Below are some frequently asked questions about the seismic hazard in the central United States. The FAQ's cover a wide range of both technical and non-technical areas.
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Frequently Asked Questions about Earthquakes
Different magnitude scales were developed to take into account
different types of available data (e.g., recordings from different
types of instruments and seismic wave types most pronounced at
different distances). Although it technically has a very specific
meaning, “Richter magnitude” generally is used incorrectly by the press
to mean a generic “magnitude”. “Moment magnitude” best reflects the
earthquake’s physical characteristics (like the area of fault surface
that broke and amount of movement that occurred across it) The USGS has
now decided to use only moment magnitude, but just says “magnitude”
when reporting to the press and public.
one can “predict” earthquakes. Scientists can only estimate the likelihood, or
probability, of earthquake occurrence and thus, it is impossible to say
that an earthquake will or will not occur at any particular instance of
time. For example, if there’s a 25% chance of an earthquake occurring,
this also means that there’s a 75% chance of it not occurring. It is
also important to keep in mind that these estimates will change as
scientists learn more about the earthquake hazard in the central U.S.
Liquefaction occurs when loose, sandy, water saturated soils are
strongly shaken. The soils lose their capacity to bear any weight and
can flow like a liquid. This process is accompanied by high pore water
pressures that can force sand, water, and mud upward, which creates a phenomenon known as a sand blow. Many factors
affect how susceptible materials are to liquefaction, but some of the
most important requirements are the degree of water saturation, the
size of the grains, and how well cemented they are.
most important lesson is that mitigation, primarily in appropriate
building practices, pays off. If we compare the loss of life and damage
in recent California earthquakes to those of comparable sized
earthquakes in other countries, the lesson is obvious. The 1994
Northridge, CA and 1995 Kobe, Japan earthquakes were of comparable
magnitude and both occurred in urban areas. Although Japan has taken
many measures to mitigate earthquake damage and some of the damage
differences may be due to factors other than building design, many
older buildings collapsed as a result of the Kobe earthquake and there
were thousands of fatalities. The building stock in Los Angeles is much
younger, and much of it has been built with earthquakes in mind. Few
people died as a result of the Northridge earthquake. Unlike California
most of the building stock in the central U.S. has been built without
earthquakes in mind.
The amplitude and other characteristics of shaking required to cause a
dam to fail are really an engineering issue. The shaking will be
determined not only by the size of the earthquake, but also by it’s
proximity to the dam (e.g., a closer smaller earthquake may cause
greater shaking than a larger more distant one).
Frequently Asked Questions about Earthquakes in the Central United States
Paleoseismic (geologic) studies conducted over the last few years have shown that sequences of earthquakes that were in the 7.5-8 magnitude range have occurred at least twice before, in approximately 900 and 1450 AD. This implies a recurrence interval of about 500 years.
New ways of looking at the historical record of earthquakes have also caused scientists to revise estimates of the recurrence time of moderate earthquakes, but only slightly. Earthquakes of magnitude 6.0 have a 100-150 year recurrence interval. This does not mean that they occur on a set schedule, but historically speaking, occur every 100-150 years.
Many people, including the press, quote probabilities of earthquakes that are nearly 25 years old. Scientists have learned a tremendous amount about central U.S. earthquakes since that time. One of the things they have learned is that coming up with probabilities is much more difficult than previously thought. If historical seismicity combined with the new information on recurrence of large earthquakes is used, scientists estimate a 25-40% chance of a magnitude 6.0 and greater earthquake in the next 50 years and about a 7-10% probability of a repeat of the 1811-1812 earthquakes in the same time period.
However, it is VERY important to note that these estimates alone do not include information about WHERE the earthquakes might occur and therefore what shaking might affect any given location. More useful are the estimates of the likely amount of ground shaking that can be expected, contained in the National Seismic Hazard maps. The ground shaking estimated accounts for both the likely ranges of recurrence intervals and locations.
They should not have any effect on preparedness measures, which should
be based on the estimated hazard. Scientists emphasize that ‘hazard’
refers to be the amount of earthquake ground shaking expected during
some time period with some specified probability, and that it accounts
for the range of probable earthquakes magnitudes, recurrence intervals,
locations, local conditions, etc.
Deformation of the land surface directly over a fault that moves may manifest as very localized uplift or subsidence, or lateral distortions of up to several meters (for a very large earthquake). Shaking can cause ground failure of various types, including liquefaction and landsliding. These would have significant effect on the landscape in terms of damming streams, spewing sand and mud into fields, and causing areas near bluffs and rivers to slide and form a broken up surface.
There is not documented evidence that this has happened in the past, so there is no current belief in the scientific community that this will happen in the future. But it is not impossible.
This is an area of some controversy and little scientific data. Scientists know there were large prehistoric earthquakes in southern Illinois and Indiana. The size and recurrence of these earthquakes needs to be investigated more thoroughly. Estimates in the National Seismic Hazard maps are based on projecting the frequency of historical small earthquakes up to larger ones. These estimates are in reasonable agreement with earlier estimates based primarily on prehistoric liquefaction observations, but more investigation needs to be done.
Frequently Asked Questions about the 1811-1812 New Madrid Earthquakes
One of the 1812 earthquakes occurred on a fault that actually crossed the river three times. The uplift along this fault formed a scarp or cliff that caused both a dam and waterfalls at different locations. The damming of the river would have temporarily backed the river up, which may account for the descriptions of the river boat pilots.
Scientists previously thought the main shocks ranged from magnitude 8.0 to 8.5. This was based on historical accounts of the events. Today, based on what is known about the geology of the region, it is believed that the earthquakes ranged from magnitude 7.5 to 8.0.