#18Measuring Earthquakes

How are earthquakes studied? Seismologists use seismometers, or

seismographs, an instrument that measure vibrations in the ground

(mm/s)

Seismogram- the paper record of the vibrations or seismic waves on a seismometer or seismograph

When are waves recorded? When recording waves

P is fastest so it is recorded first S is second, L are surface waves that are the slowest and

show up last

What are seismographs used for? To comprehend the location and magnitude of

earthquakes They record vertical motions, and

horizontal motions (east-west or north-south)

What is magnitude? Magnitude- strength or size of ground motion

An earthquake's magnitude or strength is a value of the energy that is released at the focus

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 Mexicali compared to how it felt San Diego?

What is magnitude? Magnitude- strength or size of ground motion

An earthquake's magnitude or strength is a value of the energy that is released at the focus

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 Mexicali compared to how it felt San Diego? Yes! Even though it felt different in San Diego, the Mexicali earthquake was still rated as a 7.2

How does a magnitude 6 compare to a 5 ?

- the waves are 10 times greater from a magnitude 6 than those from a magnitude 5 earthquake

- magnitude 6 releases about 30 times more energy than a magnitude 5

The Richter scale is logarithmic

Richter Magnitude

Strength

(amplitude)

Energy Released

5 X Y6 10X 30Y7 100X 1000Y

What are the 3 earthquake scales?

What are the 3 earthquake scales?

The Richter scale measures the strength of ground motion

Moment magnitude measures strength based on: the size of the area of the fault that moves, the distance the fault moved, and the local rigidity of rock

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

What is intensity? In the Mercalli scale, an intensity level I cannot

be felt, while a level XII will totally destroy an area

Intensity measures the shaking of an earthquake, which varies with location

Intensity is based on Magnitude Local types of rocks Duration- length of time Distance from epicenter

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

2.5 or below is not felt by most people

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

Loma Prieta was the epicenter of the 1989 earthquake that hit San Francisco

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

2.5 or below is not felt by most people

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

Loma Prieta was the epicenter of the 1989 earthquake that hit San Francisco = 6.9 on the moment magnitude scale, or 7.0 on the Richter = 63 dead, 3,757 injured, and

made thousands homeless **The largest ‘quake recorded in the U.S., magnitude 9.2,

occurred in Alaska in 1964 **The world's largest recorded earthquake, magnitude 9.5,

occurred in 1960 in Chile

http://www.sciencecourseware.com/eec/Earthquake/EpicenterMagnitude/Tutorials/SP.swf

The arrival of the P and S waves is used for finding the epicenter- the point above the focus

How can we find out where an earthquake started?

A seismogram can be used to find the distance between the P-waves and S-waves

If the epicenter if far away from the station, the P and S waves are far apart on the seismogram

As time goes on, the “gap” or separation between the lines increases because the P-wave moves faster than the S-wave.

S-P interval- the difference in arrival time between the P waves and S waves (the diagonal line is the average)

On a map, a circle is drawn to show the distance- how far the seismograph is from the epicenter

How can I find the epicenter? Step 1, subtract the S-wave from the P-wave on a seismogram

P-wave ^----------^ S-wave S – P = seconds 40 – 0 = 40 sec.

Step 2

Find the seconds on the the S-P interval graph

Find where it meets the diagonal line and match it to the distanceS-P interval of 40 sec = 390 km

NOTE You need to do these step 3 times for the epicenter

The radius of each circle represents the distance from the epicenter

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

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

There can be only one epicenter, an exact point where three circles meet

On a map, a circle is drawn to show the distance- how far the seismograph is from the epicenter

The epicenter is some point on the edge of the circle…What is the exact epicenter, here?

What causes earthquakes? It’s plate movement…

Faults try to move past each other…

building pressure can bend or deform rock…

or, cause earthquakes;

movement of the rocks

Bedarliex, France

Each year the southern California area has about 10,000 earthquakes. Most of them are so small that they are not felt. Only several hundred are greater than magnitude 3.0, and only about 15-20 are greater than magnitude 4.0. If there is a large earthquake, however, the aftershock sequence will produce many more earthquakes of all magnitudes for many months. The magnitude of an earthquake is a measured value of the earthquake size. The magnitude is the same no matter where you are, or how strong or weak the shaking was in various locations.

The intensity of an earthquake is a measure of the shaking created by the earthquake, and this value does vary with location. http://images.google.com/imgres?imgurl=http://www.ngdir.ir/sitelinks/kids/Image/erathquake

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Different scales

Intensity is based on Magnitude Distance from epicenter Local types of rocks (geology) Duration or length of time

With small earthquakes Richter and moment magnitude scales are similar

Moment magnitude is more accurate during large earthquakes

The difference in S and P arrival time and distance can graphed into a single line

What kind of range of magnitude? 9.5 is the highest moment magnitude on record, while 2.5 or below is not felt by most people

In the Mercalli scale, an intensity level I cannot be felt, while a XII will totally destroy an area

The 8.9 earthquake in March 2011, 80mi. off of the coast of Sendai, caused a 30ft tsunami that traveled several miles inland

The 6.7 Northridge earthquake in 1994, caused about $20 billion in damage, making it one of the costliest natural disasters in the US

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

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

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

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