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.

PA eq’s

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: