rheologic and structural controls on the deformation of okmok volcano alaska fems insar and ambient noise tomography 1 interferometric synthetic aperture radar insar data indicate that the caldera of okmok volcano alaska subsided more than a meter during its eruption in 1997 the large deformation suggests a relatively shallow magma reservoir beneath okmok seismic tomography using ambient ocean noise reveals two low velocity zones lvzs the shallow lvz corresponds to a region of weak fluid saturated materials within the caldera and extends from the caldera surface to a depth of 2 km the deep lvz clearly indicates the presence of the magma reservoir beneath okmok that is significantly deeper 4 km depth compared to previous geodetic based estimates 3 km depth the deep lvz associated with the magma reservoir suggests magma remains in a molten state between eruptions we construct finite element models fems to simulate deformation caused by mass extraction from a magma reservoir that is surrounded by a viscoelastic rind of country rock embedded in an elastic domain that is partitioned to account for the weak caldera materials observed with tomography this configuration allows us to reduce the estimated magma reservoir depressurization to within lithostatic constraints while simultaneously maintaining the magnitude of deformation required to predict the insar data more precisely the insar data are best predicted by an fem simulating a rind viscosity of 7 5 1016 pa s and a mass flux of 4 2 109 kg d from the magma reservoir the shallow weak layer within the caldera provides a coeruption stress regime and neutral buoyancy horizon that support lateral magma propagation from the central magma reservoir to extrusion near the rim of the caldera

surface wave tomography of the western united states from ambient seismic noise rayleigh and love wave phase velocity maps summary we present the results of rayleigh wave and love wave phase velocity tomography in the western united states using ambient seismic noise observed at over 250 broad band stations from the earthscope usarray transportable array and regional networks all available threecomponent time series for the 12 month span between 2005 november 1 and 2006 october 31 have been cross correlated to yield estimated empirical rayleigh and love wave green s functions the love wave signals were observed with higher average signal to noise ratio snr than rayleigh wave signals and hence cannot be fully explained by the scattering of rayleigh waves phase velocity dispersion curves for both rayleigh and love waves between 5 and 40 speriod were measured for each interstation path by applying frequency time analysis the average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station triplets we find that empirical green s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross correlation without explicit knowledge of the source distribution the average traveltime uncertainty is less than 1 s at periods shorter than 24 s we present rayleigh and love wave phase speed maps at periods of 8 12 16 and 20 s the maps show clear correlations with major geological structures and qualitative agreement with previous results based on rayleigh wave group speeds

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

evidence from aeromagnetics on the configuration of matachewan dykes and the tectonic evolution of the kapuskasing structural zone ontario canada by digital image processing of federal provincial aeromagnetic survey data for the south central superior province we have obtained an improved picture of the distribution of dykes in the huge matachewan mafic dyke swarm 2454 ma we deduce from it a picture of post emplacement deformation in the vicinity of the uplifted granulite gneisses of the kapuskasing structural zone ksz matachewan dykes are emplaced in three subswarms the two easterly subswarms are clearly truncated by the ksz s eastern boundary faults the western subswarm shows an open z bend as it crosses the ksz but it does not reveal any major fault offset on the plausible assumption supported by paleomagnetic data that the subswarms were originally intruded radially the horizontal strain suffered by the ksz since emplacement of the dykes is mainly a northeast southwest trending band of dextral transcurrent deformation which in the northeast is discontinuous and concentrated in a fault horizontal offset 60 80 km and in the south

ice physical properties structural characteristics and stratigraphy in hobson s choice ice island and implications for the growth history of east ward hunt ice shelf canadian high arctic

arctic ice shelves and ice islands origin growth and disintegration physical characteristics structural stratigraphic variability and dynamics ice shelves are thick floating ice masses most often associated with antarctica where they are seaward extensions of the grounded antarctic ice sheet and sources of many icebergs however there are also ice shelves in the arctic primarily located along the north coast of ellesmere island in the canadian high arctic the only ice shelves in north america and the most extensive in the north polar region the ellesmere ice shelves originate from glaciers and from sea ice and are the source of ice islands the tabular icebergs of the arctic ocean the present state of knowledge and understanding of these ice features is summarized in this paper it includes historical background to the discovery and early study of ice shelves and ice islands including the use of ice islands as floating laboratories for polar geophysical research growth mechanisms and age the former extent and the twentieth century disintegration of the ellesmere ice shelves and the processes and mechanisms of ice island calving are summarized surface features thickness thermal regime and the size shape and numbers of ice islands are discussed the structural stratigraphic variability of ice islands and ice shelves and the complex nature of their growth and development are described large scale and small scale dynamics of ice islands are described and the results of modeling their drift and recurrence intervals are presented the conclusion identifies some unanswered questions and future research opportunities and needs

locating tremor using stacked products of correlations we introduce a back projection method to locate tremor sources using products of cross correlation envelopes of time series between seismic stations for a given subset of n stations we calculate

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

geometry of the kapuskasing structure from a lithoprobe pilot reflection survey a short 10 km pilot seismic reflection profile recorded in the kapuskasing structural zone near foleyet ontario reveals reflections with travel times of at least 5 0 s about 16 km depth an event dipping west from about 1 7 to 3 4 s probably corresponds to the ivanhoe lake cataclastic zone the true geologic dip on this boundary is calculated to be about 35 38 west northwest and is consistent with interpretations of this zone as a west dipping crustal scale thrust fault east dipping reflections are observed with longer travel times than the fault zone arrival and are likely from within the greenstone terrane of the abitibi subprovince

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

the tectonic relationship of two early proterozoic dyke swarms to the kapuskasing structural zone a paleomagnetic and petrographic study a tea coloured cloudiness in groundmass feldspars characterizes dykes of two early proterozoic swarms 2 45 ga matachewan 2 04 ga kapuskasing that intrude amphibolite to granulite host rocks with

early proterozoic activity on archean faults in the western superior province evidence from pseudotachylite major transcurrent faults in the superior province developed in the late archean at the close of the kenoran orogeny reactivation of some of these faults late in the early proterozoic is indicated by rb sr analyses of pseudotachylite from the rainy lake seine river and quetico faults in the rainy lake region of minnesota and ontario fault veins of pseudotachylite and immediately adjacent country rock at two localities yielded subparallel isochrons that are pooled for an age of 1947 23 ma k ar and rb sr biotite ages register earlier regional cooling of the terrane at about 2500 ma with no evidence of younger thermal overprinting at temperatures exceeding 300 c accordingly the 1947 23 ma age is interpreted as dating the formation of the pseudotachylite reactivation of existing faults at this time was caused by stresses transmitted from margins of the superior province where compressional tectonic events were occurring

interpretation of rayleigh wave ellipticity observed with multicomponent passive seismic interferometry at hekla volcano iceland the 2010 eruption of eyjafjallajokull has drawn increased attention to iceland s eastern volcanic zone evz due to the threat it poses to the heavily used air traffic corridors of the northern atlantic ocean within the evz hekla is historically one of the most active volcanoes and has exhibited a decadal eruption pattern for the past 40 years hekla most recently erupted in 2000 and is thus ripe for another decadal eruption because hekla is generally aseismic except for a brief time period hours leading up to an eruption monitoring has previously depended on precursory deformation signals linde et al 1993 as a result seismic tomography of the internal structure of the volcano using phase arrivals of local earthquakes is not possible motivated by hekla s practically aseismic behavior in inter eruptive periods we installed a temporary network of four broadband seismometers around the volcanic edifice in late august 2010 with the intention of investigating the applicability of passive seismic

regional variation in paleomagnetic polarity of the matachewan dyke swarm related to the kapuskasing structural zone ontario the 2 45 ga matachewan dykes from the abitibi subprovince of the canadian shield yield a mean paleomagnetic pole of 42 n 58 e 95 3 n sites 36 which is a composite of new and previously published data domains of paleomagnetic polarity are defined an area of dykes predominantly of reversed magnetization in the abitibi subprovince contrasts with an area of exclusively normal dykes to the north the polarity domains are separated by faults related to the 1 95 ga uplift and exposure of the lower crust in the kapuskasing structural zone and therefore reflect hudsonian age tectonics in the archean superior province

evidence for a very long term trend in geomagnetic secular variation reconstructions of palaeosecular variation suggest that the earth s magnetic field reversed less frequently 2 82 to 2 45 billion years ago relative to the cenozoic era this suggests a long term trend of decreasing geodynamo stability since the archaean eon the earth s inner core is believed to inhibit rapid fluctuations in the geomagnetic field from developing into full polarity reversals1 2 consequently during the precambrian the smaller size of the inner core might suggest that polarity reversals could occur more readily it is therefore surprising that there are indications that reversals were rare during this period3 4 here we use new and existing palaeomagnetic data from three continents to examine the stability of the earth s magnetic field from 2 82 to 2 45 billion years ago we show that on average geomagnetic secular variation the field variations produced by normal geodynamo action during the late archaean and early proterozoic was different from that of the past 200 million years specifically the apparent variability of the geomagnetic pole as viewed at low and mid latitudes was reduced relative to the past 200 million years according to both dynamo simulations4 and more recent palaeomagnetic field observations5 the observed pattern of secular variation suggests a lower frequency of polarity reversals 2 5 billion years ago this may imply that the geodynamo is becoming progressively less stable over long timescales consistent with some numerical simulations6 7 possibly as a result of changing outer core geometry that has accompanied inner core growth

variations in seismic velocity and attenuation associated with seismogenesis a numerical verification using ambient noise abstract seismic velocity variations associated with the seismogenic process have been studied worldwide for almost half a century since the dilatancy hypothesis proposed in early 1970s however the reports in seismogenesis associated variations in attenuation are rare reports on simultaneous variation of velocity and attenuation are even rarer the conventional way to obtain the seismogenic temporal variation in velocity and attenuation is through the observation of travel time and amplitude variations of microseismicity in a seismogenic zone anyhow for some major earthquakes there may not always be microseismicity prior to the mainshock thus obtaining a complete record of seismic velocity and attenuation variation from microseismicity is severely limited if pre mainshock microseismicity exists in contrast seismic ambient noise is an ideal source for crustal stimulation for monitoring temporal variations in velocity and attenuation in a seismogenic zone in this paper the seismogenesis associated seismic velocity and attenuation variations shown as some observable parameters in ambient noise measurements are verified using a numerical simulation approach first based on the temporal variation in seismic velocity observed along the longmenshan fault associated with the 2008 wenchuan earthquake we divided the seismogenic process into six phases upon velocity drop stages in an elliptic area in the crust second using finite difference time domain method we generated 30 minute low frequency ambient noise over a vertical profile of 200 45 km and recorded it with a 90 station array on the surface next we processed the synthetic ambient noise records to get the auto correlation function acf cross correlation function ccf rayleigh wave dispersion curve and horizontal to vertical spectral ratio h v finally we examined the temporal variation of these parameters versus the phases of the seismogenic process and found the most pronounced changes occur in the phase with the largest velocity drop and the recovery phase directly before the mainshock

broad scale proterozoic deformation of the central superior province revealed by paleomagnetism of the 2 45 ga matachewan dyke swarm an extensive paleomagnetic study of the 2 45 ga matachewan dyke swarm of the north american superior province suggests that the interior of an archean shield can undergo broad scale distortion as a result of later proterozoic orogenic activity around the craton margins data collected from over 300 sites of which 137 are reported here for the first time reveal that the dykes contain a dual polarity primary remanence that varies across the swarm in both inclination and declination these regional variations are statistically significant at the 95 confidence level and cannot be attributed to remagnetization or to magnetic anisotropy inclination variation is probably due to real or apparent polar wander during the emplacement of the swarm and may in part explain the declination variation as well however for dykes within and northwest of the kapuskasing structural zone ksz a positive correlation is found between regionally averaged values of declination and dyke trend here the dykes appear to hav

a block model of distributed deformation by faulting it is clear from palaeomagnetic evidence that large and hitherto unexpected rotations about a vertical axis are common in regions of distributed continental deformation we propose a simple two dimensional model to illustrate how fault movement and block rotation within a zone of distributed deformation may be related to the relative motion of the rigid plates that bound the zone one surprizing feature of this model is that the component of strike slip motion on the faults within the deforming zone is in the opposite sense to the strike slip component across the zone as a whole thus slip vectors within the zone are not the same as that between the bounding rigid plates in contrast to deformation at oceanic plate boundaries examples of active fault geometries similar to those of our model can be seen in greece iran and the western usa

airborne uhf radar sounding of glaciers and ice shelves northern ellesmere island arctic canada more than 2000 line kilometres of ice were sounded in 1981 using an 840 mhz pulsed radar system the surveyed regions include mt oxford ice cap disraeli glacier milne glacier ward hunt ice shelf and milne ice shelf in northern ellesmere island arctic canada the maximum ice thickness that was reliably recorded was 750 m for milne glacier in addition to ice thickness interpretation the power reflection coefficient prc and propagation loss rate were estimated from the data measured basal prc s differ for ice shelves floating glaciers and grounded glaciers spatial variations of prc have been used to infer a composite structure for the ice shelves ward hunt ice shelf proved difficult to sound probably because of its known saline and brackish chemistry excellent results were obtained for milne ice shelf and its depth was found to average roughly 70 m in places attaining 100 m bottom side crevasses were observed within milne ice shelf the characteristic ridge and trough topography of the ice

high grade metamorphism in the chapleau foleyet area ontario archean rocks superior province kapuskasing structural zone in migmatitic mafic gneiss paragneiss dioritic and tonalitic rocks a lower grade garnet clinopyroxene plagioclase gt cpx pl zone and patchy higher grade orthopyroxene zones are distinguished based on mineral melt equilibria minimum conditions for the gt cpx pl zone are 750 degrees c 6 kbar a h 2 o 0 5 0 7 and for the orthopyroxene zone 770 degrees c 6 kbar a h 2 o 800 degrees c pressure values are 5 4 8 4 kbar and define a nne trending area of relatively high p in the kapuskasing zone supporting the interpretation of a tilted crustal section modified journal abstract

proterozoic magnetic overprinting of archaean rocks in the canadian superior province we have measured the various components of natural remanent magnetization nrm exhibited by mafic extrusive and intrusive rocks from the abitibi subprovince of the canadian precambrian shield in addition to providing what seems to be a primary archaean pole for the region the results testify to three episodes of magnetic overprinting at approximately 2 600 2 100 and 1 800 myr respectively the two earlier episodes coincide with the intrusion of matachewan and abitibi diabase dyke swarms here and elsewhere in the superior province magnetic overprinting in these cases extends far beyond areas of surficial dyke exposure the third episode postdates magmatic activity in the abitibi region but coincides in age with the 1 900 1 700 myr hudsonian orogeny in the churchill and bear provinces and the penokean orogeny in the southern province although magnetic overprints presumed to be of this age were known previously from abitibi dykes1 and nipissing sills2 4 our data are the first evidence of a 1 800 myr magnetic overprinting on a regional scale in the superior province

snowfall and oxygen isotope variations off the north coast of ellesmere island n w t canada snow pack along the lan d fast ice fringe off the north coast of ellesmere island was generally characterized by depth hoar overl ain by dense snow and wind slab mean snow depth in the study area was 0 54 m 1982 85 and the mean 6180 value of the sno w pack was 31 3 0 00 isotope data were not obtained previously for this geographic region and therefore complement a previous study of 6180 variations in high arctic snow koer ner 1979 the data are consistent with an arctic ocean moisture source the 6180 profiles show seasonal variat ions with winter snow being more depleted in 180 than fall and spring snow however the 6180 profiles are dominated by a trend to higher 6180 values with increasing depth this is attributed to a decrease in 6180 values as conden sation temperatures fall during the autumn winter acc umulation period during this time there is also a chan ge from relatively open to almost complete ice cover in the arctic ocean the change in evaporation conditions and consequent effect on 6 values gives rise to a sharp dis continuity in the 6180 prof iles and a bi m odal 6180 freq uency dist ribution the bi modal distribution is reinforced by a secon dary isotope fracti onation that occurs during dept h hoar formation this isotope effect leads to a wider 6180 range but does not signific antly alter the mean 6180 value

a millennial scale record of arctic ocean sea ice variability and the demise of the ellesmere island ice shelves 1 sea ice ice shelves at the apex of north america 80 n constitute the oldest sea ice in the northern hemisphere we document the establishment and subsequent stability of the ward hunt ice shelf and multiyear landfast sea ice in adjacent fiords using 69 radiocarbon dates obtained on holocene driftwood deposited prior to coastal blockage these dates 47 of which are new record a hiatus in driftwood deposition beginning 5500 cal yr bp marking the inception of widespread multiyear landfast sea ice across northern ellesmere island this chronology together with historical observations of ice shelf breakup 1950 to present provides the only millennial scale record of arctic ocean sea ice variability to which the past three decades of satellite surveillance can be compared removal of the remaining ice shelves would be unprecedented in the last 5500 years this highlights the impact of ongoing 20th and 21st century climate warming that continues to break up the remaining ice shelves and soon may cause historically ice filled fiords nearby to open seasonally

a double correlation tremor location method a double correlation method is introduced to locate tremor sources based on stacks of complex doubly correlated tremor records of multiple triplets of seismographs back projected to hypothetical s

thin thrust sheet formation of the kapuskasing structural zone revealed by lithoprobe seismic reflection data regional and high resolution seismic reflection data across the kapuskasing structural zone in ontario canada image at least three significant thrust faults that are low angle merge into a flat detachment on the west and together were responsible for the uplift of amphibolite and granulite facies rocks their geometry resembles a ramp and flat style of deformation that results in a thin upper plate above the 10 12 km about 4 0 s detachment northwest southeast horizontal shortening is estimated to be at least 55 km this large amount of shortening implies that much of the superior province was detached during the formation of the kapuskasing structural zone

relative age of otto stock and matachewan dykes from paleomagnetism and implications for the precambrian polar wander path north trending diabase dykes within and near the otto syenite stock of the kirkland lake area of the superior province carry a stable paleomagnetic remanence identical to that of 2454 2 ma matachewan dykes far from the stock a positive baked contact test for the remanence of a dyke within the stock demonstrates that the remanence is primary thus 1 these dykes belong to the matachewan swarm 2 the otto stock has not magnetically overprinted matachewan dykes in its contact aureole as previously reported and 3 the stock must be older than 2454 ma the magnetic remanence associated with the otto stock previously thought to be primary must now be considered of uncertain age nevertheless the positive baked contact test for the remanence of the matachewan dyke within the stock demonstrates that the remanence associated with the stock predates intrusion of the matachewan swarm the conventional polar wander path for precambrian north america decreases in age from the matachewan paleopole to the otto

paleomagnetism and orientation of precambrian dykes eastern lake superior region and their use in estimates of crustal tilting archean rocks form the eastern margin of the 1 1 ga old central north american rift along the eastern shore of lake superior and have been tilted westwards in response to rifting paleomagnetic and structural data from 2 6 ga old matachewan dykes suggest a westward crustal tilt of about 60 which agrees well with dips recorded in nearby keweenawan volcanics that rest directly on basement rocks the matachewan dyke swarm occurs throughout the east shore region of lake superior whereas keweenawan supracrustal sequences which give a more precise estimate of tilt are restricted to a few isolated shoreline patches estimates of crustal tilt can be obtained from the dykes on a regional basis thus generating a more complete picture of basement deformation adjacent to a major intracratonic rift

lava dome collapse detected using passive seismic interferometry 1 the collapse of the lava dome at the soufriere hills volcano on montserrat in july 2003 is the largest recorded in historical times i use noise correlation green s functions to measure the changes in seismic properties that resulted from this collapse continuous three component seismic data recorded at two pairs of stations were cross correlated to retrieve three component green s functions along two paths that intersect the volcanic edifice before and after the dome collapse particle motion analysis shows that the green s functions are dominated by rayleigh waves and are consistent with the expected green s tensor for a vertical point force source at one station recorded by a three component receiver at the other following the collapse there is a clear decorrelation and phase shift in the green s functions corresponding to a change in velocity of approximately 0 5 that can be interpreted in terms of the unloading of the lava dome

massive ancient sea ice strata and preserved physical structural characteristics in the ward hunt ice shelf two sea ice layers one measured as 9 m thick the other at least 12 m thick and estimated to be 24 5 m thick have been located by ice core drilling in the west ward hunt ice shelf to examine the preservation of physical structu ral characteristics over long time intervals the crystal structure and brine volumes in the sea ice which possibly dates back to about 3000 bp have been studied the structural characteristics are immediately recognizable as those of undeformed congelation sea ice accreted by stefan growth brine volumes in the ancient sea ice are higher than those in modern multi year ice at the same temperature the preservation of brine over a time span of hundreds to thousands of years is attributed to an absence of surface meltwater to effect brine flushing and the very slow even negligible action of gravity drainage brine pocket migration and brine expulsion the congelation structures indicate that sea ice can grow by the stefan accretion mechanism to thicknesses exceeding the equilibrium thickness 2 5 5 m of most undeformed multi year ice the observed physical structu ral characteristics of the ward hunt sea ice strongly suggest that many of the properties attained by sea ice are permanent and not affected by slow acting physical processes

seismicity around the hekla and torfajokull volcanoes iceland during a volcanically quiet period 1991 1995 the volcano hekla in south iceland had its latest eruption in january march 1991 the eruption was accompanied and followed by considerable seismic activity this study examines the seismicity in the hekla region 63 42 64 18 n 18 30 20 12 w during a period when the high activity related to the eruption had ceased from july 1991 to october 1995 the aim is to define the level of the normal background seismicity of the area that can be compared to the eruption related activity the hekla volcano proper was generally aseismic during the study period the most prominent earthquake cluster is in the neighbouring torfajokull volcano the epicentres are concentrated in the western part of the caldera and west of it the hypocentres are located at all depths from the surface down to 14 km with highest activity at 5 12 km inside this cluster in the northwest part of the caldera is a spherical volume void of earthquakes approximately 4 km in diameter and centred at 8 km depth this is interpreted as a cooling magma body small low frequency events of volcanic origin were occasionally recorded at torfajokull this activity has mainly occurred in swarms and was most abundant during the first year of the study period presumably reflecting some kind of connection to the 1991 hekla eruption our study area also includes the easternmost section of the south iceland seismic zone a transform zone characterized by bookshelf faulting on transverse faults two lineaments of epicentres were identified roughly corresponding to mapped faults of the south iceland seismic zone the hypocentres are relatively deep mainly at 6 12 km matching the general trend of hypocentral depth increasing toward the east the seismicity is highest in the area of the mapped faults however the epicentres extend beyond them and indicate greater width of the south iceland seismic zone or 20 30 km rather than approximately 10 km as indicated by the length of the surface faults the seismicity in the volcanic systems of hekla and vatnafjoll shows some characteristics of the south iceland seismic zone epicentres are concentrated into two n s lineaments one of which coincides with the location of the 1987 vatnafjoll earthquake mw 5 9 a strike slip event on a n to s trending fault the hypocentres of the hekla vatnafjoll events are mainly at 8 13 km depth which indicates a continuation of the depth trend of the earthquakes of the south iceland seismic zone the events located at hekla proper and immediately north of it are all of low frequency character which can be held as an indication of volcanic origin on the other hand they show clear s arrivals at observing stations like normal high frequency tectonic earthquakes

paleomagnetism structure and longitudinal correlation of middle precambrian dykes from northwestern ontario and minnesota a primary magnetization about 2 1 ga old is well preserved in the kenora kabetogama dyke swarm in minnesota and ontario its direction the mean for 12 dykes incorporating results from 30 paleomagnetic sites is 115 9 55 6 yielding a paleomagnetic pole position of latitude 42 7 s longitude 4 2 e dm 7 3 dp 5 2 the primary nature of the magnetization can be demonstrated using geochemistry to correlate individual dykes over distances that can exceed 300 km whereas the remanence direction varies between dykes it remains constant along a dyke sufficient proof that it was formed during initial cooling of the intrusion a weak longitudinal increase in hydrous alteration mgo content and degree of magnetic overprinting is observed as the swarm is followed to the south and may reflect increasing proximity to a precambrian continental margin that underwent rifting sedimentation and deformation during the time interval 2 2 1 6 ga bp although the cause of the longitudinal changes in terms of ma

the 2011 unrest at katla volcano characterization and interpretation of the tremor sources abstract a 23 hour tremor burst was recorded on july 8 9th 2011 at the katla subglacial volcano one of the most active and hazardous volcanoes in iceland this was associated with deepening of cauldrons on the ice cap and a glacial flood that caused damage to infrastructure increased earthquake activity within the caldera started a few days before and lasted for months afterwards and new seismic activity started on the southern flank no visible eruption broke the ice and the question arose as to whether this episode relates to a minor subglacial eruption with the tremor being generated by volcanic processes or by the flood the tremor signal consisted of bursts with varying amplitude and duration we have identified and described three different tremor phases based on amplitude and frequency features a tremor phase associated with the flood was recorded only at stations closest to the river that flooded correlating in time with rising water level observed at gauging stations using back projection of double cross correlations two other phases have been located near the active ice cauldrons and are interpreted to be caused by volcanic or hydrothermal processes the greatly increased seismicity and evidence of rapid melting of the glacier may be explained by a minor sub glacial eruption a less plausible interpretation is that the tremor was generated by hydrothermal boiling and or explosions with no magma involved this may have been induced by pressure drop triggered by the release of water when the glacial flood started all interpretations require an increase of heat released by the volcano

ambient noise tomography of katla volcano south iceland abstract a shear wave velocity model of subglacial katla volcano southern iceland has been developed using ambient seismic noise tomography based on data from a temporary network operating between may 2011 and august 2013 and permanent stations around the volcano phase velocity dispersion curves were obtained using cross correlations of vertical components of 136 station pairs and non linearly inverted for phase velocity maps between 1 7 and 7 5 s local dispersion curves were inverted for shear velocity variation with depth using a grid search imposing a fixed ice layer at the top the resulting one dimensional 1 d velocity models were combined to obtain a pseudo three dimensional 3 d model with estimated lateral resolution of 8 km and depth resolution varying from close to 1 km near the surface to about 8 km at 10 km depth shear wave velocities are generally higher within the katla central volcano than in its surroundings the most striking feature of the model is a high velocity anomaly beneath the caldera at 6 km depth interpreted to be due to cumulates resulting from differentiation of shallower magma intrusions and remelting of subsiding upper crust no shallow low velocity anomaly is resolved beneath the central caldera but a low velocity region is found at 2 4 km depth beneath the western half of the caldera v p v s ratios estimated from average velocity depth profiles from surface wave data and higher frequency p wave data are anomalously high 1 9 compared to average icelandic crust particularly in the top 2 3 km this is argued not to be an artifact due to lateral refraction or topography instead this anomaly could be explained as an artifact caused by velocity dispersion due to attenuation and a difference in frequency content and possibly to a degree by the compositional difference between the transalkalic fe ti basalts of katla and average tholeiitic icelandic crust