partitioning of convergence in northwest sub himalaya estimation of late quaternary uplift and convergence rates across the kangra reentrant north india the kangra reentrant constitutes a 80 km wide zone of fold thrust belt made of cenozoic strata of the foreland basin in nw sub himalaya earlier workers estimated the total long term shortening rate of 14 2 mm year by balanced cross section between the main boundary thrust and the himalayan frontal thrust geologically estimated rate is nearly consistent with the gps derived slip rate of 14 1 mm year there are active faults developed within 4 8 km depth of the sub himalayan fold thrust belt of the reentrant dating the strath surfaces of the abandoned fluvial terraces and fans above the thrust faults the uplift bedrock incision rates are computed the dips of thrust faults are measured in field and from available seismic depth profiles from the acquired data late quaternary shortening rates on the jawalamukhi thrust jt the soan thrust st and the himalayan frontal thrust hft are estimated the shortening rates on the jt are 3 5 4 2 mm year over a period 32 30 ka the st yields a shortening rate of 3 0 mm year for 29 ka the corresponding shortening and slip rates estimated on the hft are 6 0 and 6 9 mm year during a period 42 ka on the back thrust of janauri anticline the shortening and slip rates are 2 0 and 2 2 mm year respectively for the same period the results constrained the shortening to be distributed largely across a 50 km wide zone between the jt and the hft the emergence of surface rupture of a great and mega earthquakes recorded on the reactivated hft implies 100 km width of the rupture the ruptures of large earthquakes like the 1905 kangra and 2005 kashmir remained restricted to the hinterland the present study indicates that the high magnitude earthquakes can occur between the locking line and the active thrusts

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

seismic source modeling seismic source modeling is taken to mean the modeling of seismograms to investigate kinematic properties of the earthquake source current trends such as the use of other data types e g geodetic measurements of coseismic deformation and the explicit consideration of source dynamics when modeling seismograms are beginning to blur this definition but i will adhere to it in this review i have divided seismic source modeling into teleseismic regional and near source modeling teleseismic modeling has the advantage of global coverage for earthquakes of magnitude 5 5 and larger but does not provide much more than point source parameters regional distance modeling has the advantage of a more rapid assessment of earthquake source properties but is complicated by relatively complex wave propagation the coverage is also limited to areas of broadband regional networks typically continental areas of developed countries near source modeling has the potential for detailed imaging of source properties but very few earthquakes occur in regions with sufficient instrumentation to allow this i have also included a section on historical earthquakes because of the additional issues that seismologists face in working with pre wwssn data and because of the important issues such as earthquake recurrence that require historical data in addition to reviewing the progress in seismic source modeling during the last four years i have also briefly reviewed some of the progress in understanding earthquake physics and hazards that has come about because of basic research in earthquake source modeling

volcanic ash clusters tephra rafts and scavengers abstract much of the volcanic ash deposited in the vicinity of pullman washington from the may 18th eruption of mt st helens in 1980 fell in the form of composite ash clusters 0 25 to 0 5 mm in diameter particles in the clusters ranged from sub micron to more than 40 microns in size the porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward the result is poorly sorted ash deposits as far as 644 km east missoula within individual clusters textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation the clusters are very fragile and may never be preserved in the stratigraphic record but it is suggested that this kind of ash transport may well have occurred in the geologic past documentation of this ash cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine grained volcanic ash the poor size sorting in some ancient ash deposits may thus be explained

joint inversion of regional and teleseismic earthquake waveforms a least squares joint inversion technique for regional and teleseismic waveforms is presented the mean square error between seismograms and synthetics is minimized using true amplitudes matching true amplitudes in modeling requires meaningful estimates of modeling uncertainties and of seismogram signal to noise ratios this also permits calculating linearized uncertainties on the solution based on accuracy and resolution we use a priori estimates of earthquake parameters to stabilize unresolved parameters and for comparison with a posteriori uncertainties we verify the technique on synthetic data and on the 1983 borah peak idaho m 7 3 earthquake we demonstrate the inversion on the august 1954 rainbow mountain nevada m 6 8 earthquake and find parameters consistent with previous studies

formation of high grade ignimbrites part ii a pyroclastic suspension current model with implications also for low grade ignimbrites analogue experiments in part i led to the conclusion that pyroclastic flows depositing very high grade ignimbrite move as dilute suspension currents in the thermo fluid dynamical model developed the degree of cooling of expanded turbulent pyroclastic flows dynamically evolves in response to entrainment of air and mass loss to sedimentation initial conditions of the currents are derived from column collapse modeling for magmas with an initial h2o content of 1 3 wt erupting through circular vents and caldera ring fissures the flows spread either longitudinally or radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content temperature bottom slope and coarser initial grain size progressive dilution by entrainment and sedimentation causes pyroclastic currents to transform into buoyant ash plumes at the runout distance the ash plumes reach stratospheric heights and distribute 30 80 of the erupted material as widespread co ignimbrite ash pyroclastic suspension currents with initial mass fluxes of 107 1012 kg s can spread for tens of kilometers with only limited cooling although they move as supercritical strongly entraining currents for the eruption conditions considered here with increasing eruption mass flux cooling during passage through the fountain diminishes while cooling during flow transport increases the net effect is that eruption temperature exerts the prime control on emplacement temperature pyroclastic suspension currents can form welded ignimbrite across their entire extent if eruption temperature is to 1 3 tmw the minimum welding temperature high eruption rates a large fraction of fine ash and a ring fissure vent favor the formation of extensive high grade ignimbrite for very hot eruptions producing sticky partially molten pyroclasts analysis of particle aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation which reduces runout as a result very high grade ignimbrites cannot spread more than a few tens of kilometers from their source in cooler pyroclastic currents particles do not aggregate and the sedimentation process may involve re entrainment of particles which potentially leads to more extensive cooling and longer runout such effects however are only significant when net erosion of substrate occurs model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite masses and aspect ratios the model also provides an alternative explanation of the observed decrease in h lratios with ignimbrite mass

deflection of oceanic transform motion at a continental margin as deduced from waveform inversion of the 1939 accra ghana earthquake abstract waveform inversion of teleseismic p waves of the 1939 accra ghana earthquake m 6 5 suggests that the earthquake was composed of two events the smaller event m 6 1 occurring 9 5 s before the onset of the larger m 6 4 event the smaller event has a focal mechanism that suggests it occurred immediately north of the intersection of the akwapim and coastal boundary fault inversion results also indicate that the larger event could not have occurred on the reactivated akwapim fault system as suggested by previous researchers instead the larger event appears to have occurred along a strike slip fault that parallels the coastline of ghana these results along with the trend of historic earthquakes indicate that not all the motion along the romanche or chain fracture zones is being transmitted inland along the akwapim fault system but that some is being deflected tangentially along a fault that parallels the ghana coastline

volcanic plume electrification experimental investigation of a fracture charging mechanism although ashfall from particulate volcanic plumes is known to be highly electrically charged little is known about the charging mechanism we describe experiments designed to investigate the particle charges generated from the fracture of pumice small silicate particles were produced in the laboratory during collisions between two samples cut from pumice clasts the net charge magnitudes detected on these particles are similar to those previously measured on ashfall from volcanic plumes 10 5 to 10 6 c kg 1 this net charge is also shown to be the result of a small imbalance between the sums of individual particle charges of both polarities which are up to several orders of magnitude larger than the net charge the magnitude of both the net and single polarity specific charges were only weakly affected by changes of relative humidity but single polarity charges increased steadily with increasing sample impact velocities the dominant charging process during the experiments was that of material fracture the charging mechanism is thus interpreted to be fractoemission the release of nuclear particles from fresh crack surfaces occurring during the production of the silicate particles this implies that the electrification of volcanic plumes could be the result of brittle fragmentation of magma or pumice clasts within the upper regions of the conduit and in the jet region of the plume

a geologically constrained monte carlo approach to modeling exposure ages from profiles of cosmogenic nuclides an example from lees ferry arizona we present a user friendly and versatile monte carlo simulator for modeling profiles of in situ terrestrial cosmogenic nuclides tcns our program available online at http geochronology earthsciences dal ca downloads models html permits the incorporation of site specific geologic knowledge to calculate most probable values for exposure age erosion rate and inherited nuclide concentration while providing a rigorous treatment of their uncertainties the simulator is demonstrated with 10be data from a fluvial terrace at lees ferry arizona interpreted constraints on erosion based on local soil properties and terrace morphology yield a most probable exposure age and inheritance of 83 9 14 1 19 1 ka and 9 49 2 52 1 21 104 atoms g 1 respectively 2 without the ability to apply some constraint to either erosion rate or age shallow depth profiles of any cosmogenic nuclide except for nuclides produced via thermal and epithermal neutron capture e g 36cl cannot be optimized to resolve either parameter contrasting simulations of 10be data from both sand and pebble sized clasts within the same deposit indicate grain size can significantly affect the ability to model ages with tcn depth profiles and when possible sand not pebbles should be used for depth profile exposure dating

characterization of shape and terminal velocity of tephra particles erupted during the 2002 eruption of etna volcano italy in this paper we present a complete morphological characterization of the ash particles erupted on 18 december 2002 from etna volcano italy the work is based on the acquisition and processing of bidimensional digital images carried out by scanning electron microscopy sem to obtain shape parameters by image analysis we measure aspect ratio ar form factor ff compactness cc and rectangularity rt of 2065 ash particles with size between 0 026 and 1 122 mm we evaluate the variation of these parameters as a function of the grain size ash particles with a diameter of 0 250 mm are subelongate we find that on average particles with a diameter of 0 50 mm are angular using this morphological analysis and an empirical relation between the drag coefficient cd and reynolds number re of wilson and huang earth planet sci lett 44 311 324 1979 we calculate the terminal settling velocities vwh the comparisons between these velocities and those calculated with the formula of kunii and levenspiel fluidization engineering wiley new york pp 97 1969 vkl which considers ash particles as spheres show that vkl are in average 1 28 greater than vwh hence we quantify the systematic error on the spatial distribution of the mass computed around the volcano carried out by tephra dispersal models when the particles are assumed to be spherical

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

earthquake activity in southeastern ghana 1977 1980 although ghana is remote from the major earthquake zones it is moderately active seismically with a history of earthquakes damaging the capital accra seismic recording with a variety of instruments during 1977 to 1980 has enabled us to develop a crustal velocity model and locate epicenters most of the earthquakes occur in an area to the west of accra around the junction of two major fault systems the east west trending coastal boundary fault and the nne trending akwapim fault zone fault plane solutions suggest a mixture of normal faulting on the akwapim fault zone in agreement with geological evidence and strike slip faulting in response to a regional ese tensional stress perhaps caused by strike slip movement along the romanche fracture zone in view of the high stress suggested by the low b value for the recorded events precautions against earthquake hazard in accra would be timely

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

morphology of ash aggregates from wet pyroclastic surges of the 1982 eruption of el chichon volcano mexico the detailed stratigraphic study of the pyroclastic surge units s1 iu and s3 produced during the most violent phases of the 1982 eruption of el chichon volcano contains a complex succession of hydromagmatic events triggered by the interaction of different proportions of magma and external water component analyses of the horizons within single units reveal that almost all wet and cohesive horizons contain ash aggregates based on their morphology and internal structure four different types of aggregates were distinguished a accretionary lapilli b armored lapilli c irregular aggregates and d cylindrical aggregates the first three types have been described in the volcanological literature field and experimental studies cylindrical forms are reported here for the first time these hollow cylindrical aggregates consist of concentric layers of crystals and glass fragments set in a finer grained matrix they formed around millimeter size foliage fragments that are locally preserved in the interior of the aggregates as scorched or completely carbonized vestiges sem analyses suggest different mechanisms of formation for the four types of aggregates irregular aggregates and armored lapilli formed nearly instantaneously whereas accretionary lapilli and cylindrical aggregates resulted from progressive aggregation of ash in different regions of the eruptive cloud

structural interpretation of the great earthquakes of the last millennium in the central himalaya abstract a major question about the himalaya remains open does a great earthquake like the mw 8 1 1934 earthquake release all the strain stored by the tibet india convergence during the preceding interseismic period and only that strain or can it also release a background store of energy that remained unreleased through one or more earlier earthquakes and so potentially engender giant events or a relatively random sequence of events to consider this question the history of the great earthquakes of the last millennium is investigated here by combining data provided by the historical archives of kathmandu trenches through surface ruptures isoseismal damage mapping seismites and the instrumental record in the kathmandu basin the location of the epicenter of the 1934 earthquake was determined from the arrival of high energy p waves that caused sedimentary dikes and ground fractures perpendicular to the epicenter azimuth the epicenter of the mw 7 6 1833 earthquake can therefore be determined analogously from dike orientation and its location to the ne of kathmandu indicates an overlap with the mw 8 1 1934 rupture the 1934 earthquake released strain not released by the 1833 earthquake comparison of the historical records of earthquakes in kathmandu with 14c ages from paleo seismic trenches along the himalayan front suggests that 1 the 1344 kathmandu event ruptured the surface as far away as kumaon and was therefore a giant mw 8 6 earthquake and 2 the 1255 event that destroyed kathmandu is attested by surface ruptures in central and western nepal and by seismites in soft sediment as far away as kumaon geometric and rheologic controls for the different types of ruptures during the medium mw 7 great mw 8 and giant mw 8 4 earthquakes are illustrated in structural cross sections it is found that the epicenters of great himalayan earthquakes are located on the basal thrust farther north or close to the locked zone which is defined from geodetic measurements of regional deformation during the interseismic period this suggests that great earthquakes initiate in a wide transition zone between exclusively brittle and exclusively creeping regimes the extent of which depends on the dip of the main himalayan thrust the succession of the great earthquakes during the last millennium has released all the 20 m millennial himalayan convergence even in the central seismic gap which has been locked since 1505 the millennial seismic release rate is close to the convergence rate nonetheless no evidence of a succession of characteristic earthquakes has been found the 1100 1833 and 1934 earthquakes in the eastern himalaya are characterized neither by constant displacement nor by constant recurrence furthermore some great earthquakes do not release all the strain elastically stored by the himalayan and tibetan upper crust after the 1255 event there was still the potential for a slip of several meters for the mw 8 1 1505 event this suggests a rather random release of seismic energy great earthquakes could occur anytime and in any part of the central himalaya furthermore a future giant earthquake of mw 8 6 cannot be excluded

source parameters of earthquakes along the coastal margin of west africa and comparisons with earthquakes in other coastal margin settings abstract the coastal margins of western africa and most of the eastern americas are considered to be stable continental interiors where large earthquakes occur infrequently since these regions have similar geologic and tectonic histories it might seem reasonable to expect similarities between their large earthquakes on the other hand current plate forces acting on these regions differ and may lead to regional differences in earthquakes to examine the similarities of coastal margin earthquakes we first determined the source parameters for six earthquakes of m 5 5 occurring along the coastal margin of west africa a region that has not previously been studied in detail results of this analysis indicate that the earthquakes are shallow 8 15 km strike slip events along high angle faults dips generally 80 with nodal planes having orientations similar to those of faults mapped in the exposed precambrian basement five out of seven events show complexities in their source time functions east of the cameroon line most p axes trend northeast southwest to east west earthquakes west of the cameroon line show less coherency of stress field with p axes for onshore events trending northwest southeast and for offshore events north south to northeast southwest in contrast m 5 5 earthquakes of the eastern coastal margin of north and south america have reverse mechanisms with more moderate dips on nodal planes 45 70 and over 50 have east west oriented p axes which reflect stresses caused by the motion of north and south america relative to europe and africa in north america earthquakes appear to occur on faults that cannot be associated with surficial geology occur shallower or deeper than west african events and have shorter rupture lengths for the equivalent moment these differences suggest that west african events may not serve as useful analogs for events expected along the american coastal margin

frontal and piggy back seismic ruptures in the external thrust belt of western nepal abstract this paper gives structural and morphological evidence for meter scale episodic displacement pulses along the mft and mdt during a complex sequence of thrusting in the sub himalayan fold and thrust belt the studied cross section is located in western nepal at a latitude of e 82 20 along this section the main dun thrust mdt is in a piggy back setting and comprises three splays that are spaced by less than 200 m the splays display the following sequence of motion 1 the medium splay was active 2 the internal splay was active and had a dip slip displacement of 3 m that could be the surface rupture of an earthquake 3 a period of quiescence occurred for all the splays of the mdt 4 the external splay is active the motion along the medium and internal thrust predates 70 kyr whereas motion along the external splay postdates 5468 5214 yr before j c the main frontal thrust mft is the most external himalayan thrust ten encased strath terraces are found at its hangingwall sudden 1 5 m uplift events could have induced their abandonment and could have been linked to 2 m slip events along the mft a value close to the lower bound inferred for co seismic slip along the basal detachment mht during m 8 earthquakes from the offset of a terrace level a displacement of 8 m is inferred between 1224 1280 yr after j c and 1828 1883 yr after j c it could be linked to a succession of several seisms along the mft this study suggests that a several thrusts that branch off the basal decollement are active faults b great earthquakes that occur along the outer part of the basal decollement of the himalayan thrust belt reach the surface at different location c only one thrust moves during one earthquake d the sequence of thrusting is successively out of sequence and in sequence and changes at an intermediate time scale between earthquake cycle and finite geometry of the thrust system

the status of central seismic gap a perspective based on the spatial and temporal aspects of the large himalayan earthquakes abstract the central himalaya considered as a prominent seismic gap is generally believed to be the most vulnerable segment due for a great plate boundary earthquake m 8 two significant historical earthquakes are known to have occurred in this region in ad 1505 and 1803 interpreted by some workers as great earthquakes occurrences of these events question the validity of central himalaya being a seismic gap here we analyze the macroseismal data from the central himalaya and the gangetic plains and suggest that neither of these earthquakes can be qualified as a plate boundary type event and the size of at least the 1803 does not exceed mw 7 7 our studies further indicate that the central himalayan frontal thrusts may be undergoing a quiescence of 1000 years in terms of generation of great earthquakes assuming a steady elastic strain accumulation due to plate convergence and its simple translation as periodic great earthquakes such long quiescence may appear intriguing we suggest that the long term quiescence and the spatial and temporal clustering that characterize the distribution of large earthquakes in the himalaya is a manifestation of the tectonic deformation associated with a highly evolved fold and thrust belt our analyses also indicate that the large moderate earthquakes are mostly concentrated on higher level thrusts these structures probably accommodate most of the seismogenic slip in the himalaya and the rest of the strain could possibly be expended on passive folding consequently great earthquakes m 8 are relatively infrequent as in the case of central himalaya apparently leading to long periods of quiescence

probabilistic tephra fall simulation for the auckland region new zealand abstract the auckland region new zealand is at significant risk from tephra falls originating both from the local auckland volcanic field avf and several distant large volume centres we use geological data and observations of historical eruptions to develop a catalogue of simulated tephra dispersal patterns for the auckland region using the ashfall model simulated patterns relate to individual eruptive phases and can be combined to simulate events that vary in volume and duration avf eruption parameters were chosen to replicate fine tephra fall dispersed from small volume phreatomagmatic eruptions modelling of eruptions from distant volcanoes was carried out in two stages first the sensitivity of modelled tephra thickness to various model parameters was tested by far the largest thickness variations were the result of varying wind conditions followed by eruption volume the second stage involved modelling volumes that best replicated thicknesses of preserved events within the auckland region allowing the catalogue to be created only about 1 of simulations from distant volcanoes reached the auckland region suggesting that preserved tephra layers are either the result of reasonably long lived eruptions or a number of eruptive phases for eruptions originating within the avf mean modelled tephra thickness on land is small approximately 1 6 mm near the coast and thinning rapidly inland mean andesitic thicknesses are dominated by eruptions from egmont volcano which typically deposit tephra over a narrow section of the auckland region with a maximum thickness of approximately 1 7 mm rhyolitic eruptions are more widespread with mean thickness varying from 30 mm in the south of the region to 9 mm in the north when all centres are combined and weighted by their conditional likelihood of occurrence mean thickness ranges from 7 mm in the south to 1 5 mm in the north however maximum thicknesses of 150 mm for the avf 12 mm for andesitic centres and 830 mm for rhyolitic centres all have the potential to cause much greater damage within parts of the auckland region

late quaternary out of sequence deformation in the innermost kangra reentrant nw himalaya of india seismic potential appraisal from 10 be dated fluvial terraces abstract the kangra reentrant is a convex to the northeast u shaped structure in the nw himalaya where the sub himalayan fold and thrust belt is 90 km wide this region has not been struck by large earthquakes since the 1905 mw 7 8 kangra earthquake out of sequence deformation has been reported at the millennial timescale along intracrustal thrusts within this reentrant such as the jawalamukhi thrust up to now the occurrence of out of sequence deformation along inner thrusts within the kangra reentrant during the late quaternary has not yet been addressed in this study the results of a neotectonic survey undertaken in this reentrant are presented the studied zone is located between the beas and the neogad rivers and encompasses from the jawalamukhi thrust to the main boundary thrust two terraces that are deformed by branches of the medlicott wadia thrust locally named the palampur thrust are identified this is evidenced in the field by metric scale fault scarps by using 10be dating the ages of these terraces were constrained to ca 7 5 and ca 6 2 ka this is clear evidence of the late quaternary out of sequence deformation in the innermost part of this reentrant implying that strain is distributed along all the arc orthogonal extent of the local fold and thrust belt over this timespan a cumulative slip rate of ca 1 mm yr along the studied thrusts which represents 10 of the bulk strain accommodated by the whole reentrant for this timespan is calculated in spite of the marginal appearance of this figure this deformation rate is attributed to 7

production of cosmogenic radionuclides at great depth a multi element approach abstract for the last two decades in situ produced cosmogenic nuclides are increasingly applied in earth sciences to quantify surface processes in parallel significant reduction of the analytical uncertainties linked to advances in accelerator mass spectrometry ams allows more precise measurements however among all the published works on cosmogenic nuclides only few studies are dedicated to a better understanding of their production systematic or to a better constrain of the physical parameters involved in their production thus an approach to investigate in situ produced cosmogenic nuclides 10 be 26 al and 36 cl along a single 11 meter long core drilled from the surface and composed of carbonates and quartzose conglomerates has been launched these measurements have been used to quantify muon induced productions based on natural samples for each studied nuclide contrary to the current most oftenly used calculation of muon induced production parameters which are based on irradiation experiments at discrete energies productions based on natural samples are considering the entire energy range of particles reaching the ground surface the evolution of 36 cl concentrations with depth needs to agree with those parameters deduced from 10 be and 26 al data this is optimized when considering a fast muon induced 36 cl production contribution and a spallation production rate at sea level high latitude slhl of 42 0 2 0 atoms 36 cl g ca 1 a 1 1 sigma uncertainty

timing and nature of volcanic particle clusters based on field and numerical investigations aggregation processes are known to play an important role in volcanic particle dispersal and sedimentation they are also a primary source of uncertainty in ash dispersal forecasting since fundamental questions such as the timing and deposition dynamics of volcanic aggregates still remain unanswered here we applied a state of the art combination of field and numerical strategies to characterize volcanic aggregates we introduce a new category of aggregates observed with high speed high resolution videos namely cored clusters cored clusters are mostly sub spherical fragile aggregates that have never been observed in the deposits nor on adhesive tape as they typically break at impact with the ground they consist of a core particle 200 500 m fully covered by a thick shell of particles 90 m the low preservation potential of cored clusters in ash deposits explains the poor documentation in the literature and the low consideration attributed so far cored clusters can also better explain the deposition of fine ash in proximal and medial regions and the polymodality observed in many ash deposits in addition numerical inversions show how cored clusters can rapidly form within 175s from eruption onset finally our observations represent the first field based evidence of the so called rafting effect in which the sedimentation of coarse ash in cored clusters is delayed due to aggregation

the immediate environmental effects of tephra emission the earth s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere these tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents the interaction of tephra with or in receiving environments may induce an array of physical chemical and biological effects the consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent small eruptions while larger and rarer events may have acute regional scale impacts it is therefore necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems we collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post eruptive effects of tephra emission into the atmosphere onto vegetation soil or ice snow surfaces and in aquatic systems we further consider the repercussions that may be induced in the weeks to years afterwards in the atmosphere tephra can influence cloud properties and air chemistry by acting as ice nuclei in or by offering sites for heterogeneous reactions respectively tephra on vegetation causes physical damage and sustained coverage may elicit longer term physiological responses tephra deposits on soils may alter their capacity to exchange gas water and heat with the atmosphere or may have a specific chemical effect such as nutrient input or acidification on sensitive soils tephra deposition onto snow or ice may affect ablation rates rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged months or years erosion from the tephra covered catchment in the first weeks after deposition tephra leaching may affect river chemistry the abundance and speciation of phytoplankton populations in lakes may be altered by tephra induced changes in water chemistry or sediment water nutrient cycling in the oceans tephra deposition may fertilise fe limited waters with potential impacts on the global carbon cycle embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies combined with consideration of the interconnectivity of induced impacts within and between different receiving environments

fall out and deposition of volcanic ash during the 1979 explosive eruption of the soufriere of st vincent abstract the 1979 explosive activity of soufriere volcano st vincent commenced on april 13th and continued for two weeks producing eleven eruption columns the activity produced air fall ash with abundant accretionary lapilli plus minor pyroclastic flows and base surge deposits the dispersal of the columns was observed by an sms 1 geostationary satellite all but two of the plumes were dispersed entirely to the east the last event on april 26th formed a minor lobe to the east and a major lobe to the south southeast the april 26th column was observed to rise to 8 km within five minutes and ascended to a maximum of 14 km in the next hour the magma discharge rate was greater than 3000 m3 s the plume expanded at a rate of 4000 km2 hr and was dispersed at a mean velocity of 15 m s the ash deposits show bimodal grain size distribution with 95 of the ash collected being sub 1 mm diameter modelling of the ash fall out has allowed interpretation of grainsize parameters in a quantitative manner the fall velocities of ash particles appear to have been reduced by turbulence and downwind ascent of the plume which is attributed to fallout of ash reducing the plume density the coarse population of the april 26th deposit displayed a systematic change in md and away from source the fine mode displays uniform md and everywhere and is interpreted as having been deposited in the form of accretionary lapilli

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

historical seismicity and implications for diffuse plate convergence in the northeast indian ocean large historical earthquakes provide constraints on the geographical extent and kinematics of the slow diffuse plate boundary extending from the central indian ridge to the sumatra trench the fault parameters of pre 1963 earthquakes are derived using a grid search for the best fit between observed and synthetic body waveforms surface wave spectral amplitudes and polarity and polarization constraints and location uncertainties are determined using a bootstrap resampling method the march 9 1928 ninetyeast ridge m 7 7 event shows left lateral strike slip faulting along a plane approximately coincident with the orientation of the ninetyeast ridge two small events which occurred at the location of the 1928 event in 1980 show focal mechanisms nearly identical to that determined for the 1928 earthquake and apparently reactivated the same fault the 1928 waveforms can also be fit by a thrust faulting solution with ene striking fault planes but a variety of evidence including the recent strike slip earthquakes at the same location suggest the strike slip solution these earthquakes are located along the western scarp of the ridge and do not represent reactivation of the ninetyeast fracture zone to the east of the edifice which was a major plate boundary in the early cenozoic the january 23 1949 m 6 75 and march 22 1955 m 7 0 wharton basin earthquakes although only 350 km apart show different strike slip and thrust faulting mechanisms all earthquakes show a regionally consistent pattern of horizontal compressional stress which changes from nw se in the wharton basin to n s in the central indian basin the compressional axes are oriented perpendicular to linear gravity anomalies associated with lithospheric flexure indicating little change in stress orientation since the onset of folding seismic deformation along the diffuse boundary consists of sinistral strike slip faulting associated with n s features such as the ninetyeast ridge and the 86 e fracture zone and thrust or reverse faulting in regions between fracture zones assuming transform motion the approximate slip rate along the northern ninetyeast ridge calculated from the summed moments of earthquakes is 2 8 mm yr this value is an order of magnitude less than previous estimates and suggests that at most only a small fraction of the current relative plate motion 7 mm yr occurs along the ridge the geographic distribution of seismicity suggests that the wharton basin east of the ninetyeast ridge is nearly as seismically active as the central indian basin to the west this result along with observed long wavelength gravity anomalies in the wharton basin indicates that the india australia plate boundary intersects the sumatra trench along a diffusely deforming region several thousand kilometers long rather than along a discrete transform zone

sulfur as a binding agent of aggregates in explosive eruptions understanding the mechanisms that govern ash aggregation is of critical importance in volcanology aggregation reduces the residence time of ash 2 mm in the atmosphere strongly by enhancing the sedimentation of finer grained material generated during explosive eruptions to date experimental studies have focused on the expectation that water provides the strongest bonds between particles to form spherical to oblate aggregates typically less than or equal to a few mm occasionally up to several cm preserved in pyroclastic deposits under water rich conditions individual accreted particles rarely exceed 1 mm in pyroclastic density current deposits produced during the 1982 eruption of el chichon mexico which emitted 7 5 tg of so2 in the atmosphere aggregate particles one to several millimeters strongly cemented by a s rich film are common they exhibit similarities with aggregates found in sulfur cones at poas volcano costa rica we propose that sulfur is the binder between the silicate grains such a binding capacity implies a relatively fluid behavior of sulfur such as might be expected in the low viscosity temperature regime just above its melting point if so then the explosive ejection of sulfur during eruptions combined with its ability to act as a cement for particles 2 mm would imply that size fractions of lapilli can be efficiently removed from eruptive clouds a few kilometers from the vent such an aggregation mechanism would have important implications for pyroclast dispersal models and hazard assessment

a study of the source parameters of some large earthquakes of northeastern north america waveforms of body and surface waves for six moderately large north eastern north american earthquakes are analyzed for source information the earthquakes studied are the 1925 charlevoix quebec earthquake ms 6 4 the 1935 timiskaming quebec earthquake ms 6 0 the 1939 charlevoix quebec earthquake mb 5 4 the 1940 ossipee new hampshire earthquakes ml 5 3 and 5 1 and the 1944 cornwall ontario massena new york earthquake m 5 1 the focal depths for all of the earthquakes for which determinations could be made 1925 1935 1939 and december 20 1940 were about 8 to 10 km the waveforms of the 1925 1935 1939 and 1940 events are consistent with thrust mechanisms although there is some ambiguity in the strike direction for each of the events the favored focal mechanism for the 1925 and 1939 events had northeast southwest nodal planes for the 1935 event the available data suggest that the earthquake occurred on a northwest southeast oriented fault plane the strike of the nodal planes for the december 20 1940 event is quite uncertain but a north south orientation is preferred no body waves that could be modeled were collected for the december 24 1940 or 1944 events the stress drops for the events scatter widely but are comparable to those from other intraplate earthquakes there is a difference in source complexity from event to event with the december 20 1940 event having had one simple source while the 1935 shock had three separate energy releases the source time functions found for the 1935 1939 and december 20 1940 earthquakes from the short period teleseismic body waves resemble those for some shocks from california the focal mechanisms found here suggest that the minimum principal deviatoric stress is vertical and that the horizontal principal deviatoric stresses both compressional are controlled primarily by plate driving forces

source mechanisms of moderate size earthquakes and stress orientation in mid plate south america summary fault plane solutions of six mid plate earthquakes with magnitudes ranging from 4 8 to 5 5 mb were determined using p wave first motion data and the relative amplitudes of short period teleseismic depth phases pp and sp these results together with previous studies show a predominance of reverse and strike slip faulting indicating horizontal compressional stresses in mid plate south america the orientation of the p axes however is not uniform in western and southwestern brazil the p axes are oriented approximately e w probably due to the nazca south america plate convergence in eastern brazil the direction of the p axes show more scatter but an average e w direction is consistent with the available data on the contrary in northern brazil the p axes of two events in the amazon basin trend approximately n s a direction perpendicular to the direction of absolute plate motion as well as to the relative plate motions nazca south america and south america africa

the aerodynamic behaviour of volcanic aggregates a large proportion of solid material transported within the atmosphere during volcanic eruptions consists of particles less than 500 m in diameter the majority of these particles become incorporated into a wide range of aggregate types the aerodynamic behaviour of which has not been determined by either direct observation or in the laboratory in the absence of such data theoretical models of fallout from volcanic plumes make necessarily crude assumptions about aggregate densities and fall velocities larger volcanic ejecta often consists of pumice of lower than bulk density experimental data are presented for the fall velocities of porous aggregates and single particles determined in systems analogous to that of ejecta falling from a volcanic plume it is demonstrated that the fall of aggregates may be modelled in identical fashion to single particles by using a reduced aggregate density dependent on the porosity and a size corresponding to an enclosing sphere particles incorporated into aggregates attain a substantially higher fall velocity than single particles this is due to the larger physical dimensions of the aggregate which overcomes the effect of lower aggregate density additionally the internal porosity of the aggregate allows some flow of fluid through the aggregate and this results in a small increase in fall velocity the increase in fall velocity of particles incorporated into aggregates rather than falling individually results in the enhanced removal of fine material from volcanic plumes

origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife aggregation of airborne particles is an important way in which the atmosphere is cleansed of fine dust particles such as following explosive eruptions and meteorite impacts we identify successive stages in the growth history of particle aggregates based upon well preserved ash aggregate bearing pyroclastic layers on tenerife the layers are persistently organized into couplets made up of a lower ignimbrite layer and an upper widespread coignimbrite ash fall layer the upper part of each ignimbrite contains whole and fragmented concentric laminated accretionary lapilli whereas the overlying coignimbrite ash fall layer lacks accretionary lapilli and is composed of framework supported smaller and nonlaminated ash pellets sometimes slightly deformed or partly disaggregated the pellets resemble the cores of the larger accretionary lapilli in the underlying ignimbrite layer these field relations are repeated numerous times in several different successions and they indicate that ash pellets not accretionary lapilli form within the coignimbrite ash plumes some pellets fell directly to the ground producing coignimbrite ash fall layers but others settled into pyroclastic density currents where they accreted successive concentric laminations of fine ash as they circulated through the variously turbulent levels of the stratified current and heat of the lower part of the current dried and partly lithified them into brittle accretionary lapilli the fully formed whole and broken accretionary lapilli were then deposited from the current along with ash and pumice lapilli numerous ignimbrite veneers on tenerife have the form of ash layers a few centimeters thick that drape topography and locally contain matrix supported accretionary lapilli most volcanoes lack laterally continuous field exposure and such accretionary lapilli bearing layers might be mistaken for ash fall deposits we highlight the value of careful distinction between different types of ash aggregate facies when interpreting the origin of pyroclastic deposits for example during hazard assessments

seismicity and stresses in the brazilian passive margin abstract seismicity and stresses along the brazilian continental margin show different patterns between the northeastern and southeastern regions in the northeastern margin earthquakes tend to occur onshore under a strike slip regime with horizontal compression parallel to the northern coast line in the southeastern margin higher seismicity is observed offshore in areas where the continental crust was highly extended during the south atlantic rifting in the mesozoic three new events in the southeast continental shelf are shown to have reverse faulting mechanisms from waveform modeling of short period teleseismic p waves multiple depth phases reflections from the top and bottom of the water layer and double reflections in the water layer could be identified and better constrained the hypocenters to middle and upper crustal depths in south america models of intraplate stresses caused by plate boundary forces and spreading effects due to the continental oceanic crustal transition indicate higher compressional stresses in the se offshore area as compared with the continental area in agreement with the observed higher seismicity and reverse faulting mechanisms the combination of regional stresses local flexural effects from thick sedimentary loads and a presumably weaker crust from mesozoic thinning explains the main patterns of seismicity in the northeastern and southeastern brazilian margins

the seismicity seismotectonics and earthquake hazards of libya with detailed analysis of the 1935 april 19 m 7 1 earthquake sequence summary we have investigated the seismicity of libya in order to understand better the earthquake hazards the tectonics of the region and the present day evolution of the sirt basin we first relocated all events occurring between 1931 and 1988 using a bootstrap location technique we then determined focal mechanisms from firstmotion data finally we used waveform modelling techniques to determine source parameters for the 1935 main shock and its two largest aftershocks relocation of the 1930s events are in some cases significantly different from those given by the international seismological summary iss the first motion results suggest a combination of strike slip and normal faulting for the 1935 and 1939 sequences the waveform modelling results of the 1935 mainshock suggest a strike slip mechanism with one fault plane similar to the strike of the eastern edge of the hun graben about 5 hr after the mainshock an aftershock mb 6 0 occurred about 40 km to the south which was followed about 9 hr later by another aftershock mb 6 4 in the same area waveform modelling results for this earthquake sequence suggest focal depths ranged from 15 21km with rupture lengths of 14 16 km an average displacement of 1 8 m is estimated for the mainshock but no surface rupture has been noted for the event in the vicinity of the hun graben area the number of large earthquakes that have been reported through history is an indication that the area is seismically active most of the activity appears to be concentrated along the eastern margin of the nw se trending hun graben the focal mechanisms suggest that the maximum compressive stress in northwestern libya is oriented wnw in the north eastern part of the country the stress direction changes to a more ne sw orientation as evidenced by the 1963 february 21 al marj and the 1967 al jabal al akhdar mainshocks

seismic moment assessment of earthquakes in stable continental regions ii historical seismicity summary this part i study in conjunction with part 11 develops a method to determine within specified uncertainty bounds the seismic moment and thus moment magnitude of all earthquakes of stable continental regions scr for which instrumental or intensity data exist its basis is polynomial regression analysis using a database of scr earthquakes with direct seismic moment determination the independent variables include modern teleseismic magnitudes and regional magnitudes part i and isoseismal areas or number of recording stations part 11 part i11 is an application of the methodology of parts i and i1 to several major historical earthquakes all data used in the regressions are assigned individual uncertainties estimated from the literature or from experience formal confidence limits 68 per cent or 95 per cent on both the regression formulas and the predicted seismic moment values are then possible via error propagation analysis the most complete development is for the teleseismic magnitudes m and mb for both the final regression for log m is a quadratic formula that closely emulates the relationship between amplitude magnitudes and log m expected from dislocation theory and source scaling arguments regressions are also derived for the regional magnitudes mlg and m because there are many scr events mainly pre 1964 that have no teleseismic magnitudes prediction uncertainties from teleseismic magnitudes in moment magnitude units are in the ao 18 0 28 range and from regional magnitudes in the ko 23 0 38 range over a wide magnitude band finally the methodology developed here is generic even though the database is specific application to plate boundary oceanic intraplate or active continental intraplate regions should be straightforward

experimental investigation of volcanic particle aggregation in the absence of a liquid phase 1 understanding the dispersal and deposition of fine grained silicate particles from volcanic plumes is key to interpreting ash fall deposits and predicting hazards for future eruptions it is known that many of these particles fall incorporated into delicate dry aggregates whose sedimentation characteristics have not been previously investigated here we present the results of laboratory experiments on aggregates of small dry silicate particles produced by the fragmentation of pumice collected from the 18 may 1980 mount st helens fall deposit the aggregation process is driven by electrostatic charges naturally imparted to the particles during the fracture process for particle fall distances of 1 m images of the in flight aggregates show that they commonly have irregular shapes and are up to 800 m in size strobe photography was used to determine aggregate fall velocities and by representing aggregates as falling spheres suggested that they had densities of 100 200 kg m 3 comparable densities were obtained from experiments where equivalent fall velocities were assumed for aggregates and single particles which had been transported similar distances within a horizontal airflow these dispersal experiments produced bimodal particle size distributions similar to those observed in the 18 may 1980 mount st helens deposits and suggest that the aggregates were composed mainly of particles 70 m in diameter our experimental results are in agreement with aggregate size and density estimates previously used within several theoretical plume sedimentation models in order to explain some features of natural ash deposits

seismotectonics of the guinean earthquake of december 22 1983 field geological and seismological investigations conducted after the guinean earth quake on december 22 1983 show a shallow dextral strike slip fault with a large normal component the nw se horizontal compressive stress deduced from this event is consistent with measurements made at other places in this region old fissures some of them reactivated during this earthquake show that this intraplate area cons titutes a localized weakness zone in the crust

aggregation dominated ash settling from the eyjafjallajokull volcanic cloud illuminated by field and laboratory high speed imaging the recent eyjafjallajokull iceland eruption strikingly underlined the vulnerability of a globalized society to the atmospheric dispersal of volcanic clouds from even moderate size eruptions ash aggregation controls volcanic clouds dispersal by prematurely removing fine particles from the cloud and depositing them more proximally physical parameters of ash aggregates have been modeled and derived from ash fallout deposits of past eruptions yet aggregate sedimentation has eluded direct measurement limiting our ability to predict the dispersal of volcanic clouds here we use field based high speed video analysis together with laboratory experiments to provide the first in situ investigation and parameterization of the physical features and settling dynamics of ash aggregates from a volcanic cloud in may 2010 high speed video footage was obtained of both ash particles and aggregates settling from the eyjafjallajokull volcano eruption cloud at a distance of 7 km from the vent fallout samples were collected simultaneously experimental laboratory determinations of the density morphology and settling velocity of individual ash particles enable their distinction from aggregates the combination of field and experimental analyses allows a full characterization of the size settling velocity drag coefficient and density distributions of ash aggregates as well as the size distribution of their component particles we conclude that ash aggregation resulted in a tenfold increase in mass sedimentation rate from the cloud aggravating the ash hazard locally and modifying cloud dispersal regionally this study provides a valuable tool for monitoring explosive eruptions capable of providing robust input parameters for models of cloud dispersal and consequent hazard forecast

volcanic particle aggregation in explosive eruption columns part i parameterization of the microphysics of hydrometeors and ash abstract the aggregation of volcanic ash particles within the eruption column of explosive eruptions has been observed at many volcanoes it influences the residence time of ash in the atmosphere and the radiative properties of the umbrella cloud however the information on the processes leading to aggregate formation are still either lacking or very incomplete we examine the fate of ash particles through numerical experiments with the plume model atham active tracer high resolution atmospheric model in order to determine the conditions that promote ash particle aggregation in this paper we describe the microphysics and parameterization of ash and hydrometeors in a companion paper this issue we use this information in a series of numerical experiments the parameterization includes the condensation of water vapor in the rising eruption column the formation of liquid and solid hydrometeors and the effect of latent heat release on the eruption column dynamics are considered the interactions of hydrometeors and volcanic ash within the eruption column that lead to aggregate formation are simulated for the first time within a rising eruption column the microphysical parameterization follows a modal approach the hydrometeors are described by two size classes each of which is divided into a liquid and a frozen category by analogy with the hydrometeor classification we specify four categories of volcanic ash particles we imply that volcanic particles are active as condensation nuclei for water and ice formation ash can be contained in all categories of hydrometeors thus forming mixed particles of any composition reaching from mud rain to accretionary lapilli collisions are caused by gravitational capture of particles with different fall velocity coalescence of hydrometeor ash aggregates is assumed to be a function of the hydrometeor mass fraction within the mixed particles the parameterization also includes simplified descriptions of electrostatics and salinity effects

auroral zone thermospheric dynamics 1 averages forty four nights of thermospheric neutral wind and temperature measurements were obtained with a ground based fabry perot spectrometer at college alaska 65 invariant latitude mostly during high to moderate levels of solar activity the spectrometer scanned the o i 15 867 k 6300 a emission line yielding line profile measurements that originate from an altitude range between 170 and 280 km the observed average wind direction is to the northwest in the early evening changing to the south near midnight and then to the southeast in the morning the overall wind pattern generally persists but the wind speed increases with increasing geomagnetic activity a change in the zonal wind direction occurs earlier in magnetic local time with increasing magnetic activity the meridional wind pattern also shifts equatorward with the auroral oval during increasing magnetic activity consequently the average wind pattern in the north during low geomagnetic activity is rather similar to the moderate activity average in the south and the moderate geomagnetic activity average wind pattern in the north is similar to the high geomagnetic activity average in the south the observed average thermospheric temperature is found to be governed by geomagnetic activity and the previous day s 10 7 cm solar flux

the formation of high grade ignimbrites i experiments on high and low concentration transport systems containing sticky particles high grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature or even solidus temperature corresponding pyroclastic flow particles range from plastic to partially liquid and are able to aggregate or coalesce this contrasts with particles in pyroclastic flows producing unwelded ignimbrite which are capable of elastic grain interactions the low aspect ratio and great areal extent of high grade ignimbrites requires transport in a particulate state either by a high concentration mass flow facilitated by fluidizing gas reducing internal friction or by b expanded turbulent flow of low but downward increasing concentration this paper presents experiments designed to investigate the effects of plastic to liquid particles on these two contrasting transport mechanisms gas fluidization experiments using polyethyleneglycole peg powders heated above minimum sintering tms and melting tm temperatures cover a wide range of fluidization velocities umf ua 0 6 ut but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash where particle volume concentration outside the bubbles is high 10 1 when the powders reach a critical temperature tm t tms defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized beds as well as in experiments using mixtures of high and low tm 30 wt peg powders when t tms of the lower tm powder this indicates that extended particulate transport at t tms is not possible at such high particle concentrations in the turbulent flow experiments liquid sprays of molten peg or water vertically injected into a high re 104 horizontal air flow form a low concentration 10 5 to 10 4 turbulent suspension current proximal formation of partially coalesced aggregates which settle faster than individual particles causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid particle sedimentation from turbulent suspensions as particles become finer downstream and coalescence efficiency decreases in response to cooling more distally formed aggregates become too small and rare to modify sedimentation rate decay from that of suspension flows containing solid particles the key difference between the two transport systems is particle concentration c since particle collision rate rcoll c2 collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency 1 ecoal 0 is larger than 10 3 low concentration suspensions on the other hand require much higher values of ecoal for significant aggregation to occur dilute pyroclastic flows will have higher particle volume fractions 10 3 than the experimental currents but then viscous pyroclasts should have lower coalescence efficiencies than peg droplets experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high grade ignimbrite sheets

the regional intraplate stress field in south america a compilation of lithospheric stress directions for continental south america and the inferred major patterns of the regional intraplate stress field are presented stress orientations are based primarily on earthquake focal mechanisms and quaternary fault slip inversion published in the literature four new focal mechanisms based on short period p wave modeling and selected centroid moment tensor solutions published by u s geological survey consistent with p wave first motions at south american and other world wide standard seismograph network stations are also included in the data base the observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the south american plate in the andean plateau altitudes greater than 3000 m n s extensional stresses predominate e w compressional stresses are observed in the sub andean and platform regions extending up to about 1000 km east of the andes the maximum horizontal stress shmax is uniformly oriented in the e w direction throughout western south america averages of the shmax direction at grid points spaced 2 5 o were taken as representing the regional field the e w direction of this regional field is not affected by the change of strike of the andean chain nor by the contact of a flat subducted slab underneath the regional shmax direction is oriented 15 o more clockwise than the direction of the nazca plate convergence the difference between the regional shmax orientations and the absolute plate motion may be only about 6 o this may indicate that contact forces with the nazca plate may not be the only major contributor to the intraplate stresses in western south america the eastern limit of this andean stress province seems to coincide with aseismic regions in the upper amazon basin and in the paraml basin in the central amazonian region seismicity and stress data suggest a seismic province with n s compressional stresses the origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle amazon basin in northeastern brazil seismicity is characterized by upper crustal strike slip earthquakes bordering the potiguar marginal basin a model is proposed for this region in which the stress field is the result of a superposition of regional e w compressional stresses and local extensional stresses oriented perpendicular to the continental margin possibly related to density contrasts and sediment loading in the continental shelf

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

the siwaliks of western nepal ii mechanics of the thrust wedge abstract comparison between numerical models and structural data is used for a better understanding of the evolution of the siwalik thrust belt of western nepal the numerical model involves discontinuities within a critical wedge model a kinematic forward model of serial cross sections and a linear diffusion algorithm to simulate erosion and sedimentation in western nepal large piggy back basins duns are located above thick thrust sheets that involve more than 5500 m of the neogene siwalik group whereas piggy back basin sedimentation is less developed above thinner thrust sheets 4300 m thick numerical model results suggest that thrust sheet thickness and extension of wedge top basins are both related to an increase of the basal decollement dip beneath the duns the west dang transfer zone wdtz is a n ne trending tectonic lineament that limits the westward extent of the large piggy back basins of mid western nepal and is linked to a thickening of the himalayan wedge eastward the wdtz also affects the seismotectonics pattern the geometry of the thrust front the lateral extent of lesser himalayan thrust sheets and the subsidence of the foreland basin during middle siwalik sedimentation numerical models suggest that the individualisation of the piggy back basins at the transition between the middle siwalik and upper siwaliks followed the deposition of the middle siwaliks that induced a geometry of the foreland basin close to the critical taper as wdtz induces an e w thickning of the himalayan wedge it could also induce a northward shift of the leading edge of the ductile deformation above the basal detachment in greater himalayas of far western nepal field data locally suggest episodic out off sequence thrusting in the frontal thrust belt of western nepal whereas numerical results suggests that episodic out off sequence reactivation could be a general characteristic of the himalayan wedge evolution often hidden by erosion