Stations In California Research Articles

Complex Rayleigh waves resulting from deep sedimentary basins

Two earthquakes in northern Mexico recorded by TriNet stations in southern California show complex surface wave propagation through the Salton Trough. Stations contained in a narrow corridor extending from the Salton Trough north towards the Central Valley, California show recorded waveforms with two Rayleigh wave arrivals. The time separation between the Rayleigh wave arrivals is nearly constant beyond the northern edge of the Salton Trough. We are able to reproduce both arrivals with a simple 1-D model of a low velocity, deep basin in the Salton Trough. The slower Rayleigh wave is propagating within the sediments of the basin, while the faster Rayleigh wave propagates mainly in faster, basement rock. Further, we reproduce the constant time separation through the implementation of a simple 2-D model. This 2-D model has a low velocity, deep basin near the source and a crust without a basin at distances greater than 200 km.

A Bayesian coregionalization approach for multivariate pollutant data

Spatial data collection increasingly turns to vector valued measurements at spatial locations. An example is the observation of pollutant measurements. Typically, several different pollutants are observed at the same sampled location, referred to as a monitoring station or gauged site. Usually, interest lies in the modeling of the joint process for the levels of the different pollutants and in the prediction of pollutant levels at ungauged sites. In this case, it is important to take into account not only the spatial correlation but also the correlation among the different variables at each gauged site. Since, conceptually, there is a potentially observable measurement vector at every location in the study region, a multivariate spatial process becomes a natural modeling choice. In using a Gaussian process, the main challenge is the specification of a valid and flexible cross‐covariance function. This paper proposes a rich class of covariance functions developed through the so‐called linear coregionalization model [see, e.g., Wackernagel, 1998] for multivariate spatial observations. Following the ideas in the work of, for example, Royle and Berliner [1999], we can reparameterize a multivariate spatial model using suitable univariate conditional spatial processes, facilitating the computation. We provide explicit details, including the computation of the range associated with the different component processes. As an example, we fit our model to a particular day average of CO, NO, and NO2 for a set of monitoring stations in California, USA.

Time-domain Pure-state Polarization Analysis of Surface Waves Traversing California

— A time-domain pure-state polarization analysis method is used to characterize surface waves traversing California parallel to the plate boundary. The method is applied to data recorded at four broadband stations in California from twenty-six large, shallow earthquakes which occurred since 1988, yielding polarization parameters such as the ellipticity, Euler angles, instantaneous periods, and wave incident azimuths. The earthquakes are located along the circum-Pacific margin and the ray paths cluster into two groups, with great-circle paths connecting stations MHC and PAS or CMB and GSC. The first path (MHC-PAS) is in the vicinity of the San Andreas Fault System (SAFS), and the second (CMB-GSC) traverses the Sierra Nevada Batholith parallel to and east of the SAFS. Both Rayleigh and Love wave data show refractions due to lateral velocity heterogeneities under the path, indicating that accurate phase velocity and attenuation analysis requires array measurements. T he Rayleigh waves are strongly affected by low velocity anomalies beneath Central California, with ray paths bending eastward as waves travel toward the south, while Love waves are less affected, providing observables to constrain the depth extent of anomalies. Strong lateral gradients in the lithospheric structure between the continent and the ocean are the likely cause of the path deflections.

Azimuthal variation in seismic anisotropy of the southern California uppermost mantle

The Backus [1965] theory of seismic anisotropy is used to obtain expressions for the velocities and polarizations of quasi‐P and S waves for rays making small angles to the symmetry axes of an orthorhombic elastic tensor. Expressions are found for P wave polarization and S wave splitting parameters as a function of incidence angle θ and azimuth z. For small θ, splitting and P wave velocity exhibit predominantly a two z variation. P wave velocity squared is given by vp2 = v02 + c1θ2 + c2cos(2z)θ2, where the ci are combinations of coefficients of the elasticity tensor. The eigenvalues corresponding to the fast and slow S waves are given by similar expressions. Variation in P wave horizontal polarization exhibits a 2z variation; δzp = −Δz sin(2z) with little dependence on θ. SKKS and SKS splitting parameters exhibit an azimuthal variation given by ϕ = ϕ0 + d1 sin(2z) θ2 and δt = δt0 + e1θ2 + e2 cos(2z)θ2, where ϕ is the fast direction and δt is the time delay. We report 2z variation in splitting parameters at the Global Seismic Network stations in southern California. We use other estimates of seismic anisotropy to invert for depth‐averaged values of the elasticity tensor. The resulting tensor adequately describes azimuthal variation in P wave horizontal polarization [Schulte‐Pelkum et al., 2001], Pn, Rayleigh and Love wave velocities, as well as SKKS and SKS splitting, suggesting they all arise from a common source of anisotropy in the upper 200 km of the mantle.

The Recent Rainfall Climatology of the Mediterranean Californias

In this work, recent (1948‐2001) rainfall data in a southwestern California station (San Diego) and a northwestern Baja California station (Ensenada) within a region called Mediterranean California, around 33 8N, 1178W, are studied. Cumulative annual means are used as indicators of climatological variability; but the entire datasets are analyzed by modeling the histogram of each set as a Weibull distribution probability density function, f . The climatology of both stations, defined simply as the arithmetic average, is compared with their theoretical mean; that is, the first moment of f . It is assumed that this comparison would be indicative of the reliability of the available rainfall climatologies. If these assumptions hold, in particular if the data is indeed Weibull distributed, it can be concluded that the climatological annual mean precipitation in this region is slightly overestimated at this time.

Wavelet transform: A tool for the interpretation of upper mantle converted phases at high frequency

P to S converted receiver functions recorded at VBB FDSN California stations are studied in the frequency range of 0.1 to 1 Hz. Microseismic noise is maximum in this frequency range, but signal processing by the wavelet transform enables us: (1) to enhance seismic phases associated with seismic velocity gradients and discontinuities at the base of the upper mantle, (2) to extract accurate arrival times, and (3) to obtain an accurate insight into the frequency content. In the case of California, one seismic discontinuity and two zones of high gradients in the depth range of 625 to 720km are recurrently observed for two different data sets.

Mantle control of plate boundary deformation

The seismic wavefield propagating along the recently instrumented Pacific‐North American plate boundary (California) displays remarkable variation, with regional shear waves arriving at coastal stations up to 20 seconds earlier than equidistant stations in eastern California. Broadband modeling of this data reveals that coastal paths sample fast upper mantle typical of Miocene‐aged ocean plate (>50 Km thickness). Inland paths sample slower uppermost mantle, with the seismic lithosphere, or lid, measuring less than 5 Km thick, characteristic of the Basin and Range extensional province. The boundary in the uppermost mantle between these provinces is sharp, expressing the juxtaposition of the stronger Pacific plate with weaker continental North America. The lid step coincides with regionally maximum dextral strain rates measured with GPS, suggesting the uppermost mantle modulates long term, regional‐scale continental margin deformation and evolution.

Present day kinematics of the Eastern California Shear Zone from a geodetically constrained block model

We use Global Positioning System (GPS) data from 1993–2000 to determine horizontal velocities of 65 stations in eastern California and western Nevada between 35° and 37° N. We relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km. The velocity of the Sierra Nevada block with respect to Nevada is 11.1±0.3 mm/yr, with slip partitioned across the Death Valley, (2.8±0.5 mm/yr), Panamint Valley (2.5±0.8 mm/yr), and Airport Lake/Owens Valley (5.3±0.7/4.6±0.5 mm/yr) faults. The western Mojave block rotates at 2.1±0.8°/My clockwise, with 3.7±0.7 mm/yr of left lateral motion across the western Garlock Fault. We infer 11±2 mm/yr of right lateral motion across the Mojave region of the Eastern California Shear Zone.

The SCEC Phase III Strong-Motion Database

As part of the Southern California Earthquake Center (SCEC) Phase III effort to include site effects in hazard models for southern California, a regional database of strong-motion observations was developed. The observations consist of the peak ground acceleration (PGA) and 5% damped response spectral acceleration (SA) at 0.3-, 1.0-, and 3.0-sec periods from 28 earthquakes and 281 stations. A total of 449 pairs of horizontal PGA and SA observations that were taken from the SCEC Strong-Motion Database (SMDB) are presented here. The phase III database includes earthquakes with moment magnitudes larger than 5.0 and stations in southern California with locations between 32° and 36° north latitude. Observations from buildings with more than two stories and dam abutments or crests are excluded from the database. Observations with distances of 150 km or greater were also excluded. The agencies that provided the data to SMDB are the U.S. Geological Survey, California Strong Motion Instrumentation Program, University of Southern California, and the Los Angeles Department of Water and Power. The database also contains site classification information for each station. A first general classification is based on the 1:750,000 California map of Quaternary, Tertiary, and Mesozoic geologic units by Jennings (1977) as modified by Park and Elrick (1998). A second more detailed classification is based on Quaternary mapping in the Los Angeles region by Tinsley and Fumal (1985) as modified by Park and Elrick (1998). A third classification is based on the correlation of surface geology with shear-wave velocity in the upper 30 m (Wills et al., 2000). An arbitrary “depth-to-basement” parameter is assigned to stations that are located within the boundary of the 3D velocity model used by Olsen (2000), which is based on the SCEC 3D velocity model (version 1). This parameter is defined as the depth to the 2.5km/sec velocity isosurface. Special parameters associated with particular attenuation relations, such as different distance measures, a hanging wall flag, and fault-type flag are also assigned to each observation. These data are all presented within the tables and figures of this article, and also have been made available via a downloadable file on the Internet ( http://smdb.crustal.ucsb.edu/~phase3 ).

Moho depth variation in southern California from teleseismic receiver functions

The number of broadband three‐component seismic stations in southern California has more than tripled recently. In this study we use the teleseismic receiver function technique to determine the crustal thicknesses and Vp/Vs ratios for these stations and map out the lateral variation of Moho depth under southern California. It is shown that a receiver function can provide a very good “point” measurement of crustal thickness under a broadband station and is not sensitive to crustal P velocity. However, the crustal thickness estimated only from the delay time of the Moho P‐to‐S converted phase trades off strongly with the crustal Vp/Vs ratio. The ambiguity can be reduced significantly by incorporating the later multiple converted phases, namely, the PpPs and PpSs+PsPs. We propose a stacking algorithm which sums the amplitudes of receiver function at the predicted arrival times of these phases by different crustal thicknesses H and Vp/Vs ratios. This transforms the time domain receiver functions directly into the H‐Vp/Vs domain without need to identify these phases and to pick their arrival times. The best estimations of crustal thickness and Vp/Vs ratio are found when the three phases are stacked coherently. By stacking receiver functions from different distances and directions, effects of lateral structural variation are suppressed, and an average crustal model is obtained. Applying this technique to 84 digital broadband stations in southern California reveals that the Moho depth is 29 km on average and varies from 21 to 37 km. Deeper Mohos are found under the eastern Transverse Range, the Peninsular Range, and the Sierra Nevada Range. The central Transverse Range, however, does not have a crustal root. Thin crusts exist in the Inner California Borderland (21–22 km) and the Salton Trough (22 km). The Moho is relatively flat at the average depth in the western and central Mojave Desert and becomes shallower to the east under the Eastern California Shear Zone (ECSZ). Southern California crust has an average Vp/Vs ratio of 1.78, with higher ratios of 1.8 to 1.85 in the mountain ranges with Mesozoic basement and lower ratios in the Mojave Block except for the ECSZ, where the ratio increases.

JPL contribution to the VSOP mission

Abstract The Jet Propulsion Laboratory (JPL) is a participant in the VSOP Space VLBI mission, an extensive international collaboration led by Japan's Institute of Space and Astronautical Science (ISAS). The JPL effort is funded by the U.S. National Aeronautics and Space Administration (NASA). To obtain data from the orbital element of the VSOP mission, the Japanese HALCA satellite, the Deep Space Network (DSN) of JPL has built a new set of three 11-meter tracking stations in California, Spain, and Australia. These stations have supported over 1000 HALCA passes during the first year of operation, and supply science, telemetry, and Doppler data for the mission. JPL is using the Doppler data to estimate the satellite orbital parameters that are needed by the tracking stations and VLBI data correlators. The DSN also modified their three 70-meter antennas to provide ground VLBI observing support for the mission. In addition to operational support, JPL has had significant involvement in the mission planning and scientific support aspects of the mission, including mission design, international data flow, scientific scheduling, and data analysis during in-orbit checkout. JPL has also aided the scientific community in the use of VSOP by developing a user software package, writing a guide for proposers, and establishing a proposer help desk.

Public Finance and Transit-Oriented Planning: New Evidence from Southern California

Local governments seem to be continually strapped for funds. While the revenue-generating role of their planners is often discussed, it is rarely investigated in any detail. We address this research gap by considering the fiscal nature of land use policy in transitoriented development. A massive and influential literature has explored the potential for leveraging rail system investments by locating high density residential developments near commuter rail stations. The feasibility and focus of these strategies have been questioned, however, in the face of evidence that local government support for these projects is mixed at best. To explain this behavior, we examine the role basic fiscal conditions play in the decision to zone land near all existing and proposed commuter rail stations in southern California. The analysis indicates that stationarea zoning depends significantly on community public finances. The results underscore how the practice of transitoriented development must account not only for travel behavior and the broader goals of any given urban design, but also for the selfinterested nature of municipal planning.

Continuous GPS measurements of contemporary deformation across the Northern Basin and Range Province

We have acquired and analyzed data from the northern Basin and Range (NBAR) continuous GPS network since July 1996. The RMS residual with respect to the best fitting lines through the individual station position estimates is 2–3 mm in the horizontal and 6–10 mm in the vertical. After the first 395 days of operation, uncertainties in horizontal velocity estimates are 1–2 mm/yr (1‐σ). Relative motion among NBAR sites located in eastern Nevada and in Utah is small, but east‐west extension is significant assuming uniform strain accumulation across the whole network. The relative motion observed across the Wasatch fault zone is 2 ±2 mm/yr, east‐west. Relative motions among stations in western Nevada and California, in contrast, are dominated by northwest, right‐lateral shear. We infer an integrated deformation across the northern Basin and Range of 11 ± 2 mm/yr, northwest. These rates are consistent with previous geodetic measurements. Our GPS velocity estimates, however, reveal a possibly abrupt transition from east‐west extension in eastern Nevada and Utah to right‐lateral shear in western Nevada. This transition is roughly coincident with the central Nevada seismic belt and is consistent with the right‐oblique focal mechanisms of the 1954 Dixie Valley and Fairview Peak earthquakes. The transition also appears to correlate spatially with a transition in upper mantle structure.

Seismic structure using body wave arrivals at SOSUS hydrophone arrays

Body wave arrivals at SOSUS hydrophone arrays are used to determine the seismic velocity structure of the Juan de Fuca ridge and Cascadia subduction zone regions. Since 1991 NOAA has been recording data from the Navy’s SOSUS hydrophone arrays. This has provided a unique set of continuously recorded hydrophone data from oceanic stations. Body wave or P-phase arrivals from large earthquakes are detected by the SOSUS hydrophones. Using body wave arrivals Pn velocities of 8.0 km/s are measured for paths purely beneath the oceanic plates and for paths originating beneath the North American continental plate the Pn velocity is 7.7 km/s. For paths along the subduction zone velocities of 7.6–7.7 km/s are measured. The data from the SOSUS arrays are combined with data from the Pacific Northwest Seismic Network (PNSN), a network of seismic stations in Washington, Oregon and northern California, to construct a tomographic image of the crust and uppermost mantle for the Cascadia subduction zone. The model extends from the oceanic plate (131°W) to the North American continental plate (120°W) and from 40°N to 45°N. The tomographic inversion is a joint inversion for seismic structure and epicenter location using both the body wave and T-phase data. The model images the transition from oceanic to continental plate and the initiation of subduction of the Gorda plate. The discrepancy between event epicenters located using T-phase arrivals alone with the same epicenters located with P- and T-phase arrivals are analyzed to learn more about the process of T-phase generation.

Localized boundary layer below the mid‐Pacific velocity anomaly identified from a PcP precursor

Dense record sections from deep earthquakes in Fiji and Argentina recorded on hundreds of short‐period stations in California at distances of 81° to 85° are used to investigate the detailed P wave velocity structure above the core‐mantle boundary (CMB). In the Fiji data a secondary phase arriving 2 to 4 s after the direct P is identified as a precursor to PcP. This phase provides good evidence for a reflection off the top of a thin low‐velocity layer above the CMB. Comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5%–10% compared to the overlying mantle. A record section from an Argentina event does not show the PcP precursor, indicating that the low‐velocity layer is not a global feature. This thin low‐velocity layer is in the same place as a much larger S wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the CMB under the mid‐Pacific.

P′P′ Precursors Under Africa: Evidence for Mid-Mantle Reflectors

Observations of precursors to P′P′ from a recent exceptionally large deep earthquake in the Fiji Islands (moment magnitude = 7.6) at an array of broadband stations in California revealed mid-mantle reflectors near depths of 785 kilometers and 1200 kilometers under the southern African rift. Such observations, previously reported primarily in subduction zones, suggest that these reflectors may have a global character. Our analysis, which also indicated a sharp, uplifted 670-kilometer discontinuity, demonstrates the power of sparse regional broadband arrays for the study of weak, frequency-dependent features in deep-Earth structure.

Precision global temperatures from satellites and urban warming effects of non-satellite data

Data sets designed for operational use are now being employed to monitor the earth on decadal time scales, an application outside of their original purpose. One data set, from the Microwave Sounding Unit, measures atmospheric temperature in deep layers since late 1978. Procedures for verifying and validating these data by two tests are presented: one by comparison with an independent satellite and one by comparison with radiosonde data. The results show that data from such an instrument are precise and therefore ideal for global studies. Temperatures measured by surface thermometers have considerable error for long-term studies because of various factors. Widely varying results are shown for California stations, where the few sites chosen for global temperature trend estimation are shown not to be representative of the average trend measured by over 100 stations in the state. Such a result calls into question the ability of scattered, selected surface networks to provide a true picture of long-term trends. The global temperature trend of the lower troposphere measured by the Microwave Sounding Units from January 1979 to March 1994 is −0.06°C per decade.

Site amplification from coda waves: Validation and application to S-wave site response

Abstract The subject of this article is to see if the response to S waves at a specific site can be determined from coda waves. We used aftershock records of the 1992 Landers and Big Bear earthquakes observed at four TERRAscope stations (PAS, PFO, SVD, and GSC) in southern California. The wide dynamic range of the TERRAscope instrument provided a broadband seismogram for S waves, and, at the same time, coda waves over a long lapse-time window. We analyzed a total of 708 three-component waveforms from 65 events whose magnitudes ranged from 3.3 to 5.4. The station SVD is located on Quaternary alluvium, and the remaining stations are on rocks of Cretaceous age. We first evaluated the coda site-amplification factor using a spectral ratio method (May-eda et al. , 1991; Koyanagi et al. , 1992) for frequency bands centered at 0.75, 1.0, 1.5, 2.0, 3.0, 4.0, and 6.0 Hz for lapse-time windows after 70 sec from the origin time. We found that the site-amplification factor for the radial component is similar to that for the transverse component. This similarity was expected because codas consist of backscattered waves coming from all directions into the station, resulting in a directionally averaged site response. We then evaluated the S -wave site-amplification factor using an inversion method (Andrews, 1986) on the direct S -wave portion of the same data set. We found that the coda site response agrees with the S -wave site response within a factor of about 1.5, supporting the earlier findings by Tsujiura (1978) and justifying the use of coda waves in determining site-specific amplification for S waves. This agreement between S -wave site response and coda site response is understandable if we assume that coda waves are dominated by S waves, as theoretically predicted by Aki (1992) and Zeng (1993).

SKS splitting beneath southern California

Measurements of SKS phase splitting were obtained from nineteen seismic stations in southern California. The fast polarization directions are 53° at the southern end of the Great Valley, 82±8° in the western Transverse Ranges and northern Peninsular Ranges, 95±4° in Mojave Desert, and 70° on San Clemente Island. The splitting time ranges from 0.8 to 1.8 seconds, which is consistent with an anisotropic layer of 100 to 200 km thick for 4% anisotropy.

Large-Scale Atmospheric Forcing of Recent Trends toward Early Snowmelt Runoff in California

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April–June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude, suggesting that the temperature trend is sufficient to explain the runoff-timing trends. The immediate cause of the trend toward warmer winters in California is a concurrent, long-term fluctuation in winter atmospheric circulations over the North Pacific Ocean and North America that is not immediately distinguishable from natural atmospheric variability. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation of winter circulations shifted to a part of the central North Pacific Ocean that is teleconnected to California temperatures. Since the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the west coast of North America. These shifts in atmospheric circulations are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures.