Quantum mechanical potential surfaces and calculations on minerals and molecular clusters

Recently, ab-initio quantum mechanical potential surfaces calculated for silicate hydroxyacid molecules were used to extract covalent potentials for use in mineral physics calculations (Lasaga and Gibbs 1987). The calculations showed that these potentials are capable of generating the structure and physical properties of silicate minerals. In this paper we explore in more detail the suitability of various covalent potentials in mimicking the topography of the ab-initio potential surfaces. We also extend the use of such quantum-derived potentials in generating the structures of hydroxyacid dimers, trimers, and pentamers of silicate tetrahedra and in studying the structure and the dynamical properties of minerals and glasses.     

Phytoplankton production and seasonal biomass variation of seagrass,Ruppia maritima L., in a tropical Mexican lagoon with an ephemeral inlet

Plankton metabolism andRuppia maritima biomass were measured seasonally during 1982–83 in El Verde Lagoon, a small coastal lagoon with an ephemeral inlet on the Pacific Coast of Mexico. Total net aquatic primary production was 521 g C m^?2 y^?1. The water column was slightly heterotrophic, with an annual P/R ratio of 0.89. Our analysis indicates that tropical and subtropical coastal lagoons with restricted or seasonal inlets have generally higher net aquatic primary productivity levels than lagoons with permanently open inlets. We hypothesize that this is due to retention of nutrients and plankton stocks during the dry season. The seasonal pattern of water column metabolism was related to rainfall and riverflow, with higher values generally occurring during the wet season. Net production and respiration were about three times lower during the 1982 dry season as compared to the 1983 dry season which received considerable rains due to abnormal climatic conditions. The biomass ofR. maritima ranged from zero to 620 g dry wt m^?2. Growth occurred only during the dry season and there were two distinct biomass peaks representing two separate crops. The second crop was heavily epiphytized with nitrogen-fixing algae. There was an apparent succession in dominance of water column productivity over the year, withRuppia dominating during the dry season and phytoplankton more important during the wet season.     

The objective determination of the instantaneous predominant frequency of seismic signals and inferences onQ of coda waves

A technique to detect spectrum variations versus time along seismic signals is applied to coda waves of local earthquakes (Friuli, Northern Italy). The technique consists of an autoregressive modeling and utilizes nonlinear spectral analysis where the spectrum of stochastic processes is estimated as the transfer function of the filter that whitens the process under analysis. This approach appears to be particularly well suited to those investigations where automatic measurements of the instantaneous frequency have to be carried out on digital data. The detection of variations of the instantaneous frequency along the coda allows computation of seismic-Q in the lithosphere and its frequency dependence: the result obtained is $$Q = 100f^{0.4} $$ which appears to be strongly consistent with that, based on the estimate of the coda amplitude decay in the band including the most significant frequencies of the signals under analysis.     

Anisotropy of magnetic susceptibility parameters: Guidelines for their rational selection

Twenty-eight parameters used to characterize measurements of the anisotropy of magnetic susceptibility are compared theoretically in this work by introducing the concept of the field of susceptibility tensors, which allows the representation of parameters as families of lines in a plane. It is demonstrated that the foliation and lineation parameters are but a special case of the shape parameters, implying that the resolution of these two rock fabric elements using AMS measurements alone is more an artifact of the numerical range of definition of some parameters than a quantification of two physically independent features. Also, it is shown that parameters presumably of the same type do not necessarily yield equivalent interpretation of results in a qualitative sense, and therefore, caution should be strongly exercised when parameters are to be selected. Paramters quantifying the degree of anisotropy are, in general, equivalent to each other because of the very small departure observed in natural rocks from the isotropic case. However, a final consideration of the possible ability to differentiate rock types and a convenient range of values allowing expression of the degree of anisotropy in a well-defined percentage are pointed out as the main factors to be considered before selecting one parameter within this class.     

Seismic activity related to the 1991 eruption of Colima Volcano,Mexico

Ten years after the last effusive eruption and at least 15 years of seismic quiescence, volcanic seismic activity started at Colima volcano on 14 February 1991, with a seismic crisis which reached counts of more than 100 per day and showed a diversity of earthquake types. Four other distinct seismic crises followed, before a mild effusive eruption in April 1991. The second crisis preceded the extrusion of an andesitic scoriaceous lava lobe, first reported on 1 March; during this crisis an interesting temporary concentration of seismic foci below the crater was observed shortly before the extrusion was detected. The third crisis was constituted by shallow seismicity, featuring possible mild degassing explosion-induced activity in the form of hiccups (episodes of simple wavelets that repeat with diminishing amplitude), and accompanied by increased fumarolic activity. The growth of the new lava dome was accompanied by changing seismicity. On 16 April during the fifth crisis which consisted of some relatively large, shallow, volcanic earthquakes and numerous avalanches of older dome material, part of the newly extruded dome, which had grown towards the edge of the old dome, collapsed, producing the largest avalanches and ash flows. Afterwards, block lava began to flow slowly along the SW flank of the volcano, generating frequent small incandescent avalanches. The seismicity associated with the stages of this eruptive activity shows some interesting features: most earthquake foci were located north of the summit, some of them relatively deep (7–11 km below the summit level), underneath the saddle between the Colima and the older Nevado volcanoes. An apparently seismic quiet region appears between 4 and 7 km below the summit level. In June, harmonic tremors were detected for the first time, but no changes in the eruptive activity could be correlated with them. After June, the seismicity decreasing trend was established, and the effusive activity stopped on September 1991.     

Modelling the spatial and temporal distribution of rainfall intensity at local scale

Summary Accurate estimates of rainfall intensity distribution with high temporal and spatial resolution are necessary in most urban hydrological studies, such as planning, simulation or control of sewer networks. Traditionally, these data are obtained from intensity-duration-frequency (IDF) curves at sites with long rainfall intensity time-series, however, little attention is given to the spatial features of precipitation. In this paper, a mathematical model of a local scale storm that takes account of the spatial variability of rainfall and rain-cell movement is proposed. The model has been calibrated with a dense network of raingauges and a long rainfall intensity timeseries (60 years) and its parameters have been calculated for convective storms of return periods up to 15 years with their most frequently-observed rain-cell velocities (1 to 4 m/s).This work has been supported by the DGICYT, Project NAT91-0596.With 6 Figures     

Association of subsurface radon changes in Alaska and the northeastern United States with earthquakes

Changes of radon concentration in the ground have been reported to correlate with seismic activity in many places over widely varying distances from the earthquake epicenters, sometimes over 100's and occasionally over 1000's of km depending on the magnitude M of the earthquakes. For proper use of such correlations in forecasting the location and size of impending earthquakes, it would be useful to establish a relation between M and the maximum distance x_M at which radon can be significantly altered by preseismic effects.We have monitored radon in the ground continuously at Blue Mountain Lake, NY starting Dec. 3. 1975 and at three locations in Alaska—Icy Bay, Yakataga, and Sand Point (starting Nov. 12, 1979; March 5, 1980; and June 29, 1980 respectively). Sample correlations from visual examination of the recent radon record and tentative use of a proposed scaling relation show a number of possible earthquake-related signals at these locations. The data are compatible with scaling relations that were derived from two separate models of premonitory elastic strains. In this work x_M = 10^0.48M, where x_M is in units of km and M ≥ 3. Since tilt and strain signals correlate similarly with magnitude and distance, it is likely that most earthquake-related radon signals are mechanically induced.     

Convection of geothermal fluids in the timanfaya volcanic area (Lanzarote,Canary Islands)

Summary A mathematical model has been derived to study the superficial thermal anomalies to be found in Lanzarote (605°C at 13 m depth) in association with the convection of geothermal fluids. The model is valid for a wide range of conditions, in particular for those found beneath the Timanfaya volcano (active between 1730 and 1736). Geological and geophysical data suggest that the heat source is related to a cylindrical magma body with a radius of 200±100 m and a top temperature of 850±100°C at a depth of 4±1 km.Energy is transported through fractures by magmatic volatiles and/or by water vapour coming from a deeply located water table: in such a convection system, a fluid flow of 10 l/m² day, which corresponds to a thermal flux of 130 W/m², is sufficient to explain the temperature anomalies observed at the surface. The relationships between gas flow and the surface temperatures, as well as the thermal gradients in the conducting fracture are also discussed.     

Seismic Q for the lithosphere of the Montenegro region (Yugoslavia): frequency,depth and time windowing effects

The availability of accelerometric data for the Montenegro earthquake of 15th April 1979 makes it possible to investigate seismic Q of the lithosphere in that region, in particular, its dependence on frequency, on the depth reached by seismic waves, and on the length of time windows in which signals are processed. Two different spectral methods, S phase energy ratio and coda envelope decay, are applied, respectively, to direct and scattered shear waves. Similar results are obtained using different portions of the recordings, i.e., coda waves for the envelope decay fit and the S wave train, with a significant duration of ～ 10 s, for the energy ratios. The same apparent Q (Q ～ 40 f, where f is the frequency expressed in Hz) that is found for other neighbouring central Mediterranean regions (e.g., Ancona, on the central Italian Adriatic coast; Valnerina, in the central Apennines; Irpinia, in the southern Apennines) is also found for the southern Yugoslavian coast, in the band 1–25 Hz up to a maximum range of ～ 120 km from the focus. This strong frequency dependence is probably connected with the type of small-scale heterogeneity and the same geological age and level of tectonic activity peculiar to all these seismotectonic areas.In order to compare the apparent Q of the whole S wave train, ～ 10 s long, with the (intrinsic) apparent Q of the single direct S wave (usually 1 s or less), the maximum entropy method is applied in the energy spectrum computation for shorter wave trains. The use of shorter time windows does not reveal any significant variation in the tendency of Q to increase linearly with frequency as the length of the time window containing the sample of the S waves decreases. This seems to indicate that scattering-dependent Q is generally inseparable from intrinsic Q in the lithosphere when estimates based on variations with distance of the seismic signal spectrum are used. While the type of linear growth with frequency does not seem to undergo any variations (it remains of the Q = qf type), the data show there are a considerable decrease in the coefficient of proportionality Q with decreasing duration of the window of S waves analysed, probably as a result of variations in seismic attenuation with depth.