lecture8 forecasting earthquakes

8/12/2019 Lecture8 Forecasting Earthquakes

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FORECASTING EARTHQUAKES

Lecture-8

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Forecasting Earthquakes: Recent Developments

Changes in Seismic Velocities

Earthquakes are often accompanied by temporal changes in seismic wavevelocities in the region

Radon Emission

Emission of radon gas as a quake precursor is recently being explored by the

geophysicists for developing a worldwide seismic early warning system

The Van Method

The method is based on the detection of "seismic electric signals" (SES) via atelemetric network of conductive metal rods inserted in the ground.

Researchers have claimed to be able to predict earthquakes of magnitudelarger than 5 using this method.

Geodetic Measurements

Laser geodimeter measures changes in distance across the fault betweenpoints. Changes in distances may indicate a precursor to an upcomingearthquake. 3


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Seismic gap

A segment of an active plate boundary that, relative to rest of the boundary,has not recently ruptured and is considered to be more likely to produce anearthquake in the future is called a seismic gap.

The seismic gap theory states that a segment of a plate boundary that has notruptured recently has the greatest chance of rupturing in the future relative toother segments that have experienced large earthquakes. This is based on therecognition that tectonic plates move relative to one another at anapproximately constant speed and the assumption that the slip of plate

boundary faults occurs primarily during major earthquakes.

Some earthquake forecasting models use seismic gap for predictions.


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Prediction of Earthquakes

Earthquake prediction is a prediction that an earthquake of a

specific magnitude will occur in a particular place at a particulartime range. Despite considerable research efforts byseismologists, scientifically reproducible predictions cannot yetbe made to a specific day or month.

Most useful way of predicting a future earthquake is in terms ofprobabilities. For well-understood faults seismic hazardassessment maps can estimate the probability that anearthquake of a given size will affect a given location over a

certain number of years.Experts do advise general earthquake preparedness, especially in

areas known to experience frequent or large quakes, to preventinjury, death, and property damage if a quake occurs with or

without warning. 5


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Prediction of Earthquakes

Seismic hazard map of

the San Francisco BayArea, showing theprobability of a majorearthquake occurring by2032

6Source: USGS public domain


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Earthquake prediction has taken a scientific turn in late 1970s.

The first successful prediction was made in China in winter 1975for the city of Haicheng (population about 1 million).

Scientists observed changes in land elevation and ground waterlevels in that region over a period of time. A regional increase inforeshocks had triggered a low-level alert.

Based on the reports from scientists, Chinese officials had orderedthe evacuation of the city. On February 4, 1975, earthquake ofmagnitude 7.3 struck the region. Only very small fraction (2,041people) died in this event. The number of fatalities and injurieswould have exceeded 150,000 if no earthquake prediction andevacuation had been made.

First Successful Prediction

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The energy released during the earthquake travels inthe form of seismic waves

Modern Seismographs can sense the occurrence,intensity and duration of these waves in differentdirections.

Seismogram is the visual record of arrival time andmagnitude of shaking associated with seismic wave,generated by a seismograph.

Sensing the Earthquakes

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First SeismographThe first seismograph called Di-Dong-Di was invented by

Cheng Heng (132 A.D.) of China.

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Source: wikipedia


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The ancient Chinese seismograph consist of a specialvase that had eight sculpted dragons mountedaround the vase in eight primary directions. Each

dragon held in its mouth a metal ball. When theground shook, some of the balls would fall from themouths of the dragons into the waiting mouths of thesculpted frogs to show how the ground had moved.

First Seismograph

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Earlier Seismographs

Vertical

Horizontal 11

Source: google images


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Earlier Seismographs

12Source: wikipedia


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Principle of Seismographs

Seismographs are designed so that slight earth vibrations move the instrument.The suspended mass remains at rest as the spring absorbs some of the motion.The stylus (pen) affixed to the mass, records the relative motion between itselfand the rest of the instrument, thus recording the ground motion.

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Modern Seismometer

14Source: wikipedia


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Principle of Modern SeismometerModern seismometers use electronics.

In some systems, the mass is held nearly motionlessrelative to the frame by an electronic negative feedbackloop. The motion of the mass relative to the frame ismeasured, and the feedback loop applies a magnetic or

electrostatic force to keep the mass nearly motionless.The voltage needed to produce this force is the output ofthe seismometer, which is recorded digitally.

In other systems the weight is allowed to move, and its

motion produces a voltage in a coil attached to the massand moving through the magnetic field of a magnetattached to the frame. Professional seismic observatoriesusually have instruments measuring three axes: north-south, east-west, and the vertical.

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Seismogram

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Source: google images


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PROTECTING AGAINST EARTHQUAKE DAMAGE

Prepare a Seismic Risk Map for the globe which identifies rocktypes, liquefaction potential, landslide potential.

Extensive geologic surveying has to be done to identify allactive faults, including hidden faults.

Earthquake Resistant Design of StructuresEnact building codes to design and build earthquake-resistantstructures in high seismic risk areas. wood, steel andreinforced concrete are preferred as they tend to move with

the shaking ground (unreinforced concrete and heavy masonrytend to move independently and in opposition to the shaking,battering one another until the structure collapses)

Critical facilities such as nuclear power plants and dams should bebuilt on stable ground and as far as possible from active faults. 17


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http://www.bbc.co.uk/news/science-environment-17487482 (Article: Could GPS

be used to predict earthquakes?)

http://earthquake.usgs.gov/research/parkfield/pressrelease.html (Official press

release of Parkfield earthquake prediction)

http://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060

(Video of animals sensing earthquakes early)

Rikitake, T. (1966). A five year plan for earthquake prediction research in Japan.

Tectonophysics 3 : 1 15

Tsunogai, U. & Wakita, H. (1995). Precursory chemical changes in ground

water: Kobe earthquake, Japan. Science, 269 (5220): 61 63

References

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http://www.bbc.co.uk/news/science-environment-17487482http://earthquake.usgs.gov/research/parkfield/pressrelease.htmlhttp://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060http://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060http://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060http://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060http://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060http://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060http://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060http://abcnews.go.com/WNT/video/animals-sense-earthquakes-14375060http://earthquake.usgs.gov/research/parkfield/pressrelease.htmlhttp://www.bbc.co.uk/news/science-environment-17487482http://www.bbc.co.uk/news/science-environment-17487482http://www.bbc.co.uk/news/science-environment-17487482http://www.bbc.co.uk/news/science-environment-17487482http://www.bbc.co.uk/news/science-environment-17487482

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