HOW DID THE MEERS FAULT SCARP FORM - PALEOEARTHQUAKE OR ASEISMIC CREEP - A SOIL MECHANICAL PERSPECTIVE

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) along the fault were folded a s well as ruptured. In some places, almost all of the deformation is a ccommodated by ductile folding of the soils. Having this kind of defor mation 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 slo w deformation (aseismic fault creep). To determine how the scarp was f ormed, I have developed a multidisciplinary study which involved geolo gical, soil mechanical, and soil micromorphological techniques. Geolog ical mapping delineated the deformation, stratigraphy, and any feature s that might be associated with the faulting. The mapping was also nee ded to reconstruct the sequence of events that formed the scarp at the study site. Consolidation tests using the Casagrande (1936) method fo r 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 stares of stresse s needed to slowly deform or shear the soils in triaxial and direct dr ained shear tests. The direction of maximum principal stress was deter mined by Mohr's circle and soil micromorphological analyses. The resul ts of soil mechanical analysis show that the faulting of the Quaternar y soils along the fault had to be sudden or fast, which indicates that the scarp was probably created seismically; During faulting, the soil s were anisotropically consolidated or compacted, thereby, recording i n. their structure the states of stresses that caused faulting to occu r. The direction of the maximum principal horizontal stress is between N62 degrees E and N72 degrees E indicating rotation of principal stre ss axes. The technique developed for this study can be used in similar areas where active faults offset Quaternary soils with relatively unc hanging moisture contents below some depth. Also, for shallow depths, this technique might give more reliable young tectonic stress measurem ents (both magnitude and orientation) than other techniques such as ov ercoring, hydraulic fracturing and wellbore breakouts because it is us ed on geologically recent units which, unlike lithified rocks, have no t yet existed through millions of years of deformation. (C) 1997 Elsev ier Science B.V.