microseismic signatures of non linear pore fluid pressure diffusion summary most fractured porous rocks show a strong dependence of their fluid transport properties on the effective pressure and thus on the pore fluid pressure we investigate the case where the hydraulic transport properties are exponentially dependent on pore fluid pressure as a consequence the process of pore fluid pressure diffusion is governed by a non linear diffusion equation solutions of this equation that describe fluid injection experiments are analysed and the associated microseismic signatures are explored in particular it is shown that this process of non linear pore fluid pressure diffusion allows us to define two signatures one which is the triggering front corresponding to the triggering of microseismic events without changing the fluid transport properties significantly the other signature is the so called fracturing domain it triggers the majority of microseismic events and alters the fluid transport properties significantly evidence for the microseismic signature of a fracturing domain at the fenton hill hydrofrac experiment is given

subtropical anticyclones and summer monsoons the summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans in winter the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography here the mechanics of the summer and winter lowertropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations by prescribing in the model the heatings associated with several of the world s monsoons it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the kelvin wave response to the monsoon heating over the continent to the west a poleward flowing low level jet into a monsoon such as the great plains jet is required for sverdrup vorticity balance this jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region the low level jet into north america induced by its monsoon heating is augmented by a remote response to the asian monsoon heating the rossby wave response to the west of subtropical monsoon heating interacting with the midlatitude westerlies produces a region of adiabatic descent it is demonstrated here that a local diabatic enhancement can lead to a strengthening of the descent longitudinal mountain chains act to block the westerly flow and also tend to produce descent in this region below the descent sverdrup vorticity balance implies equatorward flow that closes off the subtropical anticyclone to the west and induces cool upwelling in the ocean through ekman transport feedbacks involving for example sea surface temperatures may further enhance the descent in these regions the conclusion is that the mediterranean type climates of regions such as california and chile may be induced remotely by the monsoon to the east hence it can be argued that the subtropical circulation in summer comprises a set of weakly interacting monsoon systems each involving monsoon rains a low level poleward jet a subtropical anticyclone to the east and descent and equatorward flow to the west in winter it is demonstrated how the nonlinear interaction between the strong zonal mean circulation associated with the winter hadley cell and the mountains can define many of the large scale features of the subtropical circulation the blocking effect of the longitudinal mountain chains is shown to be very important subsequent diabatic effects such as a local diabatic enhancement would appear to be essential for producing the observed amplitude of these features

salinity driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the north atlantic abstract an isopycnic coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation the model performance is illustrated by a 30 year spinup run with coarse horizontal resolution 2 mesh in a domain with north atlantic topography extending from 10 to 60 n latitude the vertical structure encompasses 10 isopycnic layers in steps of 0 2 units capped by a thermodynamically active mixed layer from an initially isohaline state with isopycnals prescribed by zonally averaged climatology the model is forced by seasonally varying wind stress radiative and freshwater fluxes and by a thermal relaxation process at the surface after a mechanical spinup time of about 15 years a quasi stationary pattern of mean circulation and annual variability ensues characterized by pronounced subtropical mode water formation

probability of a given magnitude earthquake induced by a fluid injection 1 fluid injections in geothermic and hydrocarbon reservoirs induce small earthquakes 3 m 2 occasionally however earthquakes with larger magnitudes m 4 occur we investigate magnitude distributions and show that for a constant injection pressure the probability to induce an earthquake with a magnitude larger than a given value increases with injection time corresponding to a bi logarithmical law with a proportionality coefficient close to one we find that the process of pressure diffusion in a poroelastic medium with randomly distributed sub critical cracks obeying a gutenberg richter relation well explains our observations the magnitude distribution is mainly inherited from the statistics of pre existing fracture systems the number of earthquakes greater than a given magnitude also increases with the strength of the injection source and the tectonic activity of the injection site our formulation provides a way to estimate expected magnitudes of induced earthquakes it can be used to avoid significant earthquakes by correspondingly planning fluid injections

eddies and the annular modes of climate variability the dominant modes of month to month varia tion in the troposphere are realistically simulated in a gen eral circulation model whose lower boundary is prescribed with realistic topography and seasonally varying climato logical sea surface temperatures in both hemispheres these internal modes describe a nearly zonally symmetric north south shifting of the zonal jets as anomalous westerlies vac illate between high 50 60 a nd low 30 40 latitudes the eddy structures evolve with the jets and the corre sponding eddy momentum forcings support the shifts in jet position stationary synoptic wave variations mainly ac count for the total eddy forcings in the northern south ern hemisphere pronounced eddy variations occur over the north atlantic sector

deep towed gravity surveys in the southern california continental borderland we surveyed two sites in the southern california continental borderland with a newly developed instrument a towed deep ocean gravimeter a gravity sensor that can be towed a few tens of meters above the sea floor during its development phase we used the instrument to survey two regions off the coast of southern california the first was along two tracks in the san diego trough the second was over a seamount like feature named emery knoll results of the trough survey reveal a basin with a geometry consistent with seismic data we observed no significant density contrast across the san diego trough fault in the near surface sediments the survey of emery knoll shed light on the question of the origin of this structure modeling the knoll to determine its bulk density suggested a nearly uniform structure surrounded by sedimentary basins with a more massive central intrusive body derived densities of 2850 kg m3 for the knoll and 3050 kg m3 for the central intrusion assume that no deep unmodeled sources e

how the north atlantic multidecadal oscillation may have influenced the indian summer monsoon during the past two millennia 1 observations have indicated that the north atlantic sst variations have persistent effects on the indian summer monsoon rainfall at multidecadal and longer timescales the question of how such influences may have been achieved is addressed in this study using both instrumentation data of the 20th century and proxy records of the last 2000 years major results show that the north atlantic sst anomalies strongly affect the tibetan plateau surface temperature and heat sources the latter cause changes in the meridional temperature gradient between tibetan plateau and the tropical indian ocean through the thermal wind effect the meridional temperature gradient anomalies change the lower tropospheric zonal winds and the monsoon circulation and rainfall citation feng s and q hu 2008 how the north atlantic multidecadal oscillation may have influenced the indian summer monsoon during the past two millennia geophys res lett 35 l01707 doi 10 1029 2007gl032484

characterization of hydraulic properties of rocks using probability of fluid induced microearthquakes the use of borehole fluid injections is typical for exploration and development of hydrocarbon or geothermal reservoirs such injections often induce small magnitude earthquakes the nature of processes leading to triggering of such microseismicity is still not completely understood here we consider induced microseismicity using as examples two case studies of geothermal reservoirs in crystalline rocks and one case study of a tight gas sandstone reservoir in all three cases we found that the probability of induced earthquakes occurring is very well described by the relaxation law of pressure perturbation in fluids filling the pore space in rocks this strongly supports the hypothesis of seismicity triggered by pore pressure moreover this opens additional possibilities of using passive seismic monitoring to characterize hydraulic properties of rocks on the reservoir scale with high precision

circumglobal teleconnections the jet stream waveguide and the north atlantic oscillation abstract monthly and seasonally averaged upper tropospheric northern hemisphere winter fields are examined to determine whether the waveguiding effect of the time averaged tropospheric jets on low frequency disturbances that is predicted by theory does affect the behavior of these disturbances it is found that indeed disturbances in the vicinity of the mean jets particularly the jet that stretches across south asia are fundamentally different from those that reside in regions where the mean winds have weaker meridional gradients like the mid pacific patterns of variability in the jets tend to be smaller scale and to consist of zonally oriented chains of anomalies while variability in the mid pacific is composed of patterns with distinct meridional orientation because they are meridionally trapped and zonally elongated patterns associated with the jet stream waveguide connect activity at points that are much farther apart than do patterns in other regions of the globe within the south asian waveg

changes in compressional and shear wave velocities and dynamic moduli during operation of a hot dry rock geothermal system compressional and shear wave velocities measured during cross well seismic surveys are found to decrease during operation of a hot dry rock geothermal system the relative p and s wave velocity decrease is very close to values predicted by o connell and budiansky 1974 who modeled velocities in rocks containing fluid saturated and dry cracks because our data fit their model quite well we attribute the velocity decrease to an increased density of fluid filled fractures in the reservoir fracture density parameters of o connell and budiansky range as high as 0 365 near the main heat transfer areas of the geothermal reservoir the dynamic shear and young s modulus are significantly lower near the principal heat transfer surfaces but the bulk modulus is largely unaffected these observations suggest that the velocity differences are caused by fully saturated fractures we demonstrate that the fractures inferred by this survey may have lengths as large as 70 cm these fractures may have been caused by shear failure microcracking associated with thermal drawdown or thermal stress cracking

why is there an early spring cooling shift downstream of the tibetan plateau abstract the temperature shift over the eastern flank of the tibetan plateau is examined using the last 50 yr of chinese surface station observations it was found that a strong cooling shift occurs in early spring march and april and late summer july august and september in contrast to the warming shift in other seasons the cause of the march april ma cooling is investigated in this study the ma cooling shift on the lee side of the tibetan plateau is found to be not a local phenomenon but rather it is associated with an eastward extension of a cooling signal originating from north africa that is related to the north atlantic oscillation nao in the previous winter the midtropospheric westerlies over the north atlantic and north africa tend to intensify during positive nao phases the enhanced westerlies after passing over the tibetan plateau result in strengthened ascending motion against the lee side of the plateau which favors the formation of midlevel stratiform clouds the increased a

shift in the summer rainfall over the yangtze river valley in the late 1970s 1 the summer rainfall over the middle lower valley of the yangtze river and over the whole eastern china experienced a notable regime shift in about 1979 this change is consistent with a simultaneous jump like change in the 500 hpa geopotential height 500 over the northern pacific the rainfall over the yangtze river valley is closely related to the 500 averaged over the area 20 25 n 125 140 e with a correlation coefficient of 0 66 for the period 1958 1999 since 1980 the subtropical northwestern pacific high snph has enlarged intensified and extended southwestward the changes in the snph are strongly associated with the variations of the sea surface temperatures ssts of the eastern tropical pacific and tropical indian ocean the anomalies of these ssts responsible primarily for the shift of the summer rainfall over the yangtze river through the changes in snph precede the 500 signals with different leading times

characterization of a steamed oil reservoir using cross well seismology cross well seismic tomography is developing into an important tool for reservoir management and within the last few years there have been some notable advances in out understanding its imaging capability tomography has evolved from its roots in medicine to encompass a broad range of geophysical applications which include earthquake hypocenter location mining and nuclear waste disposal in addition to oil field operations the fundamental technical requirements for the latter have been demonstrated high frequency seismic waves capable of traveling long interwell distances can be generated without damaging the borehole and mathematical inversion techniques can give reliable images as long as problems associated with nonuniform and incomplete sampling are handled correctly

contrasting eastern pacific and central pacific types of enso abstract surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of el nino southern oscillation enso in the tropical pacific an eastern pacific ep type and a central pacific cp type an analysis method combining empirical orthogonal function eof analysis and linear regression is used to separate these two types correlation and composite analyses based on the principal components of the eof were performed to examine the structure evolution and teleconnection of these two enso types the ep type of enso is found to have its sst anomaly center located in the eastern equatorial pacific attached to the coast of south america this type of enso is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical indian ocean in contrast the cp type of enso has most of its surface wind sst and subsurface anomalies confined in the central pacific and tends to onset develop and decay in situ

asian jet waveguide and a downstream extension of the north atlantic oscillation abstract anomalous atmospheric fields associated with the north atlantic oscillation nao are analyzed on interannual and intraseasonal time scales in order to examine the extent to which the nao is a regional phenomenon analyses on the interannual time scale reveal that the nao signal is relatively confined to the euro atlantic sector in december while it extends toward east asia and the north pacific in february the difference is most clearly seen in the meridional wind anomaly which shows a wave train along the asian jet collocated with an anomalous vorticity source near the jet entrance diagnoses using a linear barotropic model indicate that this wave train is interpreted as quasi stationary rossby waves trapped on the asian jet waveguide and effectively excited by the anomalous upper level convergence over the mediterranean sea it is found that when the nao accompanies the mediterranean convergence mc anomaly most frequently seen in february the nao indeed has a much wider horizontal str

nonlinear teleseismic tomography at long valley caldera using three dimensional minimum travel time ray tracing the authors explore the impact of three dimensional minimum travel time ray tracing on nonlinear teleseismic inversion this problem has particular significance when trying to image strongly contrasting low velocity bodies such as magma chambers because strongly refracted and or diffracted rays may precede the direct p wave arrival traditionally used in straight ray seismic tomography they use a simplex based ray tracer to compute the three dimensional minimum travel time ray paths and employ an interative technique to cope with nonlinearity results from synthetic data show that their algorithm results in better model reconstructions compared with traditional straight ray inversions the authors reexamine the teleseismic data collected at long valley caldera by the u s geological survey the most prominent feature of their result is a 25 30 low velocity zone centered at 11 5 km depth beneath the northwestern quandrant of the caldera beneath this at a depth of 24 5 km is a more diffuse 15 low velocity zone in general the low velocities tend to deepen to the south and east the authors interpret the shallow feature to be the residual long valley caldera magma chamber while the deeper feature may represent basaltic magmas ponded in the midcrust the deeper position of the prominent low velocity region in comparison to earliermore tomographic images is a result of using three dimensional rays rather than straight rays in the ray tracing the magnitude of the low velocity anomaly is a factor of approximately 3 times larger than earlier models from linear arrival time inversions and is consistent with models based on observations of ray bending at sites within the caldera these results imply the presence of anywhere from 7 to 100 partial melt beneath the caldera 40 refs 1 fig 1 tab less

a detailed view of the injection induced seismicity in a natural gas reservoir in zigong southwestern sichuan basin china 1 seismicity at a gas reservoir located in the relatively stable sichuan basin china mirrors the injection pressure of unwanted water suggesting that the seismicity is injection induced injection under high pressure on a routine basis began on 9 january 2009 and continued to july 2011 during the injection period over 120 000 m3 of water was pumped under a wellhead pressure of up to 6 2 mpa into the limestone formation of permian 2 45 to 2 55 km beneath the surface the injection induced more than 7000 surface recorded earthquakes including 2 m4 the largest one was ml4 4 20 m3 and more than 100 m2 events data observed by a nearby local seismic network and five temporal stations provide a detailed view of the spatiotemporal distribution of the induced earthquakes most events were limited to depths ranging from 2 5 to 4 km which is consistent with the limestone formation of permian in a map view hypocenters are concentrated in a nnw extended ellipsoidal zone approximately 6 km long and approximately 2 km wide centered approximately at the injection well multisources of evidence such as the shear mechanism pattern of hypocenter distribution and small elevated pore pressure as compared with the least principal stress in the region show that the induced earthquakes occurred as a result of lowering of the effective normal stress on known or unknown preexisting blind faults which are critically loaded under the regional stress field epidemic type aftershock sequence modeling results indicate that injection inducing and earthquake triggering are both important during earlier periods of injection while later periods are dominated by forced injection induced seismicity

improved relative locations of clustered earthquakes using constrained multiple event location a new method for improving relative locations of clustered earthquakes is presented and applied to a suite of microearthquakes induced by hydraulic fracturing the method is based on the assumption that clustering of earthquake hypocenters is obscured by the uncorrelated scatter of individual hypocenters the method is implemented as an additional constraint in a joint hypocenter determination jhd scheme the method shifts event hypocenters toward the center of mass of the events within some volume surrounding the event location if the rms misfit between predicted and measured arrival times does not increase significantly the method uses the same basic assumption of jones and stewart 1997 which is that there is greater clustering in actual earthquake locations than there is in locations determined using conventional techniques our method differs in that it is implemented as part of the jhd process so it operates on raw travel time data rather than on derived hypocenters the method produces hypocenters from a demonstration field dataset that are similar to those obtained by phillips et al 1997 from time consuming precise manual repicking of relative arrival times of events the clustering constraint can easily be incorporated as an additional constraint in earthquake location velocity tomography codes and may lead to improved velocity structure determination and earthquake location pattern identification and interpretation

interannual and interdecadal variations of the east asian summer monsoon and tropical pacific ssts part i roles of the subtropical ridge the interannual relationship between the east asian summer monsoon and the tropical pacific ssts is studied using rainfall data in the yangtze river valley and the ncep reanalysis for 1951 96 the datasets are also partitioned into two periods 1951 77 and 1978 96 to study the interdecadal variations of this relationship a wet summer monsoon is preceded by a warm equatorial eastern pacific in the previous winter and followed by a cold equatorial eastern pacific in the following fall this relationship involves primarily the rainfall during the pre mei yu mei yu season may june but not the post mei yu season july august in a wet monsoon year the western north pacific subtropical ridge is stronger as a result of positive feedback that involves the anomalous hadley and walker circulations an atmospheric rossby wave response to the western pacific complementary cooling and the evaporation wind feedback this ridge extends farther to the west from the previous winter to the following fall resulting in an 850 hpa anomalous anticyclone near the southeast coast of china this anticyclone 1 blocks the pre mei yu and mei yu fronts from moving southward thereby extending the time that the fronts produce stationary rainfall 2 enhances the pressure gradient to its northwest resulting in a more intense front and 3 induces anomalous warming of the south china sea surface through increased downwelling which leads to a higher moisture supply to the rain area a positive feedback from the strong monsoon rainfall also appears to occur leading to an intensified anomalous anticyclone near the monsoon region this sst subtropical ridge monsoon rainfall relationship is observed in both the interannual timescale within each interdecadal period and in the interdecadal scale the sst anomalies sstas change sign in northern spring and resemble a tropospheric biennial oscillation tbo pattern during the first interdecadal period 1951 77 in the second interdecadal period 1978 96 the sign change occurs in northern fall and the tbo pattern in the equatorial eastern pacific sst is replaced by longer timescales this interdecadal variation of the monsoon sst relationship results from the interdecadal change of the background state of the coupled ocean atmosphere system this difference gives rise to the different degrees of importance of the feedback from the anomalous circulations near the monsoon region to the equatorial eastern pacific in a wet monsoon year the anomalous easterly winds south of the monsoon enhanced anomalous anticyclone start to propagate slowly eastward toward the eastern pacific in may and june apparently as a result of an atmosphere ocean coupled wave motion these anomalous easterlies carry with them a cooling effect on the ocean surface in 1951 77 this effect is insignificant as the equatorial eastern pacific sstas already change from warm to cold in northern spring probably as a result of negative feedback processes discussed in enso mechanisms in 1978 96 the equatorial eastern pacific has a warmer mean sst a stronger positive feedback between ssta and the walker circulation during a warm phase tends to keep the ssta warm until northern fall when the eastward propagating anomalous easterly winds reach the eastern pacific and reverse the ssta

back front of seismicity induced by non linear pore pressure diffusion borehole fluid injections are accompanied by microseismic activity not only during but also after termination of the fluid injection previously this phenomenon has been analysed assuming that the main triggering mechanism is governed by a linear pressure diffusion in a hydraulically isotropic medium in this context the socalled back front of seismicity has been introduced which allows to characterize the hydraulic transport from the spatiotemporal distribution of post injection induced events however rocks are generally anisotropic and in addition fluid injections can strongly enhance permeability in this case permeability becomes a function of pressure for such situations we carry out a comprehensive study about the behaviour and parametrization of the back front based on a model of a factorized anisotropic pressure dependence of permeability we present an approach to reconstruct the principal components of the diffusivity tensor we apply this approach to real microseismic data and show that the back front characterizes the least hydraulic transport to investigate the back front of non linear pore fluid pressure diffusion we numerically consider a power law and an exponential dependent diffusivity to account for a post injection enhanced hydraulic state of the rock we introduce a model of a frozen i e nearly unchanged after the stimulation medium diffusivity and generate synthetic seismicity we find that for a weak non linearity and 3d exponential diffusion the linear diffusion back front is still applicable this finding is in agreement with microseismic data from ogachi and fenton hill however for a strong non linear fluid rock interaction such as hydraulic fracturing the back front can significantly deviate from a time dependence of a linear diffusion back front this is demonstrated for a data set from the horn river basin hence the behaviour of the back front is a strong indicator of a non linear fluid rock interaction

variability of the observed midlatitude storm tracks in relation to low frequency changes in the circulation pattern abstract the principal modes of month to month variability of the wintertime storm tracks over the north pacific and north atlantic are identified by empirical orthogonal function analysis of the root mean square statistics of bandpass 2 5 6 day filtered geopotential height data for 19 yr one of the two leading modes depicts fluctuations in the level of synoptic scale activity without any noticeable spatial displacement of the storm track axes whereas the other mode is associated with meridional shifts of the storm tracks from their time averaged positions higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions it is demonstrated that the leading storm track modes are linked to some of the best known monthly averaged teleconnection patterns the dipolar western pacific and western atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans whereas the m

phase asymmetric downstream development of the north atlantic oscillation and its impact on the east asian winter monsoon 1 the phase asymmetry in a downstream pattern of the north atlantic oscillation nao is examined during boreal winter time we compared the downstream anomaly patterns associated with the positive and negative persistent nao events over several days both phases of the persistent nao events accompany quasi stationary wave trains in downstream upper levels however a distinct low level anomaly appears only for the negative persistent nao events the low level anomaly migrates eastward as a surface anticyclone that is confined to the low troposphere a few days later the surface anticyclone reaches central siberia and induces cold advection over east asia therefore the phase dependent downstream development of the nao expands the impact of the nao to the east asian region we also discuss the possible interaction between the upper and lower level anomalies a necessary condition for the amplification of the siberian high

crustal structure of deception island volcano from p wave seismic tomography tectonic and volcanic implications 1 deception island 62 59 s 60 41 w is an active volcano located in the bransfield strait between the antarctic peninsula and the south shetland islands the island is composed of rocks that date from 0 75 ma to historical eruptions 1842 1967 1969 and 1970 and nowadays most of its activity is represented by vigorous hydrothermal circulation slight resurgence of the inner bay floor and intense seismicity with frequent volcano tectonic and long period events in january 2005 an extensive seismic survey took place in and around the island to collect high quality data for a high resolution p wave velocity tomography study a total of 95 land and 14 ocean bottom seismometers were deployed and more than 6600 air gun shots were fired as a result of this experiment more than 70 000 travel time data were used to obtain the velocity model which resolves strong p wave velocity contrasts down to 5 km depth the joint interpretation of the vp distribution together with the results of geological geochemical and other geophysical magnetic and gravimetric measurements allows us to map and interpret several volcanic features of the island and surroundings the most striking feature is the low p wave velocity beneath the caldera floor which represents the seismic image of an extensive region of magma beneath a sediment filled basin another low velocity zone to the east of deception island corresponds to seafloor sedimentary deposits while high velocities to the northwest are interpreted as the crystalline basement of the south shetland islands platform in general in the tomographic image we observe ne sw and nw se distributions of velocity contrasts that are compatible with the regional tectonic directions and suggest that the volcanic evolution of deception island is strongly conditioned by the bransfield basin geodynamics

interpretation of seismic data from hydraulic fracturing experiments at the fenton hill new mexico hot dry rock geothermal site an attempt was made to synthesize the results of active seismic experiments carried out by the los alamos national laboratory s hot dry rock project staff for determining the geometrical and physical properties of the fracture system produced by hydraulic fracturing in a hot low permeability rock interpretation of data from several reflection transmission and attenuation experiments using seismic probes in the frequency range from a few hundred to more than 10 sup 4 hz led us to postulate the existence of a highly complex fracture system consisting of major discrete vertical cracks intersected by several inclined joints which are surrounded by a large volume of rock containing small scale cracks we found an interesting coincidence between the mean square fluctuation of p arrival time and q sup 1 of coda waves both are increased by nearly the same amount 35approx 40 when fluid pressure in the reservoir is increased another coincidence is the scale length of the inhomogeneity of roughly 3 m obtained from the frequency dependence of attenuation and also from the spatial variation of p arrival time these results suggest that the seismic attenuation in the fractured region is caused primarily by scattering

a method to allow temporal variation of velocity in travel time tomography using microearthquakes induced during hydraulic fracturing abstract hydraulic injections produce fluid filled fractures that reduce the seismic velocity of the rock compared to intact rock the travel times of microearthquakes induced by the injections may be used to discern changes in the rock velocities as well as locating the microearthquakes determining the volumes of rock where the velocities have changed provides indirect evidence for the location of the injected fluid and the character of the changes produced in the fractured rock available data are generally insufficient to resolve both the spatial and temporal changes within the rock to extract information about temporal changes and to obtain an improved image of the velocity structure we chose a parameterization scheme in which the velocities of each block are allowed to change from the background velocity only after a threshold number of microearthquakes have occurred in the block regularizing by constraining the velocity of all the altered blocks to be similar helps stabilize the inversion the regularization can be relaxed somewhat to allow the velocity of an altered block to be different from other altered blocks if the travel time data are compelling the parameterization scheme is justified since observations show that the volume of the seismically stimulated rock increases linearly with the volume of the injected fluid we applied the method to data collected in a region of precambrian crystalline rock that was injected with 21 600 m 3 of water we use travel times from a total of 3886 microearthquakes that were induced by the injection the mean rms travel time residual decreases about 7 the velocity structure contains a low velocity zone located near the injection region other distinct low velocity zones are identified the pattern of microearthquake locations found using our method appears to contain more structure than the pattern found in locations determined using a homogeneous velocity structure two clear low velocity regions found near the point where water was injected into the rock are separated by a region whose velocity did not change the region of unaltered velocity had a large number of microearthquakes

can hydraulic fracture induced microearthquakes show where the fluid went abstract mapping microearthquakes induced by hydraulic fracturing can delineate fracture systems produced by the stimulation and can provide spatial resolution as fine as 10 to 20 m microseismicity provides the only detailed 3 d data about the fracture system away from boreholes yet since the microseismicity results from slip on joints or fractures it may not provide a reliable view of the conductive fractures or joints a more direct approach to identifying fluid pathways is needed the presence of fluids within the rock reduces the seismic wave velocities particularly for shear waves and can provide a direct seismic indication of major fluid paths although seismic tomographic techniques can determine the velocity structure of rock masses only relatively coarse spatial resolution of perhaps 100 200 m has been achieved because of the limited number of observation sites this resolution is not adequate for modeling fracture systems finer spatial resolution may be achieved by imaging of seismic scatterers using their signatures in the waveforms of the microearthquakes this approach exploits secondary arrivals contained within the seismic coda of the microearthquakes these secondary arrivals are presumed to arise because of scattering of seismic waves by some of the major individual fluid paths within the fracture system imaging of the strong scattering features may allow identification of the major fluid paths

detailed joint structure in a geothermal reservoir from studies of induced microearthquake clusters microearthquake clusters form distinct planar patterns within five study regions of a geothermal reservoir undergoing hydraulic fracturing at fenton hill new mexico the patterns define individual slipping joint surfaces of dimension 40 120 m containing 80 150 events each sharp straight edges truncate the clusters we interpret these as marking intersections with aseismic joints each edge orientation is consistent with an intersection between the active joint and a plane oriented parallel to one of the other clusters we identify therefore it appears that cluster shapes constrain the geometry of seismic and aseismic joints both could be important components of the fluid flow network the distribution of inferred slip plane orientations is consistent with but fails to provide sufficient constraint to differentiate conclusively between two very different stress field estimates one measured using pressurization and wellbore breakouts the other using focal mechanisms of the largest microearthquakes an impermeable joint model requiring pore pressure in excess of the normal stress on a joint before slip can occur was inconsistent with many of the inferred slip plane orientations the high quality locations were possible because events from the same cluster generated nearly similar waveforms permitting the precise determination of relative arrival times standard deviations of arrival time residuals fall between 0 1 and 1 1 ms for these clusters major axes and aspect ratios of the 90 confidence ellipsoids range from 6 to 28 m and 1 5 to 8 respectively small events dominate the seismic energy release and thoroughly populate the identified active joints allowing the hypocenters to reflect details of the joint structure to further investigate the reservoir structure we applied a source array slant stack technique to waveforms from the well located clusters yielding directions that scattered energy left each cluster by studying paths of scattered waves we expected to pinpoint impedance contrasts that might have indicated concentrations of fluid filled joints however results show that scattered energy in the s wave coda left the source region in the same direction as the direct s wave direct waves may have excited borehole tube waves that became trapped in the vicinity of the geophone tool overwhelming any energy scattered from the reservoir

changes in p wave velocity during operation of a hot dry rock geothermal system p wave arrival times have been picked on seismograms recorded during two dual well seismic experiments separated in time by one year that were carried out in the los alamos scientific laboratory hot dry rock geothermal energy reservoir average in situ p wave velocity is computed from arrival times measured along segments of the borehole velocity decreases as temperature drops owing to heat extraction from the reservoir the velocity changes linearly with the change in temperature at a rate of 1 07 km s 100 c the linear decrease in velocity leads to the important conclusion that p wave velocity can be used as an effective measure of change in in situ temperature during operation of the geothermal system the change in velocity is attributed to an increase in the microcrack porosity which is a result of the heat extraction from the rock

the signature of the annular modes in the tropical troposphere the extratropical annular modes are coupled with a distinct pattern of climate anomalies that spans the circulation of the tropical troposphere the signature of the annular modes in the tropical troposphere exhibits a high degree of equatorial symmetry it is associated with upper tropospheric zonal wind anomalies centered about the equator midtropospheric temperature anomalies located 208n and 208s and opposing mean meridional circulation anomalies that span the subtropics of both hemispheres the linkages between the annular modes and the tropical circulation are only evident during the cold season months and are most robust in association with the northern hemisphere annular mode nam the coupling between the annular modes and the circulation of the tropical troposphere is consistent with forcing by waves originating at extratropical latitudes both annular modes are characterized by anomalies in the eddy momentum flux convergence at tropical latitudes that act to reinforce the changes in the tropical wind and temperature fields the most pronounced tropical anomalies lag indices of the annular modes by 2 weeks and are found over the eastern tropical pacific where climatological westerlies permit extratropical waves to propagate into the deep tropics the linkages between the nam and the tropical tropospheric circulation are

nao impact towards the springtime snow disappearance in the western eurasian continent 1 the atmospheric control over the disappearance of snow in western eurasia and its relationship to the north atlantic oscillation nao are investigated while the nao has a strong effect on the lower tropospheric temperature in the region its ability to force control over the snow extent is confined to western europe in january and february successive composite differences and heat budget analysis reveals a northeastward propagation of snow cover anomaly maintained by an anomalous zonal heat advection related to the nao from january to march and the intrusion of warm climatological southerly maintaining the snow anomaly until april

integration of microseismic monitoring data into coupled flow and geomechanical models with ensemble kalman filter hydraulic stimulation of low permeability rocks in enhanced geothermal systems shale resources and co2 storage aquifers can trigger microseismic events also known as microearthquakes meqs the distribution of microseismic source locations in the reservoir may reveal important information about the distribution of hydraulic and geomechanical rock properties in this paper we present a framework for conditioning heterogeneous rock permeability and geomechanical property distributions on microseismic data to simulate the multiphysics processes in these systems we combine a fully coupled flow and geomechanical model with the mohr coulomb type rock failure criterion the resulting multiphysics simulation constitutes the forecast model that relates microseismic source locations to reservoir rock properties we adopt this forward model in an ensemble kalman filter enkf data assimilation framework to jointly estimate reservoir permeability and geomechanical property distributions from injection induced microseismic response measurements we show that integration of a large number of spatially correlated microseismic data with practical ensemble sizes can lead to severe underestimation of ensemble spread and eventually ensemble collapse to mitigate the variance underestimation issue two low rank data representation schemes are presented and discussed in the first approach microseismic data are projected onto a low dimensional subspace defined by the left singular vectors of the perturbed observation matrix the second method uses a coarser grid for representing the microseismic data a series of numerical experiments is presented to evaluate the performance of the proposed methods and to illustrate their applicability for assimilating microseismic data into coupled flow and geomechanical forward models to estimate multiphysics rock properties

seismic response to injection well stimulation in a high temperature high permeability reservoir author s hopp c sewell s mroczek s savage m townend j abstract 2019 american geophysical union all rights reserved fluid injection into the earth s crust can induce seismic events that cause damage to local infrastructure but also offer valuable insight into seismogenesis the factors that influence the magnitude location and number of induced events remain poorly understood but include injection flow rate and pressure as well as reservoir temperature and permeability the relationship between injection parameters and injection induced seismicity in high temperature high permeability reservoirs has not been extensively studied here we focus on the ngatamariki geothermal field in the central taup volcanic zone new zealand where three stimulation injection tests have occurred since 2012 we present a catalog of seismicity from 2012 to 2015 created using a matched filter detection technique we analyze the stress state in the reservoir during the injection tests from first motion derived focal mechanisms yielding an average direction of maximum horizontal compressive stress shmax consistent with the regional ne sw trend however there is significant variation in the direction of maximum compressive stress 1 which may reflect geological differences between wells we use the ratio of injection flow rate to overpressure referred to as injectivity index as a proxy for near well permeability and compare changes in injectivity index to spatiotemporal characteristics of seismicity accompanying each test observed increases in injectivity index are generally poorly correlated with seismicity suggesting that the locations of microearthquakes are not coincident with the zone of stimulation i e increased permeability our findings augment a growing body of work suggesting that aseismic opening or slip rather than seismic shear is the active process driving well stimulation in many environments

spring arctic oscillation east asian summer monsoon connection through circulation changes over the western north pacific in the present study the links between spring arctic oscillation ao and east asian summer monsoon easm was investigated with focus on the importance of the north pacific atmospheric circulation and sea surface temperature sst to reduce the statistical uncertainty we analyzed high pass filtered data with the inter annual time scales and excluded the el nino southern oscillation signals in the climate fields using a linear fitting method the significant relationship between spring ao and easm are supported by the changes of multi monsoon components including monsoon indices precipitation and three dimensional atmospheric circulations following a stronger positive spring ao an anomalous cyclonic circulation at 850 hpa appears in southeastern asia and the western north pacific in summer with the easterly anomalies spanning from the pacific to asian continent along 25 n 30 n and the westerly anomalies south of 15 n at the same time the summer western north pacific subtropical high becomes weaker consistently the positive precipitation anomalies are developed over a broad region south of 30 n stretching from southern china to the western pacific and the negative precipitation anomalies appear in the lower valley of the yangtze river and southern japan the anomalous cyclone in the western north pacific persisting from spring to summer plays a key role in modulating easm and monsoon precipitation by a positive air sea feedback mechanism during spring the ao associated atmospheric circulation change produces warmer ssts between 150 e 180 near the equator the anomalous sensible and latent heating in turn intensifies the cyclone through a gill type response of the atmosphere through this positive feedback the tropical atmosphere and sst patterns sustain their strength from spring to summer that consequently modifies the monsoon trough and the western north pacific subtropical high and eventually the easm precipitation moreover the sst response to ao circulation is supported by the numerical simulations of an ocean model and the anomalous atmospheric circulation over the western north pacific is also reproduced by the dedicated numerical simulations using the coupled atmosphere ocean model the observation evidence and numerical simulations suggest the spring ao can impact the easm via triggering tropical air sea feedback over the western north pacific

transient eddy forcing of the time mean flow as identified by geopotential tendencies abstract the forcing of the time mean flow by transient eddies is examined within the framework of a quasi geostrophic equation relating the geopotential tendency to the convergence of transient eddy transports of heat and vorticity the forcing functions of this equation are computed using observed circulation statistics for the wintertime northern hemisphere and solutions are sought for the three dimensional structure of geopotential and temperature tendencies associated with eddies of different time scales in general the geopotential tendencies associated with vorticity fluxes are of the same sign within a given atmospheric column whereas the polarity of the geopotential tendencies associated with heat fluxes in the lower troposphere is opposite to that in the upper troposphere the geostrophic wind tendencies associated with synoptic scale eddies with periods between 2 5 and 6 days are strongest in the vicinity of the oceanic storm tracks the enhanced poleward heat transports by active disturbanc

interdecadal variations in northern hemisphere winter storm track intensity abstract in this paper the interannual variations in the northern hemisphere winter storm tracks have been examined based on 51 winters december january february of ncep ncar reanalysis data the leading empirical orthogonal function eof corresponds to the simultaneous strengthening weakening of both the pacific and atlantic storm tracks interannual and month to month variations in the pacific and atlantic storm tracks are found to be significantly correlated the principal component associated with the leading hemispheric eof exhibits pronounced interdecadal variability there appears to be a transition during the early 1970s from a weak storm track state prior to 1972 73 to a strong storm track state subsequently decadal mean storm track intensity during the 1990s is about 30 stronger than that during the late 1960s and early 1970s the relationship between variations in storm track intensity and low frequency seasonal mean flow anomalies has also been examined it is shown that storm track va

el nino modoki and its possible teleconnection 1 using observed data sets mainly for the period 1979 2005 we find that anomalous warming events different from conventional el nino events occur in the central equatorial pacific this unique warming in the central equatorial pacific associated with a horseshoe pattern is flanked by a colder sea surface temperature anomaly ssta on both sides along the equator empirical orthogonal function eof analysis of monthly tropical pacific ssta shows that these events are represented by the second mode that explains 12 of the variance since a majority of such events are not part of el nino evolution the phenomenon is named as el nino modoki pseudo el nino modoki is a classical japanese word which means a similar but different thing the el nino modoki involves ocean atmosphere coupled processes which include a unique tripolar sea level pressure pattern during the evolution analogous to the southern oscillation in the case of el nino hence the total entity is named as el nino southern oscillation enso modoki the enso modoki events significantly influence the temperature and precipitation over many parts of the globe depending on the season the impacts over regions such as the far east including japan new zealand western coast of united states etc are opposite to those of the conventional enso the difference maps between the two periods of 1979 2004 and 1958 1978 for various oceanic atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline this appears to be a cause of more frequent and persistent occurrence of the enso modoki event during recent decades

dependence of the structure of the northern hemisphere annular mode on the polarity of enso 1 the structure of the northern hemisphere annular mode nam is shown to be significantly different during warm and cold winters of the enso cycle during warm winters the arctic center of action of the annular mode is more prominent and extends deeper into siberia its related surface air temperature signal is stronger and the compensating outer ring in the sea level pressure field is much broader encompassing the entire tropics during cold winters the nam signature is much more pronounced over the pacific sector the sea level pressure pattern exhibits a north pacific center of action positively correlated with the atlantic center and pronounced deflections of the storm track are also evident it is suggested that the former features are associated with the equatorward displacement of the jet stream and the storm tracks during warm winters of the enso cycle and the latter is related to the the more pronounced break in the subtropical jet stream over the pacific sector during cold winters

a physical mechanism for north atlantic sst influence on the indian summer monsoon a link between the atlantic multidecadal oscillation amo and multidecadal variability of the indian summer monsoon rainfall is unraveled and a long sought physical mechanism linking atlantic climate and monsoon has been identified the amo produces persistent weakening strengthening of the meridional gradient of tropospheric temperature tt by setting up negative positive tt anomaly over eurasia during northern late summer autumn resulting in early late withdrawal of the south west monsoon and persistent decrease increase of seasonal monsoon rainfall on inter annual time scales strong north atlantic oscillation nao or north annular mode nam influences the monsoon by producing similar tt anomaly over eurasia the amo achieves the interdecadal modulation of the monsoon by modulating the frequency of occurrence of strong nao nam events this mechanism also provides a basis for explaining the observed teleconnection between north atlantic temperature and the asian monsoon in paleoclimatic proxies citation goswami b n m s madhusoodanan c p neema and d sengupta 2006 a physical mechanism for north atlantic sst influence on the indian summer monsoon

dynamical and boundary forcing characteristics of regional components of the asian summer monsoon abstract in this paper the authors present a description of the internal dynamics and boundary forcing characteristics of two major subcomponents of the asian summer monsoon asm that is the south asian monsoon sam and the east southeast asian monsoon eam the description is based on a new monsoon climate paradigm in which the variability of asm is considered as the outcome of the interplay of a fast and an intermediate monsoon subsystem under the influence of slow external forcings two sets of regional monsoon indices derived from dynamically consistent rainfall and wind data are used in this study results show that the internal dynamics of sam are representative of a classical monsoon system in which the anomalous circulation is governed by rossby wave dynamics where anomalous vorticity induced by an off equatorial heat source is balanced by the advection of planetary vorticity on the other hand the internal dynamics of eam are characterized by a hybrid monsoon system featuring m

statistics of fracture strength and fluid induced microseismicity 1 in this paper we develop an approach for estimating the strength of rocks by analyzing fluid induced microseismicity the strength corresponds to the value of critical pressure in the pore space that must be exceeded in order to activate preexisting fractures i e to trigger earthquakes we assume that during hydraulic injection experiments in boreholes microseismicity is mainly triggered by a diffusive process of pore pressure perturbation an analytical solution can be applied to find time dependent pore pressure perturbations in rocks caused by fluid injections characteristics of the spatiotemporal evolution of microseismic clouds can be then used to estimate minimum and maximum pressures necessary to trigger earthquakes moreover we present a method for reconstruction of the full spectrum of rock strength that is we show how to estimate the probability density function of the critical pressure we verify the approach using numerical data and apply it to real data of injection induced microseismicity from two hot dry rock tests in crystalline rocks and one hydraulic fracturing experiment in a sedimentary rock the results show that such an analysis of microseismicity is able to provide us with a completely new feature of natural fractured rocks in situ namely the statistics of their strength we find that very low critical pressures in the range 0 001 1 mpa characterize the strength of preexisting cracks for all data sets analyzed the range of critical pressures is broadly distributed within 3 to 4 orders of magnitude however the probability density functions of critical pressure change very quickly from the zero level to significant values and from significant values to the zero level at the lower and upper limits of their nonvanishing value ranges respectively the lower bound of critical pressure is possibly defined by the magnitude of tidal induced stresses permanently occurring and relaxing in the earth

spatial orientation and distribution of reservoir fractures from scattered seismic energy wepresentthedetailsofanewmethodfordeterminingthereflection and scattering characteristics of seismic energy from subsurface fractured formations the method is based upon observations we have made from 3d finite difference modeling of the reflected and scattered seismic energy over discrete systems of vertical fractures regularly spaced discrete vertical fracture corridors impart a coda signature which is a ringing tail of scatteredenergy toanyseismicwaveswhicharetransmittedthrough or reflected off of them this signature varies in amplitude and coherence as a function of several parameters including 1 the difference in angle between the orientation of the fractures and the acquisition direction 2 the fracture spacing 3 the wavelength of the illuminating seismic energy and 4 the compliance or stiffness of the fractures this coda energy is most coherent when the acquisition direction is parallel to the strike of thefractures ithasthelargestamplitudewhentheseismicwavelengths are tuned to the fracture spacing and when the fractures have low stiffness our method uses surface seismic reflection tracestoderiveatransferfunctionthatquantifiesthechangeinan apparent source wavelet before and after propagating through a fracturedinterval thetransferfunctionforanintervalwithnoor low amounts of scattering will be more spikelike and temporally compact the transfer function for an interval with high scattering will ring and be less temporally compact when a 3d survey is acquired with a full range of azimuths the variation in the derived transfer functions allows us to identify subsurface areas with high fracturing and to determine the strike of those fractures wecalibratedthemethodwithmodeldataandthenapplied ittotheemiliofieldwithafracturedreservoir themethodyielded results which agree with known field measurements and previously published fracture orientations derived from ps anisotropy

seismic imaging using microearthquakes induced by hydraulic fracturing abstractseismic imaging using microearthquakes induced by hydraulic fracturing produces a three dimensional 3 d s wave velocity model of the fractured zone improves the calculated locations of the microearthquakes and may lead to better estimates of fractureplane orientations fracture density and water flow paths such information is important for predicting the amount of heat energy that may be extracted from geothermal reservoir a fractured zone was created at the los alamos hot dry rock reservoir in north central new mexico within otherwise impermeable basement rock by injecting 21000 m3 of water into a borehole under high pressure at a depth of 3 5 km induced microearthquakes were observed using four borehole seismometers the p wave and s wave arrival times have been inverted to find the 3 d velocity structures and the microearthquake locations and origin times the inversion was implemented using the separation of parameters technique and constraints were incorporated to require smooth vel

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

interdecadal changes in the major modes of asian australian monsoon variability strengthening relationship with enso since the late 1970s abstract the present paper develops an integral view of the year to year variability across the entire asian australian monsoon a am system which covers one third of the global tropics between 40 and 160 e using season reliant empirical orthogonal function s eof analysis the authors identified two major modes of variability for the period 1956 2004 the first exhibits a prominent biennial tendency and concurs with the turnabout of el nino southern oscillation enso providing a new perspective of the seasonally evolving spatiotemporal structure for tropospheric biennial oscillation the second mode leads enso by one year the remote el nino forcing the monsoon warm pool ocean interaction and the influence of the annual cycle are three fundamental factors for understanding the behavior of the first mode the monsoon ocean interaction is characterized by a positive feedback between the off equatorial convectively coupled rossby waves and the underlying sea surface temperature sst dipole anoma

how to measure the strength of the east asian summer monsoon defining the intensity of the east asian summer monsoon easm has been extremely controversial this paper elaborates on the meanings of 25 existing easm indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979 2006 period the existing indices can be classified into five categories the east west thermal contrast north south thermal contrast shear vorticity of zonal winds southwesterly monsoon and south china sea monsoon the last four types of indices reflect various aspects of the leading mode of interannual variability of the easm rainfall and circulations which correspond to the decaying el nino while the first category reflects the second mode that corresponds to the developing el nino the authors recommend that the easm strength can be represented by the principal component of the leading mode of the interannual variability which provides a unified index for the majority of the existing indices this new index is extremely robust captures a large portion 50 of the total variance of the precipitation and three dimensional circulation and has unique advantages over all the existing indices the authors also recommend a simple index the reversed wang and fan index which is nearly identical to the leading principal component of the easm and greatly facilitates real time monitoring the proposed index highlights the significance of the mei yu baiu changma rainfall in gauging the strength of the easm the mei yu which is produced in the primary rain bearing system the east asian ea subtropical front better represents the variability of the easm circulation system this new index reverses the traditional chinese meaning of a strong easm which corresponds to a deficient mei yu that is associated with an abnormal northward extension of southerly over northern china the new definition is consistent with the meaning used in other monsoon regions worldwide where abundant rainfall within the major local rain bearing monsoon system is considered to be a strong monsoon

the 2013 earthquake swarm in helike greece seismic activity at the root of old normal faults the corinth rift in central greece has been studied extensively during the past decades as it is one of the most seismically active regions in europe it is characterized by normal faulting and extension rates between 6 and 15 mm yr 1 in an approximately n10e direction on 2013 may 21 an earthquake swarm was initiated with a series of small events 4 km southeast of aigion city in the next days the seismic activity became more intense with outbursts of several stronger events of magnitude between 3 3 and 3 7 the seismicity migrated towards the east during june followed by a sudden activation of the western part of the swarm on july 15th more than 1500 events have been detected and manually analysed during the period between 2013 may 21 and august 31 using over 15 local stations in epicentral distances up to 30 km and a local velocity model determined by an error minimization method waveform similarity based analysis was performed revealing several distinct multiplets within the earthquake swarm high resolution relocation was applied using the double difference algorithm hypodd incorporating both catalogue and cross correlation differential traveltime data which managed to separate the initial seismic cloud into several smaller densely concentrated spatial clusters of strongly correlated events focal mechanism solutions for over 170 events were determined using p wave first motion polarities while regional waveform modelling was applied for the calculation of moment tensors for the 18 largest events of the sequence selected events belonging to common spatial groups were considered for the calculation of composite mechanisms to characterize different parts of the swarm the solutions are mainly in agreement with the regional nne ssw extension representing typical normal faulting on 30 50 north dipping planes while a few exhibit slip in an nne ssw direction on a roughly subhorizontal plane moment magnitudes were calculated by spectral analysis of s waves yielding b values between 1 1 and 1 2 in their frequency magnitude distribution the seismic moment release history indicates swarm like activity during the first phase which could have acted as a preparatory stage for the second phase after 12 july that presented a more typical main shock aftershock behaviour the spatiotemporal analysis reveals that the swarm has occurred in a volume that is likely related with the extension at depth of the nne dipping pirgaki normal fault outcropping 8 km to the south the slow velocity of eastward migration of the epicentres during june implies triggering by fluids the situation appears different in the second phase of the sequence which was probably triggered by a build up of stress during the first one the relatively deep hypocentres of the 2013 swarm compared to the shallower seismic layer within the rift and their coincidence with the steeply dipping pirgaki fault favour an immature rift detachment model previous results from instrumental data indicate that approximately the same region had been activated during july august 1991 the availability of the dense permanent seismological network data thus allowed for a detailed analysis of this crisis a better understanding of its mechanical context and of the earlier events

locating microearthquakes induced by hydraulic fracturing in crystalline rock microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3 5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced the induced microseismicity was monitored by a network of five vorehole seismic stations a total of about 800 induced events were reliably located from arrival times event locations show a tabular distribution that strikes 350 sup 0 and dips 65 sup 0 east subparallel to the injection well the injection was intended to produce a fracture system that would hydraulically connect two subparallel wells a lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore the 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties the injected fluid appears to have stimulated a zone of rock rather than simply a single fracture the distribution of microseismic events presumably envelopes the zone of fluid paths created by the fracture experiment

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake