Geophysical Exploration of Faults, Fissures, and Fractures at Four Sites in Mexicali, Baja California, Mexico

Wednesday, 17 December 2014
Octavio Lázaro-Mancilla1, Antonio Gonzalez-Fernandez2, Alma Contreras-Corvera1, Joann M Stock3, Delia Moreno-Ayala4, Jorge Ramirez-Hernandez1, Concepcion Carreon-Diazconti1, Dina A Larios Lopez5 and Jaime Reyes Lopez1, (1)Instituto de Ingeniería, Universidad Autonoma de Baja California, Ciencias de la Tierra y del Medio Ambiente, Mexicali, Mexico, (2)Centro de Investigación Científica y de Educación Superior de Ensenada, San Diego, CA, United States, (3)California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, (4)SIDUE, Cartografia, Mexicali, Mexico, (5)Ohio Univ, Athens, OH, United States
We conducted field geophysical measurements in areas in the City of Mexicali that are associated with geological faults, fissures, and fractures.

The study sites are:

1) Instituto Tecnologico de Mexicali

2) The buried trace of the Michoacan de Ocampo fault in the urban zone

3) Rio Nuevo

4) A site reported by Frez (2013) with ground rupture SW of Cerro Prieto

At Site 1, seismic reflection profiling used a cable with 24 geophones at 1 m spacing. The source was a 3.6 kg sledge hammer, with 3 impacts per shot point. 347 shot points at 2 m spacing provided 6 fold coverage along a straight line with minimal elevation changes. Sample rate was 2000/s, and record length 1 s; reflections were seen down to 0.3 s TWTT. Processing included: frequency filter, fk filter, predictive deconvolution, geometry, velocity analysis, NMO and stacking. Lateral changes in the seismic section are due to surface modification and/or the presence of faults.

At site 2, we measured 222Radon in 36 locations along 17 profiles across the fault, using inherent alpha spectrometry with a Durridge RAD7 detector. Each site was measured at a depth of 60 cm, with 31 five-minute readings in a 3 hour period, interspersed with 10 minute of background purge and 3 five-minute background measurements. In a profile parallel to the fault, 78% of the readings were > 100 pCi/L, confirming the presence of the fault along the swath surveyed.

At Site 3 we compiled observations of post-earthquake cracks, conducted reconnaissance, and measured some profiles using 100 MHz GPR. These observations showed that the cracks are associated with ground failure due to earthquake shaking.

At Site 4 our new 222Radon gas measurements complemented a pre-existing profile that had high 222Radon values lacking a structural explanation. Related to this we found that this region has two NW-SE trending features: a magnetic anomaly low of 360 nT (Evans, Summer and Castillo, 1972) and a graben reported by the Mexican Geological Survey in 2003.

Earthquake patterns and seismic studies in a wider area north of the border indicate a possible buried fault a few km west of the Imperial fault, as well as short NE-striking faults controlling basement structure. These results highlight some very useful methods for finding fault traces in a flat region covered with agricultural fields or disturbed by anthropogenic activity.