In this topic you will learn how scientists study the layers of Earth.

What lies below Earth's thin surface? Earth cannot be cut open to see the inside. Information can be obtained in several different ways. One way scientists learn about Earth is by studying earthquakes. Earthquakes are movements or vibrations in Earth. Scientists "listen to" and "feel" Earth's movements by using instruments called seismographs. A seismograph detects, measures, and records the energy of earthquake vibrations.

Pressure within Earth can cause rocks in its outer layer to break. If the rocks along a break move, the break is called a fault. The place where movement begins is called the focus. The focus may be 700 kilometers (about 450 miles) below Earth's surface.

When an earthquake begins, pressure causes rocks to break and move. Energy is released as vibrations called seismic waves. Seismic waves travel out from the focus in all directions. The waves move through Earth and are felt as shakings and vibrations. The farther waves travel from the focus, the weaker they become. Seismographs in different places record the strength of the waves. By comparing these records, scientists can track waves and determine their speed and direction of travel.

There are several different kinds of seismic waves. Scientists who study these earthquake waves have learned that the waves travel at different speeds through solids, liquids, and gases.

Primary waves, called P waves, are the fastest seismic waves. Secondary waves, called S waves, travel more slowly than P waves. Surface waves, or L waves, are the slowest of all waves. L waves cause the damage that often comes from an earthquake.

Scientists have used information from earthquakes to find out what Earth is like on the inside. The solid outer layer of Earth is the crust. This layer is deeper under continents than under oceans. The mantle is the layer of rock lying below the crust. Rocks in this region can move or flow slowly because of great pressures and high temperatures. The outer core is a liquid layer of Earth that is probably made of melted iron. It lies below the mantle. The inner core is a sphere of solid material at Earth's center.

The information that scientists get from instruments to learn about an earthquake can help them warn people of a possible earthquake.