geometry and physical properties of the socorro new mexico magma bodies analysis of multichannel seismic reflection profiles gathered by the consortium for continental reflection profiling cocorp constrains the geometry and material properties of a proposed magma body located within the rio grande rift a new spectral ratio method is used to find the reflectivities of the proposed midcrustal intrusion large and spatially variable reflectivities support the interpretation of the reflector as an intrusive molten magma complex modeling of the reflectivity suggests that in one location the intrusion took the form of several thin 30 to 40 m thick layers seismic wave attenuation within the rift s midcrust is low

possible causes for the seismic activity observed in cotopaxi volcano ecuador continuous monitoring of cotopaxi volcano from january 1989 to september 1997 shows that it is subject to a steady shallow long period lp activity lp events are concentrated in a column with a diameter of about 3 km and a height of about 12 km between an elevation of 4 km and 8 km depth they do not occur in swarms nor have there been periods of complete calm high frequency volcano tectonic vt events are mostly located on the northeastern side of the lp column with the maximum activity of both types of events coinciding in depth lp activity cannot be related to unrest of the volcano the most likely explanation for the continuous occurrence of lp events beneath cotopaxi is that they are produced by the interaction of glacier thaw water and hot material at shallow depths the relative spatial distribution of both type of events in two adjacent zones suggest that vt and lp activities are interconnected

possible evidence for subducted sedimentary materials beneath central california seismic reflection and refraction data from the coast ranges of central california west of the san andreas fault suggest the presence of a low velocity 4 5 5 0 km s wedge at a depth of about 15 22 km that dips east beneath rocks of both the franciscan assemblage and the salinian block we propose that the wedge represents sedimentary materials that were subducted in an offshore trench within the past 23 m y the refraction data also suggest the presence of a second low velocity zone that extends at least to mid crustal depths beneath the san andreas fault zone

crustal structure of the diablo and gabilan ranges central california a reinterpretation of existing data the compressional wave velocity structure of the crust of the coast ranges of central california was modeled from seismic refraction data reported previously for both the diablo range east of the san andreas fault and the gabilan range west of the fault two alternative velocity models were derived for each range by iterative two dimensional ray tracing each pair of models shows a sedimentary layer and an underlying crust composed of three or four layers for the diablo range the velocities in the sediments range from 2 9 to 4 8 km sec and the depth to basement varies from 0 2 km within the central diablo range to a maximum of 3 4 km north of the livermore valley in the gabilan range the velocities in the sediments overlying the granitic basement are higher north of the gabilan range 3 6 to 4 6 km sec than to the south 2 2 to 3 8 km sec and the depth to basement both north and south of the range is less than 2 km below the sediments and fractured near surface material a resolvable difference in the crustal velocity structure on opposite sides of the san andreas fault indicates compositional differences at depth the upper crust has an average velocity of 5 7 km sec at depths between 3 and 16 3 km in the diablo range and an average velocity of 6 1 km sec at depths between 2 and 9 1 km in the gabilan range the lower crust has an average velocity of 6 9 km sec at depths between 16 3 and 30 km in the diablo range and an average velocity of 6 5 km sec at depths between 10 and 24 km in the gabilan range the models also show that the depth to moho differs by several kilometers between the two ranges in the diablo range models the crust mantle boundary becomes shallower from south to north rising from a depth of 30 to 26 km in the gabilan range models the moho is at a depth of 24 to 26 km depending on the velocity assumed at the base of the crust laboratory measurements of rock velocities and the mapped surface geology allow us to interpret the velocity models in terms of crustal composition we conclude that the diablo range probably consists largely of metagraywacke to a depth of 16 3 km and gabbroic material below this depth the crust of the gabilan range probably consists of granitic material to a depth of 9 1 km and gneissic material below this depth franciscan rocks are not regionally present in the crust of the gabilan range if the gneissic lower crust consists of the same rocks as are found at the surface in the gabilan range then the presumed large scale lateral motion of the salinian block has taken place at or below the crust mantle boundary rather than along a mid crustal slip plane

accretion and subduction tectonics in the chugach mountains and copper river basin alaska initial results of the trans alaska crustal transect geologic seismic gravity and magnetic data from the northern chugach mountains and southern copper river basin alaska indicate that the chugach terrane cgt and the composite peninsular wrangellia terrane pet wrt are thin 10 km rootless sheets bounded on the south by north dipping thrust faults that sole into a shallow horizontal low velocity zone the cgt has been thrust at least 40 km beneath the pet wrt along the border ranges fault system brfs adjacent to the brfs uplift and erosion of 30 40 km since jurassic time have exposed blueschist facies rocks in the cgt and mafic and ultramafic cumulate rocks in the pet wrt four paired north dipping layers of low and high seismic velocities extend beneath the northern cgt and southern pet wrt and may be slices of subducted oceanic crust and upper mantle the upper two pairs may now be joined to the continental plate 15 references 5 figures

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

veins in the lockport dolostone evidence for an acadian fluid circulation system the orientation and spatial distribution of veins in the lockport dolostone attest to a fluid circulation system active during the acadian orogeny in western new york and southern ontario outcrops east of the clarendon linden fault zone are cut by a prominent east northeast systematic calcite filled vein set whereas these systematic veins are absent west of the fault zone except in two quarries systematic veins display distinct characteristics the mean vein orientation rotates clockwise from 067 in the east to 086 farther west veins do not propagate into the basal lockport group and calcite vein 18 o pdb values are significantly lighter to the east of the clarendon linden fault zone maximum horizontal stress s h trend lines drawn parallel to the strike of the systematic veins are incompatible with s h inferred from alleghanian plateau and other post paleozoic structures however because east west s h trend lines are compatible with an acadian tectonic event in western new england our interpretation is that systematic veins in the lockport group are a cratonward signature of the acadian orogeny

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

seismic reflections from the basal contacts of batholiths although seismic reflection data in batholithic terranes are scarce datasets available to us from three different regions show surprising similarities all three profiles one across precambrian rocks in texas and two across mesozoic batholiths in california and nevada show strong subhorizontal reflections at depths of 6 10 km these reflections are interpreted to originate from the base of the batholiths and indicate that these batholiths are tabular in shape the large amplitudes of the reflections require a marked contrast in acoustic impedance product of velocity and density between the batholith and the underlying rocks two of the profiles show reflective zones having appreciable thicknesses 0 5 2 5 km while the third profile shows a single continuous reflection the four most favored candidates for sub batholithic reflections are 1 underplated gabbro 2 cumulate layers 3 migmatites and 4 thrust faults cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance

appalachian stress study 1 a detailed description of in situ stress variations in devonian shales of the appalachian plateau we describe an experiment to measure variations in the state of stress within a horizontally bedded devonian shale sandstone limestone sequence in western new york a total of 75 stress measurements were made in three wells a kilometer or so apart using a wireline supported hydraulic fracturing system the stress profiles indicate that a major drop in horizontal stress level occurs in the generally massive shales this drop occurs principally across the lowermost member of a group of sand beds and corresponds to an offset in sh and sh of 3 5 and 9 mpa respectively above the sands thrust regime conditions prevail although the amount by which sh exceeds sv is undetermined since instantaneous shut in pressures isips were clipped at the level of sv due to fracture rotation below the sands the regime is strike slip with both horizontal stresses showing lateral uniformity despite substantial variations in topography the magnitude of sh in the sand beds themselves and a lower limestone remains at least as great as sv despite the decline in shale stress hence stress contrasts between these beds and neighboring shales become pronounced with depth the contrast in sh and sh between the lowermost sand and the immediately underlying shale is at least 6 and 14 5 mpa respectively sh levels in the lower strike slip regime are about 1 75 times greater than sh and are less than the value required to initiate slippage on favourably oriented frictional interfaces for the upper thrust regime and the sand and limestone beds however the inferred lower bound on sh is close to the slippage threshold for a coulomb friction coefficient of 0 6 the orientation of sh is ene with a standard deviation of 20 fracture traces were usually splayed occasionally spanning 30 of well bore no systematic correlation between mean orientation and lithology is evident significantly different orientations were obtained for adjacent tests in which almost identical islps were observed suggesting that the fractures quickly reorient themselves to propagate normal to the least principal stress direction similarly vertical traces were observed in those tests where isips apparently reflect sv suggesting that rotation to horizontal was rapid

synthetic seismograms for a complex crustal model the algorithm of the original reflectivity method has been vectorized and implemented on a cdc cyber 205 computer calculation times are shortened by a factor of 20 to 30 compared with a general purpose computer with a capacity of several million floating point operations per second mflop the rapid calculation of synthetic seismograms for complex models high frequency sources and all offset ranges is a provision for modeling not only particular phases but the whole observed wavefield as an example we model refraction data of the black forest southwest germany and are able to derive rather tight constraints on the physical properties of the lower crust

seismic evidence of mylonite reflectivity and deep structure in the kettle dome metamorphic core complex washington the first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the kettle dome metamorphic core complex northeastern washington the mylonite zone developed at mid crustal levels and has undergone a minimum of retrogression and cataclasis during uplift downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5 km depth as a complex zone of multicyclic reflections the seismic data also show that the crust beneath the kettle dome is characterized by abundant strongly reflective moderately dipping layers down to at least 18 km these layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both models for the structure and genesis of metamorphic core complexes must take these data into account

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

deep fault plane geometry inferred from multiplet relative relocation beneath the south flank of kilauea dense microearthquake swarms occur in the upper south flank of kilauea providing multiplets composed of hundreds of events the similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters a simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically relocation of these events greatly improves the definition of the seismic image of the fault that generates them indeed relative relocations define a plane dipping about 6 northward although corresponding absolute locations are widely dispersed in the swarm a composite focal mechanism built from events providing a correct spatial sampling of the multiplet also gives a well constrained northward dip of about 5 to the near horizontal plane this technique thus collapses the clouds of hypocenters of single event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies we cannot conclude that all south flank earthquakes collapse to a single plane there may locally be several planes perhaps with different dips and depths throughout the south flank volume the 6 northward dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of hawaii this plane is probably an important feature that characterizes the basal slip layer below the upper south flank of kilauea volcano differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane the present work illustrates how high precision relative relocations of similar events in dense swarms combined with the analysis of geodetic measurements can help to describe deep fault plane geometry systematic selection and extensive relative relocation of similar earthquakes could be attempted in other well instrumented highly seismic areas to provide reliable basic information especially useful for understanding of earthquake generation processes

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

geometry of the kapuskasing structure from a lithoprobe pilot reflection survey a short 10 km pilot seismic reflection profile recorded in the kapuskasing structural zone near foleyet ontario reveals reflections with travel times of at least 5 0 s about 16 km depth an event dipping west from about 1 7 to 3 4 s probably corresponds to the ivanhoe lake cataclastic zone the true geologic dip on this boundary is calculated to be about 35 38 west northwest and is consistent with interpretations of this zone as a west dipping crustal scale thrust fault east dipping reflections are observed with longer travel times than the fault zone arrival and are likely from within the greenstone terrane of the abitibi subprovince

crustal structure of eastern nevada from cocorp deep seismic reflection data the western nevada segments of the cocorp 40 n deep seismic reflection survey of the north american cordillera reveal the geometry of structures of cenozoic and possibly earlier ages to travel times of 10 s corresponding to depths of 30 km the most striking feature of the data is a band of prominent reflections typically at traveltimes of 9 5 to 10 5 s that are present discontinuously across the entire data set few reflections are observed from beneath the base of this reflective zone which is interpreted as the crust mantle transition this reflection mono is inferred to be continuous across the survey area varying gradually in depth but without resolvable offsets it appears to have taken its present form or position during basin range crustal extension the middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west these reflections may represent intrusions or shear zones related to basin range or pre basin range extension but some are likely to be inherited from earlier compressional deformation reflections from the upper crust are interpreted as images of basin fill strata basin range normal faults and mesozoic and paleozoic thrusts related to back arc thrusting and accretion of oceanic and arc related rocks

crustal evolution of the rhinegraben area 1 exploring the lower crust in the rhinegraben rift by unified geophysical experiments abstract unified geophysical investigations of the lithosphere in the rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process the new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust 1 1 the lower crust as the layer beneath the conrad discontinuity with a p wave velocity of about 6 5 km s or greater refraction seismics 2 2 the laminated band of reflections as seen in the near vertical reflection seismic experiments in many parts of the continents 3 3 the ductile part of the crust below the brittle ductile transition devoid of earthquakes in seismically active regions 4 4 the electrical conductivity of the lower crust indicative of dry or wet conditions or still unknown conduction phenomena in the rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process in which the crust of the rhinegraben rift system has been subjected to three different natural dynamic processes 1 uplift by 2 to 3 km with subsequent erosion of the rhinegraben shoulders black forest and vosges mountains caused decompression possibly leading to the formation of a low velocity high electrical conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the black forest 2 the brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust which became about 5 to 7 km thinner below the graben than below the shoulders 3 the deepest hypocentres in the black forest dinkelberg area if projected onto the neighbouring reflection profile would be located 7 to 8 km within the laminated lower crust beneath the southern black forest indicating a discrepancy between the top of the lower crust as defined by the brittle ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band the rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations

automatic hypocenter determination of volcano induced seismic transients based on wavefield coherence an application to the 1998 eruption of mt merapi indonesia in order to monitor the seismic activity of mt merapi indonesia over a long period of time we installed a permanent array of both broadband and short period seismometers during the summer of 1997 considering the requirements of an automatic classification and localization system for seismic monitoring and surveillance at active volcanoes we split this network into three small aperture arrays distributed around the volcano we introduce here a newly developed method to determine the hypocenters in an automatic non linear manner using the coherence of seismic waves observed at the different arrays to test this method we analyze a swarm of vt b events recorded by the network the first step in this algorithm is based on a modified smoothed coherence transform in the second step we perform a semblance analysis applied to the 3d problem evaluating the quality of the estimated relative onset times after more than one year of dormancy mt merapi renewed its activity at the end of june 1998 this gave us the opportunity to analyze all stages of dome growth collapse and new intrusion of magma using the associated seismicity in a post processing sense this also allowed us to calibrate and test our newly developed automatic monitoring system using the more pronounced waveforms of vt b events by detecting and classifying different event types automatically we are able to localize a large number of vt b events which occurred just before the initial eruption we are also able to resolve some properties of the wavefield at mt merapi which are essential for further interpretations finally the results show that the source region of the vt b type seismicity just before the 1998 eruption is closely related to the region of subsequent high volcanic activity and therefore may represent a promising tool to forecast future eruptions

an advanced signal processing technique for deriving grain size information of bedload transport from impact plate vibration measurements a reliable characterization of bedload transport is required to gauge the engineering and theoretical issues related to the dynamics of sediments transport in rivers however while significant advances have been made in the development of monitoring techniques robust quantitative predictive relationships have proven difficult to derive in this article we develop a dedicated signal processing technique aimed at improving the usage of impact plate measurements for material transport characterization our set up consists of a piezoelectric hydrophone mounted on the bottom side of a stainless steel plate thus acting as a sediment vibration sensor while the classical analysis with such systems is usually limited to rather simple procedures such as impact counting a large amount of useful information is contained in the actual waveform of the impact signal which conveys the force and the contact time that the bedload imposes on the plate an advanced signal processing technique called first arrival atomic decomposition is used to improve the characterization of bedload transport by analysing the amplitude and frequency attributes of each single impact this new processing approach proves to be well suited for bedload transport monitoring using plate systems and allows us to establish a relationship between the median grain size d50 and the impact signal properties this link is first observed and validated with controlled flume experiments and then applied to continuous impact records in a small gravel bed river during a flood event the estimated d50 offers a novel possibility to observe the time varying grain size distribution of bedload transport copyright 2014 john wiley sons ltd

earthquake swarm activity revealed from high resolution relative hypocenters clustering of microearthquakes abstract waveform similarity in an earthquake swarm enables precise arrival time differences of p and s waves through cross spectral methods to be obtained a numerical simulation guarantees the time resolution of the order of 1 ms for the time differences this precise arrival time difference is used to determine relative hypocenters of microearthquakes occurring in a swarm source region with a spatial resolution of the order of 10 m this precise hypocenter determination millisecond hypocenter determination is applied to the data using temporary seismic observation in the ashio region japan where swarm activities are especially high relative hypocenters of 153 events are determined with high spatial resolution it is revealed that the swarm activities consist of several clusters of events a cluster is a group of events concentrated within a small area in a short time interval the cluster activities in one swarm never overlap one another sharing some part of the swarm source region of about 100 m 1 km in length one cluster forms a fault like planar area with a length of about 40 150 m the event cluster swarm activity would be a manifestation of the hierarchy structure of earthquake sources

p wave velocity features of the lithosphere under the eromanga basin eastern australia including a prominent mid crustal conrad discontinuity abstract the crustal structure of the central eromanga basin in the northern part of the australian tasman geosyncline revealed by coincident seismic reflection and refraction shooting contrasts with some neighbouring regions of the continent the depth to the crust mantle boundary moho of 36 41 km is much less than that under the north australian craton to the northwest 50 55 km and the lachlan fold belt to the southeast 43 51 km but is similar to that under the drummond and bowen basins to the east the seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the north australian and lachlan provinces in particular there is a sharp velocity increase at mid crustal depths 21 24 km which has not been observed with such clarity elsewhere in australia the conrad discontinuity in the lower crust the many discontinuous sub horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust further emphasising the differences between the upper and lower crust the crust mantle boundary moho is characterised by an increase in velocity from 7 1 7 7 km s to a value of 8 15 0 04 km s the depth to the moho under the canaway ridge a prominent basement high is shallower by about 5 km than the regional moho depth there is also no mid crustal horizon under the canaway ridge but there is a very sharp velocity increase at the moho depth of 34 km the ridge could be interpreted as a horst structure extending to at least moho depths but it could also have a different intra crustal structure from the surrounding area the sub crustal lithosphere has features which have been interpreted from limited data as being caused by a velocity gradient at 56 57 km depth with a low velocity zone above it because of the contrasting crustal thicknesses and velocity gradients the lithosphere of the central eromanga basin cannot be considered as an extension of the exposed lachlan fold belt or the north australian craton the lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m consistent with an upper crustal basement of tightly folded meta sedimentary and meta volcanic rocks the crustal structure is consistent with a pericratonic or arc back arc basin being cratonised in an episode of convergent tectonics in the early palaeozoic the seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment

joint initiation in bedded clastic rocks abstract two general types of sedimentary structures serve as initiation points ip for cross fold joints in siltstone beds of the ithaca formation appalachian plateau new york planar bed boundaries and geometrically complex features such as bedforms trace fossils and soft sediment deformation structures the relative abundance of each ip type varies with the stratigraphic position of the jointed body when they are mechanically isolated by thicker shales siltstone bodies have a higher percentage of ip associated with complex features this association supports the hypothesis that sedimentary structures serve to concentrate stress thus becoming preferred sites for primary joint initiation secondary joint initiation occurs at planar bed boundaries through interaction with pre existing joints in adjacent siltstone beds that are mechanically coupled to form composite joints the propagation path of joints passing through planar bed boundaries of composite joints can be traced backward to primary ip at complex sedimentary structures modeling suggests that when beds are mechanically coupled the modest joint tip stress concentration across a shale layer e g

is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of north america on the appalachian plateau of new york ene striking joints parallel the mid continent contemporary stress field and do not correlate with other structures joints within ohio indiana michigan and west virginia also parallel the mid continent maximum horizontal compression as indicated by hydraulic fracture measurements in northern indiana where the contemporary stress field deviates significantly from the orientation of the mid continent stress the attitude of the joints deviates accordingly in addition to joints structures on several scales from subcontinent size fracture systems to microfabrics in shales apparently parallel the mid continent stress field from this i conclude that these joints are mode i cracks whose orientation corresponds to the contemporary tectonic stress field in itself the formation of these structures constitutes a tectonic event that has often been overlooked in favor of tectonic events that produce the more spectacular but less pervasive faults and folds

lithoprobe southern vancouver island cenozoic subduction complex imaged by deep seismic reflections reply the lithoprobe seismic reflection project on vancouver island was designed to study the large scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting juan de fuca plate in this paper we interpret two lithoprobe profiles from southernmost vancouver island that were shot across three important terrane bounding faults leech river san juan and survey mountain to determine their subsurface geometry and relationship to deeper structures associated with modem subduction the structure beneath the island can be divided into an upper crustal region consisting of several accreted terranes and a deeper region that represents a landward extension of the modern offshore subduction complex in the upper region the survey mountain and leech river faults are imaged as northeast dipping thrusts that separate wrangellia a large mesozoic paleozoic terrane from two smaller accreted terranes the leech river schist mesozoic rock

moho beneath the north sea compared on normal incidence and wide angle seismic records the mohorovicic discontinuity the moho is properly defined as the step in the velocity of horizontally travelling p waves from about 7 km s 1 to about 8 1 km s 1 its detailed structure varies from place to place but it is universally interpreted as the transition from crust to mantle normal incidence reflections originating at about moho depth have been reported sporadi cally during the past 20 yr during the past 10 yr continuous seismic reflection profiles have been obtained to this depth both on the continents and in the deep sea and at the same time striking improvements have been made in the methods available for interpretation of refraction experiments here we report a comparison between the moho determined by a long range refraction experiment across the north sea and the results of a short normal incidence reflection profile shot along the same line fig 1 we believe this to be the first time that such a direct comparison has been made

characterizing stress fields in the upper crust using joint orientation distributions linear elastic fracture mechanics predicts that joint orientation is controlled by the stress field in which the joints propagate thus joint sets are effective proxies for stress trajectories during joint growth complexity in joint orientation indicates stress trajectory variability a phenomenon quantified using an eigenvalue method that measures dispersion of joint normal vectors i e poles around the mean vector ratios between the eigenvalues of a joint orientation tensor give the clustering strength and the shape factors of the distribution a joint set that forms in a relatively isotropic rock subject to a rectilinear stress field should exhibit strong clustering and small random orientation variation that can be described by the fisher statistical model however most joint orientation distributions in bedded rocks have non random variation greater in strike than in dip this relative stability of the vertical stress orientation is strongest when joints are bounded by bedding interfaces reflecting the tendency for deflection in the local stress field arising from the growth of side cracks joint segments and adjacent joints in joint zones even when joint growth across bedding interfaces indicates negligible strength anisotropy joint orientation distributions reflect less joint joint interaction during vertical growth than during horizontal growth as strike variation grows due to the presence of a non rectilinear stress field the orientation data better fit a kent statistical model joint sets formed during fold development or in rocks with irregular bedding boundaries are more weakly clustered with fisher like orientation distributions orientation distributions for joint sets formed throughout a stress rotation have kent like shapes that indicate the magnitude of stress trajectory variation and clustering strength that depends on the joint density at each increment of the stress rotation

structure of the new madrid seismic source zone in southeastern missouri and northeastern arkansas new seismic reflection data in the southwestern part of the new madrid seismic zone nmsz show that a major zone of disrupted reflectors and a large antiform coincide with a prominent trend of earthquake epicenters between caruthersville missouri and marked tree arkansas we speculate that the antiform may be caused by igneous intrusions the strong correlation between the seismicity in this trend and the extent of the disrupted zone provides a geologic basis for defining the source zone for large earthquakes in this part of the nmsz

thin skinned tectonics in the northern rhenish massif germany a network of profiles in the hercynian mountain system geo traverse rhenoherzynikum is needed to determine details of the geological structure and find the reasons for tectonic movements outstanding features within the mountain system such as the large fault zone along the southern boundary of the hunsruck1 and the left flank of the rhinegraben have already been investigated by reflection seismology2 3 we describe here a new reflection refraction experiment carried out in 1978 in the most northern part of the rhenish massif across the stavelot venn massif near aachen fig 1 while layers in the lower crust dip to the nnw a strong upper reflector at 3 4 km depth shows a slight dip to the sse and indicates a prominent fault zone near aachen known in belgium as the faille du midi here a comparable and apparently identical reflector was found some 100 and 175 km to the west of our profile we interpreted this reflector as a prominent and well lubricated thrust fault along which a huge horizontal nappe displacement took place during the last stages of the variscan orogeny

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

a deep seismic profile of 800 km length recorded in southern queensland australia summary in 1984 the australian bureau of mineral resources and the geological survey of queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the eromanga basin to the beenleigh block east of the clarence moreton basin a relatively transparent upper crustal basement with an underlying more reflective lower crust is characteristic of much of the region prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the clarence moreton basin and the adjacent beenleigh block and provide some of the most interesting features of the entire survey a wide angle reflection refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the nebine ridge the only clear deep reflectors are interpreted as p to sv or sv to p converted reflections from a mid crustal boundary at a depth of about 17 km the combined nebine ridge data provide well constrained p and s wave velocity models of the upper crust and suggest a crustal structure quite different from that beneath the adjacent mesozoic basins

a case for neotectonic joints along the niagara escarpment in a search for proof that some joints on the appalachian plateau are neotectonic orientation data were collected on more than 1800 fractures in exposures of the lockport dolomite in western new york and southern ontario of greatest interest was a late forming ene joint set present in the central and western portions of the field area these joints increase in abundance with proximity to the niagara escarpment the shape of the east west trending niagara escarpment is defined by a series of asymmetric reentrants with a strong ene linear trend reflecting the bedrock joint pattern the joints appear to have formed near the earth s surface in response to low tensile stresses developed in bedrock adjacent to the retreating escarpment since the current shape and position of the escarpment were controlled by glacial activity during the last 3 m y it is possible that the ene joints and reentrants are neotectonic features

the seismic reflection character of the continental mohorovicic discontinuity the continental mohorovicic discontinuity is most often interpreted as a step function velocity boundary however on deep crustal seismic reflection profiles reflections at depths where refraction data places the moho have laminated character and are laterally discontinuous these observations on continental reflection data point to a model of the moho that has thin layered rather than block structure previous workers who have synthetically generated seismic responses from various crust mantle boundary models have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed moho arrivals geologic interpretations of the thin layering include relatively undeformed metasediments cumulate layering tectonic banding and lenses of partial melt to obtain more direct geologic evidence a synthetic seismogram is generated from geologic cross sections of the ivrea verbano zone an exposed section through the lower crust and upper mantle the synthetic shows laminations similar to those observed on reflection data without more direct evidence no single geologic interpretation of the moho is reasonable indeed the moho may be a laterally variable boundary its composition and structure dependent upon the geologic history of the overlying crust

joint interaction with embedded concretions joint loading configurations inferred from propagation paths abstract the interaction between propagating joints and embedded concretions in a devonian black shale near seneca lake ny permits identification of the loading configurations responsible for two joint sets of different ages striking at nearly the same orientation the earlier set consists of systematic joints cut by later alleghanian joints of the appalachian plateau the later set consists of non systematic curving cross joints that abut these same alleghanian joints field evidence shows that concretions functioned as stiff inclusions in a compliant black shale as a consequence of this elastic contrast local perturbations in the remote stress field persisted around the concretions during burial tectonic deformation and exhumation these stress perturbations influenced joint propagation paths of both joint sets our conclusions about loading configurations are based on finite element modeling of the effect of the local stress perturbation on concretion modified joint propagation modeling shows that the local stress perturbation from a thermoelastic loading was responsible for deflecting cross joints away from concretions in a curved propagation path near the concretion this load configuration also led to arrest of cross joints before they penetrated the shale concretion interface at greater distances from the concretion the propagation path of cross joints was controlled by the contemporary tectonic stress field the interface between concretions and the surrounding shale was strongly bonded as indicated by the crossing of the interface by some of the systematic ene joints higher compressive stress levels within the concretions relative to the shale suppressed joint development in the concretion causing the arrest of those joints once they had driven across the interface and a short distance into the concretion numerical modeling shows that interface penetration by the systematic ene joints is consistent with a fluid load the same loading configuration postulated for the subsequent alleghanian joints the traces of the systematic ene joints align on opposite sides of concretions rather than curving toward the concretion as predicted by two dimensional models of the fluid load co planar traces are indicative of large planar joints propagating in plane around the concretion making it energetically inefficient for the crack front to curve as it enters the local stress perturbation near the concretion a fluid load for the systematic ene joints came from high pore pressure during the pre alleghanian stages of burial of the devonian catskill delta complex

cocorp seismic reflection profiling in the ouachita mountains of western arkansas geometry and geologic interpretation cocorp seismic reflection profiling in the ouachita mountains of western arkansas indicates that 1 carboniferous foreland basin deposits within the arkoma basin thicken dramatically toward the south reaching an aggregate thickness in excess of 12 km 4 5 s two way travel time beneath the southern edge of the frontal thrust zone 2 evidence for northward directed low angle thrusting within this clastic sequence is prominent with a probable decollement surface lying at or near the contact with underlying early paleozoic shelf carbonates 3 beneath the benton uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km 2 8 s this structure appears to mimic the anticlinorial shape of the benton uplift indicated by the surface outcrop pattern 4 beneath the southern ouachitas south dipping stratified events are observed to depths in excess of 14 km 5 0 s 5 deeper in the section a prominent gently north dipping reflection occurs at approximately 22 km depth 7 6 s beneath the northern coastal plain southern ouachitas extrapolation of data along strike in the ouachita belt and consideration of large scale structure observed in other collisional orogenic belts suggest that the reflection events observed beneath the benton uplift represent early paleozoic shelf carbonates correlative with those that floor the arkoma basin to the north this interpretation requires that the early paleozoic deep water sediments exposed in the core of the ouachitas be allochthonous and also implies significant basement uplift beneath the core zone to the south the prominent package of layered reflections occurring beneath the southern ouachitas and northern coastal plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and or metasedimentary strata at relatively shallow levels these strata are correlative with the carboniferous flysch cropping out in the southern ouachitas and at somewhat deeper levels they are correlative with the early to middle paleozoic deep water sediments exposed on the benton uplift

early jointing in coal and black shale evidence for an appalachian wide stress field as a prelude to the alleghanian orogeny early ene striking joints present coordinates within both pennsylvanian coal and devonian black shale of the central and southern appalachians reflect an approximately rectilinear stress field with a dimension 1500 km this appalachian wide stress field awsf dates from the time of joint propagation when both the coal and shale were buried to the oil window during the 10 15 m y period straddling the pennsylvanian permian boundary the awsf was generated during the final assembly of pangea as a consequence of plate boundary tractions arising from late stage oblique convergence where maximum horizontal stress s h of the awsf was parallel to the direction of closure between gondwana and laurentia after closure the awsf persisted during dextral slip of peri gondwanan microcontinents when sh appears to have crosscut plate scale trans current faults at around 30 sup o following 10 m y of dextral slip during tightening of gondwana against laurentia the awsf was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline shaped alleghanian map pattern seen today

constraints on temporal variations in velocity near anza california from analysis of similar event pairs similar earthquake pairs recorded by the anza seismic network in southern california are used as repeatable sources to place an upper limit on temporal changes in seismic velocity which occurred in the vicinity of the anza seismic gap in the last 9 yr relative arrival times for each pair of events are found using a cross correlation method and relative locations are calculated to verify that the pairs have nearly identical hypocenters the time separation between events in these pairs varies from less than a day to almost 7 yr the long term changes in seismic travel times as measured from the pairs with the longest time separation are not significantly greater than the noise level estimated from the short time separation event pairs almost all p wave paths show less than 0 06 0 007 sec change in travel time and all s wave paths have less than 0 03 0 004 sec change sensitivity tests place an upper bound on travel time changes that could be compensated by hypocenter mislocation at 0 2 there is no evidence that localized stress accumulation causes measurable changes in seismic velocity in the anza region

seismic reflection imaging of the subducting juan de fuca plate mckenzie and parker s early suggestion1 that the juan de fuca plate is underthrusting north america has since been confirmed by numerous studies evidence for plate convergence along this junction includes a sediment filled trench2 7 intense compressional deformation of the continental slope and shelf sediments2 7 seafloor magnetic anomalies that extend beneath the continental slope2 4 inland andesitic volcanoes8 paired low high heat flow and gravity anomalies9 10 active onshore deformation11 12 onshore crustal earthquakes with compressional axes parallel to the direction of plate convergence13 sinistral strike slip earthquakes along the nootka fault fig 1 14 and a well defined benioff wadati zone6 15 16 here we report the results of a seismic reflection survey that has delineated two slabs of oceanic lithosphere underlying vancouver island one that is currently being subducted and one that is underplated these findings lead us to speculate that successive underplating of oceanic lithosphere may be an important process in the evolution and growth of continents

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

interpretation of migrated seismic reflection profiles across the northern appalachians in maine summary a simple line migration technique has been developed and applied to deep crustal reflection data collected in the northern appalachians of quebec and central maine preliminary interpretation of these data combined with the results of gravity and refraction studies shows thinning of the crust in two distinct steps beneath the silurian devonian merrimack synclinorium and shows evidence for mesozoic crustal stretching and dike intrusion

ten years of seismic monitoring over a gas field abstract ten years of seismic recording allows us to locate more than 800 tremors with magnitudes ranging from 1 0 to 4 2 in a small 10 by 10 by 5 km volume near a major gas field lacq france the seismic activity began 10 yr after gas extraction began nearly all of the epicenters are located within the lateral extent of the gas field the only perturbation of the stress or strain fields which has the same spatial and temporal scale as the seismicity is the 500 bar drop in gas pressure due to 20 yr of gas exploitation demonstrating that these events are induced the network established to monitor this activity consists of eight telemetered stations including a well bottom geophone located at 3200 m depth using three dimensional relocation techniques we computed new hypocenter positions for more than 280 events these better located hypocenters form a diffuse zone that mimics the local dome structure taking the regional tectonic stresses into account we suggest that these ruptures may be the result of unfolding of the dome structure

loading rate impact on fracturing pattern lessons from hydrocarbon recovery lacq gas field france using scaling parameters of induced seismicity in size and space domains seismic b value and spatial correlation dimension d respectively we analyze the fracturing pattern of the gas reservoir of lacq in correlation with the production rate on the basis of previous observations and a previous model segall et al 1994 we assume that the cause of induced seismicity is a poroelastic stressing such that the production rate can be viewed as a stress rate for the rocks adjacent to the reservoir during the 20 year study period the production rate has decreased dramatically leading to a change of poroelastic stress rate from 4 5 10 3 to 1 5 10 3 mpa yr accordingly we observe a significant decrease in the size of activated fractures and a reduction of the spatial clustering of fracturing both arguing for a stress rate impact on the fracturing pattern in the gas field we interpret these changes in terms of a change in the deformation mode of stressed rocks from a more brittle to a more ductile behavior as the stress rate was decreased this hypothesis is supported by independent evidence from a number of experimental numerical and field studies the loading rate thus appears as a possible candidate to drive changes of b and d in time and space the apparent sensitivity of the earth s crust fracturing pattern to loading rate variations of much lower frequency than the tidal variations suggests that the earth s crust deformation mode itself be frequency dependent also our observations may have strong implications in contexts where the loading rate is induced by human intervention e g hydrocarbon recovery mines suggesting the possibility of adjusting the loading rate so as to shape the fracturing pattern locally according to human needs

curving cross joints and the lithospheric stress field in eastern north america cross joints are late formed nonsystematic fractures that extend across intervals between systematic joints traces of such cross joints are seen on bedding plane surfaces of devonian catskill clastic sedimentary rocks of the appalachian plateau of western new york state where the maximum horizontal principal stress s h is oriented n65 e as indicated by in situ stress measurements between pairs of closely spaced systematic joints traces of cross joints are commonly planar and orthogonal to the preexisting joints however in the mid region between some widely spaced systematic joints in western new york cross joints strike parallel to the s h of the present lithospheric stress field but then curve to abut the preexisting joints at right angles a curving trace reflects a local perturbation of the regional stress field in the vicinity of preexisting joints and the perpendicular termination indicates that the preexisting joints were open depending on their age the strike of the mid region of curving cross joints denotes the orientation of either the neotectonic stress field or its tertiary predecessor in the north american lithosphere

characterization of seismic waveforms and classification of seismic events using chirplet atomic decomposition example from the lacq gas field western pyrenees france summary in the present paper we present a generalization of the wavelet transform known as chirplet transform specially designed to quantify the morphological attributes of individual seismic sections packets constituting the seismic waveforms the proposed transform relies on an atomic decomposition of individual seismograms based on local multiscale chirps swept frequency wave packets of various shape and duration we developed an algorithm that provides an optimal representation of the waveform packets in terms of i arrival time ii central frequency iii modulus iv phase v duration vi envelope shape and vii frequency modulation compacting the information contained in each seismogram into a reduced set of parameters particularly well suited to describe seismic waveforms in the present work we focus on the ability of atomic decomposition to classify seismic events we illustrate the developed methodology and resulting hierarchical classification scheme agglomerative clustering displayed as a dendrogram to seismograms of the induced seismicity recorded in the lacq gas field between 1989 and 1997 by a local seismic network for the present case study the resulting classification reveals different levels of similarity between seismic events of a same swarm accurate analysis of the subsequences of seismic events associated to an injection well shows temporal changes in the morphological attributes of the recorded seismic waveforms these changes are highly correlated with water over pressure records of the reservoir demonstrating the capability of the method to guide investigation of the underlying processes properties of propagation media source rupture processes and in a general manner the physical properties of the reservoir

subhorizontal fractures in a granitic pluton their detection and implications for radioactive waste disposal several countries are actively investigating the feasibility of using crystalline rock bodies for the ultimate disposal of radioactive nuclear waste as part of the concept assessment phase in the canadian nuclear fuel waste management program a multidisciplinary research investigation is being conducted across the lac du bonnet batholith in southeastern manitoba no radioactive materials other than sealed sources or tracers are to be emplaced within this rock body the results of a high resolution seismic reflection survey together with information from a number of deep boreholes demonstrates that major subhorizontal fractures occur at depths of up to 800 m within the batholith an integrated interpretation of the seismic data with selected borehole logs shows that warm water is flowing up the major fractures from depth a natural corollary of these results is that a three dimensional seismic reflection survey combined with an appropriate exploratory drilling program has the potential for delineatin

high precision relative locations of earthquakes at mount st helens washington several earthquake sequences have occurred at mount st helens as clusters of shallow events with nearly identical waveforms multiplets we present a technique that allows the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques a multiplet is studied in pairs of events called doublets each doublet is made up of a reference event and one other event from the multiplet the cross spectrum of a short window beginning just before the p arrival is used to compute differences in arrival times between seismograms from the same station for each doublet these time differences computed with a precision of 1 2 ms are then used to locate each earthquake relative to the reference event engineering explosions with well known locations are used to test the relocation technique explosions can be relocated with accuracy better than 20 m for events within 250 m of the reference explosion the relocation technique is applied to earthquakes preceding the september 10 1984 and the may june 1985 dome building eruptions of mount st helens forty events that occurred during a 12 hour period in september 1984 are located in a volume approximately 30 m in diameter beneath the lava dome the earthquakes following the extrusion do not occur in multiplets and appear to be distributed over a much larger volume in the may june 1985 eruption we recognize three multiplet sets that took place successively in time the multiplets are interpreted as repeated failure or slip within a relatively small volume due to high strain rates around the magma supply conduit

poroelastic stressing and induced seismicity near the lacq gas field southwestern france o hundreds of shallow small to moderate earthquakes have occurred near the lacq deep gas field in southwestern france since 1969 these earthquakes are clearly separated from tectonic seismicity occurring in the pyrenees 25 km to the southwest the induced seismicity began when the reservoir pressure had declined by 30 mpa repeated levehng over the field shows localized subsidence reaching a maximum of 60 mm in 1989 segall 1989 suggested that poroelastic stressing associated with volumetric contraction of the reservoir rocks is responsible for induced seismicity associated with fluid extraction to test this model we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields we find that the relationship between average reservoir pressure drop and subsidence is remarkably linear lending support to the linear poroelastic model displacements and stresses are computed based on a priori knowledge of the reservoir geometry materim properties and reservoir pressure changes the computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling stress perturbations accompanying gas extraction computed using the same parameters are found to be 0 2 mpa or less changes in coulomb failure stress are computed assuming that shp occurs on optimally oriented planes the predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compresslye stress axis is vertical accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

fractures in mudrocks advances in constraining timing and understanding mechanisms abstract recent advances in fracture timing techniques have led to new insights about sequences and mechanisms of fracture formation in mudrocks methods to constrain fracture timing in mudrocks include i field evidence including crosscutting relationships and evidence of compaction ii petrographic evidence including signs of soft sediment deformation and diagnostic mineral assemblages or textures iii stable isotopic evidence iv thermometric dating using burial history modelling and v radiometric age dating in this contribution we briefly review each method with salient examples from the literature many documented examples of mudrock hosted fractures formed during early burial a non intuitive finding that demands a re assessment of our assumptions about what types of materials can develop opening mode fractures in general the style and distribution of fractures varies dramatically based on fracture timing highlighting the importance of constraining timing for predicting fracture patterns and interpreting their mechanisms including the role of fluid flow

interrelation between induced seismic instabilities and complex geological structure from 1982 to 1989 more than 520 local seismic events were recorded by a permanent network of 9 telemetered stations over an active gas field lacq france the geological data of 46 boreholes coupled to several tens of kilometers of seismic profiles concentrated in a small volume of 15 15 10 km3 allow the development of a very precise velocity structure exhibiting a dome geometry limited by faulted strips the discretized velocity model includes almost 20000 blocks with 500 m 500 m 250 m minimum dimensions a relocation technique using this a priori detailed 3d velocity model allows precise locations of induced earthquakes 351 relocated hypocenters show that of the seismic activity concentrated on geometrical discontinuities of the local dome structure the locations of seismic events are correlated with geomechanical layers of the structure spatial and temporal migration of seismic ruptures mimics the different permeability rings of the reservoir which are progressively depleted by gas extraction most of these events are at present confined close to major thrust planes which limit the dome structure laterally and in depth onto preexisting faults in the basement below the gas field the occurence of these deep clusters demonstrates that fluid manipulation in the sedimentary covers can activate preexisting basement faults this result changes previous assessments of local seismic risk and provides new insights into fault mechanisms in areas with complex geological structure

horizontal slip along alleghanian joints of the appalachian plateau evidence showing that mild penetrative strain does little to change the pristine appearance of early joints abstract some alleghanian joints in black shales of the geneseo and middlesex formations of the catskill delta complex finger lakes district new york slipped horizontally up to 8 cm horizontal slip is measured by the offset of ene striking joints alleghanian joints striking 330 350 display a right lateral slip with an average value of 1 9 cm while joints striking 004 010 slip in the left lateral sense with an average value of 1 3 cm the maximum horizontal stress sh driving this slip falls between 350 and 004 the orientation of local alleghanian layer parallel shortening as indicated by both disjunctive and pencil cleavage by commonality of orientation we infer that slip on alleghanian joints is driven contemporaneously with layer parallel shortening if so the offset ene striking joints predate the alleghanian stress field these observations mean that both pre alleghanian and early alleghanian joints persist through a period of penetrative strain

the vancouver island seismic project a co crust onshore offshore study of a convergent margin the seismic structure of the british columbia continental margin has been investigated using four reversed refraction profiles the profile across strike extended 350 km from the volcanic arc on the continent to the deep ocean of the juan de fuca plate the three profiles along strike were located on vancouver island on the continental shelf and in the deep ocean on the juan de fuca plate interpretation of the profile along vancouver island yields a well constrained model for the upper crust with velocity increasing from 5 3 km s at the surface to 6 4 km s at 2 km depth to 6 75 km s at 15 5 km depth where the velocity increases sharply to 7 km s the velocity structure of the deep crust and the crustal thickness are poorly constrained four possible velocity functions based on ambiguous first arrivals and or secondary phases interpreted as moho reflections are presented the preferred one includes a deep crustal low velocity zone with a crustal thickness of 37 km models with a constant 7 1 km

loading paths to joint propagation during a tectonic cycle an example from the appalachian plateau u s a abstract based on the timing of joint propagation during the history of burial lithification deformation and denudation of clastic rocks within sedimentary basins four types of joints may be distinguished tectonic hydraulic unloading and release tectonic and hydraulic joints form at depth prior to uplift in response to abnormal fluid pressures whereas unloading and release joints form near the surface in response to thermal elastic contraction accompanying erosion and uplift tectonic joints are distinguished from hydraulic joints in that tectonic compaction is a mechanism for achieving abnormal pore pressures leading to the propagation of the former whereas compaction by overburden loading leads to the abnormal pore pressures in the latter case the orientation of unloading joints is controlled by either a residual or contemporary tectonic stress whereas the orientation of release joints is controlled by a rock fabric examples of some of these joints are found within the devonian catskill delta of the appalachian plateau new york during the alleghanian orogeny tectonic joints cross fold joints formed under abnormal pore pressure as indicated by the observation that joints propagated in the siltstones before they developed in shales and by the cross cutting relationships of folds cleavage and joints this sequence is compatible with oil company hydraulic fracture data which show that the least principal stress within sandstone layers is less than that in the intercalated shale layers plumose structures indicate that the joints within siltstones propagated as discontinuous rupture events each of which affected less than a meter of bed length the discontinuous rupturing is compatible with models for natural hydraulic fracturing release joints strike joints post date the alleghanian orogeny as indicated by abutting relationships within the deeper parts of the devonian clastic section unloading joints are orthogonal to the contemporary tectonic stress field

seismic refraction investigation of the black forest abstract within the framework of site surveys of the deep continental drilling program ktb deep seismic sounding experiments were carried out in the black forest southwest germany in august 1984 these site surveys comprised seismic refraction as well as seismic reflection measurements this paper will concentrate on the interpretation of the refraction data which were observed along a 240 km long line approximately following the morphological axis of the black forest in a n s direction and extending in the south into the swiss molasse basin from the available seismic refraction data a detailed velocity depth model of the crust and uppermost mantle was derived a clear phase refracted in the basement with a velocity of 5 9 km s is observed for all shots this phase is followed by a reflection from the upper boundary of a low velocity zone this zone is very accentuated under the northern and central black forest and is characterized by a velocity decrease from 6 2 to 5 4 km s between depths of about 8 and 14 km towards the south the velocity inversion gradually loses its intensity under the center of the southern black forest velocities of about 6 km s prevail throughout the upper crust the lower crust which starts at the lower boundary of the low velocity zone shows a very complicated response in the record sections due to long reverberations between travel times of 1 5 3 5 s reduced by 6 0 km s at an epicentral distance of approximately 30 70 km a classical phase correlation is not possible based on a combined interpretation of near angle and wide angle reflection data the lower crust is characterized by a stack of thin layers with alternating high and low velocities the average velocity of the laminated zone is 6 7 km s the reflection from the moho at a depth of 25 26 km is the most prominent phase at epicentral distances greater than 60 km

near surface in situ stress 4 residual stress in the tully limestone appalachian plateau new york the direction of maximum expansion during strain relaxation of the tully limestone at ludlowville new york is oriented about no4 e and thus is within a couple of degrees of the strike of both set ia cross fold joints and the compression direction indicated by the teeth of tectonic stylolites these joints and stylolites are members of a suite of structures accommodating approximately 9 layer parallel shortening during the main phrase of the alleghanian orogeny within the appalachian mountains because of the parallelism of the maximum expansion with the main phase alleghanian compression and because our measurements were made in joint bounded blocks we suggest that the expansion represents the relief of a residual stress locked into the tully limestone during the alleghanian orogeny the magnitude of the strain relaxation indicates that the residual differential stress was 14 mpa using a flow law for pressure solution we infer that the tully limestone deformed at a strain rate of about 3 10 15s 1 and thus the layer parallel shortening observed in the tully limestone may have required an aggregate deformation interval of only 1 m y

a three dimensional crustal model of southwest germany derived from seismic refraction data abstract from densely covered seismic refraction data obtained in 1978 urach experiment and 1984 schwarzer zollern wald experiment and from seismic reflection data and results from previous refraction investigations a three dimensional crustal model of southwest germany was derived travel time and amplitude information of seismic refraction data were interpreted with two dimensional forward modeling ray tracing to calculate two crustal cross sections in southwest germany these results fill a gap in the existing data and enabled the construction of a detailed three dimensional crustal model while seismically the upper crust is laterally homogeneous 5 9 6 0 km s throughout the area the middle and lower crust show pronounced lateral variations in thickness velocity and reflectivity the moho is a flat surface at a relatively shallow depth 25 26 km we classify the middle and lower crust of southwest germany into two characteristic crustal types type i consists of a mid crustal low velocity zone 5 4 5 8 km s overlying a thick 10 km high velocity 6 6 6 8 km s lower crust type ii has no prominent mid crustal low velocity zone and a thin

lithoprobe southern vancouver island seismic reflection sees through wrangellia to the juan de fuca plate multichannel seismic reflection profiles obtained on vancouver island show that above a zone of decoupling between the north american and juan de fuca plates wrangellia is underplated by two accreted terranes of probable oceanic origin both the zone of decoupling and wrangellia are disrupted by easterly dipping faults some of which are thrusts the crustal structure of wrangellia comprises paleozoic and mesozoic volcanic and sedimentary rocks disposed as roof pendants upon and large irregular masses within a ubiquitous jurassic plutonic and metamorphic complex

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

studies of the continental crust by near vertical reflection methods a review abstract the development of near vertical reflection methods for investigating the continental crust is strongly related to progress that has been made in seismic prospecting and in wide angle observation in crustal refraction studies owing to the good structural resolution of near vertical reflection methods many details concerning first order interfaces in the upper and lower crust have been established while reflectors in the upper crust often appear to resemble thrust faults or diffractions from dome like structures increased reflectivity of the lower crust often provides indications of strong interactions with the underlying mantle differences in reflection density in u s and w german surveys may be attributed mainly to the different tectonic styles rather than to the use of different techniques the occurrence of some reflectors with negative polarity within the band of reflections in the lower crust supports the concept of a laminated crust mantle transition a negative reflection polarity has also been detected at a low velocity gouge containing thrust fault in the upper crust the velocity resolution of near vertical techniques can be improved over that of wide angle methods by using spread lengths greater than 15 km regarding the accretion of the crust both autochthonous creation from the underlying mantle and horizontal allochthonous accretion along thrust faults play a major role the detection of sediments below such thrust planes and of magma bodies along other fault zones provides an interesting new view for prospecting surveys

death valley bright spot a midcrustal magma body in the southern great basin california a previously unrecognized midcrustal magma body may have been detected by cocorp deep seismic reflection profiles in the death valley region of the southern great basin high amplitude relatively broad band reflections at 6 s 15 km are attributed to partially molten material within a subhorizontal intrusion this bright spot extends laterally at least 15 km beneath central death valley a moderately dipping normal fault can be traced from the inferred magma chamber upward to a 690 000 yr old basaltic cinder cone the fault zone is inferred to have been a magma conduit during the formation of the cinder cone vertical variations in crustal reflection character suggest that the death valley magma body may have been emplaced along a zone of decoupling that separates a faulted brittle upper crust from a more ductile and or intruded lower crust the death valley bright spot is similar to reflections recorded by cocorp in 1977 in the rio grande rift where both geophysical and geodetic evidence support the inference of a tabular magma chamber at 20 km depth

seismic imaging of the elberton granite inner piedmont georgia using cocorp southern appalachian data seismic imaging of the elberton granite in the georgia inner piedmont was undertaken to define its shape and to determine its relation to the southern appalachian thrust the recent consortium for continental reflection profiling cocorp lines in the southern appalachians suggest a major thrusting of a crystalline block over sediments although the extent of this thrust is subject to interpretation the fine grained elberton of late devonian age is the only major pluton crossed by the seismic traverse and is not well defined on the unmigrated common depth point cdp seismic section migration by beam forming reveals considerable variation in the direction of returning signal and provides a clearer image of the laminated reflections ubiquitous in the southern appalachians which have been interpreted as the equivalents of the valley and ridge sediments beneath an allochthonous sheet the upper 5 km portions of the migrated and cdp sections lack events probably because of the necessary stripping mute of a large amplitude early arrival from the seismic records before further processing filtering and transformation of the data to the intercept slowness domain to eliminate this interfering arrival are attempted to enhance the upper portion of the seismic sections although these approaches do not give a consistent picture of the shallow section a pattern of scatterers located at depths of 2 3 5 km emerges these may correlate with the base of the intrusive body this identification of the granite base along with the presence of the thrust related reflectors beneath the pluton suggests a prethrust or synthrust emplacement of the granite however tectonic interpretations of the inner piedmont based on the elberton granite are severely limited by assumptions that the pluton age corresponds to time of cooling that the pluton is not fed by a narrow seismically elusive feeder and that the thrust has not been reactivated after emplacement of the intrusive body if all these assumptions hold then the late devonian age of the granite is the earliest possible time of thrusting

precursors to a magnitude 4 8 earthquake in the virgin islands spatial clustering of small earthquakes anomalous focal mechanisms and earthquake doublets abstract waveforms of small earthquakes that preceded a magnitude 4 8 earthquake in the virgin islands were studied using digital recordings from a local seismic network during the 10 months prior to the main shock a spatial cluster of five events 1 4 m 2 5 occurred within 3 km of the impending shock variations in the p wave first motions at one station as well as p sv amplitude ratios indicate that the focal mechanisms of the events within the cluster changed 2 months before the main shock the focal mechanism of the later events and of two foreshocks that occurred the day of the main event differed from the typical focal mechanism observed for the area the main event and most of its aftershocks exhibited the normal set of first motions recorded for events in this region the cluster events and the foreshocks occurred in pairs each pair with very similar waveforms cross correlations of these waveforms reveal that the earthquakes in each pair are located within about 110 m of each other each pair can be interpreted as an earthquake doublet that is two similarly sized events with adjacent rupture zones that are related in time the occurrence of doublets is unusual for events in this region and may be indicative of high stress in the vicinity of the impending shock no significant differences were observed between the spectra of the cluster events the foreshocks or the aftershocks

seismicity at white island volcano new zealand a revised classification and inferences about source mechanism the classification of earthquakes at white island volcano new zealand has been revised to address problems in existing classification schemes to better reflect new data and to try to focus more on source processes seismicity generated by the direct involvement of magmatic or hydrothermal fluids are referred to as volcanic and that generated by fault movement in response to stresses caused by those fluids regional stresses thermal effects and so on are referred to as volcano tectonic spasmodic bursts form a separate category as we have insufficient information to classify them as volcanic or volcano tectonic volcanic seismicity is divided into short duration long period volcanic earthquakes long duration volcanic earthquakes and harmonic and non harmonic volcanic tremor while volcano tectonic seismicity is divided into shallow and deep volcano tectonic earthquakes harmonic volcanic tremor is related to sub surface intrusive processes while non harmonic volcanic tremor originates close to active craters at shallow depth and usually occurs during eruptive activity short duration long period volcanic earthquakes come from a single source close to the active craters but originate deeper than non harmonic volcanic tremor and are not related to eruptive activity long duration volcanic earthquakes often accompany larger discrete eruptions the waveform of these events consists of an initial low frequency part from a deep source and a later cigar shaped part of mixed frequencies from a shallow crater source

crack propagation sequence and pore fluid conditions during fault bend folding in the appalachian valley and ridge central pennsylvania bedding parallel bpv strike sv and cross fold cfv veins represent a sequence of polyphase fracturing during the development of a fault bend fold in lower paleozoic carbonate beds in the appalachian valley and ridge province pennsylvania brecciation and layer parallel shearing played important roles in the development of the earliest vein set bpv which propagated prior to folding fluid inclusions in calcite bpv are highly saline brines 23 4 wt naci trapped at conditions close to lithostatic p 180 mpa t 267 c while passing through the lower kink plane of a fault bend fold strike joints propagated in dolomitic beds located on the extensional side of neutral surfaces stylolitization of strike joint surfaces accompanied slip of the hanging wall up a ramp renewed extension upon passing through a second kink plane led to propagation of antitaxial sv along the stylolitized joints by the crack seal process slightly less saline fluids 22 4 wt naci were trapped in sv at fluid pressures 144 mpa and temperatures 217 c with the concurrent formation of lateral ramps the carbonates moved to the upper flat and were subjected to strike parallel extension as manifested by the propagation of antitaxial cfv due to further mixing of fresh waters the salinity of fluids forming cfv decreased 20 5 wt nacl with trapping conditions at p 116 mpa and t 179 c the fluid evolution path from bpv to cfv through sv shows a modest decrease in salinity with a sharp decrease in possible trapping pressures

tracking the burial and tectonic history of devonian shale of the appalachian basin by analysis of joint intersection style the most pervasive systematic joints hosted by devonian black shale of the appalachian plateau include east northeast joints and younger northwest striking cross fold joints both sets were driven exclusively by fluid pressure generated as a consequence of hydrocarbon related maturation supplemented by subsequent tectonic compaction during the alleghanian tectonic cycle in the more deeply buried proximal region of the catskill delta joints of both sets cross cut however in stratigraphically equivalent black shale of the distal shallower region of the delta to the west 25 of the joint intersections are crosscutting whereas 75 of the intersections are defined by east northeast joints curving into and abutting cross fold joints east northeast joints in the distal shale propagated early but were neither as long nor as pervasive as similarly oriented joints in the more deeply buried proximal delta deposits however these same joints are much better developed more closely spaced stratigraphically lower in the distal shale succession where essentially all intersections are crosscutting it is likely that relatively high contact stress an effective stress on east northeast joints in the more deeply buried parts of the delta enabled the unimpeded transmission of elastic stress concentration at the tip of cross fold joints across the older joints in the distal delta exhumation of the devonian shale sequence following establishment of the contemporary stress field in middle tertiary time to within 1 km of the earth s surface resulted in renewed propagation of east northeast joints the propagation path of the reactivated joints curved into and terminated against cross fold joints at angles of 60 90 the curving to termination intersection style is a manifestation of crack tip stress field blunting as a consequence of slippage along the interface of cross fold joints enabled by exhumation induced relaxation of contact stress on these latter joints it is noteworthy that the orientation of the densely formed east northeast joints parallel to the maximum horizontal stress direction of the contemporary lithospheric stress field likely imparts a meaningful permeability anisotropy to these hydrocarbon source rocks

estimation of arrival times from seismic waves a manifold based approach summary we propose a new method to analyse seismic time series and estimate the arrival times of seismic waves our approach combines two ingredients the time series are first lifted into a highdimensional space using time delay embedding the resulting phase space is then parametrized using a non linear method based on the eigenvectors of the graph laplacian we validate our approach using a data set of seismic events that occurred in idaho montana wyoming and utah between 2005 and 2006 our approach outperforms methods based on singular spectrum analysis wavelet analysis and short term average long term average sta lta

the distribution of layer parallel shortening fabrics in the appalachian foreland of new york and pennsylvania evidence for two non coaxial phases of the alleghanian orogeny

determining the size and shape of blocks from linear sampling for geotechnical rock mass classification and assessment classifying and assessing geotechnical aspects of rock masses involves combining parameters in various ways guided by empirical considerations to derive quantitative geotechnical parameters geological structures and the deformation history of rocks underpin the nature of rock masses the kinematics of a deforming rock mass may occur as sliding along throughgoing discontinuities or as distributed sliding on block faces distributed sliding will tend to disrupt the continuity of planar structures such that data on the size and shape of blocks is needed rather than relying on discontinuity orientation data alone orientation and spacing data can be combined to provide a geometric analysis of block systems from linear samples such as drill core dihedral angles and spacing of sequential pairs of discontinuities provides a sample of the size and shape of blocks that can be interpreted stereologically further detail can be derived by combining neighbouring intersections that enclose or partially enclose individual blocks the shape and size of a block can be represented on a stereograph with the enclosing faces shown as poles and their perpendicular distance from an arbitrary point inside the block shown as a number identifying the size and shape of specific blocks rather than relying on statistical methods is beneficial to critical aspects of design such as analysing keyblocks that would be exposed during excavations the detailed characterization of block size and shape is also a step toward interpreting the kinematics of rock mass deformation and the analysis of rock masses as ultra close packed dilatant granular systems

migration of deformation fronts during progressive deformation evidence from detailed structural studies in the pennsylvania anthracite region u s a abstract detailed study of selected outcrops in the pennsylvania anthracite region has resulted in identification of five at least partly continuous stages of alleghanian progressive deformation these structural stages are in sequence a layer parallel shortening and top to the foreland layer parallel shear b a second episode of layer parallel shortening c flexural slip and flexural flow folding d fold modification and late thrust faulting and e late veining and fracturing at each outcrop an average shortening direction was determined for each observed structural stage the mean regional shortening direction for each structural stage was calculated and maps showing the spatial distribution of the shortening directions for each stage were constructed the mean regional shortening directions for the five structural stages successively rotated 25 30 clockwise during the evolution of the fold and thrust belt shortening directions for structural stages b and c over the region show a systematic clockwise change in orientation from hinterland to foreland indicating that these structural stages of progressive deformation were time transgressive on a regional scale

intracontinental seismicity strength of crustal units and the seismic signature of fault zones the distribution of intracontinental seismicity in the upper crust is compared with theoretically obtained strength depth curves based on recent laboratory data from cold and brittle frictional sliding as well as from high temperature creep data the lower crust is seen as a channel of enhanced ductility like jelly in a sandwich this rheological picture is compatible with the formation of mylonites in the lower crust and the appearance of cataclasites in the rigid upper crust sub horizontal zones of detachment prefer the upper part of the ductile lower crust in the continents resulting in the formation of crustal nappes during compressional phases the most active fault zones seem to exhibit reduced values of brittle elastic and inelastic parameters

fringe cracks key structures for the interpretation of the progressive alleghanian deformation of the appalachian plateau vertical joints in devonian clastic sedimentary rocks of the finger lakes area of new york state are ornamented with arrays of fringe cracks that reveal the complex deformational history of the appalachian plateau detachment sheet during the alleghanian orogeny three types of fringe cracks were mapped gradual twist hackles abrupt twist hackles and kinks gradual twist hackles are curviplanar en echelon fringe cracks that propagate with an overall vertical direction within the bed hosting the parent crack and are found in all clastic lithologies of the detachment sheet abrupt twist hackles propagate as planar features in thick shale beds above or below the siltstone beds hosting parent joints kinks propagate horizontally as planar surfaces from the tips of parent joints in siltstone beds the breakdown of the parent joint into either gradual or abrupt twist hackles depends on the orientation and magnitude of the remote stress field internal fluid pressure and the elastic properties of the bed the twist angle of gradual twist hackles is larger in coarser clastic beds indicating that stress and internal pressure are more important parameters than elastic properties in controlling breakdown assuming that the vertical stress axis s v equals 78 mpa at 3 km burial depth the difference in twist angle between sandstone and shale beds is used to estimate the maximum horizontal stress difference in the shale beds as s h s h 2 5 mpa when s h s h 12 mpa in sandstone beds the twist angle of the fringe cracks and the abutting relationships of parent joints give an indication of the overall change in stress field orientation within the detachment sheet during alleghanian tectonics these parent joints indicate a regional clockwise stress rotation of alleghanian age concordant with the twist angle of fringe cracks throughout the western part of the study area a counterclockwise twist angle in the eastern portion indicates a local stress attributed to drag where no salt was available to detach the eastern edge of the plateau sheet the clockwise change in stress orientation is consistent with the rotation in stress orientation found in the anthracite belt of the pennsylvania valley and ridge but is opposite to the sense of rotation in the southwestern portion of the detachment sheet western pennsylvania and west virginia the two regional rotation domains are separated by the juniata culmination

high resolution seismic reflection profiles reveal fracture zones within a homogeneous granite batholith several countries are investigating the use of plutonic rock bodies for the ultimate disposal of radioactive waste material1 among criteria2 3 that must be met before a rock body is mined for a repository is that it must have a low in situ permeability as the permeability of most unfractured plutonic rocks is very low3 the problem reduces to finding those bodies which do not contain major connected fractures that would allow radioactive material to be transported by water to the biosphere we report here that a high resolution seismic reflection survey across the central region of an archaean granitic pluton which on the basis of other data might be defined as relatively homogeneous identifies major sub horizontal fracture zones at depth such fracture zones which could be a major hazard for the containment of radioactive wastes may therefore be detected before selection of a repository site by a combination of high resolution seismic reflection surveying and suitably located deep drilling

seismic measurements of the internal properties of fault zones the internal properties within and adjacent to fault zones are reviewed principally on the basis of laboratory borehole and seismic refraction and reflection data the deformation of rocks by faulting ranges from intragrain microcracking to severe alteration saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity but from borehole measurements densely fractured rocks do show significantly reduced velocities the amount of reduction generally proportional to the fracture density highly fractured rock and thick fault gouge along the creeping portion of the san andreas fault are evidenced by a pronounced seismic low velocity zone lvz which is either very thin or absent along locked portions of the fault thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone high pore pressure within the pronounced lvz may be conductive to fault creep deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated seismically anisotropic mylonites

location and configuration of magma bodies beneath long valley california determined from anomalous earthquake signals recordings of 281 small earthquakes which occurred recently near the southern edge of long valley caldera at depths from 1 to 15 km are used to map the subsurface geometry of shear wave attenuating bodies in the caldera signatures of these events recorded northwest north northeast and east of long valley with ray paths through the caldera are often anomalous in that s wave arrivals have very low amplitudes and high frequency p and s wave energy is missing for the same station event combinations the volcanic and geothermal history of the region suggests that ray paths for these anomalous signals have passed through magma which absorbed much of the s wave energy and also attenuated the higher frequencies with over 1200 normal and anomalous ray paths through the caldera we have located two apparently massive magma bodies in the central and northwest caldera as well as two more dispersed magma areas in the southern caldera and beneath crowley lake the central magma body extends from 4 5 to at least 13 km beneath the surface and the northwest body is as shallow as 5 5 km these two magma bodies are probably connected below about 8 km secondary arrivals seen on select seismograms are tentatively interpreted as reverberations inside the central magma chamber and are consistent with the interpreted lateral extent of the magma the northwest and central magma bodies lie beneath the northwest and southeast corners of the caldera s resurgent dome and probably connect beneath the dome s center also the shallower parts of these magma bodies and the southern anomalous area lie beneath the surface expression of the medial graben faults suggesting that structural features seen at the surface of the valley may be related to magma location at depth