external modulation of volcanic activity summary time and again records of seismovolcanic activity that show a diurnal and semi diurnal modulation have been related to the earth s tidal stress field as an external triggering mechanism several seismic data sets from different volcanoes are examined including stromboli volcano italy mt ruapehu new zealand and merapi and batur indonesia where in some cases weather data are also incorporated a comparison with theoretical body tides in the spectral domain reveals strong evidence against a correlation between seismovolcanic activity and tidal stress however other parameters such as temperature and barometric pressure show the same diurnal and semi diurnal modulation and are therefore candidates for a possible external modulation of volcanic activity several mechanisms are discussed for stromboli a clear correlation between volcanic activity and barometric pressure is shown

spectral and dynamical hints on the time scale of preparation of the 5 april 2003 explosion at stromboli volcano stromboli volcano is well known for its continuous strombolian activity moreover the volcano occasionally shows effusive phases the latest in 1985 1986 on 28 december 2002 stromboli entered a new effusive phase accompanied by different paroxysmal events that led to considerable hazards for inhabitants and tourists on the island of stromboli on 30 december 2002 a major sector collapse affected the sciara del fuoco slope and initiated a tsunami on 5 april 2003 a powerful explosion which can be compared in size with the most recent explosion in 1930 covered a large part of the normally tourist accessible summit area with bombs as this explosion was not forecasted although the island was by then effectively monitored by a dense deployment of instruments in this paper we tackle the problem of highlighting the time scale of preparation of this event and conduct a search for possible precursors for this purpose we analyze the seismic data preceding the paroxysm with spectral and dynamical methods

seismic features of the june 1999 tectonic swarm in the stromboli volcano region italy abstract crustal tectonic seismicity in the southern tyrrhenian sea is characterized by the high occurrence rates of earthquakes to the west of the alignment of salina lipari and vulcano islands in the aeolian archipelago only a few earthquakes affect the crustal region east of these islands whereas intermediate and deep seismicity plays a relevant role based on this evidence two aspects of the seismic swarm recorded at the aeolian island seismic network between june 6 and 17 1999 looked anomalous the first aspect concerned the number of earthquakes 78 that affected the stromboli submarine edifice in a short time interval secondly despite the low maximum magnitude md 3 2 reached the cumulative strain release was conspicuous in comparison with previous swarms in this region we localized the swarm about 6 km northeast of stromboli at a depth between 8 and 12 km the source region was identified using standard methods of hypocentral location as well as azimuth analysis it is worth noting that the volcanic activity at stromboli did not change significantly during the swarm nor throughout the following months therefore the seismic swarm had no link with volcanic activity observed at the surface most of the earthquakes shared similar waveform and frequency content and can be divided into families we identified some earthquakes with magnitude up to md 3 having relatively low frequency content at different seismic stations this anomalous feature leads us to hypothesize the presence of fluid circulation and or propagation of seismic waves in a ductile medium our hypothesis is in agreement with studies on marine geology which highlight various forms of submarine volcanism in the southern basin of the tyrrhenian sea

real time seismic amplitude measurement rsam a volcano monitoring and prediction tool seismicity is one of the most commonly monitored phenomena used to determine the state of a volcano and for the prediction of volcanic eruptions although several real time earthquake detection and data acquisition systems exist few continuously measure seismic amplitude in circumstances where individual events are difficult to recognize or where volcanic tremor is prevalent analog seismic records provide a quick visual overview of activity however continuous rapid quantitative analysis to define the intensity of seismic activity for the purpose of predicing volcanic eruptions is not always possible because of clipping that results from the limited dynamic range of analog recorders at the cascades volcano observatory an inexpensive 8 bit analog to digital system controlled by a laptop computer is used to provide 1 min average amplitude information from eight telemetered seismic stations the absolute voltage level for each station is digitized averaged and appended in near real time to a data file on a multiuser computer system raw realtime seismic amplitude measurement rsam data or transformed rsam data are then plotted on a common time base with other available volcano monitoring information such as tilt changes in earthquake activity associated with dome building episodes weather and instrumental difficulties are recognized as distinct patterns in the rsam data set rsam data for domebuilding episodes gradually develop into exponential increases that terminate just before the time of magma extrusion mount st helens crater earthquakes show up as isolated spikes on amplitude plots for crater seismic stations but seldom for more distant stations weather related noise shows up as low level long term disturbances on all seismic stations regardless of distance from the volcano implemented in mid 1985 the rsam system has proved valuable in providing up to date information on seismic activity for three mount st helens eruptive episodes from 1985 to 1986 may 1985 may 1986 and october 1986 tiltmeter data the only other telemetered geophysical information that was available for the three dome building episodes is compared to rsam data to show that the increase in rsam data was related to the transport of magma to the surface thus if tiltmeter data is not available rsam data can be used to predict future magmatic eruptions at mount st helens we also recognize the limitations of rsam data two examples of rsam data associated with phreatic or shallow phreatomagmatic explosions were not preceded by the same increases in rsam data or changes in tilt associated with the three dome building eruptions

effects of bed discordance on flow dynamics at open channel confluences although a discordance in bed elevation between mainstream and tributary channels is a common feature of most river confluences this height differential is largely ignored in many models of junction flow dynamics here we present new laboratory flume data that demonstrate the significant influence of bed discordance on flow at an acute angled confluence a difference in bed height between mainstream and tributary channels is shown to obliterate flow deflection at the bed and create a distortion of the mixing layer between the flows resulting in fluid upwelling at the downstream junction corner this upwelling is responsible for the absence of a flow separation zone near the bed at the downstream junction corner and the lack of a zone of marked flow acceleration in the postconfluence channel these experiments reveal complex flow fields generated by bed discordance and suggest the need for major revisions to current models of river channel confluence flow dynamics and their application to sediment routing at these sites

bed morphology and sedimentology at the confluence of unequal depth channels abstract spatial and temporal variations in the bed geometry and bed material size of a sand bed river confluence with unequal depth channels were monitored during a sequence of floods additionally two other sand bed confluences were surveyed to test the replicability of these observations the confluences studied here have only one avalanche face which corresponds to the front of a tributary mouth bar in the shallower channel and do not exhibit a marked scour zone three distinct morpho sedimentological zones are present 1 an area around the upstream corner of the junction where the sediments are generally finer than the mean 2 a maximum depth zone with coarser than average particles and 3 a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream changes in relative discharge between the two channels had little effect on the grain size of the downstream junction corner bar but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar the downstream junction corner bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels the contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone

observations of body wave multipathing from broadband seismograms evidence for lower mantle slab penetration beneath the sea of okhotsk we present observations of anomalously broad and complex sh waveforms from two deep focus earthquakes in the southern kurils to digital world wide standard seismograph network and regional seismic test network stations in north america by making use of broadband three component seismograms and relying on a variety of phases one can limit the class of possible explanations for these phases in particular it is shown that the anomalous arrivals are not due to source complexity near receiver structure a triplication from a lower mantle velocity gradient a low q zone in the mid lower mantle or shear wave splitting we show that these arrivals correspond to relatively fast paths and have enhanced amplitudes indicating that they are most probably caused by multipathing due to interaction with a high velocity lower mantle feature below the source region we examine the hypothesis that this feature may be a lower mantle extension of the kuril slab ray tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the north american stations used in this study for a realistic slab model observations of reversed polarity arrivals are consistent with such an explanation where it appears that two rays comprising the arrival emerge from different points on the focal sphere one test of this explanation is that slab induced multipathing should not be seen along back of slab oceanic azimuths examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the sh waveforms identification of slab induced multipathing associated with deep focus events has two important consequences first it appears to require slab penetration beyond the 650 km discontinuity at least 300 400 km into the lower mantle under the sea of okhotsk second these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities low q zones near receiver structure or source complexity

the depths of the deepest deep earthquakes the maximum depth of seismic activity is a fundamental observation providing a constraint on models of mantle dynamics although most recent investigations of mantle seismicity and dynamics state that seismicity extends to about 700 km focal depths of 720 km have been reported for large earthquakes as well as depths greater than 800 km for small events we have examined focal depths of events in several catalogs and conclude that 1 the deepest reliable focal depths are at about 670 680 km and events with focal depths beneath 600 km occur in several widely separated geographic regions 2 observations supporting focal depths exceeding 680 km are usually few poor or inconsistent 3 the reduction of seismic activity beneath about 650 km is quite abrupt however we cannot determine whether activity stops completely at 670 680 km or the maximum size of events decreases gradually beneath 650 km with magnitudes of 6 5 7 0 occasionally occurring at 650 km magnitudes of 5 5 6 0 at 680 km 4 0 4 5 at 695 km etc several models can explain the absence of seismic activity in the lower mantle two models are consistent with the abrupt termination of activity at about 650 680 km in widely separated areas a barrier which resists penetration by the subducting lithosphere or a phase transition which modifies the properties of the subducted material as it goes into the lower mantle

tidal modulation of seismic noise and volcanic tremor 1 we present seismic data from fogo volcano cape verde republic that document a tidal control on the amplitude of seismic noise and volcanic tremor the effect is detectable in the raw data by simple eye inspection we identified the main periodicity in the noise modulation which coincides with the tidal component m2 the signal duration of 40 days together with a sampling rate of 1440 samples per day seismic r m s time series allowed the resolution of m2 from the neighbor solar frequencies that are often related to environmental effects spectral analysis shows that the modulation affects selectively the frequency range 2 0 hz 3 0 hz which we interpret as volcanic tremor white noise from 1 5 hz to 7 hz is also modulated comparison with meteorological data reinforces the conclusion that the effect is not due to environmental parameters

field investigation of three dimensional flow structure at stream confluences 2 turbulence stream confluences are among the most highly turbulent locations in fluvial systems this paper examines the three dimensional structure of turbulence at three stream confluences in east central illinois the analysis focuses on the characteristics of turbulence both within the shear layer and in the ambient flow results show that at the upstream end of each confluence the shear layer occupies a limited portion of the flow cross sectional area but turbulence kinetic energy within this layer is 2 3 times greater than the turbulence kinetic energy of the ambient flow which has turbulence characteristics similar to those for flow in straight channels turbulence within the shear layer can be characterized as quasi two dimensional in the sense that large scale turbulence generated by transverse shear is predominantly two dimensional whereas small scale turbulence associated with bed friction is three dimensional spectral analysis suggests that the structure of fluid motion within the shear layer differs for confluences with symmetrical versus asymmetrical planforms the shear layer dissipates rapidly as flow enters the downstream channel even though a well defined mixing interface persists at downstream locations

numerical simulation of three dimensional time averaged flow structure at river channel confluences current confluence research emphasizes three broad controls on flow structure generation 1 planform curvature 2 topographic steering and 3 anisotropic turbulence associated with flow separation and shear layer dynamics the relative importance of these processes in explaining observed flow structures is controversial a situation that may be related to the fact that different investigators have examined different confluence configurations this paper uses a three dimensional numerical model with a fully elliptic solution a free surface treatment and a turbulence model based on a renormalized group rng to help to provide a physically based explanation of the controls upon flow structure generation for both a laboratory rectangular and a field confluence the confluence of the kaskaskia river and copper slough and to identify the particular conditions under which particular flow structures are observed results suggest that an analogy with back to back meanders is possible for symmetrical configurations but that there will be progressive divergence from this state as confluence asymmetry increases in asymmetric situations a dual cell structure may be limited to the immediate vicinity of the junction because of the effects of streamline curvature and topographic steering these differences can be explained by consideration of the dynamic pressure field which may be specific to each confluence configuration as such this study partially reconciles differing views over what controls time averaged flow structures in river channel confluences although further research into the interaction of these processes with instantaneous velocity fluctuations is required

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

investigation of controls on secondary circulation in a simple confluence geometry using a three dimensional numerical model recent research into river channel confluences has identified confluence geometry and particularly bed discordance as a control on confluence flow structures and mixing processes and this has been illustrated using both field measurements in natural confluences and laboratory measurements of simplified confluences generalization of the results obtained from these experiments is limited by the number of confluence geometries that can be examined in a reasonable amount of time this limitation may be overcome by numerical models in which confluence geometry is more readily varied and data acquired more rapidly this paper aims to i validate the application of a three dimensional numerical model to a simple confluence geometry ii simulate the effects of different boundary condition values upon flow structures and iii interpret the implications of these simulations for river channel confluence dynamics the model used in this research solves the three dimensional form of the navier stokes equations and is used to simulate the flow in a parallel confluence of unequal depth channels and to investigate the effect of different combinations of velocity and depth ratio between the two tributaries the results generally agree with empirical evidence that secondary circulation is generated in the absence of streamline curvature but only for specific combinations of depth and velocity ratio this research shows how understanding of the interaction of these controls is enhanced if pressure gradients are considered the velocity ratio is the prime determinant of the cross stream pressure gradient that initiates cross stream velocities however for significant secondary circulation to form cross stream velocities must lead to significant transfer of fluid in the cross stream direction this depends on the vertical extent of the cross stream pressure gradient which is controlled by the depth ratio in this study strong secondary circulation occurred for a depth differential of 25 or more as long as the velocity in the shallower tributary was at least as great as that in the deeper channel this provides an important context for interpretation of previous work and for the design of new experiments in both the field and the laboratory 1998 john wiley sons ltd

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

mixing layer distortion at the confluence of channels of different depth channel confluences have received considerable recent attention in the fields of geomorphology1 3 sedimentology4 9 and hydraulic engineering10 13 where channels join rapid changes in fluid velocity turbulence intensity channel hydraulic geometry and bed geometry may occur14 17 previous models of junction dynamics have tended to assume that the confluent channels are of equal depth10 18 a condition that may be found only rarely in natural junctions more often the depths of confluent channels are different19 this discordance is commonly the result of the forma tion at the mouth of each confluent channel of bars that possess steep avalanche faces which dip into a central junction scour1 2 5 20 here we suggest that when channels of different depth join the mixing layer that forms at their confluence is distorted by interacting with a separation zone which forms in the lee of the mouth of the shallower channel this mixing layer distortion imparts a complex three dimensional flow to the junction promoting vertical fluid upwelling and significantly enhancing rapid mixing within the flow of the shallow tributary while retarding the rate of mixing across the flow of the deeper channel suspended sediments or contaminants within the deeper channel may be transferred rapidly across the flow of the shallower tributary upwelling downstream of the junction

p and s wave travel time inversions for subducting slab under the island arcs of the northwest pacific we have observed slablike high p and s velocity anomalies around the wadati benioff zone under island arcs of the northwest pacific through travel time tomographic inversions nineteen years of international seismological centre travel time residuals for events and stations in this large region are used analyses of resolution and noise show that the images are generally well resolved the images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth near the bottom of the upper mantle fingering of the slabs including segmenting and spreading is indicated the fast anomalies associated with the japan izu bonin and mariana subduction zones tend to flatten to subhorizontal at depth while downward spreading may occur under parts of the mariana and kurile arcs the fast anomalies below 700 km are not in the shape of a single coherent sheet the principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high velocity slab even when it bends to subhorizontal at depth the depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the 670 km discontinuity

continuous hidden markov models application to automatic earthquake detection and classification at las canadas caldera tenerife abstract a possible interaction of volcano tectonic earthquakes with the continuous seismic noise recorded in the volcanic island of tenerife was recently suggested also recently the zone close to las canadas caldera shows unusual high number of near

anisotropy and shear velocity heterogeneities in the upper mantle long period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle the dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure the data are corrected for the effect of surface layers and both love and rayleigh waves are used shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km the shear wave velocity distribution to 200 km depth correlates with surface tectonics except in a few anomalous regions below that depth the correlation vanishes cold subducted material shows up weakly at 350 km as fast s wave anomalies in the transition region a large scale pattern appears with fast mantle in the south atlantic s anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones the pacific shows a nw se fabric

deep seismicity and modes of deformation in tonga subduction zone the geometry of deep 400 km earthquakes in the subducting western edge of the pacific plate when related to the local distribution of seismic activity reveals a complex pattern of planes of shear failure enabling the modes of deformation by which the descending plate shortens and thickens to be identified the deepest part of the slab is displaced laterally suggesting that it fails to penetrate the discontinuity at 670 km

the effect of the descending lithosphere beneath the tonga island arc on p wave travel time residuals at the warramunga seismic array abstract p wave travel time residuals at the warramunga seismic array wra in the northern territory australia have been studied from 49 earthquakes with epicenters south of 19 s in the fiji tonga region focal depths are between 42 and 679 km as determined from pp p using the jeffreys bullen and the herrin travel time tables the epicentral parameters have been redetermined by considering only normal seismic stations in the location procedure these are those stations where p wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the tonga trench epicenters of deep earthquakes below 300 km have been relocated by using stations at 25 only epicenters from shallower depth earthquakes have been recalculated without using stations between 35

thermal seismic and infrasonic evidences of variable degassing rates at stromboli volcano abstract during june 1999 we measured the amplitude and rate number of events per second at which gas exited the vent at stromboli volcano as discrete gas bursts or puffs this allowed us to identify two styles of gas burst puffing activity the first is characterized by frequent rapidly rising puffs the second by less frequent slowly rising puffs each style persisted over 5 40 min long durations and was associated with a high and low number of strombolian explosions per hour respectively each period was also associated with characteristic delay times between the arrival of the infrasonic and thermal signals during strombolian explosions the delays were longer during vigorous puffing periods to explain our observations we propose a model in which the degassing process cycles between vigorous and weak degassing phases during vigorous degassing phases bubble layers ascend the conduit at a frequency of 0 5 1 0 s 1 this high degassing level reflects a gas rich magma column and leads to an increased rate in the formation of shallow foams and hence an increase in puffing and explosive activity as well as a higher free surface level and or gas jet velocity during weak phases bubble layers ascend the conduit at a reduced frequency of 0 2 0 3 s 1 during such times the magma column is poor in gas this leads to a decreased rate of foam layer formation and hence a reduction in puffing and explosive activity as well as a lower free surface level and or gas jet velocity variations in puffing activity can thus be used to track changes in the rate at which the shallow system is supplied by fresh gas rich magma our observations indicate that the two degassing styles last from 5 to 40 min and that the switch from one to the other occurs over a matter of minutes

a double seismic antenna experiment at teide volcano existence of local seismicity and lack of evidences of volcanic tremor data analyzed in the present work correspond to a 40 days field experiment carried out in teide volcano canary islands spain with two short period small aperture dense seismic antennas in 1994 the objective of this experiment was to detect analyze and locate the local seismicity we analyzed also the background seismic noise to investigate the possible presence of volcanic tremor from a set of 76 events we selected 21 of them in base of their good signal to noise ratio and their possibility to locate their seismic source by using the seismic antennas a visual classification based on the s p time and seismogram shape has permitted to establish three groups of events local seismicity s p time between 3 and 5 s very local earthquakes s p time smaller than 3 s and artificial explosions these earthquakes have been located by applying the zero lag crosscorrelation technique and the inverse ray tracing procedure those earthquakes that were recorded simultaneously by both seismic antennas were also located by intersecting both back azimuths the analysis of the seismicity has revealed that the amount of seismicity in teide volcano is moderate this seismicity could be distributed in three main areas inside the caldera edifice below the teide pico viejo complex in the eastern border of the caldera edifice and offshore of the island at present this activity is the only indicator of the volcano dynamics the analysis of the back ground seismic noise has revealed that at frequencies lower than 2 hz the oceanic load signal is predominant over other signals even over local earthquakes with a magnitude of 2 0 due to this although if in the teide area were present a weak volcanic tremor or other volcanic signals with predominant peaks below 2 hz to observe them would be a very difficult task q 2000 elsevier science b v all rights reserved

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

possible interaction between tectonic events and seismic noise at las canadas volcanic caldera tenerife spain continuous seismic noise is recorded in the volcanic island of tenerife canary islands spain the origin of this noise strongly augmented by anthropogenic contamination is still under debate in this paper we discuss the relationship between this noise and the occurrence of local tectonic events in the same area in particular transitions are sought in the time evolution of dynamic parameters computed on the seismic noise and examples are shown where abrupt transitions may be associated with the occurrence of tectonic events these transitions provide further evidence of the existence of a natural origin for at least part of the seismic noise which is strongly contaminated if not dominated by anthropogenic sources

observations of non volcanic tremor during the northern cascadia slow slip event in february 2002 1 we locate in the cascadia subduction zone non volcanic tremors from an episodic tremor and slip event in february 2002 the tremors occurred during two 10 day periods separated by a short lull tremors that occurred during the first period are broadly distributed between 10 km and 40 km depth at the southern end of vancouver island and over a 50 km wide region measured normal to the margin during the second period of tremor activity most tremors were concentrated in a narrower zone and many tremors occurred near the subduction megathrust at approximately 35 km depth this relatively large number of tremors near the subduction megathrust is in contrast to the broad depth distribution of the subsequent march 2003 tremor sequence the change in the pattern of tremor activity observed during the february 2002 sequence may indicate a change in the slow slip process or in its migration along the margin

three dimensional seismic structure beneath southwest japan the subducting philippine sea plate abstract a three dimensional seismic structure beneath southwest japan has been investigated in detail by applying an inversion method this analysis clearly reveals the present and possibly the past subduction of the philippine sea plate beneath southwest japan beneath the izu peninsula the philippine sea plate does not appear to subduct but collides against the continental asian plate here and its thickness appears thinnest at most about 30 km from the tokai to the shikoku regions the high velocity zone corresponds well to the zone of subcrustal earthquakes down to a depth of about 60 70 km indicating that the subduction of the present philippine sea plate does not reach below this depth here however a small high velocity portion appears at depths between 100 km and 150 km beneath a part of the kinki region and probably extends to the chugoku region where no corresponding seismic activities exist these high velocities might be an indication of the remnants of the past subduction of the philippine sea plate in the kyushu region the high velocity zone continues down to a depth of about 150 km or even to 200 km in the southern part corresponding to a steeply inclined seismic zone the philippine sea plate in this region appears to be thicker than in regions east of the kyushu palau ridge it was also found that there exist narrow low velocity zones intervening within the philippine sea plate where two aseismic ridges the kyushu palau and shichito iwojima ridges extend northwards and the nankai trough shows warping these low velocity zones give support to the buoyant subduction hypothesis it also appears that lateral heterogeneities existing within and around the subducting plate are closely related to the mode of rupture of large earthquakes occurring around there

the las canadas caldera tenerife canary islands an overlapping collapse caldera generated by magma chamber migration abstract the las canadas caldera is one of the most important geological structures of tenerife stratigraphic structural volcanological petrological geochronological and geophysical data suggest that the las canadas caldera resulted from multiple vertical collapse episodes that occurred during the construction of the las canadas edifice upper group three long term 200 ka cycles of phonolitic explosive activity each culminating with a caldera collapse have been identified in the upper group during the construction of the upper group the focus of felsic volcanism migrated from west to east using the results of field observations experimental analogue models and numerical studies we propose that the formation of the overlapping las canadas collapse caldera is related to the migration of the associated magma chamber our model implies that each collapse of this overlapping caldera partly or completely destroyed the feeding magma chamber this destruction led to changes in the local stress field that favoured the formation of a new chamber at one side of the previous one resulting in magma chamber migration the proposed model accounts for the formation of the las canadas caldera in particular it explains the geometrical relationships stratigraphy and chronology of the caldera wall deposits comparison with other overlapping collapse calderas suggests that our model may apply to other overlapping calderas

velocity structure in the izu bonin seismic zone and the depth of the olivine spinel phase transition in the slab in this study a method is presented for determining the in situ elastic wave velocities of slabs simultaneously with the relative relocations of earthquakes the method is an extended arrival time difference atd scheme that incorporates a realistic model several assumptions are made about the characteristics of ray paths in and around the slab in order to take advantage of the realism of the model without completely sacrificing the simplicity inherent in the atd technique the errors introduced by these approximate rays are estimated by comparing them with more exact rays calculated with a three dimensional ray tracing routine in general the comparisons suggest that these errors are usually much less than 0 1 s the inverse method is tested on the same set of hypothetical data the results of these tests show that the parameters that influence the arrival time differences the most are the amplitude of the slowness anomaly within the slab and the azimuths and takeoff angles for rays leaving the master event the arrival time differences are relatively insensitive to the other parameters describing the slab strike dip half width and the position of the master event which implies that the scheme is fairly insensitive to the particular choice of analytic function to describe the slowness variation within the slab convergence of the inverse routine is rapid in the tests usually less than five iterations are required before a stable solution is reached it appears that coupling between solutions for slab parameters and ray directions is insignificant unlike the standard atd approach solutions for hypocenters and velocities are not directly coupled in this scheme although well located events are required to insure that the hypocenters are not unduly biased by variations in structure once the velocity structure of the slab is determined that information can be used routinely by this scheme to relocate less well recorded events this method was applied to arrival time data from earthquakes located between 200 and 600 km depth in the izu bonin subduction zone structure in the 200 300 km depth range could not be resolved and results in the 500 600 km depth range proved to be too sensitive to noise data from events between 27 n and 31 n show that p wave velocities within the slab from 400 to 500 km depth are 4 6 higher than those of the ambient mantle between 300 and 400 km depth the slab structure varies regionally north of about 32 5 n the p wave velocity within the slab is 3 4 higher than ambient from 180 to 375 km depth in contrast the velocities south of 32 5 n increase from 6 7 higher than ambient in the 375 410 km depth range to 8 10 higher in the 325 375 km depth range and appear to be 9 higher than ambient in the 270 325 km depth range the determination of the high velocities in this region between 325 and 375 km depth proved to be independent of the selection of data and of the choice of starting model above 325 km depth however the velocity determinations are somewhat dependent on the choice of starting model as a result of this dependence no upper bound on the depth of the high velocities could be resolved a plausible explanation for the exceptionally high p wave velocities within the slab south of 32 5 n is that the olivine spinel phase change boundary is elevated in this part of the slab while north of 32 5 n it is not significantly elevated it is not known if the phase change is elevated south of 31 n but the velocities between 400 and 500 km depth in this region imply that it is not depressed the high velocity zone south of 32 5 n lies in an area that marks the beginning of a seismic gap increasing in breadth to the south between deep and shallow seismicity the dip of the slab also becomes gradually steeper to the south of this region in contrast the lower velocities north of 32 5 n occur in a region where the seismicity is fairly continuous and is aligned along a shallow dipping plane if the seismic gap south of 32 5 n represents a region of high velocity then the increasingly vertical dip of the slab in the south could be due to a substantial gravitational force provided by the excess mass extending above the ambient olivine spinel phase boundary the decreasing width of the gap to the north suggests that the evolution of the izu bonin seismic zone has been governed by a propagating instability from the south

identification of variable convective regimes at erta ale lava lake abstract during february 17 19 2002 we collected a combined thermal and seismic data set for persistent lava lake activity at erta ale volcano ethiopia these data indicate that the lake cycled between periods characterized by low 0 01 0 08 m s 1 and high 0 1 0 4 m s 1 surface velocities typically lasting tens to hundreds of minutes these periods of high and low velocity motion define periods of vigorous and sluggish convection respectively spectral analysis revealed that vigorous convection periods were characterized by high frequencies and energies in the thermal data and an increased presence of high frequency energy in the seismic data the data show that vigorous periods were characterized by formation of hot short lived plastic crusts and sluggish periods by cooler longer lived brittle crusts here the higher transit velocity across the lake surface from upwelling zones of crust formation to downwelling zones of crust destruction during the vigorous periods decreases the crust lifetime this in turn decreases the total cooling and thickening experienced by a plate of crust moving across the lake surface two scenarios can be envisaged to explain such convection cycles the first relates variable convection rates to changes in the volume flux and rheology of magma entering the lake in the second cyclic convection is set up by the generation of convective instabilities within the lake in this case cooling of a surface layer generates a slow moving viscous increasingly dense convection layer at the lake surface which is consumed and replaced during overturn

the theoretical foundations and potential for large eddy simulation les in fluvial geomorphic and sedimentological research abstract large eddy simulation les is a method for resolving the time dependent structure of high reynolds number turbulent flows with les it is possible to model and track the behaviour of coherent turbulent structures and study their effect on the flow field hence les is potentially an important research tool in the fluvial sciences where flow mixing sediment entrainment and sediment transport are all affected by the presence of coherent vortices and their interactions with channel boundaries and other flow structures this paper introduces the les methodology discusses a variety of ways for representing small scale processes within les the subgrid scale modelling problem and provides some examples of early work into the use of les in a fluvial context a number of advances in computational power and numerical methods are required before les can be effectively applied at the river reach scale this paper considers some recent developments and their potential for providing validated large eddy simulations of river flow at the channel scale

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

global seismicity of 1981 centroid moment tensor solutions for 542 earthquakes abstract centroid moment tensor solutions are presented for 542 earthquakes that occurred during 1981

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

discussion of three dimensional numerical study of flows in open channel junctions by jianchun huang larry j weber and yong g lai

p wave travel times from deep and intermediate depth earthquakes to local seismic stations and the subducted slab of oceanic lithosphere beneath the tonga island arc to investigate the seismic structure of the tonga subduction zone mobile seismic stations were temporarily operated on the four main islands of the tonga group data from intermediate depth and deep earthquakes were analyzed using p wave arrival times only to improve on single event locations as published in bulletins a joint hypocentral determination method was applied to three groups of deep earthquakes and two groups of intermediate depth earthquakes the resulting p wave travel time residuals observed in the tonga island arc are generally negative and reveal a characteristic pattern a notable feature is the observation that residuals from deep earthquakes located near 18 s are consistently less negative than residuals from deep earthquakes located south of 20 5 s it cannot completely be ruled out that part of this difference is caused by systematic depth errors that may arise as a result of slab propagation effects on the upgoing p leg of pp but only a minor part of the observed difference can be explained in this way the major part is probably caused by two factors first p waves traveling from the southern part of the tonga subduction zone to the northernmost stations located in the islands of vava u and ha apai spend more time in the high velocity slab than p waves originating in the northern part of the tonga seismic zone second ray tracing through a number of models of the subduction zone suggests that p waves from deep earthquakes located south of 20 5 s have their propagation paths close to the high velocity core of the slab whereas p waves from deep and perhaps also from intermediate depth earthquakes located near 18 s are likely to miss parts of the high velocity slab on their way up to the island arc this is most pronounced for the seismic station tga located in the island of tongatapu which is some 30 km further away from the trench than the other stations of the network the magnitude of residual differences 2 3 s at tga observed from different groups of deep earthquakes is best explained by a sharp bend in the dip of the subducting plate in the northern part of tonga near a depth of 200 km ray tracing through such a model shows that p waves from deep earthquakes near 18 s traveling to tga escape the slab at a depth of about 250 km and from there on propagate through the mantle overlying the slab if the interpretation is correct p wave velocities are indicated that are 5 7 higher in the slab above 250 km than in the overlying mantle independent of this interpretation the magnitude of negative residuals from deep earthquakes south of 20 5 s is best matched by velocities in the slab that are uniformly higher by 6 relative to the herrin model in another model where p wave velocities vary linearly over horizontal slab width from maximum values in the seismic zone to the velocities in the herrin model at the slab boundaries the data are equally well fitted if the velocity anomaly reaches a maximum value of 8 relative to the herrin model allowance for a crustal correction term of 0 5 s reduces these estimates by 1 modification of the velocity models to include the case of a velocity anomaly in the slab that decreases linearly with depth to 0 over the range from 400 to 700 km increases the estimates by 1 the data do not allow a proper assessment of the question of whether seismic velocities approach normal mantle values at depths below 400 km or remain substantially higher than in standard models of the earth

time averaged flow structure in the central region of a stream confluence previous process oriented field studies of stream confluences have focused mainly on fluvial dynamics at or immediately downstream of the location where the confluent flows enter the downstream channel this study examines in detail the spatial evolution of the time averaged downstream velocity cross stream velocity and temperature fields between the junction apex where the flows initially meet and the entrance to the downstream channel a well defined vertically oriented mixing interface exists within this portion of the confluence suggesting that lateral mixing of the incoming flows is limited the downstream velocity field near the junction apex is characterized by two high velocity cores separated by an intervening region of low velocity or recirculating fluid in the downstream direction the high velocity cores move inwards towards the mixing interface and high velocity fluid progressively extends downwards into a zone of scour resulting in an increase in flow velocity in the centre of the confluence the cross stream velocity field is dominated by flow convergence but also includes a component associated with a consistent pattern of secondary circulation this pattern is characterized by two surface convergent helical cells one on each side of the mixing interface the helical cells appear to be the mechanism by which high momentum fluid near the surface is advected downwards into the zone of scour for transport ineffective flows the dimensions and intensities of the cells are controlled by the momentum ratio of the confluent streams and by the extant bed morphology within the confluence although the flow structure of formative events was not measured directly in this study documented patterns of erosion and deposition within the central region of the confluence suggest that these events are dynamically similar to the measured flows except for the fact that formative flows are not constrained by but can reshape the bed morphology the results of this investigation are consistent with and augment previous findings on time averaged flow structure in the downstream portion of the confluence 1998 john wiley sons ltd

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

effect of bed morphology on flow mixing length at river confluences mixing processes at river confluences have an important bearing on problems such as pollutant dispersal and the management of river systems but remain poorly understood previous studies1 11 have indicated that flow mixing downstream of a confluence is a slow process typically completed at distances greater than 100 times the channel width bed morphology is now emerging as a critical factor flume experiments have shown that the width to depth ratios12 and height discordance13 14 between the confluent channels should influence mixing rates12 14 but the magnitude of these effects in real rivers is not known here we report measure ments from three river confluences of moderate size which show that bed discordance can markedly increase mixing rates for the rivers studied mixing is complete at distances five to ten times shorter than those reported previously

waveform effects of a high velocity subducted slab creager and jordan 1986 propose that penetration of subducting slabs under the kurile islands and other subduction zones to depths of at least 1000 km is necessary to explain travel time anomalies of deep earthquakes such penetration would also be expected to affect the amplitudes and waveforms of the body waves from earthquakes synthetic seismograms appropriate for a record section in a plane perpendicular to the strike of the slab are presented using a coupled finite difference and kirchhoff method our shear wave version of the compressional wave velocity structure of creager and jordan 1986 produces an amplitude decrease up to a factor of four and waveform broadening up to 20 seconds for sh arrivals with a take off angle pointing straight down the slab slabs that extend only 300 km below the earthquake but are half as thick and twice as anomalously fast as creager and jordan s 1986 velocity model will roughly preserve the travel time variation pattern and show less waveform broadening but produce first arrivals that are emergent slabs that become thicker with depth show less waveform broadening reconciliation of the amplitude waveform distortion and timing of body waves from deep events is necessary to understand the geometry of slabs in the upper mantle and near and below the 650 km discontinuity

the seismic noise at las canadas volcanic caldera tenerife spain persistence characterization and possible relationship with regional tectonic events abstract the origin of the continuous seismic noise recorded during the last few years in the volcanic island of tenerife canary islands spain is under debate due to the important implications in terms of hazard the seismic noise is strongly affected by anthropogenic contamination but previous work has shown that nevertheless a significant fraction of this noise has a natural origin an origin that appears to be related to the generation of local tectonic events within or close to the island in this paper the variogram tool is used to characterize the so called persistence of the seismic noise this analysis can quantify how much memory the seismic noise has of its recent past this has important implications in terms of how informative the recorded noise can be about its future evolution and this in turn puts important constraints on how likely it is for the seismic noise to be the source of potential precursors highlighting variations in the volcanic activity that is supposed to generate part of this seismic noise in particular the memory characterization can help to distinguish the naturally generated seismic noise from the anthropogenic contamination although only a few clear examples have been observed until now the possibility that this memory can be affected by the occurrence of local tectonic events is also discussed

modeling of residual spheres for subduction zone earthquakes 1 apparent slab penetration signatures in the nw pacific caused by deep diffuse mantle anomalies we have computed focal residual spheres for 145 subduction zone earthquakes along the northwest edge of the pacific using regional and global mantle velocity models from tomographic inversions the mantle models explain much of the observed residual sphere data and to a certain extent suggest the location of mantle velocity heterogeneities which are responsible for various residual sphere patterns for most deep events considered the fast slablike residual sphere anomalies are caused by diffuse heterogeneities mainly of deep lower mantle and receiver mantle origin rather than by an extension of the slab the region immediately below the deepest earthquakes depths of 650 1500 km has an effect usually smaller than or comparable to the effect of other regions of the mantle without a proper account of the teleseismic effect attributing the long wavelength anomalies of the residual sphere to near source slab effects alone or even primarily is not valid the fast bands in many observed residual spheres agree with seismicity trends once the deep mantle and receiver mantle effects are removed these may give the approximate orientation but not the depth extent of near source fast velocities for most deep earthquakes under japan the predominant fast band is subhorizontal rather than near vertical this type feature would be overlooked in conventional residual sphere studies using only steeply diving rays and cosine weighting of the data

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

ambrym vanuatu july august 2000 spectral and dynamical transitions on the hours to days timescale abstract ambrym is a large basaltic volcano with a 12 km wide 1900 year old caldera located in the vanuatu archipelago formerly known as new hebrides two cones are currently active in the caldera marum and benbow each with several active vents that hosted visible lava lakes until 1999 on 26 november 1999 an earthquake m w 7 5 located near the northern part of the island caused important changes to the vent morphology during july 2000 we installed a short period seismic station on the eastern rim of benbow crater in the western side of the caldera the station acquired continuous data for about 1 month during this period volcanic tremor was permanently recorded together with discrete events in this paper first results from these data are discussed focusing attention on the existence of significant transitions in the long term volcano dynamics studied both with spectrally derived dominant and average frequencies and more properly dynamical embedding dimensions and delay time parameters during the observation period on 9 august 2000 another significant earthquake was recorded m b 6 3 about 65 km north of the volcano the influence of this and other regional tectonic events on the volcano dynamics are discussed

stratigraphy structure and geochronology of the las canadas caldera tenerife canary islands after a long period of subaerial fissure fed extrusions of basaltic magmas 12 to 3 ma volcanic activity was then concentrated in the central part of tenerife phonolitic magma chambers formed and a central volcanic complex was constructed the las canadas edifice the formation of a large depression the las canadas caldera truncated the top of the edifice the active twin strato cones teide pico viejo are sited in this depression the history of the las canadas caldera and edifice are established from stratigraphy geochronology k ar dates and volcanological studies two different groups are recognized separated by a major unconformity the lower group is dated at 2 to 3 ma and includes the products of several volcanic centres which together represent several cycles the upper group ranges from 1 56 to 0 17 ma and includes three different formations representing three long term 100 to 300 ka volcanic cycles the periods of dormancy between each formation were of 120 to 250 ka duration the las canadas caldera is a multicyclic caldera which formed over the period 1 18 0 17 ma each cycle of activity represented by a formation culminated in caldera collapse which affected different sectors of the las canadas edifice geological observation and geochronology support an origin by collapse into a magma chamber the minimum volume of pyroclastic ejecta is substantially greater than the present caldera depression volume 45 km3 but approaches the inferred volume of the original caldera depression 140 km3 after the formation of the caldera sector collapses could also occur at the northern flank of the volcano causing the disappearance of the northern side of the caldera wall

can tectonic events change volcanic tremor at stromboli 1 besides the well known strombolian activity stromboli shows the presence of a continuous volcanic tremor tectonic events in the area are rare and no evidence of a local tectonic event changing the characteristics of the tremor was previously known in april and may 2006 a swarm of tectonic events was recorded located very close to the island of stromboli possibly within the same volcanic edifice in this paper we analyse the time evolution of several dynamical parameters computed on the volcanic tremor and show examples where abrupt transitions can be associated to the occurrence of such local tectonic events this suggests that these events may be related to the volcanic activity i e be proper volcano tectonic events