Energy released by earthquakes varies of orders of magnitude. There are a cou-
ple of dozen major earthquakes (Richter magnitude 7.0 or greater) expected each
year and over a million smaller ones. Most earthquakes are not discernible without
the aid of instrumentation. Seismograph observations are converted in various
ways to measures of earthquake intensities.
The Richter scale and its modification is the earthquake measurement scheme
most familiar to the public. This scale is one of energy release and has no theo-
retical upper limit. What is usually unappreciated is that the magnitudes are rated
on a logarithmic scale and so every increase of 1.0 represents an energy increase
of 32 times. So, a major earthquake of magnitude 7.0 releases more than a magni-
tude 5.0 event. The Richter scale has been replaced by the moment magnitude
scale, which gives somewhat better estimates of the strengths of large quakes.
The modified Mercalli scale is based on observable damage rated from I to XII.
An earthquake of magnitude I will not be felt by most of the population except,
for instance, if one is on top of a tall building. Magnitude I earthquakes do not
leave behind damage. Magnitude Vevents are felt by nearly everyone with broken
windows and dishes falling from shelves. Magnitude XII quakes are unimagi-
nably damaging but, fortunately, rare. In a magnitude XII event, the destruction
is complete with rolling waves observed on the landscape and objects thrown in
the air.
There is a definite geography to the large earthquakes. They tend to occur on
tectonic plate boundaries although they occasionally occur associated with hot
spots and other weaknesses in the crust. The ten largest quakes of recorded history
have occurred on the plate boundaries ringing the Pacific Ocean. In the United
States, Alaska and California have the greatest propensity for experiencing large
earthquakes. This is because of their proximity to ocean bottom where the Pacific
Plate is subducting underneath the North American Plate.
Deaths are common in large earthquakes and there are reliable written records
pointing to several million deaths withinhistoric times. Some earthquakes have
killed more than three-quarters of a million people. In the United States, the most
fatal event was the San Francisco temblor of 1906 in which 3,000 people were lost.
Earthquake deaths are not limited to the land close to the focus but can also be
thousands of kilometers away in the case of earthquake-generated tsunamis. In
2011, an earthquake of 9.1 magnitude rocked the eastern coast of Japan’s Honshu
Island. Most of the thousands of deaths were caused by the ensuing tsunami,
which devastated areas near the coast.
The largest earthquake in recorded history in the United States occurred off of
southern Alaska in 1964. Its estimated moment magnitude was 9.2. The onshore
areas were not heavily populated and the death toll was only 131. However, some
towns were destroyed through the resulting tsunamis (tidal waves). Anchorage
Earthquakes 109