Shearing along faults and stratigraphic joints controlled by land subsidence
and hydraulic gradient in the Valley of Queretaro, Mexico
D. Carreon-Freyre, M. Cerca, G. Ochoa-Gonzalez, F. R. Zuñiga
Laboratorio de Mecánica de Geosistemas, Centro de Geociencias,
Universidad Nacional Autónoma de México (UNAM), Queretaro C.P., México
P. Teatini
Dept. of Civil, Environmental and Architectural Engineering,
University of Padova, Padova, Italy
Slip of nearly vertical faults or horizontal stratigraphic
joints has provoked the shearing of at least 16 well
casings in a period of over 10 years in the Valley of Queretaro
aquifer, Mexico. Evidence integrated from field observations,
remote surface-deformation monitoring, in-situ monitoring,
stratigraphic correlation, and numerical modeling indicate that
groundwater depletion and land subsidence induce shearing.
Two main factors conditioning the stress distribution and the
location of sheared well casings have been identified: (1) slip
on fault planes, and (2) slip on stratigraphic joints.
Additionally, the distribution of piezometric gradients may
be a factor that enhances shearing. Slip on faults can be generated
either by the compaction of sedimentary units (passive
faulting) or by slip of blocks delimited by pre-existing faults
(reactivation). Major piezometric-level declines and the distribution
of hydraulic gradients can also be associated with slip
at stratigraphic joints. Faults and hydraulic contrasts in the
heterogeneous rock sequence, along with groundwater extraction,
influence the distribution of the gradients and delimit the
compartments of groundwater in the aquifer. Analogue
modeling allowed assessment of the distribution of stress-strain
and displacements associated with the increase of the
vertical stress. Fault-bounded aquifers in grabens are common
in the central part of Mexico and the results obtained can be
applied to other subsiding, structurally controlled aquifer systems
elsewhere.
