stress perturbations associated with active faults penetrated by boreholes possible evidence for near complete stress drop and a new technique for stress magnitude measurement detailed studies of stress induced wellbore breakouts in wells drilled through active faults reveal stress field discontinuities that are apparently associated with recent fault movements these discontinuities are expressed as localized rotations in wellbore breakout orientation in the vicinity of the fault penetrated by the borehole this phenomenon is observed in a variety of tectonic environments and rock types utilizing cases where relatively complete knowledge of the horizontal principal stresses is available from in situ measurements we use three dimensional dislocation modeling to demonstrate that these discontinuities can be explained as the superposition of a reference stress state and a perturbation caused by movement on preexisting faults case studies from normal strike slip and reverse faulting stress states indicate that nearly complete stress drop is required to match the observed breakout orientation anomalies hydraulic fracturing data independently confirm the occurrence of near complete stress drop on some faults penetrated by drilling modeling of the observed interactions between breakouts and fractures can also be used to obtain information about the magnitude of in situ stress

state of stress in the southern tyrrhenian subduction zone from fault plane solutions summary in this paper we present revised locations and original focal mechanisms computed for intermediate and deep earthquakes that occurred within the southern tyrrhenian subduction zone between 1988 and 1994 in order to improve our knowledge of the state of stress for this compressional margin in particular we define the stress distribution within a large portion of the descending slab between 40 and about 450 km depth the seismicity distribution reveals a continuous 40 50 km thick slab that abruptly increases its dip from subhorizontal in the ionian sea to a constant 70 dip in the tyrrhenian we computed focal mechanisms for events with magnitudes ranging from 2 7 and 5 7 obtaining the distribution of p and t axes for many events for which centroid moment tensor cmt solutions are not available thus enabling the sampling of a larger depth range compared to previous studies we define three portions of the slab characterized by different distributions of p and t axes a general down dip compression is found between 165 and 370 km depth whereas in the upper part of the slab 40 165 km depth the fault plane solutions are strongly heterogeneous below 370 km the p axes of the few deep events located further to the north have a shallower dip and are not aligned with the 70 dipping slab possibly suggesting that they belong to a separated piece of subducted lithosphere there is a good correspondence between the depth range in which the p axes plunge closer to the slab dip 70 and the interval characterized by the highest seismic energy release 190 370 km

the 1997 umbria marche italy earthquake sequence a first look at the main shocks and aftershocks a long sequence of earthquakes six with magnitudes between 5 and 6 struck central italy starting on september 26 1997 causing severe damages and loss of human lives the seismogenic structure consists of a nw se elongated fault zone extending for about 40 km the focal mechanisms of the largest shocks reveal normal faulting with ne sw extension perpendicular to the trend of the apennines consistently with the quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions preliminary data on the main shocks and aftershocks show that extension in this region of the apennines is accomplished by normal faults dipping at low angle 40 to the southwest and confined in the upper 8 km of the crust these normal faults might have reactivated thrust planes of the pliocene compressional tectonics the aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km long fault segments adjacent and slightly offset from one another

the recent crustal stress field in central europe trajectories and finite element modeling the recent crustal stress field of central europe and especially of the adjoining areas to the east is presented in terms of the directions of maximum horizontal stress shmax the analysis is based on fault plane solutions in situ stress measurements geologic fault slip determinations and repeated precise geodetic triangulations a bending of the direction of shmax from the well known nw se direction in the western part of the study area to directions of ne sw to e w in the eastern part is shown first results on the recent crustal stress field of the study area were presented by grunthal and stromeyer 1986 who substantiated this tendency of bending in the central and eastern parts of the study area by few observations only therefore one aim of this paper was to compile observations on the areas with few data points generally these additional data confirm the previously established pattern in some areas especially in the pannonian basin the stress features became more complicated compared with those solely based on a few data points the second part of the paper presents steady state elastic finite element model calculations to provide some possible explanations of the observed stress orientations as a result of plate driving forces simulation of the north atlantic seafloor spreading and the northward directed motion of the african plate by appropriate boundary loads produces a pattern of shmax directions for the western part of the eurasian plate which is compatible with the broad scale observed stress directions subregional anomalies such as the fanlike stress pattern perpendicular to the arc of the western alps or the radial directions around the pannonian basin can be explained only when additional stress producing features are introduced overmodulating the regional field rigorous introduction of physically constrained model parameters for all these features was not feasible to date therefore the preliminary empirical model calculations presented in this paper are attempts to discuss which features have the most influence on the stress field they are in a regional scale the north atlantic seafloor spreading the northward motion of the african and the arabian plate and obviously subregionally increased stiffness of the apulian promontary as well as of the bohemian massif

estimates of stress directions by inversion of earthquake fault plane solutions in sicily summary the tectonic stress orientation is estimated in the lithosphere of northern sicily the southern tyrrhenian sea and southern calabria and in the wadati benioff zone below the tyrrhenian by inversion of fault plane solutions of earthquakes covering a magnitude range from 2 5 to 7 1 focal mechanisms of 97 earthquakes are taken from the literature after a critical evaluation of their data quality an average misfit of f 13 indicates that the set of all shallow 50 km earthquakes is generated by a heterogeneous stress field for three subsets based on regional and magnitude separation f was small enough 2 8 lfs5 9 to support the assumption of a homogeneous stress direction for an additional subset with f 7 4 such a condition is close to being fulfilled even though some heterogeneity appears to be present the number of earthquakes in these subsets ranged from nine to 22 and the uncertainties of the principal stress directions were generally of the order of 20 at the 90 per cent confidence level the earthquakes with a425 define a regional stress field with the greatest principal stress ul dipping at a shallow angle to the south in north eastern sicily and south western calabria the stress field estimated by earthquakes is extensional with o3 in a direction of wnw and a near vertical ol in agreement with the graben tectonics mapped geologically in this area in western sicily the c1 direction is oriented wnw but this result is judged less reliable than the others based on the broader confidence limits of the solution and the average misfit of 7 4 the earthquakes in the wadati benioff zone define ol dipping at about 70 to the nw subparallel to the zone with o2 horizontal and striking parallel to the zone

the dynamics of back arc extension an experimental approach to the opening of the tyrrhenian sea the e w opening tyrrhenian sea developed after the cretaceous palaeogene alpine collision nearly perpendicular to the motion of the african plate as a back arc of the adria ionian westward subduction three driving mechanisms have been proposed to explain the dynamic evolution of the tyrrhenian apennine system 1 the northward indentation of the african plate 2 the retreating subduction of the adria ionian lithosphere and 3 the gravitational collapse of the alpine post collisional wedge in order to define the relative contribution of each of these mechanisms in the neogene dynamic of the tyrrhenian apennine system we performed 3 d laboratory experiments in which we simulated a retreating subduction process in a compressional regime oriented perpendicularly to the direction of subduction in this framework we also tested the influence of the gravitational collapse of the overriding plate experiments were constructed using dry sand and silicone putties to simulate brittle upper crust and ductile lower crust upper mantle respectively these layers floated on a high density low viscosity glucose syrup which simulated the asthenosphere the main conclusion of our experiments is that large scale continental extension similar to that observed in the tyrrhenian area could be reproduced perpendicular to the shortening direction induced by the indentation of the african plate in this framework extensional processes are indeed possible if the trench retreat velocity is higher than the rate of shortening induced by the advancing african plate our experimental results indicate that this high trench retreat velocity could be explained by the coexistence of the gravitational collapse of the post alpine wedge with a slab pull process linked to the retreating subduction of the adria ionian plate while the first mechanism is predominant in the northern tyrrhenian area the second one seems to be important in the latest stage of extension and oceanic accretion of the southern tyrrhenian area

fault plane solutions and seismicity of the italian peninsula abstract a new fault plane solution map of the italian peninsula is presented in this paper the earthquakes analyzed are included in the period 1905 1980 with magnitudes ranging 4 7 75 earthquakes are located in the crust while 31 are related to the deep and intermediate zone of the calabrian arc the large seismic events of the italian peninsula are generally associated with normal faulting while strike slip motion is mostly related to small earthquakes located along lateral segments of the mountain chain the deep and intermediate earthquakes of the tyrrhenian sea indicate predominant down dip compression and strike slip motion at the boundaries of this benioff zone this last is interpreted as a remnant of a subduction zone active since oligocene extending to 500 km depth with a very small lateral size about 300 km the present tectonics of this benioff zone is strongly conditioned by the lateral bending more so than the gravitational sinking process the coexistence of thrust and normal faulting motion associated to the earthquakes within a few tens of kilometers of each other seems to be explained by the strong lateral inhomogeneities of the crustal rocks present in this region more so than to the depth of the seismogenetic zone and the nature of the faulting process

recent tectonic evolution and present stress in the northern apennines italy the present day stress field and its recent tectonic evolution in the northern apennines are reconstructed from borehole breakout analysis and focal mechanisms of crustal earthquakes and through the comparison with paleostress data we have considered 86 wells for breakout analysis with depths down to 6 7 km 125 fault plane solutions of crustal earthquakes with m 5 that occurred between 1988 and 1995 in the northern and central apennines and data of stronger earthquakes m 6 reported in other studies the tyrrhenian coastal region and the apenninic belt are characterized by shmin direction mainly trending ne sw with predominantly normal fault plane solutions along the outer front of the belt and the adriatic offshore shmin is oriented nw se and focal solutions are thrust or strike slip with maximum compression around ne sw conversely south of 43 n breakouts evidence an orthogonal direction of horizontal compression nw se following the southern apennine trend where a widespread ne sw extension was recognized by previous investigations comparing these results to the recent tectonic evolution inferred from structural geology we argue that the extension compression pair characteristic of the post tortonian evolution of the mountain belt has been migrating in time from late miocene to present only in the northern sector of the arc whereas the southern sector underwent a generalized extension at least since middle pleistocene the striking correspondence between the active compression front and the region with evidence of a remnant subducted slab suggests that the migrating extension compression pair has been controlled by progressive retreat of the slab

compressional velocity structure and anisotropy in the uppermost mantle beneath italy and surrounding regions travel times of about 39 000 pn arrivals recorded from regional earthquakes by the italian telemetered seismic network and by stations of nearby countries are inverted to image lateral variations of seismic velocity and anisotropy at subcrustal depth in italy and surrounding regions this method allows simultaneous imaging of variations of pn velocity and anisotropy as well as crustal thickness variations the po plain the adriatic sea and the ionian sea have normal to high pn velocities in contrast lower velocities 7 9 8 0 kin s are imaged in italy beneath the western alps the northern apennines and eastern sicily and nearby calabria as well as in northern albania and beneath the pannonian basin low pn velocities beneath the northern apennines correlate with present day extension and may have resulted from thermal anomalies in the uppermost mantle possibly due to delamination processes low velocities are consistent with the high attenuation zone inferred in the uppermost mantle beneath the internal apennine units and the tyrrhenian margin of the peninsula by mele et al 1996 1997 on the contrary low velocities beneath the western alps may be an apparent effect due to the abrupt thickening of the crustal roots pn velocity is anisotropic in the study area with a maximum amplitude of 0 2 km s the largest anisotropic velocity anomalies are observed along the major arc structures of italy i e the northern apennines and the calabrian arc indicating that these features are controlled by uppermost mantle processes the anisotropy anomaly along the calabrian arc extends as far as albania but ends abruptly north of this area suggesting that a lithospheric discontinuity is present along the northern albanian border

active stress map of italy we present a new map of the present day stress field in italy obtained from all the available data the map reports 200 horizontal stress directions inferred from 109 borehole breakout data 44 centroid moment tensor solutions 34 other focal mechanisms most of which are from polarity distributions seven stress inversions of microearthquake data two averages of t and p axes of earthquake focal mechanisms in zones of diffuse seismic activity and four fault slip data the integration of breakout data which yield horizontal stress directions with fault plane solutions which reflect the stress regime allows us to obtain an improved map of the present day stress in italy this stress field map can be used for a better comprehension of active tectonic processes for seismic hazard assessment and to foresee the behavior of faults recognized with other methods stress directions obtained from different data although relative to different depth intervals e g 0 7 km for breakouts and 0 20 km for most of the earthquakes and to different tectonic units are consistent since many regions in italy are characterized by an extensional stress regime we report the minimum horizontal stress shmin orientations the map shows that an extensional regime affects most of the apenninic belt conversely a compressional or transpressional regime characterizes the eastern alps the eastern side of the northern apennines and the southern tyrrhenian to northern sicily zone an abrupt change in stress directions marks the transition between northern and southern apennines suggesting that the two arcs are characterized by a different tectonic setting and recent evolution in this paper we report all the data analyzed to date with their geographic coordinates and average stress directions and we describe the main stress provinces in italy in the framework of the tectonic evolution of the region

stress tensor orientation derived from fault plane solutions in the southwestern alps gephart and forsyth s method has been applied to estimate stress orientations from earthquake fault plane solutions of the southwestern alps a region where the tectonic stress regime is known to be fairly complex fault plane solutions have been either taken from the literature or computed using data from local and regional networks data refer to seismic events of magnitude in the range 2 5 5 3 which occurred in the last decades at depths between 0 and 25 km two zones with a different stress orientation have been identified in the studied area 44 0 45 5 n 6 5 8 5 e the western zone corresponding to the crest of the alpine belt where a high dip maximum compressive stress is found and the eastern zone alps chain to po plain transition characterized by an almost horizontal e w 1 and a nearly vertical 3 hypocenters of earthquakes used for stress inversion lie in the depth ranges 0 15 km and 5 25 km in the western and eastern zones respectively the transition between the two stress domains is very sharp and this is also indicated by space distribution of earthquake individual misfits to the respective stress models the findings of the present study are a good match for tectonic models which assume e w compression derived from the adria europe interaction and producing 1 major thrusting processes in the eastern side of the chain and 2 secondary tensional effects at very shallow depth beneath the alpine belt crest western zone of the area studied in this work

the role of slab detachment processes in the opening of the western central mediterranean basins some geological and geophysical evidence abstract a review of the geological and geophysical data from the central and western mediterranean region and the present day upper mantle structure derived from tomographic studies are utilized in order to define the oligocene recent geodynamic evolution for the area in line with previous work we suggest that the miocene quaternary opening of the western and central mediterranean basins is the result of back arc extension due to the roll back toward the southeast of a northwestward subducting african slab in a geodynamic setting pinned between the alpine and betic collisional zones we find however that this general pattern is complicated by four different detachment events which occurred beneath the alps early oligocene the betic chain aquitanian northern africa langhian and the apennines late miocene pliocene we show that each of these events determines a major tectonic reorganization within the european plate

the importance of subduction for the modern stress field in the tyrrhenian area the tyrrhenian sea was formed by lithospheric extension in a context of continental convergence between africa and eurasia at present a compilation of strain and stress indicators in this region by rebai et al 1992 shows that an extensive regime is still predominant in the tyrrhenian sea while northern italy and southern sicily are undergoing compression this tectonic configuration is the result of two major contributions which act on a very heterogeneous lithosphere the africa eurasia collision on the one hand and the subduction along the calabrian arc on the other the latter is expected to create a trench suction force on the overriding tyrrhenian plate the objective of this paper is to investigate the relative importance of these contributions using a numerical model developed by p bird 1989 based on an improved version of the thin plate approach which models lithospheric deformation in an horizontal plane the results of this model demonstrate the necessity of considering the forces related to subduction to understand the modern stress and strain field an extrusion mechanism alone cannot explain present day observations the salient features of the current tectonic regime are reproduced by the model which however predicts more limited extension than expected further work will be needed to find a better tuning of the parameters and ascertain the quality of available data

passive seismology and deep structure in central italy in the last decade temporary teleseismic transects have become a powerful tool for investigating the crustal and upper mantle structure in order to gain a clearer picture of the lithosphere asthenosphere structure in peninsular italy between 1994 and 1996 we have deployed three teleseismic transects in northern central and southern apennines in the framework of the project geo modap european community contract ev5v ct94 0464 some hundreds of teleseisms were recorded at each deployment which lasted between 3 and 4 months although many analyses are still in progress the availability of this high quality data allowed us to refine tomographic images of the lithosphere asthenosphere structure with an improved resolution in the northern and central apennines and to study the deformation of the upper mantle looking at seismic anisotropy through shear wave splitting analysis also a study of the depth and geometry of the moho through the receiver function technique is in progress tomographic results from the northernmost 1994 and the central 1995 teleseismic experiments confirm that a high velocity anomaly hva does exist in the upper 200 250 km and is confined to the northern apenninic arc this hva already interpreted as a fragment of subducted lithosphere is better defined by the new temporary data compared to previous works based only on data from permanent stations no clear high velocity anomalies are detected in the upper 250 km below the central apennines suggesting either a slab window due to a detachment below southern peninsular italy or a thinner perhaps continental slab of adriatic lithosphere not detectable by standard tomography we found clear evidence of seismic anisotropy in the uppermost mantle related to the main tectonic processes which affected the studied regions either ne sw compressional deformation of the lithosphere beneath the mountain belt or arc parallel asthenospheric flow both giving nw se fast polarization direction and successive extensional deformation e w trending in the back arc basin of northern tyrrhenian and tuscany preliminary results of receiver function studies in the northern apennines show that the moho depth is well defined in the tyrrhenian and adriatic regions while its geometry underneath the mountain belt is not yet well constrained due to the observed high complexity

surface rupture of the 1857 southern italian earthquake in october 1862 robert mallet published an extensive study of a large earthquake that occurred on 16 december 1857 150 km south east of naples this event is one of the most destructive ever recorded in italy although continental earthquakes of similar size are almost invariably associated with surface faulting mallet did not identify any surface rupture and subsequent workers have also failed to find surface traces using satellite images and aerial photographs as a guide we examined two prominent quaternary normal fault systems in the field flanking the val d agri and the val di diano both trend nw se and dip to the sw the distribution of damage suggests that one or both moved in 1857 on one fault segment in val d agri we found a 2 5 m high scarp whose youthfulness and height suggest it is due to the 1857 earthquake comparison of the observed slip with rates derived from geomorphic evidence suggests a recurrence time for 1857 type events of about 3000 years while the new data we provide are consistent with the event having a magnitude of ms 7 0 there are also grounds for supposing it might have been bigger

a proposal for the kinematic modelling of w dipping subductions possible applications to the tyrrhenian apennines system in w dipping subduction zones there is a general eastward progression of the back arc basin accretionary wedge foredeep complex with the forward progression early stages of the complex are revealed by slices of upper crust and sedimentary cover abandoned to the west left floating above a new section of mantle a major shear zone should form at the new moho separating upper crust slices of earlier accretionary stages and the eastward flowing mantle the mantle wedging at the top of the subduction plane could be responsible for the uplift of the central parts of the belt the retreating of the subduction hinge is interpreted as due to the push generated by the eastward mantle flow detected in the hot spot reference frame the foredeep depth is mainly a function of the radius of curvature of the subduction hinge the frontal wedge is constructed from the stacking of the upper layers of the subducting plate and the syntectonic clastics that fill the foredeep which are progressively involved in thrusting and later by extension in order to preserve volume balance the lithosphere of the eastern plate before subduction has to be the same size as that which has been subducted due to the longer length of the arc with respect to the original length of the linear margin between the two converging plates laterally stretched subducted lithosphere is predicted at depth w dipping subductions usually have a short life probably due to their inherent capability to produce new lateral heterogeneities of the lithosphere the thin back arc which are a key factor in controlling and generating new subductions both e and w dipping this model is applied to the apennines tyrrhenian sea system

modern tectonic regime in the tyrrhenian area observations and models summary the rapid opening since 10 ma of the tyrrhenian sea in a context of convergence between eurasia and africa remains a puzzling geodynamic problem one of the most plausible scenarios proposed by malinverno ryan 1986 attributes extension to southward and eastward migration of the trench resulting from subduction of the african adriatic lithosphere below the european lithosphere at present subduction only continues along the calabrian arc a compilation of recent strain and stress indicators by rebai philip taboada 1992 indicates that an extensional regime still prevails in the tyrrhenian sea whereas compression dominates north and south of this area in an earlier paper bassi sabadini 1994 used a numerical model to demonstrate the necessity of a trench suction force acting along the subduction trench to produce the present extension in the tyrrhenian domain this study however did not discuss in detail the tectonic regime in the different areas composing the domain nor the match between the observations and the model in this paper we first review the constraints provided by the observations compiled by rebaiet al 1992 and by more recent measurements we then investigate how much of the observed tectonic regime can be understood in terms of plate interaction forces using thin plate models bird 1989 the geometry and heterogeneous lithospheric properties of the models are constrained by observations the best constrained boundary conditions are held constant the remaining are regarded as parameters that we tune in order to fit the observations our results indicate that the present day tectonic regime in the tyrrhenian area can be explained as a consequence of trench retreat associated with the subduction of ionian oceanic lithosphere under the calabrian arc combined with a rotation of the apennines possibly related to a rotation of the adriatic microplate this study supports therefore the model of malinverno ryan 1986 and suggests moreover that rotation of the apennines is an ongoing process which is supported by seismological and geodetic data but remains to be understood in terms of geodynamics a discrepancy between observations and models is observed in western sicily which can be attributed to modelling limitations the weak extensional activity of the corsica sardinia block however is not explained by this model

nitrogen diffusion in silicate minerals with implications for nitrogen transport and cycling in the lithosphere abstract diffusion laws for nitrogen in orthoclase quartz olivine and clinopyroxene were determined using a combination of experimental strategies that included in diffusion from c h o n vapor in diffusion from molten nh4cl and ion implantation followed by heating to mobilize n most experiments were conducted at pressures near 1 bar but seven experiments were also run in a piston cylinder device at ptotal 1 gpa the c h o n source experiments were wet h2o present those on ion implanted samples were dry all n sources were highly enriched in 15n to avoid spurious 14n signals resulting from surface contamination use of 15n also enabled depth profiling by nuclear reaction analysis nra using the 15n p 12c reaction which has a detection limit of 10 ppm nitrogen profiles in three samples were characterized by both nra and sims with similar results a total of 17 to 28 diffusion coefficient measurements were made on each of the four minerals 83 in all yielding results that conform for each mineral to an arrhenius relation of the form d d0exp ea rt the following values for the pre exponential constant d0 and activation energy ea were obtained mineral log d0 m2 s ea kj mol t range c orthoclase 13 9 0 4 95 4 6 6 500 900 quartz 12 4 0 3 147 3 5 7 550 1150 olivine 14 2 0 3 135 2 6 6 650 1400 clinopyroxene 14 1 0 3 135 6 7 4 750 1300 the various experimental strategies yield generally indistinguishable results at a given temperature and no significant effects of total pressure or h2o are discernible in the overall diffusion data set experiments that involved in diffusion from external n sources provide qualitative insight into the compatibility of n in the structures of the four minerals nitrogen concentrations attain the highest levels in orthoclase 100 8000 ppm atomic with quartz olivine and clinopyroxene falling in the tens to hundreds of ppm range the new diffusion laws enable us to model the diffusive release of n acquired at depth in the crust during fluid absent exhumation and cooling in general orthoclase is the least retentive of the minerals investigated and will lose most of its n during cooling from 600 c at tectonically typical rates olivine and clinopyroxene are the most retentive of n quartz is intermediate in retentivity between the mafic minerals and feldspar but in all cases the extent of n exchange with the surroundings depends critically on the specific t t path of the mineral of interest in subduction settings where n bearing fluid is released to the mantle wedge diffusion is sufficiently fast to homogenize individual mafic mineral grains with respect to n over geologically plausible time scales but equilibration on the outcrop or regional scale via volume diffusion is precluded by our data

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

a reconnaissance laser raman and fourier transform infrared survey of silcretes from the kalahari desert botswana this paper documents the rst application of laser raman and fourier transform infrared vibrational spectroscopic techniques raman ft ir to the analysis of silcrete analyses are undertaken at a reconnaissance level on bulk silcrete samples of varying ages drawn from a range of geomorphological settings across the kalahari desert of botswana results suggest that raman ft ir combined with inspection of petrographic thin sections provides an effective tool for establishing the range and type of silica polymorphs present within samples analyses of raman and mid infrared spectra indicate that the majority of silcretes are dominated by quartz with subordinate moganite there is however considerable inter sample variability ranging from silcretes consisting of almost pure quartz to those containing complex admixtures of disordered microcrystalline or non crystalline silica in association with quartz and clays taken as a whole the results are supportive of a similar environment of formation for all samples with development taking place under broadly semi arid conditions raman ft ir is also shown to have a number of bene ts over other conventional techniques for silcrete analysis such as x ray diffraction firstly inspection of the oh region of the infrared spectrum of bulk samples is shown to have potential for establishing the provenance of quartz clasts within silcrete host sediments through the identi cation of high temperature conditions of crystal growth in detrital quartz grains raman and mid infrared analyses also permit clay identi cation within silcrete samples where such minerals are not readily resolvable in thin section the technique further allows the relative maturity of silcrete cements to be established particularly for recently formed silcretes these are found typically to contain less mature silica polymorphs and incorporate larger quantities of molecular water than quartz dominated silcretes comparisons of the range of silica polymorphs identi ed using raman ft ir with those established by petrographic investigation can also reveal evidence of silica paragenesis this is the case in two stratigraphically older samples from the kalahari margin in which less stable silica polymorphs appear to have transformed over time to chalcedony and microquartz whilst the whole rock approach used in this survey generates some problems of interpretation the overall results suggest that the use of micro raman and micro ft ir techniques should allow even more precise information about the environments of formation and developmental histories of silcrete pro les to be produced in the future copyright 2004 john wiley sons ltd

modern tectonic stress field in the mediterranean region evidence for variation in stress directions at different scales summary a compilation of more than one thousand stress indicators which include in situ stress measurements focal mechanisms microtectonic and other geological data allowed us to reconstruct the modern stress field in the mediterranean region and the surrounding area average stress directions at different scales have been reconstructed by means of a linear interpolation method this method takes into account the distribution scale and quality of stress data the results of the interpolation at plate scale allow us to recognize slightly deformed regions such as the northwestern european platform where average maximum horizontal stress direction is oriented roughly nnw sse subparallel to absolute and relative plate velocity directions other regions such as the caucasus alps and pyrenees where recent tectonic deformation and seismicity are present display important variations of stress directions the reconstruction of the average stress directions at different scales within the french alps pointed out that the average stress field pattern may vary from one scale to another nevertheless variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale

paleoseismicity of the ovindoli pezza fault central apennines italy a history including a large previously unrecorded earthquake in the middle ages 860 1300 a d geomorphic and trenching investigations along the ovindoli pezza fault show that this normal fault is one of the major seismogenic faults in the central apennines we found clear geological evidence for three holocene surface faulting earthquakes on this fault the most recent earthquake occurred in the middle ages between 860 and 1300 a d the penultimate occurred about 1900 b c or shortly after and the oldest probably occurred between 3300 and 5000 b c although the most recent surface faulting earthquake occurred during historical time no evidence for it have been found in the historical record slip per event ranges between 2 and 3 m and the length of the rupture is at least 12 20 km suggesting m 6 5 7 0 for the paleoearthquakes the dip slip rate determined from trenching is 0 7 1 2 mm yr and is consistent with the long term slip rate of 0 9 2 5 mm yr lower values preferred obtained from displaced geomorphic features trench data combined with long term slip rate estimates suggest the recurrence interval is longer than a millennium and possibly as long as 3300 years the time elapsed since the most recent earthquake is 700 1130 years the seismic behavior of the ovindoli pezza fault is consistent with other well known seismogenic faults of the central and southern apennines the lack of mention or mislocation of the most recent event on the ovindoli pezza fault in the historical record of the past two millennia should be attributed mainly to the unsettled cultural and social conditions and poor economic state that characterized the middle ages especially in thinly populated regions such as the central apennines this example highlights an intrinsic limit of the historical data and raises the possibility that other regions considered seismically quiet on the basis of a long historical record may in reality have had large earthquakes that were not recorded

moment tensor analysis of the central italy earthquake sequence of september october 1997 the larger earthquakes in the umbria marche central italy seismic sequence of september october 1997 are analyzed using long period seismograms from the mediterranean seismographic network mednet and additional data from the global seismographic network gsn we modify the harvard centroid moment tensor cmt algorithm to allow moment tensor inversion of long period waveforms primarily rayleigh and love waves for small earthquakes at local to regional distances 5 5 in the sequence moment tensors have previously been determined using teleseismic waveforms and standard methods of analysis our results agree well with those of earlier studies we determine additional moment tensors for the largest foreshock and 10 aftershocks with mw 4 2 the earthquakes are characterized by normal faulting mechanisms with a ne sw tension axis and the presumed fault plane dips towards the sw only one of the fourteen events studied has a different faulting geometry indicating instead right lateral strike slip faulting on a plane oriented approximately e w or left lateral faulting on a plane oriented n s the september 26 mainshock 09 40 ut accounts for only approximately 50 of the total moment release in the sequence

interplay between subduction and continental convergence a three dimensional dynamic model for the central mediterranean summary the mediterranean region is attracting considerable attention due to the complexities of its tectonic setting which is considered worldwide a unique natural laboratory for studying the occurrence of extensional tectonics in a general context of continental convergence the tyrrhenian apennine system is controlled by the west dipping subduction of the adria ionian lithosphere and by the near north south convergence between the african and eurasian plates we provide the first 3 d dynamic model of the central mediterranean that quantifies the effects of subduction and convergence on surface deformation in simplified geometry the axis of the model extends from sicily to the alps along the subduction hinge line a convergence rate of 1 cm yr 1parallel to the subduction hinge has been applied to the tyrrhenian block in agreement with global plate motion models density contrasts within the slab cause the gravitational sinking and roll back of the slab in the southern tyrrhenian domain modelling results show a gradual decrease of hinge retreat from south to north with values ranging between 8 and 2 mm yr 1 indicating that the arculate geometry of the hinge line along the italian peninsula is ultimately controlled by the interplay between subduction and convergence the pattern of vertical velocity along directions perpendicular to the hinge with subsidence in the foredeeps and uplift at the eastern border of the tyrrhenian domain is maintained along the whole italian peninsula with higher values in the southern areas

tomographic constraints on the geodynamic evolution of the italian region in this paper we present p wave tomographic images of the mantle beneath italy obtained by inverting 6000 teleseismic p and pkp wave arrival times accurately repicked recorded in the time period 1988 1994 by the stations of the national seismic network of the istituto nazionale di geofisica we pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set the data were inverted with the well established aki christofferson husebye tomographic technique different reference models and residuals computation have been tried to verify the stability of the results the high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the alps and the apennines looking for their lateral and vertical continuity down to 800 km depth the main finding of this study is a continuous high velocity body located between 250 and 670 km depth beneath the entire apenninic system dipping toward the tyrrhenian area which continues upward segmented in two main anomalies in the northern apenninic and the calabrian arcs we interpret this high velocity feature as the subducted oceanic lithosphere between the eurasian and african plates dipping down to the upper lower mantle boundary beneath the tyrrhenian sea the retrieved images of the lithosphere subducting beneath apennines are reliable in terms of thickness about 80 90 km and p wave velocity contrast 2 4 higher than the normal mantle furthermore our tomographic images which focus on the deep geometry and continuity of the velocity structures provide new keys to understanding the geodynamic evolution of the italian region the segmentation of the high velocity slab upward suggests a complex evolution of the arc trench system and the initially continuous subduction of the ionian adriatic plate progressively developed in subordinate arcs probably due to lateral heterogeneity of the subducting lithosphere

global cmt analysis of moderate earthquakes mw 4 5 using intermediate period surface waves a new method for calculating centroid moment tensor cmt solutions for moderate sized earthquakes is presented and tested in the new algorithm which is a modification to the standard harvard cmt method intermediate period surface waves are included in the inversion for source parameters the dispersion of teleseismic surface waves in the period range 40 to 150 sec is predicted by interpolation of recent global phase velocity maps the method is tested on a set of 20 moderate and large 5 2 mw 7 2 earthquakes with known mechanisms the retrieved mechanisms are similar to the previous results even when noise is added to the seismograms to simulate earthquakes of a smaller magnitude mw 4 5 the inversion algorithm is applied to the 1993 klamath falls mainshock and aftershock sequence and the results compare well with earlier results a catalog of mw 4 5 focal mechanisms for greece and surrounding areas for the first six months of 1995 is derived using the new method of analysis

subcrustal earthquakes in the northern apennines italy evidence for a still active subduction previously unreported subcrustal earthquakes have been located up to 90 km depth beneath the northern apennines italy these earthquakes occur beneath a zone of abundant upper crustal seismicity mostly confined to the upper 20 km of the crust although there are relatively few well located subcrustal earthquakes there appears to be a general southwestward deepening of the hypocenters from the adriatic to the tyrrhenian sea consistent with other geophysical data which suggest that the adriatic lithosphere is presently subducting beneath the northern apennines

constraints on the seismotectonics of the central mediterranean from very long baseline interferometry summary very long baseline interferometry vlbi determined site velocities from seven stations in western europe reveal a stable continental platform north of the alps deformations between sweden germany and spain can not exceed 2 mm yr south of the alps significant motions are occurring with respect to stable europe two sites east of the apennine mountains on peninsular italy have north easttrending velocities which increase from 2 mm yr l in the north at medicina to 6 mm yr l in the south at matera in contrast the vlbi site in the south eastern corner of sicily is moving 7 mm yr i to the north north west these velocities are largely explained if southern sicily is attached to a north westerly moving african plate and the eastern portion of the italian peninsula forms part of a hypothesized adriatic sea crustal block which is rotating counter clockwise with respect to europe about a pole near the alps such an explanation is consistent with the styles of the larger historical earthquakes of the region which show ne sw extension across the apennines north south convergence across the alps and ne sw compression in coastal yugoslavia the adria plate model generates 1 5 2 2 x 10l8 nm yr l of potential earthquake moment along the northern and central apennines historical records suggest that 30 60 per cent of this moment is released seismically based on a direct strain rate measurement recurrence intervals for italian earthquakes south of medicina are estimated to be 12 46 yr for m6 5 quakes and 35 143 years for m7 0 quakes

a variable slip fault model for the 1908 messina straits italy earthquake by inversion of levelling data summary a variable slip fault model is presented for the 1908 december 28 messina italy earthquake it has been obtained by inversion of levelling data using a gradient method fault location and mechanism are consistent with an approximately n s orientation and normal faulting on a low angle plane dipping 39 with about 45 of strike slip component the main difference from a homogeneous slip model is represented by a higher slip area up to 3 m located close to the city of reggio calabria it is interpretable as a major asperity which is related to the main fracture episode this is in agreement with some macroseismic evidence and the same structure could be responsible for the nearby recent seismic activity the area which effectively slipped is all confined within the messina straits this result differs from the homogeneous slip model recently proposed by capuano et al 1988

regional patterns of tectonic stress in europe nearly 1500 stress orientation determinations are now available for europe the data come from earthquake focal mechanisms overcoring measurements well bore breakouts hydraulic fracturing measurements and young fault slip studies and sample the stress field from the surface to seismogenic depths three distinct regional patterns of maximum compressive horizontal stress shmax orientation can be defined from these data a consistent nw to nnw shmax stress orientation in western europe a wnw ese shmax orientation in scandinavia similar to western europe but with a larger variability of shmax orientations and a consistent e w shmax orientation and n s extension in the aegean sea and western anatolia the different stress fields can be attributed to plate driving forces acting on the boundaries of the eurasian plate locally modified by lithospheric properties in different regions on average the orientation of maximum stress in western europe is subparallel to the direction of relative plate motion between africa and europe and is rotated 17 clockwise from the direction of absolute plate motion the uniformly oriented stress field in western europe coincides with thin to medium lithospheric thickness approximately 50 90 km and high heat flow values 80 m w m2 in western europe a predominance of strike slip focal mechanisms implies that the intermediate principal stress is vertical the more irregular horizontal stress orientations in scandinavia coincide with thick continental lithosphere 110 170 km and low heat flow 50 m w m2 the cold thick lithosphere in this region may result in lower mean stresses associated with far field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography the stress field of the aegean sea and western anatolia is consistent with n s extension in a back arc setting behind the hellenic trench subduction zone the stress field is influenced in places by regional geologic structures e g in the western alps where shmax directions show a slight tendency toward a radial stress pattern not all major geologic structures however appear to affect the shmax orientation e g in the vicinity of the rhine rift system horizontal stress orientations are continuous

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km