molecular and isotopic composition of free hydrocarbon gases from sicily italy 1 chemical and isotopic data have been used as geochemical tracers for a genetic characterization of hydrocarbon gases from a total of eleven manifestations located in eastern and central southern sicily italy the molecular analysis shows that almost all the samples are enriched in methane up to 93 2 vol with the exception of four gas samples collected around mt etna showing high mantle derived co2 content methane isotope signatures suggest that these are thermogenic gases or a mixture between thermogenic gases and microbial gases although samples from some mud volcanoes in southern sicily macalube di aragona show isotope signatures consistent with a mixing model between thermogenic and microbial by combining the molecular compositions c1 c2 c3 and the methane isotope ratios 13c1 such a process seems to be excluded therefore the occurrence of secondary post genetic processes should be invoked two main hypotheses have been considered the first hypothesis includes that the gas is produced by microbial activity and altered post genetically by microbial oxidation of methane while according to the second hypothesis thermogenic gas have modified their molecular ratios due to vertical migration

a body wave analysis of the 1966 gisborne new zealand earthquake abstract the ml 6 2 gisborne earthquake of march 4 1966 occurred along the east coast of the north island new zealand modeling of p and s body waves shows that the focal mechanism of this event is consistent with northwestward thrusting of the pacific plate beneath the north island 249 25 and 131 the focal depth is constrained to 18 km significantly less than the values of 25 30 km computed from local network data estimates of the scalar moment source duration and stress drop for the event are 4 1024 dyne cm 2 3 s and 20 120 bar respectively cross correlation errors of synthetic to observed waveforms were computed for all possible p and t axis locations and slip vector orientations and contoured on a projection of the focal sphere the error contour at which the synthetic waveforms distinctly diverged from the observed waveforms was established by eye the procedure shows that the analysis of long period body waves at least in this case provides much better constraint on focal mechanism orientation than does first motion data alone

evaluation of possible gas microseepage mechanisms petroleum microseepage anomalies over petroleum accumulations are commonly explained by rapid vertical migration of colloidal gas bubbles through fracture networks this article is a theoretical anal ysis of this mechanism and of continuous gas phase flow in frac tures the gas bubble ascent mechanism is much slower than re ported microseepage velocities so it cannot account for observed microseepage in contrast continuous gas phase flow through frac tures can equal or exceed reported microseepage velocity while maintaining total flux low enough so that petroleum accumulations can exist for geological lengths of time fracture entry pressure for bubbles is more than twice that of a continuous gas phase so con tinuous gas phase migration also requires a lower pressure threshold before initiating seepage vertical microseepage is therefore best explained by the same mechanism interpreted for macroseepage although this article provides a theoretically justified mecha nism for microseepage it also shows why interpretation of surface microseepage signals is problematic fracture geometry controls seepage velocity and flux so geochemical anomalies may indicate an increase in fracture aperture as well as possible subsurface ac cumulations larger fractures require very low gas capillary entry pressures so in some settings surface seepage could result from fractures over stratal migration pathways as well as over petroleum accumulations

fore arc deformation and underplating at the northern hikurangi margin new zealand geophysical investigations of the northern hikurangi subduction zone northeast of new zealand image fore arc and surrounding upper lithospheric structures a seismic velocity vp field is determined from seismic wide angle data and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling we found that the subducting hikurangi plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics limestone and chert the upper plateau crust is characterized by vp 4 9 6 7 km s overlying the lower crust with vp 7 1 km s gravity modeling yields a plateau thickness around 10 km the reactivated raukumara fore arc basin is 10 km deep deposited on 5 10 km thick australian crust the fore arc mantle of vp 8 km s appears unaffected by subduction hydration processes the east cape ridge fore arc high is underlain by a 3 5 km deep strongly magnetic 3 3 a m high velocity zone interpreted as part of the onshore matakaoa volcanic allochthon and or uplifted raukumara basin basement of probable oceanic crustal origin beneath the trench slope we interpret low seismic velocity high attenuation low density fore arc material as accreted and recycled suggesting that underplating and uplift destabilizes east cape ridge triggering two sided mass wasting mass balance calculations indicate that the proposed accreted and recycled material represents 25 100 of all incoming sediment and any remainder could be accounted for through erosion of older accreted material into surrounding basins we suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore arc highs have not properly been accounted for

shallow subduction tectonics in the raukumara peninsula new zealand as illuminated by earthquake focal mechanisms the raukumara peninsula affords an excellent opportunity to study the subduction process as subduction of the buoyant hikurangi plateau on the pacific plate has resulted in exposure of the forearc above the shallow part of the subduction thrust here we report on the focal mechanisms of 117 earthquakes of m l 2 4 4 9 and shallower than 80 km recorded during a 5 month deployment of 36 portable seismographs on the peninsula mechanisms have been constrained using both first motion polarity data and amplitudes of seismogram envelopes downdip tensional strain predominates in the subducted plate with taxes of events in both the upper and lower planes of the dipping seismic zone generally paralleling the local dip of the zone trenchward extensional strain is seen in the uppermost part of the overlying australian plate in line with geodetic and geological results this can be related to extension and gravity sliding of surficial rocks due to uplift of the raukumara range resulting from underplating of subducted sediment there is a marked change in earthquake mechanisms along strike in the lower part of the overlying plate and at the plate interface a concentration of low angle thrust events at the plate interface in the northeastern half of the peninsula suggests that the plate interface is less coupled there than to the southwest this along strike change in plate coupling corresponds closely to a change in the crustal structure of the overlying plate and also to a change in tectonic rotation domain determined paleomagnetically

a new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere a new estimate of global methane emission into the atmosphere from mud volcanoes mvs on land and shallow seafloor is presented the estimate considered a lower limit is based on 1 new direct measurements of flux including both venting of methane and diffuse microseepage around craters and vents and 2 a classification of mv sizes in terms of area km2 based on a compilation of data from 120 mvs the methane flux to the atmosphere is conservatively estimated between 6 and 9 mt y 1 this emission from mvs is 3 6 of the natural methane sources and is comparable with ocean and hydrate sources officially considered in the atmospheric methane budget the total geologic source including mvs seepage from seafloor microseepage in hydrocarbon prone areas and geothermal sources would amount to 35 45 mt y 1 the authors believe it is time to add this parameter in the intergovernmental panel on climate change official tables of atmospheric methane sources

deformation of the japanese islands and seismic coupling an interpretation based on gsi permanent gps observations summary the entire area of the japanese islands has been covered by the permanent gps observation network of the geographical survey institute since 1994 in this paper we use a solution for the vectors of motion during 1995 for a selection of 116 stations to discuss the origin of the observed deformation field we refer the displacement field to eurasia using the vlbi determined motion of kashima and demonstrate that other choices such as the okhotsk or north american plates for north japan are not compatible with the data 1 yr gps velocities are much higher than geological constraints would allow because these short term measurements include transient elastic deformation however the good qualitative agreement between the observed geodetic deformation tensors and those inferred from active faults and earthquakes suggests that the quaternary permanent deformation is essentially the result of the transfer of part of the subduction induced elastic deformation into permanent plastic deformation we then compute the elastic deformation of the japanese islands caused by interseismic loading of the pacific and philippine subduction planes the geometry of the coupled zone and its downward extension are determined from the distribution of earthquakes for the pacific slab for the philippine slab we use the geometry proposed by hyndman et al 1995 these elastic models account for most of the observed velocity field if the subduction movement of the philippine sea plate is 100 per cent locked and if that of the pacific plate is 75 85 per cent locked we note that the boundaries of the areas where significant elastic deformation is predicted more than 10 mm yr 1 of motion with respect to eurasia coincide with the main zones of permanent deformation the eastern japan sea deformation zone for the pacific subduction elastic deformation field and the setouchi mtl deformation zone for the nankai field each zone probably accommodates 10 15 mm yr 1 of motion in the long term convergence in the eastern japan sea strike slip in the setouchi mtl zone to account for this deformation the evect of elastic loading from the trench must be combined with 5 10 mm yr 1 of motion of the amur plate with respect to eurasia because loading during the subduction earthquake cycle causes an increase in stress in the eastern japan sea and setouchi mtl deformation zones the probability of earthquake occurrence in these zones may be higher near the end of the cycle

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

shallow morphology of the subducted pacific plate along the hikurangi margin new zealand abstract along the east coast of the north island new zealand the pacific plate is subducted beneath the australian plate the seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks a synthesis of the microearthquake seismicity the depths of thrust events the depth of s to p conversions and deep reflection seismic studies leads to a coherent picture of the subducting slab both in terms of its mechanical structure and in terms of the general morphology the slab is initially subducted at a low angle of a few degrees is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth the radius of curvature varies along strike from about 280 km just north of wellington to 240 km north of hawke bay the slab shape between depths of 15 45 km is thus approximately conical it also approximates within the errors of hypocentre location a surface of constant gaussian curvature as would be geometrically expected from the simple bending of a thin spherical lithospheric shell observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the pacific plate with a lower level of activity within the subducted mantle normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane

focal mechanisms of large earthquakes in the north island of new zealand slip partitioning at an oblique active margin we have used body wave modelling to determine the source parameters of 22 moderate to large earthquakes that have occurred along the hikurangi subduction margin and elsewhere in the north island of new zealand since 1964 we have also included events from the harvard cmt catalogue since 1977 as that catalogue contains smaller events than can be modelled with body waves we have found that shallow earthquakes occurring in the back arc taupo volcanic zone and its extension to the north show predominantly normal faulting with nodal planes parallel to the regional fabric and t axes indicating extension in a direction in agreement with geodetic measurements a normal faulting solution for the 1974 opunake earthquake is compatible with the mapping of active faults in the offshore taranaki region and the initiation depth of 17 km is close to the expected brittle ductile transition for that area earthquakes within the subducted plate at the southwestern end of the margin are dominated by down dip tension that is rotated towards the deepest part of the subducting plate further north the events in the upper part of the wadati benioff zone 200 km show pure down dip tension while this association is less obvious for the deeper events suggesting that the aseismic part of the subducting slab inferred from plate reconstructions has reached the 670 km discontinuity normal faulting earthquakes associated with plate bending below the region of interplate contact occur along the length of the margin while shallow normal faulting events also occur in the trench indicating that some slab pull propagates to that region we infer that large interplate earthquakes are not imminent because in other similar areas they are preceded by outer rise compressional events that have not been recorded in the region in historical times slip vectors of interplate thrust earthquakes indicate that the oblique plate motion across the margin is fully partitioned that is the plate interface accommodates very little transcurrent motion which is instead accommodated by faults in the overlying australian plate extensional geodetic strain has been measured across the margin in the northeast which has been regarded as unusual in view of the plate convergence we suggest that the topography at this part of the margin is too steep to be supported by the current horizontal forces due to friction on the plate interface this may have occurred due to a reduction in friction because of the subduction of seamounts and the subsequent entrapment of sediments this also being consistent with the idea of underplating at this part of the margin or due to the subduction of less buoyant crust other margins such as japan and mid america where there is known to be subduction of sediments also show extension in the forearc compressional strain observed across the margin in the southwest is more usual for a convergent margin but it does not show the partitioning we see in focal mechanisms these data might only agree after the long term compressional strain accumulation has been averaged over a complete seismic cycle large historical and palaeo earthquakes have occurred along the length of the margin associated with deformation in both the axial and the coastal ranges in addition along the length of the margin the rate of recent interplate thrust activity is low compared to margins that have a high proportion of aseismic slip these factors indicate that the entire length of this margin is capable of producing large earthquakes

predicting the geodetic signature of mw 8 slow slip events 1 elastic dislocation models of geodetic measurements above subduction zones have led to the identification of mw 6 0 7 2 slow slip events sses that release elastic strain over periods of days to months but great mw 8 sses have remained unidentified we extrapolate observations of sse duration and slip magnitude to show that slip velocity decreases with event magnitude and predict that the slip velocity of mw 8 sses is 50 mm yr the slip velocity for great sses may never exceed the plate convergence rate and thus never produce a reversal in trench perpendicular motion instead geodetically constrained estimates of apparent partial elastic coupling on subduction zone interfaces worldwide may be direct observations of ongoing mw 8 silent earthquakes with durations of decades to centuries

contemporary horizontal velocity and strain rate fields of the pacific australian plate boundary zone through new zealand we have inverted velocity solutions from nine geodetic networks distributed across new zealand to derive present day continuous horizontal velocity and strain rate fields at the earth s surface throughout the country the nine networks contain a total of 362 global positioning system gps stations that have been observed at least twice and at least a year apart between 1991 and 1998 the model velocity field is expanded as bicubic spline interpolation functions defined within a curvilinear grid that covers the country and extends into the assumed rigid australian and pacific plates to west and east the inversion jointly minimizes the magnitudes of fitted strain rates and the misfit to the observed velocity data the spline technique allows high spatial resolution of strain rate variations especially in regions with spatially dense gps data expansion of the model velocity field on the surface of a sphere allows arbitrarily large areas to be studied previously known aspects of new zealand plate boundary deformation are highlighted with improved resolution including back arc extension in the taupo volcanic zone rotation of the hikurangi forearc away from this zone in the north island and a band of high shear strain rate under the southern alps to the southeast of the alpine fault our results reveal several new features including a region of enhanced shear straining apparently associated with strike slip faults in the southern north island and a band of contractional straining subparallel and well east of the alpine fault that is similar to features found in numerical and sandbox models of continental collision

terrestrial methane seeps and mud volcanoes a global perspective of gas origin abstract a global database of gas composition and methane stable isotopes of 143 terrestrial mud volcanoes from 12 countries and 60 seeps independent from mud volcanism from eight countries was compiled and examined in order to provide the first worldwide statistics on the origin of methane seeping at the earth s surface sixteen seep data were coupled with their associated subsurface reservoirs the surface seepage data indicate that at least 76 of the mud volcanoes release thermogenic gas with only 4 biogenic and 20 with mixed character the average 201 data of methane concentration and methane carbon isotope ratios 13 c 1 of mud volcanoes are 90 v v and 46 4 respectively the other types of seeps which are independent from mud volcanism have an average 13 c 1 value that is slightly higher 42 9 gases from mud volcanoes are generally lighter more methane less ethane and propane than their associated reservoir gases suggesting a molecular fractionation during advective fluid migration other types of seeps especially dry seeps maintain the reservoir c 1 c 2 c 3 bernard ratio mud volcanoes behave like a natural refinery and the origin of gas more isotopically enriched than 50 and with c 1 c 2 c 3 500 should be attributed to a thermogenic source rather than partial oxidation of biogenic gas some data that appear biogenic in the bernard diagram can be explained by molecular fractionation of mixed gas consequently the bernard parameter may be misleading when applied to mud volcanoes since it does not always reflect the original gas composition the mechanisms of the molecular advective segregation should be studied quantitatively by specific models and experiments

terrestrial and gps measurements of deformation across the taupo back arc and hikurangi forearc regions in new zealand geodetic measurements of deformation across the taupo back arc and hikurangi forearc regions of new zealand are derived from global positioning system gps measurements made in 1990 and 1991 and triangulation observations made in the 1920s 1950s and 1970s the gps horizontal coordinate differences have precisions of 5 6 mm and the triangulation observations have precisions of 0 7 1 2 arc sec these different kinds of observation allow simultaneous estimation of strain parameters and single epoch coordinates for a total of 184 stations at 68 of which gps observations were made under the assumption that no shift of the scale or orientation of the gps reference frame is embodied in the data from 13 gps stations within the taupo volcanic zone tvz common to the 1990 and 1991 surveys the rotational dilatational and shear components of the deformation rate tensor within the tvz can be estimated for that 1 year interval the principal extension rate from 1990 to 1991 is 0 21 0 09 10 6 yr 68 confidence at an azimuth of 124 13 corresponding to 8 4 mm yr extension over 40 km neither the orthogonal principal extension rate 0 02 0 03 10 6 yr nor the rotation rate 0 06 0 04 rad yr is significant the dilatation rate of 0 23 0 09 10 6 yr is therefore produced by uniaxial extension the data from 75 stations distributed from behind the back arc region and well into the forearc region which were surveyed in at least two of the triangulation or gps epochs between the 1920s and 1991 allow the spatial variation of the shear components of the deformation rate tensor to be estimated the maximum engineering shear rates lie in the range 0 1 0 2 10 6 yr within the tvz and maintain these values with similar orientations in the forearc region to the east the western and southern margins of active extension are reasonably well determined by changes in orientation and magnitude of the shear component these results provide confirmation of the previously less well determined deformation field and in comparison with subduction models indicate that the surface deformation is reflecting variation from north to south of coupling of the subduction plate interface and that both trench suction and gravitational collapse probably contribute to the extension in the forearc region

spatial and temporal strain rate variations at the northern hikurangi margin new zealand we have calculated shear strain rates in the northeastern part of the north island of new zealand using geodetic data collected in the mid 1920s and 1995 in addition we use observations from a part of the network across raukumara peninsula measured in 1976 to look for temporal variation in shear strain rates in that area the strain rates were estimated using a simultaneous reduction method assuming uniform strain in space and time within each subnetwork we find the largest spatial variation in shear strain rates from the mid 1920s to 1995 occurs across raukumara peninsula increasing from 0 06 0 04 ppm yr in the west to 0 18 0 03 ppm yr in the east 1 confidence the orientation of the azimuth of relative extension shows significant temporal variation across raukumara peninsula from the mid 1920s to 1976 the maximum shear strain rate was 0 22 0 05 ppm yr with the azimuth of relative extension oriented at 129 7 from 1976 to 1995 the maximum shear strain rate was 0 14 0 05 ppm yr with the azimuth of relative extension oriented at 13 12 we show that a model where the subduction interface is locked down to 15 or 30 km depth and slipping at nuvel ia plate rates below that depth can explain the strain rates and orientation observed during 1976 1995 to fit the observations from the mid 1920s to 1976 we suggest that additional dip slip occurred on the shallow part of the subduction zone above 30 km from this we conclude that the plate boundary below raukumara peninsula is locked down to 15 or 30 km depth and the aseismic shallow slip episode that occurred during the mid 1920s 1976 was an isolated event possibly induced by m 7 historical earthquakes in the area

rapid postseismic transients in subduction zones from continuous gps 1 continuous gps time series from three of four recently measured large subduction earthquakes document triggered rapid postseismic fault creep representing an additional moment release upward of 25 over the weeks following their main shocks data from two mw 8 0 and mw 8 4 events constrain the postseismic centroids to lie down dip from the lower limit of coseismic faulting and show that afterslip along the primary coseismic asperities is significantly less important than triggered deep creep time series for another mw 7 7 event show 30 postseismic energy release but here we cannot differentiate between afterslip and triggered deeper creep a fourth mw 8 1 event which occurred in the broad chilean seismogenic zone shows no postseismic deformation despite coseismic offsets in excess of 1 m for the three events which are followed by postseismic deformation stress transferred to the inferred centroids at 34 60 and 36 km depths by their respective main shock asperities increased reverse shear stress by 0 5 0 8 and 0 2 bar with a comparatively small decrease in normal stress 0 01 bar constraining the coulomb stress increase required to force slip along the metastable plate interface deep triggered slip of this nature is invisible without continuous geodesy but on the basis of these earthquakes would appear to constitute an important mode of strain release from beneath the seismogenic zones of convergent margins these events captured by some of the first permanent gps networks show that deep moment release is often modulated by seismogenic rupture updip and underscore the need for continuous geodesy to fully quantify the spectrum of moment release in great earthquakes index terms 7209 seismology earthquake dynamics and mechanics 3040 marine geology and geophysics plate tectonics 8150 8155 8157 8158 8150 tectonophysics plate boundary general 3040 7230 seismology seismicity and seismotectonics keywords subduction zone earthquake postseismic moment release

slow slip on the northern hikurangi subduction interface new zealand 1 in october 2002 a surface displacement episode of 20 30 mm magnitude was observed over a 10 day period on two continuous global positioning system gps instruments near gisborne north island new zealand we interpret this to result from slow slip on the northern hikurangi subduction interface using ten years of regional campaign gps 1995 2004 and recent continuous gps data we estimate the recurrence interval for similar events to be 2 3 yrs in november 2004 a similar slow slip event occurred within this recurrence period the 2002 event can be modeled by 18 cm of slow slip near the down dip end of the seismogenic zone on the subduction interface offshore of gisborne the campaign gps data show that the 2002 slow slip event had little effect on regional strain patterns

active mud volcanoes on and offshore eastern makran pakistan to study the activity source and driving force of the venting of fluidized mud in the coastal makran we carried out reconnaissance surveys of two active onshore mud volcano fields chandragup and jebel u ghurab and of a newly born march 1999 offshore mud volcano malan island all studied on and offshore mud volcanoes line up along one southwest northeast trending structural lineament the axis of the dhak anticline isotopic data point to a bacterial origin of the gas mainly methane mixed benthic foraminiferal faunas and calcareous nannofloras suggest that the source level of the extruded mud is at a sub surface depth of 2 3 km observed mud discharge varied between 0 and 1 4 m3 h 1 and gas discharge rates between negligible amounts to 1 m3 s 1 mud temperatures in the crater lake of chandragup i are near ambient temperatures they rise slightly 1 5 c during episodes of modest mud outflow an area of 160 000 m3 of soft mud was vigorously extruded from the sea floor at a water depth of 10 m within days after 15 march 1999 forming malan island the island was destroyed within a few months after its birth by deep reaching wave action during the sw monsoon this was possibly aided by local subsidence of the mud volcano structure due to the volume loss following rapid degassing and mud extrusion

gravitational viscoelastic postseismic relaxation on a layered spherical earth viscoelastic relaxation of a ductile asthenosphere underlying a purely elastic plate is a strong candidate process for explaining anomalous rates of crustal deformation observed following large earthquakes the nongravitational treatment of pollitz 1992 which is valid on a global scale and includes the effects of compressibility is here extended to permit the calculation of gravitational viscoelastic relaxation for a specified spherically layered viscoelastic rheology following an earthquake in an elastic layer the simple approximations we adopt make the resulting treatment particularly suitable for near field calculations for an asthenosphere with a maxwell rheology the effect of gravitational coupling is manifested only many relaxation times after the earthquake as obtained by previous investigators its effect is generally to speed up the long wavelength component of the relaxation process and attenuate the overall vertical displacement pattern several subtle features common to the relaxation behavior from several different fault types thrust rift and strike slip are identified the effect of gravitational coupling on horizontal displacements is consistent with flexure of the upper elastic plate driven by the corresponding effects on the vertical displacement stress diffusion away from the source region generally exhibits pulse like behavior which is dispersive in both space and time if the asthenosphere is confined to a relatively narrow channel then the dispersion branches governing relaxation are radically altered and stress diffusion effects far from the coseismic rupture zone exhibit a complicated time dependence reflecting the competing tendencies of toroidal and spheroidal mode relaxation

reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents we use seismic reflection and refraction data to determine crustal structure to map a fore arc basin containing 12 km of sediment and to image the subduction thrust at 35 km depth seismic reflection megasequences within the basin are correlated with onshore geology megasequence x late cretaceous and paleogene marine passive margin sediments megasequence y a similar to 10 000 km 3 submarine landslide emplaced during subduction initiation at 22 ma and megasequence z a neogene subduction margin megasequence the moho lies at 17 km beneath the basin center and at 35 km at the southern margin beneath the western basin margin we interpret reflective units as deformed gondwana fore arc sediment that was thrust in cretaceous time over oceanic crust 7 km thick raukumara basin has normal faults at its western margin and is uplifted along its eastern and southern margins raukumara basin represents a rigid fore arc block 150 km long which contrasts with widespread faulting and large neogene vertical axis rotations farther south taper of the western edge of allochthonous unit y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward as proposed for the correlative onshore allochthon spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore arc crustal thickness the trench slope has many small extensional faults and lacks coherent internal reflections suggesting collapse of indurated rock rather than accretion of 1 km of sediment from the downgoing plate the lack of volcanic intrusion east of the active arc and stratigraphic evidence for the broadening of east cape ridge with time suggests net fore arc accretion since 22 ma we propose a cyclical fore arc kinematic rock moves down a subduction channel to near the base of the crust where underplating drives rock uplift oversteepens the trench slope and causes collapse toward the trench and subduction channel cyclical rock particle paths led to persistent trench slope subsidence during net accretion existing global estimates of fore arc loss are systematically too high because they assume vertical particle paths citation sutherland r et al 2009 reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents tectonics 28 tc5017 doi 10 1029 2008tc002356

holocene paleoseismicity in the fold and thrust belt of the hikurangi subduction zone eastern north island new zealand abstract along the 500 km long eastern coastline of the north island new zealand there are at most seven distinct holocene marine terraces aged 7 ka b p or less the highest of the terraces is 27 m above present day mean sea level the coastal region is subdivided into fourteen distinct subregions based on radiocarbon ages of marine deposits overlying wave cut shore platforms and geographic distribution of similarly aged terraces holocene marine terraces are the result of uplift associated with large earthquakes co seismic deformation this conclusion is based on characteristic stepped terrace morphology clustering of ages of terrace deposits within subrogions and the occurrence of co seismic uplift in historic time differential uplift across structures and distinct age variations at subregion boundaries are also characteristic recurrence intervals of uplift in any one terrace sequence vary from 0 4 ka to 2 0 ka and individual amounts of uplift vary from 1 0 m to 4 0 m in the past 2 5 ka ages from all terraces within subrogions indicate at least 21 paleoseismic events affecting coastal areas of eastern north island in that period these events cluster in time and in separate parts of eastern north island several earthquakes occurred 0 3 0 6 1 0 1 5 2 1 and 2 3 ka b p our data strongly support the concept of segmentation of deformation along the subduction margin the likely cause of coastal uplift is movement on steep reverse faults local structures rather than the subduction thrust that propagate from or near the subduction thrust some 20 25 km below the region earthquakes of moment magnitude 7 3 8 0 are estimated to be associated with these fault movements

evolution of the taupo hikurangi subduction system abstract the present day taupo hikurangi subduction system is a southward extension of the tonga kermadec arc system into a sediment rich continental margin environment it consists of a shallow structural trench the hikurangi trough a 150 km wide imbricate thrust controlled accretionary borderland the continental slope shelf and coastal hills of eastern north island a frontal ridge the main greywacke ranges of north island and a volcanic arc and marginal basin the taupo volcanic zone structural elements become progressively more elevated and subduction more oblique towards the south the whole nne trending system is truncated at a largely strike slip transform boundary that extends along the southwestern part of the hikurangi trough and the hope fault of south island to the main alpine fault the volcanic arc is 200 270 km from the structural trench and comprises a nne trending chain of andesite dacite volcanoes extending along the eastern side of the taupo volcanic zone most of the andesites are olivine bearing and have been erupted within the last 50 000 years it is suggested the taupo hikurangi margin has evolved by rotation of accretionary elements from an original nw trending subduction system north of new zealand the older elements of the prism were associated with subduction of a re entrant of the pacific plate and perhaps the south fiji basin in mid tertiary times they subsequently became separated from their nw trending volcanic arc by dextral strike slip movement along curved faults east of the main greywacke ranges during the plio pleistocene oblique subduction and accretion intensified as the taupo hikurangi margin rotated into line with the nne trending kermadec system and a marginal basin was developed along a similar trend to form the taupo volcanic zone within the last 50 000 years olivine bearing andesite volcanism has commenced along the eastern side of the taupo volcanic zone

gravity and magnetic models of the midcontinent rift in eastern lake superior gravity and magnetic models of the midcontinent rift mcr in eastern lake superior supplement recent structural and stratigraphic interpretations based on the seismic reflection method an algorithm developed to accommodate spatially varying direction and magnitude of magnetization within a magnetic source is used in both forward and inverse modeling procedures structural attitudes of rift filling basalts derived from seismic reflection sections are used to rotate the keweenawan remanent magnetization vectors in the direction of deformation an iterative linear inversion routine calculates magnitudes of induced and remanent magnetizations as well as normal and reversed polarity basalt flow distributions the results indicate that the koenigsberger ratios of these basalts generally range from 1 to 3 which is in agreement with values obtained from rock property measurements the models also suggest that the greater volume of the keweenawan basalt section in eastern lake superior is reversely polarized a

interseismic locking on the hikurangi subduction zone uncertainties from slow slip events interseismic locking on the hikurangi subduction zone in new zealand is examined in light of alternative assumed locking distributions and the impact of transients slow slip and volcanic sources on temporal and spatial resolution the modern pattern of locking in the north is poorly resolved and based on simulations of possible transient behavior may be an ephemeral feature of the subduction cycle while there appears to be some contemporary locking in the northern half of the hikurangi subduction zone hsz its location is model dependent and hence its relationship to structure slow slip or any transition zone there is unclear simulations of site velocities using the 14 year history of transient events reveal that in the timescale of the interseismic period the northern half of the hsz could be either locked or unlocked and this may not be resolvable for decades in the southern half there is strong contemporary locking in the 15 to 40 km depth range but again the slow slip history leads to uncertainty in the long term pattern slow slip events not only reduce the long term locking by aseismic slip but also greatly hinder our ability to see it it is within the range of possible models that the slip deficit rate at the hsz is more uniform along strike and the modern appearance is controlled by the particular pattern of transients over the past 10 to 20 years when the gps data were collected similarly uncertainties in surface velocities will be large at any subduction zone with large transients

shocks in a box an analogue model of subduction earthquake cycles with application to seismotectonic forearc evolution 1 we introduce and test an experimental approach to simulate elastoplastic megathrust earthquake cycles using an analogue model and apply it to study the seismotectonic evolution of subduction zones the quasi two dimensional analogue model features rate and state dependent elastic frictional plastic and viscoelastic material properties and is scaled for gravity inertia elasticity friction and viscosity the experiments are monitored with a high resolution strain analysis tool based on digital image correlation particle imaging velocimetry piv providing deformation time series comparable to seismologic geodetic and geologic observations in order to separate elastic and nonelastic effects inherent the experimental deformation patterns we integrate elastic dislocation modeling edm into a hybrid approach we use the analogue earthquake slip and interseismic locking distribution as edm dislocation input and forward model the coseismic and interseismic elastic response the residual which remains when the edm prediction is subtracted from the experimental deformation pattern highlights the accumulation of permanent deformation in the model the setup generates analogue earthquake sequences with realistic source mechanisms and elastic forearc response and recurrence patterns and reproduces principal earthquake scaling relations by applying the model to an accretionary type plate margin we demonstrate how strain localization at the rupture peripheries may lead to a seismotectonically segmented forearc including a tectonically stable shelf and coastal high 20 plate convergence accommodated by internal shortening overlying the area of large megathrust earthquake slip fifty to 75 of plate convergence is accommodated by internal shortening in the slope region where earthquake slip tapers out toward the trench the inner forearc region remains undeformed and represents a basin

source parameters of large historical 1918 1962 earthquakes south island new zealand we have studied 18 earthquakes of m w 5 5 within the north island new zealand region to determine fault parameters for earthquake hazards and regional tectonic studies since most large m w 6 5 earthquakes in the north island occurred prior to 1961 these events provide important information to supplement studies of more recent seismicity our results indicate that no large plate interface earthquakes have occurred in the central and southern north island for the past 80 years although six strike slip earthquakes of m w 6 8 occurred within the australian upper plate during an extremely active 25 year period that began in 1917 only the m w 7 2 strike slip events were associated with surface faulting indicating the difficulties that may arise in attempting to identify active faults within this region two earthquakes m w 6 9 7 1 off the northeastern north island in 1947 appear to have occurred along the plate interface and were associated with local tsunamis having runup heights of up to 10 m two m w 6 8 events also occurred within the pacific lower plate highlighting the hazards related to intraslab events slip vectors for the earthquakes studied suggest that the majority of transcurrent motion along the plate margin is accommodated within the australian plate similar to the results obtained from studies of more recent smaller earthquakes pure thrusting occurs along the plate interface and t axes of intraslab events indicate downdip tension in the pacific plate

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

subduction and back arc activity at the hikurangi convergent margin new zealand the hikurangi margin is a region of oblique subduction with northwest dipping intermediate depth seismicity extending southwest from the kermadec system to about 42 s the current episode of subduction is at least 16 20 ma old the plate convergence rate varies along the margin from about 60 mm a at the south end of the kermadec trench to about 45 mm a at 42 s the age of the pacific lithosphere adjacent to the hikurangi trench is not known

origin of nitrogen rich natural gases in the california great valley evidence from helium carbon and nitrogen isotope ratios abstract dry natural gases produced from the sacramento and northern san joaquin basins contain up to 87 nitrogen 15n measurements for gases with n2 8 vary from 0 9 to 3 5 methane 13c values vary from 61 to 15 3 he 4 he ratios indicate the presence of mantle derived helium in addition to radiogenic crustal helium and suggest that the gases may originate from mixing between three principal components a hydrocarbon rich gas with 3 he 4 he 0 1r a derived from the microbial and thermogenic alteration of sedimentary organic matter a hydrocarbonrich gas with 3 he 4 he 2 75r a derived from the localized pyrolysis of sediments by magmatic volatiles and a nitrogenrich gas with 3 he 4 he 1r a the nitrogen rich component is characterized by methane 13c less negative than 25 and n 2 ar ratios as high as 22 000 nitrogen concentrations tend to increase towards the basement along the shallow eastern margin of the valley suggesting that the nitrogen rich component is derived from metasedimentary rocks deep within the crust the coast ranges fraciscan complex which was thrust well beneath the great valley during late mesozoic and early cenozoic subduction appears to be the most likely source

a back arc basin formed within continental lithosphere the central volcanic region of new zealand abstract the central volcanic region is an area of quaternary volcanism and tensional tectonics within the north island of new zealand that appears to be a direct continuation of a young oceanic back arc basin the havre trough into the continental structure of new zealand volcanism is predominantly rhyolitic yet the region possesses many of the geophysical characteristics often associated with oceanic back arc basins upper mantle seismic velocities are low pn and sn velocities of 7 4 and 3 95 km s respectively the crust is anomalously thin for its continental setting and the heat flow almost all of which is expressed in the discharge of hot water through geothermal systems is about 700 mw m2 i e some twelve times greater than normal continental heat flow in order to account for such a high heat flow a large scale mass transfer of heat is required it is proposed therefore that the central volcanic region represents a site of active back arc spreading within a continental lithosphere

genetic characterization of natural gases natural gases can be characterized genetically using four properties c2 concentration carbon and hydrogen isotope variations in methane and carbon isotope variation in ethane three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters in these diagrams compositional fields have been defined for primary gases biogenic thermogenic associated and thermogenic nonassociated and for gases which result from mixing of these gases these fields are strictly empirical and comprise compositional variations found in about 500 natural gases the isotopic and compositional variations in natural gases can be described in terms of 1 processes during formation of the gases such as bacterial fermentation or maturation of organic matter and 2 processes during secondary migration mixing of primary gases is an important and common process migration of gases predominantly affects the c2 concentration whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged allowing an assessment of the origin of migrated gases and properties of their source rocks the formation of gas from humic organic matter and coals is not yet clear from published data the diagrams use data from various basins and areas interstitial gases from the gulf of california are entirely of bacterial origin traces of thermogenic gases are formed only in the vicinity of dolerite sills gases in the south german molasse basin and in the vienna basin are of bacterial mixed and thermogenic origins data from the north italian po basin provide examples for genetic characterization of migrated gases

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

absence of strain accumulation in the western shumagin segment of the alaska subduction zone gps measurements in the western shumagin segment of the alaska subduction zone show no significant strain the elastic dislocation model that best fits the data has the entire plate interface slipping freely the simplest explanation is that steady slip at the plate convergence rate is occurring on the entire plate interface although we cannot rule out the existence of a shallow locked patch close to the trench large enough to generate a m 7 earthquake the 99 confidence upper bound on the down dip width of the locked patch is 55 km assuming the trench is the the updip limit of the locked zone unless there is significant temporal variability in the locking of the plate interface or viscoelastic effects from a past earthquake the shumagin segment cannot by itself release enough moment to generate a great m 8 earthquake

consequences of stress changes following the 1891 nobi earthquake japan abstract great earthquakes along the philippine sea interplate boundary central honshu usually rupture a large portion of the nankai trough and suruga trough however the 1944 tonankai and 1946 nankaido earthquakes the most recent great philippine sea events did not rupture the suruga trough mogi proposed that strain release from the 1891 nobi earthquake is responsible for the apparently premature northeast termination of the 1944 rupture zone using a recently derived fault model of the 1891 earthquake combined with a suitable rheological model of central honshu we quantify both the inland and offshore consequences of the nobi earthquake north of the median tectonic line the co seismic plus postseismic stress changes of the nobi earthquake promoted the failure of faults which subsequently ruptured in the decades following 1891 in contrast along the philippine sea interplate boundary the co seismic plus postseismic strain release of the nobi earthquake during the decades following 1891 has delayed the anticipated tokai earthquake by at least 20 yr

slow slip and frictional transition at low temperature at the hikurangi subduction zone temperature changes with depth do not appear to be a primary control for either slow slip or fault locking processes at the hikurangi margin north island new zealand both slow slip events and the geodetically observed transition from fault locking to free slip at depth occur at temperatures as low as 100 c shallow portions of faults exhibit brittle stick slip behaviour that gives way to more stable sliding with increasing depth limiting the depths to which earthquake inducing slip can occur the increase of temperature with depth is often assumed to transform friction in the fault from velocity weakening unstable to velocity strengthening stable and to change conditions from brittle to ductile1 2 3 4 5 a temperature of 350 c has been suggested to mark the base of the frictionally unstable portion of the fault which becomes free slipping at depth where temperatures rise above 450 c refs 2 4 6 here we show that both slow slip events and the geodetically observed transition from fault locking to free slip at the hikurangi subduction zone east of the north island new zealand occur at temperatures as low as 100 c suggesting that temperature is not a primary control on either slow slip or fault locking processes at the hikurangi margin although globally shallow slow slip events are rare five out of seven events observed so far at the hikurangi margin are less than 15 km deep

significance of hydrocarbon seepage relative to petroleum generation and entrapment near surface indications of migrating hydrocarbons provide the petroleum systems analyst critical information about source organic matter type maturation organic maturity migration migration pathway delineation and in selected geologic settings specific prospect hydrocarbon charge all petroliferous basins exhibit some type of near surface signal but the hydrocarbon leakage to surface is not always detectable with conventional seep detection methods understanding the petroleum seepage system hence petroleum dynamics of a basin is key to understanding and using near surface geochemical methods for basin assessment and prospect evaluation the relationships between near surface hydrocarbon seepage and subsurface petroleum generation and entrapment are often complex the petroleum seepage system contain four key elements seepage activity qualitative expressions of relative leakage rates active vs passive and episodic vs continuous seepage type concentration of migrated thermogenic hydrocarbon relative to in situ material macro vs micro migration focus major direction of bulk flow leakage relative to the subsurface hydrocarbon generation and or entrapment and near surface seep disturbances near surface processes which can greatly alter or block seepage signals the rate and volume of hydrocarbon seepage to the surface greatly control near surface geological and biological responses and thus are the best method of sampling and analysis to detect hydrocarbon leakage effectively to properly predict subsurface petroleum properties interpretation of near surface geochemical data must recognize many potential problems including recent organic matter input transported hydrocarbons bacterial alteration mixing contamination and fractionation effects surface geochemical data should always be integrated with other geological data calibration datasets to determine the utility of near surface geochemical techniques within particular basinal settings are essential when evaluating prospects for hydrocarbon charge