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

Abstract:

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.