constraints on present day plate motions south of 30 s from satellite altimetry abstract gridded free air gravity anomalies derived from geosat and seasat altimetric observations are used to locate all mid ocean ridge transform faults south of 30 s and wherever possible assign azimuths and uncertainties useful for plate kinematic analysis a total of 131 ridge offsets longer than 30 km were located of which 114 are clearly transform faults with well defined valleys azimuthal uncertainties range from 0 6 for the longest transform faults to nearly 10 for the transform faults with lengths close to 30 km to assess whether the altimetrically derived azimuths are representative of present day plate slip directions azimuths of transform faults mapped with side scan sonar and side looking or conventional bathymetric systems were compared to their altimetrically derived counterparts comparison of these two independently estimated sets of azimuths using the x2 test shows no significant difference indicating that the altimetrically derived azimuths can be used where bathymetrically derived azimuths are not available we also find no evidence that large age offset transform faults are less representative of present day slip directions than short age offset transforms models constructed using the altimetrically derived azimuths fit the nuvel 1 spreading rates transform azimuths and earthquake slip vectors nearly as well as nuvel 1 as well as decreasing the model uncertainties thus the altimetrically derived azimuths offer improved accuracy and decreased uncertainties in estimates of global plate velocities without increasing non closures of plate circuits beyond the level already present in nuvel 1 in practical terms plate velocities predicted by models that substitute the altimetrically derived azimuths for all nuvel 1 transform azimuths from below 30 s never differ from velocities predicted by nuvel 1 by more than 0 3 mm yr 1 and 4 however the several degree azimuthal misfits to altimetrically derived azimuths from the africa antarctic and pacific antarctic plate boundaries are statistically significant and suggest there are several degree inaccuracies in the predictions of the nuvel 1 angular velocities the new catalog of southern hemisphere transform fault azimuths should prove useful for at least the following 1 improvement of models of deformation of the australian plate west of the macquarie ridge complex 2 improvement of estimates of motion between the australian and pacific plates 3 revision of upward bounds on the amount of seafloor deformation south of africa related to rifting of the african plate and 4 studies of a possible deforming zone within the south american plate east of the south sandwich trench

source scaling relationship for m 4 6 8 9 earthquakes specifically for earthquakes in the collision zone of taiwan we investigated the source scaling of earthquakes mw 4 6 8 9 mostly from the taiwan orogenic belt and made a global compilation of source parameters to examine the scaling self similarity finite fault slip models 12 dip slip and 7 strike slip using mainly dense strong motion data and teleseismic data from taiwan were utilized seven additional earthquakes m 7 were included for further ex amination of scaling of large events we determined the effective length and width for the scaling study was m0 l 2 and m0 l 3 for the events less than and larger than the seismic moment of 10 20 nm respectively regardless of the fault types suggesting a nonself similar scaling for small to moderate events and a self similar scaling for large events although the events showed variation in stress drops with the exception of three events with high stress drops most of the events had stress drops of 10 100 bars the observed bilinear relation is well explained by the derived magnitude area equation of shaw 2009 when we considered only events with stress drops of 10 100 bars and a seismogenic thickness of 35 km the bilinear feature of the regressed magnitude area scaling holds for ruptured areas up to about 1000 km 2 for our seismogenic thickness of 35 km for the events having rupture areas larger than that the average slip becomes proportional to the rupture length the distinct high stress drop events from blind faults in the western foothill of taiwan yield local high peak ground accelerations pgas when compared to the next generation attenuation model regardless of the relative small magnitudes of these events the high pgas give the region higher seismic hazard potential and thus require special attention for seismic hazard mitigation

selection of earthquake scaling relationships for seismic hazard analysis a fundamentally important but typically abbreviated component of seismic hazard analysis is the selection of earthquake scaling relationships these are typically regressions of historical earthquake datasets in which magnitude is estimated from parameters such as fault rupture length and area the mix of historical data from different tectonic environments and the different forms of the regression equations can result in large differences in magnitude estimates for a given fault rupture length or area we compile a worldwide set of regressions and make a first order shortlisting of regressions according to their relevance to a range of tectonic regimes plate tectonic setting and fault slip type in existence around the world regression relevance is based largely on the geographical distribution age and quantity quality of earthquake data used to develop them our compilation is limited to regressions of magnitude or seismic moment on fault rupture area or length and our shortlisted regressions show a large magnitude range up to a full magnitude unit for a given rupture length or area across the various tectonic regimes these large differences in magnitude estimates underline the importance of choosing regressions carefully for seismic hazard application in different tectonic environments

plate tectonic development of the south west indian ocean a revised reconstruction of east antarctica and africa a revised sequence of reconstructions for east antarctica relative to africa is generated from published seafloor spreading and fracture zone anomalies these date from the cretaceous tertiary boundary to the late jurassic and lead to a revised predrift reconstruction for east and west gondwanaland euler pole 1 67 s 35 99 e rotation angle 53 43 this reconstruction derived wholly from marine data places east antarctica adjacent to the lebombo mountains and south of and subparallel to the sabi acid volcanic suite supporting the postulate that these rocks represent mesozoic plate boundaries this refit implies that part of the coastal plain of mozambique is underlain by oceanic or very highly extended continental crust the reconstruction is supported by the alignment of 1 the cape fold belt with its antarctic equivalents and 2 the western limit of pan african reactivation of the mozambique mobile belt with the western limit of the sverdrupfjella metamorphic suite antarctica initially moved along a transform fault lying immediately east of and subparallel to the lebombo mountains the eastern face of the tugela cone the southeastern face of the falkland plateau in its reconstructed position and the eastern flank of the agulhas plateau in its reconstructed position at m10 time a triple junction formed near the tip of the falkland plateau implying that the oldest lineated magnetic anomalies in the eastern weddell sea associated with this triple junction cannot predate m10 at m2 time the mozambique ridge rifted from the astrid ridge in a ridge jump episode such an event may also explain the presence of magnetic anomalies in the southern mozambique basin gunnerus ridge conrad rise and del cano rise are all tectonically associated with the madagascar ridge

an investigation of the influence of data and model inputs on assimilative mapping of ionospheric electrodynamics the geospace environment modeling gem substorm challenge event of november 24 1996 has been used as a test case to investigate the influence of different data and model inputs on the assimilative mapping of ionospheric electrodynamics amie outputs during that period the interplanetary magnetic field imf went from northward to southward and then returned to northward in the later part of the day a moderate substorm with al 600 nt took place the amie convection patterns derived from the super dual auroral radar network superdarn data alone are generally similar to those derived using ground magnetometer alone especially during the relatively stable southward imf period however some differences are found during the northward imf period and during the substorm namely the reversed convection configuration near local noon imaged by superdarn is absent in the magnetometer observations while the strong convection on the nightside recorded by the magnetometers during the substorm expansion phase is not seen by the radars different conductance models do not seem to have a big effect on the large scale distributions of ionospheric convection and joule heating but they do alter the cross polar cap potential drop and the hemispheric integrated joule heating rate by nearly a factor of 2 when the amie derived electric potential drop was compared with the in situ measurements along the satellite track it is found that amie underestimated the potential drop by 20 kv amounting to a 22 underestimation the analysis of the amie results based on the superdarn and ground magnetometer data reiterates the view that the response of ionospheric convection to a sudden imf southward turning is global and nearly simultaneous

interseismic strain accumulation and the earthquake potential on the southern san andreas fault system the southern section of the san andreas fault is the only part of the fault that hasn t ruptured in historic times for this reason it is believed to pose the largest seismic risk in california although whether tectonic stress is actually accumulating there is a topic of heated debate using data from radar interferometry satellites yuri fialko has produced a detailed map of surface deformation in the area that shows that elastic stress is accumulating at a fairly high rate during the past 300 years a displacement of 6 to 9 metres has built up this suggests that the southern san andreas fault is nearing the end of the interseismic phase of the earthquake cycle and that increased seismic activity of some kind can be expected the san andreas fault in california is a mature continental transform fault that accommodates a significant fraction of motion between the north american and pacific plates the two most recent great earthquakes on this fault ruptured its northern and central sections in 1906 and 1857 respectively the southern section of the fault however has not produced a great earthquake in historic times for at least 250 years assuming the average slip rate of a few centimetres per year typical of the rest of the san andreas fault the minimum amount of slip deficit accrued on the southern section is of the order of 7 10 metres comparable to the maximum co seismic offset ever documented on the fault1 2 here i present high resolution measurements of interseismic deformation across the southern san andreas fault system using a well populated catalogue of space borne synthetic aperture radar data the data reveal a nearly equal partitioning of deformation between the southern san andreas and san jacinto faults with a pronounced asymmetry in strain accumulation with respect to the geologically mapped fault traces the observed strain rates confirm that the southern section of the san andreas fault may be approaching the end of the interseismic phase of the earthquake cycle

foreshock triggering of the 1 april 2014 mw 8 2 iquique chile earthquake abstract on april 1st 2014 a mw 8 2 u s geological survey moment magnitude earthquake occurred in the subduction zone offshore northern chile in the two weeks leading up to the earthquake a sequence of foreshocks starting with a mw 6 7 earthquake on march 16th and including three more mw 6 0 events occurred predominantly south of the april 1st mainshock epicenter and up dip of the area of significant slip during the mainshock using earthquake locations and source parameters derived in a previous study hayes et al 2014 and a coulomb failure stress change analysis of these events we assess in detail the hypothesis that the earthquakes occurred as a cascading sequence each event successively triggering the next ultimately triggering the rupture of the mainshock following the initial mw 6 7 event each of the three largest foreshocks mw 6 4 6 2 and 6 3 as well as the hypocenter of the mainshock occurred in a region of positive coulomb stress change produced by the preceding events indicating these events were brought closer to failure by the prior seismicity in addition we reexamine the possibility that aseismic slip occurred and what role it may have played in loading the plate boundary using horizontal gps displacements from along the northern chile coast prior to the mainshock we find that the foreshock seismicity alone likely does not account for the observed signals we perform a grid search for the location and magnitude of an aseismic slip patch that can account for the difference between observed signals and foreshock related displacement and find that a slow slip region with slip corresponding to a mw 6 8 earthquake located coincident with or up dip of the foreshock seismicity can best explain this discrepancy additionally such a slow slip region positively loads the mainshock hypocentral area enhancing the positive loading produced by the foreshock seismicity

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

earthquakes in the davie ridge madagascar region and the southern nubian somalian plate boundary the largest earthquake sequence mb up to 6 4 and m0 up to 5 1018 nm to have struck eastern africa within the past 60 years occurred in may and june of 1985 off the coast of the tanzania mozambique border the epicenters are located on the northern extension of the davie ridge a prominent north south trending bathymetrie feature accompanied by distinct gravity anomalies a joint hypocenter location of the 14 largest events in this sequence appears as a cluster of epicenters with no apparent lineation al teleseismic distances p and sh waves generated by the two largest events of this sequence have nearly identical waveforms on the long period records of the worldwide standard seismograph network a formal inversion of the data shows a pure normal faulting mechanism on nodal planes striking slightly west of north at least two point sources are necessary to model each event one at a depth of 40 10 km and the other at about 20 5 km on a regional scale normal faulting seems to characterize the scattered seismicity throughout the region between madagascar and the eastern arm of the east african rift zone even though oceanic lithosphere dominates part of the region and no topographic expression of a rift system can be identified since normal faulting events of up to 25 5 km deep also characterize the scattered seismicity west of the topographic expression of the western arm of the rift zone at about the same latitude we propose that the southern termination of the nubian somalian plate boundary consists of a diffuse zone of east west extension up to 2000 km wide

slumps canyons and related features on the continental margin off east london se africa sw indian ocean abstract bathymetric and seismic reflection surveys in the natal valley east of east london show that the most important factors in the neogene development of the outer continental margin have been large scale slumping and penecontemporaneous canyon formation slope wasting and allochthonous mass movements have shifted the foot of the continental slope up to 25 km basinward and have constructed extensive continental rise lobes shelf break breaching by canyon heads is considered an important factor in transferring sediment entrained in the agulhas current from the continental shelf to the deep ocean basin a preliminary sediment budget suggests that the continental rise lobes could have been constructed during the last 2 69 m y following an episode of massive slumping

an early cretaceous extinct spreading center in the northern natal valley abstract we have identified an extinct e w spreading center in the northern natal valley on the basis of magnetic anomalies which was active from chron m11 133 ma to 125 3 ma just before chron m2 124 ma in the early cretaceous seafloor spreading in the northern natal valley accounts for approximately 170 km of north south motion between the mozambique ridge and africa this extension resolves the predicted overlap of the continental central and southern mozambique ridge and antarctica in the chron m2 to m11 reconstructions from mesozoic finite rotation parameters for africa and antarctica in addition the magnetic data reveal that the mozambique ridge was an independent microplate from at least 133 to 125 ma the northern natal valley extinct spreading center connects to the spreading center separating the mozambique basin and the riiser larsen sea to the east it follows that the northern mozambique ridge was either formed after the emplacement of the surrounding oceanic crust or it is the product of a very robust spreading center to the west the extinct spreading center connects to the spreading center separating the southern natal valley and georgia basin via a transform fault prior to chron m11 there is still a problem with the overlap of mozambique ridge if it is assumed to be fixed with respect to either the african or antarctic plates some of the overlap can be accounted for by jurassic deformation of the mozambique ridge mozambique basin and dronning maud land it appears though that the mozambique ridge was an independent microplate from the breakup of gondwana 160 ma until it became part of the african plate 125 ma

folding of oceanic lithosphere folding of the lithosphere just south of the bay of bengal appears as 1 undulations in acoustic basement topography and 2 as linear geoid undulations in the seasat altimeter data from the seasat data we find that the east west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km the horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mgal elastic models of oceanic lithosphere have in the past been used to demonstrate the implausibility of lithosphere buckling or folding in response to compression these elastic models typically predict that compressive stresses of about 5 gpa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern indian ocean 40 70 ma these stresses exceed the strength of lithospheric rock we use an elastic plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12 of the elastic buckling stress we further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km i e the wavelength observed from seasat our results reinforce earlier speculation that this folding may be related to the himalayan orogeny

the somalia plate and the east african rift system present day kinematics summary the motion of the somalia plate relative to the nubia africa arabia and antarctica platcs is re evaluated using a new inversion method based on a monte carlo technique and a least absolute value misfit criterion a subset of the nuvel 1 data set with additional data along the levant fault and in the red sea is used the results confirm that the motion of arabia with respect to africa is significantly different from the motion relative to somalia it is further shown that the data along the sw indian ridge are compatible with a pole of relative motion between africa and somalia located close to the hypothetical diffuse triple junction between the ridge and thc east african rift the resulting africa somalia motion is then compatible with the geological structures and seismological data along the east african rift system assuming a separate somalia plate thus solves kinematic and geological problems around the afar triple junction and along the east african rift

high resolution seismic images of potentially seismogenic structures beneath the northwest canterbury plains new zealand 1 the transpressional boundary between the australian and pacific plates in the central south island of new zealand comprises the alpine fault and a broad region of distributed strain concentrated in the southern alps but encompassing regions further to the east including the northwest canterbury plains low to moderate levels of seismicity e g 2 m 5 events since 1974 and 2 m 4 0 in 2009 and holocene sediments offset or disrupted along rare exposed active fault segments are evidence for ongoing tectonism in the northwest plains the surface topography of which is remarkably flat and even because the geology underlying the late quaternary alluvial fan deposits that carpet most of the plains is not established the detailed tectonic evolution of this region and the potential for larger earthquakes is only poorly understood to address these issues we have processed and interpreted high resolution 2 5 m subsurface sampling interval seismic data acquired along lines strategically located relative to extensive rock exposures to the north west and southwest and rare exposures to the east geological information provided by these rock exposures offer important constraints on the interpretation of the seismic data the processed seismic reflection sections image a variably thick layer of generally undisturbed younger i e 59 ka quaternary sedimentary sequence that shows evidence of moderate faulting and folding during and subsequent to deposition these quaternary units are in unconformable contact with late cretaceous tertiary interbedded sedimentary and volcanic rocks that are highly faulted folded and tilted the lowest imaged unit is largely reflection free permian triassic basement rocks quaternary age deformation has affected all the rocks underlying the younger alluvial sediments and there is evidence for ongoing deformation eight primary and numerous secondary faults as well as a major anticlinal fold are revealed on the seismic sections folded sedimentary and volcanic units are observed in the hanging walls and footwalls of most faults five of the primary faults represent plausible extensions of mapped faults three of which are active the major anticlinal fold is the probable continuation of known active structure a magnitude 7 1 earthquake occurred on 4 september 2010 near the southeastern edge of our study area this predominantly right lateral strike slip event and numerous aftershocks ten with magnitudes 5 within one week of the main event highlight the primary message of our paper that the generally flat and topographically featureless canterbury plains is underlain by a network of active faults that have the potential to generate significant earthquakes

crustal structure across the southwest newfoundland transform margin a seismic refraction survey providing deep crustal structure information of the continent ocean boundary across the south west newfoundland transform margin was carried out using large air gun sources and ocean bottom seismometer receivers continental crust 30 km thick beneath the southern grand banks p wave velocity 6 2 6 5 km s thins oceanward to a 25 km wide transition zone in the transition zone paleozoic basement of the grand banks 5 5 5 7 km s is replaced by a basement of oceanic volcanics and synrift sediments 4 5 5 5 km s seaward of the transition zone the crust is oceanic in character with a velocity gradient from 4 7 to 6 5 km s and a thickness of 7 8 km oceanic layer 3 is absent no significant thickness of intermediate velocity 7 km s material is present at the continent ocean transition indicating that no under plating of continental crust has taken place the continent ocean transition across the transform margin is much narrower than across rifted margins supporting the

high resolution relocation of aftershocks of the mw 7 1 darfield new zealand earthquake and implications for fault activity 1 low slip rate regions often represent under recognized hazards and understanding the progression of seismicity when faults in such areas rupture will help us to better understand earthquake rupture patterns the 3 september 2010 utc mw 7 1 darfield earthquake revealed a formerly unrecognized set of faults in the canterbury region of new zealand an area that had previously been mapped as one of the lower hazard areas in the country in this study we analyze the first four months of its aftershock sequence to identify active faults and temporal changes in seismicity along them we jointly invert for three dimensional p wave and s wave velocities and hypocentral locations using data for 2840 aftershocks recorded at 36 temporary and permanent seismic stations within 70 km of the main shock epicenter these relocations delineate eight individual faults active prior to the 22 february 2011 mw 6 3 christchurch earthquake the largest aftershock of the darfield earthquake two of these faults are in the christchurch region one of which corresponds to geodetically determined rupture planes of the christchurch earthquake using focal mechanisms calculated from first motion polarities we find mainly strike slip faulting events with some reverse and normal faulting events as well we compare the orientations of these faults to the prevailing regional stress directions to identify which faults may have been active prior to the darfield earthquake and which may be newly developed

new insights into the tectonic inversion of north canterbury and the regional structural context of the 2010 2011 canterbury earthquake sequence new zealand the 2010 2011 canterbury earthquake sequence highlighted the existence of previously unknown active faults beneath the north canterbury plains and pegasus bay south island new zealand we provide new insights into the geometry and kinematics of ongoing deformation by analyzing marine seismic data to produce new maps of regional faults and cross sectional reconstructions of deformation history active faulting and folding extends up to 30 km offshore and involves reactivation of sets of late cretaceous paleogene normal faults under nw se tectonic compression the active faults consist predominantly of ne sw striking se dipping reverse faults and less commonly e w to nw se faults suitably oriented for strike slip reactivation additionally newly developing reverse faults obliquely segment and overprint the inherited basement fabric and impose geometric and kinematic complexities revealed by mapping and reverse displacement profiles of markers the quaternary reverse slip rates decrease from 0 1 0 3 mm yr beneath northern pegasus bay to 0 05 mm yr approaching banks peninsula fault growth modeling involving trishear fault propagation folding mechanisms successfully restores an evolutionary sequence of progressive fault inversion revealing a history of reactivated individual faults tectonic inversion and overprinting processes beneath pegasus bay are immature and 1 2 0 4 ma old with no evidence of systematic spatial migration of deformation our marine data analyses give insights into the structural context of the 2010 2011 canterbury earthquake sequence while the combined onshore to offshore data provide an excellent illustration of fault growth associated with immature inversion tectonics in which selective fault reactivation results from compressive stress imposed across a complex network of inherited faults

intraplate deformation and closure of the australia antarctica africa plate circuit to determine the current motion between the australian antarctic and african plates and to test whether this plate circuit obeys closure all plate motion data available along the southeast southwest and central indian ridges are analyzed and reduced to 67 spreading rates 38 transform fault azimuths and 135 earthquake slip vectors carlsberg ridge data were excluded because they record india africa not australia africa motion new data include 10 new transform azimuths along the southwest indian ridge many slip vectors and a dense aeromagnetic survey along the southeast indian ridge all published and many unpublished magnetic profiles are modeled to determine rates consistently over a 3 m y time averaging interval for three reasons 1 magnetic profiles from the southeast and central indian ridges suggest recent spreading rate changes 2 some published rates differ for identical profiles and 3 prior studies have not used identical criteria and time scales to determine spreading rates the new rates differ from published rates by as much as 5 mm yr transform fault azimuths are estimated from bathymetry seasat altimetric data epicenter distributions and offsets of magnetic lineations azimuths of plate motion are also estimated from the horizontal projections of slip vectors from the centroid moment tensor solutions and other focal mechanism studies our new australia antarctica and australia africa euler vectors differ from all prior euler vectors at the 95 confidence level along the southeast and central indian ridges our model gives rates differing by 4 7 mm yr from those of prior models from a systematic analysis of the plate motion data we find no evidence for a nubia somalia antarctica triple junction along the southwest indian ridge we also find no evidence for a triple junction previously proposed to be at 80 e along the southeast indian ridge any deformation within this seismically active region of the australian plate adds up to less than a few millimeters per year along each boundary data are fit well by a single euler vector except for a 5 systematic misfit to azimuthal data along the eastern southeast indian ridge near a seismically active region of the australian plate south of tasmania although seismicity and the azimuthal misfit suggest the australian plate is deforming measurably possibly by distributed deformation or by the westward motion of a small microplate southeast of tasmania the significance of the misfit is marginal and is small enough that systematic errors may cause it these alternative explanations could be distinguished by surveying these transform faults with modern seafloor mapping techniques prior studies found that enforcing plate circuit closure causes systematic misfits to rates along the southeast indian ridge here we find that enforcing closure causes no systematic misfits to any data moreover only insignificant nonclosure is found by an f ratio test which numerical experiments suggest could detect deformation exceeding 2 mm yr we conclude that indian ocean plate circuit nonclosure and the deformation it suggests are much smaller than thought before the absence of significant nonclosure argues against the usefulness of a model of deformation distributed throughout an indo australian plate but favors a model in which the significant deformation occurs in a diffuse plate boundary along the equatorial indian ocean between the central indian ridge and the sumatra trench

structure of the transkei basin and natal valley southwest indian ocean from seismic reflection and potential field data abstract marine geophysical data from the southern natal valley and northern transkei basin offshore southeast africa were used to study the structure of the crust and sedimentary cover in the area the data includes seismic reflection gravity and magnetics and provides information on the acoustic basement geometry where available features of the sedimentary cover and the basin s development previously mapped mesozoic magnetic anomalies over a part of the basin are now recognized over wider areas of the basin the ability to extend the correlation to the southeast within the natal valley further confirms an oceanic origin for this region and provides an opportunity to amplify the existing plate boundary reconstructions the stratigraphic structure of the southern natal valley and the northern transkei basin reflects processes of the ocean crust formation and subsequent evolution the highly variable relief of the acoustic basement may relate to the crust formation in the immediate vicinity of the continental transform margin renewed submarine seismicity and neotectonic activity in the area is probably related to the diffuse boundary between the nubia and somalia plates 2 5 d crustal models show that a 1 7 3 2 km thick sediment sequence overlies a 6 3 1 2 km thick normal oceanic crust in the deep southern natal valley and transkei basin the oceanic crust in the study area is heterogeneous made up of blocks of laterally varying remanent magnetization 0 5 3 5 a m and density 2850 2900 kg m3 strong modifications of accretionary processes near ridge fracture zone intersections may be a reason of such heterogeneity

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

geodetic measurements of crustal deformation in the western mediterranean and europe geodetic measurements of crustal deformation over large areas deforming at slow rates 5 mm yr over more than 1000 km such as the western mediterranean and western europe are still a challenge because 1 these rates are close to the current resolution of the geodetic techniques 2 inaccuracies in the reference frame implementation may be on the same order as the tectonic velocities we present a new velocity field for western europe and the western mediterranean derived from a rigorous combination of 1 a selection of sites from the itrf2000 solution 2 a subset of sites from the european permanent gps network solution 3 a solution of the french national geodetic permanent gps network rgp and 4 a solution of a permanent gps network in the western alps regal the resulting velocity field describes horizontal crustal motion at 64 sites in western europe with an accuracy on the order of 1 mm yr or better its analysis shows that central europe behaves rigidly at a 0 4 mm yr level and can therefore be used to define a stable europe reference frame in that reference frame we find that most of europe including areas west of the rhine graben the iberian peninsula the ligurian basin and the corsica sardinian block behaves rigidly at a 0 5 mm yr level in a second step we map recently published geodetic results in the reference frame previously defined geodetic data confirm a counterclockwise rotation of the adriatic microplate with respect to stable europe that appears to control the strain pattern along its boundaries active deformation in the alps apennines and dinarides is probably driven by the independent motion of the adriatic plate rather than by the africa eurasia convergence the analysis of a global gps solution and recently published new estimates for the african plate kinematics indicate that the africa eurasia plate motion may be significantly different from the nuvel1a values in particular geodetic solutions show that the convergence rate between africa and stable europe may be 30 60 slower than the nuvelia prediction and rotated 10 30 counterclockwise in the mediterranean

using regional moment tensors to constrain the kinematics and stress evolution of the 2010 2013 canterbury earthquake sequence south island new zealand abstract on september 3 2010 a m w 7 0 u s geological survey moment magnitude earthquake ruptured across the canterbury plains in south island new zealand since then new zealand gns science has recorded over 10 000 aftershocks m l 2 0 and larger including three destructive m w 6 0 earthquakes near christchurch we treat the canterbury earthquake sequence as an intraplate earthquake sequence and compare its kinematics to an andersonian model for fault slip in a uniform stress field we determined moment magnitudes and double couple solutions for 150 earthquakes having m w 3 7 and larger through the use of a waveform inversion technique using data from broadband seismic stations on south island new zealand the majority 126 of these double couple solutions have strike slip focal mechanisms with right lateral slip on ene fault planes or equivalently left lateral slip on sse fault planes the remaining focal mechanisms indicate reverse faulting except for two normal faulting events the strike slip segments have compatible orientations for slip in a stress field with a horizontal 1 oriented n115 e and horizontal 3 the preference for right lateral strike slip earthquakes suggests that these structures are inherited from previous stages of deformation reverse slip is interpreted to have occurred on previously existing structures in regions with an absence of existing structures optimally oriented for strike slip deformation despite the variations in slip direction and faulting style most aftershocks had nearly the same p axis orientation consistent with the regional 1 there is no evidence for significant changes in these stress orientations throughout the canterbury earthquake sequence

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

kinematic constraints on distributed lithospheric deformation in the equatorial indian ocean from present motion between the australian and indian plates from an expanded accurate and up to date data set comprising 110 spreading rates 46 transform azimuths and 151 earthquake slip vectors from the indian ocean and gulf of aden we determine a new rigid plate model describing the motion since 3 ma between india and australia the euler vector 0 313 m y about 5 s 78 e lies near the middle of the equatorial diffuse plate boundary dividing the indian from the australian plate and predicts a rate of north south shortening along 85 e of 4 3 mm yr only 30 as fast as predicted by our prior model the new model also predicts north south extension of 6 2 mm yr at 68 e along the western segment of the diffuse plate boundary where our prior model predicted north south contraction using data only along the carlsberg and central indian ridges and no other plate boundaries we show that plate motion data cannot be fit by a single euler vector however the data are well fit by two euler vectors when an east west striking india australia plate boundary is assumed to intersect the central indian ridge near the equator the best location along the central indian ridge for this triple junction is 6 s 3 s with 95 confidence limits of 9 s 4 n just west of a region of intense seismicity the sense of deformation as recorded in earthquake focal mechanisms reverse faults mapped with reflection seismic data and undulations in basement topography surface gravity and the geoid agrees well with the north south extension predicted in the western part and the north south shortening predicted in the eastern part of the india australia boundary the predicted rate of north south shortening between 79 e and 86 e is consistent with the rate of shortening inferred from observed faulting and folding shortening taken up by faulting is 6 to 100 times greater than that taken up by the spectacular basement folds little of the rapid north south shortening predicted east of 86 e is taken up by crustal thickening instead lithosphere is transported northeastward toward the sumatra trench through strike slip faulting and possible clockwise rotation the failure of the convergent segment of the diffuse plate boundary to form a subducting trench provides some new observations that must be satisfied by models for the initiation of subduction a model of separate rigid indian and australian plates divided by a diffuse plate boundary appears to be valid and useful because the predictions of the model agree with independent data and because the predicted velocity of relative motion is statistically significant and comparable with that across other plate boundaries

mesozoic magnetic anomalies in the southern natal valley the recognition of mesozoic magnetic anomalies m0 m12 within the submarine natal valley south west indian ocean provides a further constraint on the relative motion between south america falkland plateau and africa during the early opening phase of west gondwanaland fig 1 colinear with m12 magnetic anomaly g ref 1 is delineated north of the south tugela ridge2 an acoustic basement high which is suspected to have been emplaced within continental crust immediately north of the continental oceanic crust boundary cob during the initial rifting phase starting from a revised west gondwana reconstruction3 and using palaeo poles of rotation1 3 the falkland plateau can be rotated into the natal valley superposing the corresponding georgia basin and natal valley magnetic anomalies this achieves an optimum continuity of inter continental structural lineaments3 by overlapping of an anomaly intermediate to m10 and m11 m10n4 this suggests that the natal valley cob is located between anomalies m10 and m11 implying that in this zone of south west gondwana seafloor spreading commenced at 125 myr

continuing megathrust earthquake potential in chile after the 2014 iquique earthquake the 2014 iquique event was not the earthquake that had been expected to fill the regional seismic gap given that significant sections of the northern chile subduction zone have not ruptured in almost 150 years it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 iquique sequence two groups publishing in this issue of nature analyse the seismic context of the iquique earthquake that occurred off the coast of northern chile on 1 april 2014 in a seismic zone that had been quiescent since a significant event in 1877 gavin hayes et al identify areas of remaining or elevated earthquake hazard along the megathrust fault in the region and conclude that the 2014 iquique event was not the earthquake that had been anticipated given that significant sections of the northern chile subduction zone have not ruptured in almost 150 years they suggest that it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 iquique sequence bernd schurr et al show that the april 2014 earthquake broke a central fraction of the northern chile seismic gap the last major segment of the south american plate boundary that had yet to rupture in the past century from july 2013 up to the april earthquake they identify three seismic clusters along this part of the plate boundary each lasting a few weeks with earthquakes of increasing peak magnitudes they conclude that these seismic clusters and their slip transients reflect a gradual weakening of the central part of the seismic gap that was instrumental in initiating the final failure the seismic gap theory1 identifies regions of elevated hazard based on a lack of recent seismicity in comparison with other portions of a fault it has successfully explained past earthquakes see for example ref 2 and is useful for qualitatively describing where large earthquakes might occur a large earthquake had been expected in the subduction zone adjacent to northern chile3 4 5 6 which had not ruptured in a megathrust earthquake since a m 8 8 event in 1877 on 1 april 2014 a m 8 2 earthquake occurred within this seismic gap here we present an assessment of the seismotectonics of the march april 2014 iquique sequence including analyses of earthquake relocations moment tensors finite fault models moment deficit calculations and cumulative coulomb stress transfer this ensemble of information allows us to place the sequence within the context of regional seismicity and to identify areas of remaining and or elevated hazard our results constrain the size and spatial extent of rupture and indicate that this was not the earthquake that had been anticipated significant sections of the northern chile subduction zone have not ruptured in almost 150 years so it is likely that future megathrust earthquakes will occur to the south and potentially to the north of the 2014 iquique sequence

mapping using the tsyganenko long magnetospheric model and its relationship to viking auroral images the tsyganenko long magnetospheric model 1987 has been used in conjunction with ultra violet images taken by the viking spacecraft to investigate the relationship of the auroral distribution to different magnetospheric regions the model describes the large scale structure of the magnetosphere reasonably well for dipole tilt angles near zero but it appears to break down at higher tilt angles even so a wide variety of auroral configurations can be accurately described by the model it appears that the open closed field line boundary is a poor indicator of auroral arc systems with the possible exception of high latitude polar arcs the auroral distribution typically called the oval maps to a region in the equatorial plane quite close to the earth and can be approximately located by mapping the model current density maximum from the equatorial plane into the ionosphere although the model may break down along the flanks of the magnetotail the large scale auroral distribution generally reflects variations in the near earth region and can be modeled quite effectively

revisiting the canterbury earthquake sequence after the 14 february 2016 mw 5 7 event on 14 february 2016 an mw 5 7 gns science moment magnitude earthquake ruptured offshore east of christchurch new zealand this earthquake occurred in an area that had previously experienced significant seismicity from 2010 to 2012 during the canterbury earthquake sequence starting with the 2010 mw 7 0 darfield earthquake and including four mw 6 0 earthquakes near christchurch we determine source parameters for the february 2016 event and its aftershocks relocate the recent events along with the canterbury earthquakes and compute coulomb stress changes resolved onto the recent events and throughout the greater christchurch region because the february 2016 earthquake occurred close to previous seismicity the coulomb stress changes resolved onto its nodal planes are uncertain however in the greater christchurch region there are areas that remain positively loaded including beneath the city of christchurch the recent earthquake and regional stress changes suggest that faults in these regions may pose a continuing seismic hazard

the initial gondwana break up a synthesis based on new potential field data of the africa antarctica corridor abstract the initial opening of the africa antarctica corridor in the heart of gondwana is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the jurassic therefore in 2014 new ship borne magnetic data were systematically acquired in the northern mozambique basin and across beira high which we merged with all accessible magnetic data in the mozambique basin herein distinct magnetic lineations are observed which allow a refined identification of a whole set of jurassic magnetic spreading anomalies constraining the timing of the onset of oceanization beginning at m38n 2n 164 1 ma in combination with high resolution potential field data from the conjugate antarctic margin well expressed fracture zones can be traced throughout the africa antarctica corridor and allow the precise rotation of antarctica back to africa the initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account within a tight gondwana fit the beira high can be restored along the major sinistral namama orvin shear zone of the east african antarctic orogen the beira high represents a continental block which was detached from antarctica by 157 ma at the latest simultaneously the antarctic plate cleared the area of the mcp however the crustal nature of the southern mcp remains ambiguous the northern natal valley and the mozambique ridge consist of thick oceanic crust being emplaced between m26r m18n 157 1 144 ma and m18n m6n 144 131 7 ma respectively about the half of this crust was won from the antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the explora wedge a refined kinematic break up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the africa antarctica corridor and the somali basin

the mozambique ridge a document of massive multistage magmatism the mozambique ridge a prominent basement high in the southwestern indian ocean consists of four major geomorphological segments associated with numerous phases of volcanic activity in the lower cretaceous the nature and origin of the mozambique ridge have been intensely debated with one hypothesis suggesting a large igneous province origin high resolution seismic reflection data reveal a large number of extrusion centres with a random distribution throughout the southern mozambique ridge and the nearby transkei rise intrabasement reflections emerge from the extrusion centres and are interpreted to represent massive lava flow sequences such lava flow sequences are characteristic of eruptions leading to the formation of continental and oceanic flood basalt provinces hence supporting a large igneous province origin of the mozambique ridge we observe evidence for widespread post sedimentary magmatic activity that we correlate with a southward propagation of the east african rift system based on our volumetric analysis of the southern mozambique ridge we infer a rapid sequential emplacement between 131 and 125 ma which is similar to the short formation periods of other large igneous provinces like the agulhas plateau

substorm changes of the electrodynamic quantities in the polar ionosphere cdaw 9 on the basis of ground magnetometer data from 75 northern hemisphere stations and the ionospheric conductivity distribution estimated from viking satellite observations of auroral images various electrodynamic quantities in the polar ionosphere are calculated for the april 1 1986 coordinated data analysis workshop cdaw 9 substorm since the scandinavia and russia chains of magnetometers were located in the premidnight midnight sector during this interval and the estimated conductivity distribution is instantaneous our data set provides us with a unique opportunity to examine some long standing problems associated with the substorm expansion onset several important findings of this study are summarized as follows 1 before the expansion onset of the substorm intensifications of ionospheric currents or the cross polar cap potential are very weak in this particular example both quantities begin to increase notably only with the initiation of the substorm expansion onset 2 the intensified westward electrojet flows along the poleward half of the enhanced ionospheric conductivity belt in the midnight sector during the expansion phase while its equatorward half is occupied by a weak eastward electrojet 3 the joule heating rate and the energy input rate of auroral particles are quite comparable preceding the expansion onset during the expansion phase of the substorm however joule heating shows a marked intensification but the latter increases only moderately indicating that the joule dissipation is more effective than auroral particle energy input during substorm times 4 the hall currents are not completely divergence free the corresponding field aligned currents show highly localized structures during the maximum epoch of the substorm with the upward current being located in the region of the steepest conductivity gradient on the poleward side of the westward electrojet in the midnight sector this is indirect evidence that the so called imperfect cowling channel is effective behind the westward traveling surge

statistical tests for closure of plate motion circuits the standard method of testing whether data describing the relative motion of three plates meeting at a triple junction are consistent with rigid plate tectonics uses the condition of plate circuit closure the euler vectors or angular velocities describing the relative motion of each plate pair must sum to zero deviations from closure suggest that the data are systematically in error or that the plate boundaries have been identified incorrectly or that the plates are non rigid we examine two methods one based on a chi square test and the other based on an f ratio test of testing whether plate motion circuits close we find the latter test is useful but the former is of limited value because of the common practice of systematically overestimating errors in plate motion data we apply the f ratio test to minster and jordan s data for the boundaries dividing the pacific cocos and nazca plates which prove consistent with plate circuit closure application of the test to their data for the boundaries dividing the australian antarctic and african plates shows nonclosure even after deletion of data from the carlsberg ridge which recently has been suggested to divide the african plate from an indo arabian plate this nonclosure suggests systematic errors in the data or internal deformation of one or more of the plates meeting at the indian ocean triple junction

finite amplitude folding of a continuously viscosity stratified lithosphere we study the development of finite amplitude folding of the lithosphere through the application of finite element models characterized by both discontinuous layer over half space and continuous strength envelope viscosity distributions both models include strongly strain rate dependent rheologies that approximate slip on pervasive faults folding with layers of form contrasting viscosities is driven solely bye magnitudes of discontinuities in viscosity at layer interfaces however folds may also develop in a medium with a continuously varying viscosity distribution the net driving force for folding in a region of continuous viscosity variation is the same as the driving force across a discontinuity with the same net viscosity contrast continuous and discontinuous viscosity variations give essentially identical fold growth rates if the depth over which the viscosity varies is small compared to the wavelength of folding or the depth of the viscosity variation in the uniform viscosity layer model flexural bending of the layer results in a state of extension on topographic highs and compression in topographic lows in contrast the strength envelope model is characterized by significant penetrative shortening near the surface that results in a state of compression everywhere along the fold this model predicts layer thickening beneath fold troughs as observed in the intmpladeformation zone in the central indian ocean buocy forces retard the rate of fold amplification but are inadequato prevent folding for a range of realistic lithospheric theological structures

mapping of the ionospheric field aligned currents to the equatorial magnetosphere the assimilative mapping of ionospheric electrodynamics amie derived ionospheric field aligned current patterns during the ninth coordinated data analysis workshop substorm event have been mapped to the magnetospheric equatorial plane using the data based magnetospheric magnetic field model tsyganenko 1995 we find that at the onset of the substorm expansion phase a pair of field aligned currents emerges about 8 re tailward of the earth near the x axis this pair of field aligned currents is consistent with the substorm current wedge model it lines up in the east west direction across the near earth tail current and has the sense of the nightside region 1 current i e flowing into the ionosphere on the east side and out of the ionosphere on the west side the ionospheric counterpart of the substorm wedge currents in the midnight sector on the other hand has a northeast to southwest alignment during the expansion phase the substorm wedge currents intensify and expand in spatial size however their peaks remain nearly at the same location in the equatorial magnetosphere about 8 re tailward from the earth we also find that the intensification of the substorm wedge currents during the expansion phase is accompanied by the intensification of the region 2 sense currents located closer to the earth and the boundary between the region 2 and substorm wedge currents lies around x 6 re

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

monitoring the earthquake source process in north america with the implementation of the usgs national earthquake information center prompt assessment of global earthquakes for response system pager rapid determination of earthquake moment magnitude is essential especially for earthquakes that are felt within the contiguous united states we report an implementation of moment tensor processing for application to broad seismically active areas of north america this effort focuses on the selection of regional crustal velocity models codification of data quality tests and the development of procedures for rapid computation of the seismic moment tensor we systematically apply these techniques to earthquakes with reported magnitude greater than 3 5 in continental north america that are not associated with a tectonic plate boundary using the 0 02 0 10 hz passband we can usually determine with few exceptions moment tensor solutions for earthquakes with m w as small as 3 7 the threshold is significantly influenced by the density of stations the location of the earthquake relative to the seismic stations and of course the signal to noise ratio with the existing permanent broadband stations in north america operated for rapid earthquake response the seismic moment tensor of most earthquakes that are m w 4 or larger can be routinely computed as expected the nonuniform spatial pattern of these solutions reflects the seismicity pattern however the orientation of the direction of maximum compressive stress and the predominant style of faulting is spatially coherent across large regions of the continent

gps constraints on africa nubia and arabia plate motions summary we use continuously recording gps cgps and survey mode gps sgps observations to determine euler vectors for relative motion of the african nubian arabian and eurasian plates we present a well constrained eurasia nubia euler vector derived from 23 igs sites in europe and four cgps and three sgps sites on the nubian plate 0 95 4 8 n 21 8 4 3 e 0 06 0 005 myr 1 we see no significant 1m m yr 1 internal deformation of the nubian plate the gps nubian eurasian euler vector differs significantly from nuvel 1a 21 0 4 2 n 20 6 0 6 e 0 12 0 015 myr 1 implying more westward motion of africa relative to eurasia and slower convergence in the eastern mediterranean the arabia eurasia and arabia nubia gps euler vectors are less well determined based on only one cgps and three sgps sites on the arabian plate the preliminary arabia eurasia and arabia nubia euler vectors are 27 4 1 0 n 18 4 2 5 e 0 40 0 04 myr 1 and 30 5 1 0 n 25 7 2 3 e 0 37 0 04 myr 1 respectively the gps arabia nubia euler vector differs significantly from nuvel 1a 24 1 1 7 n 24 0 3 5 e 0 40 0 05 myr 1 but is statistically consistent at the 95 per cent confidence level with the revised euler vector reported by chu gordon based on a re evaluation of magnetic anomalies in the red sea 31 5 1 2 n 23 0 2 7 e 0 40 0 05 myr 1 the motion implied in the gulf of aqaba and on the dead sea fault dsf by the new gps nubia arabia euler vector i e ignoring possible sinai block motion and possible internal plate deformation grades from pure left lateral strike slip in the gulf and on the southern dsf with increasing compression on the central and northern dsf with relative motion increasing from 5 6 to 7 5 mm yr 1 1m m yr 1 from south to north along the northern dsf i e north of the lebanon restraining bend motion is partitioned between 6 1m m yr 1 left lateral motion parallel to the fault trace and 4 1m m yr 1 faultnormal compression relative motions on other plate boundaries including the anatolian and aegean microplates derived from the gps euler vectors agree qualitatively with the sense of motion indicated by focal mechanisms for large crustal earthquakes m 6 where data are available on fault slip rates on plate bounding faults north anatolian fault east anatolian fault dead sea fault red sea rift they are generally lower than but not significantly different from the full plate motion estimates suggesting that the majority of relative plate motion is accommodated on these structures

east african earthquake body wave inversion with implications for continental structure and deformation summary source parameters for five african intraplate events are obtained from body wave inversion for the moment tensor parameters for the events are as follows location date mb strike dip rake depth ethiopia 4 05 69 5 8 327 1 69 2 45 0 4 zaire 3 20 66 6 3 44 6 44 9 81 5 10 zambia 12 2 68 5 9 20 3 40 2 264 8 6 zambia 5 15 68 5 7 49 1 39 8 263 2 28 s africa 9 29 69 5 6 123 5 89 4 0 58 4 the ethiopia and zaire events exhibit normal faulting as would be expected for their rift zone locations the 1968 may 15 zambia event is unusual in that p pp separation times place its focus in the lower crust 28 km which may be below the brittle ductile transition zone implying high strain rates inversion yielded a high angle normal fault the depth and focal mechanism of this event along with the mechanism for the 1968 december 2 zambia event suggest southwestward rift extension teleseismic long period vertical p waves from the south africa event exhibit distortions that may be attributed to lateral inhomogeneities in the source structure

historical seismicity and implications for diffuse plate convergence in the northeast indian ocean large historical earthquakes provide constraints on the geographical extent and kinematics of the slow diffuse plate boundary extending from the central indian ridge to the sumatra trench the fault parameters of pre 1963 earthquakes are derived using a grid search for the best fit between observed and synthetic body waveforms surface wave spectral amplitudes and polarity and polarization constraints and location uncertainties are determined using a bootstrap resampling method the march 9 1928 ninetyeast ridge m 7 7 event shows left lateral strike slip faulting along a plane approximately coincident with the orientation of the ninetyeast ridge two small events which occurred at the location of the 1928 event in 1980 show focal mechanisms nearly identical to that determined for the 1928 earthquake and apparently reactivated the same fault the 1928 waveforms can also be fit by a thrust faulting solution with ene striking fault planes but a variety of evidence including the recent strike slip earthquakes at the same location suggest the strike slip solution these earthquakes are located along the western scarp of the ridge and do not represent reactivation of the ninetyeast fracture zone to the east of the edifice which was a major plate boundary in the early cenozoic the january 23 1949 m 6 75 and march 22 1955 m 7 0 wharton basin earthquakes although only 350 km apart show different strike slip and thrust faulting mechanisms all earthquakes show a regionally consistent pattern of horizontal compressional stress which changes from nw se in the wharton basin to n s in the central indian basin the compressional axes are oriented perpendicular to linear gravity anomalies associated with lithospheric flexure indicating little change in stress orientation since the onset of folding seismic deformation along the diffuse boundary consists of sinistral strike slip faulting associated with n s features such as the ninetyeast ridge and the 86 e fracture zone and thrust or reverse faulting in regions between fracture zones assuming transform motion the approximate slip rate along the northern ninetyeast ridge calculated from the summed moments of earthquakes is 2 8 mm yr this value is an order of magnitude less than previous estimates and suggests that at most only a small fraction of the current relative plate motion 7 mm yr occurs along the ridge the geographic distribution of seismicity suggests that the wharton basin east of the ninetyeast ridge is nearly as seismically active as the central indian basin to the west this result along with observed long wavelength gravity anomalies in the wharton basin indicates that the india australia plate boundary intersects the sumatra trench along a diffusely deforming region several thousand kilometers long rather than along a discrete transform zone

african stress pattern from formal inversion of focal mechanism data abstract the kinematic models and the associated orientation of extensional stress of the east african rift system have been subjected to much debate since a long time in the past decades the proposed models relied on the interpretation of the overall rift geometry geological fault slip data and the few focal mechanisms available these models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space in the recent years there has been a significant increase in the number of focal mechanisms available for the entire rift system and it is now possible to estimate the present day stress field in relative detail based on seismotectonic data alone we compile 347 focal mechanism data from the global harvard cmt catalogue and various other sources and grouped 332 of them in 24 distinct regions boxes on the basis of their geographical proximity kinematic homogeneity and tectonic setting for each box and for the same data set reduced stress tensors have been obtained by formal stress inversion using both the tensor program delvaux sperner 2003 and the slick method michael 1984 1987 both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes which stem for the robustness of the approach the obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces including plate boundary forces and 2nd and 3rd order effects as gravitational potential of topography intra lithospheric processes and the influence of structural heterogeneities of the rift structures the evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns

long aftershock sequences within continents and implications for earthquake hazard assessment one of the most powerful features of plate tectonics is that the known plate motions give insight into both the locations and average recurrence interval of future large earthquakes on plate boundaries plate tectonics gives no insight however into where and when earthquakes will occur within plates because the interiors of ideal plates should not deform as a result within plate interiors assessments of earthquake hazards rely heavily on the assumption that the locations of small earthquakes shown by the short historical record reflect continuing deformation that will cause future large earthquakes here however we show that many of these recent earthquakes are probably aftershocks of large earthquakes that occurred hundreds of years ago we present a simple model predicting that the length of aftershock sequences varies inversely with the rate at which faults are loaded aftershock sequences within the slowly deforming continents are predicted to be significantly longer than the decade typically observed at rapidly loaded plate boundaries these predictions are in accord with observations so the common practice of treating continental earthquakes as steady state seismicity overestimates the hazard in presently active areas and underestimates it elsewhere

stress triggering and the canterbury earthquake sequence the canterbury earthquake sequence which includes the devastating christchurch event of 2011 february has to date led to losses of around 40 billion nz dollars the location and severity of the earthquakes was a surprise to most inhabitants as the seismic hazard model was dominated by an expected mw 8 earthquake on the alpine fault and an mw 7 5 earthquake on the porters pass fault 150 and 80 km to the west of christchurch the sequence to date has included an mw 7 1 earthquake and 3 mw 5 9 events which migrated from west to east here we investigate whether the later events are consistent with stress triggering and whether a simple stress map produced shortly after the first earthquake would have accurately indicated the regions where the subsequent activity occurred we find that 100 per cent of m 5 5 earthquakes occurred in positive stress areas computed using a slip model for the first event that was available within 10 d of its occurrence we further find that the stress changes at the starting points of major slip patches of post darfield main events are consistent with triggering although this is not always true at the hypocentral locations our results suggest that coulomb stress changes contributed to the evolution of the canterbury sequence and we note additional areas of increased stress in the christchurch region and on the porters pass fault

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

cumulative coulomb stress triggering as an explanation for the canterbury new zealand aftershock sequence initial conditions are everything using 2 years of aftershock data and three fault plane solutions for each of the initial m7 1 darfield earthquake and the larger m 6 aftershocks we conduct a detailed examination of coulomb stress transfer in the canterbury 2010 2011 earthquake sequence moment tensor solutions exist for 283 of the events with m ge 3 6 while 713 other events of m ge 3 6 have only hypocentre and magnitude information available we look at various methods for deciding between the two possible mechanisms for the 283 events with moment tensor solutions including conformation to observed surface faulting and maximum delta cff transfer from the darfield main shock for the remaining events imputation methods for the mechanism including nearest neighbour kernel smoothing and optimal plane methods are considered fault length width and depth are arrived at via a suite of scaling relations a large 50 70 proportion of the faults considered were calculated to have initial loading in excess of the final stress drop the majority of faults that accumulated positive delta cff during the sequence were encouraged by the main shock failure but on the other hand of the faults that failed during the sequence more than 50 of faults appeared to have accumulated a negative delta cff from all preceding failures during the sequence these results were qualitatively insensitive to any of the factors considered we conclude that there is much unknown about how coulomb stress triggering works in practice

a statistical relationship between the geosynchronous magnetic field and substorm electrojet magnitude in this paper the authors examine the relationship between geosynchronous magnetic field variations during substorms measured by goes 5 and the auroral electrojet as measured by ae and poste de la baleine as in previous studies the authors find that the more taillike the field prior to the local onset the greater the dipolarization of the field during the substorm they also find that the greater the deviation of the field from a dipolar configuration the larger the change in ae during the event this implies that stronger cross tail currents prior to the substorm are associated with larger substorm associated westward electrojets and thus more intense substorms previous work has shown that in order to produce the observed taillike fields at geosynchronous altitude the intense cross tail current that builds up during the growth phase must be localized in the near earth le 10 r sub e region since the westward electrojet is the ionospheric leg of the substorm current wedge this result implies that the substorm associated westward electrojet is drawn from the near earth region in fact the authors find that most of the current diversion occurs in the near earth magnetotail furthermore they estimate that a diversion about half of the near earth cross tail currentmore can account for the current in the northern and southern westward electrojets associated with the substorm current wedge 25 refs 9 figs less

the motion and boundary between the capricorn and australian plates the motions between the somalian antarctic and australian plates the three plates believed to meet at the rodrigues triple junction in the indian ocean are inconsistent with the assumption that all three plates are rigid the discrepancy is best explained if the australian plate contains two component plates thus the traditionally defined indo australian plate consists of three component plates and multiple diffuse plate boundaries the pattern of present deformation indicates that the boundaries between the three component plates are two unconnected zones accommodating divergence and a larger zone which we interpret as three diffuse convergent plate boundaries and a diffuse triple junction

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

current plate motions across the red sea summary a new objective method is used to estimate precisely 64 rates of seafloor spreading since chron 2a 3 2 ma from all the red sea magnetic profiles available from 15 9 to 26 n the fastest spreading rate 16 mm yr 1 occurs near 18 n whereas the slowest rate 10 mm yr 1 occurs at 25 5 n and is consistent with the rate predicted from arabia nubia data to the south the standard deviation of the spreading rates is 0 8 mm yr 1 much smaller than the median standard deviation of 4 mm yr 1 previously assigned to spreading rates in the global plate motion model nuvel 1 demets et al 1990 the fit to the spreading rates as well as the locations of earthquakes in and near the red sea indicate that spreading south of approximately 17 7 n is less than the full rate of spreading between the arabian and nubian plates the red sea spreading centre is instead usefully interpreted as the boundary between the arabian plate and a danakil microplate that includes the subaerial danakil block and a larger oceanic portion of lithosphere despite the absence of reliable azimuths of transform faults in the red sea all components of the angular velocity of arabia relative to nubia are usefully constrained from just the 45 relevant spreading rates the new compact 95 per cent confidence region of the angular velocity excludes prior estimates based on only four and two spreading rates by chase 1978 and jestin et al 1994 respectively 19 spreading rates in the southern red sea are used to estimate the angular velocity between the danakil microplate and arabia an approach based on singular value decomposition shows that without slip vectors only two of the three components of the angular velocity of the danakil microplate relative to arabia or nubia are usefully constrained but that all three components are usefully constrained if one earthquake slip vector is included

seasat derived gravity constraints on stress and deformation in the northeastern indian ocean we use seasat derived gravity data to investigate crustal deformation in the ne indian ocean gravity highs reflecting crustal undulations vary in orientation from e w in the central indian basin to ne sw in the wharton basin the undulations vary in trend similarly to the variation of the maximum compression directions predicted by a plate driving force model cloetingh and wortel 1985 and are essentially restricted to the area of predicted compression for both principal horizontal stresses this agreement implies that the stress model describes the basic features of the deformation observed in the gravity as well as the seismicity the gravity data also provide insight into two enigmatic tectonic features the transition in the morphology of the 90 e ridge at 10 s from a continuous high to a complex blocky structure appears related to the deformation since undulations in the central indian basin can be projected eastward to blocks on the ridge the morphology formerly interpreted as a fossil feature may reflect the recent deformation the discrepancy between the trends of the southernmost 85 e ridge and the 90 e ridge previously thought to exclude similar hot spot track origins appears to result from treating a crustal undulation as part of the 85 e ridge earthquakes do not appear to be preferentially located with respect to the peaks and troughs of the undulations

new angles on south atlantic opening existing models of relative motion between the south america and africa plates significantly misrepresent the azimuth of lower cretaceous seafloor spreading in the south atlantic ocean an improved model is derived from fits of fracture zones magnetic reversal anomalies and the edges of a rifted large igneous province at the northeast georgia rise and agulhas plateau an absolute date for this rifting event is not known but by assuming least change in spreading rates during magnetic chron c34 rifting can be estimated to have occurred at 100 ma this modelling demands a plate divergence history that involves diachronous opening of the south atlantic consistent with published estimates of the ages of breakup from sedimentary basins on the south american and african passive margins the diachronous opening lasts approximately 40 million years during which time it must be accommodated by significant intracontinental deformation a reconstruction using the intra c34 rotation also illustrates the earliest possibility for direct deep water connection between the central and south atlantic oceans one further consequence of this model is that a total reconstruction derived from it and closure of the central atlantic between north america and africa suggests that the venezuelan and gulf of mexico basins may be conjugates formed during the earliest opening of the central atlantic and not separate marginal basins

a revised fit of south america and south central africa abstract a new set of parameters for the total plate tectonic reconstruction of south america and south central africa is presented euler pole 46 75 n 32 65 w rotation angle 56 40 this fit is constrained by at least three pre drift tectonic features crossing from one continent to the other 1 the geophysically defined eastern and western boundaries of the submarine jurassic outeniqua basin south africa and the falkland plateau basin 2 the late precambrian transcurrent fault and mylonite belts of pernambuco brazil and foumban west africa and 3 the triassic northern tectonic front of the cape fold belt and the major morphological feature on the falkland plateau with which it is closely lined up isotopic ages of falkland plateau gneisses correspond to cape pluton and cape fold belt ages suggesting their palaeoposition was within the realm of the cape fold belt in addition the bathymetrically and geophysically defined northeastern apex of the falkland plateau fits into the re entrant angle defined on the south african margin by the steep southeast facing sheared agulhas margin and the southern face of the tugela cone simultaneously known precambrian outcrops in northeastern brazil and in the gulf of benin area of west africa are juxtaposed rather than overlapped reconstructions producing a closer fit of these cratonic areas are considered untenable

present day motion along the owen fracture zone and dalrymple trough in the arabian sea all plate motion data available for describing the present day motion between the arabian and indian plates are analyzed to understand the present day tectonic role of the presumed plate boundary the owen fracture zone and dalrymple trough in the northern arabian sea all prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary in particular published global plate motion models predict some convergence across the owen fracture zone and dalrymple trough convergence along the owen fracture zone is contradicted by earthquake focal mechanisms which show right lateral strike slip convergence along the dalrymple trough is contradicted by observed normal faulting in the trough and by normal and strike slip focal mechanisms for earthquakes occurring along or near the trough the sense and rate of motion is further constrained by spreading rates along the sheba ridge northwest of the owen fracture zone and along the carlsberg ridge southeast of the owen fracture zone to estimate the present day motion we determine 43 3 m y average spreading rates from sheba ridge carlsberg ridge and central indian ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the arabia india boundary we determine an arabia india euler vector that predicts 2 mm yr of right lateral strike slip motion along the owen fracture zone with a 95 confidence interval of 0 7 mm yr which excludes the faster rates of slips predicted by several prior models the direction of motion is 35 50 counterclockwise from that predicted by published global plate motion models if we omit the data along the arabia india boundary the motion differs insignificantly from zero suggesting that magnetic profiles from the carlsberg and sheba ridges give spreading rates too imprecise to detect the slow arabia india motion if the owen fracture zone and dalrymple trough are regarded as a plate boundary it is the slowest slipping plate boundary known on earth with motion about 100 times slower than the fastest slipping faults along the east pacific rise

isostatic response of the lithosphere beneath the mozambique ridge sw indian ocean and geodynamic implications summary the sw indian ocean is characterized by the presence of several aseismic features the mozambique ridge an elongated feature lying roughly parallel to the se coast of africa is by far the least known of those structures mainly due to the scarcity of marine data to date the crustal nature and the origin of the ridge are still controversial points since knowledge of the origin of the mozambique ridge is important for a better understanding of the evolution of the sw indian ocean the isostatic response of the lithosphere beneath the ridge is analysed in order to characterize its effective elastic thickness and the emplacement process of the feature two different approaches are applied the direct computation of the geoid anomaly over the ridge by means of a 2 5 dimensional method and the computation of the admittances between the bathymetry and both the geoid and gravity anomalies both approaches point to a local isostatic response of the lithosphere the crustal thickness beneath the mozambique ridge ranges from 17 to 30 km and the average density from 2 78 to 2 7 103 kg m 3 varying with locality in good agreement with the few existing refraction profiles based on our results on the geochemical similarity between the basalts cored at the dsdp site 249 and the mor basalts erlank reid 1974 thompson et al 1982 and on the present knowledge of the sw indian ocean kinematics martin hartnady 1986 we propose an on ridge origin for the mozambique ridge the ridge would have been formed by the anomalous activity of a spreading axis linking the northern mozambique and the transkei basins accretion centres between m10 and m2 times at the m2 epoch a ridge jump event would have caused the spreading to cease the astrid ridge a poorly known aseismic structure lying close to the coast of antarctica near 15 e may be the antarctic counterpart of the mozambique ridge formed by the same accretionary phenomenon the two ridges breaking apart at m2 time due to the ridge jump event however any admittance computation at such high latitudes is impossible at present due to the very poor quality of both bathymetry and altimetry and so this hypothesis cannot be verified

tobi sidescan sonar imagery of the very slow spreading southwest indian ridge evidence for along axis magma distribution new deep tow sidescan sonar data from the southwest indian ridge reveal complex volcanic tectonic interrelationships in the axial zone of this ultra slow spreading ridge while some constructional volcanic features resemble examples documented at the slow spreading mid atlantic ridge such as axial volcanic ridges hummocky and smooth lava flows their distribution and dimensions differ markedly the largest axial volcanic ridges occur at segment centres but fresh looking volcanic constructions also occur at segment ends and in the deep basins marking the non transform discontinuities the orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes this observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes with associated fractures opening in response to magma pressure may control local emplacements of axial volcanic ridges at obliquely spreading ridges this non systematic pattern of ridge characteristics suggests an along axis variation between focused and distributed magmatic supply a model which is supported by our interpretation of low amplitude mantle bouguer anomalies calculated for the area we propose that a change of the axial segmentation pattern from two segments to the present day three segments may have introduced additional instability into the crustal accretion process

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

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos

the 2010 2011 canterbury earthquake sequence environmental effects seismic triggering thresholds and geologic legacy abstract seismic shaking and tectonic deformation during strong earthquakes can trigger widespread environmental effects the severity and extent of a given effect relates to the characteristics of the causative earthquake and the intrinsic properties of the affected media documentation of earthquake environmental effects in well instrumented historical earthquakes can enable seismologic triggering thresholds to be estimated across a spectrum of geologic topographic and hydrologic site conditions and implemented into seismic hazard assessments geotechnical engineering designs palaeoseismic interpretations and forecasts of the impacts of future earthquakes the 2010 2011 canterbury earthquake sequence ces including the moment magnitude m w 7 1 darfield earthquake and m w 6 2 6 0 5 9 and 5 8 aftershocks occurred on a suite of previously unidentified primarily blind active faults in the eastern south island of new zealand the ces is one of earth s best recorded historical earthquake sequences the location of the ces proximal to and beneath a major urban centre enabled rapid and detailed collection of vast amounts of field geospatial geotechnical hydrologic biologic and seismologic data and allowed incremental and cumulative environmental responses to seismic forcing to be documented throughout a protracted earthquake sequence the ces caused multiple instances of tectonic surface deformation 3 events surface manifestations of liquefaction 11 events lateral spreading 6 events rockfall 6 events cliff collapse 3 events subsidence 4 events and hydrological 10s of events and biological shifts 3 events the terrestrial area affected by strong shaking e g peak ground acceleration pga 0 1 0 3 g and the maximum distances between earthquake rupture and environmental response r rup both generally increased with increased earthquake m w but were also influenced by earthquake location and source characteristics however the severity of a given environmental response at any given site related predominantly to ground shaking characteristics pga peak ground velocities and site conditions water table depth soil type geomorphic and topographic setting rather than earthquake m w in most cases the most severe liquefaction rockfall cliff collapse subsidence flooding tree damage and biologic habitat changes were triggered by proximal moderate magnitude m w 6 2 earthquakes on blind faults ces environmental effects will be incompletely preserved in the geologic record and variably diagnostic of spatial and temporal earthquake clustering liquefaction feeder dikes in areas of severe and recurrent liquefaction will provide the best preserved and potentially most diagnostic ces features rockfall talus deposits and boulders will be well preserved and potentially diagnostic of the strong intensity of ces shaking but challenging to decipher in terms of single versus multiple events most other phenomena will be transient e g distal groundwater responses not uniquely diagnostic of earthquakes e g flooding or more ambiguous e g biologic changes preliminary palaeoseismic investigations in the ces region indicate recurrence of liquefaction in susceptible sediments of 100 to 300 yr recurrence of severe rockfall event s of ca 6000 to 8000 yr and recurrence of surface rupturing on the largest ces source fault of ca 20 000 to 30 000 yr these data highlight the importance of utilising multiple proxy datasets in palaeoearthquake studies the severity of environmental effects triggered during the strongest ces earthquakes was as great as or equivalent to any historic or prehistoric effects recorded in the geologic record we suggest that the shaking caused by rupture of local blind faults in the ces comprised a worst case seismic shaking scenario for parts of the christchurch urban area moderate m w blind fault earthquakes may contribute the highest proportion of seismic hazard be the most important drivers of landscape evolution and dominate the palaeoseismic record in some locations on earth including locations distal from any identified active faults a high scientific priority should be placed on improving the spatial extent and quality of off fault shaking records of strong earthquakes particularly near major urban centres