glass from the cretaceous tertiary boundary in haiti tektite like glasses preserved at the cretaceous tertiary boundary at beloc in haiti provide clear evidence of an impact event the glass composition suggests that the impact occurred on a continental shelf region generating a silica rich glass with chemical composition that reflects the melting of continental crustal rocks and a calcium rich glass produced by the fusion of marl sediments these findings indicate that catastrophic release to the atmosphere of 1015 moles of c02 from vaporized marl occurred during the impact

the 1989 macquarie ridge earthquake seismic moment estimation from long period free oscillations we have analyzed the long period 1000 s part of the earth s free oscillation spectrum of the 1989 macquarie ridge earthquake this event stands out as the largest in the last decade and as the first great earthquake to have been recorded digitally on the new generation of very broadband seismic sensors around the globe its spectrum was modeled for five spectral peaks 0s4 0s6 0s9 1s8 and 5s3 taking into account the effects of lateral heterogeneity rotation and ellipticity which cause coupling within the spectral peaks self coupling and between different multiplets multiplet coupling the effects of self coupling were calculated following the formulation of woodhouse and girnius 1982 multiplet coupling only weakly affected the spectral peaks that were used a large discrepancy in phase and amplitude between the calculated spectrum and the data was observed which suggests that the seismic moment was 50 larger than that inferred from surface wave analysis satake and kanamori 1990 an error analysis was performed in order to detect inaccuracies that might have resulted in this discrepancy centroid time source finiteness correction and directivity effects are likely to cause a significant bias both in the moment and phase determination we saw no direct evidence that a precursory event could have been responsible for the additional energy release as was suggested by ihmle et al 1993

teleseismic detection of a slow precursor to the great 1989 macquarie ridge earthquake low frequency spectra for the 1989 macquarie ridge earthquake magnitude 8 2 show an amplitude increase and a phase delay decrease below 6 millihertz that require a short term slow precursor this earthquake can be modeled as a compound event in which a fast rupturing ordinary earthquake was initiated by an episode of slow smooth deformation that began more than 100 seconds before the main shock the moment released in the slow precursor was large about 3 x 10 20 newton meters equivalent to an event of magnitude 7 6 the data are consistent with the precursor being generated in a region of the oceanic upper mantle below the main rupture

seismic moment and duration of recent large and great earthquakes the methodology developed to verify the existence of a low frequency precursor to the great 1960 chilean earthquake is applied to international deployment of accelerometers ida station data in the 1 6 mhz band to determine the seismic moment and temporal centroid of 22 recent large and great earthquakes including the 1989 macquarie ridge earthquake which has been reported to have a low frequency precursor imhle et al 1993 no clear evidence of precursors to any of these events is observed in the 1 6 mhz band the seismic moment of large unilateral earthquakes of a given type of faulting is shown to be approximately proportional to the square of the duration these data are observed to be well fit by this relation

tektites in cretaceous tertiary boundary rocks on haiti and their bearing on the alvarez impact extinction hypothesis relic tektites are associated with a pt group metal abundance anomaly and shocked minerals in a thin marl bed that marks the k t boundary on haiti the presence of these three impact produced materials at the precise k t boundary enormously strengthens the alvarez impact extinction hypothesis the tektites occur in smectite spherules that have external shapes typical of tektites their chemical and physical properties are broadly similar to those of other tektite groups except that the haitian tektites have lower si and higher fe ca and na on average they contain more sc v cu zn ga sr sn and ba and less cr ni co b mn and hf than other tektite groups amounts of rare earth elements ree in the tektites indicate that their progenitor materials were not melted mafic or ultramafic oceanic crust rather they were sedimentary deposits having a bulk composition of andesite rare tektites contain unusually high amounts of cao 20 and s 0 4 and these data suggest that some target materials consisted of caso4 anhydrite beds occur in the subsurface at two candidate impact sites chicxulub and manson sm nd isotopic data for the tektites indicate that the melted precursor sediments were most likely deposited less than 400 m y between the late cretaceous and silurian periods major chemical changes accompanied the diagenetic change of glass to smectite the haitian tektites are the first datable impact products in k t boundary rocks and 40ar 39ar ages of the glass show that the k t boundary and impact event are coeval at 64 5 0 1 ma

low frequency source characteristics of the great 1960 chilean earthquake the low frequency nature of the great 1960 chilean earthquake the earth s largest recorded event is characterized and a source model composed of three events with a combined seismic moment of 5 5 1023 n m and a duration of 1500 s is presented the results show conclusively that large scale slip which is only observed at low frequencies preceded the main shock complex amplitude measurements of normal mode data in the band 1 0 5 0 mhz recorded at eight vertical component stations comprise the data set in this band the earthquake source is assumed to be described by a line source propagating at a constant rupture velocity the data set is the basis for a sequence of least squares inversions to determine the seismic moment rupture velocity spatial slip distribution and the temporal slip function the data are best explained by a source model that begins 1150 s before the main shock with an event of moment 1 9 1023 n m and rise time of 300 s a main shock with moment of 3 2 1023 n m and for which most of the low frequency energy was released beginning about 50 s after the short period origin time and an event 350 s later with moment of 0 4 1023 n m the uncertainty in the moment estimate of the great 1960 chilean earthquake is about 50 and is due primarily to the probable range of fault plane dip which is not well constrained at this time

source complexity of the 1988 armenian earthquake evidence for a slow after slip event we analyzed teleseismic p and s waves using a multiple event deconvolution method to investigate the source process of the spitak armenia earthquake of december 7 1988 teleseismic long period body waves exhibit complex waveforms significantly more complex than those normally seen for an event of this size we identified two groups of subevents one is a group of strike slip events during the first 20 s the other is a dip slip event initiating at about 30 s after the initial rupture the tensor sum of all the subevents is essentially strike slip strike 302 dip 59 rake 143 with a scalar moment of 1 47 10 19 n m the seismic moment of the dip slip event strike 89 dip 60 rake 88 is 5 6 10 18 n m nearly 40 of the total and the moment release rate is slower than the other subevents with a strike slip mechanism we call this event a slow after slip event the mechanism of the after sup event is consistent with the present day deformation pattern in the caucasus where north south compression is predominant due to the continental collision between the arabian plate and the russian platform this result suggests that in a region where both strike slip motion and crustal shortening are taking place like southern california a large strike slip event may accompany large thrust events and vice versa in order to determine the total coseismic deformation pattern it is essential to resolve these multiple events in time space and mechanism

body wave deconvolution for variable source parameters application to the 1978 december 6 kuriles earthquake summary a method to determine the depth and mechanism of subevents in earthquakes with complex rupture is developed complex body waves are deconvolved using a damped lanczos inversion to retrieve the source time function some constraints on the mechanism and depth of the first subevent are assumed though not required the data set is deconvolved using the green s function for the first subevent to obtain a best fitting source time function this source time function is then windowed in time around the first pulse and the corresponding synthetic seismogram is subtracted from the data this process is repeated on residual data to determine mechanisms and depths of subsequent subevents this method is applied to the 1978 december 6 kuril islands earthquake a complex event which ruptured vertically from 100 to 220km in depth and represents tearing of the pacific plate as it subducts beneath the kuril and honshu arcs using long period wwssn data we found three main subevents with the mechanism o 150 80 and 20 at depths of 120 165 and 210km in this particular case one nodal plane is very well constrained and there is some a priori evidence for significant vertical rupture

variable rupture mode of the subduction zone along the ecuador colombia coast three large earthquakes occurred within the rupture zone of the 1906 colombia ecuador earthquake m w 8 8 in 1942 m s 7 9 1958 ms 7 8 and 1979 m s 7 7 we compared the size and mechanism of these earthquakes by using long period surface waves tsunami data and macroseismic data the 1979 event is a thrust event with a seismic moment of 2 9 10 28 dyne cm and represents subduction of the nazca plate beneath south america the rupture length and direction are 230 km and n40 e respectively examination of old seismograms indicates that the 1906 event is also a thrust event which ruptured in the northeast direction the seismic moment estimated from the tsunami data and the size of the rupture zone is 2 10 29 dyne cm the 1942 and 1958 events are much smaller about 1 5 to 1 10 of the 1979 event in the seismic moment than the 1979 event we conclude that the sum of the seismic moments of the 1942 1958 and 1979 events is only formula of that of the 1906 event despite the fact that the sequence of the 1942 1958 and 1979 events ruptured approximately the same segment as the 1906 event this difference could be explained by an asperity model in which the fault zone is held by a discrete distribution of asperities with weak zones in between the weak zone normally behaves aseismically but slips abruptly only when it is driven by failure of the asperities a small earthquake represents failure of one asperity and the rupture zone is pinned at both ends by adjacent asperities so that the effective width and the amount of slip are relatively small a great earthquake represents failure of more than one asperity and consequently involves much larger width and slip

high grade iron ore at windarling yilgarn craton a product of syn orogenic deformation hypogene hydrothermal alteration and supergene modification in an archean bif basalt lithostratigraphy banded iron formation bif hosted iron ore deposits in the windarling range are located in the lower greenstone succession of the marda diemals greenstone belt southern cross domain yilgarn craton and constitute a total hematite martite goethite ore resource of minimum 52 mt at 60 wt fe 0 07 p banded iron formation is interlayered with high mg basalts at windarling and precipitated during episodes of volcanic quiescence trace element content and the rare earth element ree ratios y ho 42 to 45 sm yb 1 5 together with positive la and gd anomalies in least altered hematite magnetite metachert bif indicate the precipitation from archean seawater that was fertilised by hydrothermal vent fluids with a basaltic hree y signature hypogene iron ore in sub greenschist facies metamorphosed bif formed during three distinct stages ore stage 1 was a syn to post metamorphic syn d1 fe ca mg ni co p ree metasomatism that produced local ni ree rich fe dolomite magnetite alteration in bif hydrothermal alteration was induced by hot fluid flow controlled by brittle ductile reactivation of bif basalt margins and crosscutting d1 faults the ni co rich content of dolomite and a shift in ree ratios in carbonate altered bif towards archean mafic rock signature y ho to 31 to 40 sm yb to 1 to 2 and gd gd to 1 2 to 1 4 suggest that high mg basalts in the windarling range were the primary source of introduced metals during ore stage 2 a syn deformational and likely acidic and oxidised fluid flow along bif basalt margins and within d1 faults leached carbonate and precipitated lepidoblastic and anhedral granoblastic hematite high grade magnetite hematite ore is formed during this stage ore stage 3 hydrothermal specular hematite spch fe dolomite quartz alteration was controlled by a late orogenic brittle compressional transpressional stage d4 the regional scale shear zone related d3 is not preserved in windarling this minor event remobilised iron oxides carbonate and quartz to form veins and breccia but did not generate significant volumes of iron ore ore stage 4 involved mesozoic to recent supergene oxidation and hydration in a weathering environment reaching down to depths of 100 to maximum 200 m below surface supergene ore formation involved goethite replacement of dolomite and quartz as well as martitisation important ground preparation for supergene modification and upgrade were mainly the formation of steep d1 to d4 structures steep bif basalt margins and particularly the syn d1 to syn d2 carbonate alteration of bif that is most susceptible to supergene dissolution the windarling deposits are structurally controlled supergene modified hydrothermal iron ore systems that share comparable physical chemical and ore forming characteristics to other iron ore deposits in the yilgarn craton e g koolyanobbing beebyn in the weld range mt gibson however the remarkable variety in pre syn and post deformational ore textures relative to d1 and d2 has not been described elsewhere in the yilgarn and are similar to the ore deposits in high strain zones such as of brazil quadrilatero ferrifero or iron quadrangle and nigeria the overall similarity of alteration stages i e the sequence of hydrothermal carbonate introduction and hypogene leaching with other greenstone belt hosted iron ore deposits supports the interpretation that syn orogenic bif alteration and upgrade was crucial in the formation of hypogene supergene iron ore deposits in the yilgarn craton and possibly in other archean paleoproterozoic greenstone belt settings worldwide

mineralogic evidence for an impact event at the cretaceous tertiary boundary a thin claystone layer found in nonmarine rocks at the palynological cretaceous tertiary boundary in eastern montana contains an anomalously high value of iridium the nonclay fraction is mostly quartz with minor feldspar and some of these grains display planar features these planar features are related to specific crystallographic directions in the quartz lattice the shocked quartz grains also exhibit asterism and have lowered refractive indices all these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth the shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile

seismicity associated with the great colombia ecuador earthquakes of 1942 1958 and 1979 implications for barrier models of earthquake rupture abstract we have relocated earthquakes along the colombia ecuador coast with the method of joint hypocenter determination in order to examine the nature of fault heterogeneities that controlled northward propagation of plate boundary rupture from the source region of the earthquake of 14 may 1942 to the source region of the 19 january 1958 earthquake and eventually to the source region of the earthquake of 12 december 1979 the entire region of the 1942 1958 and 1979 shocks had previously ruptured in the single great earthquake of 31 january 1906 the relocated hypocenters lie on the same plane to within the approximately 20 km uncertainty of the focal depths also the main shocks nucleated at nearly the same distance from the colombia trench these observations suggest that the heterogeneities between the 1942 and 1958 ruptures and between the 1958 and 1979 ruptures do not correspond to a major distortion of the downgoing slab but rather to either minor distortions of the slab or to regions of high friction or low available strain energy on a continuous fault surface in particular the heterogeneity between the 1958 and 1979 rupture zones seems to have been a high strength barrier with dimensions much smaller than the dimensions of either of the rupture zones the 1942 and 1958 earthquakes had source dimensions no larger than the 1979 main shock but they had stronger aftershock sequences than the 1979 earthquake this observation suggests that the arrest of earthquake rupture in 1979 left the plate boundary segment that had ruptured in 1906 in a state of lower stress than it had been following the 1942 and 1958 earthquakes long term seismicity in the decades preceding the 1979 earthquake occurred mostly outside or on the boundaries of the rupture area defined by the distribution of 1979 aftershocks the intense aftershock activity that followed the 1958 main shock within tens of kilometers of the eventual 1979 hypocenter may correspond to a long term precursory seismic swarm for the 1979 earthquake

estimating particle sizes concentrations and total mass of ash in volcanic clouds using weather radar observations of the march 19 1982 ash eruption of mount st helens made by the national weather service nws portland oregon on 5 cm radar were used to estimate the volume of the ash cloud 2000 500 km3 the concentration of ash 0 2 0 6 g m 3 and the total mass of ash erupted 3 10 1011 g the position of the cloud was also tracked by radar particle sizes in the ash cloud were estimated from settling velocities suggested by decreases in maximum ash cloud height with time as it moved away from the volcano the march 19 1982 ash blanket was sampled and mapped ash fallout times and accumulation rates were reconstructed from ground observations grain size distributions for various samples were used to obtain particle concentration 0 2 g m 3 total ashfall mass 1 3 1011 g and radar reflectivity factor 4 5 mm6 m 3 for the ash cloud our preferred estimate for total ashfall mass 4 1011 g is that obtained from the product of the ash cloud volume determined by radar 2000 500 km3 and the particle concentration inferred from ashfall data 0 2 g m 3 previously published ashfall data for the may 18 1980 mount st helens eruption has been studied using our ashfall inversion technique to estimate 6 hour mean particle concentration 3 g m 3 the size distribution total ashfall mass 5 1014 g and radar reflectivity factors 7 60 mm6 m 3 for the ash cloud a somewhat higher value 9 g m 3 for particle concentration was estimated from radar observations harris et al 1981 for an ash cloud formed during the peak eruption rate at mount st helens the two independent estimates are consistent given the many uncertainties of the problem the reflectivity factors for very dense ash clouds 3 9 g m 3 are several orders of magnitude smaller than for severe weather considered routinely detectable by airborne weather radar and dangerous for aviation because volcanic ash clouds with particle concentrations of at least 0 2 g m 3 are produced in extremely small eruptions in terms of total ashfall mass of duration less than 1 minute volcanic ash clouds must be considered an extremely serious hazard to in flight aircraft regardless of eruption magnitude these factors should be considered in hazard evaluations for known volcanoes located near air routes radar observations and calculations can provide scientists monitoring eruptive activity with significant information for estimating duration of eruption particle concentrations in ash clouds total mass of solid material erupted magma eruption rate potential ashfall mass ashfall locations and accumulation rates and duration and amounts of ashfall detailed analysis of ashfall data and nws radar observations of ash clouds from mount st helens demonstrate that weather radar can yield such timely information during and following volcanic eruptions

analysis of the relationship between seismic moment and fault length for large crustal strike slip earthquakes between 1977 92 we have used an uniform data set to determine the scaling relation between seismic moment m0 and fault length for 34 large crustal strike slip earthquakes in the period 1977 92 the seismic moments used were obtained by the centroid moment tensor analysis the aftershocks of each event were relocated in this study and the fault length estimated from their spatial extent for large events we find that in spite of some scatter in the data the trend of this uni form data set indicates that m0 c 2 our data set does not show any obvious change of slope around m0 6 8 x 102o n m as was seen in a previous non uniform data set in

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

the manson impact structure 40ar 39ar age and its distal impact ejecta in the pierre shale in southeastern south dakota the 40 ar 39 ar ages of a sanidine clast from a melt matrix breccia of the manson iowa impact structure mis indicate that the mis formed 73 8 0 3 million years ago ma and is not coincident with the cretaceous tertiary boundary 64 43 0 05 ma the mis sanidine is 9 million years older than 40 ar 39 ar age spectra of mis shock metamorphosed microcline and melt matrix breccia interpreted earlier to be 64 to 65 ma grains of shock metamorphosed quartz feldspar and zircon were found in the crow creek member upper campanian at a biostratigraphic level constrained by radiometric ages in the pierre shale of south dakota that are consistent with the 40 ar 39 ar age of 73 8 0 3 ma for mis reported herein

the november 20 1960 peru tsunami earthquake source mechanism of a slow event the november 20 1960 peru earthquake excited a tsunami that was anomalously large for an earthquake of moderate magnitude m 6 75 pas body wave first motions combined with observed love to rayleigh wave spectral ratios indicate slip along a shallowly dipping thrust fault rather than a single force mechanism suggestive of a landslide deconvolution of sh waveforms shows a time function consisting of 4 subevents with a total duration of 110 s using this time function body waveform inversion yields a very shallowly dipping thrust faulting mechanism strike 0 dip 6 slip 100 and a seismic moment of 1 9 1027 dyn cm however the time function obtained from the band limited body waveforms cannot fit the surface wave data which show a large increase in amplitude with period and a moment of 3 4 1027 dyn cm both types of data can be modeled by combining the body wave time function and a 130 s cosine time function representing a longer period component this event represents the largest earthquake to occur along this relatively aseismic portion of the peruvian coast the tsunami excitation is not anomalous relative to the measured seismic moment and moment magnitude mw 7 6 the disparity between tsunami height and surface wave magnitude results from underestimation of the earthquake size by conventional magnitude scales due to the long source duration

a reassessment of the rupture characteristics of oceanic transform earthquakes 1 we investigate the long period source spectra of oceanic transform earthquakes and find that previously proposed slow rupture components can be explained as artifacts generated by the modeling procedure we use low frequency 20 mhz rayleigh and love waves to calculate the amplitude spectra of five earthquakes on the romanche and chain transform faults in the equatorial mid atlantic ocean we find that errors and approximations in the centroid depth focal mechanism and earth structure at the source have significant effects on the shape of the source spectra if global catalog values and an average crustal model are assumed the spectra exhibit apparent anomalous energy at long periods which has previously been interpreted as a result of slow rupture we recalculate the source spectra using precise independently determined depths and moment tensors and a more realistic oceanic crustal structure in the source region the resulting source spectra are flat at long periods with no indication of anomalous long period energy our results imply that oceanic transform earthquakes do not commonly have detectable slow rupture components

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

generation and dispersal of fine ash and dust by volcanic eruptions abstract the volcanic eruptions which generate the greatest quantities of fine ash and dust are those of ignimbrite forming plinian vulcanian and phreatomagmatic types these are also the eruptions which produce the widest dispersal of this material attributed to the superior height attained by their eruptive columns however much of the fine ash and dust may be rapidly flushed out of the eruptive plume by water particularly in phreatomagmatic eruptions recent studies made on the dispersal and grain size of pyroclastic deposits produced by examples of plinian and phreatomagmatic types have yielded estimates of the quantities of material generated in each grain size class besides the extent of their dispersal not all of the fine volcanic particles are produced by fragmentation at the eruptive vent in ignimbrite eruptions there is good evidence for their large scale generation in and loss from the moving ash flows

changes in frequency size relationship from small to large earthquakes the constant b value observed in frequency magnitude distributions of earthquakes has been taken as an indication of self similarity at all magnitudes hence earthquake properties should scale uniformly in the same way from small to large earthquakes it has often been observed however that the seismic moment released in small earthquakes scales differently with rupture length than it does for large events1 2 where the crossover between small and large events is defined at a rupture dimension equal to the down dip width of the seismogenic zone a possible explanation for this contradiction is that frequency size distributions are biased by small earthquakes small events dominate most global earthquake catalogues because of the short time period covered another source of bias in size distributions is the saturation of earthquake magnitudes for large events here we correct biases in calculations of b values and present evidence for a change in b value in frequency size distributions we find that a break in self similarity from small to large earthquakes occurs at a point where the dimension of the event equals the down dip width of the seismogenic layer

tsunami earthquakes slow thrust faulting events in the accretionary wedge the november 20 1960 peru october 20 1963 kurile and june 10 1975 kurile earthquakes are classified as tsunami earthquakes based on anomalously large tsunami excitation relative to earthquake magnitude long period surface wave analysis indicates double couple faulting mechanisms for all three events rather than single force mechanisms indicative of submarine landslides the earthquakes have shallow depths 15 km and are located near the trench axis and seaward of most other thrust zone events beneath the accretionary prism body waveform inversion indicates very shallowly dipping thrust faulting mechanisms for the three events with dip angles of 6 8 surface wave spectral amplitudes and deconvolution of sh waveforms suggests anomalously long source durations and large seismic moments relative to ms specifically the 1963 kurile event ms 7 2 shows a duration of 85 s and a moment of 6 0 1027 dyn cm mw 7 8 the 1975 kurile event ms 7 0 shows a duration of 60 s and a moment of 2 0 1027 dyn cm mw 7 5 and the 1960 peru event ms 6 75 shows a time function consisting of four subevents with a total duration of 110 130 s and a seismic moment of 3 4 1027 dyn cm mw 7 6 estimated rupture velocities are about 1 km s or less but there is no evidence of unusually low stress drops the august 1 1968 philippines event previously classified as a tsunami earthquake shows none of the anomalous source properties and teleseismic tsunami height measurements are sparse we do not consider this event a tsunami earthquake most of the anomalous tsunami excitation results from underestimation of earthquake size by ms due to the long source duration the tsunami heights are not significantly anomalous relative to seismic moment the slow nature of these events may result from rupture through the sedimentary rock along the basal decollement of the accretionary prism standard scaling laws when adjusted for the slow seismic velocity in the source region show an mw ms relationship similar to that observed for the tsunami earthquakes and predict ms saturation at about 7 3 rather than 8 0 for typical events

the 1992 nicaragua earthquake a slow tsunami earthquake associated with subducted sediments the 1992 nicaragua earthquake was a tsunami earthquake that is it generated tsunamis disproportionately large for its surface wave magnitude m s the moment magnitude m w determined from long period 250 s surface waves was 7 6 significantly larger than the 20 s m s of 7 this m s m w disparity is also characteristic of tsunami earthquakes the nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms we conclude that the nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the cocos and north american plates and because of the absence of sediments on the trench floor offshore of nicaragua the slip propagated up dip all the way to the ocean bottom exciting large tsunamis the occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction zone thrust earthquakes our results reinforce the idea that tsunami warning systems using long period 100 s waves are necessary to reduce the hazard from this type of earthquake

fingerprinting the k t impact site and determining the time of impact by upb dating of single shocked zircons from distal ejecta abstract u pb isotopic dating of single 1 3 g zircons from k t distal ejecta from a site in the raton basin colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site data for the least shocked zircons are slightly displaced from the 544 5 ma primary age for a component of the target site white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65 5 3 0 ma such shocked and granular zircons have never been reported from any source including explosive volcanic rocks zircon is refractory and has one of the highest thermal blocking temperatures hence it can record both shock features and primary and secondary ages without modification by post crystallization processes unlike shocked quartz which can come from almost anywhere on the earth s crust shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers with u pb zircon dating it is now possible to correlate ejecta layers derived from the same target site test the single versus multiple impact hypothesis and identify the target source of impact ejecta the ages obtained in this study indicate that the manson impact site iowa which has basement rocks that are mid proterozoic in age cannot be the source of k t distal ejecta the k t distal ejecta probably originated from a single impact site because most grains have the same primary age

40ar 39ar age of cretaceous tertiary boundary tektites from haiti 40ar 39ar dating of tektites discovered recently in cretaceous tertiary k t boundary marine sedimentary rocks on haiti indicates that the k t boundary and impact event are coeval at 64 5 0 1 million years ago sanidine from a bentonite that lies directly above the k t boundary in continental coal bearing sedimentary rocks of montana was also dated and has a 40ar 39ar age of 64 6 0 2 million years ago which is indistinguishable statistically from the age of the tektites

the chicxulub impact event and its environmental consequences at the cretaceous tertiary boundary abstract an impact mass extinction hypothesis for the cretaceous tertiary k t boundary transition has been confirmed with multiple lines of evidence beginning with the discovery of impact derived ir in k t boundary sediments and culminating in the discovery of the chicxulub impact crater likewise a link between the chicxulub impact crater and k t boundary sediments has been confirmed with multiple lines of evidence including stratigraphic petrological geochemical and isotopic data the environmental effects of the chicxulub impact event were global in their extent largely because of the interaction of ejected impact debris with the atmosphere the environmental consequences of the chicxulub impact event and their association with the k t boundary mass extinction event indicate that impact cratering processes can affect both the geologic and biologic evolution of our planet