We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ∼2.5 in areas with dense data coverage. We determined 6739 velocities from time series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15,772 velocities from 233 (mostly) published studies onto our core solution to obtain 22,511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25° longitude by 0.2° latitude in dimension). The remainder of the Earth's surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS-derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid-body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self-consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no-net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for Mw=7.5 characteristic earthquakes.

A geodetic plate motion and Global Strain Rate Model

SUMMARY In this paper we present a global model (GSRM-1) of both horizontal velocities on the Earth’s surface and horizontal strain rates for almost all deforming plate boundary zones. A model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field. In our model we perform a least-squares fit between model velocities and observed geodetic velocities, as well as between model strain rates and observed geological strain rates. Model velocities and strain rates are interpolated over a spherical Earth using bi-cubic Bessel splines. We include 3000 geodetic velocities from 50 different, mostly published, studies. Geological strain rates are obtained for central Asia only and they are inferred from Quaternary fault slip rates. For all areas where no geological information is included a priori constraints are placed on the style and direction (but not magnitude) of the model strain rate field. These constraints are taken from a seismic strain rate field inferred from (normalized) focal mechanisms of shallow earthquakes. We present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones. Generally, the strain rate tensor field is consistent with geological and seismological data. With the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs. We find that in general there is a good agreement between the present-day plate motions we obtain and longterm plate motions, but a few significant differences exist. The rotation rates for the Indian, Arabian and Nubian plates relative to Eurasia are 30, 13 and 50 per cent slower than the NUVEL1A estimate, respectively, and the rotation rate for the Nazca Plate relative to South America is 17 per cent slower. On the other hand, Caribbean‐North America motion is 76 per cent faster than the NUVEL-1A estimate. While crustal blocks in the India‐Eurasia collision zone move significantly and self-consistently with respect to bounding plates, only a very small motion is predicted between the Nubian and Somalian plates. By integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in, for instance, redefining a no-net-rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation.

/pdf/an-integrated-global-model-of-present-day-plate-motions-and-57ic0m6oev.pdf

An integrated global model of present-day plate motions and plate boundary deformation

SUMMARY
We present a new geodetic velocity solution for Italy and the surrounding areas, obtained from an analysis of continuous and survey-mode Global Positioning System observations collected between 1991 and 2002. We have combined local, regional and global networks into a common reference frame velocity solution, providing a new detailed picture of the regional-scale deformation field in the central Mediterranean. Our velocity estimates are computed with respect to a new stable Eurasian reference frame, constraining the kinematics of the greater African–Eurasian plate boundary system in the study area. We provide strain-rate values for the main seismotectonic districts, which are in good agreement with the seismic deformation inferred from earthquake focal mechanisms. The southern Tyrrhenian area, Calabria, the Apennines, the southeastern Alps, the southern Dinarides and the Albanides display deformation rates at the order of 20–30 nanostrain yr−1. The Corsica–Sardinia block moves according to Eurasian Plate motions, and there is no indication that the opening of the Tyrrhenian is still active. The Pelagian and Sicilian domains are separated by a northwest–southeast discontinuity, the Sicily Channel rifted area, and marginally significant relative motion between the Pelagian Plateau and the African Plate is also observed. The southern Tyrrhenian is affected by north–south compression and accommodates up to 50 per cent of the Africa–Eurasia relative plate motion, whereas the Calabrian Arc exhibits ∼2 mm yr−1 northwest–southeast extension. The observed deformation pattern suggests the presence of a major approximately north–south tectonic discontinuity separating the Sicilian and Calabrian domains. An extensional boundary observed along peninsular Italy coincides with the distribution of seismogenic faults along the axis of the Apennines. This boundary separates a Tyrrhenian and an Adriatic domain with diverging velocities, orientated north–NNW-ward and northeastward, respectively. The Apennines are extending perpendicularly to the chain axis at rates of less than 3 mm yr−1, and only in the outer northern Apennines indications of active shortening are observed. Insignificant deformation is observed in the western Po Plain and the western Alps, while the central and eastern Alps display north–south shortening. The eastern Adriatic domain is shortening perpendicular to the Dinaric front with strain rates increasing from north to south. The locus of collision between the Aegean/Balkan system and the Adriatic and Ionian domains is marked by the Kephalonia fault system, which accommodates up to 20±1 mm yr−1 of right-lateral motion. The deformation pattern observed in the peri-Adriatic domain is well described by a counter-clockwise rotation of the Adriatic microplate around a pole located in the western Alps.

Crustal velocity and strain-rate fields in Italy and surrounding regions: new results from the analysis of permanent and non-permanent GPS networks

We use the GPS velocity field (1988–1998) for eastern Mediterranean and Asia Minor to determine the crustal deformation strain rate field in an area bounded by 35°N and 43°N, and 20°E and 48°E. We calculate the normal and shear strain rate components associated with the major faults and compare these qualitatively with seismological data. Uncertainties in the calculation of the strain rates reach 50 nstrain yr−1 in sparsely observed parts of Anatolia, whereas we estimate errors <20 nstrain yr−1 in the Aegean and Marmara regions. The largest compressional strain rate components in the eastern part of the study area occur along the Greater Caucasus mountain front reaching 70 nstrain yr−1. (1 nstrain yr−1 = 0.0317 × 10−15 s−1). The North Anatolian Fault Zone is the clearest feature in the shear strain rate field. It is expressed as a pronounced dextral strike-slip fault zone, reaching rates of up to 170 nstrain yr−1. This holds true also for the Izmit area, where the August 17, 1999, earthquake occurred. Central Anatolia is almost strain-free, whereas extension prevails in western Anatolia. The principal axes of extension vary around the N-S direction with strain rates of up to 85 nstrain yr−1. These extensional areas coincide with graben features and normal faulting earthquakes. The central and southwestern Aegean Sea is strain-free with values far below 40 nstrain yr−1. The seismic cluster around the Dodekanissa islands, southeastern Aegean Sea, coincides with NW-SE oriented extension, attaining strain rates of up to 90 nstrain yr−1. This area of extension also exhibits recent active volcanism. The entire Hellenic arc shows compressional strain rates perpendicular to the arc. The Pliny-Strabo troughs along the eastern segment of the arc show left-lateral shear strain rates reaching 80 nstrain yr−1. Significant extension is found in central Greece, with a NNE-SSW oriented maximum of 120 nstrain yr−1 centered around the Gulf of Corinthos. The Kephalonia Fault Zone in NW Greece is a distinct dextral fault zone, separating Apulia from the rapidly moving Aegean microplate. Right-lateral shear strain rates reach 150 nstrain yr−1.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900238

GPS‐derived strain rate field within the boundary zones of the Eurasian, African, and Arabian Plates

Crustal motion and deformation in Greece from a decade of GPS measurements, 1993–2003

New constraints on the rapid crustal motion of the Aegean region: recent results inferred from GPS measurements (1993–1998) across the West Hellenic Arc, Greece

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New constraints on the rapid crustal motion of the Aegean region: recent results inferred from GPS measurements (1993–1998) across the West Hellenic Arc, Greece