Earthquakes Chapter 12 Section 2

100% Chance of an Earthquake Today! Somewhere today, an earthquake will occur. A major portion of the world’s quakes center around the edges of lithospheric plates.

Frequency of Occurrence DescriptorMagnitudeAverage Annually Great8 +1 Major7 – Strong6 – Moderate5 – Light4 – 4.96,200 (est.) Minor3 – 3.949,000 (est.) Very minorLess than 3Mag ,000/day Mag ,000/day

Think - Pair - Share What is an earthquake?

Divergent boundaries are associated plates that move in opposite directions. This faulting creates a narrow band of numerous, shallow earthquakes. Movement of the Plates Convergent boundaries have broad zones of earthquakes. Shallowest are near the surface, deepest under volcanic mountains.

An earthquake is any seismic vibration of Earth caused by the rapid release of energy. Causes of Earthquakes A strain is deformation in response to a stress. Deformation: A change in shape. Deformation

Stress is the force per unit area that acts on a material. (1) compressive stress (2) tension stress (3) shear stress Types of Stress (4) torsion stress

Elastic deformation: a material deforms as a stress is applied, but returns to its original shape when the stress is removed. Example = rubber band Types of Deformation Plastic deformation: a material deforms, or changes shape, as a stress is applied and remains in the new shape when the stress is released. Example = modeling clay

Demonstrating Four Types of Stress 1. Which type of stress did you demonstrate?

The point on Earth’s surface directly above the focus is the epicenter. Earthquake waves travel out in all directions from a point where strain energy is released. This point is the focus. Earthquake Waves

Waves Body Waves P = Primary, particle movement is in the direction of travel S = Secondary, particle movement is perpendicular to direction of travel Surface Waves L = Love, particle movement is side-to-side R – Rayleigh, particle movement rolls like ocean waves

Surface Waves

Earthquake Measurement The Modified Mercalli scale ranks earthquakes in a range from I-XII. XII being the worst and uses eyewitness observation and post-earthquake assessments to assign an intensity value.

Earthquake Measurement Richter magnitude is intended to give a measure of the energy released during the earthquake. The Richter magnitude scale uses the amplitude of the largest earthquake wave.

Question 1 Which of the following is NOT a type of stress in rock? A. compression B. epicenter C. shearing D. tension Section Check

Answer The answer is B. The epicenter is the point on Earth’s surface located directly above the earthquake’s center. Section Check

Question 2 Where do P- and S-waves occur in relation to surface waves? Answer Seismic waves travel away from the epicenter in all directions. P- waves travel the fastest through rock material. S-waves move through the rock and cause particles to vibrate. Both P- and S- waves travel through the Earth’s interior while surface waves move along Earth’s surface. Section Check

Question 3 Why is it difficult to predict earthquakes? Section Check Answer Geologists can monitor changes in Earth that are associated with earthquakes. Measuring devices have been developed to assess changes in groundwater level and rock layers; however, no single change in Earth occurs for all earthquakes.