Publications

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Book Chapter
Legg, M, Agnew D.  1979.  The 1862 earthquake in San Diego. Earthquake and Other perils: San Diego region. ( Abbott PL, Elliott WJ, Eds.).:139-141., San Diego: San Diego Association of Geologists Abstract
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Agnew, D.  1987.  The continuous measurement of crustal deformation. Methods of experimental physics 24, Part B, Geophysics. Field measurements. ( Sammis CG, Henyey TL, Celotta R, Eds.).:409-439., London; New-York: Academic press ; Abstract
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Agnew, DC.  2007.  Earth Tides. Treatise on Geophysics and Geodesy. ( Herring TA, Ed.).:163-195., New York: Elsevier Abstract
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Agnew, D. C; Legg, SM; C.  1979.  Earthquake history of San Diego. Earthquake and Other perils: San Diego region. ( Abbott PL, Elliott WJ, Eds.).:123-138., San Diego: San Diego Association of Geologists Abstract
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Agnew, D.  1998.  Gravity since 1800. Sciences of the Earth: An Encyclopedia of places, People and Phenomenon. ( Good G, Ed.).:403-406.: Garland Publishing Abstract
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Agnew, DC.  2002.  History of Seismology. IASPEI international handbook of earthwuake engineering seismology. ( Lee WHK, Ed.).:3-13.: Academic Press Abstract
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Agnew, D.  1991.  How complete is the pre-instrumental record of earthquakes in southern California? Environmental perils, San Diego Region. ( Abbott PL, Elliott WJ, Eds.).:75-88., [San Diego, Calif.]: Published for the Geological Society of America Annual Meeting by the San Diego Association of Geologists Abstract
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Agnew, D.  1998.  Instruments, Gravity. Sciences of the Earth: An Encyclopedia of places, People and Phenomenon. ( Good G, Ed.).:453-455.: Garland Publishing Abstract
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Agnew, D.  1989.  Seismic instrumentation. The Encyclopedia of solid earth geophysics. ( James DE, Ed.).:1033-1037., New York: Van Nostrand Reinhold Abstract
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Agnew, D.  1989.  Seismology: History. The Encyclopedia of solid earth geophysics. ( James DE, Ed.).:1198-1202., New York: Van Nostrand Reinhold Abstract
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Agnew, D.  1998.  Tides, Earth. Sciences of the Earth: An Encyclopedia of places, People and Phenomenon. ( Good G, Ed.).:810-812.: Garland Publishing Abstract
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Agnew, D.  1979.  Tsunami history of San Diego. Earthquake and Other perils: San Diego region. ( Abbott PL, Elliott WJ, Eds.).:117-122., San Diego: San Diego Association of Geologists Abstract
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Journal Article
Agnew, DC.  1978.  1852 Fort Yuma Earthquake - 2 Additional Accounts. Bulletin of the Seismological Society of America. 68:1761-1762. AbstractWebsite
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Agnew, DC, Wyatt FK.  1989.  The 1987 Superstition Hills Earthquake Sequence - Strains and Tilts at Pinon Flat Observatory. Bulletin of the Seismological Society of America. 79:480-492. AbstractWebsite
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Vidale, JE, Agnew DC, Johnston MJS, Oppenheimer DH.  1998.  Absence of earthquake correlation with Earth tides: An indication of high preseismic fault stress rate. Journal of Geophysical Research-Solid Earth. 103:24567-24572.   10.1029/98jb00594   AbstractWebsite

Because the rate of stress change from the Earth tides exceeds that from tectonic stress accumulation, tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level. We analyze the tidal stresses and stress rates on the fault planes and at the times of 13,042 earthquakes which are so close to the San Andreas and Calaveras faults in California that we may take the fault plane to be known. We find that the stresses and stress rates from Earth tides at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times. While the rate of earthquakes when the tidal stress promotes failure is 2% higher than when the stress does not, this difference in rate is not statistically significant. This lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0.15 bar/h just preceding seismic failure, much above the long-term tectonic stress rate of 10(-4) bar/h.

Gomberg, J, Agnew D.  1996.  The accuracy of seismic estimates of dynamic strains: An evaluation using strainmeter and seismometer data from Pinon Flat Observatory, California. Bulletin of the Seismological Society of America. 86:212-220. AbstractWebsite

The dynamic strains associated with seismic waves may play a significant role in earthquake triggering, hydrological and magmatic changes, earthquake damage, and ground failure. We determine how accurately dynamic strains may be estimated from seismometer data and elastic-wave theory by comparing such estimated strains with strains measured on a three-component long-base strainmeter system at Pinon Flat, California. We quantify the uncertainties and errors through cross-spectral analysis of data from three regional earthquakes (the M(0) = 4 x 10(17) N-m St. George, Utah; M(0) = 4 X 10(17) N-m Little Skull Mountain, Nevada; and M(0) 1 x 10(19) N-m Northridge, California, events at distances of 470, 345, and 206 km, respectively). Our analysis indicates that in most cases the phase of the estimated strain matches that of the observed strain quite well (to within the uncertainties, which are about +/-0.1 to +/-0.2 cycles). However, the amplitudes are often systematically off, at levels exceeding the uncertainties (about 20%); in one case, the predicted strain amplitudes are nearly twice those observed. We also observe significant epsilon(phi phi) strains (phi = tangential direction), which should be zero theoretically; in the worst case, the rms epsilon(phi phi) Strain exceeds the other nonzero components. These nonzero epsilon(phi phi) strains cannot be caused by deviations of the surface-wave propagation paths from the expected azimuth or by departures from the plane-wave approximation. We believe that distortion of the strain field by topography or material heterogeneities give rise to these complexities.

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.

Rolandone, F, Burgmann R, Agnew DC, Johanson IA, Templeton DC, d'Alessio MA, Titus SJ, DeMets C, Tikoff B.  2008.  Aseismic slip and fault-normal strain along the central creeping section of the San Andreas fault. Geophysical Research Letters. 35   10.1029/2008gl034437   AbstractWebsite

We use GPS data to measure the aseismic slip along the central San Andreas fault (CSAF) and the deformation across adjacent faults. Comparison of EDM and GPS data sets implies that, except for small-scale transients, the fault motion has been steady over the last 40 years. We add 42 new GPS velocities along the CSAF to constrain the regional strain distribution. Shear strain rates are less than 0.083 +/- 0.010 mu strain/yr adjacent to the creeping SAF, with 1 - 4.5 mm/yr of contraction across the Coast Ranges. Dislocation modeling of the data gives a deep, long-term slip rate of 31 - 35 mm/yr and a shallow (0 - 12 km) creep rate of 28 mm/yr along the central portion of the CSAF, consistent with surface creep measurements. The lower shallow slip rate may be due to the effect of partial locking along the CSAF or reflect reduced creep rates late in the earthquake cycle of the adjoining SAF rupture zones.

Agnew, DC.  2007.  Before PBO: an overview of continuous strain and tilt measurements in the United States. Journal of the geodetic Society of Japan. 53:157-182. Abstract
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Wyatt, F, Agnew D, Linde A, Sacks IS.  1983.  Borehole Stranimeter studies in Pinon flat observatory. Carnegie Institute of Washington, Yearbook 82, Washington DC. :533-538. Abstract
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Agnew, DC.  2010.  Comment on "Changes of Reporting Rates in the Southern California Earthquake Catalog, Introduced by a New Definition of M(L)" by Thessa Tormann, Stefan Wiemer, and Egill Hauksson. Bulletin of the Seismological Society of America. 100:3320-3324.   10.1785/0120100027   AbstractWebsite

Earthquake catalogs can be inhomogeneous because of changes in the definition of earthquake magnitude. Provided that a sufficient number of events have magnitudes defined in more than one system, it is possible to apply a Monte Carlo method to the observed joint distribution to convert sets of magnitudes from one system to another, improving any statistical analysis of the catalog. I demonstrate the method for the southern California catalog, in which the definition of local magnitude has recently been changed. Monte Carlo magnitude mapping appears to eliminate temporal changes that are otherwise present.

Donner, S, Lin CJ, Hadziioannou C, Gebauer A, Vernon F, Agnew DC, Igel H, Schreiber U, Wassermann J.  2017.  Comparing direct observation of strain, rotation, and displacement with array estimates at Pinon Flat Observatory, California. Seismological Research Letters. 88:1107-1116.   10.1785/0220160216   AbstractWebsite

The unique instrument setting at the Pinon Flat Observatory in California is used to simultaneously measure 10 out of the 12 components, completely describing the seismic-wave field. We compare the direct measurements of rotation and strain for the 13 September 2015 M-w 6.7 Gulf of California earthquake with array-derived observations using this configuration for the first time. In general, we find a very good fit between the observations of the two measurements with cross-correlation coefficients up to 0.99. These promising results indicate that the direct and array-derived measurements of rotation and strain are consistent. For the array-based measurement, we derived a relation to estimate the frequency range within which the array-derived observations provide reliable results. This relation depends on the phase velocity of the study area and the calibration error, as well as on the size of the array.

King, NE, Agnew DC, Wyatt F.  1988.  Comparing Strain Events - A Case-Study for the Homestead Valley Earthquakes. Bulletin of the Seismological Society of America. 78:1693-1706. AbstractWebsite
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Wyatt, F, Bilham R, Beavan J, Sylvester AG, Owen T, Harvey A, Macdonald C, Jackson DD, Agnew DC.  1984.  Comparing Tiltmeters for Crustal Deformation Measurement - A Preliminary Report. Geophysical Research Letters. 11:963-966.   10.1029/GL011i010p00963   AbstractWebsite

A collection of high-precision tiltmeters is being operated at Piñon Flat Observatory, southern California, both to compare instruments and to measure tectonic deformation. We report on 1.2 years of data from four of these: two Michelson-Gale long fluid tiltmeters, one long center-pressure tiltmeter, and a shallow borehole tiltmeter. The three long-base instruments are all located on the same baseline, with a precise leveling line running between their end-monuments. At nontidal frequencies, only the two Michelson-Gale instruments show some coherence (γ² = .3 for periods of 2 to 4 days), while the center-pressure instrument is correlated with air temperature at periods from a few days to a few weeks. The most stable tilt record shows a secular rate of 0.28 µrad/a, which may be real. Over much longer times, leveling to specially stabilized benchmarks should confirm this. Comparing instruments has identified more and less successful measurement techniques; it appears that low-noise data will most probably be produced only by relatively complex and expensive instruments, though even for these, the operating costs over any reasonable lifetime will exceed the capital cost. Even the best existing sensors must be improved to measure continuous tectonic motions.