Earthquake Magnitude and Intensity

1. Earthquake Magnitude and Intensity • Magnitude is the measurement of the amount of energy released during an earthquake. • The Richter scale is a numerical scale based on the size of the largest seismic waves generated by a quake that is used to describe its magnitude. • Each successive number in the scale represents an increase in seismic-wave size, or amplitude, of a factor of 10. • Each increase in magnitude corresponds to about a 32-fold increase in seismic energy.

2. How are the Size and Strength of an Earthquake Measured? • Magnitude • Richter scale measures total amount of energy released by an earthquake; independent of intensity • Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

4. Earthquake Magnitude and Intensity Modified Mercalli Scale • The modified Mercalli scale, which measures the amount of damage done to the structures involved, is used to determine the intensity of an earthquake. • This scale uses the Roman numerals I to XII to designate the degree of intensity. • Specific effects or damage correspond to specific numerals; the higher the numeral, the worse the damage.

5. Earthquake Magnitude and Intensity

6. Earthquake Magnitude and Intensity Depth of Focus • Earthquake intensity is related to earthquake magnitude. • The depth of the quake’s focus is another factor that determines the intensity of an earthquake. • An earthquake can be classified as shallow, intermediate, or deep, depending on the location of the quake’s focus. • A deep-focus earthquake produces smaller vibrations at the epicenter than a shallow-focus quake.

7. Classification According to Focus Depths

8. What are the Destructive Effects of Earthquakes? Damage in Oakland, CA, 1989 • Building collapse • Fire • Tsunami • Ground failure

9. Earthquake Destruction Amplitude, duration, and damage increases in poorly consolidated rocks.

10. EARTH DESTRUCTION Seiches - These are the rhythmic sloshing of water in lakes, reservoirs, and enclosed basins. The waves can weaken reservoir walls and cause destruction. Liquefaction - Unconsolidated materials saturated with water turn into a mobile fluid.

11. Earth Destruction • Landslides and ground subsidence – Whenever the land moves quickly, as with landslides, there is the potential for a lot of damage and potential loss of life. • Fire – Ruptured gas lines from earthquakes is one of the major hazards. • Ground shaking versus material type – More ground shaking occurs in poorly consolidated (loose) sediments than solid bedrock.

12. Earth Destruction Ground Rupture ‐ This refers to areas where the land splits apart causing a rupture. These are often long linear features.

13. Tsunamis • A tsunami is a large ocean wave generated by vertical motions of the seafloor during an earthquake. • These motions displace the entire column of water overlying the fault, creating bulges and depressions in the water. • Long waves that can travel at speed over 800 km/h (500 mph). • When the waves enter shallow water they may form huge breakers with heights occasionally exceeding 30 m (19 ft.).

14. Formation of a Tsunami Tsunami waves can travel at a speed of over 500 mph.

15. Can Earthquakes be Predicted? Earthquake Precursors • changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the ground • seismic dilatancy model • seismic gaps

16. Can Earthquakes be Predicted? Earthquake Prediction Programs • include laboratory and field studies of rocks before, during, and after earthquakes • monitor activity along major faults • produce risk assessments