Back to School at FamilyEducation.com

EARTHQUAKES

Earthquakes are caused by movements of the giant tectonic plates that form Earth’s crust. SEISMOLOGY is the study of earthquakes. Most occur at cracks called FAULTS, at the boundaries where the plates meet. Every minute, the ground shakes somewhere in the world, but these vibrations are usually minor tremors that are barely noticed. When a major earthquake strikes, the ground shakes violently, and buildings and bridges topple.

SEISMIC WAVES

As the plates slowly shift, rocks are put under pressure. They stick, then stretch and, as the strain gets too great, they shatter and jolt into new positions. Seismic (shock) waves radiate from the earthquake’s focus, underground. The epicentre, above the focus, suffers the worst damage.

FAULT

Faults are deep cracks in rocks, mostly caused by movement at plate margins. Deep earthquakes strike in subduction zones where two plates collide and one slides below the other. Shallow earthquakes occur mostly where two plates grind past one another. The rocks may be shifted only a few centimetres, but over millions of years, this can add up to hundreds of kilometres of movement sideways, and up to 30 km (19 miles) of vertical movement.

NORMAL DIP-SLIP FAULT

The rocks along a fault may move up or down, sideways or diagonally, depending on the angle of the fault plane. The angle of the fault plane to the horizontal is known as the dip. In a normal fault, also known as a dip-slip fault, the rocks shift straight down or up, following the line of dip.

REVERSE FAULT

The distance that the rocks slip up or down during a quake or tremor is called the throw. In a reverse fault, pressure causes one block of rock to overhang another. As the rocks shift, the block is forced farther up and over the other. A reverse fault with a fault plane of 45° or less is called a thrust fault.

OBLIQUE-SLIP FAULT

In a strike-slip fault, rocks scrape sideways past one another. The amount of sideways slip is called the heave. The San Andreas Fault, which runs along the west coast of North America, is a famous example. The rocks in an oblique-slip fault slide past each other, and also up and down in a diagonal movement.

SEISMOLOGY

Seismologists study earthquakes. They also examine the behaviour of seismic waves passing through the Earth to find out about its structure. Instruments called seismographs measure the intensity of seismic waves. The magnitude (size) of earthquakes can be rated by measuring either these waves, on the Richter scale, or the damage caused – the Mercalli scale. Earthquakes cannot be prevented, but they can sometimes be accurately predicted.

Copyright © 2007 Dorling Kindersley

stay connected

Sign up for our free email newsletters and receive the latest advice and information on all things parenting.

Enter your email address to sign up or manage your account.

Facebook icon Twitter icon Follow Us on Pinterest

editor’s picks

highlights

Join BIC on our mission to save handwriting and Fight For Your Write! Writing helps kids become better readers, boosts their confidence and sparks their creativity. Visit BICFightForYourWrite.com to sign our petition to save handwriting!

11 Coolest Lunch Boxes for Kids
Send your child's lunch to school in style! Check out our picks for the 11 best lunch boxes with great features from BPA-free accessories to spill-resistant fabric.

7 Important Back-to-School Safety Tips
Follow these back-to-school safety tips to make sure your child stays safe on the way to school, in the classroom, and while on the playground.

Kindergarten Readiness App Wins Gold
Our Kindergarten Readiness app won the Gold Award of Excellence in the educational category at the 2014 Communicator Awards. This valuable checklist comes with games and activities to help your child practice the essential skills she needs for kindergarten. Download the Kindergarten Readiness app today!