Earthquakes!!Earthquakes!!

Focus and EpicenterFocus and Epicenter

Focus – where the earthquake happens

Epicenter – the spot on the surface above the focus

Elastic Elastic ReboundRebound

Stress causes the ground begins to be deformed. An earthquake occurs when a rock passes its point of maximum elasticity. After it breaks, it regains its original shape (but not position.)

Seismograph Seismograph and inertiaand inertia

Notice that it is the mass that is moving, not the drum. A heavy mass is used (more inertia) and is suspended so that it is less effected by whatever it is attached to. See the animation next.

Seismic WavesSeismic Waves

P(Primary) – body wave

S(Secondary) – body wave

L(Long) – surface wave

Seismogram- recording from Seismogram- recording from seismographseismographP wave arrives

first

S wave then arrives

The time between them tells you how far away they came from. Notice that it is 5 minutes from P to S wave.

The graph on the next page shows how to do this

5 minutes

Different speeds of P and S wavesDifferent speeds of P and S waves

First, look at an example at the times for P and S wave travel. How long does it take for a P wave to travel 5000 km? An S wave?

P wave – 8 minutes

S wave – 14 minutes and 40 seconds

Finding distance from seismogramFinding distance from seismogramThe earlier seismogram has an S-P travel time difference of 5 minutes. So, we must find the place on the graph where the difference between the P and S curves is 5 minutes and find the corresponding distance, as shown here. Start by using your y axis to measure 5 minutes, as shown.

Measure 5 minutes here

Then move it to where it fits exactly between the lines.

And read the distance here as 2900 km.

Distance GraphDistance Graph

Another graph allows you to find the distance to the epicenter using a line that shows the difference in arrival time between P and S waves. The one shown can only be used for distances from 300 – 750 km. Simply read the distance off the line, as shown.

Let us see how far an epicenter is if the difference between P and S waves is 60 seconds.

About 580 km.

Finding the EpicenterFinding the Epicenter

Three seismograph stations are needed.

Why?

Each location gives us a distance, so we can draw a circle there. Then the intersection of 3 circles gives us the epicenter.

Finding Finding MagnitudeMagnitude

Use the distance you calculated from the seismogram and graph. Then find the amplitude of the S-wave on the seismogram. Put these two quantities on the opposite sides of this special graph called a nomogram. Connect them with a line, and this gives you the magnitude.

Let’s try one here.Let’s try one here.

The difference in arrival times between the P and S wave here is about 35 sec. (These graphs are all in seconds.)

Use the time difference of 35 seconds to find the distance to the epicenter.

About 340 kmThe amplitude of the S wave is about 350 mm.

Finding the MagnitudeFinding the MagnitudeNow mark the distance and Now mark the distance and

amplitude on the graph amplitude on the graph here - called a here - called a nomogram. (Watch the nomogram. (Watch the graph carefully as we graph carefully as we click here)click here)

And then connect them with a line.

Where the line intersects the magnitude line, you have found the magnitude. This one is about 6.7, right?

We will do some for practice here.

The shadow zoneThe shadow zone Notice in this diagram Notice in this diagram

that S waves are that S waves are absorbed by the liquid absorbed by the liquid outer core. P waves are outer core. P waves are refracted by the outer refracted by the outer core, because waves core, because waves refract (bend) when they refract (bend) when they enter a new medium at enter a new medium at an angle. This is how an angle. This is how geologists found out that geologists found out that the outer core is liquid.the outer core is liquid.

Shadow zone 2Shadow zone 2So, match the correct seismogram below with its location

1

2

3

4

5

D

A

E

C

B

Changing earthquake depth at Changing earthquake depth at subduction zonessubduction zones

Why do the earthquakes vary in depth?

Which way is the plate subducting at the Tonga Trench? To the west

Which way is the plate subducting at the Peru-Chile Trench? To the east

We will do this in an activity later.

Let’s look at the Tonga Trench Let’s look at the Tonga Trench Deepening earthquakes at a subduction zone Deepening earthquakes at a subduction zone

TsunamiTsunami

Old Tsunami PhotoOld Tsunami Photo

The Big Tsunami Dec 26, 2004The Big Tsunami Dec 26, 2004

One of the greatest disastersOne of the greatest disasters

Safety tip. If you see the ocean floor, and the ocean has retreated, head for high ground as fast as possible.

How many earthquakes/how big?How many earthquakes/how big?

Historical EarthquakesHistorical Earthquakes

Intensity – damage doneIntensity – damage done