Frequently Asked Questions about

Seismic Hazard in the Central US.

- July, 2002 -

Have the estimates of the recurrence intervals for 1811-1812 type earthquakes changed? 
Yes.  Paleoseismic (geologic) studies conducted over the last few years have shown that sequences of earthquakes of comparable size to that in 1811-1812 have occurred at least twice before, in approximately 900 and 1450 AD.  This implies a recurrence interval of about 500 years. 

Does it also mean that the reoccurrence interval for a magnitude 6 event has also changed?
New ways of looking at the historical record of earthquakes have also caused us to revise estimates of the recurrence time of moderate earthquakes, but only slightly.  This change has not been enough to affect the hazard significantly.  By ‘hazard’ we mean the amount of earthquake ground shaking expected during some time period with some specified probability.  Estimates of this at any give location depend on possible shaking from all likely earthquakes. They also depend on many more factors including how far away the earthquake is, local soil conditions, etc. 

Given this and other new information, can one estimate the probability of damaging earthquakes in the New Madrid seismic zone?
Many people, including the press, quote probabilities of earthquakes that were estimated in 1985.  We have learned a tremendous amount since that time. One of the things we have learned is that coming up with probabilities is much more difficult than we used to think. If we use the data on historical seismicity combined with the new information on recurrence of large earthquakes, and make the same assumptions that go into the National Seismic Hazard maps, we would 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. 
 

What effect do these changes in recurrence estimates have on preparedness measures?
They should not have any effect on preparedness measures, which should be based on the estimated hazard.  Again, we 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.

In what terms do we need to “couch” these new revised predictions? Who made the decision to change these estimates? Do we need to mention the change or just use the current information?
No one can “predict” earthquakes! We 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. When asked about the likelihood of earthquakes occurring, we suggest saying something like  “Nobody can predict earthquakes, but given what we know now, our best estimates of the likelihood of earthquakes is…”  It is also important to keep in mind that these estimates will change as we learn more! 

Does everyone within USGS agree on the cause and effects of a future New Madrid event?  Is it possible to have a general statement issued on the hazard that planners can use?   When differences within the research community exist and spill out into the non-research environment, how or who will clarify the issues?
No one knows what causes New Madrid earthquakes.  However, there are ideas that are being researched.   Although there is great uncertainty regarding the cause of earthquakes, scientists generally do agree on what happens when they do occur ? that is, the likely levels of ground shaking associated with the waves earthquakes emit.  These levels are reflected in the National Seismic Hazard Maps, which represent the products of a long consensus building process.  These maps also account for the uncertainties in our understanding.

Differences of opinion within the research community invariably will arise.  Generally these are not critical to people outside the research arena.  When they are, the USGS sometimes has held workshops to try to come to a consensus and other times has announced our own internal consensus.  Other researchers, of course, are not bound by what we announce. Generally, we have met with the CUSEC State Geologists and been able to come to agreement at least between the State Surveys and the USGS.  In most situations, the State Surveys are the ones responsible to the State Governors and the USGS works closely with them.

Has anyone reviewed the materials from the paper published in “Science” in 1999 that says that either 1811-1812-size New Madrid earthquakes happen less frequently than has been assumed or that that they were smaller than assumed and that the hazard is overestimated? If so, does the paper make any positive point? 
The controversial Science paper went through the standard journal review process (2 reviewers). More recent documents that one of the authors has been associated with and that have caused controversy have not undergone peer review.  Many of the data, analyses and interpretations from the Science paper were reviewed at a workshop of experts and although no one took issue with the basic data they collected and processed, the interpretations were found to be inconsistent with the consensus. Basically, the authors treated New Madrid as if it were a plate boundary (like the San Andreas fault system), which is demonstrably incorrect. The occurrence of large earthquakes (magnitude greater than 7) every 500 years, documented from paleoliquefaction evidence, and the huge affected area from the large earthquakes in 1811-1812 clearly indicate that the New Madrid region has high seismic hazard.

Why is moment magnitude used by scientists but not by the media and other non-scientists?  What is the difference between moment magnitude and Richter magnitude?
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). Different magnitude scales are usually valid only within a certain magnitude range and when based on specific types of recording instruments.  Magnitudes estimated using different scales should be equal where the scales overlap, however. 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 say “magnitude” when reporting to the press and public.

What is the most credible event emergency preparedness measures should be based on for the central US?
This depends on what level of risk emergency managers are able — politically, morally, or legally —to accept.  The USGS does not make these kinds of decisions; its job is to provide the best estimates of the probabilities of ground shaking amplitudes (i.e., provide the scientific basis on which others make decisions).  Although some of the inputs have changed, estimates of the hazard (shaking) have not changed appreciably.  This is largely because the shaking depends on many factors, so that changes in a few factors often have little affect.

Do the National seismic hazard maps account for soil conditions in each community?
No!  In fact, soil conditions can change the hazard quite significantly.  This is the reason for the CUSEC State Geologist 1:24,000 mapping project and the USGS Memphis/Shelby County mapping project. In practice, building codes have soil amplification factors that are applied to the rock-site ground motions given in the National Seismic Hazard maps. The amplification factors are chosen based on characteristics of the site of interest. However, these are generic soil amplification factors that may not always be appropriate for the thick sedimentary deposits found in the Mississippi Valley and Atlantic coastal plain.

Does the USGS still maintain a prediction council? 
No, although there are ongoing discussions about reinstating it.

FEMA bases the amount of funds that they provide each state to address its earthquake hazard on the USGS National Seismic Hazard maps.  The states in turn use the map to determine how many counties fall within a certain area of risk.  Will the next generation of the National Seismic Maps change the area of concern for the central US?  If so what is this change based on?
The revised maps incorporate new results generated since the mid-1990s when the current generation of maps was made. Draft versions of the new National Seismic Hazard maps exhibit relatively small changes in the hazard estimates for the central and eastern U.S compared to the 1996 maps. These changes are largely due to our increased knowledge of large-earthquake chronologies for the New Madrid and Charleston, S.C. areas, a change in the method used to account for the uncertainties in input parameters (i.e. use of a logic tree), and the use of additional relations describing how seismic waves dissipate with distance. However, the overall pattern of earthquake hazard for the CEUS remains the same in the draft maps as the 1996 maps. The draft maps and documentation are posted on the website http://geohazards.cr.usgs.gov/eq/

How many counties in SW Indiana are proposed for inclusion in the AASHTO Bridge Design Manual (now only three)?
This is a question for AASHTO.  The USGS provides the same maps to AASHTO that are provided for anyone else. 

What is the potential for a New Madrid event causing a secondary reaction on an adjoining fault such as one in the Wabash Valley?
All we know is that this has not happened in the past few 1811-1812-sized sequences.

What are the latest estimates of intensity from New Madrid earthquakes? 
Intensity is a measure of the effects of shaking; i.e., the amount of damage to structures and the ground caused by earthquake shaking.  The USGS National Seismic Hazard maps only provide estimates of the shaking, not its effects. 

After a major earthquake in the New Madrid or Wabash Valley seismic zone, what changes to the landscape would we most likely see?
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.

Can you explain liquefaction? What conditions would increase or decrease the amount of liquefaction?
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, often forming the signature sand blows of the New Madrid seismic zone. 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.

After the 1811/1812 earthquakes there were reports that the Mississippi River flowed backward. Can you explain this phenomenon and what is it called?
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.

What have we learned from the recent quakes (especially in Japan) and the damage that occurred in earthquake designed bridges and buildings.
The 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 US has been built without earthquakes in mind. 

How large of an earthquake does it take to make an earthen dam fail?
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). 

Has the USGS considered issuing a general booklet about earthquake insurance?  Many property owners and insurance agents seem confused.  Some general information would aid property owners in making a rough cost-benefit analysis.  Such an analysis is difficult because of the infrequency of events that would cause damage exceeding the sometimes large percentage deductibles on structures.
To make such a booklet or other similar product useful for a particular business sector requires collaborations between the business and the USGS.  There are ongoing efforts to work with the insurance companies, but currently neither group has been able/willing to invest the resources to develop products targeted at insurance issues.   It is also important to keep in mind that the USGS only can provide the scientific information and at present, cannot do risk assessment (cost or damage assessment) or recommend policies. 

What is the most likely occurrence from the Wabash Valley fault both in intensity, probability and location and how is it likely to affect our Indiana infrastructure?
This is an area of some controversy and few data.  We 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.