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

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

magma reservoirs from the upper crust to the moho inferred from high resolution vp and vs models beneath mount st helens washington state usa the size frequency and intensity of volcanic eruptions are strongly controlled by the volume and connectivity of magma within the crust several geophysical and geochemical studies have produced a comprehensive model of the magmatic system to depths near 7 km beneath mount st helens washington state usa currently the most active volcano in the cascade range data limitations have precluded imaging below this depth to observe the entire primary shallow magma reservoir as well as its connection to deeper zones of magma accumulation in the crust the inversion of p and s wave traveltime data collected during the active source component of the imush imaging magma under st helens project reveals a high p wave vp s wave vs velocity anomaly beneath mount st helens between depths of 4 and 13 km which we interpret as the primary upper middle crustal magma reservoir beneath and southeast of this shallow reservoir a low vp velocity column extends from 15 km depth to the moho deep long period events near the boundary of this column indicate that this anomaly is associated with the injection of magmatic fluids southeast of mount st helens an upper middle crustal high vp vs body beneath the indian heaven volcanic field may also have a magmatic origin both of these high vp vs bodies are at the boundaries of the low vp middle lower crustal column and both are directly above high vp middle lower crustal regions that may represent cumulates associated with recent quaternary or paleogene neogene cascade magmatism seismicity immediately following the 18 may 1980 eruption terminates near the top of the inferred middle lower crustal cumulates and directly adjacent to the inferred middle lower crustal magma reservoir these spatial relationships suggest that the boundaries of these high density cumulates play an important role in both vertical and lateral transport of magma through the crust

continuous kurtosis based migration for seismic event detection and location with application to piton de la fournaise volcano la reunion abstract we present an automatic earthquake detection and location technique based on migration of continuous waveform data data are preprocessed using a kurtosis estimator in order to enhance the first arrival information then migrated onto a predefined search grid using precalculated p wave travel times and finally stacked local maxima in the resulting 4d space time grid indicate the locations and origin times of seismic events we applied our technique to earthquake swarms occurring on piton de la fournaise volcano la reunion france we located 5000 events from 12 different swarms that occurred between 2009 and 2011 our automated locations are consistent with those performed using manual picks and indicate that the seismicity concentrates around sea level multiplet analysis of the detected events and subsequent double difference relocation produce sharper images of the earthquake swarms our code waveloc is released in open source online material figures of seismicity distributions from waveloc synthetic test and stack amplitude values versus magnitudes

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

seismicity of rockfalls and avalanches at three cascade range volcanoes implications for seismic detection of hazardous mass movements abstract this study reviews seismograms from 14 rockfalls and avalanches of moderate to large volume 10 4 to 10 7 m 3 at mount st helens mount adams and mount rainier in the cascade range of washington to investigate how variations in source volume source materials track materials and failure modes affect avalanche seismicity the largest signals were generated by rockfalls at mount st helens and debris avalanches at mount rainier that initiated as block failures at mount st helens several rockfalls that originated as block failures near the crater rim descended slopes with similar profiles and appear to show a linear or nearly linear relation between source volume and seismogram amplitude those that fell over an extended period of time were poorly recorded at the reference seismic station and did not show this correlation multiple seismograms from three of the four instrumentally recorded debris avalanches at mount rainier indicate a tendency of unstable slopes there to fail progressively a pattern that has important implications for the mass movement hazard at mount rainier seismic evidence shows that the little tahoma rock fall avalanches in december 1963 occurred on 6 december over a week earlier than previously reported mass movements that have been seismically detected span the spectrum of rapid flowage events from water surges and debris flows to dry rock debris ice and snow avalanches the success in identifying and in some cases locating these events in near real time demonstrates that local and regional seismic networks can play a valuable role in reducing hazards from large rapid mass movements

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

shallow velocity structure and seismic site effects at arenal volcano costa rica abstract we use the spatial autocorrelation spac method with improved inversion algorithms to estimate the love and rayleigh dispersion curves at two sites at the west and northeast flanks of arenal volcano costa rica at the west flank site the rayleigh waves phase velocities vary from 765 m s 1 at 1 hz to 300 m s 1 at 12 hz and those of love waves between 780 and 295 m s 1 in the same frequency band at the northeast flank site the rayleigh wave velocities range from 1386 to 300 m s 1 and those of love from 1983 to 315 m s 1 from dispersion curves we derive shallow strong variations in amplitude of the spectral peaks are observed for the seismic events along two radial arrays these site effects are analysed using the h v spectral ratio method and s wave theoretical transfer functions results show that the wave amplifications are related to resonance effects of shallow structure when a resonance of the shallow layers occurs the correlation coefficients between close stations increase at the corresponding frequency the site effects may thus produce spurious results with the spac method the h v spectral ratio used in complement of the spac method can help detecting the site effects and testing the plane layer hypothesis furthermore the theoretical transfer functions calculated for the estimated velocity models is also useful to validate the models

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

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

analysis of teleseismic p waves with a 5200 station array in long beach california evidence for an abrupt boundary to inner borderland rifting we analyze teleseismic p waves from four mw 6 5 earthquakes recorded by a petroleum industry survey in long beach california the survey used a 2 d array with up to 5200 seismometers 120 m mean spacing and 7 10 km aperture at frequencies near 1 hz p wave travel times and amplitudes exhibit coherent lateral variations over scales as short as 400 m including locally delayed travel times and increased amplitudes at the crest of the long beach anticline deeper heterogeneity is indicated by p wave phase velocities that deviate from reference model predictions for events from southwestern azimuths we postulate that a sharp northeastward increase in moho depth from the inner borderland ib to mainland southern california causes the anomalous phase velocities elastic forward modeling finds the travel times are fit well by a moho that dips 65 to the northeast and flattens 10 km southwest of the newport inglewood fault zone constraining the felsic thickness of mainland crust to 28 km requires an 8 km thick layer with a p velocity of 7 km s beneath it which could result from basal accretion of former farallon ocean crust or magmatic underplating during miocene volcanism forward models with a 65 moho dip predict a p to s conversion with a phase velocity of 5 km s deconvolution of the array s mean p wave signal isolates a similar later arriving phase the steep crust thickness transition supports a locally abrupt boundary to ib rifting our results highlight the utility of dense short period arrays for passive imaging at near surface to uppermost mantle depths

high resolution 3d shallow crustal structure in long beach california application of ambient noise tomography on a dense seismic array ambient noise tomography has proven to be effective in resolving shallow earth structure we applied ambient noise tomography on a dense seismic array in long beach california the array was composed of more than 5200 stations with an average spacing close to 100 m three weeks of passive ambient noise were crosscorrelated between each station pair which resulted in more than 13 5 million crosscorrelations within the area clear fundamental mode rayleigh waves were observed between 0 5 and 4 hz which were most sensitive to structure above 1 km depth for each station pair we applied frequency time analysis to determine the phase traveltime dispersion and for each frequency we applied eikonal tomography to determine the rayleigh wave phase velocity map the eikonal tomography accounted for ray bending by tracking the wavefront and allowed uncertainties to be estimated through statistical analysis the compilation of phase velocity maps was then used to invert for 3d shear velocity structure the inverted model showed clear correlation with the known geologic features such as the shallow south north velocity dichotomy and a deeper fast anomaly associated with the newport inglewood fault zone our results can potentially be used to complement traditional active source studies

fault textures in volcanic conduits evidence for seismic trigger mechanisms during silicic eruptions it is proposed that fault textures in two dissected rhyolitic conduits in iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows detailed field and petrographic analysis of such textures may shed light on the origin of long period and hybrid volcanic earthquakes at active volcanoes there is evidence at each conduit investigated for multiple seismogenic cycles each of which involved four distinct evolutionary phases in phase 1 shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow forming the angular fracture networks that initiated faulting cycles transient pressure gradients were generated as the fractures opened which led to fluidisation and clastic deposition of fine grained particles that were derived from the fracture walls by abrasion fracture networks then progressively coalesced and rotated during subsequent slip phase 2 developing into cataclasite zones with evidence for multiple localised slip events fluidisation and grain size reduction phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction controlled stick slip behaviour increasing cohesion of cataclasites then led to aseismic distributed ductile deformation phase 3 and generated deformed cataclasite zones which are enriched in metallic oxide microlites and resemble glassy pseudotachylite continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite rich flow bands in obsidian phase 4 overall the mixed brittle ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle ductile transition with the rheological response mediated by strain rate variations and frictional heating fault processes in highly viscous magma are compared with those elsewhere in the crust and this comparison is used to appraise existing models of volcano seismic activity based on the textures observed it is suggested that patterns of long period and hybrid earthquakes at silicic lava domes reflect friction controlled stick slip movement and eventual healing of fault zones in magma which are an accelerated and smaller scale analogue of tectonic faults

influence of crystallised igneous intrusions on fault nucleation and reactivation during continental extension abstract continental rifting is commonly accommodated by the nucleation of normal faults slip on pre existing fault surfaces and or magmatic intrusion because crystallised igneous intrusions are pervasive in many rift basins and are commonly more competent i e higher shear strengths and young s moduli than the host rock it is theoretically plausible that they locally intersect and modify the mechanical properties of pre existing normal faults we illustrate the influence that crystallised igneous intrusions may have on fault reactivation using a conceptual model and observations from field and subsurface datasets our results show that igneous rocks may initially resist failure and promote the preferential reactivation of favourably oriented pre existing faults that are not spatially associated with solidified intrusions fault segments situated along strike from laterally restricted fault intrusion intersections may similarly be reactivated this spatial and temporal control on strain distribution may generate 1 supra intrusion folds in the hanging wall 2 new dip slip faults adjacent to the igneous body or 3 sub vertical oblique slip faults oriented parallel to the extension direction importantly stress accumulation within igneous intrusions may eventually initiate failure and further localise strain the results of our study have important implications for the structural of sedimentary basins and the subsurface migration of hydrocarbons and mineral bearing fluids

a switch from si oh 4 to no 3 depletion in the glacial southern ocean 1 phytoplankton in the antarctic deplete silicic acid si oh 4 to a far greater extent than they do nitrate no3 this pattern can be reversed by the addition of iron which dramatically lowers diatom si oh 4 no3 uptake ratios higher iron supply during glacial times would thus drive the antarctic towards no3 depletion with excess si oh 4 remaining in surface waters new 30si and 15n records from antarctic sediments confirm diminished si oh 4 use and enhanced no3 depletion during the last three glaciations the present low si oh 4 water is transported northward to at least the subtropics we postulate that the glacial high si oh 4 water similarly may have been transported to the subtropics and beyond this input of si oh 4 may have caused diatoms to displace coccolithophores at low latitudes weakening the carbonate pump and increasing the depth of organic matter remineralization these effects may have lowered glacial atmospheric pco2 by as much as 60 ppm

a methodological approach towards high resolution surface wave imaging of the san jacinto fault zone using ambient noise recordings at a spatially dense array we present a new technique for deriving detailed information on seismic velocities of the subsurface material from continuous ambient noise recorded by spatially dense seismic arrays this method uses iterative double beamforming between various subarrays to extract surface wave contributions from the ambient noise data in complex environments with unfavourable noise source distributions the iterative double beamforming extraction makes it possible to retrieve large amounts of rayleigh wave traveltime information in a wide frequency band the method is applied to data recorded by a highly dense nodal array with 1108 vertical geophones centred on the damage zone of the clark branch of the san jacinto fault zone south of anza california the array covers a region of 650 700 m 2 with instrument spacing of 10 30 m and continuous recording at 500 samples s 1 over 30 d in 2014 using this iterative double beamforming on subarrays of 25 sensors and cross correlations between all of the station pairs we separate surface waves from body waves that are abundant in the raw cross correlation data focusing solely on surface waves maps of traveltimes are obtained at different frequencies with unprecedented accuracy at each point of a 15 m spacing grid group velocity inversions at 2 4 hz reveal depth and lateral variations in the structural properties within and around the san jacinto fault zone in the study area this method can be used over wider frequency ranges and can be combined with other imaging techniques such as eikonal tomography to provide unprecedented detailed structural images of the subsurface material

three dimensional p wave velocity structure and precise earthquake relocation at great sitkin volcano alaska abstract waveform cross correlation with bispectrum verification is combined with double difference tomography to increase the precision of earthquake locations and constrain regional 3d p wave velocity heterogeneity at great sitkin volcano alaska from 1999 through 2005 the alaska volcano observatory avo recorded 1700 earthquakes in the vicinity of great sitkin including two m l 4 3 earthquakes that are among the largest events in the avo catalog the majority of earthquakes occurred during 2002 and formed two temporally and spatially separate event sequences the first sequence began on 17 march 2002 and was centered 20 km west of the volcano the second sequence occurred on the southeast flank of great sitkin and began 28 may 2002 it was preceded by two episodes of volcanic tremor earthquake relocations of this activity on the southeast flank define a vertical planar feature oriented radially from the summit and in the direction of the assumed regional maximum compressive stress due to convergence along the alaska subduction zone this swarm may have been caused or accompanied by the emplacement of a dike relocations of the mainshock aftershock sequence occurring west of great sitkin are consistent with rupture on a strike slip fault tomographic images support the presence of a vertically dipping fault striking parallel to the direction of convergence in this region the remaining catalog hypocenters relocate along discrete features beneath the volcano summit here low p wave velocities possibly indicate the presence of magma beneath the volcano

automatic detection and rapid determination of earthquake magnitude by wavelet multiscale analysis of the primary arrival earthquake early warning systems must save lives it is of great importance that networked systems of seismometers be equipped with reliable tools to make rapid determinations of earthquake magnitude in the few to tens of seconds before the damaging ground motion occurs a new fully automated algorithm based on the discrete wavelet transform detects as well as analyzes the incoming first arrival with great accuracy and precision estimating the final magnitude to within a single unit from the first few seconds of the p wave

real time earthquake location using kirchhoff reconstruction real time location of earthquakes can be achieved by using direct imaging of the recorded wave field based on a kirchhoff reconstruction method similar to that used in the migration of seismic reflection data the standard method of event location requires the wave arrival at each sensor to be picked and associated with an event by using direct imaging the event is identified once in the imaged wave field the computation is independent of the level of seismic activity and can be carried out on a typical desktop computer the procedure has been successfully demonstrated in two and three dimensions using data from the southern california seismic network trinet at higher resolutions the reconstruction method can identify finite source effects further work considers extending the method by implementing full elastic theory and solving for moment tensors at all locations in the mesh

seismic stereometry reveals preparatory behavior and source kinematics of intermediate size earthquakes although moderate size earthquakes are poorly studied by lack of near fault observations they can provide key information about larger damaging earthquakes here we propose a new approach inspired by double difference relocation that uses high coherency waveforms recorded at neighboring sensors to study the preparation phase and dynamics of moderate size earthquakes we validate this technique by analyzing the 2016 m w 5 2 borrego springs earthquake in southern california and find consistent rupture velocities of 2 km s highlighting two main rupture asperities the analysis of the 2019 ml5 2 le teil earthquake in france reveals slow nucleation at depth that migrates to the surface and propagates northward with a velocity of 2 8 km s highlighting two main rupture events also imaged by insar by providing unprecedented resolution in our observation of the rupture dynamics this approach will be useful in better understanding the preparation phase and rupture of both tectonic and induced earthquakes plain language summary small and moderate size earthquakes are much more numerous yet studied in much less detail than large and damaging earthquakes this is because to study small earthquakes precisely one needs seismometers very close to the rupture which is rarely the case it is still unclear if small and large earthquakes start the same way thus studying more systematically small earthquakes could help to answer this question here we show that we can use two close by seismometers as we would do with 3 d glasses to observe with a stereoscopic effect the evolution of intermediate size earthquakes when they rupture along a fault we can retrieve the direction of rupture the speed at which the fault breaks and the length of the rupture important parameters that help to characterize the earthquakes using this new method we can sometimes also observe the behavior of the fault before the main rupture which will be useful to better understand how earthquakes start

reverse time imaging of a doublet of microearthquakes in the three gorges reservoir region summary when a doublet of closely spaced microearthquakes occur nearly simultaneously their overlappingwaveformscomplicatethepickingoffirst arrivaltimesandchallengetraveltime based location methods to estimate the location and origin time of each event in the doublet the reverse time imaging rti method locates earthquakes by backpropagating the reconstructed wave fronts of p and s waves to the source locations thereby removing the need to pick first arrivals using an elastic rti method we successfully estimated the locations and origin times of a doublet of microearthquakes an ml0 1 event reported by local earthquake monitoring system and second microearthquake that occurred 0 7s earlier that was missed by the official monitoring network in the three gorges reservoir region china we introduce a time depth constrain to estimate the origin time and to evaluate how robust the imaged source location is with respect to possible errors in the velocity model for our case a model with slightly lower velocities will result in an earlier origin time but will have less effect on the estimated source depth results of this study suggest that by applying the rti method it is possible to locate doublets with overlapping first arrivals