Crustal Deformation Rates Research Articles

Earthquake cycle dominates contemporary crustal deformation in Central and Southern Andes

With the advent of global positioning system (GPS) technology and its applications to high precision geodesy, earth scientists have gained an unprecedented opportunity to study the kinematics and dynamics of present-day deformation processes along plate boundary zones. However, until now our knowledge of the deformation processes associated with the Andean subduction along the western coast of South America has been limited. Here we present first estimates of present-day crustal deformation rates in the central and southern sections of the Andes, between latitudes 22°S and 42°S. We find that the deformation in the central section of our study area is dominated by the interseismic phase of an earthquake deformation cycle, caused by 100% coupling of the thrust interface between the subducting Nazca and the overriding South America plates. The estimated depth of coupling is not uniform along strike: north of 30°S it is ∼33 km deep, while south of 35°S it reaches 50 km depth. In addition, we observed postseismic relaxation effects in the northern part of our network in the area of the 1995 M w8.0 Antofagasta earthquake. South of 38°S, we detected a similar deformation pattern as in the Antofagasta area which we attribute to postseismic relaxation effects of the 1960 M w9.5 Valdivia earthquake.

Crustal deformation rates in central and eastern U.S. inferred from GPS

Analysis of continuous GPS observations between 1996 and 2000 at 62 stations distributed throughout the central and eastern United States suggests that the area is generally stable. Seven of the 62 stations show anomalous velocities, but there is reason to suspect their monument stability. Assuming the remaining 55 stations are stable with respect to interior North America, we have found the North America‐ITRF97 Euler vector (−1.88°±1.04°N, 77.67°±0.39°W, 0.201°±0.004° Myr−1) that minimizes the RMS station velocity. Referred to fixed North America, all of these velocities are less than 3.2 mm yr−1. Motion of several stations suggests the Mississippi embayment may be moving southward away from the rest of the continent at a rate of 1.7±0.9 mm yr−1. The motion of the embayment produces a large gradient in velocity which, in turn, implies the highest seismic moment accumulation rate that we found. Although the highest rate is only marginally significant, the fact that it occurs near New Madrid, where earthquake risk is thought to be high, argues that the anomaly may be real. Nevertheless, the identification of the anomaly remains tentative.

BIFROST project: 3-D crustal deformation rates derived from GPS confirm postglacial rebound in Fennoscandia

Since autumn 1993 the BIFROST project has provided daily GPS solutions of geodetic positions from a network of more than 40 stations covering a large area of the Baltic shield. This area is expected to show large vertical motion due to glacial isostatic rebound following the deglaciation at the end of the Pleistocene. This paper will discuss the inference of three-dimensional rates of crustal motion at the GPS stations with respect to (1) a plate-fixed average for the horizontal components; (2) a geocentric reference in order to infer absolute changes of sea level from vertical crustal motion and models of geoidal rebound. We show that the horizontal strain rate pattern is largely dominated by unilateral extension and not exhibiting horizontal shear to an important extent. In regard to the vertical motion a crucial issue is the stability of the geocentre in the GPS frame. We show results from an Empirical Orthogonal Function analysis that attenuates regionally correlated noise. In all components our observations suggest reasonably close agreement with forward computions on the basis of postglacial isostatic adjustment. A dominant tectonic signal would lead to a certain fraction of the batch of baselines to exhibit shortening. A tectonic process leading to a similar pattern of horizontal motion as expected from postglacial rebound can safely be dismissed in the context of the currently accepted plate tectonic setting. Thus, our baseline rate comparison will be a critical first order test of the prevailing style of deformation.

次の南海トラフ巨大地震に備えて 東海地域における近年の地殻変動及び地震活動の変化に関する数値シミュレーションによる検討

Recently, Tada (1996) and Matsumura (2000) reported that crustal deformation rate and seismicity pattern have interestingly changed in the Tokai area, central Japan. We formulate recurring earthquake model in the Tokai area based on the rate- and state dependent frictional law derived from laboratory experiments of rock friction and on the location of a seismically locked region. Using the model parameters based on previous studies, we estimate the time when the next Tokai Earthquake will occur and the location of plate interface by fitting simulated calculation to observed data. Some of the results explain the recent decrease in the rate of uplift change especially at the three observational points.We find out that the mechanism of microearthquakes is well explained by the rate change of stress field. The present model shows the vertical rotation of the axes of principal stresses and predicts the precursory change of seismic activity in the region, where the distance from the assumed trench along the relative plate motion direction is about 150 km, in the depth range of 10-20 km. The present seismicity with P axis almost horizontal will become more inactive several years before the occurrence of a great earthquake, and/or the direction of P axis rotates abruptly about 80 degrees so as to be orthogonal to plate interface.

Preliminary division of active block boundary in Chinese mainland based on recent vertical crustal movement

The characteristics of vertical crustal movement in Chinese mainland are studied, and the ideas and principles for dividing active blocks according to the map of recent vertical crustal deformation rate in China is proposed. As a result, Chinese mainland is divided into 2 first-grade blocks, the east and west block, 6 second-grade blocks, the Northeast China, North China, East China, Xizang (Tibet), Gansu-Qinghai-Xizang (Gan-Qing-Zang) and Xinjiang block, and 16 third-grade blocks. The boundaries of active blocks divided in this paper are generally consistent with the pattern of neotectonic movement with some local difference. It shows that the recent crustal activity in Chinese mainland inherits the neotectonic movement occurred since the Quaternary, while some new activity tendencies appear.

Seismotectonics and rates of active crustal deformation in the Burmese arc and adjacent regions

The close vicinity of the Burmese subduction zone to the Himalayan collision zone across northeast India produces complex tectonics giving rise to a high level of seismicity. Using the hypocentral data of shallow earthquakes ( h≤70 km) for the period 1897–1995, a large number of focal mechanism solutions and other geophysical data in correlation with major morphotectonic features in the Burmese arc and the adjoining areas, we identified 12 broad seismogenic zones of relatively homogeneous deformation. Crustal deformation rates have been determined for each one of these sources based on summation of moment tensors. The results indicate that along the Kopili–Bomdila fault zone in eastern Himalaya, the deformation is taken up as a compression of 0.12±0.01 mm/yr along N16° and an extension of 0.05±0.004 mm/yr along N104° direction. The deformation velocities show a NS compression of 18.9±2.5 mm/yr and an EW extension of 17.1±2.2 mm/yr in the Shillong Plateau region, while a compression of 5.4±2.8 mm/yr along N33° is observed in the Tripura fold belt and the Bengal basin region. The vertical component in the Shillong Plateau shows crustal thickening of 2.4±0.3 mm/yr. The deformation velocities in Indo–Burman ranges show a compression of 0.19±0.02 mm/yr along N11° and an extension of 0.17±0.01 mm/yr along N101° in the Naga hills region, a compression of 3.3±0.4 mm/yr along N20° and an extension of 3.1±0.36 mm/yr along N110° in the Chin hills region and a compression of 0.21±0.3 mm/yr in N20° and an extension of 0.18±0.03 mm/yr along N110° in the Arakan–Yoma region. The dominance of strike-slip motions with the P axis oriented on an average along N17° indicate that the Burma platelet may be getting dragged along with the Indian plate and the motion of these two together is accommodated along the Sagaing fault. The velocities estimated along Sagaing transform fault in the back-arc region suggest that the deformation is taken up as an extension of 29.5±4.7 mm/yr along N344° and a compression of 12.4±1.9 mm/yr along N74° in the northern part of the fault zone, and a compression of 17.4±2.3 mm/yr along N71° and an extension of 59.8±8.0 mm/yr along N341° in the southern part of the fault zone. The average shear motion of about 13.7 mm/yr is observed along the Sagaing fault. The deformation observed in the southern part of the syntaxis zone along the Mishmi thrust indicate a compression of 0.63±0.08 mm/yr in N58° and an extension of 0.6±0.07 mm/yr in N328° direction. The region of Shan Plateau, west of Red River fault, shows a compression of 17.7±2.6 mm/yr along N36° and an extension of 16.1±2.4 mm/yr along N126°.

The Strength of a Continent

As [Houseman][1] discusses in this Perspective, recent advances in measuring crustal deformation rates, particularly using Global Positioning System (GPS) observations in conjunction with estimates of lithospheric gravitational potential energy, allow the strength of continents to be estimated. Flesch et al . (page [834][2]) have devised a method for separating the two key components of the stress field driving crustal deformation and have used this method to estimate the distribution of lithospheric strenghts of the southwestern United States. [1]: http://www.sciencemag.org/cgi/content/full/287/5454/814 [2]: http://www.sciencemag.org/cgi/content/short/287/5454/834

Rapid deformation rates along the Wasatch Fault Zone, Utah, from first GPS measurements with implications for earthquake hazard

Anomalously high rates of crustal deformation have been measured at the Basin‐Range transition to the Rocky Mountains along the Wasatch fault zone, Utah, by repeated Global Positioning System (GPS) measurements. Four GPS field campaigns (1992–1995) and comparisons with older (1962–1991) geodetic data have revealed east‐west extensional strain at a rate of 0.05 ± 0.02 µstrain/yr corresponding to a 2.7 ±1.3 mm/yr rate of horizontal displacement across a 55‐km wide area. This rate is more than 20% of the total ∼12 mm/yr extension rate across of the ∼800‐km wide Basin and Range province. It is also two to three times larger than the average Late Quaternary fault slip rate on the Wasatch fault and tens of times larger than the displacement rates inferred from the cumulative seismic moments of historic earthquakes. While we do not yet know the source of this unexpected contemporary deformation, possible mechanisms include homogeneous crustal extension, loading of the Wasatch and adjacent faults, and pressure solution creep. If the Wasatch fault is being loaded by this high strain rate, it increases the expected peak ground acceleration significantly from standard values. These new findings demonstrate the importance of GPS in earthquake hazard assessment.

Geological and geophysical evidence for large palaeo-earthquakes with surface faulting in the Roer Graben (northwest Europe)

From the analysis of geological, geodetic and geophysical data we provide clear evidence of seismogenic faults capable of producing large earthquakes in intraplate Europe. Previous studies (Paulissen, Vandenberghe & Gullentops 1985; Van den Berg et al. 1994; Geluk et al. 1994) have yielded some constraints on the rate of crustal deformation along the Roer Valley, a graben structure crossing the Netherlands, Belgium and Germany, and have allowed us to address the fundamental questions: can intraplate earthquakes rupture the surface in this part of ‘stable’ continental Europe, and if so, what is their return period? Detailed palaeoseismic investigations have been carried out in Belgium along a 10 km long fault scarp which is the morphological expression of the Feldbiss Fault, the southwestern border fault of the Roer Graben Camelbeeck & Meghraoui 1996). The scarp is multiple and the frontal fault scarp offsets young deposits and alluvial terraces. Field investigations using geological and geomorphological methodologies combined with geophysical prospecting provide evidence of Holocene seismic surface faulting. From 14C dating it is suggested that the last earthquake along the fault scarp occurred between 610 AD and 890 AD. Levelling profiles across the scarp suggest that it produced a vertical coseismic displacement of 0.5-1 m along the scarp. If we suppose that the last surface-faulting earthquake ruptured the whole seismogenic layer (17 km thickness) over a minimum length (10 km) corresponding to the length of the Bree scarp with an average slip of 0.6 m, its seismic moment was at least 3.1×1018 N m (MW=6.3) for an average rigidity μ=3×1010 Pa. By estimating the offset of the main terrace of the Maas River by slippage along the Feldbiss Fault, we calculate the average Late Pleistocene vertical deformation rate as 0.08±0.04 mm yr−1. Palaeoseismic information combining the trench and geomorphic observations suggests the occurrence of two surface-faulting earthquakes during the last 20 kyr. A third dates before 28–35 kyr BP. Then, if the time distribution of earthquakes is uniform, a return period of 12±5 ka and a vertical deformation rate of 0.06±0.04 mm yr−1 are inferred.

Gravitational viscoelastic postseismic relaxation on a layered spherical Earth

Viscoelastic relaxation of a ductile asthenosphere underlying a purely elastic plate is a strong candidate process for explaining anomalous rates of crustal deformation observed following large earthquakes. The nongravitational treatment of Pollitz [1992], which is valid on a global scale and includes the effects of compressibility, is here extended to permit the calculation of gravitational viscoelastic relaxation for a specified spherically layered viscoelastic rheology following an earthquake in an elastic layer. The simple approximations we adopt make the resulting treatment particularly suitable for near‐field calculations. For an asthenosphere with a Maxwell rheology, the effect of gravitational coupling is manifested only many relaxation times after the earthquake, as obtained by previous investigators. Its effect is generally to speed up the long‐wavelength component of the relaxation process and attenuate the overall vertical displacement pattern. Several subtle features common to the relaxation behavior from several different fault types (thrust, rift, and strike‐slip) are identified. The effect of gravitational coupling on horizontal displacements is consistent with flexure of the upper elastic plate driven by the corresponding effects on the vertical displacement. Stress diffusion away from the source region generally exhibits pulse‐like behavior which is dispersive in both space and time. If the asthenosphere is confined to a relatively narrow channel, then the dispersion branches governing relaxation are radically altered, and stress diffusion effects far from the coseismic rupture zone exhibit a complicated time dependence reflecting the competing tendencies of toroidal and spheroidal mode relaxation.

Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates

THE Tien Shan—a high, seismically active intracontinental mountain belt, 1,000–2,000 km north of the Himalaya—has grown as a result of India's collision with Asia1. The crustal shortening (~ 200 ± 50 km; refs 2, 3) and thickening that gave rise to the Tien Shan accommodates only a small fraction of India's total penetration into Asia (2,000—3,000km), and the temporal relationship of deformation in this belt to the India–Asia collision remains unclear. Here we report geodetic measurements of the Tien Shan, using the Global Positioning System (GPS), that indicate that the current crustal shortening rate is nearly half of India's convergence rate with Eurasia in this area4. We infer a total shortening rate for the Tien Shan of ~20 mm yr−1, which is approximately twice that inferred previously from the extrapolation of slip rates in the Holocene3 and earthquake-induced displacements during this century5, suggesting that the rate of mountain building in this region has accelerated several-fold since the onset of collision ~50–55 Myr ago6,7. If, as we argue, the current shortening rate can be extrapolated to geological timescales, then our results suggest that most of the Tien Shan has been constructed during the past 10 Myr, perhaps in response to an increased horizontal force following an abrupt rise of the Tibetan plateau8,9.

Active folding of Pleistocene unconformities on the edge of the Australian‐Pacific plate boundary zone, offshore North Canterbury, New Zealand

In North Canterbury, New Zealand, the southeastern edge of an active fold and thrust belt lies 20 km offshore beneath the continental shelf. High‐resolution marine seismic reflection profiles and a detailed Quaternary sequence stratigraphy are integrated to map and characterise the seismic potential of actively growing folds. Eleven large‐scale folds occur within the upper few hundred metres of a Pliocene to Recent succession of silty mudstone, representing the upper part of a sedimentary cover sequence up to 2 km thick. The folds are gentle, NE‐SW trending, overlapping, asymmetric structures approximately 10–32 km in length, which verge consistently to the northwest in accordance with major thrust faults and folds exposed in nearby coastal hills. The folds are inferred to overlie a system of blind, southeast dipping thrust faults that are accommodating a small component of regional NW‐SE crustal shortening in North Canterbury and to develop by coseismic uplift during thrust fault earthquakes. The critical interactions between the rates of folding, regional tilting, coastal uplift, outer shelf subsidence, and the local sedimentation response to high‐amplitude Quaternary sea‐level cycles have led to the development and preservation of a stack of folded sedimentary sequences, an essentially smooth, gently sloping seafloor throughout most of the off‐shore deformation zone, and net uplift close to zero at the deformation front. Eleven unconformity‐bounded sedimentary units of middle Pleistocene to Recent age (< 0.75 Ma) constrain the rates and timing of folding and enable comparisons with onshore deformation rates, examination of the coastal tectonic gradient, and speculation about the rates of upper crustal deformation beneath the shelf. Deposition, folding, and coastal uplift have occurred contemporaneously throughout the last 0.75 m.y., and all measurable deformation can be accommodated within approximately 0.8 m.y. Fold amplitude growth rates of 0.02 m/kyr to 0.14 m/kyr (typically 0.05–0.09 m/kyr) near the deformation front are low and up to 25 times lower than some actively growing folds exposing basement rocks onshore. There is a significant decline in strain rate across the coastal zone and inner shelf, toward the off‐shore deformation front. It is inferred that blind thrust faults beneath the offshore folds have slip rates typically of the order of 0.1–0.9 mm/yr and that the probable recurrence interval of moderately large magnitude (M 6.2–6.8 ± 0.3) thrust earthquakes beneath individual folds is of the order of several tens of thousands of years.

On the physical model of earthquake precursor fields and the mechanism of precursors’ time-space distribution (III) — anomalies of seismicity and crustal deformation and their mechanisms when a strong earthquake is in preparation

By studying the seismicity pattern before 37 earthquakes withM⩾6.0 in North China and the pattern of crustal deformation in the Capital Area from 1954 to 1992, some abnormal characteristics of these patterns before strong earthquakes have been extracted. A comparison has been made between the anomalies of these two kinds of patterns. From the results we can know the following. (1) Before a strong earthquake, the seismicity will strengthen and the crustal deformation rate will increase. (2) Several years before a strong earthquake, there will be seismic gaps and deformation gaps around the epicenter of the quake. (3) The dynamic parameters of patterns all show a decrease in information dimension. This means that the crustal deformation has become more and more localized with time and it gives an important indication showing that a strong earthquake is in preparation. At the end of the paper, the physical mechanisms of the abnormal patterns of seismicity and crustal deformation have been explained in a unified way in terms of the earthquake-generating model of a inhomogeneous strongbody in inhomogeneous media.

The influence of a finite glaciation phase on predictions of post-glacial isostatic adjustment

We predict relative sea level (RSL) variations, present day three-dimensional (3D) crustal deformation rates and baseline velocities, and a set of anomalies in the gravitational field of the planet, using a suite of models for the space-time geometry of the Pleistocene ice sheets. These models are distinguished on the basis of the details of the glaciation phase, and they include the cases of an infinite glaciation phase (i.e., an assumption of isostatic equilibrium at the time of the last glacial maximum — LGM), a steady uniform glaciation, and a glaciation in which the bulk of the accumulation is limited to the final stages of the growth phase. Comparison of the observational records of postglacial RSL change with predictions based on the assumption of isostatic equilibrium at LGM have commonly been used to infer both the space-time geometry of the final deglaciation event and the mantle viscosity. We conclude that both these applications may be influenced significantly by the incorporation of a finite glacial cycle. As an example, RSL predictions, which assume isostatic equilibrium at LGM, and those which incorporate a relatively rapid glaciation phase, can differ by a factor of 2 or more at some sites within previously glaciated regions when an Earth model characterized by a lower mantle viscosity of 10 22 Pa s is adopted. We have also found that differences in the adopted model for the glaciation event are capable of explaining entirely the discrepancies ( ∼ 0.2 mm/yr) between previously published predictions of present day tangential velocities in North America. These discrepancies are characterized by a dominant long-wavelength signal and, hence, they do not necessarily influence (strongly) predictions of baseline velocities. Variations in the glaciation model can also have an important effect on predictions of peak gravity anomalies over previously glaciated regions, and secular variations in the zonal harmonics of the Earth's geoid. In both cases the effect may be sufficient to alter significantly the signal from other geophysical processes which may need to be invoked to reconcile the observational constraints.

Field of vertical crustal deformation in North China and the feature of its dynamic evolution

Based on the data of precise leveling surveys for 30-odd years and using unified adjustment method, unified starting point of calculation and unified network, contour maps of the vertical crustal deformation rate in North China during the periods of 1965–1975, 1975–1979, 1979–1983 and 1983–1988 have been drawn. Meanwhile, the evolution of vertical crustal deformations has also been studied from a dynamic viewpoint. The results of analysis show that there is an obvious correspondence between the regular variations of the vertical crustal deformation field and the seismic cycles. Furthermore, the paper has also inferred that this correspondence might be related to the micro-variations of the regional stress/strain field. Finally, some problems related to the vertical deformation field and earthquake prediction have been discussed.

The influence of regional stress and magmatic input on styles of monogenetic and polygenetic volcanism

A new model is proposed to relate the development of monogenetic and polygenetic volcanoes to magmatic input and regional stress in the lithosphere. Output‐stress diagrams relate magmatic output rate and crustal deformation rate (or strain rate). The magmatic output rates of polygenetic volcanoes and monogenetic volcano fields, including lava fields and central (axial) volcanoes on the mid‐oceanic ridge systems, are estimated. Crustal deformation rates obtained from the literature are used as indicators of differential stress (Δσ). These data are normalized over time periods of 104 years, and over areas of 103 km2. The ratio of output rate to input rate, inferred from ophiolite sections, and from crustal deformation around volcanoes, is about 0.1 to 0.3. Based on the average ratio, an input‐stress diagram may be obtained at some volcanoes. Polygenetic volcanoes are plotted in the region of both larger output rate and smaller Δσ. Monogenetic volcano fields are plotted on a rift trend, or in the region of a small output (≤1 km3/104 yr/103 km2). In some regions, polygenetic volcanoes and monogenetic volcanoes coexist, for example, during 0.1 m.y. periods, and within the area of 30 km × 30 km. The output‐stress diagram indicates that coexistence of these volcano types results from variation in differential stress, or variation in production rate of magma in the mantle at different levels over periods of time less than 0.1 m.y. This is supported using extrusive volumes and crustal deformation data from the eastern volcanic zone, Iceland, Taupo Volcanic Zone, New Zealand, TransMexican Volcanic Belt, Mexico, and the volcanic field on and around the Izu peninsula, Japan. The observed relationship between deformation rate, magmatic output rate, and volcano type supports the crack interaction theory of magma‐filled cracks. The output‐stress diagram also indicates that the balance between local stress induced by magma accumulation and regional stress, and stress relaxation, govern the structure of a volcano.

Normal faults associated with volcanic activity arc

Volcanic centers (volcanoes, fumaroles or solfatara fields), epicenters of strong shallow earthquakes (with focal depths up to 20 km) and epicenters of intermediate depth strong earthquakes (with focal depths between 120 and 160 km) in the southern Aegean volcanic arc can be grouped into five, well defined, linear clusters trending about N60°E. This lineation of shallow earthquakes and volcanic activity is attributed to five corresponding normal faults which are named after the five corresponding volcanic centers (Sousaki, Methana, Milos, Santorini and Nisyros). This is supported by a similar trend of the geomorphological features (grabens and islands) and of geophysical features (Bouguer anomalies), as well as by other seismological data (fault plane solutions and the origins of tsunamis) and geological information on the Santorini caldera. The greater volcanic activity in the eastern volcanic centers (Santorini and Nysiros) compared to the western volcanic centers (Sousaki, Methana and Milos) is attributed to the higher rate of extensional crustal deformation. In the eastern part of the volcanic arc it is 26 mm/yr: in the west it is 2 mm/yr. The delineation of the epicenters of the intermediate depth earthquakes along the same five lines indicates the existence of five corresponding rupture zones in the lower (leading) part of the descending lithospheric slab (at depths of 120–180 km). These deep zones are probably the sources of hot material which is ascending vertically upwards and intrudes into the crust along its fracture zones. The orientation of these zones explains the focusing of the macro-seismic results of these deep shocks at narrow regions of the sedimentary arc (Peloponnesus, Crete, etc).

Rates of active crustal deformation in the Aegean and the surrounding area

Active crustal deformation is calculated for 26 zones of shallow seismicity in the Aegean and the surrounding regions. The data analysis is based on a procedure developed in a previous paper (Papazachos and Kiratzi, 1993). This procedure takes advantage of all the available historical and instrumental data for the calculation of the “size” of the deformation in a seismic zone and of all reliable fault plane solutions which are available for the broader seismic belt for the determination of the “shape” of the deformation. The Aegean and its surroundings has been divided into 11 such seismic belts which may consist of one or more seismic zones that share earthquakes with similar focal mechanisms. This data analysis showed that along the coastal region of Albania, Yugoslavia and western Greece the deformation is taken up by compression in a direction perpendicular to the coast line (47°E) at a rate of about 2 mm/a. In the Ionian islands (Leukada, Cephalonia, Zante) compression occurs at a rate of 10 mm/a in an almost EW direction (N83°E) and extension at a rate of 11 mm/a in an almost NS direction (N174°E). Along the convex side of the Hellenicarc (south of Peloponese, Crete, Rodos), the upper crust is compressed at a rate of about 6 mm/a in a direction N34°E. In the Aegean Sea and the surrounding lands (mainland of western and northern Greece, southern Yugoslavia and Bulgaria, western Turkey) the seismic deformation is taken up by an almost NS extension at an average rate of 5 mm/a. In northwestern Anatolia and the northern Aegean fault zones deformation is controlled by the westward movement along the North Anatolian fault. Northern Anatolia is undergoing a N115°E compression at a rate of 22 mm/a which is relieved by a N25°E extension at a rate of 19 mm/a, and the Northern Aegean is undergoing EW compression at a rate of 16 mm/a and NS extension at a rate of 8 mm/a. The vertical crustal thickening along the external compressional zones ranges from 0.2 to 0.5 mm/a with an average of 0.3 mm/a and the vertical crustal thinning in the inner back-arc extensional area ranges from 0.1 to 2.4 mm/a with an average equal to 0.8 mm/a. In the western part of the area and between the external compressional field and the internal extensional field, a belt exists where the deformation is expressed as extension at an average rate of 1.6 mm/a in a N112°E direction.

Deformation in the Santa Barbara Channel from GPS measurements 1987–1991

Geodesy is one of the few techniques that can be used to estimate strain rates across the submerged structures in the Santa Barbara Channel. By resolving relative surface velocities, the Global Positioning System (GPS) can be used to infer crustal deformation rates independent of geologic models. GPS measurements from the period 1987–1991 have been used to derive velocities for a 8 station network bounding the channel. These estimates are based on 10 GPS experiments each of several days in length over this 4.5 year period. Precision of individual determinations of intersite vectors range from 3 to 7 mm for the horizontal components and average 20 mm in the vertical. The GPS data indicate high rates of shortening in the eastern channel (6±1 mm/yr at N16±3E) and low rates of deformation in the western channel. Comparisons with previous geodetic studies suggest that deformation in the region has not been uniform over the last 100 years.

Is a destructive earthquake imminent in southeastern Sicily?

Southeastern Sicily, which has frequently experienced ruinous earthquakes in historic times, has only been site of minor activity in the last two centuries. Two precision levelling surveys that we carried out in 1983 and 1989 detected high rates of crustal deformation. The usual hypothesis of tectonic stationarity applied to both the time series of historical earthquakes and deformation rates would favour the imminence of an I ⩾ IX event. On the other hand, the characteristic patterns of recent seismicity strongly oppose this conclusion, suggesting that the apparent contradiction is caused by a tectonic nonstationarity, an issue that seems also confirmed by palaeogeodetic data.