Earth Science Ch. 8 Earthquakes

An earthquake is the vibrations of earth produced by rapid release of energy in the lithosphere. Earthquakes are caused by movements along faults

Parts of an earthquake Focus: point within the earth where an earthquake starts below the surface Epicenter: location on the surface directly above the focus. Seismic waves: energy produced by an earthquake that travels in all directions from the focus.

Elastic Rebound Hypothesis Probable cause of earthquakes Earthquakes are produced by release of energy stored in rock that has been subjected to great pressure. When the strength of the rock is exceeded it suddenly breaks releasing energy as seismic waves. The rock breaks at its weakest point. This is called the focus. After an earthquake, the rocks along the fault spring back to their original position. This is called elastic rebound.

An aftershock is an earthquake that occurs after a major earthquake. Foreshocks are small earthquakes that occur before a major earthquake.

Measuring Earthquakes Earthquakes produce 2 main types of seismic waves: surface waves and body waves Body waves P waves primary waves/compression waves travel in the same direction the earthquake travels can travel through solids, liquids, or gases First waves to arrive at the surface

S waves secondary / transverse waves particles move at right angles to the direction of the earthquake Can only travel through solids Second waves to arrive at the surface

Surface waves travel slower than body waves Most destructive Last to arrive at the surface 2 types: Love: side to side movement Rayleigh: rolling waves

Body Waves P and S waves Surface Waves Love and Rayleigh

Seismograph: instrument that records seismic waves Seismogram: a time record of ground motion during an earthquake All 3 types of seismic waves are shown

Intensity and Magnitude Intensity is the measure of damage an earthquake causes The Mercalli Scale is used to measure intensity.

Magnitude is the measure of the amount of energy released by an earthquake Richter Scale Each number on the scale represents a 10 fold increase in the wave height Only useful for small shallow earthquakes Moment Magnitude Measures the amount of movement along a fault. The most widely used only scale that estimates energy released by any earthquake.

Locating an Earthquake Calculate the difference in arrival times between P and S waves. The greater the time interval the greater the distance to the epicenter. A time travel graph is used to find the distance to the epicenter. Data is collected from 3 seismograph stations to find the epicenter.

Tsunamis Series of waves formed by an earthquake on the ocean floor or by underwater landslides. The height of tsunami waves increases as the water becomes more shallow. A tsunami warning system is used to alert people that live in coastal regions around the Pacific Ocean (Ring of Fire).

Earthquake Safety Drop, cover, and hold If indoors crouch beneath a sturdy table or desk Crouch against an inner wall or stand in a door frame. Avoid furniture that might fall If outside move to an open area and sit down Avoid vehicles, power lines, trees, and buildings

Earth’s Layered Structure Most of the knowledge about the earth’s interior comes from the study of seismic waves. As the speed of seismic waves changes, the waves refract or bend Scientists study the speed and paths of seismic waves to determine the chemical composition of rocks inside the earth

3 Major Layers of Earth Crust Thin rocky outer layer of the earth There are 2 types of crust: oceanic and continental Oceanic crust is composed mostly of the igneous rock basalt The most common rock in continental crust is granite Rocks in oceanic crust are more dense and younger than those in continents The continental crust is thicker

The 8 most abundant elements in the earth’s crust: Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium

Mantle Largest layer of earth Solid The boundary between the crust and mantle represents a change in the physical properties and a change in seismic waves Has the same chemical composition as the crust.

Core Composed mostly of iron and nickel Most dense area due to extreme pressure

Layers Defined by Physical Properties Temperature, pressure, and density all increase with depth The temperature and pressure determine the physical properties of the rock Solid, putty, liquid

Lithosphere Earth’s outer most layer Consists of the crust and the upper mantle Cool rigid shell

Asthenosphere Lower Mantle Softer weaker rock Rocks are close enough to the melting point that they are easily deformed Found entirely in the mantle Rocks in this layer are solid but able to flow like putty Lower Mantle More rigid but still very hot and capable of gradual flow

Outer Core Inner Core Liquid layer Flow of liquid iron generates earth’s magnetic field Inner Core Extremely high temperatures Exists in a solid state due to immense pressure The heat of the core is caused by: Radioactive decay Friction Formation of the Earth

Boundaries Between Layers Mohorovicic Discontinuity (Moho) Boundary between the crust and mantle Speed of seismic waves decreases. Gutenberg Discontinuity Boundary between the mantle and core Speed of seismic waves decreases

P waves of earthquakes bend when they reach the liquid outer core P waves of earthquakes bend when they reach the liquid outer core. This causes a region called a shadow zone. P waves bend when the material changes. S waves cannot travel through the outer core which shows that this region is liquid.

There are 3 Types of Earthquakes that occur at convergent plate boundaries Shallow focus 0 – 70 km deep Most destructive Intermediate focus 70 – 300 km deep Deep Focus Below 300 km