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

seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

seismic imaging of subduction zone metamorphism combined analysis of high resolution seismic images of the alaska and cascadia subduction zones reveals where metamorphic fl uids are released both images show the subducted oceanic crust as a dipping low velocity layer with a clear termination depth however in alaska the crust is thicker 15 20 km compared to 8 km and terminates at greater depth 120 km com pared to 40 km than in cascadia based on metamorphic reaction estimates and geodynamic models we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water bearing minerals and that the location of this reaction is dependent on the thermal structure of the subducted slab

reply to comment on geophysical approach to the study of a periglacial blockfield in a mountain area ztracene kameny eastern sudetes czech republic by stan et al 2017 geomorphology 293 380 390 abstract the authors present the answer to the discussion paper presented by uxa et al 2018 the remarks provided will explain ambiguities regarding the use of geophysical methods used to detect ice in typically nonconducive environments that are potentially unfavourable to its occurrence namely the quartzite block cover ztracene kameny 1245 m asl the authors discuss the most important remarks regarding ice formation and preservation discussion points focus on the following 1 permafrost in low altitude blockfields and related environmental settings 2 geophysical interpretation 3 extreme quartz quartzite resistivities hypotheses and 4 additional comments

recurrence behavior of short term slow slip and correlated nonvolcanic tremor episodes in western shikoku southwest japan 1 from the national research institute for earth science and disaster prevention hi net tiltmeter data we determine time dependent source processes of seven repeating short term slow slip events sse in the western shikoku region southwest japan over the period from 2002 to 2007 the tiltmeters record clear signals of sses characterized by a slow rise a week long duration and a magnitude on the order of 10 2 10 1 rad we apply a time dependent slip inversion method to the tilt records for the seven sses for each sse a slip area migrates in the strike direction of the subducting slab at a rate of 10 km d 1 comparing the slow slip area with epicenters of accompanying nonvolcanic tremors in subday time resolution we find that the two phenomena are located on a smaller area than previously reported and migrate as a group a moment rate function for each sse exhibits good correlation with the temporal change in tremor activity a comparison of the slip distributions of all of the analyzed sses shows that there is a patch of slow slip area 30 km in diameter that is shared by all of the episodes in addition the location of the slip patch is in good agreement with the epicenters of tremors and very low frequency earthquakes these lines of evidence suggest that the identified slip patch is one of the main slow slip patches in the region and a recurrence of sses may be controlled by the patch and that these slow earthquakes might be different manifestations of a single slip process on the deep plate interface

seismic properties and mineral crystallographic preferred orientations from ebsd data results from a crustal scale detachment system aegean region abstract the crystallographic preferred orientations cpos were measured on a suite of samples representative of different structural depths along the west cycladic detachment system greece electron backscatter diffraction ebsd analyses were conducted on calcitic and mica schists impure quartzites and a blueschist and the average seismic properties of the rocks were calculated with the voigt reuss hill average of the single minerals elastic stiffness tensor the calcitic and quartzitic rocks have p and s wave velocity anisotropies avp avs averaging 8 1 and 7 1 respectively the anisotropy increases with depth represented by the blueschist with avp averaging 20 3 and avs averaging 14 5 due to the content of aligned glaucophane and mica which strongly control the seismic properties of the rocks localised anisotropies of very high magnitudes are caused by the presence of mica schists as they possess the strongest anisotropies with values of 25 for avp and avs the direction of the fast and slow p wave velocities occurs parallel and perpendicular to the foliation respectively for most samples the fast propagation has the same ne sw orientation as the lithospheric stretching direction experienced in the cyclades since the late oligocene the maximum shear wave anisotropy is subhorizontal similarly concordant with mineral alignment that developed during extension in the aegean radial anisotropy in the aegean mid crust is strongly favoured to azimuthal anisotropy by our results

the 5 september 2012 nicoya costa rica mw 7 6 earthquake rupture process from joint inversion of high rate gps strong motion and teleseismic p wave data and its relationship to adjacent plate boundary interface properties 1 on 5 september 2012 a large thrust earthquake mw 7 6 ruptured a densely instrumented seismic gap on the shallow dipping plate boundary beneath the nicoya peninsula costa rica ground motion recordings directly above the rupture zone provide a unique opportunity to study the detailed source process of a large shallow megathrust earthquake using very nearby land observations hypocenter relocation using local seismic network data indicates that the event initiated with small emergent seismic waves from a hypocenter 10 km offshore 13 km deep on the megathrust a joint finite fault inversion using high rate gps strong motion ground velocity recordings gps static offsets and teleseismic p waves reveals that the primary slip zone slip 1 m is located beneath the peninsula the rupture propagated downdip from the hypocenter with a rupture velocity of 3 0 km s the primary slip zone extends 70 km along strike and 30 km along dip with an average slip of 2 m the associated static stress drop is 3 mpa the seismic moment is 3 5 1020 nm giving mw 7 6 the coseismic large slip patch directly overlaps an onshore interseismic locked region indicated by geodetic observations and extends downdip to the intersection with the upper plate moho at deeper depths below the upper plate moho seismic tremor and low frequency earthquakes have been observed most tremor locates in adjacent areas of the megathrust that have little coseismic slip a region of prior slow slip deformation to the southeast also has no significant coseismic slip or aftershocks an offshore locked patch indicated by geodetic observations does not appear to have experienced coseismic slip and aftershocks do not overlap this region allowing the potential for a comparable size rupture offshore in the future

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

elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the nankai trough subduction megathrust 1 recent seismic reflection and ocean bottom seismometer obs studies reveal broad regions of low seismic velocity along the nankai subduction plate boundary megathrust offshore sw japan these low velocity zones lvz s extend 55 km landward from the trench corresponding to depths of 10 km below sea floor here we estimate the in situ pore pressure and stress state within these lvz s by combining p wave velocities obtained from the geophysical surveys with new well constrained empirical relations between p wave velocity porosity and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment we document excess pore pressures of 17 87 mpa that extend 55 km into the subduction zone indicating that trapped pore fluids support 45 91 of the overburden stress along the base of the upper plate and surrounding major fault zones the resulting effective stresses in the lvz are limited to 1 3 of the values expected for non overpressured conditions these low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary the region of lowest effective stress coincides with precisely located very low frequency earthquakes providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure

thermal regime of the costa rican convergent margin 2 thermal models of the shallow middle america subduction zone offshore costa rica at the costa rica margin along the middle america trench along strike variations in heat flow are well mapped these variations can be understood in terms of either ventilated fluid flow where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer we model fluid flow within the subducting aquifer using nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust the sensitivity of these models to the initial thermal state of the plate and styles of fluid flow either ventilated or insulated is explored heat flow measurements on cool crust accreted at the east pacific rise are consistent with ventilated hydrothermal cooling that continues with subduction these models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation heat flow transects on warm crust accreted at the cocos nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100 c and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust

variety and spatial heterogeneity of tectonic tremor worldwide 1 deep tectonic tremor occurs at various sites worldwide and the source characteristics are heterogeneous even at small scales in this study tremor sources were determined using data sets of seismic waveforms from various locations some of which are not well recognized as being a site of tremor activity the regions of interest are subduction zones at nankai and kyushu japan cascadia in western north america mexico southern chile and new zealand tremor locations are consistent with the geometry of subducting plates and tremor depth tends to be shallower where younger plate is subducting tremor duration shows a negative correlation with tidal sensitivity as reported previously in western shikoku in the nankai subduction zone the duration is also related to the width of the tremor zone in the dip direction of the subducting plate linear structures resembling striations are observed in hypocenter distributions in western shikoku and in the probability of short term migration direction these spatial heterogeneities in tremor activity may be controlled by the heterogeneous frictional properties of the downgoing slab due to long term plate subduction thereby providing information on the maturity of the plate interface

nonvolcanic tremor observed in the mexican subduction zone nonvolcanic tremor nvt activity is revealed as episodes of higher spectral amplitude at 1 8 hz in daily spectrograms from the continuous seismological records in guerrero mexico the analyzed data cover a period of 2001 2007 when in 2001 2002 a large slow slip event sse had occurred in the guerrero oaxaca region and then a new large sse occurred in 2006 the tremor burst is dominated by s waves more than 100 strong nvt bursts were recorded in the narrow band of 40 150 km 2 to the south of iguala city and parallel to the coastline depths of nvt hypocenters are mostly scattered in the continental crust between 5 and 40 km depth tremor activity is higher during the 2001 2002 and 2006 sse compared with that for the quiet period of 2003 2005 while resistivity pattern in guerrero does not correlate directly with the nvt distribution gravity and magnetic anomaly modeling favors a hypothesis that the nvt is apparently related to the dehydration and serpentinization processes

a tremor and slip event on the cocos caribbean subduction zone as measured by a global positioning system gps and seismic network on the nicoya peninsula costa rica in may 2007 a network of global positioning systems gps and seismic stations on the nicoya peninsula of northern costa rica recorded a slow slip event accompanied by seismic tremor the close proximity of the nicoya peninsula to the seismogenic part of the cocos caribbean subduction plate boundary makes it a good location to study such events several centimeters of southwest motion were recorded by the gps stations over a period of several days to several weeks and the seismic stations recorded three distinct episodes of tremor during the same time span inversion of the surface displacement data for the depth and pattern of slip on the plate interface shows peak slip at a depth of 25 30 km downdip of the main seismogenic zone estimated temperatures here are 250 300 c lower than in other subduction zones where events of this nature have been previously identified there may also be a shallower patch of slip at 6 km depth these results are significant in that they are the first to suggest that slow slip can occur at the updip transition from stick slip to stable sliding and that a critical temperature threshold is not required for slow slip tremor and low frequency earthquake locations are more difficult to determine our results suggest they occur on or near the plate interface at the same depth range as the deep slow slip but not spatially colocated

flat slab thermal structure and evolution beneath central mexico recent seismic and magnetotelluric experiments aimed at better characterizing the shape and state of the subducting slab and continental crust beneath central mexico exposed significant differences with conclusions of previous studies a new slab geometry is revealed in which the subducting cocos slab is perfectly flat between 120 to 290 km from the trench after which it plunges into the asthenosphere at a dip angle of 65 in sharp contrast with the previously proposed 20 dip angle seismic tomography studies show negative p wave velocity anomalies 2 to 4 in the mantle wedge beneath the mexican volcanic belt and positive anomalies 2 to 3 for the subducted cocos slab magnetotelluric experiments exposed a very low resistivity area 1 10 m located within the continental crust just below the mexican volcanic arc finally several spots of non volcanic tremors nvts have been recorded inside the continental crust above the flat slab segment while all these experiments provide a better picture of the subduction system beneath central mexico several key processes need further investigation in this study we take advantage of these new observations to better constrain the thermal structure beneath central mexico two different thermal models are computed for a mantle potential temperature t p of 1 350 and 1 450 c respectively the new thermal structures are then converted into p wave velocity anomalies and compared with the observed v p anomalies we found that a t p of 1 450 c produced larger v p anomalies that do not fit the observations however using a t p of only 1 350 c our predicted v p anomalies are positive 2 to 3 for the cold slab and negative 2 to 4 in the mantle wedge these v p estimates are consistent with the observed seismic tomography from p wave arrivals and therefore we conclude that a t p of 1 350 c is a better estimate for the mantle potential temperature beneath central mexico the new thermal model in conjunction with phase diagrams for sediments hydrated basalt and lithospheric mantle have been used to estimate the amount and location of fluids released from the subducting cocos slab several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat slab we found a good correlation between the pattern of these dehydration pulses and the location of nvts suggesting that slab dehydration is responsible for triggering the tremors we suggest that nvt bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening a process that has been going on since 15 ma ago when the cocos slab entered into a flat slab regime such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15 ma allowing the slab to easily roll back the continuous low resistivity region recorded beneath the volcanic front in central mexico might represent the evidence of slab dehydration and crust weakening over time

a slow earthquake sequence on the san andreas fault earthquakes typically release stored strain energy on timescales of the order of seconds limited by the velocity of sound in rock over the past 20 years observations1 13 and laboratory experiments14 have indicated that rupture can also occur more slowly with durations up to hours such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones the detection of events with larger timescales requires near field deformation measurements in december 1992 two borehole strainmeters close to the san andreas fault in california recorded a slow strain event of about a week in duration and we show here that the strain changes were produced by a slow earthquake sequence equivalent magnitude 4 8 with complexity similar to that of regular earthquakes the largest earthquakes associated with these slow events were small local magnitude 3 7 and contributed negligible strain release the importance of slow earthquakes in the seismogenic process remains an open question but these observations extend the observed timescale for slow events by two orders of magnitude

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

subducting slab ultra slow velocity layer coincident with silent earthquakes in southern mexico great earthquakes have repeatedly occurred on the plate interface in a few shallow dipping subduction zones where the subducting and overriding plates are strongly locked silent earthquakes or slow slip events were recently discovered at the down dip extension of the locked zone and interact with the earthquake cycle here we show that locally observed converted sp arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern mexico reveal that the ultra slow velocity layer usl varies spatially 3 to 5 kilometers with an s wave velocity of 2 0 to 2 7 kilometers per second most slow slip patches coincide with the presence of the usl and they are bounded by the absence of the usl the extent of the usl delineates the zone of transitional frictional behavior

the synchronous occurrence of shallow tremor and very low frequency earthquakes offshore of the nicoya peninsula costa rica 1 the occurrence of transient shallow slow slip at seismogenic zones has important implications for earthquake and tsunami hazards here we provide evidence that a tremor and slow slip event occurred at shallow depth offshore of the nicoya peninsula costa rica in august 2008 the temporal coincidence of offshore tremor very low frequency earthquakes vlfes motions consistent with slow slip on the plate interface on western coastal gps stations and a pressure transient in an iodp borehole all indicate slow slip occurring at shallow depths large ocean loading stresses on the shallow plate interface modulate tremor activity with the peak coulomb stress forced by semi diurnal ocean tides correlating with tremor productivity based on beamforming data we constrain that the vlfe activity occurs in the same region as the tremor and slow slip the presence of slow slip at shallow depth has important implications for the up dip extent of earthquake rupture the proximity of the 5 september 2012 mw 7 6 megathrust earthquake to slow slip tremor and vlfe activity in the 2008 event suggests abrupt frictional transitions from locked to conditionally stable behavior on the plate interface offshore of the nicoya peninsula

a time dependent glaciological model of the weichselian ice sheet a time dependent model is used to study the evolution of the weichselian ice sheet the results indicate three stages of the glaciation a mountain centered ice sheet between 110 000 and 80 000 bp followed by a phase with an extension similar to or slightly larger than the younger dryas zone the last and maximum extent was reached about 30 000 20 000 bp and the main deglaciation began about 15 000 bp the model is based on a finite element method solution of the continuity equation the climatic records from the camp century and summit ice cores are used for climate control and the model is calibrated using geological deglaciation evidence in scandinavia the assumption of a general sliding condition for elevations below 100 m with an enhanced sliding zone through the center of the baltic sea and gulf of bothnia yield broad agreement for marginal positions in sweden and finland during a simulated termination

non volcanic tremor associated with the march 2010 gisborne slow slip event at the hikurangi subduction margin new zealand 1 non volcanic tremor accompanies slow slip events at most circum pacific subduction zones previously the hikurangi subduction margin new zealand and the boso peninsula of japan have been noted as exceptions where slow slip which occurs at shallow depth compared to other subduction zones does not generate seismic tremor the lack of tremor observations raises questions whether tremor and slow slip are manifestations of the same process and whether different temperature and pressure conditions influence the physical mechanism of tremor production analysis of seismic data during the march 2010 gisborne slow slip event new zealand reveals that this slow slip event at the northern hikurangi margin is accompanied by tremor with distinctive characteristics compared to local microearthquakes both the gisborne area of new zealand and the boso peninsula of japan are underlain by thick accumulations of sedimentary rocks that likely inhibit the observation of low amplitude tremor due to high attenuation discovery of tremor in new zealand suggests that tremor may always accompany slow slip and that its apparent absence at the boso peninsula the hikurangi margin prior to this study and any future regions discovered to experience slow slip without concurrent tremor may be the effect of highly attenuating sediments

crustal deformation measurements in guerrero mexico 1 gps measurements of crustal deformation in guerrero southern mexico include surveys collected between 1992 and 2001 as well as continuous gps measurements at a few sites these geodetic observations are used to calculate interseismic deformation rates and assess the presence and possible location of transient deformation during the period encompassing 1992 25 to 2001 75 the data are used to examine transient deformation in 1998 previously described from data at a single site by lowry et al 2001 survey measurements and continuous data from a site near popocatepetl volcano confirm the 1998 transient and survey data also suggest another transient occurred following the 14 september 1995 mw 7 3 copala earthquake all of the available gps position estimates have been inverted for a combined model of slip during each event plus the steady state slip on the plate interface modeling of the steady state deformation rates confirms that the guerrero seismic gap is partially frictionally locked at depths shallower than about 25 km and accumulating strain that may eventually be released in a great earthquake the data also suggest that there is frictional coupling to much greater 40 km depths which releases more frequently in aseismic slip events the locations and sizes of the transient events are only partially constrained by the available data however the transient models which best fit the gps coordinate time series suggest that aseismic slip was centered downdip of the seismogenic portion of the plate bounding thrust in both events and the moment release had equivalent magnitudes mw 7 1 1 3 1 0 in 1995 1996 and 7 1 0 4 0 1 in 1998

imaging the source region of cascadia tremor and intermediate depth earthquakes the subduction of hydrated oceanic crust releases volatiles that weaken the plate boundary interface trigger earthquakes and regulate transient phenomena such as episodic tremor and slip ets it is not clear how dehydration can separately induce earthquakes within the subducting plate and ets partly because few data exist on their relationship to subduction zone structures we present results of a seismic experiment in the washington cascades united states that images a region producing both earthquake types migration of scattered teleseis mic waves provides images of low velocity subducting crust at depths 40 45 km with sharp boundaries above and below it the sharp upper boundary indicates a layer of weak sediment or an overpressured fault zone that terminates abruptly downdip at 40 45 km depth regular earthquakes are at the top of the mantle within the downgoing plate everywhere the plate is 95 km deep but ets only exists where the sharp upper boundary occurs the ets location supports models of slow slip that require near lithostatic fluid pressure whereas regular earthquakes nucleate closer to the origin of metamorphic dehydration very low shear stresses on the plate boundary may limit seismicity to ets and similar phenomena

low frequency earthquakes at the southern cascadia margin we use seismic waveform data from the mendocino experiment to detect low frequency earthquakes lfes beneath northern california during the april 2008 tremor and slip episode in southern cascadia 59 templates were generated using iterative network cross correlation and stacking and grouped into 34 distinct lfe families the main front of tremor epicenters migrates along strike at 9 km d 1 we also find one instance of rapid tremor reversal observed to propagate in the opposite direction at 10 20 km h 1 as in other regions of cascadia lfe hypocenters from this study lie several kilometers above a recent plate interface model south of cascadia lfes were discovered on the maacama and bucknell creek faults the bucknell creek fault may be the youngest fault yet observed to host lfes these fault zones also host shallow earthquake swarms with repeating events that are distinct from lfes in their spectral and recurrence characteristics

localized seismic anisotropy associated with long term slow slip events beneath southern mexico 1 slow slip events sses are recently discovered in the transition zone downdip of the locked zone where megathrust earthquakes repeatedly occur at subduction zone interface structural changes from the locked zone to the transition zone and the nature of in situ deformation in the transition zone however remain unclear we model teleseismic converted p to s waves to investigate seismic structural transition and anisotropy associated with shear deformation near the deep locked zone and the transition zone beneath southern mexico the deep locked zone appears to be consistent with normal oceanic crust velocity in the transition zone we find that seismic anisotropy larger than 5 is associated with the ultra slow velocity layer usl near the top of the subducted oceanic crust the foliation plane is plunging 20 10 degrees steeper than the plate interface and it is consistent with crystallographic preferred orientation developed in s c mylonites we suggest that long term sses likely coincide with a near lithostatic semiductile talc rich shear zone

high vp vs ratio saturated cracks or anisotropy effects 1 we measured vp vs ratios of thermally cracked westerly granite thermally cracked carrara marble and 4 porosity fontainebleau sandstone for an effective mean pressure ranging from 2 to 95 mpa samples were fluidsaturated alternatively with argon gas and water 5 mpa constant pore pressure the experimental results show that at ultrasonic frequencies vp vs ratio of water saturated specimen never exceeded 2 15 even at effective mean pressure as low as 2 mpa or for a lithology for which the poisson s ratio of minerals is as high as 0 3 calcite in order to check these results against theoretical models we examine first a randomly oriented cracked medium with dispersion but without anisotropy and second a medium with horizontally aligned cracks with anisotropy but without dispersion the numerical results show that experimental data agree well with the first model at high frequency vp vs ratios range from 1 6 to 1 8 in the dry case and from 1 6 to 2 2 in the saturated case the second model predicts both vp sv and vp sh to vary from 1 2 to 3 5 depending on the raypath angle relative to the crack fabric in addition perpendicular to the crack fabric a high vp vs ratio is predicted in the absence of shear wave splitting from these results we argue the possibility that high vp vs ratio 2 2 as recently imaged by seismic tomography in subduction zones may come from zones presenting important crack anisotropy the cumulative effects of crack anisotropy and high pore fluid pressure are required to get vp vs ratios above 2 2 citation wang x q a schubnel j fortin e c david y gueguen and h k ge 2012 high vp vs ratio saturated cracks or anisotropy effects geophys res lett 39 l11307 doi 10 1029 2012gl051742

tidal modulation of nonvolcanic tremor episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of japan and cascadia in cascadia such episodes typically last a few weeks and differ from normal earthquakes in their source location and moment duration scaling the three most recent episodes in the puget sound southern vancouver island portion of the cascadia subduction zone were exceptionally well recorded in each episode we saw clear pulsing of tremor activity with periods of 12 4 and 24 to 25 hours the same as the principal lunar and lunisolar tides this indicates that the small stresses associated with the solid earth and ocean tides influence the genesis of tremor much more effectively than they do the genesis of normal earthquakes because the lithostatic stresses are 10 5 times larger than those associated with the tides we argue that tremor occurs on very weak faults

possible relationship between nonvolcanic tremor and the 1998 2001 slow slip event south central alaska 1 we document the occurrence of nonvolcanic tremor nvt in south central alaska for 3 months of each year between 1999 and 2001 to provide a first order attempt to understand the relationship between nvt and a large slow slip event that occurred over a 2 to 3 year period between 1998 and 2001 the majority of the nvt signals observed after the onset of the slow slip event are bursts lasting between 10 and 15 min with frequencies ranging from 1 to 6 hz by the summer of 2001 the quantity of nvt has decreased dramatically from the previous summers and episodes rarely last up to 10 min the decrease in nvt activity in the summer of 2001 is coincident with the end of the slow slip event there is a clear spatial correlation between the observed nvt and the slow slip event with most of the nvt events located on the downdip edge of the region of highest slip depth estimates range widely over the region with approximately half concentrated near the interpreted north america yakutat pacific plate interface and the remainder located above the plate interface however owing to inadequate station coverage and poor knowledge of the local structure determining accurate depths is difficult and thus the relationship between the nvt and the plate interface is problematic this is the first study giving evidence that the relationship between slow slip events and nvt seen in cascadia and southwest japan may also exist in alaska

nature of the low velocity zone in cascadia from receiver function waveform inversion abstract a dipping low velocity zone lvz is a ubiquitous structural element of subduction zones worldwide in this study we map seismic attributes characterizing the lvz beneath the cascadia subduction zone from northern vancouver island to northern california using receiver function waveform inversion throughout this region the lvz is characterized by high v p v s ratios mean 2 77 strong s velocity contrasts 50 and thicknesses averaging 3 37 km the lvz is immediately underlain by a second weaker layer exhibiting moderate v p v s ratios mean 1 85 with mean thickness of 4 62 km we interpret the combined structure in terms of subducting oceanic crust based on classical structural petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores the lvz is identified with pervasively hydrated high porosity pillow basalts and sheeted dikes of layer 2 with possible contributions from sediments layer 1 fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above and a low porosity gabbroic mafic cumulate dominated layer 3 below

persistent tremor within the northern costa rica seismogenic zone 1 we identify tremor using a spectral detection method and characterize its occurrence over a period of four years 2006 2009 in the vicinity of the nicoya peninsula costa rica although a few major tremor events accompanied by geodetic slow slip occur much of the tremor record consists of minor episodes with short duration and no detectable geodetic slip its persistent occurrence suggests that some portion of the interface is experiencing slow slip nearly continuously driving small patches to fail in accelerated slip locations indicate that much of the tremor occurs at shallow depth in freely slipping regions of the seismogenic zone this result is significant in that locations of slow slip and tremor at other subduction zones are largely limited to the downdip frictional transition tremor locations may help to refine the heterogeneous distribution of locked and freely slipping patches within the costa rica seismogenic zone

the 2006 slow slip event and nonvolcanic tremor in the mexican subduction zone the last decade featured an explosive sequence of discoveries of slow slip events sse and nonvolcanic tremor nvt in different subduction zones and continental faults many observations show that sse is usually associated with an increased nvt activity but it is not clear yet if those events are the result of the same process or are independent expressions of a common underlying seismotectonic source a large sse in central mexico occurred in 2006 during the meso american subduction experiment mase which provided continuous observations of the nvt for the years 2005 2007 gps and abundant seismic data show that although the nvt energy increased notably during the 2006 sse the two phenomena were separated spatially and not completely synchronized in time significant nvt episodes occur during the period between sses suggesting again that large slow slip events and nvt observed in the mexican subduction zone are of different origins the results presented here contribute to uncovering the nature of these two separate phenomena that have been indistinguishable in some other regions

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

hydrologic control of forearc strength and seismicity in the costa rican subduction zone the subduction zone beneath costa rica experiences infrequent large earthquakes in its northwestern part whereas slow slip dominates in the southeast seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour implying that spatial gradients in fluid content may control subduction zone seismicity

identifying low frequency earthquakes in central cascadia using cross station correlation abstract we use stations in the willamette valley and oregon coast ranges to identify low frequency earthquakes that occurred during the august september 2009 episodic tremor and slip event while autocorrelation techniques are often successful at identifying lfe templates in central cascadia this technique systematically fails due to the lack of densely spaced high quality stations instead we use cross station methods that have been successful at identifying lfes in northern cascadia to register initial candidate templates network cross correlation to register additional lfe detections and stacking to refine the identified templates using this procedure we detect nearly 16 thousand events comprising a total of 18 lfe families located in central oregon between 30 and 40 km depth either at or near the plate boundary the time history of detections between families is consistent with the slip front in the 2009 sse migrating from north to south at a velocity of 5 km day the templates we identify have more complicated waveforms than those previously identified in northern cascadia these differences in waveform character likely a consequence of the small number of contributing stations

sahke geophysical transect reveals crustal and subduction zone structure at the southern hikurangi margin new zealand 1 the seismic array hikurangi experiment sahke investigated the structure of the forearc and subduction plate boundary beneath the southern north island along a 350 km transect tomographic inversion of first arrival travel times was used to derive a well resolved 15 20 km deep p wave image of the crust the refracted phases and migrated reflection events image subducting slab geometry and crustal structure in the west australian plate moho depth decreases westward across the taranaki fault system from 35 to 28 30 km in the east subducted pacific plate oceanic crust is recognized to have a positive velocity gradient but becomes less distinct beneath the tararua ranges where the interface increases in dip at about 15 km depth from 15 this bend in the subducted plate is associated with vertical clusters in seismicity splay fault branching and low velocity high attenuation material that we interpret to be an underplated subduction sedimentary channel we infer that a step down in the decollement transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the tararua ranges reflections from the wairarapa fault show that it is listric and soles into the top of underplated sediments which in turn abut the moho of the overriding plate at 32 km depth near the downdip end of the strongly locked zone the change in dip of the hikurangi subduction interface is spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface

the 2006 aseismic slow slip event in guerrero mexico new results from gps 1 crustal deformation measurements from guerrero mexico were made with continuous gps instrumentation this network spans 75 km along the coast and a 275 km transect from the coast to mexico city which is perpendicular to the middle america trench a large aseismic slip transient occurred from april to december 2006 yielding horizontal displacements of nearly 6 cm in the direction opposite to that recorded interseismically this transient slip episode closely follows a series of m5 deep intraplate events that occurred in the northwestern portion of the guerrero region both horizontal and vertical displacements are inverted for the guerrero gps network to determine slip along the subduction interface using a simple fault model that accommodates changes in the subduction zone geometry it is found that slip is concentrated on interior fault patches and propagated to the southeast this slip event has a minimum equivalent mw of 7 5

deep low frequency earthquakes in tectonic tremor along the alaska aleutian subduction zone 1 we characterize and locate tremor not associated with volcanoes along the alaska aleutian subduction zone using continuous seismic data recorded by the alaska volcano observatory and the alaska earthquake information center from 2005 to present visual inspection of waveform spectra and time series reveal dozens of 10 to 20 min bursts of tremor along the length of the alaska aleutian subduction zone we use autocorrelation to demonstrate that these tremor signals are composed of hundreds of repeating low frequency earthquakes lfes the tremor activity we characterize is localized in four segments from east to west kodiak island shumagin gap unalaska and andreanof islands although the geometry age thermal structure frictional and other relevant properties of the alaska aleutian subduction zone are poorly known these characteristics are likely to differ systematically from east to west locations near kodiak island are the most reliable because station coverage is more complete lfe hypocenters in this region are located on the plate interface near the down dip limit of the 1964 mw 9 2 alaska earthquake rupture area lfe hypocenters in the remaining areas along the arc are also located down dip of the most recent mw 8 megathrust earthquakes although these locations are less well constrained our results support the hypothesis that tremor activity marks the down dip rupture limit for great megathrust earthquakes in this subduction zone lastly there is no correlation between the presence of tremor and particular aspects of over riding or subducting plate geology or coupling it appears that lfes are a fundamental characteristic of the alaska aleutian subduction zone

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

dynamic deformation of the accretionary prism excites very low frequency earthquakes 1 we have detected anomalous very low frequency earthquakes within the accretionary prism along the nankai trough southwestern japan centroid moment tensor inversion analysis reveals that the earthquake hypocenters are distributed at 10 km depth above the upper surface of the subducting philippine sea plate and within 50 70 km landward of the trough axis the focal mechanisms indicate reverse faulting their hypocenters are distributed beneath a deformation zone of an accretionary prism in sea floor topography these observations suggest that the occurrence of very low frequency earthquakes is related to numerous reverse fault systems within the accretionary prism and that the earthquakes reflect the dynamics of deformation within this accretionary prism

seismic imaging of the cocos plate subduction zone system in central mexico broadband data from the meso america subduction experiment mase line in central mexico were used to image the subducted cocos plate and the overriding continental lithosphere beneath central mexico using a generalized radon transform based migration our images provide insight into the process of subducting relatively young oceanic lithosphere and its complex geometry beneath continental north america the converted and reverberated phase image shows complete horizontal tectonic underplating of the cocos oceanic lithosphere beneath the north american continental lithosphere with a clear image of a very thin low velocity oceanic crust 7 8 km which dips at 15 20 degrees at acapulco then flattens approximately 300 km from the middle america trench farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the trans mexican volcanic belt tmvb where the slab bends underneath the tmvb the migrated image depicts the transition from subducted oceanic moho to continental moho at 230 km from the coast neither of which were clearly resolved in previous seismic images the deeper seismic structure beneath the tmvb shows a prominent negative discontinuity fast to slow at 65 75 km within the upper mantle this feature which spans horizontally beneath the arc 100 km may delineate the top of a layer of ponded partial melt

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

episodic slow slip events and rate and state friction 1 there are several ways of generating episodic slow slip events in models of rate and state friction here i explore the possibility that they arise on velocity weakening faults whose length is tuned in some sense unlike spring block sliders which have a unique critical stiffness for instability elastically deformable faults have multiple length scales stiffnesses relevant to nucleation slow slip events occur when the fault is large enough to initiate an event but too small for that event to reach dynamic instability the available window of lengths depends upon the effective fracture energy of the growing nucleation zone for the aging evolution law this fracture energy increases rapidly with slip speed and the range of permissible lengths increases nearly as b b a where a and b are the coefficients of the velocity and state dependence of the frictional strength this range becomes very large for faults that are near velocity neutral a b however existing lab data firmly favor the slip law as a predictor of nucleation style and for this law the effective fracture energy increases much less rapidly with slip speed the range of fault lengths capable of hosting slow slip events in this case seems too small to explain why they are common to so many subduction zones slow slip can be stabilized over an exceedingly large range of fault lengths if the fracture energy increases with slip speed much more rapidly than for the aging law an appealing mechanism is inelastic dilation of the fault zone coupled with pore pressure reduction and diffusive recovery

structure and composition of the plate boundary slip zone for the 2011 tohoku oki earthquake the mechanics of great subduction earthquakes are influenced by the frictional properties structure and composition of the plate boundary fault we present observations of the structure and composition of the shallow source fault of the 2011 tohoku oki earthquake and tsunami from boreholes drilled by the integrated ocean drilling program expedition 343 and 343t logging while drilling and core sample observations show a single major plate boundary fault accommodated the large slip of the tohoku oki earthquake rupture as well as nearly all the cumulative interplate motion at the drill site the localization of deformation onto a limited thickness less than 5 meters of pelagic clay is the defining characteristic of the shallow earthquake fault suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes

teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs a review abstract more than a decade after the discovery of deep episodic slow slip and tremor or slow earthquakes at subduction zones much research has been carried out to investigate the structural and seismic properties of the environment in which they occur slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner where three major structural elements are in contact the subducting oceanic crust the overriding forearc crust and the continental mantle in this region thermo petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions and their consumption by hydrating the mantle wedge these fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore fluid pressure and or reducing friction in fault gouges resolving the fine scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging and most advances have been made using teleseismic scattering techniques e g receiver functions in this paper we review the teleseismic structure of six well studied subduction zones three hot i e cascadia southwest japan central mexico and three cool i e costa rica alaska and hikurangi that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior we conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior and that near lithostatic pore fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence

gps constraints on 34 slow slip events within the cascadia subduction zone 1997 2005 1 refinements to gps analyses in which we factor geodetic time series to better estimate both reference frames and transient deformation resolve 34 slow slip events located throughout the cascadia subduction zone from 1997 through 2005 timing of transient onset is determined with wavelet transformation of geodetic time series thirty continuous stations are included in this study ranging from northern california to southwestern british columbia our improvements in analysis better resolve the largest creep events and also identify many smaller events at 48 5 n latitude a 14 month average recurrence interval has been observed over eight events since 1997 farther north along vancouver island a host of smaller events with a distinct 14 month periodicity also occurs in southern washington state some of the largest transient displacements are observed but lack any obvious periodicity in their recurrence along central oregon an 18 month recurrence is evident while in northern california an 11 month periodicity continues through 2005 we invert gps offsets of the 12 best recorded events for thrust slip along the plate interface using a cross validation scheme to derive optimal smoothing parameters these 12 events have equivalent moment magnitudes between 6 3 and 6 8 and have 2 3 cm of slip unlike other subduction zones no long duration events are observed and cumulative surface deformation is consistently less than 0 6 cm the many newly resolved smaller transient events in cascadia show that slow slip events occur frequently with gps best capturing only the largest events it is likely that slow slip events occur more frequently at levels not detectable with gps

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

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

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

deep electrical structure of the northern cascadia british columbia canada subduction zone implications for the distribution of fluids long period magnetotelluric data have been used to image the deep electrical structure of the cascadia subduction zone in british columbia canada zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab a shallow zone of high conductivity beneath vancouver island is likely due to fluids that are trapped above the subducting plate east of this structure is a conductive 0 03 s m forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized a free fluid phase is required to account for this enhanced conductivity elevated conductivities are observed in the upper mantle throughout the backarc 0 01 s m and strongly support the hypothesis of a shallow convecting asthenosphere this enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals or by a few percent partial melting 4

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

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

tidal modulation and triggering of low frequency earthquakes in northern cascadia we analyze the influence of earth and ocean tides on the triggering of low frequency earthquakes lfes in northern cascadia using three lfe catalogs for southern vancouver island and washington state from episodic tremor and slip events between 2003 and 2013 sensitivities of lfe families to tidally induced fault normal stress updip shear stress udss and corresponding time derivatives are computed and their geographic variability is mapped we find localized areas showing higher sensitivity to udss than their surroundings suggesting that tidal sensitivity depends on laterally heterogeneous physical properties such as variable pore fluid pressures and frictional properties along the plate interface we observe that sensitivity of lfes to udss rises dramatically from near zero on the first day of strong activity to a maximum 4 days later in addition the peak lfe rate transitions from a correlation with peak tidal shear stress rate to a correlation with peak tidal shear stress through large slow slip events we identify 64 rapid tremor reversals rtrs that start a few days after the main slip front the rtrs have an average stress drop of 0 8 kpa and a majority 72 occurs during periods of large positive udss the combined observations imply that rtrs play an important role in slow slip processes and that modulation of creep rate due to tidal stress and tidal triggering of secondary events are jointly responsible for the observed tidal sensitivity

shear veins observed within anisotropic fabric at high angles to the maximum compressive stress some faults seem to slip at unusually high angles 45 relative to the orientation of the greatest principal compressive stress1 2 3 4 5 this implies that these faults are extremely weak compared with the surrounding rock6 laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre existing frictionally weak surface7 8 9 weakening from chemical reactions10 elevated fluid pressure11 12 13 or dissolution precipitation creep14 15 here we describe shear veins within the chrystalls beach accretionary melange new zealand the melange is a highly sheared assemblage of relatively competent rock within a cleaved anisotropic mudstone matrix the orientation of the shear veins compared with the direction of hydrothermal extension veins that formed contemporaneously indicates that they were active at an angle of 80 5 to the greatest principal compressive stress we show that the shear veins developed incrementally along the cleavage planes of the matrix thus we suggest that episodic slip was facilitated by the anisotropic internal fabric in a fluid overpressured heterogeneous shear zone a similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings we therefore conclude that incremental slip along a pre existing planar fabric coupled to high fluid pressure and dissolution precipitation creep may explain active slip on severely misoriented faults

temporal variations of non volcanic tremor nvt locations in the mexican subduction zone finding the nvt sweet spot epicentral locations of non volcanic tremors nvt in the mexican subduction zone are determined from the peak of the energy spatial distribution and examined over time nvt is found to occur persistently at a distance of 215 km from the trench which we term the sweet spot because this region probably has the proper conditions i e temperature pressure and fluid content for the nvt to occur with minimum shear slip high energy nvt episodes are also observed every few months extending 190 km to 220 km from the trench with durations of a few weeks during the 2006 slow slip event sse the duration and the recurrence rate of the nvt episodes increased low energy episodes were also observed independent from the high energy episodes 150 km to 190 km from the trench during the 2006 sse both the high and low energy episodes were made up of many individual nvt s that had a range of energy release rates however the highest energy release rates of the high energy episodes were consistently double those of the low energy episodes and the persistent activity at the sweet spot we suggest that all of the high energy episodes are evidence of small short repeat interval sse given this model the increased recurrence rate of the high energy nvt episodes during the 2006 long term sse implies that short term sse s also increase during the sse and are therefore triggered by the sse

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

the fate of the downgoing oceanic plate insight from the northern cascadia subduction zone abstract in this study we use teleseismic receiver function analysis to image the seismic structure of the juan de fuca oceanic plate during its subduction beneath the north american plate seismic data have been recorded at 58 seismic stations deployed along the northern cascadia subduction zone harmonic decomposition of the receiver function data set along a trench normal profile allows us to image both the isotropic and the anisotropic structure of the plate slab our images highlight the presence of a highly anisotropic region at 40 70 km depths across the cascadia subduction zone the detected seismic anisotropy is interpreted to be related to both metamorphic facies e g blueschists and fluid released during the dehydration of the subducting mantle the processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab i e basalts gabbro layer and upper mantle as the oceanic plate sinks in the upper mantle such variations make it almost impossible to recognize the plate boundary as a characteristic velocity jump at depth neither positive nor negative along the cascadia subduction zone based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved we propose a 4 stage model of the evolution of the juan de fuca oceanic plate during its subduction beneath the north american plate

using systematically characterized low frequency earthquakes as a fault probe in guerrero mexico studies of low frequency earthquakes lfes have focused on detecting events within previously identified tectonic tremor however the principal lfe detection tools of matched filter searches are intrinsically incapable of detecting events that have not already been characterized previously as a template event in this study we therefore focus on generating the largest number possible of lfe templates by uniformly applying a recently developed lfe template detection method to a 2 5 yearlong data set in guerrero mexico using each of the detected templates in a matched filter search we then form event families that each represents a single source we finally develop simple empirical statistics to select the event families that represent lfes our resulting catalog contains 1120 unique lfe sources and a total of 1 849 486 detected lfes over the 2 5 yearlong data set the locations of the lfe sources are then divided into subcatalogs based on their distance from the subduction trench considering each lfe as a small unit of slip along the subduction interface we observe discrete episodes of lfe activity in the region associated with large slow slip events this is in direct contrast to the near continuous activity observed 35 km farther downdip within the previously identified lfe tremor sweet spot

deep tremor in new zealand triggered by the 2010 mw8 8 chile earthquake 1 deep non volcanic tremor nvt is usually associated with episodic slow slip events new zealand is one notable exception where numerous slow slip events have been identified yet nvt has remained undetected here we present the first known case of triggered nvt at new zealand s hikurangi subduction margin following the mw8 8 chilean earthquake of february 27 2010 we identify coherent high frequency tremor signals that are in phase with and modulated by the passing rayleigh waves this is consistent with the surface wave triggering potential for strike normal incidence on a low angle thrust fault after constraining the tremor depth on the plate interface we locate the tremor source within 20 km of the source area of episodic slow slip the tremor location is also near the edge of a region with high seismic attenuation that marks the boundary between dehydrated subducted slab and inferred hydrated underplated sediment we speculate that reduced interface friction and high fluid pressures provided by fluid rich underplated sediment facilitates the tremor generation

probing the transition between seismically coupled and decoupled segments along an ancient subduction interface the transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip as well as the focus site of episodic tremor and slip features exhumed remnants of the former alpine subduction zone found in the swiss alps allow analyzing fluid and deformation processes near the transition zone region 30 40 km paleodepth the dent blanche thrust dbt is a lower blueschist facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge we report field observations from the dbt region where multiple several tens of meters thick foliated cataclastic networks are interlayered within the basal dbt mylonites petrological results and microstructural observations indicate that the various cataclasis events took place at near peak metamorphic conditions 400 500 c 1 1 1 3 gpa during subduction of the tethyan seafloor in eocene times 42 48 ma some of these networks exhibit mutual crosscutting relationships between mylonites foliated cataclasites and vein systems indicating mutual overprinting between brittle deformation and ductile creep whole rock chemical compositions in situ 40ar 39ar age data of recrystallized phengite and sr isotopic signatures reveal that dbt rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the dbt from the rock fabrics we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates 10 12 s 1 accompanied by fluctuation of supralithostatic and quasi lithostatic fluid pressures 1 0 95 dbt brittle plastic deformation switches highlight the diversity of deformation processes and fluid rock interactions in the transition zone region of the subduction interface

newly observed deep slow slip events at the central hikurangi margin new zealand implications for downdip variability of slow slip and tremor and relationship to seismic structure 1 we present previously undocumented deep 25 45 km depth moderate duration 2 3 months slow slip events sses directly downdip of known shallow 15 km depth short term 2 3 weeks sses at the central hikurangi margin new zealand the moderate duration sses produce 2 6 mm of horizontal displacement at continuous global positioning system stations in 2006 and 2008 similar to cascadia sses they involve 2 5 cm of slip over 5000 km2 of the subduction interface tremor occurs just downdip of the deep sse source area we also present results from a large shallow 12 km sse in 2013 just updip of the 2006 2008 sses the location of deep sses between shallow short term sses and tremor indicates strong downdip variation of subduction interface slip behavior at central hikurangi the deep sses occur where qp and vp vs data suggest an accumulation of fluid rich underplated sediment at the interface consistent with the idea that sses occur under high fluid pressure

slow slip and frictional transition at low temperature at the hikurangi subduction zone temperature changes with depth do not appear to be a primary control for either slow slip or fault locking processes at the hikurangi margin north island new zealand both slow slip events and the geodetically observed transition from fault locking to free slip at depth occur at temperatures as low as 100 c shallow portions of faults exhibit brittle stick slip behaviour that gives way to more stable sliding with increasing depth limiting the depths to which earthquake inducing slip can occur the increase of temperature with depth is often assumed to transform friction in the fault from velocity weakening unstable to velocity strengthening stable and to change conditions from brittle to ductile1 2 3 4 5 a temperature of 350 c has been suggested to mark the base of the frictionally unstable portion of the fault which becomes free slipping at depth where temperatures rise above 450 c refs 2 4 6 here we show that both slow slip events and the geodetically observed transition from fault locking to free slip at the hikurangi subduction zone east of the north island new zealand occur at temperatures as low as 100 c suggesting that temperature is not a primary control on either slow slip or fault locking processes at the hikurangi margin although globally shallow slow slip events are rare five out of seven events observed so far at the hikurangi margin are less than 15 km deep

space time correlation of slip and tremor during the 2009 cascadia slow slip event 1 episodic tremor and slip ets involving transient deformations accompanied by emergent low frequency tremor occurs in subduction zones around the world ets events increase the shear stress on locked megathrusts and may potentially trigger damaging earthquakes despite the clear association of tremor and slip the physical relationship between them is unresolved tremor appears to result from slip on small asperities on the plate interface due to either creep on the surrounding fault or stress increases ahead of the propagating slow slip front previous studies of migrating slow slip events have not had sufficient spatial and temporal resolution to differentiate between these two models to address this we invert gps data from the august 2009 ets event in central cascadia for the space time evolution of fault slip rate we find a correlation in both space and time between tremor epicenters and the independently determined position of high fault slip rate this supports the first hypothesis that tremor asperities are loaded directly by slow slip rather than by stress increases ahead of the slip front and provides new insights into the mechanics of ets

alaska megathrust 2 imaging the megathrust zone and yakutat pacific plate interface in the alaska subduction zone we image the slab underneath a 450 km long transect of the alaska subduction zone to investigate 1 the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity and 2 the interplate coupled zone where the great 1964 earthquake mw 9 2 exhibited the largest amount of rupture the joint teleseismic migration of two array data sets based on receiver functions rfs reveals a prominent shallow dipping low velocity layer at 25 30 km depth in southern alaska modeling of rf amplitudes suggests the existence of a thin layer vs of 2 1 2 6 km s that is 20 40 slower than underlying oceanic crustal velocities and is sandwiched between the subducted slab and the overriding plate the observed megathrust layer with vp vs of 1 9 2 3 may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel our image also includes an unusually thick low velocity crust subducting with a 20 dip down to 130 km depth at 200 km inland beneath central alaska the unusual nature of this subducted segment results from the subduction of the yakutat terrane crust our imaged western edge of the yakutat terrane aligns with the western end of a geodetically locked patch with high slip deficit and coincides with the boundary of aftershock events from the 1964 earthquake it appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

slow slip events in costa rica detected by continuous gps observations 2002 2011 1 a network of continuously recording gps stations has operated in the nicoya peninsula of northern costa rica since 2002 we processed all available data from this network for the period of 2002 2011 to investigate the occurrence of slow slip events sse on the subduction interface between the cocos and caribbean plates in order to overcome signal masking by high levels of tropospheric noise we developed a new technique that facilitates detection of transient events in the presence of noise we identified five significant sses during the 2002 2011 period with event middle times in 2003 2005 2007 2009 and 2011 with an average recurrence interval of 21 6 months time series analysis shows that transient deformation imparts a signature similar to random walk removal of the sses and regional common mode errors from the time series reduced velocity uncertainty by nearly an order of magnitude limited available data for the 2003 2005 and 2011 events preclude detailed characterization of these events however good spatiotemporal coverage of the 2007 and 2009 events suggest that both events had irregular duration and distribution in the 2007 event slow slip started in the northwest coastal area and migrated southeastward over a period of 1 month the 2009 event had a significantly longer event duration and larger surface displacement stations in the northwest area observed two separate sses in 2008 6 and 2009 4 correlating well with the tremor episodes offshore indicating a shallow sse slip patch with shorter recurrence interval significant differences between the 2009 and 2007 events lead us to question the simple recurrence interval model for the sse in nicoya

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

permeability evolution in quartz fault gouges under hydrothermal conditions 1 the permeability k of fine grained quartz aggregates were measured in situ during hot pressing hping experiments to explore the evolution of fluid transport properties of fault zones during the interseismic period experiments were conducted at temperatures of 150 c and between 700 and 850 c with confining and pore water pressures of 250 and 150 mpa respectively significant permeability reduction was observed between 700 and 850 c with permeability reduction rates r 1 t ln kto kt ranging from approximately 6 10 5 s 1 at 700 c to a maximum of approximately 7 4 10 4 s 1 at 850 c permeability decreased exponentially with time and the permeability reduction rate increased with increasing temperature increasing differential stress and decreasing grain size analysis of the permeability porosity relationships indicates that permeability in the simulated gouge at high temperature shuts off at a critical porosity of 0 045 0 004 the presence of microstructures such as grain interpenetration grain shape truncation arrays of fluid inclusions and development of quartz overgrowths on grains indicate that k reduction was controlled by dissolution precipitation creep processes extrapolation of the permeability reduction rates measured in this study to temperatures typical of the continental seismogenic regime highlights the strongly time dependent nature of permeability in natural fault wear products at depths of nucleation of major earthquakes within the recurrence time of large earthquakes quartz rich fault zones in the fluid active midcrustal to lower continental crustal regimes can evolve from high permeability conduits to low permeability seals episodic changes in the fluid transport properties of faults during the interseismic period are likely to impact on the pore pressure evolution of fault wear products

three dimensional elastic dislocation modeling of the postseismic response to the 1964 alaska earthquake 1 gps velocity observations from the kenai peninsula and kodiak island alaska display a pattern of spatial variability suggesting the presence of multiple active processes on the underlying pacific north american plate interface velocities of sites on the eastern kenai peninsula and kodiak island are consistent with elastic strain accumulation at the plate interface whereas velocities of sites on the western kenai peninsula and cook inlet are oriented in the opposite direction and suggest an ongoing postseismic response to the 1964 alaska earthquake we can reproduce the observed velocities using a three dimensional elastic dislocation model to estimate the variation in coupling between the pacific and north american plates two different inversion schemes are examined to check the robustness of the results we find that the gps data can be satisfied by the presence of a locked area near southwest prince william sound a locked area near southwest kodiak island and an area of postseismic reverse slip beneath and north of the western kenai peninsula the shallow plate interface trenchward of the western kenai peninsula does not appear to be locked the locked areas correspond to the prince william sound and kodiak island asperities that ruptured in 1964 and the area of postseismic slip lies downdip of the 1964 rupture area the correspondence between the present shallow coupling distribution and the 1964 slip distribution suggests that the locked regions repeat from one earthquake cycle to another

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

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

nature of crustal terranes and the moho in northern costa rica from receiver function analysis 1 the central american subduction zone in northern costa rica shows along strike variations in both the incoming and overriding plates by analyzing the subducting oceanic moho m1 and the upper plate moho m2 with receiver functions we investigate the variability in the hydration state of the subducting cocos plate and the nature of crustal terranes within the overriding caribbean plate we calculate high quality p and pp wave receiver functions using broadband data of the global seismology network geoscope project and the crseize pocosol and corisubmod experiments in addition we estimate the depth h and vertically averaged vp vs to moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the h and estimates our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the nicoya peninsula as compared to a serpentinized oceanic mantle subducting beneath the northern part in the caribbean plate we describe the nature of the mesquito nicoya and chorotega terranes by integrating new and published vp vs estimates both the nicoya and chorotega terranes display high vp vs 1 80 1 92 consistent with their oceanic character in contrast the oceanic mesquito terrane mostly displays lower vp vs 1 62 1 80 more compatible with continental crust which may indicate that subduction zone magmatism is modifying the crust to display continental character our estimates show that the deepest m2 42 km is observed in the volcanic arc region whereas the shallowest m2 27 33 km is observed in parts of the fore arc and back arc regions