Measuring Measuring EarthquakesEarthquakes

(1) How are earthquakes studied? seismometer – or, seismograph, an – or, seismograph, an

instrument that measures ground vibrationsinstrument that measures ground vibrations

seismogram – like a telegram, a paper record – like a telegram, a paper record of the vibrations or seismic waves on a of the vibrations or seismic waves on a seismometer or seismographseismometer or seismograph

(2) How big was that ‘quake? Magnitude – a number value for the

earthquake’s strength or size of ground motion

(2) How big was that ‘quake? Magnitude – a number value for the

earthquake’s strength or size of ground motion

Magnitude is the same no matter where you are, or how strong or weak the shaking was in various locations

EXAMPLE- Did the April 4, 2010 earthquake feel different in San Diego, compared to Mexicali?

(2) How big was that ‘quake? Magnitude – a number value for the earthquake’s – a number value for the earthquake’s

strength or size of ground motionstrength or size of ground motion

Magnitude is the same no matter where you are, or how strong or weak the shaking was in various locations

EXAMPLE- Did the April 4, 2010 earthquake feel different in San Diego, compared to Mexicali? Yes!

Though it felt different in San Diego, the Mexicali earthquake has been rated at a 7.2 magnitude

(3) What are 3 kinds of earthquake scales?1) The 1) The Richter scale- measures the strength of - measures the strength of

ground motion ground motion at the surfaceat the surface

(3) What are 3 kinds of earthquake scales?1) The 1) The Richter scale- measures the strength of - measures the strength of

ground motion ground motion at the surfaceat the surface

2) 2) Moment magnitude- measures strength based - measures strength based on: on: the the sizesize of the area of the fault that moves, of the area of the fault that moves, the the distancedistance the fault moved, the fault moved, and the local and the local rigidity of rockrigidity of rock

(3) What are 3 kinds of earthquake scales?1) The 1) The Richter scale- measures the strength of - measures the strength of

ground motion ground motion at the surfaceat the surface

2) 2) Moment magnitude- measures strength based - measures strength based on: on: the the sizesize of the area of the fault that moves, of the area of the fault that moves, the the distancedistance the fault moved, the fault moved, and the local and the local rigidity of rockrigidity of rock

(3) What are 3 kinds of earthquake scales? 1) The 1) The Richter scale- measures the strength of - measures the strength of

ground motion ground motion at the surfaceat the surface

2) 2) Moment magnitude- measures strength based on: - measures strength based on: the the sizesize of the area of the fault that moves, of the area of the fault that moves, the the distancedistance the fault moved, the fault moved, and the local and the local rigidity of rockrigidity of rock

3) The 3) The Mercalli scale- expresses earthquake - expresses earthquake intensityintensity or damage, and uses Roman numerals or damage, and uses Roman numerals to describe the effects of each intensity levelto describe the effects of each intensity level

(4) How does a magnitude 6 compare to a 5 on the

Richter?The waves are 10 timesThe waves are 10 times greater from a magnitude 6 greater from a magnitude 6 than those from a magnitude 5 earthquakethan those from a magnitude 5 earthquake

Richter

MagnitudeStrength

(amplitude)

Energy Released

5 X Y

6 10X 30Y

7 100X 1000Y

(4) How does a magnitude 6 compare to a 5 on the

Richter?The waves are 10 timesThe waves are 10 times greater from a magnitude 6 greater from a magnitude 6 than those from a magnitude 5 earthquakethan those from a magnitude 5 earthquake

A magnitude 6 releases about A magnitude 6 releases about 30 times30 times more energy more energy than a magnitude 5 than a magnitude 5

Richter Magnitude

Strength

(amplitude)

Energy Released

5 X Y

6 10X 30Y

7 100X 1000Y

Moment magnitude- measures - measures strength based on: strength based on: the the sizesize of the area of the fault that moves, of the area of the fault that moves, the the distancedistance the fault moved, the fault moved, and the local and the local rigidity of rockrigidity of rock

(5) How do the earthquake scales compare? For larger earthquakes, moment magnitude is

more accurate than the Richter

(5) How do the earthquake scales compare? For larger earthquakes, moment magnitude is

more accurate than the Richter

9.5 is the highest moment magnitude on record, while the largest Richter earthquakes are about 8.8

(5) How do the earthquake scales compare? For larger earthquakes, moment magnitude is

more accurate than the Richter

9.5 is the highest moment magnitude on record, while the largest Richter earthquakes are about 8.8

The largest ‘quake on the San Andreas in the last 200 years, was in 1906 in San Francisco = 7.9 on the moment magnitude scale, 8.3 on the Richter

Loma Prieta was the epicenter of the 1989 earthquake that hit San Francisco = 6.9 on the moment magnitude scale, … 7.0 on the Richter

(6) With the Mercalli, intensity depends on your location; level I cannot be felt, while level XII is total destruction

(6) With the Mercalli, intensity depends on your location; level I cannot be felt, while level XII is total destruction

(7) How far away was that earthquake?

S-P interval- graphs the difference in travel time of both - graphs the difference in travel time of both the P and S wavesthe P and S waves

(7) How far away was that earthquake?

S-P interval- graphs the difference in travel time of both - graphs the difference in travel time of both the P and S waves, or, a diagonal line of their averagethe P and S waves, or, a diagonal line of their average

(8) Step 1 Step 1, Subtract the S-wave from the P-wave on a , Subtract the S-wave from the P-wave on a seismogramseismogram

P-wave P-wave ------------ ------------ S-wave, subtract S-wave, subtract S – P = 50 – 0 = 50 seconds S – P = 50 – 0 = 50 seconds

(9)

Step 2Step 2

Find where the time meets the distance on the diagonal Find where the time meets the distance on the diagonal line (the average of S – P) and match it to the distance line (the average of S – P) and match it to the distance S-P interval of 50 sec. = 490 km S-P interval of 50 sec. = 490 km

(10) On a map, a circle is On a map, a circle is

drawn to show the drawn to show the distance- how far distance- how far the seismograph is the seismograph is from the epicenterfrom the epicenter

The epicenter could The epicenter could be anywhere on the be anywhere on the edge of that circleedge of that circle

We need 2 more We need 2 more seismographs to seismographs to find the earthquakefind the earthquake

(10) On a map, a circle is On a map, a circle is

drawn to show the drawn to show the distance- how far distance- how far the seismograph is the seismograph is from the epicenterfrom the epicenter

The epicenter could The epicenter could be anywhere on the be anywhere on the edge of that circleedge of that circle

We need 2 more We need 2 more seismographs to seismographs to find the earthquakefind the earthquake

(11) The radius of each circle The radius of each circle represents the distance represents the distance from the epicenter from the epicenter

If only 2 stations are used, If only 2 stations are used, we end up with 2 possible we end up with 2 possible epicenters !epicenters !

[ Without Station C we [ Without Station C we wouldn't know for sure]wouldn't know for sure]

There can be only one There can be only one epicenter, and to find it, epicenter, and to find it, you need 3 seismographsyou need 3 seismographs

http://blogs.agu.org/mountainbeltway/category/japan/

Penn. State Univ. Penn. State Univ. http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/waves_and_interior.html

http://earthquake.usgs.gov/regional/nca/http://earthquake.usgs.gov/regional/nca/1906/18april/magnitude.php1906/18april/magnitude.php

How far away was that earthquake? Step 1 Step 1, Subtract the S-wave from the P-wave on a , Subtract the S-wave from the P-wave on a seismogramseismogram

P-wave P-wave ------ ------ S-wave, subtract S-wave, subtract S – P = 40 – 0 = 40 seconds S – P = 40 – 0 = 40 seconds

Seismograms can be used to find the distance Step 1Step 1, Subtract the S wave from the P wave , Subtract the S wave from the P wave

S wave 6 S wave 6 P wave 3 P wave 3 S – P =6 – 3 = 3

Step 2Step 2

Find where the time meets the distance on the diagonal Find where the time meets the distance on the diagonal line (the average of S – P) and match it to the distance line (the average of S – P) and match it to the distance S-P interval of 3 min. = 1500 km S-P interval of 3 min. = 1500 km

What kind of range?What kind of range? 9.5 is the highest 9.5 is the highest moment magnitudemoment magnitude on record, while 2.5 or on record, while 2.5 or

below is not felt by most peoplebelow is not felt by most people In the In the MercalliMercalli scale, an scale, an intensity intensity level Ilevel I cannot be felt, while a cannot be felt, while a

XIIXII will totally destroy an area will totally destroy an area The 8.9 earthquake in March 2011, 80mi. off of the coast of The 8.9 earthquake in March 2011, 80mi. off of the coast of

Sendai, caused a 30ft tsunami that traveled several miles inlandSendai, caused a 30ft tsunami that traveled several miles inland The 6.7 Northridge earthquake in 1994, caused about $20 The 6.7 Northridge earthquake in 1994, caused about $20

billion in damage, making it one of the costliest natural disasters billion in damage, making it one of the costliest natural disasters in the US in the US

Loma Prieta was the Loma Prieta was the epicenterepicenter of the 1989 earthquake that hit of the 1989 earthquake that hit San Francisco = 6.9 on the San Francisco = 6.9 on the moment magnitudemoment magnitude scale, scale, or or 7.0 on the 7.0 on the RichterRichter = 63 dead, 3,757 injured, and several = 63 dead, 3,757 injured, and several thousand made homelessthousand made homeless

The largest ‘quake on the San Andreas in the last 200 years, The largest ‘quake on the San Andreas in the last 200 years, was in 1906 in San Francisco = 8.25 on the Richterwas in 1906 in San Francisco = 8.25 on the Richter

**The largest ‘quake recorded in the U.S., magnitude 9.2, **The largest ‘quake recorded in the U.S., magnitude 9.2, occurred in Alaska in 1964occurred in Alaska in 1964

**The world's largest recorded earthquake, magnitude 9.5, **The world's largest recorded earthquake, magnitude 9.5, occurred in 1960 in Chileoccurred in 1960 in Chile