uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

the 1989 macquarie ridge earthquake and its contribution the regional seismic moment budget the macquarie ridge earthquake 1989 may 23 mw 8 2 occurred in a region of complex tectonics that marks the southern continuation of the plate boundary pac aus that runs through new zealand the first motion focal mechanism from wwssn and regional station polarities indicates dominantly strike slip movement strike 025 dip 80 rake 180 probably dextral on the northeast striking nodal plane the plate boundary south of new zealand appears to be segmented and marked by linear bathymetric highs with distinctive trends the distribution of shallow seismicity follows these bathymetric trends the moment tensors for the largest earthquakes ms 6 0 occurring on five distinctive segments of the plate boundary for the period 1964 1989 have been summed and compared with the deformation predicted by the plate motions recurrence times of the largest earthquakes in each segment determined by assuming that all deformation occurs seismically along the macquarie ridge complex mrc generally show longer recurrence times than those determined from the historic record these results suggest that during the last 70 years there has been more seismic moment release along the mrc than predicted by the plate motions the preferred explanation is that this century is a seismically overactive period which is part of a natural episodicity of quiescence and overactivity alternative explanations include complex plate interactions resulting from proximity of the plate boundary to the instantaneous pole of rotation and or the existence of a southeast australian microplate

long period seismic radiation from the may 23 1989 macquarie ridge earthquake evidence for coseismic slip in the mantle long period seismic source parameters of the may 23 1989 macquarie ridge earthquake are determined using a surface wave inversion procedure that incorporates detailed source time functions obtained from shorter period body waves the seismic source model obtained using this method is consistent with observations of both body waves and long period rayleigh waves from the earthquake the macquarie ridge earthquake rupture has a centroid time 28 s and a right lateral strike slip fault mechanism with a rake of 175 on a vertical fault plane striking n38 e this mechanism is consistent with p wave first motions of the event inversions performed for various earth models demonstrate that the choice of surface wave attenuation model in particular affects the estimates of centroid depth and seismic moment significantly allowing for uncertainty in attenuation the long period rayleigh waves periods from 150 to 300 s indicate that the macquarie ridge earthquake had a seismic moment of 1 9 0 2 1021 n m and a corresponding centroid depth of 21 6 km the static stress drop calculated using the depth and seismic moment is 0 7 0 5 107 pa 70 50 bars the centroid depth combined with the lack of resolvable directivity of the earthquake rupture suggests that significant slip occurred beneath the moho which has a maximum depth of about 16 km in the epicentral region the mantle slip component may have preferentially radiated long period seismic energy given the shallow centroid depths and low moments determined for the event from the shorter period body wave observations

crustal structure and neotectonics of the puysegur oblique subduction zone new zealand abstract eocene oligocene rifting followed by miocene quaternary strike slip and obliquely convergent tectonics are responsible for crustal thinning in the solander basin and subsequent development of the puysegur oblique subduction zone southwest new zealand highly oblique convergence at a rate of 35 mm yr is being partitioned between oblique subduction at the puysegur trench thrusting within the trench slope strike slip faulting on the puysegur bank and minor shortening within the solander basin subduction is young and a volcanic arc is not yet well developed we present seismic reflection data recorded across the puysegur subduction zone and solander basin three strike slip fault zones are recognised on the basis of seismic and bathymetric data in the puysegur bank region north of the profile some displacement on the eastern fault is transferred eastward to the hauroko fault but most strike slip motion is inferred to be west of fiordland south of the profile strike slip faults merge with faults of the puysegur ridge active thrusting dominates the trench slope where a thick sedimentary section is evident and bottom simulating reflectors are common the relatively steep average slope angle of 6 suggests the sediments are moderately well cemented possibly similar to sediments found in the solander basin the small amount of sediment found on the subducting plate is consistent with little or no accretion at the margin patches of reflectivity we interpret as sediment at the top of the subducted plate suggest that the slab has a moderate dip of 10 20 to 20 km depth in the solander basin seismic reflection data clearly image the sedimentary section and show well developed crustal and upper mantle reflections to depths of 30 35 km 12 13 s twt on the basis of lower crustal reflectivity and gravity modelling the base of the crust beneath the adjacent stewart island shelf is at 30 km 9 s twt and rises to a depth of 20 km 8 s twt beneath the solander basin prominent dipping reflections which we interpret as an active fault can be traced to 30 km depth 12 s twt where they merge into a zone of strong reflectivity in the upper mantle

geometry and mechanism of faulting of the 1980 el asnam algeria earthquake from inversion of teleseismic body waves and comparison with field observations the el asnam earthquake of october 10 1980 ms 7 3 provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models the event produced extensive surface faulting in addition to the main tectonic deformation which is clearly a thrust widespread secondary normal faulting was observed at the surface in the southwest region the surface break of a thrust fault could be traced for 24 km in the northeast part of the fault zone normal faults with throws of several meters were observed although no clear thrust type surface breaks were observed in this region geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation in this paper the rupture process of the el asnam earthquake is investigated by inversion of teleseismic p and sh waves the effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood north eastern segment of the fault zone during the earthquake rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes the seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters seismic moment 3 9 1019 n m depth 6 km strike 220 dip 46 and rake 72 the duration of rupture on this fault segment was approximately 10 s which together with the observed fault length of 24 km indicates a rupture velocity of 2 4 km s unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region the normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region guided by the evidence from aftershocks and geodetic measurements a solution with a thrust type mechanism was also sought using a priori constraints based on the field observations the observed strike and slip angle we obtain the following thrust source mechanism for the northeast segment seismic moment 3 6 1019 n m depth 6 km strike 230 dip 20 and rake 91 this solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth inversions are nonunique with respect to the seismic moment associated with normal faulting the ambiguity being caused by the limited bandwidth of the observed body waves the moment estimated from the surface waves at periods about 300 s however favors thrust faulting to be dominant in the northeast region

the oligocene miocene pacific australia plate boundary south of new zealand evolution from oceanic spreading to strike slip faulting abstract since the eocene the pacific australia plate boundary south of new zealand has evolved from a spreading system into a transform boundary swath data acquired in the southeast tasman oceanic crust between the macquarie ridge complex and the resolution ridge system show that the spreading fabric changes orientation southwards along the puysegur trench striking successively n60 e n85 e and n120 e this reflects the reorganisation of the plate boundary in response to changes in relative plate motion a comparison of these orientations with the positions of the pacific australia relative poles of rotation enables us to estimate the age of stoc where there are no identified magnetic anomalies the youngest age of the oceanic crust is ca 12 ma at the south end of the puysegur trench this age is consistent with spreading rates and the amount of crust generated since 31 ma curved fracture zones on either side of the macquarie ridge complex suggest a continuous reorientation of transform faults between 31 ma and ca 15 ma small scale seafloor morphology shows a 13 change of orientation in the l atalante fracture zone that indicates incremental rather than continuous changes in azimuth of the transform faults patterns of fanning ridges indicate that periods of asymmetric spreading accompanied the spreading segment reorientations using swath data and plate reconstruction models we infer that between 31 and 12 ma the plate boundary reorganisation resulted in a continuous increase in the ratio of the cumulative length of transform faults over the cumulative length of spreading segments along the whole plate boundary this indicates that since 14 15 ma the plate boundary has become progressively predominantly transcurrent allowing strike slip motion to develop along a line of merging transform faults that connected to the intracontinental alpine fault

earthquakes gravity and the origin of the bali basin an example of a nascent continental fold and thrust belt we infer from the bathymetry and gravity field and from the source mechanisms and depths of the eight largest earthquakes in the bali region that the bali basin is a downwarp in the crust of the sunda shelf produced and maintained by thrusting along the flores back arc thrust zone earthquake source mechanisms and focal depths are inferred from the inversion of long period p and sh waves for all events and short period p waves for two of the events centroidal depths that give the best fit to the seismograms range from 10 to 18 km but uncertainties in depth allow a range from 7 to 24 km the p wave nodal planes that dip south at 13 to 35 7 strike roughly parallel to the volcanic arc and are consistent with thrusting of crust of the bali basin beneath it the positions of the earthquakes with respect to crustal features inferred from seismic and gravity data suggest that the earthquakes occur in the basement along the western end of the flores thrust zone the slip direction for the back arc thrust zone inferred from the orientation of the earthquake slip vectors indicates that the thrusting in the bali basin is probably part of the overall plate convergence as it roughly coincides with the convergence direction between the sunda arc and the indian ocean plate summation of seismic moments of earthquakes between 1960 and 1985 suggests a minimum rate of convergence across the thrust zone of 4 2 mm a the presence of back arc thrusting suggests that some coupling between the indian ocean plate and the sunda arc occurs but mechanisms such as continental collision or a shallow subduction of the indian ocean plate probably can be ruled out the present tectonic setting and structure of the bali basin is comparable to the early forelands of the andes or western north america in that a fold and thrust belt is forming on the continental side of an arc trench system at which oceanic lithosphere is being subducted the bali basin is flanked by the tertiary java basin to the west and the oceanic flores basin to the east and thus provides an actualistic setting for the development of a fold and thrust belt in which structure and timing of deformation can change significantly along strike on the scale a few hundred kilometers

preliminary geodetic strain measurement from the south fiordland region of new zealand using repeat gps surveys implications for subduction on the puysegur trench comparing 1996 gps data and data collected only one year earlier demonstrates that 50 of the instantaneous relative plate motion is accommodated between the west coast of the south island and the eastern side of the waiau basin a distance of about 75 km the observed relative velocity vectors are nearly parallel to the inferred orientation of the plate boundary and rotated more than 30 clockwise compared to the relative plate motion vector a difference that is significant at the 95 level of confidence modeling shows that the observed deformation rates are consistent with a model where nearly all the relative plate motion occurs on the plate boundary thrust as defined by the benioff zone associated with subduction on the puysegur trench

the structural evolution of active fault and fold systems in central otago new zealand evidence revealed by drainage patterns abstract central otago in new zealand is an area of active continental shortening in which a peneplain surface cut into schist has been deformed by folds which are developed above buried reverse faults we use the drainage patterns in this region to demonstrate various processes in fold and fault growth and interaction that would be difficult to identify by other means in particular we show 1 how simple asymmetric folds can develop into box folds 2 how apparently continuous ridges were formed by the coalescing of quite separate propagating fold and fault segments 3 evidence for the relative ages or relative uplift rates of adjacent structures and 4 evidence for the propagation direction of folds or faults as they grow the few quantitative estimates we obtain for fault propagation rates suggest an increase in length of 10 50 m per earthquake on faults about 20 km long these estimates are very uncertain but are similar in magnitude to an estimate made in nevada for a normal fault of similar size and are also similar to predicted estimates from theoretical growth models they raise the question of whether fault growth earthquake recurrence rates and climate change can interact to produce semi regular discrete features in an active landscape

a numerical study of flow circulations in the central valley of california and formation mechanisms of the fresno eddy abstract in this study the authors have conducted a series of numerical experiments to investigate the flow circulations in the central valley of california and the formation mechanisms of the fresno eddy the authors have found the following under an adiabatic northwesterly low froude number flow over the central valley two cyclonic vortices form in the basin one is located on the lee slope of the northern coastal range while the other is located to the south of the san joaquin valley the first may be identified as the sacramento eddy while the second may be identified as the fresno eddy although the fresno eddy is located slightly farther to the south the formation of the sacramento eddy may be explained by either the generation of potential vorticity smith or the generation of vorticity due to baroclinicity smolarkiewicz and rotunno on the lee slope in a low froude number flow the sacramento eddy may also be classified as a lee mesocylone since it is collocated with a lee mesolow lin et

focal mechanisms of large earthquakes in the south island of new zealand implications for the accommodation of pacific australia plate motion b the plate motion model nuvel 1 predicts oblique convergence between the pacific and australian plates in the south island of new zealand we used p and sh body waveform analysis to constrain the focal mechanisms of the 15 largest earthquakes ms 5 8 that have occurred in this region since 1964 in order to see how the plate motion is accommodated at the southern end of the alpine fault convergence is achieved by oblique slip movement along a concentrated zone of deformation in the southern offshore region one event may be related to thrusting of the australian plate beneath the pacific plate and another strike slip event probably demonstrates movement on an active strike slip fault system parallel to but offset from the southern limit of the alpine fault this geometry provides a possible mechanism for the rapid uplift of the fiordland region deformation in the northern south island is more distributed in the south west marlborough region partitioning occurs between strike slip faulting in the se and reverse faulting farther nw in the buller region we suggest that the partitioning developed as a consequence of an increasing component of shortening that was accommodated by slip on reactivated pre existing normal faults in the buller region shortening in the buller region may have deflected the ne end of the alpine fault towards the nw forming the prominent bend the marlborough fault system with its youngest and most active faults to the se probably developed in an attempt to maintain a through going strike slip structure as each of the strike slip faults was transported towards the north west partitioning of the opposite polarity with reverse faulting se of the strike slip faulting occurs in north east marlborough the boundary between the two different styles of partitioning in ne and sw marlborough appears to coincide with a change in the nature of the downgoing slab and a change in strike of faults of the marlborough fault system a normal faulting earthquake on the northern edge of the chatham rise probably results from a complex interaction of the buoyant continental crust in that region with the subduction zone and the overlying marlborough fault system

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

transition from strike slip faulting to oblique subduction active tectonics at the puysegur margin south new zealand abstract south of new zealand the pacific australia pac aus plate boundary runs along the intracontinental alpine fault the puysegur subduction front and the intraoceanic puysegur fault the puysegur fault is located along puysegur ridge which terminates at ca 47 s against the continental puysegur bank in a complex zone of deformation called the snares zone at puysegur trench the australian plate subducts beneath puysegur bank and the fiordland massif east of fiordland and puysegur bank the moonlight fault system mfs represents the eocene strike slip plate boundary interpretation of seafloor morphology and seismic reflection profiles acquired over puysegur bank and the snares zone allows study of the transition from intraoceanic strike slip faulting along the puysegur ridge to oblique subduction at the puysegur trench and to better understand the genetic link between the puysegur fault and the mfs seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 geodynz survey and single beam echo soundings acquired by the nz royal navy the snares zone is the key transition zone from strike slip faulting to subduction it divides into three sectors namely east nw and sw sectors a conspicuous 3600 m deep trough the snares trough separates the nw and east sectors the east sector is characterised by the ne termination of puysegur ridge into right stepping en echelon ridges that accommodate a change of strike from the puysegur fault to the mfs between 48 s and 47 s in the nw sector and the snares trough a series of transpressional faults splay northwards from the puysegur fault between 49 50 s and 48 s thrusts develop progressively at puysegur trench into a decollement north of 48 s the snares trough develops between two splays of the puysegur fault indicating superficial extension associated with the subsidence of puysegur ridge seismic reflection profiles and bathymetric maps show a series of transpressional faults that splay northwards across the snares fault and terminate at the top of the puysegur trench slope between ca 48 s and 46 30 s the relative plate motion appears to be distributed over the puysegur subduction zone and the strike slip faults located on the edge of the upper plate conversely north of ca 46 s a lack of active strike slip faulting along the mfs and across most of puysegur bank indicates that the subduction in the northern part of puysegur trench accounts for most of the oblique convergence hence active transpression in the snares fault zone indicates that the relative pac aus plate motion is transferred from strike slip faulting along the puysegur fault to subduction at puysegur trench the progressive transition from thrusts at puysegur trench and strike slip faulting at the puysegur fault to oblique subduction at puysegur trench suggests that the subduction interface progressively developed from a western shallow splay of the puysegur fault it implies that the transfer fault links the subduction interface at depth a tectonic sliver is identified between puysegur trench and the puysegur fault its northwards motion relative to the pacific plate implies that is might collide with puysegur bank

global derivation of marine gravity anomalies from seasat geosat ers 1 and topex poseidon altimeter data summary marine gravity anomalies over the area 82 s to 82 n and 0 e to 360 e on a 2 x 2 grid have been derived from seasat geosat ers 1 and topex poseidon altimeter data the inverse vening meinesz formula with a 1 d fft method was used to compute gravity anomalies from gridded north south and west east geoid gradients in a remove restore procedure with the egm96 gravity model as the reference field this paper presents a new gridding technique and an altimeter data management system that help to extract data efficiently in the 12 areas where ship measured gravity and satellite derived gravity were compared rms agreements of 5 14 mgal were obtained special discussions of the gravity anomalies over selected inland seas the arctic antarctica and coastal waters are presented the derived gravity anomalies show detailed tectonic structures of the world ocean the global data set will be continuously updated with more altimeter data included in the derivation

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

estimates of sea surface height and near surface alongshore coastal currents from combinations of altimeters and tide gauges 1 present methods used to retrieve altimeter data do not provide reliable estimates of sea surface height ssh in the nearshore region resulting in a measurement gap of 25 50 km next to the coast in the present work gridded ssh fields produced by archiving validation and interpretation of satellite oceanographic data aviso in the offshore region are combined with coastal tide gauge time series of ssh to improve estimation in that gap along the west coast of the united states in the northern california current system between 40 and 45n and 123 8 and 126w to assess the increase in skill provided by this procedure the geostrophic alongshore currents calculated from the new ssh fields in the gap region are compared to three in situ nearshore current measurements resulting in correlation coefficients of 0 73 0 83 and standard deviations of the differences of 11 6 12 6 cm s substantially improved from the aviso only results when the ekman current components are estimated and added to the geostrophic currents comparisons to the 10 m deep acoustic doppler current profiler velocities are only slightly improved the ekman components make a more significant contribution when compared to hf radar surface current measurements providing correlations of 0 94 and standard deviations of the differences of 6 4 9 5 cm s these results represent a dramatic improvement in the quality of the ssh fields and estimated alongshore currents when additional realistic ssh data from the coastal region are added here we use coastal tide gauges to provide the additional ssh data but also discuss more general approaches for altimeter ssh retrievals in coastal regions where tide gauge data are not available

morphostructure of an incipient subduction zone along a transform plate boundary puysegur ridge and trench multibeam bathymetric and geophysical data reveal a major strike slip fault that extends along the summit of the puysegur ridge east of the puysegur trench the northward structural development of this ridge trench system illustrates the evolution of an incipient subduction zone along a transform plate boundary that has been subjected to increasing transverse shortening during the past 10 m y at the southern end of the trench where subduction has not yet started the puysegur ridge has a narrow 50 km steep sided cross section and the axial strike slip fault separates a shallow 125 625 m flat topped eastern crest from a deeper 400 1600 m western crest these characteristics indicate differential uplift during the initial stage of shortening on the lower plate an incipient 5 2 km deep trench developed in conjunction with normal and reverse faults suggesting strong interplate coupling across the trench northward the ridge broadens linearly to 80 km wide its western flank has locally collapsed and the ridge summit has subsided possibly by 1 5 km suggesting that the interplate coupling decreases and that a benioff zone is being formed concomitant to the northward ridge evolution the trench deepens to 6 2 km and normal fault throws increase along its outer wall indicating greater flexure of the downgoing plate

centroid moment tensor solutions for january march 1997 abstract centroid moment tensor solutions are presented for 162 earthquakes that occurred during the first quarter of 1997 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward dziewonski a m woodward j h 1992 acoustic imaging at the planetary scale in h emert h p harjes eds acoustical imaging plenum new york 19 785 797 a new model of anelastic attenuation of durek and ekstrom durek j j ekstrom g 1996 a radial model of anelasticity consistent with long period surface wave attenuation bull seismol soc am 86 144 158 is used to predict the decay of the waveforms

reactivation of an oceanic fracture by the macquarie ridge earthquake of 1989 theoretical calculations show1 that the occurrence of an earthquake leads to regions of increased shear stress on both sides of the ruptured fault if this stress increase is sufficiently large or if neighbouring faults are close to their failure stress the result may be triggering of earthquakes on pre existing faults in the region a search for this effect2 produced only a few examples all in continental areas where clusters of aftershocks were located in the regions of increased stress the linear dimensions of these clusters were generally less than 20 km and less than one half of the fault length that ruptured in the main shock here i report an example of this phenomenon in which the fault that is reactivated is a nearly 175 km long segment of an oceanic fracture and its length is comparable to that of the main shock the 1989 macquarie ridge earthquake

crustal deformation during 1994 1998 due to oblique continental collision in the central southern alps new zealand and implications for seismic potential of the alpine fault the positions of 115 ground marks in a 150 100 km area of oblique continental collision in the central southern alps new zealand have been measured by global positioning system gps two to four times between 1994 and 1998 contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and alpine fault across strike more than 60 of the strain occurs within a band from 5 km nw to 20 km se of the alpine fault but significant strain continues at least a further 60 km se to near the edge of the southern alps foothills projections of the fault parallel and fault normal components of velocity onto an alpine faultnormal profile show that about 85 of the nuvel 1a model relative plate motion is observed within the gps network the surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50 70 of the relative plate rate in a region deeper than about 5 8 km on the down dip extension of the se dipping alpine fault material shallower than this is behaving elastically and thus storing elastic strain in the region of the alpine fault the longer wavelength displacements can be modeled either as distributed deformation beneath the southern alps or by localization of elastic strain around the upper end of a discrete nw dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the se boundary of the mountains if extrapolated to the surface strain determined from a small scale survey network crossing the alpine fault indicates no significant near surface aseismic fault slip on the central alpine fault over the past 25 years our results are consistent with independent geological evidence that the central section of the alpine fault is capable of producing large to great earthquakes

seismic and geodetic constraints on plate boundary deformation across the northern macquarie ridge and southern south island of new zealand summary in the southern new zealand region the boundary between the australian and pacific plates changes from oblique continental convergence in the central south island of new zealand to interoceanic transcurrent motion along the macquarie ridge source parameters of recent post 1964 upper crustal events mw 5 3 in the region 43 53 s 155 175 e are determined using p and sh waveform analysis in order to investigate the nature of strain accommodated seismically throughout the region repeated occupation of three geodetic networks across the southern south island provides estimates of contemporary crustal strain seismicity over the last 35 years shows uneven spatial and temporal distribution with clusters of events occurring in regions along the plate boundary that exhibit structural variation focal mechanisms of individual earthquakes vary widely but are consistent with the accommodation of long term estimates of the relative plate motion across the southern south island geodetically measured principal contraction directions are consistent with p axis orientations the mode of accommodation of the relative plate motion varies throughout the region influenced by the change in crustal composition on either side of the plate boundary the underlying structure as a relic of previous relative motions and the variation of obliquity of the relative motion with respect to these structures

the macquarie ridge earthquake of 1989 summary a study of the seismicity on the central portion of the macquarie ridge before and after the 1989 may 23 macquarie ridge earthquake mw 8 2 and the solution of the inverse problem to obtain the moment release time history during the earthquake are presented the aftershock study shows that there were remarkably few and small aftershocks on the main fault the aftershocks were distributed along a 220 km portion of the india australia pacific plate boundary and indicate that the motion was bilateral on a vertical fault in addition the earthquake reactivated a 175 km section of a fault to its west which had been dormant since at least 1964 and 44 per cent of the aftershocks occurred on this feature moreover the largest aftershocks in the five month period following the event occurred not on the main fault plane but on the reactivated one based on available bathymetry gravity and magnetic anomaly data from the region this reactivated feature is interpreted as being an old oceanic fracture zone and hence a pre existing zone of weakness the aftershock distribution and their focal mechanisms are interpreted in terms of the tectonics of the region and suggest that a triangular piece of crust to the west of the macquarie ridge is being squeezed to the north and west the centroid moment tensor solution was obtained using both gdsn and geoscope data simultaneously it shows that the motion was primarily strike slip on a vertical fault with a moment of mo 1 5 x lo2 n m and a centroid depth of 19 5 km the moment release distribution and time history were obtained by inversion of broad band body waves recorded digitally by the gdsn and geoscope networks the rupture is found to be bilateral with the propagation first starting towards the north east and a few seconds later towards the south west the moment release was low in the hypocentral area with the higher areas of moment located towards the north east and south west of this the average rupture speed was approximately the shear wave speed no simple relation between the moment release pattern and the distribution of aftershocks on the fault plane was found instead the aftershock distribution is seen to be dominated by the intersection of the reactivated fault and the main fault plane with most aftershocks occurring to the north of this intersection

the santa cruz eddy part i observations and statistics a shallow cyclonic circulation that occurs in the summertime over the monterey bay california is investigated since it is often centered offshore from the city of santa cruz and has never been studied in detail before it is named the santa cruz eddy sce in this study its horizontal size is 10 40 km and its vertical extent is 100 500 m the sce is important for local weather because it causes surface winds along the santa cruz coast to blow from the east instead of from the northwest the latter being the climatological summer pattern for this area as a consequence of the eddy cool and moist air is advected from the south and southeast into the santa cruz area bringing both relief from the heat and fog to the city the sce is unique in its high frequency of occurrence most vortices along the western american coast form only during unusual weather events whereas the sce forms 78 79 of the days during the summer the sce frequency was determined after analyzing two years of data with empirical orthogonal functions eofs from a limited observational network and satellite imagery an explanation of the formation mechanism of the sce will be provided in part ii of this study

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

variation in fault behaviour in different tectonic provinces of new zealand abstract fault behaviour appears to be different in different tectonic provinces of new zealand uplifted holocene marine terraces along the east coast of the north island suggest time predictable behaviour on reverse faults of the hikurangi subduction margin major dextral faults of the strike slip fault province in central new zealand appear to show characteristic slip behaviour short normal faults in the central volcanic region show highly variable increments of displacement finally reverse faults of the central otago and northwest nelson region may have intermittently characteristic patterns of activity each of these behaviour patterns has been observed elsewhere although where there are good data to constrain the actual dates of palaeoearthquakes a uniform slip or coupled model best explains most situations temporal and spatial clustering of fault rupture and major earthquakes are increasingly being recognized it is crucial to obtain better dates and to determine the extent of individual rupture segments in order to better characterize fault behaviour non characteristic fault behaviour has serious implications for current methods of seismic hazard evaluation seismic hazard studies should utilize specific fault data in preference to assuming a particular fault behaviour model

tearing of the subducted australian plate during the te anau new zealand earthquake of 1988 june 3 summary the ml 6 1 earthquake of 1988 june 3 was widely felt in the south island of new zealand and caused landslides in fiordland it occurred at a depth of 57 km near 45 10 s 167 17 e in a region where the australian plate is subducting beneath the pacific plate immediately after the event portable seismographs were installed in the te anau area data from a well recorded subset of aftershocks have been used to invert for the seismic velocities of the lower crust and upper mantle of the region and for station terms the large range in station terms obtained 3 3 s for p waves emphasizes the structural complexity of the fiordland region the aftershocks as relocated with the new velocity model occur in a cigar shaped zone which extends from about 40 to 70 km in depth and dips southeast at approximately 65 the orientation of the aftershock zone in relation to the focal mechanism of the mainshoek suggests that the earthquake involved down dip tearing of the crust of the subducted australian plate with the ne part of the plate having moved up and to the east southeast relative to the sw part down dip tearing of the crust of the australian plate is also inferred for an earthquake of ml 5 9 which occurred on 1988 july 19 this was located directly down the dip of the subducted plate from the june 3 event at a depth of 122 km taken together with other seismological data these two events suggest the existence of a major tear in the subducted plate and that the shallow part of the plate to the northeast of this tear is resisting subduction the resistance may result from subduction of a region of relatively thick crust this would provide an explanation for the uneven distribution of both shallow and intermediate depth seismicity in fiordland the differing dips of the subducted plate along the margin fault plane solutions and spatial variations in the b value of intermediate depth events subduction of a region of relatively thick crust also provides a mechanism for tilting up the western part of the fiordland block

crustal deformation of the marlborough fault zone in the south island of new zealand geodetic constraints over the interval 1982 1994 crustal deformation across the marlborough fault zone in the south island of new zealand has been investigated by resurveying with the global positioning system gps a triangulation and trilateration network across part of the zone the principal strain rates which mostly have errors between 5 and 10 vary systematically across the region the principal axis of horizontal contraction gradually swings from se nw southeast of the hope fault to almost east west in the west the component of velocity parallel to the strike of the faults can account for 95 of the relative motion between the australian and pacific plates this component exhibits little variation west of the alpine wairau fault and shows an almost linear variation from the wairau fault to the east coast the horizontal strain rate northwest of the wairau fault is a small east west uniaxial contraction 0 1 ppm yr 1 if it is assumed that the crustal blocks bounded by the major faults are primarily driven by basal shear tractions with stresses on the faults being relatively insignificant then the slip rates expected on the faults may be derived from the observed velocity variations across the fault zone these predicted slip rates are in good agreement with geologically observed slip rates the larger slip rates on the hope fault are related to the observation that the strain field extends southeast of the fault over a distance large compared with the spacing between the faults the agreement between predicted and geological slip rates lends support to the idea that the observed pattern of deformation reflects that in the lower lithosphere

the kinematics of the plate boundary zone through new zealand a comparison of short and long term deformations summary surveys of shear strain rates derived from repeated triangulation cover about one quarter of the area of the plate boundary zone through new zealand and are well distributed within it using a method developed by haines relating shear strains to the total strain and velocity fields the dilatational and rotational strain rates and the velocities within the deformed zone are derived the plate boundary zone is shown to be two dimensional for the most part in that gradients of the velocity along the length of the zone are small or zero the shear strain rates are not uniform however they reach a maximum along a central axis and decrease gradually to zero toward the sides nor are the two independent components of strain one involving shortening or extension perpendicular and the other shear parallel to the zone equally developed everywhere there is a clear geographic separation with shortening developed over the subduction thrust and shear developed behind in a zone of wrench faulting time variations in the strain field in part of the region above the subduction interface are related to changes in the degree of coupling across the interface a velocity field is computed the relative velocity of the two plates from this data agree very well with that derived from seafloor spreading data these strain and velocity fields derived from geodetic studies give the short term kinematics of the plate boundary zone the long term kinematics of the zone are estimated from 1 uplift and subsidence data which are related to changes in crustal thickness and hence to dilatational strain and 2 variation in the declination of primary magnetization in rocks of known age related to the rotational strain the rate of rotational strain has increased in time from about 2 to 3 myr 1 10 myr ago to about 9 myr 1 today the increase can be understood as a secondary effect of a developing compressional component increasing the coupling between the plates so that more of the motion parallel to the zone is accommodated by distributed shear and less by aseismic slip the rates of strain averaged over the last 1 myr or so and at a scale length of a few tens of kilometres are closely similar in distribution and magnitude to the strain rates derived from short term kinematics while therefore at scale lengths of a few hundred metres or so faulting is very important and the deformation is discontinuous with unstrained blocks faulted with large but uncertain slip against adjacent blocks the deformation at these longer scale lengths is continuous and regular the two consequences of this study are that 1 strain from geodetic data can be directly related to the average long term deformation and therefore can be an important tool in the study of deformation in regions of current tectonic activity and 2 at scale lengths of a few tens of kilometres the deformation of a region can be described as the deformation of a continuum and we can ignore the effects of faulting

jetlet formation from diabatic forcing with applications to the 1994 palm sunday tornado outbreak abstract the three dimensional responses of simple stably stratified barotropic and baroclinic flows to prescribed diabatic forcing are investigated using a dry hydrostatic primitive equation numerical model the north carolina state university geophysical fluid dynamics model a time dependent diabatic forcing is utilized to isolate the effects of latent heat release in a midlatitude convective system examination of the mass momentum adjustments to the diabatic forcing is performed with a focus on the development of an isolated midlevel wind maximum the results of both cases suggest the formation of a midlevel wind maximum in the form of a perturbation meso scale cyclone which later propagates downstream as the heating is decreased the scale of the perturbation cyclone remains at a sub rossby radius of deformation length scale therefore the mass perturbations adjust to the wind perturbations as the mesocyclone propagates downstream transverse vertical circulations which favor ascent on the r

rain shadow development during the growth of mountain ranges an atmospheric dynamics perspective 1 an idealized atmospheric model is used to explore the links between climate and topography in the development of orographic rain shadows during orogenesis the atmospheric dynamics theory of density stratified fluid flow over topography is used to interpret the results the controlling nondimensional parameter is nh u where n is the brunt vaisala frequency a measure of atmospheric stability h is the terrain relief and u is the initial horizontal wind speed rain shadow development is found to be a nonlinear and nonunique function of both topography and atmospheric state indicating that geological records of orographic aridity cannot be interpreted in terms of relief alone when upstream topography exceeds nh u 1 during surface uplift downstream orographic precipitation vanishes and downstream orographic cloud mass decreases by as much as 90 upstream blocking of air flow can generate a forward projecting rain shadow in which a relatively low ridge nh u 1 may be decoupled from the atmospheric flow by a zone of flow stagnation extending upstream of the high terrain such an effect may occur if the valley separating the two ranges is narrower than the length scale of flow stagnation in the model configuration used here lateral widening of a relatively low nh u 1 range increases downstream cloud mass by up to a factor of 3 these results help to refine interpretations of climate tectonic interactions in shaping the geological record of the sierra nevada and andes

seismic strain rates in regions of distributed continental deformation we compare the rates and configurations of seismic deformation determined from moment tensor mechanisms of earthquakes in five continental regions with the rates and configurations expected if the seismicity reflected the relative plate motions in the regions the five regions the aegean iran california western new guinea and central asia are characterized by high levels of seismicity distributed over hundreds to thousands of kilometers from the accepted plate boundaries the earthquake mechanisms are compiled primarily from the catalog of centroid moment tensor solutions which is nearly complete over the interval 1977 1987 for earthquakes greater than ms 5 the thickness of the seismogenic layer in each region is constrained by the results of modeling broadband body wave phases and of previous earthquake studies in the aegean california and western new guinea the observed configurations of deformation agree with a simple model of distributed accommodation of relative plate motion in iran and central asia the moment tensors indicate that there is appreciable strain parallel to the strikes of the belts we augment our 10 year estimates of seismic deformation with the available historical moment and magnitude estimates for the five regions and find that moderate and large earthquakes account for between 25 and 70 of the deformation of the seismogenic layer

constraints on macquarie ridge tectonics provided by harvard focal mechanisms and teleseismic earthquake locations in this study we evaluate teleseismically determined focal mechanisms and epicenters for earthquakes along the macquarie ridge complex mrc from 45os to 61 os and 155oe to 168oe a region characterized by some previous investigators as undergoing subduction initiation from 65 centroid moment tensors reported by harvard we develop statistical guidelines for choosing 26 which represent better determined more reliable focal mechanisms for tectonic analysis although thrust mechanisms occur in the north near fiordland elsewhere along the mrc the better determined mechanisms virtually all indicate that present day motion along most of the mrc is strike slip this is consistent with sidescan sonar and multichannel reflection data collected between 50os and 57os on 1994 and 1996 cruises the active plate boundar3 zone appears to be quite narrow 5 km wide and no active compressional features can be observed on the seafloor if we determine a rotation pole for plate boundary motion using only slip vectors from better determined harvard mechanisms along the mrc the best fitting instantaneous pole is at 57 4os 179 4oe about 2 5 o north of the nuvel i australian pacific pole which averages motion over the last 3 0 my if the mrc pole was formerly farther south than at present this could explain the existence of relict features associated with crustal shortening such as bathymetric highs and troughs yet the absence of active features such as thrust faults etc suggests ongoing compression or subduction initiation we also carefully read arrival times for p phases for 53 earthquakes at 16 teleseismic stations selected to represent a range of azimuths surrounding the earthquakes we relocated these earthquakes using standard joint epicentral determination jed methods while most of the better quality relocations lie on or very close to the australia pacific boundar3 as determined on the 1994 cruise a few epicenters occur well away from the boundar3 apparently on cenozoic fracture zones thus on the macquarie ridge complex and other major strike slip boundaries e g in california it appears that the very largest earthquakes occur on the principal plate boundar3 fault but that other earthquakes some quite large may occur away from the boundary along zones of preexisting weakness