Chapter 19 Earthquakes What are Earthquakes Natural vibrations

Chapter 19: Earthquakes

What are Earthquakes? • Natural vibrations of the ground caused by movement in fractures in Earth’s crust or sometimes volcanic • Irregular surfaces in eruptions rocks snag and lock • Most occur because of causing stress to build movements along faults in the rocks • When rocks meet their elastic limit they break causing an Earthquake

What is Stress and Strain • Fractures form when stress exceeds the strength of the rock involved • Strain occurs in response to stress and causes the material to deform Three types of stress • Compression – squeezes • Tension – pulls apart • Shear – Causes twisting

What Is a Fault • A fracture or system of fractures along which movement occurs • 3 types of faults – Reverse fault – Normal fault – Strike slip fault

What are seismic waves? Vibrations of the ground during an earthquake P and S waves are called body waves because they pass through Earths interior 3 types of seismic waves – Primary (P) – squeeze and pull rocks along the same direction the wave travels – Secondary (S) – cause rocks to move at right angels to the direction of the waves – Surface - cause up-anddown and side –to-side as they pass through rocks

What is the focus and epicenter of an earthquake • The focus of an earthquake is the point of failure of rocks at the depth where an earthquake originates • The epicenter of an earthquake is the point on earth directly above the focus

Seismology is the study of earthquake waves • Seismometers or seismographs are sensitive instruments that detect and record the vibrations of an earthquake • A seismogram is the record produced by the seismometer • The different speeds of the P and S waves are used to determine the epicenter of an earthquake

Clues to Earths Interior • Seismic waves are reflected and refracted as they strike different materials – P waves travel through the mantel fairly straight – P waves are refracted or bent at Earths core – S waves do not enter Earths core because they do not travel through liquid

Clues to Earths Interior • Analysis of these waves has enables scientist to determine the structure and composition of Earths interior – Disappearance of S waves leads scientist to believe Earth’s outer core must be liquid – Refraction of P waves indicates the inner core is solid

Seismology and Earths Interior

Measuring and locating Earthquakes • Magnitude is a measure of the energy released during a earthquake and can be measured on the Richter Scale. • Intensity is a measure of damage caused by a quake and is measured by the Modified Mercalli Scale – Uses roman numerals – Higher the number the greater the damage

Measuring and locating Earthquakes • Data from at least 3 • Most Earthquakes occur seismic stations are in areas associated with needed to determine an plate boundaries called earthquakes epicenter seismic belts

Earthquakes and Society • Earthquakes can cause structural damage, landslides, soil liquefaction, fissures, fault scarps, uplift or subsidence and tsunamis • Tsunamis is a large destructive ocean wave caused by an underwater earthquake.

Tsunami 2004

Probability of an Earthquake • The probability that an area will have an earthquake is based on the history of a region and the rate of strain that builds on rocks