What Is An Earthquake?

Here is a basic primer on the complex global processes that create earthquakes, from substrata fault lines to Mercalli measurements.

Those who have the misfortune of experiencing an earthquake often say that it is the scariest event they have ever lived through. The floor moves; the walls shake; the very Earth becomes unstable. But what causes these great upheavals? Just what is an earthquake?

--Plate Tectonics --

Earthquakes are an unavoidable consequence of the very geology of the planet. The surface of the Earth is not whole, but is instead made up of a dozen or so large PLATES, collectively called the LITHOSPHERE, that rest and float atop the semi-molten rock ASTHENOSPHERE. The plates of the lithosphere are constantly in motion. Even now, as you read this, the plate you're standing on is moving slowly, taking you with it; we call this phenomenon "continental drift."

The plates of the lithosphere are tightly packed, and they edge up against one another. We call these edges PLATE BOUNDARIES. Smaller cracks on the surface of plates also exist; we call these cracks FAULT LINES. As the plates drift, plate boundaries and fault lines grind against one another in a variety of ways. This is where we must look to understand earthquakes.

Simply put, earthquakes are the vibrations caused by grinding of a fault line or plate boundary. While plate movement, and therefore grinding, happens constantly, it is only when the movements are sudden and violent, or when the stress in the lithosphere builds, that earthquakes occur.

-- Making Waves --

The shaking of the earth during an earthquake is actually the result of SEISMIC WAVES generated at fault lines and plate boundaries. Much like when a pebble is dropped into a pool, the very ground begins to ripple from the point of origin, called the EPICENTER, of the quake; the further from the epicenter, the lesser the strength of these waves. It is these waves that create the "quake."

There are two kinds of seismic waves that we experience during a quake. The first type is COMPRESSION WAVES, also known as "Primary" or "P" waves. These quick-moving waves cause a sudden, sharp thud when they arrive, and are often the first herald of a quake. They cause little damage on their own.

The second type is TRANSVERSE WAVES, also known as "Shear" or "S" waves. These waves arrive a few seconds after P waves, and create the rocking, shaking motions that most people associate with an earthquake. S waves cause most of the primary damage during a quake event.

Finally, the earthquake itself can be preceded by FORESHOCKS, or followed by AFTERSHOCKS. These are lesser P wave and S wave events that can occur hours, minutes, or even days before and after the quake itself. They can be damaging in their own right; aftershocks, especially, can send buildings or natural structures already damaged in the primary quake tumbling down.

-- Measuring an Earthquake --

We often hear talk of the RICHTER SCALE as the standard of measuring the MAGNITUDE, or the relative size of an earthquake, as measured by a seismograph. This scale, invented by scientist Charles Richter in 1935, was the standard quake measurement scale for most of the 20th century. It was common to hear a quake described as being of certain strength "on the Richter Scale," i.e. "The quake was a 6.2 on the Richer Scale."

The scale was flawed, however; it did not accurately rate the strongest quakes. The Richter Scale has since been replaced by the MOMENT MAGNITUDE SCALE, developed in the late 1970s. Today, quakes are simply described as being of certain "magnitude," i.e. "It was a 6.2 magnitude earthquake."

In addition, earthquakes are classified by their INTENSITY, or the observable damage that the quake causes. There is more than one standardized measure, though the most commonly used is the MODIFIED MERCALLI SCALE. Mercalli measurements are ranked via Roman numerals; the weakest intensity quakes, I & II, may go unnoticed by most people, while the most intense, XI & XII, cause near total destruction of structures and landscape.

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