imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

magma storage and ascent at lipari island aeolian archipelago southern italy at 223 81 ka the role of crustal processes and tectonic influence fluid inclusion studies together with volcanological and petrochemical data allow reconstruction of the magma feeding system of basaltic andesitic to andesitic activity during the oldest and intermediate stages of development of lipari island 223 81 ka a major magma storage zone is active during the overall investigated time span at depths of 22 km close to the crust mantle moho transition at which mantle derived mafic magmas tend to accumulate due to neutral buoyancy conditions beneath central type volcanoes m mazzacaruso m s angelo m chirica costa d agosto a shallower magma reservoir is located within the upper crust at 5 5 3 5 km associated with a major lithological discontinuity for fissural type volcanoes timpone ospedale monterosa m chirica tectonic structures are suggested to influence further magma ascent and storage at mid crustal depths 14 km with no ponding at shallower levels partial crustal melting processes at the roofs of the deep magma reservoirs 17 km are invoked to explain the origin of cordierite bearing lavas beneath m s angelo and m chirica costa d agosto volcanoes which were active during the intermediate stages of development of lipari 105 81 ka the generation of felsic anatectic melts in the lower crust could have created density and rheologic barriers to impede the passage of mafic melts and promote their ponding with influence on the subsequent evolution of lipari volcano

seismic imaging of arc magma and fluids under the central part of northeastern japan abstract a dense seismic network was temporarily deployed during 1997 1999 in the central part of northeastern japan by japanese university research groups using the local earthquake arrival time data recorded by the network we have determined high resolution three dimensional images of p wave velocity vp s wave velocity vs and vp vs ratio in this area our results show that low vp low vs and high vp vs areas are extensively distributed in the uppermost mantle along the volcanic front in the lower crust low vp low vs and high vp vs areas are visible but they are confined to the individual volcanic areas in contrast the upper crust of volcanic areas shows low vp low vs and low vp vs rather than high vp vs these observations suggest that partial melting materials exist in the uppermost mantle along the volcanic front and they spread up to the midcrust of the volcanic areas low vp low vs and low vp vs in the upper crust of the volcanic areas suggest the presence of h2o rather than melt there deep low frequency microearthquakes perhaps caused by the rapid movement of fluids are located mostly at the edge of partial melting zones particularly at their upper surface distinct s wave reflectors bright spots with large reflection coefficients are distributed in the midcrust with slightly low vp vs suggesting that most of them are filled with h2o

shear wave anisotropy and stress direction in and near long valley caldera california 1979 1988 we observed shear wave splitting birefringence for two temporary surface deployments of three component digital seismographs that were in place before and after m 6 earthquakes near the long valley caldera california in one of these deployments the data sample precedes the large events of the may 1980 mammoth lakes earthquake sequence by 6 months and cover the two most active months of the may 1980 aftershock sequence one of the stations wit from this deployment was reoccupied with identical instrumentation in 1988 another deployment preceded the 1986 chalfant valley mainshock by 2 days and recorded events for 6 days the polarization of the faster shear wave changes from n30 w at mammoth to due north at chalfant valley and in both cases is parallel to the strike of nearby surface faults and to the mean direction of p axes determined from focal mechanism groupings observations from nearby stations yield fast directions nearly at 23 from each other and time separations of fast and slow shear waves show considerable station to station variation showing no correlation with earthquake station distance or earthquake depth these observations suggest that the observed anisotropy results primarily from near station presumably shallow effects rather than from widespread aligned microcracks the records at long valley station wit show a slight variation of average fast shear wave polarization from 1979 to 1980 but with the limited amount of data available this difference is not statistically significant time separation measurements of up to 0 48 s at station wit yield estimates of 15 30 velocity anisotropy among the highest reported for local earthquakes secular variation in the time separation of fast and slow shear waves between events recorded at station wit in 1979 and 1980 is swamped by the scatter in the data if it is present the five events recorded in 1988 showed similar s wave polarizations and time separations to those in 1980 but there are not enough events for a statistical comparison unlocated events at long valley station mrs show a slight increase in time separation after the may 1980 m 6 0 earthquakes but the increase is not statistically significant

measurement and modeling of ablation of the bottom of supraglacial lakes in western greenland 1 we report measurements of ablation rates of the bottom of two supraglacial lakes and of temperatures at different depths collected during the summers of 2010 and 2011 in west greenland to our knowledge this is the first time that such data sets are reported and discussed in the literature the measured ablation rates at the bottom of the two lakes are of the order of 6 cm day versus a rate of 2 5 3 cm day in the case of bare ice of surrounding areas though our measurements suggest the presence of a vertical temperature gradient it is not possible to draw final conclusions as the measured gradient is smaller than the accuracy of our temperature sensors in situ measurements are compared with the results of a thermodynamic model forced with the outputs of a regional climate model in general the model is able to satisfactorily reproduce the measured quantities with rmse of the order of 3 4 cm for the ablation and 1 5 c in the case of water temperature our results confirm that the ablation at the bottom of supraglacial lakes plays an important role on the overall lake volume with the ablation in the case of ice covered by a lake being 110 135 of that over bare ice at nearby locations beside ice sheet hydrological implications melting at the bottom of a supraglacial lake might affect estimates of lake volume from spaceborne visible and near infrared measurements

deep low frequency microearthquakes in or around seismic low velocity zones beneath active volcanoes in northeastern japan abstract precise hypocenter relocations have been made for shallow intraplate microearthquakes beneath northeastern japan by applying source region station corrections the relocated hypocenter distribution shows that most of the events are confined to the upper 15 km of the crust which forms the brittle seismogenic zone in this volcanic arc it also shows that exceptionally deep microearthquakes well below the base of the seismic aseismic boundary occur at twelve locations of northeastern japan they occur in the lowermost crust or in the uppermost mantle where rheological properties of rocks are presumed to be in the ductile regime all the 153 events presently detected in the lowermost crust or in the uppermost mantle have anomalously low predominant frequencies 1 5 5 hz both for p and s waves in contrast to those 8 20 hz of normal shallow events in the brittle seismogenic zone the magnitudes of these events are small m 2 2 moreover all of these events are located beneath active volcanoes and or near p wave low velocity zones in the uppermost mantle the proximity of these events to the active volcanoes and other anomalous features suggest that these deep low frequency events are generated by magmatic activity of mantle diapirs in the uppermost mantle

crustal structure beneath aso caldera southwest japan as derived from receiver function analysis abstract aso volcano experienced a huge pyroclastic eruption 90 thousand years ago and formed a large caldera 18 km 25 km in order to test the hypothesis of a magma body in the mid and lower crust that has been suggested geophysically and geochemically we investigated seismic velocity discontinuities and velocity structure beneath aso caldera using receiver functions and a genetic algorithm inversion we confirm the existence of the moho at depths between 30 km and 35 km and a large velocity anomaly should exist in the deep portion of the crust beneath aso caldera from imaging of receiver functions observed only at stations outside the caldera as a result of a more detailed examination with ga inversion a low velocity layer is detected at depths between 10 km and 24 km beneath the western part of the caldera s wave velocity of the layer is estimated to be 2 0 2 4 km s we estimate that the low velocity layer contains at most 15 melt or 30 aqueous fluid the layer exists near the conrad and at the same depths as the swarm of the low frequency earthquakes and a compressional and dilatational deformation source which are expected to be caused by fluid movement beneath the middle eastern part of the caldera fluid contained in the layer might be related with huge pyroclastic eruptions of aso volcano

automatic measurement of shear wave splitting and applications to time varying anisotropy at mount ruapehu volcano new zealand 1 we present an automatic shear wave splitting measurement tool for local earthquakes with the sole manual step of choosing an s arrival time we apply the technique to three data sets recorded on mount ruapehu volcano in new zealand that have previously been determined to have fast polarizations that vary in time and with earthquake depth the technique uses an eigenvalue minimization technique applied over multiple measurement windows the dominant period of each waveform sets minimum and maximum window lengths cluster analysis determines the best solution among all the windows and quality grading criteria assess the results automatically when the same filters are used for events determined to be high quality from manual studies the automatic technique returns virtually identical results scatter increases when the automatic technique chooses the best filter but the average automatic results remain consistent with the manual results when the automatic technique is used on sets that include data previously judged as poor quality some stations yield distributions of fast polarizations that include peaks that were not present in previously published results the difference may stem from two factors automatic grading lets through some measurements that independent analysts consider poor quality but also unconscious bias in the manual selection process may downgrade measurements that do not fit expectations nonetheless the new objective analysis confirms changes in the average fast polarizations between 1994 and 2002 and between shallow and deep events therefore this new technique is valuable for objective assessment of anisotropy and its variation in time

three dimensional structure of vp vs and vp vs beneath northeastern japan implications for arc magmatism and fluids we estimated both p v p and s wave velocity v s structures beneath northeastern japan by applying a tomographic method to 169 712 p and 103 993 s wave arrival time data from 4338 local events the average value of v p v s ratio is 1 69 in the upper crust 1 75 in the lower crust and 1 77 in the uppermost mantle these differences in v p v s ratio may mainly reflect the lithological variations with depth low v p low v s and high v p v s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge here the low v p low v s and high v p v s values are interpreted because of partial melting materials which imply the presence of a vast amount of melt in the uppermost mantle in the lower crust low v p low v s and high v p v s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas which suggests that melting of lower crustal materials occurs just beneath each active volcano in contrast the upper crust beneath active volcanoes exhibits low v p low v s and low v p v s these results suggest that the partial melting zones under northeastern japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes the low v p low v s and low v p v s features in the upper crust suggest the existence of h 2 o rather than melt right beneath active volcanoes

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

local shear wave observations in joao câmara northeast brazil 1 we present shear wave splitting data from a 10 km aperture eight station digital three component seismograph network which operated from december 1992 to august 1994 near joao c mara 5 33 s 35 51 w to record shear waves from continuing microearthquake activity persisting 6 years after a main shock in november 1986 previous aftershock recording had shown remarkably impulsive simple seismograms and hypocentral errors of typically 200 m in a structure of near uniform wave speed but only one station was three component the pattern of polarization directions shown is consistent with the trend of precambrian foliation observed in the field it is therefore concluded that these shear wave observations are controlled by the seismic anisotropy associated with the precambrian foliation and hence bear the signature of the paleo stress field i e the precambrian deformation regime this conclusion has important implications for the interpretation of shear wave splitting observations made in crustal crystalline rock because other authors have asserted that shear wave splitting normally results from the effect of stress aligned fluid filled cracks extensive dilatancy anisotropy or eda eda is by contrast a signature of the present day stress field in this region there is firm evidence that the present day maximum compressive stress is e w this evidence comes from earthquake focal mechanisms and borehole breakout data and is summarized in the world stress map we therefore conclude that our present data are in maximum conflict with an interpretation in terms of eda our interpretation is supported by the observation that delay times are consistent with those expected from modeling of elastic waves in the crust

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

shear wave splitting vp vs and gps during a time of enhanced activity at aso caldera kyushu 1 we performed shear wave splitting analyses and calculated vp vs ratios on local earthquakes recorded at seven stations around aso volcano japan between 2001 and 2008 the mean vp vs of 1 61 0 01 from all stations is among the lowest average values found anywhere in the world a temporal change of shear wave splitting fast polarization is observed at one station additionally spatial variations of shear wave splitting delay times and vp vs ratios show a correlation at three stations we suggest that stress controlled anisotropy dominates at these stations gps baselines and 2d strain reveal extension in the caldera in 2003 and compression before and after 2003 strain shortening axes indicate either a variation in regional stress or a local stress source that overprints the regional field in 2003 baseline changes correlate with periods of increased low frequency seismic activity located deeper than 15 km compaction and degassing of a previously reported sill at approximately 15 km depth could lead to constantly high pressure of volcanic gases over a wide area and explain the generally low vp vs ratios and the observed compression in the caldera we propose that a small volume of magma intruded into the sill in 2003 caused extension at the surface and triggered ash eruptions between 2003 and 2005 associated localized migration of gases in overpressured cracks with aspect ratios potentially 0 5 in the upper crust with porosities 0 1 could be the source of changes in seismic anisotropy and explain the slight variations observed in vp vs

chemical monitoring of volcanic gas using remote ft ir spectroscopy at several active volcanoes in japan abstract chemical compositions of volcanic gases of several japanese active volcanoes have been monitored from distant safe places since the beginning of the 1990s using an ft ir spectral radiometer for absorption measurements an infrared light source behind volcanic gas emissions is necessary in a volcanic environment in the early observations infrared radiation from hot lava domes unzen volcano and hot ground heated by high temperature fumaroles usu aso and satsuma iwojima volcanoes were used as infrared light sources however these sources were not available in many cases this remote ft ir method became more commonly applied to chemical monitoring of volcanic gases emitted from the summit or slopes of active volcanoes using scattered solar infrared light as infrared light sources sakurajima miyakejima and asama volcanoes to date eight species have been measured using this method so2 hcl hf co co2 cos sif4 and h2o the observations indicate that volcanic gases for each volcano have different chemical composition on a so2 hcl hf ternary diagram in spite of similar tectonic settings suggesting that vapor melt volume ratios during volcanic gas formation differ among volcanoes during more than 15 years of monitoring chemical changes in volcanic gases attributable to ascent of magma were observed only at asama where hcl so2 and hf hcl ratios in the eruptive period were higher than those in non eruptive period because of scrubbing of more soluble components in surface hydrothermal systems in the non eruptive stage or solubility controlled fractionation processes results show that these parameters are the most prospective ones among the various parameters measured using the remote ft ir method to monitor volcanic activities

evidence for water filled cracks in earthquake source regions 1 we identify lowered vp vs ratios near earthquake source regions in southern california using observations from a seismic tomography model and high resolution local vp vs estimates using waveform cross correlation data from within similar event clusters the median tomographic vp vs ratio is 1 716 0 008 at all the relocated crustal earthquake locations compared to the background median value of 1 729 0 007 for the tomography model although the error estimates overlap slightly the median in situ vp vs ratio of 1 673 0 022 within the similar event clusters suggests that tomographic studies are overestimating vp vs at source regions interpretation of vp vs anomalies is complicated by the scatter in values obtained for individual clusters and in comparisons to absolute vp and vs velocities in the tomography model however the low vp vs ratios measured for the seismicity clusters are hard to explain with known rocks and suggest the presence of water filled cracks with several percent porosity in earthquake source regions in southern california which likely has an effect on faulting and earthquake activity

overpressure detection from compressional and shear wave data by analyzing experimental data we show that in many room dry rocks the poisson s ratio pr decreases with decreasing differential pressure confining minus pore pressure this means that in gas saturated rocks pr decreases with increasing pore pressure we confirm the generality of the observed effect by theoretically reproducing it via effective medium modeling this effect can be used as a new tool for overpressure detection from surface seismic cross well sonic logs and measurements ahead of the drill bit

deep low frequency earthquake swarm in the mid crust beneath mount fuji japan in 2000 and 2001 beneath mount fuji the highest active volcano in japan deep low frequency dlf earthquake activity has been monitored since the early 1980s the dlf earthquakes occurred in the mid crustal depth range and burst type activity lasting from several minutes to 30 min was detected 10 to 20 times in an ordinary year the dlf earthquake activity increased sharply in the period from october 2000 to may 2001 showing swarm like activity the occurrence rate during the dlf earthquake swarm was approximately 20 times higher than the usual activity and the wave energy released during the swarm period was twice as high as the total wave energy during the past 20 years the dlf earthquakes in the period from 1987 to 2001 were relocated by estimating station corrections in order to reduce the effect of the change of seismic station distribution the epicenters of most dlf earthquakes occurred in an elongated region with a long axis of about 5 km whose center is located 2 3 km ne from the summit a few percent of the dlf earthquakes however occurred around the summit area significantly apart from the main epicenter region the focal depths of well located dlf events range from 10 to 20 km during the high activity period in 2000 and 2001 most dlf events occurred within this main hypocenter area the sharp increase of dlf earthquake activity at mount fuji started immediately after magma discharge and intrusion events in the miyake jima and kozu shima regions in july and august 2000 the tectonic and volcanic activity changes around the area suggest that the dlf earthquake swarm at mount fuji was triggered by the change of state of the deep magmatic system around mount fuji

shear wave polarization anisotropy observed in a rift zone in japan abstract we have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in central kyushu japan we have recognized two different directions in the polarization of the faster shear waves about e w directions at three stations located in the rift zone and about n s at two stations possibly located outside and close to the edge of the rift zone the former case e w is consistent with a hypothesis that shear wave splitting is caused by vertical cracks or fractures striking in the e w direction which have formed under the present tectonic stress inside a rift zone extending in the n s direction with the maximum horizontal compressive stress striking in the direction about e w and the minimum compressive stress vertical in contrast the latter case n s is inconsistent with the above hypothesis this inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone alternatively it may result from a complicated stress regime within the crust close to the edge of the rift zone arrival time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0 12 s while those from the upper mantle events are less than about 0 25 s the relatively small arrival time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust

geological constraints on the 2003 2005 ash emissions from the nakadake crater lake aso volcano japan abstract the active crater of nakadake at aso volcano southwestern japan has been occupied by a lake during its dormant periods multiple ash emissions were observed from the crater lake between july 2003 and august 2005 the largest events occurred on 10 july 2003 and 14 january 2004 on 10 july 2003 ash 41 tons was dispersed throughout an area extending 14 km east northeast of the vent and on 14 january 2004 ash 32 tons extended 8 km to the east southeast thereafter small ash emissions were frequent at the crater lake in which the water level fell considerably from april to august 2005 during this period major ash emitting events producing mostly white aggregated ash occurred on june 10 12 june 21 4 2 tons and july 25 1 2 tons ash emissions at the nakadake crater in 2003 2005 were classified into three types gentle release of white aggregated ash from fumaroles inside the crater lake e g daytime of 14 april 10 12 june and 25 july 2005 emission of black ash from an almost dried up vent 21 june 2005 and short time ca 20 s small explosion triggered gas and ash emission through the crater lake 10 july 2003 14 january 2004 and 14 april 20h41m 2005 all products from these ash emissions consisted of fine grained

formation of caldera periphery faults an experimental study changing stresses in multi stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism s associated with caldera formation and the corresponding zones of structural weakness we evaluate characteristic structures resulting from doming chamber inflation evacuation collapse chamber deflation and cyclic resurgence inflation and deflation and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation the structural dome so produced is characterised bysteepening in the periphery whereas the broadening apex subsides pure evacuation causes the chamber roof to collapse along adjacent bell shaped reverse faults the distribution of concentric faults is influenced by the initial edifice morphology steep and irregular initial flanks result in a tilted or chaotic caldera floor the third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation following doming caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome the flanks of the edifice relax resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults the latter form locally uplifted and tiltedwedges half grabens that grade into horst and graben structures this superimposed fault pattern also extends inside the caldera we suggest that major pressure deviations in magma chamber s are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first order indication of the processes and mechanisms involved in caldera formation

hydrothermal system beneath aso volcano as inferred from self potential mapping and resistivity structure abstract we conducted self potential sp surveys sequentially from part to part over the central cones of aso volcano since august 1998 by december 2001 the compiled sp map revealed large sp anomalies on the central cones the main feature of the sp map is a w shaped profile along the ns transect over the central cones it is probable that this characteristic sp profile is produced by the combination of hydrothermal upwelling in the middle and topographic effect a positive anomaly showing a large concentric pattern has appeared after correcting the topographic effect to evaluate this sp anomaly we implemented a numerical code that calculates electric potential produced by arbitrarily positioned current sources and sinks in any three dimensional resistivity structure a layered structure obtained from a time domain electromagnetic tdem field experiment was used for the resistivity model the estimated current source is 300 a being located in a conductive layer around the sea level meanwhile sinks were estimated to sit on a circular area corresponding to the marginal part of the conductive layer water and heat budget study gives a lower limit of water mass transfer from depth to the bottom of the crater lake of nakadake this value was used to estimate the equivalent current in either case of electro kinetic ek mizutani h ishido t 1976 a new interpretation of magnetic field variation associated with the matsushiro earthquakes j geomag geoelectr 28 179 188 or rapid fluid disruption rfd process johnston m j s byerlee j d lockner d 2001 rapid fluid disruption a source for self potential anomalies on volcanoes j geophys res 106 b3 4327 4335 this comparison suggests that the former process is preferable to explain the observed sp anomaly from these results we infer a large scale hydrothermal system beneath the central cones of aso volcano in which the fluid flow initiates from the surrounding area converging to the central vent to transport the heat and materials up to the crater lake of nakadake through a vapor filled conduit

volcanic and structural history of the hohi volcanic zone central kyushu japan more than 5000 km3 of magmatic material was erupted in pliocene pleistocene times in a volcano tectonic depression i e the hohi volcanic zone hvz in central kyushu japan the eruptive deposits consist mainly of andesite lava flows and large scale pyroclastic flow deposits their eruptions were accompanied by the formation of an ew oriented graben 70 km 45 km under regional ns extensional stress pre tertiary basement rocks are absent on the surface of the graben but occur at depth having subsided up to 3 km radiometric ages of volcanic rocks on the surface show zoned isochrons from 5 ma at the margin to 0 3 ma in the center of the hvz the youngest center of age zonation coincides with a 30 mgal negative bouguer gravity anomaly radiometric ages of rocks from drill cores are older toward the bottom of the graben reaching a maximum of at least 4 ma volcanic activity concentrated over time toward the center of the graben and buried successively erupted material areas of active volcanism in the hvz became smaller and changed in style during the 5 ma history of activity volcanism of the early stage 5 2 ma was characterized by voluminous eruptions of andesitic lava flows that formed lava plateaus and were intruded by ew oriented feeder dikes perhaps related to fissure eruptions in contrast late stage volcanism 2 0 ma resulted primarily in andesitic to dacitic lava domes with features of monogenetic volcanoes produced at low eruption rates the hvz shows unimodal volcanism dominated by andesitic and dacitic lavas with a small amount of rhyolite and only traces of basalt these characteristics differ from those that typify volcanism in most other extensional areas erupted material in the hvz is of the calc alkali and high alkali tholeiite series and shows no significant chemical changes over 5 ma except for an increase in k2o after 1 6 ma the net horizontal displacement along normal faults indicates that the hvz widened by about 10 20 across the graben at an average rate of 0 1 cm yr i interpret the hvz to be neither a pull apart structure of the pre tertiary basement nor the result of propagation of the okinawa trough but rather the earliest stage of rifting when vertical subsidence caused by normal faulting is compensated by filling with volcanic material

continuous gps array and present day crustal deformation of japan a gps array with about 1 000 permanent stations is under operation in japan the gps array revealed coseismic deformations associated with large earthquakes and ongoing secular deformation in the japanese islands based on daily coordinate data of the gps stations strain rate distribution is estimated most regions with a large strain rate are related to plate boundaries and active volcanoes in addition the niigata kobe tectonic zone nktz is recognized as a region of large strain rate along the japan sea coast and in the northern chubu and kinki districts this newly found tectonic zone may be related to a hypothetical boundary between the eurasian or amurian and the okhotsk or north america plates precise observation of crustal deformation provides important boundary conditions on numerical modeling of earthquakes and other crustal activities appropriate computation methods of continuous deformation field are directly applicable to data assimilation for such numerical simulations

structure related and stress induced shear wave velocity anisotropy observations from microearthquakes near the calaveras fault in central california we attempt to distinguish between structure related and stress induced shear wave velocity anisotropy in the upper crust using microearthquakes m 2 6 that occurred before and after the tres pinos california earthquake m 5 5 of 26 january 1986 the tres pinos earthquake occurred on the quien sabe fault a northwest southeast trending right lateral strike slip fault approximately 10 km east of the calaveras fault in central california digital three component seismograms are available from 1984 to 1992 from the quien sabe ranch station hqr located only 7 3 km north northeast of the epicenter of the main event we studied polarization directions and delay times using a method that combines cross correlation techniques and particle motion plots to enhance the significance of our interpretations we reduced uncertainties related to the hypocenter location of the earthquakes by source relocation based on joint hypocenter determination and cross correlation in the study area we found two zones with different but stable polarization directions around the quien sabe fault system shear wave polarizations of over 30 microearthquakes clearly indicate that the slow shear wave direction is normal to the northwest southeast trend of this vertical right lateral strike slip fault thus velocity anisotropy appears to be caused by the fault system itself however anisotropy associated with a cluster of microearthquakes located about 4 km to the east is quite different in this zone the fast and slow shear polarization directions are correlative with the maximum and minimum horizontal stress directions respectively thus in this case it appears the crustal velocity anisotropy is controlled by the state of stress in the crust the observation of these two polarization directions at one station suggests that the shallow crust below the station hqr does not influence shear wave polarization directions

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

vp vs ratios in the yellowstone national park region wyoming abstract in this paper we study the variation of v p v s and poisson s ratio in the yellowstone national park region using earthquakes which were well recorded by a local seismic network we find that the average v p v s value within the geothermally active yellowstone caldera is about 7 lower than in the area outside the caldera within the caldera itself there may be a further 2 7 reduction of v p v s in the hydrothermally active norris geyser basin the upper and lower geyser basins and the yellowstone lake and mud volcano regions after considering various possible causes for v p v s changes such as geologic and structural differences thermal effects partial melting and hydrothermal activity we conclude that the most plausible explanation for the observed v p v s reduction is the presence of hot water at temperatures and pore pressures near the water steam transition in the caldera geothermal reservoirs

tectonics of an arc arc junction an example from kyushu island at the junction of the southwest japan arc and the ryukyu arc abstract three distinctive geologic events occurred synchronously during the last 6 m y on kyushu island lying at the junction of the southwest japan arc and the ryukyu arc these events included 1 dextral fault displacement along a 100 km section of the median tectonic line 2 formation of a rectangular volcano tectonic depression 70 km long and 40 km wide and 3 approximately 30 of counterclockwise crustal rotation we propose an integrated tectonic model that explains these three tectonic features in terms of subduction of the philippine sea plate which resumed at around 6 ma after a halt of more than 5 m y oblique subduction of the philippine sea plate beneath the southwest japan arc detached a fore arc sliver that was displaced along a dextral fault the median tectonic line north of the nankai trough a volcano tectonic depression the hohi volcanic zone was formed by the pull apart structure on the western margin of the median tectonic line by contrast normal subduction of the philippine sea plate under the ryukyu arc caused back arc spreading observed as crustal extension in the northern okinawa trough the crustal extension caused the counterclockwise rotation for the northernmost ryukyu arc including southern kyushu