Global Positioning System Velocity Field Research Articles

Mechanics of V-shaped conjugate strike-slip faults and the corresponding continuum mode of continental deformation

V-shaped conjugate strike-slip faults occur widely on Earth, Venus, and Mars in the solar system. They commonly lie at 60°–75° in map view from the maximum compressive stress (σ 1 ) direction. This fault pattern cannot be explained directly by the Coulomb fracture criterion, which predicts the formation of X-shaped shear fractures at 30° from the σ 1 direction. Possible explanations of this odd fault geometry include rotation of early formed Coulomb fractures or reactivation of preexisting weakness. Here, we show that none of these mechanisms is feasible for the formation of a late Cenozoic conjugate strike-slip fault system in central Tibet. Instead, its initiation can be best explained by distributed deformation during the formation of two parallel and adjoining shear zones that have opposing senses of shear. Our suggestion is based on the current global positioning system (GPS) velocity field in Tibet, which can be divided into two east-trending shear zones: a northern left-slip zone consisting of active ENE-striking left-slip faults, and a southern right-slip zone consisting of active WNW-striking right-slip faults. The correlation between the GPS strain field and the fault pattern suggests that the central Tibet conjugate faults may have initiated as two sets of Riedel shears in the two parallel but separate shear zones. Because the two east-trending shear zones also experience north-south contraction, we refer to this mechanism of conjugate-fault formation as paired general-shear (PGS) deformation. Assuming a Newtonian fluid, the observed Tibetan GPS velocity field requires the paired shear zones to have formed either by gravitational spreading of the Tibetan lithosphere or a horizontal shear at the base of the upper crust or mantle lithosphere. We demonstrate the feasibility of the two inferred mechanisms for the formation of V-shaped conjugate faults using simple sandbox experiments. Our paired general-shear (PGS) model implies that the combined effect of the state of strain and the state of stress favors only one set (i.e., Riedel shear) of Coulomb conjugate shear fractures under general shear flow. It also requires continuum deformation rather than discrete extrusion tectonics as the most dominant mode of deformation during the late Cenozoic development of the central Tibetan Plateau.

Geologic versus geodetic deformation adjacent to the San Andreas fault, central California

We combine geologic and global positioning system (GPS) data to characterize the style and magnitude of off-fault deformation across the San Andreas fault system in central California. Geologic structures record ∼12 km of both fault-parallel and fault-perpendicular displacements across creeping and locked portions of the San Andreas fault. Analysis of 150 GPS site velocities suggests that the borderlands record 4–6 mm/yr of fault-parallel and 3–5 mm/yr of fault-perpendicular motion alongside the creeping segment, where elastic strain is minimized. The distribution of both long-term geologic and short-term geodetic deformation is affected by basement type, where more deformation is concentrated northeast of the San Andreas fault on Franciscan basement. We suggest that at least half the fault-parallel GPS deformation measured by GPS bordering the creeping segment must be accommodated by geologic structures; this permanent deformation needs to be incorporated into dynamic models of the fault system. Elastic modeling of the San Andreas fault in central California, which incorporates its well-known transition from locked to creeping behavior near Parkfield, predicts first-order variations in the GPS velocity field along the fault and corresponding variations in dilatational strain rates. The strain rate pattern is dominated by a large contractional region northeast of the transition from locked to creeping behavior and a large extensional region southwest of the transition. The former coincides with the Coalinga and Kettleman Hills anticlines, the growth and development of which seem to have occurred under at least two kinematic conditions. We suggest that the onset of fault creep in central California promoted the growth of these folds. By implication, fault creep has been active over geologic time scales.

The RING network: improvement of a GPS velocity field in the central Mediterranean

Since 2004, a continuous Global Positioning System (GPS) network has been operated by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) to investigate active tectonic processes in Italy and the surrounding regions, which are still largely debated. This important infrastructure is known as Rete Integrata Nazionale GPS (RING) network, and it consists of about 130 stations that are deployed all over Italy. The development and realization of a stable GPS monumentation, its integration with seismological instruments, and the choice of both satellite and internet data transmission, make this network one of the most innovative and reliable CGPS networks in the world. The technologically advanced development of the RING network has been accompanied by the development of different data processing strategies, which are mainly dependent on the use of different GPS analysis software. The different software-related solutions are here compared at different scales for this large network, and the consistency is evaluated and quantified within an RMS value of 0.3 mm/yr.

Kinematics of the Western Africa-Eurasia plate boundary from focal mechanisms and GPS data

SUMMARY The Western Mediterranean displays a complex pattern of crustal deformation distributed along tectonically active belts developed in the framework of slow oblique plate convergence. We used earthquake and Global Positioning System (GPS) data to study the present-day kinematics and tectonics of the Africa-Eurasia plate boundary in this region. Crustal seismicity and focal mechanisms, analysed in terms of seismic moment release and seismic deformation, outline the geometry of major seismic belts and characterize their tectonics and kinematics. Continuous GPS data have been analysed to determine Euler vectors for the Nubian and Eurasian plates and to provide the global frame for a new Mediterranean GPS velocity field, obtained by merging continuous and campaign observations collected in the 1991‐2005 time span. GPS velocities and displacements predicted by the Nubia-Eurasia rotation pole provide estimates of the deformation accommodated across the tectonically active belts. The rather simple deformation occurring in the Atlantic region, characterized by extension about perpendicular to the Middle Atlantic and Terceira ridges and right-lateral motion along the Gloria transform fault, turns into a complex pattern of deformation, occurring along broader seismic belts, where continental lithosphere is involved. Our analysis reveals a more complex fragmentation of the plate boundary than previously proposed. The roughly E-W trending mainly compressive segments (i.e. southwestern Iberia, northern Algeria and southern Tyrrhenian), where plate convergence is largely accomodated across rather localized deformation zones, and partially transferred northward to the adjacent domains (i.e. the Algero-Balearic and Tyrrhenian basins), are interrupted by regions of more distributed deformation (i.e. the Rif-Alboran-Betics, Tunisia-Libya and eastern Sicily) or limited seismicity (i.e. the Strait of Sicily), which are characterized by less homogeneous tectonics regimes (mainly transcurrent to extensional). In correspondence of the observed breaks, tectonic structures with different orientation interfere, and we find belts with only limited deformation (i.e. the High and Middle Atlas, the Tunisian Atlas and the offshore Tunisia-Libya belt) that extends from the plate boundary into the Nubian plate, along pre-existing tectonic lineaments. Our analysis suggest that the Sicilian-Pelagian domain is moving independently from Nubia, according to the presence of a right-lateral and extensional decoupling zone corresponding to the Tunisia-Libya and Strait of Sicily deformation zone. Despite the space variability of active tectonic regimes, plate convergence still governs most of the seismotectonic and kinematic setting up to the central Aeolian region. In general, local complexities derive from pre-existing structural features, inherited from the tectonic evolution of the Mediterranean region. On the contrary, along Calabria and the Apennines the contribution of the subducted Ionian oceanic lithosphere and the occurrence of microplates (i.e. Adria) appear to substantially modify both tectonics and kinematics. Finally, GPS data across the Gibraltar Arc and the Tyrrhenian-Calabria domain support the hypothesis that slab rollback in these regions is mostly slowed down or stopped.

New constraints on the active tectonic deformation of the Aegean

Site velocities from six separate Global Positioning System (GPS) networks comprising 374 stations have been referred to a single common Eurasia‐fixed reference frame to map the velocity distribution over the entire Aegean. We use the GPS velocity field to identify deforming regions, rigid elements, and potential microplate boundaries, and build upon previous work by others to initially specify rigid elements in central Greece, the South Aegean, Anatolia, and the Sea of Marmara. We apply an iterative approach, tentatively defining microplate boundaries, determining best fit rigid rotations, examining misfit patterns, and revising the boundaries to achieve a better match between model and data. Short‐term seismic cycle effects are minor contaminants of the data that we remove when necessary to isolate the long‐term kinematics. We find that present day Aegean deformation is due to the relative motions of four microplates and straining in several isolated zones internal to them. The RMS misfit of model to data is about 2‐sigma, very good when compared to the typical match between coseismic fault models and GPS data. The simplicity of the microplate description of the deformation and its good fit to the GPS data are surprising and were not anticipated by previous work, which had suggested either many rigid elements or broad deforming zones that comprise much of the Aegean region. The isolated deforming zones are also unexpected and cannot be explained by the kinematics of the microplate motions. Strain rates within internally deforming zones are extensional and range from 30 to 50 nanostrain/year (nstrain/year, 10−9/year), 1 to 2 orders of magnitude lower than rates observed across the major microplate boundaries. Lower strain rates may exist elsewhere within the microplates but are only resolved in Anatolia, where extension of 13 ± 4 nstrain/year is required by the data. Our results suggest that despite the detailed complexity of active continental deformation revealed by seismicity, active faulting, fault geomorphology, and earthquake fault plane solutions, continental tectonics, at least in the Aegean, is to first order very similar to global plate tectonics and obeys the same simple kinematic rules. Although the widespread distribution of Aegean seismicity and active faulting might suggest a rather spatially homogeneous seismic hazard, the focusing of deformation near microplate boundaries implies the highest hazard is comparably localized.

A Field of Annual Accumulation of Strain Energy Density and its Tectonic Activity in North China by GPS Measurements

AbstractGlobal Positioning System (GPS) observations from three measurement campaigns from 1995 to 1999 are used in a study of the tectonic activity and earthquake hazards in North China. These GPS data were firstly processed together with about 13 Global IGS stations using the GAMIT software to get the baseline solutions, then crustal horizontal motion in ITRF97 frame and with respect to Eurasia were obtained based on these baseline solutions and their variance‐covariance matrix using the GPS network adjustment software PowerADJ developed by Wuhan University. Using the GPS velocity field we estimate the annual accumulation of strain rate field and annual accumulation of density of strain energy field. These resulting fields show the following: (1) most stations move at a rate of (3–12 mm/a)±3.4mm/a east‐southeastwards with respect to Eurasia. (2) Tangshan‐Hejian‐Xingtai is an active seismotectonic compression border. Datong‐Taiyuan is an active extension border with slight dextral rotation. (3) Beijing–Tianjin–Bohai Bay region and Xingtai region are of higher shear strain rate than other regions. Zhangjiakou–Beijing–Tianjin–Bohai Bay region and Jinan region are of much higher annual accumulation of strain energy density than other regions in North China, these regions are of higher amount of the energy storage. The Bohai Bay region is also located in a transition zone from positive to negative surface strain rate, it may be a region of earthquake risk in the future.

Transient and permanent deformation of central Japan estimated by GPS

Abstract We analyze continuous global positioning system (GPS) data to estimate the kinematics of the Philippine Sea plate subduction below southern central Japan. Using a back slip dislocation model, we invert the strain rate tensors derived from the GPS velocity field to obtain the velocity of the downgoing slab with respect to the Tokai and Kanto forearcs along the eastern Nankai-Suruga and Sagami troughs, respectively. In both cases, these velocities are smaller and slightly rotated with respect to the Philippine Sea plate subduction velocity in central Nankai. This result implies strain partitioning between the subduction thrusts and on land and/or offshore deformation zones in both the eastern Nankai-Suruga and the Sagami area. In southern Kanto, we predict ∼20 mm yr −1 of dextral shear, in agreement with motion observed along the Boso strike-slip fault. In the Tokai region, the slip deficit accumulated since the last earthquake (1854) ranges between 6 and 3 m. Considering the extent of the locked zone, it would correspond to a magnitude M W =8.15 earthquake, if the slip deficit were to be released now during a single event.

Full interseismic locking of the Nankai and Japan‐west Kurile subduction zones: An analysis of uniform elastic strain accumulation in Japan constrained by permanent GPS

We analyze permanent Global Positioning System (GPS) data obtained over Japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the Pacific and Philippine Sea plates below the Japanese islands. We first derive the GPS strain rate fields that characterize the crustal deformation of southern and northern Japan and invert them to determine the effective subduction velocity along the central Nankai trough on one side and the Japan‐west Kurile trench on the other. These “reference free” velocities are close to those predicted by plate motion models with respect to Eurasia. We conclude that the Eurasian reference frame gives a good approximation to the subduction motion and that to first order, both subduction zones were fully locked during the period of measurements. We then test whether the coupling ratio shows local variations within the seismogenic zones. To do this, we divide the subduction interface into 35 km×30 km elements that we model by point source groups, and we invert the GPS velocity field referenced to Eurasia to derive the coupling ratio (between 0 and 1) on each fault element. The results are coherent over the 3 years and confirm that both the central Nankai and the Japan‐west Kurile seismogenic zones are homogeneously fully locked. Most of the coupling ratios are close to 1 and a few are close to 0; intermediate values are rare. The zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 Sanriku‐Oki interplate earthquake (Japan trench) or to a small overestimation of the actual lower limit of the locked zone. We conclude that within the resolution of the GPS data and the model, (1) partial coupling did not exist during these 3 years along the Nankai and Japan‐west Kurile trenches; (2) the small seismic coupling ratio previously derived from earthquakes analysis for the Japan and Kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip; and (3) the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading. Finally, we discuss the nonelastic residual deformation within the frame of the long‐term deformation of the Japanese islands.

GPS and geologic estimates of the tectonic activity in the Marmara Sea region, NW Anatolia

A dense network of 52 Global Positioning System (GPS) sites is used to determine the velocity field and strain rate pattern at the western end of the North Anatolian Fault Zone (NAFZ) in the Marmara Sea region, NW Anatolia. Based on four GPS campaigns carried out biannually between 1990 and 1996, the detailed kinematic field of crustal motion is calculated. The southernmost station shows an average rate of 22±3 mm/yr oriented westward relative to Istanbul (Eurasia). This value is indicative of the dextral strike‐slip motion of NW Anatolia relative to the Black Sea. The GPS velocity field reveals that the sites located on the Thrace‐Black Sea block show negligible motion relative to Istanbul. The GPS‐based deformation pattern of three sections crossing the westward prolongation of the NAFZ is compared with neotectonic data. Both the GPS and the neotectonic data give clear evidence that most of the deformation occurs along a relatively narrow E‐W oriented zone. This belt extends from the single fault trace of the NAFZ through the Gulf of Izmit, the Marmara Sea, the Şarköy region, and the Gulf of Saros into the North Aegean Trough (NAT). In addition, the cross sections show the increasing influence of the N‐S oriented extension in the Aegean Sea region. From the comparison of the directions and rates of shear and normal strain deduced from GPS data with geologic and seismic observations we conclude that the most active zone of dextral shear follows a line along Mudurnu Valley, the northern Marmara Sea and the Saros Trough where it joins the NAT.