constraints on the deep structure and dynamic processes beneath the alps and adjacent regions from an analysis of gravity anomalies summary gravity anomalies including terrain corrections over the western alps and the molasse basin are within 30 mgal of those calculated assuming local airy isostatic equilibrium but isostatic anomalies over regions nw and se of the alps exceed 50 mgal therefore mass excesses and deficits that must be supported by the strength of the lithosphere or by dynamically induced stress in the asthenosphere appear to be greater beneath the areas surrounding the alps than beneath the alpine chain itself attempts to account for the gravity gradient over the molasse basin and sub alpine chains and the small bouguer anomalies measured over the elevated areas of the vosges and black forest by the flexure of an elastic plate require an absurd set of parameters the high elevations of the vosges and the black forest are presumably due to high temperature in the underlying uppermost mantle but the deviations from isostatic equilibrium as well as the high temperature probably result from active upwelling in the asthenosphere beneath this area the absence of large isostatic anomalies over the alps and the failure of an elastic model to account for the gravity anomalies over the molasse basin suggest that the dynamic processes that flexed the european plate down to form the molasse basin and that built the alpine chain have waned the late cenozoic uplift of the molasse basin and the alps might be a consequence of a diminution or termination of downwelling of mantle material beneath the alps

temporal variations in coda duration of local earthquakes in the wellington region new zealand temporal variations of coda duration relative to event magnitude for local earthquakes near wellington have been investigated the region is one of plate convergence and subduction the data consist of routinely made observations for events from 1978 through 1985 1552 events magnitude 1 6 to 5 2 depth 0 5 to 90 0 km the observed average over time correspondence between duration as measured from the origin time and magnitude is reasonably well predicted by the single backscattering theory of coda formation for events of magnitude 4 or less for larger events the observed durations are longer than predicted this theory predicts that a temporal increase in scattering attenuation will reduce the coda duration relative to magnitude thus any temporal changes in the duration magnitude relation can be interpreted in terms of changes inq however it is necessary to consider spatial biases since the observed durations are relatively long for shallow events and for events of all depths at stations situated in the south eastern half of the region usually believed part of the accretionary border for both these situations other evidence would suggest that the scattering coefficient would be relatively high lowq these observations may be due to a relatively high component of surface wave scattering and the importance of multiple scattering in the later part of the coda in regions of lowq as suggested by finite element studies of coda formation despite the lack of any significant earthquakes during the 1978 1985 period there is nevertheless a significant temporal change in duration observed in the wellington region a change from relatively long to relatively short codas occurred in mid 1981 this change correlates well with changes in the rate of activity b value radon emission and ground tilt as derived from lake levels it is not yet clear how all these parameters are related physically but an episode of aseismic slip or creep along the plate interface below the region may have been the cause

late tectonic and metamorphic evolution of the piedmont accretionary wedge queyras schistes lustres western alps evidences for tilting during alpine collision this paper addresses the contribution of extension in the upper crust during syncollisional exhumation of subducted material we present structural data based on the analysis of ductile brittle ductile and brittle deformation features in metapelitic schists and meta basalts from the schistes lustres nappe stack in the piedmont zone of the western alps to link the structures with synkinematic pressure and temperature estimates geothermobarometry was performed using fluid inclusion investigation in quartz together with temperature estimations based on metamorphic chlorite composition samples used for geothermobarometry were collected taking into account the structural framework within veins associated with extensional structures observed at the regional scale the early generation of veins associated with boudinage occurred at 415 345 c and 6 2 5 kbar and subsequent incipient normal faulting occurred at temperatures of 315 235 c and pressures 1 7 kbar the transition from ductile to brittle extension occurred between ca 27 ma and ca 10 ma these results indicate a continuous extension in the upper crust during exhumation and continuous deformation associated with regional scale tilting of the exhumed schistes lustres tectonic pile the extensional tectonic regime in greenschistfacies conditions remains active today in most of the internal alpine zones as the alpine collision continues

shear wave anisotropy and stress direction in and near long valley caldera california 1979 1988 we observed shear wave splitting birefringence for two temporary surface deployments of three component digital seismographs that were in place before and after m 6 earthquakes near the long valley caldera california in one of these deployments the data sample precedes the large events of the may 1980 mammoth lakes earthquake sequence by 6 months and cover the two most active months of the may 1980 aftershock sequence one of the stations wit from this deployment was reoccupied with identical instrumentation in 1988 another deployment preceded the 1986 chalfant valley mainshock by 2 days and recorded events for 6 days the polarization of the faster shear wave changes from n30 w at mammoth to due north at chalfant valley and in both cases is parallel to the strike of nearby surface faults and to the mean direction of p axes determined from focal mechanism groupings observations from nearby stations yield fast directions nearly at 23 from each other and time separations of fast and slow shear waves show considerable station to station variation showing no correlation with earthquake station distance or earthquake depth these observations suggest that the observed anisotropy results primarily from near station presumably shallow effects rather than from widespread aligned microcracks the records at long valley station wit show a slight variation of average fast shear wave polarization from 1979 to 1980 but with the limited amount of data available this difference is not statistically significant time separation measurements of up to 0 48 s at station wit yield estimates of 15 30 velocity anisotropy among the highest reported for local earthquakes secular variation in the time separation of fast and slow shear waves between events recorded at station wit in 1979 and 1980 is swamped by the scatter in the data if it is present the five events recorded in 1988 showed similar s wave polarizations and time separations to those in 1980 but there are not enough events for a statistical comparison unlocated events at long valley station mrs show a slight increase in time separation after the may 1980 m 6 0 earthquakes but the increase is not statistically significant

geodetic measurements of plate motions across the central gulf of california 1982 1986 a trilateration network was set up in early 1982 by a team of french and mexican institutions across the central part of the gulf of california in order to study the plate boundary related movements in this transition area between the san andreas fault system and the east pacific rise the reobservation of this network in march 1986 provides a first set of data on the present day deformations in this area both surveys used aga8 laser geodimeter measurements between 11 stations located on elevated points of baja california and sonora and on the islands between the peninsula and mainland coasts deformation patterns during the 1982 1986 interval obtained through three different methods indicate mainly a right lateral shear movement in the gulf axis direction n46 w between baja california peninsula and angel de la guarda island 17 4 cm of dextral slip occurred between the coast of sonora and the central islands of the gulf the mean displacement amount to about 23 12 cm in the southwestern part of the network weaker movements seem to have occurred and are smaller than estimated errors this may indicate either that the boundary is locked in this part or is deflected towards the ese these results which give an estimation of the relative plate velocity of 8 3 cm a are consistent with the generally accepted relative movement between north american and pacific plates about 6 cm a

a crustal scale cross section of the south western alps combining geophysical and geological imagery a geotransect across the south western alps from the pelvoux massif external french alps to the dora maira massif internal italian alps through the monviso ophiolitic complex was investigated in the framework of the geo france 3d alpes programme a new interpretative crustal scale section across the south western alps is proposed combining geological and geophysical 2d 3d data the apulian mantle i e the ivrea body might be divided into two rigid pieces separated by the downward prolongation of the penninic frontal thrust these mantle indenters drive the decoupling of the european crust beneath the high to ultra high pressure metamorphic nappes the deep structure results from the stacking of crustal slices extracted from the european lithosphere the proposed struc tural model provides a basis for discussing the evidence of the crustal scale partitioning of the current strain pattern as well as the location of the seismicity

observation of shear wave splitting from natural events south moat of long valley caldera california june 29 to august 12 1982 three component seismograms from local earthquakes recorded by three university of wisconsin stations in the south moat of long valley caldera in 1982 have been analyzed for shear wave splitting the average polarization direction of the first arriving shear waves observed at the two stations south of the resurgent dome is nnw sse time delays between the fast and slow shear waves are proportional to hypocentral distances and the maximum velocity anisotropy inferred is 6 4 the average shear wave polarization observed at the station above the resurgent dome is north south the time delays there are not a function of hypocentral distance and the normalized time delays ms km from events at the southwest boundary of the caldera are larger than those at other azimuths indicating maximum 9 6 velocity anisotropy the east west to ene wsw variation of the tensional stress fields implied by the shear wave polarizations indicates the interaction between the inflating caldera and the sierra nevada range front faults under the generally wnw ese tension expected from the regional tectonics

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

neogene to ongoing normal faulting in the inner western alps a major evolution of the late alpine tectonics 1 widespread brittle extension occurs in the internal zones of the southwestern alps forming a dense fault network which overprints the compressional structures the extension has been followed by transcurrent motions in terms of paleostress this extension is radial to the belt close to the crustal penninic front and multidirectional in the eastern part of the internal zones the paleostress field of the strike slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults globally we propose that the internal zones of the southwestern alps underwent a single neogene transtensive tectonic regime to strengthen our structural results the current tectonics has been analyzed using seismotectonic tools this approach allowed the recognition of several active faults moreover the ongoing tectonics is coherent with the observed brittle deformation the fault network controls the present day seismicity the combined seismotectonic and structural approaches give coherency and continuity from the neogene to ongoing extensional tectonics in the southwestern alps in the framework of the whole western alps a synthesis of our results with neotectonic related data allowed us to provide an accurate map of the neogene to present kinematics of the belt extension which appears as a major feature of the internal zones has been coeval with the propagation of thrusts in the outer zones during neogene times the geodynamic processes which rule this major tectonic evolution of the belt remain a matter of debate

erosion driven uplift of the modern central alps abstract we present a compilation of data of modern tectono geomorphic processes in the central european alps which suggest that observed rock uplift is a response to climate driven denudation this interpretation is predominantly based on the recent quantification of basin averaged late holocene denudation rates that are so similar to the pattern and rates of rock uplift rates as determined by geodetic leveling furthermore a gps data based synthesis of adriatic microplate kinematics suggests that the central alps are currently not in a state of active convergence finally we illustrate that the central alps have acted as a closed system for holocene redistribution of sediment in which the peri alpine lakes have operated as a sink for the erosional products of the inner central alps while various hypotheses have been put forward to explain central alpine rock uplift e g lithospheric forcing by convergence mantle processes or ice melting we show with an elastic model of lithospheric deformation that the correlation between erosion and rock uplift rates reflects a positive feedback between denudation and the associated isostatic response to unloading thus erosion does not passively respond to advection of crustal material as might be the case in actively converging orogens rather we suggest that the geomorphic response of the alpine topography to glacial and fluvial erosion and the resulting disequilibrium for modern channelized and associated hillslope processes explains much of the pattern of modern denudation and hence rock uplift therefore in a non convergent orogen such as the central european alps the observed vertical rock uplift is primarily a consequence of passive unloading due to erosion

stress tensor orientation derived from fault plane solutions in the southwestern alps gephart and forsyth s method has been applied to estimate stress orientations from earthquake fault plane solutions of the southwestern alps a region where the tectonic stress regime is known to be fairly complex fault plane solutions have been either taken from the literature or computed using data from local and regional networks data refer to seismic events of magnitude in the range 2 5 5 3 which occurred in the last decades at depths between 0 and 25 km two zones with a different stress orientation have been identified in the studied area 44 0 45 5 n 6 5 8 5 e the western zone corresponding to the crest of the alpine belt where a high dip maximum compressive stress is found and the eastern zone alps chain to po plain transition characterized by an almost horizontal e w 1 and a nearly vertical 3 hypocenters of earthquakes used for stress inversion lie in the depth ranges 0 15 km and 5 25 km in the western and eastern zones respectively the transition between the two stress domains is very sharp and this is also indicated by space distribution of earthquake individual misfits to the respective stress models the findings of the present study are a good match for tectonic models which assume e w compression derived from the adria europe interaction and producing 1 major thrusting processes in the eastern side of the chain and 2 secondary tensional effects at very shallow depth beneath the alpine belt crest western zone of the area studied in this work

shear wave splitting and anisotropy in the charlevoix seismic zone quebec during the month of august 1984 an experiment was undertaken in the charlevoix seismic zone to test the hypothesis that shear wave splitting could be observed in a seismically active region seismograms recorded by digital 3 component seismographs located very close to the epicentres of three earthquakes showed shear wave splitting the amount of splitting was estimated to be 1 6 to 2 of the shear wave velocity from particle motion diagrams the average azimuth of the direction of polarization was 30 these observations can be explained by the presence of vertical cracks striking 30 east of north with a minimum crack density of e 0 03

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

the basis for earthquake prediction summary recent advances in understanding the behaviour of shear waves propagating in the crust make the routine prediction of earthquakes seem practicable accumulating evidence suggests that most of the earth s crust is pervaded by distributions of fluid filled cracks and microcracks that are aligned by the contemporary stress field so that the cracked rockmass is effectively anisotropic to seismic waves this causes shear waves to split and shear wave splitting is observed whenever shear waves propagating along suitable raypaths in the crust are recorded by three component instruments these distributions of cracks are known as extensive dilatancy anisotropy or eda many characteristics of the crack and stress geometry can be monitored by analyzing shear waves propagating through the cracked rockmass observations of temporal variations of the behaviour of shear wave splitting in seismic gaps confirm these hypotheses and suggest that stress changes before earthquakes may be monitored by analyzing shear waves in particular monitoring earthquake preparation zones with three component shear wave vertical seismic profiles could lead to techniques for the routine prediction of earthquakes

active deformation in the inner western alps inferred from comparison between 1972 classical and 1996 gps geodetic surveys abstract eighteen geodetic points surveyed in 1972 by the french national geographic institute ign were remeasured by gps in 1996 in the brianconnais and piemont zones east of the pelvoux massif french western alps a displacement vector set was determined for the two surveys common points calculations of the strain rate tensors associated with 15 triangular cells of the network have been performed only four of them show a strain rate significant at a 95 level of confidence these data suggest an e w extension of about 2 4 mm yr between the western and eastern part of the network pelvoux external crystalline massif and queyras blueschists respectively associated with n s shortening this active deformation agrees with neotectonic and seismotectonic data the measured tectonic motion seems to be distributed throughout the central part of the brianconnais zone where the seismic activity is concentrated the local seismicity has been precisely surveyed since 1989 it is moderate ml

evidence for active tilting of the nw german basin from correlations between fluvial landscape and geological subground the catchment basin of the river hunte lower saxony nw german basin was studied on a mesoscale length of 90 km to investigate the influence of the geological subground on modern morphology a geo information system gis was used to calculate linear correlation coefficients between the depth of geological strata base zechstein to base quaternary and the height of the modern landscape holocene alluvial plain lower weichselian terrace catchment basin and watershed high linear correlation coefficients between the base of tertiary and the height of the modern topography catchment basin r2 0 87 lower weichselian terrace r2 0 95 and holocene alluvial plain r2 0 95 indicate control of the modern topography by the depth of the geological subsurface via tilting of the entire basin most likely northward tilting of the nw german basin forces the river hunte to flow in a northerly direction by relative uplift of the hinterland wiehengebirge rhenish massif and subsidence of the north sea area

cooling history of the dauphinoise zone western alps france deduced from the thermopaleomagnetic record geodynamic implications a post tectonic thermoremanent magnetization carried by pyrrhotite has been previously demonstrated in the dauphinoise zone of the french western alps the paleomagnetic signal is interpreted to record successive partial thermoremanent magnetizations acquired in normal and reverse geomagnetic fields during the slow cooling of the epimetamorphic marly limestones a comparison between the derived magnetic polarity sequence versus temperature thermopaleomagnetic record and the magnetostratigraphic time scale taking into account radiochronological constraints allows us to derive 21 age temperature couples between 24 1 ma 302 c and 20 2 ma 182 c for the bourg d oisans area combining these thermopaleomagnetic data with other p t t and structural data a geodynamical model is proposed for the tectono metamorphic evolution of the dauphinoise zone the thickening contemporaneous with the main syncleavage folding occurs around 26 5 ma depending on the hypothesis considered to explain the excess of heat the bourg d oisans area was buried to 8 6 km fluids hypothesis or 12 5 km tectonic hypothesis in both cases these depths imply the existence of overlying units which might come from the internal zones actually cropping out eastward of the penninic front a tectonic denudation is proposed just before 24 1 ma in order to explain our observations after this denudation a faster cooling occurs it is probably related to the dissipation of the thermal anomaly induced by the tectonic denudation and with the increase of the erosion due to the underthrust of a more external crustal slice this interpretation is supported by the similar fluctuations of cooling rate derived here and subsidence of the molasse basin afterwards the cooling slowed down during this time the studied area seems to be passively moved westward in the hanging wall of the external crystalline massifs thrust without significant deformations

timing of alpine fault gouges k ar ages from clay rich fault gouges in the european alps are consistent internally with established field constraints and with fission track ages demonstrating the applicability of this method for direct dating of brittle deformation illite grown by retrograde hydration of granitic and or high grade metamorphic protoliths is the major k bearing mineral in the fractions that have been separated 0 1 to 6 10 m the ages obtained are bracketed by apatite and zircon fission ages from adjacent localities this together with the interpreted cooling curve for one well constrained sample from the simplon fault indicates that illite grew or the k ar isotopic system in illite closed at temperatures in the range 120 150 c consistent with the stability of illite alone rather than illite smectite average ages obtained for fractions finer than 2 m were 1 5 2 2 6 ma 2 for 11 analyses from the detachment and footwall of the simplon fault 2 8 8 0 6 ma for four analyses from the centovalli fault at trontano 3 19 9 2 6 ma for two analyses from a major riedel fault related to the periadriatic fault offsetting the tertiary bergell tonalite at gesero 4 21 5 5 5 ma for two analyses from a zone parallel to the periadriatic fault in the adjacent southern alps in val morobbia and 5 14 5 3 9 ma for two analyses from a gouge zone on the northern rim of the tertiary mauls tonalite in the eastern alps the ages from the simplon centovalli and periadriatic faults are not identical and do not lend support to models linking these three faults as a single coeval structure

active tectonism in the central alps contrasting stress regimes north and south of the rhone valley an integrated interpretation of seismicity fault plane solutions and deep seismic reflection data suggests that the ne sw to nw se trending rhone simplon fault zone and the gently s dipping basal penninic thrust separate fundamentally different stress regimes in the western swiss alps north of the rhone simplon fault zone strike slip earthquakes on steep dipping faults within the helvetic nappes are a consequence of regional nw se compression and ne sw extension to the south vertical maximum stress and n s extension are responsible for normal mechanism earthquakes that occur entirely within the penninic nappes above the basal penninic thrust such normal faulting likely results from extension associated with southward movements collapse of the penninic nappes and or continued uplift and relative northward displacements of the underlying alpine massifs geological mapping and fission track dating suggest that the two distinct stress regimes have controlled tectonism in the western swiss alps since at least the neogene

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

widespread extension in the core of the western alps revealed by earthquake analysis the western alps are an active collision belt whose current stress field is inhomogeneous muller et al 1992 we report new seismological data which significantly improve our knowledge of this stress field about 1600 earthquakes which occurred in the western alps during the last 10 years were precisely located and 79 new focal solutions were computed the analysis of this database shows that widespread extension affects all the internal zones of the belt to better constrain the associated stress regime six stress tensors were computed using the gephart and forsyth 1984 method they show that the current tectonics of the western alps are contrasted with close variation in the stress regime transpression to the front of the belt contrasting with extension in the core of the belt the extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone suggesting extensional reactivation of this inherited crustal discontinuity such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision related tectonics we propose that intrabelt buoyancy forces such as those produced by a slab retreat or slab break off interfere with the boundary forces driven by the ongoing europe africa convergence

miocene lateral extrusion in the inner western alps revealed by dynamic fault analysis 1 from early miocene to the present day the core parts of the western european alps experienced brittle extensional deformations mostly in a strike parallel direction here we present new data constraining the brittle deformation of the vanoise area french alps and a synthesis of 312 paleostress tensors in the whole arc of the internal western alps the data show a continuous change in the direction of extension from n065 simplon area to n s vanoise area and to nnw sse briancon area the abundance of orogen perpendicular 3 axes increases from the north to the south in the brianconnais area an extensional reactivation of the basal penninic thrust seems to be the origin of the e w to ne sw oriented 3 in light of these new data and the regional paleostress synthesis we propose a predominant orogen parallel extension in the internal zone as a whole this orogen parallel extension is related to the indentation rotation of the apulian microplate and to the opening of the ligurian sea during the lower middle miocene the locally observed orogen perpendicular extension is interpreted as an effect of the exhumation of the internal crystalline massifs the uplift of the external crystalline massifs and or the present day geodynamics postorogenic gravitational collapse some transcurrent tectonics older than the extension in the valais area and younger than the extension farther south is observed in the entire inner western alps strike slip movements are correlated with the apulian rotation and local permutation of stress axes

extensional neotectonics around the bend of the western central alps an overview the western alps active tectonics is characterized by ongoing widespread extension in the highest parts of the belt and transpressive compressive tectonics along its borders we examine these contrasting tectonic regimes using a multidisciplinary approach including seismotectonics numerical modeling gps morphotectonics fieldwork and brittle deformation analysis extension appears to be the dominant process in the present day tectonic activity in the western alps affecting its internal areas all along the arc shortening in contrast is limited to small areas located along at the outer borders of the chain strike slip is observed throughout the alpine realm and in the foreland the stress orientation pattern is radial for 3 in the inner extensional zones and for 1 in the outer transcurrent tranpressional ones extensional areas can be correlated with the parts of the belt with the thickest crust quantification of seismic strain in tectonically homogeneous areas shows that only 10 20 of the geodesy documented deformation can be explained by the alpine seismicity we propose that alpine active tectonics are ruled by isostasy buoyancy forces rather than the ongoing shortening along the alpine europe adria collision zone this interpretation is corroborated by numerical modeling the neogene extensional structures in the alps formed under increasingly brittle conditions a synthesis of paleostress tensors for the internal parts of the west alpine arc documents major orogen parallel extension with a continuous change in 3 directions from ene wsw in the simplon area to n s in the vanoise area and to nnw sse in the briancon area minor orogen perpendicular extension increases from n to s this second signal correlates with the present day geodynamics as revealed by focal plane mechanisms analysis the orogen parallel extension could be related to the opening of the ligurian sea during the early middle miocene and to compression rotation of the adriatic indenter inducing lateral extrusion

aseismic deformation in the alps gps vs seismic strain quantification neotectonics of the western and central alps is characterized by ongoing widespread extension in the highest zones of the chain and transcurrent compressive tectonics at the external limits of the belt the overall geodetically measured deformations also indicate extension across the western alps there is a good qualitative coherency between seismotectonic and geodetic approaches here we attempt to quantify the seismic part of the deformation the seismic strain is compared to the deformation derived from geodesy in sub areas of homogeneous seismic stress strain we computed the total seismic moment tensor and related strain tensor this study provides new quantitative elements about the ongoing geodynamic processes in the alpine belt the important discrepancies obtained between seismic strains and geodetically measured deformations raise the issue of aseismic deformation in the alps which could be related to elastic loading creeping and or a slower ductile style deformation

evidence from shear wave splitting for the restriction of seismic anisotropy to the upper crust the existence of anistropy in the propagation speed of seismic waves through the earth s crust has been attributed to the non random alignment of microcracks in the crust1 for sparsely distributed parallel vertical cracks perturbation theory predicts the splitting of a shear wave into two orthogonally polarized waves2 4 here we present extremely clear evidence for shear wave splitting in three component seismograms from crustal and sub crustal microearthquakes recorded in the shikoku district of japan quantitative analysis of travel time differences between split shear waves leads to the conclusion that the observed anisotropy is limited to the upper crust shallower than 10 15 km this model is consistent with recent results from reflection seismology5 and implies that microcrack density or orientation changes from the upper to the lower crust

strain rates in northwestern italy from spatially smoothed seismicity 1 this work presents seismic strain rate maps for the western alps and northern apennines northern italy as derived from an earthquake catalog collecting both historical and instrumental data strain rates are calculated on the basis of the rate of seismic moment release using the anderson method unlike previous applications which determined the total strain rate associated with specific seismogenic sources we have employed an innovative zoneless approach based on a spatially smoothed seismicity method in addition a monte carlo simulation procedure is applied to allow for uncertainty in the input data e g magnitude to moment conversion seismogenic thickness maximum earthquake magnitude strain rate maps are developed by summing the moments of the earthquakes reported in the catalog and by using two different earthquake recurrence relations our results indicate that deformation rates are quite high ranging from about 2 to 12 10 9 yr 1 in the northern apennines and from 0 5 to 6 10 9 yr 1 in the western alps these values however are 1 order less than those derived from global positioning system measurements suggesting that a portion of the recent deformation in northwestern italy is related to aseismic processes the discrepancies between seismic and geodetic strain rates may also indicate that the record of seismicity may not provide a sufficient time window for assessment of secular rates of moment release or secular deformation rates and rates of recurrence of large magnitude earthquakes in the study area

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

does long term gps in the western alps finally confirm earthquake mechanisms the availability of gps survey data spanning 22 years along with several independent velocity solutions including up to 16 years of permanent gps data presents a unique opportunity to search for persistent and thus reliable deformation patterns in the western alps which in turn allow a reinterpretation of the active tectonics of this region while gps velocities are still too uncertain to be interpreted on an individual basis the analysis of range perpendicular gps velocity profiles clearly highlights zones of extension in the center of the belt 15 3 to 3 1 nanostrain year from north to south with shortening in the forelands the contrasting geodetic deformation pattern is coherent with earthquake focal mechanisms and related strain stress patterns over the entire western alps the gps results finally provide a reliable and robust quantification of the regional strain rates the observed vertical motions of 2 0 to 0 5 mm year of uplift from north to south in the core of the western alps is interpreted to result from buoyancy forces related to postglacial rebound erosional unloading and or viscosity anomalies in the crustal and lithospheric root spatial decorrelation between vertical and horizontal seismicity related deformation calls for a combination of processes to explain the complex present day dynamics of the western alps

shear wave polarization anisotropy observed in a rift zone in japan abstract we have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in central kyushu japan we have recognized two different directions in the polarization of the faster shear waves about e w directions at three stations located in the rift zone and about n s at two stations possibly located outside and close to the edge of the rift zone the former case e w is consistent with a hypothesis that shear wave splitting is caused by vertical cracks or fractures striking in the e w direction which have formed under the present tectonic stress inside a rift zone extending in the n s direction with the maximum horizontal compressive stress striking in the direction about e w and the minimum compressive stress vertical in contrast the latter case n s is inconsistent with the above hypothesis this inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone alternatively it may result from a complicated stress regime within the crust close to the edge of the rift zone arrival time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0 12 s while those from the upper mantle events are less than about 0 25 s the relatively small arrival time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust

seismic anisotropy due to preferred mineral orientation observed in shallow crustal rocks in southern alaska laboratory velocity measurements and structural field relations explain the observation of a significant seismic anisotropy within highly foliated rocks in southern alaska the orientation of the principal compressive stress indicates that this anisotropy is not related to extensive dilatancy but can be satisfactorily explained by the foliation of the metasedimentary rocks laboratory measurements indicate a significant anisotropy within these rocks even at pressures well above those thought to close microcracks these observations document that anisotropy related to preferred mineral orientation may be widespread in continental rocks thus pervasive foliation can play a more important role in causing anisotropy than dilatancy and inferences of principal stress orientations solely from anisotropy may be erroneous

shear wave polarization anisotropy in the upper mantle beneath honshu japan shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth s surface is investigated using three component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in honshu japan eighty nine high quality seismograms were selected from a period of 3 years the data used in this study are restricted such that incidence angles are smaller than the critical angle of 30 to the earth s surface in order to avoid phase shifts in the shear wave train to find directions of the maximum and minimum velocities in split shear waves where shear waves are resolved into two phases with the maximum time separation each set of the two horizontal component seismograms is rotated in the horizontal plane the split shear waves thus obtained are again recombined after the correction of anisotropy and the anisotropy corrected particle motion is compared with the focal mechanism for a cross check of the observed anisotropy directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station the stereograms and the cross sections of seismic ray paths show that 1 the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations and 2 the anisotropic region is separated into two parts one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0 to 30 from the north north anisotropy and the other in the south with it trending 90 to 120 south anisotropy the maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions the horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west honshu however the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles if we assume the vertical extent of the anisotropic material to be 100 km the maximum velocity contrast would be 4 if we adopt a crack alignment model for the observed anisotropy cracks are inferred to be distributed vertically trending 0 to 30 in the north and 90 to 120 in the south if we assume an olivine alignment model olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0 to 30 and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90 to 120 and the slip plane being horizontal

from passive margin to continental collision a tectonic scenario for the western alps general transverse section just south of the pelvoux massif european margin of tethys an ancient atlantic type margin it was subject to extension from triassic to the beginning of late cretaceous times and to collision thereafter as the result of convergence of the eurasian and african continents paleogeographic evolution is outlined a new alpine tectogenetic reconstruction is proposed an attempt is made to establish how the previous extensional structures influenced the later tectonic evolution modified journal abstract

mechanical behavior of western alpine structures inferred from statistical analysis of seismicity 1 two seismic arcs 150 km and 120 km long located in the western alps 50 km apart from each other are characterized by different statistical patterns in energy and space domains for the two arcs we found power law distributions in energy domain with a lower b value and fall off for large events for the easternmost arc only and roughly similar spatial damage by comparing the shape and the statistical parameters of these distributions with those provided by numerical models of rock damage we suggest a relatively more brittle behavior for one area with respect to the other we discuss the implications of the observed distributions in terms of tectonics and of mechanics of faulting we suggest three candidates to drive the relative changes in seismicity distribution between the two arcs the earthquake depth the host rock composition and the inherited fracturing patterns

the scattering of shear waves in the crust the two major sources of scattering for shear waves in the crust interactions with the topography at the surface and the effective anisotropy of aligned cracks throughout the rockmass introduce first order changes to the shear wave particle motion at the surface shear waves are scattered by the topography within a wavelength or two of the recording site so that unless the effective incidence angle is less than the critical angle sin 1vs vp the recorded waveforms may bear little relationship to the waveforms of the incident wave within the rockmass shear waves are scattered by extensive dilatancy anisotropy eda the distribution of stress aligned fluid filled cracks microcracks and preferentially oriented pore space pervading most rocks in the crust analysis of this shear wave splitting yields new information about the internal structure of thein situ rockmass which is not otherwise available

an automated analytical method to determine shear wave splitting abstract we introduce and discuss an automated analytical method for determination of shear wave splitting to determine the polarization direction of the first arriving shear wave fast wave we evaluate the linearity of the particle motion of shear waves by maximizing as a function of azimuth the aspect ratio i e the ratio of the projections of the particle displacement on orthogonal axes in the horizontal plane in a time window that contains only the first shear wave arrival the azimuth at which the maximum aspect ratio occurs is taken as the polarization direction of the fast shear wave then the time separation between the split shear waves is determined by time shifting the component with the slow shear wave arrival toward the fast shear wave arrival until maximum linearity in particle motion is observed the polarization direction of the fast shear wave can be accurately predicted for seismograms having good signal to noise ratios as are typical of local earthquake recordings similar procedures are used to evaluate particle motion in a vertical plane in the time interval including the first shear wave arrival to examine the apparent incident angle in order to reveal the likelihood of interfering s p conversions

present day deformation of the dauphine alpine and subalpine massifs se france the positions of 20 geodetic points surveyed in 1946 1952 by the institut geographique national in the grenoble area western alps were remeasured in 1993 1994 using gps we evaluate the displacements of the common points of the networks between the two surveys and calculate the strain rate tensors for different sets of three adjacent points the significant n70 shortening direction observed at the boundary of the belledonne and taillefer massifs suggests that the dextral strike slip motion between the external crystalline massifs ecm and the subalpine domain is still active geological evidence suggests that this deformation which is compatible with the few focal mechanisms available for the area occurred throughout the late neogene our data also suggest a roughly 3 5 mm yr 1 active shortening concentrated at the external front of the subalpine chains this thrust motion results in a principal axis of compression orientated n130 oblique to the direction of shortening observed in the ecm this change in direction is also seen in in situ stress measurements this motion may mark the southward continuation of the active fault observed in the jura mountains by jouanne menard darmendrail 1995

evidence of synextension tilting and doming during final exhumation from analysis of multistage faults queyras schistes lustres western alps abstract during the neogene the internal arc of the western alps underwent extension behind its inverted paleogene frontal thrust while shortening affected the external arc in the core of the internal arc doming of eclogite bearing gneissic nappes has formed the dora maira massif the blueschist bearing schistes lustres of queyras which overlie the western flank of the dora maira dome allow observation of how the neogene extension developed in ductile to brittle conditions during the evolution from chocolate tablet boudinage to cross trend normal faulting the extension kept a tendency to be multidirectional the analysis of variably oriented fault families allows considering them as globally associated with the same long lived tectonic regime focusing on faults bearing several generations of slickenlines and using an adapted right dihedra analysis we propose that faulting accompanied the westwards tilting of the eastern queyras structure this tilting is consistent with the doming of the dora maira massif itself associated with tectonic denudation along its western flank therefore two processes operated simultaneously during final exhumation regional thinning through widespread multitrend normal faulting and more localised tectonic denudation and doming both are consistent with a neogene regime dominated by vertical compression the role of an ascending deep indenter below the internal arc remains to be documented

evidence for shallow and pervasive seismic anisotropy in the wellington region new zealand the shear waves of local earthquakes were recorded during a 5 month deployment of seven three component digital seismographs on the wellington peninsula new zealand the seismographs were spaced an average of less than 5 km apart and over 300 local earthquakes were recorded with phase arrivals within the shear wave window a significant number 37 of the earthquakes recorded showed clear evidence of shear wave splitting identifiable fast and slow shear wave arrivals with similar pulse shapes consistent polarization directions at particular stations were also observed even when poor signal to noise ratio or scattering meant that no slow shear wave arrival could be identified however there were large station to station differences in the polarization directions correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double couple focal mechanism the cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy large station to station differences in the polarization alignments ranging from 61 24 to 137 18 suggest that most anisotropy is confined to the top 2 3 km of the crust however there is evidence from one station for a small amount of pervasive anisotropy if such a trend existed on the other stations it could not be identified because of the large scatter in the data points the measured delay times between split shear waves vary from 0 02 to 0 22 s with a mean value of 0 1 0 06 s this translates to a near surface shear wave velocity anisotropy of about 10 with up to 2 pervasive anisotropy possible throughout the crust this data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important there is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment

widespread post nappe normal faulting in the internal western alps a new constraint on arc dynamics abstract field data collected in the brianconnais and piemont zones at the centre of the western alpine arc demonstrate a high density of normal faults normal displacements are pervasive from the millimetric to the kilometric scales they occur both parallel and at high angles to the arc overprinting all the alpine folds and thrusts and the associated schistosities the orientation of fault striae implies that the pile of metamorphic nappes se of the pelvoux massif has undergone a general extension extension tends to be multidirectional in the brianconnais zone orogen parallel extension prevails in most of the piemont schistes lustres to the west in brianconnais zone brittle extension is associated with a reversal of movement along the brianconnais frontal thrust that started before the end of the oligocene in the piemont schistes lustres the onset of normal faulting occurred early in the miocene and continued the ductile extension that accompanied the exhumation of hp lt metamorphic thrust sheets during the palaeogene in both zones extension remains currently active resulting in the regional seismicity since more than 20 ma ago this normal faulting in the internal arc coexists with an outward propagation of folds and thrusts in the external arc we discuss its possible significance in the context to the dynamic evolution of the alps

mantle flow uplift of western anatolia and the aegean interpretations from geophysical analyses and geodynamic modeling 1 the western anatolian and aegean region demonstrates a complex geologic history of horizontal and vertical tectonics active normal faulting and exhumation zones indicate that western anatolia has experienced significant extension since the oligocene early miocene 30 ma our geophysical analyses demonstrate that the region is also uplifted relative to an elevation that would be expected given an isostatic response to the lithosphere structure namely topography residuals indicate a residual uplift of about 1500 m over 200 km sections of western anatolia and the aegean admittance functions between free air gravity and topography indicate that the regional topography is isostatically uncompensated and as it approaches 50 mgal km at the longest wavelengths the uncompensated topography is likely owing to an underlying mantle flow component using forward geodynamic modelling we consider an idealized section of western anatolian lithosphere based on tomographic inversions and examine the magnitude and pattern of surface topography to reconcile with the geophysical observables the models consistently show a plateau type uplift and horizontal extension through western anatolia with an amplitude and wavelength consistent with the residual topography calculations together the geophysical analyses and modelling provide independent quantitative evidence that the thin anatolian aegean lithosphere is being buoyed upwards by underlying mantle flow the mantle flow may be associated with active lithosphere delamination beneath the region a process that would also explain the ongoing crustal extension

a shear wave polarization study in the wellington region new zealand a month of digital data from two three component seismograph stations near wellington new zealand was analyzed as part of a feasibility study for a major project to investigate shear wave splitting although the total number of earthquakes studies was small 14 some suggestive results were obtained almost all events recorded within the shear wave window showed a phase reversal of the horizontal components after one or two shear wave cycles suggesting that there are actually two shear wave arrivals the measured polarization of the first shear wave arrivals was n 31 plus minus 11 e this polarization alignment cannot be explained by focal mechanisms and it is unlikely to be due to topography because of the station to station correlation the present evidence suggests the most likely cause is crustal anistropy due to the geological structure at shallow depth rather than stress aligned micro cracks

shear wave splitting above small earthquakes in the kinki district of japan abstract we have identified shear wave splitting diagnostic of the effective anisotropy induced by aligned microcracks in the wavetrains of micro earthquakes at four stations of the abuyama network in the kinki district of japan we find that the directions of polarization of the faster split shear waves are nearly parallel for all azimuths of arrival and for all angles of incidence less than the critical angle at three of the four stations these directions of polarization are consistent with the axis of maximum compression obtained from earthquake fault plane mechanisms and also agree with the directions of the general trends of geological structures which represent the orientations of the cleavage or lamination these results suggest that crack induced anisotropy is present in the brittle upper crust beneath japan as has been found elsewhere but we could not distinguish whether this reflects the distributions of cracks induced by the present stress field or results from the general trends of surface geology although the delay times between faster and slower shear waves are difficult to estimate reliably because of their high sensitivity to internal interfaces the delay times can be interpreted as the result of a distribution of parallel vertical cracks with a crack density of about 0 04 the consistency or lack of consistency of the directions of the shear wave polarizations at the four stations demonstrates the effects of surface topography and near surface layering on the shear wave polarizations

are the alps collapsing abstract orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy this transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen not all extensional features in collisional mountain belts need however reflect orogenic collapse normal faulting thrust faulting and strike slip faulting are all active in different parts of the alps today and reflect complex local responses to ongoing europe adria convergence the western alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process elsewhere in the alps normal faults are oriented at a high angle to orogen trend and were primarily active in oligocene and miocene time most present day activity in the central and eastern alps is on strike slip faults that are accommodati

analysis of seismicity and micro seismicity associated with the october november 2010 sampeyre swarm southwestern alps abstract the study presents a detailed analysis of the swarm that occurred in the area surrounding sampeyre piedmont italy during the period october november 2010 the crisis caught the attention of seismologists because of its intense seismic productivity that has never been observed in the past during the swarm 550 earthquakes were recorded and located by the regional seismic network of northwestern italy rsni http www distav unige it geofisica in the study area however including the micro seismicity mainly earthquakes of magnitude lower than 0 detected through an sta lta short term average long term average based algorithm the number of events goes up to approximately 3250 among all these events approximately two hundred exceeded a local magnitude m l of 1 0 but only two earthquakes exceeded m l 3 0 reaching up to m l 3 2 following a general description of the swarm evolution the study presents the results of a waveform similarity analysis and double difference relocation aimed at identifying groups of earthquakes belonging to common genetic sources a statistical analysis of the resulting earthquake families has revealed a variation of the scaling exponent b value of the gutenberg and richter size frequency model with depth in particular our results have evidenced a slightly higher b value at shallower depths where a distinct family of earthquakes was identified through seismogram cross correlation this may reflect variability in the distribution of fractures in the rock volume involved in the swarm

evidence for the splitting of shear waves from waveform and focal mechanism analyses abstract a temporary seismograph station was sited above the aftershock area of an intermediate crustal earthquake m 4 9 in the kinki district japan the results from analysing digital three component seismograms and numerically determining focal mechanisms of aftershocks have provided clear evidence for the splitting of shear waves attributed to crustal anisotropy faster shear waves from most of the aftershocks are polarized wnw ese regardless of their azimuths and incident angles such shear wave motions are often significantly inconsistent with the focal mechanisms of the aftershocks determined using p wave first motions for many of the events slower shear waves polarized orthogonally to the faster shear wave direction nne ssw are observed to arrive about 0 1 s later two vectors showing particle motions of faster and slower shear waves in the horizontal plane are summed to reconstruct polarizations of shear waves before splitting for events with sufficiently high signal to noise ratios of shear waves and well determined focal mechanisms the reconstructed polarizations are consistent with the focal mechanisms within the range of uncertainty such consistency is not clearly detected for the other events possibly because of large noise effects and poorly constrained fault plane solutions the direction of faster shear wave polarization wnw ese generally coincides with that observed at three other stations located within 15 km of the temporary station in the study area the fabric of crustal anisotropy is almost uniform over the scale of 15 km but appears to exhibit small local fluctuations

geodetic measurement of tectonic deformation in the santa maria fold and thrust belt california we have analyzed geodetic observations to resolve tectonic deformation across the santa maria fold and thrust belt northwest of santa barbara california the geodetic network forms a braced quadrilateral with 40 km sides whose southwest corner is the vandenberg very long baseline interferometry station the data include 1 historical triangulation from the 1880s 1920s and 1950s 2 electronic distance measurement trilateration from 1971 and 1985 and 3 global positioning system observations from 1986 and 1987 we combine the three different types of data to estimate two dimensional station positions and strain rate parameters simultaneously using a model which constrains the relative velocity field to be linear in space and constant in time we find significant strain rates the maximum compressive strain is oriented n17 e 5 e and the compressive strain rate in this direction is 0 13 0 03 strain yr under the assumption that the unresolved rotational component of the velocity field is zero we estimate that the integrated rate of deformation across the basin is 7 1 mm yr oriented at n03 e 13 this vector can be decomposed into 6 2 mm yr of crustal shortening on the general structural trend of n30 e and 3 1 mm yr of right lateral shear across this axis on the basis of these values and earthquake focal mechanisms in the area we infer that the deformation occurs on northwest trending folds and thrusts within the belt these results are consistent with the rate and direction of deformation across the central california coast ranges inferred by balancing pacific north america plate motion against san andreas slip and basin and range extension they imply that the santa maria fold and thrust belt is the primary active element in transforming motion from the coast ranges to the western transverse ranges and the santa barbara channel