RECORDING EARTHQUAKES
1.A SEISMOGRAPH IS AN INSTRUMENT THAT IS
USED TO DETECT AND RECORD SEISMIC WAVES.
a.A SEISMOGRAPH RECORDS MOTION BY TRACING
WAVE-SHAPED LINES ON A PAPER OR BY TRANSLATING
THE MOTION INTO ELECTRONIC SIGNALS.
RECORDING EARTHQUAKES
b.A SEISMOGRAPH CONSISTS OF THREE MAIN
RECORDING DEVICES THAT MEASURE MOTION
IN THE FOLLOWING DIRECTIONS.
i.VERTICAL MOTION OF THE GROUND.
ii.HORIZONTAL MOTION IN EAST-WEST DIRECTION
iii.HORIZONTAL MOTION IN NORTH-SOUTH
DIRECTION
SEISMIC WAVES
2.SCIENTISTS HAVE DETERMINED THAT
EARTHQUAKES GENERALLY PRODUCE THREE
MAJOR TYPES OF SEISMIC WAVES.
a.EACH TYPE OF WAVE TRAVELS AT A DIFFERENT SPEED
AND CAUSES DIFFERENT MOTION IN THE EARTH’S
CRUST.
PRIMARY WAVES
i.PRIMARY WAVES, OR P-WAVES, MOVE THE FASTEST
AND ARE THEREFORE THE FIRST TO BE RECORDED
BY A SEISMOGRAPH.
1.P-WAVES CAN TRAVEL THROUGH SOLIDS AND LIQUIDS.
2.THE MORE RIGID THE MATERIAL THE FASTER THE P-WAVE
TRAVELS.
3.P-WAVES ARE COMPRESSION WAVES WHICH MEANS THEY
CAUSE ROCK PARTICLES TO MOVE TOGETHER AND APART
ALONG THE DIRECTION OF THE WAVES.
SECONDARY WAVES
ii.SECONDARY WAVES, OR S-WAVES, ARE THE SECOND
WAVES TO BE RECORDED BY A SEISMOGRAPH.
1.S-WAVES CAN ONLY TRAVEL THROUGH SOLID MATERIAL.
2.S-WAVES ARE SHEAR WAVES WHICH MEANS THAT THEY
CAUSE ROCK PARTICLES TO MOVE AT RIGHT ANGLES TO
THE DIRECTION IN WHICH THE WAVES ARE TRAVELING.
SURFACE WAVES
iii.SURFACE WAVES ARE CREATED WHEN P-WAVES
AND S-WAVES REACH THE EARTH’S SURFACE.
1.SURFACE WAVES ARE THE SLOWEST MOVING WAVES AND
ARE THE LAST TO BE RECORDED ON A SEISMOGRAPH.
SURFACE WAVES TRAVEL OVER THE EARTH’S SURFACE
SIMILAR TO OCEAN WAVES.
SURFACE WAVES ARE DESTRUCTIVE WHEN TRAVELING
THROUGH LOOSE EARTH.
2.
3.
LOCATING AN EARTHQUAKE
3. LOCATING AN EARTHQUAKE
a.THE DIFFERENCE BETWEEN THE ARRIVAL TIMES OF THE PWAVES AND S-WAVES CAN BE USED TO DETERMINE THE
EPICENTER OF AN EARTHQUAKE.
P-WAVES TRAVEL ABOUT 1.7 TIMES FASTER THAN S-WAVES.
b.
i. THEREFORE, IF THE S-WAVES ARRIVE SHORTLY AFTER THE P-WAVES WE
KNOW THAT THE EARTHQUAKE OCCURRED CLOSE, AND IF THE S-WAVES
ARRIVE A LONG TIME AFTER THE P-WAVES WE KNOW THE EARTHQUAKE
OCCURRED FAR AWAY.
LOCATING AN EARTHQUAKE
c.TO DETERMINE THE EXACT LOCATION OF THE EPICENTER
SCIENTISTS PLOT THE DIFFERENCE BETWEEN THE ARRIVAL
TIMES OF THE TWO WAVES AND COMPARE THEM TO A
STANDARD GRAPH THAT TURNS THE TIMES INTO DISTANCES
FROM THE EPICENTER.
i. IN ORDER TO DETERMINE THE DISTANCE YOU MUST HAVE INFORMATION
FROM AT LEAST THREE SEISMOGRAPH STATIONS AT DIFFERENT
LOCATIONS.
CIRCLES ARE DRAWN ON A MAP USING THE DISTANCES RECORDED.
iii.WHERE THE CIRCLES INTERSECT IS THE EPICENTER OF THE
EARTHQUAKE.
ii.
EARTHQUAKE MEASUREMENT
4. EARTHQUAKE MEASUREMENT
a.MAGNITUDE IS A MEASURE OF THE ENERGY RELEASED BY AN
EARTHQUAKE AND IS ALSO DESCRIBED AS THE AMOUNT OF
GROUND MOTION.
i. MAGNITUDE IS MEASURED USING THE RICHTER SCALE OR THE MOMENT
MAGNITUDE SCALE.
1.THE RICHTER SCALE IS RELATED TO THE EFFECT OF THE EARTHQUAKE
WHILE THE MOMENT MAGNITUDE SCALE IS RELATED TO THE CAUSE OF THE
EARTHQUAKE.
EARTHQUAKE MEASUREMENT
ii.EARTHQUAKES WITH A MAGNITUDE LESS THAN A 2.5 ARE
CALLED MICROQUAKES AND ARE USUALLY NOT FELT BY
PEOPLE.
THE MERCALLI SCALE EXPRESSES THE INTENSITY OF AN
EARTHQUAKE, OR THE AMOUNT OF DAMAGE IT CAUSES
THROUGH ROMAN NUMERALS AND DESCRIPTION OF
DAMAGE.
iii.
1.THE MERCALLI SCALE DESCRIBES AN EARTHQUAKE WITH A SCALE
FROM I TO XII, WHERE A RATING OF II IS A LOW INTENSITY.