estimation of locations and migration of debris flows on izu oshima island japan on 16 october 2013 by the distribution of high frequency seismic amplitudes abstract in the early morning on 16 october 2013 large debris flows resulted in over 30 people dead on izu oshima island japan which were induced by heavy rainfall from the approaching typhoon 1326 wipha we successfully estimated the locations and migration processes of five large events of the debris flows using the spatial distribution of high frequency seismic amplitudes recorded by a seismic network on the island the flows occurred on the western flank of the island almost at the same place as the site where large traces of debris flows were identified after the disaster during each event of debris flows the estimated locations migrated downstream with increasing time from the caldera rim of miharayama volcano in the center of the island to its western side with a speed of up to 30 m s the estimated time series of source amplitudes are different from event to event exhibiting a large variety of flow sequences while they seem to have repeated at a relatively narrow area over several tens of minutes the present approach may be utilized for early detection and warning for prevention and reduction of the present type of disasters in the future

source amplitudes of volcano seismic signals determined by the amplitude source location method as a quantitative measure of event size article i nfo the amplitude source location asl method which uses high frequency amplitudes under the assumption of isotropic s wave radiation has been shown to be useful for locating the sources of various types of volcano seismic signals we tested the asl method by using synthetic seismograms and examined the source amplitudes determined by this method for various types of volcano seismic signals observed at different volcanoes our synthetic tests indicated that although asl results are not strongly influenced by velocity structure and noise they do depend on site amplification factors at individual stations we first applied the asl method to volcano tectonic vt earthquakes at taal volcano philippines our asl results for the largest vt earthquake showed that a frequency range of 7 12 hz and a q value of 50 were appropriate for the source location determination using these values we systematically estimated source locations and amplitudes of vt earthquakes at taal we next applied the asl method to long period events at cotopaxi volcano and to explosions at tungurahua volcano in ecuador we proposed a practical approach to minimize the effects of site amplifications among different volcano seismic networks and compared the source amplitudes of these various volcano seismic events with their seismic magnitudes we found a proportional relation between seismic magnitude and the logarithm of the source amplitude the asl method can be used to de termine source locations of small events for which onset measurements are difficult and thus can estimate the sizes of events over a wider range of sizes compared with conventional hypocenter determination ap proaches previously there has been no parameter widely used to quantify the sources of volcano seismic signals this study showed that the source amplitude determined by the asl method may be a useful quan titative measure of volcano seismic event size

source mechanism of very long period signals accompanying dome growth activity at merapi volcano indonesia 1 very long period vlp pulses with period of 6 7s displaying similar waveforms were identified in 1998 from broadband seismographs around the summit crater these pulses accompanied most of multiphase mp earthquakes a type of long period event locally defined at merapi volcano source mechanisms for several vlp pulses were examined by applying moment tensor inversion to the waveform data solutions were consistent with a crack striking 70 and dipping 50 sw 100m under the active dome suggest pressurized gas transport involving accumulation and sudden release of 10 60 m3 of gas in the crack over a 6s interval

source mechanisms of explosions at stromboli volcano italy determined from moment tensor inversions of very long period data 1 seismic data recorded in the 2 30 s band at stromboli volcano italy are analyzed to quantify the source mechanisms of strombolian explosions during september 1997 to determine the source centroid location and source mechanism we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account two source centroids are identified each representative of the distinct event types associated with explosive eruptions from two different vents the observed waveforms are well reproduced by our inversion and the two source centroids that best fit the data are offset 220 and 260 m beneath and 160 m northwest of the active vents the source mechanisms include both moment tensor and single force components the principal axes of the moment tensor have amplitude ratios 1 1 2 which can be interpreted as representative of a crack if one assumes the rock matrix at the source to have a poisson ratio n 1 3 a value appropriate for hot rock both imaged cracks dip 60 to the northwest and strike northeast southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness as expressed by lineaments dikes and brittle structures for our data set the volume changes estimated from the moments are 200 m 3 for the largest explosion from each vent together with the volumetric source is a dominantly vertical force with a magnitude of 10 8 n consistent with the inferred movement of the magma column perched above the source centroid in response to the piston like rise of a slug of gas in the conduit index terms 7215 seismology earthquake parameters 7280 seismology volcano seismology 8419 8414 volcanology eruption mechanisms keywords very long period seismicity moment tensor inversions eruption mehanics

seismic amplitude ratio analysis of the 2014 2015 bar arbunga holuhraun dike propagation and eruption magma is transported in brittle rock through dikes and sills this movement may be accompanied by the release of seismic energy that can be tracked from the earth s surface locating dikes and deciphering their dynamics is therefore of prime importance in understanding and potentially forecasting volcanic eruptions the seismic amplitude ratio analysis sara method aims to track melt propagation using the amplitudes recorded across a seismic network without picking the arrival times of individual earthquake phases this study validates this methodology by comparing sara locations filtered between 2 and 16hz with the earthquake locations same frequency band recorded during the 2014 2015 bar arbunga holuhraun dike intrusion and eruption in iceland integrating both approaches also provides the opportunity to investigate the spatiotemporal characteristics of magma migration during the dike intrusion and ensuing eruption during the intrusion sara locations correspond remarkably well to the locations of earthquakes several exceptions are however observed 1 a low frequency signal was possibly associated with a subglacial eruption on 23 august 2 a systematic retreat of the seismicity was also observed to the back of each active segment during stalled phases and was associated with a larger spatial extent of the seismic energy source this behavior may be controlled by the dike s shape and or by dike inflation 3 during the eruption sara locations consistently focused at the eruptive site 4 tremor rich signal close to ice cauldrons occurred on 3 september this study demonstrates the power of the sara methodology provided robust site amplification quality factors and seismic velocities are available plain language summary locating earthquakes usually implies picking phase arrivals p and s waves another technique called seismic amplitude ratio analysis sara was recently introduced to locate them only by using the amplitude recorded at different pairs of seismic stations however this technique was never proven to be true this study shows that the earthquake locations derived by sara compares remarkably well with the locations of 30 000 seismic events triggered when magma migrated in the icelandic crust prior to the 2014 2015 holuhraun eruption but the results also provide new insight into the magma dynamics that led to the largest eruption of the last two centuries in europe we show that ground vibration was continuously triggered during the 2week period preceding the eruption when magma forced its way toward the eruption site but also during the eruption itself several intriguing features were observed including low frequency vibrations possibly associated with eruption below the ice or large patches of seismic activity when the magma stopped propagating toward the eruption site this methodology performs very well provided some parameters are available and allows to gain insights into the complex dynamics associated with magma movements

source mechanism of vulcanian degassing at popocatepetl volcano mexico determined from waveform inversions of very long period signals 1 the source mechanism of very long period vlp signals accompanying volcanic degassing bursts at popocatepetl is analyzed in the 15 70 s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium the waveforms of two eruptions 23 april and 23 may 2000 representative of mild vulcanian activity are well reproduced by our inversion which takes into account volcano topography the source centroid is positioned 1500 m below the western perimeter of the summit crater and the modeled source is composed of a shallow dipping crack sill with easterly dip of 10 intersecting a steeply dipping crack northeast striking dike dipping 83 northwest whose surface extension bisects the vent both cracks undergo a similar sequence of inflation deflation and reinflation reflecting a cycle of pressurization depressurization and repressurization within a time interval of 3 5 min the largest moment release occurs in the sill showing a maximum volume change of 500 1000 m3 pressure drop of 3 5 mpa and amplitude of recovered pressure equal to 1 2 times the amplitude of the pressure drop in contrast the maximum volume change in the dike is less 200 300 m3 with a corresponding pressure drop of 1 2 mpa and pressure recovery equal to the pressure drop accompanying these volumetric sources are single force components with magnitudes of 108 n consistent with melt advection in response to pressure transients the source time histories of the volumetric components of the source indicate that significant mass movement starts within the sill and triggers a mass movement response in the dike within a few seconds such source behavior is consistent with the opening of a pathway for escape of pent up gases from slow pressurization of the sill driven by magma crystallization the opening of this pathway and associated rapid evacuation of volcanic gases induces the pressure drop pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles assuming a penny shaped crack at ambient pressure of 40 mpa the observed pressure and volume variations can be modeled with the following attributes crack radius 100 m crack aperture 5 m bubble number density 1010 1012 m 3 initial bubble radius 10 6 m final bubble radius 10 5 m and net decrease of gas concentration in the melt 0 01 wt

paleomagnetic study of the eastern klamath terrane california and implications for the tectonic history of the klamath mountains province paleomagnetic study of permian through jurassic volcanic and sedimentary strata of the eastern klamath terrane has shown the remanent magnetization of many of these rocks to be prefolding and most likely primary similarities in magnetic declinations recorded by coeval strata over a broad area are consistent with the hypothesis that the terrane in general has behaved as a single rigid block paleomagnetic data indicate that the volcanic island arc represented by this terrane the nucleus of the province was facing toward the present southwest during late paleozoic time although its orientation during earlier periods is unknown whether the arc was separated from the north american craton by a small marginal basin or originated far offshore cannot be determined from paleomagnetic data the declination anomalies for both permian and triassic strata are similar average 106 12 so we infer that clockwise rotation of the late paleozoic arc did not begin until latest triassic or earliest jurassic time the arc may have completed its initial rotation with respect to stable north america by middle jurassic time after some retrograde motion the arc was again facing west by the late jurassic by which time some of the more westerly terranes of the province had become attached to the eastern klamath terrane the composite klamath mountains terranes continued to rotate until the final 60 of clockwise rotation was nearly complete by the early cretaceous coincidence of the waning stages of rotation at about 136 ma with the beginning of deposition of the basal great valley sequence onto the klamath basement probably represents the completion of accretion of the klamath mountains terranes to the north american continent nearly all the rotation occurred while the klamath mountains terranes were part of a converging oceanic plate with only about 20 of rotation in mid tertiary time during basin and range extension no data currently available show evidence for any significant latitudinal displacement of any klamath mountains terranes relative to cratonic north america

insights into fluid transport mechanisms at white island from analysis of coupled very long period vlp long period lp and high frequency hf earthquakes the august 2012 to october 2013 white island unrest sequence included 5 explosive volcanic eruptions and emplacement of a small dome these events were linked to an overall increase in so2 and h2s gas flux and rsam seismic tremor which began in late 2011 prior to this unrest a small swarm of 25 events was observed on 19 21 august 2011 and captured on a temporary seismic array including 14 broadband sensors each event comprised coupled pulses having distinct high frequency hf 2 5 hz long period lp 0 5 1 1 hz and very long period vlp 0 03 0 125 hz or 8 30 s earthquakes for each coupled event we compute the source locations origin times and related uncertainties by application of standard arrival time locations for the hf earthquakes and waveform semblance for the lp and vlp earthquakes results suggest that the events are centred beneath the active vent at depths generally 1 5 km the hf and lp earthquakes have shallow depths 1 km while vlp have slightly deeper source locations 0 8 1 5 km emergent onsets for lp and vlp sources make an analysis of the absolute origin times problematic but waveform matching of vlp to lp and hf components suggests that the main vlp pulse precedes the hf and lp source processes waveform inversion for the vlp source is consistent with the rupture of a high angle east west oriented crack opening either in a purely tensile or shear tensile manner the moment of the isotropic component is estimated at 1 2 1012 nm and the corresponding volumetric change is in the range 145 450 m3 results are interpreted as an upward migration of fluids which first excite the vlp from a high angle crack in the magma carapace followed by the excitation of lp and hf source processes in the overlying hydrothermal system

location of tremor sources and estimation of lava output using tremor source amplitude on the piton de la fournaise volcano 2 estimation of lava output abstract we propose several relations for estimating the amounts of lava emitted during eruptions of the piton de la fournaise volcano based on the use of tremor source amplitude sources for eruption tremor are located in several frequency bands with a location method based on the use of seismic amplitudes corrected for recording site effects sources are usually found at shallow depth and close to the eruptive vents suggesting that tremor is directly generated by the eruptive activity simultaneously with the location of the tremor sources our program gives the source amplitude of the phenomenon we assume a linear or a quadratic relation between the lava output and the tremor source amplitude both relations are calibrated using data for the march 1998 eruption and tested using the volumes emitted during the july and september 1999 and february 2000 eruptions we found that the 5 10 hz frequency band gives lava output much higher than for lower frequencies suggesting that for that band the tremor generation depends on the eruption site in addition to the lava output or that tremor at those frequencies is not directly caused by the outflow of lava but maybe rather by gas the best estimations are obtained using the quadratic relation for frequencies below 5 hz with an agreement better than 15 with the field observations for the july and september 1999 eruptions and 30 for the february 2000 eruption our results show that the cumulative amplitude of tremor recorded during eruptions of the piton de la fournaise is directly related to the amounts of lava erupted

rates and timing of vertical axis block rotations across the central sierra nevada walker lane transition in the bodie hills california nevada 1 we use paleomagnetic data from tertiary volcanic rocks to address the rates and timing of vertical axis block rotations across the central sierra nevada walker lane transition in the bodie hills california nevada samples from the upper miocene 9 ma eureka valley tuff suggest clockwise vertical axis block rotations between ne striking left lateral faults in the bridgeport and mono basins results in the bodie hills suggest clockwise rotations r r 95 confidence limits of 74 8 since early to middle miocene 12 20 ma 42 11 since late miocene 8 9 ma and 14 10 since pliocene 3 ma time with no detectable northward translation the data are compatible with a relatively steady rotation rate of 5 2 ma 1 2 since the middle miocene over the three examined timescales the average rotation rates have probably not varied by more than a factor of two over time spans equal to half of the total time interval our paleomagnetic data suggest that block rotations in the region of the mina deflection began prior to late miocene time 9 ma and perhaps since the middle miocene if rotation rates were relatively constant block rotation in the bodie hills is similar in age and long term average rate to rotations in the transverse ranges of southern california associated with early transtensional dextral shear deformation we speculate that the age of rotations in the bodie hills indicates dextral shear and strain accommodation within the central walker lane belt resulting from coupling of the pacific and north america plates

the relative contribution of accretion shear and extension to cenozoic tectonic rotation in the pacific northwest large cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the pacific northwest and many tectonic models have been proposed to explain them including 1 rotation of accreted oceanic microplates during docking 2 dextral shear between north america and northward moving oceanic plates to the west and 3 microplate rotation in front of an expanding basin and range province stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations coast range structures basin and range extension and paleomagnetic data from middle miocene 15 ma coast range rocks indicate that dextral shear is responsible for at least 40 of the post 15 ma rotation of the coast range and that basin and range extension is responsible for the remainder reconstructions based on extrapolation of this ratio back to 37 and 50 ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has been a significant contributor to rotation during most of tertiary time changes in the dextral shear rotation rate over the past 50 m y correlate directly with changes in the velocity of the farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost basin and range region and suggest 17 extension since 15 ma 39 since 37 ma and 72 since 50 ma near latitude 42 n

source mechanism of a very long period event at mt ontake central japan response of a hydrothermal system to magma intrusion beneath the summit abstract a very long period vlp volcanic event which occurred on 25 january 2007 beneath mt ontake central japan was recorded by nearby broadband seismometers as well as the japanese nationwide hi net and f net seismic networks the vlp waveforms show a pulse like shape with a duration of approximately 1 min the vlp event was accompanied by a burst sequence of long period lp events we carried out waveform inversion to determine the source mechanism and location of the vlp event our analysis indicates volumetric changes in an inclined crack seismic moment of 10 14 nm at the vlp source which is located at 0 6 km above sea level beneath the summit of mt ontake and above dikes determined from gps data the source time functions of the vlp event showed a pulse like triangular shape indicating inflation followed by deflation of the crack at the source we interpreted the result of the waveform inversion as follows gases released from the intruded magma heated an aquifer system beneath the summit of mt ontake a break of unstable superheated water in a hydrothermal crack in the system caused vaporization of water and associated crack inflation which was followed by discharge of steam from the crack resulting in its collapse these processes caused the vlp event which may have been a precursor of a small phreatic eruption that occurred in late march 2007

volcanic glass as a proxy for cenozoic elevation and climate in the cascade mountains oregon usa abstract after deposition volcanic glass hydrates with ambient water recording the average hydrogen isotope ratio d or 2h of local meteoric water during the hydration period previous researchers have used ancient glass d values to reconstruct paleotopography and paleoclimate while others have questioned the long term reliability of the proxy as a recorder of ancient meteoric water in this study we sampled volcanic glasses ranging in age 33 ma to 1 ma and local meteoric water converted to glass d values throughout the study area we propose a paleoenvironmental interpretation of volcanic glass results that resolves previously published isotopic data and agrees well with the petrologic structural and stratigraphic record namely the oregon cascades have been a significant topographic barrier since at least the mid miocene and likely as far back as the oligocene since reaching a topographic maximum during the eruption of columbia river flood basalts in the mid miocene surface elevations in oregon have decreased while the northern cascades in washington continue to rise

paleomagnetism of an east west transect across the cascade arc in southern washington implications for regional tectonism paleomagnetic data from a transect across the cascade arc in southern washington were collected to test models of vertical axis rotations for this part of the pacific northwest and to provide information on the tectonic history of the st helens seismic zone the 75 site mean directions are divided into three groups based on isotopic fission track and stratigraphic ages group 1 consists of samples from rocks deposited between 39 and 30 ma and shows a mean clockwise vertical axis rotation of 34 13 group 2 consists of samples from rocks deposited between 30 and 24 ma and shows a mean clockwise rotation of 17 11 and group 3 consists of samples from rocks between 24 and 16 ma and shows a mean clockwise rotation of 20 12 although these three values of rotation are statistically indistinguishable at the 95 confidence level we interpret them in combination with other data to indicate differential rotation across the st helens seismic zone shz the available paleomagnetic data for eocene oligocene rocks west of the seismic zone show clockwise vertical axis rotations of 30 8 and 35 9 consistent with the value for group 1 the rotational values for groups 2 and 3 and the intrusive suite of kidd creek 13 ma despite their different ages have similar values east of the shz comparing these groups of data east and west of the seismic zone indicates a differential rotation of 10 3 across it in addition the rates of rotation in southern washington are similar to those for rocks in the western cascades of oregon and indicate that rotational deformation of miocene age extends northward into southern washington and eastward into the axis of the arc

gas slug ascent through changes in conduit diameter laboratory insights into a volcano seismic source process in low viscosity magmas seismic signals generated during the flow and degassing of low viscosity magmas include long period lp and very long period vlp events whose sources are often attributed to dynamic fluid processes within the conduit we present the results of laboratory experiments designed to investigate whether the passage of a gas slug through regions of changing conduit diameter could act as a suitable source mechanism a vertical liquid filled glass tube featuring a concentric diameter change was used to provide canonical insights into potentially deep or shallow seismic sources as gas slugs ascend the tube we observe systematic pressure changes varying with slug size liquid depth tube diameter and liquid viscosity gas slugs undergoing an abrupt flow pattern change upon entering a section of significantly increased tube diameter induce a transient pressure decrease in and above the flare and an associated pressure increase below it which stimulates acoustic and inertial resonant oscillations when the liquid flow is not dominantly controlled by viscosity net vertical forces on the apparatus are also detected the net force is a function of the magnitude of the pressure transients generated and the tube geometry which dictates where and hence when the traveling pressure pulses can couple into the tube in contrast to interpretations of related volcano seismic data where a single downward force is assumed to result from an upward acceleration of the center of mass in the conduit our experiments suggest that significant downward forces can result from the rapid deceleration of relatively small volumes of downward moving liquid

tectonics and seismicity of the southern washington cascade range abstract geophysical geological and seismicity data are combined to develop a transpressional strain model for the southern washington cascades region we use this model to explain oblique fold and fault systems transverse faults and a linear seismic zone just west of mt rainier known as the western rainier zone we also attempt to explain a concentration of earthquakes that connects the northwest trending mount st helens seismic zone to the north trending western rainier zone our tectonic model illustrates the pervasive effects of accretionary processes combined with subsequent transpressive forces generated by oblique subduction on eocene to present crustal processes such as seismicity and volcanism

changes to the volcanic outgassing mechanism and very long period seismicity from 2007 to 2011 at mt asama japan abstract volcanic gases can provide significant information on the state of magma which is important for interpreting and predicting volcanic activity we present a study of the seismicity and volcanic gases from mt asama japan from 2007 to 2011 to elucidate the change in the mechanism of volcanic gas supply for very long period vlp seismic events two types of degassing occur at mt asama persistent degassing and a gas burst ash free eruption following a vlp seismic event we used the linear relationship between the seismic moment and sulfur dioxide so 2 emissions and seismic records to estimate the quantity of so 2 gas in the ash free eruption which causes the vlp seismic events the amount of so 2 and volcanic gas composition data were used to calculate the amount of volcanic gases including h 2 o co 2 and other species we compared the volcanic gas emission rates of ash free eruptions eruptive gas emission rates and total volcanic gas emission rates including the persistent degassing the ratio of the eruptive gas emission rate to the total gas emission rate eruptive total gas ratios showed a drastic change with eruption an increase occurred immediately before the 2008 and after the 2009 eruptions the vlp seismic events are likely caused by sudden outgassing at a shallow depth that is induced by the propagation of the gas phase expansion and rupture we suggest that the rates of sudden outgassing are controlled by the distribution of gas phases in the conduit the variability of the eruptive gas emission rates suggests changes in the gas phase distribution in the conduit

p wave velocity structure beneath asama volcano japan inferred from active source seismic experiment an active seismic survey with unprecedented density of seismometers found a high velocity zone to the west of the summit of asama volcano japan the high velocity zone coincides with an area of magma filled crack dike intrusion associated with the 2004 eruptions inferred from precise earthquake relocations and ground deformation modeling it also coincides with an area of high resistivity surrounded by low resistivity indicating that the solidification of magma due to repeating intrusions is responsible for the high velocity this thus endorses the magma pathway previously speculated by seismic and geodetic observations these findings demonstrate that dense seismic exploration combined with geophysical monitoring is an effective way to understand the dynamics of volcanic eruptions

siluro devonian paleomagnetic results from the tuva terrane southern siberia russia implications for the paleogeography of siberia new paleomagnetic data from seven sections of siluro devonian age sedimentary sequences in the tuva terrane south siberia russia reveal the presence of up to three different components of magnetization component a is of recent or late mesozoic cenozoic origin and component b identified unambiguously in 25 samples from one section yields a sample mean direction declination inclination of 300 6 54 7 k 42 2 95 4 5 in geographic and 283 9 63 5 k 45 4 95 4 3 in stratigraphic coordinates however since the results of the fold test are inconclusive no further attempt has been made to assign an age to this magnetization component c with unblocking temperatures of 500 680 c is identified in all seven sections and with dual polarity after bedding correction the section mean directions all plot along a common small circle in stereographic projection with a mean inclination of 42 1 n 7 sites k 184 2 95 2 9 it passes the inclination only fold test the within site fold test both on the 95 significance level and the reversal test and is thus interpreted as being primary in origin and siluro devonian in age adopting the normal polarity option for component c yields a paleolatitude of 24 n 2 for the tuva terrane on the basis of geological evidence the tuva terrane was clearly accreted to siberia by early silurian times the results obtained in this study therefore indicate lower paleolatitudes for siberia than those predicted from previously published apparent polar wander paths for the siberian platform and demonstrate that siberia was positioned between the equator and 30 n during latest silurian earliest devonian times these new results are also supported by paleoecological and paleoclimatological evidence the dispersion of declinations along a small circle is indicative of clockwise block rotations within the tuva terrane of up to 72 with respect to siberia

characterization of scattered seismic wavefields simulated in heterogeneous media with topography 1 we performed numerical simulations of seismic waveforms with frequencies up to 10 hz in heterogeneous media with topography to investigate the effects of topography and structural heterogeneity on seismic scattering we used the simulated waveforms to test the source location method assuming isotropic radiation of s waves for long period events and tremor at volcanoes the assumption of isotropic radiation has been shown previously to be valid in a high frequency band because of the path effect caused by the scattering of seismic waves our simulation results showed that distortion of the wavefields increased as the correlation distance of structural heterogeneity a decreased as the frequency increased and as the travel distance increased topography alone considerably distorted the wavefields however we found that strong scattering due to topography was suppressed if the correlation distance of structural heterogeneity was longer than the s wave wavelength isotropic radiation of s waves by scattering due to topography was not achieved in our simulations our results indicated that scattering due to structural heterogeneity becomes stronger than that due to topography at ka 1 and a 50 m where k is the wave number of the s waves this suggests that strong short scale structural heterogeneity is required to achieve isotropic radiation of s waves although we could not reproduce isotropic radiation our results support the validity of the assumption of isotropic radiation in the source location method

seasonal variations in the formation of al and si rich fe stromatolites in the highly polluted acid mine drainage of agua agria creek tharsis sw spain abstract the study of the different biotic and abiotic processes involved in the current formation of fe stromatolites in rivers affected by acid mine drainage amd is essential not only to understand this unique and extreme environment but also to achieve a better understanding of the past iron formation on the earth and on mars the fe stromatolites studied in the highly polluted amd in tharsis sw spain revealed a unique mineral assemblage with jarosite and rostite as the most unexpected fe and al phases and goethite and schwertmannite as the main constituents seasonal variations in this region greatly affected the composition of the precipitates and the distribution of diatoms within the fe stromatolites drought and flood events were also recognized in the sedimentary textures and in the mineral assemblages observed of particular layers within the fe stromatolites the characterization of the chemical and mineralogical composition of fe stromatolites in tharsis as well as the different formation mechanisms proposed complement the existing literature on newly formed fe stromatolites and provides new observations to increase our understanding of those extreme environments

theoretical basis of the amplitude source location method for volcano seismic signals the source location method using high frequency seismic amplitudes based on the assumption of isotropic radiation of s waves has been used successfully to locate seismic events and tremor at volcanoes this amplitude source location asl method overcomes the limitations of traditional hypocenter determination methods that use onset arrival times and has great potential to improve volcano seismic monitoring and the investigation of source processes of volcano seismic signals however theoretical justification of the basic assumption used in the asl method has not been provided in previous studies in this study we tested the asl method by using seismic waveforms simulated with a finite difference method in realistic heterogeneous volcanic structures with intrinsic attenuation and topography our results showed that asl determinations were successful when using waveforms characterized by multiple scattering that can be approximated by the diffusion model we found that the energy solution of the diffusion model provided an amplitude distance relation that is similar to that used in the asl method our results suggest that the asl method is applicable to high frequency seismograms in highly heterogeneous media with transport mean free paths of 103 m or smaller and strong intrinsic attenuation these medium parameters are consistent with those estimated at various volcanoes our study validates the basic assumption of the asl method and justifies its use to locate the sources of high frequency seismic signals observed at volcanoes

a late cretaceous paleomagnetic pole from koto rhyolite southwest japan implications for eastern margin deformation of asia 1 a paleomagnetic pole was obtained from late cretaceous rhyolitic ash flow tuffs and associated ring dikes which are collectively called the koto rhyolite in the eastern part of southwest japan characteristic remanent magnetizations were isolated from 33 sites through thermal demagnetization normal and reversed polarity magnetizations were found and are mutually antipodal although the magnetization directions become more dispersed after tilt correction the clearly defined polarity sequence and the positive contact test between ash flow tuffs and ring dikes ensure that the characteristic magnetizations of ash flow tuffs predate the intrusion of associated ring dikes since no regional tilting is expected we regard the in situ magnetization directions to represent the late cretaceous 70 ma paleomagnetic field d 84 6 i 57 5 95 5 5 in eastern southwest japan the paleomagnetic pole is not significantly different from the coeval poles of western southwest japan this result gives a further support to a microplate model for the miocene clockwise rotation of southwest japan by canceling out the effect of the miocene rotation we were able to obtain a paleomagnetic pole of 70 ma 219 7 e 63 0 n 95 12 2 for southwest japan when the japan arc was incorporated in the eastern margin of asia paleomagnetic poles of late cretaceous age from korea south china coast and restored southwest japan are statistically indistinguishable from each other but significantly differ from that of the continental interior of east asia the observed rotation of about 20 probably results from shear deformation exerted by northeast trending strike slip faults along the eastern continental margin of asia

imaging the dynamics of magma propagation using radiated seismic intensity 1 at shallow depth beneath the earth s surface magma propagates through strongly heterogeneous volcanic material inversion of buoyancy and or solidification have strong impacts on the dynamics of propagation without any change of magma supply in this paper we study the spatial and time evolution of magma intrusions using induced seismicity we propose a new method based on ratio analysis of estimates of radiated seismic intensities recorded at different stations during seismic swarms by applying this method to the january 2010 piton de la fournaise volcano eruption we image complex dike propagation dynamics which strongly differ from a model of constant velocity dike propagation we provide a new method to image in real time the dynamics of dike propagation and to infer the position of eruptive fissures

new seismic images of the cascadia subduction zone from cruise so108 orwell abstract in april and may 1996 a geophysical study of the cascadia continental margin off oregon and washington was conducted aboard the german r v sonne this cooperative experiment by geomar and the usgs acquired wide angle reflection and refraction seismic data using ocean bottom seismometers obs and hydrophones obh and multichannel seismic reflection mcs data the main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the cascadia subduction zone and to image the downgoing plate coincident mcs and wide angle profiles along two tracks are presented here the plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca 13 km depth thus the downgoing plate dips more shallowly than previously assumed the dip of the plate changes from 2 to 4 at the eastern boundary of the wedge on the northern profile where approximately 3 km of sediment is entering the subduction zone on the southern profile where the incoming sedimentary section is about 2 2 km thick the plate dips about 0 5 to 1 5 near the deformation front and increases to 3 5 further landwards on both profiles the deformation of the accretionary wedge has produced six ridges on the seafloor three of which represent active faulting as indicated by growth folding the ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement thus the entire incoming sediment package is being accreted at least two phases of accretion are evident and the rocks of the older accretionary phase s forms the backstop for the younger phase which started around 1 5 ma ago this documents that the 30 to 50 km wide frontal part of the accretionary wedge which is characterized by landward vergent thrusts is a pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods velocities increase quite rapidly within the wedge both landward and downward at the toe of the deformation front velocities are higher than 4 0 km s indicating extensive dewatering of deep oceanic sediment further landward considerable velocity variation is found which indicates major breaks throughout the accretionary history

evidence for the importance of small faults on block rotation field and earthquake data are presented which show that small faults can be important during block rotation small here means those faults which are at least an order of magnitude smaller than the largest faults at a particular scale of observation the nw se trending dextral faults of sw england have millimetres to kilometres of displacement the largest of these faults such as the sticklepath lustleigh fault do not appear to cause more than a few degrees of rotation from the general e w strike of beds tens of degrees of rotation are often visible however on faults with millimetres of displacement similarly the apparent rotation of beds in the badajoz cordoba shear zone spain increases as the resolution of the faults is increased the power law scaling relationship of earthquake magnitudes in the san andreas fault zone illustrates that small faults can also be important in regions of active block rotation small faults can allow deformation within rotating blocks and can allow high displacement gradients to occur on the block bounding faults this would reduce the need for void creation which is a requirement of rigid block rotation models a tentative model is presented which incorporates the concept that fault behaviour is fractal emphasising the contribution of small faults it is suggested that future studies of block rotation should rigorously test the contribution of small faults and that particular care is needed when comparing palaeomagnetic data with regional scale structures

ascending seismic source during an explosive eruption at tungurahua volcano ecuador 1 application of seismic techniques to an explosion event at tungurahua volcano ecuador provided clear images to elucidate its source process a source location method using high frequency seismic amplitudes with an s wave velocity of 2000 m s indicates that the event was triggered at a depth of 6 km below the summit and the source ascended toward the summit at a speed of about 1600 m s waveform inversion of low frequency signals at the event onset points to an isotropic mechanism with initial deflation at a similar depth of 6 km the ascending source suggests that a pressure wave propagated along the magma conduit triggering fragmentation of magma at shallow depths rapid decompression of magma in a shock tube has been considered to be an important mechanism for explosive eruptions triggered by ruptures at the magma surface however our study suggests that explosive eruptions are triggered by pressure disturbances in magma at depth

two types of volcanic tremor changed with eruption style during 1986 izu oshima eruption volcanic tremor provides clues to magma migration pathways so that tremor source location is expected to be an efficient tool for tracking dynamic behavior of magma in evolution of eruptive activity however clear evidence which connects between temporal variation in volcanic tremor and evolution of eruption style is still lacking we have analyzed volcanic tremors occurred during 1986 izu oshima eruption using recently digitized data the results present a clear link between eruption styles waveform variations and source locations of the tremors moreover precursory activity of the tremors that indicates injection of magma below fissures has been clarified 5 days prior to the fissure eruptions this demonstrates predominance of tremor activity as an adaptive monitoring tool in volcanic eruption

calibrating the amplitude source location asl method by using active seismic sources an example from te maari volcano tongariro national park new zealand a frequently applied amplitude source location asl method is here calibrated and optimized by using active seismic sources located at the surface of a dry stream channel the asl produced location discrepancies larger than 1 0 km laterally and 500 m in depth by using independently determined velocity model attenuation and site amplification factors afs sensitivity tests for asl input parameters show that attenuation and velocity have moderate influence on the location but are easy to independently constrain afs are shown to strongly influence the location and their application may introduce substantial location uncertainties model uncertainties are accommodated with either lateral or depth changes depending on the input parameters station corrections and the source station geometry

diagenetic variation in the oregon coast range implications for rock strength soil production hillslope form and landscape evolution the mechanisms by which lithology modulates geomorphic processes are poorly known in the oregon coast range ocr rhythmically bedded sandstones of the eocene tyee formation underlie steep soil mantled hillslopes with relatively uniform ridge valley spacing these characteristic landforms are perturbed where diagenetic variations manifest as resistant cliffs here we use petrology rock mechanics and lidar to characterize grain scale variations in rock properties and their influence on rock strength hillslope processes and landscape morphology in two adjacent watersheds petrographic analyses suggest that a suite of diagenetic products in the resistant bedrock account for a 2 5 times increase in tensile strength relative to typical tyee bedrock our reference catchment exhibits negligible resistant outcrops and consistent hillslope gradients and longitudinal valley profiles by contrast the adjacent catchment teems with resistant 1 to 10 m thick noncontiguous sandstone beds that form hanging valleys with gentle upstream hillslopes and anomalously narrow valleys mechanical and topographic analyses suggest that the low fracture density characteristic of these resistant beds may render them relatively impervious to comminution by tree root activity the dominant ocr soil production mechanism based on both hillslope gradient and hilltop curvature erosion models we estimate that hillslopes perched above resistant beds erode at approximately half the pace of hillslopes unencumbered by downstream knickpoints the diagenetic variations likely influence relief at the watershed scale depositional position and diagenetic processes appear to control the occurrence of resistant beds providing a framework to quantify how seemingly subtle variations in rock properties can impose first order controls on landscape form and evolution

late cretaceous paleomagnetism and clockwise rotation of the silver bell mountains south central arizona late cretaceous ash flow volcanism in the silver bell mountains of southern arizona 32 3 n 248 5 e was associated with caldera formation and porphyry copper mineralization oriented samples from 34 sites in volcanic volcaniclastic and intrusive units related to this episode of igneous activity 73 69 ma yield a mean paleomagnetic direction of i 61 2 d 24 0 95 7 6 primary remanent magnetizations are indicated by the presence of both normal and reversed polarities and by the significantly improved grouping 95 confidence level of site mean directions for a widespread tuff unit after structural correction a comparison of this direction with a direction at silver bell calculated from contemporaneous units for stable north america i 58 8 d 342 3 95 7 7 indicates a late cretaceous paleolatitude anomaly of 2 4 7 6 not significant at the 95 confidence level and a declination anomaly of 41 7 14 3 for the silver bell mountains previously determined paleomagnetic data for southeastern arizona suggest that this apparent clockwise rotation in the silver bell mountains is a local phenomenon although preliminary the average paleomagnetic direction for oligocene and lower miocene rocks in the silver bell area i 43 8 d 357 3 95 16 5 is similar to that calculated for stable north america i 50 2 d 352 2 95 3 9 implying that the observed rotation in the silver bell mountains occurred before 26 ma and was most likely associated with dextral strike slip movement along the ragged top and related wnw trending faults bounding the silver bell mountain block these data in conjunction with plate reconstructions and other paleomagnetic data from southwestern north america imply that wnw trending strike slip faults may have played an important role during late cretaceous to early tertiary deformation in southern arizona

evidence for distributed clockwise rotation of the crust in the northwestern united states from fault geometries and focal mechanisms paleomagnetic and gps data indicate that washington and oregon have rotated clockwise for the past 16 myr late cenozoic and quaternary fault geometries seismicity lineaments and focal mechanisms provide evidence that this rotation is accommodated by north directed thrusting and right lateral strike slip faulting in washington and sw to w directed normal faulting and right lateral strike slip faulting to the east several curvilinear nw to nnw trending high angle strike slip faults and seismicity lineaments in washington and nw oregon define a geologic pole 117 7 w 47 9 n of rotation relative to north america many faults and focal mechanisms throughout northwestern u s and southwestern british columbia have orientations consistent with this geologic pole as do gps surface velocities corrected for elastic cascadia subduction zone coupling large quaternary normal faults radial to the geologic pole which appear to accommodate crustal rotation via crustal extension are widespread and can be found along the lewis and clark zone in montana within the centennial fault system north of the snake river plain in idaho and montana to the west of the wasatch front in utah and within the northern basin and range in oregon and nevada distributed strike slip faults are most prominent in western washington and oregon and may serve to transfer slip between faults throughout the northwestern u s

broadband seismic monitoring of active volcanoes using deterministic and stochastic approaches 1 we systematically used two approaches to analyze broadband seismic signals for monitoring active volcanoes one is waveform inversion of very long period vlp signals assuming possible source mechanisms the other is a source location method of long period lp events and tremor using their amplitudes the deterministic approach of the waveform inversion is useful to constrain the source mechanism and location but is basically only applicable to vlp signals with periods longer than a few seconds the source location method assumes isotropic radiation of s waves and uses seismic amplitudes corrected for site amplifications this simple approach provides reasonable source locations for various seismic signals such as a vlp event accompanying lp signals an explosion event and tremor associated with lahars and pyroclastic flows observed at five or fewer stations our results indicate that a frequency band of about 5 12 hz and a q factor of about 60 are appropriate for the determination of the source locations in this frequency band the assumption of isotropic radiation may become valid because of the path effect caused by the scattering of seismic waves the source location method may be categorized as a stochastic approach based on the nature of scattering waves systematic use of these two approaches provides a way to better utilize broadband seismic signals observed at a limited number of stations for improved monitoring of active volcanoes

geochronology of age progressive volcanism of the oregon high lava plains implications for the plume interpretation of yellowstone 1 the high lava plains province hlp is a late cenozoic bimodal volcanic field at the northern margin of the basin and range province in southeastern oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the yellowstone snake river plain ysrp trend we present 40ar 39ar ages for 19 rhyolite domes 5 rhyolite ash flow tuffs and 34 basaltic lavas from the hlp the previously identified trend of westward migration of hlp rhyolites is confirmed the rate of propagation is 33 km m y from 10 to 5 ma slowing to 13 km m y after 5 ma the duration of silicic volcanism at any locus is 2 m y three older hlp dacite domes yielded ages of 15 5 ma basalts are not age progressive we identify several episodes of increased basaltic activity at 7 5 7 8 5 3 5 9 and 2 3 ma with the younger episode likely continuing into the recent the hlp and ysrp trends emerged from the axis of middle miocene basaltic volcanism of the columbia river and steens basalts we propose a model in which 1 miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin 2 the hlp trend is the result of westward flow originating at the craton margin and 3 the ysrp trend is the trace of the motion of the north american plate over the tail of the plume

characteristics and source modeling of broadband seismic signals associated with the hydrothermal system at satsuma iwojima volcano japan abstract the broadband seismic data observed at satsuma iwojima volcanic island in southwestern japan show a wide variety of interesting features one of them is the periodic and frequency dependent amplitude modulation of volcanic tremors another interesting feature is the occurrence of very long period vlp seismic pulses synchronized with the tremor amplitude modulation both the tremor amplitude modulation and the vlp pulses have fairly regular intervals of 46 50 min suggesting a spatially fixed non destructive repetitive source we performed waveform inversions to reveal the source mechanism of the vlp pulses assuming a point source embedded in a medium that realistically represented the topography of the island numerical tests showed that the green s functions used in the inversion were significantly affected by the topography the observed vlp pulses were well explained by the rapid expansion of an inclined crack 100 m beneath the floor of the summit crater which had a seismic moment of 3 10 10 nm both the characteristic amplitude modulation and the vlp pulses are explained systematically by a model consisting of a water filled crack and a network of gas paths the water filled crack was heated above the boiling temperature of the water in it a sudden break of the unstable superheated state caused sudden vaporization and a pressure increase which was observed as the vlp pulse the network of gas paths consisted of swollen portions and constricted portions which worked as tremor generators when vapor from the water filled crack passed through

comment on deflection of paleomagnetic directions due to magnetization of the underlying terrain by c baag c helsley shi zhe xu and b liernert

quickly erupted volcanic sections of the steens basalt columbia river basalt group secular variation tectonic rotation and the steens mountain reversal 1 the steens basalt now considered part of the columbia river basalt group crbg contains the earliest eruptions of this magmatic episode lava flows of the steens basalt cover about 50 000 km2 of the oregon plateau in sections up to 1000 m thick the large number of continuously exposed quickly erupted lava flows some sections contain over 200 flows allows for small loops in the magnetic field direction paths to be detected for volcanic rocks this detail and fidelity are rarely found outside of the holocene and yield estimates of eruption durations at our four sections of 2 5 ka for 260 m at pueblo mountains 0 5 to 1 5 ka for 190 m at summit springs 1 3 ka for 170 m at north mickey and 3 ka for 160 m at guano rim that only one reversal of the geomagnetic field occurred during the eruption of the steens basalt the steens reversal at approximately 16 6 ma is supported by comparing 40ar 39ar ages and magnetic polarities to the geomagnetic polarity timescale at summit springs two 40ar 39ar ages from normal polarity flows 16 72 0 29 ma 16 61 and 16 92 0 52 ma 16 82 equals 2 error place their eruptions after the steens reversal while at pueblo mountains an 40ar 39ar age of 16 72 0 21 ma 16 61 from a reverse polarity flow places its eruption before the steens reversal paleomagnetic field directions yielded 50 nontransitional directional group poles which combined with 26 from steens mountain provide a paleomagnetic pole for the oregon plateau of 85 7 n 318 4 e k 15 1 a95 4 3 comparison of this new pole with a reference pole derived from crbg flows from eastern washington and a synthetic reference pole for north america derived from global data implies relative clockwise rotation of the oregon plateau of 7 4 5 0 or 14 5 5 4 respectively probably due to northward decreasing extension of the basin and range

extension tectonics the neogene opening of the north south trending basins of central thailand paleomagnetic samples were collected from late neogene basalt flows from thailand all of these flows are horizontal and are relatively unaltered in thin section these rocks possess a stable magnetization which is believed to be primary samples from 48 lava flows were collected from sites located within the khorat plateau the chao phraya phitsanulok basin and the mountainous terrane west of the chao phraya phitsanulok basin these data were combined with previously reported late neogene data from five flows from western thailand although the average inclination from the 53 sites is indistiguishable from the expected dipole inclination the average declination has a net clockwise rotation of 13 5 5 8 from the geocentric dipole field furthermore the mean declination values from the 29 flows from the khorat plateau are indistinguishable from the present dipole field direction dm 4 3 7 5 and indistinguishable from the mean declination from 28 late neogene volcanic flows from vietnam in contrast the mean declinations from 24 flows collected from central and western thailand are deflected significantly clockwise dm 24 4 7 7 from the geocentric dipole field direction the differential rotation between western and central thailand versus the khorat plateau suggests that indochina is composed of at least two structural blocks which underwent a different rotational history these observations when combined with geologic and geophysical data from the chao phraya phitsanulok basin gulf of thailand and the intermontane basins of western thailand suggest that the rotations are recording a late neogene phase of e w extension of these basins we suggest that the formation of these basins and the related basaltic volcanism developed in reponse to subduction of the indian plate under western burma we envision the tectonics of this region is similar in style to the basin and range region of the western united states last we have observed field relationships from some of the rhyolites located in the central basin although these rhyolites are reported to be mesozoic or paleozoic in age our field observations and a k ar age date show that at least some of these rhyolites are younger than the basalts we suggest that the rhyolites form a bimodal suite with the basaltic rocks which were erupted in the later stages of the extension

distortion of the apparent s wave radiation pattern in the high frequency wavefield tottori ken seibu japan earthquake of 2000 summary the distortion properties of the apparent s wave radiation pattern in the high frequency seismic wavefield of over f 2 hz is investigated using a large number of waveform records of the main shock and 29 aftershocks of the tottori ken seibu japan magnitude mw 6 6 earthquake in 2000 the dense seismic records from the kik net strong motion network show a clear four lobe pattern in the apparent s wave radiation pattern in the low frequency wavefield of f 2 hz and shows an almost isotropic distribution in all directions as the frequency increases above 5 hz the distortion of the apparent s wave radiation pattern in the high frequency wavefield increases as travel distance increases therefore the path effect caused by the scattering of seismic waves due to small scale heterogeneities in the crust is a major cause of distortion of the radiation pattern this hypothesis is examined by a 2 d finite difference method simulation of seismic waves in heterogeneous structure models the results of simulations clearly demonstrate the collapse of the s wave front due to seismic wave scattering in the heterogeneous structure by comparing the observed wavefield and the results of simulations using different sorts of stochastic heterogeneous models the most preferable model that can explain the observation is described by a von karman autocorrelation function with correlation distance of a 3 5 km order of 0 5 and rms value of e 0 07 however our simple stochastic random heterogeneity model proposed herein somewhat overestimates the scattering of low frequency signals below 2 hz

relation between single very long period pulses and volcanic gas emissions at mt asama japan 1 multiple volcanic observations conducted at mt asama japan provide evidence of a link between single very long period vlp seismic pulses and volcanic gas emissions so2 flux measurements were conducted on 2 june 2009 when mt asama was producing ash free eruptions with vlp pulses gas bursts from a vent at the crater bottom following the vlp pulses provided an excellent opportunity to examine the relation directly the so2 emission for each eruption was calculated by integrating high temporal so2 flux data obtained by the so2 imaging system and subtracting the contribution from quiescent degassing from fumaroles around the crater bottom a seismic moment of vlp pulse was estimated by the waveform inversion we observed seven eruptions and obtained the proportional relation between vlp pulse moment and so2 emission the relation determined is consistent with the vlp source model these observational results are the first report of a quantitative comparison between single vlp pulse moment and volcanic gas emission

denali fault system of southern alaska an interior strike slip structure responding to dextral and sinistral shear coupling the denali fault system dfs extends for 1200 km from southeast to south central alaska the dfs has been generally regarded as a right lateral strike slip fault along which post late mesozoic offsets of up to 400 km have been suggested the offset history of the dfs is relatively unconstrained particularly at its western end for this study we calculated relative motion vectors at discrete points along the length of the dfs based on the well understood kinematic interaction between the north american pacific and kula plates and the following assumptions 1 the arcuate geometry of the dfs has existed essentially unchanged since the late cretaceous 2 the yukon tanana terrane and other terranes north of the dfs were fixed in situ prior to the accretion of the southern alaskan terranes and 3 tangential and normal relative motion vector components calculated for points along the dfs using the plate model of kelley 1993 describe the plate kinematics of the dfs since the late cretaceous the consequent kinematic model for the dfs predicts that left lateral stresses have acted upon the western end of the dfs for much of its history and conflicting senses of shear exist between the eastern and western ends of the system the offset history of the western end of the denali fault system should be significantly different than the history of the central and eastern sections consequently individual crustal blocks in southeast and southwest alaska may have undergone respectively clockwise and counterclockwise rotations the sense of rotation predicted by our model is in agreement with rotations determined by paleomagnetic studies and provides an alternative model to the alaskan orocline hypotheses

very long period pulses at asama volcano central japan inferred from dense seismic observations summary we observed seismic events consisting of several one sided velocity pulses with durations of 5 30 s hereafter called very long period or vlp pulses using up to 14 broad band seismometers installed near the summit area of asama volcano japan particle motions do not point to a single source location despite their rectilinearities suggesting a non isotropic source mechanism together with path effect we conducted moment tensor inversion analyses for these events using the green s functions of not only translational motions but also of tilt motions the best fitting source locations are located north of the crater centre at depths of 100 150 m below the crater bottom the principal value ratios of moment tensor solutions can be approximated as 5 3 2 which is indicative of a combination of a tensile crack and a cylinder with the strike and dip angle of the crack both being nearly 70 and symmetry axis of the cylinder being inclined by nearly 30 from the vertical the time histories of moment tensors are indicative of an initial sudden pressurization and subsequent gradual depressurization at the source region we sought to model vlp pulses at asama volcano by considering the inflow and outflow of gas at the source cavity compensated by boiling in another deeper reservoir

eruptive history of south sister oregon cascades abstract south sister is southernmost and highest of the three sisters three geologically dissimilar stratovolcanoes that together form a spectacular 20 km reach along the cascade crest in oregon north sister is a monotonously mafic edifice as old as middle pleistocene middle sister a basalt andesite dacite cone built between 48 and 14 ka and south sister is a basalt free edifice that alternated rhyolitic and intermediate modes from 50 ka to 2 ka largely contemporaneous with middle sister detailed mapping 330 chemical analyses and 42 radioisotopic ages show that the oldest exposed south sister lavas were initially rhyolitic 50 ka by 37 ka rhyolitic lava flows and domes 72 74 sio2 began alternating with radially emplaced dacite 63 68 sio2 and andesite 59 63 sio2 lava flows construction of a broad cone of silicic andesite dacite 61 64 sio2 culminated 30 ka in a dominantly explosive sequence that began with crater forming andesitic eruptions that left fragmental deposits at least 200 m thick this was followed at 27 ka by growth of a steeply dipping summit cone of agglutinate dominated andesite 56 60 5 sio2 and formation of a summit crater 800 m wide this crater was soon filled and overtopped by a thick dacite lava flow and then by 150 m of dacitic pyroclastic ejecta small volume dacite lavas 63 67 sio2 locally cap the pyroclastic pile a final sheet of mafic agglutinate 54 56 sio2 the most mafic product of south sister erupted from and drapes the small 300 m wide present day summit crater ending a summit building sequence that lasted until 22 ka a 20 kyr long hiatus was broken by rhyolite eruptions that produced 1 the rock mesa coulee tephra and satellite domelets 73 5 sio2 and 2 the devils chain of 20 domes and short coulees 72 3 72 8 sio2 from n s vent alignments on south sister s flanks the compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the south sister locus throughout its lifetime south sister is part of a reach of the cascades unusually active in the last 50 kyr characterized by high vent density n s vent alignments and numerous eruptive units of true rhyolite 72 sio2 that distinguishes it from much of the quaternary cascade arc these are eruptive expressions of the complex confluence of arc and intraplate magmatic tectonic regimes

direction of maximum horizontal compression in western oregon determined by borehole breakouts elliptical borehole enlargements or breakouts caused by systematic spalling of a borehole wall were identified in 18 wells drilled in the coast range and willamette valley of western oregon the breakouts generally indicate a nnw to nne orientation of maximum horizontal compression hmax that agrees with the predominant direction of hmax determined from earthquake focal mechanisms from post middle miocene structural features and from alignments of holocene volcanic centers in the pacific northwest however this orientation is inconsistent with the n50 e convergence between the juan de fuca and north american plates determined by riddihough 1984 from juan de fuca plate magnetic lineations as young as 730 ka the brunhes matuyama boundary the predominant nnw to nne orientation of hmax may be due to the complex interaction of a northwestward moving pacific plate driving into the gorda and juan de fuca plates and indirectly transmitting n s compression across the strongly coupled cascadia subduction zone into the overriding north american plate spence 1989 alternatively the predominant nnw to nne orientation of hmax may be due to a landward counterclockwise rotation of the direction of hmax from n50 e compression offshore to n s compression in the coast range

magnetic stratigraphy and tectonic rotation of the middle eocene coaledo formation southwestern oregon summary the coaledo formation in coos bay on the southern coast of oregon consists of over 2000 m of deltaic sediments that filled a forearc basin during the middle eocene this unit yields molluscs of the cowlitz coaledo fauna one of the oldest and best studied faunas in the pacific northwest but its precise age has been controversial over 100 magnetic samples spanning the formation were analysed using both thermal and alternating field demagnetization they yielded a stable remanence held mainly in magnetite which passed a reversal test and showed a tectonic rotation of 105 5 this is greater than any other tectonic rotation so far reported from the region it is consistent with the previously observed trends that the tectonic rotations in the coast ranges of oregon and washington increase with increasing age of the rock unit and increase towards the southwest the magnetic polarity pattern in the coaledo formation best correlates with chrons c18r c20r 40 0 44 0 ma which is significantly older than the magnetic pattern in the cowlitz formation 36 5 38 3 ma this suggests that the cowlitz coaledo fauna is very long lived at least 36 5 44 0 ma spanning most of the middle eocene

late quaternary stream incision and uplift in the forearc of the cascadia subduction zone western oregon documentation of a latest pleistocene earliest holocene episode of strath formation and fluvial aggradation in the oregon coast range provides a datum from which long term bedrock stream incision rates are determined variations in long term incision rates probably reflect cumulative differential uplift in the forearc of the cascadia subduction zone although factors such as bedrock and climatic controls and isostatic adjustments to erosion obscure the precise relationship between surface uplift and stream incision patterns of differential incision are most striking near the latitude of newport where a steep gradient divides a region of higher rates 0 6 0 9 mm yr in the northern coast range from a region of lower rates 0 1 0 3 mm yr in the central coast range the steep incision gradient is nearly coincident with abrupt changes in marine terrace 80 125 kyr uplift rates the locations of quaternary faults and the southern flank of a saddle of low historic 40 70 years uplift the exact causes of these variable patterns of incision uplift are unknown analogies with uplift patterns in other subduction zones and comparisons with other neotectonic data in the region indicate that patterns of differential incision probably are caused by variations in permanent strain accumulation along the cascadia subduction zone such variations may be related to differences in seismic moment release during individual earthquakes to changes in plate geometry or rates of wedge accretion to segmentation of earthquake ruptures and or to deformation on active structures in the north american plate and accretionary wedge