CUSEC FAQ's and Information

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

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 recurrence 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, analysis 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.

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