a comparison of eastern north american seismic strain rates to glacial rebound strain rates glacial rebound strain rates computed using a simple laurentide glacial loading model are of the order of 10 9 per year within the region of glaciation and extending several hundred kilometers beyond the horizontal strain rates receive approximately equal contributions from horizontal and vertical velocities a consequence of the spherical geometry adopted for the earth model in the eastern united states and southeastern canada the computed strain rates are 1 3 orders of magnitude greater than an estimate of the average seismic strain rate anderson 1986 and 1 order of magnitude greater than predicted erosional strain rates the predicted glacial rebound strain rates are not in general oriented in such a way as to augment the observed state of deviatoric stress possibly explaining why the seismic strain rates are much smaller than the glacial rebound strain rates an exception to this may be seismically active regions in the st lawrence valley

the northeastern ohio earthquake of 31 january 1986 was it induced abstract on 31 january 1986 at 11 46 est an earthquake of m b 5 0 occurred about 40 km east of cleveland ohio and about 17 km south of the perry nuclear power plant the earthquake was felt over a broad area including 11 states the district of columbia and parts of ontario canada caused intensity vi vii at distances of 15 km and generated relatively high accelerations 0 18 g of short duration at the perry plant thirteen aftershocks were detected as of 15 april with six occurring within the first 8 days two of the aftershocks were felt magnitudes for the aftershocks ranged from about 0 5 to 2 5 focal depths for all of the earthquakes ranged from 2 to 6 km except for one small earthquake all of the aftershocks occurred in a very tight cluster with a north northeast orientation focal mechanisms of the aftershocks exhibit predominantly oblique right slip motion on nearly vertical nodal planes oriented n15 to 45 e with a nearly horizontal p axis north of east three deep waste disposal wells are currently operating within 15 km of the epicentral region and have been responsible for the injection of nearly 1 2 billion liters of fluid at pressures reaching 112 bars above ambient at a nominal depth of 1 8 km estimates of stress inferred from commercial hydrofracturing measurements suggest that the state of stress in northeastern ohio is close to the theoretical threshold for failure along favorably oriented preexisting fractures this implies that effective stress conditions near the bottom of the two most active wells may be at or near the critical level for incipient failure two and possibly three earthquakes have occurred within less than 5 km from the wells since 1983 the relative distance to the main shock epicenter and its aftershocks about 12 km the lack of large numbers of small earthquakes typical of many induced sequences the history of small to moderate earthquakes in the region prior to the initiation of injection and the attenuation of the pressure field with distance from the injection wells however all argue for a natural origin for the 1986 earthquakes in contrast the proximity to failure conditions at the bottom of the well and the probable spatial association of at least one earthquake suggest that triggering by well activities cannot be precluded

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

paleoliquefaction study of the clarendon linden fault system western new york state abstract the lack of earthquake induced liquefaction features in late wisconsin and holocene sediments in genesee wyoming and allegany counties suggests that the clarendon linden fault system clf did not generate large moment magnitude m 6 earthquakes during the past 12 000 years given that it was the likely source of the 1929 m 4 9 attica earthquake however the clarenden linden fault system probably is capable of producing future m 5 events during this study we reviewed newspaper accounts of the 1929 attica earthquake searched for earthquake induced liquefaction features in sand and gravel pits and along tens of kilometers of river cutbanks evaluated numerous soft sediment deformation structures compiled geotechnical data and performed liquefaction potential analysis of saturated sandy sediments we found that the 1929 m 4 9 attica earthquake probably did not induce liquefaction in its epicentral area and may have been generated by the western branch of the clarendon linden fault system most soft sediment deformation structures found during reconnaissance did not resemble earthquake induced liquefaction features and even the few that did could be attributed to non seismic processes our analysis suggests that the magnitude threshold for liquefaction is between m 5 2 and 6 that a large m 6 earthquake would liquefy sediments at many sites in the area and that a moderate earthquake m 5 5 9 would liquefy sediments at some sites but perhaps not at enough sites to have been found during reconnaissance we conclude that the clarendon linden fault system could have produced small and moderate earthquakes but probably not large events during the late wisconsin and holocene

linear zones seismicity and the possibility of a major earthquake in the intraplate western lake ontario area of eastern north america seismic hazard assessments conducted in eastern canada rely on seismological data which are essential but alone inadequate that is because the earthquake record is too short to provide a representative picture of where large earthquakes have occurred in the past consequently seismic hazard may be underestimated in areas such as that encompassing western lake ontario that are devoid of documented large earthquakes to attempt to ascertain the likelihood of a major earthquake occurring in that highly populated and industrialized area three regionally extensive geophysically expressed lineaments were investigated and the results were combined with available seismological information the most conspicuous is the niagara pickering linear zone within which there have been at least two if not three periods of brittle faulting including displacements compatible with the current stress field it also appears to represent the same major structure as the akron magnetic boundary in eastern ohio the site of the mb 4 9 leroy earthquake the second is the georgian bay linear zone which extends from georgian bay to new york state it displays evidence of recent outcrop scale faulting an alignment of earthquakes along its southwestern boundary and a possible spatial relationship to other earthquakes including two of m 5 lastly there is the hamilton lake erie lineament which is parallel and proximal to a possible fault and coincides with a linear array of small to moderate earthquakes all three converge on the western lake ontario area which has a higher frequency of seismicity than the adjacent areas thus the western lake ontario area might have a greater potential to experience a major earthquake than heretofore believed resume les evaluations des risques sismiques dans l est du canada sont fondees sur les donnees sismologiques dont

neotectonic faulting in metropolitan toronto implications for earthquake hazard assessment in the lake ontario region persistent small scale seismicity around the western end of lake ontario together with larger events in eastern north america have focused attention on the seismotectonic and neotectonic conditions of southern ontario and adjacent new york state newly discovered normal faults that offset both bedrock and overlying quaternary sediments by as much as 1 25 m were discovered along the rouge valley in metropolitan toronto this location is within a zone marked by the intersection of three major lineaments and an elevated level of seismicity these faults result from reactivation of east west oriented fractures in ordovician shales structures are exposed in section only and have no surface expression suggesting that others may have gone undetected the origin of the faults is not yet well established although they may represent the effects of uplift consequent upon isostatic rebound aided by high horizontal compressive stress nevertheless their development in the recent geologic past has important implications for regional seismic hazard assessment because they occur within 7 km of the pickering nuclear power plant

an ancient rift complex and its relation to contemporary seismicity in the new madrid seismic zone gravity magnetic geologic and seismicity data have been combined in a seismotectonic analysis of the new madrid seismic zone previous studies have presented evidence for several rift zones in this area upper mississippi embayment including the reelfoot rift a late precambrian early paleozoic failed arm which extends n ne from the ancient continental margin in this study we propose that the northern terminus of the reelfoot rift forms a rift complex which appears to correlate well with the seismicity in the area this correlation suggests that faults associated with this rift complex are being reactivated in the contemporary stress field ene compression if this interpretation is valid it represents a seismotectonic model which can be used to predict the extent of future seismicity in the new madrid seismic zone the proposed rift complex also provides a coherent model for the tectonic development of this region of the north american midcontinent

is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of north america on the appalachian plateau of new york ene striking joints parallel the mid continent contemporary stress field and do not correlate with other structures joints within ohio indiana michigan and west virginia also parallel the mid continent maximum horizontal compression as indicated by hydraulic fracture measurements in northern indiana where the contemporary stress field deviates significantly from the orientation of the mid continent stress the attitude of the joints deviates accordingly in addition to joints structures on several scales from subcontinent size fracture systems to microfabrics in shales apparently parallel the mid continent stress field from this i conclude that these joints are mode i cracks whose orientation corresponds to the contemporary tectonic stress field in itself the formation of these structures constitutes a tectonic event that has often been overlooked in favor of tectonic events that produce the more spectacular but less pervasive faults and folds

a re examination of the seismicity of michigan abstract according to reports on felt shocks and those recorded by instruments 105 seismic events have been reported as occurring within michigan from the 1700 s to the present of these 41 instrumental events are due to chemical explosions detonated in experiments during 1964 1966 of the remaining events most of those reported from the upper peninsula are related to mining in the lower peninsula most events can be shown or strongly inferred to be cryoseisms atmospheric shock waves from explosions or aircraft or errors in location or reporting at least four and perhaps up to eight events are thought to be tectonic earthquakes and occur in the southernmost part of the state primarily in western lake erie and the southwestern corner of the state or in lake superior this study indicates that the seismicity level within michigan is very low and that there is no evidence to substantiate either a tectonically active michigan basin seismic zone or the extension of the midcontinent seismic zone into keweenaw peninsula in addition these results suggest that earthquake catalogs for weakly seismic regions may be contaminated with non tectonic and even non seismic events thirty earthquakes occurring outside the state have also been reported in michigan these lie primarily along a line connecting the st lawrence seaway and the new madrid zone of missouri

a case for neotectonic joints along the niagara escarpment in a search for proof that some joints on the appalachian plateau are neotectonic orientation data were collected on more than 1800 fractures in exposures of the lockport dolomite in western new york and southern ontario of greatest interest was a late forming ene joint set present in the central and western portions of the field area these joints increase in abundance with proximity to the niagara escarpment the shape of the east west trending niagara escarpment is defined by a series of asymmetric reentrants with a strong ene linear trend reflecting the bedrock joint pattern the joints appear to have formed near the earth s surface in response to low tensile stresses developed in bedrock adjacent to the retreating escarpment since the current shape and position of the escarpment were controlled by glacial activity during the last 3 m y it is possible that the ene joints and reentrants are neotectonic features

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

earthquakes and the cratonward limit of iapetan faulting in eastern north america the cratonward limit of large normal faults in the passive margin of the late proterozoic to early paleozoic iapetus ocean is a fundamental boundary for the assessment of seismic hazards in populous eastern north america earthquakes at five localities in the iapetan margin have been attributed to compressional reactivation of iapetan normal faults the northwesternmost locations of known iapetan faults both seismic and currently aseismic lie along a line from labrador to alabama this line coincides approximately with the northwestward transition from a more seismically active continental rim to a generally less active cratonic interior thus the northwestern boundary of iapetan faults separates two large regions of mostly different seismic hazard

transient pumping well flow in weakly heterogeneous formations 1 the solution of pumping well flow serves for identifying aquifer s properties most results available in the literature assume homogeneous aquifers or at most stratified structures this study investigates the unsteady well flow in heterogeneous aquifers by modeling the hydraulic conductivity as a three dimensional stationary random function of axisymmetric anisotropy and gaussian correlations the main result of the investigation is the mean drawdown in a transient flow toward a fully penetrating well pumping at a constant rate the solution is obtained in terms of one quadrature over time and is easily calculated closed form approximations of the mean head are developed for moderate and large times the mean drawdowns derived in the study generalize the theis and copper and jacob distributions for aquifers of constant transmissivity it is shown that the transmissivity which is identified by using the cooper and jacob asymptotic is precisely the effective conductivity for uniform horizontal flow khefu this is compared with the finding that the equivalent conductivity for the steady state flow approaches khefu with distance from the well indelman et al 1996

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

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

curving cross joints and the lithospheric stress field in eastern north america cross joints are late formed nonsystematic fractures that extend across intervals between systematic joints traces of such cross joints are seen on bedding plane surfaces of devonian catskill clastic sedimentary rocks of the appalachian plateau of western new york state where the maximum horizontal principal stress s h is oriented n65 e as indicated by in situ stress measurements between pairs of closely spaced systematic joints traces of cross joints are commonly planar and orthogonal to the preexisting joints however in the mid region between some widely spaced systematic joints in western new york cross joints strike parallel to the s h of the present lithospheric stress field but then curve to abut the preexisting joints at right angles a curving trace reflects a local perturbation of the regional stress field in the vicinity of preexisting joints and the perpendicular termination indicates that the preexisting joints were open depending on their age the strike of the mid region of curving cross joints denotes the orientation of either the neotectonic stress field or its tertiary predecessor in the north american lithosphere

strike slip faulting at thebes gap missouri and illinois implications for new madrid tectonism numerous nne and ne striking strike slip faults and associated normal faults folds and transtensional grabens occur in the thebes gap area of missouri and illinois these structures developed along the northwestern margin of the buried reelfoot rift of precambrian cambrian age at the northern edge of the mississippi embayment they have had a long lived and complex structural history this is an area of recent moderate seismicity approximately 45 km north of the new madrid seismic zone stratigraphic evidence suggests that these faults were active during the middle ordovician they were subsequently reactivated between the early devonian and late cretaceous probably in response to both the acadian and ouachita orogenies deformation during this period was characterized by strongly faulted and folded ordovician through devonian rocks in places these deformed rocks are overlain with angular unconformity by undeformed cretaceous strata fault motion is interpreted as dominantly strike slip a still younger period of reactivation involved late cretaceous and cenozoic formations as young as the miocene or pliocene mounds gravel these formations have experienced both minor high angle normal faulting and subsequent major right lateral strike slip faulting en echelon north south folds ene striking normal faults regional fracture patterns and drag folds indicate the right lateral motion for this major episode of faulting which predates deposition of quaternary loess several nondefinitive lines of evidence suggest quaternary faulting similar fault orientations and kinematics as well as recent seismicity and proximity clearly suggest a structural relationship between deformation at thebes gap and tectonism associated with the new madrid area

2300 yr history of seismicity from silting events in lake tadoussac charlevoix quebec silt layers in organic rich lake sediments are interpreted to represent the historic earthquakes of 1638 1663 1791 1870 and 1925 of the charlevoix region of eastern canada this interpretation is based on the relative spacing of the layers and the accelerator mass spectrometer sup 14 c dating of a twig in one of the cores the silt layers were presumably caused by landslides on tributary streams and by resuspension of the sediment the cores contain up to 22 layers 1 to 45 mm thick over a depth equivalent to about 2300 yr if there is a correlation between thickness and magnitude there would have been four events of richter magnitude 7 5 or greater in a d 1663 1320 60 and 320 b c the 18 other layers could correspond to earthquakes of magnitude 6 to 7 the recurrence interval from 300 b c to a d 800 is 120 yr followed until a d 1500 by a quiet period when there was a single large event the recurrence interval from a d 1500 to the present is 75 yr

statistical trends in the intraplate stress field the world stress map wsm database contains thousands of intraplate stress indicators with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion previous studies which relied almost exclusively on visual inspection of the data established the existence of broad regions of uniform maximum horizontal compressive stress orientation shmax and stress regimes on a regional scale in the present study we present a statistical analysis of the wsm stress indicators with the aim of quantifying trends in both the shmax orientations and stress regimes the analysis was carried out within 5 5 bins which provide a resolution of several hundred kilometers only the 4537 high quality wsm indicators with rating of a to c were used in the analysis we present results for two types of analysis on the information contained within the bins first we evaluate the spatial distribution of the average stress regime normal strike slip or thrust second we apply the rayleigh test a standard statistical method in the analysis of directional data to the distribution of shmax orientations to test the null hypothesis that the orientations are random an important aspect of our study is the quantification of the conclusions drawn from visual inspection of the world stress map our results indicate that broad regions of uniform shmax orientations exist in most continental regions at high confidence levels 90 and 95 and are less robust in the slowest moving continental plates we also quantify the predominance of strike slip and compressional stress regimes in continental regions importantly our analysis provides information about trends in the shmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data for example in western north america and continental australia furthermore we find a strong correlation between average shmax orientations and both the ridge push torque and the absolute plate velocity azimuths our observation that a greater number of shmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force

formation of regional cross fold joints in the northern appalachian plateau abstract fold axis parallel elongation associated with the development of arcuate fold and thrust belts is proposed as a causal mechanism for syn orogenic cross fold joints such a mechanism can be coupled with other joint propagation models providing a widely applicable resolution to the enigmatic origin of regional systematic joints the fold axis parallel elongation model is compatible with kinematic indicators as recorded by a sequence of cross fold joints and related deformational fabrics in the central and northern appalachian plateau modeling of an arcuate tectonic boundary subjected to normal compressive loading demonstrates that tangential tensile stress can be large enough to initiate cross fold joints on the convex side of the tectonic boundary simulated stress trajectories from boundary element modeling bear a strong resemblance to the stress trajectories inferred from the regional cross fold joint patterns in the central and northern appalachian plateau modeling also displays a cratonward decrease in both the tangential stress and the tangential strain such a decrease is consistent with the deformation styles observed in the central and northern appalachian plateau

earthquakes in the eastern great lakes basin from a regional perspective abstract this paper presents a summary of the seismicity and its relation to stress and geologic structures in the eastern great lakes basin eglb and compares it with that of other regions in the central and eastern north america cena the earthquakes scattered throughout the eglb are occurring at a rate somewhat less than that of the appalachians and along the atlantic seaboard paleoseismology studies suggest that the lower seismicity rate may be characteristic of the eglb since the late wisconsin north of the eglb earthquakes have primarily thrust mechanisms while to the south of the eglb most earthquakes are strike slip throughout the region including the eglb the average p axes of the earthquakes are oriented ne sw and are aligned with the direction of the current plate driving stress on a regional basis earthquakes are centered primarily in the precambrian basement beneath the paleozoic cover many of the earthquakes in the eglb have occurred in areas of preexisting faults at least some of which may have been active during past episodes of continental rifting for individual faults that have been studied in some detail however it is not clear whether earthquakes represent reactivations of local preexisting structures or nucleation of new ruptures in or near the old fault zones

the earthquake potential of the new madrid seismic zone the fault system responsible for new madrid seismicity has generated temporally clustered very large earthquakes in a d 900 100 years and a d 1450 150 years as well as in 1811 1812 given the uncertainties in dating liquefaction features the time between the past three new madrid events may be as short as 200 years and as long as 800 years with an average of 500 years this advance in understanding the late holocene history of the new madrid seismic zone and thus the contemporary tectonic behavior of the associated fault system was made through studies of hundreds of earthquake induced liquefaction features at more than 250 sites across the new madrid region we have found evidence that prehistoric sand blows like those that formed during the 1811 1812 earthquakes are probably com pound structures resulting from multiple earthquakes closely clustered in time or earthquake sequences from the spatial distribution and size of sand blows and their sedimentary units we infer the source zones and estimate the magnitudes of earth quakes within each sequence and thereby characterize the detailed behavior of the fault system it appears that fault rupture was complex and that the central branch of the seismic zone produced very large earthquakes during the a d 900 and a d 1450 events as well as in 1811 1812 on the basis of a minimum recurrence rate of 200 years we are now entering the period during which the next 1811 1812 type event could occur

multiple late holocene earthquakes along the reelfoot fault central new madrid seismic zone the reelfoot fault is an east vergent reverse fault underlying the lake county uplift a low amplitude late holocene anticline bordered on the east by the 32 km long reelfoot scarp fluvial deposits across the scarp define an 8 m high east facing monocline most near surface deformation along the scarp is accommodated via folding rather than faulting we interpret the scarp as a fault propagation fold developed over a wcst dipping reverse fault interpreted from shallow seismic reflection data trench exposurcs provide evidence for three episodes of deformation along the reelfoot fault within thc past approximately 2400 years between a d 780 and 1000 between a d 1260 and 1650 and during a d 1812 our best estimate of the average recurrence interval for deformation along the scarp is 400 500 years each episode of deformation had a slightly different style the third most recent event produced a small graben a few tens of centimeters deep in the hanging wall of the reverse fault the second most recent earthquake produced about 1 3 m of throw in the graben as well as folding along the updip projection of the reverse fault and development of the scarp thesc relations suggest that graben development increased through time concomitant with growth of the monocline or that the events are of different magnitude the 1811 1812 episode of deformation produced abundant liquefaction prominent folding of fluvial strata along the scarp and minor faulting in the graben

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

historic and prehistoric earthquake induced liquefaction in newbury massachusetts sand dikes and sills in glaciomarine sediments record two liquefaction events in newbury massachusetts the meizoseismal area of the a d 1727 felt area magnitude 5 0 earthquake during the 1727 earthquake newbury experienced ground shaking on the order of modified mercalli intensity vii and underwent ground failure typical of liquefaction according to accounts of the earthquake ground cracks with separations of up to 0 6 m formed sand and water vented through at least ten ground cracks land was locally elevated and firm ground was changed to quagmire crosscutting relations and radiocarbon dating of woody material associated with liquefaction features exposed by trenching indicate that at least two moderate to large earthquakes have occurred in the newbury area in the past 4 ka the relatively unweathered younger features are consistent in style and location with ground failure described for the 1727 earthquake the more weathered older features probably formed during a prehistoric earthquake glaciomarine and other deposits that may be prone to liquefaction are widely distributed in northeastern north america they provide the opportunity to develop criteria for identifying paleoliquefaction events and to better determine earthquake hazard in the region