volcano dome dynamics at mount st helens deformation and intermittent subsidence monitored by seismicity and camera imagery pixel offsets the surface deformation field measured at volcanic domes provides insights into the effects of magmatic processes gravity and gas driven processes and the development and distribution of internal dome structures here we study short term dome deformation associated with earthquakes at mount st helens recorded by a permanent optical camera and seismic monitoring network we use digital image correlation dic to compute the displacement field between successive images and compare the results to the occurrence and characteristics of seismic events during a 6 week period of dome growth in 2006 the results reveal that dome growth at mount st helens was repeatedly interrupted by short term meter scale downward displacements at the dome surface which were associated in time with low frequency large magnitude seismic events followed by a tremor like signal the tremor was only recorded by the seismic stations closest to the dome we find a correlation between the magnitudes of the camera derived displacements and the spectral amplitudes of the associated tremor we use the dic results from two cameras and a high resolution topographic model to derive full 3 d displacement maps which reveals internal dome structures and the effect of the seismic activity on daily surface velocities we postulate that the tremor is recording the gravity driven response of the upper dome due to mechanical collapse or depressurization and fault controlled slumping our results highlight the different scales and structural expressions during growth and disintegration of lava domes and the relationships between seismic and deformation signals

pleniglacial sedimentation process reconstruction on laminated lacustrine sediments from lava dammed paleolake alf west eifel volcanic field germany abstract differentiating between regularly seasonal irregular and event based clastic sedimentation is difficult if sedimentation structures resemble and dating methods are imprecise in this study clastic light and dark laminae from lava dammed paleolake alf in the late pleistocene in the quaternary west eifel volcanic field are analyzed to clarify how they formed and if they are of annual origin and comparable to assumed periglacial varves from neighboring lake holzmaar therefore a multiproxy approach is applied combining sediment thin section analysis which focuses on composition and structure with 14c dates the results are compared to recently formed annually laminated clastic sediments of e g the high canadian arctic observed sedimentation structures reveal sediment delivery by over and interflows and deposition from suspension forming two characteristic microfacies type i graded laminae and type ii laminae with graded sublayers additionally erosional bases and event deposits indicate episodic underflows thus lamination is potentially seasonal but is significantly veiled by extreme runoff causing erosion and resuspension processes or a mixed water body preventing sediment delivery into the lake basin however sedimentation processes between watershed and lake could be reconstructed by comparing recent and paleosediment structures

kinematics of the philippine sea plate in this study new instantaneous rotation vectors describing the motion of the philippine sea plate relative to surrounding plates are determined that most nearly satisfy all available geological geophysical and seismological observations along the plate margins pacific philippine and caroline pacific poles are generated from a set of trial eurasia philippine poles of rotation using published determinations of the eurasia pacific and caroline philippine poles the pacific philippine caroline pacific and eurasia philippine poles are then checked for agreement with 1 slip vectors determined from focal mechanisms of earthquakes along the ryukyu trench and the nankai trough 2 convergence at the sagami trough 3 extension in the mariana trough 4 extension behind the bonin trench 5 extension in the western sorol trough and 6 convergence at the mussau trench trial eurasia philippine poles are eliminated if predicted plate motions do not match observed plate motions the locations and rotation rates of the three best fitting poles are eurasia philippine 37 0 n 141 0 e 1 60 ma pacific philippine 3 8 n 130 4 e 1 68 ma caroline pacific 16 9 n 141 4 e 0 54 ma the plate motions predicted by these new poles are compared with the spatial distribution of earthquakes and new focal mechanism solutions in the complex region where the philippine pacific and caroline plates intersect intermediate and deep focus earthquake hypocenters along the southern mariana arc show a southward decrease in the length of the seismic zone which qualitatively agrees with the predicted convergence rate as the pole of rotation is approached and as the effect of back arc spreading in the mariana trough disappears the absence of volcanism and intermediate depth seismicity along the southernmost mariana yap and palau sections of the plate boundary is attributed to very low convergence rates a new focal mechanism solution along the trench south of guam is the first clear underthrusting solution found along the mariana trench and agrees with the convergence direction implied by the pacific philippine pole of rotation no clear underthrusting focal mechanism solutions were found along the yap and palau trenches although three high angle thrust solutions demonstrate that compression is occurring in the upper plate perpendicular to the plate boundary

the 2004 2008 dome building eruption at mount st helens washington epilogue the 2004 2008 dome building eruption at mount st helens ended during winter 2007 2008 at a time when field observations were hampered by persistent bad weather as a result recognizing the end of the eruption was challenging but important for scientists trying to understand how and why long lived eruptions end and for public officials and land managers responsible for hazards mitigation and access restrictions in hindsight the end of the eruption was presaged by a slight increase in seismicity in december 2007 that culminated on january 12 13 2008 with a burst of more than 500 events most of which occurred in association with several tremor like signals and a spasmodic burst of long period earthquakes at about the same time a series of regular localized small amplitude tilt events thousands of which had been recorded during earlier phases of the eruption came to an end thereafter seismicity declined to 10 20 events per day until january 27 28 when a spasmodic burst of about 50 volcano tectonic earthquakes occurred over a span of 3 h this was followed by a brief return of repetitive drumbeat earthquakes that characterized much of the eruption by january 31 however seismicity had declined to 1 2 earthquakes per day a rate similar to pre eruption levels we attribute the tilt and seismic observations to convulsive stagnation of a semisolid magma plug in the upper part of the conduit the upward movement of the plug ceased when the excess driving pressure which had gradually decreased throughout the eruption as a result of reservoir deflation and increasing overburden from the growing dome was overcome by increasing friction as a result of cooling and crystallization of the plug

gps ground deformation patterns at mount st helens washington usa from 2004 to 2010 terra nova 24 148 155 2012 abstract gps measurements on mount st helens spanning the 2004 00 2010 85 time interval encompassing the 2004 2008 dome building eruption were analysed first a local reference frame was derived to isolate the volcanic ground deformation from the background tectonic pattern related to the interactions between the pacific juan de fuca and north american plates secondly the resulting ground deformation patterns were modelled to constrain magmatic sources results suggest a vertically elongated magma chamber centred at 8 9 km depth beneath the summit area that deflated during the beginning of the dome growth and inflated since january 2008 indicating that the same primary magma reservoir was active both during and after the eruption in a magmatic system that is persistent over the time

physicochemical models of effusive rhyolitic eruptions constrained with insar and dem data a case study of the 2011 2012 cordon caulle eruption abstract the 9 month long 2011 2012 eruption of cordon caulle southern andes chile is the best instrumentally recorded rhyolitic eruption to date and the first time that the effusion of a rhyolitic flow has been observed in detail we use interferometric synthetic aperture radar insar with time lapse digital elevation models dems and numerical models to study the dynamics of coupled magma reservoir deflation and lava effusion insar recorded 2 2 2 5 m of subsidence after the first three days of the eruption which can be modeled using a spheroidal magma reservoir at a depth of 5 km 20 km long and with a pressure drop of 20 30 mpa the source is elongated in the nw se direction and its large dimensions imply a large plumbing system active throughout the eruption and spanning neighboring volcanoes with a slight change in the geometry halfway through the effusive phase tandem x and pleiades dems record the extrusion of both the rhyolitic lava flow and the intrusion of a shallow laccolith around the eruptive vent after the third day of the eruption with a total volume of 1 45 km3 dre the laccolith was emplaced during the first month of the eruption during both the explosive and effusive stages of the eruption both the reservoir pressure drop and the extruded volume time series follow quasi exponential trends and can be explained by a model that couples the reservoir pressure decrease time and pressure dependent variations in the magma properties inside of the reservoir and conduit flow this model predicts both the temporal evolution and amplitude of both time series during the effusive phase and a magma compressibility of 10 10 pa 1 half the reported compressibility of the magma of the sub plinian explosive phase further we estimate that the reservoir contained 1 3 wt dissolved h2o at the onset of lava effusion with no exsolved co2 and h2o in the reservoir throughout the effusive phase this implies that the magma was significantly degassed after the explosive phase the remaining volatiles in the magma after the explosive stage might have caused magma fragmentation consistent with the hybrid explosive effusive style observed during the waning of the eruption

why do island arcs form cusps at their junctions an interesting feature of an island arc chain is a cusp of the arcuate trench at the arc arc junction where the wadati benioff zone and the volcanic front are also sharpened landward this cusp has been interpreted as a result of the collision of an aseismic ridge with the trench we propose a different mechanism intrinsic to the subduction of a spherical shell of lithosphere when the spherical lithosphere is bent inward at a subduction zone buckling takes place and the slab is deformed into a wavy configuration making the trench chainlike we analyzed this buckling phenomenon of spherical shells by a finite element method wliich has been developed in the fields of architecture and civil engineering our calculations show that the lithospheric shell buckles with a wavelength roughly consistent with the length of one unit of island arcs i e the distance between the two neighboring junctions the resultant undulation is not purely sinusoidal but is a succession of arcs with cusps in between very similar to the shape of the actual arc trench system in the world this agreement strongly suggests that the arcuateness of the trenches and the cuspate form of their junctions are the manifestation of buckling of the spherical lithosphere although the collision of seamounts and aseismic ridges against the trench may promote the initiation of buckling

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

complexities in the tectonic evolution of the west philippine basin abstract the oceanic west philippine basin can be subdivided into a main basin a northwest sub basin and a southern sub basin these basins have undergone different tectonic histories magnetic anomalies 21 17 are present in the main basin and indicate that it formed by two limbed seafloor spreading the now extinct spreading center is located within a zone of rough topography associated with the northwest trending central basin fault cbf the morphology of the cbf is primarily the result of post spreading tectonism the magnetic anomaly pattern of the main basin does not extend into the southern sub basin or the northwest sub basin geometrical constraints indicate that the southern sub basin did not form as part of the southern limb of the main basin the southern sub basin may have formed immediately before a ridge jump to the main basin spreading axis alternatively possible shallower crustal depth in the southern sub basin may indicate that the sub basin is younger than the main basin and formed separately from it in the northwest sub basin which originated as part of the main basin the dominant topographic trends are perpendicular to those in the main basin this difference as well as higher heat flow and shallower depth in the northwest sub basin indicate that it has undergone a deformational event which did not affect the main basin this deformation may be related to subduction along the east luzon margin in mid tertiary time the gagua ridge an uplifted sliver of oceanic crust along the western edge of the northwest sub basin may have originated during this event

a remarkable late saalian mis 6 loess dust accumulation in the lower danube at harletz bulgaria abstract while numerous high resolution studies concerning last glacial aeolian sequences are available for europe the approach of the penultimate glacial in this geographical area is still poorly developed in order to bridge this gap this study focuses on the bulgarian sequence of harletz along the danube river where extremely high sedimentation rates allow the depiction of high resolution signals during mis 6 at harletz in nw bulgaria on the western bank of the ogosta river tributary of the danube a 20 m thick loess palaeosols section was cleaned and sampled for a multi disciplinary study and detailed pedostratigraphic approach high resolution continuous bulk sampling 5 cm was carried out to characterise sedimentary grain size magnetic properties including magnetic susceptibility and its frequency dependence colour reflectance 1 cm and organic carbon geochronological control is based on 16 samples collected for osl and met pirir dating using a cyclo stratigraphic approach of the sequence combined with dating constraints provided by both met pirir dates and the age of a tephra layer occurring at a depth of 12 m within the main loess unit we can demonstrate that the harletz section exhibits a 10 m thick late saalian marine isotope stage 6 mis 6 loess accumulation unique in europe the lower part of the main loess unit is 4 m thick and overlies a basal brown soil complex allocated to mis 7 which includes an exceptionally thick 4 m and detailed succession of loess and four incipient soil horizons never described in european loess until now the closest and best dated high resolution palaeoenvironmental archive suitable for comparison comes from lake ohrid located about 400 km to the sw of harletz the ohrid palynological record shows a progressive step by step evolution in climate and in environmental change during the transition between mis 7 and mis 6 from which a parallel with the harletz pedosedimentary succession can be proposed during the younger part of mis 6 160 129ka steppe vegetation with abundant herbs artemisia is dominant in the lake ohrid record in good accordance with a global enhancement of aeolian dynamics especially well recorded in sections located close to the danube river from serbia to bulgaria and romania l2 loess according to interpretations stemming from this study the silts and fine sands building the harletz loess section would have been transported from the danube braided river system located at that time at about 4 5 km to the nw based on our data the main loess units are characterised by a very low to a total absence of coarse sand particles by contrast during the eemian interglacial mis 5e and to a lesser extent throughout mis 5 and during mis 3 interstadials the long distance transport of silt and fine sand particles is stopped and a weak aeolian sedimentation is likely driven by north easterly winds transporting coarse sand grains from the proximal ogosta river sandy banks finally the weak development of last glacial loess 4 m max likely results from a rapid infilling of the sedimentary trap during the saalian then followed by a strong anthropogenic erosion of the topsoil and of the upper part of the loess profile since the early holocene neolithic

possible structural control on the 2011 eruption of puyehue cordon caulle volcanic complex southern chile determined by insar gps and seismicity

paleomagnetism of the bonin islands and its tectonic significance abstract paleomagnetic samples were collected from six of the bonin islands located on the northeastern margin 27 n 142 e of the philippine sea these volcanic islands are of eocene age both lava flows and dikes were collected at each of 34 sites with chichi jima the largest island being sampled most extensively mean directions of the 27 sites on chichi jima are divisible into two groups showing antipolar relationships at the 95 confidence level a between site mean direction after tilt correction of 22 of the 27 sites is i 7 d 110 95 14 while that of the remaining sites is i 10 d 273 95 32 the seven sites on the other three bonin islands as a whole seem to cluster well around the mean of i 3 d 213 with a95 19 which differs significantly from both mean directions determined for chichi jima the low inclinations and deflected declinations of the bonin islands are interpreted as being due to tectonic rotation the islands have undergone a northward migration of at least 30 from the equatorial region together with possibly clockwise rotation of 30 to over 90 around the vertical axis since the northward drift and clockwise rotation are consistent with the tectonic movement inferred for the west philippine basin by previous studies the bonin islands appear to have been part of the basin before opening of the shikoku and parece vela basins during the miocene

spherical shell tectonics buckling of subducting lithosphere abstract the lithospheric plate is a spherical shell rather than a plane plate a spherical shell must buckle when it is bent inward we examined the possibility of lithospheric buckling upon subduction the lithosphere with its subducted portion is simulated by a hemispherical elastic shell bent inward at its circumferential edge by a uniform radial load the buoyancy force acting on the lithosphere seaward of the trench is simulated by clamping the shell along a parallel the deformable portion between the loaded and clamped edges corresponds to the subducting slab of lithosphere buckling analyses were made using the techniques of a linear stability analysis and a finite element method the wavelength of buckling depends on the thickness of the shell and the length of its deformable portion a longer wavelength is associated with a thicker shell and a longer deformable portion by scaling the shell thickness and the length of the deformable portion to the elastic thickness of lithosphere and the length of subducting slab respectively the wavelength of buckling can be compared favourably to the length of one unit of arcuate trench in a chain like continuation of island arcs the load to be applied for initiation of buckling is called the critical load a lower critical load is associated with a thinner shell with a longer deformable portion the excess weight of subducting slab provides a load large enough to initiate lithospheric buckling at a relatively early stage of subduction radial displacement of the circumferential edge at the critical state is an order of magnitude smaller than the depth of the leading edge of the wadati benioff zone indicating that most of the present subducting lithospheres are under a postbuckling state undulation of the shell in the postbuckling state is not purely sinusoidal but a successive continuation of arcs with cusps in between invoking the continuation of arcuate deep sea trenches with cusps at their junctions the cuspate feature of island arc chains is thus a natural consequence of lithospheric buckling and does not require any such irregularities as seamounts colliding with a trench collision of seamounts however can aid buckling greatly if they are aligned along a trench at an interval not very different from the inherent buckling wavelength

the tsunami phenomenon abstract with human activity increasingly concentrating on coasts tsunamis from japanese tsu harbour nami wave are a major natural hazard to today s society stimulated by disastrous tsunami impacts in recent years for instance in south east asia 2004 or in japan 2011 tsunami science has significantly flourished which has brought great advances in hazard assessment and mitigation plans based on tsunami research of the last decades this paper provides a thorough treatise on the tsunami phenomenon from a geoscientific point of view starting with the wave features tsunamis are introduced as long shallow water waves or wave trains crossing entire oceans without major energy loss at the coast tsunamis typically show wave shoaling funnelling and resonance effects as well as a significant run up and backflow tsunami waves are caused by a sudden displacement of the water column due to a number of various trigger mechanisms such are earthquakes as the main trigger submarine and subaerial mass wastings volcanic activity atmospheric disturbances meteotsunamis and cosmic impacts as is demonstrated by giving corresponding examples from the past tsunamis are known to have a significant sedimentary and geomorphological off and onshore response so called tsunamites form allochthonous high energy deposits that are left at the coast during tsunami landfall tsunami deposits show typical sedimentary features as basal erosional unconformities fining upward and landward a high content of marine fossils rip up clasts from underlying units and mud caps all reflecting the hydrodynamic processes during inundation the on and offshore behaviour of tsunamis and related sedimentary processes can be simulated using hydro and morphodynamic numerical models the paper provides an overview of the basic tsunami modelling techniques including discretisation guidelines for appropriate temporal and spatial resolution as well as the nesting method furthermore the boussinesq approximation a simplification of the three dimensional navier stokes equations is presented as a basic theory behind numerical tsunami models which adequately reflects the non linear dispersive wave behaviour of tsunamis the fully non linear boussinesq equations allow the simulation of tsunamis e g in the form of n waves based on the various subtopics presented recommendations for future multidisciplinary tsunami research are made it is especially discussed how the combination of sedimentary and geomorphological tsunami field traces and numerical modelling techniques can contribute to derive locally relevant tsunami sources and to improve the assessment of tsunami hazards considering the individual pre history and physiogeographical setting of a specific region

landscape instability at the end of mis 3 in western central europe evidence from a multi proxy study on a loess palaeosol sequence from the eastern lower rhine embayment germany abstract the lower rhine embayment hosts important loess palaeosol sequences lps within the western european loess belt yielding valuable information on landscape evolution and palaeoclimatic dynamics the study focusses on the palaeoenvironmental development based on a lps from the eastern shoulder of the lower rhine embayment dusseldorf grafenberg the palaeoenvironmental development within the study area is presented and discussed based on high resolution grain size analyses selected environmental magnetic parameters and geochemical analyses complemented by luminescence age estimates differences in grain size distribution gsd as well as the u ratio clearly reflect main stratigraphical units within the lps in addition the centered log ratio transformation of gsd gsd clr which is sensitive to post depositional grain size variations traces units of intensified soil formation despite no magnetic enhancement can be observed within the grafenberg lps environmental magnetic parameters combined with selected geochemical analyses sr rb provide direct information on the modification of silicates and magnetic minerals in the course of pedogenesis and diagenesis overall the grafenberg lps contributes to the understanding of environmental conditions within the lower rhine embayment particularly for the late mis 3 and the transition to mis 2 the lps reflects major shifts in the geomorphic system which are compared to palaeoenvironmental records from different archives and different geographic regions in north western europe indicating a direct relation to north atlantic climate oscillations during the last glacial period it is shown that the hesbaye unconformity at the mis 3 2 boundary is bound to an erosional event which can be traced over wide areas within the northern european loess belt

k ar dating on volcanic rocks in the bonin islands and its tectonic implication abstract the k ar dating method was applied to the volcanic rocks the boninite and related rocks from the bonin islands four fresh samples gave k ar ages of about 40 ma these ages suggest that the main volcanism of the bonin islands occurred about 40 ma ago as a result of the initiation of subduction of the pacific plate at its western margin results of the k ar dating on other rocks suggest that some volcanism or thermal disturbance may have occurred 5 10 ma ago at least in chichijima

late maastrichtian warming in the boreal realm calcareous nannofossil evidence from denmark abstract the climate of the mid to late cretaceous was probably one of the warmest of the phanerozoic but by the late maastrichtian distinctly cool high latitude regions had developed studies from low latitude sites have demonstrated that the late maastrichtian cooling was punctuated by two pulses of warming but the response of nannoplankton to climatic fluctuation in the mid to high northern latitudes is not well documented a high resolution nannofossil study was undertaken on subsurface chalk sections of denmark which provide a stratigraphically complete record of late maastrichtian sedimentation of nannofossil ooze in an extensive mid to high latitude carbonate province the quantitative nannofossil data from cored sections through the upper maastrichtian chalk reveal subtle but significant shifts in taxa interpreted as having been sensitive to temperature fluctuations arkhangelskiella cymbiformis kamptnerius magnificus lucianorhabdus cayeuxii nephrolithus frequens and watznaueria barnesiae and indicate two late maastrichtian warming events correlatable with those of low latitude records and a decrease in surface water temperatures in the latest maastrichtian

paleomagnetic data from the philippine sea plate and their tectonic significance new paleomagnetic data from guam and saipan have been obtained and are used in conjunction with previously published results from around the philippine sea plate to interpret the origin and history of the philippine sea plate a lower miocene direction i 3 5 d 54 5 a95 17 4 k 20 3 and a middle oligocene direction i 5 3 d 68 4 a95 12 8 k 52 4 have been obtained from the palau islands only one reliable result a middle oligocene direction i 15 1 d 66 1 a95 11 1 k 20 1 has been obtained from guam many characteristic remanent magnetic directions have been obtained from the lower eocene facpi formation however the results are too scattered k 4 5 to be considered reliable two reliable directions have been obtained from saipan a middle miocene result i 30 7 d 28 1 a95 8 4 k 64 6 and a lower eocene result i 12 0 d 43 0 a95 12 5 k 16 0 two lower eocene results from chichijima i 9 5 d 105 3 a95 12 6 k 6 1 and anijima i 0 8 d 91 7 a95 16 9 k 4 9 are the reliable results from the bonin islands there are two end member interpretations for these data 1 small scale local rotation of blocks along the plate margin or 2 rotation of the philippine sea plate as a whole to distinguish between these interpretations we reconstructed the east philippine sea province back to its prerifting configuration the rotation caused by deformation can be estimated by comparing the paleomagnetism of guam saipan and the bonin islands to palau prior to reconstruction the older declination values are scattered closing the marginal basins reduces the declination scatter and all of the islands show increasing clockwise deflection with time this suggests that the philippine sea plate has rotated up to 80 clockwise and moved northward around 20 since the eocene and deformational rotation is a second order feature the finite pole of rotation of the philippine sea plate relative to eurasia must be somewhere near the southeast comer of the plate because this is the only location that is compatible with both the change in orientation and paleolatitude for the plate these data cannot distinguish between a backarc origin or trapped crust origin for the west philippine sea province the existing models require major modification to be consistent with the paleomagnetic data

physics based models of ground deformation and extrusion rate at effusively erupting volcanoes 1 we present a model of effusive silicic volcanic eruptions which relates magma chamber and conduit physics to time dependent data sets including ground deformation and extrusion rate the model involves a deflating chamber which supplies newtonian magma through a cylindrical conduit solidification is approximated as occurring at fixed depth producing a solid plug that slips along its margins with rate dependent friction changes in tractions acting on the chamber and conduit walls are used to compute surface deformations given appropriate material properties and initial conditions the model predicts the full evolution of an eruption allowing us to examine the dependence of observables on initial chamber volume overpressure and volatile content employing multiple data sets in combination with a physics based model allows for better constraints on these parameters than is possible using kinematic idealizations modeling posteruptive deformation provides an improved constraint on the rate of influx into the magma chamber from deeper sources we compare numerical results to analytical approximations and to data from the 2004 2008 eruption of mount st helens for nominal parameters the balance between magma chamber pressure and frictional resistance of the solid plug controls the evolution of the eruption with little contribution from the fluid magma below the idealized crystallization depth while rate dependent plug friction influences the time dependent evolution of the eruption it has no control on the final chamber pressure or extruded volume

single station automated detection of transient deformation in gps time series with the relative strength index a case study of cascadian slow slip the discovery of slow slip events over the past decades has changed our understanding of tectonic hazards and the earthquake cycle proper geodetic characterization of slow slip events is necessary for studies of regional interseismic coseismic and postseismic deformation and miscalculations can affect our understanding of the regional stress field and tectonic hazard because of the proliferation of gps data over the last two decades an automated algorithm is required to analyze the signals and model the deformation on a station by station basis using the relative strength index rsi a financial momentum oscillator we test the ability to detect events of various sizes and durations we first determine the statistics of the rsi under different noise conditions and then use this information as the basis for the automated transient detection algorithm by testing different synthetic signals we then apply the technique to daily gps displacement time series from 213 stations along the cascadia subduction zone to form a record of transient deformation between 2005 and 2016 our estimates of the spatial extent duration and propagation of major episodic tremor and slip ets events are consistent with previous studies we use the automated detections to remodel the displacement time series and obtain transient deformation rates over the past decade and discuss the tectonic implications finally we analyze the correlation between transient detections and tremor showing good agreement between the two at slab depths commonly associated with ets events

upper plate deformation and trench retreat modulated by subduction driven shallow asthenospheric flows upper plate deformation within a subduction zone depends on the complex relationship between surface plate motions trench motion slab pull and asthenospheric flow previous modelling studies suggest that trench motion rates should be related to slab buoyancy but this relationship is neither clear nor verified by observations of natural subduction systems trench motion is also thought to induce upper plate deformation however no clear correlation has been identified between the direction of trench motion and the mode of upper plate deformation in this study we construct 2 d thermo mechanical models to explore the relationship between slab pull trench retreat and upper plate deformation focusing on subduction systems with retreating trenches we start with quasi steady state subduction and introduce a positive density anomaly into the slab to transiently increase slab pull we vary both the value of the density anomaly and the properties of the upper plate to isolate key controls on trench retreat and upper plate deformation our models indicate that asthenospheric flow responds to changes in slab pull and influences both trench retreat and upper plate deformation we propose that trench retreat depends on the competition between shallow and opposite asthenospheric flows below the subducting and upper plates and that a fast sub slab flow can hamper trench retreat even when slab buoyancy is high after a transient slab pull increase the mode of upper plate deformation partly depends on the upper plate s ability to translate horizontally an upper plate with a ridge at its trailing edge deforms by shortening while a fixed upper plate may deform by extension finally in some cases upper plate deformation seems to allow trench retreat if the upper plate is weak enough to be deformed by the basal shear from underlying asthenospheric flow our results provide insights into retreating subduction systems with contrasting upper plate deformation modes such as the compressive andes and the extensional aegean

evidence for degassing of fresh magma during the 2004 2008 eruption of mount st helens subtle signals from the hydrothermal system abstract results from chemical and isotopic analyses of water and gas collected between 2002 and 2016 from sites on and around mount st helens are used to assess magmatic degassing related to the 2004 2008 eruption during 2005 the chemistry of hot springs in the breach of mount st helens showed no obvious response to the eruption and over the next few years changes were subtle giving only slight indications of perturbations in the system by 2010 however water chemistry temperatures and isotope compositions d and 18o clearly indicated some inputs of volatiles and heat associated with the eruption but the changes were such that they could be attributed to a pre existing gas depleted magma an increase of 1 5 in the 13c values of dissolved carbon in the springs was noted in 2006 and continued through 2009 a change that was mirrored by a similar shift in 13c co2 in bubble gas emissions these changes require input of a new source of carbon to the hydrothermal system and provide clear evidence of co2 from an undegassed body of magma rising trends in 3he 4he ratios in gas also accompanied the increases in 13c since 2011 maximum rc ra values are 6 4 and are distinctly higher than 5 samples collected between 1986 and 2002 and provide additional evidence for some involvement of new magma as early as 2006 and possibly earlier given the unknown time needed for co2 and he to traverse the system and arrive at the springs

tree ring evidence for a two year interval between recent prehistoric explosive eruptions of mount st helens distinctive annual ring patterns in trees in a forest stand 10 km east of mount st helens indicate that the volcano9s voluminous layer we tephra was erupted after the a d 1481 growth season late in 1481 or early during 1482 combined with earlier research that established a d 1480 as the date of eruption of the underlying similar layer wn tephra the new tree ring data provide evidence that these two major explosive tephra eruptions about 500 yr ago were separated by about 2 yr during that eruptive sequence a voluminous explosive eruption was therefore followed by more than 1 yr of minor explosive activity before another voluminous eruption occurred this record suggests that mount st helens might produce a second large explosive eruption during its current period of activity examinations of the tephra deposits that correlate with narrowring patterns in trees at mount st helens also demonstrate that distinctive ring patterns can form in trees across a large range of tephra layer thicknesses the examinations also suggest that tephra coarseness interacts with tephra layer thickness in determining the effects that tephra fallout will have upon forest trees

on magma chamber evolution during slow effusive eruptions 1 we explore some of the controls on the evolution of a slow effusive eruption imposed by processes in a shallow magma reservoir we envisage that the emplacement of basaltic magma into a shallow evolved crustal reservoir generates sufficient overpressure to trigger the slow effusive eruption of the overlying layer of evolved magma the subsequent evolution of the system then depends on a number of factors including 1 the volatile contents of the two magmas 2 the rate of any continuing influx of basalt 3 the cooling heating rates of the two layers of magma and 4 the eruption rate of the silicic magma where we assume that the crystals and bubbles remain in the layer in which they formed any exsolved volatiles in the magma increase the compressibility of the magma in a waning system this tends to increase the volume of material which needs to be erupted to relieve a given overpressure by an amount of order 10 100 and hence increases the duration of the eruption typical calculations suggest that eruptions from magma chambers of size 1 10 km3 may persist for times of order 1 10 years if the main body of silicic magma is saturated in volatiles in addition cooling of the basalt leads to formation of dense crystals which lowers the pressure however if the magma becomes saturated exsolution of volatile gases occurs tending to increase the pressure again in a waning system this tends to extend the eruption duration and increase the mass erupted from the chamber for chamber volumes of 1 10 km3 cooling rates of 10 6 k s can increase the eruption volume by a factor of order 10 rapid cooling may also lead to an initial increase in the eruption rate while the chamber pressure is controlled by the rate of production of new bubbles if there is an influx of basalt to the chamber which is maintained during the eruption then the evolution of the eruption is primarily dependent on this input rate although the cooling of the basalt may increase the eruption rate to values in excess of this input rate especially at an early stage in the eruption the complex and nonlinear evolution of the eruption rate predicted by our simple model illustrates the sensitivity of the eruption rate to the magma volatile content and magma cooling rates such understanding is key in developing quantitative models for hazard assessments of slow effusive eruptions

recent unrest 2002 2015 imaged by space geodesy at the highest risk chilean volcanoes villarrica llaima and calbuco southern andes abstract villarrica llaima and calbuco volcanoes are the most active and dangerous volcanoes in the southern andes and we use interferometric synthetic aperture radar insar observations from multiple satellites ers 2 envisat alos radarsat 2 cosmo skymed terrasar x sentinel 1a and alos 2 to constrain ground deformation that spans episodes of unrest and eruption at all three volcanoes between 2002 and 2015 we find episodes of ground deformation at each volcano which we invert using analytic elastic half space models to make some of the first geophysical inferences about the source depths of potential magma chambers at llaima we interpret that the vei 2 april 3 2009 eruption was preceded by 6 15 cm of precursory ground uplift one month before from a source 5 km below the surface on the western side of the edifice the vei 2 march 3 2015 villarrica eruption was followed by a short lived uplift of 5 cm in the se part of the volcano from a source depth of 6 km the vei 4 april 22 23 2015 calbuco eruption produced 12 cm of coeruptive subsidence from a source depth 8 11 km and offset 2 km s from the summit importantly we do not find clear evidence that the january 1 2008 the march 3 2015 and april 22 2015 eruptions at llaima villarrica and calbuco volcanoes were preceded by either transient or continuous ground uplift there are several possible explanations for the lack of precursory deformation at each volcano it is possible that any precursory deformation occurred only hours before the eruption e g at calbuco pre eruptive inflation was canceled by co eruptive subsidence as we inferred happened during the april 2009 llaima eruption the pre eruptive deformation was too small to be detectable in areas with persistent topography correlated phase delays pressurized source are deep or that open vent volcanoes like villarrica and llaima do not pressurize at all three volcanoes x and c band interferograms decorrelate in a few weeks due to vegetation snow and ice and have persistent atmospheric phase delays that we find cannot be reliably removed with available global weather models the low number of sar acquisitions therefore makes it challenging to reliably measure unaliased deformation we recommend a multi satellite observing strategy with short repeat periods frequently acquired high resolution digital elevation models and with acquisitions during every satellite overflight that may improve the temporal resolution of measurements

volcano deformation and eruption forecasting recent advances in global positioning system gps tilt and interferometric synthetic aperture radar insar have greatly increased the availability of volcano deformation data these measurements combined with appropriate source models can be used to estimate magma chamber depth and to provide information on chamber shape and volume change however kinematic models cannot constrain magma chamber volume and provide no predictive

improved constraints on the estimated size and volatile content of the mount st helens magma system from the 2004 2008 history of dome growth and deformation 1 the history of dome growth and geodetic deflation during the 2004 2008 mount st helens eruption can be fit to theoretical curves with parameters such as reservoir volume bubble content initial overpressure and magma rheology here assumed to be newtonian viscous with or without a solid plug in the conduit center data from 2004 2008 are consistent with eruption from a 10 25 km3 reservoir containing 0 5 2 bubbles an initial overpressure of 10 20 mpa and no significant sustained recharge during the eruption we used curve fits to project the eruption s final duration and volume early projections predicted a final volume only about half of the actual value but projections increased with each measurement implying a temporal increase in reservoir volume or compressibility a simple interpretation is that early effusion was driven by a 5 10 km3 integrated core of fluid magma this core expanded with time through creep of semi solid magma and host rock

late pleistocene landscape dynamics in saxony germany paleoenvironmental reconstruction using loess paleosol sequences abstract loess archives are of paramount importance for reconstructing regional paleoenvironmental conditions of past glacial periods the saxonian loess region is an area of transition between the western and the eastern european loess belt with this contribution a documented loess paleosol composite profile for the saxonian loess region is extended and a new chronostratigraphy established by high resolution osl dating of two profiles is described in addition for the first time osl age estimates for the loess paleosol sequence at seilitz are presented for the quartz and polymineral fine grain fraction based on the presented composite profile climatic and environmental conditions e g wind speed temperature are deduced based on high resolution osl dating it is possible to identify a hiatus spanning ca 30 ka this gap is located in the gleina complex where an underlying layer shows an age of ca 60 ka and the upper layer an age of ca 30 ka additionally two periods of strong loess accumulation with ages of ca 70 60 ka and 30 18 ka could be identified there is a general trend of grain size coarsening up towards the late glacial which shows a maximum at ca 21 ka correlations with french western german and polish loess sections are discussed

the intensity and magnitude of holocene plinian eruptions from mount st helens volcano dispersal characteristics of the t we wn pu ps ye yn and yb plinian fall deposits of mount st helens have been measured at 80 sites downwind of the volcano in order to model eruption dynamics and atmospheric transport isopleth contours for the sizes of maximum pumice and lithic clasts are used to calculate peak eruption column heights and intensities magma discharge based on a theoretical model of tephra dispersal new proximal thickness measurements are combined with an empirical distal extrapolation based on studies of 53 plinian deposits to calculate the magnitude erupted mass of each eruption layer yn 3510 y r b p represents the highest intensity and largest magnitude eruption at mount st helens in post glacial times modeling suggests column height grew to about 31 km before gradually declining at the end of the plinian phase 26 hours several intraplinian surge deposits are present in the upper part of the fall layer close to the volcano and up to 15 km to the northeast of mount st helens peak intensity of the plinian phase was 108 kg s and the total erupted volume was 4 km3 dre of magma small plinian style eruptions are represented by layers such as ps and pu of the pine creek eruptive period 3000 2500 yr b p and have intensities of only 106 kg s when compared with plinian eruptions from other volcanoes the holocene eruptions of mount st helens span from the lower to the middle part of the known range in intensity and magnitude and are typical of events derived from intermediate sized stratovolcanoes there is also a general correlation between the intensity of plinian eruptions within eruptive cycles and the repose period prior to each cycle this relationship may be related to a time dependent process for the accumulation of differentiated and volatile rich magma within the chamber beneath mount st helens

intraoceanic subduction of heterogeneous oceanic lithosphere in narrow basins 2d numerical modeling abstract we provide 2d thermomechanical numerical models to constrain the subduction process of narrow 600 km wide oceanic basins resulting in formation and exhumation of serpentinite bearing high pressure hp and ultrahigh pressure uhp complexes presently exposed as dismembered massifs in orogenic belts we simulated subduction of heterogeneous i e non layered oceanic lithosphere testing different serpentinite rheologies and varying other major parameters initial slab dip convergence rate age and structure of oceanic lithosphere distance subduction zone continental margin weak zone geometry we show that serpentinite rheology is the most important parameter affecting both the geometry and behavior of serpentinite channels that form in the mantle wedge as the result of upward migration of slab fluids a strong serpentinite power law rheology is responsible for the continuous accretion and underthrusting of slab slices at the base of mantle wedge to form planar or wedge shape serpentinitic channels the great part of sediments is buried only during continental crust subduction but is never exhumed moreover slab slices do not mix with mantle wedge rocks exhumation of mafic ultramafic rocks occurs from pressure not exceeding 15 kbar on the other hand a weak newtonian behavior of serpentine best describes subduction and exhumation dynamics this rheology produces channels that usually widen to depth slab and sediment slices are easily incorporated in mantle wedge serpentinite during ongoing convergence in this case slab and mantle wedge serpentinite are mixed together and can be closely associated in the channel the models running a newtonian rheology highlight that slices of the oceanic overriding plate can be eroded and dragged in the channel till great depth a weak power law serpentinite rheology produces a transitional behavior between that of strong power law and newtonian rheology the serpentinitic melange produced using a newtonian rheology is finally exhumed from pressure 20 kbar during the final stages of continental crust subduction and collision the rise to shallow depth of the hp melange is driven by concurrent serpentinite buoyancy slab roll back and asthenospheric mantle upwelling moreover we propose that the subduction exhumation dynamics observed in the model acted during closure of the narrow alpine tethys ocean in particular we suggest that some slices presently outcropping in hp ophiolitic massifs of the western alps voltri and monviso massifs were tectonically coupled at slab mantle interface and exhumed during collision in a soft low viscosity serpentinite channel we finally investigated intensity and dynamics of magmatic processes during subduction our numerical experiments suggest that even subduction of narrow oceanic basins may indeed allow the development of a magmatic arc in such relatively small short living arcs steeper faster and younger slabs produce earlier onset of the volcanism after the beginning of subduction

paleomagnetic results from palau west caroline islands a constraint on philippine sea plate motion paleomagnetic results from the palau islands indicate that the islands have undergone 60 70 of clockwise rotation since mid oligocene time this post mid oligocene rotation is interpreted to represent the motion of the philippine sea plate and not a local rotation for the following reasons 1 lack of morphologic expression of the differential rotation along the palau kyushu ridge 2 similarity to predicted rotations obtained by using phase shifted magnetic anomaly data and 3 ubiquitous clockwise rotations seen in paleomagnetic data from around the philippine sea plate this strong clockwise rotation of the philippine sea plate provides a mechanism for oblique subduction and related transcurrent motion along the margin of the philippine archipelago

the role of boundary conditions in numerical models of subduction zone dynamics abstract numerical studies of subduction zone dynamics on a regional scale usually use a limited spatial extent for their models and therefore need to define boundary conditions on all model edges these boundary conditions imply a choice for the mechanical and thermal state of the surrounding regions which may influence the evolution of the model system we investigate the role of the surface and lateral boundary conditions for simple mechanical subduction models using a standard finite element method we find that subduction is only possible if the slab can decouple from the surface this decoupling can be achieved by a true free surface a free slip surface with a weak crust for the subducting plate or a free slip surface with a layer of low viscosity low density material sticky air between the model surface and the crust models of slab dynamics that employ a free slip surface reproduce trench migration slab sinking velocities and slab geometry of models with a free surface as long as they use either a weak crust which can be viscous viscoelastic and or brittle or a sticky air layer the vertical topography will however not be reproduced for free slip models without a sticky air layer for ocean ocean convergent models we find that the application of inflow boundary conditions at the edges of the subducting or overriding lithosphere controls trench motion and the geometry of the subducting slab inflow on the overriding side causes trench retreat and a slab resting on the lower mantle whereas inflow restricted to the subducting side can cause trench advance and a slab which folds on the lower mantle

petrology geochemistry and tectonic implications of volcanics dredged from the intersection of the yap and mariana trenches abstract rocks dredged from the forearc very close to the intersection of the yap and mariana trenches include a suite of highly depleted arc tholeiites and several samples of transitional to slightly alkaline basalt the tholeiites range from magnesian quartz tholeiites with 0 46 0 6 tio2 to andesites with up to 62 sio2 and 8 2 feo all show pronounced lree depletion and have very low contents of ba and sr they are postulated to have been produced by partial melting of upper mantle peridotite residual after morb extraction following influx of hydrous fluids from the subducted slab while these fluids were responsible for small enrichments in ba k rb and sr in melts generated lree were not involved in the metasomatism and the strong lree depletion probably reflects the unmodified depleted source peridotite the second lava suite includes slightly ne normative ti augite bearing basalts with convex upward ree patterns showing slight lree depletion la sm n 0 76 the chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening a k ar age on one sample 7 8 1 3m y is in good agreement with the initial opening of the mariana trough the tectonic significance of the dredged arc tholeiite suite is less obvious a k ar age of10 8 0 4 my on one andesite and the occurrence of similar lavas in dredges from at least 300 km along the length of the yap arc suggest that subduction was occurring beneath the yap arc in the late miocene after overthrusting of the yap greenschist allochthon and while calc alkaline arc magmatism was occurring further north on the west mariana ridge we suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the yap forearc following subduction beneath this area of young hot sorol trough crust these arc tholeiites represent a magma type transitional between more typical arc tholeiites e g tongan and high mg andesites and boninites

on the variations of flow rate in non explosive lava eruptions abstract a physical model is developed to determine factors which influence the dynamics of non explosive lava eruptions in the model magma rises in a laminar regime from an overpressured chamber surrounded by elastic rock and erupts at the surface where the accumulation of lava may alter the vent pressure in a flow rate versus volume diagram an eruption follows a path which is sensitive to relative changes in flow variables such as viscosity conduit dimensions and thickness of lava over the vent the path does not depend on the unknown elastic properties of the country rock or the chamber dimensions with appropriate volcanological data an observed flow rate versus volume path can be interpreted the approach is applied to three well documented eruptions in each case the eruption rate was partially influenced by decreasing chamber pressure the 1988 1990 eruption of lonquimay chile exhibits an early phase of conduit shrinkage which lasted about 100 days probably due to magma solidification against country rock the 1979 eruption of la soufriere de saint vincent west indies was initially affected by lava dome growth and later passed through a phase possibly caused by constriction of the conduit the 1943 1952 eruption of paricutin mexico was affected by changes of lava level in the cinder cone and magma properties in these three eruptions the pre eruptive chamber overpressures were similar 10 20 mpa even though the erupted volumes differed by as much as two orders of magnitude

ground deformations and volcanic processes as imaged by cgps data at mt etna italy between 2003 and 2008 1 we analyze the kinematic and crustal deformations of mt etna from 2003 to 2008 as imaged by the mt etna continuous gps cgps network etn net through a careful analysis of gps time series six coherent phases of crustal deformations have been identified three inflation phases and three deflation phases superimposed on a major inflation of the volcanic edifice since 2001 the inversions of gps velocities have enabled 1 a better understanding of the evolution of the volcanic sources acting beneath the volcano 2 analysis of the strain rate patterns and 3 a delineation of potential coupling between volcanic sources and the observed ground deformations the modeling of the pressure sources has shown a separation between inflation and deflation sources the deflation sources show an upward migration from 5 5 toward 2 0 km b s l while the inflation sources are located within 5 5 and 4 0 km b s l our results indicate that the kinematic and ground deformations of the mid upper eastern flank are driven by the interplay between the effect of the magmatic sources and a southeastward motion furthermore clockwise rotations have been detected that prevailed over the eastern motion of the flank during the inflation phases preceding the 2004 2005 and 2006 eruptions finally the accordance between the higher geodetic shear strain rates and the area with the highest seismic energy release shows that measured geodetic shear strain rates can provide useful information on the potential occurrence of seismic activity

spherical shell tectonics on the buckling of the lithosphere at subduction zones abstract the buckling phenomena of the subducting lithosphere due to the sphericity of the earth are studied by a nonlinear finite element method fem and the effects of geometrical nonlinearity are examined the subduction of the lithosphere is modelled by a hemispherical shell squeezed at its circumferential edge in scaling the geometry of the subducting lithosphere two parameters are employed one is the thickness of the hemispherical shell i e the thickness of the lithosphere and the other is the length of its deformable portion i e the length of the descending slab to simulate the bending of the lithosphere at the trench uniform inward load is applied to the free circumferential edge of the deformable portion which corresponds to the leading edge of the subducting lithosphere at the opposite edge of the deformable portion i e at the trench a built in boundary condition is imposed no displacements and no rotations are allowed with a load greater than some critical value the deformable portion of the shell buckles and undulates azimuthally the investigation of this buckling phenomenon can be summarized as follows 1 1 buckling with a shorter wavelength is associated with a thinner and shorter shell mechanical irregularities simulating lithospheric tearing or seamount collision do not alter the buckling wavelength very much after appropriate scaling the lengths of subduction zones coincide with the buckling wavelength calculated by f e m 2 2 sharp cusps pointing outward from the shell appear during the postbuckling undulation without any irregularities in shell property and in the applied load the shape of this cuspidal undulation is quite similar to that actually observed at an arc arc junction 3 3 buckling can take place in the stress environment likely to exist in the earth these results strongly suggest that the arcuate shapes of subduction zones are in fact generated by buckling of the subducting lithosphere

hot pressing in conduit faults during lava dome extrusion insights from mount st helens 2004 2008 abstract rhyodacitic volcanoes such as mount st helens msh soufriere hills mount unzen and mount pelee erupt spines mantled by layers of magma derived cataclasite and fault gouge msh produced seven lava spines from 2004 2008 composed of low porosity compositionally uniform crystalline dacite dome extrusion was attended by continuous drumbeat seismicity derived from faulting along the conduit margin at 0 5 1 km depth and evidenced by the enveloping gouge layers we describe the properties of the gouge derived fault rocks including laboratory measurements of porosity and permeability the gouge varies from unconsolidated powder to lithified low porosity low permeability fault rocks we reconstruct the subsurface ascent of the msh magma using published field observations and create a model that reconciles the diverse properties of the gouge with conditions in the conduit during ascent i e velocity temperature we show lithification of the gouge to be driven by hot pressing processes wherein the combination of elevated temperature confining pressure and dwell time cause densification and solid state sintering of the comminuted crystal rich glass poor gouge the degree of gouge lithification corresponds with residence time in the conduit such that well lithified materials reflect extended times in the subsurface due to slower ascent rates with this insight we suggest that gouge competence can be used as a first order estimate of lava ascent rates furthermore we posit gouge lithification which reduces porosity and permeability inhibits volcanic outgassing thereby increasing the potential for explosive events at spine producing volcanoes

a comparison of rift zone tectonics in iceland and hawaii observations of eroded volcanic rift zones indicate that dikes in iceland are typically several times thicker than those in hawaii geodetic and seismic observations of active rifts however suggest that dike heights in the two regions are similar provided the elastic properties of the rift zones are the same this implies that dikes are intruded with higher driving pressures magma pressure minus compressive stress perpendicular to the dike plane in iceland than hawaii a second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in iceland from this it can be infered that a lower percentage of dikes breach the surface in iceland than in hawaii thus although dikes in iceland are intruded with higher driving pressures they possess lower absolute magma pressures than in hawaii these differences can be interpreted in terms of the tectonic settings in the two regions in iceland a steady remote extension reduces the horizontal stress perpendicular to the rift zone allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available in hawaii a more continuous magma supply on the timescale over which the dike induced stresses are relaxed and perhaps a greater role for intrusions in driving long term rift extension ensure that the rift compressive stress is not relaxed significantly before the next dike is intruded thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur if the mechanism for relaxing the rift compressive stress were less efficient still then an even higher percentage of dikes would erupt and at times the rift zone trend could become an unfavorable orientation for dike intrusion such might be the case at mauna loa which lacks large rift zone faults and fissures and possesses numerous radial vents outside its two main rift zones

evidence from gravity data for an intrusive complex beneath mount st helens on the basis of gravity data aided by aeromagnetic magnetotelluric side looking radar and geologic information we tentatively identify a large shallow intrusive complex beneath mount st helens the complex is roughly 5 6 km thick and has apparently intruded a buried and compressed mesozoic forearc sedimentary sequence the lateral extent of the intrusive complex is uncertain and we give three alternative models varying from about 18 by 10 km to as much as 18 by 22 km the western boundary of the inferred mount st helens intrusion abuts several of a number of tertiary and younger plutons that crop out in the area the mount st helens intrusion apparently is adjacent to sedimentary rock or shallow volcanic rock along most of the remainder of its boundary a ringlike drainage pattern around mount st helens suggests subsidence caused by removing magma from deep beneath mount st helens and adding this mass to the mount st helens edifice and its underlying intrusion our analysis indicates that the average bulk density of the volcanic rock comprising mount st helens is about 2 15 g cm3

origin and evolution of the west philippine basin a new interpretation abstract combined analyses of the magnetic lineations and seafloor structures of the west philippine basin show that it was formed by spreading from the central basin spreading center in two distinctly different spreading phases before and after subduction was initiated along the palau kyushu trend at 45 ma b p these episodes are recognized by differences in the strike of the magnetic lineations spreading rates density and strike of fracture zones and basement relief from 60 45 ma b p spreading was ne sw relative to present orientations at a half rate of 44 mm a at about 45 ma b p the spreading direction changed to a more n s direction with a reconfiguration of the central basin spreading center into numerous short e w segments offset by closely spaced n s transform faults spreading slowed to a half rate of 18 mm a and ceased at 35 ma b p thus the west philippine basin originated at 45 ma b p by the trapping of normal ocean crust west of the initial subduction along the palau kyushu trend the 45 35 ma b p period represents the dying phase of spreading on the central basin spreading center following isolation of the west philippine basin from the plate driving forces of the pacific