GROUND FISSURING AS A PALEOSEISMIC INDICATOR

Seismogenic ground fissures occur at scattered places in the USA. Mapping them may offer another feature for paleoseismic studies because in some situations they are much more widespread and readily found than faults, and the fissures can occur in field settings not conducive to developing liquefaction or other paleoseismic indicators.

Fissures of seismotectonic origin typically are tabular and dip steeply, their walls pinch together downward, and they were infilled from the surface. In many field settings are features of non-tectonic origin that resemble those of tectonism. Determining the origin of suspect features can require establishing their regional pattern, character of the infillings, and 3-D morphology. Where associated with major faulting, fissures are commonly (typically?) more abundant and larger close to the fault. Our observations indicate the pattern is best developed in tectonic settings having widespread deformations and/or widespread and parallel faults – such as the strike-slip fault settings apparently commonplace and active in the central and eastern USA.

In southernmost Illinois, near Metropolis, good examples of fault-induced ground fissures having widespread development occur in alluvial stream banks and are visible on digital orthophotos. Two types of fissures fillings occur there: non-banded and vertically banded, with the banded type likely caused by creep or numerous minor fault movements. The filled fissures are in a region of karst, but the tabular morphology of the features rules out a collapse origin.

In eastern Washington, around Walla Walla, glaciofluvial deposits host numerous and widespread filled fissures. However, the probable subaqueous origin for the infillings in combination with absence of liquefaction features, and presence nearby of numerous very precariously balanced rocks, indicates the filled fissures originated by hydraulic fracturing during outburst floods from Lake Missoula, rather than earthquakes.

In southeastern South Carolina and Georgia, fissures possibly with fillings have been interpreted as caused by severe weathering, although a tectonic origin is plausible given the cross cutting nature of the fissures.

All types of features mentioned above are shown at http://www.usi.edu/science/geology/jdurbin/obermeier/photos.htm