sediment dewatering in the makran accretionary prism abstract sediments within the makran accretionary prism are considerably more compacted than the material entering the subduction zone due to the additional tectonic stresses resulting from plate convergence and accretionary processes tectonic compaction of the offscraped sediments has been documented by deriving in situ porosities in the upper 4 km from their seismic velocities wide angle seismic reflection refraction profiles were analysed from a transect crossing both the makran accretionary prism and the undeformed abyssal plain sediments to the south the 6 7 km thick abyssal plain sediments exhibit normal compaction with depth the sediments incorporated into the accretionary prism have higher seismic velocities at all depths corresponding to reductions in porosity of 30 40 mapping the porosity depth gradient across the prism shows that there is a marked change in porosity at the seaward toe of the prism the frontal fold there is then little further change until further folding begins to occur 70 km landward of the toe the dewatering and consolidation of the sediments is an important control on deformation occurring within the sedimentary prism

the 1994 sefidabeh earthquakes in eastern iran blind thrusting and bedding plane slip on a growing anticline and active tectonics of the sistan suture zone summary in 1994 a sequence of five earthquakes with mw 5 5 6 2 occurred in the sistan belt of eastern iran all of them involving motion on blind thrusts with centroid depths of 5 10 km coseismic ruptures at the surface involved bedding plane slip on a growing hanging wall anticline displaying geomorphological evidence of uplift and lateral propagation the 1994 earthquakes were associated with a nw trending thrust system that splays off the northern termination of a major n s right lateral strike slip fault elevation changes along the anticline ridge suggest that displacement on the underlying thrust dies out to the nw away from its intersection with the strike slip fault this is a common fault configuration in eastern iran and accommodates oblique ne sw shortening across the n s deforming zone probably by anticlockwise rotations about a vertical axis this style of fault kinematics may be transitional to a more evolved state that involves partitioning of the strike slip and convergent motion onto separate subparallel faults

the makran earthquake of 1983 april 18 a possible analogue to the puget sound earthquake of 1965 summary on 1983 april 18 a large earthquake mb 6 5 m0 1 2x1019 nm occurred in the makran region near the pakistan iran border this earthquake is the only event of mb 6 2 in the entire makran region since the occurrence of the great makran earthquake m 8 0 8 3 in 1945 we determined the source parameters of the 1983 event by formally inverting its p and sh waves recorded by the world wide standard seismograph network wwssn and the canadian seismograph network csn at teleseismic distances the best fitting centroidal solution shows predominantly normal faulting at a depth of about 65 5 km these results provide an additional constraint on the configuration of the down going slab previously inferred from the occurrence of three small to moderate sized events located 100 300 km to the west of the event reported here we also discuss the possibility of drawing an analogy between the occurrence of the 1983 makran event and that of the puget sound earthquake of 1965 m0 1 4x1019nm mb 6 5 and between the seismotectonic settings of the two regions if the very large m 8 0 8 3 earthquake off the makran coast in 1945 is a low angle thrust due to slip along the plate interface its occurrence calls for the possibility of similar seismic risk in the puget sound region however in contrast to inferences made from geological observations in the field about a dozen p wave first motions read from historical seismograms and those reported by the international seismological summary suggest that the 1945 makran earthquake has a large component of normal faulting which can be attributed to the relatively old age of the subducted lithosphere

great thrust earthquakes and aseismic slip along the plate boundary of the makran subduction zone the makran subduction zone of iran and pakistan exhibits strong variation in seismicity between its eastern and western segments and has one of the world s largest forearcs we determine the source parameters for 14 earthquakes at makran including the great mw 8 1 earthquake of 1945 the only instrumentally recorded great earthquake at makran we determine the loci of seismic and aseismic slip along the plate boundary and we assess the effects of the large forearc and accretionary wedge on the style of plate boundary slip we apply body waveform inversions and for small magnitude events use first motions of p waves to estimate earthquake source parameters for the 1945 event we also employ dislocation modeling of uplift data we find that the earthquake of 1945 in eastern makran is an interplate thrust event that ruptured approximately one fifth the length of the subduction zone nine smaller events in eastern makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock seaward of these thrust earthquakes lies the shallowest 70 80 km of the plate boundary we find that this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes this aseismic zone as in other subduction zones lies within that part of the accretionary wedge that consists of largely uconsolidated sediments seismic velocities less than 4 0 km s the existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern makran become sufficiently well consolidated and de watered about 70 km from the deformation front or older lithified rocks are present within the forearc so that stick slip sliding behavior becomes possible this study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes in contrast to the east the plate boundary in western makran has no clear record of historic great events nor has modem instrumentation detected any shallow thrust events for at least the past 25 years most earthquakes in western makran occur within the downgoing plate at intermediate depths the large change in seismicity between eastern and western makran along with two shallow events that exhibit right lateral strike slip motion in central makran suggest segmentation of the subduction zone two paleozoic continental blocks dominate the overriding plate the boundary between them is approximately coincident with the transition in seismicity although relative motion between these blocks may account for some of the differing seismic behavior the continuity of the deformation front and of other tectonic features along the subduction zone suggests that the rate of subduction does not change appreciably from east to west the absence of plate boundary events in western makran indicates either that entirely aseismic subduction occurs or that the plate boundary is currently locked and experiences great earthquakes with long repeat times evidence is presently inconclusive concerning which of these two hypotheses is most correct the presence of well defined late holocene marine terraces along portions of the coasts of eastern and western makran could be interpreted as evidence that both sections of the arc are capable of generating large plate boundary earthquakes if that hypothesis is correct then western makran could produce a great earthquake or it could rupture as a number of segments in somewhat smaller magnitude events alternatively it is possible that western makran is significantly different from eastern makran and experiences largely aseismic slip at all times a knowledge of the velocity structure and nature of the state of consolidation or lithification of rocks at depth in the interior portion of the forearc of western makran should help to ascertain whether that portion of the plate boundary moves aseismically or ruptures in large to great earthquakes a resolution of this question has important implications for seismic hazard not only for western makran but also for other margins such as the cascadia subduction zone of western north america where historical thrust events have not occurred

heat flow and age of the gulf of oman we present the results of twenty heat flow stations in the gulf of oman which are used to infer the first reliable age estimates for the basin a mean surface heat flux of 42 6 3 6 mw m 2 exhibits no significant regional variation after correction for thick and rapidly deposited sediments this yields an age of 70 to 100 ma according to oceanic thermal models this age is also consistent with the sediment corrected basement depths of 5 5 6 0 km and with formation during the cretaceous quiet zone the latter can explain the absence of magnetic sea floor spreading lineations heat flow measurements are also used to confirm the presence of gas hyrdate layers the measured thermal gradient yields a depth for the solid to free gas phase transition which is the same as that deduced from bright spots seen on seismic reflection profiles

seismotectonic and seismicity of makran a bimodal subduction zone se iran abstract the present study combined spatial distribution and mechanism of seismicity seismicity parameters geomorphology in satellite imagery and tectonic features in makran inland to show that the makran low angle thrust fault is divided into two separate segments depth dispersal of earthquakes especially relocated events shows that the sinking plate is steeper in the west than in the east of makran significant facts were identified from the variation of moho depth between west and east slip rates of the inland faults and crustal shortenings in the north of makran the sistan suture zone plays a significant role in dividing the overriding plate into a non deforming helmand block and a less deforming lut block these blocks have different resistances against the northward subduction large and great events such as the 1989 chabahar mw 5 5 and 1945 gwadur mw 8 1 occurred in the south of subduction fronts of the western and eastern makran respectively they occurred in shallow depth and were the pieces of evidence of weak and strong coupling between the oceanic and continental crusts on a low angle and a very low angle reverse faults respectively spectral analysis and depth determination of the instrumental and pre instrumental earthquakes respectively helped to separate some plate boundary and intraplate events this separation geometry and non characteristic behavior of the subduction zone revealed from spatial and temporal distribution of the earthquakes probably dedicate a mature aleutian type for the subduction zone in the east missing great m 6 1 earthquakes less slip rate of the continental faults and steeper dipping of the sinking plate in the west probably indicate a kurile type for the iranian part of subduction makran is a bimodal subduction because geomorphological and seismic pieces of evidence indicate a nnw surficial and right lateral fracture zone in the continental crust and a nw deep and left lateral fracture zone in the oceanic crust respectively the fractures and geometry of subductions showed that the eastern makran has seismotectonically a wider area for potential coupling of a larger earthquake than the western makran

tsunami hazard assessment of coastal south africa based on mega earthquakes of remote subduction zones the nuclear structural engineering pty and the national research foundation through the technology and human resources for industry programme project thrip tp2011061400009

geometry and mechanism of faulting of the 1980 el asnam algeria earthquake from inversion of teleseismic body waves and comparison with field observations the el asnam earthquake of october 10 1980 ms 7 3 provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models the event produced extensive surface faulting in addition to the main tectonic deformation which is clearly a thrust widespread secondary normal faulting was observed at the surface in the southwest region the surface break of a thrust fault could be traced for 24 km in the northeast part of the fault zone normal faults with throws of several meters were observed although no clear thrust type surface breaks were observed in this region geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation in this paper the rupture process of the el asnam earthquake is investigated by inversion of teleseismic p and sh waves the effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood north eastern segment of the fault zone during the earthquake rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes the seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters seismic moment 3 9 1019 n m depth 6 km strike 220 dip 46 and rake 72 the duration of rupture on this fault segment was approximately 10 s which together with the observed fault length of 24 km indicates a rupture velocity of 2 4 km s unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region the normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region guided by the evidence from aftershocks and geodetic measurements a solution with a thrust type mechanism was also sought using a priori constraints based on the field observations the observed strike and slip angle we obtain the following thrust source mechanism for the northeast segment seismic moment 3 6 1019 n m depth 6 km strike 230 dip 20 and rake 91 this solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth inversions are nonunique with respect to the seismic moment associated with normal faulting the ambiguity being caused by the limited bandwidth of the observed body waves the moment estimated from the surface waves at periods about 300 s however favors thrust faulting to be dominant in the northeast region

neotectonic deformation in central eurasia a geodynamic model approach funding was granted by the spanish government through the project mite cgl2014 59516 p and alpimed piecsic 201530e082

eastern turkish high plateau as a small turkic type orogen implications for post collisional crust forming processes in turkic type orogens abstract post collisional magmatism may be generated by extensive crustal melting in tibet type collisional environments or by falling out of slabs from under giant subduction accretion complexes in turkic type collisional orogens giving rise to decompression melting of the asthenospheric mantle replacing the removed oceanic lithosphere in turkic type post collisional magmatism the magmatic products are dominantly alkalic to peralkalic and greatly resemble those of extensional regions giving rise to much confusion especially in interpreting old collisional orogenic belts such magmatic regions are also host to a variety of economically valuable ore deposits including gold one place in the world where today active turkic type post collisional magmatism is present is the eastern anatolian high plateau produced after the terminal arabia eurasia collision in the late miocene the plateau is mostly underlain by the late cretaceous to oligocene east anatolian accretionary complex which formed south of the rhodope pontide magmatic arc this subduction accretion complex has been further shortening since the collision but it has also since been domed and became almost entirely covered by at least 15 000 km3 of volcanic rocks the volcanic rocks are calc alkalic in the north transitional in the middle and alkalic in the south of the plateau where the crust is thinnest today less than 38 km the volcanics are derived almost entirely from an enriched mantle the ages of the volcanics also become younger from north to south from about 11 ma to possibly 17th century ad we interpret the origin of the magmatic rocks as the result of decompression melting of the asthenospheric mantle sucked towards the exposed base of the east anatolian accretionary complex as the oceanic lithosphere beneath it fell out the lower density of the hot asthospheric material was the cause of the doming we believe that similar processes dominated the post collisional tectonics of such vast turkic type orogens as the altaids of central asia the late devonian early carboniferous lachlan belt in southeastern australia and the pan african collage of northeast afro arabia it is likely that archaean collisions were dominated by turkic type post collisional events rather than tibetan ones that only became common in the proterozoic

seismotectonics of southern iran the oman line seismotectonic characteristics of the transition area between the zagros continental collision zone and the makran ocean continent convergence zone the oman line are carefully examined a northeast trending zone of earthquakes terminates the zagros belt of seismicity just west of the oman line this seismic zone extends northeastward beyond the main zagros thrust and coincides with a surface escarpment clearly visible on landsat imagery events with accurately determined depths along this seismic zone occurred in the upper 20 km of the crust and possibly show a shallow 10 northeastward dip east of the oman line the zendan minab fault system is relatively aseismic the oman line region may be characterized by underthrusting of a wedge of arabian shelf edge beneath iranian crust or by an indentation of arabia into the iranian crustal block as a promontory available seismological and geological data cannot uniquely distinguish between these two possible tectonic settings the area located to the northwest of the oman line region now a zone of continental collision appears to have been characterized by an episode of neogene subduction a well located intermediate depth earthquake occurred in this area to the northeast of the main zagros thrust on november 9 1970 it was accurately located at 107 km depth beneath a line of quaternary volcanoes its focal mechanism may indicate downdip tension with nodal planes that strike closely parallel to the trend of the zagros arc comparison of this event and other neotectonic features in this part of iran with those in other active convergent zones suggests that a descending oceanic lithosphere beneath the zagros volcanic arc may still be attached to the colliding arabian plate

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

the 1983 goodnow earthquake in the central adirondacks new york rupture of a simple circular crack on 7 october 1983 a magnitude 5 1 mb earthquake occurred in the central adirondack mountains near the town of goodnow new york the earthquake was well recorded both by regional stations and by a large number of digital and analog seismographs in north and south america and europe these regional and teleseismic data are complemented by accurate locations of aftershocks recorded by a portable network from 17 hours after the main shock providing an unusually complete dataset to study the source process of an intraplate earthquake the results of an analysis involving the formal inversion of the vertical component rayleigh waves and of short period p waves show that the earthquake was due to reverse faulting with a centroidal depth of 7 5 km striking north south and dipping at 60 to the west the scalar seismic moment is 1 9 1023 dyn cm for p waves for paths from northeastern united states plausible values of t attenuation range from 0 4 to 0 7 sec and for these values the estimates of source duration range from 0 60 to 0 35 sec assuming a circular crack model with a rupture velocity of 3 0 km sec the bounds on source duration give upper and lower limits for the fault radius of 0 9 and 0 5 km and for the stress drop of 670 and 115 bars the preferred value of t 0 6 sec yields a source duration of 0 45 sec a radius of 0 7 km and a stress drop of 265 bars this fault radius agrees with that inferred from the spatial distribution of aftershocks the inferred stress drop of the goodnow earthquake is higher than generally observed for interplate events if the goodnow earthquake is representative of events in eastern north america the high stress drop may partially explain the large felt areas produced by these relatively small intracontinental earthquakes

compiling an earthquake catalogue for the arabian plate western asia abstract the arabian plate is surrounded by regions of relatively high seismicity accounting for this seismicity is of great importance for seismic hazard and risk assessments seismic zoning and land use in this study a homogenous earthquake catalogue of moment magnitude mw for the arabian plate is provided the comprehensive and homogenous earthquake catalogue provided in the current study spatially involves the entire arabian peninsula and neighboring areas covering all earthquake sources that can generate substantial hazard for the arabian plate mainland the catalogue extends in time from 19 to 2015 with a total number of 13 156 events of which 497 are historical events four polygons covering the entire arabian plate were delineated and different data sources including special studies local regional and international catalogues were used to prepare the earthquake catalogue moment magnitudes mw that provided by original sources were given the highest magnitude type priority and introduced to the catalogues with their references earthquakes with magnitude differ from mw were converted into this scale applying empirical relationships derived in the current or in previous studies the four polygons catalogues were included in two comprehensive earthquake catalogues constituting the historical and instrumental periods duplicate events were identified and discarded from the current catalogue the present earthquake catalogue was declustered in order to contain only independent events and investigated for the completeness with time of different magnitude spans

the 1960 chile earthquake inversion for slip distribution from surface deformation summary a total of 166 observations of sea level change 130 measurements of elevation difference and 16 determinations of horizontal strain provide an excellent view of the quasi static source process of the great 1960 chilean earthquake these surface deformation data were employed in classical uniform slip fault models as well as more recently developed models that allow spatial variability of slip the best uniform slip planar usp model is 850km long 130km wide and dips 20 seventeen metres of fault displacement contributed to a usp moment of 9 4 times 1022 n m the variable slip planar vsp model concentrates slip on a 900 km long 150 km wide band parallel to the coast several peaks of slip with dimensions of 50 100 km appear in this band and are thought to represent major subduction zone asperities important fractures of the oceanic lithosphere bound the 1960 rupture and are offered as a potential source of fault segmentation within the chilean subduction zone the vsp moment for 1960 earthquake totals 9 5 times 1022 n m about one fifth of the value estimated for the foreshock mainshock sequence from seismic methods except for areas out to sea geodetic resolution on the fault is fairly uniform thus it is unlikely that slip missed by the network could increase the vsp moment much beyond 1 8 times 1023 n m several patches of moment isolated from the main body at 80 110 km depth are found down dip in the vsp model and are presumably indicative of aseismic slip one patch at the northern end of the rupture is probably associated with the initiation phase of the mainshock although the time sequence of the relationship is unknown tide gauge records suggest that another patch between 40 and 43 s responsible for the observed strain and uplifts inland at those latitudes is not of coseismic origin but derives from in place post seismic creep over several years apparently great 1960 type events are not typical members of the 128 yr earthquake cycle in south central chile the nazca south america boundary here is characterized by a variable rupture mode in which major asperities are completely broken by great earthquakes only once in four or five earthquake cycles the more frequent large earthquakes that geographically overlap the great events fill in between the locked zones

large scale sediment underplating in the makran accretionary prism southwest pakistan field relations in the emergent part of the makran accretionary prism show that a mid miocene to early pliocene slope and shelf sedimentary sequence was deposited directly on abyssal plain turbidites without any detectable stratigraphic or structural discordance sedimentological evidence for rapid shoaling however indicates that the underlying sediment column was tectonically thickened by a factor of between two and three during this period this can be explained by large scale underthrusting and underplating of sediment a mode of accretion that is also favored by mass balance considerations the visible deformation in the coastal makran occurred when the region was 70 100 km north of the contemporary prism front

validation of taxon specific sampling by novice collectors for studying drilling predation in fossil bivalves abstract although bulk sampling has been considered to provide more robust information on predation in the fossil record than taxon specific targeted sampling previous work suggested that targeted sampling even by a trained novice can yield results comparable to bulk sampling the present study further investigates potential bias introduced by collectors with different experience levels by comparing drilling predation metrics for samples made by five novice and three veteran collectors we demonstrate that trained novices produce results comparable to those of experienced collectors and that results of targeted sampling are comparable to bulk sampling targeted sampling was performed in the lower waccamaw formation lower pleistocene at register quarry near old dock nc five replicate taxon specific samples were obtained by each collector for the bivalves astarte concentrica cyclocardia granulata and lirophora latilirata shell length and thickness were measured and position and size of drillholes were determined frequency of left valves drilling frequency prey effectiveness and size selectivity were determined for each taxon replicate for each variable the results for the novice collectors were compared to those from bulk samples and all veteran collectors nearly all significant differences in the unstandardized data occurred in the size related variables length and thickness when data were size standardized as in many other drilling predation studies only 3 3 of 174 tests for statistical differences in collection methods were significant and none of the 157 tests for differences in novice and veteran sampling was significant previous results were not anomalous use of targeted sampling in studying drilling predation is again validated and collector expertise is shown to have minimal impact

focal mechanism solutions for earthquakes in iran abstract focal mechanism solutions derived by using p wave first motion directions read from seismograms of worldwide standardized stations network wwssn are presented for 36 earthquakes in iran in many cases the first motion data admit more than one solution these solutions along with previously published solutions for 18 other earthquakes are graphically summarized on a tectonic map of iran

magnitudes of large shallow earthquakes from 1904 to 1980 abstract in order to obtain a uniform magnitude catalogue surface wave magnitudes ms and broad band body wave magnitudes mb have been determined for large shallow earthquakes from 1904 to 1980 in making the catalogue homogeneous the author consistently adheres to the original definitions of ms and mb given by gutenberg 1945 and gutenberg and richter 1956 the determinations of ms and mb are all based on the amplitude and period data listed in gutenberg and richter s unpublished notes bulletins from stations worldwide and other basic information mb is measured on broad band instruments in periods of 8 s consistency of the magnitude determinations from these different sources is carefully checked in detail more than 900 shallow shocks of magnitude 7 and over are catalogued the meaning of the magnitude scales in various catalogues is examined in terms of ms and mb most of the magnitudes listed by gutenberg and richter 1954 in their seismicity of the earth are basically ms for large shocks shallower than 40 km but are basically mb for large shocks at depths of 40 60 km the surface wave magnitudes given by earthquake data reports are higher than ms by 0 2 unit whether the combined horizontal amplitude or the vertical amplitude is used mb and the currently used 1 s body wave magnitude are measured at different periods and should not be directly compared

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

rupture process of the andreanof islands earthquake of may 7 1986 the andreanof islands earthquake of may 7 1986 ms 7 7 is the largest earthquake to have occurred along the aleutian arc in the past 20 yr source parameters of the mainshock three large aftershocks 6 1 ms 6 7 and a foreshock ms 6 0 are determined by inverting digitally recorded teleseismic waveforms and amplitudes long period p and sh waves are inverted simultaneously to determine the strike dip rake centroid depth source time function and seismic moment of each earthquake complexities in the rupture process of the mainshock are modeled using broadband 5 to 0 01 hz p wave displacement waveforms modeling the mainshock as a propagating line source the long period and broadband waveforms indicate that the locus of largest moment release propagated bilaterally along the arc with a well resolved velocity of 1 5 km sec the source mechanism is consistent with faulting on a low angle 23 1 thrust fault the fault s strike 260 10 and the rake of the slip vector on the fault 93 8 are less well resolved and trade off with each other acceptable centroid depths range between 10 and 25 km within this range of depths the misfit of the synthetic to the observed long period and broadband waveforms varies by only 6 per cent while moment estimates vary by as much as 64 per cent for a centroid depth of 20 km the estimated seismic moment is 1 1 1028 dyne cm 85 per cent of the moment estimated from surface wave analysis ninety per cent of the moment release was generated by three patches on the fault surface all located within 90 km of the nucleation point very little moment however was released near the epicenter of the mainshock the epicentral locations source mechanisms and depths of the foreshock and one of the aftershocks modeled by us are consistent with faulting on the same thrust plane as the mainshock the other two modeled aftershocks are located north of the island arc and have source depths of less than 15 km indicating that they occurred within the overriding plate the focal mechanisms of these events indicate almost pure strike slip faulting but the waveform data do not contain sufficient information to distinguish which of the couples of the double couple mechanism corresponds to the fault plane joint relocations of several of the aftershocks from one of these strike slip earthquakes suggests that right lateral strike slip faulting occurred on a northeasterly trending fault plane

seismic potential associated with subduction in the northwestern united states despite good evidence of present day convergence of the juan de fuca and north american plates there has been remarkably little historical seismic activity along the shallow part of the juan de fuca subduction zone although we cannot completely rule out the possibility that the plate motion is being accommodated by aseismic creep we find that the juan de fuca subduction zone shares many features with other subduction zones that have experienced great earthquakes

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

constraints on plate motions in southern pakistan and the northern arabian sea from the focal mechanisms of small earthquakes the focal mechanism and depth were determined for nine small earthquakes m0 1025 dyn cm m 5 5 that occurred in southern pakistan and the northern arabian sea from an analysis of the vertical component of rayleigh waves in combination with limited first motion data focal parameters were determined from the rayleigh waves by using an event pair method of analysis for earthquakes that are located very close to each other 50 km the event pair method is able to remove a significant proportion of propagation effects at all periods in the range of interest 20 50 s for events separated by more than 100 km the propagation effects are reduced for only the longer periods 40 50 s the earthquakes that were studied provide evidence for a model of plate interactions in the vicinity of the southern pakistan triple junction the owen fracture zone is a transform fault that accommodates right lateral motion between the indian and arabian plates the plate boundary in the vicinity of the murray ridge is also partially made up of transform segments that strike subparallel to the owen fracture zone spreading centers may also exist in the vicinity of the murray ridge but were not documented by seismic or other evidence the slip azimuths for earthquakes along this boundary are significantly more northerly than those predicted by various regional and worldwide models of plate motion the arabian plate is being subducted beneath the eurasian plate along the southern coast of pakistan slip vectors for earthquakes along this boundary trend northnortheasterly in general agreement with predicted directions left lateral motion is documented along the boundary between the indian and eurasian plates in southern pakistan the predicted direction of relative motion between these plates is not significantly different from that observed two of the earthquakes studied appear to be intraplate in nature the depth and focal mechanism of one intraplate event which may have occurred within the accretionary prism along the makran coast however are based on limited data the other intraplate earthquake occurred within the sedimentary deposits along the western edge of the indian subcontinent near the indian eurasian plate boundary appendix is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 009 2 50 payment must accompany order

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

factors affecting seismic moment release rates in subduction zones the amount of shallow seismic activity in subduction zones varies greatly from region to region we quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones to calculate the moment release we sum the seismic moment for all interplate thrust type events with surface wave magnitude ms 7 0 occurring from 1904 to 1980 we present a time history plot of the seismic moment release for each subduction zone these exhibit the differences in the seismic release patterns for subduction zones where the time window of our study is not representative the total moment release is corrected using information on repeat times the moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling these parameters include the age of the subducting lithosphere absolute velocities of the upper and subducting plates convergence velocity and length maximum depth and dip of the wadati benioff zone the moment release rate decreases as the age of the subducting lithosphere increases when the zones belonging to a single subducting plate are considered this age versus moment release relation is consistent for the zones in which the pacific cocos philippine sea and indian plates are subducting the moment release rates for the subduction zones in which the pacific plate is subducting are much higher than for zones of other plates with similar age the age versus moment release relation holds among the zones which belong to one subducting plate however zones with similar ages but belonging to different plates do not have the same moment release rates this suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget zones with retreating upper plates tend to have lower moment release rates the moment release rate does not increase with convergence velocity no simple relationship was found between these two parameters the moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate these are the two independent subduction zone parameters among the variables considered the other variables depend on these two parameters

the evolution of chabahar beach ridge system in se iran in response to holocene relative sea level changes abstract the chabahar strandplain chs stretches along the chabahar bay at a width of more than 5 km along the northern coast of the gulf of oman and se iran an area that has been subjected to tectonic uplift as a part of the makran accretionary prism the chs comprises of beach ridges inter ridge swales sand dunes tidal channels and fluvial deposits we present the first documentation of the spatial distribution and the internal architecture of the chs based on topographic surveys sediment percussive coring trenching and ground penetrating radar gpr transects radiocarbon dating on marine shells from foreshore deposits of 8 representative beach ridges yielded ages between 4800 and 270 cal years bp at respective distances of 4800 to 670 m from the present shoreline we interpret the boundary between the foreshore beach and the foredune deposits as indicator of past sea levels by analogy to present shore processes this boundary is readily recognizable in sediment profiles from cores and trenches as well as gpr reflectors based on the age model and depositional features we estimate relative sea level fall of up to 15 m over the past 4800 years considering that the eustatic sea level changes for this period are negligible for the makran coast this relative sea level fall is related to tectonic uplift across the coastal makran the elevation of palaeo shorelines with similar ages decrease from east to west of the chs which suggest that alongshore variations in uplift rates are likely related to different movements of coastal fault blocks

geomorphological analysis of the drainage system on the growing makran accretionary wedge abstract the morphology of six adjacent major catchments draining the onshore makran accretionary wedge in southeast iran and southwest pakistan was studied to examine how the channel pattern and the length profiles may reflect the recent and active growth of the wedge qualitative field surveys were combined with the quantitative analysis of channel steepness and concavity measured from digital elevation models these profiles were compared with modelled profiles using a stream power approach assuming homogeneously uplifting uniform rock substratum results show a distinct difference between the studied western and eastern catchments the three western rivers are in morphological equilibrium whereas the two eastern rivers exhibit profiles with prominent convexities and knickpoints thus notably diverging from equilibrium concave up shapes all the studied catchments share the same base level flow on similar lithologies and developed under uniform climate conditions therefore we interpret the morphometric differences in terms of differential rock uplift rate as a response to local tectonic activity this interpretation is consistent with both uplift rates of marine terraces along the coast of makran and the recorded seismicity the geomorphological analysis presented here extends coastal information to wide inland areas and documents longer term tectonic behaviour than the seismotectonic record hence the steeper surface slope and faster surface uplift rates recorded by the eastern catchments compared to equilibrium of the western catchments are regional long term signals relevant for a wide part of the makran accretionary wedge we attribute the regional geomorphic difference to quaternary variations in tectonic regimes that forced differential uplift rates of the wedge surface we hypothesize that the different tectonic regimes are related to different subduction rates

mm a variable period mantle magnitude this paper lays the theoretical groundwork for a variable period mantle magnitude mm based on the measurement of the spectral amplitude x of very long period rayleigh waves we retain the concept of magnitude by restricting ourselves to single station measurements ignoring the focal mechanism and the exact depth of the shallow earthquakes considered our measurements are made at a series of periods in all cases greater than 40 s and the largest value is retained this procedure effectively avoids the well known interference effects leading to saturation of magnitude scales defined at a fixed period two corrections are used a period dependent distance correction cd and a source correction cs also period dependent compensating for the variation of the excitation of rayleigh waves with period both of these corrections are fully predictable on the basis of standard surface wave excitation and dispersion theory the result is a formula of the type mm log10 x cd cs c0 in which all coefficients including the constant c0 are justifiable on sound theoretical grounds the analysis of a data set of 256 records from the broadband seismograph at papeete tahiti the ultra long period system at pasadena and stations of the geoscope network shows that the mean error in the estimation of the seismic moment is on the order of 0 1 0 2 units of magnitude with the standard deviation at each station also on the order of 0 2 units of magnitude no significant trend with either distance period or station can be identified the method can also be transposed to the time domain under some simple assumptions which are justifiable theoretically for typical teleseismic distances across the pacific basin both versions of the method lend themselves well to automation thus by providing a real time estimate of the seismic moment of distant earthquakes mm has considerable potential for tsunami warning purposes its concept can easily be extended to love waves and also to intermediate and deep earthquakes

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

iranian regional centroid moment tensor catalog solutions for 2012 2017 abstract in 2012 the installation of broadband seismic stations in iran allowed the beginning of the moment tensor project of iranian earthquakes by the iranian seismological center irsc in a period of five years 194 moment tensors with 3 7 mw 7 6 were computed based on analysis of seismograms recorded at local and regional distances included in the catalog are the moment tensors of 83 events not modeled by gcmt a reasonable consistency exists between the mechanisms of common events in the two catalogs the average moment magnitude difference is 0 1 and the kagan angle is below 20 for 61 of the earthquakes the p b and t axes difference is up to 15 the moment tensor catalog of iran shows a decrease of the magnitude of completeness by 0 3 after this study the number of accepted solutions as a fraction of attempts of the project with the average of 55 increased from 40 in 2012 to 68 in 2016 due to the expansions of the seismic networks in iran the dataset includes earthquakes from all seismic zones in iran with the dominance of thrust faulting the calculated depths in this study are consistent with the subregion seismotectonic conditions of iran the information on seven earthquakes with mw 6 0 and 53 events with 5 0 mw

slip vectors and fault mechanics in the makran accretionary wedge southwest pakistan the makran is a broadly east west trending arcuate accretionary wedge currently forming where the oceanic floor of the gulf of oman is being subducted north northeastward beneath the south asian margin two traverses across the makran coast ranges extending about 80 km inland from the coastline which lies about 100 km north of the wedge front show that the structure of this region is dominated by large back rotated south directed thrust faults and associated folds related structures include duplexes fault gouge containing reidel shears and melange along fault zones these structures are cut by later north directed backthrusts out of sequence south directed thrusts and apparently conjugate sets of nne trending sinistral and nw trending dextral strike slip faults slip vectors were determined from slickenlines fiber lineations gouge fabrics and riedel shears vectors from early thrusts and bedding parallel slip surfaces corrected for tectonic rotation form a broad south directed concentration lying between the normal to the regional strike varying from 160 to 180 and the present plate convergence vector 200 locally the concentration is bimodal this pattern may represent the interaction between plate forces and body forces in the accretionary wedge the late thrusts have widely variable slip vectors and stair step off the early thrusts they overlap in orientation and slip vector with gently dipping sinistral strike slip faults suggesting that they also stair step in plan off the latter the steep strike slip faults represent the final tectonic stage involving minor n s shortening and longitudinal extension these three stages probably occurred in sequence in any one area but migrated diachronously southward as the wedge grew

osl dating and gpr mapping of palaeotsunami inundation a 4000 year history of indian ocean tsunamis as recorded in sri lanka to evaluate and mitigate tsunami hazard as long as possible records of inundations and dates of past events are needed coastal sediments deposited by tsunamis tsunamites can potentially provide this information however of the three key elements needed for reconstruction of palaeotsunamis identification of sediments dating and finding the inundation distance the latter remains the most difficult the existing methods for estimating the extent of a palaeotsunami inundation rely on extensive excavation which is not always possible here by analysing tsunamites from sri lanka identified using sedimentological and paleontological characteristics we show that their internal dielectric properties differ significantly from surrounding sediments the significant difference in the value of dielectric constant of the otherwise almost indistinguishable sediments is due to higher water content of tsunamites the contrasts were found to be sharp and not to erode over thousands of years they cause sizeable electromagnetic wave reflections from tsunamite sediments which permit the use of ground penetrating radar gpr to trace their extent and morphology in this study of the 2004 boxing day indian ocean tsunami we use gpr in two locations in sri lanka to trace four identified major palaeotsunami deposits for at least 400 m inland investigation inland was constrained by inaccessible security zones the subsurface extent of tsunamites not available without extensive excavation provides a good proxy for inundation the deposits were dated using the established method of optically stimulated luminescence osl this dating partly corroborated by available historical records and independent studies contributes to the global picture of tsunami hazard in the indian ocean the proposed method of combined gpr osl based reconstruction of palaeotsunami deposits enables estimates of inundation recurrence and therefore tsunami hazard for any sandy coast with identifiable tsunamite deposits the method could be also used for anchoring and synchronizing chronologies of ancient civilisations adjacent to the ocean shores

high interseismic coupling in the eastern makran pakistan subduction zone estimating the extent of interseismic coupling along subduction zone megathrusts is essential for quantitative assessments of seismic and tsunami hazards up to now quantifying the seismogenic potential of the eastern makran subduction zone at the northern edge of the indian ocean has remained elusive due to a paucity of geodetic observations furthermore non tectonic processes obscure the signature of accumulating elastic strain historical earthquakes of magnitudes greater than 7 have been reported in particular the 1945 mw 8 1 earthquake resulted in a significant tsunami that swept the shores of the arabian sea and the indian ocean a quantitative estimate of elastic strain accumulation along the subduction plate boundary in eastern makran is needed to confront previous indirect and contradictory conclusions about the seismic potential in the region here we infer the distribution of interseismic coupling on the eastern makran megathrust from time series of satellite interferometric synthetic aperture radar insar images acquired between 2003 and 2010 applying a consistent series of corrections to extract the low amplitude long wavelength deformation signal associated with elastic strain on the megathrust we find high interseismic coupling i e the megathrust does not slip and elastic strain accumulates in the central section of eastern makran where the 1945 earthquake occurred while lower coupling coincides spatially with the subduction of the sonne fault zone the inferred accumulation of elastic strain since the 1945 earthquake is consistent with the future occurrence of magnitude 7 earthquakes and we cannot exclude the possibility of a multi segment rupture mw 8 however the likelihood for such scenarios might be modulated by partitioning of plate convergence between slip on the megathrust and internal deformation of the overlying actively deforming accretionary wedge

changing mechanical response during continental collision active examples from the foreland thrust belts of pakistan abstract we have used data from teleseismic seismic reflection and field geologic studies along with both geomechanical and gravity modeling to contrast the tectonics of four active orogenic wedges in pakistan the kashmir himalaya the salt range potwar plateau foldbelt the sulaiman range and the makran accretionary wedge in makran oceanic crust is still being subducted and a thick pile of sediments is being accreted and underplated undercompaction and excess pore pressures can explain the narrow cross sectional taper and frontal aseismicity of this wedge beneath the sulaiman wedge continental crust is just starting to be underthrust indirect evidence suggests that fine grained carbonate rocks found in abundance deep in the stratigraphic section may be deforming ductilely at the base of the sulaiman wedge and provide a zone of ductile detachment the collision has proceeded to a much more mature stage in the salt range potwar plateau foldbelt and the kashmir himalaya isostatic response to underthrusting of continental crust has kept the sedimentary pile quite thin in both of these wedges so in that respect the two foldbelts are similar however thick eocambrian salt beneath the salt range and potwar plateau permits that foldbelt to be much wider in map view with a thinner cross sectional taper and a mixture of thrust vergence directions a major normal fault in basement causes the salt range to rise in front of the mildly deformed molasse basin of the southern potwar plateau much of the diversity among these mountain belts can be understood in terms of differences in the maturity of the collision process in each area the resulting thickness of the sedimentary pile encountered at the deformation front and the presence or absence of large contrasts in strength between the various layers of the stratigraphic section and basement relief

three dimensional structure of the upper mantle beneath the atlantic ocean inferred from long period rayleigh waves 1 group and phase velocity distributions group and phase velocity distributions of rayleigh wave fundamental modes are determined in the atlantic ocean using a method of regionalization without a priori constraints for a period range from 50 to 200 s eighty six direct source station paths are studied and 38 out of the best data from weidner s 1974 phase velocities are added to our data set for periods lower than 100 s the importance of shallow layer corrections is described in detail the general features of the distributions agree with global models and smaller scale heterogeneities are revealed a spatial resolution of 1500 km is needed to detect fine lateral heterogeneities in the atlantic the small lateral extent of young oceanic regions and strong lateral velocity gradients do not allow to quantify velocity versus age relationships using our data set on the other hand the regionalization without a priori constraints of phase velocities confirms the presence of a high velocity anomaly beneath the central part of the atlantic for periods greater than 100 s

sediment compaction and fluid migration in the makran accretionary prism the makran continental margin in the gulf of oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland a recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge as seen in previous sonobuoy wide angle data accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment a prominent bottom simulating reflector appears 500 800 m beneath the sea bed several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile the hydrate reflector appears to shallow in the region of some faults suggesting a temperature anomaly due to the presence of warm pore fluids a heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly the inferred presence of overpressured pore fluids in the makran suggests that accreted sediments have a low permeability the seismic evidence suggests a two stage compaction process with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones

large holocene tsunamis in the northern arabian sea abstract tsunamis have repeatedly hit the shores of oman northern arabian sea in historical times e g 1945 2013 these events had small

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

thrust mechanics in highly overpressured accretionary wedges rapid tectonic loading of wet sediment in accretionary wedges is likely to cause the fluid pressure to rise until it is sufficient to cause dilatant fracturing dewatering of sediment that has been underthrust and accreted beneath the wedge can produce a large steady supply of such highly overpressured fluid dilatant fracturing will create escape routes so the fluid pressure is likely to be buffered at the value required for the transition between shear and oblique tensile dilatant fracture which is slightly in excess of the load pressure if the maximum compression is nearly horizontal this in turn buffers the strength of the wedge at the cohesive strength which is not pressure dependent and will not vary greatly throughout the wedge near the wedge front the strength is likely to be that of the cohesion on existing thrust faults in the wedge the shear resistance on the base of the wedge will also be fairly constant and related to the cohesive strength of the weak sediment layer that acts as the basal detachment these assumptions allow the application of a simple plastic continuum model which successfully predicts the observed gently convex taper of accretionary wedges they also allow the use of a mechanical model for frontal imbrication which predicts the spacing of imbricate thrusts at the front of the wedge from the strength and thickness of the imbricated layer the ramp angle and the factional resistances on the bounding faults and decollement surfaces reconstruction of thrust geometries from seismic data across the makran and other accretionary wedges allows the determination of the geometrical parameters and the thrust imbrication model allows the mechanical parameters to be calculated in terms of an assumed cohesive strength for the incoming sediment layer the calculated values are reasonable and successful forward modelling of the profile of the makran shows that the postulate of a buffered value of fluid pressure in excess of the load pressure is consistent with observation

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

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

the accommodation of arabia eurasia plate convergence in iran continental convergence between arabia and eurasia is taken up by distributed deformation in iran at wavelengths large compared with the thickness of the lithosphere this deformation is best described by a continuous velocity field the only quantitative source of information on the spatial distribution of strain rates within iran is the record of earthquakes we find that we can reproduce the style of deformation observed in the seismicity by simply minimizing the rate of work in a continuous viscous medium that has to accommodate the arabia eurasia plate motion between the defined shapes of iran s rigid borders when in addition we specify central iran azerbaijan and the southern caspian basin to be relatively rigid blocks within the deforming zone then the fit to the style of the observed strain rate distribution is even better we conclude that much of the pattern of deformation in iran is predetermined by the shape of its rigid borders and by the disposition of relatively rigid blocks within it this is likely also to have been a common occurrence in older orogenic belts we confirm earlier suggestions that earthquakes between 1909 and 1992 can account for only a small part 10 20 of the total deformation required by the convergence between the arabia and eurasia plates we then show that the whole plate motion can be accommodated by a velocity field with the same orientations and relative magnitudes of principal strain rates seen in the earthquakes but with larger absolute magnitudes there is therefore no requirement that the large proportion of aseismic deformation in iran is substantially different in style orientation or distribution from that released seismically in the earthquakes

geology of the onshore makran accretionary wedge synthesis and tectonic interpretation abstract this contribution is a synthesis of studies carried out since 2004 in the makran accretionary wedge exposed in se iran these studies were intended to improve our understanding of the behaviour of accretionary wedges and of the significance of the makran wedge in the frame of tethys tectonics reassessing the stratigraphy and structure of the iranian makran has allowed distinguishing four main litho tectonic units the lithological content of these four units is briefly described to discuss the sedimentary environments which record the general filling of an oceanic basin with clastic sediments derived from the north the petrology of ophiolites exposed in the backstop region to the north of the accretionary wedge itself demonstrates the existence of an oceanic basin formed in the jurassic late cretaceous to the north of continental slivers which had separated from central iran there is petrological evidence for a subduction related magmatic arc during the late cretaceous which sourced much of the clastic material in the cretaceous lower miocene turbidites present throughout the wedge the literature survey shows that the late cretaceous to present evolution of the pakistan part of the makran fold and trust belt is in its major attributes similar to that of the iranian makran these observations and results are discussed in a tectonic model that integrates knowledge on the oman mountains on the other side of the current oman subduction

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

tectonic processes along the front of modern convergent margins research of the past decade at convergent margins oceanic crust descends beneath continental crust to form a complex crustal juncture this juncture begins at an ocean trench and can be followed deep into the earth as a zone of high seismicity created as the two crustal plates move past one another the places where oceanic crust descends into the earth known as subduction zones are highly dynamic features modern examples contain analogues for processes that created the now static remains of ancient margins exposed on land at the fronts of subduction zones where the descending oceanic crust is still relatively shallow exploration geophysical techniques can define the structure of sedimentary strata modern seismic reflection techniques can resolve 100 m intervals in the upper 2 6 km of a sedimentary section at trench depths drill cores of such sediment from trenches have been successfully recovered between about 0 5 and 1 2 km below the seafloor this review focuses on the advances in knowledge of the fronts of modern convergent margins where the structure and tectonic history can be so observed the review is in part historical because new findings of the past decade became increasingly difficult to explain without extensive modifi

mud diapirism within indus fan sediments murray ridge gulf of oman summary diapiric structures showing striking spatial periodicity have been widely observed at shallow levels on single and multichannel seismic reflection profiles within indus fan sediments the study area is some 200 to 400 km from the mouth of the indus at the fan s northern margin in the region of the murray ridge neighbouring diapirs are generally associated with the same acoustically transparent layer they form at depths less than 500 in below the seafloor and have the form of gentle waves on the upper surface of this parent layer the acoustic appearance of the features may be controlled by the presence of methane which has been widely reported in the area the diapirs are formed by gravitational instability of overpressured muds within the rapidly deposited sedimentary sequence simple 2 d two layer gravity instability modelling of diapir wavelengths and growth rates has determined values of approximately 104 to 106 for the ratio of equivalent viscosity of the overburden sediments to that of the diapiric layer and a maximum equivalent viscosity of 1011 to 1012 pa s for the diapiric layer itself