simulation of seismic cycles of buried intersecting reverse faults to understand how reverse fault systems interact 1 perform a numerical simulation of seismic cycles the model fault system in a two dimensional uniform elastic half space consists of a shallowly dipping listric main reverse fault and a subfault with a higher dip angle a fixed slip rate is imposed on deeper parts of the main fault and frictional stress on the remaining parts of the main fault and on the subfault obeys a rate and state dependent friction law the main results of the numerical simulation are as follows 1 because of fault interactions there exists moderate scatter in recurrence intervals of large earthquakes on the main fault and the subfault 2 the cumulative slip amounts on deeper parts of the main fault on shallower parts of the main fault and on the subfault are different from one another their values are such that the stress concentration at the fault intersection is a minimum 3 a large earthquake on the main fault increases shear stress and decreases normal stress on the subfault causing slip on deeper parts of the subfault a subfault earthquake sometimes induces slip on the main fault result 3 is consistent with some seismic observations that two conjugate fault planes were active during an earthquake sequence results 1 and 2 may be important for long term evaluation of seismic potential

models for midcontinent tectonism an update during the past decade significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical geologic data base which permits an integration of seismicity data crustal structures and current stress directions critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns the direction of the maximum horizontal compressive stress is generally directed east northeast which supports an origin associated with ridge push forces related to movement of the north american plate crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data these data have been used to relate current seismicity and associated tectonism to one of two models increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field the zone of weakness model as the dominant control on contemporary tectonism and the local basement inhomogeneity model as a mechanism for minor low energy release earthquake activity

topography and tectonics of the central new madrid seismic zone results of numerical experiments using a three dimensional boundary element program we present results of a series of numerical experiments designed to test hypothetical mechanisms that drive deformation in the new madrid seismic zone experiments are constrained by subtle topography and the distribution of seismicity in the region we use a new boundary element algorithm that permits calculation of the three dimensional deformation field surface displacement fields are calculated for the new madrid zone under both far field plate tectonic scale and locally derived driving strains results demonstrate that surface displacement fields cannot distinguish between either a far field simple or pure shear strain field or one that involves a deep shear zone beneath the upper crustal faults thus neither geomorphic nor geodetic studies alone are expected to reveal the ultimate driving mechanism behind the present day deformation we have also tested hypotheses about strain accommodation within the new madrid contractional step over by including linking faults two southwest dipping and one vertical recently inferred from microearthquake data only those models with step over faults are able to predict the observed topography surface displacement fields for long term relaxed deformation predict the distribution of uplift and subsidence in the contractional step over remarkably well generation of these displacement fields appear to require slip on both the two northeast trending vertical faults and the two dipping faults in the step over region with very minor displacements occurring during the interseismic period when the northeast trending vertical faults are locked these models suggest that the gently dipping central step over fault is a reverse fault and that the steeper fault extending to the southeast of the step over acts as a normal fault over the long term

modeling of slow slip events along the deep subduction zone in the kii peninsula and tokai regions southwest japan 1 in the subduction zone of southwest japan short term slow slip events sses occur with low frequency tremors lfts at intervals of several months recently lfts have been located with high resolution and their activities have been examined in detail by setting the generation zones of sses such that these zones contain the lft hypocenters we simulate sses on a 3d plate interface beneath the kii peninsula and tokai regions by using a rate and state dependent friction law with a small cut off velocity for the evolution effect our numerical results show that recurrence intervals of sses in the southern and central kii peninsula in the northern kii peninsula and in the tokai region are 2 5 3 0 4 8 5 6 and 3 5 4 5 months respectively which are consistent with observed sse activity our simulation also produces a multisegment event that propagates from the kii to tokai segments at a speed of 10 km day which is consistent with observations the results suggest that generation zones of lfts coincide with sse regions and that these two events are different manifestations of the same slip process along the subduction zone we also perform 3d modeling of faster events accompanied by short term sses by considering local circular patches with a smaller critical displacement surrounded by sse zones with a larger critical displacement local circular patches are set at the observed lft locations the simulation reproduces fast events that propagate at a speed of 100 200 km day during a single sse event

tectonic stress within the new madrid seismic zone refraction data indicate a significant high density rift pillow beneath the new madrid seismic zone we present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present day stress field and we consider the implications for intraplate seismicity these models were run for a loading time of 100 m y to account for relaxation and transfer of stress since the last reactivation of the rift in the mid mesozoic results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes the model predicts a maximum compression of 30 40 mpa above the rift pillow in the center of the rift axis if the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10 30 in the direction of shmax maximum horizontal compression near the rift axis the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces the addition of the local stress associated with the rift pillow however results in an approximately 30 reduction in the resolved maximum horizontal shear stress thus while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure reduction in the resolved shear stress requires a separate mechanism for strength reduction results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m y and may still play a role in present day deformation and seismicity of the new madrid seismic zone local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones

aeromagnetic mapping of precambrian geological structures that controlled the 1968 meckering earthquake ms 6 8 implications for intraplate seismicity in western australia abstract occurring in the southwest of western australia the 1968 meckering earthquake ms 6 8 resulted in the formation of an extensive surface rupture complex comprising faults with a range of orientations and reverse and dextral lateral offsets modeling of the seismological characteristics of the source showed that reverse failure occurred on a north south striking east dipping surface but how this is related to the local precambrian bedrock geology is not clear interpretation of new aeromagnetic data has allowed concealed bedrock lithology and structure to be mapped in previously unachievable detail these data show that the surface faulting correlates closely with linear magnetic anomalies interpreted as dykes faults and lithological contacts the complicated pattern of surface faulting contrasts with the more simple seismological fault model but can be explained in terms of the reactivation of northeasterly dykes and faults and northwesterly stratigraphic trending features in a stress regime with an east west oriented maximum principal stress space problems created where these two trends converge led to the creation reactivation of a linking north south trending thrust fault which accommodated the greatest displacements recorded for the 1968 event the district scale distribution of epicentres in the 3 years encompassing the meckering event shows the same northeasterly and northwesterly trends as seen in the aeromagnetic data the implied basement features controlling the seismicity will be prone to strike slip failure in the regional east west oriented stress field it is speculated that smaller events in the meckering area will tend to be strike slip and these account for most of the strain larger events all known examples of which involve predominantly thrusting are caused by stress build up where strike slip faults with the two orthogonal trends intersect this hypothesis provides an explanation for the lack of topography in the region which is incompatible with the high level of seismic activity and a predominance of thrust faulting

controls on intra plate seismicity in southwestern australia abstract although the southwest seismic zone swsz located about 150 km to the east of perth in southwestern australia is one of the most seismically active areas in australia there is little understanding as to why the earthquakes are occurring an analysis of geophysical geological and geodetic data from the area suggests that the swsz coincides with a precambrian terrane boundary seismic data show that the terrane boundary zone dips at a shallow angle in a northeasterly direction reactivation of this zone of weakness in the contemporary stress field east west maximum horizontal stress is interpreted to be the first order control on seismicity in the region gravity data show that the terrane boundary is offset by near orthogonal structures which are interpreted as faults at least one of these trends corresponds with a linear zone of epicentres the temporal and spatial distributions of epicentres associated with the 1968 meckering earthquake ml 6 9 suggest that the second order distribution of seismicity in the swsz can be explained by the intersection model whereby stresses are amplified by space problems associated with displacements on crosscutting faults it is speculated that a zone of high density and high seismic velocity in the lower crust may also be a second order control on the local seismicity however confirmation awaits better delineation of the extent of this zone

geophysical and isotopic mapping of preexisting crustal structures that influenced the location and development of the san jacinto fault zone southern california we examine the role of preexisting crustal structure within the peninsular ranges batholith on determining the location of the san jacinto fault zone by analysis of geophysical anomalies and initial strontium ratio data a 1000 km long boundary within the peninsular ranges batholith separating relatively mafic dense and magnetic rocks of the western peninsular ranges batholith from the more felsic less dense and weakly magnetic rocks of the eastern peninsular ranges batholith strikes north northwest toward the san jacinto fault zone modeling of the gravity and magnetic field anomalies caused by this boundary indicates that it extends to depths of at least 20 km the anomalies do not cross the san jacinto fault zone but instead trend northwesterly and coincide with the fault zone a 75 km long gradient in initial strontium ratios sr i in the eastern peninsular ranges batholith coincides with the san jacinto fault zone here rocks east of the fault are characterized by sr i greater than 0 706 indicating a source of largely continental crust sedimentary materials or different lithosphere we argue that the physical property contrast produced by the peninsular ranges batholith boundary provided a mechanically favorable path for the san jacinto fault zone bypassing the san gorgonio structural knot as slip was transferred from the san andreas fault 1 0 1 5 ma two historical m6 7 earthquakes may have nucleated along the peninsular ranges batholith discontinuity in san jacinto valley suggesting that peninsular ranges batholith crustal structure may continue to affect how strain is accommodated along the san jacinto fault zone

quasi dynamic 3d modeling of the generation and afterslip of a tohoku oki earthquake considering thermal pressurization and frictional properties of the shallow plate boundary the generation of the 2011 tohoku oki earthquake has been modeled by many authors by considering a dynamic weakening mechanism such as thermal pressurization tp because the effects of tp on afterslip have not been investigated this study develops a 3d quasi dynamic model of the earthquake cycle to investigate afterslip of the tohoku oki earthquake considering tp and the geometry of the plate boundary we employ several velocity weakening vw patches for mw 7 class events and two large shallow vw patches the frictional properties are set as velocity strengthening vs outside the vw patches the results show that during megathrust earthquakes fast slip propagates to the surrounding vs regions near the vw patches owing to weakening by tp following mw 9 events large afterslips occur in regions below the northern shallow rupture area in the off fukushima region close to the japan trench which is consistent with observations in the vs region near the vw patches during the early afterslip period frictional behavior exhibits less vs with increasing slip velocity due to pore pressure reduction we also consider the frictional properties of the shallow plate boundary fault off tohoku which exhibits a transition from vw to vs from low to high slip velocities the results show the occurrence of slow slip events sses at intervals of a few decades at the shallow plate boundary during megathrust events the vw property at low slip velocity promotes slip along the shallow sse region more than the case with vs property throughout the entire velocity range

fault model for preseismic deformation at parkfield california we construct an earthquake instability model to estimate precursory faulting and ground deformation before the next moderate m 5 5 6 earthquake on the san andreas fault near parkfield california the quasi static model simulates fault slip fault shear stress and ground deformation for all stages of repeated earthquake cycles unstable slip which is the analog of a moderate parkfield mainshock is caused by overall failure of a postulated strong area or patch of the fault zone the brittle patch and surrounding weaker viscous fault are defined by the sign of a spatially varying coefficient of a slip rate dependent fault law stable failure of the patch leading up to mainshock failure causes slip rate to increase at depth amplitudes and timescales of the resulting preseismic deformation anomalies are found for the current earthquake cycle at parkfield starting just after the 1966 mainshock in two ways in the first way we vary parameter values in a sequence of simulations so as to find the simulation giving best agreement with locations moments and recurrence times of past moderate earthquakes and also with fault creep and trilateration line length changes since the 1966 mainshock in the second way we assign values of most parameters from laboratory friction experiments and a temperature depth profile inferred from heat flow data overall agreement between model results and observations is comparable for the two methods in all simulations preseismic deformation anomalies arise from slip rate increase near the model earthquake focus and start several years or less before instability preseismic anomalies in surface fault creep and line length are always too small to detect in current measurements however for some simulations constrained mainly by field data the predicted anomalies in borehole dilatation are large enough to detect

the growth of orogenic belts and the role of crustal heterogeneities in decollement tectonics a series of finite element models of contractional deformation are presented that examine the role preexisting mechanical heterogeneities within the lithosphere play in controlling the partitioning of stress and strain during orogenesis the models focus particularly on orogens that develop near previously rifted continental margins although many of the observations are relevant to other fold and thrust belts the models indicate that orogen evolution is strongly influenced by crustal architecture during the first 150 to 200 km of shortening major decollements develop at midcrustal and lower crustal levels partitioning strain into upper crustal lower crustal and mantle strain domains the magnitude and spatial distribution of strain within each domain are nearly uniform but differ significantly between domains the decollements extend throughout the entire width of the orogen providing a successively increasing degree of decoupling between deformation in the shallow crust and deeper levels the midcrustal decollement terminates beneath the leading edge of the thrust front but the deep crustal decollement extends more than 500 km beyond the foreland this allows up to 5 10 strain to develop well in front of the orogenic belt in the interior of the orogen deformation extends through all levels of the crust beginning during the earliest stages of contraction deformation propagates continentward with increasing shortening with the foreland regions undergoing the most intense shortening in the shallow crust the loci of maximum total shortening within the upper and lower crust are laterally offset with the thickest crust lying continentward of the greatest elevations and the site of most intense surface deformation this pattern of crustal thickness variation is a primary feature of orogenesis in the models and is not related to postorogenic collapse after 150 to 200 km of shortening sufficient topography develops to cause the balance between rock strength gravitational forces and shear stress on the basal detachment to dominate the style of deformation preexisting mechanical heterogeneities thereafter exert little influence on orogenic processes

stress observations and driving force models for the south american plate stress observations are compared to predictions of driving force models for the south american plate stress observations have the potential to give detailed information about the forces driving the plates in that sets of forces with the same net torque can produce quite different stress patterns for a two dimensional vertical section model of a plate there are three conditions for dynamical equilibrium the net forces and the net torque about any point must all be zero these conditions are used to estimate the magnitudes of forces as a function of age with ridge push and slab pull as active forces and plate drag and slab resistance as passive forces the results are consistent with earlier published models and indicate that the primary balance for an oceanic plate is between slab pull and the resisting drag forces we generalize the model for the possible forces driving the south american plate and compare with observed stress measurements and kinematics the south american plate has a small slab pull component amounting to about one fifth the ridge push force with the other components reasonably isolated geographically thus the plate balance is largely between ridge push and plate drag this gives an opportunity of resolving the effects of trench suction or collision at the subduction zones most remarkable is the nearly uniform e w orientation of the regional stress field shmax directions which is extensional in the high andes a second feature is the n s orientations in the amazon basin the two dimensional horizontal stress patterns are compared for successive models of combinations of driving forces models with ridge push slab pull at the scotia and caribbean arcs and trench suction near the west coast balanced by plate drag produce similar stress patterns to those observed the magnitude of a possible trench suction force is estimated using a model with an eddy between the nazca slab and the overlying south american plate

rapid intraplate strain accumulation in the new madrid seismic zone remeasurement of a triangulation network in the southern part of the new madrid seismic zone with the global positioning system has revealed rapid crustal strain accumulation since the 1950s this area experienced three large moment magnitudes 8 earthquakes in 1811 to 1812 the orientation and sense of shear is consistent with right lateral strike slip motion along a northeast trending fault zone as indicated by current seismicity detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time

local stress concentration in the seismic belt along the japan sea coast inferred from precise focal mechanisms implications for the stress accumulation process on intraplate earthquake faults 1 spatial changes in the azimuth of the maximum principal stress 1 axis are found in the intraplate region of southwest japan it is inferred from stress inversions of numerous precise focal mechanisms that the azimuths of the 1 axis in the seismic belt along the japan sea coast are oriented in n110 e n130 e directions while in the surrounding region they are aligned in almost the ew direction n90 e n100 e maximum horizontal compressional stresses in an ew direction are widely observed in the shallow crust in the inland plate along the nankai trough however the maximum horizontal stress in a wnw ese direction is seen only in the seismic belt this spatial change in the stress field in and around the seismic belt is explained by the deformation of an aseismic fault or a ductile fault zone in the lower crust beneath the seismic belt

sinking mafic body in a reactivated lower crust a mechanism for stress concentration at the new madrid seismic zone we propose a geodynamic model for stress concentration in the new madrid seismic zone nmsz the model postulates that a high density mafic body situated in the deep crust directly beneath the most seismically active part of the nmsz began sinking several thousands of years ago when the lower crust was suddenly weakened based on the fact that deformation rates in the nmsz have accelerated over the past 9 k y we envision the source of this perturbation to be related to the last north american deglaciation excess mass of the mafic body exerts a downward pull on the elastic upper crust leading to a cycle of primary thrust faulting with secondary strike slip faulting after which continued sinking of the mafic body reloads the upper crust and renews the process this model is consistent with the youth of activity the generation of a sequence of earthquakes and the velocity evolution during interseismic periods which depend upon the density contrast of the mafic body with respect to the surrounding crust its volume and the viscosity of the lower crust

are light dark coupled laminae in lacustrine shale seasonally controlled a case study using astronomical tuning from 42 2 to 45 4 ma in the dongying depression bohai bay basin eastern china abstract as a typical sedimentary structure in fine grained rocks laminae are widely distributed in shales and mudstones in sedimentary basins the shahejie formation 42 2 45 4 ma formed during the eocene of the paleogene in the dongying depression of china is recognized as a typical area to study lacustrine shale according to the composition of different lamina four types of couplets are identified including carbonate clay couplets carbonate organic couplets clay organic couplets and carbonate clay organic triplets all couplets are composed of light and dark layers a combination of core images microscopic observations mineral compositions geochemical data carbon and oxygen isotopes and strontium isotopes verifies that the laminae are primarily developed in a saline and anoxic or even euxinic environment with a high organic matter om flux through astronomical cycle analysis of the natural gamma ray gr curve from sample site well ny1 the data suggest that the lacustrine shale laminae are formed annually with an average duration of 1 34 yr the formation of laminae is affected by the season which is closely related to the seasonal growth and death of algae during the period of algae growth a large number of light colored carbonate laminae are deposited whereas when the algae dies organic matter accumulates at the bottom of lakes and forms dark organic rich laminae the interbedded and lenticular laminae which are subdivided by layering characteristics are primarily affected by diagenesis in postdepositional processes study of the laminae is helpful in understanding the formation process of lacustrine shale and it provides invaluable sources of information for paleoclimate reconstruction

stressing of the new madrid seismic zone by a lower crust detachment fault a new mechanical model for the cause of the new madrid seismic zone in the central united states is analyzed the model contains a subhorizontal detachment fault which is assumed to be near the domed top surface of locally thickened anomalous lower crust rift pillow regional horizontal compression induces slip on the fault and the slip creates a stress concentration in the upper crust above the rift pillow dome in the coseismic stage of the model earthquake cycle where the three largest magnitude 7 8 earthquakes in 1811 1812 are represented by a single model mainshock on a vertical northeast trending fault the model mainshock has a moment equivalent to a magnitude 8 event during the interseismic stage corresponding to the present time slip on the detachment fault exerts a right lateral shear stress on the locked vertical fault whose failure produces the model mainshock the sense of shear is generally consistent with the overall sense of slip of 1811 1812 and later earthquakes predicted rates of horizontal strain at the ground surface are about 10 7 year 1 and are comparable to some observed rates the model implies that rift pillow geometry is a significant influence on the maximum possible earthquake magnitude

seismic moment assessment of earthquakes in stable continental regions iii new madrid 1811 1812 charleston 1886 and lisbon 1755 summary the sizes of three major or great historical earthquakes are reassessed using the isoseismal area regression tools developed in parts i and ii of this study of stable continental region scr seismicity the earthquakes are 1811 new madrid central united states and its following sequence 1886 charleston coastal south carolina and 1755 lisbon oceanic intraplate off the continental shelf of portugal the analysis confirms the large size of these events and for the first time places constraints on the uncertainty of their seismic moment release because of the exceptionally low seismic wave attenuation of eastern north america ena a separate north american regression of seismic moment on isoseismal area was developed additionally the unknown western extents of the new madrid isoseismal areas were calibrated with the patterns of the m 6 3 6 6 1843 and 1895 earthquakes application of part ii analysis procedures with these corrections yields new madrid size estimates expressed as moment magnitude of m 8 1 0 31 for the 1811 december 16 m 7 8 0 33 for the 1812 january 23 and m 8 0 0 33 for the 1812 february 7 principal events the charleston earthquake s magnitude decreases from m 7 4 to m 7 3 0 26 after compensation for the effect of coastal plain sediments on its inner isoseismals intensity regressions for lisbon are calibrated against the isoseismal pattern of the nearly co located m 7 8 1969 st vincent earthquake which in this case increases the predicted size of lisbon from m 8 4 to m 8 7 0 39 these size estimates are supported by data from independent phenomena extent and severity of liquefaction the maximum distance of induced landslides and for lisbon tsunami wave amplitudes estimated source parameters are controlled by crustal or lithospheric temperature which governs the depth extent of brittle faulting using estimated continental and oceanic geotherms viable fault lengths are 30 80 km for charleston 120 180 km for 1811 new madrid and 180 280 km for lisbon for average displacements of 2 4 m 8 11 m and 10 14 m respectively and for average static stress and strain drops at the estimated seismic moments of this study the 1811 new madrid and the 1755 lisbon events are respectively the largest known scr and oceanic lithosphere earthquakes

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

tectonic development of the new madrid rift complex mississippi embayment north america abstract geological and geophysical studies of the new madrid seismic zone have revealed a buried late precambrian rift beneath the upper mississippi embayment area the rift has influenced the tectonics and geologic history of the area since late precambrian time and is presently associated with the contemporary earthquake activity of the new madrid seismic zone the rift formed during late precambrian to earliest cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions the configuration of the buried rift is interpreted from gravity magnetic seismic refraction seismic reflection and stratigraphic studies the increased mass of the crust in the rift zone which is reflected by regional positive gravity anomalies over the upper mississippi embayment area has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex the correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures the slippage is due to reactivation of the structure by the contemporary nearly e w regional compressive stress which is the result of plate motions

refinement of thrust faulting models for the central new madrid seismic zone abstract we present the results of the joint relocation of events recorded during 1989 1992 by the panda network in the central new madrid seismic zone the near surface material in the study area is a gently dipping layer of poorly consolidated sediments with low p wave velocity and high v p v s estimated values 1 8 km s 1 and 3 the sediments are underlain by high velocity paleozoic rocks under the network the difference in sediment thickness is only 0 6 km but because of the low velocities the location of the events using layered models is affected by errors application of the joint hypocentral determination jhd technique to a subset of 580 events shows that the single event locations may be in error by as much as 1 km in depth depending on where the events are located analysis of synthetic data generated for a realistic 3 d velocity model supports the jhd results the analysis of synthetic data also suggests that a v p v s 2 3 is more appropriate for the post paleozoic mississippi embayment sediments based on the jhd locations we present a new interpretation of the seismicity with two en echelon sw dipping thrust faults connected by a west dipping thrust fault these faults appear associated with the reelfoot scarp and its northern extension the kentucky bend scarp

the distribution of stress with depth in the lithosphere thermo rheological and geodynamic constraints horizontal tectonic forces arising from the plate tectonic process give rise to horizontal stress within the lithosphere whose depth distribution is controlled by plastic and brittle strength a constant force model of lithosphere deformation incorporating the brittle viscoelastic response of each infinitesimal component of lithosphere allows for the transient computation of lithosphere stress and overcomes the steady state approximation of the constant strain rate model for the constant force model creep in the lower crust and mantle leads to stress decay in these regions and to stress amplification in the upper lithosphere through stress redistribution the lithosphere stress depth relationship is primarily controlled by rheology which depends on geothermal gradient crustal composition and thickness for continental lithosphere low strength regions exist within and at the base of the crust due to vertical contrasts in composition and therefore rheology changes in tectonic force with time give reversals in the sign of stress with depth so that lithosphere has a memory of tectonic force history continental lithosphere strength calculated by the constant force model is comparable with estimated levels of intraplate tectonic force horizontal stress within oceanic lithosphere arises due to plate boundary forces and cooling of oceanic lithosphere application of the increasing ridge push force with age to a cooling thermo rheological model of oceanic lithosphere predicts compressive horizontal stresses whose magnitude increases with age and which decrease almost linearly with depth a thermo rheological model of thermal stress for cooling oceanic lithosphere incorporating plastic and brittle deformation predicts horizontal compression in the upper lithosphere above deeper tension cooling and ridge push combine to produce a compressive stress field in the ocean basins capable of producing brittle failure

geologic structures related to new madrid earthquakes near memphis tennessee based on gravity and magnetic interpretations new inversions of gravity and magnetic data in the region north of memphis tennessee and south of latitude 36 define boundaries of regional structures and igneous complexes in the upper crust microseismicity patterns near interpreted boundaries suggest that igneous complexes influence the locations of microseismicity a weak seismicity cluster occurs near one intrusion covington pluton at the intersection of the southwest margin of the missouri batholith and the southeast margin of the reelfoot rift a narrow seismicity trend along the reelfoot rift axis becomes diffuse near a second intrusion osceola intrusive complex and changes direction to an area along the northwest flank of the intrusion the axial seismicity trend also contains a tight cluster of earthquakes located just outside the osceola intrusive complex the mechanical explanation of the two seismicity patterns is uncertain but the first cluster may be caused by stress concentration due to the high elastic stiffness and strength of the covington intrusion the spatially changing seismicity pattern near the osceola complex may be caused by the preceding factors plus interaction with faulting along the rift axis the axial seismicity strand itself is one of several connected and interacting active strands that may produce stress concentrations at strand ends and junctions the microseismicity clusters at the peripheries of the two intrusions lead us to conclude that these stress concentrations or stressed volumes may be locations of future moderate to large earthquakes near memphis

fluid enhanced melting during prograde metamorphism anatexis is a commonly recognized feature of high grade metamorphism but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path a suite of small volume deformed two mica leucogranites has been recognized within the high himalayan crystalline series of the garhwal himalaya these granites are consistently more siliceous than minimum melt granite compositions and are characterized by low rb sr ratios high ba low abundances of hfs elements and positive eu anomalies such trace element characteristics contrast strongly with the geochemistry of the well studied early miocene leucogranites of the high himalaya derived from fluid absent melting sm nd garnet dating of one deformed granite indicates a crystallization age of 39 3 ma c 15 ma before the emplacement of the more voluminous high himalayan leucogranites whilst some entrainment of restitic phases cannot be excluded trace element signatures suggest a low temperature 2 o activity positive eu anomalies and unusually low rb sr ratios are indicative of rapid disequilibrium melting fluid enhanced melting may be a common feature of prograde upper amphibolite facies metamorphism of orogenic belts predating peak metamorphism by at least 15 ma these melts will only crystallize within this period if they segregate from their protoliths subsequent dating of long lived melts would indicate erroneously young ages for the prograde melting events however melts formed in this way may be recognized by their distinctive trace element chemistry the persistence of early formed melts within an orogen provides insights into the prograde heating path and may be critical in controlling the rheology of the middle crust and hence its deformational history

deep crustal structure in the southwestern yilgarn craton western australia abstract the archaean rocks of the yilgarn craton comprise the largest geological entity in western australia and one of the largest areas of archaean crust anywhere in the world however poor outcrop has prevented a thorough understanding of the structure of the crust in the region a subject of particular interest being the relationship between high grade metamorphic rocks in the west and lower grade granitoid greenstone terrains further east interpretation of previously unpublished seismic refraction data combined with the re interpretation of published data allows crustal velocity structure in the southwest of the craton to be resolved overall the structure is similar to shield areas elsewhere in the world the data defining a two layered crust with an average thickness of about 35 km however significant lateral variations in velocity structure are also observed and these coincide with a terrane boundary previously postulated on the basis of geological observations and seismic reflection data the terrane boundary dips to the east extends through the entire crust and is notable for an area of anomalously high seismic velocity in the lower crust two possible interpretations of the high velocity zone are proposed firstly it is due to the presence of mafic to ultramafic intrusions secondly and the preferred interpretation the high velocity zone is a fault bounded mega sliver which may be a suspect terrane in its own right perhaps with oceanic affinities the results of the seismic experiment strongly support tectonic models for yilgarn craton based on terrane tectonics the possibility that the terrane boundary is being reactivated by contemporary stresses also provides a plausible explanation for the local intra plate seismicity

quantitative investigations of the missouri gravity low a possible expression of a large late precambrian batholith intersecting the new madrid seismic zone analysis of gravity and magnetic anomaly data helps characterize the geometry and physical properties of the source of the missouri gravity low an important cratonic feature of substantial width about 125 km and length 600 km filtered anomaly maps show that this prominent feature extends nw from the reelfoot rift to the midcontinent rift system geologic reasoning and the simultaneous inversion of the gravity and magnetic data lead to an interpretation that the gravity anomaly reflects an upper crustal 11 km thick batholith with either near vertical or outward dipping boundaries considering the modeled characteristics of the batholith structural fabric of missouri and relations of the batholith with plutons and regions of alteration a tectonic model for the formation of the batholith is proposed the model includes a mantle plume that heated the crust during late precambrian and melted portions of lower and middle crust from which the low density granitic rocks forming the batholith were partly derived the batholith called the missouri batholith may be currently related to the release of seismic energy in the new madrid seismic zone earthquake concentrations occur at the intersection of the missouri batholith and the new madrid seismic zone three qualitative mechanical models are suggested to explain this relationship with seismicity

crustal structure beneath the wabash valley seismic zone from the joint inversion of receiver functions and surface wave dispersion implications for continental rifts and intraplate seismicity

fault geometry and earthquakes in continental interiors abstract although large earthquakes in continental interiors are much less frequent than those along plate boundaries they have been responsible for a disproportionate amount of destruction i present a testable model for strain accumulation and seismicity in the upper crust of continental interiors intersecting faults provide a focus for strain build up resulting in local pockets of high strain accumulation rigid plates typically deform at strain rates 10 10 to 10 9 strain yr 1 but at intersecting fault zones strain accumulates at anomalously high rates of 10 8 to 10 7 strain yr 1 i e the strain rates are comparable with those at plate boundaries the resulting large earthquakes are associated with relatively short return times hundreds of years typically one of the intersecting faults appears to split and to offset the other main fault defined as a fault on which the larger offset occurs forming two parallel strands separated by a jog i also observe a rotation of one leg of the main fault with respect to the other the region of intersection is usually associated with intense fracturing increased microseismicity and uplift

localization of intraplate deformation through fluid assisted faulting in the lower crust the flinders ranges south australia abstract in this paper we present a hypothesis for localized intraplate deformation in the continental crust of south central australia that involves fluid assisted reactivation of faults in the mid to lower crust using data from a temporary seismometer deployment in the flinders ranges we show that earthquakes relocated in a 3d velocity model cluster in elongated low vp vs anomalies that extend to depths exceeding 20 km and are aligned with the axis of the flinders ranges in the northern flinders ranges these low vp vs anomalies can be interpreted as fractured neoproterozoic to cambrian sediments that separate two cratonic blocks the gawler craton to the west and the curnamona province in the east previous studies of helium isotopes in springs to the north of the area provide evidence of mantle derived fluids that may influence faulting at depth our focal mechanism and stress inversion results show a regionally compressive stress field that provides no evidence for stress concentration we also argue that mechanisms for localized faulting such as thermal weakening and isostatic rebound also fail to account for the occurrence of earthquakes at mid to lower crustal depth in this area of high heat flow and that the focused seismicity can only be explained by high pore fluid pressure in the lower crust

paleoseismology of the southeastern reelfoot rift in western tennessee and implications for intraplate fault zone evolution 1 the southeastern reelfoot rift margin is expressed as a series of collinear scarps across western tennessee and shallow s wave reflection profiles coring and trenches show late quaternary faulting along this lineament at the north end our seismic profile and coring suggest 1 5 m of wisconsin down to the nw faulting coring and trenches across the central lineament show 3 m of up to the nw and 8 15 m of right lateral late wisconsin holocene faulting averaged late quaternary right lateral slip has been 0 85 0 37 mm yr on one fault plane at this locality suggesting that total slip across all southeast rift margin fault planes may accommodate much of the regional strain our seismic profile at this site shows up to the west faulting of eocene strata folded down to the west indicating structural inversion seismic profiles electrical surveys coring and a trench across the southern lineament segment show holocene faulting trenching exposed a vertical fault with 0 5 m of late holocene down to the nw displacement nearby coring and electrical surveys show 0 5 m of late holocene up to the nw faulting we conclude the southeastern reelfoot rift margin is a right lateral system with high angle faults showing both up to the nw and down to the nw separations age constraints permit an earthquake circa 2500 to 2000 years b p on the southwestern central segment of this fault system that ruptured 80 km we suggest the northeastern segment of the southeastern rift margin turned off in holocene when the reelfoot step over thrust turned on

spatiotemporal evolution of slow slip events in a nonplanar fault model for northern cascadia subduction zone slow slip events sses are identified as the quasi stable fault deformation in the deep transition zone from locked to continuous sliding in many subduction zones in the well instrumented cascadia margin a class of mw6 0 slow slip events arise beneath port angeles every 14 months as inferred from two decades of continuous geodetic monitoring the along strike bending of the incoming oceanic plate beneath north washington is a unique geometric feature whose influence on slow slip processes is still unknown here we incorporate a realistic fault geometry of northern cascadia in the framework of rate and state dependent friction law to simulate the spatiotemporal evolution of slow slip events on a nonplanar subduction fault the modeled sses capture the major characteristics revealed by gps observations the central 150 km long fault segment beneath port angeles acts as a repetitive slip patch where sses appear every 1 5 years with a maximum slip of 2 5 cm two minor slip patches with smaller areas and cumulative slips straddle this central slip patch the along strike segmentation of slow slip is inversely related to the local fault dip and strike angles of the slow slip zone suggesting strong geometrical control on the slow slip process this correlation holds even after removing the effect of w h ratio between velocity weakening sse fault width and characteristic nucleation size besides the gps detectable fast spreading phase we find that each sse cycle consists of deep pre sse preparation and post sse relaxation phases which may be the driving mechanism for the deep tremor activity between major sse episodes discovered in cascadia

mapping p wave azimuthal anisotropy of the new madrid seismic zone abstract we determine a detailed tomographic model of 3 d p wave azimuthal anisotropy in the crust and upper mantle beneath the new madrid seismic zone nmsz using a large number of high quality arrival time data of 877 local earthquakes and relative travel time data of 1511 teleseismic events recorded at 62 usarray stations the tomographic inversion is conducted by using a limited memory broyden fletcher goldfarb shanno algorithm for bound constrained optimization because damping regularization is not required in this inversion scheme the obtained results are less affected by human subjectivity a prominent low velocity zone with ne sw fast velocity directions of azimuthal anisotropy down to 160 km depth beneath the nmsz is imaged which may reflect upwelling mantle materials probably triggered by passage of the bermuda hotspot from the southwest of the mississippi embayment our results also reveal a subhorizontal high velocity anomaly around the moho discontinuity with a width of 150 200 km confirming the existence and defining the geometry of a lens shaped high density rift pillow near the moho which may result from the mantle upwelling nw se fast velocity directions of azimuthal anisotropy appear in the lower crust beneath the mississippi embayment this result may reflect recent weakening of a detachment fault above the rift pillow which drives recurrently sinking of the rift pillow and as a consequence causing large repeating intraplate earthquakes in the nmsz

reconciling short recurrence intervals with minor deformation in the new madrid seismic zone at least three great earthquakes occurred in the new madrid seismic zone in 1811 and 1812 estimates of present day strain rates suggest that such events may have a repeat time of 1000 years or less paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years however pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed this suggests that the seismic zone is a young feature possibly as young as several tens of thousands of years old and no more than a few million years old

modeling earthquake sequences along the manila subduction zone effects of three dimensional fault geometry abstract to assess the potential of catastrophic megathrust earthquakes m w 8 along the manila trench the eastern boundary of the south china sea we incorporate a 3d non planar fault geometry in the framework of rate state friction to simulate earthquake rupture sequences along the fault segment between 15 n 19 n of northern luzon our simulation results demonstrate that the first order fault geometry heterogeneity the transitional segment possibly related to the subducting scarborough seamount chain connecting the steeper south segment and the flatter north segment controls earthquake rupture behaviors the strong along strike curvature at the transitional segment typically leads to partial ruptures of m w 8 3 and m w 7 8 along the southern and northern segments respectively the entire fault occasionally ruptures in m w 8 8 events when the cumulative stress in the transitional segment is sufficiently high to overcome the geometrical inhibition fault shear stress evolution represented by the s ratio is clearly modulated by the width of seismogenic zone w at a constant plate convergence rate a larger w indicates on average lower interseismic stress loading rate and longer rupture recurrence period and could slow down or sometimes stop ruptures that initiated from a narrower portion moreover the modeled interseismic slip rate before whole fault rupture events is comparable with the coupling state that was inferred from the interplate seismicity distribution suggesting the manila trench could potentially rupture in a m8 earthquake