Stations In California Research Articles

Study of Infrasound Propagation from the Shuttle Atlantis using a large Seismic Network

Inclement weather in Florida forced the shuttle Atlantis to land at Edwards Air Force base in southern California on June 22, 2007, passing near three infrasound stations and several hundred seismic stations in northern Mexico, southern California, and Nevada. The high signal-to-noise ratio, broad receiver coverage, and Atlantis' positional information allowed us to test our ability to model infrasound propagation through the atmosphere to regional distances. We predicted shadow zones and arrival times using rays launched at right angles to the conical shock front surrounding the shuttle, and traced through a standard atmospheric model, as well as a global G2S model. Comparison of predictions vs observations shows agreement over much of the study area for both atmospheric specifications. To the east of the shuttle trajectory, there were no detections beyond the primary acoustic carpet. Infrasound energy was detected hundreds of kilometers to the west and northwest of the shuttle trajectory, consistent with the predictions of ducting due to the westward summer-time stratospheric jet. Both atmospheric models predict regions of ensonification alternating with shadow zones to the northwest. However, infrasound energy was detected tens of kilometers beyond predicted zones of ensonification, possibly due to uncertainties in stratospheric wind speeds.

Comparison of methods for evaluating acute and chronic toxicity in marine sediments

Sublethal test methods are being used with increasing frequency to measure sediment toxicity, but little is known about the relative sensitivity of these tests compared to the more commonly used acute tests. The present study was conducted to compare the sensitivity of several acute and sublethal methods and to investigate their correlations with sediment chemistry and benthic community condition. Six sublethal methods (amphipod: Leptocheirus plumulosus survival, growth, and reproduction; polychaete: Neanthes arenaceodentata survival and growth; benthic copepod: Amphiascus tenuiremis life cycle; seed clam: Mercenaria mercenaria growth; oyster: Crassostrea virginica lysosome destabilization; and sediment-water interface testing with mussel embryos, Mytilus galloprovincialis) and two acute methods (amphipod survival with Eohaustorius estuarius and L. plumulosus) were used to test split sediment samples from stations in California. The test with Amphiascus proved to be the most sensitive sublethal test and the most sensitive overall, identifying 90% of the stations as toxic. The Leptocheirus 10-d test was the most sensitive of the acute tests, identifying 60% of the stations as toxic. In general, the sublethal tests were not more sensitive to sediments than the acute tests, with the sublethal tests finding an average of 35% of the stations to be toxic while the acute found 44%. Of the sublethal tests, only the Amphiascus endpoints and Neanthes growth significantly (p <or= 0.05) correlated with sediment chemical concentrations. Poor correspondence occurred between the toxicity endpoints and the indicators of benthic community condition. Differences in test characteristics such as mode of exposure, species-specific contaminant sensitivity, changes in contaminant bioavailability, and influence of noncontaminant stressors on the benthos may have been responsible for variation in response among the tests and low correspondence with benthic community condition. The influence of these factors cannot be easily predicted, underscoring the need to use multiple toxicity methods, in combination with other lines of evidence, to provide an accurate and confident assessment of sediment toxicity.

Traveltime measurements from noise correlation: stability and detection of instrumental time-shifts

We test the feasibility of using Green's functions extracted from records of ambient seismic noise to monitor temporal changes in the Earth crust properties by repeated measurements at regional distances. We use about 11 yr of continuous recordings to extract surface waves between three pairs of stations in California. The correlations are computed in a moving 1-month window and we analyse the temporal evolution of measured interstation traveltimes. The comparison of the arrival times in the positive and negative correlation time of Rayleigh and Love waves allows us to separate time-shifts associated with any form of physical change in the medium, those resulting from clock drift or other instrumental errors, and those due to change in the localization of the noise sources. This separation is based on the principle of time symmetry. When possible, we perform our analysis in two different period bands: 5–10 and 10–20 s. The results indicate that significant instrumental time errors (0.5 s) are present in the data. These time-shifts can be measured and tested by closure relation and finally corrected independently of any velocity model. The traveltime series show a periodic oscillation that we interpret as the signature of the seasonal variation of the region of origin of the seismic noise. Between 1999 and 2005, the final arrival time fluctuations have a variance of the order of 0.01 s. This allows us to measure interstation traveltimes with errors smaller than 0.3 per cent of the interstation traveltime and smaller than 1 per cent of the used wave period. This level of accuracy was not sufficient to detect clear physical variation of crustal velocity during the considered 11 yr between the three stations in California. Such changes may be more easily detectable when considering pairs of stations more closely located to each other and in the vicinity of tectonically active faults or volcanoes.

A year of microseisms in southern California

Microseisms are due to continuous harmonic forcing by ocean waves, whose sources vary in time, frequency, and azimuth. Using frequency‐domain array beamforming, this variation is studied using one‐year of continuous seismic data from 155 stations in southern California. Detailed analysis of data delineates spatiotemporal variations of sources for the primary and secondary microseisms. Both types of microseisms are generated near the coasts but the locations of excitation are different and change with season. Often sources are multiply located but can be spread out over wider areas, especially in the case of secondary microseisms. Distant storms can also be seen occasionally in the frequency range between the primary and secondary microseisms where spectral amplitudes from nearby sources are low.

Validation of climate model output using Bayesian statistical methods

The growing interest in and emphasis on high spatial resolution estimates of future climate has demonstrated the need to apply regional climate models (RCMs) to that problem. As a consequence, the need for validation of these models, an assessment of how well an RCM reproduces a known climate, has also grown. Validation is often performed by comparing RCM output to gridded climate datasets and/or station data. The primary disadvantage of using gridded climate datasets is that the spatial resolution is almost always different and generally coarser than climate model output. We have used a Bayesian statistical model derived from observational data to validate RCM output. We used surface air temperature (SAT) data from 109 observational stations in California, all with records of approximately 50 years in length, and created a statistical model based on this data. The statistical model takes into account the elevation of the station, distance from coastline, and the NOAA climate region in which the station resides. Analysis indicates that the statistical model provides reliable estimates of the mean monthly SAT at any given station. In our method, the uncertainty in the estimates produced by the statistical model are directly determined by obtaining probability density functions for predicted SATs. This statistical model is then used to estimate average SATs corresponding to each of the climate model grid cells. These estimates are compared to the output of the RCM to assess how well the RCM matches the observed climate as defined by the statistical model. Overall, the match between the RCM output and the statistical model is good, with some deficiencies likely due in part to the representation of topography in the RCM.

Architecture, performance, and scalability of a real-time global positioning system data grid

We describe the architecture of our real time SensorGrid system, which supports the real-time message routing and processing of Global Positioning System data. The current system processes 1 Hz position data from 85 stations in Southern and Central California. Our system is built on a computer network-enabled, topic-based publish/subscribe system and is extensible to other data sources. In order to determine the performance and the upper limits of scalability, we have conducted and present here a set of systematic test evaluations of our implementation. Through these tests, we are able to show that performance does not degrade over time, that the system will handle up to 1000 simultaneous data providers or data consumers with a single message broker, and that multiple message brokers can be linked to provide scalability beyond 1000 providers or consumers.

The impact of rush hour traffic and mix on the ozone weekend effect in southern California

The ozone weekend effect refers to the counterintuitive observations showing weekend ozone concentrations frequently to be higher than or comparable to those observed on weekdays. Ozone dynamics are closely linked to the timing, magnitude and fleet mix of transportation activities, primary sources of ozone precursor emissions. To examine the effects of traffic activity on the ozone weekend effect, a statistical analysis was conducted of the weekly patterns of time dependent light-duty vehicle and heavy-duty truck volumes observed at 27 weigh-in-motion stations in southern California. The results show statistically significant variations in traffic flows by day of week, by vehicle type, and by location with respect to the Los Angeles metropolitan area. These variations in traffic, when converted to variations in running exhaust emissions, tend to support four of the seven California Air Resources Board’s ozone weekend effect hypotheses.

How Far, by Which Route and Why? A Spatial Analysis of Pedestrian Preference

This paper reports on a survey of pedestrian trips to transit that examined the trip lengths and route choices made by people walking to five rail transit stations in California and Oregon. In highly motorized countries such as the US, policy-makers are beginning to recognize that shifting some travel from auto trips to walking trips can help the country achieve important policy objectives such as combating obesity and reducing the air pollution and oil dependency that result from auto use. However, researchers know very little about pedestrian behaviour and the role of the built and aesthetic environment in influencing pedestrian trips to transit. As communities wrestle with the interconnected issues of obesity, sprawl, and quality of life, planners need to understand how far Americans will walk to transit and the environmental factors that influence them. This survey of 328 pedestrians walking to rail stations, primarily on weekday mornings, found that they were willing to walk an average of half a mile to the rail station and that minimizing the distance walked was the most important factor influencing their choice of route. The people surveyed also frequently mentioned safety factors as important in route choice. Aesthetic elements of the built environment, on the other hand, were rarely mentioned as important route choice factors. The paper concludes by using these survey findings to recommend strategies that planners, designers, and policy-makers can use to design successful transit and pedestrian-oriented developments.

When Katrina hit California

Beamforming of seismic noise recorded on 150 Southern California stations was used to identify body and surface waves generated by Katrina. Surface wave microseisms are commonly associated with oceanic storms; there are no previous comprehensive body wave observations. The temporal evolution of the surface and body waves was different, indicating a different source mechanism for the two wave types. The body‐waves originated in shallow water east of New Orleans and propagated deep inside the Earth. The surface waves have source location that varies with frequency with the lowest frequency surface waves originating west of the hurricane track and the higher frequency ones to the east. The seismic observations are consistent with ocean wave hindcasts and provide clear association of microseism noise with storm activity.

Promoting Organ Donation to Hispanics: The Role of the Media and Medicine

This study assesses the impact of a paid media advertising campaign employing Spanish language, culturally sensitive television and radio spots airing on major Hispanic stations in southern California. An advertising tracking study with a baseline and three postintervention telephone surveys was conducted from 2001 through 2003 among 500 randomly selected self-identified, primarily Spanish language dominant adult Hispanics. Measures of organ donation attitudes and behaviors (decision and declared intent to donate organs) improved significantly (P < .05) in 2001 and 2002, then leveled off or declined in 2003. Among the reasons given for not making a decision to donate was fear that medical personnel might withhold care from identified organ donors, suggesting lack of knowledge and distrust of the health care system. Few respondents talked to health care professionals or contacted the organ procurement agency for information either before or after the campaign. Findings from this study indicate a need for ongoing public education in the Hispanic community about organ transplantation and donation. Health professionals need to become more engaged in encouraging Hispanic patients to learn about organ transplantation and donation, and to inform their families that they have made the personal decision to donate.

Green’s functions extraction and surface-wave tomography from microseisms in southern California

We use crosscorrelations of seismic noise data from 151 stations in southern California to extract the group velocities of surface waves between the station pairs for the purpose of determining the surface-wave velocity structure. We developed an automated procedure for estimating the Green’s functions and subsequent tomographic inversion from the 11,325 station pairs based on the characteristics of the noise field. We eliminate specific events by a procedure that does not introduce any spurious spectral distortion in the band of interest, 0.05–[Formula: see text]. Further, we only used the emerging arrival structure above a threshold signal-to-noise ratio. The result is that mostly station pairs with their axes oriented toward the sea are used, consistent with the noise having a microseism origin. Finally, it is the time derivative of the correlation function that is actually related to the Green’s function. The emergence of the time-domain Green’s function is proportional to the square root of the recording time and inversely proportional to the square root of the distance between stations. The tomographic inversion yields a surface-wave velocity map that compares favorably with more conventional and elaborate experimental procedures.

Instantaneous geodetic positioning with 10–50 Hz GPS measurements: Noise characteristics and implications for monitoring networks

Recent earthquake recordings from high‐rate (1 Hz) continuous GPS stations in California indicate that even higher temporal resolution provided by modern GPS receivers is desirable. We measured seven baselines during time intervals devoid of detectable transient signals at sampling rates of 1–50 Hz with geodetic receivers from four manufacturers to investigate the noise characteristics of these data. Our tests over short distances (meters) to typical station spacing (tens of kilometers) of regional GPS networks show no loss of spatial resolution compared to 1‐Hz samples. Measurement noise is red with the typical ramp profile of log‐log spectra below about 0.5 Hz. Above this frequency, noise is essentially white. Low‐pass filtering of high‐rate positions achieves improved spatial resolution compared to decimated raw samples. Averaging 20‐Hz measurements to 2‐Hz samples on a 40‐km baseline, for example, yields about 0.5 mm horizontal and about 3–4 mm vertical accuracy at high frequencies. These estimates are a factor of 2–2.5 better than for 2‐Hz raw samples. The improvements in spatial resolution due to averaging at high frequencies are substantial and approach the theoretical “square‐root‐of‐n” expectation for independent samples. However, since noise spectral densities rise rapidly below about 0.5‐Hz, low‐pass filtering is only effective above this frequency. These results have important implications for the design of continuous GPS networks for crustal deformation and structural monitoring and for positioning and attitude determination of dynamic platforms.

Methodology and Results of Calculating Central California Surface Temperature Trends: Evidence of Human-Induced Climate Change?

Abstract A procedure is described to construct time series of regional surface temperatures and is then applied to interior central California stations to test the hypothesis that century-scale trend differences between irrigated and nonirrigated regions may be identified. The procedure requires documentation of every point in time at which a discontinuity in a station record may have occurred through (a) the examination of metadata forms (e.g., station moves) and (b) simple statistical tests. From this “homogeneous segments” of temperature records for each station are defined. Biases are determined for each segment relative to all others through a method employing mathematical graph theory. The debiased segments are then merged, forming a complete regional time series. Time series of daily maximum and minimum temperatures for stations in the irrigated San Joaquin Valley (Valley) and nearby nonirrigated Sierra Nevada (Sierra) were generated for 1910–2003. Results show that twentieth-century Valley minimum temperatures are warming at a highly significant rate in all seasons, being greatest in summer and fall (&amp;gt; +0.25°C decade−1). The Valley trend of annual mean temperatures is +0.07° ± 0.07°C decade−1. Sierra summer and fall minimum temperatures appear to be cooling, but at a less significant rate, while the trend of annual mean Sierra temperatures is an unremarkable −0.02° ± 0.10°C decade−1. A working hypothesis is that the relative positive trends in Valley minus Sierra minima (&amp;gt;0.4°C decade−1 for summer and fall) are related to the altered surface environment brought about by the growth of irrigated agriculture, essentially changing a high-albedo desert into a darker, moister, vegetated plain.

Anomalous Seismic Amplitudes Measured in the Los Angeles Basin Interpreted as a Basin-Edge Diffraction Catastrophe

The Los Angeles Basin Passive Seismic Experiment (LABPSE) involved the installation of an array of 18 seismic stations along a line crossing the Los Angeles basin from the foothills of the San Gabriel Mountains through the Puente Hills to the coast. At 3-5 km spacing between stations the array has much higher resolution than the permanent network of stations in southern California. This resolution was found to be important for analyzing the factors that govern the amplitude variation across the basin. We inverted spectra of P- and S-body-wave seismograms from local earthquakes (ML 2.1-4.8) for site effects, attenuation, and corner frequency factor using a standard model that assumes geometric spreading varying as inverse distance, exponential attenuation, and an x 2 source model. The S-wave attenuation was sep- arable into basin and bedrock contributions. In addition to the body-wave analysis, S-wave coda were analyzed for coda Q and coda-determined site effects. We find S- wave Q (QS) in bedrock is higher than in the basin. High-frequency QS is higher than low-frequency QS. Coda Q (Qc) is higher than QS. P-wave Q (QP) was not separable into basement and bedrock values, so we determined an average value only. The corner frequencies for P and S waves were found to be nearly the same. The standard model fit over 97% of the S-wave data, but data from six clustered events incident along the basin edge within a restricted range of incidence and azimuth angles gen- erated anomalous amplitudes of up to a factor of 5 higher than predicted. We test whether such basin-edge focusing might be modeled by catastrophe theory. After ruling out site, attenuation, and radiation effects, we conclude a caustic modeled as a diffraction catastrophe could explain both the frequency and spatial dependence of the anomalous variation.

High-Resolution Surface-Wave Tomography from Ambient Seismic Noise

Cross-correlation of 1 month of ambient seismic noise recorded at USArray stations in California yields hundreds of short-period surface-wave group-speed measurements on interstation paths. We used these measurements to construct tomographic images of the principal geological units of California, with low-speed anomalies corresponding to the main sedimentary basins and high-speed anomalies corresponding to the igneous cores of the major mountain ranges. This method can improve the resolution and fidelity of crustal images obtained from surface-wave analyses.

Nonparametric factorial analysis of daily weigh-in-motion traffic: implications for the ozone ?weekend effect? in Southern California

Abstract The Ozone Weekend Effect (OWE) has become increasingly more frequent and widespread in southern California since the mid-1970s. Although a number of hypotheses have been suggested to explain the effect, there remains uncertainty associated with the root factors contributing to elevated weekend ozone concentrations. Targeting the time window of the 1997 Southern California Ozone Study (SCOS97), this paper examines traffic activity data for 14 vehicle classes at 27 weigh-in-motion (WIM) stations in southern California. Nonparametric factorial analyses of light-duty vehicle (LDV) and heavy-duty truck (HDT) traffic volumes indicate significant differences in daily volumes by day of week and between the weekly patterns of daily LDV and HDT volumes. Across WIM stations, the daily LDV volume was highest on Friday and decreased by 10% on weekends compared to that on midweek days. In contrast, daily HDT volumes showed dramatic weekend drops of 53% on Saturday and 64% on Sunday. As a result, LDV to HDT ratios increased by 145% on weekends. Nonparametric tests also suggest that weekly traffic patterns varied significantly between WIM stations located close to (central) and far from (peripheral) the Los Angeles Metro area. Weekend increases in LDV/HDT ratios were more pronounced at central WIM sites due to greater weekend declines of HDT relative to LDV traffic. The implications of these weekly traffic patterns for the OWE in southern California were investigated by estimating daily WIM traffic on-road running exhaust emissions of total organic gas (TOG) and oxides of nitrogen (NOx) using EMFAC2002 emission factors. The results support the California Air Resource Board's (CARB's) NOx reduction hypothesis that greater weekend NOx reductions relative to volatile organic compound (VOC) emissions, in combinations with the VOC-limited ozone system, contribute to the OWE observed in the region. The results from this study can be used to develop weekend on-road mobile emission inventories for the purpose of air quality modeling.

GRAY WHALES BORN NORTH OF MEXICO: INDICATOR OF RECOVERY OR CONSEQUENCE OF REGIME SHIFT?

Every winter, most gray whales (Eschrichtius robustus) of the eastern North Pacific stock migrate from feeding areas in the Arctic to warm, shallow lagoons in Mexico, covering a distance of 15 000-20 000 km roundtrip. It is hypothesized that this migration to warmer climes is undertaken to reduce the whales' thermoregulatory energy requirement during winter when food resources are low. Calves are particularly vulnerable as they have yet to acquire a thick layer of blubber. Prior to the mid-1970s, newborn calves were seen primarily in Mexico's lagoons. However, since 1980, shore-based observers have reported increased numbers of calf sightings north of Mexico. Calves were greatly under-represented in the shore-based records as rarely did more than one independent observer at a time recognize the presence of a calf and a strong nearshore preference was not evident from the aerial data. Although cows with calves were difficult to detect, significant increases in average annual calf counts occurred at two counting stations in California, USA; counts increased in the late 1970s at a station near San Diego (southern California) and in the mid-1980s at a station near Carmel (central California). This trend is probably more than an increased emphasis on reporting calf sightings over the years for two reasons: (1) The first reports of calves stranding north of Mexico during the southbound migration occurred after 1976; and (2) calves were absent during many of the earlier censuses, and when they were seen, most appeared near the end of each migration. In subsequent years, calf sightings spread through the respective seasons, first at the southern stations (sometime after 1969) and then farther north (sometime after 1980). Increased calf counts at the northern stations were strongly correlated with warmer sea surface temperature anomalies. The interannual increase in calf sightings may be related to the increased abundance of the population, to changes in ocean climate, or to both factors. A one-week shift in the timing of the south- bound migration since 1980 placed the mean passage date for pregnant females near Carmel at 8 or 9 January, coinciding with earlier estimates of median calving date (10-13 January). Assuming the median parturition date has not changed, this would mean that nearly half of the calving now occurs north of Carmel.

Lateral variation of the D″ discontinuity beneath the Cocos Plate

Broadband shear wave signals from 15 deep South American earthquakes reveal small‐scale variations of the D″ shear velocity discontinuity beneath the Cocos Plate. Deconvolution by average event source wavelets allows multiple event and California station combinations to be double‐array stacked in 4 geographically separate reflection point bins with lateral dimensions of about 200 km. Reflections from the top of D″ are apparent in all 4 bins, with significant variations in timing and strength relative to ScS. The data are compatible with a discontinuous shear velocity increase 264 km above the core‐mantle boundary that varies laterally in strength from 0.9% to 2.6%, with corresponding lateral velocity variations within D″.

Weekend/weekday differences of ozone, NOx, Co, VOCs, PM10 and the light scatter during ozone season in southern California

Abstract The weekend effect on ozone, a phenomenon with high ozone concentration at weekend, was analyzed using the data measured at 7 photochemical assessment monitoring stations (PAMS), (1) volatile organic compounds (VOCs) monitoring station and (2) routine ambient air quality monitoring stations in southern California during ozone season from 1995 to 2001. Weekend/weekday differences of ozone, ozone precursors and PM10 concentrations and the light scatter, composition and reactivity of VOCs in morning traffic rush time and afternoon peak ozone time were compared. The possible causes for the weekend effect were discussed. The significant weekend effect was observed in southern California except in areas close to beach and far downwind direction from L.A. Downtown. On average, the peak O3 and max 8 h average O3 concentrations at weekend were 20% and 22%, respectively, higher than those on weekday while the concentrations of O3 precursors and PM10 and the light scatter were lower at weekday in southern California. In morning traffic rush time, the average concentrations of NOx, CO, NMOC and PM10 and the light scatter at weekend were about 37%, 18%, 15%, 14% and 9%, respectively, lower than those on weekday. In afternoon peak ozone time, the average concentrations of NOx, CO, NMOC and PM10 and the light scatter at weekend were about 29%, 13%, 24%, 17% and 17%, respectively, lower than those on weekday. The reactivities of VOCs in morning traffic rush time and afternoon peak ozone time at weekend were 17% and 26%, respectively, lower than those on weekday. VOCs sensitivity for ozone formation and a decrease NOx emissions at weekend can well explain the weekend effect in southern California. Less scatter of sunlight due to lower fine particle concentration at weekend results in enhanced ozone formation might be another possible cause for the weekend effect.

Detection of arbitrarily large dynamic ground motions with a dense high‐rate GPS network

We describe the detection of teleseismic surface waves from the 3 November 2002 Mw 7.9 Denali fault earthquake in Alaska with a dense network of 1 Hz GPS stations in southern California, about 3900 km from the event. Relative horizontal displacements with amplitudes in excess of 15 mm and duration of 700 seconds agree with integrated velocities recorded by nearby broadband seismometers with an rms difference of 2–3 mm. The displacements are derived from independent 1 Hz instantaneous positions demonstrating that a GPS network can provide direct measurements of arbitrarily large dynamic and static ground horizontal displacements at periods longer than 1 s and amplitudes above 2 mm, with an inherent precision (signal to noise) that improves indefinitely with amplitude without clipping and in real time. High‐rate, real‐time GPS networks can enhance earthquake detection and seismic risk mitigation and support other applications such as intelligent transportation and civil infrastructure monitoring.