upper crust beneath the central illinois basin united states newly available industry seismic reflection data provide critical information for understanding the structure and origin of the upper crust 0 12 km depth beneath the central illinois basin and the seismic tectonic framework north of the new madrid seismic zone in the central mississippi valley mapping of reflector sequences furnishes the first broad three dimensional perspective of the structure of precambrian basement beneath the central united states midcontinent the highly coherent basement reflectivity is expressed as a synformal wedge of dipping and subhorizontal reflections situated beneath the center of the illinois basin that thickens and deepens to the northeast e g 0 to 3 km thickness along a 123 km south to north line the thickening trend of the wedge qualitatively mimics the northward thickening of the late cambrian mt simon sandstone however other paleozoic units in the illinois basin generally thicken southward into the basin center the seismic data also reveal an anomalous subsequence defined by a spoon shaped distribution of disrupted reflections located along the southern margin of the wedge the boundaries of this subsequence are marked by distinct steeply dipping reflections possible thrust faults that continue or project up to antiformal disruptions of lower paleozoic marker reflectors suggesting paleozoic or possibly later tectonic reactivation of precambrian structure the areal extent of the subsequence appears to roughly correspond to an anomalous concentration of larger magnitude upper to middle crustal earthquakes there are multiple hypotheses for the origin of the precambrian reflectivity including basaltic flows or sills interlayered with clastic sediments and or emplaced within felsic igneous rocks such explanations are analogous to nearby keweenawan rift related volcanism and sedimentation which initiated during proterozoic rifting and were followed eventually by reverse faulting along the rift margins caused by grenville compression

three dimensional crustal structure of eastern north america extracted from ambient noise 1 group velocity dispersion curves of surface waves extracted from ambient seismic noise are inverted to find 3 d shear wave structure of the crust beneath eastern north america the 3 d model consists of one sediment layer and another six layers with fixed depths at 5 7 5 10 15 25 and 43 km velocities of the seven layers together with the thickness of the sediment layer are determined by the simulated annealing method we found that almost all failed ancient rifting events e g the reelfoot rift ouachita triple junction and the midcontinent rift and rifting related events e g the ozark uplift and the nashville dome are associated with high velocity bodies in the middle and lower crust our results also suggest the existence of a triple junction like high velocity body centered around the new madrid and the wabash valley seismic zones with the reelfoot rift the ozark uplift and the nashville dome being on its southwestern northwestern and southeastern arms respectively we also found that the western limb of the midcontinent rift mcr extends southwestward to western oklahoma and texas and the eastern limb of the mcr extends southeastward into western ohio the appalachian mountains are characterized by high velocity upper crust underlain with relatively low velocity middle and lower crust all major seismic zones are associated with either divergent or convergent events the new madrid seismic zone and wabash valley seismic zone are clearly associated with the failed reelfoot rift both the eastern tennessee seismic zone and ouachita orogen seismic zone are located along convergent boundaries

seismic velocity model of the central united states version 1 description and simulation of the 18 april 2008 mt carmel illinois earthquake abstract we have developed a new three dimensional seismic velocity model of the central united states cusvm that includes the new madrid seismic zone nmsz and covers parts of arkansas mississippi alabama illinois missouri kentucky and tennessee the model represents a compilation of decades of crustal research consisting of seismic aeromagnetic and gravity profiles geologic mapping geophysical and geological borehole logs and inversions of the regional seismic properties the density p and s wave velocities are synthesized in a stand alone spatial database that can be queried to generate the required input for numerical seismic wave propagation simulations we test and calibrate the cusvm by simulating ground motions of the 18 april 2008 m w 5 4 mt carmel illinois earthquake and comparing the results with observed records within the model area the selected stations in the comparisons reflect different geological site conditions and cover distances ranging from 10 to 430 km from the epicenter the results based on a qualitative and quantitative goodness of fit gof characterization indicate that both within and outside the mississippi embayment the cusvm reasonably reproduces 1 the body and surface wave arrival times and 2 the observed regional variations in ground motion amplitude cumulative energy duration and frequency content up to a frequency of 1 0 hz in addition we discuss the probable structural causes for the ground motion patterns in the central united states that we observed in the recorded motions of the 18 april mt carmel earthquake online material simulated and observed waveforms and response spectral acceleration ground motion prediction equation comparison

crustal structure of the appalachian highlands in tennessee abstract crustal structure of the southern appalachians and adjacent interior low plateaus in tennessee is derived from seismic refraction measurements observed by the u s geological survey in 1965 along reversed lines normal nw se and parallel ne sw to the structure of the appalachian highlands major geologic divisions its easternmost part is located approximately 80 km southwest of the westernmost part of the cocorp seismic reflection traverse within the blue ridge province the velocity depth models derived for both observational directions consist of three crustal layers with surprisingly high velocities being about 6 1 6 2 km s in the upper crust down to 7 10 km depth 6 7 6 8 km s for the middle crust between about 17 and 34 km and varying from 7 1 to 7 4 km s for the lower crust at about 40 47 km depth the boundaries between the three crustal layers as well as the crust mantle boundary are transition zones of up to 11 km thickness similar to old orogens in other parts of the earth the main result is a thick crust at places in excess of 50 km with high average velocity and a broad crust mantle transition zone

no analog planktonic foraminiferal faunas in the glacial southern south china sea implications for the magnitude of glacial cooling in the western pacific warm pool sea surface temperature sst estimates in the sediment core md01 2390 based on planktonic foraminiferal species abundances using five different transfer function techniques suggest nearly unchanged or unusually higher temperatures in the tropical southern south china sea scs during the last glacial maximum lgm relative to modern temperatures these results are in contrast to substantial cooling of 2 5 c inferred by geochemical uk 37 mg ca ratios and terrestrial proxies from the western tropical pacific region using multivariate statistics we show that the glacial southern scs harboured unique planktonic foraminiferal assemblages that have no modern analogs analyses of faunal variation through the core reveal that planktonic foraminiferal assemblages responded to temperature changes inferred from mg ca data but that this signal is subdued by superimposed variations in the relative abundance of pulleniatina obliquiloculata and neogloboquadrina pachyderma dextral these species occur in glacial samples at proportions that are not observed in the calibration data set the glacial high abundance of n pachyderma dextral are interpreted to reflect a seasonal winter inflow of cold surface water from the northeast via the bashi strait due to the combined effects of an intensified winter monsoon a southward shift of the polar front and the eastward migration of the kuroshio current in contrast processes controlling the high relative abundances of p obliquiloculata during the lgm may be unique to the southern scs we propose a scenario involving a stronger winter mixing or enhanced upwelling due to an intensified winter monsoon that prevented shallow dwelling warm indicators to establish larger populations during the lgm our results indicate that a no analog behaviour of planktonic foraminifera faunas is responsible for the warm glacial conditions in this part of the western pacific warm pool as implied by foraminiferal transfer functions and that a more significant surface cooling in the region as implied by terrestrial and geochemical mg ca ratios alkenone unsaturation index marine proxies is a more likely scenario

microtremor observations of deep sediment resonance in metropolitan memphis tennessee we measured site resonance periods throughout metropolitan memphis tennessee the site resonance was derived from recordings of ambient microtremor and is a direct measurement of seismic ground shaking in the memphis metropolitan area the period t0 in s is predicted by the empirical relation t0 0 39899 7 8922 10 3h 3 0634 10 6h2 350 h 1100 m where h is the thickness in m of the unconsolidated sediments that overly hard paleozoic rocks assuming that a quarter wavelength relationship governs the relationship between resonant period sediment thickness and average shear wave speed we estimate the average shear wave speed of unconsolidated sediments as a function of sediment thicknesses vs 521 15 0 37459h 350 h 1100 m the scatter in the observations and discordance with earlier work suggests that caution is warranted in applying these values of wave speed indiscriminately to sites in the absence of further detailed work

a cocorp deep reflection profile across the buried reelfoot rift south central united states abstract a cocorp deep reflection survey across the northern mississippi embayment reveals essential features of the late precambrian early paleozoic reelfoot rift stratified reflections delineating the sedimentary fill of the rift basin are of variable thickness the base ranging from 2 0 to a maximum of 3 5 s twt 4 to 8 km depth in contrast the base of phanerozoic the sedimentary section immediately east of the rift is at 1 4 s 2 8 km depth correlation with a basement penetrating well indicates that the base of the upper cretaceous holocene mississippi embayment supergroup base of the cambrian ordovician knox group and the lower upper cambrian bonneterre formation are basin wide seismic stratigraphic markers clastic sedimentary strata of the lamotte formation which immediately overlie precambrian basement show clear angular discordance with the overlying subhorizontal bonneterre formation indicating that the base of the bonneterre marks the transition from active extension to passive subsidence within the reelfoot rift palinspastic restoration of the basement suggests relatively modest extension associated with rift formation 17 the blytheville arch an axial antiformal feature as well as a number of lesser structures indicative of multiple episodes of fault reactivation are also evident on the cocorp profile the crystalline basement beneath the reelfoot rift is relatively devoid of reflections down to 5 0 s 12 15 km depth beyond which it passes gradationally into a complexly reflective diffractive middle and lower crust the lower crust however does not exhibit the distinctly laminated character commonly observed in mesozoic cenozoic extensional provinces the crust mantle transition beneath the rift is characterized by a gradational downward cessation of crustal reflectivity at 12 13 s 40 km depth followed by an unreflective zone which in turn is followed by a distinct horizontal reflection at 14 5 s the latter is not observed on existing profiles outside the rift at present it is not possible to determine strictly whether the deep horizon represents moho or alternatively a discrete feature sill within the uppermost mantle however its spatial association with the reelfoot rift suggests that the two are genetically related regional relations together with the existence of dipping fabric in the deep crust beneath the rift suggest the possibility that the reelfoot rift formed along the southern extension of the circa 1 ga grenville front

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

contour mapping of relic structures in the precambrian basement of the reelfoot rift north american midcontinent a new contour map of the basement of the reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features the major features are two asymmetric intrarift basins bounded by three structural highs and the rift margins the basins are oriented normal to the northeast trend of the rift two of the highs appear to be ridges of undetermined width that extend across the rift the third high is an isolated dome or platform located between the basins the minor features are three linear structures of low relief oriented subparallel to the trend of the rift two of these located within the rift basins may divide the rift basins into paired subbasins these mapped features may be the remnants of initial extensional rifting half graben faulting and basement subsidence the rift basins are interpreted as having formed as opposing half graben and the structural highs are interpreted as having formed as associated accommodation zones some of these features appear to be reactivated seismogenic structures within the modern midcontinent compressional stress regime a detailed knowledge of the geometries of the reelfoot rift s basement features therefore is essential when evaluating their seismic risk potential

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

regional variations in near surface shear wave velocity in the greater memphis area abstract site specific and regional analyses were conducted for an area that includes southwestern tennessee eastern arkansas and northwestern mississippi to establish characteristic shear wave velocity vs profiles based on measured profiles compiled from several researchers site specific studies were conducted at two locations near memphis tennessee to determine the uncertainty and randomness in measuring vs due to test related factors and local variability the result of this analysis was subsequently used as a criterion for establishing the range of variability for the characteristic vs profiles measured vs profiles were aggregated based on the age and type of geologic deposits in the memphis area to obtain characteristic profiles holocene alluvial deposits were found to have a fairly uniform shear wave velocity profile in the mississippi river floodplain pleistocene deposits characterized by a surficial layer of loess in the terrace regions of mississippi and tennessee were found to be more variable the average shear wave velocity in the upper 30 m was calculated for each measured profile based on the recommendations of the 1997 national earthquake hazards reduction program nehrp provisions to assess the susceptibility of a deposit to ground motion amplification the lowest and highest average near surface shear wave velocities were identified in the mississippi river alluvium and the pleistocene deposits respectively based on the nehrp site classes all deposits may be classified as d stiff soil however due to the thickness of soft soil deposits in the alluvial plain these deposits may be classified as e more than 3 m of soft clay based on the site class description rather than the shear wave velocity therefore a re evaluation of the nehrp site classes is proposed for the central us to more accurately represent the actual site conditions

upper mississippi embayment shallow seismic velocities measured in situ abstract vertical seismic compressional and shear wave p and s wave profiles were collected from three shallow boreholes in sediment of the upper mississippi embayment the site of the 60 m hole at shelby forest tennessee is on bluffs forming the eastern edge of the mississippi alluvial plain the bluffs are composed of pleistocene loess pliocene pleistocene alluvial clay and sand deposits and tertiary deltaic marine sediment the 36 m hole at marked tree arkansas and the 27 m hole at risco missouri are in holocene mississippi river floodplain sand silt and gravel deposits at each site impulsive p and s waves were generated by man made sources at the surface while a three component geophone was locked downhole at 0 91 m intervals consistent with their very similar geology the two floodplain locations have nearly identical s wave velocity vs profiles the lowest vs values are about 130 m s 1 and the highest values are about 300 m s 1 at these sites the shear wave velocity profile at shelby forest is very similar within the pleistocene loess 12 m thick in deeper older material vs exceeds 400 m s 1 at marked tree and at risco the compressional wave velocity vp values above the water table are as low as about 230 m s 1 and rise to about 1 9 km s 1 below the water table at shelby forest vp values in the unsaturated loess are as low as 302 m s 1 vp values below the water table are about 1 8 km s 1 for the two floodplain sites the vp vs ratio increases rapidly across the water table depth for the shelby forest site the largest increase in the vp vs ratio occurs at 20 m depth the boundary between the pliocene pleistocene clay and sand deposits and the eocene shallow marine clay and silt deposits until recently seismic velocity data for the embayment basin came from eartquake studies crustal scale seismic refraction and reflection profiles sonic logs and from analysis of dispersed earthquake surface waves since 1991 seismic data for shallow sediment obtained from reflection refraction crosshole and downhole techniques have been obtained for sites at the northern end of the embayment basin the present borehole data however are measured from sites representative of large areas in the mississippi embayment therefore they fill a gap in information needed for modeling the response of the embayment to destructive seismic shaking

toward a consistent model for strain accrual and release for the new madrid seismic zone central united states 1 at the heart of the conundrum of seismogenesis in the new madrid seismic zone is the apparently substantial discrepancy between low strain rate and high recent seismic moment release in this study we revisit the magnitudes of the four principal 1811 1812 earthquakes using intensity values determined from individual assessments from four experts using these values and the grid search method of bakun and wentworth 1997 we estimate magnitudes around 7 0 for all four events values that are significantly lower than previously published magnitude estimates based on macroseismic intensities we further show that the strain rate predicted from postglacial rebound is sufficient to produce a sequence with the moment release of one mmax6 8 every 500 years a rate that is much lower than previous estimates of late holocene moment release however mw6 8 is at the low end of the uncertainty range inferred from analysis of intensities for the largest 1811 1812 event we show that mw6 8 is also a reasonable value for the largest main shock given a plausible rupture scenario one can also construct a range of consistent models that permit a somewhat higher mmax with a longer average recurrence rate it is thus possible to reconcile predicted strain and seismic moment release rates with alternative models one in which 1811 1812 sequences occur every 500 years with the largest events being mmax 6 8 or one in which sequences occur on average less frequently with mmax of 7 0 both models predict that the late holocene rate of activity will continue for the next few to 10 thousand years

three dimensional p wave velocity structure in the new madrid seismic zone a three dimensional p wave velocity model for the new madrid seismic zone nmsz has been developed using a nonlinear travel time tomography method the inversion involved 709 earthquakes recorded by digital three component portable array for numerical data acquisition stations deployed in the nmsz over the time period 1989 through 1992 analysis of ray coverage and inversion of a synthetic data set showed that the model has high resolution to a depth of 11 km low velocity anomalies correlate with the prominent northwest and northeast trending arms of seismicity lowest velocity anomalies 8 occur at the intersection of these arms and are associated with shallow seismicity and a high swarm rate these low velocity regions are interpreted to be the result of increased fluid pressure associated with structurally disrupted rocks beneath the blytheville pascola arch complex small regions of high velocity limited to the upper few km of the crust correlate with gravity and magnetic anomalies and are interpreted to be igneous intrusions high velocity anomalies also parallel the edges of the reelfoot rift margin and probably are associated with crystalline precambrian rocks that form the margin of the reelfoot rift graben most earthquakes are associated with low velocity regions and avoid regions of high velocity

orogeny and stratigraphy numerical models of the paleozoic in the eastern interior of north america a computer model is used to estimate patterns of lithospheric deformation caused by obduction and removal of loads within orogenic belts regions of lithospheric downwarp become sedimentary depocenters and regions of lithospheric upwarp suffer erosion by following the deposition and erosion of sedimentary packages through a number of model time steps it is possible to calculate a synthetic stratigraphy for a given study area this model is applied to the post early ordovician stratigraphy of the eastern interior region of north america the loading that drives lithospheric deformation is associated with the development of the appalachian and ouachita orogens and the independent development of the michigan and illinois intracratonic basins model results are shown to be consistent with presently observed stratigraphy lithology and cumulative erosion and are then used to estimate the configuration of given sediment packages at discrete intervals following their deposition these estimates are presented in the form of isopach maps stratigraphic cross sections and time stratigraphic diagrams as a function of time over the last 470 ma by tracking the burial and erosion history of sedimentary units we are able to establish a cause and effect relationship between particular tectonic events and the extent of preservation of sedimentary units in particular we suggest that the pennsylvanian permian alleghanian orogeny produced major sediment cover over much of the study area

biofabrics as dynamic indicators in nummulite accumulations biofabric interpretation of skeletal accumulations consisting of biologically standardized nummulite tests larger foraminifera is based upon 1 the ratio of small bodied a forms megalospheric and large bodied b forms microspheric and 2 on their packing and imbrication features in the eocene mokattam formation of egypt nummulites gizehensis occurs in limestones with four basic biofabrics in wackestones the nummulites float throughout the matrix and a forms strongly dominate over b forms approximating the ratio reported for undisturbed assemblages packstones however reflect various degrees of sedimentary reworking and winnowing of the original substratum by physical processes for packstones with edge wise imbrication and a forms strongly dominating over b forms in situ winnowing of fine grained matrix by waves can be inferred in contrast packstones composed of imbricated b forms suggest hydraulic sorting and selective removal of a forms by currents packstones composed almost entirely of a forms represent transport sorted allochthonous deposits biofabric analysis thus allows us to understand better the dynamics of skeletal accumulations such as nummulite banks whose commonly complex and extensive buildups can form hydrocarbon reservoirs

nehrp soil classifications for estimating site dependent seismic coefficients in the upper mississippi embayment abstract local soil conditions have a profound influence on the characteristics of ground shaking during an earthquake exceptionally deep soil deposits on the order of 100 1000 m deep are found in the upper mississippi embayment of the central united states shear waves sh from earthquakes in the new madrid seismic zone are expected to be strongly affected by the sharp impedance contrasts at the bedrock sediment interface attenuation of seismic waves in the soil column and the sh wave velocities of the more poorly consolidated near surface 50 m soils sh wave velocities of the near surface soils at nearly 400 sites in the upper mississippi embayment were determined using conventional seismic sh wave refraction and reflection techniques based on the average sh wave velocities of the upper 30 m of the soils sites in the mississippi river floodplain portion of the study area are predominantly classified as site class d 180 360 m s in accordance with the 1997 nehrp provisions sites away from the active floodplains in western kentucky and western tennessee the sh wave velocities of the upper 30 m of soils typically ranged from mid 200 to mid 300 m s several sites in western kentucky had averaged sh wave velocities greater than 360 m s thereby qualifying them as site class c 360 760 m s in accordance with the 1997 nehrp provisions one dimensional site effects including amplification and dynamic site period were calculated for a representative suite of sites across the upper mississippi embayment at latitude 38 5 although seismic attenuation is greater in the mississippi river floodplain i e thicker lower velocity material the site effects tend to be greater than in the uplands of western tennessee because of larger impedance contrasts within the near surface soils

structure and evolution of the central gulf of mexico continental margin and coastal plain southeast united states as in many continental margins mesozoic rifting in the gulf of mexico trended subparallel to older tectonic structures created by opening and closure of a prior ocean basin unlike the situation in most other margins however extension in the gulf of mexico did not involve widespread reactivation of these older tectonic features this difference may be related to the unusual facts that 1 the antecedent passive continental margin was a transform rather than a rift margin and 2 the prior orogeny did not substantially thicken the crust in order to characterize the interaction between mesozoic rifting and prior tectonic events two geophysical transects were constructed across the central gulf of mexico coastal plain and continental shelf the transects show that the early paleozoic passive continental margin buried beneath the late paleozoic ouachita orogen was little affected by compressional tectonism or mesozoic extension during opening of the gulf of mexico the thickness of the crust on this ancient margin decreases from 35 km to 10 km over a distance of only 50 km consistent with previous interpretations that this was a transform margin the ouachita fold and thrust belt is shown to be a thin skinned doubly vergent orogen that formed above a well preserved southward dipping subduction system relict cambrian oceanic crust may exist beneath the southern part of the orogen but its presence is not required by the data the thin crust and shallow mantle in the ouachita suture created a zone of relatively high strength that acted as a barrier to mesozoic extension which is restricted to regions south of the orogen extension was initially distributed over a broad area that included the mississippi interior salt basin and regions south of the wiggins arch extension became progressively more focused south of the wiggins arch with time culminating in the onset of seafloor spreading beneath the modern continental rise mesozoic extension factors are 1 4 1 9 beneath the mississippi interior salt basin 2 9 immediately south of the wiggins arch and 4 adjacent to the oldest oceanic crust in the central gulf of mexico

faulting parameters of earthquakes in the new madrid missouri region abstract the advent of high resolution digital seismic recording and advances in computer technology enable the combination of traditional regional seismic network observations with direct seismogram modeling to improve estimates of small earthquake faulting geometry depth and size we illustrate a combined modeling approach using observations from three earthquakes that occurred within the environs of the new madrid seismic zone two missouri earthquakes from september 26 1990 and may 4 1991 and the southern illinois earthquake of february 5 1994 we also re examine the faulting geometry for two events from the 1960s that are inconsistent with the current estimate of the regional stress field based on direct modeling of the long period seismograms associated with these events we revise earlier estimates of the earthquake parameters for the march 3 1963 and july 21 1967 missouri earthquakes comparing the new and revised results with existing earthquake mechanisms in the region we find that tension axes are generally aligned in a n s to nw se direction while the compression axes trend in a ne to e direction an interesting exception to this pattern are the march 3 1963 and two nearby earthquakes that lie within a well defined 30 km long left step in seismicity near new madrid

broadband microtremor observation of basin resonance in the mississippi embayment central us we report systematic variations in the fundamental period of seismic site resonance within the mississippi embayment the resonance periods are revealed by peaks in horizontal to vertical h v spectral ratios of microtremor measurements and increase with the depth to the sediment basement interface the sediment depth varies slowly enough spatially that at any site it can be modeled as a one dimensional problem revealing structural details of the sedimentary column the strong impedance contrast and gentle dip of the base of the embayment sediments may trap seismic energy within the soft sediments increasing durations and amplitudes of strong ground motions broadband recordings of microtremor within the mississippi embayment provide a simple method to study the seismic response of the basin

three dimensional geologic map of the hayward fault northern california correlation of rock units with variations in seismicity creep rate and fault dip in order to better understand mechanisms of active faults we studied relationships between fault behavior and rock units along the hayward fault using a three dimensional geologic map the three dimensional map constructed from hypocenters potential field data and surface map data provided a geologic map of each fault surface showing rock units on either side of the fault truncated by the fault the two fault surface maps were superimposed to create a rock rock juxtaposition map the three maps were compared with seismicity including aseismic patches surface creep and fault dip along the fault by using visualization software to explore three dimensional relationships fault behavior appears to be correlated to the fault surface maps but not to the rock rock juxtaposition map suggesting that properties of individual wall rock units including rock strength play an important role in fault behavior although preliminary these results suggest that any attempt to understand the detailed distribution of earthquakes or creep along a fault should include consideration of the rock types that abut the fault surface including the incorporation of observations of physical properties of the rock bodies that intersect the fault at depth