Earthquakes and Epicenters

The Earth is composed of several layers. We can separate these layers by physical properties OR by composition (what they’re made of).

Crust, Mantle, Core are the simple separations.

The crust and the mantle closest to the crust together are called the lithosphere.

The mantle beneath the lithosphere is plastic – meaning it flows like toothpaste. It is hot and the rock is molten. It is called the asthenosphere.

Lithosphere: 0-1000o C (cold): made up of mostly Aluminum (Al), Silica (Si) and Oxygen (O). Brittle.

Asthenosphere: 1000-1500oC: (warm) - made up mostly of Magnesium (Mg) and Iron (Fe). “Plastic” – flows like toothpaste.

Lower Mantle: 1500-5000 0 C (hot) - Iron and Magnesium - rigid

Outer Core: 5000-6200 0 C: (VERY HOT) – made up of mostly Iron and nickel – liquid.

Inner Core: 6200-7000 0 C: (VERY HOT) - made up mostly of Nickel (Ni)- The inner core is solid due to very high pressures.

Remember the ways that heat energy are transferred:

1. Radiation: heat energy transferred across an open space

2. Conduction: heat energy transferred from molecule to molecule by touching

3. Convection! – heat energy transferred in a cycle through a fluid.

The mantle in the asthenosphere is heated from below, rises to the surface, and then spreads out and sinks back down.

Lithosphere

Asthenosphere

Lower mantle

Outer core

Inner core

QUESTION: Where else in our studies of Earth have we seen something heat up, rise, and spread out only to cool and sink back down?

Air currents and weather!! Remember??

(Regents Exam is in less than a month!!!)

Convection currents in the asthenosphere cause the lithosphere above to crack and move. This cracks the surface of the Earth into big “puzzle pieces”. We call them Plates, and the way that these plates move is called plate tectonics.

Think of it like a hard-boiled egg:

The shell is the crust

the white is the mantle

and the yolk is the core.

When the lithosphere (shell) cracks and the pieces move, they can move in relation to each other in three ways:

1. They can crash into each other

2. They can slide past each other

3. They can move away from each other.

These movements take place over millions of years. Plates tend to only move around a few centimeters per year. This movement means that the surface of the Earth looked a lot different millions of years ago – and will look a lot different millions of years from now!

As you can imagine, all this movement and cracking can build up tremendous amounts of energy on the rocks of the lithosphere. When that energy is released, the Earth shakes violently… An Earthquake!

Definitions:

Earthquake – a natural rapid shaking of the lithosphere caused when rocks are displaced due to the release of energy stored in the rocks.

Focus – the place where an Earthquake originates

Epicenter – the place on the Earth’s Surface directly above the focus of an Earthquake

Seismic wave – energy waves given off by an earthquake

Seismograph – an instrument used to detect and record seismic waves

Earthquakes create waves that travel through the earth. They are much like the waves created in the water of a pond when a drop of rain lands….

Seismic waves travel away from the epicenter in all directions.

Seismic stations record movements in the lithosphere with seismographs, and create seismograms – pictures of the waves arriving at the station.

Earthquakes shake the lithosphere in a few different ways, and because of this, several different types of seismic waves are produced. The fastest of these waves are called P-waves (Primary waves). P-waves move like an accordion – stretching and tightening the lithosphere.

The next fastest waves are called S-waves (secondary waves). These waves move up and down much like waves on the ocean.

The slowest waves are called surface waves. There are two types. Rayleigh and Love waves.

These waves are responsible for most of the damage in an earthquake – but they arrive after the P and S waves.

This seismograph shows the arrival of the P and S waves.

Notice that the P-waves arrive first and then the S-waves come.

(If we waited long enough, we’d see surface waves next)

So – as the waves propagate (move) away from the epicenter, the P-waves keep getting further and further from the S-Waves.

1 minute 5 minutes

10 minutes 15 minutes

Pete the P-wave rabbit.

Pete is fast! He ALWAYS BEATS SOLOMON IN A RACE.

Solomon the S-wave tortoise.

Solomon is wise – but slow. He NEVER BEATS PETE IN A RACE.

After 1 minute

After 5 minutes

After 10 minutes

This is the P-wave and S-wave travel time graph from the Earth Science Reference Table.

Answer these questions from the graph:

1. How long does it take a P-wave to travel 2 x 103 km?

2. How long does it take a S-wave to travel the same distance?

3. 7 minutes after an Earthquake, how far apart are the P and S waves?

4. How far apart are they after 13 minutes?

We use this difference in travel time to determine the epicenter of an Earthquake. To do this, we need data from at least 3 seismic stations.

Let’s say an earthquake occurs in California. Seismographs in Delano, Oxnard, and Yuma record P and S waves. Each location determines how much time went by between the arrival of the P wave and the arrival of the S wave. By doing this, they each know how far the epicenter is from them – BUT… It could be in any direction!

But if all three locations determine how far it is, they can create a circle of possible locations – and where the three circles connect – there is the epicenter!

First, Oxnard determines their circle of possible locations…

Then Delano…

Then Yuma…

They intersect here – at the Epicenter!!