displacements and tilts from dip slip faults and magma chambers beneath irregular surface topography surface displacements and tilts due to buried deformation sources are influenced by topography the leading order corrections due to topography of arbitrary profile but small slope are determined for dip slip faults edge dislocations and magma bodies lines of inflation in a two dimensional elastic medium the vertical displacement correction is simply the product of the topography and the horizontal normal strain due to the source in a flat half space the correction for a normal fault beneath an idealized basin and range topography is a slight increase in the uplift on the range side the effect of topography for a line of inflation beneath a symmetric volcano is to reduce the central uplift failure to account for topographic influences can bias estimates of source depth and geometry

influence of topography on coseismic displacements induced by the friuli 1976 and the irpinia 1980 earthquakes italy analyzed through a two dimensional hybrid model 1 we investigate the perturbations introduced by the local topography on the coseismic displacement components induced by crustal seismic faults the approach proposed by tinti and armigliato 2002 is applicable to homogeneous two dimensional domains with arbitrary topography of the free surface it uses both analytical results valid for homogeneous spaces and half spaces bounded by a flat free surface ffs and a numerical code solving the equations of equilibrium of linear elasticity through a finite element fe scheme a major characteristic of our hybrid method is that it allows for the correct representation of the displacement components in correspondence with the fault which does not need to be explicitly introduced in the fe mesh we show that in the case of homogeneous domains this makes our approach preferable to pure fe schemes our hybrid approach is here applied to two different disastrous events of the recent italian seismic history for which topography is expected to play an important role namely the 6 may 1976 ms 6 5 friuli and the 23 november 1980 ms 6 9 irpinia basilicata earthquakes for each event we selected hypotheses for the parent fault proposed in the literature and compared the coseismic displacements computed through the widely used analytical models dealing with ffs and through our hybrid approach accounting for the effect of realistic topographies the most relevant effects are observed in the case of the irpinia earthquake depending on the profile chosen the computed misfits can be as high as 19 and 13 for the horizontal and vertical displacements respectively with absolute differences up to 9 and 10 cm

the optimal estimation of earthquake parameters abstract the precision with which various earthquake parameters can be determined using an existing or a proposed network of seismographic stations has recently been of considerable interest analytical solutions for the optimal resolution are presented for the epicenter hypocenter focal mechanism seismic moment and local magnitude estimation problems the algebraic method is to decompose orthogonally the equations of condition for each problem using the singular value theorem the orthonormal equations and the data covariance then yield the uncertainties in the estimations of the unknown earthquake parameters possible biases due to significant linear correlations among the observed data are included in the analysis as an example the ability of data from the berkeley three station network of broadband seismographs to determine the above earthquake parameters is discussed the parameters of the ml 5 9 coyote lake earthquake of august 6 1979 are determined in detail in order to use all of the resolution that a network has for determining earthquake parameters it is necessary to calibrate the network for regional earthquakes which have well determined parameters if the network is not calibrated the use of relatively simple velocity models will lead to significant systematic errors in the estimation of some of the parameters

microearthquakes near the sleepy hollow oil field southwestern nebraska between april 1982 and june 1984 311 earthquakes were detected and 176 earthquakes were located in the vicinity of the sleepy hollow oil field southwestern nebraska the oil field is being injected with water to enhance oil production and the earthquakes may be related to production there is geologic evidence however to suggest that the earthquakes are tectonic in origin and are related to uplift along the axis of the cambridge arch on which the field is located first motion data from these events do not yield a well constrained fault plane solution furthermore first motion detections are inconsistent for earthquakes with nearly coincident hypocenters indicating that more than one faulting mechanism is active in the oil field injection pressures do not correlate well with earthquake occurrence

the not so simple relationship between seismicity and oil production in the permian basin west texas we have relocated seismicity occurring in the permian basin of west texas between 1975 and 1979 using three dimensional velocity models constructed from well log information and compared the locations to detailed geological and geophysical models of specific oil fields the seismicity appears to be related to a variety of causes including migration of naturally occurring overpressured fluids tectonic activity reservoir production and enhanced recovery operations many earthquakes may represent a combination of these effects although the permian basin is the site of numerous oil and gas fields only a limited number of fields appears to be associated with seismicity we find that there are similarities in the structural setting in the fields associated with seismicity furthermore fields within the delaware basin that are associated with seismicity are located in regions of high vertical and lateral fluid pressure gradients related to an overpressured zone within the delaware basin

testing self organized criticality by induced seismicity we examine the hypothesis proposed in recent years by several authors that the crust is in a self organized critical soc state this hypothesis has been suggested on the basis of the observation of power law distributions such as the gutenberg richter law for earthquakes and the fault length distribution and of the fractal geometry of sets of earthquake epicenters and of fault patterns these self similar properties are shared by simplified models of the crust exhibiting a spontaneous organization toward a critical point characterized by similar scale invariant properties the term critical is here used in the sense of phase transitions such as the curie point in magnetism the usefulness of a hypothesis is measured by its predictive and explanatory power outside the range of observations that have helped defined it we thus explore how the soc concept can help in understanding the observed earthquake clustering on relatively narrow fault domains and the phenomenon of induced seismicity we review the major reported cases of induced seismicity in various parts of the world and find that both pore pressure changes p and mass transfers m leading to incremental deviatoric stresses of 1 mpa are sufficient to trigger seismic instabilities in the uppermost crust with magnitude ranging up to 7 0 in otherwise historically aseismic areas once triggered stress variations of at least 1 order of magnitude less but still larger than the 0 01 mpa tidal stress are enough to sustain seismic activity we argue that these observations are in accord with the soc hypothesis as they show that a significant fraction of the crust is not far from instability and can thus be made unstable by minute perturbations this property is shared by simplified models of soc not all perturbations however trigger seismic activity this is also compatible with the soc hypothesis which embodies naturally the existence of large heterogeneities in the stress field the induced seismicity is found to obey generally the gutenberg richter law up to a magnitude cutoff which correlates well with the width of the local seismogenic bed ranging in size from that of mine pillars for mining induced seismicity to the thickness of brittle sedimentary beds in the vicinity of dams or depleted hydrocarbon reservoirs in conclusion the properties of induced seismicity and their rationalization in terms of the soc concept provide further evidence that potential seismic hazards extend over a much larger area than that where earthquakes are frequent

investigation of microearthquakes macroseismic data and liquefaction associated with the 1867 wamego earthquake in eastern kansas historical felt earthquakes and instrumentally recorded microseismicity in eastern kansas and western missouri have been attributed to the movement of the nemaha ridge and humboldt fault zone nrhf our investigations of the nrhf have concentrated on relocation of microearthquakes in the kansas catalog re evaluation of the 1867 earthquake felt reports and field studies to determine the presence or absence of sedimentary evidence of earthquake induced liquefaction microearthquakes in the kansas catalog have been relocated in a joint inversion with 3d seismic velocity variations this improved set of hypocenters affirms the loose association of seismicity with the basement nrhf structure even though seismicity does not seem to follow a single well defined fault feature overall the association of microearthquakes with the nrhf suggests that it remains a potential source of large earthquakes our field investigations confirm that sedimentary deposits with moderate liquefaction susceptibility are present in the vicinity of wamego and wabaunsee kansas soft sediment deformation features including flame and dish structures are present in the late holocene floodplain deposits of the kansas river these features may have formed by liquefaction or by processes unrelated to seismic activity such as sediment dewatering we found several clastic dikes that may be attributed to seismically induced liquefaction our initial results suggest that liquefaction features are present but may not be pervasive in this region these data imply that the 5 2 magnitude of the 1867 wamego earthquake may characterize the seismic source in this location however additional field investigations are still necessary to complete the assessment for liquefaction features

an accurate and efficient method for including the effects of topography in three dimensional elastic models of ground deformation with applications to radar interferometry topography has a large effect on the results predicted by elastic surface deformation models in regions of significant relief in some cases topography may have more influence on the predicted deformation field than do model source parameters we have developed an approximate analytical technique for including topographic effects that retains most of the computational simplicity of elastic half space models while providing an accurate representation of topographic effects we use a series expansion of the elastic half space solution with a small slope approximation yielding a set of higher order corrections the integrated effect of these corrections is evaluated using fourier methods we investigate the effectiveness of our method by comparing predicted results for a tilted triaxial ellipsoid with those predicted by finite element models the resulting displacements and displacement gradients are in good agreement with finite element results both for relatively smooth topography synthetically generated and for the greater relief in the vicinity of mount etna volcano we then compare the results of our method with those predicted by traditional elastic half space models using different reference elevations and with a previously proposed method of estimating topographic effects our new method provides a significantly better fit to the finite element results than do the other methods our method is able to accurately portray both the number and the horizontal pattern of fringes in a synthetic interferogram when compared with finite element results in particular the method accurately represents the broadening of fringes that is observed in regions of high relief as well as reproducing the location of the fringe center and the topographically induced deviation of the fringes from a regular pattern elastic half space models typically show a fringe center that is displaced with respect to the finite element results indicating that parameter inversions based on such a model would provide incorrect estimates of the horizontal source position the lack of topographically generated fringe distortions in elastic half space results would also likely lead to inaccuracies in the predicted magnitude and orientation of proposed deformation sources our new technique over comes these limitations and should be useful when evaluating either traditional geodetic results or the greater areal coverage provided by interferometric observations

the relocation of microearthquakes in the northern mississippi embayment three component seismograms recorded by a small array of digital instruments in the northern mississippi embayment consistently show a high amplitude phase on the vertical component that arrives approximately 0 8 s before the shear wave on the basis of its timing and apparent velocity this phase is identified as an s p conversion from the boundary between the unconsolidated cenozoic sediments and the underlying paleozoic rocks synthetic models of ground motion in the mississippi embayment indicate that vertical displacement amplitudes are higher for s p conversions than for transmitted shear waves at all angles of incidence the models and their agreement with observations of the three component seismograms suggest that true shear waves cannot be reliably identified from vertical component seismograms recorded in this area the travel times of converted phases are used together with crustal velocity models derived from a recent seismic refraction survey to relocate approximately 500 microearthquakes recorded by the central mississippi valley regional seismic network since the network data are recorded by vertical component seismographs we assume that the s phases observed at the stations sited on unconsolidated sediments are s p conversions this assumption significantly reduces the average rms residual and provides well constrained hypocentral locations for 350 earthquakes the relocated microearthquakes cluster tightly along previously identified epicentral trends focal depths range from 0 5 to 22 km but events deeper than 14 km are rare and events shallower than 3 km are confined almost exclusively to the area between ridgely tennessee and new madrid missouri focal mechanisms are generally consistent with the results of previous studies the axial seismicity trend which extends from caruthersville missouri to marked tree arkansas is characterized by right lateral strike slip motion on a northeast trending nearly vertical fault plane the events between ridgely tennessee and new madrid missouri commonly show strike slip focal mechanisms with minor components of both normal and reverse dip slip

moderate to large seismicity induced by hydrocarbon production it is well known that various human activities have the potential to generate seismic activity examples range from subsurface waste injection and reservoir impoundment in the vicinity of large dams to the development of mining geothermal or hydrocarbon resources recently induced seismicity in connection to geologic carbon sequestration projects has emerged as a new field of interest this review discusses seismicity induced by hydrocarbon production in particular i focus on published cases for which earthquakes of moderate to large magnitudes in other words earthquakes that can be felt on the surface have been reported i also discuss current theoretical approaches to model this phenomenon the emphasis on moderate to large magnitudes is intended to complement the other contributions in this issue which focus primarily on microseismicity evidently it is important to understand the conditions under which hydrocarbon production may induce seismic activity in order to ensure that field operations can

sea ice topography of the arctic ocean in the region 70 degrees w to 25 degrees e in october 1976 a cooperative experiment was made to survey the sea ice topography in the european sector of the arctic ocean h m submarine sovereign acquired 4000 km of ice draft data by using an upward looking sonar while a canadian forces aircraft flew along the submarine s track and acquired 2200 km of ice elevation data by using a laser profilometer the two types of profile were processed in corresponding 100 km section lengths and the following statistical analyses and comparisons were made i probability density functions o f ice draft and elevation each distribution shows a peak for young ice and for undeformed multi year ice at large ice thicknesses the distributions take the analytical form of a negative exponential the mean drafts enable two distinct geographical ice regimes to be identified there is an offshore zone of very heavy pressure ridging extending up to 400 km from the coasts of ellesmere island and of north greenland with mean ice draft in the range 5 0 to 7 5 m while out in the central arctic ocean the mean ice draft is lower 3 9 5 1 m and the characteristics of the ice cover remain homogeneous over a length scale of 1000 km the transition between the two regimes is abrupt taking place in less than 25 km data from the same part of the central arctic taken in march 1971 showed a mean ice draft 0 3 m lower while data from the central beaufort sea showed a mean draft more than 0 8 m lower ii level ice distributions ice with a local gradient of less than 1 in 40 was defined as level ice and used as an indicator of the quantity and thickness distribution of undeformed i e thermodynamically grown ice in the arctic ocean the distribution has a mode at 3 0 3 1 m draft and level ice percentages are in the range 30 40 bottom side and 70 80 top side in the offshore zone and 45 55 bottom and 85 95 top in the central arctic thus about half of the arctic ice cover consists of deformed ice iii pressure ridge spacings the spacings of ridge keels fit a negative exponential distribution characteristic of randomness except at close spacings where there is a deficit of keels explained as a geometrical effect and at very large spacings where there is an excess due to the contribution of polynyas the distribution of sail spacings exhibits these two effects but also differs from a random distribution at moderate sail separations iv ridge elevations and drafts keel drafts fit a law of form p h d oc exp ah 2 d h except for an excess of keels at drafts beyond 20 m there is a positive correlation between mean keel draft and keel frequency there are 3 5 4 5 keels per kilometre with draft exceeding 9 m in the offshore zone and 2 3 in the central arctic sail elevations fit a law of form p h d h x exp a h d h with a positive correlation between mean sail elevation and sail frequency the sail elevation and ice elevation distributions can be related by assuming that all thick ice is contained in pressure ridges of triangular cross section v keel sail comparison for the 21 corresponding 100 km sections there are positive correlations between mean sail height and mean keel draft and between keel frequency and sail frequency from these it is possible to convert a sail distribution computed from a laser profile into a keel distribution enabling sea ice bulk characteristics to be derived from airborne surveys alone vi leads and polynyas a lead was defined as a continuous sequence of depth points greater than 5 m long and not exceeding 1 m draft the number density n d of leads per kilometre of width d m fits the power law n d 15 d 2 exceptionally wide leads were concentrated in the offshore zone and in the marginal ice zone close to the open water of the greenland sea

top bottom multisensor remote sensing of arctic sea ice the arctic sea ice cover has been studied using near simultaneous observations by passive and active synthetic aperture radar sar microwave sensors upward looking and sidescan sonars a lidar profilometer and an infrared sensor data from two aircraft and a submarine over an approximately 100 km track of central arctic sea ice were registered and analyzed to evaluate the characteristics of the ice cover and the utility of each sensor in ice studies the results of comparative and correlation analyses are as follows the probability density functions of ice draft from sonar and elevation from lidar were found to be almost identical when isostasy is taken into account this result suggests that the basic ice thickness distribution can be derived from the surface topography measurements alone reasonable correlation was found between sar backscatter and ice draft or elevation especially when scales were adjusted such that 15 20 sar pixels were averaged however surface roughness derived directly from standard deviations in the lidar elevation data was found to be poorly correlated to the sar backscatter these results indicate that the sar values are affected more by scattering from the ice than from the snow covered surface the active and passive microwave sensors are shown to generally complement each other as the two sensors are especially sensitive to different physical properties of the sea ice undeformed first year ice showed low backscatter values but high brightness temperatures while some multiyear ice showed high backscatter values and low brightness temperature however surfaces identified as multiyear ice by the passive system have a large spread in the unaveraged sar backscatter indicating limitations when using a one channel sar for ice type identification at the highest resolution also ridged ice identified by sonar and sar data covers a large range of passive microwave emissivity suggesting considerable variability in the age and salinity of this type of ice significant variations about 0 11 in the minimum emissivity of consolidated multiyear ice are observed in different regions of the arctic using the high resolution 30 m passive microwave data this suggests that regional variations in texture and scattering characteristics of multiyear ice in the arctic are present likely influenced by different histories of formation of the ice in different regions

self induced deformation on the fault plane during an earthquake part ii continuous tangential displacements the problem of the coseismic deformation induced by an earthquake on its own fault plane is investigated here the analysis concentrates on the on fault displacement field accompanying the occurrence of an earthquake in response to a prescribed uniform shear slip and it is carried out on the basis of the classical analytical model by okada 1992 for a rectangular fault buried in an elastic homogeneous half space delimited by a planar free surface the analysis is subdivided into two separate papers the first dealing with normal and the second with tangential on fault displacements in this first paper concerning the study of the normal displacement component the contributions of the source and of the correction introduced by the free surface are investigated separately and their dependence on the fault characteristics is thoroughly discussed particular attention is also devoted to the effects of the normal displacement on the fault surface geometry it will be shown that the main effect is that of deforming the fault itself with deformation consisting chiefly in a rotation of the plane and in a bending of the fault edges the rotation angle is negligibly small on the order of 1 10 rad for a single earthquake although repeated seismic events occurring on the same fault might result in rotations of several degrees over geological time scales

topographic effect on tilt strain and displacement measurements deformation and stress fields are locally modified by the presense of topography an approximate analytical method is used to give simple expressions for the effect of arbitrarily shaped topography on vertical horizontal and surface tilt measurements this provides a means of making corrections to observations without the use of finite element techniques corrections to tilt fields are found to be directly proportional to the local slope of the topography comparison is made between expressions for tilt and those for strain and displacement although the topographic effects on secular tilt strain and vertical displacement are not yet measurable we emphasize that the topographic effect on precision tidal tilt and strain measurements can be significant

the underside of arctic sea ice imaged by sidescan sonar despite extensive submarine operations in the arctic little is yet known about the three dimensional morphology of the ice underside several studies have been done using upward looking sonar1 7 which yields a linear profile of ice draft from which the distributions of features such as pressure ridges and leads may be derived such a profile gives no information about the spatial morphology or appearance of the ice bottom surface in contrast the upper ice surface has been well documented from airborne and satellite borne sensors such as synthetic aperture radar passive microwave infrared and visual imaging here i report the first extensive set of imagery of the underside of sea ice in the arctic ocean sidescan sonar images show a distinct difference between the appearance of first year ice and of multi year ice first year ice has a smooth bottom except for a network of cracks while multi year ice is covered with an array of blisters or bulges the clear discrimination between ice types enables sidescan sonar to be used as a means of validating airborne passive microwave data

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

the underside topography of sea ice over the continental shelf of the beaufort sea in the winter of 1990 a spatial section of draft across 348 km of sea ice was obtained by moored subsea sonars in the beaufort sea in the winter of 1990 the ice field was a mixture of first year and multiyear types with mean draft of the order of 3 m level ice of 0 to 4 m draft occupied approximately 50 of the section within this fraction draft values of 0 and 1 5 m were most common while at 2 to 3 m draft a noticeable contribution was made by rafted first year ice deformed ice was observed to a maximum draft of nearly 27 m keels within the section were identified using both the rayleigh criterion and a new criterion based on level ice both criteria defined exponentially distributed populations but with different e folding scales the observations revealed an ice cover whose statistical properties are consistent with other data from the periphery of the polar pack in the beaufort sea but which are much more severe than those of ice found in the seasonal ice zone only a few tens of kilometres to the south the cross sectional shape of individual keels was highly variable but on average the distribution of ice within a keel was an exponential function of draft the topography of both first year and old ice subsections of the data was fractal in character with a dimension of about 1 5 for both ice types the average mass of sea ice per unit area was estimated as 2540 2970 kg m 2 with 68 73 of this mass contributed by deformed ice the large uncertainties in these estimates arise because the porosity of keels and the burden of snow or melt water upon the ice are not known

statistical properties of arctic sea ice morphology derived from sidescan sonar images abstract using a transparent overlay technique the authors have classified a 140km long stretch of sidescan sonar imagery of the arctic sea ice underside 700m swath width into regions occupied by unique ice types the categories used were multi year ice first year ice pressure ridge apparently open dark lead apparently refrozen bright lead and randomly deformed ice the technique was validated by comparing these classifications with data obtained simultaneously using a narrow beam upward looking sonar the pattern of ice types thus observed was tested statistically for associations between adjacent regions the distribution of lead widths spacings and orientations and the distribution of ridge spacings and orientations significantly non random sequences of ice types were observed lead orientations and spacings were clustered and were set to preferred values and ridge spacings fitted a negative exponential distribution the extent of uniformly deformed ice with no apparent linearity was much greater on the underside of the ice than has ever been observed on the upper surface

mechanics of seismic instabilities induced by the recovery of hydrocarbons we review earthquake distributions associated with hydrocarbon fields in the context of pore pressure diffusion models poroelastic stress transfer and isostasy theory these three mechanisms trigger or induce seismic instabilities at both local scale d 5 km and at regional scale d 20 km the modeled changes in stress are small 1 mpa whatever the tectonic setting each mechanism corresponds to different production processes 1 local hydraulic fracturing due to fluid injection induces seismic slip on cracks ml 3 within the injected reservoir through decreasing the effective stress 2 pure fluid withdrawal causes pore pressure to decrease within the reservoir it triggers adjustments of the geological structure to perturbations related to the reservoir response to depletion poroelastic mechanisms transfer this stress change from the reservoir to the surrounding levels whereml 5 seismic instabilities occur either above or below the reservoir 3 massive hydrocarbon recovery induces crustal readjustments due to the removal of load from the upper crust it can induce larger earthquakes ml 6 at greater distance from the hydrocarbon fields than the two other mechanisms

travel time changes in a tomography array caused by a sea ice cover abstract in september 1988 an acoustic tomography array was deployed in the greenland sea in the region 74 76 n 1 e 5 w we have modelled travel time changes which may be produced in this array by the presence or growth of a sea ice cover three types of icefield are considered a typical east greenland marginal ice zone miz thickness distribution using as input data a sonar profile from a british submarine a field of discrete floes without pressure ridging and a pancake frazils mix growing on the open sea firstly we calculated travel time changes caused by an ice sheet uniform thickness the travel time decreases with increasing ice thickness by 2 10 ms for 3 m thickness at a frequency of 250 hz and a range of 200 km but the magnitude depends on grazing angle and strongly on compressional and shear velocities in ice with some values for velocity the relationship is no longer monotonic in a real icefield we surmised that the modal ice thickness is responsible for the coherent reflected ray which is detected secondly we calculated the progressive travel time decrease resulting from the increase in mixed layer salinity caused by salt rejection from a growing ice cover we conclude that this is negligible for miz type ice covers but is detectable for rapidly growing pancake fields and in the case of shallow rays may reach 3 5 ms per day finally we calculated the scattering loss associated with ice bottom roughness in miz type icefields finding that for steep rays it is about 15 db on average at 200 km range so that signal to noise ratios in such arrays need to be raised to ensure adequate timing precision if this is achieved we conclude that acoustic tomography can be used as a means of monitoring modal ice thickness in the greenland sea

on the spacing and draft distributions for pressure ridge keels the negative exponential distribution is reconsidered as a description of pressure ridge spacings and drafts it is found to give an excellent fit for drafts but not for spacings the spacing distribution is found to be fitted much better by a three parameter lognormal distribution of which the third parameter the threshold is dependent on the beam width of the instrument used to collect the ice bottom profile

gravity induced stresses near topography of small slope topographic modification of gravity induced near surface stresses results in significant departures from a lithostatic state a perturbation scheme provides approximate analytical solutions for plane strain of an elastic half space with an irregularly shaped free surface of small characteristic slope e the leading order effect of the topography is equivalent to that of a distributed normal load on a plane boundary and the correction at order e is due to a distributed shear traction on a plane boundary in the near surface region these two effects contribute at the same order to departures from the lithostatic stress field the contribution of the shearing has been neglected in most previous analyses we present explicit solutions for several particular geometries a symmetric ridge or valley an adjacent ridge and valley a shelf and a sharp crested ridge notable features include 1 for a sufficiently steep ridge the horizontal normal stress is compressive at the crest and decreases possibly becoming tensile with depth 2 large horizontal tension is induced in a valley bottom at greater depth these effects vanish and the stresses approach lithostatic similar analysis is applied to a half space under a far field tectonic compression or tension the result shows that the leading order effect of topography is equivalent to a distributed shear traction on a plane surface regional horizontal compression can be significantly reduced or even change to tension in the neighborhood of a topographic high

present state of tectonic stress of the friuli area eastern southern alps stress orientations are determined for the friuli area from focal mechanisms using the inversion technique of gephart and forsyth 1984 the data consist of 244 fault plane solutions selected for the period 1988 1995 where the data coverage was poor this period was extended to 1978 1996 local magnitude ranges from 2 0 to 4 3 and the depths extend from 4 to 16 km the friuli area is subdivided into five seismotectonic zones on the basis of tectonic characteristics seismic activity and focal mechanisms and for each zone a stress tensor is determined the results show that the active tectonic stress in the friuli area is heterogeneous and conditioned by the complex structural pattern which is characterized by two indented tectonic wedges the western and central parts of the area are mainly affected by a compressional state of stress while in the eastern part a strike slip state of stress prevails the orientation of the principal stresses varies from the western to the eastern part of the area and differences are also noticeable with depth in the central part of the friuli area the maximum compressional stress is n s trending at shallow depths 10 km changing to nw se at deeper levels the maximum compressional stress is nw se oriented in the western part while in the eastern part it is n s oriented

surface faulting in the southern italian campania basilicata earthquake of 23 november 1980 the campania basilicata earthquake of 23 november 1980 was the largest ms 6 9 and most destructive earthquake that has occurred in this region of southern italy for over 100 years killing more than 3 000 people the consensus among seismologists geologists and earthquake engineers who have studied1 10 this event is in keeping with previous italian earthquakes that it did not produce faulting at the surface this has led to the establishment of a number of models2 3 11 13 requiring an unusual tectonic setting for the southern apennines however on a recent visit to the epicentral region of the earthquake may 1984 we discovered more than 10 km of surface faulting that is consistent with focal mechanisms for this event based on first motion polarities2 4 5 12 and long period waveform studies14 16 which show that it involved normal faulting with one nodal plane dipping at 60 towards the north east our observations of this the first well documented earthquake fault break to be reported in italy suggest that the deformation associated with earthquakes in the southern apennines takes place in the upper 10 15 km of the earth s crust on steep planar normal faults which break the surface as is widely observed in other areas of active extension outside italy

possibility of induced seismicity in the vicinity of the sleepy hollow oil field southwestern nebraska an array of four seismographs operating in northwestern kansas and southwestern nebraska detected 31 earthquakes in the magnitude range 0 6 to 2 9 from march 1979 to march 1980 of these 22 were recorded at enough stations to allow accurate location and at least three of them were felt by local residents sixteen of the located events occurred in the immediate vicinity of the most productive oil field in nebraska the sleepy hollow field a mail canvass revealed a well constrained felt area about 10 km across which coincides remarkably well with the oil field water injection has been used as a method of enhanced recovery in this field since 1966 some of the earthquakes occurred on possible downward extensions of faults in the deeper sedimentary rocks and granite basement implying that some of the seismicity may be induced deeper events and other supporting data however suggest current tectonic activity on the chadron cambridge arch the recently installed eight station sleepy hollow seismic network should provide the data necessary to test these hypotheses