Highlight the three text aids that are not already colored. EARTHQUAKES

Introduction

Earthquakes are inevitable in certain areas of the world and possible in most areas. They are among the most unsettling and deadly events possible in human experience. This chapter covers the causes and types of earthquakes, how earthquakes are measured, and how forecasting of earthquakes is done.

Inside the Earth

To understand what causes earthquakes, it is useful to first understand the structure of the earth itself. The study of earthquakes has revealed much to scientists about the earth's interior.

The earth is comprised of three main layers: the crust, mantle, and core. The crust is comparatively thin at between three and 35 miles thick. The layer under the crust is the mantle and it is much deeper, about 1,800 miles thick. The center of the earth is the core, the diameter of which measures approximately 4,300 miles. Through the study of the energy released by earthquakes, geologists have discovered that at least part of the core is a molten liquid.

Plate Tectonics

Earthquakes occur when plates shift. A plate is a large, inflexible piece of the crust that is movable. This movement may be sideways, up and down, or variations of these directions. There are six major plates.

A plate boundary is where most earthquakes occur because faults, weak areas in the crust, appear at plate boundaries. Stress is caused by plates meeting head-on or sliding around each other. Energy waves are released and the earth shakes and rolls as a result.

Sometimes, but rarely, an earthquake can strike inside a plate. Usually earthquakes occur along fault lines at plate boundaries.

Waves and Energy