ground motion modeling of the 1906 san francisco earthquake part ii ground motion estimates for the 1906 earthquake and scenario events abstract we estimate the ground motions produced by the 1906 san francisco earthquake making use of the recently developed song et al 2008 source model that combines the available geodetic and seismic observations and recently constructed 3d geologic and seismic velocity models our estimates of the ground motions for the 1906 earthquake are consistent across five ground motion modeling groups employing different wave propagation codes and simulation domains the simulations successfully reproduce the main features of the boatwright and bundock 2005 shakemap but tend to over predict the intensity of shaking by 0 1 0 5 modified mercalli intensity mmi units velocity waveforms at sites throughout the san francisco bay area exhibit characteristics consistent with rupture directivity local geologic conditions e g sedimentary basins and the large size of the event e g durations of strong shaking lasting tens of seconds we also compute ground motions for seven hypothetical scenarios rupturing the same extent of the northern san andreas fault considering three additional hypocenters and an additional random distribution of slip rupture directivity exerts the strongest influence on the variations in shaking although sedimentary basins do consistently contribute to the response in some locations such as santa rosa livermore and san jose these scenarios suggest that future large earthquakes on the northern san andreas fault may subject the current san francisco bay urban area to stronger shaking than a repeat of the 1906 earthquake ruptures propagating southward towards san francisco appear to expose more of the urban area to a given intensity level than do ruptures propagating northward

litho1 0 an updated crust and lithospheric model of the earth we present the litho1 0 model which is a 1 tessellated model of the crust and uppermost mantle of the earth extending into the upper mantle to include the lithospheric lid and underlying asthenosphere the model is parameterized laterally by tessellated nodes and vertically as a series of geophysically identified layers such as water ice sediments crystalline crust lithospheric lid and asthenosphere litho1 0 is created by constructing an appropriate starting model and perturbing it to fit high resolution surface wave dispersion maps love and rayleigh group and phase over a wide frequency band 5 40 mhz we examine and discuss the model with respect to key lithospheric parameters such as average crustal velocity crustal thickness upper mantle velocity and lithospheric thickness we then compare the constructed model to those from a number of select studies at regional and global scales and find general consistency it appears that litho1 0 represents a reasonable starting model of the earth s shallow structure crust and uppermost mantle for the purposes in which these models are used such as traveltime tomography or in efforts to create a 3 d reference earth model the model matches surface wave dispersion over a frequency band wider than the band used in the inversion there are several avenues for improving the model in the future by including attenuation and anisotropy as well as making use of surface waves at higher frequency

an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

effects of deviatoric stress on undrained pore pressure response to fault slip the influence of deviatoric stress on coseismic water level changes is examined using an empirical relation between induced pore pressure and stress skempton 1954 characterized undrained pore pressure response in clayey soils to triaxial loading using two coefficients a and b the undrained pore pressure response under axial loading can be either larger a 1 3 or smaller a 1 3 than that predicted from the mean normal stress alone for indiana limestone the undrained pore pressure response under axial loading is characterized by a 1 3 skempton s equation is generalized to a polyaxial state of stress by making the undrained pore pressure a linear combination of the mean normal stress and the octahedral shear stress which is proportional to the square root of the second invariant of the deviatoric stress tensor dependence of the undrained pore pressure on the deviatoric stress components is consistent with coseismic water level changes observed on the trace of the fault plane near parkfield california for an a value less than 1 3 regions of pore pressure decrease are enlarged relative to regions of pore pressure increase

magmatic processes revealed by textural and compositional trends in merapi dome lavas syn eruptive degassing of volcanoes may lead to syn eruptive crystallization of groundmass phases we have investigated this process using textural and compositional analysis of dome material from merapi volcano central java indonesia samples included dome lavas from the 1986 88 1992 93 1994 and 1995 effusive periods as well as pyroclastic material deposited by the november 1994 dome collapse with total crystallinities commonly in excess of 70 phenocrysts 1 microlites the liquids present in merapi andesites are highly evolved rhyolitic at the time of eruption feldspar microlites in dome rocks consist of plagioclase cores ab63an29or8 surrounded by alkali feldspar rims ab53an5or42 compositional pairs which are not in equilibrium a change in the phase relations of the ternary feldspar system caused by degassing best explains the observed transition in feldspar composition a small proportion of highly vesicular airfall tephra grains from the 1994 collapse have less evolved glass compositions than typical dome material and contain rimless plagioclase microlites suggesting that the 1994 collapse event incorporated less degassed partially liquid magma in addition to fully solidified dome rock as decompression drives volatile exsolution rates of degassing and resultant microlite crystallization may be governed by magma ascent rate microlite crystallinity is nearly identical among the 1995 dome samples an indication that similar microlite growth conditions ph2 o and temperature were achieved throughout this extrusive period however microlite number density varied by more than a factor of four in these samples and generally increased with distance from the vent low vent ward microlite number densities and greater microlite concentrations down flow probably reflect progressively decreasing rates of undercooling at the time of crystal nucleation during extrusion of the 1995 dome comparison between dome extrusion episodes indicates a correlation between lava effusion rate and microlite number density suggesting that extrusion slowed during 1995 crystal textures and compositions in the 1992 93 and 1994 domes share the range exhibited by the 1995 dome suggesting that transitions in crystallization conditions i e rates of undercooling determined by effusion rate are cyclic q 2000 elsevier science b v all rights reserved

the earthquake prediction experiment at parkfield california since 1985 a focused earthquake prediction experiment has been in progress along the san andreas fault near the town of parkfield in central california parkfield has experienced six moderate earthquakes since 1857 at average intervals of 22 years the most recent a magnitude 6 event in 1966 the probability of another moderate earthquake soon appears high but studies assigning it a 95 chance of occurring before 1993 now appear to have been over simplified the identification of a parkfield fault segment was initially based on geometric features in the surface trace of the san andreas fault but more recent microearthquake studies have demonstrated that those features do not extend to seismogenic depths on the other hand geodetic measurements are consistent with the existence of a locked patch on the fault beneath parkfield that has presently accumulated a slip deficit equal to the slip in the 1966 earthquake a magnitude 4 7 earthquake in october 1992 brought the parkfield experiment to its highest level of alert with a 72 hour public warning that there was a 37 chance of a magnitude 6 event however this warning proved to be a false alarm most data collected at parkfield indicate that strain is accumulating at a constant rate on this part of the san andreas fault but some interesting departures from this behavior have been recorded here we outline the scientific arguments bearing on when the next parkfield earthquake is likely to occur and summarize geophysical observations to date

textural heterogeneities in pumices from the climactic eruption of mount pinatubo 15 june 1991 and implications for magma ascent dynamics the climactic event of mount pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism we have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions white pumice has higher vesicularity deformed and highly coalesced vesicles with thin walls euhedral phenocrysts and microlite free groundmass gray pumice shows lower vesicularity wider ranges in vesicle number density limited coalescence vesicles with thick walls that are less deformed phenocrysts and microphenocrysts with abundant solution pitting and groundmass containing ubiquitous microlites and crystal fragments the presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors in particular the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety

correlations of volcanic ash texture with explosion earthquakes from vulcanian eruptions at sakurajima volcano japan abstract we compare the texture of volcanic ash with the maximum amplitude of explosion earthquakes a eq for vulcanian eruptions from sakurajima volcano we analyze the volcanic ash emitted by 17 vulcanian eruptions from 1974 to 1987 using a stereoscopic microscope we classify the glassy particles into smooth surface particles s type particles and non smooth surface particles ns type particles according to their surface conditions gloss or non gloss appearance as an indicator of the freshness of the particles s type particles are further classified into v type particles those including vesicles and nv type particles those without vesicles by means of examinations under a polarized microscopic of polished thin sections cross correlated examinations against seismological data show that 1 the number fraction of s type particles s fraction has a positive correlation with a eq 2 the number ratio of nv type particles to v type particles the n v number ratio has a positive correlation with a eq and 3 for explosions accompanied with bl type earthquake swarms the n v number ratio has a negative correlation with the duration of the bl swarms bl swarms refer to the phenomenon of numerous bl type earthquakes occurring within a few days prior to an increase in explosive activity kamo k 1978 some phenomena before the summit crater eruptions at sakura zima volcano bull volcanol soc japan 23 53 64 the positive correlation between the n v number ratio and a eq could indicate that a large amount of separated gas from fresh magma results in a large a eq plagioclase microlite textual analysis of nv type particles from five explosive events without bl swarms shows that the plagioclase microlite number density mnd and the l w length width ratio have a positive correlation with a eq a comparison between textural data mnd l w ratio crystallinity and the result of a decompression induced crystallization experiment couch s sparks r s j carroll m r 2003 the kinetics of degassing induced crystallization at soufriere hills volcano montserrat j petrol 44 1477 1502 suggests that a plagioclase microlite texture of volcanic ash from eruptions without bl swarms could be generated by a decompression of 100 160 mpa if the mnd is controlled by the water exsolution rate from melt the positive correlation between the mnd and a eq may suggest that a eq becomes large when the effective decompression is large and the water exsolution rate is high from 6 2 10 5 to 1 9 10 4 wt s the estimated magma ascent rate ranges from 0 11 to 0 35 m s which is one order of magnitude faster than that of an effusive eruption and one to three orders slower than those for a sub plinian eruption this suggests that the ascent rate of magma plays an important role in the occurrence of vulcanian eruptions we propose a simple model for vulcanian eruptions at sakurajima volcano that takes into account the correlation between the s fraction and a eq

regional seismic wave propagation lg and sn and pn attenuation in the arabian plate and surrounding regions an edited version of this paper was published by blackwell publishing copyright 2004 blackwell publishing see also http www blackwell synergy com doi abs 10 1111 j 1365 246x 2004 02246 x http atlas geo cornell edu middleeastnorthafrica publications al damegh2004 htm

microlite textures and volatile contents of obsidian from the inyo volcanic chain california 1 we test the hypothesis that microlite textures record progressive degassing during magma ascent we compare microlite number density nv and h2o concentrations in pyroclastic and effusive obsidian obs from the inyo volcanic chain ca comparisons reveal a weak correlation between microlite nv and h2o however most obsidians obss of a given volatile content exhibit a wide range of nv factors other than degassing must also influence final groundmass texture we postulate that obs textures record different ascent rates and residence histories prior to eruption from the conduit we conclude that microlites do not sensitively indicate the extent of magmatic degassing rather they provide qualitative indications of magma ascent and residence time

transient pore pressure response to confining stress excursions in berea sandstone flooded with an aqueous solution of co2 we measured the pore pressure response due to carbon dioxide co2 gas bubble nucleation and growth in a berea sandstone core flooded with an initially subsaturated aqueous solution of co2 in response to a rapid drop in confining stress under conditions representative of a confined aquifer a portion of the co2 in the earth s crust derived from volcanic magmatic and biogenic sources dissolves in groundwater sudden reductions in confining stress in the earth s crust occur due to dilational strain generated by the propagation of seismic rayleigh and p waves or aseismic slip in the near field of earthquakes a drop in confining stress produces a proportional drop in pore fluid pressure when the pore fluid contains dissolved co2 the pore pressure responds to a drop in confining stress like it does in the dissolved gas free case until the pore pressure falls below the bubble pressure gas bubble nucleation and diffusive growth in the pore space trigger spontaneous transient buildup of the pore fluid pressure and reduction of effective stress we measured the rate of pore fluid pressure buildup in the 100 s immediately following the confining stress drop as a function of the saturation with respect to co2 at the lowest pore pressure realized during the confining stress drop using five different co2 partial pressures the rate scales with the saturation with respect to dissolved co2 from 10 kpa min at 1 25 to 166 kpa min at 1 8 the net pore pressure rise was as large as 0 7 mpa 100 psi over 5 h

abrupt change in magma liquidus temperature because of volatile loss or magma mixing effects on nucleation crystal growth and thermal history of the magma the liquidus temperature of magma that reaches shallow levels introduction during its ascent may change abruptly as a result of the release of on its way from the source region into the earth s volatiles or through magma mixing or a combination of both crust or even onto the earth s surface magma travelling immediately after this abrupt change of the liquidus temperature through the mantle and crust evolves and one of the occurs a disequilibrium is observed in the melt and here it is shown goals in petrology is to characterize the various processes that the melt rapidly re equilibrates to its thermodynamic equilibrium that occur during this passage of special interest in the a quantitative model for cooling and crystallization of a simple framework of this study are shallow level processes such two component model melt is used to investigate such re equilibration as magma crust interaction magma mixing deprocesses the relaxation time defined as the time required for the volatilization and depressurization that change the geosystem to re equilibrate after varying degrees of liquidus temperature chemical signature of the melt its crystal content its perturbations is found to be 3 or significantly less than the liquidus temperature its flow behaviour and its erupttime it takes to crystallize 40 of the melt regardless of the ability whereas crustal contamination of magma can be amplitude of the perturbation associated with the re equilibration traced through for example trace element and isotope process is an increase in crystal fraction that can easily reach studies e g wilson 1989 and is discussed extensively 10 depending on the amplitude of the perturbation during the in the other papers of this special volume cooling compensation of the liquidus temperature perturbation the temdepressurization and possibly magma mixing strongly perature of the melt remains nearly constant or may even increase slightly depending on the latent heat budget possibly heat of mixing affect the thermal history of magma rising in the mantle and heat absorbed during volatile exsolution which suggests and crust for example any pressure decrease in volatilecrystallization of the melt without cooling saturated systems not uncommon in natural systems especially for more evolved compositions will lead to volatile exsolution and consequent increased liquidus temperature and change in the melting range one of the major consequences of such behaviour is isothermal crystallization which was recognized almost 40 years ago by tuttle bowen 1958 pp 67 and 68 and the

dynamics of coupled diffusive and decompressive bubble growth in magmatic systems bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises we have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km while the model does not attempt to simulate natural eruptions it sheds light on the processes which control eruptive behavior under various conditions ascent rates are defined such that a constant rate of decompression dp dt is maintained a variety of initial ascent rates are considered in the model from 1 m s to 100 m s for basalts and from a few centimeters per second to 10 m s for rhyolite at the base of the conduit the model results indicate that for any reasonable ascent rate basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m s but it can erupt with high oversaturation at greater ascent rates the latter may lead to explosive eruptions if the ascent rate of rhyolite is 10 m s or more then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible for the case of slow magma rise bubble growth is limited by decompression for the case of rapid magma rise bubble growth is limited by diffusion the results of our simple model do not accurately simulate natural volcanic eruptions but suggest that subsequent more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing energetics and hazards of active volcanoes

evolution of melt vapor surface tension in silicic volcanic systems experiments with hydrous melts 1 we evaluate the melt vapor surface tension of natural water saturated dacite melt at 200 mpa 950 1055 c and 4 8 5 7 wt h2o we experimentally determine the critical supersaturation pressure for bubble nucleation as a function of dissolved water and then solve for at those conditions using classical nucleation theory the solutions obtained give dacite melt vapor surface tensions that vary inversely with dissolved water from 0 042 0 003 j m 2 at 5 7 wt h2o to 0 060 0 007 j m 2 at 5 2 wt h2o to 0 073 0 003 j m 2 at 4 8 wt h2o combining our dacite results with data from published hydrous haplogranite and high silica rhyolite experiments reveals that melt vapor surface tension also varies inversely with the concentration of mafic melt components e g cao feototal mgo we develop a thermodynamic context for these observations in which melt vapor surface tension is represented by a balance of work terms controlled by melt structure overall our results suggest that cooling crystallization and vapor exsolution cause systematic changes in that should be considered in dynamic modeling of magmatic processes

hydrologic effects on water level changes associated with episodic fault creep near parkfield california as part of the parkfield california earthquake prediction experiment water level is monitored in a well 460 m from the main trace of the san andreas fault on middle mountain in the preparation zone of the anticipated parkfield earthquake the well configuration allows water level to be monitored in two fluid reservoirs at depths of 85 and 250 m below land surface during 1987 water level changes were recorded during 12 of the 18 episodes of accelerated fault creep detected by a creep meter spanning the fault trace 750 m northwest of the well the creep related water level changes in the shallow reservoir have durations of less than 1 day whereas in the deeper reservoir the changes persist for as long as 2 months these data suggest that the transient nature of the water level changes in the shallow interval is due to vertical flow to the water table and is not evidence that creep events propagate past the well phase leads of earth tidal constituents in the water level data from the shallow interval relative to the same constituents in the local volume strain tide support the interpretation of significant flow to the water table at periods of 1 day or less the form of the water level changes in the deep interval is affected by horizontal flow to the well bore this effect can be removed from the water level records using a theoretical response curve constrained by the phases of earth tidal constituents in the deep interval relative to the local volume strain tide for the events where the signal in the shallow interval has been large enough to measure the sizes of the simultaneous water level changes in the two reservoirs are consistent with the same amounts of volume strain occurring at both depths

mnd microlite number density water exsolution rate meter abstract microlites in effusive or pyroclastic rocks are possible indicators of water exsolution in particular the microlite number density mnd is considered to be a function of the rate of water exsolution from melt in this paper we have constructed a mnd water exsolution rate meter based on the recent results of theory experiments and the natural observation of crystallization kinetics the mnd method accounts for the effects of melt composition and water content on the diffusivity of crytallizing components in melt by using this meter we can estimate the water exsolution rate at the microlite nucleation depth from a mnd value provided the crystal phase plagioclase or clinopyroxene is known we applied the meter to the case of the 1991 1995 dome eruption at unzen and the 1986b subplinian eruption at izu oshima we obtained the water exsolution rates in the range of 6 1 10 6 to 2 8 10 5 wt s approximately at 70 mpa for unzen plagioclase mnd 1014 to 1015 m 3 and 1 1 10 3 to 1 1 10 1 wt s for izu oshima pyroxene mnd 1015 to 1017 m3 under the assumption of equilibrium vesiculation and steady state flow the corresponding decompression rate and ascent velocity are calculated in the range of 240 to 1100 pa s and 0 014 to 0 068 m s for unzen and 1 2 104 to 1 3 106 pa s and 1 3 to 133 m s for izu oshima this contrast in the ascent velocity at the microlite nucleation depth is closely related to the reason why unzen and izu oshima revealed the different eruption styles namely dome growth and explosive eruption respectively

vesiculation and crystallization under instantaneous decompression numerical study and comparison with laboratory experiments abstract vesiculation and crystallization in ascending magmas are key processes that control the eruption behavior and they interplay each other through the water exsolution process we conducted a numerical study in order to quantitatively understand the water exsolution and crystallization processes in natural eruptions decompression history is unknown and in laboratory experiments the amount of decompression is constant with time the numerical results which take into account homogeneous or heterogeneous nucleation and growth of bubbles with varying diffusivity of water viscosity and the amount of decompression provide a quantitative understanding of their control on bubble formation and water exsolution in the constant amount of decompression the bubble nucleation in the homogeneous nucleation can be divided into two regimes the diffusion control regime and viscosity control regime depending on the modified peclet number and the effective supersaturation in the cases of both homogeneous and heterogeneous nucleations the bubble growth is controlled by diffusion or viscosity depending on the modified peclet number and bubble number density the water exsolution rate which is controlled by the modified peclet number in the viscosity control regime and by the bubble number density and diffusive driving force in the diffusion control regime acts as an effective cooling rate in a decompression induced crystallization process a comparison of the numerical results with the results of laboratory experiments suggests that water exsolution proceeds by the diffusion limited growth of bubbles under disequilibrium vesiculation through the heterogeneous nucleation of bubbles and this in turn controls the crystallization kinetics of microlite with the homogeneous nucleation of microlite and the diffusion limited growth of crystal the several orders of variation of microlite number density with the amount of decompression in laboratory experiments can be interpreted as the effect of the amount of decompression on the driving force for the diffusive bubble growth that controls the water exsolution rate

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

sensitivity of crustal deformation instruments to changes in secular rate a variety of instruments including borehole strainmeters water wells creepmeters laser ranging and differential magnetometers are used to monitor crustal deformation in areas that are prone to geologic hazards such as earthquakes and volcanic eruptions in monitoring the deformation one typically examines the data for either a change in rate or a simple offset in the record however one needs to place a statistical confidence level that the detected signal differs from the background noise calculation of the statistical confidence level may be done using the formalism of the matched filter whose output is the signal to noise ratio two ingredients are needed to form a matched filter 1 the power density spectrum of the instrument and 2 the functional form of the signal that we desire to detect using the available crustal deformation data from the parkfield california network the background noise for individual instruments as a function of frequency f is estimated using the traditional method of the power density spectra except for two color laser distance ranging data the power spectra for most of the instruments have a frequency dependence of f n where 2 n 3 the confidence level with which a hypothesized signal is present is determined directly from the signal to noise ratio with the numerator being a function of the signal and the denominator being a function of the power spectrum using a creepmeter as an example a 0 04 mm change occurring over 1 hour a 0 06 mm occurring over 10 hours or 0 20 mm over 100 hours are all signals for which 2 and therefore have only a 5 confidence that these signals could be background noise

a study on the mechanism of coseismic groundwater changes interpretation by a groundwater model composed of multiple aquifers with different strain responses in order to research the coseismic and postseismic groundwater changes for a groundwater system consisting of multiple aquifers we studied the mechanism of coseismic groundwater changes at yudani hot spring which is an artesian well supplied by at least three aquifers in tottori prefecture japan we observed the changes in discharge and water temperature at this site in response to earthquakes tides and atmospheric pressure changes for the groundwater system of yudani hot spring a realistic model composed of three aquifers having different frequency dependencies of groundwater responses to crustal strain and atmospheric pressure was adopted the tidal and barometric responses of the discharges from the three aquifers which were cyclic responses over a limited frequency band were calculated and the physical parameters of the three aquifers were estimated by fitting a theoretical model to the observed responses then using the strain response over the whole frequency determined by the estimated physical parameters the theoretical responses of the discharges from the three aquifers to step like volumetric strain changes were calculated as a result the three aquifer model with different frequency dependencies quantitatively explained the observed groundwater changes at yudani hot spring due to earthquakes in addition the values of the coseismic volumetric strain steps were estimated based on groundwater changes at yudani hot spring these values did not agree with the theoretical coseismic volumetric strain changes at yudani hot spring due to the earthquake fault slips which were calculated under the assumption that the earth s crust is a homogeneous elastic half space medium it is possible that a local strain release such as aseismic slip at a fault near yudani hot spring might have caused the large strain changes estimated from the coseismic groundwater changes

the march 11 2002 masafi united arab emirates earthquake insights into the seismotectonics of the northern oman mountains a moderate m approx 5 earthquake struck the northeastern united arab emirates uae and northern oman on march 11 2002 the event was felt over a wide area of the northern emirates and was accompanied by smaller felt events before and after the march 11 main shock the event was large enough to be detected and located by global networks at teleseismic distances we estimated focal mechanism and depth from broadband complete regional waveform modeling we report a normal mechanism with a slight right lateral strike slip component consistent with the large scale tectonics the normal component suggests relaxation of obducted crust of the semail ophilite specifically the khor fakkan block while the right lateral strike slip component of the mechanism is consistent with shear across the oman line felt earthquakes are rare in the region however no regional seismic network exists in the uae to determine local seismicity this event offers a unique opportunity to study the active tectonics of the region as well as inform future studies of seismic hazard in the uae and northern oman

syneruptive mixing degassing and crystallization at redoubt volcano eruption of december 1989 to may 1990 abstract the 1989 1990 eruption of redoubt volcano included initial energetic vertical venting of pumiceous tephra followed by shedding of dense pyroclastic flows from successive unstable domes the suite of quenched magmatic materials thus produced coupled with detailed seismic observations provides a record of establishment and subsequent evolution of an active magma column in the upper crust the initial eruptive event involved a dacite magma with rhyolite melt and an andesite magma with dacite melt the dacite magma contains reaction rim free hornblende high cl concentrations 0 12 0 02 wt of melt and is microlite free all evidence for rapid ascent from depth in contrast the andesite component is microlite rich though this characteristic may have arisen by mixing between the two magmas rather than by slow ascent abundant dense juvenile lava like clasts with low h2o 0 1 0 3 wt were ejected in the initial eruption event implying that the magma column had reached the near surface and its upper portion had undergone degassing and bubble collapse prior to explosive disruption later eruptive events contain evidence of progressive blending of the two magmatic components as well as mixing between fresh reservoir derived and old conduit resident magma magma apparently ascended to the surface from the 6 10 km deep source chamber in a series of pulses rather than by steady flow or gradual emptying of a shallowly intruded body increased shallow long period seismicity which may indicate subsurface degassing from the flowing magma column occurred 5 days before each major eruptive event petrologic evidence indicates that each eruption contained some magma that ascended quickly from the chamber with time the proportion of new magma in eruptive batches may have decreased however even the final dome contains magma that left the chamber no more than a few days before extrusion together the pulses apparently record passage of an andesite plume through redoubt s dacite magma reservoir

delayed disequilibrium degassing in rhyolite magma decompression experiments and implications for explosive volcanism recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface in this study a series of decompression experiments using crystal free rhyolite melt were conducted to examine the development of large supersaturations due to delayed homogenous spontaneous bubble nucleation melts were saturated at 900 c and 200 mpa with either 5 2 wt dissolved h2o or with 4 2 wt h2o and 640 ppm co2 and isothermally decompressed at linear rates of either 0 003 0 025 or 8 5 mpa s to final pressures between 25 and 175 mpa additional isobaric saturation experiments 900 c 200 25 mpa using pure h2o or mixed h2o co2 fluids establish reference equilibrium solubility curves values homogenous nucleation is triggered in both h2o only and h2o co2 experiments once the supersaturation pressure pss reaches 120 150 mpa and the melt contains two times its equilibrium water contents bubble number density and nucleation rate depend on the supersaturation pressure with values on the order of 102 cm3 and 1 cm3 s for pss 120 mpa 106 cm3 and 103 105 cm3 s for pss 130 150 mpa and 107 cm3 and 106 cm3 s for pss 160 175 mpa nucleation rates are consistent with classical nucleation theory and infer an activation energy for nucleation of 1 5 10 18 j nucleus a critical bubble radius of 2 10 9 m and an effective surface tension for rhyolite at 5 2 wt h2o and 900 c of 0 10 0 11 n m the long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved h2o and co2 contents persist to the end of our runs the disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low pss homogenous and heterogenous nucleation rates are comparable however as are bubble number densities so that at a given decompression rate it appears that nucleation mechanism rather than nucleation rate determines degassing efficiency by fixing the pressure depth at which vesiculation commences and hence the time available for equilibration prior to eruption although real systems are probably never truly crystal free our results show that rhyolitic magmas containing up to 104 crystals cm3 and perhaps as high as 106 crystals cm3 are controlled by homogenous rather than heterogenous nucleation during ascent

a reappraisal of earthquake focal mechanisms and active shortening in the zagros mountains of iran summary the zagros mountains of sw iran are one of the most seismically active intra continental fold and thrust belts on earth and an important element in the active tectonics of the middle east surface faulting associated with earthquakes is extremely rare and so most information about the active faulting comes from earthquakes we use long period teleseismic p and sh body waves to determine the orientation and depth of faulting in 16 new earthquakes and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the zagros we use this information to investigate the style and distribution of active faulting in the zagros and how it contributes to the n s shortening of the arabia eurasia collision when the data are ranked in quality and carefully evaluated simple patterns are seen that are not apparent when routine catalogue data are taken at face value an important change in the fault configuration occurs along strike of the belt in the nw overall convergence is oblique to the trend of the belt and the surface anticlines and is achieved by a spatial separation partitioning of the orthogonal strike slip and shortening components on separate parallel fault systems by contrast in the se overall convergence is orthogonal to the regional strike and achieved purely by thrusting in the central zagros between these two structural regimes deformation involves parallel strike slip faults that rotate about vertical axes allowing extension along the strike of the belt the overall configuration is similar to that seen in other curved shortening belts such as the himalaya and the java sumatra trench all the zagros earthquakes we have been able to check have centroids shallower than 20 km and are confined to the upper crust many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover which is active even beneath areas of known shallow structural decollement such as the dezful embayment the dominant style of shortening is high angle reverse faulting with dips 30 though some lower angle thrusting occurs in places active thrust and reverse faulting is relatively confined to the lower topography on the sw edge of the belt today and only strike slip faulting affects the higher topography profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the simple folded belt but has progressively migrated sw over the last 5 ma there is no evidence for a seismically active structural decollement such as a low angle thrust beneath the zagros nor is there any seismic evidence for active subduction either beneath the zagros or beneath central iran instead the arabian margin seems to have shortened by distributed thickening of the basement only in the syntaxis of the oman line at the se end of the zagros is there any evidence for a low angle thrust of regional extent here earthquakes continue 50 km north of the zagros thrust line the geological suture between the arabian margin and central iran reaching depths of 30 km and may represent thrusting of arabian basement beneath central iran to this extent

determination of aquifer transmissivity from earth tide analysis the water level in an open well tapping an artesian aquifer responds to pressure head disturbance caused by earth tide dilatation of the aquifer because a finite amount of time is needed for water to flow into and out of the well there exists a phase shift or time lag between the tidal dilatation of the aquifer and the water level response in the well we derive an analytical solution that expresses the phase shift as a function of the aquifer transmissivity storage coefficient well radius and the period of the harmonic disturbance this solution is rather insensitive to the storage coefficient thus if the phase shift is known for a harmonic disturbance the transmissivity can be calculated given a rough estimate of the storage coefficient theoretical analysis shows that a significant phase shift may be present even if the disturbance is slowly varying as in the case of earth tides this opens the possibility of estimating aquifer transmissivity from water level records that show earth tide fluctuations a case study using data from a site near parkfield california is presented to illustrate application of the theory phase shifts of the o1 25 82 hour period and m2 12 42 hour period tidal constituents are chosen for analysis because they are free of systematic contamination by fluctuations in barometric pressure a brief error analysis suggests that the computed o1 phase shift is subject to large uncertainty while the computed m2 phase shift is substantially more accurate based on an assumed storage coefficient range of 10 4 to 10 6 the estimated transmissivity range is 8 10 6 to 2 10 5 m2 s while hydraulic tests have not been performed to validate these estimates the range is consistent with the transmissivity value determined by other investigators from analysis of the water level response to an earthquake

modeling seismically induced deformation and fluid flow in the nankai subduction zone 1 fluid pressure changes induced by seismic strains in the nankai subduction zone were investigated through numerical modeling seismic strains resulting from dislocations along fault surfaces were coupled to pore pressure generation and subsequent transient fluid flow was simulated this study is distinct from previous efforts that modeled homogeneous systems effects of variable mechanical and hydrologic properties were investigated by assigning different mechanical and hydrological properties to marine sediments the decollement zone and the upper oceanic crust model sensitivity studies suggest that for a reasonable range of parameter scenarios transient pressure head signals caused by discrete dislocations of a few meters in the updip region of the seismogenic zone can be observed over large areas of the margin from within shallow depths of the sediment wedge to the oceanic crust below the decollement

a coupled inversion of pressure and surface displacement a coupled inversion of transient pressure observations and surface displacement measurements provides an efficient technique for estimating subsurface permeability variations the methodology has the advantage of utilizing surface observations which are typically much less expensive than measurements requiring boreholes furthermore unlike many other geophysical observables the relationship between surface deformation and reservoir pore fluid volume changes is relatively well understood our treatment enables us to partition the estimation problem into a sequence of three linear subproblems an application of the approach to a set of tilt and borehole pressure data from the raymond field site in california illustrates its efficiency and utility the observations are associated with a well test in which fluid is withdrawn from a shallow fracture zone during the test 13 tiltmeters recorded the movement of the ground surface simultaneously nine transducers measured pressure changes in boreholes intersecting the fracture system we are able to image a high permeability north trending channel located within the fracture zone the existence and orientation of this high permeability feature is substantiated by a semiquantitative analysis of some 4000 transient pressure curves

ground motion modeling in the eastern caucasus in this study we analyzed the performance of a preliminary three dimensional 3d velocity model of the eastern caucasus covering most of the azerbaijan the model was developed in support to long period ground motion simulations and seismic hazard assessment from regional earthquakes in azerbaijan the model s performance was investigated by simulating ground motion from the damaging mw 5 9 2012 zaqatala earthquake which was well recorded throughout the region by broadband seismic instruments in our simulations we use a parallelized finite difference method of fourth order accuracy the comparison between the simulated and recorded ground motion velocity in the modeled period range of 3 20 s shows that in general the 3d velocity model performs well areas in which the model needs improvements are located mainly in the central part of the kura basin and in the caspian sea coastal areas comparisons of simulated ground motion using our 3d velocity model and corresponding 1d regional velocity model were used to locate areas with strong 3d wave propagation effects in areas with complex underground structure the 1d model fails to produce the observed ground motion amplitude and duration and spatial extend of ground motion amplification caused by wave propagation effects

syn and post fragmentation textures in submarine pyroclasts from lō ihi seamount hawai i abstract the extent and timing of vesiculation and microlite formation in ascending basaltic melts are relatively well understood for subaerial eruptions but they have not been studied thoroughly for submarine eruptions this is largely due to a lack of samples of appropriate volume and grain size we present a quantitative textural evaluation of coarse lapilli from two cone forming deposits at 1 km depth on l ihi seamount hawai i collected during a sampling intensive submersible dive series in 2006 lapilli from the deposits named the northern and southern cones have modal bulk vesicularities of 55 and 42 respectively and vesicle populations dominated by sub spherical non interconnected vesicles with number densities on the order of 10 4 to 10 5 cm 3 clasts of modal vesicularity have margin parallel zonation with seawater quenched rims interpreted to preserve syn fragmentation magmatic textures in microlite free sideromelane glass enclosing post fragmentation tachylitic interiors with vesicle and microlite textures that progressively coarsen from rim to core degassing scenarios are linked to syn fragmentation vesicle textures to demonstrate that both magmas degassed in dominantly closed systems diffusion controlled cooling rates of basaltic pyroclasts in contact with water are linked to post fragmentation evolution of vesicle and microlite textures to make inferences about transportation and dispersal of the pyroclasts in low submarine jets the textural analysis shows that the northern and southern cone eruptions were not strictly analogous to any classical subaerial eruption type they were uniquely submarine with ambient seawater playing a crucial role in the processes of degassing vesiculation crystallization and dispersal

homogeneous bubble nucleation in rhyolitic magmas an experimental study of the effect of h2o and co2 1 rapid decompression experiments were performed to study homogeneous bubble nucleation in a crystal free rhyolitic liquid at 800 c bubble nucleation was produced by lowering the pressure at 1 10 mpa s 1 from an initial value between 200 and 295 mpa to a final value below the volatile saturation pressure psat six volatile compositions with 4 1 7 7 wt h2o and 10 1200 ppm co2 were investigated for each composition we determined the critical pressure phon below which homogeneous nucleation can proceed the samples quenched below phon showed a nucleated core with a large number of uniformly spaced bubbles with decreasing pressure bubble number densities increased from 1015 m 3 the degree of supersaturation required for homogeneous nucleation phon psat phon increased with decreasing h2o content or increasing co2 content huge values of phon 135 to 310 mpa were measured in the h2o poor compositions 4 1 4 6 wt h2o 50 1100 ppm co2 much lower values from 60 to 160 mpa were obtained in the h2o rich compositions 7 0 7 7 wt h2o 10 630 ppm co2 the high phon in liquids with 4 5 wt h2o should result in the buildup of large degrees of supersaturation during magma ascent a very late nucleation event and a rapid explosive vesiculation by contrast rhyolitic liquids with much larger water contents have higher saturation pressures and much lower phon bubble nucleation may therefore occur at depth in the volcanic conduit favoring a subsequent near equilibrium degassing

the response of water level in a well to a time series of atmospheric loading under confined conditions the water level in a well that penetrates a confined aquifer can fluctuate in response to changes in atmospheric pressure the response varies with the well casing and screen dimensions the transmissivity and compressibility of the aquifer and to a small extent its storativity recently this loading response problem has been solved in terms of a frequency response function that characterizes how attenuation and phase shifts in the response signal vary with frequency the counterpart of this solution in the time domain is an impulse response function this solution has the immediate appeal that it can be used directly to filter raw loading and water level records via serial convolution where the response at any time is a weighted aggregate of the previous loading signal the impulse response function derives from well known solutions to the slug test problem when based on the cooper bredehoeft papadopulos solution the response function is precise but computationally intensive based on the hvorslev solution the response function is computationally simple but applicable only to aquifers with small storativity simulations and a field example clearly illustrate the dominating influence of transmissivity in dampening and lagging the response of a confined aquifer to a loading signal and how both effects increase with higher frequency the results also illustrate the weak influence of storativity in modulating the response signal

bnd bubble number density decompression rate meter for explosive volcanic eruptions abstract this decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions a comparison between the results of experiments and a model for bubble nucleation shows that bubble number density bnd can be calculated as a function of decompression rate for given physical properties such as the diffusivity of water in a silicate melt and interfacial tension within an acceptable margin of error conversely in this paper we propose a method hereafter referred to as bnd decompression rate meter to estimate the decompression rate by using bnd data of natural pumice samples from volcanic eruptions as a result of the application of the bnd decompression rate meter to pumice and scoria from explosive eruptions it is found that the decompression rates increase in the range from 10 6 to 10 8 pa s with the eruption column heights these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates the absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows in order to explain such high rates of decompression and the correlation with discharge rate we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit

possible strain events reflected in water levels in wells along san jacinto fault zone southern california water levels have been monitored in wells along the san jacinto fault zone since 1977 the three largest earthquakes to occur within 30 km of the segment of the san jacinto fault zone being monitored with continuous recorders showed magnitudesm of 4 5 4 8 and 5 5 two wells in borrego valley 31 to 32 km southeast of them 5 5 earthquake on 25 february 1980 showed anomalous spikes recording a probable strain event 88 hours before the earthquake two other wells 12 km northwest of the epicenter showed no water level anomalies no water level anomalies preceded them 4 8 earthquake near anza on 15 june 1982 anomalous water level fluctuations occurred in a well near ocotillo wells 13 km northeast of them 4 5 earthquake on 22 march 1982 19 to 23 days prior to the earthquake similar fluctuations in other wells have not been followed by sizable earthquakes a simultaneous drop in water level occurred in four wells on 8 september 1982 this possible strain event was not associated with a sizable earthquake the presumed strain events occur only in wells that show earth tides and may have been the result of creep on strands of the san jacinto fault zone although water level anomalies have occurred in only one or two wells prior to two out of three moderate m 4 5 5 5 earthquakes the simultaneous drop in water level on 8 september 1982 and the spikes in two wells before them 5 5 earthquake on 25 february 1980 suggest that wells responsive to earth tides may detect strain events

correlated transient fluid pulsing and seismic tremor in the costa rica subduction zone continuous measurements of fluid flow were made over a six month period across the nicoya peninsula costa rica pacific convergent margin utilizing osmotically driven fluid flow meters designed to quantify both inflow and outflow rates on the order of 10 5 to 3 cm d significant transience in flow was observed through the surface of the forearc three periods of correlated flow signals were seen on the subduction forearc among three instruments located in the out of sequence thrust oost zone over along margin strike distances of 30 km amplitudes of ground velocity recorded on collocated ocean bottom seismometers obs increase during the three correlated flow events the seismic signal has frequency characteristics that resemble volcanic and non volcanic tremor we hypothesize that repeated plate boundary slow slip events potentially originating at the up dip limit of the seismogenic zone generate the observed signals within the toe of the forearc we propose a model in which the poro elastic stress strain field around a series of creep dislocations simultaneously forces flow through fracture networks in the forearc and oceanic basement rocks and induces diffuse flow through the shallow sediments the former generates the seismic tremor like noise recorded by the obss and the latter generates the flow transients recorded by the fluid flow meters we suggest that high sensitivity fluid flow meters can be utilized to detect transient tectonic strain events in offshore environments where traditional geodetic techniques lack resolution or are not possible d 2005 elsevier b v all rights reserved

propagation characteristics of short period sn and lg in the middle east abstract we present propagation characteristics of the short period regional seismic phases sn and lg for the middle east digital waveform data from the iranian long period array ilpa and the global seismic network gsn stations abkt alibek turkmenistan anto ankara turkey keg kottamya egypt and kiv kislovodsk georgia were used in this study three component rotated seismograms were bandpass filtered 0 5 to 5 0 hz and examined for the presence and strength amplitude of sn and lg relative to the pn and pg phases the short period sn and lg waveforms show great variability sn propagates efficiently for paths crossing the mediterranean black and caspian seas as well as for paths passing north of the greater caucasus alborz and kopet dag mountains generally sn is not observed or propagates inefficiently for paths with segments beneath the turkish plateau and northern iranian plateau consistent with a previous study by kadinsky cade et al 1981 this study finds that the zone of inefficient sn propagation coincides with a zone of low pn velocities pn tomography lg is consistently not observed for paths crossing the mediterranean black and caspian seas lg propagates efficiently for paths crossing the northern arabian shield generally lg propagates efficiently for paths contained within the turkish iranian plateau paths in the middle east greater than about 2000 km consistently show no or little lg energy the correspondence of zones of inefficient sn propagation and low pn velocities suggests that high temperatures and partial melt lie at shallow depths in the upper mantle beneath the turkish plateau and northern iranian plateau the presence of water presumably due to subduction of hydrated oceanic crust lowers the solidus temperature and enhances partial melt lg is blocked by oceanic crustal structures as well as by structural and topographic variations associated with tectonic boundaries

analysis of water level changes induced by fault creep at parkfield california this paper demonstrates that the recovery rates of water level changes due to fault creep can be used to infer the position of the slipping zone relative to the well this technique is applied to five slip related water level changes observed in a well 460 m from the san andreas fault near parkfield california during january 1989 to july 1990 the water level changes are all characterized by a rapid drop in less than 8 hours and a slow recovery of 15 to 30 days the water level drop and the position of the well constrain right lateral slip to be predominantly to the southeast of the well the recoveries are modeled by pore fluid diffusion due to a plane strain dislocation in a porous fluid saturated elastic solid satisfactory agreement with the observations is obtained by assuming slip occurs instantaneously and extends indefinitely to the southeast of the well the extent of the slip northwest of the well which ranges from zero to 322 m is determined by fitting the variations in recovery times among the five events calculations are carried out for the limiting cases of both an impermeable and a permeable fault the permeability of the fault has little effect on inferences about the slip but the inferred horizontal diffusivity differs for the two cases 0 15 m2 s for the impermeable and 0 06 m2 s for the permeable for two of the events the magnitude and distribution of slip inferred from the water level changes agree well with that measured on two nearby creepmeters for a third event the agreement is moderately good discrepancies for the remaining two events suggest that the surface slip measured by the creepmeters may differ significantly from that near the 240 m depth of the well

geoelectric signals in china and the earthquake generation process we present original apparent resistivity records for stations located in the chinese provinces of hebei and sichuan along with seismicity patterns fault maps and limited hydrologic data these records are part of a 30 year database of geoelectric signals that have been recorded in china by a nationwide network of more than 100 stations set up by the state seismological bureau of the people s republic of china the database includes the time period during which the m 7 8 1976 tangshan and m 7 2 1976 songpan earthquakes occurred resistivity data recorded in and around the epicentral region of the tangshan event show coherent changes both temporal and in areal distribution years and months before and after the earthquakes a regional change in water table occurred more than 3 years prior to the tangshan earthquake the resistivity and water table signals appear to correlate in the 2 to 3 year period prior to the earthquake rainfall records from the region around tangshan extending to the cities of beijing and tianjin do not appear to account for the resistivity changes regional seismicity and active fault patterns in north china indicate that deformation occurs mainly along the borders of large tectonic blocks the source region for the geoelectric signals appears to lie within a large shear zone at the edge of a tectonic block undergoing rotation the tangshan epicentral area is thus only part of a larger zone of deformation we suggest that dilational strain associated with the opening of fractures may lower resistivity and water table for shallow crustal depths such as those monitored by the chinese geoelectric network dilation and resistivity changes are expected to begin at stress levels well below the peak values of the seismic cycle our study suggests that in some cases the areal extent of geoelectric signals may be significantly larger than that of the eventual earthquake source dimension the chinese geoelectric database offers an opportunity to improve our understanding of possible geoelectric precursors to earthquakes

pore pressure changes induced by slip on permeable and impermeable faults pore pressure changes due to a ramp introduction of slip on permeable and impermeable faults in a fluid saturated rock mass are calculated for the purpose of evaluating water well level fluctuations the calculations demonstrate the importance of coupling between deformation and fluid diffusion at observation points less than 5 4ct0 where c is the diffusivity and t0 is the rise time the decay of pore pressure in the results here is due entirely to fluid mass diffusion an approach that neglects diffusion and assumes that the pore pressure is proportional to the mean normal stress would predict a ramp pore pressure response at distances greater than 5 4ct0 the pore pressure decays so slowly that the neglect of diffusion may be appropriate for both permeable and impermeable faults the pore pressure decays more rapidly for shorter slip zone lengths and longer rise times however the pore pressure change calculated for the impermeable fault is larger particularly for observation points near the fault and decays less rapidly than for the permeable fault these differences suggest that fault permeability can be a significant factor in the response of water wells near faults and care should be used in inferring details of the slip distribution if hydrologic conditions are not known these results are applied to a water well level change observed by lippincott et al a satisfactory fit to the data is obtained by uniform slip over a fault length of about 1 5 km and a rise time of 8 hours although the slip magnitude is not well constrained by the fit the range of possible values includes the 0 5 to 1 0 cm inferred by lippincott et al using a different approach

kinetics of bubble nucleation in a rhyolitic melt an experimental study of the effect of ascent rate in order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma we performed three series of experiments decompressed at rates of either 1000 167 or 27 8 kpa s the experiments were carried out in an externally heated pressure vessel at 800 c and in the pressure range 260 59 mpa the starting material was a crystal free and bubble free rhyolitic glass containing 7 0 wt dissolved h2o in all the decompression experiments homogeneous bubble nucleation began at 90 2 mpa that is 150 mpa below the water saturation pressure of the silicate liquid 240 mpa the degree of supersaturation phon required to trigger homogeneous bubble nucleation was almost independent of decompression rate phon is the difference between the saturation pressure and the nucleation pressure nucleation pressure decreased by 3 mpa for a 36 fold increase in decompression rate these results are in good agreement with the classical theory of nucleation assuming a rhyolite h2o surface tension of 0 106 n m 1 our major experimental finding is that after a short nucleation event the nucleation rate dropped and the bubble number density n reached a stationary value that was strongly sensitive to decompression rate 6 8 mm 3 at 27 8 kpa s 470 mm 3 at 167 kpa s and 5800 mm 3 at 1000 kpa s the smaller value of n at low decompression rate was compensated by a larger mean bubble size so that at a given pressure vesicularity was almost independent of decompression rate the experimental values of n can be reproduced within a factor 0 3 1 4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas the nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation once a critical value of n is attained diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure the strong correlation between bubble number density and decompression rate has fundamental volcanological implications if we extrapolate the experimental data to the typical ascent rates of silicic magmas we obtain bubble number densities 10 3 101 mm 3 for homogeneous nucleation 10 1 103 mm 3 for heterogeneous nucleation that are orders of magnitude smaller than those measured in most natural pumices we therefore propose that the large values of n in silicic pumices may be due to two successive nucleation events 1 a first event which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles and 2 a second nucleation event yielding a very large number of small bubbles and presumably related to the dramatic increase of decompression rate that precedes fragmentation the small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates

long term fault creep observations in central california the u s geological survey usgs has been monitoring aseismic fault slip in central california for more than 10 years as part of an earthquake prediction experiment since 1968 the usgs creep network has grown from one creep meter at the cienega winery south of hollister to a 44 station network that stretches from hayward east of san francisco bay to palmdale in southern california in general the long term slip pattern is most variable on sections of the faults where several magnitude 4 and larger earthquakes occurred during the recording period e g calaveras fault near hollister and san andreas fault between san juan bautista and bear valley these fault sections are the most difficult to characterize with a single long term slip rate in contrast sections of the faults that are seismically relatively quiet e g san andreas fault between bear valley and parkfield produce the steadiest creep records and are easiest to fit with a single long term slip rate appendix is available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j82 004 1 00 payment must accompany order

heterogeneous bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure abstract the nucleation of h 2 o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 800 c and either 50 or 125 mpa and then decompressed by 20 91 mpa at temperatures that ranged between 550 and 700 c and held at the lower pressures for 10 720 s before being quenched bubbles nucleated in number densities n b that vary between 3 10 7 and 2 10 8 cm 3 blocky shaped magnetite or the ends of needle shaped hematite acted as sites for nucleation but only if a minimum super saturation was exceeded which increases with increasing melt viscosity bubbles did not nucleate along the lengths of hematite needles nor on plagioclase both the beginning and ending times of the nucleation event increases with increasing melt viscosity using nucleation theory predictions neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature water diffusivity or viscosity instead the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor resulting from the increasing polymerization of the melt at lower temperatures and water contents because only 10 8 bubbles cm 3 nucleate on magnetite in the rhyolite melt used almost regardless of experimental conditions results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions

decompression experiments as an insight into ascent rates of silicic magmas an experimental study of h2o exsolution bubble growth and microlite crystallisation during ascent decompression of silicic magmas in the volcanic conduit is presented isobaric and decompression experiments were performed on a rhyolitic melt at 860 c nno 1 h2o saturation and pressures between 15 and 170 mpa two sets of decompression experiments were performed with decompression rates varying between 0 001 and 960 mpa min 1 from 150 to 50 mpa high pressure decompression and 2 from 50 to 15 mpa low pressure decompression the experiments highlight incomplete h2o exsolution for decompression rates 100 mpa min incomplete bubble growth for decompression rates 0 1 mpa min crystal nucleation time lags and incomplete chemical re equilibration to final pressures the observed crystallisation process i e growth versus nucleation depends on the decompression range indeed decompression induced crystallisation during high pressure decompressions is dominated by growth of existing crystals whereas during low pressure decompressions crystal nucleation is the dominating process this study provides a means to infer magma ascent rates in eruptions of silicic magmas through a combined petrologic and experimental approach

earthquake ground motion prediction equations for eastern north america new earthquake ground motion relations for hard rock and soil sites in eastern north america ena including estimates of their aleatory uncertainty vari ability have been developed based on a stochastic finite fault model the model incorporates new information obtained from ena seismographic data gathered over the past 10 years including three component broadband data that provide new in formation on ena source and path effects our new prediction equations are similar to the previous ground motion prediction equations of atkinson and boore 1995 which were based on a stochastic point source model the main difference is that high frequency amplitudes f 5 hz are less than previously predicted by about a factor of 1 6 within 100 km because of a slightly lower average stress parameter 140 bars versus 180 bars and a steeper near source attenuation at frequencies less than 5 hz the predicted ground motions from the new equations are generally within 25 of those predicted by atkinson and boore 1995 the prediction equations agree well with available ena ground motion data as evidenced by near zero average residuals within a factor of 1 2 for all frequencies and the lack of any significant residual trends with distance however there is a tendency to positive residuals for moderate events at high frequencies in the distance range from 30 to 100 km by as much as a factor of 2 this indicates epistemic uncertainty in the prediction model the positive residuals for moderate events at 100 km could be eliminated by an increased stress parameter at the cost of producing negative residuals in other magnitude distance ranges adjustment factors to the equations are provided that may be used to model this effect

a numerical experiment of crystallization for a binary eutectic system with application to igneous textures numerical experiments of crystallization in a binary eutectic system were carried out to understand the origin of textural variation in igneous rocks in this study the number density of crystals was intensively examined as a parameter of texture the constant cooling rate experiment no latent heat release yields a higher number density of crystals by 1 order of magnitude than the constant heat loss experiment full latent heat release differences in growth mechanism diffusion limited with capillary effect versus reaction limited without capillary effect result in the different dependence of cooling rate on number density of crystals for diffusion limited growth with the capillary effect for spherical crystals the cooling rate dependence of number density of olivine clinopyroxene plagioclase and opaque minerals seems to be consistent with the experimental data with superliquidus initial condition for plagioclase experimental data with subliquidus initial temperature and the observational data are not consistent with the present numerical results for both growth processes suggesting that the plagioclase growth is governed by a different mechanism of nucleation and growth from homogeneous nucleation and both diffusion limited growth with capillary effect and simple reaction limited growth without capillary effect assuming the conductive cooling of a dike it is predicted that for the diffusion limited growth the number density of crystals is proportional to y 3 where y is distance from the cooling boundary this is in good agreement with some of the observational data by gray 1978 except for plagioclase the consistency among numerical natural and experimental data highly suggests that at the nucleation stage with superliquidus initial condition crystals grow by the diffusion limited process taking the spherical shape for olivine pyroxene plagioclase and some oxides the difference between volcanic and plutonic textures is caused by the difference of cooling rate by 2 orders of magnitude the variation from intergranular ophitic to poikilitic textures results from the difference in cooling rate dependence on number density between the plagioclase and the other minerals the formation of porphyritic texture must satisfy the limited condition the very small efficiency of latent heat production nearly constant cooling rate small values of interfacial energy of the first precipitating phase and smaller value of interfacial energy of the second phase

eruption dynamics inferred from microlite crystallization experiments application to plinian and dome forming eruptions of mt pelee martinique lesser antilles decompression experiments have been conducted to simulate syn eruptive crystallization in volcanic conduits to infer magma ascent rates and conditions during dome forming and plinian eruptions of silicic arc volcanoes the experiments were carried out on starting material with the composition of mt pele e rhyolitic interstitial melt 76 wt sio2 and consisted of three consecutive steps hydration decompression and annealing hydration saturated and undersaturated was performed at 8508c and 200mpa and was followed by isothermal decompression either linearly or stepwise to a final pressure pf of 30 or 5 10mpa decompression rates range from 0 003 to 25mpamin 1 decompression durations of 15 min to 40 days two samples were cooled by 258c and 508c during a 3 day step at pf subsequent to decompression the samples were held for up to 15 days at pf the experiments generated three types of crystals as products of pre syn and post decompression crystallization the experiments basically differ from previous studies in that they are specifically designed to discriminate crystal nucleation from growth and to evaluate the influence of pre decompression crystals on the decompression induced crystallization the effects of pre decompression crystals decompression rate undercooling pf and terminal cooling have been determined on plagioclase nucleation growth morphology and composition the main results suggest a positive correlation between decompression rate and the number density of plagioclase crystals nucleated at pf and highlight the effect of pre decompression crystals in further decompression induced crystallization the relations between the decompression conditions and the plagioclase characteristics have been used to infer mt pele e eruption dynamics suggesting that 1 plinian magmas ascend from the reservoir within less than 1h 1 10 m s 2 dome and block and ash flow magmas ascend within more than 2 5 days giving time for syn decompression crystallization around pre existent microlites 3 dome magmas provide evidence for long stagnation and cooling at low pressure and 4 surge magmas ascend without significant crystallization within less than 4 days and massively nucleate plagioclase at very low pressure the extent and violence of dome destruction may depend on the size or age of the dome with large or old domes favouring mildly explosive block and ash flows whereas small or young protodomes may generate highly explosive surges

on the second nucleation of bubbles in magmas under sudden decompression abstract i conducted numerical experiments to evaluate the condition for the second nucleation of bubbles when a sudden change in the decompression rate occurs in magmas with preexisting bubbles i took into account homogeneous nucleation and diffusive bubble growth results from numerical experiments with variable number densities and sizes of preexisting bubbles were able to provide the limiting condition for the decompression rate above which the second nucleation effectively takes place the limiting condition as a function of number density and size of preexisting bubbles can be divided into two regimes that are controlled by the mass ratio of dissolved volatiles to exsolved volatiles as preexisting bubbles at the initial state in the case with a realistically high mass ratio namely with a relatively small preexisting bubble volume the detectable second nucleation requires decompression rates that are higher by a factor of about 10 than those that are hypothetically required to form the preexisting bubbles by consulting a simple scaling argument we derive the expression for the limiting decompression rate as a function of the number density and size of preexisting bubbles the graphical display of the expression allows us to evaluate whether the second nucleation will effectively take place for practical cases with a given number density and size of preexisting bubbles in natural pumice and scoria the preexisting bubbles may be observed as larger pheno bubbles that provide buoyancy at the storage regions which is needed to trigger the eruption such pheno bubbles are isolated in smaller matrix bubbles that are formed by the rapid decompression that takes place during the ascent of magma to the surface

numerical study of nucleation and growth of bubbles in viscous magmas the nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formation which accounts for effects of viscosity as well as diffusivity interfacial tension and decompression rate the numerical solutions show two regimes in the nucleation and growth process a diffusion controlled regime and a viscosity controlled regime mainly depending on the decompression rate initial saturation pressure and viscosity the basic mechanism common to both regimes is that growth governs nucleation through depletion of degassing components in basaltic eruptions the vesiculation is essentially controlled by diffusion and the viscosity controlled regime is limited to very high decompression rate and very small water content when andesitic magma saturated by water at 10 mpa is decompressed through the propagation of rarefraction wave induced by a landslide as took place in the mount st helens 1980 eruption the vesiculation is controlled by the viscosity up to 100 m depth on the other hand in a rhyolitic magma for the same situation vesiculation is controlled by the viscosity over the whole depth of the magma column in the viscosity controlled regime the vesicularity may be 90 or less as seen in silicic pumice whereas in the diffusion controlledmore regime the vesicularity equals or exceeds 98 such as in reticulite in hawaiian basalt an observed variation of the number density of bubbles by several orders of magnitude in plinian eruptions and the correlation with the sio2 content can be attributed approximately to the dependence of diffusivity of viscosity on sio2 content and temperature assuming the apparent correlation between sio2 content and temperature of magma less

about the potential of wells to reflect stress variations within inhomogeneous crust abstract kumpel h j 1992 about the potential of wells to reflect stress variations within inhomogeneous crust in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 317 336 numerous pre co and post seismic well level changes that are believed to reflect stress variations in the crust have been observed in various parts of the world so far analysis of these data has not favoured a distinct model of the processes that could be involved the benefit from the signals is in fact limited because understanding of the underlying physical mechanisms is poor the paper discusses the impact that inhomogeneities in the brittle crust can have on the observations in particular the following problems are considered 1 could the well level signals be induced by the invasion of pressurized pore fluid at some place 2 could the hydrological conditions close to the observation well modulate any remote pore pressure variation 3 which features of the crust are most likely to dominate the process of stress transfer in the crust 4 how should a stress sensitive well be designed the results can be summarized as follows 1 stress diffusion due to the invasion of pressurized pore fluid can lead to significant well level changes only when the source region is at closer distance than a few kilometres probably less than a few hundred metres from the well 2 local inhomogeneities in the vicinity of the observation well may strongly influence the well s response to remote pressure changes even sign reversal of the anomaly may be encountered 3 there is increasing evidence that the wide range of stiffness values of natural rocks from 100 mpa or less till 100 gpa manifested in a fractal distribution of fault like weak zones within relatively stiff formations dominates stress transfer in the brittle crust uncertainty about the effective rheology for time scales from 10 4 to 103 years limits the inversion of many geophysical observations at present 4 of various configurations a well tapping an extensive hydraulic fracture could act best as a sensitive stress sensor provided that the fracture volume is isolated from the pore space of the host rock however experimental evidence of the practicability of such configuration is still due the weakness of conventional wells as stress sensors results from the fact that the process of fluid flow often lacks a sufficient control on attenuation mechanisms for low frequency well level variations regardless of whether the wells end in stiff or weak formations to a large extent the presented findings are compiled from publications of investigators working in various fields

the crustal and upper mantle structure of the interior arabian platform summary the crustal and upper mantle velocity structure of the interior arabian platform is derived using the spectral analysis of long period p wave amplitude ratios the ratio of the vertical to the horizontal component is utilized to obtain crustal transfer functions using the thomson haskell matrix formulation for horizontally layered crustal models 20 earthquakes recorded at the long period station ryd between azimuths n20 w and n150 e were selected for the analysis based on the following criteria focal depths in the range 5 to 215 km body wave magnitudes greater than 5 0 and epicentral distances in the range 7 to 97 a careful quality check of the data left us with six events out of 29 that had short epicentral distances 20 to be analysed the selection criterion for the final model in the forward modelling process was based on the correlation coeycient between observed and theoretical transfer function the model suggested that the crust consists of five distinct layers the upper crustal layer has a p wave velocity of about 5 6 km s 1 and is about 3 km thick the second layer has a velocity of about 6 3 km s 1 and is 10 km thick the third layer has a velocity of 6 6 km s 1 and is 8 km thick the fourth layer has a velocity of 6 9 km s 1 and is 15 km thick the lower layer has a velocity of about 7 6 km s 1 and is 10 km thick for the mohorovicic discontinuity a velocity of 8 3 km s 1 for the upper mantle and 46 km depth are indicated

laboratory measurements anomalous 0 1 0 5 hz streaming potential under geochemical changes implications for electrotelluric precursors to earthquakes streaming potentials resulting from flow of various salt solutions in rock were measured on saturated sediments fontainebleau sandstones the streaming potential v was found to be proportional to the driving pore pressure p pulses of amplitude 15 40 mv in the frequency range of 0 1 to 0 5 hz were observed when the conductivity of the injected water was decreased and the fluid flow rate was relatively low corresponding to a darcian velocity of 17 to 30 cm h the amplitudes of these pulses are 47 to 133 of the corresponding steady components of the v values such geochemically induced effects may possibly be responsible for the frequency signals from 0 1 to 0 5 hz that were sometimes observed before an earthquake

experimental study of water degassing from phonolite melts implications for volatile oversaturation during magmatic ascent abstract we have experimentally studied degassing of phonolite melts between 50 and 200 mpa and 825 and 850 c our results indicate that decompressed phonolite melts contain equilibrium water contents regardless of decompression rate or final pressure in addition the total porosities of our experiments agree with predicted equilibrium values regardless of final pressure or decompression rate combined observations of mean bubble diameters porosities and arrangement of bubbles in our experiments indicate that degassing occurs preferentially in the large bubble fraction of the population because of this retardation of growth in the small bubble population occurs this may lead to bimodal bubble size distributions in completely degassed phonolites our results indicate that water saturated phonolites at temperatures 800 c can degas in equilibrium while rhyolites follow a disequilibrium trend at decompression rates of 0 5 1 0 mpa s thus hotter water saturated phonolitic magmas are likely to maintain equilibrium magma water contents during ascent at rates up to 20 m s which are typical of highly explosive eruptions these results may apply to eruptions such as the 79 ad eruption of vesuvius which is the type location for plinian style explosive eruptions however caution must be used when applying these results to lower temperature more differentiated and water undersaturated phonolite magmas if fragmentation can occur at depths of 2 km or greater in the conduit our results indicate that highly explosive eruptions of phonolitic magma may occur in the absence of water oversaturation in the magma during ascent fast ascent rates and high shear stresses in the conduit may provide a better explanation for fragmentation in this case

lithospheric structure of the arabian shield and platform from complete regional waveform modelling and surface wave group velocities summary regional seismic waveforms reveal significant differences in the structure of the arabian shield and the arabian platform we estimate lithospheric velocity structure by modelling regional waveforms recorded by the 1995 1997 saudi arabian temporary broadband deployment using a grid search scheme we employ a new method whereby we narrow the waveform modelling grid search by first fitting the fundamental mode love and rayleigh wave group velocities the group velocities constrain the average crustal thickness and velocities as well as the crustal velocity gradients because the group velocity fitting is computationally much faster than the synthetic seismogram calculation this method allows us to determine good average starting models quickly waveform fits of the jon and sn body wave arrivals constrain the mantle velocities the resulting lithospheric structures indicate that the arabian platform has an average crustal thickness of 40 km with relatively low crustal velocities average crustal p and s wave velocities of 6 07 and 3 50 km s 1 respectively without a strong velocity gradient the moho is shallower 36 km and crustal velocities are 6 per cent higher with a velocity increase with depth for the arabian shield fast crustal velocities of the arabian shield result from a predominantly mafic composition in the lower crust lower velocities in the arabian platform crust indicate a bulk felsic composition consistent with orogenesis of this former active margin p and s wave velocities immediately below the moho are slower in the arabian shield than in the arabian platform 7 9 and 4 30 km s 1 and 8 10 and 4 55 km s respectively this indicates that the poisson s ratios for the uppermost mantle of the arabian shield and platform are 0 29 and 0 27 respectively the lower mantle velocities and higher poisson s ratio beneath the arabian shield probably arise from a partially molten mantle associated with red sea spreading and continental volcanism although we cannot constrain the lateral extent of a zone of partially molten mantle

ground motion in kuwait from regional and local earthquakes potential effects on tall buildings in recent years the construction of tall buildings has been increasing in many countries including kuwait and other gulf states these tall buildings are especially sensitive to ground shaking due to long period seismic surface waves although kuwait is relatively aseismic it has been affected by large mw 6 regional earthquakes in the zagros fold thrust belt zftb accurate ground motion prediction for large earthquakes is important to assess the seismic hazard to tall buildings in this study we first analyze the observed ground motions due to two earthquakes widely felt in kuwait the 08 18 2014 mw 6 2 earthquake 360 km nne of kuwait city and the 11 12 2017 mw 7 3 earthquake 642 km nne of kuwait city the peak spectral displacement periods of the ground motion from the 08 18 2014 mw 6 2 earthquake matched well with the ambient vibration spectra of the tallest building the al hamra tower we calculate the ground motions from potential regional and local earthquakes we use a velocity model obtained by matching the observed seismograms of the 2014 and 2017 earthquakes we calculate ground motions in kuwait due to a regional mw 7 5 earthquake and a local mw 5 0 earthquakes our study shows that a significant source of seismic hazard to tall buildings in kuwait comes from the regional tectonic earthquakes however local earthquakes have the potential to generate high peak ground accelerations 98 cm s2 close to their epicenters

long period ground motion in the arabian gulf from earthquakes in the zagros mountains thrust belt the arabian gulf is adjacent to the zagros mountains one of the most seismically active regions in the world we observe that broadband seismic records of zagros earthquakes recorded on the arabian side of the gulf display long duration surface waves while shorter periods 10 km of the gulf basin the long period energy is enhanced and transmitted efficiently consequently large earthquakes in the zagros could result in amplified ground motions at long periods 2 10 s relative to average behavior such ground motions are of concern for large engineered structures such as tall buildings and long bridges with resonant periods in the same period range here we present results of investigations of the characteristics of ground motions recorded on the western shore of the gulf from selected earthquakes in the zagros mountains region exceptionally long duration seismic waves as compared with standard models are shown to occur with periods of 2 10 s this may be due to waveguide effects in the deep sedimentary basin structure of the arabian platform in addition to analyzing recorded ground motion we performed 3d wave propagation simulations using a finite difference method and experimental velocity models of the gulf with different shallow sedimentary layers structures the simulation results confirm our hypothesis that long period waves with extremely long duration and relatively large amplitudes are caused by the geometry of the basin sedimentary layers and to some extent by shallow earthquake depths combined effects of basin edge geometry with sharp velocity contrasts and shallow sources 10 km on the eastern side of the arabian gulf can cause large long period ground motion on the western side of the gulf in contrast the short period content of ground motion 2 s at long distances is relatively weak this is mainly due to wave propagation scattering and attenuation in the shallow sedimentary layers of the gulf basin

crustal structure of iraq from receiver functions and surface wave dispersion implications for understanding the deformation history of the arabian eurasian collision summary we report the crustal structure for two locations in iraq estimated by joint inversion of p wave receiver functions rfs and surface rayleigh wave group velocity dispersion rfs were computed from teleseismic recordings at two temporary broad band seismic stations located in mosul msl in the zagros fold belt and baghdad bhd in the mesopotamian foredeep group velocity dispersion curves at the sites were derived from continental scale tomography the inversion results show that the crustal thicknesses are 39 km at msl and 43 km at bhd we observe a strong psmoho at bhd consistent with a sharp moho discontinuity however at msl we observe a weak psmoho suggesting a transitional moho where crustal thickening is likely to be occurring in the deep crust both sites reveal low velocity surface layers consistent with sedimentary thickness of about 3 km at station msl and 7 km at bhd and agreeing well with the previous reports ignoring the sediments the crystalline crustal velocities and thicknesses are remarkably similar at both stations the similarity of crustal structure suggests that the crust of the northeastern proto arabian platform was uniform before subsidence and deposition of the sediments in the cenozoic if crystalline crustal structure is uniform across the northern arabian platform then crustal thickness variations in the zagros fold belt and thrust zone should reveal the history of deformation and crustal shortening in the arabian eurasian collision zone and not reflect pre existing crustal thickness variations in the arabian plate

bubble nucleation in rhyolitic melts experiments at high pressure temperature and water content abstract we report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release two rhyolitic obsidians one containing less than 1 of microlites of fe ti oxides and the other about 20 of various crystals were hydrated at 150 mpa and 780 850 c after saturation was reached 5 3 5 5 wt water pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time before the final rapid quenching of the experiments the results demonstrate the importance of heterogeneous nucleation microlites of fe ti oxides are very efficient as sites for bubble nucleation in their presence modest nucleation was observed even after decompression by 6 10 8 bubbles cm 3 in the absence of microlites no nucleation occurred at p mpa at p 10 mpa bubbles also nucleated on crystals of biotite zircon and apatite modest nucleation 10 3 10 5 cm 3 took place even in crystal free samples but it was still heterogeneous when p exceeded 80 mpa nucleation in crystal free samples became extensive 10 5 10 7 cm 3 the lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous nucleation rates were controlled mainly by the availability of sites rates were faster than 10 6 cm 3 s 1 when microlites were present and faster than 10 5 cm 3 s 1 in the absence of microlites at p 70 mpa narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop the experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation when a large number of efficient sites e g fe ti oxide are present bubble nucleation requires very little supersaturation and to a good approximation gas and magma are in equilibrium in magmas that are crystal free or contain crystals that are inefficient at nucleating bubbles very high degrees of supersaturation are required in order to initiate nucleation these two modes of exsolution may lead to contrasting styles of convection pressure build up and eruption

response of the water level in a well to earth tides and atmospheric loading under unconfined conditions the response of the water level in a well to earth tides and atmospheric loading under unconfined conditions can be explained if the water level is controlled by the aquifer response averaged over the saturated depth of the well because vertical averaging tends to diminish the influence of the water table the response is qualitatively similar to the response of a well under partially confined conditions when the influence of well bore storage can be ignored the response to earth tides is strongly governed by a dimensionless aquifer frequency q u the response to atmospheric loading is strongly governed by two dimensionless vertical fluid flow parameters a dimensionless unsaturated zone frequency r and a dimensionless aquifer frequency qu the differences between q u and qu are generally small for aquifers which are highly sensitive to earth tides when q u and qu are large the response of the well to earth tides and atmospheric loading approaches the static response of the aquifer under confined conditions at small values of q u and qu well response to earth tides and atmospheric loading is strongly influenced by water table drainage when r is large relative to qu the response to atmospheric loading is strongly influenced by attenuation and phase shift of the pneumatic pressure signal in the unsaturated zone the presence of partial penetration retards phase advance in well response to earth tides and atmospheric loading when the theoretical response of a phreatic well to earth tides and atmospheric loading is fit to the well response inferred from cross spectral estimation it is possible to obtain estimates of the pneumatic diffusivity of the unsaturated zone and the vertical hydraulic conductivity of the aquifer

crystallization of microlites and degassing during magma ascent constraints on the fluid mechanical behavior of magma during the tenjo eruption on kozu island japan bubble and crystal textures provide information with regard to the kinetics of the vesiculation and crystallization processes they also provide insights into the fluid mechanical behavior of magma in a conduit we performed textural bubble and crystal and compositional analyses of pyroclasts that were obtained from the tenjo pyroclastic flow which resulted on account of the eruption in 838 a d on kozu island about 200 km south of tokyo japan pyroclasts in one flow unit 300 2 060 kg m3 average density 1330 kg m3 can be classified into three types on the basis of vesicle textures type i pyroclasts have small isolated spherical bubbles with higher vesicularities 67 77 vol and number density 10 8 11 7 log m 3 type ii pyroclasts have vesicularities similar to type i 61 69 vol but most bubbles exhibit evidences of bubble coalescence and lower number densities than type i 8 9 9 5 log m 3 type iii pyroclasts contain highly deformed bubbles with lower vesicularities 16 34 vol and number densities 8 2 9 0 log m 3 the microlite volume fraction dre converted also changes consistently across type i type ii and type iii as 0 06 0 08 and 0 10 0 15 respectively however the number density of the microlites remains nearly invariant in all the pyroclast types these facts indicate that the variation in the microlite volume fraction is controlled not by the number density i e nucleation process but by the size i e growth process the growth history of each type of microlite was different water content determinations show that the three types of pumices have similar h2o contents 2 6 0 2 wt this fact implies that all three types were quenched at nearly the same depth 35 5 mpa assuming that the magma was water saturated in the conduit if the crystal sizes are limited only by growth time a variation in this parameter can be related to the residence time which is attributed to the flow heterogeneity in the conduit by assuming a laminar poiseuille type flow these textural observations can be explained by the difference in ascent velocity and shearing motion across the conduit which in turn results in the differences in growth times of crystals degrees of deformation and bubble coalescence consequently for crystals in the inner part of the conduit the crystal growth time from nucleation to quenching is shorter than that near the conduit wall the vesicle texture variation of bubbles in types i ii and iii results from the difference in the deformation history implying that the effect of degassing occurred primarily towards the conduit wall

decompression experiments identify kinetic controls on explosive silicic eruptions 1 eruption intensity is largely controlled by decompression induced release of water rich gas dissolved in magma it is not simply the amount of gas that dictates how forcefully magma is propelled upwards during an eruption but also the rate of degassing which is partly a function of the supersaturation pressure pcritical triggering gas bubble nucleation high temperature and pressure decompression experiments using rhyolite and dacite melt reveal compositionally dependent differences in the pcritical of degassing that may explain why rhyolites have fueled some of the most explosive eruptions on record

experimental constraints on degassing of magma isothermal bubble growth during continuous decompression from high pressure numerical models predict that rapid ascent of hydrous magma can lead to supersaturation of dissolved volatile constituents possibly leading to explosive eruption we have performed controlled decompression experiments to investigate the ascent rates required to maintain bubble melt equilibrium high silica rhyolitic melts were saturated with water at 200 mpa and 825 c decompressed to lower pressures at constant rates of 0 025 0 25 0 5 and 1 0 mpa s 1 and then rapidly quenched isobarically other samples were saturated with water over the pressure range investigated to determine equilibrium water solubility in order to quantify degassing efficiency during decompression at a decompression rate of 0 025 mpa s 1 melt vapor equilibrium was maintained over the entire pressure range examined 200 to 17 5 mpa a single bubble nucleation event occurred in response to decompression and quenched bubble sizes can be modeled by a equilibrium bubble growth model that takes into account the number density of bubbles at decompression rates of 0 25 0 5 and 1 0 mpa s 1 rhyolitic melts could not degas in equilibrium when pressure decreased from 200 mpa to 140 mpa and water supersaturation p in the melt reached up to 60 mpa with higher values at faster decompression rates further pressure release resulted in near equilibrium degassing and p dropped significantly in each case p decreased when bubbles exceeded 10 vol a single heterogeneous bubble nucleation event occurred in each experiment when p 20 mpa no other bubbles nucleated despite p reaching 60 mpa which is probably too low to trigger homogeneous nucleation compared to estimates for magma decompression rates during lava dome eruptions our results indicate that magmas can degas efficiently throughout their ascent to the surface in explosive eruptions decompression rates may exceed those of this study and hence melts may become supersaturated with water such fast decompressions are expected however only when magma is highly vesicular which would aid approach to equilibrium degassing

regional waveform propagation in the arabian peninsula regional waveform propagation is characterized in the arabian peninsula using data from a temporary network of broadband seismometers between november 1995 and march 1997 332 regional delta 15 events were recorded from nine stations deployed across the arabian shield regional phase propagation was analyzed in two ways by individual inspection of the waveforms and by stacking of waveforms inspection of the waveforms revealed consistent variations in individual seismograms according to the region of origin waveforms from events in the gulf of aqaba northwest of the network possess weak pn pg and sn but show a prominent lg phase in contrast clear pn sn and lg are observed for events located in the zagros a region northeast of the network events near the straits of hormuz also display pn and sn but lack a strong high frequency lg southern red sea and african earthquakes have moderate amplitude body phases with some lg for the stacks the data were high pass filtered at 1 hz rectified binned and then stacked by time distance or by time slowness the time distance stacks show clear differences between regions that correspond to the variations observed in individual seismograms the time slowness stacks allow comparison of relative phase velocities and amplitudes pn velocity under the network was estimated to be 8 0 0 2 km s consistent with data from prior refraction profiles the area of inefficient pn and sn propagation coincides with an area of holocene volcanism and suggests that anomalous upper mantle underlies much of the arabian shield

ultra low frequency electromagnetic measurements associated with the 1998 mw 5 1 san juan bautista california earthquake and implications for mechanisms of electromagnetic earthquake precursors abstract ultra low frequency ulf 0 01 10 hz magnetic field anomalies prior to m 6 0 earthquakes have been reported in various regions of the world here we test whether smaller magnitude earthquakes have associated ulf anomalies by using continuous measurements made 9 km above and 2 km ne of the epicenter of the m w 5 1 8 12 98 san juan bautista ca earthquake half hour spectral averages of the magnetic field data in nine different ulf frequency bands show no long term days to weeks magnetic field anomaly prior to the earthquake in addition magnetic and electric field polarization shows no anomalous behavior clearly associated with seismic activity an 0 02 nt increase in activity for 2 h prior to the earthquake is close to the background noise levels precluding identification of a precursor we present scaling calculations to show if precursory ulf anomalies are related to the size of the earthquake the observed 0 02 nt increase is the approximately expected magnetic anomaly prior to the san juan bautista sbj earthquake based on the anomaly observed prior to the 1989 loma prieta lp earthquake our reexamination of previously proposed dilatant conductive and piezomagnetic mechanisms for ulf precursors shows that these effects are consistent with the observations prior to the sjb earthquake based on the observations i e lack of a strong precursor in contrast previously proposed electrokinetic and magnetohydrodynamic mhd mechanisms do not appear to be appropriate for this particular earthquake this analysis further supports the hypothesis that precursory ulf signals are related to the size of the earthquake

interpretation of changes in water level accompanying fault creep and implications for earthquake prediction quantitative calculations for the effect of a fault creep event on observations of changes in water level in wells provide an approach to the tectonic interpretation of these phenomena for the pore pressure field associated with an idealized creep event having an exponential displacement versus time curve an analytic expression has been obtained in terms of exponential integral functions the pore pressure versus time curves for observation points near the fault are pulselike a sharp pressure increase or decrease depending on the direction of propagation is followed by more gradual decay to the normal level after the creep event the time function of the water level change may be obtained by applying the filter derived by a g johnson and others to determine the influence of atmospheric pressure on water level to the analytic pore pressure versus time curves the resulting water level curves show a fairly rapid increase or decrease and then a very gradual return to normal the results of this analytic model do not reproduce the steplike changes in water level observed by johnson and others if the procedure used to obtain the water level from the pore pressure is correct these results suggest that steplike changes in water level are not produced by smoothly propagating creep events but by creep events that propagate discontinously by changes in the bulk properties of the region around the well or by some other mechanism in addition simplistic calculations show that significant pressure field variations and water level changes near the surface may be expected to accompany a propagating creep event on a buried fault water level changes of as much as several meters may be expected at the surface for a creep event having a dislocation amplitude of l m on a semi infinite dislocation surface extending downward from a depth of 10 km and propagating horizontally at a rate of 10 km day the maximum near surface effect should be observed at a horizontal distance from the fault about equal to the depth of the top of the dislocation surface these results are consistent with the observations made in china of large water level changes preceding large earthquakes if some sort of aseismic creep event at depth precedes these earthquakes

anomalous streamflow and groundwater level changes before the 1999 m7 6 chi chi earthquake in taiwan possible mechanisms streamflow recorded by a stream gauge located 4 km from the epicenter of the 1999 m7 6 chi chi earthquake in central taiwan showed a large and rapid anomalous increase of 124 m3 s starting 4 days before the earthquake this increase was followed by a comparable co seismic drop to below the background level for 8 months in addition groundwater levels recorded at a well 1 5 km east of the seismogenic fault showed an anomalous rise 2 days before the earthquake and then a unique 4 cm drop beginning 3 h before the earthquake the anomalous streamflow increase is attributed to gravity driven groundwater discharge into the creek through the openings of existing fractures in the steep creek banks crossed by the upstream shueilikun fault zone as a result of pre earthquake crustal buckling the continued tectonic movement and buckling together with the downward flow of water in the crust may have triggered the occurrence of some shallow slow slip events in the shueilikun and other nearby fault zones when these events propagate down dip to decollement where the faults merges with the seismogenic chelungpu fault they may have triggered other slow slip events propagating toward the asperity at the hypocenter and the chelungpu fault these events may then have caused the observed groundwater level anomaly and helped to trigger the earthquake

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

three dimensional simulation of the near fault ground motion for the 1995 hyogo ken nanbu kobe japan earthquake abstract the 17 january 1995 hyogo ken nanbu earthquake is a typical example showing that the ground motions along basin edge faults can be very destructive in this study we simulate the near fault ground motion from this earthquake based on a kinematic fault model and a simplified 3d velocity structure of the kobe area the kinematic earthquake rupture and the wave propagation are modeled using a 3d finite difference method fdm our simulation identifies the basin edge effect as an important factor that influenced the ground motion amplification pattern in the kobe area we found that the coupling of the source directivity and basin edge effects causes impulsive ground motions with extremely high amplitude at periods greater than 1 sec and in a narrow zone offset less than 1 km from the basin edge the combination of these effects acted to create a fairly continuous band of amplification that extends about 30 km in an elongated zone parallel to the basin edge boundary in some areas localized site effects might have been as important as the abovementioned effects but they cannot explain the continuity of the extended east west zone of damage

heterogeneous bubble nucleation on fe ti oxide crystals in high silica rhyolitic melts the nucleation of h2o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 740 800 c and 50 175 mpa decompressed by 20 70 mpa and held at the lower pressures for 10 s before being quenched the hydration conditions were subliquidus and all samples contain blocky magnetite needle shaped hematite plagioclase magnetite is abundant at 800 c and high pressures whereas hematite becomes more abundant at lower temperatures and pressures bubbles nucleated in a single event in all samples with the number density nt of bubbles varying between 2 107 and 1 109 cm 3 at low degrees of supersaturation one to a few bubbles nucleate on faces of magnetite but at medium to high degrees of supersaturation multiple bubbles nucleate on single magnetite grains on hematite one to a few bubbles nucleated at the ends of the needle shaped crystals at medium supersaturations but formed along their entire lengths at high supersaturations nt increases as water diffusivity decreases indicating that the number of bubbles nucleated is influenced by their growth which depletes the melt with respect to h2o and lowers supersaturation if volcanic eruptions are triggered by bubble formation in magmas stored in shallow level magma chambers then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed whereas homogeneous nucleation is unlikely to trigger eruptions

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

recent movement on the garlock fault as suggested by water level fluctuations in a well in fremont valley california water levels have been continuously recorded since march 1978 in a well in fremont valley where several strands of the adjacent garlock fault zone have exhibited both left lateral displacement and components of normal displacement differences in water levels indicate that a fault segment lies between the observation well and a nearby irrigation well during the 4 year recording period six sharp fluctuations or spikes were noted these fluctuations occurring over 2 to 4 day periods have amplitudes of 15 30 cm they appear to be the result of creep events on a nearby fault two types of creep events are plausible 1 normal slip on an en echelon trace of the garlock fault less than 300 m south of the well with the north side up relative to fremont valley or 2 left lateral slip on the same fault because of the nature of the fluctuations we favor the latter interpretation dislocation models utilizing exponential arc tangent and skewed cosine functions were used to analyze the water level fluctuations associated pressure distribution and fault displacements the results suggest that creep on the fault ranges from several millimeters to a centimeter for individual events estimates of cumulative creep for the period 1978 1982 range from 20 to 50 mm depending on the particular model employed

the crustal structure of the western arabian platform from the spectral analysis of long period p wave amplitude ratios abstract the crustal structure of the western arabian platform is derived using the spectral analysis of long period p wave amplitude ratios the ratio of the vertical to the horizontal component is used to obtain the crustal transfer function based on thickness variations crustal velocities densities and the angle of emergence at the lower crust and upper mantle interface eleven well defined earthquakes recorded at the long period ryd station during the period from 1985 to 1994 were selected for analysis based on the following criteria focal depths with a range between 7 and 89 km body wave magnitudes greater than 4 7 epicentral distances with a range from 8 8 to 26 5 and back azimuthal coverage from 196 to 340 spectral analysis calculations were based on the comparison of the observed spectral ratios with those computed from theoretical p wave motion obtained using the thomson haskell matrix formulation for horizontally layered crustal models the selection of the most suitable model was based on the identification of the theoretical model which exhibits the highest cross correlation coefficient with the observed transfer function ratio by comparing the spectral peak positions of the observed and theoretical values the thickness and velocity can be resolved within 3 km and 1 km s respectively of the observed values the spectral analysis of long period p waves can detect a thin layer near the surface of about 1 6 km thick and a velocity contrast of about 10 with that of the underlying layer a strong velocity gradient of about 0 05 km s per km was found in the upper crust and 0 02 km s per km in the lower crust the derived crustal model is not unique due to the theoretical assumptions horizontal layering constant densities and velocities in each layer quality of the data and complexities of the crustal structure the crustal model suggests that the crust consists of five distinct layers the upper crustal layer has a p wave velocity of about 5 6 km s and is about 1 6 km thick the second layer has a velocity of about 6 2 km s and is 10 2 km thick the third layer shows a velocity of 6 6 km s and is 6 8 km thick the fourth layer has a velocity of about 6 8 km s and is 12 3 km thick the lower crustal layer has a velocity of about 7 5 km s and is 9 3 km thick the mohorovicic discontinuity beneath the western arabian platform indicates a velocity of 8 2 km s of the upper mantle and 42 km depth

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

bubble growth in highly viscous melts theory experiments and autoexplosivity of dome lavas we examine the physics of growth of water bubbles in highly viscous melts during the initial stages diffusive mass transfer of water into the bubble keeps the internal pressure in the bubbles close to the initial pressure at nucleation growth is controlled by melt viscosity and supersaturation pressure and radial growth under constant pressure is approximately exponential at later stages internal pressure falls radial growth decelerates and follows the square root of time at this stage it is controlled by diffusion the time of transition between the two stages is controlled by the decompression melt viscosity and the peclet number of the system the model closely fit experimental data of bubble growth in viscous melts with low water content close fit is also obtained for new experiments at high supersaturation high peclet numbers and high variable viscosity near surface degassed silicic melts are viscous enough so that viscosity controlled growth may last for very long times using the model we demonstrate that bubbles which nucleate shortly before fragmentation cannot grow fast enough to be important during fragmentation we suggest that tiny bubbles observed in melt pockets between large bubbles in pumice represent a second nucleation event shortly before or after fragmentation the presence of such bubbles is an indicator of the conditions at fragmentation the water content of lavas extruded at lava domes is a key factor in their evolution melts of low water content 0 2 wt are too viscid and bubbles nucleated in them will not grow to an appreciable size bubbles may grow in melts with 0 4 wt water the internal pressure in such bubbles may be preserved for days and the energy stored in the bubbles may be important during the disintegration of dome rocks and the formation of pyroclastic flows

major and trace element and sr isotope constraints on fluid circulation in the barbados accretionary complex part ii circulation rates and fluxes abstract 87 sr 86 sr ratios of pore fluids recovered from diapiric structures east of the deformation front of the barbados accretionary prism remain quasi constant from the surface to depths of 8 m the sr isotopic ratios are non radiogenic compared with seawater which implies that non local fluid was expelled at such a high rate that its sr isotopic composition was dominated by the source fluid and not by pore fluid or seawater chloride depletion recorded in the fluids corresponds to a five fold dilution in the deepest sample from the atalante diatreme this also strongly suggests upward flow the comparison of measured chloride contents and sr isotope depth profiles with theoretical values obtained using a steady state advection diffusive model gives some constraints on the fluid circulation rate in the two diatremes atalante and cyclope and in one mud diapir c the velocities are variable but the diatremes which have the largest thermal and chemical anomalies yield the highest estimated advection rates the fluid circulation rate in the centre part of the largest diatreme atalante 0 47 km 2 total surface is estimated at about 1 m yr the fluid flux from the deepest to shallowest levels of the atalante diatreme was estimated on the basis of the fluid circulation rate calculations fluid release is clearly favoured if active conduits are present as they are in the diatremes but not in the mud diapirs however even though this steady state advection diffusion model provides a rough estimation of flow rates modification of the fluid pressure and therefore fluid volume during earthquakes may superimpose large but transient fluid expulsions

a multistep approach for joint modeling of surface wave dispersion and teleseismic receiver functions implications for lithospheric structure of the arabian peninsula 1 we present a multiple step procedure for joint modeling of surface wave group velocity dispersion curves and teleseismic receiver functions for lithospheric velocity structure the method relies on an initial grid search for a simple crustal structure followed by a formal iterative inversion an additional grid search for shear wave velocity in the mantle and finally forward modeling of transverse isotropy to resolve love rayleigh surface wave dispersion discrepancy it considers longer period surface wave group velocity swgv dispersion allowing for the resolution of deeper structure compared to previous joint inversions the grid search for simple crustal structure is facilitated using a library of precomputed receiver functions and swgv dispersion curves the iterative inversion improves fit to the data by increasing the number of layers in the crust when necessary in order to fit the swgv for periods greater than about 50 s we perform a grid search over mantle velocities including the mantle lid and low velocity zone keeping the crustal structure fixed to the values from the previous step in some cases a clear love rayleigh discrepancy prevents a simultaneous fit of the group velocities with an isotropic model the love rayleigh discrepancy can be resolved by allowing shear wave transverse isotropy with a vertical symmetry axis vsh vsv differences in the uppermost mantle the method is applied to 10 stations in the arabian peninsula sampling various tectonic environments including active continental rifting and stable regions the resulting shear velocity models confirm rapid crustal thinning of the arabian shield toward the red sea however we do not find strong evidence for crustal thickening toward the arabian platform our results suggest that the mantle lithosphere thickness varies regionally but that the mantle shear velocities beneath the arabian shield and red sea coast are generally anomalously low furthermore our results indicate the presence of strong polarization anisotropy up to about 10 in the lithospheric upper mantle in the vicinity of as well as farther away from the red sea our modeling yields vsv vsh in the southwestern part of the arabian peninsula consistent with vertical flow and vsh vsv in the northwestern part of the arabian peninsula and the continental interior consistent with horizontal flow indicating that the mantle flow pattern is not uniform along the axis of the red sea