aridification of central asia and uplift of the altai and hangay mountains mongolia stable isotope evidence central asia has become increasingly arid during the cenozoic though the mechanisms behind this aridification remain unresolved much attention has focused on the influence and uplift history of the tibetan plateau however the role of ranges linked to india asia convergence but well north of the plateau including the altai sayan and hangay in creating the arid climate of central asia is poorly understood today these ranges create a prominent rain shadow effectively separating the boreal forest to the north from the deserts of central asia to explore the role of these mountains in modifying climate since the late eocene we measured carbon and oxygen stable isotopes in paleosol carbonates from three basins along a 650 km long transect at the northern edge of the gobi desert in mongolia and in the lee of the altai and hangay mountains we combine these data with modern air parcel back trajectory modeling to understand regional moisture transport pathways at each basin in all basins 13c increases with the largest increase in western mongolia the first 13c increase occurs in central and southwestern mongolia in the oligocene 13c again increases from the upper miocene to the quaternary in western and southwestern mongolia we use a 1 d soil diffusion model to demonstrate that these 13c increases are linked to declines in soil respiration driven by dramatic increases in aridity using modern day empirical relations between mean annual precipitation and soil respiration we estimate that precipitation has likely more than halved over the neogene given the importance of the hangay and altai in steering moisture in mongolia we attribute these changes to differential surface uplift of the hangay and altai surface uplift in the hangay began by the early oligocene blocking siberian moisture and aridifying the northern gobi in contrast surface uplift of the altai began in the late miocene blocking moisture from reaching western mongolia thus the northern gobi became increasingly arid east to west since the late eocene likely driven by orographic development in the hangay during the oligocene and the altai in the late miocene through pliocene

a two asperity fault model with wave radiation abstract we study the dynamics of a fault containing two asperities having different strengths and subject to a constant strain rate the fault is modeled by a discrete dynamical system with two degrees of freedom that are the slip deficits of the asperities the radiation of elastic waves during the slipping modes is taken into account by introducing a term proportional to slip rate in the equations of motion we give a complete analytical solution of the four dynamic modes of the system any seismic event can be expressed as a sequence of modes for which the moment rate the spectrum and the total seismic moment can be calculated we consider the energy budget of the event and calculate the seismic efficiency the presence of radiation may remarkably change the evolution of the system from a given state since it moves the boundaries between the different subsets of the sticking region in addition the slip amplitude in a seismic event is smaller while the slip duration is longer in the presence of radiation the shape of the moment rate function depends on the seismic efficiency and the seismic moment decreases with increasing efficiency at constant radiated energy as an application of the model we consider the 1964 great alaska earthquake and calculate the mode durations the slip distribution the moment rate function and the seismic moment of the earthquake obtaining values that are consistent with observations

complex events in a fault model with interacting asperities abstract the dynamics of a fault with heterogeneous friction is studied by employing a discrete fault model with two asperities of different strengths the average values of stress friction and slip on each asperity are considered and the state of the fault is described by the slip deficits of the asperities as functions of time the fault has three different slipping modes corresponding to the asperities slipping one at a time or simultaneously any seismic event produced by the fault is a sequence of n slipping modes according to initial conditions seismic events can be different sequences of slipping modes implying different moment rates and seismic moments each event can be represented geometrically in the state space by an orbit that is the union of n damped lissajous curves we focus our interest on events that are sequences of two or more slipping modes they show a complex stress interchange between the asperities and a complex temporal pattern of slip rate the initial stress distribution producing these events is not uniform on the fault we calculate the stress drop the moment rate and the frequency spectrum of the events showing how these quantities depend on initial conditions these events have the greatest seismic moments that can be produced by fault slip as an example we model the moment rate of the 1992 landers california earthquake that can be described as the consecutive failure of two asperities one of which has a double strength than the other and evaluate the evolution of stress distribution on the fault during the event

2d thermomechanical modelling of continent arc continent collision abstract arc continent collision is a key process of continental growth through accretion of newly grown magmatic arc crust to older continental margin we present 2d petrological thermo mechanical models of arc continent collision and investigate geodynamic regimes of this process the model includes spontaneous slab bending dehydration of subducted crust aqueous fluid transport partial melting of the crustal and mantle rocks and magmatic crustal growth stemming from melt extraction processes results point to two end member types of subsequent arc continent collisional orogens i orogens with remnants of accretion prism detached fragments of the overriding plate and magmatic rocks formed from molten subducted sediments and ii orogens mainly consisting of the closed back arc basin suture detached fragments of the overriding plate with leftovers of the accretion prism and quasi insignificant amount of sediment derived magmatic rocks transitional orogens between these two endmembers include both the suture of the collapsed back arc basin and variable amounts of magmatic production the orogenic variability mainly reflects the age of the subducting oceanic plate older therefore colder and denser oceanic plates trigger subduction retreat which in turn triggers necking of the overriding plate and opening of a backarc basin in which new oceanic lithosphere is formed from voluminous decompression melting of the rising hot asthenosphere in this case subducted sediments are not heated enough to melt and generate magmatic plumes on the other hand young and less dense slabs do not retreat which hampers opening of a backarc basin in the overriding plate while subducted sediments may reach their melting temperature and develop trans lithospheric plumes we have also investigated the influences of convergence rate and volcanic plutonic rocks ratio in newly forming lithosphere the predicted gross scale orogenic structures find similarities with some natural orogens in particular with deeply eroded orogens such as the variscides in the bohemian massif

interseismic strain accumulation measured by gps in the seismic gap between constitucion and concepcion in chile the concepcion constitucion area 35 37 s in south central chile is very likely a mature seismic gap since no large subduction earthquake has occurred there since 1835 three campaigns of global positioning system gps measurements were carried out in this area in 1996 1999 and 2002 we observed a network of about 40 sites including two east west transects ranging from the coastal area to the argentina border and one north south profile along the coast our measurements are consistent with the nazca south america relative angular velocity 55 9 n 95 2 w 0 610 ma discussed by vigny et al 2008 this issue which predicts a convergence of 68 mm year oriented 79 n at the chilean trench near

3d s wave velocity pattern in the upper mantle beneath the continent of asia from rayleigh wave data abstract group velocities of rayleigh waves in the 10 150 s period range along about 1200 paths across central asia and siberia are used to obtain group velocity maps of the region to image the lateral variations of group velocities we used the tomography method developed earlier for spherical surface pepi 122 2000 19 the method is supplemented by the opportunity to estimate the resolving power of the data the locally averaged dispersion curves have been then inverted to vertical s wave velocity profiles up to depth of 500 km at different sites in the region which thus image a 3d velocity distribution in order to reduce the number of unknown parameters we assumed the relationships between s wave and p wave velocities and densities to be those of the prem model the 3d s wave velocity pattern is displayed in the form of maps at depths from 50 to 350 km with a step of 50 km and of 2d vertical velocity sections along some profiles crossing different tectonic units the most remarkable feature of the 3d velocity distribution is the presence of a high velocity rise at the depths of 250 350 km beneath southern siberia and western china which borders upon the low velocity asthenosphere along the longitude about 105 e collision of the high and low velocity substances results in pressing out the viscous asthenosphere material in the weakened zone of the baikal rift where the lithosphere is thinned the rise of the asthenosphere in this zone probably produces mantle upwelling and leads to uplift of the khangay plateau

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

magma compensated crustal thinning in continental rift zones continental rifts are elongated depressions in the earth s surface often more than 1 000 km long 20 km wide and 10 km deep filled with sediments and volcanic rocks conventional models of rift zones include three characteristic features surface manifestation as an elongated topographic trough moho shallowing due to crustal thinning and reduced seismic velocity in the uppermost mantle hans thybo and christoffer nielsen present a new seismic profile that demonstrates that the baikal rift zone in siberia displays only one of these characteristic features the elongated surface trough there is a localized zone in the lower crust with exceptionally high seismic velocity and high reflectivity the expected moho uplift appears to have been compensated by magmatic intrusion into the lower crust producing the observed high velocity zone this finding points to a previously unknown role for magmatism in rifting this study presents a seismic profile across southern lake baikal in siberia demonstrating that only a surface manifestation is observed there these findings may point toward a new role for magmatism in the rifting process continental rift zones are long narrow tectonic depressions in the earth s surface where the entire lithosphere has been modified in extension1 rifting can eventually lead to rupture of the continental lithosphere and creation of new oceanic lithosphere or alternatively lead to formation of wide sedimentary basins around failed rift zones conventional models of rift zones include three characteristic features surface manifestation as an elongated topographic trough moho shallowing due to crustal thinning and reduced seismic velocity in the uppermost mantle due to decompression melting or heating from the earth s interior2 3 4 here we demonstrate that only the surface manifestation is observed at the baikal rift zone whereas the crustal and mantle characteristics can be ruled out by a new seismic profile across southern lake baikal in siberia instead we observe a localized zone in the lower crust which has exceptionally high seismic velocity and is highly reflective we suggest that the expected moho uplift was compensated by magmatic intrusion into the lower crust producing the observed high velocity zone this finding demonstrates a previously unknown role for magmatism in rifting processes with significant implications for estimation of stretching factors and modelling of sedimentary basins around failed rift structures

upper mantle structure beneath the siberian craton and surrounding areas based on regional tomographic inversion of p and pp travel times abstract we present a model of 3d seismic anomalies in the upper mantle beneath a part of northern asia that includes the siberian craton the west siberian plate the mountains framing siberia altai sayan and baikal and mongolia we used the differential travel times of pp and p phases to investigate the deep seismic structure beneath a large part of the siberian region where neither stations nor earthquake distributions are sufficient to apply other body wave tomographic schemes to show the reliability of the results based on inversion if the pp p residuals we performed various tests including 1 tests with odd even data 2 synthetic tests with checkerboard and realistic patterns and 3 inversions in other areas with clear structures furthermore for the southern part of the study area we present another model based on the inverse tomographic scheme its which uses travel times recorded by worldwide stations from events located in the study area in the overlapping areas these two independent models are generally consistent the main model of this study is obtained by merging the results of the pp p and its schemes this model shows a clear differentiation between the high velocity siberian craton and low velocity structures beneath the mountains framing siberia the area of cenozoic magmatism in western sayan is clearly associated with a low velocity anomaly at a depth of 600 700 km this low velocity pattern is located beneath the cratonic area while at shallower depths it seems to flow around the bottom of the thick craton lithosphere to the sw margin of the craton

asthenospheric imprints on the lithosphere in central mongolia and southern siberia from a joint inversion of gravity and seismology mobal experiment summary we present a joint inversion of gravity and teleseismic data to enlighten the lithospheric structures of the baikal mongolia region an area characterized by high topographic contrasts sporadic cenozoic volcanism extension and large transcurrent faulting in the vicinity of the baikal rift central asia the study uses a 1000 km long seismic transect that cross cuts the main tectonic structures from north to south namely the siberian platform tunka basin hangay dome and gobi altai belt the siberian platform depicts a high velocity lithosphere down to about 150 km we evidence strong velocity contrasts within the crust below the hangay dome and the tunka depression interpreted as a thickened crust a low velocity density region is located at various depths below the hangay dome thanks to the dense spatial coverage of gravity data we are able to define the 3 d geometry of this particular low velocity density anomaly the hangay dome anomalous body extends from 60 to 225 km depth at his largest point and slightly thins to no more than 40 km at its easternmost end a deep low velocity zone below 150 km with only a weak negative density contrast is observed on the eastern part of the transect we propose that the late cenozoic uplift of the whole mongolian plateau and associated rifting magmatism high heat flow and lithospheric thinning are not externally driven by the india asia collision but results from the interaction of two mantle plumes with the overlying lithosphere conversely recent faulting may be the major expression of the india asia collision in this region as well as n s striking rift basins that connect and interact with major strike slip faults such as the bolnai fault the comparison between time residuals of this experiment and the baikal rift seismic data shows an undeniable disproportion for the hangay anomaly the hangay time residuals are far more positive than through the baikal arguing for an asthenospheric signature that is not seen beneath the baikal rift furthermore the southern edge of the thick siberian cratonic lithosphere may favour the rise of a sublithospheric mantle flow at the contact with the baikal mongolia lithosphere

the dynamic reference frame of rivers and apparent transience in incision rates incision rates derived from river terraces are commonly used to infer rock uplift rates however an apparent dependence of incision rate on measured time interval may confound directly relating incision to uplift the time dependent incision rates are a sadler effect that have been argued to result from a stochastic distribution of hiatal intervals in river incision potentially reducing the utility of incision records for interpreting unsteadiness in tectonic processes here we show that time dependent incision rates can arise from a simple systematic bias in the distance measurement used to calculate incision rate and thus stochastic causes are not required we present a conceptual model that describes the dynamic history of streambed elevation over cycles of terrace formation illustrating that measured incision rate is time dependent because the stream channel reference frame is not fixed with respect to the geoid because it is challenging to reconstruct the full elevation history for a river channel most researchers use the modern streambed elevation as a reference datum but we demonstrate that doing so imposes a bias that manifests as an apparent dependence of rate on measured time interval fortunately correction of this bias is straightforward and allows river incision data to be used in studies of tectonic or climatic unsteadiness

shear driven upwelling induced by lateral viscosity variations and asthenospheric shear a mechanism for intraplate volcanism abstract volcanism occurring away from ridges and subduction zones does not have an obvious plate tectonic explanation but instead must arise from sub lithospheric processes that generate upwelling flow and decompression melting several convective processes such as mantle plumes convective instability edge driven convection and richter rolls produce upwelling via the action of gravity on density heterogeneities in the mantle here we investigate an alternative mechanism the shear driven upwelling sdu which instead generates upwelling solely through the action of asthenospheric shear flow on viscosity heterogeneity using a numerical flow model we examine the effect of viscosity heterogeneity on viscous shear flow induced within an asthenospheric layer we demonstrate that for certain geometries and viscosity ratios circulatory flow develops within a cavity or step embedded into the lithospheric base or within a low viscosity pocket embedded within the asthenospheric layer for asthenosphere shearing at 5 cm yr we estimate that sdu can produce upwelling rates of up to 0 2 cm yr within a continental rift 0 5 cm yr along the vertical edge of a craton or 1 0 cm yr within a pocket of low viscosity asthenosphere in the last case the pocket must feature an aspect ratio of more than 5 occupy 20 60 of the asthenosphere s thickness and be at least 100 times less viscous than the surrounding asthenosphere such viscosity heterogeneity may be associated with thermal chemical melting volatile or grain size anomalies and is consistent with tomographic constraints on asthenospheric variability we estimate that sdu may generate up to 2 5 km myr of melt that is potentially eruptible as surface volcanism this is faster than eruption rates observed at some locations of continental basaltic volcanism we conclude that sdu could provide an explanation for intraplate volcanism occurring above rapidly shearing asthenosphere for example in the basin and range region of north america

teleseismic traveltimes topography and the lithospheric structure across central mongolia 1 teleseismic travel times recorded along a 1000 km long n s transect across central mongolia are used together with topography and gravity data to constrain the deep lithospheric structure of this region time residuals appear positively correlated with the topography suggesting that p wave velocity changes correspond to density variations which in turn cause an isostatic uplift of the topography using a simple local isostasy model and a monte carlo inversion for the crust and asthenosphere velocities and density velocity conversion factors we determine best fitting moho and lithosphere asthenosphere boundary lab geometries which satisfyingly reproduce the observed topography and gravity data the model is validated using previously published 1d s wave velocity models obtained from receiver function analyses our results indicate that most of the long wavelength topography is supported by the asthenosphere buoyancy the hangai dome in the southern half of the transect appears mainly supported by a rather shallow 60 70 km asthenospheric uplift whereas the siberian platform at the extreme north of the profile is underlain by a very thick 200 km lithosphere misfits to the observed bouguer gravity anomaly occur near major faults probably because of lithospheric flexure

vertical mantle flow associated with a lithospheric drip beneath the great basin rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional scale downwelling of lithospheric mantle however because lithospheric drips sinking plumes of cold and dense lithosphere are relatively small and transient features direct evidence of their existence has been difficult to obtain moreover the significant vertical mantle flow that should be associated with such structures has not been detected here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the great basin region of the western united states the feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1 and a localized temperature increase of 10 the drip tilts to the northeast opposite to the motion of the north american plate in the hotspot reference frame and thereby indicates northeast directed regional mantle flow seismic anisotropy data for the great basin region of the western united states coupled with tomographic images help delineate a northeast dipping lithospheric drip numerical experiments suggest that the drip could have formed owing to gravitational instability triggered by a density increase of about 1 and a temperature increase of about 10

long term dynamics of a fault with two asperities of different strengths summary a fault made of two segments or asperities having different strengths and subject to a constant strain rate is considered the fault is modelled by a discrete dynamical system made of two blockscoupledbyaspringandpulledatconstantvelocityonaroughplane wegiveacomplete analytical solution for the evolution of the system the long term behaviour of the fault is studied by calculating the orbits of the system in the phase space the dynamics of the system has four different modes and produces a variety of behaviours the asperities may slip one at a time originating a medium size earthquake or simultaneously originating a large earthquake in the latter case the focus may be at one asperity or at the other in some cases the weaker asperity may slip twice both before and after the failure of the stronger asperity the time pattern of the seismic events generated by the fault is controlled by the stress distribution on the asperities and may include many different behaviours we devise suitable correspondence rules allowing application of the discrete model to real fault systems as an example the model is applied to the 1964 great alaska earthquake which was originated by the failure of two asperities and a possible evolution of this fault system is discussed

mantle structure and rifting processes in the baikal mongolia region geophysical data and evidence from xenoliths in volcanic rocks abstract the origin of the baikal rift zone brz has been debated between the advocates of passive and active rifting since the 1970s a re assessment of the relevant geological and geophysical data from russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many active rifting models results of a large number of early and recent studies favour the role of far field forces in the opening and development of the brz this study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern siberia and central mongolia these xenoliths are direct samples of the upper mantle in the vicinity of the brz of particular importance are suites of garnet bearing xenoliths that have been used to construct p t composition lithospheric cross sections in the region for the depth range of 35 80 km xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient siberian platform sampled by kimberlites and beneath younger mobile belts south of the platform the uppermost mantle in southern siberia and central mongolia is much hotter at similar levels than the mantle in the siberian craton and also has significantly higher contents of basaltic major elements ca al na and iron higher fe si and fe mg the combination of the moderately high geothermal gradient and the fertile compositions in the off cratonic mantle appears to be a determining factor controlling differences in sub moho seismic velocities relative to the siberian craton chemical and isotopic compositions of the off cratonic xenoliths indicate small scale and regional mantle heterogeneities attributed to various partial melting and enrichment events consistent with long term evolution in the lithospheric mantle age estimates of mantle events based on os sr nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long term crust mantle coupling the ratios of 143 144nd in many lree depleted xenoliths are higher than those in morb or oib source regions and are not consistent with a recent origin from asthenospheric mantle mantle xenoliths nearest to the rift basins 30 50 km south of southern lake baikal show no unequivocal evidence for strong heating unusual stress and deformation solid state flow magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the moho comparisons between xenoliths from older and younger volcanic rocks east of lake baikal together with observations on phase transformations and mineral zoning in individual xenoliths have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small scale ascent of deep mantle material overall the petrographic p t chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies which found no evidence for a large scale asthenospheric upwarp beneath the rift and lend support to passive rifting mechanism for the brz

coseismic and postseismic slip of the 2004 parkfield earthquake from space geodetic data we invert interferometric synthetic aperture radar insar data jointly with campaign and continuous global positioning system gps data for slip in the coseismic and postseismic periods of the 2004 parkfield earthquake the insar dataset consists of eight interferograms from data collected by the envisat and radarsat satellites spanning the time of the earthquake and variable amounts of the postseismic period the two datasets complement each other with the insar providing dense sampling of motion in the range direction of the satellite and the gps providing more sparse but three dimensional measurements of ground motion the model assumes exponential decay of the postseismic slip with a decay time constant of 0 087 years determined from time series modeling of continuous gps and creepmeter data we find a geodetic moment magnitude of m 6 2 for a 1 day coseismic model and m w 6 1 for the entire postseismic period the coseismic rupture occurred mainly in two slip asperities one near the hypocenter and the other 15 20 km north postseismic slip occurred on the shallow portions of the fault and near the rupture areas of two m 5 0 aftershocks a comparison of the geodetic slip models with seismic moment estimates suggests that the coseismic moment release of the parkfield earthquake is as little as 25 of the total this underlines the importance of aseismic slip in the slip budget for the parkfield segment online material complete data tables and supplemental tables

palaeomagnetism and k ar dating of cretaceous basalts from mongolia we report a combined geochronology and palaeomagnetic study of cretaceous igneous rocks from shovon 44 4 n 103 8 e and arts bogd 44 3 n 102 2 e localities in the gobi desert south mongolia k ar dating based on seven rock samples with two independent measurements for each sample allows us to propose an age of 94 7 1 3 ma for shovon locality and a 98 2 1 4 to 118 3 1 7 ma age range for arts bogd stepwise thermal and af demagnetization generally isolated a high temperature component htc of magnetization for both shovon and arts bogds basalts eventually following a low temperature component ltc in some samples the htc directions display normal polarity consistent with the cretaceous long normal superchron rock magnetic analysis identifies fine grained pseudo single domain psd magnetite and titanomagnetite as primary carriers of the remanence mean htc palaeomagnetic direction is dm 8 2 im 63 7 n 18 flows k 41 1 95 5 5 for shovon and dm 12 1 im 66 4 n 27 flows k 53 0 95 3 9 for arts bogd because of their similar ages we combine data from shovon and data previously obtained from khurmen uul 92 0 4 0 ma recomputed in geographic coordinates and not in tilt corrected ones as in our previous interpretation at the shovon locality the combined final average palaeomagnetic direction for shovon khurmen uul is dm 7 4 im 62 7 n 23 flows k 41 4 95 4 8 the corresponding palaeopoles computed from these htc lie at 84 7 n 195 0 e dp dm 5 8 7 5 for shovon khurmen uul average age 93 4 2 6 ma and 80 5 n 159 0 e dp dm 5 2 6 3 for arts bogd average age 104 6 6 6 ma these poles are consistent with those from the european apparent polar wander path apwp at 90 100 and 110 ma and other published pole from the mongol okhotsk suture zone amuria and north china blocks this confirms the lack of a discernable latitudinal motion between amuria and siberia since their final accretion by the late jurassic early cretaceous and reinforces the idea that europe apwp can be used as a reference for siberia by the mid cretaceous we finally propose a mid cretaceous mean palaeomagnetic pole for the siberia amuria north china block assemblage which lies at 86 4 n 191 1 e n 10 k 74 9 a95 5 8

post seismic viscoelastic deformation and stress transfer after the 1960 m9 5 valdivia chile earthquake effects on the 2010 m8 8 maule chile earthquake author posting the authors 2014 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 197 2014 697 704 doi 10 1093 gji ggu048

robust features of the source process for the 2004 parkfield california earthquake from strong motion seismograms summary we explore a recently developed procedure for kinematic inversion based on an elliptical subfault approximation in this method the slip is modelled by a small set of elliptical patches each ellipse having a gaussian distribution of slip we invert near field strong ground motion forthe2004september28mw 6 0parkfield california earthquake thedatasetconsistsof10 digital three component 18 s long displacement seismograms the best model gives a moment of 1 21 10 18 n m with slip on two distinct ellipses one with a high slip amplitude of 0 91 m located 20 km northwest of the hypocentre the average rupture speed of the rupture process is 2 7 kms 1 we find no slip in the top 5 km at this depth a lineation of small aftershocks marks the transition from creeping above to locked below in the interseismic period the highslip patch coincides spatially with the hypocentre of the 1966 mw6 0 parkfield california earthquake the larger earthquakes prior to the 2004 parkfield earthquake and the aftershocks of the 2004 earthquake mw 3 also lie around this high slip patch where our model images a sharp slip gradient this observation suggests the presence of a permanent asperity that breaks during large earthquakes and has important implications for the slip deficit observed on the parkfield segment which is necessary for reliable seismic hazard assessment

the baikal rift zone the effect of mantle plumes on older structure abstract the main chain of sw ne striking cenozoic half grabens of the baikal rift zone brz follows the frontal parts of early paleozoic thrusts which have northwestern and northern vergency most of the large rift half grabens are bounded by normal faults at the northwestern and northern sides we suggest that the rift basins were formed as a result of transformation of ancient thrusts into normal listric faults during cenozoic extension seismic velocities in the uppermost mantle beneath the whole rift zone are less than those in the mantle beneath the platform this suggests thinning of the lithosphere under the rift zone by asthenosphere upwarp the geometry of this upwarp and the southeastward spread of its material control the crustal extension in the rift zone this nw se extension cannot be blocked by sw ne compression generated by pressure from the indian lithospheric block against central asia the geochemical and isotopic data from late cenozoic volcanics suggest that the hot material in the asthenospheric upwarp is probably provided by mantle plumes to distinguish and locate these plumes we use regional isostatic gravity anomalies calculated under the assumption that topography is only partially compensated by moho depth variations variations of the lithosphere asthenosphere discontinuity depth play a significant role in isostatic compensation we construct three dimensional gravity models of the plume tails the results of this analysis of the gravity field are in agreement with the seismic data the group velocities of long period rayleigh waves are reduced in the areas where most of the recognized plumes are located and azimuthal seismic anisotropy shows that these plumes influence the flow directions in the mantle above their tails the baikal rift formation like the kenya rio grande and rhine continental rifts achauer u granet m 1997 complexity of continental rifts as revealed by seismic tomography and gravity modeling in jacob a w b delvaux d khan m a eds lithosphere structure evolution and sedimentation in continental rifts proceedings of the igcp 400 meeting dublin march 20 22 1997 institute of advanced studies dublin pp 161 171 is controlled by the three following factors i mantle plumes ii older prerift linear lithosphere structures favorably positioned relative to the plumes and iii favorable orientation of the far field forces

teleseismic inversion for rupture process of the 27 february 2010 chile mw 8 8 earthquake the 27 february 2010 chile m w 8 8 earthquake is the fifth largest earthquake to strike during the age of seismological instrumentation the faulting geometry slip distribution seismic moment and moment rate function are estimated from broadband teleseismic p sh and rayleigh wave signals we explore some of the trade offs in the rupture process estimation due to model parameterizations limited teleseismic sampling of seismic phase velocities and uncertainty in fault geometry the average slip over the 81 500 km 2 rupture area is about 5 m with slip concentrations down dip up dip and southwest and up dip and north of the hypocenter relatively little slip occurred up dip offshore of the hypocenter the average rupture velocity is 2 0 2 5 km s

effect of stress perturbations on the dynamics of a complex fault a plane fault containing two asperities subject to a constant strain rate by the motion of tectonic plates is considered the fault is modelled as a discrete dynamical system where the average values of stress friction and slip on each asperity are considered the state of the fault is described by the slip deficits of the asperities we study the behaviour of the system in the presence of stress perturbations that are supposed to be due to dislocations of neighbouring faults the fault complexity entails consequences that are not present in the case of a homogeneous fault a stress perturbation not only changes the occurrence time of the following earthquake but may also sensitively change the slip amplitude and area hence the seismic moment of the earthquake as well as the position of its hypocentre the greatest changes take place when simultaneous slip of asperities is involved a coulomb stress value can be assigned to each asperity the change in the difference between the coulomb stresses of the two asperities is a measure of how much the system gets closer to or farther from the condition for simultaneous slip as an example we consider the effect of the 1960 great chilean earthquake on the two asperity fault that produced the 2010 maule earthquake and calculate the changes in the moment rate and in the total seismic moment it results that in the absence of the 1960 earthquake the maule earthquake would have occurred several decades later and would have involved a different sequence of modes so that the moment rate function would have been very different with a longer duration and a greater seismic moment

earthquake triggering by static and dynamic stress changes 1 in this study we aim to understand the effect of static and dynamic stress changes in promoting earthquake failures on secondary faults toward this goal we solve the equation of motion of a spring slider dynamic system including inertia and using rate and state dependent constitutive laws we separately investigate the dynamic response of this fault analog system to a sudden stress change represented either as a stress step or as a stress pulse which are used to model permanent static and transient dynamic stress perturbations the induced earthquake failure does not occur immediately at the application of the coseismic stress change but it is delayed in time we define this time interval as the triggering delay for a given stress perturbation we analyze the dependence of triggering delays on different system conditions and constitutive parameters our results clearly show that the effects of static and dynamic stress changes are quite different while a static stress change is able to advance as well as to delay an induced instability depending on its sign a dynamic stress pulse is only able to promote a nearly instantaneous failure provided its amplitude is positive and large enough with respect to the direct effect of friction in other words dynamic stress changes can only cause nearly instantaneous failures without any relevant triggering delay these results should be considered in interpreting the seismicity rate changes caused by large earthquakes at least as long as seismic events are interpreted as sliding instabilities obeying rate and state dependent friction laws

intraplate volcanism due to convective instability of stagnant slabs in the mantle transition zone the study of volcanism can further our understanding of earth s mantle processes and composition continental intraplate volcanism commonly occurs above subducted slabs that stagnate in the mantle transition zone mtz such as in europe eastern china and western north america here we use two dimensional numerical models to explore the evolution of stagnant slabs in the mtz and their potential to sustain mantle upwellings that can support volcanism we find that weak slabs may go convectively unstable within tens of million years upwellings rise out of the relatively warm underbelly of the slab are entrained by ambient mantle flow and reach the base of the lithosphere the first and most vigorous upwellings rise adjacent to lateral heterogeneity within the slab ultimately convective instability also acts to separate the compositional components of the slab harzburgite and eclogite from each other with harzburgite rising into the upper mantle and eclogite sinking into the lower mantle such a physical filtering process may sustain a long term compositional gradient across the mtz

mongolian plateau evidence for a late cenozoic mantle plume under central asia the 2500 x 700 km mongolian plateau average elevation 2000 m is situated between the altai orogen and the siberian craton and occupies much of mongolia and transbaikalia in russia the plateau is characterized by 1 basin and range topography and two major domes hentai 600 x 300 km and hangai 800 x 550 km where altitudes reach 3905 m 2 lithosphere that is thinner than adjacent areas minimum 50 km 3 elevated heat flow up to 120 mw m2 4 dominantly alkaline basaltic volcanism in the form of cones lava fields and volcanic plateaus mostly of miocene quaternary age and 5 rifts including baikal main evolution in the pliocene quaternary tunka oligocene early miocene and hobsogol pliocene quaternary existing models explain these features in terms of diapiric upwelling of a mantle asthenolith below the main rifts and or as a long distance effect of the india asia collision we propose that the late cenozoic uplift of the whole mongolian plateau and associated rifting magmatism high heat flow and lithospherec thinning are not externally driven by the india asia collision but are the expression of the interaction of a mantle plume with overlying lithosphere some rifts link and interact with major strike slip faults such as the bolnai such faults may be the major expression of the india asia collision in this region

active intracontinental transpressional mountain building in the mongolian altai defining a new class of orogen abstract mountain ranges that are actively forming around the western and northern perimeter of the indo eurasia collisional deformational field such as the mongolian altai comprise a unique class of intracontinental intraplate transpressional orogen with structural and basinal elements that are distinct from contractional and extensional orogens late cenozoic uplift and mountain building in the mongolian altai is dominated by regional scale dextral strike slip faults that link with thrust and oblique slip faults within a 300 km wide deforming belt sandwiched between the more rigid junggar basin block and hangay precambrian craton dominant orogenic elements in the mongolian altai include double restraining bends terminal restraining bends partial restraining bends single thrust ridges thrust ridges linked by strike slip faults and triangular block uplifts in areas of conjugate strike slip faults the overall pattern is similar to a regional strike slip duplex array however the significant amount of contractional and oblique slip displacement within the range and large number of historical oblique slip seismic events renders the term transpressional duplex more accurate intramontane and range flanking basins can be classified as ramp basins half ramp basins open sided thrust basins pull apart basins and strike slip basins neither a classic fold and thrust orogenic wedge geometry nor a bounding foredeep exists the manner in which upper crustal transpressional deformation is balanced in the lower crust is unknown however crustal thickening by lower crustal inflation and northward outflow of lower crustal material are consistent with existing geological and geodetic data and could account for late cenozoic regional epeirogenic uplift in the russian altai and sayan regions

wide band analysis of the 3 march 1985 central chile earthquake overall source process and rupture history we apply a linear finite fault waveform inversion scheme to the near source strong motion records the teleseismic body waves and the long period rayleigh waves recorded for the 3 march 1985 chile earthquake to recover the mainshock rupture history the data contain periods between about 2 and 350 sec and are inverted by allowing a variable dislocation rise time at each point on the fault the results indicate that the mainshock had a seismic moment of 1 5 1028 dyne cm mw 8 0 and ruptured mainly updip and to the south of the hypocenter for a distance of about 150 km along the nazca south america plate boundary a smaller northward component of propagation is also evident giving a total rupture length of about 200 km the total source duration of the mainshock is 70 sec with the majority of the slip occurring within the first 40 sec in a broad 100 km wide zone in the northern half of the rupture area slip in this region extends from a depth of 55 km to within about 10 km of the surface and contains two areas of maximum slip 2 3 and 2 9 m with rise times of approximately 14 sec slip in the southern portion of the fault reaches lower peak values 1 8 m and extends downdip to depths no greater than 30 km an independent variable rise time inversion of the teleseismic body waves alone yields similar results indicating that a significant component of slow fault motion is not required for this earthquake the mainshock was preceded by several smaller precursors the largest of which is an mw 6 6 thrust earthquake occurring at a depth of 22 km in the shallow 15 dipping portion of the plate interface

paleotopography and erosion rates in the central hangay dome mongolia landscape evolution since the mid miocene abstract standing over 2 km above the surrounding topography and flanked by orogen scale strike slip faults the hangay dome in central mongolia is characterized by long wavelength high topography basaltic eruptions spanning 30 million years and an abundance of flat topped summit plateaus however despite decades of research the origin and timing of the intraplate hangay dome uplift continues to be debated using landsat imagery gis and cosmogenic beryllium 10 we employ geomorphic investigations of 1 paleotopography preserved beneath basalt flows of known age 2 erosion rates at various temporal scales and 3 the relative contribution of glacial activity to total erosion to provide insight into the nature of landscape evolution in the egiin davaa region of the central hangay dome since the middle miocene reconstruction of paleo valleys cut into paleozoic basement rock that exhibit a degree of local relief 600 m similar to the modern landscape sluggish mean erosion rates

upper mantle flow beneath and around the hangay dome central mongolia mongolia represents the northernmost area affected by the india asia collision and it is actively deformed along transpressive belts closely associated with large scale strike slip faults the active and past mantle flow beneath this region is however poorly known in order to investigate deep mantle deformation beneath central mongolia and its relation with the surrounding major structures such as the siberian craton the gobi altay belt and the baikal rift a ns trending profile of broadband seismic stations has been deployed in the summer 2003 from the southern siberian craton to the gobi altay range crossing the entire hangay dome mantle flow is deduced from the splitting of teleseismic shear waves such as sks phases in eastern mongolia the permanent station uln in ulaanbaatar reveals the presence of two anisotropic layers the upper one being oriented ne sw close to the trend of lithospheric structures and the lower one nw se close to the trend of eurasia absolute plate motion along the ns profile in central mongolia seismic anisotropy deduced from sks splitting reveals a homogeneous nw se trending structure fully consistent with the observations made in the altay sayan in western mongolia the observed delay times of 1 5 to more than 2 0 s favor consistent mantle flow over large mantle thicknesses since the lithosphere is less than 100 km thick beneath central mongolia and since the observed fast directions are parallel to the trend of the lithospheric structures but also close to the trend of the absolute plate motion we propose that both the lithosphere and the asthenosphere may join their anisotropic effects beneath central mongolia to explain the large delay times although gps vectors represent the instantaneous displacement of the earth s surface and sks splitting the time and vertical integration of finite strain at depth we use the opportunity of the dense geodetic measurements available in this region to discuss the anisotropy pattern in term of present day deformation in the eurasia fixed reference frame gps and sks both depict a similar trend beneath central mongolia suggesting a lithospheric block escaping toward the east that could orient olivine a axes in the upper mantle within a transpressive tectonic regime a different behaviour is observed in western mongolia the gps vectors trend ns close to the regional compression direction whereas the fast sks directions trend ew suggesting a tectonic regime close to a mode of axial shortening generating the development of an ew trending foliation at depth we therefore propose that mongolia is a place where active and frozen lithospheric deformation may add their effects together with the sublithospheric flow in the three sources of anisotropy inferred a fundamental role is played by the siberian craton that acted as an undeformable core of the continent through time the frozen paleozoic and mesozoic structures wrap around the craton building up the fast anisotropic direction pattern the present day sublithospheric flow induced by the plate motion is likely deflecting around its deep roots finally the present day tectonic regime appears to be controlled by the presence of the craton to the north

mantle driven uplift of hangai dome new seismic constraints from adjoint tomography the origin of hangai dome an unusual large scale high elevation low relief landform in central mongolia remains enigmatic partly due to lack of constraints on its underlying seismic structure using adjoint tomography a full waveform tomographic technique and a large seismic waveform data set in east asia we discover beneath the dome a deep low shear wave speed low v conduit indicating a slightly warmer 54 k to 127 k upwelling from the transition zone this upwelling is spatially linked to a broader uppermost mantle low v region underlying the dome further observations of high compressional to shear wave speed ratios and positive radial anisotropy in the low v region suggest partial melting and horizontal melt transport we propose that the mantle upwelling induced decompression melting in the uppermost mantle and that excess heat associated with melt transport modified the lithosphere that isostatically compensates the surface uplift at upper mantle depths 80 km

helium isotopes provide no evidence for deep mantle involvement in widespread cenozoic volcanism across central asia abstract small volume alkali basaltic volcanism has occurred intermittently for the past 30 my across a vast area of thick continental crust from southern siberia through mongolia to northeast china with a lack of evidence for basin and range type crustal extension or rifting models to explain the widely dispersed yet long lived volcanism tend to favour involvement of one or more mantle plume s we examine the range of 3he 4he isotope values in olivine phenocrysts from basalts and their entrained mantle xenoliths from hamar daban in southern siberia and hangai in central mongolia in order to examine whether upwelling lower mantle appears to be present beneath central asia and thus test the validity of the plume model for this region our results show that the maximum 3he 4he value for the siberian basalts is 8 12 0 2ra and the maximum value for mongolian basalts is 9 5 0 5ra these values suggest that there is no significant contribution from a high 3he 4he primordial component that would strongly argue a lower mantle source overlap with commonly reported values for morb leads us to propose that the source of the magmatism derives from the shallow asthenosphere alternative models to a deeply sourced mantle plume that may be able to explain the magmatism include a shallow thermal anomaly confined to the upper mantle but either fed laterally or caused by thermal blanketing of the large asian landmass replacement or delamination of the lowermost lithosphere in response to tectonic stresses or large scale mantle disturbance or overturn caused by a protracted history of subduction beneath central asia that ended regionally with the jurassic closure of the mongol okhotsk ocean but continues further afield with the present indo asia collision

the 2010 mw 8 8 maule megathrust earthquake of central chile monitored by gps large earthquakes produce crustal deformation that can be quantified by geodetic measurements allowing for the determination of the slip distribution on the fault we used data from global positioning system gps networks in central chile to infer the static deformation and the kinematics of the 2010 moment magnitude m w 8 8 maule megathrust earthquake from elastic modeling we found a total rupture length of 500 kilometers where slip up to 15 meters concentrated on two main asperities situated on both sides of the epicenter we found that rupture reached shallow depths probably extending up to the trench resolvable afterslip occurred in regions of low coseismic slip the low frequency hypocenter is relocated 40 kilometers southwest of initial estimates rupture propagated bilaterally at about 3 1 kilometers per second with possible but not fully resolved velocity variations

a seismological study of the 1835 seismic gap in south central chile we study the possible seismic gap in the concepcion constitucion region of south central chile and the nature of the m 7 8 earthquake of january 1939 from 1 march to 31 may 1996 a seismic network of 26 short period digital instruments was deployed in this area we located 379 hypocenters with rms travel time residuals of less than 0 50 s using an approximate velocity distribution using the velest program we improved the velocity model and located 240 high precision hypocenters with residuals less than 0 2 s the large majority of earthquakes occurred along the wadati benioff zone along the upper part of the downgoing slab under central chile a few shallow events were recorded near the chain of active volcanos on the andes these events are similar to those of las melozas near santiago a few events took place at the boundary between the coastal ranges and the central valley well constrained fault plane solutions could be computed for 32 of the 240 well located events most of the earthquakes located on the wadati benioff zone had slab pull fault mechanism due to tensional stresses sub parallel to the downgoing slab this slab pull mechanism is the same as that of eight earthquakes of magnitude around 6 that are listed in the cmt catalog of harvard university for the period 1980 1998 this is also the mechanism inferred for the large 1939 chilean earthquake a very small number of events in the benioff zone had slab push mechanisms that is events whose pressureaxis is aligned with the slab these events are found in double layered wadati benioff zones such as in northern chile or japan our spatial resolution is not good enough to detect the presence of a double layer but we suspect there may be one 2002 elsevier science b v all rights reserved

effects of fault heterogeneity on seismic energy and spectrum abstract we study the effects of friction heterogeneity on the dynamics of a seismogenic fault to this aim we consider a fault model containing two asperities with different static frictions and a rate dependent dynamic friction we consider the seismic events produced by the consecutive failure of the two asperities and study their properties as functions of the ratio between static frictions in particular we calculate the moment rate the stress evolution during fault slip the average stress drop the partitioning of energy release the seismic energy the far field waveforms and the spectrum of seismic waves these quantities depend to various extent on the friction distribution on the fault in particular the stress distribution on the fault is always strongly heterogeneous at the beginning of the seismic event seismic energy and frictional heat decrease with increasing friction heterogeneity while seismic efficiency is constant we obtain an equation relating seismic efficiency to the parameters of the friction law showing that the efficiency is maximum for smaller values of dynamic friction the seismic spectrum depends on the friction distribution as to the positions and the values of the minima however under the model assumption that the slip durations are the same for both asperities the corner frequency is independent of the friction distribution but it depends on the friction law and on the coupling between asperities the model provides a relation between the total radiated energy and the seismic moment that is consistent with the empirical relation between the two quantities the fault model with one asperity is also considered as a particular case the model is applied to the 1965 rat islands alaska earthquake and shows the role of fault heterogeneity in controlling the spatial distribution of stress drop as well as the time dependence and the final amount of radiated energy

patterns of intraplate volcanism controlled by asthenospheric shear volcanism observed far from plate boundaries in the interior of oceanic and continental plates may result from flow in the underlying mantle comparison between a numerical model of mantle flow and the spatial distribution of intraplate volcanism indicates that rapid shear motion in the mantle may drive melting that causes intraplate eruptions most of earth s volcanism occurs at plate boundaries in association with subduction or rifting a few high volume volcanic fields are observed both at plate boundaries and within plates fed by plumes upwelling from the deep mantle1 the remaining volcanism is observed away from plate boundaries it is typically basaltic effusive and low volume occurring within continental interiors2 3 4 5 6 7 or creating seamounts on the ocean floor8 9 10 11 this intraplate volcanism has been attributed to various localized processes12 such as cracking of the lithosphere8 13 14 small scale convection in the mantle beneath the lithosphere15 16 17 or shear induced melting of low viscosity pockets of asthenospheric mantle that have become embedded along the base of the lithosphere18 here we compare the locations of observed intraplate volcanism with global patterns of mantle flow from a numerical model we find a correlation between recent continental and oceanic intraplate volcanism and areas of the asthenosphere that are experiencing rapid shear due to mantle convection we detect particularly high correlations in the interior of the continents of western north america eastern australia southern europe and antarctica as well as west of the east pacific rise in the pacific ocean we conclude that intraplate volcanism associated with mantle convection is best explained by melting caused by shear flow within the asthenosphere whereas other localized processes are less important

slip distribution of the february 27 2010 mw 8 8 maule earthquake central chile from static and high rate gps insar and broadband teleseismic data 1 the shallow depth underthrust earthquake of february 27 2010 mw 8 8 ruptured the subduction plate interface in central chile between 34 s and 38 s we retrieve the spatial and temporal distribution of slip during this mega earthquake through a joint inversion of teleseismic records insar and high rate gps hrgps data additionally our model is shown to agree with broadband surface waves rupture initiated at about 32 km depth and propagated bilaterally resulting in two main slip zones located ssw and nne of the hypocenter nucleation did not take place within or at the edge of one of these main asperities but in between during the first 30s slip propagated predominantly southwards later on the rupture evolved more slowly and more symmetrically eventually the northern asperity became predominant with maximum slip reaching about 20 m most of the seismic moment was released within 110s a relatively short time explained by the bilateral propagation the overall average rupture velocity is 2 6 km s but propagation occurred initially faster towards the south 3 2 km s large slip did not reach the trench a result consistent with the moderate size of the tsunami down dip rupture stopped at about 50 km depth in agreement with the lower limit of the locked zone inferred by ruegg et al 2009 from pre seismic gps data

cenozoic normal faulting and regional doming in the southern hangay region central mongolia implications for the origin of the baikal rift province abstract the hangay dome in central mongolia is a mountainous region covering more than 200 000 km2 with numerous flat topped peaks over 3000 m that preserve a late cretaceous paleogene erosion surface doming began in the middle oligocene producing more than 2000 m of regional topographic uplift the range represents an important kinematic link between the baikal rift province to the north and altai transpressional ranges to the south and west structural field investigations of major faults visible on satellite imagery indicate that the southern hangay dome region is dominated by late cenozoic normal faults that bound small half graben most faults are ne striking and faults at the highest elevations are the most recently active most graben appear to be isolated systems that constitute small sediment sinks perched on the flanks and crests of the dome the first order feature is the dome itself and most sediment eroded off of the dome is deposited in the mongolian valley of lakes or is carried northwards by the selenga river and its extensive tributaries the basement of the dome is a precambrian craton although the shape and dimensions of the craton are poorly constrained to the north and east late cenozoic uplift of the southern dome region appears to be confined to the area underlain by cratonic basement whereas the altai region to the south and west is underlain by mechanically weaker palaeozoic arc and accretionary belts with respect to the regional northeast directed shmax the hangay craton appears to have acted as a rigid passive indentor focusing late cenozoic transpressional deformation around its west and southern margins models invoking a late cenozoic plume as the driving force for doming and widespread alkaline volcanism on the dome are not strongly supported by geochemical and isotopic data on neogene recent volcanics and the spatial correlation between areas that are domed and older cratonic crust appears too coincidental to be ignored convective removal of an overthickened lithospheric root leading to adiabatically decompressed asthenosphere could explain regional doming and volcanism however major crustal thickening last occurred in the permian in the southern hangay region and the time lag between thickening and postulated root removal and plateau uplift 200 myr appears too long an alternative model is explored based on speculated lithospheric mantle flow patterns driven by india s continued northeastward indentation it is suggested that lithospheric mantle flow diversion around the overthickened hangay craton crustal keel could cause lithospheric thinning beneath the craton and passive asthenospheric upwarp leading to regional topographic uplift and decompression melting alkali volcanism in general the angular relationship between precambrian craton boundaries and the prevailing northeasterly shmax appears to control the kinematics of late cenozoic deformation throughout the hangay altai sayan and baikal regions

deep structure and mechanical behavior of the lithosphere in the hangai hovsgol region mongolia new constraints from gravity modeling we investigate the deep structure and mechanical behavior of the lithosphere beneath the hangai ho vsgo l region central mongolia asia in order to explain the origin and support of large scale doming in this deforming area we propose a gravity and topography based model which accounts for constraints provided by other independent results from xenolith and tomography studies deviations of the measured gravity from the theoretical airy compensation model are examined a long wavelength low gravity anomaly is spatially correlated with low pressure and shear velocity anomalies in the mantle and with the extent of cenozoic volcanic outcrops we interpret it as a deep seated low density asthenosphere and model its effect on the bouguer gravity signal using a 600 km wide light asthenospheric body density reduction 310 kg m 33 located between 100 and 200 km north and south of the hangai ho vsgo l dome short wavelength highs and lows in the bouguer gravity field are clearly correlated with fault activity they seem to reflect opposite senses of flexure of a rigid lithosphere across two major active faults the sayan and bogd transpressional systems and are modeled by moho deflections of 10 and 5 km respectively finally a shortwavelength 200 km high amplitude 350 mgal gravity residual remains beneath the highest part of the mountain bulge namely the hangai dome based on previously published xenolith analyses we interpret it as an anomalous low density body which may represent underplated cumulates or mafic granulites at the uppermost mantle we conclude that upper mantle dynamics necessarily play an important role in the origin and evolution of the hangai ho vsgo l dome but without requiring significant thinning of the lithosphere fl 2002 elsevier science b v all rights reserved

non hotspot volcano chains produced by migration of shear driven upwelling toward the east pacific rise while most oceanic volcanism is associated with the passive rise of hot mantle beneath the spreading axes of mid ocean ridges mor volcanism occurring off axis reflects intraplate upper mantle dynamics and composition yet is poorly understood off the south east pacific rise sepr volcanism along the pukapuka hotu matua and sojourn ridges has been attributed to various mechanisms but none can reconcile its spatial temporal and geochemical characteristics our three dimensional numerical models show that asthenospheric shear can excite upwelling and decompression melting at the tip of low viscosity fingers that are propelled eastward by vigorous sublithospheric flow this shear driven upwelling is able to sustain intraplate volcanism that progresses toward the mor spreads laterally close to the axis and weakly continues on the opposite plate these predictions can explain the anomalously fast eastward progression of volcanism and its spatial distribution near the sepr moreover for a heterogeneous mantle source involving a fertile component the predicted systematics of volcanism can explain the geochemical trend along pukapuka and the enriched anomaly of sepr mid oceanic ridge basalt at 16 20 5 s our study highlights the role of horizontal asthenospheric flow and mantle heterogeneity in producing linear chains of intraplate volcanism independent of a deep rooted buoyancy source

asperity distributions and large earthquake occurrence in subduction zones abstract ruff l j 1992 asperity distributions and large earthquake occurrence in subduction zones in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 61 83 plate tectonics and the seismic gap hypothesis provide the framework for long term earthquake forecasting of plate boundary earthquakes unfortunately detailed examination reveals that earthquake recurrence times and rupture length vary between successive earthquake cycles in the same subduction zone furthermore larger coseismic slip is commonly associated with larger rupture length hence large earthquake occurrence in subduction zones is characterized by variability in 1 recurrence times 2 rupture length and 3 coseismic slip these facts plus many other observations indicate that there are significant spatial variations in the strength of the plate interface one simple description of these variations and their role in the earthquake cycle is the asperity model where the large strong regions of the plate interface are called asperities and the large earthquakes occur when the large asperities break the asperity model of earthquake occurrence is able to qualitatively explain several features of large plate boundary earthquakes to go beyond general qualitative notions i pose the following scientific test are the observed asperity distributions and a simple model of their interaction self consistent with the above three observed features of large earthquake occurrence the distribution of the major asperities along plate boundary segments has now been determined for several subduction zones rupture process studies of adjacent large and great earthquakes have provided reliable estimates of the along strike asperity lengths and separations for several adjacent asperities in the kurile islands colombia and peru subduction zones the simplest mechanical model for asperity interaction is to idealize two adjacent asperities as frictional sliders that are connected by main springs to the upper plate by a coupling spring to each other and maintain frictional contact with a conveyer belt the lower plate that moves with a constant velocity an earthquake occurs when the net force on the asperity frictional slider reaches some specified level the failure force and spring constants are determined by the observed asperity distribution and simple models of elastic interaction two different macroscopic failure criteria are used this simple mechanical model displays a remarkable range of behavior from simple to complex when the two asperities are identical in all their properties sequences of identical earthquakes are produced for the more realistic case of non identical asperities earthquake sequences show great variety using system variables from the observed asperity distributions the earthquake sequences typically display 1 variable recurrence times 2 variable rupture length i e a combination of single asperity and double asperity failures and for one of the failure criteria 3 larger coseismic slip for double asperity failures statistical summaries of thousands of simulated earthquake sequences for asperity pairs in the kuriles colombia and peru subduction zones are broadly consistent with the observed features of large earthquake occurrence in these subduction zones the main conclusion is that the asperity model provides a self consistent explanation for fault zone heterogeneity the rupture process and recurrence times and rupture mode of large earthquake sequences via a simple model for adjacent asperity interaction in addition a conclusion independent of any particular model for fault zone heterogeneity is that simple deterministic models of fault zone interaction can explain complex patterns of large earthquake occurrence in subduction zones

asthenospheric flow and origin of volcanism in the baikal rift area the origin of low volume hotspot like volcanism often observed in continental rift areas is debated as is the nature of the flow in the mantle beneath in this paper we assemble seismic constraints on the mantle flow below the baikal rift zone we combine new evidence from upper mantle tomography and from a radially anisotropic shear velocity profile measured beneath and se of the rift with published shear wave splitting constraints on azimuthal anisotropy fast directions of azimuthal anisotropy are perpendicular to the rift on both of its sides stable siberian platform lithosphere on the northern side extends down to 200 km on the southern side the lithosphere is only 60 70 km thick this asymmetry with the thick cratonic root forming a barrier to convective flow inhibits a mid ocean ridge like flow pattern the radially anisotropic profile yields evidence for horizontal flow in the upper asthenosphere 60 100 km se of the rift but no evidence for vertical flow these results are consistent with horizontal asthenospheric flow nw to se perpendicular to the rift from beneath the thick siberian towards the thin baikal mongolian lithosphere when asthenosphere ascends from the 200 km depth beneath the craton to shallower depths beneath the rift decompression melting of embedded veins of enriched rock can produce the hotspot like basaltic volcanism observed the occurrence of such veins of enriched mantle in sub cratonic asthenosphere has been predicted by geochemical mantle models we propose that sub horizontal asthenospheric flow from beneath cratons is a common cause for hotspot like volcanism in continental rift zones

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

low seismic velocity layers in the earth s crust beneath eastern siberia russia and central mongolia receiver function data and their possible geological implication analysis of teleseismic receiver functions at digital stations along the bratsk irkutsk ulanbaatar undurshil profile suggests that low velocity layers in the earth s crust exist not only beneath the baikal rift zone where such a layer was found earlier by deep seismic sounding dss but also beneath early paleozoic sayan baikal paleozoic mongolian early mesozoic mongolia okhotsk fold areas and beneath the siberian platform the reliability of detection of the low velocity layers by receiver function analysis has been checked by numerical modeling the results of this modeling demonstrate that receiver functions can reveal the low velocity layers in the crust if the initial model starting approximation is close to real velocity distribution and if the model medium is divided into thin layers averaged dss velocity model without low velocity layers was used as starting approximation for the inversion of observed receiver functions the low velocity layers are interpreted to reflect inhomogeneities of the earth s crust formed during its evolution most of these layers are presumed to correspond to thick mylonite zones related to large pre cenozoic thrusts the mylonites possess a great seismic anisotropy caused by the mineral orientation formed by the ductile flow in large thrust zones they can result in low velocity layers only for seismic waves whose rays are oriented perpendicular to the mylonite foliation i e in the direction of the minimum velocity the velocities along the foliation direction can be rather high therefore the low angle mylonite zones can be distinguished by the receiver function method which uses the waves from the teleseismic events with nearly vertically oriented rays the suggestion that the low velocity layers mark low angle thrusts is in agreement with gravity and geological data the amount of overthrusting is estimated to be as large as several hundred kilometers multichannel seismic profiling can be used to verify the existence and the deep geometry of the presumed thrusts d 2002 elsevier science b v all rights reserved

geomorphic and tectonic process rates effects of measured time interval analysis of the original numerical components of diverse geomorphic and tectonic process rates indicates that process rate is not independent of measured time interval this implies that direct comparison of geologic process rates measured over variable time intervals may not be valid and that conclusions based on those comparisons regarding changes in rates through geologic time may be misleading in some cases calculation of statistically derived scaling functions is one approach that can be used to account for this measurement interval bias and to improve comparisons of geologic process rate data when this approach is used a 10 4 or 10 5 yr variation in the measured time interval accounts for approximately one order of magnitude change in the measured process rate the magnitude of the correction can substantially affect conclusions of studies that compare uncorrected process rates calculated from variable time intervals

slip distribution and seismic moment of the 2010 and 1960 chilean earthquakes inferred from tsunami waveforms and coastal geodetic data the slip distribution and seismic moment of the 2010 and 1960 chilean earthquakes were estimated from tsunami and coastal geodetic data these two earthquakes generated transoceanic tsunamis and the waveforms were recorded around the pacific ocean in addition coseismic coastal uplift and subsidence were measured around the source areas for the 27 february 2010 maule earthquake inversion of the tsunami waveforms recorded at nearby coastal tide gauge and deep ocean assessment and reporting of tsunamis dart stations combined with coastal geodetic data suggest two asperities a northern one beneath the coast of constitucion and a southern one around the arauco peninsula the total fault length is approximately 400 km with seismic moment of 1 7 1022 nm mw 8 8 the offshore dart tsunami waveforms require fault slips beneath the coasts but the exact locations are better estimated by coastal geodetic data the 22 may 1960 earthquake produced very large 30 m slip off valdivia joint inversion of tsunami waveforms at tide gauge stations in south america with coastal geodetic and leveling data shows total fault length of 800 km and seismic moment of 7 2 1022 nm mw 9 2 the seismic moment estimated from tsunami or joint inversion is similar to previous estimates from geodetic data but much smaller than the results from seismic data analysis