RECORDING EARTHQUAKES

CHAPTER 6.2

RECORDING EARTHQUAKES1.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 EARTHQUAKESb.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 DIRECTIONiii.HORIZONTAL MOTION IN NORTH-SOUTH

DIRECTION

SEISMIC WAVES2.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 WAVESi.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 WAVESii.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 WAVESiii.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. 2.SURFACE WAVES TRAVEL OVER THE EARTH’S SURFACE

SIMILAR TO OCEAN WAVES.3.SURFACE WAVES ARE DESTRUCTIVE WHEN TRAVELING

THROUGH LOOSE EARTH.

LOCATING AN EARTHQUAKE3.LOCATING AN EARTHQUAKE

a.THE DIFFERENCE BETWEEN THE ARRIVAL TIMES OF THE P-WAVES AND S-WAVES CAN BE USED TO DETERMINE THE EPICENTER OF AN EARTHQUAKE.

b.P-WAVES TRAVEL ABOUT 1.7 TIMES FASTER THAN S-WAVES. 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 EARTHQUAKEc.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.

ii.CIRCLES ARE DRAWN ON A MAP USING THE DISTANCES RECORDED.iii.WHERE THE CIRCLES INTERSECT IS THE EPICENTER OF THE

EARTHQUAKE.

EARTHQUAKE MEASUREMENT4.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 MEASUREMENTii.EARTHQUAKES WITH A MAGNITUDE LESS THAN A 2.5 ARE

CALLED MICROQUAKES AND ARE USUALLY NOT FELT BY PEOPLE.

iii.THE MERCALLI SCALE EXPRESSES THE INTENSITY OF AN EARTHQUAKE, OR THE AMOUNT OF DAMAGE IT CAUSES THROUGH ROMAN NUMERALS AND DESCRIPTION OF DAMAGE.

1.THE MERCALLI SCALE DESCRIBES AN EARTHQUAKE WITH A SCALE FROM I TO XII, WHERE A RATING OF II IS A LOW INTENSITY.