measurements of gravity wave activity within and around the arctic stratospheric vortex lidar measurements of gravity wave activity have been conducted at eureka in the high arctic since 1993 the waves are detected by the fluctuations they induce in temperature it has been found that the amount of wave energy in the upper stratosphere is related to the position of the stratospheric polar vortex in each of the four winters reported here the wave activity was a maximum within the westerly jet at the edge of the vortex a minimum inside the vortex near its centre and intermediate outside the vortex the spectra of wave induced fluctuations show that it is at the longest resolved vertical wavelengths 8 to 15 km that wave energy is being influenced by the background meteorological conditions these findings are interpreted in terms of the doppler shifting and critical level filtering that is imposed by the background wind profile

basin structure influences on the propagation of teleseismic waves in the santa clara valley california we have investigated ground motion amplification in the santa clara valley scv using teleseismic p waves observed during the 1998 deployment of 41 short period seismometers the santa clara valley seismic experiment scvse lindh et al 1999 fletcher et al 2003 recorded many local and regional earthquakes and seven large m w 6 4 teleseisms measured teleseismic p wave arrival time delays relative p wave amplification and p wave energy were used in the analysis the relative p wave amplification is found to correlate strongly with the arrival time delays in addition the p wave energy is found to correlate with the observed teleseismic delays we also compared observed p wave arrival time delays and p wave amplification with synthetics computed by using 3d finite difference simulations of the teleseismic wave field to model these parameters using both the university of california berkeley ucb stidham et al 1999 stidham 1999 and the u s geological survey usgs brocher et al 1997 jachens et al 1997 3d velocity models the results indicate that arrival time delays on the order of 0 25 sec correlate strongly with the reported basin depths in the two models we find that the correlation between the arrival time delays and basin depth is strongest for the usgs model however the ucb velocity model yields wave amplification that better matches the data the finite difference simulations indicate that in general the observations may be reproduced by either of the 3d velocity models although refinements to the proposed 3d structure for the scv are needed

strong ground motion and damage in the taipei basin from the moho reflected seismic waves during the march 31 2002 hualien taiwan earthquake 1 the march 31 2002 hualien offshore earthquake ml6 8 mw7 0 caused minor damage near its epicenter but significant damage in taipei about 110 km from the epicenter the recorded pga values around the taipei basin vary in a factor of 5 the high pga values in the southeastern part of the basin correspond to the most significant damage area during the earthquake strong ground motion recordings from this event recorded at stations around the taipei basin exhibit several distinct large amplitude seismic arrivals that can be interpreted as the pmp smp and sms phases the largest amplitude of the critically reflected s wave from the moho sms and the amplification of soft shallow sediments unique for each site are responsible for the maximum peak ground motion and the significant damage in the taipei basin during the hualien offshore earthquake

on the role of inertial instability in stratosphere troposphere exchange near midlatitude cyclones abstractin simulations of midlatitude cyclones with the university of wisconsin nonhydrostatic modeling system uwnms mesoscale regions with large negative absolute vorticity commonly occur in the upper troposphere and lower stratosphere utls overlying thin layers of air with stratospheric values of ozone and potential vorticity pv these locally enhanced stratosphere troposphere exchange ste events are related to upstream convection by tracing negative equivalent potential vorticity epv anomalies along back trajectories detailed agreement between the patterns of negative absolute vorticity pv and epv each indicators of inertial instability in the utls is shown to occur in association with enhanced ste signatures results are presented for two midlatitude cyclones in the upper midwest where convection develops between the subpolar and subtropical jets mesoscale regions of negative epv air originate upstream in the boundary layer as they are transported through convection epv becomes incre

simulation of inertia gravity waves in a poleward breaking rossby wave abstract poleward breaking rossby waves often induce an upper level jet streak over northern europe dominant inertia gravity wave packets are observed downstream of this jet the physical processes of their generation and propagation in such a configuration are investigated with a mesoscale model the study is focused on an observational campaign from 17 to 19 december 1999 over northern germany different simulations with the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 have been performed for a high resolution process study three domains were set up that encompass the evolution of rossby waves and that of inertia gravity waves to minimize the impact of model damping the horizontal and vertical resolution has been adjusted appropriately with a novel statistical approach the properties of inertia gravity wave packets have been estimated this method uses the horizontal divergence field and takes into account the spatial e

analysis of strong phases other than p and s from a microearthquake survey in the wellington region new zealand a few subcrustal earthquakes located by means of a microearthquake survey in the wellington region at the indian and pacific plate boundary exhibit strong phases between first p and first s arrivals these phases have been identified as having been converted from s to p sp or p to s ps at the mohorovicic discontinuities of the indian and pacific plates examples of these phases are shown here two events are selected and simple models are assumed to draw ray paths of these phases as possible explanations of mode conversion

a seismic structure study in the kaoping area southwestern taiwan the difference between s wave and s to p wave conversion s p phase arrival times is enhanced with rectilinear motion detector filtering to describe alluvial sediment thickness in the kaohsiung pingtung kaoping plains area a more complete understanding of the underground structures of the kaoping area is provided in this paper and explains why the surrounding regions in taiwan experience more earthquakes than the kaoping area data are based on seismic activity recorded by the portable array for numerical data acquisition panda for the period from 1995 to 1997 the difference between s wave and s p phase arrival times shows that the sedimentary layer is thicker along the west and southwest coasts p wave travel time residuals high frequency attenuation parameters kappa and quality factor q p q s and coda waves confirm this result we also determined the orientation of the chaochou fault using the first motion of p wave arrivals to the east of the chaochou fault stress trends southeast northwest while to the west it trends northeast southwest the change in stress trends east and west of chaochou fault suggests the presence of a highly fluid accretionary wedge in the kaoping area the chaochou fault forms a seismically active tectonic boundary with the uplift of the hanging wall leading to westward tilting of the basement of the kaoping plains we demonstrate that these features are the reason there are relatively few earthquakes in the kaoping area the presence of a highly fluid accretionary wedge is indicated by a thick alluvial layer in the west and southwest kaoping coasts the peikung high acts as the indenter that may allow seismic energy to escape and reduce the number of earthquakes in the region online material figures illustrating calculations of kappa q c p and s wave spectra q p and q s from ground motion data

three dimensional s wave velocity structure and vp vs ratios in the new madrid seismic zone a three dimensional s wave velocity model for the new madrid seismic zone nmsz has been developed using nonlinear travel time tomography the inversion utilized 5544 s wave arrival times from 720 earthquakes recorded by digital three component stations deployed in the nmsz over the time period 1989 through 1992 we imaged s wave velocity anomalies ranging from 5 to 8 relative to the starting one dimensional velocity model lowest s wave velocities are found south of ridgley tennessee in an area characterized by a high earthquake swarm rate and shallower than normal hypocenters two centers of higher than average s wave velocity are located west of the mississippi river north of caruthersville missouri the s wave model is similar to a p wave velocity model generated using the same earthquake data set the similarity in ray coverage in both the p and s wave solutions allowed calculation of vp vs ratios most of the seismicity in the nw trending central arm of the nmsz is associated with normal vp vs values that border regions with high vp vs north of ridgely high vp vs values are associated with higher than average compressional and shear wave velocities and are interpreted to be due to mafic intrusions along the axis and edges of the reelfoot rift se of ridgley the end of the central arm coincides with high vp vs values that are due to a significant shear wave low velocity zone and are interpreted to be the result of highly fractured and fluid saturated crust

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

constraining sediment thickness in the san francisco bay area using observed resonances and p to s conversions between october 20 1989 and october 27 1989 aftershocks of the loma prieta earthquake were recorded at five locations on a variety of site conditions in the west oakland california region the author shows evidence of p to s conversions that have comparable characteristics on both mud and alluvium sites suggesting that these conversions are generated at the base of the alluvium layer the inferred transfer function for the sedimentary layer and the travel time observations for direct and converted phases can be used to constrain the depth of the sedimentary layers both the amplitude and many characteristics of the observed transfer functions on mud and alluvium are well predicted with one dimensional modelling the results suggest that in some cases separate resonance peaks result from multiple sediment layers

a statistical study of gravity waves in the polar regions based on operational radiosonde data gravity waves in the lower polar stratosphere are examined using operational radiosonde observations gathered from 33 stations over a period of 10 years both the potential and kinetic energies of the gravity waves vary annually and reach maxima in winter in the arctic and in spring in the antarctic in the antarctic spring a region of large gravity wave energy propagates downward following the movement of a zone of high static stability associated with southern hemispheric warming moreover the enhanced energy region and the high stability zone coincide in the horizontal plane and move gradually from 135 e 50 s to 45 w 70 s over the south pole the vertical and horizontal directions of wave propagation are examined using hodograph analysis in the vertical most gravity waves transfer energy upward in the arctic while the percentage of downward energy propagation is relatively high in winter and spring in the antarctic horizontally gravity waves propagate westward relative to the mean wind in the arctic while in the antarctic the dominant direction varies from station to station the correlation between gravity wave energy in the lower stratosphere and the mean wind is also examined in the arctic gravity wave energy is highly correlated with the surface wind though in the antarctic it correlates with the stratospheric wind these results suggest that gravity waves observed in the arctic are forced by topography whereas in antarctica some sources may exist in the stratosphere one such source candidate is likely to be the polar night jet

inertial available kinetic energy and the dynamics of tropical plume formation abstract tropical plumes are identified in satellite data as elongated cloud bands originating from convective activity along the intertropical convergence zone itcz often extending far into the subtropics and middle latitudes many previous studies consider tropical plumes as a product of quasigeostrophic or convergent forcing here the authors consider the view that a tropical plume is the upper branch of an enhanced thermally direct circulation driven by latent heat released along the itcz in this way tropical plume formation is strongly tied to deep cumulus convection and inertial processes observations of plume development show that as a midlatitude wave nears a subsequent plume genesis region a northward advection of upper tropospheric low potential vorticity potential vorticity unit 1 occurs as anticyclonic flow intensifies southeast of the midlatitude wave as this low potential vorticity pv ridges over and straddles the itcz plume genesis occurs plume development occurs about 1 2

study of the h v spectral ratio method for determining average shear wave velocities in the mississippi embayment abstract the shear wave velocity v s structure of the deep sediments in the mississippi embayment is of interest to engineers and seismologists due to its influence on seismic wave propagation and site response during an earthquake although the shallow v s structure top 30 to 60 m is generally well characterized information on the v s structure at greater depths to 1000 m in some regions is very limited past studies have developed relationships between the average v s v s ave of post paleozoic sediments in the embayment as a function of sediment thickness one of these studies used converted body wave arrivals while the other used ambient noise measurements processed with the horizontal to vertical spectral ratio hvsr method interestingly these two approaches yielded relationships that differed by about 25 to 30 the objective of this study is to investigate the cause of the discrepancy between these relationships and identify the more reliable relationship for the v s structure in the mississippi embayment experimental measurements of ambient noise were performed at eleven deep soil sites in the mississippi embayment and analyzed using the hvsr method the results from the experimental study yielded v s ave values that were consistent with findings from the previous hvsr study however simulations of body and surface wave propagation performed using estimated full depth v s profiles at each site showed that the methodology used to develop the velocity thickness relationship specifically the approximate relationship used to relate site frequency sediment thickness and v s produces an overestimation of the true v s ave for the profile by 25 to 30

planetary wave and gravity wave influence on the occurrence of polar stratospheric clouds over davis station antarctica seen in lidar and radiosonde observations 1 we present rayleigh lidar observations of polar stratospheric clouds psc from davis station 68 6 s 78 0 e antarctica from the 2001 2002 2003 and 2004 austral winter seasons the psc are detected as enhanced backscatter returns at lower stratospheric heights we use operational radiosonde balloon data to compare the psc detections with stratospheric temperature and wave properties as documented by others we find that the occurrence of psc is closely tied to average stratospheric temperatures which in turn are closely related to the location of davis with respect to the antarctic vortex with psc being seen when the temperature falls to near or below the calculated formation temperature of nitric acid trihydrate there is clear evidence in the sonde data for gravity wave and planetary wave modulation of the stratospheric temperature we see small scale wavelike structure in the psc profiles which likely arises from gravity waves but find little direct correlation between a given vertical lidar psc profile and near contemporaneous sonde profile probably because the sonde flight is not an instantaneous sounding at the exact spatial location of the lidar the planetary wave induced temperature variations are of the order of 10 times the size of the gravity wave variations and have a greater and more direct influence in psc occurrence over davis

imaging shallow crustal structure in the upper mississippi embayment using local earthquake waveform data a 1d normal moveout nmo corrected and stacked pseudoprofiling method was applied to analyze the characteristic features shown on primary p and s wave coda and on sp waveforms from local microearthquakes in an attempt to image prominent reflectors and to resolve shallow crustal velocity structure 5 km in the upper mississippi embayment acoustic well log data were used to constrain the p wave velocity in the upper 5 km events at close distances and with clear p and s arrivals were selected to ensure reliable nmo correction for reflections and transmissions the observed reflections and transmissions are important controlling factors on modeling waveforms we analyzed local earthquake data recorded at all broadband and one short period station of the cooperative new madrid seismic network despite polarity differences among p s and sp waveforms consistent reflectors in the sedimentary section can be imaged across the three wave types correlation with a basement penetrating well indicates that reflectors at the base of the upper cretaceous holocene mississippi embayment supergroup the base of the cambrian ordovician knox group and the high velocity lower upper cambrian bonneterre formation are shown in pseudoprofiles among stations in the upper mississippi embayment our study finds that a one layer homogeneous velocity model of sediments in the ranges of 1 95 2 42 km s for v p and 0 60 0 73 km s for v s overlying a half space of paleozoic rocks with velocities in the ranges of 6 0 6 2 km s for v p and 3 26 3 6 km s for v s can represent shallow crustal structure in the upper mississippi embayment differential times of p ppphp and s ssshs appear linearly proportional to sediment thicknesses which best fits a one layer sediment structure with average v p 2 042 0 041 km s and v s 0 709 0 051 km s in the least squares sense predicted by the wave propagation effects online material figures illustrating seismograms process procedure imaged reflectors and resolved velocity structure for six broadband stations and one short period station

the effect of orographic gravity waves on antarctic polar stratospheric cloud occurrence and composition 1 a seasonal analysis of the relationship between mesoscale orographic gravity wave activity and polar stratospheric cloud psc composition occurrence around the whole of antarctica is presented gravity wave variances are derived from temperature measurements made with the constellation observing system for meteorology ionosphere and climate cosmic global positioning system radio occultation gps ro satellites data from the cloud aerosol lidar with orthogonal polarization caliop instrument onboard the cloud aerosol lidar and infrared pathfinder satellite observations calipso satellite are used to determine the psc composition class distribution and spatial volume the results show intermittent large wave activity above the antarctic peninsula which is coincident with large volumes of h2o ice pscs these ice psc volumes advect downstream where increases in nitric acid trihydrate nat psc volumes occur supporting the mountain wave seeding hypothesis during winter 2007 in the latitude range 60 s 70 s near the edge of the vortex and where temperatures are close to psc formation thresholds 30 of all pscs are attributable to orographic gravity waves in the separate composition classes around 50 of both h2o ice pscs and a high nat number density liquid nat mixture class of pscs are due to these waves while we show that planetary waves are the major determinant of psc presence at temperatures close to the nat formation threshold we also demonstrate the important role of mesoscale intermittent orographic gravity wave activity in accounting for the composition and distribution of pscs around antarctica

gravity wave characteristics in the southern hemisphere revealed by a high resolution middle atmosphere general circulation model abstractgravity wave characteristics in the middle to high latitude southern hemisphere are analyzed using simulation data over 3 yr from a high resolution middle atmosphere general circulation model without using any gravity wave parameterizations gravity waves have large amplitudes in winter and are mainly distributed in the region surrounding the polar vortex in the middle and upper stratosphere while the gravity wave energy is generally weak in summer the wave energy distribution in winter is not zonally uniform but it is large leeward of the southern andes and antarctic peninsula linear theory in the three dimensional framework indicates that orographic gravity waves are advected leeward significantly by the mean wind component perpendicular to the wavenumber vector results of ray tracing and cross correlation analyses are consistent with this theoretical expectation the leeward energy propagation extends to several thousand kilometers which explains part of the gravity wave distribution aro

stratospheric gravity wave simulation over greenland during 24 january 2005 1 the advanced regional prediction system forecast model is extended up to the stratopause and over the entire hemisphere to simulate gravity waves during 24 january 2005 with a 15 km horizontal resolution the simulation produces dominant gravity wave features near eastern greenland that are associated mainly with orographic forcing by the greenland terrain the simulated wave temperature perturbations compare favorably with radiance perturbations from nasa atmospheric infrared sounder observations in the upper stratosphere 40 50 km vertical overturning of the isentropes suggests the occurrence of wave breaking just east of greenland that leads to a tremendous reduction of wave amplitudes the associated flux divergence produces horizontal flow deceleration of 12 120 m s 1 day 1 and coincides with areas of depleted stratospheric wind speed suggesting strong interactions between orographic gravity waves and the polar vortex a simulation using the coarser 50 km horizontal resolution produces gravity waves of significantly weaker amplitudes

model for the tectonic evolution of the mississippi embayment and its contemporary seismicity aeromagnetic and gravity surveys of the northern mississippi embayment have delineated a northeast trending basement depression about 70 km wide and more than 300 km long the feature the mississippi valley graben is remarkably linear over its known length has nearly parallel sides and has estimated depths relative to surrounding basement of more than 2 km both gravity and magnetic anomalies show an alignment of dense magnetic plutons along the boundaries of the graben estimated magnetization directions for the plutons indicate a mesozoic age the graben of probable earliest paleozoic age underlies the northwest side of the embayment and contains the area of major seismicity in particular much of the instrumentally located contemporary seismicity and the estimated locations of the large magnitude 1811 1812 earthquakes are in the central part of the graben we interpret the early paleozoic graben the mesozoic plutonism and the sedimentation cycles as evidence of stretching events seismicity is attributed to strain in and below the central graben area which would appear to be a focal point of deformation in the stretching model we suggest that the sequence of events can be explained by the action of a rotating horizontal stress field on a crustal flaw that persisted well back in pre cambrian time the postulated rotating stress field is presumably controlled by the system that drives the global plates

estimates of shear wave q and κ0 for unconsolidated and semiconsolidated sediments in eastern north america measured and calculated values of the effective quality factor qef and the site attenuation parameter 0 for unconsolidated and semiconsolidated sediments in eastern north america ena indicate that the latter is strongly dependent on sediment thickness estimates of 0 for national earthquake hazard reduction program nehrp bc site profiles sediment plus hard rock in the mississippi embayment and the atlantic coastal plain were found to increase from about 9 to 31 msec for sedi ment thicknesses ranging from 116 to 600 m stochastic simulations using the 175 m thick hypothetical nehrp bc site profile used to estimate ena ground motions in the nationalseismichazardmapsbytheu s geologicalsurvey usgs indicatethat 0 20 msec provides a smaller estimate of amplification that agrees more closely with the low strain short period site coefficients in the nehrp recommended provisions for seismic regulations for new buildings and other structures nehrpprovisions than the10msecvalueusedbytheusgs alinearregressionofthe 0 estimatescompiledin this study indicates that 0 20 msec corresponds to a relatively thick bc sediment thickness of 460 116 m these same stochastic simulations indicate that the rela tivelyshallowusgssiteprofileprovidesestimatesofamplificationthataresmallerthan thelow strainlong periodsitecoefficientsinthenehrpprovisions thedependenceof bothsiteattenuationandsiteamplificationonsedimentthicknesssuggeststhattheuseof a single reference site condition for hazard mapping might not be appropriate instead theseresultsimplythateitheraregionalsetofreferencesiteprofilesshouldbedeveloped or that a more uniform site condition such as hard rock should be used to define a more stable reference site condition in ena

orographically forced stratospheric waves over northern scandinavia simulations were carried out using the two dimensional version of a non hydrostatic numerical model in order to calculate the mesoscale response to flow across the scandinavian mountain range the initial conditions were deduced from an upstream sounding of 27 january 1992 during the easoe campaign over and to the east of the mountain crest regions of mesoscale extent developed which experienced cooling up to 7 k air parcel trajectories passed these areas in approximately 10 min

an observational test of the origin of accelerating moment release before large earthquakes 1 a progressive increase of seismic activity distributed over a wide region around a future earthquake epicenter is termed accelerating moment release amr this phenomenon has been observed in several studies over the last 15 years although there is no consensus about the physical origin of the effect in a recent hypothesis known as the stress accumulation sa model the amr is thought to result from the last stage of loading in the earthquake cycle in this view the increasing seismicity is due to minor stress release as the whole region becomes sufficiently stressed for the major event to occur the stress accumulation model makes specific predictions about the distribution of events in an amr sequence because the amr is predicted to be a result of loading on the main fault the precursory activity should be concentrated in the positive lobes of the far field stresses calculated by a backslip dislocation model of the main shock to test this model amr is first found in optimal circular regions around the epicenters of each of the m w 6 5 earthquakes in central and southern california since 1950 a backslip dislocation model is then used to determine which of the precursory events occur in the regions predicted by stress accumulation amr is shown to occur preferentially in the lobes of the backslip stress field predicted by the stress accumulation model

comment on relationship between accelerating seismicity and quiescence two precursors to large earthquakes by arnaud mignan and rita di giovambattista

local earthquake wave propagation through mississippi embayment sediments part i body wave phases and local site responses p and s body waves from microearthquakes in the new madrid seismic zone nmsz are investigated at selected sites in an effort to understand wave propagation from future large earthquakes earthquake body waveforms display distinctive features that constrain the nature of p and s wave local site responses and wave propagation within the unconsolidated mississippi embayment sediments modeling of the waveforms demonstrates that a near surface low velocity zone is characteristic of structure within the upper 60 m of the sedimentary column and produces large p and s wave resonance effects that can be used to infer near site conditions site resonance effects change because of velocity heterogeneity between individual receivers but imply that embayment sediments will substantially amplify ground motions at high frequencies site resonance affects p and s wave amplitude spectra and can bias estimates of source and anelastic attenuation parameters travel times of observed body wave phases such as p ppphp the first p wave reverberation within the entire sedimentary column ps sp s and ssphp can be used to estimate the average wave slownesses and poisson s ratio within the embayment sediments an average poisson s ratio of 0 44 is obtained for the central nmsz under station pebm detailed s wave velocities are derived for a nafe drake sediment model using acoustic well logs and the travel time constraints of observed seismic phases v p v s ratios vary from 5 5 near the surface to approximately 2 4 at the base of the sediments use of the well log data in wave calculations also explains much of the nature of p and s wave coda within the waveforms and shows that 1d heterogeneity is a first order influence on seismic wave propagation within the mississippi embayment manuscript received 11 march 2003

resolving focal depth in sparse network with local depth phase spl a case study for the 2011 mineral virginia earthquake sequenceresolving focal depth in sparse network with local depth phase spl

sediment thickness variations of the tangshan fault zone in north china from a dense seismic array and microtremor survey abstract reliable knowledge of sediment thickness is beneficial for investigating buried active faults and improving seismic hazard assessments especially in regions with recent strong earthquakes in this study we investigate the basement of sediments around the 1976 ms 7 8 tangshan earthquake region north china using the microtremor horizontal to vertical spectral ratio h v method for the ambient vibration survey we deploy a dense seismic array comprising 152 seismographs with inter station distances of approximately 1 4 km and obtain the resonance frequency from each seismograph which indicates the sediment thickness the h v method provides a potential tool for inferring the shallow subsurface structure based on the relations between resonance frequency and sediment thickness from permanent borehole seismic stations records we recover the sediment thickness around the tangshan fault zone which varies from less than 200 m in the northern region to more than 800 m in the southern region our results are generally consistent with those of previous drilling and geological studies variations in the thickness of sediments indicate that the buried fault system has experienced significant modifications over time which is partially due to predominant tectonic control by the ne sw trending tangshan fault the obtained thickness of sediments provide an important basis for earthquake strong motion simulations sediment crustal corrections for travel time tomography and active fault investigations in the tangshan fault zone

evidence for critical level filtering of atmospheric gravity waves radiosonde measurements were used to investigate gravity wave activity within the lower stratosphere above the canadian high arctic the sondes were launched from the weather station at eureka 80 n 86 w on ellesmere island gravity waves were detected by the fluctuations they induced in temperature and the associated perturbation potential energy density was used to gauge the amount of wave activity it was found that over a 15 month period there were isolated episodes of enhanced wave activity rather than any distinct seasonal cycle profiles of wind velocity were also measured and used here to show that periods of high wave activity occurred only when there would be no or very little critical level filtering of the stationary waves generated by flow over the rough terrain in the vicinity of eureka it was also found that the potential energy spectral density was enhanced at all resolvable scales during the periods of high wave activity

mesoscale simulations of gravity waves during the 2008 2009 major stratospheric sudden warming 1 a series of 24 h mesoscale simulations of 10 km horizontal and 400 m vertical resolution are performed to examine the characteristics and forcing of gravity waves gws relative to planetary waves pws during the 2008 2009 major stratospheric sudden warming ssw just prior to ssw occurrence widespread westward propagating gws are found along the vortex s edge and associated predominantly with major topographical features and strong near surface winds momentum forcing due to gws surpasses pw forcing in the upper stratosphere and tends to decelerate the polar westerly jet in excess of 30 m s 1 d 1 with ssw onset pws dominate the momentum forcing providing decelerative effects in excess of 50 m s 1 d 1 throughout the upper polar stratosphere gws related to topography become less widespread largely due to incipient wind reversal as the vortex starts to elongate during the ssw maturation and early recovery the polar vortex eventually splits and both wave signatures and forcing greatly subside nonetheless during ssw westward and eastward propagating gws are found in the polar region and may be generated in situ by flow adjustment processes in the stratosphere or by secondary gw breaking the simulated large scale features agree well with those resolved in satellite observations and analysis products

qp qs relations in the sedimentary basin of the upper mississippi embayment using converted phases abstract three component digital seismograms recorded by the 40 panda portable array for numerical data acquisition stations in the new madrid seismic zone were analyzed to study seismic wave attenuation in the sedimentary basin using the spectral ratio method a prominent s to p s p converted phase was generated at the boundary between the uppermost sedimentary basin and the underlying paleozoic rocks the direct s wave on the horizontal component is characterized by a lower frequency content than that of the converted s p wave on the vertical component the differences in frequency content between the direct s and the s p converted waves can be attributed to the different attenuation effects between p and s waves in the unconsolidated sediments the spectral ratio between the low frequency s wave and the high frequency converted s p wave from the bottom of the sediments can be used to yield a relationship between q p and q s in the sediments results from panda stations with well constrained spectral ratios in the frequency range from 2 to 25 hz give the q p value ranging from 25 to 60 and q s from 25 to 30 for the sedimentary basin in the upper mississippi embayment

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

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

sources of gravity waves in the lower stratosphere above south pole 1 five year 2001 2005 high resolution radiosonde data were processed to obtain the gravity wave gw variabilities in the lower stratosphere over south pole sp our results show that gw activities in the lowermost section 10 15 km are strongest in may and september and weakest in the austral summer whereas in the altitude range of 15 25 km strongest weakest gw kinetic energy is observed around september january we also explored the relationships of gws to the synoptic scale variations in the troposphere and the ageostrophic motions in the upper troposphere over the antarctic which are expected to be significant mechanisms for gw generation a ray tracing model grograt was used to explore the relationship between gw propagation and the background field in the altitude of 15 25 km the annual cycle of gw activity resembles that of adjustment process below 15 km the annual cycle of gw activity has two peaks in may and september our analysis suggests that these two peaks are due to the variation of topographic gw generation and filtering of background atmosphere due to critical level filtering in the lowermost section topographic gws cannot propagate upward which makes the shape of gw annual cycle at higher altitudes closer to the annual cycle of adjustment processes the analysis suggests that the minimum of gw activity at sp during the austral summer may be due to the combination of weaker wave generation from adjustment processes associated with synoptic scale systems flow over topography and unfavorable background field for gw propagation

evolving towards a critical point a review of accelerating seismic moment energy release prior to large and great earthquakes there is growing evidence that some proportion of large and great earthquakes are preceded by a period of accelerating seismic activity of moderate sized earthquakes these moderate earthquakes occur during the years to decades prior to the occurrence of the large or great event and over a region larger than its rupture zone the size of the region in which these moderate earthquakes occur scales with the size of the ensuing mainshock at least in continental regions a number of numerical simulation studies of faults and fault systems also exhibit similar behavior the combined observational and simulation evidence suggests that the period of increased moment release in moderate earthquakes signals the establishment of long wavelength correlations in the regional stress field the central hypothesis in the critical point model for regional seismicity is that it is only during these time periods that a region of the earth s crust is truly in or near a self organized critical soc state such that small earthquakes are capable of cascading into much larger events the occurrence of a large or great earthquake appears to dissipate a sufficient proportion of the accumulated regional strain to destroy these long wavelength stress correlations and bring the region out of a soc state continued tectonic strain accumulation and stress transfer during smaller earthquakes eventually re establishes the long wavelength stress correlations that allow for the occurrence of larger events these increases in activity occur over longer periods and larger regions than quiescence which is usually observed within the rupture zone of a coming large event the two phenomena appear to have different physical bases and are not incompatible with one another

polarization analysis of high frequency three component seismic data abstract we present a method to determine the polarization of body waves from three component high frequency data and examples of its application the method is based on the principal component approach one advantage of this approach is that the polarization state can be determined for small time windows compared with the predominant period of the wave this is particularly useful for identifying converted waves within the crust the stability of the result is analyzed with synthetic cases by adding simultaneous arrivals from waves and random noise the method works well with both synthetic and local data in the detection of the polarization of the wave by separating arrivals from different directions from the local data some seismic phases related to crustal conversions are observed that require strong lateral variations

the significance of the crustal velocity model in local earthquake locations from a case example of a panda experiment in the central united states reliable local earthquake locations depend on many factors including the quality of earthquake data the spatial distribution of network stations the location algorithms and the local and regional seismic velocity models the uncertainties of p and s wave arrival times at various seismic stations are in large part responsible for the uncertainty in the determination of crustal velocity models and earthquake locations we took the advantage of the high quality and high dynamic range panda data from a 33 month seismic array experiment in the central united states to investigate the importance of crustal velocity models especially the s wave model on earthquake locations several crustal models are used to relocate a group of selected earthquakes from the panda database it is clear that earthquake depths are determined less reliably than the epicenters when the shear wave velocity model is not properly determined the mislocations in some examples are significant that the importance of determining reliable p and s velocity models independently cannot be overlooked if the crustal p and s wave velocity structures in the region can be well determined reliable earthquake locations are possible even with only a few accurate p and s readings

evaluation of deep sediment velocity structure in the new madrid seismic zone detailed knowledge of the physical properties of the sediments filling the mississippi embayment has proven critical to both unravel the tectonic frame work operating in the region and assess the seismic hazards posed by the new madrid seismic zone in this article we show that independent geotechnical estimates for p and s wave velocities are compatible with a sedimentary model of k feldspar clasts embeded in water and we test its validity by modeling receiver functions at a number of broadband stations by constraining the bulk sediment thicknesses beneath each station from independent reflection profiling estimates we have been able to recover the depth to the top of the cretaceous from the receiver function data at individual stations our receiver function modeling thus provides confidence in the velocity and density structures extrapolated from in situ geotechnical measurements in the upper mississippi embayment

resolving shallow shear wave velocity structure beneath station cbn by waveform modeling of the mw 5 8 mineral virginia earthquake sequence abstract the 23 august 2011 m w 5 8 mineral virginia earthquake was the largest earthquake in the central and eastern united states in the past 100 years and it was well recorded by strong motion and broadband seismometers thus providing abundant ground motion data for earthquake ground motion studies the largest recorded ground motion among advanced national seismic system stations was at station cbn but the site effects at station cbn need to be understood because the strong motions recorded there do not represent bedrock site conditions on the radial component of these recordings the initial p wave is very weak and is followed by a strong signal 0 3 s later which is interpreted to be the s wave converted from the p wave ps at the interface between the sediments and bedrock we first estimate the subsurface shear wave velocity by modeling the ratio of the radial to vertical components of the initial p waves and then resolve the velocity structure and thickness of the unconsolidated sediments by modeling the ps wave the subsurface shear wave velocity is found to be approximately 300 m s consistent with field survey results the shear wave velocity at the bottom of the sediments is constrained to be in the range of 320 780 m s from which the thickness of the layer is estimated to be 100 230 m the estimates of shear wave velocity structure and thickness of the sediments are further improved by modeling the oscillatory waveforms between the p and s waves this study suggests that site characterization for the estimation of ground motion amplification at a site can be achieved inexpensively and noninvasively by modeling local p waveforms recorded at the site

a simple algorithm for local earthquake location using 3d vp and vs models test examples in the central united states and in central eastern taiwan traditional local earthquake location using a horizontally layered homogeneous velocity model is limited in its resolution and reliability due to the existence of frequently overlooked 3d complexity of the real earth during traditional 3d seismic tomography simultaneous earthquake relocation using the resultant 3d velocity model has produced reliable earthquake locations however only a small subset of events are typically used and thus relocated in the inversion the rest of the events in a catalog must then be relocated using the 3d models the repeated calculation of travel times across 3d v p and v s models is also not efficient and not practical for a routine network earthquake location when the very time consuming exact 3d raytracing is used because high resolution earthquake data are now available from many modern seismic networks representative high resolution 3d v p and v s models for a region can be better determined by taking advantage of recently available high speed computer technology and large disk space we implemented a simple algorithm to efficiently locate every local earthquake using the best available regional 3d v p and v s models once the v p and v s information for all cubic cells in a 3d grid model are determined p and s travel times from each grid point to all seismic stations can be calculated and stored on disk files for later usage during the iteration process for earthquake location travel times from a trial hypocenter to all recording stations can be determined simply by a linear interpolation from those of the adjacent eight grid points available in the previously stored disk files without the need for raytracing the iterations continue until the hypocenter adjustments at the end of the last iteration are below the given criteria and the travel time residual or the difference between the observed and the calculated travel times is a minimum therefore any local earthquake can be efficiently and reliably located using the available 3d velocity models this simple location program has been applied to relocate earthquakes in the new madrid seismic zone nmsz of the central united states and in the central eastern taiwan region preliminary results in both regions reveal that earthquake hypocenters can be efficiently relocated in spite of the very significant lateral structural variations tests with data from taiwan further demonstrate that the resolution of seismic tomography and the relocated seismicity is sensitive to relative distribution of seismic network stations and background seismicity thus this single event location program can be applied to relocate all earthquakes in a seismic network catalog and more importantly to allow routine earthquake location for any seismic network using the available 3d velocity models

reflection and transmission imaging of the upper crust using local earthquake seismograms we utilize the characteristic features of primary p and sh wave coda and sp waveforms from local microearthquake data and perform prestacking 1d migration in an attempt to image prominent reflectors in the upper crust of the new madrid seismic zone nmsz this methodology applies to data from broadband stations parm and penm of the cooperative new madrid seismic network cnmsn nearby exploration well log acoustic data of the wilson 2 14 and dow chemical wilson number 1 wells are used to constrain the upper 4 km of p wave velocity model despite polarity differences among p sh and sp waveforms this technique demonstrates that consistent reflectors in the deep sedimentary section can be imaged commonly among the three wave types there are excellent correlations associated with the base of the upper cretaceous holocene mississippi embayment supergroup and the base of knox group which were also reported in a study of nearby seismic reflection profiles by hamilton and zoback 1982 the bonne terre formation seems to be a prominent seismic stratigraphic marker associated with an interface displayed in the profiling we find that earthquake event s p times must be 3 sec or more to resolve reflectors at about 4 km depth possible basement at about 4 0 to 4 5 km appears on the reflected p wave image for parm and sh wave image for penm we conclude that the velocity structure of the upper 4 km crust beneath parm can be represented by a constant velocity of graphic 1 and graphic 2 for the unconsolidated sediments and graphic 3 and graphic 4 for the paleozoic sedimentary rocks a constant velocity of graphic 5 and graphic 6 for the unconsolidated sediments and graphic 7 and graphic 8 for the paleozoic sedimentary rocks can represent structure beneath penm 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif 5 embed inline graphic 5 gif 6 embed inline graphic 6 gif 7 embed inline graphic 7 gif 8 embed inline graphic 8 gif

effects of irregular structure of the mississippi embayment on ground motion amplification the objective of this study is to evaluate effects of the presently available 3d velocity model of the mississippi embayment structure on the amplification of seismic waves by using simulated finite difference seismograms effects of both 2d and 3d embayment basement structures were considered the 3d model included information of the near surface velocities that were derived from the existing 1d velocity models the 2d crustal model was taken from catchings 1999 which extended from saint louis missouri to memphis tennessee finite difference seismograms were simulated for point sources embedded at both ends of the 2d structure these seismograms were examined to distinguish features like peak amplitude amplifications and duration of seismograms when the seismic waves propagated from the mississippi embayment toward the illinois basin and vice versa to establish a working 3d structure model of the embayment we compiled geologic information of the region on material properties of the shallow structure and used the 3d model developed at the center of earthquake research institute ceri memphis as the starting model the 3d model was used to generate finite difference seismograms along several profiles for a m w 7 2 scenario earthquake occurring on the new madrid fault zone an equivalent 1d model which included the basin materials was also used to compare the 3d versus equivalent 1d ground motions simulated using the finite difference method to establish the amplification factors due to the surface sediments in the 3d model finite difference seismograms were also computed for a 1d hard rock reference model these 1d and 3d responses of the mississippi embayment were used for estimating ground motion amplification at sites where the depth to the basement is deeper than 500 m our investigation suggests that the deep structure of the mississippi embayment has little impact on long period ground motion amplitudes t 2 sec for large earthquakes that rupture in the central part of the basin this supports the hypothesis that for engineering purposes ground motions simulated based on the equivalent 1d crustal model are adequate for representing ground motions from future large earthquakes m w 7 occurring in the new madrid seismic zone

refinement of thrust faulting models for the central new madrid seismic zone abstract we present the results of the joint relocation of events recorded during 1989 1992 by the panda network in the central new madrid seismic zone the near surface material in the study area is a gently dipping layer of poorly consolidated sediments with low p wave velocity and high v p v s estimated values 1 8 km s 1 and 3 the sediments are underlain by high velocity paleozoic rocks under the network the difference in sediment thickness is only 0 6 km but because of the low velocities the location of the events using layered models is affected by errors application of the joint hypocentral determination jhd technique to a subset of 580 events shows that the single event locations may be in error by as much as 1 km in depth depending on where the events are located analysis of synthetic data generated for a realistic 3 d velocity model supports the jhd results the analysis of synthetic data also suggests that a v p v s 2 3 is more appropriate for the post paleozoic mississippi embayment sediments based on the jhd locations we present a new interpretation of the seismicity with two en echelon sw dipping thrust faults connected by a west dipping thrust fault these faults appear associated with the reelfoot scarp and its northern extension the kentucky bend scarp

nonorographic generation of arctic polar stratospheric clouds during december 1999 1 during december 1999 polar stratospheric clouds pscs were observed in the absence of conditions conducive to generation by topographic gravity waves the possibility is explored that pscs can be generated by inertia gravity waves igw radiating from breaking synoptic scale rossby waves on the polar front jet the aerosol features on 7 and 12 december are selected for comparison with theory and with simulations using the university of wisconsin nonhydrostatic modeling system uwnms consistent with rossby adjustment theory a common feature in the uwnms simulations is radiation of igw from the tropopause polar front jet especially from sectors which are evolving rapidly in the rossby wave breaking process packets of gravity wave energy radiate upward and poleward into the cold pool while individual wave crests propagate poleward and downward causing mesoscale variations in vertical motion and temperature on 12 december the eastbound dc 8 lidar observations exhibited a fairly uniform field of six waves in aerosol enhancement in the 14 20 km layer consistent with vertical displacement by a field of igw propagating antiparallel to the flow with characteristic horizontal and vertical wavelengths of 300 and 10 km uwnms simulations show emanation of a field of igw upward and southwestward from a northward incursion of the polar front jet the orientation and evolution of the aerosol features on 7 december are consistent with a single psc induced by an igw packet propagating from a breaking rossby wave over western russia toward the northeast into the coldest part of the base of the polar vortex with characteristic period 9 hours vertical wavelength 12 km and horizontal wavelength 1000 km linear theory shows that for both of these cases igw energy propagates upward at 1 km hour and horizontally at 100 km hour with characteristic trace speed 30 m s the spatial orientation of the psc along igw phase lines is contrasted with the nearly horizontal filamentary structures in the psc which are indicative of flow streamlines it is suggested that vertical displacement is a crucial factor in determining whether a psc will form and that most pscs are relatable to specific synoptic and mesoscale motions

gravity waves appearing in a high resolution gcm simulation global characteristics of gravity waves in the lower stratosphere are examined using a gcm with high resolution in both the horizontal t106 corresponding to about 120 km and the vertical 600 m the bottom boundary condition of the model is that of an aquaplanet with perpetual february sea surface temperature the simulated gravity waves are in good agreement with mesosphere stratosphere troposphere mst radar observations at a middle latitude on the gravity wave structure and on the frequency spectra as a function of height the frequency spectra of simulated wind and temperature fluctuations are also examined as a function of latitude large values of spectral density are observed at frequencies higher than the inertial frequency f in a weak wind region around 20 km which is consistent with the characteristics of internal gravity waves an isolated peak is observed near f for horizontal wind spectra at latitudes higher than 108 while the energy is distributed in a wide range of frequency at lower latitudes where f approaches zero further analysis is performed of those fluctuations having periods shorter than 24 h and those having vertical wavelengths smaller than 5 km these are frequently analyzed as gravity waves using observation data the distribution of energy and momentum fluxes in the latitude height section is examined the result indicates that short period waves mostly propagate upward and poleward from the equatorial region the wave energy reaches about 508 lat at the 27 km altitude a negative positive maximum of vertical flux of meridional momentum y9w9 is seen above the subtropical jet in the northern southern hemisphere for small vertical scale gravity waves this is consistent with the preferred equatorward propagation of the wave indicated by a statistical analysis based on mst radar observations the ratio of potential to kinetic energy maximizes over the equator and decreases poleward the eliassen palm flux divergence associated with gravity waves is decelerative above the subtropical jet albeit small

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

improved tests reveal that the accelerating moment release hypothesis is statistically insignificant 1 we test the hypothesis that accelerating moment release amr is a precursor to large earthquakes using data from california nevada and sumatra spurious cases of amr can arise from data fitting because the time period area and sometimes magnitude range analyzed before each main shock are often optimized to produce the strongest amr signal optimizing the search criteria can identify apparent amr even if no robust signal exists for both 1950 2006 california nevada m 6 5 earthquakes and the 2004 m9 3 sumatra earthquake we can find two contradictory patterns in the pre main shock earthquakes by data fitting amr and decelerating moment release we compare the apparent amr found in the real data to the apparent amr found in four types of synthetic catalogs with no inherent amr when spatiotemporal clustering is included in the simulations similar amr signals are found by data fitting in both the real and synthetic data sets even though the synthetic data sets contain no real amr these tests demonstrate that apparent amr may arise from a combination of data fitting and normal foreshock and aftershock activity in principle data fitting artifacts could be avoided if the free parameters were determined from scaling relationships between the duration and spatial extent of the amr pattern and the magnitude of the earthquake that follows it however we demonstrate that previously proposed scaling relationships are unstable statistical artifacts caused by the use of a minimum magnitude for the earthquake catalog that scales with the main shock magnitude some recent amr studies have used spatial regions based on hypothetical stress loading patterns rather than circles to select the data we show that previous tests were biased and that unbiased tests do not find this change to the method to be an improvement the use of declustered catalogs has also been proposed to eliminate the effect of clustering but we demonstrate that this does not increase the statistical significance of amr given the ease with which data fitting can find desired patterns in seismicity future studies of amr like observations must include complete tests against synthetic catalogs that include spatiotemporal clustering

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

intrusions and anomalous vp vs ratios associated with the new madrid seismic zone 1 detailed p wave velocity vp and s wave velocity vs models and vp vs ratios were determined for a major portion of the new madrid seismic zone using arrival times recorded by the new madrid seismic network and portable array for numerical data acquisition panda stations we performed a simultaneous inversion for p and s wave velocities and hypocentral locations yielding the most detailed tomographic image of the upper portion of the crust to date low vp and high vs anomalies resulted in low vp vs ratios that correspond to the major arms of seismicity north of the intersection of the cottonwood grove blytheville arch cg ba fault with the reelfoot fault the unusual low vp vs values can be attributed to the presence of quartz rich rocks two regions contain anomalous vp and vs values and vp vs ratios that cannot be attributed to variations in rock composition and are probably produced by overpressured fluids one region is located on the hanging wall of the northern portion of the reelfoot fault and is aseismic the other region corresponds to a portion of the southern reelfoot fault that experiences swarm activity a distinct velocity contrast exists across the cg ba fault at depths exceeding 4 65 km basement rocks southeast of the fault have vp values that are 4 6 slower than values for rocks located to the northwest the most logical explanation is that the fault follows a preexisting structural feature or lithologic change in basement rocks

shear wave velocity of the sedimentary basin in the upper mississippi embayment using s to p converted waves from mid october 1989 to august 1992 40 three component panda portable array for numerical data acquisition stations were deployed in the central new madrid seismic zone three component digital seismograms recorded by the panda stations in the region are characterized by 1 the very weak direct s arrivals on the vertical component which can be identified unambiguously from the two horizontal components and 2 at least two prominent secondary arrivals between the direct p and s arrivals one sp dominant on the vertical component and another ps with smaller amplitude on the two horizontal components travel time differences between the sp and s and between the p and ps are the same for different earthquakes recorded at the same station but are different at different stations even for the same event polarization analyses of three component seismograms and travel time measurements confirm the interpretation that these two secondary arrivals are the p to s ps and s to p sp converted waves that occur at the bottom of the sedimentary basin beneath each station since abundant well log data are available in the upper mississippi embayment the thickness of the sediments beneath each seismic station can be estimated travel time differences between the direct and the converted waves can be used to calculate average shear wave velocity for the sediments beneath each station the estimated shear wave velocities of the sediments beneath panda stations vary from 0 45 to 0 67 km sec the higher shear wave velocity associated with thicker sediments can be interpreted as a consequence of increasing compaction of unconsolidated sediments due to increasing overburden

array observations of microseismic noise and the nature of h v in the mississippi embayment abstract ambient ground motion data were collected using phased seismic arrays in fall 2002 and spring 2007 within the mississippi embayment and at a single station external to the embayment these data allowed us to determine the wave field composition of ambient noise for understanding wave propagation mechanisms giving rise to spectral peaks using nakamura s h v technique ambient ground motions in the frequency band of 0 1 0 33 hz 10 3 sec period were dominated by spatially localized rayleigh and love wave microseisms generated by high ocean waves along the north american seaboard in the time periods of analysis seismic waves important in forming the h v peak near 4 sec period are composed of relatively high phase velocity rayleigh and love waves that convert to propagating homogeneous shear waves in the thick unconsolidated sediments of the embayment the h v resonant period is controlled by both constructive and destructive interference of these shear waves a simple relationship for the h v peak is given using a propagator matrix formulation that predicts the resonance frequency of a layered medium for surface wave motion at the base of the system the amplitude of the observed h v peak however does not give an accurate estimate of shear wave amplification because it depends on the slowness of the incident wave the inconsistency in estimated average shear wave velocities using the h v method and differential travel times of local earthquake sp phases in the mississippi embayment may be explained by misidentification of sp wave conversion points from deeper interfaces

determination of shallow shear wave velocity at mississippi embayment sites using vertical seismic profiling data we used vertical seismic profiling vsp data collected in shallow bore holes about 40 60 m deep to determine the shear wave velocity at four sites in the mississippi embayment in southwestern tennessee the source was an air powered hammer that produces repeatable sh waves which were recorded by source monitor geophones deployed on the surface very close to the source three approaches were used to determine interval velocities an approximate zero offset method a layer stripping method and a waveform matching method the first two methods use arrival time picks whereas the latter is based on the fit of synthetic vsp data to the first half cycle approximately of each trace the advantage of this method over the other two is that it uses a segment of the data rather than a single data point there fore the velocities determined using the waveform matching method are better con strained and are not affected by picking errors which may translate into significant spurious velocity variations the source wavelets recorded by one of the monitor geophones and the velocity model computed with the layer stripping method were used to generate synthetic vertical seismic profiling data for comparison with the actual data then the model velocities were modified interactively one layer at a time until a satisfactory match was achieved this required including attenuation in the computation of the synthetic data the four sites investigated in this study can be divided into two groups low velocity sites shelby farms and covington and high velocity sites brownsville and jackson these last two sites are at larger dis tances from the embayment axis than the other two which means that the difference in velocities probably corresponds to the presence of different geologic units good agreements between the lithology in the boreholes and the velocity profiles were obtained for all the four sites

investigation of 3 d basin structures in the izmit bay area turkey by single station microtremor and gravimetric methods summary we investigate 3 d basin structures and site resonance frequencies in the izmit bay area of turkey by new geophysical surveys based on 239 single station microtremor and 405 point gravity measurements a fundamental resonance frequency map of the izmit bay was produced from the main peak in the horizontal to vertical component spectral ratio hvsr of microtremors the hvsr analysis of the microtremor data reveals single double broad peaked or no peak type hvsr curves varying in accordance with the surface geology and spatial extent of the three basins present in the izmit bay area in the deepest sections of the izmit golcuk and derince basins of the izmit bay the fundamental resonance frequencies are dominantly 0 24 0 30 hz these resonance frequencies should be taken into consideration along with higher mode frequencies to construct earthquake resistant structures in the izmit bay area the 3 d gravimetric bedrock depth map of the izmit basin shows that the basin has an asymmetric shape with its deepest section coinciding with the surface trace of the north anatolian fault the northern shoulder of the basin has a gentle dip on the kocaeli peneplain side and the southern shoulder is much steeper and it is bounded by the samanli mountains we derive a power law relationship that correlates the fundamental site resonance frequencies with the sedimentary cover thickness obtained from the gravity and shear wave velocity data in the izmit bay we use this relationship to estimate bedrock depths beneath golcuk and derince basins our estimation of maximum basin depths is 1400 m for the izmit and is 800 m for the golcuk and derince basins finally we have analysed a converted sp phase from a local earthquake recording made at site cmp to calculate and verify the sediment thickness estimations obtained from our gravimetric and microtremor analyses this calculation shows close agreement with that of the gravimetric and microtremor results our results show that the basins in the izmit bay area have a very thick sedimentary cover with very low shear velocities underlined by hard bedrock forming a sharp impedance contrast we anticipate that these results will be a key contribution to the quantitative assessment of seismic hazard for the izmit bay area before the occurrence of strong earthquakes in the marmara region

stratospheric temperature anomalies and mountain waves a three dimensional simulation using a multi scale weather prediction model a non hydrostatic weather prediction model with an interactive grid nesting facility mm5 is used to infer the three dimensional mesoscale temperature field over the scandinavian mountains during 23 january 1991 six hours after the initialization from operational analyses warm and cold anomalies of mesoscale extent developed on isentropic surfaces at stratospheric heights over the windward slope of the mountains and further downstream mean mesoscale temperature fluctuations along isentropes amount to more than 4 k while minima and maxima can differ by more than 30 k consistent with height variations exceeding 3 km taking into account the three dimensional motion field cooling rates up to 20 k h are estimated

polar stratospheric ice cloud above spitsbergen 1 within the extremely cold and stable polar vortex of winter 2004 2005 a polar stratospheric ice cloud was observed from ny alesund spitsbergen on 26 january 2005 the observation of a cloud with backscatter ratios up to 23 and volume depolarization larger than 50 is unique in our 15 year ground based lidar data record simultaneous balloon borne water vapor measurements indicate the presence of mesoscale ice clouds nearby normally low horizontal wind speeds inside the inner vortex prevent vertical wave propagation however the rare coincidence of different meteorological processes occurring during a poleward breaking rossby wave event caused favorable conditions for the vertical propagation of mountain waves excited by the flow past spitsbergen detailed meteorological analysis shows that the ice particle formation processes on 26 january 2005 were most likely provoked by mesoscale stratospheric temperature anomalies leading to a local reduction in water vapor

three dimensional seismic velocity model for the unconsolidated mississippi embayment sediments from h v ambient noise measurements abstract the horizontal to vertical h v technique by nakamura 1989 was applied to data from 30 new field stations and 64 other broadband temporary and permanent seismic stations within the mississippi embayment of the central united states to develop a 3d model of unconsolidated sediment shear wave velocity structure using the dart 1992 map of sediment thickness as a basis two self consistent models of average shear wave velocity versus sediment thickness were developed by utilizing the theoretical linear relationship between the frequency of the h v peak and shear wave velocity one model was based on the observation that the h v peak period t p s versus sediment thickness h m was seen to be approximately linear with the relationship t p 0 003266 h 1 084 the second model was developed by considering peak frequency f p versus sediment thickness parameterized to follow ln f p 8 325 10 7 h 2 0 00232 h 0 01796 overall the models show low average shear wave velocity near the edge of the mississippi embayment with velocities increasing with increasing sediment thickness consistent with increased sediment compaction these models will be useful in studies of site resonance and amplification for earthquake shaking hazards and for wave propagation computations for the region

processes leading to the formation of mesoscale waves in the midwest cyclone of 15 december 1987 abstract on 15 december 1987 several long lived large amplitude mesoscale wave disturbances accompanied a rapidly developing extratropical cyclone in the midwest united states previous observational and modeling studies have suggested that the disturbances had large amplitudes and long lifetimes as a result of a wave cisk type instability occurring within an imperfect wave duct and were initiated by convection however infrared ir satellite imagery and radar echoes shortly before the wave disturbances formed suggest that convection was not the primary feature in the wave genesis region at that time instead a meso scale comma shaped cloud was present and appeared to evolve into the wave disturbances the origins of the comma cloud can be traced back to a cloud streak and precipitation maximum in the left exit region of an approaching jet streak over northern mexico 15 h earlier in this study satellite observations are examined in conjunction with numerical simulations of the case to explore a n

velocity structure of the northern mississippi embayment sediments part i teleseismic p wave spectral ratios analysis abstract the mississippi embayment overlies the new madrid seismic zone in the central united states and consists of a thick succession of unconsolidated sediments that covers the paleozoic basement rock the high velocity contrast at the basement sediment interface and the near vertical ray paths of teleseismic p waves generate very large p to s conversions on the radial component and large p reverberations on the vertical component this characteristic of p wave propagation is used to study p and s wave resonance in the sedimentary layer horizontal to vertical h v and vertical to horizontal v h power spectral ratios are calculated for a time window around the teleseismic p wave arrivals at broadband stations of the usarray transportable array northern embayment lithosphere experiment and the new madrid cooperative seismic network in the embayment using a map of sediment thickness we developed models for average p and s wave velocity versus sediment thickness the resulting shear wave velocity model matches very well with the model obtained from an h v ambient noise study in the region the shear wave velocity model shows low velocity near the edges of the embayment with average velocities increasing with increasing sediment thickness consistent with increased sediment compaction fundamental resonance frequency for s waves ranges from 0 2 to 0 4 hz for the embayment sediments electronic supplement two figures showing the synthetic test results and a table that includes peak resonance frequency and average v p and v s

explosion source strong ground motions in the mississippi embayment two strong motion arrays were deployed for the october 2002 embay ment seismic excitation experiment to study the spatial variation of strong ground motions in the deep unconsolidated sediments of the mississippi embayment because there are no comparable strong motion data from natural earthquakes in the area each linear array consisted of eight three component k2 accelerographs spaced 15 m apart situated 1 2 and 2 5 km from 2268 kg and 1134 kg borehole explosion sources respectively the array data show distinct body wave and surface wave arrivals that propagate within the thick unconsolidated sedimentary column the high velocity basement rocks and small scale structure near the surface time domain coherence of body wave and surface wave arrivals is computed for acceleration velocity and displacement time windows coherence is high for relatively low frequency vertical component rayleigh waves and high frequency p waves propagating across the ar ray prominent high frequency ps conversions seen on radial components a proxy for the direct s wave from earthquake sources lose coherence quickly over the 105 m length of the array transverse component signals are least coherent for any ground motion and appear to be highly scattered horizontal phase velocity is com puted by using the ratio of particle velocity to estimates of the strain based on a plane wave propagation model the resulting time dependent phase velocity map is a useful way to infer the propagation mechanisms of individual seismic phases and time windows of three component waveforms displacement gradient analysis is a complementary technique for processing general spatial array data to obtain hori zontal slowness information

gravity wave characteristics in the lower atmosphere at south pole a 4 year 1993 1996 temperature and wind data set obtained from over 2000 high resolution balloon soundings at south pole is used to study gravity wave characteristics in the troposphere and lower stratosphere extensive analyses of energy density spectra and static stability are performed to present a comprehensive view of the gravity wave characteristics in the lower atmosphere at south pole our results show that the gravity waves are ubiquitous and often fairly strong at south pole even though the generation mechanisms are not clear gravity wave characteristics are in general similar to those obtained at other high latitude southern hemisphere stations potential energies vary between about 0 5 j kg and 5 j kg with season and altitude variations in kinetic energies are not well correlated with potential energy variations and range from 1 j kg to 11 j kg we observe significant seasonal variations of the slope and magnitude of the vertical wavenumber spectrum of temperature fluctuations especially in the stratosphere in general the gravity waves in the stratosphere are stronger weaker in austral spring fall wave activity in the troposphere has little seasonal dependence stability analysis shows that instabilities are more likely to occur in the troposphere than in the stratosphere the probability of wave instability is 13 7 in the troposphere and 5 4 in the stratosphere this is due to the less stable stratification in the troposphere where the buoyancy period averages 8 3 min compared to 4 9 min in the stratosphere dynamic shear instability is more likely to occur than convective instability 11 versus 2 6 in the troposphere and 4 7 versus 0 7 in the stratosphere due to the prevailing strong wind shear the instability probabilities vary seasonally with the austral winter exhibiting the highest probability of instabilities dynamic and convective instabilities combined in both the troposphere and stratosphere