Chapter 1B Main Ideas

1. What Is an Earthquake?

An earthquake is the shaking that results when a deformed body of rock snaps back to its original shape through the process of elastic rebound. This movement occurs along a fault. The rupture is initiated at a point but quickly spreads across the area of a fault.

2. Where do earthquakes occur

Most earthquakes are associated with plate boundaries and those boundaries have characteristic patterns of earthquakes. However, some earthquakes do occur in the middle of plates, these are called intraplate earthquakes.

3. Seismic Waves

Earthquakes produce seismic waves. Seismic waves are categorized as:

Body waves: P-waves (compression wave, arrives first/travels fastest, travels through solids and liquids) and S-waves (shear wave, arrives second, only travels through solids)

Surface waves: Love waves (horizontal shear wave on the surface) and Rayleigh waves (rolling wave that make the ground roll).  Surface waves arrive after body waves.

4. Locating earthquake epicenters

Seismic waves can be measured by a seismograph (instrument) and make a seismogram (squiggly line record). The S-P arrival time difference can be used to determine the distance from an epicenter to the seismic station. Seismograms from three stations can be used to find an earthquake epicenter in a process called trilateration.

5. Magnitude and Intensity

The amplitudes of seismic waves are used to determine the amount of energy released by an earthquake – its magnitude. For the moment magnitude scale used today, the amount of energy released by an earthquake is proportional to the size of the rupture surface, the amount of displacement, and the strength of the rock. Intensity is a measure of the amount of shaking that occurs, and damage done at locations that experience an earthquake. Intensity will vary depending on the distance to the epicenter, the depth of the earthquake, and the type of geological materials present.

6. Earthquake Hazards

Earthquakes cause ground shaking and displacement. These can lead to building damage and collapse which is the most serious consequence of most large earthquakes. The amount of damage is related to the type and size of buildings, how they are constructed, and the nature of the material on which they are built. Other important consequences are liquefaction, landslides, fires, and tsunamis.

7. Assessing Seismic Risk

There is no reliable technology for predicting earthquakes, but the probability of one happening within a certain time period can be forecast. We can minimize earthquake impacts by educating the public and enforcing building codes. Seismic risk to human life can be calculated by considering what hazards are likely to affect an area, the vulnerability buildings in the area have to earthquake shaking, and how many people are living in the area. Risk assessment is used to help make sure emergency plans and supplies are where they need to be in the event of an earthquake.