emplacement of mantle peridotite in the lower continental crust ivrea verbano zone northwest italy the ivrea verbano zone has been cited repeatedly as an exposed pre alpine crust mantle transition on the basis in part of the presence of lenses of mantle peridotite and a possible petrologic moho a 10 km thick mafic complex has been described as an intrusion at the contact between the mantle and a thick crustal sequence of metamorphosed wacke and pelitic rocks the significance of these features is now challenged by new mapping that demonstrates that mantle peridotites in the southern ivrea verbano zone were lenses tectonically interfingered with metasedimentary rocks prior to intrusion of the gabbroic complex although it remains possible that the ivrea verbano section exposes part of a complex crust mantle interface within which crustal and mantle rocks were interleaved the association of peridotite and metasedimentary rocks is also consistent with assembly in an accretionary wedge in either case present exposures are from an unknown distance above the pre alpine contiguous mantle and reference to the section as a complete crust mantle transition could be misleading

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

new observational evidence of global seismic effects of basin forming impacts on the moon from lunar reconnaissance orbiter lunar orbiter laser altimeter data 1 new maps of kilometer scale topographic roughness and concavity of the moon reveal a very distinctive roughness signature of the proximal ejecta deposits of the orientale basin the hevelius formation no other lunar impact basin even the just preceding imbrium basin is characterized by this type of signature although most have similar types of ejecta units and secondary crater structures the preservation of this distinctive signature and its lack in basins formed prior to orientale is interpreted to be the result of seismically induced smoothing caused by this latest major basin forming event intense seismic waves accompanying the orientale basin forming event preceded the emplacement of its ejecta in time and operated to shake and smooth steep and rough topography associated with earlier basin deposits such as imbrium orientale ejecta emplaced immediately following the passage of the seismic waves formed the distinctive roughness signature that has been preserved for almost 4 billion years

evolution of pangea paleomagnetic constraints from the southern alps italy abstract a new early late triassic paleopole for adria has been obtained from the val sabbia sandstone in the southern alps as early permian and jurassic cretaceous paleomagnetic data from para autochthonous regions of adria such as the southern alps are consistent with african apwps 1 2 paleomagnetic data from this region can be used to bolster the west gondwana apwp in the poorly known late permian triassic time interval the southern alpine paleopoles are integrated with the west gondwana and laurussia apwps of van der voo 1 and used to generate a tectonic model for the evolution of pangea the early permian overall mean paleopole for west gondwana and adria in conjunction with the coeval laurussia paleopole support pangea b of morel and irving 3 the late permian early triassic and the middle late triassic paleopoles from adria and laurussia support pangea a 2 of van der voo and french 4 the phase of transcurrent motion between laurasia and gondwana 5 that caused the pangea b to a 2 transition occurred essentially in the permian at the end of variscan orogeny with an average relative velocity of approximately 10 cm yr finally the late triassic early jurassic paleopoles from west gondwana and laurussia agree with pangea a 1 of bullard et al 6 the widely accepted pangea configuration at the time of the jurassic breakup

crustal fracturing and chert dike formation triggered by large meteorite impacts ca 3 260 ga barberton greenstone belt south africa the ca 3260 ma contact between the largely volcanic onverwacht group and overlying largely sedimentary fig tree group in the barberton greenstone belt south africa is widely marked by chert dikes that extend downward for up to 100 m into underlying sedimentary and volcanic rocks of the mendon formation onverwacht group in the barite valley area these dikes formed as open fractures that were filled by both precipitative fill and the downward flowage of liquefied carbonaceous sediments and ash at the top of the mendon formation spherules that formed during a large meteorite or asteroid impact event occur in a wave and or current deposited unit spherule bed s2 which widely marks the onverwacht fig tree contact and as loose grains and masses within some chert dikes up to 50 m below the contact four main types of chert dikes and veins are recognized type 1 irregular dikes up to 8 m wide that extend downward across as much as 100 m of stratigraphy type 2 small vertical dikes most 1 m wide which are restricted to the lower half of the mendon chert section type 3 small crosscutting veins most 50 cm across filled with precipitative silica and type 4 small irregular to bedding parallel to irregular veins mostly 10 cm wide filled with translucent precipitative silica type 2 dikes formed first and reflect a short lived seismic event that locally decoupled the sedimentary section at the top of the mendon formation from underlying volcanic rocks and opened narrow vertical tension fractures in the lower lithified part of the sedimentary section later seismic events triggered formation of the larger type 1 fractures throughout the sedimentary and upper volcanic section widespread liquefaction of soft uppermost mendon sediments and flowage of the liquefied sediments and loose impact generated spherules into the open fractures late stage tsunamis everywhere eroded and reworked the spherule layer the coincidence of crustal disruption dike formation spherule deposition and tsunami activity suggests that all were related to the s2 impact or impact cluster crustal disruption at this time also formed local relief that provided clastic sediment to the postimpact fig tree group including a small conglomeratic fan delta in the barite valley area remobilization and further movement of debris in the subsurface continued for some time locally the deposition of dense baritic sediments over soft dike materials induced remobilization of material in the dike causing foundering of s2 and 1 2 m of overlying baritic sediments into the dike spherule beds occur at the base of the fig tree group over wide areas of the barberton belt marking the abrupt change from 300 m y of predominantly anorogenic mafic and komatiitic volcanism of the onverwacht group to orogenic clastic sedimentation and associated felsic volcanism of the fig tree group this area never again returned to onverwacht style mafic and ultramafic volcanism but evolved 100 m y later into the kaapvaal craton these results indicate that this major transition in crustal evolution coincided with and was perhaps triggered by major impact events ca 3260 3240 ma

a late triassic age for the rochechouart impact structure france argon 40 argon 39 laser spot fusion dating of pseudotachylyte from the 25 km diameter roche chouart impact structure of western central france yields a matrix age of 214 c 8 ma 20 field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process prob ably during transient cavity collapse this new age is considerably older than the previously accepted age of 186 8 ma for this structure which was obtained from hydrothermally altered melt sheet samples the new age is in accordance with earlier paleomagnetic and fission track data which indicated that rochechouart was formed during the late triassic moreover the new determination is in agreement with the regional geolog ical setting and field relations of the structure the new age of 214 2 8 ma falls within the norian stage of the triassic system

large scale folding in the upper part of the ivrea verbano zone nw italy new geological mapping has led to a new interpretation of the large scale superimposed folding in the upper part of the ivrea verbano zone italian alps the region is widely held to represent an upended section through lower continental crust of northern italy the dominant fold structure extending some 40 km along strike is the massone tight to isoclinal antiform with a hinge line strongly curved through 115 this folds pre existing large scale folds that formed during regional migmatization probably during the hercynian orogeny to form a type 2 interference geometry the region then suffered post orogenic mafic magmatic underplating and other magmatism accompanied by crustal stretching with contact metamorphism and migmatization causing the imposition of the final pattern of metamorphic isograds the ivrea verbano zone was brought into contact with the overlying metamorphic rocks of the serie dei laghi on a major shear zone sub solidus stretching continued though displacements on low angle high temperature shear zones most of the ivrea verbano zone was finally tilted to the vertical and emplaced into its present position after the mesozoic era and probably during alpine orogenesis forming the vertical limb of a crustal scale double kink

the finnefjeld domain maniitsoq structure west greenland differential rheological features and mechanical homogenisation in response to impacting the 35 by 50 km large mesoarchaean finnefjeld domain near maniitsoq in the north atlantic craton of southern west greenland constitutes the central part of the previously proposed deeply eroded maniitsoq impact structure with an age of 3 0 ga the finnefjeld domain is an exceedingly homogeneous quartzo feldspathic rock mass which superficially appears to be a late orogenic deep crustal intrusive granitoid pluton and which was described as such for decades however new observations confirm and qualify the first observations from 1962 of these rocks as cataclastic the finnefjeld domain is characterised by a highly unusual mixed rheological behaviour plagioclase displays brittle behaviour with cataclasis quartz was ductilely deformed and k feldspar was melted the deformation and homogenisation of the finnefjeld domain was caused by an intense event of heating and deformation which was coseismic in nature and comprised numerous increments of pure shear strain this type of intense brittle regional deformation and concomitant direct mineral melting in the deep crust is unknown from endogenic orogenic events and is ascribed to deep crustal effects of impacting c 2014 elsevier b v all rights reserved

pulverized fault rocks and damage asymmetry along the arima takatsuki tectonic line japan abstract we present field and laboratory data on pulverized rocks at the hakusui kyo outcrop of the arima takatsuki tectonic line attl which is a dextral strike slip fault with 17 km displacement juxtaposing granite to the south against rhyolite to the north the majority of slip at the surface is localized to a clay rich gouge fault core 8 10 cm in width surrounded by a coarsening outwards fault breccias up to 3 m wide fault damage is highly asymmetric with respect to the slipping zone the granite south of the fault has a pulverized damage zone up to 200 m wide while the rhyolite to the north has only about 3 m wide non pulverized fault breccia the degree of pulverization in the granite decreases approximately logarithmically with normal distance from the slip zone the highly fractured pulverized rocks exhibit several distinct textural characteristics in thin section grains appear to be highly comminuted but the original grain shapes and margins are recognizable microfractures tend to be tensile in no preferred orientation grain fragments display little to no rotation and lack evidence of in situ shear consequently at macroscale the rocks appear to preserve original granitic textures despite being highly fractured and friable the observed pulverization and rock damage asymmetry are most consistent with generation mechanism involving ruptures on a bimaterial interface with statistically preferred propagation direction leading to damage primarily on the side with higher seismic velocity at depth this is supported by laboratory measurements of p wave ultrasonic velocities on intact samples which indicate that the granites have consistently higher velocity than the rhyolite with increasing confining pressure

using principal components analysis and na cl ratios to trace groundwater circulation in a volcanic island the example of reunion abstract the development of the limited groundwater resources of a remote tropical island in particularly rugged volcanic terrain requires a thorough understanding of the hydrogeology the cost of a classical approach by borehole investigations often precludes a reasoned approach resulting in a somewhat haphazard development of resources wuth varying success this paper describes the use of multivariate statistical analysis to trace groundwater circulation in volcanic terrains the method involves the use of principal components analysis pca based on both structural and hydrochemical parameters of 243 springs of reunion western indian ocean as a result we propose the use of na cl analysis in differentiating three types of volcanic aquifers this analysis is consistent with a geological and hydrogeological conceptual model developed from a combined hydrochemical and geological reconnaissance of 27 springs

chemical and physical characteristics of pulverized tejon lookout granite adjacent to the san andreas and garlock faults implications for earthquake physics we present new detailed analyses of samples of pulverized tejon lookout granite collected from sections adjacent to the san andreas and garlock faults in southern california the tejon lookout granite is pulverized in all exposures within about 100 m from both faults chemical analyses indicate no or little weathering in the collected samples although xrd analysis shows the presence of smectite illite and minor kaolinite in the clay size fraction weathering products may dominate in the less than 1 micron fraction the average grain size in all samples of pulverized tejon lookout granite ranges between 26 and 208 microns silt to fine sand with the particle size distribution in part a function of proximity to the primary slip zone the san andreas fault samples that we studied are generally finer grained than those collected from adjacent to the garlock fault the particle size distribution for each studied sample from both faults follows a pseudo power law with a continuously changing exponent which suggests that pulverization is not simply a consequence of direct shear the average particle size that we determined for our samples is considerably coarser than reported in previous investigations which we attribute to possible measurement errors in the prior work our data and observations suggest that dynamic fracturing in the wall rock of the san andreas and garlock faults only accounts for about 1 or less of the earthquake energy budget

structural geometry lower crustal magmatic underplating and lithospheric stretching in the ivrea verbano zone northern italy abstract the ivrea verbano zone is believed to represent an up ended cross section through the lower continental crust as it existed at the end of the hercynian orogeny in the southern alps structurally oriented geological mapping has been carried out in the central and northern parts of the ivrea verbano zone a region some 30 km along strike the geology of the southern half of the zone is dominated by a large basic ultrabasic complex the mafic formation which is in contact with a strongly banded metasedimentary metabasic sequence to the north particular attention was paid to a the structural relationship between the rocks of the mafic formation and their envelope of high grade metasedimentary and metabasic rocks b the geometrical configuration throughout the ivrea verbano zone of high temperature shear zones which accommodated post hercynian crustal extension and c the geometry of late low temperature faulting the effects of which have been removed in order to produce a restoration of the structure as it existed during the post hercynian extensional phase the intrusion of large volumes of basic magma ca 50 of the outcrop area to form the rocks of the mafic formation appears to be coeval with the onset of extension ca 280 ma the main basic body has a laccolithic form which was originally more than 10 km thick overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot immediately subsolidus or partially molten body incorporating its envelope of hot metasediments into a large originally recumbent fold a geometrical association with a high temperature low angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion the ivrea verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating which may aid in the interpretation of present day deep seismic profiles it also demonstrates the geometry of a network of conjugate high temperature low angle shear zones in a well layered lower crustal section

expected seismic shaking in los angeles reduced by san andreas fault zone plasticity computer simulations of large m 7 8 earthquakes rupturing the southern san andreas fault from se to nw e g shakeout widely used for earthquake drills have predicted strong long period ground motions in the densely populated los angeles basin due to channeling of waves through a series of interconnected sedimentary basins recently the importance of this waveguide amplification effect for seismic shaking in the los angeles basin has also been confirmed from observations of the ambient seismic field by simulating the shakeout earthquake scenario based on a kinematic source description for a medium governed by drucker prager plasticity we show that nonlinear material behavior could reduce the earlier predictions of large long period ground motions in the los angeles basin by up to 70 as compared to viscoelastic solutions these reductions are primarily due to yielding near the fault although yielding may also occur in the shallow low velocity deposits of the los angeles basin if cohesions are close to zero fault zone plasticity remains important even for conservative values of cohesions suggesting that current simulations assuming a linear response of rocks are overpredicting ground motions during future large earthquakes on the southern san andreas fault

the 1971 san fernando earthquake a double event evidence is presented which suggest that the 1971 san fernando earthquake may have been a double event that occurred on two separate subparallel thrust faults it is postulated that the initial event took place at depth on the sierra madre fault zone which runs along the base of the san gabriel mountains rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km a second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the sierra madre fault zone the surface trace of this fault coincides with the san fernando fault zone which was the principal fault associated with surface rupture it is postulated that rupture propagated from a depth of 8 km to the free surface the moments of the first and second events are approximately 0 7 10 26 dyne cm and 1 0 10 26 dyne cm respectively this model is found to explain the combined data sets of strong ground motions teleseismic p and s waveforms and static offsets better than previous models which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth nevertheless many features of the observed motions remain unexplained and considerable uncertainty still exists regarding the faulting history of the san fernando earthquake

characterization of pulverized granitoids in a shallow core along the san andreas fault littlerock ca summary we present results on the composition structure and particle size distribution psd of pulverized and damaged granitic rocks in a 42 m deep core adjacent to the san andreas fault near littlerock ca the cored section is composed of pulverized granites and granodiorites and is cut by numerous mesoscopic secondary shears the analysis employs xrd xrf thin sections and semi automated particle size analyser methods including a novel calibration method the mean particle size for the majority of samples falls between 50 and 470 m the psds can be fitted by a power law with d values ranging between 2 5 and 3 1 as well as by a superposition of two gaussians fracture surface energy calculations based on the observed particle distributions provide very low values indicating that the part of the total earthquake energy budget expended for breaking or shattering rocks is small shear deformation is likely to dominate near secondary faults the most pronounced fault related alteration occurs along the secondary shears and is a function of both composition and depth the alteration to clay appears to be the result of fluid rock interaction and brittle deformation under low temperature conditions rather than of surface related weathering the particle size reduction noted in the core reflects multiple mechanisms of comminution the zones of pulverization that lack significant weathering likely result from repeating episodes of dynamic dilation and contraction

moderate earthquake teleseismic depth estimations new methods and use of the comprehensive nuclear test ban treaty organization network data abstract we investigate the possibility to estimate the depths of earthquakes of moderate magnitude 3 5 m m b 3 6 ardeche earthquake france 3 august 2011 in the case of the po plain earthquake italy 27 january 2012 this provides another example of the great interest in teleseismic analysis where the detection of the moho reflected phase pmp shows without doubt an upper mantle event finally the murcia event spain 11 may 2011 shows the limits of these methods for very shallow events

physics of crustal fracturing and chert dike formation triggered by asteroid impact 3 26 ga barberton greenstone belt south africa archean asteroid impacts reflected in the presence of spherule beds in the 3 2 3 5 ga barberton greenstone belt bgb south africa generated extreme seismic waves spherule bed s2 provides a field example it locally lies at the contact between the onverwacht and fig tree groups in the bgb which formed as a result of the impact of asteroid possibly 50 km diameter scaling calculations indicate that very strong seismic waves traveled several crater diameters from the impact site where they widely damaged onverwacht rocks over much of the bgb lithified sediments near the top of the onverwacht group failed with opening mode fractures the underlying volcanic sequence then failed with normal faults and opening mode fractures surficial unlithified sediments liquefied and behaved as a fluid these liquefied sediments and some impact produced spherules filled near surface fractures today represented by swarms of chert dikes strong impact related tsunamis then swept the seafloor p waves and rayleigh waves from the impact greatly exceeded the amplitudes of typical earthquake waves the duration of extreme shaking was also far longer probably hundreds of seconds than that from strong earthquakes dynamic strains of 10 3 occurred from the surface and downward throughout the lithosphere shaking weakened the onverwacht volcanic edifice and the surface layers locally moved downhill from gravity accommodated by faults and open mode fractures coast parallel opening mode fractures on the fore arc coast of chile formed as a result of megathrust events are the closest modern analogs it is even conceivable that dynamic stresses throughout the lithosphere initiated subduction beneath the onverwacht rocks

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

microearthquake spectra from the anza california seismic network site response and source scaling abstract we analyzed spectra of local microearthquakes recorded by the anza california seismic network to isolate the effects of site response and to investigate the scaling of source parameters for small earthquakes spectra of microearthquakes m m o 19 dyne cm at anza have shapes characteristic of the receiver sites and are generally independent of the source region thus the site response is a major conditioner of the observed spectral shape to remove the effects of site response from the spectra of a m 3 event and isolate its source spectrum we divided by the spectra of an adjacent aftershock used as an empirical green9s function event the spectral ratios indicate that the apparent corner frequencies of small earthquakes m o 19 dyne cm observed at even the high f max stations on hard rock are much lower than the source corner frequencies the spectral ratios are consistent with stress drop remaining constant with decreasing seismic moment for events with moments as small as 10 18 dyne cm the spectral ratios display remarkable agreement between sites which showed vast differences in their original spectra indicating that the spectral division effectively removed the site response the source spectrum of the m 3 event has a high frequency spectral fall off of about 2 an apparent dependence of high frequency fall off with seismic moment in the original spectra can also be explained by the effects of site response the difference between the p and s wave corner frequencies and high frequency roll offs in the observed spectra for these events is the result of the site response and is not a source property the shapes of the spectra of microearthquakes at anza can largely be explained by attenuation at shallow depth with a frequency independent q for some sites near surface resonances are also apparent in the spectra of microearthquakes it is indicated by t values determined for each site that q p q s for the shallow low q layer further evidence of low near surface q is observed in the anomalous spectra of an unusually shallow earthquake source depth 1 km in the network the spectra from this shallow event are depleted in high frequency energy at most stations relative to those of deeper events this observation can be explained by a low q surficial zone for stations of the network situated on alluvium this low q layer has a maximum thickness of about 3 km and maximum p and s wave q values of 30 to 50

source parameters for small events associated with the 1986 north palm springs california earthquake determined using empirical green functions using small events as empirical green functions which include path site and instrument effects on the p waveforms source parameters were estimated for 25 ml 3 4 to 4 4 events associated with the 1986 north palm springs earthquake the static stress drops ranged from 3 to 80 bars for moments of 0 7 to 11 1021 dyne cm there was a spatial pattern to the stress drops of the aftershocks which showed increasing values along the fault plane toward the northwest compared to relatively low values near the hypocenter of the mainshock the highest values were outside the main area of slip and are believed to reflect a loaded area of the fault that still has an higher level of stress which was not released during the main shock the relatively high stress drop of a preshock in 1983 located close to the hypocenter of the main shock is also interpreted as evidence that stress drops of magnitude 3 and 4 events can be used as indicators of the stress level in a particular area

shock melting of k feldspar and interlacing with cataclastically deformed plagioclase in granitic rocks at toqqusap nunaa southern west greenland implications for the genesis of the maniitsoq structure abstract folded sheets of mesoarchaean leucocratic plagioclase k feldspar mesoperthite bearing granitic rocks in the toqqusap nunaa area of the maniitsoq structure west greenland are characterised by their very fine grain sizes and microstructures without normal igneous or planar linear tectonic fabrics quartz forms equidimensional and branching ductilely deformed aggregates and bifurcating panels with protrusions constrictions and chains of ball shaped grains with healed radiating intergranular fractures plagioclase an10 20 was cataclastically deformed and comminuted whereas k feldspar and mesoperthite are devoid of cataclastic microstructures k feldspar forms dispersed highly irregular grains with numerous cusps and saddles indicating almost ubiquitous direct shock melting of this mineral it is commonly located along former fractures in plagioclase resulting in an interlaced feldspar microstructure with contact shapes indicating subsequent melting of plagioclase directly adjacent to k feldspar mesoperthite forms separate rounded and irregular grains with protrusions and cusped margins indicating crystallisation from melts some mesoperthite grains are texturally and compositionally heterogeneous and contain internal lenses of k feldspar and or plagioclase other mesoperthite grains comprise coarsened unzipped areas presumably due to localised fluid controlled dissolution reprecipitation processes the ternary feldspar precursor of the mesoperthite is interpreted as having crystallised from variably effectively mixed k feldspar shock melts and plagioclase contact melts direct melting of k feldspar but no whole rock melting requires shock metamorphism with a short lived temperature excursion to above the melting temperature of k feldspar 1300 c the presence of three different feldspar species and absence of chemical zonation magmatic mantling or metamorphic coronas furthermore hinders interpretations solely by means of endogenic magmatic or ultrametamorphic processes the feldspar microstructures with direct k feldspar melts in the leucocratic orthogneisses of the toqqusap nunaa area thus confirm that the recently discovered maniitsoq structure represents the deep crustal remnants of a giant mesoarchaean meteorite impact

a centennial reappraisal of the vredefort pseudotachylytes shaken not stirred by meteorite impact major pseudotachylyte zones constitute a spectacular component of the renowned c 2 023 ga vredefort impact structure south africa but it has always been difficult to explain how they were formed in his original account in 1916 shand interpreted the pseudotachylyte as due to cataclasis and frictional heating but pointed out two enigmas that have remained since there were no associated major faults and the pseudotachylyte volumes he observed were far greater than in similar rocks located within faults elsewhere on earth new observations show that the vredefort pseudotachylyte zones were indeed formed by cataclasis and frictional heating not by faulting but owing to impact induced seismic shaking initiated around temporarily loosened blocks in dendritic fracture systems progressive cataclasis of such loose blocks by intense high frequency oscillations of the country rock at the beginning of the cratering process led to size reduction rounding and comminution and frictional melting of feldspars and biotite in the comminuted parts most pseudotachylyte was thus not injected from anywhere but produced in situ the process of seismic shaking is well known from impacts on the moon and asteroids terrestrial earthquakes and nuclear tests but has largely been overlooked in terrestrial cratering except in the theoretical concept of acoustic fluidization

spatiotemporal variation of stress drop during the 2008 mogul nevada earthquake swarm we estimate stress drops for 148 shallow 6 km earthquakes in the complex 2008 mogul nevada swarm using empirical green s function derived spectral ratios near source temporary broadband seismometers deployed before the mw4 9 main shock provide high quality records of many foreshocks and aftershocks and an ideal opportunity to investigate uncertainties in corner frequency measurement as well as stress drop variation related to space time depth mechanism and magnitude we explore uncertainties related to source model measurement approach cross correlation limit and frequency bandwidth p s wave results range from 0 2 0 15 0 3 0 15 to 36 20 58 7 mpa a variation greater than the error range of each individual estimate although this variation is not explained simply by any one parameter spatiotemporal variation along the main shock fault plane is distinct coherent clusters of high and low earthquakes are seen and high foreshocks correlate with an area of reduced aftershock productivity these observations are best explained by a difference in rheology along the fault plane average s of 3 9 1 1 4 0 1 1 mpa using p s are similar to those found for earthquakes in a variety of settings implying that these shallow potentially fluid driven earthquakes do not have systematically lower than average tectonic earthquakes 4 mpa and therefore have similar or higher due to proximity to the surface expected ground motions compared to typical earthquakes the unprecedented detail achieved for these shallow small magnitude earthquakes confirms that when measured precisely is a valuable observation of physically meaningful fault zone properties and earthquake behavior

searching for giant ancient impact structures on earth the mesoarchaean maniitsoq structure west greenland a 100km scale circular region in the archaean north atlantic craton centred at 651150n 511500w near maniitsoq town in west greenland comprises a set of highly unusual geological features that were created during a single event involving intense crushing and heating and are incompatible with crustal orogenic processes th presently exposed features of the maniitsoq structure were buried 20 25 km below the surface when this event occurred at c 3ga during waning convergent orogeny these features include a large aeromagnetic anomaly a central 35x50 km2 large area of comminuted quartzo feldspathic material regional scale circular deformation widespread random fractures with feather like textures intense fracture cleavage amphibolite granite matrix breccias unrelated to faulting or intrusions formation and common fluidisation of microbreccias abundant evidence of direct k feldspar and plagioclase melting superimposed on already migmatised rocks deformation of quartz by oc4 slip formation of planar elements in quartz and plagioclase and emplacement of crustally contaminated ultramafic intrusions and regional scale hydrothermal alteration under amphibolite facies conditions the diagnostic tools employed to identify impacting in the upper crust are inadequate for structures preserved deep within the continental crust nevertheless the inferred scale strain rates and temperatures necessary to create the maniitsoq structure rule out a terrestrial origin of the structure

rupture dynamics with energy loss outside the slip zone 1 energy loss in a fault damage zone outside the slip zone contributes to the fracture energy that determines rupture velocity of an earthquake a nonelastic two dimensional dynamic calculation is done in which the slip zone is modeled as a fault plane and material off the fault is subject to a coulomb yield condition in a mode 2 crack like solution in which an abrupt uniform drop of shear traction on the fault spreads from a point coulomb yielding occurs on the extensional side of the fault plastic strain is distributed with uniform magnitude along the fault and it has a thickness normal to the fault proportional to propagation distance energy loss off the fault is also proportional to propagation distance and it can become much larger than energy loss on the fault specified by the fault constitutive relation the slip velocity function could be produced in an equivalent elastic problem by a slip weakening friction law with breakdown slip dc increasing with distance fracture energy g and equivalent dc will be different in ruptures with different initiation points and stress drops so they are not constitutive properties they are determined by the dynamic solution that arrives at a particular point peak slip velocity is however a property of a fault location nonelastic response can be mimicked by imposing a limit on slip velocity on a fault in an elastic medium

shock induced kelyphite formation in the core of a complex impact crater we present a compositional and textural analysis of shock induced microtextures in garnet porphyroblasts in migmatitic garnet cordierite biotite paragneisses from the centre of the vredefort impact structure south africa detailed imaging and major element analysis of deformation features in and adjacent to the garnet porphyroblasts record a complex heterogeneous distribution of shock effects at the microscale as the most competent silicate mineral in the assemblage with the highest hugoniot elastic limit and a wide pressure temperature stability field the porphyroblastic garnet preserves a more diverse shock deformation response compared to minerals such as quartz and feldspar which underwent more comprehensive shock metamorphism and subsequent annealing the garnet porphyroblasts display pre impact fractures that are overprinted by later intra granular hertzian and distinctive planar fractures associated with the impact event shock induced strain localization occurred along internal slip planes and defects including pre existing fractures and inclusion boundaries in the garnet symplectitic kelyphitic coronas commonly enclose the garnet porphyroblasts and inhabit intra granular fractures the kelyphite assemblage in fractures with open communication beyond garnet grain boundaries is characterized by orthopyroxene cordierite sapphirine conversely the kelyphite assemblage in closed off intra granular fractures is highly variable comprising spatially restricted combinations of a secondary garnet phase with a majoritic component al rich orthopyroxene sapphirine and cordierite the impedance contrast between garnet porphyroblasts and their inclusions further facilitated the formation of shock induced features al rich orthopyroxene coronas together the textural and mineralogical data suggest that these features provide a record of oscillatory shock perturbations initiated under confining pressure beneath the transient crater floor this occurred as the shocked rock volume underwent post shock expansion forming the core of the central uplift and was followed by variable textural re equilibration this study thus provides a microtextural and mineralogical perspective of the shock regime within confined crust immediately prior to and during central uplift formation

constraints on provenance stratigraphic correlation and structural context of the volta basin ghana from detrital zircon geochronology an amazonian connection abstract we have dated 1060 detrital zircon grains from the neoproterozoic to lower palaeozoic volta basin ghana and from sandstones in the adjoining thrust sheets belonging to the pan african dahomeyide belt all dated zircons in the lower voltaian bombouaka group are older than 1000 ma while samples from the middle voltaian oti group and the upper voltaian obosum group contain numerous zircons of 600 1000 ma the samples we have studied from the dahomeyide thrust sheets buem and togo structural units have zircon age spectra similar to those from the bombouaka group confirming a correlation of the investigated sandstones with lower voltaian strata the bombouaka group was deposited between 1000 and 600 ma perhaps shortly after 1000 ma as suggested by earlier rb sr data on clay minerals deposition of the oti and obosum groups took place shortly after 600 ma perhaps continuing into the lower palaeozoic most samples contain palaeoproterozoic zircons with ages of 2000 2200 ma that probably have been derived from the surrounding crystalline birimian basement archaean zircons present in smaller proportions may have come from archaean rocks in the west african craton most zircons of 1000 1900 ma were probably derived from sources outside the west african craton the amazonian craton being a plausible source region zircons with ages around 1200 ma are believed to have been derived from grenvillian orogenic belts perhaps those that fringe the amazonian craton if this is so no major seaways could have been present between these belts and the west african craton during the early neoproterozoic zircons younger than 1000 ma were probably derived from an eastern continental block that collided with the west african craton during the pan african orogeny 600 ma ago

the pb isotopic systematics during crustal contamination of subcrustal magmas the hercynian magmatism in the serie dei laghi southern alps italy abstract the pb isotopic composition and pb u and th concentrations of a series of hercynian granite plutons and mafic intermediate stocks and dykes belonging to the serie dei laghi western southern alps have been analyzed the initial pb isotope ratios of both the granites and the dykes are typical of the lower crust and are spread over a significant range 206 pb 204 pb 280 17 883 18 530 207 pb 204 pb 280 15 597 15 710 and 208 pb 204 pb 280 38 077 38 708 four dykes all at the same locality display evidence of post emplacement fluid circulation with mobilization of u and pb in spite of the pb isotopic crustal signature of the studied rock series the 87 sr 86 sr ratios reach values as low as 0 704 in the dykes and as low as 0 707 in the granites suggesting a mantle contribution in the genesis of both lithologies indeed when compared with the metasediments from the same area the magmatic rocks have somewhat lower initial 207 pb 204 pb ratios which weakly trend towards the less radiogenic pb isotopic composition of the mantle the relationships between the pb isotopic composition and the pb content along with the pb and sr isotopic covariation suggest that the suite under examination was generated from a mantle derived primary magma that underwent extensive crustal contamination a primary magma similar to the neighbouring ivrea mafic complex has been inferred further assimilation of crustal material is particularly evident in the dykes while the granites appear to have been generated mainly by crystal fractionation processes from the intermediate magma already contaminated

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

multistage accretion and exhumation of the continental crust ivrea crustal section italy and switzerland the ivrea crustal section exposes in map view all levels of the southern alpine continental crust from ultramafic mafic and felsic granulite facies rocks of the deep crust ivrea verbano zone through medium grade basement rocks strona ceneri zone and val colla zone to unmetamorphosed permo mesozoic sediments the oldest part of the crustal section is preserved in the medium grade basement units which are interpreted to be the overprinted remains of an ordovician 440 480 ma magmatic arc or forearc complex during variscan subduction this arc was technically underplated by a carboniferous accretion subduction complex 320 355 ma containing metasediments and slivers of rheic oceanic crust presently found in the ivrea verbano zone during the late stages of variscan convergence 290 320 ma lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions this was broadly coeval with extensional exhumation and erosion of the variscan overprinted ordovician crust presently exposed in the strona ceneri and val colla zones post variscan transtensional tectonics 270 290 ma were associated with renewed magmatic underplating mylonitic shearing and incipient exhumation of the lower crust in the ivrea verbano zone this coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust early mesozoic tethyan rifting of the southern alpine crust 180 230 ma reduced crustal thickness to 10 km or less in the lower crust most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing the lower crust was exhumed along a large noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust the composite structure resulting from this complex evolution is probably typical of thinned late variscan continental crust on the passive margins of western europe alpine faulting and folding 20 50 ma fragmented the crustal section the originally deepest levels of the crustal section in the ivrea verbano zone as well as some segments of the basement cover contact were steepened whereas other parts of the crustal section particularly the strona ceneri zone underwent only minor to moderate alpine rotation

the principal components statistical method as a complementary approach to geochemical methods in water quality factor identification application to the coastal plain aquifer of israel abstract in general schoeller and piper diagrams as well as other classical techniques facilitate the description of the various geochemical groups of water and help to explain quality changes in an aquifer we propose the use mainly of principal components analysis for identification of relevant groups of water and the factors that bring about a change in their quality the major advantage of this method is its suitability for simultaneous analysis of a great number of variables and observations it is here being applied for the investigation of the dan metropolitan region of israel s coastal plain the variables involved include major ions as well as the physical factors of depth of the well intake filter below sea level distance from the sea and aquifer recharge the primary purpose of this work is to demonstrate the use and reliability of principal components analysis pca as a method complementary to classical approaches for hydrogeochemical research subsidiary purposes involve determination of the major water groups characterizing the coastal plain aquifer identification of some of the principal variables that influence changes in water quality in the aquifer and description of some of the geochemical phenomena contributing to change in aquifer water quality the following results were achieved for the test area 1 1 two major groups of water were identified a low salinity calcium bicarbonate water occurring in the phreatic portion of the coastal plain aquifer and a more saline sodium chloride water characterizing the neighboring cenomanian aquifer and the confined portions of the coastal plain aquifer 2 2 the major water input sources such as rain injection water waste water irrigation etc may vary in their influence upon resulting water quality this water quality is affected by such physical factors as distance from the sea depth of well intake filters below sea level proximity to streams and the lithology of the saturated and unsaturated zones 3 3 the evolutionary alteration in content of specific ions with time can be clearly visualized on the same graph by means of pca the universal applicability of such a statistical approach to other hydrological settings is noted

variation in amphibole and plagioclase composition with deformation investigation of the mineral chemistry of a shear zone in an amphibolite facies metagabbro has shown that the composition of the amphibole and plagioclase varies with deformation chemical analyses of the rocks indicate that the shear zone approximates an isochemical system the amphibole varies progressively from an initial magnesio hornblende to a ferroan pargasitic hornblende in the shear zone alkalies ti and fe2 fe2 mg increasing while the plagioclase becomes more anorthitic varying from labradorite to bytownite these changes in composition of amphibole and plagioclase from metabasic rocks are normally associated with increasing temperature in prograde regional metamorphic terrains such chemical variations observed in localized shear zones would suggest that shearing stress in regional metamorphism may play an important role in producing the documented discrepancies in amphibole chemistry between different metamorphic belts

the global effects of impact induced seismic activity on fractured asteroid surface morphology abstract impact induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids in this study we use a series of linked seismic and geomorphic models to investigate the process in detail we begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid and we use this theory to model the global vibrations experienced on the surface of an asteroid following an impact these synthetic seismograms are then applied to a model of regolith resting on a slope and the resulting downslope motion is computed for a full range of impactor sizes next this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure showing how topographic erosion accumulates as a function of time and the number of impacts finally these results are applied in a stochastic cratering model for the surface of an eros like body same volume and surface area as the asteroid with craters formed by impacts and then erased by the effects of superposing craters ejecta coverage and seismic shakedown this simulation shows good agreement with the observed 433 eros cratering record at a main belt exposure age of 400 200 myr including the observed paucity of small craters the lowered equilibrium numbers loss rate production rate for craters less than 100 m in diameter is a direct result of seismic erasure which requires less than a meter of mobilized regolith to reproduce the near observations this study also points to an upper limit on asteroid size for experiencing global surface modifying seismic effects from individual impacts of about 70 100 km depending upon asteroid seismic properties larger asteroids will experience only localized regional seismic effects from individual impacts

analysis of broadband seismic recordings of landslide using empirical green s function the 2010 mount meager landslide generated strong seismic waves we used seismic signals generated by a smaller landslide that occurred within 2 min of the end of the main landslide as the empirical green s function egf with which to obtain landslide dynamics from high frequency signals we then obtained the apparent force time function of the landslide based on the egf by inverting the higher frequency seismic signals the apparent force time function shows that the source material of the main landslide came from four collapses and that the second collapse involved multiple small collapses we also used the egf to simulate the vertical seismogram recorded at one station and found that the synthesized seismogram fits the response of the bulk mobilization of the sliding materials remarkably well our study demonstrates that the use of the egf can significantly broaden quantitative insights into landslide dynamics plain language summary seismic records reflect the effects of landslide dynamics and the path of seismic waves through the earth because the calculation of the high frequency component of the effect of a wave s path through the earth requires a detailed characterization of the velocity and attenuation structure the effects of landslide dynamics can only be estimated by inverting the long period seismic signals to study the high frequency seismic signals generated from the mount meager landslide we considered the seismic signals generated by a smaller landslide as the effects of the path of seismic waves we used the seismic signals generated by the smaller landslide to simulate the signal generated by the main landslide to overcome the effects of a potential overlap in the frequency domain between signals from the bulk and those from smaller particles our results show that the seismic signals generated by a small landslide can be used to improve our understanding of those generated by the main landslide

on the structure of the ivrea verbano zone northern italy and its implications for present day lower continental crust geometry the ivrea verbano zone in northern italy represents a section through the lower continental crust which has been tilted and emplaced into its present position during the alpine orogeny recent and on going structurally oriented geological mapping in this region is providing new information about the geometry of the complex the central part of the zone is dominated by a large basic complex the mafic formation which is intrusive into the surrounding gneisses the foliation within the envelope of gneisses is deflected around the intrusive complex as if by ballooning but in the region south west of monte capio both units are folded together into a tight to isoclinal steeply plunging fold with an amplitude of c 10 km this fold locally inverts the stratigraphy of the layered basic group of the complex and is thought to be the result of gravitational collapse following intrusion and inflation of a large magma body into the lower crust several high temperature shear zones have now been traced within the country rock for distances up to 20 km the geometry of these and their relationship to the basic complex suggests that at least some of the extensional collapse of the mafic body is related to uplift caused by intrusion of this body close parallels can be drawn between the observed structure in the ivrea verbano zone after removing the effects of late low temperature faulting and folding related to emplacement of the rocks into their present position and those inferred from deep seismic reflection profiling in areas of current extension such as parts of the us basin and range province

determination of rupture duration and stress drop for earthquakes in southern california a simple technique is developed for determining the rupture duration and stress drop of earthquakes between magnitudes 3 5 and 4 0 using the time between the p wave onset and the first zero crossing 1 2 on seismograms from local seismic networks this method is applied to 10 main shocks in southern california to investigate regional variations in stress drop the initial pulse widths of 65 foreshocks or aftershocks of these events were measured values of 1 2 for small earthquakes below about magnitude 2 2 are generally observed to remain constant with decreasing magnitude in four sequences studied the relative pulse width of a particular main shock m 3 5 at a given station is found to be correlated with the relative pulse width of its aftershocks recorded at that station these observations are interpreted to signify that the waveforms of these small events m 2 2 are essentially the impulse response of the path between the source and receiver values of 1 2 determined from small foreshocks and aftershocks are therefore subtracted in effect deconvolved from those of each main shock to obtain an estimate of the rupture duration of the main shock which is corrected for path effects significant variations in rupture duration and stress drop are observed for the main shocks studied aftershock locations and azimuthal variations in 1 2 both indicate that the rupture zone of one earthquake expanded unilaterally a factor of 10 variation in stress drop is calculated for two adjacent events of similar seismic moments occurring 1 hr apart on the san jacinto fault system the first event in this pair had the highest stress drop of the events studied 860 bars and was followed within 8 months by a magnitude 5 5 earthquake 2 km away

amphibolite facies pseudotachylytes in premosello metagabbro and felsic mylonites ivrea zone italy abstract pseudotachylytes solidified friction induced melts produced during seismic slip that formed under high temperature ambient conditions might retain information on earthquake mechanics beneath the brittle ductile elasto frictional viscous plastic transition in the crust thin sharply bounded pseudotachylytes and ultramylonites occur together inside the granulite facies premosello metagabbro and inside country felsic mylonites of the ivrea zone italian southern alps petrographic and microstructural data indicate that the studied pseudotachylytes and ultramylonites are coeval and developed under amphibolite facies 500 650 c metamorphic conditions the occurrence of foliated pseudotachylytes and of local remnants of cataclastic microstructures within ultramylonites suggests that pseudotachylytes might have represented planar heterogeneities for nucleation of ultramylonites the association of coeval pseudotachylyte and mylonites is interpreted as caused by cyclic stages of coseismic high strain rate transients related to downward propagation of seismic ruptures from the upper brittle crust followed by post seismic stress relaxation by ductile flow

source rupture plane determination from directivity doppler effect for small earthquakes recorded by local networks abstract fault plane solutions from first motion polarity of p waves give two nodal planes among which it is not possible to distinguish the fault plane from the auxiliary one we found that for earthquakes recorded by a dense local network it is possible to identify the rupture direction from the doppler effect produced by directivity this principle was successfully applied using three different simple techniques direct inspection and time and frequency domains analyses to one m l 3 6 and six smaller 1 2 m l 2 0 well recorded earthquakes from northern baja california mexico the results indicate activity in faults conjugate to those currently mapped in this region

rupture characterization of a low magnitude earthquake of central apennines italy abstract this paper describes a detailed study of the rupture mechanism of a m l 3 aftershock of the 1984 abruzzo central italy earthquake m s 5 8 to study the rupture extension shape and directivity the time durations of p and s far field pulses were analyzed in different directions corresponding to six recording stations the path corrected far field displacement p and s pulses at six seismic stations were determined using the empirical green function method the fault plane solution has been estimated by joint inversion of p polarities and s polarizations measured on three component records finally we used a method based on the plot of isochrons associated with the observed pulse widths and projected on each nodal plane the comparison of isochron plots for the two nodal planes suggests a rupture fault plane striking along the apenninic direction a weak source directivity up and west ward on the fault plane was observed a maximum ruptured surface of 0 4 km 2 was estimated assuming a sub shear rupture process the estimate of the seismic moment from spectral measurements gives the static stress drop ranging from 1 to 66 bar for the rupture velocity ranging between 0 2 and where is the shear wave velocity at the source

geological observations of damage asymmetry in the structure of the san jacinto san andreas and punchbowl faults in southern california a possible indicator for preferred rupture propagation direction we present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the san andreas san jacinto and punchbowl faults in southern california the observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters a fault zone scale of meters to tens of meters and related geomorphologic features the observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters in all of those scales more damage is found on the side of the fault with faster seismic velocities at seismogenic depths the observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks the data are consistent with a preferred northwestward propagation direction for ruptures on all three faults if our results are supported by additional observations asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section the property of a preferred rupture direction can explain anomalous behavior of historic rupture events and may have profound implications for many aspects of earthquake physics on large faults

the evolution of the serie dei laghi strona ceneri and scisti dei laghi the upper component of the ivrea verbano crustal section southern alps north italy and ticino switzerland abstract the serie dei laghi is the upper intermediate component of the exposed crustal section of massiccio dei laghi in the southern alps the latter including also the ivrea verbano zone its oldest event is a sedimentary cycle at least of early paleozoic age it starts with a pelitic sequence scisti dei laghi that later suffered a tectono metamorphic event of rather high pressure this event is recorded by rare mineral and textural relics detected in the scisti dei laghi of the m gambarogno area later an arenaceous sequence strona ceneri was deposited it is now represented at the base by basaltic metatuffites resulting from the reworking of an ophiolite belt then followed by paragneisses with detrital phenoclasts ceneri gneiss and by fine grained gneisses gneiss minuti during the ordovician conspicuous tonalitic to granitic masses were emplaced the induced thermal disturbance and the circulation of pegmatitic stage magmatic residuals caused contact metamorphism and granitization in the sedimentary country rocks during the hercynian a main prograde event generated the regional foliation with isoclinal geometry and transposed character a syn kinematic stage produced medium grade parageneses of relatively high pressure static recrystallization and sillimanite growth then followed during the uplift due to the combined action of hydration and decompression margarite bearing coronitic textures grew around kyanite a retrograde metamorphism was then recorded by the appearance of greenschist facies parageneses at particular structural sites the formation of the cossato mergozzo brissago cmb and pogallo fault system against the ivrea verbano zone as well as the schlingen folds in the northeast is of late hercynian age along the cmb fault calc alkaline dykes and stocks appinites were intruded probably towards the end of the upheaval but deep enough to induce partial melting in their country rocks between val d ossola and valsesia the hercynian evolution is concluded by the early permian magmatic cycle that gave rise to both extrusive rhyolitic sheets and to granitic epiplutonic bodies accompanied by mafic dykes in mesozoic times the serie dei laghi was dissected by many extensional faults these partly reactivated the hercynian lineaments such as the cmb and pogallo lines during the alpine orogenic cycle the serie dei laghi was affected by s verging cataclastic overthrusts

microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels the cataclasite pseudotachylyte connection in pelitic schists of the nason terrane of washington state pseudotachylyte can be demonstrated to have developed subsequent to the formation of foliation parallel cataclasite zones numerous microstructural lines of evidence point to a genetic link between pseudotachylyte and cataclasite and chemical evidence indicates that cataclasis played an important role in controlling pseudotachylyte chemistry besides optical and textural differences between the cataclasite and pseudotachylyte which may occur together there is unambiguous evidence of a melt origin for the pseudotachylyte while such evidence within the cataclasite is absent tem evidence indicates that glass is present in at least some of the veins as demonstrated by a lack of electron diffraction microlites present in the glass lack dislocations high temperature microlites of a mineralogy and composition not present in the host schist occur in various habits which depend on the distance from the vein margin also present are assimilation textures apparently quenched sulfide droplets and vesicles and amygdules in a pseudotachylyte vein studied in detail five microstructural domains occur these include vein margin cataclasite a transitional dark pseudotachylyte vein interior glass opaque and sulfide rich blebs and translucent haloes surrounding some of the blebs the cryptocrystalline cataclasite matrix grades into the dark pseudotachylyte which optically grades into the glass after adjustment for silica the host schist and cataclasite matrix are chemically very similar the dark pseudotachylyte and glass with the exception of na2o and cao are chemically quite similar to the cataclasite matrix the main difference may be the result of the segregation of the opaque plagioclase blebs thus the pseudotachylyte chemistry is strongly dependent upon the chemistry of the material comprising the cataclasite matrix which in turn is dependent on mineral susceptibility to incorporation into the cataclasite matrix fluids infiltrated along the cataclasite zones prior to pseudotachylyte generation as recorded by hydration reactions sulfide veining na2o loss and fluid inclusions this demonstrates that pseudotachylytes can form under fluid rich conditions as opposed to the commonly assumed dry conditions

earthquake ruptures with strongly rate weakening friction and off fault plasticity part 1 planar faults abstract observations demonstrate that faults are fractal surfaces with deviations from planarity at all scales we study dynamic rupture propagation on self similar faults having root mean square rms height fluctuations of order 10 3 to 10 2 times the profile length our 2d plane strain models feature strongly rate weakening fault friction and off fault drucker prager viscoplasticity the latter bounds otherwise unreasonably large stress concentrations in the vicinity of bends our choice of a cohesionless yield function prevents tensile stress states and thus fault opening a consequence of strongly rate weakening friction is the existence of a critical background stress level above which self sustaining rupture propagation in the form of self healing slip pulses first becomes possible around this level at which natural faults are expected to operate ruptures become extremely sensitive to fault roughness and exhibit substantial fluctuations in rupture velocity except for shallow inclinations of the maximum compressive stress to the fault less than about 20 the fluctuations are anticorrelated with the local fault slope these accelerations and decelerations of the rupture together with naturally emerging slip heterogeneity excite waves of all wavelengths and result in ground acceleration spectra that are flat at high frequency consistent with observed strong motion records

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

the rupture extent of low frequency earthquakes near parkfield ca the low frequency earthquakes lfes that constitute tectonic tremor are often inferred to be slow to have durations of 0 2 to 0 5 s a factor of 10 to 100 longer than those of typical m w 1 2 earthquakes here we examine lfes near parkfield ca in order to assess several proposed explanations for lfes long durations we determine lfe rupture areas and location distributions using a new approach similar to directivity analysis where we examine how signals coming from various locations within lfes finite rupture extents create differences in the apparent source time functions recorded at various stations we use synthetic ruptures to determine how much the lfe signals recorded at each station would be modified by spatial variations of the source station travel time within the rupture area given various possible rupture diameters and then compare those synthetics with the data our synthetics show that the methodology can identify inter station variations created by heterogeneous slip distributions or complex rupture edges and thus lets us estimate lfe rupture extents for unilateral or bilateral ruptures to obtain robust estimates of the sources similarity across stations we stack signals from thousands of lfes using an empirical green s function approach to isolate the lfes apparent source time functions from the path effects our analysis of lfes in parkfield implies that lfes apparent source time functions are similar across stations at frequencies up to 8 to 16 hz depending on the family the inter station coherence observed at these relatively high frequencies or short wave lengths down to 0 2 to 0 5 km suggest that lfes in each of the 7 families examined occur on asperities they are clustered in patches with sub 1 km diameters the individual lfes rupture diameters are estimated to be smaller than 1 1 km for all families and smaller than 0 5 km and 1 km for the two shallowest families which were previously found to have 0 2 s durations coupling the diameters with the durations suggests that it is possible to model these m w 1 2 lfes with earthquake like rupture speeds around 70 of the shear wave speed however that rupture speed matches the data only at the edge of our uncertainty estimates for the family with highest coherence the data for that family are better matched if lfes have rupture velocities smaller than 40 of the shear wave speed or if lfes have different rupture dynamics they could have long rise times contain composite sub ruptures or have slip distributions that persist from event to event

fault plane determinations for three small earthquakes along the san jacinto fault california search for cross faults using rupture directivity and aftershock locations three earthquakes ml 4 2 4 3 were studied to determine which of the two nodal planes of the focal mechanism was the rupture plane all three events occurred near the san jacinto fault zone during december 1989 and february 1990 and had a large component of strike slip motion with nodal planes oriented in the northeast and northwest directions smaller earthquakes were used as empirical green functions to deconvolve the path and site complications from the short period p waveforms the resultant far field source time functions were inverted for slip on a finite fault and tested to see which nodal plane provided a better fit to the data the results of the waveform inversion showed that the earthquake near the anza gap december 2 1989 and the earthquake near san bernardino december 28 1989 ruptured northwest trending planes the earthquake near buck ridge february 18 1990 appeared to rupture a northeast trending plane but the waveform results for this event were not considered as reliable available aftershock data for the three events were also examined using master event relocations the aftershock sequence following the buck ridge event showed a clear lineation trending toward the northeast the results of this study indicate that the anza and san bernardino earthquakes ruptured areas on or parallel to the san jacinto fault while the buck ridge event ruptured a northeast trending cross fault in addition to determining the fault plane the waveform analysis resolved details of the slip distribution for these relatively small earthquakes

artificial generation of pseudotachylyte using friction welding apparatus simulation of melting on a fault plane a 150 m thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 r p m under an axial load of 330 kg for 11 s using friction welding apparatus scanning electron microscopy and electron microprobe analysis reveal that the melt layer comprises sub angular to rounded porphyroclasts of clinopyroxene feldspar and ilmenite 20 m diameter derived from the host metadolerite set within a silicate glass matrix thermal calculations confirm that melting occurred at the rock interface and that mean surface temperatures in excess of 1400 c were attained the fused layer shows many textural similarities with pseudotachylyte described from fault zones morphologically the fused layer consists of a series of stacks of porphyroclasts welded together by melt to form build ups oriented at right angles to the friction surface there is also evidence of gouging ploughing and plucking as well as transfer and adhesion of material having occurred between the rock faces the mean surface velocity attained by the metadolerite 0 24 m s 1 and duration of the experiment are comparable with velocities and rise times of typical single jerk earthquakes occurring during stick slip seismic faulting within brittle crust i e slip rates of 0 1 0 5 m s 1 for say 1 10s in these respects the experiment successfully simulated frictional fusion on a fault plane in the absence of an intergranular fluid power dissipation during the experiment was about mw m 2 comparable only to very low values for earthquakes e g 1 100 mw m 2 for displacement rates of 0 1 0 5 m s 1 at shear stresses of 100 1000 bars this indicates that melting on fault planes during earthquakes should be commonplace field evidence however does not support this contention either pseudotachylyte is not being recognized in exhumed ancient seismic fault zones or melting only occurs under very special circumstances

comparative tectonic and dynamic analysis of cratons orogens basins and metallogeny a special volume to honor the career of brian f windley

source parameters of the 23 april 1992 m 6 1 joshua tree california earthquake and its aftershocks empirical green s function analysis of geos and terrascope data source parameters of the m 6 1 23 april 1992 joshua tree mainshock and 86 m 1 8 to 4 9 aftershocks are determined using an empirical green s function methodology for the aftershocks deconvolved p and s wave spectra are calculated for 126 pairs of closely spaced events recorded on portable geos stations s wave spectra from the two horizontal components are averaged the deconvolved spectra are fit by a ratio of omega square source models yielding an optimal least squares corner frequency for both the large and the small event in each pair we find no resolved difference between the inferred p and s wave corner frequencies using the standard brune model for stress drop we also find no resolved nonconstant scaling of stress drop with moment although we also conclude that detailed scaling systematics would be difficult to resolve in particular a weak increase of stress drop with moment over a limited moment magnitude cannot be ruled out for magnitudes smaller than m 3 to 3 5 the inferred stress drop values will be limited by the maximum observable corner frequency value of 60 hz for the mainshock source time functions are obtained from mainshock recordings at three terrascope stations pfo pas and gsc using an m 4 3 foreshock as an empirical green s function the results indicate a fairly simple single pulse source time function with clear south to north directivity and an inferred rupture radius of 5 to 6 km the deconvolved source time functions are inverted to obtain a finite rupture model that gives a robust estimate of rupture dimension early aftershocks are found to lie along the perimeters of regions with high mainshock slip the inferred mainshock stress drop value 56 bars is within the range determined for the aftershocks our derived mainshock source spectra do not show resolvable deviation from the omega square model

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

nonlinear attenuation of s waves and love waves within ambient rock we obtain scaling relationships for nonlinear attenuation of s waves and love waves within sedimentary basins to assist numerical modeling these relationships constrain the past peak ground velocity pgv of strong 3 4 s love waves from san andreas events within greater los angeles as well as the maximum pgv of future waves that can propagate without strong nonlinear attenuation during each event the shaking episode cracks the stiff shallow rock over multiple events this repeated damage in the upper few hundred meters leads to self organization of the shear modulus dynamic strain is pgv divided by phase velocity and dynamic stress is strain times the shear modulus the frictional yield stress is proportional to depth times the effective coefficient of friction at the eventual quasi steady self organized state the shear modulus increases linearly with depth allowing inference of past typical pgv where rock over the damaged depth range barely reaches frictional failure still greater future pgv would cause frictional failure throughout the damaged zone nonlinearly attenuating the wave assuming self organization has taken place estimated maximum past pgv within greater los angeles basins is 0 4 2 6 m s 1 the upper part of this range includes regions of accumulating sediments with low s wave velocity that may have not yet compacted rather than having been damaged by strong shaking published numerical models indicate that strong love waves from the san andreas fault pass through whittier narrows within this corridor deep drawdown of the water table from its currently shallow and preindustrial levels would nearly double pgv of love waves reaching downtown los angeles

pseudotachylytes in the balmuccia peridotite ivrea zone as markers of the exhumation of the southern alpine continental crust terra nova 22 70 77 2010 abstract two types of pseudotachylytes are observed in the balmuccia peridotite of the ivrea zone southern alps italy a type pseudotachylytes correspond to previously studied occurrences and were formed under temperatures comprising between 550 and 900 c and pressures comprising between 0 6 and 1 2 gpa these conditions were met in the ivrea crust between 350 and 270 ma suggesting that a type pseudotachylytes were formed during variscan tectonics or permian transtensional tectonics b type pseudotachylytes post date a type pseudotachylytes textural characteristics of b type veins suggest a formation in the upper continental crust at depths of about 5 10 km or less petrological constraints indicate that the exhumation of the ivrea crust at such shallow depths was achieved later than c 70 ma thus providing a maximum age of 70 ma for b type veins pseudotachylytes appear as markers of the poly orogenic evolution of the alpine belt

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

origin of large volume pseudotachylite in terrestrial impact structures large volume pseudotachylite bodies in impact structures are dike like and consist of angular and rounded wall rock fragments enveloped by a microcrystalline and sporadically glassy matrix that crystallized from a melt knowledge of the formation of pseudotachylite bodies is important for understanding mechanics of complex crater formation most current hypotheses of pseudotachylite formation inherently assume that fragmentation and melt generation occur during a single process based on the structure of pseudotachylite bodies at sudbury canada and vredefort south africa we show that these processes differ in time and space we demonstrate that the centimeter to kilometer scale bodies are effectively fragment and melt fi lled tension fractures that formed by differential rotation of target rock during cratering highly variable pseudotachylite characteristics can be accounted for by a single process i e drainage of initially superheated impact melt into tension fractures of the crater fl oor

source process and tectonic implication of the january 20 2007 odaesan earthquake south korea abstract the source process for the 20th of january 2007 mw 4 5 odaesan earthquake in south korea is investigated in the low and high frequency bands using velocity and acceleration waveform data recorded by the korea meteorological administration seismographic network at distances less than 70 km from the epicenter synthetic green functions are adopted for the low frequency band of 0 1 0 3 hz by using the wave number integration technique and the one dimensional velocity model beneath the epicentral area an iterative technique was performed by a grid search across the strike dip rake and focal depth of rupture nucleation parameters to find the best fit double couple mechanism to resolve the nodal plane ambiguity the spatiotemporal slip distribution on the fault surface was recovered using a non negative least square algorithm for each set of the grid searched parameters the focal depth of 10 km was determined through the grid search for depths in the range of 6 14 km the best fit double couple mechanism obtained from the finite source model indicates a vertical strike slip faulting mechanism the nw faulting plane gives comparatively smaller root mean squares rms error than its auxiliary plane slip pattern event provides simple source process due to the effect of low frequency that acted as a point source model three empirical green functions are adopted to investigate the source process in the high frequency band a set of slip models was recovered on both nodal planes of the focal mechanism with various rupture velocities in the range of 2 0 4 0 km s although there is a small difference between the rms errors produced by the two orthogonal nodal planes the sw dipping plane gives a smaller rms error than its auxiliary plane the slip distribution is relatively assessable by the oblique pattern recovered around the hypocenter in the high frequency analysis indicating a complex rupture scenario for such moderate sized earthquake similar to those reported for large earthquakes

crustal modelling of the ivrea verbano zone in northern italy re examined coseismic cataclasis versus extensional shear zones and sideways rotation abstract new field and microstructural observations in the ivrea verbano zone ivz northern italy reveal extensive post hercynian cataclasis indentation structures microtectonic escape structures microcataclastic injectites and occasional true pseudotachylytes the deformation occurred in situ and was governed by pure shear these observations are incompatible with prevailing crustal models of the ivz most of these models assume the presence of lower crustal originally flat lying extensional shear zones sideways block rotation and exposure of the petrological moho although the original structural observations from the central ivz suggested that the post hercynian deformation was cataclastic rather than shear dominated other already published information also contradicts the sideways rotation of a crustal block comprising the ivz and the adjacent serie dei laghi sdl after the early permian as assumed in the models the ivz and sdl have different detrital zircon populations and are separated by a steep shear zone and a transcurrent fault not rotated listric faults as assumed in the models rotation is also prevented by a flat lying zone of miarolitic cavities in the early permian baveno granite and by contemporaneous vertical basic dykes the widespread in situ cataclasis is interpreted as due to coseismic deformation by high frequency seismic waves generated by earthquakes along the insubric fault and exerting oscillatory compression dilation and torsion in a 10 km wide zone along the footwall of the insubric line the in situ post hercynian cataclasis absence of lower crustal extensional shear zones and lack of sideways rotation of the ivz and sdl call for a new and simpler crustal model of the region

aftershock sequences of the 1982 miramichi new brunswick earthquakes the mb 5 7 miramichi new brunswick earthquake of 9 january 1982 was located by comparison to the aftershock distribution in central new brunswick canada at 47 00 n 66 60 w with a focal depth of 7 km and an uncertainty of 3 km on each of the spatial coordinates the main shock was followed by a long and complex aftershock sequence with four events m 4 5 and more than 800 events m 1 0 until 30 june 1982 the aftershocks were studied by three temporary field networks set up in the epicentral areas from 10 to 20 january 2 to 7 april and 17 to 23 june 1982 and by the permanent canadian seismograph networks the aftershock activity was concentrated in a volume approximately 6 km ns 6 km ew 8 km deep and arranged in a north trending conjugate v pattern the miramichi earthquakes were predominatly thrust earthquakes occurring on north trending mid to high angle faults in the upper 8 km of the crust in the presence of an east west compressive horizontal regional stress field detailed field surveys have not yet been able to associate the earthquakes with any known preexisting faults the main shock had a moment of 2 2 0 7 1024 dyne cm and a stress drop of 35 to 70 bars the rupture started at 7 km depth on a plane striking 195 dipping 50 down to the west and propagated up to the east with a rake angle of 120 aftershocks deeper than 3 5 km had composite mechanisms very similar to the main shock s but shallower events took place on more steeply dipping planes up to 78 dip the rupture planes of the main shock and its principal aftershocks may have been similarly curved the magnitude frequency relationship b 0 8 of the aftershocks was comparable to the regional one the aftershock activity decayed as t 0 8 and had the highest rate of activity following the principal aftershock on 11 january that aftershock mb 5 4 appears to have ruptured the east dipping plane conjugate to the main shock s plane two other principal aftershocks mb 5 1 on 9 january and mb 5 0 on 31 march were associated with the main shock rupture plane while the fourth large event mb 4 7 on 16 june occurred 30 km west of the other activity and was seemingly unrelated however all the earthquakes were apparently associated with the same granitic terrain the miramichi earthquakes did not produce any significant surface faulting one small 25 mm surface displacement was found but it was identified as a secondary stress induced movement along a preexisting joint

the athens earthquake of 7 september 1999 based on detailed aftershock monitoring the first model of the athens earthquake is formulated which is consistent with global regional and local strong motion data of the mainshock and fits with geological setting the 30 station temporary network located 450 aftershocks during the first 20 observation days the aftershocks identified the mainshock fault plane dipping 52 and striking 117 consistently with the teleseismic fault plane solution a formal upward continuation of the fault plane intersects the surface close to the fili fault numerical modeling of the broadband regional data at 10 stations epicentral distances 140 to 370 km estimates the centroidal source depth of 10 km and yields an average source duration of 5 to 6 sec the interstation variability of the apparent duration indicates source directivity the empirical green9s function modeling at the closest broadband station suggests a fault length of 20 or 10 km both the numerical and empirical modelings give a very short rise time of 0 1 to 0 3 sec the short rise time seems to favor the nearly complete stress release of an asperity a 10 km asperity stress drop of 2 7 m pa is in agreement with a gap identified during the first 12 observation days between two aftershock clusters the strong motion accelerograms in athens also indicate a short apparent duration due to directivity about 3 sec and confirm an abrupt rupture beginning there is no evidence for an abrupt stopping the short rise time and short apparent source duration were two principal factors determining the damaging ground motions in athens

the development of pseudotachylyte in the ivrea verbano zone of the italian alps abstract pseudotachylyte was formed as part of the retrogressive tectonometamorphic evolution of the ivrea verbano zone during its post hercynian uplift from the lower crust during the mesozoic and cenozoic the pseudotachylytes preferentially occur in mafic rocks and in paragneisses and were generated along brittle faults which locally cut older ductile shear zones fracturing and the selective melting of minerals resulted in an aphanitic matrix containing host rock fragments and newly formed minerals e g biotite quartz sericite feldspar friction melt induced hydraulic fracturing of the adjacent rocks facilitated the formation of pseudotachylyte veins the veins occur as networks or as single 0 1 m to several metres thick injections with the geometry of pull apart structures in zones with a high density of contemporaneously formed pseudotachylyte veins the fractured wall rock lost cohesion and became intrusive masses of angular to rounded fragments became mobilized when the macroscopically homogeneous pseudotachylyte matrix exceeded about 30 of the total rock volume geochemical data show that the pseudotachylytes developed from the immediately adjacent host rocks scanning and transmission electron microscopy indicate that the enrichment or depletion of individual elements in the pseudotachylyte relative to the host rock depended on the amount of selectively melted minerals that were incorporated amphiboles plagioclase quartz and or sheet silicates and on the degree of secondary alteration the development of pseudotachylyte took place in the upper crust during retrogressive metamorphism which commenced in the ductile deformation field with the formation of high temperature mylonites followed by the generation of low temperature mylonites and finally by brittle deformation

combination of precise leveling and insar data to constrain source parameters of the mw 6 5 26 december 2003 bam earthquake we used new precise leveling data acquired 40 days after the bam earthquake in combination with radar interferometry observations from both ascending and descending orbits to investigate static deformation associated with the 2003 bam earthquake we invert this geodetic data set to gain insight into the fault geometry and slip distribution of the rupture the best fitting dislocation model is a steeply east dipping right lateral strike slip fault that has a size of 11 by 8 km and strikes n2 w we find that such smooth geometry fits available geodetic data better than previously proposed models for this earthquake our distributed slip model indicates a maximum strike slip of 3 m occurring about 3 to 5 km deep the slip magnitude and depth of faulting taper to the north where the fault approaches the bam city inclusion of crustal layering increases the amount of maximum slip inferred at depth by about 4

mechanics and mechanisms of magmatic underplating inferences from mafic veins in deep crustal mylonite dioritic to gabbro dioritic veins with extreme length to width ratios 1000 1 are localized along an amphibolite facies shear zone the cmb line between exposed segments of originally middle and lower continental crust strona ceneri and ivrea verbano zones northern italy the geometry of these veins and their mutual cross cutting relationships with the mylonitic foliation indicate that veining was coeval with noncoaxial flattening of the lower crust in early permian time the veins formed as closely spaced extensional shear fractures and propagated parallel to the originally gently to moderately dipping 30o mylonitic foliation vein opening at high angles 60 90o to the inferred1 direction and subparallel to the pre existing planar fabric requires that melt pressure slightly exceeded the lithostatic pressure and that differential stress was low 10 20 mpa in the vicinity of the veins the interaction of regions of tensile stress concentration at vein tips caused the concordant veins to curve and link up across the mylonitic foliation once interconnected the veins served as conduits for the rapid movement of mafic melt along the shear zone thermal modelling constrains the mafic melt in the narrowest 1 mm wide veins to have crystallized almost instantaneously such veins extend no more than a meter from host veins into the country rock indicating that the minimum rate of vein tip propagation and melt flow was at least several m s maximum crystallization times of only hundreds to thousands of years for even the thickest mafic veins 10 100 m in the ivz are short compared to the 15 20 ma duration of early permian crustal attenuation and magmatism in the southern alps this suggests that veining in the lower crust occurred episodically during extended periods of mylonitic creep concordant vein networks within deep crustal shear zones that are inclined as the cmb line may have been can also channel overpressurized mafic melt from deeper sources e g lower crustal magma chambers into cooler intermediate crustal rock this locally widens the depth interval of combined viscous and brittle deformation within the crust and can trigger partial melting of the country rock 1999 elsevier science b v all rights reserved

empirical green s functions from small earthquakes a waveform study of locally recorded aftershocks of the 1971 san fernando earthquake seismograms from 52 aftershocks of the 1971 san fernando earthquake recorded at 25 stations distributed across the san fernando valley are examined to identify empirical green s functions and characterize the dependence of their waveforms on moment focal mechanism source and recording site spatial variations recording site geology and recorded frequency band recording distances ranged from 3 0 to 33 0 km hypocentral separations ranged from 0 22 to 28 4 km and recording site separations ranged from 0 185 to 24 2 km the recording site geologies are diorite gneiss marine and nonmarine sediments and alluvium of varying thicknesses waveforms of events with moment below about 1 5 1021 dyn cm are independent of the source time function and are termed empirical green s functions waveforms recorded at a particular station from events located within 1 0 to 3 0 km of each other depending upon site geology with very similar focal mechanism solutions are nearly identical for frequencies up to 10 hz there is no correlation to waveforms between recording sites at least 1 2 km apart and waveforms are clearly distinctive for two sites 0 185 km apart the geologic conditions of the recording site dominate the character of empirical green s functions even for source spatial separations of up to 20 0 km the empirical green s functions at a particular site are consistent in frequency content amplification and energy distribution therefore it is shown that empirical green s functions can be used to obtain site response functions the observations of empirical green s functions are used as a basis for developing the theory for using empirical green s functions in deconvolution for source pulses and synthesis of seismograms of larger earthquakes

pseudotachylite in impact structures generation by friction melting and shock brecciation a review and discussion abstract fault related pseudotachylites are generated as the result of brittle or brittle ductile deformation related to seismic faulting it is generally accepted among tectonic workers that while cataclasis plays a role in pseudotachylite formation friction melting is involved in addition pseudotachylite and pseudotachylite like breccias have been repeatedly described from the basement exposures of impact and cryptoexplosion structures and as veinlets in shocked meteorites and lunar samples the relevant literature is reviewed with regard to controversial usage of the term pseudotachylite and attention is drawn to several problematic aspects 1 the term pseudotachylite is currently used both as a purely descriptive term and with genetic implications 2 no unambiguous criteria for comparison and distinction of tectonically and impact generated pseudotachylites have been identified 3 it needs to be considered that in impact structures pseudotachylite could be generated by two distinct processes namely by shock brecciation melting and by friction melting pseudotachylite in impact structures could be the result of either of these two processes or of both 4 it is recommended to adhere more stringently when describing breccias from impact or cryptoexplosion structures to nomenclature accepted for the description of fault rocks in tectonic environments this would help to avoid confusion when discriminating between melt rocks pseudotachylite impact melt rock fragmental breccias cataclasite fragmental impact breccia and mylonitic rocks 5 no criteria for the distinction of melt bearing impact breccias impact melt rock and pseudotachylite have been established either in order to solve these problems close interaction between structural geologists and impact workers is warranted it is suggested that the understanding of breccias in impact structures would be improved by paying particular attention to the temporal relationships between different breccia types this would facilitate recognition of possibly different formational processes for impact produced melt and fragmental breccias and their distinction from pre and post shock deformation products

pangea b and the late paleozoic ice age abstract the late paleozoic ice age lpia was the penultimate major glaciation of the phanerozoic published compilations indicate it occurred in two main phases one centered in the late carboniferous 315 ma and the other in the early permian 295 ma before waning over the rest of the early permian and into the middle permian 290 ma to 275 ma and culminating with the final demise of alpine style ice sheets in eastern australia in the late permian 260 to 255 ma recent global climate modeling has drawn attention to silicate weathering co2 consumption of an initially high greater variscan edifice residing within a static pangea a configuration as the leading cause of reduction of atmospheric co2 concentrations below glaciation thresholds here we show that the best available and least biased paleomagnetic reference poles place the collision between laurasia and gondwana that produced the greater variscan orogen in a more dynamic position within a pangea b configuration that had about 30 more continental area in the prime equatorial humid belt for weathering and which drifted northward into the tropical arid belt as it transformed to pangea a by the late permian the presence of widespread equatorial coal basins with euramerica flora in the footprint of the greater variscan orogen during the late carboniferous is more compatible with a heterogeneous horst and graben morphology characterized by uplifted crystalline massifs acting as loci of intense silicate weathering co2 consumption and supplying sediment for proximal basins as venues of organic carbon burial than a contiguous high mountain plateau as assumed in recent climate modeling of the lpia and its demise the culminating phase of the lpia occurred at about 275 ma with the transformation from pangea b to pangea a and the attendant reduction of continental area in the equatorial humid belt as well as with continued northward drift that placed what remained of the greater variscan orogen into the zechstein arid belt in the late permian by which time the geologic landscape was largely blanketed with siliciclastics the resulting warming from reduced silicate weathering and thus increasing pco2 was interrupted at 260 ma with a cooling trend that coincided with emplacement of the emeishan large igneous province on the equatorial south china craton as well as the drift of the cimmerian continental blocks through the equatorial humid belt due to opening of the neo tethys a return to ice age conditions from the increase in silicate weathering uptake of co2 was avoided by drift of the emeishan large igneous province out of the equatorial belt that in conjunction with massive outgassing from emplacement of the siberian traps in high latitudes at the end of the permian 252 ma helped steer the climate system to sustained non glacial conditions

pseudotachylites in the eastern peninsular ranges of california abstract a continuous zone containing pseudotachylites in the eastern peninsular ranges of california extends over more than 15 km from deep canyon in the north to at least toro canyon in the south pseudotachylites are found in rocks of tonalitic to dioritic composition while the overall compositions of host rock and pseudotachylite veins are similar veins are characteristically enriched in fe and ti and depleted in si many veins are cataclastic and all contain fragments but a large number have a groundmass with skeletal and spherulitic microlites of calcic plagioclase an 40 50 biotite and ilmenite indicative of partial melting the pseudotachylites formed mainly during a late brittle event 56 62 ma and postdate the high strain ductile mylonitic deformation in the santa rosa mylonite zone 65 87 ma as ascertained by 40ar 39ar ages the size of veins ranges from less than 1 mm to more than 10 cm in thickness and aspect ratios display a broad log normal distribution three quarters of the veins are more or less parallel to the foliation the orientation of the veins is consistent with a top to the west displacement similar to that for the ductile mylonites from the sizes energies required for melting are estimated to range between 105 j for smaller veins and 1010 j for larger veins interestingly the energy distribution and geometry of these pseudotachylites correspond closely with energy distributions of current microseismic events along the san andreas fault at parkfield suggesting that basic mechanisms may be similar and due to intrinsic mechanical properties of rocks in particular these indicate that local stress concentrations rather than average crustal stresses are relevant for earthquakes

source pulse enhancement by deconvolution of an empirical green s function observations of the earthquake source time function are enhanced if path recording site and instrument complexities can be removed from seismograms assuming that a small earthquake has a simple source its seismogram can be treated as an empirical green s function and deconvolved from the seismogram of a larger and or more complex earthquake by spectral division when the deconvolution is well posed the quotient spectrum represents the apparent source time function of the larger event this study shows that with high quality locally recorded earthquake data it is feasible to fourier transform the quotient and obtain a useful result in the time domain in practice the deconvolution can be stabilized by one of several simple techniques in this paper the method is implemented and tested on high quality digital recordings of aftershocks of the jan 9 1982 miramichi new brunswick earthquake in particular seismograms from a jan 17 aftershock 017 13 33 gmt local mag 3 5 exhibit path or site effects which complicate the determination of source parameters after deconvolution the apparent far field source of this event is a simple pulse in displacement with duration 0 07 second for both p and s

source properties of earthquakes near the salton sea triggered by the 16 october 1999 m 7 1 hector mine california earthquake we analyze the source properties of a sequence of triggered earthquakes that occurred near the salton sea in southern california in the immediate aftermath of the m 7 1 hector mine earthquake of 16 october 1999 the sequence produced a number of early events that were not initially located by the regional network including two moderate earthquakes the first within 30 sec of the p wave arrival and a second approximately 10 minutes after the mainshock we use available amplitude and waveform data from these events to estimate magnitudes to be approximately 4 7 and 4 4 respectively and to obtain crude estimates of their locations the sequence of small events following the initial m 4 7 earthquake is clustered and suggestive of a local aftershock sequence using both broadband trinet data and analog data from the southern california seismic network scsn we also investigate the spectral characteristics of the m 4 4 event and other triggered earthquakes using empirical green s function egf analysis we find that the source spectra of the events are consistent with expectations for tectonic brittle shear failure earthquakes and infer stress drop values of 0 1 to 6 mpa for six m 2 1 to m 4 4 events the estimated stress drop values are within the range observed for tectonic earthquakes elsewhere they are relatively low compared to typically observed stress drop values which is consistent with expectations for faulting in an extensional high heat flow regime the results therefore suggest that at least in this case triggered earthquakes are associated with a brittle shear failure mechanism this further suggests that triggered earthquakes may tend to occur in geothermal volcanic regions because shear failure occurs at and can be triggered by relatively low stresses in extensional regimes

multivariate statistical analysis of geochemical data as indicative of the hydrogeochemical evolution of groundwater in a sedimentary rock aquifer system summary the study of groundwater hydrogeochemistry of the paleozoic basses laurentides sedimentary rock aquifer system in quebec produced a large geochemical dataset groundwater samples were collected at 153 sites over a 1500 km2 study area and analyzed for major and minor ions the large number of data can lead to difficulties in the integration interpretation and representation of the results two multivariate statistical methods hierarchical cluster analysis hca and principal components analysis pca were applied to a subgroup of the dataset to evaluate their usefulness to classify the groundwater samples and to identify geochemical processes controlling groundwater geochemistry this subgroup consisted of 144 samples and 14 parameters ca2 mg2 na k hco 3 cl so 4 2 fe2 mn2 br sr2 f ba2 hs seven geochemically distinct clusters c1 c7 resulted from the hca samples from clusters c3 c4 c6 and c7 are mostly located in preferential recharge areas the majority of these samples have ca mg hco3 recharge groundwater c3 c6 c7 and na hco3 evolved groundwater c4 samples from the other three clusters c1 c2 c5 are characteristic of an aquifer system under confined conditions the majority of these samples have na hco3 evolved groundwater c1 c5 and na cl ancient groundwater that exhibits elevated concentrations in br c2 in addition to recognizing the importance of hydrogeological conditions on groundwater geochemistry the distribution of clusters also showed the importance of the geological formations on minor and trace elements such as fe2 mn2 sr2 f and ba2 the first five components of the pca account for 78 3 of the total variance in the dataset component 1 is defined by highly positive loadings in na cl and br and is related to groundwater mixing with champlain sea water and solute diffusion from the marine clay aquitard the high positive loadings in ca2 and mg2 of component 2 suggest the importance of dissolution of carbonate rocks in this aquifer system from their characteristic loadings the first two components are defined as the salinity and hardness components respectively components 3 5 are related to more local and geological effects the integration of the hca and the pca with conventional classification of groundwater types as well as with the hydrogeological and geological contexts allowed the division of the region into four main geochemical areas providing an improved regional picture of the aquifer system dynamics and hydrogeochemical evolution of groundwater the following factors were recognized as influencing the evolution of groundwater identified in every geochemical area 1 geological characteristics including sedimentary rock type and till mineralogy 2 hydrogeological characteristics represented by the level of confinement and the hydraulic gradient and 3 the geological history including the latest glaciation and the champlain sea invasion with its integrated approach this hydrogeochemical study contributes to the characterization and understanding of complex groundwater flow systems and provides an example of the long term geochemical evolution of hydrogeological systems after a major perturbation in this case seawater invasion

magmatic underplating extension and crustal reequilibration insights from a cross section through the ivrea zone and strona ceneri zone northern italy the thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the ivrea verbano zone and strona ceneri zone in northern italy geochronological and petrologic data are compared to a two dimensional thermal kinematic model field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite to amphibolite facies metamorphism as well as reequilibration of mineral chemical and isotopic systems magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution as information on the earlier history is preserved only at upper crustal levels

hydrologic and geologic factors controlling surface and groundwater chemistry in indian wells owens valley area southeastern california usa the indian wells owens valley area is located in the semi arid basin and range province which is characterized by alternating mountains and alluvial basins surface water resources are limited in this arid region and water demand is mainly met by groundwater pumpage in a classic basin and range groundwater system water flows from recharge areas in the mountains to discharge areas in adjacent valleys discharge areas are generally occupied by playas where large amounts of salt deposition occur due to evaporating groundwater hydrochemical data from a total of 1368 spring surface and well water samples collected over an 80 year period were used to evaluate water quality and to determine processes that control water chemistry q mode hierarchical cluster analysis hca was employed for partitioning the water samples into hydrochemical facies also known as water groups or water types five major water groups resulted from the hca analysis the samples from the area were classified as recharge area waters ca na hco3 water and na ca hco3 water transition zone waters na hco3 cl water and discharge area waters na cl water and more concentrated na cl water spatial plots of the major statistical groups show that the samples that belong to the same group are located in close proximity to one another suggesting the same processes and or flowpaths inverse geochemical models of the statistical groups were developed using phreeqc to elucidate the chemical reactions controlling water chemistry the inverse geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area in a broad sense the reactions responsible for the hydrochemical evolution in the area fall into four categories 1 silicate weathering reactions 2 dissolution of salts 3 precipitation of calcite amorphous silica and clay minerals and 4 ion exchange

petrogenesis and tectonic significance of amphibolites interlayered with metasedimentary gneisses in the ivrea zone southern alps northwest italy abstract amphibolites occurring as elongate lenses and boudins within the metasedimentary kinzigite formation of the ivrea zone are all of igneous parentage a detailed study of their trace element characteristics shows that they can be divided into two groups type 1 amphibolites occur as layers up to 100 m thick and have trace element distributions showing depletion in incompatible elements typical of normal n type mid ocean ridge basalts type 2 amphibolites occur as thinner layers and boudins and have higher levels of the more incompatible elements similar to transitional or enriched e type ocean basalts although metamorphism appears to have resulted in loss of sio 2 and variable enrichment in na k rb ba sr and locally ca the less mobile elements including the rare earth elements have preserved the primary basaltic geochemical characteristics the tectonic model which best explains the assemblage of slivers of morb like amphibolites within extremely highly deformed semi pelitic metasediments is that of an accretionary sedimentary wedge formed at a palaeozoic subduction zone

three dimensional geological modelling and multivariate statistical analysis of water chemistry data to analyse and visualise aquifer structure and groundwater composition in the wairau plain marlborough district new zealand summary concerns regarding groundwater contamination with nitrate and the long term sustainability of groundwater resources have prompted the development of a multi layered three dimensional 3d geological model to characterise the aquifer geometry of the wairau plain marlborough district new zealand the 3d geological model which consists of eight litho stratigraphic units has been subsequently used to synthesise hydrogeological and hydrogeochemical data for different aquifers in an approach that aims to demonstrate how integration of water chemistry data within the physical framework of a 3d geological model can help to better understand and conceptualise groundwater systems in complex geological settings multivariate statistical techniques e g principal component analysis and hierarchical cluster analysis were applied to groundwater chemistry data to identify hydrochemical facies which are characteristic of distinct evolutionary pathways and a common hydrologic history of groundwaters principal component analysis on hydrochemical data demonstrated that natural water rock interactions redox potential and human agricultural impact are the key controls of groundwater quality in the wairau plain hierarchical cluster analysis revealed distinct hydrochemical water quality groups in the wairau plain groundwater system visualisation of the results of the multivariate statistical analyses and distribution of groundwater nitrate concentrations in the context of aquifer lithology highlighted the link between groundwater chemistry and the lithology of host aquifers the methodology followed in this study can be applied in a variety of hydrogeological settings to synthesise geological hydrogeological and hydrochemical data and present them in a format readily understood by a wide range of stakeholders this enables a more efficient communication of the results of scientific studies to the wider community

gouge formation by dynamic pulverization during earthquake rupture it is proposed here that fault gouge forms by rock pulverization within the tip region of a fast propagating earthquake due to the intensity of stress and rates of strain in this zone to examine this mechanism we calculate the deformation conditions of a dynamic shear fracture propagating close to the limiting rayleigh wave velocity the mechanical conditions close to the fracture tip are extreme tensile stresses approach 5 gpa volumetric strain rates exceed 105 s 1 and volumetric expansion alternates with volumetric contraction it is expected that a localized zone a few millimeters wide will be pulverized under these conditions this mechanism of dynamic gouge formation provides new insight to recent observations of the texture of gouge from the san andreas fault zone and from rupture zones in south african mines further as such extreme conditions develop only during fast propagating earthquakes presence or absence of pulverized gouge may serve as an indicator of earthquake propagation velocity

hydrogeological processes in a fluviokarstic area inferred from the analysis of natural hydrogeochemical tracers the case study of eastern serrania de ronda s spain in riverine karst areas surface water interacts with the groundwater flowing through carbonate aquifers understanding the interference of water fluxes in this type of environments is essential to effectively manage the groundwater resources particularly in areas where karst aquifers comprise the main source of drinking water in this research the interaction between surface water and groundwater in the turon river valley eastern serrania de ronda s spain and the functioning of sierra blanquilla karst aquifer have been characterised by means of hydrodynamic monitoring and the combined use of physical chemical parameters and natural hydrogeochemical tracers the results show connected gaining streams in the river stretches close to the permanent springs while in the surroundings of the overflow springs the river reaches behave as gaining high flow and as perched losing low flow soil natural tracers permit tracing runoff arising from the upper part of the hydrological watershed via river which interacts with the groundwater drained in the main discharge area of the aquifer furthermore the analysis of hydrogeochemical data and chemographs demonstrate that sierra blanquilla aquifer behaves as a typical conduit flow system in which the unsaturated zone predominantly participates during high flow periods while the saturated zone is more active in low flow when the groundwater inputs maintain the turon river discharge the more detailed understanding of how the hydrogeochemical characteristics of groundwater vary over time and space has direct consequences on karst spring protection and in the environmental preservation of groundwater ecodependent systems

the geological and hydrogeological framework of the panabako kodjari and bimbilla formations of the voltaian supergroup revelations from groundwater hydrochemical data abstract this research demonstrates the utility of groundwater hydrochemical data for facilitating lithological mapping in a sedimentary terrain factor analysis and q mode hierarchical cluster analysis hca were applied to groundwater hydrochemical data from the nasia basin in northern ghana to determine the major sources of hydrochemical variation and the main spatial groundwater associations groups these analyses were complemented by mass balance hydrochemical modeling from phreeqc and petrographical analysis of rock samples from the area rock samples from the study area were subjected to petrographical and geochemical analyses from a petrographic microscope and scanning electron microscope sem respectively four principal factors were distinguished from the factor analysis highlighting four principal processes responsible for the observed hydrochemistry in the area these four factors cumulatively account for over 79 of the total variance in the dataset factor scores were computed from the factor loadings matrix through a regression process to highlight the spatial variations in the significance of each of the factors in the terrain on the basis of the parameter loadings the first factor which has been noted to be pervasive throughout the terrain has been interpreted to represent silicate mineral weathering both the geochemical data and petrographical analysis indicate the presence of silicate minerals indicating the plausibility of silicate mineral weathering the process of silicate mineral weathering appears to be highly pervasive throughout the basin indicating high factor scores in places the trend of spatial variations in the scores of factor 1 clearly mimics the known geology of the terrain indicating that the geology is the main controlling factor in the hydrochemistry of groundwater in the basin this pattern exhibited by the factor scores indicates that where adequate groundwater hydrochemical data is available factor analysis would prove useful in mapping out lithological variations in an area two of the remaining factors have been observed to represent other specific geogenic processes mass balance hydrochemical modeling suggests that albite may be in active dissolution presenting pervasively negative saturation indices undersaturation while kaolinite is extensively supersaturated five groundwater associations distinguished from the q mode hca display a spatial pattern which identifies with the underlying geology

source tomography by simulated annealing using broad band surface waves and geodetic data application to the mw 8 1 chile 1995 event summary we invert surface wave and geodetic data for the spatio temporal complexity of slip during the mw 8 1 chile 1995 event by simulated annealing this quasi global inversion method allows for a wide exploration of model space and retains the non linearity of the source tomography problem complex source spectra are obtained from 5 to 45 mhz from first and second orbit fundamental mode rayleigh waves using an empirical green s function cross correlation technique coseismic displacement vectors were measured at 10 gps sites near antofagasta they are part of a french chilean experiment which monitors the northern chile seismic gap the spectra together with the geodetic data are inverted for the moment distribution on a 2 d dipping fault under the physical constraints of slip positivity and causality marginal a posteriori distributions of the model parameters are obtained from several independently inverted solutions in general features of the slip model are well resolved data are well fitted by a purely unilateral southward rupture with a nearly uniform velocity around 2 5 3 0 km s 1 and a total duration of 65 s several regions of moment release were imaged one near the hypocentre a major one 80 km south of it and a minor one 160 km south of it the major patch of moment release seemed to have propagated to relatively shallow depths near the trench 100 km ssw of the epicentre the region of major slip is located updip of the 1987 mw 7 5 earthquake suggesting a causal relationship most of the slip occurred updip of the hypocentre 36 km but the entire coupled plate interface 20 40 km ruptured during the chile 1995 event

slow and fast ruptures on a laboratory fault controlled by loading characteristics recent geodetic observations indicate that a single fault may slip slowly and silently or fast and seismically in different circumstances we report laboratory experiments that demonstrate how the mode of faulting can alternate between fast stick slip events 70 to 500 mm s sliding rates and slow silent events 0 001 to 30 mm s as a result of loading conditions rather than friction properties or stiffness of the loading machine the 760 mm long granite sample is close to the critical nucleation length scale for unstable slip so small variations in nucleation properties result in measurable differences in slip events slow events occur when instability cannot fully nucleate before reaching the sample ends dynamic events occur after long healing times or abrupt increases in loading rate which suggests that these factors shrink the spatial and temporal extents of the nucleation zone arrays of slip strain and ground motion sensors installed on the sample allow us to quantify seismic coupling and study details of premonitory slip and afterslip we find that seismic coupling decreases when slip rates fall below about 70 mm s the slow slip events we observe are primarily aseismic less than 1 of the seismic coupling of faster events and produce swarms of very small m 6 to m 8 events these mechanical and seismic interactions suggest that faults with transitional behavior where creep small earthquakes and tremor are often observed could become seismically coupled if loaded rapidly either by a slow slip front or dynamic rupture of an earthquake that nucleated elsewhere