Earthquakes Earthquakes What is an earthquake Earthquakes are

Earthquakes

Earthquakes What is an earthquake? • Earthquakes are ground movements that occur when blocks of rock in Earth move suddenly and release energy. • As plates of the lithosphere move, the stress on rocks at or near the edges of the plates increases. This stress causes cracks or faults to form. • When rocks along a fault break and move, energy is released as seismic waves that cause the ground to move. • Most earthquakes occur near a tectonic plate boundary (where two or more tectonic plates meet).

Earthquakes What causes earthquakes? • Stress on the rock, usually when rock rubs up against other rock, causes deformation, which is the process by which the rock becomes deformed and changes shape. • As the stress on rock increases, the energy stored in it increases. • When the stress is released, the rock may return to its original shape. • The return of rock to its original shape after elastic deformation is called elastic rebound. Elastic rebound causes the ground to shake.

Earthquakes What is an earthquake? • The focus is a place within Earth along a fault where the first motion of an earthquake occurs. • The place on Earth’s surface directly above the focus is called the epicenter. • Seismic waves flow outward from the focus in all directions.

Earthquakes What happens during an earthquake? • When rocks along a fault slip, the energy released travels away from the focus and through Earth in all directions as seismic waves. • Seismic waves are vibrations that cause different types of ground motion. • The strength of an earthquake is based on the energy that is released as rocks break and return to an undeformed shape.

Earthquakes What are two kinds of seismic waves? • There are two kinds of seismic waves: body waves and surface waves. Each kind travels through Earth in different ways and at different speeds. • Seismic waves carry energy, and their speed depends on the material through which they travel. • Body waves are seismic waves that travel through Earth’s interior. They are further divided into P waves and S waves.

Earthquakes What are two kinds of seismic waves? • P waves, also called pressure waves or primary waves, travel faster than S waves and surface waves. • P waves can travel through solids, liquids, and gases. They cause rock to move back and forth in the direction the wave is traveling. • S waves, also called shear waves or secondary waves, move rock from side to side. They can travel through solids, but not liquids or gases. • The difference between the arrival times of P waves and S waves is called lag time. The lag time with be shorter the closer the epicenter is to the seismic station.

P or primary waves S or secondary waves

Calculating Lag Time

Earthquakes What are two kinds of seismic waves? • Seismic waves that travel along Earth’s surface are called surface waves. • Body waves travel more rapidly than surface waves do, but surface waves, being focused on Earth’s surface, cause more damage. • Surface waves produce two types of ground motion: up-and-down and back-and-forth in a similar fashion to S waves, but do not affect much of the rock below like S waves do.

Earthquakes What are two kinds of seismic waves? • There are two types of surface waves, Love waves and Rayleigh waves. • Love waves, named after Augustus Edward Hough Love, move the ground from side to side at right angles to the direction of the wave. The horizontal shaking of Love waves is particularly damaging to the foundations of structures. • Rayleigh waves, named after Lord Rayleigh, move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.

The different types of surface waves.

Earthquakes How are seismic waves measured? • Scientists use instruments called seismometers to record seismic waves. When seismic waves reach a seismometer, it produces a seismogram. • A seismogram is a tracing of earthquake motion. It also records the arrival times of seismic waves at a seismometer station. • Seismograms are plotted on a graph, which is then used to pinpoint the earthquake’s epicenter.

Types of Seismometers Seismograms

Earthquakes How is an earthquake’s epicenter located? • In the S-P time method, scientists use the lag time between P and S waves to determine how far the waves have traveled from the epicenter. • In a process called triangulation, the epicenter can be located by drawing circles around at least three seismometer stations on a map. • The radius of each circle equals the distance from that station to the earthquake’s epicenter. The point of intersection of the circles is the epicenter.

= Epicenter

Earthquakes How is earthquake magnitude measured? • The height of the waves on a seismogram indicates the amount of ground motion. • Ground motion can be used to calculate magnitude, the measure of energy released by an earthquake. • The larger the magnitude, the stronger the earthquake.

Earthquakes How is earthquake magnitude measured? • The Richter Scale measures the ground motion from an earthquake to find the earthquake’s strength. • An increase in the magnitude by one unit corresponds to a ten-fold increase in ground motion.

Earthquakes How is earthquake intensity measured? • The effects of an earthquake and how the earthquake is felt by people are known as the earthquake’s intensity. • Magnitude measures how much energy is released by an earthquake. Intensity measures the effects of an earthquake at Earth’s surface. • The Modified Mercalli scale is used to describe an earthquake’s intensity. Intensity values are usually highest near the epicenter.

Earthquakes What factors determine the effects of an earthquake? • Four factors determine the effects of an earthquake: magnitude, local geology, distance from the epicenter, and type of construction used. • Magnitude: An earthquake’s magnitude is directly related to its strength. Stronger earthquakes cause more damage than weaker earthquakes. • Geology: Loose sediment and/or rock and sediments containing water will cause settling of particles damaging structures. • Distance: The closer the structure to the epicenter, the more damage that can be caused. • Construction: Wood structures can be damaged by earthquakes more easily than brick or cement.

Earthquakes What factors determine the effects of an earthquake? • Soil liquefaction occurs when a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change.

Earthquakes Where do earthquakes happen? • At divergent boundaries, tension stress causes normal faults to form. Earthquakes tend to be shallow because the crust is thin.

Earthquakes Where do earthquakes happen? • At convergent boundaries, rock is squeezed, and the stress is called compression. Reverse faults are formed, and earthquakes can be strong and deep.

Earthquakes Where do earthquakes happen? • At transform boundaries, shear stress pushes tectonic plates in opposite directions. Earthquakes tend to be relatively shallow.

Earthquakes What are some effects of earthquakes? • Most earthquakes do not cause damage, but some strong earthquakes can cause major damage and loss of life, especially in areas closest to the epicenter. • When the shaking of an earthquake is more than structures can withstand, major destruction can occur. • Much of the injury and loss of life after an earthquake is caused by structures that collapse.

Earthquakes What are some effects of earthquakes? • An earthquake under the ocean cause a vertical movement of the sea floor, displacing an enormous amount of water and generating a tsunami. • A tsunami is a series of extremely long waves that can travel across the ocean at speeds of up to 800 km/h (~500 mph). • As the waves reach the shoreline, the height of the waves increases. The huge waves can cause major destruction.