Did you ever wonder what causes earthquakes and how much damage they cause?

By David McClelland

Since moving to Michigan in 1978, I can recall only one earthquake (EQ) in this area and that occurred in Lake Erie in 1979. It caused a crack in a wall in my employer's building in Bloomfield Hills. I was in my auto at the time and didn't feel it.

What causes EQ's? An EQ is caused by the shaking of the ground due to an abrupt shift of rock along a fracture in the Earth, called a fault. Within seconds, an EQ releases stress that has slowly accumulated within the rock, sometimes over hundreds of years. Most EQ's are caused by slow movements deep within the Earth that push against the Earth's brittle, relatively thin, outer layer, causing the rocks to fracture suddenly. In 1871, Koto Bunjiro, a geologist in Japan, studying a 60-mile long fault whose two sides shifted about 15 feet in the great Japanese EQ of that year, was the first to suggest that EQ's are caused by faults.

Most EQ's occur at depths of less than 50 miles from the earth's surface. However, the deepest EQ's occur as deep as 400 miles below the surface.

The world's deadliest recorded EQ occurred in 1557 in central China. It struck a region where most people lived in caves carved from soft rock. Those dwellings collapsed during the EQ, killing an estimated 830,000 people. In 1976, another deadly EQ struck Tangshan, China, an important industrial city in northern China, and more than 250,000 people were killed.

In the United States, Florida and North Dakota have the fewest EQ's. Alaska and California have the most. Alaska averages seven per year. Alaska has an EQ with a magnitude of 8 or greater on the average of every 14 years. The 8 is a reading on the Richter Scale (RS) used to measure the magnitude of an EQ. It was developed by Charles Richter at the California Institute of Technology in 1935. What does an RS recording means? The 1906 San Francisco EQ had an RS reading of 7.8. The 1964 Anchorage, Alaska EQ had a RS reading of 9.2, a difference of over 125 times more energy being released which accounts for why the Anchorage EQ was felt over an area of 500,000 square miles. In terms of deaths, there were 700 resulting from the San Francisco EQ as opposed to 114 from the Anchorage EQ due to their relative populations at the time.

While in the Air Force, I was on Temporary Duty at Elmendorf Air Force Base near Anchorage in 1964 shortly after that EQ and saw, what had been a large section of the downtown area, that was then just a huge, empty crater. I was impressed by the amount of damage an EQ can do.

To give you an idea of the number of EQ's in the United States per year and their magnitude, lets look at a sample of each 10th year since 1970:

Yr/Mag. 8 – 9.9 7-7.9 6-6.9 5-5.9 Total Est. Deaths

1970 0 0 4 48 52 0
1980 0 2 8 72 82 1
1990 0 0 2 64 66 0
2000 0 0 6 63 69 0
2010 0 1 7 55 63 0

Can we predict EQ's? An EQ prediction is a prediction that an EQ of a specific magnitude will occur in a particular place at a particular time (or ranges thereof).
Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month. For well-understood faults, seismic hazard assessment maps can estimate the probability that an EQ of a given size will affect a given location over a certain number of years. The ability to predict EQ's either on an individual basis or on a statistical basis is still remote.

We are fortunate to live in an area that has few EQ's.