1. Unit 7-3: Measuring an Earthquake

2. 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.

3. 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.

4. 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.

6. Richter Scale Richter magnitudes DescriptionEarthquake effects Frequency of occurrence Less than 2.0MicroMicroearthquakes, not felt.About 8,000 per day 2.0-2.9 Minor Generally not felt, but recorded.About 1,000 per day 3.0-3.9Often felt, but rarely causes damage.49,000 per year (est.) 4.0-4.9LightNoticeable shaking of indoor items, rattling noises. Significant damage unlikely.6,200 per year (est.) 5.0-5.9Moderate Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800 per year 6.0-6.9Strong Can be destructive in areas up to about 160 kilometres (100 mi) across in populated areas. 120 per year 7.0-7.9MajorCan 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 9.0-9.9 Devastating in areas several thousand miles across. 1 per 20 years 10.0+EpicNever recorded. Extremely rare (Unknown)

7. 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.

8. 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.

9. 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.

10. 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.

12. 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.

13. 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?

14. 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.

15. 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.

16. 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.

17. 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.