Earthquakes

Earthquakes Our Definition: a sudden shaking of the ground because of movement within the earth’s crust.

Why do Earthquakes Happen? Earthquakes are the Earth's natural means of releasing stress. Earthquakes are the Earth's natural means of releasing stress.

What are Earthquakes? As the plates move they put forces on themselves and each other. As the plates move they put forces on themselves and each other. When the force is large enough, the crust is forced to break. When the force is large enough, the crust is forced to break. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake.

Earthquakes come from stressed out plates!

What causes the stress? When two plates interact at their boundaries they put forces on each other. When two plates interact at their boundaries they put forces on each other. There are three main forces that drive deformation within the Earth and create the stress that causes earthquakes. There are three main forces that drive deformation within the Earth and create the stress that causes earthquakes. –Compressional Stress –Tensional Stress –Shear Stress

Elasticity When forces are applied to rocks, such as pulling, pushing, twisting, or compression, they change their shape. When forces are applied to rocks, such as pulling, pushing, twisting, or compression, they change their shape. Once the rock reaches its elastic limit it breaks and returns to an undeformed shape. Once the rock reaches its elastic limit it breaks and returns to an undeformed shape. We can use the elastic properties of rock to explain how energy is stored and released within the Earth’s crust. We can use the elastic properties of rock to explain how energy is stored and released within the Earth’s crust.

Elasticity Rubber Band Vs. Bubble Gum

Elastic Rebound Theory As two plates move past each other they get stuck and then slip; the plates suddenly move, release energy and snap back into place. As two plates move past each other they get stuck and then slip; the plates suddenly move, release energy and snap back into place.

Seismic Waves There are three types of waves that are created when stress is released as energy in earthquakes: P, S, and surface waves. There are three types of waves that are created when stress is released as energy in earthquakes: P, S, and surface waves.

P - Waves These are the fastest of the three waves and the first detected by seismographs. These are the fastest of the three waves and the first detected by seismographs. Move at about 3.5 miles/second Move at about 3.5 miles/second They can travel through both liquid and solid rock. They can travel through both liquid and solid rock. P waves are longitudinal waves, which means that they compress and expand matter as they move through it. P waves are longitudinal waves, which means that they compress and expand matter as they move through it.

S - Waves They directly follow the P wave (2 nd to arrive) They directly follow the P wave (2 nd to arrive) Move at about 2.1 miles/second Move at about 2.1 miles/second S waves cannot travel through liquid because, while liquid can be compressed, it can't shear. S waves cannot travel through liquid because, while liquid can be compressed, it can't shear. S waves are transverse waves, which means they move matter side to side (perpendicular to the direction it’s going in). S waves are transverse waves, which means they move matter side to side (perpendicular to the direction it’s going in).

Body Waves Both P and S waves are called body waves because they move within the Earth's interior. Both P and S waves are called body waves because they move within the Earth's interior.

Surface Waves –Travel just below or along the ground’s surface –Most damaging waves! Move at about 1.2 miles/second Move at about 1.2 miles/second Two Types: Two Types: –L – Waves –R - Waves

Surface Waves R – waves, or Rayleigh waves, move in an elliptical motion, producing both a vertical and horizontal component of motion in the direction of wave propagation. R – waves, or Rayleigh waves, move in an elliptical motion, producing both a vertical and horizontal component of motion in the direction of wave propagation. L – waves, or Love Waves, move parallel to the Earth's surface and perpendicular to the direction of wave propagation. L – waves, or Love Waves, move parallel to the Earth's surface and perpendicular to the direction of wave propagation.

Faults Any stress on the plates can cause an earthquake if the elastic limit is reached. Any stress on the plates can cause an earthquake if the elastic limit is reached. Each type of stress, however, results in a specific type of fault. Each type of stress, however, results in a specific type of fault.

Normal Fault = It is normal for gravity to pull things down. It is normal for gravity to pull things down.

Reverse Fault AKA Thrust Fault AKA Thrust Fault = =

Strike Slip = No change in elevation, rock slides past one another. No change in elevation, rock slides past one another.

Focus and Epicenter The point within Earth where faulting begins is the focus. The point within Earth where faulting begins is the focus. The point directly above the focus on the surface is the epicenter. The point directly above the focus on the surface is the epicenter.

Types of Earthquakes Shallow focus - Foci are less than 70 km depth. Most destructive earthquakes. Shallow focus - Foci are less than 70 km depth. Most destructive earthquakes. Intermediate focus - Foci are between 70 and 300 km depth. Intermediate focus - Foci are between 70 and 300 km depth. Deep focus - Foci are greater than 300 km. Deep focus - Foci are greater than 300 km.

The Benioff Zone is the active seismic zone in a subduction zone. The main Benioff Zone is km below where deformation of the slab produces a concentration of earthquakes

How do we measure Earthquakes? Seismograph

Finding the Epicenter A seismograph is a device that measures seismic waves. A seismograph is a device that measures seismic waves. We can use the wave characteristics to find the epicenter of an earthquake We can use the wave characteristics to find the epicenter of an earthquake Before we look at how to find the epicenter lets make sure we know how to read a seismograph Before we look at how to find the epicenter lets make sure we know how to read a seismograph

Seismograph Arrival of P Wave Arrival of S Wave (Arrival of S Wave) – (Arrival of P Wave) =

How is an Earthquake’s Epicenter Located? Three seismograph stations are needed to locate the epicenter of an earthquake Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter The intersection of the circles locates the epicenter This process is known as Triangulation. This process is known as Triangulation.

Finding the Richter Magnitude 1. Find the amplitude of the earthquake 2. Find the distance from the epicenter by using the S-P interval. (you did this already!) 3. Use a NOMOGRAM. Make two points and connect the dots! Where your line intersects the middle line is your Richter Magnitude.