Chapter 8, Section 1 – What are Earthquakes? Seismology – The study of earthquakes. Most earthquakes occur near the edges of tectonic plates where giant pieces of the Earth’s crust are moving over plastic rock. Earthquakes occur at or near fault lines where the sliding action of these tectonic plates causes vibrations felt as earthquakes. Deformation – stress caused by the bending, tilting or folding of rock layers causing them to change shape. Plastic Deformation – rocks bend but do not break; these do NOT cause earthquakes. Elastic Deformation – rocks stretch and then break creating the vibrations of seismic waves that cause earthquakes and then return to their pre-deformed shape. Strike/Slip Faults – are the cause of most earthquakes. These faults occur at transform plate boundaries. Earthquake Zones – Areas where large numbers of faults are located parallel to tectonic plate boundaries.

Seismic Waves – Energy waves that travel in all directions through the Earth. Types of Seismic waves: 1. Body Waves – seismic waves that travel through the interior of the Earth. a. Primary or P Waves – These are the fastest moving seismic waves thus, the first to be detected. These waves travel with a “back & forth” motion. b. Secondary or S (shear) Waves – These are the second fastest, hence the second waves to be detected. S waves travel with a side to side motion. 2. Surface or L (long) waves – These are the slowest of the waves and travel along the Earth’s surface.

Chapter 8, Section 2 – Earthquake Measurement Seismograph – The instrument that records the time and intensity of the seismic waves and helps locate the epicenter of seismic events. Seismogram – The tracing or paper record of the earthquake motion created by the seismograph. The most direct method of locating the epicenter of an earthquake is to measure the time difference between the arrival of the S waves and the P waves (S – P Time Method) as recorded on the seismogram. Focus – The origin or starting point deep within the Earth where the earthquake begins along the fault. Epicenter – The point on the Earth’s surface located directly above the focus.

Chapter 8, Section 2 – Earthquake Measurement Richter Magnitude Scale – Measures the strength of the earthquake by measuring the amount of ground motion. Magnitude – A measure of the strength of the Earthquake. Each single number increase on the Richter Scale increases the strength of the earthquake by a factor of 10. For example, a magnitude 6 earthquake is 10 times greater than a 5 and 100 times greater than a 4. The highest number on this scale is a 9. Intensity – How much ground shaking is felt by people or how much damage a quake causes. This is measured on the Mercalli Intensity Scale. A single earthquake can have a variety of intensities levels with the highest near the epicenter decreasing outwards. The highest number on this scale is XI.

Chapter 8, Section 3 – Earthquakes & Society Earthquake Hazard – How likely an area is to have an earthquake based on past and present seismic activity. The west coast of the U.S. has the highest probability in our country. Earthquakes prediction is very difficult but scientists do observe patterns and are able to make generalized predictions. Gap Hypothesis – A hypothesis that states sites along active faults with few earthquakes will have strong earthquakes in the future. Seismic Gaps – Areas along a fault where a relatively few number of earthquakes have occurred.

Chapter 8, Section 3 – Earthquakes & Society Retrofitting – The process of making older buildings earthquake resistant by reinforcing their foundations with steel. Earthquake Resistant – The addition of weights to dampen seismic vibrations, cross-bracing and shock absorbers to reduce the “push-pull” effect of earthquakes that causes building to sway. Before the Quake: 1. Place heavier objects on lower shelves. 2. Develop a plan to get away from possible falling objects as well as a meeting place. 3. Stockpile food and water. During the quake: 1. If indoors, get under a heavy table to protect yourself from falling objects. 2. If outside, stay away from anything that could fall on you, finding a clear area is best. 3. If in a car, stop and stay inside the car.