on the geologic structure at the epicenter of the 1886 charleston south carolina earthquake the study focuses on evidence of cenozoic faulting in the epicentral area of the 1886 charleston south carolina earthquake and its connection with mesozoic structure the seismic data consist of several reflection profiles collected near summerville south carolina in the period 1975 1983 reprocessing of the data reveals an extensive early mesozoic extensional basin approximately 20 km in width between summerville and charleston the basin is delineated by the geometry of reflections that image early mesozoic volcanic and sedimentary rocks and by positive magnetic and gravity anomalies cenozoic compressional reactivation of mesozoic extensional faults is imaged in the interior of the basin the northwestern boundary of the basin is marked by a sharp gradient in the magnetic field folded cretaceous and tertiary atlantic coastal plain sediments in association with diffractions and truncated reflections from the early mesozoic section at four locations along this magnetic gradient indicate that the northwestern basin boundary is faulted instrumentally located earthquakes are clustered at the location of the faults imaged in the interior of the basin and in proximity to the northwestern basin margin modeling of magnetic and gravity data indicates that the upper crust beneath the seismically imaged structural basin is composed largely of mafic rocks to a depth of at least 4 km we propose that the charleston earthquake occurred due to compressional reactivation of a mesozoic fault in a localized zone of intense early mesozoic continental rifting

uppermost inner core seismic structure new insights from body waveform inversion abstract differential travel times and waveforms of pkikp and pkikp phases in the 129 141 distance range deconvolved for the effects of source time functions and average mantle attenuation operators are inverted for velocity and attenuation in the uppermost 80 km of the inner core and the velocity gradient and attenuation in the lowermost 200 km of the outer core f region results confirm degree one velocity structure in the inner core whose fast and slow regions appear to be separated by transitions with a velocity contrast of about 1 although attenuation in the inner core also exhibits degree one pattern a high attenuation region q p 1 0 002 0 0067 extends to the central pacific which has been previously considered a part of the western hemisphere characterized by slower velocity and lower attenuation the rest of the inner core exhibits a positive correlation between velocity and attenuation as observed previously an observed mantle like relationship between velocity and attenuation in the central pacific can be interpreted as originating from higher than average homologous temperature consistent with a model of inner core solidification driven by outer core convection coupled to core mantle boundary thermal heterogeneity alternative mechanisms including differences in grain size and impurity concentration and lateral differences in solidification rate might be required to explain the structure of the inner core the f region velocity and attenuation are best explained by the ak135 f velocity model and q p 1 0 0 001 respectively

evidence and implications of an extensive early mesozoic rift basin and basalt diabase sequence beneath the southeast coastal plain regional seismic reflection profiles across the georgia and south carolina coastal plain provide a detailed view into the internal structure and stratigraphy of the buried triassic jurassic south georgia basin reflection data indicate that the basin is a complex composite of smaller individual basins which vary drastically along strike these basins are relatively wide along north south profile maximum 100 km but appear to be more restricted along strike in cross section the basins are asymmetric being typically bounded on their south sides by an interpreted north dipping master normal falult the thickness of interpreted basin fill below the cretaceous tertiary overlap also varies drastically locally reaching 6 km analysis and correlation of the reflection profiles when integrated with drill hole and offshore reflection data suggest the presence of an areally extensive basalt flow and or diabase sill sequence of probable early middle jurassic age covering a large part of the triassic jurassic south georgia basin the sequence is traceable on seismic reflection profiles across much of the south georgia basin and may extend as far as from western georgia to offshore south carolina the implied areal extent of this sequence would be more than 100 000 km 2 ranking it among the great basalt flows and or sills of the world on seismic reflection profiles over the south georgia basin the basalt diabase sequence is also important in marking a post rift unconformity which separates tilted and apparently faulted lower mesozoic rift related strata below from a thinner more widespread and generally unfaulted sequence above this configuration is interpreted to mean that basalt extrusion and or hypabyssal intrusion occurred after the main episode of late triassic early jurassic normal faulting and associated localized subsidence syn rift phase and prior to a later jurassic episode of more widespread uniform subsidence post rift phase accompanied by relatively minor normal faulting this two stage evolutionary sequence is analogous to that of many rift basins throughout the world and suggests a hypothesis that the south georgia basin was an area of incipient crustal spreading associated with the early formation of the central atlantic ocean

recurrence rates of large earthquakes in the south carolina coastal plain based on paleoliquefaction data we present a reanalysis of results of 15 years of paleoliquefaction investigations in the south carolina coastal plain all earlier radiocarbon age data and locations of organic material collected by various investigators were reviewed and recalibrated to obtain a uniform data set the calibrated dates and the spatial extent of the sandblows having similar dates were used to estimate ages and magnitudes of prehistoric earthquake episodes the results of this analysis suggest seven episodes episodes a g of prehistoric liquefaction in the past 6000 years and two possible scenarios for their occurrence in the first scenario three seismic sources exist within the coastal plain of south carolina at charleston a b e and g with magnitudes m 71 georgetown c and f and bluffton d with magnitudes m 6 in the second scenario episodes c and d are combined into one episode episode c9 in this scenario all earthquakes occurred at charleston and with m 71 episodes a and b seem to be more representative of the earthquake cycle and suggest a recurrence time of 500 600 years for m 71 earthquakes at charleston the recurrence times and magnitudes for episodes c and d are estimated at 2000 years and 6 0 respectively the older episodes are less frequent a fact that may be attributable to times of low ground water table before 6000 years b p the ground water table was too low to permit observable liquefaction features to develop at the surface

cenozoic faulting in the vicinity of the charleston south carolina 1886 earthquake multichannel seismic reflection profiles in the meizoseismal area of the 1886 charleston earthquake and nearby offshore area show evidence of cenozoic faulting deformation observed on two land profiles defines a northeast striking reverse fault zone named the cooke fault a jurassic age basalt layer at 750 m depth has 50 m vertical displacement southeast side down overlying upper cretaceous and cenozoic beds have decreasing displacement with decreasing depth these displacements coupled with larger 190 m displacements of reflectors below the basalt indicate continuing cenozoic movement of a post basalt flow pre late cretaceous fault into eocene time or later the fault extends into a cluster of 1973 1978 epicenters this suggests a possible causal relationship with seismicity marine multichannel seismic reflection data combined with single channel high resolution reflection profiles reveal cenozoic faulting beneath the continental shelf near charleston a reverse fault 12 km offshore named the helena banks fault extends for at least 30 km and is observed as shallow as 10 m below the sea bottom with the most recent movement in post miocene or pliocene time there is no known seismicity in the area of this fault the reverse movement on the helena banks and cooke faults is consistent with a northwest southeast compressional stress regime elucidation of the relation if any between reactivated triassic or older structures and or a decollement defined by diffractions at 11 4 1 5 km depth on the one hand and seismicity on the other will be important in assessing earthquake risk in many areas of similar tectonic setting outside of the charleston region in the atlantic coastal and continental margin area

constraints on the structure and tectonic development of the early mesozoic south georgia rift southeastern united states seismic reflection data processing and interpretation seismic reflection data collected by the consortium for continental reflection profiling cocorp over the buried triassic jurassic south georgia rift reprocessed in order to improve the upper 5 s of reflection time provide a window into the regional deep structure of the rift and delineate variations in basin structure and depositional styles located at the southernmost extent of the north american east coast early mesozoic rift system the structure of the south georgia rift basin has previously been known only from scattered drilling and potential field interpretation leaving the internal and boundary as well as the deep structure unknown the reflection data reveal that the south georgia basin consists of a broad complex terrane of isolated synrift grabens with intervening structural highs in contrast with earlier concepts of a single narrow trough or an amorphous sag basin two main subbasin depocenters are observed in west and west central georgia the largest of which is more than 100 km in width and the deepest at least 7 km at its inferred base analysis of the reflection data indicates that the distinct structural and seismic stratigraphic character of the reflection packages is analogous to other better studied extensional provinces e g north american basin and range the reprocessing of the reflection data provides a view of contrasting styles of faulting and subsidence within the rift the structural style of the subbasins varies drastically along strike changing from 1 a narrow asymmetric graben bordered by a planar and more deeply penetrating normal fault to 2 a broad and roughly symmetric graben bordered by a shallower dipping possibly listric fault along which extension may have been accommodated at depth by a low angle detachment or by a more complex system of high angle faults only limited crustal extension is suggested by the narrower graben and planar fault 1 whereas much greater extension would be expected from the wider graben bordered by a listric low angle fault 2 such a dual style of basin formation is predicted by segmented plate models of variable extension along continental margins and intracratonic rifts and may imply a significant change in crustal rheology along the developing rift integration of the interpretation of the regional structure of the south georgia basin with that of the deeper crustal structure as inferred from the 16 s reflection time cocorp data indicates that the synrift basins developed mainly over the upper plate of the alleghanian late paleozoic suture between north america and west africa the major subbasin border faults dip northward in an antithetic relation to the predominantly northward vergence of the suture zone the border faults thus do not appear to have reactivated antecedant structure although the development of the south georgia rift appears to have approximately coincided with early mesozoic thinning of the crust in the southeastern united states no horizontally layered or laminated lower crust is observed as has been associated with other more fully rifted terranes e g north american basin and range continental shelf around british isles however the moho discontinuity appears to have formed as a dynamic feature superimposed on late paleozoic compressional structure probably associated with early mesozoic rifting these findings thus support the interpretation that the south georgia rift developed as a major early mesozoic depocenter and possible incipient spreading center associated with the initial formation of the central atlantic ocean

magnitudes and locations of the 1811 1812 new madrid missouri and the 1886 charleston south carolina earthquakes we estimate locations and moment magnitudes m and their uncertainties for the three largest events in the 1811 1812 sequence near new madrid missouri and for the 1 september 1886 event near charleston south carolina the intensity magnitude m i our preferred estimate of m is 7 6 for the 16 december 1811 event that occurred in the new madrid seismic zone nmsz on the bootheel lineament or on the blytheville seismic zone m i is 7 5 for the 23 january 1812 event for a location on the new madrid north zone of the nmsz and 7 8 for the 7 february 1812 event that occurred on the reelfoot blind thrust of the nmsz our preferred locations for these events are located on those nmsz segments preferred by johnston and schweig 1996 our estimates of m are 0 1 0 4 m units less than those of johnston 1996b and 0 3 0 5 m units greater than those of hough et al 2000 m i is 6 9 for the 1 september 1886 event for a location at the summerville middleton place cluster of recent small earthquakes located about 30 km northwest of charleston

scattered p p waves observed at short distances we detect previously unreported 1 hz scattered waves at epicentral dis tances between 30 and 50 and at times between 2300 and 2450 s after the earthquake origin these waves likely result from off azimuth scattering of pkpbc to pkpbc in the upper mantle and crust and provide a new tool for mapping variations in fine scale 10 km mantle heterogeneity array beams from the large aperture seismic array lasa clearly image the scattered energy gradually emerging from the noise and reaching its peak amplitude about 80 s later and returning to the noise level after 150 s stacks of transverse versus radial slowness t r show two peaks at about 2 2 and 2 2 s indicating the waves arrive along the major arc path 180 to 360 and significantly off azimuth we propose a mantle and surface pkpbc to pkpbc scattering mechanism for these observations because 1 it agrees with the initiation time and distinctive slowness signature of the scattered waves and 2 it follows a scattering path analogous to previously observed deep mantle pkkpscat tering chang and cleary 1981 the observed upper mantle scattered waves and pkkp waves fit into a broader set of scattered waves that we call p 0 dp 0 which can scatter from any depth d in the mantle

inner core fine scale structure from scattered waves recorded by lasa 1 recent observations of inner core scattering ics waves provide evidence that the outermost 300 km of the inner core has strong heterogeneities with a length scale of a few kilometers these waves follow a path similar to that of the inner core reflected waves pkikp and were originally observed in data from 16 events in the distance range 58 to 73 recorded by the large aperture seismic array lasa here we present additional observations of the ics waves from a total of 78 events recorded by lasa at distances from 18 to 98 we use a modified version of the generic array processing software package to identify ics waves on the basis of travel time back azimuth ray parameter amplitude and coherence there are 44 events that produce clear ics waves we then perform forward modeling of the observed ics waves using a monte carlo seismic phonon method that allows for multiple scattering along the raypath most of the ics waves appear without a visible pkikp phase initially grow in time and have a spindleshaped envelope the duration risetime and decay rates of the observed ics waves can be best explained by small scale volumetric heterogeneities in the outermost few hundred kilometers of the inner core the average qc value for the 44 events is 600 most clear ics waves are found for raypaths sampling the pacific ocean and asia and relatively few observations are from the atlantic ocean roughly consistent with the recently observed hemispheric difference in the inner core structure

mesozoic and cenozoic faulting imaged at the epicenter of the 1886 charleston south carolina earthquake a seismic reflection profile collected in 1981 southeast of summerville south carolina was reprocessed to investigate the recent observation of unusual features the reprocessing included geometrical spreading and attenuation correction minimum phase filtering and spiking deconvolution which led to improved velocity analysis and residual static correction the result gives an improved image of the shallow crust in the epicentral area of the 1886 charleston earthquake there is clear evidence of a down to the east steeply dipping fault with approximately 200 m of vertical offset displacing lower mesozoic sedimentary and volcanic rocks diffraction from fault edges is responsible for the observation of unusual events with abnormal move out which led to the discovery of the faulting the overlying cretaceous and tertiary sedimentary section shows approximately 10 m of reverse up to the east displacement resolved by the data to within 100 m of the ground surface two other near vertical faults with down to the east offset of lower mesozoic units are inferred from the data immediately to the northwest of the major fault the locality is on the north side of the ashley river near the place known as gregg s landing and it coincides with the epicenters of modern seismic activity and maximum shaking effects documented in 1886 the charleston earthquake was very likely associated with this faulting

seismological evidence for differential rotation of the earth s inner core the travel times of seismic waves that traverse the earth s inner core show a small but systematic variation over the past three decades this variation is best explained by a rotation of the inner core that moves the symmetry axis of its known seismic anisotropy the inferred rotation rate is on the order of 1 per year faster than the daily rotation of the mantle and crust

necessary conditions for intraplate seismic zones in north america the cause of intraplate seismic zones persists as an important scientific and societal question most intraplate earthquakes are concentrated in specific seismic zones along or adjacent to large scale basement structures e g rifts or sutures at ancient plate boundaries within continental crust the major intraplate seismic zones are limited to specific segments and are not distributed along the lengths of the ancient structures we present a new hypothesis that major intraplate seismic zones are restricted to places where concentrated crustal deformation ccd is overprinted on large scale basement structures examples where ccd affects the stability of specific parts of large scale structures in response to present day stress conditions include the most active seismic zones in central and eastern north america charlevoix eastern tennessee and new madrid our hypothesis has important implications for the assessment of seismic hazards

depth dependence of anisotropy of earth s inner core both body wave pkp travel times creager 1992 song and helmberger 1993a mcsweeney and creager 1993 shearer 1994 and fits to the splitting of core modes tromp 1993 show general agreement that the top 300 km of inner core is very anisotropic the anisotropy displays axial symmetry around the earth s spin axis with the polar direction 3 faster than the equatorial direction one key problem now is the depth dependence of the inner core anisotropy here we extend our polar path studies to include both long period and short period modeling for the pkp phases at ranges 120 to 173 arrivals from the top of the inner core pkikp and reflections from the inner core boundary pkikp can be observed distinctly in short period records at ranges 130 to 140 and as waveform distortions in the long period records at ranges 130 to 146 these waveforms provide a new set of data for examining the topmost 150 km of the inner core which is not well sampled by the previous body wave travel times record sections of waves traversing the inner core nearly parallel to the earth s spin axis polar paths from three events two beneath the south sandwich islands and one along the macquarie ridge recorded at world wide standardized seismograph network canadian network and long range seismic measurements stations are analyzed our results suggest that the top 150 km of the inner core is only weakly anisotropic less than 1 with strong evidence indicating that the top 60 km is not anisotropic at all

liquefaction potential assessment of pleistocene beach sands near charleston south carolina abstract liquefaction potential of four pleistocene beach sand deposits in the greater charleston area south carolina is assessed the assessment is based on a review of 51 sites of conspicuous craterlets and horizontal ground displacement that occurred in beach sand deposits during the 1886 charleston earthquake and an analysis of 82 seismic cone penetration tests with pore pressure measurements of the 51 ground failure sites 23 are associated with the ten mile hill beds 13 with the wando formation 13 with the silver bluff terrace and younger deposits that lie adjacent to the harbor rivers and creeks and two with the ladson formation liquefaction potential is analyzed using the seismic cone data with and without correction for age related processes diagenesis and then expressed in terms of the liquefaction potential index lpi probability curves are developed from the lpi calculations for different earthquake ground shaking parameters the probability curves for the wando formation overpred

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

the propagation and diurnal cycles of deep convection in northern tropical africa the propagation and diurnal cycle of organized convection in northern tropical africa are examined using five years 1999 2003 of digital infrared imagery for may august reduced dimension techniques are used to document the properties of cold clouds proxies for deep convection and precipitation large scale environments are diagnosed from global analyses organized convection in africa consists of coherent sequences or episodes which span an average distance of about 1000 km and last about 25 h a substantial fraction of events exhibits systematic propagation at regional to continental scales while undergoing decay and regeneration episodes with 36 h duration and 1472 km span recur at a one per day interval most episodes have phase speed of 10 20 m s 1 which is faster than most african easterly waves convective episodes tend to initiate in the lee of high terrain consistent with thermal forcing from elevated heat sources average diurnal frequency maxima result from the superposition of local diurnal maximum with the delayed phase arrival of systems propagating from the east propagation occurs with moderate low to mid tropospheric shear which varies with the african easterly jet migration and west african monsoon phases frequent deep convection occurs with local shear maxima near high terrain for the peak monsoon period and for 10 w 10 e where easterly waves and convective systems are frequent 35 of cold cloud episodes occur east of the wave trough compared with about 24 to the west based on the coherent behaviour of organized propagating convection inferences may be made regarding the prediction of precipitation beyond one or two days copyright 2008 royal meteorological society

mineral physics of iron and of the core mineral physics is a field of materials physics and materials chemistry confined to the study of materials pertinent to the earth and planetary interiors in the last two decades emphasis has been given to research on the values of physical properties in materials appropriate to the earth s mantle and core braginsky and roberts 1995 listed the physical properties that play a key role in their analysis of convection of the earth s core and the dynamo these connect mineral physics to geomagnetism of the core this review will be concerned primarily with progress made in the last four years in refining values of these properties

structure and dynamics of earth s inner core abstract the seismological exploration of the earth s inner core has revealed unexpected and puzzling structural complexities its elastic anisotropy is now well established and has been shown to vary spatially there is a well defined hemispherical dichotomy in anisotropy and attenuation and significant variations in the radial direction with weaker anisotropy in the uppermost regions and possibly different properties in an innermost inner core of radius 300 600 km perhaps even more puzzling is the observation of a stably stratified layer at the base of the outer core which has recently been proposed to result from melting of inner core material if confirmed the presence of this layer has major implications for our understanding of convection in the outer core inner core differential rotation with respect to the mantle remains controversial we will review here recent progress in understanding the dynamics of the inner core a number of different models have been proposed for the development of the inner core s structure and formation of the stable layer at the base of the outer core and particular care will be taken to clarify under which conditions the proposed mechanisms can be active

q for p waves in the sediments of the virginia coastal plain the seismic quality factor q for p waves in atlantic coastal plain sediments is estimated using data from the 2004 u s geological survey seismic survey in eastern virginia the estimates are based on spectral ratios derived from reflections and sediment guided p waves in late cretaceous and tertiary sediments within the annular trough of the late eocene chesapeake bay impact structure the estimates of q for the frequency range of 10 150 hz are from 75 to 100 with the best estimate of 80 based on multichannel stacking of spectral ratios from receivers in the offset range of 200 2000 m this result is approximately a factor of 2 larger than the results previously reported for the charleston south carolina area and it is approximately one half of that recently reported for the mississippi embayment

the structure of the base of the outer core inferred from seismic waves diffracted around the inner core 1 we systematically searched for seismograms of waves diffracted around the inner core pkpcdiff from all the temporary seismic arrays with data currently available at the iris dmc as well as some permanent regional seismic arrays including f net in japan and grf in germany to assemble the largest high quality pkpcdiff database ever created pkpcdiff waves preferentially sample the base of the outer core and so contain important clues about earth structure in this region we measured pkpdf pkpcdiff differential traveltimes and pkpcdiff pkpdf amplitude ratios in the distance range of 154 160 and modeled the observations using grid searches and full wave theory synthetic seismograms the optimum model found by fitting the differential traveltimes has relatively low velocity at the base of the outer core as in ak135 which is consistent with many previous traveltime studies however the optimum model found by fitting the amplitude ratios pkpcdiff pkpdf does not exhibit this feature and instead is closer to prem the discrepancy may be explained by two likely causes one is that small scale topography or roughness on the icb tends to scatter energy away from pkpcdiff waves by generating trailing coda waves the other is that there exists a thin layer with relatively low q at the base of the outer core this might be expected if there are suspended solid particles at the base of the outer core as proposed decades ago both mechanisms could generate smaller pkpcdiff amplitudes without significantly affecting pkpcdiff traveltimes

refining the formation and early evolution of the eastern north american margin new insights from multiscale magnetic anomaly analyses to investigate the oceanic lithosphere formation and early seafloor spreading history of the north atlantic ocean we examine multiscale magnetic anomaly data from the jurassic early cretaceous age eastern north american margin enam between 31 40 n we integrate newly acquired sea surface magnetic anomaly and seismic reflection data with publicly available aeromagnetic and composite magnetic anomaly grids satellite derived gravity anomaly and satellite derived and shipboard bathymetry data we evaluate these data sets to 1 refine magnetic anomaly correlations throughout the enam and assign updated ages and chron numbers to m0 m25 and eight pre m25 anomalies 2 identify five correlatable magnetic anomalies between the east coast magnetic anomaly ecma and blake spur magnetic anomaly bsma which may document the earliest atlantic seafloor spreading or syn rift magmatism 3 suggest pre existing margin structure and rifting segmentation may have influenced the seafloor spreading regimes in the atlantic jurassic quiet zone jqz 4 suggest that if the bsma source is oceanic crust the bsma may be m series magnetic anomaly m42 168 5 ma 5 examine the along and across margin variation in seafloor spreading rates and spreading center orientations from the bsma to m25 suggesting asymmetric crustal accretion accommodated the straightening of the ridge from the bend in the ecma to the more linear m25 and 6 observe anomalously high amplitude magnetic anomalies near the hudson fan which may be related to a short lived propagating rift segment that could have helped accommodate the crustal alignment during the early atlantic opening

constraints on inner core anisotropy from pkp df travel times pkp df travel times reported by the international seismological centre isc are analyzed to study the depth dependence of inner core anisotropy plots of pkp df travel time residuals versus range for different subsets of the data clearly show large anomalies at ranges beyond about 149 for those paths with inner core ray angles within 30 of the rotation axis on average the times are about 3 to 5 s earlier for these paths than for rays at other angles consistent with recent waveform measurements of anomalous pkp df times by creager 1992 and song and helmberger 1993 and indicative of inner core anisotropy in which the p velocity is about 3 faster for n s paths than e w paths the size of these apparent anomalies in the isc data are much larger than those inferred from several previous analyses of isc pkp df data which indicated 0 5 to 2 s anomalies at these ranges the earlier studies applied binning and averaging schemes to the data using windows that were overly tight thus excluding some of the most anomalous data points and biasing the results toward smaller anomalies to avoid this problem i apply a method for computing summary ray residuals that finds the maxima in smoothed histograms of the residuals and does not rely on a fixed interval to window the data points standard errors on each summary ray residual are estimated using a bootstrap technique that randomly resamples the data plots of the summary ray residuals for several range intervals reveal a small number of independent paths nearly parallel to the rotation axis for which the arrivals are 3 to 5 s early although the data are sparse for paths within 30 of the rotation axis anisotropy does not appear to diminish with depth within the inner core and may cause a much larger anomaly near 180 than the 2 s value obtained in several prior studies of isc data the uniform anisotropy models of creager 1992 and song and helmberger 1993 fit the bulk of the data reasonably well however the observed residuals at ranges between 132 and 140 are smaller than those predicted by most anisotropy models suggesting that the anisotropy may diminish within the outermost 50 km of the inner core

site response models for charleston south carolina and vicinity developed from shallow geotechnical investigations the study models the response of near surface materials in charleston south carolina and the adjacent area geotechnical investigations at 281 locations were made available by local engineering firms the data used for dynamic site response analysis were derived from shear wave velocity measurements at 52 loca tions site response was quantified as the ratio of surface motion to hypothetical hard rock basement outcrop motion scenario earthquake motions were developed with the stochastic model acceleration response ratios for 5 critical oscillator damping were computed for 12 frequencies ranging from 0 1 to 30 hz and for peak ground acceleration two features determine the general nature of site response in the study area the impedance contrast between mesozoic basement and cretaceous sediments and the shallow impedance contrast between quaternary and tertiary sediments average s wave velocities in the quaternary are relatively uniform and range from 150 to 250 m sec they are not strongly correlated with surface geology the velocities of the immediately underlying tertiary sediments range from 300 to 500 m sec because of the uniformity of velocity in the quaternary depth to the quaternary tertiary contact appears to be the most important variable leading to differences in calculated site response this surface is irregular and varies in depth from near surface at inland sites to approximately 30 m at sites near the coast as a consequence estimated site response in the frequency band 1 10 hz varies by as much as a factor of 3 site response at frequencies less than 1 hz is dominated by the first few resonant har monics of the entire sedimentary section with fundamental frequency near 0 2 hz

preserved extent of jurassic flood basalt in the south georgia rift a new interpretation of the j horizon at the end of the triassic 200 m y ago the central atlantic magmatic province camp one of the largest igneous provinces in the world was emplaced within a very short period of time the flows sills and dikes that mark the event are predominantly preserved in triassic rift basins along the atlantic margins conventional wisdom implies that the areally largest of the camp flows is preserved in the south georgia rift a triassic rift basin buried beneath the atlantic coastal plain the extent of this flow has been mapped on the basis of a prominent seismic reflection referred to as the j horizon this seismic horizon has been used as a time marker for estimating the end of rifting in the southern united states and the beginning of seafloor spreading reanalysis of existing well and seismic data however shows that the extent of the flood basalt is limited to a few areas and that the j horizon coincides with the base of the coastal plain this reopens the question of how the camp relates to the rift drift transition of eastern north america

fine scale heterogeneity in the earth s inner core the seismological properties of the earth s inner core have become of particular interest as we understand more about its composition and thermal state1 2 observations of anisotropy and velocity heterogeneity in the inner core are beginning to reveal how it has grown and whether it convects3 4 the attenuation of seismic waves in the inner core is strong and studies of seismic body waves5 6 have found that this high attenuation is consistent with either scattering or intrinsic attenuation5 the outermost portion of the inner core has been inferred to possess layering and to be less anisotropic than at greater depths7 8 9 10 here we present observations of seismic waves scattered in the inner core which follow the expected arrival time of the body wave reflection from the inner core boundary the amplitude of these scattered waves can be explained by stiffness variations of 1 2 with a scale length of 2 kilometres across the outermost 300 km of the inner core these variations might be caused by variations in composition by pods of partial melt in a mostly solid matrix or by variations in the orientation or strength of seismic anisotropy

modern seismicity and the fault responsible for the 1886 charleston south carolina earthquake abstract an eight station seismic network was installed in august 2011 in the epicentral area of the 1886 charleston south carolina earthquake within and near the town of summerville the network operated for one year and located 134 earthquakes with duration magnitudes ranging from 1 8 to 2 6 the earthquakes occurred in a tabular zone striking n186 e with dip of about 43 to the west the focal depths range from 13 km to the top of the early mesozoic section at a depth 1 km the dimensions of the seismic zone are 23 km along strike and 15 km down dip many of the earthquakes are concentrated in the 2 6 5 km depth range near shallow faults imaged on seismic reflection profiles adjacent to the ashley river our hypocenter relocations of 228 earthquakes occurring in the period 1977 2005 are consistent with the results obtained from the recent one year station deployment the 48 well constrained focal mechanisms derived from the one year deployment are predominantly reverse motion and 30 exhibit north south trending nodal planes consistent with the orientation of the seismic zone the seismicity near summerville has characteristics in common with the aftershock sequence of the 23 august 2011 mineral virginia earthquake we interpret these results to indicate that the modern seismicity is the lingering aftershock sequence of the 1886 shock and that the mainshock occurred on a south striking west dipping fault plane with predominantly reverse motion possibly with a component of right lateral strike slip most moment release probably occurred at depths greater than 6 km online material tables listing hypocenters and focal mechanisms and an animation showing hypocenter profiles oriented along different azimuths

seismic evidence for an inner core transition zone seismic waves that traverse earth s inner core along north south paths produce unusually broad pulse shapes at long periods compared with waves along east west paths and reflections from below the inner core boundary at short periods the observations provide compelling evidence for a seismic velocity discontinuity along north south paths about 200 kilometers below the inner core boundary separating an isotropic upper inner core from an anisotropic lower inner core the triplication associated with such a structure might be responsible for reported waveform complexity of short period inner core arrivals along north south paths and if the depth of the boundary is laterally variable their large travel time variation

the nature of the earth s core the properties of the earth s core are overviewed with emphasis on seismologically determined regions and pressures and seismologically measured density elastic wave velocities and gravitational acceleration attention is given to solid state convection of the inner core and it is noted that though seismological results do not conclusively prove that the inner core is convective the occurrence and magnitude of seismic anisotropy are explained by the effects of solid state convection igneous petrology and geochemistry of the inner core a layer at the base of the mantle and contact metasomatism at the core mantle boundary and evolution of the core mantle system are discussed it is pointed out that high pressure melting experiments indicate that the temperature of the core is ranging from 4500 to 6500 k and a major implication of such high temperature is that the tectonics and convection of the mantle as well as the resulting geological processes observed at the surface are powered by heat from the core as a result of the high temperatures along with the compositional contrast between silicates and iron alloy the core mantle boundary is considered to be most chemically active region of the earth