Having lived in southern California, I am all too familiar earthquakes. They are frightening when they happen, but the more you know about them can really help. Earthquakes seem to come out of nowhere, and attempts to predict often seem to fail. The state of California in the United States probably has the most earthquakes in the world, maybe as many as 10,000 per year. Most earthquakes are very small and hardly noticeable but others cause massive destruction. One of the world’s strongest earthquakes took place in Alaska in 1964.
First of all, earthquakes do not just suddenly happen. They are a result of the natural movement of the earth, and happen when two or more tectonic plates move against each other. So far scientists know that the planet’s surface is covered with at least twenty tectonic plates which are constantly on the move. An earthquake occurs when one or more of these plates move against, go over or under another plate. The resulting force causes what is known as an earthquake tremor. A tremor acts more or less like a wave motion which moves through the earth.
When an earthquake has occurred, you will often hear scientists talk about the earthquake having a hypocenter and an epicenter. The hypocenter of an earthquake is located underneath the earth’s surface where the tremor begins. The term is not commonly used, but we will often hear a news broadcaster explaining the location of the epicenter. The epicenter is located immediately above the hypocenter on the surface of the planet.
You may also have heard the word magnitude mentioned. The magnitude is measured on the Richter scale, and the highest magnitude is 10, and is the measured value of the size of the earthquake. Small earthquakes around the value of magnitude 2 cannot be felt, and the majority of earthquakes which occur around the world are about a magnitude 4. A magnitude 4 earthquake may be more noticeable inside a building than outside. Objects on shelves or walls may shake during a magnitude 4 earthquake. The biggest earthquake in history measured a magnitute of 8.5.
Most of us believe we can hear an earthquake, but as a matter of fact we can’t. The human ear can detect sounds in the frequency of 20 to 20,000 Hertz. A large majority of earthquakes which occur have a frequency of less than 20 hertz which could perhaps explain why pets are more sensitive to earthquakes. Pet owners in areas of the world with frequent earthquakes often claim their pets can “sense” the quake. More than likely, the pet can hear the actual sound wave of the initial tremor. What we humans hear is the movement of buildings and shaking home contents.
Larger earthquakes are always followed by aftershocks. They will occur along the same fault line where the plates met, but may not occur in the exact same place. Some aftershocks may be stronger than the first quake, and scientists then designate the stronger aftershock as the earthquake.
Earthquakes may be unpleasant and even dangerous but, without earthquakes the earth’s surface would not look like it does today, and we would not have separate continents.