Session Information
| Session | Poster Session | | Date | Monday (2008-04-07) | | Time | 5:00 PM - 7:00 PM | | Room | Grand Terrace |
Presentation Information
| Presenter | Don Bragg | | Title | A comparison of circular anomalies from an historical Arkansas landscape | | Affiliation | USDA Forest Service, Southern Research Station | | Authors | Don Bragg, Robert Weih | | Keywords | Bottomland, Gulf Coastal Plain, Liquefaction, Pimple mounds, Sand blows | | Presentation Type | Poster | Abstract:
Decades of agricultural land use have obscured many features of the landscapes in southern and eastern Arkansas, including some that reflect strong, large-scale seismic events. The analysis of historical aerial photographs allows us to look for and assess these features prior to their obliteration. As an example, a nearly 60,000 hectare landscape taken from 1939 aerial photographs of Ashley County, Arkansas was examined for differences between two observed sets of circular anomalies of natural origins. In the plowed bottomlands of this image, these circular features have been identified as “sand blows,” a liquefaction feature arising from one or more strong earthquakes. The circular features visible in the upland prairie have been called “pimple mounds,” and are generally thought to arise from other natural phenomena. We randomly subsampled an equally distributed number (half from the uplands, half from the bottomlands) of 25-hectare cells across this landscape and then compared circular feature patterns between those in the prairie and those of the bottomland. From a preliminary analysis of the prairie, there were an average of about 2.3 circular anomalies per hectare, significantly fewer (P < 0.001) than found in the bottomlands (average of 5.8/ha). Spatially, both types of the circular features were fairly regularly distributed, although in the bottomlands a number of distinct strings of sand blows were observed. Feature size in both locations varied, with most of the obvious anomalies ranging from 10 to 20 m across. With the exception of feature density between these samples, many similarities in appearance and pattern are suggested from these aerial photographs and may hint of a common origin. If true, this could greatly expand the area of the West Gulf Coastal Plain that has been impacted by prehistoric earthquakes. |
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