Earthquake Prediction
Animal Behavior
Seismic Gaps
Animal Behavior
• There are many reports of unusual animal behavior before
earthquakes
– Behavior intensifies as the time of the earthquake approaches
– Most common in some form of increased restlessness
Haicheng Quake of February 1975
• Successfully predicted
– Evacuated the city
• First instances reported in mid-December 1974
– Snakes came out of hibernation and froze on the ground surface
– Rats came our of hiding
– Earthquake swarm at the end of December
• Thousands of reports in January 1975
Haicheng Quake of February 1975
• Still more reports at the beginning of February
– Effects concentrated on larger animals—cows, horses, dogs, and pigs
Animal Behavior in China
• Chinese involve the general public in earthquake prediction
– Accept all reports uncritically
– People are rural, increasing the likelihood of reports of unusual animal
behavior
• There are at least 10 earthquakes in China that have reports of
unusual animal behavior prior to the earthquake
• Reports are for the general public and do not conform to normal
scientific standards
What might animals detect?
• Changes in atmospheric electric field
– Large changes produced by weather effects
– Daily variation that also varies from place to place
– Connection with earthquakes is not well established
• Changes in the Earth’s magnetic field
– There are large changes on scales of hundredths of a second to years
– Animals would have to be able to see through this noise
– Magnetic effects decay with the cube of the distance
What might animals detect?
• Sounds
– Sounds associated with a main shock arrive to late to be used for
prediction
– Animals might be able to respond to ultrasonic sounds generated by
small earthquakes prior to the main event
• Electrostatic charging of aerosol particles
– Could also be electric currents in water
– Stress changes in materials sensitive to piezoelectric effect could
produce low-frequency electromagnetic radiation
What might animals detect?
– Problems with peizoelectric effect is how to get charge into the air
– One suggestion is that stress causes positive charges to be expelled
from the crust into the air where eddy currents further disperse them
Scientific Study
• Stanford University Outdoor Primate Facility
– Increased restlessness the day prior to each of two quakes
– Lack of use of sleeping sector
Overall Conclusions
• Possible cues
– Things that are intrinsically annoying or fear producing
– Things that produce a response by association with a previous
earthquake or other event
• Different animals respond differently
– Different species may detect different kinds of cues
– Cue may involve a pattern of signals or a difference in magnitude
– Responses may not be noticed by casual observer
Overall Conclusions
• Animals may be sensitive to very small changes
• Most reports are startle responses
– These are not learned in connection with earthquakes
– Stimuli may exceed general noise level in the animal’s habitat
– May be a complex pattern of changes in several physical variables all
occurring at once
– Most are not unusual behavior but rather intensified or at unique times
Interesting Behavior
Seismic Gaps
• Movement of plates is the ultimate cause of many earthquakes
– Many more earthquakes will occur along plate boundaries than within
plates
– Different types of boundaries produce different types of earthquakes
– Along a single plate boundary slip should be constant over many years
• This implies that there should not be gaps in seismicity along plate boundaries
• Gaps are places where earthquakes are more likely
Seismic Gaps
Seismic Gaps
Summary
• Animals
– Interesting observations
– Perhaps useful for very short term predictions
• Seismic gaps
– One of the better methods for estimating risk
– Not useful in terms of date and time predictions
– Useful for prediction of next most likely area for an earthquake
Seismic Gaps
Experimental Work
• William Brace at MIT
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Squeezed samples
Initially volume decreased
Then volume increased
Sample broke
Suggests that new cracks form before the sample breaks
Development of Microfractures
• Squeezed Sample
• Instrumented with little “seismic sensors”
• Most earthquakes located along future plane of rupture
• Suggests the opening of new cracks prior to actual failure
• This could cause volume increase or dilatency
Earthquake Frequency
• Scholtz began to look at the number of earthquakes preceding rupture of a
sample
• Number continued to increase until rupture
• Probably due to formation of new cracks
Precursors
• P wave velocity
– Time for signal from blasts to reach seismograph station decreased
– Shortly after it returned to normal the earthquake occurred
– Thought to be due to opening of new cracks (dilatency)
– Influx of water returned velocity to normal
Precursors
• Tilting
– Formation of new fractures (dilatency) causes rocks to bulge or tilt
– Tilt levels off as water begins to fill cracks
– Measured with tiltmeters
Precursors
• Radon Emission
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Radon is a short-lived gas (isotope decays very quickly)
Radon is released from rocks as they fracture
Radon dissipates quickly
Indicates formation of new fractures
Radon problems
Precursors
• Electrical Resistivity
– Resistance to current passing through the rocks
– Increasing water lowers resistivity
– Suggests formation of new cracks and influx of water
Precursors
• Number of small earthquakes
– Initially increases
– Decreases as new cracks create voids and lower pore water pressure
– Influx of water raises water pressure
– Number of quakes increases until failure
Precursors
• Initially looked very promising
• Further work suggested that each one occurred for a few classes
of earthquakes, but none were generally reliable
• People still observe these precursors, but our hopes for using
them are not terribly bright any more