evaporitic and biosiliceous cyclic sedimentation in the miocene of the gulf of suez depositional and diagenetic aspects abstract the middle to late miocene evaporite bearing formations which outcrop along the gebel zeit and gems highs in the southwestern gulf of suez egypt consist of thick calcium sulphate beds rhythmically interbedded with marlstones siltstones and claystones containing various amounts of biogenic silica as well as pure diatomites the calcium sulphate beds are composed mainly of gypsum and anhydrite subaqueous crustallisation in subpermanent brine ponds of marine origin predominated during deposition of the selenite and laminated gypsum early diagenetic interstitial growth of nodular anhydrite occurred episodically when the water level dropped in response to increasing salinity associated with the precipitation of halite in the deeper troughs nevertheless most of the nodular facies resulted from the burial conversion of gypsum to anhydrite which has been sometimes rehydrated into gypsum a last phase of gypsum dehydration has occurred at the surface related to the present hot and dry climatic conditions the biosiliceous deposits contain either well preserved opal a or diagenetically altered opal ct clinoptilolite quartz and even clays diatom frustules such diagenetic changes are commonly described in the oceanic biosiliceous deposits the fossil content of these diatomitic layers implies shallow marine conditions on the shelves of highs induced by block faulting the cyclic succession of evaporited and biosiliceous deposits recorded the alternation of high sea level stands associated with high organic productivity probably enhanced by seasonal input of nutrients and low sea level stands associated with hypersaline conditions and evaporite deposition locally the sharp contact between the diatomites and gypsum argues for a rapid evolution of salinity towards high concentration up to gypsum saturation and more the formation of finely laminated diatomitic sediments is known to require either high organic productivity or anoxic conditions in bottom intermediate waters and eventually both processes in the gebel zeit and gemsa areas a local association of the biosiliceous sediments with diagenetic carbonates and native sulphur resulting from processes of bacterial sulphate reduction confirms that anoxic conditions occurred in the deep water body or in the sediments large amounts of organic compounds were involved in these diagenetic processes immature organic matter was consumed during an early diagenetic stage of transformation it is assumed that this organic matter was supplied either by the biosilica rich interbeds or the organic rich laminated carbonates the initial organic content of the biogenic deposits was impoverished as early as the first stages of sedimentation and diagenesis although a large part of the organic matter could have been destroyed by these early diagenetic processes and further oxidation in outcrops these sediments can be considered as potential source rocks in nearby less exposed sequences

stress sensitivity of fault seismicity a comparison between limited offset oblique and major strike slip faults we present a new three dimensional inventory of the southern san francisco bay area faults and use it to calculate stress applied principally by the 1989 m 7 1 loma prieta earthquake and to compare fault seismicity rates before and after 1989 the major high angle right lateral faults exhibit a different response to the stress change than do minor oblique right lateral thrust faults seismicity on oblique slip faults in the southern santa clara valley thrust belt increased where the faults were unclamped the strong dependence of seismicity change on normal stress change implies a high coefficient of static friction in contrast we observe that faults with significant offset 50 100 km behave differently microseismicity on the hayward fault diminished where right lateral shear stress was reduced and where it was unclamped by the loma prieta earthquake we observe a similar response on the san andreas fault zone in southern california after the landers earthquake sequence additionally the offshore san gregorio fault shows a seismicity rate increase where right lateral oblique shear stress was increased by the loma prieta earthquake despite also being clamped by it these responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones we can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes whereas in more limited offset faults fluids could rapidly escape the difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes since many of these events occur both inside and outside major fault zones

geometry and kinematics of convergent conjugate vein array systems abstract conjugate arrays of quartz veins in the neranleigh fernvale turbidite beds at norries head eastern australia occur in configurations in which the trends of the veins in a principal section converge towards the acute bisector of the conjugate arrays a convergent configuration such a configuration is common in vein arrays and has been attributed to initiation of the veins as antithetic shear fractures based on geometric arguments the morphology of the veins in this study indicates that the veins are hosted by extension fractures which formed by en echelon breakdown of faults previous models of extension fracture arrays emphasise that all fractures lie parallel to the bisector of conjugate arrays and that arrays develop after initiation of extension fractures and concurrent with fracture propagation sigmoidal vein shapes have been attributed to concurrent shear strain and fracture propagation an alternative model is proposed for the geometry of conjugate arrays formed by en echelon breakdown of faults the conjugate angle between parent faults is established before the en echelon extension fractures are formed the fracture array angle depends on the local displacement of the parent fault thus there is no necessity for the fractures in different arrays to be parallel if the fracture array angle is greater than half the conjugate angle between parent faults a convergent configuration of fractures is produced the kinematics of opening of veins in this study involved bending of rock bridges between fractures producing a gradation from planar to sigmoidal shapes in serial sections without evidence of concurrent fracture propagation

seismic activity of the san francisco bay region moment magnitude m with objective confidence level uncertainties are estimated for felt san francisco bay region earthquakes using bakun and went worth s 1997 analysis strategy for seismic intensity observations the frequency magnitude distribution is well described for m 5 5 events since 1850 by a gutenberg richter relation with a b value of 0 90 the seismic moment rate m yr since 1836 is 2 68 1018 n m yr 95 confidence range 1 29 1018 n m yr to 4 07 1018 n m yr the seismic moment rate since 1850 is nearly the same m yr in the 56 years before 1906 is about 10 times that in the 70 years after 1906 in contrast m yr since 1977 is about equal that in the 56 years before 1906 80 1 14 of the plate motion moment accumulation rate is available for release in earthquakes the historical m yr and the portion of the plate motion moment accumulation rate available for release in earthquakes are used in a seismic cycle model to estimate the rate of seismic activity in the twenty first century high and low rates of future seismic activity are both permissible given the range of possible seismic cycle recurrence times t and the uncertainties in the historical m and in the percentage of plate motion available for release in earthquakes if the historical seismic moment rate is not greater than the estimated 2 68 1018 n m yr and the percentage of the plate motion moment accumulation available for release in earthquakes is not less than the estimated 80 then for all t the rate of seismic moment release from now until the next 1906 sized shock will be comparable to the rate from 1836 to 1905 when m 6 1 2 shocks occurred every 15 to 20 years

the relation of a coastal environment to early diagenetic clinoptilolite zeolite formation new data from the late cretaceous european basin abstract clinoptilolite an authigenic mineral of the zeolite group is commonly considered as a product of rhyolitic volcanic glass transformation during diagenesis of carbonate pelagic hemipelagic sediments of cretaceous paleogene age another subordinate way of clinoptilolite formation that has been suggested in the literature is its precipitation during diagenetic transformation of carbonate sediments enriched with biogenic silica the observations concerning transformation of the latter into zeolite are scarce and mostly based on cretaceous pelagic hemipelagic and shallow marine deposits and post cretaceous sediments in which there was no volcanic material the mechanism of diagenetic clinoptilolite formation in volcanic free sediments as well as the role of the depositional setting in its precipitation is not well understood in carbonates of the nw part of the european basin west side of the ysogory dobrogea archipelago studied here the clinoptilolite is associated with opal ct from the dissolution of sponge spicules and a detrital clays are documented for the first time in a cretaceous coastal environment the carbonates were deposited in a coastal environment which was under the influence of weak currents delivering sponge spicules biogenic opal from the offshore zone and the terrestrial input was transported by rivers from a neighboring island the lush plant vegetation with numerous pteridophytes and conifers within a warm sub tropical or tropical and humid climate enhanced the terrestrial weathering of clays and feldspars this unique combination of elevated silica concentrations from sponge spicules dissolution and aluminum availability from terrestrial weathering of feldspars in a subtropical climate during the early diagenesis combined with alkalinity triggered by organic matter decomposition enhanced the clinoptilolite formation a few centimeters below the seabed the investigation demonstrates a novel route by which pore fluids enriched with silica and aluminum initiate clinoptilolite formation during the very early stage of diagenesis a few centimeters below the seafloor the clinoptilolite distribution in late cretaceous silica rich deposits with no volcanic input of the european basin is always associated with opal ct from sponge spicules and elevated terrestrial input clays taking into account the distinctive campanian maastrichtian sedimentation in the epicontinental european basin composed almost exclusively of pure pelagic carbonates the depositional settings in which the silica and aluminum were available in pore waters was restricted to the part of the basin influenced by terrestrial input this leads to the conclusion that clinoptilolite formation in volcanic free sediment is an indicator of not only enhanced sponge silica availability but also the close proximity of the source area

middle eocene to early oligocene paleoceanography of the antarctic ocean maud rise odp leg 113 site 689 change from a low to a high productivity ocean the sediment record from the middle eocene about 45 ma to the early oligocene about 33 ma on maud rise site 689 weddell sea paleodepth 1400 1650 m has been studied by means of a coarse fraction analysis in order to reconstruct variations in surface water productivity and bottom water masses productivity has been estimated using carbonate dissolution and accumulation rates of radiolaria diatoms benthonic and planktonic foraminifers sponge debris ostracods and echinoids accumulation rates of bolboforma spp vary parallel to these proxies productivity increased with time three episodes were found in which productivity increased sharply and productivity varied cyclically with a period of 430 ky the middle eocene ca 45 44 37 26 ma was a period of generally low productivity no opal was deposited and carbonate preservation was excellent during periods of a slight increase in productivity accumulation rates of benthonic and planktonic foraminifers ostracods and echinoids increased in spite of an increase in carbonate dissolution in the period ca 41 62 43 07 ma productivity rose in three distinct pulses clinoptilolite formed from biogenous opal is a common mineral in these sediments these three productivity maxima coincide with a mica rich unit mica probably originated from erosion on antarctica and was transported by an intermediate water depth current in the late eocene ca 36 9 34 95 ma productivity increased considerably and opal skeletons were deposited carbonate dissolution increased but planktonic foraminifer accumulation rates were not markedly decreased in periods of higher productivity because productivity increase exceeded or equalled rises in dissolution rates increasing opal accumulation rates coincided with increases in the rates for benthonic foraminifers sponge debris ostracods and echinoids in the late eocene early oligocene period 34 95 33 05 ma a further increase in productivity led to a strong dissolution of planktonic foraminifers in high productivity periods the productivity increase is probably due to the increasing pole equator temperature gradient which led to stronger atmospheric and oceanic turnover and mixing in the oceans in the uppermost late eocene two unusual layers suggest changes in bottom water masses affecting the seafloor at site 689 a sediment interval with especially well preserved carbonate microfossils underlies an interval that has poor carbonate preservation despite evidence of low productivity these intervals suggest a change in bottom water conditions from warm saline deep water wsdw which favors carbonate preservation to antarctic bottom water aabw which promotes carbonate dissolution cooling of surface waters over a period of 300 ky led to a thickening of the deep aabw layer and an upward shift of the overlying wsdw until the aabw finally reached the top of the maud rise in the area of site 689 this period of aabw influence on top of the maud rise lasted 200 ky

climatic and sea level control of jurassic pliensbachian clay mineral sedimentation in the cardigan bay basin llanbedr mochras farm borehole wales

authigenic zeolites in late pleistocene sediments of the south atlantic angola basin two types of authigenic zeolites from late pleistocene hemipelagic sediments of the south atlantic were examined by xrd and sem analysis the xrd characteristics and the thermal behaviour of the zeolites suggest that type 1 is composed of clinoptilolite type 2 was identified as mordenite based on morphological features and qualitative elemental composition their authigenic nature is suggested from the shape of the crystals and their growth on and within foraminifera tests volcanic ash material acting as precursor minerals to zeolite formation could not be found nor were the zeolites directly related to siliceous remains an authigenesis from pore waters must be assumed mineral stabilities for amorphous sio2 and zeolites were calculated with the computer program solmineq88 interstitial waters proved to be oversaturated throughout the core in respect to all zeolites considered biogenic opal represents the limiting solid phase for si concentrations in pore waters therefore the dissolution of opal is regarded as the main source of si for interstitial waters below 9 m core depth where opal is only present in traces detrital feldspars and the zeolites themselves are affected by dissolution this may indicate the metastable nature of the zeolites also corroborated by the presence of two almost isochemical members of the same zeolite group

seismic hazard in the marmara sea region following the 17 august 1999 izmit earthquake on 17 august 1999 a destructive magnitude 7 4 earthquake occurred 100 km east of istanbul near the city of izmit on the north anatolian fault this 1 600 km long plate boundary 1 2 slips at an average rate of 2 3 cm yr 1 refs 3 5 and historically has been the site of many devastating earthquakes 6 7 this century alone it has ruptured over 900 km of its length 6 models of earthquake induced stress change 8 combined with active fault maps 9 had been used to forecast that the epicentral area of the 1999 izmit event was indeed a likely location for the occurrence of a large earthquake 9 10 here we show that the 1999 event itself significantly modifies the stress distribution resulting from previous fault interactions 9 10 our new stress models take into account all events in the region with magnitudes greater than 6 having occurred since 1700 ref 7 as well as secular interseismic stress change constrained by gps data 11 these models provide a consistent picture of the long term spatio temporal behaviour of the north anatolian fault and indicate that two events of magnitude equal to or greater than the izmit earthquake are likely to occur within the next decades beneath the marmara sea south of istanbul

triggering of earthquake aftershocks by dynamic stresses it is thought that small static stress changes due to permanent fault displacement can alter the likelihood of or trigger earthquakes on nearby faults many studies of triggering in the near field particularly of aftershocks rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault here we report a comparison of the aftershock pattern of the moment magnitude mw 7 3 landers earthquake not only with static stress changes but also with transient oscillatory stress changes transmitted as seismic waves that is dynamic stresses dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone these dynamically weakened faults may fail after the seismic waves have passed by and might even cause earthquakes that would not otherwise have occurred we find similar asymmetries in the aftershock and dynamic stress patterns the latter being due to rupture propagation whereas the static stress changes lack this asymmetry previous studies have shown that dynamic stresses can promote failure at remote distances but here we show that they can also do so nearby

clinoptilolite as a new proxy of enhanced biogenic silica productivity in lower miocene carbonate sediments of the bahamas platform isotopic and thermodynamic evidence abstract massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor mbsf in lower miocene well compacted carbonate periplatform sediments from the great bahama bank ocean drilling program odp leg 166 site 1007 the diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix as well as mg smectites in carbonate rich deposits the formation of the zeolite requires a supply of silica thus the objective of the study is to determine the origin of the silica supply its diagenetic evolution and consequently the related implications on interpretation of the sedimentary record in terms of local or global paleoceanographic change for lack of evidence for any volcaniclastic input or traces of si enriched deep fluids circulation an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow water carbonate sequences geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite as well as its contemporaneous formation with the carbonate deposition sr model ages of 19 6 23 2 ma the test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage seawater rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica which reacts with clay minerals the amount of silica opal a involved in the reaction has to be significant enough at least 10 wt to account for the observed content of clinoptilolite occurring at the most zeolite rich level modeling also shows that the observed amount of clinoptilolite 19 reflects an in situ and short term reaction due to the high reactivity of primary biogenic silica opal a until its complete depletion the episodic occurrence of these well lithified zeolite rich levels is consistent with the occurrence of seismic reflectors particularly the p2 seismic sequence boundary located at 1115 mbsf depth and dated as 23 2 ma the age range of most zeolitic sedimentary levels biostratigraphic ages of 21 5 22 ma correlates well with that of the early miocene glaciation mi 1 and mi 1a global events thus the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply or in other sensitive marine areas is potentially a significant new proxy for paleoproductivity and oceanic global events such as the miocene events which are usually recognized in deep sea pelagic sediments and high latitude deposits

physical model of aftershock temporal behaviour abstract the aim of the model is to describe the temporal behaviour of cumulative seismic moment released in aftershock sequences it therefore constitutes an alternative approach with respect to the omori and utsu laws which model the number of events against time static fatigue is assumed to be the principal explanation of the aftershock temporal behaviour as a result aftershocks can originate at a stress intensity factor lower than the fracture toughness under the condition that the mainshock causes a redistribution of stress the initial stress condition of the aftershock sequence at mainshock origin time t0 can be considered as the superposition of the stress before the mainshock and of the stress step d caused by the dynamic rupture of the mainshock the model can be written in the form s t d rt ln t where s t is the cumulative stress drop at time t after the mainshock t the temperature r the universal gas constant and a constant dependent on material type the model is derived from the empirical static fatigue law t s exp u rt widely applied in fracture mechanics is the stress s a constant and the other parameters as above the fit of the model to five aftershock sequences obtained by linear regression analysis is quite good with the r2 greater than 0 85 for all the sequences but the imperial valley sequence where r2 0 7 the results seem to favour the hypothesis that the decay of the rate of seismic moment release depends on temperature in agreement with other evidences based on the evaluation of the decay rate of omori law but in contrast to some static fatigue results obtained in laboratory tests on granite rocks the scarcity of field data on underground temperature does not allow any definitive conclusion about this subject

triggered slip observations of the 17 august 1999 izmit turkey earthquake using radar interferometry we use synthetic aperture radar interferometry insar to map the displacement field of the 17 august 1999 izmit earthquake which largely conforms to that predicted for an elastic upper crust we determine the earthquake source parameters and show that slip continues farther west than the mapped fault ruptures we also show that additional sub surface displacements occurred on parallel strands of the north anatolian fault zone we argue that this was caused by changes in static stress accompanying the mainshock or by the dynamic release of regional stresses

significance of stress transfer in time dependent earthquake probability calculations 1 a sudden change in stress is seen to modify earthquake rates but should it also revise earthquake probability data used to derive input parameters permit an array of forecasts so how large a static stress change is required to cause a statistically significant earthquake probability change to answer that question effects of parameter and philosophical choices are examined through all phases of sample calculations drawing at random from distributions of recurrence aperiodicity pairs identifies many that recreate long paleoseismic and historic earthquake catalogs probability density functions built from the recurrence aperiodicity pairs give the range of possible earthquake forecasts under a point process renewal model consequences of choices made in stress transfer calculations such as different slip models fault rake dip and friction are tracked for interactions among large faults calculated peak stress changes may be localized with most of the receiving fault area changed less than the mean thus to avoid overstating probability change on segments stress change values should be drawn from a distribution reflecting the spatial pattern rather than using the segment mean disparity resulting from interaction probability methodology is also examined for a fault with a well understood earthquake history a minimum stress change to stressing rate ratio of 10 1 to 20 1 is required to significantly skew probabilities with 80 85 confidence that ratio must be closer to 50 1 to exceed 90 95 confidence levels thus revision to earthquake probability is achievable when a perturbing event is very close to the fault in question or the tectonic stressing rate is low

diagenesis of upper jurassic concretions from the antarctic peninsula abstract early diagenetic calcite concretions are widespread in mudstone tuff sequences deposited in late jurassic oxygen deficient basins of the antarctic peninsula although obscured or destroyed in host rocks original components and sedimentary structures are well preserved in concretions in spite of significant mineralogical and geochemical change during growth early diagenesis led to a rapid and almost total variation in the mineralogy within the concretions independently of the original host lithologies siliceous particles were replaced by chlorite and zeolites in suboxic alkaline and moderately reducing conditions on the sea floor and in the first centimeters of burial these conditions also led to localized early formation of phosphate concretions pyrite developed subsequently because of high sulfide abundance in the sulfate reduction zone most calcite precipitation took place in the lower part of the sulfate reduction zone in a strongly alkaline environment where sulfide content had been lowered by pyrite precipitation in this stage the remaining siliceous elements were dissolved or replaced by calcite later diagenetic changes include enlargement of zeolites minor changes in clay minerals and the formation of calcite veins the wide range of 13c values in cement and vein calcite 1 47o oo and 20 23o oo pdb is explained by mixed carbon sources from organic matter oxidation and dissolution of calcareous shells the low 18o of calcite 2 34o oo to 19 72o oo pdb is the result of recrystallization in contact with high temperature diagenetic or hydrothermal fluids flushed through micro fractures in the concretions which also produced extensive vein formation

sensitivity of static stress calculations to the earthquake slip distribution 1 the apparent strong correlation between coulomb stress changes and the spatial distribution of aftershocks suggests the possibility of making near real time estimations of areas at risk of experiencing off fault aftershocks in order to do this in practice a number of issues must first be addressed including the extent to which the main shock slip must be known in detail before a meaningful stress map can be constructed here we investigate this issue by constructing a time ordered sequence of slip solutions for the landers earthquake computing coulomb stress changes for each solution and quantitatively comparing the stress field with the observed aftershocks by 1 resolving the coulomb stress change onto the aftershock nodal planes and calculating the percentage of events consistent with triggering and 2 constructing a two dimensional map of coulomb stress and computing the correlation coefficient between the positive and negative areas and the locations of the aftershocks we find that slip solutions based on empirical relations and either focal mechanism or moment tensor data produce stress fields inconsistent with the observed spatial distribution of aftershocks whereas slip solutions incorporating the correct rupture geometry but greatly simplified slip produce stress fields consistent with the aftershock distribution when very near fault events are excluded we further find that resolving stress perturbations onto earthquake nodal planes and computing the percentage of events experiencing positive coulomb stress provides a poor measure of success because of the limited range of structures on which events occur and the compatibility of the main shock stress field with these structures our results support the hypothesis that coulomb stress changes affect the spatial distribution of aftershocks and suggest that meaningful calculations of coulomb stress can be made as soon as an earthquake s rupture geometry is well constrained

time dependent distributed afterslip on and deep below the izmit earthquake rupture surface deformation transients measured with the global positioning system during the 87 days between the 17 august 1999 izmit earthquake and the 12 november 1999 duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture elastic model inversions for time dependent distributed fault slip using a network inversion filter approach show that afterslip was highest between and below the regions of maximum coseismic slip and propa gated downward to or even below the base of the crust maximum afterslip rates decayed from greater than 2 m yr immediately after the i zmit earthquake to about 1 2 m yr just prior to the duzce earthquake maximum afterslip occurred below the eastern karadere rupture segment and near the i zmit hypocenter afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone these observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby mw 7 2 12 november duzce earthquake

paleoceanographic and paleoclimatic evolution in the weddell sea antarctica during the middle eocene late oligocene from a coarse sediment fraction and clay mineral data odp site 689 abstract paleoceanographic and paleoclimatic variations from the middle eocene to the late oligocene in the antarctic area atlantic sector have been deduced from a coarse fraction and clay mineral analysis of odp site 689 leg 113 sediments in the middle eocene smectite rich sediments were a consequence of warm climate with alternating wet and dry seasons on the antarctic margins productivity in the maud rise area of the southern ocean was low no opal was deposited preservation of carbonate is very good carbonate preservation shows cyclic changes clinoptilolite is a common mineral at the middle late eocene boundary 40 8 ma productivity suddenly increased enough to allow preservation of opal skeletons clinoptilolite disappeared where opal appeared illite appearance points to cooling of antarctic climates the presence of detrital kaolinite indicates a change in oceanic circulation in the late eocene cyclic changes in productivity cycle length 0 42 ma were associated with clay mineral compositional variations sediments from colder highly productive periods generally contain more illite and less smectite than warmer ones the early oligocene was a period of very high productivity highest opal accumulation rates correspond to strongest carbonate dissolution cyclic changes in productivity with a mean cycle duration of 0 4 0 5 myr were associated with cyclic changes in clay mineral composition highest productivity intervals are generally those with highest smectite content because of strong erosion of sediments of the antarctic margins sediments from less productive warmer periods most of the time contain more illite in the late oligocene the intensity of physical weathering on antarctica was stronger as documented by increasing fluxes of illite muscovite amphiboles and talc ice expanded on the east antarctic continent lower productivity than in the early oligocene is indicated by lower accumulation rates of opal and weaker carbonate dissolution cyclic variations in productivity correlate to cyclic variations in illite and smectite contents as in the early oligocene

suppression of large earthquakes by stress shadows a comparison of coulomb and rate and state failure stress shadows generated by california s two most recent great earthquakes 1857 fort tejon and 1906 san francisco substantially modified 19th and 20th century earthquake history in the los angeles basin and in the san francisco bay area simple coulomb failure calculations which assume that earthquakes can be modeled as static dislocations in an elastic half space have done quite well at approximating how long the stress shadows or relaxing effects should last and at predicting where subsequent large earthquakes will not occur there has however been at least one apparent exception to the predictions of such simple models the 1911 m 6 0 earthquake near morgan hill california occurred at a relaxed site on the calaveras fault we examine how the more complex rate and state friction formalism based on laboratory experiments might have allowed the 1911 earthquake rate and state time to failure calculations are consistent with the occurrence of the 1911 event just 5 years after 1906 if the calaveras fault was already close to failure before the effects of 1906 we also examine the likelihood that the entire 78 years of relative quiet only four m 6 earthquakes in the bay area after 1906 is consistent with rate and state assumptions given that the previous 7 decades produced 18 m 6 earthquakes combinations of rate and state variables can be found that are consistent with this pattern of large bay area earthquakes assuming that the rate of earthquakes in the 7 decades before 1906 would have continued had 1906 not occurred these results demonstrate that rate and state offers a consistent explanation for the 78 year quiescence and the 1911 anomaly although they do not rule out several alternate explanations

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

authigenic zeolites and their relation to silica diagenesis in odp site 661 sediments leg 108 eastern equatorial atlantic at odp site 661 from the continental margin of northwest africa authigenic clinoptilolites were investigated in detail based on chemical data crystal size and shape and their intergrowth with associated mineral phases two types of clinoptilolite type a and b occur representing different diagenetic conditions under which the formation of zeolites took place clinoptilolite type a was found in a sediment section comprises numerous hiatuses and stratigraphically condensed sediments and could have been formed during early diagenesis in contrast clinoptilolite type b was precipitated in campanian sediments during later diagenesis after the transformation from opal a to opal ct had taken place at site 661 the silica source for the authigenesis of both clinoptilolite types is biogenic opal volcanic ash material occurs but in low amounts and thus is not a prerequisite for the formation of clinoptilolite at site 661

diagenetic clinoptilolite and opal ct from the middle eocene wittering formation isle of wight u k abstract thinly interbedded estuarine sandstones and mudstones in the middle eocene wittering formation bracklesham group isle of wight contain authigenic clinoptilolite opal ct and possibly authigenic smectite over a thickness of 8 5 m the clinoptilolite and opal ct cements have resulted in these beds being the only ones tightly cemented in the wittering formation the rest being soft sands and muds the unit contains abundant siliceous sponge spicules many of which are preserved as voids and were the likely source of the opal ct the presence of an organic rich rooted paleosol histosol immediately overlying the unit is believed to have contributed significantly to the early diagenetic history of the estuarine sediments in particular the formation of zeolite although the ph and dissolved silica of unmodified seawater are too low for zeolite formation high concentrations of dissolved silica are believed to have resulted from dissolution of siliceous sponge spicules opal a and formation of silica organic acid complexes organic acid dissolution of feldspars absent from the zeolite bearing interval is the most probable source of alumina and cations for zeolite formation ree data show that volcanic ash was never present and cannot therefore have been the source of the zeolite this investigation therefore demonstrates a novel route by which the pore fluid chemistry required for zeolite formation can be achieved

global omori law decay of triggered earthquakes large aftershocks outside the classical aftershock zone 1 triggered earthquakes can be large damaging and lethal as evidenced by the1999 shocks in turkey and the 2001 earthquakes in el salvador in this study earthquakes with ms 7 0 from the harvard centroid moment tensor cmt catalog are modeled as dislocations to calculate shear stress changes on subsequent earthquake rupture planes near enough to be affected about 61 of earthquakes that occurred near defined as having shear stress change 0 01 mpa the ms 7 0 shocks are associated with calculated shear stress increases while 39 are associated with shear stress decreases if earthquakes associated with calculated shear stress increases are interpreted as triggered then such events make up at least 8 of the cmt catalog globally these triggered earthquakes obey an omori law rate decay that lasts between 7 11 years after the main shock earthquakes associated with calculated shear stress increases occur at higher rates than background up to 240 km away from the main shock centroid omori s law is one of the few time predictable patterns evident in the global occurrence of earthquakes if large triggered earthquakes habitually obey omori s law then their hazard can be more readily assessed the characteristic rate change with time and spatial distribution can be used to rapidly assess the likelihood of triggered earthquakes following events of ms 7 0 i show an example application to the m 7 7 13 january 2001 el salvador earthquake where use of global statistics appears to provide a better rapid hazard estimate than coulomb stress change calculations