monitoring volcano slope instability with synthetic aperture radar a review and new data from pacaya guatemala and stromboli italy volcanoes abstract volcano slope instability manifests in many forms ranging from steady state to punctuated movement or shallow erosion to deep seated spreading the interplay of gravity magmatic or hydrothermal fluids and often active tectonics on a volcanic edifice results in complex spatial and temporal variations in deformation kinematics while this makes the recognition assessment and monitoring of volcano slope instability challenging advancements in synthetic aperture radar sar technology have significantly accelerated our knowledge of instability phenomena and our ability to assess their hazards this review discusses the applications and challenges of sar imagery to various slope instabilities at volcanoes around the world sar amplitude images are powerful tools for mapping areas of geomorphological changes these snapshots can be combined using interferometric sar insar to create multi temporal deformation maps that provide unique information on the evolution of slope failures space borne insar has become an economic way to detect changes at volcanoes at very high resolution ground based insar gbinsar can produce frequent sar images on the order of seconds to minutes propelling insar from monitoring to surveillance and early warning applications however interpreting insar derived ground deformation signals related to volcano slope instability is still challenging deformation from magma rise or variations in hydrothermal systems may be inseparable from persistent deep seated flank motion similarly shallow or localized ground motion may occur in overlap with thermal contraction or subsidence of newly emplaced lava or tephra deposits if triggered to failure landslides can vary over several magnitudes from small volume rock falls that pose only a localized hazard to large volume debris avalanches capable of travelling tens of kilometers away from the source these collapses can have cascading hazards if accompanied by directed blasts or tsunamis emphasizing the need for continued advancement of our understanding of these events and their consequences to highlight the utility of sar for measuring and monitoring mass movement two case studies of stromboli italy and pacaya guatemala volcanoes are discussed in detail where recent instability events persistent volcanic activity and ground truth constraints have resulted in excellent case histories in applying sar imagery to understand these potentially hazardous slope instabilities

temporal changes of shallow seismic velocity around the karadere duzce branch of the north anatolian fault and strong ground motion we analyze temporal variations of seismic velocity along the karadere duzce branch of the north anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 mw7 4 izmit and mw7 1 duzce earthquakes the analysis employs 36 sets of highly repeating earthquakes each containing 4 18 events the events in each cluster are relocated by detailed multi step analysis and are likely to rupture approximately the same fault patch at different times the decay rates of the repeating events in individual clusters are compatible with the omori s law for the decay rate of regional aftershocks a sliding window waveform cross correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events we find clear step like delays in the direct s and early s coda waves sharp seismic velocity reduction immediately after the duzce main shock followed by gradual logarithmic type recoveries a gradual increase of seismic velocities is also observed before the duzce main shock probably reflecting post seismic recovery from the earlier izmit main shock the temporal behavior is similar at each station for clusters at various source locations indicating that the temporal changes of material properties occur in the top most portion of the crust the effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones and generally decrease with normal distance from the fault a strong correlation between the co seismic delays and intensities of the strong ground motion generated by the duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material

the stratigraphic distribution of ni rich spinels in cretaceous tertiary boundary rocks at el kef tunisia caravaca spain and hole 761c leg 122 abstract ni rich spinels have been found throughhout the world at the cretaceous tertiary boundary these minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris suggesting that the cretaceous tertiary boundary spinels result from an extraterrestrial event their stratigraphic distribution which has been observed at the cretaceous tertiary boundary sites of el kef tunisia caravaca spain and hole 761c odp leg 122 indian ocean are markedly different from site to site and are all much less extended than the ir anomalies regional differences in ni rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions the wider stratigraphic dispersion of ir is attributed to postdepositional chemical processes at el kef the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate the 1 2 mm thick ni rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years this upper limit is consistent only with the collision of an asteroid or a comet at the close of the cretaceous

failure of volcano slopes magmatic intrusions can initiate and sustain massive and catastrophic volcano collapse their role is twofold involving both driving and resisting forces first flank stability is diminished by magmastatic and magma overpressures and steepened slopes that accompany intrusion second excess pore pressures in potential failure zones can be generated as a result of intrusion related mechanical or thermal straining of the rock fluid medium pressurized retrograde boiling in high level magma chambers or hydrothermal fluid circulation also earthquakes may aid collapse through inertial forces and shaking induced pore pressure generation these excess pore pressures reduce the sliding resistance as shown for wedge shaped slide blocks for selected cases the destabilizing influence of mechanically induced pore pressures is maximized as the intruded width or corresponding overpressure of the intrusion is increased the destabilizing influence of thermally induced pore pressures is conditioned by the seve

the volcanic and magmatic evolution of volcan ollague a high k late quaternary stratovolcano in the andean central volcanic zone abstract volcan ollague is a high k calc alkaline composite volcano constructed upon extremely thick crust in the andean central volcanic zone volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the vinta loma series and an overlying coalescing dome and coulee sequence of the chasca orkho series following cone construction the upper western flank of ollague collapsed toward the west leaving a collapse amphitheater about 3 5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano the deposit is similar to the debris avalanche deposit produced during the may 18 1980 eruption of mount st helens u s a and was probably formed in a similar manner it presently covers an area of 100 km2 and extends 16 km from the summit subsequent to the collapse event the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse amphitheater additional post collapse activity included construction of a dacitic dome and coulee of the la celosa series on the northwest flank field relations indicate that vents for the vinta loma and post collapse series were located at or near the summit of the cone the vinta loma series is characterized by an anhydrous two pyroxene assemblage vents for the la celosa and chasca orkho series are located on the flanks and strike n55 w radial to the volcano the pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks a possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation in addition to the lava flows at ollague magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas all eruptive activity at ollague predates the last glacial episode 11 000 a b p because post collapse lava flows are overlain by moraine and are incised by glacial valleys present activity is restricted to emission of a persistent 100 m high fumarolic steam plume from a vent located within the summit andesite dome sr and nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation in contrast narrow ranges in 143nd 144nd and 87sr 86sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination

miocene quaternary structural evolution of the uyuni atacama region andes of chile and bolivia abstract we describe the miocene quaternary geological structural evolution of the region between the salar de uyuni and de atacama andes of chile and bolivia we recognized four main tectonic events based on fold geometry fault kinematics and stratigraphic data the oldest event of miocene age is characterized by folding and reverse faulting of the sedimentary successions with an e w direction of shortening in the northern part of the studied area and a wnw ese shortening in the southern part the following two events of pliocene age are characterized by lower shortening amounts they occurred first by reverse faulting with a nw se trending greatest principal stress 1 computed with striated fault planes and a vertical least principal stress 3 followed by pervasive strike slip faulting with the same nw se trending 1 and a horizontal ne sw 3 the fourth event dating to the late pliocene quaternary is characterized by normal faulting the 3 still trends ne sw whereas the intermediate principal stress 2 exchanged with 1 volcanism accompanied both the contractional transcurrent and extensional tectonic phases the mio pliocene compression appears directly linked to a rapid convergence and an apparently important coupling between the continental and oceanic plates the e w to wnw ese direction of shortening of the miocene structures and the nw se 1 of the pliocene structures seem to be more linked to an intra andean re orientation of structures following the wnw directed absolute motion of the south american plate the extensional deformations can be interpreted as related to gravity forces affecting the highest parts of the volcanic belt in a sort of asymmetrical sw ward collapse of the belt

magnetostratigraphy of the cretaceous tertiary boundary at agost spain a detailed magnetostratigraphic investigation of the agost section spain containing the cretaceous tertiary k t boundary is reported thermal demagnetization contrary to alternating field demagnetization succeeds in revealing the polarity of the characteristic remanent magnetization although an overlap in blocking temperature spectrum exists with a normal polarity secondary magnetization component the k t boundary occurs at two thirds from the base of a reversed polarity zone and by comparison with earlier results 1 this polarity zone is correlated to chron c29r linear regression of sedimentation rates in other sections with an established magnetostratigraphy and containing the k t boundary yields an age of 66 45 ma for this boundary on the basis of this age and the magnetic reversal ages of the berggren et al 13 polarity time scale new first appearance fad ages around the k t boundary are suggested for the planktonic speciesmorozovella trinidadensis bolli 66 04 ma morozovella pseudobulloides plummer 66 34 ma eoglobigerina taurica bang 66 41 ma and globigerina eugubina luterbacher premoli silva 66 43 ma

contractional tectonics and magma paths in volcanoes abstract this study aims to contribute a possible explanation for magma migration within volcanoes located in contractional tectonic settings based on field data and physically scaled experiments the data demonstrate the occurrence of large stratovolcanoes in areas of coeval reverse faulting in spite of the widely accepted idea that volcanism can develop only in extensional transcurrent tectonic settings the experiments simulate the propagation of deformation from substrate reverse faults with different attitudes and locations into volcanoes the substrate fault splits into two main shear zones within the volcano a shallow dipping one with reverse motion propagates towards the lower volcano flank and a steeper dipping one with normal motion propagates upwards in plan view the reverse fault zone is arcuate and convex outwards whereas the normal fault zone is rectilinear structural field surveys at volcanoes located in contractional settings show similar features the plio quaternary trohunco and los cardos centinela volcanic complexes argentina lie above plio quaternary reverse faults the late pleistocene holocene el reventador volcano ecuador is also located in a coeval contractional tectonic belt these volcanoes show curvilinear reverse faults along one flank and rectilinear extensional fracture zones across the crater area consistent with the experiments these data consistently suggest that magma migrates along the substrate reverse fault and is channelled along the normal fault zone across the volcano

interplate quasi static slip off sanriku ne japan estimated from repeating earthquakes 1 space time distribution of quasi static slip on the plate boundary off sanriku northeastern ne japan was estimated by using small repeating earthquakes we found 2509 repeating earthquakes occurring on the plate boundary for the last 17 years based on the interpretation that these events are caused by repeating slips of small asperities surrounded by stable sliding area we regarded these repeating events as creepmeters embedded on the plate boundary the results show that deeper portions of the plate boundary are sliding almost stably and earthquakes with magnitudes more than 6 are accompanied by quasi static slips in the surrounding areas

petrology of calc alkaline lavas at volc n ollague and the origin of compositional diversity at central andean stratovolcanoes

earthquakes recurrence and interoccurrence times the purpose of this paper is to discuss the statistical distributions of recurrence times of earthquakes recurrence times are the time intervals between successive earthquakes at a specified location on a specified fault although a number of statistical distributions have been proposed for recurrence times we argue in favor of the weibull distribution the weibull distribution is the only distribution that has a scale invariant hazard function we consider three sets of characteristic earthquakes on the san andreas fault 1 the parkfield earthquakes 2 the sequence of earthquakes identified by paleoseismic studies at the wrightwood site and 3 an example of a sequence of micro repeating earthquakes at a site near san juan bautista in each case we make a comparison with the applicable weibull distribution the number of earthquakes in each of these sequences is too small to make definitive conclusions to overcome this difficulty we consider a sequence of earthquakes obtained from a one million year virtual california simulation of san andreas earthquakes very good agreement with a weibull distribution is found we also obtain recurrence statistics for two other model studies the first is a modified forest fire model and the second is a slider block model in both cases good agreements with weibull distributions are obtained our conclusion is that the weibull distribution is the preferred distribution for estimating the risk of future earthquakes on the san andreas fault and elsewhere

reactivation of basement faults beneath volcanoes a new model of flank collapse in order to explain the presence of voluminous volcanic debris avalanche deposits around a stratovolcano reactivation of vertical faults beneath a volcanic cone has been tested using analogue models reactivation of a single vertical fault beneath a cone generates a normal fault and an upturning of the layers creating a bulge on the flank the upturning induces a flank collapse characterized by a typical horseshoe shaped scar called an avalanche caldera reactivation of two vertical faults beneath a cone also generates a normal fault and a summit bulge this bulge may result from the movement along a reverse fault a large collapse is generated within the angle created by the two vertical faults the angle of the collapse can be up to 140 whereas this angle is typically 120 for a dome intrusion collapse is instantaneous and is favoured by the presence of ductile layers ash and pumice formations in the example considered in a stratovolcano complex the model may be applicable to volcanoes in a state of dormancy or extinction in regions with active regional tectonism we suggest this mechanism of collapse in the case of the cantal stratovolcano massif central france to explain the presence of voluminous volcanic debris avalanche deposits around this volcano

earthquake cycles in a model of interacting fault patches complex behavior at transition from seismic to aseismic slip numerical simulations of complex earthquake cycles are conducted using a model with two patches asperities on a fault where friction obeys a rate and state dependent law the patches are represented by regions of steady state velocity weakening friction and are in contact with each other or are separated by a region of steady state velocity strengthening friction simulated earthquake cycles are examined by changing the characteristic slip distance of friction in one patch while that in the other patch is maintained at a constant as the characteristic slip distance in a patch increases the slip at the patch tends to be aseismic and rupture tends to be delayed as compared to rupture at the other patch or arrested due to an increase in fracture energy as a result the slip pattern changes in a complicated manner with the change of the parameter a complex multiperiodic or aperiodic slip pattern occurs in the simulation and this pattern tends to occur near the boundaries between regions of different slip behavior characteristics such as the boundary between seismic and aseismic slip in the patch with a larger characteristic slip distance iteration maps of the recurrence intervals of simulated earthquakes for an aperiodic sequence are sometimes expressed by simple curves suggesting that the simulated earthquake cycles exhibit deterministic chaos in the two patch models of the present study an aperiodic slip pattern is observed for narrower parameter ranges than in a model of a two degree of freedom spring block system which is usually regarded as a simplified model of a fault with two patches online material simulated histories of moment release rate and recurrence interval of simulated slip events in the two patches and seismic coupling coefficients

chicxulub multiring impact basin size and other characteristics derived from gravity analysis the buried chicxulub impact structure in mexico which is linked to the cretaceous tertiary k t boundary layer may be significantly larger than previously suspected reprocessed gravity data over northern yucatan reveal three major rings and parts of a fourth ring spaced similarly to those observed at multiring impact basins on other planets the outer ring probably corresponding to the basin s topographic rim is almost 300 kilometers in diameter indicating that chicxulub may be one of the largest impact structures produced in the inner solar system since the period of early bombardment ended nearly 4 billion years ago

the impact of analogue material properties on the geometry kinematics and dynamics of convergent sand wedges abstract simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks we present data on the frictional strength of different sand types employing static and dynamic shear tests the sand types analysed are characterised by an elastic frictional plastic mechanical behaviour with a transient strain hardening and strain softening phase prior to transition to stable sliding this is in conflict with the standard assumption of an ideal cohesionless coulomb material with constant frictional properties the influence of the identified transient material properties on the kinematics growth mechanisms and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary depending on material compaction between wedges with well defined segments i e frontal deformation zone frontal imbrication zone and internal accumulation zone with straight slopes and wedges with a continuous convex topographic profile for most materials only the frontal part of the wedge is critical during experimental runs taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults wedge segmentation is controlled by a bulk strength increase toward the rear of the wedge due to fault rotation in mechanically less favourable orientations and plastic material hardening the limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs on this basis we suggest that critical taper analysis of wedges must be restricted to specific kinematic segments comparison of the experimental results with the nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges moreover we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges

geological and geotechnical characterization of the debris avalanche and pyroclastic deposits of cotopaxi volcano ecuador a contribute to instability related hazard studies abstract the huge volcanic debris avalanche occurred at 4 5 ka is a major event in the evolution of the cotopaxi volcano ecuador the present volcanic hazard in the cotopaxi region is related to lahars generated by volcanic eruptions and concurrent ice melting this paper presents the geological and geotechnical field and laboratory characterization of the 4 5 ka cotopaxi debris avalanche deposit and of the younger unconsolidated pyroclastic deposits representing the probable source of future shallow landslides the debris avalanche formed a deposit with a well developed hummocky topography and climbed a difference in height of about 260 m along the slopes of the adjacent sincholagua volcano the debris avalanche deposit includes four lithofacies megablock block mixed and sheared facies that represent different flow regimes and degrees of substratum involvement the facies distribution suggests that in the proximal area the debris avalanche slid predominantly confined to the valleys along the n and ne flank of the volcanic cone emplacing a stack of megablocks when the flow reached the break in slope at the base of the edifice it became unconfined and spread laterally over most of the area of the rio pita valley a dynamic block fragmentation and dilation occurred during the debris avalanche transport emplacing the block facies the incorporation of the older chalupas ignimbrite is responsible for the mixed facies and the sheared facies geotechnical results include a full range grain size characterization which enabled to make broader considerations on possible variability among the sampled facies consolidated drained triaxial compression tests carried out on the fine fraction the investigated post 4 5 pyroclastic deposits constitute a 5 12 m thick sequence of poorly consolidated materials that are interlayered with lava flows their geotechnical analyses have evidenced a strong variability in grain size distribution reflecting the depositional processes and a generally high porosity consolidated drained triaxial compression tests delineated a similar shear stress strain behaviour among the different units where shear strength is only due to friction angle failure surfaces are always well developed indicating that the poorly consolidated pyroclastic cover could undergo failure leading to the formation of a gravity driven instability phenomena like granular or debris flows which are mainly controlled by the fine fraction this work underlies the general necessity for a site specific and interdisciplinary approach in the characterization of volcanic successions to provide reliable data for gravitational instability studies

impacts of catastrophic volcanic collapse on the erosion and morphology of a distal fluvial landscape hautapu river mount ruapehu new zealand debris avalanches caused by the collapse of volcanic flanks pose a great risk to inhabited areas and may permanently change the surrounding landscape and its drainage systems in this research we explored the interplay between a debris avalanche and a tectonically uplifting surrounding landscape providing insights into the long term consequences of volcanic edifice failures exposures of coarse volcaniclastic sediments along the hautapu river 50 km southeast of mount ruapehu new zealand show evidence of the largest known collapse event of the stratovolcano which was followed by a vigorous regrowth phase that produced numerous pyroclastic eruptions and pumice rich lahars similar diamicton deposits are exposed within the river catchment adjacent to the west cover bed stratigraphy and geochemical correlation of andesitic lava blocks within the debris avalanche deposit with dated lavas exposed on the cone indicate that deposition occurred between 125 and 150 ka the collapse took place during the shift from a glacial to an interglacial climate when glaciers on the cone were in retreat and high pore water pressures combined with deep hydrothermal alteration weakened the cone in addition collapse may have been accompanied by magmatic unrest the 2 3 km3 debris avalanche inundated an area of 260 km2 and entered the proto hautapu catchment where it was channelized within the deeply entrenched valley mass wasting events associated with postcollapse volcanism continued to be channeled into the proto hautapu river for another 10 k y producing long runout lahars subsequently the river catchment was isolated from the volcano by incision of the intervening whangaehu river into the proximal volcaniclastic sediments accompanied by regional faulting and graben deepening around ruapehu at present the volcaniclastic deposits form a distinctive plateau on the highest topographic elevation within the hautapu valley forming a reversed topography caused by preferred incision of the hautapu river into softer late tertiary sediments concurrent with constant uplift

the longer it has been since the last earthquake the longer the expected time till the next we adopt a iognormal distribution for earthquake interval times and we use a locally determined rather than a generic coefficient of variation to estimate the probability of occurrence of characteristic earthquakes we extend previous methods in two ways first we account for the aseismic period since the last event the seismic drought in updating the parameter estimates second in calculating the earthquake probability we allow for uncertainties in the mean recurrence time and its variance by averaging over their likelihood both extensions can strongly influence the calculated earthquake probabilities especially for long droughts in regions with few documented earthquakes as time passes the recurrence time and variance estimates increase if no additional events occur leading eventually to an affirmative answer to the question in the title the earthquake risk estimate begins to drop when the drought exceeds the estimated recurrence time for the parkfield area of california the probability of a magnitude 6 event in the next 5 years is about 34 per cent much lower than previous estimates furthermore the estimated 5 year probability will decrease with every uneventful year after 1988 for the coachella valley segment of the san andreas fault the uncertainties are large and we estimate the probability of a large event in the next 30 years to be 9 per cent again much smaller than previous estimates on the mojave pallett creek segment the catalog includes 10 events and the present drought is just approaching the recurrence interval so the estimated risk is revised very little by our methods

subaerial submarine evidence of structures feeding magma to stromboli volcano italy and relations with edifice flank failure and creep abstract structural field data including parameters of 109 sheets strike mostly n10 55 e average dip angle mostly 70 90 average thickness two peaks at 1 2 m and 4 2 m and of 14 eruptive fissures and vents as well as bathymetric and numerical data suggest that stromboli experienced the development of a volcanic rift zone across the cone summit with a tendency to widen towards the lower submarine flanks during the last 100 ka inside this rift zone intrusive sheets mostly strike ne and become more abundant more steeply dipping and thicker on approaching the rift axis this rift persistence reflects the edifice s state of stress with a nw se horizontal least principal stress on the magma intrusion paths after 13 ka ago sheets mainly intruded with a horseshoe shaped geometry in plan view along the boundaries of the northwestern unstable volcano flank where four main lateral collapses occurred during the last 13 ka they also intruded with a ne strike across the volcano summit and the northeastern volcano flank the horseshoe shaped geometry is influenced by the lateral creep of the volcano flank which provides space for intrusion along the transtensional boundaries of the unstable sector where thinner sheets were emplaced when the collapse depression was filled up by new deposits ne striking thicker dykes emplaced across the northwestern volcano flank indicating a rapid re establishment of regional tectonic forces across the cone unbuttressing due to the present sciara del fuoco depression and ongoing flank creeping can account for intrusions also inside the sciara del fuoco the latter in 2007

variations in rupture process with recurrence interval in a repeated small earthquake in theory and in laboratory experiments friction on sliding surfaces such as rock glass and metal increases with time since the previous episode of slip1 this time dependence is a central pillar of the friction laws widely used to model earthquake phenomena2 3 on natural faults other properties such as rupture velocity4 5 porosity and fluid pressure6 11 may also vary with the recurrence interval eighteen repetitions of the same small earthquake separated by intervals ranging from a few days to several years allow us to test these laboratory predictions in situ the events with the longest time since the previous earthquake tend to have about 15 larger seismic moment than those with the shortest intervals although this trend is weak in addition the rupture durations of the events with the longest recurrence intervals are more than a factor of two shorter than for the events with the shortest intervals both decreased duration and increased friction are consistent with progressive fault healing during the time of stationary contact

constituent material properties and clast size distribution of volcanic breccia abstract volcanic breccia is a representative heterogeneous rock as a preliminary study for determining its mechanical properties in laboratory and field experiments its material properties and structural features were described three dimensional clast size distribution was also estimated for the study site using the theory of stereology with this distribution determined a numerical simulation was performed to investigate the relationship between the volumetric proportion of clasts in a sample and the areal proportion and maximum length of clasts on a lateral surface of the sample the following observations were made 1 coefficients of variation for volumetric proportion are dependent on the size of the sample and became large when the size of the sample is small 2 samples with a small coefficient of variation can be selected at any size if a restriction is placed on the ratio dmax d0 of the maximum length of clasts to the size of the sample and 3 the volumetric proportion of clasts in each sample can be estimated from the observed areal proportions within an error range that depends on the whole volumetric proportion

toughness and failure of volcanic edifices abstract major volcanic edifices are of two main types stratovolcanoes composite edifices and basaltic edifices shield volcanoes these types of edifices show several remarkable differences for example while some basaltic edifices erupt very frequently many stratovolcanoes erupt very infrequently also most basaltic edifices have gentle slopes commonly 2 12 whereas stratovolcanoes have steeper slopes particularly in their uppermost parts where the slopes may reach 35 42 there have been various attempts to explain these differences but here the focus is on edifice strength or rather toughness toughness of a material is a measure of its resistance to fracture alternatively for a tough material large amounts of energy are needed to cause failure most quoted material toughness critical energy release rate values for rocks are 20 400 j m 2 for extension fractures such as dykes and 0 01 0 03 mj m 2 for shear fractures such as dip slip faults the related property of critical stress intensity or fracture toughness is for most rocks 0 5 3 mpa m 1 2 these values for material and fracture toughness refer to small essentially homogeneous laboratory rock samples here i present equations for calculating the material toughness in volcanic edifices based on the measured or estimated dimensions of associated large structures such as dykes and faults for typical dykes the calculated material toughness is 1 3 47 mj m 2 with a corresponding stress intensity of 114 690 mpa m 1 2 similarly for a 3 8 km long dip slip fault in an edifice the calculated material toughness is 2 3 mj m 2 and the corresponding stress intensity 150 mpa m 1 2 these results compare well with previous material toughness estimates for large strike slip faults and stress intensity estimates for large dykes generalising these results and using field data as well as materials science results i conclude that stratovolcanoes normally have a much higher material toughness than basaltic edifices this is primarily because of different internal structures a basaltic edifice is made of layers and contacts all of which have essentially similar mechanical properties whereas a stratovolcano is made of layers and contacts many of which have widely different properties accordingly it requires much more energy is more difficult for a fracture to propagate through a stratovolcano than through a basaltic edifice thus contrasting toughnesses may be one principal reason why these edifices differ in slope and in failure frequencies including the number of dyke fed eruptions caldera collapses and landslides

towards a universal rule on the recurrence interval scaling of repeating earthquakes 1 a recent analysis of recurrence properties of small repeating earthquakes on a creeping oblique thrust fault in eastern taiwan reveals a weak variation in recurrence interval tr with seismic moment mo compared to the scaling of tr with mo from repeating earthquake data near parkfield in california the repeating data from eastern taiwan has recurrence intervals that are 2 times shorter also in northeastern japan tr of repeating quakes are 4 times shorter than those expected from the parkfield scaling law when adjusted to account for differences in the geodetically derived slip rates for the three fault zones however the tr mo scaling is remarkably consistent among the three regions it suggests that the tectonic loading rate is likely the most important factor that controls the repeat time it also suggests that there seems to exist a universal rule on recurrence interval scaling of repeating earthquakes in diverse tectonic settings

the structure of composite volcanoes unravelled by analogue modelling a review abstract volcanic edifices are unstable accumulations of eruptive and intrusive products with complex internal structures a documentation of these structures facilitates the assessing of the various hazards associated with volcanoes e g flanks instabilities and eruptive events however structures are not easily assessed in the field as volcanoes are regularly covered by eruptive products and are rapidly eroded in the case of recent and active volcanoes internal structures are generally extrapolated from the external morphology of the edifice which can be documented using digital elevation models dem and geodetic data interpretation of such data greatly benefits from analogue models which are idealised representations of reality used to test the impact that various processes may have on the shape of a volcanic edifice this short review focuses on the contribution of analogue models to our comprehension of the internal structure of composite volcanic cones the other volcanic morphologies e g caldera are excluded from this review as they have distinct structure and evolution that have already been thoroughly reviewed the reviewed models were used to investigate the impact that regional faults magma injections and ductile substratum among other processes have on the internal structure and surface morphology of many volcanic edifices these models were also used to locate unstable flanks and eruption sites

composite volcanoes in the south eastern part of izmir balikesir transfer zone western anatolia turkey abstract during the early middle miocene western anatolia several volcanic fields occur along a ne sw trending shear zone known as izmir balikesir transfer zone this is a deformed crustal scale sinistral strike slip fault zone crossing the bornova flysch and extending along the nw boundary of the menderes massif by accommodating the differential deformation between the cycladic and menderes core complexes within the aegean extensional system here we discuss the volcanic activity in yamanlar and yuntdagi fields that is closely related to the extensional tectonics of the izmir balikesir transfer zone and in the same time with the episodic core complex denudation of the menderes massif this study documents two composite volcanoes yamanlar and yuntdagi whose present vent area is strongly eroded and cut by a variety of strike slip and normal fault systems the transcurrent nw se being the dominant one the erosional remnants of the vent areas resembling a shallow crater intrusive complex illustrate the presence of numerous dykes or variably sized neck like intrusions and lava flows typically associated with hydrothermal alteration processes propylitic and argillic such vent areas were observed in both the examined volcanic fields having 6 km in diameter and being much more eroded toward the south along the nw se fault system lava flows and lava domes are sometimes associated with proximal block and ash flow deposits in the cone building association part besides lava flows and remnants of lava domes rare block and ash and pumice rich pyroclastic flow deposits as well as a series of debris flow deposits have been observed the rocks display a porphyritic texture and contain various proportions of plagioclase clinopyroxene orthopyroxene amphibole rare biotite and corroded quartz the examined rocks fall at the limit between calc alkaline to alkaline field and plot predominantly in high k andesite and dacite fields and one is rhyolite the trace element distribution suggests fractional crystallization processes and mixing in upper crustal magma chambers and suggests a metasomatized lithospheric mantle lower crust source this preliminary volcanological petrological and geochronological base study allowed documenting the yamanlar and yuntdagi as composite volcanoes generated during post collisional early middle miocene transtensional tectonic movements

relationship between the flank sliding of the south east crater mt etna italy and the paroxysmal event of november 16 2006 over the last 2 years the south east crater sec at mt etna italy has been characterised by the intermittent emission of lava flows often accompanied by paroxysmal episodes with violent strombolian activity and or lava fountains one of the most intense and intriguing episodes occurred on november 16 2006 during strombolian and effusive activity from the very top of the sec at the eastern base of the crater a violent and short lasting outburst generated a 300 m high eruptive curtain of finely fragmented magma and steam which collapsed in a few seconds giving rise to a small but significant pyroclastic flow the paroxysm was preceded by progressive sliding of the eastern flank of the sec edifice which led to the formation of a large niche previously published models explain the outburst as due to rapid vaporisation of water contained in sediments on contact with hot lava flowing along the flank of the sec or alternatively to gravitational collapse of a destabilised portion of the eastern flank of the crater however several lines of evidence suggest that these models do not adequately explain the paroxysm which we propose was associated with the explosive emission of magma during the rapid opening of an ese wnw oriented fracture at the eastern base of the sec moreover geochemical data of the products associated with this episode show that they are the most primitive and gas rich among those erupted during the entire 2006 period we suggest that the paroxysm of november 16 2006 which occurred at base of the sec was a magma outburst due to rapid ground fracturing related to a drop in confining pressure upon the underlying dyke consequent to the sliding of the eastern flank of the sec cone specifically the unloaded shallow portion of the feeding dyke did not restrain the arrival of a new batch of gas rich magma which worked its way out by fracturing the base of the niche at its weakest point this paroxysm appears significant because it reveals the potential development of syn eruptive dynamics acting directly on the uppermost portion of the plumbing system which is able to generate pyroclastic flows even on basaltic volcanoes such as mount etna

mexican site for k t impact crater research throughout the caribbean suggests that the geophysical anomalies in the yucatan first noted by penfield and camargo 1981 and called the chicxulub crater could be the site of the impact purported to have caused the k t extinctions a semicircular ring of sink holes known locally as cenotes which correlates with the geophysical anomalies has been identified and it is argued that the origin of the cenote ring is related to postimpact subsidence of the chicxulub crater rim if there is indeed a crater the region within the cenote ring corresponds to its floor and the crater rim diameter is probably larger than 200 km if confirmed as a site of impact the chicxulub crater would be the largest terrestrial impact crater known which is consistent with the uniqueness of the k t global catastrophe

rescaled earthquake recurrence time statistics application to microrepeaters slip on major faults primarily occurs during characteristic earthquakes the recurrence statistics of characteristic earthquakes play an important role in seismic hazard assessment a major problem in determining applicable statistics is the short sequences of characteristic earthquakes that are available worldwide in this paper we introduce a rescaling technique in which sequences can be superimposed to establish larger numbers of data points we consider the weibull and log normal distributions in both cases we rescale the data using means and standard deviations we test our approach utilizing sequences of microrepeaters microearthquakes which recur in the same location on a fault it seems plausible to regard these earthquakes as a miniature version of the classic characteristic earthquakes microrepeaters are much more frequent than major earthquakes leading to longer sequences for analysis in this paper we present results for the analysis of recurrence times for several microrepeater sequences from parkfield ca as well as ne japan we find that once the respective sequence can be considered to be of sufficient stationarity the statistics can be well fitted by either a weibull or a log normal distribution we clearly demonstrate this fact by our technique of rescaled combination we conclude that the recurrence statistics of the microrepeater sequences we consider are similar to the recurrence statistics of characteristic earthquakes on major faults

magma induced strain localization in centrifuge models of transfer zones abstract scaled centrifuge experiments have been used to investigate the dynamic relations between deformation and magma distribution in rift related transfer zones the physical models were built using suitable analogue materials such as sand to represent the brittle upper crust various kinds of silicone mixtures to simulate the lower crust and upper mantle and glycerol to reproduce magma models simulated the development of transfer zones across pre existing glycerol reservoirs placed at the base of the analogue continental crust in plan view different geometries dimensions and positions of subcrustal reservoirs were reproduced in three different sets of experiments to compare results models were also performed without magma simulating glycerol set 1 experiments incorporating a narrow rectangular glycerol reservoir show that the low viscosity material is able to localise deformation into the overlying crust giving rise to discrete transfer zones this concentrated surface deformation corresponds at depth to major magma accumulation set 2 experiments with an initial wide squared glycerol reservoir show instead that deformation is distributed across the whole model surface corresponding at depth to relatively minor magma accumulation set 3 experiments explored various positions of a small squared reservoir that invariably localised faulting in the overlying analogue brittle crust at the onset of model deformation the overall model behaviour suggests that magma distribution at depth can effectively control the strain distribution in the overlying crust and the deformative pattern of transfer zones strain distribution in turn may control magma emplacement as localized deformation would favour major accumulation of magma at transfer zones coupled to a strong thermal weakening of the country rocks this process may ultimately lead to a positive feedback interaction between magma and deformation

deformation of the substratum of a large shield volcano triggering factor for past flank collapses in the old volcanic edifice of la gomera canary islands the basal complex of la gomera canary islands has been analyzed with the aim of deciphering its structural evolution spanning the middle to late miocene detailed structural maps and cross sections and the results of a systematic measurement of dike orientation and fault slip data are presented in this work the main structural features found in the basal complex of la gomera are four dike sets and two large normal faults guillama and montana de alcala faults several extensional episodes have been identified the main deformation phase corresponds to a nnw sse to nw se directed extension that generated first a dense swarm of mafic dikes and then a large scale collapse of the basal complex with displacement along the guillama and montana de alcala faults and associated rotation of large blocks of la gomera basement it is interpreted that the supposed staircase geometry of the guillama fault is responsible for the observed arrangement of rotated dikes due to development of kilometric scale fault bend folds with rounded hinges deformation of the basal complex shows a complete geometric kinematic and chronologic consistency with the large volcanic flank collapses that affected the units belonging to the early growth stages of la gomera subaerial shield volcano the results of this work support the models that invoke the importance of the large scale geodynamic setting on volcano destabilization more attention should be paid to the structural and geophysical characteristics of volcano basements in order to better evaluate the danger of large catastrophic volcanic landslide events

multiple sector collapses at stromboli volcano italy how they work this paper demonstrates that four large sector collapses have affected the nw flank of the stromboli volcano in the past 13 ka alternating with growth phases in order to contribute to the evaluation of the critical conditions which trigger lateral collapses a reconstruction of the geometry of each collapse of the volcano edifice in the four stages that preceded the relative collapse events is also presented and a computation of the landslide volume this reconstruction is based on new field data plotted in three dimensions prior to the initial 13 ka collapse the volcano was 1125 100 m high above sea level the collapse had a volume of 2 23 0 87 km3 whereas the pre collapse volcano volume was 218 8 7 7 km3 the next edifice that failed was 900 70 m high a s l the collapse volume was 1 0 54 km3 with a precollapse volcano volume of 201 4 5 4 km3 the edifice then grew to 1000 60 m a s l the third collapse had a volume of 1 08 0 39 km3 and occurred within a volcano with a volume of 209 1 4 6 km3 this was followed by a new growth phase followed by the last collapse with a volume of 0 73 0 22 km3 the volcano volume was about the same as the present one the present active crater zone is at 780 m a s l in the first three collapses sliding surfaces cut the main magma conduit in the last collapse the upper scarp coincided with the conduit location dyking along a main ne trending weakness zone across the volcano summit exerted a lateral force for collapse inception the decrease of the landslide volumes with the age and the concentric scarps of the four collapses suggest that the younger sliding planes tended to become more superficial and to decrease the areal extent this is interpreted as due to a successively weaker eruptive products from dominantly lavas to dominantly pyroclastics and b the feedback effects between collapses and dykes that injected along the lateral segments of the first collapse slide plane

identifying rift zones on volcanoes an example from la reunion island indian ocean we describe a methodology for identifying complex rift zones on recent or active volcanoes where structures hidden by recent deposits and logistical conditions might prevent carrying out detailed fieldwork la reunion island was chosen as a test site we used georeferenced topographic maps aerial photos and digital terrain models to perform a statistical analysis of several morphometric parameters of pyroclastic cones this provides a great deal of geometric information that can help in distinguishing the localisation and orientation of buried magma feeding fractures which constitute the surface expression of rift zones it also allowed the construction of a complete gis database of the pyroclastic cones la reunion is a perfect example where past and active volcanic rift zones are mostly expressed by clusters of monogenic centres the data has been validated in the field and compared and integrated with the distribution and geometry of dyke swarms results show the presence of several main and secondary rift segments of different ages locations and orientations whose origin is discussed considering regional tectonics local geomorphology and volcano deformation

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

tectonics of nisyros island greece by field and offshore data and analogue modelling abstract we use new on land and offshore structural data and scaled analogue models to analyse the fault tectonic pattern at nisyros island greece which has an active caldera that shows a complicated network of faults fractures and volcano tectonic structures we measured 157 faults that show dominant dip slip normal motions along planes mainly striking ne sw nne ssw and nnw sse inside the caldera dykes necks and morphometric parameters of volcanic domes explosion craters and fumarole pits indicate the control by ne striking discontinuities on magma and gas paths the nnw and ne striking faults bound a major block that underwent repeated downthrow and uplift movements during the late pleistocene holocene experiments with scaled models of caldera resurgence and two magma chambers indicate the formation of an hourglass shaped fault pattern as seen in plan view with an asymmetric increase in the fault offset and a widening of the fault divergence towards the volcano flank all these data suggest that regional fault tectonics and stress state strongly guided magma upwelling and the emplacement of volcanic centres whereas periodical bulging due to the overpressure of a second magma chamber located northwest of the caldera combined with faulting due to tectonic stresses can account for the overall deformation field

periodic pulsing of characteristic microearthquakes on the san andreas fault deep fault slip information from characteristically repeating microearthquakes reveals previously unrecognized patterns of extensive large amplitude long duration quasiperiodic repetition of aseismic events along much of a 175 kilometer segment of the central san andreas fault pulsing occurs both in conjunction with and independent of transient slip from larger earthquakes it extends to depths of 10 to 11 kilometers but may be deeper and it may be related to similar phenomena occurring in subduction zones over much of the study area pulse onset periods also show a higher probability of larger earthquakes which may provide useful information for earthquake forecasting

scaled experiments of volcanic spreading experiments were conducted to study the spreading of volcanic constructs volcanoes are simulated by a sand cone and the volcanic substratum is simulated by a sand layer brittle substratum overlying a silicone layer ductile substratum similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless numbers experiments determine values which predict whether or not spreading takes place of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano 2 and the brittle ductile ratio of the substratum 3 2 indicates that the volcano must be large enough to break the substratum before spreading occurs whereas 3 controls the style of deformation during spreading these dimensionless numbers change with time reaching values that tend toward those observed for stable configurations experimental values are compared with those from well constrained natural examples it is found that an essential requirement for volcanic spreading is the presence of a low viscosity layer within the substratum flow of the weak layer away from the excess load is responsible for the spreading the overlying edifice displays radial intersecting grabens due to concentric stretching dissected summit areas concentric zones of thrusts and folds form in the substratum around the edifice and diapirs of the ductile substratum rise within the fault zones

stability analysis of hawaiian island flanks using insight gained from strength testing of the hsdp core abstract hawaiian island flank failures are recognized as the largest landslide events on earth reaching volumes of several thousand cubic kilometers and lengths of over 200 km and occurring on an average of once every 100 000 years the 3 1 km deep hawaii scientific drilling project hsdp enabled an investigation of the rock mass strength variations on the island of hawaii schiffman p watters r j thompson n walton a w 2006 hyaloclastites and the slope stability of hawaiian volcanoes insights from the hawaiian scientific drilling project s 3 km drill core journal of volcanology and geothermal research 151 1 3 217 228 this study builds on that of schiffman et al schiffman p watters r j thompson n walton a w 2006 hyaloclastites and the slope stability of hawaiian volcanoes insights from the hawaiian scientific drilling project s 3 km drill core journal of volcanology and geothermal research 151 1 3 217 228 by considering more in depth rock mass classification and strength testing methods of the hsdp core geotechnical core logging techniques combined with laboratory strength testing methods show that rock strength differences exist within the edifice comparing the rock strength parameters obtained from the various volcano lithologies identified weak zones suggesting the possible location of future slip surfaces for large flank failures relatively weak rock layers were recognized within poorly consolidated hyaloclastite zones with increases in strength based on degree of alteration subaerial and submarine basalt flows are found to be significantly stronger with the aid of digital elevation models cross sections have been developed of key flank areas on the island of hawaii limit equilibrium slope stability analyses are performed on each cross section using various failure criteria for the rock mass strength calculations based on the stability analyses the majority of the slopes analyzed are considered stable in cases where instability i e failure is predicted decreased rock mass quality strength of the altered and highly poorly consolidated lithologies is found to have a significant influence these lithologies are present throughout the hawaiian islands representing potential failure surfaces for large flank collapses failure criterion input parameters are considered in sensitivity analyses as are the influences of certain external stability factors such as sea level variation and seismic loading

major changes in volcano behaviour after a sector collapse insights from stromboli italy in recent years increasing numbers of volcanoes are being recognized as having undergone large sector collapses how far such events can change the succeeding behaviour of the volcano is poorly known despite the importance for natural hazards and economic applications by analysing the last 100 kyr geological history of stromboli italy which has involved multiple collapses this paper illuminates the feedback effect between sector collapse and magma upwelling and emplacement the collapsed zone of stromboli is episodically unbuttressed draining magma preferentially with formation of eruptive centres in the collapse amphitheatre and dyking along the collapse shoulders after a collapse there were no further linear eruptions across the cone on opposing middle lower slopes but these occurred again after rebuilding of the cone the time averaged eruption rate increased immediately before the first collapse and remained higher during the succeeding period of frequent collapses

stability of a collapsing volcano stromboli italy limit equilibrium analysis and numerical modelling abstract stromboli is an active island volcano belonging to the aeolian archipelago italy in the last 13 ka it experienced four lateral collapses affecting its northwestern flank with its most recent volcanic crisis december 2002 associated with landslides and related tsunami events this paper presents the first stability analysis of the stromboli volcanic edifice the main input is the geotechnical model of the volcano defined on the basis of stratigraphical lithological material properties and structural data collected from in situ surveys and laboratory tests two dimensional stability analysis was performed by limit equilibrium methods lem and finite difference modelling flac 4 0 code mainly focusing on the subaerial part of stromboli s nw flank sciara del fuoco the variability of the safety factor was studied by deterministic sensitivity and probabilistic analysis focusing on the effect of external forces such as magma pressure and seismicity as potential triggering mechanisms of lateral collapse the lem analyses were developed considering a maximum depth of 150 350 m for the sliding surface to which correspond collapse volumes of 95 000 185 000 m 3 m respectively the study shows that without external forces the investigated rock mass is stable and that the tectonic seismicity of the area alone does not destabilize the studied slope on the contrary magma pressure in dykes can represent a destabilizing factor numerical modelling results are concordant with those from lem in addition the simulation reveals that deformations and superficial landslides preannounce and contribute to retrogressive plasticization and maybe to failure surface deepening shallow submarine landslides represent a possible triggering mechanism and the landslide events of 30 december 2002 consistently fit the simulated evolution flac has revealed to be a useful tool for modelling such a complex system it allowed to calibrate the response of the geotechnical model test the validity of the assumptions simulate the stress strain evolution and prepare a possible model for future scenarios

periodic slow slip triggers megathrust zone earthquakes in northeastern japan both aseismic and seismic slip accommodate relative motion across partially coupled plate boundary faults in northeastern japan aseismic slip occurs in the form of decelerating afterslip after large interplate earthquakes and as relatively steady slip on uncoupled areas of the subduction thrust here we report on a previously unrecognized quasi periodic slow slip behavior that is widespread in the megathrust zone the repeat intervals of the slow slip range from 1 to 6 years and often coincide with or precede clusters of large magnitude m 5 earthquakes including the 2011 m 9 tohoku oki earthquake these results suggest that inherently periodic slow slip events result in periodic stress perturbations and modulate the occurrence time of larger earthquakes the periodicity in the slow slip rate has the potential to help refine time dependent earthquake forecasts

morphology of pyroclastic cones and tectonics the relationships between morphology and spatial distribution of 1315 quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings tepic rift mexico ethiopian rift mexican volcanic belt canary archipelago and mount etna field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone the strike of the feeding plane is directly related to 1 the elongation of cone base and crater 2 the location of depressions on the crater rim and 3 the alignment of pyroclastic cones in relation to a given vent spacing in addition the strike and dip of faults affect the direction of cone breaching these relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9 and are especially sensitive to fault escarpment and cone height lava and cone density and fault orientation with respect to the dip of the volcanic substrate topography relations 1 and 2 become more pronounced for regions undergoing extensional tectonics where edifices also have a larger dimension whereas breaching in the direction of the fault dip is more widespread in regions under extension breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics

extensional tectonics and volcano lateral collapses insights from ollague volcano chile bolivia and analogue modelling understanding the process of volcanic lateral collapse which may be governed by numerous possible factors can be significantly improved by identifying the role of each factor here we test the perturbation induced in a volcano by tectonic normal faulting in the basement magma inflation and multiple flank deformation based on fieldwork and analogue experiments ollague volcano chile bolivia provides a good example of a sector collapse with exposed substrate extensional tectonics and our experiments have been done with conditions consistent with field data results show that none of the possible tectonic conditions led to complete lateral failure in agreement also with the field data sector collapse of the model cone occurred only when a shallow magmatic intrusion within the volcano was reproduced we also found out that volcano flank deformation is enhanced if a previous flank destabilization occurred interleaved by new cone growth terra nova 18 282 289 2006

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

the effect of volcanic constructs on rift fault patterns volcanoes in rift situations often show distinctive hour glass fault patterns with increased fault throw as the volcano is approached we present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass for faults to be reorientated there must be a ductile layer below the volcano this can be hot crust at mid ocean ridges continental rifts or weak sedimentary strata increased volcano size or mass and lower brittle ductile ratios lead to increased fault curvature whereas an increased regional extension rate decreases the effect volcanoes on one side of a rift may capture it forming the axis of a new rift with curved or en echelon zones on either side by concentrating extension magma is more easily erupted so volcanoes may erupt more magma of less evolved composition a positive feedback between increased extension and magma eruption rate will lead to rift narrowing which can favor the formation of oceanic crust

characteristic small earthquake sequence off sanriku northeastern honshu japan 1 we found eight similar earthquakes of m4 8 regularly occurring from 1957 to 1995 on the plate boundary off sanriku northeastern japan the mean recurrence interval of the events is 5 35 years and its standard deviation is only 0 53 years these events have the same focal mechanisms of low angle thrust fault type and their source sizes are estimated to be around 1 km the slip of each event is comparable to the cumulative amount of relative plate motion in the inter seismic period thus these events are probably caused by repeating slip at a small but strong asperity surrounded by creeping zone on the plate boundary the next event was expected to occur by the end of november 2001 with 99 probability and actually m4 7 event occurred on november 13 2001

faulting assisted lateral collapses and influence on shallow magma feeding system at ollague volcano central volcanic zone chile bolivia andes abstract the role played by the regional tectonics and relationships between volcanic structures and their shallow magmatic feeding system are of paramount importance in understanding multiple lateral collapse events and thus for hazard assessment in active volcanoes ollague 21 18 s 68 11 w andean central volcanic zone chile bolivia border is an active andesite dacite composite volcano developed on a regional nw striking extensional fault system that bisects the volcanic edifice geological and stratigraphic studies on the summit part of the cone together with new 40ar 39ar dating and petrographic and geochemical major trace elements analyses have allowed us to delineate the volcanic history of ollague and recognize multiple deformation and lateral collapse events the evolution of ollague appears to be characterized by four stages of volcano building separated by three main events of deformation of the cone these deformation events developed progressively by downthrow of the sw sector of the volcano 800 ka along the nw striking normal fault propagating from the substrate to a final catastrophic failure 300 ka of the sw sector and debris avalanche emplacement both nw trending regional tectonics and weakness of the edifice caused by the successive deformation and collapse events are responsible for both the geometric variations of the shallow magma feeding system and the coexistence of summit and parasitic dacitic vents central and lateral vents shifted in a sw trending direction i e perpendicular to the tectonic trend and along the axis of the collapse movement and developed along a nw trending alignment i e parallel to the main tectonic trend and collapse scarps

systematic variations in recurrence interval and moment of repeating aftershocks 1 the recurrence intervals for 194 repeating clusters on the calaveras fault follow a power law decay relation with elapsed time after the 1984 m6 2 morgan hill california mainshock the decay rates of repeating aftershocks in the immediate vicinity of a high slip patch that failed during the mainshock systematically exceed those that are farther away the trend between relative moment and recurrence interval which is a measure of the fault healing rate varies systematically with depth and changes from negative to positive value as the distance between the repeating aftershock and the mainshock slip patch increases we speculate that high strain rate in the early postseismic period may cause transient embrittlement and strengthening of the deep repeating clusters in the areas adjacent to the mainshock slip patch resulting in large moments that decrease with time as the strain rate diminishes our observations suggest that systematic behavior of repeating aftershocks reflect variations in the fault zone rheology

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

styles of volcano induced deformation numerical models of substratum flexure spreading and extrusion the gravitational deformation of volcanoes is largely controlled by ductile layers of substrata using numerical finite element modelling we investigate the role of ductile layer thickness and viscosity on such deformation to characterise the deformation we introduce two dimensionless ratios a volcano radius ductile layer thickness and b viscosity of ductile substratum failure strength of volcano we find that the volcanic edifice spreads laterally when underlain by thin ductile layers a 1 while thicker ductile layers lead to inward flexure a 1 the deformation style is related to the switch from predominantly horizontal to vertical flow in the ductile layer with increasing thickness increasing a structures produced by lateral spreading include concentric thrust belts around the volcano base and radial normal faulting in the cone itself in contrast flexure on thick ductile substrata leads to concentric normal faults around the base and compression in the cone in addition we show that lower viscosities in the ductile layer low b lead to faster rates of movement and also affect the deformation style considering a thin ductile layer if viscosity is high compared to the failure strength of the volcano high b then deformation is coupled and spreading is produced however if the viscosity is low low b substratum is effectively decoupled from the volcano and extrudes from underneath it in this latter case evidence is likely to be found for basement compression but corresponding spreading features in the volcano will be absent as the cone is subject to a compressive stress regime similar to that produced by flexure at volcanoes where basement extrusion is operating high volcano stresses and outward substratum movement may combine to produce catastrophic sector collapse an analysis of deformation features at a volcano can provide information about the type of basement below it a useful tool for remote sensing and planetary geology also knowledge of substratum geology can be used to predict styles of deformation operating at volcanoes where features have not yet become well developed or are obscured

contemporary recent extension and compression in the central andes abstract although extension in the high andes vs compression in the lowlands has already been widely discussed in the literature for the first time we recognized both extensional and contractional structures that developed contemporaneously during late pliocene quaternary times in a wide area of the central andean chain about 90 000 km 2 where crustal earthquake data are missing this area comprises north eastern chile south western bolivia and north western argentina and extends from the puna plateau to the altiplano volcanic belt late pliocene quaternary folds with hinge lines trending nne ssw to n s are mostly located along the westernmost part of the volcanic belt and the eastern part of the western cordillera locally there are coeval reverse faults parallel to the folds which reach up to the surface particularly the miscanti ridge tolocha fault and la casualidad ridge may be the morphostructural expression of tens km long fault propagation folds which locally show topographic scarps hundreds of meters high north and east of the contractional structures we found evidence of late pliocene quaternary normal faults striking n s in the southern part of the study area and nw se in the northern part well developed grabens are present in the higher areas of the volcanic belt and in the transition zone with the puna plateau the surface rupture zones of normal fault swarms range 8 24 km in length with single fault strands up to 18 km long which are typical of tectonic structures the distribution in space and time of the studied contractional and extensional structures indicates that they originated in the same time period we thus address the challenging question regarding the possible origin of the stress sources by analysing possible causes such as volcanotectonics high topography orogeny collapse and gravitational spreading of the orogen in relation also with the role played by inherited structures we finally analyse the relations between the different structures and magma upwelling and the potential for seismic hazard

a gravitational spreading origin for the socompa debris avalanche abstract socompa volcano arguably provides the world s best exposed example of a sector collapse derived debris avalanche deposit new observations lead us to re interpret the origin of the sector collapse we show that it was triggered by failure of active thrust anticlines in sediments and ignimbrites underlying the volcano the thrust anticlines were a result of gravitational spreading of substrata under the volcano load about 80 of the resulting avalanche deposit is composed of substrata formerly residing under the volcano and in the anticlines the collapse scar can be traced up to 5 km from the edifice truncating two spreading related anticlines which collapsed in the event outcrops near the volcano preserve evidence of edifice material being carried along on top of mobilised substrata on the north side of the scar the avalanche motion was initially at right angles to the failure edge structural relations indicate that immediately prior to collapse the substrata disintegrated became effectively liquidised and were ejected from beneath the edifice catastrophic mobilisation of substrata probably resulted from breakdown of ignimbrite clasts and cement it may have occurred through progressive rock fracture by high shear strain during spreading material ejected from under socompa formed a layer on which volcanic edifice debris was transported this interpretation of events explains the puzzling observation that avalanche units with the lowest gravitational potential energy moved the furthest it can also account for avalanche motion normal to the collapse scar walls ignimbrites and other rock types probably capable of similar behaviour underlie many other volcanoes identification of spreading at other sites could therefore be a first step towards assessment of the potential for this style of catastrophic sector collapse

development of characteristic volcanic debris avalanche deposit structures new insight from distinct element simulations large scale catastrophic collapse of volcanic edifices is a relatively common phenomenon in the geologic record however the processes that occur during debris avalanche emplacement remain poorly understood and must generally be inferred from analysis of avalanche deposits in the field which are recognized to contain a suite of recurrent features such as conical surface hummocks and toreva block ridges this study uses the distinct element numerical modelling method to investigate debris avalanche emplacement processes because it allows brittle deformation of the failure mass to be examined after describing the material calibration and model setup necessary for modelling the geomechanical behaviour of a failed edifice flank a sequence of emplacement snapshots is described the simulated debris avalanche is seen to evolve from initial block sliding through extension with horst and graben structures defined by propagation and offset of large scale discontinuities to a final stage in which the avalanche flows down valley the numerical model is consistent with emplacement theory in the literature and allows for a more precise view of the processes at work during failure and the deposits created thereby for example toreva blocks are formed in the medial and proximal sections of the failure through top down propagation of normally offset discontinuities developed from upper surface tension a mechanism for surface hummock formation is also recognized involving the retention of surface blocks as these and other characteristic deposit features are formed from a flank with homogeneous material properties brittle deformation of the initially listric shaped failure mass and associated macroscopic structural evolution are recognized as important factors in failure evolution and subsequent deposit morphology

recurrence time and frequency slip statistics of slip events on the creeping section of the san andreas fault in central california summary an important question in seismic hazard assessments is the frequency size and recurrence interval statistics at a point on a fault does a point on a fault obey the same statistics as earthquakes in a region do this is a difficult question to answer because the number of repetitive earthquakes on a particular fault that have been observed is small in order to overcome this difficulty we consider slip events on the creeping section of the san andreas fault in central california sequences of up to 100 events are obtained from creepmeter records we compare the statistical distribution of recurrence times with the brownian passage time lognormal and weibull distributions and using goodness of fit tests find that the weibull is the preferred distribution we also consider the frequency amplitude distribution of slip events we find that the data clearly do not obey a gutenberg richter distribution instead there is a uniform distribution of event sizes for a large fraction of events

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

extension induced by volcanic loading in regional strike slip zones volcanoes and strike slip pull apart structures are commonly found together we use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike slip zones and the converse effect of regional strike slip faults on volcano structure theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension analogue models indicate that volcanoes in strike slip zones develop extensional pull apart structures a graben typically forms normal to the minimum regional stress axis 3 at a cone cones emplaced onto preexisting faults increase that fault9s tensional component if the substratum is weak enough to permit volcano spreading thrusts form normal to the maximum stress axis 1 a feedback situation can arise i e loading related extension allows more magma influx more eruption and thus increased loading this process can create large volcano tectonic depressions volcano structures may also be used as indicators of regional strike slip movements

fault healing inferred from time dependent variations in source properties of repeating earthquakes we analyze two sets of repeating earthquakes on the calaveras fault to estimate in situ rates of fault strengthening healing earthquake recurrence intervals tr range from 3 to 803 days variations in relative moment and duration are combined to study changes in stress drop rupture dimension rupture veloc ity and particle velocity as a function of t healing rates and source variations are compared with predictions of laboratory derived friction laws two interpretations of event duration z are used one in which is given by the ratio of slip to particle ve locity and one in which it scales as rupture dimension divided by rupture velocity our data indicate that faults strengthen during the interseismic period we infer that source dimension de creases with t r due to aseismic creep within the region surround ing the repeating events stress drop increases 1 3mpa per dec ade increase in twhich represents an increase of a factor of 2 3 relative to events with tbetween 10 and 100 days this rate of fault healing is consistent with extrapolations of laboratory meas urements of healing rates if fault strength is high on order of 60mpa and stress drop is roughly 10 of this value

experiments on vertical basement fault reactivation below volcanoes scaled modeling has been conducted to study the role played by the reactivation of vertical basement faults in the destabilization process of overlying volcanoes results show that basement fault reactivation induces the formation of faults within the volcanic edifice these faults delimite a central block which is extruded from between the two undeformed lateral parts of the edifice collapse of this central block can occur when heterogeneous cones are used in experiments revealing that mechanical interfaces are of paramount importance in triggering rockslide avalanches collapse structures may display horseshoe shapes in map view and are then surprisingly similar to avalanche scars resulting from failure created by magmatic intrusions in natural volcanoes large scale destabilisation of half the surface of the cone may also occur in a single event depending on the position of the reactivated basement fault below the edifice it is emphasized that the process under consideration may occur on a dormant volcano as well provided that a fault is reactivated below it

predictability of repeating earthquakes near parkfield california summary we analyse sequences of repeating microearthquakes that were identified by applying waveform coherency methods to data from the parkfield high resolution seismic network because by definition all events in a sequence have similar magnitudes and locations the temporal behaviour of these sequences is naturally isolated which coupled with the high occurrence rates of small events makes these data ideal for studying interevent time distributions to characterize the temporal predictability of these sequences we perform retrospective forecast experiments using hundreds of earthquakes we apply three variants of a simple algorithm that produces sequence specific time varying hazard functions and we find that the sequences are predictable we discuss limitations of these data and more generally challenges in identifying repeating events and we outline the potential implications of our results for understanding the occurrence of large earthquakes

fault slip rates at depth from recurrence intervals of repeating microearthquakes unique attributes in sequences of recurring similar microearthquakes at parkfield california provide a means for inferring slip rate at depth throughout the active fault surface from the time intervals between sequence events application of the method using an 11 year microseismicity record revealed systematic spatial and temporal changes in the slip rate that were synchronous with earthquake activity and other independent measures of fault zone slip if this phenomenon is found to be generally common behavior in active faults it forms the basis for a method to monitor the changing strain field throughout a seismogenic fault zone

age stratigraphy and deposition of near k t siliciclastic deposits in mexico relation to bolide impact examination of 10 k t boundary sections in northeastern and east central mexico and new data presented from 7 sections permit the following conclusions 1 the globally recognized k t boundary and mass extinction in planktic foraminifera is stratigraphically above and separated by a thin marl layer of maastrichtian age from the siliciclastic deposit that is commonly interpreted as a short term hours to days k t impact generated tsunami deposit a similar relationship between the k t boundary and siliciclastic or breccia deposits is observed at brazos river in texas beloc in haiti and poty quarry in brazil 2 stratigraphic control indicates that deposition of the siliciclastic member occurred sometime during the last 150 k y of the maastrichtian and ended at least several thousand years prior to the k t boundary 3 at least four discrete horizons of bioturbation have been observed within the siliciclastic deposit that indicate episodic colonization by invertebrates over an extended time period 4 the glass and spherule rich unit which has been linked to the haiti spherule layer and the chicxulub structure is at the base of the siliciclastic deposit and thus significantly predates the k t boundary event the stratigraphic separation of the k t boundary and siliciclastic deposits and the evidence of long term deposition between them suggests the presence of two events 1 a globally recognized k t boundary impact event marked by ir anomaly and the mass extinction and 2 a caribbean event impact or volcanic and probably linked to the chicxulub structure that predates the k t boundary and is marked by glass and siliciclastic or breccia deposits

gradual mass extinction species survivorship and long term environmental changes across the cretaceous tertiary boundary in high latitudes stable isotope and planktic foraminiferal analyses across the cretaceous tertiary k t boundary transition at nye klov indicate longterm oceanic instability associated with global sea level fluctuations a gradual mass extinction and decreased magnitude of the 13 c shift in high latitudes oceanic instability which began at least 100 kyr before the k t boundary and continued for about 300 kyr into the tertiary was accompanied by a gradual faunal turnover the maximum sea level lowstand during latest maastrichtian is recorded about 75 cm below the k t boundary a sea level rise first in evidence at 20 cm to 40 cm below the boundary continued into zone p0 boundary clay this sea level rise was accompanied by 2 c of cooling in surface and bottom waters prior to the k t boundary 13 c values remained relatively stable up to 10 cm below the k t boundary a negative shift of 0 5 to 1 0 per mil occurred in the boundary clay in both planktic and benthic foraminifera the surface to deep 13 c gradient remained nearly unchanged in contrast to low latitudes where this gradient is virtually eliminated no sudden mass extinction occurred in this cosmopolitan high latitude fauna and nearly all cretaceous taxa thrived well into the tertiary when they gradually disappeared shallow seas dominated by cretaceous survivor taxa and a well developed oxygen minimum zone prevailed during the earliest tertiary short term sea level lowstands are marked by hiatuses at the top of zones p0 and p1a about 50 kyr and 230 kyr after the k t boundary respectively rising sea level reestablished normal marine conditions about 300 kyr to 350 kyr after the k t boundary coincident with the first post k t boundary recovery of the tertiary fauna and extinction of cretaceous survivors during this time high latitude regions temporarily acted as centers of origin and dispersal for planktic foraminifera long term oceanic instability gradual faunal turnover absence of a sudden mass extinction and greatly diminished 13 c shift in high latitudes suggest that a k t boundary bolide impact was not the primary cause for the k t boundary faunal transition moreover these data strongly imply that the destructive effects of the bolide impact would have been greatest in low latitudes and negligible in high latitudes

geochemistry of impact glasses from the k t boundary in haiti relation to smectites and a new type of glass we have individually analyzed twelve black and seven yellow glasses from the k t boundary section at beloc haiti plus three smectite mantles around black glasses the main chemical differences between black and yellow glasses are higher s ca mg zn as br sb and au but lower k na and si abundances in the yellow glasses we have found high cao yellow glasses with low s contents 0 08 0 17 wt which may be explained by fusion of carbonate rich sediments microprobe studies of individual glasses show that the black glasses are very homogeneous in their composition at the 10 100 m level while yellow glasses show much more variability and different intra sample elemental correlations one of the black glasses has higher sio2 and k2o abundances but lower concentrations of all other major elements this suggests the existence of a third glass type which we have named the high si k variety hsi k glass the glass shows areas consisting of pure sio2 maybe lechatelierite and some schlieren with lower sio2 content the chemical composition indicates that the hsi k glass may have originated from melting of a sedimentary source rock such as shale or argillite severe changes in major and trace element composition occur during alteration of the glass as documented by analyses of adhering smectites the ree patterns in the glass are similar to average upper crust but the smectites exhibit much lower abundances and flat patterns not unlike the patterns observed in claystones at numerous k t boundary sites the suggestion that the chicxulub structure in yucatan may be the source crater for the k t impact glasses is not fully supported by the currently available data impact glasses and tektites are produced from the surface layers of their targets at chicxulub the target stratigraphy comprised mainly carbonates and evaporites although we cannot exclude chicxulub as a source the present data do not provide any firm chemical evidence for a connection we conclude however that the haiti glasses have formed by impact and have later undergone alteration by low temperature hydrothermal processes to produce clays the boundary claystones at k t boundary sites may very well be alteration products of these impact glasses

near k t age of clastic deposits from texas to brazil impact volcanism and or sea level lowstand near k t boundary clastic deposits from texas mexico haiti guatemala and brazil often described as impact generated tsunami deposits are stratigraphically below well defined k t boundary horizons and appear not to be causally related to the k t boundary event stratigraphic evidence indicates that their deposition began during the last 170 200 kyr of the maastrichtian coincident with a major eustatic sea level lowstand that lowered sea level by as much as 70 100 m clastic deposition ended a few tens of thousands of years before the k t boundary during a rapidly rising sea level the presence of glass in clastic deposits in haiti northeastern mexico and yucatan suggests that the sea level lowstand coincided with a time of major volcanism or pre k t boundary bolide impact

dynamics and physical parameters of the lastarria debris avalanche central andes abstract volcanic debris avalanches are extremely destructive phenomena with the potential to travel many kilometers from their source region either as rockslides or as mass flows given that they may even be triggered at inactive volcanoes their hazard is often underestimated understanding the dynamics of such mass movements is essential for evaluating and mitigating hazards a number of case studies have been carried out around the world but there is still a need for further studies of flow dominated avalanches which remain poorly constrained these studies would have high educational value providing striking examples to teach decision makers and at risk populations about the hazard in this study we investigate the 7500 cal year b p lastarria debris avalanche it is a 7 km long deposit with exceptional preservation of both the flow structures and the collapse scar detailed fieldwork morphometric mapping of over 600 surface features and numerical modelling was carried out to constrain the avalanche s trigger and flow parameters numerical models and field scaling relationships are in good agreement suggesting maximum velocities of 210 to 270 km h 1 negligible basal friction low cohesion 50 kpa and an intermediate friction coefficient structures are dominantly oriented parallel to transport direction suggesting minimal influence from a smooth paleotopography lastarria provides an example of a shallow flank failure initiated along stratigraphic planes that allowed low strength pyroclastic strata to disaggregate rapidly and then quickly accelerate to flow as a granular material at high velocity beyond the base of the volcano overall lastarria provides excellent constraints on granular avalanche initiation and flow which are valuable for hazard assessments and for the study of less well preserved flow deposits elsewhere the question of which precursory signs may warn of such a flank failure remains open and is important to address in future studies

the dimensions of the chicxulub impact crater and impact melt sheet the chicxulub impact crater which is the principal source of impact debris in cretaceous tertiary boundary sediments is currently buried by a 1 km thick sequence of carbonate sediments because the crater is not exposed for direct scrutiny its size has remained uncertain and in particular estimates ranging from 100 to 300 km have been made on the basis of gravity and magnetic field data in this study the physical properties of the k t boundary impact ejecta and chicxulub impact melt sheet have been used to test the geophysical estimates of the impact crater s size on the basis of the measured thickness of impact ejecta in haiti and north america the calculated volume of the impact melt sheet and the chemical and isotopic composition of the melt sheet the chicxulub impact crater is inferred to be 180 km in diameter and to contain a 3 to 7 km thick melt sheet and breccia sequence within a centrally located 100 km diameter region that corresponds to the limits of the transient cavity

gravity and magnetic field modeling and structure of the chicxulub crater mexico the chicxulub crater is an 180 km diameter peak ring crater based on drill hole logs and samples potential fields seismic reflection profiles and surface fracture patterns a structural cross section produced based on these constraints has the features expected for a large complex impact crater the bouguer gravity anomaly consists of a broad 90 km radius 30 mgal low with a central 20 km radius 20 mgal high and two 5 mgal concentric lows at 35 and 60 km radius the gravity anomaly is disrupted by large scale basement anomalies and possibly by large scale slumping and backwash erosion effects the magnetic field anomaly over the crater consists of three zones an outer zone from 45 to 90 km radius of low amplitude short wavelength anomalies with an irregular perimeter a middle zone from 20 to 45 km radius of high amplitude short wavelength anomalies slightly elongated nnw sse and an inner 20 km radius single large amplitude anomaly magnetic field modeling indicates that the melt pool averages 90 km in diameter and the melt volume in the crater is estimated at 20 000 km3 the melt pool size constrains the collapsed transient cavity diameter to 90 km gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but 40 km in diameter and underlies or protrudes into the melt pool the preliminary structural cross section indicates that the inferred peak ring is decoupled from the structural uplift the geometry and bouguer gravity signature of the crater indicate that no significant uplift of the moho or relaxation of the crater has occurred

the effect of internal gas pressurization on volcanic edifice stability evolution towards a critical state results from simple physical and numerical models investigating the effects of increased internal pore fluid pressures of a mohr coulomb volcanic edifice are presented physical experiments make use of a heap built from angular sand on top of a stiff substrate of variable angle with the provision for injection of internal fluid gas pressures into the base the resulting failure geometries arising from internal pressurization of the model appear similar to some natural examples of sector collapse two dimensional limit equilibrium models analysing 42 500 possible failure surfaces were run with internal pressures p0 in the range 5 35 mpa and show that the potential critical failure surface migrates to increasingly deeper levels with increasing internal pressure although internal pressurization alone is unlikely to reduce the factor of safety fs below unity the edifice is driven towards a state of criticality that will render in susceptible to any internal or external perturbations

tektite bearing deep water clastic unit at the cretaceous tertiary boundary in northeastern mexico the hypothesis of cretaceous tertiary k t boundary impact on yucatan mexico predicts that nearby sites should show evidence of proximal impact ejecta and disturbance by giant waves an outcrop along the arroyo el mimbral in northeastern mexico contains a layered clastic unit up to 3 m thick that interrupts a biostratigraphically complete pelagic marl sequence deposited at more than 400 m water depth the marls were found to be unsuitable for determining magnetostratigraphy but foraminiferal biostratigraphy places the clastic unit precisely at the k t boundary we interpret this clastic unit as the deposit of a megawave or tsunami produced by an extraterrestrial impact the elastic unit comprises three main subunits 1 the basal spherule bed contains glass in the form of tektites and microtektites glass spherules replaced by chlorite smectite and calcite and quartz grains showing probable shock features this bed is interpreted as a channelized deposit of proximal ejects 2 a set of lenticular massive graded laminated beds contains intraclasts and abundant plant debris and may be the result of megawave backwash that carried coarse debris from shallow parts of the continental margin into deeper water 3 at the top several thin ripple beds composed of fine sand are separated by clay drapes they are interpreted as deposits of oscillating currents perhaps a seiche an iridium anomaly 921 23 pg g is observed at the top of the ripple beds our observations at the mimbral locality support the hypothesis of a k t impact on nearby yucatan

time dependent recurrence of strong earthquake shaking near plate boundaries a lake sediment perspective abstract accurate probabilistic seismic hazard analysis requires a good knowledge of the recurrence parameters of the strongest earthquakes in a region due to the typical short temporal span of instrumental and historical data it is often unclear whether one should adopt a time dependent or time independent poissonian recurrence model the choice of which has large repercussions on the probability estimates for new strong events the rapidly growing discipline of lacustrine paleoseismology aims at producing long continuous records of strong seismic shaking which integrate the activity of all significant seismic sources in a region and allow a reliable determination of recurrence patterns the typical continuous sedimentation regime in lakes can lead to complete sensitive paleoseismic records and a reduced temporal uncertainty in recurrence intervals here i present a worldwide compilation of published long lacustrine paleoseismic records grouped per tectonic domain and statistically explore the variability of their recurrence intervals expressed by the coefficient of variation cov a cov 1 indicates a time independent process whereas cov 1 may be related to changes in the sensitivity of the lacustrine paleoseismograph or to real earthquake clustering due to e g stress transfer between neighboring faults the most useful lacustrine paleoseismic records can be retrieved in high seismicity settings where many paleoseismic events can be recorded in a relatively short time span i e

postseismic response of repeating aftershocks the recurrence intervals of repeating earthquakes on the san andreas fault in the loma prieta aftershock zone follow the characteristic 1 t decay of omori s law a model in which these earthquakes occur on isolated patches of the fault that fail in stick slip with creep around them can explain this observation in this model the recurrence interval is inversely proportional to the loading rate due to creep logarithmic velocity strengthening friction predicts 1 t decay in creep rate following the mainshock the time dependence of recurrence is inconsistent with a simple viscous constitutive relationship which predicts an exponential decay of loading rate thus our observations imply postseismic slip at seismogenic depth under a power law rheology the time dependence of postseismic deformation measured geodetically may be diagnostic of whether postseismic deformation is caused by creep or possible viscoelastic deformation at greater depths

volcanic hazards from bezymianny and bandai type eruptions major slope failures are a significant degradational process at volcanoes slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years the historic record provides evidence for at least six of these events in the past century several historic debris avalanches exceed 1 km3 in volume holocene avalanches an order of magnitude larger have traveled 50 100 km from the source volcano and affected areas of 500 1500 km2 historic eruptions associated with major slope failures include those with a magmatic component bezymianny type and those solely phreatic bandai type the associated gravitational failures remove major segments of the volcanoes creating massive horseshoe shaped depressions commonly of caldera size the paroxysmal phase of a bezymianny type eruption may include powerful lateral explosions and pumiceous pyroclastic flows it is often followed by construction of lava dome or pyroclastic cone in the new crater bandai type eruptions begin and end with the paroxysmal phase during which slope failure removes a portion of the edifice massive volcanic landslides can also occur without related explosive eruptions as at the unzen volcano in 1792 the main potential hazards from these events derive from lateral blasts the debris avalanche itself and avalanche induced tsunamis lateral blasts produced by sudden decompression of hydrothermal and or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s 1 the ratio of area covered to distance traveled for the mount st helens and bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume the potential for large scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice as at mount st helens and bezymianny debris avalanches can move faster than 100 ms 1 and travel tens of kilometers when not confined by valley walls avalanches can affect wide areas beyond the volcano s flanks tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions the probable travel distance l of avalanches can be estimated by considering the potential vertical drop h data from a catalog of around 200 debris avalanches indicates that the h l rations for avalanches with volumes of 0 1 1 km3 average 0 13 and range 0 09 0 18 for avalanches exceeding 1 km3 h l ratios average 0 09 and range 0 5 0 13 large scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts the nature and duration of precursory activity vary widely and the timing of slope faliure greatly affects the type of associated eruption bandai type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity

volcanic edifice stability during cryptodome intrusion limit equilibrium analyses were applied to the 1980 mount st helens and 1956 bezymianny failures in order to examine the influence on stability of structural deformation produced by cryptodome emplacement weakening structures associated with the cryptodome include outward dipping normal faults bounding a summit graben and a flat shear zone at the base of the bulged flank generated by lateral push of the magma together with the head of the magmatic body itself these structures serve directly to localize failure along a critical surface with low stability deep within the interior of the edifice this critical surface with the safety coefficient reduced by 25 30 is then very sensitive to stability condition variation in particular to the pore pressure ratio ru and seismicity coefficient n for ru 0 3 or n 0 2 the deep surface suffers catastrophic failure removing a large volume of the edifice flank in the case of mount st helens failure occurred within a material with angle of friction 40 cohesion in the range 105 106 pa and probably significant water pore pressure on 18 may 1980 detachment of slide block i occurred along a newly formed rupture surface passing through the crest of the bulge although sliding of block i may have been helped by the basal shear zone significant pore pressure and a triggering earthquake were required ru 0 3 and n 0 2 detachment of the second block was guided by the summit normal fault the front of the cryptodome and the basal shear zone this occurred along a deep critical surface which was on the verge of failure even before the 18 may 1980 earthquake the stability of equivalent surfaces at bezymianny volcano appears significantly higher thus although magma had already reached the surface weaker materials or higher pore pressure and or seismic conditions were probably required to reach the rupture threshold from our analysis we find that deep seated sector collapses formed by removing the edifice summit cannot generally result from a single slide cryptodome induced deformation does however provide a deep potential slip surface as previously thought it may assist deep seated sector collapse because it favors multiple retrogressive slides this leads to explosive depressurization of the magmatic and hydrothermal systems which undermines the edifice summit and produces secondary collapses and explosive blasts