CUSEC FAQ's and Information

Frequently Asked Questions (FAQ's)
about
Seismic Hazard in the Central U.S.

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.

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