paleoseismology along the 1980 surface rupture of the irpinia fault implications for earthquake recurrence in the southern apennines italy the irpinia fault was the source of the ms 6 9 1980 irpinia earthquake and produced the first unequivocal historical surface faulting in italy trenching of the 1980 fault scarp at piano di pecore a flat intermontane basin about 5 km south of the 1980 instrumental epicenter provides the first data on earthquake recurrence intervals slip per event and slip rate on a major normal fault in the southern apennines fault zone the trenches exposed evidence of four pre 1980 paleoearthquakes that occurred during the past 8600 years a best estimate average recurrence interval is 2150 years although the time interval between individual events varies by as much as a factor of 2 each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation which suggests that the 1980 event is characteristic for the irpinia fault slip per event values average 61 cm the net vertical displacement of 2 12 2 36 m since 8600 cal year b p observed in the trenches gives a vertical slip rate of 0 25 0 35 mm yr a dip slip rate of 0 29 0 40 mm yr and an extension rate of 0 14 0 20 mm yr although fault behavior data are only available for the irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern apennines they suggest that 1 fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude m7 earthquakes and 2 the holocene rate of extension across the apennines is 1 mm yr the 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation

tomographic images of the el asnam fault zone and the evolution of a seismogenic thrust related fold the p wave velocity structure of the upper 9 km of the el asnam algeria fault zone the site of the ms7 3 earthquake october 10 1980 is obtained by inverting arrival times from 310 selected aftershocks computed crustal structure and relocation of aftershocks yield new insight on the structural geology of the area and help to clarify the evolution of fault related folding and the complexity of the rupture episodes during the el asnam earthquake the correlation between tomograms and geologic structures observed at the surface and inferred at depth delineates local heterogeneities of the upper crust there is evidence for distinct large scale structural trends consisting of two main thrusts with antithetic reverse faults seismic tomography provides reasonable constraints on the el asnam fault related fold geometry pointing out a complex preexisting structure along which the rupture evolution is controlled by the heterogeneous crust we propose that the main shock originated along an inverted normal fault bordering the pre neogene basement lateral lithologic heterogeneities obtained by seismic tomography reflect the fault segmentation and geometrical discontinuities along strike in agreement with the tectonic pattern

seismic potential revealed by surface folding 1983 coalinga california earthquake the 2 may 1983 coalinga california earthquake magnitude 6 5 failed to rupture through surface deposits and instead elastically folded the top few kilometers of the crust the subsurface rate of fault slip and the earthquake repeat time are estimated from seismic geodetic and geologic data three larger earthquakes up to magnitude 7 5 during the past 20 years are also shown to have struck on reverse faults concealed beneath active folds

the effect of wind shear and curvature on the gravity wave drag produced by a ridge the analytical model proposed by teixeira miranda and valente is modified to calculate the gravity wave drag exerted by a stratified flow over a 2d mountain ridge the drag is found to be more strongly affected by the vertical variation of the background velocity than for an axisymmetric mountain in the hydrostatic approximation the corrections to the drag due to this effect do not depend on the detailed shape of the ridge as long as this is exactly 2d besides the drag all the perturbed quantities of the flow at the surface including the pressure may be calculated analytically

inversion for slip distribution using teleseismic p waveforms north palm springs borah peak and michoacan earthquakes we have inverted the teleseismic p waveforms recorded by stations of the global digital seismograph network for the 8 july 1986 north palm springs california the 28 october 1983 borah peak idaho and the 19 september 1985 michoacan mexico earthquakes to recover the distribution of slip on each of the faults using a point by point inversion method with smoothing and positivity constraints in the inversion procedure a fault plane with fixed strike and dip is placed in the region of the earthquake hypocenter and divided into a large number of subfaults rupture is assumed to propagate at a constant velocity away from the hypocenter and synthetic ground motions for pure strike slip and dip slip dislocations are calculated at the teleseismic stations for each subfault the observed seismograms are then inverted to obtain the distribution of strike slip and dip slip displacement for the earthquake results of the inversion indicate that the global digital seismograph network data are useful for deriving fault dislocation models for moderate to large events however a wide range of frequencies which includes periods shorter than those within the passband of the long period global digital seismograph network instruments is necessary to infer the distribution of slip on the earthquake fault although the long period waveforms define the size dimensions and seismic moment of the earthquake data at shorter periods provide additional constraints on the variation of slip on the fault dislocation models obtained for all three earthquakes are consistent with a heterogeneous rupture process where failure is controlled largely by the size and location of high strength asperity regions

three dimensional properties of andes mountain waves observed by satellite a case study the southern andes region has been clearly identified in previous satellite and balloon observations and in global models as a hot spot of small scale gravity wave activity with monthly mean momentum fluxes exceeding 10 times background values in fall winter and spring seasons this makes this region a focus of interest for global circulation and climate studies we analyze a case study on 8 may 2006 combining observations from the atmospheric infrared sounder instrument on the aqua satellite and the high resolution dynamics limb sounder instrument of the aura satellite to form a three dimensional picture of the wave field the observations show a widespread wave pattern over the southern andes extending eastward over the south atlantic simulations with the weather research forecasting model clearly identify the waves as orographic in origin but the observed wave pattern is far from the simple two dimensional wave field forced by steady flow over a mountain ridge the morphology of the pattern is consistent with three dimensional linear theoretical calculations of downstream propagation and latitudinal focusing of mountain waves into the stratospheric jet the observations confirm the importance of this process in the stratosphere and we find the process also occurring in the global analysis and forecasts from the european centre for medium range weather forecasting our analysis evaluates some strengths and weaknesses of current orographic wave drag parameterizations in global models and the relevance of parameterization assumptions in global models with high resolution copyright 2011 by the american geophysical union

three dimensional vp and vp vs models of the upper crust in the friuli area northeastern italy summary 3 d images of p velocity and p t os velocity ratio have been produced for the upper crust of the friuli area northeastern italy using local earthquake tomography the data consist of 2565 p and 930 s arrival times of high quality the best fitting v p and v p v s 1 d models were computed before the 3 d inversion v p was measured on two rock samples representative of the investigated upper layers of the friuli crust the tomographic v p model was used for modelling the gravity anomalies by converting the velocity values into densities along three vertical cross sections the computed gravity anomalies were optimized with respect to the observed gravity anomalies the crust investigated is characterized by sharp lateral and deep v p and v p v s anomalies that are associated with the complex geological structure high v p v s values are associated with highly fractured zones related to the main faulting pattern the relocated seismicity is generally associated with sharp variations in the v p v s anomalies the v p images show a high velocity body below 6 km depth in the central part of the friuli area marked also by strong v p v s heterogeneities and this is interpreted as a tectonic wedge comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the friuli area

analysis of scale exaggeration on seismic profiles 1 seismic profiles have become an important medium for illustrating interpreted structural geometry and style in areas of steep dip and complex geologic structure however these profiles often yield distorted structural views due to vertical or horizontal exaggeration geological misinterpretation may result if the effects of this exaggeration are not recognized seismic profiles from the central rocky mountain basins illustrate the geometrical distortion produced by significant scale exaggeration to assist in analysis of these profiles three nomograms have been constructed the nomograms show graphically the relationships between exaggeration factors exaggerated dips unexaggerated dips and correction factors which are derived from the ratio of exaggerated orthogonal thicknesses to true orthogonal thicknesses with knowledge of the horizontal scale the nomograms also can be used to estimate average velocities on exaggerated profiles in areas of steep dip and uniform stratigraphic thicknesses

a three dimensional mountain wave imaged in satellite radiance throughout the stratosphere evidence of the effects of directional wind shear swath scanned thermal radiances from the advanced microwave sounding units amsu a on the noaa and aqua satellites are used to image the horizontal temperature structure of a long wavelength mountain wave that formed over southern scandinavia on 14 january 2003 data from all six stratospheric channels show this wave propagating through the full depth of the stratosphere in channels 9 11 altitudes 20 90 hpa the imaged wave has a phase structure broadly consistent with a stationary wave radiated by the southeastward flow over and above the quasi elliptical terrain of southern norway channel 12 radiances at 10 hpa however show a remarkable abrupt change in imaged wave structure the horizontal wavelength contracts phase lines rotate anticlockwise by 30 40 and peak activity migrates to the south to lie over denmark and northern germany similar structure persists in channels 13 and 14 at 2 5 hpa with progessively increasing radiance amplitudes these features are stable over the 10 hours of amsu a measurements from five separate overpasses and are validated against independent radiances acquired by the atmospheric infrared sounder airs and retrieved airs amsu a temperature profiles from the aqua overpass this change at the channel 11 12 interface coincides with an onset of anticlockwise rotation backing and intensification of background stratospheric winds with height fourier ray and spatial ray modelling incorporating these directionally sheared winds and simplified orographic forcing reproduce the salient features of the observations but only after wave induced temperature perturbations have been converted to channel radiances using a forward model the differential visibility of various components of this three dimensional mountain wave to the amsu a channel weighting functions has a first order impact on the observations at heights above 10 hpa once that is factored in the combined observations and modelling provide direct experimental support for the shutts model s predictions of how backing wind vectors affect the vertical evolution of three dimensional mountain waves implications of these observations for orographic gravity wave drag parametrization are briefly discussed copyright 2007 royal meteorological society

on the origins of mesospheric gravity waves 1 using hourly data from a three year simulation based on a gravity wave resolving general circulation model we have first inferred a global view of gravity wave sources and propagation affecting significantly the momentum balance in the mesosphere the meridional cross section of momentum fluxes suggests that there are a few dominant propagation paths originating from the subtropics in summer and the middle to high latitudes in winter these gravity waves are focused into the mesospheric jets in their respective seasons acting effectively to decelerate the jets the difference in the source latitudes likely contributes to the hemispheric asymmetries of the jets the horizontal distribution of the momentum fluxes indicates that the dominant sources are steep mountains and tropospheric westerly jets in winter and vigorous monsoon convection in summer the monsoon regions are the most important window to the middle atmosphere in summer because of the easterlies associated with the monsoon circulation

local source tomography applications to italian areas we describe three study cases in which we used local earthquake and shot travel time residuals to investigate the upper crustal structure of three regions in italy we inverted for velocity and hypocentral parameters using a damped least squares technique making use of parameter velocity and hypocentre separation the three studied regions are in italy namely a the vulsinian volcanic complex latium where there is an active geothermal field b the irpinia campania lucania region in the southern appennines site of the strongest earthquake in italy for at least 65 years november 1980 ms 6 9 c the friuli region in northeastern italy where another strong earthquake ms 6 5 occurred in 1976 the computed shallow velocity models generally correspond with surface geological structures for the three studied areas the main results are respectively a a low velocity anomaly detected in the centre of the vulsinian volcanic complex at a depth of 5 8 km probably due to anomalous heat flow caused by a partially molten or cooling intrusive body b the identification of a deep 10 km discontinuity in the crust beneath the irpinia fault zone approximately corresponding with the fault extension at depth c the detection of a wedge of high velocity high density material at seismogenic depth 5 10 km beneath the friuli region interpreted as a buried thrust of the metamorphic basement

identification of wrench faults using subsurface structural data criteria and pitfalls 1 many structures imaged with reflection seismic can resemble a wrench fault system because of the great structural diversity of strike slip faults and the wide range of contractional and extensional features associated with these faults because of these resemblances numerous faults are misidentified as wrench faults by workers using subsurface structural data credible identification requires combinations of profile and map criteria 1 narrow long straight throughgoing solitary master fault or zone of deformation 2 steep to moderate dip of the master fault at depth 3 offset of the top of basement 4 changes in relative upthrown side separation sense and or fault dip direction at depth or along the strike of the master fault 5 narrow fault slices within the master fault zone that steepen and join at depth negative and positive flower structures 6 different separation sense and orientation of relative upthrown side at these fault slices and 7 coeval en echelon flanking structures the identification should be confirmed with other criteria that refute alternative interpretations the main pitfalls to a reliable identification are not considering alternative structural styles and the misinterpretation of features or patterns that superficially resemble wrench systems the profile geometries of basement involved reverse or normal faults particularly those with structural inversion can resemble the single strand of convergent or divergent wrench faults profile geometries of flower structures can be mimicked by narrow contractional horsts or narrow extensional grabens complexly faulted anticlines or synclines splay faults fault zones intruded by salt or shale and overprinted structures pitfalls in interpreting map relationships are possible when structural patterns combine several tectonic influences are controlled by rock anisotropy or include elements hat are not coeval and if the patterns characteristic of other styles are misinterpreted many erroneous identifications of wrench faults can be eliminated by demonstrating that the investigated fault or feature lacks a straight trace lateral length or throughgoing continuity

aftershocks of the campania lucania italy earthquake of 23 november 1980 abstract the result of a study of 1130 aftershocks of the 1980 campania lucania earthquake are presented the data were collected primarily with a dense array of portable smoked drum seismographs deployed in the epicentral region a few days after the main shock tests performed on subsets of the data to establish the location accuracy are described in detail and epicentral maps and cross sections are presented and discussed one hundred and sixty well constrained fault plane solutions using upward traveling rays are presented the distribution of aftershocks forms an elongate cloud which does not define a plane this is not because the events are poorly located a subset of events located with epicentral errors of less than 1 km and depth errors less than 2 km forms a similar cloud to the complete aftershock set the aftershock mechanisms are very varied some are similar to the main event but many differ profoundly these include events on shallow or vertical planes and thrust events the latter include some events which require contraction in the direction in which the main event indicates extension the aftershocks of all mechanisms are intermixed in space in such a way that no simple system of faults can explain them a model to explain the aftershocks and the lack of surface ruptures in the main event is offered it is proposed that the main shock cuts between two aseismic detachment surfaces and that the aftershocks are a consequence of the complicated deformation that such a geometry demands whatever interpretation is adopted the results are a challenge to the commonly accepted view that aftershocks even approximately define simple fault planes or that aftershock mechanisms replicate the mechanism of the main shock defects in interpretations of the main shock and aftershocks process arise largely from a lack of information about the geometry of structures at depth in the epicentral region suitable deep reflection profiles could help to resolve this problem

structural trend analysis by axial surface mapping discussion original article shaw j h s c hook and j suppre 1994 aapg bulletin v 78 p 700 721

dip analysis as a tool for estimating regional kinematics in extensional terranes abstract in basin analyses that rely almost solely on seismic reflection data to configure the underlying structure it has become common practice to use extensional fault geometries to infer kinematic histories this procedure relies on the existence of fault geometries predicted by ideal orthogonal extension and assumes the structure is well constrained in areas where data are sparse or of equivocal quality this often leads to circular analysis that is ideal orthogonal extension is assumed to have been operative then under constrained fault geometries are mapped according to model predictions fault geometries so derived are subsequently used as evidence that orthogonal extension was operative and that the direction of extension is determinable we develop an alternative tool for inferring kinematics which is independent of fault geometries the method relies on the empirical evidence that the dip of the hangingwall s is systematic and predictable in extensional systems even when fault geometries are non ideal although not without significant limitations dip analysis can provide additional constraints for structural and kinematic interpretations especially where seismic reflection data are the main source of information and dip domains define discrete crustal blocks

alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave drag parametrization systematic westerly biases in the northern hemisphere wintertime flow of the meteorological office 15 layer operational model and 11 layer general circulation model are described evidence that the failure to parametrize subgrid scale orographic gravity wave drag may account for such biases is presented this evidence is taken from aircraft studies surface pressure drag measurements and studies of the zonally averaged momentum budget a parametrization scheme is described in which the surface stress is proportional to the near surface wind speed and static stability and to the variance of subgrid scale orography the stress is absorbed in the vertical by considering the influence of such gravity wave activity on static stability and vertical wind shear a richardson number dependent wave breaking formulation is devised and the vertical stress profile determined by a saturation hypothesis whereby the breaking waves are maintained at marginal stability it is shown that wave breaking preferentially occurs in the boundary layer and in the lower stratosphere results from a simple zonally symmetric model show how the adjustment to thermal wind balance with a wave drag in the stratosphere warms polar regions by adiabatic descent and decelerates the mean westerlies in the troposphere the influence of the parametrization scheme on integrations of the 11 layer model is described and found to be generally beneficial in a discussion of the reasons why this problem has only recently emerged it is suggested that the satisfactory northern hemisphere winter circulations of previous coarser general circulation models were due to a compensation implied by underestimating both the surface drag and the horizontal flux of momentum by explicitly resolved large scale eddies

momentum flux estimates for south georgia island mountain waves in the stratosphere observed via satellite abstract we show high resolution satellite observations of mountain wave events in the stratosphere above south georgia island in the remote southern atlantic ocean and compute the wave momentum fluxes for these events the fluxes are large and they imply important drag forces on the circulation small island orography is generally neglected in mountain wave parameterizations used in global climate models because limited model resolution treats the grid cell containing the island as ocean rather than land our results show that satellite observations can be used to quantitatively constrain mountain wave momentum fluxes and they suggest that mountain waves from island topography may be an important missing source of drag on the atmospheric circulation

ground motion waveforms and source spectral scaling from close distance accelerograms in a compressional regime area friuli northeastern italy abstract an aftershock sequence was recorded by an sma 1 instrument installed at gemona del friuli on a massive limestone site during the period from may to october 1976 the sequence consisted of small to moderately large 10 21 m 0 10 25 dyne cm dominant thrust earthquakes that occurred within 15 km of the station the instrument was triggered by p waves for 24 events the resulting records are particularly suitable to the investigation of source properties for the friuli earthquakes acceleration velocity and displacement waveforms have been analyzed in order to investigate source properties durations t and areas 0 of the shear wave pulses have been measured from the displacement time histories and compared with the values of corner frequency f c and seismic moment m 0 estimated in the frequency domain from the acceleration spectra of the same events the comparison between these two set of data reveals that attenuation correction is important also for correct estimations of source parameters from ground motions at close focal distance and moderate magnitudes after correcting for attenuation stress drops are approximately constant 300 bars on the average in the investigated seismic moment range for this sequence the behavior of peak ground motions versus earthquakes size is consistent with the trend obtained from stochastic simulations assuming an omega squared spectral model with a constant stress drop of 300 bars the good results obtained by applying to friuli earthquakes an omega squared model with a constant stress drop of 300 bars are further confirmed by the analysis of spectral ratios for different size earthquakes of the sequence this analysis demonstrates that the finding of this spectral scaling model is not biased to first order by whole path or near site propagation properties whose effect can be significant when data from a single station are investigated

structural control on fault geometry example of the grevena ms 6 6 normal faulting earthquake in this paper we present a three dimensional velocity structure of the kozani grevena region a relatively low seismicity area of the western margin of the internal hellenides which was struck by a ms 6 6 earthquake on may 13 1995 p wave arrival times from 656 aftershocks are inverted to resolve the velocity structure of the upper 12 km of the crust the 1995 ruptured fault dheskati fault lies at the southeastern end of a 35 km long normal fault the servia fault the crustal heterogeneities identified by tomography help to constrain the geometry and structure of the 1995 fault and the rupture evolution during the mainshock the aftershocks and the velocity structure define a fault dipping 35 toward nw which from 5 km depth splays upward into two nearly parallel shallow faults the slip during the mainshock and the aftershocks all were concentrated along the deeper portion of the fault while the limited slip observed at the surface occurred on the northern of the two shallower splays the paleochori fault it appears that the lithological heterogeneities of the shallow crust are responsible for the splaying of the main fault upward our work stresses the importance of crustal structure in determining fault geometry and segmentation and rupture evolution

impacts of horizontal propagation of orographic gravity waves on the wave drag in the stratosphere and lower mesosphere the impact of horizontal propagation of mountain waves on the orographic gravity wave drag ogwd in the stratosphere and lower mesosphere of the northern hemisphere is evaluated for the first time using a fine resolution 1 arc min terrain and 2 5 2 5 european centre for medium range weather forecasts era interim reanalysis data during 2011 2016 two sets of ogwd are calculated offline according to a traditional parameterization scheme without horizontal propagation and a newly proposed scheme with horizontal propagation in both cases the zonal mean ogwds show similar spatial patterns and undergo a notable seasonal variation in winter the ogwd is mainly distributed in the upper stratosphere and lower mesosphere of middle to high latitudes whereas the summertime ogwd is confined in the lower stratosphere comparison between the two sets of ogwd reveal that the horizontal propagation of mountain waves tends to decrease increase the ogwd in the lower stratosphere middle to upper stratosphere and lower mesosphere consequently including the horizontal propagation of mountain waves in the parameterization of ogwd can reduce the excessive ogwd in the lower stratosphere and strengthen the insufficient gravity wave forcing in the mesosphere which are the known problems of traditional ogwd schemes the impact of horizontal propagation is more prominent in winter than in summer with the ogwd in western tibetan plateau rocky mountains and greenland notably affected

the compilation of the instrumental seismic catalogue of italy 1975 1984 abstract we compile a homogeneous and complete catalogue of the seismicity of the italian region for 1975 1984 the period marking the transition from standard analogue seismometry to the new digital era the work is developed in three phases 1 the creation of a uniform digital databank of all seismic station readings unifying the database available at the istituto nazionale di geofisica with the catalogue of the international seismological centre 2 the preparation of numerical procedures for automatic association of arrival data and for hypocentre location using arrivals from local regional and teleseismic stations in a spherical geometry 3 the introduction of lateral heterogeneity by calibrating regional travel time curves and station corrections the first two phases have been completed providing a new instrumental catalogue obtained using a spherical earth model the third phase is presented here in a preliminary stage

the irpinia italy 1980 earthquake detailed analysis of a complex normal faulting a detailed analysis of near source strong motion and leveling data together with the results of teleseismic waveform modeling by westaway and jackson 1987 and aftershock studies by deschamps and king 1984 allows a satisfactory kinematic description of the complex normal faulting associated with the magnitude ms 6 9 november 23 1980 irpinia earthquake southern italy the three main rupture episodes starting at about 0 20 and 40 s are here associated with better constrained source parameters than in previous studies we first evaluated the triggering time for the 11 closest accelerometers by using the well recorded 40 s strong motion phases this gave the absolute time and location of the dominant episodes of faulting the first rupture propagated mainly toward the northwest on a ne dipping normal fault at a mean velocity close to 3 km s for about 20 km and continued 15 km further on smaller subfaults it was associated with surface breakage in the southeastern part the second rupture started from the southeastern end of the first rupture about 18 s after it and propagated about 20 km toward the southeast on a low angle normal fault dipping 20 ne it was associated with secondary faulting on steeper planes reaching the surface the third and last episode at 39 s nucleated near the first hypocenter at shallower depth and the rupture possibly propagated on a 10 to 15 km long normal fault striking se antithetic to the first activated fault a clear correlation appears between the strength of the geological formations and the existence of surface breakage and shallow aftershock activity

characterization of earthquake induced ground motion from the l aquila seismic sequence of 2009 italy summary based only on weak motion data we carried out a combined study on region specific source scaling and crustal attenuation in the central apennines italy our goal was to obtain a reappraisal of the existing predictive relationships for the ground motion and to test them against the strong motion data peak ground acceleration pga peak ground velocity pgv and spectral acceleration sa gathered during the mw 6 15 l aquila earthquake 2009 april 6 01 32 utc the l aquila main shock was not part of the predictive study and the validation test was an extrapolation to one magnitude unit above the largest earthquake of the calibration data set the regional attenuation was determined through a set of regressions on a data set of 12 777 high quality high gain waveforms with excellent s n ratios 4259 vertical and 8518 horizontal time histories seismograms were selected from the recordings of 170 foreshocks and aftershocks of the sequence the complete set of all earthquakes with ml 3 0 from 2008 october 1 to 2010 may 10 all waveforms were downloaded from the iside web page http iside rm ingv it iside standard index jsp a web site maintained by the istituto nazionale di geofisica e vulcanologia ingv weak motion data were used to obtain a moment tensor solution as well as a coda based moment rate source spectrum for each one of the 170 events of the l aquila sequence 2 8 mw 6 15 source spectra were used to verify the good agreement with the source scaling of the colfiorito seismic sequence of 1997 1998 recently described by malagnini et al 2008 finally results on source excitation and crustal attenuation were used to produce the absolute site terms for the 23 stations located within 80 km of the epicentral area the complete set of spectral corrections crustal attenuation and absolute site effects was used to implement a fast and accurate tool for the automatic computation of moment magnitudes in the central apennines

speculations on the geometry of the initiation and termination processes of earthquake rupture and its relation to morphology and geological structure earthquake initiation and termination processes are commonly described in terms of barriers and asperities barriers fall into two classes geometric barriers are associated with places where the orientation of a failure surface changes and relaxation barriers where stress is low because aseismic creep processes outpace tectonic loading geometric barriers fall into conservative and nonconservative subgroups according to whether finite fault motion can proceed without the creation of new structures or whether it demands the creation of new faulting or void space the multiple faulting or fragmentation associated with some nonconservative barriers can disrupt fault planes and form asperities by means of selected examples it is shown that a description in terms of these barriers can help one to visualise the processes of earthquake rupture and its relation to the geological environment

synoptic responses to mountain gravity waves encountering directional critical levels abstract a heuristic model is used to study the synoptic response to mountain gravity waves gws absorbed at directional critical levels the model is a semigeostrophic version of the eady model for baroclinic instability adapted by smith to study lee cyclogenesis the gws exert a force on the large scale flow where they encounter directional critical levels this force is taken into account in the model herein and produces potential vorticity pv anomalies in the midtroposphere first the authors consider the case of an idealized mountain range such that the orographic variance is well separated between small and large scale contributions in the absence of tropopause the pv produced by the gw force has a surface impact that is significant compared to the surface response due to the large scales for a cold front the gw force produces a trough over the mountain and a larger amplitude ridge immediately downstream it opposes somehow to the response due to the large scales of the mountain range whic

the parametrization of drag induced by stratified flow over anisotropic orography a new parametrization of drag arising from the flow over unresolved topography ut in a general circulation model gcm is presented it is comprised of three principle components a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation a parametrization of the drag associated with low level wave breaking and a parametrization of low level drag associated with upstream blocking and lee vortex dynamics novel features of the scheme include a new procedure for defining the ut in each gcm grid cell which takes account of the gcm resolution and includes only the scales represented by the parametrization scheme a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the ut in each gcm grid cell and a new application of form drag in the lowest levels which can change the direction of the low level flow so that it is more parallel to unresolved two dimensional topographic ridges the new scheme is tested in the canadian centre for climate modelling and analysis third generation atmospheric gcm at horizontal resolutions of t47 and t63 five year seasonal means of present day climate show that the new scheme improves mean sea level pressures or mass distribution and improves the tropospheric circulation when compared with the gravity wave drag scheme used currently in the gcm the benefits are most pronounced during northern hemisphere winter it is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity wave field with two waves rather than just one allows 30 50 more gravity wave momentum flux up into the middle atmosphere depending on the season the additional momentum flux into the middle atmosphere is expected to have a beneficial impact on gcms that employ a more realistic representation of the stratosphere

effects of horizontal geometrical spreading on the parameterization of orographic gravity wave drag part i numerical transform solutions abstractnumerical transform solutions for hydrostatic gravity waves generated by both uniform and sheared flow over elliptical obstacles are used to quantify effects of horizontal geometrical spreading on amplitude evolution with height both vertical displacement and steepness amplitudes are considered because of their close connections to drag parameterizations in weather and climate models novel diagnostics quantify the location and value of the largest wavefield amplitudes most likely to break at each altitude these horizontal locations do not stray far from the obstacle peak even at high altitudes resulting vertical profiles of wave amplitude are normalized to remove density and refraction effects thereby quantifying the horizontal geometrical spreading contribution currently absent from parameterizations horizontal geometrical spreading produces monotonic amplitude decreases with height through wave action conservation as waves propagate into progressively larger horizontal areas accumulated

a new subgrid scale orographic drag parametrization its formulation and testing a scheme for the representation of subgrid scale orography sso in numerical weather prediction and climate models is presented the new scheme arose in part from a desire to represent nonlinear low level mountain drag effects not currently parametrized an important feature of the scheme is that it deals explicitly with a low level flow which is blocked when the effective height of the subgrid scale orography is sufficiently high in this new scheme it is assumed that for this blocked flow separation occurs at the mountain flanks resulting in a form drag this drag is parametrized on model levels which are intersected by the sso and provides a dynamically based replacement for envelope orography the upper part of the low level flow goes over the orography and generates gravity waves at the model resolutions considered t106 and t213 it is assumed that the length scales characteristic of the sso are sufficiently small for the coriolis force to be neglected the various parameters of the scheme are adjusted using an off line procedure in which the scheme is used to estimate the mountain drag and the momentum profiles above the pyrenees and these estimates are validated with the pyrex data forecasts using t106 and t213 resolutions with this new scheme and with mean orography show that the forecast mountain drag consistently reproduces the drag measured during pyrex whenever the flow component normal to the ridge is large isentropic flow diagnostics further show that the new scheme has a realistic impact on the flow dynamics reinforcing the low level wake observed in mesoscale analyses of the flow with this new scheme and a mean orography the ecmwf model outperformed in forecast skill a version of the model which had an envelope orography and the old gravity wave drag scheme while no longer suffering any disadvantages of envelope orography the proposed low level drag parametrization should also be relevant at model horizontal resolutions much higher than t213

aftershock patterns and main shock faulting we have compared aftershock patterns following several moderate to large earthquakes with the corresponding distributions of coseismic slip obtained from previous analyses of the recorded strong ground motion and teleseismic waveforms well located aftershock hypocenters are projected onto the main shock fault plane and their positions are examined relative to the zones of coseismic displacement indicated by the estimated distributions of main shock slip we also examine the aftershock focal mechanisms when these data are available in an attempt to identify possible patterns of secondary faulting within the aftershock zone our results are consistent with a hypothesis of aftershock occurrence that requires a secondary redistribution of stress following primary failure on the earthquake fault aftershocks following the earthquakes examined in this study occur mostly outside of or near the edges of the source areas indicated by the patterns of main shock slip the spatial distribution of aftershocks reflects either a continuation of slip in the outer regions of the areas of maximum coseismic displacement or the activation of subsidiary faults within the volume surrounding the boundaries of main shock rupture

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast