On The Continuity To The Northwest Of The Cerro Prieto Fault

RADON CONCENTRATION MEASUREMENTS BIASED TO CERRO PRIETO FAULT FOR VERIFY ITS CONTINUITY TO THE NORTHWEST OF THE MEXICALI VALLEY

O. Lázaro-Mancilla1, D. López2, J. Reyes-López1, C. Carreón-Diazconti1, J. Ramírez-Hernández1
1Departamento de Hidrología, Geofísica e Impacto Ambiental
Instituto de Ingeniería, Universidad Autónoma de Baja California, Campus Mexicali
2Department of GeologicalSciences, Ohio University
olazaro@iing.mxl.uabc.mx

Abstract

The need to know the exact location in the field of the fault traces in Mexicali has been an important affair due that the topography in this valley is almost flat and fault traces are hidden by plow zone, for this reason, the southern and northern ends of the San Jacinto and Cerro Prieto fault zones, respectively, are not welldefined beneath the thick sequence of late Holocene Lake Cahuilla deposits. The purpose of this study was to verify if Cerro Prieto fault is the continuation to the southeast of the San Jacinto Fault proposed by Hogan in 2002 who based his analysis on pre-agriculture geomorphy, relocation and analysis of regional microseismicity, and trench exposures from a paleoseismic site in Laguna Xochimilco,Mexicali. In this study, four radon (222Rn) profiles were carried out in the Mexicali Valley, first, to the SW-NE of Cerro Prieto Volcano, second, to the W-E along the highway Libramiento San Luis Río Colorado-Tecate, third, to the W-E of Laguna Xochimilco and fourth, to the W-E of the Colonia Progreso.

The Radon results allow us to identify in the Cerro Prieto profile three peaks where the valuesexceed 60 picocuries per liter (pCi/L), these regions can be associated to fault traces, one of them associated to the Cerro Prieto Fault (60 pCi/L) and other related with Michoacán de Ocampo Fault (239 pCi/L). The profile Libramiento San Luis Río Colorado-Tecate, shows four peaks above 200 pCi/L, the highest peak is related to the Michoacán de Ocampo fault. The profile of the Laguna Xochimilco,site used by Hogan et al., (2002), permit us observe three peaks above the 140 pCi/L, we can associate the peak of 184 pCi/L the Michoacán de Ocampo Fault, but none of them to the Cerro Prieto Fault. Finally in spite of the Colonia Progreso is the shortest profile with only five stations, it shows one peak with a value of 270 pCi/L that we can correlate with the Cerro Prieto Fault. The resultsof this study allow us to think in the possibility that the Michoacán de Ocampo Fault is the Continuation to the South of the San Jacinto Fault, not the Cerro Prieto Fault.


Introduction

The need to know the exact location in the field of the fault traces in the Mexicali Valley has been an important affair for geologist and earth scientists due that the topography there is almost flat andfault traces are hidden by plow zone that cover the thick sequence of late Holocene Lake Cahuilla deposits. For to solve this problem we have followed a two steps calibration procedure, first: we have used 100 MHz Ground Penetrating Radar images comparing them with trench logs (Thomas, 1965), both of the Colonia Castro site where the traces of Imperial fault are known, and second: we have usedan outdoor 222Rn Concentration profile comparing it with the GPR image of the same site. The results of this study permitted us relate the radon features profiles with those of GPR images where the fault traces are located (Lázaro-Mancilla, et al, 2008). After the calibration procedure we have used the lateral GPR images and 222Rn Concentration profile for to infer another traces.
The purpose ofthis study was to prove how good are our results using only 222Rn Concentration method for to verify if Cerro Prieto Fault is the continuation to the southeast of the San Jacinto Fault proposed by Hogan et al. (2002); they based their proposal on the results of three points analysis, first: the analysis of the New River trench site where is exposed a broad, diffuse zone of vertical...