seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

crustal structure of northern italy from the ellipticity of rayleigh waves abstract northern italy is a diverse geological region including the wide and thick po plain sedimentary basin which is bounded by the alps and the apennines the seismically slow shallow structure of the po plain is difficult to retrieve with classical seismic measurements such as surface wave dispersion yet the detailed structure of the region greatly affects seismic wave propagation and hence seismic ground shaking here we invert rayleigh wave ellipticity measurements in the period range 10 60 s for 95 stations in northern italy using a fully non linear approach to constrain vertical v s v p and density profiles of the crust beneath each station the ellipticity of rayleigh wave ground motion is primarily sensitive to shear wave velocity beneath the recording station which reduces along path contamination effects we use the 3d layering structure in mambo a previous model based on a compilation of geological and geophysical information for the po plain and surrounding regions of northern italy and employ ellipticity data to constrain v s v p and density within its layers we show that ellipticity data from ballistic teleseismic wave trains alone constrain the crustal structure well this leads to mambo e an updated seismic model of the region s crust that inherits information available from previous seismic prospection and geological studies while fitting new seismic data well mambo e brings new insights into lateral heterogeneity in the region s subsurface compared to mambo it shows overall faster seismic anomalies in the region s quaternary pliocene and oligo miocene layers and better delineates the seismic structures of the po plain at depth two low velocity regions are mapped in the mesozoic layer in the western and eastern parts of the plain which seem to correspond to the monferrato sedimentary basin and to the ferrara romagna thrust system respectively

in situ behaviour of a stiff layered clay subject to thermal loading observations and interpretation the paper presents an interpretation of an in situ heating test carried out on opalinus clay in the mont terri underground laboratory opalinus clay is a stiff strongly bedded mesozoic clay of marine origin when subjected to thermal loading saturated stiff clays exhibit a strong pore pressure response that significantly affects the hydraulic and mechanical behaviour of the material the observations gathered in the in situ test have provided an opportunity to examine the integrated thermo hydro mechanical thm response of this sedimentary clay coupled thm numerical analyses have been carried out to provide a structured framework for interpretation and to enhance understanding of thm clay behaviour numerical analyses have been based on a coupled theoretical formulation that incorporates a constitutive law especially developed for this type of material the law includes degradation of bonding by damage by performing three dimensional computations it has been possible to incorporate anisotropy of m

crustal structure beneath portugal from teleseismic rayleigh wave ellipticity up until now portugal lacked a countrywide shear velocity model sampling short length scale crustal structure which limits interpretations of seismicity and tectonics and predictions of strong ground motion in turn such interpretations and predictions are important to help mitigate risk of destruction from future large on and offshore earthquakes similar to those that portugal has experienced in the past e g the mw 8 5 8 7 tsunamigenic event in 1755 in this study we measured teleseismic rayleigh wave ellipticity rwe from 33 permanent and temporary seismic stations in portugal with wave periods between 15 s and 60 s and inverted it for 1 d models of shear wave velocity vs structure beneath each station using a fully non linear monte carlo method because rwe is strongly sensitive to the uppermost few kilometres of the crust both rwe measurements and vs models are spatially correlated with surface geology in portugal for instance we find that sedimentary basins produced by rifting that had begun in the mesozoic such as the lusitanian basin lb and the lower tagus sado basin ltsb are characterised by higher rwe lower vs interestingly we observe similar rwe and vs values in the interior of the central iberian zone ciz which is a metamorphic belt of paleozoic age together with reduced crustal thickness previously estimated for the same parts of the ciz this suggests that the ciz might have experienced an episode of extension possibly simultaneous to mesozoic rifting the galicia tras os montes zone gtmz that has undergone polyphased deformation since the paleozoic is characterised by the lowest rwe highest vs in portugal ossa morena zone and the south portuguese zone exhibit intermediate vs values when compared to that of basins and the gtmz our crustal vs model can be used to provide new insights into the tectonics seismicity and strong ground motion in portugal

a 3 d model of the crust and uppermost mantle beneath the central and western us by joint inversion of receiver functions and surface wave dispersion 1 rayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the usarray transportable array west of 100 w longitude for data recorded in the years 2005 through 2010 to produce a 3 d model of shear wave speeds beneath the central and western us to a depth of 150 km eikonal tomography is applied to ambient noise data to produce about 300 000 rayleigh wave phase speed curves and helmholtz tomography is applied to data following 1550 ms 5 0 earthquakes so that rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region harmonic stripping generates back azimuth independent receiver functions with uncertainty estimates for each of the stations a nonlinear bayesian monte carlo method is used to estimate a distribution of shear wave speed vs models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties the assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated moho depths improving the determination of the shear velocity jump across moho and improving the resolution of the depth of anomalies in the uppermost mantle a great variety of geological and tectonic features are revealed in the 3 d model that form the basis for future detailed local to regional scale analysis and interpretation

the yellowstone magmatic system from the mantle plume to the upper crust the yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world an understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk using a joint local and teleseismic earthquake p wave seismic inversion we revealed a basaltic lower crustal magma body that provides a magmatic link between the yellowstone mantle plume and the previously imaged upper crustal magma reservoir this lower crustal magma body has a volume of 46 000 cubic kilometers 4 5 times that of the upper crustal magma reservoir and contains a melt fraction of 2 these estimates are critical to understanding the evolution of bimodal basaltic rhyolitic volcanism explaining the magnitude of co2 discharge and constraining dynamic models of the magmatic system for volcanic hazard assessment

anisotropy and vp vs in the uppermost mantle beneath the western united states from joint analysis of pn and sn phases pn and sn phases are valuable for resolving velocity structure in the mantle lid as they propagate horizontally right below the moho relatively few sn tomography attempts have been made compared to pn because sn is often highly attenuated or buried in p wave coda usarray has greatly increased data coverage for regional phases and both pn and sn are routinely picked by network analysts here we jointly invert pn and sn arrival time residuals with a modified time term analysis and a regularized tomography method and present new maps of crustal thickness uppermost mantle p velocity perturbations vp vs ratios and azimuthal anisotropy strength and orientation beneath the western united states the results indicate partially molten mantle below the snake river plain and the colorado plateau the seismic structure of the top 40 km of the mantle below the colorado plateau differs from that seen at greater depths in other studies such as surface wave or teleseismic body wave tomography whereas the snake river plain anomaly just below the moho is comparable to structures seen at about 200 km depth pn fast axes provide complementary information to sks shear wave splitting observations and our analysis indicates that in several regions in the western united states the orientation of azimuthal anisotropy changes with depth in the upper mantle however we have so far been unable to resolve shear wave splitting directly in sn waveforms which seem to be dominated by sn sv energy

analysis of the thm behaviour in a clay based engineered barrier system ebs modelling of the he e experiment mont terri url to understand the complex thermo hydro mechanical thm processes and their evolution in a clay based engineered barrier systems ebs during the closure phase of a geological repository for radioactive waste a 1 2 scale in situ heating experiment he e experiment has been ongoing since 2011 in the mont terri rock laboratory based on the experimental data a fully coupled thm 3d simulation using the finite element program opengeosys was carried out the main objectives of the simulation were to interpret the experimental observations to understand the thermally induced thm interactions and to analyse the different material properties five experimental phases were numerically interpreted in order to take into account the changes in the temporary material properties during experimental operations these included the tunnel excavation the ventilation the emplacement of the bentonite ebs the heating phase and the phase after shutdown of the heater a non isothermal richards flow model was used to take into account the evaporation and vapour diffusion during the heating the material behaviours of ebs in association with the saturation dependent thermal conductivity and the water retention behaviour under high temperatures were analysed in detail the strong thermal hydraulic and mechanical anisotropic properties of the opalinus clay were described by a transversely isotropic model

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

lithosphere asthenosphere interaction beneath the western united states from the joint inversion of body wave traveltimes and surface wave phase velocities summary the relation between the complex geological history of the western margin of the north american plate and the processes in the mantle is still not fully documented and understood several pre usarray local seismic studies showed how the characteristics of key geological features such as the colorado plateau and the yellowstone snake river plains are linked to their deep mantle structure recent body wave models based on the deployment of the high density large aperture usarray have provided far more details on the mantle structure while surface wave tomography ballistic waves and noise correlations informs us on the shallow structure here we combine constraints from these two data sets to image and study the link between the geology of the western united states the shallow structure of the earth and the convective processes in mantle our multiphase dna10 s model provides new constraints on the extent of the archean lithosphere imaged as a large deeply rooted fast body that encompasses the stable great plains and a large portion of the northern and central rocky mountains widespread slow anomalies are found in the lower crust and upper mantle suggesting that low density rocks isostatically sustain part of the high topography of the western united states the yellowstone anomaly is imaged as a large slow body rising from the lower mantle intruding the overlying lithosphere and controlling locally the seismicity and the topography the large e w extent of the usarray used in this study allows imaging the slab graveyard a sequence of farallon fragments aligned with the currently subducting juan de fuca slab north of the mendocino triple junction the lithospheric root of the colorado plateau has apparently been weakened and partly removed through dripping the distribution of the slower regions around the colorado plateau and other rigid blocks follows closely the trend of cenozoic volcanic fields and ancient lithospheric sutures suggesting that the later exert a control on the locus of magmato tectonic activity today the dna velocity models are available for download and slicing at http dna berkeley edu

the anatomy of uppermost mantle shear wave speed anomalies in the western u s from surface wave amplification abstract we build swus amp a three dimensional shear wave speed model of the uppermost mantle of the western u s using rayleigh wave amplification measurements in the period range of 35 125 s from teleseismic earthquakes this represents the first ever attempt to invert for velocity structures using rayleigh wave amplification data alone we use over 350 000 rayleigh wave amplitude measurements which are inverted using a monte carlo technique including uncertainty quantification being a local seismic observable rayleigh wave amplification is little affected by path averaged effects and in principle has stronger depth resolution than classical seismic observables such as surface wave dispersion data swus amp confirms shallow mantle heterogeneities found in previous models in the top 100 km of the mantle we observe low velocity anomalies associated with yellowstone and the basin range province as well as a fast velocity anomaly underneath the colorado plateau where a strong velocity gradient at its edges shows a drastic contrast with its surroundings swus amp also gives additional insights into the current state of the uppermost mantle in the region we image a high velocity anomaly beneath the high topography wyoming province with a maximum depth extent of about 150 170 km which is shallower than in previous tomographic models and resolves previous inconsistencies with geological information beneath the snake river plain a finger like low velocity anomaly dips to the west suggesting lateral flow in the region below about 150 km depth swus amp shows a north south dichotomy in shear wave speed structure with the northern region showing mostly high velocity anomalies whereas the southern region shows low velocity anomalies this is consistent with the continuous subduction history of the western u s and with the recent extension and uplift of the southern region

tomography reveals buoyant asthenosphere accumulating beneath the juan de fuca plate the boundary between earth s strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth perhaps indicating a thin weak layer that may strongly influence plate motion dynamics the behavior of such a layer at subduction zones remains unexplored we present a tomographic model derived from on and offshore seismic experiments that reveals a strong low velocity feature beneath the subducting juan de fuca slab along the entire cascadia subduction zone through simple geodynamic arguments we propose that this low velocity feature is the accumulation of material from a thin weak buoyant layer present beneath the entire oceanic lithosphere the presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics

imaging crustal and upper mantle structure beneath the colorado plateau using finite frequency rayleigh wave tomography a new 3 d shear velocity model of the crust and upper mantle beneath the colorado plateau and surrounding regions of the southwestern united states was made with finite frequency rayleigh wave tomography using earthscope usarray data the goal of our study is to examine the colorado plateau lithospheric modification that has resulted from cenozoic tectonism and magmatism we have inverted for the isotropic vs model from a grid of rayleigh wave dispersion curves obtained by a modified two plane wave method for periods from 20 to 167 s we map the lithosphere asthenosphere boundary under the colorado plateau by identifying the middle of the shallowest upper mantle negative vs gradient the depths of the lithosphere asthenosphere boundary inferred here agree well with receiver function estimates made independently the strong lateral heterogeneity of shear velocity can be mainly attributed to 200 400 k variations in temperature together with 1 partial melt fraction in the shallow upper mantle the resulting vs structures clearly image the upper mantle low velocity zones under the colorado plateau margins that are associated with magmatic encroachment these upper mantle low velocity zones resulted from the convective removal of the colorado plateau lithosphere that had been rehydrated by subduction released water refertilizing and destabilizing it this convective erosion by the asthenosphere at the low viscosity part of the lithosphere is driven by the large step in lithospheric thickness and the thermal gradient across the boundary between the plateau and the extended basin and range since the mid cenozoic at a rate similar to that of magmatic migration into the plateau from the southeast south and northwest moreover the rayleigh wave tomography model images parts of a high velocity drip in the western colorado plateau and thus provides additional seismic evidence for ongoing convective downwelling of the lithosphere that was initially suggested by receiver functions and body wave tomography the widespread edge convective erosion which the regional delamination style downwelling processes are a 3 d manifestation of could provide additional buoyancy sources to support the excess uplift at the margins of the plateau

joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations we present a new global whole mantle model of isotropic and radially anisotropic s velocity structure sglobe rani based on 43 000 000 surface wave and 420 000 body wave travel time measurements which is expanded in spherical harmonic basis functions up to degree 35 we incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure this is possible since we utilize short period group velocity data with a period range down to 16 s which are strongly sensitive to the crust the isotropic s velocity model shares common features with previous global s velocity models and shows excellent consistency with several high resolution upper mantle models our anisotropic model also agrees well with previous regional studies anomalous features in our anisotropic model are faster sv velocity anomalies along subduction zones at transition zone depths and faster sh velocity beneath slabs in the lower mantle the derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts which has been difficult to constrain due to poor access compared with continental crusts

local amplification of rayleigh waves in the continental united states observed on the usarray abstract we develop a method based on ratios of amplitudes measured at adjacent stations to determine local amplification of surface waves across an array of seismic stations we isolate the effects of local structure from those of the earthquake and propagation by systematic averaging of ratios corresponding to many sources we apply the method to data recorded on the usarray for the years 2006 2011 and determine amplification factors at each station of the array for rayleigh waves at periods between 35 s and 125 s local amplification factors are spatially coherent and display variations of 10 at a period of 125 s and greater variations at shorter periods maps of local amplification exhibit spatial correlation with topography and geologic structures in the western and central united states at long periods the observed amplification factors correlate well with predictions from a regional crust and mantle model of north america at short periods correlations are weaker suggesting that the local amplification factors can be useful for constraining shallow structure better

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

clays in natural and engineered barriers for radioactive waste confinement an introduction the concept of engineered geological disposal has been developed for the safe long term management of long lived radioactive waste this involves emplacement of radioactive waste in deep geological repositories that contain and isolate the waste and consequently protect humans and the environment the multiple barrier concept is the cornerstone of all proposed schemes for the geological disposal of radioactive waste based on the principle that uncertainties in performance can be minimized by conservatism in design the concept invokes a series of complementary barriers both engineered and natural geological between the waste and the surface environment each successive barrier represents an additional impediment to the movement of radionuclides regarding engineered barriers for lowand intermediate level waste the waste may be incorporated in a relatively stable and inert matrix such as cement bitumen lead alloy or polymer resin the choice varying depending on the waste management organization glass may be used in the case of certain high level reprocessing wastes owing to the very low leach rate of glass in groundwater vitrification is widely accepted to be one of the best methods of immobilizing the aqueous products from the reprocessing of spent fuel many waste containers will provide some form of physical barrier to groundwater however because of the relatively small volumes of waste involved spent fuel vitrified waste and other highly active wastes will be totally encapsulated in corrosion resistant metal canisters that are designed to prevent groundwater entry for extended time periods in excess of 100 000 years depending on the disposal concept engineered barriers may comprise the buffer backfill medium enclosing the waste containers the tunnel borehole liner and the backfill and high integrity seals placed in the repository access ways or emplacement boreholes the buffer backfill medium enclosing the waste will often also provide both a physical and a chemical barrier to radionuclide migration the functions of the engineered chemical barriers are to reduce the rate of corrosion of the waste containers and thus extend their life to limit the rate of hydraulic transport to limit the release of radionuclides from the waste form to the far field geosphere after container failure to limit the migration of radionuclides along the pathway provided by the access tunnels and shafts of a repository or the boreholes in the case of a deep borehole emplacement

different slip systems controlling crystallographic preferred orientation and intracrystalline deformation of amphibole in mylonites from the neyriz mantle diapir iran abstract a deformed layered gabbro and a mylonitic gabbro sample from the marginal shear zone of the neyriz mantle diapir in iran were analyzed using electron backscatter diffraction ebsd both samples have the common amphibole crystallographic preferred orientation cpo in which 100 lies perpendicular to foliation and parallel to lineation amphibole grains in the layered gabbro sample have little internal deformation whereas in the mylonitic gabbro sample the amphibole grains are strongly distorted and contain low angle grain boundaries there is a subtle change in cpo as a function of grain size in the mylonitic gabbro coarse grains porphyroclasts have a 100 cpo oriented with the main foliation reference frame whilst fine grains have a 100 cpo oriented with the c shear bands detailed analysis of porphyroclast distortions and subgrain boundary trace analysis suggests that hard slip systems most particularly 110 control intracrystalline deformation schmid factor analysis suggest that these slip systems are not involved in foliation formation but are linked kinematically to c shear bands it is unlikely that the slip systems that control intracrystalline deformation are important in cpo formation we interpret that subgrain rotation recrystallization lead to grain size reduction and the elongate recrystallized grains were rotated towards the c shear bands by grain boundary sliding this rigid body rotation possibly in combination with easy slip on 100 are considered the main cause of cpo formation amphibole zonation patterns in the layered gabbro sample suggest that oriented growth of amphibole may have contributed to cpo

upper mantle origin of the yellowstone hotspot fundamental features of the geology and tectonic setting of the northeast propagating yellowstone hotspot are not explained by a simple deep mantle plume hypothesis and within that framework must be attributed to coincidence or be explained by auxiliary hypotheses these features include the persistence of basaltic magmatism along the hotspot track the origin of the hotspot during a regional middle miocene tectonic reorganization a similar and coeval zone of northwestward magmatic propagation the occurrence of both zones of magmatic propagation along a first order tectonic boundary and control of the hotspot track by preexisting structures seismic imaging provides no evidence for and several contraindications of a vertically extensive plume like structure beneath yellowstone or a broad trailing plume head beneath the eastern snake river plain the high helium isotope ratios observed at yellowstone and other hotspots are commonly assumed to arise from the lower mantle but upper mantle processes can explain the observations the available evidence thus renders an upper mantle origin for the yellowstone system the preferred model there is no evidence that the system extends deeper than 200 km and some evidence that it does not a model whereby the yellowstone system reflects feedback between upper mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep mantle plume hypothesis

crust and upper mantle electrical conductivity beneath the yellowstone hotspot track combining long period magnetotelluric data from the spatially uniform earthscope usarray and higher resolution profiles we obtain a regional three dimensional electrical resistivity model in the snake river plain and yellowstone areas idaho and wyoming united states and provide new constraints on the large scale distribution of melt and fluids beneath the yellowstone hotspot track contrary to what would be expected from standard mantle plume models the electromagnetic data suggest that there is little or no melt in the lower crust and upper mantle directly beneath yellowstone caldera instead low mantle resistivities 10 m and below which we infer to result from 1 3 partial melt are found 40 80 km beneath the eastern snake river plain extending at least 200 km southwest of the caldera beneath the area of modern basaltic magmatism the reduced resistivities extend upward into the mid crust primarily around the edges of the snake river plain suggesting upward migration of melt and or fluid is concentrated in these areas the anomaly also shallows toward yellowstone where higher temperatures enhance permeability and allow melts to ascend into the crust the top of the conductive layer is at its shallowest in the upper crust directly beneath the modern yellowstone supervolcano

a kinematic model for the late cenozoic development of southern california crust and upper mantle a model is presented for the young and ongoing kinematic deformation of southern california crust and upper mantle that accounts for both the known late cenozoic geologic record and upper mantle structure in this model the high velocity upper mantle transverse ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere using typical scaling relations we estimate this anomaly to be 500 c colder and 1 more dense than average southern california mantle of the same depth we infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly the low velocity upper mantle anomaly beneath the salton trough region is attributed to high temperatures and 1 4 partial melt related to adiabatic decompression during mantle upwelling our model has crustal motions in the central transverse ranges region differing from mantle motions while the subcrustal lithosphere south of the transverse ranges moves toward and sinks beneath the transverse ranges the corresponding crust largely avoids convergence by adopting a more westerly direction of motion the differing velocity fields necessitates of a decoupling zone immediately south of the transverse ranges near the moho we interpret the upper mantle anomalies as results of small scale convection beneath southern california to which we attribute much of the recent tectonic activity in the region