paleolatitudes determined from paleomagnetic data from vertical cores most of the paleomagnetic data available from oceanic plates consist of measurements of inclination but not declination from azimuthally unoriented vertical cores similarly the basic paleomagnetic data to be analyzed from crustal blocks that have rotated by varying amounts about vertical axes are a set of paleoinclinations a new method is proposed for determining the paleocolatitude of a sampling site and the associated confidence limits paleomagnetic inclinations are first converted to paleocolatitudes and averaged to produce a mean colatitude after showing that this mean is biased we derive a paleocolatitude correction to remove the bias using a model for geomagnetic secular variation based on the latitude dependence of secular variation during the past 5 million years after testing the validity of the model by applying it to paleomagnetic data from quaternary lavas for which both inclination and declination are known we apply the method to inclination only paleomagnetic data from suiko seamount the most complete data set from a drilling site the method is then applied to paleomagnetic data from all deep sea drilling project sites for the pacific plate to obtain for each site an unbiased mean paleocolatitude and its confidence limits these paleocolatitudes and confidence limits are then used as input for a least squares analysis to obtain early and late cretaceous poles for the pacific plate and 95 confidence limits for these poles it is shown that the scatter of individual site paleocolatitudes about the best fit pole is consistent with the confidence limits at each site as determined by the new method of analysis

comparison between the hot spot and geomagnetic field reference frames hot spots are located on the earth s surface and are caused by hot plumes of upwelling mantle material they manifest themselves by volcanic activity which changes its location in response to the movement of the lithosphere over the mantle plume it has been suggested1 3 that hot spots are relatively stationary with respect to each other and also that they represent a frame of reference with which to measure the absolute movement of the lithospheric plates over the surface of the earth another frame of reference can be partially defined by the position of the mean magnetic dipole field of the earth as the dipolar field is believed to be closely related to the spin axis this frame of reference automatically gives information about the palaeolatitude and orientation of the lithospheric plates but since the position of the ancient dipole field of the earth does not give any information about palaeolongitude and since in many cases the definition of the field is relatively poor due to less than adequate rock collections during the time of interest hot spot reference frames have been favoured by many people we now report a comparison of the two frames of reference and show that over the past 200 myr there has been relative movement between the two frames of reference of about 20 and that this motion has not been constant with time

self similar earthquake triggering bath s law and foreshock aftershock magnitudes simulations theory and results for southern california 1 bath s law the observation that the largest aftershock is on average 1 2 magnitudes smaller than its main shock independent of main shock size suggests some degree of self similarity in earthquake triggering this behavior can largely be explained with triggering models in which the increased triggering caused by larger magnitude events is exactly compensated for by their decreased numbers and these models can account for many features of real seismicity catalogs the bath s law magnitude difference of 1 2 places a useful constraint on aftershock productivity in these models a more general test of triggering self similarity is to plot foreshock and aftershock rates as a function of magnitude m relative to the main shock magnitude mmax of the largest event in the sequence both computer simulations and theory show that these dn dm curves should be nearly coincident regardless of main shock magnitude the aftershock dn dm curves have the same gutenberg richter b value as the underlying distribution but the foreshock dn dm curves have the same b value only for foreshock magnitudes less than about mmax 3 for larger foreshock values the dn dm curve flattens and converges with the aftershock dn dm curve at m mmax this effect can explain observations of anomalously low b values in some foreshock sequences and the decrease in apparent aftershock to foreshock ratios for small magnitude main shocks observed apparent foreshock and aftershock dn dm curves for events close in space and time to m 2 5 to 5 5 main shocks in southern california appear roughly self similar but differ from triggering simulations is several key respects 1 the aftershock b values are significantly lower than that of the complete catalog 2 the number of aftershocks is too large to be consistent with bath s law and 3 the foreshock to aftershock ratio is too large to be consistent with bath s law these observations indicate for southern california that triggering self similarity is not obeyed for these small main shocks or that the space time clustering is not primarily caused by earthquake to earthquake triggering

paleomagnetic and rock magnetic results from the twin creek formation middle jurassic wyoming abstract a magnetization which passes the fold test has been observed in 73 limestone samples 10 sites from the middle jurassic twin creek formation the pole calculated from the site mean poles is located at 68 4 n 145 0 e k 31 8 a 95 8 7 this pole lies in a segment of the north american apparent polar wander apw path for which there are only a few reliable poles in the literature the results corroborate earlier studies which conclude that the jurassic segment of the apw path does not include the present north pole however the position of the twin creek pole suggests that significantly more apw took place prior to the late jurassic than previous studies indicated

nontrivial decay of aftershock density with distance in southern california the decay of the aftershock density with distance plays an important role in the discussion of the dominant underlying cause of earthquake triggering here we provide evidence that its form is more complicated than typically assumed and that in particular a transition in the power law decay occurs at length scales comparable to the thickness of the crust this is supported by an analysis of a very recent high resolution catalog for southern california sc and surrogate catalogs generated by the epidemic type aftershock sequence etas model which take into account inhomogeneous background activity short term aftershock incompleteness anisotropic triggering and variations in the observational magnitude threshold our findings indicate specifically that the asymptotic decay in the aftershock density with distance is characterized by an exponent larger than 2 which is much bigger than the observed exponent of approximately 1 35 observed for shorter distances ranging from the main shock rupture length up to a length scale comparable to the thickness of the crust this has also important consequences for time dependent seismic hazard assessment based on the etas model

paleomagnetism of jurassic volcanic rocks in the patagonia mountains southeastern arizona implications for the north american 170 ma reference pole paleomagnetic study of middle jurassic volcanic and volcaniclastic rocks in southeastern arizona yields a paleomagnetic pole located at 61 8 n 116 0 e k 49 6 95 6 2 these rocks from corral canyon represent an autochthonous segment of the jurassic cordilleran magmatic arc and were deposited directly on upper paleozoic carbonates of the cratonic miogeocline rock magnetic experiments demonstrate that the natural remanent magnetization of these rocks is dominantly a single component magnetization carried in both magnetite and hematite phases and is interpreted as a primary thermoremanent magnetism rb sr isotope geochemistry on whole rock samples from eight welded ash flow tuffs yields an isochron age of 172 2 5 8 2 ma the corral canyon pole falls between poles from newark trend group ii 179 3 ma and glance conglomerate 151 2 ma

paleomagnetism of cretaceous volcanic rocks from central chile and implications for the tectonics of the andes the mean direction of remanent magnetization in well dated structurally and petrologically simple cretaceous ash flow tuffs from the central valley of chile near santiago indicates that the region has undergone a small about 14 clockwise rotation two other paleomagnetic studies on cretaceous rocks from coastal chile give a similar result we tentatively attribute these rotations to dextral shear caused by oblique subduction of the nazca plate beneath south america in contrast cretaceous rocks from peru seem from paleomagnetic evidence to have been rotated counterclockwise relative to the stable interior of the continent this sense of apparent rotation agrees with the sense of deflection of geologic trends from north south to north northwest at about the peru chile border the arica deflection on the basis of these observations several authors have suggested that the arica deflection is an orocline however uncertainty in all such paleomagnetic interpretations is introduced by what seems to be a very streaked distribution of cretaceous reference poles from the south american craton the cause of this streaking is not known

paleomagnetism of the upper silurian and lower devonian carbonates of new york state evidence for secondary magnetizations residing in magnetite abstract paleomagnetic directions for the upper silurian and lower devonian carbonates of the helderberg escarpment new york state differ from expected late silurian and early devonian directions for cratonic north america the mean direction d 165 i 10 paleopole at 50 n 129 e is similar to late carboniferous and early permian results negative fold tests and a lack of reversals suggest that the magnetization is secondary however low coercivities low blocking temperatures the thermomagnetic curves tc near 570 c and the acquisition of isothermal remanent magnetizations all suggest that the remanence is carried by magnetite if a detrital origin of these magnetites is assumed the secondary nature of the remanence would argue for thermal resetting as a result of deep burial of the rocks however no evidence for such thermal resetting is seen in the alteration of conodonts more likely perhaps is a chemical or thermochemical origin of the remanence this would require the magnetites to be authigenic

paleomagnetism of lower devonian volcanics and devonian dykes from northcentral new brunswick canada abstract some 50 oriented samples 120 specimens have been collected on eight sites of volcanic rocks from the lower devonian dalhousie group of northern new brunswick and devonian andesitic to basic dykes from central new brunswick univectorial and occasional multivectorial components were extracted from the various samples results after af and thermal demagnetization compare relatively well in the volcanics and tuffs two components of magnetization have been isolated a d 33 i 58 95 7 3 k 236 for four sites and b d 66 i 53 for three sites the grouping of component a is improved after tilt correction but the fold test is not significantly positive at the 95 confidence level component a is interpreted as being primary while component b is unresolved and appears to be the resultant magnetization of a late paleozoic and a recent component the pole position obtained for tilt corrected component a is 268 e 1 s dp 6 5 dm 8 8 the paleolatitude calculated for component a is 39 s the paleopole of in situ component a is located close to those of the early middle devonian formations from quebec new brunswick and new england states while the paleopole of tilt corrected component a is similar to lower devonian poles of rock units from the canadian arctic archipelago if component a is primary as we believe it to be then the western half of the northern appalachians had already docked onto the north american craton by early devonian time alternatively if component a is secondary the same conclusion applies but the juxtaposition took place in middle devonian time

absolute dating of dedolomitization by means of paleomagnetic techniques paleomagnetic analysis in conjunction with petrographic studies is an approach whereby absolute dates can be placed on diagenetic events hematite is associated with dedolomite in the lower ordovician kindblade formation in south central oklahoma and is interpreted as a by product of the dedolomitization of ferroan dolomite paleomagnetic results date the time of dedolomitization as late pennsylvanian early permian dedolomitization was probably caused by oxidizing fluids with high calcium contents that migrated through the kindblade formation following uplift of the arbuckle mountains

a paleomagnetic estimate of the age and thermal history of the kentland indiana cryptoexplosion structure the kentland indiana cryptoexplosion structure consists of a centrally uplifted zone of steeply dipping fault blocks surrounded by a ring depression and a low amplitude structural high with a radius of 12 km the age of the deformation is constrained stratigraphically to the interval between mississippian and quaternary paleomagnetic samples were collected from seven carbonate formations and one siltstone unit of these only the middle ordovician quimby s mill limestone contains a stable magnetization with consistent directions stepwise thermal and alternating field demagnetization shows the presence of a northwesterly moderately steep characteristic remanence with a median destructive field of 10 to 30 mt and maximum unblocking temperatures of 300 c the pole position calculated from the mean direction falls near late cretaceous paleomagnetic poles dispersion increases when a bedding correction is applied indicating post deformational magnetization because the magnetization fails the fold test it

palaeomagnetism of the early carboniferous deer lake group western newfoundland no evidence for mid carboniferous displacement of acadia early carboniferous visean to possibly earliest namurian sedimentary rocks of the deer lake group of western newfoundland rest unconformably on grenvillian basement rocks of the canadian shield which form the western border of the early palaeozoic appalachian orogen in addition to magnetically soft magnetizations directed along the present field three families of magnetization directions are found two of them referred to as n north and s south are antiparallel and prefolding and were probably acquired during the depositional process n and s are roughly of equal frequency they have a mean direction irrespective of sign of 0 7 35 2 k 40 95 8 9 and a palaeopole 21 5 n 121 8 e 10 3 6 0 corresponding to a palaeolatitude of 20 6 s this agrees with the palaeolatitude 17 5 s determined from the somewhat older early carboniferous tournaisian terenceville formation of the avalon platform on the eastern side of the appalachian orogen in newfoundland the third magnetization referred to as h roughly horizontal has a mean direction 156 8 13 3 k 37 95 10 1 and a palaeopole 45 4 n 140 3 e 10 3 5 3 corresponding to a palaeolatitude of 7 4 n we interpret this to be an early kiaman latest carboniferous to early permian overprint probably acquired chemically the palaeolatitude determined from the h overprint agrees with that determined from early carboniferous rocks of cratonic north america west of the appalachians therefore we argue early carboniferous palaeofield determinations for cratonic north america have been strongly biased by unremoved kiaman overprints because of this and because of the good agreement between early carboniferous palaeolatitudes obtained from opposite margins of the appalachian orogen we suggest that there is at present no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the appalachians acadia relative to cratonic north america during the carboniferous

seismicity rate immediately before and after main shock rupture from high frequency waveforms in japan 1 we analyze seismicity rate immediately before and after 82 main shocks with the magnitudes ranging from 3 to 5 using waveforms recorded by the hi net borehole array in japan by scrutinizing high frequency signals we detect 5 times as many aftershocks in the first 200 s as in the japan meteorological agency catalogue after correcting for the changing completeness level immediately after the main shock the aftershock rate shows a crossover from a slower decay with an omori s law exponent p 0 58 0 08 between 20 and 900 s after the main shock to a faster decay with p 0 92 0 04 after 900 s the foreshock seismicity rate follows an inverse omori s law with p 0 73 0 08 from several tens of days up to several hundred seconds before the main shock the seismicity rate in the 200 s immediately before the main shock appears steady with p 0 35 0 50 these observations can be explained by the epidemic type aftershock sequence etas model and the rate and state model for a heterogeneous stress field on the main shock rupture plane alternatively nonseismic stress changes near the source region such as episodic aseismic slip or pore fluid pressure fluctuations may be invoked to explain the observation of small p values immediately before and after the main shock

reply to comment on paleomagnetism of a polarity transition in the lower triassic chugwater formation wyoming by emilio herrero bervera and charles e helsley

root mean square velocities of the continents with respect to the hot spots since the early jurassic the root mean square velocities of the major continents with respect to the hot spots were estimated for the past 180 m y by combining previously published plate reconstructions these velocities agree well with velocities relative to the spin axis inferred from paleomagnetic data from 169 m y b p to l44 m y b p the hot spot data indicate that laurasia moved rapidly 70 mm yr but the reconstructions permit a velocity as low as 50 mm yr we also estimate that from 63 m y b p to 48 m y b p india moved rapidly 140 20 30 mm yr faster than the present velocity of any plate and more than twice as fast as the present velocities of several subducting oceanic plates those attached to downgoing slabs along a substantial fraction of their margin the data require no other continent to have exceeded a velocity of 45 mm yr during the last 180 m y in agreement with previous studies based on paleomagnetic data and in agreement with previous studies based on no nettorque analyses we conclude that the continents have moved faster occasionally much faster than at present from this we infer that continental lithosphere is unlikely to have much greater resistance to plate motion than oceanic lithosphere instead oceanic plates tend to move more rapidly than continental plates because the subduction and rapid motion of a plate that includes no continents tends to persist longer than the subduction and rapid motion of a plate that includes one or more continents on the basis of the very rapid early tertiary motion of india we conclude that the velocities of subducting plates can vary by a factor of 2 therefore models of the driving forces of the plates that predict a uniform velocity of subducting slabs are invalid

post tectonic magnetizations from the cordova gabbro ontario and palaeozoic reactivation in the grenville province summary the cordova gabbro of southern ontario intrudes 1300 myr old volcanic rocks of the hastings lowlands in the grenville structural province three distinct vector magnetizations a b and c have been isolated using a combination of stable endpoints subtracted vectors from orthogonal vector plots and converging remagnetization circles the a magnetization with mean direction d 294 i 55 5 k 42 ag5 5 5 n 18 sites is a high coercivity high blocking temperature remanence recorded by 49 samples the b magnetization was isolated in 33 samples and has a mean direction d 305 5 1 1 5 k 24 ag5 9 5 n 11 sites b has lower coercivities and blocking temperatures than a where the two are superimposed the a and b palaeopoles 151 e 10 5 s dp 6 dm 8 and 165 5 e 24 n dp so dm 9 5 fall on the grenville track around 900 and 820 ma respectively the a and b magnetizations thus date from uplift and cooling following the grenvillian orogeny the third magnetization the c component has been isolated in 23 samples its mean direction is d 180 i 27 5 k 18 ag5 10 5 n 12 sites the c is a low coercivity low blocking temperature overprint of a and b its palaeopole 102 e 31 n dp 6 5 dm 12 is unlike post 1300 precambrian poles for cratonic north america but matches silurian and late ordovician poles 40ar 39ar plateau ages of 446 and 447 ma determined by lopez martinez and york for plagioclases from one of the cordova samples confirm this age assignment the c magnetization therefore records a previously unrecognized mild thermal or hydrothermal event that occurred in palaeozoic time long after the grenvillian orogeny

reply to comment by s hainzl on self similar earthquake triggering bath s law and foreshock aftershock magnitudes simulations theory and results for southern california 1 increasedcatalogincompleteness followingmainshocks can indeed result in artificially low b value estimates for aftershock sequences in their early stages hanizl 2013 nicely documents how this effect could explain the low b values seen for aftershocks of m 2 5 to 5 5 mainshocks in southern california documented in shearer 2012a however catalog incompleteness is most clearly seen following large earthquakes where it is generally attributed to network and or analyst overload and the overlapping codas of the main shock and its larger aftershocks e g kagan 2 004 peng et al 2007 these effects will be less important for smaller main shocks which have shorter codas and individually produce many fewer aftershocks above any threshold magnitude thus it is not clear if a relative lack of smaller aftershocks inthe 12hoursfollowingm2 5to5 5 mainshocks insouthern california is an artifact of catalog incompleteness or a real property of the earth resolving this issue may require new observational studies to search for the missing aftershocks that the catalog incompleteness model predicts should exist 2 one diagnostic property of catalog incompleteness is a flattening in the magnitude versus frequency curve at smaller magnitudes this can be seen in the concave down curvature of the synthetic aftershock dn dm curves in figures 1 and 2b of hanizl 2013 however the curvature is fairly subtle and comparisons to the corresponding data curves are inconclusive some amount of curvature is visible in the data plotted in hainzl s figure 3b but interestingly does not appear in the corresponding data curves plotted in figure 8 of shearer 2012a further study is warranted but resolving the exact shape of the dn dm curves may be difficult given the small differences that need to be resolved 3 i agree with hanizl 2013 that bath s law is subject to some uncertainty with respect to the space time windows used for its computation and that models with higher triggering productivities than those nominally consistent with baths law should be explored comparisons between the absolute numbers of aftershocks seen in the data versus those predicted by triggering simulations should ideally use the same time space windowing method for both data and synthetics thus it is potentially misleading to compare synthetic aftershock sequences with spatially windowed catalog aftershocks as is done in figure 3 of hanizl 2013 i attempted to take these data windowing effects into account in shearer 2012a which concludes that observed aftershock numbers for 2 5 m 5 5 main shocks are too large to be compatible with bath s law in a follow up paper shearer 2012b more completely explores the space time clustering of california earthquakes and reaches a similar conclusion that at least some of the temporal clustering of seismicity observed at short scales 0 5 5km does not appear to be caused by local earthquake triggering but instead reflects an underlying physical process that temporarily increases the seismicity rate such as is often hypothesized to drive earthquake swarms this conclusion is based on comparisons between data and synthetic triggering models over a wide range of scales which identify a number of models versus data differences but perhaps the strongest evidence supporting the conclusion is the anomalously high foreshock to aftershock ratio seen for the smaller earthquakes whichisa point ofagreement withhanizl 2013 4 additional tests of earthquake to earthquake triggering models versus seismic observations are warranted at a range of magnitudes and time distance scales it is important to understand where current models work and where they fail because their limitations provide clues about the underlying physical changes that drive earthquake activity

paleomagnetism of upper devonian reefal limestones canning basin western australia two conflicting models have been proposed for the middle paleozoic position of gondwana with respect to euramerica a pole from morocco msissi norite that may be of late devonian age suggests that a broad ocean 3 000 km separated the northern continents from gondwana whereas results from mauritania gneiguira supergroup indicate that this ocean had closed by devonian time detailed thermal and alternating field demagnetization experiments have been carried out on red and gray limestones from seven sites 89 samples in an essentially undeformed late devonian reef complex northern canning basin western australia after minor tilt correction the site mean characteristic directions average 42 7 28 9 declination inclination with k 61 6 and 95 7 8 these results position northwestern australia at 15 s latitude in the late devonian a paleolatitude consistent with the presence of a well developed reefal sequence an early age is inferred for these magnetizations on the basis of the occurrence of mixed polarities and the fact that no post devonian directions for australia resemble the observed declinations and inclinations if the smith and hallam reconstruction for gondwana is used the paleomagnetic pole calculated for the late devonian of western australia agrees with the msissi norite result from morocco and suggests that an ocean still separated gondwana from euramerica in the late devonian the pole measured from the gneiguira supergroup in mauritania may be based on remagnetizations of late paleozoic age the directions determined from canning basin limestones are similar in inclination to but differ by 40 in declination from paleomagnetic results from the upper devonian comerong volcanics lachlan fold belt southeastern australia canning basin samples are from the stable craton and comerong volcanics samples are from the tasman fold belt the discrepancy between the two australian results may be best explained by tectonic rotation of the comerong volcanics locality

statistical features of foreshocks in instrumental and etas catalogs we study the spatial distribution of earthquakes in temporal intervals before and after the occurrence of large shocks mainshocks in the magnitude range m in 2 5 for four different regional catalogs we find that the spatial organization of pre shock seismicity depends on the mainshock magnitude and is independent of the lower magnitude threshold these properties are found to be a stable feature of regional catalogs and cannot be reproduced by epidemic type aftershock sequence models our findings suggest that the area fractured during the mainshock is encoded in the foreshock spatial organization and therefore enhance the prognostic value of foreshocks

paleomagnetism and late diagenesis of jurassic carbonates from the jura mountains switzerland and france a crucial fold test for magnetizations measured in bajocian through kimmeridgian limestones from the jura mountain region shows that rare blue gray limestones found in this sequence preserve a prefolding magnetization in contrast tan limestones which predominate in this sequence contain secondary magnetizations acquired after folding in the late miocene the postfolding magnetizations in tan limestones appear to be carried in goethite and because reversed as well as normal polarity directions are present the magnetization in these limestones is probably a post miocene pre brunhes 700 000 yr chemical remanent magnetization crm field relations between tan and blue gray limestones which are present in some cases in the same rock unit for example the hauptrogenstein suggest that the tan limestones may have formed from alteration of blue limestones whereas goethite pseudomorphs after pyrite are found in tan limestones the occurrence of pyrite in the blue limestones supports this model indicating that goethite could have formed from oxidation of pyrite abundant meteoric water probably was available for such an oxidation process during uplift of these strata subsequent to deformation natural remanent magnetization in blue limestones appears to be carried in magnetite the positive fold test for the characteristic magnetization in these rocks is significant at the 99 confidence level k values for 24 site mean directions before and after tilt correction are 9 3 and 43 2 respectively the simplest interpretation for the origin of observed remanence in magnetite in the blue limestones is a primary magnetization of depositional or biogenic origin characteristic directions in these rocks yield a mean paleopole position of 77 7n 148 4e k 26 0 a95 5 9 this pole is in reasonable agreement with one other upper jurassic pole from western europe and with other middle to upper jurassic poles from the atlantic bordering continents after rotation of these continents to a reconstructed configuration for middle to late jurassic time

paleomagnetism of late jurassic rocks in the northern canelo hills southeastern arizona the canelo hills volcanics are exposed in the canelo hills a northwest trending range in santa cruz county southeast arizona the formation is composed of silicic tuffs and flows as well as volcaniclastic conglomerates and sandstones strikes of the rocks are generally to the northwest with moderate dips to the southwest and northeast apparent age results from the sequence studied paleomagnetically include two published isotopic dates of 147 6 ma k ar biotite and 149 11 ma whole rock rb sr and a rb sr isochron age reported here which indicates an age of 151 2 ma paleomagnetic samples were collected from 17 cooling units in the northern canelo hills samples from most of these units responded to alternating field af demagnetization and secondary components were generally erased by peak af between 10 and 50 mt samples from five sites showed no response to af demagnetization thermal demagnetization of samples from these units produced no significant changes in direction of natural remanent magnetization nrm although within site clustering of nrm directions was improved data from two sites were rejected because of failure to isolate a well determined characteristic nrm of the remaining 15 sites 10 sites were of normal polarity while five sites showed reversed polarity intensities of the characteristic nrm ranged from 4 10 3 to 3 10 1 a m the data from these 15 cooling units yield a formation mean direction of i 29 9 d 334 9 with k 33 4 and 95 6 7 the resulting paleomagnetic pole is at 62 2 n 130 3 dp 4 1 dm 7 4 this pole is between poles obtained from the summerville and lower morrison formations the canelo hills pole is thus consistent both in position and age with the late jurassic episode of rapid apparent polar wander originally defined by paleomagnetic data from the summerville and morrison formations

a review of paleomagnetic results from the carboniferous of north america the concept of carboniferous geomagnetic field horizon markers results from the east coast carboniferous basin contain an important time stratigraphic horizon of possible global significance late carboniferous units are found to have mostly reversed magnetizations while older carboniferous units may have normal reverse or most frequently dual polarity magnetizations this feature records the change in geomagnetic field from a period of common polarity changes to a period of mainly reversed polarity the kiaman magnetic interval recognition of the stratigraphic position of this polarity feature provides a time line for correlation of units within and among carboniferous basins in the east coast carboniferous basin of atlantic canada this horizon has now been located at the riversdale group level early pennsylvanian namurian westphalian boundary since remanence acquisition of red sediments is often a lengthy process possibly spanning field reversal time scales red bed units pre dating and post dating this time line can in most instances be readily distinguished the former generally recording both polarities and the latter just one reverse polarity a polar displacement of at least 16 is recorded in the results from the east coast carboniferous basin paleolatitude maps show that the area was located near the equator and was displaced northwards by about 8 during the pennsylvanian it cannot be ascertained if those observations are representative of the whole of north america or merely the atlantic seaboard poles derived from other areas of north america do not all fit the regular pattern observed on the atlantic coast using the horizon marker as a tool it is suggested that those poles are post time line in age so that none of them is representative of the position of north america prior to the time line

paleomagnetism and structural history of the ghost range intrusive complex central abitibi belt ontario further evidence for the late archean geomagnetic field of north america on the basis of their remanence properties the lithologies of the late archean 2710 2703 ma ghost range complex an east west trending layered mafic ultramafic extrusive sequence in the central

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

remagnetizations in late permian and early triassic rocks from southern africa and their implications for pangea reconstructions a paleomagnetic study of late paleozoic and early mesozoic sedimentary rocks from southern africa suggests wide spread remagnetization of these rocks samples of the mofdiahogolo formation in botswana and of the lower beaufort group in south africa were treated using thermal alternating field and chemical demagnetization the mofdiahogolo redbeds show a univectoral decay of the remanence revealing a characteristic direction of

influence of non dipole field on determination of plio pleistocene true polar wander some studies of global paleomagnetic data have found an offset of the magnetic pole during the plio pleistocene which has been interpreted as indicating a period of rapid true polar wander with a rate of movement comparable to the present day rate of polar motion deduced from astronomical observations we show that much of the polar offset determined from the paleomagnetic data may be due to deviations in pole position caused by persistent non dipole zonal components of the geomagnetic field a correction of paleomagnetic poles for the long term non dipole field reduces the polar offset and thus suggests a s lower or shorter episode of true polar wander over the past 5 million years

a paleozoic pangaea paleozoic paleogeographies should be consistent with all available reliable data however comparison of three different devonian paleogeographies that are based largely or wholly on the data of remanent magnetism show them to be inconsistent in many regards when these three paleogeographies are provided with possible ocean surface current circulation patterns and have added to them lithofacies and biogeographic data they also are shown to be inconsistent with such data a pangaeic reconstruction positioned in the southern hemisphere permits the lithofacies and biogeographical data to be reconciled in a plausible manner

a middle late silurian paleomagnetic pole from limestone reefs of the wabash formation indiana u s a a middle late silurian paleomagnetic pole has been obtained from reefal limestones in north central indiana eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization one additional site gave the same characteristic direction after great circle analysis these 12 site level directions were used to calculate the mean paleomagnetic pole reef flank beds dip steeply about 38 away from reef centers field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12 a 12 tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement indicating that the magnetization was acquired early the presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence the mean of the site paleomagnetic poles with sites from steeply inclined flank beds corrected for 12 of tilt is 16 8 n 124 9 e k 74 a95 5 1 in good agreement with the middle silurian rose hill pole for north america

errors in minimum plate velocity determined from paleomagnetic data in prior studies paleomagnetic data have been used to determine ancient minimum velocities of the continents the greatest minimum velocities 50 km m y exceed the present day absolute velocities of the major continents this result is important because the observation that present day oceanic plates move faster than present day continental plates can be treated as a major constraint on plate dynamics however these prior studies are limited because they ignore known errors in the paleomagnetic data in this paper we present a technique for incorporating these errors and using them to estimate 95 confidence limits on the minimum velocities this new technique was applied to pennsylvanian to cretaceous paleomagnetic data from north america and to the ancient geometry of laurasia when errors are considered minimum velocities are 10 to 50 km m y lower than when errors are ignored additional uncertainties owing to a hypothesized clockwise rotation of the colorado plateau a source of many of the data are considered and found to alter the minimum velocities only negligibly allowing for uncertainties in the pole positions uncertainties in ages and the uncertainty due to possible rotation of the colorado plateau we estimate that laurasia moved at least 48 km m y from early jurassic to early cretaceous time and at least 43 km m y from pennsylvanian through late triassic time there is only a 5 risk that the true minimum velocities are lower than these lower bounds these velocities are less than the peak velocities over short intervals found in prior analyses but exceed the average velocities previously found for triassic and jurassic time these newly estimated velocities are still much higher than the present velocities of the continents and as in the prior studies imply that the slow motion characteristic of major continents at present is not a fundamental attribute of plate motions

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

global plate motions relative to the hot spots 64 to 56 ma we examined the early tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole in our reconstruction the velocities of the pacific and african plates were inferred directly from their hot spot tracks and the velocities of other plates with respect to the hot spots were calculated from the motion of the pacific or african plate and by relative motions estimated from seafloor spreading data the relative motion between assumed separate north pacific and south pacific chatham rise plates was estimated by assuming the hot spots in the atlantic ocean basin have been fixed with respect to those in the pacific ocean basin thereby avoiding the use of a poorly defined relative motion circuit we found that the characteristics of early tertiary plate motions with respect to the hot spots resemble those at present the root mean square velocity of every major continental plate with respect to the hot spots during the early tertiary exceeds its present velocity but with the exception of the indian plate is less than the root mean square velocity of every early tertiary oceanic plate equatorial lithosphere moved faster than polar lithosphere during the early tertiary but the difference is less than at present over the interval 64 to 56 ma the no net torque absolute velocities differ from the assumed fixed hot spot velocities by only 0 04 ma which is insignificant thus two dissimilar approaches fixed hot spots and no net torque of the lithosphere when applied to identical early tertiary relative plate velocities yield similar absolute plate velocities we attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here

late cretaceous apparent polar wander of the pacific plate evidence for a rapid shift of the pacific hotspots with respect to the spin axis paleomagnetic data from the pacific plate show that the average azimuth of the pacific plate decreased by 12 between 90 ma and 81 ma whereas trends of late cretaceous seamount chains suggest that the pacific plate moved northwestward relative to the hotspots with little change in azimuth during the same time interval this difference is used to infer a 13 4 shift of the spin axis relative to the hotspots in the pacific ocean basin this shift occurred within 5 m y of the occurrence of anomalous behavior of the paleomagnetic field recorded in anomalies 33 and 34 this coincidence in timing suggests there may be a causal link between these three phenomena

paleomagnetism of jurassic rocks in the western sierra nevada metamoprhic belt and its bearing on the structural evolution of the sierra nevada block the western metamorphic belt of the sierra nevada consists of two eugeosynclinal terranes separated by the melones and sonora faults subvertical bedded mesozoic volcanic rocks metamorphosed to low greenschist facies predominate to the west whereas paleozoic metamorphic rocks of higher grade and greater structural complexity predominate to the east in order to study the structural development of the faults 121 samples of basalt and diabase were collected for paleomagnetic analysis from three jurassic formations the logtown ridge and penon blanco formations west of the melones fault and the sonora dike swarm to the east of the sonora fault a northwesterly downward directed magnetization occurs in each unit three fold tests and a conglomerate test on the two formations west of the faults show that the magnetization is secondary postdating nevadan late jurassic folding and is probably coeval with peak metamorphism an average of five paleomagnetic poles from the sierra nevada three derived from the secondary magnetizations given herein and two previously published all of probable kimmeridgian age yields 67 2 n 161 2 e and 95 6 5 southeasterly magnetizations also occur in the logtown ridge formation and sonora dike swarm directions from the sonora dikes are approximately antipodal to the secondary directions and are reversed magnetizations from the logtown ridge formation yield similar results only if corrected for the tilt of bedding the logtown ridge magnetizations tilt corrected yield a pole position near to that expected for north america the data from the sonora dikes require a tilt correction of 25 30 toward the south southwest about a horizontal axis parallel to the regional structure in order to yield a north american pole position we conclude that the eastern wall rocks of the melones and sonora faults have been rotated 25 30 in response to nevadan deformation in contrast to the western wall rocks which have been rotated about 90

space time clustering of seismicity in california and the distance dependence of earthquake triggering 1 using two recent high resolution earthquake catalogs i examine clustering in california seismicity by plotting the average rate of earthquakes as a function of both space and time from target events of m 2 to 5 comparisons between pre and post target event activity can be used to resolve earthquake to earthquake triggering associated with target events of different magnitudes the results are more complicated than predicted by computer simulations of earthquake triggering that begin with background events occurring at random times in particular at least some of the temporal clustering of seismicity at short scales 0 1 to 5 km does not appear to be caused by local earthquake triggering but instead reflects an underlying physical process that temporarily increases the seismicity rate such as is often hypothesized to drive earthquake swarms earthquake triggering for m 4 5 earthquakes is only resolvable in average seismicity rates at times less than about one day and to distances of less than about 10 km and its linear density decreases as r 1 5 to r 2 5 significantly steeper than some previous studies have found

magnetostratigraphy paleomagnetism and remanence acquisition in the triassic chugwater formation of wyoming the magnetic reversal sequence determined at five sections of the chugwater formation can be correlated over a distance of more than 200 km a detailed study of 203 cores constituting a grid of samples taken from a normal to reverse transition zone at the red grade section shows that the transition is characterized by a fairly smooth 180 direction change accompanied by an intensity decrease in about 80 cm of section the transition occurs in the same way in all vertical columns within the grid and in other columns at the same stratigraphic level up to 80 m from the grid and appears identical to the transition reported by herrero bervera and helsley at the same lithologic horizon about 1 km from the grid thermal and chemical demagnetization of grid samples removes a secondary remanence in the general direction of the earth s present magnetic field and does not reveal the presence of normal and reverse components in the same sample the transition appears to have been magnetized during a reversal of the earth s field samples probably became magnetized in a few hundred years or less while they were close to the deposition surface paleomagnetic pole positions for all five sites are very closely clustered and there appears to be no consistent long term polar wander from bottom to top in the sections the mean pole position 46 6 n 113 5 e 95 1 88 is significantly different from pole positions for the early triassic moenkopi formation from the colorado plateau suggesting that post triassic rotation has occurred between the plateau and the basement uplifts of wyoming

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

implications of early devonian poles from the canadian arctic archipelago for the north american apparent polar wander path lower devonian red beds from the upper member of the peel sound formation at prince of wales island canadian arctic yield two different paleopoles at 25n 099e and at 01n 091e the first one being

paleomagnetic evidence for crustal and thin skinned rotations in the european hercynides devonian and carboniferous paleomagnetic data from the european hercynides show a coherent pattern in inclination but significant dispersion in declination the declination anomalies with respect to expected declinations for stable europe indicate a strong correlation with the regional changes in the structural trend of the mountain belt detailed analysis of the paleomagnetic and structural data revealed the highest degree of correlation between changes in strike and declination anomalies for external parts of the orogen where thin skinned nappe emplacement has been most prevalent the correlation between changes in strike and declination anomalies from interior parts of the hercynides is less well defined and displays a greater degree of scatter nevertheless the partly secondary character of the arcuate shape can be demonstrated thus the internal hercynides are at least in part a true orocline with secondary bending in the sense of s w carey we infer that the bending of the european hercynides resulted from the combination of multiple deep reaching deformations of the european lithosphere as well as the effects of thin skinned rotations the decreasing degree of rotations toward the crystalline interior of the belt appears to favor a geodynamic model involving indentations and or buttressing effects the indentation of hercynian europe by a microplate or an african promontory during the hercynian orogeny might be one of the principal causes for the secondary bending and the associated nappe rotations

paleomagnetism of late archean metavolcanics and metasediments abitibi orogen canada volcanics of the blake river group and sediments and volcanics of the timiskaming group a paleomagnetic study 159 samples 30 sites was done on calc alkaline volcanics of the blake river group dated at 2703 2 ma and occupying the central portion of a major synclinorium in the abi

british paleomagnetism iapetus ocean and the great glen fault new paleomagnetic data for britain establish the width of the iapetus ocean as 1000 km in mid ordovician time but do not yet allow precise estimates of the rate of its subsequent closure the new lower paleozoic data from northern scotland are similar to data from the rest of britain but are not compatible with north america data on the bullard et al reassembly if this discrepancy is correct lines of major transcurrent dislocation have yet to be identified either in the north atlantic continental shelves and margins or in eastern north america and the great glen fault was not the main locus of offset between britain and north america the tectonic complexities associated with this configuration require further evaluation

paleomagnetism of certain constituents of the bay st george sub basin western newfoundland abstract paleomagnetic studies have been made of certain constituents of the bay st george sub basin specifically results are reported from the spout falls formation tournaisian the jeffreys village member of the robinsons river formation visean and the searston formation namurian westphalian the following magnetizations have been isolated spout falls a tournaisian with d 343 5 i 22 7 k 61 2 95 7 1 and the corresponding pole at 28 6 n 139 5 e 4 5 8 5 spout falls b kiaman with d 166 7 i 12 2 k 51 7 95 10 7 and the corresponding pole at 34 5 s 42 7 w 5 5 10 9 jeffreys village a visean with d 351 2 i 27 3 k 54 0 95 7 6 and the corresponding pole at 26 5 n 130 7 e 4 5 8 3 searston a namurian with d 161 7 i 11 7 k 107 95 7 4 and the corresponding pole at 33 9 s 37 2 w 3 8 7 5 and searston c with d 111 6 i 13 8 k 28 8 95 14 5 and the corresponding pole at 19 6 s 19 0 e 7 6 14 8 after comparison with paleopoles of similar ages derived from eastern and western newfoundland rocks from constituents of the east coast basin and for interior north america it is concluded that 1 it is unlikely that any large scale relative motion took place since the early carboniferous between eastern and western newfoundland 2 it is unlikely that any north south relative motion took place between the east coast basin and the bay st george sub basin and 3 the bay st george sub basin results do not support the earlier proposed displaced terrane hypothesis of the northern appalachians in as much as the motions during the carboniferous are not supported there is evidence of the northward motion of the appalachians and north america as a whole during the carboniferous the magnetostratigraphic horizon marker in the carboniferous separating a dominant normal and reversed magnetization on the older side and an entirely reversed kiaman magnetization on the younger side may be placed in the bay st george sub basin at the base of the searston formation

paleomagnetism of the north american precambrian a look at the data base more than half of the approximately 650 paleopoles reported from all precambrian cratons have been obtained from north america the observations have spurred a proliferation of apparent polar wander paths to support different hypotheses for the tectonic evolution of the continent many of the reasons for this development can be traced to the inadequacy of the data base with its average of only 1 6 poles 10 ma 1 as compared to 7 poles 10 1 ma for the last 300 ma even when it is assumed that a primary pole has been derived from each rock unit sampled the total number of primary poles excluding the 900 1200 ma interval yields an average of only 0 4 pole 10 ma 1 an inspection of the data base shows a non systematic sampling of a non continuous fragmented paleomagnetic record with gaps as large as tens and even hundreds of ma more importantly a search through the literature reveals that very few poles have an age known to better than 50 ma and for many poles the age bracket could be as large as a few hundreds of ma this is partly owing to the fact that radiometric and paleomagnetic sites rarely originate from the same localities it is shown that this limited data base with all its deficiencies and uncertainties cannot be used to support certain hypotheses or for the drawing of a meaningful apparent polar path for the whole of the precambrian it appears that because of the enormous time scale precambrian paleomagnetism should rather be directed towards the construction of segments of polar paths such as those in the 900 1200 ma interval comparison of well dated contemporaneous segments from different parts or structural provinces could be used most effectively to delineate relative movements if any and reconstruct the mosaic of the past continent for certain time intervals one of the recent developments of precambrian paleomagnetism has been to show that the apparent pole did not remain constrained to the equatorial region but made several excursions into high latitudes

magnetostratigraphy of the red sandstone creek section vail colorado samples for paleomagnetic study were collected from the area of red sandstone creek lat 39 73 n long 253 6 near vail colorado in hopes of locating the stratigraphic base of the kiaman reversed interval to within a few meters previous workers had sampled the same upper pennsylvanian and permian minturn and maroon formations in this section and had placed the base of the kiaman at 91m to 213m above the minturn formation the interval from 276m below to 420m above the maroon minturn contact was sampled in the present study at one meter intervals where possible of the 171 samples collected only 2 had near normal components the 2 were not consecutive and the normal components were removed easily by thermal cleaning or were shown to be a viscous overprint of the present field the previous workers were apparently misled by a lack of magnetic cleaning of their samples the conclusion of this study is that the base of the kiaman must be stratigraphically lower in this section the pole position from the sampled section is lat 42 6 n long 115 4 e a95 2 75 for n 145 samples

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

paleomagnetic study of thrust sheet rotation during foreland impingement in the wyoming idaho overthrust belt upper jurassic and lower cretaceous strata in the darby and absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the wyoming idaho overthrust belt no significant rotations were found lack of rotation in these thrust sheets suggests that previously reported rotation of the prospect thrust sheet was not transmitted to the older darby and absaroka thrust sheets indicating that the prospect thrust sheet only experienced local rotations in response to collision with the teton gros ventre uplift the upper jurassic stump formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64 n 170 e k 18 a95 15 in good agreement with other published upper jurassic poles for cratonic north america samples from all lower cretaceous sites are believed to possess a synfolding remagnetization of chemical origin

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

late palaeozoic to early mesozoic evolution of pangaea several possible configurations of the pangaea supercontinent have been suggested for the interval from late carboniferous to early triassic here we re examine the palaeomagnetic basis for these models emphasizing the trends of the paths of apparent polar wander for the individual components of the supercontinent rather than simply averaging poles of presumed similar age two of the alternatives pangaea b and c may result from averaging poles of dissimilar age along common polar wander paths giving rise to spurious tectonic displacements the most likely model appears to be the formation in late devonian time of a modified pangaea a2 followed by evolution to the traditional configuration a during the triassic

cretaceous and early tertiary paleomagnetic results from the antarctic peninsula paleomagnetic results are presented for early tertiary and cretaceous igneous and volcanic units from the northern antarctic peninsula when combined with previously published data they permit the construction of a polar wander path that is very similar to one derived for the east antarctic craton by rotating tertiary and cretaceous poles from india and australia into the antarctic frame of reference when considered in this context the paleomagnetic data from the orville coast which was previously interpreted as evidence in favor of oroclinal bending or relative rotation of the southern antarctic peninsula with respect to the northern part may be considered as an in situ resulte the implications of the new data are as follows 1 the antarctic peninsula down to the orville coast has been a part of the antarctic plate since 100 m y ago 2 movement of the antarctic peninsula required to avoid overlap in reconstructions of gondwanaland must have occurred before 100 m y ago

paleomagnetism of the late precambrian fourchu group cape breton island nova scotia volcanogenic sediments of the fourchu group and a gabbroic intrusion which are found in the avalonian terrane of south eastern cape breton island have been sampled for paleomagnetic analysis upo

short term properties of earthquake catalogs and models of earthquake source i review the short term properties of earthquake catalogs in particular the time and size distributions and the completeness of the early part of aftershock sequences for strong shallow earthquakes i determine the parameters of the omori and gutenberg richter laws for aftershocks close in time to a mainshock aftershock sequences of large earthquakes in southern california 1952 kern county 1992 joshua tree landers big bear sequence 1994 northridge and 1999 hector mine recorded in the caltech catalog are analyzed to demonstrate that at the beginning of these series many small earthquakes are absent from the catalog the number of missing earthquakes increases with the magnitude range of a catalog and for some data sets exceeds the number of aftershocks close to a mainshock listed in a catalog comparing global earthquake catalogs harvard centroid moment tensor and preliminary determination of epicenter with local data sets indicates that the catalogs based on longer period waves miss many early aftershocks even when their magnitudes are well above the stated magnitude threshold such short term incompleteness may introduce significant biases to the statistical analysis of the seismicity pattern in particular for branching models of earthquake occurrence incorporating the omori law for such models the likelihood function strongly depends on close aftershocks i review the techniques to alleviate this problem analyzing the source rupture process of several recent large earthquakes suggests that rupture propagation is highly inhomogeneous in space time and focal mechanism these random variations in the rupture process can be viewed as an extension of the aftershock stochastic generating mechanism toward the origin time of a mainshock i review various models of the earthquake rupture process and suggest that fractal distributions of microevents in time space and focal mechanism constitute the development of an earthquake the final identification of an individual earthquake depends on both objective and subjective factors manuscript received 21 may 2003

paleomagnetism and magnetic mineralogy of the nahant gabbro and tonalite eastern massachusetts paleomagnetic analyses of the nahant gabbro and an associated tonalite of eastern massachusetts reveal five separate populations of paleomagnetic directions the gabbro and tonalite exposed in the northernmost portion of the boston basin were selected for this study because of the lack of significant macroscopic evidence of penetrative metamorphism or deformation the tonalite has not been radiometrically dated however an ordovician date has been reported for the gabbro detailed petrographic data indicate that the five populations of paleomagnetic directions obtained for the gabbro and tonalite correspond to five styles of magnetic mineral alteration a mean alteration index was calculated for each site in the nahant gabbro and tonalite there is an apparent relationship between magnetite alteration and normalized intensity at a given demagnetization level with normalized intensity decreasing as magnetite alteration increases this relationship illustrates the interdependence between paleomagnetic pr

a paleocene north american paleomagnetic pole incorporating declination only data a problem in determining apparent polar wander paths for the continents is how to incorporate the valid declinations from paleomagnetic studies that have measured shallowly biased inclinations i e inclination error it is proposed that in such cases pole positions and confidence limits be determined using a method previously applied only to paleomagnetic data from oceanic plates applying this method to paleomagnetic data from 67 55 ma rocks of north america a paleomagnetic pole 78 7 n 185 1 e and 95 confidence ellipse 6 0 major semiaxis striking 105 clockwise from north and minor semiaxis of 3 7 were determined an analysis of the information content of the data shows that the single declination only datum from the nacimiento formation new mexico contains 31 of the information which would have been wasted by conventional paleomagnetic analysis if excluded or caused a bias if the inclination had also been included

a late eocene paleomagnetic pole for the pacific plate abstract seamount magnetic anomaly inversions as well as dsdp paleomagnetic and equatorial sediment facies data constrain a paleomagnetic pole for the pacific plate of late eocene age the location of the pole at 77 5 n 21 2 e implies 12 5 1 6 of apparent polar wander for the pacific plate during the last 41 5 m y the late eocene pole is significantly different from the pacific maastrichtian pole at the 95 confidence level and indicates 7 2 of apparent polar motion of the pacific between 69 and 41 m y b p the data source locations for the late eocene pole are scattered over a large area of the north pacific and thus the consistency of the data supports the hypothesis that the north central pacific plate has been rigid since the eocene the agreement of the late eocene pole with the motion predicted for the pacific from hotspot models suggests that relative motion between the spin axis and hotspots has been small since that time additionally this finding dictates that the significant amounts of hotspot versus spin axis motion inferred by other authors to have occurred since the cretaceous must have instead occurred at a faster rate and concluded before the eocene

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

paleomagnetism of siluro devonian and cambrian granitic rocks from the avalon zone in cape breton island nova scotia as part of a major study of the avalon zone in cape breton island we describe the paleomagnetism of radiometrically well studied granitic rocks of two generations representing the acadian 350 45

paleomagnetism of a polarity transition in the lower triassic chug water formation wyoming a total of 156 oriented specimens taken at stratigraphic intervals of 3 to 20 cm have been collected from the red peak member of the chugwater formation at three localities approximately 25 km southeast of dubois wyoming the stratigraphic interval containing the polarity transition in the remanent magnetic field is about 1 0 m all specimens were thermally demagnetized at 500 550 620 and 660 c and show directions of magnetization of normal reversed and transitional character as well as a remarkable decrease of the remanent intensity in the transition zone the section sampled is divided into five intensity periods two periods recording the dipolar field one recording a dipolar field of decreasing intensity one recording a dipolar field of increasing intensity and a period of low intensity recording the field present during the 180 directional swing a virtual geomagnetic pole vgp path obtained for a n r polarity transition makes a fairly smooth transit with the path confined to a sector of longitude between 85 w and 110 e the reversal s onset is first indicated by a slow movement of the vgp across northern latitudes the vgp then lingers to two positions 60 n and 40 n 35 e to 70 e and 20 n and 40 n 335 e to 5 e suggesting brief periods of field stability or nondipolar field dominance the onset of the transition appears to be more rapid than the recovery the vgp path observed at a northern hemisphere site is located about 90 from the site the pole position derived from stable normal and reversed portions of this study 115 5 e 46 4 n 95 4 1 dp 2 2 dm 4 2 lies approximately in the center of the main group of poles reported for other triassic units

paleomagnetism of early cambrian redbeds on cape breton island nova scotia results are reported from steeply dipping red sandstones collected at nine sites 45 samples of the lower cambrian morrison river and maccodrum formations on southeast cape breton island which forms part of the avalon terrane demagnetization studies indicate minimal overlap of stability spectra in thermal alternating field and chemical treatments a stable component hematite with high unblocking temperatures 680 c is dominant at all sites its mean direction corrected for geologic tilt is d 281 i 32 k 97 giving a paleomagnetic pole pole m at 20 n 146 w dp dm 3 6 the corresponding in situ direction is d 147 i 59 k 97 with a pole m at 2 s 39 w dp dm 6 8 poles m and m were compared with published paleozoic poles for the avalon zone and cratonic north america to test whether the magnetization originated before or after the taconic ordovician folding it could have been acquired during deposition or early diagenesis early to middle cambrian in whic

paleomagnetism of the deadman s bay diabase dikes from northeastern central newfoundland twenty six oriented samples were collected from six diabase dikes cutting the deadman s bay granite of northeastern central newfoundland for the purpose of paleomagnetic investigations the dikes probably form part of a general swarm of diabase dikes in this area of devonian age 40ar 39ar age of 370 10 ma stepwise thermal demagnetization of individual specimens followed by vector subtraction based on zijderveld plots revealed two characteristic magnetizations one a poorly grouped magnetization of 78 0 95 17 4 16 carried by multidomain magnetite and a better grouped magnetization of 4 0 95 10 0 42 residing in hematite a baked contact test of the host rock deadman s bay granite intruded by the diabase dikes revealed a steep well grouped magnetization which is not dissimilar to that carried by magnetite in the dikes the magnetization of the host rock seems to be well grouped irrespective of distance away from the dike thus making the contact test inconclusive

comment on self similar earthquake triggering bath s law and foreshock aftershock magnitudes simulations theory and results for southern california by p m shearer 1 shearer 2012 finds three differences of the seismicity clustering in southern california compared to self similar triggering models i a significantly lower b value for the aftershocks ii a too large aftershock number and iii a too large foreshock aftershock ratio to be consistent with the bath law based on these observations the author concluded that the observed seismicity is not in agreement with self similarity triggering and or the observed clustering is not primarily caused by earthquake to earthquake triggering however i show that the observed lower b value is likely related to incomplete recordings after mainshocks and that the apparently too large aftershock number does not disprove the self similarity thus only the enhanced foreshock to aftershock ratio seems to indicate some discrepancy to self similar triggering

paleomagnetism of the late ordovician thouars massif vendee province france a paleomagnetic study of the late ordovician thouars massif reveals very steep components of magnetization directed upward and downward and with thermally discrete blocking temperatures of 550 to 580 c this characteristic magnetization yields a mean direction with declination inclination 340 83 with k 27 and 95 7 based on 15 sites the south paleopole is located at 34 n 5 e a partial magnetic overprint of hercynian age is observed in sites close to the south armorican shear zone the 15 sites are spread over some 20 by 30 km and the internal consistency of the site mean directions indicates that differential tilting of parts of the massif has not occurred since it appears to be unlikely that the entire massif was tilted we infer that the characteristic direction is primary and that the paleopole adequately represents the local magnetic field in the late ordovician the paleopole for the thouars massif supports previously obtained results from other ordovician rocks in hercynian europe but it is the first result to pass modern reliability criteria the high ordovician paleolatitudes of the armorican massif other areas in hercynian europe and north western africa indicate that gondwana and armorica were adjacent until late ordovician time low paleolatitudes for late devonian time suggest that armorica underwent rapid northward motion at a rate of about 10 cm yr during the silurian and earliest devonian armorica is thought to have collided with north america and the baltic shield russian platform before or during the early devonian forming the old red continent the collisions produced the caledonian and acadian orogenic belts

cosmic ray exposure and pre atmospheric size of the gebel kamil iron meteorite cosmogenic he ne and ar as well as the radionuclides 10be 26al 36cl 41ca 53mn and 60fe have been determined on samples from the gebel kamil ungrouped ni rich iron meteorite by noble gas mass spectrometry and accelerator mass spectrometry ams respectively the meteorite is associated with the kamil crater in southern egypt which is about 45 m in diameter samples originate from an individual large fragment individual as well as from shrapnel concentrations of all cosmogenic nuclides stable and radioactive are lower by a factor 3 4 in the shrapnel samples than in the individual assuming negligible 36cl decay during terrestrial residence indicated by the young crater age 85 cm i e a pre atmospheric mass 20 tons with a preferred radius of 115 120 cm 50 60 tons the analyzed samples came from a depth of approximately 20 cm individual and approximately 50 80 cm shrapnel the size of the gebel kamil meteoroid determined in this work is close to estimates based on impact cratering models combined with expectations for ablation during passage through the atmosphere folco et al 2010 2011

some paleomagnetic constraints on the tectonic evolution of the coastal cordillera of central chile paleomagnetic samples collected from 19 sites in radioisotopically dated jurassic mafic dykes and plutons in the coastal cordillera of chile between latitude 32 15 and 33 30 s yield paleomagnetic poles calculated by averaging the virtual geomagnetic poles from approximately coeval sites at latitude 85 9 s longitude 36 7 e n 8 a95 11 9 for sites dated at close to 170 ma and at latitude 80 3 s longitude 207 8 e n 10 a95 9 5 for sites dated at close to 145 ma these paleomagnetic poles from the coast of chile coincide with poles from coeval sequences within the south american craton some of the igneous rocks that constitute these sampling sites crosscut a high grade metamorphic complex that is distinct from other segments of the coastal cordillera in age structural style and metamorphic grade this complex cannot be correlated with metamorphic rocks anywhere else in central western south america and could be considered a potentially exotic allochthonous terrane if this terrane has been added to the western south american margin then accretion occurred prior to 170 ma paleomagnetic results presented here indicate that no major rotation or large scale latitudinal translation of crustal blocks has taken place in this part of central chile since middle jurassic time

paleomagnetism and rock magnetism of the mississippian leadville carbonate formation and implications for the age of sub regional dolomitization the mississippian leadville formation of central colorado is dolomitized near the sawatch uplift and hosts numerous pb zn ag mineral deposits paleomagnetic and rock magnetic data from several dolomite types away from centers of tertiary intrusive activity imply a late paleozoic late mississippian age for the dolomitization e g d 148 i 11 k 86 95 8 6 n 3 localities 31 samples in dolomitized units the magnetization is of diagenetic origin and was probably acquired during or soon after dolomitization tertiary thermochemical activity has overprinted dolomites close to intrusive centers data indicate that limestones removed from igneous activity were also remagnetized at the same time