New Mexico Geological Society Annual Spring Meeting — Abstracts

The San Marcial earth fissure, which appeared during a heavy rainstorm in August 1981, crosses Interstate 25 along the western edge of the Rio Grande valley about 50 km south of Socorro. The fissure was widened by erosion immediately upstream of an embankment for southbound Interstate 25, then under construction, suggesting that blockage of the arroyo may have contributed to its growth. On a large scale the trace of the fissure is straight, following a small, unnamed arroyo, and nearly 1500 m in length. Strike of the trace, which is not affected by topography, is nonhwest-southeast. Closer examination of 1:250 aerial photographs, taken one month after the fissure appeared, shows that the trace is composed of many short segments meters to tens of meters in length. Some segments are crudely en echelon, but with no consistent sense of stepping along the entire length of the trace. In places where erosion widened the fissure before the photographs were taken, non-overlapping en echelon segments seem to have coalesced to form larger segments. In places were erosion was not as severe and finer features are visible, deflection of segment tips agrees with published analyses of non-overlapping linear elastic crack interaction. The growth of separate, in some cases en echelon, segments into longer crack traces suggests to us that the fissure propagated upward from depth.

Strong seismic reflectors at two-way times of 0.12, 0.14, and 0.25 s define a graben approximately 300 m wide, which is bounded by steeply-dipping normal faults. These three reflectors define layers with P-wave velocities, inferred by previous investigators using seismic refraction surveys, of approximately 800 m/s from 0 to 48 m in depth, 1900 m/s from 48 to 67 m in depth, and 3650 m/s below 67 m in depth. There is no evidence of recent fault scarps along either of the graben margins, and weak reflectors at two-way times less than 0.10 s do not appear to be broken by faults. The average Bouguer gravity profile, calculated from five readings taken over two days at each seismic shotpoint, shows a -1 mgal anomaly over the nonheastern boundary of the graben, which coincides with both an 0.04 s vertical offset (to 0.29 s) of the 0.25 s reflector and a topographic depression within the arroyo. Halfwidth analysis of the anomaly gives a maximum depth of 175 m, or about halfway between the 0.14 and 0.25 s reflectors. Based upon this geophysical evidence, we believe that the gravity anomaly is the result of brecciation along a fault passing through the 3650 m/s layer, and that shallow, lower velocity layers were draped passively over deeper faults to form a small monocline along each side of the graben.

Location of the fissure within the graben contradicts existing models for the formation of fissures by concavedownward bending of thin alluvium layers over bedrock projections. These models predict that fissures should form by propagating downward from the ground surface in areas of concave-downward topography, where the surface is in tension. In contrast, the San Marcial fissure is located in an area where topography, seismic reflectors, and the gravity profile are all gently concave-upward to straight, where the ground surface should be in compression, and appears to have propagated upward. Analysis of elastic stresses and displacements developed in a two-dimensional gravity-loaded span, which we use as an analog for a soil layer lying over a bedrock graben, shows that minimum principal stress (taking compressive srresses to be negative) will be tensile to neutral throughout the lower portion of the span. Magnitude of the tension decreases with layer thickness, and will increase if outward-directed shear is applied to the base of the layer. For layers with length thickness ratios greater than about 10:1, which can be analyzed using elementary beam theory, the absolute value of fiber stress along the upper and lower boundaries will decrease with the cube of thickness. Tensile to neutral normal stress would decrease soil shear strength-as well as capacity to resist seepage forces-and we speculate that the San Marcial fissure must have originated in a zone of tension at depth and grew upward by piping. As piping continued, layer thickness would have decreased continuously, thereby increasing tensile stresses at shallower and shallower depths until the ground surface was broken and surface runoff continued to enlarge the fissure.