scale dependence of seismic energy to moment ratio for strike slip earthquakes in japan we analyzed four pairs of a large m w 6 and a small m w 3 5 to 4 shallow strike slip earthquakes to investigate the scale dependence of the seismic energy to moment ratio an important macroscopic parameter which reflects the basic physical process of seismic slip these earthquakes occurred in the south western part of japan and high quality close in records epicentral distance 50 km are available for both the small and large earthquakes the paired events have almost the same focal mechanism and hypocenter location we used the spectral ratio of the paired events in order to remove the effects of attenuation along the wave propagation path and the station site response we then estimated the seismic energy from the source spectra estimated from the spectral ratio the energy to moment ratio increases with the earthquake size this scale dependence is very similar to that found earlier for earthquakes in southern california

diversity in earth s early felsic crust paleoarchean peraluminous granites of the barberton greenstone belt earth9s oldest preserved granitoid crust dates back to the paleoarchean and consists predominantly of sodic tonalite trondhjemite granodiorite ttg granitoids that arose through the partial melting of hydrated metabasalts in contrast granites sensu stricto typically appear relatively late in the plutonic record of the old cratons however the existence of hadean zircons with mineral inclusion suites that are consistent with crystallization from peraluminous granitic magmas indicates that granitic rocks formed part of the earliest felsic crust although we have direct evidence this earliest felsic crust is not preserved here we present evidence of an unusual variety of markedly low cao k 2 o rich rutile bearing peraluminous granite and rhyolite that was produced concurrently with ttg magmas during three magmatic cycles in the barberton greenstone belt bgb southern africa this material is not preserved as in situ rock units but occurs as clasts within a younger conglomerate within these rocks plagioclase feldspar is a rare inclusion in zircon relative to alkali feldspar and has low anorthite contents an 2 o and sr content as well as the peraluminous character of the magmas is a consequence of phengite melting in a metagraywacke source at pressures in excess of those of plagioclase stability this process contributed to each episode of continental crustal growth through the paleoarchean to mesoarchean in the bgb despite leaving no plutonic record at the typical mid crustal level of exposure that the ttg plutons around the belt represent

dynamical fault rupture processes in heterogeneous media abstract the fault rupture processes in a horizontally layered medium and also in a three dimensionally heterogeneous structure are investigated on three dimensional spontaneous dynamic shear crack models the wave equations for three dimensional space are solved numerically by a finite difference scheme under the appropriate boundary conditions and the finite stress fracture criterion the heterogeneous properties of the elastic medium particularly the existence of low velocity zones give remarkable effects on the rupture process yielding appreciably decelerated rupture velocities and large fault displacements in and around the zones and strong motions in the near field the large fault displacements are enhanced when the rupture breaks the ground surface the static seismic moment in the case with a low velocity zone is essentially the same as in a homogeneous half space an attempt is made to simulate the rupture process of a moderate size earthquake by applying the above shear crack model in a heterogeneous medium with depth dependent and laterally heterogeneous stress drop the model appears to explain the observed features to a satisfactory degree

the state of stress on some faults of the san andreas system as inferred from near field strong motion data we present a simple method to calculate the stress produced on an earthquake fault during rupture this method allows the complete evaluation of the stress spatio temporal history over the fault we apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the san andreas fault system over the last 20 years the imperial valley earthquake of 1979 the morgan hill earthquake of 1984 the loma prieta earthquake of 1989 and the northridge earthquake of 1994 we use for this study the tomographic models of the fault rupture obtained from the inversion of the near field seismic data recorded during these earthquakes the results obtained show that the static and the dynamic stress drops vary greatly over the fault the peak values obtained for the four earthquakes studied range from about 20 to 100 mpa these high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault the regions of the fault which break with a high stress drop are also the regions where slip is large this suggests that most of the slip produced in a large earthquake takes place over the strong areas of the fault low slip regions tend to break with low stress drops after the earthquake the shear stress is increased over a significant portion of the fault which corresponds to low slip regions aftershock activity tends to be concentrated in these areas of stress increase the apparent strength of the fault before the earthquake that is the local shear stress increase which is required for rupture is also extremely heterogeneous the rupture velocity seems to be inversely correlated with this apparent fault strength the rupture accelerating over the weak areas of the fault and slowing down over the high strength areas

complex source processes and the interaction of moderate earthquakes during the earthquake swarm in the hida mountains japan 1998 abstract this paper presents a set of detailed source models of 18 moderate m4 5 earthquakes that were part of the swarm activity beneath the hida mountains in central japan in 1998 to reveal the detailed rupture process this study uses a waveform inversion procedure with the empirical green function egf most of our models show a complicated rupture process that includes several separate rupture areas and that involves ruptures that in some events seem to repeat at almost the same location in our models we measured as characteristic values the maximums of slip slip rate stress drop and average rupture propagation velocity in comparison with the models for larger events determined by similar methods the maximum slip scales with the cubic squares of the seismic moment for large stress drop events only the slip rate has an upper limit of about 1 2 m s and the stress drop is distributed widely from 1 to 50 mpa the average stress drops estimated using the corner frequency are almost uniform at about 1 mpa the first event in each swarm cluster tends to have a large maximum stress drop which reflects the change in the stress field during swarm activity the average rupture velocity determined by the least squares method is relatively slow and shows a negative relationship with the maximum stress drop this finding cannot be explained by simple crack models and instead suggests a very complicated mechanism like the interactive fault system the interaction between the slip areas of neighbor events resulted in a non overlapping slip distribution this finding indicates that previous rupture areas restrict the size and location of later events even in such complex swarm activities

ion microprobe ages of zircons from early archaean granite pebbles and greywacke barberton greenstone belt southern africa abstract we report single grain and grain domain u pb zircon ages for three granitoid clasts from the moodies conglomerate and for detrital zircons from a fig tree greywacke from clastic sediments in the early archaean barberton greenstone belt of southern africa the crystallization ages obtained for the moodies clasts vary between 3570 6 ma and 3518 11 ma while the fig tree zircons show a surprisingly uniform source terrain age of 3453 9 ma that may date the stratigraphically underlying onverwacht group felsic metavolcanics the discordancy patterns are ascribed to a combination of variable pb loss at unspecified times in the past and new zircon growth and or recrystallization the shrimp data record details of isotopic variation even within single zircons that result in precise definition of upper concordia intercept ages our results suggest an age range between 3460 and 3570 ma for the source region of the barberton belt clastic metasediments presumably the ancient gneiss complex of swaziland and felsic volcanics of the overwacht group this source included geochemically evolved granitoids that predate the oldest greenstones by some 100 ma

continental growth and convergence related arc plutonism in the mesoarchaean evidence from the barberton granitoid greenstone terrain south africa u pb zircon ages from trondhjemitic gneisses of the badplaas domain located in the southwestern parts of the mesoarchaean barberton granitoid greenstone terrain of south africa document a hitherto largely unrecognized plutonic event the geochronological results indicate the addition of juvenile felsic crust over a period of 60 myr between ca 3290 and 3230ma and prior to the main collisional event in the granitoid greenstone terrain at 3230 ma the timing and duration of plutonism together with the structural and compositional heterogeneity of the badplaas domain suggest that the badplaas domain represents part of a convergence related magmatic arc on a regional scale the spatial and temporal relationships between plutonism metamorphism and deformation are interpreted to preserve an almost complete inventory of a mesoarchean arc trench system this includes 1 the 3290 3230ma convergence related magmatic arc 2 a largely coeval deposition of the back arc type volcano sedimentary 3260 3225ma fig tree group of the barberton greenstone belt 3 relics of the underplated crust preserved in high p low t rocks to the immediate east of the magmatic arc juxtaposed against 4 high t medium p rocks of the overlying plate along the major crustal structure of the inyoni shear zone these findings corroborate the notion that thermal and rheological conditions on the early earth supported at least locally crustal growth and the extraction of buoyant felsic crust along convergence related magmatic arcs

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

quantifying rates of dome and keel formation in the barberton granitoid greenstone belt south africa abstract the barberton granitoid greenstone belt is a classic dome and keel province characterized by kilometer scale gneiss domes and elongate keels of largely folded supracrustal rocks combined u pb shrimp data and structural mapping demonstrate that the geometry of the barberton belt reflects events that occurred over 30 million year interval from ca 3230 and 3203 ma early deformation with nw se shortening in the upper crust was accompanied by emplacement of tonalite trondhjemite granodiorite ttg magmas at 3234 12 and 3226 9 ma much of the structural grain of the greenstone belt relates to a long episode of post orogenic extension with ne directed extension in the lower crust leading to exhumation of high grade gneisses in the southern barberton terrane advective heat transfer during emplacement of kilometer scale ttg plutons around the margins of the greenstone belt facilitated the infolding of the relatively denser and colder greenstone sequence the end of this tectonic cycle is punctuated by the emplacement of the undeformed 3203 7 ma dalmein pluton which sharply truncates not only anticlines and synclines in the greenstone belt but also the dominant fabric in the high grade gneisses

accretionary history of the archean barberton greenstone belt 3 55 3 22 ga southern africa the 3 55 3 22 ga barberton greenstone belt south africa and swaziland and surrounding coeval plutons can be divided into four tectono stratigraphic blocks that become younger toward the northwest each block formed through early mafic to ultramafic volcanism onverwacht group probably in oceanic extensional island or plateau settings volcanism was followed by magmatic quiescence and deposition of fine grained sediments possibly in an intraplate setting late evolution involved underplating of the mafic crust by tonalitic intrusions along a subduction related magmatic arc yielding a thickened buoyant protocontinental block the growth of larger continental domains occurred both through magmatic accretion as new protocontinental blocks developed along the margins of older blocks and when previously separate blocks were amalgamated through tectonic accretion evolution of the barberton belt may reflect an early archean plate tectonic cycle that characterized a world with few or no large stabilized blocks of sialic crust

estimation of dynamic rupture parameters from the radiated seismic energy and apparent stress we propose a new procedure for estimating the critical slip weakening distance dc by evaluating the spatio temporal distribution of the apparent stress calculated from the spatio temporal distribution of the slip velocity function on the fault plane obtained from a kinematic inversion of the earthquake source the idea is based on the fact that the apparent stress can be related to the difference between the earthquake average stress and the frictional dynamical stress on the fault plane during the rupture process from the cumulative slip apparent stress relationship we estimate the critical slip dc of the 1992 landers earthquake we find that the critical slip weakening plays an important role in controlling the rupture velocity in the case of the landers earthquake the very low rupture velocity in the central segment could be explained by the large dc value obtained for the main asperity on that segment

timing of deposition and deformation of the moodies group barberton greenstone belt south africa very high resolution of archaean surface processes abstract thin and largely discontinuous volcanic tuff beds in the alluvial fluvial and marginal marine siliciclastic strata of the middle archaean moodies group barberton greenstone belt bgb contain rare to abundant zircon dating of magmatic zircon grains from five tuff beds and one cross cutting dike yield ages which span the entire spectrum of bgb history from ca 3550 ma to 3215 ma and show variable degrees of post depositional alteration until ca 2500 ma the youngest pristine zircon grains in all tuff samples analyzed by ca id tims chemical abrasion ion dissolution thermal ion mass spectrometry yield upper intercept and concordia ages between 3227 6 0 3 and 3216 0 3 ma and those grains analyzed by la sf icp ms laser ablation sector field inductively coupled plasma mass spectrometry and shrimp sensitive high resolution ion microprobe yield ages between 3228 9 and 3214 9 ma most of these ages are either identical within error or significantly older than a ca id tims age of 3223 4 0 3 ma obtained from zircon grains of a tuff horizon of the schoongezicht formation at the top of the underlying fig tree group we interpret this age to mark the time of volcanic eruption whereas the significantly older and overlapping ages of the stratigraphically higher moodies tuff zircon grains point either to a high degree of magmatic inheritance and or a detrital contribution from surrounding sedimentary rocks during or after ash tuff deposition the minimum age for deposition of moodies group sediments and deformation is provided by a la icp ms zircon u pb age of 3219 9 ma obtained from an undeformed felsic dike which crosscuts folded moodies strata this age overlaps with a precise ca id tims u pb zircon age of 3216 0 0 3 ma obtained from a tuff horizon of the upper moodies group overall the new age data collectively imply that the up to 3 7 km thick sandstone dominated moodies group was deposited and deformed within

origin and evolution of a peraluminous silicic ignimbrite suite the violet town volcanics the violet town volcanics are a 373 ma old comagmatic s type volcanic sequence mainly comprising crystal rich intracaldera ignimbrites rock types vary from rhyolites to rhyodacites all containing magmatic cordierite and garnet phenocrysts variation in the suite is primarily due to fractionation of early crystallized quartz plagioclase and biotite plus minor accessory phases in a high level magma chamber prior to eruption early magmatic crystallization occurred at around 4 kb and 850 c with melt water contents between 2 8 and 4 wt this high temperature markedly water undersaturated restite poor granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain leaving a granulite facies residue

teleseismic estimates of the energy radiated by shallow earthquakes the energy flux contained in the p wave groups p pp sp or the s wave groups s ps ss radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently by defining generalized radiation patterns which incorporate this neutral interference the wave groups are analyzed as though they were comprised of a single phase measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body wave attenuation and the frequency band of the recording the corrected measurements are then used to estimate the seismic energy radiated by the earthquake this analysis is applied to digital recordings of the teleseismic wave groups radiated by the may 2 1983 coalinga california earthquake and the october 28 1983 borah peak idaho earthquake for the coalinga earthquake an estimate of es 1 6 0 4 1021 dyn cm was determined from six p wave groups while the sh wave group recorded at station col returned an estimate of es 1 2 1021 dyn cm for the borah peak earthquake an estimate ofes 3 2 0 5 1021 dyn cm was determined from seven p wave groups the distribution of the isoseismals for the borah peak earthquake indicate that the energy radiated by this event was focussed to the northwest in the direction of rupture propagation correcting the teleseismic estimate for this focussing gives es 3 6 0 5 1021 dyn cm combining these estimates of the radiated energy with broad band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the coalinga and the borah peak earthquakes respectively

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

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

a high frequency secondary event during the 2004 parkfield earthquake by using seismic records of the 2004 magnitude 6 0 parkfield earthquake we identified a burst of high frequency seismic radiation that occurred about 13 kilometers northwest of the hypocenter and 5 seconds after rupture initiation we imaged this event in three dimensions by using a waveform back projection method as well as by timing distinct arrivals visible on many of the seismograms the high frequency event is located near the south edge of a large slip patch seen in most seismic and geodetic inversions indicating that slip may have grown abruptly at this point the time history obtained from full waveform back projection suggests a rupture velocity of 2 5 kilometers per second energy estimates for the subevent together with long period slip inversions indicate a lower average stress drop for the northern part of the parkfield earthquake compared with that for the region near its hypocenter which is in agreement with stress drop estimates obtained from small magnitude aftershocks

u pb hf isotope systematics of detrital zircons in high grade paragneisses of the ancient gneiss complex swaziland evidence for two periods of juvenile crust formation paleo and mesoarchaean sediment deposition and 3 23 ga terrane accretion abstract we present the first results of combined u pb and lu hf isotope analyses of detrital zircon grains in high grade paragneisses luboya kubuta granulites from the ca 3 66 3 22 ga ancient gneiss complex agc in swaziland uranium pb ages demonstrate that sedimentation along the se margin of the proto kaapvaal craton took place during two time intervals during the paleoarchaean ca 3 53 and 3 43 ga and mesoarchaean ca 3 22 and 3 16 ga the hf isotope data show that the hinterland to the paleoarchaean metasediments experienced juvenile crust formation from a depleted mantle source at ca 3 53 ga hf 3 53ga values between 0 7 and 4 1 and by reworking of more ancient crust t dm 3 8 3 6 ga in contrast detrital zircons in mesoarchaean metasediments record juvenile crust formation at 3 32 ga hf 3 32ga values between 1 5 and 6 0 t dm 3 45 3 32 ga followed by internal reworking of this crust until 3 22 ga a small number of detrital zircons indicate limited input from older paleoarchaean agc crust compared to sediments of the barberton greenstone belt i e fig tree and moodies group the mesoarchaean metasediments of swaziland are more juvenile in character and show greater complexity in their detrital zircon age spectra pointing towards a different provenance our new u pb hf isotope data together with field and petrological information from the study area suggest that the paleoarchaean metasediments were largely derived from the agc whereas the mesoarchaean metasediments were derived from a ca 3 45 3 32 ga juvenile island arc terrane located se of the agc which became accreted onto the kaapvaal craton at ca 3 23 ga i e the onset of burial and high grade metamorphism in the area

inferring rate and state friction parameters from a rupture model of the 1995 hyogo ken nanbu kobe japan earthquake we consider the applicability of laboratory derived rate and state variable friction laws to the dynamic rupture of the 1995 kobe earthquake we analyze the shear stress and slip evolution of ide and takeo s 1997 dislocation model fitting the inferred stress change time histories by calculating the dynamic load and the instantaneous friction at a series of points within the rupture area for points exhibiting a fast weakening behavior the dieterich ruina friction law with values of dc 0 01 0 05 m for critical slip fits the stress change time series well this range of dc is 10 20 times smaller than the slip distance over which the stress is released dc which previous studies have equated with the slip weakening distance the limited resolution and low pass character of the strong motion inversion degrades the resolution of the frictional parameters and suggests that the actual dc is less than this value stress time series at points characterized by a slow weakening behavior are well fitted by the dieterich ruina friction law with values of dc 0 01 0 05 m the apparent fracture energy gc can be estimated from waveform inversions more stably than the other friction parameters we obtain a gc 1 5 l06 j m 2 for the 1995 kobe earthquake in agreement with estimates for previous earthquakes from this estimate and a plausible upper bound for the local rock strength we infer a lower bound for dc of about 0 008 m

u pb and hf isotope data of detrital zircons from the barberton greenstone belt constraints on provenance and archaean crustal evolution combined u pb and hf isotope analyses of detrital zircons from the fig tree and moodies groups of the barberton greenstone belt south africa yield similar hf isotope compositions and age populations thus pointing to a similar provenance zircon populations of fig tree group greywacke and moodies group quartzarenite are both dominated by age clusters at 3 53 3 47 and 3 28 ga and a minor cluster at 3 36 ga the moodies quartzarenite sample additionally contains a younger age population at 3 23 3 19 ga hafnium isotope data indicate that the source area of both sediments was affected by new crust formation from depleted mantle sources at 3 53 3 47 and perhaps at 3 36 ga hf t between 1 7 and 4 5 accompanied by partial reworking of an eoarchaean crustal component as old as 3 75 3 95 ga in contrast crustal reworking was the predominant process between 3 28 and 3 22 ga hf t between 6 0 and 0 9 probably related to subduction and collision of terranes along the inyoka fault system the zircon u pb and hf isotope datasets favour a southern provenance for the fig tree and moodies sediments comprising granitoids in the vicinity of the southern barberton greenstone belt and in swaziland this finding is in contrast to the sedimentary record of the moodies group which mostly suggests a northern and along strike provenance this discrepancy may be due to reworking of sediments during extensive syn and postorogenic strike slip faulting and high uplift or subsidence between 3 26 and 3 19 ga supplementary material results of in situ u pb and lu hf isotope zircon analyses are available at www geolsoc org uk sup18561

triassic mineralization with cretaceous overprint in the dahu au mo deposit xiaoqinling gold province constraints from shrimp monazite u th pb geochronology abstract the xiaoqinling terrane in central china is the second largest gold producing area in china previous application of age constraints has yielded ages for the gold mineralization ranging from paleoproterozoic through triassic indosinian to jurassic and cretaceous yanshanian shrimp u th pb dating of hydrothermal monazite in a mo mineralized quartz vein from the dahu au mo deposit in the northern xiaoqinling terrane identifies two mineralization episodes hydrothermal monazite intergrown with molybdenite yields an initial precipitation age of 216 5 ma which is identical to but more precise than the molybdenite re os isochron age of 218 41 ma a younger generation of monazite is distinguished by data falling on a mixing array it is inferred that the younger monazite was formed at the expense of the older ones during the yanshanian probably during a single event that occurred at 125 ma combining these results with available geochronological data and geological evidence we conclude that molybdenum mineralization accompanied by monazite deposition took place in the indosinian followed by a yanshanian tectono hydrothermal event that resulted in large scale gold mineralization in the xiaoqinling terrane our observation demonstrates that monazite is sensitive to hydrothermal overprint and thus is useful for dating fluid infiltration event

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

tourmaline b isotopes as tracers of fluid sources in silicified palaeoarchaean oceanic crust of the mendon formation barberton greenstone belt south africa pervasive silicification is a common phenomenon in early archaean volcano sedimentary sequences reflecting the interaction between si rich fluids and the archaean oceanic crust in the palaeoarchaean barberton greenstone belt south africa the silicified zones locally contain abundant tourmaline in order to constrain the source of fluids responsible for the alteration six tourmaline bearing samples from the 3 3 ga mendon formation were investigated by in situ analyses of mineral chemistry and boron isotopes the samples comprise a silicified finely laminated sedimentary chert and five highly altered and silicified komatiites electron microprobe analysis empa indicates that tourmaline is mostly dravitic with one sample between dravite and mg foitite tourmaline has a weakly developed patchy colour and chemical zonation with variations in al fe and mg composition some samples show strong zoning in cr concentration with cr enrichment in the core and locally the high cr domains are at contacts with other cr rich phases such as magnesiochromite and chromian muscovite fuchsite overall the boron isotope composition 11b ranges from 20 7 to 10 2 two groups of tourmaline are distinguished based on the variation in 11b ratios within single samples 1 those with small 4 range in 11b values spinifex textured komatiites and silicified sediment and 2 those with a large range in 11b values of up to 18 brecciated and foliated samples positive boron isotope values 6 to 10 are found in one spinifex textured komatiite and are interpreted as being derived from seawater interacting with archaean oceanic crust in shallow hydrothermal systems the intermediate boron isotope values 5 to 8 found in all other samples are similar to morb and may represent boron derived from the archaean oceanic crust the rare and exceptionally light boron isotope values 10 to 21 were found in rims of group 2 tourmaline these values cannot be produced by fractionation alone and point to the presence of another fluid source the source of the light boron has yet to be identified but possible candidates are remobilized boron either derived from granitic crust or from marine evaporites that precipitated from seawater enriched in 10b in any case the isotopically light boron appears to have been present during seafloor hydrothermal alteration and deposition of the mendon formation at ca 3 3 ga

determination of earthquake energy release and ml using terrascope we estimated the energy radiated by earthquakes in southern california using on scale very broadband recordings from terrascope the method we used involves time integration of the squared ground motion velocity and empirical determination of the distance attenuation function and the station corrections the time integral is typically taken over a duration of 2 min after the p wave arrival the attenuation curve for the energy integral we obtained is given by q r cr n exp kr r 2 2 h ref 2 with c 0 49710 n 1 0322 k 0 0035 km 1 and h ref 8 km where is the epicentral distance a similar method was used to determine m l using terrascope data the station corrections for m l are determined such that the m l values determined from terrascope agree with those from the traditional optical wood anderson seismographs for 1 5 6 5 m l saturates the ratio e s m 0 m 0 seismic moment a measure of the average stress drop for six earthquakes the 1989 montebello earthquake m l 4 6 the 1989 pasadena earthquake m l 4 9 the 1990 upland earthquake m l 5 2 the 1991 sierra madre earthquake m l 5 8 the 1992 joshua tree earthquake m l 6 1 and the 1992 landers earthquake m w 7 3 are about 10 times larger than those of the others that include the aftershocks of the 1987 whittier narrows earthquake the sierra madre earthquake the joshua tree earthquake and the two earthquakes on the san jacinto fault the difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times the aftershocks which occurred on the fault plane where the mainshock slippage occurred had a very short time to heal hence a low stress drop the repeat time of the major earthquakes on the frontal fault systems in the transverse ranges in southern california is believed to be very long a few thousand years hence the events in the transverse ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times such as the san andreas fault and the san jacinto fault the observation that very high stress drop events occur in the transverse ranges and the los angeles basin has important implications for the regional seismic potential the occurrence of these high stress drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events characterization of earthquakes in terms of the e s m 0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the los angeles basin and the transverse ranges

seismic energy release in mexican subduction zone earthquakes we estimated radiated seismic energy es from 18 shallow thrust mexican subduction zone earthquakes 4 1022 m 1 1 1028 dyne cm 11 h 37 km using digital accelerograms from the guerrero accelerograph array and neglecting stations with large site effects es is computed by integrating squared velocity spectra after applying geometrical spreading and q corrections we discarded epicentral recordings for the largest michoacan event we find that log es m 4 152 0 275 which gives a median value of es m and apparent stress a of 7 1 10 5 and 24 bars respectively the median es m value is in accordance with gutenberg and richter s g r 1956 formula for es in which es m 5 10 5 is implicit worldwide es m data where es is computed from local records mostly fall between 5 10 5 and 5 10 4 for events with m 1022 dyne cm on the other hand es m values generally lie between 5 10 6 and 5 10 5 if es is estimated from teleseismic records especially anomalous are the es m data from kikuchi and fukao 1988 for large and great earthquakes which fall near 5 10 6 thus while es from local data suggests that the g r relation seldom overestimates seismic energy release the teleseismic data point to the contrary the cause of this discrepancy may lie in the difficulty of resolving incoherent radiation from the fault and inappropriate choice of t in the analysis of teleseismic data

an apparent mechanism dependence of radiated seismic energy we develop an extension to the method of boatwright and choy 1986 for determining the radiated seismic energy es that accounts for factors that bias the estimate we apply our technique to 204 events worldwide during the period 1992 1999 and find that the apparent stress is on average largest for strike slip events 0 70 mpa while for reverse and normal events it is significantly smaller 0 15 and 0 25 mpa respectively these results support the mechanism dependence of es reported by choy and boatwright 1995 although we find that once likely sources of bias are accounted for the mechanism dependence is not as strong as found previously the source of the mechanism dependence is unclear but one possibility is that it reflects a mechanism dependent difference in the stress drop this hypothesis is suggested by the scaling of slip with width in large strike slip earthquakes and makes two predictions which could be used to test it the first is that the discrepancy should disappear for the very largest dip slip earthquakes as the length of the fault greatly exceeds the downdip extent the second is that the discrepancy ought to disappear for smaller earthquakes the first can not yet be tested due to a lack of recent very large dip slip earthquakes the second is supported by the lack of mechanism dependence to es for smaller earthquakes an alternative hypothesis is that the apparent mechanism dependence could result if faults are opaque during rupture blocking seismic radiation across them brune 1996 this could cause radiated seismic energy to be trapped preferentially in the crust near the source volume for dipping faults there remains however a large discrepancy between estimates of es obtained from teleseismic versus regional data this discrepancy indicates a problem with teleseismic and or regional estimates of the seismic energy and must be resolved before a definite conclusion can be drawn

detrital zircon geochronology of sandstones of the 3 6 3 2 ga barberton greenstone belt no evidence for older continental crust

global patterns of radiated seismic energy and apparent stress radiated energies from shallow earthquakes with magnitudes 5 8 that occurred between 1986 and 1991 are used to examine global patterns of energy release and apparent stress in contrast to traditional methods which have relied upon empirical formulas these energies are computed through direct spectral analysis of broadband seismic waveforms velocity squared spectra of body waves are integrated after they have been corrected for effects arising from depth phases frequency dependent attenuation and focal mechanism the least squares regression fit of energy es to surface wave magnitude ms for a global set of 397 earthquakes yields log es 4 4 1 5ms which implies that the gutenberg richter relationship overestimates the energies of earthquakes the least squares fit between es and seismic moment m0 is given by the relationship es 1 6 10 5 m0 which yields 0 47 mpa as the average global value of apparent stress however the regression lines of both es ms and es m0 yield poor empirical predictors for the actual energy radiated by any given earthquake the scatter of data is more than an order of magnitude about each of the regression lines on the other hand global variations between es and mo while large are not random when subsets of es m0 are plotted as a function of seismic region and faulting type the scatter in data is substantially reduced the es m0 fits for many seismic regions and tectonic environments are very distinctive and a characteristic apparent stress c can be derived the lowest apparent stresses 3 0 mpa are associated with strike slip earthquakes that occur at oceanic ridge ridge transforms and in intraplate environments seaward of island arcs intermediate values of apparent stress 1 5 a 3 0 mpa are associated with strike slip earthquakes at incipient or transitional plate boundaries in general the dominant mode of failure for a tectonic environment is associated with the faulting type that has the lowest apparent stress an energy magnitude me can complement moment magnitude mw in describing the size of an earthquake me being derived from velocity power spectra is a measure of seismic potential for damage mw being derived from the low frequency asymptote of displacement spectra is more physically related to the final static displacement of an earthquake when earthquake size is ranked by moment a list of the largest events is dominated by earthquakes with thrust mechanisms when earthquake size is ranked by energy the list of the largest events is dominated by strike slip earthquakes

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

petrography and geochemistry of granitoid and metamorphite pebbles from the early archaean moodies group barberton mountainland south africa pebbles of potassic granitoids and metamorphites constitute up to 5 of the basal conglomerate of the moodies group in a ratio of 2 1 the granitoid pebbles frequently show micrographic quartz feldspar intergrowth whereas the metamorphites of a modal composition similar to that of the granitoids are characterized by large quartz grains which could represent original quartz phenocrysts in felsic volcanic precursors the granitoids show high k2o sr k2o na2o and k rb small enrichment of light ree large negative eu anomalies and slightly depleted and fractionated heavy ree compared to the granitoids the metamorphites show higher fe2o3 tio2 and cr concentrations greater enrichment of light ree and also large negative eu anomalies there is little similarity between the moodies pebbles and the majority of the rocks of the ancient gneiss complex of swaziland agc there is only some similarity of the ree distribution patterns between the pebbles and the mkhondo metamorphic suite possibly an areally restricted phase of the agc the geochemical data and especially the large negative eu anomalies suggest that the moodies pebbles were derived from granites which represent residual magmas from which much plagioclase had been removed the granites crystallized at depths of 7 km from magmas with low h2o pressures in a rather thick sialic crust it appears possible that the pre moodies granitoids originated through partial melting of low al2o3 siliceous gneisses of the agc a chronologic connection of the formation of the granitoids with the late onverwacht group volcanicity is possible

the geochemical record of the last 17 000 years in the guaymas basin gulf of california abstract sediments deposited on the western slope of the guaymas basin in the central gulf of california are composed predominantly of detrital clastic material and biogenic silica biopal with minor organic material average of 2 8 organic carbon and calcium carbonate the caco 3 is derived from calcareous plankton and is highly variable ranging from 0 to 16 in general the caco 3 content of the sediments varies inversely with the biopal content reflecting the relative abundance of calcareous and siliceous plankton in the photic zone siliceous plankton dominate when winds are predominantly out of the northwest producing strong upwelling calcareous plankton indicates weak southeasterly winds that bring warm tropical pacific surface water into the gulf based mainly on relative abundances of biopal and caco 3 the sediments deposited over the last 17 000 years in the western guaymas basin can be divided into five intervals in general the sediments in the intervals with high biopal and low caco 3 are laminated but this is not always true unlike most other continental margins of the world with well developed oxygen minimum zones where highest concentrations of organic carbon and redox sensitive trace metals occur in laminated sediments the laminated sediments on the anoxic slope of the western guaymas basin do not always have the highest concentrations of organic carbon and trace metals such as mo and cd

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

timing of multiple hydrothermal events in the iron oxide copper gold deposits of the southern copper belt carajas province brazil the southern copper belt carajas province brazil hosts several iron oxide copper gold iocg deposits including sossego cristalino alvo 118 bacuri bacaba castanha and visconde mapping and u pb sensitive high resolution ion microprobe shrimp iie zircon geochronology allowed the characterization of the host rocks situated within regional wnw ese shear zones they encompass mesoarchean 3 08 2 85 ga ttg orthogneiss granites and remains of greenstone belts neoarchean ca 2 74 ga granite shallow emplaced porphyries and granophyric granite coeval with gabbro and paleoproterozoic 1 88 ga porphyry dykes extensive hydrothermal zones include albite scapolite biotite scapolite tourmaline magnetite alteration and proximal potassium feldspar chlorite epidote and chalcopyrite formation u pb laser ablation multicollector inductively coupled mass spectrometry la mc icp ms analysis of ore related monazite and re os ntims analysis of molybdenite suggest multiple neoarchean 2 76 and 2 72 2 68 ga and paleoproterozoic 2 06 ga hydrothermal events at the bacaba and bacuri deposits these results combined with available geochronological data from the literature indicate recurrence of hydrothermal systems in the southern copper belt including 1 90 1 88 ga ore formation in the sossego curral ore bodies and the alvo 118 deposit although early hydrothermal evolution at 2 76 ga points to fluid migration coeval with the carajas basin formation the main episode of iocg genesis 2 72 2 68 ga is related to basin inversion coupled with neoarchean ca 2 7 ga felsic magmatism the data suggest that the iocg deposits in the southern copper belt and those in the northern copper belt 2 57 ga salobo and igarape bahia alemao deposits do not share a common metallogenic evolution therefore the association of all iocg deposits of the carajas province with a single extensive hydrothermal system is precluded

spatial distribution of high frequency energy radiation on the fault of the 1995 hyogo ken nanbu japan earthquake mw 6 9 on the basis of the seismogram envelope inversion abstract we studied the generation and propagation of high frequency above 1 hz s wave energy from the 1995 hyogo ken nanbu kobe japan earthquake m w 6 9 by analyzing seismogram envelopes of the mainshock and aftershocks we first investigated the propagation characteristics of high frequency s wave energy in the heterogeneous lithosphere around the source region by applying the multiple lapse time window analysis method to aftershock records we estimated two parameters that quantitatively characterize the heterogeneity of the medium the total scattering coefficient and the intrinsic absorption of the medium for s waves observed envelopes of aftershocks were well reproduced by the envelope green functions synthesized based on the radiative transfer theory with the obtained parameters next we applied the envelope inversion method to 13 strong motion records of the mainshock we divided the mainshock fault plane of 49 21 km into 21 subfaults of 7 7 km square and estimated the spatial distribution of the high frequency energy radiation on that plane the average constant rupture velocity and the duration of energy radiation for each subfault were determined by grid searching to be 3 0 km sec and 5 0 sec respectively energy radiated from the whole fault plane was estimated as 4 9 10 14 j for 1 to 2 hz 3 3 10 14 j for 2 to 4 hz 1 5 10 14 j for 4 to 8 hz 8 9 10 12 j for 8 to 16 hz and 9 8 10 14 j in all four frequency bands we found that strong energy was mainly radiated from three regions on the mainshock fault plane around the initial rupture point near the surface at awaji island and a shallow portion beneath kobe we interpret that energetic portions were associated with rupture acceleration a fault surface break and rupture termination respectively

estimation of radiated energy of finite source earthquake models we estimated seismically radiated energy for kinematic earthquake rupture models instead of integrating far field waveforms we used an expression for radiated energy with slip and stress on the fault plane using an analytic crack model of sato and hirasawa 1973 we show that the finite difference method can estimate the radiated energy fairly well however if the rupture propagation velocity is near the s wave velocity and the grid interval is coarse we may underestimate the radiated energy substantially we analyzed five models of three earthquakes of m w 5 7 for the 1995 kobe earthquake we used three different kinematic models and obtained consistent values of radiated energy of 2 4 3 6 1014 j there are common features among the three models 1 more efficient energy radiation from the deeper regions of the fault and 2 little energy radiation beneath the city of kobe the models of the kobe earthquake and the other two earthquakes show that energy radiation is concentrated near the hypocenter and the initiation point of asperities we also find the area of negative energy radiation energy absorption near the edge of large slipped areas although our estimates tend to be smaller by about a factor of 3 than previous estimates the difference is reasonable because the frequency range is not complete and we cannot deal with detailed rupture behavior such as rapid rupture acceleration and deceleration manuscript received 18 january 2002

nb ta fractionation by partial melting at the titanite rutile transition during the evolution of the earth distinct geochemical reservoirs with different nb ta ratios have developed archean granitoids of the tonalite trondhjemite granodiorite ttg suite which represent the earth s early continental crust show larger nb ta variations than any other earth reservoir this implies that significant nb ta fractionation must have occurred during early crust formation while the underlying mechanism behind is still unclear here we present a new model on how nb may be fractionated from ta during partial melting of subducted oceanic crust our data show that nb ta ratios in melts derived from rutile and titanite bearing eclogite are largely controlled by the modal relative abundances of rutile and titanite in the source high modal ratios of titanite over rutile generate melts with very high nb ta 60 whereas low modal titanite rutile produces melts with much lower nb ta 30 very low nb ta 16 occur when all ti phases are consumed at very high degrees of melting as the modal ratio of titanite to rutile is a function of pressure the nb ta of melts is a function of melting depth our new model helps to explain the extreme variation of nb ta observed in many ttgs and thus how nb and ta were fractionated during the early evolution of the earth furthermore the model also indicates that simple one stage melting models for mafic crust are not sufficient to explain the formation of ttgs

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

a method for mapping apparent stress and energy radiation applied to the 1994 northridge earthquake fault zone using the northridge earthquake as an example we demonstrate a new technique able to resolve apparent stress within subfaults of a larger fault plane from the model of wald et al 1996 we estimated apparent stress for each subfault using where g is the modulus of rigidity is the shear wave speed and is the average slip rate the image of apparent stress mapped over the northridge fault plane supports the idea that the stresses causing fault slip are inhomogeneous but limited by the strength of the crust indeed over the depth range 5 to 17 km maximum values of apparent stress for a given depth interval agree with a max 0 06s z where s is the laboratory estimate of crustal strength as a function of depth z the seismic energy from each subfault was estimated from the product ada where a is subfault area and d its slip over the fault zone we found that the radiated energy is quite variable spatially with more than 50 of the total coming from just 15 of the subfaults

moment energy stress drop and source spectra of western united states earthquakes from regional coda envelopes we present a new method to estimate stable seismic source parameters such as energy moment and orowan stress drop using regional coda envelopes from as few as one broadband station we use the method to compute path and site corrected seismic moment rate spectra for 117 recent western united states earthquakes empirical green s function corrections were applied to our surface and body wave coda envelope measurements to generate s wave source spectra these source spectra provide stable single station estimates of radiated seismic energy es and seismic moment mo that for common events are in excellent agreement with network averaged estimates obtained using local and regional data teleseismic moment estimates are compatible with our regional results but teleseismic energy estimates appear to be nearly an order of magnitude low we estimated the seismic moment of events ranging between mw 2 2 and 7 3 and energy estimates for which we had measured at least 70 of the total energy generally events above mw 3 3 we use these estimates to examine the behavior of derived parameters such as the orowan stress drop 2 es mo while the earthquakes we studied have a small range in orowan stress drop generally between 0 1 and 20 mpa they show a strong tendency for orowan stress drop to increase with moment approximately as mo0 25 we believe this is a source effect and is not due to inadequate bandwidth or attenuation correction and note that this trend appears to continue for microearthquakes as described in a recent deep borehole study in southern california many of the large high stress drop earthquakes show complexity in their moment rate spectra near the corner frequency and cannot be fit by a simple square model instead above the first corner frequency the spectral decay ranges between f 1 0 and f 1 5 this leads to larger estimates of radiated energy than predicted with a simple square model and has implications for seismic hazard estimation coda envelopes have three main advantages over direct arrivals for estimating seismic moment and energy 1 coda amplitudes vary little with geology and source radiation anisotropy and allow accurate single station applications 2 path corrected coda amplitude measurements can be applied to very large regions allowing a comparison of source parameters throughout the western united states using a common methodology and stations 3 because long period coda can last for hours for large local and regional events it allows the analysis of seismograms with clipped early arrivals

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

determination of constitutive relations of fault slip based on seismic wave analysis constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields no actual relation during a natural earthquake has been determined the 1995 kobe earthquake is suitable for detailed kinematic analysis and this enables the first evaluation of constitutive relations for a natural earthquake in this study we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane an inversion method based on bayes theorem is employed to obtain a spatiotemporal slip distribution and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints this slip distribution is then used as part of the boundary condition in the finite difference calculation the time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane they show slip weakening relations almost everywhere on the fault plane while slip rate dependency is not clear the slip weakening behavior has a clear depth dependency indicating that the slip weakening rate d du is smaller in the shallow crust than that in the deep crust this may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems

source model of the 1995 hyogo ken nanbu earthquake and simulation of near source ground motion the 1995 hyogo ken nanbu earthquake struck the heavily populated kobe and adjacent cities in western japan more than 6400 people were killed and more than 150 000 buildings were destroyed the characteristics of mainshock ground motions in the heavily damaged area are needed to understand how buildings and bridges performed and why they reached failure unfortunately very few strong ground motions were recorded in the heavily damaged area during the mainshock in this study we attempt to estimate mainshock ground motions by using the em pirical green s function method egf method first we assume an initial source model with the asperities based on the rupture process obtained by inversion of strong ground motion records for simplicity we consider each asperity as a sub event with uniform stress drop in a finite extent then the initial model was improved by matching the synthetic and observed ground motions using a trial and error pro cedure the final model consists of three subevents subevent 1 with stress drop of 163 bars under the akashi strait around the rupture starting point subevent 2 with stress drop of 86 bars under the nojima fault in awaji island and subevent 3 with stress drop of 86 bars under kobe finally we estimate strong ground motions using aftershock records at sites where the mainshock was not recorded the near source motions in kobe synthesized with the best fit model are characterized by two large pulses with a duration of 1 to 3 sec the pulses are caused by forward rupture directivity effects from subevents 1 and 3 peak horizontal acceleration and velocity of the synthesized motions at the heavily damaged sites are about 1000 cm sec 2 and 130 crrdsec respectively while those at a rock site in the near source region are about 300 crrdsec 2 and 60 cm sec

provenance and tectonic implications of the 3 28 3 23 ga fig tree group central barberton greenstone belt south africa abstract sedimentary rocks provide a valuable record of non uniformitarian paleoarchean tectonic processes the rocks of the 3 28 3 23 ga mapepe formation mark the initiation of orogenesis in the barberton greenstone belt south africa however reconstruction of the mapepe basin s is challenging because the rocks occur in fault bounded structural belts that are characterized by lateral heterogeneity and whose correlation remains uncertain we have studied the petrography geochemistry and detrital zircon geochronology from outcrop and drill core samples from two adjacent structural belts the manzimnyama syncline and the eastern barite valley area to evaluate the timing of basin formation correlate between these two adjacent areas and provide constraints on the character of tectonic uplift in the eastern barite valley area sedimentation was initiated at about 3260 ma the section includes a lower deep water unit of mainly mudstone overlain by a shoaling upward sequence of fan delta sandstone and conglomerate the sandstone throughout the formation was derived by erosion of local uplifts of the underlying greenstone belt sequence except at the top of the section where sedimentation was dominated by dacitic volcaniclastic sediments dated at 3239 ma in contrast rocks of the mapepe formation in the manzimnyama syncline area 1 2 km south of the eastern barite valley area record the initiation of sedimentation at about 3277 ma with deposition of a rhyolitic tuffaceous unit followed by persistent deep water sedimentation of banded iron formation banded ferruginous chert and turbiditic sandstone siliciclastic sediments are immature and were derived by erosion of older supracrustal greenstone belt rocks no fig tree age volcanism younger than 3260 ma is recorded in this belt overall mapepe sedimentary rocks record the formation of numerous local uplifts of underlying supracrustal rocks with periodic volcanic activity and the dispersal of debris to form mainly small fan deltas flanked by deep water systems the contrasting ages lithologies depositional settings and provenance of the sedimentary rocks in the now adjacent eastern barite valley and manzimnyama syncline belts suggest that they were deposited in either different parts of a large complex fig tree basin or represent separate basins that have been juxtaposed during post fig tree tectonism and shortening

inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley california earthquake a least squares point by point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 imperial valley california earthquake the imperial fault is represented by a plane embedded in a half space where the elastic properties vary with depth the inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right lateral strike slip and normal dip slip components of motion inversions are run for different fault dips and for both constant and variable rupture velocity models effects of different data sets are also investigated inversions are compared which use the strong ground motions alone the teleseismic body waves alone and simultaneously the strong ground motion and teleseismic records the inversions are stabilized by adding both smoothing and positivity constraints the moment is estimated to be 5 0 10 25 dyne cm and the fault dip 90 5 dislocation in the hypocentral region south of the united states mexican border is relatively small and almost dies out near the border dislocation then increases sharply north of the border to a maximum of about 2 m under interstate 8 dipslip motion is minor compared to strike slip motion and is concentrated in the sediments the best fitting constant rupture velocity is 80 per cent of the local shear wave velocity however there is a suggestion that the rupture front accelerated from the hypocenter northward the 1979 imperial valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border

climatic and geochemical implications of archean pedogenic gypsum in the moodies group 3 2 ga barberton greenstone belt south africa abstract lithic sandstones of braided fluvial to supratidal facies in the paleoarchean moodies group 3 22 ga barberton greenstone belt south africa include several regionally traceable units with common to abundant in places rock forming nodular concretions of megaquartz pseudomorphs after gypsum barite and calcite concretionary accumulations are stratiform and commonly associated with aqueously reworked fine grained tuffaceous sediment of originally rhyodacitic composition and can grow to fist sized agglomerates in crusts tens of m in lateral extent weathering of tuffaceous material and feldspar delivered alkali cations such as ca ba and k while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a co2 rich atmosphere sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic s or so2 concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone dominated by seasonal fluctuations of the groundwater level under evaporitic conditions the concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols their formation and composition constrain the local occurrence of sulfate in the archean atmo and hydrosphere their interaction with the emerging biosphere archean weathering regime local climate and vadose zone hydrodynamics

la icp ms dating of zircons from meso and neoarchean granitoids of the pietersburg block south africa crustal evolution at the northern margin of the kaapvaal craton abstract in order to unravel the late archean magmatic evolution of the northern kaapvaal craton we performed la icp ms u pb isotopic analyses on zircon grains from 15 granitoid samples of the pietersburg block including tonalitic and granodioritic gneisses ttg as well as weakly to not deformed high k monzogranites granodiorites and diorites our new age data coupled to already published results show that the pietersburg block is schematically made up of two lithologically and chronologically distinct domains namely 1 gneiss units that cover most of the surface of this terrane and comprising the formerly defined goudplaats hout river groot letaba gneisses as well as the so called duiwelskloof batholith where crust formation and reworking took place between 3350 and 2840 ma and 2 late stage high k plutons and batholiths that emplaced at 2780 ma and 2690 ma in details the tectono magmatic evolution of the pietersburg block can be divided into five successive episodes 1 the first crust forming event is related to the emplacement of juvenile ttg magmas in the range 3150 3350 ma in our dataset it is only represented by inherited zircon cores from younger granitoids 2 two ttg samples from the goudplaats hout river gneiss unit and the duiwelskloof area yielded emplacement ages of 2950 ma this magmatic event is very widespread in the whole pietersburg block and we propose that it represents an important period of crustal growth linked with the accretion of the pietersburg block to the northern edge of the older nucleus ca 3600 3100 ma of the kaapvaal craton 3 we obtained an intrusion age of 2840 ma for two samples of high k biotite bearing granites that are intimately associated with and probably formed by melting of the 2950 ma old ttgs thus this episode represents an important event of crust reworking that is likely associated with a long lasting deformation event 4 further intracrustal melting led to the development of large batholiths of high k biotite granites e g turfloop batholith and their satellite intrusions the latter being represented by one of our samples dated at 2780 ma 5 the intrusion of high k calc alkaline granitoids of possible mixed crust mantle origin mashashane matlala moletsi and matok plutons took place at 2690 ma they are likely associated with collision between the pietersburg block and the central zone of the limpopo belt together with localized strain along shear zones e g hout river shear zone and granulite facies metamorphism that both occurred in the same time