a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

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

moment release rate of cascadia tremor constrained by gps 1 a comparison of gps and seismic analyses of 23 distinct episodic tremor and slip events located throughout the cascadia subduction zone over an 11 year period yields a highly linear relationship between moment release as estimated from gps and total duration of nonvolcanic tremor as summed from regional seismic arrays the events last 1 5 weeks typically produce 5 mm of static forearc deformation and show cumulative totals of tremor that range from 40 to 280 h moment released by each event is estimated by inverting gps measured deformation which is sensitive to all rates of tremor synchronous faulting including aseismic creep for total slip along the north american juan de fuca plate interface tremor which is shown to be largely invariant in amplitude and frequency content both between events and with respect to its duration is quantified using several different parameterizations that agree to within 10 all known cascadia events detected since 1997 which collectively span the cascadia arc from northern california to vancouver island canada release moment during tremor at a rate of 5 2 0 4 1016 n m per hour of recorded tremor this relationship enables estimation of moment dissipation via seismic monitoring of tremor along the deeper cascadia subduction zone that poses the greatest threat to its major metropolitan centers

mechanism of deep low frequency earthquakes further evidence that deep non volcanic tremor is generated by shear slip on the plate interface 1 we study the mechanism of low frequency earthquakes lfes in the nankai trough in western shikoku japan precise locations have previously suggested that they represent shear slip on the plate boundary in this paper we examine the mechanism of these events due to the low signal to noise ratio we analyze stacked lfe waveforms and compare them with the waveforms of nearby earthquakes of known mechanism within the subducting philippine sea plate analysis of both the focal mechanism using p wave first motions and the moment tensor using s waveforms indicates that lfes represent shear slip on a low angle thrust fault dipping to the northwest namely the plate interface together with reports that deep tremor consists of a swarm of lfes our results suggest that deep tremor is generated directly by shear slip on the plate interface and as such represents a seismic signature of the accompanying slow slip events

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

imbricated slip rate processes during slow slip transients imaged by low frequency earthquakes low frequency earthquakes lfes often occur in conjunction with transient strain episodes or slow slip events sses in subduction zones their focal mechanism and location consistent with shear failure on the plate interface argue for a model where lfes are discrete dynamic ruptures in an otherwise slowly slipping interface sses are mostly observed by surface geodetic instruments with limited resolution and it is likely that only the largest ones are detected the time synchronization of lfes and sses suggests that we could use the recorded lfes to constrain the evolution of sses and notably of the geodetically undetected small ones however inferring slow slip rate from the temporal evolution of lfe activity is complicated by the strong temporal clustering of lfes here we apply dedicated statistical tools to retrieve the temporal evolution of sse slip rates from the time history of lfe occurrences in two subduction zones mexico and cascadia and in the deep portion of the san andreas fault at parkfield we find temporal characteristics of lfes that are similar across these three different regions the longer term episodic slip transients present in these datasets show a slip rate decay with time after the passage of the sse front possibly as t 1 4 they are composed of multiple short term transients with steeper slip rate decay as t with between 1 4 and 2 we also find that the maximum slip rate of sses has a continuous distribution our results indicate that creeping faults host intermittent deformation at various scales resulting from the imbricated occurrence of numerous slow slip events of various amplitudes

very low frequency earthquakes within accretionary prisms are very low stress drop earthquakes 1 we report characteristic p wave spectra and stress drops of very low frequency vlf earthquakes that occurred within the accretionary prism along the nankai trough southwestern japan although many vlf earthquakes showed no distinct phases on a raw seismogram a few showed slightly distinct phases the arrival times of some phases were consistent with the calculated p wave arrival times the corner frequencies of the vlf earthquakes were calculated using the stacked p wave spectra assuming an omega square spectrum stress drops were very low i e in the range of 0 1 10 kpa which corresponds to 0 1 1 of those of ordinary earthquakes such extremely low stress drops of the vlf earthquakes suggest that the fault strength within the accretionary prism may have weakened due to the existence of a fluid within the thrust fault system of the accretionary prism

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

reconsidering earthquake scaling the relationship scaling between scalar moment m0 and duration t potentially provides key constraints on the physics governing fault slip the prevailing interpretation of m0 t observations proposes different scaling for fast earthquakes and slow mostly aseismic slip populations and thus fundamentally different driving mechanisms we show that a single model of slip events within bounded slip zones may explain nearly all fast and slow slip m0 t observations and both slip populations have a change in scaling where the slip area growth changes from 2 d when too small to sense the boundaries to 1 d when large enough to be bounded we present new fast and slow slip m0 t observations that sample the change in scaling in each population which are consistent with our interpretation we suggest that a continuous but bimodal distribution of slip modes exists and m0 t observations alone may not imply a fundamental difference between fast and slow slip

geophysics mediating plate convergence movement of earth s tectonic plates generates low frequency seismic events these events are not background noise but represent slower and deeper earthquakes

relation between mainshock rupture process and omori s law for aftershock moment release rate summary we compare the source time functions i e moment release rates of three large california mainshocks with the seismic moment release rates during their aftershock sequences aftershock moment release rates computed by summing aftershock moments in time intervals follow a power law time dependence similar to omori s law from minutes to months after the mainshock furthermore in contrast to the previously observed saturation in numbers of aftershocks shortly after the mainshock rupture no such saturation is seen in the aftershock moment release rates which are dominated by the largest aftershocks we argue that the observed saturation in aftershock numbers described by the time offset parameter c in omori s law is likely an artefact due to the underreporting of small aftershocks which is related to the difficulty of detecting large numbers of small aftershocks in the mainshock coda we further propose that it is more natural for c to be negative i e singularity follows the onset of mainshock rupture than positive singularity precedes onset of rupture to make a more general comparison of mainshock rupture process and aftershock moment rates we then scale mainshock time functions to equalize the effects of the varied seismic moments for the three california mainshocks we compare the scaled time functions with similarly scaled aftershock moment rates finally we compare global averages of scaled time functions of many shallow events to the average scaled aftershock moment release rate for six california mainshocks in each of these comparisons the extrapolation using omori s law of the aftershock moment rates back in time to the onset of the mainshock rupture indicates that the temporal intensity of the aftershock moment release is about 1 5 orders of magnitude less than the maximum reached by the mainshock rupture this may be due to the differing amplitudes and relative importance of static and dynamic stresses in aftershock initiation compared to mainshock rupture propagation

variations in earthquake size distribution across different stress regimes the earthquake size distribution follows in most instances a power law1 2 with the slope of this power law the b value commonly used to describe the relative occurrence of large and small events a high b value indicates a larger proportion of small earthquakes and vice versa statistically significant variations of b values have been measured in laboratory experiments mines and various tectonic regimes such as subducting slabs near magma chambers along fault zones and in aftershock zones3 however it has remained uncertain whether these differences are due to differing stress regimes as it was questionable that samples in small volumes such as in laboratory specimens mines and the shallow earth s crust are representative of earthquakes in general given the lack of physical understanding of these differences the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b 1 is a universal constant for earthquakes in general5 here we show that the b value varies systematically for different styles of faulting we find that normal faulting events have the highest b values thrust events the lowest and strike slip events intermediate values given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress

multi event simulations of slow slip events for a cascadia like subduction zone 1 we employ the earthquake simulator rsqsim which incorporates rate and state dependent friction to investigate characteristics of slow slip events sses along a cascadia like mega thrust the simulations consist of 100 000 sses with equivalent moment magnitudes mw4 0 7 0 the largest simulated sses mw6 4 7 0 have inter event times of 19 months durations of 10 40 days mean slips of 2 2 4 1 cm and along strike propagation speeds of 7 20 km day which are comparable to observations from cascadia the simulations show quiescence after mw 6 4 sses followed by a progressive increase in both the magnitude and frequency of sses prior to the next large event small sses below mw5 6 develop in an incoherent manner and have irregular geometries while larger sses show highly coherent growth of the slip region and spontaneous but transient event to event segmentation the change in event characteristics at mw5 6 corresponds to a break in the scaling of seismic moment with slip fault area and duration

interplay of seismic and aseismic deformations during earthquake swarms an experimental approach observations of earthquake swarms and slow propagating ruptures on related faults suggest a close relation between the two phenomena earthquakes are the signature of fast unstable ruptures initiated on localized asperities while slow aseismic deformations are experienced on large stable segments of the fault plane the spatial proximity and the temporal coincidence of both fault mechanical responses highlight the variability of fault rheology however the mechanism relating earthquakes and aseismic processes is still elusive due to the difficulty of imaging these phenomena of large spatiotemporal variability at depth here we present laboratory experiments that explore in great detail the deformation processes of heterogeneous interfaces in the brittle creep regime we track the evolution of an interfacial crack over 7 orders of magnitude in time and 5 orders of magnitude in space using optical and acoustic sensors we explore the response of the system to slow transient loads and show that slow deformation episodes are systematically accompanied by acoustic emissions due to local fracture energy disorder features of acoustic emission activities and deformation rate distributions of our experimental system are similar to those in natural faults on the basis of an activation energy model we link our results to the rate and state friction model and suggest an active role of local creep deformation in driving the seismic activity of earthquake swarms

interface locking along the subduction megathrust from b value mapping near nicoya peninsula costa rica 1 to understand controlling factors for seismogenesis and strain accumulation in subduction megathrusts we examine seismicity patterns across nicoya costa rica to determine the overall and spatial variability in the earthquake frequency magnitude distribution along the interface the mean reduction in earthquake activity with magnitude b value is higher 1 06 than global subduction zone averages 0 6 0 8 suggesting the interface here is weakly coupled strong spatial variations in b are anticorrelated 0 53 with geodetic estimates of interface locking of e norabuena et al 2004 high b prevails in two regions including the subducted fisher seamount chain a broad zone of reduced b is observed at and offshore the central nicoya coast extending towards an imaged locked patch these results suggest b value studies may be useful in identifying regions of increased interface locking in subduction zones which may indicate regions capable of large slip in future large earthquakes

an earthquake like magnitude frequency distribution of slow slip in northern cascadia 1 major episodic tremor and slip ets events with mw 6 4 to 6 7 repeat every 15 2 months within the cascadia subduction zone under the olympic peninsula although these major ets events are observed to release strain smaller tremor swarms without detectable geodetic deformation are more frequent an automatic search from 2006 2009 reveals 20 000 five minute windows containing tremor which cluster in space and time into 96 tremor swarms the 93 inter ets tremor swarms account for 45 of the total duration of tremor detection during the last three ets cycles the number of tremor swarms n exceeding duration follow a power law distribution n 0 66 if duration is proportional to moment release the slip inferred from these swarms follows a standard gutenberg richter logarithmic frequency magnitude relation with the major ets events and smaller inter ets swarms lying on the same trend this relationship implies that 1 inter ets slip is fundamentally similar to the major events just smaller and more frequent and 2 despite fundamental differences in moment duration scaling the slow slip magnitude frequency distribution is the same as normal earthquakes with a b value of 1

spatiotemporal variations in the b value of earthquake magnitude frequency distributions classification and causes abstract interpretation of the b value of earthquake frequency magnitude distributions has received considerable attention in recent decades this paper provides a comprehensive review of previous investigations of spatial and temporal variations in b value including their classification and possible causes based on least squares regression of seismicity data compiled from the neic iris and isc catalogs we find an average value of 1 02 0 03 for the whole earth and its two hemispheres consistent with the general view that in seismically active regions the long term average value is close to unity nevertheless wide ranging b variations 0 3 b 2 5 have been reported in the literature this variability has been interpreted to arise from one or more of the following factors prevailing stress state crustal heterogeneity focal depth pore pressure geothermal gradient tectonic setting petrological environmental geophysical characteristics clustering of events incomplete catalog data and or method of calculation excluding the latter all of these factors appear to be linked directly or indirectly with the effective state of stress although time dependent changes in b value are well documented conflicting observations reveal either a precursory increase or decrease in b value before major earthquakes our compilation of published analyses suggests that statistically significant b variations occur globally on various timescales including annual monthly and perhaps diurnal taken together our review suggests that b variations are most plausibly linked with changes in effective stress

dispersal in the ordovician speciation patterns and paleobiogeographic analyses of brachiopods and trilobites abstract the middle to late ordovician was a time of profound biotic diversification paleoecological change and major climate shifts yet studies examining speciation mechanisms and drivers of dispersal are lacking in this study we use bayesian phylogenetics and maximum likelihood analyses in the r package biogeobears to reanalyze ten published data matrices of brachiopods and trilobites and produce time calibrated species level phylogenetic hypotheses with estimated biogeographic histories recovered speciation and biogeographic patterns were examined within four time slices to test for changes in speciation type across major tectonic and paleoclimatic events statistical model comparison showed that biogeographic models that incorporate long distance founder event speciation best fit the data for most clades which indicates that this speciation type along with vicariance and traditional dispersal were important for paleozoic benthic invertebrates speciation by dispersal was common throughout the study interval but notably elevated during times of climate change vicariance events occurred synchronously among brachiopod and trilobite lineages indicating that tectonic climate and ocean processes affected benthic and planktotrophic larvae similarly middle ordovician inter oceanic dispersal in trilobite lineages was influenced by surface currents along with volcanic island arcs acting as stepping stones between areas indicating most trilobite species may have had a planktic protaspid stage these factors also influenced brachiopod dispersal across oceanic basins among laurentia avalonia and baltica these results indicate that gyre spin up and intensification of surface currents were important dispersal mechanisms during this time within laurentia surface currents hurricane tracks and upwelling zones controlled dispersal among basins increased speciation during the middle ordovician provides support for climatic facilitators for diversification during the great ordovician biodiversification event similarly increased speciation in laurentian brachiopod lineages during the hirnantian indicates that some taxa experienced speciation in relation to major climate changes overall this study demonstrates the substantial power and potential for likelihood based methods for elucidating biotic patterns during the history of life

periodic slow earthquakes from the cascadia subduction zone continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking and they are thus known as slow or silent earthquakes because subduction zones generate the largest earthquakes

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

bridging the gap between seismically and geodetically detected slow earthquakes 1 recently observed unusual seismic events deep low frequency earthquakes and tremor very low frequency earthquakes and slow slip events form a family of slow earthquakes that are governed by a scaling law different from ordinary earthquakes guided by this scaling law we have observed previously unknown events with source duration of 20 200 s and moment magnitude of 3 4 under the kii peninsula in western japan these events radiate seismic energy in the 2 8 hz band in direct proportion to their seismic moment rate though the constant of proportionality is 4 5 orders of magnitude smaller than for ordinary earthquakes slow earthquakes in this region may be comprised of numerous shear slip events which in aggregate are manifest seismically as tremor or isolated longer events up to 200 s and geodetically as slow slip events

slow slip events and seismic tremor at circum pacific subduction zones 1 it has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements however only recently has a fuller spectrum of strain release processes including normal slow and silent earthquakes or slow slip events and continuous and episodic slip been observed and generated by numerical simulations of the earthquake cycle despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive the concurrence of seismic tremor with slow slip episodes in cascadia and southwestern japan provides insight into the process of slow slip a perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface alternatively evidence is accumulating to support the notion that tremor results from shear failure during slow slip global observations of the location spatial extent magnitude duration slip rate and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation most slow slip events occur just downdip of the seismogenic zone consistent with rate and state dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation at a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault bringing it closer to or farther from earthquake failure respectively this paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide with a focus on circum pacific subduction zones trends in global observations of slow slip events suggest that 1 slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it 2 different frictional properties likely control fast versus slow slip 3 the depth range of slow slip may be related to the thermal properties of the plate interface and 4 the equivalent seismic moment of slow slip events is proportional to their duration mo different from the mo 3 scaling observed for earthquakes

scaling relationship between the duration and the amplitude of non volcanic deep low frequency tremors 1 we investigate a duration amplitude relation of non volcanic deep low frequency dlf tremors in the tokai region southwest japan to constrain the source process of the tremors we apply two models to the distribution one is an exponential model as a scale bound distribution and the other a power law model as a scale invariant distribution the exponential model shows a better fit to the duration amplitude distribution of the tremors than a power law model implying that the dlf tremors are caused by a scale bound source process the source process of the dlf tremors therefore differs from those for earthquakes we suggest that the non volcanic dlf tremor is possibly caused by a fixed source dimension with variable excess pressure of fluid or variable stress drop

similar scaling laws for earthquakes and cascadia slow slip events faults can slip not only episodically during earthquakes but also during transient aseismic slip events1 5 often called slow slip events previous studies based on observations compiled from various tectonic settings6 8 have suggested that the moment of slow slip events is proportional to their duration instead of following the duration cubed scaling found for earthquakes9 this finding has spurred efforts to unravel the cause of the difference in scaling6 10 14 thanks to a new catalogue of slow slip events on the cascadia megathrust based on the inversion of surface deformation measurements between 2007 and 201715 we find that a cubic moment duration scaling law is more likely like regular earthquakes slow slip events also have a moment that is proportional to a3 2 where a is the rupture area and obey the gutenberg richter relationship between frequency and magnitude finally these slow slip events show pulse like ruptures similar to seismic ruptures the scaling properties of slow slip events are thus strikingly similar to those of regular earthquakes suggesting that they are governed by similar dynamic properties a new catalogue of slow slip events on the cascadia megathrust shows that a cubic moment duration scaling law is likely with scaling properties strikingly similar to regular earthquakes

universality of the seismic moment frequency relation we analyze the seismic moment frequency relation in various depth ranges and for different seismic regions using flinn engdahl s regionalization of global seismicity three earthquake lists of centroid moment tensor data have been used the harvard catalog the usgs catalog and the huang et al 1997 catalog of deep earthquakes the results confirm the universality of the values and the maximum moment for shallow earthquakes in continental regions as well as at and near continental boundaries moreover we show that although fluctuations in earthquake size distribution increase with depth the values for earthquakes in the depth range of 0 500 km exhibit no statistically significant regional variations the regional variations are significant only for deep events near the 660 km boundary for declustered shallow earthquake catalogs and deeper events we show that the worldwide values have the same value of 0 60 0 02 this finding suggests that the value is a universal constant we investigate the statistical correlations between the numbers of seismic events in different depth ranges and the correlation of the tectonic deformation rate and seismic activity the number of earthquakes above a certain threshold level per year the high level of these correlations suggests that seismic activity indicates tectonic deformation rate in subduction zones combined with the universality of the value this finding implies little if any variation in maximum earthquake seismic moment among various subduction zones if we assume that earthquakes of maximum size are similar in different depth ranges and the seismic efficiency coefficient is close to 100 for shallow seismicity then we can estimate for deeper earthquakes for intermediate earthquakes 5 and 1 for deep events these results may lead to new theoretical understanding of the earthquake process and better estimates of seismic hazard

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

noble gases bulk chemistry and petrography of olivine rich achondrites eagles nest and lewis cliff 88763 comparison to brachinites eagles nest and lewis cliff lew 88763 are olivine rich achondrites that have been suggested to be related to brachina and alh 84025 both are ultramafic although lew 88763 has 10 plagioclase which eagles nest lacks olivine in eagles nest fo68 is virtually identical in composition to that in brachina fo68 70 and that in lew 88763 is only slightly different fo63 64 the meteorites cosmic ray exposure ages differ from each other and from brachina and alh 84025 like brachina eagles nest has a substantial amount of radiogenic 129xe and an apparent k ar age of 4 5 ga lewis cliff 88763 on the other hand has no detectable radiogenic 129xe an apparent k ar age of 4 5 ga and a substantial amount of trapped xe the chemical composition of lew 88763 is within a factor of two of chondritic for virtually every element analyzed including siderophiles which suggests that it was melted or at least heated and recrystallized in a closed system analyses of the chemical composition of eagles nest are hampered by extensive terrestrial weathering but it appears to be broadly chondritic also although it like brachina is depleted in siderophiles our data suggest that eagles nest is genetically related to brachina but that lew 88763 is not which is a conclusion that is consistent with the o isotopic data of clayton and mayeda 1996

slow slip hidden in the noise the intermittence of tectonic release referred to as slow slip events the transient aseismic slip that occurs along plate boundaries can be indirectly characterized through colocated seismicity such as tectonic tremor and low frequency earthquakes lfes using the timing of cataloged lfe and tremor activity in guerrero mexico and northern cascadia i decompose the inter aseismic gps displacement defined as the surface deformation between previously detected slow slip events into separate regimes of tectonic loading and release in such a way previously undetected slow slip events that produce less than a millimeter of surface deformation are extracted from the geodetic noise these new observations demonstrate that the inter aseismic period is not quiescent and that slow slip occurs much more often than previously thought this suggests that the plate interface where slow slip and tremor occur is in fact strongly coupled and that slow aseismic release occurs over a wide spectrum of time scales

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

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

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses