earthquake nests as natural laboratories for the study of intermediate depth earthquake mechanics article the physical mechanism of intermediate depth earthquakes is still under debate in contrast to conditions in the crust and shallow lithosphere at temperatures and pressures corresponding to depths 50 km rocks ought to yield by creep or flow rather than brittle failure some physical process has to enable brittle or brittle like failure for intermediate depth earthquakes the two leading candidates for that are dehydration embrittlement and ther mal shear runaway given their great depth intermediate depth earthquake processes can t be observed direct ly insteadwe must relyona combination ofseismologyand thestudy of laboratory analogs tounderstand them earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity in this paper we focus on three intermediate depth earthquake nests vrancea hindu kush and bucaramanga and what they reveal about the mechanics of intermediate depth earthquakes we review published studies of tectonic setting focal mechanisms precise earthquake locations and earthquake source physics at these locations with an emphasis on the bucaramanga nest all three nests are associated with subducting litho sphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity in contrast the bucaramanga nest has a larger b value larger variability of focal mechanisms and shows no evidence of aftershock sequences unlike the other two we also report for the first time finding a signifi cant number of repeating earthquakes some with reverse polarity

interplay between subduction retreat and lateral extrusion tectonics of the western carpathians 1 analysis of brittle microstructures reveals three tertiary deformation events in the western carpathians 1 late paleogene bedding parallel extension 2 oligocene to middle miocene nne ssw compression and ese wnw extension 3 post mid miocene nw se extension these regional deformations applied to crustal blocks in the intra carpathian region resulted from forces which were induced by the coupled plate tectonic processes of subduction retreat beneath the carpathian arc and lateral extrusion from the eastern alps toward the carpathian region material flow from the alps spread toward east and ne guided by sinistral strike slip zones along the ne trending continental margin subduction retreat was terminated by mid miocene soft collision of the inner western carpathians with the european foreland ongoing subduction retreat beneath the eastern carpathians caused post mid miocene back arc extension forming the pannonian basin extension spread into the western carpathians by reactivating the extrusion related strike slip faults as normal faults

observational constraints on the fracture energy of subduction zone earthquakes we relate seismologically observable parameters such as radiated energy seismic moment rupture area and rupture speed to the dynamics of faulting to achieve this objective we computed the radiated energy for 23 subduction zone earthquakes recorded between 1992 and 2001 most of these earthquakes have a magnitude m w 7 5 but we also included some smaller m w 6 7 well studied subduction zone earthquakes and six crustal earthquakes we compiled static stress drop estimates for these 29 earthquakes from literature and used a slip weakening model to determine the radiation efficiency of these earthquakes we also determined the rupture speed of these earthquakes from literature from fracture mechanics fracture energy and hence radiation efficiency can be related to the rupture speed the radiation efficiencies estimated from the partitioning of energy as given by the slip weakening model are consistent with the rupture speed estimated for these earthquakes most earthquakes have radiation efficiencies between 0 25 and 1 and are hence efficient in generating seismic waves but tsunami earthquakes and two deep earthquakes the 1994 bolivia and the 1999 russia china border earthquakes have very small radiation efficiencies 0 25 and hence dissipate a large amount of energy during faulting we suggest that differences in the radiation efficiencies of different types of earthquakes could be due to fundamental differences in their rupture mechanics in deep events the energy is probably dissipated in thermal processes in the fault zone while it is possible that the morphology of the trench causes large energy dissipation during the rupture process of tsunami earthquakes

improving local earthquake locations using the l1 norm and waveform cross correlation application to the whittier narrows california aftershock sequence experiments with different earthquake location methods applied to aftershocks of the october 1 1987 whittier narrows earthquake in california ml 5 9 suggest that local event locations can be greatly improved through the use of the l1 norm station corrections and waveform cross correlation the whittier narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the southern california seismic network scsn a telemetered network of several hundred short period seismographs scsn travel time picks and waveforms obtained through the southern california earthquake center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock using a smoothed version of the standard southern california velocity model and the existing travel time picks improved location accuracy is obtained through use of the l1 norm rather than the conventional least squares l2 norm approach presumably due to the more robust response of the former to outliers in the data a large additional improvement results from the use of station terms to account for three dimensional velocity structure outside of the event cluster to achieve greater location accuracy waveforms for these events are resampled and low pass filtered and the p and s wave cross correlation functions are computed at each station for every event pair for those events with similar waveforms differential times may be obtained from the cross correlation functions these times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked locations obtained from the adjusted picks show a further improvement in accuracy location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick improvements in location accuracy are indicated by the reduced scatter in the residuals smaller estimated location errors and the increased tendency of the locations to cluster along well defined fault planes median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location although the relative locations within localized clusters of similar events are better constrained seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with a ml 5 3 aftershock these fault planes appear to cross and activity began on the secondary fault plane prior to the large aftershock

source parameters of intermediate depth vrancea romania earthquakes from empirical green s functions modeling abstract several source parameters source dimensions slip particle velocity static and dynamic stress drop are determined for the moderate size october 27th 2004 mw 5 8 and the large august 30th 1986 mw 7 1 and march 4th 1977 mw 7 4 vrancea romania intermediate depth earthquakes for this purpose the empirical green s functions method of irikura e g irikura k 1983 semi empirical estimation of strong ground motions during large earthquakes bull dis prev res inst kyoto univ 33 part 2 no 298 63 104 irikura k 1986 prediction of strong acceleration motions using empirical green s function in proceedings of the 7th japan earthquake engineering symposium 151 156 irikura k 1999 techniques for the simulation of strong ground motion and deterministic seismic hazard analysis in proceedings of the advanced study course seismotectonic and microzonation techniques in earthquake engineering integrated training in earthquake risk reduction practices kefallinia 453 554 is used to generate synthetic time series from recordings of smaller events with 4 mw 5 in order to estimate several parameters characterizing the so called strong motion generation area which is defined as an extended area with homogeneous slip and rise time and for crustal earthquakes corresponds to an asperity of about 100 bar stress release miyake h t iwata and k irikura 2003 source characterization for broadband ground motion simulation kinematic heterogeneous source model and strong motion generation area bull seism soc am 93 2531 2545 the parameters are obtained by acceleration envelope and displacement waveform inversion for the 2004 and 1986 events and msk intensity pattern inversion for the 1977 event using a genetic algorithm the strong motion recordings of the analyzed vrancea earthquakes as well as the msk intensity pattern of the 1977 earthquake can be well reproduced using relatively small strong motion generation areas which corresponds to small asperities with high stress drops 300 1200 bar and high particle velocities 3 5 m s these results imply a very efficient high frequency radiation which has to be taken into account for strong ground motion prediction and indicate that the intermediate depth vrancea earthquakes are inherently different from crustal events

tectonic framework of the himalaya karakoram and tibet and problems of their evolution the himalaya the karakoram and tibet were assembled by the successive accretion to asia of continental and arc terranes during the mesozoic and early tertiary the jinsha and banggong sutures in tibet join continental terranes separated from gondwana ophiolites were obducted onto the shelf of southern tibet in the jurassic before the formation of the banggong suture the kohistan ladakh terrane contains an island arc that was accreted in the late cretaceous on the shyok suture and consequently evolved into an andean type batholith further east this trans himalayan batholith developed on the southern active margin of tibet without the prior development of an island arc ophiolites were obducted onto the shelf of india in the late cretaceous to lower palaeocene before the closing of tethys and the formation of the indus yarlung zangbo suture at about 50 ma post collisional northward indentation of india at ca 5 cm a 1 since the eocene has redeformed this accreted terrane collage palaeomagnetic evidence suggests this indentation has given rise to some 2000 km of intracontinental shortening expressions of this shortening are the uplift of mid crustal gneisses in the karakoram on a late tertiary breakback thrust folding of palaeogene redbeds in tibet south directed thrust imbrication of the foreland and shelf of the indian plate north directed back thrusts along the indus suture zone post miocene spreading and uplift of thickened tibet giving rise to n s extensional faults and strike slip faults which allowed eastward escape of tibetan fault blocks

recent developments in centrifuge modelling of tectonic processes equipment model construction techniques and rheology of model materials abstract the experimental tectonics laboratory at queen s university is equipped with a large capacity centrifuge that is capable of subjecting tectonic models measuring 127 76 mm in plan and up to 51 mm in depth to accelerations as high as 20 000 g this high capacity greatly extends the range of potential model materials and permits the use of relatively stiff and or brittle substances a number of new techniques of model construction have been devised that permit internal and surface strain patterns and kinematic evolution to be monitored in detail one particularly useful technique which will find application in non centrifuged experiments as well allows the preparation of highly uniform anisotropic multilayers composed of alternating layers of plasticine and silicone putty with individual layer thicknesses as low as 20 m and with controllable ratio between thicknesses of the relatively competent and incompetent units examples of models constructed using these new techniques are illustrated one particular type of the commonly used model material silicone putty has been subjected to a series of rheological test the results indicate that at strain rates in the range 10 6 10 3 s 1 applicable to the centrifuge experiments the silicone putty exhibits power law rheology with n 7 2 at higher strain rates the material appears to tend towards linear behaviour available rheological data and dimensional analysis using standard scaling laws and appropriate model ratios suggest that the microlaminated plasticine silicone putty multilayer is a suitable analogue in centrifuged experiments for interbedded sequences of indurate limestone and incompetent shale the excellent degree of dynamic similitude attained is demonstrated by the realistic form of fold and fault structures developed in models constructed of this material

fault plane orientations of intermediate depth earthquakes in the middle america trench 1 intermediate depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones the orientation of presubduction faults is particularly well known offshore of middle america where seismic reflection profiles show outer rise faults dipping toward the trench and extending 20 km into the lithosphere if water is transported along these faults and incorporated into hydrous minerals the faults may be reactivated later when the minerals dehydrate in this case the fault plane orientations should be the same in the outer rise and at depth after accounting for the angle of subduction to test this hypothesis we analyze the directivity of 54 large mw 5 7 earthquakes between 35 and 220 km depth in the middle america trench for 12 of these earthquakes the directivity vector allows us to identify the fault plane of the focal mechanism between 35 and 85 km depth we observe both subhorizontal and subvertical fault planes the subvertical fault planes are consistent with the reactivation of outer rise faults whereas the subhorizontal fault planes suggest the formation of new faults deeper than 85 km we only observe subhorizontal faults indicating that the outer rise faults are no longer being reactivated the similarity with previous results from the colder tonga kermadec subduction zone suggests that the mechanism generating these earthquakes and controlling fault plane orientations depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate depth seismicity exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes

tectonic implications of tomographic images of subducted lithosphere beneath northwestern south america we used seismic tomography to investigate the complex structure of the upper mantle below northwestern south america images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted caribbean pacific lithosphere beneath the study area beneath western venezuela and colombia we distinguish two slabs a maracaibo and a redefined bucaramanga slab the maracaibo slab coinciding with most of the bucaramanga slab previously defined by w d pennington dips in a direction of 150 at an angle of 17 to a depth of 275 km and correlates to the subducted late cretaceous oceanic plateau of the caribbean plate farther south a second slab dips at an angle of 50 in a direction of 125 to a depth of at least 500 km and correlates to the subducted oceanic crust of the nazca plate and the downdip extension of the panama island arc we refer to this slab as the redefined bucaramanga slab because it is different from the bucaramanga slab segment defined by pennington the area of the south american plate overriding both slabs is characterized by the absence of an active volcanic arc an anomalously wide topographically uplifted and tectonically active area and the northward escape of the maracaibo block along active strike slip faults in support of earlier studies we attribute this to the underthrusting of the caribbean oceanic plateau our shallowly dipping maracaibo slab along the base of the south american lithosphere and to the recent collision of the panama island arc rafted in on more steeply dipping crust of the nazca plate our redefined bucaramanga slab

an insight into the bucaramanga nest abstract we used the local seismicity for the period of 1993 to 2001 in the northeast of colombia to show the existence of two slabs in the north and south of the bucaramanga nest the northern slab has a dip angle of about 25 and the southern slab has a 50 dip angle while the dip in the bucaramanga nest is about 29 in order to explain the nature of the bucaramanga nest we proposed the scenario of collision between these two slabs using a 3d finite element model fem we show that collision can concentrate modify and perturb the stress field the active process of dehydration embrittlement at intermediate depths and the concentrated stress field in the collision zone may explain the high rate of seismic activity inside the bucaramanga nest the perturbed and modified stress field resulting from the simultaneous effect of collision between two subducted slabs and subduction of the lithosphere under its own weight can explain the variation in the focal mechanism of micro earthquakes and the complexity in the source of the moderate size earthquakes in the bucaramanga nest

the frontier mountain meteorite trap antarctica the frontier mountain blue ice field is an important antarctic meteorite trap which has yielded 472 meteorite specimens since its discovery in 1984 remote sensing analyses and field campaigns from 1993 to 1999 have furnished new glaciological data on ice flow ice thickness bedrock topography ice ablation and surface mass transport by wind along with detailed descriptions of the field situation at the trap this solid set of data combined with an updated meteorite distribution map and terrestrial ages available from literature allows us to better describe the nature of the concentration mechanism in particular we observe that the meteorite trap forms in a blue ice field 1 located upstream of an absolute and a shallow sub ice barriers 2 characterized by compressive ice flow with horizontal velocities decreasing from 100 to 10 cm year on approaching the obstacle 3 undergoing mean ablation rates of 6 5 cm year 4 nourished by a limited snow accumulation zone extending 20 km upstream of the blue ice area we also draw the following conclusions 1 the origin of the meteorite trap can be explained according to the present day glaciological situation 2 the meteorite concentration develops according to the general principles of the ice flow model 3 the accumulation model can be described as stagnant ice or slow moving ice against an absolute and submerged barriers according to the descriptive schemes present in literature 4 the frontier mountain ice field is an effective trap for meteorites weighing more than 200 g for smaller masses the combination of wind and glacial drift may remove meteorites in less than a few tens of thousands of years 5 although the activation age of the frontier mountain trap is not yet constrained we infer that one of the most important findsites may be as old as 50 ka predating the last glacial maximum

on late oligocene to pliocene depocentre migrations and the evolution of the carpathian pannonian system abstract the late oligocene to pliocene evolution of foredeep basins of the eastern alps carpathians fold and thrust belt is marked by pronounced internal to external and lateral depocentre shifts the latter shifts covering a present along arc distance of 1700 km portray accelerating rates of foredeep depocentre migration particularly so in the late early and in the middle miocene from about 7 to about 45 cm yr lateral depocentre migration came to a close at the beginning of the late miocene the subsequent late miocene to pliocene foredeep infill history was characterized by exponentially increasing accumulation rates in the intersection area of the east european ukrainian and moesian platforms successive steps in the evolution of the foredeep basins had pronounced counterparts in the intra carpathian area the beginning of acceleration of foredeep depocentre migration in latest early miocene times was coeval with the inception of intra carpathian extensional tectonics the extremely rapid middle miocene depocentre shift coupled with a change in direction of foredeep depocentre migration corresponded with maximum extension in the intra carpathian area the ensuing end of foredeep depocentre migration around the middle late miocene transition was coeval with the end of extension of the intra carpathian area which in turn was followed by the inception of overall thermal subsidence at about 11 5 ma the timing of and spatial relationships between discrete coeval events in arc and intra arc evolution put unambiguous geological constraints on geodynamic modelling of the evolution of the carpathian pannonian system it is speculated that the geological observations may best be understood in terms of the surficial effects of lateral migration of slab detachment such effects would mirror the dominant role of a concentrating slab pull taken to result in e g a time progressive acceleration of foredeep depocentre migration

grip deuterium excess reveals rapid and orbital scale changes in greenland moisture origin the northern hemisphere hydrological cycle is a key factor coupling ice sheets ocean circulation and polar amplification of climate change here we present a northern hemisphere deuterium excess profile covering one climatic cycle constructed with the use of 18 o and d greenland ice core project grip records past changes in greenland source and site temperatures are quantified with precipitation seasonality taken into account the imprint of obliquity is evidenced in the site to source temperature gradient at orbital scale at the millennial time scale grip source temperature changes reflect southward shifts of the geographical locations of moisture sources during cold events and these rapid shifts are associated with large scale changes in atmospheric circulation

earthquake mechanics and deformation in the tonga kermadec subduction zone from fault plane orientations of intermediate and deep focus earthquakes 1 we make use of rupture directivity to analyze 82 deep earthquakes 100 km depth in the tonga kermadec subduction zone identifying the fault planes for 25 of them we are able to place new constraints on both the physical mechanism of intermediate and deep focus earthquakes and deformation within the subducting slab we find that half of deep earthquakes with mw 6 have detectable directivity we compare the obtained fault orientations with those expected for the reactivation of outer rise normal faults and with those expected for the creation of new faults in response to the ambient stress field earthquakes 300 km depth match the patterns expected for the creation of a new system of faults we observe both subhorizontal and subvertical fault planes consistent with a downdip compressional stress field slip along these faults causes the slab to thicken rupture propagation shows no systematic directional pattern in contrast at intermediate depths 100 300 km all ruptures propagate subhorizontally and all identified fault planes whether in the upper or lower region of the double seismic zone are subhorizontal rupture propagation tends to be directed away from the top surface of the slab after accounting for the angle of subduction the subhorizontal fault plane orientation is inconsistent with the orientation of outer rise normal faults allowing us to rule out mechanisms that require the reactivation of these large surface faults subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation if all deformation takes place on these subhorizontal faults it would cause the slab to thin assuming the slab is incompressible this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths

structural imprints at the front of the choco panama indenter field data from the north cauca valley basin central colombia abstract the northern andes are a complex area where tectonics is dominated by the interaction between three major plates and accessory blocks in particular the choco panama and northern andes blocks the studied cauca valley basin is located at the front of the choco panama indenter where the major romeral fault system active since the cretaceous changes its kinematics from right lateral in the south to left lateral in the north structural studies were performed at various scales dem observations in the central cordillera between 4 and 5 7 n aerial photograph analyses and field work in the folded oligo miocene rocks of the serrania de santa barbara and in the flat lying pleistocene quindio risaralda volcaniclastic sediments interfingering with the lacustrine to fluviatile sediments of the zarzal formation the data acquired allowed the detection of structures with a similar orientation at every scale and in all lithologies these families of structures are arranged similarly to riedel shears in a right lateral shear zone and are superimposed on the cretaceous romeral suture they appear in the central cordillera north of 4 5 n and define a broad zone where 060 oriented right lateral distributed shear strain affects the continental crust the romeral fault system stays active and strain partitioning occurs among both systems the southern limit of the distributed shear strain affecting the central cordillera corresponds to the e w trending garrapatas ibague shear zone constituted by several right stepping en echelon right lateral active faults and some lineaments north of this shear zone the romeral fault system strike changes from nne to n paleostress calculations gave a wnw ese trending maximum horizontal stress and 69 of compressive tensors the orientation of 1 is consistent with the orientation of the right lateral distributed shear strain and the compressive state characterizing the romeral fault system in the area it bisects the synthetic and antithetic riedels and is sub perpendicular to the active romeral fault system it is proposed that the continued movement of the choco panama indenter may be responsible for the 060 oriented right lateral distributed shear strain and may have closed the northern part of the cauca valley thereby forming the cauca valley basin conjugate extensional faults observed at surface in the flat lying sediments of the zarzal formation and quindio risaralda volcaniclastic fan are associatedwith soft sediment deformations these faults are attributed to lateral spreading of the superficial layers during earthquakes and testify to the continuous tectonic activity from pleistocene to present finally results presented here bring newinformation about the understanding of the seismic hazard in this area whereas the romeral fault systemwas so far thought to be themost likely source of earthquakes themore recent cross cutting fault systems described herein are another potential hazard to be considered

crustal structure of the caribbean northeastern south america arc continent collision zone 1 we present the results of a 568 km long regional wide angle seismic profile conducted in the southeastern caribbean that crosses an active island arc a remnant arc two basins possibly floored by oceanic crust an allochthonous terrane of forearc affinity and the passive margin of northern south america the velocity structures of the late cretaceous aves ridge remnant arc and miocene and younger lesser antilles arc are remarkably similar which implies that magmatic processes have remained moderately steady over time crustal thickness is 26 km at the aves ridge and 24 km at the lesser antilles arc in comparison to the izu bonin and aleutian arcs the lesser antilles arc is thinner and has no evidence for a lower crustal cumulate layer which is consistent with the estimated low magma production rates of the lesser antilles arc crustal thickness beneath the grenada and tobago basins is 4 10 km and the velocity structure suggests that these basins could be floored by oceanic crust a decrease of 1 km s in average seismic velocity of the upper crust is observed from nw to se across the north coast fault zone we argue that this marks the suture between the far traveled caribbean arc and the passive margin of the south american continent current strike slip motion between the caribbean and south american plates is located 30 km to the south and thus material originally deposited on the south american passive margin has now been transferred to the caribbean plate

modeling the evolution of continental subduction processes in the pamir hindu kush region several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the india eurasia collision zone with steep northward subduction of indian lithosphere beneath the hindu kush and southward subduction of asian lithosphere under the pamir we investigate the geometry and the timing of these subduction processes seismic tomography images are used to constrain the geometry of the two slabs under the pamir and the hindu kush by measuring the tomographically inferred length of the indian slab under the hindu kush region and by comparison with paleomagnetic reconstructions we estimate that the process of indian slab break off most likely occurred at the early stage of the collision at ca 44 48 ma we infer that after slab break off western india continued its northward motion until its northern boundary reached the present hindu kush region where it began to subduct we estimate an age for the initiation of subduction in this region of about 8 ma we apply 2d thermo kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the hindu kush and pamir regions for the hindu kush we introduce parameters reproducing fast and near vertical subduction whereas in the pamir region we use the results from published studies to reproduce slower subduction modeling results indicate that faster subduction under the hindu kush results in a deeper brittle region which is consistent with observed deeper seismicity than in the pamir region

scaling of source parameters for vrancea romania intermediate depth earthquakes the stress drop scaling of the vrancea romania intermediate depth earthquakes with seismic moments from 10 to 20 dyne cm is investigated the empirical green s function technique is used in order to retrieve the seismic source parameters for the smaller earthquakes 1021 1026 dyne cm available source parameters for the major vrancea earthquakes that occurred in 1940 1977 1986 and 1990 mw 6 9 as determined from teleseismic and local data were included a circular source model approximates well the deconvolved source time functions and source spectra for the analyzed earthquakes in case of the large earthquakes an equivalent circular area is considered corresponding to the zone of aftershocks the static stress drop is determined from seismic moment and source radius for brune s model a dependence of the stress drop on the seismic moment as m140 is outlined over the entire magnitude range

stress state seismicity and subduction geometries of the descending lithosphere below the hindukush and pamir abstract the paper presents an analysis of spatial distribution of 6600 earthquake events which occurred during the period 1964 to 1999 between latitude 34 to 40 n and longitude 68 to 76 e this large volume event is reported in the international seismological centre isc catalog in addition to this a total of 248 focal mechanism solutions are considered to derive a generalised predominant stress prevailing in the descending lithosphere below the hindukush and pamir regions the analysis of spatial distribution shows that the epicentres of the events at shallow level depth 170 km the epicentres below the hindukush are mainly concentrated in a triangular shaped zone and the mean points of concentration of the epicentres appear to be shifted towards southwest at increasing depth the distribution of epicentres at the intermediate and deeper layers of the pamir is observed to be diffused except a cluster of few events in each layer appears to be shifted towards south southeast at increasing depth the distribution of hypocentres changes its concentration from lesser to considerably higher at about 70 km depth and further takes a minimum at about 170 km depth below the hindukush and pamir the present study further involves in analysing the composite group effect of stresses associated with the descending lithosphere below the hindukush and pamir after deriving the best fit generalized predominant directions of stresses it shows that the intermediate depth seismic zone below the hindukush is acted upon by maximum compressive stresses p axes from two directions while the deeper depth zone from three directions and may convincingly be correlated with the changing shape of the respective seismic zones another interesting phenomenon observed here is the change in direction of maximum compressive stresses in clockwise fashion from intermediate to deep seismic zones below the hindukush at shallow depths below the pamir the maximum and minimum t axes compressive stresses are acting almost along nnw sse and ene wsw and are oriented horizontally t axes for few events at these depths show almost vertical orientation the observed down dip extension is predominantly parallel with the descending lithosphere below the hindukush the entire analysis along with the observed scattering of p and t axes of some events at intermediate depths might be indicating a slight contortion of the middle layer below the hindukush the spatial distribution of seismicity and the generalised stress pattern of both the regions infer the existence of two isolated subducting lithosphere it perhaps has created the eastward expulsion or lateral extrusion of tibet along the major strike slip faults like karakorum altyn tagh kunlun and red river finally the whole analysis confirms the existence of shield like continental rigid slab at depths greater than 170 km below the hindukush

an enhanced image of the pamir hindu kush seismic zone from relocated earthquake hypocentres summary we determine the shape of the seismic zone in the pamir hindu kush region de ned by 30 42 0 n 68 78 0 e by obtaining improved hypocentral locations with 90 per cent con dence limits of less than 30 km the depth error bar for most of the earthquakes of about 6000 shallow and intermediate depth earthquakes available s and depth phase arrival times are also used together with the p wave arrival times in the joint hypocentre determination technique to obtain the best possible hypocentral locations the study region is divided into three depth ranges 0 60 60 160 and 160 km the 0 60 km depth zone is then subdivided laterally into 19 blocks with the deeper regions divided into two blocks each the improved delineation of the seismic zone obtained by using the relocated hypocentres implies that the intermediate depth seismicity in the pamir hindu kush region is most simply explained by a single s shaped seismic zone 700 km long and no more than 30 km wide and with most activity concentrated at 100 300 km depth the main features observed are 1 the eastward steepening of the north dipping hindu kush seismic zone through to its overturning at its eastern end beneath the pamirs where it dips to the southeast 2 the curvature and forking of the subducting slab at depths greater than 200 km within the eastern part of the hindu kush seismic zone 3 the very abrupt cut oi in intermediate depth seismicity at 90 110 km depth with no extension to shallower depths under the pamirs and with a persistent gap between the intermediate and shallow seismicity in the northern pamirs and 4 the unusual horizontal t axes for intermediate depth earthquakes of the pamir seismic zone which align with its curvature this study shows that the seismic zone under the hindu kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity whereas the pamir region has horizontal t axes that follow the trend of the contorted seismic zone this suggests that the pamir seismic zone is a slab deformed due to ow in the upper mantle

characterization of seismogenic zones of romania although the time and magnitude range covered by available seismological data is limited several significant regional trends are outlined in the seismogenic zones of romania vrancea region which is by far the most seismically active area has a persistent rate of occurrence of intermediate depth earthquakes clustered in a very confined focal volume and a clear compressive stress regime the deformation field as deduced from the available fault plane solutions is drastically reduced in the crust where the maximum magnitude is below 6 5 except shabla zone in bulgaria the system of major faults developed in a nw se direction in the carpathians foredeep area is certainly linked to the subduction process in vrancea although they seem not to play a significantly active role as could be expected for an active subduction process the existing data indicate an extensional deformation regime over the foredeep area and southern carpathians while a predominant compressive regime is outlined at the contact between the eastern margin of the pannonian depression and carpathians orogen in agreement with the bending tendency of the maximum horizontal compression orientation of the crustal stress field from ne sw in western and central europe to e w in the intra carpathian region grunthal and strohmeyer 1992

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

a note on the depth recurrence and strain release of large vrancea earthquakes the large 1940 vrancea event with body wave magnitude mb 7 3 was investigated in terms of the depth of rupture initiation and extent it was found that the rupture started at 150 km depth and propagated downwards the ruptured zones of the 1977 1986 and 1990 vrancea events were estimated from the distribution of aftershocks located with the joint hypocentre determination method their depth ranges are 90 110 km 130 150 km and 70 90 km respectively because the body wave magnitudes mb were saturated for the last three events and the surface wave magnitudes ms were underestimated not being corrected for depth we chose the moment magnitudes to quantify the four events 7 7 1940 7 4 1977 7 1 1986 and 6 9 1990 respectively the depth interval 110 130 km is found to be unbroken since at least 1838 or 1802 and therefore is expected to accommodate the next large moment release mw 7 0 to 7 4 the strain in the vertical and in the se nw directions is about 0 0003 giving a moderate strain rate of about 10 13 s 1 a correlation between the depth interval of the large events and the year of occurrence in a century is suggested offering a physical basis to the observed quasi cyclicity

s wave attenuation characteristics beneath the vrancea region in romania new insights from the inversion of ground motion spectra the s wave attenuation characteristics beneath the vrancea region in romania are analyzed from the spectra frequency range 0 5 20 hz of more than 850 recordings at 43 accelerometric stations of 55 intermediate depth earthquakes m 4 0 7 1 that occurred in the vrancea seismogenic zone the method commonly chosen for this type of investigation in the case of crustal earthquakes is the generalized inversion technique git e g andrews 1986 castro et al 1990 yet the vrancea dataset is entirely different from common crustal datasets because of the strong clustering of the hypocenters within a very small focal volume there are only few crossing ray paths from sources to receivers as a consequence inhomogeneities in the attenuation properties are not averaged out which leads to unphysical results if the standard git approach is adopted the problem is discussed qualitatively by performing tests with synthetic data and solved quantitatively by adapting the git technique in view of these peculiarities with the optimally adapted inversion scheme it is possible to unravel differences in the attenuation characteristics between two or more sets of stations the results show that the attenuation of seismic waves is roughly comparable in the low frequency range 4 5 hz but stronger by up to an order of magnitude at higher frequencies within the carpathian mountain arc as compared with the foreland area modeling this strongly frequency dependent lateral variation of seismic attenuation by a significantly lower q beneath vrancea 1 provides a very good fit of observed strong motion characteristics 2 sheds new light on the distribution of intensities of the previous strong earthquakes 3 will have strong implications for future hazard assessment and 4 is fully compatible with structural models from deep seismic sounding tomography and teleseismic attenuation

ice flow dynamics and meteorite collection in antarctica abstract typical targets for meteorite searches in antarctica are blue ice fields in front of major bedrock obstacles where the meteorite bearing ice slows down is uplifted and exhumed by ablation however in some areas collection occurs in the lee of topographic barriers as documented in the downstream side of the frontier mountain a major emerged bedrock barrier in this study we present an analogue modelling study of ice flow at frontier mountain which has been chosen as example study area because an exceptionally complete set of glaciological data is available to investigate the causes leading to meteorite concentration results of modelling show how the boundary conditions mainly the bedrock topography control ice flow and how this flow coupled to high ablation may give rise to a stable meteorite trap downstream of major topographic obstacles modelling results support the contention that as erosion is not a mountain builder ablation does not represent a driving force in ice flow dynamics although it plays a significant role in shaving stranding surfaces and exhuming meteorites

coseismic and postseismic velocity changes measured by repeating earthquakes 1 repeating earthquakes that rupture approximately the same fault patch and have nearly identical waveforms are a useful tool for measuring temporal changes in wave propagation in the earth s crust since source and path effects are common to all earthquakes in a repeating earthquake sequence multiplet differences in their waveforms can be attributed to changes in the characteristics of the medium we have identified over 20 multiplets containing between 5 and 40 repeating events in the aftershock zones of the 1989 loma prieta and 1984 morgan hill california earthquakes postmain shock events reveal delays of phases in the early s wave coda of as much as 0 2 s relative to premain shock events the delay amounts to a path averaged coseismic velocity decrease of about 1 5 for p waves and 3 5 for s waves since most of the multiplets are aftershocks and follow omori s law we have excellent temporal sampling in the immediate postmain shock period we find that the amplitude of the velocity decrease decays logarithmically in time following the main shock in some cases it returns to the premain shock values while in others it does not similar results are obtained for the morgan hill main shock because the fractional change in s wave velocity is greater than the fractional change in p wave velocity it suggests that the opening or connection of fluid filled fractures is the underlying cause the magnitude of the velocity change implies that low effective pressures are present in the source region of the velocity change our results suggest that the changes are predominantly near the stations and shallow but we cannot exclude the possibility that changes occur at greater depth as well if the variations are shallow we may be detecting the lingering effects of nonlinearity during main shock strong ground motion if the variations are deep it suggests that pore pressures at seismogenic depths are high which would likely play a key role in the earthquake process

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

polarities of p and s waves and shear wave splitting observed from the bucaramanga nest colombia the bucaramanga nest colombia is an intense seismic source centered at 6 48 n 73 10 w and 161 km depth beneath the eastern andes during a 3 week period 161 microearthquakes mb 4 3 were recorded by a 260 by 140 km array of 12 three component university of wisconsin uw seismographs centered above the nest eighty eight percent of the events recorded were from the nest providing a unique opportunity to study natural events that have different source parameters but nearly the same ray paths shear wave splitting analysis shows that the average polarization direction of the first arriving shear waves from the nest events at nine of the seismographs is n12 e 18 after corrections based on the amount of discrepancy between the observed average p wave azimuth and the expected event to station azimuth average s wave polarization is n6 e 15 variation in time delays with azimuth and epicentral distance appears to be due to the varying angles between the ray path and the principal axes of the anisotropic system shear wave splitting observed from 17 events along the wadati benioff zone outside the nest shows that the time delays increase with increasing depth and defines a wedge shaped anisotropic volume above the subducting plate the anisotropic layer has a maximum thickness of approximately 60 km at the depth of the nest and is thinning updip the anisotropy most likely results from aligned anisotropic minerals such as olivine and orthopyroxene in a localized mantle flow in the lithosphere above the wadati benioff zone a region weakened by active fluid migration and mobilized by the heating and shearing along the suducting slab

active tectonics of the pamirs and karakorum the source parameters of moderate to large sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three dimensional configuration of the subducted lithosphere in the pamir karakorum intracontinental convergence zone the continental collision has resulted in intense deformation in the upper crust the deformation is distributed over a broad area and is absorbed by thrust and strike slip faulting along the northern margin of the pamirs where the deformation due to continental collision is most intense the northern pamir thrust and several major fault zones mark the present boundary most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region along the pamir front thrust faulting dominates while on the western and eastern edges of the pamirs the style of deformation is characterized by oblique thrusting with a component of strike slip motion thrust type events beneath the tadjik depression indicate that both the sedimentary rocks and the basement are involved in shortening right lateral strike slip motion is observed on the eastern edge of the pamirs where they border the tarim basin and in the talas fergana kun lun fault zone a southward dipping seismic zone beneath the pamirs and karakorum indicates that the asian lithosphere has been subducted along the pamir front to a depth of at least 200 km this interpretation is consistent with published geological and geophysical data focal mechanism solutions of some intermediate depth events beneath the pamirs show strike slip faulting with approximately n s horizontally oriented p axes indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation a 90 km deep event beneath the karakorum is interpreted as occurring in the subducted asian lithosphere the fact that the p axis of this event is oriented parallel to the descending plate suggests downdip compression within the asian lithosphere the overall tectonics of the region is interpreted as a consequence of the underthrusting asian lithosphere being impinged upon by the shallow northward underthrusting indian lithosphere

repeating deep earthquakes evidence for fault reactivation at great depth we have identified three groups of deep earthquakes showing nearly identical waveforms in the tonga slab relocation with a cross correlation method shows that each cluster is composed of 10 to 30 earthquakes along a plane 10 to 30 kilometers in length some of the earthquakes are colocated demonstrating repeated rupture of the same fault and one pair of events shows identical rupture complexity suggesting that the temporal and spatial rupture pattern was repeated recurrence intervals show an inverse time distribution indicating a strong temporal control over fault reactivation runaway thermal shear instabilities may explain temporally clustered earthquakes with similar waveforms located along slip zones weakened by shear heating earthquake doublets that occur within a few hours are consistent with events recurring before the thermal energy of the initial rupture can diffuse away

dehydration kinetics of talc and 10 a phase consequences for subduction zone seismicity abstract the process of dehydration embrittlement is usually proposed as an explanation for the presence of intermediate depth earthquakes in subduction zones it assumes that the release of water by hydrous mineral breakdown is fast enough to provoke brittle failure we performed high pressure high temperature dehydration experiments of talc and 10 a phase coupled with in situ measurement of reaction kinetics using synchrotron x ray diffraction newly developed x ray transparent pressure sealed titanium capsule ensured a closed thermochemical environment from isothermal kinetics data fitted to the avrami s equation and from the texture of reaction products we conclude that dehydration rates of these minerals are limited by diffusion predicted minimum rates of fluid release range from 10 4 to 9 10 6 m 3 fluid m 3 rock s 1 and are fast enough to provoke hydraulic rupture since maxwell relaxation rate of rocks relevant of subduction zones are slower than the rate of fluid release these rates are comparable between talc 10 a phase and antigorite also perrillat j p daniel i koga k t reynard b cardon h crichton w a 2005 kinetics of antigorite dehydration a real time x ray diffraction study earth planet sci lett 236 899 913 consequently we suggest that the dehydration of hydrous minerals may eventually be fast enough to trigger the intermediate depth earthquakes and that the deepest among intermediate depth earthquakes may actually locate the limits for dehydration of hydrous minerals in the downgoing lithosphere

seismological constraints on the mechanism of deep earthquakes temperature dependence of deep earthquake source properties abstract seismological observations are of vital importance for understanding the mechanism of deep earthquakes most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms deep earthquake aftershock sequences show temporal decay rates and magnitude frequency relations b values that are similar to shallow earthquakes with the aftershocks occurring preferentially along the mainshock fault planes in addition deep earthquake rupture velocities are similar to those of shallow earthquakes however there are also some observations that are clearly distinctive relative to shallow earthquakes stress drops of deep earthquakes show a large variation but are larger on average than shallow earthquakes different deep seismic zones show very different b values in contrast to shallow earthquakes which show similar b values worldwide in addition deep earthquakes show fewer aftershocks than shallow earthquakes a variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab both deep earthquake b values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs suggesting that these factors are temperature controlled to an extent much greater than with shallow earthquakes large deep earthquakes in warm slabs show slower rupture velocities larger stress drops and lower seismic efficiencies than similar earthquakes in cold slabs the width of deep seismic zones is also probably temperature controlled but deep earthquake rupture can propagate outside the normal limits of benioff zone seismicity simple thermal models for the tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200 c warmer than the temperature that limits smaller earthquakes in the slab these observations can be used to evaluate physical models for deep earthquakes including brittle slip along fluid weakened faults transformational faulting and thermal and perhaps melt lubricated shear instabilities it is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model since hydrated materials are expected in only a narrow depth zone at the top of the slab and the fault planes do not have the expected orientations for reactivated faults the lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model in which events should be confined within a narrow metastable wedge seismological studies have also failed to find evidence for the existence of metastable olivine in slabs the temperature dependence of deep earthquakes argues in favor of a temperature activated phenomenon such as thermal shear instabilities and perhaps fault zone melting

catch a falling star meteorites and old ice a model for the process of meteorite concentration in blue ice regions of the antarctic ice sheet is proposed based on data from near the allan hills and the assumptions that both meteorite influx and glacial flow have been constant the meteorite influx is calculated to be 60 x 10 6 kilogram per square kilometer per year and the age of the exposed ice to be 0 to 600 000 years varying with distance from the allan hills these results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites providing support for the assumption of steady flow and meteorite influx this may be the oldest sequence of ice in stratigraphic order yet discovered and the results imply that this part of the east antarctic ice sheet has been approximately steady during this time interval

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

radiated seismic energy and earthquake source duration variations from teleseismic source time functions for shallow subduction zone thrust earthquakes 1 earthquake source time functions deconvolved from teleseismic broadband p wave recordings are used to examine rupture variations for 417 underthrusting earthquakes located on the interplate interface in circum pacific subduction zones moment scaled duration of significant moment release varies with depth with longer duration events occurring in the shallowest 20 km of the megathrusts the source time functions are also used to estimate radiated seismic energy two estimates are obtained a simple scaled triangle substitution for the moment release history provides a minimum estimate while integration of the time function shape provides an estimate limited only by the bandwidth of the teleseismic deconvolutions while these energy calculations underestimate total energy they enable systematic comparisons of rupture process within and between subduction zones we do not find significant depth dependence for radiated energy overall but some regions do show mild trends of increasing energy seismic moment ratios e mo with increasing source depth that correspond to rupture duration variations in those regions the observations of longer rupture duration at shallow depth with moderate e mo may be due to heterogeneous friction and structural features on the shallow plate interface index terms 7215 seismology earthquake parameters 7230 seismology seismicity and seismotectonics 8164 tectonophysics stresses crust and lithosphere keywords radiated energy shallow subduction source parameters

delamination or slab detachment beneath vrancea new arguments from local earthquake tomography 1 vrancea located at the southeastern carpathians arc bend is one of the areas in the alpine himalayan belt that features strong earthquakes occurring at intermediate depths 60 200 km in this study we investigated the crustal and lithospheric structure beneath the vrancea seismic area using a local earthquake tomography approach we used an updated and revised catalog spanning from 1982 to 2006 that uses data from both permanent and temporary networks in the target area simultaneous tomographic inversion for the vp and vs anomalies and the vp vs ratio and source locations was done using the lotos code the reliability and robustness of the results were rigorously checked using various tests e g by studying the role of different parameters on the results of the inversion performing the inversion using random data subsets and synthetic modeling the tomography results clearly indicate the presence of a high velocity material beneath vrancea at a depth interval of about 60 200 km that coincides with the distribution of intermediate depth seismicity this result agrees generally with previous tomographic studies we compare two scenarios leading to this structure 1 subduction and slab detachment and 2 drop forming or delamination the latter mechanism presumes that the thickening of the crust due to continent continent collision causes transformation of the mafic lower crust into denser eclogite this material accumulates until it reaches a critical mass at which point it forms a large drop that begins to fall down we propose that the high velocity anomaly we observe in our tomogram might represent the descending eclogitic lower crust material enveloped by the entrained lithosphere it is possible that a similar delamination process can be observed in other parts of the alpine himalayan belt such as in the pamir hindu kush area

source parameters of a m4 8 and its accompanying repeating earthquakes off kamaishi ne japan implications for the hierarchical structure of asperities and earthquake cycle 1 we determine the source parameters of a m4 9 0 1 characteristic earthquake sequence and its accompanying microearthquakes at 50 km depth on the subduction plate boundary offshore of kamaishi ne japan the microearthquakes tend to occur more frequently in the latter half of the recurrence intervals of the m4 9 0 1 events our results show that the microearthquakes are repeating events and they are located not only around but also within the slip area for the 2001 m4 8 event from the hierarchical structure of slip areas and smaller stress drops for the microearthquakes compared to the m4 8 event we infer the small repeating earthquakes rupture relatively weak patches in and around the slip area for the m4 8 event and their activity reflects a stress concentration process and or change in frictional property healing at the area we also infer the patches for the m4 9 0 1 and other repeating earthquakes undergo aseismic slip during their interseismic period

role of shallow phase changes in the subduction of oceanic crust detailed studies of the seismicity of several subduction zones demonstrate that shallow dipping thrust zones turn to steeper angles at depths of about 40 kilometers an increased downward body force resulting from shallow phase changes in subducted oceanic crust may be the cause of this increased dip angle in addition the volume reduction associated with phase changes may produce sufficiently large stresses in neighboring rocks to cause the seismicity of the upper benioff zone

seismic anisotropy above a subducting plate shear wave splitting observed in northeastern colombia has provided evidence of seismic anisotropy in a shear zone immediately above a subducting plate in an upper mantle composed mainly of olivine 57 and orthopyroxene 17 the splitting can be interpreted by wave propagation in an anisotropic medium of orthorhombic symmetry that results from alignment of these intrinsically anisotropic minerals the mechanism of alignment is most likely the shearing associated with the subduction aided by fluids migrating from the subducting plate when the plate exceeds 100 km in depth

physical modeling of the influence of bedrock topography and ablation on ice flow and meteorite concentration in antarctica 1 three dimensional laboratory physical experiments have been used to investigate the influence of bedrock topography and ablation on ice flow different models were tested in a plexiglas box where a transparent silicone simulating ice in nature was allowed to flow experimental results show how the flow field in terms of both flow lines and velocity and variations in the topography of the free surface and internal layers of the ice are strongly influenced by the presence and height of bedrock obstacles in particular the buttressing effect forces the ice to slow down rise up and avoid the obstacle the higher the bedrock barrier the more pronounced the process only limited uplift of internal layers is observed in these experiments in order to exhume deep material embedded in the ice ablation simulated by physically removing portions of silicone from the model surface to maintain a constant topographic depression must be included in the physical models in this case the analogue ice replenishes the area of material removal thereby allowing deep layers to move vertically to the surface and severely altering the local ice flow pattern this process is analogous to the ice flow model proposed in the literature for the origin of meteorite concentrations in blue ice areas of the antarctic plateau

aftershock sequences of moderate sized intermediate and deep earthquakes in the tonga subduction zone intermediate and deep focus earthquakes are generally thought to produce few aftershocks the two year operation of a network of broadband seismographs in the tonga fiji region provides an opportunity to monitor the aftershock activity of moderate sized earthquakes in addition to the march 9 1994 mw 7 6 event that shows 144 aftershocks we found that all nine intermediate and deep earthquakes with mw 6 0 showed some aftershock activity suggesting that aftershock sequences are ubiquitous on average deep tonga earthquakes show an order of magnitude fewer aftershocks above a given magnitude than shallow california earthquakes of similar moment the largest aftershocks of each sequence have an mb that is 2 0 units smaller on average than the mw of the mainshock whereas the california earthquakes show a mean difference of 1 1 since b values of the deeper sequences appear to be similar to shallow sequences the difference in aftershock productivity results from the larger size difference between the mainshock and the larger aftershocks the most prolific sequences show an activity level not far below shallow earthquakes of the same moment this is in contrast with observations of little aftershock activity for deep earthquakes in south america japan and kuriles and suggests that the tonga subduction zone is unusually prolific in aftershock production perhaps due to cold slab temperatures

the physical mechanisms of deep moonquakes and intermediate depth earthquakes how similar and how different abstract we here review the characteristics of deep moonquakes dmq and conclude that they share many common features with certain categories of intermediate depth earthquakes namely the pressure temperature conditions where they occur their occurrence in highly localized nests their tendency to occur repeatedly at the same or nearly the same location and their distribution of sizes b values we thus explore various physical mechanisms proposed for earthquakes that might also explain deep moonquakes these include the possibility that there are partial melts or fluid phases in the lunar mantle that permit dmq to occur at pressures where ordinary brittle fracture is not possible the possibility that stress concentration increases tidal stresses at certain locations where moonquakes occur repeatedly and the possibility that dmq occur at relatively low stresses because rock strength is lowered by a fatigue process induced by the repetitive application of tidal stresses the similarities between dmq and terrestrial earthquakes imply that research focusing on the properties of both phenomena may help us understand the physical origins of both

on the glaciological meteorological and climatological significance of antarctic blue ice areas this paper presents the current state of our knowledge about antarctic blue ice areas scattered widely over the antarctic continent yet covering only approximately 1 of its surface area blue ice areas have some peculiar characteristics that make them of particular interest for planetary geologists glaciologists meteorologists and climatologists for instance blue ice areas are subject to net ablation mainly by sublimation since any accumulation is easily swept off their smooth surfaces by winds moreover they are significantly warmer than their surroundings and the wind over them is generally more gusty an important feature of blue ice is its relatively low albedo 0 56 compared with that of snow 0 80 blue ice is probably considerably smoother than snow in the aerodynamic sense with roughness lengths as low as 3 10 6 m which indicates that blue ice is probably the only permanent natural surface over which the air flow is mainly in the aerodynamic smooth regime further since it is likely that very old ice is exposed at the surface of some blue ice areas this ice would be of great interest for paleoclimatic studies it has also been suggested that blue ice areas are sensitive to climate change this would make them useful as antarctic climate indicators because they can easily be monitored by satellite finally this paper addresses several gaps in our knowledge about blue ice areas and gives suggestions for future blue ice area research

the pamir hindu kush seismic zone as a strain marker for flow in the upper mantle we introduce a new hypothesis to explain the unusual pattern of intermediate depth earthquakes in the pamir hindu kush region of central asia in our model the seismicity of the zone is defined by a remnant piece of oceanic crust that became neutrally buoyant and now hangs in the mantle underneath the center of the active mountain belt we interpret an abrupt cutoff in seismicity shallower than 70 km under the pamir as evidence that the slab has become decoupled from the surface deformation and we suggest that the remnant slab has acted as a strain marker for flow in the upper mantle the contortion of the slab can be explained by superimposed vertical and lateral simple shear related to the western edge of the indian plate this model requires relatively low strain rates of the order of 10 15 s 1 consistent with strain rates estimated from earthquake moment release in benioff zones in contrast the double facing subduction zone model applied by a number of previous authors requires strain rates of the order of 10 13 s 1 which are much higher than inferred strain rates in subducting slabs the results are important for understanding mantle dynamics providing a unique way to measure strain in the upper mantle the implied flow field also has significant implications for the kinematics of the entire collisional plate boundary in tibet and the tien shan

instability of deformation this chapter addresses the mechanisms by which instabilities can develop in rocks in the nominally ductile regime under conditions that the authors believe are relevant to earth we will not address brittle failure or frictional sliding processes in detail but much of our discussion will be couched in terms of knowledge gained in studies of these processes as a consequence we will begin by discussing why brittle failure and frictional sliding cannot be the mechanism by which rocks become unstable at depth despite the strong evidence that they are the underlying mechanism of earthquakes at shallow depths e g scholz 1990 2002 brittle shear failure in the normal sense is fundamentally a tensile process tensile microcracks must first nucleate and self organize via interaction of the stress concentrations at their tips as a consequence brittle failure and frictional sliding are strongly inhibited by pressure because work must be done against the pressure to open the mode i cracks the inhibition is so strong that the stress required to create a fault or initiate sliding on an existing fault becomes greater than the room temperature flow stress of many rocks at pressures equivalent to only a few tens of kilometers in earth increasing temperature has little effect on brittle processes but in contrast the ductile flow stress of rocks falls exponentially with temperature because both pressure and temperature increase with depth in earth earthquakes by unassisted brittle fracture mechanisms or frictional sliding can only occur at depths less than 30 50 km nevertheless earthquakes occur to depths of almost 700 km in earth and seismological evidence demonstrates unequivocally that they occur by displacement across a surface or narrow zone i e they occur by faulting this conundrum can be explained qualitatively by certain specific types of mineral reactions occurring under certain restricted conditions the

current states of stress in the northern andes as indicated by focal mechanisms of earthquakes abstract systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern andes reveals relatively homogeneous patterns of crustal stress in three main regions the first region presently under the influence of the caribbean plate includes the northern segment of the eastern cordillera of colombia and the western flank of the central cordillera north of 4 n it is characterized by wnw ese compression of dominantly reverse type that deflects to nw se in the merida andes of venezuela where it becomes mainly strike slip in type a major bend of the eastern thrust front of the eastern cordillera near its junction with the merida andes coincides with a local deflection of the stress regime sw ne compression suggesting local accommodation of the thrust belt to a rigid indenter in this area the second region includes the sw pacific coast of colombia and ecuador currently under the influence of the nazca plate in this area approximately e w compression is mainly reverse in type it deflects to wsw ene in the northern andes south of 4 n where it is accommodated by right lateral displacement of the romeral fault complex and the eastern front of the northern andes the third and most complex region is the area of the triple junction between the south american nazca and caribbean plates it reveals two major stress regimes both mainly strike slip in type the first regime involves sw ne compression related to the interaction between the nazca and caribbean plates and the panama micro plate typically accommodated in an e w left lateral shear zone the second regime involves nw se compression mainly related to the interaction between the caribbean plate and the north andes block which induces left lateral displacement on the uramita and romeral faults north of 4 n deep seismicity about 150 170 km concentrates in the bucaramanga nest and cauca valley areas the inversion reveals a rather homogeneous attitude of the minimum stress axis which dips towards the e this extension is consistent with the present plunge of the nazca and caribbean slabs suggesting that a broken slab may be torn under gravitational stresses in the bucaramanga nest this model is compatible with current blocking of the subduction in the western northern andes inhibiting the eastward displacement of slabs which are forced to break and sink in to the asthenosphere under their own weight

single link cluster analysis of earthquake aftershocks decay laws and regional variations using single link cluster analysis the authors investigate how various properties of aftershock sequences depend on their tectonic regime and focal depth for international seismological centre earthquakes of m sub b ge 4 8 they find that earthquakes deeper than 70 km have the fewest and smallest aftershock sequences even after accounting for differences in detectability and maximum magnitude they find that ridge transform earthquakes have smaller aftershock sequences that shallow subduction zone earthquakes among different subduction zones they find that zones with high moment release rates possess larger aftershock sequences comparing ridge transform zones they find those with slower spreading rates possess larger aftershock sequences by transposing origin times of several different aftershock sequences as if all had main shocks occurring at time zero they are able to study the properties of aftershock sequences which individually have too few aftershocks to study by other means secondary events determined by single link cluster analysis follow a modified omori s power law decay for time separations of 0 1 day to 20 days from the parent event with p values ranging from 0 539 plus minus 0 022 intermediate and deep focus earthquakes to 0 928 plus minus 0 024 ridge transform earthquakes they find that earthquake foreshocks and multiplets also follow a modified omori s law atmore greater times from the main shock the decay is steeper than a power law decay more like an exponential decay aftershocks in the adak catalog m sub b ge 2 0 show a marked decrease in activity between 40 and 50 km depth they speculate that the observed differences in number of aftershocks and p values may be caused by variations in fault heterogeneity or in fluid pressures less

implications of spatial and temporal development of the aftershock sequence for the mw 8 3 june 9 1994 deep bolivian earthquake on june 9 1994 the mw 8 3 bolivia earthquake 636 km depth occurred in a region which had not experienced significant deep seismicity for at least 30 years the mainshock and aftershocks were recorded in bolivia on the banjo and seda broadband seismic arrays and on the san calixto network we used the joint hypocenter determination method to determine the relative location of the aftershocks we have identified no foreshocks and 89 aftershocks m 2 2 for the 20 day period following the mainshock the frequency of aftershock occurrence decreased rapidly with only one or two aftershocks per day occuring after day two the temporal decay of aftershock activity is similar to shallow aftershock sequences but the number of aftershocks is two orders of magnitude less additionally a mb 6 apparently triggered earthquake occurred just 10 minutes after the mainshock about 330 km east southeast of the mainshock at a depth of 671 km the aftershock sequence occurred north and east of the mainshock and extends to a depth of 665 km the aftershocks define a slab striking n68 w and dipping 45 ne the strike dip and location of the aftershock zone are consistent with this seismicity being confined within the downward extension of the subducted nazca plate the location and orientation of the aftershock sequence indicate that the subducted nazca plate bends between the nnw striking zone of deep seismicity in western brazil and the n s striking zone of seismicity in central bolivia a tear in the deep slab is not necessitated by the data a subset of the aftershock hypocenters cluster along a subhorizontal plane near the depth of the mainshock favoring a horizontal fault plane the horizontal dimensions of the mainshock beck et al this issue silver et al 1995 and slab defined by the aftershocks are approximately equal indicating that the mainshock ruptured through the slab

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

comprehensive analysis of earthquake source spectra in southern california we compute and analyze p wave spectra from earthquakes in southern california between 1989 and 2001 using a method that isolates source receiver and path dependent terms we correct observed source spectra for attenuation using both fixed and spatially varying empirical green s function methods estimated brune type stress drops for over 60 000 m l 1 5 to 3 1 earthquakes range from 0 2 to 20 mpa with no dependence on moment or local b value median computed stress drop increases with depth in the upper crust from about 0 6 mpa at the surface to about 2 2 mpa at 8 km where it levels off and remains nearly constant in the midcrust down to about 20 km however the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop spatially coherent variations in median stress drop are observed with generally low values for the imperial valley and northridge aftershocks and higher values for the eastern transverse ranges and the north end of the san jacinto fault we find no correlation between observed stress drop and distance from the san andreas and other major faults significant along strike variations in stress drop exist for aftershocks of the 1992 landers earthquake which may correlate with differences in main shock slip

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

microseismicity and focal mechanisms of the intermediate depth bucaramanga nest colombia the bucaramanga nest of intermediate depth seismicity in northern colombia was investigated from microearthquakes m sub b less than or equal to4 3 recorded from june 20 to july 6 1979 by a local array of 12 digital three component seismographs of 161 events studied 142 are from the nest and 19 define a thin wadati benioff w b zone mainly from 110 to 190 km depth the w b zone part of the bucaramanga segment strikes n10 sup 0 e with a dip increasing wtih depth from 30 sup 0 e to near vertical beneath the nest the 89 best located events from within the nest define a volume of about 8 km nw by 4 km ne by 4 km depth centered at 161 km depth with relative location accuracies of 0 5 to 1 0 km double couple fault plane solutions of 59 events were determined from an objective procedure using p wave polarities and s p amplitude ratios there is no dominant trend in focal mechanisms nor any reasonable correlation with regional stress patterns or previously determined mechanisms from teleseismic data the subduction and assimilation of a linear feature such as an island arc undergoing a phase transformation may explain the existence of the bucaramanga nest however the extraordinarilymore high rate of seismicity suggests that the nest may be moving rate several orders of magnitude faster than the rate of relative plate motion alone the intense seismicity and variation in focal mechanisms suggest that the nest represents some aspect of stress release associated with the ascent of fluids perhaps brine or magma that will ultimately result in the initiation of volcanic activity over this recently emplaced w b zone copyright american geophysical union 1987 less

insights into the mechanism of intermediate depth earthquakes from source properties as imaged by back projection of multiple seismic phases 1 this study investigates the spatial and temporal distribution of energy release of large intermediate depth earthquakes using a modified back projection technique first used to study the 2004 sumatra andaman megathrust event multiple seismic phases are included in the back projection analysis which provides the capability to determine the energy distribution with respect to depth and time a total of 22 intermediate depth earthquakes with moment magnitudes greater than or equal to 6 5 are investigated with hypocentral depths between 100 and 300 km for most of these events the vertical extent of energy release is either below the resolution of this study 5 km or slightly above 15 km this observation agrees with previous studies that find large intermediate depth earthquakes have subhorizontal rupture planes the results also show a significant portion of the events have multiple rupture planes that are well separated in depth the closeness in time of the ruptures on separate planes and the distance between the planes suggest dynamic triggering where the p waves from the first rupture initiate rupture on the second plane we propose that a dehydration embrittlement mechanism combined with preferentially hydrated subhorizontal faults can explain the observations of dominant subhorizontal rupture planes and the frequent occurrence of rupture complexity involving multiple subevents

do intermediate and deep focus earthquakes occur on preexisting weak zones an examination of the tonga subduction zone it has been proposed that intermediate and deep focus earthquakes occur on preexisting planes of weakness that were created at shallow depth the purpose of this study is to test this hypothesis we examined fault plane solutions for 360 events that occurred in the tonga subduction zone from january 1976 to february 1999 for events in the slab each fault plane solution is rotated by the slab dip angle so that the slab is in its original horizontal position we then compare fault plane solutions for events in the slab with those for events at the outer rise we find that an asymmetric fault system corresponding to that found for outer rise events persists down to about 450 km depth this suggests that earthquakes down to this depth are caused by the reactivation of preexisting faults created in the oceanic plate before subduction a similar pattern though less well constrained is found in the kurile subduction zone down to 200 km depth for earthquakes deeper than 450 km the fault plane solutions are much more scattered in some places the fault plane pattern suggests the creation of new fault planes along orientations of maximum shear with zero internal friction the pattern of fault plane solutions in particular localized volumes cannot be easily explained by the creation of new faults in the direction of maximum shear under a coherent regional tectonic stress field thus either a highly heterogeneous stress field exists or preexisting planes of weakness also account for some deep earthquakes

velocity structure of the vrancea region romania abstract a set of 26 earthquakes recorded by a newly installed telemetered network was used for a simultaneous inversion of hypocenters and velocities the travel times and their partial derivatives were computed for flat lying layers of constant velocity gradients after a velocity model was found and the 26 foci relocated three different velocity models for three different zones covered by the network were succesfully inverted with the foci kept fixed the procedure was applied for first arriving p waves and also tried for s wave arrivals two low velocity zones were put into evidence

earthquakes and plastic deformation of anhydrous slab mantle in double wadati benioff zones double wadati benioff seismic zones dsz with two parallel planes of seismicity separated by 15 30 km are a global feature of subduction zones in the 50 200 km depth range upper plane seismicity is generally attributed to dehydration of the oceanic crust but the origin of the lower seismicity plane is debated serpentine or hydrous phase dehydration embrittlement is a commonly advocated mechanism that implies significant slab mantle hydration high resolution seismic tomography revealed low seismic velocities in the lower seismicity plane that are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization earthquakes correlate with anisotropic planar shear zones and favor a shear instability mechanism as the cause of lower plane seismicity without requiring the presence of water in the center of subducting slabs the contribution of the subducted lithospheric mantle to the water budget of subduction zones is thus likely limited to the first 2 3 kilometers beneath oceanic crust

estimate of earthquake hazard in the vrancea romania region a maximum likelihood method is used to estimate the earthquake hazard parameters maximum magnitudemmax annual activity rate and theb value of the gutenberg richter equation in the vrancea romania region the applied procedure permits the use of mixed catalogs with incomplete historical as well as complete instrumental parts the consideration of variable detection thresholds and the incorporation of earthquake magnitude uncertainty

effect of slab temperature on deep earthquake aftershock productivity and magnitude frequency relations deep earthquakes generally show fewer aftershocks and a larger variability in magnitude frequency relations than shallow earthquakes1 5 these characteristics and the factors that control them may place constraints on the mechanism of deep earthquakes which is still uncertain6 8 here we show that systematic variations in aftershock productivity and magnitude frequency relations are related to the temperature of the downgoing slab as inferred from the vertical velocity and age of the subducting lithosphere deep earthquakes with substantial aftershock sequences are found only in colder slabs and earthquakes in warmer slabs consistently show few aftershocks magnitude frequency relations for deep earthquakes also show systematic variations as a function of slab thermal structure with warmer slabs showing fewer small earthquakes lower b values the statistical properties of deep earthquakes thus appear to be temperature sensitive to an extent not observed for shallow earthquakes and not adequately explained by simple geometrical limitations on fault width

interrelations between intermediate depth earthquakes and fluid flow within subducting oceanic plates constraints from eclogite facies pseudotachylytes field evidence preserved in ancient subducted oceanic crust documents that eclogite facies frictional failure with melting pseudotachylyte formation during intermediate depth earthquakes was accompanied and followed not preceded by infiltration of external fluids and progressive vein formation in the eclogites eclogitization began during seismic failure and fluid passage through the shear zone subsequent fluid flow produced hydraulic fracturing and continuous vein formation during ongoing burial we suggest that this kind of shear zone may allow channellized fluid flow within and out of slabs

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

unravelling the high altitude nansen blue ice field meteorite trap east antarctica and implications for regional palaeo conditions abstract antarctic blue ice zones the most productive locations for meteorite recovery on earth contain old ice that is easily accessible and available in large quantities however the mechanisms behind these meteorite traps remain a topic of ongoing debate here we propose an interdisciplinary approach to improve our understanding of a meteorite trap in dronning maud land east antarctica on the nansen blue ice field meteorite trap 2600 3100 m above sea level where more than half of the asuka meteorites have been collected based on 185 surface blue ice samples one of the largest observed spatial patterns in oxygen isotopic variation to date is found relying on meteorites for which the terrestrial ages are determined using 14c and 36cl this surface ice is interpreted to date from the last interglacial up to the present day by combining state of the art satellite derived surface velocities surface mass balance modelling and ice flow modelling we estimate that about 75 85 of the meteorites found on the ice field were supplied by ice flow after entering the ice sheet in an accumulation area of a few hundred square kilometres located south upstream of the ice field less than 0 4 new meteorites per year are supplied to the ice field through ice flow suggesting that the hundreds of meteorites found 25 years after the first visit to this ice field mostly represent meteorites that were previously not found rather than newly supplied meteorites by combining these findings the infall rate of meteorites from space is estimated which is in line with values from the literature but situated at the higher end of the range a comparison of the oxygen isotopic variation of the surface blue ice to that of the european project for ice coring in antarctica epica dronning maud land edml ice core located 750 km to the west at the same elevation suggests that the regional changes in topography have been relatively limited since the last interglacial supporting theories of an overall stable east antarctic ice sheet eais over this time period

brittle ductile transition and associated seismicity experimental and numerical studies and relationship with the b value the acoustic emission ae and the mechanical behavior of granite samples during triaxial compression tests have been analyzed the size of ae events displays power law distributions conforming to the gutenberg richter law observed for earthquakes which is characterized by the b value as the confining pressure increases the macroscopic behavior becomes more ductile for all different stages of the rock mechanical behavior linear nonlinear prepeak nonlinear postpeak shearing there is a systematic decrease of the b value with increasing confining pressure a numerical model based on progressive elastic damage and the finite element method allows simulations of the main experimental observations on ae and of a wide range of macroscopic behaviors from brittleness to ductility the model reproduces a decrease in the b value that appears to be related to the type of macroscopic behavior brittle ductile rather than to the confining pressure both experimental and numerical results suggest a relationship between the b value and the brittle ductile transition moreover these results are consistent with recent earthquake observations and give new insight into the behavior of the earth s crust

meteorites constrain the age of antarctic ice at the frontier mountain blue ice field northern victoria land abstract we show that meteorites can provide chronological constraints upon the age of the ice cropping out at the frontier mountain meteorite trap antarctica when their terrestrial age is placed in a glaciological context amongst the over 700 meteorites found so far frontier mountain fro 84001 99028 93005 and 93054 were most likely not wind drifted across the ice field since their masses 772 1665 g are much heavier than the local 200 g wind transport threshold the four meteorites were found along a stretch of ice where a representative section of the frontier mountain blue ice crops out based on the bedding of englacial tephra layers the structure of the ice along the section appears to be essentially an up glacier dipping monocline the 14 c terrestrial age of fro 8401 99028 and 93005 are 13 2 21 3 and 27 2 ky respectively the 41 ca 36 cl age of fro 93054 is 40 10 ky the terrestrial ages of the four meteorites increase from the top to the bottom layers of the monocline this geographic distribution is best explained by delivery of meteorites at the ice surface through the ice flow model i e englacial transport from the snow accumulation zone and exhumation in the blue ice area through ablation rather than direct fall since the effect of ablation in decoupling terrestrial ages of meteorites and the age of the ice on which they sit must have been minor most likely 7 ky based on the local ice dynamics we conclude that the age of the bulk of the ice body currently under ablation at frontier mountain is up to 50 ky old this result has implications on both the meteorite concentrations mechanism at frontier mountain and the regional ice dynamics

antarctic blue ice moraines analogue for northern hemisphere ice sheets abstract this paper reviews the distribution character and age of blue ice moraines in antarctica and asks whether there are implications for the study of former pleistocene ice sheets in the northern hemisphere blue ice forms where acceleration of downslope katabatic winds removes snow and causes ice to ablate upward ice flow compensates for the surface ablation and in some places brings rock debris to the surface to form a blue ice moraine blue ice moraines occur where topography focuses katabatic winds notably outlet glaciers cutting through mountains and in the lee of nunataks and escarpments nearer the coast many antarctic blue ice moraines have been accumulating for millions of years the cyclic growth and decay of pleistocene ice sheets and the dominance of surface ablation near the ice sheet margins are clearly different yet there are aspects that apply to pleistocene ice sheets based on antarctic blue ice deposits equivalent deposits associated with former pleistocene ice sheets are likely to 1 lie in topographic sediment traps such as in side valleys or embayments next to outlet glaciers 2 occur in the lee of mountains 3 display a morphology indicating ice flow into the embayment or towards the mountain front 4 include a wide range of lithologies derived from the inland ice sheet yet consist wholly of local debris in places 5 accumulate a thick deposit perhaps over successive glaciations further 6 the location and intensity of moraine formation will change spatially and vertically in response to changes in the relative elevation of the surrounding mountains and its effect on ice flow the extent to which these criteria will help in interpreting the behaviour of pleistocene ice sheets is uncertain but we use examples from the greenland laurentide and eurasian ice sheets to suggest that the concept of enhanced ablation by katabatic winds encouraging surface moraine formation helps resolve several puzzles

kinematics of retreating subduction along the carpathian arc romania kinematic axes and resultant vectors of displacement along the carpathian arc and the apuseni mountains aid in the reconstruction of retreating subduction along the carpathians the recent vertical position of the subducted slab below the vrancea area of the eastern carpathians represents the final rollback stage of a small fragment of oceanic lithosphere formerly situated between the moesian and east european plates the ages and locations of the eruption centers of the andesitic volcanic chain along the carpathian arc in the overriding plate as well as the thrust directions are used as markers to reconstruct the rollback area of the subducted slab between the moesian and east european plates

frequency distribution and quantification of deep earthquakes the frequency distribution of deep earthquakes d 350 km is analyzed on a regional scale using magnitude data from 4660 events reported in the international seismological center bulletin 1964 1982 and 391 centroid moment tensor solutions 1977 1987 we use the moment magnitude mw to define b values comparable with studies of shallow seismicity scaling laws relating mw mb and mo are derived mw 1 6mb 3 2 mb log mo 4 3 2 4 which compare very favorably with published laws uncertainties and biases of the mb and mo data sets are considered the slope of the frequency distribution is evaluated over a restricted magnitude range the results are very robust and independent from the magnitude range used from the mb magnitude data we obtain regional b values in the range 0 4 b 1 22 with a worldwide average value b 0 87 these results are confirmed by the analysis of the seismic moment data the steepest frequency distribution is observed in the tonga islands region b 1 22 although 60 of the worldwide deep seismicity occurs here only few events since 1977 had seismic moment exceeding 1019 n m the high fractal dimension d 2 4 implies a volume partially filled with fault planes in agreement with the observations on the seismic morphology of the region at the other extreme south america is characterized by the largest recorded deep events and an almost complete absence of small ones b 0 4

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

tomographic imaging of deep velocity structure beneath the eastern and southern carpathians romania implications for continental collision the vrancea seismic zone located beneath the elbow of the eastern and southern carpathians is characterized by intermediate depth seismicity associated with subduction and plate collision among many strong earthquakes that have occurred in the region the 1977 vrancea earthquake march 4 1977 ms 7 1 depth of 86 km caused extensive damage and fatalities the vrancea seismic zone is defined by a tabular region of seismicity that extends from the surface to a depth of 200 km the small size about 80 km in length and 40 km in width and geometry of the seismic zone have made it difficult to interpret the kinematics of subduction and continental collision in the romanian carpathians to better understand the collision subduction environment body wave seismic tomography was used to image the deep structure beneath the eastern and southern carpathians in romania a total of 7837 p and 2292 s arrival times from 433 shallow and intermediate depth earthquakes recorded by local and regional seismic stations were inverted for the three dimensional 3 d seismic velocity structure the travel times and ray paths were calculated by an efficient 3 d ray tracing technique by utilizing the lsqr algorithm p and s wave velocity perturbations were determined from the surface down to a depth of 200 km the high resolution p wave tomographic images reveal a pattern of broad heterogeneity in the velocity structure of the region one of the most important features is a high velocity body existing beneath the carpathian arc at depths between 100 and 170 km this high velocity body outlines a slab dipping near vertically where most of the intermediate depth earthquakes occur beneath the transylvanian basin a high velocity zone is dipping toward the se and may represent the base of lithosphere which is in convergence with the subducted slab low velocity bodies are found along the forearc of the carpathian foredeep down to depths of 70 to 80 km these low velocity bodies may be correlated with the shallow sedimentary layer in the foreland and may represent crustal materials wedged and underthrust along with the subducted lithosphere