shear velocity structure in the aegean area obtained by inversion of rayleigh waves summary the purpose of this work is to derive a 3 d tomographic image of the shear wave velocity structure of the crust uppermost mantle in the aegean area using the group velocities of rayleigh wave fundamental mode the database consists of 185 regional earthquakes recorded at broad band stations that were installed for a period of 6 month in the aegean area within the framework of a large scale experiment in a previous work karagianni et al 2002 an averaged group velocity has been determined using the method of frequency time analysis ftan for each epicentre station ray path and the data were used in order to determine the local group velocities for different periods over the area covered by the seismic ray paths taking into account additional resolution results obtained for the local group velocities a grid of 0 5 was adopted for the aegean area and a local dispersion curve was defined for each gridpoint more than 80 local dispersion curves were finally inverted using a non linear inversion approach deriving the corresponding 1 d shear velocity models the interpolation of these models resulted in a 3 d s wave tomographic image of the crust and uppermost mantle in the broader aegean area as expected as a result of the complex tectonic setting of the aegean area strong lateral variations of the s wave velocities of the crust and uppermost mantle of the studied area are found in the southern aegean sea as well as in a large part of the central aegean sea a thin crust of approximately 20 22 km is observed whereas the remaining aegean sea area exhibits a crustal thickness less than 28 30 km on the contrary a crustal thickness of 40 46 km is observed in western greece along the hellenides mountain range whereas in the eastern continental greece the crust has a typical thickness of approximately 30 34 km for shallow depths 10 km low s wave velocities are observed under the sedimentary basins of the north aegean sea the gulf of thermaikos axios basin and western greece at depths ranging from 10 to 20 km low s wave velocities are mainly found in western greece under peloponnesus as well as in rhodes this low velocity zone seems to extend along the hellenic arc and can be correlated to the hellenides mountain range and the alpine orogenesis in agreement with previous p wave tomographic results in the southern aegean sea very low s wave velocities 3 6 4 0 km s 1 are observed at depths of approximately 30 40 km just below the moho discontinuity while in the rest of the inner aegean sea and continental greece the uppermost mantle is characterized by velocities around 4 3 4 4 km s 1 this low velocity zone in the southern aegean sea can be associated with the high temperatures and the presence of significant percentage partial melt in the mantle wedge of the southern aegean subduction zone in agreement with previous studies

three dimensional crustal and upper mantle structure beneath chalkidiki northern greece abstract we invert teleseismic relative residuals recorded in the thessaloniki seismological network for the crustal and upper mantle structure beneath chalkidiki northern greece where extension is observed to have occurred since eocene time after conducting several tests to insure the reliability of the results a low velocity anomaly 5 8 is observed which is located beneath chalkidiki for the top two layers 0 35 km and 35 85 km this anomaly is probably related to the fact that the crust is thicker here than beneath the neighboring basins for the two other layers with depths between 85 and 145 km and between 145 and 205 km relatively low velocities 3 5 are observed beneath the thermaico gulf and the kavala basin these results are compared with lyberis s 1985 8 calculated shallowing of the isotherms due to extension and thinning of the lithosphere we conclude that the velocity anomalies that we observe are likely to be due to the intrusion of hot material from the asthenosphere into the gap in the extended lithosphere

tsunami generation by a rapid entrance of pyroclastic flow into the sea during the 1883 krakatau eruption indonesia 1 the 1883 eruption of krakatau volcano in indonesia was one of the most explosive volcanic events in history it was a marine caldera forming eruption that resulted in voluminous ignimbrite deposits and huge tsunamis we have used numerical simulations to investigate three major mechanisms for tsunami generation caldera collapse phreatomagmatic explosion and pyroclastic flow and have constrained the source parameters computed tsunami characteristics for each hypothesis are compared with observations at locations along the coasts of the sunda strait where tsunami data were obtained immediately after the eruption for the pyroclastic flow hypothesis two types of two layer shallow water models dense and light type models were used under different initial conditions pyroclastic flows are erupted from a circular source following a sine function that assumes waning and waxing phases caldera collapse was performed using a simple piston like plunger model in which collapse duration was assumed to be up to 1 h the phreatomagmatic explosion hypothesis was examined using simple empirical models for underwater explosions in shallow water with explosion energy between 1016 and 1017 j the results show that when a pyroclastic flow with a volume of 5 km3 and an average discharge rate of the order of 107 m3 s enters the sea the computed tsunami heights are broadly consistent with historical records in coastal areas including a tide gauge record at batavia now jakarta we conclude that a pyroclastic flow entering the sea is the most plausible mechanism of the 1883 krakatau tsunami

a geophysical interpretation of the 1883 krakatau eruption abstract from the present submarine topography in the vicinity of the krakatau islands it is concluded that the focus of the large explosions was situated to the northwest of the present rakata island the channel between krakatau and sebesi islands was completely blocked by banks of volcanic material immediately after the eruptions and it is suggested that this material was mainly lithic fragments the explosion sequences of krakatau are deduced from the records of sea waves and air waves observed at jakarta the large tsunami was caused by the most violent explosion simultaneously with the largest air waves it is inferred that the origin of the tsunami was a sudden upheaval of the seawater due to a violent explosion and that the height of the tsunami near its source was 30 40 m energy of the explosion is estimated from analyses of the air waves as one order of magnitude greater than that of the 1956 bezymianny eruption thus the 1883 krakatau eruption may have caused explosive removal of more than 10 km3 of material the subsurface structure of krakatau islands after the 1883 eruption is deduced from gravity anomalies it is concluded that at the bottom of the caldera there are deposits of low density in the shape of an inverted cone 8 km in diameter and 1 km in depth from the residual gravity anomaly observed over the caldera one can estimate the mass deficiency there this allows estimates to be made as to the amount of ejecta although large uncertainties remain these data indicate that explosive removal of material was the main process responsible for the disappearance of the northern half of the former rakata krakatau island in the 1883 eruption

objective regionalization of rayleigh wave dispersion data by clustering algorithms an application to the mediterranean basin abstract the main target of the present study is an objective and automated regionalization of rayleigh wave dispersion data for the mediterranean basin without a priori seismotectonic constraints and to determine the corresponding regional shear velocity structures the database used is formed by almost 200 rayleigh wavetrains corresponding to 42 regional events with surface wave magnitude greater than 4 5 recorded at the mednet very broad band stations in the mediterranean area path averaged group velocities for the rayleigh wave fundamental mode are derived for each available epicentre station trajectory crossing the mediterranean basin after this a principal component analysis and a clustering process are applied to local group velocities obtained for 13 different periods from 10 to 70 s in order to classify the mediterranean basin into several homogeneous regions the stochastic inversion of the averaged group velocity dispersion curve obtained for each region provides the respective shear velocity structures down to a depth of 150 160 km the characteristics of these areas and their possible correlation with the main seismotectonic features of the mediterranean region are discussed the regional models reveal significant lateral changes in the elastic structure with the main differences concerning particularly the upper 35 40 km within this depth range low shear velocities varying from 2 8 to 3 9 km s 1 characterize the eastern mediterranean whereas higher velocities ranging from 3 0 to 4 2 km s 1 are deduced for the western mediterranean these results suggest a thicker crust in the eastern part but with a greater thickness of sedimentary layers however for depths of between 80 and 110 km lower shear velocities are obtained in the western part while higher shear velocities are derived for the eastern mediterranean sea in the aegean sea greece the south of italy sicily and tunisia this velocity pattern suggests an averaged thicker lithosphere under the latter areas as the top of the asthenosphere is detected at a mean depth of 75 km for the remaining regions this thicker lithosphere can be related to processes associated with the convergence of the eurasian and african plates and subduction under the calabrian and hellenic arcs

generation mechanism of tsunamis from the 1883 krakatau eruption the 1883 eruption of krakatau generated a destructive tsunami which claimed about 34 000 lives we compute the generation and propagation of tsunamis from three models previously proposed they are 1 large scale caldera collapse of the northern part of krakatau island 2 emplacement of pyroclastic flow deposits 3 submarine explosion we numerically solve the non linear shallow water equations on actual bathymetry all three models displace the same volume 11 5 km 3 of water in different ways we compare the results of our computation with the observed tsunami waveforms at batavia and tsunami wave heights along the coasts of java and sumatra the comparison shows that the submarine explosion model of 1 to 5 min duration best explains the generation of the largest tsunami

a re analysis of the late bronze age eruption and tsunami of santorini greece and the implications for the volcano tsunami hazard the paroxysmal eruption of santorini ca 3500 bp referred to as the late bronze age lba eruption probably generated multiple tsunami their occurrence and impacts being cited frequently in scientific papers and articles this paper examines what is known about any lba tsunami noting possible mechanisms of generation and identifying sedimentological traces firstly the eruption sequence is outlined providing the context in which tsunami genesis may have occurred secondly the arguments forwarded for the tsunami and a summary of the evidence is given thirdly results of a new geological investigation for lba tsunami deposits at 41 coastal sites from crete and kos are presented the data are used to test the hypothesis that the lba eruption generated an east mediterranean wide tsunami it will be seen that no terrestrial geological evidence is identified the paper re examines the original arguments presented for lba tsunami challenging them because their founding assumptions are flawed together the new data and the re analysis of the original tsunami hypothesis indicate that there is insufficient evidence to demonstrate that any significant far field tsunami propagated throughout the entire east mediterranean as frequently purported some terrestrial evidence exists to suggest localised near field tsunami inundation there is good submarine evidence however to suggest that a tsunami was focused to the w and wsw the results have important implications for understanding the volcano related tsunami hazard within the region and elsewhere

p and s deep velocity structure of the hellenic area obtained by robust nonlinear inversion of travel times we derived a new model for the p and s velocity structure of the lithosphere in the subduction area of the hellenic arc and the aegean sea from the inversion of travel times of local events the inversion technique applied is nonlinear since three dimensional ray tracing is incorporated at the same time an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results the p s coherency is controlled by an additional damping of the vp vs ratio the study focuses mainly on the structure of the hellenic subduction in the southern aegean region interesting features and details of the subducted slab can be recognized in the final tomographic images on the western part the shallower section of the slab is dipping at a very small angle 10 this changes to approximately 25 for the deeper part of the subduction resulting in a prominent kink at a depth of about 70 km which is in accordance with the general characteristics of the associated benioff zone the smaller eastern part of the slab is not resolved with the same detail but a clearly steeper subduction zone can be recognized moreover detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments finally the results confirm the recent suggestion concerning the existence of a low velocity crustal layer at shallow depths 10 15 km in the accretionary prism of the alpine belt

numerical predictions of surface effects during the 2029 close approach of asteroid 99942 apophis asteroid 99942 apophis close approach in 2029 will be one of the most significant small body encounter events in the near future and offers a good opportunity for in situ exploration to determine the asteroid s surface properties and measure any tidal effects that might alter its regolith configuration resurfacing mechanics has become a new focus for asteroid researchers due to its important implications for interpreting surface observations including space weathering effects this paper provides a prediction for the tidal effects during the 2029 encounter with an emphasis on whether surface refreshing due to regolith movement will occur the potential shape modification of the object due to the tidal encounter is first confirmed to be negligibly small with systematic simulations thus only the external perturbations are taken into account for this work despite this seismic shaking induced by shifting blocks might still play a weak role and we will look into this mechanism in future work a two stage approach is developed to model the responses of asteroid surface particles the regolith based on the soft sphere implementation of the parallel n body gravity tree code pkdgrav a full body model of apophis is sent past the earth on the predicted trajectory to generate the data of all forces acting at a target point on the surface a sandpile constructed in the local frame is then used to approximate the regolith materials all the forces the sandpile feels during the encounter are imposed as external perturbations to mimic the regolith s behavior in the full scenario the local mechanical environment on the asteroid surface is represented in detail leading to an estimation of the change in global surface environment due to the encounter typical patterns of perturbation are presented that depend on the asteroid orientation and sense of rotation at perigee we find that catastrophic avalanches of regolith materials may not occur during the 2029 encounter due to the small level of tidal perturbation although slight landslides might still be triggered in positions where a sandpile s structure is weak simulations are performed at different locations on apophis surface and with different body and spin axis orientations the results show that the small scale avalanches are widely distributed and manifest independently of the asteroid orientation and the sandpile location we also include simulation results of much closer encounters of the apophis with earth than what is predicted to occur in 2029 showing that much more drastic resurfacing takes place in these cases

microseismic activity and seismotectonics of heraklion area central crete island greece abstract a seismological network of 10 portable analogue stations was installed in the area of heraklion central crete from september to december 1995 during this period more than 1000 events were recorded by at least 4 stations with magnitudes ranging from 0 5 to 4 6 and depths up to 70 km analysis of 336 well located events revealed high seismic activity in the onshore area seismicity is shallow

deep sea tsunami deposits in the eastern mediterranean new evidence and depositional models the tsunami wave induced by the collapse of the santorini caldera after the bronze age minoan eruption 3500 bp produced turbidites and large volume mega turbidites in the abyssal plains of the ionian sea as well as on the floor of small basins of the mediterranean and calabrian ridges characterized by the so called cobblestone topography since the first discovery in 1978 a holocene mud layer which has been termed homogenite and which typically shows a graded interval at its base has been identified and correlated in over 50 deep sea cores recovered in the eastern mediterranean four types of homogenite can be distinguished each related to a particular depositional setting 1 a closed cobblestone these are from a few decimetres to several metres thick pelagic turbidites of local provenance exclusively found at the bottom of small sized ponded basins of the mediterranean and calabrian ridges a debris flow may be present at the base of the turbidite where the vertical relief of the basins is over 200 m 2 b abyssal plain a 10 to 20 m thick megaturbidite recorded in the ionian and sirte abyssal plains the source area is the african continental margin possibly the continental shelf it can be easily identified as a transparent acoustic layer that shows recent deformation across the deformation front of the mediterranean ridge accretionary prism the volume of the minoan homogenite in the ionian abyssal plain has been calculated at a minimum of 11 km3 3 c open cobblestone this type is exclusively found on the outer slope of the mediterranean ridge near the deformation front unlike types a and b the base of the turbidite here is erosional instead of depositional this homogenite has been deposited in small basins and occasionally on topographic highs of the mediterranean ridge by the up slope flow of turbidity currents of african provenance that formed the abyssal plain deposit type b 4 d a fourth depositional model has been identified homogenites present in deep anoxic basins of the mediterranean ridge this setting is substantially similar to type a but the vertical relief of the basin is much higher up to 800 m and the deposition occurs in high density anoxic brines which modify the settling rate and hence the resulting sedimentological characters

a scenario of the 1883 krakatau tsunami abstract the 1883 krakatau tsunami provides an important clue for solving the mechanisms of the eruption of krakatau first sea waves of volcanic origin are basically classified the travel times of the crest of the largest tsunami in the 1883 krakatau eruption are recalculated taking the wave heights and the sea depths along the path into consideration as a special case of sea waves air waves coupled with those of the sea are examined and the necessary conditions of their generation are discussed the sea depths must be around 5 km the sea wave paths require long stretches of sea and the magnitudes of volcanic explosions must be large to produce the air sea waves the information collected suggests that the tsunamis registered at batavia jakarta were not air sea waves some results of the recent reflection survey of the subbottom materials and a supplemental gravity survey in the krakatau area are introduced to provide further evidence for explosive mechanisms the origin times of the onset and the crest of the tsunami and the air wave both registered at batavia are calculated from their travel times adopting these origin times the author reconstructs a scenario of the tsunami in order to generate a tsunami as long as about 60 km wave length successive developments of surges and water domes seem to be necessary

gravity fields and the interpretation of volcanic structures geological discrimination and temporal evolution abstract a review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes to a first approximation all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice positive anomalies with wavelengths less than 20 km and amplitudes up to ca 30 mgal characterise mainly basaltic volcanoes from various tectonic settings and are caused by a relatively dense intrusive complex magma body which contrasts with its surroundings either because the body is more mafic than average or more likely because near surface previously erupted materials are uncompacted negative anomalies with wavelengths up to 100 km and amplitudes up to ca 60 mgal occur over much larger volcanic calderas many of which have erupted highly silicic pyroclastic ash and pumice uncompacted silicic caldera infill with a possible contribution from low density magma bodies is responsible for the observed anomalies there is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction related basaltic andesite to mature high level silicic calderas repeated microgravity observations over active volcanoes enable magma movements variations in magma input and changes in magma density to be monitored closely dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events

incremental collapse and stratocone growth within a funnel shaped caldera guayabo costa rica abstract correlations between the surface geology and detailed borehole stratigraphy density logs particularly in the geothermally active area illustrate that the present morphology and structure of the guayabo caldera is the product of at least four subsidence events much of the sub caldera tuff lava sequence also exposed in the topographic caldera walls is the product of several ca 2 ma old overlapping stratocones formerly present in the area but later subjected to subsidence and edifice collapse the v shaped gravity anomaly across the caldera is due to a funnel shaped cross section developed as follows the earliest and innermost now concealed down faulted zone was filled by a low density pumice unit which extended southwest along an early arc normal graben at least two subsequent subsidence events produced the present scarps and outflow tuff breccias but the erupted volume is markedly lower than the subsidence volume the missing magma may have been diverted to the east where at the same time further stratocone buildup was occurring the gravity signature over this eastern caldera margin suggests the presence of a large dense sub volcanic intrusive complex further edifice collapse and graben formation due to a combination of gravitational failure and arc normal strain are relatively recent and continuing processes the evolution of the guayabo caldera therefore involved a series of subsidence events punctuated by stratocone buildup and collapse the resolution of concentric ring faulted block structures does not support a chaotic collapse model but instead indicates the incremental development by widening of a funnel shaped caldera

the origin of the 1883 krakatau tsunamis abstract hypotheses for the origin of the krakatau tsunamis of 27 august 1883 are reviewed the 151 minute sea wave travel time between krakatau and batavia jakarta first proposed by verbeek cannot be substantiated thus hypotheses for the origin of the tsunamis which rest on very accurate timings of the origins of the air and sea waves such as that of latter 1981 also cannot be substantiated there was one exceptionally large sea wave tsunami on 27 august and many smaller ones most of the smaller waves appear to have been air sea coupled waves rather than true tsunamis of the few sea waves that were probably true tsunamis only one may have originated by emplacement of a pyroclastic flow at a point nearer batavia than krakatau the catastrophic event in the eruption of august 27th was preceded some 40 minutes earlier by a subordinate event which also propagated an air sea coupled wave and a tsunami the largest heard explosion produced an air sea coupled wave but was not associated with a tsunami although the precise time relations between the origins of the air and sea waves cannot be definitely established the most likely scenario for the catastrophic phase of the eruption is broadly similar to that originally proposed by verbeek in which collapse of part of the original volcanic edifice propagated a major volcanic explosion

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

explosive volcanism vei 6 without caldera formation insight from huaynaputina volcano southern peru through examination of the vent region of volcan huaynaputina peru we address why some major explosive eruptions do not produce an equivalent caldera at the eruption site here in 1600 more than 11 km3 dre vei 6 were erupted in three stages without developing a volumetrically equivalent caldera fieldwork and analysis of aerial photographs reveal evidence for cryptic collapse in the form of two small subsidence structures the first is a small non coherent collapse that is superimposed on a cored out vent this structure is delimited by a partial ring of steep faults estimated at 0 85 by 0 95 km collapse was non coherent with an inwardly tilted terrace in the north and a southern sector broken up along a pre existing local fault displacement was variable along this fault but subsidence of approximately 70 m was found and caused the formation of restricted extensional gashes in the periphery the second subsidence structure developed at the margin of a dome the structure has a diameter of 0 56 km and crosscuts the non coherent collapse structure subsidence of the dome occurred along a series of up to seven concentric listric faults that together accommodate approximately 14 m of subsidence both subsidence structures total 0 043 km3 in volume and are much smaller than the 11 km3 of erupted magma crosscutting relationships show that subsidence occurred during stages ii and iii when 2 km3 was erupted and not during the main plinian eruption of stage i 8 8 km3

source mechanisms of volcanic tsunamis volcanic tsunamis are generated by a variety of mechanisms including volcano tectonic earthquakes slope instabilities pyroclastic flows underwater explosions shock waves and caldera collapse in this review we focus on the lessons that can be learnt from past events and address the influence of parameters such as volume flux of mass flows explosion energy or duration of caldera collapse on tsunami generation the diversity of waves in terms of amplitude period form dispersion etc poses difficulties for integration and harmonization of sources to be used for numerical models and probabilistic tsunami hazard maps in many cases monitoring and warning of volcanic tsunamis remain challenging further technical and scientific developments being necessary and must be coupled with policies of population preparedness

tomography of the crust and upper mantle in southeast europe compressional velocity structure of the crust and the upper mantle in southeastern europe broader aegean area is studied by inverting residuals of the first p arrivals from earthquakes in this region 16 e 31 e 34 n 43 n the data used are from regional events recorded by the permanent network of stations during the period 1971 1987 enriched with data from experiments with portable seismographs in four regions of this broad area this study confirms the strong variations of crustal thickness in this area as well as the subduction of the eastern mediterranean lithosphere under the southern aegean and gives further detailed information on the crustal and upper mantle structure of the area important new information is the existence of a low velocity crustal layer in western greece and albania and that the velocity anomaly in the mantle under the southern aegean extends much farther and deeper to the northeast than the benioff zone of the intermediate depth earthquakes indicates furthermore evidence is presented about the possible existence of older subduction in the northern aegean and about the influence of the tectonic regime on the velocity field

on the generation of oceanic surface waves by underwater volcanic explosions abstract the oceanic surface waves generated by submarine volcanism are modeled as the response of a compressible ocean to an underwater point explosion at an arbitrary depth over an elastic half space we generalize the work of kajiura and miyoshi by allowing the elastic sea floor to influence both the sound and surface gravity waves the classic work of press and ewing is extended to allow for the movement of the oceanic free surface from numerical inversion techniques we find that most of the energy is radiated away as surface gravity waves a mixture of direct and reflected p compressional and s type distortional head waves may produce high frequency large amplitude oscillations in the sea surface

seismic imaging of santorini subsurface constraints on caldera collapse and present day magma recharge abstract volcanic calderas are surface depressions formed by roof collapse following evacuation of magma from an underlying reservoir the mechanisms of caldera formation are debated and predict differences in the evolution of the caldera floor and distinct styles of magma recharge here we use a dense active source seismic tomography study to reveal the sub surface physical properties of the santorini caldera in order to understand caldera formation we find a 3 km wide cylindrical low velocity anomaly in the upper 3 km beneath the north central portion of the caldera that lies directly above the pressure source of the 2011 2012 inflation we interpret this anomaly as a low density volume caused by excess porosities of between 4 and 28 with pore spaces filled with hot seawater vents that were formed during the first three phases of the 3 6 ka late bronze age lba eruption are located close to the edge of the imaged structure the correlation between older volcanic vents and the low velocity anomaly suggests that this feature may be long lived we infer that collapse of a limited area of the caldera floor resulted in a high porosity low density cylindrical volume which formed by either chaotic collapse along reverse faults wholesale subsidence and infilling with tuffs and ignimbrites phreatomagmatic fracturing or a combination of these processes phase 4 eruptive vents are located along the margins of the topographic caldera and the velocity structure indicates that coherent down drop of the wider topographic caldera followed the more limited collapse in the northern caldera this progressive collapse sequence is consistent with models for multi stage formation of nested calderas along conjugate reverse and normal faults the upper crustal density differences inferred from the seismic velocity model predict differences in subsurface gravitational loading that correlate with the location of 2011 2012 edifice inflation this result supports the hypothesis that sub surface density anomalies may influence present day magma recharge events we postulate that past collapses and the resulting topographical and density variations at santorini influence magma focusing between eruptive cycles a feedback process that may be important in other volcanoes

shear wave attenuation tomography of the lithosphere asthenosphere system beneath the mediterranean region abstract 3 d tomographic images of the shear wave attenuation structure of the lithosphere asthenosphere system beneath the mediterranean region are derived from the inversion of anelastic attenuation coefficients obtained for the fundamental mode of nearly 3100 rayleigh wavetrains recorded at very broad stations placed in the mediterranean region the two station method is used to compute 124 interstation path averaged attenuation coefficients within the 15 120 s period range an adaptation to attenuation coefficients of the yanovskaya ditmar formulation specifically devoted to path averaged surface wave dispersion data and body waves permits to obtain the local attenuation coefficients and their uncertainties for all the period range at every node of a grid covering the studied region maps of stretching azimuths and mean radii of the resolution ellipses are also obtained for every period all of these parameters are strongly related to the spatial path coverage data quality and data coherency a local attenuation dispersion curve is defined then in each grid node and the stochastic inversion of these curves yields 1 d models of inverse shear wave intrinsic quality factor q 1 the shear wave velocity models obtained for every grid node in an elastic tomography developed in a previous work for the same domain are taken as reference models for the inversion process the kriging contouring algorithm is used to represent the resulting 3 d q 1 structure by means of horizontal and vertical cross sections down to 160 km depth the main patterns of this anelastic structure are mostly in agreement with the structural and tectonic features obtained in the reference shear velocity tomography the local attenuation maps manifest notable lateral changes for periods of 20 and 40 s whereas more moderate spatial variation is observed for longer periods the 3 d anelastic models show that at shallow levels the attenuation in the western mediterranean is usually slightly higher than in the central eastern part the area from corsica sardinia islands to italy presents high q 1 values of 40 0 70 0 10 3 down to 75 km depth whereas q 1 does not exceed 20 0 10 3 in the rest of the mediterranean basin except for the south eastern part where slightly higher values are obtained 20 0 30 0 10 3 those high q 1 values suggest the presence of asthenospheric material at shallow levels which would be related to distensive tectonic processes responsible for the recent opening of extensional basins as the tyrrhenian sea and the ligurian provencal basin in the eastern mediterranean the vertical cross sections at different latitudes and longitudes reveal at deep levels some structural patterns consistent with signs of subduction which are linked to the convergence between the african and eurasian plates

pyroclastic flows and surges over water an example from the 1883 krakatau eruption pyroclastic deposits from the 1883 eruption of krakatau are described from areas northeast of the volcano on the islands of sebesi sebuku and lagoendi and the southeast coast of sumatra massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the krakatau islands the distal deposits exhibit a decrease in sorting coefficient median grain size and thickness with increasing distance from krakatau crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards the deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 august at approximately 10 a m energetic flows spread out away from the volcano at speeds in excess of 100 km h and traveled up to 80 km from source the flows retained temperatures high enough to burn victims on the sw coast of sumatra historical accounts from ships in the sunda straits constrain the area affected by the flows to a minimum of 4x103 km2 at the distal edge of this area the flows were relatively dilute and turbulent yet carried enough material to deposit several tens of centimeters of tephra the great mobility of the krakatau flows from the 10 a m activity may be the result of enhanced runout over the sea it is proposed that the generation of steam at the flow seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective the buoyant distal edge of these ash and steam laden clouds lifted off into the atmosphere leading to cooling condensation and mud rain

lithospheric structure of the aegean obtained from p and s receiver functions combined p and s receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the aegean region are used to map the geometry of the subducted african and the overriding aegean plates we image the moho of the subducting african plate at depths ranging from 40 km beneath southern crete and the western peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern greece however the dip of the moho of the subducting african plate is shallower beneath the peloponnesus than beneath crete and rhodes and flattens out beneath the northern aegean observed p to s conversions at stations located in the forearc indicate a reversed velocity contrast at the moho boundary of the aegean plate whereas this boundary is observed as a normal velocity contrast by the s to p conversions our modeling suggests that the presence of a large amount of serpentinite more than 30 in the forearc mantle wedge which generally occurs in the subduction zones may be the reason for the reverse sign of the p to s conversion coefficient moho depths for the aegean plate show that the southern part of the aegean crustal thickness of 20 22 km has been strongly influenced by extension while the northern aegean sea which at present undergoes the highest crustal deformation shows a relatively thicker crust 25 28 km this may imply a recent initiation of the present kinematics in the aegean western greece crustal thickness of 32 40 km is unaffected by the recent extension but underwent crustal thickening during the hellenides mountains building event the depths of the aegean moho beneath the margin of the peloponnesus and crete 25 28 and 25 33 km respectively show that these areas are also likely to be affected by the aegean extension even though the cyclades crustal thickness of 26 30 km were not significantly involved in this episode the aegean lithosphere asthenosphere boundary lab mapped with s receiver functions is about 150 km deep beneath mainland greece whereas the lab of the subducted african plate dips from 100 km beneath crete and the southern aegean sea to about 225 km under the volcanic arc this implies a thickness of 60 65 km for the subducted african lithosphere suggesting that the aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the cyclades

the 1883 eruption of krakatau the 1883 eruption of krakatau was a modest ignimbrite forming event the deposits are primarily coarse grained dacitic non welded ignimbrite large explosions produced pyroclastic flows that entered the sea generating destructive tsunami grain size studies of the ignimbrite suggest that these explosions were not driven by magma seawater interaction the total bulk volume of pyroclastic deposits including co ignimbrite ash is estimated to be 18 21 km3

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

coupling eruption and tsunami records the krakatau 1883 case study indonesia the well documented 1883 eruption of krakatau volcano indonesia offers an opportunity to couple the eruption s history with the tsunami record the aim of this paper is not to re analyse the scenario for the 1883 eruption but to demonstrate that the study of tsunami deposits provides information for reconstructing past eruptions indeed though the characteristics of volcanogenic tsunami deposits are similar to those of other tsunami deposits they may include juvenile material e g fresh pumice or be interbedded with distal pyroclastic deposits ash fall surges due to their simultaneity with the eruption five kinds of sedimentary and volcanic facies related to the 1883 events were identified along the coasts of java and sumatra 1 bioclastic tsunami sands and 2 pumiceous tsunami sands deposited respectively before and during the plinian phase 26 27 august 3 rounded pumice lapilli reworked by tsunami 4 pumiceous ash fall deposits and 5 pyroclastic surge deposits only in sumatra the stratigraphic record on the coasts of java and sumatra which agrees particularly well with observations of the 1883 events is tentatively linked to the proximal stratigraphy of the eruption

s velocity structure and radial anisotropy in the aegean region from surface wave dispersion summary the lithospheric structure of the aegean region is investigated by analysis of rayleigh wave fundamental mode dispersion measurements isotropic 1 d models for almost 100 two station ray paths across the region display distinct variations in the moho depth and crustal s wave velocities the descending slab of the subducting african plate can be resolved down to 120 km depth beneath the volcanic arc three different regions are distinguished in terms of moho depth 1 the forearc with large crustal thicknesses between 38 and 48 km and an average of 43 km 2 the northern aegean with an average moho depth of 28 km and 3 the southern aegean central volcanic arc i e cyclades and sea of crete with an even thinner crust of around 25 km lateral variations in structure between 25 and 55 km depth indicate a marked difference between the western and eastern forearc collocated with pronounced changes in trench and slab geometry as well as published deformation rates s velocities between 25 and 55 km depth are low everywhere beneath the forearc but increase from the peleponnesus to crete an abrupt change occurs between western and central crete in terms of the visibility of the aegean moho and the seismic structure of the lithospheric mantle wedge an aegean mantle wedge with s velocities above 4 4 km s 1 is only observed to the east of central crete whereas to the west velocities of less than 4 0 km s 1 occur down to the plate contact these low velocities above the slab may indicate the presence of a melange of metamorphic rocks at the depths a low velocity asthenospheric layer is observed beneath the sea of crete and the cyclades below 40 km depth between the thinned lithosphere above and the slab below the observed radial anisotropy in the northern part of the aegean is likely to be due to preferred orientation of anisotropic minerals within the lower crust possibly caused by lateral ductile flow associated with recent lithospheric extension

detection of magma bodies beneath krakatau volcano indonesia from anomalous shear waves abstract the seismograms of 14 local earthquakes recorded by a temporary network in the sunda strait were used to construct an image of shear wave attenuating bodies in the vicinity of the krakatau volcanic complex seismograms for all ray paths going beneath the krakatau complex show a diminution of the amplitude of s waves these diminutions cannot be attributed to source mechanisms or station effects combination of all ray paths although in limited number demonstrates that the attenuating body is located in two zones which are probably disconnected although the limited set of data available does not allow us to draw a definite conclusion the upper zone is about 9 km deep and the lower one is at least 22 km deep the upper zone contains probably numerous irregular pockets of magma the deep zone appears to be a very wide body related to the extensional nature of the sunda strait the existence of a large deep reservoir should be taken into account for the mitigation of future hazards

seismic wave attenuation in the upper mantle beneath the aegean region a detailed study of p and s waves from earthquakes located along the hellenic arc and recorded by the greek seismological network shows an abnormal distribution of the seismic waves in the central aegean sea the data indicate a zone of anomalously high seismic wave attenuation in the upper mantle beneath the cyclades plateau and the inner part of the volcanic arc several independent observations suggest the presence of magma beneath the cyclades massif

application of the gauss theorem to the study of silicic calderas the calderas of la primavera los azufres and los humeros mexico abstract we explored applications including limitations of gauss s theorem to the study of silicic calderas first it enables us to determine the mass deficiency from calderas mass deficiency itself has also other potential applications it enables to make qualitative comparisons between calderas we can use the mass deficiency to test in a quick way and as a preliminary step to a formal gravity inversion for the feasibility of caldera types of simple geometry i e piston subsidence and funnel models this application can be done in a straightforward way once the mass deficiency has been determined for this purpose the mass deficiency is converted to the volume of material missing at the caldera subsequently for example this volume and the respective caldera diameter enable us to estimate the height of the cylinder fitting the piston subsidence model if the obtained parameters are congruent with the known geology and geophysical information then the model may be considered further in the inversion of the gravity data for the detailed structure other simple models i e the funnel model can also be analyzed in this way in particular when working with a piston subsidence caldera type the gauss theorem enables us to estimate the caldera collapse very difficult to obtain based on geologic information alone these possible uses of gauss s theorem are illustrated with the calderas of la primavera los azufres and los humeros caldera mexico the obtained mass deficiency from these calderas follow the linear mass deficiency diameter trend observed for other calderas in particular because of their diameters and mass deficiencies la primavera and krakatau calderas can be considered equiparable this comparison is of the most importance considering that la primavera is located in the neighbourhood of a metropolis guadalajara city since geophysical studies have already established a piston subsidence model for these calderas we assessed gauss s theorem used to estimate their collapses the respective estimated subsidences for la primavera 600 m los humeros 500 m and los azufres calderas 800 and 1200 m respectively are supported by borehole data these relatively small subsidences in relation to many calderas in western us can be explained by the presence of cylindrical to sub spherical magma chambers of smaller size than those underlying the western us calderas of yellowstone valles and long valley for example in an epicontinental setting of young quaternary rocks against precambrian cratonic basement in western us a reassessment of all the available geological and geophysical information was also done in particular the caldera limits were checked and corrected when necessary based on available 2 d gravity modeling gravity gradients and the location of the volcanic vents regional tectonic lineaments might explain many of the particular details of their structures la primavera caldera might have formed through a piecemeal subsidence controlled by the pre caldera tectonics los humeros caldera is an example of a caldera that evolved by incremental growth as the vulsiniani calderas in italy the los azufres caldera comprises two contiguous calderas emplaced in a regional tectonic depression

tsunamis of volcanic origin summary of causes with particular reference to krakatoa 1883 known tsunamis of volcanic origin are reviewed and classified according to their causes earthquakes accompanying eruptions excluding tectonic events which apparently triggered eruptions pyroclastic flows and submarine explosions have each accounted for about 20 of cases ten causes of volcanic tsunamis are discussed from the risk point of view those due to landslides are particularly dangerous eruptions at calderas are more likely to generate tsunamis than eruptions elsewhere of those killed directly by volcanic eruptions nearly a quarter have died as a result of tsunamis by transfer of energy to sea waves a violent eruption which would be comparatively harmless on land extends greatly the radius over which destruction occurs

from structure to erosion controlled subsiding calderas evidence thresholds and mechanics collapse calderas evolve by increasing their depth diameter ratio to properly characterize caldera evolution a structural s d ratio between structural subsidence and ring fault diameter ss ds and a topographic s d ratio between topographic caldera depth and topographic caldera width st dt are considered we review the evolution of the a d 2000 miyakejima caldera with two concentric ring faults at earlier collapsing stages and erosion of its wall accumulating debris on the floor at later collapsing stages while st dt and ss ds show a similar increase at initial stages when ss ds 0 33 the ss ds becomes significantly different from st dt while continuous caldera subsidence monotonically increases ss ds the erosion of the wall and the filling of the floor decrease st dt this evolution finds close similarities with recent caldera collapses of krakatau 1883 katmai 1912 fernandina 1968 tolbachik 1975 1976 pinatubo 1991 and dolomieu 2007 analog experiments mimic the observed variation evolving from a depression controlled by the activity of the double ring faults to that controlled by the erosion of the wall and sedimentation at the floor these natural and modeling results show that the control on the shape of mature calderas ss ds 0 07 and approaching ss ds 0 3 0 4 passes from a mainly structural to a mainly erosional control both st dt and ss ds are needed to describe the evolution of a collapse and the processes accompanying it evaluating st dt and ss ds allows proper description of the precise evolutionary stage of a caldera and of the relative importance of the structural and erosional processes and allows making semiquantitative comparisons between evolutionary stages

3 d velocity structure beneath the crust and upper mantle of aegean sea region the region of the aegean sea and the surrounding areas in the eastern mediterranean lies on the boundary zone between the eurasian and the african plates it is a zone of widespread extensive deformation and therefore reveals a high level of seismicity

evidence of incremental growth in the vulsinian calderas central italy abstract a comparative work on the vulsini calderas has been carried out fundamentally from volcanological field study drilling and gravity anomalies data on the basis of a detailed stratigraphy four volcanic complexes have been recognized in the vulsini volcanic district from the oldest to the youngest paleo bolsena bolsena montefiascone and latera the latter two complexes having a contemporaneous evolution in their activity each complex gave rise to several little calderas caused by piecemeal collapses due to moderate size explosive eruptions or coherent subsidence of a superficial piston like block nevertheless the extensional volcano tectonic regime affecting this area could have accounted for as much of the total subsidence as the caldera forming eruptions bolsena and latera calderas respectively 16 km and 8 km in average diameter represent the main depressions of the vulsini district and their topographic features are still well preserved several collapses forming the main poligenetic calderas of bolsena and latera are masked at present by the bolsena lake the younger volcanic covering the lacustrine deposits and erosion in a minor amount only the little montefiascone and vepe calderas which represent the youngest collapses of the vulsini area have not been masked bolsena caldera could be considered a depression caused mostly by a subsidence hinged to the southwest by down sagging and by collapses which occurred after the most important explosive eruptions of the paleo bolsena bolsena and montefiascone volcanic complexes meanwhile latera caldera is mostly due to a piecemeal collapse linked to several moderate size ignimbrite units which were emplaced throughout the explosive activity of the latera complex therefore the vulsinian calderas did not form by catastrophic eruptions because several and frequent moderate size eruptions downsagging and tectonic hinged subsidence concur indicating an incremental growth

revised estimates for the volume of the late bronze age minoan eruption santorini greece the late bronze age minoan eruption of santorini greece occurred from within an existing caldera low temperature pyroclastic flow emplacement on shallow slopes outside the caldera can only be consistent with the caldera being filled with eruption products that are not preserved field observations and seismic reflection surveys suggest that this missing material has been downfaulted the volume of the caldera infill is estimated as 18 26 km 3 dense rock equivalent dre increasing the total of the minoan eruption deposits to 78 86 km 3 dre and making it the largest known holocene eruption this study highlights a general mechanism whereby caldera eruption volumes could be significantly underestimated

tsunamis generated by eruptions from mount st augustine volcano alaska during an eruption of the alaskan volcano mount st augustine in the spring of 1986 there was concern about the possibility that a tsunami might be generated by the collapse of a portion of the volcano into the shallow water of cook inlet a similar edifice collapse of the volcano and ensuing sea wave occurred during an eruption in 1883 other sea waves resulting in great loss of life and property have been generated by the eruption of coastal volcanos around the world although mount st augustine remained intact during this eruptive cycle a possible recurrence of the 1883 events spurred a numerical simulation of the 1883 sea wave this simulation which yielded a forecast of potential wave heights and travel times was based on a method that could be applied generally to other coastal volcanos

the hazards of eruptions through lakes and seawater abstract eruptions through crater lakes or shallow seawater referred to here as subaqueous eruptions present hazards from hydromagmatic explosions such as base surges lahars and tsunamis which may not exist at volcanoes on dry land we have systematically compiled information from eruptions through surface water in order to understand the circumstances under which these hazards occur and what disastrous effects they have caused in the past subaqueous eruptions represent only 8 of all recorded eruptions but have produced about 20 of all fatalities associated with volcanic activity in historical time excluding eruptions that have resulted in about a hundred deaths or less lahars have killed people in the largest number of historical subaqueous eruptions 8 followed by pyroclastic flows excluding base surges 5 tsunamis 4 and base surges 2 subaqueous eruptions have produced lahars primarily on high 1000 m steep sided volcanoes containing small

three dimensional seismic attenuation structure beneath the aegean region and its tectonic implication abstract the three dimensional attenuation structure beneath the aegean sea and the surrounding regions is determined by inversion of seismic intensity data a large number of seismic intensity data have been accumulated in a uniform scale in the aegean region where the seismic activity is much higher than that of the other mediterranean regions nearly 7000 seismic intensity data from 50 earthquakes that have occurred in these regions are used to determine seismic attenuation structure and source acceleration of the earthquakes the resultant structure reveals a remarkable contrast of attenuation in the top layer depth 0 40 km low q is dominant in the aegean sea while high q is dominant in the surrounding land areas except for the southwestern turkey the low q regions correspond to areas of neogene quaternary grabens where the high seismicity of shallow earthquakes appears in the upper mantle high q zones corresponding to the subducting african plate dominate along the hellenic arc the irregular shape of high q zones might reflect the split or disintegration of the african slab some low q spots corresponding to the distribution of volcanoes exist along the volcanic arc the low q spots might correspond to diapirs causing the subduction volcanism