $15
million to build and launch.
Meanwhile, a few technical challenges remain to be solved. At satellite
altitudes, space itself is full of noise, compromising the data gathered.
The data must be digitally processed with filters and pattern-matching
software, still being refined. And down on the ground, man-made noise
fills
the electromagnetic spectrum. Researchers are attempting to use
differential
processing of two distant sensors to reduce or eliminate such
interference.
We expect these problems, both technical and financial, to be worked out
within the next 10 years. Then governments in active earthquake areas such
as California, China, Japan, Russia, and Taiwan could install warning
systems as early as 2015, saving lives and minimizing the chaos of
earthquakes.
About the Authors
Tom Bleier is CEO of QuakeFinder, in Palo Alto, Calif. He previously spent
37 years developing, building, and testing defense and commercial
satellites
and ground-control systems, most recently for Stellar Solutions Inc., a
satellite-systems engineering company, also in Palo Alto.
Friedemann Freund is a senior researcher at NASA Ames Research Center,
in Mountain
View, Calif., and is also a professor in the physics department of San
Jose
State University, in California. His research focuses on how stress can
cause electric current in rocks.
To Probe Further
QuakeFinder's earthquake forecasting research, using ground-based and
satellite-based electromagnetic monitoring techniques, is described at
http://www.quakefinder.com.
Electromagnetic signals created by the fracturing of rocks before
earthquakes are analyzed at
http://science.nasa.gov/headlines/y2003/11aug_earthquakes.htm.
France's DEMETER satellite's monitoring of earthquake signals is discussed
at http://smsc.cnes.fr/DEMETER/GP_actualite.htm.
On 7/18/06, Rovicky Dwi Putrohari <[EMAIL PROTECTED]> wrote:
>
> Saya ndak tau apakah bener ada bola api ini. Sulit skali buat saya
> percaya sesuatu yg muncul bersamaan diatara org-org ini. Aku sendiri
> masih menganggap mitos spt mitos UFO, dimana smua menyatakan mahluk
> berkepala panjang. Dalam hal gempa crita2 bola api, awan, cahaya, dll
> seoalh2 muncul sbg fenomena fisis.
>
> Tanpa adanya rekaman data fisis yg memiliki repeatability yg kuat saya
> kok masih menganggap hal-hal aneh tsb sebagai mitos ttg gempa, sampai
> nantinya science menjelaskan scr fisis.
> Ada yg punya pemikiran lain ?
>
> Rdp
>
> ---------- Forwarded message ----------
> From: Mahfud <[EMAIL PROTECTED]>
> Date: Tue, 18 Jul 2006 00:16:20 +0000 (UTC)
> Subject: [Dongeng Geologi] Comment: "Model West Java Tsunami 17 July
2006"
> To: [EMAIL PROTECTED]
>
>
> New comment on your post #222 "Model West Java Tsunami 17 July 2006"
>
> Author : Mahfud (IP: 202.162.212.17 , 17.212.iconpln.net.id)
>
> E-mail : [EMAIL PROTECTED]
>
> URI :
>
> Whois : http://ws.arin.net/cgi-bin/whois.pl?queryinput=202.162.212.17
>
> Comment:
>
> yth Pak ROvicky,
>
> Menurut tayangan rekaman video amatir, terlihat adanya bola api besar
> di atas permukaan laut. Terjadi di gempa jogja dan katanya juga di
> gempa Pangandaran.
>
> Mohon dijelaskan fenomena ini, mengingat adanya banyak spekulasi
> tentang fenomena ini.
>
>
>
> You can see all comments on this post here:
>
>
>
http://rovicky.wordpress.com/2006/07/17/model-west-java-tsunami-17-july-2006/#comments
>
>
>
> To delete this comment, visit:
>
>
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>
> To mark this comment as spam, visit:
>
>
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>
>
>
> --
> http://rovicky.wordpress.com/
>
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