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

the hawke s bay new zealand earthquake of 1982 september 02 and deformation in the interior of the subducted pacific plate abstract the most damaging earthquake to occur in new zealand in 1982 was the hawke s bay earthquake of september 02 m b 5 6 an aftershock study using portable microearthquake recorders has established that this event occurred some 20 km south of hastings at a depth of about 47 km this depth places the event in the mantle of the subducted pacific plate this earthquake is a member of a recently recognized class of intraplate shocks occurring along the strike of the subducted plate near the east coast of the north island such events are characterized by normal faulting mechanisms when taken together with activity in the crust of the subducted plate this subcrustal activity resembles a double planed seismic zone restricted in the down dip direction of the subducted plate several lines of evidence suggest that deformation in the mantle of the subducted plate is controlled by the state of coupling of the plate interface at shallow depth 1 1 the subcrustal activity occurs directly beneath the source region of major interplate earthquakes along the east coast of the north island and there appears to be a pairing of interplate and subcrustal events 2 2 stress directions determined for the subcrustal events are concordant with geodetic strain directions and 3 3 the amount of subcrustal activity appears to be related to the state of coupling of the plate interface

seamount resolution in satellite derived bathymetry 1 i compare satellite derived global relief data on 13 seamounts in the northeast pacific ocean to echosounder derived multibeam values from the gulf of alaska seamount expeditions and the west coast seamounts and ridges multibeam surveys for peak height latitude longitude and midsection area i find smith and sandwell s 1997 global sea floor topography relatively accurate for peak geoposition but generally deeper than multibeam by 192 m 132 m cell size alone can explain 50 90 of peak height variation suggesting higher resolution geodetic altimetry could improve the accuracy of these estimates nevertheless midsection areas overlap by 80 11 the altimetric technique clearly resolves the presence and general form of numerous unnamed deep seamounts though predicted seamount peak height estimates for small features may vary from their true depth by up to 50 these findings support high end estimates of global seamount abundance since small seamount features 1200 m relief revealed by satellite may in actuality be quite substantial features 2500 m relief

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

coseismic uplift of holocene marine terraces in the pakarae river area eastern north island new zealand abstract holocene marine terraces along 15 km of the northeastern coast of north island record episodic tectonic uplift a maximum of seven terraces are arranged in staircase fashion and lie about 20 km above the subduction interface between the pacific and australian plates the highest t1 corresponds with the maximum of the holocene marine transgression about 6700 14 c yr b p younger terraces are marine abrasion platforms overlain by thin beach deposits radiocarbon ages of marine shells from beach deposits indicate that uplift above marine conditions occurred ca 6700 t1 5500 t2 3900 t3 2500 4 1600 t5 1000 t6 and slightly younger than 600 t7 yr b p uplift probably occurred coseismically the maximum late holocene uplift rate in the study area is 8 mm yr altitudinal distribution of terraces suggests deformation exists as a ca 20 km elongate dome broken at the southern end by the pakarae fault which trends across the dome rupture on this fault has accompanied the growth of the dome but is not responsible for it bathymetric profiling suggests that an active fault parallel to and about 5 km offshore is probably responsible for the episodic coastal uplift

large intermediate depth earthquakes and the subduction process this study provides an overview of intermediate depth earthquake phenomena placing emphasis on the larger tectonically significant events and exploring the relation of intermediate depth earthquakes to shallower seismicity especially we examine whether intermediate depth events reflect the state of interplate coupling at subduction zones and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes historic record of large intraplate earthquakes m b 7 0 in this century shows that the new hebrides and tonga subduction zones have the largest number of large intraplate events regions associated with bends in the subducted lithosphere also have many large events e g altiplano and new ireland we compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with m 6 and depth between 40 and 200 km the final catalog includes 335 events with 47 new focal mechanisms and is probably complete for earthquakes with m b 6 5 for events with m 6 5 nearly 48 of the events had no aftershocks and only 15 of the events had more than five aftershocks within one week of the mainshock events with more than ten aftershocks are located in regions associated with bends in the subducted slab focal mechanism solutions for intermediate depth earthquakes with m 6 8 can be grouped into four categories 1 normal fault events 44 and 2 reverse fault events 33 both with a strike nearly parallel to the trench axis 3 normal or reverse fault events with a strike significantly oblique to the trench axis 10 and 4 tear faulting events 13 the focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down dip extensional stress prevails in a gently dipping plate in contrast along decoupled subduction zones great normal fault earthquakes occur at shallow depths e g the 1977 sumbawa earthquake in indonesia type 2 events with strike subparallel to the subduction zone and most of them with a near vertical tension axis occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and consequently the change in focal mechanism from type 1 to type 2 events events of type 3 occur where the trench axis bends sharply causing horizontal parallel to the trench strike extensional or compressional intraplate stress type 4 are hinge faulting events for strongly coupled zones we observed temporal changes of intermediate depth earthquake activity associated with the occurrence of a large underthrusting event after the occurrence of a large underthrusting event the stress axis orientation of intermediate depth earthquakes changes from down dip tensional to down dip compressional e g 1960 chile 1974 peru 1982 tonga and 1952 kamchatka earthquakes or the number of large intermediate events decreases for a few years e g 1964 alaska and 1985 valparaiso earthquakes we conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate depth seismicity spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate depth events

geodetically determined strain across the southern end of the tonga kermadec hikurangi subduction zone summary a method of simultaneous reduction is presented for determining strain rates from multiple triangulation surveys where common triangulation stations have been used but the angles of the old survey have not necessarily been reobserved this method is applied to triangulation in the northern south island at the southern end of the tonga kermadec hikurangi subduction zone from a profile of shear strain across the indian pacific plate boundary the displacement of the indian plate relative to the pacific is calculated to be 54 9 mm yr at an azimuth of 84 f lo in remarkable agreement with the motion predicted by global plate tectonic models most of this motion occurs within a 15okm wide zone bounded on the east by the hikurangi trough within this zone the motion is partitioned near the hikurangi trough no slip is occurring at the upper surface of the subducting pacific plate the subduction thrust and motion is predominantly thrusting normal to the trough axis to the west is a region of predominantly dextral strike slip faulting this pattern is consistent with fitch s model of oblique subduction to the south of the profile a change is observed in the azimuth of the faulting along a line which marks the southern extent of the subduction slab indicating the end of the partitioned motion

normal faulting through subducted oceanic crust the 19 july 1985 earthquake of hawke s bay new zealand abstract source parameters of the m l 5 7 19 july 1985 earthquake of northern hawke s bay new zealand are determined by modelling the teleseismic p and s body waves a focal depth of 31 1 km scalar moment 5 6 10 17 nm and source duration of 3 0 3 5 s are estimated the focal mechanism strike 224 dip 78 rake 273 shows normal faulting on a nw dipping plane that is parallel to the strike of the subducted pacific plate thirty five aftershocks were recorded by a temporary seismograph network 21 of which were relocated using the jhd method the hypocentres define a northwest dipping plane 20 30 km which extends from the hypocentre of the mainshock to a depth of 47 km the position of the subducted plate in hawke s bay is well defined from previous micro earthquake studies in the region of the mainshock the plate interface is located at a depth of approximately 28 to 30 km placing the hypocentre of the mainshock in the crust of the pacific plate the extent and location of the aftershock distribution suggest that the mainshock ruptured downward and extended through the crust of the plate this event thus differs from other normal faulting events in the region that usually occur in the mantle of the subducted plate the scalar moment at long periods t 45s dziewonski et al 1986 exceeds that determined here by a factor of 2 suggesting a source rich in long period energy an idea supported by the relatively large aftershock area

slip partitioning along major convergent plate boundaries along plate boundaries characterized by oblique convergence earthquake slip vectors are commonly rotated toward the normal of the trench with respect to predicted plate motion vectors consequently relative plate motion along such convergent margins must be partitioned between displacements along the thrust plate interface and deformation within the forearc and back arc regions the deformation behind the trench may take the form of strike slip motion back arc extension or some combination of both we observe from our analysis of the harvard moment tensor catalog that convergent arcs characterized by back arc spreading specifically the marianas and new hebrides are characterized by a large degree of slip partitioning however the observed rates directions and location of back arc spreading are not sufficient to account for degree of partitioning observed along the respective arcs implying that the oblique component of subduction is also accommodated in part by shearing of the overriding plate in the case of the sumatran arc where partitioning is accommodated by strike slip faulting in the overriding plate the degree of partitioning is similar to that observed along the marianas but the result is viewed with caution because it is based on a predicted plate motion vector that is based on locally derived earthquake slip vectors in the case of the alaskan aleutian arc where back arc spreading is also absent the degree of partitioning is less and rotation of slip vectors toward the trench normal appears to increase linearly as a function of the obliquity of convergence if partitioning in the alaskan aleutian arc is accommodated by strike slip faulting within the upper plate the positive relationship between obliquity of convergence and the rotation of earthquake slip vectors to the trench normal may reflect that either 1 the ratio of the depth extent of strike slip faults behind the trench z s to the subduction thrust z t increases westward along the arc 2 the dip of the subduction thrust increases westward along the arc or 3 the strength of the subduction thrust decreases westward along the arc

slab pull and the seismotectonics of subducting lithosphere this synthesis links many seismic and tectonic processes at subduction zones including great subduction earthquakes to the sinking of subducted plate earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity providing details on how subduction occurs compression is localized across a locked interface thrust zone because both the ridge push and the slab pull forces are resisted there the slab pull force increases with increasing plate age thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate the 1960 chile earthquake mw 9 5 the largest earthquake to occur in historic times began its rupture at an interface bounding oceanic plate 30 m y old however this rupture initiation was associated with the locally oldest subducting lithosphere weakest coupling the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere terminating near the subducting chile rise prior to a great subduction earthquake the sinking subducted slab will cause increased tension at depths of 50 200 km with greatest tension near the shallow zone resisting plate subduction plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260 350 km provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle this explains k mogi s observations of m 7 thrust earthquakes at depths of 260 350 km immediately downdip and within 3 years prior to five great shallow earthquakes of northern japan the slab pull model explains the lower layer of double seismic zones as due to tension from the deeper sinking plate and the upper layer as due to localized in plate compression as plate motion is resisted by the bounding mantle just downdip of the interface thrust zone there occurs an aseismic 20 50 dip increase of subducted plate this slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone

global variability in subduction thrust zone forearc systems deviations of slip vector azimuths of interplate thrust earthquakes from expected plate convergence directions at oblique subduction zones provide kinematic information about the deformation of forearcs and indirect evidence on the dynamics of the plate boundary a global survey of slip vectors at major trenches of the world reveals a large variability in the kinematic response of forearcs to shear produced by oblique convergence the variability in forearc deformation inferred from slip vector deflections is suggested to be caused by variations in forearc rheology rather than in the stresses acting on subduction zone thrust faults estimated apparent macroscopic rheologies range from elastic to perfectly plastic or viscous forearc rheologies inferred from slip vectors do not correlate with age of the subducting lithosphere but continental forearcs or old arcs appear to deform less than oceanic or young arcs the inferred absence of forearc deformation at continental arcs from this study is counter to inferences drawn from compiled geologic information on forearc faults correlations of the apparent forearc rheology with backarc spreading convergence rate slab dip arc curvature and downdip length of the thrust contact are poor however great subduction zone earthquakes occur where forearcs are apparently more elastic i e less deformed by oblique convergence which suggests that the mechanical properties of forearcs rather than stress magnitude on thrust faults control both the kinematic behavior of forearcs and where great subduction zone earthquakes occur

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

systematics of deep focus earthquakes along the kuril kamchatka arc and their implications on mantle dynamics subduction along the kuril kamchatka arc has been proposed as a primary example in which subducted lithosphere plunges steeply into the lower mantle with little deformation the occurrence of several unusual earthquakes along this arc during the last few years places new constraints on the configuration and the state of strain of the down going slab we systematically determined the source parameters of 27 large to moderate sized earthquakes mb 5 5 that occurred between 1963 and 1990 at depths greater than 200 km along this subduction zone to our surprise our results show that the configuration and strain due to earthquakes in subducted lithosphere are highly variable along both the strike and dip of the arc approximately 75 km farther west of the deepest earthquakes in the northern portion of the arc the earthquake of august 14 1988 occurred at a comparable depth of 636 17 km thus at least part of the slab has been displaced in a direction perpendicular to the overall strike of the wadati benioff zone earthquakes in the central part of the arc delineate shoaling of the subducted slab at a depth of approximately 570 km reminiscent of the configuration of subducted lithosphere along the tonga arc both observations indicate that the subducted slab is deforming probably in response to resistance encountered near the 670 km discontinuity for all earthquakes north of 48 n focal mechanisms of all events deeper than 200 km show a remarkably consistent pattern of downdip compression strain in the southern kuril the sakhalin island earthquake m0 8 1019n m of may 12 1990 is one of three large deep focus earthquakes that have occurred over 150 km farther west of the well defined wadati benioff zone the relationship between the source region of these events and the rest of the subducted slab is equivocal due to the lack of background seismicity nonetheless for earthquakes south of 48 n net downdip extension dominates between depths of 200 450 km before switching to downdip compression below 540 km thus the strain field within the slab is segmented both along strike and dip if the slab in southern kuril kamchatka can serve as a stress guide down to depths of 615 km the changeover from extension to compression occurs at an unusually large depth within the transition zone of the upper mantle alternatively the deep focus events west of the island of sakhalin could have occurred in a detached piece of lithosphere as a whole the configuration and state of strain of the subducted slab along the kuril kamchatka arc are highly variable close to a microcosm of variations among different deep subduction systems observed on a global scale in particular the subducted lithosphere is apparently undergoing considerable deformation as it approaches the base of the upper mantle our results do not preclude the possibility of subducted lithosphere ever reaching the lower mantle nevertheless it seems unlikely that the down going slab along the kuril kamchatka arc penetrates into the lower mantle unhindered

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

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

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

eruptive hummocks building blocks of the upper ocean crust the spreading axis at many slow spreading mid ocean ridges is marked by an axial volcanic ridge in this study we use a combination of high resolution remote sensing methods to elucidate the detailed nature of volcanoes in such a ridge we find that the hummocks described in previous sidescan sonar studies are dome or cone shaped edifices 5 150 m high with diameters of 30 330 m we estimate they form quickly in single eruptions each of which may produce several hummocks hummock collapse is common and hummocks of all heights are prone to failure collapses generally occur down the regional seafloor slope suggesting control by local topography approximately 33 of hummocks lose 40 of their volume by collapse so 12 of all material erupted on the axial volcanic ridge is rapidly converted to talus the higher porosity of these deposits may increase average upper crustal porosity by several percent contributing 0 5 km s 1 to seismic velocity decrease in the upper oceanic crust and may be one of the dominant mechanisms for increasing porosity in upper slow spreading oceanic crust

strain partitioning between structural domains in the forearc of the hikurangi subduction zone new zealand the pacific plate obliquely converges with the australian plate at latitude 39 50 s along the hikurangi margin off the east coast of the north island of new zealand an extensive and youthful subaerially exposed forearc on the east coast of the north island in the hawke s bay area provides the opportunity to document contemporaneous forearc deformation in this obliquely convergent margin setting geologic mapping and analysis of strain at both mesoscale and megascale indicates that strain is partitioning into domains of extension contraction and strike slip the domains are elongate and trend parallel to the margin measurements of net shortening and transcurrent slip in the forearc show that the obliquely convergent motion is transferred across the plate interface deformation rates calculated for the past 1 2 m y for structures in all six forearc domains account for 50 70 of the margin parallel motion required by pacific australian plate convergence and about 6 of the plate motion perpendicular to the plate boundary at the surface in the forearc this obliquely convergent motion is manifest not by transpressional faults but rather by paired structural domains that consist of a strike slip fault zone and an accompanying contractional fault and fold zone on the trenchward side there are two such transcurrent faulting and contraction couplets one where the backstop daylights at the arcward edge of the forearc and another couplet trenchward of a relatively undisturbed forearc basin the small amount of shortening relative to strike slip in the onshore part of the forearc suggests that shortening perpendicular to the plate boundary may be concentrated offshore and that most of the component of plate motion perpendicular to the plate boundary may be accommodated by slip along the subduction zone megathrust

seismicity structure and tectonics of the wellington region new zealand summary the wellington region lies on the border between the pacific and australian plates the former being subducted the plate interface lies at shallow depth under the region 10 60 km so that detailed and long term monitoring of the seismicity of the zone where large thrust earthquakes are expected to occur is possible hypocentres for events recorded by a 12 station seismic network for the period 1978 1982 7231 events have been refined by the use of a threedimensional velocity model derived from a simultaneous inversion of arrival time data from 129 local events for both hypocentres and velocity the hypocentres show that the subducting pacific plate is defined by a nw dipping zone of relatively intense activity the plate interface as given by the upper envelope of this activity is offset vertically by 7 km on average along a nw se line through cook strait parallel to the direction of absolute motion of the pacific plate rather than to the direction of convergence the strike of the interface at deeper levels also changes across this line this distortion may partly reflect effects of the transition to continent continent type convergence further south within the subducting pacific plate the seismic activity defines two parallel zones separated by about 20 km the lower zone being much less active and perhaps merging with the upper zone at a depth of 80 90 km composite focal mechanisms of events in the upper zone imply predominantly down dip tension with a compression axis perpendicular to the plate interface the mechanism in the lower zone also implies down dip tension but the compression axis has rotated to be horizontal and along strike indicating a component of lateral shearing within the plate these two types of mechanism correlate well with those found previously for events at depths of 150 km or more thus it seems that the large negative buoyancy of the subducted lithosphere dominates the internal stress field throughout the length of the seismically active region the lithosphere acting as a stress guide there is no evidence for a significant number of events with mechanisms indicating thrusting along the plate interface the seismic velocity in the region corresponding to the upper seismic band is close to that typical for oceanic crust which here is probably considerably thicker than normal there is no evidence for a substantial increase in velocity down

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

corals on seamount peaks provide evidence of current acceleration over deep sea topography geological and physical studies of seamounts have suggested the existence of distinct deep sea habitats characterized by exposed rocky bottom and a unique current regime1 9 however few biological data have been collected for deep seamounts10 12 here we present some of the first quantitative observations of hard bottom non hydrothermal fauna in the deep sea these observations show that black corals antipatharians and horny corals gorgonians present on the slopes of a multi peaked seamount are more abundant near peaks compared with mid slope sites at corresponding depths on narrow peaks corals are most abundant on the crest whereas on wide peaks coral densities are highest at the edge of the crest the abundance of corals also increases on knobs and pinnacles physical models and observations2 4 9 13 15 together with our direct measurements suggest that the seamount topography affects the local current regime corals appear to benefit from flow acceleration and some of their patterns of distribution can be explained by current conditions these results suggest that suspension feeders have some potential as indicators of prevailing currents at deep hard bottom sites

possible source models for the 1855 wairarapa earthquake new zealand the magnitude 8 wairarapa new zealand earthquake of 1855 was associated with surface rupture along the wairarapa fault and regional uplift of the southwest of the north island forward elastic dislocation modelling shows that movement on a steeply dipping wairarapa fault alone cannot account for the recorded deformation data modelling of movement on the subduction interface that underlies the wellington region as well as the wairarapa fault also fails to produce a satisfactory ill to the data although a complex wairarapa fault model may be able to explain the deformation pattern if its location subsurface geometry and slip distribution could be independently constrained the best effort supported by available data a flexed model incorporating a left side step of 8 km at the surface incorrectly locates the deformation the best fit to the data is obtained from a listric wairarapa fault model involving rupture on 0 to 50 km width of the deeper part of the subduction interface the shallower pan of the subduction interface east of the wairarapa fault apparently did not rupture in 1855 and the uplift mechanism for the overlying aorangi range remains unexplained partitioning of strike slip and dip slip components of the relative plate motions may involve separate earthquakes seismological verification of listric fault rupture mechanisms is required to determine the plausibility of the listric model presented here because its implications arc that the 1855 earthquake did not completely account for the relative plate motion in the region

average pleistocene climatic patterns in the southern central andes controls on mountain glaciation and paleoclimate implications abstract despite elevations of 5000 6800 m modern glaciers occur along the southern puna plateau and the northern sierras pampeanas in the southern central andes the modern snowline rises from 5100 m in sierra aconquija to 5800 m in the puna as a result of a westward decrease in precipitation from 450 to less than 100 mm yr during the pleistocene these arid highlands experienced multiple cirque and valley glaciation that likely postdate the last interglacial period although lack of age control prevents an absolute chronology glaciation in the puna and along the eastern puna edge produced a 300 m pleistocene snowline psl depression while in the sierras pampeanas the psl depression was at least 900 m the greater psl depression in the sierras pampeanas is best explained by a combination of cooling and increase of easterly moisture whereas the psl depression in the puna appears more sensitive to moisture increases than temperature previously glaciations in this region have been explained by increa

body waveforms and source parameters of some moderate sized earthquakes near north island new zealand p wave first motion and synthetic seismogram analysis of p and sh waveforms recorded at teleseismic distances on the wwssn are used to estimate source parameters of seven of the largest earthquakes 6 1 mb 6 3 that occurred in the vicinity of north island new zealand since 1965 the source parameters of three other mb 6 1 events determined outside of this study are included and considered in the final analysis four of the earthquakes occurred at shallow depths 20 km of which three were located within and to the north of north island two of the shallow events show strike slip and normal focal mechanisms with t axes oriented in a manner consistent with their location in an area of known back arc extension one of the shallow events occurred in northern south island and shows a reverse type mechanism indicating horizontal contraction of the crust in an easterly azimuth six events occurred at intermediate depths h 39 to 195 km of which five exhibit thrust mechanisms with t axes consistently oriented near vertical in the light of previously published plate tectonic models the near vertical orientation of t axes of the intermediate depth events may be used to infer that the southern kermadec plate boundary immediately north of north island is not strongly coupled and hence not likely capable of producing great earthquakes a similar inference cannot be made for the section of the hikurangi margin adjacent to north island since the intermediate depth events considered in this study lie to the north of this segment of the plate boundary

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

diverse slow slip behavior at the hikurangi subduction margin new zealand 1 the installation of a continuous gps cgps network in new zealand since 2002 has enabled the discovery of 15 slow slip events at the hikurangi subduction margin our analysis and interpretation of the cgps data reveal a marked diversity in characteristics of slow slip events sses in the north island with durations varying from 6 days to 1 5 years equivalent moment release between mw 6 3 7 2 and recurrence intervals of repeating sses on the order of 2 years to more than 5 years the duration and magnitude characteristics of hikurangi sses appear to be related to the depth where they occur the deepest longest duration and largest sses occur at the southern hikurangi margin near the downdip limit of deep down to 40 km depth strong interseismic coupling the shallowest shortest duration smallest and most frequent sses occur at the northern and central hikurangi margin near the downdip limit of unusually shallow 10 15 km depth interseismic coupling the direction of slip on the interface in the hikurangi sses is consistent with slip partitioning at the hikurangi subduction margin we also show that moment accumulation rates on the interface for the time periods between sses are 40 higher than for the period of more than a decade averaging through the sses indicating that sses compose a major portion of the overall moment release budget of the hikurangi subduction interface

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

rheological control of wadati benioff zone seismicity intermediate and deep focus earthquakes in wadati benioff zones are thought to occur in the cold interiors of downgoing slabs which are significantly stronger than the warmer mantle given that earthquakes in oceanic lithosphere appear restricted by an isotherm and hence a given value of lithospheric strength we investigate whether a similar formulation is useful for subducting plates strength in downgoing slabs should be affected by both pressure and temperature an effect previously treated using a depth dependent limiting temperature for seismicity wortel 1982 we find this limiting temperature implies that a possible limiting strength increases strongly with depth unless either the temperatures were too low or the activation volume too large comparison of the analytic model used by wortel with numerical thermal models appears to exclude the first possibility we explore the second possibility by using the numerical thermal model to compute strength contours for flow law constants reported from laboratory experiments and find that the expected pressure strengthening is large enough that the slab should have considerable strength well below the deepest seismicity we conclude that if laboratory results are applicable to these conditions either a strongly depth dependent limiting strength exists or factors in addition to strength control the distribution of subduction zone earthquakes

global distribution of seamounts from seasat profiles bathymetrie profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins but only a small fraction of the seafloor has been sampled by ships at the present exploration rate it will take several centuries to map significant portions of the seafloor topography satellite altimetry which maps the topography of the equipotential sea surface is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years using a model of a gaussian shaped seamount loading a thin elastic lithosphere we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile the model predicts that the seamount diameter is equal to the peak to trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed moreover the model suggests that these two measurements are relatively insensitive to the cross track location of the seamount we confirm these model predictions using seasat altimeter profiles crossing 14 well surveyed seamounts in the pacific we then apply the measurement technique to 26 106 million kilometers of seasat profiles resulting in a new global set of seamount locations approximately one quarter of the seamounts identified in seasat profiles were previously uncharted modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid therefore the set of locations is not a straightforward sampling of the total seamount population but is weighted toward seamounts which are poorly compensated a preliminary analysis indicates considerable variations in population density and type across the oceans most notable among them are the absence of seamounts in the atlantic variations in population density across large age offset fracture zones in the pacific the prevalence of small signatures in the indian ocean and the existence of linear trends in the large seamounts of the west pacific

structural evolution along the inner forearc of the obliquely convergent hikurangi margin new zealand the accretionary margin of the hikurangi forearc on the southeast coast of the north island of new zealand is part of the leading edge of the australian plate which is overriding the obliquely converging pacific plate we investigate the last 10 m y of deformation history of the innermost western quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the eketahuna area on the east coast of the north island the eketahuna area is ideal for such a study because emergence of the margin in the quaternary has exposed a complete late neogene rock record this record has allowed us to chronicle the strain history from 10 ma to about 2 5 ma this forearc region was the locus of subsidence and marine deposition in the latest pliocene this part of the margin began to shorten through folding and reverse faulting bringing an end to basin filling the period of shortening was brief and by the late pleistocene reverse faulting had ceased and was immediately succeeded by dextral strike slip faulting in some cases along the same faults presently the dominant strain regime in the inner quarter of the forearc is strike slip faulting this structural history illustrates that over time the pattern of strain partitioning has changed in the hikurangi forearc the switch from crustal shortening to dextral shear along the major faults in this area in the last 1 m y may be a response to more than 10 of clockwise rotation in the southern hikurangi forearc in pliocene and pleistocene time this rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging australian and pacific plates in this area at the southernmost end of the kermadec hikurangi subduction system the inboard portion of this young accretionary margin is exceptionally well exposed today probably in part because of the late neogene subduction of relatively thick buoyant crust of the hikurangi chatham plateau

a reassessment of outer rise seismicity and its implications for the mechanics of oceanic lithosphere we use body waveform modelling to constrain the source parameters of earthquakes occurring globally in oceanic lithosphere beneath the subduction zone outer rise and outer trench slope these data are then used to map the stress state in the lithosphere of the downgoing plate as it bends into the subduction zone our results provide new constraints on the faulting of oceanic lithosphere at the outer rise which is important for understanding the transmission of plate driving forces through the subduction system in all cases shallow normal faulting earthquakes are observed at the top of the plate and are separated in depth from any deeper thrust faulting earthquakes no temporal variation associated with large thrust faulting earthquakes on the subduction interface is seen in the depth extent of each type of faulting at the outer rise the transition depth from trench normal extension to compression is found to vary in agreement with models in which deformation is driven by the combination of in plane stresses and bending stresses resulting principally from slab pull combining the seismologically derived constraints on the thickness of the elastic core of the plate with estimates of the plate curvature we place upper bounds on the strength of the lithosphere at the outer rise which is required to be 300 mpa for a constant yield stress model or governed by an effective coefficient of friction of 0 3

geochemistry and paleomagnetism of late cretaceous mafic dikes in kerala southwest coast of india in relation to large igneous provinces and mantle plumes in the indian ocean region new geochemical and paleomagnetic results are presented on two late cretaceous dikes of the 85 90 ma leucogabbroic and doleritic dikes and the 65 70 ma dolerites in kerala india the dikes are rich in incompatible elements have fractionated patterns with light rare earth element enrichment and are akin geochemically to cretaceous basalts on the east coast of madagascar the magmas were formed at garnet lherzolite depths above the marion plume constituting part of a large igneous province in madagascar in contrast the 65 70 ma dolerites are moderately depleted in incompatible elements with almost flat rare earth element patterns and resemble the upper formations of the deccan traps and the tholeiitic dikes of the seychelles these dolerites were formed by melting of spinel lherzolite over the reunion plume paleomagnetic data from the dikes and the other coeval igneous units from south india provide the 90 ma pole latitude 24 longitude 293 a 95 5 9 n 18 sites for india the 65 70 ma dolerites possess both normal and reverse polarities and the mean pole latitude 36 longitude 283 a 95 5 7 n 10 sites compares well with the deccan superpole paleolatitude estimates indicate 5 southward migration for the marion plume and a northward migration for the reunion plume in conformity with global mantle circulation models however distinguishing migration of the reunion plume from the effects of true polar wander is difficult furthermore the geodynamic reconstructions extending the shear zones of southern madagascar into south india are not tenable

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

effects of near source bathymetry on teleseismic p waveforms teleseismic p waveforms from submarine earthquakes show water reverberations that are often poorly modeled by synthetic seismograms this study suggests that considerable misfit results from the non horizontal nature of the seafloor in the epicentral region and examines this effect for a planar dipping water crust interface geometric ray theory synthetics calculated for a double couple source show that p waveforms are extremely sensitive to seafloor orientation because of the large impedance contrast across the boundary seafloor dips as small as 0 5 significantly modify the ray paths affecting the take off angle at the source reflection and transmission coefficients at the interface geometric spreading and travel time to test the applicability of such modeling p waveforms from the march 29 1976 cocos ridge intraplate earthquake were studied the waveforms many of which show prominent water multiples poorly modeled with flat lying bathymetry were inverted for the best fitting planar ocean bottom orientation using a grid search technique synthetics computed for the best fitting seafloor orientation match the data much better than synthetics for a flat lying structure and reproduce the observed azimuthal dependence of water multiple amplitudes the derived ocean bottom dip angle and direction are in agreement with bathymetric charts suggesting p waveforms provide constraints on the seafloor orientation in the epicentral region green s functions incorporating the effects of bathymetry should provide better resolution of the source parameters of submarine earthquakes

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

global distribution of seamounts inferred from gridded geosat ers 1 altimetry using a forward modeling approach based on an axisymmetric gaussian seamount i characterize the global seamount distribution by locating circular maxima in the gridded vertical gravity gradient field derived from altimetry collected by the geosat and ers 1 satellite missions the global seamount distribution is long tailed and resembles a power law distribution for seamounts in the height range 2 7 km smaller seamounts are not well isolated by my technique nor are they well resolved in the gridded data several factors are likely to influence the height of volcanic seamounts such as melt availability magma driving pressure and plate thickness the observed relationship between seamount gravimetric amplitudes and the age of the underlying seafloor implies that there is an upper limit on seamount heights whether a seamount will reach that height depends most likely on supply side factors such as melt availability and magma driving pressure but the limiting height itself seems more likely to be controlled by the strength of the oceanic plate specifically compressional stresses directly beneath the seamount as a consequence of the lithosphere s flexural response to loading may eventually exceed the magma driving pressure and prevent magma from reaching the surface thus limiting the growth of the seamount because oceanic plate strength primarily is controlled by plate age the limit on seamount height is inferred to be a simple function of plate age at the time of seamount emplacement using analytical solutions i present a simple flexural model that predicts the observed global height age relationship

earthquake mechanisms in the hellenic trench near crete summary in the hellenic trench south of crete convergence between the southern aegean sea and africa occurs at a rate of at least 60 mm yr 1 previously published first motion fault plane solutions show a variety of different fault orientations and types but are not well constrained furthermore the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist nonetheless the mechanisms of these earthquakes have strongly influenced views of the tectonics in the hellenic trench we have improved estimates of the fault parameters and focal depths for 14 of these earthquakes using long period p and sh waveforms the earthquake mechanisms fall into four groups a normal faults with a n s strike in the over riding material above the subduction zone b low angle thrusts with an e w strike at a depth of about 40 km c high angle reverse faults with the same strike but shallower focal depths than b d events within the suducting lithosphere with approximately e w p axes the thrusting in groups b and c is probably the mechanism by which the sediments of the mediterranean sea underplate and uplift crete these events have slip vectors in the direction 025 12 which represents the convergence direction between crete and africa along the sw facing boundary of the hellenic trench one of the events in group d occurred beneath the mediterranean ridge and involved high angle reverse faulting with a wnw ese p axis almost perpendicular to the direction of shortening deduced from folds at the surface the mediterranean sea floor in this region appears to be in a state of e w compression for reasons that are not clear

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

observations on gulf of alaska seamount chains by multi beam sonar abstract geomorphic and age data are presented for the dellwood denson dickins giacomini and ely seamounts the tsimshian seachannel and the southern juan de fuca ridge with brown bear bear cub grizzly bear and cobb seamounts formational speculations extrapolated to a regional scale allow the strikes and outer limits of the seamount chains to be interpreted six of these chains are shown in the gulf of alaska none of which conform to the pratt welker or kodiak bowie in the literature different strikes show the chains plate to have rotated 23 about 17 m y ago morphology also shows that there are four less guyots in the gulf than previously thought and that at least in the gulf of alaska guyot heights do not necessarily reflect sealevel during erosion

seismic coupling and uncoupling at subduction zones seismic coupling has been used as a qualitative measure of the interaction between the two plates at subduction zones kanamori 1971 introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern pacific subduction zones a quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables age of the subducting lithosphere and convergence rate the largest earthquakes occur in zones with young lithosphere and fast convergence rates while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane asperities are strong regions that resist the motion between the two plates the zones with the largest earthquakes have very large asperities while the zones with smaller earthquakes have small scattered asperities this observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface while the variation in asperity size is used to establish a connection between earthquake size and tectonic stress it also implies that plate age and rate affect the asperity distribution plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory i e the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity indirect influences are many including oceanic plate topography and the amount of subducted sediments all subduction zones are apparently uncoupled below a depth of about 40 km and we propose that the basalt to eclogite phase change in the down going oceanic crust may be largely responsible this phase change should start at a depth of 30 35 km and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change

sizes and ages of seamounts using remote sensing implications for intraplate volcanism satellite altimetry was used to identify and characterize pacific intraplate seamounts the gravimetric amplitudes of seamounts appear to be related to the age difference between the sea floor and seamounts by inverting this relation pseudo ages can be obtained for undated seamounts these pseudo ages imply that excursions in seamount volcanism generally correlate with times of formation of large oceanic plateaus

seismic coupling and outer rise earthquakes variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events this variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone outer rise seismicity is strongly correlated with variations in interplate coupling reflecting the stress state of the interplate coupled zone over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling these events occur within the downgoing i e oceanic plate near the bathymetric trench axes and generally fall into the categories of tensional normal or compressional thrust with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench in uncoupled subduction zones only tensional outer rise earthquakes occur which indicates that the outer rise is dominated by tensional stresses associated with plate bending and or slab pull forces in strongly coupled subduction zones both tensional and compressional outer rise events are found these events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle in the strongly coupled regions tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone in four instances compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2 4 7 and 19 years after the associated outer rise event the remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown the occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region thirty compressional outer rise events have been located in trench segments of middle and south america the kurile islands the tonga and kermadec islands the new hebrides arc and the solomon islands regions in both the southern kamchatka and northern new hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional following a previous large underthrusting event to compressional at present thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present requiring only the occurrence of the next underthrusting event to complete the cycle the occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale

shallow subduction tectonics and fault interaction the weber new zealand earthquake sequence of 1990 1992 a series of four large earthquakes with epicenters very close to one another occurred in 1990 1992 in the weber region of the southeast coast of the north island new zealand the region is one of oblique plate convergence and subduction the plate interface being at about 20 km depth the first event february 19 1990 ml 6 1 mw 6 2 occurred within the upper part of the subducting pacific plate on a steeply northwest dipping normal fault as defined by relocated aftershocks the mechanism of this event reflects downdip tension within the subducting plate due to slab pull forces as is common in new zealand the second event may 13 1990 ml 6 2 mw 6 4 occurred on an imbricate fault in the overlying australian plate dipping shallowly northwest with components of both thrusting and dextral strike slip motion the slip vector is similar to the direction of plate convergence showing no arc normal rotation as observed for subduction thrust events in other areas of oblique convergence it is unlikely that the spatial and temporal closeness of these events is coincidence a similar pair occurred to the south in 1942 but the coupling mechanism is not clear perhaps being related to aseismic slip downdip on the subduction interface two subsequent events both of ml 5 5 occurred on august 15 1990 and march 2 1992 and extended the aftershock zone of the deeper first main shock to the northeast matching the extent of the shallower second main shock mechanisms of aftershocks of the two larger events are quite variable but as a whole are similar to the relevant mainshock the time history of the aftershocks of the first deeper event shows a period of quiescence beginning 35 days before the second shallow event the mean magnitude of those events that did occur was high this observation cannot be explained by station biases weather or changes in analysis methods and may be due to stress changes induced by precursory aseismic slip which would discourage aftershock occurrence

varied responses to subduction in nankai trough and japan trench forearcs responses of sediment entering trenches range from accretion at the foot of the inner slope to subduction into the mantle with inner trench slopes variously displaying uplift subsidence growth and retreat leg 87 is the latest of the dsdp voyages which have provided many clues to the behaviour of subduction zones and was devoted to the study of subduction processes in two trenches off japan the nankai trough fig 1 on the basis of very high quality reflection profiles1 2 is accreting most or all its trench fill which is being stripped off the underlying shikoku basin haemipelagites along a decollement near their contact fig 2a because these seismic profiles define the large scale geometry so clearly the objective of the leg 87 drilling programme was to investigate the structural fabric and physical properties of sediments as they enter the subduction zone using these data which we now report we hoped to deduce or constrain the stress conditions accompanying deformation