Duncan Carr Agnew

The complete (3-D) surface displacement field in the epicentral area of the 1999 Mw7.1 Hector Mine earthquake, California, from space geodetic observations

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_w7.1 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.

Seismic waves increase permeability

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. Distant earthquakes may even increase the permeability in faults. Most of these hydrological observations can be explained by some form of permeability increase. 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° in the phase of the water level response to the dilatational volumetric strain of the semidiurnal tidal components of wells at the Piñon Flat Observatory in Southern California. After the earthquakes, the phase gradually returns to the background value at a rate of less than 0.1° 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. 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.

Space geodetic measurement of crustal deformation in central and southern California, 1984-1992

A laboratory type of analyzer for quantitatively determining the percent third element content of a hydrocarbon sample. A unique rhodium/americium radioactive source is disclosed.

NLOADF: A program for computing ocean‐tide loading

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.

Uplift and subsidence associated with the great Aceh-Andaman earthquake of 2004

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 ≈ 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 ~2.5°N, to Preparis Island, Myanmar (Burma), at ~14.9°N. Thus the rupture is ~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.

The time‐domain behavior of power‐law noises

The power spectra of many geophysical phenomena are well approximated by a power-law dependence on frequency or wavenumber. I derive a simple expression for the root-mean square variability of a process with such a spectrum over an interval of time or space. The resulting expression yields the power-law time dependence characteristic of fractal processes, but can be generalized to give the temporal variability for more general spectral behaviors. The method is applied to spectra of crustal strain (to show what size of strain events can be detected over periods of months to seconds) and of sea level (to show the difficulty of extracting long-term rates from short records).

Absence of earthquake correlation with Earth tides: An indication of high preseismic fault stress rate

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.

Southern California Permanent GPS Geodetic Array: Continuous measurements of regional crustal deformation between the 1992 Landers and 1994 Northridge earthquakes

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 (Mw=7.3), Landers and January 17, 1994 (Mw=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 earthquakes 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.

Monument motion and measurements of crustal velocities

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 measurement 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 campaigns, the previous two conclusions remain unchanged. 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.

Application of the global positioning system to crustal deformation measurement: 1. Precision and accuracy

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 108; the east-west component shows a similar precision at the shortest distances, 2.1 mm, with a larger length dependence, 1.3 parts in 108. 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 108, 5.2mm+2.8 parts in 108, 11.7mm+13 parts in 108 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.