evolution of oceanic lithosphere a driving force study of the nazca plate the role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the nazca plate results indicate that ridge push forces when distributed over that portion of the oceanic lithosphere giving rise to the force predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest in particular the component of stress normal to the ridge vanishes near the ridge for the new modeling predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on nazca intraplate stress stress states have also been calculated for a variety of slab pull models which include possible dependence on age rate and angle of subduction the predicted stresses vary substantially near the trench among these models with less variation farther from the trench combinations of ridge push and slab pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab pull over ridge push simple viscous modeling with only distributed ridge forces predicts relative motion between the nazca and south american plates that is in fairly poor agreement with the observed direction the fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the antarctic plate

control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

variations in column height and magma discharge during the may 18 1980 eruption of mount st helens abstract peak eruption column heights for the b1 b2 b3 and b4 units of the may 18 1980 fall deposit from mount st helens have been determined from pumice and lithic clast sizes and models of tephra dispersal column heights determined from the fall deposit agree well with those determined by radar measurements b1 and b2 units were derived from plinian activity between 0900 and about 1215 hrs b3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs a brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height 19 km during deposition of b4 variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass balance calculations based on the volume of pyroclastic flows and coignimbrite ash peak magma discharge occurred during the period 1215 1630 hrs when pyroclastic flows were generated by collapse of low fountains through the crater breach pyroclastic flow deposits and the widely dispersed co ignimbrite ash account for 77 of the total erupted mass with only 23 derived from plinian discharge a shift in eruptive style at noon on may 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw up and tapping of deeper levels in the magma reservoir as predicted by theoretical models of magma withdrawal return to plinian activity late in the afternoon when magma discharge decreased is consistent with theoretical predictions of eruption column behavior the dominant generation of pyroclastic flows during the may 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable convective eruption column to a collapsing one

correction to an extensive region of off ridge normal faulting earthquakes in the southern indian ocean by eric a bergman john l nabelek and sean c solomon

acoustic response of gas hydrate formation in sediments from south china sea abstract gas hydrate has been recognized as a potential energy resource in south china sea scs understanding the acoustic response of gas hydrate formation in the scs sediments is essential for regional gas hydrate investigation and quantification the sediments were obtained from gravity core sampling at e 115 12 52363 n 19 48 40299 gas hydrate was formed within a gas water saturated scs sediments system combination of a new bender element technique and coated time domain reflectometry tdr was carried out to study the acoustic response of hydrate occurrence in scs sediments the results show the acoustic signal becomes weak when hydrate saturation sh is lower than 14 the acoustic velocities vp vs of the sediments increase with sh during hydrate formation and vs increases relatively faster when sh is higher than 14 these results indicate that tiny hydrate particles may firstly float in the pore fluid which causes a significant acoustic attenuation but has little influence on shear modulus as time lapses and sh approaches 14 numerous particles coalesce together and contact with sediment particles as a result vs has a sharp increase when hydrate saturation exceeds 14 several velocity models were validated with the experimental data which suggests a combination of the bgtl biot gassmann theory modified by lee model and the weighted equation is suitable to estimate sh in scs

non planar fault models complexities induced by crustal layering in transcurrent faulting processes summary crack models suitable to describe transcurrent faulting in the presence of a softer shallow layer are presented we employ the asymptotic theory of generalized cauchy kernel equations to study the singular behaviour of a strike slip crack crossing the welded interface between two different media if a planar crack cuts vertically across a material discontinuity the dislocation density is bounded at the interface but a stress drop discontinuity condition must be met in which the stress drop is proportional to the local rigidity such a condition cannot be fulfilled in several cases two antiplane strain models are proposed to avoid this difficulty in the first model fault bending model the fault surface is affected by a sharp change of the dip angle at the intersection with the interface an unbounded singularity in the dislocation density distribution and in the stress field appears at the interface and its dependence from model parameters is studied a generalized stress drop condition is obtained which can be interpreted in terms of the interaction of the crack with the welded boundary condition in the second model fault branching model the crack is split into three interacting sections with the lower section vertical and the other two sections inclined with respect to the vertical in this case the order of the singularity at the interface remains undetermined this result may be interpreted in terms of the further degree of freedom introduced by the presence of a second fracture in the upper layer in the case of branching two conditions are obtained for the stress drop on the three sections according to both models the generalized stress drop conditions may account for a stress drop ratio between the soft layer and the hard basement lower than the rigidity ratio

seismotectonics of northeastern united states and adjacent canada data for local earthquakes recorded by a network of stations in northeastern united states and adjacent canada were analyzed to study the seismicity the relationship between earthquakes and known faults the state of stress and crustal and upper mantle velocity structure in addition portable seismographs were deployed in the field to study aftershocks as a result accurate locations for about 364 local earthquakes 2 mb 5 and 22 focal mechanism solutions were determined a comparison of the spatial distribution of these events 1970 1979 with historical earthquakes 1534 1959 reveals that seismic activity in the northeast is relatively stationary in space those areas that have had little or no seismicity historically are relatively aseismic today whereas the historically active areas are also active today the instrumental locations historical seismicity and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces 1 the adirondack western quebec province is a northwesterly trending zone of seismic activity about 200 km wide and at least 500 km long extending from the se adirondacks into western quebec canada thrust faulting on planes striking nnw to nw appears to predominate and the inferred axis of maximum horizontal compression is largely uniform and trends wsw nearly parallel to the calculated absolute plate motion of north america little or no seismicity is found where anorthosite outcrops at the surface correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep ne or sw gradient in bouguer anomalies this zone does not appear to extend southeastward to boston as proposed by some workers 2 the appalachian province is a northeasterly trending zone of seismic activity extending from northern virginia to new brunswick canada highangle reverse or thrust faulting on n to ne trending planes appears to predominate the western margin of this province however appears to be relatively aseismic we attribute this relative lack of activity to one or more of the following the presence of igneous activity postdating rifting of africa from north america the occurrence of intense metamorphism during the acadian orogeny which may have annealed preexisting faults and the predominance of ductile as opposed to brittle deformation in the geologic past the inferred axis of maximum horizontal compression along the eastern margin of the appalachians is rather uniform and trends w wnw almost perpendicular to the magnetic lineations offshore we suggest that this w wnw compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers

three dimensional nonplanar simulation of the 1992 landers earthquake 1 we have simulated the rupture process of the 1992 landers california earthquake using a realistic nonplanar fault geometry including fault bends and branches through a three dimensional boundary integral equation method considering an external loading system due to local tectonics as well as a nonplanar fault structure we could reproduce the characteristic features of this earthquake such as spontaneous rupture transfer between faults pattern of slip distribution and propagation of rupture front without assuming any laterally heterogeneous distribution of parameters we could also reproduce seismic waveforms near the faults especially in the forward direction of rupture propagation where rupture directivity controlled by rupture transfer between faults significantly affects ground motion

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

the transfer of slip between two en echelon strike slip faults a case study from the 1992 landers earthquake southern california detailed mapping of the fresh surficial ruptures in the right step between the homestead valley fault and the emerson fault two right lateral en echelon faults that broke in the 1992 landers earthquake indicates that the transfer of dextral slip between the faults is accommodated primarily by a series of obliquely striking right lateral strike slip cross faults right lateral slip on the cross faults and counterclockwise rotation of the interior blocks are sufficient to transfer virtually 100 of the dextral slip and to accommodate the extension across the jog asymmetry of the dextral slip curves along the cross faults indicates that some of them may have been induced to fail by slip on the homestead valley fault while others were induced to fail by slip on the emerson fault comparison of the magnitude of slip in 1992 to bedrock and geomorphic offsets suggests that the homestead valley fault and several secondary faults have experienced about 100 nominal landers events and that the stepover structure originated at the same time as the homestead valley fault about 1 m y ago because the emerson fault has an order of magnitude greater total offset but only slightly greater surficial slip in 1992 we conclude that it is a significantly older fault repetition of 1992 like events should eventually lead to a merging of the homestead valley fault with the emerson

the mechanism of continental splitting abstract continental splitting is the response of the continental lithosphere under favourable conditions to deviatoric tension and a hot partially fused upper mantle beneath it has commonly been suggested that rift systems such as that of east africa may possibly represent splitting in the process of occurring here the hot upper mantle has probably formed by convective upwelling from deep in the mantle allowing the escape of heat from the earth s interior by hotspot activity this causes volcanism and isostatic uplift of the heated and thinned lithosphere and the crustal tension subsequently develops in response to the surface load caused by the uplift and the upthrust of the isostatic compensation at depth it is suggested here that the crustal tension causing continental splitting does not normally originate as in east africa but rather by the suction force acting on the overriding plates at trenches at periods in earth history when subduction occurs on the opposite sides of a large continental mass such as pangaea the critical evidence on the splitting mechanism is mostly deeply buried at passive margins and inaccessible to study some margins such as the north biscay one were apparently formed under much quieter volcanic and tectonic conditions than continental rift zones whereas others such as those of the icelandic region underwent extensive continental volcanism indicating a hot upper mantle beneath the icelandic types of margin probably acted as foci for continental splitting by dyke emplacement with dykes propagating laterally from them through the more quietly developed parts of the margin to link with existing plate boundaries normal plate motions occur in response to the steady escape of heat from the deep interior through formation and cooling of the oceanic lithosphere which can be recycled whereas continental splitting is seen as the result of the spasmodic escape of heat from the deep interior through the continental lithosphere

intraplate lithosphere strength and heat flow the magnitude levels of intraplate stresses and the strength of intraplate lithosphere are critical for understanding the mechanisms of intraplate deformation the decay of stress in the ductile lower lithosphere and its resulting amplification in the brittle upper lithosphere constitute an important mechanism for providing stress levels in the upper brittle lithosphere sufficient for fracture such stress transfer and the subsequent lithosphere deformation are controlled by lithosphere rheology a model is examined here for the deformation of viscoelastic lithosphere with temperature and stress dependent rheology subjected to laterally applied stress for different lithosphere thermal structures the geotherm is found to be of critical importance in determining the strength of the lithosphere and the extent of the resulting lithosphere deformation for lithosphere with heat flow 75 mw m 2 0 25 kbar tensile stress is sufficient to cause geologically significant deformation while for compressional deformation either greater stress levels or a hotter lithosphere are necessary

mechanisms of crustal deformation thirty sixthwilliam smith lecture the energy source for crustal deformation is isotopic heating and secular cooling of the mantle in a true solid heat would be lost to the surface by conduction however solid state creep processes allow the earth s solid mantle to exhibit a fluid behaviour thus thermal convection can convert heating into directed motion variations in temperature lead to variations in densitythrough thermal expansion and contraction although the resultant body forces are vertical horizontal variations in temperature lead to horizontal body forces at an ocean ridge the variations in the thickness of the lithosphere lead to the elevation of the ridge the result is a substantial horizontal ridge push force this force along with the trench pull force on the descending lithosphere is the primary force driving plate tectonics lithospheric thinning must occur beneath continental rifts and plateau uplifts i suggest that lithospheric thinning is caused by the diapiric penetration of hot asthenospheric rock to the crust mantle boundary this penetration may also cause lithospheric stoping and plateau uplifts lithospheric thinning leads to large tensional stresses above the zone of thinning these forces are generally much larger than the compressional forces generated by crustal thinning the forces generated by lithospheric thinning are quite large and may be responsible for the propagation of continental rifts in island arcs and andean type orogenic belts both lithospheric thinning and crustal thickening result in large tensional forces probably the most complex zone of crustal deformation is the continental collision variations in types of continental collisions are discussed

early cretaceous volcanism on the northeastern american margin and implications for plate tectonics volcanic and igneous rocks interbedded with lower cretaceous sedimentary rocks were penetrated in four exploratory oil wells in the orpheus graben scotian shelf offshore eastern canada the lower unit of the volcanogenic sequence is composed of basalt as sills or feeder dikes or flows a few metres to a few tens of metres thick the upper unit consists of tens of metres of pyroclastic and volcaniclastic rocks in which volcanic tephra including glass is mixed with quartz rich sediment interbedded with thin lava flows these rocks show dips of as much as 36 suggesting that they occur as volcanic cones the basalts are geochemically similar to within plate oceanic alkali basalts particularly in their rare earth element ree patterns and trace element composition radiometric ages of 102 to 125 5 ma for the basalts are in good agreement with ages of interbedded sediments dated by palynomorphs as aptian the volcanic rocks are located on continental crust along the possible extension of two oceanic crustal features the newfoundland azores fracture zone and the fogo seamount chain the orpheus graben alkaline volcanism may be an expression of a transform fault propagating into the continent as a result of rejuvenation during plate motion changes in the late lower cretaceous when continental spreading was initiated in the southern atlantic and labrador greenland regions mesozoic volcanism on the atlantic continental margin was episodic with a periodicity of 50 70 ma it is broadly time correlative with major episodes of intraplate oceanic volcanism in both the atlantic and pacific oceans indicating that the volcanism is mantle driven

offset boreholes in the rocky mountains of alberta canada recent northeast directed updip bedding plane displacements of several centimetres are indicated by borehole offsets in road cuts at eight localities in the southern canadian rocky mountains these displacements are thought to record release of tectonic stresses because the displacements are congruent with the present day regional stress field

petrology of the grenville swarm of gabbro dikes canadian precambrian shield the grenville tholeiitic dikes of late proterozoic or early paleozoic age cut marbles and gneisses of the grenville province within and adjacent to the ottawa rift structure where traversed the swarm consists of about 40 large dikes up to 100 m wide representing a crustal extension of 1 km the principal minerals are olivine augite pigeonite plagioclase magnetite ilmenite k feldspar and quartz crystals of pigeonite around olivine and complex augite pigeonite composite grains suggest that the reactions olivine melt pigeonite and pigeonite melt augite have taken place conspicuous zoning and grain to grain variation in the composition of augite are consistent with fractional crystallization but crystal melt equilibrium during crystal growth is indicated by a restricted range in pyroxene paleotemperatures of 1180 1060 c rock textures subophitic combined ophitic subophitic and equigranular in dike centres are related to dike width and are determined principally by the influence of

influence of the fragmentation process on the dynamics of vulcanian eruptions an experimental approach abstract the dynamics of magma fragmentation is a controlling factor in the behavior of explosive volcanic eruptions if porous magma is sufficiently decompressed a fragmentation front develops and travels through the magma at a certain speed fragmentation speed while the resulting particles are being ejected to investigate the influence of the fragmentation process on eruption dynamics we have performed fragmentation experiments in a shock tube apparatus using natural volcanic samples with diverse porosities and different applied pressures 4 20 mpa for each experiment we simultaneously measured the fragmentation speed and ejection velocities the results are consistent with a theoretical model based on a 1 d shock tube theory considering the conservation laws across the fragmentation front our results show that a certain pressure threshold has to be exceeded for fragmentation and ejection of the particles to take place and that the fragmentation speed determines the initial conditions of the expansion of the gas particle mixture the fragmentation process has a controlling influence on the velocity density and mass discharge rate per unit area of the gas particle mixture all factors which can affect eruption dynamics significantly the model presented herein may help describe the dynamics of vulcanian eruptions and improve hazard assessment

origin of the lithospheric tension causing basin formation most fault controlled basin formation within plate interiors occurs by normal faulting in response to horizontal deviatoric tension in the continental crust it is suggested that the tension originates either from the plate boundary forces acting at trenches or as a result of isostatically compensated uplifted regions such as east africa the tension produced by both mechanisms is greatest in high heat flow regions where the upper elastic part of the lithosphere is thinned and weakened particularly widespread tension in the continental lithosphere occurs when subduction takes place on opposite sides of a large continental mass such as pangaea in the early mesozoic where it led to widespread graben formation and in association with hot spot activity to continental splitting

global slab deformation and centroid moment tensor constraints on viscosity we analyze moment tensor solutions from deep subduction zone earthquakes to determine global slab deformation patterns inferred strain rates are compared to predicted deformation patterns from fluid models to help constrain the first order radial and lateral viscosity structure of the earth while all slabs that reach the lower mantle are compressed at their tip intermediate depth patterns are more complex we compute 3 d spherical flow with various slab rheologies and compare the angular misfit between the compressive eigenvectors of the resultant stress field and global centroid moment tensor gcmt solutions we find that upper mantle slab viscosities of 10 100 and lower mantle viscosities of 30 100 times the upper mantle produce the best match to gcmts a 0 1 viscosity reduction in the asthenosphere seems preferred slab geometry and lower mantle viscosity exert significant control on deformation inclusion of the phase changes at 410 km and 660 km increases extensional deformation at intermediate depth and compressional deformation at the lower mantle improving the match to gcmts for strong slabs our conclusions are fairly insensitive to surface boundary conditions however models which include net rotations of the surface with respect to the lower mantle produce compression at intermediate depths for west directed slabs and extension for east directed slabs without allowing for regional variations these models yield the best match to gcmts while significant deviations between model and seismicity remain our results show that seismicity provides an underutilized constraint for slab dynamics

3 d numerical simulations of eruption column collapse effects of vent size on pressure balanced jet plumes abstract buoyant columns or pyroclastic flows form during explosive volcanic eruptions in the transition state these two eruption styles can develop simultaneously we investigated the critical condition that separates the two eruption styles referred to as the column collapse condition by performing a series of three dimensional numerical simulations in the simulation results we identify two types of flow regime a turbulent jet that efficiently entrains ambient air jet type and a fountain with a high concentration of the ejected material fountain type hence there are two types of column collapse jet type and fountain type which type of collapse occurs at the column collapse condition depends on whether the critical mass discharge rate for column collapse mdr cc is larger or smaller than that for the generation of a fountain mdr jf for a given exit velocity temperature controls the relative magnitude of mdr cc relative to mdr jf and hence the type of collapse for given magma properties e g temperature and water content the column collapse condition is expressed by a critical value of the richardson number ri cc g 0 l 0 w 0 2 where g 0 is the source buoyancy l 0 is the vent radius and w 0 is the exit velocity when the jet type collapse occurs at the column collapse condition ri cc is independent of the exit velocity when the fountain type collapse occurs at the column collapse condition on the other hand ri cc decreases as the exit velocity increases as the exit velocity exceeds the sound velocity a robust flow structure with a series of standing shock waves develops in the fountain which suppresses entrainment of ambient air and enhances column collapse

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

moho beneath the north sea compared on normal incidence and wide angle seismic records the mohorovicic discontinuity the moho is properly defined as the step in the velocity of horizontally travelling p waves from about 7 km s 1 to about 8 1 km s 1 its detailed structure varies from place to place but it is universally interpreted as the transition from crust to mantle normal incidence reflections originating at about moho depth have been reported sporadi cally during the past 20 yr during the past 10 yr continuous seismic reflection profiles have been obtained to this depth both on the continents and in the deep sea and at the same time striking improvements have been made in the methods available for interpretation of refraction experiments here we report a comparison between the moho determined by a long range refraction experiment across the north sea and the results of a short normal incidence reflection profile shot along the same line fig 1 we believe this to be the first time that such a direct comparison has been made

explosive volcanic eruptions vii the ranges of pyroclasts ejected in transient volcanic explosions an earlier treatment of the motions of pyroclasts produced in discrete volcanic explosions assumed that clasts were ejected into a stationary atmosphere we argue here that the air in the vicinity of an explosion site will itself be moving radially away from the source with a speed comparable to that of the clasts especially at early times after the start of the motion as a result the initial relative velocity of clasts and air will be small in the earlier treatment this relative velocity was inevitably large leading to an overestimate of the atmospheric drag force at early times and a consequent underestimate of the final clast range we set up a simple model of the explosion process which allows us to link the initial clast and air velocities and to find an approximation to the subsequent air flow field integration of the equations of motion for clasts with a wide range of sizes densities initial speeds and initial elevation angles allows the ranges of these clasts to be found clast range data from three documented explosive eruptions are re analysed to yield initial velocities and the implications of these velocities for pre explosion pressure conditions are discussed

stress field associated with the rupture of the 1992 landers california earthquake and its implications concerning the fault strength at the onset of the earthquake we investigate the space and time history of the shear stress produced on the fault during the 1992 landers earthquake the stress is directly calculated from the tomographic image of slip on the fault derived from near source strong motion data the results obtained shed some light on why the earthquake rupture cascaded along a series of previously distinct fault segments to produce the largest earthquake in california in over 40 years rupture on the 30 km long northernmost segment of the fault was triggered by a large dynamic increase of the stress field of the order of 20 to 30 mpa produced by the rupturing of the adjacent fault segments such a large increase was necessary to overcome the static friction on this strand of the fault unfavorably oriented in today s tectonic stress field this misorientation eventually led to the arrest of rupture the same mechanism explains why rupture broke only a small portion of the johnson valley fault on which the earthquake originally started before jumping to an adjacent fault more favorably oriented we conclude from these results that the dynamic stress field could not sustain and drive the rupture along the strongly misoriented nw se strands of the preexisting fault system instead the dynamic stress field produced new fractures favorably oriented in a n s direction and connecting parts of the old fault system

high resolution image of calaveras fault seismicity 1 by measuring relative earthquake arrival times using waveform cross correlation and locating earthquakes using the double difference technique we are able to reduce hypocentral errors by 1 to 2 orders of magnitude over routine locations for nearly 8000 events along a 35 km section of the calaveras fault this represents 92 of all seismicity since 1984 and includes the rupture zone of the m 6 2 1984 morgan hill california earthquake the relocated seismicity forms highly organized structures that were previously obscured by location errors there are abundant repeating earthquake sequences as well as linear clusters of earthquakes large voids in seismicity appear with dimensions of kilometers that have been aseismic over the 30 year time interval suggesting that these portions of the fault are either locked or creeping the area of greatest slip in the morgan hill main shock coincides with the most prominent of these voids suggesting that this part of the fault may be locked between large earthquakes we find that the calaveras fault at depth is extremely thin with an average upper bound on fault zone width of 75 m given the location error however this width is not resolvably different from zero the relocations reveal active secondary faults which we use to solve for the stress field in the immediate vicinity of the calaveras fault we find that the maximum compressive stress is at a high angle only 13 from the fault normal supporting previous interpretations that this fault is weak

dynamic 3d simulations of earthquakes on en echelon faults one of the mysteries of earthquake mechanics is why earthquakes stop this process determines the difference between small and devastating ruptures one possibility is that fault geometry controls earthquake size we test this hypothesis using a numerical algorithm that simulates spontaneous rupture propagation in a three dimensional medium and apply our knowledge to two california fault zones we find that the size difference between the 1934 and 1966 parkfield california earthquakes may be the product of a stepover at the southern end of the 1934 earthquake and show how the 1992 landers california earthquake followed physically reasonable expectations when it jumped across en echelon faults to become a large event if there are no linking structures such as transfer faults then strike slip earthquakes are unlikely to propagate through stepover s 5 km wide

upper crustal stresses and vertical stress migration in eastern canada diversified phenomena are contributing to our knowledge of various aspects of the stress field and the origin of the crustal stresses in eastern canada both in situ stress measurements to 2100 m and earthquake fault plane solutions in the upper crust 5 20 km indicate that the maximum horizontal stress is greater than the vertical absence of earthquakes in the lower crust 20 km implies a comparatively lower deviatoric horizontal stress in this layer stress directions as determined from both in situ measurements and earthquake fault plane solutions indicate an overprint confined to the ene octant which correlates with the direction predicted by modeling of plate tectonic stresses thus spreading mid atlantic ridge stress is considered to be one of the important contributors to the stress field in eastern canada viscoelastic relaxation of this stress is considered to enhance the deviatoric stress level in the upper crust while simultaneously reducing that in the lower crust the crust acts as a stress guide within which stresses in the upper crust that are lost through brittle failure i e earthquake stress drop can be replenished from the lower crust moreover the directionality predicted by modeling of spreading mid atlantic ridge stress is maintained incomplete postglacial rebound also contributes to deviatoric compression other possible contributors to the upper crustal stress field are basal drag and membrane stress but the relative contributions from these sources are not properly understood

dynamics of strombolian ash plumes from thermal video motion morphology and air entrainment 1 imaging volcanic plumes is essential to provide an observational basis for understanding and modeling plume dynamics during june july 2004 150 strombolian ash plumes were imaged at stromboli volcano italy with a forward looking infrared radiometer flir thermal video camera 30 hz of these 25 80 plumes were suited for different levels of quantitative analyses in this study some simple analyses are applied to constrain basic parameters for the dynamics of strombolian plumes during their initial ascent 130 m plume rise rates covered both gas thrust 15 m s 1 and buoyant regimes 15 m s 1 which in turn controlled lateral spreading rates and air entrainment rates the half angle of lateral spreading of the plume front averaged 7 3 1 6 for gas thrust regimes and 13 5 1 6 for buoyant regimes equating to mean air entrainment coefficients of 0 06 0 12 for gas thrust regimes and 0 22 0 03 for buoyant regimes these factors were also linked to plume morphologies which included jets starting plumes and thermals a rooted thermal form was observed and presumed as an intermediary between starting plumes and discrete thermals plume rise could be approximated by a power law dependence with time rooted thermals spread and entrained air at rates approaching those of a discrete thermal but rose at a rate similar to that of a starting plume phenomena including helical motion and sedimentation were visible in the flir imagery these results demonstrate that emergent plume behavior is progressive and highly transient furthermore this study offers empirical reinforcement that entrainment dynamics are intrinsically different in 1 gas thrust versus buoyantly driven regimes and 2 plume fronts versus steady plumes

deep structure of a fault discontinuity relocated aftershocks at a fault discontinuity that ruptured in the 1992 landers earthquake show that there is no simple connection between the homestead valley and emerson faults at depth rather faulting within the discontinuity is as complex as observed at the surface both aftershocks and mainshock slip on the far side of the discontinuity that is on the emerson fault were confined primarily to depths above 10 km whereas slip and aftershock activity on the near side of the discontinuity extended to 17 km similar behavior was also observed at a fault discontinuity in the 1979 coyote lake earthquake and may be attributable to the greater effectiveness of dynamic stress in inducing fault rupture at shallow depths

growth linkage and termination processes of a 10 km long strike slip fault in jointed granite the gemini fault zone sierra nevada california abstract field based structural analysis of an exhumed 10 km long strike slip fault zone elucidates processes of growth linkage and termination along moderately sized strike slip fault zones in granitic rocks the gemini fault zone is a 9 3 km long left lateral fault system that was active at depths of 8 11 km within the transpressive late cretaceous sierran magmatic arc the fault zone cuts four granitic plutons and is composed of three steeply dipping northeast and southwest striking noncoplanar segments that nucleated and grew along preexisting cooling joints the fault core is bounded by subparallel fault planes that separate highly fractured epidote chlorite and quartz breccias from undeformed protolith the slip profile along the gemini fault zone shows that the fault zone consists of three 2 3 km long segments separated by two zones of local slip minima slip is highest 131 m on the western third of the fault zone and tapers to zero at the eastern termination slip vectors plunge shallowly west southwest and show significant variability along strike and across segment boundaries four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at or below greenschist facies conditions taken together we interpret the fault zone to be a segmented linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations complex fault segment interactions and asymmetric slip distance profiles

state of stress from focal mechanisms before and after the 1992 landers earthquake sequence the state of stress in the eastern california shear zone ecsz changed significantly because of the occurrence of the 1992 m w 6 1 joshua tree and the m w 7 3 landers earthquakes to quantify this change focal mechanisms from the 1975 galway lake sequence the 1979 homestead valley sequence background seismicity from 1981 to 1991 and the 1992 landers sequence are inverted for the state of stress in all cases the intermediate principal stress axis s2 remained vertical and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes s 1 and s 3 and small variations in the value of the relative stress magnitudes in general the stress state in the ecsz has s 1 trending east of north and 0 43 to 0 65 suggesting that the ecsz is a moderate stress refractor and the style of faulting is transtensional south of the pinto mountain fault in the region of the 1992 joshua tree earthquake the stress state determined from the 1981 to 1991 background seismicity changed on 23 april and again on 28 june 1992 in the central zone s 1 rotated from n14 5 e to n28 5 e on 23 april and back again to n16 5 e on 28 june thus the landers mainshock in effect recharged some of the shear stress in the region of the m w 6 1 joshua tree earthquake comparison of the state of stress before and after 28 june 1992 along the landers mainshock rupture zone showed that the mainshock changed the stress orientation the s1 trend rotated 7 to 20 clockwise and became progressively more fault normal from south to north along the emerson camp rock faults the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor the complex distribution of p and t axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock the san bernardino mountains region of the m w 6 2 big bear earthquake has a distinctively different state of stress as compared to the landers region with s 1 trending n3 5 w this region did not show any significant change in the state of stress following the 1992 m w 6 2 big bear sequence

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

dynamics of fault interaction parallel strike slip faults we use a two dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures our results indicate that a strike slip earthquake is unlikely to jump a fault step wider than 5 km in correlation with field observations of moderate to great sized earthquakes we also find that dynamically propagating ruptures can jump both compressional and dilational fault steps although wider dilational fault steps can be jumped dilational steps tend to delay the rupture for a longer time than compressional steps do this delay leads to a slower apparent rupture velocity in the vicinity of dilational steps these dry cases assumed hydrostatic or greater pore pressures but did not include the effects of changing pore pressures in an additional study we simulated the dynamic effects of a fault rupture on undrained pore fluids to test sibson s 1985 1986 suggestion that wet dilational steps are a barrier to rupture propagation our numerical results validate sibson s hypothesis by demonstrating that the effect of the rupture on the undrained pore fluids is to inhibit the rupture from jumping dilational stepovers the basis of our result differs from sibson s hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

regions of reduced static stress drop near fault tips for large strike slip earthquakes abstract we present a case study of slip distributions for the 1992 landers m w 7 3 and the 1999 hector mine m w 7 1 earthquakes in california s mojave desert slip distributions as determined from geophysical inversion of geodetic strong ground motion and teleseismic data are complex heterogeneous and often exhibit linearly tapering or concave upward patterns toward segment tips distinctly different from the elliptical slip distributions characteristic of uniform stress drop mechanical interaction of discontinuous fault segments fully explains the reduced slip near the southern termination of the camp rock emerson fault segment of the landers rupture however numerical models demonstrate that such interactions are insufficient to explain slip distributions observed at other segment terminations numerical models demonstrate that long 5 25 km zones of reduced stress drop in the vicinity of some rupture segment terminations can explain the slip distributions for these large earthquakes zones of reduced stress drop are implemented as regions of increased coulomb strength slip distributions are improved 30 70 relative to models with uniform stress drop regions of reduced stress drop appear to play a relatively greater role near segment tips at which rupture terminates than near segment tips at which rupture jumps to a nearby fault segment similar results are obtained implementing discrete stepwise and spatially linear reductions of stress drop plausible mechanical explanations for such zones of reduced stress drop include heterogeneous fault strength or friction spatial or temporal changes in pore pressure geometric complexity of the fault surface heterogeneity of normal tractions resolved on the fault surface inelastic deformation and dynamic rupture effects

the origin of tectonic stress in the lithosphere abstract the sources of lithospheric stress and their distinctive features are briefly reviewed it is suggested that there are two main categories of lithospheric stress renewable stress which persists despite continuing stress relaxation and non renewable stress which can be dissipated by relief of the initial strain the two most important types of renewable stress arise from plate boundary forces and from isostatically compensated loads non renewable stress systems include bending stresses membrane stresses and thermal stresses an important phenomenon generating large stresses at shallow depth is stress amplification caused by lower lithospheric creep this applies to renewable stresses but not to the non renewable type it is suggested that only renewable stresses contribute significantly to tectonic activity however bending and thermal stresses are locally important in subducting lithosphere

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

geologic evidence for recurrent moderate to large earthquakes near charleston south carolina multiple generations of earthquake induced sand blows in quaternary sediments and soils near charleston south carolina are evidence of recurrent moderate to large earthquakes in that area the large 1886 earthquake the only historic earthquake known to have produced sand blows at charleston probably caused the youngest observed blows older late quaternary sand blows in the charleston area indicate at least two prehistoric earthquakes with shaking severities comparable to the 1886 event

the co seismic slip distribution of the landers earthquake we derived a model for the co seismic slip distribution on the faults which ruptured during the landers earthquake sequence of 28 june 1992 the model is based on the inversion of surface geodetic measurements primarily vector displacements measured using the global positioning system gps the inversion procedure assumes that the slip distribution is to some extent smooth and purely right lateral strike slip for a given fault geometry a family of so lutions of varying smoothness can be generated we choose the optimal model from this family based on cross validation which measures the predictive power of the data and the trade off of misfit and roughness solutions which give roughly equal weight to misfit and smoothness are preferred and have certain features in common 1 there are two main patches of slip on the johnson valley fault and on the homestead valley emerson and camp rock faults 2 virtually all slip is in the upper 10 to 12 km and 3 the model reproduces the general features of the geologically measured sur face displacements without prior constraints on the surface slip in all models regardless of smoothing very little slip is required on the fault that represents the big bear event and the total moment of the landers event is 9 1019 n m the nearly simultaneous rupture of multiple distinct faults suggests that much of the crust in this region must have been close to failure prior to the earthquake

on the state of stress in new england as determined from earthquake focal mechanisms assuming only that slip occurs in the direction of resolved shear stress on fault planes we have found a wide range of uniform stress tensors each of which is consistent with ten diverse focal mechanisms for recent moderate size earthquakes in new england and vicinity thus it is possible that the variations in focal mechanisms are due to the occurrence of specific pre existing zones of weakness rather than to inhomogeneous stresses acceptable stresses are broadly consistent with those established for other parts of the eastern united states namely northeast or east northeast directed maximum compression and nearly vertical minimum compression

integration of surface slip and aftershocks to constrain the 3d structure of faults involved in the m 7 3 landers earthquake southern california investigation of the mechanics of complex earthquakes requires insight into fault geometry we use linear elastic quasi static mechanical models of faults involved in the m 7 3 landers earthquake to test the influence of fault continuity dip and strike on fault slip and the perturbed stress field in regions of complex surface deformation and dense aftershock activity subsurface fault structure is constrained with both geological and geophysical data by comparing observed right lateral surface slip with slip along model faults and by comparing focal mechanisms with model coulomb planes at the locations of large aftershocks the refined structure of major faults at landers includes a southward extension of the johnson valley fault jvf at depth beyond the mapped fault trace a 75 w dip along the central johnson valley fault and a near surface discontinuity along the southern homestead valley fault hvf the final model better captures observed slip deficits along the johnson valley and homestead valley faults improves quantitative fit to the observed offset data and captures the local stress state in 9 of 12 locations studied relating aftershock orientations to local stresses resulting from mainshock fault slip provides a mechanical basis for deciphering between the two nodal planes provided by each focal mechanism slip along nonplanar faults with friction of 0 6 produces model failure planes in a variety of orientations indicating that heterogeneous aftershocks do not necessarily result from low fault friction mechanical selection of nodal planes suggests that aftershocks do not consistently reflect the orientation of their cluster or the orientation and slip direction of mainshock faults

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

explosive volcanic eruptions v observations of plume dynamics during the 1979 soufriere eruption st vincent summary during the 1979 eruption of the soufritre of st vincent west indies a 14min period of explosive activity occurred on april 22 the first three minutes of this activity were filmed five individual explosions formed distinct plumes which fed an eruption column which eventually ascended to a height of over 18 km the ascent velocities of the fronts of these plumes ranged from 8 5 to 61 7m s the half angle of spreading of the plume fronts ranged from 21 5 to 24 one of the plumes was observed to 8 km height and decelerated steadily from 5 1 5 to 23 m s l the main column fed by individual explosions was however observed to accelerate from 8 5 to 58 m s l in the same height interval a theoretical treatment of volcanic plume motion is presented measurements of dimensions velocities and spreading rates from the film are used to estimate plume parameters such as temperature particle content and volume discharge rate of magma from the theoretical relationships these calculations show that the individual plumes became hotter and richer in juvenile ash with time the acceleration of the main eruption column was the result of being fed by increasingly hotter and more ash rich explosions an average volume discharge rate of 12600m3s is estimated from the analysis of the plume motions this value agrees closely with an estimate of discharge rate based on the heat flux required to form an 18 km high column this agreement suggests that theories of convective motions in plumes can be successfully adapted to the volcanic case as long as the effect of hot entrained ash particles is considered

the picton and varty lake ultramafic dikes jurassic magmatism in the st lawrence platform near belleville ontario the picton and varty lake ultramafic dikes that intrude ordovician limestone near belleville ontario consist of olivine or serpentinized pseudomorphs and ti phlogopite phenocrysts set in a groundmass of spinel perovskite serpentine calcite apatite and chlorite phlogopite exhibits a complex chemical variation and late stage ba enrichment up to 6 8 wt bao spinels are chemically zoned and commonly exhibit an atoll texture in which a magnesian aluminous chromite core is separated by a silicate zone from late stage magnesian aluminous titanomagnetite chemically similar to kimberlitic spinel atoll spinels from the picton dike also have a zone of titanian pleonaste surrounding a central grain of magnesian aluminous chromite at the centre of the atoll structures both dikes are extremely silica undersaturated with high k na and mg mg fe2 ratios petrographic and chemical features of the picton and varty lake dikes preclude simple classification as both dikes have aspects indicative of kimberl

satellite observations and interpretation of the 1991 mount pinatubo eruption plumes we demonstrate the use of hourly geostationary meteorological satellite gms and noaa polar orbiting advanced very high resolution radiometer avhrr satellite images of the june 1991 mount pinatubo volcanic eruption plumes in providing details of the timing changing eruptive style and umbrella plume movement and dynamics a chronology of explosive events which culminated in a 9 hour long climactic phase on june 15 has been determined from satellite data and favorably compared to ground based and other ancillary information maximum eruption column altitudes of 40 km and column top undercooling in excess of 55 c below the ambient atmosphere occurred during the climactic phase phases dominated by plinian versus coignimbrite eruptive plumes can be distinguished on the images analysis of the spreading umbrella plume during the climactic phase suggests that over the first 4 5 hours the plume spread laterally as a gravitational intrusion before becoming advected in the ambient winds we compare the rate of plume growth to a model of a radially symmetric continuously fed intrusion to describe the motion the rate of plume growth shows good agreement with this simple model for the first 4 5 hours from column top heights we determine eruption rates and obtain an independent estimate of the total volume of magma erupted from june 12 16 of 5 5 km3 dense rock equivalent dre in good agreement with an independent estimate of the total eruptive volume made from deposits on land and submarine ash fall of up to 5 3 km3 dre we also show that secondary explosions in the pinatubo ignimbrite deposits produced ash plumes reaching altitudes as high as 19 km in september 1991 this study indicates that realistic interpretation and monitoring of major eruption plumes can be accomplished by analysis of high temporal resolution satellite data

a study of joints and stress release buckles in palaeozoic rocks of the niagara peninsula southern ontario joint sets in palaeozoic bedrock of the niagara peninsula occur at 005 045 085 and 135 orientations there is no consistent change in joint orientation with lithology rock age or location within the area drainage pattern analysis using airphotos and topographic maps indicates that glacial drift effectively masks underlying joint systems rare calcite and gypsum mineralization occurs along joints of 085 and 135 trend stress relief buckles indicate a contemporary maximum principal stress along an 050 trend joint systems in the niagara peninsula are similar in trend to those on the appalachian plateau

characterisation of the 1997 vulcanian explosions of soufriere hills volcano montserrat by video analysis the activity of convention at montserrat soufriere hills volcano montserrat during the period 1995 1999 included numerous violent explosions two major cycles of vulcanian explosions occurred in 1997 a first of 13 explosions between 4 and 12 august and a second of 75 between 22 september and 21 october the explosions were short lived events lasting a few tens of seconds during which partial fountain collapse generated pyroclastic surges and pyroclastic flows and buoyant plumes ascended 3 15 km into the atmosphere each explosion discharged on average 3 105 m3 dense rock equivalent dre of magma draining the conduit to depths of 1 2 km the paper focuses on the first few seconds of three explosions of the 75 that occurred in september october 1997 6 october 1997 at 17 50 7 october 1997 at 16 02 and 9 october 1997 at 12 32 physical parameters such as exit velocities magmatic water contents and magma pressures at fragmentation are estimated by following and modelling the ascent of individual momentum dominated finger jets visible on videos during the initial stages of each explosion the model treats each finger jet as an incompressible flow sustained by a steady flux of gas and particles during the few seconds of ascent and produces results that compare favourably with those using a multiphase compressible code run using similar eruptive parameters each explosion reveals a progressive increase in eruptive intensity with time jet exit velocities increasing from 40 m s 1 at the beginning of the explosion up to 140 m s 1 after a few seconds modelling suggests that the first magma to exit was largely degassed whereas that discharged after a few seconds contained up to 2 wt water magma overpressures up to 10 mpa are estimated to have existed in the conduit immediately prior to each explosion progressive increases in jet exit velocity with time over the first few seconds of each explosion provide direct evidence for strong pre eruptive gradients in water content and magma pressure in the upper reaches probably 100 500 m of the conduit fountain collapse occurred during the first 10 20 s of each explosion because the discharging jets had bulk densities up to 100 times that of the atmosphere and were unable to entrain enough air to become buoyant such high eruptive densities were due to the presence of partially degassed magma in the conduit

upwarp of anomalous asthenosphere beneath the rio grande rift continental rifts are possible analogues of mid ocean ridges although major plate tectonic features are less clearly observed1 current thermal models of mid ocean ridges2 4 consist of solid lithospheric plates overlying the hotter less viscous asthenosphere with plate thickness increasing away from the ridge axis the lithospheric lower boundary lies at or near the melting point isotherm so that at greater depths higher temperatures account for lower viscosity lower seismic velocities and possibly partial melting upwarp of this boundary at the ridge axis concentrates heat there thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium at slow spreading ridges a major central graben forms owing to the mechanics of magma injection into the crust5 topography heat flow gravity and seismic studies support these models on the continents a low velocity channel has been observed although it is poorly developed beneath ancient cratons6 9 plate tectonic models have been applied to continental basins and margins10 12 but further similarities to the oceanic models remain elusive topographic uplift is often ascribed to airy type isostatic compensation caused by crustal thickening rather than thermal compensation in the asthenosphere here we discuss the rio grande rift in southwestern united states teleseismic p wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

fault branching and rupture directivity 1 could the directivity of a complex earthquake be inferred from the ruptured fault branches it created typically branches develop in forward orientation making acute angles relative to the propagation direction direct backward branching of the same style as the main rupture e g both right lateral is disallowed by the stress field at the rupture front here we propose another mechanism of backward branching in that mechanism rupture stops along one fault strand radiates stress to a neighboring strand nucleates there and develops bilaterally generating a backward branch such makes diagnosing directivity of a past earthquake difficult without detailed knowledge of the branching process as a field example in the landers 1992 earthquake rupture stopped at the northern end of the kickapoo fault jumped onto the homestead valley fault and developed bilaterally there nnw to continue the main rupture but also sse for 4 km forming a backward branch we develop theoretical principles underlying such rupture transitions partly from elastostatic stress analysis and then simulate the landers example numerically using a two dimensional elastodynamic boundary integral equation formulation incorporating slip weakening rupture this reproduces the proposed backward branching mechanism based on realistic if simplified fault geometries prestress orientation corresponding to the region standard lab friction values for peak strength and fracture energies characteristic of the landers event we also show that the seismic s ratio controls the jumpable distance and that curving of a fault toward its compressional side like locally along the southeastern homestead valley fault induces near tip increase of compressive normal stress that slows rupture propagation

estimation of exit velocity of volcanic plume from analysis of vortex structures abstract we propose a simple method for estimating the exit velocities of volcanic eruptions from the observation of volcanic plumes for this purpose we used a model of a vortex ring of an experimental jet which was developed in the engineering field to validate the model for the vortex structures of volcanic plumes we applied it to plumes generated in 3 d numerical simulations in 11 cases where exit velocity 66 8 200 5 m s is given as a boundary condition we successfully estimated it with 7 underestimation by analyzing the size and motion of the leading vortex ring that forms at the plume front using the same procedure we could also estimate the exit velocity by analyzing the trailing vortices that develop behind the vortex ring 14 underestimation from these results we conclude that i the model of the vortex ring proposed by the jet engineering studies is appropriate for the vortex ring at the front of simulated volcanic plumes and ii the model is also applicable to the trailing vortices of the plumes these conclusions indicate that we can estimate the time evolution of the exit velocity for a series of eruptions from observations of the vortex structures of the actual volcanic plumes by applying that method to an eruption of sakurajima volcano on february 15 2011 we found that following an increase during the first 10 s of the eruption the exit velocity remained constant at 40 m s up to 80 s after the onset of the eruption our method will be useful in understanding the time evolution of eruptive events such as the transitional behavior from stable column to column collapse from observations of volcanic plumes

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

the opening of the atlantic the mesozoic new england igneous province and mechanisms of continental breakup abstract plume and tensionally induced rifting models have been proposed as causes of continental rifting and breakup mantle plumes should produce early lithospheric doming events associated with alkaline magmatism however the triassic alkaline rocks on the east north american e n a margin are structurally localized the uplifts are mostly jurassic in age and voluminous plateau magmatism post dates the initiation of taphrogenesis by 10 to 20 ma in addition there are no chains of anorogenic igneous rocks leading to any of the postulated triassic plumes these facts argue against the plume generated rifting model for the e n a margin but are consistent with the predicted effects of the tensionally induced rifting model the genesis of linear chains of alkaline rocks has been attributed to passage of a lithospheric plate over a mantle plume this explanation has been proposed for the cretaceous monteregian and white mountain intrusive series and the kelvin seamount chain however these rocks are distributed along structurally controlled e w trending lineaments the monteregians are localized by the presence of a paleorift ottawa bonnechere graben in the underlying grenvillian basement and the kelvin seamounts are emplaced along the early jurassic kelvin fracture zone the structures localizing the southern white mountain lineament are less well defined the spatial temporal and geochemical relationships along and between these lineaments are consistent with an origin as a southeastward propagating tensional event the intra plate stresses are probably the result of shifts in the pole of rotation of the north american plate due to irregular northward extensions of seafloor spreading during the cretaceous

slip distribution of the 1992 landers earthquake and its implications for earthquake source mechanics we use near source 10 to 164 km displacement seismograms to image the slip distribution and rupture history of the 28 june 1992 mw 7 3 landers california earthquake aftershock seismograms from similar distances are modeled to find the velocity model and frequency range 0 05 to 0 25 hz over which theoretical green s functions are most accurate and the measure of fit is used as an upper bound on theoretical error in the mainshock inversion we represent the rupture surface with three planar segments divided into 3 by 3 km elements extending from the surface to 18 km depth and solve for the slip distribution and the rupture time model that minimizes misfit to both recorded seismograms and mapped surface displacement in a least squares sense we investigate two faulting models one with a uniformly short 3 sec rise time everywhere on the fault surface and the other with a variable rise time 2 to 6 sec we perform sensitivity tests using synthetic data from a hypothetical earthquake to evaluate model resolution and solution stability in the sensitivity tests both fault models recover similar slip and rupture features but neither is capable of imaging details of the rise time for the landers earthquake we find an average rupture velocity of 2 5 km sec and use this average for a starting model in a linearized inversion for rupture time in our solutions of slip amplitude distribution the southernmost johnson valley fault segment has 20 of the total seismic moment 6 to 8 1019 n m with small displacements near the hypocenter the homestead valley segment contributes half of the moment with the largest slip amplitudes 25 to 35 km northwest of the hypocenter at 4 to 12 km depth and the camp rock emerson segment contributes the remaining moment with the largest slip amplitude 35 to 50 km northwest of the hypocenter in the shallow crust 9 km there is some evidence that the rupture front is delayed as it encounters high slip regions suggesting that prior to the mainshock these areas were further from failure owing either to greater strength or lower prestress

turbulent dynamics of the 18 may 1980 mount st helens eruption column volcanic eruption columns that fail to entrain and heat enough air to become buoyant collapse and generate devastating pyroclastic flows turbulent eddies along the column margins entrain and mix air into the column interior allowing the column to become buoyant currently the turbulent velocity fields of volcanic eruption columns are unknown and therefore numerical models of eruption columns remain untested against geologic observations through extensive reevaluation of video and photographs of the 18 may 1980 eruption of mount st helens united states we report the first measurements of the turbulent velocity field of a volcanic column during the buoyant b2 phase of eruption eddies along the column margins were larger and the fluctuating turbulent component of velocity was lower than during partial column collapse in the b3 phase of eruption we propose that the turbulent structure of the column margins reflects the thickness of and velocity gradients within the column boundary layer in communication with the atmosphere thus eddy size scales with the column radius when the entire column becomes buoyant whereas eddy size is controlled by the thickness of a buoyant annulus surrounding a dense collapsing core during periods of partial column collapse

a numerical study of turbulent mixing in eruption clouds using a three dimensional fluid dynamics model 1 the dynamics of eruption clouds in explosive volcanic eruptions are governed by entrainment of ambient air into eruption clouds by turbulent mixing we develop a new numerical pseudo gas model of an eruption cloud by employing three dimensional coordinates a third order accuracy scheme and a fine grid size in order to investigate the behavior of entrainment due to turbulent mixing the quantitative features of entrainment are measured by a proportionality constant relating the inflow velocity at the edge of the flow to the average vertical velocity i e the entrainment coefficient our model has successfully reproduced the quantitative features of entrainment observed in the laboratory experiments as well as fundamental features of the dynamics of eruption clouds such as the generation of eruption columns and or pyroclastic flows the value of the entrainment coefficient for eruption clouds is estimated from the column height and critical condition for column collapse by comparing results of our model with those of previous one dimensional models it is suggested that the value of the entrainment coefficient for an eruption cloud is approximately constant although the value estimated from the critical condition for column collapse k 0 07 is slightly smaller than that based on the column height k 0 1 this difference reflects the vertical change of flow structure in the eruption cloud the eruption cloud in the upper region exhibits a meandering instability which leads to efficient mixing whereas the cloud near the vent maintains a concentric structure with an inner dense core surrounded by an outer shear region our model is consistent with previous one dimensional models for steady eruption clouds supported by the laboratory experiments and it is also applicable to unsteady and transient features of actual eruption clouds

foreshock sequence of the 1992 landers california earthquake and its implications for earthquake nucleation the june 28 1992 landers california earthquake mw 7 3 was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events in this study we examine the geometry and temporal development of the foreshocks using high precision locations based on cross correlation of waveforms recorded at nearby stations by aligning waveforms rather than trying to obtain travel time picks for each event independently we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals we perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100 m horizontally and less than 200 m vertically with the improved locations the geometry of the foreshock sequence becomes clear the landers foreshocks occurred at a right step of about 500 m in the mainshock fault plane the nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step this geometry is confirmed by the focal mechanisms of the foreshock sequence which are right lateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault and are rotated clockwise for foreshocks that occur within the step the extent of the foreshock sequence is approximately 1 km both vertically and horizontally modeling of the coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other this result is particularly clear for the mw 4 4 immediate foreshock since stress transfer in the sequence appears not to have played a significant role in its development we infer an underlying aseismic nucleation process probably aseismic creep other studies have shown that earthquake nucleation may be controlled by fault zone irregularities this appears to be true in the case of the landers earthquake although the size of the irregularity is so small that it is not detectable by standard location techniques

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

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

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

stress relief buckles in the mcfarland quarry ottawa six quarry floor buckles in a west end ottawa bedrock quarry are described and a semi quantitative estimate of the stress field is inferred from them the maximum principal stress component is hori

paleomagnetism of the hearst dike swarm and implications for the tectonic history of the kapuskasing structural zone northern ontario prominent north to northwest trending dikes herein termed the hearst swarm west of the kapuskasing structural zone ksz in northern ontario yield a paleomagnetic pole position of 48 6 n 69 0 e dm 6 9 dp 3 6 60 samples from 12 dikes using paleomagnetic petrographic and structural criteria the hearst swarm is correlated with the 2 6 ga matachewan swarm east of the ksz a positive correlation is observed between the trend and paleomagnetic declination of hearst matachewan dikes and may result from an 15 post 2 6 ga rotation of terranes across the ksz paleomagnetic and structural data from the hearst dikes are consistent with the regional westward tilting of the crust near the southern ksz as first proposed in 1983 by percival and card

strike slip fault geometry in turkey and its influence on earthquake activity the geometry of turkish strike slip faults has been reviewed and described from this data set it appears that fault geometry the distribution of discontinuities such as bends and stepovers along the main fault trace plays an important role in controlling the location of large earthquake rupture segments along the fault zones large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30 more important than the size of each discontinuity however is the total geometric pattern i e the distribution of adjacent bends and stepovers based not only on distance from one another but also on relative discontinuity size certain geometric patterns restraining single or double bends are particularly common and can be viewed as responsible for strain accumulation along portions of the fault zone fault geometry not only plays a role in the extent of earthquake rupture but also in characteristics of earthquake behavior for example large earthquake epicenters often occur near restraining bends or double bends furthermore aftershocks and swarm activity can often be associated with releasing areas areas along the strike slip fault that are undergoing a relatively large amount of extension

crustal doming and the mechanism of continental rifting abstract bott m h p 1980 crustal doming and the mechanism of continental rifting in j h illies editor mechanism of graben formation tectonophysics 73 1 8 a mechanical explanation of the relationship between crustal uparching and graben formation is suggested based on the geologically supported assumption that doming precedes rifting rifting and graben formation can occur under conditions of crustal tension by the vening meinesz wedge subsidence mechanism modified to apply to the uppermost 20 km or thereabouts of the continental crust if sediment filled troughs of around 5 km depth are to be formed by this mechanism then a persistent tension of about 200 mpa 2 kbar is required such a stress system may result from the combined effect of the topographic load of the uplifted region and the upthrust caused by the underlying low density upper mantle region below if the upper 10 to 20 km of the crust is elastic but the underlying region can deform slowly by visco elastic creep then stress differences of the order of 200 mpa can occur in the upper elastic crust by this process it is suggested that such a stress system rather than one arising from bending of the uparched crust or from plate boundary stresses may be the primary cause of the rifting

quantifying focal mechanism heterogeneity for fault zones in central and southern california summary we present a statistical analysis of focal mechanism orientations for nine california fault zones with the goal of quantifying variations of fault zone heterogeneity at seismogenic depths the focal mechanism data are generated from first motion polarities for earthquakes in the time period 1983 2004 magnitude range 0 5 and depth range 0 15 km only mechanisms with good quality solutions are used we define fault zones using 20 km wide rectangles and use summations of normalized potency tensors to describe the distribution of double couple orientations for each fault zone focal mechanism heterogeneity is quantified using two measures computed from the tensors that relate to the scatter in orientations and rotational asymmetry or skewness of the distribution we illustrate the use of these quantities by showing relative differences in the focal mechanism heterogeneity characteristics for different fault zones these differences are shown to relate to properties of the fault zone surface traces such that increased scatter correlates with fault trace complexity and rotational asymmetry correlates with the dominant fault trace azimuth these correlations indicate a link between the long term evolution of a fault zone over many earthquake cycles and its seismic behaviour over a 20 yr time period analysis of the partitioning of san jacinto fault zone focal mechanisms into different faulting styles further indicates that heterogeneity is dominantly controlled by structural properties of the fault zone rather than time or magnitude related properties of the seismicity

fault plane solutions of intraplate earthquakes in brazil some constraints on the regional stress field abstract determination of intraplate stress fields is fundamental for a better understanding of the forces driving the plates in the south american plate few earthquakes away from the plate boundaries have been recorded by enough stations to have the nodal planes well determined the fault plane solutions of four recent events in central and northern brazil were determined these solutions were compared with previously determined solutions of other events in central and southeast brazil a statistically reliable estimation of the stress field for all of brazil is not yet possible on the basis of these data near the atlantic coast however the data are compatible with a regional stress field with the maximum compressive stress oriented parallel to the expected direction of ridge push or of south america s absolute plate motion mendiguren and richter 1978 farther to the west the stress field changes the direction of the maximum compressive stress seems to be aligned n s rather than e w almost all earthquakes analysed show reverse or strike slip focal mechanisms one event in central brazil however shows normal faulting thus the stress field in brazil is certainly not uniform and in some areas local sources of stress may predominate over regional fields

paleoseismic evidence for recurrence of earthquakes near charleston south carolina a destructive earthquake that occurred in 1886 near charleston south carolina was associated with widespread liquefaction of shallow sand structures and their extravasation to the surface several seismically induced paleoliquefaction structures preserved within the shallow sediments in the meizoseismal area of the 1886 event were identified field evidence and radiocarbon dates suggest that at least two earthquakes of magnitudes greater than 6 2 preceded the 1886 event in the past 3000 to 3700 years the evidence yielded an initial estimate of about 1500 to 1800 years for the maximum recurrence of destructive intraplate earthquakes in the charleston region

the 1999 izmit turkey earthquake a 3d dynamic stress transfer model of intraearthquake triggering before the august 1999 izmit kocaeli turkey earthquake theoretical studies of earthquake ruptures and geological observations had provided estimates of how far an earthquake might jump to get to a neighboring fault both numerical simulations and geological observations suggested that 5 km might be the upper limit if there were no transfer faults the izmit earthquake appears to have followed these expectations it did not jump across any step over wider than 5 km and was instead stopped by a narrower step over at its eastern end and possibly by a stress shadow caused by a historic large earthquake at its western end our 3d spontaneous rupture simulations of the 1999 izmit earthquake provide two new insights 1 the west to east striking fault segments of this part of the north anatolian fault are oriented so as to be low stress faults and 2 the easternmost segment involved in the august 1999 rupture may be dipping an interesting feature of the izmit earthquake is that a 5 km long gap in surface rupture and an adjacent 25 restraining bend in the fault zone did not stop the earthquake the latter observation is a warning that significant fault bends in strike slip faults may not arrest future earthquakes manuscript received 30 august 2000

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults