a numerical method for determining the state of stress using focal mechanisms of earthquake populations application to tibetan teleseisms and microseismicity of southern peru the numerical method described in this paper enables the study in terms of stress of the kinematics of seismic faults provided by focal mechanisms this method assumes a mean state of stress in the source region and is based on the simple mechanical model used for fault population analysis which supposes slip in the direction of the resolved shear stress acting on the fault plane the proposed algorithm first defines compressional and tensional zones resulting from superimposition of the compressional and tensional quadrants limited by the nodal planes this enables one to test the data homogeneity furthermore this restricts the space where the principal stress axes have to be searched then for each principal stress reference whose location is constrained by above confined zones the r value chosen equal to 2 1 3 1 is computed which fits the slip vector on each nodal plane this permits one to select a set of preferred seismic fault planes from a set of auxiliary planes finally a state of stress is computed from the preferred seismic fault plane set using the non linear simplex method already applied to fault populations this algorithm is constructed so as to avoid two major difficulties misleading estimation of the deviatoric stress tensor which may result from excessive emphasis by minimizing the residuals by a least squares method if some of the data are wrong and a lengthy prospection of the stress references over all the space of directions this calculation does not take into account possible changes of the strike and dip of the nodal planes however we use rotations of nodal planes in agreement with the first arrival data to test the compatibility of these data with the computed state of stress tibetan teleseisms and southern peruvian microseisms are analyzed and results are compared with kinematics of superficial recent and active faults measured in field in the same regions this suggests that major seismic events may give a good evaluation of the regional mean state of stress focal mechanisms of the small events are also modelled by a mean stress deviator but this differs significantly from the regional state of stress in southern peru deduced from quaternary faulting this may be due to imprecise location of the small events however some movements are strongly different from those predicted by the computed mean stress deviator this suggests that small fault kinematics surely depend on the regional state of stress but that local effects may control the small deformations

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

interaction between regional and magma induced stresses and their impact on volcano tectonic seismicity abstract recent seismological observations have reported volcano tectonic vt earthquakes with fault plane solutions exhibiting a change of 90 in their pressure axes relative to the regional stress field interestingly they are recorded mainly during periods preceding eruptive activity and coexisting with those vts showing a regional trend this study explains the occurrence of such trends in vt seismicity and discusses the possible patterns of earthquake locations related to the interaction of regional and magma induced stresses caused by pressurization or depressurization of magmatic sources our analysis shows that in the presence of a dominant regional stress field faulting will occur on faults whose associated slip direction is close to or in agreement with the background regional stress failure on faults with an opposite slip direction is unlikely to occur as magma pressure starts counter acting the regional stresses the likelihood of faults to slip in either a regional or opposite sense of slip relative to regional maximum compression increases allowing the co existence of possible failure with both slip tendencies however the spatial distribution of possible faulting differs as the pressure is progressively increased the stress patterns gradually approach those corresponding to the absence of a regional stress field the presented modeling results have implications for volcanic monitoring routines aiming to detect changes in stress patterns they will ultimately help to improve the correct interpretation of volcano tectonic seismicity

review of eruptive activity at tianchi volcano changbaishan northeast china implications for possible future eruptions one of the largest explosive eruptions in the past several thousand years occurred at tianchi volcano also known as changbaishan on the china north korea border this historically active polygenetic central volcano consists of three parts a lower basaltic shield an upper trachytic composite cone and young comendite ash flows the millennium eruption occurred between 938 and 946 ad and was preceded by two smaller and chemically different rhyolitic pumice deposits there has been at least one additional small eruption in the last three centuries from 2002 to 2005 seismicity deformation and the helium and hydrogen gas contents of spring waters all increased markedly causing regional concern we attribute this event to magma recharge or volatile exhalation or both at depth followed by two episodes of addition of magmatic fluids into the overlying aquifer without a phreatic eruption the estimated present magma accumulation rate is too low by itself to account for the 2002 2005 unrest the most serious volcanic hazards are ash eruption and flows and lahars the available geological information and volcano monitoring data provide a baseline for comprehensive assessment of future episodes of unrest and possible eruptive activity

stress tensor analysis of the 1998 1999 tectonic swarm of northern quito related to the volcanic swarm of guagua pichincha volcano ecuador the phreatic activity and the subsequent dacitic dome growth in 1998 1999 at guagua pichincha volcano ecuador were associated with two seismic swarms one located in the northern part of quito population 1 500 000 and another one just below the active volcano about 15 20 km sw from the first one quito swarm tectonic events have high frequencies from 1 to 10 15 hz we registered more than 3200 events among which 2354 events of 1 4 m l 4 2 between june 1998 and december 1999 at the 2 and 17 km depth the volcanic events below the guagua pichincha caldera have high from 1 to 10 15 hz and low less than 3 hz frequencies approximately 130 000 events were registered between september 1998 and december 1999 at the 2 4 and 3 5 km depth here we study the stress tensors of these two swarms deduced from the polarities of p first motions and compare them to the regional stress tensor deduced from cmt harvard focal mechanisms the quito swarm stress tensor is relatively close to the regional stress tensor the 1 axis was oriented n117 e close to the n102 e direction of the plate motion found by the gps measurement and 3 is nearly vertical the difference may be due to the action of the closely active guagua pichincha volcano the guagua pichincha stress tensor is very different from the regional tectonic one the 1 axis of the volcano is oriented n214 e almost perpendicular to the 1 of the swarm of quito and 3 is almost horizontal even if these two tensors are different they can be explained in a more general tectonic scheme the almost horizontal direction of 3 just below the volcano is compatible with an extensional horizontal direction that may be expected in the shallow extrados part of a compressional region and consistent with an opening of the top of the guagua pichincha volcano the movement of the fluids magma gas and or groundwater produced by the closely active guagua pichincha volcano seems to have an influence in the acceleration of the generation of seismic events

seismogenic stress field beneath the tatun volcano group northern taiwan article i nfo the tatun volcano group tvg represents the main volcanic center in northern taiwan and based on various observations it is considered a potentially active volcano tvg has been monitored since 2003 by a seismic network that consists of eight stations equipped with three component short period and broadband seismometers in this study we use waveform data of high frequency earthquakes in order to investigate the stress field orientation beneath the tvg area the focal mechanisms of 35 selected events have been derived using p wave polarities and amplitude ratios assuming a double couple source these fault plane solutions and various subsamples derived from them were subsequently inverted for the best fitting stress tensor using a linear inversion method the results show stress homogeneity beneath tvg for depths larger than 3 km while the stress tensor is characterized by a subvertical 1 and a subhorizontal nw se trending 3 axis consistent with the regional stress field in northern taiwan on the other hand chihsinshan which is an area of vigorous hydrothermal activity exhibits a localized stress field with horizontal nw se trending 3 axis and ne sw trending horizontal 1 such an axes orientation is likely to be causing opening of microcracks and thus favour the ascent and circulation of fluids in the upper crust shear wave splitting measurements seem to confirm these results showing fast polarization directions along nne ne subparallel to the main fracture system in tvg shear wave anisotropy averages about 2 9 and is probably caused by an anisotropic volume of fluid saturated cracks within the upper 2 5 km of the crust

the 1998 1999 seismic series at deception island volcano antarctica abstract during the 1998 1999 antarctic summer the pattern of seismic activity at deception island volcano changed significantly the change was characterized by the occurrence of an intense swarm of volcano tectonic vt earthquakes more than 2000 vt earthquakes with s p times smaller than 4 s were recorded in the period january february 1999 pure volcanic events were also detected especially long period lp events volcanic tremor and some hybrid events seismic monitoring was performed using two short period small aperture arrays among other instruments based on their signal to noise ratios we selected 863 vt earthquakes 350 lp events and tremor episodes and 9 hybrid events for analysis we estimated apparent slowness and back azimuth for all events using the zero lag cross correlation array technique combining this information with s p times and other indirect evidence we identified two different source regions lp seismicity is located less than 1 1 5 km southwest of the fumarole array site these events are likely to have a hydrothermal origin vt earthquakes and hybrid events are located at depths of 0 3 10 km in an area under the bay of deception island the area extends from the fumarole array to the northeast with epicentral distances that range from 0 5 to 12 km most hypocenters are clustered in a small volume of around 8 km3 the sources of the lp seismicity and the vt earthquakes are spatially distinct which indicates that they are not produced by the same mechanisms moment magnitude analyses of the vt earthquakes provide an average magnitude of 0 5 and very low average stress drop around 1 bar a study of first motion of the p waves suggests that the events in this small source region should have a variety of source mechanisms this is supported by the existence of families of events with the same waveforms the occurrence of repeating fracture processes with low stress drop and small fault dimensions can be explained by the lubrication of pre existing zones of weakness by pressurized fluids the most probable hypothesis that explains the generation of this seismic series at deception island is a seismic series caused by the stress generated by the uplift of the source area due to a magmatic injection in depth we favor this hypothesis since it is compatible with the majority of the characteristics of the seismicity and explains the spatial and temporal behavior of the series

preliminary report on the activity at unzen volcano japan november 1990 november 1991 dacite lava domes and pyroclastic flows abstract the eruption of unzen volcano commenced on 17 november 1990 phreatic and phreatomagmatic eruptions occurred by early may 1991 no large scale explosive eruptions preceded the extrusion of lava domes lava domes appeared in a summit crater on 20 may 1991 and they grew on the steep slope of mt fugen at unzen volcano rockfalls from the margins of the domes frequently generated pyroclastic flows major pyroclastic flows occurred on 3 june 8 june and 15 september 1991 the 3 june pyroclastic flow killed forty three persons many of the pyroclastic flows seem to have resulted from the simple rockfalls except one flow on 8 june which was accompanied by an explosion from the crater many of the rockfalls that generated pyroclastic flows were witnessed as of november 1991 unzen volcano was still active with a nearly constant magma supply rate of about 0 3 106 m3 d the total magma output exceeded 45 106 m3 by the beginning of november 1991 the volume of the lava domes is more than 23 106 m3

new radiometric and petrological constraints on the evolution of the pichincha volcanic complex ecuador fieldwork radiometric 40ar 39ar and 14c ages and whole rock geochemistry allow a reconstruction of eruptive stages at the active mainly dacitic pichincha volcanic complex pvc whose eruptions have repeatedly threatened quito most recently from 1999 to 2001 after the emplacement of basal lavas dated at 1100 to 900 ka the eruptive activity of the old rucu pichincha volcano lasted from 850 ka to 150 ka before present bp and resulted in a 15 20 km wide edifice which comprises three main building stages 1 a lower stratocone lower rucu 160 km3 in volume developed from 850 to 600 ka 2 this edifice was capped by a steeper sided and less voluminous cone the upper rucu 40 50 km3 the history of which started 450 430 ka ago and ended around 250 ka with a sector collapse 3 a smaller 8 10 km3 but more explosive edifice grew in the avalanche amphitheatre and ended rucu pichincha s history about 150 ka ago the guagua pichincha volcano ggp was developed from 60 ka on the western flank of rucu with four growth stages separated by major catastrophic events 1 from 60 to 47 ka a basal effusive stratocone developed terminating with a large ash and pumice flow event 2 this basal volcano was followed by a long lasting dome building stage and related explosive episodes the latter occurring between 28 30 and 22 23 ka these first two stages formed the main ggp 30 km3 a large part of which was removed by a major collapse 11 ka bp 3 sustained explosive activity and viscous lava extrusions gave rise to a new edifice toaza 4 5 km3 in volume which in turn collapsed around 4 ka bp 4 the ensuing amphitheatre was partly filled by the 1 km3 cristal dome which is the historically active centre of the pichincha complex the average output rate for the whole pvc is 0 29 km3 ka nevertheless the chronostratigraphic resolution we obtained for lower rucu pichincha and for the two main edifices of guagua pichincha main ggp and toaza leads to eruptive rates of 0 60 0 65 km3 ka during these construction stages these output rates are compared to those of other mainly dacitic volcanoes from continental arcs our study also supports an overall sio2 and large ion lithophile elements enrichment as the pvc develops in particular distinctive geochemical signatures indicate the involvement of a new magma batch at the transition between rucu and guagua at the ggp the same phenomenon occurs at each major collapse event marking the onset of the ensuing magmatic stage since the 11 ka bp collapse event this magmatic behaviour has led to increasingly explosive activity four explosive cycles of between 100 and 200 years long have taken place at the cristal dome in the past 3 7 ka and repose intervals between these cycles have tended to decrease with time as a consequence we suggest that the 1999 2001 eruptive period may have initiated a new eruptive cycle that might pose a future hazard to quito 2 million inhabitants

earthquake swarms and dike intrusions off the east coast of izu peninsula central japan abstract relationships between earthquake swarms and dike intrusions are investigated in the monogenetic volcano region off the east coast of the izu peninsula central japan hypocentral distribution of an earthquake swarm associated with the 1989 ito oki submarine eruption and those of eight major earthquake swarms for 7 years before the eruption in the same region are precisely relocated comparison of the hypocentral distribution with a dike model obtained from crustal movement measurements by okada and yamamoto 1991 ensures that hypocentral areas of the earthquake swarms can be used as reliable indicators of intruded dikes the epicentral area of each earthquake swarm shows an elliptic shape with a 3 5 km long axis and a 2 4 km short axis the long axis trending from a nw se to wnw ese direction the total hypocentral distribution of the nine swarms forms a seismic zone of about 20 km long and about 5 km wide with each of the swarm areas overlapping focal migrations from deep 10 km to shallow 3 km depths are found in the two largest events in 1988 and 1989 indicating that dikes propagated from deep to shallow areas in the upper crust these features of the hypocentral distributions suggest that the seismic swarms were induced by dike intrusions which ascended from a deep seated magma reservoir with dike like geometry located beneath the seismic zone solidification time of a dike with thickness comparable to that of the dike model of okada and yamamoto 1 45 m at the bottom of the dike is several days indicating that the dikes inducing the earthquake swarms solidified almost during the swarm period the physical mechanisms of the dike induced earthquake swarms are investigated for the izu region the focal depth migration and fault type of the earthquake swarm suggest that stress change in the region surrounding the dike due to dike emplacement is the most important factor for the cause of the earthquakes

emplacement mechanisms of a dyke swarm across the brittle ductile transition and the geodynamic implications for magma rich margins abstract igneous dykes are the main magma transport pathways through the earth s crust and they are considered to contribute to tectonic extension in volcanic rifts dykes are typically considered to result from brittle fracturing even in the ductile crust a common assumption is that dyke orientation is controlled by tectonic stresses such that dykes in rifts are expected to be vertical and perpendicular to extension here we report on detailed field observations of a spectacularly well exposed dyke swarm to show that dykes were not systematically emplaced by purely brittle processes and that dyke orientation may differ from the dominant tectonic stress orientations the dyke complex formed near the brittle ductile transition during opening of the iapetus ocean and is now exposed in the scandinavian caledonides distinct dyke morphologies related to different emplacement mechanisms has been recognized 1 brittle dykes that exhibit straight contacts with the host rock sharp tips en echelon segments with bridges exhibiting angular fragments 2 brittle ductile dykes that exhibit undulating contacts rounded tips ductile folding in the host rock and contemporaneous brittle and ductile features 3 ductile dykes that exhibit rounded shapes and mingling between the soft ductile host rock and the intruding mafic magma the brittle dykes exhibit two distinct orientations separated by c 30 that are mutually cross cutting suggesting that the dyke swam did not consist of only vertical sheets perpendicular to regional extension as expected in rifts we were able to use the well exposed host rock layers as markers to perform a kinematic restoration to quantify the average strain accommodating the emplacement of the dyke complex it accommodated for 100 extension but counter intuitively it also accommodated for 27 crustal thickening we infer that the magma influx rate was higher than the tectonic stretching rate implying that magma was emplaced forcefully as supported by field observations finally our observations suggest that the fast emplacement of the dyke swarm triggered a rapid shallowing of the brittle ductile transition and lead to a considerable weakening of the crust the interpretations presented here could potentially have large implications for surface topography and seismicity in active rifts and volcanic areas around the world

volcanism in a compressional andean setting a structural and geochronological study of tromen volcano neuquen province argentina we document evidence for growth of an active volcano in a compressional andean setting our data are surface structures and 39ar 40ar ages of volcanic products on tromen volcano tromen is an active back arc volcano in the andean foothills of neuquen province argentina its volcanic products are unconformable upon mesozoic strata of the neuquen basin the volcano straddles a n s trending pop up which formed during e w shortening the main underlying structures are eastward verging thrusts their traces curve around the eastern foot of the volcano minor folds and faults also occur in the volcanic cover of tromen as a result of e w shortening new 39ar 40ar ages for these volcanic rocks are younger than 2 27 0 10 ma and show that tromen has been active almost continuously from the late pliocene to the holocene we conclude that volcanism and thrusting have been coeval and that magma must have reached the surface in a tectonic setting of horizontal compression our results have wider implications for magmatic processes in such settings

a process based model of pre eruption seismicity patterns and its use for eruption forecasting at dormant stratovolcanoes abstract using long term data sets the eruptive size timing and style can typically be successfully forecast for dormant volcanoes using current monitoring data and knowledge of precursory eruptive patterns the usaid usgs volcano disaster assistance program vdap utilizes seismic data over multiple eruption cycles and across volcano types to successfully forecast eruption size style and onset times as well as changes within ongoing eruptions from millions of seismic event observations we show that seismic data a proxy for rate dependent strain changes in the magmatic system combined with a process based conceptual geologic model enables eruption forecasting especially for stratovolcanoes dormant 20 years we show that this model often works for some open systems ones that sufficiently reseal to plug the conduit and also where a deep intrusion occurs after phreatic or phreatomagmatic explosions have begun this model is divided into four seismicity and geologic stages we describe each of the four stages of seismicity in detail and how they relate to the ongoing geologic process of that stage stage 1 deep intrusion and seismicity occurring at 10 to 40 km depth below the volcanic edifice stage 2 distal vt seismicity induced at distances from 2 to 30 km laterally from the summit by magma intruding into and inflating the upper crustal magma storage region stage 3 vent clearing seismicity associated with the initial vent creation and clearing phase of eruptive activity stage 4 repetitive event seismicity associated with the final ascent of magma from 2 km to the surface we present 36 examples of this seismic progression from 26 volcanoes we believe that this progression generally correlates with the magma flux and the eventual explosivity and is invaluable for forecasting eruptions

crumbling of dacite dome lava and generation of pyroclastic flows at unzen volcano recent modelling of volcanic eruptions has shown that the efficiency of subsurface degassing of magmas determines whether magma erupts explosively or effuses quietly1 2 slow uprise of magma is often accompanied by effective degassing leading to the extrusion of lava flows and domes although lava dome extrusion is one of the less explosive modes of eruption it is often accompanied by explosive pyroclastic activities3 5 the 1991 eruption of unzen volcano provided an opportunity to observe at close range several types of small scale pyroclastic flow glowing avalanches originating from lava domes most of the pyroclastic flows are of merapi type caused by blocks falling from a collapsing dome others are of pelean type originating in an explosion from the side of a dome the lavas apparently show variable degrees of self explosivity we suggest that variable degrees of degassing of the magma produced a wide range of excess pore pressures in the extruded lava domes resulting in both merapi type and pelean type pyroclastic flows from the domes

seismic petrologic and geodetic analyses of the 1999 dome forming eruption of guagua pichincha volcano ecuador abstract guagua pichincha located 14 km west of quito ecuador is a stratovolcano bisected by a horseshoe shaped caldera in 1999 after some months of phreatic activity guagua pichincha entered into an eruptive period characterized by the extrusion of several dacitic domes vulcanian eruptions and pyroclastic flows we estimated the three dimensional 3 d p wave velocity structure beneath guagua pichincha using a tomographic inversion method based on finite difference calculations of first arrival times hypocenters of volcano tectonic vt earthquakes and long period lp events were relocated using the 3 d p wave velocity model a low velocity anomaly exists beneath the caldera and may represent an active volcanic conduit petrologic analysis of eruptive products indicates a magma storage region beneath the caldera having a vertical extent of 7 8 km with the upper boundary at about sea level this zone coincides with the source region of deeper vt earthquakes indicating that a primary magma body exists in this region lp swarms occurred in a cyclic pattern synchronous with ground deformation during magma extrusions the correlation between seismicity and ground deformation suggests that both respond to pressure changes caused by the cyclic eruptive behavior of lava domes

the 1989 earthquake swarm beneath mammoth mountain california an initial look at the 4 may through 30 september activity abstract mammoth mountain is a 50 000 to 200 000 year old cumulovolcano standing on the southwestern rim of long valley in eastern california on 4 may 1989 two m 1 earthquakes beneath the south flank of the mountain marked the onset of a swarm that has continued for more than 6 months in addition to its longevity noteworthy aspects of this persistent swarm include 1 an exponential like increase in the rate of activity through the first month 2 a vertically oriented planar distribution of hypocenters at depths between 6 and 9 km with a north northeast strike roughly perpendicular to the average t axis orientation for the swarm earthquakes 3 recurring spasmodic bursts rapid fire sequences of similar sized earthquakes with overlapping coda and occasional earthquakes with enhanced low frequency energy 4 a uniform temporal distribution of the four largest m 3 events over the first 4 months of the swarm with a cumulative seismic moment for the entire sequence through 30 september corresponding to a single m 4 earthquake 5 a b value of 1 2 and 6 submicrostrain perturbations on the nearby borehole dilatometer the first of which led the onset of swarm activity by more than 2 weeks these aspects of the swarm together with its location along the southern extension of the youthful mono inyo volcanic chain which last erupted 500 to 600 years ago point to a magmatic source for the modest but persistent influx of strain energy into the crust beneath mammoth mountain

stress failure and fluid flow deduced from earthquakes accompanying eruptions at piton de la fournaise volcano abstract in the present episode of eruptive activity evidence from seismicity for sustained magma inflow from depth into the edifice of piton de la fournaise is lacking pre eruptive main deformation and shallow seismicity help to identify very small volumes of magma that are in motion beneath the rim of the dolomieu summit crater and oriented along the azimuth of the future vents small magma pockets may reside in the cone above sea level or may be expelled repeatedly due to crystallisation in a small low velocity aseismic region below sea level under the high velocity central plug of the cone in which pre eruptive earthquake swarms are located in cross section the hypocentres define two steep sheets diverging from the aseismic zone at sea level towards 1 5 km above sea level or 1 km beneath the 2632 m high cone however failure induced by increased pressure in the suggested chamber does not account for the observed focal mechanisms the occurrence and timing of magma transport are attested by eruption and seismic activity may be related to magma transport focal mechanisms document strike slip not normal faulting or tensile failure vertical propagation of the edge of a feeder dike may enhance strike slip motion above the edge in a region where effective normal stress is decreased by thermally induced groundwater flow the strike slip mechanisms could also be caused by a tensile shear widening of the horizontal section of vertical conduits fournaise strike slip earthquakes occur in two orientations with p axes orthogonal between them within a single pre eruptive event earthquakes are distributed in the same volume but mechanisms switch from one to another type systematically with time indicating a reversal of stress conditions the orientations of p axes with respect to the epicentral trend suggest that in the later parts of events leading to eruptions a compression of the medium occurs after a dilation in the first part the activated zone might respond successively to the arrival and the departure of the magma on its way from the reservoir at depth to the vent radial to the cone

fissural volcanism polygenetic volcanic fields and crustal thickness in the payen volcanic complex on the central andes foreland mendoza argentina shield volcanoes caldera bearing stratovolcanoes and monogenetic cones compose the large fissural payen volcanic complex located in the andes foreland between latitude 35 s and 38 s the late pliocene pleistocene and recent volcanic activity along e w trending eruptive fissures produced basaltic lavas showing a within plate geochemical signature the spatial distribution of fractures and monogenetic vents is characterized by self similar clustering with well defined power law distributions vents have average spacing of 1 27 km and fractal exponent d 1 33 defined in the range 0 7 49 3 km the fractal exponent of fractures is 1 62 in the range 1 5 48 1 km the upper cutoffs of fractures and vent fractal distributions about 48 49 km scale to the crustal thickness in the area as derived from geophysical data this analysis determines fractured media crust thickness associated with basaltic retroarc eruptions we propose that the payen volcanic complex was and is still active under an e w crustal shortening regime

the december 1994 seismic swarm near aoba ambae volcano vanuatu and its relationship with the volcanic processes abstract a seismic swarm which occurred in december 1994 near the aoba volcano vanuatu is investigated in detail by analyzing local and regional broad band seismic records the seismic area is situated in a poorly instrumented region although major seismic and volcanic events are possible the data collected by four permanent short period stations the nearest one being installed in the espiritu santo island located 80 km from the volcano allowed us to locate epicenters with a 2 3 km precision and to document the present local seismicity the quality of available regional broadband data allows us to determine reliable focal parameters though the magnitude of the strongest event is 4 8 data have been selected from the permanent geoscope and iris stations located in the south western pacific region and from two temporary networks one installed in new caledonia and vanuatu by i r d and the e o s t and another one installed in fiji and tonga islands by the washington university st louis usa results obtained either with local epicentral distance d km body waves or regional 500 km surface waves lead us to conclude that the swarm is associated with the volcanic edifice but is not of volcanic type according to the definitions given by minakami phys volcanol 1974 1 and schick bull volcanol 44 1981 491 the observed mechanisms evidence an important local influence of the volcano itself on the stress field and the observed near horizontal p axis argues against a strong increase of phreatic activity in addition seismic and volcanic observations are analyzed in the framework of the interpretation of earthquake swarms occurring in the vicinity of active volcanoes

volcano tectonic earthquakes a new tool for estimating intrusive volumes and forecasting eruptions abstract we present data on 136 high frequency earthquakes and swarms termed volcano tectonic vt seismicity which preceded 111 eruptions at 83 volcanoes plus data on vt swarms that preceded intrusions at 21 other volcanoes we find that vt seismicity is usually the earliest reported seismic precursor for eruptions at volcanoes that have been dormant for decades or more and precedes eruptions of all magma types from basaltic to rhyolitic and all explosivities from vei 0 to ultraplinian vei 6 at such previously long dormant volcanoes because large eruptions occur most commonly during resumption of activity at long dormant volcanoes vt seismicity is an important precursor for the earth s most dangerous eruptions vt seismicity precedes all explosive eruptions of vei 5 and most if not all vei 4 eruptions in our data set surprisingly we find that the vt seismicity originates at distal locations on tectonic fault structures at distances of one or two to tens of kilometers laterally from the site of the eventual eruption and rarely if ever starts beneath the eruption site itself the distal vt swarms generally occur at depths almost equal to the horizontal distance of the swarm from the summit out to about 15 km distance beyond which hypocenter depths level out we summarize several important characteristics of this distal vt seismicity including swarm like nature onset days to years prior to the beginning of magmatic eruptions peaking of activity at the time of the initial eruption whether phreatic or magmatic and large non double couple component to focal mechanisms most importantly we show that the intruded magma volume can be simply estimated from the cumulative seismic moment of the vt seismicity from log10 v 0 77 log moment 5 32 with volume v in cubic meters and seismic moment in newton meters because the cumulative seismic moment can be approximated from the size of just the few largest events and is quite insensitive to precise locations the intruded magma volume can be quickly and easily estimated with few short period seismic stations notable cases in which distal vt events preceded eruptions at long dormant volcanoes include nevado del ruiz 1984 1985 pinatubo 1991 unzen 1989 1995 soufriere hills 1995 shishaldin 1989 1999 tacana 1985 1986 pacaya 1980 1984 rabaul 1994 and cotopaxi 2001 additional cases are recognized at frequently active volcanoes including popocateptl 2001 2003 and mauna loa 1984 we present four case studies pinatubo soufriere hills unzen and tacana in which we demonstrate the above mentioned vt characteristics prior to eruptions using regional data recorded by neic we recognized in near real time that a huge distal vt swarm was occurring deduced that a proportionately huge magmatic intrusion was taking place beneath the long dormant sulu range new britain island papua new guinea that it was likely to lead to eruptive activity and warned rabaul volcano observatory days before a phreatic eruption occurred this confirms the value of this technique for eruption forecasting we also present a counter example where we deduced that a vt swarm at volcan cosiguina nicaragua indicated a small intrusion insufficient to reach the surface and erupt finally we discuss limitations of the method and propose a mechanism by which this distal vt seismicity is triggered by magmatic intrusion

tectonic segmentation of the north andean margin impact of the carnegie ridge collision the north andean convergent margin is a region of intense crustal deformation with six great subduction earthquakes mw 7 8 this century the regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the andes segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction this segmentation is related to heterogeneity on the subducting nazca plate in particular the influence of the carnegie ridge collision is investigated four distinct seismotectonic regions can be distinguished region 1 from 6on to 2 5on with steep ese dipping subduction and a narrow volcanic arc region 2 from 2 5on to 1os showing an intermediate depth seismic gap and a broad volcanic arc region 3 from 1os to 2os with steep ne dipping subduction and a narrow volcanic arc region 4 south of 2os with flat subduction and no modern volcanic arc the carnegie ridge has been colliding with the margin since at least 2 ma based on examination of the basement uplift signal along trench parallel transects the subducted prolongation of carnegie ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed broad volcanic arc these findings conflict with previous tectonic models suggesting that the carnegie ridge entered the trench at 1 ma furthermore the anomalous geochemical adakitic signature of the volcanoes in the broad ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant shallowly subducting carnegie ridge from segments of steep subduction in regions 1 and 3 it is further suggested that carnegie ridge supports a local flat slab segment similar to that observed in peru the impact of the carnegie ridge collision on the upper plate causes transpressional deformation extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the san andreas fault system the pattern of instrumental and historical seismicity indicates 1 great earthquakes on the northern and southern flanks of the colliding ridge 2 a slight reduction in observed seismicity at the trench ridge intersection 3 increased stress far into the continent and 4 a nne displacement of the n andes block to be further effects of the collision 1999 elsevier science b v all rights reserved

rates of similar micro earthquakes comparison between campi flegrei southern italy and other volcanic areas abstract the precise relocation of micro earthquakes using waveform cross correlation is an efficient tool for defining the spatial characteristics of seismogenic structures in volcanic areas to be applied and to provide meaningful results the technique requires however the presence of sufficient amounts of similar events we examined about 870 earthquakes recorded at the campi flegrei cf caldera by a temporary digital network during the end of the 1982 1984 bradiseismic crisis the classification of these earthquakes indicates a surprising low amount of similar events especially according to the clustered aspect of the seismicity the relocation technique allows only to identify 3 elongated patterns that may be interpreted as small faults which are dipping inward toward the center of the caldera to determine up to which point this low similarity rate is unusual we examined the rates for several data sets coming from various volcanic areas hawaii mount pinatubo philippines and piton de la fournaise la reunion island the results show for all areas percentages of similar events significantly higher than for cf and indicate that the low rate observed there is not just a matter of low detection threshold or network characteristics at cf most of the seismicity appears to be diffused and generated along small fractures rather than along well defined fault structures the low similarity rate is in good agreement with a seismicity generated in a fractured medium by overpressure in the hydrothermal system and low deviatoric local stress

seismicity and stress tensor inversion in the central washington cascade mountains abstract tectonic stress in the pacific northwest washington is dominated by a n s major compressive axis 1 and a minor compressive axis 3 which varies from e w to near vertical some variations in this pattern occur in different parts of the region in this study we used 550 earthquakes in the central washington cascade mountains to study in detail the uniformity of the stress tensor in this volcanic arc earthquakes from the pacific northwest seismograph network pnsn catalog were divided into several subsets based on epicentral and depth groupings and stress tensor inversions using the gephart and forsyth technique were computed for each group as in previous similar studies the maximum compressive stress axis 1 is nearly horizontal and trending n s to nne ssw in all but one subset shallower events directly under mount rainier have a near vertical 1 for other subsets the minimum compressive stress axis 3 deviates from vertical to horizontal for different groups of events in particular events in the depth range of 10 to 14 km in the western rainier seismic zone wrsz have near vertical 3 direction whereas other depth ranges in this area show a near horizontal e w 3 orientation we hypothesize that the change in orientation of 3 in the 10 to 14 km depth range in the wrsz may be due to the influence of the nearby mount rainier magmatic system independent evidence for magma at this depth comes in the form of a few deep long period lp events

seismological significance of the 1977 1978 eruptions and the magma intrusion process of usu volcano hokkaido abstract an earthquake swarm and the major pumice eruptions in august 1977 which followed marked the start of the dacitic doming activity of usu volcano in southwestern hokkaido japan the sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data the distribution of the abundant hypocenters shows clearly an earthquake free zone beneath the summit crater the hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations considered due to magma intrusion into this earthquake free zone the earthquake mechanism solutions are mostly of dip slip type and are interpreted in terms of the doming deformations the existence of earthquake families earthquakes with similar waveforms is the main cause of the peculiar occurrence of earthquakes in space time and magnitude the concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm and the observed episodic deformations

quaternary state of stress in the northern andes and the restraining bend model for the ecuadorian andes inversion of shallow focal mechanisms in the northern andes together with a neotectonic analysis of the ecuadorian andes shows that the state of stress is homogeneous in most of the northern andes e w trending 1 however north of 5 n 1 is roughly nw se trending this difference in state of stress is due to the force balance between the nazca caribbean and south american plates south of 5 n the state of stress appears to be mainly controlled by the nazca south american plate interaction while north of 5 n it appears to be mainly controlled by the caribbean south american plate interaction this state of stress difference from south to north is consistent with dextral motions along faults trending parallel to the chain south of 5 n and sinistral motions along faults trending parallel to the chain north of 5 n this inversion of shallow focal mechanisms also illustrates the incomplete strain partitioning of the oblique convergence between the nazca and south american plates along the ecuadorian southern colombian trench the convergence obliquity that increases northward controls the active transcurrent fault slip rate in the upper plate finally this convergence obliquity is responsible for the location of the seismicity in the upper plate in addition the andean block is considered in this paper as a large triple zone under constriction with one of its borders caribbean behaving partly as a free border

helium concentrations and isotope compositions in 10 km deep groundwaters abstract helium isotopes are useful for tracing terrestrial water movement and calculating residence times which have important implications for radioactive waste disposal and carbon capture and storage projects helium concentrations are generally measured directly in water samples however this is not always possible in this scenario the he concentration equilibrium between pore waters and he accessible volumes e g fluid inclusions hereafter hav in minerals e g quartz can be utilized to determine he concentrations in this study the he partial pressure was measured in havs of quartz grains collected from cores of the kola super deep borehole ksdb using isothermal he extraction saturation experiments a large fraction of he was released from these samples during the first interval of isothermal heating indicating a destruction of the unstable havs likely due to decrepitation of fluid inclusions we present a new approach to calculate he partial pressures using only the he fraction released from the stable havs this approach gives a he partial pressure phe of 1 5 atm at depths of 10 km when combined with solubility data this value suggests a high he concentration in the deep pore waters nearby the ksdb 1 3 10 6 mol cc h2o 1 indicating a long residence time of he atoms in a deep water mineral system close to the age of the regional metamorphism 1700 ma the 3he 4he in the deep ksdb quartz samples and the pore waters are 3 10 8 typical of radiogenic he generated in 10 km deep archean rocks of the ksdb importantly external fluxes are not needed to explain he isotope abundances in rocks minerals and pore waters at these depths

underdamped responses of a well to nearby swarm earthquakes off the coast of ito city central japan 1995 we observed coseismic and noncoseismic water level changes of an underdamped well aquifer system at epicentral distances of 5 km during the 1995 earthquake swarm activity off the coast of ito city shizuoka prefecture central japan each water level change consists of a static offset with a linear trend component followed by a damped harmonic oscillation the characteristic frequency and decay parameter of these oscillations are 18 3 s and 0 024 s 1 respectively we detected 14 such events within 8 days during the activity the offset values are mostly 2 mm or less but there were two exceptionally large offsets of 4 and 6 mm the observed waveforms are consistent with an analytical expression for the theoretical well level response to step like pressure head change in an underdamped well aquifer system this consistency allowed us to estimate the parameters of the well aquifer system and the offset component of pressure head changes from the observed hydrographs the seismic records at the nearest station indicate that among the 14 events only seven including the two exceptionally large events were apparently synchronous with swarm earthquakes among these seven events the largest two are quantitatively explained as the consequence of the coseismic volumetric strain changes the other five are interpreted as either the effect of coseismic strain change near a node of the volumetric strain field or some other coseismic effects the seven well events not synchronous with earthquakes indicate the occurrence of abrupt local pressure disturbances in the aquifer within a distance of a few tens of meters from the well

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

the origin of volcano tectonic earthquake swarms volcano tectonic vt seismicity is commonly recorded prior to and during eruptions vt seismicity may reflect stresses induced by dike propagation as indicated by propagating hypocenters and fault plane solutions fps reflecting regional stresses or stresses induced by dike inflation indicated by randomly distributed hypocenters and fps with pressure axes rotated 90 to regional maximum compression although numerical models of dike induced stresses indicate that both regimes should occur during dike emplacement this has not yet been observed according to published seismic data instead 90 rotated fps are observed in some cases while propagating hypocenters mark dike formation in other cases we suggest that differences in the seismic expression of upper crustal magma migration may result from differences in the regional tectonic setting and in the nature of magma wall rock interactions ultimately it may be possible to use information contained in vt seismicity to forecast changes in a volcano s behavior by establishing the characteristic stress field response for a given volcano or through a deeper understanding of the complex relationships between vt seismicity local crustal stresses and the physical mechanisms of magma migration

multiple seismogenic processes for high frequency earthquakes at katmai national park alaska evidence from stress tensor inversions of fault plane solutions the volcanological significance of seismicity within katmai national park has been debated since the first seismograph was installed in 1963 in part because katmai seismicity consists almost entirely of high frequency earthquakes that can be caused by a wide range of processes i investigate this issue by determining 140 well constrained first motion fault plane solutions for shallow depth 9 km earthquakes occurring between 1995 and 2001 and inverting these solutions for the stress tensor in different regions within the park earthquakes removed by several kilometers from the volcanic axis occur in a stress field characterized by horizontally oriented 1 and 3 axes with 1 rotated slightly 12 relative to the nuvel1a subduction vector indicating that these earthquakes are occurring in response to regional tectonic forces on the other hand stress tensors for earthquake clusters beneath several katmai cluster volcanoes have vertically oriented 1 axes indicating that these events are occurring in response to local not regional processes at martin mageik vertically oriented 1 is most consistent with failure under edifice loading conditions in conjunction with localized pore pressure increases associated with hydrothermal circulation cells at trident novarupta it is consistent with a number of possible models including occurrence along fractures formed during the 1912 eruption that now serve as horizontal conduits for migrating fluids and or volatiles from nearby degassing and cooling magma bodies at mount katmai it is most consistent with continued seismicity along ring fracture systems created in the 1912 eruption perhaps enhanced by circulating hydrothermal fluids and or seepage from the caldera filling lake manuscript received 25 april 2002

precursory swarms of long period events at redoubt volcano 1989 1990 alaska their origin and use as a forecasting tool abstract during the eruption of redoubt volcano from december 1989 through april 1990 the alaska volcano observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long period lp seismic events dominant period of about 0 5 s the initial eruption on december 14 occurred after 23 years of quiescence and was heralded by a 23 hour swarm of lp events that ended abruptly with the eruption after a series of vent clearing explosions over the next few days dome growth began on december 21 another swarm with lp events similar to those of the first began on the 26th and ended in a major tephra eruption on january 2 eruptions continued over the next two weeks and then ceased until february 15 when a large eruption initiated a long phase of repetitive dome building and dome destroying episodes that continued into april warnings were issued before the major events on december 14 and january 2 but as the eruptive sequence continued after january 2 the energy of the swarms decreased and forecasting became more difficult a significant but less intense swarm preceded the february 15 eruption which was not forecast this eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until march 4 when the first of three new stations was installed within 3 km of the active vent from march 4 to the end of the sequence on april 21 there were eight eruptions six of which were preceded by detectable swarms of lp events although weak these swarms provided the basis for warnings issued before the eruptions on march 23 and april 6 the initial swarm on december 13 had the following features 1 short duration 23 hours 2 a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm followed by a decline of activity during the 5 hours preceding the eruption 3 a magnitude range from 0 4 to 1 6 4 nearly identical lp signatures with a dominant period near 0 5 s 5 dilatational first motions everywhere and 6 a stationary source location at a depth of 1 4 km beneath the crater this occurrence of long period events suggests a model involving the interaction of magma with groundwater in which magmatic gases steam and water drive a fixed conduit at a stationary point throughout the swarm the initiation of that sequence of events is analogous to the failure of a pressure relief valve connecting a lower supercharged magma dominated reservoir to a shallow hydrothermal system a three dimensional model of a vibrating fluid filled crack recently developed by chouet is found to be compatible with the seismic data and yields the following parameters for the lp source crack length 280 380 m crack width 140 190 m crack thickness 0 05 0 20 m crack stiffness 100 200 sound speed of fluid 0 8 1 3 km s compressional wave speed of rock 5 1 km s density ratio of fluid to rock 0 4 and ratio of bulk modulus of fluid to rigidity of rock 0 03 0 07 the fluid filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0 4 to 40 bar such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack

structural controls of volcanism in transversal chains resheared faults and neotectonics in the cordon caulle puyehue area 40 5 s southern andes abstract the nature of the relationship between tectonics and volcanism in a specific area is the result of the interaction between the crustal structures of the basement and ongoing regional stress field in the southern andes different kinematic environments represented by two nearly orthogonal active fissure systems show apparently incompatible arrays of volcanic chains moreover a local quaternary compressive regime is inferred from basement faults however these mutually orthogonal chains which are also oblique to the volcanic front have contrasting magma evolution and eruptive styles that can be related with their tectonic setting whereas northeast trending monogenetic cones carran los venados can be interpreted as resulting from magma upwelling along regional tension cracks in a quaternary dextral transpressive compressive domain northwest striking fissure system cordon caulle volcanic complex can be the result of extensional fault reactivation and long magma residence in the upper crust different kinematic environments cause contrasting magma evolution and eruptive styles a two way coupling mechanism is suggested for to explain local depart of the regional stress field geometric array of extensional features and final magmatic evolution in these volcanic domains of the southern andes

shallow seismogenic zone detected from an offshore onshore temporary seismic network in the esmeraldas area northern ecuador for a given site many factors control the seismic risk earthquake magnitude hypocentral distance rupture mechanism site effects and site vulnerability are among the most important this article deals with one of these factors the depth of the seismogenic zone in the northern ecuadorian subduction system beneath a highly vulnerable site the city of esmeraldas and its industrial complex the ecuadorian oil refinery and shipping terminal to address this problem we analyzed data from a three weeks passive seismological experiment conducted in the spring of 1998 using 13 ocean bottom seismometers and 10 portable land stations a preliminary interpretation of wide angle data obtained in the fall of 2000 in the manta area 100 km south of the study area unambiguously indicates the presence of a velocity inversion in the ecuadorian margin velocity structure this velocity inversion is characterized by a shadow zone of 1 s on the record sections and is interpreted as the result of a velocity contrast between the upper plate structure and the sedimentary and basaltic layer ii of the subducted oceanic nazca plate one dimensional velocity models are deduced from these wide angle data and are used for earthquake location in the esmeraldas area this highly improved the hypocentral parameter determinations the updip limit of the seismogenic zone is found at a depth of 12 km 35 km eastward of the trench and the depth of the seismogenic zone below the esmeraldas city is found at 20 km this shallow depth of the seismogenic zone dramatically increases the seismic hazard of the area

rise and emplacement of magma during horizontal shortening of the brittle crust insights from experimental modeling 1 magmatic activity tends to concentrate at plate margins at divergent margins extensional tectonics provide steep conduits for magma to reach the surface at rapidly convergent margins such as the andes one might imagine that horizontal compression prevents the rise of magma nevertheless volcanoes are also common in order to study the mechanisms by which magma rises in a compressional context we resorted to laboratory experiments in which a brittle crust was shortened while magma was intruding our model materials were 1 cohesive fine grained silica powder representing brittle crust and 2 molten low viscosity vegetable oil representing magma in general horizontal shortening and injection were coeval but independent processes thrust faults accommodated the shortening while overpressured oil formed hydraulic fractures in those experiments where there was no shortening injection resulted in a saucer shaped intrusive body in the other experiments where there was shortening oil formed a basal sill before rising along thrust faults once in place the sill lubricated the base of the model so that arcuate thrusts formed at the leading edge of a plateau uplift of the plateau promoted further intrusion of oil at depth in general the pattern of deformation and intrusion depended on the kinematic ratio r between rates of shortening and injection the lengths of the basal sill and plateau increased with decreasing r on the basis of these results we have reexamined two natural examples of magmatic complexes which were emplaced in compressional tectonic settings tromen volcano in argentina and the boulder batholith of montana

stress tensor and fault plane solutions for a population of earthquakes an algorithm for the simultaneous estimation of the orientation and shape of the stress tensor and the individual fault plane solutions for a population of earthquakes is studied it corresponds to a synthesis of the methods used by brillinger et al 1980 to obtain focal mechanisms and by armijo and cisternas 1978 for stress tensor analysis in microtectonics the input data are the polarities of the p arrival and take off angles for the set of source station pairs the method distinguishes in general which one of the nodal planes corresponds to the fault and gives the direction of the slip the application to the aftershock sequence of the 1980 arudy earthquake western pyrenees shows that the observations may be explained by a single stress tensor producing a n32 e extension with a likelihood of 95 per cent

seismic evolution of the 1989 1990 eruption sequence of redoubt volcano alaska abstract redoubt volcano in south central alaska erupted between december 1989 and june 1990 in a sequence of events characterized by large tephra eruptions pyroclastic flows lahars and debris flows and episodes of dome growth the eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater notable features of the eruption seismicity include 1 small long period events beginning in september 1989 which increased slowly in number during november and early december 2 an intense swarm of long period events which preceded the initial eruptions on december 14 by 23 hours 3 shallow swarms 0 to 3 km of volcano tectonic events following each eruption on december 15 4 a persistent cluster of deep 6 to 10 km volcano tectonic earthquakes initiated by the eruptions on december 15 which continued throughout and beyond the eruption 5 an intense swarm of long period events which preceded the eruptions on january 2 and 6 nine additional intervals of increased long period seismicity each of which preceded a tephra eruption hypocenters of volcano tectonic earthquakes suggest the presence of a magma source region at 6 10 km depth earthquakes at these depths were initiated by the tephra eruptions on december 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths the locations and time history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989 1990 eruption this source region appears to be connected to the surface by a narrow pipe like conduit as inferred from the hypocenters of volcano tectonic earthquakes concentrations of shallow volcano tectonic earthquakes followed each of the tephra eruptions on december 15 these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths this shallow zone was the source area of the majority of long period seismicity through the remainder of the eruption the long period seismicity likely reflects the pressurization of the shallow portions of the magmatic system

active tectonics in the lesser caucasus coexistence of compressive and extensional structures detailed spot image analysis which completed field data and a microseismicity study was done on the central caucasian mountain belt georgia armenia and eastern turkey north of the front of the arabian collision in order to clarify the relationships between compression extension and volcanism in fact this region is characterized by relatively complex active tectonics associating both n s compressive e w thrusts and folds and e w extensional n s normal faults and dikes structures accompanied by considerable neogene to quaternary volcanism and ne sw left lateral and nw se right lateral strike slip faults these different structures are all intricately imbricated and can be observed at different scales the general layout implies an important variation in the stress state for instance this passes from a n s compression in the spitak earthquake fault zone to an e w extension at the aboul samsar volcanic axis these results agree well with the n s convergence between arabia and eurasia and the e w lateral expulsion of the anatolian and iranian blocks a detailed cartographic network of active structures was drawn and their kinematic relationships were specified in places recent offsets on strike slip faults were estimated at about 500 to 1000 m suitable sites for future determination of fault velocity displacements were also selected this study shows that some strike slip faults behave partly like faults which transform the e w extension of the n s striking normal faults into a n s compression along the e w thrusts one of the characteristics of this region is the continuous important volcanic activity at least from the jurassic until now this persistence evidences a lithospheric thinning which remained in spite of the recent arabian eurasian collision because of the e w extension linked to the opposite lateral expulsion of the anatolian and iranian blocks this geodynamic evolution can explain the juxtaposition and superimposition of volcanic structures and thrusts in a general compressive context

structure and evolution of volcanic plumbing systems in fold and thrust belts a case study of the cerro negro de tricao malal neuquen province argentina magma ascent and emplacement in compressional tectonic settings remain poorly understood geophysical studies show that volcanic plumbing systems in compressional environments are vertically partitioned into a deep level subject to regional compression and a shallow level subject to local extension such vertical partitioning has also been documented for the plumbing systems of mud volcanoes implying common yet unresolved underlying processes in order to better constrain the mechanisms governing this depth partitioning of emplacement mechanisms we studied the structure and evolution of the cerro negro intrusive complex emplaced in the chos malal fold and thrust belt in the foothills of the neuquen andes argentina the cerro negro intrusive complex consists of sills and n s striking dikes that crosscut the sills the most prominent structures in the study area are n s trending folds and both e and w vergent thrusts we provide new u pb ages of 11 63 0 20 ma and 11 58 0 18 ma for sills and 11 55 0 06 ma for a dike which show that the cerro negro intrusive complex was emplaced in a short period of time our ages and field observations demonstrate that the emplacement of the cerro negro intrusive complex was coeval with the tectonic development of the chos malal fold and thrust belt this implies that the dikes were emplaced perpendicular to the main shortening direction the systematic locations of the dikes at the anticlinal hinges suggest that their emplacement was controlled by local shallow stresses related to outer arc stretching at the anticlinal hinge we conclude that folding related outer arc stretching is one mechanism responsible for the vertical partitioning of igneous plumbing systems in compressional tectonic settings

hydrothermal and magmatic reservoirs at lazufre volcanic area revealed by a high resolution seismic noise tomography we determine here for the first time the geometry and location of the hydrothermal and magmatic reservoirs in the lazufre volcanic area this furthers the understanding of the origin of one of the largest worldwide volcanic uplift regions both in space and amplitude the exact locations and shapes of the sources generating a double wide uplift region in the lazufre found by past deformation data insar and gps and generating hydrothermal and magmatic fluids found by geochemical gas analysis have not been well delimited in this study we use seismological data to perform a 3 d high resolution s wave velocity model which allows defining better the locations and shapes of the sources of the deformations and the hydrothermal and magmatic reservoirs we find three anomalies two of them with s wave velocity of about 1 2 1 8 km s are located below the lastarria volcano the shallow one 1 km below the volcano base has a funnel like shape the deeper one is located between a depth of 3 and 6 km below the volcano base both are strongly elliptical in an ew direction and separated by a 2 3 km thick zone with vs of 1 5 2 km s as far as these anomalies are located under the hydrothermal activity of lastarria volcano they are interpreted as a double hydrothermal the shallow part and magmatic source the deeper part the latter can feed the former this double hydrothermal and magmatic source is in agreement with previous geochemical deformation gps and insar and magneto telluric studies in particular it explains the double origin of the gases hydrothermal and magmatic the third low velocity zone with s wave velocity of about 2 3 km s located at 5 km depth and deeper is centered beneath an area of surface uplift as determined by insar data we compare the seismic tomography and insar results to propose that this low velocity zone is at the top of a large reservoir hosting hydrothermal fluids and possibly also magma

crust upper mantle seismic velocity structure across southeastern china abstract one in line wide angle seismic profile was conducted in 1990 in the course of the southeastern china continental dynamics project aimed at the study of the contact between the cathaysia block and the yangtze block this 380 km long profile extended in nw se direction from tunxi anhui province to wenzhou zhejiang province five in line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line we have used two dimensional ray tracing to model p and s wave arrivals and provide constraints on the velocity structure of the upper crust middle crust lower crust moho discontinuity and the top part of the lithospheric mantle p wave velocity s wave velocity and v p v s ratio are mapped the crust is 36 km thick on average albeit it gradually thins from the northwest end to the southeast end offshore of the profile the average crustal velocity is 6 26 km s for p waves but 3 6 km s for s waves a relatively narrow low velocity layer of about 4 km of thickness with p and s wave velocities of 6 2 km s and 3 5 km s respectively marks the bottom of the middle crust at a depth of 23 km northwest and 17 km southeast at the crust mantle transition the p and s wave velocity change quickly from 7 4 to 7 8 km s northwest and 8 0 to 8 2 km s southeast and from 3 9 to 4 2 km s northwest and 3 9 to 4 5 km s southeast respectively this result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately the average v p v s ratio ranges from 1 68 1 8 for the upper crust to 1 75 for the middle and 1 75 1 85 for lower crust with the interpretation of the wide angle seismic data jiangshan shaoxin fault is considered as the boundary between the yangtze and the cathaysia block

temporal and spatial variation of the stress state of popocatepetl volcano mexico abstract the fault plane solutions fps of volcano tectonic events vts can help in the determination of the stress state of a volcano before during and after an eruptive period hence for popocatepetl we have calculated the fps from 1996 to 2003 and we have compared the results with its volcanic activity for popocatepetl there are two main areas of vts one below the crater and another in the southeastern zone for the below crater we have 398 solutions and for the southeastern zone 64 the fps below the crater have pressure axes p with a preferential orientation of s70 w n70 e which is consistent with the regional stress state for the tension axes t a radial tension is observed although a great amount of axes exists with a direction of nw se most of the fps are normal solutions and these are present at depths mainly between 0 and 3 km asl although we have periods where an increase is observed in the number of reverse events approximately between 3 and 0 km asl the variability of the solutions with depth is probably associated with the intrusion of magma inside the volcano the group of events located in the southeastern part of the volcano have pressure axes p with directions of s n and w e and the same orientations are observed for the tension axes t most of the mechanisms in this area are strike slip solutions which follow two possible faults which were activated during the early magma intrusions inside of the volcano

multifractal dimension and b value analysis of the 1998 1999 quito swarm related to guagua pichincha volcano activity ecuador 1 temporal variations of spatial multifractal dimensions of quito swarm seismicity ecuador are related to the august 1998 to december 1999 guagua pichincha volcanic activity multifractal dimensions decrease a few days before the five main seismic energy peaks and increase again before or just after these peaks this behavior reveals a self organization of earthquakes that precedes main seismic energy peaks the quito swarm is also characterized by a bimodal gutenberg richter law with two b values b1 0 95 0 15 and b2 1 48 0 15 this bimodality may reveal the superposition of two different processes one related to a classical elastic rupture and the other related to hydraulic fracture leading to high b values resulting from magma and overpressurized groundwater movements groundwater expulsion may be driven by heat released from deeper upgoing magma and manifested on the surface by the occurrence of phreatic explosions beginning 7 august 1998 and by the formation of eight dacitic lava domes beginning 25 september 1999