chapter 8 earthquakes 1994- northridge, ca san francisco-1989
TRANSCRIPT
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Chapter 8 Earthquakes
1994- Northridge, CA San Francisco-1989
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Section 8-1
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
What causes Earthquakes?
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Elastic rebound
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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.
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Earthquakes in Last 30 Days Worldwide
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Earthquakes in the last week.
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Types of Faults
Normal Fault-tension pulls rocks apart
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Fault Video
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Reverse Fault-compression pushes rock, one piece is forced up and over another.
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Strike-slip Fault-friction from shearing force builds up and rocks move when elastic limit is reached.
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Seismic waves
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Focus/Epicenter
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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
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Seismograph Seismogram
An analysis of a seismogram tells seismologists 2 things:
•How powerful the Earthquake is.
•How far away the epicenter of the EQ is.
Earthquakes are detected by a device called a seismograph
Seismogram- paper record of EQ shaking.
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Seismogram
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Finding Epicenter
• Use P-wave arrival times• Use S-P lag times
3 Times are needed to pinpoint epicenter of EQ
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8-2 EQ Measurement
How do seismologists calculate the magnitude of an EQ?
Richter Scale- measures EQ’s by describing how much energy is released by the EQ.
Mercalli Scale- old method of measuring EQ’s by damage caused. Not accurate due to building strengths in different areas.
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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!
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Predicting EQ’sVery 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
SMALL MOVEMENT ALONG A FAULT CAN LEAD TO LARGE EARTHQUAKES.
3. Seismic gap- looks for areas that are building up stress along fault.
4. Satellite observations- use of GPS to monitor movement.
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Structures are strongest when they are built on BEDROCK.
Soft Earth leads to LIQUEFACTION.
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Base Isolators and Cross-bracingbest ways to strengthen buildings
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EQ Safety
EQ Preparation:
What to do during and after EQ: