test ban treaty verification with regional data a review this paper summarizes the use of regional 30 seismic data in a test ban context for detecting locating identifying and determining the yield of underground nuclear explosions in many areas of the world eastern north america africa eastern ussr lg is the largest amplitude wave recorded on standard seismograph systems and thus is the most appropriate phase for monitoring small magnitude events excellent location capability for near regional events has been demonstrated at the norwegian small aperture array noress using lg and p waves lg and other regional phases may contain information on source depth but such information has not been exploited to date fifteen classes of regional discriminants have been identified including 1 1 first motion 2 2 ms mb 3 3 p2 p1 ratio 4 4 excitation of short period sh waves 5 5 lg rg amplitude ratios 6 6 pn lg pg lg and pmax lg amplitude ratios 7 7 lg group velocity and energy ratios in lg 8 8 excitation of sn 9 9 third moment of frequency 10 10 generation of higher mode surface waves 11 11 peak amplitudes of love and rayleigh waves and long period surface wave energy density 12 12 prevailing period of long period love waves 13 13 spectral ratio long period s waves to rayleigh waves 14 14 spectral ratio long period love waves to rayleigh waves 15 15 frequency of the peak spectral amplitudes and spectral ratios in pn pg s and lg each of these proposed discriminants has with differing degrees of success separated some explosions from some earthquakes however most have been tested only on limited data usually from one geographic region and only one or two recording stations no systematic analyses have been done to determine the best individual discriminant or combination of them preliminary evaluation of the use of lg for yield determination suggests that regional waves hold promise in this application theoretical studies have contributed significantly to the understanding of propagation characteristics of regional waves but further studies are required emphasizing modeling for realistic anisotropic sources the major conclusion of this study is that a systematic and comparative evaluation of all the proposed regional discriminants is now required utilizing a common data base derived from all present day test sites this evaluation would suggest the optimal discrimination procedure using regional waves and would also define areas of needed research without such an integrated evaluation it is still possible to speculate using existing results on the most promising regional discriminants

guided wave propagation in laterally varying media ii lg waves in north western europe summary lg wave observations at norsar from a sequence of explosions straddling the central graben in the north sea show that the phase is almost extinguished by passing through this structure for models of the graben structure based on seismic refraction experiments with crustal thinning beneath a zone of thickened sediments numerical modelling of lg propagation shows very poor transmission the inclusion of attenuation simulating the effects of scattering from faults and other complex structures further diminishes the transmittivity the theoretical effect of a crustal pinch is to redistribute s energy over a wide range of groups velocities a pattern that fits well with the observations three components recordings at norsar of explosive charges fired in water covering a wide range of azimuths show significant transverse components to the seismograms the transverse energy builds up with travel time and for lg is of the same order as the vertical component these results are consistent with the theoretical prediction of progressive net transfer of energy from rayleigh to love modes as the wavetrain propagates through a three dimensionally heterogeneous medium

possible resonance effect in the distribution of earthquake damage in mexico city the destruction caused by the earthquake of 19 september 1985 in mexico city shows three remarkable features1 first a concentration of damage in the former lake bed second a peculiar distribution of high and low damage areas alternating within a few city blocks and third a selectivity for buildings between five and fifteen storeys high while the last point is understood by civil engineers2 and the first confirms the relevance of the soft ground the second feature is the most puzzling to a physicist such a pattern evokes the idea of a standing wave with low and high damage areas corresponding to nodes and antinodes this suggests a resonance phenomenon of the waterlogged ground moving collectively from the navier equations of classical elasticity it is known3 that a transfer of the incoming seismic wave energy to longitudinal waves takes place if a sharp change from hard to soft ground exists solving then the two dimensional poisson equation for the longitudinal waves within the ancient lake boundaries we show that for the observed seismic wave period of 2 s a standing wave solution exists which explains qualitatively the damage distribution pattern

the oaxaca mexico earthquake of 1931 lithospheric normal faulting in the subducted cocos plate a study of the great earthquake in oaxaca mexico on 15 january 1931 shows that it was a normal fault earthquake striking east west the lack of evidence of surface faulting near the source region1 and the focal depth of 40 km inferred from body wave modelling suggest that the earthquake occurred within the subducted slab the large size of the earthquake ms 8 0 strongly suggests that the rupture broke through the entire lithosphere of the subducted cocos plate other large normal fault earthquakes in the japanese2 aleutian3 peruvian4 and sumba5 arcs show lithospheric faulting seawards of the trench or very close to it the 1931 event however occurred farther down dip in the slab 150 km landward of the trench below the bottom of what appears to be the seismogenic plate interface we speculate here that large although infrequent normal fault earthquakes such as the 1931 oaxaca earthquake may have a role in decoupling at depth the subducted slab from the upper part of the slab and perhaps from the overriding mexican continental plate the epicentral location of the 1858 michoacan earthquake6 ms 7 5 suggests that it may represent another example of lithospheric normal faulting in the middle american arc such earthquakes for which the periods of recurrence are unknown constitute an extremely high seismic risk to population centres of mexico

modal variability in the southeast pacific basin energetics of the 2009 event abstract we study the barotropic variability in the southeast pacific basin in particular focusing on the extreme event during the fourth quarter of 2009 a 3 year integration of a barotropic shallow water model forced with wind stress anomalies generates localized variability that is similar in spatial extent and amplitude as the observed anomalous event an eigenmode analysis of the same model shows the presence of several free modes in the southeast pacific but projection of the modal patterns on the model output shows that their amplitudes are low instead the mode is interpreted as an almost free mode the modal excitation accounts for a considerable fraction 23 on average of the kinetic energy input by the wind stress in the southeast pacific basin increasing to 38 for the anomalous event in 2009 surprisingly a similar but weaker event during the third quarter of 2008 appears to have been more significant from an energetics point of view with almost 50 of the energy being input into the mode key areas of energetic dissipation appear to be the eltanin fracture zone the crest of the east pacific rise and the chile rise east pacific rise intersection

detection of the earth rotation response to a rapid fluctuation of southern ocean circulation in november 2009 1 at seasonal and shorter periods the solid earth and its overlying geophysical fluids form a closed dynamical system which except for tidal forcing conserves its total angular momentum while atmospheric effects dominate changes in the earth s rate of rotation and hence length of day lod on these time scales the addition of oceanic angular momentum oam estimates has been shown to improve closure of the lod budget in a statistical sense here we demonstrate for the first time the signature of a specific sub monthly ocean current fluctuation on the earth s rotation rate coinciding with recently reported anomalies which developed in southeast pacific surface temperature and bottom pressure fields during late 2009 our results show that concurrent variations in the antarctic circumpolar current acc which saw a sharp drop and recovery in zonal transport during a two week period in november were strong enough to cause a detectable change in lod following the removal of atmospheric angular momentum aam computed from the modern era retrospective analysis for research and applications merra database the strong oam variations driving the lod aam changes were diagnosed from ocean state estimates of the consortium for estimating the circulation and climate of the ocean ecco and involved roughly equal contributions from the current and pressure terms with in situ confirmation for the latter provided by tide corrected bottom pressure recorder data from the south drake passage site of the antarctic circumpolar current levels by altimetry and island measurements acclaim network

interannual bottom pressure signals in the australian antarctic and bellingshausen basins abstractanalyses of large scale 750 km ocean bottom pressure pb fields derived from the gravity recovery and climate experiment grace and from an estimating the circulation climate of the ocean ecco state estimate reveal enhanced interannual variability partially connected to the antarctic oscillation in regions of the australian antarctic basin and the bellingshausen basin with pb magnitudes comparable to those of sea level and good correlation between the grace and ecco pb series consistent with the theory of gill and niiler the patterns of stronger pb variability are partly related to enhanced local wind curl forcing and weakened gradients in h f where h is ocean depth and f is the coriolis parameter despite weaker h f gradients motions against them are sufficiently strong to play a role in balancing the local wind input topographic effects are as or more important than changes in f additionally and contrary to the dominance of barotropic processes at subannual time scales barocl

antarctic circumpolar current transport variability during 2003 05 from grace abstract gravity recovery and climate experiment grace gravity data spanning january 2003 november 2005 are used as proxies for ocean bottom pressure bp averaged over 1 month spherical gaussian caps 500 km in radius and along paths bracketing the antarctic circumpolar current s various fronts the grace bp signals are compared with those derived from the estimating the circulation and climate of the ocean ecco ocean modeling assimilation system and to a non boussinesq version of the regional ocean model system roms the discrepancy found between grace and the models is 1 7 cmh2o 1 cmh2o 1 hpa slightly lower than the 1 9 cmh2o estimated by the authors independently from propagation of grace errors the northern signals are weak and uncorrelated among basins the southern signals are strong with a common seasonality the seasonal cycle grace data observed in the pacific and indian ocean sectors of the acc are consistent with annual and semiannual amplitudes of 3 6 and 0 6 cmh2o 1 1 and 0

time variable gravity observations of ice sheet mass balance precision and limitations of the grace satellite data time variable gravity data from the gravity recovery and climate experiment grace mission have been available since 2002 to estimate the mass balance of the greenland and antarctic ice sheets we analyze current progress and uncertainties in grace estimates of ice sheet mass balance we discuss the impacts of errors associated with spherical harmonic truncation spatial averaging temporal sampling and leakage from other time dependent signals e g glacial isostatic adjustment gia the largest sources of error for antarctica are the gia correction the omission of l 1 terms nontidal changes in ocean mass and measurement errors for greenland the errors come mostly from the uncertainty in the scaling factor using release 5 0 rl05 grace fields for january 2003 through november 2012 we find a mass change of 258 41 gt yr for greenland with an acceleration of 31 6 gt yr2 and a loss that migrated clockwise around the ice sheet margin to progressively affect the entire periphery for antarctica we report changes of 83 49 and 147 80 gt yr for two gia models with an acceleration of 12 9 gt yr2 and a dominance from the southeast pacific sector of west antarctica and the antarctic peninsula

the zihuatanejo mexico earthquake of 1994 december 10 m 6 6 source characteristics and tectonic implications summary an analysis of the zihuatanejo mexico earthquake of 1994 december 10 m 6 6 based on teleseismic and near source data shows that it was a normal faulting intermediate depth h 50 k 5 km event it was located about 30 km inland within the subducted cocos plate the preferred fault plane has an azimuth of 130 a dip of 79 and a rake of 86 the rupture consisted of two subevents which were separated in time by about 2s with the second subevent occurring downdip of the first the measured stress drop was relatively high requiring a acr of about a kilobar to explain the high frequency level of the near source spectra a rough estimate of the thickness of the seismogenic part of the oceanic lithosphere below zihuatanejo based on the depth and the rupture extent of this event is 40 km this event and the oaxaca earthquake of 1931 january 15 m 7 8 are the two significant normal faulting intermediate depth shocks whose epicentres are closest to the coast both of these earthquakes were preceded by several large to great shallow lowangle thrust earthquakes occurring updip the observations in other subduction zones show just the opposite normal faulting events precede not succeed updip thrust shocks indeed the thrust events soon after their occurrence are expected to cause compression in the slab thus inhibiting the occurrence of normal faulting events to explain the occurrence of the zihuatanejo earthquake we note that the cocos plate after an initial shallow angle subduction unbends and becomes subhorizontal in the region of the unbending the bottom of the slab is in horizontal extension we speculate that the large updip seismic slip during shallow low angle thrust events increases the buckling of the slab resulting in an incremental tensional stress at the bottom of the slab and causing normal faulting earthquakes this explanation may also hold for the 1931 oaxaca event

analysis of large scale ocean bottom pressure variability in the north pacific 1 we use the leading empirical orthogonal functions eofs of ocean bottom pressure obp derived from an ocean model and the technique of eof reconstruction to reduce noise in the large scale obp variations derived from the gravity recovery and climate experiment grace the subsequent obp variations from the model and grace are then examined in the north pacific between january 2003 and may 2007 although annual and semiannual variations are similar grace observes large interannual fluctuations poleward of 30 where obp increases from a low of nearly 3 cm below normal in early 2003 to normal throughout 2004 and 2005 then an increase of nearly the same magnitude in 2006 these fluctuations have also been observed in obp inferred from satellite altimetry corrected for steric variations computed from argo float data since grace and steric corrected altimetry are completely independent observations of obp we conclude that the model has errors or deficiencies in predicting the interannual obp fluctuations in the north pacific

the distance dependence of regional phase discriminants abstract amplitude ratios of p and s phases recorded at regional distances have been suggested as potential discriminants for the character of seismic sources because of the differences expected in the radiation patterns of earthquakes and well contained explosions the most useful reference phase for p waves beyond 200 km from the source is pn and for s possible choices are the mantle phase sn and the crustally guided wave lg each of these phases has a different interaction with the seismic structure of the crust and mantle and such structural effects will impose their own patterns on the radiation characteristics from the source the range dependent component in lg pn and sn pn ratios have been investigated for a dense set of three component records covering the distance range out to 700 km for an explosive shot in water in southern sweden which formed part of the 1979 fennolora experiment a stable measure of amplitudes and amplitude ratios is provided by using the vector resultant of ground motion the square root of the total energy however even for a common source there are significant variations in the amplitude ratios for lg pn and sn pn of a factor of three or more as a function of range three component record sections allow a detailed examination of the way in which individual records relate to the whole wavefields the role of multiple reflections in building the pg and lg phases can be clearly seen as well as the complex substructure of the pn and sn phases returned from the uppermost mantle

evaluation of new grace time variable gravity data over the ocean 1 monthly grace gravity field models from the three science processing centers csr gfz and jpl are analyzed for the period from february 2003 to april 2005 over the ocean the data are used to estimate maps of the mass component of sea level at smoothing radii of 500 km and 750 km in addition to using new gravity field models a filter has been applied to estimate and remove systematic errors in the coefficients that cause erroneous patterns in the maps of equivalent water level the filter is described and its effects are discussed the grace maps have been evaluated using a residual analysis with maps of altimeter sea level from jason 1 corrected for steric variations using the world ocean atlas 2001 monthly climatology the mean uncertainty of grace maps determined from an average of data from all 3 processing centers is estimated to be less than 1 8 cm rms at 750 km smoothing and 2 4 cm at 500 km smoothing which is better than was found previously using the first generation grace gravity fields

expected improvements in determining continental hydrology ice mass variations ocean bottom pressure signals and earthquakes using two pairs of dedicated satellites for temporal gravity recovery 1 the gravity recovery and climate experiment mission has demonstrated the ability to quantify global mass variations at large spatial scales with monthly to sub monthly temporal resolution future missions of this type taking advantage of improved measurement technologies will be limited by temporal aliasing errors we suggest the addition of a second pair of satellites to reduce these errors using an optimized mission architecture consisting of a polar pair of satellites coupled with a lower inclined pair of satellites 72 both in 13 day repeating orbits we quantify the expected scientific improvements that having two pairs of satellites will provide over one pair numerical simulations to spherical harmonic degree 100 are run over one full year analysis using empirical orthogonal functions reveals that two satellite pairs determine annual mass variations in small basins which are undetected using one pair of satellites averaging kernels are used to show that two satellite pairs offer an 80 reduction in the level of error in determining mass variations in 53 hydrological basins and 12 greenland basins over the year after standard grace post processing techniques have been applied to the one pair solutions it is seen that two satellite pairs with no post processing still offer a 25 75 improvement in determining the mass variations spatiospectral localization analysis is used to show increased spatial resolution and higher signal to noise ratios in recovering hydrology in the amazon river basin ocean bottom pressure signals in the southeast pacific basin and a simulated earthquake signal representative of the 2010 maule chile earthquake

is there truly a hard rock site in the valley of mexico to understand the cause of observed amplification of seismic waves even at hill zone sites in the valley of mexico digital accelerographs have been installed at three especially chosen sites two of these sites madi and texc located on hard pleistocene lava a few meters in thickness overlying oligocene andesites were expected to be free of site effects analysis of the data recorded by these and other accelerographs during three moderate shallow subduction zone events however shows significant amplification at madi and texc between 0 2 and 0 6 hz with respect to established attenuation relations the cause of the amplification at hill zone sites in the valley of mexico including madi and texc may be pervasive low s wave velocity in and complex structure of the upper layers of the volcanic rocks if so then there may not be a truly hard rock site in the valley

strong ground motion from the michoacan mexico earthquake the network of strong motion accelerographs in mexico includes instruments that were installed under an international cooperative research program in sites selected for the high potenial of a large earthquake the 19 september 1985 earthquake magnitude 8 1 occurred in a seismic gap where an earthquake was expected as a result there is an excellent descripton of the ground motions that caused the disaster

increasing rates of ice mass loss from the greenland and antarctic ice sheets revealed by grace 1 we use monthly measurements of time variable gravity from the grace gravity recovery and climate experiment satellite gravity mission to determine the ice mass loss for the greenland and antarctic ice sheets during the period between april 2002 and february 2009 we find that during this time period the mass loss of the ice sheets is not a constant but accelerating with time i e that the grace observations are better represented by a quadratic trend than by a linear one implying that the ice sheets contribution to sea level becomes larger with time in greenland the mass loss increased from 137 gt yr in 2002 2003 to 286 gt yr in 2007 2009 i e an acceleration of 30 11 gt yr2 in 2002 2009 in antarctica the mass loss increased from 104 gt yr in 2002 2006 to 246 gt yr in 2006 2009 i e an acceleration of 26 14 gt yr2 in 2002 2009 the observed acceleration in ice sheet mass loss helps reconcile grace ice mass estimates obtained for different time periods

a note on the barotropic response of sea level to time dependent wind forcing this study examines the extent to which sea level variations at periods between 30 days and 1 year and spatial scales greater than 1000 km can be described by the wind driven linear barotropic vorticity dynamics the topex poseidon altimetric observations of sea level and the wind products of the national meteorological center are used as the database for the study each term of the linear barotropic vorticity equation was evaluated by averaging over regions of 10 10 in most of the open ocean the result of the analysis suggests that the sea level variabilities at the scales considered cannot be fully described by the equation the apparent net vorticity change is more than what can be explained by the local wind stress curl in the few regions where the wind stress curl is strong enough to balance the vorticity budget predominantly in the northeast pacific and the southeast pacific the balance is basically achieved in terms of the time dependent topographic sverdrup relation namely the balance between the advection of the planetary vorticity plus the topography induced vorticity and the forcing by the wind stress curl

on the nature of regional seismic phases iii the influence of crustal heterogeneity on the wavefield for subduction earthquakes the 1985 michoacan and 1995 copala guerrero mexico earthquakes summary the most prominent feature of the regional seismic wavefield from about 150 to over 1000 km is usually the lg phase this arrival represents trapped s wave propagation within the crust as a superposition of multiple reflections and its amplitude is quite sensitive to the lateral variation in the crust along a propagation path in an environment where the events occur in a subduction zone such as the western coast of mexico quite complex influences on the character of the regional wavefield arise from the presence of the subduction zone the great 1985 michoacan earthquake mw 8 1 which occurred in the mexican subduction zone was one of the most destructive earthquakes in modern history and its notable character was that at mexico city located over 350 km from the epicentre there was strong ground shaking almost comparable to that in the epicentral region that lasted for several minutes considerable effort has been expended to explain the origin of the unusual observed waves that caused the severe damage in the capital city during the destructive earthquake the nature of the propagation process in this region can be understood in part by using the detailed strong motion records from the 1995 copala guerrero mw 7 4 earthquake near the coast to the south of mexico city which also had an enhanced amplitude in the valley of mexico numerical modelling of both p and s seismic waves in 2 d and 3 d heterogeneous crustal models for western mexico using the pseudospectral method provides direct insight into the nature of the propagation processes through the use of sequences of snapshots of the wavefield and synthetic seismograms at the surface a comparison of different models allows the influences of different aspects of the structure to be isolated 2 d and 3 d modelling of the 1985 michoacan and 1995 copala earthquakes clearly demonstrates that the origin of the long duration of strong ground shaking comes from the sn and lg wave trains these s wave arrivals are produced efficiently from shallow subduction earthquakes and are strongly enhanced during their propagation within the laterally heterogeneous waveguide produced by the subduction of the cocos plate beneath the mexican mainland the amplitude and duration of the lg coda is also strongly reinforced by transmission through the mexican volcanic belt from the amplification of s waves in the low velocity surficial layer associated withs to p conversions in the volcanic zone the further amplification of the large and long lg wave train impinging on the shallow structure in the basin of mexico city with very soft soil underlain by nearly rigid bedrock with a strong impedance contrast gives rise to the destructive strong ground shaking from the mexican subduction earthquakes

impact of assimilating bottom pressure anomalies from grace on ocean circulation estimates 1 the impact of assimilating global ocean bottom pressure obp information from the gravity recovery and climate experiment grace gravity anomalies on the circulation estimate was investigated for this an estimate of the ocean circulation is being inferred by extending the 50 year long german part of the estimating the circulation and climate of the ocean gecco ocean synthesis into recent years the assimilation system is an improved version of the previous gecco optimization which now includes a sea ice model has enhanced resolution on a truly global domain including the arctic ocean by analyzing differences to a synthesis that additionally assimilated obp the grace data was found to provide complementary information to standard ocean data sets including satellite altimetry when assimilated although in principle standard ocean data sets include the obp information the reason why this cannot be extracted is the much larger prior errors for hydrographic and altimeter data in comparison to obp data owing to the fact that only the former two need to include the unresolved eddy signal the largest impact of gravity data is found to be on the barotropic circulation particularly in the subtropical gyres and the polar latitudes remaining differences between the simulated and observed obp information are associated with meridional stripes in the grace gravity maps and with the leakage of terrestrial hydrological information into the ocean additional differences close to the continental boundaries are related to the self attraction and loading processes that are not included in the models

geometry of subduction and depth of the seismogenic zone in the guerrero gap mexico the western coast of guerrero in southern mexico has been identified as a seismic gap on the middle american trench in which no large earthquakes have occurred at least since 1908 it has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake a permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region the seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast the resulting geometry of the subduction zone shows that the cocos plate dips at a shallow angle beneath the north american plate to a depth of 40 km from there the subducted slab is bent upward following a subhorizontal trajectory extending inland at a depth of 50 km this plate geometry is reminiscent of that found in peru and central argentina two other regions where a young oceanic plate is being subducted in mexico however the slab underplates an overriding plate which is only half as thick as that observed in south america a possible oceanic origin of the allochtonous terranes comprising southern mexico may explain the presence of the anomalously thin lithosphere in this region

record warming in the south pacific and western antarctica associated with the strong central pacific el nino in 2009 10 1 satellite data for the past three decades reveal a record high sea surface temperature sst anomaly within a large mid latitude region of the south central pacific scp during the mature phase of the 2009 10 el nino with a peak magnitude that is 5 times the standard deviation of local sst anomaly and is warmer than the concurrent tropical pacific sst anomaly the scp oceanic warming was confined to the upper 50 meters and is associated with an extreme and persistent anticyclone wind changes associated with the anticyclone caused the oceanic warming with surface heat flux and ocean processes playing equally important roles the anticyclone diverted circumpolar westerlies and warm air towards antarctica austral summer sst in the bellingshausen sea also reached a three decade high the extreme atmospheric and oceanic anomalies in the south pacific may have been fueled by the 2009 10 el nino because of its record high sst anomaly in the central equatorial pacific

generation of inhomogeneous p waves in a layered medium abstract the generation of inhomogeneous horizontally propagating p waves in a stratified elastic medium were analyzed for incident sv waves beyond the critical angle using realistic values of s and p wave velocities and considering both clay and alluvial sedimentary layers overlying a bedrock half space we obtained enormous amplifications for the horizontal displacements at the free surface these amplifications are of the order of several thousands for the fundamental mode of the system which systematically dominates all other frequencies

bottom pressure changes around antarctica and wind driven meridional flows 1 spatially averaged bottom pressure anomalies near antarctica south of 60 s calculated from grace data are well correlated with those produced by the ecco project using least squares optimization methods to fit an ocean model to most available data both grace and ecco results indicate mass exchange primarily between the southern ocean and the pacific and the importance of zonal wind stress to this exchange the ecco flow fields show that the near surface meridional ekman transport directly driven by the zonal wind stress is nearly balanced by return flows below the shallowest topography at 60 s 1300 m at this drake passage latitude with the return transport being slightly lagged in time relative to the ekman transport such time lags which may result from geostrophic adjustment at depth cause the small associated net transport across 60 s to be 90 out of phase with the wind this in turn can explain why zonal wind stress and bottom pressure anomalies around antarctica tend to be anticorrelated in both grace and ecco results

geometry of the benioff zone and state of stress in the overriding plate in central mexico analysis of depths and focal mechanisms of 16 small earthquakes in central mexico along with previous data shows that a the subducted cocos plate becomes subhorizontal between 110 and 275 km from the trench reaching a depth of about 50 km and b the bottom part of the overriding continental plate is in tensional stress regime neotectonic structures in central mexico and stress orientations estimated from borehole elongations cinder cone alignments and fault slip analysis in the trans mexican volcanic belt which lies in the northeastern portion of the area under study also indicate the same stress regime absolute motion of the north american plate in the region has a component normal to the trench of 20 mm yr which if true would result in compressional stress in the upper plate if the trench position is fixed in an absolute frame of reference if we allow for seaward retreat of the trench then the tensional stress in the overriding plate and the observed geometry of the benioff zone can be explained this however implies that seaward retreat of a trench is possible even for a young 15 m y old subducting slab alternatively tectonic erosion of the leading edge of the continent which has been proposed to explain truncated igneous and metamorphic continental terrane could give rise to the tensional stress but would not explain the geometry of the benioff zone

a record high ocean bottom pressure in the south pacific observed by grace in late 2009 to early 2010 the gravity recovery and climate experiment grace satellite pair observed a record increase in ocean bottom pressure obp over a large mid latitude region of the south east pacific its magnitude is substantially larger than other oceanic events in the southern hemisphere found in the entire grace data records 2003 2010 on multi month time scales the obp data help to understand the nature of a similar signal in sea surface height ssh anomaly observed by altimetry the ssh increase is mainly due to mass convergence analysis of the barotropic vorticity equation using scatterometer data atmospheric reanalysis product and grace and altimeter an atmospheric reanalysis product observations suggests that the observed obp ssh signal was primarily caused by wind stress curl associated with a strong and persistent anticyclone in late 2009 in combination with effects of planetary vorticity gradient bottom topography and friction

quantitative analysis of the inclined low velocity zone in the mantle wedge of northeastern japan a systematic change of melt filled pore shapes with depth and its implications for melt migration abstract travel time tomography beneath the northeastern japan arc reveals that an inclined low velocity zone exists in the mantle wedge sub parallel to the down dip direction of the slab this zone distributed continuously along the arc as a single inclined sheet has been considered as the main source of arc magma a quantitative interpretation of both p and s wave velocity structures precisely determined from the recent travel time tomography is done in terms of thermal heterogeneity and fluid content the combined analysis of both p and s wave velocity structures makes it possible to independently derive the information on the shape and volume fraction of the fluid filled pores the reference p and s wave velocities representing the host rock velocities at reference temperature t 0 are estimated from the tomographic data while an alternative method based on high temperature and high pressure elasticity of upper mantle minerals is used to confirm the validity of the present method the calculated velocity anomalies are corrected for the thermal effect using the three dimensional thermal structure estimated from the p wave attenuation data showing that the observed low velocity anomalies cannot be explained by the thermal effect alone the remaining velocity anomalies are explained by the existence of melt filled pores and effective aspect ratio and volume fraction of the pores are determined the results show a systematic change in melt filled pore shapes with depth suggesting the existence of 3 6 vol melts as grain boundary tubules at a depth of 90 km 0 04 0 05 vol melts as thin cracks or dikes with aspect ratio of 0 001 at a depth of 65 km and 1 2 vol melts as cracks or dikes with aspect ratio of 0 02 0 04 at a depth of 40 km

variability of the ross gyre southern ocean drivers and responses revealed by satellite altimetry year round variability in the ross gyre rg antarctica during 2011 2015 is derived using radar altimetry the rg is characterized by a bounded recirculating component and a westward throughflow to the south two modes of variability of the sea surface height and ocean surface stress curl are revealed the first represents a large scale sea surface height change forced by the antarctic oscillation the second represents semiannual variability in gyre area and strength driven by fluctuations in sea level pressure associated with the amundsen sea low variability in the throughflow is also linked to the amundsen sea low an adequate description of the oceanic circulation is achieved only when sea ice drag is accounted for in the ocean surface stress the drivers of rg variability elucidated here have significant implications for our understanding of the oceanic forcing of antarctic ice sheet melting and for the downstream propagation of its ocean freshening footprint

the influence of critical moho reflections on strong ground motions recorded in san francisco and oakland during the 1989 loma prieta earthquake the amplitudes of strong ground motions from the loma prieta earthquake recorded in the san francisco and oakland areas exceeded the levels predicted by standard empirical attenuation relations preliminary analysis of accelerograms having known trigger times strongly suggests that the elevation of ground motion amplitudes in the distance range of approximately 40 to 100 km was due to critical reflections from the base of the crust these reflections which are identified on the basis of their arrival times and phase velocity and by comparison with simulated accelerograms were large and occurred at relatively close range because of the deep focal depth of the earthquake and the strong velocity gradient at the base of the crust these motions were further amplified presumably by impedance contrast effects at soft soil sites in san francisco and oakland the effect of the critical reflections in amplifying peak accelerations of the loma prieta earthquake in the san francisco and oakland regions was as large as the effect of soft soil site conditions focal depth has an important influence on strong motion attenuation at distances beyond about 40 km and empirical attenuation relations derived from shallow crustal earthquakes may underpredict the ground motions of deeper crustal events in this distance range further analyses using an expanded data base that includes recordings of aftershocks are required to rigorously test the proposed explanation of the ground motions recorded in san francisco and oakland and the conclusions drawn from that explanation

modeling short period crustal phases p lg for long range refraction profiles abstract the short period seismic phases known as p and lg are often recorded at distances of 200 1000 km on long range refraction profiles and are usually the largest amplitude features on record sections for this distance range p and lg propagate as multiply reflected compressional and shear waves in a crustal waveguide whose principal boundaries are the moho and the free surface equivalently they can be interpreted as the interference pattern produced by a superposition of higher mode p sv and sh waves propagating in a leaky waveguide for compressional waves the waveguide efficiency is a strong function of frequency and depends on the presence or absence of low velocity layers within a few kilometers of the surface such as deep sedimentary sections commonly found in active tectonic areas such low velocity surface layers create constructive interference effects for upcoming p waves incident at near grazing angles at the free surface and lead to efficient p propagation several good examples of strong p phases can be found on long range refraction profiles for the tectonically active western united states the 550 km profile eastward from shoal to delta ut is analyzed here we have used a modified reflectivity method computer program to model crustal phases for the shoal delta profile the reflectivity technique accounts for all body and surface waves contributing to the short period seismograms it is found that the synthetic waveforms realistically model the observed p characteristics in this case the decay of p amplitudes with distance appears to be dominated by surface reflection leakage from the waveguide rather than by anelastic attenuation due to q of crustal rocks

coefficients for sea surface wind stress heat flux and wind profiles as a function of wind speed and temperature surface layer coefficients for wind profiles wind stress and heat flux in typical open sea conditions are briefly reviewed businger dyer flux gradient relationships and a charnock wind stress formula fit the empirical data and are dimensionally consistent these have been solved by an iterative method and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data

lg wave propagation and crustal structure differences near denmark and the north sea summary recordings of regional earthquakes near denmark and the north sea have been collected in an attempt to find the lg wave propagation properties in the area propagation paths of lengths between 300 and 1100km are investigated the amplitudes of the lg waves and the other seismic phases in the seismograms are compared the propagation paths are classified according to whether the lg wave propagation compared to p and s wave propagation is good intermediate or poor it is noted that there is a distinct regional pattern in the lg wave attenuation the lg wave propagation is poor across the north sea the north sea is surrounded by areas of good continental lg wave propagation except towards the south east in the netherlands and western germany an area in skagerrak is characterized by good but slow propagation of a surface wave similar to lg since this is exactly the area of a cluster of earthquakes in skagerrak it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area just off the coast of north western denmark the lg wave propagation is as good in the northern part of denmark as it is in the fennoscandian shield the extent to which the propagation is poor across the north sea is dependent on the frequency of the lg waves observed with different seismographs the lg waves are more attenuated relative to the other seismic phases the higher the frequency is between 0 25 and 2 hz as the structural differences between the fennoscandian bedrock area and the norwegian danish basin with 8 10 km of sediments has no effect on the lg wave propagation it is suggested that the structural anomalies causing the poor lg wave propagation across the north sea are deep crustal anomalies possibly significant variations in crustal thickness it is likely that such deep crustal differences are located along the most prominent upper crustal differences in the sedimentary grabens in the middle of the north sea the poor propagation across the north sea is similar to that observed across a sedimentary basin in eastern greenland which also developed mainly before seafloor spreading separated greenland from europe the frequency dependent propagation of the lg waves is explainable in terms of surface wave propagation across structural boundaries as well as in terms of anelastic attenuation

wind forced intraseasonal sea level variability of the extratropical oceans abstract seven years worth of sea level observations from the topex poseidon altimeters and wind observations from the european remote sensing satellites ers 1 2 scatterometers were used to investigate the dynamics of large scale intraseasonal sea level variability at mid and high latitudes coherent patterns of sea level variability with spatial scales of 1000 km and timescales of 20 days to 1 year are identified in three particular regions the bellingshausen basin west of drake passage the australian antarctic basin and the central north pacific ocean 30 50 n near the date line significant coherence is found between wind stress curl and the sea level variability a simple barotropic vorticity equation in which the time rate of relative vorticity variation is balanced by the wind stress curl and a damping term is used to simulate the sea level anomalies with a significant degree of correlation with the observations although the coherence between the simulation and the observation is signifi

ground motion in mexico city during the april 25 1989 guerrero earthquake abstract instrumental observations of ground motion in mexico city during the april 25 1989 guerrero earthquake were analyzed our aim was to understand various aspects of the seismic response of the valley that had not been completely resolved such understanding of the basic mechanisms that control the seismic behavior of the valley sediments is crucial in any modeling attempt the study of vertical motion for this event which was shown to be practically unaffected by site conditions lead to the identification of a prominent long period rayleigh wave this together with the availability of absolute time for some stations allowed the establishment of a common time reference for all recordings horizontal motion in contrast was significantly amplified with large increases in duration at lake bed sites in order to interpret the observed complexity of ground motion we studied two simplified models of soft alluvial valleys one of these is two dimensional and it is excited by plane s waves with variable polarization and incidence angles this model allows three dimensional response the other is a three dimensional axi symmetric flat valley with a rigid base computations were performed in the frequency domain by means of a boundary integral method for the two dimensional model and using a collocation least squares technique for the three dimensional one seismograms were obtained through fourier synthesis it was found that the irregular soft layer response produces polarization patterns which are similar to the observations suggesting that the latter are a consequence of three dimensional effects

causes of large scale sea level variations in the southern ocean analyses of sea level and a barotropic model 1 we analyze a decade of sea surface height ssh measurements in the southern ocean from the topex poseidon and ers altimeters with a focus on the variability at timescales 2 years among the different processes contributing to large scale ssh variations the barotropic response to the winds dominates poleward of 50 s while thermosteric processes dominate equatorward except for resonant basins for the barotropic modes and regions of intense eddy activity a finite element barotropic model has been developed to analyze the vorticity budget the ssh from the model agrees well with observations the leading barotropic mode which is annular and is confined near antarctica is responsible for most of the barotropic circumpolar transport it is coherent with the zonally integrated eastward wind stress consistent with a free mode response although previously evidenced in bottom pressure data this mode is only partially seen in altimeter data because of ice coverage it nevertheless distinctly appears above the pacific ridges where it expands meridionally up to midlatitudes in the rest of the domain several regions coherent with the local wind stress curl are found these are regions isolated by f h contours mostly deep basins an analysis of the vorticity budget shows that generally topographic sverdrup balance is the leading process for periods 50 days but in some regions resonant basins diffusive and nonstationary terms are important a model experiment shows that transients redistribute energy along f h waveguides contributing to drain resonant regions as was hypothesized in previous works

numerical simulation of high and low frequency lg wave propagation summary lg waves are sometimes extinguished when traversing geological structures such as mountain ranges or grabens previous investigations have applied numerical full waveform simulations at relatively low frequencies on the order of 1 hz to investigate the relationship of different structural models and the amplitude anomalies of the lg waves these studies often fail to link specific known structures to the amplitude anomalies however the observed waveforms also often contain data of significantly higher frequency than the full waveform simulations we compare such low frequency simulations computed with the boundary integral equation bie technique with synthetic seismograms generated by dynamic ray tracing drt beginning with a plane layered model of the crust in central france we show that the general characteristics of the lg wave train over distances ranging from 100 km to around 500 km can be modelled by computing a relatively small number of rays reflected between the free surface and the moho matching the details of the waveforms is more difficult but can be improved by the addition of reflections between other interfaces in the model considering a one layer model of the crust with a step like change in moho depth that leads to a rapid decrease in crustal thickness we show again that the bie and drt solutions are very similar in character significantly both solutions predict that the lg wave will not be extinguished on traversing the moho step finally we consider a more complete and detailed model of the pyrenees though it is more difficult to match exactly the ray and integral equation solutions the general trends continue to be the same and the lg waves are still predicted to cross the tectonic structures associated with the mountains therefore it appears that observed amplitude changes must be caused by something other than the general structure of the pyrenees such as scattering by smaller scale features within the lower crust