accelerated stress buildup on the southern san andreas fault and surrounding regions caused by mojave desert earthquakes a sequence of four mw 6 earthquakes including the 1992 mw 5 7 3 landers and mw 5 7 1 hector mine earthquakes occurred in the mojave desert in the 1990s in close proximity to the southern san andreas fault inducing stress changes on several of its segments we calculate that coseismic slip combined with postseismic relaxation of viscous lower crust and or upper mantle has led to a coulomb stress increase of 2 3 3 5 bar on the san bernardino mountain segment of the southern san andreas fault between 1992 and 2001 with a projected increase of 3 6 4 9 bar by the year 2020 in comparison the calculated coseismic stress increase is 1 8 bar for this segment this accelerated buildup of stresses is predicted to bring the san bernardino mountain segment which last ruptured more than 190 yr ago closer to a potentially major rupture meanwhile we project a net stress decrease of as much as 23 5 bar between 1992 and 2020 for the western coachella valley segment if the fault is governed by low effective friction or an increase of 1 5 bar if the fault is governed by high effective friction coulomb stresses are calculated to decrease on the mojave segment by as much as 21 bar between 1992 and 2020 accelerated stress buildup is also predicted to occur on parts of the san jacinto elsinore and calico faults the pattern of the observed post landers aftershock clustering and the calculated coulomb stress buildup on the calico fault is similar to that noted in the hector mine region prior to the 1999 mw 5 7 1 earthquake these results imply that the stress changes caused by an earthquake may still play a role in triggering future quakes in neighboring crust many years later through viscoelastic processes

bolides and biostratigraphy address as retiring president of the geological society of america october 1982 the stratigraphic record is punctuated by episodes of mass extinction these must be accounted for by major changes in the evolution of the earth three groups of forces have acted on the earth through time and may be adduced as the ultimate causes of many geological phenomena including extinctions these are internal earth changes solar system changes and astronomical accidents immediate causes are many and may stem from more than one ultimate cause among astronomical accidents asteroid or cometary impacts by bodies ranging up to 10 km in diameter or more occur at a known rate and must be accepted as an ongoing phenomenon evidence for large body impacts exists locally in the presence of craters and ejecta but regional or worldwide evidence is harder to assess it is probable that geochemical sedimentological and biological effects will prove the most reliable indicators although each involves difficulties in preservation and interpretation mass extinctions and animal or plant diversity are commonly demonstrated by plotting numbers of taxa that disappeared against an interval of time for example genera or families per stage or series a better measure of the real effect on life of an extinction event is to estimate changes in the biomass a measure of the number or volume of organisms and their extent the difference in biomass between two taxa of equal rank can be and frequently is many orders of magnitude changes in the physical environment are seen as the driving force of evolution and may be slow acting or sudden only accurate biochronological and sedimentological studies can determine how sudden an event has been evidence for some extinctions is found by changes at a bedding plane in each section examined if the possibility of gaps in sedimentation in such sections can be eliminated and the horizon accurately correlated then this constitutes strong evidence for a sudden event and an astronomical accident must be considered the phanerozoic record contains at least five major extinctions late ordovician late frasnian mid late devonian late permian late trias and late cretaceous detailed evidence on the time of disappearance of the taxa affected is hard to come by and much more work is required before any hypotheses of extinction can be substantiated although the cretaceous tertiary extinction is marked by noble metal anomalies at many sections the reason for such anomalies remains controversial there appears to be a substantial extinction of biomass at the same horizon in considering the five mass extinctions the likelihood that a large body impact has contributed to each extinction event is assessed as follows probability high enough to take seriously late frasnian late cretaceous possibility late ordovician late trias least likely late permian

geodetically constrained models of viscoelastic stress transfer and earthquake triggering along the north anatolian fault over the past 80 years 8 mw 6 7 strike slip earthquakes west of 40 longitude have ruptured the north anatolian fault naf from east to west the series began with the 1939 erzincan earthquake in eastern turkey and the most recent 1999 mw 7 4 izmit earthquake extended the pattern of ruptures into the sea of marmara in western turkey the mean time between seismic events in this westward progression is 8 5 11 years 67 confidence interval much greater than the timescale of seismic wave propagation seconds to minutes the delayed triggering of these earthquakes may be explained by the propagation of earthquake generated diffusive viscoelastic fronts within the upper mantle that slowly increase the coulomb failure stress change cfs at adjacent hypocenters here we develop three dimensional stress transfer models with an elastic upper crust coupled to a viscoelastic burgers rheology mantle both the maxwell m 4 1018 1 1019 pa s and kelvin k 1 1018 1 1019 pa s viscosities are constrained by studies of geodetic observations before and after the 1999 izmit earthquake we combine this geodetically constrained rheological model with the observed sequence of large earthquakes since 1939 to calculate the time evolution of cfs changes along the north anatolian fault due to viscoelastic stress transfer apparent threshold values of mean cfs at which the earthquakes in the eight decade sequence occur are between 0 02 to 3 15 mpa and may exceed the magnitude of static cfs values by as much as 177 by 2023 we infer that the mean time dependent stress change along the northern naf strand in the marmara sea near istanbul which may have previously ruptured in 1766 may reach the mean apparent time dependent stress thresholds of the previous naf earthquakes

post seismic and interseismic fault creep i model description we present a model of localized aseismic fault creep during the full interseismic period including both transient and steady fault creep in response to a sequence of imposed coseismic slip events and tectonic loading we consider the behaviour of models with linear viscous non linear viscous rate dependent friction and rate and state dependent friction fault rheologies both the transient post seismic creep and the pattern of steady interseismic creep rates surrounding asperities depend on recent coseismic slip and fault rheologies in these models post seismic fault creep is manifest as pulses of elevated creep rates that propagate from the coseismic slip these pulses feature sharper fronts and are longer lived in models with rate state friction compared to other models with small characteristic slip distances in rate state friction models interseismic creep is similar to that in models with rate dependent friction faults except for the earliest periods of post seismic creep our model can be used to constrain fault rheologies from geodetic observations in cases where the coseismic slip history is relatively well known when only considering surface deformation over a short period of time there are strong trade offs between fault rheology and the details of the imposed coseismic slip geodetic observations over longer times following an earthquake will reduce these trade offs while simultaneous modelling of interseismic and post seismic observations provide the strongest constraints on fault rheologies

coseismic displacements from the hector mine california earthquake results from survey mode global positioning system measurements we describe the collection and processing of global positioning system gps data from 77 locations around the hector mine earthquake which we use to estimate coseismic displacements related to this shock the existence of pre event gps data some collected to monitor postseismic displacements from the 1992 landers earthquake and some to establish survey control in the meizoseismal area provided a relatively dense coverage close to the rupture zone the data available were collected mostly within the 2 years prior to the 1999 earthquake we reobserved many points within a few days after the shock and all within 6 months after we include corrections for interseismic motion to provide the best value possible for coseismic motion caused by this earthquake the displacements in general display the pattern expected for a strike slip fault though a few show significant vertical motion the maximum horizontal displacement observed was 2 m one station between fault ruptures showed little horizontal motion but significant uplift

search for impact remains at the frasnian famennian boundary in the stratotype area southern france abstract in order to detect whether the end frasnian worldwide biotic crisis is related to an extraterrestrial impact the global stratotype section of the frasnian famennian boundary and auxiliary sections within the stratotype area have been examined for impact indicators iridium ni rich spinel bearing spherules and glassy microtektites this area is particularly well suited to the search for discrete events because it exhibits biostratigraphically continuous sections of sedimentologically homogenous off shore deposits different environmental settings on oxygenated deep water seamounts such as the stratotype section at coumiac and in oxygen depleted depressions la serre section are available the latter is investigated in more detail because it is the least condensed across the boundary which is determined by the first occurrence of the typical morphotype of palmatolepis triangularis the indicator of the first famennian conodont biozone samples from the biostratigraphically defined boundary and adjacent levels failed to provide significantly high ir values and no ni rich spinel or microtektite has been recovered this is in contradiction with the results of earlier investigations carried out by h geldsetzer on the same section in contrast the values of ir concentrations that we measured are always very low or not detectable the small overabundances observed in some samples which are about two orders of magnitude lower than what is currently observed at the cretaceous tertiary boundary are probably due to the accumulation of the normal flux of cosmic dust during periods of relatively low depositional rates or to a terrestrial origin at present we have no evidence that an extraterrestrial impact occurred at the f f transition

seismic remote sensing for the earthquake source process and near source strong shaking a case study of the october 16 1999 hector mine earthquake a new methodology to automatically determine the earthquake source process and near source strong ground motions using regional and local distance data was used to study the october 16 1999 hector mine earthquake mw7 1 broadband displacement data were inverted for the seismic moment tensor and to resolve the fault plane ambiguity independent of aftershock location and surface faulting information the identified nnw striking fault plane was used to invert for the distribution of fault slip over a planar surface the slip model was used to predict near fault peak ground velocity which was found to agree well with the trinet shakemap and also with near fault observations that were not available at the time of the initial analysis our method if automated is capable of providing near source strong ground motion information within 30 minutes of the earthquake origin time even in cases where there are few near fault recording instruments

stress modulation on the san andreas fault by interseismic fault system interactions during the interseismic phase of the earthquake cycle between large earthquakes stress on faults evolves in response to elastic strain accumulation driven by tectonic plate motions because earthquake cycle processes induce non local stress changes the interseismic stress accumulation rate on one fault is influenced by the behavior of all nearby faults using a geodetically constrained block model we show that the total interseismic elastic strain field generated by fault interactions within southern california may increase stressing rates on the mojave and san bernardino sections of the san andreas fault within the big bend region by as much as 38 relative to estimates from isolated san andreas models assuming steady fault system behavior since the c e 1857 fort tejon earthquake shear stress accumulated on these sections due only to interaction with faults other than the san andreas reaches 1 mpa 3 times larger than the coseismic and postseismic stress changes induced by recent southern california earthquakes stress increases along big bend sections coincide with the greatest earthquake frequency inferred from a 1500 yr long paleoseismic record and may affect earthquake recurrence intervals within geometrically complex fault systems including the sections of the san andreas fault closest to metropolitan los angeles

the 1995 kobe japan earthquake a long delayed aftershock of the offshore 1944 tonankai and 1946 nankaido earthquakes the 1995 kobe earthquake produced predominantly right lateral slip along the northeast southwest trending nojima fault approximately 200 km landward of the nankai trough southwest japan the last major earthquakes along the nankai trough the m 8 0 1944 tonanakai and m 8 2 1946 nankaido earthquakes involved about 4 m of interplate slip each releasing about 100 years of accumulated stress from the northwest southeast convergence of the philippine sea phs plate by calculating the change in coulomb failure stress on the nojima fault due to 1944 46 to 1995 postseismic relaxation of a thin low viscosity channel beneath southwest japan we show that stress diffusion from the 1940s interplate earthquakes increased the likelihood of future failure on the nojima fault by slowly but progressively unloading the normal stress on the fault over a 50 yr period

an iridium anomaly in the middle lower jurassic of the venetian region northern italy a significant iridium enrichment has been found in a jurassic marine sequence about 180 million years old outcropping in the alps of the venetian region the maximum iridium concentration of 3 2 0 2 ng g 1 occurs in a brown crust 2 3 mm thick containing essentially iron hydroxide this crust characterized by a nearly total absence of detritic components lies on top of upper lias limestones and is overlaid with bajocian bathonian limestones the origin of the ir anomaly is not clearly understood yet but could be explained by either a drop in the sedimentation rate resulting in the concentration within a thin sediment layer of the normally infalling cosmic dust and or by an increase of cosmic material infall following an asteroid or comet impact s

evidence from crater ages for periodic impacts on the earth recent evidence has indicated that the impact of a comet or asteroid may have been responsible for mass extinction at the ends of both the cretaceous1 and the eocene2 4 quantitative analysis by raup and sepkoski5 showed that mass extinctions occur with a 26 myr period similar to the period seen in qualitative pelagic records by fischer and arthur6 to account for the possibility of periodic comet showers davis et al 7 proposed that such showers could be triggered by an unseen solar companion star as it passes through perihelion on a moderately eccentric orbit to test a prediction implicit in this model we examined records of large impact craters on the earth we report here that most of the craters occur in a 28 4 myr cycle within measurement errors this period and its phase are the same as those found in the fossil mass extinctions the probability that such agreement is accidental is 1 in 103

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

secondary aftershocks and their importance for aftershock forecasting the potential locations of aftershocks which can be large and damaging are often forecast by calculating where the mainshock increased stress we find however that the mainshock induced stress field is often rapidly altered by aftershock induced stresses we find that the percentage of aftershocks that are secondary aftershocks or aftershocks triggered by previous aftershocks increases with time after the mainshock if we only consider aftershock sequences in which all aftershocks are smaller than the mainshock the percentage of aftershocks that are secondary also increases with mainshock magnitude using the california earthquake catalog and monte carlo trials we estimate that on average more than 50 of aftershocks produced 8 or more days after m 5 mainshocks and more than 50 of all aftershocks produced by m 7 mainshocks that have aftershock sequences lasting at least 15 days are triggered by previous aftershocks these results suggest that previous aftershock times and locations may be important predictors for new aftershocks we find that for four large aftershock sequences in california an updated forecast method using previous aftershock data and neglecting mainshock induced stress changes can outperform forecasts made by calculating the static coulomb stress change induced solely by the mainshock manuscript received 20 november 2002

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

large earthquake triggering clustering and the synchronization of faults large earthquakes are sometimes observed to trigger other large earthquakes on nearby faults the magnitudes of the calculated coulomb stress transfers presumed to cause the triggering are 10 2 10 3 of the earthquake stress drops the earthquake stress drops and the triggering delay times are similarly small with respect to the natural recurrence time of the earthquakes this requires that both faults be simultaneously very close to the ends of their seismic cycles paleoseismological data show that for the same regions prior earthquakes have occurred in clusters of ruptures of several faults separated by long quiescent periods both observations suggest that synchronization is occurring between faults theory and observations indicate that synchronization can occur between nearby faults with positive stress coupling and intrinsic slip velocities within an entrainment threshold in the south iceland seismic zone the central nevada seismic belt and the eastern california shear zone several synchronous clusters that apparently act independently can be recognized this behavior is the 3d equivalent of the phase locking that results in the seismic cycle of individual faults being dominated by large characteristic earthquakes and for synchronization of fault segments along a given fault rupture patterns of repeated individual earthquakes or earthquake clusters are not identical in either the 2d or 3d cases the state of this system which exhibits strong indications of synchrony without exact repetition may be called fuzzy synchrony

mantle flow beneath a continental strike slip fault postseismic deformation after the 1999 hector mine earthquake two recent large earthquakes in the mojave desert california the magnitude 7 3 1992 landers and magnitude 7 1 1999 hector mine earthquakes have each been followed by elevated crustal strain rates over periods of months and years geodetic data collected after the hector mine earthquake exhibit a temporally decaying horizontal velocity field and a quadrant uplift pattern opposite to that expected for localized shear beneath the earthquake rupture we interpret the origin of this accelerated crustal deformation to be vigorous flow in the upper mantle in response to the stress changes generated by the earthquake our results suggest that transient flow in the upper mantle is a fundamental component of the earthquake cycle and that the lower crust is a coherent stress guide coupling the upper crust with the upper mantle

late quaternary rate of slip along the san jacinto fault zone near anza southern california the anza section of the san jacinto fault southern california has not experienced a groundbreaking earthquake since at least 1918 and probably since before 1899 leading previous workers to designate this 20 km long fault segment as a seismic gap study of displaced pleistocene and holocene alluvial fan and fluvial deposits across the san jacinto fault near anza california dated using 14c and soil development age control indicates a minimum right lateral slip rate of 9 2 2 mm yr since 9 5 ka and a slip rate of 11 9 5 mm yr since 14 ka 12 9 5 mm yr since 17 ka and 13 10 6 mm yr since 50 ka these estimates agree with a previously determined 700 ka to present minimum slip rate of 10 2 mm yr based on an estimated average slip rate of 12 mm yr from the longer term slip rate estimates about 0 8 m of potential slip has accumulated if the 1918 earthquake broke the anza segment over 1 1 m of potential slip has accumulated if the last earthquake occurred prior to 1899 these data attest to the potential for an earthquake m6 5 for this section of the fault

temperature and biotic crises in the marine realm climatic change has been a prominent cause of marine mass extinction but areal restriction of seafloor during global regression has not late eocene and pliocene pleistocene cooling for example caused major extinctions but profound global oligocene and pleistocene regressions had little or no direct effect on benthic diversity recurrent themes of pre cenozoic marine crises suggest that global temperature change also served as a major and perhaps dominant agent of extinction in these events 1 mass extinctions have frequently been concentrated in the tropics which seem to have become a refrigerated trap from which there has been no escape biotas previously occupying high latitudes have shifted equatorward to replace disappearing tropical biotas 2 some crises were not instantaneous but followed protracted and pulsatile temporal patterns as would be predicted for complex global climatic crises 3 several mass extinctions coincided with recognized intervals of climatic cooling

water column anoxia enhanced productivity and concomitant changes in δ13c and δ34s across the frasnian famennian boundary kowala holy cross mountains poland the investigation of the trace element and organic geochemistry of the frasnian famennian boundary section at kowala holy cross mountains poland shows that the lower water column was oxygen deficient during late frasnian and early famennian times the abundance and carbon isotopic composition of diaryl isoprenoids biomarkers indicative for green sulfur bacteria prove that euxinic waters reached into the photic zone at least episodically total organic carbon toc contents show two maxima that are time equivalent to the kellwasser horizons deposited in shallower water settings enhanced toc concentrations are explained by a higher primary productivity presumably as a consequence of an enhanced nutrient supply from the continent the increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to toc increased at the frasnian famennian transition the sulfur isotopic composition of pyritic and organically bound sulfur shows a 27 excursion across the boundary the observation that the 34s values of organic bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source likely early diagenetic sulfide a change in the 13c of total dissolved inorganic carbon as a consequence of an enhanced burial of 12c enriched organic carbon is indicated by a 3 excursion measured for toc as well as for individual n alkanes and isoprenoids the burial of large amounts of organic carbon is expected to result in a decrease in pco2 and should affect the photosynthetic carbon isotope fractionation ep the fact that we observe no change in ep can be explained by the circumstance that ep was most probably at maximum values as a consequence of high atmospheric and oceanic dissolved co2 concentrations during the devonian

triggering effect of m 4 5 earthquakes on the earthquake cycle of repeating events at parkfield california stress perturbations influence earthquake recurrence and are of funda mental importance to understanding the earthquake cycle and determining earthquake hazard the large population of repeating earthquakes on the san andreas fault at parkfield california provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes using 187 m 0 4 to 1 7 repeating earthquake sequences from the high resolution seismic network cat alog we find that the time to recurrence of repeating events subsequent to nearby m 4 5 earthquakes is shortened suggesting triggering by major events the triggering effect is found to be most evident within a distance of 5 km corresponding to static coseismic stress changes of 0 6 26 6 kpa and decays with distance we also find coherently reduced recurrence intervals from 1993 to 1998 this enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s online material figures of event chronologies of res estimates of dt and coseismic static stress changes

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

segmentation of the northern san jacinto fault zone southern california accurate forecasting of large earthquakes in the san jacinto fault zone depends on the correct determination of fault segmentation we have searched for discontinuities in the fault zone using as data geological maps of the fault traces relocated earthquake hypocenters focal mechanisms slip rates and historic large earthquake rupture zones we identify nine principal discontinuities in the northern san jacinto fault zone these are characterized as structural discontinuities bends steps branches intersections geodetic discontinuities slip rate and lithologic discontinuities most of the discontinuities have more than one defining characteristic we also analyzed the local strain field orientation along the fault zone significant lateral discontinuities in the strain field orientation are coincident with many of the structural and geodetic discontinuities we identify segment lengths defined from the data vary from about 7 to 35 km this range of segment lengths is consistent with the lengths of large historical earthquake ruptures however evidence from small earthquake hypocenter patterns suggests the possibility that in some areas large earthquakes may rupture multiple adjacent segments in these cases maximum magnitudes may be larger than those observed historically because of the short historic record and the sparse paleoseismic record probabilistic estimates of earthquake recurrence for this fault zone have depended heavily on assumed characteristic lengths of individual rupture segments ours and previous delineations of segment lengths have varied considerably indicating that assumed segment length is a major source of uncertainty in the recurrence calculations this source of uncertainty should be taken into account in calculations of the reliability of the probabilistic hazard estimates

controls of shear zone rheology and tectonic loading on postseismic creep 1 postseismic deformation is well documented in geodetic data collected in the aftermath of large earthquakes in the postseismic time interval gps is most sensitive to creep in the lower crust or upper mantle activated by earthquake generated stress perturbations in these regions deformation may be localized on an aseismic frictional surface or on a ductile shear zone these two hypotheses imply specific rheologies and therefore time dependence of postseismic creep hence postseismic creep constitutes a potential probe into the rheology of aseismic regions of the lithosphere i present a simple shear zone model of postseismic creep in which the rheology of the creeping element can be varied in the absence of tectonic loading during the postseismic time interval the displacement history of the shear zone obeying a power law rheology with stress exponent n follows an analytical relaxation curve parameterized by 1 n for a frictional surface postseismic creep follows the same relaxation law in the limit 1 n 0 a rough estimate of the apparent stress exponent can be obtained from continuous gps records application to data collected after the 1994 sanriku earthquake yields 1 n 0 1 which is consistent with dislocation creep mechanisms however the records of two other subduction zone events the 2001 peru event and the 1997 kronotski earthquake and a continental strike slip earthquake the 1999 izmit earthquake require negative 1 n rather than characterizing the shear zone rheology these negative exponents indicate that reloading of the shear zone by tectonic forces is important numerical simulations of postseismic deformation with nonnegligible reloading produce curves that are well fit by the generalized relaxation law with 1 n 0 although the actual stress exponent of the rheology is positive while this prevents rheology from being tightly constrained by the studied gps records it indicates that reloading is important in the postseismic time interval in other words the stress perturbation induced by an earthquake is comparable to the stress supported by ductile shear zones in the interseismic period

delayed seismicity rate changes controlled by static stress transfer on 15 june 2010 a m w5 7 earthquake occurred near ocotillo california in the yuha desert this event was the largest aftershock of the 4 april 2010 m w7 2 el mayor cucapah emc earthquake in this region the emc mainshock and subsequent ocotillo aftershock provide an opportunity to test the coulomb failure hypothesis cfs we explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative cfs change imparted by the ocotillo event based on simple cfs calculations we divide the yuha desert into three subregions one triggering zone and two stress shadow zones we find the nominal triggering zone displays immediate triggering one stress shadowed region experiences immediate quiescence and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown we quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of cfs change with the rate state earthquake rate formulation of dieterich 1994 assuming that each subregion contains a mixture of nucleation sources that experienced a cfs change of differing signs our modeling reproduces the observations including the observed delay in the stress shadow effect in the third region following the ocotillo aftershock the delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations we find that rate state models of time dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity

iridium anomaly in the upper devonian of the canning basin western australia a moderate iridium anomaly about 20 times the local background has been found in upper devonian rocks in the canning basin it occurs at or near the frasnian famennian boundary which is known to be associated with a major massextinction event of global extent the anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions its occurrence suggests a causal link with some form of meteoroid impact moreover carbon isotope data indicate that a large reduction in biomass could have occurred at this level however the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium frutexites iridium platinum iron manganese cobalt arsenic antimony and cerium are preferentially concentrated in filaments of this organism with concentrations ranging from two to five times that of the matrix it is possible that frutexites extracted these elements directly from seawater without the need for their derivation from an extraterrestrial source

three dimensional finite element modeling of stress evolution around the xiaojiang fault system in the southeastern tibetan plateau during the past 500 years abstract the left lateral strike slip xiaojiang fault system is a main boundary between the southeastern tibetan plateau and the south china block and accumulates several large m 7 earthquakes in the past 500 years and a dozen of moderate 5 7 earthquake ruptures occurred in the past 500 years are simulated with split node technique in seismogenic upper crust we investigate the coulomb failure stress changes mainly on vertical planes parallel to the different strands of the xiaojiang fault system and at the epicenters of large m 7 earthquakes in the past 500 years and of moderate 5 7 earthquakes that occurred in the past 500 years both from their coseismic and postseismic effects among which the postseismic effect results in the stressing rate being obviously accelerated near epicenters of the large m 7 earthquakes after their occurrence thus the percentage of positive coulomb failure stress changes of 0 01 mpa at the epicenters increases from 37 to 50 for the large m 7 earthquakes and from 43 to 64 for the moderate 5

implications of deformation following the 2002 denali alaska earthquake for postseismic relaxation processes and lithospheric rheology 1 during the first 2 years following the 2002 mw 7 9 denali alaska strike slip earthquake a large array of global positioning system gps receivers recorded rapid postseismic surface motions extending at least 300 km from the rupture and at rates of more than 100 mm yr in the near field here we use three dimensional 3 d viscoelastic finite element models to infer the mechanisms responsible for these postseismic observations we consider afterslip both from an inversion of gps displacements and from stress driven forward models poroelastic rebound and viscoelastic flow in the lower crust and upper mantle several conclusions can be drawn 1 no single mechanism can explain the postseismic observations 2 significant postseismic flow below a depth of 60 km is required to explain observed far field motions best explained by a weak upper mantle with a depth dependent effective viscosity that ranges from 1019 pa s at the moho 50 km depth to 3 4 1018 pa s at 100 km depth 3 shallow afterslip within the upper crust occurs adjacent to and beneath the regions of largest coseismic slip 4 there is a contribution from deformation in the middle and lower crust from either lower crustal flow or stress driven slip afterslip is preferred over broad viscoelastic flow owing to the existence of seismic velocity discontinuities across the fault at depth though our modeling does not favor either mechanism if the process is viscoelastic relaxation the viscosity is a factor of 3 greater than the inferred mantle viscosity 5 poroelastic rebound probably contributed to the observed postseismic deformation in the immediate vicinity of the denali totschunda junction these conclusions lead us to infer an alaskan mechanical lithosphere that is about 60 km thick overlying a weak asthenosphere and a denali fault that cuts through the entire lithosphere with shear accommodated by faulting in the top 20 km and time dependent aseismic shear below

viscoelastic flow in the lower crust after the 1992 landers california earthquake space geodesy showed that broad scale postseismic deformation occurred after the 1992 landers earthquake three dimensional modeling shows that afterslip can only explain one horizontal component of the postseismic deformation whereas viscoelastic flow can explain the horizontal and near vertical displacements the viscosity of a weak about 10 km thick layer in the lower crust beneath the rupture zone that controls the rebound is about 10 18 pascal seconds the viscoelastic behavior of the lower crust may help to explain the extensional structures observed in the basin and range province and it may be used for the analysis of earthquake hazard

osmium isotopic evidence against an impact at the frasnian famennian boundary two sections across the frasnian famennian boundary were analyzed for re and os concentrations and 187os 188os ratios to evaluate evidence for a meteoritic input coincident with this boundary and its associated mass extinction these sections are from a siltstone and shale sequence at irish gulf in new york us and a calcareous shale and ferromanganese oxide sequence at la serre in france the irish gulf section with an initial 187os 188os of about 0 49 does not show the characteristic meteoritic os imprint with an 187os 188os of about 0 13 the integrity of the section with regard to re and os retention is indicated by the construction of an isochron with an age of 388 41 ma consistent with independently determined ages for the frasnian famennian boundary although the la serre section with os concentrations as high as 33 ppb and re concentrations ranging from 1 4 to 7 4 ppb might be expected to show excellent evidence for a meteoritic contribution the highly radiogenic isotopic composition 187os 188os 2 42 3 61 instead suggests recent massive re loss or addition of radiogenic os these possibilities prevent the reconstruction of an initial 187os 188os value for the boundary for this site assuming reasonable re concentrations prior to loss the os isotopic composition is inconsistent with a large meteoritic component this inconsistency emphasizes that platinum group element pge concentrations are not reliable indicators of extraterrestrial addition

stratigraphy of the upper cretaceous and lower tertiary strata in the tethyan himalayas of tibet tingri area china the 1500 m thick marine strata of the tethys himalaya of the zhepure mountain tingri tibet comprise the upper albian to eocene and represent the sedimentary development of the passive northern continental margin of the indian plate investigations of foraminifera have led to a detailed biozonation which is compared with the west tethyan record five stratigraphic units can be distinguished the gamba group upper albian lower santonian represents the development from a basin and slope to an outer shelf environment in the following zhepure shanbei formation lower santonian middle maastrichtian outer shelf deposits continue pebbles in the top layers point to beginning redeposition on a continental slope intensified redeposition continues within the zhepure shanpo formation middle maastrichtian lower paleocene the series is capped by sandstones of the jidula formation danian deposited from a seaward prograding delta plain the overall succession of these units represents a sea level high at the cenomanian turonian boundary followed from the turonian to danian by an overall shallowing upward megasequence this is followed by a final transgression regression cycle during the paleocene and eocene documented in the zhepure shan formation upper danian lutetian and by upper eocene continental deposits the section represents the narrowing and closure of the tethys as a result of the convergence between northward drifting india and eurasia the plate collision started in the lower maastrichtian and caused rapid changes in sedimentation patterns affected by tectonic subsidence and uplift stronger subsidence and deposition took place from the middle maastrichtian to the lower paleocene the final closure of remnant tethys in the tingri area took place in the lutetian

modes 2d method of detecting strike shift shear zones one of the goals of using the global positioning system gps and other geodetic survey techniques in tectonics has been to detect boundaries such as faults or shear zones between rigid or mildly deforming crustal masses the calculation of infinitesimal strains and rotations with gps data has been widely used to detect shear zones but it has been largely unsuccessful because infinitesimal strain and rotation although useful in many other ways is non diagnostic of shear zones or faults our approach is to work with components of deformation not strain and to design specifically a diagnostic method of detecting shear zones this paper introduces the first part of our method the detection of two dimensional strike shift shear zones modes 2d the modes 2d method has three elements 1 determination of the orientation of a suspected strike shift shear zone by analyzing components of a deformation tensor derived from a data set of displacements in an arbitrary coordinate system 2 resolution of the deformation tensor into the coordinate system parallel and normal to the detected shear zone and 3 exploration of the resolved data set for evidence for a belt of inhomogeneous deformation which is an essential characteristic of a shear zone the operation of modes 2d is illustrated herein with a theoretical survey network across an ideal shear zone developed with a buried dislocation fault and with a survey network afforded by the crossing of the kaynasli viaduct by the 1999 duzce bolu earthquake rupture in turkey

carbon and sulfur isotope anomalies across the frasnian famennian extinction boundary alberta canada stable isotopes of carbonate and organic carbon from two upper devonian sections in alberta canada indicate a heavy carbon event across the frasnian famennian f f boundary and a negative 13 c shift at the boundary at cinquefoil mountain 13 c values in carbonate are anomalously high 3 5 to 4 5 pdb peedee belemnite within an interval 10 m below to 23 m above the f f boundary representing about 1 m y two standard conodont zones compared to background values of 1 above and below this interval superimposed on this positive excursion is a marked 13 c decrease of about 1 at precisely the f f boundary at medicine lake a similar 13 c excursion in organic carbon from a background of 29 to a high of 26 5 is recorded across the f f boundary the 34 s data for iron sulfide in these sections show unusually high values as much as 15 to 20 cdt canon diablo troilite in the f f boundary interval these data together with those from other areas indicate a period of major global changes e g greenhouse and marine anoxia that took place well before and after the sudden biomass killing event at the f f boundary existing evidence suggests that the f f mass extinction was probably caused by a large extraterrestrial impact on earth at a time when the global ecosystem was already severely stressed

middle palaeozoic extinction events faunal and isotopic data abstract at least 9 and possibly as many as 12 extinction events of global or near global impact can be discriminated in the mid palaeozoic earliest silurian to early carboniferous on the basis of brachiopod coral conodont and ammonoid data and the history of carbonate build ups isotopic data from whole rock samples are presented for three of these events based on australian carbonate sequences constrained by conodont data these data represent the initial phase of a more extensive investigation using c o s sr and nd isotopic signatures derived from conodonts articulate and inarticulate brachiopod shell and fish remains from numerous australian and european sequences the aim of the project is to identify isotopic responses to extinction events and address causes for these changes in limestone sections analysed so far variations in carbon isotope compositions on a whole rock scale are most marked at horizons that can be correlated with times of significant reduction in biomass and diversity this is despite the fact that the whole rocks are in fact multicomponent systems with respect to carbon in part arising from diagenetic rearrangements of carbon distribution within the scale of the whole rock thus the carbon isotope data do provide evidence on a whole rock scale for fundamental changes in the global carbon cycle that are correlative with extinction events it is however unlikely that the magnitude of the isotopic shift will be precisely documented from such whole rock analyses the oxygen isotope results are less obviously related to the times of carbon isotope excursion this contrast in apparent resilience to post depositional modification of pristine isotope compositions exemplifies the extreme caution needed in evaluating isotopic data from carbonates

coulomb stress changes from earthquakes to underground excavation failures abstract evidence from shallow tunnels and mines indicates that in certain cases deformation and failure are not confined to the vicinity of the excavation front as is widely believed this situation has recently been described as lateral strain propagation from a source to remote parts of the excavation through low strength rocks a similar situation is observed in deep excavations mainly expressed by rock bursts and micro earthquakes we suggest that slow propagation or seismic radiation of deformation away from the excavation front can be explained on the basis of the theory of coulomb failure stress changes in analogy to faulting in large earthquakes local increase of stresses due to the excavation or failure in a certain point leads to a slight increase of stresses around this point if adjacent points are already at a critical level for failure this slight increase triggers a laterally domino style propagating new failure yet in contrast to earthquakes triggered deformation in mining can in principle be predicted and counteracted

no geochemical evidence for an asteroidal impact at late devonian mass extinction horizon one way to test the proposed causal relationship between mass biotic extinction and asteroidal impact is to search for the geochemical signature of such an event at additional mass extinction horizons other than the cretaceous tertiary k t boundary we report here the results of an extensive search for an indium ir anomaly across the late devonian frasnian famennian f f extinction horizon 367 myr bp over a decade ago mclaren1 proposed that this extinction was caused by an astrophysical catastrophe we have examined three sedimentary sequences in new york state the standard reference section for eastern north america and one in belgium the type locality which cross the f f boundary no ir anomaly was found in any of the analysed stratigraphie sections the highest values of ir detected ranged from only 0 2 to 2 of those reported for the marine and terrestrial ir analyses at the k t boundary further devonian pyrite rich sediments similar to those at the k t boundary do not contain high ir concentrations

evolution of stress in southern california for the past 200 years from coseismic postseismic and interseismic stress changes summary the seismicity of southern california results from stresses that arise from the relative motion of the pacific and north american plates being accommodated along the san andreas fault saf system and the eastern california shear zone ecsz here we calculate how the stress field in southern california has evolved over the past two centuries due to interseismic loading as inferred from current gps observations of surface velocities from redistributions of static stress induced by large m w 6 5 earthquakes since the 1812 wrightwood quake and postseismic viscoelastic relaxation associated with these events that serves to transfer coseismic stresses from the deep warm lower crust and upper mantle to the overlying seismogenic upper crust we calculate coulomb stress changes on vertical strike slip faults striking parallel to the saf and at the hypocenters on the rupture planes of all m w 6 events over the past two centuries our results suggest that the 1857 m w 8 2 fort tejon earthquake by far the largest event to have occurred in the region over the past two centuries had a profound influence on the state of stress in southern california during the 19th century inducing significant stress increases to the north parkfield region and adjoining creeping saf and south southern saf and san jacinto fault and stress relief across the southern ecsz these stress changes were then greatly magnified by postseismic relaxation through the early part of the 20th century slow interseismic build up of stress further loads all major strike slip faults and works to reload the areas of the ecsz where stress was relieved by the 1857 quake our calculations suggest that only 56 of hypocenters were pushed closer to failure by preceding coseismic stress changes suggesting that the occurrence of large earthquakes is not strongly determined by coseismic coulomb stress changes this percentage rises to 70 when postseismic stress changes are also considered our calculations demonstrate the importance of postseismic viscoelastic relaxation in the redistribution of stress following large earthquakes we find however that postseismic processes associated with events more than about a decade old are near completion and thus do not significantly influence the regional velocity field presently observed in southern california

stress loading from viscous flow in the lower crust and triggering of aftershocks following the 1994 northridge california earthquake following the m w 6 7 northridge earthquake significant postseismic displacements were resolved with gps using a three dimensional viscoelastic model we suggest that this deformation is mainly driven by viscous flow in the lower crust such flow can transfer stress to the upper crust and load the rupture zone of the main shock at a decaying rate most aftershocks within the rupture zone especially those that occurred after the first several weeks of the main shock may have been triggered by continuous stress loading from viscous flow the long term decay time of aftershocks about 2 years approximately matches the decay of viscoelastic loading and thus is controlled by the viscosity of the lower crust our model provides a physical interpretation of the observed correlation between aftershock decay rate and surface heat flow

a forest fire and soil erosion event during the late devonian mass extinction abstract the late devonian mass extinction occurred in a stepwise manner and culminated close to the frasnian famennian f f boundary 372 million years ago organic molecular indices from marine sedimentary rocks at the sinsin section belgium indicate that the sequence of combustion of land vegetation soil erosion and anoxia euxinia occurred close to this boundary the increased concentrations of biomarkers indicating forest fire and soil erosion measured in the sinsin section suggest that fire became widespread at this time leading to various damaging consequences increased runoff and oceanic anoxia that caused marine extinctions magnetic susceptibility data in the sinsin section indicate a relatively dry climate spanning the f f boundary which would have encouraged forest fires the study of organic biomarkers presents several lines of evidence to link forest fire and soil erosion to the late devonian mass extinction

time dependent changes in failure stress following thrust earthquakes two dimensional 2 d viscoelastic finite element models were used to calculate the time dependent changes in coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension results suggest that thrust earthquakes cause a coseismic increase in coulomb stress along antithetic lobes normal to the slip plane following a quake creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust under certain conditions transfer of stress may lead to a further buildup of high coulomb stress along the base of the upper crust potentially shortening the time to failure of other faults in the region the conditions under which an antithetic lobe of high coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following the lower crust or upper mantle has an effective viscosity not greater than 1019 pa s the thrust fault has a moderate dip angle 40 50 the brittle ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion and the crust has a low apparent coefficient of friction 0 2 postseismic increases in coulomb stress within the upper crust may also be caused by aseismic creep on the fault stress changes due to this mechanism are maximized with a high apparent coefficient of friction analysis of experimentally determined non newtonian flow laws suggests that wet granitic quartz and feldspar aggregates may have a viscosity of the order of 1019 pa s the calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate results of this 2 d analysis suggest a potentially important role of viscous flow in controlling time dependent postseismic stress changes that warrant further investigation using 3 d viscoelastic analysis

glassy microspherules microtektites from an upper devonian limestone the properties of microspherules recovered from an upper devonian marine limestone immediately overlain by a geochemical anomaly of siderophile and chalcophile elements are similar to those of impact derived microtektites these microspherules are glass have splash form shapes contain spherical vesicles and lechatelierite inclusions and show oxide compositional variations similar to those in known microtektites these characteristics suggest that these upper devonian microspherules have an impact origin a bolide impact may have occurred about 365 million years ago on the south china plate and caused a faunal extinction on eastern gondwana

stress triggering of the 1999 hector mine earthquake by transient deformation following the 1992 landers earthquake the m 7 3 june 28 1992 landers and m 7 1 october 16 1999 hector mine earthquakes california both right lateral strike slip events on nnw trending subvertical faults occurred in close proximity in space and time in a region where recurrence times for surface rupturing earthquakes are thousands of years this sug gests a causal role for the landers earthquake in triggering the hector mine earth quake previous modeling of the static stress change associated with the landers earthquake shows that the area of peak hector mine slip lies where the coulomb failure stress promoting right lateral strike slip failure was high but the nucleation point of the hector mine rupture was neutrally to weakly promoted depending on the assumed coefficient of friction possible explanations that could account for the 7 year delay between the two ruptures include background tectonic stressing dissi pation of fluid pressure gradients rate and state dependent friction effects and post landers viscoelastic relaxation of the lower crust and upper mantle by employing a viscoelastic model calibrated by geodetic data collected during the time period between the landers and hector mine events we calculate that postseismic relaxa tion produced a transient increase in coulomb failure stress of about 0 7 bars on the impending hector mine rupture surface the increase is greatest over the broad surface that includes the 1999 nucleation point and the site of peak slip further north since stress changes of magnitude greater than or equal to 0 1 bar are associated with documented causal fault interactions elsewhere viscoelastic relaxation likely con tributed to the triggering of the hector mine earthquake this interpretation relies on the assumption that the faults occupying the central mojave desert i e both the landers and hector mine rupturing faults were critically stressed just prior to the landers earthquake

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

molecular and petrographic indicators of redox conditions and bacterial communities after the f f mass extinction kowala holy cross mountains poland abstract pyrite framboid diameter analysis and organic geochemistry of the triangularis crepida boundary section at kowala holy cross mountains poland imply suboxic to oxic and sporadically euxinic bottom waters during the lower famennian in addition morphological web like structures typical for microbial mats as well as the recognition of 2 methylhopanes and monomethyl alkane cyanobacteria biomarkers is evidenced of microbial activity after the global frasnian famennian f f extinction event the presence of cyanobacterial mats also suggests suboxic to oxic environments and at the same time photic bottom water conditions however isorenieratane and its derivatives were detected in almost all samples the presence of this well known biomarker of green sulfur bacteria implies that euxinic conditions were present in the upper part of the water column at least intermittently or that temporal euxinia occurred in the water column presence of euxinic conditions is confirmed by the occurrence of small sized pyrite framboids which were particularly dominant in the lower part of the section the shift towards low 13 c values in both micritic limestones and in sedimentary organic matter seen at the beginning of the period of diminished photic zone might reflect an influx of newly respired co 2 to surface waters caused by enhanced respiration at depth after the f f transition in the checiny zbrza basin

irregular recurrence of large earthquakes along the san andreas fault evidence from trees old trees growing along the san andreas fault near wrightwood california record in their annual ring width patterns the effects of a major earthquake in the fall or winter of 1812 to 1813 paleoseismic data and historical information indicate that this event was the san juan capistrano earthquake of 8 december 1812 with a magnitude of 7 5 the discovery that at least 12 kilometers of the mojave segment of the san andreas fault ruptured in 1812 only 44 years before the great january 1857 rupture demonstrates that intervals between large earthquakes on this part of the fault are highly variable this variability increases the uncertainty of forecasting destructive earthquakes on the basis of past behavior and accentuates the need for a more fundamental knowledge of san andreas fault dynamics

a more precise chronology of earthquakes produced by the san andreas fault in southern california improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the san andreas fault that are recorded in the sediments at pallett creek previous dates of these events had 95 confidence errors of 50 100 calendar years new error limits are less than 23 calendar years for all but two of the dated events this greater precision is due to larger sample size longer counting time lower background noise levels more precise conversion of radiocarbon ages to calendric dates and better stratigraphic constraints and statistical techniques the new date ranges with one exception fall within the broader ranges estimated previously but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years variability about the mean interval is much greater than was suspected previously five of the nine intervals are shorter than a century three of the remaining four intervals are about two to three centuries long despite the wide range of these intervals a pattern in the occurrence of large earthquakes at pallett creek is apparent in the new data the past 10 earthquakes occur in four clusters each of which consists of two or three events earthquakes within the clusters are separated by periods of several decades but the clusters are separated by dormant periods of two to three centuries this pattern may reflect important mechanical aspects of the fault s behavior if this pattern continues into the future the current period of dormancy will probably be greater than two centuries this would mean that the section of the fault represented by the pallett creek site is currently in the middle of one of its longer periods of repose between clusters and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in california the greater precision of dates now available for large earthquakes recorded at the pallett creek site enables speculative correlation of events between paleoseismic sites along the southern half of the san andreas fault a history of great earthquakes with overlapping rupture zones along the mojave section of the fault remains one of the more attractive possibilities

evidence of power law flow in the mojave desert mantle studies of the earth s response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power n refs 2 3 most geodynamic models of flow in the lower crust and upper mantle however resort to newtonian linear stress strain rate relations here we show that a power law model of viscous flow in the mantle with n 3 5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 landers and 1999 hector mine earthquakes in southern california a power law rheology implies that viscosity varies spatially with stress causing localization of strain and varies temporally as stress evolves rendering newtonian models untenable our findings are consistent with laboratory derived flow law parameters for hot and wet olivine the most abundant mineral in the upper mantle and support the contention that at least beneath the mojave desert the upper mantle is weaker than the lower crust

sequence stratigraphy of the frasnian famennian transitional strata a comparison between south china and southern belgium the sedimentological evolution of the frasnian famennian transitional strata in south china and southern belgium has been investigated a similar trend in the deepening and shallowing of the sedimentation environment occurs in the two palaeogeographically distinct areas the stratigraphic succession has been subdivided into depositional sequences a general deepening occurs in the palmatolepis rhenana conodont zone during the most rapid rise in sea level a transgressive systems tract formed this transgressive systems tract is followed by a highstand systems tract in the late p rhenana zone a major sequence boundary within the latter zone is indicated by an unconformity in intraplatform and platform areas and by a conformity in the basinal area the uppermost sediments within the late rhenana zone correspond to a lowstand systems tract a rapid flooding took place near the base of the palmatolepis linguiformis zone and dark shales and limestones formed a second sequence boundary coincides with or is very close to the frasnian famennian boundary base p triangularis zone the global extinction event at the frasnian famennian boundary coincides with an important eustatic fall in sea level the proposed stratigraphic subdivision should allow worldwide correlations of shallow and deep water deposits

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

mobility of continental mantle evidence from postseismic geodetic observations following the 1992 landers earthquake the crust around the rupture zone of the 1992 landers earthquake has continued to deform in the years following the earthquake at rates 3 times greater than pre earthquake rates we use a combination of global positioning system gps and synthetic aperture radar insar data collected during a 3 year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities in order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation the combination of gps and insar data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models the data particularly require the presence of a highly ductile uppermost mantle beneath the central mojave domain with temperature between the wet and dry basalt solidus this is consistent with independent seismic and geochemical inferences of a regionally warm uppermost mantle further consideration of seismic velocity variations in conjunction with faulting patterns within the mojave desert suggests that the primary faulting characteristics of the mojave desert namely the pervasive late cenozoic deformation within the eastern california shear zone versus the near absence of faults in the western mojave domain are controlled by the rheology of the uppermost mantle

forecasting the evolution of seismicity in southern california animations built on earthquake stress transfer 1 we develop a forecast model to reproduce the distribution of main shocks aftershocks and surrounding seismicity observed during 1986 2003 in a 300 310 km area centered on the 1992 m 7 3 landers earthquake to parse the catalog into frames with equal numbers of aftershocks we animate seismicity in log time increments that lengthen after each main shock this reveals aftershock zone migration expansion and densification we implement a rate state algorithm that incorporates the static stress transferred by each m 6 shock and then evolves coulomb stress changes amplify the background seismicity so small stress changes produce large changes in seismicity rate in areas of high background seismicity similarly seismicity rate declines in the stress shadows are evident only in areas with previously high seismicity rates thus a key constituent of the model is the background seismicity rate which we smooth from 1981 to 1986 seismicity the mean correlation coefficient between observed and predicted m 1 4 shocks the minimum magnitude of completeness is 0 52 for 1986 2003 and 0 63 for 1992 2003 a control standard aftershock model yields 0 54 and 0 52 for the same periods four m 6 0 shocks struck during the test period three are located at sites where the expected seismicity rate falls above the 92 percentile and one is located above the 75 percentile the model thus reproduces much but certainly not all of the observed spatial and temporal seismicity from which we infer that the decaying effect of stress transferred by successive main shocks influences seismicity for decades finally we offer a m 5 earthquake forecast for 2005 2015 assigning probabilities to 324 10 10 km cells

a unique geochemical record at the permian triassic boundary a 330 metre core drilled through the marine permian triassic boundary in the carnic alps of austria allows closely correlated studies of geochemistry petrography and palaeontology across the boundary the isotope shifts and metal concentrations are extended multiple and complex and do not resemble those seen at the cretaceous tertiary boundary both the carbon isotope shifts and the chemical events including an indium anomaly may have causes related to a major regression of the sea

conodont survival and low iridium abundances across the permian triassic boundary in south china the permian triassic sedimentary sequence of china includes one of the most complete and fossiliferous paleozoic mesozoic boundaries known closely spaced sampling across the boundary which is an important extinction event for most organisms has produced good conodont faunas that show little diversity change a drop in conodont abundance is the only apparent response to the extinction event a low concentration of iridium in the boundary clay 0 002 part per billion 20 percent as well as in samples immediately below and above that range from 0 004 to 0 034 part per billion do not support the proposal of an extraterrestrial impact event at this boundary in china

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

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

depositional bias and environmental change important factors in sequence stratigraphy abstract differences among depositional systems here called depositional bias strongly influence sequence patterns siliciclastics and shallow water carbonates for instance shed most of their sediment during opposite phases of a sea level cycle lowstand shedding and highstand shedding respectively furthermore the two systems generate their own system specific relief on the sea floor disperse their sediment load along different avenues and differ in the way they are deactivated reefs and carbonate platforms can be drowned whereas siliciclastic deposition can be shut off and renewed at any depth as a consequence of these differences pronounced unconformities drowning unconformities develop where carbonate platforms are terminated and buried by siliciclastics the siliciclastic to carbonate transition tends to be more gradual drowned platforms and drowning unconformities appeared world wide in great abundance in the miocene cretaceous valanginian turonian jurassic toarcian and devonian frasnian famennian examples of drowning unconformities interpreted as sequence boundaries include those of the early cretaceous platforms off new jersey and off morocco the mid cretaceous unconformity in the gulf of mexico and miocene unconformities on top of reefs in the far east the eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms either the rates of rise are an order of magnitude lower than the growth potential of platforms or cycle amplitudes are too small or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone the critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems thus drowning unconformities demonstrate the importance of environmental change as a control on sequence development other examples of dominantly environment controlled sequences include slope deposits shaped by shifting currents e g florida and blake plateau and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland e g gulf of mexico environmental change must be considered as a third independent factor that competes with eustatic and tectonically driven regional changes of sea level for control of sequences the definition of sequences and sequence boundaries should be broad enough to include the possibility of non sea level controls

a perspective on the evidence for periodic cometary impacts on earth abstract recent suggestions of periodic cometary showers coincident with mass extinctions have prompted a review of the terrestrial cratering record from the point of view of completeness of search and crater retention the record used to argue for statistical periodicities is incomplete and may not be representative estimates of crater ages are of variable accuracy and precision and in some cases different dating methods yield discordant results it is possible to define through time series analysis a number of periodicities of different magnitude 29 m y 21 m y 18 5 m y 13 5 m y and phase depending on which craters are considered the most representative sample of the record arguing against periodic cometary showers is the observation that estimates of the average cratering rate in recent time and observations on apollo bodies are equivalent in addition siderophile element data from impact melt rocks suggest a variely of projectile types were responsible for relatively large terrestrial craters given these observations it is concluded that statements regarding periodic cometary showers are not categorical and require evidence beyond arguments based on time series analyses of an incomplete cratering record

consequences of stress changes following the 1891 nobi earthquake japan abstract great earthquakes along the philippine sea interplate boundary central honshu usually rupture a large portion of the nankai trough and suruga trough however the 1944 tonankai and 1946 nankaido earthquakes the most recent great philippine sea events did not rupture the suruga trough mogi proposed that strain release from the 1891 nobi earthquake is responsible for the apparently premature northeast termination of the 1944 rupture zone using a recently derived fault model of the 1891 earthquake combined with a suitable rheological model of central honshu we quantify both the inland and offshore consequences of the nobi earthquake north of the median tectonic line the co seismic plus postseismic stress changes of the nobi earthquake promoted the failure of faults which subsequently ruptured in the decades following 1891 in contrast along the philippine sea interplate boundary the co seismic plus postseismic strain release of the nobi earthquake during the decades following 1891 has delayed the anticipated tokai earthquake by at least 20 yr

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

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents

compressional structures on mars abstract wrinkle ridges accommodate very low amounts of shortening strain around immense volcanic constructs and within impact basins on mars they originate from stresses distributed uniformly throughout the brittle lithosphere and are consistently located in stratified deposits including lava flows and sediment most recent models interpret wrinkle ridges as the surface manifestation of folding above underlying blind thrusts that accommodate similarly low strain and likely penetrate tens of kilometers into the brittle crust alternative models suggest shortening accommodated by some wrinkle ridges is confined to only the upper few kilometers of the crust the interpretation of the geometry of blind thrusts on mars appears to be quite varied and remains controversial although some models suggest they may ultimately flatten into the brittle ductile transition in the middle to lower crust small scale crenulations superposed on ridges are interpreted as produced by high level back thrusts nucleating at

fault slip rates effects of elastic heterogeneity on geodetic data and the strength of the lower crust in the salton trough region southern california 1 in southernmost california the salton trough sedimentary basin lies between the subparallel san jacinto and san andreas faults that together with the elsinore fault accommodate 80 of the 50 mm yr of pacific north american relative motion the slip rates on the san andreas and san jacinto faults have been a matter of recent debate and one source of uncertainty has been the influence of crustal strength heterogeneity due to elastically weak basin sediments on geodetic data to evaluate these effects we have modeled regional kinematics with elastic and viscoelastic finite element models that incorporate laterally and vertically varying crustal material properties we find that in general the effects of the sedimentary basin on surface deformation are relatively small and that to explain the geodetic velocity data the slip rate on the san andreas must be higher than that on the san jacinto fault consistent with traditional geologic estimates we also conclude that a relatively strong lower crust of viscosity 5 x 10 19 pa s exists in this region

modeling of short period surface wave propagation in southern california abstract for the 1992 landers mainshock the lateral heterogeneity of the lithosphere combined with an extended earthquake source may account for the complexity of the strong motion observations that cannot be easily interpreted by a simplified point source model in a laterally homogeneous medium both the effect of an extended source and that of lateral heterogeneity are examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane in a 3d medium surface wave synthetics are constructed using surface wave gaussian beam method which allows for a mild degree of lateral heterogeneity we analyze the amplitude and waveform data of the landers strong motion recordings in southern california in order to identify the path effects and the effect of an extended source the overall source parameters of the landers earthquake are obtained by comparing synthetics with recordings at near source stations which lie within relatively homogeneous path structures this procedure yields a unilateral rupture propagating northwesterly over two distinct rupture segments with somewhat different focal mechanisms the surface wave phase velocity map is modified to fit the data recorded at larger hypocentral distances the final improved model generates a very good agreement between observations and synthetics in the presence of lateral heterogeneity the focusing defocusing and multipath interference are found to contribute significantly to the strong motion shaking in a 3d basin

poroelastic triggering in the 9 22 january 2008 nima gaize tibet earthquake sequence a sequence of three earthquakes of ms 6 4 5 9 and 5 4 occurred in southern central tibet on 9 16 and 22 january 2008 interferometric satellite aperture radar insar observations indicate that the two largest events occurred on two northeast trending northwest dipping normal faults separated by 9 5 km with slip reaching 1 2 m on the main fault and 0 75 m on the second we develop a three dimensional finite element model to calculate the coulomb stress change due to poroelastic deformation in the crust after the first event using a uniform permeability of 8 10 15 m 2 for faults and the host rock leads to negligible temporal pore pressure change in the two weeks after the first event however assuming higher permeability along faults 10 14 to 10 11 m 2 the second fault channels the flow of pore fluids upward from the deeper region where the pore pressure increase was the largest immediately after the first event in this case the poroelastic coulomb stress on the second fault increased suddenly by 1 10 5 pa after the first earthquake and continued to increase by as much as 70 in the subsequent week furthermore the distribution of coulomb stress change on the second fault correlates precisely with the slip distribution inferred for the second event these findings suggest that the first earthquake not only triggered the second event but the coulomb stress change it induced on the second fault plane controlled the extent of the second rupture