geochemical and sr nd isotopic constraints on the genesis of the cenozoic linzizong volcanic successions southern tibet abstract the linzizong volcanic successions that crop out in the lhasa terrane southern tibet have been conventionally regarded as the products of northward subduction of the neotethyan oceanic slab beneath south asia this study reports geochemical data of 100 volcanic rocks from the lhasa terrane to better constrain the temporal spatial distribution and petrogenesis of the linzizong volcanism the linzizong volcanic successions consist dominantly of calc alkaline rocks that erupted from ca 69 to 43 ma and show typical arc lava geochemical features marked with lile enrichment and hfse depletion in the spidergram their sr and nd isotope ratios e nd t 3 3 to 2 4 i sr 0 7048 0 7072 are generally similar to those of the associated gangdese i type granitoids the linzizong volcanism is characterized by a flare up period ca 50 ma that shows significant geochemical variations manifested by the coexistence of five types of volcanic rocks 1 the main suite of calc alkaline rocks sio 2 45 80 wt la 12 45 ppm e nd t 3 8 to 4 9 i sr 0 7037 0 7105 that could be interpreted by partial melting of the mantle wedge followed by assimilation and fractional crystallization afc processes with 10 crustal contamination and or magma mixing 2 the low k low ree suite sio 2 48 61 wt k 2 o 0 5 1 1 wt la 7 10 ppm that has the highest nd isotope ratios e nd t 5 9 to 3 5 suggesting a juvenile mantle origin possibly related to decompression melting of the asthenosphere 3 the shoshonitic suite sio 2 53 71 wt k 2 o 3 8 6 7 wt e nd t 2 8 to 6 1 from small degree melting of the metasomatized lithospheric mantle 4 the high ree suite sio 2 61 75 wt la 36 87 ppm e nd t 2 6 to 3 2 originating from remelting of a newly underplated basaltic lower crust and 5 the evolved suite of rhyolitic flows and ignimbrites sio 2 65 wt that have the lowest e nd t values of 14 to 18 representing remelting products of the basement or continental crust of the lhasa terrane such geochemical heterogeneities are attributed to breakoff of the subducted neotethyan slab under southern tibet that occurred in the early stage of the india asia collision

vector constraints on western u s deformation from space geodesy neotectonics and plate motions the rate of slip vector on the san andreas fault in central california estimated from geodetic and holocene geological data 34 3 mm yr n41 w 2 is inconsistent with the prediction of rigid plate models such as rm2 56 3 mm yr n35 w 2 this well known san andreas discrepancy is diagnostic of plate deformation distributed both east of the fault in the basin and range and west of the fault along the california continental margin we construct constraints on the integrated deformation rates across these two regions consistent with 1 the kinematical boundary conditions imposed by the rigid plate model 2 neotectonic and paleoseismic estimates of deformation rates 3 ground based geodetic measurements and 4 rates of change observed by very long baseline interferometry along seven baselines to western u s sites the space geodetic data on basin and range extension taken over a 4 year interval are compatible with geological observations averaged over the holocene the best estimate of its integrated deformation rate provided by the joint inversion of both data types is 9 7 2 1 mm yr n56 w 10 too small and in the wrong direction to account entirely for the san andreas discrepancy we therefore attribute much of it to deformation west of the sierra nevada great valley block accommodated for example on faults such as the san gregorio hosgri system of coastal california the integral of this deformation estimated by subtracting the basin and range contribution from the discrepancy vector requires significant right lateral shear parallel to the san andreas 13 5 mm yr and some compression perpendicular to it 9 3 mm yr

seismicity in the st elias region of northwestern canada and southeastern alaska abstract low level monitoring in the st elias region between september 1978 and march 1981 has identified two principal zones of seismicity that are consistent with the historical pattern of seismicity at higher magnitude levels the most active zone follows the plate boundary along the coast of southeast alaska a recurrence relation based on completeness over varying time intervals is log n m 0 74 m 3 77 a less active zone is observed generally following the duke river and dalton segments of the denali fault system through the southwest yukon instead of continuing into chatham strait this trend of seismicity turns south into the glacier bay region to connect with the fairweather fault north of cross sound the seismicity and high rate of uplift observed in the glacier bay region is probably the result of convergence across the fairweather fault north of cross sound the recurrence relation for the denali zone is log n m 0 90 m 3 77 a similiar recurrence curve is found for activity with magnitudes down to about 2 0 observed over the 39 month low level monitoring program the rate of activity in the denali zone appeared to decrease for a period of about 1 yr following the 28 february 1979 m 7 2 st elias earthquake the rate of aftershock activity was also found to vary with the b value decreasing throughout the sequence only minor seismicity is observed northeast of the denali fault system in the southwest yukon and east of chatham strait in northwest british columbia a prominent cluster east of juneau near the british columbia border may be related to volcanic activity a 5 week microearthquake survey over a 40 km segment of the denali fault system found activity in a highly faulted zone about 15 km wide at depths of less than about 15 km a composite p nodal solution indicates motion on either east west or north south striking planes neither of which are consistent with the general trend of the denali system in the area an average slip rate of no more than about 1 mm yr is estimated from the seismicity for the denali fault system through the southwest yukon an estimate of the minimum present accumulated elastic strain suggests the potential for a magnitude 6 1 2 earthquake

strain accumulation in western united states this review is principally concerned with recent geodetic strain measure ments in western united states undertaken by the us geological survey as part of the earthquake studies program and as a consequence is heavily biased toward the author s own publications most of the publications reporting crustal strain measurements in western united states prior to about 1968 have been compiled in one volume national geodetic survey 1973 and more recent work with complete bibliographies is summarized in three successive quadrennial reports to the international union of geodesy and geophysics meade 1971 savage 1975 thatcher 1979b the following conventions are employed in this paper strain a dimensionless quantity is reported in units of 10 6 extension is taken as positive to distinguish between engineering and tensor shear strain we denote the former by y and quote the units as j lrad whereas the latter is denoted by e and given the units of j lstrain uncertainties in all cases are quoted as one standard deviation at the present time geodetic techniques furnish the only reliable measure of strain accumulation in the seismic regions of the world the superiority of geodetic techniques derives principally from the long baseline of the measurements difficulties in coupling fiducial marks to the earth have generally made short baseline measurements of strain accumulation questionable wyatt 1982 however long baselines introduce the dis advantage that the measured strain is not closely localized the current technique of measuring strain accumulation involves the

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

strain accumulation along the denali fault at the nenana river and delta river crossings alaska surveys of trilateration networks across the denali fault at the nenana river in 1982 1984 and 1988 and at the delta river in 1975 1979 1982 and 1984 indicate a minor 0 10 0 04 strain yr northeastward uniaxial extension the component of right lateral shear strain accumulation across the fault is not significant at the two standard deviation level at the delta river network the strain accumulation rate decreases rapidly with distance from the fault but evidence for a similar decrease with distance from the fault is lacking at the nenana river network the strain accumulation rates inferred from trilateration are consistent with the very long baseline interferometry vlbi measurement reported by ma et al 1990 and support their contention that significant right lateral shear is not accumulating along the denali fault at the present time savage et al 1981 had earlier concluded erroneously that preliminary geodetic measurements at the delta river network demonstrated right lateral shear strain accumulation the absence of significant right lateral deformation across the denali fault in the 1975 1988 interval is in marked contrast with the abundant geomorphic evidence for holocene right lateral secular slip at the rate of 10 20 mm yr on the denali fault in this sector

tectonic block motion and glacial isostatic adjustment in southeast alaska and adjacent canada constrained by gps measurements 1 we use data from campaign and continuous gps sites in southeast alaska and the neighboring region of canada to constrain a regional tectonic block model that estimates block angular velocities and derives a self consistent set of fault slip rates from the block motions present day tectonics in southeast alaska is strongly influenced by the collision of the yakutat block our model predicts a velocity of 50 3 0 8 mm a toward n22 9 0 6 w for that block our results suggest that the eastern edge of the yakutat block is deforming along this edge the fairweather fault accommodates a large portion of the pacific north america relative plate motion through 42 9 0 9 mm a of dextral slip further south along the queen charlotte fault our model predicts an average of 43 9 0 6 mm a of dextral slip and a southward increasing amount of transpression strain from the yakutat collision is transferred far to the east of the strike slip system this strain transfer causes the region north of glacier bay to undergo a clockwise rotation south of glacier bay and inboard of the queen charlotte fault a smaller but clearly defined clockwise rotation is observed the heterogeneous block motion north and south of glacier bay may indicate the area is undergoing internal deformation and could explain regional patterns of diffuse seismicity the northern cordillera of canada displays a small northeasterly motion our block model suggests that the entire southeastern alaska northwestern canada margin is mobile

deformation in the yakataga seismic gap southern alaska 1980 1986 a 60 by 40 km trilateration network in the yakataga seismic gap was surveyed in 1980 1982 1984 and 1986 with precise electro optical distance measuring equipment to measure strain accumulation the overall deformation is roughly approximated by a 0 24 0 03 strain yr n32 w 2 4 uniaxial contraction that is uniform in time however the spatial distribution of deformation shows some concentration of convergence in the neighborhood of the chugach st elias fault and of right lateral shear across the contact fault a simple dislocation model of the plate interaction in the yakataga gap fits the observed deformation reasonably well but seems to require that the motion of the pacific plate relative to the north american plate be directed more nearly n36 w than n15 w the generally accepted direction of relative motion for this location however the direction of plate motion inferred from the dislocation model depends upon details of the interaction at the plate boundary that may not have been modeled accurately a nearby but smaller trilateration network at icy bay was surveyed in 1982 1984 and 1986 this network spans the southwest corner of the rupture zone of the 1979 st elias earthquake the deformation at icy bay consists of left lateral shear across a northeast trending zone the relation of this deformation to strain accumulation in the yakataga gap postseismic relaxation associated with the 1979 earthquake or rebound from the unloading associated with the rapid recession of the guyot glacier is not understood

new constraints on the motion of the fairweather fault alaska from gps observations 1 gps velocities from sites near to the fairweather fault in southern alaska are combined with line length data from geodetic surveys by the u s geological survey usgs to estimate the slip rate and locking depth of the fairweather fault using dislocation theory we perform a weighted least squares inversion of the geodetic data and obtain a best fitting slip rate of 45 6 2 0 mm yr and locking depth of 9 0 0 8 km the slip rate we estimate is one of the highest observed across any strike slip fault we also include the dalton creek segment of the denali fault in our model and estimate a slip rate of 3 8 1 4 mm yr for this section of the fault this is substantially lower than the rate observed for the denali fault segment to the northwest which ruptured in the november 3 2002 mw 7 9 denali fault earthquake

exhumation at orogenic indentor corners under long term glacial conditions example of the st elias orogen southern alaska abstract syntaxial bends in convergent plate boundaries or indentor corners display some of the most intriguing deformation patterns on earth and are type localities for aneurysms of coupled erosion thermal weakening and strain the st elias orogen in alaska is a small young convergent system that has been dominated by a glacial climate for much of its history and exhibits two prominent indentor corners that are not well understood we have added 40 new apatite u th he ages to the already extensive dataset for the low temperature cooling history of this orogen to constrain the pattern of exhumation in these indentor corners ages from the western syntaxis show minor variation across the structural hinge suggesting that the bend has little effect on the pattern of exhumation and that structures including the bagley fault connect smoothly from the orogen core to the subduction zone to the southwest rock uplift on the north flank of the range appears to increase steadily towards the eastern syntaxis which represents the apex in the right angle bend between a transform fault in the south and the collision zone in the west based on age elevation relationships zones of relative rock uplift can be defined in which the mt logan massif or the area just north of the eastern syntaxis experienced 4 8 km greater rock uplift than background levels northwest of the western syntaxis a bulge in relative rock uplift is symmetric about the hinge in the eastern indentor corner however rates of denudation in this bulge are not as rapid as the core of the fold and thrust belt and are lower than those implied by detrital cooling ages from beneath the seward glacier this implies that a large bull s eye of ultra rapid 5 mm yr exhumation does not occur and that the subpopulation of young detrital ages may be sourced from a narrow transpressional zone along the fairweather fault unlike the himalayan syntaxes it thus appears that an aneurysm of coupled erosion strain has not developed in either indentor corner of the st elias orogen this may indicate a limit to the degree to which glacial erosion can partition strain in convergent orogens similarly the likely existence of a through going dextral fault the totschunda fault through the eastern syntaxis implies that tectonics rather than surface processes exerts the main control on strain partitioning in these corners

the determination of displacement fields from geodetic data along a strike slip fault observations of angles or distances between stations of a geodetic network are commonly used to infer information about the movement of the surface of the earth the absence of any observations external to the network leads to an ambiguous displacement field existing techniques for eliminating this ambiguity are all unsatisfactory in some respect the best technique an inner coordinate solution is not appropriate for networks located in a strike slip fault environment the inner coordinate solution zeros the rotation of all stations about their center of mass along a strike slip fault like the san andreas however motion normal to the fault is less likely than motion parallel to the fault the solution presented here an outer coordinate solution finds the rotation of the network that minimizes the components of displacement normal to the fault since motion along a strike slip fault is generally expected to be parallel to the fault the displacements obtained with the outer coordinate solution are more reasonable than those obtained with other techniques examination of a trilateration network near san francisco bay demonstrates the large effect that the choice of adjustment technique can have on the inferred relative motion of the two sides of the fault the inner coordinate solution gave a rate of about 1 mm yr whereas the preferred outer coordinate solution rate was 36 mm yr

deformation during terrane accretion in the saint elias orogen alaska the saint elias orogen of southern alaska and adjacent canada is a complex belt of mountains formed by collision and accretion of the yakutat terrane into the transition zone from transform faulting to subduction in the northeast pacific the orogen is an active analog for tectonic processes that formed much of the north american cordillera and is also an important site to study 1 the relationships between climate and tectonics and 2 structures that generate large to great magnitude earthquakes the yakutat terrane is a fragment of the north american plate margin that is partly subducted beneath and partly accreted to the continental margin of southern alaska interaction between the yakutat terrane and the north american and pacific plates causes significant differences in the style of deformation within the terrane deformation in the eastern part of the terrane is caused by strike slip faulting along the fairweather transform fault and by reverse faulting beneath the coastal mountains but there is little deformation immediately offshore the central part of the orogen is marked by thrusting of the yakutat terrane beneath the north american plate along the chugach saint elias fault and development of a wide thin skinned fold and thrust belt strike slip faulting in this segment may be localized in the hanging wall of the chugach saint elias fault or dissipated by thrust faulting beneath a north northeast trending belt of active deformation that cuts obliquely across the eastern end of the fold and thrust belt superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen where the sedimentary cover of the yakutat terrane is accreted into the upper plate of the aleutian subduction zone these three structural segments are separated by transverse tectonic boundaries that cut across the yakutat terrane and also coincide with the courses of piedmont glaciers that flow from the topographic backbone of the saint elias mountains onto the coastal plain the malaspina fault pamplona structural zone separates the eastern and central parts of the orogen and is marked by reverse faulting and folding onshore most of this boundary is buried beneath the western or agassiz lobe of the malaspina piedmont glacier the boundary between the central fold and thrust belt and western zone of superimposed folding lies beneath the middle and lower course of the bering piedmont glacier

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

strain accumulation across the fairweather and totschunda faults alaska deformation across the fairweather and totschunda faults is estimated from repeated surveys of geodetic networks that span those faults surveys in 1967 1983 and 1986 of networks crossing the fairweather fault indicate that right lateral engineering shear strain is accumulating across the 1958 earthquake rupture at a rate of 1 6 0 2 rad yr this rate of deformation is one of the highest observed across a strike slip fault system and it is several times greater than the secular rate of deformation across locked parts of the san andreas fault the rate of strain accumulation across the fairweather fault was the same in the two intervals 1967 1983 and 1983 1986 and is thus believed to be a secular rate rather than a transient associated with the 1958 earthquake the 1983 1986 line length changes are best fit by a model fault that is locked from the surface to a depth of 7 to 9 km and is slipping in a right lateral sense at a rate of 41 to 51 mm yr below that depth models with the fault locked to any depth of 4 km fit the data almost equally well the best fitting models indicate a slip rate comparable to the 48 to 58 mm yr average late holocene slip rate indicated by offset glacial moraines and to the 54 mm yr component of relative motion between the pacific and north american plates that is parallel to the fairweather fault no significant contraction was measured across the fairweather fault only a small part of the relative plate motion is transferred across the saint elias mountains to the colinear totschunda fault surveys in 1909 1912 and 1985 of a geodetic network that extends for 72 km across the totschunda fault indicate right lateral shear strain is accumulating at a rate of 0 11 0 06 rad yr this rate of strain accumulation suggests a slip rate of about 10 mm yr on the totschunda fault a value at the lower end of the average holocene slip rate of 10 to 20 mm yr

late cenozoic offsets on the offshore connection between the fairweather and queen charlotte faults off southeast alaska abstract closely spaced seismic reflection profiles permit us to extend the onshore fairweather fault across the continental margin off southeast alaska to the queen charlotte fault evidence for quaternary movement along this major plate boundary includes 20 40 m high sea floor scarps offset subsurface seismic reflectors sea floor bathymetry that indicates as much as 70 km of right lateral offset in the last 1 m y 5 6 cm yr of movement on the onshore fairweather fault and recorded seismicity along the fault the fairweather queen charlotte fault system similar in dimension to the san andreas fault system is a dextral transform fault that truncates the continental margin of north america much of the late cenozoic motion between the pacific and north american plates has been taken up along the fault system the fairweather queen charlotte fault evolved to its present form after passage of the yakutat block the youngest of the allochthonous terranes that form southern alaska

improving hydrological information acquisition from dem processing in floodplains extraction of hydrological information from digital elevation models dems is a required step when conducting any spatially distributed hydrological modelling in particular automated methods are proposed to extract the drainage structure from the dem however a realistic river network is not always derived from conventional dem processing methods indeed inaccuracy occurs in flat areas corresponding to floodplains in these areas additional sources of information are required to extract the correct drainage direction from the dem in this study it is demonstrated that traditional approaches of dem preprocessing such as the commonly known stream burning fail to provide correct maps of drainage directions and catchment areas when the extension of flat areas is large a new method is proposed to take advantage of available imagery data this method is based on a double dem burning process dem is first burned in the main rivers using the channel network and then in the floodplain using the spatial distribution of floodplains provided by classified satellite images in this sense the method has been referred to as the floodplain burning approach or simply fb approach spatial distribution of floodplains is derived from a multitemporal sar image classification a system of equations is used to vary the elevation offset required to be burnt in each cell representing the floodplain according to the minimal distance from the channel network which is calculated by a distance transformation the fb approach was applied to a sub basin located within the larger amazon river basin the region is characterized by large floodplain extensions basin delineation maps derived from the new method were compared with those obtained from the traditional stream burning method and highlighted more realistic results copyright 2008 john wiley sons ltd

tectonic model of the pacific north american plate boundary in the gulf of alaska from broadband analysis of the 1979 st elias alaska earthquake and its aftershocks the st elias alaska earthquake of 28 february 1979 ms 7 2 is reanalyzed using broadband teleseismic body waves and long period surface waves because of unresolved questions about its depth focal mechanism seismic moment and location in a seismic gap teleseismic waveforms are simultaneously inverted to determine the source mechanism seismic moment rupture history and centroid depth these data are well modeled with a point source propagating in the ese direction with an average kinematic rupture velocity of 2 5 km s the best fitting source mechanism indicates underthrusting on a ne dipping plane the mainshock depth of 24 km and the depth of aftershocks determined from inversions are consistent with locations on the gently dipping main thrust of the pacific north american plate boundary these depths are substantially different from those of earlier body wave studies and regional seismic network aftershock depth determinations but are in accord with the harvard centroid moment tensor and international seismological centre determinations the seismic moment determined from body waves is 9 4 1019 n m mw 7 3 the spatial and temporal distribution of moment release indicates that the st elias earthquake was a complex rupture consisting of two distinct subevents within 38 s of the initial onset followed by low moment release during the next 34 s earlier studies indicated an unusual amount of surface wave energy at very long periods 200 s that led some workers to suggest that st elias was a slow earthquake our broadband modeling does not require more than 34 s of additional moment release after the first two subevents moreover we are able to match the phase and amplitude of 200 s love and rayleigh waves with a thrust fault point source of moment 1 3 1020 n m mw 7 4 located at the body wave centroid the moment difference is not discernible with body waves for moment evenly distributed over 72 s thus the st elias earthquake is not slow with respect to 200 s surface waves but is complex with regard to the broadband body waves upper plate structure apparently controlled the gross characteristics of rupture the rupture direction parallels mapped upper plate faults rupture propagated unilaterally to the ese with little initial moment release as a shallow north dipping thrust that later changed to more steeply ne dipping with a large right lateral strike slip component the locations and source mechanisms of these subevents and locations of aftershocks define a shallow dipping surface at the eastern edge of the pacific plate moreover the component of strike slip motion increases with time in the mainshock implying that the transition to strike slip faulting occurs along the plate interface the estimated nucleation point of the second subevent coincides with a large concentration of aftershocks interpreted as representing a barrier to continuous rupture associated with the northern most boundary of the yakutat terrane joint relocation of aftershocks suggests that the main plate boundary may be offset vertically by 5 10 km as a result of this structure the southern part of the aftershock zone while containing many aftershocks appears not to have ruptured coseismically but may have failed later by aseismic creep as seen in geodetic measurements faults associated with the malaspina fault system the onshore extension of the aleutian trench appear to be the surface expression of the underthrusting plate boundary however upper plate deformation is widespread because of the collision of the yakutat terrane the convergence direction may explain the lack of a highly active wadati benioff zone downdip of the st elias zone the neotectonic deformation of the chugach st elias mountains is probably related to collision and subduction of the yakutat terrane a terrane in the process of accreting and subducting will cause considerable upper plate deformation over a wide zone once subduction of a terrane has begun deformation may then become localized

new gps constraints on the motion of the yakutat block global positioning system gps measurements were made at yakutat on the yakutat terrane of southern alaska to investigate the motion of the yakutat block with respect to the north american plate and to help constrain motion along the fairweather fault the velocity of yakutat derived from the gps data is 44 1 1 9 mm yr toward n37 4 w relative to stable north america the magnitude of this velocity is similar to that of the pacific plate predicted by nuvel1a although there is a significant difference in the azimuth of these two vectors the motion of yakutat relative to north america is almost exactly parallel to the strike of the fairweather fault suggesting that most deformation inboard of yakutat is right lateral strike slip on the fairweather fault or faults parallel to it and that significant motion normal to the fairweather fault occurs offshore of yakutat the gps velocity at yakutat is also used to help constrain the slip rate and locking depth of the fairweather fault

computer simulations of alaskan neotectonics ongoing deformation and fault slip neotectonics in the alaskan bering sea region were modeled with thin plate finite element methods by computing anelastic strain rate stress and velocity fields which represent time averages over several seismic cycles models included variable crust and lithosphere thickness heat flow and elevation and incorporated all faults known or thought to be active three sets of experiments totaling 46 models were computed with systematic variations of shear traction on the megathrust fault friction within north america internal friction of lithospheric blocks and mantle creep strength model quality was scored by comparing predictions to geologic data on fault slip rates the most compressive horizontal principal stress direction and geodimeter and very long baseline interferometry vlbi baseline rates best results were obtained with a driving traction of only 10 15 mpa on the aleutian megathrust and fault friction of only 0 17 within north america this is clearly a low stress model the fanning of away from the syntaxis was reproduced but does not imply high deviatoric stress magnitudes within the north american plate the aleutian forearc west of kodiak island moves westward along strike at rates which increase to 65 mm yr 1 in the komandorsky region however no crust is presently escaping westward from the alaskan syntaxis instead it is shortened and stacked by thrusting principally in the vicinity of the chugach and wrangell mountains because stresses in this model are low and are delicately balanced against topography it is likely that the present regime represents a transient response to plio pleistocene glacial mass redistribution and differs from typical tectonic flow in the tertiary

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

measurement of horizontal motions in alaska using very long baseline interferometry we have analyzed 6 years of very long baseline interferometry vlbi data beginning in 1984 from a network of sites in alaska the yukon and the conterminous united states the analysis of these 53 data sets includes constrained continuous piecewise linear estimation of the site atmospheres and clocks application of values and correlations from a self consistent earth orientation series derived from other vlbi data and simultaneous estimation of site positions and velocities from a solution that includes 21 007 observations with a fit of 40 ps in delay and 64 fs s in delay rate we have determined the horizontal velocities of seven sites with precisions of 0 5 2 mm yr 1 standard deviation the three vlbi sites within the alaska aleutian subduction boundary zone yakataga kodiak and sand point have higher instantaneous velocities relative to eastern north america than the interior sites of alaska fairbanks and sourdough the estimated velocity of yakataga relative to fairbanks is 37 8 1 3 mm yr at n27 w 2 modeling of the yakataga data suggests that the observed motion is the result of elastic straining of the overriding plate due to a locked main thrust zone with a component of oblique slip the estimated velocity of kodiak relative to fairbanks is 13 6 0 8 mm yr at n41 w 3 this result is lower than the rate predicted on the basis of simple elastic dislocation models the estimated velocity of sand point relative to fairbanks is 6 4 0 9 mm yr at n65 w 8 the component of this velocity in the plate direction 3 5 1 1 mm yr is not consistent with either an aseismic slip model i e no strain accumulation in the overriding plate or a model in which the site is undergoing compressional strain 10 9 mm yr at n28 w due to simple subduction at the predicted orientation of pacific plate convergence in a north america fixed reference frame the estimated velocities of fairbanks 1 5 0 5 mm yr at s12 e 20 and sourdough 3 9 1 0 mm yr at s13 w 13 are both southward these results may imply motion of this region relative to stable north america but more observations are required to reduce the uncertainties in the azimuths the time evolution of the estimated site positions from vlbi in alaska are in general consistent with constant velocity however in 1988 the yakataga site showed a significant departure from the position predicted from a constant velocity model obtained from the previously estimated position this departure may be static displacement associated with two major strike slip earthquakes that occurred in the gulf of alaska between the 1987 and 1988 measurement sessions

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period