source characteristics of the nw himalaya and its adjoining region geodynamical implications abstract in this study we propose a novel interpretation of source parameters estimated using brune model for the nw himalaya and its adjoining region analysis of 157 events 2 2 mw 4 9 resulted in the scaling relationship between seismic moment m0 and corner frequency fc for the region as m0 8 1022 fc 2 89 which is in good unison to the garhwal and the kumaon himalaya with analogous seismogenic potential variability of kappa 0 01 s 0 08 s with varying number of events 2 5 m 5 0 at different depths is indicative of nature and extent of structural heterogeneity beneath the source zone which controls the propensity of background seismicity that provides enough testimony to complex geodynamical set up of the nw himalaya and its adjoining region source radii of source zone varying from 148 m to 975 m with static stress drops calculated for a given constant stress of 0 1 bar 1 bar 10 bar and 100 bar correspond to an average stress drop varied from 0 4 bar to 99 9 bar seismic moments of events vary from 2 64 1019 dyne cm to 2 32 1023 dyne cm with increase of the source radii we infer that variability in stress drop for seismic moment mo is attributed to different seismic sources of varying strengths associated with different structural heterogeneities stress drop of the events has good correspondence with source depth having the linear relationship of shear stress regime in the brittle crust with stress drop and source depth suggesting the presence of intricate subsurface seismogenic sources beneath the nw himalaya and its adjoining region

young ores in old rocks proterozoic iron mineralisation in mesoarchean banded iron formation northern pilbara craton australia abstract the origin of bedded iron ore deposits developed in greenstone belt hosted algoma type banded iron formations of the archean pilbara craton has largely been overlooked during the last three decades two of the key problems in studying these deposits are a lack of information about the structural and stratigraphic setting of the ore bodies and an absence of geochronological data from the ores in this paper we present geological maps for nearly a dozen former mines in the shay gap and goldsworthy belts on the northeastern margin of the craton and the first u pb geochronology for xenotime intergrown with hematite ore iron ore mineralisation in the studied deposits is controlled by a combination of steeply dipping ne and se trending faults and associated dolerite dykes simultaneous dextral oblique slip movement on se trending faults and sinistral normal oblique slip movement on ne trending faults during initial ore formation are probably related to e w extension uranium lead dating of xenotime from the ores using the sensitive high resolution ion microprobe shrimp suggests that iron mineralisation was the cumulative result of several proterozoic hydrothermal events the first at c 2250 ma followed by others at c 2180 ma c 1670 ma and c 1000 ma the cause of the first growth event is not clear but the other age peaks coincide with well documented episodes of orogenic activity at 2210 2145 ma 1680 1620 ma and 1030 950 ma along the southern margin of the pilbara craton and the capricorn orogen farther south these results suggest that high grade hematite deposits are a product of protracted episodic reactivation of a structural architecture that developed during the mesoarchean the development of hematite mineralisation along major structures in mesoarchean bifs after 2250 ma implies that fluid infiltration and oxidative alteration commenced within 100 myr of the start of the great oxidation event at c 2350 ma

synorogenic hydrothermal origin for giant hamersley iron oxide ore bodies geologic mapping basin analysis and calculated fluid compositions indicate that giant orebodies of microplaty hematite and possibly martite goethite in the hamersley province of western australia were formed by heated fluids driven by early paleoproterozoic orogenesis detrital grains of microplaty hematite in the mcgrath trough a foreland basin in front of the northward advancing ophthalmian fold belt constrain the age of the earliest microplaty hematite ore formation to 200 c and locally up to 400 c were involved regional circulation of hydrothermal fluids including heated surface water through reduced banded iron formations occurred during or soon after the ophthalmian orogeny we speculate that martite goethite orebodies previously considered mesozoic cenozoic could also be related to heated paleoproterozoic meteoric fluids migrating northward away from the ophthalmian fold belt

fault plane orientations of small earthquakes of the 1997 umbria marche italy seismic sequence from p wave polarities and rise times summary a two step technique has been developed with the aim of retrieving the fault plane orientation of a small magnitude earthquake the technique uses a set of non linear equations which relate the rise times of first p waves to source parameters source dimension l dip and strike of the fault and intrinsic qp at the first step of the technique the inversion of p polarities provides two fault plane orientations for each focal mechanism solution at the second step the inversion of rise times is carried out to retrieve l and qp by fixing and to the values of the fault plane orientation inferred at the first step a robust analysis based on the random deviates technique allows us to evaluate if one fault plane exists which systematically better fits data and can be considered the true fault plane a parameter is introduced to quantify the level of resolution of the true fault plane a probabilistic approach based on the assumption that errors on data are gaussian distributed allows us to a posteriori validate or reject the true fault plane the technique has been applied to 47 small earthquakes 1 3 ml 3 9 occurred below the town of sellano during the 1997 umbria marche central italy seismic crisis the strike of the inferred fault planes is generally along the apennine direction fault plane solutions can be mainly subdivided into two groups a first group of sw dipping faults and a second group of ne dipping faults these results agree with the present day knowledge of the fault systems in the area an average qp equal to 354 63 has been estimated in agreement with previous attenuation studies

qc qβ qi and qs attenuation parameters in the umbria marche italy region the attenuation of coda and s waves has been inferred for the umbria marche region central italy using seismic waveforms collected during the 1997 seismic crisis the selected dataset is composed of 343 small magnitude 1 4 ml 4 2 earthquakes recorded at a temporary array composed of 23 seismic stations the sato 1977 method based on the assumption of single isotropic scattering has been used to infer qc considering three different lapse times 20 30 and 40 s the coda normalization method aki 1980 has been used to infer q both qc and q show a clear frequency dependence with a different frequency dependent parameter the frequency dependence of qc is comparable with that previously found in the same area and around it using the method of wennerberg 1993 intrinsic and scattering attenuation have been separated intrinsic attenuation is found to be close to coda attenuation and dominates over scattering dissipation coda q increases with increasing lapse time but at a rate smaller than that observed in other areas of the earth coda and intrinsic attenuation in the umbria marche region are very high compared to other seismic active regions of the earth the retrieved high values of intrinsic attenuation may be explained in terms of the previously hypothesized fluid pressurized regime of the crust in the central apennines miller et al 2004

source parameters estimated from the aftershocks of the 1997 umbria marche italy seismic sequence we derived the source parameters of 563 aftershocks 1 4 m l 4 5 of the 1997 umbria marche seismic sequence by analyzing three component digital recordings of a temporary network and developed a local magnitude scale using the whole available data set more than 25 000 waveforms from synthetic wood anderson seismograms then considering a subnetwork composed of nine portable stations more than 12 000 earthquake spectra were corrected for the near surface attenuation parameter k and the path attenuation the corrected spectra were used to compute the seismic moment the source radius and stress drop assuming an 2 brune source model we calculate the relationship log m 17 46 1 12 m l between seismic moment and local magnitude for the studied area the source dimension versus seismic moment relationship did not show any breakdown in the similarity of the rupture process even when events of magnitude less than 2 2 the threshold value for the completeness of the considered catalog were considered the average stress drop obtained analyzing the s wave spectra was 38 10 bar and the source dimension ranged between about 40 and 500 m

the 1997 umbria marche italy earthquake sequence a first look at the main shocks and aftershocks a long sequence of earthquakes six with magnitudes between 5 and 6 struck central italy starting on september 26 1997 causing severe damages and loss of human lives the seismogenic structure consists of a nw se elongated fault zone extending for about 40 km the focal mechanisms of the largest shocks reveal normal faulting with ne sw extension perpendicular to the trend of the apennines consistently with the quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions preliminary data on the main shocks and aftershocks show that extension in this region of the apennines is accomplished by normal faults dipping at low angle 40 to the southwest and confined in the upper 8 km of the crust these normal faults might have reactivated thrust planes of the pliocene compressional tectonics the aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km long fault segments adjacent and slightly offset from one another

palaeomagnetism and magnetic anisotropy of proterozoic banded iron formations and iron ores of the hamersley basin western australia abstract rock magnetic properties and palaeomagnetism of weakly metamorphosed banded iron formations bifs of the palaeoproterozoic hamersley group western australia and proterozoic bif derived iron ores have been investigated the bif units sampled here are slightly younger than 2500 ma at paraburdoo mount tom price and mount newman iron ore formation was completed before 1850 ma sampling was mainly from the mount tom price and paraburdoo mining areas and for the first time a palaeomagnetic fold test on fresh unweathered and unaltered bif samples has allowed the nature of the remanence of the bifs to be defined the remanence of the bifs is carried by late diagenetic low grade metamorphic magnetite after primary haematite this remanence is pre folding and is unlikely to be greatly affected by the high anisotropy because the palaeofield inclination was demonstrably low determination of palaeofield directions from measured remanence directions is complicated by self demagnetization effects in strongly magnetic highly anisotropic bif specimens we present a method for correcting measured directions for the effects of self demagnetization and anisotropy for typical bifs the effect of magnetic anisotropy on measured remanence inclinations and inferred palaeolatitudes is minor for low palaeolatitudes but can lead to large errors in calculated palaeopoles for intermediate to moderately steep palaeolatitudes anisotropy also causes cones of confidence to be underestimated due to compression of the range of inclinations in principle deflection of post folding remanence towards the bedding plane by high magnetic anisotropy can produce an apparent syn folding signature with best agreement between directions from different fold limbs after partial unfolding thus high anisotropy cannot only bias estimated palaeofield directions and cause underestimation of errors but can also mislead interpretation of the time of remanence acquisition the anisotropy of anhysteretic remanent magnetization arm probably yields an upper limit to the anisotropy of the chemical remanent magnetization crm carried by the bifs therefore from the anisotropy of arm a maximum inclination deflection of 9 is suggested for the sampled bifs this corresponds to less than 5 change of palaeolatitude the palaeomagnetic pole position calculated for bifs at paraburdoo is 40 9 s 225 0 e dp 2 9 dm 5 8 after tilt correction but without correction for anisotropy other pole positions reported include that from flat lying bifs from wittenoom at 36 4 s 218 9 e dp 4 6 dm 9 1 from mount tom price iron ore at 37 4 s 220 3 e dp 5 7 dm 11 3 and from paraburdoo ore at 36 4 s 209 9 e dp 4 7 dm 8 8 the poles from the bifs the paraburdoo ore and the part of the tom price deposit that was sampled in this study are indistinguishable from each other and from the mount jope volcanics overprint pole the magnetization of the bifs was probably acquired during burial metamorphism of the hamersley group soon before the main folding and uplift event in the southern part of the hamersley province this tectonic event exposed magnetite rich bifs to near surface oxidizing conditions producing extensive martite goethite orebodies and also appears to have produced the syn folding overprint magnetization recorded by the mount jope volcanics of the underlying fortescue group ages of magnetization are tentatively interpreted as 2200 100 ma for the bifs 2000 100 ma for the supergene enrichment of bif to martite goethite ore recorded by the parabudoo and mount tom price orebodies and 1950 100 ma for the metamorphic martite microplaty haematite ore recorded as an overprint by the tom price orebody and as the only surviving magnetization of the mount newman orebody

estimation of source parameters of local earthquakes originated in shillong mikir plateau and its adjoining region of northeastern india abstract we estimate the source parameters seismic moment source radius stress drop and source displacement and scaling laws for local earthquakes that occurred in the shillong mikir plateau assam valley and arunachal himalaya in northeast india during 2001 2008 the source parameters were determined using the spectral analysis of p waves from the vertical component seismograms after correction for attenuation seismic moments are observed within the range from 9 51 10 12 to stress drop ranges from 4 10 5 to 9 10 7 pa for the brune model and 7 10 5 to 1 10 8 pa for the madariaga model seismic events in this study are prominent with an average stress drop of 0 1 10 mpa the effect of site geology may be a contributing factor for such a variation in stress drop source dimensions are however found to be smaller within the major part of the plateau it is suggested that local earthquakes in the region are associated with a brittle shear failure mechanism on fault segments and or the presence of weakened zones and earthquakes are triggered by low deviatoric stress empirical relations between m w m l and m 0 m l are developed leading to the future prediction of calibration coefficients for the local earthquakes in the shillong mikir plateau and its adjoining region online material tables of source parameters

redox state of the archean atmosphere evidence from detrital heavy minerals in ca 3250 2750 ma sandstones from the pilbara craton australia the presence of detrital uraninite and pyrite in fluvial placers of the witwatersrand basin south africa has been used to infer low levels of atmospheric oxygen during the archean 2500 ma however recent studies advocate a hydrothermal origin for these minerals thereby limiting their value as constraints on the composition of the early atmosphere in contrast ca 3250 2750 ma fluvial siliciclastic sediments from the pilbara craton in australia have never undergone significant hydrothermal alteration and their heavy minerals are of unequivocal detrital origin these heavy minerals include the redox sensitive phases pyrite uraninite and gersdorffite along with more inert zircon rutile chromite and monazite locally siderite is a major constituent to 90 of the heavy mineral population with grains displaying evidence for several episodes of erosion rounding and subsequent authigenic overgrowth detrital siderite is very rare in post archean sandstones largely due to its instability in oxidizing environments however its frequent survival of prolonged transport in well mixed and therefore well aerated archean river waters that contained little organic matter strongly implies that the contemporary atmosphere was indeed much less oxidizing than at present moreover concentrations of reduced sulfur species must have been very low in surface fluids for siderite to survive repeated transportation events without pyritization

stratigraphy of the late palaeoproterozoic 2 03 ga wooly dolomite ashburton province western australia a carbonate platform developed in a failed rift basin abstract this paper describes the sequence stratigraphy of the 2 03 ga wooly dolomite the uppermost stratigraphic unit of the horseshoe basin strata of which unconformably overlie the 2772 2410 ma hamersley province unconformably truncate folded 2210 ma dolerite sills and post date the 2195 2145 ma ophthalmia orogeny the horseshoe basin was an intracontinental rift that initiated at 2 05 ga with deposition of 360 m of fluvial and shallow marine siliciclastic sandstones beasley river quartzite followed by eruption of 2 7 km of flood basalt cheela springs basalt and culminated with deposition of 325 m of platformal carbonates and associated volcaniclastic and siliciclastic sediments wooly dolomite the three formations of the horseshoe basin stack conformably with tectonically driven unconformities developed only at a late stage within the wooly dolomite depositional systems of the wooly dolomite appear to have been compartmentalised but connected to an ocean five depositional sequences are identified depositional sequence 1 which developed conformably on the cheela springs basalt records the establishment of a carbonate platform coeval with volcaniclastic sedimentation but it was developed only in the southeastern most sector of the basin resedimented tuffs volcaniclastic siltstones are present in all of the depositional sequences but are most abundant in ds1 depositional sequence 2 cuts deeply into the cheela springs basalt and has a lower section of siliciclastic sedimentary rocks that is overlain by carbonate platform deposits the wyloo dome at an interpreted rift triple junction was an uplifted area until the late stage of ds2 implying that accommodation generation was also fault compartmentalised depositional sequences 3 4 and 5 have predominantly carbonate platform deposits and are also unconformity bound whereas ds4 and ds5 preserve a shelf slope transition subsidence in the horseshoe basin ended with basin inversion during the d1 panhandle deformation event which predated emplacement of an 2008 ma dolerite dyke swarm a subsequent rifting event recorded by the mcgrath basin led to a rift drift transition that finally initiated a west facing atlantic type continental margin

p and s wave attenuation in the region of marche italy we have found empirical functions to describe the amplitude decay with distance of p and s waves in the marche region of italy these functions result from the analysis of spectral amplitudes of 161 three component digital records from 26 earthquakes recorded by the marchesan seismograph network rsm jointly run by the italian seismological survey of rome ssn and the geophysical observatory of macerata ogsm the events have magnitudes in the range 1 8 m 3 6 and were recorded at hypocentral distances between 9 and 125 km in order to study the possible variations of the quality factor q with the direction of wave propagation q anisotropy we made independent estimates of the attenuation for p sv and sh waves using the longitudinal vertically polarized and transversely polarized components respectively for the whole distance range and in the frequency band analyzed 1 f 25 hz the frequency dependence of q can be approximated as qp 108 4 f0 70 for the p waves qsv 105 6 f0 65 for sv waves and qsh 86 1 f0 70 for sh waves the small difference between the sh and sv spectral decay is within the statistical uncertainties of the estimates suggesting that q anisotropy is negligible however when only deeper wave paths are used in the analysis we observed that at low frequencies f 4 hz the values of qsv are significantly above the estimates of qsh this observation provides additional evidence of the seismic anisotropy reported by margheriti et al geophys res lett 23 2721 2724 1996 for the region the low q anisotropy is possibly related with the weakly deformed adriatic foreland the high attenuation observed for both p and s waves can be explained by the double low velocity layer reported by ponziani et al tectonophysics 252 391 418 1995 since the seismic energy can be trapped between these layers in addition the low values of q obtained for distance ranges between 5 and 50 km can be due to the presence of hot material in the lower crust beneath the northern apennines as suggested by mele et al j geophys res 102 11863 11875 1997

site response of strong motion stations in the umbria central italy region we used near field and regional records from 16 earthquakes m l 4 6 5 9 located in the epicentral area of the 1997 umbria marche sequence to estimate the site response of 40 strong motion stations the distribution of hypocenters covers a distance range from 5 to 100 km we estimated site amplification factors using two techniques namely a generalized spectral inversion method and horizontal to vertical component spectral ratios hvsrs of ground acceleration for the first approach we inverted observed spectral amplitudes for site q and source using a reference site since the site response estimated using hvsrs assumes that the vertical component of motion is amplification free we also separated source and site effects by inverting the vertical component of the acceleration spectra we found that although most of the stations do not show important amplifications on the vertical component a few of them have a significant vertical amplification at low frequencies f q representative of the s wave train shows a frequency dependence that can be approximated by the relation q s 31 2 f 1 2 between 0 3 and 9 5 hz however at high frequencies f 10 hz this dependence weakens and q s takes an approximately constant value of 438

the ml scale in southern california measurements 9 941 of peak amplitudes on wood anderson instruments or simulated wood anderson instruments in the southern california seismographic network for 972 earthquakes primarily located in southern california were studied with the aim of determining a new distance correction curve for use in determining the local magnitude m l events in the mammoth lakes area were found to give an unusual attenuation pattern and were excluded from the analysis as were readings from any one earthquake at distances beyond the first occurrence of amplitudes less than 0 3 mm the remaining 7 355 amplitudes from 814 earthquakes yielded the following equation for m l distance correction log a 0 log a 0 1 110log r 100 0 00189 r 100 3 0 where r is hypocentral distance in kilometers a new set of station corrections was also determined from the analysis the standard deviation of the m l residuals obtained by using this curve and the station corrections was 0 21 the data used to derive the equation came from earthquakes with hypocentral distances ranging from about 10 to 700 km and focal depths down to 20 km with most depths less than 10 km the log a 0 values from this equation are similar to the standard values listed in richter 1958 for 50 200 km the effect at close distances is consistent with that found in several other studies and is simply due to a difference in the observed 1 r geometrical spreading for body waves and the 1 r 2 spreading assumed by gutenberg and richter in the construction of the log a 0 table m l s computed from our curve and those reported in the caltech catalog show a systematic dependence on magnitude small earthquakes have larger magnitudes than in the catalog and large earthquakes have smaller magnitudes by as much as 0 6 units to a large extent these systematic differences are due to the nonuniform distribution of data in magnitude distance space small earthquakes are preferentially recorded at close distances relative to large earthquakes for large earthquakes however the difference in the two magnitudes is not solely due to the new correction for attenuation magnitudes computed using richter s log a 0 curve are also low relative to the catalog values the differences in that case may be due to subjective judgment on the part of those determining the catalog magnitudes the use of data other than the caltech wood anderson seismographs the use of different station corrections or the use of teleseismic magnitude determinations whatever their cause the departures at large magnitude may explain a 1 0 0 7 proportionality found by luco 1982 between m l s determined from real wood anderson records and those from records synthesized from strong motion instruments if it were not for the biases in reported magnitudes luco s finding would imply a magnitude dependent shape in the attenuation curves we studied residuals in three magnitude classes 2 0 m l 3 5 3 5 m l 5 5 and 5 5 m l 7 0 and found no support for such a magnitude dependence based on our results we propose that local magnitude scales be defined such that m l 3 correspond to 10 mm of motion on a wood anderson instrument at 17 km hypocentral distance rather than 1 mm of motion at 100 km this is consistent with the original definition of magnitude in southern california and will allow more meaningful comparison of earthquakes in regions having very different attenuation of waves within the first 100 km

local magnitude scale and seismicity rate for tanzania east africa abstract a local magnitude scale is developed for tanzania east africa using data collected by the 1994 1995 tanzania broadband seismic experiment the waveform data from 1189 local and regional earthquakes located within east africa were corrected for instrument response and convolved with the nominal wood anderson torsion seismograph response appropriate for the original definition of local magnitude a total of 24 710 maximum s wave amplitudes were incorporated into an iterative regression for individual event local magnitudes 38 horizontal component station factors and 2 linear distance dependent factors in the log a 0 term of the equation for m l the resulting distance correction log a 0 is given by log a 0 0 776 log r 17 0 000902 r 17 2 0 where r is hypocentral distance in kilometers this distance correction yields much less ground motion attenuation than observed for southern california and is similar to that observed for local s waves and regional lg waves propagating in eastern north america normalizing the distance correction using the standard constraint of 1 mm ground motion at 100 km for a magnitude 3 0 earthquake results in a bias of nearly half of a magnitude unit between m b and m l with m l being larger normalizing using the constraint of 10 mm of motion at 17 km for m l 3 0 removes the bias in magnitude measures and indicates that structure within tanzania is relatively high q the seismicity rate of tanzania for 1994 1995 was examined using the gutenberg richter seismicity distribution and is seen to follow the relation log n 4 63 0 84 m l where n is the number of earthquakes per year of local magnitude m l or greater the catalog of events used in this study is seen to be complete to magnitude 2 to 2 5

initiation and arrest of earthquake ruptures due to elongated overstressed regions the initiation of natural and induced earthquakes is promoted in fault areas where the shear stress is close to fault strength in many important situations these overstressed fault areas or asperities are very elongated for example in the case of a fault intersecting a reservoir subject to fluid injection or the stress concentration along the bottom of a seismogenic zone induced by deep fault creep theoretical estimates of the minimum overstressed asperity size leading to runaway rupture and of the final size of self arrested ruptures are only available for 2d problems and for 3d problems with an asperity aspect ratio close to one in this study we determine how the nucleation of ruptures on elongated asperities and their ensuing arrest depend on the size and aspect ratio of the asperity and on the background stress based on a systematic set of 3d dynamic rupture simulations assuming linear slip weakening friction we find that if the shortest asperity side is smaller than the 2d critical length the problem effectively reduces to a 2d problem in which rupture nucleation and arrest are controlled by the shortest length of the asperity otherwise nucleation and rupture arrest are controlled by the asperity area with a minor exception for asperities with shortest side slightly larger than the 2d critical length arrested ruptures are smaller than predicted by the asperity area the fact that rupture arrest is dominantly controlled by area even for elongated asperities corroborates the finding that observed maximum magnitudes of earthquakes induced by fluid injection are consistent with the theoretical relation between the magnitude of the largest self arrested rupture and the injected volume galis et al 2017 in context of induced seismicity our simulations provide plausible scenarios that could be either favourable or challenging for traffic light systems and provide mechanical insights into the conditions leading to these situations

evidence for the variation of stress drop between normal and thrust faulting earthquakes in italy the horizontal components of 45 strong motion accelerograms recorded in friuli italy from may to september 1976 have been analyzed in order to estimate the stress drop of eight thrust faulting earthquakes two different stress parameters the brune stress drop and the apparent stress a have been considered the friuli main shock has a strong stress drop 800 bars while the aftershocks have stress drops which range from 250 to 400 bars five accelerograms written by the montenegro yugoslavia earthquake of april 1979 have also been analyzed this thrust faulting earthquake had a stress drop of 900 bars the stress drops of these thrust faulting earthquakes are compared to the stress drops determined by rovelli et al 1988a for nine normal faulting earthquakes which occurred in the apennines region from 1979 to 1984 the radiated energies and apparent stresses have also been calculated for the set of apennine earthquakes the thrust faulting earthquakes in the alpine dynaride region exhibit higher values for both the brune and the apparent stress than the normal faulting earthquakes in the apennines region excluding the friuli and montenegro main shocks the brune stress drops of the thrust faults are approximately 3 times those of the normal faults while the apparent stresses are twice as large this difference is statistically significant at a confidence level of 95 the same difference also emerges from the scaling of the peak ground motions recorded in these two regions suggesting an enhanced severity of the seismic input in the alpine dynaride environment compared with the apennines

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

giant iron ore deposits of the hamersley province related to the breakup of paleoproterozoic australia new insights from in situ shrimp dating of baddeleyite from mafic intrusions banded iron formations of the ca 2770 2405 ma hamersley province of western australia were locally upgraded to high grade hematite ores during the early paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen ore genesis was associated with the stratigraphic break between the lower and upper wyloo groups of the ashburton province and has been variously linked to the ophthalmian orogeny late orogenic extensional collapse and anorogenic continental extension small spot in situ pb pb dating of baddeleyite by sensitive high resolution ion microprobe shrimp has resolved the ages of two key suites of mafic intrusions constraining for the first time the tectonic evolution of the ashburton province and the age and setting of iron ore formation mafic sills dated as ca 2208 ma were folded during the ophthalmian orogeny and then cut by the unconformity at the base of the lower wyloo group a mafic dike swarm that intrudes the lower wyloo group and has a close genetic relationship to iron ore is ca 2008 ma slightly younger than a new syneruptive 2031 6 ma zircon age for the lower wyloo group these new ages constrain the ophthalmian orogeny to the period between ca 2208 and 2031 ma before lower wyloo group extension sedimentation and flood basalt volcanism the ca 2008 ma dikes pre s ent a new maximum age for iron ore genesis and deposition of the upper wyloo group thereby linking ore genesis to a ca 2050 2000 ma period of continental extension similarly recorded by paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca 2320 ma

the 1997 1998 umbria marche sequence central italy source path and site effects estimated from strong motion data recorded in the epicentral area 1 we apply a nonparametric spectral inversion scheme to calculate source spectra s wave attenuation and site transfer functions from strong motion records in the epicentral region of the 1997 1998 umbria marche seismic sequence central italy we use records from moderate size earthquakes 4 6 ml 5 9 to parameterize the spectral amplitude decay in the distance range from 5 to 40 km we find that the average quality factor q can be approximated by q f 49 f0 9 between 0 5 and 8 hz and the geometrical spreading by r 0 9 at high frequencies f 8 hz the dependence of q on frequency weakens and it takes an approximate constant value of 318 we fit the source spectra to the square model and calculate an average stress drop of 2 1 mpa the average value is consistent with the previous estimates from the weak events 1 4 ml 4 5 of the umbria marche seismic sequence the most remarkable site effects are found in correspondence of large sedimentary basins filled by alternation of sandy clayey deposits the estimated spectral parameters are used to simulate acceleration spectra recorded during several earthquakes of the umbria marche sequence both point source and finite fault effects are considered furthermore attenuation relationships for peak ground velocity and ground acceleration are estimated using synthetic data and compared to existing relationships

the mulgandinnah shear zone an archean crustal scale strike slip zone eastern pilbara western australia abstract a large part of the deformation in the archean pilbara granitoid greenstone terrain is localized in relatively narrow shear zones the mulgandinnah shear zone msz is a major one of these with a width up to 8 km that can be followed for over 70 km along strike in the shaw batholith in the eastern pilbara it forms part of the mulgandinnah lineament that can be traced to the lalla rookh basin and the carlindi batholith in the north giving it a total length of over 150 km the msz contains both mylonites and ultramylonites both of which have foliations that are subvertical to steeply dipping with the ultramylonitic foliation overprinting the mylonitic foliation to form more localized zones of high strain stretching lineations are mostly subhorizontal to shallow south plunging for both the mylonites and ultramylonites but can vary between 50 n and 50 s both foliations formed under upper greenschist to amphibolite grade conditions the msz displays a consistent sinistral sense of shear as indicated by and type porphyroclasts s c fabrics and rotated pegmatite veins the mylonites and ultramylonites are interpreted to have developed as part of a progressive deformation event leading to strain localization in the ultramylonites timing constraints indicate that the msz has been active at ca 2930 ma but it is likely to have been a long lived structure that was reactivated at that stage when it acted as a conjugate shear during a nw se crustal shortening in the eastern pilbara the final juxtaposition of the various domains in the pilbara granitoid greenstone terrain occurred during this deformation phase

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

an egf technique to infer the rupture velocity history of a small magnitude earthquake 1 an empirical green s function egf technique has been developed to detect the rupture velocity history of a small earthquake the assumed source model is a circular crack that is characterized by a single and unipolar moment rate function mrf the deconvolution is treated as an inverse problem in the time domain which involves an assumed form of the moment rate function mrf the source parameters of the mrf are determined by adopting a global nonlinear inversion scheme a thorough synthetic study on both synthetic and real seismograms allowed us to evaluate the degree of reliability of the retrieved model parameters the technique was applied to four small events that occurred in the umbria marche region italy in 1997 to test the hypothesis of a single rupture process the inversion results were compared with those arising from another egf technique which assumes a multiple rupture process for each event the best fit model was selected using the corrected akaike information criterion for all the considered events the most interesting result is that the selected best fit model favors the hypothesis of a single faulting process with a clear variability of the rupture velocity during the process for the studied events the maximum rupture speed can even approach the p wave velocity at the source as theoretically foreseen in studies of the physics of the rupture and recently observed for high magnitude earthquakes

stress drop variations and source scaling relations of moderate earthquakes of the indian tectonic plate we investigate the source characteristics of 53 earthquakes in and around the indian subcontinent through a grid search technique applied to the p and s wave displacement spectra of regional waveforms registered at 42 broadband seismic stations in addition we estimate the p and s wave attenuation factors representative of the various tectonic regions that constitute the indian shield we discuss the sensitivity of the displacement spectrum to various parameters and demonstrate that for a brune type source model the seismic moment is the best estimated parameter compared to others such as the corner frequency our estimates of moment magnitudes are within the range of 1014 1020 n m with the crack radii being less than 12 km the nonuniform distribution of the static stress drop along the himalaya burmese arc and the western indian regions indicates that the central himalayas are prone to frequent failures owing to the lesser stress drop in contrast with the northwestern himalaya and burmese arc regions our results suggest that median stress drops are independent of the earthquake mechanism online material list of events and associated estimated parameters

source parameters of the december 2011 january 2012 earthquake sequence in southern carpathians romania abstract the seismicity at the contact between the getic depression and the south carpathians is part of the overall seismicity characterizing the contact of the moesian platform and the south carpathians orogen the december 2011 january 2012 earthquake sequence that occurred close to tg jiu city provides the best data set for the seismic activity in the region the seismic source parameters are estimated for 15 events of the sequence using the empirical green s function and spectral ratio techniques we selected 3 main events and 12 associated collocated small events as empirical green s functions to calculate the spectral ratios and determine the relative source time functions estimates of the source duration and corner frequency imply stress drop values in the range of 6 112 mpa relative small radius of the source and high stress drops suggest an intraplate type behaviour the scaling relationships investigated comply well with similar relationships in other regions in the world and in other seismogenic areas in the south carpathians region

determination of δσ and κ0 from response spectra of large earthquakes in greece we fit an 09 2 model to response spectra from eight recent greek earth quakes ranging in size from m 5 8 to m 6 9 the diminution parameter k o was determined for each site with a value near 0 06 for a typical soil site the stress parameter aa showed little variation from earthquake to earthquake and had a mean value of 56 bars over all earthquakes predictions of peak velocity peak acceleration rupture duration and fault length using the derived stress parameters are consistent with observations frequency dependent site amplifications were included in all es timates the combined effect of amplification and attenuation had a maximum value close to a factor of 2 5 for a typical soil site relative to the motions at the surface of a perfectly elastic uniform half space composed of materials near the source the results form the foundation for predictions of strong motions in greece for distances and magnitudes other than those for which data are available

the magnitude frequency distribution of earthquakes recorded with deep seismometers at cajon pass southern california abstract the cumulative b value the slope of the gutenberg richter relationship between earthquake occurrence rate and magnitude is commonly found to be constant 1 network catalogues however reveal a decrease at small magnitudes b value is not just an artifact of catalogue incompleteness but that small earthquakes are really not as numerous as a constant b value extrapolated from larger events would predict in the cajon pass area southern california the b value of seismicity recorded by the local network scsn appears to decrease below about m l 1 6 in order to investigate whether this decrease is real or simply represents the network detection threshold we use seismicity recorded by the deep 1 5 and 2 5 km seismometers deployed in the cajon pass scientific drillhole between april 1992 and october 1994 the maximum amplitudes recorded downhole are compared to scsn magnitudes for events recorded by the network to determine the relationship between amplitude and m l as a function of hypocentral distance from the borehole magnitudes are then calculated for 1300 earthquakes which occurred within 40 km of the borehole magnitude frequency curves are calculated for those events within 18 km of the borehole and a constant b value is observed to m l 0 5

improved calibration procedures and new standards for u pb and th pb dating of phanerozoic xenotime by ion microprobe abstract xenotime is a widely occurring mineral that is amenable to u pb and th pb dating but often is found as micrometre sized crystals that can only be dated by in situ microanalytical techniques determining accurate ages for phanerozoic samples and assessing concordance in older samples requires accurate determination of pb u and pb th however ion microprobe data for these ratios are affected by the highly variable trace element composition of xenotime we have identified calibration procedures including matrix corrections for the effects of the dominant trace elements u th and ree that provide an accuracy of 1 for pb u and

attenuation and source parameters of shallow microearthquakes at mt etna volcano italy we estimated the attenuation laws of high frequency seismic waves in the shallow crust depth 5 km and earthquake source parameters by using a se lected data set of 320 shallow events 2 6 md 4 2 recorded at mt etna volcano during the last two flank eruptions occurring in 2001 and 2002 2003 the quality factor q was estimated from spectra of p and s waves for 24 stations of the local permanent network by applying a spectral ratio technique the results show varia tions in both qp and qs as a function of frequency according to the power law q q0f n with n ranging between 0 3 and 1 3 for p waves and between 0 2 and 0 9 for s waves as typical of volcanic environments strong azimuthal variations of qp were also found suggesting the presence of local strong lateral heterogeneities and or of fluid filled cracked volumes after correction for attenuation we estimated the source parameters seismic moment source radius and stress drop of a subset of 66 shallow events under the assumption of a circular dislocation the estimated seismic moments m0 range from 10 13 to 10 15 n m the source radii r are confined between 100 and 1000 m and stress drop dr ranges between 0 2 mpa and about 4 mpa combining the source parameters obtained in this study with those calculated by pataneet al 1997 for an old data set of smaller microearthquakes 10 9 m0 10 14 n m recorded in the same area we re evaluated the scaling relationship between seismic moment m 0 and corner frequency f c for the earthquakes with m 0 ranging between 10 13 and 10 15 n m we confirm that microearthquakes at mt etna seem not to obey a

two episodes of regional scale precambrian hydrothermal alteration in the eastern pilbara western australia abstract two regional scale hydrothermal events have been identified and mapped in the precambrian rocks of the eastern pilbara western australia the first post dates lithification and tilting of the 2630 6 ma carawine dolomite and pre dates the 1317 11 ma manganese group it involved epithermal mostly silica alteration with subordinate carbonate and minor chlorite and hematite the most significant result of this event was the development of a regional scale silica carapace known as the pinjian chert breccia on the carawine dolomite ranging from a few metres to about 100 m thick the second event comprised two phases the first phase involved regional scale hematite quartz mineralisation that was broadly synchronous with deposition of the manganese group the second involved regional scale dissolution of carawine dolomite and local precipitation of barite and manganese oxide it post dates lithification of the lower part of the manganese group and was either synchronous with or post dated deposition of the upper part of the group this study highlights the important roles of bedding parallel aquicludes and unconformities in precambrian hydrothermal systems the role of physical and chemical variations in lithologies in controlling the locations of alteration and mineralisation and shows that ancient crustal fractures acted repeatedly as fluid conduits it also shows that precambrian hydrothermal events in non magmatic rocks in non orogenic settings can be intense and widespread affecting thousands of square kilometres further by dating such events links can be made between relatively undeformed sedimentary successions and seemingly unrelated precambrian orogens the study also challenges a long standing supergene based model for the formation of silica and manganese in the eastern pilbara and adds a new component to the pilbara iron ore story

hydrothermal origin for the 2 billion year old mount tom price giant iron ore deposit hamersley province western australia giant iron ore deposits such as those in the hamersley province of northwestern australia may contain more than a billion tonnes of almost pure iron oxides and are the world s major source of iron it is generally accepted that these deposits result from supergene oxidation of host banded iron formation bif accompanied by leaching of silicate and carbonate minerals new textural evidence however shows that formation of iron ore at one of those deposits mount tom price involved initial high temperature crystallisation of magnetite siderite iron silicate assemblages this was followed by development of hematite and ferroan dolomite bearing assemblages with subsequent oxidation of magnetite leaching of carbonates and silicates and crystallisation of further hematite preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids initial hematite crystallisation occurred at about 250 c and high fluid pressures and continued as temperatures decreased although the largely hydrothermal origin for mineralisation at mount tom price is in conflict with previously proposed supergene models it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation

a pre 2 2 ga age for giant hematite ores of the hamersley province australia one current model for the origin of giant hematite ores of the hamersley province requires a two step process formation of martite and goethite by supergene enrichment followed by burial metamorphism to convert goethite to microplaty hematite our new structural stratigraphic and geoehronological data suggest that at least some microplaty hematite ores were present before metamorphism owing to burial under the lower wyloo group a shrimp u pb zircon age from volcaniclastic breccia within the lower wyloo group suggests that hematite detritus in that group was derived from hematite ores older than 2209 or 15 ma the action of pre 2 2 ga oxidizing fluids is implied although their origin supergene versus hypogene remains controversial

rupture directivity of the major shocks in the 1997 umbria marche central italy sequence from regional broadband waveforms we derive the relative moment rate function for the main shocks in the fall 1997 central italy seismic sequence mw 5 7 6 0 5 6 by applying an empirical green function method by using three conveniently placed broad band mednet stations we obtain clear source time functions indicating directivity effects for all of the three earthquakes with southeastward rupture propagation for the two southernmost events and an opposite direction for the other and largest one taking a simple source model forward modeling is utilized for determining fault parameters such as length rupture propagation velocity and direction and slip distribution

tranquillityite the last lunar mineral comes down to earth tranquillityite fe 2 8 zry 2 ti 3 si 3 o 24 was first discovered in mare basalts collected during the apollo 11 lunar mission to the sea of tranquillity the mineral has since been found exclusively in returned lunar samples and lunar meteorites with no terrestrial counterpart we have now identified tranquillityite in six dolerite dikes and sills from western australia terrestrial tranquillityite commonly occurs as clusters of fox red laths closely associated with baddeleyite and zirconolite in quartz and k feldspar intergrowths in late stage interstices between plagioclase and pyroxene its composition is relatively uniform comprising mostly si zr ti and fe with minor al mg mn ca nb hf y and rare earth elements its habit and chemistry are consistent with tranquillityite in lunar basalts and it has a face centered cubic subcell similar to that of annealed lunar tranquillityite unlike coexisting baddeleyite and zirconolite it is commonly altered to a secondary intergrowth of submicron phases comprising mainly si ti and ca with minor zr in situ sensitive high resolution ion microprobe shrimp u pb geochronology of tranquillityite from sills intruding the eel creek formation northeastern pilbara craton yields a 207 pb 206 pb age of 1064 14 ma this age indicates that the previously undated sills belong to the ca 1070 ma warakurna large igneous province extending the geographic range of this mafic complex the date also provides a new minimum age 1 05 ga for the intruded sedimentary rocks which were previously thought to be neoproterozoic examination of dolerite from western australia suggests that tranquillityite is a relatively widespread albeit volumetrically minor accessory mineral and where sufficiently coarse it represents an exceptional new u pb geochronometer

review secular tectonic evolution of archean continental crust interplay between horizontal and vertical processes in the formation of the pilbara craton australia the archean pilbara craton contains five geologically distinct terranes the east pilbara karratha sholl regal and kurrana terranes all of which are unconformably overlain by the 3 02 to 2 93 ga de grey superbasin the 3 53 3 17 ga east pilbara terrane ep represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3 53 3 43 3 35 3 29 and 3 27 3 24 ga each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3 72 ga and melting of crust to generate first tonalite trondhjemite granodiorite ttg and then progressively more evolved granitic magmas in each case plume magmatism was accompanied by uplift and crustal extension the combination of conductive heating from below thermal blanketing from above and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust these mantle melting events caused severe depletion of the subcontinental lithospheric mantle making the ep a stable buoyant unsubductable continent by c 3 2 ga extension accompanying the latest event led to rifting of the protocontinent margins at between 3 2 and 3 17 ga after 3 2 ga horizontal tectonic forces dominated over vertical forces as revealed by the geology of the three terranes karratha sholl and regal of the west pilbara superterrane the c 3 12 ga whundo group of the sholl terrane is a fault bounded 10 km thick volcanic succession with geochemical characteristics of modern oceanic arcs including boninites and evidence for flux melting that indicate steep archean subduction at 3 07 ga the 3 12 ga sholl terrane 3 27 ga karratha terrane and c 3 2 ga regal terrane accreted together and onto the ep during the prinsep orogeny this was followed by development of the de grey superbasin an intracontinental sag basin and widespread plutonism 2 99 2 93 ga as a result of orogenic relaxation and slab break off craton wide compressional deformation at 2 95 2 93 ga culminated with 2 91 ga accretion of the 3 18 ga kurrana terrane with the ep this compression caused amplification of the dome and keel structure in the ep final cratonization was effected by emplacement of 2 89 2 83 ga post tectonic granites

in situ u pb dating and element mapping of three generations of monazite unravelling cryptic tectonothermal events in low grade terranes abstract in situ u pb dating of monazite and xenotime in sedimentary rocks from the mid archean soanesville group in the pilbara craton yields ages for provenance diagenesis and multiple low grade metamorphic events detrital monazite and xenotime grains give dates 3250 ma whereas diagenetic xenotime provides a new minimum age of 3190 10 ma for deposition of the basal soanesville group previously constrained between 3235 ma and 2955 ma metamorphic monazite provides evidence for three episodes of growth at 2 88 2 16 and 1 65 ga element mapping of monazite for la sm y and th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ u pb dating by sensitive high resolution ion microprobe shrimp specifically la and sm distributions closely correlate with different generations of monazite the presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology it also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire potentially multi aged crystals the ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton the 2 88 ga age coincides with a phase of regional deformation metamorphism and gold mineralization along a major crustal lineament whereas the most common monazite age population at 2 16 ga corresponds with the migration of a foreland fold and thrust belt across the craton the youngest age 1 65 ga coincides with an episode of tectonic reworking in the capricorn orogen along the southern pilbara margin the prolonged history of monazite growth may in part relate to channelized fluid flow during reactivation of long lived n to ne trending crustal structures that transect the craton despite repeated episodes of metamorphism the isotopic system in each generation of monazite remained unperturbed yielding precise dates the ability of monazite to record three separate events and in some instances two events in a single crystal distinguishes it from most other low temperature mineral chronometers which are readily reset during metamorphic overprinting low temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far field fluid flow related to tectonic processes the previously unrecognized history of crustal fluid flow in the pilbara craton has implications for chemical mineralogical and isotopic studies seeking to understand conditions on the early earth

comprehensive analysis of earthquake source spectra in southern california we compute and analyze p wave spectra from earthquakes in southern california between 1989 and 2001 using a method that isolates source receiver and path dependent terms we correct observed source spectra for attenuation using both fixed and spatially varying empirical green s function methods estimated brune type stress drops for over 60 000 m l 1 5 to 3 1 earthquakes range from 0 2 to 20 mpa with no dependence on moment or local b value median computed stress drop increases with depth in the upper crust from about 0 6 mpa at the surface to about 2 2 mpa at 8 km where it levels off and remains nearly constant in the midcrust down to about 20 km however the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop spatially coherent variations in median stress drop are observed with generally low values for the imperial valley and northridge aftershocks and higher values for the eastern transverse ranges and the north end of the san jacinto fault we find no correlation between observed stress drop and distance from the san andreas and other major faults significant along strike variations in stress drop exist for aftershocks of the 1992 landers earthquake which may correlate with differences in main shock slip

tectonic settings of archaean 3325 2775 ma crustal supracrustal belts in the west pilbara block abstract the archaean pilbara block comprises six fault bounded tectonostratigraphic domains the west pilbara block contains from east to west domains 4 5 and 6 review of stratigraphic magmatic structural and geochronologic data and new field mapping indicates that the crustal and supracrustal rock records correlate to three megacycles megacycle ivu 3325 3135 ma is recorded by an intra arc basin and arc granitoids in domain 6 and a back arc basin in domains 4 and 5 the domains were an arc continent orogen from 3200 3155 ma during which the outboard are setting was accreted to the back arc continental margin although domain 6 is allochthonous with respect to domains 5 and 4 it was not exotic with respect to the tectonic settings of the pilbara continent megacycle vl 3135 2955 ma is recorded by in decreasing age i a back arc basin and arc granitoids in domain 5 ii a deep marine retroarc or remnant ocean basin in domain 4 and a plutonic belt in domain 5 iii a plutonic belt in domains 5 and 6 and possibly also in domain 4 and iv craton wide deformation belts coeval with the whim creek basin and granitoid plutons in domain 5 the whim creek basin was a half graben that developed at a releasing bend or overstep along the boundary fault to domains 4 and 5 and to the northeast domains 4 and 6 the basin was closed by sinistral transpression as the boundary fault between domains 4 and 6 propagated to the southwest from the releasing bend and thereby generating a restraining bend geometry fundamental strike slip faults which are interpreted to have been tectonic escape responses to a cause external to the preserved pilbara block juxtaposed domains craton wide the initial stage of megacycle vu 2955 2775 ma is recorded by the eroded remnants of a rift province preserved elements include the negri basin domain 5 only mafic ultramafic sub volcanic layered intrusions and granitic plutons and related high level sills two periods of granitoid intrusion are preserved base metal sulphide deposits hosted by the whim creek basin were probably sourced from extensional geofluids in the rift province a plume origin is likely for the komatiitic parent magmas to sub volcanic intrusions and to basalts of the negri basin plume magmatism is interpreted to have been coincident with but not the cause of lithospheric extension because plumerelated volcanism post dates the first period of plutonic intrusion by 25 million years extension was followed by orogenic deformation that converted domains 4 5 and 6 to a hinterland fold thrust belt deformation is interpreted to have been caused by collision orogeny during terrane accretion along the western pilbara margin collision orogeny was the last event of a long history of pacific type tectonics along the margins of the archaean pilbara continent

matrix effects and calibration limitations in ion probe u pb and th pb dating of monazite abstract monazite can be used for u pb and th pb dating of a wide variety of rocks and geological processes particularly metamorphism in many instances ion microprobe analysis is the preferred method for dating monazite combining good analytical precision with m scale spatial resolution however ion microprobe dates can be compromised by matrix effects on the ionisation efficiencies of different secondary ionic species which can bias monazite pb tho n and pb uo n data by as much as 40 and 30 respectively for known monazite compositions these effects appear to be entirely attributable to the sputtering process in which case similar effects exist in data from all ion probes the effects on data ratios are modified sometimes exacerbated or reversed when data are subjected to the correlation based pb th and pb u calibrations that are now commonly used one dimensional calibrations of 206 pb 270 uo 2 and 208 pb 264 tho 2 are recommended corrections derived from data for a suite of reference materials with diverse compositions can reduce biases in 208 pb 232 th ages to 206 pb 238 u ages to 204 pb can be addressed without iterative data reduction

prolonged history of episodic fluid flow in giant hematite ore bodies evidence from in situ u pb geochronology of hydrothermal xenotime abstract absolute ages for hydrothermal mineralization and fluid flow are critical for understanding the geological processes that concentrate metals in the earth s crust yet many ore deposits remain undated because suitable mineral chronometers have not been found the origin of giant hematite ore deposits which are hosted in precambrian banded iron formations bifs remains contentious several models have been formulated based on different sources and timing for the mineralizing fluids supergene metamorphic syn orogenic late orogenic extensional collapse and syn extensional precise geochronology of the ore offers a means of discriminating between these models in this study two u pb chronometers xenotime and monazite have been identified in high grade hematite ore bodies from the mount tom price mine in the hamersley province northwestern australia both phosphate minerals occur as inclusions within the hematite ore and as coarser crystals intergrown with martite hematite pseudomorph after magnetite and microplaty hematite indicating that the xenotime and monazite precipitated during mineralization in situ u pb dating by ion microprobe indicates that both phosphate minerals grew during multiple discrete events our results suggest that ore genesis may have commenced as early as 2 15 ga with subsequent hydrothermal remobilization and or mineralization at 2 05 ga 1 84 ga 1 67 ga 1 59 ga 1 54 ga 1 48 ga and 0 85 ga the location of the ore bodies along ancient fault systems and the coincidence of at least some of the u pb phosphate dates with episodes of tectonothermal activity in the adjacent proterozoic capricorn orogen implies that fluids were channelled through major structures in the southern pilbara craton during discrete phases of tectonic compression and extension our results show that the hematite ore bodies formed at sites of repeated focussed hydrothermal fluid flow in contrast to the aforementioned models our results imply that iron ore formation was probably a long lived multi stage process spanning more than one billion years

source parameters of earthquakes recorded near the itoiz dam northern spain we calculate the source parameters and attenuation from earthquakes recorded near the itoiz dam from 2004 to 2009 with magnitudes ranging between 1 2 and 5 2 we use a genetic algorithm in order to fit the three component p wave spectra with the spectral level corner frequency and attenuation factor as searching parameters the obtained moments range from 1 72 1011 to 2 65 1015 nm the radii span from 0 09 to 1 00 km and the stress drops vary from 0 006 to 29 462 mpa the maximum value for the q attenuation factor is 794 and the minimum value is 53 we find a good agreement between empirical and theoretical relationships between moment and magnitude there seems to be a breakdown of self similarity but it could be due to the method used we group the data by means of a self organizing map and the clusters found are related by their magnitude and not by other considerations

source parameter scaling and radiation efficiency of microearthquakes along the irpinia fault zone in southern apennines italy we analyzed the p and s wave displacement spectra of 717 microearthquakes in the moment range 4 109 2 1014 n m recorded at the dense networks operating in southern apennines italy and deployed along the 1980 ms 6 9 irpinia earthquake fault zone source attenuation and site parameters are estimated by using a parametric modeling approach which is combined with a multistep nonlinear inversion strategy we found that in the analyzed frequency band an attenuation model with constant q has to be preferred to frequency dependent q models consistent estimates of the median p and s quality factors q p 167 90 296 and q s 226 114 417 are obtained from two different techniques and relatively high values of qs qp median value 1 3 0 8 2 1 are found in the same depth range where high vp vs and a peak in seismicity distribution are observed this is the evidence for a highly fractured partially or completely fluid saturated medium embedding the irpinia fault zone down to crustal depths of 15 20 km a nearly constant stress drop 1 4mpa 0 4 5 0 and apparent stress a 0 1mpa 0 03 0 4 scaling of p and s corner frequencies and seismic energies is observed above a seismic moment value of about 1011 n m the measured radiation efficiency is low sw 0 06 0 030 13 e g the radiated energy is only a small fraction of the whole energy spent by friction and fracture development a large positive dynamic overshoot high dynamic shear strength can be the dominant mechanism controlling the microearthquake fractures along the irpinia fault zone

source parameters of microearthquakes in southeastern sicily italy we computed the source parameters of 135 local earthquakes that occurred in southeastern sicily italy between 1994 and 2001 magnitudes ranged from 0 6 to 2 5 the source parameters were determined from the spectral analysis of p waves after corrections for attenuation and site effects p wave quality qp and site s factors are known in the two frequency ranges 3 9 hz and 16 27 hz giampiccolo et al 2003 because the correction for qp and s over the whole spectrum could not be performed a new approach to estimate spectral parameters was applied we defined the new notion of holed spectrum to specify a spectrum lacking corrected spectral amplitude values within a certain frequency range whereas we referred to a whole spectrum to specify when the entire analyzed frequency band was used a detailed statistical analysis showed that dealing with the holed spectrum did not lead to significant differences in the estimate of spectral parameters displacement holed spectra corrected by the attenuation and site response were then used to estimate source parameters seismic moments ranged from 1017 to 1020 dyne cm and source dimensions from 150 to 500 m in general the stress drop values obtained analyzing p wave spectra were very low because they mainly span the range of 0 05 5 bars

moment tensor analysis of the central italy earthquake sequence of september october 1997 the larger earthquakes in the umbria marche central italy seismic sequence of september october 1997 are analyzed using long period seismograms from the mediterranean seismographic network mednet and additional data from the global seismographic network gsn we modify the harvard centroid moment tensor cmt algorithm to allow moment tensor inversion of long period waveforms primarily rayleigh and love waves for small earthquakes at local to regional distances 5 5 in the sequence moment tensors have previously been determined using teleseismic waveforms and standard methods of analysis our results agree well with those of earlier studies we determine additional moment tensors for the largest foreshock and 10 aftershocks with mw 4 2 the earthquakes are characterized by normal faulting mechanisms with a ne sw tension axis and the presumed fault plane dips towards the sw only one of the fourteen events studied has a different faulting geometry indicating instead right lateral strike slip faulting on a plane oriented approximately e w or left lateral faulting on a plane oriented n s the september 26 mainshock 09 40 ut accounts for only approximately 50 of the total moment release in the sequence

source parameters of microearthquakes at mount st helens usa summary we estimate the source parameters for a selection of microearthquakes that occurred at mount st helens in the period 1995 1998 excluding the activity of 2004 september this time period includes the most intense episode of earthquake activity since the last dome building eruption in 1986 october 200 seismograms were processed to obtain seismic moments source radii stress drops and average fault slip the source parameters were determined from the spectral analysis of p waves after correction for attenuation and site effects in particular p wave quality qp and site s factors have been previously calculated in the frequency ranges 2 7 hz and 18 30 hz because it was impossible to perform corrections for qp and s over the whole spectrum we applied a new approach based on the notion of holed spectrum to estimate spectral parameters the term holed spectrum indicates a spectrum lacking corrected spectral amplitude values at certain frequencies we carried out a statistical study to verify that dealing with the holed spectrum does not lead to significant differences in the estimates of spectral parameters we also investigated the dependence of spectral parameters low frequency level corner frequency and high frequency decay on the bandwidth of spectral hole and defined the threshold values for three different spectral models displacement holed spectra corrected by attenuation and site response are then used to determine spectral parameters in order to calculate seismic source parameters seismic moments range from 1017 to 1019 dyne cm source dimensions from 100 to 350 m and average fault slip from 0 003 to 0 1 cm self similarity seems to break down in that stress drops are very low 0 1 1 bars we postulate that seismicity is associated with a brittle shear failure mechanism occurring in a highly heterogeneous material under a relatively low stress regime

bootstrap inversion for vertical and lateral variations of the s wave structure and the υp υs ratio from shallow earthquakes in the rhinegraben seismic zone germany using s wave arrival times of 400 earthquakes in the southern rhinegraben 1 d and 3 d inversions for the shear s wave crustal structure of that region are performed these models are then combined with previously obtained 1 d and 3 d p wave models to compute vertical and lateral variations of the p s poisson s ratio such an independent p and s wave inversion approach circumvents many of the limitations of the classical wadati diagram method usually used for the determination of crustal p s ratios for the statistical evaluation of the 1 d models we have used a new monte carlo bootstrap resampling technique which has been found useful in many statistical estimation problems where the underlying statistics of the data is poorly known the 1 d bootstrap p s crustal models exhibit significant vertical variations whereas the upper sedimentary and crystalline layer of the crust is characterized by a significant increase of p s over the reference value p s in the upper and middle crust is lowered this correlates well with several findings from wadati diagrams for p s in other seismically active zones and might be explained through the gradual closing of saturated microcracks in that depth range however the p s reduction can also be interpreted in terms of a particular petrological composition such as orthoclase feldspar without the indispensable need for fluids a lowering of p gus is also observed for the lower crust which might reflect the strong lamination found here from previous deep seismic sounding experiments for the upper mantle a strong increase of p s is observed which is primarily triggered by a decrease of s and is best explained by an eclogite petrological assemblage for the 3 d s wave models significant reductions of more than 100 were obtained for the unexplained travel time residuals variations of p s in the upper and middle crust are mostly induced by changes in p rather than s for the upper crust slight decreases of p s are observed for the rhinegraben but increases are detected for the adjacent vosges and black forest areas in the middle crust the refraction seismic deduced reduction of p s under parts of the black forest can neither be corroborated nor rejected however because of some experimental ambiguity on the effects of petrological composition and interstitial pore fluids this does not rule out the presence of the latter in the lower crust a local reduction of p s is observed under the kaiserstuhl and graben black forest shoulder and has been explained in terms of a metamorphic dehydrating phase change of the hydrous amphibolite facies to the anhydrous granulite facies depending on the pore pressure of the dehydrated water and the fractional amount of interstitial co2 granulite facies metamorphism occurs in the 650 800 c temperature range temperatures at the upper end of this interval can be generated by the mechanism of accretionary magmatic underplating as it has been proposed for some extensional rift zones where asthenospheric upwelling as in the rhinegraben occurs

geochronology of a late archaean flood basalt province in the pilbara craton australia constraints on basin evolution volcanic and sedimentary accumulation and continental drift rates abstract eleven high precision 2 5 million years shrimp zircon u pb ages have been obtained from felsic rocks within a single stratigraphic section of late archaean volcanic and sedimentary rocks in the east pilbara of western australia the stratigraphic succession nullagine and mount jope supersequences in sequence stratigraphic terminology fortescue group in lithostratigraphic terminology is interpreted to be the rock record of three major geotectonic cycles that formed in an extensional rift related environment between about 2772 and 2715 ma the geochronology is constrained by a detailed stratigraphic framework based on unconformities and supported by a preliminary magnetostratigraphy field mapping geochemical and petrographic studies have shown that previously unrecognised thin felsic tuff bands are interbedded in subaerial flood basalt piles and mafic tuffs while flood basalts and proximal felsic volcanic rocks comprise by volume most of the volcanogenic components of the succession felsic volcanism is now known to have been active periodically through each geotectonic cycle the succession covers a time period of about 57 million years the lower 1400 m of a thick 1700 m clastic sedimentary succession from the oldest geotectonic cycle was deposited at a rate of about 100 m per million years over a mean time period of 14 million years in contrast a younger 150 m thick cogenetic tuff basalt unit accumulated in less than 3 million years and others probably accumulated at similar rates comparable to those of phanerozoic flood basalts unconformities in the succession are shown to be of variable duration and one unconformity marking the boundary between the first and second geotectonic cycles may represent a time gap of more than 10 million years the unconformity based stratigraphic framework the new geochronology and palaeomagnetic studies j geophys res 108 2003 b12 2551 emp 2 1 to 2 21 have been combined to determine a possible late archaean continental drift rate for one part of the succession implying a period of motion as fast as or up to five times faster than any known from the phanerozoic

sequence stratigraphy of the archaean supracrustal belts of the pilbara block western australia abstract the stratigraphic succession of the 3580 2770 m y old granite greenstone terrain of the pilbara block is re assessed by the first and second order sequence stratigraphic techniques that are the standard for phanerozic terrains with the view to testing the conformity of stratigraphic cause and response a holistic tectonostratigraphic framework is presented that pairs the supracrustal and crustal rock records the resulting sequence scheme is tested against the cause of first order sequence cycles viz global sea level responses to supercontinent breakup and assembly three first order inter regional unconformities divide the stratigraphic succession of the supracrustal belts into four units of first order megasequence status in order of decreasing age these are named the warrawoona gorge creek roebourne and mount negri megasequences a total of 17 second order supersequences are also named although some may represent supersequence sets the megasequences and or their genetic crustal equivalents represent the preserved rock record of four megacycles of forearc arc and or back arc geotectonic evolution associated with the convergent margins of a pilbara continent individual supersequences are the rock record of separate second order basins or basin phases and reflect the different geotectonic components of each megasequence the four megacycles had respective longevities from oldest to youngest of 150 m y 230 m y 160 m y and 180 m y they are identical in their stratigraphic successions and predicted first order sea level cycles to phanerozoic first order tectono eustatic cycles and are interpreted as the same response to global tectonic change the warrawoona and gorge creek megacycles and the roebourne and mount negri megacycles are tectonogenetic pairs of the east pilbara and the west pilbara megacycle sets respectively the east pilbara megacycle set evolved over a period of 380 m y from 3490 ma to 3110 ma whereas the west pilbara megacycle set evolved over a period of 340 m y from 3110 ma to 2770 ma the two megacycle sets are identical in their longevity geotectonic components and predicted sea level cycles to convergent margin wilson and supercontinent cycles and are thereby interpreted individually as the oldest established records of the supercontinent cycle the implication of this interpretation is that a steady state global tectonic regime has been in operation from early in earth s history which has important consequences for theoretical modelling of the earth s early tectonic style and secular changes in its geochemical regime

source attenuation and site parameters of the 1997 umbria marche seismic sequence from the inversion of p wave spectra a comparison between constant qp and frequency dependent qp models 1 the attenuation of p waves the site responses and the source parameters seismic moment corner frequency source dimension of 490 seismic events that occurred during the 1997 colfiorito umbria marche seismic sequence have been inferred from the inversion of p wave velocity spectra the boatwright source model has been assumed to model the source spectra a global nonlinear inversion scheme was developed to avoid any a priori selection of the initial qp and the corner frequency to establish if a frequency dependent qp model fits the data better than a constant qp model two inversion results have been compared application of the akaike information criterion indicates that the constant qp model represents the best compromise between model simplicity and data misfit the station qp values are small in the range of 50 to 200 a one dimensional qp model is obtained by back projecting the inverted t our results indicate both well defined near site attenuation effects at some sites and heterogeneity in the inelastic properties of the crust with the exception of the amplification response at five seismic stations most of the recording sites did not show amplification peaks at particular frequencies the stress drop clearly increases as a function of the seismic moment which indicates a deviation from self similarity whereas it does not show an increase with depth probably owing to the effects of fluid pressurization in the crust a stress drop of about 39 mpa is inferred the relationship between the seismic moment and the local magnitude for p waves has been calibrated

grenvillian aged orogenesis in the palaeoproterozoic gascoyne complex western australia 1030 950 ma reworking of the proterozoic capricorn orogen in situ shrimp u pb geochronology of monazite and xenotime in pelitic schists from the central gascoyne complex western australia shows that greenschist to amphibolite facies metamorphism occurred between c 1030 and c 990 ma monazite from an undeformed rare element pegmatite from the same belt gives a 207pb 206pb age of c 950 ma suggesting that peak metamorphism and deformation was followed by pegmatite intrusion and coeval granite magmatism metamorphism in the central gascoyne complex was previously interpreted as barrovian largely based on the identification of kyanite in peak metamorphic assemblages and has been attributed to intense crustal shortening and substantial tectonic thickening during palaeoproterozoic continent continent collision however the stable al2sio5 polymorph has been identified in this study as andalusite rather than kyanite and the prograde assemblages of staurolite garnet andalusite biotite muscovite quartz indicate temperatures of 500 550 c and pressures of 3 4 kbar these data show that the palaeoproterozoic gascoyne complex underwent an episode of grenvillian aged intracontinental reworking concentrated in a nw se striking corridor during the edmundian orogeny until now the edmundian orogeny was thought to have involved only reactivation of structures in the gascoyne complex along with deformation and very low to low grade metamorphism of mesoproterozoic cover rocks some time between 1070 and 755 ma however we suggest that it involved regional amphibolite facies metamorphism and deformation granite magmatism and pegmatite intrusion between c 1030 and c 950 ma therefore the capricorn orogen experienced a major phase of tectonic reworking c 600 myr later than previously recognized our results emphasize the importance of in situ geochronology integrated with petrological studies in order to link the metamorphic history of a terrane with causally related tectonic events

determination of near surface attenuation with κ parameter to obtain the seismic moment stress drop source dimension and seismic energy for microearthquakes in the granada basin southern spain abstract a set of 43 microearthquakes of the granada basin with magnitude duration m d from 1 4 to 3 5 have been spectrally analysed the digital data used in this study was recorded by two short period digital seismic network five permanent stations using andalusian seismic network red sismica de andalucia rsa and three stations of a portable digital seismic network red sismica portatil rsp the displacement spectra for p and s waves were analysed with brune s j geophys res 75 1970 4997 j geophys res 76 1971 5002 source model and the spectra parameters were determined by snoke s model to correct spectra from path attenuation coda q was used for the spectra correction from near surface attenuation parameter was calculated obtaining it from acceleration spectra the values of parameter range from 0 01 to 0 04 s for p waves and from 0 006 to 0 04 s to s waves once the spectra of these attenuation effects was corrected we obtained the seismic moments that range from 5 45 10 17 dyne cm to 1 53 10 20 dyne cm the source radii are between 0 13 and 0 39 km the seismic energy ranges from 7 17 10 3 j to 1 13 10 8 j the stress drop values were below 4 bars the scaling relations between seismic moment and stress drop indicated decreasing stress drop with decreasing seismic moment