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

rupture area and displacement of past cascadia great earthquakes from coastal coseismic subsidence coastal marshes record a 6500 yr history of coseismic displacements in great earthquakes at the cascadia subduction zone we compiled estimates of coseismic displacement for past megathrust events based on correlations with megathrust triggered turbidites and estimated megathrust slip based on comparisons of marsh displacements with dislocation model predictions age correlated marsh data are compatible with event rupture extents defi ned by the published turbidite record and a 6500 yr mean recurrence interval that increases northward from 230 to 480 yr within the constraints of the marsh data the width of the coseismic rupture zone generally agrees with the downdip width of the interseismic locked zone inferred from geodetic and thermal data in southernmost cascadia where the model does not include the complex deformation near the mendocino triple junction the coastal data may be better fi t by a model with an 25 narrower rupture than that inferred from regional geophysical data at each coastal marsh site coseismic displacements are roughly similar from event to event independent of the time since the previous event slip in the a d 1700 earthquake was consistent with the preceding interval of strain accumulation 200 yr only at the northern and southern ends of the margin but it was apparently much higher in southern washington and northern oregon possibly indicating postseismic contamination and or catch up coseismic slip to make up for a defi cit in the preceding event overall agreement between the dislocation models and the marsh data for most of the margin implies that such models can be usefully applied to rupture and ground shaking predictions

seismicity at the cascadia plate boundary beneath the oregon continental shelf since 2003 39 small earthquakes have been detected off the coast of central oregon in the nominally locked part of the cascadia subduction zone where very little seismic activity has been recorded in spite of a paleoseismic record of great subduction events although the regional earthquake bulletin reports depths of 29 and 28 km for the two largest events m w 4 9 and m w 4 7 which occurred in 2004 analysis by trehu et al 2008 indicates that they were low angle thrust events that occurred on the plate boundary at depths of 9 11 and 16 km respectively because of sparse onshore station coverage most of the smaller events have large location uncertainties double difference relative location of 30 of these earthquakes reveals two tight clusters approximately 30 km apart each cluster is associated with one of the two larger events within each cluster relocation reduces the hypocenter depth spread from 15 km to 3 km with uncertainties on the order of 0 1 km the relocations combined with independent absolute hypocenter locations for two 2008 events using a deployment of land and ocean bottom seismometers suggest that the seismicity occurred at plate boundary depths possibly on the cascadia megathrust this concentrated activity in the seismogenic zone may represent patches on the fault plane with anomalous frictional characteristics possibly caused by subducted topographic features which can affect the propagation of a large megathrust rupture

free surface effects on the propagation of dynamic rupture dynamic rupture of reverse and normal fault intersecting the surface are investigated in the case of a normal fault nucleating at depth and propagating upwards coupling of rupture radiated stress and free boundary conditions at the surface may induce a shallow secondary nucleation anticipating up to a few seconds the arrival of the main rupture front indeed the free surface induces normal stress fluctuations modifying the fault frictional strength no significant effect on rupture velocity is observed in the case of reverse faulting these incidences are explained by a stress analysis and illustrated by some numerical simulations in the case of dynamic normal faulting in a homogeneous half space the described effects could explain some observations of high frequency radiation close to the surface in documented shallow earthquakes like in kalamata 1986 bouin 1994

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

fully coupled simulations of megathrust earthquakes and tsunamis in the japan trench nankai trough and cascadia subduction zone subduction zone earthquakes can produce significant seafloor deformation and devastating tsunamis real subduction zones display remarkable diversity in fault geometry and structure and accordingly exhibit a variety of styles of earthquake rupture and tsunamigenic behavior we perform fully coupled earthquake and tsunami simulations for three subduction zones the japan trench the nankai trough and the cascadia subduction zone we use data from seismic surveys drilling expeditions and laboratory experiments to construct detailed 2d models of the subduction zones with realistic geometry structure friction and prestress greater prestress and rate and state friction parameters that are more velocity weakening generally lead to enhanced slip seafloor deformation and tsunami amplitude the japan trench s small sedimentary prism enhances shallow slip but has only a small effect on tsunami height in nankai where there is a prominent splay fault frictional parameters and off fault material properties both influence the choice of rupture pathway in complex ways the splay generates tsunami waves more efficiently than the decollement rupture in cascadia is buried beneath the seafloor but causes a tsunami that is highly complex due to the rough seafloor bathymetry neglecting compliant sediment layers leads to substantially different rupture behavior and tsunami height we demonstrate that horizontal seafloor displacement is a major contributor to tsunami generation in all subduction zones studied we document how the nonhydrostatic response of the ocean at short wavelengths smooths the initial tsunami source relative to commonly used approach for setting tsunami initial conditions finally we determine self consistent tsunami initial conditions by isolating tsunami waves from seismic and acoustic waves at a final simulation time and backpropagating them to their initial state using an adjoint method we find no evidence to support claims that horizontal momentum transfer from the solid earth to the ocean is important in tsunami generation

kinematic rupture scenarios and synthetic displacement data an example application to the cascadia subduction zone scenario ruptures and ground motion simulation are important tools for studies of expected earthquake and tsunami hazards during future events this is particularly important for large mw8 and very large mw8 5 events for which observations are still limited in particular synthetic waveforms are important to test the response of earthquake and tsunami warning systems to large events these systems are not often exercised in this manner we will show an application of the karhunen loeve kl expansion to generate stochastic slip distributions of large events with an example application to the cascadia subduction zone we will discuss how to extend the static slip distributions obtained from the k l expansion to produce kinematic rupture models and generate synthetic long period displacement data at the sampling rates of traditional gnss stations we will validate the waveforms produced by this method by comparison to a displacement based ground motion prediction equation gmpe obtained from gnss measurements of large earthquakes worldwide

plate boundary earthquakes and tsunamis of the past 5500 yr sixes river estuary southern oregon eleven plate boundary earthquakes over the past 5500 yr have left a stratigraphic signature in coastal wetland sediments at the lower sixes river valley in south coastal oregon within a 1 8 km2 abandoned meander valley 10 buried wetland soils record gradual and abrupt relative sea level changes back in time to 6000 yr ago an additional youngest buried soil at the mouth of the sixes river subsided during the a d 1700 cascadia earthquake multiple lines of evidence indicate that tectonic subsidence caused soil burial including permanent relative sea level rise following burial lateral continuity of buried soil horizons over hundreds of meters diatom assemblages showing that sea level rose abruptly at least 0 5 m and sand deposits on top of buried soils demonstrating coincidence of coseismic subsidence and tsunami inundation for at least two of the buried soils liquefaction of sediment accompanied subsidence the 11 soil burial events took place between 300 and 5400 yr ago yielding an average recurrence interval of plateboundary earthquakes of 510 yr comparing paleoseismic sites in southern washington and south coastal oregon with the sixes river site for the past 3500 yr indicates that the number and timing of recorded plate boundary earthquakes are not the same at all sites in particular a sixes earthquake at 2000 yr ago lacks a likely correlative in southern washington therefore unlike the a d 1700 cascadia earthquake some cascadia plate boundary earthquakes do not rupture the entire subduction zone from southern oregon to southern washington in the lower sixes river valley the upperplate cape blanco anticline deforms sediment of late pleistocene and holocene age directly above the subduction zone differential tectonic subsidence occurred during two of the plate boundary earthquakes when a blind upper plate reverse fault for which the cape blanco anticline is the surface fold slipped coseismically with rupture of the plate boundary during these two earthquakes sites 2 km from the anticline axis subsided 0 5 m more than sites 1 km from the axis

on the dynamics and water mass transformation of a boundary current connecting alpha and beta oceans abstracta subpolar marginal sea like the nordic seas is a transition zone between the temperature stratified subtropics the alpha ocean and the salinity stratified polar regions the beta ocean

a model for the thickness and salinity of the upper layer in the arctic ocean and the relationship between the ice thickness and some external parameters abstract this paper presents a dynamical model for the salinity and thickness of the upper layer in the arctic the parameters are the river runoff to the arctic the buoyancy supply through the bering strait the export of ice from the arctic and a parameter characterizing the vertical mixing an ice model is formulated having the following two important properties 1 the horizontal surface area of the exported ice is essentially determined by external parameters the wind field over the arctic and 2 there is a relationship between the ice thickness and the fraction of open water in the arctic the model for the upper layer and the ice model are used together with a heat budget for the arctic also including the effect of different albedo for ice and open water a relationship between the freshwater supply and the ice thickness is derived also investigated are the effects on the ice thickness of a changed export of ice area and changed properties of the flow through the bering strait it is found t

linked 3 d modelling of megathrust earthquake tsunami events from subduction to tsunami run up e h m t u and a a g acknowledge additional support by the german research foundation dfg projects no ga 2465 2 1 ga 2465 3 1 by bacatec project no a4 and baylat by konwihr the bavarian competence network for technical and scientific high performance computing project newwave by kaust crg gast grant no ors 2016 crg5 3027 and fragen grant no ors 2017 crg6 3389 02 by the european union s horizon 2020 research and innovation program exahype grant no 671698 cheese grant no 823844 and tear grant no 852992

holocene earthquake records from the cascadia subduction zone and northern san andreas fault based on precise dating of offshore turbidites abstract we present preliminary evidence for a 10 000 year earthquake record from two major fault systems based on sediment cores collected along the continental margins of western north america new stratigraphic evidence from cascadia demonstrates that 13 earthquakes ruptured the entire margin from vancouver island to at least the california border since the eruption of the mazama ash 7700 years ago the 13 events above this prominent stratigraphic marker have an average repeat time of 600 years and the youngest event 300 years ago coincides with the coastal record we also extend the record of past earthquakes to the base of the holocene at least 9800 years ago during which 18 events correlate along the same region the sequence of holocene events in cascadia appears to contain a repeating pattern of events a tantalizing first look at what may be the long term behavior of a major fault system the northern california margin cores show a cyclic record of turbidite beds that may represent holoce

broadband simulations for mw 7 8 southern san andreas earthquakes ground motion sensitivity to rupture speed using the high performance computing resources of the southern california earthquake center we simulate broadband 0 10 hz ground motions for three m w 7 8 rupture scenarios of the southern san andreas fault the scenarios incorporate a kinematic rupture description with the average rupture speed along the large slip portions of the fault set at 0 96 0 89 and 0 84 times the local shear wave velocity consistent with previous simulations a southern hypocenter efficiently channels energy into the los angeles region along the string of basins south of the san gabriel mountains however we find the basin ground motion levels are quite sensitive to the prescribed rupture speed with peak ground velocities at some sites varying by over a factor of two for variations in average rupture speed of about 15 these results have important implications for estimating seismic hazards in southern california and emphasize the need for improved understanding of earthquake rupture processes

slow slip predictions based on granite and gabbro friction data compared to gps measurements in northern cascadia received 7 october 2008 revised 17 april 2009 accepted 12 june 2009 published 29 september 2009 1 for episodic slow slip transients in subduction zones a large uncertainty in comparing surface deformations predicted by forward modeling based on rate and state friction to gps measurements lies in our limited knowledge of the frictional properties and fluid pore pressure along shallow subduction faults in this study we apply the laboratory rate and state friction data of granite and gabbro gouges under hydrothermal conditions to a cascadia like 2 d model to produce spontaneous aseismic transients and we compare the resulting intertransient and transient surface deformations to gps observations along the northern cascadia margin an inferred region along dip of elevated fluid pressure is constrained by seismological observations where available and by thermal and petrological models for the cascadia and sw japan subduction zones for the assumed

non volcanic tremor resulting from the combined effect of earth tides and slow slip events swarms of non volcanic tremor in southeastern japan are associated with slow slip events and tend to occur with a periodicity of 12 or 24 h this periodicity can be reproduced by a combination of stresses due to earth tides and transient stress changes caused by slow slip events non volcanic tremors may therefore be useful for understanding stress relaxation at the subduction zone interface slow slip events1 are accompanied by swarms of non volcanic tremor2 3 along the subduction zone of the philippine sea plate in southwest japan the swarms often occur with a periodicity of about 12 or 24 h refs 4 5 these episodic events are considered to be a manifestation of stress relaxation at the subducting plate interface1 6 7 here we analyse seismic data to record the locations and durations of non volcanic tremor swarms that occurred in may 2005 and february 2006 we evaluate the magnitude of stress changes produced by slow slip events as well as the effective normal stress on the plate interface where slow slip events occur we find that the observed periodicity in tremor occurrence originates from a combined effect of the periodic stress due to earth tides and the transient stress due to slow slip events our calculations show that non volcanic tremor is sensitive to stress change this phenomenon can therefore be effective in monitoring the process of stress relaxation at subducting plate interfaces

surface displacements in the 1906 san francisco and 1989 loma prieta earthquakes the horizontal displacements accompanying the 1906 san francisco earthquake and the 1989 loma prieta earthquake are computed from geodetic survey measurements the 1906 earthquake displacement field is entirely consistent with right lateral strike slip on the san andreas fault in contrast the 1989 loma prieta earthquake exhibited subequal components of strike slip and reverse faulting this result together with other seismic and geologic data may indicate that the two earthquakes occurred on two different fault planes

seismic structure of the southern cascadia subduction zone and accretionary prism north of the mendocino triple junction four multichannel seismic reflection profiles collected as part of the mendocino triple junction seismic experiment image the toe of the southern cascadia accretionary prism today 250 600 m of sediment is subducting with the gorda plate and 1500 3200 m is accreting to the northern california margin faults imaged west and east of the deformation front show mixed structural vergence a north south trending 20 km long portion of the central margin is landward vergent for the outer 6 8 km of the toe of the prism this region of landward vergence exhibits no frontal thrust is unusually steep and narrow and is likely caused by a seaward dipping backstop close to the deformation front the lack of margin wide preferred seaward vergence and wedge taper analysis suggests the prism has low basal shear stress the three southern lines image wedge shaped fragments of oceanic crust 1 1 7 3 km in width and 250 700 m thick near the deformation front these wedges suggest shortening and thickening of the upper oceanic crust discontinuities in the seafloor west of the prism provide evidence for mass wasting in the form of slump blocks and debris fans the southernmost profile extends 75 km west of the prism imaging numerous faults that offset both the gorda basin oceanic crust and overlying sediments these high angle faults bounding basement highs are interpreted as strike slip faults reactivating structures originally formed at the spreading ridge northeast or northwest trending strike slip faults within the basin are consistent with published focal mechanism solutions and are likely caused by north south gorda pacific plate convergence

scaling relationships of source parameters for slow slip events to better understand the physical mechanisms of slow slip events sses detected worldwide we explore the scaling relationships of various source parameters and compare them with similar scaling laws for earthquakes these scaling relationships highlight differences and similarities between slow slip events and earthquakes and hold implications for the degree of heterogeneity and fault healing characteristics the static stress drop remains constant for different sized events as is observed for earthquakes however the static stress drop of slow slip events is within a range of 0 01 1 0 mpa 1 2 orders of magnitude lower than that found for earthquakes which could be related to the low stress state on the fault the average rupture velocity ranging from kilometers per second to kilometers per day decreases linearly with increasing seismic moment in log log space unlike earthquakes that are nearly constant this inverse relationship of rupture velocity with seismic moment could be related to the heterogeneity of fault properties slow slip events typically have ratios of event duration over dislocation rise time less than 3 while earthquakes have ratios greater than 3 this indicates that slow slip events are less pulselike than earthquakes in their mode of propagation and suggests that the healing behind the rupture front is delayed the recurrence statistics of slow slip events on the northern cascadia subduction zone are weakly time predictable and moderately antislip predictable that is the event size and preevent recurrence interval are anticorrelated which may indicate that healing between events strengthens the fault with time online material table of source parameters and data sources

stress rotation across the cascadia megathrust requires a weak subduction plate boundary at seismogenic depths abstract the mendocino triple junction region is the most seismically active part of the cascadia subduction zone the northward moving pacific plate collides with the subducting gorda plate causing intense internal deformation within it here we show that the stress field rotates rapidly with depth across the thrust interface from a strike slip regime within the subducting plate reflecting the pacific plate collision to a thrust regime in the overriding plate we utilize a dense focal mechanism dataset including observations from the cascadia initiative ocean bottom seismograph experiment to constrain the stress orientations to quantify the implications of this rotation for the strength of the plate boundary we designed an inversion that solves for the absolute stress tensors in a three layer model subject to assumptions about the strength of the subducting mantle our results indicate that the shear stress on the plate boundary fault is likely no more than about 50 mpa at 20 km depth regardless of the assumed mantle strength we infer a relatively weak megathrust fault with an effective friction coefficient of 0 to 0 2 at seismogenic depths such a low value for the effective friction coefficient requires a combination of high fluid pressures and or fault zone minerals with low inherent friction in the region where a great earthquake is expected in cascadia

the glacial oceans abstract the accumulation of carbonate and carbonaceous matter increased during the glacial age due to intensified sink water formation in the northernmost atlantic ocean and the intensified upwellings in the indo pacific oceans as a consequence of that there was a decrease of the co2 concentration in the ocean as well as of the co2 content in the atmosphere as a whole the accumulation area of calcareous matter was enlarged by ca 20 during glacial times the decreased accumulation area in the north atlantic was well compensated by a large increase in the indo pacific oceans the northernmost part of the atlantic ocean was quasi stagnant during the most intense deglacial times thus causing a reduced exchange between surface water and deep water which in its turn led to a decrease of the accumulation of carbon as caco3 and organic matter the concentration of co2 rose in the seawater and the pco2 increased as well

active faulting and block rotations in the western transverse ranges california the geometry of faulting slip vectors of major earthquakes and results of very long baseline interferometry vbli are consistent with active clockwise rotation of crustal blocks in the western transverse ranges about vertical axes oblique left lateral thrust faults trending roughly east west across the western transverse ranges apparently separate long east west blocks of upper crust and the large components of lea lateral shear on east striking planes suggest that the blocks rotate with respect to the san andreas fault in the mojave region between the western transverse ranges and the mojave desert paleomagnetic declinations indicate some 35 and possibly 60 of clockwise rotation since about 10 ma and more importantly the relative velocity between the only two frequently measured vbli sites in the western transverse ranges is consistent with a rotation rate of 6 ma 3 ma during a 5 year period in the 1980s azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the san andreas fault across the western mojave desert indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation extension of material parallel to that segment of the fault plate motions late quaternary offsets on the san andreas and san jacinto faults and geodetic measurements in the mojave region yield a calculated direction of relative motion between the san gabriel mountains and the pacific plate across the western transverse ranges that also is perpendicular to the san andreas fault in the mojave region rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates the rotation rate of 2 8 5 3 ma calculated assuming that the long east west blocks rotate in response to a uniform compression of a viscous substratum to the western transverse ranges however overlaps the range of observed average rotation rates thus the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it

the north pacific a global scale estuary abstract the atmospheric net flow of water from the atlantic to the pacific ocean is supposed to maintain the salinity difference between the two oceans assuming the existence of a subsurface level of no horizontal pressure gradient in the ocean the mean sea level in the northern pacific must be higher than in the arctic ocean this mean sea level difference is supposed to drive the observed mean flow through the bering strait the estimated flow of freshwater through the bering strait is approximately equal to the estimated atmospheric net flow of water from the north atlantic to the north pacific this justifies the formulation of a simple estuary model for the north pacific in which the brackish water exits through the bering strait the salinity difference between the two oceans is shown to be controlled by the topography of the bering strait the estuary model gives residence times for water in the upper layer 1000 m thick of approximately 1000 years and in the lower layer of approximately 400

measuring earthquake source parameters in the mendocino triple junction region using a dense obs array implications for fault strength variations abstract subduction zones produce earthquakes on a set of faults that operate under a wide variety of conditions resulting from considerable variations in depth temperature rock type and fluid pressure these variations likely lead to variation in the stress levels that drives particular earthquakes and that in turn effects the magnitude of seismic shaking they produce in the mendocino triple junction mtj region intraplate faults within the mantle of the subducting plate fail regularly in energetic earthquakes while the adjacent thrust interface of the cascadia subduction zone remains seismically quiet despite the likelihood that it operates at much lower levels of stress and strength in 2012 as part of the cascadia initiative community experiment an ocean bottom seismometer obs array was deployed in the mtj area providing unusually dense data covering both the inter and intra plate earthquakes combining these data with onshore networks we detect and relocate 1137 earthquakes with a three dimensional velocity model we perform detailed spectral and time domain analysis to study variations in earthquake source properties between the different types of faults we observe a wide variability of stress drops and systematic lateral and depth variations in the earthquake source spectra resulting from the different types of tectonic fault systems in this region intraplate faults within the subducted oceanic mantle the mendocino transform plate boundary fault and the thrust interface of the cascadia subduction zone some of the depth variability of source spectra can be explained by the expected increase in rupture velocity with depth however the overall variation in stress drop estimates is consistent with the highest stress drop earthquakes occurring in the depth range predicted by strength envelopes moreover the earthquakes in the vicinity of the thrust interface likely including some within the subducted oceanic crust show clearly lower stress drops and depletion of high frequency radiation for their size compared to events in the oceanic mantle and along the transform fault at a given depth our results suggest that the variability in earthquake source spectra that can be expected in the cascadia subduction zone is systematic and that the thrust interface operates at a lower stress level than the core of the subducted plate a few kilometers away

rates of late cenozoic tectonism in the vallecito fish creek basin western imperial valley california the kinetics of continental rifting are recorded in the late neogene sediments of the western imperial valley california the gulf of california opened in its present form about 4 m y ago capturing the colorado river some 5 km of deltaic and fluvial sediments accumulated in the vallecito fish creek area during the interval 4 3 0 9 m y ago initial sedimentation rates were 5 5 mm yr 1 which diminished in an exponential fashion to 0 5 mm yr 1 when isostatic adjustments are accounted for this implies tectonic subsidence rates of 1 5 mm yr 1 decreasing systematically to 0 1 mm yr 1 after 0 9 m y ago right lateral shearing along the elsinore fault zone cut across the basin causing it to be tilted and uplifted at a mean rate of 5 9 mm yr 1 the basin was concurrently rotated as a unit 35 clockwise the dissected pediments and badlands that now characterize the area were formed and eroded in the past 0 9 m y

evolution of the northern santa cruz mountains by advection of crust past a san andreas fault bend the late quaternary marine terraces near santa cruz california reflect uplift associated with the nearby restraining bend on the san andreas fault excellent correspondence of the coseismic vertical displacement field caused by the 17 october 1989 magnitude 7 1 loma prieta earthquake and the present elevations of these terraces allows calculation of maximum long term uplift rates 1 to 2 kilometers west of the san andreas fault of 0 8 millimeters per year over several million years this uplift in concert with the right lateral translation of the resulting topography and with continual attack by geomorphic processes can account for the general topography of the northern santa cruz mountains

the 14 november 2001 kokoxili tibet earthquake high frequency seismic radiation originating from the transitions between sub rayleigh and supershear rupture velocity regimes seismic array based analysis of the major kokoxili earthquake tibet 14 november 2001 yields an unambiguous reconstruction of the seismic rupture history and relates it to the generated seismic radiation we demonstrate that after a classical sub rayleigh velocity stage the rupture speed has jumped to supershear values close to compressional wave velocity over a 175 km long fault segment before abruptly slowing down in the late part of the earthquake the transition locations between these three phases are correlated with the fault geometry and are associated with the most energetic radiation this observation proves that the rupture velocity changes as theoretically predicted are a primary source of high frequency seismic radiation this result requires reconsidering the origins of seismic damage generally attributed to slip variations

influences of precipitation on water mass transformation and deep convection abstractthe influences of precipitation on water mass transformation and the strength of the meridional overturning circulation in marginal seas are studied using theoretical and idealized numerical models nondimensional equations are developed for the temperature and salinity anomalies of deep convective water masses making explicit their dependence on both geometric parameters such as basin area sill depth and latitude as well as on the strength of atmospheric forcing in addition to the properties of the convective water the theory also predicts the magnitude of precipitation required to shut down deep convection and switch the circulation into the haline mode high resolution numerical model calculations compare well with the theory for the properties of the convective water mass the strength of the meridional overturning circulation and also the shutdown of deep convection however the numerical model also shows that for precipitation levels that exceed this critical threshold the circulat

how subduction interface roughness influences the occurrence of large interplate earthquakes the role of seafloor roughness on the seismogenic behavior of subduction zones has been increasingly addressed over the past years although their exact relationship remains unclear do subducting features like seamounts fracture zones or submarine ridges act as barriers preventing ruptures from propagating or do they initiate megathrust earthquakes instead we address this question using a global approach taking into account all oceanic subduction zones and a 117 year time window of megathrust earthquake recording we first compile a global database subquake that provides the location of a rupture epicenter the overall rupture area and the region where the largest displacement occurs the seismic asperity for m w 7 5 subduction interplate earthquakes with these data we made a quantitative comparison with the seafloor roughness seaward of the trench which is assumed to be a reasonable proxy for the subduction interface roughness we compare the spatial occurrence of megathrust ruptures seismic asperities and epicenters with two roughness parameters the short wavelength roughness r sw 12 20 km and the long wavelength roughness r lw 80 100 km we observe that ruptures with m w 7 5 tend to occur preferentially on smooth subducting seafloor at long wavelengths which is especially clear for the m w 8 5 events at both short and long wavelengths seismic asperities show a more amplified relation with smooth seafloor than rupture segments in general for the epicenter correlation we see a slight difference in roughness signal which suggests that there might be a physical relationship between rupture nucleation and subduction interface roughness plain language summary subduction zones are regions on earth where an oceanic plate dives below another plate earthquakes that occur along the contact between plates in such regions are among the largest and most destructive on earth to better understand where these large earthquakes are most likely to occur we look at the effect of seafloor roughness a rough seafloor is often characterized by many topographic features such as seamounts or ridges while a smooth seafloor is generally more flat on a global scale we compared the roughness of the incoming seafloor of the downgoing plate with the occurrence of large earthquakes in each subduction zone we find that the seafloor in front of large earthquakes is generally smoother than in areas where no large earthquakes have occurred this is the clearest for very large earthquakes with magnitudes larger than 8 5 investigating which parameters play a role in the location of earthquakes helps us to understand where future earthquakes are more likely to occur

mega earthquakes rupture flat megathrusts the 2004 sumatra andaman and 2011 tohoku oki earthquakes highlighted gaps in our understanding of mega earthquake rupture processes and the factors controlling their global distribution a fast convergence rate and young buoyant lithosphere are not required to produce mega earthquakes we calculated the curvature along the major subduction zones of the world showing that mega earthquakes preferentially rupture flat low curvature interfaces a simplified analytic model demonstrates that heterogeneity in shear strength increases with curvature shear strength on flat megathrusts is more homogeneous and hence more likely to be exceeded simultaneously over large areas than on highly curved faults

paleomagnetic investigation of late neogene vertical axis rotation and remagnetization in central coastal california outcrops of shallow marine sedimentary rocks of the neogene purisima formation and santa cruz mudstone occur throughout the santa cruz mountains and along the coast of santa cruz and san mateo counties in california within the santa cruz mountains the purisima formation has been extensively folded and offset by reverse faults along the coast the purisima formation and santa cruz mudstone are only gently folded although the purisima formation is cut by the san gregorio fault zone previous paleomagnetic studies have shown that vertical axis rotation has occurred within some parts of the san gregorio fault zone as part of a kinematic study of the san gregorio fault zone this paleomagnetic investigation of the purisima formation and santa cruz mudstone was conducted to determine the extent of vertical axis rotation the characteristic paleomagnetic directions of the purisima formation and santa cruz mudstone which we interpret as the primary directions indicate that a clockwise vertical axis rotation of 35 to 60 has occurred throughout the san gregorio fault zone whereas no rotation has occurred away from the fault zone this result suggests that vertical axis rotation is fundamentally related to shear across the san gregorio fault zone our work also indicates that while the purisima formation and santa cruz mudstone along the coast have retained their primary magnetizations the purisima formation within the santa cruz mountains has been remagnetized after folding

paleoceanography of the mid pleistocene the fossil record of the pleistocene calcareous nannoplankton indicates that during the mid pleistocene 0 93 1 25 my occurred an episode of overwhelming dominance of smallgephyrocapsa during this episode normally abundant large size specimens of this genus mainlygephyrocapsa oceanica were virtually excluded from the phytoplankton of tropical and subtropical oceans the best modern analog of this dominantly smallgephyrocapsa assemblage is the subpolaremiliania huxleyi assemblage which implies that nutrient content was significantly greater and water temperature was lower in the photic water column of the tropical oceans than they are today increased equatorial upwelling in the oceans on a scale much greater than today may explain the above pattern to achieve such broad equatorial upwelling there must be a source and a drive for cold dense water the arctic ocean which was probably seasonally free of ice during this interval of the mid pleistocene is capable of providing the requisite source as well as a drive for the inferred equatorial upwelling the energy balance of a predominantly ice free arctic ocean requires an approximately three to seven fold increase of hydrospheric heat transport from the north atlantic to the arctic ocean which dictates a corresponding or even greater increase in the volume of warm water entering the arctic ocean at the surface and cold dense water exiting at depth to the north atlantic such enhanced dense water formation in the arctic ocean could drive the intensified equatorial upwelling implied by the smallgephyrocapsa dominance interval if the above scenario is correct then the climate of the earth s northern hemisphere during the mid pleistocene may have been very different from the younger pleistocene climate one manifestation of this difference may be the mid pleistocene shift in climatic cycle periodicity from 40 ky to 100 ky another important aspect is that the enhanced greenhouse effect expected during the next century because of an increase of atmospheric co2 is thought to lead directly to melting of the arctic ocean ice cover and of the greenland ice sheet thus the greenhouse arctic ocean and its attendant ocean circulation would resemble the inferred mid pleistocene conditions

transports of nordic seas water masses and excess sf6 through fram strait to the arctic ocean to determine the exchanges between the nordic seas and the arctic ocean through fram strait is one of the most important aspects and one of the major challenges in describing the circulation in the arctic mediterranean sea especially the northward transport of arctic intermediate water aiw from the nordic seas into the arctic ocean is little known in the two ship study of the circulation in the nordic seas arctic ocean 2002 the swedish icebreaker oden operated in the ice covered areas in and north of fram strait and in the western margins of greenland and iceland seas while rv knorr of woods hole worked in the ice free part of the nordic seas here two hydrographic sections obtained by oden augmented by tracer and velocity measurements with lowered acoustic doppler current profiler ladcp are examined the first section reaching from the svalbard shelf across the yermak plateau covers the region north of svalbard where inflow to the arctic ocean takes place the second western section spans the outflow area extending from west of the yermak plateau onto the greenland shelf geostrophic and ladcp derived velocities are both used to estimate the exchanges of water masses between the nordic seas and the arctic ocean the geostrophic computations indicate a total flow of 3 6 sv entering the arctic on the eastern section the southward flow on the western section is found to be 5 1 sv the total inflow to the arctic ocean obtained using the ladcp derived velocities is much larger 13 6 sv and the southward transport on the western section is 13 7 sv equal to the northward transport north of svalbard sulphur hexafluoride sf6 originating from a tracer release experiment in the greenland sea in 1996 has become a marker for the circulation of aiw from the geostrophic velocities we obtain 0 5 sv and from the ladcp derived velocities 2 8 sv of aiw flowing into the arctic the annual transport of sf6 into the arctic ocean derived from geostrophy is 5 kg year which is of the same magnitude as the observed total annual transport into the north atlantic while the ladcp measurements 19 kg year imply that it is substantially larger little sf6 was found on the western section confirming the dominance of the arctic ocean water masses and indicating that the major recirculation in fram strait takes place farther to the south

the paleomagnetic field from equatorial deep sea sediments axial symmetry and polarity asymmetry paleomagnetic data from 89 equatorial deep sea sediment cores indicate that the configuration of the time averaged geomagnetic field depends strongly on polarity state but that it remains within 1 degree of axial symmery throughout the pliocene and pleistocene last 5 million years the relative magnitude of the nondipole field was greater by almost a factor of 2 during reverse than during normal polarity intervals these results thus support earlier suggestions that there may be a standing nonreversing component of the geomagnetic dynamo

a 2 d hybrid technique to model the effect of topography on coseismic displacements application to the umbria marche central italy 1997 earthquake sequence summary we face the problem of modelling the influence of the irregular topography of the earth s crust on coseismic displacements the modelling technique we propose is valid for a homogeneous elastic 2 d domain and is based on a two step approach that uses 1 the traditional analytical results valid for elastic spaces and elastic half spaces with a flat free surface ffs and 2 a finite element fe numerical code to compute the disturbance induced by the local topography attention is particularly focused on 1 the comparison between the results obtained through our method and the analytical formulae valid for ffs geometries which are used extensively today in both forward and inverse modelling even in the presence of irregular topography and 2 the advantages of our hybrid approach over pure fe models after discussing the theoretical experiment of a gaussian topography we present an application to modelling the displacement fields induced by selected events of the earthquake sequence that hit the umbriamarche region in the central apennines italy in 1997 devoting major attention to the effect of the apennines chain topography in the source region in all cases the topography is found to play an important role and to introduce non negligible corrections to the coseismic surface displacement components computed through the ffs approximation the main effect is that topography not only affects the magnitude of the surface signal but it may also lead to highfrequency disturbances that tend to grow with the shallowness of the fault

central cascadia subduction zone creep central cascadia between 43 n and 46 n has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes suggesting elevated persistent fault creep in this section of the subduction zone we estimate subduction thrust decade scale locking and crustal block rotations from three component continuous global positioning system gps time series from 1997 to 2013 as well as 80 year tide gauge and leveling derived uplift rates geodetic observations indicate coastal central oregon is rising at a slower rate than coastal washington southern oregon and northern california modeled locking distributions suggest a wide locking transition zone that extends inland under central oregon paleoseismic records of multiple great earthquakes along cascadia indicate less subsidence in central oregon the cascade thrust under central oregon may be partially creeping for at least 6500 years the length of the paleoseismic record reducing interseismic uplift and resulting in reduced coseismic subsidence large accretions of eocene age basalt siletzia terrane between 43 n and 46 n may be less permeable compared to surrounding terranes potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep in a separate inversion three component gps time series from 1 july 2005 to 1 january 2011 are used to estimate upper plate deformation locking between slow slip events sses slip from 16 sses and an earthquake mechanism cumulative sses and tectonic tremor are weakest between 43 n and 46 n where partial fault creep is increased and siletzia terrane is thick suggesting that surrounding rock properties may influence the mode of slip

slow slip phenomena in cascadia from 2007 and beyond a review recent technological advances combined with more detailed analyses of seismologic and geodetic observations have fundamentally changed our understanding of the ways in which tectonic stresses arising from plate motions are accommodated by slip on faults the traditional view that relative plate motions are accommodated by a simple cycle of stress accumulation and release on locked plate boundary faults has been revolutionized by the serendipitous discovery and recognition of the significance of slow slip phenomena mostly in the deeper reaches of subduction zones the cascadia subduction zone located in the pacific northwest of the conterminous united states and adjacent canada is an archetype of exploration and learning about slow slip phenomena these phenomena are manifest as geodetically observed aseismic transient deformations accompanied by a previously unrecognized class of seismic signals although secondary failure processes may be involved in generating the seismic signals the primary origins of both aseismic and seismic phenomena appear to be episodic fault slip probably facilitated by fluids on a plate interface that is critically stressed or weakened in cascadia this transient slip evolves more slowly and over more prolonged durations relative to the slip in earthquakes and it occurs between the 30 and 40 km depth contours of the plate interface where information was previously elusive although there is some underlying organization that relaxes nearly all the accrued plate motion stresses along the entirety of cascadia we now infer that slow slip evolves in complex patterns indicative of propagating stress fronts our new understanding provides key constraints not only on the region where the slow slip originates but also on the probable characteristics of future megathrust earthquakes in cascadia herein we review the most significant scientific issues and progress related to understanding slow slip phenomena in cascadia and highlight some of their societal implications we provide a comprehensive review from the big picture as inferred from studies of regional scale monitoring data to the details revealed by innovative focused experiments and new instrumentation we focus on what has been learned largely since 2007 when several major investments in monitoring and temporary deployments dramatically increased the quality and quantity of available data

the seismic coupling of subduction zones revisited 1 the nature of seismic coupling for many of the world s subduction zones has been reevaluated geodetic estimates of seismic coupling obtained from gps measurements of upper plate deformation during the interseismic period are summarized we compared those with new estimates of seismic coupling obtained from seismological data the results show that with a few notable exceptions the two methods agree to within about 10 the seismological estimates have been greatly improved over those made 20 30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence in the intervening years of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record the data also again with a few notable exceptions support the frictional instability theory of seismic coupling and in particular the test of that theory made by scholz and campos 1995 overall the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value there is also considerable variation of coupling within individual subduction zones earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

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

coupling on the northern cascadia subduction zone from geodetic measurements and physics based models kinematic inversions of gps and tide gauge leveling data display an unresolved gap between the downdip limit of the locked megathrust and the top of the episodic tremor and slip ets zone in northern cascadia this work combines physics based models of slow slip events with both mean ets displacements and decadal averaged deformation rates to explain the gap and determine how interseismic stress accumulates on the megathrust while physics based predictions match the average ets displacements they significantly misfit long term rates implying faster slip rates within both the gap and the ets region heterogeneous green s functions or velocity strengthening friction within the gap cannot explain the decadal rates the observed uplift rates require steeper gradients in slip rate at the base of the locked zone we invert for the smallest possible shear stress rate on the creeping megathrust below a locked zone that satisfactorily fits the data a nonzero shear stress rate within the ets zone reaching 2 5 kpa yr at a depth of 25 30 km is required finally of all the models that adequately fit both horizontal and vertical data only those with deep locking depths around 21 km significantly improve the fit to the uplift rates

deep water formation and quaternary glaciations a large fraction of climatic varability on the quaternary time scale can be explained by nonlinear interactions between the radiation balance of the global atmosphere ocean system and the mass balance of northern hemisphere ice sheets recent analyses of paleoclimatic proxy data indicate a further important contribution to this variability from changes in deep water formation occurring in the north atlantic subpolar sea we study the effects of these changes on variations in global temperature and ice volume characteristic of the late quaternary the novel framework of boolean delay equations bdes is used to formulate a conceptual model of the climatic system under study and to analyze this formal model selfsustained oscillations in the intensity of the atlantic ocean s thermohaline circulation result from the interaction of sea ice formation with the waxing and waning of continental ice sheets the comparison of model results with paleoclimatic records suggests a considerable slowing down of the abyssal circulation during glacial episodes

broadband synthetic seismograms for magnitude 9 earthquakes on the cascadia megathrust based on 3d simulations and stochastic synthetics part 1 methodology and overall resultsmethodology and overall results

near source ground motion from steady state dynamic rupture pulses 1 ground motion from two dimensional steady state dynamic ruptures is examined for both subshear and supershear rupture velocities synthetic seismograms demonstrate that coherent high frequency information about the source process rapidly attenuates with distance from the fault for subshear ruptures such records provide almost no resolution of the spatial extent of the stress breakdown zone at supershear speeds s waves radiate away from the fault preserving the full source spectrum and carrying an exact history of the slip velocity on both the fault parallel and fault normal components of motion whose amplitudes are given by a function of rupture speed that vanishes at times the s wave speed the energy liberated from the strain field by the passage of a supershear rupture is partitioned into fracture energy dissipated within the fault zone and far field s wave radiation the partition depends on both the rupture velocity and the size of the breakdown zone

slow slip event within a gap between tremor and locked zones in the nankai subduction zone we report on two small long term slow slip events sses within a gap between tremor and locked zones in the nankai subduction zone southwest japan the sses were detected by subtracting the steady state component and postseismic effects of large earthquakes from long term and high density global navigation satellite system data both sses occurred in adjacent regions of the bungo channel following long term sses in the bungo channel in 2003 and 2010 the estimated slip was 1 5 cm year that lasted at least 1 2 years after 2004 and 2011 partly accommodating plate convergence as the low frequency tremor in the downdip region is activated at the same time as the bungo channel long term sse a spatiotemporal correlation was observed between the detected sses and long term tremor activity in the downdip region this correlation indicates along dip interaction of the slips on the subducting plate interface

high frequency paleovariability in climate and co2 levels from vostok ice core records the high resolution of the vostok records provides a unique look at the causes of paleoclimatic variability during the last complete glacial cycle the records present strong evidence for the interaction between orbital forcing and internal physico chemical mechanisms of variability this interaction appears to account for the great wealth of spectral features found in the records

magnetostratigraphy of the late neogene purisima formation santa cruz county california abstract the magnetic polarity zonation of a late neogene sedimentary sequence in santa cruz county california has provided a chronologic framework for studies of the sedimentologic and tectonic processes involved in an episode of basin formation in the vicinity of the san andreas fault system in central coastal california the zonation is based on the analysis of samples from 79 horizons in a 300 m thick section of the purisima formation and a portion of the overlying aromas formation although rock magnetic studies support the hypothesis that the primary carrier of the remanence is magnetite many samples contain a secondary overprint which cannot be completely removed by alternating field demagnetization nevertheless systematic analysis of the behavior of the samples during demagnetization has led to an unambiguous determination of the polarity of each horizon and to the development of a magnetic polarity zonation containing thirteen magnetozones these magnetozones can be correlated to the magnetic polarity time scale using biostratigraphic constraints provided by diatoms in the lower portion of the section and invertebrates and vertebrates in the upper portion the studied section is found to span the interval from the epoch 5 epoch 6 boundary 6 07 mya to the matuyama gauss boundary 2 47 mya with a hiatus corresponding to the upper part of the gilbert epoch 4 5 mya to 3 5 mya this hiatus does not coincide with major regressions in the global sea level curve and is interpreted as a period of tectonic uplift the compression which generated this uplift was probably caused by interplay between the san andreas fault east of the study area and the san gregorio hosgri fault west of it

denitrification in continental shelf sediments has major impact on the oceanic nitrogen budget denitrification rates in sediments within the oxygen deficient waters off mexico and from the gulf of maine were investigated on the basis of interstitial nutrient profiles nitrate fluxes into the sediments were calculated from gradients across the sediment water interface and vertical molecular diffusion coefficients and averaged 0 151 mexico and 0 0920 gulf of maine pmol no 3 cm 2 s 1 these are minimum values since these gradients may have been underestimated in the gulf of maine bottom water irrigation by macrobenthos increases the nitrate supply well above this estimate in addition only 15 22 of the expected ammonium is present in gulf of maine sediments perhaps because of removal by a rapid coupling of nitrification with denitrification this large apparent loss of the regenerated ammonium appears to be ubiquitous in shelf sediments with oxygenated bottom water the global denitrification rate in continental shelf sediments was reassessed to be 50 tg n yr 1 1 tg 1012 g demonstrating that sediments are an important sink for oceanic nitrogen globally current nitrogen losses from the oceans may exceed inputs by 60 90 tg n yr 1 over the glacial interglacial cycle the global sedimentary denitrification rate probably varied commensurately with the changing continental shelf area an oscillating oceanic nitrogen budget over these time scales could occur given the sequence of 1 scouring and dumping of terrestrial nitrogen into the oceans during glacial advance 2 removal of oceanic combined nitrogen to the atmosphere by denitrification following glacial retreat and 3 reincorporation of this n into terrestrial biomass during the interglacial period

an analysis of the effects of thermal demagnetization on magnetic carriers we have attempted to determine why samples of the mio pliocene purisima formation are not amenable to thermal demagnetization analyses of the changes in direction intensity susceptibility color and saturation magnetization as well as examination of individual magnetic grains using a scanning electron microscope have all contributed to the development of a consistent hypothesis for the fate of iron during thermal demagnetization in this particular case we believe that oxidation exsolution of ilmenite and then direct dissolution of nearly pure magnetite result from progressively higher heating levels our approach should be useful in understanding the origin and nature of the magnetization of many lithologies including those for which thermal demagnetization is the method of choice

on the initiation of sustained slip weakening ruptures by localized stresses numerical simulations of dynamic earthquake rupture require an artificial initiation procedure if they are not integrated in long term earthquake cycle simulations a widely applied procedure involves an overstressed asperity a localized region stressed beyond the static frictional strength the physical properties of the asperity size shape and overstress may significantly impact rupture propagation in particular to induce a sustained rupture the asperity size needs to exceed a critical value although criteria for estimating the critical nucleation size under linear slip weakening friction have been proposed for 2 d and 3 d problems based on simplifying assumptions they do not provide general rules for designing 3 d numerical simulations we conduct a parametric study to estimate parameters of the asperity that minimize numerical artefacts e g changes of rupture shape and speed artificial supershear transition higher slip rate amplitudes we examine the critical size of square circular and elliptical asperities as a function of asperity overstress and background off asperity stress for a given overstress we find that asperity area controls rupture initiation while asperity shape is of lesser importance the critical area obtained from our numerical results contrasts with published theoretical estimates when background stress is low therefore we derive two new theoretical estimates of the critical size under low background stress while also accounting for overstress our numerical results suggest that setting the asperity overstress and area close to their critical values eliminates strong numerical artefacts even when the overstress is large we also find that properly chosen asperity size or overstress may significantly shorten the duration of the initiation overall our results provide guidelines for determining the size of the asperity and overstress to minimize the effects of the forced initiation on the subsequent spontaneous rupture propagation

the importance of site selection sediment supply and hydrodynamics a case study of submarine paleoseismology on the northern cascadia margin washington usa abstract the washington continental margin presents both a classic test of submarine paleoseismology and an opportunity to explore advancement of the field through analysis of sediment dispersal in a heterogeneous system new and archive core bathymetric backscatter and seismic reflection data from the washington cascadia margin show that during high stand conditions the northernmost canyons barkley nitinat juan de fuca jdf and to some extent quillayute are relict systems with little holocene recharge the remaining canyons quinault grays guide and willapa are recharged to a varying degrees by northward transport of columbia river derived sediment all systems are nonetheless active conduits for turbidity currents during the holocene which are weaker and more restricted than their pleistocene counterparts sedimentologic and ct analysis supported by radiocarbon ages micropaleontology and the mazama ash datum show that the holocene sedimentary sequence consists of a series of sand to mud turbidites in the active portions of all systems interbedded with hemipelagic sediment however the pleistocene astoria and nitinat fans are largely inactive in the holocene with turbidity current activity limited to the proximal parts of the main channels within active systems the turbidite record is modulated by local landsliding and growth of active folds and faults in the relict systems the turbidite record begins in the middle canyon reaches as opposed to the actively recharged systems which have a turbidite record in the more proximal canyon reaches deposits in the relict systems increase in thickness and modal grain size downstream suggesting multiple sources and or entrainment of material along the canyon walls hydrodynamic models using monte carlo model searches for best fitting flow parameters show the most likely pathways by which holocene and pleistocene currents flowed through the complex accretionary wedge to the abyssal plain model results supported by heavy mineral distributions show that the northern canyon systems barkley nitinat jdf quillayute are independent of the southern systems quinault guide grays willapa during the holocene supporting the classic confluence test balancing of sediment volumes required suggests sediment supplied to the slope canyons and abyssal channels in turbidity current events is 2 5 times greater than that supplied by recharge to the canyon heads by external sources this implies a process that is independent of recharge that scavenges the plentiful relict and hemipelagic sediment supply on the continental slope lithostratigraphic correlation and age models suggest similar sequences were deposited in most systems independently and likely synchronously the correlated turbidite sequence suggests deposition of 23 holocene and 16 post mazama turbidites in most parts of the canyon systems with only modest variation mostly attributable to evidence of bypassing and erosion the best candidate for triggering of these sequences remains great earthquakes and the sequence correlates well to onshore and other offshore paleoseismologic evidence the new data including new cores in northern oregon suggest revisions to segmented rupture boundaries several partial ruptures segment b likely extended further north to juan de fuca canyon one event was resolved into two and one washington only event was added most segment c ruptures were found to extend further northward to the latitude of astoria canyon mean recurrence times are therefore revised to 434 years for the washington coast and 340 years for the central and northern oregon coast conditional probabilities in the next 50 years also are revised to 10 17 for the washington coast and 16 22 for the central and northern oregon coast in cascadia sumatra and other locales globally core siting spatial sample density high resolution analysis of core and regional geophysics and understanding of flow dynamics are critical to capturing a paleoseismic record sedimentary records vary across turbidity current systems of varying sediment supply physiography and sediment sources variations in the record will reflect these factors yet may still present a consistent paleoseismic history this analysis highlights the importance of a thorough understanding of flow paths hydrodynamics and the interplay of sedimentation and local tectonics when testing for evidence of earthquakes in the sedimentary record of active margins

rapid post miocene tectonic rotation associated with the san gregorio fault zone in central california paleomagnetic measurements of samples from the mio pliocene purisima formation demonstrate that the pomponio tectonic block of central coastal california has rotated clockwise by approximately 35 to 55 within the last 2 5 million years the most likely interpretation of this data is that the pomponio block is broken into several small blocks which have rotated by various amounts the data suggest that rotations contribute to vertical deformation and secondary faulting within the central san andreas fault system and that they play an important role in the accommodation of shear along the fault system

slip weakening models of the 2011 tohoku oki earthquake and constraints on stress drop and fracture energy we present 2d dynamic rupture models of the 2011 tohoku oki earthquake based on linear slip weakening friction we use different types of available observations to constrain our model parameters the distribution of stress drop is determined by the final slip distribution from slip inversions as three groups of along dip slip distribution are suggested by different slip inversions we present three slip weakening models in each model we assume uniform critical slip distance eastward from the hypocenter but several asperities with smaller critical slip distance westward from the hypocenter the values of critical slip distance are constrained by the ratio of deep to shallow high frequency slip rate power inferred from back projection source imaging our slip weakening models are consistent with the final slip slip rate rupture velocity and high frequency power ratio inferred for this earthquake the average static stress drop calculated from the models is in the range of 4 5 7 mpa though large spatial variations of static stress drop exist to prevent high frequency radiation in the region eastward from the hypocenter the fracture energy needed there is in the order of 10 mj m2 and the average up dip rupture speed cannot exceed 2 km s the radiation efficiency calculated from our models is higher than that inferred from seismic data suggesting the role of additional dissipation processes we find that the structure of the subduction wedge contributes significantly to the up dip rupture propagation and the resulting large slip at shallow depth

fossil seep structures of the monterey bay region and tectonic structural controls on fluid flow in an active transform margin abstract the subsurface plumbing system of ancient seeps of hydrocarbon rich gases and fluids is partially preserved by seep deposits in miocene rocks of the coast ranges and on the seafloor of the monterey bay region in central california the fossil seep related deposits are of two types authigenic carbonates and intrusions of oil saturated sandstones both types occur in fractured and faulted siliceous mudstone and porcelanite layers of the organic rich miocene santa cruz mudstone they are associated with tectonic structures indicating deformation and are located in the proximity of faults the morphology of the seep carbonates is characterized by strong asymmetries and by holes pipes and other types of fluid conduits with orientations parallel to the directions of the local fracture and fault sets thus suggesting structural control over pathways of fluid migration the dominant direction of the conduits in the seep carbonates and of the intrusive dikes northeast southwest differs from the dominant orientation of the tectonic structures of the san andreas fault system northwest southeast previous interpretations of these ancient seep related deposits suggested that lateral changes of the thickness of the cenozoic marine host sediments favored fluid migration from basin depocenters located offshore toward basin margins different driving forces for fluids have been hypothesized including hydrocarbon generation differential compaction and tectonic compression most of the fossil seeps are located on structural highs of the basement and adjacent to offshore basins that form elongated depressions oriented northwest southeast previously unidentified structures that offset the basement along northeast southwest trends cross setting faults paralleling the orientations of the conduits of the seep carbonates and the dikes of oil saturated sandstones were also recognized tectonic deformation block rotations documented by local clockwise rotations of paleomagnetic directions in the coastal ranges and cross faults were probably produced during the late miocene and pliocene to accommodate the shear stress between conjugate faults during eastward shifting of the activity of the san andreas fault system comparison with previous studies on ancient seep deposits regional geology and offshore tectonic data suggest that the fluids that sourced the seeps were driven by episodes of inverse tectonics during which previously extensional depositional basins were involved in episodes of tectonic compression tectonic deformation created cross faults in rotated blocks which could have acted as important conduits for the flow of hydrocarbon rich fluids and gases and fostered the injection of liquefied sands

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

overlapping regions of coseismic and transient slow slip on the hawaiian decollement abstract earthquakes and slow slip are different faulting processes that release accumulated stress on different time scales despite nearly two decades of study it is still unclear whether the same section of fault is capable of hosting both slow aseismic and fast seismic slip here we jointly invert gps strong motion records and empirically corrected tsunami waveforms for the 2018 m7 1 hawai i earthquake our inversion results suggest the earthquake had a slow rupture speed of 1 2 km s with a maximum slip of 3 m south east of the hypocenter extending to the south west through an area known to regularly host slow slip events after exploring how the choice of fault geometry and regularization parameters affect the slip distributions of the earthquake and slow slip events we find strong evidence of overlap between the two additionally we perform numerical modeling to simulate fast and slow slip behavior due to the homogeneity of the hawaiian decollement differences in effective stress are likely an important factor that causes the diversity of slip we find that an earthquake can completely penetrate into the slow slip zone provided the effective stress differences between fast and slow slip zones is large enough to facilitate this our results reinforce the idea that an individual section of fault is capable of hosting a variety of distinct slip behaviors this is important for estimating earthquake rupture extent and for better ground motion and tsunami hazard assessment

determination of slow slip episodes and strain accumulation along the cascadia margin 1 continuous gps stations in the pacific northwest geodetic array network clearly record subduction related strain accumulation and slow slip episodes along the cascadia convergent margin many of the slow slip episodes have been correlated in time and space with seismic evidence for nonvolcanic tremor leading to the previous discovery of episodic tremor and slip ets in this study we use a hyperbolic tangent curve fitting technique for the identification of slow slip times and displacement magnitudes within the gps time series independent of seismic tremor data we then apply this technique to study the patterns of strain accumulation and release associated with ets events and characterize patterns of coupling associated with the locked and transition zones of the plate interface we demonstrate the effectiveness of this automated technique for both identification of slow slip observations and calculation of slow slip displacements recurrence patterns in the distribution of gps observations demonstrate coherence among neighboring stations over time and apparent along strike segmentation of the subduction interface when slow slip events are removed from the time series we can estimate the total site velocities between slow slip events these velocities decay as depth to the subduction interface increases but they diverge from the long term trends expected from the interseismic cycle at 30 60 km above the interface consistent with the location where slow slip displacements occur forward modeling of coupling on the plate interface reveals that in between slow slip events there is a patch of at least 30 coupling from 20 to 35 km depth which is needed to produce the observed back slip displacements intriguingly our best fitting models have a decrease in coupling down to 30 at 20 km depth followed by a peak of greater than 80 coupling at 30 35 km depth suggesting the source zone for ets events acts as a distinct locking zone that releases strain more frequently than the updip seismogenic locked zone although a zone of constant 30 coupling cannot be ruled out with this data set such a scenario indicates that frictional behavior with depth follows a more complex model than a simple temperature controlled transition we propose that coupling initially decreases with depth due to a decrease in strength of the overriding lower crust but then coupling increases again when the subducting plate comes in contact with the stronger overriding mantle

a window into the complexity of the dynamic rupture of the 2011 mw 9 tohoku oki earthquake the 2011 mw 9 tohoku oki earthquake recorded by over 1000 near field stations and multiple large aperture arrays is by far the best recorded earthquake in the history of seismology and provides unique opportunities to address fundamental issues in earthquake source dynamics here we conduct a high resolution array analysis based on recordings from the usarray and the european network the mutually consistent results from both arrays reveal rupture complexity with unprecedented resolution involving phases of diverse rupture speed and intermittent high frequency bursts within slow speed phases which suggests spatially heterogeneous material properties the earthquake initially propagates down dip with a slow initiation phase followed by sustained propagation at speeds of 3 km s the rupture then slows down to 1 5 km s for 60 seconds a rich sequence of bursts is generated along the down dip rim of this slow and roughly circular rupture front before the end of the slow phase an extremely fast rupture front detaches at about 5 km s towards the north finally a rupture front propagates towards the south running at about 2 5 km s for over 100 km key features of the rupture process are confirmed by the strong motion data recorded by k net and kik net the energetic high frequency radiation episodes within a slow rupture phase suggests a patchy image of the brittle ductile transition zone composed of discrete brittle asperities within a ductile matrix the high frequency is generated mainly at the down dip edge of the principal slip regions constrained by geodesy suggesting a variation along dip of the mechanical properties of the mega thrust fault or their spatial heterogeneity that affects rise time

broadband synthetic seismograms for magnitude 9 earthquakes on the cascadia megathrust based on 3d simulations and stochastic synthetics part 2 rupture parameters and variability

arctic ocean glacial history while there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the arctic ocean a holistic view on the northern hemisphere s late quaternary ice sh

secular change of the seasonal variation in sea level and of the pole tide in the baltic sea regular sea level observations have been performed in stockholm since 1825 yielding the longest unbroken series of monthly mean sea levels in the world applying fourier analysis to these data we have investigated the annual and semiannual sea level variations as well as the pole tide whereas the semiannual variation has a fairly constant amplitude of about 4 cm the annual variation shows a secular increase in amplitude from 8 to 10 cm this increase is statistically significant at the 99 level it is suggested that this change might be connected to secular changes of the oceanographic conditions that have been observed to occur in the northeastern north atlantic especially a movement of the oceanic polar front the pole tide also shows a secular change the amplitude for the 14 3 month chandler period was less than 2 cm below the noise level during the previous century during our own century the amplitude is 3 cm 6 times the equilibrium tide this might indicate that a meteorological forcing component close to the chandler period has changed and added variability to the sea level at this period

time and size of a giant earthquake in cascadia inferred from japanese tsunami records of january 1700 geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the cascadia subduction zone off the pacific coast of north america the most recent event or series of events is dated at about 300 years ago1 4 but the precise date and magnitude have not been determined geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two4 although this information is important for the assessment of future hazard5 we have found several tsunami records in japan from ad 1700 with no indication of a local cause historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in south america alaska or kamchatka leaving cascadia as the most likely source of this tsunami the estimated time of the earthquake is the evening about 21 00 local time of 26 january 1700 the magnitude is estimated as 9 from the tsunami heights in which case the earthquake ruptured the entire length of the cascadia subduction zone2 these estimates are consistent with native american legends that an earthquake occurred on a winter night6

spatiotemporal evolution of slow slip events in a nonplanar fault model for northern cascadia subduction zone slow slip events sses are identified as the quasi stable fault deformation in the deep transition zone from locked to continuous sliding in many subduction zones in the well instrumented cascadia margin a class of mw6 0 slow slip events arise beneath port angeles every 14 months as inferred from two decades of continuous geodetic monitoring the along strike bending of the incoming oceanic plate beneath north washington is a unique geometric feature whose influence on slow slip processes is still unknown here we incorporate a realistic fault geometry of northern cascadia in the framework of rate and state dependent friction law to simulate the spatiotemporal evolution of slow slip events on a nonplanar subduction fault the modeled sses capture the major characteristics revealed by gps observations the central 150 km long fault segment beneath port angeles acts as a repetitive slip patch where sses appear every 1 5 years with a maximum slip of 2 5 cm two minor slip patches with smaller areas and cumulative slips straddle this central slip patch the along strike segmentation of slow slip is inversely related to the local fault dip and strike angles of the slow slip zone suggesting strong geometrical control on the slow slip process this correlation holds even after removing the effect of w h ratio between velocity weakening sse fault width and characteristic nucleation size besides the gps detectable fast spreading phase we find that each sse cycle consists of deep pre sse preparation and post sse relaxation phases which may be the driving mechanism for the deep tremor activity between major sse episodes discovered in cascadia

on the hydrographic and ice conditions in the northern north atlantic during different phases of a glaciation cycle abstract a theoretical investigation of the hydrographic conditions in the northern north atlantic n of the greenland scotland ridge for different forcing conditions is undertaken first a simple dynamical model for the mediterranean circulation in the greenland and norwegian seas the subpolar sea is developed the calculated contemporary rate of deepwater formation about 2 5 10 6 m 3 s seems to be quite realistic it is concluded that the mediterranean circulation is possible as long as the cyclonal atmospheric circulation in the subpolar sea is strong enough to prevent the fresher water in the coastal currents along the eastern and western boundaries from spreading over the interior utilizing the arctic ocean model in stigebrandt 1981 the hydrographic and ice conditions in the subpolar sea and the arctic ocean during different phases of a glaciation are finally investigated as so many parameters are involved the derivations are by necessity rather uncertain however it seems possible that the hydrographic state in the subpolar sea changes between mediterranean and estuarine circulation several times during a glacial cycle

modeling the effects of source and path heterogeneity on ground motions of great earthquakes on the cascadia subduction zone using 3d simulations abstract we ran finite difference earthquake simulations for great subduction zone earthquakes in cascadia to model the effects of source and path heterogeneity for the purpose of improving strong motion predictions we developed a rupture model for large subduction zone earthquakes based on a k 2 slip spectrum and scale dependent rise times by representing the slip distribution as the sum of normal modes of a vibrating membrane finite source and path effects were important in determining the distribution of strong motions through the locations of the hypocenter subevents and crustal structures like sedimentary basins some regions in cascadia appear to be at greater risk than others during an event due to the geometry of the cascadia fault zone relative to the coast and populated regions the southern oregon coast appears to have increased risk because it is closer to the locked zone of the cascadia fault than other coastal areas and is also in the path of directivity amplification from any rupture propagating north to south in that part of the subduction zone and the basins in the puget sound area are efficiently amplified by both north and south propagating ruptures off the coast of western washington we find that the median spectral accelerations at 5 s period from the simulations are similar to that of the zhao et al 2006 ground motion prediction equation although our simulations predict higher amplitudes near the region of greatest slip and in the sedimentary basins such as the seattle basin

the arctic atlantic thermohaline circulation abstractthe atlantic ocean s thermohaline circulation is an important modulator of global climate its northern branch extends through the nordic seas to the cold arctic a region that appears to be particularly influenced by climate change a thermohaline circulation is fundamentally concerned with two degrees of freedom this is in particular the case for the inflow of warm and saline atlantic water through the nordic seas toward the arctic that is balanced by two branches of outflow the authors present an analytical model rooted in observations that constrains the strength and structure of this arctic atlantic thermohaline circulation it is found maybe surprisingly that the strength of atlantic inflow is relatively insensitive to anomalous freshwater input it mainly reflects changes in northern heat loss freshwater anomalies are predominantly balanced by the inflow s partition into estuarine and overturning circulation with southward polar outflow in the surface and dense overflow at depth res

long term nongeocentric axial dipole directions and a geomagnetic excursion from the middle pleistocene sediments of the humboldt river canyon pershing county nevada a paleomagnetic record has been obtained from the middle pleistocene sediments of the humboldt river canyon pershing county nevada the age of the sediments is constrained by two tephra layers the 610 000 year old lava creek tephra and the approximately 495 000 year old dibekulewe tephra the magnetization of the sediments is strong and stable and the similarity of data from replicate sections strongly suggests that the data set represents an accurate record of the magnetic field except for the presence of postmagnetization rotations of less than 10 the influence of postmagnetization rotations was eliminated in one of the sections hr1a by rotating the paleomagnetic directions in accordance with the direction of the lava creek welded tuff previously determined from five sampling sites near yellowstone national park after correcting the mean directions we observed nongeocentric axial dipole non gad mean directions of moderate amplitude 19 13 in declination and 22 8 in inclination in the record from the hrla section which may represent as much as 115 000 years of magnetic field behavior in the youngest part of the humboldt river canyon sediments a zone of anomalous directions was recorded which we interpret to be a detailed record of an unconfirmed geomagnetic excursion this record exhibits behavior consistent with that of previously reported polarity transitions because of the widespread nature of the tephras especially the lava creek tephra the prospects are very good for improving estimates of the magnitude and duration of the non gad paleomagnetic directions and also for confirming the excursion

simple shear of southern california during neogene time suggested by paleomagnetic declinations we have studied the paleomagnetism of various neogene age rocks in southern california as a means of determining the amounts of oligocene and younger tectonic rotation and translation which has occurred in this region our results suggest that fully 25 of this area in particular the transverse ranges has undergone extreme clockwise rotation work in southeastern california implies that 40 of clockwise rotation has occurred here although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200 the crustal block bounded by the san gabriel and san andreas fault has undergone a net clockwise rotation of 35 although the data here can be interpreted to show that an original early miocene rotation of about 50 was followed by a late miocene or pliocene counterclockwise rotation of 15 paleomagnetic results from the offshore islands suggests that san clemente santa barbara and san nicolas islands have not rotated but santa catalina has undergone about 90 of clockwise rotation all of the northern channel islands including anacapa santa cruz santa rosa and san miguel are implied to have rotated 70 or 80 clockwise this result is also found for the santa monica mountains east of anacapa island in the santa ynez range north of the channel islands paleomagnetic study of the monterey formation also indicates large clockwise rotations of near 90 these data also suggest that the santa maria basin is not rotated and that the north boundary of the rotated region is the santa ynez river fault stratigraphic control on the paleomagnetic data from the monterey formation implies that the rotation began about 16 m y ago and may be continuing today in the western region paleomagnetic inclination data from our study show that the northern channel islands in particular may have translated 15 northward since middle miocene time however equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections non dipole magnetic field behavior and right offset on the san andreas fault system palinspastic reconstruction of southern california regions for the early miocene implies that parts of the transverse ranges structures were once aligned with north trending extensional structures in the southwestern united states we propose that pacific american plate interactions both rifted the continental crust to create this pattern and rotated the western most structures within a dextral simple shear zone which had a half width of about 400 km

tectonic tremor and slow slip along the northwestern section of the mexico subduction zone abstract the southwestern coast of mexico is marked by active subduction of the rivera and cocos plates producing megathrust earthquakes that tend to recur every 50 100 yr herein we use seismic and gps data from this region to investigate the potential relationship between earthquakes tectonic non volcanic tremor and transient slip along the westernmost 200 km of the mexico subduction zone visual examination of seismograms and spectrograms throughout the 18 month long mars seismic experiment reveals clear evidence for frequent small episodes of tremor along the rivera and cocos subduction zones beneath the states of jalisco colima and michoacan using a semi automated process that identifies prominent energy bursts in envelope waveforms of this new data analyst refined relative arrival times are inverted for source locations using a 1 d velocity model the resulting northwest southeast trending linear band of tremor is located downdip from the rupture zones of the 1995 mw 8 0 colima jalisco and 2003 mw 7 2 tecoman subduction thrust earthquakes and just below the regions of afterslip triggered by these earthquakes despite the close proximity between tremor and megathrust events there is no evidence that the time since the last great earthquake influences the spatial or temporal pattern of tremor a well defined gap in the tremor beneath the western colima graben appears to mark a separation along the subducted rivera cocos plate boundary from the position time series of 19 continuous gps sites in western mexico we present the first evidence that slow slip events occur on the rivera plate subduction interface unlike the widely recorded large amplitude slow slip events on the nearly horizontal cocos plate subduction interface below southern mexico slow slip events below western mexico have small amplitudes and are recorded at relatively few mostly coastal stations the smaller slow slip beneath western mexico may be due to the steeper dip causing a narrower zone where the conditions for slow slip are met the prominent afterslip following the m 8 1995 colima jalisco and m 7 2 2003 tacoma megathrust earthquakes may also shrink the area available for episodic slow slip