evidence of a collision between the yucatan block and mexico in the miocene we present the evidence for an anomalous southwest dipping slab in southern mexico the main evidence comes from a clear receiver function image along a seismic line across the isthmus of tehuantepec and is also supported by a previous global tomographic model the slab dips at 35 is approximately 250 km in length and appears to truncate the cocos slab at about 120 km depth we hypothesize that the slab was created by subduction of oceanic lithosphere prior to the collision of the yucatan block with mexico at approximately 12 ma this scenario would explain the chiapas fold and thrust belt as the product of this collision and its age constrains the date of the event to be in the miocene

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

present day deformation of the qaidam basin with implications for intra continental tectonics abstract the qaidam basin is a plateau of low relief about 3 km in elevation and covering a large area of 1 2 105 km2 physiographically the basin marks a transitional region between the northeastern tibetan plateau 5 0 5 km in elevation and the surrounding low lying areas the northeastern and southwestern edges of the basin are bounded by the qilian shan nan shan and qimantag eastern kunlun shan mountain belts respectively at present the entire region is under northeast southwest compression based on a synthesis of recent geological and geophysical results we propose that each flanking mountain belt consists of a sequence of crustal nappes originating from ramp thrusts at mid crustal levels 15 km deep over a distance range of 700 km these two broad fold and thrust belts operating in tandem and with some uniform shortening in the intervening qaidam basin successively raised the surface elevation from below 2 km to 5 km at depth the crust is correspondingly thickened from 40 km beneath the foreland of the qilian shan to approximately 55 km under the interior of the qaidam basin and then to nearly 70 km beneath tibet in contrast to the himalayan front where earthquake faulting occurs down to a depth of 50 km in the foreland but the zone of active thrust faulting is less than 200 km in width plateau building in the qaidam region illustrates how a wide orogen is developing along northeastern tibet in the interior of eurasia

shear wave polarization anisotropy and subduction induced flow in the mantle wedge of northeastern japan abstract shear wave polarization anisotropy in the central part of northeastern ne japan was investigated we analyzed s phases of 293 intermediate depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters the obtained results show the systematic variation in splitting parameters across the arc the leading shear wave polarization directions fast direction obtained at stations in the western side of the study area are oriented nearly e w which is sub parallel to the direction of relative plate motion between the pacific plate and the north american okhotsk plate in contrast fast directions obtained at stations in the eastern side of the study area are oriented approximately n s stations in the western side have a larger average delay time compared to those in the eastern side these variations suggest that the nature of anisotropy is quite different between the eastern and the western sides both experimental studies on the deformation of olivine and numerical simulations of plate driven flow predict that the lattice preferred orientation of olivine causes the fast direction sub parallel to the flow direction for the conditions in the mantle wedge of ne japan we thus infer that the lattice preferred orientation of olivine generated by flow induced strain is the most likely candidate for anisotropy that produces splitting with e w fast direction three possible causes of anisotropy in the eastern side are considered deformation of water rich olivine in the mantle wedge trench parallel flow in the mantle wedge due to along strike dip variations in the slab and anisotropy in the crust and the slab we could not however determine which causes are dominant further systematic study is required to reveal the cause of anisotropy in the eastern side

temporal spatial rupture process of the 1999 chi chi earthquake from iris and geoscope long period waveform data using aftershocks as empirical green s functions a large earthquake m w 7 6 occurred near chi chi taiwan on 20 september 1999 utc and was followed by many moderate size aftershocks in the following days the two largest aftershocks with magnitudes m w 6 1 and 6 2 respectively were used as empirical green s functions egfs to retrieve the source time functions stfs and further image the temporal spatial rupture process of the mainshock for each station two types of stfs were retrieved one from p phases and another from s phases a total of 178 stf individuals were retrieved for source process analysis of the event from the stfs retrieved firstly similarities appeared on the stfs from most stations except that several stfs in special azimuths looked different or odd because of the focal mechanism difference between the mainshock and the egf aftershocks and secondly systematic shape variation with azimuth appeared the analysis of the stfs indicated that this event consisted of two subevents on the average the second event was about 7 sec later than the first one and the first event was about 15 larger than the second one in the moment release rate the total duration time of earthquake rupture process was about 26 sec from the image of the static slip distribution there were two slip concentrated areas on the fault plane and their centers were about 45 km away from each other the maximum slip of about 6 5 m appeared on the northern one the rupture area with the slip greater than 0 5 m was about 80 km long and 60 km wide the maximum stress drop was about 25 mpa 250 bar and the average stress drop on the entire fault was 9 2 mpa 92 bar from the snapshots of the temporal spatial variation of slip and slip rate the rupture initiated at the southern end and stopped at the northern end of the fault which suggested an overall unilateral rupture at a rupture velocity of about 2 5 km sec the depth of the initiation locus was about 20 km the rupture duration times on the subfaults were estimated a good correlation was noticed between the rupture duration time and slip amplitude of the subfaults the larger the slip amplitude the longer the rupture duration time of the subfault the maximum duration time for the subfaults was 16 sec which was about two thirds the total duration time of the earthquake rupture process in the beginning and near the end of the source process the rupture propagated like a self healing pulse of slip manuscript received 21 may 2001

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

quantifying the oldest tidal record the 3 2 ga moodies group barberton greenstone belt south africa the 3 2 ga moodies group in the barberton greenstone belt south africa contains the oldest preserved record of tides the tidal record is preserved in a tidal sand wave deposit in the lower moodies group as bundles of sandstone foresets separated by mudstone drapes detailed analysis of rhythmic foreset bundles permits quantification of the tidal record and reveals a hierarchy of diurnal fortnightly and monthly tidal periodicities thick thin pairs of foreset bundles reflect deposition from semidiurnal dominant and subordinate flood tidal currents respectively cyclic variations in foreset bundle thicknesses record longer period changes in strength of the dominant semidiurnal tidal currents consistent with neap spring neap tidal cyclicity alternating thicker and thinner neap spring neap cycles are comparable to anomalistic perigean apogean tidal signatures this quantitative record of tides in the middle archean moodies group represents by 2 2 b y the oldest such documentation tidal cyclicity recognized in the moodies sand wave deposit is comparable to that recorded in modern tidal settings and identified in the carboniferous rock record and is most compatible with a lunar orbital shape similar to that existing today

importance of model parameterization in finite fault inversions application to the 1974 mw 8 0 peru earthquake the spatial and temporal slip distributions for the october 3 1974 mw 8 0 peru subduction zone earthquake and its largest aftershock on november 9 ms 7 1 are calculated and analyzed in terms of the inversion parameterization and tectonic significance teleseismic long period world wide standard seismograph network p and sh waveforms are inverted to obtain the rupture histories we demonstrate that erroneous results are obtained if a parameterization is used that does not allow for a sufficiently complex source involving spatial variation in slip amplitude risetime and rupture time the inversion method utilizes a parameterization of the fault that allows for a discretized source risetime and rupture time well located aftershocks recorded on a local network have the same general pattern as teleseismically determined hypocenters and help to constrain the geometry of the subduction zone for the main shock a hinged fault is preferred having a shallow plane with a dip of 11 and a deeper landward plane with a dip of 30 the preferred nucleation depth lies between 11 and 15 km a bilateral rupture is obtained with two major concentrations of slip one 60 to 70 km to the northwest of the epicenter and a second 80 to 100 km to the south and southeast of the epicenter for these source regions risetimes vary from 6 to 18 s our estimates of risetimes are consistent with the time for the rupture to traverse the dominant local asperity the slip distribution for the november 9 aftershock falls within a conspicuous hole in the main shock rupture pattern near the hypocenter of the main shock the november 9 event has a simple risetime function with a duration of 2 s aftershocks recorded by the local network are shown to cluster near the hypocenter of the impending november 9 event and downdip from the largest main shock source region slip during the main shock is concentrated at shallow depths above 15 km and extends updip from the hypocenter to near the plate boundary at the trench axis the large amount of slip at shallow depths is attributed to the absence of any significant accretionary wedge of sediments and the relatively young age and high convergence rate of the subducted plate which results in good seismic coupling near the trench axis

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

imaging the moho and subducted oceanic crust at the isthmus of tehuantepec mexico from receiver functions using teleseismic data recorded along a transect which we call veox for veracruz oaxaca seismic line of 46 broadband stations installed across the isthmus of tehuantepec in southern mexico we obtained receiver functions and stacked them to study the moho topography and back projected them to visualize the subducted slab geometry beneath the isthmus we observed a back azimuth dependent moho thickness across the transect particularly beneath the los tuxtlas volcanic field also we observed the cocos plate which subducts with an angle of 26 between 140 and 310 km from the trench comparison with regional seismicity indicates that it occurs below the oceanic crust

geological constraints on the precambrian history of earth s rotation and the moon s orbit over the past decade the analysis of sedimentary cyclic rhythmites of tidal origin i e stacked thin beds or laminae usually of sandstone siltstone and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation has provided information on earth s paleorotation and the evolving lunar orbit for precambrian time before 540 ma depositional environments of tidal rhythmites range from estuarine to tidal delta with a wave protected distal ebb tidal delta setting being particularly favorable for the deposition and preservation of long detailed rhythmite records the potential sediment load of nearshore tidal currents and the effectiveness of the tide as an agent of sediment entrainment and deposition are related directly to tidal range or maximum tidal height and consequent current speed hence the thickness of successive laminae deposited by tidal currents can be a proxy tidal record with paleotidal and paleorotational values being determined by analysis of measured records of lamina and cycle thickness the validity of the findings can be investigated by testing the primary observed values for internal self consistency through application of the laws of celestial mechanics paleotidal and paleorotational values provided by late neoproterozoic 620 ma tidal rhythmites in south australia are validated by these tests and indicate 13 1 0 1 synodic lunar months yr 400 7 solar days yr a length of day of 21 9 0 4 h and a relative earth moon distance a a0 of 0 965 0 005 the mean rate of lunar recession since that time is 2 17 0 31 cm yr which is little more than half the present rate of lunar recession of 3 82 0 07 cm yr obtained by lunar laser ranging the late neoproterozoic data militate against significant overall change in earth s moment of inertia and radius at least since 620 ma cyclicity displayed by paleoproterozoic 2450 ma banded iron formation in western australia may record tidal influences on the discharge and or dispersal of submarine hydrothermal plumes and suggests 14 5 0 5 synodic months yr and a a0 0 906 0 029 the combined rhythmite data give a mean rate of lunar recession of 1 24 0 71 cm yr during most of the proterozoic 2450 620 ma suggesting that a close approach of the moon did not occur during earlier time concentrated study of precambrian tidal rhythmites promises to illuminate the evolving dynamics of the early earth moon system and may permit the lunar orbit to be traced back to near the time of the moon s origin

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

seismic anisotropy beneath the shumagin islands segment of the aleutian alaska subduction zone lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction in this study we inferred possible strain geometries in the mantle beneath the eastern aleutians by mapping seismic anisotropy with shear wave splitting we analyzed s phases of local events recorded by the shumagin seismic network and obtained splitting parameters fast direction and splitting time t and their 95 confidence regions to avoid complexity from converted phases data were restricted to paths at incidence angles less than 32 and to events deeper than 50 km the s phases sample the forearc and back arc of the shumagin segment from 159 7 w to 163 8 w at depths up to 216 km in our data set of 84 local events 44 of 127 s phases yielded constrained splitting parameters with consistent fast directions that are approximately parallel to the volcanic arc roughly 60 the remaining 83 s phases produced null measurements and the fast directions inferred from these phase polarizations agree with the fast axis alignment in the constrained measurements splitting times vary from 0 1 to 0 35 s and show an overall increase with hypocentral depth observed splitting parameters are well matched by predicted values for olivine rich mantle wedge models with 1 sv anisotropy where the olivine b axis is arc orthogonal and the a axis is vertical no arc parallel strain or arcparallel no vertical strain these results are consistent with mantle wedge strain models in which arc normal compression is accompanied by arc parallel or vertical shearing or extension

tectonic stress field of china inferred from a large number of small earthquakes mean principal tress axes were inferred for china using 9621 p wave first motion polarity readings from 5054 small earthquakes 1 ml 5 the area studied was divided into 76 subregions the mean p compressive b intermediate and t relatively tensional axes corresponding to composite focal mechanism solutions of multiple earthquakes for each subregion were determined by a grid test to all possible orientations of the p b and t axes with a step of 5 or 10 in order to get a relatively homogeneous sampling in space we have avoided using the readings from spatially and temporally clustered earthquakes we have rechecked most of the polarity readings by inspection of original seismograms the focal mechanism solutions of large m 6 individual earthquakes are also presented for comparison the results indicate the existence of an overall radial pattern of the present day maximum horizontal stress orientations throughout the continental area of china this pattern is thought to be closely related to indentor models of continental collision between the indian ocean and the eurasian plate

investigation of the rupture process of the 28 june 1992 landers earthquake utilizing terrascope abstract displacement seismograms recorded by terrascope for the 28 june 1992 landers earthquake m w 7 3 are deterministically modeled using a forward point source summation technique although the data set is sparse it was possible to robustly determine important rupture parameters such as gross slip distribution rupture velocity rise time and total source duration the relatively simple approach lends itself to rapid application following large earthquakes provided that a catalog of green9s functions appropriate for the region is available the fault used in the modeling of the landers mainshock has a length of 70 km along strike and a width of 15 km along dip a model was found in which the distribution and amplitude of slip at the surface matches the observed surface slip and provides a very good level of fit to the seismic data seismically the landers earthquake is characterized as two subevents the peak slip of the first subevent is 10 km north of the epicenter and the second is 40 km northwest along strike from the epicenter the seismic moment is distributed as 2 10 26 dyne cm to the first and 6 10 26 to the second subevent respectively it was assumed in our modeling that the distribution of seismic moment along strike was the same at all depths this assumption implies that slip at depth is 69 of that at the surface as a result of differences in the material properties in the velocity model the sensitivities of the source model to rupture velocity and dislocation rise time were examined a rupture velocity of 2 9 km sec 80 of the shear wave velocity and a rise time of 1 to 3 sec were found to satisfy the data the rise time is only a fraction of the total source process time of 24 sec and implies that slip on the fault occurred within a narrow band 3 to 10 km at any instant during the rupture

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

cenozoic shortening budget for the northeastern edge of the tibetan plateau is lower crustal flow necessary 1 two of the most popular mechanisms for thickening the crust beneath the tibetan plateau are 1 pure shear with faulting and folding in the upper crust and horizontal shortening below and 2 flow of lower or middle crust without significant shortening of the upper crust to help discriminate between the relative contributions of these two mechanisms well constrained estimates of upper crustal shortening are needed here we document the cenozoic shortening budget across the northeastern tibetan plateau margin with several balanced cross sections that exploit thermochronological and magnetostratigraphic constraints these sections indicate 11 2 1 east west shortening since middle miocene time and 9 2 3 nne ssw shortening between middle eocene and middle miocene times with little subsequent shortening of this orientation shortening rates accelerate fivefold after middle miocene time given the present day crustal thickness of 56 4 km in northeastern tibet crustal restorations that remove cenozoic shortening suggest that the northeastern tibetan crust was 45 5 km thick prior to the india asia continental collision this precollision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the tibetan plateau 44 4 km and globally 41 6 km and suggests that pure shear alone may account for cenozoic crustal thickening in northeastern tibet in contrast to eastern tibet where in the absence of significant shortening structures crustal flow has been invoked to explain the addition of crustal material since middle miocene time our results may obviate lower crustal flow as a necessary crustal thickening agent in northeastern tibet

implications of lunar orbital periodicity from the chaibasa tidal rhythmite india of late paleoproterozoic age harmonic analysis of the sandstone foreset laminae thickness series from the chaibasa tidal rhythmite india clearly shows that a normal semidiurnal tidal system with synodic month of 32 lunar days was in effect during the late paleoproterozoic 2100 1600 ma the minimum number of solar days in a lunar sidereal month was 31 published quantitative tidal rhythmite data of others in combination with data derived from the chaibasa tidal rhythmite indicate long term stability of the lunar orbit and progressive increase in the earth moon distance during the proterozoic

precambrian length of day and the validity of tidal rhythmite paleotidal values paleotidal records obtained from sedimentary tidal rhythmites may be systematically abbreviated and so may give incorrect paleotidal and paleorotational values the validity of determined values including past length of day l o d can be assessed by testing for internal self consistency through application of the laws of celestial mechanics three independent values obtained from the 620 ma elatina reynella rhythmites in south australia 14 1 sidereal months year 401 sidereal days year and 19 5 years for the lunar nodal period when employed in different equations that make allowance for lunar and solar tidal effects each give a lunar semimajor axis in the range of 96 5 96 9 of the present figure such self consistency strongly supports the validity of the derived l o d of 21 9 hours at 620 ma the validity of the estimated l o d of 20 9 hours at 900 ma revised value big cottonwood rhythmites utah and of 17 1 18 9 hours at 2 5 ga weeli wolli rhythmites western australia cannot be assessed in that way because each data set has only one directly determined value the derived mean rate of lunar retreat of 2 16 cm year since 620 ma averts a close approach of the moon at least since 3 ga and a lower rate of retreat seems likely during the proterozoic

source complexity of the 1987 whittier narrows california earthquake from the inversion of strong motion records strong motion records for the whittier narrows earthquake are inverted to obtain the history of slip both constant rupture velocity models and variable rupture velocity models are considered the results show a complex rupture process within a relatively small source volume with at least four separate concentrations of slip two sources are associated with the hypocenter the larger having a slip of 55 90 cm depending on the rupture model these sources have a radius of approximately 2 3 km and are ringed by a region of reduced slip the aftershocks fall within this low slip annulus other sources with slips from 40 to 70 cm each ring the central source region and the aftershock pattern all the sources are predominantly thrust although some minor right lateral strike slip motion is seen the overall dimensions of the whittier earthquake from the strong notion inversions is 10 km long along the strike and 6 km wide down the dip the preferred dip is 30 degree and the preferred average rupture velocity is 2 5 km s moment estimates range from 7 4 to 10 0 times 10 sup 24 dyn cm depending on the rupture model

incision rate of the yellow river in northeastern tibet constrained by 10be and 26al cosmogenic isotope dating of fluvial terraces implications for catchment evolution and plateau building unlike other large rivers flowing out of tibet the yellow river escapes from the plateau towards the ne crossing no less than five nw se striking actively growing ranges and intervening basins thick plio quaternary deposits and fluvial terraces testify to a phase of aggra dation and sediment infill up to the average surface elevation 3200 3250 m a s l of the gonghe guide and qinghai lake basins a set of seven main terraces across the gonghe basin suggests progressive down cutting of the yellow river carving the 500 m deep longyang gorge at the basin exit 10be and 26al concentrations in quartz of surface and sub surface samples of four terraces constrain the timing of incision by determining the burial age of the deposit and the exposure age of its surface modelling the depth dependence of the 10be concentration and the 26al 10be ratio allows us to constrain the onset of the ongoing phase of incision to 120 250 ka these ages suggest long term incision rates between 2 6 mm a together with the present morphology of the yellow river terraces across the gonghe basin and the longyang gorge our results imply rapid river catchment evolution and interaction between river dynamics tectonic and climate in northeastern tibet

source process of the 1990 gonghe china earthquake and tectonic stress field in the northeastern qinghai xizang tibetan plateau the m s 6 9 gonghe china earthquake of april 26 1990 is the largest earthquake to have been documented historically as well as recorded instrumentally in the northeastern qinghai xizang tibetan plateau the source process of this earthquake and the tectonic stress field in the northeastern qinghai xizang plateau are investigated using geodetic and seismic data the leveling data are used to invert the focal mechanism the shape of the slipped region and the slip distribution on the fault plane it is obtained through inversion of the leveling data that this earthquake was caused by a mainly reverse dip slipping buried fault with strike 102 dip 46 to ssw rake 86 and a seismic moment of 9 4 1018 nm the stress drop strain and energy released for this earthquake are estimated to be 4 9 mpa 7 4 10 5 and 7 0 1014 j respectively the slip distributes in a region slightly deep from nww to see with two nuclei i e knots with highly concentrated slip located in a shallower depth in the nww and a deeper depth in the see respectively

postseismic deformation mechanisms of the 1990 mw 6 4 gonghe china earthquake constrained using leveling measurements abstract we model leveling deformation data observed following the 1990 mw 6 4 gonghe earthquake in qinghai province western china to infer postseismic deformation sources and mechanisms using coseismic vertical displacements and previous study result obtained from seismic body wave inversion we update the main shock rupture model as a reverse dip slip of 0 81 m analyzing the leveling data observed one time before and six times after the quake across the seismic fault we find that 1 the near field deformation at the hanging wall side of the coseismic rupture continued to uplift over a large region after the earthquake with the highest rate measured in the first year 2 the scope of the postseismic doming region coincides with the coseismic doming region 3 time series of elevation change between adjacent benchmarks can be modeled by a logarithmic or an exponential relaxation function with the relaxation time constants estimated as 0 165 years and 1 344 years for the two models respectively to model the postseismic displacements we utilize the raw observations of elevation differences between adjacent benchmarks not their integrals with respect to a reference benchmark to constrain a dislocation model in a continuum it makes full use of the leveling data and effectively reduces biases introduced from cumulative errors due to data integration our postseismic modeling result suggests that two mechanisms operate simultaneously to produce the postseismic vertical deformation observed at the surface afterslip on the coseismic rupture fault plane of the main shock and its peripheral extension particularly upward into the sediment layer above the main rupture and viscoelastic relaxation of the lower crust and upper mantle with a viscosity of 9 10 19 pa s the result suggests more brittle and less viscous lower crust and upper mantle underneath the qaidam basin than some of previous studies envisioned

a time domain inversion technique for the tempo spatial distribution of slip on a finite fault plane with applications to recent large earthquakes in the tibetan plateau a time domain inversion technique is proposed to invert for the tempo spatial distribution of slip on an earthquake fault plane this technique is based on the idea that a finite fault plane can be divided into several subfaults each of which can be treated as a point source and that the source time function stf of the finite fault is the weighted sum of the stfs of all the subfaults this technique is applied to deduce the source processes of three recent large earthquakes in the qinghai xizang tibetan plateau the 1990 april 26 gonghe qinghai earthquake ms6 9 ruptured on a fault plane with a strike of 113 a dip of 68 and a rake of 89 the fault has a length of 45 km a width of 15 km and a total moment release of 9 4 1018 n m the rupture initiated at the wnw end of the fault and propagated unilaterally towards the ese end of the fault in total the rupturing lasted for 45 s and was divided into two episodes the first 9 s long and the second 16 s long with 20 s quiescence in between there are two distinct rupture regions nuclei on the fault plane these two nuclei with maximum slips of about 62 and 55 cm are about 25 km apart in the wnw ese direction the 1996 february 3 lijiang yunnan earthquake ms7 0 has a focal mechanism with a strike of 157 a dip of 48 and a rake of 102 the earthquake fault is about 45 km with a width of 20 km and a total moment release of 9 8 1018 n m the total duration of the rupture was 16 s is divided into two episodes the first 8 5 s long and the second 7 5 s long the inverted final slip distribution of the lijiang earthquake also shows two nuclei one nucleus with a maximum slip of 40 cm is located on the nnw portion of the fault at a shallower depth and the other with a maximum slip of 50 cm is located on the sse portion of the fault at a greater depth the 1997 november 8 mani xizang tibet earthquake ms7 9 has a focal mechanism with a strike of 250 a dip of 88 and a rake of 19 and a moment release of 3 4 1020 n m the total duration of the rupture was about 15 s the inverted final slip distribution of the mani earthquake shows three nuclei on the fault plane the first nucleus with a maximum slip of 956 cm is located at the wsw end of the fault at 10 km in depth the second with a maximum slip of 743 cm is located at the ene end of the fault 55 km away from the wsw end of the fault and at 35 km depth the third with a maximum slip of 1060 cm is about 30 km away from the wsw end of the fault and at about 40 km depth these three nuclei form a total rupture area of about 70 km in length and 60 km in depth

rapid mantle flow beneath the tonga volcanic arc abstract flow in the earth s asthenospheric mantle is typically assumed to be driven by motions of the overlying and adjacent plates suggesting flow rates comparable to plate velocities however arc parallel shear wave splitting and geochemical observations imply along strike flow in the lau basin presenting a challenge to geodynamic models of plate driven subduction systems as plate driven flow models predict arc perpendicular fast direction with simple a type mineral texturing although b type mineral texturing would result in the fast directions developing 90 relative to a type and likely occurs in the forearc mantle such texturing is unlikely in the asthenosphere wedge where higher temperatures and weak rheology likely prevail by tracking mineral texturing development within geodynamic flow models patterned after the tongan system in the lau basin that include along strike flow we find that the observations are best explained by models including rapid along arc flow within the subarc mantle the effect of melt on the anisotropy does not appear to be a primary control but needs to be investigated more fully the best fit to the observations is a model with along strike flow within a low viscosity channel beneath the arc at a rate of nearly 50 cm yr several times the plate motion velocities

reliability of lunar orbital periods extracted from ancient cyclic tidal rhythmites abstract ancient tidal periods extracted from the geologic record are useful to constrain and calibrate theoretical models of the evolution of the earth moon system analysis of rocks that contain such tidal periodicities termed cyclic rhythmites have tended to concentrate on long series and have focused on the more obvious periods for example neap spring periods have been the most widely discussed such information can be reliably extracted from bedform thickness series measured within laterally accreted tidal bundles and vertically accreted tidal laminae other lunar orbital data is also encoded in cyclic rhythmites however this additional information has been much less extensively applied to earth moon models herein short segments of well formed cyclic rhythmites are examined to determine how reliably the various tidal periods can be extracted via harmonic analysis rhythmite series analyzed here exhibit well formed neap spring tidal periods and also exhibit a prominent fortnightly inequality or a variation in the height of successive spring tides this inequality is related to the elliptical nature of the lunar orbit anomalistic month and it is demonstrated that this period can also be reliably extracted from cyclic rhythmites tropical periods which reflect the changing lunar declination that occurs during a tropical month have also been reported from cyclic rhythmites unlike synodic and anomalistic periods the apparent tropical periods extracted from some cyclic rhythmites do not always accurately correspond to the expected values in only a few cases can this period be reliably extracted from short segments of cyclic tidal rhythmite data this period is commonly only poorly encoded in rhythmites because 1 it tends to be masked by higher amplitude synodic periods and 2 it is not well expressed within predominantly semidiurnal tidal systems samples selected from the proterozoic and phanerozoic tend to be more common during specific intervals of geologic time these intervals were characterized by glacio eustatic flux and in some cases by unique continental configurations understanding these basic controls on rhythmite occurrences should aid in the discovery of additional examples

empirical green function deconvolution of broadband surface waves rupture directivity of the 1992 landers california mw 7 3 earthquake we present a simple method to determine gross temporal and spatial characteristics of faulting that can uniquely define the fault plane for a relatively large earthquake the technique involves deconvolution of teleseismic and regional broadband surface waves and or body waves of a small event an empirical green function from the corresponding signals at the same stations for a large mainshock this deconvolution corrects each mainshock seismic record for propagation and instrument response and if the focal mechanisms and centroid depths are identical deconvolution provides relative source time functions for the mainshock azimuthal variations of source time function width or subevent interference reveal the mainshock rupture directivity changes in the source mechanism during rupture can be evaluated for some classes of events our empirical green function procedure is novel in its application to surface waves and its use for large m 7 earthquakes we perform synthetic tests to establish intrinsic limitations of the method the procedure requires very little data processing and can be applied in near real time with the current distribution of seismic stations that have dial up data retrieval capability it also provides a straightforward procedure for reducing bias in source duration estimates resulting from aspherical heterogeneity and for detecting anomalous long period radiation we apply the procedure to the 28 june 1992 landers california mw 7 3 earthquake the landers earthquake source duration lasted about 25 sec and was dominated by two subevents with predominantly north northwestward rupture extending 60 to 70 km the subevents show counter clockwise rotations of at least 12 in strike that correlate well with mapped surface rupture

shear wave splitting observations at the regions of northern baja california and southern basin and range in mexico 1 recent installation of 3 component broadband digital stations around the gulf of california allowed us to make shear wave splitting observation from records of 73 sks and skks phases from 30 events recorded at 12 stations since january of 2001 stations in the southern basin and range province yield upper mantle fast shear wave direction from ne sw to e w consistent with both local direction of miocene extension and north american absolute plate motion the shear at the pacific north american plate limit seems to control tightly the anisotropy pattern of the closest station ne70 the anisotropic pattern along the peninsular range east of the former trench between the north american plate and the now subducted farallon plate is almost uniform and is interpreted as asthenospheric flow induced by the sinking fragment of the farallon plate although frozen anisotropy associated with in situ captured fragment of this plate cannot be ruled out

quantifying the oldest tidal record the 3 2 ga moodies group barberton greenstone belt south africa comment and reply eriksson and simpson 2000 1 claimed that the moodies tidal data demonstrate the potential for constraining the lunar orbital periods and orbital shape during the archean their claim however will remain undersubstantiated until the following issues are clarified first figure 5a of