Post on 26-Dec-2015
Chapter 8 Earthquakes
1994- Northridge, CA San Francisco-1989
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?
Elastic rebound
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.
Earthquakes in Last 30 Days Worldwide
Earthquakes in the last week.
Types of Faults
Normal Fault-tension pulls rocks apart
Fault Video
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.
Seismic waves
Focus/Epicenter
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
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.
Seismogram
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?
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.
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’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.
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: