spectral analysis of ground penetrating radar response to thin sedimentary layers 1 ground penetrating radar gpr systems utilized in studies of sedimentary deposits generate wavelengths tens of centimeters that are commonly much longer than the thickness of bedding often millimeters to centimeters within the target strata where this is the case radar profiles represent interference patterns simple models of radar response to sequences of thin beds such as those found in coastal deposits show potentially detectable spectral shifts toward higher frequencies in radar returns spectral analysis of radar data over barrier beach deposits at waites island south carolina shows that returns from packages with heavy mineral laminations are shifted toward higher frequencies relative to returns from more isolated contacts such spectral shifts may be useful as an indicator of fine scale layering on radar profiles

lithological anomalies in a relict coastal dune geophysical and paleoenvironmental markers 1 ground exposures of migration surfaces slipfaces of a relict holocene coastal dune along the southeastern baltic sea coast provide an ideal opportunity for establishing the causes of prominent reflections on geophysical profiles high amplitude reflections on high resolution ground penetrating radar gpr images correlate well with two major lithological anomalies 1 paleosols developed on dune slipfaces and 2 slipfaces consisting of heavy mineral concentrations hmcs paleosols serve as indicators of dune stability represent datable chronostratigraphic surfaces and help reconstruct dune paleo morphology hmcs have substantially higher magnetic susceptibility values than background quartz rich sands and where they are well developed can be also used for spatial correlation based on their occurrence at the study site these enriched horizons likely represent periods of increased wind activity storminess multiple hmcs upwind of paleosol p1 800 670 cal years bp likely reflect periods of intensified wind activity along the southeast baltic region during the medieval warm period

the sedimentary structure of linear sand dunes linear sand dunes dunes that extend parallel to each other rather than in star like or crescentic forms are the most abundant type of desert sand dune1 but because their development and their internal structure are poorly understood they are rarely recognized in the rock record2 models of linear dune development2 3 4 5 6 have not been able to take into account the sub surface structure of existing dunes but have relied instead either on the extrapolation of short term measurements of winds and sediment transport or on observations of near surface internal sedimentary structures from such studies it has not been clear if linear dunes can migrate laterally2 7 8 here we present images produced by ground penetrating radar showing the three dimensional sedimentary structure of a linear dune in the namib sand sea where some of the world s largest linear dunes are situated these profiles show clear evidence for lateral migration in a linear dune moreover the migration of a sinuous crest line along the dune produces divergent sets of cross stratification which can become stacked as the dune height increases and large linear dunes can support superimposed dunes that produce stacked sets of trough cross stratification these clear structural signatures of linear dunes should facilitate their recognition in geological records

removal of wavelet dispersion from ground penetrating radar data wavelet dispersion caused by frequency dependent attenuation is a common occurrence in ground penetrating radar gpr data and is displayed in the radar image as a characteristic blurriness that increases with depth correcting for wavelet dispersion is an important step that should be performed before gpr data are used for either qualitative interpretation or the quantitative determination of subsurface electrical properties over the bandwidth of a gpr wavelet the attenuation of electromagnetic waves in many geological materials is approximately linear with frequency as a result the change in shape of a radar pulse as it propagates through these materials can be well described using one parameter q related to the slope of the linear region assuming that all subsurface materials can be characterized by some q value the problem of estimating and correcting for wavelet dispersion becomes one of determining q in the subsurface and deconvolving its effects using an inverse q filter we present a

shumagin seismic gap alaska peninsula history of great earthquakes tectonic setting and evidence for high seismic potential the shumagin seismic gap a segment of the plate boundary along the eastern aleutian arc has not ruptured during a great earthquake since at least 1899 1903 because at least 77 years have elapsed since the shumagin gap last ruptured in a great earthquake and repeat times for the 1938 rupture zone and part of the shumagin gap are estimated to be 50 to 90 years a high probability exists for a great earthquake to occur within the shumagin gap during the next one to two decades reconsideration of the rupture zones of the aleutian earthquakes of 1938 1946 and 1948 suggests that those events did not break the interplate boundary beneath the shumagin islands thus the shumagin seismic gap extends from the western end of the 1938 rupture zone to the eastern end of that of 1946 these boundaries also coincide with transverse structural features at least the eastern half of the shumagin gap broke in great earthquakes in 1788 and 1847 and possibly in 1898 1903 the shumagin gap is probably not the result of aseismic slip rather plate motion is accommodated there seismically and episodically and can be expected to produce large earthquakes in the future although there is no definitive evidence of long term precursors of a possible future earthquake several observations suggest that the shumagin gap is in an advanced stage of the earthquake cycle both teleseismic and local network data indicate a near absence of seismic activity m 2 above a depth of 30 km along the main thrust zone within the gap this is in strong contrast to adjacent portions of the arc where seismic activity is scattered across most of the main thrust zone two earthquakes with high stress drops 600 900 bars which occurred at the base of the main thrust zone may indicate the accumulation of a considerable amount of strain energy within the gap a possible seismic gap at the eastern end of the aftershock zone of the aleutian earthquake of 1957 has been identified near unalaska island an earthquake that nucleates in the shumagin gap could also rupture the possible unalaska gap to the west the 1938 aftershock zone to the east or both with resultant magnitude up to mw 9 0 alternatively the shumagin gap alone or in one of the above combinations could rupture in a series of very large earthquakes instead of a single great shock past alaska aleutian earthquakes including those of 1788 1938 1946 1957 1964 and 1965 have generated very large tsunamis future large earthquakes in the shumagin islands region could generate wave heights of several tens of meters along shorelines near the rupture areas the shumagin gap is one of two major gaps along the united states portion of the alaska aleutian plate boundary and is one of the few areas in the united states where processes leading to a great earthquake are likely to be observed within a reasonable span of time

late pleistocene barrier island development reconstructed from genetic classification and timing of erosional surfaces paleo tokyo bay japan abstract formative processes of a retrogradational barrier island system and associated transgressive erosional surfaces are reconstructed from the upper pleistocene kioroshi formation developed in paleo tokyo bay japan the kioroshi formation provides high quality sedimentary records of a preserved barrier island system three volcanic ash beds intercalated in the formation provide a high resolution chronostratigraphic framework the barrier island system of the kioroshi formation developed in a glacio eustatically forced sea level cycle during oxygen isotope stages 6 to 5e in the early stage of the transgression paleo tokyo bay was non barred and was exposed to the pacific ocean affected by open ocean wave processes shoreface erosion in such a non barred environment developed a wave ravinement surface rs1 rs1 overlies incised valley fill deposits formed in the earliest stage of the transgression and is overlain by shoreface deposits that are further overlain by embayment deposits such as lagoonal and flood tidal delta deposits formed during succeeding barrier development during the later stage of the same transgression associated with the formation and landward migration of the barrier chain tidal currents in tidal inlet and flood tidal deltas became active and a tidal ravinement surface trs was formed trs is developed on the base of tidal inlet and flood tidal delta deposits furthermore shoreface erosion on the open ocean side of the barrier chain eroded underlying embayment deposits and formed a secondary wave ravinement surface rs2 a chronostratigraphic surface in embayment deposits defined by a volcanic ash bed intersects trs and is eroded by rs2 this relationship together with associated facies architecture indicates diachroneity of the formation of these marine erosional surfaces associated with the retrogradational barrier island system

volumetric analysis of a new england barrier system using ground penetrating radar and coring techniques ground penetrating radar gpr profiles calibrated with core data allow accurate assessments of coastal barrier volumes we applied this procedure successfully to the barrier system along saco bay maine usa as part of a sediment budget study that focused on present day sand volumes in various coastal shoreface and inner shelf lithosomes and on sand fluxes that have affected the volume or distribution of sand in these sediment bodies through time on gpr profiles the components of the barrier lithosome are readily differentiated from other facies except where the radar signal is attenuated by brackish or salty groundwater significant differences between dielectric properties of the barrier lithosome and other units commonly result in strong boundary reflectors the mostly sandy barrier sediments allow deep penetration of gpr waves in contrast to finer grained strata and till covered bedrock within the saco bay barrier system 22 pm 3 times 10 6 m 3 of sediment are unevenly distributed

anisotropic traveltime tomography velocity estimation technique using seismic data is often based on time distance equations which are independent of direction but useful results have been obtained over the years from these isotropic estimates nevertheless if velocities are significantly direction dependent then the isotropic assumption may lead to serious structural interpretation errors additionally information on angle dependence may lead to a better understanding of the lithology of the rocks under measurement vsp and cross well data may each lack the necessary aperture to estimate more than two velocity parameters for each wave type and if the data straddle a symmetry axis then these may be usefully chosen to be the direct velocities from time and distance measurements along the axis and nmo velocities from differential time offset measurements these sets of two parameters define ellipses and provide intermediate models for the variation of velocity with angle which can later be assembled and translated into estimates of the elastic moduli of the rocks under scrutiny the inversion proposed in this paper is a simple extension of well known isotropic schemes and it is conceptually identical for all wave types examples are shown to illustrate the application of the technique to cross well synthetic and field p wave data the examples demonstrate three important points more a when velocity anisotropy is estimated by iterative techniques such as conjugate gradients early termination of the iterations may produce artificial anisotropy b different components of the velocity are subject to different type of artifacts because of differences in ray coverage c even though most rocks do not have elliptical dispersion relations these elliptical schemes represent a useful intermediate step in the full characterization of the elastic properties less

spatial and temporal strain rate variations at the northern hikurangi margin new zealand we have calculated shear strain rates in the northeastern part of the north island of new zealand using geodetic data collected in the mid 1920s and 1995 in addition we use observations from a part of the network across raukumara peninsula measured in 1976 to look for temporal variation in shear strain rates in that area the strain rates were estimated using a simultaneous reduction method assuming uniform strain in space and time within each subnetwork we find the largest spatial variation in shear strain rates from the mid 1920s to 1995 occurs across raukumara peninsula increasing from 0 06 0 04 ppm yr in the west to 0 18 0 03 ppm yr in the east 1 confidence the orientation of the azimuth of relative extension shows significant temporal variation across raukumara peninsula from the mid 1920s to 1976 the maximum shear strain rate was 0 22 0 05 ppm yr with the azimuth of relative extension oriented at 129 7 from 1976 to 1995 the maximum shear strain rate was 0 14 0 05 ppm yr with the azimuth of relative extension oriented at 13 12 we show that a model where the subduction interface is locked down to 15 or 30 km depth and slipping at nuvel ia plate rates below that depth can explain the strain rates and orientation observed during 1976 1995 to fit the observations from the mid 1920s to 1976 we suggest that additional dip slip occurred on the shallow part of the subduction zone above 30 km from this we conclude that the plate boundary below raukumara peninsula is locked down to 15 or 30 km depth and the aseismic shallow slip episode that occurred during the mid 1920s 1976 was an isolated event possibly induced by m 7 historical earthquakes in the area

modeling earthquake cycles in the shumagin subduction segment alaska with seismic and geodetic constraints deformation associated with the earthquake cycle in the shumagin islands segment of the alaska aleutian subduction zone is analyzed with the use of a two dimensional finite element model the model consists of an oceanic plate dipping under an upper plate both of which respond elastically to stress fluctuations in the earthquake cycle and these are underlain by asthenospheric mantle and mantle wedge regions which respond viscoelastically it is tailored to the geometry of the shumagin islands region by using seismicity to define the position of the interplate interface and partially coupled region along it the model is preconditioned by forcing this interface to undergo periodically repeated slips up to and including the time of the may 31 1917 event ms 7 4 in that region with each chosen to be consistent with the moment and estimated rupture area of that event we investigated the dependence of model results for geodetic signals on the strength of seismic coupling between the plates and viscoelastic relaxation of deviatoric stresses in the mantle including in the mantle wedge close to the plate junction and along the aseismic downdip continuation of the thrust interface in models with significant relaxation in the wedge or downdip thrust zone results show that as the intraseismic stage matures there is a region of diminished compressional strain rates and even of locally extensional rates on the earth s surface above the downdip end of the seismically coupled zone based on the seismic estimates of the location of the coupled zone this region is in the area of the shumagin islands we find that if approximately 20 of the convergence takes place seismically compatible with the previous seismic history and if an extensive region of relaxed deviatoric stress is assumed to be present in the wedge and or along the downdip interface then deformations predicted by the model can be made consistent with the measured strain data from the shumagin islands geodetic network lisowski et al 1988 larson and lisowski 1994 as well as uplift and tilt data savage and plafker 1991 beavan 1992 our model simulations here suggest that the shumagin segment is capable of large earthquakes the hypothesis of totally aseismic subduction is not similarly consistent with all geodetic constraints

vertical resolution of thick beds thin beds and thin bed cyclothems resolution has two common definitions the first of these relates to determination of the position of a reflector in time or space resolution by this definition is inversely related to pulse width or sharpness of the wavelet and potentially directly related to bandwidth of the frequency spectrum the second definition of resolution relates to determination of the spacing of close features such as thin beds this type of resolution is determined by the rayleigh criterion which inversely relates resolution to wavelength resolution of thin beds improves with shorter wavelength or higher frequency in comparison to the rayleigh criterion the ricker and widess criteria improve the resolution potential of thin beds but they rely on models of the reflectors and require that the beds be isolated it is a long established and widely accepted axiom that the best wavelets contain bandwidths of at least a couple of octaves this results in wavelets with reduced side lobes and ringiness a more easily interpret

ground penetrating radar examination of thin tsunami beds a case study from phra thong island thailand coastal overwash deposits from tsunamis and storms have been identified and characterised from many coastal environments to date these investigations have utilised ad hoc time energy and cost intensive invasive techniques such as pits and trenches or taking core samples here we present the application of high frequency ground penetrating radar gpr to identify and characterise the 2004 indian ocean tsunami iot and palaeotsunami deposits from phra thong island thailand this site is one of the most intensively studied palaeotsunami sites globally and preserves a series of late holocene stacked sandy tsunami deposits within an organic muddy low energy backbeach environment using 100 500 and 1000 mhz gpr antennas 29 reflection profiles were collected from two swales x and y inland of the modern beach and two common mid point cmp profiles using the 200 mhz antennas were collected from swale y detailed examination of the cmps allowed accurate velocity estimates to be applied to each profile the reflection profiles included across swale profiles and a high resolution grid in swale x and were collected to investigate the feasibility of gpr to image the palaeotsunami deposits and two profiles from swale y where the tsunami deposits are poorly known the 500 mhz antennas provided the best stratigraphic resolution which was independently validated from the stratigraphy and sedimentology recovered from 17 auger cores collected along the profiles it is clear from the augers and gpr data that the different dielectric properties of the individual layers allow the identification of the iot and earlier tsunami deposits on phra thong island although applied in a coastal setting here this technique can be applied to other environments where thin sand beds are preserved in order to prioritise sites for detailed examination

full resolution 3d radar stratigraphy of complex oolitic sedimentary architecture miami limestone florida u s a abstract at the field scale m s to 100 s m it is often difficult to define the true three dimensional form of depositional units or facies within sediments or sedimentary rocks due to inherent spatial bias in traditional data sources such as outcrops and cores full resolution 3d ground penetrating radar gpr surveying has the potential to overcome this problem southwest of miami paleotopography on the upper pleistocene miami limestone formation indicates the former presence of a large oolitic barrier bar backed by extensive ooid shoals data from three closely spaced cores and two large outcrops on the barrier bar allow initial subdivision of the miami limestone in to a lower mottled oolitic facies and an upper bedded oolitic facies however detailed correlation between the cores and outcrops is problematic due to spatial aliasing a 250 mhz full resolution 3d gpr data set was collected over an area of 45 m 24 m that encompassed the location of the three cores depth of radar wave penetration was around 6 m and both horizontal and vertical resolution was approximately 0 1 m for the 6500 m3 rock volume such that the upper bedded oolitic facies was imaged without spatial aliasing radar surfaces and intervening radar facies define two main radar package types low angle sheet and sigmoidal the low angle sheet radar packages are interpreted as oolitic shoal deposits consisting of tidal bars covered by ebb orientated straight crested sand waves the overlying sigmoidal radar packages record barrier bar formation through alongshore progradation comparable sedimentary successions from both pleistocene and modern deposits in the bahamas suggest that the three evolutionary stages recorded for the miami limestone are typical of many transgressive ooid shoal complexes

factors affecting seismic moment release rates in subduction zones the amount of shallow seismic activity in subduction zones varies greatly from region to region we quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones to calculate the moment release we sum the seismic moment for all interplate thrust type events with surface wave magnitude ms 7 0 occurring from 1904 to 1980 we present a time history plot of the seismic moment release for each subduction zone these exhibit the differences in the seismic release patterns for subduction zones where the time window of our study is not representative the total moment release is corrected using information on repeat times the moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling these parameters include the age of the subducting lithosphere absolute velocities of the upper and subducting plates convergence velocity and length maximum depth and dip of the wadati benioff zone the moment release rate decreases as the age of the subducting lithosphere increases when the zones belonging to a single subducting plate are considered this age versus moment release relation is consistent for the zones in which the pacific cocos philippine sea and indian plates are subducting the moment release rates for the subduction zones in which the pacific plate is subducting are much higher than for zones of other plates with similar age the age versus moment release relation holds among the zones which belong to one subducting plate however zones with similar ages but belonging to different plates do not have the same moment release rates this suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget zones with retreating upper plates tend to have lower moment release rates the moment release rate does not increase with convergence velocity no simple relationship was found between these two parameters the moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate these are the two independent subduction zone parameters among the variables considered the other variables depend on these two parameters

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

strain accumulation in the shumagin islands results of initial gps measurements deformation in the shumagin seismic gap has been monitored with repeated trilateration edm in the 1980 1987 interval and with the global positioning system gps in the 1987 1991 interval the geodetic network extends for 100 km across the shumagin islands to the alaska peninsula results from the gps surveys are consistent with those previously reported for the edm surveys we failed to detect significant strain accumulation in the n30 w direction of plate convergence using the method of simultaneous reduction for position and strain rates we found the average rate of extension in the direction of plate convergence to be 25 25 nanostrain yr nstrain yr during the 1987 1991 interval of gps surveys compared with 20 15 nstrain yr during the 1981 1987 interval of complete edm surveys we found a marginally significant 26 12 nstrain yr extension rate in the 1981 1991 interval covered by the combined edm and gps surveys strain rates are higher but not significantly so in the part of the network closest to the trench spatial variation in the deformation is observed in the 1980 1991 average station velocities where three of the four stations closest to the trench have an arcward velocity of a few mm yr the observed strain rates are an order of magnitude lower than the 200 nstrain yr rate predicted by dislocation models

delineation of macroporous zones in the unsaturated portion of the miami limestone using ground penetrating radar miami dade county florida summary the atlantic coastal ridge in dade county is an ooid shoal formed during the late pleistocene the ridge is composed of the eogenetic karst miami limestone characterized by a heterogeneous distribution of porosity that can manifest as large touching vug macroporous features with sizes in the meters and tens of meters direct evidence for the presence of such large dissolution features is only visible when exposed to the surface which may be problematic in urbanized areas for that reason it is critical to detect any potential precursors for such dissolution features at the subsurface level the purpose of this study was to investigate the ability of ground penetrating radar to detect areas of macroporosity by quantifying changes in electromagnetic wave velocity and relate them to changes in porosity after application of the complex refractive index model crim in order to constrain volumetric water content vwc in the crim an experiment using miami limestone samples was designed to understand 1 the range of typical vwcs and 2 the effect of capillary fringe on water table elevation the results show several areas where increases in em wave travel time associated with contrasts in porosity exceeding 40 cannot be explained by changes in vwcs typically shown in the miami limestone this study may help understand porosity variability in the unsaturated part of the miami limestone and if expanded to larger scales may aid groundwater flow models by better capturing distributions of macroporous areas that may contribute to direct recharge of the biscayne aquifer

the shumagin islands alaska earthquake of 31 may 1917 abstract although macroseismic reports indicate that great earthquakes have occurred in the vicinity of the shumagin gap of the eastern aleutian arc the largest well documented event within the gap has been an m s 7 5 event that occurred in 1948 this earthquake apparently ruptured only a 75 km long segment of the 200 km long gap and as such is not considered a gap filling event relocations of turn of the century seismicity by boyd and lerner lam 1988 suggest however that another more poorly documented event nucleated near the shumagin islands on 31 may 1917 an m s of 7 9 determined from a single station bulletin has been assigned to this event although consistent with its apparent aftershock distribution macroseismic reports appear to be too small for such a large event we use surface waves to estimate the moment and rupture bounds and body waves to constrain the focal mechanism and depth of the 1917 event the average m s at 13 azimuthally well distributed stations is 7 4 0 3 much smaller than the previous estimate of 7 9 the spatial variation in individual magnitude estimates is azimuthally dependent 7 9 to the west and 6 8 to the east implying source directivity assuming a thrust fault mechanism body wave inversion yields a source depth of 29 km and m w of 7 4 in addition assuming rupture velocity and rupture propagation along arc to the west consistent with the aftershock distribution a rupture length of 75 km and m w of 7 5 are required to match long period surface waves recorded at uppsala sweden and gottingen germany these results suggest that the average slip of the 1917 event was only about 75 to 150 cm and was restricted to the easternmost portion of the gap these results when combined with rupture zones and recurrence intervals of other eastern aleutian events imply a temporally heterogeneous rupture style where the rupture bounds and consequently the sizes of earthquakes vary from cycle to cycle estimating the magnitudes or rupture extents of the next series of events based on the previous series appears to be unjustified

estimates of long term probabilities for future great earthquakes in the aleutians we compute probabilities for future great earthquakes in the aleutian arc probability distributions are fitted to recurrence periods of great aleutian evens m sub w or 7 8 since 1788 given a probability distribution and data of the last grat earthquake in each arc segment time dependent conditional probabilities are determined for future periods of interest we obtain for the next two decades high probabilities 99 to 30 for great earthquakes in the shumagin yakataga unalaska and kommandorski seismic gaps these probabilities are higher than for any other assessed region of the u s believed to be capable of great earthquakes low probabilities 17 to 9 are found for segments that ruptured most recently in 1965 1964 and 1957 recurrence periods for great earthquakes measure on average about 80 years but vary substantially whether recurrence periods for great aleutian earthquakes follow a normal log normal or any other probability distribution is not resolved because in most arc segments the known seismic record embraces at best one or two recurrences the resulting uncertainty affects the magnitude of estimated probabilities especially in seismic gaps that have not ruptured for a long time in some instances a normal distribution yields a three times higher probability than the correspondingmore log normal distribution less

ground penetrating radar of northern lacustrine deltas ground penetrating radar gpr was used in several selected deltaic sedimentary environments to better understand subsurface stratigraphy and reconstruct former depositional environments the profiles provide high resolution continuous subsurface data on facies thickness and depths orientation of major sedimentary structures postdepositional failure planes and depth of peat deposits field experiments were carried out on six river deltas records from four of the deltas exhibit sedimentary facies a record from one delta shows a possible slump and records from another delta reveal the thickness and stratigraphic relationships of peat deposits the delta types are i sandy wave influenced ii sandy immature wave influenced steeper middle and lower shoreface iii sandy braided and iv gravelly fan foreset in areas of limited subsurface control stratigraphic logs from drill core cutbank exposure or geophysical logs radar profiles can provide big picture perspectives of the subsurface

improved sea level monitors for measuring vertical crustal deformation in the shumagin seismic gap alaska we have improved the relative vertical deformation detection capability of a network of sea level gauges in the shumagin seismic gap alaska an examination of the present noise levels suggests that the network is now capable of providing relative vertical deformation data that is quieter than data from leveling the global positioning system very long baseline interferometry or satellite laser ranging

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

detection and analysis of lnapl using the instantaneous amplitude and frequency of ground penetrating radar data this paper reports the results of using the ground penetrating radar gpr method to detect light non aqueous phase liquids lnapl floating on the water table in an area where the thickness of lnapl present ranges from a few centimetres to several decimetres to understand the gpr response in this context gpr theoretical models are calculated using information from the literature and hydrogeological field data the study revealed that in the case of lnapl floating on the water table in a static condition there is an increase in the reflection amplitude from the water table due to the decrease in the capillary fringe nevertheless the amplitude of reflection from the water table can discriminate the contaminated from the non contaminated zone apart from an analysis of the real traces the analysis of some attributes of the complex trace instantaneous amplitude phase and frequency are also good tools to detect hydrocarbons floating on the water table such attributes depending on both the signal frequency and the hydrocarbon thickness can also give information about the thickness of the hydrocarbon layer it is concluded that analysing the lateral variations in signal amplitude of the real trace and in the amplitude phase and instantaneous frequency of the complex signal permits the delimiting of the area polluted by the hydrocarbon

transition from double to single wadati benioff seismic zone in the shumagin islands alaska a discontinuity exists in the moderate depth seismicity located by the shumagin seismic network in the eastern aleutian subduction zone the boundary is nearly perpendicular to the arc and has a double planed wadati benioff zone to the west and a single planed zone to the east the transition does not appear to be an artifact of mislocations as shown by three dimensional ray trace modeling and examination of phases of representative earthquakes current theory on the presence or absence of double seismic zones suggests a change in stress across the boundary if so the east and west regions may either have different aseismic to seismic slip ratios or be in different stages of the earthquake cycle

subsurface structure of a maar diatreme and associated tuff ring from a high resolution geophysical survey rattlesnake crater arizona abstract geophysical survey techniques including gravity magnetics and ground penetrating radar were utilized to study the diatreme and tuff ring at rattlesnake crater a maar in the san francisco volcanic field of northern arizona significant magnetic anomalies 1600 nt and a positive gravity anomaly 1 4 mgal are associated with the maar joint modeling of magnetic and gravity data indicate that the diatreme that underlies rattlesnake crater has volume of 0 8 1 km 3 and extends to at least 800 m depth the modeled diatreme comprises at least two zones of variable density and magnetization including a low density highly magnetized unit near the center of the diatreme interpreted to be a pyroclastic unit emplaced at sufficiently high temperature and containing sufficient juvenile fraction to acquire thermal remanent magnetization magnetic anomalies and ground penetrating radar gpr imaging demonstrate that the bedded pyroclastic deposits of the tuff ring also carry high magnetization likely produced by energetic emplacement of hot pyroclastic density currents gpr profiles on the tuff ring reveal long 100 m wavelength undulations in bedding planes elsewhere comparable bedforms have been interpreted as base surge deposits inflated by air entrainment from eruption column collapse interpretation of these geophysical data suggests that rattlesnake crater produced highly energetic phreatomagmatic activity that gave way to less explosive activity as the eruption progressed the positive gravity anomaly associated with the maar crater is interpreted to be caused by coherent bodies within the diatreme and possibly lava ponding on the crater floor these dense magnetized bodies have excess mass of 2 4 10 10 kg and occupy approximately 5 of the diatreme by volume magnetic anomalies on the crater floor are elongate nw se suggesting that the eruption may have been triggered by the interaction of ascending magma with water in fractures of this orientation gpr imaging of the tuff ring also suggests that substantial land slip may have occurred on the western rim perhaps causing part of the tuff ring to collapse into the crater strong radar reflections indicative of well developed weathering horizons are present as well the techniques employed at rattlesnake crater demonstrate the value of combining multiple geophysical techniques in areas where exposures are limited and invasive exploration is not an option

kinematics of the pacific north america plate boundary zone northern california we measured motions of 54 sites in an east west transect across northern california at 38 40 north by global positioning system gps observations over a 4 year we estimate the total slip rate on the san andreas fault system to be 39 6 0 6 1 5 mm yr 68 6 upper and lower confidence intervals from a nonlinear inversion are indicated by superscripts and subscripts slip rates on the individual faults are determined less precisely due to the high correlations between the estimated parameters our best fitting model fits the fault parallel velocities with a mean square error of 1 04 and the following estimated fault slip rates all in mm yr san andreas 17 4 3 1 2 5 ma acama 13 9 2 8 4 1 and bartlett springs 8 2 1 9 2 1 the data are fit best by models in which the san andreas fault is locked to 14 9 7 1 12 5 km the ma acama fault locked to 13 4 4 8 7 4 km except for shallow creep in the upper 5 km and the bartlett springs fault creeping at all depths the ma acama fault most likely poses a significant seismic hazard as it has a high slip rate and has accumulated a slip deficit large enough to generate a magnitude 7 earthquake we find little evidence for contraction across coast ranges except at western edge of great valley where 1 3 mm yr of shortening is permitted by the data no strain is observed within the great valley or sierra nevada except that associated with right lateral strike slip on the san andreas fault system this is consistent with models of the pacific north america plate boundary zone in which the relative plate motion is partitioned into two domains one strike slip and one dominantly extensional separated by the elastically deforming sierra nevada great valley block

spatial distribution of turn of the century seismicity along the alaska aleutian arc abstract epicenters determined for instrumentally observed earthquakes occurring around the turn of the century have figured prominently in estimates of seismic hazard along the alaska aleutian arc unfortunately because of poor timing control and a sparse station distribution the locations quoted in the older catalogs can be very unreliable more reliable epicenter estimates and quantitative assessments of the errors associated with these estimates are needed to address issues related to seismic hazard seventeen earthquakes with surface wave magnitudes above 6 8 that were recorded between 1898 and 1917 are relocated using a modified least squares criterion epicentral estimates are determined using p s and s p arrival times with the a priori constraint that the final location must be near the geographically defined arc although epicenters determined using a standard least squares criterion can be located up to 800 km from the arc the arc proximity constraint provides tectonically more meaningful locations which to a high degree of confidence fit the travel time observations as well as the unconstrained locations epicentral shifts of up to 1 200 km are observed between the relocations and those reported in the earlier catalogs seventy five per cent confidence bounds on the along arc location of each event vary from 30 to 400 km several large events m s 6 9 occurred within or near the rupture zone of the 1965 rat islands earthquake and may indicate that this zone failed in a sequence of events around the turn of the century of the four events previously suggested as being possible candidates for rupturing either the shumagin or unalaska seismic gaps none relocate near these arc segments the relocation of a large m s 7 9 earthquake which occurred in 1917 within the shumagin islands region however provides the first strong evidence of largescale seismic moment release within this intensely studied seismic gap

gpr investigation of tephra fallout cerro negro volcano nicaragua a method for constraining parameters used in tephra sedimentation models most tephra fallout models rely on the advection diffusion equation to forecast sedimentation and hence volcanic hazards here we test the application of the advection diffusion equation to tephra sedimentation using data collected on the proximal 350 to 1 200 m from the vent to medial greater than 1 200 m from the vent tephra blanket of a basaltic cinder cone cerro negro volcano located in nicaragua our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and high resolution ground penetrating radar gpr data collected in the near vent and proximal zones if the advection diffusion equation applies then the thickness of individual tephra deposits should have gaussian crosswind profiles and exponential decay with distance away from the vent at cerro negro steady trade winds coupled with brief eruptions of relatively low energy vei 2 3 create relatively simple deposits gpr data were collected along three crosswind profiles at distances of 700 1 600 m from the vent sample pits were used to estimate thickness of the 1992 tephra deposit up to 13 km from the vent horizons identified in proximal gpr profiles exhibit gaussian distributions with a high degree of statistical confidence with diffusion coefficients of 500 m2 s 1 estimated for the deposits confirming that the advection diffusion equation is capable of modeling sedimentation in the proximal zone the thinning trend downwind of the vent decreases exponentially from the cone base 350 m to 1 200 m from the vent beyond this distance deposit overthickening occurs identified in both gpr and sample pit datasets the combined data reveal three depositional regimes 1 a near vent region on the cone itself where fallout remobilizes in granular flows upon deposition 2 a proximal zone in which particles fall from a height of less than 2 km and 3 a medial zone in which particles fall from 4 to 7 km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit this overthickening of the tephra blanket defining the transition from proximal to medial depositional facies is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud a feature of deposits previously identified in larger volume eruptions we interpret this change to represent a change in diffusion law occurring at total particle fall times the fall time threshold of numerical models of 400 s thus the detailed gpr profiles and pit data collected at cerro negro help to validate current numerical models of tephra sedimentation

triple seismic zone and the regional variation of seismicity along the northern honshu arc the regional variation of seismicity along the northern honshu arc japan is studied using accurate focal depths and focal mechanism types we use focal depths determined from pp p time intervals reported in the isc bulletins for submarine earthquakes depths are corrected by considering the pp phase reported in the bulletins as the pwp phase the reflection from the ocean surface out of more than 600 well located earthquakes selected from the isc bulletins we determine the types of the focal mechanisms of 184 events using p wave first motion data based on historical seismicity of great and large earthquakes we divide the zone of thrust type earthquakes at the plate interface into two regions the shallow thrust zone 0 40kin where great earthquakes ms 8 0 occur and the deep thrust zone 40 60km where large earthquakes ms 7 4 occur the activity of great or large earthquakes shows a variation along the are in some regions both the shallow and deep thrust zones are active and in other regions only one of the thrust zones is active the seismicity of recent moderate size earthquakes mb 4 combined with the focal mechanism type shows a variation along the are which reflects the variation of the activity of great or large earthquakes where large earthquakes do not occur in the deep thrust zone neither thrust type nor down dip compression tension type events occur in and beneath the deep thrust zone where large earthquakes do occur in the deep thrust zone we find a number of thrust type earthquakes further in the latter case in some regions the down dip compression and tension type events of the double seismic zones extend seaward just beneath the deep thrust zone and form a triple planed structure of seismicity the triple seismic zone this study confirms the hypothesis of previous workers seno and pongsawat 1981 on the causal relation between the strong seismic coupling of two converging plates at the deep thrust zone and extension of the double seismic zone i e the presence or absence of activity within the slab beneath the deep seismic zone occurs when the deep thrust zone has a strong or weak coupling respectively here the weak coupling could be interpreted as either aseismic slip or as low stress buildup since the last large event occurred at the deep thrust zone triple seismic zones are found offshore of miyagi prefecture where the deep thrust zone has been broken recently in 1978 and offshore of fukushima prefecture we expect a future large earthquake at the deep thrust zone offshore of fukushima prefecture because the presence of the triple seismic zone suggests stress has been accumulating and 40 years have passed since the deep thrust zone was ruptured in 1938

absence of strain accumulation in the western shumagin segment of the alaska subduction zone gps measurements in the western shumagin segment of the alaska subduction zone show no significant strain the elastic dislocation model that best fits the data has the entire plate interface slipping freely the simplest explanation is that steady slip at the plate convergence rate is occurring on the entire plate interface although we cannot rule out the existence of a shallow locked patch close to the trench large enough to generate a m 7 earthquake the 99 confidence upper bound on the down dip width of the locked patch is 55 km assuming the trench is the the updip limit of the locked zone unless there is significant temporal variability in the locking of the plate interface or viscoelastic effects from a past earthquake the shumagin segment cannot by itself release enough moment to generate a great m 8 earthquake

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

strain accumulation in the shumagin seismic gap alaska strain accumulation in the shumagin seismic gap was estimated from repeated trilateration surveys made between 1980 and 1985 the measured rate of shear strain accumulation 0 01 0 03 rad a was an order of magnitude less than the rate 0 18 rad a predicted for seismic subduction the absence of measurable shear strain accumulation in the shumagin islands suggests that the main thrust zone there may not be locked but rather may slip stably if this is indeed the case the probability of a great shallow thrust earthquake in the shumagin gap is greatly diminished a moderate earthquake mb 6 0 occurred at the seaward end of the trilateration network 4 months after the most recent survey however no significant anomalies that could be interpreted as precursors were detected in that survey

determination of parameters of subsurface layers using gpr spectral inversion method a step by step spectral inversion algorithm is proposed in this paper to estimate the thickness depth permittivity and conductivity of layered media from ground penetrating radar gpr signals due to the distinct gpr spectral responses to media parameters different attributes of the spectra are used to estimate separately one or several parameters within each step such estimated parameters are then used as starting values in a nonlinear optimization problem for the inversion of all parameters simultaneously from the whole gpr spectral data the cost function of the optimization problem is defined as the difference between the modeled generalized reflection coefficient spectrum of subsurface layers and the actual one the optimization problem is solved by using a modified stochastic hill climbing shc algorithm the method is applied on synthetic data of a wedge model and real data from two highway gpr detections the results show that the step by step spectral inversion method can reduce the ambiguity of the inversion and improve its accuracy and efficiency

structure of the benioff zone beneath the shumagin islands alaska relocation of local earthquakes using three dimensional ray tracing seismic rays are traced through a prescribed three dimensional inhomogeneity that simulates the subducted slab below the shumagin islands region to calculate local station delays for a given hypocenter and a slab model hypocenters determined by the shumagin seismic network are then relocated using the station delays a flat layered velocity structure and a standard earthquake location computer program station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data a set of path corrections that is calculated for each grid point station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km application of this method to data from 1982 for the digitally recording shumagin seismic network shows the following results 1 a previously observed apparent increase in dip of the subducted slab at depths of 100 km disappears 2 the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45 toward north northwest at depths between 80 and 250 300 km and has a 7 higher velocity than the surrounding mantle 3 hypocenters determined from shumagin network data are located only 10 20 km south of high quality hypocenters determined from teleseismic data alone 4 qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high frequency coda possible converted phases and low amplitudes of first p arrivals for rays that travel mostly along the slab conversely for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first p arrivals that are followed by an insignificant coda and low amplitude s waves

ground penetrating radar 2 d and 3 d subsurface imaging of a coastal barrier spit long beach wa usa abstract the ability to effectively interpret and reconstruct geomorphic environments has been significantly aided by the subsurface imaging capabilities of ground penetrating radar gpr the gpr method which is based on the propagation and reflection of pulsed high frequency electromagnetic energy provides high resolution cm to m scale and shallow subsurface 0 60 m near continuous profiles of many coarser grained deposits sediments of low electrical conductivity this paper presents 2 d and 3 d gpr results from an experiment on a regressive modern barrier spit at willapa bay wa usa the medium grained sand spit is 38 km long up to 2 3 5 km wide and is influenced by a 3 7 m tidal range spring as well as high energy longshore transport and high wave energy depositional processes the spit has a freshwater aquifer recharged by rainfall the gpr acquisition system used for the test was a portable digital pulseekko system with antennae frequency ranging from 25 to 200 mhz and transmitter voltages ranging from 400 to 1000 v step sizes and antennae separation varied depending on the test requirements in addition 100 mhz antennae were used for conducting antennae orientation tests and collecting a detailed grid of data 50 50 m sampled every meter the 2 d digital profiles were processed and plotted using pulseekko software the 3 d datasets after initial processing were entered into a landmark workstation that allowed for unique 3 d perspectives of the subsurface to provide depth near surface velocity measurements were calculated from common midpoint cmp surveys results from the present study demonstrate higher resolution from the 200 mhz antennae for the top 5 6 m whereas the 25 and 50 mhz antennae show deeper penetration to 10 m for the study site 100 mhz antennae provided acceptable resolution continuity of reflections and penetration the dip profiles show a shingle like accretionary depositional pattern whereas strike profiles show a horizontal and subhorizontal nearly continuous reflection pattern results from the gpr experiment reveal upper shoreface reflections with dip towards the ocean at about 1 2 the loss of signal from below a depth of 6 8 m indicates a lithofacies change because of the storm wave base the parallel broadside and perpendicular broadside antennae orientation tests show detailed stratigraphy continuity and depth of penetration the cross polarization test exhibits reduced continuity of reflections and less depth of penetration but dipping reflections are apparent the grid pattern data provided a detailed view of 3 d geometry of individual reflections high quality data were obtained processed and directly exported into a landmark workstation for interpretation the resulting interpretations of the upper shoreface beds from the test cube 50 50 m total 2600 traces are shown as vertical sections slices horizontal sections time slices contour maps 3 d representations of individual beds and an isopach map the 3 d depositional framework allows a more detailed interpretation than widely spaced 2 d profiles

internal structure of mixed sand and gravel beach deposits revealed using ground penetrating radar mixed sand and gravel beaches are a distinctive type of coarse clastic beach ground penetrating radar gpr and photographic records of previous excavations are used to investigate the stratigraphy and internal sedimentary structure of mixed beach deposits at aldeburgh in suffolk south east england the principles of radar stratigraphy are used to describe and interpret migrated radar reflection profiles obtained from the study site the application of radar stratigraphy allows the delineation of both bounding surfaces radar surfaces and the intervening beds or bed sets radar facies the deposits of the main backshore berm ridge consist of seaward dipping bounding surfaces that are gently onlapped by seaward dipping bed sets good correspondence is observed between a sequence of beach profiles which record development of the berm ridge on the backshore and the berm ridge s internal structure the beach profile data also indicate that backshore berm ridges at aldeburgh owe their origin to discrete depositional episodes related to storm wave activity beach ridge plain deposits at the study site consist of a complex progradational sequence of foreshore berm ridge overtop and overwash deposits relict berm ridge deposits separated by seaward dipping bounding surfaces form the main depositional element beneath the beach ridge plain however the beach ridges themselves are formed predominantly of vertically stacked overtop overwash units which lie above the berm ridge deposits consequently beach ridge development in this progradational mixed beach setting must have occurred when conditions favoured overtopping and overwashing of the upper beachface interannual to decadal variations in wave climate antecedent beach morphology shoreline progradation rate and sea level are identified as the likely controlling factors in the development of such suitable conditions