Geophysical characterization of soil deformation associated with earth fissures near San Marcial and Deming, New Mexico : Proc 4th International Conference on Land Subsidence, Houston, 12–17 May 1991 P271–280. Publ IAHS Press: Wallingford, 1991

Abstract

WILLIAM C. HANEBERG New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico, U.S.A. 87801 CHARLES B. REYNOLDS Charles B. Reynolds & Associates, 4409 San Andres Avenue NE, Albuquerque, New Mexico, U.S.A. 87110 IRENE B. REYNOLDS Geological Associates, 4409 San Andres Avenue NE, Albuquerque, New Mexico, U.S.A. 87110 ABSTRACT Seismic reflection profiles across two earth fissures show that sur-ficial strata are draped over a buried normal fault scarp beneath one fissure and over a buried channel deposit beneath the other. P-wave velocity contour plots also show anomalies near both fissures. Gravity profiles suggest the pres­ence of buried structures, but provide comparatively little information about the geologic setting of the fissures. Mechanical analysis of a compressible elas­tic soil layer draped over steps shows that the most tensile stresses will de­velop along the upper free surface if the steps are wide and along the base of the layer if the steps are narrow. We conclude that geometric details of buried irregularities control whether a fissure propagates upward from depth or down­ward from the ground surface. Moreover, details of stress trajectory fields can control the position of a fissure at the earth's surface relative to bedrock irreg­ularities at depth. INTRODUCTION Earth fissures are a geologic hazard in many parts of the southwestern United States, and can be associated with land subsidence due to fluid withdrawal, neotectonic activity, or mining. Be­yond the obvious danger of unexpected cavernous openings to humans and livestock, the ap­pearance of fissures can affect the siting and construction of highways, aqueducts, or hazardous waste storage facilities. In this paper we present the results of shallow seismic and gravity sur­veys across two earth fissures in New Mexico, one apparently related to aseismic fault move­ment and the other to groundwater overdraft, and then use a simple mechanical model for flex­ure of a compressible elastic layer to interpret our observations. We define earth fissures as narrow openings of considerable length and depth formed in un-solidated sediments, engineering soils, or regolith. Although the two earth fissures we describe here apparently began in zones of tension, we have not incorporated any genetic terms into our definition. There are two reasons for this. First, we believe that earth fissures, particularly if any amount of time has passed since the fissures appeared, axe as much the products of sur­face and subsurface erosion (e.g., Slaff et al., 1989) as they are products of fracturing. Second, we believe that there is much more to be gained by using purely descriptive observations as boundary conditions for mechanical models, from which genesis can be elucidated by comparing theory and observation, than by incorporating genetic terms into our descriptions. Holzer (1984, also see references therein) reviews occurrences and postulated origins of earth fissures associated with groundwater overdraft in Arizona, California, and Nevada. In general these fissures are associated both in space and in time with the dewatering of phreatic 271