geochemistry genesis and health implications of fluoriferous groundwaters in the upper regions of ghana the most appropriate and widely used source of drinking water for the populations of the upper regions of ghana is groundwater in general groundwater quality is acceptable except for some parts of the bolgatanga and bongo districts where there are occurrences of elevated levels of natural groundwater fluoride concentrations of groundwater fluoride in excess of the world health organization who maximum guideline value 1 5 mg l in the bongo area have been known since 1978 however the effect of fluoride on people ingesting the water did not receive public and medical attention until october 1993 when health personnel were asked to investigate the cause of stained teeth in school children the investigation established that 62 of the total population of school children in the bongo area had dental fluorosis against this background a study was initiated to understand the geochemistry genesis and distribution of fluoride in relation to the geology of the area groundwater fluoride in the upper regions ranges from 0 11 to 4 60 ppm with the highest concentrations associated with the fluorine enriched bongo coarse grained hornblende granite and syenite suite the source of groundwater fluoride within the bongo granitoids is dissolution of the mineral fluorite and dissolution of and anion exchange with micaceous minerals and their clay products applying the who recommended guideline values for fluoride in drinking water reveals that 49 of wells in the area deliver water below the optimum level of 0 5 mg l f these populations are thus prone to dental caries twenty eight percent of the wells fall within the optimum interval for good dental health 0 5 1 5 mg l f twenty three percent of the wells have concentrations above the recommended maximum guideline limit of 1 5 mg l f this population is susceptible to dental and possibly skeletal fluorosis climatic conditions of the area suggest that the individual water consumption is in the order of 3 to 4 l which is higher than the who estimate of 2 l adult day in addition dietary intake for the upper region population is probably higher than who baseline values 0 2 0 5 mg day this implies that a much higher population is susceptible to developing dental and skeletal fluorosis than originally suspected geochemical symbol plot maps help geochemists understand factors controlling the distribution and uptake of fluoride in the upper regions but they are of minimal value to health officials responsible for planning epidemiological studies and dental health education programs in the region by casting fluoride data into contoured geochemical health risk maps using intake interval guidelines more closely aligned to regional climatic and dietary conditions health officals can better judge the impacts regional and population based of fluoride on segments of the population such as various sex and age groups

analysis of the completeness magnitude and seismic network coverage of japan abstract a reliable estimate of completeness magnitude m c above which all earthquakes are considered to be detected by a seismic network is vital for seismicity related studies we show a comprehensive analysis of m c in japan we use the catalog maintained by the japan meteorological agency jma and also available information on seismic stations that report to jma for computing m c we adopt a commonly used method based on the gutenberg richter frequency magnitude law presently m c 1 0 might be typical in the mainland but to have a complete catalog one needs to use earthquakes with magnitudes of 1 9 or larger comparison with the southern california seismic network scsn suggests that the recent event detectability in the mainland generally shows similar completeness levels to that in the authoritative region of scsn we argue that the current m c of japan is due to the success of network modernization over time particularly we show that the spatiotemporal change of m c closely matches the addition of the hi net borehole stations to enhancing seismic station density it started in october 1997 in southwestern japan continuing to northeastern japan until 2002 as suggested from this matching we confirm that m c inversely correlates with station density further we find that irrespective of the network change after 1997 this correlation is unchanged in time demonstrating that the influence on m c from factors beyond station density does not vary in time contrary to alaska and california wiemer and wyss 2000 our results do not attribute such factors simply to anthropogenic noise because this is due to the borehole stations that reduce ambient noise we conclude that in japan the anthropogenic noise has an insignificant effect on m c

hydrogeochemical mechanisms governing the mineralization and elevated fluoride f contents in precambrian crystalline aquifer groundwater in central benin western africa in the central part of benin western africa high fluoride left text f right contents have been reported in groundwater from precambrian crystalline bedrock aquifer which is the main source of drinking water the hydrogeochemical mechanisms leading to such elevated fluoride concentrations are usually not fully understood in this context the objective is to identify the hydrogeochemical processes governing groundwater mineralization and the origin of the high fluoride concentrations a dataset of 162 groundwater samples was collected from the aquifer consisting of a thin altered bedrock layer shallow aquifer and a deep fractured crystalline bedrock deep aquifer geochemical approaches and multivariate statistics have been used to explore the data fluoride concentrations vary between 0 00 and 7 19 mg l in groundwater samples collected in the southern part of the investigated area close to dassa zoume show the highest concentrations with more than 75 greater than the guideline value of 1 5 mg l the deep fractured aquifer shows higher concentrations of fluoride than the shallow regolith reservoir results reveal that groundwater mineralization is derived mainly from the hydrolysis of silicate minerals but it is also influenced by anthropogenic effects particularly in the shallow reservoir however fluoride has a geogenic origin primarily from the weathering of silicate minerals primarily biotite text ca 2 text na cation exchanges and text f text oh anion exchanges between groundwater and the rock matrix also occur as minor processes saturation of the water with respect to calcite and the precipitation of this mineral which reduces calcium activity also favor the release of fluoride from rocks

three dimensional p and s wave velocity structures beneath japan abstract we determined the three dimensional vp and vs structures beneath japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the japan sea and the pacific ocean as well as those at permanent stations on the japan islands as a result we obtained more precise seismic images than previous studies in the crust and the uppermost mantle southwestern honshu exhibited weaker heterogeneity than the other areas in japan corresponding to the distribution of active volcanoes stripe like heterogeneities exist in the subducting pacific slab relatively low velocity zones correspond to low seismicity areas in the pacific slab suggesting that the slab is possibly torn or thin around the areas the fact that nonvolcanic deep tremors associated with the subducting philippine sea slab beneath shikoku kii and tokai do not occur in zones of high vp high vs and low vp vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab prominent and wide low vp and vs zones exist beneath central honshu at the depth range of 30 60 km where the volcanic front related to the subducting pacific plate is located and seismicity around the philippine sea plate is very low this condition may exist because magma genesis processes related to the subducting pacific plate activate the same processes around the philippine sea plate

assessment of shallow groundwater in lake nyos catchment cameroon central africa implications for hydrogeochemical controls and uses in 1986 carbon dioxide gas exploded from lake nyos and killed about 1 800 people after that disaster various administrative and research activities have been conducted to mitigate subsequent disasters however none of those endeavors have characterized the groundwater chemistry to identify hydrogeochemical processes that control the water chemistry and the quality of the water for domestic and agricultural uses that support the lives of un official resettlers around lake nyos conventional hydrochemical techniques coupled with statistical and graphical analysis were therefore employed to establish the baseline hydrochemical conditions assess processes controlling solutes distribution in shallow groundwater in the lake nyos catchment and explore its usability groundwater samples were analyzed for their physical and chemical properties the wide ranges of electrical conductivity and total dissolved solid values reveal the heterogeneous distribution of groundwater within the watershed the relative abundance of major dissolved species was ca mg na k for cations and hco3 cl so4 no3 for anions piper diagram classified almost all water samples into mixed camg hco3 water type major ion geochemistry reveals that in addition to silicates weathering water rock interaction ion exchange processes regulate the groundwater chemistry principal component analysis supports the occurrence of water rock interaction hierarchical cluster analysis showed that the chemistry of groundwater in the study area is controlled by three main factors and suggests no hydraulic connectivity between deep lake water and groundwater in the catchment the quality assessment of the groundwater showed that groundwater parameters are within the acceptable limit of the world health organization and nigeria guidelines for drinking and domestic uses and water found to be good for irrigation

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

anomalously high b values in the south flank of kilauea volcano hawaii evidence for the distribution of magma below kilauea s east rift zone abstract the pattern of b value of the frequency magnitude relation or mean magnitude varies little in the kaoiki hilea area of hawaii and the b values are normal with b 0 8 in the top 10 km and somewhat lower values below that depth we interpret the kaoiki hilea area as relatively stable normal hawaiian crust in contrast the b values beneath kilauea s south flank are anomalously high b 1 3 1 7 at depths between 4 and 8 km with the highest values near the east rift zone but extending 5 8 km away from the rift also the anomalously high b values vary along strike parallel to the rift zone the highest b values are observed near hiiaka and pauahi craters at the bend in the rift the next highest are near makaopuhi and also near puu kaliu the mildest anomalies occur adjacent to the central section of the rift the locations of the three major and two minor b value anomalies correspond to places where shallow magma reservoirs have been proposed based on analyses of seismicity geodetic data and differentiated lava chemistry the existence of the magma reservoirs is also supported by magnetic anomalies which may be areas of dike concentration and self potential anomalies which are areas of thermal upwelling above a hot source the simplest explanation of these anomalously high b values is that they are due to the presence of active magma bodies beneath the east rift zone at depths down to 8 km in other volcanoes anomalously high b values correlate with volumes adjacent to active magma chambers this supports a model of a magma body beneath the east rift zone which may widen and thin along strike and which may reach 8 km depth and extend from kilauea s summit to a distance of at least 40 km down rift the anomalously high b values at the center of the south flank several kilometers away from the rift may be explained by unusually high pore pressure throughout the south flank or by anomalously strong heterogeneity due to extensive cracking or by both phenomena the major b value anomalies are located sse of their parent reservoirs in the direction of motion of the flank suggesting that magma reservoirs leave an imprint in the mobile flank we hypothesize that the extensive cracking may have been acquired when the anomalous parts of the south flank now several kilometers distant from the rift zone were generated at the rift zone near persistent reservoirs since their generation these volumes may have moved seaward away from the rift but earthquakes occurring in them still use the preexisting complex crack distribution along the decollement plane at 10 km depth the b values are exceptionally low b 0 5 suggesting faulting in a more homogeneous medium

geographical distribution of 3he 4he ratios in japan implications for arc tectonics and incipient magmatism the 3he 4he and 4he 20ne ratios of 115 natural gas samples including various types of gases from volcanoes hot springs mineral springs water wells petroleum fields and natural gas fields were measured using a mass spectrometer the observed 3he 4he and 4he 20ne ratios range from 7 47 10 8 to 9 65 10 6 and from 0 26 to 1100 respectively the 3he 4he ratios reflect well the geotectonic structure of the japanese islands in northeastern ne japan there is a clear geographical difference in the 3he 4he ratios between the frontal arc forearc and volcanic arc back arc regions lower ratios were found in the trench side region and higher ratios in the back arc side this result suggests that the mantle derived helium in the volcanic arc region is associated with the diapiric uprise of a magma lower 3he 4he ratios in the frontal arc region may be due to radiogenic he produced by radioactive decay of u and th in the crustal and sedimentary rocks in southwestern sw japan there is no clear geographical contrast in the 3he 4he ratios some samples in the frontal arc region show quite high 3he 4he ratios the tendency of the 3he 4he ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from ne japan the high 3he 4he ratios observed in the frontal arc region in sw japan may be indicative of renewed or incipient magmatism due to a descending young and warm slab

3he 4he ratios in well gases in the kinki district sw japan surface appearance of slab derived fluids in a non volcanic area in kii peninsula abstract a total of 27 well gas samples collected from sw japan were subjected to precise analysis of he ne and ar isotopic compositions the sampling area covers localities where mantle derived helium emanates in the fore arc of the kii peninsula and is known as the kinki spot e g sano and wakita j geophys res 90 1985 8728 8741 the kinki spot apparently is located within the fore arc with no obvious magmatic activity which contrasts to the occurrence of similarly high 3 he 4 he ratios along the volcanic front of ne japan our high density sampling revealed more clearly that the high 3 he 4 he ratio wells are distributed in the kii peninsula above the area where the young and hot philippine sea plate is subducting with a relatively steeper dip than in the adjacent area this situation favors the near trench side of the subducting slab entering eclogite facies conditions involving slab dehydration without inducing melting of the mantle wedge the aqueous fluids derived from the slab acquire mantle he during their passage through the mantle wedge thus the observed association of mantle he and the fore arc dehydration of the philippine sea plate indicates that fluid derived from the subducting slab is contributing to the occurrence of mantle he in well gases of the kii peninsula the high 3 he 4 he region of kii peninsula also coincides with the occurrence of long period tremors these are caused by the movement of fluids derived from the subducting slab localized stresses cause significant uplift in the area interpreted as indicating that intra crustal fractures provide pathways through the crust for the fluids derived from depth thus magma is not necessarily required as the carrier of mantle derived helium to the surface of the fore arc region of kii peninsula

mapping active magma chambers by b values beneath the off ito volcano japan the b value of the frequency magnitude relation and thus the mean magnitude in the off ito volcanic area is not uniform based on detailed b value tomography using about 10 000 events in an area of 10 km radius and in the upper 15 km of the crust a high b value anomaly b 15 below 7 km depth and with a radius of about 2 km located below the coast southeast of ito contrasts with lower values of typically b 0 7 north of it and at shallower depths on the basis of surface deformations a tensile crack dislocation source was located above 7 km depth thus we surmise that the magma chamber in the off ito area was located below 7 km depth the correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies the magma body may be described as consisting of two parts a lower well established chamber below 11 km depth and a shallower part 11 7 km depth the b value in the volume surrounding the magma chamber increased from 0 7 during the 1980s to 1 5 during the earthquake swarms of the 1990s this may reflect a trend of increasing crack density due to the intrusion activity

high 3he emanation and seismic swarms observed in a nonvolcanic forearc region the 3he 4he and 4he 20ne ratios of gases emergent in the southwestern part of japan have been measured by a mass spectrometer the geographical distribution pattern of the 3he 4he ratios in the kinki district a non volcanic forearc region where microseismic activity at shallow depths has continued over a long period of time is strikingly different from those found in the rest of the japanese islands including the shikoku district which lies next to kinki anomalously high 3he 4he ratios up to 9 69 10 6 are found in the kinki district all the observed phenomena high emanation of 3he of magmatic origin high temperature hot springs high terrestrial heat flow hypocentral distribution of microearthquakes a local irregularity in the dip direction of the subducting philippine sea plate and the presence of molten materials inferred from the characteristic phases of sxp and sxs reflected waves in microseismograms strongly suggest the presence of a shallow magma body beneath the area

a point process analysis of the matsushiro earthquake swarm sequence the effect of water on earthquake occurrence temporal characteristics of the famous matsushiro earthquake swarm were investigated quantitatively using point process analysis analysis of the earthquake occurrence rate revealed not only the precise and interesting process of the swarm but also the relation between pore water pressure and the strength of the epidemic effect and the modified omori type temporal decay of earthquake activity the occurrence rate function t for this swarm is represented well as

tomographic evidence for the mantle upwelling beneath southwestern japan and its implications for arc magmatism abstract the seismotectonics and volcanotectonics of southwestern sw japan have been affected by the subduction of the old pacific and the young philippine sea slabs we have attempted to determine the three dimensional p and s wave velocity structures of the upper mantle in sw japan by using a large amount of high quality arrival time data recorded by the dense seismograph network on the japanese islands to understand the tectonic framework of sw japan our results show the existence of a large low velocity anomaly below the philippine sea slab in the chugoku and shikoku districts at least down to depths of 300 km we infer that the low velocity zone is an upwelling flow from depth two shallower low velocity zones derived from this low velocity zone are detected one is toward the quaternary volcanoes in the chugoku district and the other is toward the uppermost mantle beneath the kii peninsula the former suggests that the magma for the quaternary volcanoes in the chugoku district is attributable to the upwelling flow in the upper mantle this idea supports the geochemical and petrological observations that the erupted alkalic basalt in the chugoku district should have originated from a more primitive or enriched mantle the latter is responsible for the absence of a high velocity anomaly corresponding to the subducted philippine sea slab beneath the kii peninsula the philippine sea slab could have been more hydrated locally beneath the kii peninsula than that in the surrounding regions by the interaction with the mantle upwelling consequently dehydration reactions could be involved during the progressive metamorphism accompanied by intensive intraslab seismicity lowering the seismic velocity in the slab aqueous fluids thus released to the overriding plate might be a possible source of mantle type helium and trigger a long term swarm activity in the crust of the overriding plate our results indicate the importance of the heterogeneous structure below the philippine sea slab with regard to the tectonics of sw japan

aqueous fluids derived from a subducting slab observed high 3he emanation and conductive anomaly in a non volcanic region kii peninsula southwest japan abstract it has long been recognized that the kii peninsula in the southwest japan arc is peculiar in a non volcanic region indicated by the presence of high temperature hot springs high terrestrial heat flow and high 3 he content in hot spring gases geophysical and geochemical studies were carried out to understand the geotectonic environment in the southern part of the kii peninsula most of the measured 3 he 4 he ratios are similar or higher than air indicating wide spread incorporation of mantle derived helium into meteoric water a region with rather high 3 he 4 he ratios 4 r a on the west side of the omine mountains coincides with the occurrence of high temperature hot springs a deep crustal resistivity structure across the omine mountains was imaged by wide band magnetotelluric soundings a 2 d inversion with n s strike using both tm and te modes reveals two conductors one in the upper 3 7 km depth and the other in lower crust 25 35 km depth to the west of the omine mountains the distribution of microearthquakes and low frequency tremors and the existence of seismic reflectors indicate that the large conductor in the lower crust is related to aqueous fluids derived from the philippine sea plate the upper crustal conductive zone may also reflect the aqueous fluids trapped in the upper crust which are presumably derived from the subducting slab considering the occurrence of seismic events in the subducting slab beneath the southern kii peninsula the aqueous fluids generated by dehydration of the slab mantle could plausibly include morb type helium derived from the residual lithospheric mantle therefore the high temperature hot springs and high 3 he emanations in hot spring gases and other geotectonic events in the southern kii peninsula may be due to heat flux and mantle derived helium discharged from aqueous fluid in the upper crust

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

seismic and geodetic evidence for the existence of hot materials beneath the wakayama swarm activity southwestern japan abstract a remarkable seismic swarm area exists amid the wakayama plain on the western coast of the kii peninsula in southwestern japan close to the median tectonic line mtl the hypocenters of these swarm earthquakes are very shallow with a median of 6 km this is exceptionally shallow for seismic activity in plain areas where the seismogenic layer is usually deep compared to that in mountainous regions takayama et al 2006 also the b value below 8 km in the eastern part of this swarm region is anomalously high in addition to these seismic features geonet data and leveling survey data obtained by the geospatial information authority of japan indicate that land around the swarm region are expanded and upheaved areas of the dilatation and upheaval almost overlap and their centers correspond to the site where the high b value anomaly is observed these facts strongly suggest that there are hot materials beneath the source region in the kii peninsula acid volcanic rocks outcrop widely and the wakayama swarm area is located on the southern side of the mtl in the metamorphic belt below which the relatively young hot philippine sea plate is subducting and dehydration from a surpentinized forearc mantle is expected to occur based on the seismic and geodetic observations and the tectonic background of the area in which the swarm region is situated we propose a hypothesis that a diorite or a serpentine diapir with plentiful hot fluids might be intruding beneath the source area

groundwater flows in weathered crystalline rocks impact of piezometric variations and depth dependent fracture connectivity summary groundwater in shallow weathered and fractured crystalline rock aquifers is often the only perennial water resource especially in semi arid region such as southern india understanding groundwater flows in such a context is of prime importance for sustainable aquifer management here we describe a detailed study of fracture properties and relate the hydraulic connectivity of fractures to groundwater flows at local and watershed scales investigations were carried out at a dedicated experimental hydrogeological park in andhra pradesh southern india where a large network of observation boreholes has been set up twenty height boreholes have been drilled in a small area of about 18 000 m 2 in which borehole loggings and hydraulic tests were carried out to locate the main flowing fractured zones and investigate fractures connectivity several hydraulic tests nineteen slug tests and three pumping tests performed under two water level conditions revealed contrasting behavior under high water level conditions the interface including the bottom of the saprolite and the first flowing fractured zone in the upper part of the granite controls groundwater flows at the watershed scale under low water level conditions the aquifer is characterized by lateral compartmentalization due to a decrease in the number of flowing fractures with depth depending on the water level conditions the aquifer shifts from a watershed flow system to independent local flow systems a conceptual groundwater flow model which includes depth dependent fracture connectivity is proposed to illustrate this contrasting hydrological behavior implications for watershed hydrology groundwater chemistry and aquifer vulnerability are also discussed

40ar 39ar mineral age constraints for the tectonothermal evolution of the sambagawa metamorphic belt central shikoku japan a cretaceous accretionary prism abstract the sambagawa metamorphic belt exposed in central shikoku japan is tectonostratigraphically represented by the oboke and structurally overlying besshi nappe complexes oboke units consist of metamorphic rocks largely derived from clastic protoliths sandstone and conglomerate whereas besshi units are dominated by rocks with oceanic protolith affinities shale chert and greenstone whole rock schist samples from low grade oboke units record intermediate temperature 40 ar 39 ar plateau ages of 70 77 ma which are interpreted to closely date attainment of maximum metamorphic conditions whole rock schist samples from besshi units chlorite zone record plateau ages of 85 94 ma suggesting an earlier metamorphic climax within this structural level amphibole within the albite biotite zone in besshi records 36 ar 40 ar vs 39 ar 40 ar plateau isotope correlation ages of 84 87 ma which are interpreted as dating the post metamorphic cooling through temperatures required for intracrystalline retention of argon a slightly older isotope correlation age 94 ma is recorded by amphibole from the higher grade oligioclase biotite zone contrasts in amphibole cooling ages are interpreted as reflecting thrust related metamorphic inversion and resultant earlier cooling of higher grade structural units muscovite from all structural levels of the besshi nappe complex records 40 ar 39 ar plateau ages of 76 89 ma these are interpreted to date post metamorphic cooling through appropriate closure temperatures following structural assembly of the besshi nappe complex protoliths of the besshi units originated at approx 130 ma and attained maximum metamorphic conditions during entrainment in an accretionary complex at approx 90 100 ma higher grade sequences developed at deeper levels along the hanging wall of the accretionary complex during subsequent structural uplift to shallower crustal levels the besshi nappe complex was tectonically emplaced over oboke units

crustal deformation in kyushu derived from geonet data 1 an investigation was conducted on the horizontal crustal deformation in kyushu using the coordinate data from gps earth observation network geonet for the 4 years from 1998 to 2002 for which the annual variations are corrected we examined the spatial differences in the displacement rate relative to the amurian plate by subtracting the velocity of the amurian plate at each of the global positioning system gps stations models for interplate coupling in hyuganada and the dilatational source beneath sakurajima are obtained the crustal deformation in eastern kyushu is well explained by the model of interplate coupling in hyuganada which is strong in the north and weak in the south when the effects of the interplate coupling and the dilatational source are removed a southward extensional field is recognized in and to the south of the beppu shimabara graben in western kyushu in southern kyushu to the south of around latitude 32 n a southeastward movement is conspicuous it must be noted that the displacement field is extensional indicating that southern kyushu is being dragged at the base by a flow in the mantle not moving like a raft on the flow it is probable that the flow in the mantle is driven by the rising of hot materials in the okinawa trough and a retreat of the trench where the philippine sea plate is subducting a gap in the magnitude of the eastward displacement rate around latitude 32 n implies that the velocity of the flow in the mantle decreases substantially to the north of the line the beppu shimabara graben may be a continuation of the okinawa trough but the significant southwestward displacement rate observed in the eastern part suggests that the deformation in the graben at least to the east of aso volcano is affected by the westward movement of the forearc sliver produced by the oblique subduction of the philippine sea plate on the nankai trough off shikoku island we think that the strong coupling in the bungo channel in our model is probably not real but might be a result of our assumption that all the westward displacements in northeastern kyushu a part of which are actually due to the movement of the forearc sliver are produced by interplate coupling

a quantitative approach to the dike intrusion process inferred from a joint analysis of geodetic and seismological data for the 1998 earthquake swarm off the east coast of izu peninsula central japan 1 a series of earthquake swarms off the east coast of izu peninsula central japan is typically associated with dike intrusion around active volcanoes numerous studies in this region have documented the seismic activity and ground deformations without fully resolving the process of dike intrusion this study involves a joint analysis of geodetic and seismic data from the most recent major activity in this region occurring in 1998 to reveal the process of dike intrusion quantitatively we combine data from a dense gps array including 10 temporary stations with seismic analysis published previously we estimate the location of the dikes from seismic data and the volume of intruded magma from geodetic data combined analysis of both data sets enables us to reveal the kinematic process of dike intrusion associated with the 1998 earthquake swarm the process of dike intrusion is quantitatively modeled using the time evolution of recorded dike volume and the use of seismic hypocenters as an indicator of the site of instantaneous dike intrusion on the basis of these observations we propose the following process of the dike intrusion the magma rose upward through a pathway aligned vertically and created a dike when it lost buoyancy we also evaluate the dike intrusion process quantitatively using a model of a circular dike that grows under a continuous magma supply from a deep magma reservoir from the observational data we infer the time evolution in the dike shape pressure distribution and magma viscosity in the dike the acquired viscosity increases over time and it may be reasonable because magma cools gradually however its value is too large compared to typical basaltic magma expected in this region it may represent magma within the dike that is nearly solidified otherwise the magma is supplied intermittently and the dike growth is mainly controlled by the magma supply rate from a deep reservoir the quantitative description of the dike intrusion process in nature described in this study is very rare and our results provide constraints on several parameters that have not been solved in many previous theoretical studies on the dike intrusion process

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

magmatic focusing to mid ocean ridges the role of grain size variability and non newtonian viscosity melting beneath mid ocean ridges occurs over a region that is much broader than the zone of magmatic emplacement that forms the oceanic crust magma is focused into this zone by lateral transport this focusing has typically been explained by dynamic pressure gradients associated with corner flow or by a sub lithospheric channel sloping upward toward the ridge axis here we discuss a novel mechanism for magmatic focusing lateral transport driven by gradients in compaction pressure within the asthenosphere these gradients arise from the co variation of melting rate and compaction viscosity the compaction viscosity in previous models was given as a function of melt fraction and temperature in contrast we show that the viscosity variations relevant to melt focusing arise from grain size variability and non newtonian creep the asthenospheric distribution of melt fraction predicted by our models provides an improved explanation of the electrical resistivity structure beneath one location on the east pacific rise more generally we find that although grain size and non newtonian viscosity are properties of the solid phase their effect on melt transport beneath mid ocean ridges is more profound than their effect on the mantle corner flow