extracting time domain green s function estimates from ambient seismic noise 1 it has been demonstrated experimentally and theoretically that an estimate of the green s function between two seismic stations can be obtained from the time derivative of the long time average cross correlation of ambient noise between these two stations this tdgf estimate from just the noise field includes all tensor components of the green s function and these green s function estimates can be used to infer earth structure we have computed cross correlations using 1 to 30 continuous days of ambient noise recorded by over 150 broadband seismic stations located in southern california the data processing yielded thousands of cross correlation pairs for receiver separations from 4 500 km which clearly exhibit coherent broadband propagating dispersive wavetrains across frequency band 0 1 2 hz

evidence for widespread nonlinear strong ground motion in the mw 6 9 loma prieta earthquake we exploit 55 repeating microearthquake sequences on the san andreas fault just south of the rupture zone of the 1989 m w 6 9 loma prieta earthquake to search for time dependent properties of the earth s crust using moving window waveform cross correlation we identify clear and systematic delays as large as 20 msec for the direct s wave and exceeding 50 msec in the early s wave coda following the loma prieta mainshock others have also identified phase delays velocity reductions associated with damaging earthquakes and they have suggested a myriad of possible causal mechanisms here we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an earthquake a strong correlation between the spatial patterns of s delays and the intensity of strong ground motion in the loma prieta earthquake suggests that physical damage the formation or growth of cracks during strong ground motion to the earth s shallow crust is responsible for the observed velocity reductions the strong spatial variability in s delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver the effect is stronger at stations on young soft rocks than at stations on old hard rock disproportionately larger s coda delays than p coda delays suggest that the cracks formed by the strong shaking are fluid filled in the 10 years after loma prieta the initial increase in travel times reduces logarithmically with respect to time often back to the premainshock levels we attribute this behavior to the same slow dynamic healing observed in laboratory studies of the recovery of materials from transient nonlinear strain manuscript received 14 january 2004

temporal changes of the crustal structure associated with the m6 1 earthquake on september 3 1998 and the volcanic activity of mount iwate japan we detected temporal changes of crustal structure by cross spectrum analyses of seismic waves excited by two artificial explosions that were carried out about one month before and two months after a m6 1 shallow earthquake phase spectrum analysis shows that seismic velocity of the upper crust around the focal area of the m6 1 earthquake and mount lwate decreased 0 3 1 0 during the three months since lager decreases of the seismic velocity were observed at the region close to the focal area and a volcanic pressure source we conclude that stress change in the upper crust is a most plausible candidate to cause the velocity changes among other candidates we further apply a coherence analysis to the observed seismic coda to locate regions where short wavelength heterogeneity temporally changed the result indicates a possibility of temporal changes of the heterogeneity beneath the volcano where a magmatic fluid intrusion is inferred

temporal changes of shallow seismic velocity around the karadere duzce branch of the north anatolian fault and strong ground motion we analyze temporal variations of seismic velocity along the karadere duzce branch of the north anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 mw7 4 izmit and mw7 1 duzce earthquakes the analysis employs 36 sets of highly repeating earthquakes each containing 4 18 events the events in each cluster are relocated by detailed multi step analysis and are likely to rupture approximately the same fault patch at different times the decay rates of the repeating events in individual clusters are compatible with the omori s law for the decay rate of regional aftershocks a sliding window waveform cross correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events we find clear step like delays in the direct s and early s coda waves sharp seismic velocity reduction immediately after the duzce main shock followed by gradual logarithmic type recoveries a gradual increase of seismic velocities is also observed before the duzce main shock probably reflecting post seismic recovery from the earlier izmit main shock the temporal behavior is similar at each station for clusters at various source locations indicating that the temporal changes of material properties occur in the top most portion of the crust the effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones and generally decrease with normal distance from the fault a strong correlation between the co seismic delays and intensities of the strong ground motion generated by the duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material

volcanic coseismic and seasonal changes detected at white island whakaari volcano new zealand using seismic ambient noise ambient noise interferometry is becoming increasingly popular for studying seismic velocity changes such changes contain information on the structural and mechanical properties of earth systems application to monitoring however is complicated by the large number of processes capable of inducing crustal velocity changes we demonstrate this at white island volcano over a 10 year period containing multiple well documented eruptions using individual seismic stations we detect velocity perturbations that we ascribe to volcanic activity large earthquakes and seasonality distant seismic stations capture widespread nonvolcanic changes that are also present at the volcano comparison between velocity changes recorded by distant and local stations then allows us to distinguish volcanic phenomena from seasonality through this we resolve distinct features in ambient noise derived velocity changes that relate to volcanic unrest and a phreatic eruption illustrating the strength of the approach plain language summary detecting small changes in physical properties at volcanoes is important toward effectively forecasting volcanic eruptions one approach is to monitor the speed of seismic waves at the earth s surface which are sensitive to changes in the rock they pass through at volcanoes such changes may manifest through subsurface pressure increases preceding eruptions correctly interpreting these changes however is complicated many other nonvolcanic processes are also capable of producing similar wave speed perturbations at white island volcano new zealand we detect changes associated with volcanic activity large earthquakes and weather related processes using seismometers located on the volcano we demonstrate that volcanic and nonvolcanic features can be distinguished by comparing changes recorded at the volcano with those recorded at large distances where volcanic changes are not expected to be observed through this we resolve distinct differences that can be related to volcanic activity illustrating the strength of this approach

detecting seasonal variations in seismic velocities within los angeles basin from correlations of ambient seismic noise summary we analyse 3 yr of continuous seismic records from broad band stations of the caltech regional seismic network ci in vicinity of the los angeles basin using correlations of ambient seismic noise relative velocity variations in the order of 0 1 per cent can be measured between all interstation pairs it is the first time that such an extensive study between 861 interstation pairs over such a large area has been carried out we perform these measurements using the stretching technique assuming that one of the two waveforms is merely a stretched version of the other obviously this assumption is always violated and the two waveforms are generally decorrelated due to temporal changes in the earth crust due to different sources or simply because the cross correlations are not fully converged we investigate the stability of these measurements by repeating each measurement over various time windows of equal length on average between all interstation pairs in the los angeles basin a seasonal signal in the relative velocity variation is observed with peaks and troughs during winter and summer time respectively generally the observed signal decreases with increasing interstation distance and relative traveltime perturbations can only be measured up to an interstation distance of 60 km furthermore the traveltime perturbations do not depend on azimuth of station pairs suggesting that they are not related to seasonal variations of the noise sources performing a simple regionalization by laterally averaging measurements over a subset of stations we found the sedimentary basin showing the most consistent signal and conclude that the observed seasonality might be induced either by changes in the ground water aquifer or thermo elastic strain variations that persist down to a depth of 15 22 km

suppression of near surface scattered body to surface waves a steerable and nonlinear filtering approach we present an approach based on local slope estimation for the separation of scattered surface waves from reflected body waves the direct and scattered surface waves contain a significant amount of seismic energy they present great challenges in land seismic data acquisition and processing particularly in arid regions with complex near surface heterogeneities e g dry river beds wadis large escarpments and karst features the near surface scattered body to surface waves which have comparable amplitudes to reflections can mask the seismic reflections these difficulties added to large amplitude direct and back scattered surface rayleigh waves create a major reduction in signal to noise ratio and degrade the final sub surface image quality removal of these waves can be difficult using conventional filtering methods such as an f k filter without distorting the reflected signal the filtering algorithm we present is based on predicting the spatially varying slope of the noise using steerable filters and separating the signal and noise components by applying a directional nonlinear filter oriented toward the noise direction to predict the noise and then subtract it from the data the slope estimation step using steerable filters is very efficient it requires only a linear combination of a set of basis filters at fixed orientation to synthesize an image filtered at an arbitrary orientation we apply our filtering approach to simulated data as well as to seismic data recorded in the field to suppress the scattered surface waves from reflected body waves and we demonstrate its superiority over conventional f k techniques in signal preservation and noise suppression

chronology and products of the 2000 eruption of miyakejima volcano japan lateral migration of magma away from miyakejima volcanic island japan generated summit subsidence associated with summit explosions in the summer of 2000 an earthquake swarm beneath miyakejima began on the evening of 26 june 2000 followed by a submarine eruption the next morning strong seismic activity continued under the sea from beneath the coast of miyakejima to a few tens of kilometers northwest of the island summit eruptive event began with subsidence of the summit on 8 july and both explosions and subsidence continued intermittently through july and august the most intense eruptive event occurred on 18 august and was vulcanian to subplinian in type ash lofted into the stratosphere fell over the entire island and abundant volcanic bombs were erupted at this time another large explosion took place on 29 august this generated a low temperature pyroclastic surge which covered a residential area on the northern coast of the island the total volume of tephra erupted was 9 3 106 m3 dre much smaller than the volume of the resulting caldera 6 108 m3 migration of magma away from miyakejima was associated with crustal extension northwest of miyakejima and coincident shrinkage of miyakejima island itself during july august 2000 this magma migration probably caused stoping of roof rock into the magma reservoir generating subsurface cavities filled with hydrothermal fluid and or magmatic foam and formation of a caldera oyama caldera at the summit interaction of hydrothermal fluid with ascending magma drove a series of phreatic to phreatomagmatic eruptions it is likely that new magma was supplied to the reservoir from the bottom during waning stage of magma s migration resulting in explosive discharge on 18 august the 18 august event and phreatic explosions on 29 august produced a conduit system that allowed abundant so2 emission as high as 460 kg s 1 after the major eruptive events were over at the time of writing inhabitants of the island about 3 000 have been evacuated from miyakejima for more than 3 years

geomagnetic changes associated with the dike intrusion during the 2000 miyakejima eruptive activity japan abstract a marked magnetic field change exceeding 200 nt was detected by a three component magnetometer at the beginning of the 2000 miyakejima eruptive activity the change in the magnetic field is correlated with an earthquake activity and crustal deformation which indicate a dike intrusion very close to the observation site a dike model based on the crustal deformation data shows that the primary cause of the magnetic change was the piezomagnetic effect associated with the dike emplacement and that the other potential factors including thermal and electrokinetic effects were unlikely to be its principal cause we obtained a new dike model by combining both the crustal deformation and the magnetic field change which is sensitive to the strong stress changes near the edges of dike the new dike model shows lateral and upward propagation of the dike during its intrusion and suggests that shallow cracks induced by the dike intrusion near the ground surface contributed to the changes seen in the magnetic field

monitoring of phreatic eruptions using interferometry on retrieved cross correlation function from ambient seismic noise results from mt ruapehu new zealand abstract since the last major eruption in 1995 96 mt ruapehu has erupted twice on 4 october 2006 and on 25 september 2007 these events occurred without any clear precursors and were mostly phreatic explosions the technique of interferometry on retrieved cross correlation function from ambient seismic noise irccasn is used to monitor subtle temporal changes of mt ruapehu s elastic properties the computation of cross correlation functions of seismic noise recorded at several stations around the volcano allowed us to observe variations during the 2006 eruption period the comparison between a reference cross correlation function and a current cross correlation function allows us to infer relative seismic velocity variations a 0 8 decrease of relative seismic velocity in the edifice starting two days before the 2006 eruption was observed this drop is due to a reversible and ephemeral effect which can be attributed to a pressurization of a magma pocket beneath the east flank of ruapehu due to new magma entering a small reservoir this pressure increase produced an inflation of the east flank of ruapehu and opened fractures in this area leading to a localised drop of seismic velocity we conducted the same analysis for the 2007 eruption but no significant seismic velocity variation was observed this difference is possibly due to the varying time scales of pressurization for the two events and also the irccasn time resolution an analysis of the seismicity before these two eruptions allows us to propose a conceptual model which explains the velocity drop for the 2006 eruption and the lack of velocity variations for the 2007 eruption since there was no surface deformation recorded by the geonet gps network we modelled the maximum radius and pressure change for a simple mogi point source at 5 km depth that produced no deformation we inferred the maximum fresh magma volume of 0 0017 km 3 entering the reservoir as a detectability threshold for gps ground deformation measurements

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

predicting stress induced velocity anisotropy in rocks a simple transformation using measured isotropic vp and vs versus hydrostatic pressure is presented for predicting stress induced seismic velocity anisotropy in rocks the compliant crack like portions of the pore space are characterized by generalized compressional and shear compliances that are estimated from the isotropic vp and vs the physical assumption that the compliant porosity is crack like means that the pressure dependence of the generalized compliances is governed primarily by normal tractions resolved across cracks and defects this allows the measured pressure dependence to be mapped from the hydrostatic stress state to any applied nonhydrostatic stress predicted p and s wave velocities agree reasonably well with uniaxial stress data for barre granite and massillon sandstone while it is mechanically similar to methods based on idealized ellipsoidal cracks the approach is relatively independent of any assumed crack geometry and is not limited to small crack densities

seismic scattering near the earth s surface the seismic coda is usually thought to be composed of waves scattered from the heterogeneities in the earth three classes of scattering mechanisms have been recognized repeated specular reflection of primary waves in a uniformly layered structure volume scattering of primary waves from localized volume heterogeneities and scattering of primary waves from irregularities in an otherwise generally layered structure the presence of the earth s surface complicates the description of all these scattering phenomena whenever the scattering obstacles are near or at the surface in this paper i review work which demonstrates the effects of scattering near the earth s surface emphasizing three general areas of investigation scattering of body waves from an irregular free surface scattering of body waves in irregular layers and propagation of surface waves across irregular topography or in irregular wave guides most of the effects of importance have been recognized in model studies of idealized geometries observational evidence in support of the model studies exists but is often inferred few controlled experiments to measure scattering have been performed

travel time monitoring with vibroseis abstract the general failure of searches for precursory seismic travel time variations associated with strike slip earthquakes in california has led to this investigation into the feasibility of using a controlled source seismic method to improve significantly the precision of travel time measurements and to investigate the nature of any detected travel time changes travel times have been measured over a period of several years at sites south of hollister california along the seismically active creeping zone of the san andreas fault using a single channel vibroseis system with real time on site data processing at a site near bear valley 7 km from the fault no variations in the travel time of a deep crustal reflection were observed that could be associated with local earthquakes however significant variations 0 5 to 2 5 msec of first arrival travel times observed near the cienega winery may have been associated with a series of nearby earthquakes and with a creep event major sources of measurement error have been identified in source variations and in rainfall induced variations in near surface properties the former limits the precision of the deep reflection measurements to about 0 05 per cent of the travel time and the first arrival measurements to about 0 1 per cent of the travel time the second effect is apparent as seasonal oscillations in travel time of as much as 15 to 20 msec and also in wavelet amplitude and waveform giving an implied travel time accuracy of about 0 2 per cent for the deep reflection measurements and about 1 per cent for first arrivals while these noise levels are disappointing they can be reduced significantly by improved field procedures ongoing experiments are testing such procedures

seismic interferometry turning noise into signal turning noise into useful data every geophysicist s dream and now it seems possible the field of seismic interferometry has at its foundation a shift in the way we think about the parts of the signal that are currently filtered out of most analyses complicated seismic codas the multiply scattered parts of seismic waveforms and background noise whatever is recorded when no identifiable active source is emitting and which is superimposed on all recorded data those parts of seismograms consist of waves that reflect and refract around exactly the same subsurface heterogeneities as waves excited by active sources the key to the rapid emergence of this field of research is our new understanding of how to unravel that subsurface information from these relatively complex looking waveforms and the answer turned out to be rather simple this article explains the operation of seismic interferometry and provides a few examples of its application

suppressing near receiver scattered waves from seismic land data upgoing body waves that travel through a heterogeneous near surface region can excite scattered waves when the scatteringtakesplaceclosetothereceivers secondarywaves interferewiththeupcomingreflections diminishingthecontinuity of the wavefront we estimate a near surface scattering distribution from a subset of a data record and use this scattering distribution to predict the secondary waves of the entire data record with a wave theoretical model for near receiver scattering we then subtract the predicted scattered waves from the record to obtain the wavefield that would have been measured in the absence of near surface heterogeneities we apply this method to part of a field data set acquired in an area with significant near surface heterogeneity themainresultofourprocessingschemeisthatweeffectively remove near surface scattered waves this in turn increases trace to trace coherence of reflection events moreover applicationofourmethodimprovestheresultsobtained from just an application of a dip filter because we remove parts of the scattered wave with apparent velocities that are typically accepted by the pass zone of the dipfilter based on these results we conclude that our method for suppressing near receiver scattered waves works well on densely sampled land data collected in areas with strong near surface heterogeneity

spatio temporal changes in seismic velocity associated with the 2000 activity of miyakejima volcano as inferred from cross correlation analyses of ambient noise abstract miyakejima volcano japan showed strong and interesting volcanic activities in 2000 with the occurrence of dike intrusions caldera formation and a large amount of gas emission to understand temporal changes of the volcanic structure associated with this activity we apply ambient noise correlation analyses to the seismic records of five seismic stations at miyakejima volcano from july 1999 to december 2002 we calculate cross correlation functions ccfs for eight available station pairs at frequency bands of 0 4 0 8 hz and 0 8 1 6 hz and retrieve the rayleigh waves propagating with a group velocity of about 1 0 km s comparing ccfs obtained for each day with the reference ccfs that are calculated from a few months of data we estimate seismic velocity changes before and after the 2000 activity results at the two frequency bands show seismic velocity increase and decrease shallow structure at the volcanic edifice on the flanks experienced a seismic velocity increase up to 3 3 on the other hand the regions located close to the collapsed caldera show a seismic velocity decrease down to 2 3 to understand the mechanisms that may introduce these seismic velocity increase and decrease we examine several possible mechanisms of the velocity changes stress changes due to volcanic pressure sources and caldera formation and topographic changes by using the deflation sources previously determined from gps data we calculate the stresses at depths of 0 km and 1 km at the volcano the results suggest that the seismic velocity increases observed at seismic paths on the volcanic flank are explained by the compression caused by the deflation sources associated with magma activity in 2000 stress sensitivities of the seismic velocity changes are estimated to be 1 5 10 2 to 2 2 10 2 mpa 1 and 4 6 10 3 to 2 4 10 2 mpa 1 at frequency bands of 0 4 0 8 hz and 0 8 1 6 hz respectively which are in the range of the previously reported results our three dimensional numerical calculations on seismic wave propagation show that the seismic velocity decreases observed at seismic paths crossing the collapsed caldera can be explained by topographic changes introduced by a caldera we further suggest that the seismic velocity decreases detected around the regions close to the caldera may originate from the dilated regions cause by formation of the caldera

earthquake swarms preceding the 2000 eruption of miyakejima volcano japan the first sign of magma accumulating beneath miyakejima an island volcano in the northern izu islands japan came at around 18 00 on 26 june 2000 when a swarm of earthquakes was detected by a volcano seismic network on the island earthquakes occurred initially beneath the southwest flank near the summit and gradually migrated west of the island where a submarine eruption occurred the next morning earthquakes then migrated further to the northwest between miyakejima and kozushima another volcanic island and developed to the most intense earthquake swarm ever observed in and around japanese archipelago to better image how the initial magma intrusion occurred we relocated hypocenters by using a station correction method and a double difference method the relocated epicenters are generally concentrated near the upper bound of dyke intrusions inferred from geodetic studies throughout the initial stages of the 2000 eruption at miyakejima from 26 to 27 june 2000 as for seismic activity westward off miyakejima in the morning on 27 june hypocenters from both a nationwide seismic network that were relocated by the double difference method and those from the volcano seismic network relocated by the station correction method formed a very shallow cluster that ascended slowly with time as it propagated northwestward from miyakejima this suggests that the dykes have both a radial and upward component of movement

advanced interferometric synthetic aperture radar insar time series analysis using interferograms of multiple orbit tracks a case study on miyake jima 1 insar time series analysis is an effective tool for detecting spatially and temporally complicated volcanic deformation to obtain details of such deformation we developed an advanced insar time series analysis using interferograms of multiple orbit tracks considering only right or only left looking sar observations incidence directions for different orbit tracks are mostly included in a common plane therefore slant range changes in their interferograms can be expressed by two components in the plane this approach estimates the time series of their components from interferograms of multiple orbit tracks by the least squares analysis and higher accuracy is obtained if many interferograms of different orbit tracks are available additionally this analysis can combine interferograms for different incidence angles in a case study on miyake jima we obtained a deformation time series corresponding to gps observations from palsar interferograms of six orbit tracks the obtained accuracy was better than that with the sbas approach demonstrating its effectiveness furthermore it is expected that higher accuracy would be obtained if sar observations were carried out more frequently in all orbit tracks the deformation obtained in the case study indicates uplift along the west coast and subsidence with contraction around the caldera the speed of the uplift was almost constant but the subsidence around the caldera decelerated from 2009 a flat deformation source was estimated near sea level under the caldera implying that deceleration of subsidence was related to interaction between volcanic thermal activity and the aquifer

continuous monitoring of propagation velocity of seismic wave using across 1 temporal variations in the p and s wave travel times were continuously monitored using across which is a newly developed system for utilizing accurately controlled continuous sinusoids for seismic exploration we made an experiment lasting for 15 months at a site near the nojima fault which ruptured during the 1995 kobe earthquake m7 2 the elastic waves emitted from the across vibrators located at the surface were observed with seismometers at 800 m and 1700 m depth the long term variations of about 2 ms for both the p and s waves were observed during the 15 month experiment the major cause of these variations is identified to be that of a near surface property which changes the near source vibration the variations in the deeper structures estimated after the reduction of the near source variation were about 0 5 ms an s wave delay of about 1 ms was detected due to the 2000 western tottori earthquake m6 6

analysis and removal of multiply scattered tube waves because of irregularities in or near the borehole vertical seismic profiling vsp or crosswell data can be contaminated with scattered tube waves these can have a large amplitude and can interfere with weaker upcoming reflections destroying their continuity this type of organized noise cannot always be removed with filtering methods currently in use we propose a method based on modeling the scattered tube wave field and then subtracting it from the total data set we assume that the scattering occurs close to the borehole axis and therefore use a 1 d impedance function to characterize borehole irregularities estimation of this impedance function is one of the first steps our method also accounts for multiply scattered tube waves we apply the method to an actual vsp data set and conclude that the continuity of reflected upcoming events improves significantly in a washout zone

monitoring a temporal change of seismic velocity in a volcano application to the 1992 eruption of mt merapi indonesia multiplets i e events with similar waveforms are selected from shallow earthquakes recorded on merapi volcano indonesia before the eruption of february 2nd 1992 two multiplet families are found with their sources close to the summit their seismograms are analyzed using the moving window cross spectrum technique which measures the precise time delay between seismic phases in the entire seismogram for both families of multiplets a gradual decrease in the arrival times of coda waves is observed as a function of the date prior to the eruption coda waves are becoming progressively faster up to 1 2 per cent as the time interval to the eruption shortens this observation is interpreted as the consequence of an increase in the seismic velocity inside the volcano the increase in velocity started in may 1991 and was observed until september 1991 4 months before the eruption this velocity increase may be related to an increase in pressure in the magma chamber or in the conduits and to the resulting closure of the surrounding cracks

the end ordovician glaciation and the hirnantian stage a global review and questions about late ordovician event stratigraphy abstract this paper proposes a global review of hirnantian event stratigraphy the hirnantian gssp in south china is tentatively correlated with latest ordovician strata from the peri gondwanan glacial regions problems of biostratigraphical correlation are highlighted at a worldwide scale the major biostratigraphically useful fossil groups graptolites chitinozoans brachiopods conodonts acritarchs are analysed and their limits for global correlation of the uppermost ordovician are discussed palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective late ordovician global biostratigraphical scheme as an alternative correlative tool the hice hirnantian isotopic curve excursion event is often put forward in the literature however carbon isotope chemostratigraphy shows like biostratigraphy some limits to the present state of knowledge no good independent biostratigraphical control of the hice exists in both shallow carbonate deposits and deeper shaly ones recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic 13corg and inorganic 13ccarb carbon species further complicating the use of the hice as an isochronous benchmark all of these difficulties for hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future precise late ordovician and early silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the big five extinctions

imaging of near surface heterogeneities by scattered elastic waves abstractwe have developed an elastic reverse time migration rtm approach for imaging near surface heterogeneities such as karst features using scattered waves e g body to p s and surface waves knowledge of location and strength of the scatterers helps in seismic imaging survey planning and geotechnical site characterization to model seismic wave propagation for rtm we use an elastic staggered grid finite difference scheme the scattered body to surface waves provide optimal illumination and wavenumber coverage of the near surface as they travel horizontally along the free surface we tested the elastic rtm approach on synthetic data simulated using a finite difference solver and found it to be robust

seismological studies at parkfield iv variations in controlled source waveform parameters and their correlation with seismicity 1987 to 1995 since june 1987 at parkfield california the 10 station borehole network of three component sensors has been illuminated 52 times using a shear wave vibrator in three orientations at up to eight source points in a search for temporal changes in elastic wave p and s velocities anisotropy or attenuation the monitoring interval includes the beginning and end of a severe 3 yr drought and four earthquake sequences two of which produced the only a level alerts to date in the parkfield prediction experiment a comprehensive study of the entire data set reveals a progressive travel time advance in the coda of s waves propagating in a localized region southeast of middle mountain the anomalous wave field exhibits high apparent velocities suggesting deep penetration of the fault zone although similar changes are not seen in waveforms from repeating similar microearthquakes accompanying the changes in travel time were systematic variations in spectral content and polarization of the same segments of the wave field these variations correlate well in time and space with significant features of seismicity fault creep and water levels at parkfield a preferred mechanism for the phenomenon is changing hydrologic conditions along the affected stretch of the fault zone possibly deformation induced that perturb the shallow propagating s coda in the upper few hundred meters of section

coda wave interferometry for estimating nonlinear behavior in seismic velocity in coda wave interferometry one records multiply scattered waves at a limited number of receivers to infer changes in the medium over time with this technique we have determined the nonlinear dependence of the seismic velocity in granite on temperature and the associated acoustic emissions this technique can be used in warning mode to detect the presence of temporal changes in the medium or in diagnostic mode where the temporal change in the medium is quantified

seismic evidence for rock damage and healing on the san andreas fault associated with the 2004 m 6 0 parkfield earthquake we deployed a dense linear array of 45 seismometers across and along the san andreas fault near parkfield a week after the m 6 0 parkfield earthquake on 28 september 2004 to record fault zone seismic waves generated by aftershocks and explosions seismic stations and explosions were co sited with our previous exper iment conducted in 2002 the data from repeated shots detonated in the fall of 2002 and 3 months after the 2004 m 6 0 mainshock show 1 0 1 5 decreases in seismic wave velocity within an 200 m wide zone along the fault strike and smaller changes 0 2 0 5 beyond this zone most likely due to the coseismic damage of rocks during dynamic rupture in the 2004 m 6 0 earthquake the width of the damage zone characterized by larger velocity changes is consistent with the low velocity waveguide model on the san andreas fault near parkfield that we derived from fault zone trapped waves li et al 2004 the damage zone is not symmetric but extends farther on the southwest side of the main fault trace waveform cross correlations for repeated aftershocks in 21 clusters with a total of 130 events located at different depths and distances from the array site show 0 7 1 1 increases in s wave velocity within the fault zone in 3 months starting a week after the earthquake the velocity recovery indicates that the damaged rock has been healing and regaining the strength through rigidity recovery with time most likely due to the closure of cracks opened during the mainshock we estimate that the net decrease in seismic velocities within the fault zone was at least 2 5 caused by the 2004 m 6 0 parkfield earthquake the healing rate was largest in the earlier stage of the postmainshock healing process the magnitude of fault healing varies along the rupture zone being slightly larger for the healing beneath middle mountain correlating well with an area of large mapped slip the fault healing is most promi nent at depths above 7 km

structural evolution beneath sakurajima volcano japan revealed through rounds of controlled seismic experiments abstract structural evolution beneath an active volcano is detected as the variation of seismic reflectivity through controlled seismic experiments which is interpreted as being associated with discharging magma the target of the present study is sakurajima volcano which is one of the most active volcanoes in japan six rounds of seismic experiments with controlled sources have been conducted annually at the volcano two seismic reflection profiles are obtained from the datasets for each successful round of experiments the experiments reveal clear annual variation in seismic reflectivity at a depth of 6 2 km in the northeastern part of sakurajima the reflectivity is maximum in december 2009 upon the first intrusion of magma and decreases gradually until december 2013 which coincides with the inflation and deflation cycle of sakurajima volcano reflectivity variation occurred in the embedded clear reflector at depth an evolving sandwiched structure in the intermediate layer is used as the reflector model lower velocity magma embedded in the intermediate layer and its succeeding velocity increment explain the variation range of reflectivity this is interpreted as a temperature decrease associated with discharging magma at depth the present study describes a new approach for instantaneously sensing magma properties and for monitoring active volcanoes

long term observation of in situ seismic velocity and attenuation 1 we conducted 1 year continuous measurements of in situ compressional wave velocity and attenuation across a distance of 12 m in a vault at the coast of miura bay japan using a piezoelectric transducer as the ultrasonic wave source we detected for the first time periodic variations in both velocity and attenuation with amplitudes of 0 3 and of 4 respectively corresponding to the diurnal and semidiurnal lunar and solar tides these variations correlate well with the in situ areal strain change due to tidal ocean loading estimated from the strain records about 100 m away from the observational site relative minimum velocity and relative maximum attenuation occur at every time of relative maximum areal strain expansion suggesting that opening closure of pores or cracks of in situ rocks is responsible for these periodic variations the velocity variation shows a remarkable 14 day periodicity corresponding to the spring and neap tides arising from a nonlinear response of the velocity change to tidal strain change such that the velocity change depends on how slowly the tidal dilatation departs from its peak value

nonlinear soil response a reality geotechnical models consistently indicate that the stress strain relationship of soils is nonlinear and hysteretic especially at shear strains larger than 10 5 to 10 4 nonlinear effects such as an increase in damping and reduction in shear wave velocity as excitation strength increases are commonly recognized in the dynamic loading of soils on the other hand these effects are usually ignored in seismological models of ground motion prediction because of the lack of compelling corroborative evidence from strong motion observations the situation is being changed by recently obtained data explicit evidence of strong motion deamplification accompanied by changes in resonant frequencies are found in the data from the 1985 michoacan mexico and the 1989 loma prieta california earthquakes the events recorded by the vertical and surface accelerograph arrays in taiwan as well as a number of other events throughout the world evidence of nonlinear behavior becomes apparent beyond a threshold acceleration of 100 to 200 gal nonlinearity is considerable in cohesionless soil but may be negligible in stiff soils the findings of recent years indicate that nonlinear site effects are more common than previously recognized in strong motion seismology

spatiotemporal gravity changes at miyakejima volcano japan caldera collapse explosive eruptions and magma movement 1 calderas are ubiquitous topographic features of volcanos yet caldera formation itself has not been recorded intensively by modern measurement techniques here we report the spatiotemporal gravity changes before and after caldera collapse at the miyakejima volcano japan in 2000 a gravity decrease of as much as 145 gal 1 gal 10 8 m s2 at the summit area since june 1998 was detected 2 days prior to the collapse interpreted as reflecting the formation of a large void beneath the volcano gravity changes detected after the initiation of collapse can mostly be corrected by the effect of collapsed topography from which a rapid rate of collapse of more than 1 6 107 m3 d can be inferred correcting for the effect of topography change we identified a temporal decrease in gravity from the middle of july to late august despite ground subsidence the gravity decrease is interpreted as a reduction of the density in a cylindrical conduit attributed to water inflow from an ambient aquifer that also promoted intensive magma water interaction and subsequent explosive eruptions from september to at least november 2000 gravity values at all sites increased significantly by a degree that cannot be explained by ground displacement alone we interpret this temporal evolution as primarily due to magma ascent and refilling of the conduit

sudden drop of seismic velocity after the 2004 mw 6 6 mid niigata earthquake japan observed with passive image interferometry 1 passive image interferometry pii is a seismological method employing ambient seismic noise to monitor the temporal evolution of mean shear wave velocity within the earth first the elastic green s tensor between two seismometers is computed from the cross correlation of seismic noise recorded during a certain period second the constructed seismograms of different time periods are treated as earthquake multiplets and small time shifts in their coda are used to invert a relative change in mean shear wave velocity when applied to the source region of the 2004 mw 6 6 mid niigata earthquake japan centroid depth 5 km we used noise recorded at six seismometers located at a distance of less than 25 km from the epicenter daily inversions during the 2 months before and after the earthquake show a sudden drop of mean shear wave velocity of some tenths of a percent at the time of the mid niigata earthquake using noise in two frequency bands 0 1 0 5 hz and 2 8 hz we find similar amplitudes for the velocity drop which indicates that changes are not restricted to the shallow subsurface possible interpretations of this velocity decrease are a decrease of crustal stress after the earthquake a nonlinear site response in the shallow subsurface layer due to strong ground motion or structural weakening due to the creation of new fractures in the source area of the earthquake

surface wave tomography from microseisms in southern california received 5 april 2005 revised 23 may 2005 accepted 9 june 2005 published 26 july 2005 1 since it has already been demonstrated that point topoint seismic propagation green functions can be extracted from seismic noise it should be possible to image earth structure using the ambient noise field seismic noise data from 148 broadband seismic stations in southern california were used to extract the surface wave arrival times between all station pairs in the network the seismic data were then used in a simple but densely sampled tomographic procedure to estimate the surface wave velocity structure within the frequency range of 0 1 0 2 hz for a region in southern california the result compares favorably with previous estimates obtained using more conventional and elaborate inversion procedures this demonstrates that coherent noise field between station pairs can be used for seismic imaging purposes citation sabra k g p gerstoft p roux w a kuperman and m c fehler 2005 surface wave tomography from microseisms in southern california geophys res lett 32 l14311 doi 10 1029 2005gl023155

velocity independent redatuming a new approach to the near surface problem in land seismic data processing it is well known that a complex near surface encountered in areas such as desert mountainous and glacial till environments can significantly distort primary reflections from deeper horizons of interest this has been and remains a major problem in seismic data processing

a dike intrusion model in and around miyakejima niijima and kozushima in 2000 from june 26 2000 an earthquake swarm started in miyakejima about 50 km south off honshu central japan eruptions of miyakejima and five large earthquakes with magnitudes 6 0 and above occurred for the following 2 months together with a large number of smaller earthquakes over 100 000 in this study we focused on spatio temporal crustal deformation observed at gps stations in this area in order to explain the seismicity and the crustal deformation we considered a dike intrusion model placed between miyakejima and kozushima then we attempted to obtain spatio temporal distributions the amount of the dike intrusion deflation beneath miyakejima and fault slips of the seven large events by geodetic data inversion for this purpose we divided the time series of gps data from june 12 to august 27 into 10 periods which are related to significant events in order to find location depth strike and dip of the dike plane and a depth of deflation beneath miyakejima we used the monte carlo method and tested 10 000 models for each period as a result we estimated that the total amount of the dike intrusion and the deflation beneath miyakejima reached about 1 1 10 9 and 5 4 10 8 m 3 respectively the maximum amount of the dike intrusion and the deflation beneath miyakejima reached over 3 5 10 8 m 3 during the period from july 20 to july 28 2000 and over 1 7 10 8 m 3 during the period from june 15 to june 28 2000 respectively temporal change of the amount of the dike intrusion corresponds well to that of the deflation beneath miyakejima until the middle of july however since large amount of the dike intrusion from july 20 to 28 did not correspond to that of the deflation beneath miyakejima we deduced that the magma source changed from miyakejima to kozushima and the magma might come from sub crustal magma pockets from the middle of july if we assume the open crack associated with the dike intrusion and the deflation beneath miyakejima are filled with magma with a density of 2500 kg m 3 the mass would be about 2 75 10 9 and 1 35 10 9 tons respectively we deduced that at least 1 4 10 9 tons of magma came from sub crustal magma pockets d 2002 elsevier science b v all rights reserved

caldera collapse during the 2000 eruption of miyakejima volcano japan a collapsed caldera 1 6 km in diameter and 450 m in depth was formed at the summit of miyakejima volcano during the 2000 eruption the collapsed caldera appeared on 8 july with a minor phreatic eruption 12 days after seismic activity and magma intrusion occurred northwest of the volcano growth of the caldera took from 8 july to the middle of august with seismic swarms associated with the continuous intrusion of magma northwest of the volcano the growth rate of the caldera was about 1 4 107 m3 day and the final volume of the collapsed caldera was about 6 108 m3 major phreatomagmatic eruptions produced a total of about 1 6 1010 kg 1 1 107 m3 of volcanic ash after caldera growth the caldera structure and the nature of the eruptive materials of the first collapse on 8 july suggest that the surface subsidence was caused by the upward migration of a steam filled cavity with stoping of the roof rock above the magma reservoir the diameter of the stoping column was estimated to be 600 700 m from circumferential faults that developed in the caldera floor and the collapse of the caldera wall enlarged the diameter of the caldera to 1 6 km the total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1 6 2 1 km

increase of shear wave velocity before the 1998 eruption of merapi volcano indonesia 1 we infer temporal changes in the elastic properties of the edifice of merapi volcano java indonesia before its eruption in 1998 by analyzing multiply scattered elastic waves excited by a repeatable controlled seismic source a pre eruptive increase of shear wave velocity which correlates well with pre eruptive seismicity and dome growth is revealed the method can be used as a pressure gauge for pressure changes inside of volcanoes because increasing pressures in rocks are known to cause proportionally increasing elastic wave velocities

real time monitoring of relative velocity changes using ambient seismic noise at the piton de la fournaise volcano la reunion from january 2006 to june 2007 we present the results of a real time method based on coda wave interferometry from seismic noise cross correlation functions for relative seismic velocity variations monitoring on a volcanic edifice the ambient seismic noise at the piton de la fournaise volcano on la reunion island is analyzed from january 2006 to june 2007 during this period five eruptions occurred showing a great diversity in eruption duration intensity and eruptive fissure location two different methods are used to compute the velocity variations in order to compare their stability in quasi real time routine we compare the obtained velocity variations with the surface deformation observed by gps and extensometers networks this allows us to identify and quantify three major processes at the origin of seismic wave velocity variations in the edifice firstly the observed pre eruptive summit inflation is accompanied by a decrease in seismic velocity secondly the edifice deflation following the opening of an eruptive fissure is characterized by an increase of the velocity finally the summit caldera collapse generates a strong velocity drop coda wave interferometry from seismic noise cross correlation functions in quasi real time may allow us to forecast eruption and constrain the processes taking place in the volcanic plumbing system

submarine flank eruption preceding caldera subsidence during the 2000 eruption of miyakejima volcano japan during the early part of a seismic swarm preceding eruption and caldera formation at miyakejima volcano discoloured sea surfaces were observed 1 5 km off the western coast of miyakejima on 27 june 2000 a later survey of the area using a multi beam side scan sonar and a remotely operated small submarine revealed four craters of 20 30 m diameter aligned east west in a 100 10 30 m area on the seafloor with hot water at 140 c being released from one of the centres each crater consists of submarine spatter overlain in part by scoria lapilli dredged spatter from the craters was fresh and there was no evidence of activity of marine organisms on the spatter surface indicating that the discoloured sea surface resulted from magmatic eruption on the seafloor this eruption occurred when a westward propagating seismic swarm initiated beneath miyakejima s summit passed through the area finding new magma on the seafloor demonstrates that this seismic swarm was associated with intruding magma moving outward from beneath miyakejima submarine spatter shows flattened shapes with a brittle crust formed by cooling in water and its composition is aphyric andesite of 54 wt sio2 the spatter is similar in whole rock and mineral composition to spatter erupted in 1983 however the wide range of cl in melt inclusions in plagioclase of the 27 june submarine spatter shows that it is not simply a remnant of the 1983 magma which has only high cl melt inclusions in plagioclase the mixed character of melt inclusions suggests involvement of a magma with low cl melt inclusions the magma erupted explosively on 18 august from miyakejima s summit considered as the second juvenile magma in this eruption contains low cl melt inclusions in plagioclase based on these observations and the eruption sequence we present the following model 1 a shallow magma chamber was filled with a remnant of 1983 magma that had evolved to a composition of 54 55 wt sio2 2 injection of the 18 august magma into this chamber generated a mixed magma having a wide range of cl in melt inclusions contained plagioclase the magma mixing might have occurred shortly before the submarine eruption and could have been a trigger for the initiation of the removal of magma from the chamber as an extensive dyke which eventually led to caldera subsidence