relocation and assessment of seismicity in the iran region summary more than 2000 instrumentally recorded earthquakes occurring in the iran region during the period 1918 2004 have been relocated and reassessed with special attention to focal depth using an advanced technique for 1 d earthquake location a careful review of starting depths association of teleseismic depth phases and the effects of reading errors on these phases are made and when necessary waveforms have been examined to better constrain ehb focal depths uncertainties in ehb epicentres are on the order of 10 15 km in the iran region owing to the earth s lateral heterogeneity and uneven station distribution uncertainties of reviewed ehb focal depth estimates are on the order of 10 km as compared to about 4 km for long period p and sh body waveform inversions nevertheless these ehb depth estimates are sufficiently accurate to resolve robust differences in focal depth distribution throughout the iran region and within their errors show patterns that are in agreement with the smaller number of earthquakes whose depths have been confirmed by body wave modelling or local seismic networks the importance of this result is that future earthquakes with apparently anomalous depths can easily be identified and checked if necessary most earthquakes in the iranian continental lithosphere occur in the upper crust with the crustal shortening produced by continental collision accommodated entirely by thickening and distributed deformation in the zagros mountains nearly all earthquakes are confined to the upper crust depths 20 km and there is no evidence for a seismically active subducted slab dipping ne beneath central iran by contrast in southeastern iran where the arabian seafloor is being subducted beneath the makran coast low level earthquake activity occurs in the upper crust as well as to depths of at least 150 km within a northward dipping subducting slab near the oman line a region transitional between the zagros and the makran seismicity extends to depths of up to 30 45 km in the crust consistent with low angle thrusting of arabian basement beneath central iran in north central iran along the alborz mountain belt seismic activity occurs primarily in the upper crust but with some infrequent events in the lower crust particularly in the western part of the belt the talesh where the south caspian basin underthrusts nw iran earthquakes that occur in a band across the central caspian following the apscheron balkhan sill between azerbaijan and turkmenistan have depths in the range 30 100 km deepening northwards these are thought to be connected with either incipient or remnant northeast subduction of the south caspian basin basement beneath the east west trending apscheron balkhan sill curiously in this region of genuine mantle seismicity there is no evidence for earthquakes shallower than 30 km

3d joint inversion of gravity data and rayleigh wave group velocities to resolve shear wave velocity and density structure in the makran subduction zone south east iran abstract in this study we developed a method to invert jointly rayleigh wave group velocities and gravity anomalies for velocity and density structure of the lithosphere we applied the method to the makran accretionary prism se iran the reason for using different data sets is that each of these data sets is sensitive to different parameters surface wave group velocities are sensitive mainly to shear wave velocity distribution in depth but do not well resolve density variations therefore joint inversion with gravity data increases the resolution of density distribution our approach differs from others mainly in the model parameterization instead of subdividing the model into a large number of thin layers we invert for the properties of only four layers thickness p and s wave velocities and densities and their vertical gradients in sediments upper crust lower crust and upper mantle the method is applied first to synthetic models in order to demonstrate its usefulness we then applied the method to real data to investigate the lithosphere structure beneath the makran the resulting model shows that moho depth increases from oman sea 18 33 km and makran fore arc 33 37 km to the volcanic arc 44 46 km the crustal density is high in the oman sea as should be expected for the oceanic crust we also find a high velocity anomaly in the upper mantle under the oman sea corresponding to the subducting slab the crust under the fore arc volcanic arc and back arc settings of makran subduction zone is characterized by low velocity zones

upper mantle s velocity structure and moho depth variations across zagros belt arabian eurasian plate boundary the collision of the arabian and eurasian plates in the early miocene after the subduction of neo tethys ocean beneath eurasia formed the zagros belt a seismically active continental continental plate boundary in southwest iran the zagros suture zone is an important seismotectonic boundary indicating an abrupt cutoff between the intense seismicity of the zagros and the almost aseismic central iran plateau compared with other more evolved plate boundaries little is known about the deep lithospheric structure of the zagros belt such as the fate of subducted neo tethys plate and ambiguity in the presence of intermediate and deep earthquakes under the zagros suture zone in this study we use the partitioned waveform inversion pwi method to image the upper mantle s velocity structure and moho depth variations across zagros collisional zone the resulting moho depth along the profile shows the average moho depth value of order of 40 45 km for most parts of the profile with abrupt crustal thickening in the middle of the profile up to about 65 km as expected the derived models show that the relatively old and cold arabian plate has higher velocity at depth than the younger lithosphere farther north in central iran a sharp and steep subcrustal boundary is found roughly coincident with the surficial expression of the main zagros thrust mzt separating two different mantle domains a high velocity anomaly possibly representing a fragment of subducted lithosphere has been imaged beneath central iran at a depth between 350 and 600 km which is quite similar to the case of indo asian collision in qinghai tibetan plateau these observations as well as the sudden changes of shallow velocities along the cross section with a sharp boundary under the zagros suture zone support the idea that subducted oceanic lithosphere has broken off under the region of maximum moho depth a relatively low velocity region beneath the arabian plate is also imaged in our high resolution tomography model which is interpreted as an indication of lithospheric delamination within the arabian lithosphere

the southern caspian a compressional depression floored by a trapped modified oceanic crust the south caspian intracontinental depression floored by oceanic basement is a relatively stable block with minor deformation surrounded by active fold thrust belts of arcuate form talesh alborz and kopeh dagh mountains which have undergone intense late cainozoic crustal shortening the basin is interpreted as a neogene quaternary compressional depression bounded by multi role mountain bordering reverse faults and apparently floored by a late paleozoic triassic or late mesozoic early tertiary modified oceanic crust trapped along an old geosuture it may be a relic of an old paleozoic triassic ocean or else a marginal sea developed behind a mesozoic paleogene ocean and analysis of geological and geophysical data enables a scheme to be suggested the general arcuate shape of the alborz and the talesh bordering mountain belts follows the pattern of the supposed rigid and thickened ocean crust of south caspian depression a tectono sedimentary study of the south caspian region coupled w

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

microseismicity in the region of tehran we record and analyze small earthquakes around tehran iran using the permanent seismological network of the institute of geophysics of the university of tehran and two temporary dense seismological networks installed for several weeks during 1999 and 2000 regional seismicity is distributed throughout central alborz extending from the caspian sea to the iran plain with the highest level of activity east of tehran related to the mosha and to the garmsar faults which both dip northward and have had strong historical earthquakes our focal mechanisms the first for these faults confirm a left lateral strike slip motion

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

main geophysical features related to the virunga zone western rift and their volcanological implications the virunga volcanic complex in the western rift vahey of africa is the most important volcanic structure within the east african rift system it includes eight main volcanic edifices muhavura gahinga sabinyo visoke karisimbi mikeno nyiragongo and nyamuragira fig 1 hamaguchi et al this volume during the last decade seismic magneto telluric gravity geodetic and levelling studies have been conducted in this area particularly near the nyiragongo and nyamuragira volcanoes some of the survey periods included eruptive episodes of the nyamuragira volcano e g hamaguchi et al this volume wafula et al this volume the results derived from the observational data can be summarized as follows 1 the spatial distribution of the epicenters of the current seismic activity is usually confined to ne sw oriented faults and fissures transecting the nyiragongo and nyamuragira fields the earthquakes shown in figure 1 are of a type e g wafula et al this volume their foci are mainly in the depth range of 8 13 km beneath nyamuragira and 14 16 km beneath nyiragongo tanaka 1983 the vertical distribution of earthquakes associated with eruptive episodes of nya

topography the geoid and compensation mechanisms for the southern rocky mountains the southern rockies of colorado are anomalously high elevations greater than 2800 m topographically rough implying active uplift and underlain by significant low velocity anomalies in the upper mantle that suggest an intimate relationship between mantle geodynamic processes and the surface topography the region is in isostatic equilibrium i e near zero free air gravity anomaly however the poor correlation between the high topography and crustal thickness makes the application of simple compensation models e g pure heiskanen or pratt hayford problematic knowledge of how the current topography of the rockies is isostatically compensated could provide constraints on the relative role of sublithospheric buoyancy versus lithospheric support here we evaluate the geoid and its relationship to the topography using the geoid to elevation ratio gtr in the spatial domain and the admittance in the frequency domain to constrain the mechanism of compensation we separate the upper mantle geoid anomalies from those with deeper sources through the use of spherical harmonic filtering of the egm2008 geoid we exploit the fact that at wavelengths greater than the flexural wavelength where features are isostatically compensated the geoid topography ratio can be used to estimate the depth of compensation and the elastic thickness of the lithosphere the results presented below indicate that the main tectonic provinces of the western united states have moderate geoid topography ratios between 3 5 and 5 5 m km 3 9 for the southern rockies 4 25 for the colorado plateau and 5 2 for the northern basin and range suggesting shallow levels of isostatic compensation in terms of the elastic thickness of the lithosphere our results indicate an elastic thickness of less than 20 km these value support the notion that a major portion of the buoyancy that has driven uplift resides at depths less than 100 km and that upper mantle processes such as small scale convection may play a significant role in the buoyant uplift of the southern rockies as well as other actively uplifting areas of the western united states further support for this hypothesis is provided by high coherence for the geoid topography relationship for nearly all wavelengths between 50 and 1000 km

tectonic implications of the 1989 afar earthquake sequence in august 1989 a sequence of m b 5 6 normal faulting earthquakes occurred in the area between the ghoubbet asal manda inakir and manda haroro abhe bad rifts in central afar relocation of the events suggest they occurred on major faults in the dobi graben located midway between the principal rift zones the earthquakes are evidence that extensional strain is still occurring in the area the bookshelf tectonic model for central and southern afar proposed by tapponnier et al 1990 is modified to account for ongoing extension in the area between the principal rifts as well as distributed shear

coherent radar ice thickness measurements over the greenland ice sheet we developed two 150 mhz coherent radar depth sounders for ice thickness measurements over the greenland ice sheet we developed one of these using connectorized components and the other using radio frequency integrated circuits rfics both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice we used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers we operated both radar systems on the nasa p 3b aircraft equipped with gps receivers radar data are tagged with gps derived location information and are collected in conjunction with laser altimeter measurements we have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of program for regional climate assessment parca investigations and the north greenland ice core project radar echograms and derived ice thickness data are placed on a server at the university of kansas http tornado rsl ukans edu greenlanddata htm for easy access by the scientific community we obtained good ice thickness information with an accuracy of 10 m over 90 of the flight lines flown as a part of the parca initiative in this paper we provide a brief description of the system along with samples of data over the interior along the 2000 m contour line in the south and from a few selected outlet glaciers

the north tanganyika hydrothermal fields east african rift system their tectonic control and relationship to volcanism and rift segmentation the two branches of the east african rift system include numerous hydrothermal fields which are closely related to the present fault motion and to volcanic and seismic activity in this study structural data from pemba and cape banza hydrothermal fields western branch north tanganyika zaire are discussed in terms of neotectonic phenomena different types of records such as fieldwork onshore and underwater and landsat and spot imagery are used to explain structural controls on active and fossil hydrothermal systems and their significance the pemba site is located at the intersection of 000 020 trending normal faults belonging to the uvira border fault system and a 120 130 trending transtensional fault zone and is an area of high seismicity with events of relatively large magnitude ms 6 5 the cape banza site occurs at the northern end of the ubawari peninsula horst it is bounded by two fault systems trending 015 and is characterized seismically by events of small magnitude ms 4 the hydrothermal area itself is tectonically controlled by structures striking 170 180 and 080 the analysis of both hydrothermal areas demonstrates the rejuvenation of older proterozoic structures during recent rift faulting and the location of the hydrothermal activity at the junctions of submeridian and transverse faults the fault motion is compatible with a regional direction of extension of 090 110 the cape banza and pemba hydrothermal fields may testify to magma chambers existing below the junctions of the faults they appear to form at structural nodes and may represent a future volcanic province together with the four surface volcanic provinces existing along the western branch they possibly indicate an incipient rift segmentation related to valley valley or transverse fault valley junctions contrasting with the spacing of the volcanoes measured in the eastern branch these spacings appear to express the different elastic thicknesses between the eastern and western branches of the east african rift system perhaps related to a difference in stage of evolution of the two branches

south caspian to central iran basins introduction abstract the structurally and stratigraphically complex area of northern and central iran holdsthe key to understanding the plate tectonic evolution of the south caspian central iran area theclosure of the palaeotethys the opening of the neotethys the rise and demise of the cimmerianmountain chain as well as the onset of neotethys subduction and large scale neotethyan back arcrifting all predated the formation of the more than 20 km thick ll of the south caspian basin this volume brings together work by specialists in different disciplines of the geosciences tectonics geophysics sedimentology stratigraphy palaeontology basin modelling and geo dynamics in order to elucidate the complex late palaeozoic cenozoic geodynamic history ofthe iran area and the birth of the south caspian basin the south caspian basin fig 1 is among thedeepest sedimentary basins in the world yet itstiming of formation subsidence pattern andhistory are poorly understood as are the mechan isms responsible for its geometry and evolution for example the age of the oldest sediments ofthe south caspian basin is unknown due to theirinaccessibility at great depth of burial underupper cenozoic sediments consequently under standing of the pre neogene and particularly themesozoic history is dependent on inference fromthe geology of surrounding areas one aim of themiddle east basin evolution mebe programmewas to perform new eldwork on the margins ofthe south caspian basin where inversion during thecenozoic stages of the arabia eurasia collision hasexposed the rock record in the alborz koppehdagh and binalud mountains in northern iran aswell as the eastern extent of the greater caucasus because the eastern edge of the south caspianbasin and its relationship with other iranian basinsis unknown studies of the central iranian blockswhere the remnants of mesozoic deep oceanicbasins are preserved are key to understanding thecomplex geological evolution of north central iranand the greater south caspian area

present and past climate control on fjord glaciations in greenland implications for ird deposition in the sea calving of icebergs is the dominant ablation mechanism for large outlet glaciers from the greenland ice sheet except in northernmost greenland where bottom melting from floating glaciers dominates this difference is controlled by present climate conditions glacial geological evidence indicates that the transition between the associated types of fjord glaciations moved north south in response to past climate change in cold periods local melt out of debris from the bottom of an increasing number of floating glaciers reduces the potential for iceberg transport of ird thus the marine ird signal of greenland origin is not a simple cold climate signal our findings are discussed in the context of the ongoing debate about the kind of ice transporting ird icebergs or sea ice

an improved coherent radar depth sounder the university of kansas developed a coherent radar depth sounder during the 1980s this system was originally developed for glacial ice thickness measurements in the antarctic during the field tests in the antarctic and greenland we found the system performance to be less than optimum the field tests in greenland were performed in 1993 as a part of the nasa program for arctic climate assessment parca we redesigned and rebuilt this system to improve the performance the radar uses pulse compression and coherent signal processing to obtain high sensitivity and fine along track resolution it operates at a center frequency of 150 mhz with a radio frequency bandwidth of about 17 mhz which gives a range resolution of about 5 m in ice we have been operating it from a nasa p 3 aircraft for collecting ice thickness data in conjunction with laser surface elevation measurements over the greenland ice sheet during the last 4 years we have demonstrated that this radar can measure the thickness of more than 3 km of cold ice and can obtain ice thickness information over outlet glaciers and ice margins in this paper we provide a brief survey of radar sounding of glacial ice followed by a description of the system and subsystem design and performance we also show sample results from the field experiments over the greenland ice sheet and its outlet glacicrs

pn tomographic velocity and anisotropy beneath the iran region we present tomographic velocity and anisotropy models of the uppermost mantle beneath the iran region a total of 74 375 pn phase readings from 86 stations of the iranian network and 133 stations of the international seismological centre are used in the investigation the study uses the pn travel time tomography method proposed by hearn 1996 the tomography results show some interesting anomalies the average pn velocity under the iran region is approximately 8 0 km s and the maximum velocity perturbations are approximately 3 4 high pn velocities under the zagros mountains and the caspian sea may be due to the presence of oceanic crust lithosphere material low pn velocities were found under the alborz and caucasus regions and may be due to higher temperatures or partial melting resulting from volcanoes and mid cenozoic volcanic plutonic rocks in these regions the inversion velocities support the idea that the subduction of the arabian plate into the mantle beneath the iranian plateau may have resulted in the upwelling of hot material the well resolved pn anisotropy model is jointly obtained with a velocity model for the areas with good ray path coverage in the plate collision regions zagros and alborz the fast pn anisotropy direction is oriented parallel to the collision arc and to large reverse faults due to pure shear deformation from cross fault compression and along fault extension under the caucasus regions the pn anisotropy results indicate that the preferred alignment of olivine crystals is parallel to the plate movement direction however the surface fault strike is at an angle of nearly 45 with the crustal movement direction and anisotropy these differing deformations suggest potential decoupling between the crust and upper mantle the possible decoupling and differing deformation between the crust and upper mantle are easily enhanced under high temperatures due to volcanoes and supported by low velocities beneath the caucasus we validate the existence of pn anisotropy under these regions by azimuthal averaging of the apparent pn velocity

a rapid method to map the crustal and lithospheric thickness using elevation geoid anomaly and thermal analysis application to the gibraltar arc system atlas mountains and adjacent zones we present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness the main assumptions are local isostasy and a four layered model composed of crust lithospheric mantle sea water and the asthenosphere we consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle we perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of rms error of elevation and geoid anomaly databases the application of this method to the gibraltar arc system atlas mountains and adjacent zones reveals the presence of a lithospheric thinning zone sw ne oriented this zone affects the high and middle atlas and extends from the canary islands to the eastern alboran basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western betics eastern rif rharb basin and gulf of cadiz a number of different even mutually opposite geodynamic models have been proposed to explain the origin and evolution of the study area our results suggest that a plausible slab retreating model should incorporate tear and asymmetric roll back of the subducting slab to fit the present day observed lithosphere geometry in this context the lithospheric thinning would be caused by lateral asthenospheric flow an alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the canary island and extending as far as central europe

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

integrated lithospheric modeling combining thermal gravity and local isostasy analysis application to the ne spanish geotransect a two dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal gravity and local isostasy analyses is presented gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure while absolute elevation is used to determine the lithospheric mantle thickness the calculation is performed using a finite element technique that links the different physical equations the program optionally calculates the temperature at any material boundary and with given rheological parameters the strength distribution and the total lithospheric strength in selected columns we apply the algorithm to the northeastern spanish geotransect which extends from the pyrenees to the balearic promontory and along which a strong variation in crustal and lithospheric thickness is evident we assess the use of two different inferred density models for the lithospheric mantle the first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference the second model assumes a constant density for the whole lithospheric mantle although conceptually the two hypotheses differ substantially the results obtained do not show significant differences lithospheric thicknesses of 120 130 km below the pyrenees 60 65 km in the valencia trough and 65 75 km below the balearic promontory are deduced in all cases the mean lithospheric mantle density has to be 40 60 kg m 3 higher than the asthenospheric density the algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature density elevation relationship

crustal scale structure of the south caspian basin revealed by deep seismic reflection profiling abstract two deep seismic reflection profiles from the south caspian basin provide the first normal incidence image of possibly the thickest sedimentary basin in the world significant features imaged with these seismic data are 1 a series of high amplitude folds developed within a thick 10 km neogene sedimentary section bounded discordantly below by 2 a gently n dipping intermittent reflection at 14 20 km 8 0 s 3 a prominent deep reflector at 26 28 km depth 12 8 13 0 s with a gentle northward dip 4 an underlying layered interval with discernibly lower frequency reflections down to 36 40 km 16 0 16 5 s and 5 a noticeable decrease in reflectivity below 36 40 km based on the downward termination of fault cored folds and their discordance with subhorizontal underlying reflections the reflection at 14 20 km is interpreted to be the basal detachment for structures associated with the absheron ridge the bright reflection at 26 28 km depth is interpreted as the basement cover contact and the underlying 10 km portion of the section is interpreted as crystalline basement downward diminution of reflectivity at 36 40 km is thought to represent the moho despite the absence of a clearly reflective horizon these interpretations are consistent with previous velocity models from both seismic refraction and teleseismic studies in the south caspian region that suggested a minimum sedimentary thickness of 20 km and a crustal thickness of 35 40 km both the apparent lack of local structural relief on the basement reflector and seismic velocities vp 6 8 km s obtained from earlier studies suggest this crust is not thinned continental material although thicker than typical oceanic crust 6 8 km the apparent 10 km thickness of the south caspian crystalline crust is interpreted to be of oceanic affinity for this part of the basin active seismicity down to mantle depths 80 km and gentle deepening of the crust from south to north suggest northward subduction of the south caspian oceanic lithosphere beneath the continental lithosphere of the central caspian basin

active transtension inside central alborz a new insight into northern iran southern caspian geodynamics the tectonic activity in the alborz mountain range northern iran is due both to the northward convergence of central iran toward eurasia and to the northwestward motion of the south caspian basin with respect to eurasia inducing a left lateral wrenching along this range these two mechanisms give rise to a nne ssw transpressional regime which is believed to have affected the entire range for the last 5 6 2 m y in this paper we show that the internal domain of central alborz is not affected by a transpressional regime but by an active transtension with a wnw ese extensional axis we show that this transten sion is young middle pleistocene it postdates an earlier n s compression and may have been initiated when the south caspian basin started moving consequently our results suggest that the south caspian basin motion may have taken place more recently than previously proposed

lithospheric structure of the south china sea and adjacent regions results from potential field modelling abstract this work aims to investigate the crustal and lithospheric mantle thickness of the south china sea scs and adjacent regions the crust mantle interface average crustal density and lithospheric mantle base are calculated from free air gravity anomaly and topographic data using an iterative inversion method we construct a three dimensional lithospheric model with different hierarchical layers the satellite derived gravity is used to invert the average crustal density and moho crust mantle interface undulations the average crustal density and lab lithosphere asthenosphere boundary depths are further adjusted by topographic data under the assumption of local isostasy the average difference in moho depths between this study and the seismic measurement results is

variation of moho depth in the central part of the alborz mountains northern iran summary the alborz mountains of northern iran form a belt of active crustal deformation along the southern side of the caspian sea within the broad arabian eurasia continental collision zone although the range has an average elevation of about 3000 m with the volcanic peak damavand reaching an elevations of 5671 m early gravity studies found that the crust beneath the range is no thicker than that beneath the surrounding region suggesting the range is not supported by a crustal root we determine a model for the crust of the central alborz mountains using teleseismic receiver functions from data recorded on a network of broad band seismographs temporarily deployed across the central part of the range the receiver functions from these recordings have been inverted simultaneously with fundamental mode rayleigh wave group velocity measurements in the 10 100 s period range our analysis shows a thickening of the crust from 48 km beneath the northern part of the central iranian plateau to 55 58 km below the central part of the alborz mountains then a thinning of the crust to 46 km north of the alborz mountains beneath the coastal region of the south caspian sea our seismological results show that the central alborz mountains have a moderate crustal root but of insufficient thickness to compensate the elevation of the range the analysis of free air gravity shows that the elevation of the alborz mountains is largely supported by the elastic strength of the iranian plate the south caspian plate or both

a strong seismic velocity contrast in the shallow mantle across the zagros collision zone iran summary surface wave dispersion measurements are interpreted jointly with the inversion of teleseismic p wave traveltime residuals along a dense 620 km long temporary seismic profile across the zagros to investigate its upper mantle structure the s wave model determined from rayleigh wave dispersion in the zagros fold and thrust belt has high velocities from 4 5 0 2 km s 1 below the moho to 4 9 0 25 km s 1 at 200 km depth which are comparable to a shield like structure beneath the suture region from the main zagros thrust mzt to the urumiehdokhtar volcanic arc s wave velocities are lower than beneath the zagros in the top 50 km of the upper mantle with a minimum of 4 4 0 2 km s 1 at 80 km depth from 150 km and deeper s velocities are as high as beneath the zagros we suggest that part of the velocity difference at shallow depth is due to higher mantle temperatures and or higher fluid content beneath the northern half of the profile but that velocities are too high to support the hypothesis of mantle lid delamination under the suture zone teleseismic p traveltime relative residuals display a long wavelength variation along the transect with a difference of 1 1 s between negative residuals in the zagros simple folded belt and positive residuals in central iran this difference backprojects into a 6 7 per cent lateral variation of p wave velocity in the shallow upper mantle with higher vp beneath zagros and lower vp beneath central iran the main short wavelength variation of the residual is located in the suture region with late p arrivals in the region of the mzt and early arrivals in the sanandaj sirjan zone ssz using synthetic models of vp perturbations we show that the high velocities of the arabian platform have to extend laterally at least to the ssz to fit the observed p delays this model also predicts rayleigh wave phase velocities which are within the error bars of the observed dispersion it supports the model of crustal scale overthrusting at the mzt

spatial mapping of b value heterogeneity beneath the rwenzori region albertine rift evidence of magmatic intrusions abstract we report studies conducted on populations of local crustal earthquakes depths 35 km that were triggered beneath the rwenzori region of the albertine rift and then modelled according to the gutenberg and richter 1954 law a total of about 10 000 earthquakes whose location precision is 5 km were available for analysis a glance of the entire catalogue indicated that the data were complete for magnitudes 1 7 to estimate the b value distribution in the region we used the maximum likelihood method of aki 1965 the overview b value map revealed anomalously high b values ranging between 1 00 0 06 to 1 25 0 09 between longitudes 30 00 30 50 and latitudes 0 00 1 25 on the cross section however we observed low b values 0 65 0 03 to 0 88 0 05 that we believe could possibly correspond to the presence of asperities we interpreted these low b values in asperities as signatures indicative of elevated stress levels probably due to material homogeneity in the crust the intermediate b values 0 88 0 05 to 1 04 0 05 observed represent regions that are probably experiencing either high thermal gradients due to thermal upwelling above a hot source or an indication of remnant geothermal effects after past volcanic activity in the rwenzori region high b value anomalies of ranges b 1 04 0 05 1 21 0 09 were observed at different localities we related these anomalous volumes to an increase in material heterogeneity due to the thermal cracking in nearby rocks associated with an active magma body emplaced in the crust whose source is probably in the lithospheric mantle beneath the rwenzori region the high b value signatures observed suggested a possible source of the heat flux for the geothermal activity observed in the rwenzori region

spatial variability of seismicity parameters in aftershock zones the spatial variability of the b value of the frequency magnitude relationship and the decay rate of aftershocks as described by the p value of the modified omori law is investigated by using dense spatial grids we map out the distribution of b and p values within the landers northridge morgan hill and kobe aftershock sequences considerable spatial variability is found with b values of independent subvolumes ranging from 0 6 to 1 4 and p values ranging from 0 6 to 1 8 these systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km the spatial distribution of these two parameters is compared with the slip distribution during the mainshock suggesting that the areas of largest slip release correlate with high b value regions we hypothesize that the frictional heat created during the event may influence the p value distribution within an aftershock zone while applied shear stress crack density and pore pressure govern the frequency magnitude distribution by investigating the frequency magnitude distribution separately for preseismic and postseismic periods for the morgan hill mainshock we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value which decays to premainshock values within a year surrounding areas of the aftershock zone show an approximately constant b value with time because the aftershock hazard after a mainshock depends strongly on both the b and p value we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters

non linear stochastic inversion of gravity data via quantum behaved particle swarm optimisation application to eurasia arabia collision zone zagros iran potential field data such as geoid and gravity anomalies are globally available and offer valuable information about the earth s lithosphere especially in areas where seismic data coverage is sparse for instance non linear inversion of bouguer anomalies could be used to estimate the crustal structures including variations of the crustal density and of the depth of the crust mantle boundary i e moho however due to non linearity of this inverse problem classical inversion methods would fail whenever there is no reliable initial model swarm intelligence algorithms such as particle swarm optimization pso are a promising alternative to classical inversion methods because the quality of their solutions does not depend on the initial model they do not use the derivatives of the objective function hence allowing the use of l1 norm and finally they are global search methods meaning the problem could be non convex in this paper quantum behaved particle swarm qpso a probabilistic swarm intelligence like algorithm is used to solve the non linear gravity inverse problem the method is first successfully tested on a realistic synthetic crustal model with a linear vertical density gradient and lateral density and depth variations at the base of crust in the presence of white gaussian noise then it is applied to the eigen 6c4 a combined global gravity model to estimate the depth to the base of the crust and the mean density contrast between the crust and the upper mantle lithosphere in the eurasia arabia continental collision zone along a 400 km profile crossing the zagros mountains iran the results agree well with previously published works including both seismic and potential field studies this article is protected by copyright all rights reserved

variations in earthquake size distribution across different stress regimes the earthquake size distribution follows in most instances a power law1 2 with the slope of this power law the b value commonly used to describe the relative occurrence of large and small events a high b value indicates a larger proportion of small earthquakes and vice versa statistically significant variations of b values have been measured in laboratory experiments mines and various tectonic regimes such as subducting slabs near magma chambers along fault zones and in aftershock zones3 however it has remained uncertain whether these differences are due to differing stress regimes as it was questionable that samples in small volumes such as in laboratory specimens mines and the shallow earth s crust are representative of earthquakes in general given the lack of physical understanding of these differences the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b 1 is a universal constant for earthquakes in general5 here we show that the b value varies systematically for different styles of faulting we find that normal faulting events have the highest b values thrust events the lowest and strike slip events intermediate values given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress

3d crustal and lithospheric model of the arabia eurasia collision zone three dimensional joined inversion of topography gravity and geoid data was carried out in the collision zone between the eurasian and arabian plates comprising central and eastern anatolia the black sea basin and the caucasus region the 3d algorithm first introduced by motavalli anbaran et al 2013 is based on a bayesian approach with gaussian density functions in which the targeted area is divided into vertical columns each comprised of four layers namely water of known thickness i e bathymetry if present crust lithospheric mantle and asthenosphere the inversion results are moho depth average crustal density and depth to the lithosphere asthenosphere boundary defined here as the 1350 c isotherm existing seismic moho depths were used as a priori information in order to constrain crustal thicknesses thickest crust up to 50 km was found underneath the great caucasus mountains thinnest crust 22 25 km underneath the black sea basin the east anatolian plateau is underlain by relatively thick crust up to 45 km thinning to less than 40 km towards the central anatolian plateau eastern anatolia and the eastern part of central anatolia are underlain by strongly thinned lithosphere up to 100 km thickest lithosphere up to 200 km underlies the caspian basin and the russian platform

lithospheric structure of the north china craton integrated gravity geoid and topography data abstract the lithospheric structure of ancient cratons provides important constraints on models relating to tectonic evolution and mantle dynamics here we present the 3d lithospheric structure of the north china craton ncc from a joint inversion of gravity geoid and topography data the ncc records a prolonged history of archean and paleoproterozoic accretion of crustal blocks through subduction and collision building the cratonic architecture which was subsequently differentially destroyed during mesozoic through extensive magmatism the thermal structure obtained in our study is considered to define the lithosphere asthenosphere boundary lab of the ncc and reflects the density variations within the mantle lithosphere employing the moho depths from deep seismic sounding profiles for the inversion and based on repeated computations using different parameters we estimate the moho depth lab depth and average crustal density of the craton the moho depth varies from 28 to 50 km and the lab depth varies from 105 to 205 km the lab and moho show concordant thinning from west to east of the ncc the average crustal density is 2870 kg m 3 in the western part of the ncc higher than that in the eastern part 2750 kg m 3 the results of joint inversion in our study yielded lab depth and lithospheric thinning features similar to those estimated from thermal and seismic studies although our results show different depth and variations in the thickness the lithosphere gently thins from 145 to 105 km in the eastern ncc where as the thinning is much less pronounced in the western ncc with average depth of about 175 km the joint inversion results in this study provide another perspective on the lithospheric structure from the density properties and corresponding geophysical responses in an ancient craton

seismic anisotropy of the lithosphere asthenosphere system beneath the rwenzori region of the albertine rift shear wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the upper mantle beneath the rwenzori region of the east african rift system at most stations shear wave splitting parameters obtained from individual earthquakes exhibit only minor variations with backazimuth we therefore employ a joint inversion of sks waveforms to derive hypothetical one layer parameters the corresponding fast polarizations are generally rift parallel and the average delay time is about 1 s shear phases from local events within the crust are characterized by an average delay time of 0 04 s delay times from local mantle earthquakes are in the range of 0 2 s this observation suggests that the dominant source region for seismic anisotropy beneath the rift is located within the mantle we use finite frequency waveform modeling to test different models of anisotropy within the lithosphere asthenosphere system of the rift the results show that the rift parallel fast polarizations are consistent with horizontal transverse isotropy hti anisotropy caused by rift parallel magmatic intrusions or lenses located within the lithospheric mantle as it would be expected during the early stages of continental rifting furthermore the short scale spatial variations in the fast polarizations observed in the southern part of the study area can be explained by effects due to sedimentary basins of low isotropic velocity in combination with a shift in the orientation of anisotropic fabrics in the upper mantle a uniform anisotropic layer in relation to large scale asthenospheric mantle flow is less consistent with the observed splitting parameters

late cenozoic deformation in the south caspian region effects of a rigid basement block within a collision zone abstract active deformation in the south caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone rigid basement to the south caspian basin moves with a westward component relative both to stable eurasia and iran and is beginning to subduct at its northern and western margins this motion is oblique to the approximately north south arabia eurasia convergence and causes oblique shortening to the south and northeast of the south caspian basin thrusting in the alborz and kopet dagh is accompanied by range parallel strike slip faults which are respectively left and right lateral there are also arcuate fold and thrust belts in the region for two principal reasons firstly weaker regions deform and wrap around the rigid block this occurs at the curved transition zone between the alborz and talysh ranges where thrust traces are concave towards the foreland secondly a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence west to east thrusts in the eastern talysh represent underthrusting of the south caspian basement but pass via an arcuate fan of fold trains into ssw directed thrusts in the eastern greater caucasus which accommodates part of the arabia eurasia convergence each part of the south caspian region contains one or more detachment levels which vary dependent on the pre pliocene geology buckle folds in the south caspian basin are detached from older rocks on thick mid tertiary mudrocks whereas thrust sheets in the eastern greater caucasus detach on mesozoic horizons in the future the south caspian basement may be largely eliminated by subduction leading to a situation similar to archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust

subduction history of the tethyan region derived from seismic tomography and tectonic reconstructions 1 in the mantle underneath the tethyan suture zone large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present day signature of subducted tethyan lithosphere we investigate the mesozoic cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions three different subduction scenarios for the tethyan oceanic lithosphere representative for the available tectonic reconstructions are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late mesozoic next these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images in this analysis we include among others the possible effects of ridge subduction and slab detachment after the cenozoic continental collisions absolute plate motion and slab thickening in the mantle our preferred subduction model comprises the opening of large back arc oceanic basins within the eurasian margin the model points to slab thickening by a factor of 3 in the mantle in which case the estimated volumes allow for active oceanic spreading 1 2 5 cm yr in the tethyan lithosphere during convergence our results further indicate the occurrence of early oligocene slab detachment underneath the northern zagros suture zone followed by both westward and eastward propagation of the slab tear and diachronous eocene to miocene slab detachment below the eastern to western himalayas free sinking rates of the detached material of 2 cm yr in the lower mantle provide the best fit between the tomographic mantle structure and our tethyan subduction model

crustal and upper mantle structures of makran subduction zone se iran by combined surface wave velocity analysis and gravity modeling abstract the inversion of rayleigh wave group velocity dispersion curves is challenging because it is non linear and multimodal in this study we develop and test a new rayleigh wave dispersion curve inversion scheme using the shuffled complex evolution sce algorithm incorporating this optimization algorithm into the inverse procedure not only can effectively locate the promising areas in the solution space for a global minimum but also avoids its wandering near the global minimum in the final stage of search in addition our approach differs from others in the model parameterization instead of subdividing the model into a large number of thin layers we invert for thickness velocities and densities and their vertical gradients of four layers sediments upper crust lower crust and upper mantle the proposed inverse procedure is applied to non linear inversion of fundamental mode rayleigh wave group dispersion curves for shear and compressional wave velocities at first to determine the efficiency and stability of the sce method two noise free and two noisy synthetic data sets are inverted then real data for makran region in se iran are inverted to examine the usage and robustness of the proposed approach on real surface wave data in a second step we applied 3d gravity modeling based on surface wave analysis results to obtain the density structure and thickness of each layer the reason for using both types of data sets is that gravity anomaly has a bad vertical resolution and surface wave group velocities are good for placing layer limits at depth but they are not very sensitive to densities therefore using gravity data increases the overall resolution of density distribution in a final step we used again the sce method to invert the fundamental mode rayleigh wave group dispersion curves based on the gravity results gravity results like thicknesses and sediment densities have been used to constrain the limit of search space in the sce method results show a high shear and compressional velocity under the gulf of oman which reduce to the north of the makran region the moho depth of the oman gulf is about 18 28 km and it increases to 46 48 km under the taftan bazman volcanic arc the density image shows an average crustal density with maximum values under the gulf of oman decreasing northward to the makran

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

the crustal structure of the southern caspian region the south caspian basin appears to behave as a relatively rigid aseismic block within the otherwise deforming alpine himalayan orogenic belt this anomalous character suggests that there is a fundamental compositional difference between the crust of the south caspian basin and that of the surrounding region five new estimates of crustal velocity structure beneath the southern caspian region are determined by the teleseismic receiver function method these models show that the crust in turkmenia along the trend of the apshceron balkhan sill kopet dag mountains is 50 km thick in the southwestern part of the caspian basin the crust is 33 km thick and consists of a 13 km thick sedimentary section lying on a high velocity vp 7 1 km s 1 lower crustal section in the southeastern part of the basin the crust is 30 km thick and consists of a 10 km thick sedimentary section overlying a 20 km thick low velocity vp 5 8 km s 1 crystalline crust the receiver function models are combined with velocity models from previous russian deep seismic sounding studies into a 1800 km long ese wnw trending crustal cross section across the kura depression the south caspian basin and the kopet dag mountains the most significant features of this crustal model are the 20 km variation in thickness of cenozoic sedimentary basin deposits the absence of a granitic vp 5 8 6 5 km s 1 crustal layer in the central part of the south caspian basin and 20 km of crustal thinning beneath the central part of the basin the moho beneath the south caspian basin has a broad arch like structure whose western boundary is a relatively narrow zone across which the crust thins rapidly 20 km thinning over a 100 km zone and whose eastern boundary has a more gradual change in crustal thickness 20 km thinning over a 400 km zone the velocity depth profiles derived from the receiver functions are compared to laboratory velocity estimates made at pressures and temperatures appropriate for the lower crust in turkmenia but the seismic p wave data alone do not permit differentiation between the various possible rock types the crustal model developed for the south caspian basin is consistent with the hypothesis that the crystalline crust of the basin is a section of oceanic crust that is being overthrust by the continental crust around much of its margins however we cannot rule out the possibility that the crystalline crust beneath the basin is a section of lower continental crust whose upper crustal section has been removed by erosion or faulting

the relationship between topography and gravity on earth and venus abstract the measurement of the admittance between the topography and the gravity of a planet as a function of wavelength provides important constraints on its interior processes a detailed discussion of the influence of noise on such measurements shows that at long wavelengths the admittance is best determined in the frequency domain using gravity as input and topography as output this procedure is used to estimate the admittance in the pacific and indian oceans the observed values are reduced by volcanism above plumes which produces a contribution to the long wavelength compensated topography when this effect is taken into account there is little evidence that the admittance is reduced by the presence of a low viscosity zone in the upper mantle a similar study of venus shows an excellent correlation between gravity g and topography h for wavelengths greater than 700 km with an admittance of about 50 mgal km 1 this value agrees well with that expected from numerical axisymmetric models of plumes and can be used to construct maps of residual topography by calculating h g 50 where h is in kilometers and g in milligals the power spectrum of the gravity field and its skewness with positive anomalies covering a smaller area but being larger than the negative ones are also in agreement with those of the numerical models of the major topographic features only aphrodite and ishtar terrae behave differently with a value of admittance of about one third of other large structures this difference requires their topography to be partly supported by variations in crustal thickness by analogy with similar structures on earth both are likely to be supported by volcanically thickened crust on top of rising plumes though the observed relationships do not rule out other models the behavior of the admittance at long wavelengths is important because it can constrain the thickness of the convecting layer at wavelengths of more than 3000 km the gravity data provides some evidence that the layer depth is about 1000 km over the center of beta and phoebe regiones there are short wavelength anomalies of residual topography produced by crust that is thinner than that elsewhere flanked by positive anomalies of thicker crust the regions of thin crust occur beneath rift zones this behavior can be explained if crust flows down the flanks of elevated regions above plumes and can generate coronae when the plume support disappears

seismological evidence for crustal scale thrusting in the zagros mountain belt iran summary crustal receiver functions rfs computed from the records of 45 temporary seismological stations installed on a 620 km long profile across central zagros provide the first direct evidence for crustal thickening in this mountain belt due to a rather short 14 km average station spacing the migrated section computed from radial rfs displays the moho depth variations across the belt with good spatial resolution from the coast of the persian gulf to 25 km southwest of the main zagros thrust mzt the moho is almost horizontal with slight depth variations around 45 km crustal thickness then increases abruptly to a maximum of 70 km beneath the sanandaj sirjan metamorphic zone between 50 and 90 km northeast of the surface exposure of the mzt further northeast the moho depth decreases to 42 km beneath the urumiehdokhtar magmatic assemblage and the southern part of the central iranian microcontinent the region of thickest crust is located 75 km to the northeast of the bouguer anomaly low at 220 mgals gravity modelling shows that the measured moho depth variations can be reconciled with gravity observations by assuming that the crust of zagros underthrusts the crust of central iran along the mzt considered as a crustal scale structure this hypothesis is compatible with shortening estimates by balanced cross sections of the zagros folded belt as well as with structural and petrological studies of the metamorphic sanandaj sirjan zone

3d lithosphere density structure of southern indian shield from joint inversion of gravity geoid and topography data abstract we present the 3d crustal and lithospheric structure and crustal average density distribution of southern indian shield south of 18 n sri lanka and adjoining oceans the model is based on the assumption of local isostatic equilibrium and is derived from joint inversion of free air gravity and geoid anomalies and topography data the derived crustal thickness of 10 25 km in the oceanic region increases to 34 35 km along the coast a crustal thickness of 34 38 km is obtained beneath the eastern dharwar craton and 36 45 km beneath the western dharwar craton and the southern granulite terrain sri lanka has a thinner crust of 30 35 km the lithosphere asthenosphere boundary is located at depths of 70 120 km under oceanic regions and 150 180 km below the dharwar craton and the northern block of southern granulite terrain a notably thinned lithosphere of 130 km near bangalore in the eastern dharwar craton 140 km beneath the southern block of southern granulite terrain and 130 km in sri lanka is observed the thickness of the lithosphere 130 km near bangalore is inferred as the frozen in signature of a small fossil mantle plume and or tectono compositional effect of a rifted margin and a suture considerable stretching and or convective removal of pristine lithosphere in the southern block of southern granulite terrain and adjoining sri lanka before disappearing completely in the archaean northern block of southern granulite terrain and dharwar craton is suggested

seismic imaging of the lithospheric structure of the zagros mountain belt iran we present a synthesis and a comparison of the results of two temporary passive seismic experiments installed for a few months across central and northern zagros the receiver function analysis of teleseismic earthquake records gives a high resolution image of the moho beneath the seismic transects on both cross sections the crust has an average thickness of 43 2 km beneath the zagros fold and thrust belt and the central domain the crust is thicker beneath the hanging wall of the main zagros reverse fault mzrf with a larger maximum moho depth in central 69 2 km than in northern zagros 56 2 km the thickening affects a narrower region 170 km beneath the sanandaj sirjan zone of central zagros and a wider region 320 km in northern zagros we propose that this thickening is related to overthrusting of the crust of the arabian margin by the crust of central iran along the mzrf considered as a major thrust fault cross cutting the whole crust the fault is imaged as a lowvelocity layer in the receiver function data of the northern zagros profile moreover the crustal scale thrust model reconciles the imaged seismic moho with the bouguer anomaly data measured on the central zagros transect at upper mantle depth p wave tomography confirms the previously observed strong contrast between the faster velocities of the arabian margin and the lower velocities of the iranian micro blocks our higher resolution tomography combined with surface wave analysis locates the suture in the shallow mantle of the sanandaj sirjan zone beneath central zagros the arabian upper mantle has shield like shear wave velocities while the lower velocities of the iranian upper mantle are likely due to higher temperature but these velocities are not low enough and the low velocity layer not thick enough to conclude on a delamination of the lithospheric mantle lid beneath iran the lack of a high velocity anomaly in the mantle beneath central iran suggests that the neotethyan oceanic lithosphere is probably detached from the arabian margin

mean magnitude variations of earthquakes as a function of depth different crustal stress distribution depending on tectonic setting 1 the mean magnitude of earthquakes in the gulf of corinth is found to increase strongly with depth b value decreases whereas the dip of fault planes decreases the b value difference of 0 25 between shallow and deep earthquake distributions is based on about 7 000 events and therefore is statistically highly significant the same is true in california but opposite patterns are observed in southern iceland and in western nagano japan because large mean magnitudes low b values are indicative of relatively high stress levels we propose that in the detachment layer at about 9 2 km depth earthquakes are generated at higher stresses than in the shallower parts of the crust the correlation of low b values with low faulting dips can be taken as line of evidence that low b values map high stress regimes

improvement of radar ice thickness measurements of greenland outlet glaciers using sar processing extensive aircraft based radar ice thickness measurements over the interior and outlet glacier regions of the greenland ice sheet have been obtained by the university of kansas since1993 with the latest airborne surveys conducted in may 2001 the radar has evolved during this period to a highly versatile system capable of charac terizing ice thickness over a wide variety of ice sheet conditions before 1997 the digital system was limited only capable of storing incoherent data or coherent data with a very large number of presumed signals at a low pulse repetition frequency in1998 the radar was upgraded with modern components allowing coherent datato be stored with a small number of presumed returns for1024 range cellsat a highpulse repetition frequency the new data on icethickness of greenlandoutlet glaciers arearchivedand made availableto the scientific community in the form of radar echograms and derived ice thickness at http tornado rsl ukans edu greenlanddata htm the u s national snow and ice data center nsidc also provides a link to these data and nsidc will eventually serve as the permanent archive of these data improvements in radar sensitivity in outlet glacier regions have been achieved by collecting coherent radar data and applying various signal processing techniques deep outlet glacier channels that were previously unre solvedwith incoherentdata can nowbemappedusingacoherent signal signalcondition ing and synthetic aperture radar sar processing

surface waveform tomography of the turkish iranian plateau summary the middle east is a tectonically complex region consisting of terrains as diverse as precam brian shields and young orogens the dominant tectonic feature is the turkish iranian plateau a recently elevated seismically active region along the zagros bitlis suture which results from the collision of the arabian plate with eurasia in this study we use surface waveform tomography to elucidate the upper mantle shear wave velocity structure beneath the turkish iranian plateau and adjacent regions the main large scale feature in the tomographic model is a low shear wave velocity anomaly in the uppermost mantle beneath the plateau this low velocity feature correlates with a long wavelength free air gravity anomaly and with recent volcanism whose geochemistry has a lower lithospheric mantle signature seismology gravity and volcanism all suggest the presence of a thin lithosphere and warm upper mantle beneath the turkish iranian plateau the upper mantle low shear wave velocity zone the high free air gravity and the deep lithospheric source depth for the basaltic volcanism are all consistent with a partial delamination of the lithosphere as a result of earlier lithospheric thickening resulting from the continental collision

evidence for a missing crustal root and a thin lithosphere beneath the central alborz by receiver function studies summary the alborz mountains build the northern part of the alpine himalayan orogen in western asia they are located south of the caspian sea and form an east west range across the north of iran this region is one of the most active tectonic areas as it undergoes extensive crustal deformation and shortening in the present work we used data from 11 permanent stations of the tehran telemetry seismic network to estimate the thickness of the crust and mantle lithosphere beneath the central alborz region by p and s receiver function methods results of both p and s receiver functions revealed a relatively large crustal thickness beneath this region 51 54 km which can be associated with the shortening process related to the orogenic belt no remarkable crustal thickening has been detected below the high topography of central alborz a thick crust 67 5 km is observed locally in the region beneath the damavand volcano which is possibly related to the magmatic addition at the base of the crust beneath the volcanic region the s receiver functions exhibit the existence of a seismic discontinuity in the upper mantle at a depth of 90 km which we interpreted as the base of the lithosphere the missing crustal root and the relatively thin lithosphere beneath the alborz may imply that sublithospheric mantle could be responsible for and support the elevation of the alborz

mapping the frequency magnitude distribution in asperities an improved technique to calculate recurrence times we hypothesize that highly stressed asperities may be defined by mapping anomalously low b values along the san andreas fault near parkfield the asperity under middle mountain with its b 0 46 can be distinguished from all other parts of the fault surface likewise along the calaveras fault the northern asperity of the morgan hill 1984 m6 2 rupture can be identified by its low b of 0 5 as a high stress patch along the fault we add further evidence to the observations that the b value of the frequency magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the parkfield segment of the san andreas and along the calaveras fault in both of these areas b values above and below 5 km depth are 1 2 and 0 8 respectively we propose that probabilistic recurrence times tr based on the seismicity parameters a and b should be calculated from their values within asperities only instead of from the values of the entire rupture area of the maximum expected earthquake the strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone which slips along passively when an asperity breaks therefore no information on tr is contained in the passive fault segments only in the asperities at parkfield the probabilistic estimates of tr derived from the data in the whole rupture and in the asperity only are 72 18 24 and 23 12 18 years respectively compared to the historically observed repeat time of 22 years at morgan hill the tr estimates are 122 46 76 and 78 47 110 years respectively compared to the observed repeat time of 72 years

late cenozoic reorganization of the arabia eurasia collision and the comparison of short term and long term deformation rates the arabia eurasia collision deforms an area of 3 000 000 km2 of continental crust making it one of the largest regions of convergent deformation on earth there are now estimates for the active slip rates total convergence and timing of collision related deformation of regions from western turkey to eastern iran this paper shows that extrapolating the present day slip rates of many active fault systems for 3 7 million years accounts for their total displacement this result means that the present kinematics of the arabia eurasia collision are unlikely be the same as at its start which was probably in the early miocene 16 23 ma or earlier in some but not all active fault systems short term 10 year and long term 5 million year average deformation rates are consistent there is little active thickening across the turkish iranian plateau and possibly the interior of the greater caucasus these are two areas where present shortening rates would need more than 7 million years to account for the total crustal thickening and where there are structural and or stratigraphic data for pre late miocene deformation we suggest that once thick crust up to 60 km built up in the turkish iranian plateau and the greater caucasus convergence took place more easily by crustal shortening in less elevated regions such as the zagros simple folded zone the south caspian region and foothills of the greater caucasus or in other ways such as westward transport of turkey between the north and east anatolian faults the time and duration of this changeover are not known for certain and are likely be diachronous although deformation started or intensified in many of the currently active fault systems at 5 2 ma

a systematic test of the hypothesis that the b value varies with depth in california we spatially map the shallow to deep b value ratio in the crust in california previous studies of the frequency magnitude distribution as a function of depth for selected crustal regions indicated that b decreases from b 1 1 in the 0 5 km depth range to b 0 8 in the depth range 7 15 km our detailed mapping confirms that this pattern can be established at the 99 significance level for about 32 of the entire seismically active crust about 2 of the crust displays the opposite b gradient one such area is the san francisco bay area we speculate that differences in stress levels are the main factor controlling the depth dependency of b these results confirm that the b value should not always be considered a constant in studies such as seismic hazard estimations

the accommodation of arabia eurasia plate convergence in iran continental convergence between arabia and eurasia is taken up by distributed deformation in iran at wavelengths large compared with the thickness of the lithosphere this deformation is best described by a continuous velocity field the only quantitative source of information on the spatial distribution of strain rates within iran is the record of earthquakes we find that we can reproduce the style of deformation observed in the seismicity by simply minimizing the rate of work in a continuous viscous medium that has to accommodate the arabia eurasia plate motion between the defined shapes of iran s rigid borders when in addition we specify central iran azerbaijan and the southern caspian basin to be relatively rigid blocks within the deforming zone then the fit to the style of the observed strain rate distribution is even better we conclude that much of the pattern of deformation in iran is predetermined by the shape of its rigid borders and by the disposition of relatively rigid blocks within it this is likely also to have been a common occurrence in older orogenic belts we confirm earlier suggestions that earthquakes between 1909 and 1992 can account for only a small part 10 20 of the total deformation required by the convergence between the arabia and eurasia plates we then show that the whole plate motion can be accommodated by a velocity field with the same orientations and relative magnitudes of principal strain rates seen in the earthquakes but with larger absolute magnitudes there is therefore no requirement that the large proportion of aseismic deformation in iran is substantially different in style orientation or distribution from that released seismically in the earthquakes

the structure and kinematics of the southeastern zagros fold thrust belt iran from thin skinned to thick skinned tectonics 1 we present the first complete balanced cross section across the southeastern zagros fold thrust belt zftb the following main structural features emerge from this section 1 in the south of the zftb the proterozoic to recent sedimentary sequence has been decoupled from its panafrican basement along the ductile basal evaporites and folded into a series of large detachment anticlines ongoing shortening of these structures has resulted in migration of the basal salt layers into the cores of the anticlines and propagation of forelimb thrusts 2 in the north of the zftb deep seated ramps have folded the hanging wall rocks and produced imbrications and duplex structures within the higher levels of the sedimentary sequence 3 out of sequence thrusts linked to major seismogenic basement faults have cut through the structures in the cover of the zftb a three step incremental restoration of the section shows that two main phases of deformation can be separated in the tectonic evolution of the zftb a mio pliocene thin skinned phase in the course of which most of the structures in the cover were generated followed by a pliocene to recent thick skinned phase expressed as out of sequence faulting in the cover which is currently underlined by the seismicity within the basement in plan view the initial structures of the southeastern zftb developed with a curved shape essentially controlled by the shape and thickness of the underlying proterozoic salt basin i e the jura style in the following basement involved phase out of sequence thrusts cut at oblique angles through the preexisting structures of the cover the total shortening absorbed in the cover amounts to at least 45 km corresponding to a ratio of 22

3d joint inversion modeling of the lithospheric density structure based on gravity geoid and topography data application to the alborz mountains iran and south caspian basin region abstract we present a 3d algorithm to obtain the density structure of the lithosphere from joint inversion of free air gravity geoid and topography data based on a bayesian approach with gaussian probability density functions the algorithm delivers the crustal and lithospheric thicknesses and the average crustal density stabilization of the inversion process may be obtained through parameter damping and smoothing as well as use of a priori information like crustal thicknesses from seismic profiles the algorithm is applied to synthetic models in order to demonstrate its usefulness a real data application is presented for the area of northern iran with the alborz mountains as main target and the south caspian basin the resulting model shows an important crustal root up to 55 km under the alborz mountains and a thin crust ca 30 km under the southernmost south caspian basin thickening northward to the apsheron balkan sill to 45 km central and nw iran is underlain by a thin lithosphere ca 90 100 km the lithosphere thickens under the south caspian basin until the apsheron balkan sill where it reaches more than 240 km under the stable turan platform we find a lithospheric thickness of 160 180 km

rift induced delamination of mantle lithosphere and crustal uplift a new mechanism for explaining rwenzori mountains extreme elevation with heights of 4 5 km the topography of rwenzori mountains a large horst of old crustal rocks located inside a young passive rift system poses the question why are the rwenzori mountains so high the cenozoic western rift branch of the east african rift system is situated within the late proterozoic mobile belts between the archean tanzania craton and congo craton the special geological setting of the massif at a rift node encircled by the ends of the northern western rift segments of lake albert and lake edward suggests that the mechanism responsible for the high elevation of the rwenzoris is related to the rifting process our hypothesis is based on the propagation of the rift tips surrounding the stiff old lithosphere at rwenzori region thereby triggering the delamination of the cold and dense mantle lithosphere ml root by reducing viscosity and strength of the undermost lower crust as a result this unloading induces fast isostatic pop up of the less dense crustal rwenzori block we term this rid rift induced delamination of mantle lithosphere the physical consistency of the rid hypothesis is tested numerically viscous flow of 2d models is approximated by a finite difference method with markers in an eulerian formulation the equations of conservation of mass momentum and energy are solved for a multi component system based on laboratory data of appropriate rock samples a temperature pressure and stress dependent rheology is assumed assuming a simple starting model with a locally heated ml the ml block between the weakened zones becomes unstable and sinks into the asthenosphere while the overlying continental crust rises up thus rid seems to be a viable mechanism to explain geodynamically the extreme uplift important conditions are a thermal anomaly within the ml a ductile lower crust with visco plastic rheology allowing significant strength reduction and lateral density variations the special situation of a two sided rifting or offset rift segments to decouple the ml laterally from the surrounding continental lithosphere seems to be most decisive further support for the rid mechanism may come from additional crustal thickness and an extensive stress field some parameters such as the excess temperature and yield stress are very sensitive small changes determine whether delamination takes place or not

greenland ice sheet thickness changes measured by laser altimetry precise airborne laser altimetry surveys at locations on the greenland ice sheet that had been accurately surveyed in 1980 and 1981 reveal a thickening in western greenland of up to two meters between 1980 and 1993 we cannot yet state whether this represents a long term trend or the cumulative effects of interannual variability of snow accumulation rates nevertheless the information presented here provides an indication of ice thickness changes across southern greenland in unprecedented detail laser altimetery surveys have now been made over all the major ice sheet drainage basins and will be repeated at regular intervals to provide detailed estimates of ice thickening thinning rates over the entire ice sheet

mapping asperities by minima of local recurrence time san jacinto elsinore fault zones minima of the local recurrence time tl m are correlated with the locations of historical main shocks in the san jacinto sj elsinore e fault zones of southern california we define tl m as the recurrence time calculated from the local b and a values of the frequency magnitude distribution earthquakes within 6 and 10 km from each point in the sj and e fault zones respectively we propose that anomalously low tl m values map asperities out of six historic main shocks 1899 1968 five ruptured substantial parts of four asperities mapped by our method which uses solely the modern earthquake catalog m 1 2 1981 0 1998 8 we estimate the probability for this correlation to occur at random as 10 2 to 10 3 depending on the scenario chosen it appears that the minima in tl m are tied to minima in b values more than to maxima in a values thus locations of low b values can also be used as an indication where asperities may be located and where main shocks are most likely the anza gap a segment of the san jacinto fault with low a values is mapped as an asperity by our method the heterogeneity in b values is strong throughout the study area the b values range from 0 5 to 1 8 over distances of 10 km volumes with approximately constant b values have radii of r 8 6 km our results suggest that detailed mapping of b values and local recurrence times in volumes with a few kilometers radius reveals important information about the local seismic hazard hitherto untapped by conventional seismicity analyses