tomographic imaging by reflected and refracted arrivals at the north sea we discuss some processing steps of a marine 3 d data set from the oseberg field north sea we compare the prestack depth migrated images obtained by the velocity fields provided by different tools velocity spectra reflection tomography and joint tomographic inversion of reflected and refracted arrivals the last ones are definitely better we also produced a synthetic example by modeling the estimated earth structure and the actual recording geometry and we reached similar conclusions the correlation between reflected and refracted signals may be unclear for later arrivals because of their reciprocal interference and multiple reflections we adopt a technique based on a surgical mute in the p domain which allows coupling the signals coming from the same elastic interface

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

relocations of earthquakes 1899 1917 in south central alaska i have relocated 18 earthquakes occurring in the south central alaska region between 1899 and 1917 using a bootstrap relocation technique locations of events within the yakutat region suggest that the 1899 sequence began on 4 september with a ms 7 9 event within the area of the pamplona fault zone western transition fault zone rupturing the western portion of the north american pacific plate interface a ms 7 4 event on 10 september appears to have ruptured the offshore portion of the plate interface to the east of the 4 september event this was followed by a ms 8 0 event that likely ruptured the onshore and down dip portion of the plate interface a ms 7 0 event in 1908 may have ruptured a small portion of the plate interface between the 4 september and 10 september events events occurring between 1911 and 1916 in the prince william sound region appear to be slab events occurring in similar locations to more recent seismicity within the kodiak region the 1900 earthquake of ms 7 7 has a location consistent with the rupture of the kodiak asperity which also ruptured during the 1964 great alaska earthquake other large magnitude kodiak events appear to be associated with regions of recent seismicity including the karluk lake area of southwestern kodiak island and the albatross basin located offshore southeast of kodiak island space time seismicity patterns since 1899 indicate that magnitude 6 to7 events have occurred with regularity in the kodiak island region that there has been a lack of magnitude 6 events in the prince william sound region since 1964 and that the yakutat region has remained notably quiescent at the magnitude 6 level

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

composition diagenesis and weathering of the sediments and basement of the callabonna sub basin central australia implications for landscape evolution abstract the evolution of the callabonna sub basin central australia is described in terms of sediment provenance stratigraphy sedimentology and postdepositional mineral and chemical alteration six mineral exploration drillholes in the mundi mundi plain were studied using a combination of qualitative and quantitative xrd pimatm portable infrared mineral analyzer xrf normative analysis and sem the sedimentary cover consists of 160 210 m of diagenetically modified fluvio lacustrine tertiary clay silt and sand comprising sandy mottled and gray clay units overlain by quaternary soil the basement consists of weathered proterozoic schist gneiss siltstone and shale quartz kaolinite smectite and muscovite are the dominant minerals within the sediments whereas the weathered bedrock contains quartz feldspar kaolinite illite and muscovite characteristic wavelength absorption features from pimatm show that the youngest sandy clay unit exhibits the lowest reflectance intensities because of its lower diagenetic maturity weathering of primary minerals within the basement produced an illite kaolinite rich saprolite whose clays exhibit strong reflectance intensities and well developed crystallinity the micromorphology of all secondary products reflects their environment of crystallization low porosity and anoxic conditions within the sediments resulted in poorly developed clays of variable textures colors and compositions conversely the weathered bedrock developed over protracted periods of in situ weathering resulting in better developed secondary products this study distinguishes between preburial weathering of the basement detrital mineralogy postdepositional alteration of the sedimentary cover and subsequent weathering of previously formed mineral assemblages this has widespread implications for paleotopographic weathering reconstitution and landscape evolution thus 1 the extent of weathering of basement relative to diagenesis and maturity of sediments and gradational nature of their unconformity suggests that most of the alteration of basement occurred prior to and was sustained after deposition of sediments and 2 morphotectonic evolution of the region controlled the composition of transported materials by changing the style and patterns of fluvial systems further influencing the distribution and preservation of sediments and weathered bedrock and hence regional landscape morphology

combining borehole and surface seismic data for velocity field estimation through slope tomography a joint traveltime and slope or slowness tomography method of marine surface reflections and walk away transmitted arrivals is investigated based on our previous developments of 2 d isotropic slope tomography dedicated to reflection data the transmitted arrivals are added into the inversion scheme the results of the transmitted arrival tomography are compared to those of the joint transmitted and reflected arrivals tomography on synthetic data moreover a quality control qc procedure is proposed finally walk away transmitted arrivals surface reflected arrivals and joint tomography of both arrivals are applied on real data sets from oseberg north sea these data include 2 d surface and walk away lines along the same direction the resulting velocity fields are used for pre stack depth migration allowing us to conclude that joint inversion leads to significantly improved images both in terms of focusing and event location a key point is that improvements are not restricted in the area covered by transmitted arrivals as the number of available borehole receivers is small this case study demonstrates the strong impact direct arrivals that may have on depth imaging

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

a viscoelastic and afterslip postseismic deformation model for the 1964 alaska earthquake 1 we developed a 3 d viscoelastic model in concert with an afterslip model to describe the postseismic deformation following the 1964 alaska earthquake our model incorporates a realistic geometry including an elastic slab with very low dip angle these geometric factors are important and require a reanalysis of the 1964 coseismic model our coseismic model differs from previous models in that the montague island splay fault extends farther along the kenai peninsula coast and as a result slip on the megathrust in that region is smaller we computed postseismic deformation models using a range of mantle viscosities with maxwell relaxation time of 1 to 50 years with a best estimate of 20 years the viscoelastic model explains most of the trenchward motion observed in the present velocity field but has little impact on cumulative 1965 to present uplifts after removing the viscoelastic response the largest residual cumulative uplifts were located downdip from areas of largest coseismic slip area and we explain them using an afterslip model constrained in space by the observed cumulative postseismic uplifts and in time by tide gauge records no single mechanism can explain both the 30 years of cumulative uplift and the present velocities but a combination of viscoelastic relaxation afterslip interseismic elastic deformation and motion of southern alaska relative to north america explains the first order features of the observations forty years after the earthquake the present day velocities contain a significant component of postseismic deformation so very long lived postseismic deformation plays an important role in the subduction zone earthquake cycle for great earthquakes

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

velocity analysis by iterative profile migration in conventional seismic processing velocity analysis is performed by using the normal moveout nmo equation which is based on the assumption of flat horizontal reflectors imaging by migration either before or after stack is done normally in a subsequent step using these velocities in this paper velocity analysis and imaging are combined in one step and migration itself is used as a velocity indicator because unlike nmo migration can be formulated for any velocity function migration based velocity analysis methods are capable of handling arbitrary structures i e those with lateral velocity variations in the proposed scheme each shot gather profile is migrated with an initial depth velocity model profile migration is implemented in the x domain but the actual implementation of profile migration is not critical as long as it is not done in a spatial wavenumber domain which would preclude handling of lateral velocity variations after migration with an initial velocity model the

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

changes in seismic moment rates along the rupture zone of the 1964 great alaska earthquake we examined seismic moment rates associated with the prince william sound and kodiak island asperities of the 1964 great alaska earthquake rupture zone to determine how seismicity has varied through time and space our results indicate that there has been a factor of 5 10 decrease in seismic moment rates within the prince william sound asperity region since 1964 whereas seismic moment rates have actually increased within the kodiak asperity region since 1964 primarily because of a sequence of m w 7 0 earthquakes that have occurred since 1998

rupture process of the 1964 prince william sound alaska earthquake from the combined inversion of seismic tsunami and geodetic data 1 four great earthquakes 1952 1960 1964 and 2004 have occurred since seismic monitoring began and only two since the installation of a global seismic network a reexamination of the 1964 m 9 2 prince william sound pws alaska earthquake is timely due to the 2004 sumatra earthquake because it adds constraints to the potential range of source parameters for these types of infrequent events and aids in the ability to predict the ground motions for other subduction zone including cascadia we first measured the durations of high frequency energy radiation from teleseismic p wave codas recorded by short period world wide standardized seismographic network wwssn sp instruments to put constraints on the earthquake rupture length the durations ranged between 250 and 600 s and are shorter in time between azimuths of 190 and 260 the fault length is estimated to range from 540 to 740 km rupturing at average speeds of 1 4 2 km s in a 220 238 direction we developed a multiple time window kinematic rupture model for the pws earthquake from the least squares inversion of teleseismic p waves tsunami tide gauge records and geodetic leveling survey observations based on the green s function technique we assume three major fault segments for the subduction zone dipping 6 12 based on geologic data the subfault size on the megathrust was set to 50 50 km the patton bay fault pbf imbricate thrust was assigned a 60 dip with a subfault size of 20 20 km we estimated a seismic moment of 5 52 1022 n m mw9 12 we identified three areas of major moment release on the pws segment near montague and middleton islands extending out to the trench beneath the portlock anticline on the cook segment between 58 n fracture zone and the kodiak bowie seamount chain and finally on the katmai segment off the coast of kodiak island extending to the trench our preferred rupture model has a peak slip of 14 9 m along the megathrust and 17 4 m along the pbf the addition of the pbf resulted in an 8 improvement in residuals and also had a significant effect on the overall slip distribution moment release occurred as deep as 50 km depth slab contour near the 350 c isotherm and mantle fore arc wedge which is about 10 20 km deeper than previous coupling depths

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

experimental study of soil microfabrics induced by isotropic stresses of wetting and drying this experimental study examines the relationship between texture and microfabric under isotropic stresses generated by wetting and drying this can be achieved by observing and measuring the reorganization of microfabrics plasmic fabrics and related distribution pattern rdp microfabrics are produced in the laboratory by mixing montmorillonite with various proportions of sand and silt and subjecting them to wetting and drying without confinement thin sections are cut to study their microfabrics results show broad and systematic variations in fabric with changes in texture fine matrix silt plus clay in physical continuity is developed in samples with low sand silt ratio and regardless of this ratio at high clay levels coarse porphyric and fine porphyric rdp are formed at low and high clay contents respectively plasmic fabrics are represented mainly by insepic and weakly developed mosepic and masepic fabrics in the matrix in the form of small plasma separations with random orientation and weak skelsepic plasma as embedded grain argillans on the other hand coarse matrix sand grains in physical continuity dominates the sandy samples which are associated with relatively thick and strongly birefringent free grain argillans giving well developed skelsepic fabric the rdp follows two major paths beginning with orthogranic very low clay content along the iunctic sequence at low clay silt ratio and along the chlamydic sequence at high ratio the regularities in fabric development are interpreted in the light of the dynamic properties and behaviour of the active clay fraction in a clay water system the pedogenetic implications of the results are explored and the term erdic fabric defined

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

estimation of strong ground motion from the great 1964 mw 9 2 prince william sound alaska earthquake we are generating physically plausible near field synthetic ground motions for the great 1964 prince william sound alaska earthquake compatible with available seismological data tectonic information and eyewitness accounts the first objective of this study is the simulation of the low frequency i e f 0 03 hz strong ground motions on selected locations and on a dense grid of observation points extending over the shallow dipping causative fault of the 1964 alaska earthquake in order to accomplish this task we are utilizing the slip model proposed by johnson et al 1996 based on a joint inversion of tsunami waveforms and geodetic data the calculations are carried out using the discrete wavenumber representation method bouchon and aki 1977 bouchon 1979 and the generalized transmission and reflection coefficient technique luco and apsel 1983 the second objective of this study is the reconstruction of the strong ground motion time histories and response spectra that the city of anchorage experienced during the 1964 alaska earthquake the low frequency i e f 0 03 hz ground motions are generated using the methodology described previously the intermediate frequency i e 0 03 f 0 50 hz ground motions are simulated by convolving green s functions generated by the discrete wavenumber representation method with far field radiation pulses of circular cracks sato and hirasawa 1973 dong and papageorgiou 2002a the high frequency i e 0 5 f 8 0 hz ground motions are simulated using the stochastic modeling approach the three independently derived ground motion components are then properly combined to generate synthetic broadband ground motion time histories and response spectra for the city of anchorage due to the 1964 prince william sound earthquake the third objective of this study is the validation of the synthetic strong ground motions for the 1964 alaska earthquake against observed tectonic deformation ground motion estimates inferred from descriptions of structural damage and eyewitness accounts in summary the analysis presented in this study contributes to a better understanding of the seismological aspects of the 1964 alaska earthquake and provides synthetic time histories and response spectra for engineering applications compatible with all available information i e seismological tectonic and eyewitness accounts it should be noted however that the generated strong ground motions are not necessarily unique nor do they reflect the entire uncertainty that characterizes the problem under investigation

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

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

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

stress shear deformation and micromorphological clay orientation a synthesis of various concepts summary microscopic oriented clay bodies known as plasma separations to soil micromorphologists show various degrees of symmetry and regularity in their patterns they could be regarded as sensitive and reliable indicators of stress strain regime of a soil empirical and experimental evidence signifies the importance of stress specifically that generated by in situ swelling in the genesis of plasmic fabrics hitherto most work in this field is confined to geometric characterization with limited attempts to dynamic interpretations of their origin this paper enlists appropriate concepts in soil mechanics structural geology and agricultural engineering to interpret the possible stress strain components that rearrange fabric units into plasma separations some pertinent concepts including physico chemical behaviour of the clay water system shrink swell phenomenon stress strain deformation and failure of soil are employed to explain the origin of plasmic fabrics by mechanical principles a conceptual model is suggested to synthesize the conditions changes and consequences related to the formation of plasmic fabrics strongly developed plasma separations in natural soils are attributed to the establishment of an anisotropic state of stress generated by in situ swelling in conjunction with the passive pressure of lateral confinement and vertical overburden the difference between the maximum and minimum principal stresses produces a deviatoric stress that can at certain stages of a shrink swell cycle exceed the shear strength of the soil the extra stress is relieved plastically by shear deformation which is manifested as the alignment of platy particles in domain configuration by rotation and translation along the shear planes soil mainly fails in compressive plastic mode along a large number of small shear planes which form the plasma separations the formation of different types of sepic plasmic fabrics is discussed in relation to the stress strain regimes of soils

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

eocene paleosols on king george island maritime antarctica macromorphology micromorphology and mineralogy abstract we investigated the origin of reddish layers between basalt flows in an early eocene volcanic succession in king george island maritime antarctica previous studies in the area classified these layers as regolith surfaces baked zones and paleosols but the properties of these layers and the reasons for their classification are not clarified since the early eocene epoch was one of the warmest phases on earth during the cenozoic a proper investigation of these layers is crucial to obtain additional regional information about the paleoenvironment the focus of this study is on three profiles located in the admiralty bay on king george island they were classified as paleosols based on field micromorphological and mineralogical properties main soil forming processes identified by micromorphological analyses were clay neoformation pedo bioturbation and a weak redoximorphosis the occurrence of plant residues in all profiles was a reliable indicator that these soils formed a land surface during the early eocene abundant weathered volcanic glass and pyroclasts may indicate andosolization although the main diagnostic features for andosols which are the presence of nanocrystalline minerals and organic matter accumulation were not detected or only in small amounts respectively the long term burial of the paleosol could be a reason for lacking of these diagnostic features clay minerals associated with moderately weathered volcanic soils smectite were detected in the x ray diffraction analysis a predominantly reddish colour of most of the soil horizons is given by hematite confirmed by the determination of pedogenic oxides nevertheless rubification was not solely a result of pedogenesis but also of reddening of oxyhydroxides by dehydration during diagenesis other diagenetic processes were burial compaction loss of organic matter and induration in the horizons directly buried by a basalt flow voids were filled by zeolite crystals an effect related to lava cooling another possible diagenetic effect was the geochemical rejuvenation and homogenization of the profiles despite the occurrence of these features the results are in agreement with previous studies indicating a humid temperate paleoclimate for king george island during the early eocene

velocity model estimation for depth imaging comparison of three tomography methods on a 2d real data set the estimation of velocity models is still crucial in seismic reflection imaging as it controls the quality of the depth migrated image which is the basis of geological interpretation among the numerous existing methods for velocity determination tomographic methods are very attractive for their efficiency and ability to retrieve heterogeneities of the medium we present three tomographic methods in order to estimate heterogeneous velocity models from 2d prestack pp reflection data a traveltime tomography in the time migrated domain a traveltime and slope tomography in the non migrated time domain and a slope tomography in the depth migrated domain the first method traveltime tomography in the time domain is based on continuous picked events whereas the two slope tomographic methods one in the time domain and the other in the depth domain are based on locally coherent events with no assumptions about reflector geometry or the unknown velocity field the purpose of this paper is not to describe in detail the theoretical basis and implementation of the methods but to apply and compare their output using the same marine real data set based on the estimated velocity models the migrated images and the common image gathers from the three processing routes the relative strengths and weaknesses of the methods are discussed finally similarities are indicated and potential alternative approaches are proposed

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

tomographic determination of three dimensional seismic velocity structure using well logs vertical seismic profiles and surface seismic data a tomographic technique traveltime inversion has been developed to obtain a two or three dimensional velocity structure of the subsurface from well logs vertical seismic profiles vsp and surface seismic measurements the earth was modeled by continuous curved interfaces polynomial or sinusoidal series separating regions of constant velocity or transversely isotropic velocity ray tracing for each seismic source receiver pair was performed by solving a system of nonlinear equations which satisfy the generalized snell s law surface to borehole and surface to surface rays were included a damped least squares formulation provided the updating of the earth model by minimizing the difference between the traveltimes picked from the real data and calculated traveltimes synthetic results indicated the following conclusions for noise free cases the inversion converged closely from the initial guess to the true model for either surface or vsp data adding random noise to the observations and performin

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

stereotomography of seismic data acquired on undulant topography stereotomography is a robust method for building velocity models from seismic reflection data and has been applied to offshore seismic data but there is almost no stereotomographic study with rugged topographic conditions we study the topographic effects on the slopes of locally coherent events of seismic data and propose an approach to calculating the slopes on an undulant observation surface using the horizontal and vertical components of slowness vectors estimated then we present an extended stereotomography with undulant observation surface based on the conventional one tests on synthetic data validate the extended stereotomography application to the field seismic data in a foothill belt in xinjiang of the west china shows that the extended stereotomography is an effective tool to build velocity models for prestack depth migration of seismic data acquired on rugged topography

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

clay mineralogy as a crucial factor in badland hillslope processes abstract comprehensive and important work has been done in understanding badland processes on both the big scale and more recently on smaller scale in order to understand the subtle differences between sites however most studies lack important information regarding clay mineralogy and their importance for weathering processes type and rate of weathering depend on material physico chemical properties climatic regimes and slope characteristics as weathering progresses the surface conditions change influencing erosion processes usually badland lithologies are reported as sodium rich highly dispersive materials but that is not invariably true the primary objective of this study is to show that clay minerals have an important role in defining surface and subsurface processes on badland hillslopes field studies conducted in dinosaur park badlands alberta canada chinguacousy badlands ontario canada and sestino badlands tuscany italy have shown that there are two dominant clay rich lithologies smectite rich and smectite poor mudrocks different surface crust and weathering profiles were observed on smectite rich mudrocks during two field years with distinct precipitation inputs after a dry fall winter season the popcorn surface was very well developed 2 3 cm the subsurface layer 10 15 cm thick overlaid the layer of smaller shards followed by the unweathered material after a very wet fall winter season the surface layer was denser and thinner 0 5 cm with wider cracks and a thin subsurface layer attached to the crust weathering profiles on smectite poor lithologies did not show any differences with variable climatic conditions a thin surface layer was composed of soft thin and small 0 5 0 3 0 1 cm shards a layer of larger shards overlying unweathered material was beneath it in order to better understand the described occurrences badland materials were monitored in controlled laboratory conditions during an incremental weathering experiment six samples of each material were subjected to 10 cycles of simulated rainfall which duration ranged 10 60 min very short rainfalls 10 20 min can cause swelling of clay minerals in smectite rich materials and formation of popcorn surface during drying periods during subsequent wetting drying periods the crust becomes flatter and denser and desiccation cracks become wider and deeper samples subjected to 50 and 60 min of rainfall showed maximum swelling after only one cycle of rainfall after this swift swelling samples became unstable and dispersive during subsequent wetting dispersion became dominant resulting in flatter surfaces thinner crust and narrower desiccation cracks smectite poor mudrocks appeared to be resistant to variable wetting and drying large shards broke down into smaller due to differential swelling of illite and chlorite once they were reduced to tiny flaky shards the surface became compacted the surface cracks served as the initial pathways for sediment and water movement knowing if the material is smectite rich or smectite poor can help in prediction of processes and the erosion rates the amount intensity and duration of precipitation will determine not only the rate of weathering but also the type of the weathered surface on smectite rich surfaces smectite poor materials appear to be more resistant to change in precipitation

pedology of soils within the spray zone of the victoria falls zimbabwe abstract six profiles were sampled in the victoria falls national park zimbabwe in order to investigate the influence of the abundant spray of the falls on the pedogenesis in the otherwise semi arid environment depending upon the thickness of the alluvium soils developed in the sediment with variable admixture of basalt gravel or practically directly on the underlying karroo plateau basalt layer those near the gorge are subject to a continuous spray during most of the year the more remote ones get only the natural precipitation chemical mineralogical and micromorphological analyses indicate that the parent material is a more dominant factor determining chemical mineralogical and micromorphological characteristics than precipitation the main active process is bioturbation phytolites and sponge spicules are abundant in most profiles basalt derived laumontite is observed in thin sections and in the silt fraction weathering of the basalt gravel is limited although pedogenesis has been active for at least 50 100 ka a subdivision of enaulic c f related distribution patterns is proposed

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

a classification of soil microfabrics abstract the definitions of some terms to describe the distribution pattern of micromorphological components are given and discussed five existing schemes for the characterization of related distribution patterns in soil micromorphology are summarized in a table compared and evaluated the replacement of the terms related distribution pattern by microfabric and fabric sequence by fabric groups are suggested a scheme based on existing terminology and augmented with new terms to describe the hitherto neglected poorly structured soils is proposed four fabric groups are drawn from existing terminology namely iunctic and chlamydic for coarse matrix and fragmic and matrigranic for fine matrix fabrics two new fabric groups namely planvic and equanvic are proposed to describe poorly structured soils according to the shapes and volumes of voids the six fabric groups are arranged in a synoptic diagram to illustrate a comprehensive classification of soil microfabrics a dichotomous key to aid fabric identification is provided use of standardized short forms for fabric names is suggested

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

historical seismicity 1918 1964 of the kodiak island region i have relocated 35 earthquakes and conducted waveform modeling studies of 12 events located primarily within the kodiak portion of the 1964 great alaska earthquake rupture zone and the northeasternmost portion of the 1938 semidi earthquake rupture zone these results show that there is considerable similarity between earthquakes occurring before the 1964 mainshock pre 1964 mainshock seismicity and after the 1964 mainshock post 1964 mainshock seismicity persistent seismicity has occurred for the past 85 years at the southwestern end of the 1964 rupture zone a region where global positioning system gps geodesy studies indicate the plate interface is currently locked earthquakes also occurred frequently within the kennedy entrance region where the transition between the kodiak and kenai block of the subducting pacific plate occurs the seismic moment rate for post 1964 mainshock seismicity is about two times larger than for premainshock seismicity this is in contrast to the prince william sound portion of the 1964 rupture zone where post 1964 mainshock moment rates are orders of magnitude lower than pre 1964 mainshock rates and the pattern of seismicity has changed considerably with time these observations reinforce previous studies that indicate the behavior of plate subduction beneath the kodiak island region is much different from that beneath the prince william sound region

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

crustal deformation and the seismic cycle across the kodiak islands alaska 1 the kodiak islands are located 120 to 250 km from the alaska aleutian trench and are within the southern extent of the 1964 prince william sound mw 9 2 earthquake rupture and aftershock zone here we report new campaign gps results 1993 2001 from northeastern kodiak and reprocessed gps results 1993 1997 from southwestern kodiak the rate and orientation of the horizontal velocities relative to a fixed north america range from 29 7 1 7 mm yr at n30 3 w 3 3 located 120 km from the deepest point of the trench to 8 0 1 3 mm yr at n62 4 w 9 3 located 230 km from the trench we evaluated alternate models of coseismic and interseismic slip to test the importance of the mechanisms that account for surface deformation rates near the gulf of alaska coastal region of kodiak the horizontal velocity can be accounted for primarily by the viscoelastic response to plate motion and a locked main thrust zone mtz downdip creep and to a lesser extent slip in the 1964 earthquake farther inland the dominant mechanisms that account for post 1964 uplift rates are time dependent downdip creep and a locked mtz for the horizontal velocity component southwest translation of western kodiak may be important as well on the basis of the pre 1964 and post 1964 earthquake pattern of interseismic earthquakes we suggest that between the occurrences of great earthquakes like the 1964 event more moderate to large earthquakes occur in the southwestern kodiak region than near northeastern kodiak