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Los lahares secundarios representan una de las mayores amenazas en volcanes activos o en estado de quietud, por lo tanto, es importante estimar la frecuencia, distribución y los mecanismos disparadores en diferentes volcanes para establecer escenarios de peligro. En este trabajo se analizó el lahar ocurrido el día 4 de febrero del 2010, en la barranca Nexpayantla, ubicada en el sector NW del volcán Popocatépetl. Para ello, se realizó un estudio geomorfológico y textural del depósito, así como de la precipitación y de imágenes de satélite tipo ASTER para delimitar el área de inundación y el mecanismo disparador. Los datos mostraron que este evento se generó por una precipitación de 100 mm/día e inició como una corriente fluvial, que se transformó en un flujo de escombros y finalizó como una corriente diluida. Su transformación, de corriente fluvial a flujo de escombros, se debió a la incorporación de sedimentos por erosión laminar y procesos de remoción en masa que ocurrieron en la barranca, asociados al mismo evento de precipitación. En contraste, su dilución a una corriente fluvial fue por la rápida sedimentación relacionada con cambios en la geometría del cauce, el alto contenido de troncos de árboles y el bajo contenido de material fino. La reproducción del evento mediante simulaciones numéricas permitió calcular que su profundidad pudo alcanzar 1.7 m. Al comparar la distribución del depósito en campo y del lahar en las imágenes de satélite, se observó que el depósito representa sólo el 68 % del área total de inundación del evento. Asimismo, el análisis de la lluvia indicó que una precipitación de igual magnitud a la que disparó el lahar Nexpayantla, tiene un periodo de retorno de 27 años. Finalmente, trabajos enfocados en la caracterización de lahares secundarios en el volcán Popocatépetl permitirán establecer escenarios que incluyan casos como el aquí estudiado. more...

Popocatepetl volcano produced 625 ash emissions to heights greater than1 km between 1994 and 2008. During this time, ash fall affected mostly populations within a 60 km radius around the volcano, a zone which includes 4.5 million people. We assess the effects of prolonged ash fall on human health in these areas between 1994 and 2008. To do this, we considered 94,000 non-infectious respiratory disease (NIRD) records from five health databases. This included the 98 municipalities where ash fall was frequent, plus two municipalities outside of the ash fall area which served as a control group. NIRD rates (cases/1 k inhabitants) from 1992, 2 years before the beginning of the eruptive period, tol 2008 were compared with mapped ash distributions during the same period. Wind dispersed 29% of these ash emissions to the northeast, 20% to the east, 17% to the southwest, 13% to the northwest, 9% to the southeast, and12% in other directions, producing more ash fall in the northeast and east sectors and higher NIRD rates. Results, tested with an analysis of variance (ANOVA) and covariance statistical analyses, show that changes in NIRD rates correlate with ash dispersion direction and the amount of fine ash deposited. During eruptive crises in 1994–2003 and 2005, NIRD rates increased from 2.6, reaching levels up to > 3 in municipalities where ash was sampled. In 2004 and 2006–2008 when ash plume frequency and ash concentration decreased (e.g., only two minor ash emissions in 2004), the NIRD rates remained unchanged. This was a result of the chronic effect on health of the preceding ash fall. We found that health impacts are principally related to the amount of ash, as well as the percentage of fine particles which in turn is also a function of the prolonged nature of the eruption. The fine ash from Popocatepetl is composed of up to 19.2% of particles smaller than 10 μm and up to 5.7% smaller than 2.5 μm and contains plagioclase crystals, lithic particles, glass, pyroxene crystals, cristobalite, and minor amounts of titanomagnetite and sublimates. more...

We present a Geographical-Information-System-based Risk Atlas (GIS-RA) of Mexico City, Mexico. The main purposes are to provide a local-government level tool for implementing preventive and remedial actions useful for land-use development plans and to strengthen the culture for disaster prevention. We identified the prevalent social risk to the more relevant hazards in Mexico City (CDMX): earthquakes, volcanic eruptions, floods, landslides, forest fires, and land subsidence. A total of 274 shape-file maps were generated in this project. Seismic hazard was estimated for return periods (RP) of 20, 125, 250, and 475 years. Three areas in central and northwestern CDMX were identified along the Younger Chichinautzin Monogenetic Volcanic Field with high probability of forming a new volcano. Subsidence is concentrated to the east and southeast of CDMX, where subsidence rates are among the highest worldwide. Flooding events were estimated for RP of 2, 5, 10, 50 and 100 years and the majority of them are concentrated in the central and northern sectors of the city. During the dry season (December-April), southern CDMX has very high probability of forest fire occurrence. There is high susceptibility of landslides on the west and southwest of the city. The goals of this GIS-RA are to: 1) improve the knowledge of the potential impact of local hazards; 2) provide elements for disaster prevention, mitigation, preparedness, and response; 3) benefit decision-makers with robust risk data; 4) provide information for land-use planning; and 5) support further research to reduce the impact of disasters. more...

An eruption in the Younger Chichinautzin Monogenetic Field (YCMF) represents a major volcanic hazard to Mexico City, one of the most important cultural and financial centres in Latin America where >20 million people live. The birth of a new volcano in this field could severely impact the economic and social activity of the country. To identify where and when the next eruption in the YCMF could occur, the temporal and spatial distribution of past events were analyzed, assuming that future activity will follow a similar tendency. We reconstructed ages for the non-dated monogenetic volcanoes based on cone morphometric analyses correlated with published radiometric dates (R2 = 0.901). A Grid Probability Analysis (GPA) was applied for the first time in a monogenetic volcanic field to calculate the spatio-temporal probability of a new volcano forming. The field was divided into 96 cells of 5 × 5 km, which is the maximum observed distance between volcanoes. The recurrence rate and probability of a new volcano forming was then calculated for each cell using the estimated ages and a spatio-temporal probability map was constructed for the field. There are at least three areas with higher probability, concentrated in the central and northwestern parts of the study area. These areas are close to the volcanoes with the most recent eruptions and an eruption in these zones could seriously impact Mexico City and nearby areas as did Xitle and Chichinautzin volcanoes ~2000 years ago. more...

The unique nature of monogenetic volcanism has always raised questions about its origin, longevity and spatial distribution. Detailed temporal and spatial boundaries resulted from a morphometric study, mapping, relative dating, twenty-four new 40Ar/39Ar dates, and chemical analyses for the Sierra Chichinautzin, Central Mexico. Based on these results the monogenetic cones were divided into four groups: (1) Penon Monogenetic Volcanic Group (PMVG); (2) Older Chichinautzin Monogenetic Volcanic Group (Older CMVG); (3) Younger Chichinautzin Monogenetic Volcanic Group (Younger CMVG) and (4) Sierra Santa Catarina Monogenetic Volcanic Group (SSC). The PMVG cover the largest area and marks the northern and southern boundaries of this field. The oldest monogenetic volcanism (PMVG; 1294 ± 36 to 765 ± 30 ka) started in the northern part of the area and the last eruption of this group occurred in the south. These basaltic-andesite cones are widely spaced and are aligned NE-SW (N60°E). After this activity, monogenetic volcanism stopped for ~527 ka. Monogenetic volcanism was reactivated with the birth of the Tezoyuca 1 Volcano, marking the beginning of the second volcanic group (Older CMVG; 238 ± 51 to 95 ± 12 ka) in the southern part of the area. These andesitic to basaltic andesite cones plot into two groups, one with high MgO and Nb, and the other with low MgO and Nb, suggesting diverse magma sources. The eruption of the Older CMVG ended with the eruption of Malacatepec volcano and then monogenetic volcanism stopped again for ~60 ka. At ~35 ka, monogenetic volcanism started again, this time in the eastern part of the area, close to Popocatepetl volcano, forming the Younger CMVG ( The SSC (132 ± 70 to 2 ± 56 ka) is located in the northern part of the area, in the old Chalco Lake and is separated by faults from the rest of the volcanic groups as a different range. The SSC formed closely spaced basaltic andesites to andesitic cones oriented NE-SW (N70°E). The SSC samples have high Zr, P2O5, and Nb, indicating a different magma source. The northern and southern spatial boundaries of the field (the surface area with monogenetic volcanoes) became smaller with time: 78 km for PMVG, ~40 km for the Older CMVG and ~25 km for the Younger CMVG, concentrating the volcanoes in the central part of the area. The alignment of the cones changed progressively from NNE-SSW to NE-SW to E-W through the time, associated with the changes in the stress field which appears also to have caused the gaps. Results suggest that the Sierra Chichinautzin is actually four different volcanic fields, some partially overlapping, instead of one as previously considered. The differences in age, emplacement orientation and geochemistry support this conclusion. more...