3 d imaging of marlborough new zealand subducted plate and strike slip fault systems summary we present a 3 d seismic velocity model of the northern south island new zealand using several combined passive and active source data sets this area is the transition from hikurangi subduction in the north island to oblique continental collision along the alpine fault and the southern alps and geodetically observed deformation is consistent with plate motion being primarily taken up on a series of rotating strike slip faults we carry out simultaneous inversion for vp and vp vs and hypocentres using earthquake data recorded by the permanent seismic network and three temporary seismic arrays also using active source marine airgun data recorded at onshore stations we image the subducted hikurangi slab as a continuous unbroken feature extending from the active subduction zone to the north beneath the marlborough region the 3 d hypocentres define the subducted slab as a smooth curved feature to over 250 km depth whereas below the junction of the awatere and alpine faults the seismicity abruptly ends at 100 km depth the subducting pacific plate has a thick crust including greywacke upper crust which slows subduction the interaction of the subducted plate with the translating overlying plate forms a 50 km long zone of apparent crustal thickening with vp 7 0 km s 1 observed to 40 km depth the australian plate has a high velocity block to 65 km depth aligned with the separation point pluton and trending oblique to the subducted slab this zone of thicker australian lithosphere may also hinder subduction the marlborough faults trend about 25 to the strike of the underlying subducted slab so that the relation of faults to the slab varies along the fault strikes and the overlying plate ranges in thickness from 20 to 50 km along the faults thus the fault interaction with the subducted slab varies from translating material directly above the subducting plate to bottoming into a thick ductile region the lower crust has high vp vs under the marlborough faults except for a region of haast schist which may indicate excess fluid released by the subducted plate

strain partitioning in the transition area between oblique subduction and continental collision hikurangi margin new zealand spatial differences in late quaternary structural style and deformation rates indicate a complex pattern of strain partitioning within the transition area from oblique subduction beneath southern north island to oblique continental collision in northern south island new zealand the late quaternary structure of the offshore southern hikurangi margin is determined here using seismic reflection profiles mr1 side scan sonar swath images em12dual multibeam swath bathymetry and backscatter images and sediment core and dredge samples where oblique 50 subduction of the oceanic hikurangi plateau is occurring beneath southern wairarapa region the steep dissected upper margin consists of a wedge of rocks that are being deformed by strike slip and reverse faults beneath the lower margin an accretionary wedge of quaternary age has developed rapidly by frontal accretion of a thick succession of turbidites on the pacific plate estimates of late quaternary geological strain rates have been made on two margin perpendicular transects across southern north island and the offshore margin to budget the predicted nuvel 1a plate motion more than 33 55 of the total plate motion is expressed in the offshore part of the upper plate above the subduction detachment in the central hikurangi margin between 40 s and 41 s this offshore deformation could potentially include margin parallel fault slip of up to 14 15 mm yr across the upper plate strain is partitioned into zones of folding and thrust faulting and possibly more than one zone of strike slip faulting but the partitioning may be incomplete with oblique slip thrusting an important component of the deformation the wide distribution of deformation within the upper plate partially reflects strong coupling between the subducting oceanic plateau and the leading edge of the overriding plate this coupling increases toward the southwest where the continental part of the subduction zone beneath marlborough may now be inactive or very strongly coupled toward the southwest the transpressive deformation style of the upper wairarapa margin continues into the upper marlborough margin and onshore into the strike slip marlborough fault system of northern south island in contrast the accretionary wedge narrows to the southwest and terminates in southeastern cook strait approximately above the transition from subducted oceanic crust to subducted continental crust of the pacific plate farther southwest beneath the lower marlborough margin low strain folding and thrust faulting is occurring but the style of deformation is not typical of the accretionary wedge this shortening results either from a small amount of subduction slip beneath marlborough and or from contractional strain partitioned to the southeast of the continental transpression zone whereas the initial onset of continental collision occurs in marlborough the surface expression of deformation of the pacific plate occurs south of 42 30 s normal faulting of the northwestern margin of the continental chatham rise possibly reflects flexural extension of the pacific plate crust at the southern end of the subduction zone whereas folding and thrust faulting of the north canterbury coastal region results from nw se contraction of the delaminated upper crust entering the southern alps collision zone

comparison of magnitude estimates for new zealand earthquakes moment magnitude local magnitude and teleseismic body wave magnitude new zealand is one of the more seismically active countries in the world with more than 15 000 earthquakes located each year routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude m w 3 5 4 0 has recently been implemented in new zealand nearly 330 regional moment tensor rmt solutions have been calculated for earthquakes in the new zealand region dating back to late 2003 this complements local magnitude m l the primary magnitude calculated by geonet in new zealand the rmt catalog along with 155 global centroid moment tensor global cmt catalog solutions is used to compare m w with m l for new zealand earthquakes in addition to m w and m l there are more than 330 teleseismic body wave magnitude m b values available from the united states geological survey and international seismological center catalogs for events that also have an m w these are used to examine the relationship between m w m l and m b for new zealand earthquakes there is a clear distinction in the relationship of m l to m w for shallow focus 33 km depth and deep focus 33 km depth earthquakes shallow focus earthquakes show m l to be fairly consistent with m w particularly for events with m w 4 5 and with graphic 1 deep focus earthquakes have m l consistently larger than m w more than a full magnitude unit for some events with graphic 2 m w and m b are in fairly good agreement regardless of the depth whereas m b estimates are consistently smaller than m l for deep events this suggests that m l is overestimated for deep focus events and that the large m ls are not the result of physical properties of the source 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif

andean tectonics as a cause for changing drainage patterns in miocene northern south america newdatafromneogenestratainnorthernsouthamericasuggestthatmiocenetectonisminthenortheasternandeswasresponsible for the genesis of the amazon river and changes in the drainage patterns of other major rivers such as the magdalena and the orinoco here we present a new model for the paleogeographic evolution of northern south america during the miocene in the early miocene a large part of the drainage of northwest amazonia was directed northward along the paleo orinoco river system to a deltainlakemaracaibo upliftoftheeasterncordillerainthelate middle miocene caused thefirst development of the amazon river however no connection with the atlantic was established and the amazon fed the paleo orinoco river system which drained toward the caribbean substantial andean uplift in the late miocene resultedinmajorchangesinpaleogeography theorinocochangedits course the amazon established a connection to the atlantic causing the drowning of carbonate platforms and the amazon caribbeanconnectionwasclosed thusthedrainageandpaleogeography ofnorthernsouthamericainthemiocenewerestronglycontrolled by tectonic movements in the northeastern andes

recognition of active strike slip faulting from high resolution marine seismic reflection profiles eastern marlborough fault system new zealand major strike slip faults within the eastern part of the marlborough fault system of new zealand extend offshore beneath the continental shelf into the southern end of the hikurangi margin six major submarine faults each tens of kilometers in length and exhibiting the three dimensional structural characteristics typical of strike slip deformation zones are mapped in detail using closely spaced high resolution seismic reflection profiles each fault displaces late quaternary sediments that have been interpreted within the framework of the sequence stratigraphic model and the established history of glacio eustatic sea level cyclicity some of the faults are associated with strike slip microearthquake focal mechanisms the faults are within a complex structural high that underlies much of the outer continental shelf and within the flaxbourne basin between the structural high and the coast the plan view pattern of faulting recorded in the quaternary sediments reflects only the major surface traces that have propagated upsection as contemporaneous sedimentation has blanketed most of the evolving structures regionally the late quaternary faults compose two groups that bound blocks with rhomboid surface areas on a scale of tens of square kilometers to hundreds of square kilometers one group strikes typically between 023 and 057 i e 22 56 from the plate motion vector 079 and includes dextral oblique slip thrust faults as well as inherited steeply dipping pre pliocene strike slip faults that bound thick sedimentary basins the second group comprises possibly young 1 ma steeply dipping strike slip faults that commonly exhibit normal slip separation and strike between 067 and 085 subparallel to the azimuth of the instantaneous pacific australian plate motion vector

including anisotropy in 3 d velocity inversion and application to marlborough new zealand summary two types of seismic velocity heterogeneity are often observed in a given region pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes we seek to combine these types of observations by including anisotropy in 3 d velocity inversion the fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model the ray paths are estimated for each of the shear wave splitting observations and the 3 d initial model is determined via an averaging process and a coherence analysis the initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements the traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node the ray paths are defined with approximate 3 d ray tracing and for each point along a ray path the velocity is defined as a function of the ray path azimuth at that point a blended approach is used to damp the perturbations to the magnitude of anisotropy and or perturbations to the fast direction tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable application to the marlborough region shows variations in anisotropy in the brittle crust ductile lower crust mantle and subducted slab the patterns are consistent with the shear wave splitting observations but show some additional features the maximum anisotropy is approximately 12 per cent oriented northeast between the awatere and wairau faults this high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain in the region of interaction of the ductile lower crust with the shallow subducting slab moderate anisotropy with east west orientation is imaged this is consistent with ductile deformation roughly parallel to the plate velocity

a preliminary assessment of glacial ice profiling using vlf surface impedance measurements abstract vlf surface impedance measurements have been used in the past for sub surface mapping the application of this technique to glacial ice probing is discussed theoretically and results of measurements on brewster glacier new zealand are presented results were fitted to a three layer model and a section profile is given dramatic changes in the phase of the surface impedance were observed in the vicinity of crevasses results indicate that the technique has potential as a tool for quick reliable and non invasive ice thickness measurements resum eune approche preliminaire de la determination de l epaisseur de glace il l aide de mesures d impedance de surface en vlf des mesures d impedance de surface en vlf ont deja ete utilisees dans le passe pour la cartographie en profondeur on discute du point de vue theorique l application de cette technique a la glace et on presente les resultats des mesures sur le brewster glacier nouvelle zelande les resultats sont obtenus dans l hypothese d un modele a trois couches et on presente un profil de introduction a number of geophysical techniques are used currently to measure ice thickness these include seismic soundings electrical resistivity vhf radar and gravity surveys the first three techniques require a significant amount of equip ment as the source of the signal in addition to the sensors must be provided at the exploration site in the case of seismic sounding the source is often explosive charges an undesirable aspect when investigations are conducted in national parks and other sensitive areas for electrical resistivity a long dipole antenna and good electrical contact with the ice is required this can be difficult to achieve as ice is a very poor conductor all techniques suffer from errors associated with ice anisotropy particularly if the ice is fractured and the bottom is irregular while seismic and radar techniques are time of flight measurements resistivity and gravity surveys are not and so prove more difficult to interpret at the exploration site in this paper vlf very low frequency i e in the frequency range 10 20 khz surface impedance measurements are suggested as an alternative to these techniques for a number of reasons first it is not necessary to provide a source field as vlf emissions from distant navigation trans mitters are used secondly the receiver can be a small hand held device of minimal weight and size thirdly at very low frequencies the wavelength of the radiation is very long in comparison with the ice bottom roughness in most cases and so scattering from the bottom is not a problem typically the wavelengths free space used lie in the range 10 20 km this also means that small changes in the intrinsic properties of the ice due to changes in the ice temperature the presence of impurities or the age and history of different layers are averaged out in the measure ments obtained 376 section on a observe de tres importantes variations de i impedance de surface au volslnage des crevasses les resultats montrent que cette technique se revele un outil potentiel pour des mesures d epaisseur de glaces rapide sur et sans contreverses zusammenfassung eine vorliiufige wertung der bestimmung von eisprofilen aus vlf messungen der ober fliichenimpedanz vlf messungen der impedanz an der oberfhiche wurden in der vergangenheit zur ermittlung von tiefenstrukturen benutzt die anwendung dieses verfahrens auf eiskorper von gletschern wird theoretisch diskutiert messungsergebnisse vom brewster glacier neuseeland werden vorgelegt sie werden einem drei schichten modell angepasst und in einem querschnitt dargestellt starke phasenschwankungen der oberfhichenimpedanz wurden in der nachbarschaft von spalten beobachet die ergebnisse lassen das verfahren als wirksames mittel zur schnellen zuverhissigen und schonenden messung der eisdick

very high strains recorded in mylonites along the alpine fault new zealand implications for the deep structure of plate boundary faults abstract oblique displacement on the alpine fault which forms the principal structure along the australian pacific plate boundary in south island new zealand has resulted in exhumation of a kilometre wide mylonite zone in the hanging wall adjacent to the current brittle fault trace the mylonites formed under amphibolite facies conditions at depths of ca 25 km and have been uplifted during the past 5 ma a suite of 65 70 ma pegmatite veins in the hanging wall alpine schists has been progressively deformed within the mylonite zone and sheared out over a strike length of ca 100 km measurements of the thickness distribution of the pegmatite veins within the non mylonitised schists and at three localities within the progressively strained mylonites have been used to estimate strain values within the mylonites the thicknesses approximate a log normal distribution with a mean value that is progressively reduced through the protomylonites mylonites and ultramylonites by assuming that the thickness distribution currently observed in the schists was the same for the pegmatites within the mylonites before strain a model of deformation incorporating simple shear and simultaneous pure shear is used to strain the undeformed veins until a fit is obtained with the strained distributions shear strains calculated range from 12 to 22 for the protomylonites 120 to 200 for the mylonites and 180 to 300 for the ultramylonites corresponding to pure shear values of 1 3 in each case these values are compatible with the strains predicted if most of the surface displacement on the fault over the past 5 ma were accommodated within a 1 2 km wide mylonite zone through the middle and lower crusts the results suggest that processes such as erosional focussing of deformation and thermal weakening may cause intense strain localisation within the lower crust with plate boundary deformation restricted to narrow zones rather than becoming increasingly distributed over a widening shear zone with depth

the acquisition of source path and site effects from microearthquake recordings using q tomography application to the united kingdom source site and propagation parameters are inverted from a u k database of weak motion events 2 0 m l 4 7 this results in the complete spectral parameterization of over 3200 velocimetric records of 273 events from the year 1992 to 2006 the s wave is extracted from the vertical records and is processed using a multitaper fourier transform we initially use a nonlinear least squares log space optimization to obtain estimates of the attenuation parameter for each spectrum the estimates of t are then used to geometrically constrain a depth dependent q model using a technique adapted from velocity tomography we then invert for the remaining frequency dependent parameters and a collective amplitude parameter from the velocity spectra while fixing the newly computed attenuation parameters based on raytracing through our q model the resultant amplitude parameters are then split into source moments apparent geometrical spreading and site correction factors we find a frequency independent depth dependent q structure a linear relationship proportional to 0 7 m l between moment magnitude m w and local magnitude m l is found in the range of 2 4 7 m l the majority of stress drops are found to range on the order of 0 1 10 mpa a multiple segment apparent geometrical spreading model is found to best describe the amplitude decay with distance accounting for factors such as geometrical spreading and scattering along with multiple phase interference in the analysis window site response functions are found to broadly correlate with regional geology mean amplification occurring in the cenozoic sedimentary rock sites to the southeast of england relative to the harder palaeozoic rock sites of wales and scotland we use a bootstrap analysis technique to analyze the dependence of our results on the data in order to estimate the variance of the results and check the robustness of different inversions synthetic spectra are also computed in order to obtain minimum variance and bias of model parameters associated with the method in applying a geometrical q constraint through the use of q tomography we find that the robustness of the results is significantly increased a thorough analysis of the trade offs involved in the inversion is performed using synthetic datasets we find strong trade offs between the parameters but we are able to show that this covariance is reduced when adopting the q tomography approach

seismic attenuation tomography in southwestern china insight into the evolution of crustal flow in the tibetan plateau abstract crustal flow in southwestern china is commonly attributed to the continental collision between the indian and the eurasian plates therefore studying crustal flow in this area can provide valuable insight into the collisional process the quality factor q is an indicator of seismic wave attenuation that provides constraints on lithology rheology and geodynamic processes which are vital for interpreting the nature of the crustal flow in a given region in this study we used local seismic waveforms to obtain regional attenuation models for p qp and s qs waves in the sichuan yunnan region syr the emeishan large igneous province elip is characterized by high qp and qs values in the middle yunnan block myb at depths greater than 5 km the qp and qs values in most of the fault zones surrounding the myb were lower than 300 indicating that the material within crustal faults causes high waveform attenuation our results suggest that the crustal flow was diverted into two channels in the northern myb due to resistance from the elip to illuminate the tectonic dynamics of the syr we propose a simple model that explains both the evolution of the crustal flow and the variation in the seismic wave attenuation

fluid generation and pathways beneath an active compressional orogen the new zealand southern alps inferred from magnetotelluric data 1 forty one wideband magnetotelluric mt soundings were collected in a 150 km long transect across the southern alps of the central south island of new zealand an active compressional orogen decomposed mt impedance tensors vertical magnetic field relations and reconnaissance soundings at two locations off line imply an approximately two dimensional geometry here with average regional geoelectric strike of n40 e similar to surface geologic trends two independent two dimensional inversion algorithms were applied to the mt data and both imply a concave upward u shaped middle to lower crustal conductive zone beneath the west central portion of the island the average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along strike conductive strands narrower than the transverse magnetic cross strike mode mt data can resolve the deep crustal conductor under the southern alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust fluid interconnection and electrical conduction are promoted by shear deformation the conductor rises northwestward toward the trace of the alpine fault but attains a near vertical configuration at a depth of 10 km and reaches close to the surface 5 10 km inland of the fault trace itself the transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle ductile transition in uplifting schist and approach the surface through induced hydrofractures the high grade schist becomes resistive after depletion of fluids and continues to extrude toward the alpine fault shallow extensions of the deep high conductivity are coincident with modern hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance to the southeast high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent southern alps orogen which also exhibits evidence for expulsion of high temperature fluids the higher conductivity inferred along strike possible anisotropy could reflect more efficient fluid interconnection in this higher strain direction as well as possible contributions by sheared fluid deposited graphite conductivity of the uppermost mantle of the south island is low consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by p wave delay studies

dielectric behaviour of firn and ice from the antarctic peninsula antarctica les mesures dielectriques de 14 echantillons de glace et neve preleves en antarctique pour des temperatures de 2 a 70 c et des frequences de 10 hz a 100 khz ont ete comparees a celles de la glace prelevee au groenland

distribution of seismic velocities and attenuation in the crust beneath the north anatolian fault turkey from local earthquake tomography we investigate the crustal structure beneath the western part of the north anatolian fault zone nafz an area where at least five damaging earthquakes occurred during the twentieth century this study is based on local earthquake tomography using the data from aftershocks of the izmit event 17 august 1999 m 7 4 recorded by stations of permanent and temporary networks we derive the distribution of v p v s and the v p v s ratio based on the iterative inversion for both v p v s and v p v p v s using the lotos code innovatively in this study we perform an inversion for frequency dependent s wave attenuation 1 q s the reliability of the results is assessed through synthetic tests the distributions of the resulting seismic parameters v p v s v p v s and q s highlight important geodynamical features in the study area the low velocity and high attenuation patterns mostly correlate with the fracturing zones of the nafz low velocities are also observed beneath the main sedimentary basins e g adapazari duzce and kuzuluk high velocity and low attenuation patterns correlate with blocks presumed to be rigid kocaeli armutlu and almacik blocks the rupture traces of the largest earthquakes in this area pass generally in the transition areas between high and low velocities while moderate and weak seismicity is mostly concentrated in low velocity areas based on these results we propose and discuss the role that the almacik block could have played in producing the largest earthquakes in the study area in the twentieth century

vlf surface impedance measurements for ice depth mapping an assessment of some commonly encountered interference effects the effects on very low frequency surface impedance measurements of lateral variations commonly found in ice environments have been measured and modelled numerically using the quasi static two dimensional boundary element method results indicate that surface impedance measurements made in the vicinity of crevasses oriented perpendicular to the plane of incidence and those made in the vicinity of moraines and melt streams can all show significant changes to the measured apparent resistivity it is therefore misleading to use such measurements in the interpretation of ice depth

deformed neogene basins active faulting and topography in westland distributed crustal mobility west of the alpine fault transpressive plate boundary south island new zealand abstract tectonic activity in the south island of new zealand is dominated by the alpine fault component of the australia pacific plate boundary west of the alpine fault deformation is recorded by paleogene neogene basins coeval with the evolution of the right lateral transpressive plate margin initial tectonic setting was controlled by n s normal faults developed during late cretaceous and eocene early miocene rifting following inception of the alpine fault c 25 ma reverse reactivation of the normal faults controlled tectonic segmentation that became apparent in the cover sequences at c 22 ma based on restored transects tied to stratigraphic sections seismic lines and wells we reconstruct the vertical mobility of the top basement unconformity west of alpine fault from c 37 35 ma to 22 ma subsidence was controlled by extensional faulting after 22 ma the region was affected by differential subsidence resulting from eastward crustal flexure towards the alpine fault boundary and or components of transtension transition from subsidence to uplift started at c 17 ma within a belt of basement pop ups separated by subsiding basins localised in the common footwall of oppositely dipping reverse faults from 17 to 7 3 ma reverse fault reactivation and uplift migrated to the wsw persistent reverse reactivation of the inherited faults in the present stress field is reflected by the close match between tectonic block segmentation and topography filtered at a wavelength of 25 km i e at a scale comparable to crustal thickness in the region however topography filtered at wavelength of 75 km shows marked contrasts between the elevated tasman ranges region relative to regions to the south variations in thickness and rigidity of the australian lithosphere possibly control n s longitudinal changes consistent with our estimates of increase in linear shortening from the tasman ranges to the regions located west of the alpine fault bend

tracking repeated subduction of the hikurangi plateau beneath new zealand abstract the hikurangi plateau has been subducted beneath new zealand twice firstly at ca 100 ma during n s convergence with gondwana and currently during e w convergence between the pacific and australian plates here we use a nationwide 3 d seismic velocity model derived from seismic tomography studies and seismicity relocated with this model to determine the location of the subducted western and southern edges of the plateau in the hikurangi subduction zone in the north island and northern south island we identify the western edge in the 37 140 km depth range based on the distribution of seismic velocities in the top of the subducted pacific plate changes in the thickness of the dipping seismic zone and steepening of the subducted plate downdip of the buoyant plateau in the central south island where the hikurangi plateau is subducting for a second time the plateau dips less steeply its southern edge is well defined by the collision of the delaminated lower crust of the plateau with the northern fiordland subduction zone where it bends the younger subducted australian plate to vertical the hikurangi plateau was originally rifted from the ontong java plateau and the subducted portions of the hikurangi plateau identified here fill a gap in reconstructions of this large igneous province

effect of plastic viscous layering and strain softening on mode selection during lithospheric extension 1 factors controlling the selection of deformation modes during continental extension are investigated using analytical and numerical methods we view the lithosphere as a laminate and examine a simple system with a uniform plastic layer overlying a uniform linear viscous layer the rate of energy dissipation is analyzed for pure shear ps symmetric plug sp and asymmetric plug ap extension modes and the analysis reveals that the primary control is the relative rate of dissipation in the two layers a basic difference is that the plastic layer yield strength is independent of the strain rate whereas the viscous stress depends on strain rate therefore dissipation scales linearly and quadratically with extension velocity for these respective layers when other parameters e g extension velocity and properties are held constant minimum dissipation predicts that the modes ap sp and ps will be selected in this order with increasing viscosity of the lower layer transition viscosities between modes t1 and t2 are 4 1021 pa s and 8 1022 pa s respectively for our parameters values numerical models confirm the analysis results inferred mode controls and order of mode selection when strain softening of the plastic layer occurs during extension implications for lithosphere that acts as a bonded plastic viscous laminate include the following 1 asymmetric extension ap mode is preferred when the extension rate and or effective viscosity is low or the viscous region temperature is high 2 symmetric sp mode extension is preferred for intermediate combinations of parameters and 3 overall pure shear ps mode may occur for opposite end member parameter combinations

experimental measurements of seismic attenuation in microfractured sedimentary rock ultrasonic compressional and shear wave attenuation in water saturated carrara marble increase with increasing crack density and decreasing effective pressure between 0 4 and 1 0 mhz empirical linear relationships between 1 q and crack density cd were found to be cd 1 96 or 0 63 x 1 q for compressional waves andcd 6 7 or 1 5 x 1 q for shear waves

vlf surface impedance measurements for ice depth mapping in the antarctic vlf surface im pedance measur ements were made along four trave rses on the anta rctic ice sheet in the vicinity of casey base computer modelling of the surfa ce impedance data allowed ice depth predi ctions to be made predictions which are almost independent of ice temperature for ice depths less than 800 m results agree with ice radar and other ice depth data surface impedance anomalies were observed close to moraines and crevasses in the ice sheet the technique is fast and the instr umen tation suffi ciently portable for single person operat ion suggested and tested over a temperate glacier in south island new zealand by thiel i986 descriptions of the theory equipment method of oper ation and data analysis were included in that paper no independent depth measurements were available on that occasion and so no direct check on the method was available this paper reports an attempt to verify the procedure on known ice environments in the general area of casey base antarctica during the summer of 1987 88 a surface i mpedance meter capable of operat ing in a sub zero tem perature environment was constructed follo wing the general principles outlined previously thiel 1986 casey base was used because of the considerable effort that has been made in mapping the adjacent law

p wave attenuation structure in the fault area of the 1995 kobe earthquake the three dimensional 3 d p wave attenuation structure in the kobe epicentral area is determined using aftershock records from the 1995 kobe hyogo ken nanbu earthquake waveform data from both permanent seismic networks and portable stations set up following the kobe mainshock were used the observed p wave amplitude spectra were inverted for the spectral plateau value the source corner frequency and a frequency independent t operator assuming an 2 type source model the calculated t operators were subsequently used for the computation of the 3 d absorption structure using a 3 d velocity model only high quality amplitude spectra were used for the estimation of reliable attenuation parameters resulting in a final data set of 4100 t operators two regions of high absorption were found in the upper part of the crust one is below the northern part of awaji island and reaches down to a depth of 5 km the other region northeast of kobe city is along the arima takatsuki tectonic line and extends along the rokko kashiodani and yamada faults it extends northward to the hokusetsu sanchi area in both regions a high density of fault lineations is mapped and high poisson ratios are known this suggests that partially saturated cracks cause the observed high attenuation

crustal heterogeneity and subduction processes 3 d vp vp vs and q in the southern north island new zealand summary along the southern hikurangi subduction zone in the southern north island new zealand the subduction zone changes from volcanic with moderately weak interseismic coupling to non volcanic with strong coupling in wellington to investigate how material heterogeneity relates to subduction processes we compute 3 d seismic properties using 342 earthquakes and 2 shots recorded by a 165 station seismic array the time base for some telemetered temporary stations was not stable which required revising the inversion code to allow an additional origin time related to the telemetered stations progressive inversions from 2 d to fine 3 d were carried out for vp vp vs and qp combined interpretation of these three seismic properties highlights features of the subduction zone the subducted slab is imaged as a high velocity high q feature in the overlying crust from southeast to northwest there is low velocity in the accretionary wedge high velocity in the axial ranges and low velocity in the wanganui basin high vp vs and low qp characterize the crust east of the north island dextral fault belt high vp vs is imaged above the subducted slab to 40 km depth in the northern half of the study area low qp is also prominent above the slab particularly trenchwards of the volcanic region in marked contrast to the high vp vs the low qp above the slab extends as a continuous feature up to the low qp in the shallow crust underlying the volcanic region the low qp and high vp vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release vp vs is particularly sensitive to overpressured fluid in the compressional forearc whereas qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region the rheology of the lower crust of the overlying plate is a factor in large scale subduction coupling as evidenced by the terrane boundaries and haast schist in the section near wellington the changing rheology of the lower crust of the overlying plate in the southern north island also provides a possible explanation for the southward cessation of volcanism

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

thermal structure of the subduction zone in western japan derived from seismic attenuation data 1 we use a well determined three dimensional attenuation model to infer the temperature distribution of the subduction zone in western japan the tomographic attenuation model was derived from about 19 000 t determinations for p waves recorded at 128 stations the attenuation model with a previously determined velocity model was used to infer the temperature distribution of the subduction zone from depths of 15 to 80 km using the high temperature background method we can clearly see the cooler slab 400 c to 700 c that is being subducted under the hotter mantle material 425 c to 850 c we also show that the deep low frequency earthquakes are occurring in regions just above the slab with temperatures of about 500 c to 600 c

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake

benthonic foraminifera in the upper miocene cruse formation at quinam bay trinidad western tropical atlantic ocean and their palaeoenvironmental significance the upper miocene cruse formation of trinidad yields predominantly agglutinated foraminifera the limited assemblage has previously hampered palaeoenvironmental interpretations twenty two samples taken from a basal cruse section at 0 5 m intervals from quinam bay 10 05 07 7 n 61 45 04 7 w yielded 2938 foraminifera in 33 species almost all agglutinated the absence of calcite cemented agglutinants suggests post mortem dissolution of calcareous specimens dominant spirosigmoilinella compressa indicates lower bathyal to abyssal palaeodepths although the low values of the information function h are typical of shallower water subdominant haplophragmoides carinatus and haplophragmoides sp 1 indicate low dissolved oxygen levels diversities measured using species richness s and h were especially low in the lowest 3 5 m of the section the proportional abundance of the dominant species in each sample max p i indicated three subsections being low in the middle of the section but higher at the top and bottom she analysis indicated six abundance biozones abs containing one to seven samples each of the three abs with more than three samples two had type 1 community structures and one had a type 0 community structure abs with one or two samples indicate that environmental change at the top of the interval with low diversity is rapid this is reflected in a change from abundant morphotype m3a surficial epifauna flattened at the base of the section to abundant m4a shallow infauna planispiral with m4b deep infaunal towards the top which shows a downward shift in the position of the redox front part way through the section

a physical model for widespread near surface and fault zone damage induced by earthquakes 1 i present a rigorous unifying interpretation of the well documented widespread near surface and fault zone damage induced by earthquakes by simulating three dimensional dynamic rupture propagation on a vertical strike slip fault stresses in the crust are depth dependent and material response is governed by the drucker prager yielding criterion i show that material yielding induced by seismic waves under the low confining pressure causes widespread near surface damage because the confining pressure increases with depth the yielding zone at depth is narrowly confined near the fault but its thickness broadens dramatically near the surface forming a flower like damage zone the fault zone damage at depth is induced by large dynamic stresses associated with the rupture front while is induced by strong seismic waves ahead of the rupture front near the earth s surface these results have important implications for the formation and evolution of fault zones and possibly for the dynamic triggering of earthquakes as well

detailed seismic attenuation structure beneath hokkaido northeastern japan arc arc collision process arc magmatism and seismotectonics in this study we imaged a detailed seismic attenuation structure frequency independent q 1 beneath hokkaido japan by merging waveform data from a dense permanent seismic network with those from a very dense temporary network corner frequency of each event used for t estimation was determined by the s coda wave spectral ratio method the seismic attenuation qp 1 structure is clearly imaged at depths down to about 120 km for the fore arc side of hokkaido high qp zones are imaged at depths of 10 to 80 km in the crust and mantle wedge above the pacific slab low qp zones are clearly imaged in the mantle wedge beneath the back arc areas of eastern and southern hokkaido these low qp zones extending from deeper regions extend to the moho beneath volcanoes the locations of which are consistent with those of seismic low velocity regions these results suggest that the mantle wedge upwelling flow occurs beneath hokkaido except in the area where there is a gap in the volcano chain in contrast an inhomogeneous seismic attenuation structure is clearly imaged beneath the hokkaido corner a broad low qp zone is located at depths of 0 60 km to the west of the hidaka main thrust the location almost corresponds to that of the seismic low velocity zone in the collision zone the fault planes of the 1970 m6 7 and 1982 m7 1 earthquakes are located at the edges of this broad low qp zone observations in this study indicate that our findings contribute to understanding the detailed arc arc collision process magmatism and seismotectonics beneath hokkaido

modern applications of biostratigraphy in exploration and production paleontology has played a long and distinguished role in the petroleum industry traditionally paleontology has been used for two main purposes dating of sedimentary rocks and stratigraphic correlations the primary method for achieving both objectives was the recognition of index fossils based on the observed ranges of these index fossils the stratigraphic column was divided into biozones which then became the primary units for correlation although this approach has had many successful applications this venerable methodology is not without limitations time resolution is not very precise the presence of index fossils can be controlled by lithology and or depositional environment and index fossils may have different ranges in different basins

joint local earthquake and teleseismic inversion for 3 d velocity and q in new zealand abstract we have developed a joint inversion of local earthquake and teleseismic data and applied it in new zealand to obtain improved 3 d seismic velocity qp and qs models to accommodate sharper detail where there are dense local earthquake data varied spatial resolution is enabled with linking of nodes into larger inversion elements where data are sparse the teleseismic t measurements are obtained by stacking and removing the average for the common source and path the combined inversions used data from 126 teleseisms and 2365 local earthquakes that extend to 350 km depth vp results are similar to previous studies but have enhanced detail near the slab provided by local seismicity sub slab low velocity and low q is imaged along the eastern north island which may represent asthenosphere with entrained water and melt along the western south island there is a high vp high q feature distinct from the current slab and interpreted to be a slab remnant from the period of subduction transform edge propagator fault development and trench rollback the axial south island high vp high q feature of the hikurangi plateau is imaged and is bounded on the southeast by thinner lithosphere that may be related to the period of great south basin rifting

along strike variation in subducting plate seismicity and mantle wedge attenuation related to fluid release beneath the north island new zealand abstract we analyze seismicity in the subducted slab of the hikurangi subduction zone along a 600 km length beneath the north island new zealand the volcanic character changes along strike with extremely productive rhyolitic volcanism in the central taupo volcanic zone moderate andesitic volcanism in the northern and south central zones and subduction without volcanism in the southernmost hikurangi zone we have relocated slab earthquakes with 3 d velocity models the relocated seismicity shows more detail of the varied distribution and abundance of slab seismicity which below 50 km depth may be related to embrittlement from high fluid pressure the depth of the buoyant subducted hikurangi plateau ranges from 40 to 140 km depth at depths where the subducting slab interacts with the mantle wedge along arc variation in slab seismicity is a fundamental characteristic with patches of abundant seismicity separated by low seismicity zones the largest most numerous patch at 150 220 km depth underlies a pronounced low qp zone in the mantle wedge which is associated with the rhyolite dominant taupo caldera extensive melt in the region of low qp requires high h 2 o flux from the underlying slab the abundant taupo seismicity suggests a correlation between melt production and regions of earthquake fracture permeability following embrittlement which promote migration of dehydration fluid the hydration history of the incoming slab may be a key factor in producing variations in dehydration and intraslab fluid migration broader extensive outer rise yielding and hydration may have occurred near the approach of the re entrant hikurangi plateau forming the slab section that currently has high seismicity slab seismicity deepens from 240 to 330 km along arc as the subduction rate increases from 40 mm yr the southwestern slab seismicity is bounded by an unusually narrow zone with 110 km depth extent this is inferred to be a dehydration front related to heating at a slab edge that is located 70 km further southwest

small earthquakes provide insight into plate coupling and fluid distribution in the hikurangi subduction zone new zealand abstract to better understand the nature of interplate coupling in the southern north island of new zealand we compare the distribution of relocated small earthquakes with interseismic geodetic locking of the plate interface from gps and structure within the overlying plate from tomographic inversions relocated seismicity shows a good correlation with both the distribution of strong geodetic locking and geological terranes in the overlying plate it appears that the strong permian triassic rakaia geological terrane controls coupling at the plate interface this relatively impermeable terrane also modulates the small earthquake distribution within the underlying slab promoting clustered seismicity in the slab crust and suppressing lower plane seismicity in the mantle the seismicity and structural data suggest a model where plate coupling is controlled by the ability of fluid to cross the plate interface when an impermeable terrane in the overlying plate prevents such fluid flow plate coupling appears to be strong

structure of the hanmer strike slip basin hope fault new zealand hanmer basin 10 x 20 km located in northern south island new zealand is evolving where two major segments of the dextral strike slip hope fault are projected to converge across a 6 to 7 km wide releasing step over the structural geometry and development of hanmer basin does not conform to traditional pull apart basin models the respective fault segments do not overlap but are indirectly linked along the southwest margin of the basin by an oblique normal fault the hope river segment terminates in an array of oblique normal faults along the northwestern basin range front and east west striking normal faults on the west hanmer plain faulted holocene alluvial fan surfaces indicate west hanmer basin is actively subsiding evolving under north south extension the conway segment along the southeastern margin of the basin terminates in a complex series of active fault traces small pop up ridges and graben depressions early basin fill sediments of pleistocene age are being folded elevated and dissected as the eastern part of hanmer basin is progressively inverted and destroyed by north south contraction the north margin of the basin is defined by a series of topographic steps caused by normal faulting outside of the area of the releasing step over these normal faults we interpret to reflect large scale upper crustal collapse of the hanging wall side of the hope fault new seismic reflection data and geologic mapping reveal a persistent longitudinal and lateral asymmetry to basin development four seismic stratigraphic sequences identified in the eastern sector of the basin thicken and are tilted southward with insequence lateral onlaps occurring to the north and east and also onto basement near the fault controlled basin margins the basin depocenter currently contains 1000 m of sediment adjacent to the south margin and is disrupted by faulting only at depth in the western part of the basin the sediment fill is thinner today the rate of basin deepening under transtension at the western end is matched by its progressive inversion and destruction under transpression in the eastern sector with the oldest basin fill now being recycled we propose a hybrid model for hanmer strike slip basin one in which geometric elements of a fault wedge basin downward and upward tipped spindle shaped ends are combined with those of a pull apart basin step over region between the major fault segments we also conclude that changes in fault geometry releasing and restraining bends and step overs at a variety of scales and over short distances control the development of the extensile and contractile parts of the basin and three dimensional basin asymmetry strain partitioning is complex and cannot be related simply to local reorientation of the regional stress field

taiwan mountain building insights from 2 d thermomechanical modelling of a rheologically stratified lithosphere summary the taiwan orogen has long been regarded as a case example for studying mountain building in association with subduction processes in this paper we present a fully coupled thermomechanical modelling of the taiwan collision based on a realistic viscous elastic plastic rheology it satisfactorily reproduces available thermochronometric data long short term deformation patterns heat flux and erosion sedimentation distribution across the taiwan orogeny we found that a deep seated flux of asian crustal material into the orogenic wedge should be invoked to counter balance observed exhumation and erosion in the central range however in contrast with recent thermokinematic models of exhumation and deformation suggesting that underplating plays a significant role we show that most constraints on exhumation and deformation can be more straightforwardly interpreted by the frontal accretion of the rheologically layered asian crust we finally infer that such a model is in better agreement with the basic expectation that the hot young and buoyant chinese continental margin should hardly be subducted beneath the cold old and dense oceanic plate of the philippines sea

accommodation of compressional inversion in north western south island new zealand old faults versus new in the nw south island new zealand high angle faults inherited from episodes of late cretaceousepaleocene and eocene extension have since the early miocene undergone compressional inversion in association with right lateral shearing and transpression on the alpine fault active reverse faulting and large historical earthquakes occur along nes to nneessw trending faults which at the surface dip 45e75 to both the east and west the faults truncate subparallel folds that deform the tertiary sequence overlying a composite paleozoicemesozoic crystalline basement however the deep geometry of these faults their penetration into the middle to lower crust and their relationship to the alpine fault are poorly understood the tectonic architecture of this compressional inversion province is analysed by reconstructing structural contours at the base of the oligocene carbonate sequence in the north west of the south island deformation of the oligocene carbonate sequence structural analyses in the field and subsurface data indicate a mixed style of inversion with 1 reactivation of some high angle normal faults and 2 thrusting on new moderate dipping cross cutting faults that detach slivers of basement and cause flexural folding in the sedimentary cover these faults may remain blind or concealed beneath cover sequences but are likely to control seismic rupturing in the basement at depths of w10e15 km 2006 elsevier ltd all rights reserved

three dimensional attenuation structure of the hikurangi subduction zone in the central north island new zealand summary we use waveform data from 2708 earthquakes recorded by dense seismograph deployments in the central north island of new zealand to image the 3 d attenuation structure of the hikurangi subduction zone down to ca 300 km depth attenuation images are obtained by determining the quality factor of p waves qp using a t inversion with a previously determined 3 d seismic velocity model we have included limited frequency dependence for qp with qp being frequency independent above 10 hz and having a frequency dependence of f 10 0 5 below 10 hz the qp images provide further constraint on the large scale features associated with subduction and magmatism beneath the central north island and serve to refine interpretations of crust and upper mantle structure from vp and vp vs the subducted plate is the most prominent feature in the qp images its high qp 900 1200 is consistent with the ca 120 myr old slab being relatively cold qp is better at resolving the slab than vp the coincidence of a strong gradient in qp with the upper plane of the dipping seismic zone indicates that even below 75 km depth the upper envelope of seismicity provides a good estimate of the location of the plate interface the mantle wedge is generally imaged as a relatively low qp 400 feature below 50 km depth however there are significant changes evident in the wedge along the strike of the subduction zone the most pronounced low qp in the mantle wedge occurs from ca 50 to 85 km depth beneath the productive rhyolite dominated central segment of the taupo volcanic zone suggesting a close link between volcanism and low qp in the shallow mantle wedge there is a strong correlation between low qp l owvp and high vp vs in this part of the mantle wedge suggesting that high temperature is the controlling influence on qp there within 30 km of the surface of the slab there is a large change in qp but only a modest change in vp our results are consistent with a fluid rich viscous blanket being entrained with the motion of the subducted slab down to ca 140 km depth where hydrous melting of mantle peridotite initiates trenchward of the volcanic front a strong horizontal gradient to higher qp is imaged in the mantle wedge we interpret this as a stagnant mantle nose not involved in the corner flow of the adjacent mantle the distribution of qp within both the slab and the overlying plate provides support for dehydration embrittlement being important in promoting seismicity in the mantle of the slab where qp is high at shallow depth in the forearc it is usually underlain by low qp near the plate interface this suggests that competent terranes in the forearc may act as aquicludes increasing fluid content in the rocks below and hence might control the distribution of coupling at the plate interface

seismic rheological model and reflection coefficients of the brittle ductile transition it is well established that the upper cooler part of the crust is brittle while deeper zones present ductile behaviour in some cases this brittle ductile transition is a single seismic reflector with an associated reflection coefficient we first develop a stress strain relation including the effects of crust anisotropy seismic attenuation and ductility in which deformation takes place by shear plastic flow viscoelastic anisotropy is based on the eigenstrain model and the zener and burgers mechanical models are used to model the effects of seismic attenuation velocity dispersion and steady state creep flow respectively the stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity which is obtained by the arrhenius equation and the octahedral stress criterion the p and s wave velocities decrease as depth and temperature increase due to the geothermal gradient an effect which is more pronounced for shear waves we then obtain the reflection and transmission coefficients of a single brittle ductile interface and of a ductile thin layer the pp scattering coefficient has a brewster angle a sign change in both cases and there is substantial ps conversion at intermediate angles the pp coefficient is sensitive to the layer thickness unlike the ss coefficient thick layers have a well defined brewster angle and show higher reflection amplitudes finally we compute synthetic seismograms in a homogeneous medium as a function of temperature

three dimensional attenuation model of the shallow hikurangi subduction zone in the raukumara peninsula new zealand 1 the material properties of the shallow subduction zone are imaged with data from a passive seismic deployment in the raukumara peninsula attenuation images are obtained using t values measured from 2848 spectra of p wave arrivals the t values are inverted for frequency independent three dimensional 3 d qp using a 3 d velocity model linked nodes in a preliminary 2 d inversion are used to obtain the most reliable initial model for 3 d inversions the results show the benefits of obtaining 3 d q in addition to 3 d vp and vp vs q is related to localized permeability and heterogeneity and thus 3 d q is an important tool for interpreting active tectonic regions qp in the shallow overlying plate generally corresponds to the mapped geology two high qp features 600 represent subducted seamounts or cretaceous volcanics within the overlying plate the gradient above the subducting plate is the strongest feature in the qp model with the slab having high qp 600 900 below 20 km depth and above the plate interface there is a zone of 50 decrease in qp and high vp vs about 10 km thick which represents actively subducting sediment and has distributed microearthquake activity associated with active underplating

northern california seismic attenuation 3d qp and qs models the geological complexity of northern california and its accreted terranes produces pronounced regional variations in the attenuation 1 q of earthquake waves using local earthquakes 3d q p and q s models have been obtained for this region that includes the san andreas fault system the great valley the sierra nevada batholith and the gorda subduction volcanic system path attenuation t values were determined from p and s spectra of 959 spatially distributed earthquakes magnitude 2 5 6 0 using 1254 stations from northern california earthquake data center networks and mendocino and sierra nevada temporary arrays the t data were used in q inversions using existing 3d velocity models and basic 10 km node spacing uneven data coverage was accommodated with linking nodes into larger areas for useful q images across the 3d volume results at shallow depth show very low q in the sacramento delta the eureka area and parts of the san francisco bay area in the brittle crust low q tends to be associated with regions of fault zones and microearthquakes in the ductile lower crust low q is observed below fault zones with large cumulative displacement and inferred grain size reduction with apparent broad ductile shear zones across the fault system in the coast ranges underlying active arc volcanoes low q features are apparent below 20 km depth under the long valley caldera two distinct partial melt features at 3 and 15 km depth are inferred moderately high q is associated with sierra nevada and salinian igneous rocks whereas the franciscan subduction complex shows moderately low q prominent high q relates to the great valley ophiolite the pattern of path specific attenuation rate from 3d q is generally similar to the pattern of psuedospectral acceleration residuals for the south napa earthquake indicating that it may be beneficial to incorporate 3d q models into future ground motion prediction equations for northern california online material figures of q p and q s resolution and station delays for t and table of q p v p q s and v s

rates of active faulting during late quaternary fluvial terrace formation at saxton river awatere fault new zealand a flight of faulted fluvial terraces at saxton river on the awatere fault northeast south island new zealand preserves the incremental slip history and detailed paleoearthquake chronology of this major strike slip fault here six fluvial terraces have been progressively displaced across the inland molesworth section of the fault with horizontal displacements ranging from 6 m for an ephemeral channel on the youngest terrace to 81 m for the riser above the oldest terrace new optically stimulated luminescence ages for abandonment of the two oldest terrace treads are 14 5 1 5 and 6 7 0 7 ka when combined with new measurements of incremental horizontal displacements and previous age data these new ages indicate that strike slip on this part of the awatere fault has been occurring at a near constant rate of 5 6 0 8 mm yr since ca 15 ka this rate is similar to recent slip rate estimates for an adjoining section of the same fault to the east which suggests that there is near complete slip transfer across the junction between the two fault strands comparison of the magnitudes and ages of the terrace riser displacements to the timing of paleoearthquakes on the molesworth section allows the mean per event horizontal displacement over the eight most recent surface rupture events to be estimated at 4 4 0 8 m between ca 5 ka and ca 2 ka surface rupturing earthquakes increased in frequency and decreased in their mean coseismic displacements to 2 6 m during this time the sense of local dip slip also shifted from north side up to south side up rapid incision of saxton river in the mid holocene may have caused a perturbation in the near surface stress directions acting on the fault plane beneath the saxton river valley forcing a change in the near surface fault geometry that resulted in the shift in sense of dip slip

seismic properties of anita bay dunite an exploratory study of the influence of water as a pilot study of the role of water in the attenuation of seismic waves in the earth s upper mantle we have performed a series of seismic frequency torsional forced oscillation experiments on a natural anita bay dunite containing accessory hydrous phases at high temperatures to 13008c and confining pressure pc o f 200 mpa within a gas medium high pressure apparatus both oven dried and pre fired specimens wrapped in ni fe foil within the poorly vented assembly were recovered essentially dry after 50 100 h of annealing at 13008c followed by slow staged cooling the results for those specimens indicate broadly similar absorptionband viscoelastic behaviour but with systematic differences in the frequency dependence of strain energy dissipation q 1 attributed to differences in the small volume fraction of silicate melt and its spatial distribution in contrast it has been demonstrated that a new assembly involving a welded pt capsule retains aqueous fluid during prolonged exposure to high temperaturescallowing the first high temperature torsional forced oscillation measurements under high aqueous fluid pore pressure pf at temperatures 410008c a marked reduction in shear modulus without concomitant increase in q 1 is attributed to the widespread wetting of grain boundaries resulting from grain scale hydrofracturing and the maintenance of conditions

seven million years of strike slip and related off fault deformation northeastern marlborough fault system south island new zealand the marlborough fault system mfs south island new zealand is remarkable for the oblique slip nature of its active faulting and can be placed in a framework of pacific australian pac aus plate motions and geodetic strain new zealand illustrates changing degrees of upper plate slip partitioning in an obliquely convergent margin that varies along strike in crustal type and subduction geometry in this paper we focus on the ne part of this transcurrent fault system especially the awatere fault which is the only marlborough fault to offset a major neogene basin strike slip on an ancestral eastern strand of this fault began after 7 4 6 4 ma later vertical axis rotation may have caused it to be abandoned in favor of the currently active strand since 7 ma at least 34 10 km of dextral slip has accrued on the awatere fault 6 km of slip has accrued on two nearby strands and 4 km has accrued on the currently active coastal strand comparison of miocene with late quaternary offsets suggests that rates of strike slip on the fault have been nearly constant or have possibly decelerated we interpret spatial and temporal patterns of strike slip in the mfs to reflect southward migration of transform motion resulting from increased pac aus plate convergence late neogene to present day shortening directions in ne south island are indicative of a partially slip partitioned margin slip partitioning diminishes southward as the underlying subduction interface becomes strongly coupled suggesting a simple mechanical relationship between these processes

seismic attenuation and mantle wedge temperatures in the alaska subduction zone 1 anelastic loss of seismic wave energy or seismic attenuation 1 q provides a proxy for temperature under certain conditions the q structure of the upper mantle beneath central alaska is imaged here at high resolution an active subduction zone where arc volcanism is absent to investigate mantle thermal structure the recent broadband experiment across the alaska range beaar provides the first dense broadband seismic coverage of this region the spectra of p and sh waves for regional earthquakes are inverted for path averaged attenuation operators between 0 5 and 20 hz along with earthquake source parameters these measurements fit waveforms significantly better when the frequency dependence of q is taken into account and in the mantle frequency dependence lies close to laboratory values inverting these measurements for spatial variations in q reveals a highly attenuating wedge with q 500 assuming frequency dependence comparison with p results shows that attenuation in bulk modulus is negligible within the low q wedge as expected for thermally activated attenuation mechanisms bulk attenuation is significant in the overlying crust and subducting plate indicating that q must be controlled by other processes the shallowest part of the wedge shows little attenuation as expected for a cold viscous nose that is not involved in wedge corner flow overall the spatial pattern of q beneath alaska is qualitatively similar to other subduction zones although the highest wedge attenuation is about a factor of 2 lower the q values imply that temperatures exceed 1200 c in the wedge on the basis of recent laboratory based calibrations for dry peridotite these temperatures are 100 150 c colder than we infer beneath japan or the andes possibly explaining the absence of arc volcanism in central alaska

electrical resistivity of ice from the antarctic peninsula antarctica l objectif etait de determiner le comportement electrique de la glace dans une zone ou existe une fonte estivale substantielle ainsi que des regimes thermiques contrastes des mesures electriques effectuees en trois sites le long d une ligne de courant sur le george vi ice shelf montrent que a la resistivite de la glace en profondeur est semblable a celle des autres shelfs antarctiques b la resistivite de la glace a la surface de la glace du shelf qui est soumise a la percolation et au regel de l eau de fusion est semblable a celle de la glace en profondeur et donc la glace a un caractere polaire

the changing course of the orinoco river during the neogene a review abstract recent studies have revealed that the rich fresh water vertebrate fossil fauna of the urumaco formation in northwestern falcon of late miocene age belongs biogeographically to the orinoco river system this finding together with other evidence can be used to chart the changing course of the orinoco river during the neogene and thus date the uplift of various mountain ranges that affected the change in this course presence of the misoa delta in the middle eocene of the maracaibo basin has been suggested as evidence for a large river running in a south north direction and draining the central cordillera of colombia and the guayana highlands the late middle eocene uplifting of western venezuela changed the paleogeographic setting and a new delta building shifted to the south represented by the extensive carbonera formation of late eocene to oligocene age in the llanos basin of colombia and southwestern venezuela in the earliest miocene the falcon basin situated to the east of the maracaibo basin was primarily the site of marine shale sedimentation stratigraphic and microfaunal evidence suggests the possibility of the presence of the proto orinoco river in the western margin of the basin in any case from the mid early miocene until the end of the middle miocene a fluvio deltaic sequence was deposited in the northwestern falcon with a total thickness of 4 7 km the urumaco formation overlies this sequence and its vertebrate fauna indicates that the river which built the prior delta was definitely the proto orinoco the deformation and uplift of the eastern cordillera of colombia and of the southwestern end of the merida andes caused the distal course of the river to be deflected to a west east direction a late middle miocene age for this change of course is precisely dated on marine microfaunal evidence the late miocene sedimentary record of the orinoco river delta appears east of maturin in the eastern venezuela basin and pliocene and pleistocene deltaic sediments are especially evident in trinidad and the columbus basin

fluid and deformation regime of an advancing subduction system at marlborough new zealand newly forming subduction zones on earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes the transpressional subduction regime of new zealand which is advancing laterally to the southwest below the marlborough strike slip fault system of the northern south island is an ideal setting in which to investigate these processes here we acquired a dense high quality transect of magnetotelluric soundings across the system yielding an electrical resistivity cross section to depths beyond 100 km our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops massive fluid release just inland of the trench induces fault fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates narrow strike slip faults in the shallow brittle regime of interior marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones deep subduction generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high angle thrusts that are poorly oriented for failure under dry conditions the fluid deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models

squirt flow in fully saturated rocks we estimate velocity frequency dispersion and attenuation in fully saturated rocks by employing the squirt flow mechanism of solid fluid interaction in this model pore fluid is squeezed from thin soft cracks into the surrounding large pores information about the compliance of these soft cracks at low confining pressures is extracted from high pressure velocity data the frequency dependence of squirt induced pressure in the soft cracks is linked with the porosity and permeability of the soft pore space and the characteristic squirt flow length these unknown parameters are combined into one expression that is assumed to be a fundamental rock property that does not depend on frequency the appropriate value of this expression for a given rock can be found by matching our theoretical predictions with the experimental measurements of attenuation or velocity the low frequency velocity limits as given by our model are identical to those predicted by gassmann s formula the high frequency limits may sign

distributed deformation in the lower crust and upper mantle beneath a continental strike slip fault zone marlborough fault system south island new zealand converted phases from teleseisms recorded by a seismic array spanning the northern half of the marlborough fault system south island new zealand show a continuous unbroken moho underlying a seismically anisotropic lower crust beneath the two north ernmost faults of the fault system these observations suggest that distributed deforma tion not slip on a narrow vertical fault accommodates displacement in the lower crust below the 120 480 km of right lateral slip across the wairau fault one splay of the marl borough fault system and the northward continuation of the alpine fault beneath the wairau fault the moho dips 258 308 southeast from a depth of 26 km northwest of the fault to a depth of 34 km southeast of the fault farther to the southeast ps conversions from the moho continue under the awatere fault 34 6 10 km of slip with a constant amplitude and depth of 34 km across the two faults converted energy from 16 20 km depth varies with back azimuth in a manner suggesting the presence of anisotropy in the lower crust these observations imply that one of the tenets of plate tectonics that faults defining plate boundaries pass through both crust and upper mantle does not apply to new zealand or to continents in general

imaging the fault zones of the 2000 western tottori earthquake by a new inversion method to estimate three dimensional distribution of the scattering coefficient 1 we developed an inversion method to estimate the spatial variation of scattering coefficient from observed envelopes of s coda waves synthetic envelopes are calculated on the basis of the single scattering theory assuming double couple sources and a one dimensional depth dependent velocity structure in our algorithm we evaluated scattering attenuation taking spatial variation of scattering coefficient into consideration in order to model energy attenuation by passing through the localized anomalous zones with high scattering coefficients the inversion method was applied to the seismograms in and around the focal area of the 2000 western tottori earthquake with m 7 3 obtained results show that there exist several zones with large scattering coefficient zones lszs one of the most predominant lsz is located along and around the fault zone of the m 7 3 earthquake in the upper crust this lsz suggests the existence of damaged zones in the main shock fault within this lsz an exceptionally small scattering coefficient area was detected this area corresponds to an asperity with a large amount of coseismic slip of the main shock other prominent lszs are detected in the mid to lower crust below the northwestern part of the main shock fault in addition lszs are distributed in a seismic low velocity zone near quaternary volcanoes and near hypocenters of deep low frequency microearthquakes some of the imaged lszs are probably related to local concentration of fluid in the crust

comparative tomography of reverse slip and strike slip seismotectonic provinces in the northern south island new zealand abstract actively deforming crust and upper mantle around the obliquely convergent pacific australia plate boundary in the northern south island of new zealand have been investigated by seismic tomography using data from a temporary c 50 station network plus geonet the alpine wairau fault principal component of the plate boundary at the surface transects the study area separating the buller nelson bn province of active compressional inversion involving steep reverse faulting to the northwest from the marlborough fault system ma dominated by dextral strike slip faulting to the southeast in the course of this study ma hosted the 2013 seddon earthquake sequence m5 9 6 6 and the 2016 m7 8 kaikoura earthquake active fault structures and present seismicity are associated with a heterogeneous distribution of low velocity and vp vs anomalies elsewhere there is a general association between crustal seismicity and low velocity zones along major fault structures within both the ma and bn seismotectonic provinces along which high vp vs anomalies are locally conspicuous areas of active crustal seismicity are also generally characterized by high vp vs for instance the hypocentre and aftershocks of the 2016 m7 8 kaikoura earthquake overlie a low velocity region with high vp vs the coincidence of anomalously low vp and vs and anomalously high vp vs with zones of high electrical conductivity defined by a previous mt transect is consistent with the notion that upward migration of overpressured hydrothermal fluid from the subducting slab at depth leads to a heterogeneous distribution of overpressured fluid in and around the base of the crustal seismogenic zone weakening the overlying crust and promoting seismic rupture along major fault systems it seems possible that the association of mid crustal low velocity zones with anomalously high vp vs may diagnose rupture preparation zones where frictional strength is being lowered by the build up of fluid overpressure concurrent with accumulating shear stress so that eventual fault failure is dual driven

mechanisms of seismic attenuation in the crust scattering and anelasticity in new york state south africa and southern california the envelopes of high frequency seismograms from earthquakes in new york state south africa and southern california are analyzed to determine how much of the observed attenuation of shear waves in the crust is caused by anelasticity and how much is produced by scattering from small scale heterogeneities for seismograms from new york and south africa there is a systematic enrichment of high frequency energy that develops in the coda relative to the direct s or l sub g waves the author uses the growth with source receiver distance of the high frequency energy in the coda relative to the direct wave to estimate the scattering attenuation of the crust and the relative time decay of high frequency energy in the coda to determine the anelastic attenuation using the estimate of l sub g wave q as a constraint he finds that the scattering q in the crust of new york is either independent of frequency or increases with frequency f slower than f sup 0 3 a similar analysis for south africa indicates that scattering is the major cause of the observed s wave attenuation of high frequency energy 30 hz at local distances 100 km for southern california the extent of the scattering attenuation is unclear from the data more although the relatively steep coda decay of southern california implies that anelastic attenuation is stronger there than in new york or south africa for southern california and new york state the 3 hz coda q taken from the late coda 50 110 s is comparable to the l sub g wave q however the 3 hz coda q for south africa over these lapse times is about 5 times the estimated l sub g wave q for all three areas the frequency dependence of the coda q does not match the frequency dependence of the s wave q and for new york and south africa it changes with lapse time less

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

balancing the plate motion budget in the south island new zealand using gps geological and seismological data summary the landmass of new zealand exists as a consequence of transpressional collision between the australian and pacific plates providing an excellent opportunity to quantify the kinematics of deformation at this type of tectonic boundary we interpret gps geological and seismological data describing the active deformation in the south island new zealand by using an elastic rotating block approach that automatically balances the pacific australia relative plate motion budget the data in new zealand are fit to within uncertainty when inverted simultaneously for angular velocities of rotating tectonic blocks and the degree of coupling on faults bounding the blocks we find that most of the plate motion budget has been accounted for in previous geological studies although we suggest that the porter s pass amberley fault zone in north canterbury and a zone of faults in the foothills of the southern alps may have slip rates about twice that of the geological estimates up to 5 mm yr 1 of active deformation on faults distributed within the southern alps 100 km to the east of the alpine fault is possible the role of tectonic block rotations in partitioning plate boundary deformation is less pronounced in the south island compared to the north island vertical axis rotation rates of tectonic blocks in the south island are similar to that of the pacific plate suggesting that edge forces dominate the block kinematics there the southward migrating chatham rise exerts a major influence on the evolution of the new zealand plate boundary we discuss a model for the development of the marlborough fault system and hikurangi subduction zone in the context of this migration

seismic anisotropy from local earthquakes in the transition region from a subduction to a strike slip plate boundary new zealand shear wave splitting is used to investigate anisotropy in the crust and upper mantle in a subduction zone lower half of the north island of new zealand and its transition to oblique transform faulting marlborough area northern south island in marlborough delay times show almost no increase with depth and it is most likely that the higher frequency phases used in this study respond mainly to lithospheric anisotropy in the central marlborough fault system mfs fast polarizations are subparallel to the faults anisotropy is attributed to the presence of metamorphosed schist eclogite of 30 10 km thickness and located 50 80 km beneath the mfs on the edges of the mfs fast polarizations are parallel to the maximum compressive stress direction consistent with crack induced anisotropy in the crust the shear zone which is as wide as the island in the mantle as inferred from sks phases seems to occur in a narrower zone in the crust in the lower half of the north island fast polarizations from events at all depths are oriented parallel to the strike of the hikurangi subduction zone as well as to the faults polarisations are similar to those of sks phases which mainly sample the mantle this suggests that the lithosphere and the upper mantle asthenosphere deform in a coherent strike slip shear we calculate 1 2 0 3 velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth in order to match the sks delay times this result requires the presence of anisotropic material down to 580 100 km depth or a change in anisotropy with depth or frequency dependent splitting

insights into the structure and tectonic history of the southern south island new zealand from the 3 d distribution of p and s wave attenuation

three dimensional attenuation structure of central and southern south island new zealand from local earthquakes 1 the three dimensional attenuation properties of the south island new zealand are obtained from local earthquake t spectral decay data we extend the spectral fitting method used to obtain t by enabling limited frequency dependence for q observed qp heterogeneity relates to both active geological processes and to constituent tectonic blocks with high qp for the haast schist the most pronounced low qp features in the upper crust correspond to zones of recent seismicity rather than the most significant faults the central alpine fault a dipping oblique slip fault forms the western boundary to a low qp volume in the ductile crust particularly low qp is imaged at the base of the alpine fault at 30 km depth and may be related to metamorphic fluid release in the crustal root a region of predicted high strain conversely the southern alpine fault which is a vertical strike slip fault forms the western boundary to the high qp schist region in the southern south island the plate boundary becomes a subduction zone exhibiting unusual qp character moderately high qp is associated with the subducted australian plate there is also a high qp high vp feature in the pacific mantle to 160 km depth that appears to cause the subducting slab to bend to near vertical this feature may represent mantle shortening since the miocene regional attenuation rates for fiordland earthquakes calculated from the 3 d qp model show large variations over the south island the low qp in the crustal root has a significant effect on passing raypaths

regional spectral analysis of three moderate earthquakes in northeastern north america abstract we analyze fourier spectra obtained from the horizontal components of broadband and accelerogram data from the 1997 cap rouge the 2002 ausable forks and the 2005 riviere du loup earthquakes recorded by canadian and american stations sited on rock at hypocentral distances from 23 to 602 km we check the recorded spectra closely for anomalies that might result from site resonance or source effects we use beresnev and atkinson s 1997 near surface velocity structures and boore and joyner s 1997 quarter wave method to estimate site response at hard and soft rock sites we revise the street et al 1975 model for geometrical spreading adopting a crossover distance of r o 50 km instead of 100 km we obtain an average attenuation of q 410 25 f 0 50 0 03 for s l g surface waves with ray paths in the appalachian and southeastern grenville provinces we correct the recorded spectra for attenuation and site response to estimate source spectral shape and radiated energy for these three earthquakes and the 1988 m 5 8 saguenay earthquake the brune stress drops range from 130 to 419 bars and the apparent stresses range from 39 to 63 bars the corrected source spectral shapes of these earthquakes are somewhat variable for frequencies from 0 2 to 2 hz falling slightly below the fitted brune spectra

three dimensional qp and qs tomography beneath taiwan orogenic belt implications for tectonic and thermal structure summary we determined the 3 d qp and qs structure of the taiwan orogenic belt to enhance understanding of the related tectonic and thermal structure beneath the collision zone the inversion used t values measured from the spectra of p and s waves from the dense taiwan strong motion network for moderate size earthquakes ml 4 5 5 5 to avoid source complexity the time period 1991 2007 includes the aftershock sequence of the 1999 chi chi earthquake that provides good ray coverage in central taiwan over 18 000 velocity spectra from 883 earthquakes were analysed a non linear least square technique is applied to the spectra for t determination by assuming a 2 source model for the frequency band of 1 30 hz a frequency independent q was assumed in this study the corner frequency of a specific event was fixed for the corresponding stations and a quality index was defined to assure good quality data for the inversion the results reveal the sharp variation of qp and qs across the recently ruptured chelungpu fault and the kaoping and chaochou faults in pingtung plain the q values in the hangingwall are smaller by about 85 and 110 for qp and qs respectively relative to the footwall the fault geometry is distinctly delineated by the contour of qp qs of 1 2 that extends to the depth of the geologically identified decollement structure beneath the central range the low qp low qs and high qp qs features coincide well with the aseismic zone comparison to the recent thermomechanical numerical models of taiwan shows that the low q zone corresponds to the exhumation of the lower crust the low qs regime high attenuation beneath the central ranges at the depth of 5 22 km coincides with predicted temperatures of 400 600 c the qs comparison with the major tectonic and thermal mechanical models of taiwan reveals that the shear wave attenuation model contains comprehensive rheological and thermal information of relevance to understanding mountain building processes this technique appears particularly useful for distinguishing strong and weak crustal regions in the absence of other constraints

illumination of rheological mantle heterogeneity by the m7 2 2010 el mayor cucapah earthquake 1 major intracontinental strike slip faults tend to mark boundaries between lithospheric blocks of contrasting mechanical properties along much of their length both crustal and mantle heterogeneities can form such boundaries but the role of crustal versus mantle strength contrasts for localizing strain sufficiently to generate major faults remains unclear using the crustal velocity field observed through the global positioning system gps in the epicentral area of the m7 2 2010 el mayor cucapah earthquake baja california we find that transient deformation observed after the event is anomalously small in areas of relatively high seismic velocity in the shallow upper mantle 50 km depth this pattern is best explained with a laterally heterogeneous viscoelastic structure that mimics the seismic structure the mantle of the southern colorado river desert scrd and peninsular ranges pr which bound the fault system to its east and west respectively have anomalously high viscosity and seismic velocity we hypothesize that compared with the rest of the san andreas fault saf system to its north the strike slip fault system in northern baja california is narrow because of the presence of the pr and scrd high viscosity regions which bound it