Determining Relative Age of Faulting by Using Soil Stratigraphy: Problems and Misconceptions

Abstract

Soil stratigraphy has played an increasingly important role in determining the recency of fault activity, but its application is not a panacea. Many problems can arise when attempting to use soil stratigraphy to decipher fault activity. Common problems include the following: 1) Site grading before trenching, 2) failure to consider age-related soil science information, 3) failure to understand pedochronology, 4) inadequate carbon dating, 5) the presence of weakly developed old soils, 6) pedogenesis along ancient faults, and 7) failure to recognize bisequa (two-story soils). Common misconceptions when attempting to use soil stratigraphy to date fault activity include the following: 1) A seismic trench can be properly excavated, logged, and buried on the same day; 2) The age of a soil can be obtained by dating the A horizon; 3) The age of a buried paleosol can be obtained by dating its Ab horizon; 4) The C-14 age of pedogenic calcite in a Bk or K horizon measures its mean residence time; and 5) All slickensides are evidence of tectonism. Although the five factors of soil formation make pedochronology relatively imprecise, an evaluation of surficial units and fault features from a soil science perspective often is critical for estimating risk due to fault rupture hazard. Avoiding these common problems and misconceptions involving soils is essential to professional seismic investigations.