tertiary extensional faulting and evolving ductile brittle transition zones in the northern snake range and vicinity new insights from seismic data new geologic mapping and high resolution seismic data from east central nevada shed light on the evolution of the northern snake range decollement nsrd and its relation to the present basins and ranges we suggest that the nsrd originated in oligocene time as a flat 20 km wide 7 km deep zone of decoupling between localized ductile stretching below and more widely distributed high angle normal faulting above younger east dipping normal faults cut the western part of the nsrd forming the tilted fault blocks and highly asymmetric grabens that constitute the duck creek range duck creek valley and schell creek range spring valley and folded the nsrd to its present antiformal geometry these youngest faults flatten into pervasive subhorizontal reflectors at a depth of 10 to 15 km that we interpret as the youngest ductile brittle transition

the 1986 kalamata south peloponnesus earthquake detailed study of a normal fault evidences for east west extension in the hellenic arc tectonic and seismological data collected in the field following the september 13 1986 kalamata earthquake south peloponnesus are presented and analyzed to discuss the earthquake rupture process and the regional tectonics the event occurred on the kalamata normal fault whose trace was mapped with spot images and topographic and field observations this fault is part of an approximately nnw sse en echelon system cutting through the hellenic nappes the fault striking n15 e on the average with a dip of about 50 has a minimum cumulated quaternary throw of the order of 1 km the measured coseismic slip is 6 18 cm over a length of 6 km the main shock focal mechanism obtained from long period waveform modeling strike 201 10 20 dip 45 5 rake 283 10 25 represents almost pure east west extension and is in good agreement with tectonic observations the centroid depth is constrained to 5 3 km and the seismic moment to 7 0 2 5 1017 n m over 700 aftershocks located by a 16 station network installed after the earthquake for a period of 2 weeks define two clusters separated by a gap of aftershock activity from the surface to a depth of about 10 km the main cluster to the south defines a 45 west dipping plane which lies on the downward extension of the fault mapped at the surface focal mechanisms of aftershocks on this fault plane are homogeneous and represent e w extension as the main shock in contrast the majority of focal mechanisms in the uppermost part of the foot wall show more or less e w compression probably corresponding to postseismic stress release the northern cluster of aftershocks is very dense and located away from the surface rupture within a relay zone between the kalamata and the next en echelon faults to the nw the thouria faults there focal mechanisms represent extension from about n115 to n70 and n20 corresponding mostly to fault reactivation in an area where nonrigid deformations prevail the main shock probably initiated in this relay zone 3 4 s before the rupture front reached the main fault plane and released most of the energy there the rupture presumably propagating southward the focal mechanism of the kalamata earthquake and that of the april 27 1965 earthquake located to the northwest of crete as well as the regional active normal fault pattern imply that e w extension oblique to the hellenic arc is presently the dominant tectonic regime e w stretching occurs partly on reactivated nw se faults parallel to the hellenic structures but mostly on newly formed n s normal faults across those structures the latter faults are responsible for the apparent segmentation of the hellenic belt from southern peloponnesus to crete the existence of active e w extension in this region implies a recent change in the tectonic regime and consequently a change in boundary conditions at the subduction zone probably in response to the incoming margin of africa

a study of mantle layering beneath the western pacific the reverberative interval of the seismogram defined as the portion following the surface wave train propagating along the minor arc and ending with the first body wave arrivals from the major arc provides an excellent window in which to observe subcritical reflections from mantle discontinuities on an sh polarized seismogram the intervals between scsn and sscsn wave groups zeroth order reverberations are approximately 15 min long and consist almost entirely of waves reflected one or more times from mantle discontinuities first and higher order reverberations we have developed signal enhancement and waveform inversion schemes to progressively extract information pertaining to mantle layering from the waveforms of zeroth and first order reverberations and have applied them to a data set of long period digital seismograms drawn from a number of tectonic regions in and around the western pacific the results place important constraints on the nature of the transition zone discontinuities measurements of the two way vertical shear wave travel time from the free surface to the 400 km and 650 km discontinuities 400 and 650 respectively yield estimates of transition zone travel time tz largely uncontaminated by outside heterogeneity variations in the path averaged estimates of tz are of the order of 5 4 s indicating significant lateral heterogeneity in the transition zone correlations of tz with 400 and lm travel time through the lower mantle are consistent with but do not require the thermal deflection of an exothermic phase transition near 400 km and an endothermic transition at 650 km with 12 km of long wavelength 500 5000 km topography on each the amount of topography estimated for the 650 km discontinuity is significantly less than predicted for a rigorously stratified mantle 100 km in a search for other mantle discontinuities we find evidence for a strong reflector possibly associated with the onset of partial melting in the upper mantle average depth of 86 km due to ambiguity inherent to the method this feature can also be interpreted as an impedance increase in the d region of the lower mantle 70 170 km above the core mantle boundary no evidence is found for a 220 km discontinuity or the d discontinuity proposed by lay and helmberger 1983 suggesting that if such features are present in the western pacific they are either small normal incidence sh reflection coefficient r 1 5 or intermittent or have lateral depth variations well in excess of 50 km

tracking slabs beneath northwestern pacific subduction zones article i nfo article history this study uses the amplitudes of bottom side reflected shear waves to constrain the morphology and dynam ics of subducted oceanic lithosphere beneath northwestern pacific subduction zones across honshu arc the 410 and 660 km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the wadati benioff zone their topographies are negatively correlated along slab dip showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone the base of the upper mantle shows broad depressions as well as localized zones of shallow average depths be neath korea and northeast china the 15 km peak to peak topography west of the wadati benioff zones suggests that the stagnant part of the subducted pacific plate is not as flat as previously suggested eastward slab pile up is also possible at the base of the upper mantle across southern kuril arc the shear wave reflec tion coefficients of major olivine phase boundaries fall below 5 within the wadati benioff zone the appar ent reflection gaps and the spatial connection between a strong reflector at 900 km depth may imply 1 possible compositional variations at the top and bottom of the transition zone and 2 substantial mass heat flux across the 660 km seismic discontinuity we also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths the depths and strengths of these reflectors are highly variable be tween honshu and southern kuril islands

pliocene pleistocene evolution of the tyrrhenian arc paleomagnetic determination of uplift and rotational deformation abstract the jaramillo subchron and the brunhes matuyama transition have been identified in the pelagic pleistocene marls located on the calabrian side of the messina straits italy polarity of the characteristic magnetization has been deduced from an analysis of multicomponent magnetization this magnetostratigraphic age determination combined with the paleobathymetry of the quaternary marls implies uplift rates of fault bounded blocks of 0 7 1 0 m 1000 yr a positive fold test on three sites from the trubi formation 4 3 ma shows a clockwise rotation of 36 an antipodal test on pliocene pleistocene micritic limestones 0 9 2 0 ma also indicates a rotation of 14 with the same trend both results suggest a rotation rate of about 9 ma these results are in agreement with a tectonic mechanism involving simultaneous rotation and uplifts a discussion of the possible mechanisms indicates that a compression phase oriented ene wsw could have played a dominant role

seismicity normal faulting and the geomorphological development of the gulf of corinth greece the corinth earthquakes of february and march 1981 three major destructive earthquakes of m s 6 7 6 4 6 4 occurred in the eastern part of the gulf of corinth in february and march 1981 associated normal faulting was observed on both the north and the south sides of the gulf examination of teleseismic local seismic surface faulting and geomorpbological data suggests that the first and second of these shocks activated major north dipping normal faults these faults control the topography and bathymetry and are related to the recent uplift and subsidence of the coastline it is probable that the first earthquake occurred on a faultwhich outcrops underwater and the second on a fault which is visible on land it appears that the third shock activated an antithetic normal fault dipping southwards on the north side of the gulf and that the overall structure of the gulf of corinth graben is asymmetric the northern antithetic fault clearly demonstrates the geomorphological changes associated with developing young normal faulting in addition we suggest that observations of recent coastline movement provide a powerful tool for identifying major active fault systems and predicting their historical role in the paleogeography of an area

deep upper mantle melting beneath the tasman and coral seas detected with multiple scs reverberations abstract multiple scs reverberations are used to search for mantle reflectors beneath the tasman and coral seas with a hierarchical waveform inversion migration method in addition to the major transition zone discontinuities a low velocity layer above the 410 km discontinuity is detected the top of the low velocity layer lies at an average depth of 352 km indicating that the layer could be more than 70 km thick if it persists to the 410 km discontinuity which occurs at an average depth of 420 km along paths containing the low velocity layer we attribute the low velocities to partial melt resulting from volatile induced melting the considerable thickness of the partial melt layer may require thin films of a hydrous melt with a zero degree dihedral angle surrounding grains or the combined effect on melting of the addition of both water and carbon to the deep upper mantle via subduction although the depths of the transition zone discontinuities do not indicate that the transition zone itself is rich in water the impedance contrasts do contain a subtle signature that could be related to transition zone water namely a decrease in the impedance contrast across the 410 km discontinuity and a relatively strong 520 km discontinuity

seismic observations of splitting of the mid transition zone discontinuity in earth s mantle the transition zone of earth9s mantle is delineated by globally observed discontinuities in seismic properties at depths of about 410 and 660 kilometers here we investigate the detailed structure between 410 and 660 kilometers depth by making use of regional stacks of precursors to the ss phase the previously observed discontinuity at about 520 kilometers depth is confirmed in many regions but is found to be absent in others there are a number of regions in which we find two discontinuities at about 500 and 560 kilometers depth an effect which can be interpreted as a splitting of the 520 kilometer discontinuity these observations provide seismic constraints on the sharpness and observability of mineralogical phase transitions in the mantle transition zone

a complex 660 km discontinuity beneath northeast china abstract we present a detailed seismic study of the 660 km discontinuity beneath northeast china using the receiver function technique we use seismic data collected from 24 broadband stations in northeast china analysis of these data shows that the 660 km discontinuity is locally depressed in the region from longitude 128 0 e to 130 5 e and latitude 40 0 n to 44 0 n and then it splits into multiple discontinuities in the surrounding regions the complexity of the 660 km discontinuity beneath the subduction zones is probably attributable to the interaction between the upper mantle and subducted slab we speculate that the flattening of the subducted lithosphere near the bottom of the upper mantle causes the multiple discontinuity structure and that the slab penetrating the 660 km discontinuity into the lower mantle causes the narrow 660 km depression area

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

a model for tectonic rotations about a vertical axis abstract the rotation of a rigid ellipsoidal inclusion within a highly viscous fluid orientated so that two of the principal axes remain horizontal is used as a model for the rotation of crustal inclusions in wide zones of continental deformation this model is also applicable to other geological problems involving the rotation of inclusions in a matrix the pattern of behaviour in such a model is shown to be complex in general the rotation rate of the inclusion is a function of all components of the velocity field of the deforming medium and the horizontal aspect ratio of the inclusion however for a given velocity field this aspect ratio must exceed a critical value before the inclusion can rotate continuously inclusions with lower aspect ratios will rotate for a certain range of orientations in an opposite direction to the sense of shear in the deforming zone the possibility of the inclusion changing shape during rotation adds to the complexity of behaviour

geologic structure and neotectonics of the north african continental margin south of sicily abstract marine geological and geophysical data together with drilling information indicate that the north african passive continental margin has been subjected to extension and wrenching after it collided with the northern part of sicily the area of the tripolitania basin jarrafa trough melita and medina bank and the ragusa malta plateau has formed part of a sinking passive margin since the dispersal of gondwanaland at about 180 my ago as observed from geohistory diagrams a record of rifting in a nw se direction accompanied by dextral shear along the southern troughs is observed in seismic reflection data the rifting started during the neocomian and lasted until the eocene when activity became minor a pre middle miocene period of northward subduction of oceanic crust is inferred from the geology in ne sicily uplift of the northern part of the african margin after collision in the middle miocene is seen in wells in southern sicily after the messinian a rift and dextral shear zone established itself across the african margin from the strait of sicily to the medina ridge in the lonian basin the zone is marked by up to 1 7 km deep grabens narrow active wrench faulted channels volcanic fissures and local uplifted keilhorsts such as malta this zone which varies in width from 100 to 35 km forms the southern boundary of a microplate which includes sicily we speculate that the present motion of this microplate is partly due to the eastward movement of the calabrian arc with the sicilian block over the last remaining oceanic lithosphere in the eastern mediterranean

collapse basin a new type of extensional sedimentary basin from the devonian of norway pure shear and simple shear models provide interpretations of sedimentary basins formed by extension of crust and lithosphere of normal thickness devonian basins of western norway developed during collapse of the thickened caledonian crust and return to a crust and lithosphere of normal thickness because extension did not thin the lithosphere to less than its normal thickness no thermal subsidence phase followed the initial subsidence

paleomagnetic evidence for rotation in opposite senses of adjacent blocks in northeastern aegea and western anatolia paleomagnetic data from western anatolia and from the island of lesbos demonstrate significant counterclockwise clockwise or null rotations of coherent blocks which occurred during the neotectonic period these results suggest that the brittle carapace of the lithosphere may not everywhere follow the motion of the ductile lower part but that its movement arises from adjustment to the extensional tectonic regime

discussion of and correction to deformation of the ne basin and range province the response of the lithosphere to the yellowstone plume summary this article corrects and further develops theory that is necessary in order to relate slip sense on faults in the upper crustal brittle layer to continuous plastic deformation of the underlying uppermost mantle a reasonable starting point in order to do this but which some previous theoretical models for continental deformation fail to satisfy requires that in the absence of external forces acting on the brittle layer or the underlying plastically deforming material velocity in both layers must match overall as closely as possible in a region extending by pure shear only fault slip vector azimuth gives the extension direction beneath the brittle layer which equals the azimuth of the extensional eigenvector of the strain rate tensor beneath the brittle layer in a region that takes up distributed simple shear on parallel faults that do not rotate around vertical axes and extends by pure shear in the direction perpendicular to the simple shear neither the eigenvectors of the strain rate tensor nor the slip vector azimuth on the faults gives the true extension direction however individual faults in these two types of region may appear the same and unless one knows or infers independently the overall deformation sense in a region one cannot unambiguously make use of fault slip sense information from it earlier conclusions that tectonic deformation in the ne basin and range province of the western united states is driven by flow in the yellowstone upwelling plume are confirmed by these additional investigations

first paleomagnetic results from mio pliocene series of the hellenic sedimentary arc abstract paleomagnetic measurements have been conducted on the mio pliocene series along the external sedimentary hellenic arc in order to detect the presence or absence of rotational deformations in different sections of the arc in the central eastern part of the arc crete and rhodes paleomagnetic directions are very closely aligned along the north south direction showing that this portion of the arc has not undergone significant rotation since at least 7 m y for crete and 4 m y for rhodes in the western and northwestern parts results from south peloponnesos zakinthos kephallinia and corfu indicate that this section of the arc has undergone a clockwise rotation detailed analysis of the data from the three ionian islands shows that their rotation began about 5 m y b p and has proceeded since then at roughly 5 m y giving a total amplitude of about 26 it is suggested that the beginning of this rotation is related to the lower pliocene compressive phase which most probably initiated the tectonic activity of the present day ionian margin

northeast basin and range province active tectonics an alternative view slip rates and slip vector azimuths on major active oblique normal faults are used to investigate whether circulation associated with the yellowstone upwelling plume is driving tectonic deformation in the northeast basin and range province observed deformation is consistent with this suggestion the plume is sheared to the southwest by motion of the north american plate testable predictions are made for structure and evolution of the region

transition zone thickness beneath indonesia as inferred using the receiver function method for data from the jisnet regional broadband seismic network 1 we analyzed broadband waveforms recorded by the newly deployed japan indonesia seismic network to determine the depths of the 410 km and 660 km discontinuities and thickness of the mantle transition zone beneath indonesia by the receiver function method the discontinuities have an undulation with a maximum difference of 40 km with respect to the iasp91 model the thickness of the transition zone is well correlated with seismicity and seismic velocity anomalies suggesting that the topography is caused by temperature anomalies associated with the subducted slabs beneath indonesia

source parameters of the cache valley logan utah earthquake of 30 august 1962 abstract the cache valley utah earthquake of 30 august 1962 m s 5 7 has been reexamined our revised focal mechanism with dip 43 strike 193 and rake 101 confirms oblique normal faulting with nodal planes striking approximately north and south this mechanism is based mainly on polarities from records at u s air force lrsm stations which are a valuable scientific resource inversion of long period teleseismic body waveforms indicates centroid focal depth 10 km and seismic moment 3 1 10 24 dyne cm 0 31 10 18 nm corresponding to m w 5 6 the cache valley like other north south trending extensional features in the epicentral region is bounded on its east side by a major active normal fault the east cache fault which has several km of neogene throw however our revised source coordinates for the cache valley earthquake indicate it occurred too far east to have involved slip on this fault its hypocenter was near a downdip projection of the subparallel temple ridge fault a less prominent feature with only 500 m of neogene throw oriented subparallel to the west dipping nodal plane of our focal mechanism we suggest that this fault is continuous and approximately planar between 10 km depth and the earth9s surface with 40 dip and moved in the cache valley earthquake with a component of right lateral strike slip with slip vector azimuth n 65 w

surface faulting accompanying the borah peak earthquake central idaho the ms 7 3 borah peak earthquake that struck central idaho on october 28 1983 was one of the strongest historic earthquakes in the intermountain seismic belt much of the 34 km long northwest trending zone of fault scarps and surface ruptures that formed during the earthquake follows holocene and upper pleistocene scarps of the lost river fault throw along the new fault scarps averages 0 8 m exceeds 1 0 m along 43 of their length and attains a maximum of 2 7 m along the broad and complex zone of deformation in the southern section the net slip was normal sinistral averaging 17 cm lateral slip for 100 cm of dip slip the preferred nodal plane from the focal mechanism strikes n22 w dips 59 sw and suggests a much larger component of strike slip than do the geologic data

first paleomagnetic results from neocene formations in evia skyros and the volos region and the deformation of central aegea paleomagnetic results from neogene volcanic and sedimentary formations in central aegea indicate that evia and skyros have undergone a 48 and 26 clockwise rotation respectively the volos region does not show any significant rotation in the last 3 ma the rotation of evia is about twice as large as the one measured from coeval formations in the external hellenic arc the results are interpreted in terms of a published model of distributed deformation by faulting which also provides an explanation for the significant differential rotation of evia and skyros

planar high angle faulting in the basin and range geodetic analysis of the 1983 borah peak idaho earthquake geodetic elevation changes record the broad scale deformation associated with the m 7 0 october 28 1983 borah peak idaho earthquake on the lost river fault the crest of the lost river range rose 0 2 m and adjacent thousand springs valley subsided 1 0 m in relation to reference points 45 km from the main shock epicenter the deformation was modeled by dislocations in an elastic half space a planar fault with uniform dip slip of 2 05 0 10 m dipping 47 o 2osw and extending to a vertical depth of 13 3 1 2 km fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution the geodetic moment is 2 6 0 5 x 1019 n m 2 6 0 5 x 1026 dyn cm and the estimated static stress drop is 2 9 0 4 mpa 29 4 bars tests for coseismic slip on listric faults which flatten with depth and on detachments horizontal faults or shear zones showed fits to the geodetic data that are inferior to those for planar high angle faults no detectable coseismic slip occurred on the mesozoic white knob thrust fault although the low angle thrust sheet intersects the south end of lost river fault near the 1983 mainshock epicenter if the high angle lost river fault abuts a flat lying detachment fault or shear zone such a structure must lie at depths of 12 km near the brittle ductile transition where stick slip behavior gives way to creep the depth and geometry of faulting at borah peak is similar to that inferred from seismic and geodetic evidence for the 1954 m 7 2 fairview peak nevada and the 1959 m 7 3 hebgen lake montana events suggesting that if detachments are active at these localities they are deep and most likely slip by creep

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

evidence for a thick mantle transition zone beneath the philippine sea from multiple scs waves recorded by jisnet 1 we deployed the jisnet broadband seismic network in west and central indonesia using data from jisnet and other broadband stations we estimate the thickness of the mantle transition zone beneath east and southeast asia and the philippine sea using the scs reverberation method the thickness is about 240 km beneath east and southeast asia 280 290 km beneath the western edge of the philippine sea 260 280 km beneath the central part of the philippine sea and 260 km beneath the izu bonin arc these results imply that the temperature is about 300 k lower than the global average beneath the western edge of philippine sea 200 k lower beneath the mid part of the philippine sea 130 k lower beneath the izu bonin arc and almost equal to the average beneath east and southeast asia

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

the tripoli libya earthquake of september 4 1974 implications for the active tectonics of the central mediterranean source parameters have been determined for the earthquake ms 5 6 that occurred offshore of tripoli libya on september 4 1974 one nodal plane of its focal mechanism has dip 37 strike 297 and rake 141 indicating oblique normal faulting this nodal plane is subparallel to many west northwest striking normal faults in the epicentral area and is most likely the fault plane indicating a component of right lateral strike slip with slip vector azimuth n84 e inversion of long period teleseismic body waves indicates 12 km centroid depth and 0 4 1018 n m seismic moment a much larger earthquake ms 7 0 on april 19 1935 that occurred in the same zone of active oblique normal faults 400 km farther southeast near sirte probably involved similar slip sense this zone for which the name tunisia libya seismic zone appears appropriate has overall northwest southeast extent 1000 km from northern libya to between tunisia and sicily it takes up a change in motion direction relative to stable europe from west of north inside the african plate to between n30 e and n50 e in the ionian sea between sicily southernmost peninsular italy southwest greece and libya this suggested motion direction of sicily relative to stable europe agrees with independent estimates from fault slip rates and senses elsewhere in italy

reflectivity of the 410 km discontinuity from pp and ss precursors a number of studies have confirmed the global existence of a transition zone discontinuity at 410 km depth by aligning large numbers of long period seismograms on a surface reflection phase before stacking in particular ss and pp precursors from the 410 km discontinuity termed p410p and s410s have revealed long wavelength topography of this discontinuity here we extend these techniques to examine the reflection coefficient of the 410 km discontinuity using our measurements of p410p and s410s amplitudes we constrain the impedance contrasts across the 410 km discontinuity we also show lateral variations in the p wave impedance contrast at 410 km which is typically low under north america and china and higher beneath the north pacific the s wave impedance contrast shows less variability on the regional scale however analysis of p410p and s410s amplitudes over smaller areas by binning traces into spherical caps shows that the s wave reflection coefficient varies over much shorter scale lengths than that for p waves the different patterns of variation for p410p and s410s reflection amplitudes could be due to the presence of melt water or other chemical heterogeneities in the transition zone other factors such as temperature or mantle olivine content variations could also influence precursor amplitudes but they would be expected to lead to correlated variations and so they cannot explain all the variation that we observe

ss wave sensitivity to upper mantle structure implications for the mapping of transition zone discontinuity topographies 1 studies of the depths of the upper mantle discontinuities have relied on the differential travel times between the ss wave and its precursors we examine the effect of velocity heterogeneities on these differential travel time data ray theory which is used to correct these data for the effect of velocity heterogeneities is shown to be inaccurate if the characteristic heterogeneity length scales are similar to those seen in regional models or if the station is near the ss wave radiation nodal plane

the calabrian arc and the ionian sea in the dynamic evolution of the central mediterranean in the central mediterranean area it is possible to recognize three main realms the tyrrhenian sea the pelagian block and the ionian block these domains are limited by important fracture zones that are easily recognizable on land and offshore these fracture zones can be grouped into four main trends 1 the nw se trend which is characterized by dextral horizontal movements 2 the ne sw trend which is generally characterized by sinistral lateral horizontal movements 3 the e w trend which also shows dextral shear movements and 4 the n s trend which is characterized by normal faults only the tyrrhenian sea realm is characterized by a thinned crust of oceanic type and is connected to the apenninic chain by means of listric faults from a geophysical point of view the area shows a high heat flow which follows the main structural trends and positive gravimetric anomalies which are compatible with seismic refraction data and can be explained by the proposed model the pelagian block shows a continental crust about 20 km thick which progressively increases beneath the sicilian chain here an abrupt interruption with the tyrrhenian block takes place in the pelagian realm rifting processes developed according to the nw se and n s trend the n s trend characterizes the westernmost zone of the realm where volcanic processes and a relatively high heat flow can also be observed the ionian block is characterized by the following a thinned crust 17 20 km which increases 40 km towards the southern calabrian chain a thick sedimentary sequence high positive bouguer anomalies 300 mgal and very low heat flow 50 mw m 2 geophysical and structural analyses allowed us to evaluate the post tortonian evolution of the whole area under consideration this evolution resulted in continental crust formation according to a rigid plastic deformation model according to our model the ionian block can be considered as a domain characterized by a thinned continental crust and affected by intrusive material derived from the upper mantle furthermore this domain of probably jurassic age can be considered as a pelagic basin bordered by the pelagian block to the west and by the apulian block to the east northeast this basin according to its position with respect to the apenninic and sicilian chains has not been affected by tectogenesis and still represents a site of continuous sedimentation from a kinematic point of view the ionian realm can be linked with the pelagian block shifting towards the east northeast whereas it is limited towards the north by an e w fracture zone which regulates the opening of the tyrrhenian sea and the counterclockwise rotation of the apenninic chain according to the proposed model the opening of the tyrrhenian sea can be considered as a megaextension fracture which developed parallel to the maximum principal stress direction and evolved as a triangular shaped structural feature in preparing the model the authors have taken into account a large set of data collected both on land and offshore

quaternary extension in southern tibet field observations and tectonic implications we summarize evidence for quaternary and active faulting collected in the field during three sino french expeditions to southeastern tibet 1980 1982 detailed mapping of quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the himalayas in the last 2 0 5 m y the maximum horizontal principal stress in south tibet appears to be only the intermediate principal stress 2 1 being vertical south of the chord joining the eastern and western syntaxes of the himalayan arc extensional strains are principally localized within seven regularly spaced rift zones three of which have been studied in some detail the extension direction is determined to be n96 e 7 mainly from statistical averaging of strikes of newly formed normal faults throw rates on normal faults are evaluated for different time spans 2 0 5 m y 60 40 kyr and 10 2 kyr b p using structural and topographic reliefs as well as synglacial and postglacial vertical offsets the rate of quaternary extension is about 1 m y 1 along an 1100 km long ese traverse across south tibet this corresponds to a spreading rate of 1 0 6 cm yr this rate and the divergent horizontal projections of slip vectors of earthquakes along the himalayan front constrain the rate at which rigid india underthrusts southern tibet to be 2 1 cm yr although most of the normal faults appear to be independent of and nearly orthogonal to the e w mesozoic tertiary tectonic fabric the yadong gulu rift appears to be guided for over 130 km by the older oblique ne sw nyainqentanglha range and fault zones along it this reactivated zone is the most prominent left lateral strike slip fault system in se tibet excepting this zone and the vicinity of the se extremity of the karakorum fault quaternary strike slip faulting is rare in south tibet i e south of the chord between the syntaxes of the himalayan arc north of the chord the tectonic style is different there minor conjugate strike slip faulting is widespread and appears to control quaternary normal faulting which is more diffuse and subdued than in south tibet along the chord the presence of a major zone of active right lateral en echelon strike slip faults karakorum jiali fault zone probably reflects the greater facility of eastward extrusion in north central tibet in response to the northward push of india the eastern himalayan syntaxis may be an obstacle to such extrusion movements south of the chord

aftershock relocation and frequency size distribution stress inversion and seismotectonic setting of the 7 august 2013 m 5 4 earthquake in kallidromon mountain central greece abstract on august 7 2013 a moderate earthquake noa ml 5 1 noa mw 5 4 occurred in central kallidromon mountain in the pthiotis region of central greece 2270 aftershocks were relocated using a modified 1 d velocity model for this area the b value of the aftershock sequence was b 0 85 for a completeness magnitude of mc 1 7 the rate of aftershock decay was determined at p 0 63 the spatial distribution of the aftershock sequence points towards the reactivation of a n70 10 e striking normal fault at crustal depths between 8 and 13 km a nnw sse cross section imaged the activation of a steep south dipping normal fault a stress inversion analysis of 12 focal mechanisms showed that the minimum horizontal stress is extensional at n173 e no primary surface ruptures were observed in the field however the earthquake caused severe damage in the villages of the kallidromon area the imaged fault strike and the orientation of the long axis of the aftershock sequence distribution are both at a high angle to the strike of known active faults in this area of central greece we interpret the kallidromon seismic sequence as release of extensional seismic strain on secondary steep faults inside the fokida viotia crustal block

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

an analysis of ss precursors using spectral element method seismograms summary the traveltimes of underside shear wave reflections i e ss precursors are widely used data for mapping the topography of mantle discontinuities in the upper mantle here we examine the accuracy of ray theory in estimating the contribution of shear velocity heterogeneity in the mantle to ss s400s ss s670s and s400s s670s traveltimes we analyse stacks of spectral element method waveforms computed for 34 shallow earthquakes and 16 020 globally distributed virtual seismometers the waveforms are computed for three versions of shear velocity model s20rts with variable strengths of the volumetric shear velocity perturbations within the layered structure of the preliminary reference earth model we find that ray theoretical corrections account for only 50 per cent of the traveltime variation due to large scale velocity heterogeneity for current tomographic models as represented by s20rts this translates into unpredictable long and short wavelength errors in maps of mantle discontinuity depths of about 5 km this amounts to roughly 10 per cent of published variations in mantle discontinuity depths however relative errors will be significantly larger if ray theoretical traveltime corrections are based on forthcoming models of shear velocity with much stronger heterogeneity at the smallest scales

the growth of geological structures by repeated earthquakes 2 field examples of continental dip slip faults a strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault bounded geological structures such as basins and ranges which are built by repeated cycles of deformation along strike slip faults the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign resulting in little permanent deformation except for the fault offset for dip slip faults portions of the crust are lifted and dropped and so buoyancy forces are exerted the seismic and interseismic deformations do not balance and structures grow and become subject to erosion and deposition we consider three examples for which the structure and fault geometry are well known the white wolf reverse fault in california site of the 1952 kern county m 7 3 earthquake the lost river normal fault in idaho site of the 1983 borah peak m 7 0 earthquake and the cricket mountain normal fault in utah site of quaternary slip events basin stratigraphy and seismic reflection records are used to profile the structure and coseismic deformation measured by leveling surveys is used to estimate the fault geometry to reproduce these structures we add the deformation associated with the earthquake cycle the coseismic slip and postseismic relaxation to the flexure caused by the observed sediment load treating the crust as a thin elastic plate overlying a fluid substrate the cumulative deformation is principally dependent on the elastic plate thickness modestly sensitive to the sediment substrate density difference and insensitive to the fluid viscosity for the 4 to 8 ma structures we deduce a longterm flexural rigidity of 2 15 1019 nm this is equivalent to an elastic plate thickness of 2 4 km for a young s modulus of 2 5 1010 nm 2 this value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km but where coseismic fault slip extends to a depth of 10 15 km thus much of the seismogenic crust must weaken substantially during the life of active faults causing the fault bounded basins to narrow over time

the tectonic development of the hellenic arc and the sea of crete a synthesis abstract we use field analyses together with interpretation of aerial photographs orbital imagery and sea beam data to define more accurately the geometry of fault systems in the hellenic arc the tectonics of the area are dominated by extensional processes related to normal faulting we then use sea beam data previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the cretan sea basin which is found consistent with the land fault pattern the mechanisms of the late neogene and quaternary deformation of the central and southern aegean region are analysed in terms of stress trajectories as revealed by field studies of fault populations we then examine the subsidence of the sea of crete in terms of a model of uniform stretching of the lithosphere the opening of the sea of crete was cylindrical and did not result in bending of crete this leads us to modify the southern portion of an earlier kinematic reconstruction of aegea in agreement with recent paleomagnetic results the implications of the opening of the sea of crete on the tectonic evolution of the straits to the west and east of crete are discussed finally the uplift of the outer arc is explained by underplating due to subduction

the effect of topography of upper mantle discontinuities on ss precursors using the spectral element method we explored the effect of topography of upper mantle discontinuities on the traveltimes of ss precursors recorded on transverse component seismograms the latter are routinely used to infer the topography of mantle transition zone discontinuities the step from precursory traveltimes to topographic changes is mainly done using linearised ray theory or sometimes using finite frequency kernels we simulated exact seismograms in 1 d and 3 d elastic models of the mantle in a second simulation we added topography to the discontinuities we compared the waveforms obtained with and without topography by cross correlation of the ss precursors since we did not add noise the precursors are visible in individual seismograms without the need of stacking the resulting time anomalies were then converted into topographic variations and compared to the original topographic models based on the correlation between initial and inferred models and provided that ray coverage is good we found that linearised ray theory gives a relatively good idea on the location of the uplifts and depressions of the discontinuities it seriously underestimates the amplitude of the topographic variations by a factor ranging between 2 and 7 real data depend on the 3 d elastic structure and the topography all studies to date correct for the 3 d elastic effects assuming that the traveltimes can be linearly decomposed into a structure and a discontinuity part we found a strong non linearity in this decomposition which cannot be modelled without a fully non linear inversion for elastic structure and discontinuities simultaneously

collapse of a hercynian tibetan plateau into a late palaeozoic european basin and range province the large width the importance of crustal shortening and the likelihood of thick crust in the late palaeozoic hercynian or variscan belt of southern europe suggest a tectonic evolution analogous to the evolution of the tibetan plateau and its neighbouring himalaya1 and of the altiplano and other high plateaux of the andes here we extend dewey and burke s analogy1 of the hercynian chain with tibet to include the subsequent stage of pervasive normal faulting and crustal extension and thinning which characterize the active tectonics of tibet parts of the high andes and the basin and range province of western north america it appears that normal faulting and rifting also characterize the late palaeozoic tectonic activity of the hercynian chain

stabilizing effects of bacterial biofilms eps penetration and redistribution of bed stability down the sediment profile biofilms consisting of microorganisms and their secreted extracellular polymeric substances eps serve as ecosystem engineers stabilizing sedimentary environments natural sediment bed provides an excellent substratum for biofilm growth the porous structure and rich nutrients allow the eps matrix to spread deeper into the bed a series of laboratory controlled experiments were conducted to investigate sediment colonization of bacillus subtilis and the penetration of eps into the sediment bed with incubation time in addition to eps accumulation on the bed surface eps also penetrated downwards however eps distribution developed strong vertical heterogeneity with a much higher content in the surface layer than in the bottom layer scanning electron microscope sem images of vertical layers also displayed different micro morphological properties of sediment eps matrix in addition colloidal and bound eps exhibited distinctive distribution patterns after the full incubation the bio sedimentary beds were eroded to test the variation of bed stability induced by biological effects this research provides an important reference for the prediction of sediment transport and hence deepens the understanding of the biologically mediated sediment system and broadens the scope of the burgeoning research field of biomorphodynamics

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

a block model of distributed deformation by faulting it is clear from palaeomagnetic evidence that large and hitherto unexpected rotations about a vertical axis are common in regions of distributed continental deformation we propose a simple two dimensional model to illustrate how fault movement and block rotation within a zone of distributed deformation may be related to the relative motion of the rigid plates that bound the zone one surprizing feature of this model is that the component of strike slip motion on the faults within the deforming zone is in the opposite sense to the strike slip component across the zone as a whole thus slip vectors within the zone are not the same as that between the bounding rigid plates in contrast to deformation at oceanic plate boundaries examples of active fault geometries similar to those of our model can be seen in greece iran and the western usa

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

deformation of the ne basin and range province the response of the lithosphere to the yellowstone plume summary tectonic deformation in the ne basin and range province of the western united states is concentrated in two zones the n s trending northern part of the intermountain seismic belt in northern utah se idaho and sw wyoming and the e w trending central idaho seismic zone which converge at yellowstone in nw wyoming accepting the popular view that the yellowstone volcanic field is above an upwelling plume from the deep mantle i explore the possibility that deformation of the brittle upper crust in these two seismic zones may be caused by return flow from this plume this return flow is assumed sheared sw in the direction of motion of the north american plate relative to yellowstone the snake river plain srp a linear outcrop of neogene and quaternary basalt that trends sw from yellowstone for more than 400 km bisecting the angle between the two seismic zones may be a trail formed as the plate moves sw across the upwelling plume beyond 150 km from yellowstone slip vectors on active normal faults in both deforming zones are oriented inwards at 45 towards the sw azimuth along the srp normal slip rates on these faults are typically 1 mm yr 1 implying extensional strain rate 10 15 s 1 proportions of normal slip and strike slip suggest faults in the intermountain seismic belt are rotating anticlockwise at 2 myr 1 around vertical axes and faults in the central idaho seismic zone are rotating clockwise at similar rates within 100 km of yellowstone observed sense of rotation around vertical axes in both zones appears reversed tilt rates of hanging wall beds suggest most faults in both zones are rotating at 2 myr 1 also 10 15 s 1 around horizontal axes oriented nw the relative size of rotation rate around vertical axes and horizontal extensional strain rate is shown to imply that velocity in the underlying deforming fluid is small in the direction perpendicular to the axis of the srp in an initial model this velocity is set to zero enabling the two dimensional pattern of flow at the horizontal upper boundary of this deforming fluid to be solved straightforwardly this approximation enables observed rates and senses of horizontal extension and rotation around a vertical axis to be explained as consequences of the pattern of horizontal gradients of velocity parallel to the axis of the srp in the deforming fluid beneath the brittle layer a refinement suggests that velocity perpendicular to the srp near the upper boundary of the deforming fluid is small but inward towards the srp from both sides and is accompanied by downwelling beneath the srp beyond 100 km sw of yellowstone this pattern of flow will act to move magma in the uppermost mantle beneath the srp from its sides it may thus reconcile petrological observations that srp basalts do not show deep mantle characteristics expected had they been extruded from the yellowstone plume with the existence of this plume nearby conservation of angular momentum around horizontal axes perpendicular to the srp for the combined system of the plume interacting with the north american plate leads to the deduction that the plume radius is 50 km consistent with results of other investigations and its upwelling velocity is 60 mm yr 1

active tectonics of the alpine himalayan belt between western turkey and pakistan summary over 80 new fault plane solutions combined with satellite imagery as well as both modern and historical observations of earthquake faulting are used to investigate the active tectonics of the middle east between western turkey and pakistan the deformation of the western part of this region is dominated by the movement of continental material laterally away from the lake van region in eastern turkey this movement helps to avoid crustal thickening in the van region and allows some of the shortening between arabia and eurasia to be taken up by the thrusting of continental material over oceanic type basement in the southern caspian mediterranean makran and black sea thus central turkey bounded by the north and east anatolian strike slip faults is moving west from the van region and overrides the eastern mediterranean at two intermediate depth seismic zones one extending between antalya bay and southern cyprus and the other further west in the hellenic trench the motion of northern iran eastwards from the van region is achieved mainly by a conjugate system of strike slip faults and leads to the low angle thrusting of iran over the southern caspian sea the seismicity of the caucasus shows predominantly shortening perpendicular to the regional strike but there is also some minor elongation along the strike of the belt as the causcasus overrides the caspian and black seas the deformation of the eastern part of this region is dominated by the shortening of iran against the stable borders of turkmenistan and afghanistan the north east direction of compression seen in zagros is also seen in north east iran and the kopet dag where the shortening is taken up by a combination of strike slip and thrust faulting large structural as well as palaeomagnetic rotations are likely to have occurred in ne iran as a result of this style of deformation north south strike slip faults in southern iran allow some movement of material away from the collision zone in ne iran towards the makran subduction zone where genuinely intermediate depth seismicity is seen within this broad deforming belt large areas such as central turkey nw iran azerbaijan central iran and the southern caspian appear to be almost aseismic and therefore to behave as relatively rigid blocks surrounded by active belts 200 300 km wide the motion of these blocks can usefully be described by poles of rotation the poles presented in this paper predict motions consistent with those observed and also predict the opening of the gulf of iskenderun ne of cyprus the change within the zagros mountains from strike slip faulting in the nw to intense thrusting in the se and the relatively feeble seismicity in se iran baluchistan this description also explains why the north south structures along the iran afghanistan border do not cut the east west ranges of the makran within the active belts surrounding the relatively aseismic blocks a continuum approach is needed for a description of the deformation even though motions at the surface may be concentrated on faults the evolution of fault systems within the active zones is controlled by geometric constraints such as the requirement that simultaneously active faults do not in general intersect many of the active processes discussed in this paper particularly large scale rotations and lateral movement along the regional strike are likely to have caused substantial complexities in older mountain belts and should be accounted for in any reconstructions of them

upper mantle discontinuity topography from thermal and chemical heterogeneity using high resolution stacks of precursors to the seismic phase ss we investigated seismic discontinuities associated with mineralogical phase changes approximately 410 and 660 kilometers km deep within earth beneath south america and the surrounding oceans detailed maps of phase boundary topography revealed deep 410 and 660 km discontinuities in the down dip direction of subduction inconsistent with purely isochemical olivine phase transformation in response to lowered temperatures mechanisms invoking chemical heterogeneity within the mantle transition zone were explored to explain this feature in some regions multiple reflections from the discontinuities were detected consistent with partial melt near 410 km depth and or additional phase changes near 660 km depth thus the origin of upper mantle heterogeneity has both chemical and thermal contributions and is associated with deeply rooted tectonic processes

the growth of geological structures by repeated earthquakes 1 conceptual framework in many places earthquakes with similar characteristics have been shown to recur if this is common then relatively small deformations associated with individual earthquake cycles should accumulate over time to create geological structures following this paradigm we show that existing models developed to describe leveling line changes associated with the seismic cycle can be adapted to explain geological features associated with a fault in these models an elastic layer containing the fault overlies a viscous half space with a different density fault motion associated with an earthquake results in immediate deformation followed by a long period of readjustment as stresses relax in the viscous layer and isostatic equilibrium is restored deformation is also caused as a result of the loading and unloading due to sediment deposition and erosion in this paper the parameters that control the growth of dip slip structures are identified we find that the flexural rigidity of the crust or the apparent elastic thickness provides the main control of the width of a structure the loading due to erosion and deposition of sediment determines the ratio of uplift to subsidence between the two sides of the fault the flexure due to sediment load is much more important in this respect than whether the fault is normal or reverse in character we find that in general real structures are associated with apparent elastic thicknesses of 4 km or less and thus with very low flexural rigidities

sensitivity analysis of seismic waveforms to upper mantle discontinuities using the adjoint method using spectral element simulations of wave propagation we investigated the sensitivity of seismic waveforms recorded on transverse components to upper mantle discontinuities in 1 d and 3 d background models these sensitivity kernels or frechet derivatives illustrate the spatial sensitivity to model parameters of which those for shear wave speed and the surface topography of internal boundaries are discussed in this paper we focus on the boundaries at 400 and 670 km depth of the mantle transition zone ss precursors have frequently been used to infer the topography of upper mantle discontinuities these seismic phases are underside reflections off these boundaries and are usually analysed in the distance range of 110 160 this distance range is chosen to minimize the interference from other waves we show sensitivity kernels for consecutive time windows at three characteristic epicentral distances within the 110 160 range the sensitivity kernels are computed with the adjoint method using synthetic data from our simulations we can draw three main conclusions i the exact frechet derivatives show that in all time windows and also in those centred on the ss precursors there is interference from other waves this explains the difficulty reported in the literature to correct for 3 d shear wave speed perturbations even if the 3 d structure is perfectly known ii all studies attempting to map the topography of the 400 and 670 km discontinuities to date assume that the traveltimes of ss precursors can be linearly decomposed into a 3 d elastic structure and a topography part we recently showed that such a linear decomposition is not possible for ss precursors and the sensitivity kernels presented in this paper explain why iii in agreement with previous work we show that other parts of the seismograms have greater sensitivity to upper mantle discontinuities than ss precursors especially multiply bouncing s waves exploiting the s wave triplications due to the mantle transition zone these phases can potentially improve the inference of global topographic variations of the upper mantle discontinuities in the context of full waveform inversion in a joint inversion for an elastic parameters and topography

reconciling pp and p p precursor observations of a complex 660 km seismic discontinuity high frequency precursors to p p almost invariably observe a narrow 660 km discontinuity whereas pp precursor studies at long periods struggle to detect a reflection from the 660 despite its apparent sharpness to p p to investigate these contradictory observations we compare pp and p p precursors in the same region using short period p p precursors we observe a sharp 660 km discontinuity which appears to vary in depth substantially the apparent topography on the 660 is too large to originate solely from thermal variations regardless of its cause therefore indicating chemical variations at the base of the mantle transition zone long period p p precursors show no 660 as they are sensitive to a larger area and thus average out the apparent topography in agreement with long period pp precursors instead we see some evidence in both long period data types for a reflection from 720 km depth which is likely to correspond to a phase change in the garnet system

block rotation in western turkey 1 observational evidence western turkey is a region of rapid continental extension where extension rate increasing westward causes counterclockwise vertical vorticity and hence counterclockwise rotation around vertical axes on a regional scale field observations and seismicity of the past century are used to deduce positions and slip senses of major active oblique normal faults in this region which take up extension in the upper crustal brittle layer and allow fault bounded blocks to rotate around vertical axes the principal active normal faults in western turkey strike roughly east west near the aegean coast turning gradually toward westnorthwest eastsoutheast strike across 200 km distance inland to the east slip vector azimuths indicate typical extension direction on these faults is s18 8w they truncate a second set of faults with smaller displacement and roughly orthogonal strike and slip vector azimuth as well as subdividing the brittle layer into blocks that show little internal deformation some major oblique normal faults subdivide it into domains that show different neogene rotation around vertical axes some domains contain sets of major active oblique normal faults that bound blocks that are elongated angular and shaped like tilted dominoes these faults take up extension and can also take up rotation around vertical axes provided they and the blocks between them rotate around vertical axes at the same rate where adequate paleomagnetic observations are available they indicate these domains have rotated counterclockwise as expected by up to 40 roughly equal to the change in strike of the principal set of active faults between the aegean coast and the eastern edge of the actively extending zone however evidence is scarce for the right lateral strike slip expected in some kinematic models on a set of oblique normal faults that takes up extension and counterclockwise rotation instead each fault appears to have the component of strike slip necessary to enable it given its local strike to slip in ue direction in which the region is extending two domains that have rotated clockwise are approximately circular in plan view with diameter 90 km and are bounded by faults with variable orientation each comprises a single block with no internal major active faults

waveform inversion of ss precursors an investigation of the northwestern pacific subduction zones and intraplate volcanoes in china abstract the arrival time and amplitude of underside reflections from mantle seismic discontinuities ss precursors have made major contributions to the understanding of mantle composition and dynamics in this study we introduce a nonlinear waveform inversion technique to simultaneously constrain shear velocities and discontinuity depths beneath the northwestern pacific subduction system based exclusively on a large ss precursor waveform dataset we are able to clearly delineate the morphology of the descending pacific plate which flattens at the base of the upper mantle and extends westward by 1500 km toward northern central china our grid search yields the maximum correlation between shear velocity and transition zone thickness at an angle of 30 consistent with the reported average slab dip beneath the study region the strongly positive correlation suggests predominantly thermal rather than compositional variations along the descending pacific plate the joint depth velocity solution also shows a 5 10 km depression of the 410 km discontinuity and an average decrease of 1 2 in upper mantle shear velocity beneath the intraplate volcanic fields in northeastern china this anomaly which reaches the middle of the upper mantle transition zone beneath the changbai hotspot initiates at a significantly shallower 320 km depth beneath the wudalianchi region high amplitude reflections at depths greater than 410 km suggest a water poor melt layer in possible association with 1 decompression melting from passive upwelling and or 2 active upwelling through a slab window

the neotectonics of the aegean an alternative view minor reverse faults in widely dispersed neogene outcrops in the central aegean have led several people to suggest that the regional extension which started in the lower pliocene or earlier was interrupted by short periods of compression the durations of these postulated compressional episodes are short compared to the time scale on which convective forces acting on the base of the lithosphere can change thus possible mechanisms for driving such motions are limited in this paper we offer alternative explanations for these observations and in particular show that these apparently compressional episodes are probably not regional in extent and may not be truly compressional in origin in some places they are more likely to be a consequence of the considerable rotation that is found in extensional terranes the plio quaternary paleogeography of the central aegean is satisfactorily explained by uplift in the footwall blocks of normal faults and does not require regional compression studies of the faulting associated with recent large earthquakes suggest that the sort of microtectonic analysis which is typically carried out on small faults in surficial sediments is more likely to reflect the internal deformation of blocks bounded by major fault structures than be a reliable indicator of regional stress patterns

source parameters of the 28 october 1983 borah peak idaho earthquake from body wave analysis modeling of long period body waves of the m s 7 3 borah peak idaho earthquake suggests that the earthquake was a simple rupture that nucleated at a depth of about 16 km and propagated unilaterally northwestward toward the surface the seismic moment tensor obtained from the inversion of teleseismic body wave amplitude data agrees well with the fault plane solution obtained from short period first motion data strike 138 3 dip 45 3 rake 60 5 and the observed surface faulting the scalar double couple moment of 2 1 10 26 dyne cm obtained from the inversion is comparable to a moment of 1 4 10 26 dyne cm estimated from the observed surface faulting estimates of stress drop based on these values for the moment are 17 to 12 bars respectively

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

topography of the 410 km and 660 km discontinuities beneath the japan sea and adjacent regions by analysis of multiple scs waves the northwest pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities due to the sparse distribution of seismic stations in the sea previous studies mostly focus on mantle transition zone mtz structures beneath continents or island arcs leaving the vast area of the japan sea and okhotsk sea untouched in this study we analyzed multiple scs reverberation waves and a common reflection point stacking technique was applied to enhance consistent signals beneath reflection points a topographic image of the 410 km and 660 km discontinuities is obtained beneath the japan sea and adjacent regions one dimensional and 3 d velocity models are adapted to obtain the apparent and true depth we observe a systematic pattern of depression 10 20 km and elevation 5 10 km of the 660 with the topography being roughly consistent with the shift of the olivine phase transition boundary caused by the subducting pacific plate the behavior of the 410 is more complex it is generally 5 15 km shallower at the location where the slab penetrates and deepened by 5 10 km oceanward of the slab where a low velocity anomaly is observed in tomography images moreover we observe a wide distribution of depressed 410 beneath the southern okhotsk sea and western japan sea the hydrous wadsleyite boundary caused by the high water content at the top of the mtz could explain the depression the long history trench rollback motion of pacific slab might be responsible for the widely distributed depression of the 410 ranging upward and landward from the slab

seismic shear wave reflectivity structure of the mantle beneath northeast china and the northwest pacific 1 using multiple scs reverberations we examine the mantle reflectivity structure beneath northeast china and the northwest pacific we find several upper mantle discontinuities including a melt layer with a mean thickness of 64 km atop the 410 km discontinuity present on both sides of the subducting slab near the nankai trench the transition zone contains a split 520 km discontinuity in several paths and tomographic images show stagnant slabs at this depth we believe that this may be slab related based on experimental work a negative reflector is found in one path beneath the northeast china craton at a depth of 598 km mid mantle reflectors are found in all of our paths and are present throughout a wide depth range 750 1600 km

surface faulting in the southern italian campania basilicata earthquake of 23 november 1980 the campania basilicata earthquake of 23 november 1980 was the largest ms 6 9 and most destructive earthquake that has occurred in this region of southern italy for over 100 years killing more than 3 000 people the consensus among seismologists geologists and earthquake engineers who have studied1 10 this event is in keeping with previous italian earthquakes that it did not produce faulting at the surface this has led to the establishment of a number of models2 3 11 13 requiring an unusual tectonic setting for the southern apennines however on a recent visit to the epicentral region of the earthquake may 1984 we discovered more than 10 km of surface faulting that is consistent with focal mechanisms for this event based on first motion polarities2 4 5 12 and long period waveform studies14 16 which show that it involved normal faulting with one nodal plane dipping at 60 towards the north east our observations of this the first well documented earthquake fault break to be reported in italy suggest that the deformation associated with earthquakes in the southern apennines takes place in the upper 10 15 km of the earth s crust on steep planar normal faults which break the surface as is widely observed in other areas of active extension outside italy

receiver functions in northeast china implications for slab penetration into the lower mantle in northwest pacific subduction zone abstract seismic studies of the subducting lithosphere and the upper mantle discontinuities in the northwest pacific subduction zone beneath japan and northeast china have suggested contrary subduction scenarios there was little consensus on the issue whether the subducting slab penetrates the upper mantle discontinuities into the lower mantle or it is deflected atop of the 660 km discontinuity over several hundred kilometers we calculate receiver functions from a recent seismic broadband station network located in northeast china and find topographic variations of the upper mantle discontinuities a deeper than normal 660 km discontinuity is observed over an area of 400 km and it coincides with the stagnant slab imaged by seismic tomography the 660 km discontinuity is locally depressed by more than 35 km and the transition zone is thickened by more than 20 km in the east of the region where it encounters the slab these observations provide evidence of the slab accumulating in the mantle transition zone and locally penetrating into the lower mantle

strike slip fault geometry in turkey and its influence on earthquake activity the geometry of turkish strike slip faults has been reviewed and described from this data set it appears that fault geometry the distribution of discontinuities such as bends and stepovers along the main fault trace plays an important role in controlling the location of large earthquake rupture segments along the fault zones large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30 more important than the size of each discontinuity however is the total geometric pattern i e the distribution of adjacent bends and stepovers based not only on distance from one another but also on relative discontinuity size certain geometric patterns restraining single or double bends are particularly common and can be viewed as responsible for strain accumulation along portions of the fault zone fault geometry not only plays a role in the extent of earthquake rupture but also in characteristics of earthquake behavior for example large earthquake epicenters often occur near restraining bends or double bends furthermore aftershocks and swarm activity can often be associated with releasing areas areas along the strike slip fault that are undergoing a relatively large amount of extension

seismicity and late quaternary faulting of the northern basin and range province montana and idaho abstract examination of seismicity and late quaternary faults in montana and idaho north of the snake river plain shows a geographic correspondence between high seismicity and 24 faults that have experienced surface rupture during the late quaternary the lewis and clark zone delineates the northern boundary of this tectonically active extensional region earthquakes greater than magnitude 5 5 and all identified late quaternary faults are confined to the montana idaho portion of the basin and range province south of the lewis and clark zone furthermore all 12 holocene faults are confined to a seismologic zone centennial tectonic belt parallel to the northern flank of the snake river plain in extreme southwestern montana and adjacent idaho fault trends strain data and focal mechanisms suggest that both late quaternary faulting and seismicity in the region are primarily products of two distinct stress regimes 1 overall basin and range extension along a s45 w direction relative to the mid continent and 2 localized effects of the yellowstone hot spot which appears to have the least principle stress axis oriented horizontally at about s4 w the three largest historic earthquakes north of the snake river plain are indicative of three different stress provinces the 1983 borah peak earthquake occurred in the montana idaho basin and range the 1959 hebgen lake earthquake is associated with the yellowstone hot spot area and the 1925 clarkston valley event appears to be associated with the stress field of the united states mid continent

present day kinematics of the plate boundary zone between africa and europe from the azores to the aegean anticlockwise rotation of africa relative to europe around an euler pole in the eastern atlantic causes plate convergence in the central mediterranean towards n20 w at rate considered a priori to be 5 10 mm yr 1 however previous summaries of the present day deformation sense in the central mediterranean are inconsistent with observations in southern italy recent investigations of italy and adjacent parts of north africa enable local deformation sense observed to be reconciled with sense predicted from relative plate motions suggested angular velocity of the african plate relative to europe is 0 07 myr 1 around a pole near 21 n 21 w this predicts 2 6 mm yr 1 maximum extension rate towards n64 e near the azores consistent with the minimum local accretion of new oceanic crust in the past few millions of years and 4 3 mm yr 1 plate convergence towards n20 w in the central mediterranean at 13 e further east the northern margin of the african plate can be regarded as the gargano seismic zone in the central adriatic sea the northern adriatic sea is a microplate rotating anticlockwise relative to europe at 0 3 myr 1 around a pole in northwestern italy causing northeastward shortening in yugoslavia and up to 4 mm yr 1 extension in central and northern italy the african plate is deforming internally in tunisia and northern libya extending obliquely in direction e10 s at rate up to 5 5 mm yr 1 the resultant velocity relative to europe in the ionian and southern adriatic seas 5 mm yr 1 towards n50 e explains the observed relative velocity across southern italy kinematic consistency requires present day shortening rate east of calabria associated with subduction on the tyrrhenian sea benioff zone and tyrrhenian sea extension rate to be both zero or equal to within 1 mm yr 1

gross errors in upper mantle discontinuity topography from underside reflection data summary currnet models of upper mantle discontinuity topography derived from traveltime variations of long period underside reflections pdp or sds waves may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point the use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the fresnel zone in the data geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions examples of synthetic long period pdp waveforms for reflections near subducting lithosphere show that scattering from expected small scale depth variations of the discontinuities inside slabs causes complex waveform variations for a 15 20s pdp wave these small scale depth variations with a scale length of 4 are smaller than the fresnel zone roughly 20 but larger than the pdp wavelength at 420 km depth about 150 200 km the synthetic waveforms are processed to obtain pdp traveltimes which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics the apparent discontinuity depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models it is shown that large scale wavelengths of the order of 1000 km downwarping of the 670 km discontinuity near subduction zones derived from underside reflection data can be explained as the long wavelength manifestation of a single small scale wavelengths of several hundreds of kilometres deflection of the discontinuity inside and near subducting slabs