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Q: How many earthquakes have happened in the whole world?
Photo showing earthquake damage in Auburn, Washington, in 1949.

Photo courtesy of the White River Valley Museum, Auburn, WA.

This photo shows damage caused by an earthquake that struck Auburn, Washington, in 1949.

No one knows how many earthquakes there have been through history. In fact, experts cannot say how many earthquakes have happened this year, this month, this week, or even today.

How can this be? Read on and you will find out.

As you might know, the earth’s crust is broken into large pieces, like a puzzle. These pieces are called tectonic plates. There are about a dozen large ones and several smaller ones. Each plate is 30 to 60 miles thick.

Where the edges of plates meet, there are deep cracks known as faults. (One famous example is California’s San Andreas fault.)

The plates move in relation to each other, but very, very slowly — about two to four inches a year.  As they move, they interact with each other in a few different ways. A plate might pull away from another plate, or slide under it, or slip past it.

Because their edges are rough and jagged, plates sometimes get stuck along faults. When this happens, the rest of the plate continues to slowly move, but energy builds up in the stuck area.

Over time, the energy gradually increases. Finally, it becomes great enough to overcome the friction that what keeping it stuck. It suddenly becomes unstuck and lurches in the direction the rest of the plate has been moving.

All of the stored up energy is released when this happens, and an earthquake occurs. The energy shoots outward in all directions from the fault in vibrations that scientists call seismic waves. (Say seismic like this: SIZE-mik.)

When the seismic waves reach the earth’s surface, they can cause the ground to shake. When the ground shakes, so does everything on it, including homes, buildings, trees, roads, and bridges.

Using instruments called seismographs (SIZE-mo-graffs), scientists can tell where an earthquake happened, how big it was, and how much it caused the ground to shake at different locations.

Earthquakes vary greatly in their size, which scientists call their magnitude. To categorize earthquakes, scientists use a magnitude scale that goes from 1 to 10. The higher the number, the bigger the earthquake.

The earthquakes that make the news are usually the ones that are highest in magnitude, because they cause the most damage and the most deaths. Fortunately, these earthquakes do not occur very frequently

The U.S. Geological Survey has put together the following chart showing the number of earthquakes in different magnitude ranges that occur around the world each year:

Magnitude                   Average annual number of earthquakes

8.0 and higher              1

7.0 to 7.9                     15

6.0 to 6.9                     134

5.0 to 5.9                     1,319

4.0 to 4.9                     13,000 (estimated)

3.0 to 3.9                     130,000 (estimated)

2.0 to 2.9                     1,300,000 (estimated)

Notice that the numbers get bigger as you go down the list. Weak earthquakes occur far more often than strong ones.

Many earthquakes are too weak to be felt or detected. And the chances of detection go down if a weak earthquake occurs in an area where no people live, or under the ocean, or far from any seismograph. This explains why the word estimated appears in the bottom three categories.

It also explains why no number — not even an estimate — is given for earthquakes lower than 2.0 in magnitude. Since most of these earthquakes happen without anyone noticing, it is impossible to say, or even to estimate, how many of them occur each year or have occurred through history.

(Question submitted by a 4th-grade student at North Coventry Elementary School in Pottstown, Pennsylvania.)

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