Earthquakes
are feared because they seem to strike without prior warning. Seismologists are
good at estimating the probability
of large earthquakes within timescales of years or decades: "There is a
62.4% probability that one or more earthquakes of magnitude 6.7 or larger will
hit the San Francisco Bay Area before 2032", says a US Geological Survey
website. Clearly, we should be able to do better. In fact, we probably can.
To
develop an effective earthquake
early warning system we have to first understand what happens in the
hypocenter deep in the Earth, where tectonic forces stress rocks to the
breaking point. My scientific work on positive hole charge carriers, which is
done at the SETI Institute and the NASA Ames Research Center, seems to provide
a good start.
Laboratory experiments tell us that, when a rock is stressed, it
turns into a battery. Something similar must happen in all hypocenters prior to
catastrophic failure.
A battery is a device that can deliver electric currents. However,
for a current to flow, the battery circuit must be closed. In other words, if a
stream of positive holes is to flow out of a stressed rock volume, the
electrons must follow suit.
That's where the difficulty lies. Rocks are hole conductors but
cannot conduct electrons. The electrons, co-activated in the stressed rock
volume, have to take a different path. The situation is like in an electrochemical
battery, where cations flow through the electrolyte but electrons have to hitch
a ride through the wire connecting the anode to the cathode.
It appears that, occasionally, the Earth manages
to generate powerful electric currents flowing out of the hypocenter,
indicating that the battery circuit had closed. The currents flow in pulses.
They produce potentially powerful electromagnetic signals at extremely low and
ultralow frequencies (ELF/ULF).
However, there is a catch: ELF/ULF waves coming
from below will be totally reflected when they hit the Earth's surface beyond a
certain angle. We don't know yet how large this angle is, but suspect that it
is pretty steep. This means that only ELF/ULF waves within a relatively narrow
cone will have a chance to make
it through the Earth surface.
To record those ELF/ULF signals directly one has
to be close to the epicenter. This is rare.
At the same time any ELF/ULF waves, which make
it through the Earth's surface, will be beamed straight up into the ionosphere.
Once there, they will spread within the ionospheric waveguide and travel around
the globe. Usually, it is hard to tell from where they came.
The moderate Alum Rock earthquake, magnitude 5.4, rattled the
southern San Francisco Bay in late 2007. For those who experienced it at close
quarters, it was a brief, hard jolt. Overall this event was unremarkable –
except that one of QuakeFinder's CalMagNet stations, which are spread over
California along the San Andreas Fault, was barely 2 km from the epicenter.
A new paper, just published by "Natural Hazards and Earth System
Science," describes that three suspected pre-earthquake indicators were
recorded by this QuakeFinder station: (i) short bursts of electromagnetic
radiation, 10-30 sec long, increasing in number over the last two weeks before
the quake, (ii) a 14-hours long episode of intense air ionization on the day
before the earthquake, and (iii) a continuous wave of ULF magnetic pulsations,
lasting for nearly 1 hour during the time of the most intense air ionization.
In addition, satellites picked up enhanced infrared radiation emitted from
several areas around the earthquake site. Together these observations make a
strong case that they are all related to this earthquake BEFORE it struck.
With observations like these the future for earthquake early
warning looks bright. Once the basic physical processes are understood, we can
bring to bear many different techniques, both space-bound and on the ground,
each capable of providing a different piece of the puzzle.
This paper can be downloaded from:
http://www.nat-hazards-earth-syst-sci.net/9/585/2009/nhess-9-585-2009.html
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