Regional amplification of ground motion in central Mexico. Results from coda-length magnitude data and preliminary modeling

Seismic ground motion in central Mexico is amplified relative to ground motion observed at the same epicentral distance along the Pacific Coast in a frequency band that includes destructive ground motion at Mexico City. Although several hypothesis have been advanced, at present there is no generally accepted explanation of such amplification. We have analyzed coda-length magnitude data reported by Servicio Sismológico Nacional (SSN) for events recorded during 1993 to increase our understanding of the spatial distribution of this phenomenon. Our results indicate that regional amplification: (a) is detected by magnitude residual computed at each station, relative to the average of SSN network;and (b) is likely related to the crustal structure under the central portion of the Transmexican Volcanic Belt (TMVB). Finally, preliminary wave propagation modelling (using SH wave, finite difference method) suggests that crustal heterogeneity is a possible cause of regional amplification. However, if this is so, it is required that both geometry and velocity distribution vary between the coast and Mexico City.     

The HartX-synthesis: An experimental approach to water and carbon exchange of a Scots pine plantation

Summary In May 1992 during the interdisciplinary measurement campaign HartX (Hartheim eXperiment), several independent estimates of stand water vapor flux were compared at a 12-m high Scots pine (Pinus silvestris) plantation on a flat fluvial terrace of the Rhine close to Freiburg, Germany. Weather during the HartX period was characterized by ten consecutive clear days with exceptionally high input of available energy for this time of year and with a slowly shifting diurnal pattern in atmospheric variables like vapor pressure deficit. Methods utilized to quantify components of stand water flux included porometry measurements on understory graminoid leaves and on pine needles and three different techniques for determining individual tree xylem sap flow. Micrometeorological methods included eddy covariance and eddy covariance energy balance techniques with six independent systems on two towers separated by 40 m. Additionally, Bowen ratio energy balance estimates of water flux were conducted and measurements of the gradients in water vapor, CO₂, and trace gases within and above the stand were carried out with an additional, portable 30 m high telescoping mast.Biologically-based estimates of overstory transpiration were obtained by up-scaling tree sap flow rates to stand level via cumulative sapwood area. Tree transpiration contributed between 2.2 and 2.6 mm/day to ET for a tree leaf area index (LAI) of 2.8. The pine stand had an understory dominated by sedge and grass species with overall average LAI of 1.5. Mechanistic canopy gas exchange models that quantify both water vapor and CO₂ exchange were applied to both understory and tree needle ecosystem compartments. Thus, the transpiration by graminoid species was estimated at approximately 20% of total stand ET. The modelled estimates for understory contribution to stand water flux compared well with micrometeorologically-based determinations. Maximum carbon gain was estimated from the canopy models at approximately 425 mmol/(m²day) for the tree needles and at 100 mmol/(m²day) for the understory. Carbon gain was suggested by the modelling analysis to remain relatively constant during the HartX period, while water use efficiency in carbon fixation increased with decreasing vapor pressure deficit. Biologically- and micrometeorologically-based estimates of stand water flux showed good general agreement with variation of up to 20% that reflects both errors due to the inherent assumptions associated with different methods as well as natural spatial variability in fluxes. The various methods support a reliable estimate of average ET from this homogeneous canopy during HartX of about 2.6 mm/day (a maximum of about 3.1 mm/day) with an insignificant decreasing trend in correlation with decreasing vapor pressure deficit and possibly soil moisture.Findings during HartX were embedded in local scale heterogeneity with greater roughness over the forest and much higher ET over the surrounding agricultural fields which results in weak but clearly existant circulation patterns. A variety of measurements were continued after the HartX campaign. They allow us to extend our findings for six months with changing environmental conditions, including shortage of soil moisture. Hydrological estimates of soil water extractions and micrometeorological estimates of ET by the one-propeller eddy covariance (OPEC) system were in very good agreement, supporting the use of this robust eddy covariance energy balance technique for long-term monitoring.With 5 Figures     

The available energy over a Scots Pine plantation: What's up for partitioning?

Summary Errors influencing the calculation of the available energy above a forest are discussed. The main emphasis is put on the investigation of the problems affecting the measurement of net radiation. This is done by utilizing the data set of a surface energy balance experiment which was conducted in and above a Scots Pine plantation from May 11 to 22, 1992. During that Hartheimer Experiment (HartX) there were redundant measurements of net radiation using five different radiometers of three different designs. The initially fair agreement between the net radiometer readings was considerably improved by introducing different responsivities for the shortand longwave range. The mean deviations to the relative net radiation after correction vary between — 1.4 and 1.2 Wm^–2 with standard deviations between ± 5.4 and 6.6 Wm^–2. The total error referring to the available energy is estimated to be up to ± 36 Wm^–2 (± 6%) around midday decreasing to 10 Wm^–2 during nighttime.With 3 Figures     

Explicit expression and regularization of the harmonic reproducing kernels for the earth's ellipsoid

The reproducing kernel functions and the integral formulae involving the solutions of the fixed gravimetric boundary value problems for the earth's ellipsoid are investigated in a suitable polar coordinate system defined on the boundary ellipsoid. The infinite series expressions of the reproducing kernel functions are represented explicitly up to the square of the first excentricity of the boundary surface. The obtained results show that the reproducing kernel functions for the earth's ellipsoid are, in contrast to the case of the spherical boundary, inhomogeneous and anisotropic. Moreover, the anisotropy is stronger near the pole. Subsequently the kernel functions are regularized in order to overcome the weak singularity of the integrals.     

Chaos in a quartic dynamical model

We study the orbital characteristics of a time independent, two dimensional quartic dynamical model with two exact periodic orbits that displays always closed zero velocity curves. It is shown that the stability of the periodic orbits depends on the value of the coupling parameter . Computer calculations suggest that the degree of stochasticity is small for the values of in the range 1<<3 while it grows rapidly when >3. We also compute the Lyapunov characteristic exponents for different values of the coupling parameter.     

The macroseismic field of the Balkan area

A catalogue of 356 macroseimic maps which are available for the Balkan area was compiled, including information on the source parameters of the corresponding earthquakes, the macroseismic parameters of their strength and their macroseismic field. The data analysis of this catalogue yields new empirical relations for attenuation, which can be applied for the calibration of historical events, modelling of isoseismals and seismic hazard assessment. An appropriate analysis allowed the separation and estimation of the average values of the geometrical spreading, n, and anelastic attenuation factor, c, for the examined area which were found equal to –3.227 ± 0.112 and –0.0033 ± 0.0010. Scaling relations for the focal macroseismic intensity, I_f, and the epicentral intensity I₀, versus the earthquake moment magnitude were also determined for each Balkan country. A gradual decrease of the order of 0.5 to 1 intensity unit is demonstrated for recent (after 1970) earthquakes in Greece. Finally the depths of the examined earthquakes as they robustly determined (error <5 km) on the basis of macroseismic data were found to have small values ( 10 km). However large magnitude earthquakes show higher focal depths ( 25 km), in accordance with an increase of the seismic fault dimensions for such events.     

Flux agreement above a Scots pine plantation

Summary The surface energy exchange of 12m high Scots pine plantation at Hartheim, Germany, was measured with a variety of methods during a 11-day period of fine weather in mid-May 1992. Net radiation and rate of thermal storage were measured with conventional net radiometers, soil heat flux discs and temperature-based storage models. The turbulent fluxes discussed in this report were obtained with an interchanging Bowen ratio energy budget system (BREB, at 14 m), two one-propeller eddy correlation systems (OPEC systems 1 and 2 at 17m), a 1-dimensional sonic eddy correlation system (SEC system 3) at 15 m, all on one low tower, and a 3-dimensional sonic eddy correlation system (SEC system 22) at 22 m on the high tower that was about 46 m distant. All systems measured sensible and latent heat (H and LE) directly, except for OPEC systems 1 and 2 which estimated LE as a residual term in the surface energy balance. Closure of turbulent fluxes from the two SEC systems was around 80% for daytime and 30% for night, with closure of 1-dimensional SEC system 3 exceeding that of 3-dimensional SEC system 22. The night measurements of turbulent fluxes contained considerable uncertainty, especially with the BREB system where measured gradients often yielded erroneous fluxes due to problems inherent in the method (i.e., computational instability as Bowen's ratio approaches –1). Also, both eddy correlation system designs (OPEC and SEC) appeared to underestimate |H| during stable conditions at night. In addition, both sonic systems (1- and 3-dimensional) underestimated |LE| during stable conditions. The underestimate of |H| at night generated residual estimates of OPEC LE containing a phantom dew error that erroneously decreased daily LE totals by about 10 percent. These special night problems are circumvented here by comparing results for daytime periods only, rather than for full days. To summarize, turbulent fluxes on the low tower from OPEC system 2 and the adjacent SEC system 3 were in reasonable agreement, while the BREB system appeared to overestimate H and underestimate LE; H and LE measured by SEC system 22 on the high tower were lower than from OPEC and SEC3 on the low tower. The turbulent flux measurements tended to converge, but the data exhibit unexplained differences between days, between systems, and between locations.With 7 Figures     

Helium radiation diffusion in prominences

The following is shown on the basis of a solution of the integral diffusion equations for radiation in a multilevel helium atom under low-temperature plasma conditions (T _e = 7000 K) in the Vertical slab model: (a) Neutral helium is ionized by coronal radiation mainly in the 100–300 Å spectral region; the degree of helium ionization is maximum at the boundary planes, (b) The photorecombinations to the 2³ S and 2¹ S levels and the photoionizations by the Balmer continuum of the Sun are very nearly balanced and this determines the population of these levels. The 2³ S level is destroyed by electron impacts (this reduces the brightness of the triplet lines), and the 2¹ S level decays via the escape of the quanta of 584 through the 2¹ P level, (c) Emission in the resonance line 584 (2¹ P 1¹ S) occurs due to recombination to 2¹ S with subsequent absorption of quanta of infrared radiation 20581. This is a rare case. (d) The radiation of helium is generated in the vicinity of the boundary planes in the region of penetration of radiation with 200 Å, where the density of matter decreases gradually down to the coronal value. In the subordinate lines, the radiation is conditioned by the quasi-resonance scattering of photospheric radiation, (e) The calculated absolute values of the intensities of the helium and hydrogen lines are in good agreement with the observations (see Figure 6).The helium to hydrogen number ratio is close to 0.05.     

Model estimates for the sensitivity of atmospheric centers of action to global climate changes

The sensitivity of the characteristics of atmospheric centers of action (ACAs) in the Northern Hemisphere to global climate changes is analyzed on the basis of models of different complexity, including the climate model of intermediate complexity of the Institute of Atmospheric Physics, Russian Academy of Sciences and the ECHAM4/OPYC3 and HadCM3 general circulation models of the atmosphere and ocean. The emphasis is on the analysis of trends of the change in ACA characteristics in winter, when the long-term global warming is most considerable. The global climate models are shown to be able to describe not only the intermediate regimes of ACAs but also their dynamics. In particular, ECHAM4/OPYC3 is capable of reproducing the statistically significant connection of the characteristics of the North Pacific centers of action with El Niño/La Niña events, revealed from observational data. With the use of the results of the global climate models, the possible changes in the characteristics of centers of action in the 21st century are estimated for an increased content of greenhouse gases in the atmosphere.