wave attenuation over coastal salt marshes under storm surge conditions salt marshes protect coastlines against waves wave flume experiments show that marsh vegetation causes substantial wave dissipation and prevents erosion of the underlying surface even during extreme storm surge conditions

mississippi river subaqueous delta is entering a stage of retrogradation abstract the subaqueous delta of the mississippi river the largest river system in the conterminous u s has entered a stage of retrogradation caused by multiple natural and anthropogenic activities since the 1950s the suspended sediment load of the mississippi river has decreased by 50 due primarily to the construction of 50 000 dams in the mississippi basin the impact of this decreased sediment load has been observed in subaerial environments but the impact on sedimentation and geomorphology of the subaqueous delta front has yet to be examined to identify historic trends in sedimentation patterns we compiled bathymetric datasets including historical charts industry and academic surveys and national oceanic and atmospheric administration hydrographic data collected between 1764 and 2009 the progradation rate measured at the 10 m depth contour of southwest pass which receives 69 of the suspended sediment load reaching head of passes has decreased from 67 m yr between 1874 and 1940 to 26 m yr between 1940 and 1979 with evidence of further deceleration from 1979 to 2009 at south pass and pass a loutre the delta front has entered the destructive phase with the 10 m contour retreating at rates 20 m yr at both passes since 1979 advancement of the delta front also decelerated in deeper water in some areas out to 180 m depth except locally where mudflow lobes are advancing deeper contours show a pattern of decreasing progradation rate between 1874 1940 and 1979 2005 time periods furthermore based on differences measured between available bathymetric datasets the sediment accumulation rate across the delta front decreased by 73 for the same period the retention rate of mississippi river sediment on the delta front ranged from 67 to 81 for the time periods assessed with total sediment load stored on the delta front equal to 317 54 mt yr from 1874 to 1940 145 25 mt yr from 1940 to 1979 and 87 15 mt yr from 1979 to 2005 we document for the first time that the mississippi river delta front has entered a phase of retrogradation which will likely be accelerated by future upstream activities that divert a portion of the sediment load to the upper delta for coastal protection and restoration projects the decline of the subaqueous mississippi river delta has critical implications for biogeochemical cycling subaqueous mass wasting and sediment dispersal to the coastal ocean

an attempt to reconcile subsidence rates determined from various techniques in southern louisiana abstract subsidence rates determined from geodetic releveling observations since 1920 in southern louisiana are consistently higher than subsidence rates determined from radiocarbon data in the same region over holocene timescales radiocarbon based subsidence rates are similar to numerically modeled shallow sedimentary compaction rates at similar timescales while recent geodetic observations are an order of magnitude higher possible explanations for the dramatic recent increase in regional subsidence rates suggested by the geodetic data that are considered here include 1 a recent increase in regional contributions from faulting 2 the recent contribution of a regional high impact process such as fluid withdrawal 3 accuracy in one or more of the datasets and 4 a strong dependence of subsidence rates upon the time frame over which different techniques are used faulting and regional groundwater withdrawals appear insufficient to explain the high regional geodetic rates the contribution of regional depressurization from deep fluid withdrawals remains unknown estimate errors are likely smaller than the magnitude of the discrepancies between rates from different datasets observations of subsidence rates may be biased by measurement duration

teleseismic waveform complexities caused by near trench structures and their impacts on earthquake source study application to the 2015 illapel aftershocks central chile

coupling instrumental and geological records of sea level change evidence from southern new england of an increase in the rate of sea level rise in the late 19th century 1 we construct a high resolution relative sea level record for the past 700 years by dating basal salt marsh peat samples above a glacial erratic in an eastern connecticut salt marsh to test whether or not the apparent recent acceleration in the rate of sea level rise slr is coeval with climate warming the data reveal an average slr rate of 1 0 0 2 mm year from about 1300 to 1850 a d coupling of the regional tide gauge data 1856 to present with this marsh based record indicates that the nearly three fold increase in the regional rate of slr to modern levels likely occurred in the later half of the 19th century thus the timing of the observed slr rate increase is coincident with the onset of climate warming indicating a possible link between historic slr increases and recent temperature increases

monochromatic body waves excited by great subduction zone earthquakes large quasi monochromatic body waves were excited by the 1995 chile mw 8 1 and by the 1994 kurile mw 8 3 events they are observed on vertical radial component seismograms following the direct p and pdiff arrivals at all azimuths we devise a slant stack algorithm to characterize the source of the oscillations this technique aims at locating near source isotropic scatterers using broadband data from global networks for both events we find that the oscillations emanate from the trench we show that these monochromatic waves are due to localized oscillations of the water column their period corresponds to the gravest id mode of a water layer for vertically traveling compressional waves we suggest that these monochromatic body waves may yield additional constraints on the source process of great subduction zone earthquakes

the 2012 sumatra great earthquake sequence the equatorial indian ocean is a well known place of active intraplate deformation defying the conventional view of rigid plates separated by narrow boundaries where deformation is confined on 11 april 2012 this region was hit in a couple of hours by two of the largest strike slip earthquakes ever recorded moment magnitudes mw 8 6 and 8 2 broadband seismological observations of the mw 8 6 mainshock indicate a large centroid depth 30 km and remarkable rupture complexity detailed study of the surface wave directivity and moment rate functions clearly indicates the partition of the rupture into at least two distinct subevents to account for these observations we developed a procedure to invert for multiple point source parameters the optimum source model at long period consists of two point sources separated by about 209 km with magnitudes mw 8 5 and 8 3 to explain the remaining discrepancies between predicted and observed surface waves we can refine this model by adding directivity along the wnw ese axis however we do not exclude more complicated models to analyze the mw 8 2 aftershock we removed the perturbation due to large surface wave arrivals of the mw 8 6 mainshock by subtracting the corresponding synthetics computed for the two subevent model analysis of the surface wave amplitudes suggests that the mw 8 2 aftershock had a large centroid depth between 30 km and 40 km this major earthquake sequence brings a new perspective to the seismotectonics of the equatorial indian ocean and reveals active deep lithospheric deformation

smooth and rapid slip near the japan trench during the 2011 tohoku oki earthquake revealed by a hybrid back projection method abstract we developed a new back projection method that uses teleseismic p waveforms to integrate the direct p phase with reflected phases from structural discontinuities near the source and used it to estimate the spatiotemporal distribution of the seismic energy release of the 2011 tohoku oki earthquake we projected a normalized cross correlation of observed waveforms with corresponding green s functions onto the seismic source region to obtain a high resolution image of the seismic energy release applying this method to teleseismic p waveform data of the 2011 tohoku oki earthquake we obtained spatiotemporal distributions of seismic energy release for two frequency bands a low frequency dataset and a high frequency dataset we showed that the energy radiated in the dip direction was strongly frequency dependent the area of major high frequency seismic radiation extended only downdip from the hypocenter whereas the area of major low frequency seismic radiation propagated both downdip and updip from the hypocenter we detected a large release of seismic energy near the japan trench in the area of maximum slip which was also the source area of the gigantic tsunami when we used only the low frequency dataset the timing of this large seismic energy release corresponded to an episode of smooth and rapid slip near the japan trench and reflects the strong dependence of the seismic energy distribution obtained on the frequency band of the input waveform dataset the episode of smooth and rapid slip may have been the trigger for a release of roughly all of the accumulated elastic strain in the seismic source region of the 2011 tohoku oki earthquake

episodic overbank deposition as a dominant mechanism of floodplain and delta plain aggradation the common view that frequent overbank flooding leads to gradual aggradation of alluvial strata on floodplains and delta plains has been challenged by a variety of studies that suggest that overbank aggradation occurs in a strongly episodic fashion however this remains a largely untested hypothesis due to the difficulty in establishing age models with sufficiently high resolution here we use 39 optically stimulated luminescence osl ages from proximal overbank deposits in the mississippi delta to demonstrate for the first time that alluvial aggradation over centennial to millennial time scales is predominantly episodic with aggradation rates of 1 4 cm yr that can persist for centuries osl ages from three separate study areas produce age clusters that are distinctly different yet complement each other these findings suggest that a substantial portion of the continental stratigraphic record consists of patchworks of relatively discrete centennial to millennial scale sediment bodies assembled by autogenic processes

deciphering holocene sea level history on the u s gulf coast a high resolution record from the mississippi delta published holocene relative sea level rsl curves for the u s gulf coast are in mutual conflict with some characterized by a smooth rsl rise akin to widely accepted eustatic sea level curves versus others including several recent ones that are characterized by a conspicuous stair step pattern with prolonged millennium scale rsl stillstands alternating with rapid meter scale rises in addition recent work in texas and alabama has revitalized the notion of a middle holocene rsl highstand estimated at 2 m above present mean sea level an extensive sampling program in the mississippi delta louisiana focused on the collection of basal peats that accumulated during the initial transgression of the pre existing consolidated pleistocene basement we used stable carbon isotope ratios to demonstrate that many of these samples accumulated in environments affected by frequent saltwater intrusion in the 30 cm zone between mean spring high water and mean sea level and we selected plant macrofossils that were subjected to ams 14c dating nearly 30 sea level index points from a 20 km2 study area on the eastern margin of the delta suggest that rsl rise followed a relatively smooth trend for the time interval 8000 3000 cal yr b p thus questioning the occurrence of major rsl stillstands alternating with abrupt rises given the narrow error envelope defined by our data set any sea level fluctuations if present would have amplitudes of 1 m although a true middle holocene highstand never occurred in the mississippi delta the high level of detail of our time series enables a rigorous test of this hypothesis correction of our data set for a hypothetical tectonic subsidence rate of 1 1 mm yr 1 assuming a constant subsidence rate compared to the tectonically relatively stable adjacent coast of texas leads to sea levels near 2 m above present during the time interval 6000 4000 cal yr b p however this model also implies a rsl position near 2 m around 8000 cal yr b p which is inconsistent both with data of this age from texas as well as with widely accepted sea level data from elsewhere we therefore conclude that a middle holocene highstand for the u s gulf coast is highly unlikely and that the entire area is still responding glacio isostatically by means of forebulge collapse to the melting of the laurentide ice sheet

effects of sedimentary structure and bathymetry near the source on teleseismic p waveforms from shallow subduction zone earthquakes summary the effects of sedimentary layers and non flat bathymetry on teleseismic p waveforms from shallow subduction zone earthquakes are examined by calculating synthetic seismograms the actual bathymetry and shallow sedimentary layers revealed by the seismic refraction studies are incorporated in 2 d velocity models of the oceanic trench region 2 d finite difference calculation is employed to get a complete response of the assumed non flat model structures the result of the modelling of wwssn long period seismograms shows that not only the non flat bathymetry but also thick sedimentary layers are essential to model the synthetic seismograms the lack of non flat bathymetry or sedimentary structure in the structural model often results in unrealistic synthetics in general the paths going across the trench axis are most affected this is caused by the steeply dipping ocean bottom and the thick sedimentary layers at the trench slope observed seismograms of the 1968 kurile thrust event mb 5 5 and the 1967 java normal event mb 6 0 show remarkable large amplitude later arrivals despite their moderate magnitudes a realistic model of the trench structure however reproduces remarkably well this character of later arrivals which shows that it is essential to incorporate the structure our results also indicate that we need precise knowledge on the trench structure to model the structural effects in detail on the gdsn long period seismograms the effect of the sedimentary layers is negligible because the rather narrow band response of the gdsn seismograph lies in a frequency range lower than the affected band however the effect of the non flat bathymetry is significant and can not be neglected

deltaic morphodynamics and stratigraphic evolution of middle barataria bay and middle breton sound regions louisiana usa implications for river sediment diversions abstract river sediment diversions have been recognized as a key strategy for offsetting land loss in coastal louisiana recently much attention has been focused on sediment capture from the main stem river and conveyance of that material through diversion outflows yet the performance and long term feasibility of diversions as land building agents also hinges upon the subsurface stratigraphic architecture of receiving basins a concept that has largely been unexplored a major core collection and analysis program was undertaken to study the geological properties of substrates in the middle barataria bay and middle breton sound diversion receiving basins louisiana usa over a region of 200 square kilometers in both basins 50 vibracores up to 5 5 m in length were collected in spring and summer of 2015 cores were logged for bulk density and imaged with a geotek multi sensor core logger split cores were then subsampled for granulometry organic content and 14c dating both receiving basins are characterized by 1 2 m of organic rich surficial strata underlain by 2 m of mineral rich sand and silt beds that display a greater bulk density additionally some cores contain deeper peaty strata intercalated within muds and sands 14c dates of these buried peats range between 1910 and 3203 calendar years before present demonstrating that relatively old deltaic strata exist at shallow depths age comparisons of stratigraphically similar peat beds from this study and previous delta lobe chronological models indicate that the st bernard lobe prograded eastward at a time averaged rate of 75 m yr from a geotechnical standpoint the surficial uncompacted

strong sh to love wave scattering off the southern california continental borderland seismic scattering is commonly observed and results from wave propagation in heterogeneous medium yet deterministic characterization of scatterers associated with lateral heterogeneities remains challenging in this study we analyze broadband waveforms recorded by the southern california seismic network and observe strongly scattered love waves following the arrival of teleseismic sh wave these scattered love waves travel approximately in the same azimuthal direction as the incident sh wave at a dominant period of 10 s but at an apparent velocity of 3 6 km s as compared to the 11 km s for the sh wave back projection suggests that this strong scattering is associated with pronounced bathymetric relief in the southern california continental borderland in particular the patton escarpment finite difference simulations using a simplified 2 d bathymetric and crustal model are able to predict the arrival times and amplitudes of major scatterers the modeling suggests a relatively low shear wave velocity in the continental borderland

en echelon and orthogonal fault ruptures of the 11 april 2012 great intraplate earthquakes the magnitude 8 7 earthquake that occurred off the coast of the sumatra subduction zone on 11 april 2012 is shown to have had an extraordinarily complex four fault rupture these great ruptures represent large lithospheric deformation that may eventually lead to a localized boundary between the indian and australian plates on 11 april 2012 two of the largest strike slip earthquakes ever recorded at magnitudes of 8 7 and 8 2 occurred in the northeastern indian ocean a few hundred kilometres off the coast of sumatra three groups now report the analysis of seismic data from the days and months before and after these events as well as the events themselves matthias delescluse and co authors show that these earthquakes are part of the ongoing boost of intraplate deformation between india and australia that followed the aceh 2004 and nias 2005 megathrust earthquakes they conclude that the australian plate driven by slab pull forces at the sunda trench is gradually detaching from the indian plate han yue and colleagues show that the 11 april event involved a complex four fault rupture lasting several minutes followed two hours later by a magnitude 8 2 aftershock these great ruptures on a lattice of strike slip faults that extends through the crust and into the upper mantle represent large lithospheric deformation that may eventually create a localized boundary between the indian and australian plates fred pollitz and colleagues show that in the six days following 11 april the global rate of remote earthquakes with magnitudes greater than 5 5 increased nearly fivefold and events up to magnitude 7 seem to have been triggered the unprecedented delayed triggering power of this earthquake may arise from its strike slip source geometry or because it struck at a time of an unusually low global earthquake rate and increased the number of nucleation sites that were very close to failure the indo australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary from an environment of continental collision to an island arc subduction zone1 2 on 11 april 2012 one of the largest strike slip earthquakes ever recorded seismic moment magnitude mw 8 7 occurred about 100 200 kilometres southwest of the sumatra subduction zone occurrence of great intraplate strike slip faulting located seaward of a subduction zone is unusual it results from northwest southeast compression within the plate caused by the india eurasia continental collision to the northwest together with northeast southwest extension associated with slab pull stresses as the plate underthrusts sumatra to the northeast here we use seismic wave analyses to reveal that the 11 april 2012 event had an extraordinarily complex four fault rupture lasting about 160 seconds and was followed approximately two hours later by a great mw 8 2 aftershock the mainshock rupture initially expanded bilaterally with large slip 20 30 metres on a right lateral strike slip fault trending west northwest to east southeast wnw ese and then bilateral rupture was triggered on an orthogonal left lateral strike slip fault trending north northeast to south southwest nne ssw that crosses the first fault this was followed by westward rupture on a second wnw ese strike slip fault offset about 150 kilometres towards the southwest from the first fault finally rupture was triggered on another en echelon wnw ese fault about 330 kilometres west of the epicentre crossing the ninetyeast ridge the great aftershock with an epicentre located 185 kilometres to the ssw of the mainshock epicentre ruptured bilaterally on a nne ssw fault the complex faulting limits our resolution of the slip distribution these great ruptures on a lattice of strike slip faults that extend through the crust and a further 30 40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the indian and australian plates

imaging widespread seismicity at midlower crustal depths beneath long beach ca with a dense seismic array evidence for a depth dependent earthquake size distribution we use a dense seismic array composed of 5200 vertical geophones to monitor microseismicity in long beach california poor signal to noise ratio due to anthropogenic activity is mitigated via downward continuation of the recorded wavefield the downward continued data are continuously back projected to search for coherent arrivals from sources beneath the array which reveals numerous previously undetected events the spatial distribution of seismicity is uncorrelated with the mapped fault traces or with activity in the nearby oil fields many events are located at depths larger than 20 km well below the commonly accepted seismogenic depth for that area the seismicity exhibits temporal clustering consistent with omori s law and its size distribution obeys the gutenberg richter relation above 20 km but falls off exponentially at larger depths the dense array allows detection of earthquakes two magnitude units smaller than the permanent seismic network in the area because the event size distribution above 20 km depth obeys a power law whose exponent is near one this improvement yields a hundred fold decrease in the time needed for effective characterization of seismicity in long beach

mysterious tsunami in the caribbean sea following the 2010 haiti earthquake possibly generated by dynamically triggered early aftershocks abstract dynamically triggered offshore aftershocks caused by passing seismic waves from main shocks located on land are currently not considered in tsunami warnings the m7 0 2010 haiti earthquake epicenter was located on land 27 km north of the caribbean sea and its focal mechanism was oblique strike slip nevertheless a tsunami recorded on a caribbean deep ocean assessment and reporting of tsunami dart buoy and a tide gauge produced runup heights of 1 3 m along haiti southeast coast earthquake finite fault model inversions of the dart waveform suggest that a reverse fault doublet with magnitudes of m6 8 and m6 5 located 85 km southwest of the epicenter may have excited the tsunami this doublet collocates with dynamically triggered aftershocks derived from back projection analysis that occurred 20 60 s after the main shock of the haiti earthquake the aftershocks are within a region of maximum dynamic strain predicted by the main shock on a possibly tectonically active submarine ridge southwest of haiti s southern peninsula the agreement between the tsunami finite fault source models and the seismic and tectonic evidence suggests that earthquakes on land even strike slip faults can generate tsunamis by dynamically triggering offshore aftershocks

structural and surface features of arc shaped radar echoes along an outer tropical cyclone rainband this study uses ground based dual doppler radar and surface observations to document the structural and surface features of the arc shaped radar echoes asres evident along an outer rainband of typhoon longwang as it approached northern taiwan on 1 october 2005 the particular aim of this study is to explore the possible distinction between the present case previously documented tropical cyclone rainbands tcrs and squall lines the dual doppler derived fields show that the leading precipitation of the studied asres exhibited a convective nature with a sharp horizontal gradient of reflectivity and a significant vertical extent theregionsbehindtheleadingconvectionwerecharacterizedbyband relativerear to frontflowatlowlevels and were associated with a broader area of stratiform precipitation the deep layer of front to rear flow extending from the surface to the upper troposphere was generally present ahead of the asres this flow appearstobeliftedupwardatandimmediatelyaheadoftheleadingedgeofthelow levelrear to frontflowto form rearward tilting updrafts these airflow patterns are similar to those of the convective region of squall lines but differ fundamentally from those of previously documented tcrs that were located closer to the inner core of cyclones the detailed analyses of surface fluctuations during the passage of one of the studied asresfurthershowanabruptpressurerise 2mb atemperaturedrop 48c andapronounceddeceleration of inflow air coincident with the leading heavy precipitation the evaluation presented suggests that the convectively generated cold pool may be important in influencing the structures and propagation of the studied asres

effects of peat compaction on delta evolution a review on processes responses measuring and modeling abstract peat is most compressible of all natural soils compaction of peat layers potentially leads to substantial amounts of land subsidence peat is common in many distal parts of holocene deltas which are often densely populated it is known that land subsidence due to peat compaction may have serious societal implications in such areas as it may cause damage to construction works and lead to land inundation effects of peat compaction on the natural evolution of deltas are however poorly understood whereas this might be an important control on delta evolution at both local and regional scales the main objective of this paper is to review current knowledge concerning the peat compaction process and its effect on delta evolution in holocene settings and to identify gaps in this knowledge an overview is given regarding 1 the compaction process 2 presumed and potential effects of peat compaction on delta evolution 3 field methods to quantify peat compaction and 4 numerical models to calculate the amount and rate of peat compaction peat compaction and formation influence channel belt elevation channel belt geometry and channel belt configuration last mentioned aspect mostly concerns the influence of peat compaction on avulsion which is one of the most important processes controlling delta evolution interactions between peat compaction peat formation and avulsion have seldom been studied and remain unclear partly because factors such as peat type organic matter content sediment sequence composition and groundwater table fluctuation are so far not taken into account peat compaction and formation potentially influence avulsion as 1 a decrease in accommodation space created by peat compaction underneath a channel causes superelevation and or an increase in lateral migration 2 the high cohesiveness of peat banks inhibits lateral migration which increases bed aggradation decreases sediment transport capacity and hence increases crevassing frequencies which possibly evolve into an avulsion although the low regional gradient in peatlands will hinder this and 3 peat compaction and oxidation in flood basins following groundwater table lowering leads to relief amplification of channel belts at delta scale variations in compaction rates might stimulate the occurrence of nodal avulsions to quantify effects of peat compaction on delta evolution and to determine the relative importance of different factors involved field research should be combined with numerical models describing peat compaction and formation the model should be validated and calibrated with field data

depositional dynamics in a river diversion receiving basin the case of the west bay mississippi river diversion abstract river deltas are a globally distributed class of sedimentary environment that are highly productive ecologically diverse and serve as centers for population and commerce many deltas are also in a state of environmental degradation and the mississippi river delta mrd stands out as a particularly iconic example plans to restore the mrd call for partially diverting the mississippi river which should reinitiate natural deltaic land building processes while the basic physical underpinnings of river diversions are relatively straightforward there exists a considerable controversy over whether diversions can and do deliver enough sediment to the coastal zone to build sub aerial land on restoration dependent time scales this controversy was addressed through a study of crevasse splay dynamics at the west bay mississippi river diversion the largest diversion in the mrd that was specifically constructed for coastal restoration we found that most sediments were distributed over a 13 5 km area with the maximum deposition occurring at the seaward end of this field these results indicate substantial sediment deposition downstream of project boundaries and run counter to simple sedimentary models which predict that maximum sediment deposition should occur closest to the riverbank despite this most sediments appear to be retained in the nearshore zone suggesting that the sediment retention efficiency was at the higher end of the 30 70 range suggested by some sediment budgets

combined chronological and mineral magnetic approaches to reveal age variations and stratigraphic heterogeneity in the yangtze river subaqueous delta abstract delta deposits show large spatial heterogeneity in terms of depositional rate and age which is critical to the study of delta erosion in response to the declining fluvial sediment load observed at many river mouths in the world in this study we show that the magnetic susceptibility can be an indicator to reveal age variations and stratigraphic heterogeneity in the yangtze river subaqueous delta ages of three short sediment cores 1700 years the cores at shallow water depths are younger than those from deeper sites modern deposits i e 800 a show lower and uniform values due to the reductive dissolution of ferrimagnetic minerals while younger sediments

a revised chronology for mississippi river subdeltas radiocarbon measurements by accelerator mass spectrometry relating to three of the four late holocene mississippi river subdeltas yielded consistent results and were found to differ by up to 2000 carbon 14 years from previously inferred ages these geological dataare in agreement with archaeological carbon 14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework which has implications for prehistoric human settlement patterns coastal evolution and wetland loss and sequence stratigraphic interpretations

a numerical study of stratospheric gravity waves triggered by squall lines observed during the toga coare and copt 81 experiments a 3d mesoscale model is used to study the structure and intensity of stratospheric gravity waves generated by tropical convection two prototypical cases are examined a squall line observed during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare over the pacific warm pool and a west african squall line observed during the convection profonde tropicale 1981 copt 81 experiment gravity waves generated by these two squall lines are compared with those generated by a previously investigated event over northern australia although the individual squall lines vary in intensity the stratospheric gravity waves generated by the various storms have surprisingly similar amplitudes the similarity in the wave amplitudes arises because within each storm there is a positive correlation between the updraft intensity and the height of the level of neutral buoyancy when the level of neutral buoyancy is relatively high the atmospheric density in the region of wave generation is relatively low and this reduction in density tends to weaken the convectively triggered waves thereby compensating for the stronger updraft velocities in the more intense storms the sensitivity of the azimuthal distribution of the convectively generated gravity waves to the upper tropospheric and lower stratospheric wind profile is also examined in the absence of critical level absorption the motion of the storm relative to the stratospheric winds appears to be the single most important factor determining the azimuthal distribution of the waves stationary storm relative waves similar to mountain waves may also be generated when there is a strong storm relative stratospheric wind the interaction of the gravity waves with shear layers representative of the eastward and westward phases of the quasi biennial oscillation qbo is also examined an analysis of the domain averaged momentum budget supports the claim that the drag exerted by critical level absorption of convectively generated gravity waves plays a nontrivial role in the downward propagation of the qbo

local near instantaneously dynamically triggered aftershocks of large earthquakes aftershocks are often triggered by static and or dynamic stress changes caused by mainshocks the relative importance of the two triggering mechanisms is controversial at near to intermediate distances we detected and located 48 previously unidentified large early aftershocks triggered by earthquakes with magnitudes between 7 and 8 within a few fault lengths approximately 300 kilometers during times that high amplitude surface waves arrive from the mainshock less than 200 seconds the observations indicate that near to intermediate field dynamic triggering commonly exists and fundamentally promotes aftershock occurrence the mainshocks and their nearby early aftershocks are located at major subduction zones and continental boundaries and mainshocks with all types of faulting mechanisms normal reverse and strike slip can trigger early aftershocks

drowning of the mississippi delta due to insufficient sediment supply and global sea level rise global sea level rise reduced sediment supply and subsidence threaten the stability of the mississippi delta calculations of riverine sediment load and storage indicate that 10 000 13 500 km2 of the delta could be submerged by ad 2100

fault interactions and triggering during the 10 january 2012 mw 7 2 sumatra earthquake the 10 january 2012 mw 7 2 sumatra earthquake in the wharton basin occurred 3 months before the great mw 8 6 and mw 8 2 earthquakes in the same region which had complex ruptures and are the largest strike slip earthquakes ever recorded teleseismic p wave back projection of the mw 7 2 earthquake images a unilateral rupture lasting 40 s without observable frequency dependency low frequency 0 05 0 3 hz high frequency 0 3 1 hz in addition to radiation bursts during the mw 7 2 main shock coherent energy releases from 50 to 75 s and from 100 to 125 s are observed about 143 km northeast of the main shock rupture and landward of the trench analysis of globally recorded p waves in both 0 02 0 05 hz velocity records and 1 5 hz stacked envelope functions confirms the presence of coherent sources during the time windows the observed energy bursts are likely to be large early aftershocks occurring on or near the subduction interface both dynamic and static triggering could have induced these early aftershocks as they initiated after the surface wave passed by and the coulomb stress perturbations from the mw 7 2 main shock promote earthquakes in the observed locations the earthquake sequence is a clear example of a seaward intraplate strike slip earthquake triggering landward intraplate earthquakes in the same region in contrast to previously reported normal reverse or reverse normal interactions at subduction zones

the 2012 mw 8 6 wharton basin sequence a cascade of great earthquakes generated by near orthogonal young oceanic mantle faults we improve constraints on the slip distribution and geometry of faults involved in the complex multisegment mw 8 6 april 2012 wharton basin earthquake sequence by joint inversion of high rate gps data from the sumatran gps array sugar teleseismic observations source time functions from broadband surface waves and far field static gps displacements this sequence occurred under the indian ocean 400 km offshore sumatra the events are extraordinary for their unprecedented rupture of multiple cross faults deep slip large strike slip magnitude and potential role in the formation of a discrete plate boundary between the indian and australian plates the sugar recorded static displacements of up to 22 cm along with time varying arrivals from the complex faulting which indicate that the majority of moment release was on young wnw trending right lateral faults counter to initial expectations that an old lithospheric nne trending fracture zone played the primary role the new faults are optimally oriented to accommodate the present day stress field not only was the greatest moment released on the younger faults but it was these that sustained very deep slip and high stress drop 20 mpa the rupture may have extended to depths of up to 60 km suggesting that the oceanic lithosphere in the northern wharton basin may be cold and strong enough to sustain brittle failure at such depths alternatively the rupture may have occurred with an alternative weakening mechanism such as thermal runaway

characterizing thunderstorm gust fronts near complex terrain abstractfire safety aviation wind energy and structural engineering operations are impacted by thunderstorm outflow boundaries or gust fronts gfs particularly when they occur in mountainous te

potential vorticity generation by west african squall lines abstractthe west african summer monsoon features multiple complex interactions between african easterly waves aews moist convection variable land surface properties dust aerosols and the diu

dynamical influence of microphysics in tropical squall lines a numerical study abstract based on environmental conditions of the 22 and 23 june squall lines during the convection profonde tropicale in 1981 copt81 experiment in west africa a series of numerical simulations are performed with a two dimensional nonhydrostatic cloud model to examine the dynamical effect of microphysics in tropical squall lines the role of ice phase microphysics strongly depends on ambient conditions for the environment of strong convective instability the ice phase is important regarding the system scale structure but is not important to the convective scale dynamics on the other hand the ice influence is crucial to the squall line convective system if the environment has a weak convective instability and is almost saturated at low levels

current subsidence rates due to compaction of holocene sediments in southern louisiana 1 relative contributions of geologic and anthropogenic processes to subsidence of southern louisiana are vigorously debated of these shallow sediment compaction is often considered dominant although this has never been directly observed or effectively demonstrated quantitative understanding of subsidence is important for predicting relative sea level rise storm surge flooding due to hurricanes and for successful wetland restoration despite many shallow borings few appropriate stratigraphic and geotechnical data are available for site specific calculations we overcome this by determining present compaction rates from monte carlo simulations of the incremental sedimentation and compaction of stratigraphies typical of the holocene of southern louisiana this approach generates distributions of present compaction rates that are not expected to exceed 5 mm yr but may locally locations with present subsidence rates greater than the predicted maximum probable shallow compaction rates are likely influenced by additional processes

recent global sea level acceleration started over 200 years ago 1 we present a reconstruction of global sea level gsl since 1700 calculated from tide gauge records and analyse the evolution of global sea level acceleration during the past 300 years we provide observational evidence that sea level acceleration up to the present has been about 0 01 mm yr2 and appears to have started at the end of the 18th century sea level rose by 6 cm during the 19th century and 19 cm in the 20th century superimposed on the long term acceleration are quasi periodic fluctuations with a period of about 60 years if the conditions that established the acceleration continue then sea level will rise 34 cm over the 21st century long time constants in oceanic heat content and increased ice sheet melting imply that the latest intergovernmental panel on climate change ipcc estimates of sea level are probably too low

an equilibrium profile model for retreating marsh shorelines in southeast louisiana abstract louisiana s coastal marshes are experiencing the highest wetland loss rates in the u s in part due to subsidence driven relative sea level rise these marshes are also vulnerable to the erosive power of wave attack 1 on the marsh edge adjacent to open water bodies and 2 after the marsh platform is submerged marsh shorelines in barataria bay breton sound and the active balize delta of southeastern louisiana were examined in areas where the subaerial marsh platform had disappeared since 1932 vibracore transects of marsh and adjacent bay surface sediments to 2 m depth were analyzed using geotechnical stratigraphic and radiochemical 137 cs and 210 pb methods and the subaerial to subaqueous transition of the marsh was mapped for elevation using standard stadia rod transit and fathometer measurements results indicate that marsh edge erosion of the platform takes place subaqueously until water depths of 1 5 m are reached this is observed even in interior pond regions but the shoreface elevation profiles are a function of fetch exposed open bay sites display greater incision depth and rate of the marsh platform than protected interior bay or pond sites core stratigraphy reveals that the outer part of the subaqueous platform switches from erosional to depositional as retreat proceeds covering the incised marsh deposits unconformably with estuarine shelly muds 137 cs and excess 210 pb activity indicates that these muds are deposited within a few decades of subaerial marsh loss the consistency of the cross shore profile results suggests that a single profile of equilibrium can approximate the morphology of eroding marsh edges in southeast louisiana platform stratigraphy and resistance to erosion have a limited effect on profile shape this equilibrium profile and remote sensing images of shoreline change are used to estimate the sediment yield to adjacent estuarine areas by this process on average 1 5 m3 of sediment are yielded per m shoreline length annually from both barataria bay and breton sound due to the highly organic nature of the eroded sediment 30 this supply of organic rich material could significantly impact estuarine productivity and hypoxia on the louisiana continental shelf

a 20th century acceleration in global sea level rise 1 multi century sea level records and climate models indicate an acceleration of sea level rise but no 20th century acceleration has previously been detected a reconstruction of global sea level using tide gauge data from 1950 to 2000 indicates a larger rate of rise after 1993 and other periods of rapid sea level rise but no significant acceleration over this period here we extend the reconstruction of global mean sea level back to 1870 and find a sea level rise from january 1870 to december 2004 of 195 mm a 20th century rate of sea level rise of 1 7 0 3 mm yr 1 and a significant acceleration of sea level rise of 0 013 0 006 mm yr 2 this acceleration is an important confirmation of climate change simulations which show an acceleration not previously observed if this acceleration remained constant then the 1990 to 2100 rise would range from 280 to 340 mm consistent with projections in the ipcc tar

mississippi delta subsidence primarily caused by compaction of holocene strata coastal subsidence causes sea level rise shoreline erosion and wetland loss which poses a threat to coastal populations this is especially evident in the mississippi delta in the southern united states which was devastated by hurricane katrina in 2005 the loss of protective wetlands is considered a critical factor in the extensive flood damage the causes of subsidence in coastal louisiana attributed to factors as diverse as shallow compaction and deep crustal processes remain controversial current estimates of subsidence rates vary by several orders of magnitude here we use a series of radiocarbon dated sediment cores from the mississippi delta to analyse late holocene deposits and assess compaction rates we find that millennial scale compaction rates primarily associated with peat can reach 5mm per year values that exceed recent model predictions locally and on timescales of decades to centuries rates are likely to be 10 mm or more per year we conclude that compaction of holocene strata contributes significantly to the exceptionally high rates of relative sea level rise and coastal wetland loss in the mississippi delta and is likely to cause subsidence in other organic rich and often densely populated coastal plains

hidden aftershocks of the 2011 mw 9 0 tohoku japan earthquake imaged with the backprojection method 1 the first 25 h of the aftershock sequence following the 11 march 2011 mw 9 0 tohoku japan earthquake is investigated using a backprojection method in total 600 aftershocks are imaged during this time period these aftershocks are distributed over a 500 by 300 km area and include many events in the outer rise the backprojection events are compared with the japan meteorological agency jma catalogue which is composed of earthquakes recorded by local seismic networks in japan surprisingly half of the backprojection events are not found in the jma catalogue these events cluster near the japan trench and in the outer rise and fill in gaps in the spatial distribution of the early aftershock sequence where large main shock slip is thought to have occurred these results show that the jma magnitude of completeness is very high near the trench following the 2011 tohoku main shock and earthquakes as large as magnitude 6 8 went undetected by local seismic networks

middle and late holocene sea level changes in eastern maine reconstructed from foraminiferal saltmarsh stratigraphy and ams 14c dates on basal peat abstract a relative sea level history is reconstructed for machiasport maine spanning the past 6000 calendar years and combining two different methods the first method establishes the long term 10 3 yr trend of sea level rise by dating the base of the holocene saltmarsh peat overlying a pleistocene substrate the second method uses detailed analyses of the foraminiferal stratigraphy of two saltmarsh peat cores to quantify fluctuations superimposed on the long term trend the indicative meaning of the peat the height at which the peat was deposited relative to mean tide level is calculated by a transfer function based on vertical distributions of modern foraminiferal assemblages the chronology is determined from ams 14 c dates on saltmarsh plant fragments embedded in the peat the combination of the two different approaches produces a high resolution replicable sea level record which takes into account the autocompaction of the peat sequence long term mean rates of sea level rise corrected for changes in tidal range are 0 75 mm yr between 6000 and 1500 cal yr b p and 0 43 mm yr during the past 1500 years the foraminiferal stratigraphy reveals several low amplitude fluctuations during a relatively stable period between 1100 and 400 cal yr b p and a sea level rise of 0 5 m during the past 300 years

degradation of the plaquemines sub delta and relative sea level in eastern mississippi deltaic coast during late holocene abstract the stratigraphic architecture of plaquemines sub delta lobe is identified from sedimentary cores and sub bottom seismic data collected at lower breton sound louisiana usa four lithofacies were identified in cores from bottom to top 1 organic poor mud 2 organic poor silty sand 3 massive mud 4 organic rich peat and root rich soft mud overall the lithofacies displays a typical progradational deltaic deposit at bottom with upward coarsening feature and then an upward fining deposit overlying on top stratigraphic position of the organic rich peat facies indicates that it is formed after the fluvial sediment source moves away from this region and it is recognized as a termination of progradation of the plaquemines sub delta dated peat samples show that sub delta progradation stops between 876 and 1384 yr ad during which a 20 70 cm relative sea level rise is detected the geological framework reported in this study provides critical information for future modeling on river diversions the stratigraphic architecture underlying lower breton sound implicates that top three facies of root rich soft mud organic rich peat and massive mud are likely to be eroded away before the development of new crevasse splay or sub delta when sediment diversion is in operation the deeper organic poor silty sand lithofacies however can serve as a firm foundation to build new land

global synthetic seismograms using a 2 d finite difference method two dimensional 2 d finite difference fd synthetics which fill the gap between fast 1 d analytic synthetics and time consuming full 3 d synthetics in our ability to model seismograms have been used in many studies we address several issues involving 2 d fd methods in generating global synthetic seismograms these include 1 interfacing point source excitation for earthquakes with 2 d fd methods 2 out of plane spreading corrections and 3 reducing the spherical earth to the flattened models the first issue is tackled using two methods a transparent source box approach and a moment tensor excitation approach where each has its own advantages moreover our source box excitation does not have the late time drift problem that occurred in previous studies the out of plane geometric spreading correction is accounted for by estimating the ray parameter and applying a post simulation filter to 2 d synthetics finally parameters of the earth flattening transformation are discussed and validated the effectiveness of this method is demonstrated by comparing our synthetics with frequency wavenumber summation normal mode and 3 d spectral element synthetics

modeling the 2012 wharton basin earthquakes off sumatra complete lithospheric failure a sequence of large strike slip earthquakes occurred west of sunda trench beneath the wharton basin first reports indicate that the main shock was extremely complex involving three to four subevents m w 8 with a maze of aftershocks we investigate slip models of the two largest earthquakes by joint inversion of regional and teleseismic waveform data using the m w 7 2 foreshock we developed hybrid green s functions for the regional stations to approximate the mixture of oceanic and continental paths the main shock fault geometry is defined based on the back projection results point source mechanisms aftershock distribution and fine tune of grid searches the fault system contains three faults labeled f1 89 289 for dip strike f2 74 20 and f3 60 310 the inversion indicates that the main rupture consisted of a cascade of high stress drop asperities up to 30 mpa extending as deep as 50 km the rupture propagated smoothly from one fault to the next f1 f2 and f3 in sequence with rupture velocities of 2 0 2 5 km s the whole process lasted about 200 s and the major moment release 70 took place on the n s oriented f2 the m w 8 2 aftershock happened about 2 h later on a n s oriented fault with a relatively short duration 60 s and also ruptured as deep as 50 km the slip distributions suggest that the earthquake sequence was part of a broad left lateral shear zone between the australian and indian plates and ruptured the whole lithosphere these earthquakes apparently reactivated existing fracture zones and were probably triggered by unclamping of the great sumatran earthquake of 2004

insights into the kinematic rupture of the 2015 m w 8 3 illapel chile earthquake from joint analysis of geodetic seismological tsunami and superconductive gravimeter observations

bathymetric effects on body waveforms from shallow subduction zone earthquakes and application to seismic processes in the kurile trench resolution of earthquake source characteristics is limited by structural complexities in the source region this study explores the effects of dipping near source bathymetry and structure on long period teleseismic body waveforms from shallow subduction zone earthquakes ray theory synthetics calculated for shallow double couple sources and velocity structures incorporating planar dipping interfaces suggest that p waveforms are most sensitive to water depth and sea bottom orientation for sources beneath a substantial water layer the seafloor dip of the inner trench slope can give rise to prominent later arrivals and waveform complexity not found on synthetics calculated for horizontal bathymetry sediment thickness and the dip of the sediment crust and crust mantle interfaces generally have lesser effects the high amplitude arrivals result largely from changes in the source takeoff angles of the reverberations these arrivals present difficulties for source depth and time function determination from p waveforms and may cause spurious complexity in the derived pattern of earthquake moment release sh waveforms show little effect of sea bottom orientation or local structure for shallowly dipping thrust faulting events suggesting that sh waveform analysis can stabilize source parameter studies with respect to unknown or incompletely parameterized bathymetry and structure effects of nonhorizontal structure are observed in the p waveforms from shallow kurile trench events ms 5 8 7 2 which show prominent later arrivals not observed on sh waveforms synthetics calculated for a dipping water crust interface provide a better fit to these arrivals than those calculated for a horizontal interface suggesting that the p waveforms show systematic effects of seafloor dip the synthetics are highly sensitive to the assumed orientation of the seafloor the data are fit best for seafloor orientations consistent with the dip of the inner trench slope the distorting effect of the water layer gives rise to systematic differences in p wave complexity between shallow events near the seismic front which occur beneath a thick water layer and show numerous later arrivals and deeper arcward events which occur beneath a thin water layer and show simple waveforms large reverberations in the p waveforms of the october 20 1963 and june 10 1975 kurile tsunami earthquakes have previously been used as evidence for a long source duration and a change in focal mechanism during rupture however sh waveforms suggest simple source time functions with durations of 25 and 43 s inversion of p and sh waveforms and amplitudes yields shallowly dipping thrust mechanisms with no conclusive evidence for a previously proposed steepening of the fault dip during rupture the large p wave reverberations cannot be fit using a point source and a horizontal seafloor detailed modeling of the 1963 event as an upward propagating rupture extending through low velocity sediments to the ocean bottom combined with the effects of the dipping seafloor allow the reverberation amplitudes to be matched the anomalous tsunami excitation appears to be related to the location of the sources at shallow depths within low velocity sediments

the west african squall line observed on 23 june 1981 during copt 81 kinematics and thermodynamics of the convective region abstract the squall line observed during the night of 23 24 june 1981 was one of the most intense events during the copt 81 convection profonde tropicale experiment conducted in may and june 1981 at korhogo in the northern ivory coast by french and ivorian research institutes the presquall environment possessed a very large convective instability with a stable layer in the lowest levels the presence of dry air in the midlevels which promoted the development of convective downdrafts and the unusually low altitude of the strong easterly winds which aided the development of a leading anvil above 6 km are two noteworthy characteristics of the environment of this squall line its mesoscale structure as deducted from radar data is similar to that previously observed for tropical squall lines convective updrafts and downdraft occur in the leading heavy precipitation region convective region and a mesoscale updraft and downdraft is found within and below the trailing mid to upper level anvil clouds

mid to late holocene relative sea level change in southwest britain and the influence of sediment compaction relative sea level changes in southwest britain are poorly constrained because of limitations in the quality and quantity of existing geological data as a consequence in contrast to most other regions in the uk it is not possible to reliably infer rates of land and sea level change during the last few thousand years furthermore geologically based sea level reconstructions from this area display significant misfits with relative sea level predictions based on a recent global model of the glacio isostatic adjustment process this paper presents a new record of relative sea level change for the last 4000 years derived from radiocarbon dated sea level index points in addition to providing information concerning the pattern and rates of late holocene sea level change these data are used to evaluate the suggestion that the apparent misfits between model predictions and geological reconstructions can be satisfactorily explained as a consequence of the lowering of sea level index points by sediment compaction the results indicate that a simple first order method of decompaction based on the stratigraphic position of sea level index points can eliminate much of the misfit between reconstructions and predictions and substantially reduce vertical scatter in geological data they also suggest that the influence of compaction on sea level data from this region may be larger than previously thought the simple addition of reliable stratigraphic data in combination with targeted sampling has the potential to improve our understanding of the compaction process and increase the utility of existing data this is increasingly important as the partnership between geophysical modelling and geological investigation becomes more commonplace in sea level studies around the world

propensity for erosion and deposition in a deltaic wetland complex implications for river management and coastal restoration abstract the mississippi river delta is one of the most rapidly changing area on earth with large areas experiencing land loss and smaller areas experiencing loss while some of the drivers of these changes are well known high rates of relative sea level rise reduced sediment inputs canal dredging debate exists about other drivers one area that has received substantial attention is the role of river diversions areas where sediments and water are diverted from the mississippi river into degrading wetlands with the hope of reinitiating deltaic land building processes some authors have argued that diversions lead to reduced shear strengths of wetland soils that make them more vulnerable to storm driven erosion while other authors have argued that sediments from river diversions will develop stable land this study examined this controversy in the cubits gap subdelta an analogue for a large 1420 m3 s 1 river diversion by testing the hypothesis that areas of land gain and or resilience to erosion occurred in areas that actively received river sediments and as a result had mineral rich soils with high shear strength to accomplish this a normalized difference water index ndwi was developed for landsat 7 enhanced thematic mapper etm and landsat 8 operational land imager oli images the ndwi was calculated from blue swir blue swir where swir is the shorter wavelength and yielded land water boundary maps with 30 m resolution results indicate that land gain occurred predominantly in the riverside section of this subdelta where sediments were imported from mississippi river crevasses and or dredging land loss typically occurred in the distal regions of the subdelta which had lower levels of sediment supply and greater wave exposure sediment geotechnical analyses revealed land loss pixels generally correlated sediments with to high organic contents 9 0 1 9 water contents 54 8 3 7 and salinity 6 5 2 0 psu with low shear strengths 5 7 0 8 kn m 2 and low bulk density 0 6 0 8 g cm 3 whereas land gain pixels generally correlate with low organic content 3 9 0 6 water content 38 1 4 2 high shear strength 10 9 4 1 kn m 2 and bulk density 1 00 0 1 g cm 3 this study suggests plans to restore the region by partially diverting the flow of the mississippi river will be most successful if they carry high loads of sediment and that concerns about the integrity of fresh marsh may be unwarranted if those marshes are sediment rich

mesoscale convective systems 1 mesoscale convective systems mcss have regions of both convective and stratiform precipitation and they develop mesoscale circulations as they mature the upward motion takes the form of a deep layer ascent drawn into the mcs in response to the latent heating and cooling in the convective region the ascending layer overturns as it rises but overall retains a coherent layer structure a middle level layer of inflow enters the stratiform region of the mcs from a direction determined by the large scale flow and descends in response to diabatic cooling at middle to low levels a middle level mesoscale convective vortex mcv develops in the stratiform region prolongs the mcs and may contribute to tropical cyclone development the propagation of an mcs may have a discrete component but may further be influenced by waves and disturbances generated both in response to the mcs and external to the mcs waves of a larger scale may affect the propagation velocity by phase locking with the mcs in a cooperative mode the horizontal scale of an mcs may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the rossby radius of the mcv the vertical redistribution of momentum by an mcs depends on the size of the stratiform region while the net vertical profile of heating of the large scale environment depends on the amount of stratiform rain regional variability of the stratiform rain from mcss affects the large scale circulation s response to mcs heating

two stage composite megathrust rupture of the 2015 mw8 4 illapel chile earthquake identified by spectral element inversion of teleseismic waves the mw8 4 illapel earthquake occurred on 16 september was the largest global event in 2015 this earthquake was not unexpected because the hypocenter was located in a seismic gap of the peru chile subduction zone however the source model derived from 3 d spectral element inversion of teleseismic waves reveals a distinct two stage rupture process with completely different slip characteristics as a composite megathrust event the two stages were temporally separated rupture in the first stage with a moment magnitude of mw8 32 built up energetically from the deeper locked zone and propagated in the updip direction toward the trench subsequently the rupture of the second stage with a magnitude of mw8 08 mainly occurred in the shallow subduction zone with atypical repeating slip behavior the unique spatial temporal rupture evolution presented in this source model is key to further in depth studies of earthquake physics and source dynamics in subduction systems

mitigating land loss in coastal louisiana by controlled diversion of mississippi river sand the bonnet carre spillway diverts floodwaters from the mississippi river to lake pontchartrain and was opened for 42 days during the 2011 flood according to measurements of the newly deposited sediments at least 31 46 of the river s sand load was diverted into the spillway at this time suggesting that such diversions can help mitigate coastal wetland loss

compressive sensing of frequency dependent seismic radiation from subduction zone megathrust ruptures megathrust earthquakes rupture a broad zone of the subducting plate interface in both along strike and along dip directions the along dip rupture characteristics of megathrust events e g their slip and energy radiation distribution reflect depth varying frictional properties of the slab interface here we report high resolution frequency dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive sensing sparse source recovery technique resolving generally low frequency radiation closer to the trench at shallower depths and high frequency radiation farther from the trench at greater depths together with coseismic slip models and early aftershock locations our results suggest depth varying frictional properties at the subducting plate interfaces the shallower portion of the slab interface above 15 km is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip deficient high frequency radiation and few early aftershocks the slab interface at intermediate depths 15 35 km is the main unstable seismogenic zone for the nucleation of megathrust quakes typically with large coseismic slip abundant early aftershocks and intermediate to high frequency radiation the deeper portion of the slab interface 35 45 km is seismically unstable however with small coseismic slip dominant high frequency radiation and relatively fewer aftershocks

dynamic changes and processes in the mississippi river delta research in the modern delta of the mississippi river has revealed short term changes and processes that are of significant magnitude deltaic lobes each lobe covering an area of 30 000 sq km and having an average thickness of 35 km switch sites of deposition on an average of every 1 500 yr through short periods of geologic time this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas within a single delta lobe and operating on an even higher frequency are bay fills and overbank splays bay fills having areas of 250 sq km and thickness of 15 m require only 150 yr to accumulate four major events have taken place in the modern balize delta since 1838 overbank splays are much smaller covering areas of less than 2 sq km and having thicknesses of 3 m but are associated with high floods on the river at the river mouth continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long 8 km wide and a thickness of 80 m in a period of only 200 yr differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr on the subaqueous delta platform sediment instabilities operate nearly continuously mass moving large quantities of shallow water deposits to deeper water environments via arcuate rotational slides and mudflow gullies and depositional lobes all of these changes and processes operate at differing spatial and temporal scales but all result in deposition of large volumes of sediment over extremely short periods of time

sand as a stable and sustainable resource for nourishing the mississippi river delta dams have starved the lower mississippi river of sediment over recent decades suggesting that the drowning of the delta is inevitable analysis of the rivers suspended sediment load and morphodynamic modelling suggest that the amount of sand essential for land building has not significantly decreased since dam construction with sand remaining available for several centuries

a window into the complexity of the dynamic rupture of the 2011 mw 9 tohoku oki earthquake the 2011 mw 9 tohoku oki earthquake recorded by over 1000 near field stations and multiple large aperture arrays is by far the best recorded earthquake in the history of seismology and provides unique opportunities to address fundamental issues in earthquake source dynamics here we conduct a high resolution array analysis based on recordings from the usarray and the european network the mutually consistent results from both arrays reveal rupture complexity with unprecedented resolution involving phases of diverse rupture speed and intermittent high frequency bursts within slow speed phases which suggests spatially heterogeneous material properties the earthquake initially propagates down dip with a slow initiation phase followed by sustained propagation at speeds of 3 km s the rupture then slows down to 1 5 km s for 60 seconds a rich sequence of bursts is generated along the down dip rim of this slow and roughly circular rupture front before the end of the slow phase an extremely fast rupture front detaches at about 5 km s towards the north finally a rupture front propagates towards the south running at about 2 5 km s for over 100 km key features of the rupture process are confirmed by the strong motion data recorded by k net and kik net the energetic high frequency radiation episodes within a slow rupture phase suggests a patchy image of the brittle ductile transition zone composed of discrete brittle asperities within a ductile matrix the high frequency is generated mainly at the down dip edge of the principal slip regions constrained by geodesy suggesting a variation along dip of the mechanical properties of the mega thrust fault or their spatial heterogeneity that affects rise time

rupture imaging of the mw 7 9 12 may 2008 wenchuan earthquake from back projection of teleseismic p waves 1 the mw 7 9 wenchuan earthquake of 12 may 2008 was the most destructive chinese earthquake since the 1976 tangshan event tens of thousands of people were killed hundreds of thousands were injured and millions were left homeless here we infer the detailed rupture process of the wenchuan earthquake by back projecting teleseismic p energy from several arrays of seismometers this technique has only recently become feasible and is potentially faster than traditional finite fault inversion of teleseismic body waves therefore it may reduce the notification time to emergency response agencies using the iris dmc we collected 255 vertical component broadband p waves at 30 95 from the epicenter we found that at periods of 5 s and greater nearly all of these p waves were coherent enough to be used in a global array we applied a simple down sampling heuristic to define a global subarray of 70 stations that reduced the asymmetry and sidelobes of the array response function arf we also considered three regional subarrays of seismometers in alaska australia and europe that had apertures less than 30 and p waves that were coherent to periods as short as 1 s individual arfs for these subarrays were skewed toward the subarrays however the linear sum of the regional subarray beams at 1 s produced a symmetric arf similar to that of the groomed global subarray at 5 s for both configurations we obtained the same rupture direction rupture length and rupture time we found that the wenchuan earthquake had three distinct pulses of high beam power at 0 23 and 57 s after the origin time with the pulse at 23 s being highest and that it ruptured unilaterally to the northeast for about 300 km and 110 s with an average speed of 2 8 km s it is possible that similar results can be determined for future large dip slip earthquakes within 20 30 min of the origin time using relatively sparse global networks of seismometers such as those the usgs uses to locate earthquakes in near real time

localized water reverberation phases and its impact on backprojection images coherent radiators imaged by back projections bp are commonly interpreted as part of the rupture process nevertheless artifacts introduced by structure related phases are rarely discriminated from the rupture process in this study we use a calibration event to discriminate between rupture and structure effects we re examine the waveforms and bp images of the 2012 mw 7 2 indian ocean earthquake and a calibration event mw 6 2 the p wave codas of both events present similar shape with characteristic period of approximately 10 s which are back projected as coherent radiators near the trench s wave bp doesn t image energy radiation near the trench we interpret those coda waves as localized water reverberation phases excited near the trench we perform a 2d waveform modeling using realistic bathymetry model and find that the steep near trench bathymetry traps the acoustic water waves forming localized reverberation phases these waves can be imaged as coherent near trench radiators with similar features as that in the observations we present a set of methodologies to discriminate between the rupture and propagation effects in bp images which can serve as a criterion of subevent identification

high winds generated by bow echoes part ii the relationship between the mesovortices and damaging straight line winds airborne radar analysis of a mesovortex that developed near the apex of a bow echo is presented the mesovortex was shown to play a critical role in determining the location of intense straight line wind damage at the surface the perturbation pressure gradient force in natural coordinates along the parcel path accelerated the horizontal winds however intense mesovortices modified the low level outflow and largely determined the locations where the strongest winds occurred regions of maximum winds are accounted for as a superposition of the vortex and the flow in which it is embedded the strongest winds occur on the side of the vortex where translation and rotation effects are in the same direction this model explains the observed tongue of high wind speeds that were confined to the periphery of the mesovortex the origin of the mesovortex is also examined similarities and differences of this bow echo event with recent modeling studies are presented

morphodynamics of the erosional phase of crevasse splay evolution and implications for river sediment diversion function abstract despite being a primarily depositional landform a crevasse splay experiences an initial evolutionary phase that is primarily erosional as sediment laden river water spills from a main river channel and incises a new route through the river banks and levee into an interdistributary basin or floodplain this phase sets the dimensions and the conveyance properties of the crevasse which in turn influences the continued expansion or closure of the crevasse channel however little is known about the controlling morphodynamics or how the erosional processes transition to depositional processes during this phase the objective of this study is to investigate these phenomena at the west bay sediment diversion louisiana usa using coupled field observations and numerical modeling the west bay diversion was cut into a lower mississippi river levee to mimic the function of a crevasse splay i e to divert river water and sediment to an adjacent receiving basin for land building purposes bathymetric measurements show that the diversion channel experienced significant natural morphologic evolution during the initial decade 2004 2014 hydrodynamic and sediment transport modeling suggests that this evolution initially increased the discharge of flow and sediment through the crevasse as the channel became wider and deeper and altered its orientation relative to the main river flow direction after 5 years the model results predict that further evolution led to monotonically reduced diversion discharges during this time natural and engineered sediment deposition in the receiving basin decreased predicted basin flow velocities and promoted a backwater effect that reduced the sediment transport capacity of the diversion channel observations during the final 2 years show that much of the initial erosion around the diversion had abated indicating that diversion morphology may have stabilized a modeling sensitivity analysis confirmed that the observed changes to channel geometry and orientation likely promoted flows of water and sediment through the diversion while increases in basin bed elevation would have had a contrary effect the morphodynamic evolution of the west bay diversion documented in this study presents a model indicative of the erosional phase of crevasse splay evolution in a deltaic distributary fluvial network study results offer an analogue on how an engineered river sediment diversion constructed for coastal restoration may function during its first years of operation and suggest that the desired land building processes may take time to become established

earthquake in a maze compressional rupture branching during the 2012 mw 8 6 sumatra earthquake seismological observations of the 2012 moment magnitude 8 6 sumatra earthquake reveal unprecedented complexity of dynamic rupture the surprisingly large magnitude results from the combination of deep extent high stress drop and rupture of multiple faults back projection source imaging indicates that the rupture occurred on distinct planes in an orthogonal conjugate fault system with relatively slow rupture speed the east southeast west northwest ruptures add a new dimension to the seismotectonics of the wharton basin which was previously thought to be controlled by north south strike slip faulting the rupture turned twice into the compressive quadrant against the preferred branching direction predicted by dynamic coulomb stress calculations orthogonal faulting and compressional branching indicate that rupture was controlled by a pressure insensitive strength of the deep oceanic lithosphere

organic clastic facies relationships and chronostratigraphy of the barataria interlobe basin mississippi delta plain abstract the lithofacies of barataria basin an interlobe basin of the mississippi delta plain were established from analyses of 115 boreholes five organic lithofacies are distinguished by organic matter content botanical origin and degree of decomposition organic rich facies accumulated in three stages a b and c during two depositional events stage a developed locally but stages b and c developed as blankets over abandoned delta lobes protected from marine influx by a shoreline stage b lasted from 2 300 1 000 yr bp and ended when clastic sediments from renewed deltaic sedimentation blanketed the peats expanding the basin about 50 km seaward peat accumulation of stage c lasted from 900 yr bp to the present and is currently being terminated by subsidence induced transgression high quality stage b peats accumulated beneath herbaceous freshwater floating and emergent marshes mainly above the footwall of a strike oriented reactivated growth fault above the footwall the basin was shallower than on the hanging wall where the deeper basin invited more overbank sedimentation which increased ash content and diluted peats thus both tectonic and sedimentary factors influenced organic facies the quality and quantity of barataria basin peats differ from those of many deltaic coals however these peats accumulated in only 2 300 yr immediately after a rapid sea level rise frequent delta lobe switching increased organic matter preservation potential but did not provide sufficient time for the accumulation of thick low ash peats the geometries of organic and clastic facies are similar to those of some coal bearing strata

wetland loss and the subdelta life cycle abstract the rapid deterioration of marsh habitat observed during recent years in the modern mississippi river delta is a consequence at least in part of the natural life cycle of subdeltas with life spans typically less than 200 years subdeltas or bay fill deposits are scaled down versions of major delta lobes yet provide through pulses of sediment nearly all the subaerial land in an active delta using maps charts and aerial photographs curves were constructed for rates of change in land area sediment volume and linear progradation in the four subdeltas that have formed on the modern mississippi river delta since the first accurate survey in 1838 results indicate that each subdelta 1 lasted for approximately 115 175 years 2 included both periods of growth and deterioration 3 was initiated by a crevasse or break in the natural levee system 4 showed linear advancement and volumetric growth during subaerial deterioration and 5 displayed a new pulse of subaerial growth during the high discharge decade of the 1970s contrary to popular accounts demise of the mississippi river delta through deterioration of its subdeltas is not a result of the construction of artificial levees upstream or discharge of sediment off the continental shelf edge rather it is attributable to a substantial decrease and fining of sediments being transported downstream to depositional sites within a delta that has developed through natural processes a complex and inefficient channel network for delivering these sediments

coherent seismic arrivals in the p wave coda of the 2012 m w 7 2 sumatra earthquake water reverberations or an early aftershock

polarimetric signatures and hydrometeor classification of west african squall lines during the amma campaign the doppler and polarimetric radar ronsard was deployed in kopargo benin in the present paper three squall lines sampled during that period are described it was the first time this type of system had been observed with a polarimetric radar in the west african region the strongest case occurred on 28 july 2006 and the others on 12 september and 30 june the polarimetric characteristics of these systems are described and a hydrometeor classification algorithm is applied in order to identify the microphysics of the systems finally the 3d wind field is retrieved for the 28 july event the convective part of the squall lines is generally characterized by maxima of reflectivity differential reflectivity zdr and specific differential phase kdp the results of the hydrometeor classification show that the convective region is composed of strong 30 mm h 1 to moderate rain 5 30 mm h 1 below the melting level and graupel above this zone seems to be characterized by relatively strong updraughts 5 10 m s 1 responsible both for the formation of graupel by riming and for the growth of drops due to coalescence to the rear the systems are characterized by light rain 5 mm h 1 below and snow or ice crystals above these regions present downward flow below the melting layer which prevents drop growth by condensation consistent with the light rain observed the stratiform part of these systems can be very non uniform due to the presence of old cells dissipating associated with moderate rain and thus stronger reflectivities

transgressive depositional systems of the mississippi delta plain a model for barrier shoreline and shelf sand development abstract depositional sequences generated in the mississippi river delta plain consist of a regressive and a transgressive component the transgressive component has been considerably less studied but accounts for the majority of the surface area on the lower mississippi river delta plain and up to 50 percent of the total sequence thickness in shallow water deltas the development and preservation of transgressive depositional systems in abandoned delta complexes follows the process of transgressive submergence in which the horizontal component of reworking occurs during shoreface retreat combined with a vertical component of submergence acting to preserve the sequence the evolution of transgressive depositional systems in each of the abandoned holocene mississippi river delta complex s can be summarized in a three stage model beginning with stage 1 an erosional headland and flanking barriers in this stage regressive sand deposits contained within abandoned deltaic headlands are reworked by the eroding shoreface and dispersed longshore into contiguous flanking barriers enclosing restricted interdistributary bays submergence of the delta plain during relative sea level rise generates an intradeltaic lagoon separating the former stage 1 sand body from the shoreline forming stage 2 a transgressive barrier island arc the landward migrating barrier island arc is unable to keep pace with relative sea level rise and the retreating mainland shoreline resulting in submergence and the formation of stage 3 an inner shelf shoal following submergence the former barrier island arc sand body continues to be reworked into a marine sand body on the inner continental shelf during stage 3 this sequence of coastal evolution provides direct evidence of barrier island formation with each stage producing a distinctive stratigraphic signature the current sea level rise models of shoreface retreat and in place drowning developed for the u s atlantic continental shelf do not adequately explain either the morphology or the stratigraphy of transgressive mississippi river delta sand bodies current models of mississippi deltaic stratigraphy emphasize the deep water artificially maintained balize delta which differs considerably from the shallow water shelf phase delta complexes that are the primary depositional constituents of he holocene mississippi river delta plain

crevasse splays versus avulsions a recipe for land building with levee breaches 2018 the authors natural levee breaches can not only initiate an avulsion but also under the right circumstances lead to crevasse splay formation and overbank sedimentation the formative conditions for crevasse splays are not well understood yet such river sediment diversions form an integral part of billion dollar coastal restoration projects here we use delft3d to investigate the influence of vegetation and soil consolidation on the evolution of a natural levee breach model simulations show that crevasse splays heal because floodplain aggradation reduces the water surface slope decreasing water discharge into the flood basin easily erodible and unvegetated floodplains increase the likelihood for channel avulsions denser vegetation and less potential for soil consolidation result in small crevasse splays that are not only efficient sediment traps but also short lived successful crevasse splays that generate the largest land area gain for the imported sediment require a delicate balance between water and sediment discharge vegetation root strength and soil consolidation