Strain accumulation along an oblique plate boundary: the Reykjanes Peninsula, southwest Iceland

We use annual GPS observations on the Reykjanes Peninsula (RP) from 2000 to 2006 to generate maps of surface velocities and strain rates across the active plate boundary. We find that the surface deformation on the RP is consistent with oblique plate boundary motion on a regional scale, although considerable temporal and spatial strain rate variations are observed within the plate boundary zone. A small, but consistent increase in eastward velocity is observed at several stations on the southern part of the peninsula, compared to the 1993–1998 time period. The 2000–2006 velocities can be modelled by approximating the plate boundary as a series of vertical dislocations with left-lateral motion and opening. For the RP plate boundary we estimate left-lateral motion  18⁺⁴₋₃ mm yr⁻¹  and opening of  7⁺³₋₂ mm yr⁻¹  below a locking depth of  7⁺¹₋₂ km  . The resulting deep motion of  20⁺⁴₋₃ mm yr⁻¹  in the direction of  N(100⁺⁸₋₆)°E  agrees well with the predicted relative North America–Eurasia rate. We calculate the areal and shear strain rates using velocities from two periods: 1993–1998 and 2000–2006. The deep motion along the plate boundary results in left-lateral shear strain rates, which are perturbed by shallow deformation due to the 1994–1998 inflation and elevated seismicity in the Hengill–Hrómundartindur volcanic system, geothermal fluid extraction at the Svartsengi power plant, and possibly earthquake activity on the central part of the peninsula.     

Slip rate on the Dead Sea transform fault in northern Araba valley (Jordan)

The Araba valley lies between the southern tip of the Dead Sea and the Gulf of Aqaba. This depression, blanketed with alluvial and lacustrine deposits, is cut along its entire length by the Dead Sea fault. In many places the fault is well defined by scarps, and evidence for left-lateral strike-slip faulting is abundant. The slip rate on the fault can be constrained from dated geomorphic features displaced by the fault. A large fan at the mouth of Wadi Dahal has been displaced by about 500 m since the bulk of the fanglomerates were deposited 77–140 kyr ago, as dated from cosmogenic isotope analysis (¹⁰Be in chert) of pebbles collected on the fan surface and from the age of transgressive lacustrine sediments capping the fan. Holocene alluvial surfaces are also clearly offset. By correlation with similar surfaces along the Dead Sea lake margin, we propose a chronology for their emplacement. Taken together, our observations suggest an average slip rate over the Late Pleistocene of between 2 and 6 mm yr⁻¹, with a preferred value of 4 mm yr⁻¹. This slip rate is shown to be consistent with other constraints on the kinematics of the Arabian plate, assuming a rotation rate of about 0.396° Myr⁻¹ around a pole at 31.1°N, 26.7°E relative to Africa.     

Large-scale velocity field and strain tensor in Iran inferred from GPS measurements: new insight for the present-day deformation pattern within NE Iran

A network of 26 GPS sites was implemented in Iran and Northern Oman to measure displacements in this part of the Arabia–Eurasia collision zone. We present the GPS velocity field obtained from three surveys performed in 1999 September, 2001 October and 2005 September and the deduced strain tensor. This study refines previous studies inferred from only the two first surveys. Improvements are significant in NE Iran. The present-day shortening rate across the mountain belts of NE Iran is estimated to 5 ± 1 mm yr⁻¹ at about N11°, 2 ± 1 mm yr⁻¹ of NS shortening across the eastern Kopet Dag and 3 ± 1 mm yr⁻¹ of NS shortening across Binalud and Kuh-e-Sorkh. Our GPS measurements emphasize the varying character of the Kopet Dag deformation between its southeastern part with prevailing thrusting at low rates and its northwestern part with dominant strike-slip activity at increasing rates. The principal axes of the horizontal strain tensor appears very homogeneous from the Zagros to the Alborz and the Kopet-Dag (N20°) and in eastern Iran (Makran and Lut block: N30°). Only NW Iran suffers a variable strain pattern which seems to wrap the Caspian basin. The strain tensor map underlines the existence of large homogeneous tectonic provinces in terms of style and amplitude of the deformation.     

Late- to post-orogenic exhumation of the Central Pyrenees revealed through combined thermochronological data and modelling

Apatite (U–Th)/He and fission track thermochronometry have been combined with 3D thermal modelling to constrain the late- to post-orogenic exhumation history of the Central Pyrenees, Spain. Data from four massifs immediately north and south of the present drainage divide of the mountain belt reveal a diachroneity in the transition from syn- to post-orogenic forcing of exhumation. Immediately south of the drainage divide, rapid exhumation of ∼1.5 mm year⁻¹ decelerated after ∼30 Ma to ∼0.03 mm year⁻¹. A similar transition occurred immediately north of the drainage divide at the same time. Further south, in the core of the Axial Zone antiformal stack of the Pyrenees, rapid (∼1 mm year⁻¹), syn-orogenic exhumation continued to ∼20 Ma, but slowed to ∼0.1–0.2 mm year⁻¹ soon after that time. This order of magnitude decrease in exhumation rates across the orogen records the diachronous transition into a post-orogenic state for the mountain belt. These data do not record rejuvenation of exhumation in Late Miocene or Pliocene times driven either by large-scale base-level change or an evolution to more erosive climatic conditions.     

Geodetic observations of ice flow velocities over the southern part of subglacial Lake Vostok, Antarctica, and their glaciological implications

In the austral summer seasons 2001/02 and 2002/03, Global Positioning System (GPS) data were collected in the vicinity of Vostok Station to determine ice flow velocities over Lake Vostok. Ten GPS sites are located within a radius of 30 km around Vostok Station on floating ice as well as on grounded ice to the east and to the west of the lake. Additionally, a local deformation network around the ice core drilling site 5G-1 was installed.
The derived ice flow velocity for Vostok Station is  2.00 m a⁻¹± 0.01 m a⁻¹  . Along the flowline of Vostok Station an extension rate of about 10⁻⁵ a⁻¹ (equivalent to 1 cm km⁻¹ a⁻¹) was determined. This significant velocity gradient results in a new estimate of 28 700 years for the transit time of an ice particle along the Vostok flowline from the bedrock ridge in the southwest of the lake to the eastern shoreline. With these lower velocities compared to earlier studies and, hence, larger transit times the basal accretion rate is estimated to be 4 mm a⁻¹ along a portion of the Vostok flowline. An assessment of the local accretion rate at Vostok Station using the observed geodetic quantities yields an accretion rate in the same order of magnitude. Furthermore, the comparison of our geodetic observations with results inferred from ice-penetrating radar data indicates that the ice flow may not have changed significantly for several thousand years.     

Surface vertical displacements, potential perturbations andgravity changes of a viscoelastic earth model induced by internal point dislocations

Using the viscoelastic correspondence principle, we utilize the surface coseismic spheroidal deformation fields (i.e. vertical displacements, potential perturbations and gravity changes) of SNREI earth models caused by four typical types of point dislocation, derived by Sun & Okubo (1993 ), to deduce the fundamental formulas for spheroidal fields relevant to viscoelastic earth models. In computations, we employ a strike-slip dislocation on a vertical plane buried at the bottom of the lithosphere to estimate the maximal viscous relaxation responses to this kind of source that possibly exist on the surface of the earth. We take the seismic moment as 10²²  N  m, which is characteristic of an average large earthquake. The numerical results demonstrate that, if we take the viscosity as 10¹⁹  Pa  s in the asthenosphere, and 10²¹  Pa  s in the other mantle layers, the rates of surface vertical displacements and gravity changes within about 2.5° for the 10 postseismic years are respectively 1.5–8.1  cm  yr⁻¹ and 4.0–14.9  μgal  yr⁻¹ : the viscous relaxation for this mantle viscosity profile proceeds much faster than for a constant mantle viscosity of 10²¹  Pa  s.     

Aseismic fault movement before the 1995 Kobe earthquake detected by a GPS survey: implication for preseismic stress localization?

By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr⁻¹ within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M _L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M _L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr⁻¹ at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M _L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr⁻¹ during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.     

Joint inversion of receiver function and surface wave dispersion observations

We implement a method to invert jointly teleseismic P wave receiver functions and surface wave group and phase velocities for a mutually consistent estimate of earth structure. Receiver functions are primarily sensitive to shear wave velocity contrasts and vertical traveltimes, and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages. Their combination may bridge resolution gaps associated with each individual data set. We formulate a linearized shear velocity inversion that is solved using a damped leastsquares scheme that incorporates a priori smoothness constraints for velocities in adjacent layers. The data sets are equalized for the number of data points and physical units in the inversion process. The combination of information produces a relatively simple model with a minimal number of sharp velocity contrasts. We illustrate the approach using noisefree and realistic noise simulations and conclude with an inversion of observations from the Saudi Arabian Shield. Inversion results for station SODA, located in the Arabian Shield, include a crust with a sharp gradient near the surface (shear velocity changing from 1.8 to 3.5 km s¹ in 3 km) underlain by a 5kmthick layer with a shear velocity of 3.5 km s¹ and a 27kmthick layer with a shear velocity of 3.8 km s¹, and an upper mantle with an average shear velocity of 4.7 km s¹. The crustmantle transition has a significant gradient, with velocity values varying from 3.8 to 4.7 km s¹ between 35 and 40 km depth. Our results are compatible with independent inversions for crustal structure using refraction data.     

New seismological constraints on differential rotation of the inner core from Novaya Zemlya events recorded at DRV, Antarctica

Novaya Zemlya nuclear test records at the seismic station DRV, Antarctica, are analysed in order to obtain further constraints on a possible differential rotation of the inner core with respect to the mantle. These data allow the sampling of the inner core along a nearly polar path in very stable conditions over more than two decades, from 1966 to 1990. The PKP (BC)– PKP (DF) traveltime residuals, which reflect the inner-core anisotropy and/or heterogeneities sampled along the path, exhibit a great stability through time. A computation of the residuals that are expected for various differential rotation rates and the same rotation axis as the mantle has been performed using the worldwide residual catalogue of Engdahl et al . (1997) for summary rays that include the time as an additional parameter in data stacking. Comparison of data and predictions shows that an eastward differential rotation with a rate as large as 3°  yr⁻¹, as suggested by some authors, is not possible, but an eastward rotation at 1°  yr⁻¹ or lower cannot be rejected.     

LICHEN GROWTH RATES ON GLACIER FORELANDS IN SOUTHERN NORWAY: PRELIMINARY RESULTS FROM A 25-YEAR MONITORING PROGRAMME

A unique 25-year lichen growth monitoring programme involving 2,795 individuals of the Rhizocarpon subgenus at 47 sites on 18 glacier forelands in the Jotunheimen–Jostedalsbreen regions of southern Norway is reported. The data are used to address fundamental questions relating to direct lichenometry: spatial and temporal variability in lichen growth rates, climatic effects on lichen growth rates, lichen growth models, and implications for lichenometric dating. Mean annual (diametral) growth rate ranged from 0.43 to 0.87 mm yr⁻¹ between sites, which is attributed primarily to local habitat differences. Interannual variability in annual mean growth rate exceeded 1.0 mm yr⁻¹ at some sites. Annual mean growth rates for all sites combined varied from 0.52 to 0.81 mm yr⁻¹ and was positively correlated with annual mean temperature and winter mean temperature (both r = 0.64, p <0.005) but not with summer seasonal temperature: positive correlations with annual and winter precipitation were less strong and the correlation with summer precipitation was marginally significant (r = 0.41 p <0.10). Growth-rate models characterized by annual growth rates that remain approximately constant or increase with lichen size up to at least 120 mm tended to fit the data more closely than a parabolic model. This is tentatively attributed to a long 'linear/mature' phase in the growth cycle. Comparison with growth rates inferred from indirect lichenometry suggest that such high measured growth rates could not have been maintained over the last few centuries by the largest lichens used in southern Norway for lichenometric dating. Several hypotheses, such as the effects of competition and climate change, which might explain this paradox, are discussed.     

A Maastrichtian palaeomagnetic pole for the Pacific plate from a skewness analysis of marine magnetic anomaly 32

The asymmetry (skewness) of marine magnetic anomaly 32 (72.1–73.3  Ma) on the Pacific plate has been analysed in order to estimate a new palaeomagnetic pole. Apparent effective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire Pacific plate and spanning a great-circle distance of ~12  000  km. The data were inverted to obtain a palaeomagnetic pole at 72.1°N, 26.8°E with a 95 per cent confidence ellipse having a 4.0° major semi-axis oriented 98° clockwise of north and a 1.8° minor semi-axis; the anomalous skewness is 14.2° ± 3.7°. The possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible effect on our palaeopole analysis over the range of relevant spreading half-rates, ~25 to ~90  mm  yr⁻¹ . The new pole is consistent with the northward motion for the Pacific plate indicated by coeval palaeocolatitude and palaeoequatorial data, but differs significantly from, and lies to the northeast of, coeval seamount poles. We attribute the difference to unmodelled errors in the seamount poles, mainly in the declinations. Comparison with the northward motion inferred from dated volcanoes along the Hawaiian–Emperor seamount chain indicates 13° of southward motion of the Hawaiian hotspot since 73  Ma. When the pole is reconstructed with the Pacific plate relative to the Pacific hotspots, it differs by 14°–18° from the position of the pole relative to the Indo–Atlantic hotspots. This has several possible explanations including bias in one or more of the palaeomagnetic poles, motion between the Pacific and Indo–Atlantic hotspots, and errors in plate reconstructions relative to the hotspots.     

Quaternary normal faulting in southeastern Sicily (Italy):a seismic source for the 1693 large earthquake

We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian offshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is affected by WNW–ESE regional extension producing normal faulting of the southern edge of the Siculo–Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by different ages and related to distinct tectonic processes. The older NW–SE-trending normal fault segments developed up to ≈400  kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW–SSE-trending normal fault segments and extends along the Ionian offshore zone following the NE–SW-trending Avola and Rosolini–Ispica normal faults. These faults are characterized by vertical slip rates of 0.7–3.3  mm  yr ⁻¹ and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes ( M ~ 7) could be related to a 45  km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7  mm  yr ⁻¹ is calculated, and a recurrence interval of about 550 ± 50  yr is proposed for large events similar to that of January 1693.     

Inference of mantle viscosity from GRACE and relative sea level data

Gravity Recovery And Climate Experiment (GRACE) satellite observations of secular changes in gravity near Hudson Bay, and geological measurements of relative sea level (RSL) changes over the last 10 000 yr in the same region, are used in a Monte Carlo inversion to infer-mantle viscosity structure. The GRACE secular change in gravity shows a significant positive anomaly over a broad region (>3000 km) near Hudson Bay with a maximum of ∼2.5 μGal yr⁻¹ slightly west of Hudson Bay. The pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ICE-5G deglaciation history, strongly suggesting a postglacial rebound origin for the gravity change. We find that the GRACE and RSL data are insensitive to mantle viscosity below 1800 km depth, a conclusion similar to that from previous studies that used only RSL data. For a mantle with homogeneous viscosity, the GRACE and RSL data require a viscosity between  1.4 × 10²¹  and  2.3 × 10²¹  Pa s. An inversion for two mantle viscosity layers separated at a depth of 670 km, shows an ensemble of viscosity structures compatible with the data. While the lowest misfit occurs for upper- and lower-mantle viscosities of  5.3 × 10²⁰  and  2.3 × 10²¹  Pa s, respectively, a weaker upper mantle may be compensated by a stronger lower mantle, such that there exist other models that also provide a reasonable fit to the data. We find that the GRACE and RSL data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle.     

A surge of Perseibreen, Svalbard, examined using aerial photography and ASTER high resolution satellite imagery

The identification of surge activity is important in assessing the duration of the active and quiescent phases of the surge cycle of Svalbard glaciers. Satellite and aerial photographic images are used to identify and describe the form and flow of Perseibreen, a valley glacier of 59 km² on the east coast of Spitsbergen. Heavy surface crevassing and a steep ice front, indicative of surge activity, were first observed on Perseibreen in April 2002. Examination of high resolution (15 m) Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite imagery confirmed this surge activity. Perseibreen retreated by almost 750 m between 1961 and 1990. Between 1990 and the summer of 2000, Perseibreen switched from retreat and its front began to advance. Rapid advance was underway during the period June 2000 to May 2001, with terminus advance at over 400 m yr⁻¹. Between May and August 2001 the rate increased to over 750 m yr⁻¹. The observed crevasse orientation indicates that ice was in longitudinal tension, suggesting the down-glacier transfer of mass. Ice surface velocities, derived from image correlation between ASTER images, were 2-2.5 m d⁻¹ between May and August 2001. The glacier was flowing at a relatively uniform speed with sharp velocity gradients located close to its lateral margins, a velocity structure typical of ice masses in the active phase of the surge cycle. The stress regime is extensional throughout and the surge appears to be initiated low on the glacier. This is similar to the active-phase dynamics of other Svalbard tidewater glaciers. Perseibreen has probably been inactive since at least 1870, a period of about 130 years to the present surge which defines a minimum length for the quiescent phase.     

Shallow velocity structure along the Hirapur–Mandla profile using traveltime inversion of wide-angle seismic data, and its tectonic implications

In order to investigate the velocity structure, and hence shed light on the related tectonics, across the Narmada–Son lineament, traveltimes of wide-angle seismic data along the 240 km long Hirapur–Mandla profile in central India have been inverted. A blocky, laterally heterogeneous, three-layer velocity model down to a depth of 10 km has been derived. The first layer shows a maximum thickness of the upper Vindhyans (4.5 km s⁻¹ ) of about 1.35 km and rests on top of normal crystalline basement, represented by the 5.9 km s⁻¹ velocity layer. The anomalous feature of the study is the absence of normal granitic basement in the great Vindhyan Graben, where lower Vindhyan sediments (5.3 km s⁻¹ ) were deposited during the Precambrian on high-velocity (6.3 km s⁻¹ ) metamorphic rock. The block beneath the Narmada–Son lineament represents a horst feature in which high-velocity (6.5 km s⁻¹ ) lower crustal material has risen to a depth of less than 2 km. South of the lineament, the Deccan Traps were deposited on normal basement during the upper Cretaceous period and attained a maximum thickness of about 800 m.     

Multi-interferogram method for measuring interseismic deformation: Denali Fault, Alaska

Studies of interseismic strain accumulation are crucial to our understanding of continental deformation, the earthquake cycle and seismic hazard. By mapping small amounts of ground deformation over large spatial areas, InSAR has the potential to produce continental-scale maps of strain accumulation on active faults. However, most InSAR studies to date have focused on areas where the coherence is relatively good (e.g. California, Tibet and Turkey) and most analysis techniques (stacking, small baseline subset algorithm, permanent scatterers, etc.) only include information from pixels which are coherent throughout the time-span of the study. In some areas, such as Alaska, where the deformation rate is small and coherence very variable, it is necessary to include information from pixels which are coherent in some but not all interferograms. We use a three-stage iterative algorithm based on distributed scatterer interferometry. We validate our method using synthetic data created using realistic parameters from a test site on the Denali Fault, Alaska, and present a preliminary result of  10.5 ± 5.0  mm yr⁻¹ for the slip rate on the Denali Fault based on a single track of radar data from ERS1/2.     

A review of the trends observed in British Isles mean sea level data measured by tide gauges

This report presents an up-to-date review of the British Isles (BI) Mean Sea Level (MSL) data set obtained from tide gauges, and of the long-term secular trends in BI MSL. The data are of mixed quality, and not as copious as one would like for scientific analysis given the complexity of long-term sea and land level changes around BI coasts. Nevertheless, the small number of very long records can be studied effectively, and indicate that twentieth century secular trends in BI MSL are consistent with those obtained from NW Europe as a whole and with the bottom range of estimates for global average MSL change during the past 100 years. Century-timescale low-frequency 'accelerations' in MSL of the order of 0.4–0.8  mm  yr⁻¹  century⁻¹ are obtained from the three longest records, which are also similar to estimates from mainland Europe. 'Sea level indices' for the BI are constructed which can provide responsible agencies with a guide to the 'average state' of BI sea level. Combined ocean tide and storm surge ('tide + surge') numerical modelling is demonstrated to be a valuable tool in understanding part of the variability of MSL around the BI, although modelling of long-term changes is limited at present by the lack of adequate meteorological data sets from before 1955. Nevertheless, it is shown that, even if a complete meteorological modelling data set were available, it would not be able to account for all of the MSL variability. Finally, recommendations are made for the development of tide gauge and Global Positioning System (GPS) recording in the BI for the purpose of long-term MSL monitoring in the next century.     

Present-day deformation of the Dauphine Alpine and Subalpine massifs (SE France)

The positions of 20 geodetic points surveyed in 1946-1952 by the Institut Géographique National in the Grenoble area (western Alps) were remeasured in 1993-1994 using GPS. We evaluate the displacements of the common points of the networks between the two surveys, and calculate the strain-rate tensors for different sets of three adjacent points. The significant N70° shortening direction observed at the boundary of the Belledonne and Taillefer massifs suggests that the dextral strike-slip motion between the External Crystalline Massifs (ECM) and the Subalpine domain is still active. Geological evidence suggests that this deformation, which is compatible with the few focal mechanisms available for the area, occurred throughout the late Neogene. Our data also suggest a roughly 3-5 mm yr⁻¹ active shortening concentrated at the external front of the Subalpine chains. This thrust motion results in a principal axis of compression orientated N130°, oblique to the direction of shortening observed in the ECM. This change in direction is also seen in in situ stress measurements. This motion may mark the southward continuation of the active fault observed in the Jura mountains by Jouanne, Ménard & Darmendrail (1995).     

North Atlantic Water in the Barents Sea Opening, 1997 to 1999

North Atlantic Water (NAW) is an important source of heat and salt to the Nordic seas and the Arctic Ocean. To measure the transport and variability of one branch of NAW entering the Arctic, a transect across the entrance to the Barents Sea was occupied 13 times between July 1997 and November 1999, and hydrography and currents were measured. There is large variability between the cruises, but the mean currents and the hydrography show that the main inflow takes place in Bjørnøyrenna, with a transport of 1.6 Sv of NAW into the Barents Sea. Combining the flow field with measurements of temperature and salinity, this results in mean heat and salt transports by NAW into the Barents Sea of 3.9×10¹³ W and 5.7×10⁷ kg s⁻¹, respectively. The NAW core increased in temperature and salinity by 0.7 °C yr⁻¹ and 0.04 yr⁻¹, respectively, over the observation period. Variations in the transports of heat and salt are, however, dominated by the flow field, which did not exhibit a significant change.