The primary reason the history of seismic activity in the New Madrid Seismic Zone is hard to measure is due to the geological conditions, specifically that the area is largely buried under 100-200 feet of soft river sediment. This sediment can obscure older geological features and seismic evidence, making it challenging to identify and study past earthquake activity.
While it's true that there have been significant earthquakes in the New Madrid Seismic Zone, many of them occurred in the early 19th century (1800s) and the record-keeping from that time is not as reliable as modern seismological data. However, the main issue is indeed the sediment cover, which limits direct geological and paleoseismic analysis that could provide more information on the history and frequency of earthquakes in the region. Additionally, the lack of recent major seismic events may also contribute to a limited understanding of seismic risk in the area, but the sediment cover is the primary barrier to a comprehensive historical assessment.
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