Mario Chavez

Pathogenic and Free-Living Protozoa Cultured from the Nasopharyngeal and Oral Regions of Dental Patients

Protozoa of the nose, mouth, and pharynx of 30 randomly chosen male caries patients at an odontological clinic of the National Autonomous University of Mexico, in Mexico City, were surveyed by culture from swabs. Culture tubes from swabs were observed every other day for 5 weeks. Pathogens found included Entamoeba histolytica, Naegleria fowleri, Acanthamoeba castellanii, A. culbertsoni, A. polyphaga, and Giardia lamblia. Such isolations of pathogens suggest that patients may be healthy carriers of cysts and trophic forms of protozoa responsible for several human diseases. Amoebae responsible for primary amoebic meningoencephalitis were well represented in the samples. Small pathogenic free-living amoebae have not been isolated before from healthy males in Mexico. Many species of free-living cyst-forming protozoa and some commensals without cysts were also cultured from swabs.

Neotectonic evolution of the Isthmus of Tehuantepec (southeastern Mexico)

Abstract The Isthmus of Tehuantepec of southeastern Mexico is located near the triple junction of the North American, Cocos and Caribbean plates. A neotectonic study, including fault tectonic analysis and study of sub-surface data, was performed in order to understand the tectonic evolution of this complex zone. The Plio—Quaternary fault pattern is described and the brittle structures are interpreted in terms of palaeostress orientations. We propose a model of tectonic evolution of the Isthmus of Tehuantepec since the Late Miocene (∼6 Ma) characterized by extensional tectonics. The present structure of the isthmus results from the superimposition of three distinct types of tectonism: (1) a tilting of the eastern Isthmus during the Late Miocene—Early Pliocene, along a major N—S-trending normal fault zone (the Isthmus Fault Zone), as a consequence of the deformation of the subducting slab of the Cocos plate along the subducted part of the Tehuantepec Fracture Zone; (2) the subsidence of the southern isthmus in relation to N—S extension, associated with the eastward displacement of the western Caribbean plate with respect to the southern North-American plate along the Polochic—Motagua fault system; and (3) extensions that develop in the northern half of the Isthmus related to the evolution of the passive margin of the Gulf of Mexico.

Low-Frequency 3D Wave Propagation Modeling of the 12 May 2008 Mw 7.9 Wenchuan Earthquake

The seismic potential of southern China is associated with the collision between the Indian and the Eurasian plates. This is manifested in the western Sichuan Plateau by several seismically active systems of faults, such as the Longmen Shan. The seismicity observed on the Longmen Shan fault includes recent events with magnitudes of up to 6.5, and the one of 12 May 2008 Mw 7.9 Wenchuan earth- quake. Herewith, as part of an ongoing research program, a recently optimized three- dimensional (3D) seismic wave propagation parallel finite-difference code was used to obtain low-frequency (≤ 0:3 Hz) 3D synthetic seismograms for the Wenchuan earthquake. The code was run on KanBalam (Universidad Nacional Autonoma de Mexico, Mexico) and HECToR (UK National Supercomputing Service) supercompu- ters. The modeling included the U.S. Geological Survey 40 × 315 km 2 kinematic description of the earthquakes rupture, embedded in a 2400 × 1600 × 300 km 3 phys- ical domain, spatially discretized at 1 km in the three directions and a temporal dis- cretization of 0.03 s. The compression and shear wave velocities and densities of the geologic structure used were obtained from recently published geophysical studies performed in the Sichuan region. The synthetic seismograms favorably compare with the observed ones for several station sites of the Seismological and Accelerographic Networks of China, such as MZQ, GYA, and TIY, located at about 90, 500, and 1200 km, respectively, from the epicenter of the Wenchuan event. Moreover, the com- parisons of synthetic displacements with differential radar interferometry (DinSAR) ground deformation imagery, as well as of maximum velocity synthetic patterns with Mercalli modified intensity isoseist of the 2008 Wenchuan earthquake, are acceptable. 3D visualizations of the propagation of the event were also obtained; they show the source rupture directivity effects of the Mw 7.9 Wenchuan event. Our results partially explain the extensive damage observed on the infrastructure and towns located in the neighborhood of the Wenchuan earthquake rupture zone.

Benchmark Study of a 3d Parallel Code for the Propagation of Large Subduction Earthquakes

Benchmark studies were carried out on a recently optimized parallel 3D seismic wave propagation code that uses finite differences on a staggered grid with 2ndorder operators in time and 4thorder in space. Three dual-core supercomputer platforms were used to run the parallel program using MPI. Efficiencies of 0.91 and 0.48 with 1024 cores were obtained on HECToR (UK) and KanBalam (Mexico), and 0.66 with 8192 cores on HECToR. The 3D velocity field pattern from a simulation of the 1985 Mexico earthquake (that caused the loss of up to 30000 people and about 7 billion US dollars) which has reasonable agreement with the available observations, shows coherent, well developed surface waves propagating towards Mexico City.

3D parallel elastodynamic modeling of large subduction earthquakes

The 3D finite difference modeling of the wave propagation of M>8 earthquakes in subduction zones in a realistic-size earth is very computationally intensive task. We use a parallel finite difference code that uses second order operators in time and fourth order differences in space on a staggered grid. We develop an efficient parallel program using message passing interface (MPI) and a kinematic earthquake rupture process. We achieve an efficiency of 94% with 128 (and 85% extrapolating to 1,024) processors on a dual core platform. Satisfactory results for a large subduction earthquake that occurred in Mexico in 1985 are given.