The Cuyo basin Argentina, originated in the Triassic as a continental
rift, has been subject to strong compression during the Cenozoic.
In this paper an estimate of the amount of crustal extension in the
Cuyo basin from gravity data modeling is presented. Available seismic
and magnetic information aided in modeling. A 3D model of the topography
of the lower crust-upper mantle interface, derived from the re-gional
Bouguer anomalies of wavelength ³ 135 km (gravity effect of the basin's
fill removed), allowed to estimate of a maximum mean value for the
stretching of around 16 km, with an extension coefficient of 1.08.
The primitive thinning persists at the present time, although diminished
by isostatic readjustments and the strong Cenozoic deformation which
has particularly affected the northern part of the basin.
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El
sismo de Armenia, Colombia (Mw = 6.2) del 25 de enero de 1999. Un
análisis telesísmico de ondas de cuerpo, observaciones de campo y
aspectos sismotectónicos
Hugo Monsalve-Jaramillo
Carlos Alberto Vargas-Jimenez
The 25th January, 1999, Armenia (Colombia) Earthquake, (18:19:16
UT), was originated by a left lateral strike-slip fault with normal
component, rupture lenght about 11.7 km and wide about 10.6 km, with
an estimated total rupture area of 124 km2. These studies are necessary
and of great importance to understand geodynamic processes, the identification
of seismogenic sources and in the territorial planning, in terms of
the mitigation of regional and local seismic risks. A total of 391
very well located aftershocks, were obtained, most of them distributed
in a range depth between 9 to 20 km, for the rupture area determination.
The event can be considered as a single source with a SE-NW propagation
rupture. The aftershock distribution, tectonic map, and Richter's
Parameter b Study, indicate a single rupture source, that involved
two very near faults to the Cordoba Fault, that moved after the main
event. The rupture process was modelled using teleseismic body waves
formal inversion, with broad band slip records with wave range from
5 to 0.001 Hz. The fault mechanism is consistent with a left lateral
strike-slip fault with a normal component, a dip d = 67.3° to the
east, and an azimuth f = 356° ± 10° and a slip angle l = -33.8°. An
H centroid depth about 18.6 km was obtained, with a normalized rms
about 0.51 mm. The seismic moment was calculated 2.1x1025 dinacm,
and an estimated dislocation Du = 0.484 m. The radiated energy for
nine aftershocks was estimated at about 3.0 x 1013 £ M0 £ 1.13 x 1018
N-m, estimated with the SH wave and the Brune's method.
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Inversión
de la estructura del Cráter de Chicxulub empleando Métodos de Optimización
Global
Carlos Ortiz Alemán
Jaime Urrutia Fucugauchi
Arturo Iglesias Mendoza
Magnetic field data over the Chicxulub impact crater in Yucatán
(Mexico) are modeled in this work by using global inversion methods
(simulated annealing and genetic algorithms). Aeromagnetic data over
the structure reveal three well-defined concentric patterns, with
a central 40-km diameter zone of high amplitude anomalies. Main magnetic
anomalies are associated with the existence of a melt sheet, upper
breccias and a central uplift, which present 3 or 4 orders of magnitude
contrasts relative to the surrounding carbonate units. The limited
number of magnetic property measurements has restricted our ability
to determine the characteristics and distribution of other major structural
elements of the Chicxulub crater. All available geologic and geophysical
data are used as a priori information in the construction of an input
initial guess for inverse modeling. Results indicate that the melt
sheet extends to a radial distance of ~45 km from the center of the
structure and the central uplift is irregular in shape and protrudes
into the melt pool. The sources of anomalies forming the melt pool
have depths ranging between 2 and 4 km. The magnetic anomaly sources
in the central uplift zone are located in the range from 3 to 8 km
depth, with dominant contributions from an apparent large body forming
the struc-tural uplift.
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Vientos
troposféricos observados con el radar VHF en Piura-Perú: condiciones
normales versus “El Niño 1997-1998”
Luis Alberto Flores Caucha
Jacqueline La Madrid Olivares
Jorge Luis Chau Chong Shing
In this paper, we summarize the statistical characteristics of the
horizontal winds observed with the Piura VHF radar between 1991 and
2000. The behavior of horizontal wind is shown. The information, analyzed
as a function of the height and hour for different seasons, is presented
and discussed for both normal conditions, (periods without El Niño)
and for the 1997-1998 event. In addition, the horizontal wind characteristics
are shown for the two seasons previous to El Niño 1997-1998.
Our results suggest the existence of two potential precursors of this
phenomenon.
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Modelo
funcional del proceso de alteración del agua subterránea en las cercanías
del basurero de Santa Catarina, Chalco, México
Tomás González-Morán
The current concern to study and understand environmental problems
is reflected in the approach of this paper. Geophysics, together with
hydrogeology, provides information leading to the understanding of
the alterations in groundwater quality.
This research was carried out in the sub-basin of Chalco, located
SE of Mexico City, which is an important part of the basin of Mexico.
Its geological conforma-tion is similar to the rest of the basin.
Knowledge of the main lithostratigraphic units of the basin of Mexico,
obtained through interpretation of vertical electrical sounding (VES's)
and from interpretation provided by lithologic columns of wells has
allowed an in-depth study of the subsurface.
One of the greatest landfills is located in this region. Chemical
analyses have provided knowledge on the quality of the groundwater,
showing evidence of its deterioration. The wells in Santa Catarina
and those in the vicinity of the landfill of Santa Catarina show alterations
in diverse physico-chemical parameters. The most probable cause is
the lixiviations from the Santa Catarina's landfill.
Analysis and interpretation of a geoelectrical section obtained from
verti- cal electrical sounding (VES's) on the Mexico-Puebla highway,
near the Santa Catarina landfill, allowed identification, within a
complex volcanic environment of a 9 W-m conductive zone associated
with a polluting plume generated at the final solid waste disposal
site at Santa Catarina.
