Dora Carreón-Freyre

Correlation of near-surface stratigraphy and physical properties of clayey sediments from Chalco Basin, Mexico, using Ground Penetrating Radar

Detailed measurements of water content, liquid and plastic limits, electric conductivity, grain-size distribution, specific gravity, and compressibility were performed on the upper 7 m of the lacustrine sequence from the Chalco Basin, Valley of Mexico. Eight stratigraphic units consisting of alternating layers of clay, silt, sand, and gravel of volcanic origin are described for this sequence. The analysis of contrasts in the physical properties permitted to identify potential reflectors of radar waves: (i) change in the electrical conductivity at 0.4 m depth; (ii) increment in the clay and water content at 0.8 m depth; (iii) bimodal behavior of the water content at 1.3 m depth; (iv) increment in the sand content and decrease in water content at 2.6 m depth; and (v) the presence of a pyroclastic unit at 3.7 m depth. Radargrams with frequencies of 900 and 300 MHz were collected on a grid of profiles covering the study area. Correlation of radargrams with the reference section permitted the spatial interpolation of variations in the physical properties and the near-surface stratigraphy. Contrary to the expected in these clayey sediments, electric contrast enhanced by variations in water content and grain size permitted the recording of the near-surface sedimentary structures. Distinctive radar signatures were identified between reflectors. Furthermore, lateral discontinuities of the reflectors and their vertical displacements permitted the identification of deformational features within the sequence. D 2003 Elsevier B.V. All rights reserved.

Delineating the near-surface geometry of the fracture system affecting the Valley of Querétaro, Mexico: Correlation of GPR signatures and physical properties of sediments

We present a combined Ground Penetrating Radar (GPIR) and geotechnical investigation of a fault-fracture system that affects the Valley of Queretaro. The main fracture, named Falla Central (FC), is aligned with north-south striking regional faults suggesting that the buried fault scarps influences the geometry and propagation of fractures. Although the origin of fracl uring is closely related to geological factors, in the urbanized area of Queretaro the mechanical and hydraulic equilibrium in the <subsoil is also broken by anthropogenic activities, such as overexploitation of groundwater and overloading of high compressible grounds. In order to delineate the geometry of fracl uring we collected several vertical GPR profiles perpendicular to the main pattern of the fracture, with two different prospecting frequencies, 900 and 300 MHz. Processing of radar profiles was no needed because of high contrasting properties of the prospected sub-soil; we only used in some cases background removal, topographic correction or AGC amplitude corrtection so as to enhance stratigraphic related records. Perturbations in the radar signature help to identify changes in direction, width, and vertical displacement of fractures. Notably, in the radargrams from the northern part of the FC pertiirbations are concentrated near the fracture and vertical displlacement is higher, whereas in the southern part, the perturbations are distributed in a wider area in both sides of the FC. GPR profiles collected in areas with no vertical displacement along the trace of the FC recorded discontinuities suggesting that fraciures propagate from depth to surface. Detailed measurements of specific gravity, grain-size, plasticity, water cont(ent, and electrical conductivity were performed in samples collected in two shallow trenches in order to observe physical changes in the sedimentary sequence related to electrical contrasts. The near-surface stratigraphy consists of partially saturated fluvio-lacustrine granular and pyroclastic deposits. Dark clay with high plasticity and medium to firm consistency, grading to medium plastic silt with desiccation cracks characterizes the first meters. The analysis of the GPR profiles supported with the stratigraphic reco.rd permitted to identify variations in deformation of layers that can be explained in terms of differences of thickness and compressibility. Our results suggest that the evolution of the fracture is influenced by the conditions of deposition of the geological materials and by the anthropogenic activity. The analysis of clayey sediments suggests that the fracture is also controlled by the history of loads of the materials and the stratigraphic boundary between coarse and clayey sediments. Thus, the morphology of the near surface fracturing depends upon the spatial relation between regional fracturing and the depositional conditions of sediments. We propose that one of the main factors of propagation and orientation of the FC trace in the surface is the differential deformation because of the variation of vertical compaction of sediments. GPR profiles permitted to correlate vertical and lateral variations of the geological properties for a better understanding and forecasting of the distribution and behavior of fractures.

Holocene paleo-earthquakes recorded at the transfer zone of two major faults: The Pastores and Venta de Bravo faults (Trans-Mexican Volcanic Belt)

We present evidence of five late Holocene earthquake ruptures observed at two paleoseismological trenches in the Laguna Bani sag pond (Trans-Mexican Volcanic Belt, central Mexico). The trenches exposed two fault branches of the western termination of the Pastores fault, one of the major fault systems within the central Trans-Mexican Volcanic Belt. The site was studied by combining geomorphological and structural approaches, volcanic mapping, ground-penetrating radar, and paleoseismological analysis. The study revealed that coseismic surface rupture was noncharacteristic, and that the exposed fault branches had not always moved simultaneously. The fault tip has ruptured at least 5 times within the past 4 k.y., and the rupture events followed and preceded the deposition of an ignimbrite. The close temporal relationship of the seismic rupture with the volcanic activity of the area could be the result of volcanism triggered by faulting and its associated seismicity. The relatively high recurrence of seismic events (1.1–2.6 k.y.) and the noncharacteristic fault behavior observed at this tip of the Pastores fault suggest that the fault might have been active as a primary fault rupturing along segments of variable length or depth, and/or that the fault ruptured eventually as a secondary fault. The secondary ruptures would likely be related to earthquakes produced at major neighboring faults such as the Acambay fault, which moved during the 1912 Acambay earthquake, or the Venta de Bravo fault. A relatively large slip rate estimated for this fault branch (0.23–0.37 mm/yr) leads us to contemplate the possible connection at depth between the Pastores and the Venta de Bravo faults, increasing the maximum expected magnitude for central Mexico.