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Inbal, A, Cristea-Platon T, Ampuero JP, Hillers G, Agnew D, Hough SE.  2018.  Sources of long-range anthropogenic noise in Southern California and implications for tectonic tremor detection. Bulletin of the Seismological Society of America. 108:3511-3527.   10.1785/0120180130   AbstractWebsite

We study anthropogenic noise sources seen on seismic recordings along the central section of the San Jacinto fault near Anza, southern California. The strongest signals are caused by freight trains passing through the Coachella Valley north of Anza. Train-induced transients are observed at distances of up to 50 km from the railway, with durations of up to 20 min, and spectra that are peaked between 3 and 5 Hz. Additionally, truck traffic through the Coachella Valley generates a sustained hum with a similar spectral signature as the train transients but with lower amplitude. We also find that wind turbine activity in northern Baja California introduces a seasonal modulation of 1- to 5-Hz energy across the Anza network. We show that the observed train-generated transients can be used to constrain shallow attenuation structure at Anza. Using the results from this study as well as available borehole data, we further evaluate the performance of approaches that have been used to detect nonvolcanic tremor at Anza. We conclude that signals previously identified as spontaneous tremor (Hutchison and Ghosh, 2017) were probably generated by other nontectonic sources such as trains.

Agnew, DC.  2018.  An improbable observation of the diurnal core resonance. Pure and Applied Geophysics. 175:1599-1609.   10.1007/s00024-017-1522-1   AbstractWebsite

The resonance associated with the ellipticity of the core-mantle boundary is usually measured with observations of either the Earth's nutations, or of tidal gravity, strain, or tilt. But, improbably, it can also be seen in a dataset collected and processed with older and simpler technologies: the harmonic constants for the ocean tides. One effect of the resonance is to decrease the ratio of the amplitude of the constituent to the amplitude of the constituent to 0.96 of the ratio in the equilibrium tidal potential. The compilation of ocean-tide harmonic constants prepared by the International Hydrographic Bureau between 1930 and 1980 shows considerable scatter in this ratio; however, if problematic stations and regions are removed, this dataset clearly shows a decreased ratio. While these data apply only a weak constraint to the frequency of the resonance, they also show that the effect could have been observed long before it actually was.

Larson, KM, Agnew DC.  1991.  Application of the Global Positioning System to Crustal Deformation Measurement 1. Precision and Accuracy. Journal of Geophysical Research-Solid Earth. 96:16547-16565.   10.1029/91jb01275   AbstractWebsite

In this paper we assess the precision and accuracy of interstation vectors determined using the Global Positioning System (GPS) satellites. These vectors were between stations in California separated by 50-450 km. Using data from tracking the seven block I satellites in campaigns from 1986 through 1989, we examine the precision of GPS measurements over time scales of a several days and a few years. We characterize GPS precision by constant and length dependent terms. The north-south component of the interstation vectors has a short-term precision of 1.9 mm + 0.6 parts in 10(8); the east-west component shows a similar precision at the shortest distances, 2.1 mm, with a larger length dependence, 1.3 parts in 10(8). The vertical precision has a mean value of 17 mm, with no clear length dependence. For long-term precision, we examine interstation vectors measured over a period of 2.2 to 2.7 years. When we include the recent results of Davis et al. (1989) for distances less than 50 km, we can describe long-term GPS precision for baselines less than 450 km in length as 3.4 mm + 1.2 parts in 10(8), 5.2 mm + 2.8 parts in 10(8), 11.7 mm + 13 parts in 10(8) in the north- south, east-west, and vertical components. Accuracy has been determined by comparing GPS baseline estimates with those derived from very long baseline interferometry (VLBI). A comparison of eight interstation vectors shows differences ranging from 5 to 30 mm between the mean GPS and mean VLBI estimates in the horizontal components and less than 80 mm in the vertical. A large portion of the horizontal differences can be explained by local survey errors at two sites in California. A comparison which suffers less from such errors is between the rates of change of the baselines. The horizontal rates estimated from over 4 years of VLBI data agree with those determined with 1-2 years of GPS data to within one standard deviation. In the vertical, both GPS and VLBI find insignificant vertical motion.

Larson, KM, Webb FH, Agnew DC.  1991.  Application of the Global Positioning System to Crustal Deformation Measurement 2. The Influence of Errors in Orbit Determination Networks. Journal of Geophysical Research-Solid Earth. 96:16567-16584.   10.1029/91jb01276   AbstractWebsite

Global Positioning System (GPS) measurements of a geodetic network in southern and central California have been used to investigate the errors introduced by adopting different sets of stations as fixed. Such fixed points, called fiducial stations, are necessary to eliminate the errors of imprecise satellites orbits, which otherwise would dominate the error budget for distances greater than tens of kilometers. These fiducial stations also define the reference frame of the crustal deformation network. Establishing the magnitude of the effect of changing the fiducial network is essential for crustal deformation studies, so that these artifacts of the differences between fiducial networks used for the data analyses are not interpreted as geophysical signals. Solutions for a crustal deformation network spanning distances up to 350 km were computed with a variety of fiducial networks. We use fiducial coordinates determined from very long baseline interferometry (VLBI). We compare these solutions by computing the equivalent uniform strain and rotation that best maps one solution into another. If we use a continental-scale fiducial network with good geometry, the distortions between the solutions are about 10(-8), largely independent of the exact choice of stations. The one case of a large-scale fiducial network where the distortions are larger is when the three fiducial stations chosen all lie close to a great circle. Use of a fiducial network no larger than the crustal deformation network can produce apparent strains of up to 10(-7). Our work suggests that fiducial coordinates determined from GPS data analysis may be used, although they should be determined using a consistent reference frame, such as provided by VLBI and satellite laser ranging.