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1995
Johnson, HO, Agnew DC.  1995.  Monument Motion and Measurements of Crustal Velocities. Geophysical Research Letters. 22:2905-2908.   10.1029/95gl02661   AbstractWebsite

It is usually assumed in geodetic studies that measurement errors are independent from one measurement to the next and that the rate of deformation (velocity) is constant over the duration of the experiment. Any temporal correlation between measurements can substantially affect the uncertainty in this velocity estimate when it is determined;from the time series of measurements. One source of possible long-term. correlation is motion of the geodetic monument with respect to the ''deep'' crust. Available measurements suggest that this motion introduces errors that have the form of a random walk process. We show how such errors affect the uncertainty of velocity estimates. For a geodetic experiment of set duration we calculate the velocity uncertainty as a function of the number of observations and of the relative amount of correlated and uncorrelated noise. We find that 1) neglecting long-term temporal correlations makes the uncertainty in the estimated velocities much too small, and that 2) when the correlated and independent noise sources are of similar magnitude, the expected improvement in uncertainty from having more measurements (1/root N) is not realized; there is almost no improvement in some cases. We have also examined the effect of outliers (''blunders'') on the velocity uncertainty; for a frequency of outliers typical of geodetic field the previous two conclusions remain These results suggest that long-term correlations have a large effect on estimating deformation rates; unless these correlations are small, frequent observations give little advantage. If frequent observations are planned, the amount of correlated noise due to monument instability must be kept small if the full capabilities of the measurement technique are to be realized.

Agnew, DC.  1995.  Ocean-Load Tides at the South-Pole - A Validation fo Recent Ocean-Tide Models. Geophysical Research Letters. 22:3063-3066.   10.1029/95gl03074   AbstractWebsite

Small diurnal and semidiurnal gravity tides are seen at the South Pole because of the loading by and attraction of the ocean tides. These data provide a check on the quality of ocean-tide models, especially in the southernmost ocean, which has historically been the most lacking in tidal data. Ocean-tide models developed in the 1980's did not predict the gravity tides at this location very well. Recently-developed models based on the Topex/Poseidon altimetric data and improved hydrodynamical modeling agree much better with the observations, provided that the tides beneath the ice shelves are included. The level of agreement at this remote location suggests that, loads from very local tides aside, the new generation of ocean-tide models can predict the loading tides to very high accuracy.

1996
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.

Hart, RHG, Gladwin MT, Gwyther RL, Agnew DC, Wyatt FK.  1996.  Tidal calibration of borehole strain meters: Removing the effects of small-scale inhomogeneity. Journal of Geophysical Research-Solid Earth. 101:25553-25571.   10.1029/96jb02273   AbstractWebsite

We investigate the estimation of Earth strain from borehole strain meter data in a study of tidal calibration of the Gladwin borehole tensor strain meter (BTSM) at Pinon Flat. Small-scale geological inhomogeneity is one of several effects examined that cross couple remote areal/shear strain into measured areal/shear strain. A methodology is developed for incorporating cross coupling into the strain meter calibration. Using the measured strain tides from the colocated laser strain meter (LSM) as a reference, we show that calibration of the BTSM with cross coupling removes systematic errors of up to 30% in the borehole strain meter tides. This calibration accurately relates the BTSM measurements to strains at the scale length of the LSM, about 1 km. The calibration technique provides a solution to a major criticism of all short-baseline strain measurements, namely, that tectonic strains are not representatively sampled due to small-scale inhomogeneities. The technique removes errors potentially greater than 50% in observed strain offsets from fault slip, permitting improved constraint of slip mechanisms. We show that current theoretical estimates of strain tides in the instrument locality are not yet of sufficient accuracy for cross-coupled calibration. Comparison of theoretical tides with measurements from the LSM suggest that at least half of the error is in the ocean load tide estimates.

1997
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.

Agnew, DC.  1997.  NLOADF: A program for computing ocean-tide loading. Journal of Geophysical Research-Solid Earth. 102:5109-5110.   10.1029/96jb03458   AbstractWebsite

The loading of the Earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique. In order to compute these most accurately; a combination of global and local models of the ocean tides may be needed. The program NLOADF convolves the Green functions for loading with ocean tide models using a station-centered grid with fixed dimensions, making it easy to combine different ocean models without overlap in the convolution. The program computes all the quantities of interest (gravity, displacement, tilt, and strain) and includes the case where measurements are made beneath the surface of the ocean.

1998
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, 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.  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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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.

2000
Astiz, L, Shearer PM, Agnew DC.  2000.  Precise relocations and stress change calculations for the upland earthquake sequence in southern California. Journal of Geophysical Research-Solid Earth. 105:2937-2953.   10.1029/1999jb900336   AbstractWebsite

We relocate earthquakes that occurred near the 1988 (M-L = 4.7) and the 1990 (M-L = 5.5) Upland, California, earthquakes to map the fault geometry of the poorly defined San Jose fault and to test the static:Stress triggering hypothesis for this sequence. We adopt the L1 norm, waveform cross-correlation method of Shearer [1997] to obtain precise relocations for 1573 events: between 1981 and 1997 in the Upland area. To limit computation time, we only perform waveform cross correlation on 60 of the nearest neighbors of leach relocated event. Our final relocations show two linear features. The first is imaged,by the locations of the initial month of aftershocks of the 1988 Upland earthquake, which delineate a fault with a,dip angle of similar to 45 degrees between 7 and 9 km depth, consistent with the mainshock focal mechanism. The second linear feature is a plane, dipping at about 74 degrees from 2 to 9 km depth, which is illuminated by both the 1988:and 1990 Upland sequences, in agreement with the inferred location of the San Jose fault at depth. However, below 9 km the event locations become more diffuse, giving rise to two different interpretations of the fate of the San Jose fault at depth. One possibility is that the fault shallows at depth, consistent with our relocations: but not with the focal mechanism of a M-L = 4.7 deep aftershock. Alternatively, the. fault may be offset at depth by the more shallow dipping fault strand broken during the 1988 earthquake, Using these inferred fault geometries, we compute stress changes resulting from slip during the mainshocks to test whether the relocated aftershocks are consistent with the:hypothesis that more aftershocks occur where the change in static Coulomb failure stress is positive (on faults optimally oriented for failure). This requires an extension of previous models of changes in the failure stress to three dimensions and arbitrary fault orientation. We find that patterns of change in Coulomb failure stress differ little between the different fault geometries: all are nearly symmetric about the fault and so do not match the aftershock distribution, in which most of the off-fault events occur to one side of the fault plane.

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.

2001
Agnew, DC.  2001.  Map Projections to show the possible effects of surface loading. Journal of the geodetic Society of Japan. 47:255-260. Abstract
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Fialko, Y, Simons M, Agnew D.  2001.  The complete (3-D) surface displacement field in the epicentral area of the 1999 M(w)7.1 Hector Mine earthquake, California, from space geodetic observations. Geophysical Research Letters. 28:3063-3066.   10.1029/2001gl013174   AbstractWebsite

We use Interferometric Synthetic Aperture Radar (InSAR) data to derive continuous maps for three orthogonal components of the co-seismic surface displacement field due to the 1999 M-w 7.1 Hector Mine earthquake in southern California. Vertical and horizontal displacements are both predominantly antisymmetric with respect to the fault plane, consistent with predictions of linear elastic models of deformation for a strike-slip fault. Some deviations from symmetry apparent in the surface displacement data may result from complexity in the fault geometry.

Nikolaidis, RM, Bock Y, de Jonge PJ, Shearer P, Agnew DC, vanDomselaar M.  2001.  Seismic wave observations with the Global Positioning System. Journal of Geophysical Research-Solid Earth. 106:21897-21916.   10.1029/2001jb000329   AbstractWebsite

We describe the direct measurement of ground displacement caused by the Hector Mine earthquake in southern California (M-w 7.1, October 16, 1999). We use a new method of instantaneous positioning, which estimates site coordinates from only a single epoch of Global Positioning System (GPS) data, to measure dynamic as well as static displacements at 24 stations of the Southern California Integrated GPS Network (SCIGN), with epicentral distances from 50 to 200 km. For sites outside the Los Angeles basin the observed displacements are well predicted by an elastic half-space model with a point shear dislocation; within the sedimentary basin we observe large displacements with amplitudes up to several centimeters that last as long as 3-4 min. Since we resolve the GPS phase ambiguities and determine site coordinates independently at each epoch, the GPS solution rate is the same as the receiver sampling rate. For the SCIGN data this is 0.033 Hz (once per 30 s), though sample rates up to 2 Hz are possible with the SCIGN receivers. Since the GPS phase data are largely uncorrelated at I s, a higher sampling rate would offer improved temporal resolution of ground displacement, so that in combination with inertial seismic data, instantaneous GPS positioning would in many cases significantly increase the observable frequency band for strong ground motions.

2002
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, DC, Owen S, Shen ZK, Anderson G, Svarc J, Johnson H, Austin KE, Reilinger R.  2002.  Coseismic Displacements from the Hector Mine, California, earthquake: Results from survey-mode global positioning system measurements. Bulletin of the Seismological Society of America. 92:1355-1364.   10.1785/0120000928   AbstractWebsite

We describe the collection and processing of Global Positioning System (GPS) data from 77 locations around the Hector Mine earthquake, which we use to estimate coseismic displacements related to this shock. The existence of pre-event GPS data, some collected to monitor postseismic displacements from the 1992 Landers earthquake and some to establish survey control in the meizoseismal area, provided a relatively dense coverage close to the rupture zone. The data available were collected mostly within the 2 years prior to the 1999 earthquake; we reobserved many points within a few days after the shock, and all within 6 months after. We include corrections for interseismic motion to provide the best value possible for coseismic motion caused by this earthquake. The displacements in general display the pattern expected for a strike-slip fault, though a few show significant vertical motion. The maximum horizontal displacement observed was 2 m; one station between fault ruptures showed little horizontal motion, but significant uplift.

Owen, S, Anderson G, Agnew DC, Johnson H, Hurst K, Reilinger R, Shen ZK, Svarc J, Baker T.  2002.  Early postseismic deformation from the 16 October 1999 M-w 7.1 Hector Mine, California, earthquake as measured by survey-mode GPS. Bulletin of the Seismological Society of America. 92:1423-1432.   10.1785/0120000930   AbstractWebsite

The 16 October 1999 (M-w 7.1) Hector Mine earthquake was the largest earthquake in California since the 1992 (M-w 7.3) Landers event. The Hector Mine earthquake occurred in the eastern Mojave Desert, where the density of permanent Global Positioning System (GPS) stations is relatively low. Since the earthquake, groups from the United States Geological Survey, University of Southern California, University of California, Los Angeles, University of California, San Diego, and Massachusetts Institute of Technology have made postseismic survey-mode observations to increase the spatial coverage of deformation measurements. A total of 55 sites were surveyed, with markers from a few meters to 100 km from the surface rupture. We present velocity estimates for the 32 sites that had enough repeated observations between 17 October 1999 and 26 March 2000 to provide reliable results; these survey-mode data complement the temporal and spatial coverage provided by newly installed Southern California Integrated Geodetic Network permanent GPS stations and future Interferometric Synthetic Aperture Radar postseismic results. We then use the postseismic velocity estimates to compute a simple afterslip model. Results of inversions show that the observed velocities are consistent with deep afterslip occuring underneath the coseismic rupture area.

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.

2003
Gonzalez-Garcia, JJ, Prawirodirdjo L, Bock Y, Agnew D.  2003.  Guadalupe Island, Mexico as a new constraint for Pacific plate motion. Geophysical Research Letters. 30   10.1029/2003gl017732   AbstractWebsite

[1] We use GPS data collected on Isla de Guadalupe and in northern Baja California, Mexico, to estimate site velocities relative to Pacific plate motion. The velocities of all three geodetic monuments on Guadalupe fit a rigid Pacific plate model with residuals of 1 mm/yr. Using the Guadalupe data and data from five IGS stations on the Pacific plate ( CHAT, KOKB, KWJ1, MKEA, and THTI) we estimate an angular velocity for this plate that is consistent with other recently-published estimates. Our results indicate that Isla de Guadalupe lies on the Pacific plate, and that GPS data collection on the island usefully constrains Pacific plate motion and rigidity.

Anderson, G, Agnew DC, Johnson HO.  2003.  Salton trough regional deformation estimated from combined trilateration and survey-mode GPS data. Bulletin of the Seismological Society of America. 93:2402-2414.   10.1785/0120030014   AbstractWebsite

The Salton Trough in southeastern California, United States, has one of the highest seismicity and deformation rates in southern California, including 20 earthquakes M 6 or larger since 1892. From 1972 through 1987, the U.S. Geological Survey (USGS) measured a 41-station trilateration network in this region. We remeasured 37 of the USGS baselines using survey-mode Global Positioning System methods from 1995 through 1999. We estimate the Salton Trough deformation field over a nearly 30-year period through combined analysis of baseline length time series from these two datasets. Our primary result is that strain accumulation has been steady over our observation span, at a resolution of about 0.05 mustrain/yr at 95% confidence, with no evidence for significant long-term strain transients despite the occurrence of seven large regional earthquakes during our observation period. Similar to earlier studies, we find that the regional strain field is consistent with 0.5 +/- 0.03 mustrain/yr total engineering shear strain along an axis oriented 311.6degrees +/- 23degrees east of north, approximately parallel to the strike of the major regional faults, the San Andreas and San Jacinto (all uncertainties in the text and tables are standard deviations unless otherwise noted). We also find that (1) the shear strain rate near the San Jacinto fault is at least as high as it is near the San Andreas fault, (2) the areal dilatation near the southeastern Salton Sea is significant, and (3) one station near the southeastern Salton Sea moved anomalously during the period 1987.95-1995.11.

2004
Agnew, DC.  2004.  Robert Fitzroy and the myth of the 'Marsden Square': Transatlantic rivalries in early marine meteorology. Notes and Records of the Royal Society of London. 58:21-46.   10.1098/rsnr.2003.0223   AbstractWebsite

Marine data (especially in meteorology) are often grouped geographically using a set of numbered 10degrees latitude-longitude squares known as Marsden squares, which are usually attributed to William Marsden, Secretary of the Admiralty (and Vice-President of The Royal Society), who supposedly invented them early in the nineteenth century. Available records suggest that this system was in fact probably invented by Robert FitzRoy soon after his appointment as head of the British Meteorological Office in 1854. FitzRoy felt that early English work in marine meteorology was being ignored, notably by the American Matthew Fontaine Maury, who had pioneered the collecting of marine meteorological data from ship's logs. A desire to undo this wrong led FitzRoy to emphasize earlier (though abortive) British projects by A.B. Becher (in 1831) and by Marsden (probably in the 1780s), both of which involved grouping marine data geographically, though only over limited areas. FitzRoy's treatment of this earlier work seems to have created, much later, the belief that Marsden had invented the system of 10degrees squares. Given both Maury's and FitzRoy's desire to demonstrate priority in this field, it is ironic that the first clear proposal to collect and group data from ship's logs was made by the American (and British) natural philosopher Isaac Greenwood in 1728.

2005
Agnew, DC.  2005.  GHAM: A compact global geocode suitable for sorting. Computers & Geosciences. 31:1042-1047.   10.1016/j.cageo.2005.02.007   AbstractWebsite

The GHAM code is a technique for labeling geographic locations based on their positions. It defines addresses for equal-area cells bounded by constant latitude and longitude, with arbitrarily fine precision. The cell codes are defined by applying Morton ordering to a recursive division into a 16 by 16 grid, with the resulting numbers encoded into letter-number pairs. A lexical sort of lists of points so labeled will bring near neighbors (usually) close together; tests on a variety of global datasets show that in most cases the actual closest point is adjacent in the list 50% of the time, and within 5 entries 80% of the time. (C) 2005 Elsevier Ltd. All rights reserved.

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.

Elkhoury, JE, Brodsky EE, Agnew DC.  2006.  Seismic waves increase permeability. Nature. 441:1135-1138.   10.1038/nature04798   AbstractWebsite

Earthquakes have been observed to affect hydrological systems in a variety of ways-water well levels can change dramatically, streams can become fuller and spring discharges can increase at the time of earthquakes(1-7). Distant earthquakes may even increase the permeability in faults(8). Most of these hydrological observations can be explained by some form of permeability increase(1,5). Here we use the response of water well levels to solid Earth tides to measure permeability over a 20-year period. At the time of each of seven earthquakes in Southern California, we observe transient changes of up to 24 degrees in the phase of the water level response to the dilatational volumetric strain of the semidiurnal tidal components of wells at the Pinon Flat Observatory in Southern California. After the earthquakes, the phase gradually returns to the background value at a rate of less than 0.1 degrees per day. We use a model of axisymmetric flow driven by an imposed head oscillation through a single, laterally extensive, confined, homogeneous and isotropic aquifer to relate the phase response to aquifer properties(9). We interpret the changes in phase response as due to changes in permeability. At the time of the earthquakes, the permeability at the site increases by a factor as high as three. The permeability increase depends roughly linearly on the amplitude of seismic-wave peak ground velocity in the range of 0.21-2.1 cm s(-1). Such permeability increases are of interest to hydrologists and oil reservoir engineers as they affect fluid flow and might determine long-term evolution of hydrological and oil-bearing systems. They may also be interesting to seismologists, as the resulting pore pressure changes can affect earthquakes by changing normal stresses on faults(10).