propagation of rifting along the arabia somalia plate boundary into afar it is generally accepted that the aden ridge has propagated westward from 58 e to the western tip of the gulf of aden tadjoura at the edge of afar here we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the republic of djibouti we show that it veers northward forming a narrow zone of dense faulting along the northeastern edge of the afar depression the zone includes two volcanic rifts asal ghoubbet and manda inakir connected to one another and to the submarine part of the ridge by transfer zones both rifts are composite divided into two or three disconnected parallel nw se striking subrifts all of which appear to have propagated northwestward in asal ghoubbet as in manda inakir the subrifts appear to have formed in succession through north directed jumps from subrifts more farther south at present the northernmost subrifts manda and dirko koma of the manda inakir rift form the current tip of the northward propagating arabia somalia plate boundary in afar we account for most observations by a mechanical model similar to that previously inferred for the gulf of aden in which propagation is governed by the intensity and direction of the minimum horizontal principal stress 3 we interpret the northward propagation on land almost orthogonal to that in the gulf to be related to necking of the central afar lithosphere where it is thinnest such necking may be a consequence of differential magmatic thickening greater in the center of the afar depression where the ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression the model explains why the aden ridge foregoes its wsw propagation direction constant from 58 e to asal ghoubbet at a smaller scale individual rifts and subrifts keep opening perpendicular to the arabia somalia or danakil somalia motion vector and propagate northwestward concurrently such lithospheric cracks are forced to jump northward such that the plate boundary remains inside the regional n s necking zone changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns a small angle promoting long en echelon subrifts and a high angle smaller nested subrifts within subrifts the propagation mechanism is thus similar whether in oceanic or continental lithosphere the principal change being the overall propagation path here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the aden ridge finally because the same mechanism has led rifting along the red sea to propagate southward and jump to the western edge of afar the arabia somalia and arabia nubia plate boundaries tips have missed each other and keep overlapping further leading to strain transfer by large scale bookshelf faulting

laboratory earthquakes the sub rayleigh to supershear rupture transition we report on the experimental observation of spontaneously nucleated supershear rupture and on the visualization of sub rayleigh to supershear rupture transitions in frictionally held interfaces the laboratory experiments mimic natural earthquakes the results suggest that under certain conditions supershear rupture propagation can be facilitated during large earthquake events

early and persistent supershear rupture of the 2018 magnitude 7 5 palu earthquake the speed at which an earthquake rupture propagates affects its energy balance and ground shaking impact dynamic models of supershear earthquakes which are faster than the speed of shear waves often start at subshear speed and later run faster than eshelby s speed here we present robust evidence of an early and persistent supershear rupture at the sub eshelby speed of the 2018 magnitude 7 5 palu indonesia earthquake slowness enhanced back projection of teleseismic data provides a sharp image of the rupture process along a path consistent with the surface rupture trace inferred by subpixel correlation of synthetic aperture radar and satellite optical images the rupture propagated at a sustained velocity of 4 1 km s 1 from its initiation to its end despite large fault bends the persistent supershear speed is further validated by seismological evidence of far field rayleigh mach waves the unusual features of this earthquake probe the connections between the rupture dynamics and fault structure an early supershear transition could be promoted by fault roughness near the hypocentre steady rupture propagation at a speed unexpected in homogeneous media could result from the presence of a low velocity damaged fault zone

angular velocities of nubia and somalia from continuous gps data implications on present day relative kinematics abstract this study focuses on the break up of the african tectonic plate into separate nubian and somalian blocks based on recent global positioning system gps data a new unique velocity field has been obtained by processing all available observations of permanent gps stations on africa since 1996 the quantity and distribution of the stations and the length of the time series of observations exceed that of previous studies by a considerable margin allowing one to derive a reliable estimate of the differential motion between the nubia and somalian plates which are considered as a single african block in the prevailing global tectonic plate models the estimated relative pole of rotation of somalia with respect to nubia is located at 54 8 s 37 0 e with magnitude 0 069 ma implying distinct opening in the ethiopian rift of magnitude 7 mm year and azimuth n94 e whereas in southeastern south africa this value is reduced to 2 mm year in almost the same direction this is in accordance with some of the independent geological and geophysical tectonic models of the nubia somalia plate boundary region however the spatial density of the current tracking network is still not optimal to establish the exact location of the entire somalia nubia plate boundary in particular the possible branch east of lake victoria and heading towards the mozambique channel is impossible to confirm or reject at this moment

nearfield vertical displacement in the creeping segment of the san andreas fault central california 1975 to 1994 abstract nearfield precise spirit leveling reveals progressive aseismic height changes in the creeping segment of the san andreas fault between gold hill in cholame valley and san juan bautista over time periods from 8 to 18 years height changes are greatest at each end of the creeping segment whereas the rate of horizontal creep is greatest in the center of the creeping segment both vertical and horizontal displacements take place at the surficial trace of the fault whereas surficial downwarping is evident as much as 500 m from the mapped fault trace the large height changes at san juan bautista parkfield and gold hill may be related to compaction induced subsidence where groundwater is pumped from unmonitored irrigation wells on one side of the fault at san juan bautista however the water table has remained nearly constant according to local farmers even though the height change there is the greatest among the six sites therefore it is concluded here that the vertical displacements are tectonic and result from local deviations of the direction of fault slip relative to the far field slip vector or alternatively to deceleration of horizontal creep and dissipation of horizontal strain into the transition zones between the end of the creeping fault segment and the adjacent locked segments

a supershear transition mechanism for cracks seismic data indicate that fault ruptures follow complicated paths with variable velocity because of inhomogeneities in initial stress or fracture energy we report a phenomenon unique to three dimensional cracks locally stronger fault sections rather than slowing ruptures drive them forward at velocities exceeding the shear wave speed this supershear mechanism differentiates barrier and asperity models of fault heterogeneity which previously have been regarded as indistinguishable high strength barriers concentrate energy producing potentially destructive pulses of strong ground motion

dip and depth extent of density boundaries using horizontal derivatives of upward continued gravity data a new computer method was developed to derive values of dip and vertical extent as well as location for the truncated face of a horizontal plate model the method is based on lateral offsets of the zero crossover point of the second horizontal derivative of an upward continued gravity profile application requires that the distance to the upper surface of the model be known numerical tests indicate useful results are obtained when both the length of the gravity profile and the finite strike length of a model are at least three times the maximum depth extent of the model gaussian noise with a standard deviation of 1 mgal can be tolerated given an anomaly amplitude of 70 mgal an example from the trans hudson orogen of central manitoba canada indicates that the kisseynew metasedimentary gneisses extend over a vertical distance of 16 18 km and are separated from the baldock batholith to the north by a near vertical boundary

3 d inversion of deep tow magnetic data on the atlantis ii deep red sea hydrothermal and geodynamic interpretation abstract to study the magnetic signature of hydrothermal oceanic crust areas data were collected using a magnetometer towed a few hundred meters above the sea floor within an acoustic transponder net the experiment was performed in the red sea over the atlantic ii deep near 21 20 n 38 05 e this deep created during the brunnes epoch is well known for its intense hydrothermal activity and the presence of hot brines overlying metalliferous sediments the magnetic field was inverted to obtain the distribution pattern of crustal magnetization including the effects of topography and irregular depth of the magnetic sensor the procedure however does not produce any unique solution it is approximated using probabilistic linear methods tarantola and valette 1982 which require the introduction of constraints in the form of gaussian probability density functions the resultant magnetization pattern then follows a gaussian law the mean and standard deviation are then computed the results show contrary to what has been suggested formerly rona 1978a b that the strongest magnetization as well as the most frequent hydrothermal and magmatic occurrences are found in the southwest basin the high magnetic intensities are thus associated with a recent magmatism which initiates the hydrothermal circulation two hypotheses explain this 1 1 the freshness of the basalts and the low temperature alteration are the principal causes of magnetization contrasts and represent respectively the discharge and recharge zones of the hydrothermal plumbing 2 2 in the general framework of the red sea opening by progressive tearing the formation of the atlantis ii was extremely fast according to the elastic rebound theory reid 1911 this hypothesis is applicable to some other deeps

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked

three dimensional inversion of marine magnetic anomalies on the equatorial atlantic ridge st paul fracture zone delayed magnetization in a magmatically starved spreading center the st paul fracture zone fz in the equatorial atlantic is interrupted by three intratransform ridge itr spreading centers a detailed magnetic survey corrected for the diurnal variations using a moored magnetic station six submersible dives and three bottom towed video camera tracks provide data on the most eastern itr 0degrees37 n 25degrees27 w visual observations and submersible sampling displayed a high ultramafic volcanic ratio supporting the assumption that the itr is in a magmatically starved state volcanics were mainly found on the rift valley floor from 4700 to 4000 m and as a thin cap 160 m on the top of the eastern rift crest 2700 m most of the rift walls consist essentially of serpentinized peridotites and gabbros the magnetic data show a well defined ridge centered anomaly a generalized inversion method was applied to the field data to calculate the crustal equivalent magnetization assuming that the seafloor is broken down into elementary cells of 1x1x0 5 km 3 which fit the topography the average of absolute value of equivalent magnetization is 2 7 a m 1 the width of the central normal polarity brunhes epoch is wider at least 34 km than that indicated by the nuvel 1 kinematic model 24 5 km this 40 excess is believed to be significant and is thought to be the result of prolonged chemical remanent magnetization acquired during the serpentinization of peridotites in a magmatically starved accretion segment we suggest that peridotites could continue to acquire magnetization as long as tectonic activities facilitate the circulation of seawater in the upper mantle

confirmation of arabia plate slow motion by new gps data in yemen 1 during the last 10 years a network of about 30 gps sites was measured in djibouti east africa additional points were also measured in yemen oman ethiopia iran and on la reunion island merged with data from the available international gps service permanent stations scattered on the different plates in the area eurasia anatolia africa arabia somalia this unique data set provides new insight on the current deformation in the africa somalia arabia triple junction area and on the arabian plate motion here we show that coherent motions of points in yemen bahrain oman and iran allow us to estimate a geodetically constrained angular velocity for the arabian plate 52 59 n 15 74 w 0 461 myr in itrf2000 this result differs significantly from earlier determinations and is based upon our vectors in yemen they provide new additional data and better geometry for angular velocity determination combined with the african and somalian motions this new angular velocity results in predicted spreading rates in the red sea and the gulf of aden which are 15 20 lower than those measured from oceanic magnetic anomalies and thus averaged over the last 3 myr with respect to eurasia the geodetic motion of arabia is also about 30 slower than predicted by nuvel 1a on the basis of the kinematic results presented here and on other evidence for a similar slower geodetic rate of the indian plate we suggest that the whole collision zone between africa arabia india on one hand and eurasia on the other hand has slowed down in the last 3 myr

dynamically modeling fault step overs using various friction laws it is well known that fault step overs can under some circumstances allow through going rupture and under other circumstances cause rupture to terminate however the effects of different friction law formulations on jumping rupture have not been extensively explored in this study we use the 2 d dynamic finite element method to investigate how different frictional parameterizations affect the ability of rupture to jump a step over in both extensional and compressional settings we compare linear slip weakening friction and three forms of rate and state dependent friction the aging law slip law and slip law with strong rate weakening we have found that for friction parameterizations with the same effective slip weakening distance the functional forms of such friction laws can have a significant effect on maximum jump distance with friction laws scaled to have equivalent fracture energies we find that the functional forms of such friction laws have a second order effect on jumping rupture finally with our specific parameterizations we find that delays in rupture across the step over systems can lead to a previously unseen mode of supershear transition once the rupture renucleates on the secondary fault segment even if the initial stress field precludes such a transition studies using multiple friction laws in complex geometries such as fault step overs can lead to better understanding of the dependence of rupture properties on the type of friction law utilized in models and statistical analysis

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

transient deformation in the asal ghoubbet rift djibouti since the 1978 diking event is deformation controlled by magma supply rates the asal ghoubbet rift ag rift in djibouti lies in the subaerial continuation of the aden ridge system thereby constituting a unique location to study rifting processes and mechanisms involved in continental breakup and oceanic spreading continually upgraded and expanded geodetic technology has been used to record the 1978 asal rifting event and postdiking deformation in light of recent results obtained for the manda hararo dabbahu rifting event 2005 2010 we propose that the horizontal and vertical geodetic data can be modeled with a double source involving a dike like inflation component aligned along the rift axis and a spherical pressure source located at midsegment below the fieale caldera by revisiting the codiking data we propose that the reservoir below fieale could have fed at least partially the 1978 injection and the contemporaneous ardoukoba eruption and potentially induced local subsidence due to magma draining out of the central reservoir as an alternative to previously proposed viscoelastic relaxation models we reinterpret postdiking observations using a purely elastic rheology we determine the relative contribution of a midsegment reservoir inflation and a dike like opening component together with their respective time evolutions our results suggest that interactions between steadily accumulating tectonic strain and temporal variations in melt supply to the shallow magma plumbing system below the ag rift may entirely explain the geodetic observations and that viscoelastic deformation processes played a minor role in the 30 years following the 1978 rifting event

tectonic implications of the 1989 afar earthquake sequence in august 1989 a sequence of m b 5 6 normal faulting earthquakes occurred in the area between the ghoubbet asal manda inakir and manda haroro abhe bad rifts in central afar relocation of the events suggest they occurred on major faults in the dobi graben located midway between the principal rift zones the earthquakes are evidence that extensional strain is still occurring in the area the bookshelf tectonic model for central and southern afar proposed by tapponnier et al 1990 is modified to account for ongoing extension in the area between the principal rifts as well as distributed shear

tectonic evolution of the northern african margin in tunisia from paleostress data and sedimentary record abstract a reconstruction of the tectonic evolution of the northern african margin in tunisia since the late permian combining paleostress tectonic stratigraphic and sedimentary approaches allows the characterization of several major periods corresponding to consistent stress patterns the extension lasting from the late permian to the middle triassic is contemporaneous of the rifting related to the break up of pangea during liassic times regional extensional tectonics originated the dislocation of the initial continental platform in northern tunisia the evolution of the liassic ne sw rifting led during dogger times to the north african passive continental margin whereas in southern tunisia a n s extension associated with e w trending subsiding basins lasted from the jurassic until the early cretaceous after an upper aptian early albian transpressional event ne sw to ene wsw trending extensions prevailed during late cretaceous in relationship with the general tectonic evolution of the northeastern african plate the inversions started in the late maastrichtian paleocene in northern tunisia probably as a consequence of the africa eurasia convergence two major nw se trending compressions occurred in the late eocene and in the middle late miocene alternating with extensional periods in the eocene oligocene early middle miocene and pliocene the latter compressional event led to the complete inversion of the basins of the northwestern african plate originating the maghrebide chain such a study supported by a high density of paleostress data and including complementary structural and stratigraphic approaches provides a reliable way of determining the regional tectonic evolution

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

geodetic and tectonic analyses along an active plate boundary the central gulf of california the gulf of california is traversed by the shear plate boundary between pacific and north american plates and because of several islands in its central part offers the possibility of direct geodetic measurements of plate motion a geodetic network of 150 km aperture and comprising 11 stations was measured in 1982 and 1986 by laser trilateration methods the deformations deduced from the comparison of the two epochs indicate right lateral shear strain covering the entire gulf rather than localized movements in the eastern part of the network between the axial islands and the sonoran coast significant right lateral shear deformation occurs with a relative displacement of about 23 12 cm over 4 years in the northwestern region canal de ballenas a right lateral displacement of about 17 4 cm is observed whereas in the southwestern part of the network canal sal si puedes the deformation remains very weak this suggests that south of the canal de ballenas the plate boundary is locked a tectonic analysis of neogene and quaternary faults in baja california sonora and the central islands of the gulf permitted the reconstruction of the stress pattern evolution of this area these data also indicate the predominance of right lateral motion on a nw se trending zone within a regional framework characterized by an approximately n s compression and an e w extension the geodetic results are discussed in comparison with the neotectonic analysis and the seismic data available in the area the data suggest a broad strain accumulation zone covering the totality of the central gulf of california a nw se relative velocity of about 8 3 cm yr is found between the two sides of the gulf during the 1982 1986 interval

first epoch geodetic gps measurements across the afar plate boundary zone in november 1991 a geodetic network was installed across the actively rifting plate boundary zone between africa and arabia using the microwave signals transmitted by the satellites of the global positioning system gps the relative positions of 37 stations are estimated with an average wrms scatter of 12 or 14 mm for lines shorter or longer than 100 km respectively assuming that the measurements can be repeated with the same precision in 1999 we expect to obtain horizontal uncertainties of 2 1 or 2 4 mm yr in the rate of change in the short and long lines respectively this level of precision will tightly constrain geological models for the spatial distribution of deformation around the rift in djibouti and for the far field plate motions surrounding the afar depression

seismotectonics in the republic of djibouti linking the afar depression and the gulf of aden abstract the arta seismological array around the gulf of tadjurah monitored earthquake activity for over 15 years epicentral locations and focal mechanisms are compared with tectonic evidence from surface breaks traces of active faults and the spatial distribution of swarms of aftershocks strike slip motion on near vertical faults predominates in the gulf left lateral strike slip on n150 e oriented faults is suggested at variance with the emphasis in previous models on either 1 a continuous plate boundary with spreading centres connected by ene trending right lateral transform faults 2 a propagating rift oriented approximately east west or 3 a few broad zones of extension between n150 e normal faults left lateral strike slip motion occurring on a series of parallel faults results in clockwise rotation of the blocks between the faults the type of deformation has similarities to that further west but is not obviously controlled by a distinct pair of overlapping rifts as proposed inland by tapponnier et al 1990 it is suggested that the stresses induced by the northeasterly drift of arabia alone may deform the southern part of the more easterly oriented zone of lithosphere derived of plume and ridge which separates arabia from africa

structural transect of the western transverse ranges california implications for lithospheric kinematics and seismic risk evaluation a retrodeformable cross section that integrates surface and subsurface data across the western transverse ranges california illustrates an actively developing fold and thrust belt that began forming at 2 3 ma high level thrusts are interpreted to root in a mid crustal detachment at 12 15 km depth which coincides with the maximum depth of earthquakes the cross section documents 53 km of convergence above the mid crustal detachment dividing this by the time since onset of deformation yields convergence rates of 17 6 26 5 mm yr the high density lithospheric anomaly beneath the transverse ranges is related to subduction of lower crust and lithosphere below the mid crustal detachment to balance the shallow crustal shortening thrust ramps coincide with zones of high seismicity in the transverse ranges this suggests that ramp regions have the highest potential for compressive earthquake events e g the recent whittier narrows earthquake of october 1 1987 m sub l 5 9 occurred along the eastern part of a ramp zone

deformation across the asal ghoubbet rift djibouti uplift and crustal extension 1979 1986 geodetic networks of different sizes have been established since 1973 in the asal ghoubbet rift in south western afar in order to study crustal movements across an extensive plate boundary partial or entire remeasurements of these networks during the 1979 1986 interval provide detailed data about strain accumulation and vertical movements after a seismo volcanic rifting event that occured in november 1978 along this area similar to an oceanic accreting plate boundary the results of the geodetic monitoring indicate a continuous opening of the rift zone of about 6 cm yr since 1979 the deformation is principally distributed in the inner floor of the rift with a uniaxial extensional n40 e strain rate of about 15 strain yr the results of the 1979 and the 1984 85 levellings define the development of a large uplift across the rift zone amounting 15 cm during this 6 year interval this uplift is accompanied with a relative collapse of the inner floor zone of about 4 cm which is associated with creep on the active faults bounding the rift two stages of the rifting process are successively observed in djibouti an abrupt rifting event in 1978 and then a continuous accumulation stage which is controled by the plate motion and partially by magmatic inflation and active creep faulting

crustal strain near the big bend of the san andreas fault analysis of the los padres tehachapi trilateration networks california in the region of the los padres tehachapi geodetic network the san andreas fault saf changes its orientation by over 30 degree from n 40 degree w close to that predicted by plate motion for a transform boundary to n 73 degree w the strain orientation near the saf is consistent with right lateral shear along the fault with maximum shear rate of 0 38 plus minus 0 01 mu rad yr at n 63 degree w in contrast away from the saf the strain orientations on both sides of the fault are consistent with the plate motion direction with maximum shear rate of 0 19 plus minus 0 01 mu rad yr at n 44 degree w the strain rate does not drop off rapidly away from the fault and thus the area is fit by either a broad shear zone below the saf or a single fault with a relatively deep locking depth the fit to the line length data is poor for locking depth d less than 25 km for d of 25 km a buried slip rate of 30 plus minus 6 mm yr is estimated the authors also estimated buried slip for models that include the garlock and big pine faults in addition to the saf slip rates on other faults are poorly constrained bymore the los padres tehachapi network the best fitting garlock fault model had computed left lateral slip rate of 11 plus minus 2 mm yr below 10 km buried left lateral slip of 15 plus minus 6 mm yr on the big pine fault within the western transverse ranges provides significant reduction in line length residuals however deformation there may be more complicated than a single vertical fault a subhorizontal detachment on the southern side of the saf cannot be well constrained by these data the authors investigated the location of the saf and found that a vertical fault below the surface trace fits the data much better than either a dipping fault zone located south of the surface trace less

rheologic controls on inter rifting deformation of the northern volcanic zone iceland article i nfo extensional rifts are characterized by significant lateral variations in crustal rheology as lithospheric material cools and advects away from the rift axes nevertheless most models used for modeling of crustal movements consist of horizontal layers with uniform properties this paper explores the role spatial variation of rheological properties play in modifying the style of surface deformation at rifts by comparing finite element model predictions to inferred inter rifting surface deformation in iceland s northern volcanic zone crustal deformation has been observed by satellite radar interferometry insar and gps measurements extension is observed across the entire northern rift zone whereas subsidence occurs in two distinct areas corresponding to mapped fissure swarms where major rifting took place in 1975 1984 and 1874 1876 respectively our models indicate that the observed inter rifting plate spreading deformation field is controlled by rheological variations within the en echelon arrangement of fissure segments and that a central ridge axes model does not apply uniform stretching across a plate boundary zone where fissure swarms are weaker than the surrounding crust and reflect the surface expression of the rift reproduces the characteristics of the deformation field the most realistic fissure swarm structure consists of a wedge of

september 2005 manda hararo dabbahu rifting event afar ethiopia constraints provided by geodetic data faults the volume of the 2005 dike 1 5 2 0 km 3 is not balanced by sufficient volume loss at dabbahu and gabho volcanoes 0 42 and 0 12 km 3 respectively taking into account the deflation of a suspected deep midsegment magma chamber simultaneously to dike intrusion produces a smoother opening distribution along the southern segment above the dike faults slipped by an average 3 m yielding an estimated geodetic moment of 3 5 10 19 nm one order of magnitude larger than the cumulative seismic moment released during the earthquake swarm between dabbahu and ado ale volcanic complexes significant opening occurred on the western side of the dike the anomalous location of the dike at this latitude offset to the east of the axial depression may explain this phenomenon a two stage intrusion scenario is proposed whereby rifting in the northern manda hararo rift was triggered by magma upwelling in the dabbahu area at the northern extremity of the magmatic segment although vigorous dike injection occurred during the september 2005 event the tectonic stress deficit since the previous rifting episode was not fully released leading to further intrusions in 2006 2009

numerical modelling of quaternary deformation and post rifting displacement in the asal ghoubbet rift djibouti africa over the last three decades a host of information on rifting process relating to the geological and thermal structure long time scale deformation quaternary and holocene and rifting cycle displacement across the asal ghoubbet rift has been made available these data are interpreted with a two dimensional thermo mechanical model that incorporates rheological layering of the lithosphere dyke inflation and faulting active fault locations and geometry are mainly controlled by both thermal structure and magma intrusion into the crust the distributed slip throughout the inner rift is related to the closeness of magma chamber leading to additional stress into the upper thinned crust assuming a constant arabia somalia motion of 11 mm year the variation of subsidence rate between the last 100 and 9 ka is associated with a decrease of the average injection rate from 10 to 5 mm year these values about equal to the regional opening rate suggest that both volcanism and tectonic play an equivalent role in the rifting process our modelled sequence of events gives one possible explanation for both vertical and horizontal displacements observed since the 1978 seismovolcanic crisis although part of the post rifting deformation could be due to viscous relaxation the high opening rate in the first years after the event and the abrupt velocity change in 1984 1986 argue for a large dyke inflation of 12 cm year ending in 1985 the asymmetric and constant pattern of the gps velocity since 1991 suggests that present post rifting deformation is mainly controlled by fault creep and regional stretching this study demonstrates the internal consistency of the data set highlights the role of magmatism in the mechanics of crustal stretching and reveals a complex post rifting process including magma injection fault creep and regional stretching d 2005 elsevier b v all rights reserved

strike slip faults parallel to crustal spreading axes data from iceland and the afar depression slickenside studies in regions of crustal spreading such as iceland and the afar depression east africa reveal that a significant number of faults parallel and close to rift axes are strike slip rather than normal therefore the pattern of brittle deformation in these regions does not conform to the classic two dimensional schemes of oceanic tectonics and pre oceanic rifting dip slip and strike slip faulting presumably alternated along or in the vicinity of spreading axes indicate a varying stress field and a combination of transverse and longitudinal movements in iceland strike slip faults parallel to rifts are observed both west and east of the rift system as well as in a median area between overlapping rifts the mechanisms proposed for their origin include accommodation of oblique convergence or divergence of crustal sections due to variations of spreading directions along axis and the interaction of overlapping rifts in the afar depression this kind of fault is recorded west of the rift of asal and can be imputed to reflect an interaction among rifts in the vicinity of the afar triple junction rift parallel strike slip faults cannot however be assumed to be a feature of all crustal spreading axes due to the peculiarity of the examined regions both of them are hot spot areas and the afar depression lies at a triple junction

bookshelf faulting and horizontal block rotations between overlapping rifts in southern afar lateral slip on initially rift parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts it occurs in southern afar where clockwise block rotations result from distributed dextral shear between the overlapping ghoubbet asal manda inakir and manda hararo abhe bad rifts faulting observed during the 1969 serdo earthquakes and on spot images is consistent with the shear being taken up by left lateral slip on steep nw se striking faults which formed as normal faults before extensional strain became localized in the two rifts this bookshelf faulting accounts quantitatively for the 14 5 7 5 rotation documented by paleomagnetism in the 1 8 0 4 ma old afar stratoid basalts given the 17 5 5 mm yr rate of separation between arabia and somalia

source mechanisms of three moderate california earthquakes of july 1986 abstract we studied the source processes of three moderate size earthquakes in california the m s 6 0 event of july 8 1986 near palm springs the m s 5 8 shock of july 13 1986 offshore of oceanside and the m s 6 2 event of july 21 1986 in the chalfant valley on the east side of the sierra nevada the centroidal source parameters are estimated by inversion of long period p and sh waves recorded at teleseismic distances by seismographs of worldwide networks a more detailed description of the source is then obtained by analyzing broadband seismograms the north palm springs earthquake had a simple rupture process most of the seismic moment was released in the first 3 sec in a single step the strike of the source fault plane follows the average trend of the banning fault and its dip agrees with the dip of the fault plane delineated by the aftershocks the faulting mechanism of the main shock is right lateral strike slip with a sizable reverse component that is consistent with uplift north of the banning fault the earthquake offshore of oceanside had a slightly more complex rupture process the far field source time function consists of two pulses that represent the release of most of the moment in the first 3 sec the source mechanism indicates reverse faulting with the compressional axis trending northeast almost perpendicular to the trend of mapped faults offshore the event does not appear to be associated with major mapped faults in the area but the strike of one of the nodal planes agrees with the trend of the major mapped faults offshore and its mechanism is consistent with the missing component of plate convergence not taken up by the onshore strike slip fault systems and the idea of a weak san andreas fault system the rupture process of the chalfant valley earthquake was yet more complicated the source time function indicates that in a 4 sec time window three asperities were broken the source mechanism indicates right lateral strike slip faulting with a small normal faulting component on a fault that dips approximately 50 southwest earthquake activity suggests that presently the deformation in the northern part of the owens valley which includes the chalfant valley is dominated by conjugate strike slip faults which strike obliquely to the main range bounding normal faults the waveforms indicate the presence of a low velocity zone at a depth of about 20 km in the crust in the chalfant valley a synthesis of information from waveform modeling first motion fault plane solutions and aftershock distributions indicates that the nucleation and or termination of rupture for the north palm springs and the chalfant valley events were controlled by preexisting fault geometry

fault geometry rupture dynamics and ground motion from potential earthquakes on the north anatolian fault under the sea of marmara summary using the 3 d finite element method we develop dynamic spontaneous rupture models of earthquakes on the north anatolian fault system in the sea of marmara turkey considering the geometrical complexity of the fault system in this region we find that the earthquake size rupture propagation pattern and ground motion all strongly depend on the interplay between the initial static regional pre stress field and the dynamic stress field radiated by the propagating rupture by testing several nucleation locations we observe that those far from an oblique normal fault stepover segment near istanbul lead to large through going rupture on the entire fault system whereas nucleation locations closer to the stepover segment tend to produce ruptures that die out in the stepover however this pattern can change drastically with only a 10 rotation of the regional stress field our simulations also reveal that while dynamic unclamping near fault bends can produce a new mode of supershear rupture propagation this unclamping has a much smaller effect on the speed of the peak in slip velocity along the fault finally we find that the complex fault geometry leads to a very complex and asymmetric pattern of near fault ground motion including greatly amplified ground motion on the insides of fault bends the ground motion pattern can change significantly with different hypocentres even beyond the typical effects of directivity the results of this study may have implications for seismic hazard in this region for the dynamics and ground motion of geometrically complex faults and for the interpretation of kinematic inverse rupture models

application of the global positioning system to crustal deformation measurements 3 result from the southern california borderlands five years of measurements from the global positioning system gps satellites collected between 1986 and 1991 are used to investigate deformation in the offshore regions of southern california gps provides the first practical technique to make precise geodetic measurements in the region the geodetic network is situated along the california coastline from vandenberg 120 6 w 34 6 n to san diego with additional sites on santa cruz san nicolas santa catalina santa rosa and san clemente islands the precision of horizontal interstation vectors is subcentimeter and the interstation vector rate between ovro and vandenberg agrees with the very long baseline interferometry derived rate to within one standard deviation no significant motion is observed in the western santa barbara channel between vandenberg and santa rosa island 0 5 1 6 mm yr where the quoted uncertainties are one standard deviation motions in the eastern santa barbara channel are consistent with compressional deformation of 6 1 mm yr at n16 3 e this motion is in agreement with seismicity and an independent geodetic analysis for the period 1971 1987 larsen 1991 san clemente island is moving relative to san diego at the rate of 5 9 1 8 mm yr at a direction of n38 20 w the motion between san nicolas island and san clemente island 0 8 1 5 mm yr is insignificant

an introduction to the global positioning system and some geological applications receivers equipped to measure dual frequency carrier phase signals from satellites of the global positioning system gps have been capable under special conditions of determining relative horizontal positions among stations separated by one to a few hundred kilometers with a precision of one to several millimeters since the early 1980s the major obstacles to making this capability routine extending it to all parts of the globe and extending it to longer station separations have been equipment cost limitations in the gps satellite constellation arduous data analysis uncertainties in satellite orbits uncertainties in propagation delays associated with variable tropospheric water vapor and difficulties in resolving carrier phase cycle ambiguities recent improvements have occurred in all these areas the increasing ease and reduced cost of gps data acquisition and analysis are having a significant impact on studies of near fault crustal deformation and earthquake processes until recently the province of conventional terrestrial geodetic techniques the enhanced satellite constellation improved models and establishment of global tracking networks have extended several millimeters horizontal positioning capability to station separations of 1000 km or more in virtually all parts of the world this enables study of new classes of tectonic problems that previously were difficult to attack with any geodetic technique examples include a complete kinematic description of ongoing crustal deformation in broad complex continental plate boundary zones and measurement of relative plate motion at convergent boundaries where global models may be poorly constrained

gravity contributions to the understanding of salt tectonics from the jebel cheid area dome zone northern tunisia detailed gravity measurements integrated with geological data were computed to constrain the mechanisms that were active during the emplacement of the triassic evaporite bearing folds of jebel cheid from the salt dome zone in the atlassic region the gravity analysis consists in mapping the contrasting gravity responses complete bouguer anomaly residual anomaly and derivative maps the main results display a positive amplitude gravity anomaly as the response of triassic evaporite bodies and important ne sw trending features at the boundaries between the triassic outcrops and their enveloping strata in contrast with gravity calculations of a salt dome structure usually resulting in negative gravity anomaly models the jebel cheid clearly expresses a positive gravity anomaly furthermore this result is supported by synthetic gravity interpretation

fluid controlled faulting process in the asal rift djibouti from 8 yr of radar interferometry observations the deformation in the asal rift djibouti is characterized by magmatic inflation diking distributed extension fissure opening and normal faulting an 8 yr time line of surface displacement maps covering the rift constructed using radar interferometry data acquired by the canadian satellite radarsat between 1997 and 2005 reveals the aseismic behavior of faults and its relation with bursts of microseismicity the observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the fieale volcanic center slip distribution along fault strike shows triangular sections a pattern not explained by simple elastic dislocation theory these observations suggest that the asal rift faults are in a critical failure state and respond instantly to small pressure changes in fluid filled fractures connected to the faults reducing the effective normal stress on their locked section at depth

fault propagation and climatic control of sedimentation on the ghoubbet rift floor insights from the tadjouraden cruise in the western gulf of aden summary a detailed geophysical survey of the ghoubbet al kharab djibouti clarifies the small scale morphology of the last submerged rift segment of the propagating aden ridge before it enters the afar depression the bathymetry reveals a system of antithetic normal faults striking n130 e roughly aligned with those active along the asal rift the 3 5 khz sub bottom profiler shows how the faults cut distinct layers within the recent up to 60 m thick sediment cover on the floor of the basin a large volcanic structure in the centre of the basin the ghoubbet volcano separates two sedimentary flats the organization of volcanism and the planform of faulting with en echelon subrifts along the entire asal ghoubbet rift appear to confirm the westward propagation of this segment of the plate boundary faults throughout the rift have been active continuously for the last 8400 yr but certain sediment layers show different offsets the varying offsets of these layers dated from cores previously retrieved in the southern basin imply holocene vertical slip rates of 0 3 1 4 mm yr 1 and indicate a major decrease in sedimentation rate after about 6000 yr bp and a redistribution of sediments in the deepest troughs during the period that preceded that change

geodetic estimate of coseismic slip during the 1989 loma prieta california earthquake offsets in the relative positions of geodetic stations resulting from the loma prieta earthquake can be explained with a dislocation model that includes buried oblique slip on a rupture surface extending 37 km along the strike of the san andreas fault dipping 70 degree to the sw and extending from a depth of about 5 to 17 5 km assuming uniform slip on a rectangular surface the mean values for a range of reasonable fault geometries are 1 6 plus minus 0 3 m right lateral strike slip and 1 2 plus minus 0 3 m reverse slip slip on an adjacent extension of the rupture to the southeast recorded in the aftershock sequence is not well constrained by the geodetic data the geodetic data clearly preclude rupture extending near the surface

weighted euler deconvolution of gravity data euler deconvolution is used for rapid interpretation of magnetic and gravity data it is particularly good at delineating contacts and rapid depth estimation the quality of the depth estimation depends mostly on the choice of the proper structural index and adequate sampling of the data the structural index is a function of the geometry of the causative bodies for gravity surveys station distribution is in general irregular and the gravity field is aliased this results in erroneous depth estimates by weighting the euler equations by an error function proportional to station accuracies and the interstation distance it is possible to reject solutions resulting from aliasing of the field and less accurate measurements the technique is demonstrated on bouguer anomaly data from the charlevoix region in eastern canada

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

mechanics of 3 d shear cracks between rayleigh and shear wave rupture speeds abstract though mode ii shear fractures primarily strike slip earthquakes can not only exceed the shear wave speed of the medium but can even reach the compressional wave speed steady state calculations showed that speeds between the rayleigh and shear wave speeds were not possible thus defining a forbidden zone for more than 30 years it was believed that this result in which the rupture jumps over the forbidden zone also holds for 3 d ruptures in which mode ii and mode iii mainly dip slip faulting are mixed using unprecedentedly fine spatial and temporal grids we show that even in the simple configuration of homogeneous fault properties and linear slip weakening friction law a realistic 3 d rupture which starts from rest and accelerates to some higher velocity actually does pass smoothly through this forbidden zone but very fast the energy flux from the rupture tip is always positive even within the so called forbidden zone contrary to the 2 d case finally our results show that the width of the cohesive zone initially decreases then increases as the rupture exceeds the shear wave speed and finally again decreases as the rupture accelerates to a speed of 90 of the compressional wave speed

block rotation and continental extension in afar a comparison to oceanic microplate systems the reorganization of oceanic spreading centers separating major plates often appears to occur by a process in which discrete microplates form and evolve by rift propagation to see whether such microplate behavior has implications for continental rifting we investigate the application of a microplate model to the afar region at the nubia somalia arabia triple junction studies of marine magnetic anomalies volcanic ages bathymetry and seismicity suggest that the westward propagating gulf of aden spreading center has propagated into eastern afar within the past 2 m y causing rifting and extension within the continent we derive constraints on the extension history from the geometry and timing of rift formation and from paleomagnetic data indicating that pliocene to pleistocene age rocks have undergone a clockwise rotation of 11 we suggest that the history of rifting the rotation and several other features of the regional geology can be described by combining features of an oceanic microplate model and the concept of rift localization previously proposed for afar in this scenario motion occurring on several rifts within an extensional zone preceding the propagating spreading center is gradually transferred to a single rift while motion is transferred the overlap region between the growing and dying rifts acts as one or more microplates or blocks that rotate relative to the surrounding major plates the rifting history and rotations in eastern afar are thus related to the rift propagation and localization that occurs as the plate boundary evolves provided the constraints we use are appropriate our model better describes the regional kinematics than alternative block models including one based on bookshelf faulting if the tectonics of afar are typical for continental breakup they have interesting implications for the geometry of passive margins in particular asymmetric rifted margins can be produced if the final location of the rift axis is not at the center of the zone of initially disrupted lithosphere additionally if the rate of rift propagation and the rate and location of rift localization are not uniform then along axis structural variations will result

subsurface geology of the ariana region diapir zone northern tunisia by means of gravity analysis gravity data were used to investigate subsurface geology in the ariana region from the diapir zone in tunisia our study area is located in the north eastern part of the maghrebides affected by the alpine orogeny despite the smooth topography and the low density of the quaternary series integrated geological data and gravity responses bouguer anomaly and derivative mapping help decipher the geological heterogeneities beneath the quaternary overburden the interpretation of the gravity data points to alignments of positive anomalies delineating two major orthogonal features that are thought to characterize the subsurface deep structures 1 a lineament striking north west at the emplacement of an actual horst bordered by a buried fault system directed n140 and 2 a second counterpart striking north east and is clearly expressed in the surface outcrops these principal structural features were described as the result of a regional tectonic compressive phase dating the miocene this phase was marked by reactivation and oblique slip displacements of regional faults some of which n070 are reverse and south east vergent further faulting enhanced the ascension of triassic evaporites in diapir structures this tectonic evolution may be integrated in the regional tectonic scheme of alpine orogeny induced by a collision between african and european plates

contemporary holocene and quaternary deformation of the asal rift djibouti implications for the mechanics of slow spreading ridges because the frequency and character of rifting events along mid ocean ridges are largely unknown how the repetition of such events gives rise to rift structures is unexplored the asal rift in the afar depression of djibouti africa provides the world s best subaerial analogue for young slow spreading mid ocean ridges seismic geodetic and field observations of a seismovolcanic event in 1978 at asal yield estimates of the fault and dike locations geometry displacement and volume of basalt extruded in a rifting event a 6 9 kyr old lake shore highstand at asal has been warped downward by 70 m providing a holocene measure of the vertical deformation across the rift the rift topography furnishes an older datum which we infer to be 34 6 kyr old using the holocene deformation rate we find that faults throughout the rift valley are active holocene slip rates diminish beyond 4 km from the rift axis late quaternary rates decrease beyond 6 7 km the holocene slip rates are used to estimate repeat times by taking the displacement on the faults which slipped in 1978 as characteristic we find tectonic events on individual faults recur every 200 300 years half the rift faults slipped together in the 1978 event if this is typical then groups of faults are activated every 100 150 years we suggest that half the events take place in the rift axis accompanied by volcanic extrusion the remainder occur peripheral to the neovolcanic zone and involve fault slip only both events having a repeat time of 200 300 years given the 10 km width of the rift and its 16 mm yr 1 spreading rate the mean age of the material in the rift should be 350 kyr an order of magnitude older than the inferred age of the formation of the rift topography the subsidence rate of the rift axis during the past 35 kyr is 8 9 mm yr 1 with the rate of infilling by volcanic extrusion 1 mm yr 1 the resulting net subsidence rate about equal to the half spreading rate of the rift could not be sustained for 300 kyr without significant infilling by lavas thus both observations suggest that the long term vertical deformation in the rift has not been steady state instead we suggest that there is a rifting filling cycle at asal with the most recent filling episode ending 35 kyr

the july 1986 oceanside ml 5 3 earthquake sequence in the continental borderland southern california an earthquake of m l 5 3 occurred at 32 58 7 n 117 51 5 w southwest of oceanside in san diego county at 13 47 13 july 1986 ut this main shock was followed by an extensive aftershock sequence with 55 events of m l 3 0 during july 1986 the epicenters of the main shock and aftershocks are located at the northern end of the san diego trough bahia soledad fault zone sdt bs where it changes strike from northwest to a more westerly direction through a left offset or a bend in the fault the northwest striking sdt bs is one of three strike slip fault systems that constitute the offshore agua blanca fault system the spatial distribution of the aftershocks indicates a unilateral 7 to 9 km long rupture to the east southeast away from the epicenter of the main shock the focal mechanism of the main shock also has an east southeast striking and south dipping plane with mostly reverse movement on it focal mechanisms of the m l 3 0 aftershocks show both reverse and strike slip movement the reverse focal mechanisms indicate that this sequence may have occurred on a thrust fault that provides for a left stepping offset or a bend in the san diego trough fault as movement is transferred to the west along the santa cruz catalina island escarpment some of the aftershocks that are located to the southeast of the main shock and have strike slip focal mechanisms suggest activation of the northwest trending san diego trough fault a stress inversion of the focal mechanism data shows that the maximum principal stress determined from the focal mechanisms of the main shock and 22 aftershocks that occurred within 36 hours of the main shock has an azimuth of s30 w plunging 18 the maximum principal stress determined from 30 aftershocks that occurred from 15 july to 2 october 1986 has an azimuth of s20 w plunging 18 the values the measure of the relative sizes of the principal stresses are approximately 0 07 and 0 1 respectively indicating that the intermediate and minimum principal stress are of similar magnitude the results of the stress inversion and the focal mechanisms that showed reverse faulting suggest that the inner continental borderland offshore from oceanside is not currently a pure strike slip tectonic regime but rather a strike slip mixed with reverse faulting regime when the 52 aftershock focal mechanisms are divided into four groups and the stress inversion is repeated the change in stress can be described as a progressive counter clockwise rotation of 14 of the orientation of the maximum principal stress to a more southerly direction with the greatest change in stress orientation observed shortly after the main shock the abundance of aftershocks may be related to the large temporal variation in stress orientation that in turn may have resulted from the small stress drop of the main shock

a reappraisal of the geomagnetic polarity time scale to 4 ma using data from the turkana basin east africa recalibration of the pliocene and early pleistocene geomagnetic time scale using the k ar dated fluvial sequence of the turkana basin in east africa agrees with calibrations based on astronomical calculations ages estimated here are olduvai subchron 1 78 1 96 ma reunion subchrons 2 11 2 15 ma and 2 19 2 27 ma matuyama gauss boundary 2 60 ma kaena subchron 3 02 3 09 ma mammoth subchron 3 21 3 29 ma gauss gilbert boundary 3 57 ma

the origin of intraplate deformation in the atlas system of western and central algeria from jurassic rifting to cenozoic quaternary inversion abstract analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central saharan atlas algeria this intraplate area has recorded the breakup of pangea late triassic the opening of the maghrebian tethys since the dogger and subsequently its closure oligocene to present however the two periods of strong coupling between europe and africa late lutetian and pleistocene which correspond to rapid uplifts of the atlas system and important deformations are not collision related they can be correlated to the beginning and the end of the development of the western mediterranean sea

supershear transition due to a free surface in 3 d simulations of spontaneous dynamic rupture on vertical strike slip faults supershear rupture propagation has been inferred from seismic observations for natural faults and observed in laboratory experiments we study the effect of the free surface on the transition of earthquake rupture from subshear to supershear speeds using simulations of spontaneous dynamic rupture on vertical strike slip faults we find that locally supershear rupture near the free surface can occur due to i the generalized burridge andrews mechanism that is a supershear loading field between p and sv wave arrivals generated by the main rupture front at depths and ii the phase conversion of sv to p diffracted waves at the free surface weaker supershear slip due to the generalized burridge andrews mechanism is caused by the low strength at shallow portions of the fault relative to deeper ones dominant supershear rupture is supported by the additional supershear loading field produced by phase conversion locally supershear propagation at the free surface occurs regardless of the level of prestress and can cause transition to supershear propagation over the entire seismogenic depth such global supershear transition which depends on prestress can occur under prestress levels lower than the theoretical estimates for models with no free surface although the effectiveness of supershear transition due to the free surface can be diminished by several potentially important factors it may play an important role on natural faults at least in those strike slip earthquakes that accumulate significant surface slip

on the initiation of sustained slip weakening ruptures by localized stresses numerical simulations of dynamic earthquake rupture require an artificial initiation procedure if they are not integrated in long term earthquake cycle simulations a widely applied procedure involves an overstressed asperity a localized region stressed beyond the static frictional strength the physical properties of the asperity size shape and overstress may significantly impact rupture propagation in particular to induce a sustained rupture the asperity size needs to exceed a critical value although criteria for estimating the critical nucleation size under linear slip weakening friction have been proposed for 2 d and 3 d problems based on simplifying assumptions they do not provide general rules for designing 3 d numerical simulations we conduct a parametric study to estimate parameters of the asperity that minimize numerical artefacts e g changes of rupture shape and speed artificial supershear transition higher slip rate amplitudes we examine the critical size of square circular and elliptical asperities as a function of asperity overstress and background off asperity stress for a given overstress we find that asperity area controls rupture initiation while asperity shape is of lesser importance the critical area obtained from our numerical results contrasts with published theoretical estimates when background stress is low therefore we derive two new theoretical estimates of the critical size under low background stress while also accounting for overstress our numerical results suggest that setting the asperity overstress and area close to their critical values eliminates strong numerical artefacts even when the overstress is large we also find that properly chosen asperity size or overstress may significantly shorten the duration of the initiation overall our results provide guidelines for determining the size of the asperity and overstress to minimize the effects of the forced initiation on the subsequent spontaneous rupture propagation

approximating edges of source bodies from magnetic or gravity anomalies cordell and grauch 1982 1985 discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks the final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes we attempt to automate their final step our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources the method applies to both local surveys and to continent wide compilations of magnetic and gravity data e g zietz 1982 simpson et al 1983a kane et al 1982

twenty five years of geodetic measurements along the tadjoura asal rift system djibouti east africa since most of tadjoura asal rift system sits on dry land in the afar depression near the triple junction between the arabia somalia and nubia plates it is an ideal natural laboratory for studying rifting processes we analyze these processes in light of a time series of geodetic measurements from 1978 through a network of about 30 gps sites covers the republic of djibouti additional points were also measured in yemen and ethiopia stations lying in the danakil block have almost the same velocity as arabian plate indicating that opening near the southern tip of the red sea is almost totally accommodated in the afar depression inside djibouti the asal ghoubbet rift system accommodates 16 1 mm yr of opening perpendicular to the rift axis and exhibits a pronounced asymmetry with essentially null deformation on its southwestern side and significant deformation on its northeastern side this rate slightly higher than the large scale arabia somalia motion 13 1 mm yr suggests transient variations associated with relaxation processes following the asal ghoubbet seismovolcanic sequence of 1978 inside the rift the deformation pattern exhibits a clear two dimensional pattern along the rift axis the rate decreases to the northwest suggesting propagation in the same direction perpendicular to the rift axis the focus of the opening is clearly shifted to the northeast relative to the topographic rift axis in the petit rift a rift in rift structure containing most of the active faults and the seismicity vertical motions measured by differential leveling show the same asymmetric pattern with a bulge of the northeastern shoulder although the inner floor of the rift is subsiding with respect to the shoulders all sites within the rift system show uplift at rates varying from 0 to 10 mm yr with respect to a far field reference outside the rift

near source ground motions from simulations of sustained intersonic and supersonic fault ruptures we examine the long period near source ground motions from simulations of m 7 4 events on a strike slip fault using kinematic ruptures with rupture speeds that range from subshear speeds through intersonic speeds to supersonic speeds the strong along strike shear wave directivity present in scenarios with subshear rupture speeds disappears in the scenarios with ruptures propagating faster than the shear wave speed furthermore the maximum horizontal displacements and velocities rotate from generally fault perpendicular orientations at subshear rupture speeds to generally fault parallel orientations at supersonic rupture speeds for rupture speeds just above the shear wave speed the orientations are spatially heterogeneous as a result of the random nature of our assumed slip model at locations within a few kilometers of the rupture the time histories of the polarization of the horizontal motion provide a better diagnostic with which to gauge the rupture speed than the orientation of the peak motion subshear ruptures are associated with significant fault perpendicular motion before fault parallel motion close to the fault supershear ruptures are associated with fault perpendicular motion after significant fault parallel motion consistent with previous studies we do not find evidence for prolonged supershear rupture in the long period 2 sec ground motions from the 1979 imperial valley earthquake however we are unable to resolve the issue of whether a limited portion of the rupture approximately 10 km in length propagated faster than the shear wave speed additionally a recording from the 2002 denali fault earthquake does appear to be qualitatively consistent with locally supershear rupture stronger evidence for supershear rupture in earthquakes may require very dense station coverage in order to capture these potentially distinguishing traits

strain accumulation across the asal ghoubbet rift djibouti east africa the asal ghoubbet rift in the republic of djibouti is a subaerial continuation of the gulf of aden ridge system many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm yr perpendicular to the normal faults that bound the rift periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm yr although the observed deformations could be contamined by postseismic relaxation after a rifting episode in november 1978 the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley the deformation is uniformly distributed in time and in space inside the inner floor of the rift the necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process

post rifting stress relaxation at the divergent plate boundary in northeast iceland interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the earth s surface in a diffusion like manner following tectonism at plate boundaries this process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary observations of such strain transients have been rare and have hitherto been confined to strike slip and underthrusting plate boundaries1 here we report the observation of a strain transient at the divergent spreading plate boundary in iceland a global positioning system survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion pulse shape and an amplitude three times greater than the 5 7 cm that would be expected from the average spreading rate2 a simple one dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1 1 a 0 3 m2s 1 combined with struc tural information from magnetotelluric measurements this implies a viscosity of 0 3 2 1019 pa s a value comparable to that derived for iceland from post glacial rebound23 but low compared with estimates for mantle viscosity obtained elsewhere3

thick skinned tectonics within the intracontinental easternmost atlas foreland and thrust belt tunisia meso cenozoic kinematics and implications for regional geodynamics the transition zone between western and central mediterranean domains presents a key area to investigate kinematic interactions within the adjacent orogen systems such as the easternmost atlas foreland and thrust belt analysis of gravity and seismic data revealed a highly structured basement characterizing a series of structural highs and lows delimited by high angle n s e w and nw se extensional faults this basement architecture is inherited from successive extensional events related to the openings of the triassic early cretaceous tethys oceans i e alpine tethys ligurian tethys and mesogea throughout this period this mosaic of continental blocks significantly controlled the thickness and facies distributions early stages of diapirism took place along these basement faults and allowed maximum subsidence in minibasins revealed by the development of growth strata in response to the late cretaceous eocene shortenings these extensional faults have been reactivated as trasnpressional shear zones giving rise to narrow popup structures in addition gravity modeling indicates crustal thinning and deep rooted faults affecting the crust south of the zaghouan thrust and along e w transfer zones from the late miocene a drastic change in the stress regime is attributed to the effect of the adjacent sicily channel on the study area this promotes crustal thinning basin subsidence and channeling up of mantle derived helium along lithospheric scale weak zones our results give rise to new insights into the reactivation of inherited weakness zones of southern tethys margin in response to the complex interaction between african and eurasian plates accommodated by subduction rollback collision and slab segmentation

role of preexisting faults in the geodynamic evolution of northern tunisia insights from gravity data from the medjerda valley abstract the middle medjerda valley belongs to the tellian zone of tunisia it behaves as a post orogenic basin where surface structural indices are almost completely absent it is noteworthy to recall that alpine and atlasic tectonic prints are well expressed on both sides of this basin this paper presents an integrated study comprising geology and gravity data subsurface geological modeling was based on gravity data interpretation including the complete bouguer anomaly upward continuation residual and derivatives 2 5d gravity modeling was essential for subsurface geometric features characterization in spite of its sub horizontal and smooth topography and the low density of the homogenous quaternary series covering the area well contrasted and variable gravity responses are observed reflecting the heterogeneities of the geological substratum the enhanced horizontal gravity gradient confirms structural systems that are already evidenced by surface geology it supports also the presence of new masked fault systems the main structural features inferred are i nw directed grabens located at mellegue and s ez zaouam depressions ii triassic diapirs associated to ne thrust faults of el merdja bousalem ben bechir and mjez el assa these structures are generally covered by the quaternary series the presence of both distensive and compressive structures within a compressive regime is explained by the solicitation of preexisting fault systems in the area during the nw directed shortening occurred at the upper miocene age the geodynamic evolution of northern tunisia was dominated by strike slip thrust movements through the reactivation of deep major inherited faults e g i the dextral ew oriented fault system of ghardimaou thibar and ii the sinister ns accident of dehmani bousalem ras rajel these accidents shaped the medjerda basin and contribute to its complex geological architecture

positioning results with doris on spot2 after first year of mission a new space system named doris has been developed by the centre national d etudes spatiales in france with the collaboration of the institut geographique national for precise orbit determination and positioning it is a doppler based system with a worldwide ground network of transmitting beacons and an onboard receiver the spot2 satellite launched in january 1990 carries doris as an experimental instrument in this study we present positioning results obtained from the analysis of the first 10 months of doris data through a semi dynamical approach i e the satellite orbit is improved simultaneously with the beacon positions external calibration with satellite laser ranging slr and very long baseline interferometry vlbi positions indicates that doris accuracy from a few months of data is presently 15 20 cm rms in absolute and relative baseline length 1000 km positioning for very short baselines 1000 km positioning results is in the range 10 20 cm rms for the shortest baselines 100 km the month to month repeatability is better than 5 cm in cartesian coordinates this repeatability is only 2 cm in the north and east components whereas it is 12 cm in the vertical component suggesting that tropospheric errors are important it should be possible to improve these results in the future by reducing the residual orbit error especially the atmospheric drag perturbation which was particularly troublesome in 1990 a year having very large solar activity and by improving modeling of the tropospheric correction

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

detection of crustal deformation from the landers earthquake sequence using continuous geodetic measurements the measurement of crustal motions in technically active regions is being performed increasingly by the satellite based global positioning system gps 1 2 which offers considerable advantages over conventional geodetic techniques3 4 continuously operating gps arrays with ground based receivers spaced tens of kilometres apart have been established in central japan5 6 and southern california to monitor the spatial and temporal details of crustal deformation here we report the first measurements for a major earthquake by a continuously operating gps network the permanent gps geodetic array pgga 7 9 in southern california the landers magnitude afw of 7 3 and big bear mw 6 2 earthquakes of 28 june 1992 were monitored by daily observations ten weeks of measurements centred on the earthquake events indicate significant coseismic motion at all pgga sites significant post seismic motion at one site for two weeks after the earthquakes and no significant preseismic motion these measurements demonstrate the potential of gps monitoring for precise detection of precursory and aftershock seismic deformation in the near and far field

evidence of supershear during the 2018 magnitude 7 5 palu earthquake from space geodesy a magnitude 7 5 earthquake hit the city of palu in sulawesi indonesia on 28 september 2018 at 10 02 43 coordinated universal time it was followed a few minutes later by a 4 7 m high tsunami palu is situated in a narrow pull apart basin surrounded by high mountains of up to 2 000 m altitude this morphology has been created by a releasing bend in the palu koro fault a rapidly moving left lateral strike slip fault here we present observations derived from optical and radar satellite imagery that constrain the ground surface displacements associated with the earthquake in great detail mapping of the main rupture and associated secondary structures shows that the slip initiated on a structurally complex and previously unknown fault to the north extended southwards over 180 km and passed through two major releasing bends the 30 km section of the rupture south of palu city is extremely linear and slightly offset from the mapped geological fault at the surface this part of the rupture accommodates a large and smooth surface slip of 4 7 m with no shallow slip deficit almost no aftershock seismicity was recorded from this section of the fault as these characteristics are similar to those from known supershear segments we conclude that the palu earthquake probably ruptured this segment at supershear velocities the devastating 2018 magnitude 7 5 earthquake in palu indonesia ruptured at supershear speeds according to evidence from space geodesy

evidence for a supershear transient during the 2002 denali fault earthquake elastodynamic considerations suggest that the acceleration of ruptures to supershear velocities is accompanied by the release of rayleigh waves along the fault from the stress breakdown zone these waves generate a secondary slip pulse trailing the rupture front but manifest almost entirely in ground motion perpendicular to the fault in the near source region we construct a spontaneously propagating rupture model exhibiting these features and use it to explain ground motions recorded during the 2002 denali fault earthquake at pump station 10 located 3 km from the fault we show that the initial pulses on both the fault normal and fault parallel components are due to the supershear stress release on the fault whereas the later arriving fault normal pulses result from the trailing subshear slip pulse on the fault online material mpeg movies of rupture history and ground motion

supershear rupture in a mw 6 7 aftershock of the 2013 sea of okhotsk earthquake earthquake rupture speeds exceeding the shear wave velocity have been reported for several shallow strike slip events whether supershear rupture also can occur in deep earthquakes is unclear because of their enigmatic faulting mechanism using empirical green s functions in both regional and teleseismic waveforms we observed supershear rupture during the 2013 moment magnitude m w 6 7 deep earthquake beneath the sea of okhotsk an aftershock of the large deep earthquake mw 8 3 the m w 6 7 event ruptured downward along a steeply dipping fault plane at an average speed of 8 kilometers per second suggesting efficient seismic energy generation comparing it to the highly dissipative 1994 m w 8 3 bolivia earthquake the two events represent end members of deep earthquakes in terms of energy partitioning and imply that there is more than one rupture mechanism for deep earthquakes

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed

gravity analysis of salt structures an example from the el kef ouargha region northern tunisia triassic outcrops in the atlassic zone of northern tunisia may be modelled in two ways salt bodies piercing through cretaceous terrains or triassic salt flows stratified within an albian series both models find support from gravity data and are debatable to evaluate the mass distribution changes with depth the bouguer anomaly of the el kef ouargha region was successively decomposed into regional and residual components to construct multiple pseudo depth slices and apparent density maps analyses of gravity lows clearly show a vertical continuity of less dense materials below the triassic salt outcrops these features can be explained by salt diapirism during mesozoic and cenozoic further gravity data tend to indicate less dense materials below aptian outcropping in jebel aite oued bou adila thus suggesting triassic materials occurring at depth in addition dense entities were recognized under mio pliocene and quaternary deposits which are thought to correspond to cretaceous paleoshoals currently collapsed by non outcropping faults our findings lend support to a diapir model intruding overburden rather than the salt glacier model stratified in the albian series proposed by some authors as the genetic structural model for triassic material bearing series in the north of tunisia