Right-lateral Strike-slip Motion Research Articles

The Garzon fault: active southwestern boundary of the Caribbean plate in Colombia

We propose active right-lateral strike-slip motion on the Garzon fault zone of the Neiva basin, Colombia, based on the identification of two active right-stepping releasing bend basins along the fault using stereoscopic analysis of 1/250000 SPOT images. The Garzon fault connects the Bocono-Pamplona-Guaicaramo fault zones of Venezuela and Colombia with the Romeral, Dolores and Guayaquil faults of Colombia. Together these faults form a continuous, active right-lateral fault between accreted terranes in northwestern South America and a more stable South America plate. We infer 5-km right-lateral offset of the Garzon fault based on the width of the Algeciras releasing bend basin.

Active crustal deformation in the Marmara Sea Region, NW Anatolia, Inferred from GPS measurements

Deformation in the boundary zone between the Anatolian and Eurasian plates has been determined by means of repeated Global Positioning System (GPS) measurements. Three campaigns (1990, 1992, 1994) were carried out around the Marmara Sea. The network lies within the active earthquake belt of NW Anatolia, extending from the western part of the North Anatolian Fault Zone (NAFZ) to the Aegean extensional province. Results of the dense network consisting of 52 sites clearly reveal the westward motion of Anatolia. The rates amount to 17 mm/a ±3 mm/a on average relative to Istanbul, located at the northeastern edge of the Sea of Marmara. Based on the trajectories for the displacements of the observation sites it is concluded that the Marmara Sea forms a transition zone between the Europe fixed region of Thrace/Black Sea (Eurasia) and NW Anatolia. At the western extremity of NW Anatolia the motion bends from W to WSW(247°). The strain field calculated yields an E‐W oriented right‐lateral strike‐slip movement (average 0.12 ppm/a, maximum 0.41 ppm/a) accompanied by NE‐SW extension (average −0.12 ppm/a, maximum −0.52 ppm/a) at the eastern end of the Marmara Sea and N‐S extension at the Aegean shore line. Comparisons with the directions of T axes derived from 65 fault plane solutions of earthquakes show remarkable agreement.

Inversion of regional surface-wave spectra for source parameters of aftershocks from the 1992 Petrolia earthquake sequence

AbstractRegional distance surface waves are used to study the source parameters for moderate-size aftershocks of the 25 April 1992 Petrolia earthquake sequence. The Cascadia subduction zone had been relatively seismically inactive until the onset of the mainshock (Ms = 7.1). This underthrusting event establishes that the southern end of the North America-Gorda plate boundary is seismogenic. It was followed by two separate and distinct large aftershocks (Ms = 6.6 for both) occurring at 07:41 and 11:41 on 26 April, as well as thousands of other small aftershocks. Many of the aftershocks following the second large aftershock had magnitudes in the range of 4.0 to 5.5. Using intermediate-period surface-wave spectra, we estimate focal mechanisms and depths for one foreshock and six of the larger aftershocks (Md = 4.0 to 5.5). These seven events can be separated into two groups based on temporal, spatial, and principal stress orientation characteristics. Within two days of the mainshock, four aftershocks (Md = 4 to 5) occurred within 4 hr of each other that were located offshore and along the Mendocino fault. These four aftershocks comprise one group. They are shallow, thrust events with northeast-trending P axes. We interpret these aftershocks to represent internal compression within the North American accretionary prism as a result of Gorda plate subduction. The other three events compose the second group. The shallow, strike-slip mechanism determined for the 8 March foreshock (Md = 5.3) may reflect the right-lateral strike-slip motion associated with the interaction between the northern terminus of the San Andreas fault system and the eastern terminus of the Mendocino fault. The 10 May aftershock (Md = 4.1), located on the coast and north of the Mendocino triple junction, has a thrust fault focal mechanism. This event is shallow and probably occurred within the accretionary wedge on an imbricate thrust. A normal fault focal mechanism is obtained for the 5 June aftershock (Md = 4.8), located offshore and just north of the Mendocino fault. This event exhibits a large component of normal motion, representing internal failure within a rebounding accretionary wedge. These two aftershocks and the foreshock have dissimilar locations in space and time, but they do share a north-northwest oriented P axis.

Ridge collision tectonics in terrane development

Some allochthonous terranes form along active continental margins when slivers of forearc crust (or more extensive crust) are displaced along arc-parallel strike-slip faults. Such faults can be generated or reactivated in response to either oblique subduction or ridge collision (collision between an oceanic spreading ridge and the leading edge of the forearc). The mechanical and thermal effects of ridge collision are important factors in the origin crustal development of some forearc sliver terranes. Some of the effects of ridge collision are well illustrated in the South American forearc near the Chile triple junction (46° S) where the Chile Rise is colliding today. Impingement of the Chile Rise, in conjuction with oblique subduction, has caused an elongate forearc sliver terrane to move northward away from an extensional zone at the collision site. The terrane is bounded on the east by the arc-parallel Liquiñe-Ofqui fault system (LOF) which coincides roughly with the forearc-arc boundary, and on the south by the Golfo de Penas extensional basin. Fault fabrics, recent seismicity, and paleomagnetic results indicate a component of right-lateral strike-slip movement on the LOF. Neotectonic geomorphology and pre- and post-seismic vertical strain data from the 1960 Concepcíon earthquake indicate a west-down dip-slip component of movement. Three-dimensional finite element models of ridge collision in this region substantiate these shear strains and development of an arc-parallel fault at about 150–200 km from the trench.Development of the forearc crust during Miocene and younger collision also involved intrusion of silicic magmas and emplacement of the Pliocene(?) Taitao ophiolite within about 15 km of the trench. The ophiolite and the silicic magmas constitute anomalous additions to the forearc crust, and record tectonic events leading to the origin of the allochthonous terrane carrying them. Similar ophiolite/silicic plutonic associations may help unravel the origin of other allochthonous terranes.

Global Positioning System (GPS) estimates of crustal deformation in the Marmara Sea region, Northwestern Anatolia

In the Marmara Sea region the relative motion between the Anatolian and Eurasian lithosphere has been measured by means of the modern techniques of space geodesy using the Global Positioning System (GPS). In order to resolve in detail the kinematic field within the active earthquake belts of Northwestern Anatolia, two GPS campaigns were carried out across a dense network consisting of 45 sites. Preliminary results reveal horizontal westward motion of Anatolia relative to Eurasia of 2.4 cm/y on average that runs along the western end of the North Anatolian Fault Zone. E-W oriented right-lateral strike-slip movement (average 0.06 ppm/y, maximum 0.13 ppm/y) (1ppm/y= 1 μstrain/y and NE-SW extension (average 0.06 ppm/y, maximum 0.21 ppm/y) can be obtained from the calculated strain field. In the west strike-slip motion trending WSW-ENE is observed. These findings are compared to the stress pattern derived from seismology and neotectonics.

Earthquakes on the Kazerun Line in the Zagros Mountains of Iran: strike-slip faulting within a fold-and-thrust belt

Summary The Kazerun Line is a transverse valley of about 200 km long that obliquely crosses the regular anticlines of the Zagros fold belt in SW Iran. At its northern end it is a clear fault which can be mapped on the surface. Anticline axes die out or bend towards this valley but do not cross it. Six moderate-sized earthquakes that occurred close the the Kazerun Line, and within a 25 km area involved right-lateral strike-slip motion parallel to the strike of the valley. They indicate that the Kazerun Line is the surface expression of a buried strike-slip fault. Slip vectors in these strike-slip earthquakes are different from those of neighbouring reverse-fault earthquakes, suggesting that the Kazerun Line accommodates some of the shortening between Arabia and central Iran by an elongation of the Zagros mountains parallel to strike. The centroid depths and the source dimensions of these earthquakes, combined with the lack of seismogenic surface faulting in the Zagros, suggest to us that all these earthquakes involve faulting in the metamorphic basement beneath the sedimentary cover. The sedimentary cover is almost certainly decoupled from the basement by several thick evaporite horizons. The seismicity of the Kazerun Line thus demonstrates how lateral interruptions to the regularity of a fold belt can arise from faulting in the basement, and not just from lateral ramps between the thrust sheets that deform the sedimentary cover.

Late Cainozoic geodynamic evolution of Thessaly and surroundings (central-northern Greece)

In the framework of the late Alpide deformation of Greece and of the recent and active extensional tectonism of the Aegean region, the geotectonic evolution of the Thessaly region (central-northern Greece) has been examined, using a quantitative and qualitative structural analysis; stratigraphie, sedimentological, morphotectonic and seismological data. The geometry of the faults, their architecture and the knowledge of the stress pattern are used to explain some aspects of the tectonics and crustal dynamics of Thessaly and the surrounding area. The oldest compressional phases taken into account show a mean ENE-WSW trending direction of shortening and have been defined as late Alpide (early Aquitanian and Langhian). A later (Late Miocene-Pliocene) NE-SW oriented extensional phase has been related to the Hellenic post-orogenic collapse which developed behind the collisional front between the Aegean (Eurasia) and African plates. This phenomenon diachronically migrated from the east (Central Macedonia, Thermaikos Basin) towards the west (Epirus, Albania) where it is still active. As a consequence of this second phase, the area forms a basin and range like structure. The third, and last, phase (Middle Pleistocene-present) is characterized by a N–S direction of extension and affects the entire Aegean region. It generated new E–W trending basins, superimposed on the inherited ones. This gave as a final result, the complex block pattern we can see today. The recent and active right-lateral strike-slip movements along the North Aegean Trough seem to stop in the Sporades Basin and do not affect the uppermost crust of mainland Greece. A further WNW-ESE directed extension observed occasionally in central and northern Greece could be explained by local events or as block-related deformation.

Rapid Intraplate Strain Accumulation in the New Madrid Seismic Zone

Remeasurement of a triangulation network in the southern part of the New Madrid seismic zone with the Global Positioning System has revealed rapid crustal strain accumulation since the 1950s. This area experienced three large (moment magnitudes >8) earthquakes in 1811 to 1812. The orientation and sense of shear is consistent with right-lateral strike slip motion along a northeast-trending fault zone (as indicated by current seismicity). Detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time.

Recent and active tectonics of the north-eastern Ecuadorian Andes

The north-eastern part of the Ecuadorian Andes, comprising the Interandean Valley, the Cordillera Real and part of the Sub-Andean Zone, is a region of diffuse seismicity with events reaching up to Ms = 7.0. The neotectonics of the region east of the Interandean Valley is analysed here by the integration of mesotectonic, microtectonic and seismological data. The structural setting is characterized by a predominance of N-S and NNE-SSW thrusts in the eastern region; whereas in the western region several NNE-SSW trending right-lateral strike-slip faults are present, the longest of which (the Cayambe-Chingual Fault) has a length of more than 70 km. Right-lateral faults trending NNE-SSW and some oblique thrust faults cut volcanic rocks of Late Pleistocene age and all the other fault sets. These faults offset streams of post-glacial age, are erosionally immature and have triangular facets. These data argue for a recent age for these faults. The Quatenary state of stress computed from microstructural data has a WSW-ENE greatest principal stress which is consistent with published focal mechanism solutions in this region. These consist of a combination of thrusting and right-lateral strike-slip motions along NNE-SSW striking planes. A kinematic model of the upper crust, in which westward dipping oblique thrusts and strike-slip faults coexist and converge at depth, accounts both for the observed seismicity and for the field measurements. These structures are part of a broader fault zone with a predominant right-lateral component of motion which extends along the eastern margin of the Andean Block, from the Gulf of Guayaquil, in southern Ecuador, to northern Colombia.

Multisource remotely sensed data, field checks and seismicity for the definition of active tectonics in Ecuadorian Andes

Abstract The active tectonics of the Ecuadorian Andes were analysed by the correlation of remotely sensed features, and microtectonic and seismological data. Landsat and radar imagery, and airborne stereophotos, were employed to investigate the fault pattern of scarcely accessible areas, with special reference to those lineaments showing young morphology, linear continuity and dragging effect on streams of post-glacial age. Some NNE-SSW long lineaments characterized by right-lateral strike-slip motions, were identified in the eastern Andes as probably very recent faults. Field microtectonic analysis was guided by remotely sensed data and allowed the reconstruction of the active tectonic compressional stress field characterized by a WSW-ENE trending σ. It also confirmed the recent motions along the NNE-SSW set. These data are consistent with published focal mechanism solutions in the same area. The seismicity is explained by a kinematic model of the upper crust, in which west dipping oblique thrusts and st...

Tectonic connection of the Hellenic and Cyprus arcs and related geotectonic elements

The Cyprus Arc System forms part of a well-defined, presently active, plate boundary and is a critical area for understanding the evolution of the Eastern Mediterranean Sea. The available seismic reflection profiles are utilised in order to establish the nature and origin of the main tectonic features of the arc, which include: 1. (a) The Cyprus Trench, displaying evidence of underthrusting and morphostructural features typical of subduction zones. 2. (b) The Pytheus Trench, associated with right-lateral strike-slip movement and connecting the western end of the Cyprus Trench to the Strabo Trench sector of the Hellenic Trench System. 3. (c) A predominantly transtensional fault zone connecting the easternmost part of the Cyprus Trench via the Baer Bassit overthrust zone in Syria to the Dead Sea Transform Fault System. 4. (d) A zone of complex deformation, involving thrust and transform elements, running from the Misis Mountains of southeast Turkey through the Kyrenia Range in Cyprus to the Gulf of Antalya and transected by roughly N-S major shears (notably the Cape Gelidonya-Anaximander Mountains strike-slip zone), which appears to form the western boundary of the present active system of the Cyprus Arc. The nature of the shallow structures associated with the Cyprus Arc is consistent with a geotectonic model involving some post-Miocene rotation of the South Anatolian-Cyprus Block, probably induced by “escape dynamics” resulting from continent-continent collision in eastern Turkey and the development of the Maras triple junction in southeastern Turkey. The alternation of plate boundary sectors respectively characterised by thrusting and strike-slip displacement suggests that the shallow crust in this part of the eastern Mediterranean is in a compartmented and relatively brittle state, allowing accumulated boundary stresses to be accommodated largely by displacements and deformation along secondary features.

Structural Study of the Southern Perth Basin by Geophysical Methods

Seismic, gravity and geothermal data were interpreted to examine the structure and the evolution of the southern Perth Basin. Seismic data show that the principal fault trend is north-south and that strike-slip faulting is of major importance in the formation of the basin. Gravity modelling shows that: (a) crustal thickness of the basin is approximately 30 km; (b) greater crustal thickness beneath the basin than on either side implies isostatic imbalance; (c) the Darling and Busselton Faults are steeply dipping faults which extend into the lower crust and control the structure of the southern Perth Basin. Analysis of these data shows that the evolution of the basin has had three major periods of tectonism which reactivated major faults: (a) a right-lateral strike-slip motion along the Darling Fault in the Late Permian-Early Triassic; (b) a phase of tectonism with associated left-lateral motion along the Dunsborough Fault in the Jurassic; (c) the separation of Australia from India in the Early Cretaceous which produced a predominantly tensional, and some oblique-transcurrent, style of faulting.

Present state of geodynamic properties of Kalabsha area, nortwest of Aswan Lake, Egypt

Aswan Lake is the second largest man-made lake in the world. Its filling started 1964 and reached the maximum water level in 1978. An earthquake of magnitude 5.5 took place in 1981 along the most active fault near the lake (Kalabsha fault). This earthquake was follwed by a tremendous number of smaller events that continue till now. Seismicity and the underground water table around the lake are monitored continuously through a radio-telemetered network. A local geodetic network was established around parts of the active faults in the northwestern part of the High Dam Lake, for monitoring vertical and lateral movements. The Kalabsha local geodetic network (the first one) was established around an active part of the Kalabsha fault in 1983. Precise geodetic measurements have benn carried out twice a year since 1984. On the basis of the repeated geodetic measurements, seismicity of the area and geophysical as well as geological data, the present state of the geodynamical properties of the Kalabsha area is studied. Remarkable horizontal movements were detected; they are correlated with the seismicity of the area and are attributed to the differential loading by the lake. The Kalabsha fault is a right-lateral strike-slip motion on an E-W plane. The magnitude of the movements detected along the fault is variable for the different epochs of measurements and is correlated with both seismicity and water loading in the lake.

The evaluation of satellite imagery and field spectro-radiometer data for the study of the lithology and the ancient and reactivation tectonics in the vicinity of the East African Rift

In the GARS project (UNESCO-IUGS) concerning Eastern Africa, satellite images (LANDSAT-MSS and -TM, SPOT XS and P) have been processed and geologically analysed in combination with a spectro-radiometric and a geological control survey. Generally, spectral signature are related with the nature of corresponding rocks, in the field and on the images as well. The processed images (decorrelation, principal-components analysis, combination of selected bands, texture analysis, classifications) fairly well expose the main lithological units. Structural features, seen on the imagery, allow detailed interpretations of the tectonics. At Karema (western Tanzania), a large NW-SE-trending shear zone is observed, underlined by ultramylonites displaying the effects of their left-lateral strike-slip emplacement mechanisms, superimposed on the main Ubenian (Lower Proterozoic) structuration. A shallow reactivation of this shear zone is identified as a right-lateral strike-slip movement corresponding to the Cenozoic Rift emplacement along an intracontinental transform zone. Near Gitega (Burundi), a major and deep seated Late Kibaran (Middle Proterozoic) shear zone separating two structural domains of the belt appears as a part of the (still active) N-S-trending fault zone of Central Burundi. It is paralleled to the west by the N-S-trending rift basin of northern Lake Tanganyika which is interpreted as the result of the reactivation in depth of this major ductile thrust. Rift tectonics appears thus to be controlled by major Proterozoic shear zones, reactivated respectively by strike-slip and normal faulting.

The 23 May 1989 MacQuarie Ridge Earthquake: A very broad band analysis

We use data from the GEOSCOPE global seismograph network to determine source parameters for the 23 May 1989 Macquarie Ridge earthquake. By moment tensor inversion of long‐period surface waves we obtain a seismic moment of 1.8–2.4 × 1021 N m, which makes this the largest seismic event to have occurred worldwide since the 1977 Sumba earthquake. The focal mechanism shows pure right‐lateral strike‐slip motion on a plane parallel with Macquarie Ridge and the Pacific‐Australian plate boundary. The mechanism is consistent with the strike‐slip motion predicted for this area from relative plate motion models. We invert broad band teleseismic P and SH waveforms to determine source complexity, using the results from the long‐period analysis as a priori soft constraints. The broad band waveforms show that the release of seismic energy was concentrated in the first 30 s of seismic rupture, but fault slip continued for more than 60 s. Lack of directivity in the long‐period and broad band waveforms indicates that the rupture propagated bilaterally or vertically from the nucleation point.

1974年伊豆半島沖地震の破壊過程

The 1974 Izu-Hanto-Oki earthquake is studied in detail using near-field strong motion seismograms. A waveform inversion method is applied to deduce the dislocation distribution and the characteristics of the rupture propagation during this earthquake.This earthquake involved right-lateral strike-slip motion on the almost vertical fault plane with a strike of N47°W. The rupture initiated at the central deepest part of the fault plane and propagated both sides smoothly, as a bilateral rupture propagation. The total source process time is about 11sec. A dislocation larger than 1m occurred in the region ranging from 3km to 10km in depth, and its horizontal span is about 20km around the hypocenter. Except for the southeastern end of the fault plane, dislocation smaller than 0.5m occurred in the shallower region of the fault plane. The average dislocation on the whole fault plane is 1.0m, and the total seismic moment is 7.6×1025dyne·cm. Few aftershocks took place in the area where dislocation larger than 2m occurred during the main event. Surface fractures, associated with this earthquake, appeared in the meizoseismal area. The dislocation distribution seismologically obtained in this study is consistent with the fault displacement along these surface fractures.

Mylonites from the Towaliga fault zone, central Georgia: Products of heterogeneous non-coaxial deformation

Retrograde mylonitic rocks occur in the segment of the Towaliga fault southeast of Jackson, Georgia. Mylonite protoliths are heterogeneous quartz-feldspathic gneisses (upper amphibolite facies, sillimanite zone) from the Pine Mountain and Piedmont terranes. Protoliths contain no distinctive lithological markers that can be used to determine fault displacement. Shear sense can only be determined using microstructural criteria (σ, δ, and θ-type porphyroclast systems; shear bands; asymmetric microfolded quartz ribbons; subgrain fabrics in quartz ribbons). The most common porphyroclast system developed in mylonites and ultramylonites is θ-type. θ-type systems do not have readily identifiable tails but do produce distinctive asymmetrical pile ups in the matrix such that the angle of intersection between the perturbed zone and the mylonitic foliation points in the shear direction. Differences in morphology between σ, δ, and θ-type systems can be related to differences in the recrystallization rate/deformation rate ratio ( R/ γ) reflecting changes in ductility contrast between matrix and porphyroclast that controls the amount of recrystallized material in a porphyroclast mantle that is potentially available to form tails. In early stages of mylonitization, with low ductility contrasts and large R/ γ ratios, large mantles occur around porphyroclasts and tailed systems (σ & δ) develop. In advanced stages of mylonitization, with large ductility contrasts and small R/ γ ratios, either very small, or partially or totally missing mantles occur, and θ-type systems develop. Microstructural criteria ubiquitously record a right-lateral strike-slip motion component to the deformation. Textural evidence (e.g. unidirectional shear bands; consistent vorticity recorded by porphyroclasts and asymmetrical microfolds) suggest that the mylonites are the products of non-coaxial deformation. The local presence of fabrics with coaxial symmetry result from heterogeneity in the deformation. The right-lateral offset on the Towaliga fault most likely occurred during the late Paleozoic, Alleghanian orogeny.

The earthquake hazard alert of September 1982 in Southern Tobago

AbstractAn earthquake swarm that occurred in the vicinity of the island of Tobago (West Indies) during the latter half of 1982 was monitored in the near-field by a five-station seismograph network. The monitoring of the swarm eventually led to the issuing of a potential earthquake hazard alert, 3 days prior to the major energy release (earthquake magnitude mb = 5.2). We discuss the reasons for issuing this alert. In particular, daily monitoring of the changing b value and energy release was used to constrain estimates of future earthquake behavior. The aftershock seismicity showed activity in a direction trending west to WNW. This is in good agreement with the focal mechanism of the main earthquake which showed right-lateral strike-slip motion along an E-W fault plane dipping steeply (71°) to the north. This active fault appears to form part of the previously unrecognized Southern Tobago Fault System for which there is evidence in the geology of the Late Neogene rocks of the island.

Mesozoic evolution of Northeast Asia and the collision of the okhotsk microcontinent

Tectonic models of Northeast Japan generally involve the progressive subduction of the Paleo-pacific oceanic crust during the Mesozoic and collision of the Okhotsk microcontinent at the end of the Cretaceous with Eurasia. However, these models are not based on a precise knowledge of the overall structure and succession of deformations in the collision zone. Our survey leads us to propose structural maps and cross sections of the Hokkaido Central Belt and a tectonic description of each tectonic stage. It appears that at the end of the Jurassic, until the Early Cretaceous an important event of nappe emplacement occurred. We interprete this event as the collision of the Okhotsk block with Eurasia, much earlier than in classical models. During this collision, the oceanic crust of a marginal basin was obducted with an east vergence and partly metamorphosed. Contemporaneously with the collision, an important volcanism, linked with the Pacific subduction, started and continued during the whole of the Cretaceous period. The continental margin was affected by strike-slip faults running parallel to the margin with a left-lateral motion. The main fault was the Tanlu fault which accommodated the northward movement of the eastern part of the Asian continent which led to the closing of an oceanic domain between Siberia and the Okhotsk block. This tectonic scenario ended 70 m.y. ago contemporaneously with the cessation of the volcanism in the Okhotsk-Uda Margal volcanic belt. From the Eocene to the Middle Miocene a right-lateral strike-slip motion along Sakhalin and Hokkaido gave rise to the Hidaka shear zone and finally led to the opening of the Sea of Japan.

Constraints on North American-Pacific Plate Boundary Deformation in Central California from VLBI and Ground-Based Geodetic Data

To accommodate the relative motion across the North American-Pacific plate boundary predicted by global plate solutions, significant deformation on faults other than the San Andreas is necessary. In central California, this deformation is thought to include distributed compression perpendicular to the San Andreas as well as right-lateral strike-slip motion parallel to the San Andreas on faults such as the San Gregorio/Hosgri system. A self-consistent set of VLBI observations from experiments beginning in October 1982 is used to determine the vector rate of change of station position at central California VLBI sites Ovro, Mojave, Vandenberg, Fort Ord, Presidio, and Point Reyes. To estimate VLBI station positions, a procedure is used that minimizes the uncertainties in defining a reference frame by including a priori geologic and geodetic information. The vector rate of change of station positions provides constraints on the integrated deformation rates between stations. Geologic and geophysical data suggest that the rate and mode of deformation varies on both local and regional scales. Thus, the VLBI derived results are interpreted in the context of an overall tectonic framework by examining geologic and ground-based geodetic data.