Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles

Earthquakes are no laughing matter.  They are the result of vibrations in the Earth’s outer crust.  These vibrations are caused when underground rock slips.  The Earth’s crust is made of large plates of rock.  Over time these layers of rock slowly move causing the plates to slip against each other.  These tectonic type earthquakes occur in regions where the large plates border each other – the San Andreas Fault in California is a well known fault line.

Earthquake damage to a roadEarthquakes can also be caused by volcanic activity.  These types of earthquakes are usually not as destructive as the earthquakes caused by shifting plates.  In fact, earthquakes caused by volcanic activity can actually be beneficial since they often precede a major volcanic eruption and act as a warning that something big is about to happen.  Hawaii, where the is much volcanic activity, has recorded as many as 1,000 minor earthquakes (some so small that people aren’t even aware that a earthquake just took place)  in a single day.

And surprisingly enough, earthquakes can also be caused by man made activities.  Filling of man-made lakes, underground explosive activity, or pumping of wells are examples of activities that have been know to cause earthquakes.  In 1962, Denver Colorado began experiencing earthquakes for the first time in that city’s history.  Officials finally realized that the earthquakes began occurring at the same time that the city began pumping wasted water into deep man-made wells on the eastern side of the city.  When the pumping ceased, so did the earthquakes.

The greatest danger from an earthquake is the destruction of man-made buildings, roads, or bridges.  Resulting tidal waves, often with waves as high as a 10-story building, are also a forbidding danger.  In 1896, the town of Sanrika, Japan was completely swept away by a tidal wave (which the Japanese call tsunamis).

The size of an earthquake is measure on a scale called the Richter scale.  Named after the American seismologist (a scientist who studies earthquakes) Charles Francis Richter, the Richter scale measures the movement of the land surface 60 miles from the epicenter of the earthquake.  This measurement is a logarithmic scale – a magnitude 7 earthquake moves land 10 times farther than a magnitude 6 earthquake, a magnitude 6 earthquake moves land 100 times further than a magnitude 5 earthquake, and so on.   Up until 1979, a magnitude 8.5 earthquake was thought to be the maximum.  When scientists discovered improved measuring techniques they revised this maximum – it is now believed that a 9.5 is the maximum magnitude a earthquake can produce.  Scientist now believe that the Alaskan earthquake of 1964 was a 9.2 on the Richter scale.