source mechanisms of usu volcano japan earthquakes and their tectonic implications abstract an intensive earthquake swarm and remarkable surface deformation due to dacitic doming have taken place at the usu volcano in southwestern hokkaido japan since august 1977 a major u shaped fault has formed in the summit crater and the amount of upheaval reached 180 m for 5 years in order to reveal the quantitative relation between this doming deformation and the associated earthquakes the mechanisms of the latter have been studied for this purpose broadband and wide dynamic range seismic observations were performed in a near field region of the usu volcano for the period from june 1979 to july 1980 long period seismometers and a digitally recording accelerometer system were used for relatively large earthquakes the data from the digital recording system permit very accurate calculations of ground displacements by employing the recursive filter method source parameters such as seismic moment average dislocation stress drop and fault dimensions for five earthquake families have been determined from a comparison of the observed ground displacements with synthetic ones synthetic displacements were computed using haskell s formulation for relatively deep earthquakes the discrete wavenumber dw method was used for relatively shallow earthquakes the stress drop varies widely among earthquakes in the same earthquake family the cumulative seismic deformation caused by the large earthquakes is almost consistent with the observed surface deformation it is suggested that the large earthquakes are caused by stick slip motions at the two sides of the u shaped fault

dike fault interaction during the 2004 dallol intrusion at the northern edge of the erta ale ridge afar ethiopia received 17 july 2012 revised 29 august 2012 accepted 4 september 2012 published 12 october 2012 1 during continental rifting the interaction between faulting and magmatic intrusions is not well understood using insar and seismicity we show that a 0 06 km 3 dike was intruded along the dallol segment ethiopia and was accompanied by a mw 5 5 earthquake and associated fault slip along the western flank of the rift the intrusion was fed by a previously unidentified magma chamber under dallol the total seismic moment release was 2 3 10 17 nm 10 of the geodetic moment this is a higher proportion than during the 2005 2009 dabbahu rifting episode which ranged between 1 4 of the geodetic moment a larger component of faulting occurs at dallol than at dabbahu segment a feature we interpret to be related to the proximity 10 km of the dallol segment to the rift margin where welldeveloped faults facilitate slip citation nobile a c pagli d keir t j wright a ayele j ruch and v acocella 2012 dike fault interaction during the 2004 dallol intrusion at the northern edge of the erta ale ridge afar ethiopia geophys res lett 39 l19305 doi 10 1029 2012gl053152

possible magma intrusion revealed by temporal gravity ground deformation and ground temperature observations at mount komagatake hokkaido during the 1996 1998 crisis summary mount komagatake 1131 m above sea level is a subduction related active volcano in hokkaido japan where two phreatic eruptions occurred in 1996 march and 1998 october after 54 years of dormancy we analyse four sessions of geodetic microgravity and ground temperature observations made over a two year period from november 1996 to may 1997 neither significant gravity nor significant elevation changes gps or levelling were observed from may 1997 to november 1997 we observed a slight subsidence 1 2 cm and contraction within 1 cm of the edifice a gravity increase 15 30 gal at the benchmarks inside the 2 km wide summit crater and a local temperature increase of 15 20 c at the summit crater from november 1997 to may 1998 we observed an edifice wide gravity increase of about 15 gal with no significant elevation change for benchmarks outside the summit crater besides this edifice scale variation we recorded a subsidence of about 5 6 cm and apparently no change of gravity inside the summit crater we inverted our data using models of increasing complexity elastic models are able to explain our deformation observations but they cannot explain either the gravity data or the temperature observations satisfactorily we introduced both an isothermal and a non isothermal porous medium filled with fluids to model deformation and gravity variations and we used a fissure model to invert the temperature anomaly our observations are consistent with a model of underground shallow water evaporation occurring as a result of heat rising from a hot and dense body density contrast of 200 kg m 3 of about 1011 kg at 4 5 km depth this interpretation suggests a possible intrusion of a magmatic body within the edifice

contrasted styles of rifting in the eastern gulf of aden a combined wide angle multichannel seismic and heat flow survey continental rifts and passive continental margins show fundamental along axis segmentation patterns that have been attributed to one or a number of different processes extensional fault geometry variable stretching along strike preexisting lithospheric compositional and structural heterogeneities oblique rifting and the presence or absence of eruptive volcanic centers the length and width scales of the rift stage fault bounded basin systems change during the late evolution of the new plate boundary and the role of magmatism may increase as rifting progresses to continental rupture along obliquely spreading ridges first order mid ocean ridge geometries originate during the synrift stage indicating an intimate relationship between magma production and transform fault spacing and location the gulf of aden rift is a young ocean basin in which the earliest synrift to breakup structures are well exposed onshore and covered by thin sediment layers offshore this obliquely spreading rift is considered magma poor and has several large offset transforms that originated during late stage rifting and control the first order axial segmentation of the spreading ridge widely spaced geophysical transects of passive margins that produce only isolated 2 d images of crust and uppermost mantle structure are inadequate for evaluation of competing rift evolution models using closely spaced new geophysical and geological observations from the gulf of aden we show that rift sectors between transforms have a large internal variability over short distances 10 km the ocean continent transition oct evolves from a narrow magmatic transition to wider zones where continental mantle is probably exhumed we suggest that this small scale variability may be explained 1 by the distribution of volcanism and 2 by the along strike differences in time averaged extension rate of the oblique rift system the volcanism may be associated with 1 the long offset alula fartak fracture zone which may enhance magma production on its younger side or 2 channeled flow from the afar plume material along the newly formed oct and the spreading ridge oblique extension and or hot spot interactions may thereby have a significant control on the styles of rifting and continental breakup and on the evolution of many magma poor margins

seismic energy releases from volcanoes seismic energy release during the precursory eruptive and declining stages of volcanic activities provides various information about the mechanisms of volcanic eruptions and the temporary developments of their activities hitherto the energy release patterns from precursory earthquake swarms were used to predict the eruption times especially of andesitic or dacitic volcanoes in this paper the discussion is expanded to quantify the total amount of seismic energy released at the threshold of volcanic eruptions with reference to the results observed at several volcanoes the results generally indicate that the cumulative seismic energy release from the precursory earthquake swarms exceed 1017 18ergs before eruptions at any andesitic or dacitic volvanoes this allows the seismic efficiency or the ratio of energy radiated seismically and the energy required for the volumetric expansion to be estimated by incorporating available deformation data with the seismic data the dependency of seismic efficiency on the type of volcanic activity i e non explosive outbreaks phreatic and magmatic eruptions dome formation etc was evaluated from observations at a few volcanoes that provided a variety of examples

the nature of volcanic earthquake swarm preceding the 2000 flank eruption at usu volcano hokkaido japan summary the 2000 eruption of usu volcano hokkaido occurred on the northwestern flank of volcano on march 31 at 13 07 local time after about 4 days of intensive seismic activity we study the seismic process prior to the eruption using the routine earthquake catalogue of the japan meteorological agency jma and the digital seismograms of the short period station nbbt jma earthquakes began beneath the summit and southern slope of the volcano then seismic foci clustered along two linear zones intersecting to the south of the initial epicentral zone the majority of earthquakes occurred at depth within the range between 4 and 7 km the magnitude range of located earthquakes varied between 0 8 and 4 4 the earthquake epicentres did not reach the sites of the forthcoming eruption they stopped at a distance of about 1 3 km the eruption began during the ceasing of seismic activity a study of the nature of the swarm was performed for a sample of 303 earthquakes with a magnitude interval from 2 5 to 3 4 that were representative of the swarm visual analysis allowed separating the earthquakes into two groups 286 volcano tectonic vt earthquakes and 17 long period lp earthquakes the spectral analysis of s wave so f274 vt earthquakes in the frame of brune s model showed that the vt earthquakes may be separated into two groups as characterized by high stress drops hsd and low stress drops lsd the swarm began with hsd earthquakes the lsd and lp earthquakes were recorded about 30 40 hr later simultaneously with the beginning of deformation of volcanic edifice and accompanied the hsd earthquakes up to the beginning of eruption we attribute the hsd earthquakes to the probable product of magmatic thermal fracturing of host rocks the relatively long lsd ruptures may be attributed to the slips along the pre existing fractures during magma movement

relocation of swarm events in the june 1987 earthquake sequence from the lake aswan area in egypt we have examined the digital waveform data and relocated a number of events within the june 1987 earthquake swarm which occurred beneath the northern part of lake aswan 70 km southwest of the aswan high dam in egypt this swarm occurred between june 17th and 19th with a maximum magnitude event of m l 3 5 cross correlation between a chosen master and the analyzed events has been carried out on seismograms from stations of the aswan network the cross correlation demonstrates the presence of a difference in both the p wave t p and the s wave t s arrival times at each station in the network relative to the arrival times of the master event at the same stations t p ranges between 0 15 and 0 11 second while t s ranges between 0 17 and 0 11 second the primary interpretation is that the se time differences represent an error in the manually picking arrival times then t p and t s values for each event result from a change in the hypocentral parameters from those of the master event assuming the p and s wave velocity distribution remains constant during the swarm activity this interpretation enables us to determine the relative distribution of hypocenters with respect to the hypocentral location of the master event we present the results from a swarm of 9 events demonstrating they originate from a nearly unique location rather than the zone identified from the preliminary locations which used manually picked onset times

effects of three dimensional earth structure on cmt earthquake parameters abstract we investigate errors in centroid earthquake parameters due to unmodeled structural heterogeneity we generate a simulated dataset consisting of synthetic seismograms for 50 earthquakes and 150 stations distributed globally to generate the synthetic seismograms we use a spectral element wave propagation package specfem3d globe that accounts for the earth s three dimensional structure an established centroid moment tensor inversion algorithm from the global cmt project is used to invert the synthetic dataset with and without added noise for earthquake source parameters this algorithm uses a one dimensional earth structure together with approximate corrections for three dimensional structure to model the seismograms we interpret the differences between the estimated source parameters and the parameters used to compute the synthetic dataset as errors due to unmodeled structural heterogeneity and the presence of noise we expect that the errors obtained in this study are representative of the errors in the global cmt catalogue we find that the errors in scalar moment moment tensor elements and location are small on average the depth and centroid time are however biased by a small amount we find that the error in depth can be reduced significantly by applying corrections for the difference in the velocity structure at the source and receiver locations from the earth s average structure in the cmt inversions this modification has a minimal effect on the errors in centroid time we do not find large errors in scalar moments even where the crustal thickness at the source is very different from the earth s average crustal thickness

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked

textural constraints on effusive silicic volcanism beyond the permeable foam model transitions between explosive and effusive phases of silicic volcanic eruptions have been related either to stratification of volatiles in the source magma body or to the loss of volatiles through the permeable host rock of the conduit one way to distinguish between these two models is to map and analyze the vesicular and glassy textures found in silicic lava flows in this paper we present textural observations and isotopic evidence from active and recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions after magma has reached the earth s surface this view is in contrast to the widely promoted permeable foam model which states that all volatiles escape during ascent of the magma and that all dense glassy material in lava flows forms from the collapse of pumiceous lava i e that silicic lavas emerge as highly inflated foam flows such interpretations which claim that silicic lavas are completely degassed upon extrusion and that all degassing must take place on the time scale of the eruption neglect several important pieces of evidence including the presence of obsidian in extremely small domes and of vesicular zones in the interiors of silicic flows the copious loss of volatiles through eruption plumes between eruptive phases and direct observations of surface vesiculation during growth of the mount st helens lava dome the permeable foam model also implies the unlikely requirement that explosive to effusive transitions be associated with an increase in eruption rate we present a more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent as well as late stage vesiculation we then discuss how the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days weeks or months after the lava emerges from the vent

elastic thickness control of lateral dyke intrusion at mid ocean ridges abstract magmatic accretion at slow spreading mid ocean ridges exhibits specific features although magma supply is focused at the centre of second order segments melts are episodically distributed along the rift toward segment ends by lateral dyke intrusions it has been previously suggested that an along axis downward topographic slope away from the magma source is sufficient to explain lateral dyke propagation however this cannot account for the poor correlation between dyke opening and surface elevation in the 2005 2010 series of 14 dyke intrusions of afar ethiopia using mechanical arguments constrained by both geodetic and seismological observations we propose that the large dykes that initiate near the mid segment magma source are attracted toward segment ends as a result of a thickening of the elastic brittle lithosphere in the along rift direction this attraction arises from the difference of elastic resistance between the segment centre where the lithosphere is thermally weakened by long term focusing of melts and comparatively colder hence stronger segment ends the axial topographic gradient in magmatic rifts may be more likely explained as an incidental consequence of these variations of along axis elastic brittle thickness rather than the primary cause of lateral dyke injections

seafloor spreading event in western gulf of aden during the november 2010 march 2011 period captured by regional seismic networks evidence for diking events and interactions with a nascent transform zone in november 2010 intense seismic activity including 29 events with a magnitude above 5 0 started in the western part of the gulf of aden where the structure of the oceanic spreading ridge is characterized by a series of n115 trending slow spreading segments set within an ew trending rift using signals recorded by permanent and temporary networks in djibouti and yemen we located 1122 earthquakes with a magnitude ranging from 2 1 to 5 6 from 01 november 2010 to 31 march 2011 by looking in detail at the space time distribution of the overall seismicity and both the frequency and the moment tensor of large earthquakes we reexamine the chronology of this episode in addition we also interpret the origin of the activity using high resolution bathymetric data as well as from observations of sea floor cable damage caused by high temperatures and lava flows the analysis allows us to identify distinct active areas firstly we interpret that this episode is mainly related to a diking event along a specific ridge segment located at e044 in light of previous diking episodes in nearby subaerial rift segments for which field constraints and both seismic and geodetic data exist we interpret the space time evolution of the seismicity of the first few days migration of earthquakes suggests initial magma ascent below the segment center this is followed by a southeastward dike propagation below the rift immediately followed by a northwestward dike propagation below the rift ending below the northern ridge wall the cumulative seismic moment associated with this sequence reaches 9 1 1017 nm and taking into account a very low seismic versus geodetic moment we estimate an horizontal opening of 0 58 to 2 9 m the seismic activity that followed occurred through several bursts of earthquakes aligned along the segment axis which are interpreted as short dike intrusions implying fast replenishment of the crustal magma reservoir feeding the dikes over the whole period the opening is estimated to 1 76 8 8 m across the segment a striking feature of this episode is that the seismicity remained confined within one individual segment whereas the adjacent en echelon segments were totally quiescent suggesting that the magma supply system of one segment is disconnected from those of the neighboring segments second we identify activity induced by the first intrusion with epicenters aligned along a n035 e trending 30 km long at the northwestern end of the active opening segment this group encompasses more than 7 earthquakes with magnitude larger than 5 0 and with strike slip focal mechanisms consistent with the faults identified in the bathymetry and the structural pattern of the area we propose that a transform fault is currently in formation which indicates an early stage of the ridge segmentation at the locus of the trend change of the spreading ridge which also corresponds to the boundary between a clear oceanic lithosphere and the zone of transform between continental and oceanic crust

post eruptive volcanic dome evolution as revealed by deformation and microgravity observations at usu volcano hokkaido japan abstract usu volcano hokkaido japan is a dacitic volcano known for its high production rate of lava domes and crypto domes it is thus a good target to study processes of volcanic dome evolution upheaval and or relaxation we carried out repeated gps and microgravity surveys on the three most recent domes of mt usu 1910 meiji shinzan 1943 1945 showa shinzan and 1977 1982 usu shinzan the repeat period was 1 to 2 months and extended from october 1996 to june 1997 we also compare new data with results from former studies more than 20 years after the start of usu shinzan dome growth there is still subsidence at a maximum rate of about 7 to 8 cm year the reasons for this subsidence are discussed repeated gravity surveys revealed an increase of gravity on the domes about 60 10 microgal year for usu shinzan about 15 microgal at showa shinzan and 10 to 20 microgal for meiji shinzan this gravity increase exceeds that expected due to subsidence we discuss and interpret the excess gravity change in terms of a density increase in the edifice caused by a combination of processes contraction of the edifice water level change devesiculisation cooling and magma intrusion quantification of these processes at usu volcano may help to understand the processes of evolution at domes on other volcanoes such as merapi indonesia unzen japan or montserrat west indies

energy partition in the 1982 eruption of el chichon volcano chiapas mexico abstract the 1982 eruption of el chichon volcano chiapas mexico has been extensively investigated from various standpoints including seismicity tephra geology atmospheric effects and tectonic structure in this paper the six largest of a series of explosions are isolated considering various sources of information then the seismic energy released by volcanic earthquakes and tremor is estimated using seismograms of low magnification in order to avoid saturation of seismic wave amplitudes the explosion kinetic energy is evaluated by two methods one by the initial velocity of the ejecta and the other by the air wave data recorded at a distance both the results are roughly coincident the thermal energy is classified into two components that transferred by volcanic ejecta and that consumed by convective clouds the thermal energy released by the explosions is very large compared with that of mechanical energy such as the seismic energy and explosion kinetic energy after the last major explosion of april 4 a type volcanic earthquakes swarmed till the end of april their occurrence rate decreased with time in an aftershock like sequence these earthquake swarms may have been due to post eruptive readjustment of the stress field within and near the volcano such seismic activity contrasts with other examples of similar major eruptions which were followed by growth of lava domes in the new craters after the paroxysmal eruptions the 1982 eruption of el chichon volcano has not been followed by the growth of a lava dome within the newly formed crater

sequence of faulting and deformation during the 2000 eruptions of the usu volcano hokkaido japan interpretation and image analyses of aerial photographs abstract significant faulting and deformation of the ground surface has been rarely known during volcanic eruptions usu volcano hokkaido japan is a unique example of deformation due to felsic magma intrusion usu volcano has a history of such types of eruptions as phreatic pumice eruption plinian type pyroclastic flowing and lava doming since 1663 on march 31 2000 phreatomagmatic to phreatic eruptions took place after 23 years of dormancy in the western piedmont followed by explosions on the western flank of usu volcano they were associated with significant deformation including faulting and uplift the eruptions and deformation were continuing up to the end of may 2000 we identified the faulting using total nine sets of aerial photographs taken from before the eruption march 31 2000 to more than 1 year april 27 2001 after the end of the activity and traced deformation processes through image processing using aerial photographs we found that some of the new faults and the associated phreatic eruptions were related to old faults formed during the 1977 1981 eruptive episode the image processing has revealed that the surface deformation is coincident with the area of faulting forming small grabens and the phreatic explosion vents however the faulting and main explosive eruptions did not take place in the highest uplift area but along the margin this suggests that the faulting and explosive activities were affected by small feeder channels diverging from the main magma body which caused the highest uplift

seismic character of volcanic activity at the ultraslow spreading gakkel ridge never before has a volcanic eruption on a slow or ultraslow spreading mid ocean ridge been both observed seismically and confirmed on the seafloor during the first half of 1999 a long lived volcanic spreading event occurred on the ultraslow spreading gakkel ridge in the arctic ocean the seismicity associated with this event was unprecedented in duration and magnitude for a seafloor eruption sonar images from the u s s hawkbill which passed over the area within four months of the start of activity are consistent with the presence of a large recently erupted flow and a volcanic peak directly in the area of seismic activity seismic activity began in mid january and continued vigorously for three months a reduced rate of activity persisted for an additional four months or more in total 252 events were large enough to be recorded on global seismic networks although a limited number of volcanic spreading events have been observed globally the duration and magnitude of the gakkel ridge swarm when compared with volcanic seismicity at ridges spreading at intermediate and fast spreading rates suggest that a negative power law relationship may exist between these parameters and spreading rate fault activation in response to magmatic emplacement appears to have occurred over a broad region suggesting that magma may have been tapped from mantle depths the slow migration of the largest magnitude events along the axis of the rift valley suggests multiple magmatic pulses at depth in combination with bathymetric setting and sidescan sonar confirmation the seismic data for this event have provided a unique look at the scale and character of eruption processes at ultraslow spreading rates

fracturing associated with the 1977 1978 eruption of usu volcano north japan as revealed by geophysical measurements abstract none of the ground temperature measurements inside the somma caldera of usu volcano in southwestern hokkaido japan show patterns that correlate with the development of surface faults and the hypocenter distribution of earthquake swarms observed during the 1977 1978 eruptions audiomagnetotelluric a m t soundings on the surface of the somma caldera display well defined high resistivity zones h r z within the low resistivity edifice h r z are explained as being due to rising dry steam and low resistivity zones are attributed to ionized water in the volcanic edifice inversion of data shows that elongated h r z have a shallow roof and are some hundred meters wide steam rising along narrow vertical fractures generates widely distributed vapor dominated geothermal systems because rising steam condenses near the surface the condensed steam creates a water filled zone which inhibits the free escape of succeeding steam distribution of elongated h r z correlates well with the high ground temperature zones a self potential survey reveals a positive anomaly developed over the usu edifice about 350 mv in magnitude electrokinetic coupling effect due to intensive hydrothermal convection is a probable interpretation of the anomaly because the high potential zone is closely related to a high ground temperature zone hydraulic fracturing of the upper crust at the time of eruption allowed release of elastic strain energy due to magma pressure at depths with the spectacular consequence that one side of the major fracture was uplifted about 180 m the geometry of such a process may be drawn from seismological results the roof of the edifice is pushed up by expanding magma accompanying intensive seismic activities we suggest that earthquake free zones are proof of magma filled fractures and that eruption took place when one of these fractures opened to the surface

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

conditions for detection of ground deformation induced by conduit flow and evolution 1 at mature andesitic volcanoes magma can reach the surface through the same path for several eruptions thus forming a volcanic conduit because of degassing cooling and crystallization magma viscosity increase in the upper part of the conduit may induce the formation of a viscous plug we conducted numerical simulations to quantify the deformation field caused by this plug emplacement and evolution stress continuity between newtonian magma flow and elastic crust is considered plug emplacement causes a ground inflation correlated to a decrease of the magma discharge rate a parametric study shows that surface displacements depend on three dimensionless numbers the conduit aspect ratio radius length the length ratio between the plug and the conduit and the viscosity contrast between the plug and the magma column larger displacements are obtained for high viscosity plugs emplaced in large aspect ratio conduits we find that only tiltmeters or gps located close to the vent a few hundred meters might record the plug emplacement at immediate proximity of the vent plug emplacement might even dominate the deformation signal over dome growth or magma reservoir pressurization effects for given plug thicknesses and viscosity profiles our model explains well the amplitude of tilt variations from 1 to 25 rad measured at montserrat and mt st helens we also demonstrate that at montserrat even if most of the tilt signal is due to shear stress induced by magma flow pressurization beneath the plug accounts for 20 of the signal

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

magma prospecting in usu volcano hokkaido japan using magnetotelluric soundings abstract we carried out magnetotelluric mt soundings across usu volcano hokkaido japan which has been active since 1663 the objective was to image an intrusion expected below the formation of 180m high cryptodomes formed during the 1977 78 eruption a ne sw two dimensional resistivity model has been obtained from the inversion in the frequency range from 0 01 to 10 000 hz the major features of the resistivity section are as follows 1 a 50 m thick surficial resistive 1000 10 000 m layer corresponding to the lava layer 2 a relatively conductive 10 1000 m layer from the surface to 200 m depth corresponding to quaternary sedimentary and volcanic rocks 3 beneath the quaternary formations a thick 2 4 km conductive 1 500 m layer of altered tertiary volcanics distributed widely in the volcanic edifice 4 a resistive block 500 1 000 m in the tertiary layer from 200 to 400 m in depth beneath the summit crater forming a striking contrast to the rest of the conductive tertiary layer and probably indicating a cooling intrusion at shallow depth 5 pre tertiary formations consisting of a high resistivity 1000 10 000 m layer overlying a conductive layer with the top of the pre tertiary becoming shallow to the northeast and 6 no deep conductive anomalies corresponding to a magma chamber inferred from petrological studies

january 2006 seafloor spreading event at 9 50 n east pacific rise ridge dike intrusion and transform fault interactions from regional hydroacoustic data 1 an array of autonomous underwater hydrophones is used to investigate regional seismicity associated with the 22 january 2006 seafloor spreading event on the northern east pacific rise near 9 50 n significant earthquake activity was observed beginning 3 weeks prior to the eruption where a total of 255 earthquakes were detected within the vicinity of the 9 50 n area this was followed by a series of 252 events on 22 january and a rapid decline to background seismicity levels during the subsequent 3 days because of their small magnitudes accurate locations could be derived for only 20 of these events 18 of which occurred during a 1 h period on 22 january these earthquakes cluster near 9 45 n and 9 55 n at the distal ends of the young lava flows identified posteruption where the activity displays a distinct spatial temporal pattern alternating from the north to the south and then back to the north this implies either rapid bilateral propagation along the rift or the near simultaneous injection of melt vertically from the axial magma lens short duration t wave risetimes are consistent with the eruption of lavas in the vicinity of 9 50 n on 22 january 2006 eruptions on 12 and 15 16 january also may be inferred from the risetime data however the locations of these smaller magnitude events cannot be determined accurately roughly 15 h after the last earthquakes were located adjacent to the eruption site a sequence of 16 earthquakes began to the north northeast at a distance of 25 40 km from the 9 50 n site these events are located in vicinity of the clipperton transform and its western inside corner an area from which the regional hydrophone network routinely detects seismicity coulomb stress modeling indicates that a dike intrusion spanning the known eruptive zone to the south 9 46 9 56 n would act to promote normal faulting or a combination of normal faulting and transform slip within this region with stress changes on the order of 1 10 kpa

inter eruptive volcanism at usu volcano micro earthquakes and dome subsidence abstract post eruptive crustal activity after the 2000 eruption of usu volcano was investigated by seismic and geodetic field observations remarkable features of the magmatic eruptions that occur almost every 30 years include lava dome formation and strong precursory earthquakes on the other hand rapid dome subsidence was observed by electronic distance meter edm measurement after the 1977 1982 summit eruption since the 2000 eruption seismic activity at a shallow part under the summit crater has remained at a high level relative to that after the 1977 1982 eruption although eruption occurred at the western foot of the volcano during the 2000 eruption to reveal the shallow crustal activity in the inter eruptive period around the summit area seismicity and crustal deformation have been investigated since 2006 dense temporary seismic observations and hypocenter relocation analysis using a three dimensional velocity structure model revealed that the focal area is localized along the u shaped fault that developed in the dome forming stage of the 1977 1982 eruption three major focal clusters are distributed on the southwestern side of usu shinzan cryptodome which was built up during the 1977 1982 eruption for the seven major events with magnitudes larger than 1 the focal mechanism was a large dip slip component which suggests the subsidence of usu shinzan cryptodome interferomatetric satellite aperture radar insar image analysis and repeated gps measurements revealed subsidence of the summit dome which is almost centered at the usu shinzan cryptodome the area of rapid deformation is restricted to a small area around the summit crater the estimated rate of dome subsidence relative to the crater floor is about 3 cm year these results strongly suggest that subsidence of usu shinzan is associated with the small earthquakes along the u shaped fault that surrounds the cryptodome according to prior seismic and geodetic studies it is thought that most of the magma rising under the summit crater during the 2000 eruption stopped around a depth of 2 km below sea level which is sufficiently deep relative to the focal area of the present seismicity a part of the magma intruded under the western foot and contributed to the 2000 eruption we conclude that the 2000 eruption scarcely affected the shallow crustal activity under the summit crater and that usu shinzan cryptodome is continuing to subside just as it was before the 2000 eruption the shallow volcanic earthquakes that began increasing from 1995 are closely related to the successive subsidence of the summit domes temporal change in fumarole temperature suggests a relationship between the shallow earthquakes and cooling of the magma that intruded under usu shinzan during the 1977 1982 eruption

evidence of recent volcanic activity on the ultraslow spreading gakkel ridge seafloor spreading is accommodated by volcanic and tectonic processes along the global mid ocean ridge system as spreading rate decreases the influence of volcanism also decreases and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates 1 5 cm yr 1 here we present three dimensional sonar maps of the gakkel ridge earth s slowest spreading mid ocean ridge located in the arctic basin under the arctic ocean ice canopy we acquired this data using hull mounted sonars attached to a nuclear powered submarine the uss hawkbill sidescan data for the ultraslow spreading approximately 1 0 cm yr 1 eastern gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley the western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999 suggesting that an axial eruption was imaged shortly after its occurrence these findings demonstrate that eruptions along the ultraslow spreading gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought

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

lithospheric structure of a nascent spreading ridge inferred from gravity data the western gulf of aden the aden spreading ridge somalia arabia plate boundary does not connect directly to the red sea spreading ridge it propagates toward the east african rift through the afar depression where the presence of a hot spot has been postulated from seismological and geochemical evidence the spreading direction n37 e is highly oblique to the overall trend n90 e of the ridge we present and interpret new geophysical data gathered during the tadjouraden cruise r v l atalante 1995 in the gulf of aden west of 46 e these data allow us to study the propagation of the ridge toward the afar and to discuss the processes of the seafloor spreading initiation we determine the lithospheric structure of the ridge using gravity data gathered during the cruise with the constraint of available refraction data a striking bouguer anomaly gradient together with the identification of magnetic anomalies defines the geographical extent of oceanic crust the inversion of the bouguer anomaly is performed in terms of variations of crustal thickness only and then discussed with respect to the expected thermal structure of the mantle lithosphere which should depend not only on the seafloor spreading but also on the hot spot beneath east africa our results allow us to define three distinct lithospheric domains in the western gulf of aden east of 44 45 e the lithosphere displays an oceanic character thermal subsidence recorded for the last 10 ma and constant crustal thickness between 43 30 e and 44 10 e the lithosphere is of continental type but locally thinned beneath the axial valley the central domain defined between 44 10 e and 44 45 e is characterized by a transitional lithosphere which can be seen as a stretched continental crust where thick blocks are mixed with thinned crust it displays en echelon basins that are better interpreted as extension cells rather than accretion cells

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

acoustic detection of a seafloor spreading episode on the juan de fuca ridge using military hydrophone arrays until recently no practical method has been available to continuously monitor seismicity of seafloor spreading centers the availability of the u s navy s sound surveillance system sosus for environmental research has allowed the continuous monitoring of low level seismicity of the juan de fuca ridge in the northeast pacific on june 22 1993 noaa installed a prototype system at u s naval facility whidbey island to allow real time acoustic monitoring of the juan de fuca ridge using sosus on june 26 2145 gmt a burst of low level seismic activity with accompanying harmonic tremor was observed and subsequently located near 46 15 n 129 53 w on the spreading axis of the juan de fuca ridge over the following 2 days the activity migrated to the nne along the spreading axis with the final locus of activity near 46 31 5 n 129 35 w the nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor based on this interpretation a response effort was initiated by u s and canadian research vessels and both warm water plumes and fresh lavas were subsequently identified at the site

precursory seismicity of the 1994 eruption of popocatepetl volcano central mexico popocatepetl volcano is located in the central mexican volcanic belt within a densely populated region inhabited by over 20 million people the eruptive history of this volcano indicates that it is capable of producing a wide range of eruptions including plinian events after nearly 70 years of quiescence popocatepetl reawakened in december 21 1994 the eruptive activity has continued up until the date of this submission and has been characterized by a succession of lava dome growth and destruction episodes similar to events that have apparently been typical for popocatepetl since the fourteenth century in this regime the episodes of effusive and moderately explosive activity alternate with long periods of almost total quiescence in this paper we analyze five years of volcano tectonic seismicity preceding the initial eruption of the current episode the evolution of the v t seismicity shows four distinct stages which we interpret in terms of the internal processes which precede an eruption after a long period of quiescence the thermal effects of a magma intrusion at depth the fracturing related to the slow development of magma related fluid pathways the concentration of stress causing a protracted acceleration of this process and a final relaxation or redistribution of the stress shortly before the initial eruption are reflected in the rates of v t seismic energy release a hindsight analysis of this activity shows that the acceleration of the seismicity in the third stage asymptotically forecast the time of the eruption the total seismic energy release needed to produce an eruption after a long period of quiescence is related to the volume of rock that must be fractured so imposing a characteristic threshold limit for polygenetic volcanoes limit that was reached by popocatepetl before the eruption

asperities barriers characteristic earthquakes and strong motion prediction recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip stable asperities and barriers which survive many earthquakes can explain these results the activity of volcano usu japan since 1977 which produced a dacite dome with a total upheaval of 180 m offered a great opportunity to study the recurrence behavior of a fault seismologists at hokkaido university revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall along which the dome moved upward this family can be explained by the existence of a stable asperity on the crater wall and is called the asperity type family they found however another type of family of earthquakes which share the same fault plane but show a great range of variation in the amount of slip the behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers and this family is referred to as the barrier type family this type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from california and the results reveal the stability of local stress drop among different earthquakes the agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault

propagation of rifting along the arabia somalia plate boundary the gulfs of aden and tadjoura the localization and propagation of rifting between arabia and somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern gulf of aden and the gulf of tadjoura using bathymetric magnetic seismological and structural evidence large scale southwestward propagation of the aden ridge markedly oblique to the arabia somalia relative motion vector began about 30 myr ago between the error and sharbithat ridges it was an episodic process with stages of rapid propagation mostly at rates 10 cm yr interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the arabia somalia plate the longest pause was at the shukra el sheik discontinuity 45 e where the ridge tip stalled for 13 myr between 17 and 4 ma west of that discontinuity rifting and spreading took place at an azimuth n25 10 e and rate 1 2 0 3 cm yr different from those of the global arabia somalia motion vector n39 1 73 cm yr implying an additional component of movement n65 10 e 0 7 0 2 cm yr due to rotation of the danakil microplate at shukra el sheik the typical oceanic ridge gives way to a narrow wsw trending axial trough resembling a large fissure across a shallow shelf this trough is composed of about eight rift segments which result from normal faulting and fissuring along n110 n130 e trends all the segments step to the left southwestward mostly through oblique transfer zones with en echelon normal faults only two segments show clear significant overlap there is one clear transform the maskali fault between the obock and tadjoura segments the latter segment which encroaches onland is composed of two parallel subrifts iboli ambabbo that propagated northwestward and formed in succession the most recent southwestern subrift ambabbo represents the current tip of the aden ridge we propose a mechanical model in which the large scale propagation of the ridge followed a wsw trending zone of maximum tensile stress while the small scale propagation of its nw trending segments was dictated by the orientation of that stress oblique propagation was a consequence of passive lithospheric necking of the arabia somalia plate along its narrow section in map view between socotra and the kink of the red sea ethiopian rift above the afar plume individual ridge segments oriented roughly perpendicular to plate motion like lithospheric cracks were forced to jump southward because of confinement within the necking zone self sustaining plate scale necking may explain why the aden ridge did not connect with the red sea through bab el mandeb but continued straight into afar

hydrological and geochemical change related to volcanic activity of usu volcano japan abstract mt usu is a stratovolcano located on the southern side of the toya caldera in hokkaido japan in historical times it emitted dacitic products associated to remarkable crustal deformations usu volcano erupted on march 31st 2000 after 23 yr of dormancy and this eruption was the fourth in the 20th century secular variations in water levels temperature and chemical compositions of thermal water have been observed around usu volcano the water levels decreased since october 1999 6 months prior to the eruption the decrease in water levels is a precursory change and is probably due to leakage of thermal water from the shallow aquifer hosting it no variations in temperature and chemical composition of thermal water were evident prior to the eruption however in a few wells the temperature of the thermal water increased by up to 15 c with respect to the pre eruptive values moreover the concentration of some solutes na k cl and so4 became higher after the eruption these thermal and chemical changes are ascribable to increased input of a deep hydrothermal liquid into the shallow groundwater aquifer

rift zone development around a plume related magma centre on the isle of skye scotland a model for stress inversions using the example of the isle of skye in scotland we investigate the influence of pressure variations in upper crustal magma reservoirs on the development of rift type normal faulting around central volcanoes the regional synmagmatic stress regime is of strike slip type in scotland during the lower tertiary during a prolonged period of overall high pressure in the skye magma reservoir gabbro intrusion stage crustal extension results from the injection of basaltic dykes parallel to the trend of the far field maximum stress during a subsequent period of pressure decrease in the reservoir granites intrusion stage normal faults trending parallel to the dykes are initiated these faults tilt the upper crustal blocks along with the former dyke swarm and associated lava pile finite element modelling shows that a decrease of magma pressure in a circular cavity may lead as in skye to a change from a regional strike slip to a local rift type normal stress regime

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

geometry of ionic arrangements and magnetic interactions in ordered ferri ilmenite solid solutions and its effect on low t magnetic behavior 1 the magnetic properties of metastable chemically ordered ilmenite rich fe2o3 fetio3 solid solutions in the temperature range 500 0 k represent a complex amalgam of the incompatible properties of the end members while hematite is controlled by adjacent layer antiferromagnetic interactions ilmenite is controlled by double layer antiferromagnetic interactions the complex geometry of the exchange coupled ions is visualized and tentative explanations of property changes with temperature are provided by schematic 2 d ionic images along a common 1 1 2 0 plane showing the exchange interactions in the end members and in ilm 90 80 and 70 solid solutions accompanied by parallel temperature and frequency dependent susceptibility diagrams to provide a larger scale visualization of the extent of exchange clusters and networks 3 d images of ilm 95 90 85 80 75 and 70 containing 2592 atomic positions were also constructed both types of images together provide a conceptual overview of the physical mechanisms governing this complex solid solution system and its low temperature magnetic behavior even though the geometric approach does not in itself provide a quantitative model of physical properties it is an indispensable prerequisite for designing realistic quantitative models judging their validity and interpreting model results it also supplies information on the distributions numbers and orientations of magnetic exchange interactions which can become guidelines for more sophisticated calculations

strain accommodation by slow slip and dyking in a youthful continental rift east africa continental rifts begin and develop through repeated episodes of faulting and magmatism but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented in highly evolved rifts tensile stresses from far field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions1 2 3 these rifting crises are rarely observed in thick lithosphere during the initial stages of rifting here we show that most of the strain during the july august 2007 seismic crisis in the weakly extended natron rift tanzania was released aseismically deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping this event provides compelling evidence for strain accommodation by magma intrusion in addition to slip along normal faults during the initial stages of continental rifting and before significant crustal thinning

the afar depression interpretation of the 1960 2000 earthquakes summary we studied the seismic activity of the afar depression ad and adjacent regions during the period 1960 2000 we define seven distinct seismogenic regions using geological tectonic and seismological data based on the frequency magnitude relationships we obtain b values of about 1 for the different regions the pattern of the distribution of the location of epicentres fits with the known active fault zone in the ad and the axial volcanic ridges the bab el mandab area and the danakil aysha a blocks are less active for 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated the results of the fault plane solutions for the afar depression indicate mainly strike slip and normal sense of movement originating from fault planes striking nw se these results indicate a clockwise block rotation described previously as a bookshelf model in central ad there are a few rightlateral faults east of massawa with e w striking fault planes at the southern red sea north of the danakil block the mixed focal mechanisms with axial plane striking nw se comprise several reverse faulting strike slip motion and normal faulting right lateral movement was also calculated for a cluster of seismic events between the manda hararo and alyata volcanic ridges along nw se striking faults along the n s striking faults in the escarpment at the western afar margins there are two distinct clusters of epicentres the strong earthquakes at the southern cluster exhibit normal or strike slip motions the intermediate to small earthquakes in the northern cluster exhibit reverse and strike slip motions mainly normal faults were calculated along ne sw striking faults of the ethiopian east african rift estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less implying that most of the motion is taken aseismically

magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics the volcanic rift zone in iceland is characterized by elongate volcanic systems consisting of tension fractures normal faults and volcanic fissures where most of the volcanotectonic activity takes place most volcanic systems develop central volcanoes the formation of which is still poorly understood there is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes in the central volcanoes eruptions are frequent typically one every several hundred years of small volume normally less than 0 1 km3 and fed mainly by thin average thickness of 0 5 m inclined sheets outside central volcanoes the eruptions are rare typically one every several thousand years but of large volume typically more than 1 km3 and fed mainly by thick average thickness of 4 5 m subvertical dikes using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi infinite plate these empirical relations as well as the formation of specific central volcanoes in the volcanic systems can be explained once a cavity like magma chamber has formed its existence in the rift zone concentrates tensile stress this stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system which partly explains the formation of central volcanoes and their high eruption frequencies this concentration also gives rise to a local stress field that encourages injection of small volume sheets in all directions and at various dips from their source magma chamber the magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma through inclined sheets toward a limited area at the surface where the central volcano gradually forms the inverse relationship between eruption frequency and eruption volume when the central volcano is compared with other parts of the volcanic system is also partly due to the trap like nature of the magma chamber the chamber is normally much smaller than its source mantle reservoir so that a single magma flow through a dike from the reservoir lasting perhaps many years may trigger tens of magma flows through sheets from the chamber many of which would reach the surface in the central volcano

aftershock sequences in the mid ocean ridge environment an analysis using hydroacoustic data abstract hydroacoustic data from autonomous arrays and the u s navy s sound surveillance system sosus provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow spreading mid atlantic ridge mar intermediate spreading juan de fuca ridge jdfr and fast spreading east pacific rise epr aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single link cluster slc analysis with a combined space time metric in the regions examined hydroacoustic data improve the completeness level of the earthquake catalog by 1 5 2 0 orders of magnitude allowing the decay constant p of the modified omori law mol to be determined for individual sequences a non parametric goodness of fit test indicates six of the seven sequences examined are described well by a mol model the p values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data for three sequences along the siqueiros discovery and western blanco transforms p values are estimated to be 0 94 1 29 the spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra transform spreading centers at these locations an aftershock sequence following a 7 1 m s thrust event near the northern edge of the easter microplate exhibits p 1 02 0 11 within the sequence aftershocks are located to the north of a large topographic ridge which may represent the surface expression of the shallow dipping fault that ruptured during the mainshock two aftershock sequences near 24 25 n and 16 35 n on the mar exhibit higher p values 1 74 0 23 and 2 37 1 65 although the latter estimate is not well constrained because of the small number of aftershocks larger p values along the ridge crest might reflect a hotter thermal regime in this setting additional monitoring however will be needed to determine if p value differences between the ridge and transform sequences are robust a 1999 sequence on the endeavour segment of the jdfr which has been correlated with changes in the hydrothermal system is described poorly by the mol model the failure of the mol model the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence

crustal structure of the rifted volcanic margins and uplifted plateau of western yemen from receiver function analysis we analyse p wave receiver functions across the western gulf of aden and southern red sea continental margins in western yemen to constrain crustal thickness internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism faulting and mechanical crustal thinning during continental breakup we analyse teleseismic data from 21 stations forming the temporary young conjugate margins laboratory yocmal network together with gfz and yemeni permanent stations analysis of computed receiver functions shows that 1 the thickness of unextended crust on the yemen plateau is 35 km 2 this thins to 22 km in coastal areas and reaches less than 14 km on the red sea coast where presence of a high velocity lower crust is evident the average vp vs ratio for the western yemen plateau is 1 79 increasing to 1 92 near the red sea coast and decreasing to 1 68 for those stations located on or near the granitic rocks thinning of the crust and by inference extension occurs over a 130 km wide transition zone from the red sea and gulf of aden coasts to the edges of the yemen plateau thinning of continental crust is particularly localized in a 30 km wide zone near the coastline spatially co incident with addition of magmatic underplate to the lower crust above which on the surface we observe the presence of seaward dipping reflectors sdrs and thickened oligo miocene syn rift basaltic flows our results strongly suggest the presence of high velocity mafic intrusions in the lower crust which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the sdrs our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western gulf of aden and southern red sea volcanic margins followed by a second phase of extension along the red sea margin

gradual bubble growth in dacite magma as a possible cause of the 1977 1978 long lived activity of usu volcano abstract the 1977 1978 activity of usu volcano was characterized by persistent earthquake swarms and remarkable ground deformations which continued until march 1982 essential evidence for considering the cause of the long lived activity is that the rate of seismic energy release and the upheaval rate of the new cryptodome decreased exponentially after the pumice eruptions in august 1977 associated with the phreatomagmatic eruptions in august september 1978 other valuable evidences were observed co eruptive deflation of the summit and post eruption quiescence of the seismic activity and ground deformations these observational evidences suggest that a possible cause of the long lived activity might be the gradual bubble growth in the dacite magma which had been isolated from deeper sources but partly supersaturated with volatiles even after the pumice eruptions numerical calculations based on a bubble growth model indicated that the observed time variations of the upheaval rate of the cryptodome could be well explained by the model if the water diffusivity in the dacite magma of usu volcano were of the order of 10 9 cm2 s 1 and the degree of supersaturation had been a few percent after the pumice eruptions in august 1977

centroid depths of mid ocean ridge earthquakes dependence on spreading rate from inversions of teleseismic p and sh waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962 1983 on slowly spreading mid ocean ridges all events are characterized by predominantly normal faulting on planes that dip at approximately 45 and strike parallel to the local trend of the ridge axis centroid depths range from 1 to 6 km beneath the seafloor the p waves from these earthquakes show strong water column reverberations suggesting that fault rupture extended to the seafloor under the assumption that the centroid depth marks the mean depth of fault slip earthquake faulting extended to a depth of 2 10 km for these earthquakes the water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley the maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate and the maximum seismic moment may also decrease with increasing spreading rate these results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region the concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley

geophysical characteristics of dacite volcanism the 1977 1978 eruption of usu volcano abstract the 1977 1978 eruption of usu volcano is discussed from the geophysical standpoint as a classic example of dacite volcanism the activities of dacitic volcanoes are characterized by persistent earthquake swarms and remarkable crustal deformations due to the high viscosity of the magmas the former include shocks felt near the volcanoes and the latter accompany formation of lava domes or cryptodomes the hypocenters of the earthquakes occurring beneath usu volcano have been located precisely their distribution defines an earthquake free zone which underlies the area of doming within the summit crater this zone is regarded as occupied by viscous magma the domings within the summit crater forming the cryptodomes have amounted to about 160 m in addition to uplift they showed thrusting towards the northeast as a result the northeastern foot of the volcano has contracted by about 150 m the relation between crustal deformation and earthquake occurrence is examined and it is found that the abrupt domings are accompanied by the larger earthquakes m 3 4 3 both the seismic activity and the ground deformation are shown to have a unique and common energy source the energy of activities of usu volcano consists of the explosive type the deformation type and the seismic type the second and the third are in parallel with each other in discharges and both energies are complementary to the explosive energy the explosive energy and the seismic energy have been calculated for an explosion sequence and it is concluded that the deformation energy is about 10 times greater than the seismic energy the discharge rate of the seismic energy and the upheaval rates of the cryptodomes have continued to decrease since the outburst of the eruption except for a small increase at the end of january 1978 eruptions are governed not only by the supply of the energies but also by the depth of the magma which has gradually approached the surface the last eruption occurred in october 1978 however the crustal deformations and the earthquake swarms are still proceeding as of january 1980 albeit at a lower rate of activity

characteristics of micro earthquake swarms preceding eruptions at showa crater of sakurajima volcano japan abstract the showa crater of sakurajima an andesitic volcano in japan resumed eruptive activity in june 2006 for the first time in 58 years explosive eruptions at the showa crater have been active since 2009 micro earthquake swarms have sometimes been observed in the 1 or 2 h preceding these eruptions since 2012 and have frequently occurred during periods of low eruptive activity these precursory micro earthquakes are dominated by high frequency 5 6 hz components and no clear s wave and are thus classified as bh type earthquake swarms the hypocenters of the earthquakes are located in a shallow region depth of approximately 1 km near the showa crater prior to the eruptions the onset of inflation has been observed by extensometers followed by precursory earthquake swarms 30 min to 1 h later in the case of insufficient ground contraction the precursory earthquakes reoccur when the strain reaches the same level it showed just before the eruptions occurred from the occurrence period and pattern of the precursory earthquakes it appears they may be generated within a narrow conduit leading to the showa crater

post eruptive seismic activity of mount cameroon cameroon west africa a statistical analysis abstract recent seismological studies of the cameroon volcanic line show that mt cameroon is the most active centre so a permanent seismic network of six seismographs was set up in its region between 1984 and 1986 the network was reinforced with temporary stations up till 1987 and the local seismicity was studied here we emphasise a statistical analysis of seismic events recorded by the permanent seismic stations four swarms lasting 9 to 14 months are identified at intervals of 2 3 years most earthquakes are felt intensity and magnitude respectively less than vi mm and 5 during the first three swarms and a few during repose periods the main focal regions are the northwest and southeast flanks the bimbia and bioko regions in the south of the volcano hypocentres are distributed from the surface to 60 km depth indicating crustal and subcrustal activities the subcrustal events are observed only in the southeast flank they are the most regular earthquakes with a monthly frequency of 9 to 15 events they are characteristic earthquakes with magnitude 2 8 0 1 between 1984 and 1992 their yearly mean time interval between successive events range from 50 to 86 hours for that period their occurrence can be modelled as a stationary renewal process with a 3 day period but the analysis of variance shows possible significant differences among yearly means a weibull s distribution confirms that the time intervals between successive deep events are not independent and in 1993 a swarm of deep earthquakes is recorded hence a non loglinear magnitude frequency relation the deep seismicity is thought to be associated with a zone of weakness perhaps a magmatic conduit and may have some close relationship with the magmatic activity

broadband calibration of the r v marcus g langseth four string seismic sources 1 the r v marcus g langseth is the first 3 d seismic vessel operated by the u s academic community with up to a four string 36 element source and four 6 km long solid state hydrophone arrays this vessel promises significant new insights into earth science processes the potential impact of anthropogenic sound sources on marine life is an important topic to the marine seismic community to ensure that operations fully comply with existing and future marine mammal permitting requirements a calibration experiment was conducted in the gulf of mexico in 2007 2008 results are presented from deep 1 6 km and shallow 50 m water sites obtained using the full 36 element 6600 cubic inches seismic source this array configuration will require the largest safety radii and the deep and shallow sites provide two contrasting operational environments results show that safety radii and the offset between root mean square and sound exposure level measurements were highly dependent on water depth

origin and models of oceanic transform faults abstract mid ocean ridges sectioned by transform faults represent prominent surface expressions of plate tectonics a fundamental problem of plate tectonics is how this pattern has formed and why it is maintained gross scale geometry of mid ocean ridges is often inherited from respective rifted margins indeed transform faults seem to nucleate after the beginning of the oceanic spreading and can spontaneously form at a single straight ridge both analog and numerical models of transform faults were investigated since the 1970s two main groups of analog models were developed thermomechanical freezing wax models with accreting and cooling plates and mechanical models with non accreting lithosphere freezing wax models reproduced ridge ridge transform faults inactive fracture zones rotating microplates overlapping spreading centers and other features of oceanic ridges however these models often produced open spreading centers that are dissimilar to nature mechanical models on the other hand do not accrete the lithosphere and their results are thus only applicable for relatively small amount of spreading three main types of numerical models were investigated models of stress and displacement distribution around transforms models of their thermal structure and crustal growth and models of nucleation and evolution of ridge transform fault patterns it was shown that a limited number of spreading modes can form transform faults microplates overlapping spreading centers zigzag ridges and oblique connecting spreading centers however the controversy exists whether these patterns always result from pre existing ridge offsets or can also form spontaneously at a single straight ridge during millions of year of accretion therefore two types of transform fault interpretation exist plate fragmentation structures vs plate accretion structures models of transform faults are yet relatively scarce and partly controversial consequently a number of first order questions remain standing and significant cross disciplinary efforts are needed in the future by combining natural observations analog experiments and numerical modeling

subsurface structure of usu volcano japan as revealed by detailed magnetic survey abstract a detailed magnetic survey based on 1018 stations was conducted in the usu volcanic region hokkaido japan from the study the following results were obtained 1 the main body of usu volcano is composed of strongly magnetized basaltic somma lavas and weakly magnetized crater fill 2 the southern rim of somma volcano is situated about 200 m south of the present topographic rim suggesting the existence of the buried horseshoe shaped caldera which is thought to have been formed several thousand years ago 3 parts of the mt showa shinzan lava dome formed in the 1943 1945 activity are still occupied with lavas of high temperature above the curie temperature of the ferromagnetic minerals 4 the root of the lave dome of mt showa shinzan has almost the same diameter as that of the dome exposed on the earth s surface 5 an intrusive body about 200 m in diameter may exist at shallow depth in the roof mountain of mt showa shinzan

seismic precursors to the mount st helens eruptions in 1981 and 1982 six categories of seismic events are recognized on the seismograms from stations in the vicinity of mount st helens two types of high frequency earthquakes occur near the volcano and under the volcano at depths of more than 4 kilometers medium and low frequency earthquakes occur at shallow depths less than 3 kilometers within the volcano and increase in number and size before eruptions temporal changes in the energy release of the low frequency earthquakes have been used in predicting all the eruptions since october 1980 during and after eruptions two types of low frequency emergent surface events occur including rockfalls and steam or gas bursts from the lava dome

sequence of rifting in afar manda hararo rift ethiopia 2005 2009 time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling thirteen dike intrusions in the manda hararo rift afar ethiopia from september 2005 to june 2009 studied using an extensive interferometric synthetic aperture radar insar data set provide insight into the mechanics of a major active rift kinematic inversions of insar data reveal that dikes opened by 0 8 3 5 m at an average 5 km depth with volumes of 0 04 0 2 km 3 with up to 12 m opening and a volume greater than 1 km 3 for the september 2005 megadike dikes have their source in a midsegment magma reservoir which induces a local shallowing of the brittle ductile boundary presumably due to thermal weakening of the lithosphere the smaller dikes in 2006 2009 were emplaced in regions of minimum opening of the september 2005 megadike above the central magma reservoir in contrast the most voluminous dike intrusions in 2006 2009 occurred near the locus of the peak of maximum opening of the september 2005 megadike 10 km north of the magma source this may suggest that tension on the plate boundary was highest there both prior to 2005 and possibly also after 2005 evolution and distribution of normal stress on the plate boundary throughout the rifting episode may indicate that tension near to the magma reservoir is lower than toward segment ends average relief of normal stresses of tectonic origin coeval to dike intrusions is comparable with shear stress drops for earthquakes presumably because dikes in the manda hararo rift are intruded at low magma pressure and high tectonic stress time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling

contrasting structural styles of the eastern and western margins of the southern red sea the 1988 sonne experiment abstract seismic investigations in the sudanese and yemeni sections of the red sea showed contrasting structural styles across the two surveyed areas of the opposing flanks off sudan seaward of an about 50 km broad zone of stretched continental crust different domains of oceanic crust were identified the oldest of these domains was found under an up to 6 km thick cover of sediments which include a 1 to 2 km thick pre evaporite sequence these sediments are interpreted to be part of a pull apart basin generated during the initial stages of red sea evolution towards the red sea axial trough a structural boundary separates this from younger oceanic crust which is covered only by post evaporite sediments this sequence thins progressively from about 800 m at the boundary to zero at the axial trough off yemen in contrast oceanic crust is confined to the axial trough zone which terminates southward of 16 n further south to 14 n no oceanic crust has yet been formed stretched continental crust was found across the entire flank of the southern red sea and at the tihama coastal plain further inland beginning at the escarpment the crust thickens in several major steps to a thickness of 35 km in central yemen

post rift volcanism and high heat flow at the ocean continent transition of the eastern gulf of aden a high heat flow 900 mw m 2 has been observed over a volcanic structure at the ocean continent transition in the eastern gulf of aden oman margin the anomaly is superposed to a progressive increase of heat flow across the margin and can be interpreted either by 1 heat refraction 2 fluid discharge or 3 cooling magma the two first explanations cannot be ruled out definitely by modelling analysis but require unlikely thermal conductivity or permeability values the third one implies that the latest activity of the volcano was about 100 000 years old and therefore continued c 18 ma after the break up of africa and arabia this potential mechanism is consistent with other lines of evidence of post rifting activity in the gulf of aden and could invalidate the conventional assumption that rifted margins become passive after the break up of continents

seismicity during lateral dike propagation insights from new data in the recent manda hararo dabbahu rifting episode afar ethiopia seismicity released during lateral dike intrusions in the manda hararo dabbahu rift afar ethiopia provides indirect insight into the distribution and evolution of tensile stress along this magma assisted divergent plate boundary in this paper 5 dike intrusions among the 14 that form the 2005 present rifting episode are analyzed with local and regional seismic data during dike intrusions seismicity migrates over distances of 10 15 km at velocities of 0 5 3 0 km h away from a single reservoir in the center of the rift segment confirming the analogy with a slow spreading mid ocean ridge segment comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward suggesting an asymmetry of tension in the brittle elastic lithosphere seismicity during dike injections is concentrated near the propagating crack front in contrast faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions the seismic energy released during dike intrusions in the manda hararo rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion the low level of seismic energy accompanying dike intrusions despite their significant volumes is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary

controls of conduit geometry and wallrock elasticity on lava dome eruptions abstract many lava dome building eruptions show periodic to complex non periodic pulsatory activity typical time scales associated with this activity range from hours to decades previous studies modelled the ascent of magma using a set of transient 1 d transport equations accounting for degassing induced crystallization kinetics gas exsolution and viscosity increase due to crystal growth these models assumed flow in a cylindrical conduit with a fixed cross section area since several observations suggest that extrusions are mainly fed by dykes with cylindrical geometries developing only at shallow levels here we generalised the model to the flow geometry represented by an elliptical dyke with major and minor semi axes changing with depth quasi static elastic deformation of the dyke is accounted by an analytical solution that couples cross section area with the magmatic overpressure the effects of the main dyke geometrical parameters and boundary conditions on the eruption dynamics were investigated the presence of a deformable dyke can lead to a more complex periodic behaviour with a wider range of time scales and cyclicity patterns with respect to a uniform cylindrical conduit there is a regime where the period of pulsations is controlled by the elasticity of the dyke and a regime where the period is controlled by the volume of the magma chamber intermediate regimes are possible periodic variations in discharge rate are also possible for both fixed pressure in dyke source region and fixed influx rate into the dyke our study emphasizes the strong non linearities and complex behaviours of lava dome eruptions from a forecasting and hazard perspective intrinsic uncertainties in governing parameters may make volcanic systems in some circumstances unpredictable on the other hand lava dome systems may also develop episodic and systematic behaviours so that behaviour becomes predictable for a while

analog models of oblique rifting in a cold lithosphere new lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution basin segmentation and mantle exhumation occurring during rift localization we present two models one with a preexisting oblique lithospheric weakness model b and another with no weakness zone model a both oblique rifts have an obliquity of about 40 the main results are as follows 1 the fault populations especially during the early stages of deformation are composed of faults that in strike are largely intermediate between rift parallel and perpendicular to displacement this fault population is characteristic of oblique rifts observed in previous studies 2 in later stages faults parallel to the rift become numerous in both models buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift parallel faults 3 during the final stages of extension in model b the crust is deformed by rift parallel faults while in the basins the small scale deformation pattern is composed of displacement normal faults however in model a displacement normal faults tend to accommodate most of the extension controlling its final stages they probably also control the formation of the ocean continent transition any possible mantle exhumation as well as the geometry of oceanic accretion centers these results provide an insight into the possible evolution of the gulf of aden conjugate margins which developed in an oblique context and most probably without any preexisting rift parallel localizing heterogeneity in the lithosphere

broad accommodation of rift related extension recorded by dyke intrusion in saudi arabia volcanic fields at the eastern margin of the red sea rift were regarded as seismically quiet until a swarm of 30 000 earthquakes struck in 2009 geological analyses reveal the intrusion of a volcanic dyke and indicate that rift related extension is spread over a broad region far from the rift axis

hydration and dehydration of zeolitic tuff from yucca mountain nevada naturally occurring zeolites expand and contract when hydrated or dehydrated in tuffaceous rock composed largely of such zeolites the entire rock may swell or contract significantly as the rock becomes saturated or dries out if such rock is constrained significant stresses may develop as a result of hydration or dehydration we present experimental results that substantiate this in a zeolitized non welded tuff from yucca mountain nv rock permeability governs the swelling rate since the major constituent clinoptilolite hydrates as fast as it can be exposed to water at yucca mountain where a nuclear waste repository is proposed strata of welded devitrified tuff overlie non welded zeolitic tuff should the hydration state of the units change significantly over the repository lifetime additional stresses on the same order of magnitude as now exist may develop