What can make these - 70-90 mile wide and thousands of miles long - low gravity crack lineament features so dangerous is their hidden fractures, buried in the earth's crust below. These low gravity areas are called "Rift zones" where the continental crust tried to pull itself apart, but failed. The thinning of the crust enabled lighter density young rock to intrude, thus resulting in the lower gravity characteristics in the rift zones. Other recent studies have suggested that the thinning of the crust may occur from the delamination of heavier crustal rock "falling" down into the earth's mantle. However, in the case of linear features of these long distance "lineament rift zones", the case of random delamination from "falling" into the mantle would not be able to explain these highly organized structures.
However, scientists are placed at a disadvantage, in earthquake risk assessment, since any hidden fractures are buried deep in what is called the "Midcontinental Basement". Without having any data to the size, length, and number of any hidden fractures, it renders any risk analysis "blind". Only a paleogeologic assessment of past locations, patterns, frequency, and magnitude of earthquakes are scientists able to form a type of a risk assessment. Compounding any risk assessment is understanding the rupture process of these hidden fractures. Scientists refer to these as "Intraplate earthquakes" as the quake occurs within the interior of a tectonic plate. Pressure, within the buried fracture (i.e. "fault"), may instigate an earthquake. GPS, strain sensors, and even inSAR (interferometric Synthetic Aperature Radar) are of no use to proffering answers to these questions as these fractures are inaccessible since they are deep below - while the sediment or crust above hides any telltale sign of crustal creep or stress.
Because of risk assessment uncertainty, FEMA and other emergency agencies are left to estimating the risk to America's infrastructure based on a worst case paleogeologic history (i.e. the maximum at any moment). Indeed, in 1999, FEMA listed as one of the major top four hazards in the United States, as "catastrophic", would be a giant earthquake striking the central U.S.