Continuous GPS Stations Research Articles

GPS連続観測による長町-利府断層帯およびその周辺 (東北地方中部) の地殻変動

We constructed eight continuous GPS stations across the Nagamachi-Rifu fault zone, northeastern Japan to reveal the detailed spatial pattern of crustal deformation. High strain rate around the fault zone observed by the GEONET and the occurrence of the MJMA 5.2 Sendai earthquake on September 15, 1998, at the deep edge of the seismogenic part of the fault motivated us to confirm the aseismic sliding of the fault in or below the seismogenic depth from the contemporary deformation. We analyzed the GPS data of new stations as well as existing stations, totaling up to 50 stations in central Tohoku, using GIPSY 2.6.1 software and estimated GPS site velocity and strain rate by a least square method. One component of the principal strain rate shows distinctive compression in the E-W or ESE-WNW direction in the whole region. In the forearc region, the other component of the principal strain rate are almost zero in the south of 38°15′, while NNE-SSW extension in the north of 38°15′. Strain rate distribution is spatially heterogeneous in the region. We found a high strain rate zone west of the Nagamachi-Rifu fault zone and that between Honshu and Awashima in the eastern margin of the Sea of Japan. Low strain rate zones exist in the west of the high strain region of the Nagamachi-Rifu fault zone and in the western half of Yamagata Prefecture. Strain rate distribution pattern on the hanging wall side of the Nagamachi-Rifu fault can be modeled by aseismic sliding of 10mm/yr on a horizontal detachment fault connecting to the Nagamachi-Rifu fault at a depth of about 15km beneath the Ou backbone range. However, it is difficult to constrain a fault model for the strain rate distribution from the present dataset uniquely.

GPS measurement of postseismic deformation following the 1999 Chi‐Chi, Taiwan, earthquake

In the 15‐month period (September 1999 to December 2000) after the 1999 Chi‐Chi, Taiwan, earthquake (Mw 7.6) about 80 campaign‐surveyed GPS stations in central Taiwan were occupied up to seven times. Furthermore, seven continuous GPS stations were put into operation in the epicentral region mostly within three weeks after the main shock. GPS data from these near‐field stations and some pre‐existing far‐field stations were utilized to study the postseismic deformation following the Chi‐Chi earthquake. The postseismic displacements of the GPS stations were approximated by a combination of an exponential transient decay function with a relaxation time of 86 days and a postseismic linear rate change. Stations on the hanging wall displayed west‐northwesterly horizontal displacements of up to 252 mm and uplift as large as 229 mm. Postseismic displacements are much larger to the south near the epicenter of Chi‐Chi main shock than in the north where the maximum coseismic displacement occurred. Stations on the footwall show only small displacements. The postseismic GPS data were inverted to infer the deeper fault geometry and afterslip distribution based on a four‐segment fault model with its shallow part constrained by coseismic fault geometry deduced from GPS and seismological studies. The optimal model requires the lower fault segment to be a horizontal plane at a depth of 10.4 km, consistent with a model based on GPS data taken during the first three months after the main shock and other geological or geophysical studies. The maximum afterslip (459 mm) occurs in the hypocentral region and in the northern part of the decollement. Slip of about 430 mm also occurs in the southern part of the decollement. The afterslip moment inferred from 15 months of GPS data is 4.7 × 1019 N m, approximately 2.3 times the seismic moment released by aftershocks. This implies that a major part of the postseismic deformation is aseismic.

Crustal velocity field of Mexico from continuous GPS measurements, 1993 to June 2001: Implications for the neotectonics of Mexico

We combine velocities for 14 continuous GPS stations spanning Mexico and 173 additional continuous GPS sites on the North American and Pacific plates to study the large‐scale deformation of Mexico. The new station velocities, which are derived from more than 6000 days of previously unused GPS data, provide the first ever view of the crustal velocity field of Mexico. Key results are as follows: (1) Areas north of the Mexican Volcanic Belt, not including Baja California, move with the North American plate interior within the 1–2 mm yr−1 station velocity uncertainties. Station velocities for the Mexican Basin and Range are consistent with no present‐day extension and yield an upper 95% limit of 1–3 mm yr−1 for any regional extension. (2) South of the Mexican Volcanic Belt, five of the six sites move significantly relative to the North American plate. All sites in the Yucatan Peninsula move toward the east at 3–4 mm yr−1, possibly defining an independent Yucatan block. (3) Site velocities are consistent with limits of 0–4 mm yr−1 for present slip across the Mexican Volcanic Belt. (4) Tampico, on the gulf coast, exhibits eastward motion consistent with gravity sliding known to occur in the adjacent Mexican Ridges fold belt. (5) Southeastward motion of La Paz relative to the Pacific plate is consistent with the hypothesis that the Baja Peninsula is not fully attached to the Pacific plate. (6) Residual velocities for 160 North American plate GPS stations outside of Mexico exhibit no coherent regional patterns indicative of internal plate deformation.

A comprehensive approach to monitoring volcano deformation as a window on the eruption cycle

Since the 1980 eruption of Mount St. Helens, volcanologists have made considerable progress toward predicting eruptions on the basis of precursors that typically start a few days to several months in advance. Although accurate eruption prediction is by no means routine, it may now be possible in some cases to extend the effective warning period by anticipating the onset of short‐term precursors. Three promising indicators of deep magmatic processes are (1) deep, long‐period earthquakes and tremor that indicate the ascent of magma through the crust, (2) magmatic CO2 emission rate as a proxy for magma supply rate, and (3) relatively broad, generally aseismic surface uplift caused by magmatic intrusions. In the latter case it is essential to sample the deformation field thoroughly in both time and space to adequately constrain source models. Until recently, this has been nearly impossible because high‐precision sensors could not be deployed in sufficient numbers, nor could extensive geodetic surveys be conducted often enough. Advances in instrumentation, interferometric synthetic aperture radar (InSAR), and telecommunications are helping to overcome these limitations. As a result, comprehensive geodetic monitoring of selected volcanoes is now feasible. A combination of InSAR, large‐aperture GPS surveys, microgravity surveys, and dense arrays of continuous GPS stations, strain meters, and tiltmeters can reveal both spatial and temporal patterns of ground deformation throughout the eruption cycle. Improved geodetic monitoring of many of the world's volcanoes would be a major stride toward better understanding of magmatic processes and longer‐term eruption forecasts.

Precursory transient slip during the 2001 Mw = 8.4 Peru earthquake sequence from continuous GPS

Two‐hour position estimates from a continuous GPS station located at Arequipa, Peru, document precursory deformation beginning 18 hours prior to an Mw = 7.6 aftershock of the June 23rd 2001 Mw = 8.4 earthquake. This preseismic signal appears on the north and east components as a slow displacement with an amplitude twice that of the subsequent coseismic. Analysis of three years of 18‐hour rate measurement shows this signal to be unprecedented and beyond four standard deviations from the mean rate. The best fitting centroid is directionally consistent with slow slip along the plate interface and suggests the preseismic deformation arises from creep near the aftershock rupture. This implies the Nazca‐South American plate interface slipped slowly prior to seismogenic faulting. These observations indicate the Mw = 7.6 earthquake grew out of slow slip along the plate interface and clearly demonstrate the breadth of slip rates accommodated by subduction zone plate interfaces.

Strain accumulation in and around Ou Backbone Range, northeastern Japan as observed by a dense GPS network

A dense GPS network was established in 1997 around Ou Backbone Range (OBR), northeastern Japan, by deploying 28 continuous GPS stations to complement the sparse portion of GEONET operated by the Geographical Survey Institute of Japan. The aim of the network is to investigate the present surface deformation and understand the relationship between earthquake occurrence and the deformation process of the island-arc crust. Our GPS data are analyzed using a precise point positioning strategy of GIPSY/OASIS-II. Results of GEONET in daily SINEX files have been supplied by the GSI. Producing grid data of horizontal velocities and taking spatial derivatives, we derived a map of strain rate distribution. The results show that the region between 38.8° and 39.8°N in the OBR experiences notable concentration of east-west contraction. The region coincides with the area of active seismicity, including the focal areas of large earthquakes occurring in 1896 (M7.2), 1900 (M7.0), 1962 (M6.5), 1970 (M6.2), and 1998 (M6.1). Observed strain is larger than can be explained by the total moment release of earthquakes that occurred in the same period as this study. Possible sources of strain concentration may be viscoelastic deformation due to large earthquakes and/or aseismic slip along the deeper extension of the active faults.

Technical Issues and Recommendations Related to the Installation of Continuous GPS Stations at Tide Gauges

Geodesists around the world have begun installing continuous GPS (CGPS) stations at tide gauges in order to determine the exact position of these tide gauges and, in particular, the vertical velocity of the land or the seafloor underlying each tide gauge. The goal is to make these measurements in a well-defined global reference frame. The scientific applications of these measurements include the calibration of satellite altimeters and the removal of crustal motion signals from long time series of sea level change. In this article we focus on the technical issues associated with this agenda, including site selection, instrumentation, monumentation, ancillary measurements, and the tide gauge leveling program. There is no universally best approach to building CGPS stations at tide gauges. Therefore we emphasize the various trade-offs that typically occur, and give general recommendations and rules of thumb based on recent installations and experience. Additional information can be found at the CGPS@TG website.

稠密GPS観測網により観測された東北奥羽脊梁山地の地殻変動

A dense GPS network was established in 1997 around Ou Backbone Range (OBR), northeastern Japan, by deploying 9 new continuous GPS stations so as to complement the sparse portion of GEONET (GPS Earth Observation Network) operated by Geographical Survey Institute (GSI), Japan. This new observation network is aimed to investigate the present deformation and to understand the relationship between the occurrence of intraplate earthquakes and the deformation process of an island-arc crust.GPS data are analyzed using a precise point positioning strategy of GIPSY/OASIS-II. Coordinates of the GEONET stations in daily SINEX files have been supplied by GSI. A map of the intraplate deformation field derived by combining the two data sets and by subtracting the motion of the North American Plate demonstrates displacements toward WNW along the Pacific coast. This is caused by the interplate coupling between the Pacific and North American Plates. Displacements along the Japan Sea coast, on the other hand, are considerably small in magnitude, and directed to NW and N in the southern and northern area, respectively. OBR lies halfway between the two coasts and shows the transitional pattern of both deformation characteristics.By producing grid data of displacement velocities and taking spatial derivatives, we obtained a map of strain rate distribution. We found that the region between 38.8°N and 39.8°N in OBR demonstrates notable concentration of EW contraction. The region coincides with the area of active seismicity and includes the focal areas of 1896 (M7.2), 1962 (M6.5), 1970 (M6.2), and 1998 (M6.1) events. The observed strain is larger than can be explained by the total moment release of earthquakes that occurred in the same period of this study. Possible sources of the strain concentration may be viscoelastic deformation due to the large earthquakes and/or aseismic slip along the deeper extension of active faults.

An integrated crustal velocity field for the central Andes

We present an integrated velocity field for the central Andes, derived from GPS observations collected between January 1993 and March 2001 that eliminates the velocity bias between the South America‐Nazca Plate Project (SNAPP) and central Andes GPS Project (CAP) velocity fields published by Norabuena et al. [1998] and Bevis et al. [1999]. The reference frame is realized by minimizing the motion of eight continuous GPS stations and one rover GPS station located in the stable core of the South American plate. The RMS horizontal motion of these stations is just 1.1 mm/yr. The amplitude of these residual motions is roughly compatible with expected levels of measurement error. In our new solution, five of the six SNAPP stations located just outside the orogenic belt are effectively stationary, and the velocities for adjacent CAP and SNAPP stations now agree at a level consistent with their formal uncertainties.

Co-, Post- and Pre(?)-seismic Displacements Associated with the Mw 8.4 Southern Peru Earthquake of 23 June 2001 from Continuous GPS Measurements

On 23 June 2001, at 20:33 UTC, a major earthquake occurred near the coast of southern Peru, about 190 km west of Arequipa (16.15°S, 73.40°W). A revised magnitude of 8.4 was computed for this earthquake by the United States Geological Survey, NEIC (see http://neic.usgs.gov/neis/bulletin/01_EVENTS/0l0623203313/010623203313.html for further information). The earthquake occurred along the west coast of Peru (Figure 1) at the boundary between the Nazca and South American Plates and resulted from thrust faulting along this boundary as the Nazca Plate subducts beneath the South American Plate. The two plates are converging toward each other at a rate of 78 mm/yr (DeMets et al. , 1990). Southwestern Peru has a history of very large earthquakes. The 23 June shock originated just southeast of the source of a magnitude 7.7 earthquake that occurred in November 1996. This area of the plate boundary has not recorded a major rupture since the 1868 earthquake of magnitude approximately 8.5–9 (Dorbath et al. , 1990; Comte and Pardo, 1991). The 23 June shock occurred in a previously identified seismic gap (Nishenko, 1985). The 1868 earthquake was more destructive than the 23 June earthquake and produced a tsunami that killed hundreds of people along the South American coast. The 23 June earthquake is very similar to the 1995 Mw 8.1 Antofagasta earthquake (Ruegg et al. , 1996), which ruptured a zone of about 180 × 70 km2 of the Chilean coast, along which no major historical earthquake has been recognized before. The city of Arequipa, located 190 km east of the main shock epicenter, experienced damage during the 23 June earthquake. At Arequipa several precise positioning systems are permanently operating, including continuous GPS station of the IGS, DORIS beacon, Satellite Laser Ranging. In this study we give a quick report about surface deformation …

The GIL network of continuous GPS monitoring in Israel for geodetic and geophysical applications

Wdowinski, S., Bock, Y., Forrai, Y., Melzer, Y., and Baer, G. 2001. The GIL network of continuous GPS monitoring in Israel for geodetic and geophysical applications. Isr. J. Earth Sci. 50: 39–47. GIL (GPS in Israel) is a network of 12 continuous GPS stations, of which 11 stations are fully operational and one station is to be installed in 2002. The network provides a reference frame for precise GPS measurements in Israel and serves basic and applied geophysical research, including (1) monitoring plate motion and crustal deformation across the Dead Sea Fault, (2) mapping atmospheric water vapor content, and (3) monitoring ionospheric total electron content. Results from 36 months of continuous GPS measurements reveal that the current displacement rate within the State of Israel is 1–4 mm/yr, reflecting interseismic deformation across the Dead Sea Fault due to 2–4 mm/yr of relative motion between Sinai and Arabia and possibly post-seismic deformation induced by the 1995 Nuweiba earthquake.

Adaptive filtering of continuous GPS results

An adaptive finite-duration impulse response filter, based on a least-mean-square algorithm, has been used to mitigate multipath effects, and to derive tectonic and fault movement signals from continuous global positioning system (CGPS) data. By applying the filter on both pseudo-range and carrier-phase multipath sequences from CGPS observations on consecutive days, multipath models have been reliably derived. The standard deviations of residual time series are reduced to about one-quarter on pseudo-range and to about one-half on carrier phase. The adaptive filter is then used to process baseline solutions from a five-station array. Tectonic and fault movements have been resolved, which are in good agreement with previous studies involving many more CGPS stations.

Ocean loading corrections for continuous GPS: A case study at the Canadian coastal site Holberg

The continuous GPS stations of the Western Canada Deformation Array (WCDA) are subject to movements due to the loading of the lithosphere by ocean tides. Using 4 weeks of continuous GPS data, this study examines the effects of ocean loading corrections at the coastal site HOLB. Theoretical ocean loading displacements were based on the Grenoble Global Model FES95.2, modified by detailed modeling of nearby coastal ocean cells. Positions for HOLB were obtained using the Bernese software for 3hr and 24hr solutions with hourly tropospheric delay estimates in both cases. Without ocean loading corrections, the 3hr‐computed heights had daily variations as large as 8 cm that were well correlated with tidal predictions. Application of ocean loading corrections to these solutions removed the tidal correlations and reduced the variance by a factor of two. The repeatability of the 24hr vertical positions was not significantly improved by the addition of ocean loading corrections. However, for these daily solutions without loading corrections, a significant amount of the relative vertical tidal motion of the station was absorbed in the (1hr) tropospheric delay estimates. We conclude that constraining station position for HOLB, in our case to a daily average, can bias short‐term estimates of zenith delay by up to 10 mm and, in turn, estimates of precipitable water vapour by several millimetres.

Current rates of convergence across the central Andes : Estimates from continuous GPS observations

We present velocity solutions for 16 continuous GPS (CGPS) stations located in South America in a geodetic reference frame that nominally fixes the craton or stable core of this continent. The RMS horizontal velocity of 8 stations located within the craton is 1.3 mm/yr. Three CGPS stations located in Chile between about 33° S and 20° S have velocities in the range 19–24 mm/yr. These motions manifest current rates of convergence across the southern limb of the Central Andes. The IGS station in Arequipa, Peru (at 16.5°S) in the northern limb of the orocline, however, has a velocity of just 10 mm/yr.