retrogression by deep infiltration of meteoric fluids into thrust zones during late orogenic rapid unroofing hypersaline na ca cl fluids are associated with late stage quartz veining and retrogression of garnet kyanite and other high p t phases in the vicinity of thrusts and major lithological boundaries in the scandian nappes of the troms ofoten region northern norway they record early devonian fluid infiltration during rapid exhumation in the final stages of caledonian orogenesis the d18o and dd characteristics of these late fluids provide compelling evidence for deep circulation of meteoric fluids the sub greenschist to low greenschist facies retrogression p 2 1 kbar t 300 350 c suggests infiltration to depths of 7 9 km in a regime of supra hydrostatic fluid pressure peak metamorphic quartz veins and associated fluids have dd and d18o characteristics consistent with a metamorphic origin dd 47 to 75 d18o 8 6 to 17 4 however late quartz veins and associated fluids show a broad spread of dd from 42 to 148 interpreted in terms of meteoric fluid infiltration such negative dd values are only recorded in present day high latitude or high altitude settings and since north norway was in an equatorial setting 10 s in the early devonian a high altitude origin is deduced by calculation and by comparison with modern examples the early devonian mountains of the north norwegian caledonides are interpreted to have had a topography in excess of 5 km the deep circulation of surface waters is interpreted in terms of topographically driven flow linked with a hydrothermal system induced by elevated geothermal gradients due to rapid uplift whilst the case for deep penetration of surface derived fluids has been promoted for mesozoic and younger mountain belts this study represents one of the first documented examples for a palaeozoic orogenic belt it suggests that many of the fundamental processes operating during the exhumation of mountain belts are similar irrespective of age

control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

seismic tomography of the north anatolian fault new insights into structural heterogeneity along a continental strike slip fault major funding was provided by the uk natural environment research council nerc under grant ne i028017 1 equipment was provided and supported by the nerc geophysical equipment facility seis uk this project is also supported by bogazici university scientific research projects bap under grant 6922 and turkish state planning organization dpt under the tam project number 2007k120610

seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

aftershock tectonics of the 1978 tabas e golshan iran earthquake sequence a documented active thin and thick skinned tectonic case summary in this study a locally recorded aftershock sequence of the 1978 tabas e golshan earthquake m 7 4 was accurately located out of 1560 located events 329 best located aftershocks passed a strict quality criterion these well located aftershocks which have uncertainties in epicentre and in focal depth of about 1 and 2km respectively together with the wellconstrained focal mechanisms provided a detailed picture of active continental deformation during an aftershock sequence almost all aftershocks follow very closely the pattern of the earthquake faults at the surface and lie in the hanging wall block of the active fault the hypocentres occurred mainly at depths less than 23 km with a high concentration of seismic activity between 8 14 km depth the aftershocks clearly demonstrate an active imbricate listric thrust system with fault planes flattening into a basement decollement zone and the reactivation of different basement reverse faults in response to a considerable amount of shortening of the top sedimentary cover the sense of motion was almost universally thrusting and the aftershocks shared the same tectonic causes as the main shock the study indicates that the development of the young fold thrust mountain belts necessarily involves basement shortening thin and thick skinned tectonics and that the frontal reverse faults in young active fold thrust mountain belts are the most seismically active faults geological and seismic data propose that the active frontal reverse fault systems are possibly reactivated old normal faults and may add support to the contention of reversal of fault motion during rethickening of continental crust the active thinand thick skinned tectonics documented in this study may prevail in other young and active fold thrust mountain belts which are characterized by a thick sequence of telescoped top sedimentary cover over a decollement detachment zone

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

strong motion modeling of the imperial valley earthquake of 1979 twelve three component strong motion displacement records are modeled for the 1979 imperial valley earthquake to recover the distribution of slip on the imperial fault plane the final model for which point source responses are calculated by a discrete wavenumber finite element technique uses a structure with gradients in material properties rather than layers the effects of a velocity gradient are investigated by comparing synthetics with a layer over a half space model using generalized rays it is shown that a uniform fault rupture model on a rectangular fault plane does not explain the data the preferred fault model has slip concentrated below 5 km in the basement material and between the epicenter 5 km south of the international border and highway 80 within this region there appears to be two localized areas of larger dislocations one just north of the border near bonds corner and a second under interstate 8 at meloland overpass a major arrival associated with large amplitude vertical accelerations up to 1 7 g is identified in the el centro array records this arrival has an s p time of approximately 2 3 sec at many of the array stations and is modeled as originating from a localized source 8 km to the south of the array the moment is estimated to be 5 0 10 25 dyne cm from the strong motion records which is consistent with teleseismic body wave estimates the preferred fault model is strike slip with a 90 dip the average strike is 143 however to explain vertical waveforms near the fault trace a corrugated or wiggly fault plane is introduced the average rupture velocity is in the range 2 5 to 2 7 km sec 0 8 to 0 9 times the basement shear wave velocity the preferred model has unilateral rupture propagation to the north although the data would allow a small amount of propagation to the south the estimated stress drop for the entire fault plane is only 5 to 10 bars however the stress drop over the more localized sources is about 200 bars the fault model is consistent with the pattern of seismicity and observations of aseismic creep in the imperial valley and suggests that the southern half of the imperial fault acts as a locked section which breaks periodically

geodetically determined strain across the southern end of the tonga kermadec hikurangi subduction zone summary a method of simultaneous reduction is presented for determining strain rates from multiple triangulation surveys where common triangulation stations have been used but the angles of the old survey have not necessarily been reobserved this method is applied to triangulation in the northern south island at the southern end of the tonga kermadec hikurangi subduction zone from a profile of shear strain across the indian pacific plate boundary the displacement of the indian plate relative to the pacific is calculated to be 54 9 mm yr at an azimuth of 84 f lo in remarkable agreement with the motion predicted by global plate tectonic models most of this motion occurs within a 15okm wide zone bounded on the east by the hikurangi trough within this zone the motion is partitioned near the hikurangi trough no slip is occurring at the upper surface of the subducting pacific plate the subduction thrust and motion is predominantly thrusting normal to the trough axis to the west is a region of predominantly dextral strike slip faulting this pattern is consistent with fitch s model of oblique subduction to the south of the profile a change is observed in the azimuth of the faulting along a line which marks the southern extent of the subduction slab indicating the end of the partitioned motion

longitudinal variability of the mesopause sao a longitudinal variation in the mesopause semi annual oscillation sao is documented from wind observations made by the high resolution doppler imager satellite instrument the asymmetry is dominated by a wavenumber 1 pattern with a variable phase during the easterly phase of the mesopause sao equinoxes maximum tropical easterlies occur over eastern south america and the atlantic ocean in the longitude band 300 360 during the westerly phase solstices the westerly winds descend more rapidly and are larger below 80 km in this same atlantic longitude band to the best that can be determined with sparse data the longitudinal asymmetries near the stratopause are weak and are in phase with those in the mesosphere

the 1978 earthquake sequence near thessaloniki northern greece summary this paper presents a detailed study of teleseismically and locally recorded foreshocks and aftershocks of the 1978 thessaloniki earthquake sequence and discusses the relations between the locations and mechanisms of these shocks and the three dimensional geometry of faulting in the source region sixteen teleseismically recorded events were relocated using a relative relocation method and positioned geographically using the accurate locally determined location of the largest aftershock all these shocks had depths shallower than 15 km the spatial distribution of the foreshocks showed that the activity was initiated at the central part of the epicentral region progressed westwards and for about 20 days clustered in the region of the imminent mainshock no teleseismically recorded foreshock was located west of the mainshock location in less than 1 hr after the mainshock large aftershocks occurred west of it and in later stages spanned the entire western part of the epicentral region no teleseismically recorded aftershocks were located east of the mainshock locally recorded aftershocks were accurately located and provided a more detailed picture of the seismic deformation they occurred at depths between 3 and 12 km and in three distinct clusters separated by well defined gaps the largest foreshock on may 23 had a moment of 5 6 1024dyne cm and a mean displacement of 31cm the mainshock on june 20 had a moment of 5 2 1025dyne cm a strain drop of 4 0 10 5 static stress drop of 12 bar and a mean displacement of 64 cm these values are for one of the two possible interpretations of the aftershock distribution which suggests that the largest earthquakes of the sequence occurred on two different but adjacent fault segments the first event 1 on may 23 occurred along the central fault segment and was followed 28 days later by the second event 2 on june 20 along a western fault segment finally 61 days later another smaller earthquake event 3 occurred along another fault east of event 1

thermal effects of thrust faulting abstract calculations based on simple models of overthrust sheets in crystalline basement rocks show that significant thermal effects may result from their movements if rates are sufficiently high e g plate tectonic rates the thrust sheets sufficiently thick 5 10 and 15 km are modelled here the distances moved sufficiently large and for reasonable values of the coefficient of friction along the thrust plane overthrusting can cause metamorphic mineral zonations and heat flow anomalies observable in the field regions where large scale overthrusting has occurred should be characterized by a decrease with depth of grade of metamorphic mineral assemblages and anomalously low heat flow the theoretical effects are presented as a series of maximum temperature vs depth and heat flow vs time plots

mathematical removal of regional ductile strains in central brittany evidence for wrench tectonics abstract hercynian deformation in central brittany is responsible for folding of paleozoic and earlier sediments with concurrent development of cleavage essentially e w and vertical and stretching lineation essentially horizontal in the past the regional motion was inferred to be a n s shortening more recent studies emphasize e w transcurrent motions associated with major crustal faults values and orientations of ductile strain at 29 localities in central brittany show that the regional deformation is basically a horizontal plane strain with local three dimensional perturbations we first remove the perturbations rendering the problem two dimensional then a selected domain is divided into an array of finite elements parallel to strain trajectories using strain values extant at element centroids each element is unstrained independently of its neighbours resulting gaps and overlaps are minimized using a least squares technique which allows elements to translate and rotate rigidly thus we obtain the configuration of the domain in the undeformed state to within an unknown but constant rigid rotation the domain has undergone a heterogeneous dextral simple shear with vertical shearing plane striking n125 and horizontal shear direction a band of low strain running through guer is flanked by two zones of more intense strain the banding is approximately parallel to a major transcurrent fault in the area the south armorican shear zone

earthquake recurrence and magnitude and seismic deformation of the northwestern okhotsk plate northeast russia 1 recorded seismicity from the northwestern okhotsk plate northeast asia is currently insufficient to account for the predicted slip rates along its boundaries due to plate tectonics however the magnitude frequency relationship for earthquakes from the region suggests that larger earthquakes are possible in the future and that events of mw 7 5 which should occur every 100 350 years would account for almost all the slip of the plate along its boundaries due to eurasia north america convergence we use models for seismic slip distribution along the bounding faults of okhotsk to conclude that relatively little aseismic strain release is occurring and that larger future earthquakes are likely in the region our models broadly support the idea of a single okhotsk plate with the large majority of tectonic strain released along its boundaries

experiments on adsorption and molecular sieving and inferences on primary migration in upper jurassic claystone source rocks north sea fine pores primarily slit shaped micropores in upper jurassic claystone source rocks were studied by gas adsorption at 20 c approximately 80 of the mesopore and micropore volume is penetrable to water only because of constrictions approximately 3 a wide the pore surface is nonpolar and probably characterized by kerogen covering the clay mineral surfaces water is not structured in the mesopores and micropores but readily forms a liquid phase without distinct monolayer formation n alkanes probably enter the pores end on and become adsorbed parallel with the surface these results apply to all samples studied irrespective of the degree of clay mineral diagenesis three migration types proposed for these claystone source rocks are 1 type a migration which at experimental conditions permits entry and removal of n alkanes isobutane neohexane cyclohexane and benzene in approximately equal amounts 2 type b migration which permits entry and removal of n alkanes isobutane and neohexane in approximately equal amounts but where only 20 of the pore volume and surface area is also open to cyclohexane and benzene and 3 type c migration which is intermediate between type a and type b the occurrence of the migration type is related to the reservoir oil composition in the samples studied tectonics and clay mineral diagenesis appear to have no influence on the migration type migration types may be related to past overpressured conditions but do not appear to be related to current pressures

the role of microcracking and grain boundary dilation during retrograde reactions department of geology university of southampton southampton oceanography centre southampton so14 3zh uk email ajb1 soc soton ac ukveined metamorphic rocks are common and give evidence of fracture controlled fluid flow however the processesinvolved in fluid infiltration in domains between fractures is rarely well established reaction products concentrated atgrain boundaries give an indication that diffusion of elements along grain boundaries may be important and linearfluid inclusion trails provide evidence of grain scale microcracking despite these common observations the relativeimportance of grain boundary processes and microcracking at different metamorphic grades is not well characterised etheridge et al 1983 developed a metamorphic porosity model comprising grain boundary bubbles and tubules ofwhite white 1981 but also incorporating dilatant grain boundaries and intra grain tensile cracks variations in thelocal granular stress state will initiate transient micro cracking and grain boundary dilatancy promoting enhancedporosity permeability and grain scale fluid infiltration this in turn causes retrogression the extent to which it devel ops being influenced by the degree of connectivity of the dilatant cracks and grain boundaries and whether for particu lar p t x conditions the infiltrating fluid is at equilibrium or not with the host being infiltrated with reference to examples of greenschist facies retrogression from troms norway and south harris scotland this study establishes that the processes of microcracking and grain boundary dilatancy are highly heterogeneous atthin section scale in examples where reaction products are not related to mutual contact of specific phases it is shownthat the development of reaction products varies according to grain boundary orientation and density of microcracks an example from norway shows a quartz rich garnet mica schist where a centimetre scale brittle fracture has in duced extensive microcracking and fluid infiltration in the adjacent matrix fluid inclusions defining healed microcracks perpendicular to schistosity have h

deep intraplate seismicity in the western sierra nevada central california abstract a microearthquake monitoring program was conducted from 1976 to 1979 in the eastern san joaquin valley and sierran foothills of central california this program revealed one of the deepest occurrences of intraplate earthquakes observed in the conterminous united states earthquakes in the m l range of approximately 0 to 3 2 were confined to the lower crust of the sierran block at depths from 12 to 38 km the distribution of epicenters was diffuse and widespread with some clustering in space and time two particulary active areas that were consistent with the historical seismicity record were recognized an area 15 km south of the town of madera and an area 15 km south of mariposa no association of the seismicity with faults or other geologic structures expressed at the surface was apparent based on regional seismic monitoring by the u s geological survey and the detailed monitoring performed in this study this occurrence of deep seismicity does not appear to extend farther north along the foothills than jamestown although two deep events have been observed near oroville the southern limit is not as well defined due to the lack of seismographic coverage focal mechanisms of these deep crustal earthquakes exhibited 1 strike slip faulting combined with some reverse and normal components and 2 a regional tectonic stress field characterized by a maximum compressive stress generally trending north south and a minimum compressive stress trending east west these observations support the view that 1 the sierra nevada represents a transition between a region of primarily shear deformation along the san andreas fault zone and extensional tectonics of the basin and range province and 2 the orientation of the tectonic stress field is generally uniform throughout the crust of the sierran block

the relations between mean rock stress and fluid flow in the crust with reference to vein and lode style gold deposits abstract fluid movement in the crust at the typical depths of formation of vein and lode style gold deposits will be dominated by upward flow focussing of upward flow into discrete channelways as required to form gold deposits is due to lateral gradients in fluid pressure fluid channelways should have lower fluid pressures than surrounding rock if lateral gradients are induced by variations in permeability this is however at variance with inferences from quartz vein structures of high relative fluid pressures during the formation of gold deposits focussing into zones of low mean rock stress will in contrast be associated with high relative fluid pressures in the zone of focussed fluid flow variations in mean rock stress are a direct consequence of a regional deviatoric stress acting on an inhomogeneous rock sequence analysis of stress fields shows a wide variety of potential sites of low mean stress dependent on the geometry of rock units and on patterns of faults fractures or shear zones a model of fluid focussing in the crust due to variations in mean stress is thus consistent with the large variety of structural setting of vein and lode gold deposits observed in nature use of the model through the technology of stress mapping has the potential to generate viable exploration targets

geodetic measurement of crustal deformation on the san andreas hayward and calaveras faults near san francisco california analysis of a geodetic network of 115 lines crossing the san andreas hayward and calaveras faults in the vicinity of san francisco bay and measured repeatedly between 1970 and 1980 has revealed details about the accommodation of relative plate motion in this area the most striking result is that the deformation is not uniformly distributed across the area in the east bay along the hayward and calaveras faults all motion appears to take place as slip directly on the fault with no accumulation of strain in the adjacent crust on both the calaveras and the hayward faults the rate obtained for the 1970 to 1980 period agrees with geologic rates spanning a few million years and with creep rates spanning a few decades the hayward fault slip rate is 7 1 mm yr the calaveras fault slip rate is 7 1 mm yr with perhaps half of this slip distributed across a zone a few kilometers wide probably as inelastic deformation of weak near surface material the absence of strain accumulation in the east bay is surprising since the hayward and calaveras faults have been the site of large earthquakes in the past a block located east of the calaveras fault and south of the las positas fault has been rotating clockwise at a rate of 0 3 0 1 rad yr with very little internal deformation along the san francisco peninsula no detectable slip occurs less than 1 5 mm yr at the surface but appreciable strain is accumulating near fault shear strain rates are 0 6 0 1 strain yr engineering with direction n47 w 9 the slip rate near the san andreas fault is 12 2 3 9 mm yr distributed across a broad zone the relative motion across the whole region during the period 1970 1980 is 32 1 7 4 mm yr supplement available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 011 1 00 payment must accompany order

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

a multidisciplinary study of the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part ii amphibole geobarometry and gravity modeling abstract the late mesozoic geology of the lower yangtze area is characterized by extensional sedimentary basins numerous granitic plutons and several world class ore deposits regionaly the coeval tectonic and geodynamic framework remains inadequately investigated in order to provide a more comprehensive understanding of the regional tectonic context we performed a multidisciplinary study of the emplacement mechanism of the qingyang jiuhua massif it consists of the granodioritic qingyang and the monzogranitic jiuhua plutons u pb dated at 142 1 0 ma and 131 2 6 ma respectively biotite and amphibole cooling ages are 5 8 myr younger results deduced from field structural observation petrographic and magnetic fabrics paleomagnetism show that this massif was probably vertically emplaced by permissive intrusion coeval with weak regional extension however detailed information on the characteristics of this tectonic event and its emplacement depth was not documented therefore we carried out an investigation of amphibole geobarometry and gravity modeling in order to address the emplacement mode of the plutons within their regional tectonic framework amphibole data show that the qingyang jiuhua massif was emplaced in the upper crust at a depth between 5 and 12 km gravity modeling indicates that the massif is laccolithic it is characterized by several ne sw striking linear thickening zones that we interpret as the feeder roots of the massif these results led us to conclude that 1 the qingyang jiuhua massif was fed by vertical ne sw striking tension gashes 2 consequently the late early cretaceous tectonics in the study area was characterized by nw se extension

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

physical models of earthquake instability and precursory processes this paper selectively reviews physical models of earthquake instability in these models instability arises as a result of interaction of a fault constitutive relation with deformation of the surrounding material that occurs in response to remote tectonic loading in contrast to kinematic models in which the fault slip is imposed it is calculated in physical models and consequently these models are essential for understanding precursory processes some kind of weakening behavior for the fault constitutive relation is required to produce an instability analogous to an earthquake two commonly employed idealizations discussed here are rate independent slip weakening and rate state dependent friction when these constitutive models are employed on surfaces embedded in elastic half spaces or layers possibly coupled to a viscoelastic substrate the results are capable of simulating realistically some aspects of earthquake occurrence common to all models is the prediction that earthquake instability is preceded by precursory slip which produces a departure of surface strain rate from the background level near the epicenter of a moderate to large earthquake the magnitude of this departure appears to be well within the range of current geodetic measurement accuracy and its duration is of the order of months to years however details depend on a variety of factors including the modelling of the constitutive relation near peak stress coupling of elastic crust to the asthenosphere and coupling of deformation with pore fluid diffusion

strain accommodation about strike slip fault discontinuities in granitic rock under brittle to ductile conditions abstract brittle and ductile structures adjacent to fault terminations and echelon fault steps of left lateral strike slip faults in the lake edison granodiorite of the central sierra nevada california are related to stress perturbations caused by fault slip the structures associated with fault terminations change from dilatant splay fractures only to splay fractures and ductile fabrics on opposing sides of the fault to ductile fabrics ahead of the fault with decreasing distance from a younger neighboring pluton and presumably with increased temperatures the distributions of deformation mechanisms correlate with the inferred local magnitudes and spatial distributions of the maximum tensile stress the mean stress and the maximum shear stress displacements along the faults are transferred across echelon stepovers by mineralized dilatant fractures in extensional steps to accommodate slip transfer across contractional steps rock apparently was squeezed vertically out of the step by ductile flow concentrations of mobile and immobile elements indicate that diffusive mass transfer was not significant increased mean stress in contractional steps apparently enhanced crystal plastic flow of quartz a flow law that includes a pressure effect through correlation with pressure dependent solidus temperatures is more successful than other experimentally derived flow laws in predicting the observed distribution of ductile fabrics

footwall rocks to the mid tertiary chemehuevi detachment fault a window into the middle crust in the southern cordillera crystalline rocks unroofed from the middle crust during tertiary crustal extension by the chemehuevi detachment fault record a complex history of comprises both subhorizontal and subvertically foliated mylonitic gneisses sharing a common gently sw plunging stretching lineation which are intruded by members of the voluminous laccolith shaped chemehuevi mountains plutonic suite proterozoic and mesozoic gneisses migmatites and early members of the chemehuevi mountains plutonic suite in the eastern chemehuevi mountains are deformed into a gently southwest dipping zone of protomylonite mylonite and locally ultramylonite in excess of 1 5 km thick a belt of steeply foliated mylonitic gneiss up to 5 km across traceable for 12 km along strike ne sw crops out in the northern part of the range this belt passes abruptly downward into the subhorizontal shear zone exposed in the eastern chemehuevi mountains a preliminary shear sense study indicates ne directed shear within the sub horizontal mylonitic rocks and sinistral shear parallel to the steeply foliated mylonites deformation in these two zones took place under upper greenschist to middle amphibolite facies conditions mylonitic deformation associated with the two shear zones was spanned by intrusion of the chemehuevi mountains plutonic suite cross cutting contacts within the suite indicate that ductile deformation occurred after intrusion of hornblende biotite granodiorite four zircon fractions from this unit are discordant but define a chord with a lower intercept of around 90 ma and an upper intercept of 1900 ma thirteen zircon fractions from two separate undeformed members of the suite biotite granodiorite and porphyritic biotite granodiorite are variably discordant but fractions for each unit define chords that yield latest cretaceous crystallization ages with upper intercepts of 1600 ma together these data constrain the timing of plutonism and ductile deformation with associated thick zones of fabric reorientation and mylonitic transposition to the late cretaceous superimposed on this fabric and involving all rocks types in the footwall are the localized brittle and ductile effects of the mid tertiary extensional deformation movement direction and the timing of mylonitic deformation in the chemehuevi mountains suggest that the effects of the cordilleran fold and thrust belt extends into the chemehuevi mountains region where it in part overlapped with and was heated by plutonism along the eastern margin of the mesozoic magmatic arc during late cretaceous time

tectonics of slow spreading mid ocean ridges and consequences of a variable depth to the brittle ductile transition geophysical evidence especially microearthquakes that extend into the upper mantle beneath the inner floor of the mid atlantic and gorda ridges indicates that the axial magma chamber episodically freezes beneath slow spreading ridges freezing of the axial magma chamber will result in a variable thermal structure that has important tectonic consequences most importantly large near axis faults will intersect as the brittle ductile transition deepens causing some of the faults to become locked the topographic effect of fault locking will be a variation in the width of the inner floor from narrow 10 km when a magma chamber is absent the deep microearthquakes also imply that faults at slow spreading ridges extend into the upper mantle the nature of these faults at depth is unknown but the following hypotheses are presented 1 the faults may be planar and pass downward into aseismic ductile shear zones and 2 the faults may sole into a flat detachment fault defining a sharp boundary between brittle faulting and homogeneous ductile flow such low angle normal faults are common in continental extensional domains large scale tilting 50 70 is probably common at slow spreading ridges and implies either listric faulting or rotation of planar faults and fault blocks similar to toppling dominoes calculations of temperatures at the brittle ductile transition are made assuming olivine and diabase rheologies for the mantle and crust respectively the temperature is approximately 800 c when the transition is at 8 km depth and approximately 600 when the transition is at 2 km depth the strength of the lithosphere varies dramatically with the rise and fall of the brittle ductile transition as magma chambers form and subsequently freeze strong lateral temperature gradients will be present when a magma chamber is formed resulting in localization of faulting within thin weak crust above the chamber when the magma chamber freezes lateral temperature gradients will be small and the strength of the lithosphere will be similar over a wide area this results in active faulting occurring over a width of as much as 60 km some ophiolites possibly provide insights to the tectonic processes at slow spreading ridges the entire crustal section of the josephine ophiolite was tilted by approximately 50 prior to deposition of overlying pelagic sediments this tilting probably took place by faulting suggesting that oceanic faults extended into the upper mantle possible oceanic faults have been identified in the josephine ophiolite but their oceanic origin cannot be established beyond doubt

p wave complexity and fault asperities the borrego mountain california earthquake of 1968 results from a synthetic seismogram analysis of the short period p waves from the borrego mountain earthquake of 9 april 1968 m l 6 4 are used to model the strong motion recording at el centro a short period long period deconvolution analysis of the teleseismic p waves suggested that a two source model would fit the data much better than the single source model presented by burdick and mellman 1976 forward and inverse modeling of the data demonstrated that two sources each of less than 2 sec duration the second occurring 2 2 sec after the first and both being at about 8 km depth best fit the short period waveforms from this model long period synthetics were generated which were found to be quite compatible with the data this source model was also used to synthesize the strong motion sh displacement velocity and acceleration records from el centro california the close match of synthetics and data is used to argue that short period waveforms contain much information about asperities which play a crucial role in the near source strong motions from an earthquake the borrego mountain event probably began with the failure of a fault asperity the evidence for this is the several hundred bars stress drops of the two short period sources and the probable location of these sources in a place where there was almost no aftershock activity or postseismic creep on the fault

stable isotope geochemistry and formation mechanisms of quartz veins extreme paleoaltitudes of the central alps in the neogene quartzveins ranging in size from less than 50 cm length and 5 cm width to greater than 10 m in length and 5 m in width are found throughout the central swiss alps in some cases the veins are completely filled with milky quartz while in others sometimes spectacular void filling quartz crystals are found the style of vein filling and size is controlled by host rock composition and deformation history temperatures of vein formation estimated using stable isotope thermometry and mineral equilibria cover a range of 450 c down to 150 c vein formation started at 18 to 20 ma and continued for over 10 my the oxygen isotope values of quartz veins range from 10 to 20 permil and in almost all cases are equal to those of the hosting lithology the strongly rock buffered veins imply a low fluid rock ratio and minimal fluid flow in order to explain massive nearly monomineralic quartz formation without exceptionally large fluid fluxes a mechanism of differential pressure and silica diffusion combined with pressure solution is proposed for early vein formation fluid inclusions and hydrous minerals in late formed veins have extremely low d values consistent with meteoric water infiltration the change from rock buffered static fluid to infiltration from above can be explained in terms of changes in the large scale deformation style occurring between 20 and 15 ma the rapid cooling of the central alps identified in previous studies may be explained in part by infiltration of cold meteoric waters along fracture systems down to depths of 10 km or more an average water flux of 0 15 cm3 cm 2yr 1 entering the rock and reemerging heated by 40 c is sufficient to cool rock at 10 km depth by 100 c in 5 million years the very negative d values of

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

water diffusion in quartz at high pressure tectonic implications an indirect method based on the effect of water on plastic deformation mechanisms in quartz is used to estimate water diffusion penetration depths at high pressure diffusivities calculated by this method are orders of magnitude greater than those determined at low pressure and high pressure activation energy for water diffusion is three to five times greater than low pressure values this implies very temperature sensitive hydrolytic weakening at deep crustal and upper mantle depths potentially leading to a sharp mechanical decoupling of the lithosphere

the near source ground motion of the 6 august 1979 coyote lake california earthquake a finite fault striking n24 w and extending to a depth of 10 km is proposed to explain the strong ground motion data for the 6 august 1979 coyote lake california earthquake m l 5 9 our source model suggests that right lateral faulting initiated at a depth of 8 km and ruptured toward the south with a velocity of 2 8 km sec this unilateral rupture can explain the large displacement recorded south of the epicenter however the waveform coherency across an array south and southwest of the epicenter suggests that the rupture length is less than 6 km the maximum dislocation is about 120 cm in a small area near the hypocenter and the total moment is estimated to be 3 5 10 24 dyne cm an abrupt stopping phase which corresponds to a deceleration of right lateral motion can explain the high peak acceleration recorded at array station 6 the stress drop in the hypocentral area is about 140 bars the average stress drop over the entire rupture surface is 30 bars the preferred finite source model can predict the p n1 waveforms and the beginning features in the teleseismic seismograms no clear arrivals can be observed in the near source data for the possible second and third smaller events suggested by nabelek personal communication

kinematics of the pacific north america plate boundary zone northern california we measured motions of 54 sites in an east west transect across northern california at 38 40 north by global positioning system gps observations over a 4 year we estimate the total slip rate on the san andreas fault system to be 39 6 0 6 1 5 mm yr 68 6 upper and lower confidence intervals from a nonlinear inversion are indicated by superscripts and subscripts slip rates on the individual faults are determined less precisely due to the high correlations between the estimated parameters our best fitting model fits the fault parallel velocities with a mean square error of 1 04 and the following estimated fault slip rates all in mm yr san andreas 17 4 3 1 2 5 ma acama 13 9 2 8 4 1 and bartlett springs 8 2 1 9 2 1 the data are fit best by models in which the san andreas fault is locked to 14 9 7 1 12 5 km the ma acama fault locked to 13 4 4 8 7 4 km except for shallow creep in the upper 5 km and the bartlett springs fault creeping at all depths the ma acama fault most likely poses a significant seismic hazard as it has a high slip rate and has accumulated a slip deficit large enough to generate a magnitude 7 earthquake we find little evidence for contraction across coast ranges except at western edge of great valley where 1 3 mm yr of shortening is permitted by the data no strain is observed within the great valley or sierra nevada except that associated with right lateral strike slip on the san andreas fault system this is consistent with models of the pacific north america plate boundary zone in which the relative plate motion is partitioned into two domains one strike slip and one dominantly extensional separated by the elastically deforming sierra nevada great valley block

on a possible connection between three major earthquakes in california and oil production abstract the 1983 coalinga m 6 5 the 1985 kettleman north dome m 6 1 and the 1987 whittier narrows m 5 9 earthquake sequences each occurred beneath major producing oil fields and caused an aftershock sequence whose epicentral extent coincides approximately with that of the overlying oil field despite this coincidence a causal relationship between these sequences and oil production has been discounted because the earthquakes were located at depths of the order of 10 km whereas oil production is confined to the topmost several km moreover because all three oil fields coalinga kettleman north dome and montebello are situated on anticlines that have undergone recent uplift as a consequence of horizontal tectonic compression of the crust natural earthquakes are not unexpected a mechanical connection between oil production and the earthquake sequence is suggested however by the observation that in each case the total seismic deformation was just that required to offset the force imbalance caused by oil production specifically for each of the three sequences m 0 2 m c where m 0 is the sum of the seismic moments is the modulus of rigidity m is the mass of the fluid removed due to oil production is the fraction of the upper crust that is seismogenic and c is the average upper crustal density in essence net extraction of oil and water reduces slightly the average density of the upper crust causing an isostatic imbalance the ductile lower crust deforms in response to this imbalance thus increasing the load on the seismogenic layer which fails seismically to thicken the crust so as to restore static equilibrium locally accordingly earthquakes near the base of the upper crust may be an expected outcome of major oil production from growing anticlines irrespective of the depths of the producing formations

lower crustal deformation during terrane dispersion along strike slip faults abstract after accretion terranes caught between obliquely converging plates are often displaced several hundreds of kilometers how this motion is accommodated at depth is not well known spurred by our trans alaskan crustal transect tact seismic studies of the yukon tanana terrane alaska i propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin parallel translation the locations of these ductile regions are dependent on temperature composition and strain rate for geotherms based on moderate surface heat flow 60 90 mwm 2 and reasonable geological strain rates 100 in 3 ma typical lower crustal rocks can deform ductilely the resultant subhorizontal layered fabric should be detectable with seismic techniques two terranes that have experienced hundreds of kilometers of margin parallel translation the yukon tanana terrane sampled by our seismic refraction wide angle reflection profiles and the salinian block california sampled by both seismic refraction and reflection have reflective lower crust both of these terranes have moderate to high surface heat flows have relatively thin crust and are interpreted seismically to be underlain by a quartzo feldspathic lower crust conditions favorable for ductile deformation in the lower crust interpretations of lower crustal layering for these two terranes is based on an increase in lower crustal reflectivity this layering is attributed to tectonic banding consistent with ductile deformation

quaternary flake tectonics of the california transverse ranges p residual studies suggest that the pacific american plate boundary in the upper mantle lies in the central mojave desert northeast of its crustal position at the san andreas fault focal mechanisms of some relatively deep microearthquakes suggest a flattening with depth of north dipping thrust faults resulting in a mid crustal detachment in which the central transverse ranges move south relative to the mantle this makes sense geologically if the central transverse ranges and western mojave desert are considered as tectonic flakes of brittle crystalline rocks underlain by ductile pelona schist or catalina schist which makes up the zone of decollement such ductile schists would occur today at depths comparable to those at which they were metamorphosed the detachment breaks through to the surface at the point dume malibu coast fault zone and the red mountain san cayetano santa susana sierra madre cucamonga fault zone the latter coinciding with the southern edge of the palmdale bulge the transverse ranges flake now moves west northwest relative to the western mojave flake south southwest relative to the pacific plate and northwest relative to the great valley sierra nevada the mantle boundary may rejoin the san andreas fault north of a southwest trending zone of high seismicity in the tehachapi mountains which may be produced by convergence the age of the present tectonic system may be the same as the age of the break through thrust faults these are probably no older than 2 m y and may be even younger

basement faulting and the focal depths of the larger earthquakes in the zagros mountains iran summary the present day seismicity of the zagros seems to occur on high angle reverse faults distributed across the whole width of the belt it does not indicate activity on a single inclined thrust surface and there do not seem to have been any well located events at intermediate depths modelling of the long period teleseismic body waves of seven large earthquakes presented here shows their focal depths to be in the range 8 15 km this is thought to indicate faulting in the uppermost basement beneath the sedimentary cover though the absence of published seismic refraction results renders the sediment thickness uncertain faulting of this type and depth may occur on inherited normal faults which have subsequently been reactivated as thrusts such reactivation allows considerable shortening to take place in the early stages of continental collision without the subduction or excessive thickening of continental crust

electrical conductivity variations around the palmdale section of the san andreas fault zone our analysis of a decade of observations from the palmdale california telluric array is presented the array is a system of large scale telluric field measurements used for the passive monitoring of electrical conductivity variations over a large region centered over the san andreas fault zone northeast of los angeles since 1985 systematic changes in the relationships between the telluric field measurements around the array have been observed these changes amounted to 1 to 2 by the time the array was closed down in 1989 with the fields measured along the fault and northeast of the fault showing signal decreases relative to the fields southwest of the fault because of the possible implications of these changes for conductivity variations related to tectonic activity on the san andreas we have attempted to sort out the geography of the zones that could cause the changes the simplest though nonunique interpretation of the observed variations in the telluric relationships is a steady increase in the conductivity of the lower crustal region of the fault zone from two dimensional models of the telluric response around the array we infer that an increase of about 60 milli siemens in the conductance of the lower crustal fault zone could account for the changes seen over the period 1985 to 1989 assuming changes were in a kilometer wide zone and that the lower crustal fluids were very conductive this could represent a strain of about 0 4 10 6 the surface strain would be much smaller and would be lost in surface strain noise because of the possibility that fluid pressure changes are involved these small strains could still be significant for earthquake cycle phenomena alternatively the observed changes could have been produced by increases in the conductivity of the upper crust of the mojave desert northeast of the fault or by decreases in the conductivity of the upper crust in the san gabriel mountains southwest of the fault to produce the observed telluric changes however the additional conductivity and the associated strain changes would have to be much larger the strain needed to accommodate the increased conductivity on the mojave side would have to be of the order of 6 10 4 which seems unrealistically high smaller strains could account for conductivity changes in the san gabriel mountains but these strains would have to be compressive water level changes are not considered to be a possible cause of the conductivity changes because the measurement scale is much larger than the typical scale of water level changes and no seasonally correlated changes in the telluric fields were observed

the structure of an exhumed intraplate seismogenic fault in crystalline basement abstract the 600 m thick gole larghe fault zone glfz is hosted in jointed crystalline basement and exposed across glacier polished outcrops in the italian alps ancient seismicity is attested by the widespread occurrence of cataclasites associated with pseudotachylytes solidified frictional melts formed at 9 11 km depth and ambient temperatures of 250 300 c previous work focused on the southern part of the fault zone here we quantitatively document fault zone structure across the full width of the glfz and surrounding tonalite host rocks by using a combination of structural line transects and image analysis of samples collected across fault strike these new datasets indicate that the glfz has a broadly symmetric across strike damage structure and contains distinct southern central and northern zones distinguished by large variations in fracture density distribution of pseudotachylytes volume of fault rock materials and microfracture sealing characteristics the c 100 m wide central zone is bound by two thick 2 m and laterally continuous 1 km protocataclastic to ultracataclastic horizons within and immediately surrounding the central zone fracture density is relatively high due to cataclastic fault fracture networks that reworked earlier formed pseudotachylytes the fault fracture networks were associated with pervasive microcracking and fluid rock interaction resulting in the development of a c 200 m thick alteration zone delimited by lobate fluid infiltration fronts in the c 250 m thick southern and northern zones fracture densities are much lower and pseudotachylytes systematically overprint cataclastic faults that exploited pre existing magmatic cooling joints analysis of the structure of the glfz suggests that it shares certain characteristics with the seismogenic source responsible for the 2002 au sable forks intraplate earthquake sequence in the northeastern usa including seismicity distributed across a fault zone 500 1000 m thick and large 100 mpa static stress drops associated with frictional melting

theory of time dependent rupture in the earth at a velocity x which is a well defined function of k this propagation is stable and quasi static and is referred to as subcritical crack growth we shall use the terms stable quasistatic and subcritical to mean propagation at velocities much less than the sonic velocities of the medium this behavior results from stress induced corrosion at the crack tip the principal corrodent for the present application being h20 this behavior has been firmly established in the laboratory for mode i tensile cracks in a wide variety of materials ineluding silicates and silicate glasses see cholz 1968a 1972a martin 1972 wiederhorn and bolz 1970 atkinson 1979 lawn and wilshaw 1975 knott 1973 the form of the relationship between k and x does not vary significantly with the material only the parameters in the law vary as an example we show in figure 1 data on subcritical crack growth in quartz after atkinson 1979 the empirical relationship is found to take the form