The point within Earth where rock under stress breaks is called the focus • The point directly above the focus on the surface is the epicenter The Focus and Epicenter of an Earthquake
What are Seismic Waves? • Vibrations that travel through Earth carrying the energy released during an earthquake • Energy starts at the focus and moves outward • Two types: • Body waves • P and S • Surface waves
Body Waves: P and S waves Undisturbed Material • Body waves • P or primary/pressure waves • fastest waves • travel through solids, liquids, or gases • Waves compress and expand like an accordian • S or secondary/shear waves • slower than P waves • travel through solids only • Move up and down as well as side to side P Waves S Waves
Surface Waves: R waves • Surface Waves • Travel just below or along the ground’s surface • Slower than body waves • Surface waves are the most damaging and are especially damaging to buildings. R waves L waves L waves
How is an Earthquake’s Epicenter Located? Seismic wave behavior • P waves arrive first, then S waves, then surface waves • Average speeds for all these waves is known • After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.
Where Do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the circum-Pacific belt ~15% occur in the Mediterranean-Asiatic belt ~remaining 5% occur in the interiors of plates and on spreading ridge centers • more than 150,000 quakes strong enough to be felt are recorded each year
How do you locate an epicenter? The Focus and Epicenter of an Earthquake • The point within Earth where rock under stress breaks is called the Focus • The point directly above the focus on the surface is the Epicenter. epicenter Surface B A Which seismic Station will Receive the Wave first? A or B focus
How do you locate an epicenter? You must have three different seismographs at three different Locations - then you can pinpoint the different times each picked up the earthquake wave. A scientist will do this in a lab. epicenter Surface B A focus
How is an Earthquake’s Epicenter Located? • Three seismograph stations are needed to locate the epicenter of an earthquake • A circle where the radius equals the distance to the epicenter is drawn • The intersection of the circles locates the epicenter
How is an Earthquake’s Epicenter Located? Time-distance graph shows the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves
How are the Size and Strength of an Earthquake Measured? • Magnitude: the measurement of earthquake strength based on seismic waves and movement along faults
Three ways of measuring earthquakes: • Mercalli Scale measures the intensity of an earthquake • Richter Scale is a rating of the size of seismic waves • Moment Magnitude Scale estimates the total energy released by an earthquake
The Economics and Societal Impacts of EQs • Building collapse • Fire • Tsunami • Ground failure Damage in Oakland, CA, 1989
Ground Shaking • The earthquake’s amplitude, how long it lasts, and damage increases in poorly consolidated rocks What are the Destructive Effects of Earthquakes?
Local soil conditions: loose soil shakes more violently than the surrounding rock (house built on solid rock will shake less than a house build on sandy soil) Liquefaction: earthquakes violent shaking suddenly turns loose soil into liquid mud Aftershocks: buildings weakened by an earthquake may collapse during aftershocks Tsunamis: earthquakes on the ocean floor cause large waves that can grow to the height of a six story building Earthquake Hazards
Can Earthquakes be Predicted? • Forecasting when and where earthquakes will occur and their strength is difficult. • One way seismologist forecast earthquakes in a place is to observe their past strength and frequency.
Earthquake Hazard • Earthquake hazard is a measurement of how likely an area is to have damaging earthquakes in the future. An area’s earthquake-hazard level is determined by past and present seismic activity.
Forecasting Earthquakes • A seismic gap is an area along a fault where relatively few earthquakes have occurred recently but where strong earthquakes are known to have occurred in the past. Ex: California The gap hypothesis states that sections of active faults that have had relatively few earthquakes are likely to be the sites of strong earthquakes in the future.