SMALL-SCALE SIGNATURES OF SEISMICITY

There are relatively few known seismogenic faults in the central and eastern U.S., making the search for paleoliquefaction features invaluable to paleoseismic investigations. Large-scale paleoliquefaction features commonly form in fluvial sediments and include clastic dikes, sills, and lateral spreads, and mapping their distribution can define the epicentral regions of seismogenic faults. However, geologic conditions during earthquakes aren’t always conducive for producing the major liquefaction required to develop such features, so the absence of large paleoliquefaction features doesn’t always indicate seismic quiescence. Small-scale seismogenic features within the Wabash Valley Seismic Zone such as detachments, dikes, sills, mushroom-shaped intrusions, and disrupted bedding typically form in conjunction with large-scale features, but can also form in settings where large features do not or cannot form. For example, small-scale features can form in thin-bedded sandy alluvium as well as in more plastic sediments found in lacustrine settings. Small-scale features can also form in unsaturated sediments above the water table or in sediments with relative densities too high for complete liquefaction. Small features can form at distances far beyond the meizoseismal area where shaking levels were too low for complete liquefaction but high enough to cause a significant increase in pore-water pressure.

Factors involved in creating these small-scale features vary and depend on the geologic setting. For example, sills are particularly prone to form during prolonged, low-level shaking in moderately compact sandy sediments. Mushrooms are more likely to develop where very soft plastic sediments overlie a thin sand bed. Dikes likely require complete, sudden liquefaction, as do detachments. Additional research is needed to define these controls more explicitly.

Locating small-scale features is particularly relevant to paleoseismic investigations in beginning phases because such features are commonly found far beyond the epicentral region. One or several isolated features may not be significant, but when they are abundant and widespread they are good indicators of paleoseismicity, and mapping their trends and frequency of occurrence can help locate the epicentral regions for paleoearthquakes.