average seasonal variation of the atlantic equatorial currents from historical ship drifts abstract surface currents in the tropical atlantic were studied using historical ship drift data these are the only available data capable of resolving the long term seasonal fluctuations of currents over a broad geographical region the north equatorial countercurrent was found to extend as a continuous eastward flowing current across the atlantic in the band 5 8 n from july to december the maximum average eastward velocity was 30 cm s 1 during july september east of 20 w the countercurrent continued throughout the year west of 20 w and from january to june the countercurrent disappeared and westward currents were observed average westward velocity reached 20 cm 1 in april and may centered near 40 w the annual cycle in surface velocity 20 cm s 1 amplitude agrees closely with that of geostrophic velocity inferred from variations in thermocline depth across the counter current jets centered near 2 n and 4 s separated by a minimum near 1 s the northern jet has a large 14 cm s 1 semiannual v

three dimensional geometry of the reelfoot blind thrust implications for moment release and earthquake magnitude in the new madrid seismic zone mapping of the reelfoot blind thrust using portable array for numerical data acquisition panda seismicity suggests that it is a complex fault that changes its geometry along strike the thrust appears to be bounded to the north by an east trending strike slip fault the southern end of the thrust is defined by seismicity and does not terminate at a known transverse fault the northern portion of the thrust steepens at shallow levels forming a listric concave upward shape in cross section the southern segment of the thrust is interpreted to flatten near the top of the precambrian basement although some segmentation of the blind thrust is observed from the mapping of 3 km wide strips of seismicity oriented perpendicular to the fault it does not appear to be significant enough to prevent rupture along its entire length the area of the blind thrust 1301 km 2 coupled with several estimates of displacement in the 7 february 1812 event is used to determine the moment released during the event the values of m 0 range from 6 8 10 26 to 1 4 10 27 dyne cm with preferred values between 6 8 10 26 and 8 7 10 26 dyne cm similar calculations for an earthquake in a d 1450 yield a moment release of 1 0 10 27 dyne cm computed moment magnitude for the 7 february 1812 event ranges from m w 7 2 to 7 4 with preferred values between m w 7 2 and 7 3 the moment magnitude for the a d 1450 event is computed as m w 7 3 these values for the magnitude of the 1812 earthquake are lower than previous estimates based on historical records of shaking however this does not imply a lowered seismic risk in the new madrid seismic zone as site effects in the mississippi embayment may significantly amplify the ground motion caused by earthquakes of a given magnitude

felt intensity versus instrumental ground motion a difference between california and eastern north america we examine the question of whether a single ground motion intensity correlation equation gmice is applicable to both eastern north america ena intraplate and california interplate we initially address this issue with the datasets from previous studies and separate them into central united states cus california and canada datasets we then add data from the 2008 m 5 2 mt carmel illinois and the 2005 m 4 7 riviere du loup earthquakes to the cus and canada datasets respectively for each dataset the median of ground motion values at each modified mercalli intensity mmi level and their 95 confidence limits are calculated for peak ground velocity pgv peak ground acceleration pga and spectral acceleration sa at 0 3 1 0 and 2 0 s the california median value is relatively higher than the cus and canada for pgv and sa at 1 0 and 2 0 s based on the median values and the associated uncertainty we combine all datasets for pga and 0 3 s sa in order to determine a gmice the cus and canada datasets are combined separately from california for pgv 1 0 s sa and 2 0 s sa ena earthquakes have a different spectral shape than those in california particularly at intermediate periods 1 0 s where ena spectral amplitudes are reduced which may explain the difference we see at longer periods finally a log linear fit of mmi to the median values of ground motion is used to determine gmice

near surface structural model for deformation associated with the february 7 1812 new madrid missouri earthquake the february 7 1812 new madrid missouri earthquake m moment magnitude 8 was the third and final large magnitude event to rock the northern mississippi embayment during the winter of 1811 1812 although ground shaking was so strong that it rang church bells stopped clocks buckled pavement and rocked buildings up and down the eastern seaboard little coseismic surface deformation exists today in the new madrid area the fault s that ruptured during this event have remained enigmatic we have integrated geomorphic data documenting differential surficial deformation supplemented by historical accounts of surficial deformation and earthquake induced mississippi river waterfalls and rapids with the interpretation of existing and recently acquired seismic reflection data to develop a tectonic model of the near surface structures in the new madrid missouri area this model consists of two primary components a north northwest trending thrust fault and a series of northeast trending strike slip tear faults we conclude that the reelfoot fault is a thrust fault that is at least 30 km long we also infer that tear faults in the near surface partitioned the hanging wall into subparallel blocks that have undergone differential displacement during episodes of faulting the northeast trending tear faults bound an area documented to have been uplifted at least 0 5 m during the february 7 1812 earthquake these faults also appear to bound changes in the surface density of epicenters that are within the modern seismicity which is occurring in the stepover zone of the left stepping right lateral strike slip fault system of the modern new madrid seismic zone

strain accumulation in the new madrid and wabash valley seismic zones from 14 years of continuous gps observation the mechanical behavior and hence earthquake potential of faults in continental interiors is an issue of critical importance for the resultant seismic hazard but no consensus has yet been reached on this controversial topic the debate has focused on the central and eastern united states in particular the new madrid seismic zone struck by four magnitude 7 or greater earthquakes in 1811 1812 and to a lesser extent the wabash valley seismic zone just to the north a key aspect of this issue is the rate at which strain is currently accruing on those plate interior faults a quantity that remains debated here we address this issue with an analysis of up to 14 6 years of continuous gps data from a network of 200 sites in the central united states centered on the new madrid and wabash valley seismic zones we find that the high quality sites in these regions show motions that are consistently within the 95 confidence limit of zero deformation these results place an upper bound on strain accrual on faults of 0 2 mm yr and 0 6 mm yr in the new madrid and wabash valley seismic zones respectively for the new madrid region where a paleoseismic record is available for the past 5000 years we argue that strain accrual if any does not permit the 500 900 year repeat time of paleo earthquakes observed in the upper mississippi embayment these results together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors indicate that either tectonic loading rates or fault properties vary with time in the new madrid seismic zone and possibly plate wide

slow deformation and lower seismic hazard at the new madrid seismic zone global positioning system gps measurements across the new madrid seismic zone nmsz in the central united states show little if any motion these data are consistent with platewide continuous gps data away from the nmsz which show no motion within uncertainties both these data and the frequency magnitude relation for seismicity imply that had the largest shocks in the series of earthquakes that occurred in 1811 and 1812 been magnitude 8 their recurrence interval should well exceed 2500 years longer than has been assumed alternatively the largest 1811 and 1812 earthquakes and those in the paleoseismic record may have been much smaller than typically assumed hence the hazard posed by great earthquakes in the nmsz appears to be overestimated

a complex double couple source mechanism for the ms 7 2 1929 grand banks earthquake abstract the m s 7 2 grand banks earthquake of 1929 was one of the largest and the most fatal earthquakes to have occurred in canada with most of the death and destruction having been caused by a tsunami and submarine landslide associated with the earthquake it has been suggested hasegawa and kanamori 1987 that a single force landslide mechanism was more consistent with the data than was a double couple source and that therefore the event was not an earthquake however that particular study considered only four double couple solutions and left many unanswered questions in particular with respect to the source time function and sediment volume involved here a larger number of seismograms are used to examine the full range of double couple solutions to determine more definitively the nature of the event waveform modeling using both forward and inverse methods indicates that this event was an earthquake with a complex source mechanism the first and largest subevent was a strike slip double couple event occurring on a northwest striking plane two later subevents were probably strike slip double couples on northeast striking planes but other mechanisms cannot be completely ruled out the first subevent has a well constrained focal depth of 20 2 km the second and third subevents also appear to have occurred at 20 km but are constrained only to within 5 km these depths provide further evidence that the event was not a landslide the sum of the subevent moments corresponds to an m w of 7 2 0 3 which is in close agreement with the m w of 7 1 0 1 obtained by the cmt method using long period data these m w 9s are also consistent with the m s of 7 2 0 3 and m b of 7 1 0 2 calculated directly from the seismograms modeling of the seafloor displacement for this mechanism indicates that the tsunami was generated by the landslide and not directly by the earthquake

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

a 2000 year natural record of magnitudes and frequencies for the largest upper colorado river floods near moab utah using well established procedures for paleoflood hydrology and employing optically stimulated luminescence osl geochronology we analyzed a very well preserved natural record of 44 upper colorado river extreme floods with discharges ranging from 1800 to 9200 m3s 1 these are the largest floods occurring during the last 2140 220 years and this natural record indicates that large floods are much more frequent than can be estimated by extrapolation from the stream gaging record that extends back to 1914 most of these large floods occurred during the last 500 years and the two largest floods in the record both exceeded the probable maximum flood pmf estimated at 8500 m3s 1 300 000 cfs for nearby moab utah another four floods with discharges greater than 7000 m3s 1 occurred during the last two millennia flood frequency analyses using the fldfrq3 model yields the following values depending on the manning n roughness coefficients 100 yr flood 4670 4990 m3s 1 500 yr flood 6675 7270 m3s 1 1000 yr flood 7680 8440 m3s 1 the presumed pmf discharge 8500 m3s 1 gets assigned a recurrence interval of about 1000 years and the largest historical 1884 flood 3540 m3s 1 a recurrence interval of 100 years flood frequency analysis for the moab valley based on the gaged record 1914 2012 yield 2730 m3s 1 for the 100 yr flood and 3185 m3s 1 for the 500 yr flood this underestimation of the frequency of large floods from the gage data results from effects on that record by modern regulation of upstream river flow and associated water extraction for agriculture

impact of earthquake induced liquefaction and related ground failure on a mississippian archeological site in the new madrid seismic zone central usa an investigation of earthquake induced liquefaction features and related ground failure at the east archeological site 3po610 located near the southern end of the new madrid seismic zone was crucial to the identification of cultural features and horizons and interpretation of archeological data through the examination of stratigraphic and structural relations in the archeological excavation as well as in three nearby trenches it was determined that cultural horizons and features including the floor of a sweat lodge are tilted displaced downward and buried by a vented sand deposit known as a sand blow this sand blow is up to 1 m thick and is composed of four major depositional units suggesting that it formed during a sequence of large earthquakes the compound sand blow directly overlies cultural horizons and features indicating that the earthquakes occurred near the end of or soon after occupation on the basis of radiocarbon dating of samples collected above and below the sand blow the earthquakes are estimated to have occurred between a d 1300 and 1670 a new madrid earthquake sequence ca a d 1450 150 was the probable cause of liquefaction and ground failure at the site and may have led to cultural abandonment of the site ironically the earthquakes that may have led to abandonment of the site also helped to preserve it for posterity

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

hf radar comparisons with moored estimates of current speed and direction expected differences and implications the validation of estimates of ocean surface current speed and direction from high frequency hf doppler radars can be obtained through comparisons with measurements from moored near surface current meters acoustic doppler current profilers or drifters expected differences between current meter cm and hf radar estimates of ocean surface vector currents depend on numerous sources of errors and differences such as instrument and sensor limitations sampling characteristics mooring response and geophysical variability we classify these sources of errors and differences as being associated exclusively with the current meter as being associated exclusively with the hf radar or as a result of differing methodologies in which current meters and hf radars sample the spatially and temporally varying ocean surface current vector field in this latter context we consider three geophysical processes namely the stokes drift ekman drift and baroclinicity which contribute to the differences between surface and near surface vector current measurements the performance of the hf radar is evaluated on the basis of these expected differences vector currents were collected during the high resolution remote sensing experiment ii off the coast of cape hatteras north carolina in june 1993 the results of this analysis suggest that 40 60 of the observed differences between near surface cm and hf radar velocity measurements can be explained in terms of contributions from instrument noise collocation and concurrence differences and geophysical processes the rms magnitude difference ranged from 11 to 20 cm s 1 at the four mooring sites the average angular difference ranged between 15 o and 25 o of which about 10 o is attributed to the directional error of the radar current vector estimates due to the alignment of the radial beams

paleoflood hydrology origin progress prospects from an origin in diverse studies of flood geomorphology and quaternary geology paleoflood hydrology emerged as a geophysical and an applied hydrological science during the 1970s and 1980s since acquiring its formal name in 1982 the most productive approach in paleoflood hydrology has become energy based inverse hydraulic modeling of discrete paleoflood events recorded in appropriate settings as slackwater deposits and other paleostage indicators swd psi or as various threshold indicators of non exceedence technological advances particularly in hydraulic modeling and geochronology were instrumental in moving the discipline to its present status the most recent advances include 1 new techniques for the accurate geochronology of flood sediments notably tams radiocarbon analyses and osl dating and 2 the phenomenal increase in computer power that allows complex hydraulic calculations to become feasible for routine studies from its initial demonstration in the southwestern united states swd psi paleoflood hydrology proved its widespread applicability to various landscape environments particularly important studies have been accomplished in australia china india israel south africa spain and thailand paleoflood hydrology has also generated its share of controversy in part because of the differing viewpoints and attitudes of the two scientific traditions from which it emerged quaternary geology geomorphology versus applied hydrologic hydraulic engineering nevertheless the future growth of the discipline is assured given the rapid pace of discoveries that it engenders indeed so many international studies exist that it is appropriate to pursue global syntheses to address interesting and timely questions of extreme flood phenomena in relation to global climatic change

on wind wave current interactions during the shoaling waves experiment 1 this paper presents a case study of wind wave current interaction during the shoaling waves experiment showex surface current fields off duck north carolina were measured by a high frequency ocean surface current radar oscr wind wind stress and directional wave data were obtained from several air sea interaction spar asis buoys moored in the oscr scanning domain at several times during the experiment significant coastal currents entered the experimental area high horizontal shears at the current edge resulted in the waves at the peak of wind sea spectra but not those in the higher frequency equilibrium range being shifted away from the mean wind direction this led to a significant turning of the wind stress vector away from the mean wind direction the interactions presented here have important applications in radar remote sensing and are discussed in the context of recent radar imaging models of the ocean surface

directional attributes of the ocean surface wind stress vector in a recent paper by geernaert 1988 it was suggested that the stress and wind vectors are rarely aligned and that the deviation of the stress vector direction from the wind vector is related to the surface heat flux it was argued that since cold air advection over the open ocean generally corresponds to upward heat flux the consequent rotation of the winds to the right with height during cold air advection could be responsible for observations of the stress vector to be to the right of the wind flow for unstable stratifications conversely observations of the stress vector to the left of the flow for stable stratifications were suggested to be associated with the turning of the wind vector to the left of the flow for warm air advection or downward heat flux in that study only 30 of the statistical variance of the angle between the wind and wind stress vectors could be explained by air sea heat flux and it was suggested that the orbital velocity spectrum of surface waves i e data unavailable in that study could be important for explaining at least part of the remaining variance in this study we report on measurements of the wind stress vector and wave directional spectrum collected during the synthetic aperture radar x band ocean nonlinearities experiment conducted on the chesapeake light tower during 1988 the experiment yielded data reported herein that provide convincing evidence that the stress vector direction during light winds and near zero air sea heat flux is governed in large part by the direction of long waves on the ocean surface further the measurements suggest that the stress vector has a direction which is a blend between the wind direction and the swell direction

paleohydrologic bounds and extreme flood frequency of the upper arkansas river colorado usa article i nfo article history the upper arkansas river basin has experienced notable large floods including the event of 2 6 june 1921 that devastated the city of pueblo colorado we investigated flood and paleoflood hydrology at strategic sites to determine the frequency and geographic extent of extreme floods within the basin for a dam safety application streamgage historical and paleoflood data were utilized to develop frequency curves at sites near salida cotopaxi parkdale and pueblo soil stratigraphic descriptions radiocarbon dating and hydraulic modeling were used to estimate paleoflood nonexceedance bounds at the four sites which ranged from 400 to 2200 ybp for late holocene surfaces to late pleistocene surfaces near cotopaxi peak flow data are from lower magnitude snowmelt runoff in may and june in the upper basin and from high magnitude rainfall runoff from june to august in the lower basin flood frequency curves reflect this transition near parkdale from snowmelt to extreme rainfall runoff for similar return periods paleoflood peak discharges increase from about 480 m 3 s upstream at loma linda to about 4250 m 3 s downstream near pueblo this increase is attributed to the larger rainfall component derived from lower elevations between loma linda and pueblo return periods for design floods at pueblo dam exceeded 10 000 years based on paleoflood frequency curve extrapolations

overestimation of historical earthquake magnitudes in the near east due to common site effects abstract throughout historical times human habitation in the near east was limited by its arid climate to sites with strategic advantages these sites underwent millennia of recurring destruction and rebuilding resulting in the accumulation of artificial mounds known as tells consisting of multiple archeological debris strata of ruined cities which could pile up to a height of a few tens of meters intensity based magnitude evaluation of near east historical earthquakes often utilizes reported damage from cities built on earlier archeological mounds moreover old residential quarters of many modern near eastern cities are founded on such mounds in this study we measured the subsurface shear wave velocity vs profile on top of a typical mound structure as well as the vs profile of the bedrock on which the mound was accumulated the results suggest that the mound structure is likely to amplify seismic ground motions these results imply that intensity based magnitude evaluation of near east historical earthquakes that are based on reported damage from cities built on archeological mounds might be biased and could lead to over estimations of past earthquake magnitudes furthermore many old residential quarters of modern near eastern cities are commonly founded on such mounds and might be exposed to a higher seismic hazard than nearby modern neighborhoods thus requiring special attention in seismic hazard mitigation programs and seismic design

archeological and pedological evidence for large prehistoric earthquakes in the new madrid seismic zone central united states prehistoric liquefaction features have been identified by careful observation of their structural and stratigraphic relations to native american occupation horizons and their subtle soil characteristics the ages of these liquefaction features have been estimated from radiocarbon dating of wood associated with the features and native american artifacts found within bounding occupation horizons at three sites near blytheville arkansas in the central part of the new madrid seismic zone one sand blow crater formed between a d 800 and 1400 two sand blow deposits formed between a d 800 and 1670 and three possibly four sand dikes formed since 4035 b c where not found in association with native american occupation horizons and artifacts prehistoric liquefaction features can be difficult to distinguish from features that formed during the great new madrid earthquakes of a d 1811 and 1812 this raises the possibility that prehistoric liquefaction features may have been misinterpreted during previous studies in the area nevertheless a paleoearthquake chronology is beginning to emerge for the new madrid seismic zone our findings are consistent with paleoseismological studies in the northern part of the seismic zone and suggest a recurrence interval of hundreds of years for earthquakes large enough to induce liquefaction in this region m 6 4 by mapping the age distribution of liquefaction features a more accurate assessment of the long term earthquake potential of the region will be possible

analyses of the magnitude and frequency of a 400 year flood record in the fish river basin namibia abstract ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime mainly dominated by floods in these environments floods are a water resource and a potential hazard with important socioeconomic implications the fish river 86 600 km2 is the largest ephemeral stream in namibia and recently also the focus of new development plans including construction of the largest dam in namibia the hydrological analysis to support decisions implies large uncertainties owing to spatial limited and short hydrological records since 1962 here we investigate the current and past patterns of extreme floods combining instrumental historical and palaeoflood records palaeoflood studies were performed at two reaches with preserved sedimentary evidence at the upper sector vogelkranz upstream of hardap dam 13 catchment area and in the lower part of the river in the fish river canyon national park 70 catchment area in the hardap reach the palaeoflood record identified at least eight large floods during the last 350 years with the largest flood reaching a minimum discharge of 4800 m3 s 1 150 year return period in the fish river canyon reach the sedimentary record shows at least 12 large floods over the last 400 years the largest with an estimated minimum discharge of 8700 m3 s 1 the elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages associated with discharges of 6400 m3 s 1 and 16 140 m3 s 1 for the upper and mid lower fish river respectively in the upper reach the flood frequency analysis ffa combining systematic and palaeoflood data provided lower discharges 25 of the flood quantiles than the ones using only systematic data sets in the fish river canyon reach incorporation of the palaeoflood data into the ffa results in slightly higher values in the magnitude of the higher flood quantiles 4 7 the ffa analysis using an upper limited lognormal distribution function ln4 fitted with palaeoflood data shows a good performance with a slow behaviour approaching the upper limit our flood frequency results suggest that the hardap dam should increase the spillway capacity and safety check flood of the original design in order to satisfy the dam safety criteria however the projected reevaluation figures calculated from conventional hydrological methods results in an overestimation of the safety floods according to our estimations

on the modified mercalli intensities and magnitudes of the 1811 1812 new madrid earthquakes we reexamine original felt reports from the 1811 1812 new madrid earthquakes and determine revised isoseismal maps for the three principal mainshocks in many cases we interpret lower values than those assigned by earlier studies in some cases the revisions result from an interpretation of original felt reports with an appreciation for site response issues additionally earlier studies had assigned modified mercalli intensity mmi values of v vii to a substantial number of reports that we conclude do not describe damage commensurate with intensities this high we investigate several approaches to contouring the mmi values using both analytical and subjective methods for the first mainshock on 02 15 lt december 16 1811 our preferred contouring yields m 7 2 7 3 using the area moment regressions of johnston 1996 for the 08 00 lt on january 23 1812 and 03 45 lt on february 7 1812 mainshocks we obtain m 7 0 and m 7 4 7 5 respectively our magnitude for the february mainshock is consistent with the established geometry of the reelfoot fault which all evidence suggests to have been the causative structure for this event we note that the inference of lower magnitudes for the new madrid events implies that site response plays a significant role in controlling seismic hazard at alluvial sites in the central and eastern united states we also note that our results suggest that thrusting may have been the dominant mechanism of faulting associated with the 1811 1812 sequence

reconstructing the ca 100 year flood in northern thailand the key utilities of palaeohydrological studies are the potential to extend the flood record of a catchment and improve estimates of flood frequency and magnitude in this study we use a hydraulic model to produce estimates of discharge based on psi palaeo stage indicator evidence from large floods for a study reach of the mae chaem river thailand careful calibration of this model especially manning s n was possible because of the availability of psis from several modern gauged flows organic psi evidence contained in a cave permitted dating of the largest flood for which evidence was available which is close to the 100 year flood the magnitude of this flood is twice that predicted from an extrapolation of the 49 year gauging record employing the gumbel ev1 flood frequency model officially endorsed by water management agencies in thailand this magnitude of flood is however very similar to that predicted from a power law model and suggests that extreme flood magnitude and frequency in this catchment conform to power law behaviour these results significantly increase the perception of flood risk for the area and this palaeohydrological case study represents the first for the region

warm core ring velocities inferred from nscat the nasa scatterometer nscat was designed to estimate from space the wind velocity relative to the ocean surface velocity assuming that the spatial scale of the meteorological forcing giving rise to the wind is large compared with the diameter of a warm core ring it should be possible to obtain information about the currents in the ring from the demeaned scatterometer winds to test this hypothesis 14 nscat passes obtained within three days of clear avhrr derived sea surface temperature images showing a warm core ring were analyzed for evidence of circulation in the rings the residual wind vectors for the 14 scatterometer passes were binned and averaged by radial distance from ring center the mean azimuthal component of the speed compares well with adcp data collected in one of the rings a maximum of 0 89 m s 50 km from ring center compared with 0 88 m s and 1 0 m s for the two adcp transects both also at 50 km from ring center

the marine atmospheric boundary layer jet in the coastal ocean dynamics experiment the vertical and horizontal structure of the atmospheric boundary layer near the northern california coast was investigated during spring upwelling conditions as part of the 1981 coastal ocean dynamics experiment two daytime aircraft flights were devised to measure mean and turbulent quantities for 25 km tracks along and across the continental shelf from 30 to 1200 m altitude the pacific high offshore and an inland thermal low characterized the synoptic situations and equatorward winds resulted which produced intense ocean upwelling the wind profile was found to be jet shaped maximum wind speeds were 25 m s 1 and 16 m s 1 at 400 and 100 m respectively in the two flights above and below the heights of the jet speed maxima the wind speed decreased rapidly sharp density inversions existed at the levels of the peak wind speeds in one flight the inversion also sloped down toward the coast turbulence was effectively confined below the inversions strong zonal temperature gradients above the inversion between the hot land and cool marine air caused a thermal wind for which the wind speed decreased with height below the inversion turbulent stress divergence was large and effective in transferring the horizontal momentum of the wind to the sea slowing the wind there was also a small thermal wind below the inversion but opposite to the one above owing to the well mixed air temperature following the sea surface temperature which was cold at the coast and relatively warmer offshore owing to the upwelling these effects explain the jet shaped wind profile the mean momentum equations show that an acceleration toward the coast can balance the pressure gradient coriolis and turbulent stress divergence forces and there were some observations to support this complete balance was not obtained for the turbulent kinetic energy budget inferred dissipation rates were too large

ocean currents evident in satellite wind data satellite mounted radar scatterometers designed to quantify surface winds over the ocean actually measure the relative motion between the air and the ocean surface estimates of the wind stress from conventional surface wind measurements are usually derived neglecting ocean currents however when the relative motion is used the differences in the estimated stress can be as large as 50 near the equator and may even reverse sign during an el nino this assertion is supported by the strong relationship between the surface currents measured by the tropical atmosphere ocean tao array in the pacific ocean and the differences between the winds estimated from scatterometer data and those measured by tao anemometers the fact that the scatterometer measures relative motion and not wind alone makes scatterometer derived stress a more accurate representation of the boundary condition needed for both atmospheric and oceanic models than stress fields derived neglecting ocean currents

estimation of intraplate strain accumulation in the new madrid seismic zone from repeat gps surveys the new madrid seismic zone nmsz a region within the eastern portion of the north american plate where great earthquakes occurred in 1811 1812 provides a type example of large earthquakes within a relatively stable continental interior assessing the scale and causes of deformation in such regions as well estimation of the recurrence interval for the large earthquakes within them has long been recognized as an important but challenging issue for which the advent of space based geodesy provides an important new tool here we report initial results from two global positioning system gps surveys of a geodetic network spanning the new madrid seismic zone because the expected strain signal is small the data were analyzed in three different ways first site velocities were estimated and analyzed for spatial gradients parallel and perpendicular to the trend of the major strike slip faults inferred from seismicity this analysis suggests but does not require a few millimeters per year of right lateral shear across the southern portion of the seismic zone consistent with the earthquake focal mechanisms second we divided the network into near field subnetworks and found possibly significant shear associated with approximately e w shortening within the southern nmsz and possible n s shortening across the left step connecting the presumed strike slip faults third we compared the strain observed along individual baselines to that predicted from the results of earlier near field surveys which combined gps and triangulation data in the southern part of the seismic zone we found general agreement with the earlier study although our data suggest lower strain rates hence after only 2 years of measurements the gps data suggest but do not yet require detectable strain accumulation however given that great earthquakes occur in the area and that our results are reasonably consistent with the results of other geodetic studies using different data and with earthquake mechanisms it seems likely that we are seeing strain accumulation in the southern nmsz a simple interpretation of the data as showing about 3 to 5 mm yr 1 of right lateral fault parallel motion implies a 1000 to 3300 year recurrence for great earthquakes with 5 to 10 m slip assuming all the accumulating strain is released seismically this recurrence interval may be an underestimate if the observed strains are largely postseismic effects of the 1811 1812 earthquakes a possibility suggested by the observation that plate wide data show less motion across the nmsz future surveys should resolve this issue as the longer measurement spans yield more precise velocity estimates

tectonic deformation in the new madrid seismic zone inferences from map view and cross sectional boundary element models the lack of instrumental recordings and obvious fault scarps associated with the 1811 1812 new madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting strike or dip slip number strike length and width of the major faults in the new madrid seismic zone nmsz this is accomplished through two dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains tectonic deformation is reflected in 1 the seismicity 2 the lake county uplift 3 reelfoot lake 4 the fractured rocks of the blytheville arch and 5 the st francis sunk lands many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left stepping right lateral strike slip faults that are coincident with the northeast trending zones of seismicity and the blytheville arch the morphology appears to be at least in part a consequence of major earthquakes that rupture these faults the locations of the 1811 1812 and largest post 1812 earthquakes and the models are consistent with a process in which the 1811 1812 earthquakes relieved accumulated regional shear strain causing the greatest post 1812 shear strains to exist at the ends of the fault zone modeling results also suggest that the numerous small earthquakes in the nmsz are not aftershocks of the 1811 1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field the bootheel lineament does not appear to be significant in shaping the morphology geologic structure and pattern of seismicity in the nmsz the absence of fault scarps and coherence of shallow reflectors overlying the blytheville arch suggests that the major nmsz faults may not have ruptured to the surface during the 1811 1812 earthquakes the inferred lengths of the 1811 1812 earthquake ruptures also suggest that their sizes may have been overestimated model predicted subsidence within the st francis sunk lands indicates that tectonic deformation also may have influenced alluvial processes in the nmsz

sea state dependence of the wind stress over the ocean under hurricane winds 1 the impact of the surface wave field sea state on the wind stress over the ocean is investigated with fetch dependent seas under uniform wind and with complex seas under idealized tropical cyclone winds two different approaches are employed to calculate the wind stress and the mean wind profile the near peak frequency range of the surface wave field is simulated using the wavewatch iii model the high frequency part of the surface wave field is empirically determined using a range of different tail levels the results suggest that the drag coefficient magnitude is very sensitive to the spectral tail level but is not as sensitive to the drag coefficient calculation methods the drag coefficients at 40 m s vary from 1 10 3 to 4 10 3 depending on the saturation level the misalignment angle between the wind stress vector and the wind vector is sensitive to the stress calculation method used in particular if the cross wind swell is allowed to contribute to the wind stress it tends to increase the misalignment angle our results predict enhanced sea state dependence of the drag coefficient for a fast moving tropical cyclone than for a slow moving storm or for simple fetch dependent seas this may be attributed to swell that is significantly misaligned with local wind

observational constraints on earthquake source scaling understanding the limits in resolution abstract i examine the resolution of the type of stress drop estimates that have been used to place observational constraints on the scaling of earthquake source processes i first show that apparent stress and brune stress drop are equivalent to within a constant given any source spectral decay between 1 5 and 3 i e any plausible value and so consistent scaling is expected for the two estimates i then discuss the resolution and scaling of brune stress drop estimates in the context of empirical green s function results from recent earthquake sequences including the 1992 joshua tree california mainshock and its aftershocks i show that no definitive scaling of stress drop with moment is revealed over the moment range 1019 1025 within this sequence however there is a tendency for moderate sized m 4 5 events to be characterized by high stress drops however well resolved results for recent m 6 events are inconsistent with any extrapolated stress increase with moment for the aftershocks focusing on corner frequency estimates for smaller m

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 status of central seismic gap a perspective based on the spatial and temporal aspects of the large himalayan earthquakes abstract the central himalaya considered as a prominent seismic gap is generally believed to be the most vulnerable segment due for a great plate boundary earthquake m 8 two significant historical earthquakes are known to have occurred in this region in ad 1505 and 1803 interpreted by some workers as great earthquakes occurrences of these events question the validity of central himalaya being a seismic gap here we analyze the macroseismal data from the central himalaya and the gangetic plains and suggest that neither of these earthquakes can be qualified as a plate boundary type event and the size of at least the 1803 does not exceed mw 7 7 our studies further indicate that the central himalayan frontal thrusts may be undergoing a quiescence of 1000 years in terms of generation of great earthquakes assuming a steady elastic strain accumulation due to plate convergence and its simple translation as periodic great earthquakes such long quiescence may appear intriguing we suggest that the long term quiescence and the spatial and temporal clustering that characterize the distribution of large earthquakes in the himalaya is a manifestation of the tectonic deformation associated with a highly evolved fold and thrust belt our analyses also indicate that the large moderate earthquakes are mostly concentrated on higher level thrusts these structures probably accommodate most of the seismogenic slip in the himalaya and the rest of the strain could possibly be expended on passive folding consequently great earthquakes m 8 are relatively infrequent as in the case of central himalaya apparently leading to long periods of quiescence

estimating locations and magnitudes of earthquakes in eastern north america from modified mercalli intensities we use 28 calibration events 3 7 m 7 3 from texas to the grand banks newfoundland to develop a modified mercalli intensity mmi model and associated site corrections for estimating source parameters of historical earthquakes in eastern north america the model batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document mathrm mmi 1 41 1 68 times mathbf m 0 00345 times delta 2 08 mathrm log delta end document where is the distance in kilometers from the epicenter and m is moment magnitude provides unbiased estimates of m and its uncertainty and if site corrections are used of source location the model can be used for the analysis of historical earthquakes with only a few mmi assignments we use this model mmi site corrections and bakun and wentworth s 1997 technique to estimate m and the epicenter for three important historical earthquakes the intensity magnitude m i is 6 1 for the 18 november 1755 earthquake near cape ann massachusetts 6 0 for the 5 january 1843 earthquake near marked tree arkansas and 6 0 for the 31 october 1895 earthquake the 1895 event probably occurred in southern illinois about 100 km north of the site of significant ground failure effects near charleston missouri manuscript received 14 march 2002

the fine structure of nearshore tidal and residual circulations revealed by h f radar surface current measurements abstract using the ocean surface current radar oscr developed by the rutherford appleton laboratory uk 30 days of synoptic hourly surface current vectors were obtained for 84 locations within a nearshore region some 18 kms square tidal analyses of these data show that the currents associated with the predominant m2 constituent sweep smoothly and regularly through the area unaffected by the finer topographic features moreover contours of the amplitude of the m2 semi major axis are mutually consistent to a precision of better than 0 5 cm s 1 statistical analyses of these data indicate that the standard error of oscr current measurements is less than 4 cm s 1 by contrast the major higher harmonic constituent m4 shows pronounced but ordered spatial variability relating the observed distributions for m4 and m2 is foreseen as an instructive modeling problem that should advance our knowledge of shallow water tidal interaction processes standard relationships between residual surface currents and

seasonal adjustments in the equatorial atlantic during 1983 as seen by surface moorings upper ocean seasonal variations in velocity and temperature observed with current meters on surface moorings along the equator in the atlantic during february september 1983 are shown the longitudinal variation in the duration of upwelling and downwelling responses to zonal wind stress changes are discussed these responses exhibit an integral character dependent upon longitudinal position which is consistent with forced linear long wave theory

observed directional characteristics of the wind wind stress and surface waves on the open ocean sonic anemometer data were taken during the surface waves processes program swapp in march 1990 in the north pacific measurements of the wind stress vector span several strong wind events significant angles between the wind stress vector and the mean wind vector are seen simultaneous measurements of the directional wave field were made with a surface scanning doppler sonar the data suggest that the wind stress direction is influenced by the direction of the surface waves especially for stronger winds overall the stress vector lies between the mean wind and the mean wave directions at the higher wind speeds over 8 6 m s there is a nonzero correlation between the variations in wave directions and stress directions as well finally the stress and wave component directions have similar frequency dependence over the frequency band where wave energy is nonnegligible suggesting a dynamic link

major earthquakes recorded by speleothems in midwestern u s caves historic earthquakes generated by the new madrid seismic zone represent some of the largest recorded in the united states yet prehistoric events are recognized only through deformation in late wisconsin to holocene age near surface sediments liquefaction monoclinal folding and changes in river meanders in this article we show that speleothems in caves of southwestern illinois and southeastern missouri may constitute a previously unrecognized recorder of large earthquakes in the u s midcontinent region the timing of the initiation and regrowth of stalagmites in southwestern illinois and southeastern missouri caves is consistent with the historic and prehistoric record of several known seismic events in the u s midcontinent region we conclude that dating the initiation of original stalagmite growth and later postearthquake rejuvenation constitutes a new paleoseismic method that has the potential for being applied to any region around the world in the vicinity of major seismic zones where caves exist use of this technique could expand the geographical distribution of paleoseimic data document prehistoric earthquakes and help improve interpretations of paleoearthquakes

relationships between instrumental ground motion parameters and modified mercalli intensity in eastern north america a new relationship between modified mercalli intensity mmi and peak ground velocity pgv is developed by comparing instrumentally recorded and inferred historical pgv values to observed mmi for 18 significant earthquakes from eastern north america ena of moment magnitude 3 6 7 25 this pgv mmi relationship is implemented in shakemaps in ontario to estimate ground shaking intensity instrumentally derived mmi given instrumental recordings of pgv corresponding relationships for 5 damped pseudoacceleration psa at frequencies of 1 5 and 10 hz are also developed we compare the results with previous relationships between mmi and ground motion parameters developed for california by wald et al 1999a and atkinson and sonley 2000 we conclude that empirical relationships between mmi and ground motion are significantly different in eastern north america than in california thus such relationships should be assessed for each region where shakemaps are implemented manuscript received 24 october 2003

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

geophysical surveys of earthquake induced liquefaction deposits in the new madrid seismic zone we explore the effectiveness and limitations of electrical and electromagnetic methods in imaging buried earthquake induced liquefaction deposits geophysical surveys conducted at liquefaction sites in the new madrid seismic zone nmsz in the central united states demonstrate that these subsurface imaging techniques can be useful tools in paleoseismology paleoseismological studies of liquefaction features provide one of the few means for estimating recurrence intervals of large earthquakes in the nmsz a region with widespread evidence of strong ground shaking but short instrumental record noninvasive geophysical methods minimize ground disturbance during these studies an attribute of particular importance when the studies are conducted at federally protected archaeological sites surveys such as those described here can be used to locate buried liquefaction deposits and to site trenches for detailed geologic studies

oceanic and atmospheric anomalies of tropical instability waves tropical instability waves tiws are detected in remotely sensed sea surface height ssh temperature sst and wind records of the eastern equatorial pacific analyses of tiw anomaly relationships reveal strong dynamical influence of tiws within approximately 5 of the equator the first influence is advective heat flux the primary forcing of tiw sst anomalies is advection of the meridional temperature gradient by tiw currents the second influence is modification of the wind stress and ekman pumping fields by tiw surface ocean currents by affecting surface stress and hence roughness tiw currents in this low wind region introduce a significant bias in scatterometer vector wind measurement this bias is evident in both nscat and quikscat winds the difference between wind measurements from tao moorings and scatterometers is phase locked with tiw sst oscillations these results have important implications for scatterometry and for understanding tropical dynamics thermodynamics and biogeochemistry

bayesian flood frequency analysis with paleohydrologic bound data 1 it is valuable to construct likelihood functions that rigorously incorporate measurement errors and annual peak discharge historical and paleohydrologic bound information in bayesian flood frequency analyses estimates of primary posterior modes for common three parameter frequency distributions are constructed using simulated annealing and the simplex method parameter and flood frequency probability intervals are calculated directly by systematic parameter space integration bayesian flood frequency analyses with annual peak discharge historical and paleohydrologic bound data for the santa ynez river california and the big lost river idaho demonstrate that paleohydrologic bounds reduce quantile biases by placing large observed peak discharges in their proper long term contexts and substantially narrow peak discharge confidence intervals when estimating floods with low exceedance probabilities

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

earth science new madrid in motion a new network of geodetic field stations has greatly improved monitoring of relative motion across a seismic zone in the central united states it seems that rapid deformation is occurring across this fault system a series of large earthquakes centred on the town of new madrid in the mississippi valley in the winter of 1811 12 still reverberates in the world of seismology the controversy relates to their origin over 2 000 km from the nearest plate boundary and their relevance to earthquake hazard today the population of new madrid in the early nineteenth century was tiny a similar event today would have much more serious consequences new evidence based on gps data points to the existence of rapid strain rates in the area comparable in magnitude to those across active plate boundaries and consistent with known active faults in the region

revisiting the 23 february 1892 laguna salada earthquake according to some compilations the laguna salada baja california earthquake of 23 february 1892 ranks among the largest earthquakes in california and baja california in historic times although surface rupture was not documented at the time of the earthquake recent geologic investigations have identified and mapped a rupture on the laguna salada fault that can be associated with high probability with the 1892 event mueller and rockwell 1995 the only intensity based magnitude estimate for the earthquake m 7 8 was made by strand 1980 based on an interpretation of macroseismic effects and a comparison of isoseismal areas with those from instrumentally recorded earthquakes in this study we reinterpret original accounts of the laguna salada earthquake we assign modified mercalli intensity mmi values in keeping with current practice focusing on objective descriptions of damage rather than subjective human response and not assigning mmi values to effects that are now known to be poor indicators of shaking level such as liquefaction and rockfalls the reinterpreted isoseismal contours and the estimated magnitude are both significantly smaller than those obtained earlier using the method of bakun and wentworth 1997 we obtain a magnitude estimate of m 7 2 and an optimal epicenter less than 15 km from the center of the mapped laguna salada rupture the isoseismal contours are elongated toward the northwest which is qualitatively consistent with a directivity effect assuming that the fault ruptured from southeast to northwest we suggest that the elongation may also thus reflect wave propagation effects with more efficient propagation of crustal surface lg waves in the direction of the overall regional tectonic fabric

quantifying peak discharges for historical floods abstract it is usually advantageous to use information regarding historical floods if available to define the flood frequency relation for a stream peak stages can sometimes be determined for outstanding floods that occurred many years ago before systematic gaging of streams began in the united states this information is usually not available for more than 100 200 years but in countries with long cultural histories such as china historical flood data are available at some sites as far back as 2 000 years or more it is important in flood studies to be able to assign a maximum discharge rate and an associated error range to the historical flood this paper describes the significant characteristics and uncertainties of four commonly used methods for estimating the peak discharge of a flood these methods are 1 rating curve stage discharge relation extension 2 slope conveyance 3 slope area and 4 step backwater logarithmic extensions of rating curves are based on theoretical plotting techniques that results in straight line extensions provided that channel shape and roughness do not change significantly the slope conveyance and slope area methods are based on the manning equation which requires specific data on channel size shape and roughness as well as the water surface slope for one or more cross sections in a relatively straight reach of channel the slope conveyance method is used primarily for shaping and extending rating curves whereas the slope area method is used for specific floods the step backwater method also based on the manning equation requires more cross section data than the slope area ethod but has a water surface profile convergence characteristic that negates the need for known or estimated water surface slope uncertainties in calculating peak discharge for historical floods may be quite large various investigations have shown that errors in calculating peak discharges by the slope area method under ideal conditions for recent floods i e when flood elevations slope and channel characteristics are reasonably certain may be on the order of 10 25 under less than ideal conditions where streams are hydraulically steep and rough errors may be much larger the additional uncertainties for historical floods created by the passage of time may result in even larger errors of peak discharge

initial insights from 2 5d hydraulic modeling of floods in athabasca valles mars 1 we present the first application of a 2 5d hydraulic model to catastrophic floods on mars this model simulates flow over complex topography and incorporates flood dynamics that could not be modeled in the earlier 1d models we apply this model to athabasca valles the youngest outflow channel on mars investigating previous bank full discharge estimates and utilizing the interpolated mars orbiter laser altimeter elevation map as input we confirm that the bank full assumption does not fit the observed landforms instead the channel appears more deeply incised near the source flow modeling also identifies several areas of special interest including a dry cataract that coincides with a region of predicted high erosion however artifacts in the elevation data strongly impacted estimated stages and velocities in other areas more extensive connection between the flood hydraulics and observed landforms awaits improved topographic data

differences in attenuation among the stable continental regions 1 there are systematic differences in the attenuation of damaging earthquake ground motions between different stable continental regions scrs seismic intensity and weak motion data show that the attenuation in seismic waves for eastern north america ena is less than for india africa australia and northwest europe if ena ground motion attenuation relations are used in seismic hazard models for other scrs as is commonly done then the estimated ground motions and resulting hazard may be too large if an attenuation model that averages observations from ena and the other scrs is used to estimate the magnitudes of large historical earthquakes in ena as is the case for recent estimates of m for the 1811 1812 new madrid missouri and the 1886 charleston south carolina events then the magnitude estimates for these events will be too large as will be the resulting hazard

replication of apparent nonlinear seismic response with linear wave propagation models it is necessary to understand ground motion amplification by sediment defined as the ratio of ground motions at sediment sites to those at rock sites to predict seismic loadings for earthquake engineering at sediment sites observed weak motion amplifications from magnitude 3 to 4 aftershocks of the 1994 northridge earthquake were twice as large as magnitude 6 7 mainshock amplifications amplitude dependent nonlinear amplification by sediment is one explanation however earthquake simulations with empirical impulse responses and elastic finite difference calculations with weakly heterogeneous random three dimensional 3d crustal velocity variations show that linear wave propagation can explain observed apparently nonlinear sediment responses random 3d velocity variations also reproduce the observed log normal dispersion of peak ground motions deterministic wave propagation models are not adequate to quantify the scaling and dispersion of near source ground motions

new evidence for a large earthquake in the new madrid seismic zone between a d 1400 and 1670 in an integrated geological archaeological and geophysical study in the new madrid seismic zone of the southeastern united states we documented a prehistoric sand blow and related feeder dikes at an archaeological site near steele missouri archaeological analysis combined with radiocarbon dating suggest that the earthquake induced features formed between a d 1400 and 1670 this paleoseismic study provides the best evidence to date for a large earthquake occurring in the zone within 400 yr prior to the 1811 1812 new madrid earthquake sequence to determine an optimal location for excavating at the study site we mapped surficial artifact density and conducted geophysical surveys in doing so we were able to reveal critical relationships for constraining the age of the prehistoric earthquake with minimal impact to the archaeological site

sediment induced amplification and the collapse of the nimitz freeway the amplification of ground motion by low seismic velocity surface sediments is an important factor in determining the seismic hazard specific to a given site the ms 7 1 loma prieta earthquake of 17 october 1989 was the largest event in the contiguous united states in 37 years and yielded an unparalleled volume of seismic data from the main shock and aftershock sequence1 these data can be used to image the seismic source to study detailed earth structure and to study the propagation of seismic waves both through bedrock at depth and through sediment layers near the surface near the edge of san francisco bay site conditions vary considerably on scales of hundreds of metres the collapsed section of the two tiered nimitz freeway in oakland was built on san francisco bay mud whereas stiffen alluvial sediments underlie a southern section that was damaged but did not collapse here we analyse high quality digital aftershock recordings from several sites near the nimitz freeway and conclude that soil conditions and resulting ground motion amplification may have contributed significantly to the failure of the structure