strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

correlation of changes in gravity elevation and strain in southern california measurements made once or twice a year from 1977 through 1982 show large correlated changes in gravity elevation and strain in several southern california networks precise gravity surveys indicate changes of as much as 25 microgals between surveys 6 months apart repeated surveys show that annual elevation changes as large as 100 millimeters occur along baselines 40 to 100 kilometers long laser ranging surveys reveal coherent changes in areal strain of 1 to 2 parts per million occurred over much of southern california during 1978 and 1979 although the precision of these measuring systems has been questioned the rather good agreement among them suggests that the observed changes reflect true crustal deformation

review of magnetic and electric field effects near active faults and volcanoes in the u s a abstract synchronized measurements of geomagnetic field have been recorded along 800 km of the san andreas fault and in the long valley caldera since 1974 and during eruptions on mount st helens since 1980 for shorter periods of time continuous measurements of geoelectric field measurements have been made on mount st helens and near the san andreas fault where moderate seismicity and fault slip frequently occurs significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include 1 two moderate earthquakes m l 5 8 for which magnetometers were close enough to expect observable signals about three source lengths 2 one moderate earthquake m s 7 3 for which magnetometers were installed as massive fluid outflow occurred during the post seismic phase 3 numerous fault creep events and moderate seismicity 4 a major explosive volcanic eruption and numerous minor extrusive eruptions and 5 an episode of aseismic uplift for one of the two earthquakes with m l 5 8 seismomagnetic effects of 1 3 and 0 3 nt were observed for this event magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a m l 5 2 earthquake longer term changes were recorded in one region in southern california where a moderate m l 5 9 earthquake has since occurred surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision 0 25 nt and 0 01 respectively and are consistent with near surface fault failure models of these events longer term creep is sometimes associated with corresponding longer term magnetic field perturbations correlated changes in gravity magnetic field areal strain and uplift occurred during episodes of aseismic deformation in southern california primarily between 1979 and 1983 because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models the preferred explanation appeals to short term strain episodes independently detected in each data set an unknown source of meteorologically generated noise in the strain gravity and uplift data and an unknown but correlated disturbance in the absolute magnetic data might also explain the data no clear observations of seismoelectric or tectonoelectric effects have yet been reported the eruption of mount st helens generated large oscillatory fields and 9 2 nt offset on the only surviving magnetometer a large scale traveling magnetic disturbance passed through the san andreas array from 1 to 2 h after the eruption subsequent extrusive eruptions generated small precursory magnetic changes in some cases these data are consistent with a simple volcanomagnetic model magneto gas dynamic effects and a blast excited traveling ionospheric disturbance traveling ionospheric disturbances tids also generated by earthquake related atmospheric pressure waves may explain many electromagnetic disturbances apparently associated with earthquakes local near fault magnetic field transients rarely exceed a few nt at periods of a few minutes and longer

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

the distribution of the mid to lower crustal low velocity zone beneath the northeastern tibetan plateau revealed from ambient noise tomography we collected continuous seismic data recorded between 2007 and 2010 by 208 broadband stations from the chinese provincial digital seismic networks a seismic collaborative experiment of northern tibet and the northeastern tibet seismic experiment cross correlations of vertical component records are computed to extract the rayleigh wave empirical green s functions group and phase velocities are then constructed from the empirical green s functions in 8 to 50 s period at periods 25 s more than 10 lower velocities are imaged beneath the qaidam basin and high velocities are observed beneath the nonbasin regions at periods 30 s up to 10 lower velocities are imaged in the qiangtang and songpan ganze terranes from these group and phase velocity maps a three dimensional 3 d vsv model of the crust is derived the model shows that the qiangtang and songpan ganze terranes have a very thick crust with a prominent low velocity zone lvz in the middle crust the lvz thins out in the vicinity of the eastern kunlun mountains providing a new constraint on the mode of deformation across the tibetan plateau the northwestern qilian orogen where receiver functions reveal a moho deeper than the surrounding areas also features a relatively weak midcrustal lvz which we interpret as an intracrustal response associated with the shortening between the north china craton and the tibetan plateau

seismically invisible fault zones laboratory insights into imaging faults in anisotropic rocks phyllosilicate rich rocks which commonly occur within fault zones cause seismic velocity anisotropy however anisotropy is not always taken into account in seismic imaging and the extent of the anisotropy is often unknown laboratory measurements of the velocity anisotropy of fault zone rocks and gouge from the carboneras fault zone in se spain indicate 10 15 velocity anisotropy in the gouge and 35 50 anisotropy in the mica schist protolith greater differences in velocity are observed between the fast and slow directions in the mica schist rock than between the gouge and the slow direction of the rock this implies that the orientation of the anisotropy with respect to the fault is key in imaging the fault seismically for example for fault parallel anisotropy a significantly greater velocity contrast between fault gouge and rock will occur along the fault than across it highlighting the importance of considering the foliation orientation in design of seismic experiments

quartz fabrics in the alpine fault mylonites influence of pre existing preferred orientations on fabric development during progressive uplift strong quartz crystallographic preferred orientations cpos were developed during dislocation creep in the mylonite zone exposed in the hanging wall of the dextral reverse alpine fault zone new zealand the cpos have a consistent asymmetry indicating a high ratio of simple to pure shear strain with a shear sense of dextral up to the nw consistent with the mesoscopic shear sense indicators and with slip on the active fault there is a transition from y maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace to cross girdle fabrics in the protomylonites further from the fault the strong y maxima or single girdle cpos are ascribed to high ductile shear strains under amphibolite facies conditions while the cross girdle patterns found in the protomylonites are interpreted to represent deformation under lower temperature conditions however the observed fabric transition cannot logically be attributed to variations in temperature during the last increment of deformation the highly oriented y maximum fabrics formed at high temperature contain very few grains suitably oriented for basal slip so that the slip systems activated during subsequent shear at lower temperatures during exhumation were prism or rhomb further from the fault where shear strains under high temperature conditions were lower weaker fabrics developed under these conditions were modified at higher levels in the crust into crossed girdle patterns this interpretation implies that intense localisation of shear strain along the fault zone within the lower crust must have occurred in order for the high temperature fabrics to become sufficiently intense to be preserved this in turn implies that deep seated localised shear was taking place early during the evolution of the current oblique slip plate boundary fault the results of this study also indicate that the current model that operation of certain slip systems in quartz is mostly a function of temperature is incorrect future work should consider the effects of deformation history total strain and the presence of other mineral phases

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

tectonomagnetism and tectonoelectricity the last four years has seen a considerable waning of efforts in the u s to use the inherent stress sensitivity of the magnetization of rocks in tectonically active regions as a tool for understanding and perhaps predicting earthquake activity and volcanic eruptions the primary reasons for this decreased interest derive from accumulating evidence that the mean deviatoric stress release during earthquakes and the ambient shear stress near active faults is lower than than had been previously assumed and secondly the observational limits of magnetometers separated by about 10 km after application of a variety of noise reduction techniques appear to be no better than about 0 1 nt lower estimates of the average change in deviatoric stress associated with earthquake rupture correspondingly reduce the fields calculated from tectonomagnetic models at the same time some unexpected and exciting results have been obtained which indicate many aspects of magnetic field changes near active faults and volcanoes are still not completely understood the more important of these results are 1 the observation of correlated magnetic strain uplift and gravity changes during the latter stages of a remarkable deformation episode near the san andreas fault in southern california 2 indications of anomalous change in magnetic fields along the san andreas fault to the southeast of palm springs 3 an unambiguous observation of a volcanomagnetic effect at mt st helens prior to an extrusive eruption and 4 the first observations of coseismic seismomagnetic effects on two independent magnetometers during the july 8 1986 ml 5 9 north palm springs earthquake a resurgence of interest has occurred during the quadrennium in a related area concerning electrical phenomena and electromagnetic effects related to earthquakes and volcanoes mechanisms have been proposed to explain the occurrence of earthquakes lights and the possibility of electromagnetic emission prior to the 1960 chilean earthquake has been proposed self potential anomalies have been observed before and during several eruptions on kilauea volcano in hawaii

the 1979 palmdale california strain event in retrospect although the long term 1971 1984 strain accumulation across the san andreas fault near palmdale california is simply right lateral shear at the rate of 0 20 0 01 strain a tensor shear annual measurements indicate that an anomalous contraction perpendicular to the fault accumulated in the interval 1973 1978 and was abruptly released in mid 1979 this anomaly is referred to as the 1979 palmdale strain event more frequent measurements of strain since 1979 suggest a more irregular accumulation of strain than prior to 1979 this irregularity may be due to measurement errors larger than previously estimated as a consequence of this post 1979 irregularity the 1979 strain event no longer appears as anomalous as it had in the shorter 1971 1980 record to provide independent support for existence of the 1979 palmdale strain event we review gravity measurements at palmdale by jachens 1983 and water level measurements in wells near palmdale by merifield and lamar 1981 that have previously been shown to provide a marginal correlation with the 1979 strain recovery these data taken together with the original strain observations suggest a real strain anomaly did occur near palmdale in 1979 but the evidence is not overwhelming

role of subsurface loads and regional compensation in the isostatic balance of the transverse ranges california evidence for intracontinental subduction bouguer gravity anomalies in the transverse ranges of southern california show a pronounced asymmetry in north south profiles across the mountains and a characteristic shape which is largely insensitive to variations in the topographic load along strike while individual gravity topography profile pairs could be consistent with a number of isostatic compensation mechanisms the unusual relationship between gravity and topography in the san bernardino mountains severely limits the class of acceptable models if the bouguer gravity field is caused by flexure of the crust mantle boundary the simplest solution consistent with the data in the western transverse ranges the san gabriel mountains and the san bernardino mountains models the great valley and mojave provinces to the north as a 50 km thick elastic plate supporting the mountains which are underthrust by a much weaker plate to the south the best fit to the gravity data in the san bernardino mountains requires a terminal bending moment on the southern plate beneath the mountains the magnitude of the terminal bending moment decreases to the west in the san gabriel and western transverse ranges we equate the load responsible for this terminal moment with the high velocity slablike feature mapped in the upper mantle beneath the transverse ranges using seismic tomography the coupling between the subsurface load and the overlying lithosphere suggests that the slab represents lithospheric subduction with an extremely small density contrast rather than a small scale convective feature

seismic signature of the collision between the east tibetan escape flow and the sichuan basin abstract gps displacement vectors show that the crust in east tibet is being squeezed in an easterly direction by the northward motion of the indian plate and the sichuan basin is resisting this stream and redirecting it mainly towards indochina the longmen shan containing the steepest rise to the high plateau anywhere in tibet results from the strong interaction between the east tibetan escape flow and the rigid yangtze block sichuan basin but the kinematics and dynamics of this interaction are still the subject of some debates we herein present results from a dense passive source seismic profile from the sichuan basin into eastern tibet in order to study the deep structure of this collision zone using p and s receiver function images we observe a sudden rise of the lithosphere asthenosphere boundary lab from 120 to 150 km beneath the sichuan basin and from 70 to 80 km beneath eastern tibet in contrast the depth of the crust mantle boundary moho increases from 36 to 40 km beneath the sichuan basin and from 55 to 60 km beneath eastern tibet the 410 km discontinuity is depressed below eastern tibet by about 30 km although the 660 remains at nearly the same depth throughout the lms from these observations we conclude that the mode of collision that occurs between tibet and the sichuan basin is very different to that found between india and tibet in southern tibet we observe in essence the subduction of the indian plate which penetrates northwards for several hundred kilometers under central tibet the very thin mantle part of the lithosphere beneath eastern tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow this process leads to the exceptionally steep topography at the eastern tibetan margin as a result of gravitational buoyancy this view is supported by the very unusual depression of the 410 km discontinuity beneath eastern tibet which could be caused by the dynamics of the sub vertical downward asthenospheric flow

three dimensional adjustment of trilateration data the three dimensional locations of the monuments in the usgs hollister trilateration network were adjusted to fit line length observations observed in 1977 using a bayesian approach and incorporating prior elevation estimates as data in the adjustment procedure no significant discrepancies in the measured line lengths were found but significant elevation adjustments up to 1 85 m were needed to fit the length data

interseismic deformation along the san andreas fault in southern california eight trilateration networks located along the san andreas fault in southern california have been surveyed 8 to 19 times within a 14 to 17 year interval between 1971 and 1992 the data measurements of distances between the same 10 to 32 pairs of geodetic monuments within a network in each of the surveys have been corrected for coseismic offsets from nearby earthquakes calculated from dislocation models the corrected data a total of 2027 measurements are displayed in the form of plots of measured distance versus time for each of the 167 lines measured the hypothesis that the interseismic deformation is steady is tested by examining whether deviations from linear fits to the data in the plots are within the range expected for observational error a significant deviation from steady deformation is found only for the network located neax palmdale california in that network many of the measurements made in the survey of early 1982 deviate from the trend defined by the measurements in other surveys the deviations are not of the form fixed proportional error one would generally expect from systematic survey error if the survey of early 1982 is excluded the remaining data at palmdale are consistent with steady deformation thus the apparent strain event observed near palmdale in 1982 was transient

strain accumulation in southern california 1973 1984 repeated surveys of selected lines from five trilateration networks along the san andreas fault in southern california have been used to deduce the 1973 1984 strain accumulation records at five localities the secular rate of engineering shear strain accumulation is about 0 3 rad yr with the plane of maximum shear parallel to the local strike of the san andreas fault the secular rate of accumulation of areal dilatation is negligible the data were examined to detect evidence for fluctuations in the rate of strain accumulation for this examination 19 lines were removed from the data set four because they exhibited an obvious coseismic offset and 15 others because they contained at least one very anomalous measurement that could reasonably be attributed to a survey blunder the incidence of such blunders appears to be one in every 75 measurements the remaining data consist of 104 lines with an average of 10 measurements each although the strain accumulation plots for the five networks may exhibit marginally significant temporal fluctuations we are not convinced that those fluctuations are greater than could be attributed to survey error in particular we are unable to demonstrate that the 1973 1979 southern california strain anomaly reported by savage and others is real given the uncertainty in the random and systematic errors in measurement the strain measurements in southern california are marginally consistent with linear in time strain accumulation the strain accumulation plots for the salton network clearly established that unlike the deformation reported after the 1940 imperial valley earthquake no acceleration in the shear strain rate has yet been observed following the 1979 imperial valley earthquake

frictional properties of exhumed fault gouges in dfdp 1 cores alpine fault new zealand principal slip zone gouges recovered during the deep fault drilling project dfdp 1 alpine fault new zealand were deformed in triaxial friction experiments at temperatures t of up to 350 c effective normal stresses n of up to 156 mpa and velocities between 0 01 and 3 m s chlorite white mica bearing dfdp 1a blue gouge 90 62 m sample depth is frictionally strong friction coefficient 0 61 0 76 across all experimental conditions tested t 70 350 c n 31 2 156 mpa it undergoes a transition from positive to negative rate dependence as t increases past 210 c the friction coefficient of smectite bearing dfdp 1b brown gouge 128 42 m sample depth increases from 0 49 to 0 74 with increasing temperature and pressure t 70 210 c n 31 2 93 6 mpa the positive to negative rate dependence transition occurs as t increases past 140 c these measurements indicate that in the absence of elevated pore fluid pressures dfdp 1 gouges are frictionally strong under conditions representative of the seismogenic crust

strain on the san andreas fault near palmdale california rapid aseismic change frequently repeated strain measurements near palmdale california during the period from 1971 through 1980 indicate that in addition to a uniform accumulation of right lateral shear strain engineering shear 0 35 microradian per year across the san andreas fault a 1 microstrain contraction perpendicular to the fault that accumulated gradually during the interval 1974 through 1978 was aseismically released between february and november 1979 subsequently november 1979 to march 1980 about half of the contraction was recovered this sequence of strain changes can be explained in terms of south southwestward migration of a slip event consisting of the south southwestward movement of the upper crust on a horizontal detachment surface at a depth of 10 to 30 kilometers the large strain change in 1979 corresponds to the passage of the slip event beneath the san andreas fault

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

intermediate term pre earthquake phenomena in california 1975 1986 and preliminary forecast of seismicity for the next decade intermediate term observations preceding earthquakes of magnitude 5 7 or greater in california from 1975 through 1986 suggest that 1 the sudden appearance of earthquakes in a previously inactive area indicates an increased likelihood of a significant earthquake in that area for a period from days to years 2 these larger earthquakes tend to occur towards the ends of creeping fault segments 3 one large earthquake in a region increases the likelihood of a subsequent significant event in the adjacent area and 4 marginal evidence for the occurrence of a regional deformation event suggests that such events increase the probability of earthquake occurrence throughout the entire area a common element in many of these observed patterns appears to be the transmission and amplification of tectonic stress changes by the mechanism of fault creep and suggests that surface fault creep is a sensitive indicator of changes in stress the preceding critieria are used to construct a preliminary forecast of the likely locations of significant earthquakes over the next decade

seismic and geodetic evidence for extensive long lived fault damage zones during earthquakes slip is often localized on preexisting faults but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes seismic and geodetic data from the calico fault in the eastern california shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli using seismic traveltimes trapped waves and interferometric synthetic aperture radar observations we document seismic velocities reduced by 40 50 and shear moduli reduced by 65 compared to wall rock in a 1 5 km wide zone along the calico fault observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures no large earthquake has broken the calico fault in historic time implying that fault damage persists for hundreds or perhaps thousands of years these fi ndings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces and throughout much of the seismogenic zone a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems

properties of seismic fault zone waves and their utility for imaging low velocity structures a two dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study properties of seismic waves in a laterally heterogeneous low velocity structure the waves referred to as seismic fault zone waves include head waves internal fault zone reflections and trapped waves the analysis aims to clarify the dependency of the waves on media velocities media attenuation coefficients layer widths and source receiver geometry additional calculations with extreme low velocity layers provide examples that may be relevant for volcanic and geothermal domains the interference patterns controlling seismic fault zone waves change with the number of internal reflections in the low velocity structure this number increases with propagation distance along the structure decreases with fault zone width and increases for given length scales with the velocity contrast the relative lateral position of the source within the low velocity layer modifies die length scales associated with internal reflections and influences the resulting interference pattern low values of q affect considerably the dominant period and overall duration of the waves thus there are significant tradeoffs between propagation distance along the structure fault zone width velocity contrast source location within the fault zone and q the lateral and depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong function of both coordinates the zone connecting sources generating fault zone waves and observation points with appreciable wave amplitude can be over an order of magnitude larger than the fault zone width calculations for cases with layer p wave velocity of 200 m s 1 modeling a vertical dike or crack with fluid and gas show conspicuous persisting oscillations the results resemble aspects of seismic data in volcanic domains if these waves exist in observed records their explicit recognition and modeling will help to separate source and structural effects and aid in the interpretation of volcano seismology signals although the tradeoffs in parameters governing seismic fault zone waves are significant each variable has its own signature and the parameters may be constrained by additional geophysical data simultaneous modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high resolution structural image

a plate boundary earthquake record from a wetland adjacent to the alpine fault in new zealand refines hazard estimates abstract discovery and investigation of millennial scale geological records of past large earthquakes improve understanding of earthquake frequency recurrence behaviour and likelihood of future rupture of major active faults here we present a 2000 year long seven event earthquake record from john o groats wetland adjacent to the alpine fault in new zealand one of the most active strike slip faults in the world we linked this record with the 7000 year long 22 event earthquake record from hokuri creek 20 km along strike to the north to refine estimates of earthquake frequency and recurrence behaviour for the south westland section of the plate boundary fault eight cores from john o groats wetland revealed a sequence that alternated between organic dominated and clastic dominated sediment packages transitions from a thick organic unit to a thick clastic unit that were sharp involved a significant change in depositional environment and were basin wide were interpreted as evidence of past surface rupturing earthquakes radiocarbon dates of short lived organic fractions either side of these transitions were modelled to provide estimates for earthquake ages of the seven events recognised at the john o groats site three post date the most recent event at hokuri creek two match events at hokuri creek and two events at john o groats occurred in a long interval during which the hokuri creek site may not have been recording earthquakes clearly the preferred john o groats hokuri creek earthquake record consists of 27 events since 6000 bc for which we calculate a mean recurrence interval of 291 23 years shorter than previously estimated for the south westland section of the fault and shorter than the current interseismic period the revised 50 year conditional probability of a surface rupturing earthquake on this fault section is 29 the coefficient of variation is estimated at 0 41 we suggest the low recurrence variability is likely to be a feature of other strike slip plate boundary faults similar to the alpine fault

damaged beyond repair characterising the damage zone of a fault late in its interseismic cycle the alpine fault new zealand dfdp 1 was funded by gns science victoria university of wellington the university of otago the university of auckland the university of canterbury deutsche forschungsgemeinschaft and the university of bremen natural environment research council grants ne j024449 1 ne g524160 1 and ne h012486 1 and the university of liverpool and the marsden fund of the royal society of new zealand

permeability and seismic velocity and their anisotropy across the alpine fault new zealand an insight from laboratory measurements on core from the deep fault drilling project phase 1 dfdp 1 the alpine fault a transpressional plate boundary between the australian and pacific plates is known to rupture quasi periodically with large magnitude earthquakes mw 8 the hydraulic and elastic properties of fault zones are thought to vary over the seismic cycle influencing the nature and style of earthquake rupture and associated processes we present a suite of laboratory permeability and p vp and s vs wave velocity measurements performed on fault lithologies recovered during the first phase of the deep fault drilling project dfdp 1 which sampled principal slip zone psz gouges cataclasites and fractured ultramylonites with all recovered lithologies overprinted by abundant secondary mineralization recording enhanced fluid rock interaction core material was tested in three orthogonal directions orientated relative to the down core axis and when present foliation measurements were conducted with pore pressure h2o held at 5 mpa over an effective pressure peff range of 5 105 mpa permeabilities and seismic velocities decrease with proximity to the psz with permeabilities ranging from 10 17 to 10 21 m2 and vp and vs ranging from 4400 5900 m s in the ultramylonites cataclasites and 3900 4200 m s at the psz in comparison with intact country rock protoliths the highly variable cataclastic structures and secondary phyllosilicates and carbonates have resulted in an overall reduction in permeability and seismic wave velocity as well as a reduction in anisotropy within the fault core these results concur with other similar studies on other mature tectonic faults in their interseismic period

strain effects near palmdale associated with the san fernando earthquake 1971 abstract field data from the southeast palmdale network in california are the basis for a practical strain analysis with triangulation data available for the year 1938 and 1958 and trilateration data at 1 year intervals from 1971 to date a uniform although small strain accumulation over the last forty years appears to be evident from this data set a possible soceismic strain jump of 2 strain in both shearing strain components related to the 1971 san fernando earthquake may be postulated in the observations however it was already pointed out previously that this coseismic strain jump needs further qualifications due to the corrupting influence of uncertainties in the network height differences this particular situation arises because triangular data have to be compared with trilateration data for the computation of the coseismic strain changes to clarify this situation the height differences of the southeast palmdale network were determined by trigonometric levelling in spring 1981 the effect of this new height data on the interpretation of the coseismic strain change is discussed in detail a comparison of this result with strain model calculations is presented

tectonic motions and systematic errors in leveling and trilateration data for california abstract we examined leveling and trilateration data for southern california we used mathematical models that allow for both tectonic motion and systematic errors for both data types we find evidence of systematic errors the leveling data reveal no significant tectonic motion except the direct effects of earthquakes trilateration data show steadily accumulating shear strain but previously reported episodic dilatations probably result from systematic errors

reprint of extensional shear band development on the outer margin of the alpine mylonite zone tatare stream southern alps new zealand abstract schistose mylonitic rocks in the central part of the alpine fault af at tatare stream new zealand are cut by pervasive extensional c shear bands in a well understood and young natural ductile shear zone the c shears cross cut the pre existing mesozoic aged foliation displacing it ductilely synthetic to late cenozoic motion on the af using a transect approach we evaluated changes in geometrical properties of the mm cm spaced c shear bands across a conspicuous finite strain gradient that intensifies towards the af precise c attitudes c foliation dihedral angles and c s intersections were calculated from multiple sectional observations at both outcrop and thin section scales based on these data the direction of ductile shearing in the alpine mylonite zone during shear band activity is inferred to have trended 20 clockwise down dip of the coeval pacific australia plate motion indicating some partitioning of oblique slip motion to yield an excess of dip slip relative to plate motion azimuth or some up dip ductile extrusion of the shear zone as a result of transpression or both constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary szb across all examined parts of the shear zone the mean dihedral angle between the c shears and the mylonitic foliation s remains a constant 30 1 1 the aggregated slip accommodated on the c shear bands contributed only a small bulk shear strain across the shear zone 0 6 0 8 uniformity of per shear slip on c shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain hardening rheology such that they locked up at a finite shear strain inside c bands of 12 15 shear band boudins and foliation boudins both record extension parallel to the szb as do the occurrence of extensional shear band sets that have conjugate senses of slip we infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with w k

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

crustal thickening and uplift of the tibetan plateau inferred from receiver function analysis the india eurasia collision crustal thickening and northward convergence leading to uplift of the tibetan plateau have been topics of multidisciplinary studies here we employ the h k stacking technique and depth domain receiver function to constrain the crustal thickness and p and s wave velocity ratio as well as the upper mantle discontinuity our results identify the signature of slab break off and lower crustal delamination in southern tibet which might have induced the mantle upwelling and crustal melting the convergence of the indian plate inducing northward crustal flow might be a major factor that contributes to the uplift and crustal thickening of tibet we also evaluate the e w trending cumulative compression as another dominant factor that contributes to the increase in crustal thickness in the northern tibet 2015 elsevier ltd all rights reserved

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

seismic trapped modes in the oroville and san andreas fault zones three component borehole seismic profiling of the recently active oroville california normal fault and microearthquake event recording with a near fault three component borehole seismometer on the san andreas fault at parkfield california have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals these wave trains are interpreted as fault zone trapped seismic modes parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers as shallow as 4 kilometers and extend 12 kilometers along the fault on either side of the recording station selected oroville and parkfield wave forms are modeled as the fundamental and first higher trapped sh modes of a narrow low velocity layer at the fault modeling results suggest that the oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second whereas at parkfield the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1 1 to 1 8 kilometers per second these low velocity layers are probably the rupture planes on which earthquakes occur

the crustal and upper mantle structures beneath the northeastern margin of tibet abstract teleseismic receiver functions are used to estimate the crustal thickness v p and v p v s or poisson s ratio beneath the northeastern margin of the tibetan plateau surface wave dispersion measurements are combined with teleseismic receiver functions to improve estimates of model parameters a migration method based on a local velocity model is used to image the upper mantle discontinuities according to the location of pierce points of the direct converted phases on the observed receiver functions we estimate the topography of upper mantle discontinuities the thicker crust in the altyn tagh fault hexi corridor basin may indicate the superposition of crusts during the collision between the indian and the eurasian plates beneath tianshui and its neighboring regions in the eastern margin of tibet high v p and high v p v s values close to uplifts of magma and hot springs might result from the upwelling of mantle materials induced by the collision the depressed 410 km discontinuity might be related to the warm temperature below the northeastern margin of tibet we speculate that the mantle flow beneath the eastern margin of tibet may be turning downward and subsiding into the mantle transition zone

statistical tests of aries data statistical tests are performed on project aries preliminary baseline measurements in the southern california triangle formed by the jet propulsion laboratory the owens valley radio observatory and the goldstone tracking complex during 1976 1980 in addition to conventional one dimensional tests a two dimensional test which allows for an arbitrary correlation between errors in individual components is formulated using the hotelling statistic on two out of three baselines the mean rate of change in baseline vector is statistically significant apparent motions on all three baselines are consistent with a pure shear with north south compression and east west expansion of 1 x 10 to the 7th year the aries measurements are consistent with the usgs geodolite networks in southern california and the safe laser satellite ranging experiment all three experiments are consistent with a 6 cm year motion between the pacific and north american plates and a band of diffuse shear 300 km wide except that corresponding rotation of the entire triangle is not found

seismic anisotropy and its impact on imaging the shallow alpine fault an experimental and modeling perspective the transpressional alpine fault in new zealand has created a thick shear zone with associated highly anisotropic rocks low seismic velocity zones lvz and high seismic reflectivity are recorded in the alpine fault zone but no study has explored the underlying physical rock parameters of the shallow crust that control these observations protomylonites are the volumetrically dominant lithology of the fault zone here we combine experimental measurements of p wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged numerical models that account for the porosity free real samples fabric elastic tensors from ebsd are calculated by mtex and a finite element model fem while microfractures are modeled with differential effective media dem theory at effective pressures representative of the alpine fault brittle zone experimental wave speeds are lower than those predicted by mtex fem a possible dem model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures 70 mpa such micro porosity in the form of foliation and mica basal plane parallel microfractures and grain boundaries is validated with synchrotron x ray microtomography and tem images finally by modeling anisotropy of seismic reflection coefficients with angle of incidence we demonstrate that the high reflectivity and lvz observed at the alpine fault can only be explained if this microporosity is accounted for throughout the brittle fault zone even at depths of 7 10 km

probability of earthquake recurrence with nonuniform stress rates and time dependent failure current methods for calculation of long term probabilities for the recurrence of large earthquakes on specific fault segments are based upon models of the faulting process that implicitly assume constant stress rates during the interval separating earthquakes and instantaneous failure at a critical stress threshold however observations indicate that the process of stress recovery following an earthquake involves rate variations at all time scales in addition to stress steps caused by nearby earthquakes additionally the existence of foreshocks aftershocks and possible precursory processes suggest that there may be significant time dependence of the earthquake nucleation process a method for determining the conditional probabilities for earthquake occurrence under conditions of irregular stressing is developed that could be useful at all time scales including those pertinent to short and intermediate term prediction used with models for earthquake occurrence at a stress threshold the addition of variable stressing introduces a simple scaling of the conditional probabilities by stress level and stress rate a model for the time dependent nucleation of earthquake slip has been proposed recently that is based upon laboratory observations of fault strength this failure criterion results in large but relatively short duration changes in the probability of earthquake recurrence particularly following stress steps applied to populations of earthquakes the models predicts a 1 t decay of seismicity following stress steps as observed for aftershocks and for frequency of foreshock mainshock pairs the model suggests that variations of seismicity rates of small earthquakes in the nucleation zone of the expected earthquake directly indicate variations in probability of recurrence of the large earthquake

hydraulic and acoustic properties of the active alpine fault new zealand laboratory measurements on dfdp 1 drill core abstract we report on laboratory measurements of permeability and elastic wavespeed for a suite of samples obtained by drilling across the active alpine fault on the south island of new zealand as part of the first phase of the deep fault drilling project dfdp 1 we find that clay rich cataclasite and principal slip zone psz samples exhibit low permeabilities 10 18 m 2 and that the permeability of hanging wall cataclasites increases from c 10 18 m 2 to 10 15 m 2 with distance from the fault additionally the psz exhibits a markedly lower p wave velocity and young s modulus relative to the wall rocks our laboratory data are in good agreement with in situ wireline logging measurements and are consistent with the identification of an alteration zone surrounding the psz defined by observations of core samples the properties of this zone and the low permeability of the psz likely govern transient hydrologic processes during earthquake slip including thermal pressurization and dilatancy strengthening

noncontacting benchtop measurements of the elastic properties of shales we evaluated a laser based noncontacting method to measure the elastic anisotropy of horizontal shale cores whereas conventional transducer data contained an ambiguity between phase and group velocity measurements small laser source and receiver footprints on typical core samples ensured group velocity information in our laboratory measurements with a single dense acquisition of group velocity versus group angle on a horizontal core we estimated the elastic constants c11 c33 and c55 directly from ultrasonic waveforms and c13 from a least squares fit of modeled to measured group velocities the observed significant p wave velocity and attenuation anisotropy in these dry shales were almost surely exaggerated by delamination of clay platelets and microfracturing but provided an illustration of the new laboratory measurement technique although challenges lay ahead to measure preserved shales at in situ conditions in the lab we evaluated the fundamental advantages of the proposed method over conventional transducer measurements

seismic evidence for stepwise thickening of the crust across the ne tibetan plateau we performed a receiver function analysis on teleseismic data recorded along two 550 km long profiles crossing the northeastern tibetan plateau results from time to depth migration grid search vp vs determination and simulated annealing inversion of waveforms reveal that the crust thickens from 50 km near the northern edge of the plateau to 80 km south of the jinsha suture in the qiang tang block crustal thickening occurs in staircase fashion with steps located beneath the main reactivated sutures the vp vs ratio close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern tibetan crust

small 660 km seismic discontinuity beneath tibet implies resting ground for detached lithosphere 1 using seismic profiles comprising of high resolution triplicate waveforms across apertures of over 1 000 km we show that because of high p wave speed vp near the bottom of the mantle transition zone the contrast in vp across the 660 km discontinuity beneath central tibet is small only about 70 of that beneath the northern indian shield this subhorizontal anomaly of high vp is most likely a remnant of detached mantle lithosphere that recently sank to depth thus providing key evidence for a direct connection between continental collision near the surface and deep seated dynamics in the mantle

late interseismic state of a continental plate bounding fault petrophysical results from dfdp 1 wireline logging and core analysis alpine fault new zealand 1 we present a geophysical characterization at 0 1 100 m scales of a major plate bounding continental fault in a late interseismic state the alpine fault produces mw 8 earthquakes every 200 400 years and last ruptured in 1717 ad wireline geophysical logs and rock cores extending from one side of the alpine fault to the other were acquired in two boreholes drilled in 2011 at gaunt creek during the first phase of the deep fault drilling project dfdp 1 these data document ambient conditions under which the next alpine fault earthquake will occur principal component analysis of the wireline logging data reveals that 80 of the variance is accounted for by electrical acoustic and natural gamma properties and preliminary multivariate classification enables the lithologies of sections of missing core to be reconstructed from geophysical measurements the fault zone exhibits systematic variations in properties consistent with common processes of progressive alteration and comminution near the principal slip zone superimposed on different protolith compositions our observations imply that the fault zone has the opposite sense of elastic asymmetry at 0 1 100 m scales to that of the crustal scale orogen imaged by remote geophysical methods on the basis of the fault zone scale asymmetry the bimaterial interface model of preferred earthquake rupture directions implies a northeastward direction of preferred alpine fault rupture on going characterization of the structural and hydraulic architecture of the alpine fault will improve our understanding of the relationship between in situ conditions earthquake rupture processes and the hazards posed by future earthquakes

crustal structure beneath the eastern margin of the tibetan plateau and its tectonic implications 1 two crustal cross sections through the eastern margin of the tibetan plateau are jointly determined from deep seismic sounding the e w trending line aa passes through the western sichuan plateau including the songpan garze terrane and the longmenshan fault belt and ends in the sichuan basin a part of the yangtze craton line bb has a trend of nne and crosses the songpan garze terrane two dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data our seismic velocity models indicate that the western sichuan plateau and the sichuan basin have crustal thicknesses of 62 and 43 km average crustal p wave velocities of 6 27 and 6 45 km s and lower crustal vp 6 5 km s thicknesses of 27 and 15 km respectively density models constructed from the seismic velocity models are consistent with observed bouguer gravity anomalies we infer that collision between the tibetan plateau and the yangtze craton has caused thickening of the lower crust and uplift of the western sichuan plateau we detect a low velocity layer in the upper crust of the western sichuan plateau but observe no equivalence in the sichuan basin west dipping thrusts may detach into this low velocity layer the seismic phase pmp in the western sichuan plateau has low amplitude suggesting high attenuation in the lower crust qp of 100 300 we suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western sichuan plateau

relationships between kinematic indicators and strain during syn deformational exhumation of an oblique slip transpressive plate boundary shear zone the alpine fault new zealand abstract in order to assess how different elements of plate boundary scale shear zone tectonite fabrics reflect the boundary kinematics and strain history we have examined a range of such structures within the active alpine fault mylonite zone new zealand in this oblique slip zone we find that quartz cpo fabric symmetry and individual mica grain lineations reflect incremental stretches but develop stable orientations parallel to the finite stretching direction s 1 this s 1 trends oblique to the plate motion vector because the shear zone experiences some extrusion toward the free surface conversely asymmetric structures such as shear bands and mica fish reflect simple shear dominated deformation late in the shear zone history parallel to the imposed relative plate motion vector finally linear fabrics are a poor record of the mylonitic deformation in this zone because 1 they are dominantly inherited and have been only partially re oriented during mylonitisation and 2 macroscopically they become less visible at higher strains due to fabric homogenisation generally these results indicate that particularly in heterogeneous polyphase shear zones a full understanding of the movement history can only be obtained by considering kinematic information from as many different sources as possible

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

seismic velocity and anisotropy in mylonites and the reflectivity of deep crystal fault zones compressional wave velocity anisotropy has been measured to 1 kb confining pressure in several mylonites in one cataclasite and in their protoliths no significant difference in the velocity or density was found between the protolith and the tectonically altered rock at 1 kb strong seismic anisotropy greater than 10 was present in only two of nine mylonites measured the anisotropy was due to their aphanitic texture high content of layered silicates and pervasive foliation most mylonitic rocks studied were not strongly anisotropic despite a well developed macroscopic foliation where anisotropy was present velocities usually were slowest perpendicular to the foliation and fastest parallel to both foliation and lineation these preliminary results suggest that ductile fault zones in the crust are not strong seismic reflectors unless mylonites in the fault zone are aphanitic and contain large amounts of foliated phyllosilicates it is also possible that anomalous environmental conditions such as high pore pressure in the fault zone in situ may explain observed seismic reflectivity

late quaternary slip rates and slip partitioning on the alpine fault new zealand abstract published geological data on late quaternary offsets on the alpine fault new zealand have been assembled into a common format and analysed with respect to uncertainties uncertainties arise mainly from measurement of offset features relating apparent offsets to actual fault slip and dating the offset features despite the considerable uncertainties the data form a coherent set consistent with a relatively constant rate of strike slip of 27 5 mm year between milford sound and hokitika this rate represents 70 75 of the fault parallel interplate motion north of the confluence with the hope fault the rate drops substantially dip slip rates on the other hand show considerable variation along strike rising to a maximum of more than 10 mm year in the central section and decreasing to zero at the southern end partitioning of c 25 of the interplate slip on to structures east of the alpine fault occurs in the central section consistent with predictions from critical wedge models the partitioning of all the fault normal component of displacement on to other structures in the south may be related in part to a doubling in width of the deforming wedge to the east most probably however the fault normal displacement is mainly accommodated by underthrusting of the australian plate offshore due to a change in the nature of the crust from continental to oceanic

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

forecasting southern california earthquakes since 1978 and 1979 california has had a significantly higher frequency of moderate to large earthquakes than in the preceding 25 years in the past such periods have also been associated with major destructive earthquakes of magnitude 7 or greater and the annual probability of occurrence of such an event is now 13 percent in california the increase in seismicity is associated with a marked deviation in the pattern of strain accumulation a correlation that is physically plausible although great earthquakes magnitude greater than 7 5 are too infrequent to have clear associations with any pattern of seismicity that is now observed the san andreas fault in southern california has accumulated sufficient potential displacement since the last rupture in 1857 to generate a great earthquake along part or all of its length

low velocity damaged structure of the san andreas fault at parkfield from fault zone trapped waves 1 we used dense linear seismic arrays across and along the san andreas fault saf at parkfield california to record fault zone trapped waves generated by explosions and microearthquakes in 2002 prominent trapped waves appeared at stations close to the saf main fault trace while some energy was trapped in the north strand at the array site observations and 3 d finite difference simulations of trapped waves at 2 5 hz show evidence of a damaged core zone on the main saf the zone from the surface to seismogenic depths is marked by a low velocity waveguide 150 m wide in which q is 10 50 and shear velocities are reduced by 30 40 from wall rock velocities with the greatest velocity reduction at shallow depth we interpret that this distinct low velocity zone on the main saf is a remanent of damage due to past large earthquakes on the principal fault plane at parkfield a less developed low velocity zone may be evident on the north strand that experienced minor breaks in the 1966 m6 event

fault zone guided wave generation on the locked late interseismic alpine fault new zealand fault zone guided waves fzgws have been observed for the first time within new zealand s transpressional continental plate boundary the alpine fault which is late in its typical seismic cycle ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone distinctive dispersive seismic codas 7 35 hz have been recorded on shallow borehole seismometers installed within 20 m of the principal slip zone near the central alpine fault known for low background seismicity fzgw generating microseismic events are located beyond the catchment scale partitioning of the fault indicating lateral connectivity of the low velocity zone immediately below the near surface segmentation initial modeling of the low velocity zone indicates a waveguide width of 60 200 m with a 10 40 reduction in s wave velocity similar to that inferred for the fault core of other mature plate boundary faults such as the san andreas and north anatolian faults

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

seismic images of the biggest crash on earth in plate tectonics the upper layer of earth the lithosphere consists of rigid plates that shift over geological time scales data from global positioning surveys have estimated that the entire indian subcontinent has moved over the past 50 years about 2 m to the north diving slowly underneath tibet this giant collision has been ongoing for 50 million years and has thrown up the highest mountains as well as the largest and highest plateau on earth not only is the world climate strongly influenced by this massive plateau average elevation of 5000 m but the collision also causes catastrophic earthquakes in southern central and eastern asia

late cenozoic denudation by large magnitude landslides in the eastern edge of tibetan plateau abstract eastern edge of the tibetan plateau is marked by the longmen shan which experienced pronounced denudation in late cenozoic and was previously attributed to river erosion our recent work however demonstrated that the landsliding must have played an important integral role in the surface processes various scale landslides are present in the peripheral zone of the southern longmen shan which were interpreted as overthrust related klippen in previous works we revisited these exotic blocks and demonstrated that they were actually gravity driven landslides in origin the landslides are composed mostly of thick bedded marine limestone and sandstone of paleozoic early triassic ages and were detached along some weak zones like lower silurian phyllite and unconformities three types of landslides can be categorized distributed stacked and coherent and their differential occurrence is considered to bear upon both uplift rates of the longmen shan and foreland topography outward flow of lower crustal materials from the tibetan plateau interior provides a feasible mechanism for initiating the uplift of its eastern flank in cenozoic positive feedbacks are believed to have existed between extrusion of lower crustal channel flow and surface denudation processes rapid denudation by large magnitude landslides in conjunction with coeval river erosion in the southern longmen shan might have led to predominant sub vertical extrusion of lower crustal flow channel as evidenced by nearly symmetric exhumation of the pengguan massif the northern longmen shan was comparatively less denuded with few basement rocks cropping out and landslides are absent in its front as well differential denudation between the southern and northern longmen shan might have in large part resulted from the lower crustal flow that presumably moved to the southeast and had less impact on the northern longmen shan

crustal structure and composition beneath the northeastern tibetan plateau from receiver function analysis abstract we analyzed hundreds of p wave receiver function data recorded by 53 portable seismic stations to investigate the crustal structure beneath the ne tibetan plateau the moho depths identified by this study reveal that the crustal thickness decreases laterally from the qilian orogen 63 km to east kunlun fault eklf area 44 km the moho depth becomes obviously deeper northeast of songpan ganzi terrane and beneath the qilian orogen a remarkable contrast is observed in the measured poisson s ratio between the qilian orogen and eklf area the qilian orogen is characterized by lower poisson s ratio in general the higher poisson s ratios 0 30 in the eklf area can be considered as the evidence for dominantly mafic rocks in the crust the measured low poisson s ratio and the negative correlations between vp vs and the crustal thickness beneath the qilian orogen suggest dominantly felsic crust beneath the qilian orogen which is in contrast with the mid to lower low velocity crustal model beneath the ne margin of the tibetan plateau indicating that the low velocity zone terminates beneath the east kunlun area the seismic evidence suggests that the thickened crust is probably resulted from superposition of the successive intracrustal thrusts our results reveal the lateral inhomogeneity of crustal structure in this area and are inconsistent with the scenario of an inflated crust due to extrusion of lower crust material from the central tibetan plateau

seismicity near palmdale california and its relation to strain changes we evaluate the relationships between the spatio temporal patterns and faulting mechanisms of small earthquakes and the recent temporal changes in horizontal strain observed along the big bend portion of the san andreas fault near palmdale california microearthquake activity along the entire big bend of the san andreas fault increased in november 1976 concurrent with the initiation of an earthquake swarm at juniper hills this activity then decreased abruptly to the northwest and southeast of juniper hills during the beginning of 1979 this drop in seismic activity occurred around the time that crustal dilatation was observed on the u s geological survey palmdale trilateration network focal mechanisms from the study region are predominantly thrust there are two time periods when the mechanisms are closer to strike slip than to thrust the first period december 1976 to february 1977 corresponds to the beginning of the juniper hills swarm the second period november 1978 to april 1979 approximately coincides with a change in trend of the strain data from uniaxial n s compression to dilatation

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

prehistoric dates of the most recent alpine fault earthquakes new zealand we use four methods to investigate the earthquake history over the past 600 yr of the central alpine fault new zealand trenches across the active fault trace at five locations identify two ruptures on the fault in the past 450 yr three other indirect indicators episodes of landsliding and aggradation dated either via 14 c or from forest establishment on the resulting surfaces and tree ring growth anomalies provide further evidence for these two earthquakes and a greater resolution in estimates of their timing and extent the most recent earthquake occurred in a d 1717 and the rupture extended over a section of fault between milford and the haupiri river a distance of 375 km about 1630 another earthquake occurred on the fault and this extended between at least the paringa and ahaura rivers 250 km regional episodes of landsliding and forest establishment provide evidence for a third earthquake at about 1460 over the same section of fault the pattern of earthquake recurrence is not regular but averages 200 yr and varies from 100 yr to at least 280 yr which is the lapsed time since the most recent rupture

crustal structure and continental dynamics of central china a receiver function study and implications for ultrahigh pressure metamorphism abstract the qinling tongbai hong an dabie sulu orogenic belt records the tectonic history of paleozoic convergence between the south china and north china blocks in this study the distribution of crustal thickness and p and s wave velocity ratio vp vs is obtained by using the h k stacking technique from the dabie sulu belt in central china our results show marked differences in the crustal structure between the dabie and sulu segments of the ultrahigh pressure uhp orogen the lower crust in the dabie orogenic belt is dominantly of felsic intermediate composition whereas the crust beneath the sulu segment is largely intermediate mafic the crust of the dabie orogenic belt is thicker by ca 3 5 km as compared to that of the surrounding region with the presence of an orogenic root the crustal thickness is nearly uniform in the dabie orogenic belt with a generally smooth crust mantle boundary a symmetrically thickened crust in the absence of any deep structural features similar to that of the yangtze block suggests no supportive evidence for the proposed northward subduction of the yangtze continental block beneath the north china block we propose that the collision between the yangtze and north china blocks and extrusion caused crustal shortening and thickening as well as delamination of the lower crust resulting in asthenospheric upwelling and lower crustal uhp metamorphism along the dabie orogen our results also reveal the presence of a se to nw dipping moho in the north china block beneath the tran north china orogen and eastern block suggesting the fossil architecture of the northwestward subduction of the kula plate

far field lithospheric deformation in tibet during continental collision 1 crustal deformation along the present day northern margin of the tibetan plateau has occurred since mid to late eocene time soon after india collided with eurasia assuming that on these distance and time scales the lithosphere can be approximated using a thin viscous sheet we show that far field lithospheric deformation caused by an indenting boundary is expected to start shortly after collision begins when a strong region analogous to cratonic lithosphere like that of the tarim basin is included in the calculations shear and vertical strain rates concentrate along the southern edge of the strong region resulting in enhanced deformation there in calculations scaled to the dimensions of india and the tibetan plateau shear and vertical strain rates on the northern edge of the plateau increase soon after collision begins crustal thickening accelerates later in the calculations we propose that the apparent increased exhumation and perhaps surface uplift around 10 ma may signal the arrival of the thickening front

aseismic crustal deformation in the transverse ranges of southern california abstract geodetic data at a plate boundary can reveal the pattern of subsurface displacements that accompany plate motion we model these displacements as the sum of rigid block motion and the elastic effects of frictional drag between blocks the frictional interactions are represented by uniform dislocation on each of several rectangular fault patches we then estimate the block velocities and fault parameters from geodetic data our bayesian inversion procedure employs prior estimates based on geological and seismological data we apply the method to the transverse ranges using prior data from bird and rosenstock 1984 and geodetic data from the usgs trilateration networks and nasa vlbi measurements our model consists of 12 blocks and 27 rectangular fault patches the block motion inferred from the geodetic data has the same order of magnitude as the geologic estimates and for many faults the agreement is excellent however the geodetic data imply a displacement rate of about 20 mm yr 1 across the san andreas fault while the geologic estimates exceed 30 mm yr 1 the final model implies about 6 mm yr 1 of crustal shortening normal to the trend of the san andreas fault this is about half the rate implied by the prior model most of this shortening occurs on the sierra madre cucamonga and the white wolf fault systems aseismic fault motion is a major contributor to plate motion and the thickness of the frictional surface varies considerably from one fault to another the geodetic data can help to identify faults that are suffering rapid stress accumulation in the transverse ranges those faults are the san andreas san gabriel sierra madre pleito and white wolf faults

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

anomalous water level changes and possible relation with earthquakes since late 1976 abandoned water wells have been monitored along the san andreas and san jacinto faults to detect water level changes which may be premonitory to earthquakes an m5 5 earthquake occurred 25 february 1980 within the san jacinto fault zone near anza the water level in a continuously recorded well in borrego valley 35 km from the epicenter rose 0 5 m and returned to its prior level during a four hour period beginning 88 hours before the earthquake the water level in this well had been remarkably steady compared to other wells being monitored another well in borrego valley showed a much smaller rise and fall in water level at the same time because these changes are unique for the long term records of these wells they may represent precursors to the earthquake several wells along the palmdale valyermo segment of the san andreas fault showed possibly anomalous long term water level changes which are not clearly related to rainfall between early 1979 and early 1980 water levels in six wells were 6 to 4 5 m higher and levels in two wells were about 3 m lower than would have been predicted based on the previous records the water level increases could have been the result of delayed response to seasonal rainfall following a long drought however it is interesting that the possibly anomalous water level changes first occurred at the same time as the reported changes in strain pattern from compression to expansion along the san andreas fault in the same area

erosional control on the structural evolution of a transpressional thrust complex on the alpine fault new zealand abstract the waikukupa thrust is a 4 km long oblique thrust segment of the new zealand alpine fault that has developed over the last 65 000 years emplacing mylonite and cataclasite over fluvioglacial gravel during the last 20 000 years or so a 2 km long out of sequence imbricate the hare mare thrust has formed on the east side of the waikukupa river valley the average rate of slip on the two thrusts is estimated at 22 30 mm year analysis of the fault structure in terms of critical wedge theory is consistent with imbrication resulting from a reduction in wedge taper below the critical value due to rapid river erosion the internal structure of the waikukupa thrust sheet consists of reverse faults and angular folds forming duplex like systems associated with minor strike slip faults within the limits of uncertainty the structures are consistent with strain accumulating during internal transpressional shear of the thrust sheet while the taper of the latter was subcritical prior to the transfer of fault displacement to the hare mare imbricate the thrust complex forms part of a serrated range front fault system consisting of oblique thrust sections linked by strike slip faults we introduce the term serial partitioning for this type of partitioned transpressional fault system its development is linked to the erosional processes at the range front an alternative system consisting of parallel thrust and strike slip faults we here term parallel partitioning while angle of obliquity is an important parameter in determining which partitioning model develops we suggest that erosion rate is also an influential factor

the structure of the crust and uppermost mantle beneath south china from ambient noise and earthquake tomography summary two years of continuous recordings of ambient seismic noise observed at 354 stations in south china from 2009 to 2010 are used to estimate rayleigh wave group and phase velocity maps from 6 to 40 s period these results are merged with rayleigh wave phase velocity maps from 25 to 70 s period derived from earthquakes in the same time frame eikonal tomography generates the dispersion maps which by monte carlo inversion are used to estimate a 3 d vsv model of the crust and upper mantle down to a depth of 150 km across all of south china with attendant uncertainties a clear image emerges of the west yangtze block a region of the western yangtze craton characterized by relatively thick crust 40 km overlying a seismic mantle lithosphere that extends to at least 150 km that may have been the nucleus for the formation of the yangtze craton in the archean and may present a present day obstacle to the eastward expansion of tibet the west yangtze block contrasts with the thinner crust 30 km and mantle lithosphere 70 80 km beneath the eastern yangtze craton and south china foldbelt these observed differences are consistent with processes associated with flat slab subduction in the mesozoic that may have eroded the lithosphere of the eastern yangtze craton and the south china foldbelt

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

mica controlled anisotropy within mid to upper crustal mylonites an ebsd study of mica fabrics in the alpine fault zone new zealand

migration of historical earthquakes in california most large earthquakes of magnitude 6 0 in california during 1852 1987 appear to show a southeast to northwest tendency of epicenter migration this finding is consistent with earlier findings ofsavage 1971 for a relatively few large earthquakes along the west coast of north america and ofwood andallen 1973 for smaller events along the san andreas fault in central california the average speed of migration is approximately 130 km yr which is within the range of speeds observed for other major seismic zones in the world the epicenter migration in california may be the result of some small but broad scaled episodic strain changes associated with creep waves induced by magma injections at the east pacific rise and propagating northwestwardly along a broad transform boundary between the pacific and north american plates at subseismogenic depths as proposed bysavage 1971

seismic measurements of the internal properties of fault zones the internal properties within and adjacent to fault zones are reviewed principally on the basis of laboratory borehole and seismic refraction and reflection data the deformation of rocks by faulting ranges from intragrain microcracking to severe alteration saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity but from borehole measurements densely fractured rocks do show significantly reduced velocities the amount of reduction generally proportional to the fracture density highly fractured rock and thick fault gouge along the creeping portion of the san andreas fault are evidenced by a pronounced seismic low velocity zone lvz which is either very thin or absent along locked portions of the fault thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone high pore pressure within the pronounced lvz may be conductive to fault creep deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated seismically anisotropic mylonites

precision of geodolite surveys a reply to jackson and cheng jackson and cheng have suggested that the changes in areal dilatation measured in geodolite surveys by the u s geological survey may be simply an artifact of the measuring system although systematic error could conceivably account for the observed excursions in dilatation we maintain that the specific criticisms by jackson and cheng are incorrect the excursions in dilatation cannot be attributed to the offset correction nor to proportional error associated with temperature the absence of both errors is demonstrated by using data that are particularly sensitive to the two effects

how do lineations reflect the strain history of transpressive shear zones the example of the active alpine fault zone new zealand abstract lineations within mylonites exhumed in the hanging wall of new zealand s active alpine fault zone have a complicated relationship to contemporary plate kinematics the shear zone is triclinic and macroscopic object lineations are not usually parallel to the simple shear direction despite high total simple shear strains 150 this is mostly because the lineations are inherited from pre mylonitic fabrics and have not been rotated into parallelism with the mylonitic stretching direction which pitches c 44 in the fault plane furthermore some lineations have been variably rotated depending on whether they are present in shear bands or microlithons which accommodated bulk strains with different vorticities total strains required to obtain parallelism between the finite maximum principal stretching direction calculated from transpression models and these mylonitic lineations are pure shear stretch s 1 3 5 simple shear 11 7

subduction of asian lithospheric mantle beneath tibet inferred from models of continental collision the relative northward motion of the indian subcontinent that followed the onset of continental collision with asia has produced extensive deformation of the earth s crust giving rise to the world s highest mountains in the himalayan chain and the world s largest high elevation region the tibetan plateau the formation of the broad mountain belt implies that contrary to the original tenets of plate tectonics the lithospheric plates have experienced widespread deformation far from the plate boundary1 several models have been proposed2 6 to explain the manner in which this post collisional deformation is distributed within the continental lithosphere of the indian and asian plates here we propose an alternative model in which subduction of the asian lithospheric mantle develops following the collision of india our model is supported by numerical calculations of crustal deformation and thickening and is consistent with available geological and geophysical data7 9 this picture suggests that lithospheric mantle is not deformed along with the crust and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed