Advanced sensitivity calibration of the Los Angeles strong motion array

Results are presented of recent sensitivity calibration of 76 accelerographs (SMA-1) of the Los Angeles Strong Motion Array. These have pendulum-like transducers and optical recording system. One characteristic of their design is off-axis sensitivity, which is magnified by transducer misalignment. A new calibration procedure was applied, which considers off-axis sensitivity and measures the angles of misalignment (φ and ψ), as well as the incident angle of the light beam onto the film (θ₀). These are required (1) for accurate estimation of sensitivity, and (2) for proper instrument correction of recorded accelerograms which considers also cross-axis sensitivity and misalignment. These effects are important near large acceleration peaks (approaching and exceeding 1g), e.g. like the ones recorded near the source of the 1994 Northridge earthquake (M_L=6·4). This earthquake was recorded by 65 stations of the Los Angeles Strong Motion Array, at epicentral distances from 2 to 85 km. Histograms showing distribution of the misalignment angles, light beam incidence angle θ₀ (for unloaded position) and the transducer sensitivities are presented. These indicate that the misalignment angles are typically 1–1·5°, but may also be 3–4°. Angle θ₀ (usually neglected), is mostly between ±8°, but may reach ±12°. Assuming θ₀=0 leads to systematically smaller values of the measured sensitivity (e.g. by ∼3% for θ₀=8° and ∼4% for θ₀=12°). Comparison of the newly measured sensitivities with those measured prior to installation (in 1979/1980), s^old, shows that, in general, the new values are systematically smaller. The difference is typically within 5 per cent, but in some cases is as large as 10 per cent. Other principal sources of the observed differences and their mechanisms are discussed. Those include long-term changes in the transducers (e.g. change of stiffness, reflected in changes of the natural frequency) and differences in the calibration procedure (e.g. errors associated with manual reading film records with tilt test data, and with transducer and instrument housing misalignment). The presented results may be considered typical of similar strong motion arrays worldwide. © 1998 John Wiley & Sons, Ltd.     

A Late Holocene Paleoecological Record from Torrey Pines State Reserve, California

Paleoenvironments of the Torrey Pines State Reserve were reconstructed from a 3600-yr core from Los Peñasquitos Lagoon using fossil pollen, spores, charcoal, chemical stratigraphy, particle size, and magnetic susceptibility. Late Holocene sediments were radiocarbon dated, while the historical sediments were dated using sediment chemistry, fossil pollen, and historical records. At 3600 yr B.P., the estuary was a brackish-water lagoon. By 2800 yr B.P., Poaceae (grass) pollen increased to high levels, suggesting that the rising level of the core site led to its colonization by Spartina foliosa (cord-grass), the lowest-elevation plant type within regional estuaries. An increase in pollen and spores of moisture-dependent species suggests a climate with more available moisture after 2600 yr B.P. This change is similar to that found 280 km to the north at 3250 yr B.P., implying that regional climate changes were time-transgressive from north to south. Increased postsettlement sediment input resulted from nineteenth-century land disturbances caused by grazing and fire. Sedimentation rates increased further in the twentieth century due to closure of the estuarine mouth. The endemic Pinus torreyana (Torrey pine) was present at the site throughout this 3600-yr interval but was less numerous prior to 2100 yr B.P. This history may have contributed to the low genetic diversity of this species.     

Territoriality and the Police

Despite widespread interest in the relationship between the exercise of power and control of space, few current works in geography make explicit use of the concept of territoriality. This paper does so by considering the means by which the Los Angeles Police Department pursues its law enforcement and order maintenance functions through regulating space. I draw upon fieldwork observations of a single LAPD patrol division to demonstrate that officers regularly seek to govern the citizenry through controlling the spatial parameters of permissible action. The imperative toward effective territorial control is given further impetus within the subcultural world that officers construct; indeed, officers evaluate each other's competence in large part on their ability to manage activity within the spaces for which they are responsible.     

Nonprofits and park equity in Los Angeles: a promising way forward for environmental justice

Disparities in park provision raise environmental and health justice concerns. With public agencies stepping back from providing environmental amenities in increasingly neoliberal urban regimes, nonprofits in the U.S. have assumed a prominent role in the parks and recreation sector. But very few studies have comprehensively assessed whether and how park nonprofits contribute to increasing or closing the equity gap in park provision. Focusing on Los Angeles, I analyze how park nonprofits operate and which demographic groups benefit from new and improved parks supported by nonprofits. Based on interviews with local practitioners and geospatial analyses, I find that nonprofits are leading a park equity movement in Los Angeles and helped address park disparities. To do so, nonprofits built diverse coalitions, leveraged complementary strengths, coalesced with public agencies, and helped generate public funds for parks. These findings show that equity-oriented nonprofits can successfully challenge the unjust outcomes of neoliberal governance.     

Burgess and Hoyt in Los Angeles: testing the Chicago models in an automotive-age American city

For much of the twentieth century, the “Chicago models” proposed by E. W. Burgess in the 1920s, Homer Hoyt in the 1930s, and Chauncy Harris and Edward Ullman in 1945 dominated discussions of the spatial form of cities in the United States. The changes that have subsequently occurred in American urban geography naturally raise questions about the continuing relevance of the models. In recent years, a “Los Angeles School” in geography and urban studies has dismissed the Chicago models as outdated. But the critics have provided little empirical evidence in support of their claims. Identifying exogenous amenities—those of distance from the city center, terrain, and waterfronts—as central elements in the Chicago models, we analyzed the relation of these factors to the patterns of income in Los Angeles and Chicago using spatial statistical regression. The newer, automobile-age city closely follows, while the older city of Chicago deviates substantially from, the patterns predicted in the classical Chicago models. These models may best describe the most recently built American cities and may be more relevant than ever today in explaining the dynamics of urban form.     

The Reinvention of Public Green Space

Much attention has been paid to preserving land at the urban fringe, and to the negative effects of sprawl and its costs. There is increasing recognition that enhancing green, public open spaces in cities provides a strategy to make those cities more sustainable, more livable, and more equitable. This involves a new approach to public spaces that integrates infrastructure needs, takes equity into account, and reexamines the range of uses public spaces offer. We consider the potential for urban greening through a case study in the dense inner core of Los Angeles that probed local resident attitudes and values toward a more inclusive strategy, and that measured the potential value of nature's services in the urban fabric using a GIS program.     

最新版海冰模式CICE的优势:基于北极海冰时空特征的评估

【研究目的】海冰模式CICE(Los Alamos sea ice model)作为当前国际上的主流海冰模式之一,已被耦合进了大部分地球系统模式,对该模式模拟能力的评估工作是发展地球系统模式的重要参考依据。【创新点】通过观测数据与不同版本CICE模式对北极海冰数值模拟结果进行对比分析,研究了最新版本CICE6.0模拟能力及优势。【重要结论】CICE6.0模拟结果的年际误差最小,且季节变化与观测值最为吻合。相较而言,CICE4.0严重高估了冬季海冰总面积及低估了夏季海冰总面积,而CICE5.0在冬季的误差明显大于其他版本。此外,我们也关注了三个模式对多年冰和季节冰的模拟效果,从其均方根误差空间分布看出:模拟误差主要出现在中央海区及其周边海域。CICE4.0和CICE5.0在这些区域模拟的多年冰偏少、季节冰偏多,均低估了多年冰的变化趋势,且高估了季节冰的变化趋势;CICE6.0很好地解决了这些问题,其模拟的多年冰和季节冰的趋势最接近观测值,特别在北冰洋中部。总的来说,CICE6.0模拟的北极海冰在各方面都优于其他版本。     

Phreatomagmatic to Strombolian eruptive activity of basaltic cinder cones: Montaña Los Erales, Tenerife, Canary Islands

Phreatomagmatic activity results from the interaction of magma and external water during a volcanic eruption and is a frequent eruptive phenomenon worldwide. Such ‘fuel-coolant’ reactions change the eruptive dynamics, thus generating particles that reflect the degree of explosivity. Different eruptive phases may thus be identified from deposits, allowing us to reconstruct conditions that prevailed in the past and use these to predict the level of explosivity in a given geological setting in the future.A detailed study of the deposits from Montaña Los Erales, a 70 m-high Quaternary cinder cone belonging to a rift-related chain of vents in the Bandas del Sur region, in Southeast Tenerife, was undertaken. Field observations on excavated sections and SEM analysis of tephra suggest that the eruption style changed progressively from an initial phreatomagmatic phase, through a transitional stage, to one that was entirely Strombolian. To investigate the causes and the nature of these changes in eruptive style, products from each major unit were analysed for their morphology using hand specimen observations, secondary electron microscopy, backscatter electron microscopy, and reflected light microscopy to examine fragment size variation, fragment morphologies, vesicularity, and the level of secondary hydrous alteration (e.g. palagonitisation and zeolitisation). Study results demonstrate that the initial phase of activity was largely driven by magma–water interaction, where magma may have interacted with a lens of fresh ground- or surface water. With proceeding eruptive activity the water became exhausted, giving rise to an entirely Strombolian eruptive style.Examples of phreatomagmatic activity that occur on typical rift-related basaltic vent alignments are not infrequent in the Canary Islands. These vent systems usually erupt in Strombolian fashion, producing scoria and lava flows that do not generally extend far beyond the vent area. However, aligned feeders may intersect different strata, structural features, and different hydrological situations, thus giving rise to activity that is less predictable in setting, intensity, and duration. The occurrence of phreatomagmatism in an otherwise low-explosivity basaltic eruptive environment increases the need for awareness of the geo-hydrological situation in volcanically active areas. Studying the past eruptive history is therefore essential to derive realistic scenarios for future vulnerability evaluation and risk assessment, especially in densely populated areas like the Canary Islands.     

Pb chronology and trace metal geochemistry at Los Tuxtlas, Mexico, as evidenced by a sedimentary record from the Lago Verde crater lake

Lago Verde is a fresh-water maar found on the lower slopes of San Martin volcano, at the Sierra de Los Tuxtlas, Mexico, currently the northernmost remnant of the tropical rain forest in America. ²¹⁰Pb and ¹³⁷Cs analyzed in a sediment core were used to reconstruct the historical fluxes of Ag, Cd, Cu, Pb, Hg and Zn to the site during the last ∼ 150 yr. The ²¹⁰Pb_xs-derived sediment accumulation rates, the magnetic susceptibility, C/N ratios and δ¹³C data evidenced background conditions at the lake until 1960s, when enhanced erosion related to the clearing of large forested areas at Los Tuxtlas promoted higher accumulation rates of a heavier and more magnetic sedimentary material. Recent sediments from Lago Verde were found enriched by Pb (26-fold natural concentration level [NCLs]) and moderately enriched by Cd > Cu > Zn and Hg (6-, 5-, 4- and 4-fold corresponding NCLs, respectively). The fluxes of Cu, Hg, Pb and Zn have significantly increased since 1940s, with peak ratios of total modern to pre-industrial fluxes of 11, 11, 19 and 49, respectively. The lake occupies a relatively pristine, non-industrialized basin, and therefore the increased metal fluxes might be related to long-distance aeolian transport of trace metals.     

Environmental hazards posed by the Los Angeles Basin urban oilfields: an historical perspective of lessons learned

Urban encroachment into areas historically reserved for oil and gas field operations is an ever-present problem within the Los Angeles Basin. The recent frenzy in real estate development has only intensified what can be characterized as a conflict in land usage. Subsurface mineral rights are severed from surface ownership, often resulting in developments being approved without adequate consideration of the underlying oil and gas field consequences. Also, surface operations are frequently co-located within residential areas without consideration of the health and safety consequences of emissions of toxics to air. This paper presents a review of the environmental, health and safety hazards posed by urban oilfield operations, with an emphasis upon the lessons learned from the L.A. Basin: Original Urban Oilfield Legend (see Castle and Yerkes 1976; Denton and others 2001; Endres and others 2002; Kouznetsov and others 1994; Katz and others 1994; Schumacher and Abrams 1994; and Schoell 1983). The Los Angeles Basin has provided the authors with one of the largest natural laboratories in the world for studying the consequences of these issues. The results presented are part of a long-term research program based upon the application of geoscience and petroleum engineering principles in obtaining a fundamental understanding of the root causes of the environmental hazards posed. Topics addressed include: (1) vertical migration of gas to the surface along faults and improperly completed or abandoned wellbores (e.g., due to poor cementing practices), (2) subsidence caused by the fluid production and declining reservoir pressures, (3) soil and groundwater contamination resulting from historic oil and gas field operations, and (4) air toxics resulting from surface operations. A number of case histories are discussed that illustrate the seriousness of the problem. A clear case is made for the urgent need for closer coordination and education by the petroleum industry of the local government planning departments. These departments have the principal role in determining land use policies, acting as the lead agency in performing environmental site assessments (e.g., under the California Environmental Quality Act), and in establishing mitigation measures for dealing with the long-term environmental hazards. This paper establishes prudent practices on the part of oilfield operators for the monitoring and mitigation of these hazards.