Santiago Basin Research Articles

Stochastic strong-ground motion simulation in the Santiago metropolitan region considering an Mw 7.8 intraplate intermediate-depth earthquake

Large intraplate intermediate-depth (IID) events are the most destructive to Chilean structures and they occurred throughout the country. Cities located above the hypocenters of these large earthquakes have been completely destroyed. In recent years, only small- and moderate-magnitude IID events have been reported below Santiago, the most populated Chilean city; however, the occurrence of large-magnitude IID events cannot be ruled out. In this study, we investigated the strong-ground motion generated by large-magnitude earthquakes occurring below the Santiago metropolitan region. We used a stochastic methodology to simulate synthetic records, considering IID events of magnitude Mw 7.8. To validate this method, we simulated intermediate-depth events of magnitudes Mw ∼5-6 that occurred near Santiago. We further calibrate our results by reproducing the strong-ground motion data recorded during the 2005 Tarapacá Mw 7.8 Northern Chile IID earthquake. We observe that in some areas of the Santiago Basin, high PGA values can reach values close to 1 g, in which, in addition to directivity effects and takeoff angles, the rupture distances (∼100 km) and soil type (C) play a key role in amplifying strong-ground motion. Finally, our findings highlight the importance of reproducing high-frequency time histories as a proxy to re-evaluate the seismic hazard due to large IID earthquakes.

High-Resolution Deformation Monitoring from DInSAR: Implications for Geohazards and Ground Stability in the Metropolitan Area of Santiago, Chile

Large urban areas are vulnerable to various geological hazards and anthropogenic activities that affect ground stability—a key factor in structural performance, such as buildings and infrastructure, in an inherently expanding context. Time series data from synthetic aperture radar (SAR) satellites make it possible to identify small rates of motion over large areas of the Earth’s surface with high spatial resolution, which is key to detecting high-deformation areas. Santiago de Chile’s metropolitan region comprises a large Andean foothills basin in one of the most seismically active subduction zones worldwide. The Santiago basin and its surroundings are prone to megathrust and shallow crustal earthquakes, landslides, and constant anthropogenic effects, such as the overexploitation of groundwater and land use modification, all of which constantly affect the ground stability. Here, we recorded ground deformations in the Santiago basin using a multi-temporal differential interferometric synthetic aperture radar (DInSAR) from Sentinel 1, obtaining high-resolution ground motion rates between 2018 and 2021. GNSS stations show a constant regional uplift in the metropolitan area (~10 mm/year); meanwhile, DInSAR allows for the identification of areas with anomalous local subsistence (rates < −15 mm/year) and mountain sectors with landslides with unprecedented detail. Ground deformation patterns vary depending on factors such as soil type, basin geometry, and soil/soil heterogeneities. Thus, the areas with high subsidence rates are concentrated in sectors with fine sedimentary cover and a depressing shallow water table as well as in cropping areas with excess water withdrawal. There is no evidence of detectable movement on the San Ramon Fault (the major quaternary fault in the metropolitan area) over the observational period. Our results highlight the mechanical control of the sediment characteristics of the basin and the impact of anthropogenic processes on ground stability. These results are essential to assess the stability of the Santiago basin and contribute to future infrastructure development and hazard management in highly populated areas.

Several types of triangle zones from the Subandean ranges of Peru: Fish-tails, tectonic wedges and passive-roof duplexes

Surface geology, well data and 2D seismic interpretations reveal the presence of triangle zone structures at the contact of the Subandean Ranges and the Amazonian foreland in Peru. From north to south the Santiago, Marañon, Ucayali and Madre de Dios basins have been studied. Interpretations presented in this study show a great variety of structural styles at this contact area. Simple triangle zones with one back thrust, composite ones with several back thrusts, more complex double vergent ones with pop-up structures, best described as fish-tails, tectonic wedges and passive-roof duplexes are present in the study area. The role of pre-Andean tectonics and paleogeography is key to understanding the Andean evolution, due to the control on the location and geometry of the main detachment levels. The Santiago Basin is characterized by the presence of fish-tail structures with a Permian-Triassic evaporitic basal detachment located at the base of the Pucara Gr. At the Ucayali Basin, several detachment levels interact, the basal ones located within the Ordovician, Devonian, and Carboniferous defining triangle zones or tectonic wedges. At the Madre de Dios Basin, two detachment levels define the base of the triangle zone, Permian shales in the northwest at the Inambari Imbricates area, and Lower Devonian (Cabanillas Gr.) in the southeast at the Candamo area. A roof thrust is located within the Paleogene section at both the Madre de Dios and Ucayali basins, defining passive-roof duplex structures.Several detachment levels are rooted into the Lower Paleozoic; others within the Permian-Triassic section, being locally evaporitic, and finally others are located at the base of the Cretaceous. Paleozoic shales represent the main basal detachment level of the southern Subandean ranges. The role of these detachments and the role of the sedimentation rate are discussed as factors that control the development and geometry of the triangle zones described in the study area.Except for the San Martin-Sagari area of the Ucayali Basin, the triangle zones of the Santiago and Madre de Dios basins remain unexplored even though active petroleum systems are present. Sub-thrust structures at the Azulmayo area of the Madre de Dios Basin remain promising undrilled structures.

Two-dimensional simulation of the seismic response of the Santiago Basin, Chile

Results from an ambient noise tomography, gravimetric studies, and local geophysical surveys are combined to develop three representative cross-sections of the Santiago Basin, Chile, that include the main geological features and the wave velocities of the materials. One-dimensional (1D) and two-dimensional (2D) seismic responses were simulated for each cross-section, considering input seismic signals polarized in different directions. Amplification factors calculated in the surface of the cross-sections highlight the larger amplitudes and durations of the strong-ground motions in the softer deposits of the Basin due to surface waves generated at the boundaries of the rock outcrops. 2D over 1D aggravation factors for several earthquake ground motion characteristics indicate that 2D effects are negligible in the stiff gravel deposits and that the 1D simulations cannot capture the basin edge effects in the fine-grained soils. Numerical predictions are confirmed using the analyses of earthquake records of the 2015 Mw 8.3 Illapel Earthquake.

Machine learning techniques for estimating seismic site amplification in the Santiago basin, Chile

Seismic site amplification and seismic hazard maps are crucial inputs for decision making and risk evaluation in places where seismicity imposes a significant risk to human life and infrastructure. In this work, we propose a novel machine learning (ML) based methodology to integrate qualitative and quantitative data to map the degree of seismic amplification in an area of Chile, one of the most seismically active countries on Earth. Our method uses measurements of surface shear wave velocities (Vs30) and predominant frequencies (f0) combined with gravity anomaly maps to update the geographic extension of seismic amplification classes. Additionally, we trained the predictive models to interpolate and extrapolate Vs30 and f0 to the unsampled sites. Applying this method to the Santiago basin resulted in (i) a refined seismic amplification map, and (ii) maps of Vs30 and f0 estimated with improved accuracy. The best predictions, obtained by ML techniques and validated through cross-validation, are possibly due to the inclusion of spatial covariates for algorithm training, enhancing the ability of the model to capture the spatial correlations of geological, geophysical and geotechnical data. The estimation of predominant frequencies (f0) is improved considerably by including gravity as a covariant. The accuracy of the f0 predictions apparently depends more on the choice of covariates than on the algorithm used, while the Vs30 predictions are more sensitive to the chosen algorithm. These results illustrate the great potential of machine learning predictive algorithms in digital soil mapping, which surpass traditional geostatistical techniques. The major contribution of this work is to introduce a novel methodology, based on artificial intelligence models, to extend local measurements of site-specific dynamic properties. This information can be used to quantitatively estimate seismic hazard over a regional scale.

Seasonal variation in atmospheric pollutants transport in central Chile: dynamics and consequences

Abstract. Central Chile faces atmospheric pollution issues all year long as a result of elevated concentrations of fine particulate matter during the cold months and tropospheric ozone during the warm season. In addition to public health issues, environmental problems regarding vegetation growth and water supply, as well as meteorological feedback, are at stake. Sharp spatial gradients in regional emissions, along with a complex geographical situation, make for variable and heterogeneous dynamics in the localization and long-range transport of pollutants, with seasonal differences. Based on chemistry–transport modeling with Weather Research Forecasting (WRF)–CHIMERE, this work studies the following for one winter period and one summer period: (i) the contribution of emissions from the city of Santiago to air pollution in central Chile, and (ii) the reciprocal contribution of regional pollutants transported into the Santiago basin. The underlying 3-dimensional advection patterns are investigated. We find that, on average for the winter period, 5 to 10 µg m−3 of fine particulate matter in Santiago come from regional transport, corresponding to between 13 % and 15 % of average concentrations. In turn, emissions from Santiago contribute between 5 % and 10 % of fine particulate matter pollution as far as 500 km to the north and 500 km to the south. Wintertime transport occurs mostly close to the surface. In summertime, exported precursors from Santiago, in combination with mountain–valley circulation dynamics, are found to account for most of the ozone formation in the adjacent Andes cordillera and to create a persistent plume of ozone of more than 50 ppb (parts per billion), extending along 80 km horizontally and 1.5 km vertically, and located slightly north of Santiago, several hundred meters above the ground. This work constitutes the first description of the mechanism underlying the latter phenomenon. Emissions of precursors from the capital city also affect daily maxima of surface ozone hundreds of kilometers away. In parallel, cutting emissions of precursors in the Santiago basin results in an increase in surface ozone mixing ratios in its western area.

Loop source transient electromagnetics in an urban noise environment: A case study in Santiago de Chile

We have developed a transient electromagnetic (TEM) study in the noisy urban environment of the megacity Santiago de Chile. Our investigation characterizes the electrical conductivity structure of the Santiago Basin down to 300 m depth, providing key information about the sedimentary infill, hydrogeologic aspects, and geomorphological units. In total, 52 TEM soundings were recorded over roughly 900 km2. We identified different transient characteristics and noise patterns, spatially correlated to the investigation area. This step allowed classification of the soundings into three groups: highly distorted data, transients severely affected by cultural electromagnetic noise sources, and good-quality data with low noise levels. Conventional 1D inversion techniques were then used to derive resistivity depth models. The models were interpreted using sedimentary units, alluvial fan, static groundwater level information, and also were verified using borehole data. Based on 1D models, different minimum sedimentary thicknesses are observed, ranging between 50 and 300 m in depth. The average derived sedimentary thickness is approximately 210 m. In addition, seven soundings indicated the basement. These are mostly located toward the periphery of the Santiago Basin. We found a high conductive zone toward the north and a low-conductive area toward the south of the basin. The southern area is characterized by the absence of conductive layers at a depth between 100 and 150 m. This is likely related to the presence of a thick gravel layer and the absence of shallow clay layers acting as aquitards. The resistivity distribution and the sedimentary thickness obtained for this noisy and urban environment are key inputs for groundwater research as the water supply becomes more limited, specifically in central Chile. The overall consistency of the derived subsurface models highlights the suitability of the TEM method for investigating urban noise environments.

Impact of residential combustion and transport emissions on air pollution in Santiago during winter

Santiago (33.5°S, 70.5°W), the capital of Chile, is frequently affected by extreme air pollution events during wintertime deteriorating air quality (AQ) and thus affecting the health of its population. Intense residential heating and on-road transport emissions combined with poor circulation and vertical mixing are the main factors responsible for these events. A modelling system composed of a chemistry-transport model (CHIMERE) and a meteorological model (WRF) was implemented to assess the AQ impacts of residential and transportation sources in the Santiago basin. A two-week period of July 2015 with various days with poor AQ was simulated focusing on the impact on AQ with respect to fully inhalable particles (PM2.5) and nitrogen oxides (NOX). Three emission scenarios, within the range of targeted reductions of the decontamination plan of Santiago, were tested; namely 50% reduction of residential emission, 50% reduction of transport emissions and the combination of both. An additional scenario decreasing transport emissions in 10% was carried out to examine whether a linear dependence of surface concentrations on changes in emissions exists.The system was validated against surface and vertically resolved meteorological measurements. The model reproduces the daily surface concentration variability from the AQ monitoring network of Santiago. However, the model not fully captures the emissions variations inferred from the observations which may be due to missing sources such as resuspension of dust.Results show that, during the period studied, although both residential and transportation sources contribute to observed AQ levels in Santiago, reducing transport emissions is more effective in terms of reducing the number of days with pollution events than decreasing residential combustion. This difference in impact is largely due to the spatial distribution of the emission sources. While most of the residential combustion is emitted in the outskirts of the city, most of the transport emissions occur within the city, where most of the stations from AQ monitoring network of Santiago are located. As can be expected, the largest improvement of AQ in Santiago is achieved by the combined reduction of emissions in both sectors. Sensitivity analysis with 10% reduction in transport emissions reveals a linear behavior between emissions and concentrations for NOX and approximate linear behavior for PM2.5. The absence of secondary aerosols formation and dust resuspension in the current simulation could explain this deviation from linearity for fine particles. Nevertheless, it suggests that the results can be used for mitigation policies with emissions reductions below the 50% used in this study.

Late Triassic Hexactinellid Sponges from the Pongo De Lorocache Limestones, Peru: New Evidence for Siliceous Sponge Dominance on Shallow Carbonate Platforms

Upper Triassic lyssacinosan hexactinellid sponges are described from the late Triassic Pongo de Lorocache limestones, Santiago basin, Peru. The sponges are found in the uppermost levels of the Chambara Formation. This unit exhibits sequences of the Upper Norian to Lower Rhaetian showing an unconformable contact with Jurassic sequences. A great number of complete cup-shaped sponge body fossils occur in life position showing gregarious habits, and forming biostrome accumulations. Preservation prevents definite identification. However, the specimens have the characteristics of hexactinellid hexactinosans, very closely related to the Superfamily Crepospongiodea. The Triassic-Jurassic marine crises produced an unexpected resurgence of siliceous sponges in detriment of carbonate skeletal fauna. Previous studies in Nevada (USA), Peru, Morocco, Austria and France show that this sponge event can be considered a global phenomenon, apparently associated with an important change in atmospheric pCO2. This finding in the Norian- Rhaetian levels slightly predates the main peak of siliceous sponge dominance recorded in Lower Jurassic units. This implies that at least previous appearances of hexactinellid sponges dominated shallow water carbonate boundstone intervals before the maximum peak of the extinction event.

Characterization of the depocenters and the basement structure, below the central Chile Andean Forearc: A 3D geophysical modelling in Santiago Basin area

AbstractSince the last century, several geological and geophysical studies have been developed in the Santiago Basin to understand its morphology and tectonic evolution. However, some uncertainties regarding sedimentary fill properties and possible density anomalies below the sediments/basement boundary remain. Considering that this is an area densely populated with more than 6 million inhabitants in a highly active seismotectonic environment, the physical properties of the Santiago Basin are important to study the geological and structural evolution of the Andean forearc and to characterize its seismic response and related seismic hazard. Two and three‐dimensional gravimetric models were developed, based on a database of 797 compiled and 883 newly acquired gravity stations. To produce a well‐constrained basement elevation model, a review of 499 wells and 30 transient electromagnetic soundings were used, which contribute with basement depth or minimum sedimentary thickness information. For the 2‐D modelling, a total of 49 gravimetric profiles were processed considering a homogeneous density contrast and independent regional trends. A strong positive gravity anomaly was observed in the centre of the basin, which complicated the modelling process but was carefully addressed with the available constrains. The resulting basement elevation models show complex basement geometry with, at least, eight recognizable depocenters with maximum sedimentary infill of ~ 500 m. The 3‐D density models show alignments in the basement that correlates well with important intrusive units of the Cenozoic and Mesozoic. Along with interpreted fault zones westwards and eastwards of the basin, the observations suggest a structural control of Santiago basin geometry, where recent deformation associated with the Andean contractional deformation front and old structures developed during the Cenozoic extension are superimposed to the variability of river erosion/deposition processes.

First analysis of life history and season of death of a South American gomphothere

Excavations associated with wastewater treatment facilities in the Santiago Basin of Chile uncovered remains of an adult male Stegomastodon platensis in Pleistocene fluvial sediments. The specimen included a skull with both tusks still in their alveoli. A small sample block of dentin and cementum was excised from the right tusk for analysis of compositional and structural variation recorded in the sequence of dentin layers. The sample was analyzed using serial isotope assays, microCT, and measurement of dentin increments in thin sections. These procedures allowed us to recover life history information relating to the final four years of life. Analyses show seasonal variation in composition, density, and growth rate, permitting identification of years. MicroCT features appear to recur semiannually, which has not been observed previously in proboscidean tusks; these features are interpreted to correspond to the winter-spring boundary and some other aspect of environment or behavior recurring within each year. Average annual apposition of dentin within the tusk is 10.4 mm for complete years studied, suggesting that this individual was healthy leading up to the final year of life. Recurrent periods of low rate of apposition measured in weekly dentin increments that formed during the summer are interpreted as representing periods of musth. Using the location of microCT features, annual appositional thickness, and the pattern of weekly growth within the final year of life, we interpret the season of death for this individual as being within the early autumn.

Deep characterization of the Santiago Basin using HVSR and cross-correlation of ambient seismic noise

Continuously recorded seismic ambient noise is used to investigate the intermediate and deep structure of the Santiago Basin for seismic site-response evaluation. Single-station H/V spectral ratios (HVSR) were used as a proxy to identify zones with high stiffness soils and low impedance contrast with the underlying bedrock, as a complement of available surface geology information. Frequency-domain cross-correlation of the vertical components of ambient noise were calculated and used to estimate phase velocity dispersion curves associated to station pairs over different soil deposits. In addition, noise correlation functions (NCFs) calculated from time-domain cross-correlation of the vertical seismic noise records were used to estimate group velocity dispersion curves and to verify results from the spectral method. Data processing from both methods resolves a frequency band between 0.1 and 8.0Hz, a critical band for civil infrastructure that is difficult to determine with traditional local-scale passive surface wave methods. Station pairs with high signal correlation over stiff soils in the center, south, and east part of the basin, mainly associated to flat HVSR response, yielded robust phase velocity dispersion curves that vary approximately from 3.5km/s at 0.1Hz to 1km/s at 4Hz. Shear wave velocity profiles inverted from the dispersion curves show high average shear wave velocities that also have a pronounced increase rate with depth and a lack of clear soil-bedrock interface at depths predicted by available gravimetric data.

Effect of Nearby Forest Fires on Ground Level Ozone Concentrations in Santiago, Chile

On 4 and 8 January 2014, at the height of the austral summer, intense wildfires in forests and dry pastures occurred in the Melipilla sector, located about 70 km to the southwest of Santiago, the Chilean capital, affecting more than 6 million inhabitants. Low level winds transported the forest fire plume towards Santiago causing a striking decrease in visibility and a marked increase in the concentration of both primary (PM10 and CO) and secondary (Ozone) pollutants in the urban atmosphere. In particular, ozone maximum concentrations in the Santiago basin reached hourly averages well above 80 ppb, the national air quality standard. This ozone increase took place at the three sampling sites considered in the present study. These large values can be explained in terms of high NOx concentrations and NO2/NO ratios in biomass burning emissions.

Lithobates magnaocularis Frost & Bagnara, 1974, the Northwest Mexico Leopard Frog (Anura: Ranidae): new state records for Aguascalientes and Zacatecas, Mexico

We report an adult female and subadult specimen of the frog Lithobates magnaocularis Frost & Bagnara, 1974 from tropical deciduous forest habitat in the Municipality of Calvillo, Aguascalientes and Valparaiso, Zacatecas, Mexico. These vouchers represent the first state records of L. magnaocularis for Aguascalientes and Zacatecas, and extend the known distribution of the species through the northeastern portion of the tributaries of the Río Grande de Santiago basin.

Profundidad del basamento derivado de la gravedad en la Cuenca de Santiago: implicancias para la evolución geológica, hidrogeológica, geotermia de baja entalpiía, caracterización de suelos y peligros geológicos.

A recording of 1,115 gravimetric stations, the review of 368 wells, and the petrophysics measurements of 106 samples from representative outcrops have been used for a comprehensive geological/geophysical study of Santiago Basin. 2.5D and 3D gravimetric modeling, constrained by regional geology, soil and bedrock densities, edge- basin outcrops, depth (minimum) to basement from wells, and detailed modeling of heterogeneous bedrock and mid- crustal blocks, provided a well-constrained depth to basement model. Model results indicate the presence of a relatively shallow basin with an average of 250 m depth, and three sub basins with depth in excess of 500 m, but comprising less than 30% of the basin surface. From erosion rates in central Chile we estimate a basin infill lasting between 10 to 20 Ma. Basement topography/geomorphology, undercover a structural pattern dominated by NE and NW-trending structures that can be traced out of the basin, westwards in the Coastal Cordillera and eastwards in the Main Cordillera, with second order relevance of NS structures in the eastern border of the basin. This observation, further supported by natural crustal seismicity and basement-derived-magnetic signatures, suggests that the basin origin is mainly controlled by inherited old structures oblique to the margin. Active seismicity along these traverse NE and NW structures suggest that permanent deformation, and associated seismic hazard in the basin is mostly concentrated along these structures. The dynamic response of soils, in terms of the natural resonance frequency, shows that the basement-to-sedimentary/ infilling-impedance-ratio is proportional to the amplitude of the resonance peak. On the other hand, the expected cor- relation between fundamental frequency and depth to basement is only partially supported by the empirical evidence. The difference between a greater gravimetric depth-to-basement compared to lesser seismic depth-to-basement, is attrib- uted to changes in mechanical stiffness with depth compaction with minor effects in bulk density. Finally low enthalpy geothermal resources of the Santiago Basin is analyzed considering depth to bedrock, water table estimates and simple Darcy's-temperature coupled flow modeling. Results show that high groundwater temperature is restricted to deeper parts of southern sub-basin, which improves direct uses of geothermal energy for heating purposes.

Estimating low-enthalpy geothermal energy potential for district heating in Santiago basin–Chile (33.5 °S)

This work presents the results of a regional-scale estimation of low-enthalpy geothermal resources for district heating in the Santiago basin. The purpose of this work is to identify promising areas for the development of this type of renewable energy. The estimation was based on comparison of soil thermal properties and hydrogeological parameters, using Geographic Information System (GIS). To determine the geothermal potential, Ground Source Heat Exchanger (GSHE) coupled with heat pump was used to supply a fixed demand equivalent to the energy required to heat a Chilean standard house. The main barrier for the implementation of a GSHE coupled with heat pump is the well drilling cost, therefore the potential is presented as meters to be drilled in order to install 2 types of GSHE: 1) Borehole Heat Exchanger (BHE) and 2) Groundwater Heat Exchanger (GWHE). To assess the BHE, we used specific Heat Extraction (sHE) of sediments. To evaluate a GWHE, we used depth of groundwater table and groundwater drawdown caused by pumping water to the heat pump. The depth to be drilled ranges from 35 to 105 m in case of the BHE, while in case of the GWHE it ranges from 10 to 400 m.

Combining active and passive multi-channel analysis of surface waves to improve reliability of $$\hbox {V}_\mathrm{S,30}$$ V S , 30 estimation using standard equipment

Surface wave methods are commonly used for seismic soil classification, however, their indirect nature implies a degree of uncertainty that needs to be described and constrained. To evaluate dispersive characteristics, we use active and passive sources, linear and circular arrays, and frequency-wavenumber analysis, spatial autocorrelation method and Roadside MASW analyses. We test the reliability of this approach in three different soil conditions of the Santiago basin (Chilean capital) using standard equipment. This methodology is compared with reliable results from high-energy active source and borehole information. To obtain a confident shear wave depth-profile with standard equipment, it is necessary to combine the dispersion curves obtained with low-energy active source, passive linear and circular arrays. Results obtained only through a passive linear test may not guarantee a reliable 30 m of exploration, especially when no high-traffic roads are present nearby. According to results obtained in the Santiago basin, we propose a methodology based on a combination of active and passive results.

Trends in thermoluminescence date distributions for the Angostura micro region in Central Chile

This article presents a series of thermoluminescence dates obtained from archaeological sites in the Angostura micro region, at the southern edge of the Santiago basin, an area occupied by Bato and Llolleo groups during the Early Ceramic period (200 BC–AD 1300). This series of dates, which cover a time span of around 1500 years, shows moments when dates are abundant and others when they are remarkably scant. We propose that a comparison of the relative frequency of dates can be used as an indirect measure of variations in the intensity of occupation of each of these groups in the zone. We used a summed probability analysis by decade to evaluate sets of dates according to cultural context and sectors of the study area. The results show that while the Bato and Llolleo were irrefutably contemporaneous, the Bato emerged slightly earlier, and that both –but especially the Llolleo– coexisted in later times with Aconcagua groups. At a micro regional scale significant differences as well as changes in Bato and Llolleo population dynamics were made clear. While the Bato display a natural growth curve and then declines, Llolleo increased their population significantly after AD 700, probably because of their reliance on maize crops. Not only did the chronological curves differ; the sectors where they placed their settlements varied over time depending on their specific horticultural practices, local climatic fluctuations, and each other's proximity.

Ozone distribution in the lower troposphere over complex terrain in Central Chile

Observations were performed in 12 communities of Central Chile in order to determine the horizontal gradients of ozone in the Santiago Basin and surrounding valleys. Higher ozone mixing ratios were found northeast of the Santiago Basin and included east of the Aconcagua Valley (~70 km from Santiago) suggesting that photochemical pollution produced in Santiago is capable of passing through the Chacabuco mountain chain (~1.3 km) and have impact downwind from the regions with the largest NOx and VOC emissions. To complement existing surface observations, ozonesonde and tethersonde campaigns were performed in the Santiago Basin and the Aconcagua Valley. The results suggest ozone can accumulate in layers aloft (e.g., >102 ppb at 2 km) similarly to layers observed in complex topography coastal regions like Southern California. Layers of significant ozone concentrations having a near surface origin were observed above the mixed layer and below the subsidence inversion base. We propose that the ozone in this residual layer can be transported large distances (at least to 70 km) to further penetrate into the local environment under conditions of a well‐mixed boundary layer.

Analysis and evolution of air quality monitoring networks using combined statistical information indexes

In this work, we present combined statistical indexes for evaluating air quality monitoring networks based on concepts derived from the information theory and Kullback–Liebler divergence. More precisely, we introduce: (1) the standard measure of complementary mutual information or ‘specificity’ index; (2) a new measure of information gain or ‘representativity’ index; (3) the information gaps associated with the evolution of a network and (4) the normalised information distance used in clustering analysis. All these information concepts are illustrated by applying them to 14 yr of data collected by the air quality monitoring network in Santiago de Chile (33.5 S, 70.5 W, 500 m a.s.l.). We find that downtown stations, located in a relatively flat area of the Santiago basin, generally show high ‘representativity’ and low ‘specificity’, whereas the contrary is found for a station located in a canyon to the east of the basin, consistently with known emission and circulation patterns of Santiago. We also show interesting applications of information gain to the analysis of the evolution of a network, where the choice of background information is also discussed, and of mutual information distance to the classifications of stations. Our analyses show that information as those presented here should of course be used in a complementary way when addressing the analysis of an air quality network for planning and evaluation purposes.