If you throw a pebble into a pond, you see waves moving outward in all directions. When rocks in the earth break, something similar happens; waves travel through the earth in all directions, causing the ground to shake. During a severe earthquake, the ground can rise and fall in a way that is similar to the movement of waves in the ocean. The motion of the ground causes trees and buildings to sway and fall.

Most earthquakes are caused by faulting, which occurs when sections of the earth’s crust are pushed or pulled in different directions. During faulting, rocks break and slide past one another, releasing energy. The movement in an earthquake is caused both by the movement and the vibration of rocks. As the rocks move or vibrate, they cause nearby rocks to move or vibrate. This chain reaction continues until all the energy is expended, or used up.

A break in the earth’s surface where faulting occurs is called a fault. The San Andreas Fault runs north and south from the Gulf of California to San Francisco. The land west of this fault is slowly moving north, and the land east of it is slowly moving south. However, the rocks along the fault do not all move simultaneously, nor do they all necessarily move at the same pace. During a severe earthquake in San Francisco in 1906, rocks on both sides of the San Andreas Fault moved.

Faults can be close to the earth’s surface or up to 74 kilometers deep. The point beneath the surface where rocks break and move is called the focus of an earthquake. The point on the earth’s surface directly above the focus is called the epicentre. The most violent movement during an earthquake is at its epicentre.

Rock movement and vibration can be measured by an instrument called a seismograph. The instrument has a recording sheet on a rotating drum. Above the drum, a pen is attached to a heavy object. When all is quiet, the pen draws a straight line on the recording sheet. When the earth moves, the base vibrates, but the weighted pen does not. As a result, the pen leaves a wavy-line on the drum. The degree of side-to-side movement of the wavy lien depends on the strength of the earthquake.

The Richter scale, which was designed in 1935 by Charles F. Richter, an American scientist, is used to measure how much energy an earthquake releases. This scale numbers earthquakes from 1 to 10, based on how violent they are, and each number indicates an earthquake that is ten times stronger than the preceding number, or the number that comes before. For example, an earthquake that registers 2 on the scale is ten times stronger than an earthquake that registers 1. Any quake registering more than 6.0 on the Richter scale is termed destructive. The Kobe, Japan, earthquake registered 7.2. The Northridge, California, quake registered 6.8.

When an earthquake occurs on the ocean floor, an area of the ocean floor may rise. If for example, an area the size of Indiana or Ohio rises about 2 meters, it sends vibrations through the water, and the waves caused by the vibrations form a tsunami. Tsunamis move very fast—1,000 to 1,300 kilometers per hour! Their waves have a long wavelength; a wavelength is the distance from the crest of one wave to the crest of another. As long as the waves are over the deep ocean, however, they are not very high.

As tsunamis rush toward land, the shallow water slows them down. This slowdown, in turn, causes the water to pile up, forming a towering wave that may be over 30 meters high. The power behind the wave sends it crashing against the shore and makes the water flood the land.

Scientists can quickly determine the epicentre of a quake that has occurred, but they still don’t understand earthquakes well enough to predict them. However, every earthquake that occurs provides scientists with data that bring them closer to understanding these natural disasters. From these data, scientists hope to find a pattern of natural events that are related to earthquakes and that take place before the actual quake. If scientists can identify such a pattern, perhaps they can predict earthquakes. Certain areas are more susceptible to, or easily affected by, earthquake activity, and scientists are studying these areas carefully.

In their studies, scientists use various instruments, one of which is called a tiltmeter. This device records any changes in the land’s slope that could indicate the movement of rocks sensitive gravity meters measure tiny changes in gravity that indicate an increase or decrease in the elevation of an area. Electronic devices are used to detect increased stress on underground rock that may cause the rock to break or shift. Lasers are used to detect slight shifts in the earth’s crust.

In addition to these scientific methods, scientists are also examining folklore about earthquakes. Animals have been reported as behaving strangely hours before a quake, and strange lights and loud sounds have supposedly occurred before an earthquake. Scientists are trying to find out how reliable theses signs are.

Since tsunamis occur after earthquakes, they are less difficult to predict. Earthquakes occurring on the ocean floor usually produce tsunamis, but strong quakes on land can also produce tsunamis if they disturb the ocean floor.

The most important aim in the study of tsunamis is a warning system to let people in coastal areas know when these waves are coming. Such a warning system operates in Hawaii; it sends information to the entire Pacific area. This station receives seismograph readings from many other stations. Equipped with a computer, the station determines the position of the earthquake’s epicentre and calculates, or figures out, the time that the tsunami is expected to arrive at land bordering the Pacific.

The study of earthquakes is a relatively young area of science. Yet scientists around the world are working hard to learn how to predict these disasters and reduce the loss of life.

An earthquake that registers 5.5 on the Richter scale is ten times weaker than a quake that registers ________.

An earthquake registering 8.0 would ________.

Which of the following statements is NOT true?

In which of the following areas is an earthquake most likely to result in a tsunami?

Why can’t scientists predict earthquakes?