seasonal cycle of mesoscale instability of the west spitsbergen current abstractthe west spitsbergen current wsc is a topographically steered boundary current that transports warm atlantic water northward in fram strait the 16 yr 1997 2012 current and temperature salinity measurements from moorings in the wsc at 78 50 n reveal the dynamics of mesoscale variability in the wsc and the central fram strait a strong seasonality of the fluctuations and the proposed driving mechanisms is described in winter water is advected in the wsc that has been subjected to strong atmospheric cooling in the nordic seas and as a result the stratification in the top 250 m is weak the current is also stronger than in summer and has a greater vertical shear this results in an e folding growth period for baroclinic instabilities of about half a day in winter indicating that the current has the ability to rapidly grow unstable and form eddies in summer the wsc is significantly less unstable with an e folding growth period of 2 days observations of the eddy kinetic energy eke show a p

three dimensional dynamic simulation of the 1992 landers earthquake the 1992 landers california earthquake magnitude 7 3 was modeled as the propagation of a spontaneous rupture controlled by a realistic prestress distribution with the use of a three dimensional finite difference method the dynamic rupture reproduces the general slip pattern used to compute the initial stress distribution and generates near fault ground motions at the surface similar to observations the simulated rupture propagates on the fault along a complex path with highly variable speed and rise time changing the magnitude and pattern of the stress significantly the method provides the framework to estimate earthquake rupture parameters from recorded seismic and geodetic data

planetary science a chunk of ancient mars analysis of a meteorite found in northwest africa prosaically named nwa 7533 indicates that it is the first sample of the regolith or soil of mars and is derived from the earliest martian igneous crust yet identified see letter p 513 this paper identifies the nwa 7533 meteorite from northwest africa as the first sample of martian highlands rock in the meteorite collection munir humayun and co authors show that nwa 7533 has a composition indicative of a highlands breccia it also contains zircons more than 4 4 billion years old implying that early crustal differentiation on mars occurred in the first 100 million years of its history coeval with earliest crust formation on the moon and the earth

a two asperity fault model with wave radiation abstract we study the dynamics of a fault containing two asperities having different strengths and subject to a constant strain rate the fault is modeled by a discrete dynamical system with two degrees of freedom that are the slip deficits of the asperities the radiation of elastic waves during the slipping modes is taken into account by introducing a term proportional to slip rate in the equations of motion we give a complete analytical solution of the four dynamic modes of the system any seismic event can be expressed as a sequence of modes for which the moment rate the spectrum and the total seismic moment can be calculated we consider the energy budget of the event and calculate the seismic efficiency the presence of radiation may remarkably change the evolution of the system from a given state since it moves the boundaries between the different subsets of the sticking region in addition the slip amplitude in a seismic event is smaller while the slip duration is longer in the presence of radiation the shape of the moment rate function depends on the seismic efficiency and the seismic moment decreases with increasing efficiency at constant radiated energy as an application of the model we consider the 1964 great alaska earthquake and calculate the mode durations the slip distribution the moment rate function and the seismic moment of the earthquake obtaining values that are consistent with observations

complex events in a fault model with interacting asperities abstract the dynamics of a fault with heterogeneous friction is studied by employing a discrete fault model with two asperities of different strengths the average values of stress friction and slip on each asperity are considered and the state of the fault is described by the slip deficits of the asperities as functions of time the fault has three different slipping modes corresponding to the asperities slipping one at a time or simultaneously any seismic event produced by the fault is a sequence of n slipping modes according to initial conditions seismic events can be different sequences of slipping modes implying different moment rates and seismic moments each event can be represented geometrically in the state space by an orbit that is the union of n damped lissajous curves we focus our interest on events that are sequences of two or more slipping modes they show a complex stress interchange between the asperities and a complex temporal pattern of slip rate the initial stress distribution producing these events is not uniform on the fault we calculate the stress drop the moment rate and the frequency spectrum of the events showing how these quantities depend on initial conditions these events have the greatest seismic moments that can be produced by fault slip as an example we model the moment rate of the 1992 landers california earthquake that can be described as the consecutive failure of two asperities one of which has a double strength than the other and evaluate the evolution of stress distribution on the fault during the event

mixing in the west spitsbergen current abstract twenty eight profiles of temperature and salinity down to 1 km depth have been measured in the area north from the coast of svalbard to latitude 84 40 n between 0 and 32 e longitude they show atlantic waters of the west spitsbergen current entering the arctic ocean in a bifilimentary mode one filament following the coastline and the other 300 km farther north apparently following the continental shelf break the profiles show these filaments mixing into the ambient water in a series of intrusive layers which are remarkably uniform over distances of hundreds of kilometers a triple peak structure at the atlantic water maximum is the dominant oceanographic characteristic of the area with salt fingering occurring on the upper surface of the cold intrusions and indications of double diffusive exchanges through the lower surface a system or eddies or meanders is postulated as the mechanism for the production of multiple fronts allowing the intrusive layers to extend to distances much greater t

long term dynamics of a fault with two asperities of different strengths summary a fault made of two segments or asperities having different strengths and subject to a constant strain rate is considered the fault is modelled by a discrete dynamical system made of two blockscoupledbyaspringandpulledatconstantvelocityonaroughplane wegiveacomplete analytical solution for the evolution of the system the long term behaviour of the fault is studied by calculating the orbits of the system in the phase space the dynamics of the system has four different modes and produces a variety of behaviours the asperities may slip one at a time originating a medium size earthquake or simultaneously originating a large earthquake in the latter case the focus may be at one asperity or at the other in some cases the weaker asperity may slip twice both before and after the failure of the stronger asperity the time pattern of the seismic events generated by the fault is controlled by the stress distribution on the asperities and may include many different behaviours we devise suitable correspondence rules allowing application of the discrete model to real fault systems as an example the model is applied to the 1964 great alaska earthquake which was originated by the failure of two asperities and a possible evolution of this fault system is discussed

coseismic and postseismic slip of the 2004 parkfield earthquake from space geodetic data we invert interferometric synthetic aperture radar insar data jointly with campaign and continuous global positioning system gps data for slip in the coseismic and postseismic periods of the 2004 parkfield earthquake the insar dataset consists of eight interferograms from data collected by the envisat and radarsat satellites spanning the time of the earthquake and variable amounts of the postseismic period the two datasets complement each other with the insar providing dense sampling of motion in the range direction of the satellite and the gps providing more sparse but three dimensional measurements of ground motion the model assumes exponential decay of the postseismic slip with a decay time constant of 0 087 years determined from time series modeling of continuous gps and creepmeter data we find a geodetic moment magnitude of m 6 2 for a 1 day coseismic model and m w 6 1 for the entire postseismic period the coseismic rupture occurred mainly in two slip asperities one near the hypocenter and the other 15 20 km north postseismic slip occurred on the shallow portions of the fault and near the rupture areas of two m 5 0 aftershocks a comparison of the geodetic slip models with seismic moment estimates suggests that the coseismic moment release of the parkfield earthquake is as little as 25 of the total this underlines the importance of aseismic slip in the slip budget for the parkfield segment online material complete data tables and supplemental tables

initial investigation of the landers california earthquake of 28 june 1992 using terrascope the 1992 landers earthquake m s 7 5 m w 7 3 was recorded at six terrascope stations in southern california peak accelerations ranged from 0 16 g at svd 63 km to 0 0092 g at isa 245 km decreasing with distance away from the fault zone the peak velocity showed a different pattern reflecting the rupture directivity from south to north the largest peak velocity 19 cm sec was observed at gsc 125 km moment tensor inversion of long period surface waves yielded a mechanism with m 0 1 1 10 27 dyne cm m w 7 3 dip 74 rake 176 and strike 340 inversion of teleseismic p and s waves revealed two distinct sub events of 6 and 8 sec duration and about 10 sec apart the source parameters for the first and second events are m 0 1 9 10 26 dyne cm dip 83 rake 179 strike 359 and m 0 6 1 10 26 dyne cm dip 87 rake 178 strike 333 respectively the radiated wave energy e s was estimated as 4 3 10 23 ergs the ratio e s m 0 3 9 10 4 corresponds to a stress drop of 280 bars and suggests that the landers earthquake belongs to the group of high stress drop earthquakes and occurred on a fault with a long recurrence time the rupture directivity can be seen clearly in the records from pfo 68 km located to the south and gsc located to the north of the epicenter the maximum displacement at pfo is only 13 of that at gsc despite the shorter epicentral distance to pfo than to gsc the slip distribution determined with the empirical green s function method indicates that the landers earthquake consists of two distinct sub events about 30 km apart with the second sub event to the north being about twice as large as the first one this slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field

branching of atlantic water within the greenland spitsbergen passage an estimate of recirculation atlantic water aw supplies the largest source of heat mass and salt to the arctic ocean via fram strait greenland spitsbergen passage yet it represents only a fraction of the atlantic water that resides in the greenland iceland norwegian and barents seas this is a result of both the branching of the central core of aw along its northward flow and the modification of its t s signature through air sea ice interactions and internal mixing this paper addresses the quantitative analysis of the three dominant atlantic water cores within fram strait and north of 76 n using an 11 year 1977 to 1987 hydrographic database spatial variations of heat volume and salt along its flow path of some 600 km showed that the major core of atlantic water that directly enters the arctic ocean svalbard branch did not extend past 20 e of the 9719 km3 of atlantic water existing within the region one third resided within the svalbard branch the remainder 22 and 45 were held within the return atlantic current and the yermak branches respectively restricting the analysis to a southern limit of 79 n effectively removed the return atlantic current and showed a nearly equal split between the two remaining branches work completed by bourke et al 1988 indicated that the yermak branch is largely recirculated to the south if true this analysis supports rudels 1987 model estimate of a 50 recirculation of aw within this region

effect of stress perturbations on the dynamics of a complex fault a plane fault containing two asperities subject to a constant strain rate by the motion of tectonic plates is considered the fault is modelled as a discrete dynamical system where the average values of stress friction and slip on each asperity are considered the state of the fault is described by the slip deficits of the asperities we study the behaviour of the system in the presence of stress perturbations that are supposed to be due to dislocations of neighbouring faults the fault complexity entails consequences that are not present in the case of a homogeneous fault a stress perturbation not only changes the occurrence time of the following earthquake but may also sensitively change the slip amplitude and area hence the seismic moment of the earthquake as well as the position of its hypocentre the greatest changes take place when simultaneous slip of asperities is involved a coulomb stress value can be assigned to each asperity the change in the difference between the coulomb stresses of the two asperities is a measure of how much the system gets closer to or farther from the condition for simultaneous slip as an example we consider the effect of the 1960 great chilean earthquake on the two asperity fault that produced the 2010 maule earthquake and calculate the changes in the moment rate and in the total seismic moment it results that in the absence of the 1960 earthquake the maule earthquake would have occurred several decades later and would have involved a different sequence of modes so that the moment rate function would have been very different with a longer duration and a greater seismic moment

the rupture process and tectonic implications of the great 1964 prince william sound earthquake we have determined the rupture history of the march 28 1964 prince william sound earthquake m w 9 2 from long period wwssn p wave seismograms source time functions determined from the long period p waves indicate two major pulses of moment release the first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture the second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds because of the large size of this event and thus a deficiency of on scale digitizable p wave seismograms it is impossible to uniquely invert for the location of moment release however if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault the first moment pulse most likely initiated near the epicenter at the northeastern down dip edge of the aftershock area and then spread over the fault surface in a semi circular fashion until the full width of the fault was activated the rupture then extended toward the southwest approximately 300 km ruff and kanamori 1983 the second moment pulse was located in the vicinity of kodiak island starting at 500 km southwest of the epicenter and extending to about 600 km although the aftershock area extends southwest past the second moment pulse by at least 100 km the moment release remained low we interpret the 1964 prince william sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major but smaller asperity in the kodiak island region

early spring oceanic heat fluxes and mixing observed from drift stations north of svalbard abstract from several drifting ice stations north of svalbard norway observations were made in early spring of the ocean turbulent characteristics in the upper 150 m using a microstructure profiler and close to the under ice surface using eddy correlation instrumentation the dataset is used to obtain average heat fluxes at the ice water interface in the mixed layer across the main pycnocline as well over different water masses in the region the results are contrasted with proximity to the branches of the warm and saline atlantic water current the west spitsbergen current wsc which is the main oceanic heat and salinity source both to the region and to the arctic ocean hydrographic properties show that the surface water mass modification is typically due to atmospheric cooling with relatively less influence of ice melting surface heat fluxes of o 100 w m 2 are found within the branches of the wsc and over shelf areas with elevated levels of mixing due to strong tides away from the shelves and

a high resolution hindcast of wind and waves for the north sea the norwegian sea and the barents sea 1 a combined high resolution atmospheric downscaling and wave hindcast based on the era 40 reanalysis covering the norwegian sea the north sea and the barents sea is presented the period covered is from september 1957 to august 2002 the dynamic atmospheric downscaling is performed as a series of short prognostic runs initialized from a blend of era 40 and the previous prognostic run to preserve the fine scale surface features from the high resolution model while maintaining the large scale synoptic field from era 40 the nested wam wave model hindcast consists of a coarse 50 km model covering the north atlantic forced with era 40 winds and a nested 10 11 km resolution model forced with downscaled winds a comparison against in situ and satellite observations of wind and sea state reveals significant improvement in mean values and upper percentiles of wind vectors and the significant wave height over era 40 improvement is also found in the mean wave period era 40 is biased low in wind speed and significant wave height a bias which is not reproduced by the downscaling the atmospheric downscaling also reproduces polar lows which cannot be resolved by era 40 but the lows are too weak and short lived as the downscaling is not capable of capturing their full life cycle

a simple shelf circulation model intrusion of atlantic water on the west spitsbergen shelf abstractbarotropic flow along depth contours is found in accordance with standard geostrophic theory a numerical model is developed that studies the deviation from such a flow the model gives a good approximation of the dynamical processes on the west spitsbergen shelf wss and shows that the west spitsbergen current wsc the main gateway of atlantic water aw toward the arctic connects more easily to the isfjorden trough than anywhere else along the shelf the circulation of aw in the troughs along the wss is here named the spitsbergen trough current stc from hydrographical and ocean current observations it is evident that the stc is primarily barotropic and driven by the sea surface height a connection between the along coast wind stress and the stc is established and it is demonstrated how the increased occurrence of winter cyclones in fram strait during january february accelerates and widens the wsc ultimately this results in a strengthened stc and dominance of aw on the wss the stc re

eddy driven recirculation of atlantic water in fram strait eddy resolving regional ocean model results in conjunction with synthetic float trajectories and observations provide new insights into the recirculation of the atlantic water aw in fram strait that significantly impacts the redistribution of oceanic heat between the nordic seas and the arctic ocean the simulations confirm the existence of a cyclonic gyre around the molloy hole near 80 n suggesting that most of the aw within the west spitsbergen current recirculates there while colder aw recirculates in a westward mean flow south of 79 n that primarily relates to the eastern rim of the greenland sea gyre the fraction of waters recirculating in the northern branch roughly doubles during winter coinciding with a seasonal increase of eddy activity along the yermak plateau slope that also facilitates subduction of aw beneath the ice edge in this area

thermohaline circulation in the arctic mediterranean seas the renewal of the deep north atlantic by the various overflows of the greenland scotland ridges is only one manifestation of the convective and mixing processes which occur in the various basins and shelf areas to the north the arctic ocean and the greenland iceland and norwegian seas collectively called the arctic mediterranean the traditional site of deep ventilation for these basins is the greenland sea but a growing body of evidence also points to the arctic ocean as a major source of deep water this deep water is relatively warm and saline and it appears to be a mixture of dense brine enriched shelf water with intermediate strata in the arctic ocean the deep water exits the arctic ocean along the greenland slope to mix with the greenland sea deep water conversely very cold low salinity deep water from the greenland sea enters the arctic ocean west of spitsbergen within the arctic ocean the lomonosov ridge excludes the greenland sea deep water from the canadian basin leaving the latter warm saline and rich in silica in general the entire deep water sphere of the arctic mediterranean is constrained by the greenland scotland ridges to circulate internally therefore it is certain of the intermediate waters formed in the greenland and iceland seas which ventilate the north atlantic these waters have a very short residence time in their formation areas and are therefore able to rapidly transmit surface induced signals into the deep north atlantic

hydrography of the west spitsbergen current svalbard branch autumn 2001 1 the atlantic inflow is the primary source of water and heat to the arctic ocean a large portion enters through fram strait where it splits into the yermak and svalbard branches in october to november 2001 the uscgc healy occupied five oceanographic transects across the svalbard branch west and north of spitsbergen temperature and salinity sections show the warm salty atlantic water cooling and freshening as it flows along the continental slope for 553 km the temperature in the upper 500 m of the water column decreased by 0 25 c 100 km implying a heat flux across the sea surface of up to 520 w m2 if one assumes a nominal current of speed of 0 1 m s a temperature salinity t s analysis shows that the shape of the t s curves can be explained in terms of the ratio of the atmospheric to ice melt heat flux the observations imply that ice melt accounts for progressively more surface cooling as the flow moves toward the arctic ocean an autonomous underwater vehicle completed a vertical section in the region of maximum water mass gradient and revealed layer interleaving in fine detail data from this cruise extend a long time series of atlantic layer source temperature beginning in 1910 saloranta and haugan 2001 the extended observations show that after the temperature of the atlantic layer inflow reached a peak in 1992 it cooled to a minimum in 1998 and rose again through 2001

dynamic modeling of the 1992 landers earthquake we have used observed band pass filtered accelerograms and a previously determined slip distribution to invert for the dynamic rupture propagation of the 1992 landers earthquake in our simulations dynamic rupture grows under the simultaneous control of initial stress and rupture resistance by friction which we modeled using a simple slip weakening law we used a simplified landers fault model where the fault segments were combined into a single vertical planar fault by trial and error we modified an initial stress field inferred from the kinematic slip distribution proposed by wald and heaton 1994 until dynamic rupture generated a rupture history and final slip distribution that approximately matched those determined by the kinematic inversion we found that rupture propagation was extremely sensitive to small changes in the distribution of prestress and that a delicate balance with energy release rate controls the average rupture speed for the inversion we generated synthetic 0 5 hz ground displacements using an efficient green s function propagator method axitra this method enables us to propagate the radiation generated by the dynamic rupture to distances greater than those feasible using the finite difference method the dynamic model built by trial and error inversion provides a very satisfactory fit between synthetics and strong motion data we validated this model using records from stations used in the slip inversion as well as some which were not included we also inverted for a complementary model that fits the data just as well but in which the initial stress was perfectly uniform while rupture resistance was heterogeneous this demonstrates that inversion of ground motion is nonunique

stable creeping fault segments can become destructive as a result of dynamic weakening faults in earth s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast seismic wave producing rupture events perceived as earthquakes these types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment one with stable rate strengthening friction and the other with rate weakening friction that leads to stick slip the 2011 tohoku oki earthquake with moment magnitude m w 9 0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping here we propose a model in which stable rate strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids allowing unstable slip to occur in segments that can creep between events the model parameters are based on laboratory measurements on samples from the fault of the m w 7 6 1999 chi chi earthquake the long term slip behaviour of the model which we examine using a unique numerical approach that includes all wave effects reproduces and explains a number of both long term and coseismic observations some of them seemingly contradictory about the faults at which the tohoku oki and chi chi earthquakes occurred including there being more high frequency radiation from areas of lower slip the largest seismic slip in the tohoku oki earthquake having occurred in a potentially creeping segment the overall pattern of previous events in the area and the complexity of the tohoku oki rupture the implication that earthquake rupture may break through large portions of creeping segments which are at present considered to be barriers requires a re evaluation of seismic hazard in many areas

the westward turning branch of the west spitsbergen current a salinity temperature survey of fram strait in september 1985 extending to north of 81 n demonstrated the western branch of the west spitsbergen current flowing northwest and then west mainly south of 81 n there also was a southwestward recirculation from the eastern branch of the west spitsbergen current baroclinic flow of warm water northward through 81 n was negligible or zero some doubt is cast on past flow diagrams based on core analysis which show the western branch of the west spitsbergen current coursing northward to 83 n one of a probable line of mesoscale eddies is described in some detail

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

effects of fault heterogeneity on seismic energy and spectrum abstract we study the effects of friction heterogeneity on the dynamics of a seismogenic fault to this aim we consider a fault model containing two asperities with different static frictions and a rate dependent dynamic friction we consider the seismic events produced by the consecutive failure of the two asperities and study their properties as functions of the ratio between static frictions in particular we calculate the moment rate the stress evolution during fault slip the average stress drop the partitioning of energy release the seismic energy the far field waveforms and the spectrum of seismic waves these quantities depend to various extent on the friction distribution on the fault in particular the stress distribution on the fault is always strongly heterogeneous at the beginning of the seismic event seismic energy and frictional heat decrease with increasing friction heterogeneity while seismic efficiency is constant we obtain an equation relating seismic efficiency to the parameters of the friction law showing that the efficiency is maximum for smaller values of dynamic friction the seismic spectrum depends on the friction distribution as to the positions and the values of the minima however under the model assumption that the slip durations are the same for both asperities the corner frequency is independent of the friction distribution but it depends on the friction law and on the coupling between asperities the model provides a relation between the total radiated energy and the seismic moment that is consistent with the empirical relation between the two quantities the fault model with one asperity is also considered as a particular case the model is applied to the 1965 rat islands alaska earthquake and shows the role of fault heterogeneity in controlling the spatial distribution of stress drop as well as the time dependence and the final amount of radiated energy

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

slip distribution of the february 27 2010 mw 8 8 maule earthquake central chile from static and high rate gps insar and broadband teleseismic data 1 the shallow depth underthrust earthquake of february 27 2010 mw 8 8 ruptured the subduction plate interface in central chile between 34 s and 38 s we retrieve the spatial and temporal distribution of slip during this mega earthquake through a joint inversion of teleseismic records insar and high rate gps hrgps data additionally our model is shown to agree with broadband surface waves rupture initiated at about 32 km depth and propagated bilaterally resulting in two main slip zones located ssw and nne of the hypocenter nucleation did not take place within or at the edge of one of these main asperities but in between during the first 30s slip propagated predominantly southwards later on the rupture evolved more slowly and more symmetrically eventually the northern asperity became predominant with maximum slip reaching about 20 m most of the seismic moment was released within 110s a relatively short time explained by the bilateral propagation the overall average rupture velocity is 2 6 km s but propagation occurred initially faster towards the south 3 2 km s large slip did not reach the trench a result consistent with the moderate size of the tsunami down dip rupture stopped at about 50 km depth in agreement with the lower limit of the locked zone inferred by ruegg et al 2009 from pre seismic gps data

the atlantic water boundary current in the nansen basin transport and mechanisms of lateral exchange data from a shipboard hydrographic survey near 30 e in the nansen basin of the arctic ocean are used to investigate the structure and transport of the atlantic water boundary current two high resolution synoptic crossings of the current indicate that it is roughly 30 km wide and weakly mid depth intensified using a previously determined definition of atlantic water the transport of this water mass is calculated to be 1 6 0 3 sv which is similar to the transport of atlantic water in the inner branch of the west spitsbergen current at the time of the survey a small anti cyclonic eddy of atlantic water was situated just offshore of the boundary current the data suggest that the feature was recently detached from the boundary current and due to compensating effects of temperature and salinity on the thermal wind shear the maximum swirl speed was situated below the hydrographic property core two other similar features were detected within our study domain suggesting that these eddies are common and represent an effective means of fluxing warm and salty water from the boundary current into the interior an atmospheric low pressure system transiting south of our study area resulted in southeasterly winds prior to and during the field measurements a comparison to hydrographic data from the pacific water boundary current in the canada basin under similar atmospheric forcing suggests that upwelling was taking place during the survey this provides a second mechanism related to cross stream exchange of heat and salt in this region of the nansen basin this article is protected by copyright all rights reserved

internal waves and mixing in the marginal ice zone near the yermak plateau observations were made of oceanic currents hydrography and microstructure in the southern yermak plateau in summer 2007 the location is in the marginal ice zone at the arctic front northwest of svalbard where the west spitsbergen current wsc carries the warm atlantic water into the arctic ocean time series of approximately 1 day duration from five stations upper 520 m and of an 8 day duration from a mooring are analyzed to describe the characteristics of internal waves and turbulent mixing the spectral composition of the internal wave field over the southern yermak plateau is 0 1 0 3 times the midlatitude levels and compares with the most energetic levels in the central arctic dissipation rate and eddy diffusivity below the pycnocline increase from the noise level on the cold side of the front by one order of magnitude on the warm side where 100 m thick layers with average diffusivities of 5 3 10 25 m 2 s 21 lead to heat loss from the atlantic water of 2 4 w m 22 dissipation in the upper 150 m is well above the noise level at all stations but strong stratification at the cold side of the front prohibits mixing across the pycnocline close to the shelf at the core of the svalbard branch of the wsc diffusivity increases by another factor of 3 6 here nearbottom mixing removes 15 w m 22 of heat from the atlantic layer internal wave activity and mixing show variability related to topography and hydrography thus the path of the wsc will affect the cooling and freshening of the atlantic inflow when generalized to the arctic ocean diapycnal mixing away from abyssal plains can be significant for the heat budget around the yermak plateau it is of sufficient magnitude to influence heat anomaly pulses entering the arctic ocean however diapycnal mixing alone is unlikely to be significant for regional cooling of the wsc

seismic coupling and uncoupling at subduction zones seismic coupling has been used as a qualitative measure of the interaction between the two plates at subduction zones kanamori 1971 introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern pacific subduction zones a quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables age of the subducting lithosphere and convergence rate the largest earthquakes occur in zones with young lithosphere and fast convergence rates while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane asperities are strong regions that resist the motion between the two plates the zones with the largest earthquakes have very large asperities while the zones with smaller earthquakes have small scattered asperities this observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface while the variation in asperity size is used to establish a connection between earthquake size and tectonic stress it also implies that plate age and rate affect the asperity distribution plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory i e the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity indirect influences are many including oceanic plate topography and the amount of subducted sediments all subduction zones are apparently uncoupled below a depth of about 40 km and we propose that the basalt to eclogite phase change in the down going oceanic crust may be largely responsible this phase change should start at a depth of 30 35 km and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change

asperity distributions and large earthquake occurrence in subduction zones abstract ruff l j 1992 asperity distributions and large earthquake occurrence in subduction zones in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 61 83 plate tectonics and the seismic gap hypothesis provide the framework for long term earthquake forecasting of plate boundary earthquakes unfortunately detailed examination reveals that earthquake recurrence times and rupture length vary between successive earthquake cycles in the same subduction zone furthermore larger coseismic slip is commonly associated with larger rupture length hence large earthquake occurrence in subduction zones is characterized by variability in 1 recurrence times 2 rupture length and 3 coseismic slip these facts plus many other observations indicate that there are significant spatial variations in the strength of the plate interface one simple description of these variations and their role in the earthquake cycle is the asperity model where the large strong regions of the plate interface are called asperities and the large earthquakes occur when the large asperities break the asperity model of earthquake occurrence is able to qualitatively explain several features of large plate boundary earthquakes to go beyond general qualitative notions i pose the following scientific test are the observed asperity distributions and a simple model of their interaction self consistent with the above three observed features of large earthquake occurrence the distribution of the major asperities along plate boundary segments has now been determined for several subduction zones rupture process studies of adjacent large and great earthquakes have provided reliable estimates of the along strike asperity lengths and separations for several adjacent asperities in the kurile islands colombia and peru subduction zones the simplest mechanical model for asperity interaction is to idealize two adjacent asperities as frictional sliders that are connected by main springs to the upper plate by a coupling spring to each other and maintain frictional contact with a conveyer belt the lower plate that moves with a constant velocity an earthquake occurs when the net force on the asperity frictional slider reaches some specified level the failure force and spring constants are determined by the observed asperity distribution and simple models of elastic interaction two different macroscopic failure criteria are used this simple mechanical model displays a remarkable range of behavior from simple to complex when the two asperities are identical in all their properties sequences of identical earthquakes are produced for the more realistic case of non identical asperities earthquake sequences show great variety using system variables from the observed asperity distributions the earthquake sequences typically display 1 variable recurrence times 2 variable rupture length i e a combination of single asperity and double asperity failures and for one of the failure criteria 3 larger coseismic slip for double asperity failures statistical summaries of thousands of simulated earthquake sequences for asperity pairs in the kuriles colombia and peru subduction zones are broadly consistent with the observed features of large earthquake occurrence in these subduction zones the main conclusion is that the asperity model provides a self consistent explanation for fault zone heterogeneity the rupture process and recurrence times and rupture mode of large earthquake sequences via a simple model for adjacent asperity interaction in addition a conclusion independent of any particular model for fault zone heterogeneity is that simple deterministic models of fault zone interaction can explain complex patterns of large earthquake occurrence in subduction zones

the west spitsbergen current disposition and water mass transformation recent work suggests that the west spitsbergen current which provides the principal contribution to the arctic ocean of salt and sensible heat as well as of a variety of anthropogenic tracers is a rather complex circulation feature north of 79 n where the isobaths diverge markedly the current contains two separate warm cores that follow different isobaths the western core carried by the offshore branch of the current follows the western flank of the yermak plateau and north of 80 n at least part of this flow detaches from the plateau probably to contribute to the recirculation in fram strait in contrast the inshore branch follows the shelf break into the arctic ocean it is this inshore branch that provides the primary focus in this paper during the transit of the inshore waters past northwestern spitsbergen the core properties change primarily through vertical heat flux which during ice free conditions in winter is estimated to be of the order of 200 w m 2 from the core layer alone together with some freshening within the arctic ocean this process is responsible for fully transforming the original atlantic water into arctic intermediate water within about 600 km of fram strait

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

baroclinic instability and the mesoscale eddy field around the lofoten basin the vigorous mesoscale eddy field around the lofoten basin west of northern norway is thought to be related to eddy shedding from the norwegian atlantic current flowing along the norwegian coast here we study baroclinic instability in the current with a particular focus on the influence of topography the flow over the steepest part of the continental slope is found to be the most unstable the growth characteristics cannot be understood from eady theory alone but require the consideration of interior potential vorticity gradients a study of the fully developed macroturbulent field shows that eddy kinetic energy is advected away from the generation regions and that nonlinear effects likely draw the eddy statistics away from the linear growth regime