He explains that reports from old journals and logs were used to analyze earthquakes that happened as far back as the mid-1500s. [A number of eyewitness accounts from the series of New Madrid earthquakes in 1811 and 1812 has been compiled by the Center for Earthquake Research and Investigation (CERI) at the University of Memphis, and are available to view online at http://tinyurl.com/7mnjals, for example.]
John Ake, a seismologist in the NRC’s office of research, said that since the 1980s the industry has learned a great deal about using geological information such as soil liquefaction associated with earthquakes to identify new seismic sources. Soil liquefaction is a phenomenon in which saturated soils (that is those below the water table) lose their strength when they are subjected to strong ground shaking. They often form spouts and eject materials onto the ground surface. Experts can search for these features, date them and, based on the extent of the feature, can assess how large historical earthquakes might have been.
Photograph of moderate-sized sand blow (12m long, 7m wide, 14cm thick) that formed about 40 km from epicentre of 2001 7.7M Bhuj, India earthquake, combined with schematic vertical section
The New Madrid earthquakes in 1811 and 1812 covered the region with sand blows, thousands of which remain today and can be seen as large, light-colored sandy patches in agricultural fields. Study of these features has given more evidence of ancient earthquakes for use in the model.
