shear wave polarizations near the north anatolian fault ii interpretation in terms of crack induced anisotropy summary the polarizations of shear waves recorded by networks of digital thxee component seismometers immediately above small earthquakes near the north anatolian fault in turkey display shear wave splitting on almost all shear wave seismograms recorded within the shear wave window this splitting is incompatible with source radiation patterns propagating through simple isotropic structures but is compatible with effective anisotropy of the internal structure of the rock along the ray paths this paper interprets the phenomena in terms of widespread crack induced anisotropy distributions of stress induced cracks model many features of the observations and synthetic polarization diagrams calculated for propagation through simulated cracked rock are similar to the observed patterns this evidence for widespread crack induced anisotropy lends strong support to the hypothesis of extensive dilatancy anisotropy eda suggested by laboratory experiments in subcritical crack growth the crucial evidence confirming some form of eda would be observations of temporal changes in shear wave splitting as the stress field alters the crack density and crack geometry there is some weak evidence for such temporal changes at one site but further analysis of suitable digital three component seismometer networks in seismic areas is required to confirm eda

borehole response studies at the garner valley downhole array southern california the garner valley downhole array gvda consists of a set of seven downhole strong motion instruments ranging from 0 to 500 m depth one of the objectives of this experiment is to estimate site response and study wave propagation as the energy travels from the bedrock underneath the site up through the soil column the gvda velocity structure is studied by computing synthetic accelerograms for a small event located at an epicentral distance of 10 km these synthetics simulate well the data recorded at the borehole stations in addition theoretical transfer functions are calculated using the obtained velocity model and compare well with the empirical transfer functions from 54 recorded events it is also observed that the downgoing wave effect is predominant in the first 87 m and is strongly reduced at depth using the velocity structure at gvda and the transfer function results it has also been possible to develop a simple method to compute the incident wave field which is needed in nonlinear site response for instance recently there have been many comparative studies between horizontal to vertical h v spectral ratios and traditional spectral ratios although many of these studies show that h v spectral ratios can reproduce the shape of the site response curve most show differences in the amplitude level in the case of garner valley where we have both surface and multiple borehole instruments we find that this discrepancy in amplitude of the site response estimates is because the vertical component has significant site response associated with it due to s to p conversions that begin in the weathered granite boundary at 87 m depth manuscript received 27 august 2001

origin of perylene in ancient sediments and its geological significance the distributions of polycyclic aromatic hydrocarbons pahs in sediments of three upper triassic to middle jurassic sedimentary sequences from the northern carnarvon basin australia have been investigated perylene was found to be a major pah component in the top lower to base middle jurassic sediments that are immature or at low maturity its depth age profiles are not related to the combustion derived pahs that have been believed to be produced during ancient vegetation fires before deposition this suggests a diagenetic origin for perylene the concentration of perylene in the sediments is proportional to the amount of terrestrial input decreasing with distance from the source of land sediments its carbon isotope composition is slightly heavier than higher plant derived pahs but still in the range of the terrestrially sourced pahs including higher plant pahs and combustion derived pahs as suggested previously fungi are proposed to be the major precursor carriers for perylene in sediments based on the facts that 1 perylenequinone structures have been previously suggested to be the natural precursors for perylene 2 perylenequinone pigments exist in many fungal bodies 3 fungi have played an important role during geological processes

record of postglacial organic matter delivery and burial in sediments of lake ontario a 12 m piston core obtained from lake ontario has allowed us to study the accumulation of organic matter in sediments deposited since the retreat of the laurentian ice sheet about 12 500 years ago discrepancies between radiocarbon dates of disseminated organic matter and ostracod shells emphasize the magnitude of the recycling and retention of organic carbon in the great lakes concentrations of caco3 and organic carbon decrease down core due to the presence of fine grained glaciolacustrine clays at the base of the sedimentary sequence increases in sediment grain size indicate periods with enhanced fluxes of terrigenous material cn ratios organic carbon and caco3 vary proportionally with fluctuations in terrigenous input cn ratios indicate that lacustrine algae have been the main source of organic matter to lake ontario sediments organic 13c values become heavier at the bottom of the core suggesting a shift in carbon sources for the bulk organic matter anthropogenic effects are well documented in total hydrocarbon and total fatty acid profiles of modern sediments whereas postglacial trends representing natural changes of organic matter are relatively undetectable sedimentary profiles of terrigenous aquatic ratios of n alkane and n alkanoic acids indicate that early variations in postglacial sedimentation rates may have impacted the preservation of aquatic organic matter changes in watershed vegetation and organic matter delivery to lake ontario altered these ratios as deglaciation progressed organic geochemical properties of modern sediments have recorded how anthropogenic activity has augmented algal productivity in lake ontario by increasing nutrient input

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

initial shear wave particle motions and stress constraints at the anza seismic network summary we use focal plane solutions to constrain principal stress directions in the vicinity of six anza network stations which show evidence for shallow shear wave anisotropy in the vicinity of the anza seismic gap region of the san jacinto fault faulting near all stations is consistent with approximately n s maximum compressive stress five of these stations show nearly n s initial particle motion alignment consistent with the anisotropy stress relationship expected for stress aligned microcracks however one station knw has a well defined preferred initial shear wave polarization direction of n40 w although this polarization direction differs dramatically from the local maximum compressive stress direction it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals particularly biotite thus anisotropy observed at this station most likely reflects a fixed palaeostrain alignment of anisotropic minerals and or microcracks and does not require a dependence on the current stress field

a possible terrigenous origin for perylene based on a sedimentary record of a pond lorraine france abstract the origin of perylene a five ring polycyclic aromatic hydrocarbon pah in sediments has been a matter of constant debate and its formation mechanisms remain unclear a perylene record in sediment was described and could support a clear link between perylene and terrigenous organic matter om input the distributions of pahs the variations of organic proxies such as the ratio of terrigenous to aquatic n alkanes tar hc and 5 h stanols 5 sterols ratio in sediments have been investigated sediments were sampled from a pond artificially created during the middle ages from a swampy area in the period prior to the pond creation for which high contributions of terrigenous om were evidenced perylene was found to be the predominant pah furthermore perylene content was shown to increase in response to the establishment of more reducing conditions this result supports the common idea that the main source of perylene is natural and derives from biogenic precursors under reducing conditions the creation of the pond in this wetland is marked by the deposition of a wood rich level characterized both by more oxygenated conditions and higher concentrations of perylene these high concentrations could result from the introduction of high concentrations of biogenic precursors of perylene under oxidative conditions subsequently the progressive burial of the woody level could have allowed the establishment of oxygen depleted conditions and the formation of perylene these results validate the use of perylene as a paleoenvironmental marker of terrigenous sources but it must be carefully used as a marker for syn or post depositional oxygen depleted conditions in lacustrine environments

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

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

thermogenic hydrocarbons from the offshore calypso hydrothermal field bay of plenty new zealand hydrothermal gases from shallow seafloor vents in the bay of plenty new zealand contain co2 ch4 and the higher gaseous hydrocarbons up to i n c4h10 the gases are similar to those discharged at fumaroles on the nearby white island carbon isotope compositions for co2 fall between 3 4 and 5 5 pdb and reflect a shallow magmatic carbon source the 13c values of ch4 range from 24 6 to 28 9 pdb and the d values vary between 122 and 135 smow the ch4 isotope values and the presence of the higher hydrocarbon compounds such as c2h6 and c3h8 with 13c values near 20 pdb suggest hydrocarbon production by high temperature maturation of sedimentary organic matter and mixing 1 1 of the thermogenic ch4 with abiogenic ch4 long chained hydrocarbons occur in dredged samples close to the active vents their n alkane distribution has a high to moderate odd even predominance and an extensive hopane series indicative of higher land plant waxes and prokaryotic membranes in the source substantial amounts of unsubstituted polynuclear aromatic hydrocarbons pah mark the transition from aliphatic to aromatic dominated bitumens consistent with extensive source maturation resulting from thermal stress the bitumens are interpreted as pyrolysates derived from buried near coastal vegetation and terrestrial detritus under various thermal regimes mixed with immature seafloor organic matter

sedimentary organic matter sources and depositional environment in the yegua formation brazos county texas abstract the complex depositional environment of the eocene yegua formation brazos county texas can be better understood by integrating organic matter om geochemistry with stratigraphy yegua sediments represent parasequences separated by exposure surfaces organic petrography and geochemistry biomarkers c n ratios and carbon isotopes indicate the presence of both terrestrial and marine om in transgressive sediments in contrast regressive sediments contain only terrestrial om these differences relate to contrasting om sources and depositional styles on the shelf om in the sediments is immature and the potential for generating hydrocarbons is poor the study suggests that organic geochemical data can help in distinguishing transgressive and regressive environments in sedimentary formations

palaeoenvironmental implications of coal formation in dobrudzha basin bulgaria insights from organic petrological and geochemical properties abstract the dobrudzha basin in northeast bulgaria contains 86 coal seams distributed within three coal bearing formations of late carboniferous age this study presents the organic petrological and geochemical characteristics of coals from the makedonka and krupen formations containing some of the most prominent coal seams these were characterized by means of proximate and rock eval analyses vitrinite reflectance measurements organic petrography and gcms analysis in order to establish variations in vegetation and depositional settings based on random vitrinite reflectance avg 0 7 r r and toc contents avg 80 daf the coals can be classified as low rank bituminous high volatile b c low tmax values avg 430 c further indicate a low degree of organic matter maturity the low to moderately high ash yields together with the presence of shaly partings and generally low sulfur contents are consistent with peat formation in a fluviatile sedimentary environment petrographic analysis reveals complex maceral composition with abundant liptinite and inertinite the predominance of detrovitrinite and the almost complete lack of strongly gelified macerals argue for enhanced humification of the plant tissues and peat formation in relatively dry environments above water table the distributions of n alkanes diterpenoids and steranes indicate that the organic matter originated from terrestrial vascular plants the high pristane phytane ratios point to peat deposition under oxic conditions low concentrations of hopanes argue for limited bacterial degradation of the organic matter possibly due to low ph conditions during peatification the presence of tetracyclic diterpanes and their aromatic counterparts is attributed to pre gymnosperm cordaitean input the existence of cordaiteans although as minor component of the palaeovegetational communities is further evidenced by the presence of trace amounts of aromatic arborane fernane derivatives in almost all studied samples the occurrence of combustion derived pahs in all studied samples is consistent with the abundant inertinite and indicates frequent forest fires during peat formation

frequency dependent anisotropy in wellington new zealand shear wave splitting measurements are made using teleseismic s scs and sks waveforms recorded at the gdsn broadband station snzo situated in south karori new zealand the average s and sks delay times are around 2 to 3 s among the highest in the world the average scs delay time is significantly smaller around 1 s this discrepancy appears to be due to differences in dominant frequency the most likely cause of frequency dependent anisotropy is oriented heterogeneities with a scale length much smaller than the wavelength the fast polarizations range between 21 and 79 with an average ne sw direction which is sub parallel to the trend of the local geologic structure and the strike of the hikurangi subduction zone azimuthal variations in delay time which cannot be explained by differences in period may be due to a dipping axis of symmetry or laterally varying anisotropy or a more complicated symmetry system

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

characterization of fractures in shallow granite from the modeling of the anisotropy and attenuation of seismic waves abstract we try to characterize the fracturing of the anisotropic shallow granite et al 1992 is modeled by performing numerical simulations of propagation of p and sv waves in 2d fractured media the calculations are performed using a boundary integral equation method which takes into account multiple scattering and full crack diffraction the value of the time delay measured at garner valley between the fast s wave and the slow s wave is well reproduced for different models of fractured media in order to differentiate between these models we also measure and calculate the attenuation of the transmitted s waves from the comparison between the observations and the numerical simulations we conclude and infer the following 1 the presence of fractures in granite explains the seismic anisotropy and attenuation observed at garner valley 2 the cracks and fractures are nearly vertical and are oriented in a north south direction 3 there is no characteristic crack length but rather the crack length distribution seems to be fractal 4 the density of fractures present in the shallow granite at garner valley is about 10 4 representing the total volume of the fractures over the total volume of the fractured zone

perylene an indicator of alteration processes or precursor materials abstract perylene is a polycyclic aromatic hydrocarbon pah that is found pervasively in both marine and freshwater sediments yet its origin remains obscure we have explored relationships between perylene and possible precursor materials in two 210 pb dated sediment cores from lake ontario in which contributions of aquatic and land plant organic matter and concentrations of anthropogenic pahs have been measured no significant correlations were identified between perylene occurrence and indicators of terrigenous or aquatic sources of organic matter distinct differences between perylene and anthropogenic pah sedimentary profiles rule out an industrial source for perylene at these locations the general pattern of low to absent perylene concentrations in upper oxic sediments and increasing concentrations with depth indicates that in situ diagenesis under anoxic conditions produces perylene and that its formation is kinetically controlled the results of our study suggest that perylene is formed from non specific precursor materials by biotic or abiotic transformation processes if this scenario accurately describes its formation then perylene is an indicator of depositional conditions rather than the source of organic matter

geological link between fluid inclusions dilatant microcracks and paleostress field seismic investigations using three component instruments have demonstrated shear wave splitting in widely different geological and tectonic environments shear wave analysis suggests that the inferred anisotropy can best be explained if the rocks contain distributions of subparallel subvertical fluid filled microfractures which are aligned according to the current stress field the crampin extensive dilatancy anisotropy eda model geological evidence for widespread fluid activity in the earth s crust supports this hypothesis and suggests that the myriads of planes of microscopic secondary fluid inclusions which characterise most geological materials represent fossil eda cracks field studies confirm a strong spatial relationship between the orientation of fluid inclusion arrays and the paleodirections of maximum compressive stress at elevated temperatures and pressures the healing of fluid filled cracks in the crust of whatever origin is considered to be a fairly rapid process moreover the inclusions which form as a result of crack healing respond to changes in temperature pressure and stress by changing shape and or volume this indicates a high degree of fracture compliance sufficient perhaps to react to rapid changes in the current stress field and associated shear wave splitting though individual inclusions typically 20 m will have no effect on the propagation of seismic waves with wavelengths of tens to hundreds of meters the rock volume sampled by the waves will be rendered anisotropic if the overall average of the inclusions shows a preferred orientation thus fluid inclusions may constitute one of the principal types of fluid filled microcrack described by the eda hypothesis

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

garner valley downhole array of accelerometers instrumentation and preliminary data analysis the garner valley downhole array gvda in southern california 33 41 60 n 116 40 20 w is a five element array of three component dual gain force balance accelerometers that are capable of measuring accelerations from 3 10 6 to 2 0 g over a frequency range from 0 0 to 100 hz the accelerometers are placed at depths of 0 6 15 22 and 220 m the lithostratigraphy is 19 m of soil overlying a 24 m layer of weathered granite below which is granite tonalite the array only 7 km from the san jacinto fault is located at the northern end of the anza seismic gap on the san jacinto fault where an m 6 5 or greater earthquake can be expected it is only 35 km from the indio segment of the san andreas fault which last ruptured with an m 8 earthquake around 1700 a d from its installation in july 1989 through july 1991 gvda has recorded 280 earthquakes with magnitudes ranging from 1 2 to 4 7 and epicentral distances ranging from from 0 to 110 km the maximum acceleration recorded was 89 cm sec 2 from an m l 4 2 earthquake 15 8 km hypocentral distance from gvda the average amplification of the apparent seismic moment between 220 m depth and the surface is about 13 for over five orders of magnitude in the size of the recorded earthquakes the spectral ratio 0 220 of horizontal acceleration amplitude spectrum shows a mean amplification of about 10 for the frequency range of 2 0 to 30 hz for 17 events resonance peaks exist at about 1 7 3 0 and 12 0 hz where the spectral ratio 0 220 is nearly 40 analysis of the acceleration spectra of two specific earthquakes that have nearly the same hypocenter but with different magnitudes m 4 2 and 2 5 shows that the weathered granite zone beneath the soil strongly attenuates the spectral amplitudes for frequencies greater than 40 hz the impedance of the soil relative to the weathered granite amplifies all frequencies by about a factor of 3 leading to spectral levels that are greater at the surface than at 22 m depth near the top of the weathered granite the acceleration spectra at each depth show resonant peaks at the same frequencies the spectra at each depth are well correlated for both earthquakes the acceleration spectrum at 220 m is nearly constant for frequencies less than 60 hz which implies that there is little or no attenuation

perylene occurrence alkylation and possible sources in deep ocean sediments abstract 81 deep ocean sediment samples obtained from the dsdp ipod program legs 63 66 and six gravity cores from dsdp ipod leg 64 guaymas basin gulf of california site survey cruise sio were analyzed for perylene s perylene was found to be most abundant in anoxic diatomaceous sediments containing moderate amounts of silts and or clays elevated perylene yields normalized to organic carbon contents coincided with enhanced preservation of autochthonous tetraterpenoid and tetrapyrrole pigments indicating a requirement of anoxia in the formation of perylene strongly euxinic conditions correlate with decreased perylene yields but not its total absence as found for strongly aerobic deposition and or redeposition perylene alkylation occurs in response to increasing in situ temperatures alt sub bottom depth this alkylation was found to begin with the generation of methyl c 1 perylene and to eventually yield a perylene alkyl perylene homologous series in decreasing abundance through the equivalent of c 3 trimethyl methyl ethyl propyl perylene the realm of most intense perylene alkylation for quaternary or pliocene miocene sediments corresponds with present in situ temperatures of 40 50 c or 30 35 c respectively thus the degree of perylene alkylation may represent an organic indicator for the thermal history of immature sediments geological phenomena such as reworking and igneous intrusions were found to complicate downhole perylene alkylation patterns a substituted c4h7o2 perylene was isolated from several anoxically deposited diatomaceous oozes and may represent one of the many possible perylene precursors the possible source compounds and biota for sedimentary perylene are discussed in light of the present study and review of past reports

biotite chemistry and the role of halogens in archaean greenstone hosted gold deposits a case study from geita gold mine tanzania abstract geita hill is one of the largest tanzanian gold deposits located southwest of lake victoria and 2 km along strike from the giant nyankanga deposit the deposit is hosted by metasedimentary rocks dominated by ironstones and intruded by numerous diorite dykes textural structural and geochronological observations indicate that the gold mineralisation overprints all the ductile fabrics and the late orogenic d6 reverse faulting event suggesting that gold was introduced during later d7 and d8 events the metamorphic background is characterized by biotite k feldspar actinolite pyrhotite which is overprinted by the gold related alteration consisting of sulfidation fronts veins and micro fractures dominated by quartz biotite k feldpar pyrite the almost identical mineralogy of the mineralised assemblage and the metamorphic background makes it difficult to identify the source the effect and the spatial extent of the mineralising fluid as part of this study detailed analyses of biotite were conducted in order to identify the nature of the mineralising fluid and its spatial effect across the geita hill deposit results show that the mineralised assemblage is slightly more oxidized pyrite magnetite compared to the metamorphic background pyrrhotite magnetite the intense sulfidation within the ore zone resulted in the formation of mg rich biotite which grades into more fe rich biotite away from the ore zone this change in biotite chemistry causes a shift in the swir fe oh 2250 nm absorption feature to lower wavelengths which is also correlated with an increase in gold grade within the mineralised zone the abundance of f in biotite significantly increases within the ore zone indicating the presence of f rich hydrothermal fluids during mineralisation late timing of mineralisation synchronous with the emplacement of voluminous high k granites and the presence of k and f bearing fluids indicate a genetic connection between the mineralising fluids and the late granites biotite grains from different deposits types porphyry iocg lode au appear to have distinct composition suggesting that the source of the fluids involved in the formation of each deposit type has a characteristic halogen chemistry

observation of shallow anisotropy on local earthquake records at the garner valley southern california downhole array abstract we study very shallow s wave anisotropy effects on local earthquake seismograms recorded by a vertical array of accelerometers at garner valley southern california using seismic events recorded by sensors buried at depth in the crystalline bedrock 220 m in weathered crystalline rocks 22 m and in sediments 15 6 and 0 m we analyze the s wave vertical travel times and polarizations at different depths hodograms plotted for horizontal components at s onset exhibit a clear linear and constant polarization independent of both earthquake location and mechanism this alignment varies with depth and is n 325 13 at 220 m while n 25 20 at 22 15 6 and 0 m a for the deep propagation 220 m the s wave direction of polarization at 220 m is identical to the observation obtained on a crystalline outcrop at the knw usgs stations 7 km away studies conducted on this site concluded to a rock fabric anisotropy due to the alignment of minerals and or microcracks along the lineation direction b for the shallow s fast s slow travel time delays roughly correspond to half the dominant period of the signal and the subsurface linear polarization is interpreted as a degenerated ellipse the north 20 anisotropy fast axis coincides with the maximum horizontal compressive stress associated with the san andreas fault system the shallow anisotropy extension corresponds to slow velocities associated with altered granite it suggests either a stress induced anisotropy due to pore crack deformation in an altered e g hydrothermally medium or the superposition of two crystalline units having different lineation directions these data show that in presence of slow 0 to 200 m subsurface layers shallow anisotropy strongly affects the earthquake surface records in the usual frequency range below 30 hz used in short period seismology

3d modeling of fracture induced shear wave splitting in the southern california basin splitting of shear waves generated by local earthquakes in the southern california basin and their polarization changes during propagation through layers with different orientations of anisotropy are investigated from multicomponent seismograms and their hodograms synthetic data for earthquakes in the southern california basin illustrate the effects on polarization caused by dry vertical microcracks with a crack density of 0 04 at the source and receiver locations hodograms for observed and synthetic data show that the polarization at the receiver depends only on the orientation of the anisotropy within two wavelengths of the receiver which qualitatively explains the frequency dependence of anisotropy in previous studies seismograms are simulated for a model with a range of station azimuths and distances if the anisotropy producing crack orientations do not change between the source and a receiver and the source motion is parallel to the cracks there is no shear wave splitting splitting occurs when the anisotropy orientation at the receiver or between the source and receiver is different from the source orientation

spatial variability and non linearity of strong ground motion near a fault summary we present observations of ground accelerations recorded at a small array close to the fault during the duzce earthquake and its early aftershocks the records show the strong spatial variability of ground acceleration over distances of only a few hundred metres during the main shock the peak horizontal acceleration values ranged from 0 3 to about 1 0 g at stations distant of 1 5 km only we attribute this spatial variability to a fault zone site effect as peak ground acceleration steadily increases as the distance to the fault trace decreases the spectral ratio between the ground motion recorded near the fault and the one outside the fault zone shows a shift of the spectral peak to lower frequencies with increasing peak accelerations such an observation suggests a non linear behaviour of the fault zone due to the strong ground shaking as much as a 45 per cent reduction in the shear wave velocity is necessary for the observed shifts the opening of pre existing cracks throughout the fault zone is the proposed mechanism to account for the observed shear wave reductions the observation that elastic fault zone properties are soon recovered following episodes of large strains shows that cracks and fissures close rapidly after the strong shaking is over