Bellringer# 27 What is the Richter Scale? What is the greatest magnitude on the Richter scale? Where would an earthquake with the greatest magnitude be felt?

Ch.21 S.2 Earthquakes Focus What is an earthquake? What causes and earthquake? How are earthquakes rated? How are earthquakes measured?

Key Ideas and Key Terms Where do most earthquakes occur? How do scientists learn about earthquakes and the Earth’s interior? Focus Epicenter Surface wave Seismology Richter scale Vent

What are Earthquakes? An earthquake is the vibration of Earth produced by the rapid release of energy. They are caused by slippage along a fault in Earth’s crust. The energy released is radiated in all directions from its source or focus.

Earthquakes Sensitive instruments located around the world record the event. Over 30,000 earthquakes occur wolrdwide each year. (only strong enough to be felt) Only 75 significant earthquakes occur each year. An earthquake is one of the most destructive natural forces on Earth.

7 Where do Earthquakes Occur?

Where do earthquakes occur? By looking at maps showing past seismic activity, one can see that earthquakes occur mostly at the boundaries of tectonic plates, where the plates move with respect to each other.

Earthquakes occur at plate boundaries Earthquakes are vibrations resulting from rocks sliding past each other at a fault. Seismic waves are waves of energy released during in earthquake. focus: the location within Earth along a fault at which the first motion of an earthquake occurs epicenter: the point on Earth’s surface directly above an earthquake’s starting point, or focus

Energy from earthquakes is transferred by waves. – The energy released from an earthquake is measured as shock waves. – Earthquakes generate three types of waves: Longitudinal waves, also known as P waves Transverse waves, also known as S waves Surface waves

– Longitudinal waves travel by compressing and stretching crust. They are also called primary waves (P waves). – Transverse waves travel in an up and downward movement. They are also called secondary waves (S waves).

Waves move through Earth and along its surface. – Both P waves and S waves spread out from the focus in all directions through the earth. – Surface waves move only on Earth’s surface. surface wave: a seismic wave that travels along the surface of a medium and has a stronger effect near the surface of the medium than it has in the interior

Foreshocks and Aftershocks The adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks. Aftershocks occur for several days after a quake. Sometimes they can destroy already weakened structures. Foreshocks are small earthquakes that happen after a major earthquake for days or years.

Measuring Earthquakes 〉 How do scientists learn about earthquakes and the Earth’s interior? 〉 Because energy from earthquakes is transferred by waves, scientists can measure the waves to learn about earthquakes and about the interior of Earth through which the waves travel.

Seismology and Seismographs Seismology the study of earthquakes including their origin, propagation, energy, and prediction. – Seismos= shake, ology= the study of – The Chinese were the first to study the waves. Seismographs are instruments that record seismic waves – Seismos= shake, graph= write Seismograms are records of seismic activity – Seismos= shake, gramma= what is written

Horizontal Seismograph The mass swings from side to side. It detects horizontal ground motion 2 horizontal seismographs are needed one that measures North- South and one for East-West The mass (weight) is motionless The recording drum is anchored to the bedrock and vibrates with the seismic waves

Vertical Seismograph The mass is suspended by a spring. The mass stays motionless while the Earth and support move due to inertia. It detects vertical ground motion

Three seismograph stations are necessary to locate the epicenter of an earthquake. – There are more than 1000 seismograph stations across the world. – Because P waves travel faster, the difference between the arrival of P waves and the arrival of S waves allows scientists to calculate how far away the focus is.

P-waves move faster than S-waves. So, if the distance of the earthquake to the recording station increases, the time between the arrival of P- and S-waves will increase. From this P-S travel time difference we can determine the distance of the earthquake to the seismometer. Note that we only get the distance, not the direction of the earthquake focus.

The solution is to determine the distance for more than one seismic station, and draw this on a map. Two determinations result in two possible locations (two overlapping circles have two points in common). Three or more determinations can only have one location in common. This location is called the epicenter.

Geologists use seismographs to investigate Earth’s interior. – The way P and S waves travel through Earth’s interior help scientists make a model of Earth with layers of different densities. – Scientists have used this information to develop a model of Earth’s interior structure.

The Richter scale is used to measure earthquakes. – Although the Richter scale was used popularly for much of the 20 th century, scientists today more often use other scales, such as the moment- magnitude scale. Richter scale: a scale that expresses the magnitude of earthquakes

Magnitude scales alone cannot predict how severe an earthquake will be. – Many factors, such as distance from the focus and local construction techniques, affect how much damage an earthquake will cause. Scientists are trying to predict earthquakes. – Scientists are trying to measure changes in Earth’s crust that might signal an earthquake. – The ability to predict an earthquake could save thousands of lives. – earthquakes-the-science-of-earthquake-prediction- video.htm earthquakes-the-science-of-earthquake-prediction- video.htm

San Andreas Fault The most studied fault system in the world. fault-lines-and-earthquakes-video.htm

San Francisco in flames after the 1906 earthquake.

Protecting Structures from Damage structures-from-damage-video.htm structures-from-damage-video.htm

Homework Earthquakes East of the Rockies Read and answer the questions on the back. Don’t forget about the vocab quiz Thursday!!