Chapter 8: Earthquakes 8.1 What is an Earthquake? 8.2 Measuring Earthquakes 8.3 Destruction From Earthquakes 8.4 Earth’s Layered Structure Fault Types Earthquake Earthquake An earthquake is a vibration of Earth produced by the rapid release of energy. An earthquake is a vibration of Earth produced by the rapid release of energy. Earthquakes are often caused by slippage along a break in Earth’s crust called faults. Earthquakes are often caused by slippage along a break in Earth’s crust called faults. Seismograph Normal Fault Reverse Fault Reverse Fault - Pushes together Normal Fault - Pulls apart Strike Slip fault - slides side to side Faults Napa Quake 2014 Earthquake Risk in Sacramento Fault Types Normal Fault: extension, hanging wall goes down Reverse fault: compression, hanging wall goes up San Andreas Fault at Carrizo Plain Strike Slip Fault Focus and Epicenter Focus - the point inside the Earth where the earthquake starts. Epicenter - location on Earth’s surface directly over the focus. Cause of Earthquake - Elastic Rebound Theory Draw it! Most Earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces. On your whiteboard, draw the focus, epicenter, and a sentence explaining the Elastic Rebound Theory. Show me Rocks bend under applied pressure When the strength of the rock is exceeded, it suddenly breaks Energy released as the vibration of an earthquake. Aftershocks and Foreshocks Measuring Earthquakes Aftershocks - smaller earthquakes that occur after the main quake. Can destroy already weakened structures. Seismograph - instrument that records earthquake waves. Seismogram - written record on the seismograph Foreshocks - small earthquakes that occur before the main quake Earthquake Waves Surface Waves ( L and R Waves) move up and down as well as side to side. Cause the most destruction. Only travel on the surface of the Earth Body Waves Can travel through the Earth’s Interior. Two types Primary (P Waves) - pull and compress rock in the direction of the wave motion. Secondary (S Waves) - vibrate the Earth perpendicular to the direction of wave motion. Cannot travel through Gases and Liquids Your turn! On your whiteboard, list the 2 types of surface waves and 2 types of body waves and draw a diagram showing how each moves. Which wave travels fastest? Which can go through anything in the earth? Locating an Earthquake Locating an Earthquake Use the travel time between the P and S waves. Once the distance to the epicenter is known, trace a circle on a map with that radius. From this time, determine the distance from a travel Time graph as we constructed last time. You will need information from at least 3 stations to triangulate the location. Earthquake Zones Measuring Earthquakes Most earthquakes occur in narrow zones around the world. Richter Scale - outdated, logarithmic scale. An increase in 1 on the scale equals an earthquake 10 times more powerful. Circum-Pacific Belt Mediterranean Belt Ring of Fire Moment magnitude - most widely used measurement, measures actual energy released by Earthquake. Modified Mercalli Scale - Scale based on the damage done, uses Roman numerals from I to XII. Your turn! Begin Day 2 Destruction From Earthquakes Answer on your whiteboard: Napa Quake 2014 What are 3 different scales we use to measure earthquakes? Earthquake Risk in Sacramento The damage to buildings from earthquakes depends on several factors Volcano experiment Fault Types Tsunamis Normal Fault: extension, hanging wall goes down Reverse fault: compression, hanging wall goes up 1) Building Design Brick and unreinforced stone most dangerous Wood structures are better Reinforced steel is best Intensity and duration of the vibrations Nature of the material on which the structure is built Design of the structure 2) Liquefaction Soils that are saturated with water, turn to pudding during the shaking. Buildings sink Underground tanks and pipes can float to the surface 3) Tsunamis Triggered by an earthquake Occurs when a slab of ocean floor is displaced vertically along a fault or during an undersea landslide. Sends out a wave travelling up to 900 km/hr roughly 1 meter high As wave approaches shore, it slows down and gets higher. 4) Tsunami Warning System 5) Warning center based in Honolulu Hawaii Sends warning all around the Pacific during an earthquake if a Tsunami is detected. 6) Other Dangers Landslides, rockfalls, and ground subsidence Breaking of gas and water lines Fires Aftershocks Predicting Earthquakes 7) Earth Layers Defined by Composition No reliable way to predict Earthquakes Crust We can give a probability of an Earthquake occurring based upon history and current ground measurements. Mantle Core 8 Oceanic Crust -About 7 km thick made mostly of basaltic rock density about 3.0 g/cm3 about 180 million years old or less. 9 Continental Crust 10 8 to 75 km thick, average is about 40 km Made of granitic rock Density about 2.7 g/cm3 Some rocks over 4 billion years old 11 Mantle 82% of Earth’s volume Semi-solid rocky shell Extends to depth of 2890 km Made mostly of peridotite Density of 3.4 g/cm3 12 Core 13 Made of Iron-Nickel alloy Density about 13 g/cm3 Notice the layers get denser as they go down Layers Defined by Physical Properties 14 Lithosphere - about 100 km thick, solid, rocky Asthenosphere- rock is soft and near melting point to 660 km Lower Mantle- More rigid layer (660 to 2890 km) Outer Core- Liquid (2260 km thick), molten iron creates magnetic field Inner Core- 1220 km thick. Solid layer due to pressure 15 16 Day 2 Discovering Earth’s Layers 17 In 1909, Andrija Mohorovicic showed seismic waves increase in speed at about 50 km below surface. Called Moho discontinuity. Only P waves can travel through core P waves bent at boundary of core and mantle 18 19 Discovering Earth’s Composition 20 Seismic data and drilling showed crust made of Granitic rock Ocean drilling samples show Sea floor made of basalt Lava samples show mineral content deep into the mantle similar to rocky meteorites Core thought to be similar to metallic meteorites.