Three Pleistocene glaciations and two Holocene Neoglacial advances occurred on volcano Ajusco in central Mexico. Lateral moraines of the oldest glaciation, the Marqués, above 3250 m are made of light-gray indurated till and are extensively modified by erosion. Below 3200 m the till is dark red, decomposed, and buried beneath volcanic colluvium and tephra. Very strongly to strongly developed soil profiles (Inceptisols) have formed in the Marqués till and in overlying colluvia and tephra. Large sharp-crested moraines of the second glaciation, the Santo Tomás, above 3300 m are composed of pale-brown firm till and are somewhat eroded by gullies. Below 3250 m the till is light reddish brown, cemented, and weathered. Less-strongly developed soil profiles (Inceptisols) have formed in the Santo Tomás till and in overlying colluvia and tephra. Narrow-crested moraines of yellowish-brown loose till of the third glaciation, the Albergue, are uneroded. Weakly developed soil profiles (Inceptisols) in the Albergue till have black ash in the upper horizon. Two small Neoglacial moraines of yellowish-brown bouldery till on the cirque floor of the largest valley support weakly developed soil profiles with only A and Cox horizons and no ash in the upper soil horizons. Radiocarbon dating of organic matter of the B horizons developed in tills, volcanic ash, and colluvial volcanic sand includes ages for both the soil-organic residue and the humic-acid fraction, with differences from 140 to 660 yr. The dating provides minimum ages of about 27,000 yr for the Marqués glaciation and about 25,000 yr for the Santo Tomás glaciation. Dates for the overlying tephra indicate a complex volcanic history for at least another 15,000 yr. Comparison of the Ajusco glacial sequence with that on Iztaccíhuatl to the east suggests that the Marqués and Santo Tomás glaciations may be equivalent to the Diamantes glaciation First and Second advances, the Albergue to the Alcalican glaciations, and the Neoglacial to the Ayolotepito advances. 相似文献
How atmospheric and oceanic processes control North American precipitation variability has been extensively investigated, and yet debates remain. Here we address this question in a 50 km-resolution flux-adjusted global climate model. The high spatial resolution and flux adjustment greatly improve the model’s ability to realistically simulate North American precipitation, the relevant tropical and midlatitude variability and their teleconnections. Comparing two millennium-long simulations with and without an interactive ocean, we find that the leading modes of North American precipitation variability on seasonal and longer timescales exhibit nearly identical spatial and spectral characteristics, explained fraction of total variance and associated atmospheric circulation. This finding suggests that these leading modes arise from internal atmospheric dynamics and atmosphere-land coupling. However, in the fully coupled simulation, North American precipitation variability still correlates significantly with tropical ocean variability, consistent with observations and prior literature. We find that tropical ocean variability does not create its own type of atmospheric variability but excites internal atmospheric modes of variability in midlatitudes. This oceanic impact on North American precipitation is secondary to atmospheric impacts based on correlation. However, relative to the simulation without an interactive ocean, the fully coupled simulation amplifies precipitation variance over southwest North America (SWNA) during late spring to summer by up to 90%. The amplification is caused by a stronger variability in atmospheric moisture content that is attributed to tropical Pacific sea surface temperature variability. Enhanced atmospheric moisture variations over the tropical Pacific are transported by seasonal mean southwesterly winds into SWNA, resulting in larger precipitation variance.
Volcanic eruptions may create a wide range of risks in inhabited areas and, as a consequence, major economic damage to the
surrounding territory. An example of volcanic hazard was given between 1998 and 2001 by Mt. Etna volcano, in Italy, with its
frequent paroxysmal explosive activity that caused more than a hundred fire-fountain episodes. In the period January–June
2000, in particular, 64 lava fountains took place at the Southeast Crater. During the most intense explosive phase of each
episode, a sustained column often formed, reaching up to 6 km above the eruptive vent. Then, the column started to expand
laterally causing more or less copious tephra fallout on the slopes of Etna; ash and lapilli, therefore, constituted a serious
danger for vehicular and air traffic. A software and hardware warning system was developed to mitigate the volcanic hazard
indicating the areas affected by potential ash and lapilli fallout. The alert system was mainly based on the good correspondence
between the pattern of volcanic tremor amplitude and the evolution of explosive activity. When a fixed tremor threshold was
exceeded, a semiautomatic process started to send faxes to Civil Defence and Municipalities directly affected by tephra fallout,
together with information on wind directions from the Meteorological Office. The application of this methodology, during the
last 14 eruptive episodes in 2000 and the 14 events occurred in 2001, demonstrated the good correspondence between the forecasts
on the areas affected by tephra fallout and the effective tephra distribution on land. Despite the integrity of the performance
provided by the alert system, small discrepancies occurred in the technical procedure of alerting, for which possible solutions
have been discussed. The improvement of this type of system, could become basic for the Etnean region and be proposed for
similar volcanic areas throughout the world. 相似文献
We develop new approaches to calculating 30-year probabilities for occurrence of moderate-to-large earthquakes in Italy. Geodetic
techniques and finite-element modelling, aimed to reproduce a large amount of neotectonic data using thin-shell finite element,
are used to separately calculate the expected seismicity rates inside seismogenic areas (polygons containing mapped faults
and/or suspected or modelled faults). Thirty-year earthquake probabilities obtained from the two approaches show similarities
in most of Italy: the largest probabilities are found in the southern Apennines, where they reach values between 10% and 20%
for earthquakes of MW ≥ 6.0, and lower than 10% for events with an MW ≥ 6.5. 相似文献
The Emilia, May–July 2012, earthquake hit a highly industrialized area, where some tens thousands industrial buldings, mainly single storey precast structures, are located. Due to the likelihood of strong after shocks and the high vulnerability of these structures, the authorities first asked for a generalized seismic retrofit after the strong shakings of May 20th. In order to accelerate community recovery, this requirement was later loosened, leaving out the buildings which had undergone a strong enough shaking without any damage; the strong enough shaking was defined with reference to the ultimate limit state design earthquake. To the authors’ knowledge, it is the first time that the information on the earthquake intensity and structural damage is used for such a large scale post earthquake simplified safety assessment. In short, the earthquake was used as large experimental test. This paper shows the details of the models and computations made to identify the industrial buildings which have been considered earthquake tested and therefore not compelled to mandatory seismic retrofit. Since earthquake indirect (e.g. due to economic halt) costs may be as large the direct ones, or even larger, it is believed that this method may considerably lower the earthquake total costs and speed up the social and economic recovery of a community. 相似文献
Many volcanic eruptions are shortly preceded by injection of new magma into a pre-existing, shallow (<10 km) magma chamber,
causing convection and mixing between the incoming and resident magmas. These processes may trigger dyke propagation and further
magma rise, inducing long-term (days to months) volcano deformation, seismic swarms, gravity anomalies, and changes in the
composition of volcanic plumes and fumaroles, eventually culminating in an eruption. Although new magma injection into shallow
magma chambers can lead to hazardous event, such injection is still not systematically detected and recognized. Here, we present
the results of numerical simulations of magma convection and mixing in geometrically complex magmatic systems, and describe
the multiparametric dynamics associated with buoyant magma injection. Our results reveal unexpected pressure trends and pressure
oscillations in the Ultra-Long-Period (ULP) range of minutes, related to the generation of discrete plumes of rising magma.
Very long pressure oscillation wavelengths translate into comparably ULP ground displacements with amplitudes of order 10−4–10−2 m. Thus, new magma injection into magma chambers beneath volcanoes can be revealed by ULP ground displacement measured at
the surface. 相似文献
Here, we report the first continuous data of geochemical parameters acquired directly from the active summit crater of Vulcano. This approach provides a means to better investigate deep geochemical processes associated with the degassing system of Vulcano Island. In particular, we report on soil CO2 fluxes from the upper part of Vulcano, a closed-conduit volcano, from September 2007 to October 2010. Large variations in the soil CO2 and plume SO2 fluxes (order of magnitude), coinciding with other discontinuous geochemical parameters (CO2 concentrations in fumarole gas) and physical parameters (increase of shallow seismic activity and fumarole temperatures) have been recorded. The results from this work suggest new prospects for strengthening geochemical monitoring of volcanic activity and for improving the constraints in the construction of a “geochemical model”, this being a necessary condition to better understand the functioning of volcanic systems. 相似文献