Unit 7-3:Measuring an

Earthquake

Earthquake Magnitude

In addition to locating epicenters, seismographs are useful in determining another factor of an earthquake.

Magnitude: The strength of the shaking that the quake

produces. The most widely used and recognized

magnitude scale is the Richter Scale.

Earthquake Magnitude

Richter Scale Developed in the 1940s by Charles F. Richter. It measures the amount of energy released by an

earthquake. Every one increase of one magnitude number on

the scale means that the earthquake’s strength increased by 10.

So a magnitude 2 earthquake is 10x more powerful than a magnitude 1.

Earthquake Magnitude

That is why magnitude is usually expressed as the magnitude number with a decimal after it (ex. 2.2).

This also means that a 7.0 earthquake is 100x more powerful than a 5.0 earthquake!

This is also why a 1-3 magnitude earthquake is largely ignorable, while a 4-5 can cause massive damage.

Richter ScaleRichtermagnitudes

Description Earthquake effectsFrequency of

occurrence

Less than 2.0 Micro Microearthquakes, not felt. About 8,000 per day

2.0-2.9Minor

Generally not felt, but recorded. About 1,000 per day

3.0-3.9 Often felt, but rarely causes damage. 49,000 per year (est.)

4.0-4.9 Light Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 per year (est.)

5.0-5.9 ModerateCan cause major damage to poorly constructed buildings over small regions. At

most slight damage to well-designed buildings.800 per year

6.0-6.9 StrongCan be destructive in areas up to about 160 kilometres (100 mi) across in

populated areas.120 per year

7.0-7.9 Major Can cause serious damage over larger areas. 18 per year

8.0-8.9

Great

Can cause serious damage in areas several hundred miles across. 1 per year

9.0-9.9Devastating in areas several thousand miles across.

1 per 20 years

10.0+ Epic Never recorded.Extremely rare

(Unknown)

Earthquake Damage

There are two major reasons why building collapse during an earthquake: Ground shaking. Foundation failure.

Remember, earthquakes don’t kill people. The damage caused by the earthquake (falling buildings, flooding, fire, etc.) kills people.

Earthquake Damage

Ground Shaking: The result of the waves of energy released by the

earthquake. The way the earth shakes is similar to the way a

bell shakes when it is struck. The waves move in different directions depending

on the type of wave. The buildings have to move with the earth as it

shifts.

Earthquake Damage

Ground Shaking: Most buildings can withstand large vertical

shaking without major damage. However, when the S waves come by, and the

earth moves back and forth, Most buildings cannot survive horizontal movement. This is because the different parts of the building

stretch, compress, and then break apart.

Earthquake Damage

Foundation Failure: If the soil under the building settles, then the

foundation is no longer supported. This causes the foundation to break and the building

to collapse. If the soil conditions are right, then liquefaction

may occur. Liquefaction: When the shaking of the earth makes

the solid dirt and soil act as if it were a liquid.

Earthquake Damage

Foundation Failure: Having a solid foundation is of critical importance. Buildings build upon solid rock experience very

little damage. Buildings built on filled in bogs, soft fill, or any

other soft material suffer severe damage. Most collapse completely.

Earthquake Prediction

In order to an earthquake prediction to be successful, it must correctly forecast three facts: The place where the earthquake (epicenter) will

occur. When the earthquake will occur. The magnitude of the earthquake.

But how do we correctly forecast these?

Earthquake Predictions

It has been discovered that P waves slow down by about 10%-15% for a period of time before an earthquake occurs. This period of time, however, may be a few days,

or a few years. They also discovered that the earthquake occurs

shortly after the P waves return to normal speed.

Earthquake Predictions

It was also noted that the longer the P waves were below normal, the stronger the earthquake.

Although not a perfect system, it has helped areas such as California, China, and Russia prepare for earthquakes.

Earthquake Prediction

In some areas, a slight uplift in elevation has preceded an earthquake. In Japan, an uplift was noted for ten years before

a 7.5 earthquake struck. In California, Palmdale Bulge has been closely

monitored for changes that may indicate an earthquake.

Earthquake Prediction

Other methods: A decrease in electrical resistance in the ground

may indicate an earthquake. An increase in radon in well water seems to

correlate with earthquakes. By studying these observable changes,

scientists hope to one day accurately predict earthquakes, reduce casualties and lower property damage.