Chapter 8 Earthquakes
This section explores the fundamental causes of earthquakes, emphasizing the movement and stress of tectonic plates. It details the types of faults—normal, reverse, and strike-slip—and describes various seismic waves (P-waves, S-waves, and L-waves) and their characteristics. The measurement of earthquake magnitude using the Mercalli and Richter scales is explained, alongside challenges in predicting earthquakes. Finally, it highlights the importance of building safety measures and actions to take during and after an earthquake.
Chapter 8 Earthquakes
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Presentation Transcript
Chapter 8 Earthquakes San Francisco-1989 1994- Northridge, CA
Section 8-1 What causes Earthquakes? Stress of plates moving causes rock to bend and stretch. Rocks reach their elastic limit and break, causing a sudden movement along a fault. During elastic rebound, energy is released. The energy released by the breaking of rock along a fault causes an earthquake
Where do Earthquakes occur? • Most occur along plate boundaries. • Along fault systems, some which can be far away from plate boundaries. • Around actively erupting volcanoes.
Types of Faults Normal Fault-tension pulls rocks apart
Reverse Fault-compression pushes rock, one piece is forced up and over another.
Strike-slip Fault-friction from shearing force builds up and rocks move when elastic limit is reached.
Types of Seismic Waves Primary (P-waves)- fastest of waves, travel at 6 km per second in an accordion-like motion. Weakest wave Secondary (S-waves)- travel half the speed of P-waves (3 km per second), move in a side to side motion. Stronger than P waves Surface (L-waves)- slowest of waves, move in a wave-like motion, strongest, cause the most destruction in an EQ
Earthquakes are detected by a device called a seismograph Seismograph Seismogram Seismogram- paper record of EQ shaking. • An analysis of a seismogram tells seismologists 2 things: • How powerful the Earthquake is. • How far away the epicenter of the EQ is.
Finding Epicenter • Use P-wave arrival times • Use S-P lag times 3 Times are needed to pinpoint epicenter of EQ
8-2 EQ Measurement How do seismologists calculate the magnitude of an EQ? Mercalli Scale- old method of measuring EQ’s by damage caused. Not accurate due to building strengths in different areas. Richter Scale- measures EQ’s by describing how much energy is released by the EQ.
How to find Richter Value Scale is from 0.1 to 9+ For every increase of 1.0 on Richter scale, 100 times more energy is released. 6.0 quake is 10,000 times stronger than a 4.0 Amplitude and Distance to find the Richter Value Get info from the seismogram!
Predicting EQ’s Very difficult to predict actual day, more on time span. 1. Creep meter- measures movement by tying a wire across fault to measure movement 2. Lasers- line up laser and mirror on opposite sides of fault 3. Seismic gap- looks for areas that are building up stress along fault. 4. Satellite observations- use of GPS to monitor movement. SMALL MOVEMENT ALONG A FAULT CAN LEAD TO LARGE EARTHQUAKES.
Structures are strongest when they are built on BEDROCK. Soft Earth leads to LIQUEFACTION.
Base Isolators and Cross-bracingbest ways to strengthen buildings
EQ Safety EQ Preparation: What to do during and after EQ:
