a continuum of stress strength and slip in the cascadia subduction zone movement of the down going oceanic plate in subduction zones is accommodated by earthquakes slow slip and free slip with increasing depth analysis of accompanying tremor reveals a continuum of slow slip events in the cascadia subduction zone which suggests that deep free slip of the subducted plate may cause stress to be gradually transferred up the plate interface towards the seismogenic zone

accurate tremor locations from coherent s and p waves nonvolcanic tremor is an important component of the slow slip processes which load faults from below but accurately locating tremor has proven difficult because tremor rarely contains clear p or s wave arrivals here we report the observation of coherence in the shear and compressional waves of tremor at widely separated stations which allows us to detect and accurately locate tremor events an event detector using data from two stations sees the onset of tremor activity in the cascadia tremor episodes of february 2003 july 2004 and september 2005 and confirms the previously reported south to north migration of the tremor event detectors using data from three and four stations give sand p arrival times of high accuracy the hypocenters of the tremor events fall at depths of 30 to 40 km and define a narrow plane dipping at a shallow angle to the northeast consistent with the subducting plate interface the s wave polarizations and p wave first motions define a source mechanism in agreement with the northeast convergence seen in geodetic observations of slow slip tens of thousands of locations determined by constraining the events to the plate interface show tremor sources highly clustered in space with a strongly similar pattern of sources in the three episodes examined the deeper sources generate tremor in minor episodes as well the extent to which the narrow bands of tremor sources overlap between the three major episodes suggests relative epicentral location errors as small as 1 2 km

episodic slow slip events accompanied by non volcanic tremors in southwest japan subduction zone 1 episodic slow slip events have been recognized by means of tilt changes in the western shikoku area southwest japan the crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non volcanic deep tremor activities in this area observed tilt changes can be explained by slow slip events occurring around the source area of tremors in each episode the source of the slow slip event and tremor migrate simultaneously the spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate

variability in north atlantic marine radiocarbon reservoir effects at c ad 1000 c age measurements made on samples from three archaeological sites located on north atlantic coasts were used to investigate the marine reservoir effect mre at c ad 1000 this is an important period within human cultural and palaeoenvironmental research as it is a time when norse expansion to the north atlantic islands occurred during what appears to be a period of ameliorating climatic conditions this makes improved chronological precision and accuracy at this time highly desirable the data indicate a potential latitudinal variation in mre at c ad 1000 from a dr of 1429 16 14 c yr at omey island 538 32 n to 649 13 14 c yr at undir junkarinsflotti 618 51 n the results are compared with modern assessments of mre values within the context of oceanographic and climatic regimes that provide a possible driving mechanism for spatial and temporal variation in mre

array analysis and precise source location of deep tremor in cascadia 1 we describe a new method to estimate the s p time of tremor like signals and its application to the nonvolcanic tremor recorded in july 2004 by three dense arrays in cascadia the cross correlation between vertical and horizontal components indicates that very often the high amplitude tremor signal contains sequences of p and s waves characterized by constant s p times ts p in the range 3 5 7 s a detailed observation of the three component seismograms stacked over the array stations confirms the presence of p and s wave sequences the knowledge of the ts p poses a strong constrain on the source array distance which dramatically reduces the uncertainty on source locations when used with more traditional array processing techniques data were analyzed using the zero lag cross correlation technique zlcc to estimate the propagation properties of the most correlated phases in the wavefield detailed polarization analyses were computed using the covariance matrix method in the time domain polarization parameters joint with the results of zlcc allows for the discrimination between p and s coherent waves results show that the tremor wavefield is composed mostly by shear waves although a consistent amount of coherent p waves is often observable the comparison of the back azimuth at the three arrays indicate that the source of deep tremor migrates over a wide area and often many independent sources located far from each other are active at the same time the tremor source was located by a probabilistic method that uses the results of zlcc given a velocity model when available the inclusion of the ts p time in the location procedure strongly reduces the depth range with a distribution of hypocenters very near the subduction interface this result significantly different compared with previous less precise locations makes the cascadia nonvolcanic tremor more similar to the nonvolcanic tremor recorded in japan at least in cases of measurable ts p the polarization azimuth aligned with the slow slip direction and the source located on the plate interface indicate that deep tremor and slow slip are two different manifestations of a common phenomenon related with the subduction dynamics

on corner frequencies attenuation and low frequency earthquakes we have recently suggested that the nearly constant duration of low frequency earthquakes lfes and equivalently the band limitation of tectonic tremor manifests a moment duration scaling that is fundamentally different from regular earthquakes and is most easily explained as rupture on asperities of roughly constant dimension in that work we employed qualitative arguments against potential bias by attenuation here we examine the role of attenuation more quantitatively through an analysis that avoids specification of particular source e g brune models and relies on the particle velocity spectral maximum as the definition of apparent corner frequency the analysis leads to the formal definition of a saturation frequency as the limiting value of apparent corner frequency as the true corner frequency tends infinity the saturation frequency a formal equivalent to fmax can be used to set bounds on path averaged quality factor q we apply these relations to deep crustal and intraslab earthquakes beneath vancouver island to estimate bulk crustal attenuation parameters that are subsequently used to correct apparent corner frequency measurements of lfes reported in our earlier work the attenuation bias due to bulk crustal structure is shown to be small with negligible effect on the principal conclusions of that study however a review of laboratory and seismic refraction measurements of attenuation in oceanic basalts and evidence for high p to s lfe corner frequency ratios raises the possibility that strong highly localized near source attenuation accompanying high pore fluid pressures could cause the bandlimited nature of lfes through the depletion of high frequencies

application of 34s analysis for elucidating terrestrial marine and freshwater ecosystems evidence of animal movement husbandry practices in an early viking community around lake mývatn iceland carbon and nitrogen stable isotope ratios 13c and 15n have been used widely in archaeology to investigate palaeodiet sulphur stable isotope ratios 34s have shown great promise in this regard but the potential of this technique within archaeological science has yet to be fully explored here we report 34s 13c and 15n values for 129 samples of animal bone collagen from skutusta ir an early viking age landnam settlement in north east iceland this dataset represents the most comprehensive study to date of its kind on archaeological material and the results show a clear offset in 34s values between animals deriving their dietary resources from terrestrial mean 5 6 2 8 freshwater mean 2 7 1 4 or marine mean 15 9 1 5 reservoirs with the three food groups being significantly different at 2 this offset allows reconstruction of the dietary history of domesticated herbivores and demonstrates differences in husbandry practices and animal movement trade which would be otherwise impossible using only 13c and 15n values for example several terrestrial herbivores displayed enriched bone collagen 34s values compared to the geology of the lake m vatn region indicating they may have been affected by sea spray whilst being pastured closer to the coast before being traded inland additionally the combination of heavy 15n values coupled with light 34s values within pig bone collagen suggests that these omnivores were consuming freshwater fish as a significant portion of their diet arctic foxes were also found to be consuming large quantities of freshwater resources and radiocarbon dating of both the pigs and foxes confirmed previous studies showing that a large freshwater radiocarbon 14c reservoir effect exists within the lake overall these stable isotope and 14c data have important implications for obtaining a fuller reconstruction of the diets of the early viking settlers in iceland and may allow a clearer identification of the marine and or freshwater 14c reservoir effects that are known to exist in human bone collagen

the source scanning algorithm mapping the distribution of seismic sources in time and space summary we introduce a new method which we call the source scanning algorithm ssa for imaging the distribution of seismic sources in both time and space using trial locations and origin times the method calculates the brightness function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times the spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness the greatest advantages of this method are that 1 it exploits waveform information both arrival times and relative amplitudes without the need to calculate highfrequency synthetic seismograms and 2 it requires neither pre assembled phase picking data nor any a priori assumptions about the source geometry a series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval finally we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip ets sequence in the northern cascadia subduction zone

the rupture extent of low frequency earthquakes near parkfield ca the low frequency earthquakes lfes that constitute tectonic tremor are often inferred to be slow to have durations of 0 2 to 0 5 s a factor of 10 to 100 longer than those of typical m w 1 2 earthquakes here we examine lfes near parkfield ca in order to assess several proposed explanations for lfes long durations we determine lfe rupture areas and location distributions using a new approach similar to directivity analysis where we examine how signals coming from various locations within lfes finite rupture extents create differences in the apparent source time functions recorded at various stations we use synthetic ruptures to determine how much the lfe signals recorded at each station would be modified by spatial variations of the source station travel time within the rupture area given various possible rupture diameters and then compare those synthetics with the data our synthetics show that the methodology can identify inter station variations created by heterogeneous slip distributions or complex rupture edges and thus lets us estimate lfe rupture extents for unilateral or bilateral ruptures to obtain robust estimates of the sources similarity across stations we stack signals from thousands of lfes using an empirical green s function approach to isolate the lfes apparent source time functions from the path effects our analysis of lfes in parkfield implies that lfes apparent source time functions are similar across stations at frequencies up to 8 to 16 hz depending on the family the inter station coherence observed at these relatively high frequencies or short wave lengths down to 0 2 to 0 5 km suggest that lfes in each of the 7 families examined occur on asperities they are clustered in patches with sub 1 km diameters the individual lfes rupture diameters are estimated to be smaller than 1 1 km for all families and smaller than 0 5 km and 1 km for the two shallowest families which were previously found to have 0 2 s durations coupling the diameters with the durations suggests that it is possible to model these m w 1 2 lfes with earthquake like rupture speeds around 70 of the shear wave speed however that rupture speed matches the data only at the edge of our uncertainty estimates for the family with highest coherence the data for that family are better matched if lfes have rupture velocities smaller than 40 of the shear wave speed or if lfes have different rupture dynamics they could have long rise times contain composite sub ruptures or have slip distributions that persist from event to event

cascadia tremor located near plate interface constrained by s minus p wave times nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable across a seismic network an alternative approach to identifying specific phases is to measure the lag between the s and p waves we cross correlate vertical and horizontal seismograms to reveal signals common to both but with the horizontal delayed with respect to the vertical this lagged correlation represents the time interval between vertical compressional waves and horizontal shear waves measurements of this interval combined with location techniques resolve the depth of tremor sources within 2 kilometers for recent cascadia tremor the sources locate near or on the subducting slab interface strong correlations and steady s p time differences imply that tremor consists of radiation from repeating sources

a phase coherence approach to identifying co located earthquakes and tremor we present and use a phase coherence approach to identify seismic signals that have similar path effects but different source time functions co located earthquakes and tremor the method used is a phase coherence based implementation of empirical matched field processing modified to suit tremor analysis it works by comparing the frequency domain phases of waveforms generated by two sources recorded at multiple stations we first cross correlate the records of the two sources at a single station if the sources are co located this cross correlation eliminates the phases of the green s function it leaves the relative phases of the source time functions which should be the same across all stations so long as the spatial extent of the sources are small compared with the seismic wavelength we therefore search for cross correlation phases that are consistent across stations as an indication of co located sources we also introduce a method to obtain relative locations between the two sources based on back projection of interstation phase coherence we apply this technique to analyse two tremor like signals that are thought to be composed of a number of earthquakes first we analyse a 20 s long seismic precursor to a m 3 9 earthquake in central alaska the analysis locates the precursor to within 2 km of the mainshock and it identifies several bursts of energy potentially foreshocks or groups of foreshocks within the precursor second we examine several minutes of volcanic tremor prior to an eruption at redoubt volcano we confirm that the tremor source is located close to repeating earthquakes identified earlier in the tremor sequence the amplitude of the tremor diminishes about 30 s before the eruption but the phase coherence results suggest that the tremor may persist at some level through this final interval

radiocarbon reservoir effects in human bone collagen from northern iceland human bone collagen from a series of icelandic human pagan graves was radiocarbon 14c dated to aid understanding of early settlement landnam chronologies in northern iceland these individuals potentially consumed marine protein the 14c age of samples containing marine carbon requires a correction for the marine 14c reservoir effect the proportion of non terrestrial sample carbon was quantified via measurement of carbon stable isotopes 13c using a simple mixing model based on 13c measurements of archaeofaunal samples non terrestrial carbon was also quantified in six pig bones from the archaeofaunal dataset assuming all non terrestrial carbon in human and pig bone collagen was marine derived calibrated age ranges calculated using a mixed intcal09 marine09 calibration curve were consistent with an early settlement date close to landnam but several samples returned pre landnam age ranges measurements of nitrogen stable isotopes 15n strongly suggest that many of the human bone collagen samples contain freshwater diet derived carbon icelandic freshwater systems frequently display large freshwater 14c reservoir effects of the order of 10 000 14c years and we suggest that the presence of freshwater carbon is responsible for the anomalously early ages within our dataset in pig samples the majority of non terrestrial carbon is freshwater in origin but in human samples the proportion of freshwater carbon is within the error of the marine component 10 this presents a major obstacle to assessing temporal patterns in the ages of human remains from sampled graves although the majority of grave ages are within the same broad calibrated range

rapid continuous streaking of tremor in cascadia nonvolcanic tremor is a recently discovered weak seismic signal associated with slow slip on a fault plane and has potential to answer many questions about how faults move its spatiotemporal distribution however is complex and varies over different time scales and the causal physical mechanisms remain unclear here we use a beam backprojection method to show rapid continuous slip parallel streaking of tremor over time scales of several minutes to an hour during the may 2008 episodic tremor and slip event in the cascadia subduction zone the streaks propagate across distances up to 65 km primarily parallel to the slip direction of the subduction zone both updip and downdip at velocities ranging from 30 to 200 km h we explore mainly two models that may explain such continuous tremor streaking the first involves interaction of slowly migrating creep front with slip parallel linear structures on the fault the second is pressure driven fluid flow through structurally controlled conduits on the fault both can be consistent with the observed propagation velocities and geometries although the second one requires unlikely condition in addition we put this new observation in the context of the overall variability of tremor behavior observed over different time scales

array measurements of deep tremor signals in the cascadia subduction zone 1 preliminary analysis of deep tremor recorded during july 2004 in the cascadia subduction zone shows that small aperture arrays can resolve the slowness and back azimuth of seismic waves with a useful resolution data were collected by three dense arrays of short period seismometers specifically deployed in the puget sound area under an us italy canada cooperative effort slowness analyses at the three arrays indicate that the 2 4 hz tremor wave field is composed of waves propagating with apparent velocities higher than 4 km s combining this with polarisation analysis show these waves to be transverse sh waves however p waves though smaller in amplitude can be detected by different slowness values obtained for the radial and transverse components the intersection of wave vectors determined by the back azimuth and slowness values measured at the three arrays provides a preliminary estimate of source location for a sample of the recorded deep tremor

detecting and locating precursory signals during the 2011 el hierro canary islands submarine eruption pilar sanchez pastor acknowledges funding from the spanish ministry of economy and competitiveness project misterios cgl2013 48601 c2 2 r

low frequency earthquakes below southern vancouver island 1 the nature and distribution of low frequency earthquakes lfes in subduction zones provide insight into plate boundary deformation downdip of the locked seismogenic zone we employ network autocorrelation detection to identify lfe families beneath southern vancouver island and environs an initial suite of 5775 lfes detected in 2004 and 2005 at a select set of 7 stations is grouped into 140 families using waveform cluster analysis these families are used as templates within an iterative network cross correlation scheme to detect lfes across different tremor episodes incorporate new stations and improve lfe template signal to noise ratio as in southwest japan representative lfe locations define a relatively tight dipping surface several km above the locus of intraslab seismicity within a prominent dipping low velocity zone lvz lfe polarizations for near vertical source receiver geometries possess a remarkably uniform dipolar signature indicative of point source double couple excitation focal mechanisms determined fromp wave first motions are characterized by a combination of strike slip and thrust faulting we suggest that lfes and regular intraslab seismicity occur in distinct structural and stress regimes the lvz inferred to represent weak overpressured porous and mylonitized metabasalts of oceanic crustal layer 2 separates lfes manifesting deformation within a plate boundary shear zone from intraslab earthquakes generated by tensional stresses and dehydration embrittlement within a more competent lower oceanic crustal layer 3 and underlying mantle

cascadia subducting plate fluids channelled to fore arc mantle corner ets and silica deposition in this study we first summarize the constraints that on the cascadia subduction thrust there is a 70 km gap downdip between the megathrust seismogenic zone and the episodic tremor and slip ets that lies further landward there is not a continuous transition from unstable to conditionally stable sliding seismic rupture occurs mainly offshore for this hot subduction zone ets lies onshore we then suggest what does control the downdip position of ets we conclude that fluids from dehydration of the downgoing plate focused to rise above the fore arc mantle corner are responsible for ets there is a remarkable correspondence between the position of ets and this corner along the whole margin hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore arc mantle laboratory data show that such fore arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone at the fore arc mantle corner these fluids are released upward into the more permeable overlying fore arc crust an indication of this fluid flux comes from low poisson s ratios and vp vs found above the corner that may be explained by a concentration of quartz which has exceptionally low poisson s ratio the rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner

simplified stability analysis of strain softening slopes under drawdown conditions the progressive failure of slopes is often induced by drawdown and strain softening of the soil thus the stability level of a strain softening slope under drawdown conditions should be evaluated while considering progressive failure a simplified method was developed to analyze the stability of a strain softening slope based on a new algorithm for determining the subsidiary shear deformation under drawdown conditions the parameters of the method could be easily determined the predictions of the failure behavior of the slope that were obtained using the proposed method were close to the observations of centrifuge model tests under drawdown conditions thus the method was confirmed to be effective in analyzing the stability evolution of the strain softening slopes induced by the drawdown of water level this proposed method was used to evaluate the stability of a real world slope under drawdown conditions the results showed that there is a significant evolution in the safety factor of the slope as the water level decreases the safety factor of a slope is overestimated if the progressive failure behavior is not considered the strain softening behavior and initial water level have a significant effect on the safety factor and critical slip surface of the slope

imaging slow slip fronts in cascadia with high precision cross station tremor locations 1 we apply a new method to obtain accurate locations of tremor sources beneath southern vancouver island unlike more standard cross time methods which compare waveforms from different time windows at the same station this cross station method compares waveforms from the same time window at widely separated stations it performs well relative to cross time methods when the response to an impulsive tremor source is dominated by the main arrival rather than coda and when multiple colocated sources are active within the time window examined we focus on a region roughly 10 km across that was monitored by the polaris deployment from 2003 to 2006 relative location errors appear to be 1 km allowing us to image in great detail rapid and small scale tremor migrations that arise behind the main slow slip front the secondary fronts tend to a start at or within about 1 km of the main tremor front and propagate back along strike at rates of 10 20 km h b less commonly do the reverse ending at the main front or c propagate up or downdip at or within 1 2 km of the main front estimated stress drops in the secondary events are comparable to that in the main event implying that their 25 50 times greater propagation speed results from a similarly greater slip speed

a comparative study of low frequency earthquake templates in northern cascadia abstract using network correlation methods we generate low frequency earthquake templates for a set of 4 composite arrays on vancouver island and washington state that employ data from earthscope transportable and flexible arrays plate boundary observatory polaris and permanent network canadian national seismograph network pacific northwest seismic network sources on the basis of empirical and semi analytical arguments the templates can be viewed as green s function sections corresponding to moment tensor point sources with step function time dependence in displacement low frequency earthquake hypocentres follow the general epicentral distribution of tremor and occur along tightly defined surfaces in depth with washington locations averaging slightly deeper than those on vancouver island we invert template waveforms for moment tensor mechanisms and find that data are well modelled by double couple sources for southern vancouver island with the highest quality templates the majority of mechanisms are consistent with shallow thrusting in the direction of plate motion the three other data sets with lower signal to noise levels show predominantly thrust mechanisms with more variable orientations taken together with other constraints our observations support the hypothesis that low frequency earthquakes manifest shear slip on a relatively thin plate boundary

seismic and geodetic constraints on cascadia slow slip 1 automatically detected and located tremor epicenters from episodic tremor and slip ets episodes in northern cascadia provide a high resolution map of washington s slow slip region thousands of epicenters from the past four ets events from 2004 to 2008 provide detailed map view constraints that correlate with geodetic estimates of the simultaneous slow slip each of these ets events exhibits remarkable similarity in the timing and geographic distribution of tremor density and geodetically inferred slip analysis of the latest 15 month inter ets period also reveals ageodetic tremor activity similar both in duration and extent to ets tremor epicenters from both ets and inter ets tremor are bounded between the 30 and 45 km plate interface depth contours and locate approximately 75 km east of previous estimates of the locked portion of the subducting juan de fuca plate inter ets tremor overlaps but is generally downdip of ets tremor and does not yet correlate with geodetically observed slip but this is likely because the slip is below current gps detection levels based on the tremor and slip correlation and the tremor duration and slip magnitude relationship we suggest that the well resolved sharp updip edge of tremor epicenters reflects a change in plate interface coupling properties the region updip of this boundary may accumulate stress with the potential for coseismic shear failure during a megathrust earthquake alternatively plate convergence in this region could be accommodated by continuous slow slip with no detectable tremor or by slow slip events with sufficiently long recurrence intervals that none have been detected during the past 10 years of gps observations

nonvolcanic tremors deep beneath the san andreas fault we have discovered nonvolcanic tremor activity i e long duration seismic signals with no clear p or s waves within a transform plate boundary zone along the san andreas fault near cholame california the inferred epicentral region of the 1857 fort tejon earthquake moment magnitude 7 8 the tremors occur between 20 to 40 kilometers depth below the seismogenic zone the upper 15 kilometers of earth s crust where earthquakes occur and their activity rates may correlate with variations in local earthquake activity

precise tremor source locations and amplitude variations along the lower crustal central san andreas fault 1 we precisely locate 88 tremor families along the central san andreas fault using a 3d velocity model and numerous p and s wave arrival times estimated from seismogram stacks of up to 400 events per tremor family maximum tremor amplitudes vary along the fault by at least a factor of 7 with by far the strongest sources along a 25 km section of the fault southeast of parkfield we also identify many weaker tremor families which have largely escaped prior detection together these sources extend 150 km along the fault beneath creeping transitional and locked sections of the upper crustal fault depths are mostly between 18 and 28 km in the lower crust epicenters are concentrated within 3 km of the surface trace implying a nearly vertical fault a prominent gap in detectible activity is located directly beneath the region of maximum slip in the 2004 magnitude 6 0 parkfield earthquake

an autocorrelation method to detect low frequency earthquakes within tremor 1 recent studies have shown that deep tremor in the nankai trough under western shikoku consists of a swarm of low frequency earthquakes lfes that occur as slow shear slip on the down dip extension of the primary seismogenic zone of the plate interface the similarity of tremor in other locations suggests a similar mechanism but the absence of cataloged low frequency earthquakes prevents a similar analysis in this study we develop a method for identifying lfes within tremor the method employs a matched filter algorithm similar to the technique used to infer that tremor in parts of shikoku is comprised of lfes however in this case we do not assume the origin times or locations of any lfes a priori we search for lfes using the running autocorrelation of tremor waveforms for 6 hi net stations in the vicinity of the tremor source time lags showing strong similarity in the autocorrelation represent either repeats or near repeats of lfes within the tremor we test the method on an hour of hi net recordings of tremor and demonstrates that it extracts both known and previously unidentified lfes once identified we cross correlate waveforms to measure relative arrival times and locate the lfes the results are able to explain most of the tremor as a swarm of lfes and the locations of newly identified events appear to fill a gap in the spatial distribution of known lfes this method should allow us to extend the analysis of shelly et al 2007a to parts of the nankai trough in shikoku that have sparse lfe coverage and may also allow us to extend our analysis to other regions that experience deep tremor but where lfes have not yet been identified

high resolution imaging of rapid tremor migrations beneath southern vancouver island using cross station cross correlations we develop a cross station method to detect and locate tremor and low frequency earthquakes lfes based on the original work of armbruster et al 2014 that compares waveforms from the same time window at stations separated by roughly 10 km to improve the signal to noise ratio we first rotate the horizontal components into the empirical shear wave particle motion direction the large scale rapid tremor reversals beneath southern vancouver island are best recorded by stations that exhibit pronounced shear wave splitting which obscures this optimal direction we correct for splitting using the stacked templates of 11 low frequency earthquake families obtained in this region by bostock et al 2012 we find that the style of rapid tremor migrations rtms evolves as the main front passes over a region very close to the main front numerous small scale migrations occur with recurrence intervals far shorter than tidal periods these usually propagate along the main front even when that is not parallel to dip several larger rtms propagating along the main front have prominent elongation orthogonal to the propagation direction inconsistent with the interpretation that their large propagation speed is an apparent velocity caused by the slow main front intersecting a preexisting linear structure on the plate interface farther behind the main front rtms gradually progress to being tidally modulated and have generally slower propagation speeds many reversal like rtms are observed some of which evolve from fronts initially propagating along the main front these reversals are sometimes coherent across regions of low tremor density

spatial temporal patterns of seismic tremors in northern cascadia 1 we study in detail the two consecutive episodic tremor and slip ets events that occurred in the northern cascadia subduction zone during 2003 and 2004 for both sequences the newly developed source scanning algorithm ssa is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors in map view the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30 km and 50 km depth contours of the plate interface the horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km d in cross section tremors in both sequences span a depth range of over 40 km across the interface with the majority occurring in the overriding continental crust in particular 50 55 of them are located within 2 5 km from the strong seismic reflector bands above the plate interface the lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences the source spectra of tremors clearly lack high frequency content 5 hz relative to local earthquakes we propose two possible models to explain the relationship between slip and tremors the first one regards ets tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone in the second model tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface neither model can be dismissed conclusively at this stage

temporal and spatial occurrence of deep non volcanic tremor from washington to northern california 1 regional network data reveal deep non volcanic tremor occurs in five independent sections along the u s section of the cascadia subduction zone with different recurrence intervals sizeable tremor episodes occur during periods with no measurable geodetic deformation as well as coincident with episodic slow slip events and the displacement amplitudes of the tremor with and without geodetic deformation are similar the tremor depths cover a wide range that does not coincide directly with the subducting slab interface the tremor can migrate bi directionally from the point of initiation and more than one spatially distinct tremor source can be active at a given time these results indicate that the tremor is not directly related to the slow slip event instead we suggest that the tremor is related to the presence and movement of fluids at depth which can explain the locations and episodic occurrence as well as the seismic signal characteristics themselves namely the limited frequency content small size emergent onsets and prolonged durations with and without geodetic deformation

automated detection and location of cascadia tremor 1 a new autonomous seismic location and detection methodology enables real time opportunities for high resolution spatio temporal monitoring of non volcanic tremor combining a unique cross correlation technique with epicenter clustering analysis in northern cascadia automatically yields thousands of tremor epicenters from the may 2008 episodic tremor and slip ets event while routinely detecting and locating inconspicuous inter ets tremor bursts although ets events in this area produce about two weeks of continuous tremor we find a nearly equal amount of tremor during the last 15 month inter ets period the resulting ets and inter ets epicenters occur in the slow slip region where the plate interface is 30 45 km deep and have a sharp well resolved updip boundary about 75 km east of the downdip edge of the seismogenic megathrust zone this ability to track tremor with high spatio temporal resolution facilitates automatic tremor monitoring and the mapping of the transition zone and regions of locked zone stress accumulation

testing small aperture array analysis on well located earthquakes and application to the location of deep tremor we have here analyzed local and regional earthquakes using array tech niques with the double aim of quantifying the errors associated with the estimation of propagation parameters of seismic signals and testing the suitability of a probabilistic location method for the analysis of nonimpulsive signals we have applied the zero lag cross correlation method to earthquakes recorded by three dense arrays in puget sound and vancouver island to estimate the slowness and back azimuth of direct p waves and s waves the results are compared with the slowness and back azimuth computed from the source location obtained by the analysis of data recorded by the pacific northwest seismic network pnsn this comparison has allowed a quanti fication of the errors associated with the estimation of slowness and back azimuth obtained through the analysis of array data the statistical analysis gives bp 10 and bs 8 as standard deviations for the back azimuth and sp 0 021 sec km and ss 0 033 sec km for the slowness results of the p and s phases respec tively these values are consistent with the theoretical relationship between slowness and back azimuth and their uncertainties we have tested a probabilistic source location method on the local earthquakes based on the use of the slowness estimated for two or three arrays without taking into account travel time information then we applied the probabilistic method to the deep nonvolcanic tremor recorded by the arrays during july 2004 the results of the tremor location using the probabilistic method are in good agreement with those obtained by other techniques thewide depth range of between 10 and 70 km and the source migration with time are evident in our results the method is useful for locating the source of signals characterized by the absence of pickable seismic phases

split from slip and schist crustal anisotropy beneath northern cascadia from non volcanic tremor 1 polarization and splitting analyses of minute long windows of non volcanic tremor at 3 component stations in the vicinity of southern vancouver island provide new evidence for the presence of anisotropy within the crustal portion of the north american plate beneath cascadia as reported previously tremor particle motions are predominantly horizontal and inferred to manifest an upgoing wavefield composed primarily of s waves under this assumption estimates of incidence angle and back azimuth can be deduced from orientation of the smallest principle direction of motion when subjected to standard s wave splitting analysis most stations yield systematic and reproducible fast polarization directions estimates of original polarization direction are more scattered and splitting times often exhibit multimodal distributions numerical simulations wherein band limited synthetic seismograms comprising contributions from multiple sources are analyzed for splitting neatly reproduce these characteristics and thereby provide a framework for interpretation a number of stations in our study sit astride the san juan fault which based on reflection studies dips approximately 60 to 70 to the north underlying these stations is the leech river complex consisting of strongly foliated greenschist facies phyllites with steeply dipping foliations striking parallel to fast polarization directions of split s waves detailed laboratory velocity measurements at elevated pressures show that as little as 2 to 3 km of vertically dipping leech river phyllite are required to produce the observed splitting these phyllites have some of the highest s wave anisotropies measured to date and are part of a once continuous belt of anisotropic crust extending over 2000 km along the western north american coast from southern vancouver island to southwest alaska

cascadia tremor polarization evidence for plate interface slip 1 new seismic measurements of the repeated phenomenon of episodic tremor and slip in northern cascadia indicate identical source processes of tremor and slow slip predicted polarization directions of upgoing s waves radiated from shear slip on the plate interface align with the relative motion between the juan de fuca and north american plates seismic observations from small aperture array data on the olympic peninsula of the cascadia subduction zone show uncharacteristically stable linear particle motion coincident with the passage of tremor sources beneath the array the azimuth of this horizontal ground motion matches expected polarizations from slip on the plate interface this finding suggests that cascadia tremor is shear slip on the plate interface implying that as in japan geodetically observed slow slip and seismically observed tremor are manifestations of the same phenomenon

intermittent tremor migrations beneath guerrero mexico and implications for fault healing within the slow slip zone slow slip events exhibit significant complexity in slip evolution and variations in recurrence intervals behavior that varies systematically with recurrence interval is likely to reflect different extents of fault healing between these events here we use high resolution tremor catalogs beneath guerrero mexico to investigate the mechanics of slow slip we observe complex tremor propagation styles including rapid tremor migrations propagating either along the main tremor front or backward reminiscent of those in northern cascadia we also find many migrations that originate well behind the front and repeatedly occupy the same source region during a tremor episode similar to those previously reported from shikoku japan these migrations could be driven by slow slip in the surrounding regions with recurrence intervals possibly modulated by tides the propagation speed of these migrations decreases systematically with time since the previous migration over the same source area tremor amplitudes seem consistent with changes in the propagation speeds being controlled primarily by changes in the slip speeds one interpretation is that the high propagation speeds and inferred high slip speeds during the migrations with short recurrence intervals are caused by incomplete healing within the host rock adjacent to the shear zone which could lead to high permeability and reduced dilatant strengthening of the fault gouge similar processes may operate in other slow slip source regions such as cascadia

islands of change vs islands of disaster managing pigs and birds in the anthropocene of the north atlantic the offshore islands of the north atlantic were among some of the last settled places on earth with humans reaching the faroes and iceland in the late iron age and viking period while older accounts emphasizing deforestation and soil erosion have presented this story of island colonization as yet another social ecological disaster recent archaeological and paleoenvironmental research combined with environmental history environmental humanities and bioscience is providing a more complex understanding of long term human ecodynamics in these northern islands an ongoing interdisciplinary investigation of the management of domestic pigs and wild bird populations in faroes and iceland is presented as an example of sustained resource management using local and traditional knowledge to create structures for successful wild fowl management on the millennial scale

resolving the detailed spatiotemporal slip evolution of deep tremor in western japan we study the detailed spatiotemporal behavior of deep tremor in western japan through the development and application of a new slip inversion method although many studies now recognize tremor as shear slip along the plate interface manifested in low frequency earthquake lfe swarms a conventional slip inversion analysis is not available for tremor due to insufficient knowledge of source locations and green s functions here we introduce synthetic template waveforms which are typical tremor waveforms obtained by stacking lfe seismograms at arranged points along the plate interface using these synthetic template waveforms as substitutes for green s functions we invert the continuous tremor waveforms using an iterative deconvolution approach with bayesian constraints we apply this method to two tremor burst episodes in western and central shikoku japan the estimated slip distribution from a 12 day tremor burst episode in western shikoku is heterogeneous with several patchy areas of slip along the plate interface where rapid moment releases with durations of 100 s regularly occur we attribute these heterogeneous spatiotemporal slip patterns to heterogeneous material properties along the plate interface for central shikoku where we focus on a tremor burst episode that occurred coincidentally with a very low frequency earthquake vlf we observe that the source size of the vlf is much larger than that estimated from tremor activity in western shikoku these differences in the size of the slip region may dictate the visibility of vlf signals in observed seismograms which has implications for the mechanics of slow earthquakes and subduction zone processes