Campaign Global Positioning System Research Articles

Updating Thai Reference Frame to ITRF2005 Using GPS: Diversion Between ITRF2000 and 2005 in Southeast Asia

The Thai geodetic network has been regularly observed with Global Positioning System (GPS) since 1994 through several collaborative EU-ASEAN projects such as GEODYSSEA, SEAMERGES and RTSD-Delft. This geodetic network has long been served as a reference frame for Thailand. Previous realisations of the Thai coordinate reference frame were therefore tied to the global International Terrestrial Reference Frame (ITRF) at epochs 1994, 1996 and 2000. After the occurrence of the 9.2 Mw Sumatra-Andaman earthquake on the 26th December 2004, horizontal displacements were evident at different magnitudes in many surrounding countries. The geodetic network within Thailand was also significantly deformed during the earthquake at the centimetre to decimetre level. Large co-seismic horizontal displacements were observed in the southern part of Thailand, while moderate and small displacements were seen in the central and northern parts of Thailand. The Royal Thai Survey Department (RTSD) has been carrying out multiple GPS field campaigns to monitor the post-seismic displacements. This paper will analyse the GPS observations obtained from the RTSD GPS campaigns up to the end of 2008 using the Precise Point Positioning (PPP) strategy of the GIPSY-OASIS II software. It has been demonstrated that by employing the state of the art PPP technique, the users could achieve mm-level of repeatability in the horizontal components and centimetre precision in the vertical direction, for a 24-hr data span from a static site occupied by a geodetic-quality receiver. Coordinate results obtained from each campaign are then mapped to ITRF2000 and ITRF2005 using a number of well-determined global International GNSS service (IGS) sites. By comparing coordinate results between ITRF2000 and ITRF2005, it is evident that there is a significant diversion in the north component at a rate of 1.7 mm per year over Southeast Asia region. Finally, ITRF2005 coordinate results obtained from the latest RTSD GPS campaign (November 2008) will be served as a new coordinate reference frame for Thailand.

Deformation Assessment Considering an A Priori Functional Model in a Bayesian Framework

Monitoring Earth-surface deformations with surveying instruments plays a key role in the knowledge of dynamics and the evolution of geophysical phenomena. During this stage, it is important to adopt an appropriate check of the statistical significance of point displacements. We verified the advantages of using a statistical Bayesian approach with a simple a priori geophysical model instead of the frequentist methods usually adopted. In particular, we analyzed global positioning system (GPS) coordinate time series in two applications. First, by using a dedicated device, we applied movements of known amplitude to a GPS antenna. Such known imposed values were used as a term of comparison both for the frequentist and for the Bayesian approach. Furthermore, we analyzed data from GPS campaigns carried out on a landslide located in the central part of Italy. The results of both investigations (imposed and unknown movements) show more valuable outcomes by using the Bayesian approach.

Estimation of strain accumulation of densification network in Northern Marmara Region, Turkey

Abstract. Strain analysis is one of the methods for the kinematic analysis of the repeated geodetic measurements. In order to derive strain accumulation in Marmara Region, different institutions carried out several Global Positioning System (GPS) campaigns in 1999 and 2006. The GPS campaigns were performed on the geodetic network which cover the provinces: Kirklareli, Tekirdag, Bursa, Bilecik and Adapazari. Then, the displacements of the network stations were estimated by means of analysing the GPS space geodetic measurements. For the assessment of the datum differences between 1999 and 2006 on the station coordinates, 3-D Helmert transformation was applied to the coordinates of each 1999 and 2006 datum. Then, a global test was introduced to determine the significant deformation which occurred in the geodetic GPS network. Strain accumulation with a finite element model was then computed. First, triangles were constructed for the whole network with the Delaunay method. Hereafter, strain parameters were calculated for each triangle. Maximum values of strain accumulation were found around the surroundings of Marmara Ereglisi and Izmit, whereas minimum values are around Istanbul.

Investigating mechanisms of edifice deflation, 1981–2007, at Mount Baker volcano, Washington, United States

At Mount Baker, elevated gas and heat fluxes from fumaroles in Sherman Crater indicate the presence of a degassing magma reservoir. Campaign Global Positioning System (GPS) surveys in 2006 and 2007 provide slope distance measurements of 19 trilateration lines and provide baseline positions for future GPS study on Mount Baker. Comparison of slope distance measurements acquired in 1981 and 1983 with electronic distance meters (EDM) indicates that significant surface deformation has occurred on Mount Baker during the past quarter century. Slope distances have predominantly shortened around the edifice at rates <2 mm/yr. The greatest slope length change detected is −17 ± 4 ppm on the northern flank of the volcano. A uniform surface strain rate model fit to the weighted slope change data shows contractional strain, with an areal dilation rate of −420 ± 140 nanostrain/yr. The observed strain rate is an order of magnitude greater than that expected from tectonic sources. Elastic dislocation models are used to invert for the location and strength of a point source at depth. The optimal model predicts a volume change of −11 × 106 m3, located 1500 m east–northeast of the summit, at a depth of ∼5.8 km. The model can account for most of the deformation detected, suggesting that the magmatic and hydrothermal system at Mount Baker has depressurized since 1981, from the combined result of densification and devolatilization.

Kinematics of the eastern part of the North Anatolian Fault Zone

Abstract The North Anatolian Fault Zone (NAFZ), which marks the boundary between Anatolia and the Eurasian plate, is one of the world's most seismically active structures. Although the eastern part of NAFZ has high seismic hazard, there is a lack of geodetic information about the present tectonics of this region. Even though many scientists would like to study this area, geographical and logistical problems make performing scientific research difficult. In order to investigate contemporary neotectonic deformation on the eastern NAFZ and in its neighborhood, a relatively dense Global Positioning System (GPS) monitoring network was established in 2003. Geodetic observations were performed in three GPS campaigns in an area of 350 km × 200 km with 12-month intervals. In addition, 14 new GPS stations were measured far from the deforming area. Since this region includes the intersection of the NAFZ and the East Anatolian Fault Zone (EAFZ), deformation is complex and estimating seismic hazard is difficult. One important segment is the Yedisu segment and it has not broken since the 1784 earthquake. After the 1992 Erzincan and 2003 Pulumur earthquakes, the Coulomb stress loading on the Yedisu segment of the NAFZ has increased significantly, emphasizing the need to monitor this region. We computed the horizontal velocity field with respect to Eurasia and strain rates field as well. GPS-derived velocities relative to Eurasia are in the range of 16–24 mm/year, which are consistent with the regional tectonics. The principal strain rates were derived from the velocity field. Results show that strain is accumulating between the NAFZ and EAFZ along small secondary fault branches such as the Ovacik Fault (OF).

Branco River Stage Gradient Determination and Amazon Hydrologic Studies Using GPS Water Level Measurements

This study presents the result of a demonstration of Global Positioning System (GPS) hydrologic studies in a remote area. A GPS campaign was conducted using a GPS-equipped vessel and a GPS buoy to measure water level along Rio Branco, a tributary of the Amazon. The GPS water level data agree well with river gauge data and with ENVISAT radar altimeter measurements. The GPS-estimated river stage gradient of Rio Branco is 5.75 ± 0.48 cm/km with 99% confidence, which is higher than previous estimates. This result also agrees well with ENVISAT altimetry results.

Surface deformation induced by present-day ice melting in Svalbard

SUMMARY The vertical movement of the Earth’s surface is the result of a number of internal processes in the solid Earth, tidal forces and mass redistribution in the atmosphere, oceans, terrestrial hydrosphere and cryosphere. Close to ice sheets and glaciers, the changes in the ice loads can induce large vertical motions at intraseasonal to secular timescales. The Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) antennas in Ny-A u lesund, Svalbard that started observations in 1991 and 1995, respectively, observe vertical uplift rates on the order of 8 ± 2m m yr −1 , which are considerably larger than those predicted by postglacial rebound (PGR) models (order 2 mm yr −1 ). The observations also indicate increased uplift rates starting some time in 2000. A local GPS campaign network that has been reoccupied annually since 1998, reveals a tilting away from the neighbouring glaciers. The Svalbard glaciers have been undergoing melting and retreat during the last century, with increased melting since about 2000. We compared the observed vertical motion to the motion predicted by loading models using a detailed ice model with annual time resolution as forcing. The model predictions correlate well with the observations both with respect to the interannual variations and the spatial pattern of long-term trends. The regression coefficients for predicted and observed interannual variations in height is 1.08 ± 0.38, whereas the regression coefficient for the predicted and observed spatial pattern turns out to be 1.26 ± 0.42. Estimates of the predicted secular trend in height due to PGR and present-day melting are on the order of 4.8 ± 0.3 mm yr −1 and thus smaller than the observed secular trend in height. This discrepancy between predictions and observations is likely caused by the sum of errors in the secular rates determined from observations (due to technique-dependent large-scale offsets) and incomplete or erroneous models (unaccounted tectonic vertical motion, errors in the ice load history, scale errors in the viscoelastic PGR models and the elastic models for present-day melting).

Active crustal deformation in two seismogenic zones of the Pannonian region — GPS versus seismological observations

The seismicity and the associated seismic hazard in the central part of the Pannonian region is moderate, however the vulnerability is high, as three capital cities are located near the most active seismic zones. In our analysis two seismically active areas, the Central Pannonian and Mur-Mürz zones, have been considered in order to assess the style and rate of crustal deformation using Global Positioning System (GPS) and earthquake data. We processed data of continuous and campaign GPS measurements obtained during the years 1991–2007. Velocities relative to the stable Eurasia have been computed at HGRN, CEGRN and EPN GPS sites in and around the Pannonian basin. Uniform strain rates and relative displacements were calculated for the investigated regions. GPS data confirm the mostly left lateral strike slip character of the Mur-Mürz–Vienna basin fault system and suggest a contraction between the eastward moving Alpine-North Pannonian unit and the Carpathians. The computation of the seismic strain rate was based on the Kostrov summation. The averaged unit norm seismic moment tensor, which describes the characteristic style of deformation, has been obtained from the available focal mechanism solutions, whereas the annual seismic moment release showing the rate of the deformation was estimated using the catalogues of historical and recent earthquakes. Our analysis reveals that in the Central Pannonian zone the geodetic strain rate is significantly larger than the seismic strain rate. Based on the weakness of the lithosphere, the stress magnitudes and the regional features of seismicity, we suggest that the low value of the seismic/geodetic strain rate ratio can be attributed to the aseismic release of the prevailing compressive stress and not to an overdue major earthquake. In the Mur-Mürz zone, although the uncertainty of the seismic/geodetic strain rate ratio is high, the seismic part of the deformation seems to be notably larger than in the case of the Central Pannonian zone. These results reflect the different deformation mechanism, rheology and tectonic style of the investigated zones.

Contemporary tectonic motion of the eastern Snake River Plain: A campaign global positioning system study

A comparison of precision campaign GPS data from 1995 and 2004 from 10 benchmarks on the eastern Snake River Plain (eSRP) has revealed that the province moved 2.8 ± 0.3 mm/a to the SW (232.4 ± 6.3°) relative to a fixed North American reference frame. The benchmarks had no measurable displacement relative to one another at the resolution of the GPS during the 9‐annum study, evidence that the province moves as a rigid, nonextending block. This scenario is supported by the aseismic nature of the province and the lack of measurable horizontal stress in boreholes. However, an additional small component of intraplain extension must also be invoked to account for the observed NW‐trending volcanic rift zones that transect the eSRP. We suggest that intraplain extension is too slow (<1 mm/a) to measure using our campaign GPS methods, but may be sufficient over millennial timescales to accommodate rift zone formation. Slower velocities measured on three benchmarks within the neighboring Basin and Range “seismic parabola” are consistent with this region serving as a zone of detachment between the North American craton and the faster‐moving eSRP.

Current deformation in the northern Canadian Cordillera inferred from GPS measurements

Continuous and campaign‐style Global Positioning System (GPS) measurements provide new constraints on the first‐order current deformation pattern of the northern Cordillera of NW Canada and eastern Alaska. The Yakutat block is currently colliding with North America in the corner of the Gulf of Alaska. Our data infer that relative Yakutat–North America motion is accommodated across the eastern boundary by right‐lateral motion (∼40 mm/a), mainly on the Fairweather fault, and minor shortening (∼6 mm/a). To the north, collision is taken up by shortening (∼31 mm/a) mainly on the Chugach–St. Elias fault system, with westward extrusion and possible counterclockwise rotation of the Yakutat block and Alaskan fore arc facilitated by ∼23 mm/a right‐lateral motion that is shared by several faults. The seismicity pattern indicates that plate boundary deformation is far reaching, producing strain throughout eastern Alaska, Yukon, and western Northwest Territories. Our GPS data confirm the transfer of strain from the Yakutat collision zone and enable the extent of plate boundary deformation to be mapped, thereby defining the western edge of “stable” North America. GPS sites in SW Yukon show motion of 3–10 mm/a to the NE, confirming that strain is transferred at least 400 km from the Yakutat collision. Continued transfer north toward the Mackenzie Delta and NE to the Mackenzie Mountains, indicated by current seismicity, is not yet well resolved by the campaign GPS data, which are hampered by coseismic and postseismic effects of the 2002 Mw 7.9 earthquake on the Denali fault in eastern Alaska.

Determinasi sumber tekanan dan analisis regangan utama di Gunung Api Papandayan untuk mengetahui korelasi dengan kegempaan

http://dx.doi.org/10.17014/ijog.vol2no2.20072Papandayan volcano is located in the southern part of Garut regency, about 70 km southeast of Bandung, West Java. Monitoring the activities of Papandayan volcano has been done using various methods both continously and periodically, one of them is deformation method using repeated GPS (Global Positioning System) survey. GPS survey method is basically used to obtain the pattern and speed of the deformation body of the volcano, both in horizontal and vertical directions and also could be used to determined the location and size of the strain source of deformation based on Mogi model. By describing the shallow seismic activities before and after the eruption, this could show us the correlation of deformation characteristic and its seismic activities. By the result of eight GPS campaigns show that the deformation acceleration is running rapidly, where the fluctuation of shallow seismic activities are directly followed by inflation and deflation of volcano body. Pressure source movement running up and down to southwest-northeast direction. This correlation will lead us to the more comprehensive phenomena of a volcanic eruption, especially in Papandayan volcano.

Viscoelastic relaxation and long-lasting after-slip following the 1997 Umbria-Marche (Central Italy) earthquakes

SUMMARY We combine Global Positioning System (GPS) measurements with forward modelling of vis- coelastic relaxation and after-slip to study the post-seismic deformation of the 1997 Umbria- Marche (Central Apennines) moderate shallow earthquake sequence. Campaign GPS mea- surements spanning the time period 1999-2003 are depicting a clear post-seismic deformation signal. Our results favour a normal faulting rupture model where most of the slip is located in the lower part of the seismogenic upper crust, consistent with the rupture models obtained from the inversion of strong motion data. The preferred rheological model, obtained from viscoelastic relaxation modelling, consists of an elastic upper crust, underlain by a transition zone with a viscosity of 10 18 Pa s, while the rheology of deeper layers is not relevant for the observed time-span. Shallow fault creep and after-slip at the base of the seismogenic upper crust are the first order processes behind the observed post-seismic deformation. The deep after-slip, below the fault zone at about 8 km depth, acting as a basal shear through localized time-dependent deformation, identifies a rheological discontinuity decoupling the seismogenic upper crust from the low-viscosity transition zone.

Recent crustal deformation of İzmir, Western Anatolia and surrounding regions as deduced from repeated GPS measurements and strain field

To investigate contemporary neotectonic deformation in İzmir, Western Anatolia and in its neighborhood, a relatively dense Global Positioning System (GPS) monitoring network was established in 2001. Combination of three spatially dense GPS campaigns in 2001, 2003 and 2004 with temporally dense campaigns between 1992 and 2004 resulted in a combined velocity field representing active deformation rate in the region. We computed horizontal and vertical velocity fields with respect to Earth-centered, Earth-fixed ITRF2000, to Eurasia and to Anatolia as well. The rates of principal and shear strains along with rigid-body rotation rates were derived from velocity field. Results show east–west shortening between Karaburun Peninsula and northern part of İzmir Bay together with the extension of İzmir Bay in accordance with general extension regime of Western Anatolia and Eastern Agea. East–west shortening and north–south extension of Karaburun Peninsula are closely related to right-lateral faulting and a clockwise rotation. There exists a block in the middle of the peninsula with a differential motion at a rate of 3–5 ± 1 mm/year and 5–6 ± 1 mm/year to the east and south, respectively. As is in Western Anatolia, north–south extension is dominant in almost all parts of the region despite the fact that they exhibit significantly higher rates in the middle of the peninsula. Extensional rates along Tuzla Fault lying nearly perpendicular to İzmir Bay and in its west are maximum in the region with an extension rate of 300–500 ± 80–100 nanostrain/year and confirm its active state. Extensional rates in other parts of the region are at level of 50–150 nanostrain/year as expected in the other parts of Western Anatolia.

Inversion for rheological parameters from post-seismic surface deformation associated with the 1960 Valdivia earthquake, Chile

SUMMARY Data collected during two Global Positioning System campaigns in 1994 and 1996 across Chile and western Argentina (22 stations), in the area where the M w = 9.5 1960 May 22 Valdivia earthquake took place, shows ground motion velocities that cannot be fully explained by the elastic strain accumulation during the interseismic phase of an earthquake deformation cycle. We use dislocation models to reproduce the observed velocities, with a 3-D source in a medium with one elastic layer overlying a Maxwell viscoelastic half-space, and a planar rupture surface with uniform coseismic slip. The reason for avoiding a more detailed and elaborated model is that knowledge about the Valdivia earthquake source parameters and the area where the event took place is poorly constrained. We focus, therefore, on examining the first-order postseismic deformation, and ignore finer details about the heterogeneity of the Earth. By means of a grid search inversion over more than a million different models, we derived the most likely values for some of the medium and source parameters involved in the deformation process, namely viscosity (η), thickness of the elastic layer (D), average slip on the rupture surface (U 0) and the seismic coupling coefficient (χ). According to our study, the optimum values are: η = 10 20 Pa · s, D = 46 km, U 0 = 15 m and χ = 96. A clear difference is seen between the surface deformation caused by silent-slip on the rupture surface and the one caused by postseismic relaxation processes, two possibilities proposed to explain the anomalous velocities. We find that the deformation associated with the 1960 Valdivia event can still be observed after several decades and it is the most likely explanation for the velocity component that cannot be explained by plate convergence. Our model also predicts that this deformation will still be measurable for several more decades. Our model reproduces the first-order pattern of the measured GPS velocities, showing good agreement with recent finite-element studies, with the advantage of simplicity and short computation time, allowing the extensive search for the best-fitting model.

Slow slip on the northern Hikurangi subduction interface, New Zealand

In October 2002, a surface displacement episode of 20–30 mm magnitude was observed over a ∼10 day period on two continuous Global Positioning System (GPS) instruments near Gisborne, North Island, New Zealand. We interpret this to result from slow slip on the northern Hikurangi subduction interface. Using ten years of regional campaign GPS (1995–2004) and recent continuous GPS data, we estimate the recurrence interval for similar events to be 2–3 yrs. In November 2004, a similar slow slip event occurred within this recurrence period. The 2002 event can be modeled by ∼18 cm of slow slip near the down‐dip end of the seismogenic zone on the subduction interface offshore of Gisborne. The campaign GPS data show that the 2002 slow slip event had little effect on regional strain patterns.

Time-Height Distribution of Water Vapor Derived by Moving Cell Tomography During Tsukuba GPS Campaigns

Observations using a dense network of 75 Global Positioning System (GPS) receivers were carried out in Tsukuba, Japan, in October-November 2000 and July-September 2001. We have applied a tomography technique on the slant path propagation delay of the GPS signals to derive the temporal and spatial distribution of water vapor. In particular, we have employed the moving cell method, which was originally developed by Seko et al. (2000) for a beta-mesoscale precipitation system associated with the Baiu front. Because we are interested in local-scale phenomena, we tried to determine the water vapor distribution with a time resolution of ten minutes. The tomographic analyses resolve the water vapor variations during the meteorological events studied here.

Dry Biases of Humidity Measurements from the Vaisala RS80-A and Meisei RS2-91 Radiosondes and from Ground-Based GPS

During the Tsukuba GPS (Global Positioning System) Dense Network Campaign Observations in the autumn of 2000, we performed 3 hourly upper air soundings using two types of radiosondes. One was the Meisei RS2-91, operational sonde of the Japan Meteorological Agency (JMA) made by Meisei Electric Co. Ltd. with an independent thin-film capacitive sensor, and the other was Vaisala RS80-15G with A-type Humicap humidity sensor (RS80-A hereafter). It was found that the Precipitable Water Vapor (PWV) measured by Vaisala RS80-A were clearly smaller (dry bias) by 3-4 mm (about 4-6%) than those by JMA RS2-91 and those analyzed from GPS observations. The dry bias error of Vaisala RS80-A was also confirmed with dual sonde balloon flights equipped with both Vaisala RS80-A and JMA RS2-91 instruments, and near simultaneous flights of two types of radiosondes. It was found that the dry bias error was reduced by the new tight protective cap for the humidity sensor, but substantial amount of dry bias error still remained. We further compared humidity measurements by the two radiosondes to a chilled-mirror dew-point hygrometer using a two-pressure type humidity calibration chamber. The experiment was conducted under room temperature conditions. It showed that relative humidity as measured by the Vaisala RS80- A was about 5 to 15% smaller than that of the dew-point hygrometer. The bias was larger in high humidity conditions. Although the dry bias of Vaisala RS80-A radiosondes has been pointed out in several other studies, the dry bias revealed in this study is much larger than previously reported, indicating that a problem still exists in RS80-A humidity measurements taken in moist air conditions such as Japan. The RS2-91 showed only small dry bias relative to the dew-point hygrometer (less than 4%). However, the Aerological Observatory of the JMA reported that they improved the RS2-91 humidity sensor since 1999 (Shibue et al. 2000). RS2-91 sondes manufactured before and in 1999 had a dry bias with the same order of magnitude as the Vaisala RS80-A. There have been several reports (e.g., Ohtani et al. 2000) that the PWV derived by GEONET (Japanese nationwide GPS array) and other campaign GPS observations agreed well with those by JMA or Vaisala radiosondes. The implication of the dry bias errors of the radiosonde to the previous GPS studies is that the past GPS analysis in Japan produced a drier atmosphere than actual conditions. It is recommended that the past GPS data, including the GEONET data, be reanalyzed using the most advanced GPS analysis methods.

Current tectonics of northern Cascadia from a decade of GPS measurements

Global Positioning System (GPS) measurements to study regional deformation were initiated in northern Cascadia in the late 1980s and early 1990s. On the basis of a decade of GPS data, we derive a crustal velocity field for NW Washington‐SW British Columbia. The permanent and campaign GPS velocities are defined with respect to North America in the ITRF2000 reference frame. Velocity uncertainties are estimated using a model of time series noise spectra. This new velocity field is the basis for interpretation of the tectonics of the northern Cascadia subduction system. GPS velocities are interpreted in terms of interseismic loading of the megathrust using different coupling models. Our data confirm that the upper part of the megathrust is nearly fully locked. An exponential model for the downdip transition zone gives slightly better agreement with the data compared to the common linear transition. The landward decrease of forearc strain loading is smaller than predicted by any of the current subduction interseismic models. This could be a consequence of a small (0–3 mm/yr) long‐term motion of the southern Vancouver Island forearc, with respect to North America, or of a concentration of interseismic strain across the elastically weaker Cascadia volcanic arc. In northern Vancouver Island, our velocity field supports the existence of an independent Explorer microplate currently underthrusting underneath North America, at least up to Brooks Peninsula. Further north, GPS velocities indicate transient and/or permanent deformation of northernmost Vancouver Island related to the interaction with the Explorer microplate and possibly with the Queen Charlotte transform margin.

Recent crustal deformation and the earthquake cycle along the Ecuador–Colombia subduction zone

Recent results from Global Positioning System (GPS) measurements show deformation along the coast of Ecuador and Colombia that can be linked to the rupture zone of the earthquake in 1979. A 3D elastic boundary element model is used to simulate crustal deformation observed by GPS campaigns in 1991, 1994, 1996, and 1998. Deformation in Ecuador can be explained best by 50% apparent locking on the subduction interface. Although there have not been any historic large earthquakes ( M w>7) south of the 1906 earthquake rupture zone, 50% apparent elastic locking is necessary to model the deformation observed there. In Colombia, only 30% apparent elastic locking is occurring along the subduction interface in the 1979 earthquake rupture zone ( M w 8.2), and no elastic locking is necessary to explain the crustal deformation observed at two GPS sites north of there. There is no evidence from seismicity or plate geometry that plate coupling on the subduction zone is reduced in Colombia. However, simple viscoelastic models suggest that the apparent reduction in elastic locking can be explained entirely by the response of a viscous upper mantle to the 1979 earthquake. These results suggest that elastic strain accumulation is occurring evenly throughout the study area, but postseismic relaxation masks the true total strain rate.

Transient rheology of the uppermost mantle beneath the Mojave Desert, California

Geodetic data indicate that the M7.1 Hector Mine, California, earthquake was followed by a brief period (a few weeks) of rapid deformation preceding a prolonged phase of slower deformation. We find that the signal contained in continuous and campaign global positioning system data for 2.5 years after the earthquake may be explained with a transient rheology. Quantitative modeling of these data with allowance for transient (linear biviscous) rheology in the lower crust and upper mantle demonstrates that transient rheology in the upper mantle is dominant, its material properties being typified by two characteristic relaxation times ∼0.07 and ∼2 years. The inferred mantle rheology is a Jeffreys solid in which the transient and steady-state shear moduli are equal. Consideration of a simpler viscoelastic model with a linear univiscous rheology (2 fewer parameters than a biviscous model) shows that it consistently underpredicts the amplitude of the first ∼3 months signal, and allowance for a biviscous rheology is significant at the 99.0% confidence level. Another alternative model – deep postseismic afterslip beneath the coseismic rupture – predicts a vertical velocity pattern opposite to the observed pattern at all time periods considered. Despite its plausibility, the advocated biviscous rheology model is non-unique and should be regarded as a viable alternative to the non-linear mantle rheology model for governing postseismic flow beneath the Mojave Desert.