the 1904 ms 7 3 earthquake in central alaska abstract on 27 august 1904 seismic stations from around the globe recorded an m 7 earthquake originating from central alaska very little was known about this earthquake one felt report from rampart alaska had been attributed to the notes of harry fielding reid yet its original source was unknown here we present five felt reports for the 1904 earthquake that show evidence of felt shaking across most of central alaska using the 1904 arrival time data we estimate an epicentral location near lake minchumina at the northeastern extent of the iditarod nixon fault our preferred fault for the 1904 earthquake is the right lateral iditarod nixon fault which though relatively seismically quiet generated an m 6 2 earthquake in 1935 paleoseismic investigations are needed to search for evidence of fault activity including the 1904 earthquake rupture in the tectonically complex region of the 1904 earthquake electronic supplement tables of arrival time figures of station registers visualization of nonlinloc nll solution for the 1904 alaska earthquake distribution of depths of the posterior probability for the 1904 and 1935 events epicenter and samples of the posterior probability distribution for the 1904 and 1935 earthquakes map of southward shift of epicenters and estimated epicenters for the 3 february 2000 kaltag earthquake

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

conodont color and textural alteration an index to regional metamorphism contact metamorphism and hydrothermal alteration experimental and field data are used to extend the utility of conodonts as semi quantitative thermal indices into the regimes of regional and contact metamorphism as well as hydrothermal alteration an arrhenius plot of data from induced conodont color alteration by pyrolysis in air at 1 atm was used to generate the geologic temperatures for conodont color alteration indices cai above 300 c that is for cai values of 5 through 8 such cai values occur in very low to medium grade regionally metamorphosed contact metamorphosed and hydrothermally altered rocks the uniformity or variability of cai values within a sample together with conodont texture can help to distinguish grades and environments of metamorphism particularly in metacarbonate sequences induced cai by pyrolysis in a water methane mixture at kbar results in retardation of cai and in a disparate mixture of both low and high cai values within each experimental sample in this system color alteration processes above a cai of 2 to 3 seem to change from predominantly carbonization to predominantly loss of organic matter presumably by oxidation and volatilization of oxides these experiments approximate the type of cai mixture characteristically found in conodonts recovered from hydrothermally altered rocks these data indicate that cai values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization

geothermal palaeogradients and metamorphic zonation from the conodont colour alteration index cai the conodont colour alteration index cal is potentially does not exist there are areas where cal and ki values are valuable in thermal history investigations but there are notably consistent for the anchizone boundaries however problems in its use an approach is proposed herein to anomalous correlations between cal and ki values have also overcome the problem of establishing reliable geothermal been found in some areas a cal anchizone or ancaizone is palaeogradients in areas with burial metamorphism based on defined in order to enable cal values to be used independently knowledge of the stratigraphic succession thickness the to establish a metamorphic zonation boundaries of chronostratigraphic units and the age and duration of deformation episodes although also problematical terra nova 13 79 83 2001 that a correlation between cal and kubler index ki values

seismicity of the unalaska region alaska the wadati benioff zone beneath unalaska island in the eastern aleutian arc is shown to dip at approximately 48 between the depths of 50 and 150 km above this depth the seismic zone appears to dip at a more shallow angle 12 the depth of the wadati benioff zone beneath makushin volcano on unalaska island is about 100 km events located within the upper plate near the wadati benioff zone indicate that the maximum depth of seismic coupling between the subducting and overriding plates is about 30 km this suggests that the maximum width of seismic coupling between the downgoing and overriding plates is about 75 km earthquake locations based on the local network data and 20 yr of teleseismic data pde define similar seismically active and quiescent regions a major seismicity gap exists for events of magnitudes greater than at least 4 6 in the forearc region near unalaska island this seismicity gap correlates well with the eastern edge of the tsunami source area of the great mw 8 6 1957 earthquake and a large forearc basin 75 km south of unalaska island suggesting a similar process may be controlling the location of all three phenomena only one intermediate sized mb 6 1 earthquake has occurred within the seismicity gap during the past 20 yr this event is shown to be a high stress drop 690 bars intraplate event within the subducting slab at a depth of 35 km that may be related to lateral compressive stress induced by a change in slab curvature with depth a review of the historic record from about 1770 reveals that the unalaska region may have ruptured in a great earthquake in 1878 and a large earthquake in 1902 if this region is currently locked an earthquake of magnitude 8 2 could rupture this section of the alaska aleutian arc estimates of the recurrence interval for events of this size vary from between 40 and 70 yr

geophysical and petrological evidence for the presence of an anomalous upper mantle beneath the sahara basins algeria abstract north of 20 n the northwest part of the stable african shield is characterized by an anomalous high heat flow zone 80 110 mw m 2 this east west elongated zone is oblique to major proterozoic structural units and cannot be related to differences in crustal heat production maximum heat flow occurs in the southern part of the sahara basins 100 120 mw m 2 only a mantle heat flow contribution of 60 70 mw m 2 can account for this very high surface heat flow the estimated geotherms suggest that the temperature of upper mantle is anomalously high and partial melting may be present at shallow depths this hypothesis is supported by the existence of a long wavelength negative component of the gravity field that corresponds with the high heat flow axis and with the very low lithospheric s wave velocities 4 2 4 4 km s down to 160 km obtained from surface wave studies in addition the chemistry of the scarce cenozoic melilitite lavas illizi volcanic district geographically associated with the thermal anomaly as well as the mineralogy of their inclusions phlogopitized garnet spinel peridotite and alkali clinopyroxenite show that the upper mantle beneath this region is locally highly metasomatised and partially melted from these preliminary results it appears that the northwest african upper mantle has been modified by recent thermal rejuvenation processes including gas transfer that are likely to be more extensive in the southern part of the sahara basins

shumagin seismic gap alaska peninsula history of great earthquakes tectonic setting and evidence for high seismic potential the shumagin seismic gap a segment of the plate boundary along the eastern aleutian arc has not ruptured during a great earthquake since at least 1899 1903 because at least 77 years have elapsed since the shumagin gap last ruptured in a great earthquake and repeat times for the 1938 rupture zone and part of the shumagin gap are estimated to be 50 to 90 years a high probability exists for a great earthquake to occur within the shumagin gap during the next one to two decades reconsideration of the rupture zones of the aleutian earthquakes of 1938 1946 and 1948 suggests that those events did not break the interplate boundary beneath the shumagin islands thus the shumagin seismic gap extends from the western end of the 1938 rupture zone to the eastern end of that of 1946 these boundaries also coincide with transverse structural features at least the eastern half of the shumagin gap broke in great earthquakes in 1788 and 1847 and possibly in 1898 1903 the shumagin gap is probably not the result of aseismic slip rather plate motion is accommodated there seismically and episodically and can be expected to produce large earthquakes in the future although there is no definitive evidence of long term precursors of a possible future earthquake several observations suggest that the shumagin gap is in an advanced stage of the earthquake cycle both teleseismic and local network data indicate a near absence of seismic activity m 2 above a depth of 30 km along the main thrust zone within the gap this is in strong contrast to adjacent portions of the arc where seismic activity is scattered across most of the main thrust zone two earthquakes with high stress drops 600 900 bars which occurred at the base of the main thrust zone may indicate the accumulation of a considerable amount of strain energy within the gap a possible seismic gap at the eastern end of the aftershock zone of the aleutian earthquake of 1957 has been identified near unalaska island an earthquake that nucleates in the shumagin gap could also rupture the possible unalaska gap to the west the 1938 aftershock zone to the east or both with resultant magnitude up to mw 9 0 alternatively the shumagin gap alone or in one of the above combinations could rupture in a series of very large earthquakes instead of a single great shock past alaska aleutian earthquakes including those of 1788 1938 1946 1957 1964 and 1965 have generated very large tsunamis future large earthquakes in the shumagin islands region could generate wave heights of several tens of meters along shorelines near the rupture areas the shumagin gap is one of two major gaps along the united states portion of the alaska aleutian plate boundary and is one of the few areas in the united states where processes leading to a great earthquake are likely to be observed within a reasonable span of time

thermal maturity of carbonaceous material in conodonts and the color alteration index independently identifying maximum temperature with raman spectroscopy abstract the color alteration index cai of conodont specimens is commonly used for identifying the maximum temperature to which units of sedimentary rock particularly carbonates have been heated observable color variations in these fossils are thought to be a result of the thermally induced structural evolution of organic carbonaceous matter cm such temperature history information is extremely valuable for applications in hydrocarbon exploration as well as for constraining other temperature related geological processes in sedimentary systems however the identification of cai depends on the qualitative visual assessment of color which along with a host of other potential complications may yield inaccurate determinations of maximum temperature raman spectroscopy allows thermally induced structural changes in cm to be quantified and it has been used to estimate the thermal maturity in metasedimentary rocks for almost two decades here we use raman spectroscopy of carbonaceous material rscm in conodont specimens and their mississippian to upper triassic host rocks from british columbia canada to estimate maximum temperatures based on the transformation structural reorganization of disordered carbon to graphite this study demonstrates that the maximum temperatures experienced by conodont specimens as calculated from rscm using the iterative fitting of raman spectra ifors technique correlate well with cai but lie outside the suggested ranges in some instances this may be due to complex thermal histories of these conodont specimens or the influence of diagenetic alteration not thermal histories on conodont color we recommend the application of raman spectroscopic analyses of cm in conodonts and their host rocks to obtain more confident accurate and precise estimations of maximum temperature that are independent of cai

characteristics of regional stresses in alaska and neighboring areas earthquake focal mechanisms for a selected group of crustal earthquakes located in alaska and neighboring areas have been compiled from various sources and regional characteristics of the horizontal stress have been deduced for the region from the mechanism solutions the results show that the compressional stress regime of southern coastal areas continues inland through central alaska and extends in a northeast direction the western part of alaska including chukotka and the bering sea adjoining alaska and the aleutian island chain constitute a tensional stress regime

chronology of the central atlantic magmatic province implications for the central atlantic rifting processes and the triassic jurassic biotic crisis abstract the central atlantic magmatic province camp is among the largest igneous provinces on earth emplaced synchronously with or just prior to the triassic jurassic t j boundary ca 200 ma in great part due to the controversial connection between the occurrence of camp and the events of the t j boundary the demand for better constraints on the duration and eruptive chronology of this province has increased more than 100 new 40 ar 39 ar ages have been published in the last 15 years with more than half of these in the last 3 years a careful review and selection of available ages as well as the publication of 16 new ages from the carolinas newark basin usa french guyana and morocco are presented judicious selection yields a total of 58 accepted age determinations for camp magmatism ranging from 202 to 190 ma covering every part of the camp a more complete picture develops with intrusive camp magmatism commencing as early as 202 ma extrusive activity initiated abruptly 200 ma reaching peak volume and intensity around 199 ma on the african margin the main period of camp magmatism is confirmed as brief but is suggested to consist of at least two phases over 1 5 ma with magmatism commencing along the africa north american margins and slightly later along the south american margin two volumetrically minor but distinctive magmatic peaks centered at 195 and 192 ma are mirrored in data from all three continents and highlighted by our statistical approach models describing rifting and thermal input and magma production on these timescales are explored despite significant advances in our understanding of the chronology of camp more data of better quality and broader geographical coverage are needed to completely characterize the evolution of the camp and infer its geodynamic origin in addition lack of a well defined t j boundary age as well as the absence of a relevant basis for comparison between u pb and 40 ar 39 ar data for this time period remain limiting factors to unambiguously linking camp in time with the events of the t j boundary

conodont thermal alteration patterns in palaeozoic rocks from belgium northern france and western germany in this paper thermal patterns based on conodont colour alteration indices cai have been studied in the devonian and carboniferous rocks from belgium northern france and western germany four maps with the exception of the southeastern rheinisches schiefergebirge have been compiled for the investigated area one for the eifelian and givetian a second for the frasnian another for the famennian and one for the tournaisian and visean conodonts have not been found in cambrian rocks from belgium and northern france and as they are scarce in the ordovician to lower devonian formations of this area it was impossible to integrate these cai data into an isograd map in germany in the eastern rheinisches schiefergebirge only a few cai values for the lower devonian strata are available in contrast to the ardennes where in general conodonts show cai values of 3 0 5 0 indices in areas adjacent to the anglo brabant massif e g in the western namur synclinorium and in the eifel hills range between 1 5 and 2 0 slightly higher values occur in the campine basin north of the anglo brabant massif by comparison with cai data from belgium conodont colour alteration indices from the same stratigraphic horizons in some areas of the northern and eastern rheinisches schiefergebirge are up to one index higher in most areas of the rheinisches schiefergebirge the level of metamorphism corresponds to the anchizone regional variations in cai data appear to be related to buried intrusions and associated hydrothermal activity in the southeastern rheinisches schiefergebirge cai data are not completely consistent but it is possible to discern an increase in coalification from the north to the south high index values in the southern rheinisches schiefergebirge are believed to result from regional metamorphism and heating related to numerous intrusive bodies

historical seismicity of peninsular india abstract well documented historical earthquake data from 1340 to 1983 has been analyzed and reported in this work so as to understand the seismicity and associated phenomena of peninsular india it has been the belief among earth scientists that the peninsular shield is aseismic as the region attained stability long ago however the earthquake at koyna 10 december 1967 bhadrachalam 13 april 1969 broach 23 march 1970 hyderabad 30 june 1983 etc altered this concept and it has now widely accepted that seismic activity is still continuing on a mild scale as such a need has arisen to take into consideration historical as well as recent seismological data to study the seismic status of peninsular india using the well known frequency magnitude relation of gutenberg and richter 1954 an attempt has been made to derive a and b and the ratio a b the values as obtained for peninsular india are 4 4 0 85 and 5 17 which are typical of moderately active zones the world over recurrence periods of earthquake magnitude 6 75 and 7 0 are calculated to be 21 and 35 yr respectively further the historical seismicity correlates well with the protocontinents such as dharwar aravalli and singhbhum in the peninsular shield it is observed that seismicity in a broad sense is defined by the protocontinents with low intracontinental seismicity while the lineaments and rift valleys are from zones of moderate seismicity

stress trajectories across the northeast alaska range a single earthquake on the southeast portion of the denali fault in 1981 and two earthquake swarms occurring in 1981 and 1984 on the northern flanks of the alaska range south of fairbanks indicate that the axis of maximum horizontal compressive stress is oriented in a nw se direction in the region this is in accord with earlier findings by nakamura et al 1980 that tectonic trends signal a similar orientation of maximum horizontal compressive stress trajectories here

pan african displaced terranes in the tuareg shield central sahara concepts developed in the recently published model of the air region eastern tuareg shield niger africa as a college of three displaced terranes integrated in a single geodynamic model lead us to propose a terrane map of the tuareg shield 500000 km 2 the 23 terranes recognized have their own lithological metamorphic magmatic and tectonic characteristics and are separated by subvertical strike slip megashear zones that can be traced for hundreds of kilometres or by major thrust fronts some of these boundaries have ophiolitic assemblages or molassic deposits the tuareg shield was shaped and partly accreted during the pan african orogeny 750 550 ma but not as a homogeneous body

temporal spatial reconstruction of the early frasnian late devonian anoxia in nw africa new field data from the ahnet basin algeria anoxic conditions were widespread in nw africa during the early frasnian late devonian that resulted in deposition of organic rich shales and limestones with total organic carbon toc values of up to 14 organic richness and thickness of these sediments vary laterally and organic rich vs organic poor facies boundaries are likely to have been diachronous a precise temporal spatial reconstruction of this anoxic phase in nw africa is complicated because the organic matter in outcrops is largely oxidised and biostratigraphic resolution in boreholes is generally low due to the lack of recoverable conodonts this contribution is based on eight outcrop sections at the margin of the central algerian ahnet basin where detailed spectral gammaray measurements were carried out using a handheld instrument the pre weathering organic richness in frasnian outcrop sections is approximated using the characteristic uranium enrichment in the anoxic facies that based on well studies is positively correlated with the total organic carbon content conodont biostratigraphic results from these sections suggest that the uraniumenriched interval the anoxic interval at the basin margin is most common in the basal frasnian conodont biozones 1 2 confirming previous results from the anti atlas in morocco in three of the eight localities studied the basal frasnian has not been deposited and the frasnian here commences with distinctly younger uranium enriched intervals including zones 4 11 well data from the eastern algerian berkine basin is interpreted to indicate a significantly longer anoxic phase there million yearscale diachroneities of the frasnian anoxia therefore clearly exist across the north africa shelf it is assumed that the palaeorelief might have been a major factor in controlling the onset duration and intensity of anoxia in the region d 2003 elsevier b v all rights reserved

new architectures of deep water carbonate buildups evolution of mud mounds into mud ridges middle devonian algerian sahara spectacular totally exhumed carbonate mud buildups of givetian age can be found in the southern ahnet basin of the central sahara their geometries range from isolated mud mounds to mud ridges up to 8 km long the latter are the result of lateral coalescence of closely spaced individual mounds mounds up to 40 m high and ridges up to 85 m high show original slopes of 25 to 65 frame builders in the massive core of the buildups are restricted to scattered small tabulate and solitary rugose corals the total absence of stromatoporoids colonial rugose corals and algae as well as the lack of debris at the toes of the buildups shows that they were constructed in deep water below the photic zone and storm wave base the arrangement of the buildups in north to northwest trending clusters and ridges suggests that their distribution was controlled by early variscan extensional movements inherited from precambrian patterns

buckle controlled seismogenic faulting in peninsular india as intraplate earthquakes are often not associated with major known faults their location as well as their timing is unpredictable in peninsular india the larger mx5 0 events occur mainly on reverse faults in a series of belts b400 km apart which are aligned roughly normal to the azimuth of convergence between the indian and eurasian plates the location of the belts is controlled largely by the buckling wavelength of the lithosphere and the seismogenic faults do not generate folding and sometimes result from it there is consequently no need to postulate the creation of regularly spaced normal faults in an antecedent extensional phase and the deformation is consistent with a plate driving force such as gravity glide which is unlikely to reverse its polarity and which creates structures that are influenced by plate geometry at the leading edge the thesis is potentially of value to seismic hazard mitigation as it identifies the zones that are most at risk r 2004 published by elsevier ltd

sedimentary evolution of a palaeozoic basin and ridge system the middle and upper devonian of the ahnet and mouydir algerian sahara the ahnet and mouydir regions of southern algeria are part of one of the world s largest almost undeformed exposures of palaeozoic rocks which exemplify a hitherto poorly known early variscan development of a devonian basin and ridge system this area includes a series of intracratonic basins along the northern margin of the west african craton which consists from w to e of the reggane basin azel matti ridge ahnet basin foum belrem ridge and mouydir basin the depositional and palaeogeographic interpretation is based on 71 sections in this region which for the first time were biostratigraphically calibrated by means of conodonts goniatites and brachiopods the structural evolution during devonian times was probably controlled by reactivation of ancient n s to nw se running faults in the precambrian basement which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf a hiatus during emsian times indicates widespread emergence during this interval the entire area was flooded during the earliest eifelian when the first vestiges of the azel matti ridge become evident by stratigraphic condensation the palaeogeographic differentiation is most apparent during the givetian when a shoal with reduced carbonate sedimentation was established on the azel matti ridge passing towards the west and east into basinal environments of the reggane and ahnet basins respectively the foum belrem ridge is distinguished by increased subsidence during the early givetian and by revived uplift during the late givetian in the mouydir basin further east up to 1000 m of shales were deposited during the givetian the early frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites these lithologies occur repeatedly already during the middle devonian and document intermittent anoxic conditions the basin and ridge topography is levelled by the shallowing up sequence of up to 1400 m thick upper frasnian and famennian shales which grade into a deltaic sequence of uppermost famennian tournaisian sandstones the up to now only vaguely discriminated lithostratigraphic formations of the devonian have been biostratigraphically defined in suitable type sections

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

coseismic ground deformation due to an intraplate earthquake using synthetic aperture radar interferometry the mw6 1 killari india earthquake of 29 september 1993 1 the mw6 1 killari earthquake of 29 september 1993 occurred in central india an intraplate area of low historical seismicity resulting in 11 000 fatalities and causing devastation within a 15 km by 16 km region near the epicenter 18 01 n 76 56 e this earthquake occurred on a nearly east west trending 45 south dipping reverse fault and is one of the few intraplate earthquakes to produce a primary surface rupture interferometric analysis of ers 1 and ers 2 synthetic aperture radar data reveals ground deformation attributable to the killari earthquake modeling the earthquake as a dislocation in an elastic half space points to reverse slip on a steep dipping fault though a planar fault does not fit the data well although severe temporal decorrelation effects prevent determination of all details of the ground deformation pattern the analysis establishes that the coseismic deformation occurred over a region with dimensions of approximately 30 km by 20 km and suggests possible subsidence due to soil compaction within the meizoseismal area comparison of interferometric synthetic aperture radar measured to model predicted deformation indicates that the fault ruptured by the killari earthquake increased in steepness from 45 near the ground surface to 70 at a depth of 6 km

revision of magnitudes of large shallow earthquakes 1897 1912 abstract in previous research trace amplitudes of surface wave maxima recorded by undamped milne seismographs were used to determine the surface wave magnitudes ms of large shallow earthquakes which occurred prior to 1912 for this purpose the effective gain of these instruments was calibrated by using the surface wave magnitudes ms gr which were calculated from the unpublished worksheets for seismicity of the earth of gutenberg and richter in this paper the real quality of ms gr is critically re evaluated by using independent sets of data it is found that ms gr for the period 1904 1909 is considerably overestimated the average excess from the real ms is 0 5 units for 1904 1906 0 4 for 1907 0 3 for 1908 1909 and 0 0 for 1910 1912 this overestimation is so systematic and large that the previous results are all redetermined the average effective gain of milne instruments is revised to be 21 9 previously the gain depended on ms this revision results in systematic reduction in the previously assigned magnitudes the revised values of ms for 264 shallow earthquakes with ms 6 8 and over in the period 1897 1912 inclusive are listed the present revision is large enough to preclude the possibility of the high activity of large shallow earthquakes around the turn of the century the present results have a direct effect on all the magnitude catalogues of shallow earthquakes which occurred prior to 1909

thermal and burial history of the cracow silesia region southern poland assessed by conodont cai analysis abstract conodont colour alteration cai data have been applied in the reconstruction of palaeothermal conditions in the cracow silesia region southern poland the analysis of conodont cai values supplemented by electron spin resonance esr data provided a complex maturity pattern which coincides well with the tectonostratigraphical divisions of this region during the variscan and alpine diastrophic cycles the palaeozoic rocks in the upper silesian coal basin attained the observed thermal maturity during late carboniferous or permian time just after the tectonic inversion of the basin vertical maturity trends suggest that palaeo geothermal gradients were significantly higher than the present values the irregular maturity pattern recognized in the cracow myszkow zone is related to the thermal overprint of intrusive igneous rocks on the maturity field resulting from the relatively shallow burial this pattern has not been modified by mesozoic hydrothermal events the low maturity level of devonian and early carboniferous rocks in the substrate of the carpathian foredeep is a product of the subsidence during miocene time the implications for hydrocarbon generation and further exploration are discussed the cai data are consistent with other indicators of organic metamorphism previously used in this region the study highlights also features about the conodont colour alteration such as the influence of silica mobilization on conodont colour which limit the application of the cai method

conodont thermal alteration patterns in devonian and carboniferous rocks of the ahnet and mouydir basins southern algeria abstract conodont colour alteration indices cai have been used to recognize the thermal maturation of the middle devonian to late carboniferous sedimentary infill of the ahnet and mouydir basins in southern algeria the study is based on a large set of conodont samples obtained from 69 sections and outcrops in the ahnet basin the middle and upper devonian rocks at the surface show a uniform thermal maturity level characterized by the cai values of 2 which suggests maximum burial temperatures about 50 c in contrast the carboniferous exposures yield a wide range of cai values from 1 to 3 5 besides the general tendency in maturity decreases upward in the sequence there are anomalies related to heating by small local magmatic bodies in the mouydir basin the cai values in the middle and upper devonian outcrops range from 2 to 3 5 the pattern indicates that the majority of middle devonian rocks have experienced burial temperatures of about 120 c while thermal maturation of the upper devonian strata is clearly lower with maximum heating temperatures about 50 c at the edges of the basin the observed thermal maturity has been mostly achieved during late carboniferous time prior to the variscan inversion and only locally during the mesozoic it is a product of loading by carboniferous overburden succession of which part is now removed approx 200 1300 m in the ahnet basin and up to 1200 m in the mouydir basin the analysis of palaeotemperatures suggests an average geothermal gradient of 30 c km during carboniferous time in the ahnet mouydir area in terms of hydrocarbon exploration the studied devonian succession is predominantly mature for oil and gas in contrary the carboniferous rocks are immature higher maturities are likely to occur in the subsurface

crustal and upper mantle velocity structure of the hoggar swell central sahara algeria abstract the hoggar region is known as one of the most important swells in the african continent its altitude culminates at 2908 m in the tahat hill atakor the hoggar and other massifs of central africa air eghei tibesti darfur cameroon mount form a system of domal uplifts with similar scale morphology and volcanic activity the knowledge of the structure beneath the hoggar swell will help us to understand the origin of continental swells in order to get an image of the lithosphere in this region we have performed a teleseismic field experiment the 33 short period seismic stations have been maintained for 2 1 2 month along a 700 km long nnw ssw profile this experiment crossed the central hoggar and extended northward into the in salah sahara basin which is characterized by high heat flow values of deep origin the high quality of the data recorded during this experiment allows us to perform a velocity inversion the hoggar appears to be characterized by lower mantle velocities the anomalous zone extends from the upper lithosphere to the mantle the weak velocity contrast is interpreted in agreement with gravity geothermal and petrological data as due to extensive mantle modifications inherited from cenozoic volcanic activity it confirms that the hoggar swell is not due to a large scale uplift of hot asthenospheric materials but corresponds to a now cooled off modified mantle on the contrary local low velocity zones associated with the atakor and tahalra volcanic districts show that hot materials still exist at depths in relation with recent basaltic volcanism

the 1993 killari latur central india earthquake an example of fault reactivation in the precambrian crust the september 30 1993 killari event in central india is a rare incidence of an earthquake occurring within a precambrian craton a pertinent question concerning seismicity in such regions is whether a preexisting fault exists and if so what is its reactivation interval ourstudiesinthe1993rupturezonesuggestthatthekillariearthquakeoccurred in a region of previous seismic activity older thrust sheets and fault gouge presumably formedduringpreviousepisodes wereexposedinadeeptrench thestudiesalsoindicated an obsequent fault line scarp aligned with the current rupture zone the morphological features in the area suggest mass removal of the upper part of the hanging wall on the southwestern side of the rupture existence of a prominent northwest striking structure passing through the epicentral zone is revealed in the digital landsat data these data together with the spatial trend of historic earthquakes along the northwest striking structure reinforce the argument that the earthquake at killari is related to the reactivation of a preexisting fault

rupture mechanism of the 1993 killari earthquake india constraints from aftershocks and static stress change the killari earthquake of september 29 1993 mw 6 2 in peninsular india triggered several aftershocks that were recorded by a network of 21 stations we computed the change in regional static stress caused by coseismic slip on the earthquake rupture and correlated it with the aftershocks with a view to constrain some of the rupture parameters of this earthquake we evaluated the six available estimates of fault plane solutions for this earthquake and concluded that reverse slip on a 42j dipping n112j trending fault which extends up to the surface from a depth of 7 km produces maximum correlation between the increased static stress and aftershock distribution our analysis suggests that the majority of coseismic slip occurred on the part of the rupture that lies in the depth range of 3 6 5 km d 2003 elsevier b v all rights reserved

mise en evidence d importantes variations regionales du flux de chaleur en algerie evaluation of heat flow in 230 oil wells using temperature measurements bottom hole temperature and temperature of fluids in drill stem test and various rock porosity data reveals a high heat flow average 82 19 mw m 2 associated with the algerian sahara basins the isopleth map exhibits significant regional variations overprinted by short wavelength anomalies that in general are related to the local geological structure on a regional scale we observe an essentially north south zonation that is not directly related to the major structural units except for the northern alpine domain the southern area at the border of the hoggar precambrian basement is characterized by very high heat flow values 90 130 mw m 2 the anomalies define a major axis generally east west which seems to affect the northern part of the african plate from the canaries to libya locally some relationships with extensional miocene pliocene quaternary volcanism suggest an association with recent mantle thermal events

burial history and thermal evolution of the southern and western saharan basins synthesis and comparison with the eastern and northern saharan basins a quasi two dimensional analysis of burial history and thermal evolution was carried out along eight profiles for sedimentary sections in 24 wells in the southern and western saharan basins with the aid of one dimensional nonsteady numerical reconstructions the modeling shows that thermal activation during the permian triassic jurassic was most intense in the timimoun basin and the northern part of the ahnet and mouydir basins with a heat flow of about 70 80 mw m2 and a lithosphere thickness of 40 50 km activation was more moderate in the sbaa subbasin and reggane basin with a heat flow of approximately 63 67 mw m2 and a crustal thickness of 55 60 km these values are comparable to those for the eastern saharan basins where the intensity of the triassic activation decreased considerably from north to south the present day thermal regime of the ahnet and reggane basins and the central part of the timimoun basin is comparable to that of the illizi basin where the present day lithosphere is as thin as 25 km and recent volcanism has occurred moderate extension of the lithosphere with a maximum total amplitude of about 1 16 during the cenozoic is assumed in our modeling in the reggane basin and the southern half of the timimoun basin as well as in the central and eastern part of the illizi basin thermal analysis of the saharan basins has shown that the hercynian erosion can account for only a minor part of the jumps observed in the vitrinite profiles ro intrusive activity and the associated hydrothermal heat transfer during the triassic and early jurassic accurately explain the steplike character of maturation profiles in the saharan basins in particular the lower level of catagenesis and conservation of oil deposits in the relatively hot sbaa subbasin are the result of less intensive hydrothermal intrusive heating of the basin during the jurassic as compared to that in neighboring basins this could also be ascribed to a deeper location of the lithosphere and probably deeper intrusions in the sbaa subbasin this relatively moderate thermal regime of the sbaa subbasin might be connected to its structural position of as it was a shoulder to neighboring deeper depressions

exploration history of the palaeozoic petroleum systems of north africa subsurface exploration of the palaeozoic petroleum systems of libya algeria tunisia and morocco began in the early 1950s after the discovery of a large prospective palaeozoic basin in the sahara desert province of north africa the first exploration well to test this concept was drilled in algeria in 1952 a non commercial find in 1953 was followed by major discoveries in 1956 since that time approximately 1100 new field wildcats have been drilled to test palaeozoic and basal triassic reservoir targets in a variety of discrete petroleum systems this resulted in the discovery of approximately 330 palaeozoic sourced accumulations in the algerian libyan and tunisian portions of the sahara desert by the end of 1996 ultimate recoverable reserves in oil equivalent for these discoveries were estimated to be just over 46 billion barrels using measures of activity and reserves discovered it is seen that the history of exploration of this province has followed a normal cycle familiar to the petroleum industry which can be divided into five major periods they are pre discovery discovery post discovery boom post boom and revival it is anticipated that future advances in technology will result in new ideas and more effective exploration and ultimately in the future discovery of significant oil and gas reserves

body wave and surface wave analysis of large and great earthquakes along the eastern aleutian arc 1923 1993 implications for future events the 500 km long alaska peninsula and shumagin islands segments of the aleutian arc have a moderate to high probability of rupturing in one or more large or great earthquakes in the next few decades to understand the likely modes of rupture in the next sequence of large and great events and to delineate the current geometry of the plate interface we determine focal mechanisms depths and source time functions from seismic records for the largest events since 1917 the great earthquake of november 10 1938 and seven events with surface wave magnitude ms of 6 9 to 7 5 teleseismic body waves and surface waves are used to estimate the seismic moment and gross rupture characteristics of the great earthquake of november 10 1938 along the alaska peninsula body wave inversion of five p and four sh waves gives a duration of about 110 s in which moment was released in two episodes each of about 50 s duration with the second being larger than the first the first source was located in the general vicinity of the epicenter of the mainshock and the second which occurred about 60 s later was centered about 180 km to the northeast the body wave derived seismic moment is 3 7 1021 n m mw 8 3 we corroborate the body wave results by calculating surface waves by normal mode summation and comparing them with data at periods greater than 50 s an adequate fit to observed seismograms is obtained for either a single point source or two point sources with one located about 180 km northeast of the mainshock epicenter rupture in 1938 appears to have been confined to the alaska peninsula segment uniform rupture into the shumagin region is not supported by the data the ms 6 9 earthquake of may 13 1993 ruptured a small portion of the shumagin gap the earthquake of may 14 1948 ms 7 5 occurred on a shallow dipping thrust fault with a depth of about 31 km not 60 km as originally suggested five ms 7 events in the 1938 rupture zone have locations depths and mechanisms that define a shallow dipping 16 19 plate interface which shallows to 8 under kodiak island subduction of seamounts of the gulf of alaska seamount province may explain the location moment focal mechanism and depth of the five ms 7 events the plate interface dips nearly uniformly between the alaska peninsula and shumagin segments indicating that segmentation if any is not controlled by the orientation of the plate interface a slight warp of the plate interface may form the boundary between rupture zones of great earthquakes along the kodiak and alaska peninsula segments of the arc and may explain why rupture zones of large to great earthquakes rarely cross this tectonic boundary

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

an investigation into the latur earthquake of september 29 1993 in southern india abstract the devastating m w 6 2 latur earthquake of september 29 1993 in south india has claimed an estimated 11 000 human lives with an i max of viii the earthquake was felt to an average distance of 750 km more than 125 shocks were reported to have been felt during august 1992 march 1993 out of these during october november 1992 several shocks of m 2 0 were recorded at the ngri seismic station at hyderabad which is the closest 220 km to the epicentre no such shocks occurred for at least 8 months before the latur earthquake the aftershocks were monitored by a network of up to 21 stations between october 8 1993 and january 31 1994 a majority of the aftershocks occurred within a 10 km radius from the main shock on the basis of the location of aftershocks in the first few days a plane dipping at an angle of 45 towards the southwest and striking at 135 is inferred to be the fault plane which extends to a depth of 4 5 km and on projection meets the surface in the vicinity of the observed surface rupture assuming the aftershock zone to be the rupture zone the stress drop is estimated to be 7 mpa with a maximum displacement of 1 7 m for the main earthquake a unique discovery is the high concentration of helium in the soil in the immediate vicinity of the surface rupture indicating that the rupture extends to the surface from a depth of a few km a detailed broadband magnetotelluric mt investigation revealed the presence of an anomalously high conductive zone at a depth of 6 10 km observation of a pc phase lagging behind the pg phase by about 0 6 to 0 8 s in the seismograms of aftershocks indicating a low velocity zone at 7 to 10 km depth is consistent with the mt results this highly conductive low velocity layer is inferred to be fluid filled the main stress regime in peninsular india is ne compressive stress due to plate tectonic movement erosion of the basalt cover in the deccan plateau may be adding additional compressive stress in the region the existence of a low velocity highly conductive fluid filled layer will enhance stress concentrations in the uppermost brittle part of the crust causing the earthquake

conodont based thermal maturation of paleozoic rocks in arizona color changes in conodonts are established thermal indices that have been related to the thermal window for hydrocarbon generation and preservation and that also have value for targeting mineralization potential conodont color alteration values from 141 localities in arizona appear to characterize the thermal maturation of the major paleozoic outcrop areas in the state paleozoic rocks in the colorado plateau in part of the belt of faulted plateau like rocks bordering the plateau and in a small area from 31 20 prime to 31 44 primen and from 110 00 prime to 110 40 primew in southeast arizona have thermal potential for oil paleozoic rocks in the same areas as well as in the pedregosa basin southeasternmost arizona have thermal potential for gas f these areas southeast arizona appears to have the best hydrocarbon potential because of the association of possible source beds and favorable thermal maturation levels

estimates of long term probabilities for future great earthquakes in the aleutians we compute probabilities for future great earthquakes in the aleutian arc probability distributions are fitted to recurrence periods of great aleutian evens m sub w or 7 8 since 1788 given a probability distribution and data of the last grat earthquake in each arc segment time dependent conditional probabilities are determined for future periods of interest we obtain for the next two decades high probabilities 99 to 30 for great earthquakes in the shumagin yakataga unalaska and kommandorski seismic gaps these probabilities are higher than for any other assessed region of the u s believed to be capable of great earthquakes low probabilities 17 to 9 are found for segments that ruptured most recently in 1965 1964 and 1957 recurrence periods for great earthquakes measure on average about 80 years but vary substantially whether recurrence periods for great aleutian earthquakes follow a normal log normal or any other probability distribution is not resolved because in most arc segments the known seismic record embraces at best one or two recurrences the resulting uncertainty affects the magnitude of estimated probabilities especially in seismic gaps that have not ruptured for a long time in some instances a normal distribution yields a three times higher probability than the correspondingmore log normal distribution less

tectonic stress field in the indian subcontinent a map of maximum horizontal compressive stress orientation in the indian subcontinent has been prepared using orientations derived from three different stress indicators borehole elongation breakouts in situ hydraulic fracturing measurements and earthquake focal mechanisms most part of the subcontinent appears to be characterized by a compressional stress regime thrust and strikeslip faulting imposed by plate boundary forces although shmax orientations do not in general show clear correlation with the direction of motion of the indian plate four provinces are recognized on the basis of regionally consistent orientations these are the midcontinent stress province the southern shield the bengal basin and the assam wedge their boundaries have been determined taking into consideration regional tectonics and seismicity central and northern india including the shillong plateau stretching up to the great himalaya pakistan and nepal are included in a broad midcontinent stress province characterized by nne ene oriented shmax the mean orientation of shmax in this province is n23 e subparallel to the direction of compression expected to arise from the net resistive forces at the himalayan collision zone suggesting that it is largely determined by the tectonic collision processes much of southern india mysore plateau and the high grade metamorphic terrain south of the plateau appears to be part of a second stress province characterized by nw oriented shmax these appear close to those of the intraplate stress field prevailing in the central indian ocean a third stress province was recognized in the bengal basin including parts of west bengal tripura manipur and mizoram in northeastern india and most of bangladesh this province extends eastward from the marginal fault in the western margin of the bengal basin to the indo burma subduction zone and is bounded on the north by the e w striking dauki fault shmax within the sedimentary pile of the bengal basin is oriented in e w direction while p axes of earthquakes within the basement and the crust beneath the basin and within the subducted slab of the indian plate beneath the indo burman ranges generally trend north n30 e shmax orientations within the sedimentary pile of the basin are parallel to the local approximately e w direction of the convergence of indian and burmese plates suggesting a casual relation to the resistive forces at the subduction zone in the indo burma region interestingly the stress field in the basement and the crust beneath the bengal basin and in the subducted slab is similar to the one prevailing in the midcontinent stress province assam wedge stress region occupies the northeastern corner of the indian plate including upper assam arunachal pradesh and much of nagaland this region subducts beneath a sharply bent continental collision boundary consisting of the northeastern limb of the himalayan and northern limb of the indo burman fold belts as a result the stress field in this province is depth differentiated and most likely responsible for the absence of consistent shmax directions

application of an inhomogeneous stress patch model to complex subduction zone earthquakes a discrete interaction matrix approach in recent years it has been recognized that the level of shear and normal stress along a fault can vary thus the stress is spatially and temporally inhomogeneous moreover it has also been suspected that faults might interact in some way with the result that a variety of earthquake magnitudes might be produced along a given length of fault at varying times in order to explore these ideas we have developed a quantitative formalism which we call the interaction matrix method to express the influence of one fault upon another this matrix is calculated by use of the energy change for a system of interacting cracks or faults and therefore gives energy consistent results specifically the interaction matrix relates the area averaged stress on the fault segment to the area averaged slip state on all the other fault segments in the system since any fault can be subdivided into an arbitrary number of fault segments the interaction matrix can have arbitrary dimension in fact the continuum limit is recovered as the dimension of the matrix approaches infinity we combine this matrix method with a segmentation or patch model for earthquakes in which each discrete segment of a fault has the same coseismic stress change defined as the difference between the driving stress at which healing occurs minus the driving stress at which sliding starts each time it slips we show that slip on a patch during an earthquake can vary substantially depending on how it interacts with other nearby patches in this model it is quite possible for the spatial distribution of stress on the fault following an event to be again in a spatially inhomogeneous state rather than in a uniform state as is often assumed hence the seismic moment produced by an earthquake on a given set of patches can vary substantially depending on the sequence of sliding and healing on the different patches to apply these ideas we devised a means to calculate the interaction matrix elements and used them to quantitatively examine earthquake sequences off the colombia ecuador coast and in the nankai trough near japan

determination of magnitude for large shallow earthquakes 1898 1917 abstract in order to arrive at a uniform basis for a study of the global seismicity the surface wave magnitude ms of shallow earthquakes for 1898 1917 is newly determined by using maximum amplitudes of surface waves recorded by undamped milne and omori seismographs the effective gain of these instruments needs to be calibrated with great care because of the lack of a damping device the calibration of milne instruments is based on a comparison of amplitudes measured by milne instruments with those by more modern damped instruments the effective gain varies from 7 to 20 in the range of ms from 6 to 8 5 the reported amplitudes for the years 1910 1912 are twice as large as the standard and need to be corrected by applying these results to a wealth of the milne data ms is determined for a large number of earthquakes between 1898 and 1912 to supplement the data for 1913 1917 records of undamped omori seismographs are used for the entire period from 1898 to 1917 308 shallow earthquakes with m s 7 0 are listed here both this list and the previous catalogue of abe 1981 for 1918 1980 provide a catalogue of large shallow earthquakes much more uniform than any previously established between 1899 and 1910 the annual frequency of shocks with m s 7 0 is 20 which is twice as large as the annual average of 10 during 1911 and 1980

transition from double to single wadati benioff seismic zone in the shumagin islands alaska a discontinuity exists in the moderate depth seismicity located by the shumagin seismic network in the eastern aleutian subduction zone the boundary is nearly perpendicular to the arc and has a double planed wadati benioff zone to the west and a single planed zone to the east the transition does not appear to be an artifact of mislocations as shown by three dimensional ray trace modeling and examination of phases of representative earthquakes current theory on the presence or absence of double seismic zones suggests a change in stress across the boundary if so the east and west regions may either have different aseismic to seismic slip ratios or be in different stages of the earthquake cycle

aftershocks of the 13 may 1993 shumagin alaska earthquake the 13 may 1993 ms 6 9 shumagin earthquake had an aftershock sequence of 247 earthquakes with magnitudes greater than or equal to 1 5 by 1 june 1993 of these aftershocks 79 were located by using s p travel times at the only two stations within 570 km of the mainshock epicenter the rupture area inferred from the aftershocks is about 600 km2 and we estimate for the mainshock a mean fault displacement of 1 0 m and a 28 bar stress drop the magnitude frequency plots give a b value for the aftershock sequence of about 0 4 which is low compared to the background value of approximately 0 8 the decay of the aftershock sequence followed the modified omori law with a p value of 0 79 which is also lower than the typical values of about 1 1 observed in alaska both of these facts can be interpreted as indicating relatively high ambient stress in the shumagin seismic gap and the possibility that the 13 may earthquake was a foreshock to a larger gap filling event to occur within the next few years

hercynian compressional deformations of the ahnet mouydir basin algerian saharan platform far field stress effects of the late palaeozoic orogeny n s to nw se trending faults and reverse faults occur within the palaeozoic ahnet mouydir basin of the algerian saharan platform located to the east of the west african pan african suture zone deformation and stratigraphic analysis show that this basin underwent a nne ssw to ene wsw shortening at about the carboniferous permian transition or more probably during the early permian a brief review of the more or less synchronous deformations registered in the neighbouring regions and more specifically around the west african craton illustrates the mechanical coupling between intraplate tectonics and the hercynian plate margins orogenies

the 1993 killari earthquake in central india a new fault in mesozoic basalt flows the mw 6 1 1993 killari earthquake in central india was one of the deadliest earthquakes to occur in a stable continental area it had a centroid 2 6 km deep and ruptured to the surface a good fit of teleseismic waveforms is obtained with a simple 4 2 s long pulse that releases a moment mo 1 8 1018 nm from a reverse fault rupture with strike dip and rake of 126 46 and 100 respectively a 10 km wide meizoseismal area is tightly defined by relative casualty count in the center of this area aftershock hypocenters baumbach et al 1994 outline a southwest dipping rupture that extends 5 5 8 km along strike and from the surface 8 10 km along dip these dimensions correspond with an average displacement of 1 5 0 8 m and a stress drop of 10 4 5 mpa respectively we mapped a 1 km long zone of compressional scarps which are spatially correlated with the rupture outlined by hypocenter in the central portion of this zone the scarps are multiple and face in opposite directions deformation features indicate north northeast directed shortening of at least 0 5 m a leveling profile of an irrigation canal about 3 km northwest of the scarps displays a one wavelength warp about 1 km long and 1 3 m in peak to peak amplitude that may reflect deformation associated with the rupture two trenches across the scarps exposed faults that offset the soil rock interface as much as 50 cm we found no convincing evidence suggesting that these faults existed prior to the earthquake in the basalt these faults reactivated exfoliation fractures in shear but were not associated with a zone of preexisting breccia and mineralization the hypothesis that the 1993 rupture is a new fault in the deccan traps is consistent with lack of geomorphic evidence of prior faulting and with the lack of accumulated deformation or tilting in basalt layers exposed as close as few tens of meters from the scarp no convincing clues about a seismogenic fault have yet been detected from either historic seismicity or geology in the 1993 source area but a burst of seismicity 1 year before the mainshock and 2 5 years after the first impounding of a nearby reservoir was interpreted by some as a possible precursor to a larger earthquake before the 1993 mainshock precursory seismicity and artificial effects that cause significant mechanical changes in the crust may provide an indication of future potentially damaging earthquakes in stable continental areas

spatial distribution of turn of the century seismicity along the alaska aleutian arc abstract epicenters determined for instrumentally observed earthquakes occurring around the turn of the century have figured prominently in estimates of seismic hazard along the alaska aleutian arc unfortunately because of poor timing control and a sparse station distribution the locations quoted in the older catalogs can be very unreliable more reliable epicenter estimates and quantitative assessments of the errors associated with these estimates are needed to address issues related to seismic hazard seventeen earthquakes with surface wave magnitudes above 6 8 that were recorded between 1898 and 1917 are relocated using a modified least squares criterion epicentral estimates are determined using p s and s p arrival times with the a priori constraint that the final location must be near the geographically defined arc although epicenters determined using a standard least squares criterion can be located up to 800 km from the arc the arc proximity constraint provides tectonically more meaningful locations which to a high degree of confidence fit the travel time observations as well as the unconstrained locations epicentral shifts of up to 1 200 km are observed between the relocations and those reported in the earlier catalogs seventy five per cent confidence bounds on the along arc location of each event vary from 30 to 400 km several large events m s 6 9 occurred within or near the rupture zone of the 1965 rat islands earthquake and may indicate that this zone failed in a sequence of events around the turn of the century of the four events previously suggested as being possible candidates for rupturing either the shumagin or unalaska seismic gaps none relocate near these arc segments the relocation of a large m s 7 9 earthquake which occurred in 1917 within the shumagin islands region however provides the first strong evidence of largescale seismic moment release within this intensely studied seismic gap

the hydrocarbon systems of north africa abstract north africa s hydrocarbon reserves are spread over a wide range of reservoirs and play types but derive from only a few key source rock intervals particularly those in the silurian late cretacous and eocene source rock maturity together with the timing of the generation relative to trap forming and destructive events seem to be the key controlling factors on success or failure at a basin level in most cases generation clearly post dates trap formation although a more complex relationship is evident in various parts of algeria where some traps did not form until gas was being generated source rock deposition and maturation can be tied to two main extensional tectonic cycles which ended in the compressional and destructive events of the hercynian and alpine orogenies remaining exploration potential in mature provinces may be tied largely to subtle and stratigraphic traps in addition significant frontier potential can be identified in two main fairways corresponding to regions where silurian devonian and cretaceous eocene sources respectively are likely to have generated petroleum critical factors in these areas are timing of generation and preservation the gas discoveries in the ahnet basin provide analogues for palaeozoic generation preservation models in the interior silurian devonian source fairway similarly the key to success for the mesozoic basins of the atlas region lies in demonstrating a favourable relationship between trap formation destruction and generation from late cretaceous and eocene source rocks

deep structure of the hoggar domal uplift central sahara south algeria from gravity thermal and petrological data abstract initial heat flow determinations show that no significant regional thermal disturbance is associated with the volcanic hoggar swell however the large scale negative gravity anomaly associated with this swell strongly suggests the occurrence of a low density upper mantle beneath the hoggar this is inferred from the occurrence of the depleted metasomatized and intensely recrystallized and veined peridotitic upper mantle revealed by the study of xenoliths from the cenozoic basaltic eruptions we suggest that the low density of the upper mantle is due to magmatic events of asthenospheric origin from various observations including petrographie gravimetric and thermal constraints the age of these events is estimated to be late mesozoic to early cenozoic the alkali cenozoic volcanism cannot be regarded as the cause of the swelling the occurrence of an altered lithosphere now cooled and presently marked by low density magmatic materials is consequently proposed as a possible model for the hoggar

relative importance of the hercynian and post jurassic tectonic phases in the saharan platform a palaeomagnetic study of jurassic sills in the reggane basin algeria summary in the intracontinental domain of the northwestern saharan platform the deformation of the palaeozoic sedimentary cover is mainly attributed to a far field effect of the hercynian orogeny having occurred at the african laurusian plate boundary in the late carboniferous to early permian times however geological observations from different parts of africa and arabia provide evidence that several minor but widespread tectonic events occurred later particularly during the cretaceous contrary to elsewhere in the northwestern part of africa in the reggane basin outcropping doleritic sills of early jurassic age are intruded in folded palaeozoic sediments of devonian to carboniferous ages deposited before the hercynian orogeny in this favourable situation a palaeomagnetic study of the liassic dolerite is able to provide information on the tectonic history of the surrounding area independently from geological observations the present study aims to quantify the relative proportion of tilting related to respectively the hercynian and a post intrusion phase using a fold test based on the small circle analysis this method proved to be very efficient to unravel these tectonic events it shows that in the studied area the folds were initiated during the hercynian phase but mainly amplified during the post intrusion phase which turned out to be the dominant one in the reggane basin the age of this second event is not geologically well constrained between probably late jurassic and early cretaceous it could be the far field effect of either the cimmerian phase 140 ma or more likely the austrian tectonic phase late barremian 125 ma the late barremian tectonic episode corresponds to a major event the break up of western gondwana which led to the separation of africa from south america and to the incipient fragmentation of the african plate into three major blocks the conclusion drawn from the palaeomagnetic study in the reggane basin is consistent with the geological observations and representative of the intraplate cretaceous deformations recorded in many other places in africa it emphasizes once again that stresses can be transferred far from the plate boundaries into the continental plate interiors

the world s most spectacular carbonate mud mounds middle devonian algerian sahara abstract in the southern ahnet basin of algeria a unique middle devonian underwater scenery is revealed which consists of 191 mud mounds 15 mud ridges and 2 mud atolls distributed over nine discrete areas of a shallow pelagic carbonate shelf all the buildups are totally exhumed thus perfectly exhibiting their original morphologies slope angles elevations and the pattern of the onlapping strata as well as their relationship to the surrounding off buildup sequences types of buildups include 1 small mud mounds 1 5 m high 10 20 m diameter 2 large mud mounds up to 40 m high up to 200 m diameter 3 lens shaped mounds 1 30 m thick 10 15 slopes 4 mud ridges up to 100 m high 150 8500 m long and 5 semicircular mud atolls 2500 2800 m long 20 30 m high the buil ups were constructed during a short time span in the early givetian part of the lower varcus zone probably not exceeding a few hundred thousand years sedimentation rates in coeval off buildup areas were at least 3 10 times lower the lithology of the buildups is a massive boundstone with a high proportion of irregular spar filled cavities stromatactis but only very scattered potential frame builders the carbonate mud is considered to be an in situ cyanobacterial precipitate although evidence of such is poor organic components of the buildups are not significantly different from those of the off buildup lithologies and include numerous crinoid ossicles some tabulate and solitary rugose corals brachiopods mollusc debris trilobites and very rare sponge spicule and bryozoans the total absence of stromatoporoids large tabulate and colonial rugose corals green algae and micritic envelopes around grains suggests growth of the buildups in the lower part of the photic zone 100 200 m they are onlapped by a discontinuous sequence a few meters thick of skeletal packstones and grainstones and locally orthoceratid floatstones that were deposited later in the givetian a widespread rise of sea level in the early frasnian buried both buildups and adjacent areas under shales and siltstones postdepositional alteration of the mud buildups includes 1 formation of stromatactis cavities probably by dewatering and internal erosion 2 cementation of stromatactis cavities and interparticle pore spaces by early high mg calcite and late blocky low mg calcite 3 neomorphic transformation of mg calcitic mud and aragonitic skeletal debris 4 occasional formation of neptunian dikes 5 slight erosion of buildup surfaces 6 compaction and slumping of onlapping capping beds and 7 local dolomitization the formation and growth of the mud build ups is envisaged as a sequence of stages starting with patchy coral thickets evolving into small mud mounds that coalesced into larger mounds by continuous vertical and lateral growth closely spaced mounds amalgamated into mud ridges and mud atolls regular nw se and n s patterns of some mound rows and ridges show a relationship of buildup formation to rejuvenated late precambrian tectonic lineaments which reappear in the hercynian orogeny the isotopic signature of matrix and early ements of mound rocks however does not support the idea of fluid migration and hydrothermal co2 supply during buildup growth

paleoseismicity of two historically quiescent faults in australia implications for fault behavior in stable continental regions paleoseismic studies of two historically aseismic quaternary faults in australia confirm that cratonic faults in stable continental regions scr typically have a long term behavior characterized by episodes of activity separated by quiescent intervals of at least 10 000 and commonly 100 000 years or more studies of the approximately 30 km long roopena fault in south australia and the approximately 30 km long hyden fault in western australia document multiple quaternary surface faulting events that are unevenly spaced in time the episodic clustering of events on cratonic scr faults may be related to temporal fluctuations of fault zone fluid pore pressures in a volume of strained crust the long term slip rate on cratonic scr faults is extremely low so the geomorphic expression of many cratonic scr faults is subtle and scarps may be difficult to detect because they are poorly preserved both the roopena and hyden faults are in areas of limited or no significant seismicity these and other faults that we have studied indicate that many potentially hazardous scr faults cannot be recognized solely on the basis of instrumental data or historical earthquakes although cratonic scr faults may appear to be nonhazardous because they have been historically aseismic those that are favorably oriented for movement in the current stress field can and have produced unexpected damaging earthquakes paleoseismic studies of modern and prehistoric scr faulting events provide the basis for understanding of the long term behavior of these faults and ultimately contribute to better seismic hazard assessments

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

strain accumulation in the shumagin seismic gap alaska strain accumulation in the shumagin seismic gap was estimated from repeated trilateration surveys made between 1980 and 1985 the measured rate of shear strain accumulation 0 01 0 03 rad a was an order of magnitude less than the rate 0 18 rad a predicted for seismic subduction the absence of measurable shear strain accumulation in the shumagin islands suggests that the main thrust zone there may not be locked but rather may slip stably if this is indeed the case the probability of a great shallow thrust earthquake in the shumagin gap is greatly diminished a moderate earthquake mb 6 0 occurred at the seaward end of the trilateration network 4 months after the most recent survey however no significant anomalies that could be interpreted as precursors were detected in that survey

geological investigations at killari and ter central india and implications for palaeoseismicity in the shield region abstract the 1993 killari earthquake occurred in the central part of the indian shield an area generally believed to be of low seismogenic potential this rare event provided an opportunity to understand the seismogenesis within the shield regions trench excavations in the rupture zone as well as other natural exposures in the vicinity of the epicentral region of the killari earthquake indicate episodic activity separated by long periods deep drilling in the epicentral area recorded a maximum of 6 m of slip in the deeper strata gupta et al 1998a assuming an average of about 1 m slip for a moderate earthquake as revealed by the 1993 event accumulated slip indicates at least six moderate earthquakes at killari while the timing of the penultimate event may work out to be hundreds of thousands of years or more evidence for at least one moderate earthquake 1500 yr ago was obtained from ter about 40 km northwest of killari observations at killari and ter indicate the localization of deformation along a preexisting shear zone that are being reactivated in the current stress field because of factors like low background seismicity long interseismic intervals and smaller ruptures compounded by low preservation of surface exposures behaviour of seismogenic structures in the shield regions may appear to be random in space and time observations presented in this paper suggest that the perceived randomness probably holds only for individual patches in the current time window other parts of the seismogenic fault may behave independently showing a different temporal pattern comprehensive investigations of the entire structure are needed to understand the long term behaviour of such faults and thereby improve the seismic hazard assessment in the cratonic interiors

thermal effects of igneous intrusions on maturity of organic matter a possible mechanism of intrusion abstract this paper assesses thermal maturation of organic matter near a well dated intrusion in the dsdp 41 368 hole near cape verde rise eastern atlantic and near some other intrusions from the u k u s a and east greenland the traditional numerical model with instantaneous intrusion satisfactorily describes the thermal evolution and thermal maturity aureoles of igneous bodies only if the initial temperature of intrusion is close to or even lower than the solidus temperature calculations in this model with more real values of the intrusion temperature and with consideration of latent heat effects lead to considerable over estimation of thermal influence of the intrusion models which consider formation of the intrusive body over time as distinct from instantaneous intrusion and especially models of emplacement of an intrusive body in the shell of relatively cool magmatic rocks show better agreement between the computed and observed vitrinite reflectance profiles consideration of convective heat flow due to hydrothermal activity arising in the vicinity of the intrusion during its formation and cooling helps to explain the difference between maturation profiles above and below some sills

reply to comment on the 1993 killari earthquake in central india a new fault in mesozoic basalt flows by l seeber et al

coseismic surface deformation caused by the 23 march 2012 mw 5 4 ernabella pukatja earthquake central australia implications for fault scaling relations in cratonic settings abstract on 23 march 2012 at 09 25 utc an m w 5 4 earthquake occurred in the eastern musgrave ranges of north central south australia near the community of ernabella pukatja several small communities in this remote part of central australia reported the tremor but there were no reports of injury or significant damage this was the largest earthquake recorded on mainland australia in the past 15 years and resulted in the formation of a 1 6 km long surface deformation zone that included reverse fault scarps with a maximum vertical displacement of more than 0 5 m extensive ground cracking and numerous rock falls the earthquake occurred in nonextended stable continental region scr cratonic crust more than 1900 km from the nearest plate boundary surface deformation from the ernabella earthquake provides additional constraint on relations of surface rupture length to earthquake magnitude such relations aid in interpreting australia s rich record of prehistoric seismicity and contribute to improved estimates of scr seismic hazard worldwide based upon an analysis of new and reinterpretation of existing surface rupture length data faults in nonextended stable cratonic australia appear to produce longer surface ruptures for earthquakes larger than m w 6 5 than rupture lengths estimated using existing moment to rupture length scaling relations the implication is that the estimated maximum or characteristic magnitude of paleoearthquakes in such settings may be overestimated where the estimate is based only on the length of the prehistoric fault scarp

long term seismic behavior of the focal and adjacent regions of great earthquakes during the time between two successive shocks we analyze the spatial temporal energy distribution of shallow depth h 70 km seismicity surface wave magnitude ms 7 occurring in the focal and adjacent regions of very great shallow earthquakes rupture lengths of the order of hundreds of kilometers in the circum pacific seismic belt for as many decades as possible before and after the occurrence of the main shock for this purpose we have revised the catalog of data for the world instrumental seismicity 1900 1995 by removing a series of heterogeneities in the earthquake reporting and magnitude determination this revised catalog is fairly complete for earthquakes with m 7 since early in the century our findings indicate that once a main shock aftershock sequence occurs the focal region enters a period of relative quiescence for large ms 7 earthquakes significant seismic activity ms 7 occurs during many decades prior to the next main shock this activity is fairly continuous or random in time and lasts until the onset of the main event but clusters in space around the ends of the forthcoming rupture and or in the vicinity of the principal epicenter except in the case of rare bilateral breaks the main epicenter is located at one of these ends thus major portions of the rupture zone experience no events with ms 7 during the seismic cycle larger aftershocks also cluster near the edges of the rupture and near the principal epicenter preshock activity minutes to weeks before the main shock if any occurs in general near the main epicenter we also find that the nucleation and ends of the rupture often coincide in space with recognizable geometrical and or geological inhomogeneities barriers existing in the lithosphere it appears that while strain energy accumulates high stress concentrations often occur at the ends of the future rupture zone which results in the long term foreshock activity and in the subsequent initiation of the main rupture from within those regions we suggest a simple mechanical model that explains the observed pattern of earthquake occurrence for both unilateral and bilateral ruptures in all cases of great earthquakes that have occurred this century for which a long and complete to a level of 2 to 3 orders of magnitude smaller than the moment of the main shock catalog of prior seismicity exists we can not recognize any statistically significant quiescent or doughnut patterns in the preceding seismicity at the ms 7 level