MEASURING EARTHQUAKES

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Measuring Earthquakes

Historically, scientists have used two different types of measurements to describe the size of an earthquake—intensity and magnitude.

Richter Scale

Mercalli Scale

Richter Scale• Based on the amplitude of the largest seismic wave

• Each unit of Richter magnitude equates to 10 times greater ground shaking

• Does not estimate adequately the size of very large earthquakes

Mercalli Scale• Based on an Earthquakes

intensity

• Subjective scale

• I to XII Scale

• Rating based on amount of damage that occurs

Moment Magnitude

• Derived from the amount of displacement that occurs along the fault zone

• Moment magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes.

• Measures very large earthquakes

Video: Why Moment Magnitude

• Factors that determine structural damage

1. Intensity and duration of the earthquake

- Unreinforced stone or brick buildings are the most serious safety threats

2. Nature of the material upon which a structure rests

3. The design of the structure

- The deeper the focus = less damage

What Determines Damage??

Liquefaction

Saturated material turns fluid

Surface objects may sink

Underground objects may float to surface

Cause of Tsunami

• A tsunami triggered by an earthquake occurs where a slab of the ocean floor is displaced vertically along a fault.

• A tsunami also can occur when the vibration of a quake sets an underwater landslide into motion.

http://www.pbs.org/wgbh/nova/tsunami/anat-flash.html

Landslides

• the sinking of the ground triggered by vibrations

Fire• In the San Francisco earthquake of 1906,

most of the destruction was caused by fires that started when gas and electrical lines were cut.

Use Chapter 8.2 and 8.3 of your textbook to answer the following questions at the end of your notes. Please write out the question in a different color, or you can highlight or underline the question.

1. When measuring earthquakes, when is the Richter Scale more useful and

when is moment magnitude more useful?

2. What are three factors that can contribute to the damage of buildings or

structures during an earthquake?

3. Buildings made from which type of materials tend to experience the most

damage during an earthquake?