imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

development of the accretionary prism along peru and material flux after subduction of nazca ridge results from ocean drilling program leg 112 indicated tectonic erosion of the peruvian convergent margin during subduction of the nazca ridge followed by renewed accretion against the erosional scar seismic images and swath mapping morphology show a regional geology shaped as subduction of nazca ridge migrated 800 km along the peru trench the accretionary prism is imaged at two stages of development since upper miocene time with depth migrated seismic records in which the back stop is well defined from these data material flux can be quantified after the ridge crest subducted and accretion dominated over erosion the prism grew rapidly to 10 to 15 km width during rapid growth the margin taper was large and about 60 percent of the sediment supply accreted as the prism growth slowed only 30 percent accreted despite an increased trench sediment supply since the convergence rate changed little the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted the decreased prism growth may be partially controlled by varying the structure of the back stop the force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary thus the shear stress transmitted across the plate boundary is insufficient to drive out of sequence thrust faults that would thicken the back stop or raise it beyond a limited height a constant back stop height limits accretion and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop nazca ridge subduction affected the convergent margin mass flux for about 8 ma mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest when growth of the accretionary prism slowed a greater terrigenous input through the trench was accommodated by sediment subduction the resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which may affect coupling

out of sequence thrusting in experimental coulomb wedges implications for the structural development of mega splay faults and forearc basins 1 we have investigated how an arc ward increase in bulk mechanical strength in an experimental accretionary prism influences the development timing and duration of slip on out of sequence thrusts we have monitored the structural development and kinematics in side view during the development of a frontally accreting coulomb wedge growing out in front of a critically tapered and mechanically stronger inner wedge the inner wedge initially behaved as classic backstop to deformation with the most actively slipping thrust occurring near the deformation front on the forward most thrust structures with continued growth however significant out of sequence slip on reactivated fore thrusts occurred in conjunction with slip on newly formed back thrusts in the inner wedge this out of sequence deformation resulted in punctuated rapid uplift of the model forearc basin and a noticeable break in topographic slope in the outer pro wedge cyclical out of sequence fore and back thrusting driven by ongoing frontal thrust imbrication continued with periodic recovery of taper and was followed by additional out of sequence faulting and associated basin uplift

megathrust splay faults at the focus of the prince william sound asperity alaska 1 high resolution sparker and crustal scale air gun seismic reflection data coupled with repeat bathymetric surveys document a region of repeated coseismic uplift on the portion of the alaska subduction zone that ruptured in 1964 this area defines the western limit of prince william sound differencing of vintage and modern bathymetric surveys shows that the region of greatest uplift related to the 1964 great alaska earthquake was focused along a series of subparallel faults beneath prince william sound and the adjacent gulf of alaska shelf bathymetric differencing indicates that 12 m of coseismic uplift occurred along two faults that reached the seafloor as submarine terraces on the cape cleare bank southwest of montague island sparker seismic reflection data provide cumulative holocene slip estimates as high as 9 mm yr along a series of splay thrust faults within both the inner wedge and transition zone of the accretionary prism crustal seismic data show that these megathrust splay faults root separately into the subduction zone decollement splay fault divergence from this megathrust correlates with changes in midcrustal seismic velocity and magnetic susceptibility values best explained by duplexing of the subducted yakutat terrane rocks above pacific plate rocks along the trailing edge of the yakutat terrane although each splay fault is capable of independent motion we conclude that the identified splay faults rupture in a similar pattern during successive megathrust earthquakes and that the region of greatest seismic coupling has remained consistent throughout the holocene

paleogene evolution of the kodiak islands alaska consequences of ridge trench interaction in a more southerly latitude the kodiak islands of alaska are principally composed of a late cretaceous and paleogene complex of accreted deep sea rocks an early paleocene magmatic event invaded the accretionary complex with mid ocean ridge basalt morb andesite and granodiorite geochemical evidence suggests both the andesite and granodiorite are mixtures of morb and sediment the volcanic rocks are interlayered with coarse clastic sediments and deformed with the latter the granodioritic intrusions with k ar ages as old as 62 ma postdate emplacement of the early paleocene sedimentary and volcanic rocks interaction of the subduction complex with the kula farallon ridge most simply explains this early paleocene magmatic episode paleomagnetic studies of the volcanic rocks indicate this interaction occurred at 40 3 6 north latitude which is 25 3 9 south of the expected position of the kodiak islands relative to north america the magmatic arc presently northwest of the kodiak accretionary complex was active during the late cretaceous and early paleocene diminished after the ridge trench interaction and resurged from late eocene into miocene time the magmatic resurgence is coeval with the obductive offscraping of eocene to oligocene submarine fan deposits in the kodiak islands the petrology of these fan deposits indicates they are the most likely proximal equivalent for the zodiac fan thereby limiting its relative motion with respect to the kodiak islands possible models accounting for the northward motion of the kodiak accretionary complex include 1 coupling the accretionary complex and possibly related arc to the northward moving kula plate and closing an ocean basin north of the arc in paleogene time 2 emplacing the kodiak accretionary complex at the northward limit of the paleomagnetic data with subsequent northward motion accomplished by closure of a small back arc basin and or intracontinental shortening and 3 transcurrent faulting and northward motion of the kodiak accretionary complex without the presently adjacent arc

development and applications of double difference seismic tomography double difference dd tomography is a generalization of dd location it simultaneously solves for the three dimensional velocity structure and seismic event locations dd tomography uses a combination of absolute and more accurate differential arrival times and hierarchically determines the velocity structure from larger scale to smaller scale this method is able to produce more accurate event locations and velocity structure near the source region than standard tomography which uses only absolute arrival times we conduct a stability and uncertainty analysis of dd tomography based on a synthetic data set currently three versions of the dd tomography algorithms exist tomodd tomofdd and tomoadd tomodd assumes a flat earth model and uses a pseudo bending ray tracing algorithm to find rays between events and stations while tomofdd uses a finite difference travel time algorithm and the curvature of the earth is considered both codes are based on a regularly distributed inversion grid with the former for a local scale and the latter for a regional scale in contrast tomoadd adapts the inversion mesh to match with the data distribution based on tetrahedral and voronoi diagrams we discuss examples of applying dd tomography to characterize fault zone structure image high resolution structure of subduction zones and determine the velocity structure of volcanoes

model for the origin of the yakutat block an accreting terrane in the northern gulf of alaska a composite oceanic and continental terrane the yakutat block is currently colliding with and accreting to north america in the northern gulf of alaska marine geophysical data indicate that the block has moved with the pacific plate during pliocene and quaternary time but are indeterminant about its previous movement history a model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the kula farallon spreading center beneath north america in this model the spreading center that formed the basaltic basement of the yakutat block was offset from a spreading center to the south by a left lateral transform fault 400 500 km long subduction of the northern spreading center about 45 m y ago sliced off part of the north america continental margin attached it to the kula plate and initiated northward movement of the composite terrane as the yakutat block during eocene and oligocene time migration of the southern spreading center toward north america truncated the yakutat block on the south and left a fossil fracture zone that is now seen as the transition fault the southern spreading center subducted about 25 m y ago and the yakutat block has since traveled with the pacific plate to southern alaska

paleoclimatic evidence for cenozoic migration of alaskan terranes chronostratigraphic and paleoclimatic comparisons of microfossils from deep sea cores from samples of an exploratory drill hole and from dredged rock of the gulf of alaska with coeval microfossil assemblages on the north american continent provide constraints on the northward migration of the yakutat block the prince william terrane and the pacific plate during tertiary time the comparative paleolatitudes of microfauna and flora provide three main constraints 1 the prince william terrane was in its present position with respect to north america at high latitudes 50 5 n by middle eocene time 40 42 ma consistent with models derived from paleomagnetic data 2 the adjacent yakutat block was 30 5 south of its present position in early eocene 50 ma 20 5 south in middle eocene 40 44 ma and 15 5 south in late eocene time 37 40 ma thus requiring a northward motion of about 30 since 50 ma moreover the yakutat block was at least 10 south of the prince william terrane during eocene time these data are consistent with migration of the yakutat block with the pacific and kula plates for at least the last 50 ma 3 site 192 on the pacific plate was at about 15 5 n latitude in the late cretaceous 68 ma at 30 5 n in early eocene 50 ma at 40 5 n in middle eocene 40 44 ma at 45 5 n in late eocene 37 40 ma and north of 50 5 n in latest eocene to early oligocene time 34 37 ma these paleolatitudes based on planktonic foraminiferal assemblages indicate northward drift consistent with the north america pacific plate reconstructions from about 68 ma to 40 ma engebretson 1982 however from cretaceous to early eocene time faunal data indicate significantly lower latitudinal positions and from oligocwne to early miocene time significantly higher latitudinal positions these discrepancies can be explained by the northward expansion of tropical faunas during the globally warm early tertiary and southward expansion of cold subarctic faunas as a result of global cooling during oligocene time

strike slip earthquakes in the oceanic lithosphere observations of exceptionally high apparent stress summary the radiated energies e s and seismic moments m0 for 942 globally distributed earthquakes that occurred between 1987 to 1998 are examined to find the earthquakes with the highest apparent stresses a e s m0 where is the modulus of rigidity the globally averaged a for shallow earthquakes in all tectonic environments and seismic regions is 0 3 mpa however the subset of 49 earthquakes with the highest apparent stresses a greater than about 5 0 mpa is dominated almost exclusively by strike slip earthquakes that occur in oceanic environments these earthquakes are all located in the depth range 7 29 km in the upper mantle of the young oceanic lithosphere many of these events occur near plate boundary triple junctions where there appear to be high rates of intraplate deformation indeed the small rapidly deforming gorda plate accounts for 10 of the 49 high a events the depth distribution of a which shows peak values somewhat greater than 25 mpa in the depth range 20 25 km suggests that upper bounds on this parameter are a result of the strength of the oceanic lithosphere a recently proposed envelope for apparent stress derived by taking 6 per cent of the strength inferred from laboratory experiments for young less than 30 ma deforming oceanic lithosphere agrees well with the upper bound envelope of apparent stresses over the depth range 5 30 km the corresponding depth dependent shear strength for young oceanic lithosphere attains a peak value of about 575 mpa at a depth of 21 km and then diminishes rapidly as the depth increases in addition to their high apparent stresses which suggest that the strength of the young oceanic lithosphere is highest in the depth range 10 30 km our set of high a earthquakes show other features that constrain the nature of the forces that cause interplate motion first our set of events is divided roughly equally between intraplate and transform faulting with similar depth distributions of a for the two types secondly many of the intraplate events have focal mechanisms with the t axes that are normal to the nearest ridge crest or subduction zone and p axes that are normal to the proximate transform fault these observations suggest that forces associated with the reorganization of plate boundaries play an important role in causing high a earthquakes inside oceanic plates extant transform boundaries may be misaligned with current plate motion to accommodate current plate motion the pre existing plate boundaries would have to be subjected to large horizontal transform push forces a notable example of this is the triple junction near which the second large aftershock of the 1992 april cape mendocino california sequence occurred alternatively subduction zone resistance may be enhanced by the collision of a buoyant lithosphere a process that also markedly increases the horizontal stress a notable example of this is the aleutian trench near which large events occurred in the gulf of alaska in late 1987 and the 1998 march balleny sea m 8 2 earthquake within the antarctic plate

the depth dependence of earthquake duration and implications for rupture mechanisms the duration of rupture is a fundamental characteristic of earthquakes and is important for understanding the mechanics of faulting1 2 the complexity of the seismic source and the incoherence of the high frequency seismic wavefield often inhibit the identification location and timing of features in the later part of earthquake rupture here we sum many teleseismic records from regional seismic arrays producing an unusually clear depiction of the earthquake source at short periods by suppressing background noise and coda generated near the receivers the ending as well as the beginning of rupture is clearly identifiable for most earthquakes examined measurements of 130 large earthquakes show that near 100 km depth rupture duration averages 11 s when scaled to a moment of 1026 dyn cm this decreases to 5 5 s at 650 km depth models of faulting suggest that duration should be inversely proportional to the shear wave velocity and the cube root of stress drop thus to explain the observed twofold decrease in duration with depth stress drops would have to increase by a factor of four as shear velocity increases with depth by only about 20 however observed stress drops show no strong trend with depth3 4 suggesting that the faulting process changes with depth

frontal accretion an internal clock for bivergent wedge deformation and surface uplift 1 we propose a conceptual model of frontal accretion within bivergent wedges which is based on two dimensional sandbox simulations and the analysis of particle displacement fields each frontal accretion cycle consists of a thrust initiation an underthrusting phase and a reactivation phase the location and magnitude of deformation within a bivergent wedge and its associated surface uplift vary systematically with the phase of the frontal accretion cycle and are thus predictable therefore the frontal accretion cycle can be considered as an internal clock for wedge scaled deformation and surface uplift we further demonstrate that the geometry of the deformation front and the spatial distribution of surface uplift can be used to infer the currently active phase within a frontal accretion cycle surface uplift of the axial zone and the retrowedge may reach up to half of the thickness of the incoming sedimentary layer during a frontal accretion cycle we estimate cycle duration to range between 104 and 105 years which is thus similar to the time frame of climatic cycles the proposed conceptual model provides an explanation for the commonly observed transience of deformation and surface uplift and for the discrepancy between geodetic paleoseismologic and geologic estimates of fault slip

structure of the mediterranean ridge accretionary complex from seismic velocity information seismic velocities obtained from ocean bottom hydrophone expanding spread profile and multi channel seismic data were used to compile a velocity model for the mediterranean ridge along a 220 km long transect extending from the sirte abyssal plain to the cleft region near the hellenic trough a 200 300 m thin layer of plio quaternary sediments with velocities of 1800 2200 m s 1 covers the whole ridge the messinian evaporites 4000 4500 m s 1 occur in the southwest as a tectonically thickened layer and in a basin just northeast of the crest of the ridge in the intervening region however the evaporites appear absent and the seismic velocities are generally lower arched reflectors imaged in the depth migrated section suggest that the sediments beneath the ridge crest belong to a pre messinian accretionary wedge beneath the messinian evaporites a northeastward thinning layer of probable tertiary sediments shows laterally increasing velocities from 3300 m s 1 to 4600 m s 1 assuming that the layer thinning is caused by compaction due to increased overburden alone we have calculated a porosity reduction from 15 to 4 and an associated fluid expulsion of 10 km3 km 1 along the trench axis this corresponds to c 60 of the initial fluid volume of an undeformed sediment column from the abyssal plain the almost impermeable evaporitic cap over these sediments leads to high fluid pressures at the base of the evaporites likely to make this horizon the basal decollement of the modern accretionary system a 2 5 km thick unit of probable mesozoic carbonates with velocities of 4500 4600 m s 1 is inferred at c 8 km depth the top of the oceanic crust occurs at a depth of about 10 km the results from this study have widespread implications for the understanding of the regional geological history

elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust nankai trough subduction zone pore fluid pressure is a key parameter governing both the shear strength of faults and the conditions for seismic and tsunamigenic slip on subduction zone megathrusts however quantitative constraints on this parameter based on in situ data in these or any faults have proven elusive using seismic interval velocities derived from a three dimensional seismic reflection survey we compute porosity fluid pressure and effective stress at the plate interface in the nankai trough subduction zone offshore southwestern japan to 20 km down the fault dip using well constrained locally calibrated empirical relationships between velocity porosity and effective stress we show that the fault and immediately subjacent footwall are nearly undrained implying that the subduction megathrust slips under a shear stress of

accretion in the wake of terrane collision the neogene accretionary wedge off kenai peninsula alaska subduction accretion and repeated terrane collision shaped the alaskan convergent margin the yakutat terrane is currently colliding with the continental margin below the central gulf of alaska during the neogene the terrane s western part was subducted after which a sediment wedge accreted along the northeast aleutian trench this wedge incorporates sediment eroded from the continental margin and marine sediments carried into the subduction zone on the pacific plate prestack depth migration was performed on six seismic reflection lines to resolve the structure within this accretionary wedge and its backstop the lateral extent of the structures is constrained by high resolution swath bathymetry and seismic lines collected along strike accretionary structure consists of variably sized thrust slices that were deformed against a backstop during frontal accretion and underplating toward the northeast the lower slope steepens the wedge narrows and the accreted volume decreases notwithstanding a doubling of sediment thickness in the trench in the northeasternmost transect near the area where the terrane s trailing edge subducts no frontal accretion is observed and the slope is eroded the structures imaged along the seismic lines discussed here most likely result from progressive evolution from erosion to accretion as the trailing edge of the yakutat terrane is subducting

predicted ground motions for great interface earthquakes in the cascadia subduction zone ground motions for earthquakes of moment magnitude m 7 5 9 0 on the cascadia subduction zone interface are simulated based on a stochastic finite fault model and used to estimate average response spectra for firm site conditions in vancouver victoria and seattle we also express the response spectra as ground motion prediction equations gmpes for cascadia events the simulations are calibrated by modeling the wealth of ground motion data from the m 8 1 tokachi oki earthquake sequence of japan adjustments to the calibrated model are made to consider average source attenuation and site parameters for the cascadia region we perform best estimate simulations for a preferred set of input parameters typical results suggest mean values of 5 damped pseudoacceleration in the range from about 100 to 200 cm sec2 at frequencies from 1 to 4 hz for firm ground conditions in vancouver victoria and seattle uncertainty in stress drop causes uncertainty in simulated response spectra of about 50 uncertainties in the attenuation model produce even larger uncertainties in response spectral amplitudes a factor of about 2 at 100 km becoming even larger at greater distances it is thus important to establish the regional attenuation model for ground motion simulations furthermore combining data from regions with different attenuation characteristics in particular japan and mexico into a global subduction zone database for development of global empirical gmpes may not be a sound practice time histories of acceleration for the stochastically simulated motions are provided for reference sites in vancouver victoria and seattle an alternative set of motions based on lightly modifying real recordings from the tokachi oki earthquake to match expected conditions for cascadia cities are also provided these alternative records have similar spectral content to the simulated motions but contain additional complexity and more realistic phasing the provision of alternative record sets allows users to conduct studies to determine the importance of these effects for structural response

stress rotation across the cascadia megathrust requires a weak subduction plate boundary at seismogenic depths abstract the mendocino triple junction region is the most seismically active part of the cascadia subduction zone the northward moving pacific plate collides with the subducting gorda plate causing intense internal deformation within it here we show that the stress field rotates rapidly with depth across the thrust interface from a strike slip regime within the subducting plate reflecting the pacific plate collision to a thrust regime in the overriding plate we utilize a dense focal mechanism dataset including observations from the cascadia initiative ocean bottom seismograph experiment to constrain the stress orientations to quantify the implications of this rotation for the strength of the plate boundary we designed an inversion that solves for the absolute stress tensors in a three layer model subject to assumptions about the strength of the subducting mantle our results indicate that the shear stress on the plate boundary fault is likely no more than about 50 mpa at 20 km depth regardless of the assumed mantle strength we infer a relatively weak megathrust fault with an effective friction coefficient of 0 to 0 2 at seismogenic depths such a low value for the effective friction coefficient requires a combination of high fluid pressures and or fault zone minerals with low inherent friction in the region where a great earthquake is expected in cascadia

landslides and megathrust splay faults captured by the late holocene sediment record of eastern prince william sound alaska abstract we present new marine seismic reflection profiles and bathymetric maps to characterize holocene depositional patterns submarine landslides and active faults beneath eastern and central prince william sound pws alaska which is the eastern rupture patch of the 1964 m w 9 2 earthquake we show evidence that submarine landslides many of which are likely earthquake triggered repeatedly released along the southern margin of orca bay in eastern pws we document motion on reverse faults during the 1964 great alaska earthquake and estimate late holocene slip rates for these growth faults which splay from the subduction zone megathrust regional bathymetric lineations help define the faults that extend 40 70 km in length some of which show slip rates as great as 3 75 mm yr we infer that faults mapped below eastern pws connect to faults mapped beneath central pws and possibly onto the alaska mainland via an en echelon style of faulting moderate m w 4 upper plate earthquakes since 1964 give rise to the possibility that these faults may rupture independently to potentially generate m w 7 8 earthquakes and that these earthquakes could damage local infrastructure from ground shaking submarine landslides regardless of the source of initiation could generate local tsunamis to produce large run ups along nearby shorelines in a more general sense the pws area shows that faults that splay from the underlying plate boundary present proximal perhaps independent seismic sources within the accretionary prism creating a broad zone of potential surface rupture that can extend inland 150 km or more from subduction zone trenches

measuring earthquake source parameters in the mendocino triple junction region using a dense obs array implications for fault strength variations abstract subduction zones produce earthquakes on a set of faults that operate under a wide variety of conditions resulting from considerable variations in depth temperature rock type and fluid pressure these variations likely lead to variation in the stress levels that drives particular earthquakes and that in turn effects the magnitude of seismic shaking they produce in the mendocino triple junction mtj region intraplate faults within the mantle of the subducting plate fail regularly in energetic earthquakes while the adjacent thrust interface of the cascadia subduction zone remains seismically quiet despite the likelihood that it operates at much lower levels of stress and strength in 2012 as part of the cascadia initiative community experiment an ocean bottom seismometer obs array was deployed in the mtj area providing unusually dense data covering both the inter and intra plate earthquakes combining these data with onshore networks we detect and relocate 1137 earthquakes with a three dimensional velocity model we perform detailed spectral and time domain analysis to study variations in earthquake source properties between the different types of faults we observe a wide variability of stress drops and systematic lateral and depth variations in the earthquake source spectra resulting from the different types of tectonic fault systems in this region intraplate faults within the subducted oceanic mantle the mendocino transform plate boundary fault and the thrust interface of the cascadia subduction zone some of the depth variability of source spectra can be explained by the expected increase in rupture velocity with depth however the overall variation in stress drop estimates is consistent with the highest stress drop earthquakes occurring in the depth range predicted by strength envelopes moreover the earthquakes in the vicinity of the thrust interface likely including some within the subducted oceanic crust show clearly lower stress drops and depletion of high frequency radiation for their size compared to events in the oceanic mantle and along the transform fault at a given depth our results suggest that the variability in earthquake source spectra that can be expected in the cascadia subduction zone is systematic and that the thrust interface operates at a lower stress level than the core of the subducted plate a few kilometers away

stress orientations in subduction zones and the strength of subduction megathrust faults subduction zone megathrust faults produce most of the world s largest earthquakes although the physical properties of these faults are difficult to observe directly their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them a global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward consistently making an angle of 45 to 60 with respect to the subduction megathrust fault these angles indicate that the megathrust fault is not substantially weaker than its surroundings together with several other lines of evidence this implies that subduction zone megathrusts are weak faults in a low stress environment the deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface

estimating finite source properties of small earthquake ruptures many of the most fundamental questions in earthquake science are currently limited by a lack of knowledge about small earthquake ruptures small earthquakes are difficult to study owing to the poor constraints placed on many of the interesting physical parameters by bandlimited far field seismic data traditionally dynamic models such as an expanding circular crack have been utilized to bridge the gap between the easily measurable quantities for small earthquakes and more interesting physical parameters such as stress drop and rupture velocity here i present a method for estimating the basic finite source properties of a rupture that is independent of any a priori model and utilizes the description of a finite source the second moments that far field waves are inherently sensitive to application to two magnitude 5 events in southern california demonstrates the ability of an empirical green9s function approach to estimating the second moments to resolve the fault plane ambiguity rupture length and overall directivity additional results are presented for two example m 2 7 events from the creeping section of the san andreas fault to examine the likely lower bound on event size that can be studied with surface seismometers the creeping section earthquakes have very similar rupture areas but would be improperly interpreted as significantly different using the traditional methodology one of these events presents a relatively clear interpretation of the velocity of rupture front propagation which is about 0 8 of the rayleigh speed suggesting little difference in rupture velocity between it and typical large earthquakes

crustal heterogeneity and subduction processes 3 d vp vp vs and q in the southern north island new zealand summary along the southern hikurangi subduction zone in the southern north island new zealand the subduction zone changes from volcanic with moderately weak interseismic coupling to non volcanic with strong coupling in wellington to investigate how material heterogeneity relates to subduction processes we compute 3 d seismic properties using 342 earthquakes and 2 shots recorded by a 165 station seismic array the time base for some telemetered temporary stations was not stable which required revising the inversion code to allow an additional origin time related to the telemetered stations progressive inversions from 2 d to fine 3 d were carried out for vp vp vs and qp combined interpretation of these three seismic properties highlights features of the subduction zone the subducted slab is imaged as a high velocity high q feature in the overlying crust from southeast to northwest there is low velocity in the accretionary wedge high velocity in the axial ranges and low velocity in the wanganui basin high vp vs and low qp characterize the crust east of the north island dextral fault belt high vp vs is imaged above the subducted slab to 40 km depth in the northern half of the study area low qp is also prominent above the slab particularly trenchwards of the volcanic region in marked contrast to the high vp vs the low qp above the slab extends as a continuous feature up to the low qp in the shallow crust underlying the volcanic region the low qp and high vp vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release vp vs is particularly sensitive to overpressured fluid in the compressional forearc whereas qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region the rheology of the lower crust of the overlying plate is a factor in large scale subduction coupling as evidenced by the terrane boundaries and haast schist in the section near wellington the changing rheology of the lower crust of the overlying plate in the southern north island also provides a possible explanation for the southward cessation of volcanism

mechanisms of subduction accretion along the central aleutian trench we have used migrated 24 fold seismic records to study mechanisms of subduction accretion along the central aleutian trench two major structural units underlie the landward trench slope and are separated by an acoustically defined decollement surface that can be traced at least 20 km landward of the trench axis the upper unit consists of three to four structural blocks of deformed trench fill each block possesses a consistent internal structure and is bound by major landward dipping thrust faults the structures within the blocks include monoclinal sequences antiforms synforms and faulted folds beneath the decollement nearly undeformed trench fill 1 km thick is being under thrust along with the underlying oceanic lithosphere this basal section is an intervening unit between the offscraped sediment and the rough igneous oceanic basement and may prevent the oceanic crust from being incorporated into the subduction complex at least within 20 km of the front of the subduction zone owing to bending stresses the oceanic crust fails along normal faults beneath and at least 15 km landward of the trench axis these normal faults although poorly constrained are believed to propagate upwards producing offsets within both the subducting and the accreting sediment in addition normal faulting enhances the irregular surface of the oceanic crust structural associations imaged on all of the seismic reflection profiles although possibly fortuitous suggest that the subduction of these irregular basement highs results in the temporary displacement of the overlying fault bound sediment packets and promotes the development of the structures found within the accretionary prism tectonic riffling of the accreted sediment by the passage of the basement relief may also provide a mechanism by which underthrusting sediments are decoupled from the subducting lithosphere and underplated to the base of the aleutian accretionary complex

structural style of the offscraped ligurian oceanic sequences of the northern apennines new hypothesis concerning the development of melange block in matrix fabric shaly dismembered formations crop out extensively on the eastern side of the northern apennines as overthrusted cretaceous oceanic units the lack of metamorphism and the unconformably overlain epi ligurian sequence which represents the trench slope basin sedimentary sequence suggest that these units are accreted sequences accomplished by offscraping and imbrication these units are characterized by stratal disruption lack of internal coherence and complete destruction of the original stratigraphic sequence block in matrix fabric results in symmetric boudins and systematic presence of isolated or intrafolial hinges of isoclinal folds two orthogonal sets of tight to isoclinal folds are always recognizable involving the break up and the boudinage of the competent layers now transposed parallel to the axial planes of the disrupted isoclinal folds the superposition of two generations of orthogonal tight to isoclinal folds may be locally observed either in single bed fragments or in some preserved coherent units disruption and block in matrix fabric is a consequence of polyphase folding and intense shortening this geometry and kinematic interpretation well constrain the tectonic environment where these rocks were deformed to the proximity of the prism toe and within the prism itself thus the apennine broken formations result from the disruption of a sedimentary pile lying on the subducting plate of the cretaceous to eocene trench fore arc system which generated the ligurian accretionary wedge

opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the kodiak shelf and slope alaska an industry acquired seismic reflection line from the aleutian arc to the alaska trench was prestack depth migrated and further constrained with wide angle ocean bottom hydrophone seismic velocity data and swath mapping from the seismic reflection image individual thrust slices the backstop and the forearc basin were resolved sufficiently well to balance the section tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over 150 km during the past 3 myr about 70 of the plate convergence is measured as contraction across the 40 km wide neogene accretionary prism which composes less than 10 of the forearc permanent contraction is minimal above the 1964 alaska earthquake aftershock zone indicating dominently elastic strain since elastic strain is poorly recorded in the geological record it was approximated from the historical record of the 1964 alaska earthquake the 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain consistent with the earthquake cycle concept elastic deformation is dominant beneath the shelf and upper slope rapid permanent deformation is concentrated beneath the lower slope and a transition between them occurs beneath the middle slope

double difference tomography the method and its application to the hayward fault california we have developed a double difference dd seismic tomography method that makes use of both absolute and relative arrival times by reducing systematic errors using the more accurate relative arrival times the method produces an improved velocity model simultaneously it yields event locations of a quality equivalent to those of the dd location method while avoiding simplifying assumptions of that method we test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography we also test this method on a hayward fault california earthquake dataset spanning 1984 1998 the earthquakes relocated by this method collapse to a thin line along the fault trace consistent with previous results the dd velocity model has sharper velocity contrasts near the source region than the standard tomography model

earthquake stress drops and inferred fault strength on the hayward fault east san francisco bay california we study variations in earthquake stress drop with respect to depth faulting regime creeping versus locked fault behavior and wall rock geology we use the p wave displacement spectra from borehole seismic recordings of m 1 0 4 2 earthquakes in the east san francisco bay to estimate stress drop using a stack and invert empirical green s function method the median stress drop is 8 7 mpa and most stress drops are in the range between 0 4 and 130 mpa an apparent correlation between stress drop and magnitude is entirely an artifact of the limited frequency band of 4 55 hz there is a trend of increasing stress drop with depth with a median stress drop of 5 mpa for 1 7 km depth 10 mpa for 7 13 km depth and 50 mpa deeper than 13 km we use s p amplitude ratios measured from the borehole records to better constrain the first motion focal mechanisms high stress drops are observed for a deep cluster of thrust faulting earthquakes the correlation of stress drops with depth and faulting regime implies that stress drop is related to the applied shear stress we compare the spatial distribution of stress drops on the hayward fault to a model of creeping versus locked behavior of the fault and find that high stress drops are concentrated around the major locked patch near oakland this also suggests a connection between stress drop and applied shear stress as the locked patch may experience higher applied shear stress as a result of the difference in cumulative slip or the presence of higher strength material the stress drops do not directly correlate with the strength of the proposed wall rock geology at depth suggesting that the relationship between fault strength and the strength of the wall rock is complex

slip on the suckling hills splay fault during the 1964 alaska earthquake abstract the suckling hills in southern alaska experienced localized anomalously large coseismic uplift in the m w 9 2 1964 alaska earthquake large uplift at the suckling hills can be explained by increased slip or an asperity on the alaska aleutian megathrust however this paper suggests that increased uplift may be a result of slip on the suckling hills splay fault we present a series of models that demonstrate how the inclusion of the suckling hills fault improves the fit between modeled vertical displacement and measured coseismic uplift in comparison to slip on the alaska aleutian megathrust alone our results suggest that 3 m of average slip on the suckling hills fault during the 1964 earthquake can help explain the large coseismic uplift data these results are consistent with recent studies indicating pleistocene slip on the suckling hills fault and together highlight the potential seismic and tsunami risk associated with this segment of the alaskan subduction complex

edge deep seismic reflection transect of the eastern aleutian arc trench layered lower crust reveals underplating and continental growth an edge deep crustal seismic reflection transect of the eastern aleutian arc trench traces oceanic crust and moho more than 200 km beneath the accretionary prism to depths of more than 30 km these horizons project beneath a prominent sequence of layered reflectors that extends from about 9 to 35 km beneath the mesozoic core of the prism earthquake hypocenters imply continuity of the downgoing lithosphere from the base of the layered reflectors to beneath and beyond the active augustine volcano rapid lateral growth of the prism in eocene oligocene time coincided with uplift of the mesozoic core of the prism during lateral growth maintenance of critical taper requires thickening either by internal deformation or underplating because exposed rocks show only modest postemplacement shortening thickening most likely occurred by underplating probably of the layered reflectors the overall geometry of the layered reflectors is reminiscent of nappe structures and their emplacement may represent crustal scale duplexing associated with underplating the edge reflection data and borehole results indicate that the self edge is marked by an active out of sequence thrust that separates the paleogene and neogene prisms this thrust apparently developed in response to the prism s need to maintain critical taper and demonstrates that contrasts inmore lithology can result from mechanisms other than terrane emplacement less

a geometric model to estimate slip rates from terrace rotation above an offshore listric thrust fault kaikōura new zealand abstract the kaik ura peninsula lies at the transition from subduction to continental collision at the southern end of the hikurangi subduction system this study uses a 2012 lidar survey over the kaik ura peninsula to re map a flight of four uplifted late pleistocene marine terraces at high resolution the lidar shows that the terraces are being progressively tilted landwards in a manner consistent with listric thrust faulting offshore the fault known as the kaik ura peninsula fault dips shallowly at c 30 to the northwest at depth and probably truncates the seaward segment of the hope fault in the footwall of the jordan thrust geometric analysis of the tilt using ages based on previous dating and a regional sea level curve suggests that slip rates on the kaik ura peninsula fault have ranged between 2 3 1 5 mm yr and 4 1 1 3 mm yr over the late pleistocene the elevations of late holocene fringing beaches track regional sea level changes apart from a departure indicating a single uplift event penultimate earthquake in the late 18th to earliest 19th century 1702 1838 ce followed by uplift in the 2016 kaik ura earthquake the slip rate calculated from beach uplift including the kaik ura earthquake is consistent with longer term rates the accumulation of the slip over only c 220 years following 2000 years of quiescence suggests that the offshore thrust and associated faults exhibit clustered behavior possibly as a result of keystone faulting

the long recurrence intervals of small repeating earthquakes may be due to the slow slip rates of small fault strands observations since 1998 have revealed that repeating earthquakes and particularly small repeating earthquakes occur less often than expected given their seismically derived slip and the regional fault slip rate here we test the hypothesis that small repeaters occur infrequently because they occur on fault segments or strands with low slip rates we analyze the recurrence interval moment scaling of earthquake sequences near parkfield california we find that closely spaced sequences which likely occur on the same fault strand and respond to the same slip rate follow a scaling consistent with seismic slip rates while widely spaced sequences which likely occur on different strands follow a scaling consistent with the previous counter intuitive observations these results suggest that spatially varying slip rates could create the recurrence interval scaling though we cannot exclude other explanations

accretionary prisms in subduction earthquake cycles the theory of dynamic coulomb wedge 1 we expand the theory of critically tapered coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles building on the coulomb plasticity of the classical theory we assume an elastic perfectly coulomb plastic rheology and derive exact stress solutions for stable and critical wedges the new theory postulates that the actively deforming most seaward part of an accretionary prism the outer wedge overlies the updip velocity strengthening part of the subduction fault and the less deformed inner wedge overlies the velocity weakening part the seismogenic zone during great earthquakes the outer wedge is pushed into a compressively critical state with an increase in basal and internal stresses and pore fluid pressure after the earthquake the outer wedge returns to a stable state the outer wedge geometry is controlled by the peak stress of the updip velocity strengthening part of the subduction fault achieved in largest earthquakes the inner wedge generally stays in the stable regime throughout earthquake cycles acting as an apparent backstop and providing a stable environment for the formation of forearc basins the new theory has important implications for the studies of the updip limit of the seismogenic zone the evolution of accretionary prisms and forearc basins activation of splay faults and tsunami generation evolution of the fluid regime and mechanics of frontal prisms at margins dominated by tectonic erosion

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

continuous uplift near the seaward edge of the prince william sound megathrust middleton island alaska middleton island located at the seaward edge of the continental shelf 50 km from the base of the inner wall of the aleutian trench affords an opportunity to make land based measurements of uplift near the toe of the prince william sound megathrust site of the 1964 m 9 2 alaska earthquake leveling surveys 1973 1993 on middleton island indicate roughly uniform tilting 1 rad a down to the northwest of the island and gps surveys 1993 2012 show an uplift rate of 14 mm a of the island relative to fixed north america the data are consistent with a combined coseismic and postseismic uplift in meters due to the 1964 earthquake as a function of time years after the earthquake u 3 5 1 21 log10 1 1 67 h where 3 5 is the coseismic uplift and h is 0 for 0 and 1 otherwise the current uplift on middleton island is attributed to continuous slip on a fault splaying off from the megathrust and the long term uplift is the superposition of the effects of past earthquakes each earthquake being similar to the 1964 event then the predicted uplift at time t due to a sequence of earthquakes at times ti would be iut ti from studies of strandlines associated with the uplifted terraces on middleton island plafker et al 1992 estimated the occurrence times of the last six earthquakes and measured the present day elevations of those strandlines the predicted uplift is in rough agreement with those measurements about half of the predicted uplift is due to postseismic relaxation from previous earthquakes

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to

mapping the megathrust beneath the northern gulf of alaska using wide angle seismic data in the northern gulf of alaska and prince william sound we have used wide angle seismic reflection refraction profiles earthquake studies and laboratory measurements of physical properties to determine the geometry of the prince william and yakutat terranes the aleutian megathrust and the subducting pacific plate in this complex region the yakutat terrane is underthrust beneath the prince william terrane and both terranes are interpreted to be underlain by the pacific plate wide angle seismic reflection refraction profiles recorded along five seismic lines are used to unravel this terrane geometry modeled velocities in the upper crust of the prince william terrane to 18 km depth agree closely with laboratory velocity measurements of orca group phyllites and quartzofeldspathic graywackes the chief components of the prince william terrane to hydrostatic pressures as high as 600 mpa 6 kbar a landward dipping reflector at depths of 16 24 km is interpreted as the base of the prince william terrane this reflector corresponds to the top of the wadati benioff zone seismicity and is interpreted as the megathrust immediately beneath the megathrust is a 4 km thick 6 9 km s refractor which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to be gabbro in eocene age oceanic crust of the underthrust yakutat terrane wide angle seismic data magnetic anomaly data and tectonic reconstructions indicate that the yakutat terrane has been underthrust beneath the prince william terrane for at least a few hundred kilometers wide angle seismic data are consistent with a 9 to 10 landward dip of the subducting pacific plate beneath the outer shelf and slope distinctly different from the inferred average 3 to 4 dip of the overlying 6 9 km s refractor and the wadati benioff seismic zone beneath the inner shelf our preferred interpretation of the geophysical data is that one composite plate composed of the pacific plate of a fairly uniform thickness and the yakutat plate of varying thickness is subducting beneath southern alaska

the state of stress near the mendocino triple junction from inversion of earthquake focal mechanisms focal mechanisms of 70 earthquakes occurring in the region of the mendocino triple junction between 1977 and 1995 are inverted to obtain the regional stress orientations and relative magnitudes in this technically complex area a diverse set of earthquake geometries is consistent with a single stress field characterized by north northwest horizontal maximum principal compressive stress although this stress direction is almost perpendicular to convergence between the north american and gorda plates it is consistent with the stress direction inferred within the gorda plate northwest of the triple junction a maximum compressive stress direction nearly parallel to strike of the cascadia subduction zone implies very low resolved shear stress across this plate boundary evidence for failure along the southernmost section of the cascadia subduction zone comes from the occurrence of the recent 1992 ms 7 1 cape mendocino underthrusting earthquake as well as from measurements of holocene surface uplift consistent with the 1992 coseismic uplift pattern rupture of the cascadia subduction zone under this stress regime requires that the southernmost region of the gorda north american plate boundary is weak

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

backstop geometry and accretionary mechanics of the sunda margin the convergent sunda margin off indonesia displays all geological features characteristic of an accretion dominated subduction zone a combined interpretation of prestack depth migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high resolution bathymetric data and for the first time allows the exact mapping of backstop regimes initially the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin increasing material strength of the outer high due to lithification formed a dynamic backstop which controls accretion today an out of sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area the existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc which is observed in regimes of frontal as well as of oblique subduction mass balance calculations which account for porosity changes and metamorphism indicate a subduction history dominated by accretionary processes since the late eocene accretion is associated with the low values of basal friction inferred for the sunda margin structural investigations of conjugate fault planes indicate a very weak basal detachment effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments whereas overpressuring within the frontal accretionary prism is negligible

morphotectonics and mechanics of the central makran accretionary wedge off pakistan abstract swath bathymetry images and 3 5 khz data acquired during rv sonne cruise 123 in autumn 1997 are the base for analysing the morphology and deformation styles of the huge makran accretionary wedge the continental slope off pakistan is characterised by three morphological provinces a the lower slope is built of a sequence of imbricate thrust slices which form long and narrow accretionary ridges with flanks locally as steep as 20 and between 10 and 20 on average while the regional slope is only 1 2 b the nearly flat mid slope terrace is present in the central east part of the margin narrows significantly towards the east and is absent east of 63 45 and c the upper slope up to the shelf has a regionally uniform slope as steep as about 8 and comprises a very rough topography with many gullies and canyons and evidence for mass wasting two meandering canyons crossing the entire margin down to the abyssal plain at about 63 15 63 30 can be correlated with onshore rivers they have been formed by episodic erosion through turbidity flows and are characterised by erosional and depositional portions sinuosity of the canyons is exclusively caused by the accretionary ridges functioning as obstacles for flow to be directly downhill application of coulomb rheology to the frontal part of the makran wedge implies that the mid level decollement is intrinsically extraordinary weak with a great strength contrast to the overlying sediments comparison with nankai cascadia and the western mediterranean ridge reveals new insights in the parameters controlling the shape of an accretionary wedge and the role of a mid level decollement

the mw 6 5 offshore northern california earthquake of 10 january 2010 ordinary stress drop on a high strength fault the 10 january 2010 mw 6 5 earthquake offshore northern california is one of the first intraplate earthquakes in oceanic lithosphere to be well captured by a gps network it presents an opportunity to evaluate rupture mechanics on a high strength fault static inversion of the coseismic displacements shows that the slip peaks at the same depth as the expected strength envelope where the differential stresses can be as high as 600 mpa laboratory experiments on peridotite predict dramatic dynamic weakening at these conditions the observed ordinary stress drop 2 20 mpa may indicate that the lithosphere is much weaker than strength envelope predicts or that the failure mechanisms seen in the laboratory are not occurring during the rupture the gps observations show very little postseismic signal indicating that if a shear zone exists beneath the coseismic rupture it operates at significantly greater stress levels than the coseismic stress change

aseismic slip transients emerge spontaneously in three dimensional rate and state modeling of subduction earthquake sequences 1 to investigate the possible physical mechanisms of recently observed aseismic slip events in the cascadia japan and mexico subduction zones we apply a dieterich ruina rate and state friction law to a three dimensional model of a shallow subduction fault that is loaded by imposed steady plate slip rate far downdip along the thrust interface friction properties are temperature and hence depth dependent such that sliding is stable at depths below about 30 km the system is perturbed into a significantly nonuniform slip mode along strike by introducing small along strike variations in the constitutive parameters a and a b in addition to large heterogeneous earthquake slip at seismogenic depths and associated postseismic transients we found that slip events which have clearly aseismic slip rates emerge spontaneously around the downdip end of the seismogenic zone both transients which start well after a seismic event and those which are triggered by other transients are observed from the simulations the slip velocity depth range and sometimes along strike migration speed of simulated transients are similar to the observations from natural subduction zones unstable stable transitional friction properties near the downdip end of the seismogenic zone are suggested to be an ingredient allowing such transients simulated transients can weaken the locking intensity of the updip seismogenic zone while enhancing that of the transition zone spatial temporal correlation of aseismic transients and nearby seismicity in the guerrero mexico area suggests that transients may signal a period of increased probability for nucleating their high frequency counterparts as damaging subduction thrust events

california foreshock sequences suggest aseismic triggering process 1 foreshocks are one of the few well documented precursors to large earthquakes therefore understanding their nature is very important for earthquake prediction and hazard mitigation however the triggering role of foreshocks is not yet clear it is possible that foreshocks are a self triggering cascade of events that simply happen to trigger an unusually large aftershock alternatively foreshocks might originate from an external aseismic process that ultimately triggers the mainshock in the former case the foreshocks will have limited utility for forecasting the latter case has been observed for several individual large earthquakes however it remains unclear how common it is and how to distinguish foreshock sequences from other seismicity clusters that do not lead to large earthquakes here we analyze foreshocks of three m 7 mainshocks in southern california these foreshock sequences appear similar to earthquake swarms in that they do not start with their largest events and they exhibit spatial migration of seismicity analysis of source spectra shows that all three foreshock sequences feature lower average stress drops and depletion of high frequency energy compared with the aftershocks of their corresponding mainshocks using a longer term stress drop catalog we find that the average stress drop of the landers and hector mine foreshock sequences is comparable to nearby swarms our observations suggest that these foreshock sequences are manifestations of aseismic transients occurring close to the mainshock hypocenters possibly related to localized fault zone complexity which have promoted the occurrence of both the foreshocks and the eventual mainshock

rock uplift at the transition from flat slab to normal subduction the kenai mountains southeast alaska abstract the process of flat slab subduction results in complex deformation of overlying forearcs yet how this deformation decays with distance away from the zone of underthrusting is not well understood in south central alaska flat slab subduction of the yakutat microplate drives shortening and rock uplift in a broad coastal orogenic belt defined limits of the zone of underthrusting allow testing how orogenesis responds to the transition from flat slab to normal subduction to better understand forearc deformation across this transition apatite u th he low temperature thermochronometry is used to quantify the exhumation history of the kenai mountains that are within this transition zone measured ages in the northern kenai mountains vary from 10 20 ma and merge with the exhumation pattern in the chugach mountains to the northeast where high exhumation occurs due to flat slab related deformation in the southern kenai mountains however ages increase to 30 50 ma across a transition near seward alaska above the zone from flat slab to normal subduction these ages are relatively old in comparison to ages determined in other studies in southern alaska and suggest minimal exhumation furthermore transitions in topographic expression of the coastal orogen also occur at the margin of yakutat underthrusting these observations suggest that either deformation associated with flat slab subduction requires tens of kilometers to decay with distance away from the zone of underthrusting or that orogenesis in the kenai mountains is driven by a distinct tectonic cause a potential driver of deformation is underplating of thick sediments specifically the surveyor submarine fan along the aleutian megathrust analogous to the tectonic mechanism responsible for the emergence of the kodiak island forearc if correct this may represent a recent tectonic transition in the region given the minimal exhumation of the rugged kenai mountains despite the presence of an erosion conducive glacial climate

fault parameters of the nisqually earthquake determined from moment tensor solutions and the surface deformation from gps and insar the magnitude 6 8 nisqually earthquake occurred on 28 february 2001 at a depth of 50 60 km within the subducting juan de fuca plate the fault parameters are estimated from moment tensor solutions and by comparing the surface displacements from global positioning system gps data and from satellite interferometry with predictions from elastic deformation models the simple deformation model calculates the coseismic surface displacements caused by an earthquake on an extensional rectangular fault in a uniform half space continuous gps stations within 200 km of the epicenter resolved horizontal displacements as great as 9 mm 210 and vertical displacements as great as 13 mm of subsidence near the epicenter the near vertical displacements were also determined from a differential interferogram created from synthetic aperture radar insar data from radarsat the only satellite data available for the event a maximum vertical subsidence of approximately 20 mm is observed 30 km east of the epicenter the gps insar and moment tensor solutions provide a consistent solution for the rupture parameters of the nisqually earthquake the fault has a strike of 180 and a dip of 20 and is centered at 47 10 n and 122 67 w 4 km east and 6 km south of the epicenter or has a strike of 360 and a dip of 70 and is centered at 47 10 n and 122 69 w 2 km east and 6 km south the trade off between fault area and rupture displacement is not resolved by our data but a good fit is found with the main rupture having an area of 230 km2 and an along strike length of 23 km and downdip width of 10 km the rupture area is centered at a depth of 51 km with a scalar moment of 2 0 1019 n m and with the given area an average rupture displacement of 1 4 m to refine the interpretation of the gps and insar data a 3d heterogeneous numerical model was generated having realistic shear moduli structure a 3d model of the subducting slab and a spherical earth the results are similar to those from the half space model but there is significant refinement with a deeper rupture center at 60 km manuscript received 9 april 2003

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

comprehensive analysis of earthquake source spectra in southern california we compute and analyze p wave spectra from earthquakes in southern california between 1989 and 2001 using a method that isolates source receiver and path dependent terms we correct observed source spectra for attenuation using both fixed and spatially varying empirical green s function methods estimated brune type stress drops for over 60 000 m l 1 5 to 3 1 earthquakes range from 0 2 to 20 mpa with no dependence on moment or local b value median computed stress drop increases with depth in the upper crust from about 0 6 mpa at the surface to about 2 2 mpa at 8 km where it levels off and remains nearly constant in the midcrust down to about 20 km however the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop spatially coherent variations in median stress drop are observed with generally low values for the imperial valley and northridge aftershocks and higher values for the eastern transverse ranges and the north end of the san jacinto fault we find no correlation between observed stress drop and distance from the san andreas and other major faults significant along strike variations in stress drop exist for aftershocks of the 1992 landers earthquake which may correlate with differences in main shock slip

relation between the subducting plate and seismicity associated with the great 1964 alaska earthquake tectonic studies of the great 1964 alaska earthquake have underappreciated the nature of the subducted plate in influencing seismicity we compare seismological observations in the prince william and kodiak areas that ruptured during this earthquake with the corresponding morphology and structure of the subducting plate the upper plate geology prince william terrane and velocity structure are the same in both areas in the prince william area where the yakutat terrane subducted the energy released and coupling were stronger than above the kodiak subduction zone where thick trench sediment subducts the conjecture that lower plate character or the amount of subducted sediment affects coupling helps explain variability in seismology geodetic inversions and the horizontal velocity of gps stations

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

relationship between the present day stress field and plate boundary forces in the pacific northwest the relationship between plate boundary forces and the observed stress field in the pacific northwest is established using numerical models of continental deformation because the orientation of the greatest horizontal principal stress throughout the pacific northwest differs considerably from the direction of convergence between the juan de fuca and north american plates the relationship between the stress field and forces acting along the subduction zone has been unclear to address this relationship a two dimensional finite element model developed by bird 1989 is used that incorporates critical aspects of continental deformation such as a stratified rheology and interaction between thermal and mechanical components of deformation boundary conditions are specified in terms of either velocity or shear traction depending on whether the computed shear stress at the plate boundary is less than or exceeds respectively a prescribed limit shear stress limits on the subduction and transform plate boundaries are independently varied to determine the relative effect of forces along these boundaries on intraplate deformation results from this study indicate that the shear stress limit of both subduction and transform boundaries is low and that the intraplate stress field is attributed in part to the normal component of relative plate motion along the transform boundaries however the models also indicate that although the subduction zone fault is weak a minimum shear strength 10 mpa for the fault is necessary to explain the observed stress field the balance among forces along the tectonic boundaries of north america results in a surprising degree of variation in the present day stress field

the 1992 nicaragua earthquake a slow tsunami earthquake associated with subducted sediments the 1992 nicaragua earthquake was a tsunami earthquake that is it generated tsunamis disproportionately large for its surface wave magnitude m s the moment magnitude m w determined from long period 250 s surface waves was 7 6 significantly larger than the 20 s m s of 7 this m s m w disparity is also characteristic of tsunami earthquakes the nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms we conclude that the nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the cocos and north american plates and because of the absence of sediments on the trench floor offshore of nicaragua the slip propagated up dip all the way to the ocean bottom exciting large tsunamis the occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction zone thrust earthquakes our results reinforce the idea that tsunami warning systems using long period 100 s waves are necessary to reduce the hazard from this type of earthquake

spatial and temporal stress drop variations in small earthquakes near parkfield california 1 we estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the parkfield segment of the san andreas fault in central california we use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed p wave source spectra with a spatially varying empirical green s function egf our brune type stress drop estimates vary from 0 1 to over 100 mpa with a median value of 6 75 mpa which is nearly constant with moment implying self similarity over the ml 0 5 to 3 0 range of our data the corner frequency decreases for earthquakes at shallower depths consistent with slower rupture velocities and reduced shear wave velocities in local velocity models the estimated median stress drops show significant lateral variations we find lower stress drops in the middle mountain asperity and along the creeping fault section and higher stress drops in the hypocentral region of the 2004 m6 0 parkfield earthquake the main shock did not alter the overall pattern of high and low stress drop regions however a statistical test reveals areas with significant changes in computed stress drops after the main shock which we compare to estimated absolute shear stress changes from a main shock slip model by calculating t from the spectral egf ratio we also identify areas with increased attenuation after the main shock and we are able to distinguish source effects and near source attenuation effects in the spectral analysis these results are confirmed independently from spectral ratios of repeating microearthquake clusters

stress state in the largest displacement area of the 2011 tohoku oki earthquake the 2011 moment magnitude 9 0 tohoku oki earthquake produced a maximum coseismic slip of more than 50 meters near the japan trench which could result in a completely reduced stress state in the region we tested this hypothesis by determining the in situ stress state of the frontal prism from boreholes drilled by the integrated ocean drilling program approximately 1 year after the earthquake and by inferring the pre earthquake stress state on the basis of the horizontal stress orientations and magnitudes estimated from borehole breakouts and the increase in coseismic displacement during propagation of the rupture to the trench axis in situ horizontal stress decreased during the earthquake the stress change suggests an active slip of the frontal plate interface which is consistent with coseismic fault weakening and a nearly total stress drop

the hettangian corals of the isle of skye scotland an opportunity to better understand the palaeoenvironmental conditions during the aftermath of the triassic jurassic boundary crisis at ob lusa isle of skye scotland six distinct coral beds were observed in a modern outcrop where a hettangian succession is exposed the coral associations are monogenic belonging to lepidophyllia a massive cerioid genus the lowest bed has relatively well developed colonies that form small bioconstructions whereas the other beds have small and dispersed colonies that are completely drowned in the matrix their morphology and size can vary but the general growth fabric is dominated by platy colonies this type of growth fabric is defined as a platestone the most surprising characteristic of these specimens especially for the platy corals is their growth pattern many samples do not exhibit the classical growth polarity because they are bifacial geochemical analyses 180 13c were conducted on oyster shells that were associated with the corals the results indicate that the mean palaeotemperature was approximately 22 c sedimentological analysis revealed shallow settings where the hydrodynamic energy and siliciclastic inputs fluctuated the general faunal assemblage of the outcrop had low diversity and was mainly composed of allochthonous bioclasts the corals at ob lusa clearly did not live under ideal environmental conditions for the development of corals

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

slow slip evolves into megathrust earthquakes in 2d numerical simulations 1 slow slip events sse in many subduction zones incrementally stress the adjacent locked megathrust suggesting that they could potentially either trigger or evolve into damaging earthquakes we explore this with 2d quasi dynamic simulations with rate state friction dilatancy and coupled 1d pore fluid and heat transport steady state weakening friction allows transient slip to nucleate but is inhibited by dilatant strengthening and destabilized by thermal pressurization sse spontaneously nucleate in low effective stress velocity weakening lesvw regions if the dimension of the lesvw is relatively small the sse are trapped at its updip end imparting a strong stress concentration in the locked zone after several centuries sse penetrate into the region of higher effective stress where thermal pressurization eventually leads to dynamic rupture for larger lesvw regions sse tend to increase in length with time ultimately higher slip speeds enhance thermal weakening leading to dynamic instability within the sse zone in both cases the onset of the ultimate sse is essentially indistinguishable from preceding events

teleseismic inversion for the second degree moments of earthquake space time distributions summary determining detailed descriptions of the space time distribution of an earthquake s rupture using only teleseismic data is often a difficult and non unique process however a description of an earthquake that goes beyond a point source is often needed for many seismotectonic problems we have developed a method that inverts measurements of the shifts in traveltime and amplitude of various seismic phases at global stations for the second degree polynomial moments of an event s space time distribution our values for the second moments of the 1995 mw 8 0 jalisco mexico thrust event correspond to a characteristic rupture length of 121t10 km along strike a characteristic rupture width of 76t11 km downdip a characteristic duration of 29t2 s and a direction of rupture propagation along the strike of the subduction zone to the nw for the 1995 mw 7 2 gulf of aqaba earthquake our estimates of the second moments correspond to a characteristic rupture length of 53t2 km a characteristic duration of 12t1 s and rupture propagation to the ne along the strike of the dead sea transform our estimates of the first and second moments agree well with values from local seismic and geodetic networks for both the jalisco and aqaba events we are able to resolve the fault plane ambiguity associated with the events moment tensors using our estimates of the second spatial moment

subduction zone recycling processes and the rock record of crustal suture zonesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent offshore observations at modern ocean margin subduction zones omszs reveal that bodies of accreted material are commonly volumetrically small or missing that crustal thinning and subsidence 3 5 km has occurred and that most trench axes lie close 5 30 km to the seaward tapering edge of coastal basement rock onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front these observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes respectively of sediment subduction and subduction erosion subduction erosion truncates the margin migrates the trench inboard at 2 5 km ma onshore observations at ancient crust suturing subduction zones csszs imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates during a protracted period of suturing 20 50 millio

coseismic strengthening of the shallow portion of the subduction fault and its effects on wedge taper 1 according to the dynamic coulomb wedge model coseismic strengthening of the shallowest part of the subduction interface can cause permanent deformation of the overlying outer wedge so that the cumulative effects of great interplate earthquakes control the geometry of the wedge in this work we use a numerical model which is a hybrid of the frictional contact model and the classical crack model to study how stress is coseismically transferred from the seismogenic zone to the strengthening updip zone to cause wedge deformation in this static model the critical strengthening required to prevent the rupture from breaking the trench depends on the force drop of the seismogenic zone defined as the product of the average shear stress drop and the area of the seismogenic zone in a simple model of uniform material properties with a few megapascals average stress drop over a seismogenic zone of 120 km downdip width the critical strengthening for a 30 km wide updip zone is an increase in the effective friction coefficient by about 0 05 using the coulomb wedge theory we demonstrate that this level of strengthening can readily push the overlying wedge into a critical state of failure with much greater strengthening the rupture is able to extend into the updip zone only slightly causing localized wedge compression in the area of slip termination we examined wedge geometry of 22 subduction zones in the light of the model results we found that the surface slope of these wedges is generally too high to be explained using the classical wedge theory but can be explained using the dynamic coulomb wedge model including coseismic strengthening of the shallow portion of the megathrust

mass and fluid flux during accretion at the alaskan margin prestack depth migration of a seismic reflection transect across the alaskan convergent margin clarified structure and provided an undistorted tectonic cross section between the trench floor and a backstop of eroded eocene miocene rock the accretionary prism displays three tectonic styles seaward of the deformation front trench sediment is cut by incipient faults confined to the subsurface and distributed deformation in the 10 km landward of the deformation front thrust faults develop into imbricate structures above a lower detachment in the remaining 20 km longer and more steeply dipping thrust units form a classical imbricate prism above an upper detachment a velocity model at the scale of the thrust slices was derived from depth processing and seismic refraction data the resulting true scale tectonic cross section was balanced to estimate horizontal contraction a porosity field was derived through the velocity porosity relation in five nearby drill holes it indicates porosity reduction patterns characteristic of each tectonic style seaward of the deformation front the strain rate is high and pore fluid pressure elevates but porosity reduction is minimal in the 10 km landward of the deformation front tectonism and dewatering are most rapid sea floor venting was observed only here in the classic accretionary zone strain rates decrease and dewatering is moderate here an upper detachment propagates across the lower parts of imbricate slices and doubles the underthrust mass one third of the trench sediment section is underthrust at the deformation front whereas two thirds of the sediment section is subducted beneath the backstop

vertical extent of the newborn san andreas fault at the mendocino triple junction three dimensional seismic refraction data reveal the structure of the plate boundaries at the mendocino triple junction first arrival traveltimes were inverted to obtain a three dimensional seismic velocity model the tomography results indicate a strong lateral velocity contrast in the lower crust this velocity contrast is beneath the surface trace of the san andreas fault and tracks along the fault to the triple junction and then along the mendocino fracture zone velocity values appropriate for mafic rocks are more than 5 km shallower on the pacific plate than east of the san andreas fault and 1 3 km shallower than under the cascadia accretionary prism the model suggests that the san andreas fault which is growing as the triple junction migrates northward cuts steeply through the entire crust pacific oceanic crust does not extend beneath north america at this location the slabless window south of the triple junction and east of the san andreas fault must be filled with upwelling asthenosphere the sharp corner of the pacific plate migrates northward displacing the cascadia accretionary prism and forcing the prism material to move north or east

mechanisms of subduction accretion as implied from the broken formations in the apennines italy mesoscale structural analysis of the ligurian broken formations of the northern apennines and the regional occurrence of subduction related structures indicate polyphase folding as the mechanism responsible for progressive disruption the evidence for direct correlation between regionally extensive noncylindrical folds and tectonic disruption has a strong bearing on determining how the ligurian accretionary prism evolved for example the block in matrix structure of the ligurian tectonic melanges is produced through extension developed along fold limbs in a compressive stress field as expected from offscraping and intraprism deformation the model implied by the mesoscale and macroscale analyses of the ligurian tectonic melanges provides one mechanism of accretion reconciling two deformation styles i e stratal disruption and folding commonly seen in ancient accretionary prisms furthermore this model could be relevant in active accretionary prisms especially in the intraprism part where the seismic and drilling data are sparse

olivine friction at the base of oceanic seismogenic zones 1 we investigate the strength and frictional behavior of olivine aggregates at temperatures and effective confining pressures similar to those at the base of the seismogenic zone on a typical ridge transform fault triaxial compression tests were conducted on dry olivine powder grain size 60 m at effective confining pressures between 50 and 300 mpa using argon as a pore fluid temperatures between 600 c and 1000 c and axial displacement rates from 0 06 to 60 m s axial strain rates from 3 10 6 to 3 10 3 s 1 yielding shows a negative pressure dependence consistent with predictions for shear enhanced compaction and with the observation that samples exhibit compaction during the initial stages of the experiments a combination of mechanical data and microstructural observations demonstrate that deformation was accommodated by frictional processes sample strengths were pressure dependent and nearly independent of temperature localized shear zones formed in initially homogeneous aggregates early in the experiments the frictional response to changes in loading rate is well described by rate and state constitutive laws with a transition from velocity weakening to velocity strengthening at 1000 c microstructural observations and physical models indicate that plastic yielding of asperities at high temperatures and low axial strain rates stabilizes frictional sliding extrapolation of our experimental data to geologic strain rates indicates that a transition from velocity weakening to velocity strengthening occurs at approximately 600 c consistent with the focal depths of earthquakes in the oceanic lithosphere

formation of an active thrust triangle zone associated with structural inversion in a subduction setting eastern new zealand 1 we analyze a thrust triangle zone which underlies the continental shelf of hawke bay eastern new zealand within the hikurangi subduction margin this triangle zone differs from many other examples in that it is active 90 km from the leading edge of the overriding plate and formed due to polyphase deformation involving opposed dipping thrust duplex and backthrust with the later structure forming in response to inversion of an extensional graben the component structures of the zone mainly developed sequentially rather than synchronously high quality marine seismic reflection lines tied to well and seabed samples reveal the three dimensional structure of the zone together with its 25 myr evolution and late quaternary activity the triangle zone occurs in the lateral overlap between a stack of nw dipping blind thrusts and a principal backthrust the kidnappers fault the nw dipping thrusts initiated in the early middle miocene during the early stages of subduction with subsequent thrust duplex formation producing major uplift and erosion in the late miocene early pliocene the active backthrust formed during the late miocene to early pliocene as a thin skinned listric extensional fault confined to the cover sequence structural inversion of the extensional fault commenced in the early middle pliocene produced the backthrust and marks the formation of the thrust triangle zone the thrust duplex and backthrust accrued strain following inversion however the later structure accommodated most of the surface deformation in the quaternary section balancing of the triangle zone together with a detailed analysis of reverse displacements along the backthrust reveal spatial and temporal variations of strain accumulation on the two principal components of the zone although the formation of the triangle zone is strongly influenced by regional tectonics of the subduction system these variations may also in part reflect local fault interaction for example high quaternary displacement rates on the backthrust accounts for 70 of the displacement loss that occurs on the southern segments of the overlapping lachlan fault understanding the tectonic evolution of such complex polyphase thrust triangle zones requires the preservation of growth strata that record sequential deformation history in the absence of such data synchroneity of opposed dipping thrusts in triangle zones cannot be assumed

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event

crustal structure of the peruvian continental margin from wide angle seismic studies keine zusammenfassung in englischer sprache vorhanden no summary in english language