Geoscience Reference
In-Depth Information
cing one. It is a little-known fact that the largest earthquakes on record in the United States did
not happen in California but along the Mississippi River in New Madrid, Missouri, in late 1811
and early 1812. The quakes were so powerful that they shook down chimneys 360 miles away
in Cincinnati and changed the course of the Mississippi River for more than 100 miles.
Earthquake prediction is based on the study of past earthquake patterns. It generates statist-
ics about the probability of an earthquake within a certain magnitude range occurring in a spe-
cified region over a specified period of time. A typical prediction for the San Francisco Bay
Area might read something like, “The San Francisco Bay Area is overdue for a significant
earthquake. We believe that this area has a 70 percent chance of having an earthquake in the 7.5
to 9.5 range sometime in the next thirty years.” Except in rare instances when undeniable geo-
logical signs indicate that a major quake is imminent, modern scientific methods cannot give
accurate earthquake warnings. And we think weather forecasters have a hard time being accur-
ate!
This chapter discusses earthquake preparations, specific techniques and strategies to im-
prove your chances of surviving a serious earthquake, how to improve the earthquake resist-
ance of your home, and what to do in the aftermath of an earthquake.
What to Expect When an Earthquake Strikes
In the aftermath of a major earthquake, the public utilities usually fail, and fires tend to break
out due to broken gas mains and lack of water. Most government workers are either injured
themselves, or at home caring for family members. If the earthquake is a significant one, with
widespread damage and casualties, in all likelihood you will be on your own for several days or
more. In this case, medical attention for the wounded and rescue efforts for those trapped under
debris will be the first priority, with water second, followed by shelter and food.
The magnitude of an earthquake is usually measured on the Richter scale, or the newer
“moment magnitude scale,” which are both roughly equivalent logarithmic scales—meaning
that each point of magnitude is a factor ten times as strong as the point below. For example, a
magnitude 7.0 earthquake is ten times as strong as one of magnitude 6.0, 100 times as strong as
one of magnitude 5.0, and 1,000 times as strong as one of magnitude 4.0. However, the mag-
nitude of the quake tells only part of the story. The extent of the damage will depend on a num-
ber of factors in addition to the quake's magnitude, including the geophysics of the local soil
and rock, the age and quality of the local buildings and bridges, and the local seismic building
codes (or lack thereof).
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