crustal structure of accreted terranes in southern alaska chugach mountains and copper river basin from seismic refraction results seismic refraction data were collected along a 320 km long transect line in southern alaska crossing the prince william chugach peninsular and wrangellia terranes and along several shorter lines within individual terranes velocity structure in the upper crust less than 9 km depth differs among the four terranes in contrast layers in the middle crust 9 to 25 km depth in some cases extend across projected terrane boundaries the following observations can be made 1 an intermediate velocity layer 6 4 km s at 9 km depth extends across the deep projection of the suture between the chugach and peninsular terranes suggesting that the northern chugach and southern peninsular terranes are detached and rest on a deeper terrane of unknown origin 2 the top of a gently north dipping sequence of low and high velocity layers 5 7 7 8 km s more than 10 km thick extends from near the surface in the southern chugach terrane to more than 20 km depth beneath the southern peninsular terrane this sequence truncated by the suture between the prince william and chugach terranes is interpreted to be an underplated terrane made up of fragments of the kula plate and its sedimentary overburden that were accreted during subduction in the late mesozoic and or early tertiary during or between times of accretion of the prince william and chugach terranes 3 a thick crustal root with a laminated sequence at its top extends from a depth of 19 km to as much as 57 km beneath the northern peninsular and wrangellia terranes this root extends across the deep projection of the suture between the peninsular and wrangellia terranes although resolution of this apparent crosscutting relationship is relatively poor this root may represent tectonically or possibly magmatically emplaced rocks the lower crust beneath the prince william chugach and southern peninsular terranes includes a north dipping 3 to 8 km thick section of subducting oceanic crust

calcium and titanium isotopes in refractory inclusions from cm co and cr chondrites abstract previous studies have shown that cv and cm chondrites incorporated ca al rich inclusions cais with different isotopic characteristics which may represent different snapshots in the isotopic evolution of the early solar system to better understand how the isotopic characteristics of cais vary between different chondrite groups we have studied calcium and titanium isotopes in cais from cm co and cr chondrites we show that all three chondrite groups contain cais with large anomalies in 48ca and or 50ti 10s of or 100s of e units as well as cais with no anomalies resolved beyond measurement uncertainties isotopically the anomalous co and cr chondrite cais resemble the platy hibonite crystals placs from cm chondrites but they are more mineralogically complex the new data are consistent with the well established mutual exclusivity relationship between incorporation of 26al and the presence of large anomalies in 48ca and 50ti the two highly anomalous co chondrite cais have correlated anomalies in 46ti and 50ti while most other highly anomalous cais do not this result could indicate that the reservoir with coupled 46ti and 50ti that was sampled by bulk meteorites and cv chondrite cais already existed before arrival and or homogeneous distribution of 26al in the protoplanetary disk among the studied cm chondrite cais are ten spinel hibonite inclusions shibs with known oxygen isotopic compositions our results show that these objects sampled a reservoir that was well mixed in oxygen calcium and titanium isotopes we further show that shibs tend to be slightly enriched in the heavy calcium isotopes suggesting that their formation history was different from cv chondrite cais

neogene foreland basin deposits erosional unroofing and the kinematic history of the himalayan fold thrust belt western nepal sedimentological and provenance data from the lower miocene pliocene dumri formation and siwalik group in western nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the himalaya and the cenozoic 87 sr 86 sr record of the ocean in western nepal the dumri formation is an 750 1300 m thick fluvial sandstone and overbank mudstone unit the siwalik group is 4200 m thick and consists of a lower member 850 m of 2 12 m thick fluvial channel sandstones and oxidized calcareous paleosols a middle member 2400 m of very thick 20 m channel sandstones and mainly organic rich histosols and an upper member 1000 m composed of gravelly braided river deposits paleocurrent data indicate that middle miocene pliocene rivers in western nepal flowed southward transverse to the thrust belt throughout deposition of the siwalik group no evidence was found for an axial fluvial trunk system i e the paleo ganges river in siwalik group sandstones a major increase in fluvial channel size is recorded by the transition from lower to middle siwalik members at 10 8 ma probably in response to an increase in seasonal discharge modal petrographic data from sandstones in the dumri formation and the siwalik group manifest an upsection enrichment in potassium feldspar carbonate lithic fragments and high grade metamorphic minerals modal petrographic analyses of modern river sands provide some control on potential source terranes for the miocene pliocene sandstones the dumri formation was most likely derived from erosion of sedimentary and low grade metasedimentary rocks in the tibetan tethyan himalayan zone during early miocene emplacement of the main central thrust the presence in dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the greater himalayan zone perhaps in response to tectonic unroofing by extensional detachment faults of the south tibetan detachment system during deposition of the lower siwalik group 15 11 ma emplacement of the dadeldhura thrust sheet one of the synformal crystalline thrust sheets of the southern himalaya on top of the dumri formation supplied abundant metasedimentary lithic fragments to the foreland basin a steady supply of plagioclase grains and high grade minerals was maintained by deeper erosion into the main central thrust sheet from 11 ma to the present k feldspar sand increased steadily suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower siwalik group this provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the dadeldhura thrust sheet above a large duplex in the lesser himalayan rocks since the onset of deposition of the conglomeratic upper siwalik group 4 5 ma fault slip in this duplex has been fed updip and southward into the main boundary and main frontal thrust systems we obtained 113 u pb ages on detrital zircons from modern rivers and siwalik group sandstones that cluster at 460 530 ma 850 1200 ma 1 8 2 0 ga and 2 5 ga an abundance of cambrian ordovician grains in the siwalik group suggests sources of siwalik detritus in the granites of the dadeldhura thrust sheet and possibly the greater himalayan orthogneisses the older ages are consistent with sources in the greater and lesser himalayan zones an overall upsection increase in zircons older than 1 7 ga suggests increasing aerial exposure of lesser himalayan rocks none of the detrital zircons even in the modern river samples yielded a cenozoic age that might suggest derivation from the cenozoic greater himalayan leucogranites but this may be attributable to the inheritance problems that characterize the u pb geochronology of the leucogranites when compared with recent studies of the 87 sr 86 sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the siwalik group the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the lesser himalayan zone and cambrian ordovician granitic rocks of the crystalline thrust sheets in central and eastern nepal may have played a significant role in elevating the 87 sr 86 sr ratio of middle miocene synorogenic sediments in the indo gangetic foreland basin and the bengal fan as well as global seawater

seismicity of the wrangell and aleutian wadati benioff zones and the north american plate along the trans alaska crustal transect chugach mountains and copper river basin southern alaska the configuration of the aleutian and wrangell wadati benioff zones in southern alaska between longitudes 142 w and 148 w is determined from regionally recorded earthquakes from 1971 to 1986 the earthquakes are located with a simple velocity model calibrated with seismic refraction measurements conducted for the trans alaska crustal transect tact west of the transect meridian approximately 145 20 w and beneath the chugach mountains shocks in the depth range 25 60 km compose the shallow part of the northwest to north dipping aleutian zone this zone can be traced updip and seaward beneath prince william sound east of the transect meridian and north of 61 n latitude the weakly active north northeast dipping wrangell zone extends to a depth of at least 100 km beneath the wrangell volcanoes and along strike for at least 150 km the two zones appear to be continuous at least in the depth range 20 45 km and may define adjacent limbs of a buckle in the subducted oceanic lithosphere north of 61 n focal mechanisms in the shallow aleutian zone are compatible with a uniform stress field characterized by in plane least compressive stress directed toward the steeply dipping deeper part of the aleutian zone north of cook inlet stress orientation in the wrangell zone is unresolved because of the low level of seismicity no focal mechanisms corresponding to slip on a gently northward dipping megathrust have been observed along the transect meridian a change of focal mechanism with depth near the top of the dipping seismic zone is inferred to reflect the interplate boundary beneath the chugach mountains this boundary coincides with a band of strong reflectors imaged by tact vertical seismic profiling the overriding plate appears to be aseismic beneath the northern chugach mountains but minor shallow seismicity occurs beneath the copper river basin first motion patterns for these shallow shocks suggest the most compressive stress within the overriding plate is oriented subhorizontally and approximately north south

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

slump generated megachannels in the pliocene pleistocene glaciomarine yakataga formation gulf of alaska pliocene pleistocene megachannels as much as 400 m deep and several kilometers wide are exceptionally well exposed around the gulf of alaska coastal margin in glaciomarine sedimentary strata of the yakataga formation in the icy bay region stacked megachannels of the yakataga formation are filled predominantly with debris flow diamictites crudely stratified conglomerates and thinly bedded sandstone and mudstone turbidites detailed sedimentological and micropaleontological analyses of the megachannel deposits identified two distinct facies associations that represent channel margin and central channel fill depositional settings all facies types indicate deposition by sediment gravity flow processes and high rates of sediment supply in a tectonically active setting foraminiferal biofacies suggest deposition in inner neritic to upper bathyal water depths and vigorous transport of sediment from shallow to deeper water the yakataga formation megachannels probably formed as slump scars produced by the retrogressive failure and downslope slumping of rapidly deposited outer shelf upper slope sediments the intimate association of megachannels with thrust faults and folds suggests that initial slumping was triggered by tectonic tilting and high magnitude earthquakes megachannels have a relatively immature form and were probably short lived and filled rapidly with sediment gravity flow deposits megachannels of the yakataga formation provide a well exposed analog for megachannels formed in other tectonically active settings or passive margin settings characterized by high sedimentation rates there is little evidence to suggest that megachannel development is systematically related to episodes of sea level lowstand and a sequence stratigraphic context is difficult to identify

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

crustal structure along the edge transect beneath the kodiak shelf off alaska derived from obh seismic refraction data summary along the edge alaska transect over the kodiak shelf off alaska a deep crustal seismic refraction experiment was carried out in 1994 using an airgun source with a high shot density and closely spaced ocean bottom hydrophones obhs these data clearly define a gently dipping plate boundary that gradually increases from 2 5d at the trench to 5d 250 km landwards of the trench beneath the shelf a landward increase of the velocities in the subducting oceanic crust was found the sedimentary upper plate beneath the continental slope can be divided into a young neogene accretionary prism beneath the lower slope and a more layered structure below the middle and upper slope where a body with elevated velocities 4 2 5 1 km s 1 in the lower part is interpreted as well consolidated sediment of possibly late eocene age acting as the backstop across the inner shelf between kodiak island and kenai peninsula near surface high velocities 6 0 km s 1 document a strong uplift 10 km of the mesozoic and palaeocene accretionary complex and furthermore suggest its seaward extension beneath the outer shelf the most striking feature along the edge transect is the thick low velocity zone lvz that coincides with the arched reflectors in the lower crust beneath the kodiak island kenai peninsula axis on the edge seismic reflection data this lvz is interpreted as indicating underplated low velocity rocks which contribute the same volume of new material as the frontal accretion to the continent the growth of which is rapid totalling 4200 km2 across the kodiak shelf since eocene times a calculation of the material balance shows that an underplating of underthrust sediment alone may not account for the huge volume of the underplated low velocity rocks we speculate that the underplating of continental fragment or more likely seamounts or plateau may be involved in continental growth

the east greenland continental margin 65 n since the last deglaciation changes in seafloor properties and ocean circulation nine gravity cores collected on the east greenland continental margin off kangerdlugssuaq fjord reveal a drastic decrease in average sedimentation rates from the cross shelf through the continental slope from 43 2 cm kyrs to 2 2 cm kyrs respectively since the last deglaciation beginning at 2g cm2kyrs at 4000 yrs b p both caco3 and non caco3 accumulation rates became high g cm2kyrs since 3000 yrs b p these rates distinctly decreased melting of glaciers and current activity which drastically changed in space and time controlled the terrigenous sediment input and thus the grain size distribution and the physical properties of sediments across the shelf these observations suggest that current winnowing on sediments is one of the dominant processes at present this is seen in the acoustic reflectivity and in the acoustically hard seafloor particularly at the upper continental slope in addition icebergs transported southward along the continental margin have scoured the sediment surface in water depths 350m

magnetic signature of upper plate structures and subducting seamounts at the convergent margin off costa rica offshore the pacific margin of costa rica the caribbean plate converges with the subducting cocos plate along the middle america trench the tectonics of both plates have been studied on the basis of reflection seismic gravimetric and magnetic data and swath mapping the magnetic data were acquired by geomar kiel germany during cruise so 76 of r v sonne off costa rica in 1992 two different magnetic anomaly patterns are observed off nicoya peninsula the linear magnetic anomalies trend about n50 e and can be traced from the ocean basin to the landward end of the profiles to the southeast of a major structural break trending parallel to fisher seamount and the associated ridge the strike of the magnetic anomalies is n70 e in the area southeast of fisher seamount the magnetic signatures of numerous seamounts are superimposed upon the linear anomalies we interpret the magnetic lineations as chrons 5c to 6b 16 22 5 ma magnetic modeling demonstrates a significant magnetization 1 a m of the margin wedge in the continental slope a structural unit known from seismic studies this is not compatible with the previously discussed origin of the continental slope as an accretionary wedge a composition of the margin wedge similar to the ophiolitic rocks found onshore on nicoya peninsula appears likely a number of morphological furrows observed in the landward trench slope opposite the seamount covered domain of the oceanic crust coincide with magnetic anomalies three dimensional magnetic modeling demonstrates that these anomalies can be explained well by subducting seamounts this implies that the seamounts remain intact at least magnetically for a long time interval during the subduction process thus they may indeed be important seismogenic asperities

the 1964 prince william sound earthquake joint inversion of tsunami and geodetic data the 1964 prince william sound alaska earthquake mw 9 2 ruptured a large area beneath the continental margin of alaska from prince william sound to kodiak island a joint inversion of tsunami waveforms and geodetic data consisting of vertical displacements and horizontal vectors gives a detailed slip distribution two areas of high slip correspond to seismologically determined areas of high moment release the prince william sound asperity with average slip of 18 m and the kodiak asperity with average slip of 10 m the average slip on the fault is 8 6 m and the seismic moment is estimated as 6 3 1022 n m or over 75 of the seismic moment determined from long period surface waves

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

deformation of the kenai peninsula alaska crustal deformation on the kenai peninsula in southern alaska has been studied using data obtained from global positioning system gps measurements in 1993 and 1995 and leveling observations in 1964 immediately after the prince william sound earthquake this analysis shows that the kenai peninsula has experienced as much as 900 mm uplift during the past 3 decades and that the uplift forms an 125 km wide elongate dome with its major axis trending southwest to northeast following the trend of the major tectonic features of the region the averaged uplift rate between 1964 and 1995 is as high as 30 mm yr 1 although the current uplift rate may be substantially lower the gps measurements cast further doubt on previously suspect tide gauge data for nikiski alaska which indicated rapid postseismic uplift at this site located in northwest kenai peninsula adjacent to cook inlet examination of the three dimensional gps data indicates that the eastern kenai peninsula is currently undergoing significant sse to nnw contraction in response to north american pacific plate convergence the horizontal velocities are consistent with the predictions of an elastic half space model for the interseismic deformation this result taken in combination with the small changes in uplift between 1993 and 1995 suggests that most of the present deformation is due to steady plate convergence rather than transient postseismic rebound

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

the rupture process and tectonic implications of the great 1964 prince william sound earthquake we have determined the rupture history of the march 28 1964 prince william sound earthquake m w 9 2 from long period wwssn p wave seismograms source time functions determined from the long period p waves indicate two major pulses of moment release the first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture the second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds because of the large size of this event and thus a deficiency of on scale digitizable p wave seismograms it is impossible to uniquely invert for the location of moment release however if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault the first moment pulse most likely initiated near the epicenter at the northeastern down dip edge of the aftershock area and then spread over the fault surface in a semi circular fashion until the full width of the fault was activated the rupture then extended toward the southwest approximately 300 km ruff and kanamori 1983 the second moment pulse was located in the vicinity of kodiak island starting at 500 km southwest of the epicenter and extending to about 600 km although the aftershock area extends southwest past the second moment pulse by at least 100 km the moment release remained low we interpret the 1964 prince william sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major but smaller asperity in the kodiak island region

16 m y of hotspot and nonhotspot volcanism on the patton murray seamount platform gulf of alaska age progressive linear seamount chains in the northeast pacific appear to have formed as the pacific plate passed over a set of stationary hotspots however some anomalously young ages and the lack of an enriched isotopic signature in basalts from the seamounts do not fit the standard hotspot model for example published ages 28 30 ma for basalts dredged from the patton murray seamount platform in the gulf of alaska are 2 4 m y younger than the time when the platform was above the cobb hotspot however the lowermost basalt recovered by ocean drilling on patton murray yielded a 40 ar 39 ar age of 33 ma this age exactly coincides with the time when the seamount platform was above the cobb hotspot consistent with a stationary long lived mantle plume a 27 ma alkalic basalt flow recovered 8 m above the 33 ma basalt is similar in age and composition to the previously dredged basalts and may be the alkalic capping phase typical of many hotspot volcanoes a 17 ma tholeiitic basalt sill recovered 5 m above the 27 ma basalt was emplaced long after the seamount platform moved away from the hotspot and may be associated with a period of intraplate extension anomalously young phases of volcanism on this and other hotspot seamounts suggest that they can be volcanically rejuvenated by nonhotspot causes but this rejuvenation does not rule out the hotspot model as an explanation for the initial creation of the seamount platform the lack of an enriched isotopic signature in any of these basalts shows that enriched compositions are not necessary characteristics of plume related basalts the isotopic compositions of the lower basalts are slightly more depleted than the 0 9 ma products of the cobb hotspot despite the fact that the hotspot was closer to a spreading ridge at 0 9 ma it appears that this hotspot like several others has become more enriched with time

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

coseismic slip in the 1964 prince william sound earthquake a new geodetic inversion the 1964 prince william sound earthquake march 28 1964 m w 9 2 caused crustal deformation over an area of approximately 140 000 km 2 in south central alaska in this study geodetic and geologic measurements of this surface deformation were inverted for the slip distribution on the 1964 rupture surface previous seismologic geologic and geodetic studies of this region were used to constrain the geometry of the fault surface in the kodiak island region 28 rectangular planes 50 by 50 km each oriented 218 with a dip varying from 8 nearest the aleutian trench to 9 below kodiak island define the rupture surface in the prince william sound region 39 planes with variable dimensions 40 by 50 km near the trench 64 by 50 km inland and orientation 218 in the west and 270 in the east were used to approximate the complex faulting prior information was introduced to constrain offshore dip slip values the strike slip component and slip variation between adjacent planes our results suggest a variable dip slip component with local slip maximums occurring near montague island up to 30 m further to the east near kayak island up to 14 m and trenchward of the northeast segment of kodiak island up to 17 m a single fault plane dipping 30 corresponding to the patton bay fault with a slip value of 8 m modeled the localized but large uplift on montague island the moment calculated on the basis of our geodetically derived slip model of 5 0 1029 dyne cm is 30 less than the seismic moment of 7 5 x 1029 dyne cm calculated from long pe riod surface waves kanamori 1970 but is close to the seismic moment of 5 9 1029 dyne cm obtained by kikuchi and fukao 1987

seismicity of the kodiak island region 1964 2001 and its relation to the 1964 great alaska earthquake earthquake relocation first motion and waveform modeling studies of m 5 5 earthquakes are used to examine the seismicity associated with the kodiak island region since the 1964 great alaska earthquake our results indicate that the most intense seismicity and moment release in this region occurred within the pacific plate and along the plate interface of the southwestern edge of the kodiak segment where slip during the 1964 mainshock was less than 5 m few earthquakes have occurred within the north american plate the intense seismicity correlates well with the northern edge of a zone of high plate interface coupling detected by global positioning system geodesy studies our studies suggest that during 1964 1974 faulting within the pacific plate was characterized by normal and normal oblique faulting however since 1974 reverse oblique faulting has become more common in the pacific plate seismic moment release has been over 6 times greater in the kodiak region than in the prince william sound cook inlet region during the past 37 years unlike the prince william sound region there is also a lack of down dip migration of seismicity since 1964 manuscript received 23 october 2001

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

asperity distribution and earthquake occurrence in the southern kurile islands arc the seismicity in the southern kurile islands arc is characterized by a complex overlapping of aftershock zones of a few great and many large earthquakes ms 7 0 the spatial relationships of these aftershock zones can be explained in terms of a simple pattern of fault plane heterogeneity the four largest earthquakes in this region are from north to south the 1963 kurile islands mw 8 5 the 1958 etorofu mw 8 3 the 1969 kurile islands mw 8 2 and the 1973 nemuro oki mw 7 8 events detailed body wave studies indicate that the moment release associated with each event is concentrated in a small portion of the aftershock area with scale lengths 100 km these regions of high moment release are the largest asperities the areas that surround the large asperities slip in two modes 1 coseismically during the great events and 2 as distinct large ms 7 0 earthquakes both before and after the great events the second mode involves either rupture of relatively weak regions which results in low stress drop events or rupture of smaller asperities the earthquakes produced by failure of small asperities tend to repeatedly rupture the same regions and they have impulsive source time functions indicating rupture of highly stressed regions of the fault surface by analysis of all ms 7 0 events since 1946 in this region we find a segmentation of the southern kurile islands thrust zone into strongly coupled regions with two scale lengths dominant and secondary asperities and weakly coupled regions the surrounding areas generating the low stress drop events the largest earthquakes and the smaller asperity earthquakes occur at the down dip portion of the lithospheric interface while the low stress drop events occur much closer to the trench axis this implies a difference in the degree of mechanical coupling between the subducting and overriding plates with the deeper portions having greater strength than the shallower parts

asperity distributions and large earthquake occurrence in subduction zones abstract ruff l j 1992 asperity distributions and large earthquake occurrence in subduction zones in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 61 83 plate tectonics and the seismic gap hypothesis provide the framework for long term earthquake forecasting of plate boundary earthquakes unfortunately detailed examination reveals that earthquake recurrence times and rupture length vary between successive earthquake cycles in the same subduction zone furthermore larger coseismic slip is commonly associated with larger rupture length hence large earthquake occurrence in subduction zones is characterized by variability in 1 recurrence times 2 rupture length and 3 coseismic slip these facts plus many other observations indicate that there are significant spatial variations in the strength of the plate interface one simple description of these variations and their role in the earthquake cycle is the asperity model where the large strong regions of the plate interface are called asperities and the large earthquakes occur when the large asperities break the asperity model of earthquake occurrence is able to qualitatively explain several features of large plate boundary earthquakes to go beyond general qualitative notions i pose the following scientific test are the observed asperity distributions and a simple model of their interaction self consistent with the above three observed features of large earthquake occurrence the distribution of the major asperities along plate boundary segments has now been determined for several subduction zones rupture process studies of adjacent large and great earthquakes have provided reliable estimates of the along strike asperity lengths and separations for several adjacent asperities in the kurile islands colombia and peru subduction zones the simplest mechanical model for asperity interaction is to idealize two adjacent asperities as frictional sliders that are connected by main springs to the upper plate by a coupling spring to each other and maintain frictional contact with a conveyer belt the lower plate that moves with a constant velocity an earthquake occurs when the net force on the asperity frictional slider reaches some specified level the failure force and spring constants are determined by the observed asperity distribution and simple models of elastic interaction two different macroscopic failure criteria are used this simple mechanical model displays a remarkable range of behavior from simple to complex when the two asperities are identical in all their properties sequences of identical earthquakes are produced for the more realistic case of non identical asperities earthquake sequences show great variety using system variables from the observed asperity distributions the earthquake sequences typically display 1 variable recurrence times 2 variable rupture length i e a combination of single asperity and double asperity failures and for one of the failure criteria 3 larger coseismic slip for double asperity failures statistical summaries of thousands of simulated earthquake sequences for asperity pairs in the kuriles colombia and peru subduction zones are broadly consistent with the observed features of large earthquake occurrence in these subduction zones the main conclusion is that the asperity model provides a self consistent explanation for fault zone heterogeneity the rupture process and recurrence times and rupture mode of large earthquake sequences via a simple model for adjacent asperity interaction in addition a conclusion independent of any particular model for fault zone heterogeneity is that simple deterministic models of fault zone interaction can explain complex patterns of large earthquake occurrence in subduction zones

do trench sediments affect great earthquake occurrence in subduction zones seismic energy release is dominated by the underthrusting earthquakes in subduction zones and this energy release is further concentrated in a few subduction zones while some subduction zones are characterized by the occurrence of great earthquakes others are relatively aseismic this variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties age of the subducting lithosphere and convergence rate these two properties do not explain all the variance in maximum earthquake size i propose that a third subduction zone property trench sediments explains part of the remaining variance in maximum earthquake size subduction zones are divided into two groups 1 those with excess trench sediments and 2 those with horst and graben structure at the trench thirteen of the 19 largest subduction zone events including the three largest occur in zones with excess trench sediments about half the zones with excess trench sediments are characterized by great earthquake occurrence most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age rate correlation two notable exceptions are the oregon washington and middle america zones overall the presence of excess trench sediments appears to enhance great earthquake occurrence one speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer which at elevated temperature and pressure forms a homogeneous and strong contact zone between the plates

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

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

deformation across the rupture zone of the 1964 alaska earthquake 1993 1997 a linear array of 15 geodetic monuments was installed in 1993 across the rupture zone of the 1964 alaska earthquake mw 9 2 the array extends from middleton island at the edge of the continental shelf and 80 km from the alaska aleutian trench to north of palmer alaska 380 km from the trench in the approximate direction of pacific north american plate convergence n15 5 w the array was surveyed in june 1993 may 1995 and june 1997 the changes between surveys are a measure of the deformation of the continental margin across the subduction zone in southern alaska measured relative to the interior of the north american plate the horizontal velocities on the outer plate margin are parallel to the direction of plate convergence n15 5 w and reach a maximum 58 mm yr 1 about 150 km from the trench beyond about 300 km from the trench the observed horizontal velocities are small a narrow halfwidth 50 km zone of significant uplift 10 mm yr 1 maximum is observed about 300 km from the trench coinciding roughly with the locus of maximum coseismic subsidence associated with the 1964 alaska earthquake although the deformation is roughly described by the conventional model of deformation at a subduction zone deformation due to virtual back slip on the main thrust zone at the 55 mm yr 1 plate convergence rate a better fit is given with a 65 mm yr 1 virtual back normal slip rate this higher rate is attributed to continued postseismic relaxation the model does not explain the relatively high uplift rate and low n15 5 w velocity observed at middleton island that anomalous motion is attributed to continued thrusting on postulated upward trending splays from the subduction zone beneath the island