determination of focal mechanisms of nonvolcanic tremor using s wave polarization data corrected for the effects of anisotropy we propose a method to determine the focal mechanisms of nonvolcanic tremors using anisotropy corrected s polarizations measured directly from noisy tremor signals the successful retrieval of polarization angles in noisy signals owes much to the observation that in general tremors propagate slowly and therefore do not change their location immediately this feature of tremors enables us to use longer time windows to compute polarization angles e g 1 min or longer resulting in a stack of particle motions we applied the method to a tremor episode that occurred beneath the kii peninsula in april 2013 although the majority of inferred focal mechanisms suggested that tremors manifest shear slip on the plate interface some events with strike slip components were also found to have occurred the proposed method provides us with a tool for investigating smaller scale spatial distributions and temporal variations of focal mechanisms throughout tremor zones

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

characteristics of slow earthquakes in the very low frequency band application to the cascadia subduction zone deep tectonic tremors detected in many subduction zones worldwide are often accompanied by very low frequency vlf signals detectable by broadband seismometers but usually hidden in large ambient noise by stacking broadband seismograms relative to tremor hypocentral times we can recover the vlf signals the stacked signals are then inverted to determine a moment tensor solution using a procedure previously applied to vlf signals in japan taiwan and mexico here we apply this method to the cascadia subduction zone where tremors and slow slip events are clearly observed as expected we successfully recover vlf signals for almost the entire tremor region beneath southern vancouver island and northern washington state the moment tensors are mostly well determined as low angle thrust type but source depths are poorly constrained the slip direction is slightly rotated counterclockwise with respect to the local plate motion direction probably due to bending of the subducting plate the seismic moment measured in vlf band is proportional to the seismic energy of tremors with a scaled energy of about 3 10 9 the widespread observability of vlf signals suggests that the deformation associated with tremors and sses is actually a very broadband phenomenon as suggested by stochastic models

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

very low frequency earthquakes along the ryukyu subduction zone 1 a total of 1314 very low frequency earthquakes vlfes were identified along the ryukyu trench from seismograms recorded at broadband networks in japan f net and taiwan bats in 2007 the spectra of typical vlfes have peak frequencies between 0 02 to 0 1 hz among those waveforms from 120 vlfes were inverted to obtain their centoroid moment tensor cmt solutions and locations using an examination grid to minimize a residual between the observed and synthetic waveforms within an area of 11 14 in latitude and longitude and at depths of 0 to 60 km most of the vlfes occur on shallow thrust faults that are distributed along the ryukyu trench which are similar to those earthquakes found in honshu and hokkaido japan the locations and mechanisms of vlfes may be indicative of coupled regions within the accretionary prism or at the plate interface this study highlights the need for further investigation of the ryukyu trench to identify coupled regions within it

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

timing of the nihewan formation and faunas abstract magnetostratigraphic dating of the fluvio lacustrine sequence in the nihewan basin north china has permitted the precise timing of the basin infilling and associated nihewan mammalian faunas the combined evidence of new paleomagnetic findings from the hongya and huabaogou sections of the eastern nihewan basin and previously published magnetochronological data suggests that the nihewan formation records the tectono sedimentary processes of the plio pleistocene nihewan basin and that the nihewan faunas can be placed between the matuyama brunhes geomagnetic reversal and the onset of the olduvai subchron 0 78 1 95 ma the onset and termination of the basin deposition occurred just prior to the gauss matuyama geomagnetic reversal and during the period from the last interglaciation to the late last glaciation respectively suggesting that the nihewan formation is of late pliocene to late pleistocene age the nihewan faunas comprising a series of mammalian faunas such as maliang donggutuo xiaochangliang banshan majuangou huabaogou xiashagou danangou and dongyaozitou are suggested to span a time range of about 0 8 2 0 ma the combination of our new and previously published magnetostratigraphy has significantly refined the chronology of the terrestrial nihewan formation and faunas

anisotropic viscoplasticity explains slow slip m 0 t scaling at convergent plate margins abstract in this study we quantify the mechanisms that govern two related observations with regard to deep 15 50 km subduction zone aseismic slow slip events sses i the linear scaling relationship between seismic moments m0 and event durations t and ii the direction dependent slow slip rupture speeds geological observations suggest that deep subduction slow slip shear zones are anisotropic and viscoplastic the anisotropy is due to the presence of dip parallel mafic lineaments created by seamount subduction whereas the viscoplasticity is due to deformation of mixed brittle mafic and ductile felsic materials we postulate that a dip parallel slip direction mafic lineament in an overall felsic slow slip shear zone acts as a stress guide which localizes initial slow slip rupture in the dip direction subsequent stress concentration along the dip parallel edges of the early ruptured lineament leads to along strike rupture with the rupture front propagating through the felsic shear zone the second phase slip area expansion maintains a constant dip parallel rupture zone length inherited from the length of the early ruptured mafic lineament by combining an energy balance equation with a two phase rupture model outlined above we obtain the first analytical expression of the observed linear scaling law in the form of m0 c0t where c 0 4 1 z g 2 l 2 s d gh z e cos v fw 2 l z s 2 d 2 gh 2 4 1 g l z zlg v fw v a 2 the observed value of this empirical constant is between 1011 5 and 1013 5 j s in the above expression l h z g e s and d are length depth thickness dip angle shear rigidity effective viscosity and effective coefficients of static and dynamic friction of the slow slip shear zone 1 is surface energy density of the initially ruptured mafic lineament is overriding plate density vfw and va are subducting plate and slow slip velocities and g is surface gravity our model based on the assumed shear zone anisotropy successfully predicts fast 100 km h dip parallel rupture along high viscosity 1020 pa s mafic lineaments and slow 2 10 km day strike parallel rupture through low viscosity 1017 pa s felsic materials during a deep subduction slow slip event

low frequency earthquakes in the mexican sweet spot 1 we use data from the meso america subduction experiment to detect and locate low frequency earthquakes lfes in the mexican subduction zone we use visually identified templates to perform a network waveform correlation search that produced 17 000 robustly detected lfes that form 15 distinct families stacking an lfe family s corresponding detections results in seismograms with high signal to noise ratios and clear p and s wave arrivals we use these travel times to locate the sources the resulting locations superpose a previously identified region of permanent non volcanic tremor nvt activity husker et al 2012 called this region a sweet spot suggesting that the local conditions are adequate to continuously generate nvt the lfe hypocenters have been located at a depth of 40 45 km in an area that is surrounding the upper slab plate interface we characterize their focal mechanisms by comparing their stacked seismograms to synthetic seismograms this analysis reveals a common low dipping focal mechanism

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

centroid single force inversion of seismic waves generated by landslides the centroid moment tensor cmt waveform inversion method of dziewonski et al 1981 is modified to analyze long period seismic waves generated by single force events such as the gigantic landslides associated with the 1980 eruption of mount st helens we refer to the method as centroid single force csf inversion as the result of the inversion we obtain the spatio temporal centroid and three components of a vector we call the csf vector which is the seismic point source representation of a landslide and whose direction is the opposite of the direction of sliding the sealer magnitude of the csf vector measures the overall size of the landslide and is given by mcsf md where m is the mass of the sliding object and d is the sliding distance we apply csf inversion to the long period surface waves generated by the 1980 mount st helens event the 1975 kalapana earthquake and the 1974 mantaro landslide using seismograms from the gdsn ida and hglp networks the csf solution for the st helens event is remarkably consistent with the actual geological observations and proves the efficacy of csf inversion the csf solution for the kalapana event does not fit the overall waveforms better than the cmt solution does however because the cmt solution does not explain the observed love wave radiation pattern it may be necessary to invoke a combination of both types of mechanism for this event although the geometry of the csf solution the direction of the force for the mantaro event is consistent with the actual landslide mcsf is about 5 times smaller than that expected from the mass and travel distance of the landslide estimated by geological observations this discrepancy may suggest either the relatively aseismic nature of this landslide or that the total volume of the slide was overestimated by analogy to single station cmt inversion single station csf inversion also appears to be stable and useful

magnitudes and moment duration scaling of low frequency earthquakes beneath southern vancouver island we employ 130 low frequency earthquake lfe templates representing tremor sources on the plate boundary below southern vancouver island to examine lfe magnitudes each template is assembled from hundreds to thousands of individual lfes representing over 269 000 independent detections from major episodic tremor and slip ets events between 2003 and 2013 template displacement waveforms for direct p and s waves at near epicentral distances are remarkably simple at many stations approaching the zero phase single pulse expected for a point dislocation source in a homogeneous medium high spatiotemporal precision of template match filtered detections facilitates precise alignment of individual lfe detections and analysis of waveforms upon correction for 1 d geometrical spreading attenuation free surface magnification and radiation pattern we solve a large sparse linear system for 3 d path corrections and lfe magnitudes for all detections corresponding to a single ets template the spatiotemporal distribution of magnitudes indicates that typically half the total moment release occurs within the first 12 24 h of lfe activity during an ets episode when tidal sensitivity is low the remainder is released in bursts over several days particularly as spatially extensive rapid tremor reversals rtrs during which tidal sensitivity is high rtrs are characterized by large magnitude lfes and are most strongly expressed in the updip portions of the ets transition zone and less organized at downdip levels lfe magnitude frequency relations are better described by power law than exponential distributions although they exhibit very high b values 5 we examine lfe moment duration scaling by generating templates using detections for limiting magnitude ranges mw 1 5 mw 2 0 lfe duration displays a weaker dependence upon moment than expected for self similarity suggesting that lfe asperities are limited in fault dimension and that moment variation is dominated by slip this behavior implies that lfes exhibit a scaling distinct from both large scale slow earthquakes and regular seismicity

tectonic volcanic and semi volcanic deep low frequency earthquakes in western japan abstract in western japan relatively small m the present paper characterizes the seismicity of the three types of lfes in five regions based on a more complete catalog thanks to automated event detection and precise relocation using waveform correlation the results show that isolated intraplate lfes are more similar to volcanic lfes than to tectonic lfes the volcanic and isolated intraplate lfes are insensitive to tidal stress are characterized by power law magnitude frequency statistics such as the gutenberg richter relation and are distributed mainly in small vertical clusters while the tectonic lfes are modulated by tidal stress are limited in size and are distributed along the plate boundary hence we propose that isolated intraplate lfes should be named semi volcanic lfes since tectonic lfes and volcanic lfes are thought to be related to the fluid existence and fluid movements respectively isolated intraplate semi volcanic lfes may be related to crustal fluid movements in the source area

analysis of long period seismic waves excited by the may 18 1980 eruption of mount st helens a terrestrial monopole long period 100 to 260 s love and rayleigh waves excited by the eruption of mount st helens on may 18 1980 and recorded by ida sro and asro stations were analyzed to determine the mechanism of the eruption the amplitude radiation patterns of both rayleigh and love waves are two lobed with a nodal direction in e5 s for rayleigh waves and in n5 e for love waves these radiation patterns preclude any double couple mechanism the radiation pattern the initial phase the relatively large amplitude ratio of love to rayleigh waves and the existence of clear nodes in the radiation patterns of fundamental mode and higher mode rayleigh waves suggest that the source is represented by an almost horizontal less than 15 from the horizontal single force pointed toward s5 w the surface wave spectra fall off very rapidly at periods shorter than 75 s suggesting a very slow source process although the details of the source time history could not be determined a smooth bell shaped time function f o s t 1 2 f o l cos t for 0 t 2 and f o s t 0 for t 2 with 75 s is considered appropriate on the basis of comparison between synthetic and observed seismograms and of the shape of the source spectrum the peak value of the force f 0 is about 10 18 dynes the tailing end of the source time function could not be resolved and some overshoot may be added the magnitude and the time history of the force can be explained by a northward landslide followed by a lateral blast observed at the time of the eruption two distinct events about 110 s apart can be identified on body wave and short period surface wave records the first event may correspond to the earthquake which triggered the landslide and the lateral blast the second event appears to correspond to a second large earthquake and explosion which took place about 2 minutes after the first earthquake

a high resolution diatom record of the palaeoclimates of east siberia for the last 2 5 my from lake baikal abstract two parallel drilling cores bdp96 1 200 m and bdp96 2 100 m have been taken from top of the underwater akademichesky ridge in lake baikal and dated by paleomagnetic techniques these cores are part of the baikal drilling project kuzmin et al 1997a b diatom analysis of the first 100 m of these sediments reveals that they store a 2 5 myr high resolution continuous record of the palaeoclimates of east siberia sediments belonging to warmer climates have a high content of diatom algae frustules whereas those belonging to global glaciations are diatom barren the record of upper pleistocene 500 kyr bp strongly correlates with the oceanic specmap curve and contains distinct 100 41 23 and 19 kyr periods as revealed by fourier analysis diversity of diatom species was small in pliocene abrupt and frequent changes of the climate in pleistocene resulted in frequent dramatic changes in the diatom communities the extant dominating endemics cyclotella minuta and aulacoseira baicalensis became important in lake baikal 760 kyr and less than 120 kyr bp respectively

magnetostratigraphic dating of the huojiadi paleolithic site in the nihewan basin north china the fluvio lacustrine sequences in the nihewan basin northern china provide important terrestrial archives of paleoclimate and paleoenvironment as well as an important source of paleolithic settlements and therefore early human occupation in the eastern old world magnetostratigraphic dating of the sequences has permitted the precise timing of the basin infilling and associated paleolithic and mammalian fauna sites here we report new magnetostratigraphic results that place stringent age controls on the huojiadi paleolithic site in this basin partially oxidized magnetite and hematite were identified as the major carriers for the characteristic remanent magnetizations magnetostratigraphic results show that the huojiadi sequence recorded the very late matuyama reverse chron and the jaramillo normal subchron the stone artifact layer resides in the jaramillo normal subchron yielding an estimated age of around 1 0 ma for the huojiadi paleolithic site furthermore by combining our magnetostratigraphy and previously published magnetochronology data a magnetochronological sequence of the early pleistocene paleolithic sites in the nihewan basin has been established the chronological sequence implies that early humans may have persistently colonized the high latitude temperate areas in the eastern old world such as the nihewan basin during the early pleistocene

numerical modeling of tectonically driven river dynamics and deposition in an upland incised valley abstract within upstream reaches of incised valleys fluvial sedimentation occurs where it is controlled by interaction between climate and tectonics this study focuses on a plio pleistocene fluvial paleovalley which drained the northeastern margin of siena basin northern apennines italy valley filling resulted from the interaction between river drainage and active normal faults striking perpendicular and parallel to the main valley through numerical modeling this study aims to refine temporal and spatial mesoscale deposit variations which highlight the upset of fluvial architectures derived from the interplay between the river system and uplift geomorphological and hydrodynamic parameter calibration was performed integrating field studies with paleohydraulic and paleomagnetic data the numerical model simulates the evolution of valley formation with the development of i a pre tectonic steady state system followed by ii a syntectonic aggradation and avulsion phase and iii a post tectonic relaxation phase the syntectonic phase shows fine sediment back filling upstream of the uplifted area and coarse sediment down filling downstream of the upwarping the recorded aggradations are asynchronous with upstream deposition preceding downstream deposition

vegetation succession and climate change during the early pleistocene 2 2 1 8 ma in the nihewan basin northern china abstract the nihewan formation northern china is ideal for studying environmental changes during the early pleistocene in conjunction with palaeomagnetic measurements pollen and grain size analyses were conducted on 120 samples from a 24 m long section 2 2 1 8 ma of core nha from the nihewan basin in order to reconstruct past vegetation and climatic changes the pollen assemblages were dominated by pinus indicating that the vegetation was primarily pine forest and that the climate was relatively warm and wet from 2 15 1 92 ma deciduous broad leaved tree pollen significantly increased to 10 of the total showing that more broad leaved trees grew in the study area during the warmest and wettest period in the study section from 1 92 1 78 ma coeval with the olduvai event pinus and broad leaved tree pollen types decreased the percentages of picea 20 artemisia and chenopodiaceae increased indicating that spruce forests expanded the openness of the forested areas increased and the climate became cold and dry the vegetation changes reconstructed during the olduvai period indicates that the climate in the nihewan basin was cold and dry relating to global cooling facilitated by the uplift of the tibetan plateau and the strengthening of the winter monsoon

geometry and seismic properties of the subducting cocos plate in central mexico the geometry and properties of the interface of the cocos plate beneath central mexico are determined from the receiver functions rfs utilizing data from the meso america subduction experiment mase the rf image shows that the subducting oceanic crust is shallowly dipping to the north at 15 for 80 km from acapulco and then horizontally underplates the continental crust for approximately 200 km to the trans mexican volcanic belt tmvb the crustal image also shows that there is no continental root associated with the tmvb the migrated image of the rfs shows that the slab is steeply dipping into the mantle at about 75 beneath the tmvb both the continental and oceanic moho are clearly seen in both images and modeling of the rf conversion amplitudes and timings of the underplated features reveals a thin low velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab by inverting rf amplitudes of the converted phases and their time separations we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting cocos plate and its thickness high poisson s and vp vs ratios due to anomalously low s wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure the evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates this may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust

southern hemisphere forcing of pliocene δ18o and the evolution of indo asian monsoons monsoon phase relationships consistent with direct orbital forcing across the entire indo asian region including marine and terrestrial proxies from the chinese loess plateau the south china sea and the arabian sea strong pliocene summer and winter monsoons were in phase with one another strengthened at obliquity minima and precession minima the summer monsoon was also strengthened at precession maxima yielding a semiprecession spectral signal strong pliocene monsoons at orbital extremes indicate a direct response to fast physics processes including sensible heating and cooling of the asian landmass and for the summer monsoon the export of latent heat from the southern indian ocean as northern hemisphere ice volume grew into the pleistocene the timing of strong winter and summer monsoons drifted apart becoming influenced by the combined effects of fast physics and slow physics ice volume variables the phase of strong winter monsoons shifted toward ice maxima and the phase of strong summer monsoons shifted toward ice minima

new timing constraints for the onset of laramide deformation in southwest montana challenge our understanding of the development of a thick skinned structural style during flat slab subduction

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

eruption dynamics at mount st helens imaged from broadband seismic waveforms interaction of the shallow magmatic and hydrothermal systems 1 the current eruption at mount st helens is characterized by dome building and shallow repetitive long period lp earthquakes waveform cross correlation reveals remarkable similarity for a majority of the earthquakes over periods of several weeks stacked spectra of these events display multiple peaks between 0 5 and 2 hz that are common to most stations lower amplitude very long period vlp events commonly accompany the lp events we model the source mechanisms of lp and vlp events in the 0 5 4 s and 8 40 s bands respectively using data recorded in july 2005 with a 19 station temporary broadband network the source mechanism of the lp events includes 1 a volumetric component modeled as resonance of a gently nnw dipping steam filled crack located directly beneath the actively extruding part of the new dome and within 100 m of the crater floor and 2 a vertical single force attributed to movement of the overlying dome the vlp source which also includes volumetric and single force components is 250 m deeper and nnw of the lp source at the sw edge of the 1980s lava dome the volumetric component points to the compression and expansion of a shallow magma filled sill which is subparallel to the hydrothermal crack imaged at the lp source coupled with a smaller component of expansion and compression of a dike the single force components are due to mass advection in the magma conduit the location geometry and timing of the sources suggest the vlp and lp events are caused by perturbations of a common crack system

along fault pore pressure evolution during a slow slip event in guerrero mexico abstract slow earthquakes are important constituents of the seismic cycle and are involved in the stress transfer between the viscously slipping portion of the plate interface and the seismogenic zone their occurrence is likely associated with the near lithostatic pore pressure in the slow earthquake source region where fluids might modify fault friction and whose presence is indicated by high ratios of compressional p wave velocity to shear s wave velocity observed at the interface between the subducting plate and the overlying crust here we compare two slow earthquake phenomena observed in the guerrero region of the mexican subduction zone low frequency earthquakes lfes and a slow slip event sse recorded by gps we observe variations of the lfe occurrence rates over month long time scales during a large sse that we interpret as a manifestation of transient changes in the fault shear strength we argue that these transient changes are caused by a pore pressure fluctuation that migrates updip along the subduction interface this mechanism suggests that fluids do not only passively weaken the plate interface but also play an active role in slow earthquake source regions

fault valve behavior and the hydrostatic lithostatic fluid pressure interface abstract currently deforming regions of continental crust are characterized by seismic activity in a brittle carapace which presumably overlies regions undergoing local or distributed metamorphism one may note in this regard the global correlation between active seismic belts and the distribution of co2 rich springs barnes et al 1984 in regions of moderate to high heat flow 60 100 mw m 2 the seismogenic regime representing the zone of unstable frictional sliding typically extends to depths of 10 15 km marking the onset of greenschist metamorphic conditions sibson 1983 however in regions of rapid thrust convergence 1cm yr 1 seismic activity may extend to significantly greater depths through depression of isotherms as in the thrust front of the himalaya

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

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

seven million years of wind and precipitation variability on the chinese loess plateau abstract recent studies of the eolian deposits in northern china have extended the east asian monsoon history back to the early miocene however the relative intensities of monsoonal winds and precipitation and the extent of their coupling prior to the pleistocene epoch remain poorly constrained mainly due to uncertainties in the interpretation of proxy indices generated from the mio pliocene red clay sequences here we reconstruct east asian monsoon oscillations over the past 7 ma using magnetic susceptibility and carbonate content as summer monsoon precipitation proxies and quartz grain size as a winter monsoon wind intensity index our results suggest that precipitation and wind intensity exhibited significant orbital scale variations prior to 4 2 ma followed by slightly damped variability between 4 2 and 2 75 ma subsequently east asian monsoon circulation experienced two large shifts at about 2 75 and 1 25 ma characterized by stepwise strengthening of glacial wind and interglacial precipitation a remarkable change in east asian monsoon seasonality occurred around 3 15 2 75 ma prior to 3 15 ma strong winds were positively correlated with high effective precipitation whereas after 2 75 ma strong winds were negatively correlated with heavy precipitation this shift was probably induced by a change from an insolation forced system to one strongly influenced by the combined effects of the phased uplift of the himalaya tibetan plateau and the simultaneous development of the northern hemisphere ice sheets

generation of talc in the mantle wedge and its role in subduction dynamics in central mexico geophysical evidence shows the presence of low seismic velocity material at the surface of slabs in subduction zones in the central mexican subduction zone this appears as a thin 4 km low velocity zone that absorbs nearly all of the strain the p to s velocity ratio as a function of s wave velocity distinguishes among the various candidate hydrous low strength minerals the thin layer in the flat slab region is most consistent with a layer showing enrichment in talc overlying normal morb like gabbro based on available thermodynamic data for equilibria for talc its generation at the trench is nearly impossible and hence we propose it originates from the mantle wedge during the slab flattening process coupled with trench rollback the evolution of this low strength zone has important implications for the dynamics of the slab flattening process as well as the geochemistry of the mantle wedge and arc in central mexico

tectonic control on coarse grained foreland basin sequences an example from the cordilleran foreland basin utah newly released reflection seismic and borehole data combined with sedimentological provenance and biostratigraphic data from upper cretaceous paleocene strata in the proximal part of the cordilleran foreland basin system in utah establish the nature of tectonic controls on stratigraphic sequences in the proximal to distal foreland basin during campanian time coarse grained sand and gravel were derived from the internally shortening charleston nebo salient of the sevier thrust belt a rapid regional campanian progradational event in the distal foreland basin 200 km from the thrust belt in 8 m y can be tied directly to active thrust generated growth structures and an influx of quartzose detritus derived from the charleston nebo salient eustatic sea level variation exerted a minimal role in sequence progradation

observations of volcanic tremor at mount st helens volcano digital recordings of ground motion during tremor episodes accompanying eruptions at mount st helens volcano in the state of washington on august 7 and october 16 18 1980 are studied the spectra of the vertical component waveforms contain at least two dominant peaks at 1 0 and 1 3 hz for all events recorded during both eruptions that were studied spectra of horizontal ground motion show peaks at 0 9 and 1 1 hz the relative amplitude of the two peaks changes between tremor episodes and during single tremor episodes and shows no consistent relation to amplitude of ground motion spectra of long period earthquakes are very similar to those of tremor events suggesting that tremor is composed of many long period earthquakes that occur over a period of time the unique waveform of tremor events observed at mount st helens must be due to a source effect since the relative amplitude of the two dominant peaks changes during tremor episodes the path effect on tremor waveforms is small since there are no peaks in the spectra of waveforms recorded during tectonic earthquakes occurring in the vicinity of mount st helens the consistency of the location of the spectral peaks for the wide range of tremor amplitudes means that there must be a physical length at the source that is constant independent of the amplitude of motion at the source variations in amplitude of motion generated during tremor events must be due to variations in the force driving the tremor amplitudes of ground motion varies between 0 11 and 4 7 m seismic moment rates during the two eruptions are calculated using the model of aki et al 1977 and found to vary between 6 1018 and 1 1020 dynes cm s 1 which are larger than values found by aki et al 1977 who studied amplitudes of shallow tremor events recorded during the october 1963 eruption of kilauea volcano in hawaii study of tremor amplitudes recorded at corvallis oregon leads to the conclusion that tremor accompanying the cataclysmic may 18 1980 eruption was at least one order of magnitude larger in amplitude than tremor during august and october

4 ma coarsening of sediments from baikal chinese loess plateau and south china sea and implications for the onset of nh glaciation abstract intense tectonic movement occurred along the northeastern margin of the tibetan plateau 3 4 ma during the same time period global climate changed from a period of stability to a period of frequent and abrupt changes which prevented fluvial and glacial systems from establishing a state of equilibrium thus it is difficult to directly attribute the 3 4 ma coarse grained sediment accumulation in east asia to climate change or tectonic activity this study compares late cenozoic multi proxy records from the chinese loess plateau clp to the low latitude grain size sea level and benthic 18 o records from the south china sea scs and the high latitude grain size record from lake baikal all records suggest a steady and persistent cooling since 4 ma which correlates well with those cooling records from other regions around the world this coincided in timing with the modeling results that the closure of the central american seaway cas initiated strengthening of atlantic meridional overturning circulation between 4 8 and 4 0 ma which led to both warming of the northern hemisphere nh and cooling of the southern hemisphere sh cooling of the sh would induce a marked development of the antarctic ice sheets at 4 ma pushing the intertropical convergence zone itcz northward this was superimposed on warming of the nh and brought more precipitation to the middle latitudes of the nh resulting in increases in coarse grained sediments in the sikouzi section from the western clp since 4 2 ma notably the absence of coarse grained sedimentation in the sikouzi section during 3 0 2 1 ma and the gradual decrease in coarse grained peaks since 2 1 ma reflected by the sikouzi grain size record probably resulted from stepwise increase in the nh ice volume pushing the itcz southward on the other hand development of the antarctic ice sheets would induce global cooling and enhancement of physical weathering initiating increases in sedimentation rates as well as increases in grain size from lake baikal to the clp to the scs therefore the closure of the cas during 4 8 4 0 ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4 ma a persistent and steady cooling during 4 3 ma probably made a significant contribution to the establishment of the nh ice sheets at 2 75 ma ago

very low frequency earthquakes in cascadia migrate with tremor we find very low frequency earthquakes vlfes in cascadia under northern washington during 2011 episodic tremor and slip event vlfes are rich in low frequency energy 20 50 s and depleted in higher frequencies higher than 1 hz compared to local earthquakes based on a grid search centroid moment tensor inversion we find that vlfes are located near the plate interface in the zone where tremor and slow slip are observed in addition they migrate along strike with tremor activity their moment tensor solutions show double couple sources with shallow thrust mechanisms consistent with shear slip at the plate interface their magnitude ranges between mw 3 3 and 3 7 seismic moment released by a single vlfe is comparable to the total cumulative moment released by tremor activity during an entire episodic tremor and slip event the vlfes contribute more seismic moment to this episodic tremor and slip event than cumulative tremor activity and indicate a higher seismic efficiency of slow earthquakes in cascadia than previously thought spatiotemporal correlation of vlfe and tremor activity suggests that they are the results of the same physical processes governing slow earthquakes

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

low frequency earthquakes reveal punctuated slow slip on the deep extent of the alpine fault new zealand we present the first evidence of low frequency earthquakes lfes associated with the deep extension of the transpressional alpine fault beneath the central southern alps of new zealand our database comprises a temporally continuous 36 month long catalog of 8760 lfes within 14 families to generate this catalog we first identify 14 primary template lfes within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross correlation routine the hypocentres of 12 of the 14 lfe families lie within 10 km of the inferred location of the alpine fault at depths of approximately 20 30 km in a zone of high p wave attenuation low p wave speeds and high seismic reflectivity the lfe catalog consists of persistent discrete events punctuated by swarm like bursts of activity associated with previously and newly identified tremor periods the magnitudes of the lfes range between ml 0 8 and ml 1 8 with an average of ml 0 5 we find that the frequency magnitude distribution of the lfe catalog both as a whole and within individual families is not consistent with a power law but that individual families frequency amplitude distributions approximate an exponential relationship suggestive of a characteristic length scale of failure we interpret this lfe activity to represent quasi continuous slip on the deep extent of the alpine fault with lfes highlighting asperities within an otherwise steadily creeping region of the fault

episodic tremor and slip explained by fluid enhanced microfracturing and sealing episodic tremor and slow slip events at the deep extension of plate boundary faults illuminate seismic to aseismic processes around the brittle ductile transition these events occur in volumes characterized by overpressurized fluids and by near failure shear stress conditions we present a new modeling approach based on a ductile grain size sensitive rheology with microfracturing and sealing which provides a mechanical and field based explanation of such phenomena we also model pore fluid pressure variation as a function of changes in porosity permeability and strain rate dependent fluid pumping the fluid enhanced dynamic evolution of microstructures defines cycles of ductile strain localization and implies increase in pore fluid pressure we propose that slow slip events are ductile processes related to transient strain localization while nonvolcanic tremor corresponds to fracturing of the whole rock at the peak of pore fluid pressure our model shows that the availability of fluids and the efficiency of fluid pumping control the occurrence and the p t conditions of episodic tremor and slip

magnetic signature of environmental change reflected by pleistocene lacustrine sediments from the nihewan basin north china abstract a mineral magnetic study has been conducted on pleistocene lacustrine sediments from the xujiayao paleolithic site within the nihewan basin north china the magnetic signal is characterized by a dominance of high coercivity hematite minerals that is modulated by low coercivity magnetite maghemite assemblages during interglacials a high amplitude record of the composition dependent magnetic s 0 3 t parameter varies in accord with composite marine oxygen isotope records suggesting climate modulation of magnetic composition the xujiayao site is situated at the northern outskirts of the chinese loess plateau clp and has neither received significant eolian input nor been exposed to pedogenic processes comparable to those acting on clp mineral magnetic analyses suggest that variations in both magnetite and hematite are primarily controlled by detrital fluxes rather than by authigenic production of magnetic minerals here we propose that the fluctuations of magnetic signals at xujiayao are not derived from varying compositions of eolian flux with glacial interglacial periods or minute eolian influx during interglacials but is likely dominated by runoff related processes by which some weathered pedogenized loess and soils with stronger magnetic signals have been eroded and transported to the catchment during interglacials resulting in magnetic enhancement the inferred continuous deposition from the brunhes matuyama boundary to the paleosol s 1 ca 129 ka gives the age of ca 500 ka for the sediments carrying the xujiayao paleolithic site suggesting an allogenic origin of the paleolithic artifacts and mammalian bones dated by u th method to ca 100 ka

the castlegate sandstone of the book cliffs utah sequence stratigraphy paleogeography and tectonic controls abstract earlier stratigraphic work had predicted that at the type section of the castlegate formation the castle gate near price utah the unit consists of two sequences separated by a sequence boundary representing approximately one million years of unrecorded time although the type section is well exposed it consists of a monotonous succession of braided fluvial sandstones and no obvious boundary can be identified using facies criteria this is a good example of a cryptic sequence boundary petrographic data indicate however a significant change in detrital composition 20 m above the base of the section at a through going erosion surface that is therefore interpreted as the sequence boundary revised sequence correlations together with other petrographic data and regional paleocurrent patterns provide the basis for a model of the paleogeographic evolution of the area rocks assigned to the castlegate sandstone comprise two or possibly three sequences formed at times of slow regional subsidence erosional sequence boundaries and tilts in paleoslope between each sequence record thrust loading and unloading of the basin and the growing influence of intrabasinal upwarps movement of which was beginning to be affected by laramide movements toward the end of castlegate sedimentation

subpolar link to the emergence of the modern equatorial pacific cold tongue the cold upwelling tongue of the eastern equatorial pacific is a central energetic feature of the ocean dominating both the mean state and temporal variability of climate in the tropics and beyond recent evidence for the development of the modern cold tongue during the pliocene pleistocene transition has been explained as the result of extratropical cooling that drove a shoaling of the thermocline we have found that the sub antarctic and sub arctic regions underwent substantial cooling nearly synchronous to the cold tongue development thereby providing support for this hypothesis in addition we show that sub antarctic climate changed in its response to earth s orbital variations from a subtropical to a subpolar pattern as expected if cooling shrank the warm water sphere of the ocean and thus contracted the subtropical gyres

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

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

sedimentary indicators of atmospheric activity in the northern hemisphere during the cenozoic abstract the concentrations and accumulation rates of eolian phases and the mineralogy of sediments from a north pacific core ll44 gpc3 have been used to evaluate changes in eolian sedimentation and atmospheric activity during the cenozoic the quartz content and clay mineralogy of the sediments reflect changing source areas for eolian material whereas the accumulation rates of eolian phases reflect climatic variation and wind intensity eolian sedimentation decreased from 70 to 50 m y ago at the site as it moved north out of the equatorial pacific where it was supplied with eolian debris from north america and africa low rates of eolian sediment accumulation 50 to 25 m y ago reflect the temperate humid environment of the early tertiary and the lack of vigorous atmospheric circulation at that time from 25 to 7 m y ago eolian accumulation increased as the site moved into the influence of the westerlies from 7 to 3 m y ago there was a more rapid increase in accumulation of eolian material at the site which was not accompanied by changes in mineralogy or quartz content and was therefore related to climatic change and more vigorous atmospheric circulation about 2 5 m y ago a dramatic increase in eolian sedimentation accompanied the onset of dry northern hemisphere glaciation

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

deep and shallow long period volcanic seismicity linked by fluid pressure transfer volcanic long period earthquakes are attributed to pressure fluctuations that result from unsteady mass transport in the plumbing system of volcanoes whereas most of the long period seismicity is located close to the surface the volcanic deep long period earthquakes that occur in the lower crust and uppermost mantle reflect the activity in the deep parts of magmatic systems here we present observations of long period earthquakes that occurred in 2011 2012 within the klyuchevskoy volcano group in kamchatka russia we show two distinct groups of long period sources events that occurred just below the active volcanoes and deep long period events at depths of 30 km in the vicinity of a deep magmatic reservoir we report systematic increases of the long period seismicity levels prior to volcanic eruptions with the initial activation of the deep long period sources that reflects pressurization of the deep reservoir and consequent transfer of the activity towards the surface the relatively fast migration of the long period activity suggests that a hydraulic connection is maintained between deep and shallow magmatic reservoirs the reported observations provide evidence for the pre eruptive reload of the shallow magmatic reservoirs from depth and suggest that the deep long period earthquakes could be used as a reliable early precursor of eruptions shallow volcanic earthquakes can aid eruption forecasts analysis of seismicity beneath the klyuchevskoy volcano group in russia reveals much deeper magma induced earthquakes that may serve as an early eruption indicator

failure of cap rock seals as determined from mechanical stratigraphy stress history and tensile failure analysis of exhumed analogs the sedimentologic and tectonic histories of clastic cap rocks and their inherent mechanical properties control the nature of permeable fractures within them the migration of fluid through mm to cm scale fracture networks can result in focused fluid flow allowing hydrocarbon production from unconventional reservoirs or compromising the seal integrity of fluid traps to understand the nature and distribution of subsurface fluid flow pathways through fracture networks in cap rock seals we examine four exhumed paleozoic and mesozoic seal analogs in utah we combine these outcrop analyses with subsidence analysis paleoloading histories and rock strength testing data in modified mohr coulomb griffith analyses to evaluate the effects of differential stress and rock type on fracture mode relative to the underlying sandstone reservoirs all four seal types are low permeability heterolithic sequences that show mineralized hydraulic extension fractures extensional shear fractures and shear fractures burial history models suggest that the cap rock seal analogs reached a maximum burial depth 4 km 2 5 mi and experienced a lithostatic load of up to 110 mpa 15 954 psi median tensile strength from indirect mechanical tests ranges from 2 3 mpa 334 psi in siltstone to 11 5 mpa 1668 psi in calcareous shale analysis of the pore fluid factor through time shows changes in the expected failure mode extensional shear or hydraulic extension and that failure mode depends on a combination of mechanical rock properties and differential stress as expected with increasing lithostatic load the amount of overpressure that is required to induce failure increases but is also lithology dependent

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

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

permeability anisotropy induced by the shear displacement of rough fracture walls 1 the permeability anisotropy that results from a shear displacement between the complementary self affine walls of a rough fracture is investigated experiments in which a dyed fluid radially injected into a transparent fracture displaces a transparent one are presented a clear anisotropy is observed in the presence of shear displacements and allows us to estimate the ratio of the permeabilities for flows parallel and perpendicular to a simple model which accounts for the development of channels perpendicular to qualitatively explains these results and predicts a permeability decreasing increasing linearly with the variance of the aperture field for flow parallel perpendicular to the shear displacement these predictions are then compared to the results of numerical simulations performed using a lattice boltzmann technique and to the anisotropies measured in displacement experiments

ductile creep and compaction a mechanism for transiently increasing fluid pressure in mostly sealed fault zones a simple cyclic process is proposed to explain why major strike slip fault zones including the san andreas are weak field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure the increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic the frictional slip associated with earthquakes creates porosity in the fault zone the cycle adjusts so that no net porosity is created if the fault zone remains constant width the fluid pressure within the fault zone reaches long term dynamic equilibrium with the hydrostatic pressure in the country rock one dimensional models of this process lead to repeatable and predictable earthquake cycles however even modest complexity such as two parallel fault splays with different pressure histories will lead to complicated earthquake cycles two dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes a possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip stabilizing the fault and then decreases with further slip destabilizing it

pore fluid pressure development in compacting fault gouge in theory experiments and nature the strength of fault zones is strongly dependent on pore fluid pressures within them moreover transient changes in pore fluid pressure can lead to a variety of slip behavior from creep to unstable slip manifested as earthquakes or slow slip events the frictional properties of low permeability fault gouge in nature and experiment can be affected by pore fluid pressure development through compaction within the gouge layer even when the boundaries are drained here the conditions under which significant pore fluid pressures develop are analyzed analytically numerically and experimentally friction experiments on low permeability fault gouge at different sliding velocities show progressive weakening as slip rate is increased indicating that faster experiments are incapable of draining the pore fluid pressure produced by compaction experiments are used to constrain the evolution of the permeability and pore volume needed for numerical modeling of pore fluid pressure build up the numerical results are in good agreement with the experiments indicating that the principal physical processes have been considered the model is used to analyze the effect of pore fluid pressure transients on the determination of the frictional properties illustrating that intrinsic velocity strengthening behavior can appear velocity weakening if pore fluid pressure is not given sufficient time to equilibrate the results illustrate that care must be taken when measuring experimentally the frictional characteristics of low permeability fault gouge the contribution of compaction induced pore fluid pressurization leading to weakening of natural faults is considered cyclic pressurization of pore fluid within fault gouge during successive earthquakes on larger faults may reset porosity and hence the capacity for compaction weakening

long term river response to regional doming in the price river formation central utah a profound down basin change in alluvial architecture is coincident in time and space with the early phase of uplift of the san rafael swell along the book cliffs of central utah marked thinning in the lower part of the price river formation campanian approaching the crest of the swell demonstrates that the swell was first active as a region of reduced subsidence rate before it developed topographic relief a comparison of total unit thickness percentage of sandstone and relative abundance of sheet vs isolated sand bodies in vertical sections within the price river formation and laterally equivalent farrer formation shows changes in these measures across the swell crest the abundance of isolated lenticular fluvial sand bodies increases in the area of reduced aggradation at the expense of sheetlike bodies this change in alluvial architecture over the crest of the swell is found only in units laid down when the swell was active not in overlying or underlying units this coincidence strongly suggests a link between early growth of the swell and alluvial response however the sense of change i e fewer connected sand bodies in the region of reduced aggradation is opposite to that predicted by most alluvial architecture models after comparison to other field and experimental studies it seems likely that different river systems may reveal different avulsion frequencies that ultimately produce varied but not necessarily unpredictable alluvial architectures

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

fluvial response to active extension evidence from 3d seismic data from the frio formation oligo miocene of the texas gulf of mexico coast usa tectonic deformation of the land surface is known to influence the gradient water and sediment discharge and the grain size of modern fluvial systems any change in these variables alters the equilibrium of a fluvial system potentially causing a change in channel morphology 3d seismic data from the tertiary miocene age upper frio formation kelsey field south texas in the us are used to examine changing fluvial channel morphology through time during a period of active growth of a rollover anticline in the hangingwall of a normal fault the vicksburg fault the studied interval varies between 22 and 47 m thick and spans several hundred thousand years it consists of an alternation of fluvial sandstones overbank mudstones and coal seismic extractions show the evolution of sinuous fluvial channels during a phase of growth fault activity prior to growth a single sinuous channel is imaged during growth the fluvial system became decapitated by a developing rollover anticline and a highly sinuous drainage network formed with frequent avulsion events headward propagation of streams and related stream capture increased channel sinuosity was spatially associated with increased avulsion frequency in the area down dip to the east of the rollover anticline more than 10 km from the active fault more than 25 m of relative accommodation developed on the flank of the growing rollover anticline compared with on the crest the increased channel sinuosity is interpreted as reflecting an increase in longitudinal valley slope analogous to observations made in flume experiments and modern river systems the increase in avulsion frequency is attributed to increased aggradation as the rivers adjusted back to equilibrium grade following the increase in slope

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

dehydration induced porosity waves and episodic tremor and slip episodic tremor and slip ets along the subduction interface takes place where there is abundant evidence for elevated near lithostatic pore pressures at sufficiently great depths 30 45 km that chemical dehydration reactions must act as their dominant source we simulate fluid and heat flow while tracking the location of a vertically oriented one dimensional column of material as it subducts through the slow slip and tremor zone the material in the column is transformed through a pressure dependent and temperature dependent dehydration reaction that we describe with a generalized nonlinear kinetic rate law column deformation is largely dominated by viscous creep with a closure rate that depends linearly on porosity this behavior causes the dehydration reaction to generate traveling porosity waves that transport increased fluid pressures within the slow slip region to explore the possibility that the observed periodicity of slow slip and tremor in subduction zones can be explained by the migration of such porosity waves we derive a dispersion relation that accurately describes our numerical results we also obtain an expression for how the thickness of the dehydrating layer is expected to vary as a function of the parameters in the reaction rate law although the amplitudes of pore pressure perturbations rival those that are produced by known external forcings e g tides or passing surface waves our analysis suggests that given reasonable estimates of rock viscosity permeabilities in the range 6 5 10 15 to 5 10 10 m2 are required for porosity wave trains to form at periods comparable to those of slow slip and tremor

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

numerical modeling of landquakes the thurwieser landslide that occurred in italy in 2004 and the seismic waves it generated have been simulated and compared to the seismic signal recorded a few tens of kilometers from the landslide source i e landquake the main features of the low frequency seismic signal are reproduced by the simulation topography effects on the flowing mass have a major impact on the generated seismic signal whereas they weakly affect low frequency wave propagation simulation of the seismic signal makes it possible to discriminate between possible alternative scenarios for flow dynamics and to provide first estimates of the rheological parameters during the flow as landquakes are continuously recorded by seismic networks our results provide a new way to collect data on the dynamics and rheology of natural flows

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

ab initio elastic properties of talc from 0 to 12 gpa interpretation of seismic velocities at mantle pressures and prediction of auxetic behaviour at low pressure talc is a hydrous magnesium rich layered silicate that is widely disseminated in the earth from the seafloor to over 100 km depth in ultra high pressure metamorphism of oceanic crust in this paper we determine the single crystal elastic constants at pressures from 0 to 12 gpa of talc triclinic c 1 and monoclinic c2 c polytypes using ab initio methods we find that talc has an extraordinarily high elastic anisotropy at zero pressure that reduces with increasing pressure the exceptional anisotropy is complemented by a negative poisson s ratio for many directions in crystal space calculations show that talc is not only one of very few common minerals to exhibit auxetic behaviour but the magnitude of this effect may be the largest reported so far for a mineral the compression vp and shear vs wave velocity anisotropy is 80 and 85 for the triclinic polytype at pressures where talc is known be stable in the earth up to 5 gpa the vp and vs anisotropy is reduced to about 40 for both velocities which is still a very high value vp is slow parallel to the c axis and fast perpendicular to it this remains unchanged with increasing pressure and is observed in both polytypes the shear wave splitting difference between fast and slow s wave velocities at low pressure has high values in the plane normal to the c axis with a maximum near the a axis in the triclinic and the b axis in the monoclinic polytype the c axis is the direction of minimum splitting the pattern of shear wave splitting does not change significantly with pressure the volume fraction of talc varies between 11 and 41 for hydrated mantle rocks but the lack of data on the crystallographic preferred orientation cpo precludes a detailed analysis of the impact of talc on seismic anisotropy in subduction zones however it is highly likely that cpo can easily develop in zones of deformation due to the platy habit of talc crystals for random aggregates of talc the isotropic vp vs and vp vs ratio have significantly lower values than those of antigorite and may explain low velocity regions in the mantle wedge vp vs ratios are more complex in anisotropic media because there are fast and slow s waves resulting in vp vs1 and vp vs2 ratios for every propagation direction making interpretation difficult in deformed polycrystalline talc with a cpo talc on the subduction plate boundary can strongly influence guided wave velocity as cpo would develop in this region of intense shearing the very low coefficient of friction 0 1 of talc above 100 c could also explain silent earthquakes at shallow depths ca 30 km along the subduction plate boundaries frequently responsible for tsunami

analysis of seismic body waves excited by the mount st helens eruption of may 18 1980 seismic body waves which were excited by the may 18 1980 eruption of mount st helens and recorded by the global digital seismographic network stations are analyzed to determine the nature and the time sequence of the events associated with the eruption the polarity of teleseismic p waves period 20s is identical at six stations which are distributed over a wide azimuthal range this observation together with a very small s to p amplitude ratio at 20 s suggests that the source is a nearly vertical single force a simple model shows that for seismic radiation a volcanic eruption can be represented by a single force applied in the direction opposite to the blast direction the time history of the vertical force suggests two distinct groups of events about 2 min apart each consisting of several subevents with a duration of about 25 s the magnitude of the force is approximately 2 6 10 12 n this vertical force is in contrast with the long period 150 s southward horizontal single force which has been determined by a previous study and interpreted to be due to the massive landslide an m s 5 2 earthquake initiated the eruption sequence the direction of the p wave first motion of this event observed at two nearby stations is consistent with the radiation pattern expected for the landslide and suggests that it represents the onset of the landslide the ground motions observed at station lon 67 km are dominated by rayleigh waves i e lamb pulse and provide tight constraints on the time sequence of the events

incrementally developed slickenfibers geological record of repeating low stress drop seismic events an accretionary melange of triassic age ocean floor sediments exposed in the chrystalls beach complex south island new zealand comprises competent sandstone and chert phacoids set in a cleaved mudstone matrix deformed in a continuous discontinuous style at subgreenschist conditions deformation structures include a pervasive anastomosing fault fracture mesh of multiple shearing surfaces subparallel to cleavage coated with incrementally developed quartz calcite slickenfibers microstructural observations reveal slickenfiber growth by crack seal shear slip increments of 10 100 m with incremental slip transfer of the same order accommodated by opening of extension fractures that link en echelon slip surfaces individual slip surfaces can be traced for meters to tens of meters so that the ratio of average slip u to potential rupture length l predominantly lies within the range 10 6 u l 10 5 characteristic of microearthquakes obeying constant stress drop scaling with a low stress drop 30 kpa typical of low frequency earthquakes the host rock assemblage metamorphic environment inference of near lithostatic fluid overpressures low stress drop and mixed continuous discontinuous shearing resemble conditions and characteristics of low frequency earthquakes as identified within the seismic signals recorded during episodic tremor and slow slip events at the downdip end of the seismogenic subduction thrust interface and within accretionary prisms

sediment transport model for the eocene escanilla sediment routing system implications for the uniqueness of sequence stratigraphic architectures abstract stratigraphic architectures are fundamentally controlled by the interplay at different temporal and spatial scales of accommodation and sediment supply modulated by autogenic responses of the sediment routing system and its constituent segments the flux and caliber of sediment supply is a function of climate catchment area and tectonics in the source regions and unraveling these forcing mechanisms from the observed stratigraphic architecture remains a key research challenge the mid to late eocene escanilla sediment routing system had its source regions in the south central pyrenean orogen northern spain and transported sediment from wedge top basins along tectonic strike to marine depocenters by constructing a volumetric budget of the sedimentary system it has been demonstrated that there were marked changes in the grain size distribution released from the sediment sources and also in the position of the gravel front across three 2 6 myr time intervals from 41 6 to 33 9 ma classical sequence stratigraphic interpretations would relate the movement of depositional boundaries such as the gravel front to changes of base level either in isolation or in combination with sediment supply herein we explore the possibility that the position of the gravel front was primarily driven by variability of grain size distributions released from the source regions as a result of changes in catchment uplift rate and or surface run off using a simple model of sediment transport that captures first order processes we simulate the lateral movement of gravel deposition in the proximal part of the escanilla sediment routing system movement of the gravel front is a function of both accommodation generation and the transport capacity of the sediment routing system we assume that the transport capacity is a linear function of the local slope and the water flux by assuming that the observed thickness of deposits is equivalent to the accommodation available during deposition we then use the stratigraphic architecture to constrain the change in catchment size and water flux over the three time intervals of the escanilla paleo sediment routing system multiple scenarios are investigated in order to find the most plausible tectonic and climatic history model results indicate that during the mid eocene there was an increase in catchment length and sediment flux most likely driven by tectonic uplift in the pyrenean orogen subsequent marked progradation of the gravel front during the late eocene was the consequence of reduced transport capacity due to a reduction in surface run off the latter model result is in agreement with records of pollen taxa that indicate increased climatic aridity in the late eocene the combination of a sediment transport model with a full sediment budget makes it possible to test the non uniqueness of these results