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Borsa, AA, Agnew DC, Cayan DR.  2014.  Ongoing drought-induced uplift in the western United States. Science.   10.1126/science.1260279   AbstractWebsite

The western United States has been experiencing severe drought since 2013. The solid earth response to the accompanying loss of surface and near-surface water mass should be a broad region of uplift. We use seasonally-adjusted time series from continuously operating GPS stations to measure this uplift, which we invert to estimate mass loss. The median uplift is 4 mm, with values up to 15 mm in California’s mountains. The associated pattern of mass loss, which ranges up to 50 cm of water equivalent, is consistent with observed decreases in precipitation and streamflow. We estimate the total deficit to be about 240 Gt, equivalent to a 10 cm layer of water over the entire region, or the annual mass loss from the Greenland Ice Sheet.

Bock, Y, Agnew DC, Fang P, Genrich JF, Hager BH, Herring TA, Hudnut KW, King RW, Larsen S, Minster JB, Stark K, Wdowinski S, Wyatt FK.  1993.  Detection of Crustal Deformation from the Landers Earthquake Sequence Using Continuous Geodetic Measurements. Nature. 361:337-340.   10.1038/361337a0   AbstractWebsite

THE measurement of crustal motions in tectonically active regions is being performed increasingly by the satellite-based Global Positioning System (GPS)1,2, which offers considerable advantages over conventional geodetic techniques3,4. Continuously operating GPS arrays with ground-based receivers spaced tens of kilometres apart have been established in central Japan5,6 and southern California to monitor the spatial and temporal details of crustal deformation. Here we report the first measurements for a major earthquake by a continuously operating GPS network, the Permanent GPS Geodetic Array (PGGA)7-9 in southern California. The Landers (magnitude M(w) of 7.3) and Big Bear (M(w) 6.2) earthquakes of 28 June 1992 were monitored by daily observations. Ten weeks of measurements, centred on the earthquake events, indicate significant coseismic motion at all PGGA sites, significant post-seismic motion at one site for two weeks after the earthquakes, and no significant preseismic motion. These measurements demonstrate the potential of GPS monitoring for precise detection of precursory and aftershock seismic deformation in the near and far field.

Bock, Y, Wdowinski S, Fang P, Zhang J, Williams S, Johnson H, Behr J, Genrich J, Dean J, vanDomselaar M, Agnew D, Wyatt F, Stark K, Oral B, Hudnut K, King R, Herring T, Dinardo S, Young W, Jackson D, Gurtner W.  1997.  Southern California Permanent GPS Geodetic Array: Continuous measurements of regional crustal deformation between the 1992 Landers and 1994 Northridge earthquakes. Journal of Geophysical Research-Solid Earth. 102:18013-18033.   10.1029/97jb01379   AbstractWebsite

The southern California Permanent GPS Geodetic Array (PGGA) was established in 1990 across the Pacific-North America plate boundary to continuously monitor crustal deformation. We describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19-month period between the June 28, 1992 (M-W = 7.3), Landers and January 17, 1994 (M-W = 6.7), Northridge earthquakes. We compare displacement rates at four site locations with those reported by Feigl et al. [1993], which were derived from an independent set of Global Positioning System (GPS) and very long baseline interferometry (VLBI) measurements collected over nearly a decade prior to the Landers earthquake. The velocity differences for three sites 65-100 km from the earthquake's epicenter are of order of 3-5 mm/yr and are systematically coupled with the corresponding directions of coseismic displacement. The fourth site, 300 km from the epicenter, shows no significant velocity difference. These observations suggest large-scale postseismic deformation with a relaxation time of at least 800 days. The statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets; two possible noise models fit the PGGA data equally well as described in the companion paper by Zhang et al. [this issue]; the pre-landers data are too sparse and heterogeneous to derive a reliable noise model. Under a fractal white noise model for the PGGA data we find that the velocity differences for all three sites are statistically different at the 99% significance level. A white noise plus flicker noise model results in significance levels of only 94%, 43%, and 88%. Additional investigations of the pre-landers data, and analysis of longer spans of PGGA data, could have an important effect on the significance of these results and will be addressed in future work.

Berger, J, Agnew DC, Parker RL, Farrell WE.  1979.  Seismic System Calibration 2. Cross-Spectral Calibration Using Random Binary Signals. Bulletin of the Seismological Society of America. 69:271-288. AbstractWebsite
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Barbour, AJ, Agnew DC.  2012.  Detection of Seismic Signals Using Seismometers and Strainmeters. Bulletin of the Seismological Society of America. 102:2484-2490.   10.1785/0120110298   AbstractWebsite

Using data from borehole and long-base strainmeters and from borehole and surface seismometers, we compare the seismic-wave detection capability of strainmeters and seismometers. We use noise spectra to determine the relative signal-to-noise ratios (SNRs) on different sensors, as a function of the phase velocity and frequency of a signal. For the instruments we analyze, signals with frequencies from 10(-3) to 10 Hz and phase velocities typical of (or higher than) surface and body waves will have lower SNRs on the strainmeters than on broadband seismometers. At frequencies from 0.1 to 10 Hz the borehole (short-period) seismometers have better SNRs than strainmeters for typical phase velocities; at lower frequencies strainmeter data signals would have higher SNRs.

Barbour, AJ, Agnew DC, Wyatt FK.  2015.  Coseismic strains on plate boundary observatory borehole strainmeters in Southern California. Bulletin of the Seismological Society of America. 105:431-444.   10.1785/0120140199   AbstractWebsite

Strainmeters can record offsets coincident with earthquakes, but how much these represent strain changes from elastic rebound, and how much they are contaminated by local effects, remains an open question. To study this, we use a probabilistic detection method to estimate coseismic offsets on nine borehole strainmeters (BSMs) operated by the Plate Boundary Observatory (PBO) in southern California, from 34 earthquakes with a wide range of magnitudes and distances. In general, the offsets estimated for the BSM data differ substantially from the static strain predicted by elastic dislocation theory, which is well supported by other techniques, though 10% of the observed offsets agree well with theory. For one earthquake, the BSM offsets significantly disagree with collocated long-base laser strainmeter data. Comparisons with collocated seismic data provide strong evidence that the absolute errors between the observed and predicted strains scale with the level of seismic energy density but also that relative errors (normalized by the model size) do not. We conclude that apparent strain offsets are induced by seismic waves, occurring presumably because of irreversible deformation, whether in the rock or cementing materials close to the BSMs, or in the instruments themselves. Coseismic offsets seen in PBO BSM data should therefore be viewed with caution before being used as a measure of large-scale coseismic deformation.

Barbour, AJ, Agnew DC.  2012.  Noise Levels on Plate Boundary Observatory Borehole Strainmeters in Southern California. Bulletin of the Seismological Society of America. 101:2453-2466.   10.1785/0120110062   AbstractWebsite

To establish noise levels for the borehole strainmeters of the Plate Boundary Observatory (PBO), we have analyzed data recorded by eight of these instruments, all in the Anza region of southern California. We determine time-varying power spectra for frequencies from 10(-3) to 10 Hz, using a new method that combines multitaper spectrum estimation, smoothing by local regression, and computation of cumulative distribution functions. From about 2 Hz to the Nyquist frequency of 10 Hz, the noise floor is set by instrument resolution; for frequencies between 0.1 Hz and 1 Hz, it is set by microseisms. The lowest noise level is between 0.01 and 0.1 Hz, with a rapid increase at lower frequencies. However, in most instruments this low-noise range also contains narrowband noise that appears to be caused by power supply fluctuations. We compare these results with noise spectra from other types of strainmeters, which suggest two conclusions: (1) they are in agreement with results for surficial, long-baseline instruments; and (2) other subsurface strainmeters have lower noise in the seismic band than the PBO instruments do.