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Multidisciplinary technique to differentiate paleoseismic from creep displacement of faults : tested at the Meers fault, Oklahoma
Recent studies confirm that the Meers fault in southwestern Oklahoma moved in "Late Holocene" creating a maximum 5 meter vertical and possibly appreciably more left-lateral strike slip displacements. During this deformation, the Quaternary age soil deposits along the fault folded in a ductile manner as well as ruptured. In fact, at some places along the fault, almost all of the deformation is accommodated by ductile folding of these soil layers. Having this kind of deformation with no record of an earthquake associated with the Meers fault during historical times brings up the question whether the scarp we see along the fault today was created by earthquake event(s), by slow aseismic deformation, or by a combination of both means. In order to determine how the scarp along the Meers fault was formed, a multidisciplinary approach made up of soil mechanical, soil micromorphological, and geological methods was developed and used. Consolidation tests with Casagrande's (1936) method for finding maximum effective stresses were used to determine the states of stresses imposed on the soil deposits during the past. These states of stresses were then compared to the states of stresses needed to deform or shear these soil deposits in a slow manner in triaxial and direct drained shear tests. Triaxial shear tests were also used to see if these soil deposits can fail in a ductile manner under the natural field moisture contents and the states of stresses found in the consolidation tests. Direction of maximum principal stress was determined by Mohr's circle and soil thin section analysis. The results show that the Meers fault scarp was probably created seismically by one, or possibly two or more earthquake events that occurred in Holocene time. During faulting, the soil deposits along the fault were anisotropically consolidated or compacted, recording in their structure the states of stresses that faulted them, and the pores, clays, shale clasts, and some other grains were reoriented with their long axes perpendicular to the maximum principal stress direction. Ductile folding of these soil deposits along the fault under these states of stresses and natural field moisture contents could have easily occurred if field moisture contents were similar to the present. The direction of the maximum principal stress is between N62'E and N85'E, inclined at 10' SW to the horizontal indicating rotation of principal stress axes. The approach developed and refined in this study can be used in similar areas where active faults offset Quaternary soil deposits with relatively unchanging moisture contents below some depth. Finally, this method might give more reliable young tectonic stress measurements (both magnitude and orientation) for shallow depths because it is used on geologically recent soil deposits which, unlike lithified deposits, have not yet gone through millions of years of deformation.
Cetin, Hasan (1995). Multidisciplinary technique to differentiate paleoseismic from creep displacement of faults : tested at the Meers fault, Oklahoma. Texas A&M University. Texas A&M University. Libraries. Available electronically from
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