Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 153

between historic and prehistoric sand blows can be difficult. Historic sand blows
have weakly developed soils displaying thin A horizons with a single grain or
massive soil profile structure. Prehistoric sand blows have more mature soils with
thicker A horizons and an angular, blocky or friable, or more mature structure.
Tuttle and Schweig (1996)determined that soil A horizons within sand blows in
theNew Madrid seismic zone in the central USA develop at an average rate of
0.4--0.5 mm yr − 1 .Adams (1996)drewsimilarconclusions regarding sand blows in
Canada. In cases where the sand blows are buried, the thickness of the palaeo-A
horizon can be used as a guide to the length of time the sand blow was exposed
at the ground surface.
Tuttle and Schweig's (1996)estimates of the rates of soil formation in sand
blows prove useful for dating prehistoric seismic activities in other regions where
liquefaction has occurred. However, it is always important to remember that soils
can develop at different rates under different climatic regimes. The same is true
of regional variations in other soil forming factors such as topography, parent
material, and faunal and floral variations. Independent evidence of rates of soil
formation need to be ascertained for any individual region before any reasonable
estimates of the age of prehistoric sand blows, and hence past seismic activity,
can be made.
Archaeological evidence can prove useful in dating sand blows. Tuttle and
Schweig (1996)notedthat relatively well-drained, prehistoric sand blows in the
New Madrid seismic zone were used by Native Americans as home, storage and
burial places. Chronological determinations of artefacts found in association
with sand blows, provide a minimum age for the seismic activity. Radiocarbon
and luminescence dating techniques (see Appendix A) can be used in such set-
tings. Luminescence dating is particularly useful if the sand blow is sufficiently
thick so that the buried sands have been excluded from exposure to sunlight
and cosmogenic rays. Radiocarbon dating of organic inclusions within the sand
blow from sedimentary layers overlying the liquefied sand is also possible but
this only provides a maximum age for the seismic event as organic inclusions
are likely to be older than the event itself.
It is difficult to date and distinguish sand blows in regions where earth-
quakes have frequently induced liquefaction events. It is important that the
ages of sand blows are well constrained if these liquefaction features are to be
correlated regionally to determine the source areas and magnitudes of the pre-
historic seismic events. Events that have occurred over time periods of less than
a couple of hundred years are difficult to distinguish using conventional radio-
carbon and luminescence dating. This is because of the uncertainty margins
of 50--100 years, or more, associated with such dates; however, accelerator mass
spectrometry radiocarbon dating can help to reduce these uncertainty margins.