Ch. 19 - Earthquakes Earthquakes are caused gigantic fractures in the Earth’s crust, which produce ground vibrations. The pressure (force/area) acting on the rocks involved forms fractures when this stress exceeds the strength of the rocks. The deformation of materials in response to stress is called strain. 3 Types  compression, tension, and shear. 1. Compression occurs with a decrease in volume of the material. 2. Tension occurs when the material pulls apart. 3. Shear occurs when the material twists. Fig. 19-1 (pg. 496)

Fault – movement of the Earth’s crust causing a fracture or system of fractures. 3 Types of Faults 1. Reverse fault – occurs when land compresses together horizontally 2. Normal fault – occurs when land separates from one another horizontally. 3. Strike-slip fault – occurs when land slides left and right of one another horizontally. Earthquakes are caused by movements along faults. As stress continues to build in these rocks, they reach their elastic limit, break, and produce an earthquake.

Types of Seismic Waves Seismic waves are vibrations of the ground during an earthquake. 1. Primary waves (P waves) – rocks are squeezed and pulled in the same direction in which the waves are traveling. 2. Secondary waves (S waves) – rocks move at right angles in relation to the direction of the waves. 3. Surface waves move in two directions as they pass through rock. Similar to an ocean wave.

The point were an earthquake originates is the focus. The focus is usually several kilometers below the Earth’s surface. The epicenter is the point directly above the focus at the Earth’s surface. What is the study of earthquake waves? Seismology Seismic waves provide us with information to understand the Earth’s interior. What are seismometers or seismographs? Instruments used to detect and record vibrations sent out from earthquakes. They can shake the entire Earth.

The seismogram is the paper portion of the seismograph that records the vibrations Fig. 19-8 A travel-time graph displays the distance and time it takes for p-waves and s-waves to travel. Fig. 19-9 P & S-waves travel through the crust and mantle. Only the P-wave travels through the outer and inner core of the Earth’s interior. Fig. 19-10 The disappearance of S-waves allows scientists to theorize that the Earth’s outer core must be a liquid.

Studies of seismic waves shows that the lithosphere (crust & upper mantle) is primarily igneous rocks granite, basalt, (crust) and peridotite (liquid – mantle). The Earth’s core and mantle are made up similar components to that of a meteorite – iron and nickel. Scientists study meteorites to indirectly study the Earth’s core and mantle.

Measuring & Locating Earthquakes Approximately 1 million earthquakes occur annually. Little if any cause damage and are not felt. Magnitude is the amount of energy released during an earthquake. The magnitude is rated on a scale called the Richter scale based on the size of the largest seismic wave generated by the quake. 1-10 A magnitude of 8 is ten times larger than 7 and 100 times larger than 6.

Moment magnitude scale is rated on the size of the fault rupture, the amount of movement along the fault, and the rocks’ stiffness Most seismologist use this scale. The values are estimated from several seismic waves rather than the largest generated. Modified Mercalli scale rates the earthquake based on the amount of damage done to the structures involved. It uses Roman numerals I to XII. Table 19-1 – pg. 507 MiniLab – page 508

Locating an earthquake Locations of several seismic stations are used by plotting the radius or epicentral, which is the distance away from the station which detects the greatest seismic intensity with calculations. The time it takes for the seismic waves to get to the station can be used to calculate this also. Fig 19-14 Seismic belts are the global distribution of these epicenters. Most of the seismic belts are narrow regions. Fig. 19-15 (pg. 510) Are there any trend or patterns on the map?

Earthquakes & Society Most of the damage produced by an earthquake is to buildings and deaths may occur because of it. Brick, stone, and concrete buildings damage easily compared to wooden buildings. Some buildings even rest on large rubber structures that absorb most of the vibrations. Some major earthquakes include the following: Mexico City (1985) = destroyed lots of buildings, Turkey (1999), Peru (1970) – 7.8 magnitude = due to landslides from the quake, Taiwan (1999) - 7.6 magnitude = vertical offsets up to 10 m height.

Tsunami – large ocean waves generated by vertical motions of the seafloor during an earthquake. The Indian Ocean tsunami of December 26, 2004 with a 9.0 magnitude. The death toll exceeded 225,000. Most of the earthquakes that are predicted are based on probability from the history of earthquakes in an area and the rate at which strain builds up in the rocks. Seismic gaps are active faults that haven’t experience significant earthquakes for a long period of time. Lasers  Fig. 19-20