The Measure of an Earthquake

We heard that the recent Japanese earthquake measured 8.9. What does that number mean? The number used to refer to the Richter Scale, but now refers to the moment magnitude scale, but the numbers are calculated so that they are the same in both scales. The moment scale measures the size of an earthquake in terms of the rigidity of the earth, the amount of movement and the size of the area affected. In other words, the amount of wiggle on a seismograph. Both the Richter and moment scales are logarithmic, meaning that an earthquake of size 7 is 10 times stronger than an earthquake of size 6. But the amount of energy released is another matter.

Lee Alison, Arizona State Geologist, explains on his blog:

How does the Japan earthquake of magnitude 8.9 compare to other recent large quakes?

The news media do a better job than they used to of noting that each magnitude number is 10 times that of the lower number. But most everyone assumes that refers to the relative amount of energy released by the quake – comparable to measuring the power of atomic bombs for instance.

Not true.

The magnitude is a measure of the amplitude of the seismic waves. But each 1.0 magnitude increase is equal to approximately a 32 times increase in energy release. Each increase of magnitude by 2.0 equals 32 x 32 or (about) 1,000 times increase in energy released.

The M8.9 Japan quake released the equivalent of 336 megatons of TNT. In comparison, last month’s Christchurch, New Zealand M6.3 quake was equal to 43 kilotons, and last year’s M7.0 Haiti quake was equal to 474 kilotons.

The Japan quake was about 7814 times bigger than the Christchurch quake and 709 times larger than the Haiti quake.

I’ve simplified this in regards to Richter magnitude vs moment magnitude but my intent is to emphasize the power of the Japan quake.

 The strongest recorded earthquake was in Valdivia, Chile, May 1960. It measured 9.5. The second largest, measuring 9.2, was in Alaska in 1964. The 1906 earthquake in San Francisco had a moment magnitude of 7.9. The U.S. Geological Survey says that an earthquake of about 8.0 or more occurs on average of once per year.

The Alaskan earthquake is interesting because it demonstrated a certain property of some clays that contributed to the extensive damage in Anchorage. Some clays are thixotropic, meaning that when subject to shear stress, that is, you shake them, they turn to liquid. Thixotropic substances are normally thick and viscous, but turn very liquid under shear stress. You have experienced thixotropy with a ketchup bottle.

Besides shaking and breaking, seismic sea waves, tsunamis, are the greatest danger. Tsunamis are long-wavelength ocean waves with energy extending from the sea surface to the ocean floor. When the wave reaches shallow water near the coast, all that energy is concentrated into a smaller and smaller space, hence its destructive force. In mid-ocean, a tsunami is barely noticeable.

You can see a list of the largest recorded earthquakes here.

Most earthquakes occur near the edges of tectonic plates, but there are some intra-plate quakes as well. For instance, on December, 16, 1811, a large earthquake, estimated strength 7.2-8.1, occurred near New Madrid, Missouri. New York is not immune to earthquakes either. See here for earthquake information by state.

 And some earthquakes are caused by human intervention. I experienced the Denver earthquakes of 1967-1968. The Rocky Mountain Arsenal near Denver was disposing of waste material by pumping it down more than 12,000 feet beneath the surface. That lubricated a deep range-front fault and caused is to slip.

 Here is a map from the US Geological Survey showing locations of major earthquakes since 1900. The pattern describes the boundaries of major tectonic plates and the volcanoes of Hawaii.