automated detection and location of microseismicity at mount st helens with a large n geophone array in the summer of 2014 a dense array of 904 geophones was deployed at mount st helens along the road and trail system within 15 km distance of the summit crater the array recorded continuous data for approximately 2 weeks and presents an unprecedented seismic observation of an active volcano a reverse time imaging method is applied to short term average over long term average time series data to automatically detect and locate microseismicity these efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the pacific northwest seismic network earthquake locations resolve a narrow 1 km wide vertical lineament of seismicity which extends from the surface to 4 km depth directly beneath the summit crater this feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface

upper crustal low frequency seismicity at mount st helens detected with a dense geophone array abstract mount st helens has been closely monitored by a permanent network including 16 seismometers located within a 15 km radius yet some of its diverse seismic signals remain challenging to detect and relate to deformation processes data from a dense array of 904 geophones within 15 km of mount st helens provide novel constraints on weak seismic signals observed during a two week deployment in 2014 here we report on the identification of weak low frequency lf events that were either not detected by the permanent network or were misclassified as likely rockfalls we refer to the signals as lf earthquakes because they have less power above 10 hz and longer durations compared to similar magnitude volcano tectonic vt events reverse time imaging with the entire geophone array indicates that the lf hypocenters overlap with the frequent vt seismicity in a sub vertical column beneath the summit crater at 6 km below sea level beamforming of p wave arrivals with an exceptionally dense sub array yields slowness estimates that confirm a subsurface origin for 16 lf events occurrence of vt and lf events in approximately the same volume suggests distinct source processes rather than path effects as the primary explanation for their different signals some of the lf and vt events have multiple spectral peaks that may be consistent with source mechanisms involving resonance in fluid filled cavities however the majority of lf and vt event spectra exhibit smooth amplitude decay with increasing frequency consistent with shear failure sources the depletion of high frequencies for lf events could be caused by unusually low rupture velocities or stress drops our results indicate a variety of earthquake source processes occur in a sub vertical column at 6 km depth which is thought to overly the primary upper crustal magma reservoir lf events are typically associated with eruptive or near eruptive activity and are generally not reported at depths greater than 1 km at mount st helens but based on our short duration high density experiment we suggest that upper crustal lf events may be more common than previously recognized and occur across a broader depth range

three dimensional p and s velocity structure in the coalinga region california the coalinga earthquake sequence of 1983 provided a unique opportunity to perform a three dimensional velocity and hypocenter inversion in an area of complex three dimensional structure dominated by folding and blind thrusts additionally since other varied geological and geophysical studies have been completed in this area the three dimensional inversion solution could be compared to other interpretations inversion of 7696 p and 1511 s first arrivals from earthquakes and 696 p first arrivals from refraction shots produced a three dimensional velocity model with grid spacing of 1 2 km in the hypocentral area the overall shape and location of velocity features correspond well to the mapped surface geology the three dimensional inversion yields details of folds where the resolution is good and the general shape where resolution is lower the amounts of structural relief inferred for the local folds are similar to values inferred from geologic data the three dimensional velocity solution has several distinctive features there is a linear high velocity body 6 1 6 5 km s about 25 km long from 6 to 8 km depth that may represent a fragment of coast range ophiolite a shallow low velocity zone lvz which extends for 20 km along the fold axis at about 6 km depth and correlates with lvzs observed in both the refraction and the reflection data may indicate high pore pressure caused by lateral compressive strain deeper lvzs occur within inferred franciscan material and are characterized by horizontal or southwest dipping zones of varied thickness 4 8 km wide and 5 10 km long these lvzs may represent multiple thrust faults their locations and geometry are consistent with thrust faults inferred with seismic reflection data the three dimensional velocity solution compares well to prior two dimensional seismic reflection and refraction models and observed gravity the shape of the inferred sedimentary section agrees well with the reflections from the cenozoic strata compared to the refraction model the three dimensional solution has similar velocities and similar locations of velocity features but is more detailed in the hypocentral zone where it uses more data the gravity computed from the three dimensional velocities is similar to the observed gravity in both shape and amplitude both a simple one dimensional initial model and a complex initial model derived from the refraction interpretation were tried a simple starting model gave the best results the s velocity solution has different resolution than the p velocity solution because it uses a different set of stations and it has lower resolution because it uses fewer arrival times while the general patterns of velocity variation are similar for both vp and vs the vs solution tends to have more smearing of velocity features and can have somewhat different locations of velocity features

alaska crustal deformation finite element modeling constrained by geologic and very long baseline interferometry data we compute crustal motions in alaska by calculating the finite element solution for an elastic spherical shell problem the method we use allows the finite element mesh to include faults and very long baseline interferometry vlbi baseline rates of change boundary conditions include pacific north american pa na plate motions the solution is constrained by the oblique orientation of the fairweather queen charlotte strike slip faults relative to the pa na relative motion direction and the oblique orientation from normal convergence of the eastern aleutian trench fault systems as well as strike slip motion along the denali and totschunda fault systems we explore the effects that a range of fault slip constraints and weighting of vlbi rates of change has on the solution this allows us to test the motion on faults such as the denali fault where there are conflicting reports on its present day slip rate we find a pattern of displacements which produce fault motions generally consistent with geologic observations the motion of the continuum has the general pattern of radial movement of crust to the ne away from the fairweather queen charlotte fault systems in se alaska and canada this pattern of crustal motion is absorbed across the mackenzie mountains in nw canada with strike slip motion constrained along the denali and tintina fault systems in south central alaska and the alaska forearc oblique convergence at the eastern aleutian trench and the strike slip motion of the denali fault system produce a counterclockwise pattern of motion which is partially absorbed along the contact and related fault systems in southern alaska and is partially extruded into the bering sea and into the forearc parallel the aleutian trench from the alaska peninsula westward rates of motion and fault slip are small in western and northern alaska but the motions we compute are consistent with the senses of strike slip motion inferred geologically along the kaltag kobuk trench and thompson creek faults and with the normal faulting observed in nw alaska near nome the nonrigid behavior of our finite element solution produces patterns of motion that would not have been expected from rigid block models strike slip faults can exist in a continuum that has motion mostly perpendicular to their strikes and faults can exhibit along strike differences in magnitudes and directions

three dimensional shear velocity anisotropic model of piton de la fournaise volcano la reunion island from ambient seismic noise we cross correlate 4 years of seismic noise from the seismic network of piton de la fournaise volcano la reunion island to measure the group velocity dispersion curves of rayleigh and love waves we average measurements from vertical and radial components to obtain 577 rayleigh wave dispersion curves the transverse components provided 395 love wave dispersion curves we regionalize the group velocities measurements into 2 d velocity maps between 0 4 and 8 s finally we locally inverted these maps for a pseudo 3 d anisotropic shear velocity model down to 3 km below the sea level using a neighborhood algorithm the 3 d isotropic shear wave model shows three distinct high velocity anomalies surrounded by a low velocity ring the anomaly located below the present plaine des sables could be related to an old intrusive body at the location of the former volcanic center before it migrated toward its present location the second high velocity body located below the summit of the volcano likely corresponds to the actual preferential dyke intrusion zone as highlighted by the seismicity the third high velocity anomaly located below the grandes pentes and the grand brule areas and is an imprint of the solidified magma chamber of the dismantled les alizes volcano radial anisotropy shows two main anomalies positive anisotropy above sea level highlighting the recent edifice of piton de la fournaise with an accumulation of horizontal lava flows and the second one below the sea level with a negative anisotropy corresponding to the ancient edifice of piton de la fournaise dominated by intrusions of vertical dykes

seismic scattering and absorption mapping of debris flows feeding paths and tectonic units at mount st helens volcano abstract frequency dependent peak delay times and coda quality factors have been used jointly to separate seismic absorption from scattering quantitatively in earth media at regional and continental scale to this end we measure and map these two quantities at mount st helens volcano the results show that we can locate and characterize volcanic and geological structures using their unique contribution to seismic attenuation at 3 hz a single high scattering and high absorption anomaly outlines the debris flows that followed the 1980 explosive eruption as deduced by comparison with remote sensing imagery the flows overlay a nnw sse interface separating rocks of significant varying properties down to 2 4 km and coinciding with the st helens seismic zone high scattering and high absorption anomalies corresponding to known locations of magma emplacement follow this signature under the volcano showing the important interconnections between its feeding systems and the regional tectonic boundaries with frequency increasing from 6 to 18 hz the nnw sse tectonic feeding trends rotate around an axis centered on the volcano in the direction of the regional scale magmatic arc sw ne while the aseismic high scattering region wsw of the volcano shows no evidence of high absorption the regions of highest scattering and absorption are consistently located at all frequencies under either the eastern or the south eastern flank of the volcanic edifice from the comparison with the available geological and geophysical information we infer that these anomalies mark both the location and the trend of the main feeding systems at depths greater than 4 km

magma system recharge of mount st helens from precise relative hypocenter location of microearthquakes 1 four hundred forty seven well recorded earthquakes at mount st helens during the late 1990s are relocated with high precision using a combination of a cross correlation technique and then a procedure to obtain a one dimensional velocity structure solution together with station corrections the resulting high resolution pattern of the spatial temporal evolution of the earthquake activity along with focal mechanisms of events provide additional evidence for the presence of a magma reservoir in the depth range 5 5 10 km connected to the surface by a thin vertical conduit moreover the distribution of the events below 5 5 km yielded a much clearer view of the structures that generate the seismicity at that depth some of the deeper events occur along a sequence of faults under the influence of a radial stress field however a large number of the events occur on a nne ssw striking steeply dipping fault with slip consistent with magma being periodically injected into a truncated dike on the northwest side of this fault

the yellowstone magmatic system from the mantle plume to the upper crust the yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world an understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk using a joint local and teleseismic earthquake p wave seismic inversion we revealed a basaltic lower crustal magma body that provides a magmatic link between the yellowstone mantle plume and the previously imaged upper crustal magma reservoir this lower crustal magma body has a volume of 46 000 cubic kilometers 4 5 times that of the upper crustal magma reservoir and contains a melt fraction of 2 these estimates are critical to understanding the evolution of bimodal basaltic rhyolitic volcanism explaining the magnitude of co2 discharge and constraining dynamic models of the magmatic system for volcanic hazard assessment

the magma system of mount st helens non linear high resolution p wave tomography abstract high resolution three dimensional images of p wave velocity anomalies below mt st helens washington were derived using tomographic inversion the model is a 27 5 21 20 km target volume parameterized by blocks 0 5 km per side the area included 39 stations and 5454 local events leading to 35 475 rays used in the inversion to diminish the effects of noisy data the laplacian was constrained to be small within horizontal layers providing smoothing of the model non linear effects were compensated for by iterating three dimensional ray tracing using pseudo bending between inversions and relocating earthquakes relative to the updated three dimensional model the structural differences between the linear and non linear inversions appear to be insignificant although the amplitudes of the anomalies are larger in the non linear models results indicate a low velocity anomaly 7 approximately 1 km in lateral extent from 1 5 to 3 km depths between 3 and 6 km depth the anomaly appears to spread out below 6 km depth the low velocity feature changes to a higher velocity perturbation with lower velocity perturbations flanking around the perimeter of the volcano the higher velocity material which correlates with the higher seismicity at that depth is interpreted as being a plug capping the low velocity magma chamber which begins below 9 km depth

local crustal extension at mount st helens washington seismicity and the orientation of fault planes from focal mechanisms indicate that mount st helens is located at a dextral offset along the st helens seismic zone shz earthquake swarms occurring in this offset are related to volcanic eruptions because motion on the shz is in a right lateral strike slip sense this dextral offset creates extension within a volume of the crust between the offset fault segments this offset geometry is similar to that of geothermal areas along the san andreas fault system we apply a model derived from these geothermal areas to mount st helens and find that the major differences between mount st helens and the geothermal areas can be related to the ratio of the width of the offset between fault segments l to the seismogenic depth h at mount st helens this ratio is 1 whereas in the geothermal areas the ratio is 1 we propose that when l h 1 as at mount st helens the regional minimum principal stress does not completely dominate the small volume under extension and as a consequence the opening geometry is poory established compared to the oblique crustal spreading that characterizes the geothermal areas where l h 1 late quaternary volcanic vents near mount st helens strike northeast similar to the strike of a set of pre quaternary faults and intrusive rocks that are mapped north of the volcano in addition the deepest earthquakes occurring within the extensional volume are aligned along a northeast striking fault since these northeast striking features are aligned approximately prependicular to the regional minimum principal stress we infer that the spatial position of mount st helens is controlled by the junction of the right stepping offset of the shz with the older set of fractures and that these fractures are favorably aligned with respect to the contemporary regional tectonic stress directions for the transport of magma through the brittle crust the sense of fault motions predicted by our model for local crustal extension is consistent with an apparent component of right lateral shear measured from geodetic lines around mount st helens during june and july 1980

models of ground deformation from vertical volcanic conduits with application to eruptions of mount st helens and mount etna we present simple analytical models of ground deformation from inflation of vertical volcanic conduits or pipes we compare three cases corresponding to 1 a pressurized pipe with closed end at the top that resists the internal pressure 2 a pressurized pipe with top open for which the internal deformation is mainly dislocation of the cylindrical walls and 3 a pipe shaped region that dilates for the closed pipe we use eshelby s inclusion theory to model deformation from a thin pressurized elongated prolate ellipsoid in a full space which we express in terms of double forces to satisfy surface boundary conditions we superimpose image solutions by using double forces derived from mindlin s half space solution for the point force for the open pipe we apply the volterra integral to dislocation across a cylindrical surface and generalize it to the half space using mindlin s point force solution these two solutions show marked differences from the line of dilatation solution ratios of maximum horizontal deformation to maximum vertical deformation for the pipe models are significantly greater than values for the center or line of dilatation models this and the more gradual fall off of the deformation with distance may be used as diagnostics for discriminating between pipe like sources and dilatational sources on volcanoes where both components of the deformation field are available as examples we compare the closed pipe model with deformation associated with dome building on mount st helens volcano and tilt predicted by the open pipe model with tilt measured during a period of explosive eruptive activity on mount etna sicily in 1995 comparison between model values and the measurements suggests that the effective elastic moduli of the volcanic cones are very low

magma accumulation rates and thermal histories of plutons of the sierra nevada batholith ca zircon u pb geochronology results indicate that the john muir intrusive suite of the central sierra nevada batholith california was assembled over a period of at least 12 ma between 96 and 84 ma bulk mineral thermochronology u pb zircon and titanite 40ar 39ar hornblende and biotite of rocks from multiple plutons comprising the muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure assembly of intrusive suites in the sierra nevada and elsewhere over millions of years favors growth by incremental intrusion estimated long term pluton assembly rates for the john muir intrusive suite are on the order of 0 001 km3 a 1 which is inconsistent with the rapid magma fluxes that are necessary to form large volume magma chambers capable of producing caldera forming eruptions if large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites it is doubtful that the intrusive suites represent cumulates left behind following caldera forming eruptions

tomographic imaging of local earthquake delay times for three dimensional velocity variation in western washington tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations in the puget sound region of western washington the 37 500 square km region is represented by nearly cubic blocks of 5 km per side p wave arrival time observations from 4 387 crustal earthquakes with depths of 0 to 40 km were used as sources producing 36 865 rays covering the target region a conjugate gradient method lsqr is used to invert the large sparse system of equations to diminish the effects of noisy data the laplacian is constrained to be zero within horizontal layers providing smoothing of the model the resolution is estimated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife statistical procedure results of the inversion are correlated with some known geologic features and independent geophysical measurements high p wave velocities along the eastern flank of the olympic peninsula are interpreted to reflect the subsurface extension of crescent terrane low velocities beneath the puget sound further to the east are inferred to reflect thick sediment accumulations the crescent terrane appears to extend beneath puget sound consistent with its interpretation as a major accretionary unit in themore southern puget sound basin high velocity anomalies at depths of 10 20 km are interpreted as crescent terrane and are correlated with a region of low seismicity near mt ranier high velocity anomalies may reflect buried plutons less

vp structure of mount st helens washington usa imaged with local earthquake tomography we present a new p wave velocity model for mount st helens using local earthquake data recorded by the pacific northwest seismograph stations and cascades volcano observatory since the 18 may 1980 eruption these data were augmented with records from a dense array of 19 temporary stations deployed during the second half of 2005 because the distribution of earthquakes in the study area is concentrated beneath the volcano and within two nearly linear trends we used a graded inversion scheme to compute a coarse grid model that focused on the regional structure followed by a fine grid inversion to improve spatial resolution directly beneath the volcanic edifice the coarse grid model results are largely consistent with earlier geophysical studies of the area we find high velocity anomalies nw and ne of the edifice that correspond with igneous intrusions and a prominent low velocity zone nnw of the edifice that corresponds with the linear zone of high seismicity known as the st helens seismic zone this low velocity zone may continue past mount st helens to the south at depths below 5 km directly beneath the edifice the fine grid model images a low velocity zone between about 2 and 3 5 km below sea level that may correspond to a shallow magma storage zone and although the model resolution is poor below about 6 km we found low velocities that correspond with the aseismic zone between about 5 5 and 8 km that has previously been modeled as the location of a large magma storage volume

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

tomographic inversion for three dimensional velocity structure at mount st helens using earthquake data tomographic inversion is applied to 17 659 p phase observations at 21 stations from 2023 earthquakes in the vicinity of mount st helens to study the three dimensional velocity structure block size for the inversion is 2 km horizontally and 2 km or more vertically locations of hypocenters are assumed known and are based on a reference one dimensional layered velocity structure a conjugate gradient technique lsqr is used to invert the large sparse system of equations augmented by regularization with a laplacian roughening matrix resolution is estimated by computing the impulse response of the inversion for various critical locations and uncertainties of the estimates are determined by a jackknife approach the results of the inversion show a remarkable correlation with known geological and geophysical features the spirit lake and spud mt plutons are characterized by high velocity regions extending to approximately 9 km depth the st helens seismic zone a band of diffuse seismicity extending nnw from the volcano is evident as a prominent low velocity lineation the change in character of the velocity anomalies south of st helens corresponds well with the near cessation of seismic activity there a low velocity anomaly beneath the crater from 6 to 16 km depths may represent modern magma accumulations

relationship of temperature to reservoir quality for feldspathic sandstones of southern california abstract the diagenetic alteration of a sandstone results from the combined effects of many factors in order to ascertain the role played by a particular diagenetic factor the remaining diagenetic factors must be held constant for the sandstones of the basins of southern california we have found that the effects of almost all of the principal diagenetic factors are essentially uniform the notable exception is thermal history because of this we have been able to evaluate the diagenetic imprint of temperature upon the sandstones measured reservoir property data taken on core samples of reservoir sandstones from 16 fields in the los angeles ventura and san joaquin basins were used to determine the average rate of porosity and permeability loss with depth for each field a s raight line appears to be the proper representation for the porosity versus depth profiles for the interval of interest the slope of this line is defined here as the porosity gradient porosity gradients for the fields investigated range from 1 1 to 5 8 per 1 000 ft a direct relationship exists between the porosity gradient and the present geothermal gradient for the 16 fields that have been examined as geothermal gradient increases porosity gradient increases the correlation coefficient between these variables is 0 916 for geothermal gradients between 1 6 f and 2 2 f 100 ft a similar relationship also exists between the rate of permeability loss with depth and the geothermal gradient but the average deviation from the mean permeability value is so great the relations ip has little practical significance end of article last page 250

eruption dynamics at mount st helens imaged from broadband seismic waveforms interaction of the shallow magmatic and hydrothermal systems 1 the current eruption at mount st helens is characterized by dome building and shallow repetitive long period lp earthquakes waveform cross correlation reveals remarkable similarity for a majority of the earthquakes over periods of several weeks stacked spectra of these events display multiple peaks between 0 5 and 2 hz that are common to most stations lower amplitude very long period vlp events commonly accompany the lp events we model the source mechanisms of lp and vlp events in the 0 5 4 s and 8 40 s bands respectively using data recorded in july 2005 with a 19 station temporary broadband network the source mechanism of the lp events includes 1 a volumetric component modeled as resonance of a gently nnw dipping steam filled crack located directly beneath the actively extruding part of the new dome and within 100 m of the crater floor and 2 a vertical single force attributed to movement of the overlying dome the vlp source which also includes volumetric and single force components is 250 m deeper and nnw of the lp source at the sw edge of the 1980s lava dome the volumetric component points to the compression and expansion of a shallow magma filled sill which is subparallel to the hydrothermal crack imaged at the lp source coupled with a smaller component of expansion and compression of a dike the single force components are due to mass advection in the magma conduit the location geometry and timing of the sources suggest the vlp and lp events are caused by perturbations of a common crack system

3 d velocity structure of damavand volcano iran from local earthquake tomography damavand volcano is a large intraplate quaternary composite cone overlying the active fold and thrust belt of the central alborz mountains in northern iran in this study we present the first 3 d

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

geology of the mount st helens area record of discontinuous volcanic and plutonic activity in the cascade arc of southern washington the quaternary edifice of mount st helens volcano was built upon a deeply eroded terrane of gently folded and altered volcanic and plutonic rocks that represent the core of the tertiary cascade magmatic arc these rocks constitute an east dipping homoclinal sequence several kilometers thick of subaerially erupted mafic to silicic flows and volcaniclastic strata k ar ages from this section range from about 28 to 23 ma late oligocene and earliest miocene which corresponds to an apparent lull in cascade volcanism to the north of mount rainier volcanism was essentially continuous during this period of time and neither a well defined base nor top of the section is exposed within the mapped area basalt and basaltic andesite dominate the lower part of the mid tertiary section whereas andesitic and dacitic rocks comprise most of the upper part this section was intruded by numerous mafic to silicic dikes sills and irregular plutonic bodies most no more than a few million years younger than their host rocks and subjected to pervasive burial metamorphism and widespread hydrothermal alteration large areas of hornfelsed rock surrounding even relatively small intrusions indicate that the proportion of plutonic rock becomes significantly greater at shallow depth beneath the existing erosion surface a large granitic pluton intruded the mid tertiary section north of spirit lake at about 21 ma the earl porphyry copper deposit occurs within the pluton but appears too young 17 ma to be genetically related to it in contrast to the rather continuous and voluminous oligocene to early miocene activity volcanism since then in the mount st helens area has been localized and volumetrically minor products of three younger eruptive periods have been recognized a sequence of 15 m y old pyroxene andesite flows resting unconformably on mid tertiary strata south of mount st helens widespread shallow dikes and sills of pyroxene andesite between 10 and 8 m y old and compositionally diverse rocks erupted during the past 3 m y the quaternary lavas are more potassic than the tertiary lavas and typically contain phenocrysts of hornblende and biotite which are absent from the older rocks a number of tertiary structures define a broad nne trending zone that may reflect a deep seated lithospheric flaw that has controlled the locus of cascade magmatism in southern washington for the past 25 m y mount st helens lies within this zone at the intersection of the nnw striking st helens seismic zone shz and an ene trending alignment of pleistocene silicic plug domes no surface breakage has been detected along the shz which is apparently very young the linear zone of silicic vents is probably controlled by a fault that has been interpreted from seismic records to occur directly beneath the volcano this zone parallels the directions of regional maximum horizontal compressive stress and north america juan de fuca plate convergence mount st helens is an example of a low volume tectonically controlled magmatic system in an early stage of development

regional tectonic and earthquake hazard implications of a crustal fault zone in southwestern washington the st helens seismic zone shz is a crustal earthquake zone in southwestern washington that is defined by small to moderate magnitude 2 5 to 5 5 earthquakes and focal mechanisms this zone seismically active along a distance of 90 km is interpreted as a fault zone capable of generating a moderate to large magnitude shallow earthquake at the northern end of this seismic zone about 15 km north of the cowlitz river a large area of seismic quiescence for earthquakes above magnitude 3 0 has predominated since 1977 this quiescence leaves open the question whether the shz continues northward into southern puget sound earthquake focal mechanisms along the shz indicating nearly vertical strike slip faulting with the preferred fault planes striking north south from these focal mechanisms we infer that the direction of maximum compression is northeast approximately parallel with the direction of plate convergence between the north american and juan de fuca plates we interpret the focal mechanism data as evidence for locked subduction this tectonic model raises the possibility of a large magnitude subduction earthquake neither the possibility of a moderate to large shallow crustal earthquake nor a major subduction event is part of the current earthquake hazard assessment for washington

an aeromagnetic study of mount st helens aeromagnetic data from surveys flown by the u s geological survey over mount st helens washington before and after the climactic may 18 1980 eruption were used to determine the bulk magnetic properties of the volcano and to delineate a buried source we assumed that most of the edifice of preeruption and posteruption mount st helens has a magnetization direction near the present earth s field of 69 inclination and 20 declination and calculated its intensity as 4 1 a m after subtraction of magnetic anomalies due to topogaphy magnetized with this direction and intensity the preeruption and posteruption surveys revealed nearly identical residual magnetic highs and lows indicative that their sources were not altered or removed by the may 1980 eruption the residual highs were explained by a 200 m deep source lying mostly within the edifice of mount st helens the source could be terrain that predates mount st helens such as a buried ridge or a cone or a valley filled with lava we calculated the magnetization of the material removed by the may 1980 eruption and found its intensity to be 4 2 a m in a direction near the present earth s field similar to that assumed for the volcano as a whole this similar result confirmed the validity of the magnetization vector assumed for the entire edifice

thermal structure of the low velocity zone derived from laboratory and seismic investigations laboratory measurements of seismic velocities in upper mantle peridotite can be used to constrain the thermal state in the low velocity zone the measurements carried out at high temperature and high confining pressure can be extrapolated to upper mantle conditions we compare laboratory velocities with seismic observations and estimate temperatures and partial melt fractions in the low velocity zone the comparison gives 1310 c and 7 vol partial melt at 45 km depth in 0 5 my asthenosphere beneath the iceland plateau and 1450 c and 3 vol partial melt at 85 km depth in 0 20 my asthenosphere beneath the pacific ocean the laboratory results show that up to a 6 velocity drop in the low velocity zone relative to the lithospheric velocity does not necessarily require the existence of partial melt this implies that partial melt may not exist in the low velocity zone older than 5 my and 50 my in the iceland plateau and the pacific ocean respectively

magmatic system geometry at mount st helens modeled from the stress field associated with posteruptive earthquakes earthquakes following the may 18 1980 and june 12 1980 explosive eruptions of mount st helens were concentrated at depths below 6 km the hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater the timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume the withdrawal of magma caused a pressure decrease within the reservoir and the earthquakes occurred as a brittle response to the resulting stress change focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km this anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime in the best fitting model the magma body is centered at 46 204 n 122 187 w 600 m to the north of the present dome and is under a regional shear stress of 20 bars the radius of the chamber is calculated to be between 0 65 and 0 75 km when using pressure drops within the cylinder of 220 and 150 bars respectively the magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3 posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime this fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust

seismicity and structure of akutan and makushin volcanoes alaska using joint body and surface wave tomography joint inversions of seismic data recover models that simultaneously fit multiple constraints while playing upon the strengths of each data type here we jointly invert 14 years of local earthquake body wave arrival times from the alaska volcano observatory catalog and rayleigh wave dispersion curves based upon ambient noise measurements for local vp vs and hypocentral locations at akutan and makushin volcanoes using a new joint inversion algorithm the velocity structure and relocated seismicity of both volcanoes are significantly more complex than many other volcanoes studied using similar techniques seismicity is distributed among several areas beneath or beyond the flanks of both volcanoes illuminating a variety of volcanic and tectonic features the velocity structures of the two volcanoes are exemplified by the presence of narrow high vp features in the near surface indicating likely current or remnant pathways of magma to the surface a single broad low vp region beneath each volcano is slightly offset from each summit and centered at approximately 7 km depth indicating a potential magma chamber where magma is stored over longer time periods differing recovery capabilities of the vp and vs data sets indicate that the results of these types of joint inversions must be interpreted carefully

multi scale reasonable attenuation tomography analysis murat an imaging algorithm designed for volcanic regions abstract the attenuation of body wave amplitudes with propagation distance can be used to provide detailed tomographic images of seismic interfaces fluid reservoirs and melt batches in the crust the high sensitivity of body wave energies to high scattering structures becomes an obstacle when we try to apply attenuation tomography to small scale volcanic media where we must take into account the complexities induced by strong heterogeneous scattering topography and uncertain source modeling in the recorded wave fields the murat code uses a source and site independent coda normalization method to obtain frequency dependent measurements of p to coda and s to coda energy ratios the code inverts these data for both the geometrical spreading factor and the spatially dependent quality factors q providing additional attenuation information in the regions where velocity tomography is available the high sensitivity of coda waves to highly heterogeneous structures highlights zones of anomalous scattering which may corrupt amplitude dependent attenuation measurements and where basal assumptions of linear optics may go unfulfilled a multi step tomographic inversion increases the stability of the results obtained in regions of high heterogeneity e g the volcanic edifice by the inclusion of data corresponding to either sources or stations located in regions of lower heterogeneity on the other hand a mere increase in the number of rays entirely contained in the heterogeneous structures affects both the stability and the effective resolution of the results we apply the code to two small waveform datasets recorded at an active mount st helens and at a quiescent mount vesuvius volcano the results show that the seismicity located inside or under the volcanic edifice produces an increase of the low frequency energy ratios with travel time in both areas in our interpretation the anomalous concentration of energy which affects any waveform recorded on the cone produced inside the volcanic edifice or in the feeding system of the volcano is due to seismic source or medium dependent resonance the results also provide spatial and frequency limits on the feasibility of attenuation tomography in these two regions with larger datasets

characterization of fracture systems using precise array locations of earthquake multiplets an example at deception island volcano antarctica 1 volcano tectonic earthquakes are common seismic events in active volcanic areas the stress produced by volcanic processes is released through fracturing of the shallow crust very often these earthquakes occur in multiplets with similar waveforms a fact which indicates common source characteristics in this work we introduce a method that uses array techniques to calculate precise relative locations of earthquake multiplets we use the relative slowness estimate method to determine accurately the apparent slownesses and propagation azimuths of the earthquakes relative to a selected master event we also obtain precise estimates of the s p delays this information is used to calculate precise relative locations by ray tracing in an earth model we applied this method to determine the characteristics of the fractures activated during the 1999 seismic series at the deception island volcano antarctica we selected a set of 17 earthquake multiplets initially located in a small 4 4 km region a few km ne of the array site we estimated precise locations for 14 of the clusters in most cases hypocenters were distributed in well defined planar geometries we found the best fitting planes which we interpreted as fractures in the medium for two clusters the method spatially separated the earthquakes into two subgroups thus we obtained two planes for each of these clusters resulting in a total of 16 fracture planes this is the first time that the orientations of fracture planes related to a seismic series have been obtained using a seismic array we performed several tests to check various aspects in relation to the stability of the method and concluded that the results were robust the dip angles indicate that the planes are mostly subvertical while the strike angles clearly show a nw se trend for most of the planes and a few planes with ne sw trends the geometry and position of these planes suggest that the 1999 seismic series was influenced by regional tectonics although the origin of the destabilization of the system may be related to the reactivation of a shallow magma chamber

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast