stress transfer and its implication for earthquake hazard on the kunlun fault tibet abstract the 1600 km long kunlun fault striking e w to wnw ese had long been recognized as one of the major left lateral strike slip faults bounding the tibetan plateau and ranked one of the most active faults in china continent during the past hundred years over twenty strong earthquakes occurred along and near the kunlun fault including six large earthquakes m 7 since some major highly populated and industrialized cities are close to the kunlun fault understanding of stress transfer and earthquakes migration along the kunlun fault is most important for assessing seismic hazard in this region in this study by integrating coseismic effect viscoelastic relaxation and tectonic loading we studied the evolution of the regional coulomb stress field by analyzing a sequence of strong earthquakes along the kunlun fault we studied the stress evolution over one century by analysing a sequence of five earthquakes m 7 that occurred along the kunlun fault since 1937 the model of dislocation sources embedded in a mixed elastic inelastic layered half space was used and the layered model and relevant parameters were constrained by seismic studies fault rupture locations and geometry as well as slip distribution of earthquakes were taken from field observations and seismic studies numerical results showed a good correlation between stress transfer accumulation and earthquakes occurrence all four studied earthquakes occurred after the 1937 tuosuo lake quake were encouraged by the preceding earthquakes with positive stress loading in subject to the choice of the earthquake source parameter two or three out of four events occurred in regions that experienced previous coseismic and postseismic stress changes of at least 0 01 mpa suggesting that earthquake triggering due to stress transfer has occurred along the kunlun fault the total stress change since 1937 of the kunlun fault region has lead to high levels of stress accumulated on the xidatan dongdatan segment and maqin maqu segment which have not experienced any significant large earthquake over at least several hundred or several thousand years the accumulated stress raises the potential earthquake hazard in these areas our study demonstrated the crucial importance of postseismic viscoelastic relaxation in the stress transfer and accumulation following large earthquakes

stress evolution in southern california and triggering of moderate small and micro size earthquakes we calculate the evolution of stresses in southern california extending the study of deng and sykes 1997 by increasing from 6 to 36 the number of earthquakes for which coseismic changes in stress are computed and by expanding from m 6 to m 1 8 the range of magnitudes m of events whose focal mechanism solutions are examined in the context of the evolving stress field the cumulative stress on a given date is calculated with respect to an arbitrary zero baseline just before the 1812 wrightwood earthquake by taking into account the long term stress loading associated with 98 fault segments and coseismic stress changes for 36 significant earthquakes our calculations indicate that more than 85 of m 5 earthquakes from 1932 1995 occurred in regions of positive change in coulomb failure function cff most of the remaining about 15 earthquakes that occurred in areas of negative cff fall very close to boundaries between positive and negative cff some of which are sensitive to the less well controlled slip distributions of the earliest historic events calculations also show that from 1981 until just before the 1992 landers earthquake more than 85 of small m 3 and micro size m 1 8 shocks in the seeber and armbruster 1995 catalog with mechanisms involving either nw trending right lateral or ne trending left lateral strike slip faulting occurred in regions of positive cff the ratio of encouraged to all small and micro size events reaches a high value of about 88 if an apparent coefficient of friction between 0 0 and 0 6 is used the highest percentage of earthquakes occurred in areas where stress is about 1 mpa above the 1812 baseline most 66 events occurred in regions of cff between 0 0 and 2 0 mpa the upper limit indicates that the approximate range of stress variation in the earthquake cycle is of the order of 2 0 mpa the fact that the locations of most moderate small and micro size earthquakes are still related to stress changes remaining from large historical events might be used to constrain slip distribution of some of those earthquakes and to constrain the locations of future significant events

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

late pleistocene and holocene palaeohydrology of lake rukwa tanzania inferred from diatom analysis diatom analysis has been used to provide information on the lake level fluctuations of lake rukwa during the last 21 7 ka diatom inferred chemical variables conductivity ph anion and cation ratios indicate that at the lgm lake level was low probably driven by a low p e ratio but with sufficient water to avoid desiccation following the lgm deep water conditions were obtained through a series of steps until maximum water depth was reached by at least 13 5 ka the high lake level phase lasted through the early holocene until after 6 7 ka a shallow saline lake has occupied the basin from ca 5 5 ka until the present day the pattern of reconstructed lake level fluctuation in lake rukwa is similar to that from lake tanganyika and other lakes from the equatorial and northern tropical zone we can confirm that the n tropical mode of millennial scale lake level change since the lgm extended to at least 8 s

prospects for larger or more frequent earthquakes in the los angeles metropolitan region far too few moderate earthquakes have occurred within the los angeles california metropolitan region during the 200 year long historic period to account for observed strain accumulation indicating that the historic era represents either a lull between clusters of moderate earthquakes or part of a centuries long interseismic period between much larger moment magnitude m w 7 2 to 7 6 events geologic slip rates and relations between moment magnitude average coseismic slip and rupture area show that either of these hypotheses is possible but that the latter is the more plausible of the two the average time between m w 7 2 to 7 6 earthquakes from a combination of six fault systems within the metropolitan area was estimated to be about 140 years

mobility and fractionation of rees during deep weathering of geochemically contrasting granites in a tropical setting malaysia in this study we describe the mobility and fractionation of rees in two deep up to 30 m tropical weathering profiles developed on two granites from the kuala lumpur pluton malaysia sampled at cheras and rawang on the basis of na2o and k2o both are s type granites but rawang has higher cao mgo and feo than cheras and lower sio2 with respect to al saturation rawang is i type and cheras is s type we compared the two profiles in terms of total rees magnitude and changes in ce and eu anomalies ree mobility and lree hree fractionation rawang profiles have higher ree contents display lower mobility for most except the heaviest rees and show higher lree hree fractionation than those from cheras these differences can be linked to differences in primary mineralogy and degree of weathering the latter controlling the type and volume of secondary minerals specifically bowl shaped parent rock normalised patterns in the cheras saprolites appear to be a result of apatite dissolution moreover moderate weathering evident in lower mineralogical indices of alteration mia at cheras has conserved parent rock ree patterns and fractionation factors in the saprolites by contrast more intense weathering observed in rawang profiles has produced abundant kaolinite group minerals that have preferentially retained lree which consequently display high lree hree fractionation this study provides important insights into the factors controlling ree mobility during tropical weathering and its potential as an indicator of weathering intensity

release of heavy metals during weathering of the lower cambrian black shales in western hunan china weathering of heavy metal enriched black shales may be one of the most important sources of environmental contamination in areas where black shales are distributed heavy metal release during weathering of the lower cambrian black shales lcbs in western hunan china was investigated using traditional geochemical methods and the icp ms analytical technique concentrations of 16 heavy metals 8 trace elements and p were measured for samples from selected weathering profiles at the taiping vanadium ore mine tp the matian phosphorous ore mine mt and taojiang stone coal mine tj the results show that the bedrock at these three profiles is enriched with sc v cr co ni cu zn pb th u mo cd sb tl and p based on mass balance calculation the percentages of heavy metals released in loss relative to immobile element nb were estimated the results show significant rates of release during weathering of v cr co ni cu zn u mo cd sn sb and tl for the tp profile sc cr mn co ni cu zn pb th cd and sn for the mt profile and sc mn co ni zn th cd sn and tl for the tj profile among these heavy metals co ni zn cd and sn show very similar features of release from each of the three weathering profiles the heavy metals released during weathering may affect the environment especially topsoil and surface waters and are possibly related to an observed high incidence of endemic diseases in the area

geochemical behavior under tropical weathering of the barama mazaruni greenstone belt at omai gold mine guiana shield abstract mineralogical petrographical and geochemical studies of the weathering profile have been carried out at omai au mine guyana the area is underlain by felsic to mafic volcanic and sedimentary rocks of the barama mazaruni supergroup part of the paleoproterozoic greenstone belts of the guiana shield tropical rainy climate has favoured extensive lateritization processes and formation of a deeply weathered regolith the top of the weathering profile consists of lateritic gravel or is masked by the pleistocene continental deltaic berbice formation mineralogical composition of regolith consists mainly of kaolinite goethite and quartz and subordinately sericite feldspar hematite pyrite smectite heavy minerals and uncommon mineral phases nacrite ephesite corrensite guyanaite a specific feature of the weathering profile at omai is the preservation of fresh hydrothermal pyrite in the saprolith horizon chemical changes during the weathering processes depend on various physicochemical and structural parameters consequently the depth should not be the principal criterion for comparison purposes of the geochemical behavior within the weathering profile but rather an index that measures the degree of supergene alteration that has affected each analyzed sample independently of the depth of sampling thus the mineralogical index of alteration mia can provide more accurate information about the behavior of major and trace elements in regolith as opposed to unweathered bedrock it can also aid in establishing a quantitative relationship between intensity of weathering and mobility leaching or accumulation of each element in each analyzed sample at omai some major and trace elements that are commonly considered as immobile ex tio2 zr etc during weathering could become mobile in several rock types and cannot be used to calculate the mass and volume balance in addition due to higher immobile element ratios the weathered felsic volcanic rocks plotted in identification diagrams are shifted towards more mafic rock types and a negative adjustment of 20 units is necessary for correct classification in contrast these elements could aid in defining the material source in sedimentary rocks affected by weathering generally the rare earth element ree patterns of the bedrock are preserved in the saprolith horizon this can represent a potentially useful tool for geochemical exploration in tropical terrains strong negative ce and tb anomalies are displayed by weathered pillowed andesites which are explained by the influence of the water rock ratio

records and causes of holocene salinity shifts in laguna de bay philippines abstract a 7000 year record of salinity shifts from molluscan and diatom assemblages and verified geochemical salinity proxies c s s sr ca and sr ba is reconstructed from philippines largest freshwater lake laguna de bay the salinity shifts are interplays of millennial scale sea level change centennial scale tectonic activity from the west marikina valley fault wmvf and decadal scale climatic changes currently only 1 m above mean sea level the lake is connected to manila bay through the pasig napindan river which meanders across a densely populated paranaque strip partly occupied by metropolitan manila the controversial wmvf borders the lake on its western shore our 10 5 m sediment core from the western lobe reveal that the deposits are from a marine environment deposited 6600 cal bp to 4700 cal bp during a sea level still stand above present mean sea level apmsl in southeast asia and the philippines episodic decreases in salinity with 260 y cycles were traced to the activity of the wmvf that would have accounted for at least 6 m of uplift of the paranaque strip the vertical component of movement raised a sill and emergent landmass that started to isolate the paleo laguna de bay from manila bay subsequent episodic uplifts and sea level fall decreased depths to intertidal levels by 4700 cal bp salinity to brackish but highly influenced by marine waters by 4100 cal bp and to freshwater by 3100 cal bp the latest salinity increase since mid 1960 is attributed to renewed saltwater incursion due to abnormally dry years low lake levels and pasig river delta plain subsidence

multiple large earthquakes in the past 1500 years on a fault in metropolitan manila the philippines the first 14 c based paleoseismic study of an active fault in the philippines shows that a right lateral fault on the northeast edge of metropolitan manila poses a greater seismic hazard than previously thought faulted hillslope colluvium stream channel alluvium and debris flow deposits exposed in trenches across the northern part of the west marikina valley fault record two or three surface faulting events three eroded clay rich soil b horizons suggest thousands of years between surface faulting events whereas 14 c ages on detrital charcoal constrain the entire stratigraphic sequence to the past 1300 1700 years we rely on the 14 c ages to infer faulting recurrence of hundreds rather than thousands of years minimal soil development and modern 14 c ages from colluvium overlying a faulted debris flow deposit in a nearby stream exposure point to a historic age for a probable third or fourth most recent faulting event

convergence rates across a displacement transfer zone in the western transverse ranges ventura basin california three cross sections are balanced and retrodeformed to the top of the pleistocene saugus formation and to a horizon close to the base of the jaramillo subchron to yield crustal shortening and shortening rates for the western transverse ranges of california the cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault red mountain fault to a blind thrust and from that to a combination of both a surface reverse fault san cayetano fault and the blind thrust using an age of 250 50 ka for the top of the saugus formation based on amino acid racemization of fossil mollusks crustal shortening rates appear to have abruptly accelerated through time from 3 4 3 to 5 4 mm yr between 250 and 975 ka to 20 6 to 28 1 mm yr since 250 50 ka using an age of 500 ka for the top of the saugus formation based on paleomagnetic stratigraphy there is little increase in deformation rates through time ranging from 5 5 to 8 6 mm yr between 975 500 ka and from 10 3 to 14 1 mm yr since 500 ka more crustal convergence rates determined by global positioning system surveys taken over 4 6 years indicate a shortening rate of 7 10 mm yr across the basin this is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the saugus formation less

polarization tomography for p wave velocity structure in southern california we develop a tomographic inversion method that uses teleseismic p wave polarization data to obtain velocity structure polarization inversion has some intrinsic advantages over travel time inversion it is not influenced by source location and origin time errors it is not sensitive to deep mantle velocity structure and can be used iteratively to improve the tomographic result polarization inversion is more sensitive to near station velocity structure and to velocity gradient and is complementary to travel time inversion in this sense the method is applied to california institute of technology u s geological servey southern california array data the result is generally consistent with previous work and also reveals that the high velocity feature beneath the transverse ranges is bounded between 40 and 200 km depths and possibly has a second small piece at about 300 km depth the slow velocity anomaly under the salt on trough is limited to shallow depths less than about 60 km

mis 21 and the mid pleistocene climate transition climate and sea level variation from a sediment core in osaka bay japan abstract we report climate and sea level variation for the marine oxygen isotope stage mis 21 encompassing the end of the mid pleistocene climate transition mpt based on pollen diatom and sulfur records from a 50 m thick sequence in a core from osaka bay an extremely warm climate coincided with the sea level highstand of substage 21 5 when the warm temperate element quercus cyclobalanopsis exceeds 40 of total arboreal pollen this was followed by a warm temperate to temperate and humid climate that continued until the end of mis 21 a linear age model shows that climate was dominated by precessional cyclicity with an inverse correlation between temperature and precipitation the postglacial sea level rise reached its highest peak in substage 21 5 when paleo osaka bay reached its maximum extent including kyoto and nara basins at this time pelagic diatoms were dominant in the central part of the bay sea level dropped below the osaka bay sill about 60 m at present during substage 21 4 followed by a rise above the sill in substage 21 3 and a drop at 21 2 sea level remained below the sill during substage 21 1 the thermal maximum and sea level peak occurred just after the rapid postglacial sea level rise after which there was a gradual decline in temperature and sea level accompanied by precession related oscillations these features are typical of the post mpt interglacials dominated by 100 ka cyclicity these features may be a sign of termination of the mpt

zircon upb ages of guyana greenstone gneiss terrane abstract isotopic u pb studies of zircons collected from weathered metagreywackes of the barama mazaruni supergroup of northern guyana south america demonstrate an age of origin of ca 2250 ma this is the best estimate for the age of the associated metavolcanic rocks zircons from weathered gneiss of the bartica complex adjacent to the volcanic sedimentary belts yield a similar age the contiguous greenstone gneiss terrane of eastern venezuela is also of similar age and comparable greenstone gneiss terranes of eastern suriname and french guiana are probably also of this age continental crust formation of a style closely comparable to that of the canadian archean occurred on a very widespread scale in the lower proterozoic of the guiana shield the lead losses from the weathered zircons are comparable to those from zircons from fresh rock from the adjacent terrane of venezuela and the advantages of field concentration from numerous saprolite exposures warrant use of such material in future geochronological studies of the region

the santa barbara california earthquake of 13 august 1978 the 5 1 m l santa barbara earthquake of 13 august 1978 occurred at 22h54m 52 8s utc the epicenter was located 3 km southeast of santa barbara at 34 23 9 n latitude and 119 40 9 w longitude with a focal depth of 12 7 km the main shock was followed between 13 august and 30 september by 373 aftershocks that were located with the caltech usgs array the aftershock zone extended 12 km wnw from the epicenter and was 6 km wide in the n s direction and it had a very clear temporal development during the first 20 min of activity all the aftershocks were located in a cluster 7 km wnw of the main shock epicenter during the next 24 hr the aftershock zone grew to 11 km in the wnw direction and 4 km in the n s direction during succeeding weeks the zone extended to 12 by 6 km this temporal spatial development relative to the main shock epicenter may indicate that the initial rupture propagated 7 km unilaterally to the wnw and the initial rupture plane may have been considerably smaller than the eventual aftershock zone this smaller area suggests that the stress drop may have been significantly greater than that derived from the final aftershock zone in cross section the aftershock hypocenters outline a nearly horizontal plane dipping 15 or less at 13 km depth the main shock focal mechanism indicates nne ssw compression and vertical extension the preferred fault plane strikes n80 w and dips 26 nne indicating north over south thrusting with a component of left lateral movement focal mechanisms for 40 aftershocks also indicate compression in the general n s direction for most of these events the north dipping nodal plane dips between 7 and 45 with most dipping 25 or more which is significantly steeper than the plane delineated by the hypocenters themselves these observations are consistent with a tectonic model in which much of the slip during the santa barbara earthquake occurred on a nearly horizontal plane the after shocks then might represent movement on a complex series of imbricate thrust faults that flatten into the plane of primary slip hence the santa barbara earthquake may be taken as evidence for mid crustal horizontal shearing in the western transverse ranges

mobilization and redistribution of rees and thorium in a syenitic lateritic profile a mass balance study abstract ree th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at akongo in sw cameroon ree and th mass balance calculations for the host rock minerals show that at least 70 of the lree s and 50 of the hrees are contained in allanite apatite titanite and epidote and at least 50 of the th is controlled by the same accessory minerals which represent about 2 wt of the unaltered syenite these accessory phases are destroyed during the first stages of weathering causing most of the rees and th to be rapidly released into the soil comparison of the variation in the zr ti and th content as a function of the apparent density of the different zones of the saprolite shows that th is the least mobile element the presence of secondary thorianite tho2 the etched surface on zircon grains and the presence of ti in secondary cerianite support this geochemical interpretation the concentration of thorium was therefore chosen as invariant relative to the concentration of the other elements especially the rees in mass balance calculations most of the rees are leached in the iron rich upper horizons loose nodular horizon iron crust and top of mottled clay horizon where the groundwater table moves saprolite and bottom of the mottled clay horizon the rees are fractionated and redistributed there is a juxtaposition of leached and accumulation zones with precipitation of lree aluminous hydrated phosphates this study supports the existence of two different cycles for the redistribution of elements in the soil 1 1 as dissolved ions in the saprolite horizon and 2 2 as individual particles in the upper part of the profile

did stress triggering cause the large off fault aftershocks of the 25 march 1998 mw 8 1 antarctic plate earthquake the 1998 antarctic plate earthquake produced clusters of aftershocks mw 6 4 up to 80 km from the fault rupture and up to 100 km beyond the end of the rupture because the mainshock occurred far from the nearest plate boundary and the nearest recorded earthquake it is unusually isolated from the stress perturbations caused by other earthquakes making it a good candidate for stress transfer analysis despite the absence of near field observations we tested whether the off fault aftershocks lie in regions brought closer to coulomb failure by the main rupture we evaluated four published source models for the main rupture in fourteen tests using different aftershocks sets and allowing the rupture sources to be shifted within their uncertainties 6 were significant at 99 confidence 3 at 95 confidence and 5 were not significant 95 level for the 9 successful tests the stress at the site of the aftershocks was typically increased by 1 2 bars 0 1 0 2 mpa thus the antarctic plate event together with the 1992 mw 7 3 landers and its mw 6 5 big bear aftershock 40 km from the main fault supply evidence that small stress changes might indeed trigger large earthquakes far from the main fault rupture

supergene alteration since the upper cretaceous on alkaline igneous and metasomatic rocks of the pocos de caldas ring complex minas gerais brazil abstract this contribution presents field evidence mineralogical observations and geochemical data of different modes of lateritic weathering which affected the alkaline magmatic rocks and their metasomatic alteration products since the upper cretaceous field relations demonstrate that ferralitic bauxitic weathering is confined to well drained hill crest and upper slope settings in the pocos de caldas area bauxite normally rests directly on parent rock stagnant drainage and reducing conditions beneath tropical peat deposits along valley floors and in lower slope situations favour sialitic saprolitic weathering the additional importance of high porosity and permeability is demonstrated by co occurring parent rocks with strongly contrasting response on weathering attack in normal nepheline syenites and phonolites nepheline breaks down first and nearly instantly upon weathering this results in high porosity giving water and atmospheric gases easy access to the more resistant minerals thus in well drained morphological positions bauxite is generated if the same original magmatic parent rocks are shielded against pervasive weathering attack by pre lateritic metasomatic alteration no comparable early permeability arises a fine grained and dense metasomatic assemblage of kaolinite illite microcline and smectite combined with the annealing of fractures and joints is able to prevent bauxite generation and saprolite is formed instead this metasomatically induced saprolite is significantly richer in illite than normal saprolite bauxite formation results in a strong depletion of mg ca na k si ni co cu zn rb ba sr y and ree sialitic saprolitic weathering leads to a generally less pronounced depletion of essentially the same elements silicon and ni are more efficiently retained than in bauxite mobile elements are mn ca na k mg co zn rb ba sr and ree cerium can be re precipitated under oxidizing local conditions closely associated positive and negative ce anomalies are due to inhomogenously concentrated precipitates in small scale spatial alternation with zones of nearly total depletion a general correlation between the type of laterite saprolite or bauxite and ce retainment or depletion does not exist host phases for most of the immobile trace elements are goethite and related plasmas in bauxites and saprolites immobility is exhibited by v pb ga nb zr th and u relict zircon is too rare to explain a seemingly retarded hree loss in saprolites

influence of static stress changes on earthquake locations in southern california earthquakes induce changes in static stress on neighbouring faults that may delay hasten or even trigger subsequent earthquakes1 10 the length of time over which such effects persist has a bearing on the potential contribution of stress analyses to earthquake hazard assessment but is presently unknown here we use an elastic half space model11 to estimate the static stress changes generated by damaging magnitude m 5 earthquakes in southern california over the past 26 years and to investigate the influence of these changes on subsequent earthquake activity we find that in the 1 5 year period following a m 5 earthquake any subsequent nearby m 5 earthquake almost always ruptures a fault that is loaded towards failure by the first earthquake after this period damaging earthquakes are equally likely to rupture loaded and relaxed faults our results suggest that there is a short period of time following a damaging earthquake in southern california in which simple coulomb failure stress models could be used to identify regions of increased seismic hazard

fault slip rates and earthquake histories for active faults in southern california abstract within the context of the historical record of seismicity we review the geological data bearing on the quaternary slip rates and paleoearthquake histories of active faults in southern california

lithostratigraphy geochronology and gold metallogeny in the northern guiana shield south america a review abstract with a surface area of nearly 900 000 km 2 the guiana shield represents the northern segment of the amazonian craton in south america lying for the most part between the amazon and orinoco river basins most of the guiana shield formed during protracted periods of intense magmatism metamorphism and deformation culminating with the trans amazonian tectono thermal event bracketed between 2 1 and 1 9 ga the guiana shield is among the least known precambrian terranes because it is relatively inaccessible lacks bedrock exposure due to intense weathering and is poorly documented in the international geological literature this situation has significantly improved during the last 20 years when shallow in situ gold occurrences attracted exploration and mining companies to initiate geological programs aimed at better understanding the geology and the mineral deposits of the shield the only archean terrane ca 3400 ma known to date in the guiana shield is the imataca complex in venezuela the paleoproterozoic low grade volcano sedimentary greenstone sequences and associated granitoid intrusions have yielded ages between 2 25 and 2 08 ga recent u pb age determinations of the granitoid greenstone belts suggest protracted magmatic cycles from pre to post peak regional metamorphism the younger terranes comprise anorogenic sedimentary sequences of the roraima formation as well as felsic volcanic rocks and associated intrusions of the uatuma formation mafic dikes of the avanavero suite and rapakivi type and alkali intrusions several large scale ductile shear zones have been documented in the guiana shield in northcentral venezuela the most outstanding structure documented to date the ne sw trending guri fault juxtaposes the archean imataca complex against paleoproterozoic terranes the central guiana shear zone cgsz extends from french guiana westerly towards central suriname and further west towards northcentral guyana where it matches with the makapa kuribrong shear zone mksz in french guiana the north guiana trough ngt is interpreted as a sinistral strike slip formed during the trans amazonian orogeny most gold deposits and occurrences discovered to date in the guiana shield are sited in close proximity to major structures in addition they are linked with low to medium metamorphic grade granitoid greenstone belts similar to other better explored precambrian terranes at a local scale the gold deposits are hosted within or in close proximity to quartz veins that are syn to late tectonic and to a lesser extent in stockworks breccias and lenses they are commonly located in units that behaved in a more brittle manner than the country rocks available information suggests that gold deposits are mainly epigenetic although some are associated with specific lithostratigraphic units pyrite pyrrhotite chalcopyrite galena sphalerite scheelite molybdenite and tellurides are the main metallic minerals associated with gold non metallic minerals are mainly quartz and carbonates ankerite calcite siderite associated with minor chlorite epidote albite muscovite and fuchsite silica carbonate propylitic and potassic alteration is common high erosion rates expected after the creation of an orogenic belt did not occur in the northern guiana shield shallow level deposits preserved in many settings suggest that the granitoid greenstone belts represent first order exploration targets for large tonnage low grade gold deposits

chemical signatures of palaeotsunamis a forgotten proxy article i nfo article history a range of diagnostic criteria are required to help identify palaeotsunami deposits although chemical signatures have long been used as indicators of palaeosalinity in sedimentary sequences their use in the studies of palaeotsunami deposits is rarely applied and is yet to be widely recognised the chemical composition of interstitial water in coastal ponds and sedimentary sequences in coastal marshes and lagoons has been used successfullyasanindicatoroftsunamiinundation chemicalsignaturescanalsoprovidecluesaboutthelandward limit of runup of a tsunami beyond the area of sediment deposition like most palaeotsunami proxies however the preservation of chemical signatures varies with environmental conditions

san andreas fault zone california m 5 5 earthquake history the san andreas fault zone has been a very significant source of major california earthquakes from 1812 to 1906 it generated four major earthquakes of m 7 or larger in two pairs on two major portions of the fault a pair of major earthquakes occurred on the central to southern region where the 1857 faulting overlapped the 1812 earthquake faulting a pair of major earthquakes occurred on the northern region where the 1906 faulting overlapped the 1838 earthquake faulting also earthquakes of m 7 occurred in the san francisco bay area on the hayward fault in 1868 and the santa cruz mountains near loma prieta in 1989 and on the imperial fault near the border with mexico in 1940 the 1838 earthquake s damage effects throughout the bay area from san francisco to santa clara valley and monterey were unequalled by any historical earthquake other than the 1906 event this and numerous strong possible aftershocks during the following 3 years in the san juan bautista vicinity suggest 1838 faulting from san francisco to san juan bautista cycles of seismicity and quiescence were associated with the bay area earthquakes of 1868 1906 and 1989 the 1868 earthquake on the hayward fault was preceded by 12 earthquakes of m 5 5 from 1855 to 1866 within 60 km of the hayward fault and was followed by 13 quiet years the 1906 san andreas fault event was preceded from 1881 to 1903 by 18 earthquakes of m 5 5 and was followed by quiescence with only three earthquakes of m 5 5 until 1954 the bay area has been seismically quiet at the m 5 5 level since the 1989 loma prieta earthquake and its 1990 aftershocks which contrasts with the 10 years before 1989 when five m 5 5 6 2 events occurred the loma prieta earthquake is of similar magnitude to the 1868 hayward event and could be followed by a similarly short quiet period the 1857 earthquake had immediate foreshocks in the lonoak bitterwater region 50 km northwest of parkfield in the northern end zone of the 1857 rupture extending southeast from bitterwater 70 km to parkfield the rate of seismic moment release has decreased with time since 1857 this may reflect the decay with time of the stress loading due to the 9 m 1857 fault displacements 80 km southeast of parkfield and explain why the predicted earthquake which was based on the assumption of regular recurrence of parkfield earthquakes has not yet occurred the extent of the 1812 earthquake fault rupture is not well defined jacoby et al 1988 estimated that it extended 170 km from cajon pass to tejon pass based on this estimate we present the hypothesis that the rupture occurred in two segments in december 1812 the eastern segment generated the 8 december earthquake that damaged san juan capistrano san gabriel san fernando and san buenaventura thirteen days later the western segment ruptured generating the earthquake that damaged san fernando and san buenaventura again as well as santa barbara santa ynez and purisima concepcion

cerium anomalies in lateritic profiles the ree geochemistry and mineralogy have been studied in four lateritic profiles one derived from a syenite akongo sw cameroon the others being developed on a gneissic basement and located along a soil toposequence goyoum e cameroon there is a fractionation between lree and hree in the lateritic samples during weathering the weathered residual products being enriched in lree from la to eu and depleted in hree from gd to lu sampled waters are enriched in hree in relation to the syenite host rock a positive ce anomaly has been found systematically at the top of the saprolite beneath a zone of iron oxide accumulation in the laterite up to 2000 ppm ce may be present in the akongo profile cerianite ceo2 is present as very fine coatings in non ferruginous clayey domains primary ree bearing accessory minerals are weathered at the bottom of the profile specifically allanite and apatite are transformed into florencite and rhabdophane but these phases have no ce anomaly all the data are interpreted as the result of the following processes 1 1 ree leaching in a reducing environment 2 2 oxidation of ce3 to ce4 in an oxidizing environment 3 3 deposition of cerium as cerianite whereas the other ree remain in solution

nd and sr isotope study of hydrothermal scheelite and host rocks at omai guiana shield implications for ore fluid source and flow path during the formation of orogenic gold deposits omai is a high tonnage low grade world class gold deposit located in the paleoproterozoic guiana shield it is the second most important gold deposit in the guiana shield after las cristinas venezuela and one of the largest in south america 4 0 million oz sm nd and sr isotope data are presented for host rocks and for scheelite from auriferous quartz carbonate scheelite sulfide telluride veins from the omai deposit gold bearing veins are hosted by the paleoproterozoic barama mazaruni supergroup a greenstone belt sequence consisting of mafic volcanic rocks interbedded with sedimentary rocks that are intruded by quartz feldspar porphyry and rhyolite dikes this lithologic sequence was folded and metamorphosed to lower greenschist facies during the paleoproterozoic trans amazonian orogeny the volcano sedimentary unit was intruded by a post tectonic quartz monzodiorite diorite hornblendite stock initial nd isotope ratios for the omai volcanic rocks range from nd 2 1 to 4 2 these values suggest that this part of the guiana shield was a site of new crust formation during the paleoproterozoic and was not contaminated by older archean reworked continental crust initial nd isotope ratios for the omai stock range between 0 5 and 2 3 which suggest limited contamination with previously formed continental crust although the nd isotopic ratios of gold related scheelites overlap with those of the host rocks particularly the tholeiitic basalts at the interpreted time of vein emplacement the lack of both isotopic mixing and significant nd movement during the hydrothermal process suggest that the nd isotope composition can be used to determine the isotopic characteristics of the ore fluid source area at omai the ore fluid is largely derived from a radiogenic nd source represented by mantle or lower crustal reservoirs strontium isotope ratios for the scheelites cluster tightly between 0 7019 and 0 7021 the sr isotope data suggest that unlike nd sr was significantly mobile during the hydrothermal process the fluids responsible for the omai deposit may have picked up sr along the flow path the constant low sr isotope values of scheelites probably reflect the key role that the local tholeiitic basalts played as the main source of sr in the fluids whereas nd isotopes identify the fluid source area the sr isotopes map the fluid flow paths

quaternary stratigraphy and trace element indices of the yangtze delta eastern china with special reference to marine transgressions the quaternary stratigraphy of the yangtze delta has been revealed by a deep 344 64 m continuous borehole the sediments of which are divided into four sections the early q1 middle q2 and late pleistocene q3 and the holocene q4 six lithofacies include lacustrine fluvial coastal drowned river channel deltaic and shallow marine in these time periods the fluvial and lacustrine lithofacies prevailed during q1and q2 coastal and drowned river channel lithofacies periodically appeared in q1 q2 and q3 shallow marine lithofacies dominated q3 and deltaic lithofacies occurred in q4 based on stratigraphy four transgressive events are inferred trace element sr ba b ga v content and ratios of sr ba and b ga are considered with regard to their distribution in the various lithofacies geochemical facies criteria are established based on combined diagnostic indices sr 160 ppm b 90 ppm sr ba 0 35 b ga 4 0 for the shallow marine lithofacies sr 90 ppm b 50 ppm sr ba 0 2 b ga 2 5 for the terrigenous lithofacies these indices correspond well to paleosalinity data obtained using the sedimentary phosphate method ca ca fe transgressive events identified by the geochemical indices are mostly the same as reflected by marine fossils nevertheless geochemical indices can indicate the occurrence of a weak transgression where marine fossils are rare

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

over 100 years of faults interaction stress accumulation and creeping implications on chaman fault system pakistan the curve shaped chaman transform fault between pakistan and afghanistan demarks the western boundary of the indian plate which is the locus of many catastrophic earthquakes and this is poorly studied region to understand the earthquake fault interaction and hazard assessment based on earthquake triggering theory we investigate a sequence of earthquakes comprising five earthquakes that occurred in the chaman fault system since 1892 our results elucidate that three out of four earthquakes are triggered by the preceding earthquakes the 1935 earthquake of magnitude mw 7 7 is somewhat an independent earthquake in this sequence slightly influenced by the 1892 event the 1935 earthquake significantly loaded either terminus of its rupture with positive stress which later triggered the 1975 earthquake with 35 of its rupture length where cfs is above triggering threshold value and 1990 earthquake with 44 of its rupture length in positive stress lobe on ghazaband fault in the south lobe since the 1935 earthquake significantly increased the cfs on either terminus of its rupture the 2013 earthquake with magnitude mw 7 7 occurred on hoshab fault in the positive stress zone with 23 of its rupture length where cfs is positive moreover the northern locked segment of cf the southern segment of gbf southeastern segment of hf and paf demonstrate a high value of cfs it could trigger seismicity in these regions thus requiring arrangements to mitigate any seismic hazards in future

dynamic versus static stress triggering and friction parameters inferences from the november 23 1980 irpinia earthquake this paper concentrates on the problem of fault interaction and earthquake triggering through the 1980 irpinia italy sequence more specifically this paper deals with the problem of the triggering of the second subevent by the mainshock the interaction between the two segments is modeled through a dynamic coulomb failure function the aims of this paper are first to discriminate between the dynamic and the static stress effects on the triggering if these effects exist second to estimate the fault strength relative to the initial state of stress third to determine the parameters of a slip dependent friction law that lead to the observed delay of 20 s numerical simulations show that the critical slip dc may range from 0 03 m up to 1 7 m and that the initial slope of the friction law 0 must be lower than 0 04 m 1 we show that the relative magnitude of the fault strength and the initial state of stress govern the existence and value of a dc lower threshold under which the fault always ruptures before 13 s a close to failure fault is not consistent with a critical slip dc less than 0 8 m whereas small values of dc typically 0 03 m imply a far from failure fault general results concern the effect of a dynamic stress pulse we show that an event can be triggered by a transient stress pulse and that in this case the event can have an initiation duration much longer than the pulse duration we show that it is possible to explain both the triggering and the time delay only with the effect of the transient stress pulse this may explain aftershock triggering even in regions of negative coulomb failure function or long distance triggering of earthquakes by propagating waves

structural mineralogical and geochemical studies of the paleoproterozoic omai gold deposit guyana the granitoid and greenstone hosted omai gold deposit is located in the paleoproterozoic barama mazaruni greenstone belt guiana shield at regional scale the emplacement of the deposit was controlled by the makapa kuribrong crustal shear zone at local scale the gold bearing quartz veins are associated mainly with a quartz monzodioritic intrusion omai stock fennell pit and subvolcanic quartz feldspar porphyry and rhyolite dikes and subordinately with intermediate volcanic flows and metasedimentary rocks wenot pit six gold bearing undeformed subhorizontal and subvertical vein sets can be distinguished in the two ore zones on the basis of their internal structures and textures the veins can be classified as crack and seal laminated breccia and open space filling veins the geometrical and textural relationships between the vein sets suggest that they are broadly contemporaneous the formation of most veins can be summarized by two filling stages and a late fracture filling stage related to a protracted hydrothermal process although the metallic minerals represent less than 1 percent of the vein volume their mineralogy is complex and consists of various sulfides together with tungstates native elements tellurides and sulfosalts the metallic paragenesis is defined by the au ag te w bi pb zn cu hg mo assemblage the nonopaque gangue includes mainly quart and subordinately carbonates albite sericite chlorite tourmaline rutile and epidote wall rocks of gold bearing veins are affected by synmineralization alteration the dominant alteration assemblages include carbonates sericite silica chlorite albite epidote argillic minerals pyrite and pyrrhotite there is an obvious association between the alteration assemblages the vein internal textures and the chemical nature of the host rocks vein forming scheelite has 87 sr 86 sr ratios between 0 7019 and 0 7021 and delta 18 o values between 3 8 and 4 3 per mil which suggest both consistent temperature and isotopic composition of the hydrothermal solutions during its deposition oxygen isotopes measured in vein quartz vary between 13 2 and 14 per mil similar to the delta 18 o values of carbonates avg 13 8ppm for calcite and 14 4ppm for ankerite the carbon isotopes of carbonates range between 1 7 and 4 7 per mil the delta 18 o values of the mineralizing fluids vary between 5 6 and 2 7 per mil and the delta d values between 52 and 18 per mil the isotopic composition of the hydrothermal fluids plots outside both magmatic and metamorphic water boxes therefore suggesting a significant component of surface derived water the omai deposit is late orogenic and its emplacement was controlled by the last brittle to brittle ductile stages of the trans amazonian orogeny it can be considered as a paleoproterozoic equivalent of the archean epizonal orogenic deposits described in the yilgarn wiluna racetrack and zimbabwe shamwa commoner cratons

the co seismic coulomb stress change and expected seismicity rate caused by 14 april 2010 ms 7 1 yushu china earthquake a moderate and widely felt earthquake of ms7 1 mw6 9 occurred on april 14 2010 in yushu district of qinghai province china which destroyed a large number of buildings and killed more than two thousand people the distribution of the aftershock sequence is sparse but with relatively large magnitudes ms6 3 and ms5 7 aftershocks were recorded strong aftershocks are still highly potential to occur in the near future therefore it is critical to delineate areas where potential aftershocks could occur based on static stress triggering theory we calculated the coseismic stress changes induced by the mainshock in yushu county and its surrounding regions by adopting elastic dislocation theory and a multilayered crustal model according to the rate and state variable friction law we calculated the expected seismicity rate and probability of occurrences of m 5 0 earthquake in the next ten years it is observed that the coulomb stress changes increase apparently on the segment of the wudaoliang qumalai fault the batang luoxu and dangjiang longbao segments of the ganzi yushu fault and the whole wulanwula lake yushu fault based on the theory of coulomb stress triggering the probability of earthquake occurrence on these faults would increase in the near future considering the historical seismicity the strain accumulation and the coulomb stress changes the probability of m 5 earthquake and m 6 earthquake occurrence in the entire study area in the next 10 years are 79 87 and 26 7 33 6 respectively actually a moderate earthquake of ms5 3 occurred to the northwest of the nangqian county on june 26 2011

modeling climate shift of el nino variability in the holocene a coupled ocean atmosphere general circulation model is used to investigate climatic shift of el nino in the holocene the model simulates a reduced enso intensity in the early and mid holocene in agreement with paleoclimate record the enso reduction is proposed to be caused by both an intensified asian summer monsoon and a warm water subduction from the south pacific into the equatorial thermocline

related spatio temporal changes in hypocenters and the b value in the 2017 kagoshima bay swarm activity indicating a rise of hot fluids abstract spatio temporal characteristics of the b value in relation to changes in hypocentral distribution and the occurrence rate of swarm earthquakes in the 2017 kagoshima bay activity were investigated noting stepwise changes in the area as well as the occurrence rate of swarm events we divided the activity from december 2016 to october 2017 into six stages characteristics of the activity in each stage were clearly grasped when swarm area was divided into several rectangular zones in a nnw sse direction and when hypocenters and the b value in each zone were examined in detail after the largest shock with a magnitude m of 5 3 on july 11 2017 the swarm area expanded conspicuously however the dominant factor that played a leading role in the development of swarm activity was not the occurrence of the m 5 3 shock but activity in the zone zone b just to the west of the focal region the number of events in zone b was more than two times the whole number of events that occurred in the entire swarm area except for zone b the hypocentral distribution in zone b was relatively shallow from early stages but the most notable feature was the abrupt diffusion of events to a shallower layer at the time of the m 4 4 shock on august 24 2017 the b value in the central part of zone b was noticeably high from earlier stages but remarkably high for the shallow events of the burst like activity after the m 4 4 shock from these spatio temporal characteristics of hypocenters and the b value we propose that hot fluids that had existed beneath the central part of zone b ascended at the time of the m 4 4 shock moreover hot fluids were the major cause of the 2017 kagoshima bay swarm activity

geochemical signatures up to the maximum inundation of the 2011 tohoku oki tsunami implications for the 869 ad jogan and other palaeotsunamis abstract the geochemical signature of the tohoku oki tsunami deposit and underlying soil was assessed two months five months and seven months after the 11 march 2011 tsunami inundated the sendai plain the extent of the recognisable sand deposit was traced up to 2 9 km inland while a mud deposit was found up to 4 65 km inland representing 60 and nearly 95 of the maximum tsunami inundation respectively the limit of tsunami inundation was identified 4 85 km from the shore using geochemical marine markers s and cl two months after the tsunami in the absence of any sedimentological evidence concentrations of other geochemical markers k ca sr indicative of the marine incursion and associated minerals were found to decrease landward 13 c and 15 n and c n ratios suggested a mixture of terrestrial and marine organic sources in the sediment while 34 s of sulphate reflected the marine source of water soluble salts the chemical composition of the 869 ad jogan tsunami sand deposit was characterised by high sr and rb concentrations and was comparable to that of the tohoku oki tsunami deposit suggesting that the sources of sediment may be similar marked decreases in s and cl with time indicated that rainfall resulted in the leaching of salts from the sandy sediments however both s and cl markers as well as sr were still well preserved in the muddy sediments and underlying soil beyond the limit of the recognisable sand deposit seven months after the tsunami this suggests that geochemical indicators may well be useful in identifying the extent of historical and palaeotsunamis by determining the marine origin of fine grained sediments beyond the limit of recognisable sand deposition in particular when marine microfossils are sparse or lacking as is the case on the sendai plain this would allow researchers to redraw palaeotsunami inundation maps and re assess the magnitude of events such as the jogan tsunami and other palaeotsunamis not only on the sendai plain but also elsewhere around the world this has important implications for tsunami risk assessment hazard mitigation and preparedness

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

mechanism for the enrichment of organic matter in the liushagang formation of the weixinan sag beibuwan basin china abstract eocene lacustrine organic rich shales mudstones and shales in the middle member of liushagang formation designated as ls2 are the most important source rocks in the beibuwan basin located in the northwest of the south china sea in this study we explored the mechanisms responsible for the accumulation of organic matter in source rocks from ls2 of weixinan sag by investigating the paleoclimate paleoredox paleoproductivity and lithological characteristics of the parent rocks using biomarkers and elemental geochemistry data based on our analytical data the deposition of the source rocks from ls2 occurred in a warm humid paleoclimate with moderate chemical weathering as indicated by cia indexes and sr cu ratios the sr ba ratios and gammacerane indexes indicated that the source rocks from ls2 were deposited in a freshwater paleolake the corg p ratios and redox sensitive elements rses toc bivariate diagrams demonstrated that the organic rich shales and mudstones shales deposited under anoxic and oxic suboxic conditions respectively the levels of toc copper cu molybdenum mo and c30 4 methylsterance index in the organic rich shales were higher than those of the mudstones and shales indicating greater algae bloom and paleoproductivity during the deposition of the organic rich shales compared to during the sedimentation of the mudstones and shales combined with la yb ree sio2 tio2 al2o3 tio2 nb ta and cr co ratios it is considered that the parent rocks of the organic rich shales were granite of relatively intermediate lithological compositions with greater levels of nutrients such as phosphorus p mo cu compared to those of the mudstones and shales large scale decomposition of dead algae led to oxygen depletion which created favorable conditions for the preservation of organic matter and the release of p from the sediments to water the increased content of p in water subsequently aggravated algal blooms such a positive feedback loop was conducive to the formation of organic rich shales the inter relationships of algal blooms controlled by p paleoproductivity and paleoredox conditions play an important role in the accumulation of organic matter in organic rich shales from ls2 of weixinan sag beibuwan basin

changes in state of stress on the southern san andreas fault resulting from the california earthquake sequence of april to june 1992 the april to june 1992 landers earthquake sequence in southern california modified the state of stress along nearby segments of the san andreas fault causing a 50 kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near san gorgonio pass the decrease in compressive normal stress may also have reduced fluid pressures along that fault segment as pressures are reequilibrated by diffusion that fault segment should move closer to failure with time that fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years

the geneses of sedimentary organic matter with anomalous 13c enriched isotopic composition in saline and freshwater lakes a case study of lacustrine source rocks from dongpu and qikou sags bohai bay basin eastern china abstract the carbon isotopic composition of sedimentary organic matter 13com is notably important for the genetic identification of source rock and its products the sapropelic organic matter mostly plankton with high hydrogen index is generally depleted in 13c as compared to humic organic matter mainly higher plants with lower hydrogen index however some lacustrine sediments contain sapropelic source rocks with anomalous 13c enriched isotopic ratios especially reported in freshwater lakes the purpose of this study is to discuss the geneses of lacustrine sedimentary organic matter characterized by anomalous 13c enriched ratios in saline and freshwater sediments to this end we integrate the detailed organic geochemical data including rock eval analysis data total organic carbon contents total organic carbon isotopic compositions biomarkers and trace element concentrations of 64 saline lake sediments and 39 freshwater lake sediments obtained from the paleogene shahejie formation of dongpu sag and qikou sag separately in bohai bay basin of eastern china both sags exist samples with high hydrogen index values hi 400 mg g and high total organic carbon contents toc 2 but anomalous 13c enriched isotopic ratios up to 21 1 in qikou sag the 13com values of sapropelic kerogen correlate positively with toc contents the anomalous enrichment of 13c in organic matter is associated with extremely high productivity of algae such as algae bloom in dongpu sag the enrichment of 13c in organic matter is attributed to the significant increase of drought and salinity high salinity and aridity climate may lead to the abundance of c4 plants and chlorobium as well as the enrichment of 13c in plankton

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

weathering of gneissic rocks in the upper reaches of cauvery river south india implications to neotectonics of the region in the upper reaches of the cauvery catchment area in southern india the archean peninsular gneisses exhibit a greater degree of weathering in the field relative to the massive charnockites and granites although the gneisses have been physically broken down along foliation planes fractures and compositional discontinuities secondary mineral such as chlorite smectite kaolinite and fe oxyhydroxide formation is insignificant consequently there is little chemical change even up to the stage of saprolite development only in the regolith there is some loss of ca na sr ba mg and sio2 other elements such as fe mn al cr ni and ree all show enrichment relative to tio2 in areas which have developed profiles by in situ weathering the chondrite normalized ree patterns however show little change excepting a slight flattening of the patterns because of greater hree mobility during weathering clay minerals also have flattened ree patterns because of relative hree enrichment the depletion and the enrichment of different elements appear to be related to mild leaching of primary mafic minerals by meteoric water rather than by mineral breakdown this and the extent of variation in chemical index of alteration values suggest that that the gneisses have suffered only incipient chemical weathering for the extent of physical weathering undergone by them it is likely that this contrasting weathering extents of these 2500 ma rocks is due to their exposure to surface geological processes only recently because of ongoing periodic uplift and physical erosion of this region

comparison of laser grain size analysis with pipette and sieve analysis a solution for the underestimation of the clay fraction classically the grain size of soil and sediment samples is determined by the sieve method for the coarse fractions and by the pipette method based on the stokes sedimentation rates for the fine fractions results from the two methods are compared with results from laser diffraction size analysis which is based on the forward scattering of monochromatic coherent light from a point of view of laboratory efficiency the laser sizing technique is far superior accuracy and reproducibility are shown by measurements on certified materials it appears that laser grain size measurements of certified materials correspond very well with the certificated measurements tests were also done on a set of randomly selected sediments of fluvial aeolian and lacustrine origin except for the 2 m clay fraction there is a coarsening of the mean diameter of one to two size classes 0 25 caused by the non sphericity of the particles the platy form of the clay particles induces considerable differences eight size classes between pipette and laser measurements the 2 m grain size defined by the pipette method corresponds with a grain size of 8 m defined by the laser particle sizer for the studied sediments using a higher grain size level for the clay fraction when laser analysis is applied enables workers in the geological and environmental field to compare classical pipette analysis with a laser sizing technique

quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment mobility of gold throughout the global weathering system of dondo mobi south gabon has been quantatively addressed by calculating the gold mass transfer during lateritic weathering of auriferous lisvenite archean gneiss and proterozoic schist and by proposing a thermodynamic approach to gold solubility under equatorial rainforest acidic conditions both chemical and short distance translocation processes govern the mobility of gold and gold particles during the long term evolution of collapsed open weathering systems which result from the combined interaction of rock meteoric water and biological agents free gold particles are translocated from the lisvenite derived soil towards formerly barren archean gneiss and porous proterozoic formations derived from schist to form a 200 m wide supergene gold dispersion halo which took shape over about 0 86 1 26 million years simultaneously with physical translocation of gold dissolution of gold makes the total balance of gold transfer through the whole weathering system strongly negative which is the converse of what is generally believed to occur in lateritic systems taking into account hydrolysis reactions three main gold complexes are proposed as a function of corresponding inorganic and organic ligand concentrations these are au oh h o aucioh and au oh fa these gold complexes leave the dondo mobi weathering mantle and move towards the river system in the course of both physical and chemical processes involved in the weathering of the formation

analysis of supergene ore forming processes and ground water solute transport using mass balance principles analysis of metal distribution in weathering profiles developed above ore deposits provides a simple and direct method to determine the geologic controls on surficial leaching and enrichment of metals by supergene descending fluids this approach offers an understanding of the interrelationships of metal concentration zone thicknesses and bulk rock density of the various zones within a weathered rock column analytical solutions to mass balance equations provide expressions describing the chemical evolution paths of enrichment blanket metal grade b in terms of original protore metal grade p leached zone metal grade 1 and the ratio of total leached zone column height to blanket thickness l o t b neglecting density terms for the sake of simplicity this function is expressed as b p l o t b p 1 linear data arrays in the coordinate system b versus p 1 for groups of drill holes in individual geologic domains e g wall rock types strongly suggest that enrichment blanket metal grades evolve from a protore metal grade p with a fairly constant slope of l o t b to a final value b determined by the extent of metal leaching in the zone of oxidation the constancy of the ratio l o t b within geologic domains indicates a simultaneous deepening of the zone of oxidation and thickening of the enrichment blanket under conditions of a descending ground water table analysis of metal mass balance in drilling profiles also provides a means of reconstructing paleosurficial topography at the time of active oxidation even in highly eroded terrains application of these methods using rock density and assay data from the butte district of montana shows the paleo oxidation surface to have been eroded by as much as 450 ft down to the present unconformity between weathered bedrock and overlying tertiary gravels geomorphic stability and slow erosion rates often thought necessary for effective chemical weathering are therefore questioned we propose instead a general balance of rates of erosion and descent of the ground water table for optimal enrichment preservation of the fossil weathering profiles occurs by submergence of the zone of oxidation below the paleoground water table resulting in cessation of supergene oxidation and preservation of the blanket the mass balance methods derived in this study also relate overall leaching efficiency to protore characteristics showing a strong hypogene alteration control at butte as described by mcclave 1973 lateral copper transport is evaluated by identifying local anomalies in the computed oxidation surface zones of copper introduction near faults and zones of high permeability appear as apparent topographic highs whereas zones of depletion appear as topographic depressions preliminary analysis of this type at the porphyry copper deposit at la escondida chile indicates deeper oxidation and leaching effects than at butte the assumption of vertically homogeneous protore copper grades leads to the conclusion that erosion of at least 200 m of leached capping has occurred throughout much of the la escondida district geologic domains defined by computed values of l o t b correlate well with hypogene alteration zoning computation of the magnitude and direction of overall subsurface lateral metal fluxes indicates transport distances of at least 1 km in directions consistent with present surface topography and fracture patterns there is a strong correlation of the thickest and highest grade enrichment zones with inferred positive lateral fluxes whereas the most thoroughly leached cap rocks make up zones of inferred negative lateral flux which are likely source regions alternative explanations to the lateral flux hypothesis are evaluated including assessment of variations in protore metal grade projections both in vertical and horizontal directions despite the approximations involved in the present calculations the lateral flux pattern appears to be the most likely interpretation of the present metal distribution in the supergene system at la escondida

geochemical evolution of a palaeolaterite the interbasaltic formation northern ireland abstract the interbasaltic laterites of northern ireland were formed during a period of relative volcanic inactivity by extensive chemical weathering of tertiary basalts they reach a maximum thickness of 30 m and once provided a major source of iron and aluminium ore an extensive database comprising major minor and trace elements has been compiled for 240 samples in order to study the effects of weathering in terms of the changes in whole rock chemistry and mineralogy from basalt through to iron rich crust percolating waters caused degradation of the parent basalt mineralogy and precipitation of neoformed phases principally through incongruent dissolution processes primary olivine plagioclase feldspar and augite were successively broken down and replaced by a mineral assemblage consisting of hematite gibbsite goethite anatase meta halloysite and kaolinite changes in mineralogy facilitated concomitant changes in element concentrations mass balance calculations indicate that all elements were depleted in the iron crust enrichment of al loi cr cu and v occurred in the laterite horizon while enrichment of al loi ba ce cr cu ni and rb occurred in the lithomarge notably yttrium was found to be mobile indicating that weathered basalts should not be used in discrimination of original tectonic environments the severe leaching conditions evidenced by yttrium depletion local aluminium redistribution and iron crust formation are indicative of weathering under a humid sub tropical monsoon climate

holocene sea level highstand and its implications for the vertical stability of panglao island southwest bohol philippines abstract emerged tidal notches in panglao island southwest bohol central philippines were used to investigate relative sea level changes during the holocene the elevation and profile of a laterally continuous notch were surveyed at 22 sites around panglao island the tidal notch documents higher than present mean sea level pmsl consistent with regional field observations and predictions for mid holocene time the reconstructed paleo shoreline elevations range from 0 to 1 6 m above pmsl exhibiting a general increase to the northeast the more protected part of the coast this lateral variation in the holocene paleo mean sea level msl along with changes in notch depth and profile suggests tilting of the island best explained by differential land movement last interglacial msl estimates by previous workers also show tilting to the southwest moreover the holocene notch data suggest segmentation of the island into several blocks that tilt consistently to the southwest suggesting the presence of faults lineations in aerial photographs trending northeast southwest confirm the presence of these faults these data indicate differential land movements during the holocene and late pleistocene calculated rates of uplift are 0 3 and 0 06 mm yr for the holocene and for the late pleistocene respectively these values relative to the high uplift rates from tectonically active regions imply that panglao is relatively vertically stable after accounting for tectonic tilting mid holocene sea level is estimated at approximately 0 3 0 6 m

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

triggering of 1812 santa barbara earthquake by a great san andreas shock implications for future seismic hazards in southern california we study the evolution of the stress field over the last 200 years in southern california using the stress buildup associated with major faults and stress drops associated with great earthquakes in this report we calculate the change in the coulomb failure function cff associated with the great wrightwood earthquake of dec 8 1812 on the san andreas fault for the region near santa barbara that experienced a large shock 13 days later the rupture length constrained by paleoseismic data is used for the wrightwood earthquake of 1812 a uniform displacement of 3 5 m from the surface to seismogenic depths is assumed the geologic setting and focal mechanisms of earthquakes in the santa barbara region are dominated by thrust left lateral strike slip faulting and combinations of the two for our preferred rupture model of the earthquake of dec 8 the santa barbara shock of dec 21 1812 occurred in a region of 0 01 to 0 1 mpa cff for reasonable fault orientations indicating that it may have been advanced by years to decades and triggered by the great san andreas earthquake 13 days earlier we also calculate cff on typical kinds of faults in southern california for a future candidate great earthquake along the san bernardino and coachella valley segments of san andreas fault this earthquake produces large positive values of cff for shallow dipping thrust faults beneath the greater los angeles area and for nw trending right lateral strike slip faults in three regions this suggests that shocks of magnitude similar to the m 7 1 1812 santa barbara earthquake could be triggered by the next great event on the southern san andreas fault

neotectonics of the marikina valley fault system mvfs and tectonic framework of structures in northern and central luzon philippines abstract recognition of neotectonic features along the marikina valley fault system mvfs in central luzon philippines indicates a dominantly dextral strike slip motion during its most recent activity believed to be late pleistocene to holocene in age variations in the ratios of vertical to horizontal displacements for the segments imply a dominantly dextral motion of the west marikina valley fault wmvf and oblique dextral motion for the east marikina valley fault emvf the displacement data further suggest that rupturing along the emvf involved multiple segments and occurred separately from the events along the wmvf segments estimated earthquake magnitudes for the wmvf and emvf based on single event offsets fall within the range m 7 3 7 7 the vertical slip component in the northern part of the marikina valley is associated with the development of a basin between the emvf and wmvf while the large vertical component in the southernmost segment of the emvf talim is attributed to volcanism related extension lateral advection of the block bounded by the mvfs and the philippine fault zone pfz rather than pure shear resulting from an assumed east west compression best explains the observed kinematics of the mvfs this is the result of compression during the westward drift of the philippine sea plate and northern luzon and occurs through slip along the wmvf and emvf at rates of 5 7 mm yr

palaeoenvironmental reconstruction of lacustrine source rocks in the lower 1st member of the shahejie formation in the raoyang sag and the baxian sag bohai bay basin eastern china abstract the lower 1st member es1l of the oligocene shahejie formation is the most important source rock interval in the raoyang sag and the baxian sag to reveal the controlling factors that affected the distribution of organic rich source rocks we analyzed the lower es1 es1l samples collected from the raoyang sag and the baxian sag using sedimentological and geochemical techniques to reconstruct the lacustrine palaeoenvironmental conditions most of the samples had chemical index of alteration cia values between 70 and 75 which reflect a stable and moderately weathered source under a transitional climate the distribution of depocenters and palaeo water depths indicated that the main catchment area was concentrated in the raoyang sag with a maximum palaeo water depth of 14 m and a wave base depth of 7 5 m stable isotope data from carbonates 18ocarb and 13ccarb showed that the lakes with long residence times in the raoyang sag had a positive correlation and that the lakes with short residence times in the baxian sag had no isotopic covariance during the deposition of the lower es1 the p al and p ti ratios ptotal and carbon isotope values of the organic matter 13corg in the two sags illustrated stable palaeoproductivity without much fluctuation the enrichments of mo u and ratios of c p org b ga and sr ba further indicated saline and anoxic bottom water conditions associated with stable water column stratification in well xi63 and brackish and less anoxic bottom water conditions in well nd1 during the deposition of the main source rock of the lower es1 moreover the plots of mo and u enrichment versus toc and hydrogen index values versus 13c i e the difference between the 13c values of carbonates and organic matter corroborated the inference that the preservation of organic matter was the main factor controlling the formation of organic rich source rocks in the raoyang sag and the baxian sag during the deposition of the lower es1

u pb and pb pb age constraints on paleoproterozoic magmatism deformation and gold mineralization in the omai area guyana shield abstract the omai intrusion centred au quartz vein system the largest au producer presently operating in the guyana shield was sampled for detailed u pb and pb pb geochronology and petrological investigation the age of a metavolcanic sub volcanic unit in the host rock sequence is 2120 2 ma zircon analyses from the main body dioritic rocks give u pb ages of 2094 6 2092 11 and 2096 11 10 ma magmatic titanite and apatite that grew in hornblende rich peripheral phases of the intrusion define a consistent pb pb age of 2094 1 ma in agreement with the zircon data colourless titanite and rutile from strongly altered phases of the intrusion along with low u apatite and feldspar define a significantly younger pb pb isochron age of 2002 5 ma the igneous ages agree with data from similar units in the guyana shield and west africa showing that 2100 ma was a time of significant intrusive activity the ages obtained for the deformed metavolcanic and undeformed intrusion at omai define a 26 2 ma bracket for trans amazonian deformation in central guyana previous fluid inclusion studies indicate that the mineralizing solutions at omai were too co 2 rich to form titanite and the titanite bearing sample is unmineralized suggesting that it was not altered by gold bearing solutions therefore the 2002 5 ma age is interpreted as a late hydrothermal overprint that formed titanite and reset rutile zircon and baddeleyite from a thick gabbro dyke of the avanavero suite which cuts the omai pluton define an age of 1794 4 ma ruling out the dyke as a source for the late thermal effects the hydrothermal age may record the passage of fluids released by deep crustal metamorphism due to late stage tectonic underplating as previously proposed for the superior province

rare earth element mobility during conversion of nepheline syenite into lateritic bauxite at passa quatro minais gerais brazil abstract in a lateritic bauxite formed by weathering of nepheline syenite at passa quatro minas gerais state brazil bauxites on the hill tops directly develop from the syenite bed rock while downslope a kaolinitic layer occurs between bauxite and synetie a petrological investigation was performed on undisturbed weathered rock samples collected from a representative upslope pit the undisturbed weathered rocks were chemically analysed for major trace elements including ree and zr mass balance calculations were applied and the behaviour of the ree in the passa quatro weathering system was established compared to ree reference chondrite and to ree reference parent rock in the lateritic bauxite the results suggest that the first stages of weathering induce a volumetric change of 50 i e collapse with respect to the parent rock and remove ree with a slightly larger loss of the lree except ce compared to the hree in the upper layers where bauxite is more mature a net mass gain in ree is observed relative to the underlying layers this gain takes place during the reduction of the upper layer during the downward progression of the weathering front very significant ree losses occurs during the bauxitization processes throughout the upslope profile in addition the downslope kaolinitic system is demonstrated to be depleted in ree in the same proportions as the upslope bauxite we proposed that the ree exported in solution from the whole weathering mantle have enriched neighbouring watershed sediments

tropical chemical weathering of hillslope deposits and bedrock source in the aburra valley northern colombian andes abstract tropical chemical weathering produces extensive lateritization and formation of deep weathering profiles both processes are fundamental to landscape evolution and slope instability the aburra valley of the northern colombian andes is characterized by tropical conditions the valley slopes are mostly covered by hillslope deposits originating from four basement rock suites which comprise contrasting granitoid volcanic sedimentary ophiolitic and metamorphic sources respectively tropical chemical weathering of the aburra hillslope deposits and their respective bedrock were examined using x ray fluorescence and x ray diffraction analysis to document and quantify their chemical weathering profiles compositions and mineralogical properties the chemical index of alteration cia loss on ignition loi and the mobiles index imob were used to quantify the degree of weathering of hillslope deposits and bedrock source weathering trends were analyzed using a cn k and a cnk fm diagrams the material mantling the slopes in the aburra valley records an intense weathering history chemical weathering is characterized by increased development of clay minerals kaolinite halloysite and iron and aluminum sesquioxides lateritization characterizes the final stage of the weathering profiles concentrations of cao na2o k2o decrease markedly in the weathering products compared to the fresh bedrock source whereas concentrations of al2o3 fe2o3 and mgo increase significantly cia ratios of matrix slope deposits derived from all four sources near 100 whereas those of boulder slope deposits and saprolites are lower but exceed source rock values different a cn k weathering paths are evident for each lithotype validating the correlation established between the hillslope deposits and their various parents chemical weathering indices in some samples are strongly influenced by the presence of sesquioxides as reflected by high loi anomalously low cia and varying enrichment trends on the a cnk fm diagrams consequently different chemical indices based on different criteria need to be combined to obtain best results as illustrated here by the combination of loi cia and imob the overall results suggest that tropical conditions have dominated for a long time in the northern colombian andes leading to uniformly high weathering indices in matrix slope deposits irrespective of parent lithotype prolonged warm and humid conditions could thus be responsible for the weathering and remobilization of extensive old hillslope deposits during the quaternary however in addition to the influence of climatic factors tectonism has also undoubtedly influenced slope evolution in the aburra valley

evolution of the stress field in southern california and triggering of moderate size earthquakes a 200 year perspective changes in stress in southern california are modeled from 1812 to 2025 using as input 1 stress drops associated with six large 7 0 7 5 earthquakes through 1995 and 2 stress buildup associated with major faults with slip rates 3 mm yr as constrained by geodetic paleoseismic and seismic measurements evolution of stress and the triggering of moderate to large earthquakes are treated in a tensoffal rather than a scalar manner we present snapshots of the cumulative coulomb failure function acff as a function of time for faults of various strike dip and rake throughout southern california we take acff to be zero everywhere just prior to the great shock of 1812 we find that about 95 of those well located m 6 earthquakes whose mechanisms involve either strike slip or reverse faulting are consistent with the coulomb stress evolutionary model that is they occurred in areas of positive acff the interaction between slow moving faults and stresses generated by faster moving faults significantly advanced the occurrence of the 1933 long beach and 1992 landers events in their earthquake cycles coulomb stresses near major thrust faults of the western and central transverse ranges have been accumulating for a long time future great earthquakes along the san andreas fault especially if the san bernardino and coachella valley segments rupture together can trigger moderate to large earthquakes in the transverse ranges as appears to have happened in the santa barbara earthquake that occurred 13 days after the great san andreas shock of 1812 maps of current acff provide additional guides to long term earthquake prediction

mantle derived guyanaite in a cr omphacitite xenolith from moses rock diatreme utah guyanaite naturally occurring crooh has been identified in a xenolith of cr rich omphacitite from the moses rock diatreme in the navajo volcanic field of the southwestern united states it occurs as the dominant phase in small clusters of accessory minerals intergrown with kosmochlor rich omphacite zincian chromite eskolaite and carmichaelite the assemblage is interpreted as the result of metasomatism of chromite bearing serpentinite by slab derived fluids during subduction of the farallon plate in laramide time at the time of entrainment of the xenolith the rock was undergoing prograde metamorphism with guyanaite dehydrating to eskolaite plus water this reaction and the coeval dehydration of the inferred accompanying host serpentinites which would have been much more volumetrically significant provided water for hydration of the subcontinental upper mantle contributing to uplift of the colorado plateau recognition of guyanaite as a component of a subducted slab supports recent proposals based on laboratory experiments that high pressure polymorphs of other common crustal oxy hydroxide minerals such as boehmite and goethite i e high pressure alooh and feooh can also transport and store water in the mantle

evolution of moderate seismicity in the san francisco bay region 1850 to 1993 seismicity changes related to the occurrence of large and great earthquakes the rate of seismic activity of moderate size m 5 5 earthquakes in the san francisco bay sfb region has varied considerably during the past 150 years as measured by the rate of seismic moment release seismic activity in the sfb region is observed to accelerate prior to m 7 0 earthquakes in 1868 1906 and 1989 and then to decelerate following them we examine these seismicity changes in the context of the evolution of the stress field in the sfb region as a result of strain accumulation and release using a model of dislocations in an elastic halfspace we use a coulomb failure function cff to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the sfb region we find that the occurrence of a large or great earthquake creates a stress shadow a region where the stress driving earthquake deformation is decreased interseismic strain accumulation acts to reverse this process gradually bringing faults in the sfb region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failure is possible as the stress shadow generated by a large or great earthquake disappears it migrates inward toward the fault associated with that large or great event the observed changes in the rate of occurrence of moderate earthquakes in the sfb region are broadly consistent with this model in detail the decrease in seismicity throughout most of the sfb region and a localized increase in the monterey bay region following the great 1906 earthquake is consistent with our predicted stress changes the timing and location of moderate size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by strain accumulation in the sfb region those earthquakes that are most inconsistent with our stress evolution model including the 1911 earthquake southeast of san jose are found to occur in regions where dip slip faulting is common in addition to strike slip the 1906 earthquake brought that zone of dip slip faulting closer to failure suggesting that the 1911 event may have been a reverse faulting earthquake rather than a strike slip one similar to the 1984 morgan hill earthquake the occurrence of activity on faults very close to the san andreas such as the lake elsman earthquakes of 1988 and 1989 appear to be associated with the last disappearence of the stress shadow on the loma prieta segment of the san andreas fault thus events of that type may represent an intermediate term precursor to a large earthquake such as the 1989 loma prieta event much of the moderate size earthquake activity in the sfb region appears to be modulated in time by the buildup and release of stress in large and great earthquakes a tensorial approach to earthquake prediction i e taking into account changes in the components of the stress tensor has several advantages over examining scalar changes such as those in seismic activity and moment release rates this tensorial approach allows for both activation and quiescence but in different subregions prior to as well as after large earthquakes

changes in frequency of moderate size earthquakes and coulomb failure stress before and after the landers california earthquake of 1992 changes in the frequency of moderate size events before and after the 28 june 1992 landers earthquake are investigated and their implications are discussed in the context of coulomb failure stress cfs evolution since 1812 in southern california we systematically considered circular regions and equal area annuli centered on the epicenter of the landers earthquake frequency magnitude relationships for two 10 year periods before and two 5 year periods around the landers event are compared only events with magnitude m 4 0 are included aftershocks are removed for the larger circular regions with radii of 140 to 160 km the rate and slope of the frequency magnitude distribution for moderate size events just before the mainshock appear to be anomalous compared to those for either the preceding or subsequent periods for areas closer to the 1992 epicenter however the number of events is few and the differences in the distributions are less obvious when we examined the seismic activity in annuli of equal area however the largest changes occurred about 150 km from the epicenter of the mainshock not closer as would be expected for a precursor to the landers event we also derive an index value to better quantify differences in the frequency of occurrence of moderate size events as a function of time the index value and the frequency magnitude distribution show similar spatial dependence since 1812 a large region near landers has moved closer to failure in terms of changes in cfs for faults of san andreas type these changes however are dominated by coseismic changes associated with the 1812 and 1857 earthquakes and by tectonic stress buildup related to the san andreas fault not by stress buildup associated with the landers faults themselves which are characterized by very slow long term displacements hence the most pronounced changes in the frequency of moderate size earthquakes before 1992 do not appear to be related to stress buildup to the landers sequence itself they along with the landers sequence may be indicative of a broad region that is approaching a high stress state prior to an eventual future great earthquake the failure to find a pronounced increase in moderate size shocks close in to landers is in accord with the idea that such increases on a timescale of years to decades are associated with the regional buildup of stress to large earthquake along faults of high not low long term slip rates

higher than present holocene mean sea levels in ilocos palawan and samar philippines abstract holocene paleo mean sea levels along the coasts of ilocos palawan and samar in the philippines are established using estimates from emerged tidal notches and coral reef terraces age control is provided by 17 14 c and 22 230 th 234 u dates the notches and terraces with the oldest dating back to approximately 8 2 ky have elevations from 0 1 to 6 7 m above the present mean sea level in each of the study sites adjacent coastal areas show different paleosea level histories in both the level and number of relative stillstands due to local faulting in ilocos paleosea levels show greater elevation and number of recorded stillstands due to a significant thrust component along this segment of the philippine fault in samar a general increase in the elevation of paleosea level to the east indicates tilting of the island towards the philippine fault abrupt changes in the elevations of correlative paleosea level indicators and an overall northeast and southwest tilting respectively of northern and southern palawan suggest the presence of active faults in central palawan discrete double notches suggest two emergent stillstands occurring between 7 3 and 2 4 ky the lower and higher notches respectively correspond to earlier and later stillstand events possible across site variation in the duration and timing of initiation and termination of these stillstands can be due to differences in regional tectonics and crustal response to hydro isostasy or an artifact of sampling

coulomb stress evolution along xianshuihe xiaojiang fault system since 1713 and its interaction with wenchuan earthquake may 12 2008 abstract the curved left lateral strike slip xianshuihe xiaojiang fault system xxfs in southwestern china extends at least 1400 km in the eastern margin of the tibetan plateau fieldworks confirm that the xxfs is one of the longest and most seismically active faults in china the strain released by the slip motion on the xxfs is related to the convergence between the indian and eurasian plates the entire fault system has experienced at least 35 earthquakes of m 6 in the recent 300 years and almost all segments of the system have been the locus of major historical earthquakes since the xxfs region is heavily populated over 50 million people understanding the migration of the large earthquakes in space and time is of crucial importance for the seismic hazard assessment in this region we analyze a sequence of 25 earthquakes m 6 5 that occurred along the xxfs since 1713 and investigate their influence on the 2008 mw7 9 wenchuan earthquake occurred on the adjacent longmenshan fault in our analysis the relevant parameters for the earth crust are constrained by seismic studies the locations and geometries of the earthquake faults as well as the rupture distributions are taken from field observations and seismological studies results from the coulomb failure stress modeling indicate significant interactions among the earthquakes after the 1713 earthquake 19 out of 24 earthquakes occurred in the positive stress zone of the preceding earthquakes the other 5 earthquakes located in the area without significant stress changes induced by the preceding events in particular we can identify 4 visible earthquake gaps with increasing seismic hazard along the xxfs consistent with the findings from the paleo seismological studies the seismic activity and tectonic motion on the xxfs reduced the coulomb stress accumulation at the hypocenter of 2008 mw7 9 wenchuan earthquake implying that the wenchuan earthquake might not be triggered directly by the seismic activities on the xxfs on the other hand the coulomb failure stress induced by the wenchuan earthquake has increased in a region of 125 km long segment of the xxfs northwest of kangding city

active faulting and growth folding in the eastern santa barbara channel california we develop new methods to identify blind thrust fault systems determine fault slip rates and estimate potential earthquake magnitudes and recurrence intervals in active fold and thrust belts these methods are applied to compressive folds along the offshore oak ridge and blue bottle trends which overlie active blind thrust faults in the eastern santa barbara channel these folds and their causative faults are interpreted using fault bend fold theory and are represented in balanced models and cross sections that integrate surface and subsurface data the structures are mapped using a new technique of axial surface mapping in seismic reflection grids which defines three dimensional structural geometries and shows changes in slip and subsurface fault geometry along strike analysis of syntectonic growth sediments yields pliocene and quaternary fault slip rates of 1 3 mm yr on a deep thrust 16 km and 1 3 mm yr on shallower faults 2 5 km the combined 2 6 mm yr slip rate represents only part of the 6 mm yr of shortening measured by geodesy across the channel and estimated from relative pacific north american plate motions across the transverse ranges additional shortening is probably accommodated on other active thrusts in the western transverse ranges and in the northern channel along the santa barbara coast deformed seafloor sediments and a swarm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake hazards to coastal southern california unsegmented fault surfaces are used through empirical relationships between fault surface area and rupture magnitude to estimate the sizes of potential earthquakes this analysis suggests that a ramp in the channel islands fault beneath the offshore oak ridge trend is capable of rupturing in a m s 7 2 earthquake earthquakes of this magnitude may release 2 m of slip which when combined with the estimated slip rate 1 3 mm yr yields an earthquake recurrence interval of 1500 yr for this channel islands fault ramp

post glacial evolution of the indo pacific warm pool and el nino southern oscillation recent research has revealed new insights into the temperature size and variability of the indo pacific warm pool ipwp and the nature of the el nino southern oscillation enso since the last glacial maximum lgm sea surface temperature sst reconstructions from foraminiferal mg ca alkenone and revised coral sr ca palaeothermometry agree that ssts in the ipwp during the lgm were similar to 3degreesc cooler than at present in the central portion of the ipwf the rapid post glacial rise in sst led the deglaciation by similar to 3000 years to produce near modern ssts by the early holocene in contrast further west and north post glacial shifts in ssts in the south china and sulu seas are synchronous with abrupt climate changes in the north atlantic new evidence for the nature of the little ice age in the tropics has been obtained from a 420 year record of coral sr ca and 6180 from the great barrier reef australia this indicates that ssts and salinity were higher in the 18th century than in the 20th century the results suggest that the tropical pacific played a role as a source region of water vapour during the global expansion of little ice age glaciers the onset of modern enso periodicities is identified by palaeo enso records throughout the tropical pacific region similar to 5000 years ago with an abrupt increase in enso magnitude similar to 3000 years ago individual enso events recorded by corals reveal that the precipitation response to el nino temperature anomalies was subdued in the mid holocene the apparent non linear onset of enso in the late holocene appears to reflect abruptly enhanced interaction between the southern oscillation and the pacific intertropical convergence zone comparisons of precipitation variability recorded by great barrier reef corals with enso indices for the last 350 years confirms that non stationarity of enso teleconnections is a natural characteristic of modern climate c 2003 elsevier ltd and inqua all rights reserved

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

rare earth element geochemistry and the tetrad effect abstract the tetrad effect is found in the rare earth elements la lu and results from increased stability at quarter half three quarter and complete filling of the 4f electron shell suggestions that this effect is observed in ree patterns of seawater and other geological samples marine phases the mineral kimuraite late stage granitic rocks the mineral uraninite and siliciclastic sediments are not compelling in the cases of seawater and the carbonate mineral kimuraite a discontinuity at gd may be attributed to the tetrad effect but the complexity of the overall controls on these ree distributions leaves such an interpretation uncertain at best unusual ree patterns in leucogranites and pegmatites are best explained by fractionation of trace minerals notably apatite apparent tetrad anomalies in uraninite rees disappear when normalised to upper crustal i e shale abundances analytical difficulties probably explain apparent tetrad effects in some seawater limestone and siliciclastic ree patterns

evaluating weathering of palaeosols in cameroon central africa as a tool for paleoenvironmental reconstruction abstract palaeosols formed from different igneous rocks have been identified and characterized in the humid savannas 1400 mm mean annual rainfall 22 c mean annual temperature of the adamaoua region in north central cameroon by means of macro and micromorphology geochemical analyses and bulk mineralogy the studied sequence at mangoli is about 11 m deep and is divided into three sections in terms of weathering products 1 precambrian granite located in the base 2 vesicular porphyritic basalt in the middle section 3 aphanitic basalt in the uppermost layer the products of weathering from the precambrian granite consist of the following horizons from bottom to top a saprolite with granite structure a yellowish to whitish fine saprolite and btg and bt horizons the saprolite horizon is dominated by quartz feldspars plagioclases and microcline and muscovite primary minerals except quartz show weathering while clay fills the pores and fractures and kaolinite dominates the clay mineralogy the granite weathering products are slightly acidic ph ranging from 5 9 to 6 3 and display low values of fe extracted with dithionite soluble fe fed and high chemical index of alteration cia and mineral index of alteration mia values the middle section of the sequence consists of different areas of the basalt s spheroidal weathering zone from the more advanced to the less weathered phase the clay mineralogy is dominated by kaolinite the zones with the most alteration are the fractures where thick reddish clay coatings are observable in these areas the minerals are well crystallized the kaolinite crystals share areas with fe oxides the upper section of the sequence constitutes the modern soil from the aphanitic basalt and consists of a spheroidal weathering zone divided into three horizons the lowest is a c horizon overlaid by bt horizons the c horizon shows a high degree of weathering in some areas but it is still possible to identify the primary minerals their products of weathering are slightly acidic ph 6 and have high fed cia and mia values which are indicative of intense weathering the results suggest warm and humid climates during the formation of all of the palaeosols similar to modern environmental conditions we conclude that palaeosol 1 formed from the granite represents a soil formed during the mid miocene climatic optimum when the temperatures are assumed to have been 3 6 c warmer than today features discovered during field investigations reveal that the former valley at the surface of the landscape has been filled by lava flows and today occupies the top of the plateau meanwhile the former top of the plateau is located on the slopes and lower altitudes the burial of soils by volcanic materials at mangoli increased the altitudes of the former plateau and resulted in landscape inversion slopes these palaeosols and their top layers that induced the slope inversion have not yet been described and appear as a unique example worldwide