elevation changes and the great 1960 chilean earthquake support for aseismic slip we reanalyse the near field vertical displacement data for the great chilean earthquake of 1960 to provide constraints on the down dip slip distribution a recent study of free oscillation data confirms earlier proposals of a slow precursor comparable in size to the main shock we seek evidence for this event in the displacement data spatial coverage of the data is inadequate to allow determination of a unique fault model but inversions show that thrust slip must extend over a width of about 300 km and to a depth between 60 km and 85 km well below the seismogenic zone for interplate events we propose that this deeper slip is the source of the slow event our preferred class of solutions has moments ranging from 1 6 1023 n m to 2 5 1023 n m only 30 to 45 of the free oscillation value underestimation of the near trench displacements by 50 together with a seismic overestimation of moment by 30 due to tradeoff with rise time would result in satisfactory agreement

detection of a meteoritic component in ivory coast tektites with rhenium osmium isotopes measurement of rhenium re and osmium os concentrations and os isotopic compositions in ivory coast tektites natural glasses with upper crustal compositions that are ejected great distances during meteorite impact and rocks from the inferred source crater lake bosumtwi ghana show that these tektites incorporate about 0 6 percent of a meteoritic component analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components because the target rocks already have relatively large amounts of noble metals the re os system is ideally suited for the study of meteorite impacts on old continental crust for three reasons i the isotopic compositions of the target rocks and the meteoritic impactor are significantly different ii closed system mixing of target rocks and meteorites is linear on re os isochron diagrams which thus permits identification of the loss of re or os iii osmium isotopic compositions are not likely to be altered during meteorite impact even if re and os are lost

rupture process of the 1983 japan sea akita oki earthquake using a waveform inversion method the rupture process of the 1983 japan sea earthquake mjma 7 7 is determined by applying waveform inversion of displacement type strong motion records based on the bayesian method the synthesis of the seismic motions from the main shock is made using the records from the two aftershocks mjma 7 1 and mjma 6 1 as empirical green s functions the main shock fault consists of two subfaults the aftershock m 6 1 occurring south of the main shock fault is used for the southern subfault empirical green s function and the aftershock m 7 1 occurring north of the main shock fault is used for the northern subfault in this synthesis it is not necessary to calculate propagation path and local site effects therefore the information about the source process can be extracted from the comparison between observed seismic motions and the synthetic ones without detailed information on the velocity structure along the whole path from the source to each station the inversion for the rupture process is attempted for both a line source model and an area source model in both models the main shock fault surface is divided into several elements whose sizes are determined from the scaling relations between the main shock and its aftershocks the slip displacement and the rupture starting time on each element are estimated as model parameters in the inversion a good convergence is obtained after about 10 iteration steps for the line source case and after 5 iteration steps for the area source case the results are summarized 1 the average rupture velocity is approximately 2 5 km sec for the southern subfault and 2 0 km sec for the northern subfault and 2 there is a large slip displacement near the rupture initiation point and the north edge of the fault thus there is considerable heterogeneity in rupture propagation and slip distribution over the fault plane

layered tektites of southeast asia field studies in central laos and vietnam abstract we have recovered 18 kg of layered tektites from 10 tektite bearing localities in laos and central vietnam including 5 localities around the town of muong nong laos several of these deposits originally contained several hundred kilograms of layered tektite fragments and one fragmented mass may have been as large as 1000 kg this is the largest single deposit of tektites yet reported in this region layered tektite fragments are found in isolated clusters usually associated with a pebbly laterite horizon that is 0 1 m below the surface near khe sanh vietnam we estimate the abundance of layered tektite fragments to be 100 g m2 this is greater than five times the abundance estimated for northeast thailand fiske et al 1996 in a region that extends from northeast thailand through central laos and into central vietnam we found only layered tektites which confirmed the existence of a large 50 000 km2 subfield of the australasian strewn field with only layered tektites the east west extent of the layered only subfield is well constrained but little field data exist to constrain its north south extent

a possible model for large preseismic slip on a deeper extension of a seismic rupture plane abstract i develop a numerical model for preseismic slip and episodic aseismic slip events slow earthquakes on a deeper extension of a seismogenic plate interface using a rate and state dependent friction law the deep preseismic slip and episodic slow slip were indicated by models based on geodetic observations though they cannot be explained by common rate and state dependent laws i investigate whether this incongruity can be removed by including use of additional known constraints on the frictional property the steady state friction on a model fault is taken to be velocity weakening at low velocities and velocity strengthening at high velocities the transition velocity from velocity weakening friction to velocity strengthening is assumed to be the seismic slip velocity at shallow seismic depths and decreases with depth at deeper parts simulation results for a 2d model of interplate earthquakes at a subduction zone are as follows 1 during an interseismic period between successive large earthquakes at shallow seismogenic depths some episodic aseismic slip events with durations of a few years occur on the plate interface just below the seismogenic depths 2 preseismic slip occurs at nearly the same location where the episodic aseismic slip events occur these may explain some characteristics of observed preseismic slip of the 1944 tonankai earthquake m 7 9 along the nankai trough central japan and episodic aseismic slip events in the same area

extraterrestrial impact events the record in the rocks and the stratigraphic column abstract the known terrestrial impact record is a biased sample of a much larger population of impact events the biases are due to the modifying effects of terrestrial geologic processes coupled with incomplete searches for impact structures and impact related materials terrestrial impact structures have the same basic forms as impact craters on the other planets of the inner solar system but because of post impact modification by terrestrial geologic process are recognised by the occurrence of shock metamorphic effects in some cases siderophile anomalies have been identified in impact lithologies and have been used to estimate the composition of the impacting body similar shock metamorphic effects and a siderophile anomaly in k t boundary materials are indicative of a major impact event which has been correlated with the formation of the chicxulub structure mexico evidence of a small number of other impacts occur in the stratigraphic record most commonly as tektite or microtektite horizons in some cases they are known to be accompanied by geochemical anomalies in other cases a number of ir anomalies have been reported in the stratigraphic record but there is no confirmatory evidence that they are due to impact the majority of known impact events in the stratigraphic record are from relatively recent geologic time logic dictates however that many more impacts must be recorded in terrestrial sediments and model calculations indicate that relatively small impacts d 20 km have the potential to cause atmospheric blow out and thus global dispersion of some of the impact products geochemical detection however of such events may not be easy in some cases because of relatively small absolute signals against the background of the daily infall of cosmic material in addition non chondritic bodies may result in no appreciable geochemical anomaly in view of this any claim to a geochemical signature of impact in the stratigraphic record should be accompanied by a physical search for impact materials although in the case of impacts into oceanic crust this too will be difficult given the k t experience however and the fact that large scale impact on earth is a natural consequence of the character of the solar system the potential of impacts to provide local and global marker horizons can not be ignored similarly the fact that impacts may have the potential to result in short term biologic or climatic excursions can not be dismissed arbitrarily when considering the causes of such phenomena as stable isotope anomalies in the stratigraphic record

a slow thrust slip event following the two 1996 hyuganada earthquakes beneath the bungo channel southwest japan we report a slow thrust slip event that occurred beneath the bungo channel region southwestern japan on oct and dec 1996 two hyuganada earthquakes both mw 6 7 followed by afterslips occurred in addition a crustal movement characterized by an extremely slow rise was observed around the bungo channel about 200 km north from the epicenters and continued for about 300 days long assuming a slow slip on the plate boundary we estimate its duration and surface displacements from gps time series data by curve fitting and then determine the fault slip distribution we found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the hyuganada earthquakes occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling

the amurian plate motion and current plate kinematics in eastern asia we use global positioning system gps velocity data to model eastern asian plate kinematics out of 15 stations in korea russia china and japan studied here three sites considered to be on the stable interior of the hypothetical amurian plate showed eastward velocities as fast as 9 10 mm yr with respect to the eurasian plate they were stationary relative to each other to within 1 mm yr and these velocity vectors together with those of a few additional sites were used to accurately determine the instantaneous angular velocity euler vector of the amurian plate the predicted movement between the amurian and the north american plates is consistent with slip vectors along the eastern margin of the japan sea and sakhalin which reduces the necessity to postulate the existence of the okhotsk plate the euler vector of the amurian plate predicts left lateral movement along its boundary with the south china block consistent with neotectonic estimates of the displacement at the qinling fault possibly the southern boundary of the amurian plate the amurian plate offers a platform for models of interseismic strain buildup in southwest japan by the philippine sea plate subduction at the nankai trough slip vectors along the baikal rift the boundary between the amurian and the eurasian plates are largely inconsistent with the gps based euler vector suggesting an intrinsic difficulty in using earthquake slip vectors in continental rift zones for such studies

coesite and shocked quartz discovered in the australasian and north american microtektite layers samples from the north american australasian and ivory coast microtektite layers were examined for unmelted impact ejecta 125 m size coesite and shocked quartz identified by x ray diffraction were found at all the sites where a well defined north american microtektite layer had previously been documented coesite and shocked quartz were also foundin seven out of 33 cores from the australasian strewn field however no coesite or shocked quartz was found associated with the ivory coast microtektite layer stishovite was also found at three of the australasian sites all of the australasian cores containing coesite and shocked quartz were taken from within 2000 km of indochina which is believed by most to be the source area for the australasian tektites and microtektites six of the seven cores have the highest concentrations of microtektites ever determined for australasian microtektite bearing cores the seventh core did not contain any microtektites which may be consistent with the hypothesis that the microtektites were deposited in a raylike pattern the discovery of coesite and impact ejecta associated with both the north american and australasian microtektite layers provides further support for the hypothesis of terrestrial impact origin of tektites

frictional properties of megathrust fault gouges at low sliding velocities new data on effects of normal stress and temperature abstract friction data used in modelling studies of subduction zone seismogenesis are often poorly representative of in situ conditions we investigated the influence of in situ effective stresses and temperatures on the frictional properties of simulated fault gouges prepared either from nankai odp material or illite shale at sliding velocities approaching those relevant for earthquake nucleation and sses biaxial double direct shear experiments were performed at room temperature normal stresses of 5 30 mpa and sliding velocities of 0 16 18 m s all materials exhibited velocity strengthening under these conditions along with an increase in the friction coefficient and slip hardening rate with increasing normal stress illite gouge showed increased velocity strengthening towards higher normal stresses the effect of temperature was investigated by means of ring shear experiments on illite gouge at 200 300 c an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa and sliding velocities of 1 100 m s these experiments showed a transition from velocity strengthening to velocity weakening at 250 c our results provide a possible explanation for the updip seismogenic limit within subduction zone megathrusts and imply an enhanced tendency for earthquake nucleation and sses at low effective normal stresses

spherule layers records of ancient impacts abstract a large extraterrestrial object striking earth at cosmic velocity melts and vaporizes silicate materials which can condense into highly spheroidal sand size particles that get deposited hundreds to thousands of kilometers from the point of impact these particles known as impact spherules have been detected in great abundance in a relatively small number of thin discrete layers ranging in age from less than a million years to 3 47 billion years unaltered impact spherules consist entirely of glass microtektites or a combination of glass and crystals grown in flight microkrystites impact spherule layers form very rapidly and can be very extensive even global in extent e g the cretaceous tertiary k t boundary layer so they form excellent time stratigraphic markers because they are always found in a stratigraphic context spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts a reco

ductile fracture of fine grained plagioclase in the brittle plastic transition regime implication for earthquake source nucleation abstract crush zones along strain localized zones of plastic deformation were found in a small natural brittle plastic shear zone in the abukuma mountains ne japan the optical microstructures indicate that crush zones are developed where plastic deformation is strong scanning electron microscopy sem revealed the presence of microvoids which connect together to form intergranular fractures in fine grained plagioclase layers in strain localized zones with crush zones this suggests that the microvoids coalesce in the fine grained plagioclase layers allowed ductile fracture to form crush zones during plastic deformation transmission electron microscopy tem revealed that the dislocation densities of all grains in fine grained plagioclase layers in strain localized zones with crush zones are high while evidence for grain boundary migration and dynamic recrystallization is found away from crush zones during plastic deformation of plagioclase in the brittle plastic transition regime strong plastic deformation with insufficient recrystallization may have resulted in the nucleation of microvoids which link together to form intergranular fractures this process may be an important mechanism during earthquake source nucleation

the distribution and geochemistry of platinum group elements as event markers in the phanerozoic abstract the discovery of the platinum group element pge enrichment in the cretaceous tertiary k t boundary impact layer popularized the use of pges as geochemical event markers and prompted an explosion of interest in these elements whose geochemical behavior is poorly understood instead of concentrating on the more easily analyzed pges e g iridium many studies now include the entire group analytical methods have been improved to the point where small samples 5 g can be analyzed confidently at the sub ppb level allowing the determination of the distribution of pges in the oceans atmosphere and biosphere the presence or absence of a pge anomaly is often used as a criterion for determining whether an event horizon is impact related however to date only the k t boundary yields a distinct pge anomaly with positive interelement correlations and solar interelement ratios terrestrial sources such as volcanic emissions are also being investigated as potential contributors to pge anomalies at other phanerozoic event horizons primary signatures e g interelement ratios characteristic of the pge source may be altered during various high and or low temperature processes for example fractionation during impact melting vaporization condensation remobilization redistribution and diagenesis can obscure original signatures and make determination of the pge source difficult future research might be directed towards establishing the behavior of the pges during such processes

correlated transient fluid pulsing and seismic tremor in the costa rica subduction zone continuous measurements of fluid flow were made over a six month period across the nicoya peninsula costa rica pacific convergent margin utilizing osmotically driven fluid flow meters designed to quantify both inflow and outflow rates on the order of 10 5 to 3 cm d significant transience in flow was observed through the surface of the forearc three periods of correlated flow signals were seen on the subduction forearc among three instruments located in the out of sequence thrust oost zone over along margin strike distances of 30 km amplitudes of ground velocity recorded on collocated ocean bottom seismometers obs increase during the three correlated flow events the seismic signal has frequency characteristics that resemble volcanic and non volcanic tremor we hypothesize that repeated plate boundary slow slip events potentially originating at the up dip limit of the seismogenic zone generate the observed signals within the toe of the forearc we propose a model in which the poro elastic stress strain field around a series of creep dislocations simultaneously forces flow through fracture networks in the forearc and oceanic basement rocks and induces diffuse flow through the shallow sediments the former generates the seismic tremor like noise recorded by the obss and the latter generates the flow transients recorded by the fluid flow meters we suggest that high sensitivity fluid flow meters can be utilized to detect transient tectonic strain events in offshore environments where traditional geodetic techniques lack resolution or are not possible d 2005 elsevier b v all rights reserved

the impact flood connection does it exist in a recent article in terra nova kristan tollmann and tollmann 1994 suggested that the biblical flood can be explained by seven fragments of a comet that impacted the ocean at seven locations on earth at 03 00h c e t on 23 september 9545 yr bp we demonstrate that all the geological proofs that allegedly support their conclusions are not supported by the available data on impact cratering their hypothesis is based on insufficient and ambiguous data selective citation and incomplete comprehension of previous research

excavation and analysis of layered tektites from northeast thailand results of 1994 field expedition we have analyzed the size shape composition and spatial distribution of 6 kg of layered tektite fragments excavated from a 3 m 3 m area near the town of ban huai sai in northeast thailand our analysis suggests that these fragments represent a single homogeneous mass that underwent fragmentation far in the past and has undergone little disturbance since its deposition we have also studied the stratigraphic occurrence of layered tektites exposed in situ near the town of ban huai om tektites were found along a disconformable paleo erosion surface covered by recent aeolian sand similar to other occurrences throughout southeast asia this stratigraphic relationship provides little chronostratigraphic information and thus does not support a stratigraphic age paradox for the australasian tektites the present day surface density of layered tektites in this area is 2 to 20 g m2

episodic creep events on the san andreas fault caused by pore pressure variations recent seismic and geodetic observations indicate that interseismic creep rate varies in both time and space the spatial extent of creep pinpoints locked asperities while its temporary accelerations known as slow slip events may trigger earthquakes although the conditions promoting fault creep are well studied the mechanisms for initiating episodic slow slip events are enigmatic here we investigate surface deformation measured by radar interferometry along the central san andreas fault between 2003 and 2010 to constrain the temporal evolution of creep we show that slow slip events are ensembles of localized creep bursts that aseismically rupture isolated fault compartments using a rate and state friction model we show that effective normal stress is temporally variable on the fault and support this using seismic observations we propose that compaction driven elevated pore fluid pressure in hydraulically isolated fault zone and subsequent frictional dilation cause the observed slow slip episodes we further suggest that the 2004 mw6 parkfield earthquake might have been triggered by a slow slip event which increased the coulomb failure stress by up to 0 45 bar per year this implies that while creeping segments are suggested to act as seismic rupture barriers slow slip events on these zones might promote seismicity on adjacent locked segments

large earthquakes initiate by the acceleration of slips on the downward extensions of seismogenic faults abstract in order to examine the relationship between anomalous crustal deformations and subsequent large earthquakes we compiled and examined data on preseismic anomalous crustal deformations for two earthquakes that occurred in and around the japanese islands in recent years we found that the slip on a downward extension of a seismogenic fault accelerates before the main shock and that the average slip velocity is inversely proportional to the time remaining until the main shock occurs

the mechanism of the 1983 japan sea earthquake as inferred from long period surface waves and tsunamis abstract the source process of the japan sea earthquake of may 26 1983 is studied by using the long period surface waves and tsunamis the moment tensor inversion of rayleigh waves which consider the lateral heterogeneity of the earth is developed and applied to the ida records of this event the moment tensor solution and first motion data indicate that the mechanism is dip slip and the seismic moment is 7 6 1027 dyne cm the fault is estimated to be 120 km in length 40 km in width and dips eastward with a dip angle of 30 from the aftershock distribution tsunami simulation for an actual topography is made to restrain the slip on the fault the aftershock area and the tsunami records suggest that the fault is divided into two segments the northern part strikes nnw and slips 4 m while the southern part strikes nne and slips 5 m the fault geometry the aspect ratio and the stress drop of this event are similar to those of the 1964 niigata earthquake but differ from those of the earthquakes in the pacific coast of japan

the quest for an extraterrestrial component in muong nong type and splash form australasian tektites from laos using highly siderophile elements and re os isotope systematics abstract extremely low and variable concentrations of osmium os and other highly siderophile elements hse in most tektites make it challenging to establish direct links between these impact related materials and their possible extraterrestrial contribution new os concentrations 2 43 ppt and 187os 188os ratios 0 131 0 68 in a suite of fifteen well characterized australasian tektites from laos muong nong and splash form types with variable ni enrichment indicate a maximum of 0 005 addition of a chondritic impactor this is similar to some australasian tektites from vietnam with similarly low siderophile contents but significantly lower than found in previous studies of more ni rich australasian splash form tektites and microtektites from different parts of the australasian strewn field e g indonesia south china sea the contents of hse and re os isotopic compositions of layered muong nong type australasian tektites are highly variable suggesting mingling of crustal derived siderophile element poor and extraterrestrial siderophile element rich materials the absence of a direct correlation between hse and ni contents is interpreted to result from a fractionation process related to their different vaporization condensation temperatures the low os abundance in most of the analyzed australasian tektites combined with non radiogenic 187os 188os far below average upper continental crust may provide a direct test to distinguish continental versus seawater impact scenario in the absence of any specific low os target a particular process of os loss following impact is required we envisage a scenario where evaporative loss of 90 os in the form of os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and or saline pore water embedded in sediments off shore indochina consistent with elevated contents of halogens in australasian tektites this water assisted os loss could also play significant role for central european tektites while the continental surface with limited amount of water would prevent from more efficient hse loss as could be the case for ivory coast tektites

episodic aseismic earthquake precursors shallow earthquakes are generally believed to be brittle fractures in a stressed medium with rupture velocity at a speed close to that for shear waves we know however that the earth allows failure over a wide range of timescales creep events occur on the san andreas fault system and for some earthquakes the high speed rupture is accompanied by a slower process1 5 here we report on episodic slow events which occurred before the japan sea earthquake of magnitude 7 7 on 26 may 1983 a sacks evertson borehole strain meter6 90 km from the earthquake recorded about 100 aseismic strain events in a five month period before and immediately following the earthquake none has been detected after the large aftershocks the observed signals are consistent with a precursory redistribution of stress through aseismic slip on a deep extension of the main shock fault plane such episodic aseismic events may provide a mechanism for relatively rapid stress concentration before earthquakes

extraterrestrial component associated with australasian microtektites in a core from odp site 758b major and trace element analyses of 30 sediment samples from odp core 758b across the brunhes matuyama geomagnetic polarity reversal and the australasian microtektite bearing layer show a small but significant anomaly in the elemental abundances of ir co and ni the maximum of the anomaly between 10 89 and 11 00 m below seafloor correlates with the maximum abundance of microtektites iridium abundances which show the most significant anomaly reach 0 16 ppb which is evidence for an extraterrestrial component

new sites of australasian microtektites in the central indian ocean implications for the location and size of source crater 1 we have identified 15 new australasian microtektite sites along a transect roughly n s in the central indian ocean these locations in addition to the existing 46 sites total to 61 microtektite sites in the oceans we carried out regression analysis of a selected area between 12 26 n and 98 112 e at 0 5 intervals using three microtektite data sets separately 41 sites 46 sites number of sites known up to the years 1994 and 2006 respectively and the entire data set of 61 sites this area was chosen because on the basis of various lines of evidence several investigators suggested impact craters for the strewn field within this broad area the 41 site data set defines an impact site at about 12 n and 106 e whereas the contours joining the highest values of the square of the correlation coefficient by using 46 site and 61 site data sets define an area located in ne thailand central laos the center of this site is at 18 n and 104 e therefore with progressive increase in microtektite data the location of the impact site seems to get defined more rigorously on the basis of the equations for the concentration of the ejecta and the distance from the source region the calculated crater size has a diameter range between 33 and 120 km interestingly the lower size limit of 33 km has been found closer to ground truth by verification from the microtektite data of the ivory coast strewn field

thermobarometry of metamorphosed pseudotachylyte and associated mylonite constraints on dynamic co seismic rupture depth attending caledonian extension north norway abstract the exhumed post caledonian eidsfjord and fiskfjord extensional shear zones of northern norway exhibit evidence of coseismic rupture propagating into the ductile crust as evidenced by the presence of mylonitic and metamorphosed pseudotachylyte geothermobarometric calculations on garnet bearing mineral assemblages in mylonitic gneisses associated with mylonitic pseudotachylyte and in metamorphosed pseudotachylyte permit determination of the depth and ambient temperature of seismogenic low angle ductile extension average pressures from eidsfjord 570 115 mpa at ca 650 c and fiskfjord 1120 220 mpa at ca 650 c correspond to faulting depths of 21 4 km and 41 9 km respectively the fiskfjord results agree with previous thermobarometric calculations on mylonitic cpx grt bearing pseudotachylyte at fiskfjord the calculated depths are 5 25 km below the depth of the standard seismogenic zone these results demonstrate that low angle normal faults may cut through a large portion of continental crust this occurrence of mylonitic pseudotachylyte in an extensional crustal setting is most easily explained by dynamic downward rupture into the ductile regime and or unusually high shear stresses to account for coseismic rupture at such depths implying a direct connection with a seismogenic normal fault in the upper crust

strain accumulation across the asal ghoubbet rift djibouti east africa the asal ghoubbet rift in the republic of djibouti is a subaerial continuation of the gulf of aden ridge system many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm yr perpendicular to the normal faults that bound the rift periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm yr although the observed deformations could be contamined by postseismic relaxation after a rifting episode in november 1978 the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley the deformation is uniformly distributed in time and in space inside the inner floor of the rift the necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process

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

postseismic deformation associated with the 1992 m ω 7 3 landers earthquake southern california following the 1992 m 7 3 landers earthquake a linear array of 10 geodetic monuments at roughly 5 km spacing was established across the emerson fault segment of the landers rupture the array trends perpendicular to the local strike of the fault segment and extends about 30 km on either side of it the array was surveyed by global positioning system 0 034 0 048 0 381 1 27 1 88 2 60 and 3 42 years after the landers earthquake to measure both the spatial and temporal character of the postearthquake relaxation the temporal behavior is described roughly by a short term decay time 84 23 days exponential relaxation superimposed upon an apparently linear trend because the linear trend represents motions much more rapid than the observed preseismic motions we attribute that trend to a slower decay time greater than 5 years postseismic relaxation the curvature of which cannot be resolved in the short run 3 4 years of postseismic data about 100 mm of right lateral displacement and 50 mm of fault normal displacement accumulated across the geodetic array in the 3 4 year interval covered by the postseismic surveys those displacements are attributed to postseismic right lateral slip in the depth interval 10 to 30 km on the downward extension of the rupture trace the right lateral slip amounted to about 1 m directly beneath the geodetic array and the fault normal displacement is apparently primarily a consequence of the curvature of the rupture these conclusions are based upon dislocation models fit to the observed deformation however no dislocation model was found with rms residuals as small as the expected observational error

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses

accretion rate of extraterrestrial matter iridium deposited 33 to 67 million years ago iridium measured in 149 samples of a continuous 9 meter section of pacific abyssal clay covering the time span 33 to 67 million years ago shows a well defined peak only at the cretaceous tertiary boundary in the rest of the section iridium ranges from a minimum concentration near 0 35 nanograms per gram in the paleocene to a maximum near 1 7 in the eocene between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years correction for terrestrial iridium leads to an extraterrestrial flux of9 3 nanograms of iridium per square centimeter per million years and an estimated annual global influx of 78 billion grams of chondritic matter consistent with recent estimates of the influx of dust meteorites and crater producing bodies with masses ranging from 10 13 to 1018 grams combining the recent flux of objects ranging in mass from 106 to 107 grams with the flux of 1014 to 1015 gram objects indicates that the number of objects is equal to 0 54 divided by the radius in kilometers to the 2 1 power periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years the predicted iridium maxima more than 30 times background are not present in the intervals associated with the cretaceous tertiary boundary or the tektiteproducing late eocene events

geochemistry of tektites and impact glasses glass produced by melting during impact events is called impact glass or impactite although impact crate ring produces a great variety of shock metamorphosed rocks all of which are called impactites by various authors to a differing extent we restrict the use of this term here to material completely converted into glass the degree of metamorphism seen in different classes of impact materials starting with those showing simple shock effects shock lithification shatter cones diaplectic glasses coesite stishovite lechatelierite impact breccias glass depends on the peak pressure and temperature the rock has experienced as a result of mixing brecciation and similar processes impact glasses also tend to include fragments of less shocked or less homogeneous metamorphosed rocks we limit our discussion here however to the more homogeneous members of the terrestrial impactite family a discussion of lunar impact glasses was given by delano et al 1981 another group of natural glasses are tektites which at first glance are similar to obsidian most tektites are very homogeneous often aerodynam ically shaped spherically symmetric objects that are several centimeters in size and are found in various areas referred to as tektite strewn fields in reflected light they are usually black but in transmitted light they show variations of grey brown and green the surface features gouges furrows grooves pits striations clearly distinguish them from terrestrial volcanic glasses and have probably originated from solution etching and weathering effects on earth baker 1963 lamarche et a1 1984 and from surface stresses occurring during their cooling tektites found in geographically adjacent

source of the australasian tektite strewn field a possible off shore impact site although there is a preponderance of evidence that tektites were formed by asteroid impacts on the earth no source crater has been found for the largest and youngest of the strewn fields the australasian strewn field we have examined a combined seasat geos 3 altimeter data set of sea surface heights in the northern portion of the australasian strewn field for negative gravity anomalies on the continental shelf and slope which might be related to the source crater for these tektites a large negative anomaly which we call the qui nhon slope anomaly is a sea surface depression of approximately 1 5 meters over an area of 100 km diameter centered at 13 47 n latitude and 110 37 e longitude it corresponds to a gravity anomaly of about 50 mgal a surface ship gravimeter track across most of the structure confirms the satellite measurements this site is on the continental slope in 1 to 3 km of water a sea floor topographic depression of more than 200 meters occurs in the southeast quadrant of the anomaly the center of the structure is 175 km offshore and is less than 300 km from the muong nong substrewn field and the prolific dalat site perhaps the largest concentration of tektites within the australasian field due to its location size and magnitude we propose this anomaly may be due to the impact structure that produced the australasian strewn field

geographic variation in australasian microtektite concentrations implications concerning the location and size of the source crater geographic variations in the concentration of australasian microtektites in 42 cores from the indian ocean western equatorial pacific ocean and the philippine celebes and sulo seas were used to predict the location of the australasian tektite source crater and to estimate its size the location that seems to best explain the geographic variations in microtektite concentrations is located in central cambodia at about 12 n latitude and 106 e longitude the diameter of the source crater is estimated to be between 32 and 114 km based on thickness of the microtektite layer at each site and distance from the predicted source area in cambodia the large range in estimated size of the source crater is due to a lack of knowledge about the value of the exponent that describes the radial decrease in ejecta thickness additional microtektite ejecta bearing sites closer to the source region could help resolve this problem

postseismic deformation following the landers earthquake california 28 june 1992 accelerated strain followed the landers and big bear earthquakes returning to the normal rate only after a period of several months we observed this strain throughout most of southern california using the global positioning system gps three gps receivers operating continuously in fixed positions at pinyon flat jet propulsion laboratory pasadena and goldstone all recorded postseismic deformation in a relative sense in addition we established 16 sites where we deployed portable receivers occasionally over a period of about 6 months near the rupture zones of the earthquakes anomalous postseismic displacements ranged from 55 mm near the epicenter to a few millimeters far from the fault we modeled the displacements using dislocation theory as due to variable slip on the faults that were displaced at the times of the earthquakes the model suggests that the postseismic strain released the equivalent of about 15 of the seismic moment of the mainshock while the strain released from the upper 10 km is about the same as what can be explained by direct effects of aftershocks the major contribution of strain release comes from the lower layer below 10 km depth significant afterslip or viscous relaxation must have occurred below 10 km depth to explain the observed deformation more than 100 km from the fault one interpretation is that high stress on the margin of the co seismic rupture zone drives the rupture to extend itself into urbroken rock below and along the initial rupture zone

temporal variation of crustal deformation during the days preceding a thrust type great earthquake the 1944 tonankai earthquake of magnitude 8 1 japan the temporal variation in precursory ground tilt prior to the 1944 tonankai japan earthquake which is a great thrust type earthquake along the nankai trough is discussed using the analysis of data from repeated surveys along short distance leveling routes sato 1970 pointed out that an anomalous tilt occurred one day before the earthquake at kakegawa near the northern end of the focal region of the earthquake from the analysis of additional leveling data sato s result is re examined and the temporal change in the ground tilt is deduced for the period of about ten days beginning six days before the earthquake a remarkable precursory tilt started two or three days before the earthquake the direction of the precursory tilt was up towards the south uplift on the southern nankai trough side but the coseismic tilt was up towards the southeast perpendicular to the strike of the main thrust fault of the tonankai earthquake the postseismic tilt was probably opposite of the coseismic tilt the preseismic tilt is attributed to precursory slip on part of the main fault if similar precursory deformation occurs before a future earthquake expected to occur in the adjacent tokai region the deformation may help predict the time of the tokai earthquake

on the physical mechanism of silent slip events along the deeper part of the seismogenic zone 1 to investigate the mechanism of recently observed silent slip events we simulated earthquake preparation processes using the dieterich ruina rate and state dependent friction law to ensure realistic modeling of the unstable stable transition we considered small cut off velocity to an evolution effect in the friction law for the transition zone when the cut off velocity to the evolution effect is significantly smaller than that of a direct effect steady state friction behaves as velocity weakening at low slip velocity and velocity strengthening at high slip velocity this frictional behavior was experimentally and theoretically confirmed for the unstable stable transition zone the results of our numerical simulations show that silent slips of which velocity is higher than the velocity of relative plate motion eventually propagates horizontally along the unstable stable transition over a period of several years silent slip events can be interpreted as being caused by the transitional behavior of the fault constitutive law