Publications

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2006
Meltzner, AJ, Sieh K, Abrams M, Agnew DC, Hudnut KW, Avouac JP, Natawidjaja DH.  2006.  Uplift and subsidence associated with the great Aceh-Andaman earthquake of 2004. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb003891   AbstractWebsite

Rupture of the Sunda megathrust on 26 December 2004 produced broad regions of uplift and subsidence. We define the pivot line separating these regions as a first step in defining the lateral extent and the downdip limit of rupture during that great M(w) approximate to 9.2 earthquake. In the region of the Andaman and Nicobar islands we rely exclusively on the interpretation of satellite imagery and a tidal model. At the southern limit of the great rupture we rely principally on field measurements of emerged coral microatolls. Uplift extends from the middle of Simeulue Island, Sumatra, at similar to 2.5 degrees N, to Preparis Island, Myanmar (Burma), at similar to 14.9 degrees N. Thus the rupture is similar to 1600 km long. The distance from the pivot line to the trench varies appreciably. The northern and western Andaman Islands rose, whereas the southern and eastern portion of the islands subsided. The Nicobar Islands and the west coast of Aceh province, Sumatra, subsided. Tilt at the southern end of the rupture is steep; the distance from 1.5 m of uplift to the pivot line is just 60 km. Our method of using satellite imagery to recognize changes in elevation relative to sea surface height and of using a tidal model to place quantitative bounds on coseismic uplift or subsidence is a novel approach that can be adapted to other forms of remote sensing and can be applied to other subduction zones in tropical regions.

2002
Fialko, Y, Sandwell D, Agnew D, Simons M, Shearer P, Minster B.  2002.  Deformation on nearby faults induced by the 1999 Hector Mine earthquake. Science. 297:1858-1862.   10.1126/science.1074671   AbstractWebsite

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.

2000
Sandwell, DT, Sichoix L, Agnew D, Bock Y, Minster JB.  2000.  Near real-time radar interferometry of the Mw 7.1 Hector Mine Earthquake. Geophysical Research Letters. 27:3101-3104.   10.1029/1999gl011209   AbstractWebsite

The Hector Mine Earthquake (Mw 7.1, 16 October 1999) ruptured 45 km of previously mapped and unmapped faults in the Mojave Desert. The ERS-2 satellite imaged the Mojave Desert on 15 September and again on 20 October, just 4 days after the earthquake. Using a newly-developed ground station we acquired both passes and were able to form an interferogram within 20 hours of the second overflight. Estimates of slip along the main rupture are 1-2 meters greater than slip derived from geological mapping. The gradient of the interferometric phase reveals an interesting pattern of triggered slip on adjacent faults as well as a 30 mm deep sink hole along Interstate 40.

1993
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.

Feigl, KL, Agnew DC, Bock Y, Dong D, Donnellan A, Hager BH, Herring TA, Jackson DD, Jordan TH, King RW, Larsen S, Larson KM, Murray MH, Shen ZK, Webb FH.  1993.  Space Geodetic Measurement of Crustal Deformation in Central and Southern California, 1984-1992. Journal of Geophysical Research-Solid Earth. 98:21677-21712.   10.1029/93jb02405   AbstractWebsite

We estimate the velocity field in central and southern California using Global Positioning System (GPS) observations from 1986 to 1992 and very long baseline interferometry (VLBI) observations from 1984 to 1991. OUT core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north-south, and a semimajor axis of approximately 3 mm/yr oriented east-west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position (''repeatability''), (2) independent velocity estimates from separate analyses of the GPS and VLBI data, and (3) rates of change in baseline length estimated from the joint GPS+VLBI solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model from the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 +/- 1, 5 +/- 1, 5 +/- 1, and 2 +/- 1 mm/yr, respectively. North of the Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnetic declinations. Our estimates indicate that the ''San Andreas discrepancy'' can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of 1-2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the offshore islands and the mainland.

1988
Wyatt, FK, Morrissey ST, Agnew DC.  1988.  Shallow Borehole Tilt - A Reprise. Journal of Geophysical Research-Solid Earth and Planets. 93:9197-9201.   10.1029/JB093iB08p09197   AbstractWebsite

We describe results from nearly a decade of tilt measurements produced by two arrays of shallow borehole tiltmeters: one in a semiarid environment at Piñon Flat Observatory (PFO), California (depth of burial 4.5 m) and the other in a maritime-Arctic environment at Adak, Alaska (depth of burial 2 m). Although renovation and reinstallation of the instruments at the two sites reduced thermal noise, it did not change the secular records significantly. This implies that the large tilts observed reflect instability of the ground rather than the sensor, so that deeper installations should give better results. The PFO data show large rainfall-related tilts (caused by near-surface weathering) and also periodic temperature-related tilts (from several thermoelastic effects). The Adak data are dominated by tilts from the annual temperature cycle (though by a smaller amount than at PFO), but at other frequencies they show essentially the same power levels as at PFO. Both data sets confirm earlier results that burial at such shallow depths, even in apparently stable material, is inadequate for the measurement of tectonic tilts.

1986
Agnew, DC, Berger J, Farrell WE, Gilbert JF, Masters G, Miller D.  1986.  Project IDA: a decade in review. EOS Trans. AGU. 67:203-212. Abstract
n/a
1984
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.