EarthquakesChapter 6

6.1- Forces in Earth’s CrustStress- A force that acts on rock

to change its shape or volume, works over millions of years

Types of Stress:1. Tension- pulls on crust, stretching

rock2. Compression- Squeeze rock until

folds or breaks3. Shearing- pushes a mass of rock in

2 opposite directions

Kinds of FaultsFault- a break in the rock of the crust

where rock surfaces slip past each other◦Most faults occur along plate boundaries

where the forces of plate motion push or pull the crust so much that the crust breaks

Three main types:1. Normal Faults- the fault is at an angle, so

one block lies above (hanging wall) and one block lies below (footwall)

Caused by tension in Earth’s crust that pulls rocks apart

Kinds of Faults2. Reverse Fault- Where rock of the

crust is pushed together, caused by compression

Same structure as a normal fault but blocks move in opposite directions

3. Strike-Slip Fault- Where plates move past each other, caused by shearing

Rocks on either side of the fault slip past each other sideways, with little up and down motion

If it forms the boundary between 2 plates, then called a transform boundary

Changing Earth’s SurfaceOver millions of years, the forces of

plate movement can change a flat plain into landforms such as anticlines, and synclines, folded mountains, fault-block mountains, and plateaus.

Folds- Bends in rock formed when compression shortens and thickens part of Earth’s crust◦Anticline- bends upward into an arch◦Syncline- bends downward to form a valley◦Ex. Appalachian Mtns., Himalayas, Alps

Changing Earth’s SurfaceFault-Block Mountain- When 2

normal faults cut through a block of rock; when 2 normal faults form parallel to each other, a block of rock is left lying between them, the block then moves upward◦Ex. Great Basin

Plateau- a large area of flat land elevated high above sea level◦Form when forces in Earth’s crust push

up a large, flat block of rock◦Ex. Colorado Plateau

6.2- Earthquakes and Seismic WavesEarthquake- the shaking and

trembling that results from the movement of rock beneath Earth’s surface◦Caused by the forces of plate

movement1. Produce stress in crust, adding

energy2. Stress increases along a fault until

rock breaks3. Earthquake begins

Most Earthquakes begin in lithosphere within 100 km of Earth’s surface

Focus- area beneath Earth’s surface where rock is under stress

Epicenter- the point on the surface directly above the focus

Types of Seismic WavesWaves carry energy as they

travel outwardSeismic waves- vibrations that

travel through Earth carrying the energy released during an Earthquake through the interior and across the surface

P-waves- (primary waves) seismic waves that compress and expand the ground

Types of Seismic WavesS-waves- (secondary waves)

seismic waves that vibrate from side-to-side and up and down; they shake the ground back and forth◦Cannot move through liquids

Surface Waves- move more slowly than P-waves and S-waves, but can produce sever ground movements

Measuring EarthquakesThere are at least 20 different

measures for rating earthquakesMercalli Scale- rate earthquakes

according to the level of damage at a given place

Magnitude- a number that geologists assign to an earthquake

Richter Scale- a rating of an earthquakes magnitude based on the size of the earthquake’s seismic waves

Seismograph- an instrument that records and measures seismic waves

Moment magnitude scale- a rating system that estimates the total energy released by an earthquake◦Used to rate all sizes, near or far◦Geologists study data from seismographs

Each one-point increase in magnitude releases roughly 32 times more energy

Below 3- barely notice Below 5- small, little damage Above 6- great damage 8 or above- rare

Mercalli Scale

Locating the EpicenterGeologists use seismic waves to

locate an earthquake’s epicenter◦Scientists measure the difference

between arrival times of the P waves and S waves

◦Geologists then draw at least 3 circles, the center of each circle is a particular seismograph’s location

◦The point where the three circles intersect is the location of the epicenter

6.3- Monitoring EarthquakesSeismic waves cause the seismograph’s

drum to vibrate. But the suspended weight with the pen attached moves very little. Therefore, the pen stays in place and records the drum’s vibrations◦A seismograph’s heavy weight resists motion

during a quake, but the rest of the instrument vibrates when seismic waves arrive

Seismogram- the record of an earthquake’s seismic waves◦Height of the lines is greater for more severe

EQ

Instruments That Monitor Faults

In trying to predict earthquakes, geologists have developed instruments to measure changes in elevation, tilting of the land surface, and ground movements along faults.◦Tiltmeter- measures tilting or raising of the ground◦Creep Meter- uses wire stretched across a fault to

measure horizontal movement of the ground◦Laser-Ranging Devices- uses a laser beam to

detect horizontal fault movements◦GPS satellites- Earth-orbiting satellites that

monitor changes in elevation and horizontal movement

GPS Satellites

Using Seismographic DataSeismographs and fault-

monitoring devices provide data to:1. Map faults2. Detect changes along faults3. Predict Earthquakes

6.4- Earthquake SafetyGeologists can determine

earthquake risk by locating where faults are active and where past earthquakes have occurred◦In the US, the risk is highest along

the Pacific Coast◦The eastern U.S. has a low risk

because they are far from plate boundaries, but has experienced some of most powerful quakes. Maybe because the continental plate below N. America is under stress

How Earthquakes Cause DamageShaking- can trigger landslides or

avalanches, damage or destroy structures

Liquefaction- occurs when an earthquake’s violent shaking suddenly turns loose, soft soil into liquid mud

Aftershocks- an earthquake that occurs after a larger earthquake in the same area

Tsunamis- the water displaced by the earthquake can form a large wave

Steps to Earthquake Safety

To reduce earthquake damage, new buildings must be made stronger and more flexible. Older buildings may be modified to withstand stronger quakes.

Designing Safer Buildings

The main danger is from falling objects and flying glass

Drop, Cover, and Hold