an estimate of the absolute stress tensor in kaoiki hawaii the stress tensor orientation was calculated by inversion of 81 fault plane solutions of m 3 5 0 6 earthquakes located in an area 6 by 16 km at depths of 7 2 km this crustal volume is situated on a straight line between the summits of the active volcanoes kilauea and mauna loa the orientation of the greatest principal stress was found to be near horizontal and in the line connecting the two volcanoes this is further evidence supporting the model that magmatic expansion within the feeding conduits of these volcanoes is the source of stress that causes earthquakes in southern hawaii these earthquakes are tectonic earthquakes in the sense that they occur in a brittle elastic crust at distances of a few to several tens of kilometers from the volcanoes and that they are not directly associated with the opening of cracks by intrusions we propose that this model leads to the corollary that the shear stresses responsible for earthquake failure in the shallow hawaiian crust are approximately 3 2 mpa and that the pore pressure of ground water p in the hypocentral volume must be under near lithostatic pressure this result is obtained by arguing that the greatest principal stress 1 is equal to the magma pressure in the volcanic feeder pipe but cannot exceed the least principal stress 3 by more than the tensile strength of the crust because otherwise hydrofracture would occur opening a crack against the least principal stress given the difference between the greatest and least principal stress the fact that the overburden at the 7 km hypocentral depth equals the intermediate principal stress and the ratio r 1 2 1 3 0 4 which is obtained in the inversion of fault plane solutions we estimate that 1 202 mpa 2 200 mpa 3 196 mpa 3 mpa and n 199 mpa with uncertainties on the order of several mpa it follows that for any coefficients of friction larger than 0 15 the pore fluid pressure necessary for faulting on the observed fault planes has to be larger than 90 of the lithostatic pressure these considerations suggest that many earthquakes represent fault ruptures under low ambient shear stresses in the order of the average observed stress drop of 3 mpa

descent and modification of the overflow plume in the denmark strait bulk properties of the denmark strait overflow dso plume observed in velocity and hydrography surveys undertaken in 1997 and 1998 are described despite the presence of considerable short term variability it is found that the pathway and evolution of the plume density anomaly are remarkably steady bottom stress measurements show that the pathway of the plume core matches well with a rate of descent controlled by friction the estimated entrainment rate diagnosed from the rate of plume dilution with distance shows a marked increase in entrainment at approximately 125 km from the sill leading to a net dilution consistent with previous reports of a doubling of overflow transport measured by current meter arrays the entrainment rate increase is likely related to the increased topographic slopes in the region compounded by a decrease in interface stratification as the plume is diluted and enters a denser background

seismicity in the vicinity of the snorcle corridors of the northern canadian cordillera the slave northern cordillera lithospheric evolution snorcle corridors of the northern cordillera sample some of the most and least seismically active regions of canada the earthquake history of this region is short precise determination of earthquake locations and depths is not possible even today nonetheless significant gains in our knowledge of the seismicity of this region have been made in recent years from studies of historic earth quakes microseismicity studies and advances in waveform modelling techniques combined with broadband data that allow for determination of focal mechanisms and depths for moderate earthquakes this article summarizes our current knowledge of the seismicity and seismic hazards across the region these detailed analyses have shown that i the largest historical earthquakes have occurred in regions of ongoing microseismicity ii the largest earthquakes have occurred in pairs or in swarms suggesting that stress triggering is important in this region iii the active faults are concentrated in the offshore region iv there is a concentration of seismicity in the fold and thrust belt several hun dred kilometres from the active plate margin and v there is no seismicity associated with the quaternary volcanic zone in northern british columbia potentially damaging magnitude m 5 earthquakes can be expected every few years in the vicinity of the northern cordillera the mw 7 9 denali alaska earthquake where mw is the moment magnitude was a good reminder that the effects of a large earthquake can be substantial even hundreds of kilometres from the epicentre detailed studies of seismicity earth structure and tectonics with the latter made possible in large part by the snorcle transect will allow for informed decision making for resource development and the design of safe structures and infrastructure in the northern canadian cordillera resume les corridors du projet snorcle de la cordillere septentrionale permettent d echantillonner quelques unes des regions sismiques les plus et les moins actives du canada l histoire des seismes de cette region est breve meme

seismic hazard in hawaii high rate of large earthquakes and probabilistic ground motion maps the seismic hazard and earthquake occurrence rates in hawaii are locally as high as that near the most hazardous faults elsewhere in the united states we have generated maps of peak ground acceleration pga and spectral acceleration sa at 0 2 0 3 and 1 0 sec 5 critical damping at 2 and 10 exceedance probabilities in 50 years the highest hazard is on the south side of hawaii island as indicated by the m i 7 0 m s 7 2 and m i 7 9 earthquakes which occurred there since 1868 probabilistic values of horizontal pga 2 in 50 years on hawaii s south coast exceed 1 75 g because some large earthquake aftershock zones and the geometry of flank blocks slipping on subhorizontal decollement faults are known we use a combination of spatially uniform sources in active flank blocks and smoothed seismicity in other areas to model seismicity rates of earthquakes are derived from magnitude distributions of the modern 1959 1997 catalog of the hawaiian volcano observatory s seismic network supplemented by the historic 1868 1959 catalog modern magnitudes are m l measured on a wood anderson seismograph or m s historic magnitudes may add m l measured on a milne shaw or bosch omori seismograph or m i derived from calibrated areas of mm intensities active flank areas which by far account for the highest hazard are characterized by distributions with b slopes of about 1 0 below m 5 0 and about 0 6 above m 5 0 the kinked distribution means that large earthquake rates would be grossly underestimated by extrapolating small earthquake rates and that longer catalogs are essential for estimating or verifying the rates of large earthquakes flank earthquakes thus follow a semicharacteristic model which is a combination of background seismicity and an excess number of large earthquakes flank earthquakes are geometrically confined to rupture zones on the volcano flanks by barriers such as rift zones and the seaward edge of the volcano which may be expressed by a magnitude distribution similar to that including characteristic earthquakes the island chain northwest of hawaii island is seismically and volcanically much less active we model its seismic hazard with a combination of a linearly decaying ramp fit to the cataloged seismicity and spatially smoothed seismicity with a smoothing half width of 10 km we use a combination of up to four attenuation relations for each map because for either pga or sa there is no single relation that represents ground motion for all distance and magnitude ranges great slumps and landslides visible on the ocean floor correspond to catastrophes with effective energy magnitudes m e above 8 0 a crude estimate of their frequency suggests that the probabilistic earthquake hazard is at least an order of magnitude higher for flank earthquakes than that from submarine slumps

rupture histories of eastern north american earthquakes abstract digital worldwide records of p and sh waves are used to invert for rupture histories of the recent larger eastern north american earthquakes the data include a broad bandwidth to facilitate recovery of both source details and total moment the following events are studied 9 january 1982 new brunswick 5 october and 23 december 1985 nahanni 25 november 1988 saguenay and 25 december 1989 ungava earthquakes we employ a finite fault waveform inversion scheme that discretizes the slip history as a function of position on the fault this formulation allows estimation of rise time and slip velocity in addition to the spatial distribution of slip static stress drops averaged over the entire rupture surface range from a few tens of bars to just over 100 bars and are similar to values estimated for western united states earthquakes estimates of stress drop for spatially limited asperities are in the range of a few hundred bars with the value for the saguenay earthquake asperity being the largest approaching 1 kbar the variation in stress drop is considerable but no evidence is seen for a scaling relation in which stress drop increases with moment maximum slip velocities on the fault also have a wide range from 50 cm sec to perhaps greater then 200 cm sec with the saguenay and 5 october nahanni earthquakes lying at the upper end of this range of the events studied the saguenay earthquake is unique in terms of its greater depth spatially concentrated source and large asperity stress drop

eddy formation in the denmark strait overflow on the basis of observations along the continental slope of east greenland a primitive equation open boundary model is used to study the formation of eddies in the denmark strait overflow the eddies are produced by vortex tube stretching initiated by cross slope components of the velocity field a sequence of cyclones and anticyclones is formed the cyclones are located along the deeper part of the slope and the anticyclones are along the shallower slope the eddies are almost barotropic with horizontal and vertical velocities of 30 cm s 1 and 0 1 cm s 1 respectively the cyclones migrate along the isobaths with a slight downhill component and intensify on their path until they become influenced by the flat deep basin their translation speed is typically 20 30 cm s 1 and the local change in relative vorticity is nearly entirely determined by vortex tube stretching and vorticity advection the overflow water flows mainly between the cyclonic and anticyclonic eddy chains the overflow is strongly accelerated by the eddies the existence of anticyclonic eddies along the shallower part of the east greenland continental slope predicted by the numerical model has been confirmed by observations

on the hydrography of denmark strait using 111 shipboard hydrographic sections across denmark strait occupied between 1990 and 2012 we characterize the mean conditions at the sill quantify the water mass constituents and describe the dominant features of the denmark strait overflow water dsow the mean vertical sections of temperature salinity and density reveal the presence of circulation components found upstream of the sill in particular the shelfbreak east greenland current egc and the separated egc these correspond to hydrographic fronts consistent with surface intensified southward flow deeper in the water column the isopycnals slope oppositely indicative of bottom intensified flow of dsow an end member analysis indicates that the deepest part of denmark strait is dominated by arctic origin water with only small amounts of atlantic origin water on the western side of the strait the overflow water is a mixture of both constituents with a contribution from polar surface water weakly stratified boluses of dense water are present in 41 of the occupations revealing that this is a common configuration of dsow the bolus water is primarily arctic origin water and constitutes the densest portion of the overflow the boluses have become warmer and saltier over the 22 year record which can be explained by changes in end member properties and their relative contributions to bolus composition

craters as sand traps dynamics history and morphology of modern sand transport in an active martian dune field abstract aeolian transport of sand is abundant on modern day mars as revealed by remote sensing measurements of the motion of dunes and of the meter scale ripples that mantle them we study a large scale natural sand trap within the meroe patera dune field a 1 8 km diameter crater which features a dune free shadow in its lee we compare the volume of sand trapped within this crater to the sand volume that would be expected to cover the area of the crater and its dune free shadow behind it if the crater were not present we find that the crater holds less sand than this missing volume would predict implying that sand escapes from the crater over time modern day imagery shows an apparent lack of sand escaping from the meroe crater however suggesting that changes in the wind regime at the site may have allowed sand to escape in the past the persistence of an altered dune morphology all the way to the far downwind edge of the dune field suggests consistent wind conditions over the time of the crater dune field interaction

variability of the denmark strait overflow a numerical study mesoscale variability in the denmark strait overflow is investigated using a version of the miami isopycnic coordinate ocean model that includes a recently developed intermittent and vigorous turbulent and diffusive diapycnal mixing scheme earlier idealized modeling work on the subject is further extended in that a realistic replica of the shelf slope topography and irregular coastline geometry of the denmark strait area is also employed compared with earlier studies our experiments reveal that 1 the introduction of the new diapycnal scheme and the true shelf slope topography do not affect the formation of the intense cyclonic eddies south of the sill and along the continental slope and 2 the new diapycnal scheme has a significant effect on the bottom plume in that the distribution volume transport and properties of the bottom plume appear to be more realistic and more in line with what is observed the first conclusion adds support to the vortex stretching mechanism suggested in earlier studies as the cause for the formation of the cyclonic eddies this mechanism is therefore a robust feature and provides an explanation for the observed variability in the denmark strait overflow the second conclusion underscores the importance of including realistic parameterizations of diapycnal mixing in isopycnal coordinate ocean models specifically the results show that the water that descends along the bottom south of the sill becomes lighter and is more voluminous compared to experiments without the new mixing scheme even when including the mixing scheme the bottom plume separates into distinct boluses of enhanced thickness moreover these boluses are also associated with water of anomalously high density as well as with the cyclonic eddies in the upper water column

omak rock and the 1872 pacific northwest earthquake abstract omak rock is an about 40 ton granite boulder balanced on a narrow support the rock is located in northern washington state in the general vicinity of the 1872 pacific northwest m 7 earthquake the survival of the balanced rock has in the past been used to place limits on peak acceleration and intensity at its location and thus possibly on the epicenter of the earthquake using dynamical analysis i find that the rock could have survived far greater peak ground motion than previously concluded and might well have survived within a few kilometers of the epicenter thus invalidating the earlier conclusions

a theoretical investigation of source path and site effects during the 1986 kalamata earthquake greece summary on 1986 september 13 a magnitude 6 2 ms earthquake occurred in the south peloponnese greece and severely damaged the town of kalamata in order to retrieve the acceleration levels recorded in kalamata synthetic accelerograms are computed the source is modelled by an aggregate of circular shear crack barrier model embedded first in a two layer crust model and second in a structure taking into account a velocity gradient in the uppermost crust good agreement is found between the peak accelerations obtained from synthetics and those observed in kalamata in the case of the gradient model the model also predicts rapid variations up to 50 60 per cent of peak acceleration between stations spaced only 600 m apart therefore the non uniform repartition of damage observed in kalamata may be explained at least partly as a source effect calculations also show that because of the small distance between the fault plane and the stations ground motion is dominated by the radiation of the closest sub events and will depend on their stress drop independently of the magnitude of the large event using a 2 d model we also investigate the effects related to a north south thickness variation of the surficial sedimentary deposits beneath the city expected peak spectral amplification is found to be relatively constant along the profile about 3 however the corresponding resonance frequency range is 2 4 hz for the southern part of the city where slight damage was observed and 4 8 hz for the northern part from where heavy damage was reported simultaneous consideration of the largest aftershock strong motion records and of the aftershock distribution shows that the assumption concerning the average size of elementary subevent considered in the model is correct therefore the rupture process on the kalamata fault can be interpreted in terms of a barrier model

hydraulic estimates of denmark strait overflow 1 upper bounds of the denmark strait deep water overflow from the nordic seas into the north atlantic are estimated using rotating hydraulic theory the calculations are made for the real bottom topography of the strait and are based on hydrographic sections surveyed during a dedicated field experiment in the area results are presented for zero as well as finite but constant potential vorticity and it is shown that the differences in outcome between these two approaches are only minor the calculated interface configurations are found to be in good agreement with those observed and the theoretically obtained transports conform with earlier estimates

large accelerations and tectonic setting of the may 1993 shumagin islands earthquake sequence variability of earthquake rupture can lead to large disparities between expected and observed ground motion to better understand the range of ground motions generated by subduction zone earthquakes we examine teleseismic and nearsource strong motion records from an interplate thrust earthquake sequence that occurred near the shumagin islands alaska peninsula in may 1993 the two largest events show source depths of 36 km may 13 and 42 km may 25 from inversion of teleseismic waveforms which place them in the deepest part of the interplate thrust zone this inference is verified by single station epicenter estimates derived from p wave polarization and s p times and by the focal mechanisms the 13 may 1993 mainshock ms 6 9 shows peak accelerations of 0 7 to 1 4 m sec 2 within 75 km of the epicenter consistent with scaling relations observed elsewhere the largest aftershock 25 may mw 6 1 shows peak acceleration near 8 m sec 2 at a station 25 km away the 8 m sec 2 acceleration is far larger than expected for an earthquake of this size and depth and is perhaps the largest recorded in alaska simple explanations such as site effects and rupture directivity cannot fully account for the anomalous accelerations the short rupture time of the 25 may event 2 8 sec contributes to high accelerations but cannot explain the factor of 12 variability observed in peak acceleration near the epicenter unusual focusing effects and rupture patterns are suspected but cannot be confirmed whatever the cause the unusually high accelerations demonstrate that deep and moderate sized earthquakes can occasionally produce significant ground motions in subduction zones

teleseismic and near field analysis of the nahanni earthquakes in the northwest territories canada the analysis of the nahanni earthquakes of october 5 1985 ms 6 6 and december 23 1985 ms 6 9 will have important implications for the assessment of seismic hazards in intraplate environments to maximize the information available to seismic engineers broadband data recorded teleseismically are analyzed jointly with strong motion data recorded in the near field the time domain analysis of teleseismic data yields the source mechanisms depths and complexities of rupture of each earthquake both earthquakes occurred as shallow thrusts with centroid depths 6 to 7 km and shallowly dipping fault planes that correspond well with the aftershock distributions obtained from a local survey run by the canadian geological survey the shallow nodal plane for the october 5 earthquake dips 30 to the wsw while the shallow nodal planes of the subevents for the december 23 earthquake dip an average of 23 to the wsw the october 5 earthquake has an impulsive initial rupture followed by a weak subevent of longer duration but smaller moment release the december 23 earthquake exhibits more complexity being comprised of three subevents of similar size the subevent delays derived from the teleseismic analysis are used to help interpret arrivals in records of ground velocity recorded in the near field of the december 23 earthquake the rupture geometries inferred from the joint near and far field analysis suggest that the rupture processes were unusually complicated and that the 2 g peak that occurs late in one of the near field records could be a localized phenomenon spectral analyses of the teleseismic p waves yield the following source parameters for the october 5 and december 23 earthquakes respectively the seismic moments are 1 2 and 1 8 1026 dyne cm the radiated energies are 1 8 and 2 8 1021 dyne cm and the dynamic stress drops are 65 and 50 bar the acceleration source spectra of both earthquakes exhibit an intermediate slope u from 0 03 to 0 3 hz suggesting that the earthquakes represent the failure of asperities extrapolating the teleseismic p wave spectra to estimate the near field s wave spectra yields good fits to the acceleration spectra from two strong motion records but underestimates the spectra from a third strong motion record with the strongest but possibly localized accelerations

an analysis of the 1985 nahanni earthquakes abstract two large magnitude earthquakes occurred in 1985 near the north nahanni river in the northeast cordillera northwest territories canada on 5 october m s 6 6 and 23 december m s 6 9 these events were unprecedented for this region of the cordillera and the december event is the largest earthquake ever known in the eastern part of the canadian cordillera this report presents detailed studies of the nahanni earthquakes their source parameters macroseismic effects aftershock sequences triggered rock avalanches their relationship to the geology of the epicentral area and their seismic hazard implications parameters calculated here for these two major events are parentheses indicate other studies the earthquakes occurred within a small relatively undeformed plateau the mackenzie plain in the foreland fold belt created along the northeastern cordillera during the columbian or laramide orogeny that fold belt is now subjected to northeast southwest horizontal compressive stresses possibly very similar to the original laramide stress regime and the fault plane implied by the seismic mechanisms is very similar to the laramide faults in the area however no surface fault break has been found and none of the mapped laramide faults in the epicentral area appears to be involved in the aftershock activity it is thus believed that the nahanni earthquakes occurred on a blind laramide thrust fault under the mackenzie plain the large magnitudes of the nahanni earthquakes show that the present seismic zoning for building code purposes in the eastern cordillera north of 60 n needs reassessment and suggest that the zoning in the eastern cordillera in british columbia and alberta may also need revision the felt areas of the earthquakes show that regional anelastic attenuation in the eastern canadian cordillera is significantly anisotropic and much lower along strike than across strike the felt areas also appear to scale with seismic moment rather than m b magnitude the thrust mechanisms regional stress regime shallow focal depth and high velocity sedimentary rocks in the focal region suggest that the nahanni earthquakes and particularly the near field strong ground motion recorded for the second event are ideal engineering design data for critical facilities in eastern north america

a fault model for the nahanni earthquakes from aftershock studies abstract relative locations of 323 large aftershocks m 3 0 or greater in the period from 5 october 1985 to 25 march 1988 show that the m s 6 6 event on 5 october 1985 initiated at 62 208 n 124 217 w about 2 5 km northeast of the m s 6 9 main shock on 23 december 1985 the overall aftershock distribution suggests the october rupture was primarily a west dipping low angle thrust in subsequent aftershock activity the main rupture plane was marked by a distinct quiescent area of about 200 km 2 that persisted until the 23 december event most of the stress drop and slip occurred in this area following the 23 december rupture a similar sized quiescent zone was also observed however it was only evident during the first 24 hr of the aftershock sequence and the area was about 50 per cent too small to yield the overall stress drop the additional area appeared to come from secondary rupture zones that developed coincident with the main shock rupture precise locations of 182 small m 3 0 or less aftershocks recorded during a third field survey from 12 to 21 september 1986 indicated at least one and probably three high angle faults composite mechanism solutions showed thrust faulting except in a region directly south of the main shock rupture areas where there is a bend in one of the secondary fault zones and a concentration of aftershock activity mechanism solutions calculated for five of the largest aftershocks in the same region also indicated a similar variability development of secondary fault zones explained the increased complexity of the december event and may also provide an explanation for the vertical peak acceleration exceeding 2 g that was recorded about 10 sec after the december rupture initiated

three dimensional simulations of overflows on continental slopes abstract three dimensional features and instabilities of dense overflows from marginal seas onto continental slopes are investigated using a three dimensional primitive equation numerical ocean model the numerical simulations reveal important instability and three dimensional features of the overflow plumes that have not been included in previous simulations with a one dimensional streamtube model and a two dimensional plume model it is shown that the large primary plume breaks into a number of smaller subplumes on the offshore side of the plume due to instabilities manifested as growing topographic rossby waves over the slope the observed high temporal and spatial variabilities in the denmark strait overflow could be caused by the inherent dynamic instabilities as revealed by the numerical simulations the simulations indicate that the initial overflow velocity and width the properties of the source water the planetary rotation and the slope steepness play major roles in determining the scales of

on the nature and variability of the east greenland spill jet a case study in summer 2003 abstractresults from a high resolution 2 km numerical simulation of the irminger basin during summer 2003 are presented the focus is on the east greenland spill jet a recently discovered component of the circulation in the basin the simulation compares well with observations of surface fields the denmark strait overflow dso and the hydrographic structure of typical sections in the basin the model reveals new aspects of the circulation on scales of o 0 1 10 days and o 1 100 km the model spill jet results from the cascade of dense waters over the east greenland shelf spilling can occur in various locations southwest of the strait and it is present throughout the simulation but exhibits large variations on periods of o 0 1 10 days the spill jet sometimes cannot be distinguished in the velocity field from surface eddies or from the dso the vorticity structure of the jet confirms its unstable nature with peak relative and tilting vorticity terms reaching twice the planetary vorticity term the

application of a transient reduced gravity plume model to the denmark strait overflow a hydrostatic reduced gravity two dimensional primitive equation numerical model for the simulation of transient bottom arrested gravity plumes was developed by adopting an algorithm from a wadden sea model a model that simulates the wetting and drying of mud flats sand banks etc with a movable lateral boundary the vertically integrated nonlinear rotational model accounts for the dissipation of momentum due to bottom friction and for the entrainment of water mass properties from a spatially structured but stagnant ambient water body it predicts the temporal and spatial evolution of the flow field and the water mass modification within a gravity plume that descends on an arbitrary topography previous model investigations considered a laterally integrated streamtube whereas our model resolves the plume horizontally in the streamtube approach it was assumed that the plume descends in a stationary almost geostrophic balance modified only by bottom drag and entrainment the results of this model demonstrate that this balance may be disturbed whenever the plume encounters topographic disturbances which for the initial intruding phase in particular accounts for its highly transient character the model results are in good agreement with theoretical and laboratory investigations and with observations as is demonstrated by some basic test experiments an application to the overflow of dense water through the denmark strait is presented

significant role of the north icelandic jet in the formation of denmark strait overflow water the largest dense water plume feeding the lower limb of the atlantic meridional overturning circulation from the nordic seas comes from denmark strait overflow water measurements of hydrography and water velocity north of iceland and ocean model simulations indicate that a significant part of this water is supplied by the north icelandic jet

the bright spot of an earthquake abstract umeda y 1992 the bright spot of an earthquake in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 13 22 field surveys in the source region for some large shallow earthquakes reveal that there is a special spot characterized by high acceleration aftershock gap and parallel faults system this spot is called an earthquake bright spot which occupies a relatively small region compared to the final rupture region at least two kinds of phases p1 and p2 which are responsible for the rupture initiation and the bright spot formation can be identified on the broad band seismograms for large earthquakes duration times of preliminary waves p1 p2 are proportional to the earthquake magnitudes by using this evidence from the field and seismograms a model having three rupture stages is proposed for a growth process of large earthquakes the 1st stage is an initiation and propagation of rupture the 2nd stage is formation of the bright spot which produces the high frequency seismic waves in the 3rd stage smooth dislocations again propagate in bi laterally and radiate the long period seismic waves

structure and variability of the denmark strait overflow model and observations we report on a combined modeling and observational effort to understand the denmark strait overflow dso four cruises over the course of 3 years mapped hydrographic properties and velocity fields with high spatial resolution the observations reveal the mean path of the dense water as well as the presence of strong barotropic flows energetic variability and strong bottom friction and entrainment a regional sigma coordinate numerical model of interbasin exchange using realistic bottom topography and an overflow forced only by an upstream reservoir of dense fluid is compared with the observations and used to further investigate these processes the model successfully reproduces the volume transport of dense water at the sill as well as the 1000 m descent of the dense water in the first 200 km from the sill and the intense eddies generated at 1 3 day intervals hydraulic control of the mean flow is indicated by a region supercritical to long gravity waves in the dense layer located approximately 100 km downstream of the sill in both model and observations in addition despite the differences in surface forcing both model and observations exhibit similar transitions from mostly barotropic flow at the sill to a bottom trapped baroclinic flow downstream indicating the dominant role of the overflow in determining the full water column dynamics

a faulting model for the 1992 petrolia earthquake can extreme ground acceleration be a source effect the 1992 petrolia earthquake m 7 0 was quite remarkable in that an accelerometer at cape mendocino cap recorded a high frequency pulse of almost 2 g whereas the nearby roughly 6 km away petrolia station pet recorded a maximum of only 0 6 g in our model for this event we have investigated the possibility that extreme ground acceleration with high spatial variability can be caused by a source effect specifically the rupture of a barrier asperity this type of rupture feature can produce extreme ground acceleration provided that certain geometrical constraints are met in the present study we derive a combined high and low frequency faulting model for the 1992 petrolia earthquake that is consistent with the slip distribution on the fault and that produces the high frequency pulse at cap our model consists of four parts 1 a low frequency 0 1 0 6 hz inversion that determines the slip rupture time and risetime distributions on the fault 2 an interpolation and perturbation of the above distributions that produces synthetic accelerograms between 0 6 and 3 0 hz 3 random phase signal that is scaled to the spectral amplitude level of the data between 3 0 and 12 5 hz and 4 the rupture of a barrier like asperity that produces greatly amplified radiation at cap but not at other stations we find that our model produces a good fit to the near source records of the 1992 petrolia event the results emphasize the interaction between fault geometry and rupture propagation to produce ground motion as well as the hazard associated with locations above the hanging walls of dipping faults where the geometry permits the production of extreme ground acceleration

a new path for the denmark strait overflow water from the iceland sea to denmark strait 1 denmark strait overflow water dsow is one of the main components of the thermohaline circulation there has been no consensus on where it is formed and by which way it is brought to the denmark strait it is shown here that it is brought to the sill by a hitherto unknown current that is traced from the sill back into the iceland sea north of iceland the transport of this current is sufficient to account for a major part of the transport of dsow as it has been measured at the sill if some entrainment of ambient water is assumed this supports theories suggesting that the iceland sea is the main source for the dsow and this has consequences for the way in which climate change affects the thermohaline circulation

exceptional ground motions recorded during the 26 april 2008 mw 5 0 earthquake in mogul nevada abstract an unusually shallow swarm of earthquakes in mogul a suburb in west reno nevada started 28 february 2008 and continued for several months temporary instruments in the epicentral region constrained event depths and locations and provided onscale recordings of the strong ground motions and supplemented a network of over 21 permanent advanced national seismic system anss strong motion stations deployed throughout the metropolitan reno area by the end of august the swarm had produced over 200 earthquakes with magnitude m l greater than 2 0 the largest earthquake in the sequence occurred at 06 40 utc on 26 april 2008 11 40 p m on 25 april local time with magnitude m l 4 7 m w 5 0 and a hypocentral depth of about 3 1 km in the m w 5 0 mainshock mean horizontal peak ground acceleration exceeded 0 6 g at three stations the strongest peak vector acceleration was 1164 cm sec 2 or about 1 19 g at the station mogl 0 4 km from the epicenter at a frequency of about 3 hz the mean horizontal peak velocity exceeded 12 cm sec at the four nearest stations and the peak vector velocity at the strongest was 54 cm sec the duration of the strongest shaking was about 2 sec at these stations the observed peak accelerations and peak velocities decreased more rapidly with distance than predicted to values below the median predictions at most stations in the reno metropolitan area the shallow source depth is to some degree a likely contributing factor to these unusual observations the motions in the residential neighborhood near the epicenter are significantly greater than the seismic forces required by the 1985 uniform building code zone 3 that was in effect when the mogul neighborhood was developed but damage to the wood frame structures in the neighborhood was minimal

determination of stress parameters for eight well recorded earthquakes in eastern north america we determined the stress parameter for the eight earthquakes stud ied by atkinson and boore 2006 using an updated dataset and a revised point source stochastic model that captures the effect of a finite fault we consider four geometrical spreading functions ranging from 1 r at all distances to two or three part functions the values are sensitive to the rate of geometrical spreading at close distances with 1 r 1 3 spreading implying much higher than models with 1 r spreading the important difference in ground motions of most engineering con cern however arises not from whether the geometrical spreading is 1 r 1 3 or 1 r at close distances but from whether a region of flat or increasing geometrical spreading at intermediate distances is present as long as is constrained by data that are largely at distances of 100 km 800 km the simple 1 r model fits the sparse data for the eight events as well as do more complex models determined from larger datasets where the larger datasets were used in our previous ground motion predic tion equations this suggests that uncertainty in attenuation rates is an important com ponent of epistemic uncertainty in ground motion modeling for the attenuation model used by atkinson and boore 2006 the average value of from the point source model ranges from 180 bars to 250 bars depending on whether or not the stress parameter from the 1988 saguenay earthquake is included in the average we also find that for a given earthquake is sensitive to its moment magnitude m with a change of 0 1 magnitude units producing a factor of 1 3 change in the derived

note on the very high acceleration fault zone 16 record from the 2004 parkfield earthquake a very high acceleration recording was obtained at station fault zone 16 near parkfield california during the m 6 0 parkfield earthquake of 28 september 2004 this note documents key aspects of the station the instrument and the record a preliminary reconstruction of the record is performed to produce an estimate of the true acceleration which is then integrated to obtain the estimated velocity the strong motion accelerograph at fault zone 16 is deployed at a soil site in a light fiberglass housing common in the parkfield array postearthquake testing indicates that the accelerograph operated normally instrument shake table tests indicate that the recorded acceleration is relatively accurate even at the high accelerations involved in this record that is the very high accelerations in the record are a reasonably accurate reflection of the accelerations that occurred at the site during the parkfield earthquake although the peak motion was not recorded the record itself has several unusual features the trace was lost at the top edge of the film where the excursion went beyond the recordable part of the film most unusually the traces of two accelerometers recorded on top of one another during the positive highest amplitude part of the record the accelerograph did not record the peak accelerations of the components which occurred during this time an additional aspect is that for the largest amplitude component the negative part of the acceleration was affected by momentary stalling of the recording film during the strongest shaking the constraint for the mean of the acceleration over the whole record to be zero and detailed analysis of the time trace leads to a reconstruction of the record the peak acceleration on this component is over 1 8 g and most likely over 2 g the peak velocity is estimated to be about 80 cm sec the other horizontal component goes off the film at the same time combined the horizontals yield a fault normal acceleration of over 2 2 g and most likely over 2 5 g this record is a challenge to interpret an important practical lesson is that given what has been learned about near fault accelerations any instrument deployed in the region should have a recording capacity well in excess of 2 g

spectral scaling of the 1985 to 1988 nahanni northwest territories earthquakes abstract the sequence of earthquakes that began in october 1985 near the nahanni river northwest territories canada provides an exceptional opportunity to study the variation with magnitude of the source spectra from intraplate earthquakes we study the spectral scaling for the seven largest events using p waves recorded at stations of the western canada telemetred network wctn the eastern canada telemetred network ectn and the global digital seismograph network gdsn these earthquakes range in size from m 4 8 to m 6 8 we compare predictions from various source scaling models with observed spectral ratios and source acceleration spectra for the suite of events the spectral ratios show a strong azimuthal dependence which is probably due to differences in the focal mechanisms or rupture characteristics of the events taking the average of the ratios from individual gdsn stations as the best representation of the spectral ratios an squared model with a constant stress parameter or one that increases slightly with moment magnitude gives a simultaneous fit to both the spectral ratios and to the absolute spectra the high frequency spectral levels by themselves imply a somewhat stronger dependence of the stress parameter on moment than found from fitting the overall spectra the stress parameters for the scaling laws are less than 35 bars for earthquakes as large as m 7 recently proposed scaling laws in which the corner frequency is proportional to the inverse quarter root of seismic moment for an squared model are not consistent with the data and neither are cubed models the spectral scaling for the sequence of nahanni earthquakes may not be representative of intraplate magnitude scaling in general since there is some evidence that aftershocks do not exhibit the same scaling as main shocks the high frequency spectral levels of the aftershocks of both the nahanni and the 9 january 1989 miramichi new brunswick earthquakes imply stress parameters that increase with magnitude by contrast the spectral levels for a series of main shocks in eastern north america show less magnitude dependence of the stress and generally higher stress levels when compared to aftershocks of the same magnitude

hydrographic observations in denmark strait in fall 1997 and their implications for the entrainment into the overflow plume denmark strait is the most important exit for water masses formed in the arctic mediterranean sea and supplies a substantial fraction of the north atlantic deep water observations obtained from rv aranda in august september 1997 indicate that the water crossing the 620m deep sill is mainly drawn from the intermediate waters of the east greenland current the overflow plume was stratified and capped by a less saline layer as it descended beyond 2000m the presence of a low salinity lid implies that entrainment of ambient water is small and that the downstream evolution of the plume characteristics is due to mixing within the plume between the initial overflow waters low salinity but dense water from the east greenland current flowing over the shelf may cross the shelf break south of the sill and add a less dense fraction to the overflow

the 1985 earthquake triggered north nahanni rockslide northwest territories canada the co seismic movement of a sedimentary rock mass conditioned by residual strength abstract the north nahanni rockslide volume 7 6 mm3 northwest territories canada was triggered by the mw 6 6 nahanni earthquake of october 5 1985 the earthquake was followed by a mw 6 8 earthquake at approximately the same location in december 1985 the landslide occurred in a palaeozoic carbonate sequence sliding along a thrust fault which partly follows bedding and partly cuts across bedding in the source rock mass the sliding surface thus consisted of two planes the lower plane dipped at 20 while the upper plane dipped at 35 tilting table test of block samples from the sliding surface and field measurements of large scale sliding surface roughness indicated that the friction angle of the sliding surface was 24 two dimensional 2d slope stability analysis was performed for the slope before and during the earthquake using a discontinuum numerical technique the distinct element udec code 2d static slope stability analyses indicate that the simple structured rock mass was marginally stable for sliding surface friction angles of 24 and was constrained by the residual shear strength of the pre sheared fault surface 2d dynamic analyses of the co seismic movements are conducted by comparing seismically induced slope displacements computed in both udec and slammer a sliding block analysis using real earthquake ground motions records of the december 1985 nahanni earthquake both results are in good agreement the dynamic analysis indicates that the slope becomes unstable for given seismic inputs that correspond closely to those estimated for the october earthquake at a friction angle of 24 for the sliding surface indicating that co seismic rock slope movement was only possible on low friction pre sheared surfaces at residual strength it also showed that the deformation behavior of the north nahanni rock masses was dependent on the frequency of the seismic signals because the static slope stability analysis showed that the slope was close to instability prior to seismic shaking the october 1985 nahanni earthquake operated as a trigger event that accelerated the occurrence of the slide