Measurements Of Crustal Deformation Research Articles

Geodetic determination of relative plate motion and crustal deformation across the Scotia‐South America plate boundary in eastern Tierra del Fuego

Global Positioning System (GPS) measurements provide the first direct measurement of plate motion and crustal deformation across the Scotia‐South America transform plate boundary in Tierra del Fuego. This plate boundary accommodates a part of the overall motion between South America and Antarctica. The subaerial section of the plate boundary in Tierra del Fuego, about 160 km in length, is modeled as a two dimensional, strike‐slip plate boundary with east‐west strike. Along the Magallanes‐Fagnano fault system, the principal fault of this portion of the plate boundary, relative plate motion is left‐lateral strike‐slip on a vertical fault at 6.6 ± 1.3 mm/year based on an assumed locking depth of 15 km. The site velocities on the Scotia Plate side are faster than the relative velocity by an additional 1–2 mm/yr, suggesting there may be a wider region of diffuse left‐lateral deformation in southern Patagonia. The north‐south components of the velocities, however, do not support the existence of active, large‐scale transpression or transtension between the South America and Scotia plates along this section of the plate boundary.

Large‐scale global surface mass variations inferred from GPS measurements of load‐induced deformation

Detrended crustal deformation measurements from a global network of up to 200 continuous Global Positioning System (GPS) tracking sites are inverted for low degree and order (n, m ≤ 6) spherical harmonic series of surface mass variations. An independent geophysical model of the atmosphere, oceans, and water and snow on land is also used to guide the spherical harmonic truncation and to evaluate aliasing errors in the results. Inversion uncertainties decrease significantly as spatial coverage and data quality improve with time, especially in the last few years. Consequently, GPS inverted seasonal geocenter and low degree zonal harmonics converge to those determined by measurements from satellite laser ranging (SLR). Significant n > 1 and non‐zonal variations have also been found.

Geodetic measurements of crustal deformation on NW–SE faults of the Betic Cordillera, southern Spain, 1999–2001

The Granada basin, located in the central sector of the Betic Cordillera, is one of the most seismically active zones of the Iberian Peninsula. For the first time, a geodetic network along the Padul fault and two levelling profiles (Genil and Viznar) crossing the Granada fault were operated to detect crust microdeformations in this area. Three years of terrestrial geodetic measurements are analysed to characterize the behaviour of these faults in a low to moderate strain rate environment. After the comparison of these three campaigns, we can conclude that there is no significant short-term movement of the Padul fault. Granada fault measurements show significant differences between 1999 and 2001 campaigns in Viznar profile. However, more data are needed to correlate this displacement to the tectonic activity.

Error evaluation in acoustic positioning of a single transponder for seafloor crustal deformation measurements

The observation of seafloor crustal deformation is very important to understand plate motions, nucleation processes and mechanisms of great interplate earthquakes as well as the activities of submarine volcanoes. We have been developing an observation system for seafloor crustal deformation. This system consists of two main components; (1) kinematic GPS positioning of an observation vessel and (2) accurate acoustic measurements of distances between a transponder attached on the side of the vessel (onboard station) and one located on the ocean bottom (seafloor station). In this study, we performed numerical simulations to estimate measurement errors with acoustic positioning assuming acoustic velocities in the sea water and the distribution of observation points around the single seafloor station. We found that the position of the seafloor station which we can obtain by analyzing travel-time data might have around 18-cm discrepancy with respect to its “true” position. Colombo et al. (2001) reported that the position of the vessel can be determined with about 10-cm error by kinematic GPS positioning. These results indicate that the system should be able to detect seafloor crustal deformation much larger than 28 cm, including pre-, co-, and post-seismic slips due to the large earthquakes at subduction zones, slow and silent earthquakes, etc. Therefore, we emphasize the importance of continuous observations with a nationwide geodetic observational network for seafloor crustal deformation.

Measurements and interpretations of tilt–strain gauges in seismically active areas

An overview of different aspects of deformation measurements is given. An introduction to the general theory of point-crustal deformations with an application to the stress–strain relations at the earth surface is made. The instrumentation used in crustal deformation measurements is briefly explained, emphasizing different constructions of tilt and strainmeters. The problems connected with the housing of the instruments, i.e., the cavity and topography effects, and with the ambient noise factors, as the hydrological and pressure effects, are discussed. The last part of the paper is concerned with the relation between deformation measurements and seismic events, both from a theoretical and observational standpoint. A number of examples are shown that report on pre-, co-, and post-seismic and the secular term deformation in different areas of the world.

RNGCHN: a program to calculate displacement components from dislocations in an elastic half-space with applications for modeling geodetic measurements of crustal deformation

The RNGCHN program calculates a single component of the displacement field due to a finite or point-source dislocation buried in an elastic half space. This formulation approximates the surface movements produced by earthquake faulting or volcanic intrusion. As such, it is appropriate for modeling crustal deformation measured by geodetic surveying techniques, such as spirit leveling, trilateration, Very Long Baseline Interferometry (VLBI), Global Positioning System (GPS), or especially interferometric analysis of synthetic aperture radar (SAR) images. Examples suggest that this model can fit simple coseismic earthquake signatures to within their measurement uncertainties. The program’s input parameters include fault position, depth, length, width, strike, dip, and three components of slip. The output consists of displacement components in the form of an ASCII list or a rectangular array of binary integers. The same program also provides partial derivatives of the displacement component with respect to all 10 input parameters. The FORTRAN source code for the program is in the public domain and available as the compressed tar file rngchn.tar.Z in the directory/pub/GRGS via the Internet by anonymous ftp to spike.cst. cnes.fr. This distribution includes worked examples and a MATLAB interface.

GPS measurements of crustal deformation in Skåne, Sweden, between 1989 and 1996

We estimate a network of crustal deformation in Skåne using observations of the Global Positioning System (GPS) from 1989, 1990, 1992, and 1996. The network straddles the Tornquist Zone, potentially one of the most active fault zones in Sweden. It includes 5 GPS sites spaced approximately 80 km apart in the south of Sweden. The precisions of the relative horizontal components for these stations are described by repeatabilities with approximately 3 mm in the north-south direction and about 1 mm in the east-west direction in 1992 and 1996, but with a precision of only about 10 mm for the horizontal components in 1989 after the improvement of broadcast orbits, keeping three fiducial station (Onsala, Wettzell, and Tromsö) fixed at the ITRF94 system. About 72% of the GPS integer carrier phase ambiguities were resolved for the 1992 and 1996 campaigns and merely 43% in the 1989 campaign. Three stations, Dalby, Limhamn, and Blentarp, moved with a rate of 4 to 5±0.5 mm/yr toward the SSE and ESE relative to the station Stavershult. SSE and ESE displacements of the three stations mean left lateral strike-slip shear along the Tornquist Zone in Skåne, which is consistent with geological results. The 2.0±0.7 mm/yr S displacement of station Kivik indicates right lateral displacement along the southern aseismic portion of the Protogine Zone. The results were similar when the Onsala station was fixed at the ITRF94. For a more detailed investigation of the present tectonic deformation in Skåne we suggest the GPS network be enlarged to more than 10 stations.

Optical Fiber Interferometers for Referencing Surface Benchmarks to Depth

—We have developed and operated optical fiber interferometers for monitoring displace ments within boreholes, as part of a program of crustal deformation measurement. These optical fiber strainmeters—a total of twelve instruments at two sites in southern California—were installed to sense the motion of the end-monuments of much longer baseline strainmeters and tiltmeters, allowing correction for any near-surface ground movement. One of the installations was specifically designed to investigate the distribution of deformation with depth, measuring over several borehole length-intervals from 5 m to 50 m. The displacements recorded over year-long time scales along these length intervals range up to 6 mm and show internal consistency and stability at the 50 μ m level. The use of these interferometers to provide correction signals for kilometer-scale crustal strain measurements has resulted in greatly improved records.

Crustal deformation measurements using repeat‐pass JERS 1 synthetic aperture radar interferometry near the Izu Peninsula, Japan

We have examined the precision of interferometric synthetic aperture radar (SAR) measurements of surface deformation of the Earth using 24‐cm wavelength data acquired by the Japanese Earth Resources Satellite 1 (JERS 1) spacecraft, over the Izu Peninsula, Japan. Radar images acquired from 1993 to 1994 were made into 11 independent interferograms. Apparent water vapor signatures with equivalent path delay up to 16 cm preclude reliable estimates of small deformation from only one interferogram. Averaging of interferograms improves the measurement, such that residuals between precise leveling data and corresponding radar observations are less than 1 cm RMS. There is a clear correlation between cloud cover extent on a given day and the spectral power of the water vapor signature in an interferogram formed from the corresponding day's SAR image. Also, several water vapor features associated with local topography are consistent with accumulation of moisture on the windward side of an on‐shore flow. Analysis shows that decorrelation in mixed conifer and deciduous forest is a weak function of the time between observations, imposing a roughly constant level of additional decorrelation relative to scatter‐stable urban areas. Furthermore, in high‐relief areas, correlation is high when the baseline is tens of meters but drops rapidly with increasing baseline length. With strict control of the interferometric baseline length, adequate correlation in interferograms over very long time periods is possible even in forested, high relief regions. Observed inflation over Ito city is consistent with a localized magma source at depth, suggesting a different mechanism from the previous episode in 1989.

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 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.

Active detachment faulting in the San Francisco Bay area?

Measurements of crustal deformation may help determine if mid-crustal reflectors imaged at ∼15 km depth below the San Francisco Bay area, California, are active detachment horizons that accommodate strike-slip shear and/or convergence across the region. Elastic dislocation models predict distinguishable secular surface displacements for fault geometries involving vertical shear zones or horizontal detachments only where faults step, bend, or branch along their trace. In the southern San Francisco Bay area, the San Andreas, Hayward, and Calaveras fault zones splay from a single trace of the creeping central segment of the San Andreas fault. Whereas measured horizontal surface displacements in the central and northern bay area are equally well predicted by models of strike-slip faults below the major faults and by models involving a detachment, a zone of fault-normal motion observed in the southern bay area is not well explained by models involving only vertical strike-slip faults. A model involving detachment slip parallel to the San Andreas fault appears more consistent with the observations. We find no evidence for significant San Andreas fault-normal regional contraction or detachment faulting. However, additional data and better models are required to more confidently constrain the three-dimensional active fault architecture.

Crustal deformation

Geodetic measurements of crustal deformation provide direct tests of geophysical models which are used to describe the dynamics of the Earth. Although geodetic observations have been made throughout history, only in the last several hundred years have they been sufficiently precise for geophysical studies. In the 19th century, these techniques included leveling and triangulation. Approximately 25 years ago, trilateration measurements were initiated by the USGS (United States Geological Survey) to monitor active faults in the United States. Several years later, NASA (National Aeronautics and Space Administration) begin an effort to measure plate tectonic motions on a global scale, using space geodetic techniques, VLBI (Very Long Baseline Interferometry) and SLR (Satellite Laser Ranging). The period covered by this report to the IUGG, 1991–1994, was a transition period in the field of crustal deformation. Trilateration measurements (previously the backbone of measurements across plate boundaries in the western United States and Alaska) have been abandoned. This system was labor‐intensive, involved highly trained crews to carry out the observations, and only measured the length between sites. In addition, NASA drastically cut the budgets for VLBI and SLR during this period. Fixed site VLBI systems are still operational, but mobile VLBI measurements in North America have ceased. SLR measurements continue on a global scale, but the remaining crustal deformation measurements are now being made with the Global Positioning System (GPS). Nonetheless, because of the time scales involved, older geodetic data (including leveling, triangulation, and trilateration) continue to be important for many geophysical studies.

Continuous measurements of crustal deformation for the 1992 Landers earthquake sequences : F. K. Wyatt, D. C. Agnew & M. Gladwin, Bulletin - Seismological Society of America, 84(3), 1994, pp 768–779

Continuous measurements of crustal deformation for the 1992 Landers earthquake sequences : F. K. Wyatt, D. C. Agnew & M. Gladwin, Bulletin - Seismological Society of America, 84(3), 1994, pp 768–779

GPS measurements of crustal deformation within the Pacific-Australia plate boundary zone in Irian Jaya, Indonesia

Global Positioning System (GPS) measurements made in 1991, 1992 and 1993 provide preliminary estimates of slip distribution between the Australian and Pacific plates in Irian Jaya, Indonesia. We interpret the GPS results with constraints from earthquake mechanisms and slip vectors, recent marine surveys, and geology. Three GPS sites in southeastern Irian Jaya show motions that are within 10 mm/yr of the expected motion of Australia. A coast-to-coast N-S baseline along 140.5°E crosses all known onland regions of active deformation but reveals no more than 15 mm/yr of shortening and 20 mm/yr of left-lateral shear in the 27-month period. The remaining 40 mm/yr of expected convergence between the Pacific and Australian plates probably occurs at the New Guinea trough. GPS sites on the island of Biak, at 136°E, and at Sorong, near the western tip of Bird's Head (at 131°E), both move 90–100 mm/yr in a WSW direction relative to Irian Jaya, but less than 15 mm/yr relative to each other. These sites are on either side of the Sorong fault and demonstrate that it is not presently the major boundary between the Australian and Pacific plates. Instead the plate boundary is now south of the Sorong and Biak sites. Earthquakes suggest possible structures that accomodate motion between Bird's Head and Australia but the relative importance of them remains uncertain.

Continuous measurements of crustal deformation for the 1992 Landers earthquake sequence

Abstract We describe, and attempt to interpret, continuous measurements of strains and tilts made at Pinon Flat Observatory (PFO) before, during, and after the Landers and Joshua Tree earthquake sequences. These data show substantial transient deformation following the Landers mainshock, with a total amplitude of several percent of the co-seismic deformation, and a decay time of at least several days. Comparing data from the many types of instruments at PFO allows us to infer possible sources for this deformation. The immediate postseismic transient was nearly the same size on three long-base strainmeters, suggesting either broadscale deformation or local motion near one part of the observatory. The latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source. Possible mechanisms for broad-scale deformation include postseismic fault slip, time-dependent creep in near-surface rocks, and elastic or thermal responses to water-table changes. The first two agree best with the observations from PFO, but if postseismic fault slip is the source, it must have been distributed differently than the co-seismic slip, and may have included faults other than those that ruptured seismically. If one of the other mechanisms is the main source, the PFO data imply that the postseismic-slip must have been very much smaller than the seismic slip, perhaps 2% or less. No significant preseismic deformation was observed, at a level of 2 × 10 −3 of the co-seismic deformation, for the days to minutes before the earthquake.

Global Positioning System Measurements of Crustal Deformation in the Taiwan Arc-Continent Collision Zone

Global Positioning System Measurements of Crustal Deformation in the Taiwan Arc-Continent Collision Zone

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

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. Our 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.

Application of the global positioning system to crustal deformation measurements: 3. Result from the southern California borderlands

Five years of measurements from the Global Positioning System (GPS) satellites collected between 1986 and 1991 are used to investigate deformation in the offshore regions of southern California. GPS provides the first practical technique to make precise geodetic measurements in the region. The geodetic network is situated along the California coastline from Vandenberg (120.6°W, 34.6°N) to San Diego, with additional sites on Santa Cruz, San Nicolas, Santa Catalina, Santa Rosa, and San Clemente Islands. The precision of horizontal interstation vectors is subcentimeter, and the interstation vector rate between OVRO and Vandenberg agrees with the very long baseline interferometry derived rate to within one standard deviation. No significant motion is observed in the western Santa Barbara Channel between Vandenberg and Santa Rosa Island, 0.5 ± 1.6 mm/yr, where the quoted uncertainties are one standard deviation. Motions in the eastern Santa Barbara Channel are consistent with compressional deformation of 6 ± 1 mm/yr at N16 ± 3°E. This motion is in agreement with seismicity and an independent geodetic analysis for the period 1971–1987 (Larsen, 1991). San Clemente Island is moving relative to San Diego at the rate of 5.9 ± 1.8 mm/yr at a direction of N38 ± 20°W. The motion between San Nicolas Island and San Clemente Island, 0.8 ± 1.5 mm/yr, is insignificant.

GPS measurements of crustal deformation associated with the 22 April 1991, Valle de la Estrella, Costa Rica Earthquake

We present analysis results for Global Positioning System (GPS) measurements made at sites in Costa Rica during two campaigns in February and July 1991. GPS solutions for 5 sites (Limon, Liberia, Bratsi, ETCG, and Vueltas) reveal significant horizontal and vertical displacements relative to their February positions. Horizontal displacements relative to Liberia, measured 244.7 ± 0.8, 89.2 ± 0.9, 12.4 ± 1.3, and 1.9 ± 0.9 cm at Limon, Bratsi, Vueltas, and ETCG respectively. Vertical displacements relative to Liberia measured 163 ± 2.1, 15.3 ± 3.0, −10.5 ± 4.4, and −0.6 ±2.1 cm at Limon, Bratsi, Vueltas, and ETCG respectively. We find differences in the GPS derived vertical and horizontal displacements compared to other types of geodetic measurements of uplift in the coastal region and their associated models. To address these differences we compute a dislocation model which fits the GPS measured displacements. A simple uniform planar slip model can not reconcile the differences between the coastal uplift data or the seismic moment, suggesting considerable complexity of the earthquake source.

One Year of Daily Satellite Orbit and Polar Motion Estimation for Near Real Time Crustal Deformation Monitoring

The Permanent GPS Geodetic Array (PGGA) in southern California consists of five continuously operating stations established to monitor crustal deformation in near real time. The near real time requirement has been problematic since GPS satellite ephemerides and predicted earth orientation values (IERS Bulletins A and B) have been found to be neither sufficiently timely nor accurate to achieve horizontal position accuracies of several mm on regional scales. Therefore, we have been estimating precise GPS ephemerides and polar motion since August 1991. An examination of overlapping 24-hour satellite arcs indicates worst-case orbital errors of approximately 0.2 meters in the radial components, 1 meter in the cross-track components and 2–3 meters in the along-track components. A comparison with very long baseline interferometry indicates an accuracy of less than 1 mas in our determination of 24-hour values of pole position. These products are sufficiently timely and accurate to achieve several mm long-term horizontal precision in regional scale measurements of crustal deformation in near real time, as has been demonstrated during the 28 June, 1992 Landers and Big Bear earthquakes in southern California. The PGGA stations were able to detect seismically induced, sub-centimeter-level motions with respect to a terrestrial reference frame defined by the global tracking stations.