Since the March–April 1982 eruption of El Chichòn volcano, intense hydrothermal activity has characterised the 1-km-wide summit crater. This mainly consists of mud and boiling pools, fumaroles, which are mainly located in the northwestern bank of the crater lake. During the period 1998–2000, hot springs and fumaroles discharging inside the crater and from the southeastern outer flank (Agua Caliente) were collected for chemical analyses. The observed chemical fluctuations suggest that the physico-chemical boundary conditions regulating the thermodynamic equilibria of the deep rock/fluid interactions have changed with time. The chemical composition of the lake water, characterised in the period 1983–1997 by high Na+, Cl−, Ca2+ and SO42− contents, experienced a dramatic change in 1998–1999, turning from a Na+–Cl−- to a Ca2+–SO42−-rich composition. In June 2000, a relatively sharp increase in Na+ and Cl− contents was observed. At the same time, SO2/H2S ratios and H2 and CO contents in most gas discharges increased with respect to the previous two years of observations, suggesting either a new input of deep-seated fluids or local variations of the more surficial hydrothermal system. Migration of gas manifestations, enhanced number of emission spots and variations in both gas discharge flux and outlet temperatures of the main fluid manifestations were also recorded. The magmatic-hydrothermal system of El Chichòn is probably related to interaction processes between a deep magmatic source and a surficial cold aquifer; an important role may also be played by the interaction of the deep fluids with the volcanic rocks and the sedimentary (limestone and evaporites) basement. The chemical and physical changes recorded in 1998–2000 were possibly due to variations in the permeability of the conduit system feeding the fluid discharges at surface, as testified by the migration of gas and water emanations. Two different scenarios can be put forward for the volcanic evolution of El Chichòn: (1) build-up of an infra-crater dome that may imply a future eruption in terms of tens to hundreds of years; (2) minor phreatic–phreatomagmatic events whose prediction and timing is more difficult to constrain. This suggests that, unlike the diminished volcanic activity at El Chichòn after the 1982 paroxistic event, the volcano-hydrothermal fluid discharges need to be more constantly monitored with regular and more frequent geochemical sampling and, at the same time, a permanent network of seismic stations should be installed. 相似文献
The low frequency oscillation of latent heat flux over the tropical oceans has been studied. The NCEP reanalyzed fields of
wind and humidity alongwith Reynolds SST are used to compute the instantaneous as well as monthly mean surface latent heat
fluxes (LHF) for the year 1999. The procedure of LHF computation is based on bulk method. Spectral analysis shows that significant
energy is contained in Madden Julian Oscillation band in the winds, SST, moisture and in the latent heat flux. The global
distribution of wind, humidity, SST and LHF oscillation on the time scale of 30–50 days are analyzed. Maximum amplitude of
oscillation on this time scale in all the above mentioned parameters were found over the Indian Ocean. The fluctuation of
surface wind speed and moisture controls the latent heat flux on this time scale. The fluctuation of SST on this time scale
does not seem to be important over most of the oceans. 相似文献
El Niño/Southern Oscillation (ENSO) is well known to have a worldwide impact, in particular on streamflows. As an illustration, we found a good correlation (~0.47, 5-month time delay) between the ENSO activity and the discharge of the main rivers in French Guiana. However, this calculation was made between climatic and hydrological processes at ‘isolated precise moments’. Using a new method of time series analysis, we considered the ENSO and Atlantic Ocean/river discharge correlation integrated over time. This method shows, with more than 99% confidence level, new possible delayed and combined influences between the phenomena (respectively ~20 and 40 months for ENSO and Atlantic Ocean influences). To cite this article: C. Gaucherel, C. R. Geoscience 336 (2004).相似文献
Fast-moving landslides are one of the most significant dangers deriving from slope instabilities. Landslides involving large volumes can develop in rock or debris avalanches with extreme mobility and enormous destructiveness. Nevertheless, a relevant number of casualties and damages derive from small, fast-moving landslides with flow-like behaviour.
The Las Colinas landslide occurred at Santa Tecla (El Salvador, Central America) during a strong earthquake. It slid off the northern flank of the Bálsamo ridge, and resulted in almost 500 casualties and can be considered one of the most destructive landslides ever known. Earthquake shaking was amplified by the rock mass and the steep ridge topography.
We collected original geological, geomorphological and geophysical data in the Cordillera del Bálsamo area. The involved materials, ranging from lapilli to tuff layers of different strength, have been mapped and characterized.
Slope stability analyses have been performed both under static and dynamic conditions through limit equilibrium and finite element methods.
Hazard zonation for this type of landslides requires the forecast of the movement velocity and final deposition area. We used a fully two-dimensional FEM model to simulate landslide spreading downslope. The developed code allows the use of different constitutive models and yield rules with the possibility to model and study internal deformation of the landslide mass, as well material entrainment and deposition. 相似文献
The El Ni?o Southern Oscillation (ENSO) is a cli- mate anomaly responsible for worldwide weather im- pacts ranging from droughts to floods. It is of scien- tific importance to clarify the influences of the 1997/98 El Ni?o event (for simplicity, named the ENSO) on the regional and global lightning activity. Goodman et al.[1] noticed that during the El Ni?o ma- ture phase from December 1997 to February 1998, the total lightning frequencies recorded by LIS, lightning days and lightning ho… 相似文献