How did the Meers fault scarp form? Paleoearthquake or aseismic creep?: A soil mechanical perspective

Abstract

Studies confirm that the Meers fault in southwestern Oklahoma has been active in recent times. The most recent movement occurred about 1100 y ago in late Holocene. There is as much as 5 m vertical and possibly appreciably more left-lateral strike–slip displacements on the fault. During faulting, the Quaternary soils 1 Soil, in an engineering sense, is the relatively loose agglomerate of mineral and organic materials and sediments found above the bedrock, and at a particular site, it can be residual (that is, weathered in place) or transported (moved by water, wind, glaciers, etc.) ( Holtz and Kovacs, 1981). 1 along the fault were folded as well as ruptured. In some places, almost all of the deformation is accommodated by ductile folding of the soils. Having this kind of deformation with no record of an earthquake associated with the Meers fault during historical times raises the question whether the present scarp was formed seismically by earthquake event(s), or aseismically by slow deformation (aseismic fault creep). To determine how the scarp was formed, I have developed a multidisciplinary study which involved geological, soil mechanical, and soil micromorphological techniques. Geological mapping delineated the deformation, stratigraphy, and any features that might be associated with the faulting. The mapping was also needed to reconstruct the sequence of events that formed the scarp at the study site. Consolidation tests using the Casagrande (1936)method for finding maximum effective stresses were used to determine the states of stresses imposed on the soil deposits when they were first faulted. These states of stresses were then compared to the states of stresses needed to slowly deform or shear the soils in triaxial and direct drained shear tests. The direction of maximum principal stress was determined by Mohr's circle and soil micromorphological analyses. The results of soil mechanical analysis show that the faulting of the Quaternary soils along the fault had to be sudden or fast, which indicates that the scarp was probably created seismically. During faulting, the soils were anisotropically consolidated or compacted, thereby, recording in their structure the states of stresses that caused faulting to occur. The direction of the maximum principal horizontal stress is between N62°E and N72°E indicating rotation of principal stress axes. The technique developed for this study can be used in similar areas where active faults offset Quaternary soils with relatively unchanging moisture contents below some depth. Also, for shallow depths, this technique might give more reliable young tectonic stress measurements (both magnitude and orientation) than other techniques such as overcoring, hydraulic fracturing and wellbore breakouts because it is used on geologically recent units which, unlike lithified rocks, have not yet existed through millions of years of deformation.