Measuring Earthquakes Seismology

Learning Target I will explain how earthquakes are measured and how scientists locate the epicenter of an earthquake.

Earthquakes generate waves that travel through the earth Seismic Waves Earthquakes generate waves that travel through the earth

Earthquake Detection Seismograph - instrument that records earthquake waves. a weight is suspended from a support that is attached to bedrock. When the bedrock vibrates, a pen records the earthquake on a rotating drum.

How Seismographs Work Weight is suspended from an object on bedrock, vibration causes it move.

Seismic Records Seismograms - records of the ground motion Sheets of paper removed from the rotating drum of the seismograph.

Calculating Earthquakes Earthquake - the vibration of the Earth caused by a rapid release of energy Focus - the point inside the Earth where the earthquake begins. Epicenter - the location on the surface directly above the focus

Locating Earthquakes The difference in speeds of P and S waves helps us locate the epicenter. The more time between the 1st arriving P and S waves, the greater the distance the earthquake is away. Example: an interval of 5 minutes indicates the earth-quake was 3,400 km away from the recorder.

Seismic Waves in the Earth

Locating Earthquakes How can we find an earthquake’s direction? Location can be found if three seismic stations recorded the earthquake. Draw circles around seismograph station Radius = distance from the seismograph to the epicenter (using time-distance graph) the epicenter is where the 3 circles intersect (triangulation)

Drawing circles around seismograph stations to find the epicenter Locating Earthquakes Drawing circles around seismograph stations to find the epicenter

Drawing circles around seismograph stations to find the epicenter Locating Earthquakes Drawing circles around seismograph stations to find the epicenter

Locating Earthquakes

Magnitude Magnitude - measures energy released (earthquake’s size) Richter Scale - measures magnitude using seismic records of energy release - No upper or lower limits - Largest quakes about Magnitude 9.5 (Chile 2010)

Intensity Intensity - measures how strong an earthquake feels to the observer Depends on: Distance to quake Geology Type of building (or outside?) Person Varies from place to place

Mercalli Scale Mercalli scale – rates earthquakes according to the level of damage at a given place. Mercalli Scale: 1 to 12 1 is not felt at all 12 is catastrophic

Think-Pair-Share How are the Mercalli scale and Richter scale similar? …Different? On which scale would an earthquake’s scale vary from one place to another, and why?

Comparing Magnitudes One point increase in magnitude = 32 times more energy Ex. Magnitude 6 earthquake releases about 32 x more energy than a magnitude 5 earthquake

Magnitude and Energy Magnitude Energy Explosive Power Example 9 8 7 6 U.S. Energy Use for a month Alaska 1964 Indonesia 2004 8 U.S. Energy Use for a day San Francisco, 1906 7 One Megaton World Series Earthquake, 1989 6 U.S. Energy Use for a minute Large Thunderstorm 5 One Kiloton 4 3 One ton of explosives World Trade Center Collapse

Magnitude and Energy Magnitude Energy Explosive Power Example 3 2 1 -1 One ton of Explosives World Trade Center Collapse 2 1 Topple 50-meter tree One kilogram of explosives Head-on collision at 60 mph Drop a car 10 meters Half stick of dynamite Very bad day skydiving -1 Impact of bullet One gram of explosives -2 Hammer blow -3 Dribbling a basketball