silent heralds of megathrust earthquakes d uring an earthquake rupture propagates along the fault plane within a few tens of seconds much slower rupture lasting for weeks or months has recently been observed in slip transients or slow earthquakes 1 1 2 2 these events are also dubbed silent earthquakes because

along arc segmentation and interaction of subducting ridges with the lesser antilles subduction forearc crust revealed by mcs imaging highlights we image the deep structure of the lesser antilles subduction zone by mcs profiles the complex deformation of the outer forearc crust is induced by subducting ridges we discuss also the effect of the subducting compressive nam sam plate boundary along strike variations of the seaward edge of the outer forearc crust are discovered the updip limit proxy of the seismogenic part reaches 20 km trenchwards than believed abstract we present the results from a new grid of deep penetration multichannel seismic mcs profiles over the 280 km long north central segment of the lesser antilles subduction zone the 14 dip lines and 7 strike lines image the topographical variations of i the subduction interplate decollement ii the top of the arcward subducting atlantic oceanic crust toc under the huge accretionary wedge up to 7 km thick and iii the trenchward dipping basement of the deeply buried forearc backstop of the caribbean upper plate the four northernmost long dip lines of this new mcs grid reveal several kilometre high topographic variations of the toc beneath the accretionary wedge offshore guadeloupe and antigua islands they are located in the prolongation of those mapped on the atlantic seafloor entering subduction such as the barracuda ridge this mcs grid also provides evidences on unexpected huge along strike topographical variation of the backstop basement and of the deformation style affecting the outer forearc crust and sediments their mapping clearly indicates two principal areas of active deformation in the prolongation of the major barracuda and tiburon ridges and also other forearc basement highs that correspond to the prolongation of smaller oceanic basement highs recently mapped on the atlantic seafloor although different in detail the two main deforming forearc domains share similarities in style the imaged deformation of the sedimentary stratification reveals a time and space dependent faulting by successive warping and unwarping which deformation can be readily attributed to the forearc backstop sweeping over the two obliquely oriented elongated and localized topographical ridges the induced faulting producing vertical scarps in this transport does not require a regional arc parallel extensional regime as proposed for the inner forearc domain and may support a partitioned tectonic deformation such as in the case of an outer forearc sliver a contrasted reflectivity of the sedimentary layering at the transition between the outer forearc and accretionary domains was resolved and used to define the seaward edge of the outer forearc basement interpreted as being possibly a proxy to the updip limit of the interplate seismogenic zone its mapping documents along arc variations of some tens of kilometres of the subduction backstop with respect to the negative gravity anomaly commonly taken as marking the subduction trench with the exception of the southernmost part the newly mapped updip limit reaches 25 km closer to the trench thus indicating a possible wider seismogenic zone over almost the whole length of the study area

structure of the lesser antilles subduction forearc and backstop from 3d seismic refraction tomography in 2007 the sismantilles ii experiment was conducted to constrain structure and seismicity in the central lesser antilles subduction zone the seismic refraction data recorded by a network of 27 obss over an area of 65 km 95 km provide new insights on the crustal structure of the forearc offshore martinique and dominica islands the tomographic inversion of first arrival travel times provides a 3d p wave velocity model down to 15 km basement velocity gradients depict that the forearc is made up of two distinct units a high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south short wavelength deformations of the outer forearc basement are observed beneath a 3 to 6 km thick sedimentary pile in relation with the subduction of the tiburon ridge and associated seafloor reliefs north offshore dominica island the outer forearc is 70 km wide it extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed its width decreases strongly to the south to terminate offshore martinique where the inner forearc acts as the backstop the inner forearc is likely the extension at depth of themesozoicmagmatic crust outcropping to the north in la desirade island and along the scarp of the karukera spur the outer forearc could be either the eastern prolongation of the inner forearc but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes or an oceanic terrane more recently accreted to the island arc

searching for unconventional seismic signals on a subduction zone with a submerged forearc obs offshore the lesser antilles abstract detecting unconventional seismic signals related to subduction zone processes at depth in continuous ocean bottom seismometer obs records requires the analysis and identification of noise due to instrumental problems deployment sites or sea state conditions the temporary obs deployment at the lesser antilles subduction zone provides new insights into the feasibility of detecting unconventional signals such as non volcanic tremor nvt long period lp or ultra long period ulp events analysis of noise at an array comprising several sites and types of instruments and comparison with recordings on land shows transients in the noise episodes can be identified considering the diversity of sites and instrument types and comparing the seismic signals with meteorological and oceanographic data in order to reliably detect nvt 1 10 hz originating from inside the solid earth one must first characterize noise induced by the activity of the atmosphere and hydrosphere at the sea bottom as well as on land the semidiurnal modulation of noise amplitude can be shown here not to be due to that of the nvt from a seismic source at depth which is related to the subduction interplate and whose activity is modulated by the tidal stresses as inferred for other megathrusts on emerged forearcs here the semidiurnal modulation is rather due to the effect of the tides themselves such as tidal currents since they do not affect all types and all components of the unique multi station array of obs that could be deployed on this submerged forearc the short period cut off of the strong noise due to ocean surface infragravity waves increases to longer periods with obs depth thereby increasing the observational window with low noise to lower frequencies and deep obs sites may be advantageous for detecting lp events

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

lateral extrusion in the eastern alps part 1 boundary conditions and experiments scaled for gravity lateral extrusion encompasses extensional collapse gravitational spreading away from a topographic high in an orogenic belt and tectonic escape plane strain horizontal motion of wedges driven by forces applied to their boundaries in the eastern alps it resulted from 1 an overall northerly compression apulia against eurasia 2 a strong foreland bohemian massif 3 lack of constraint along a lateral boundary carpathian region and 4 a previously thickened gravitationally unstable thermally weakened crust eastern alpine orogenic belt six indentation experiments reproduce lateral extrusion at lithospheric scale the models have two to four lithospheric layers with a mohr coulomb rheology for the upper and a viscous rheology for the lower crust the lithosphere rests upon a low viscosity asthenosphere a broad indenter a narrow deformable area and a weakly constrained eastern margin fullfill as closely as possible conditions in the eastern alps indentation produces both thickening in front of the indenter and escape of triangular wedges lateral variations in crustal thickness become attenuated by gravitational spreading the overall fault pattern includes domains of reverse strike slip oblique normal and pure normal faults strike slip faults in conjugate sets develop serially the narrow width of the deformable area and the strength of the foreland determine the angles between the sets gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin opposite the unconstrained margin the indenter front shows thrusts and folds intersecting with the conjugate strike slip sets a triangular indenter favors spreading high velocity of indentation favors escape high confinement limits lateral motion inhibits spreading and favors thickening lateral extrusion in the eastern alps is best modeled by 1 a weak lateral confinement 2 a broad and straight indenter 3 a narrow width of the deformable area and 4 a rigid foreland crustal thickening lateral escape and gravitational spreading all contribute to the overall deformation

crustal and uppermost mantle structure of the bohemian massif based on celebration 2000 data 1 the deep structure of the bohemian massif bm the largest stable outcrop of variscan rocks in central europe was studied using the data of the international seismic refraction experiment central european lithospheric experiment based on refraction celebration 2000 the data were interpreted by seismic tomographic inversion and by two dimensional 2 d trial and error forward modeling of p and s waves additional constraint on crustal structure was given by amplitude modeling using the reflectivity method and gravity modeling though consolidated the bm can be subdivided into several tectonic units separated by faults shear zones or thrusts reflecting varying influence of the cadomian and variscan orogeneses the saxothuringian barrandian moldanubian and moravian velocity models determine three types of crust mantle transition in the bm reflecting variable crustal thickness and delimiting contacts of tectonic units in depth the nw area the saxothuringian has a highly reflective lower crustal layer above moho with a strong velocity contrast at the top of this layer this reflective laminated lower crust reaches depths of 26 35 km and is characteristic for the saxothuringian unit which was subject to eastward subduction the moldanubian in the central part is characterized by the deepest 39 km and the most pronounced moho within the whole bm with a strong velocity contrast 6 9 8 1 km s 1 a thick crust mantle transition zone in the se with velocity increase from 6 8 to 7 8 km s 1 over the depth range of 23 40 km seems to be the characteristic feature of the moravian overthrusted by the moldanubian during variscan collision

a record of the last 460 thousand years of upper ocean stratification from the central walvis ridge south atlantic the upper branch of the atlantic meridional overturning circulation predominantly enters the atlantic ocean through the southeast where the subtropical gyre is exposed to the influence of the agulhas leakage al to understand how the transfer of indian ocean waters via the al affected the upper water column of this region we have generated new proxy records of planktic foraminifera from a core on the central walvis ridge on the eastern flank of the south atlantic gyre sag we analyzed the isotopic composition of subsurface dweller globigerinoides ruber sensu lato and thermocline globorotalia truncatulinoides sinistral spanning the last five pleistocene glacial interglacial g ig cycles the former displays a response to obliquity suggesting connection with high latitude forcing and a warming tendency during each glacial termination in response to the interhemispheric seesaw the 18o difference between the two species interpreted as a proxy for upper ocean stratification reveals a remarkably regular sawtooth pattern bound to g ig cyclicity it rises from interglacials until glacial terminations with fast subsequent decrease appearing to promptly respond to deglacial peaks of al stratification however bears a different structure during the last cycle being minimal at last glacial maximum and peaking at termination i we suggest this to be the result of the intensified glacial wind field over the sag and or of the invasion of the south atlantic thermocline by glacial north atlantic intermediate waters the 13c time series of the two species have similar g ig pattern whereas their difference is higher during interglacials we propose that this may be the result of the alternation of intermediate water masses in different circulation modes and of a regionally more efficient biological pump at times of high pco2

seismic activity offshore martinique and dominica islands central lesser antilles subduction zone from temporary onshore and offshore seismic networks this work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the central lesser antilles island arc from martinique to guadeloupe islands during the whole recording period extending from january to the end of august 2007 more than 1300 local seismic events were detected in this area a subset of 769 earthquakes was located precisely by using hypoellipse we also computed focal mechanisms using p wave polarities of the best azimuthally constrained earthquakes we detected earthquakes beneath the caribbean forearc and in the atlantic oceanic plate as well at depth seismicity delineates the wadati benioff zone down to 170 km depth the main seismic activity is concentrated in the lower crust and in the mantle wedge close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed we propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense thick and rigid crustal block and the lighter and more flexible outer forearc seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate

earth s free oscillations recorded by free fall obs ocean bottom seismometers at the lesser antilles subduction zone three unburied ocean bottom seismometers obs equipped with trillium 240 s broad band seismometers recorded spheroidal free oscillations of the earth out to periods over 1000 s period for the m 8 1 april 1 2007 solomon islands earthquake in contrast to broadband observatories of the global network that operate in quiet continental locations these instruments were dropped on the several km thick layer of sediments of the forearc and accretionary wedge of the lesser antilles subduction zone furthermore a high ambient noise level due to the ocean surface infragravity waves is expected to cover the frequency band of earth s normal modes band when recorded at these sites in spite of these hostile environmental conditions the frequency of clearly defined peaks of the earth s normal modes were measured after the earthquake this suggests that the recording of normal modes and long period waves can be extended to parts of the hitherto inaccessible ocean with currently available obs technology

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

tidal modulation of nonvolcanic tremor episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of japan and cascadia in cascadia such episodes typically last a few weeks and differ from normal earthquakes in their source location and moment duration scaling the three most recent episodes in the puget sound southern vancouver island portion of the cascadia subduction zone were exceptionally well recorded in each episode we saw clear pulsing of tremor activity with periods of 12 4 and 24 to 25 hours the same as the principal lunar and lunisolar tides this indicates that the small stresses associated with the solid earth and ocean tides influence the genesis of tremor much more effectively than they do the genesis of normal earthquakes because the lithostatic stresses are 10 5 times larger than those associated with the tides we argue that tremor occurs on very weak faults

shape and origin of the east alpine slab constrained by the alpass teleseismic model abstract during the last two decades teleseismic studies yielded valuable information on the structure of the upper mantle below the alpine mediterranean area subducted oceanic lithosphere forms a broad anomaly resting on but not penetrating the 670 km discontinuity more shallow slabs imaged below the alpine arc are interpreted as subducted continental lower lithosphere substantial advances in our understanding of past and active tectonic processes have been achieved due to these results however important issues like the polarity of subduction under the eastern alps and the slab geometry at the transition to the pannonian realm are still under debate the alpass teleseismic experiment was designed to address these open questions teleseismic waveforms from 80 earthquakes recorded at 75 temporary and 79 permanent stations were collected during 2005 and 2006 from these data a tomographic image of the upper mantle was generated between 60 km and 500 km depth crustal corrections additional station terms and ray bending caused by the velocity perturbations were considered a steeply to vertically dipping shallow slab below the eastern alps is clearly resolved down to a depth of 250 km it is interpreted as european lower lithosphere detached from the crust and subducted during post collision convergence between adria and europe below the pannonian realm low velocities or high mantle temperatures prevail down to 300 km depth consistent with the concept of a pannonian lithospheric fragment which underwent strike slip deformation relative to the european plate and extension during the post collision phase of the alpine orogeny between 350 km and 400 km depth a deep slab extends from below the central eastern alps to under the pannonian realm it is interpreted as subducted lithosphere of the alpine tethys at greater depth there is a continuous transition to the high velocity anomaly above the 670 km discontinuity

nloadf a program for computing ocean tide loading the loading of the earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique in order to compute these most accurately a combination of global and local models of the ocean tides may be needed the program nloadf convolves the green functions for loading with ocean tide models using a station centered grid with fixed dimensions making it easy to combine different ocean models without overlap in the convolution the program computes all the quantities of interest gravity displacement tilt and strain and includes the case where measurements are made beneath the surface of the ocean

northern cascadia episodic tremor and slip a decade of tremor observations from 1997 to 2007 1 we analyze continuous seismic and gps records collected in the last decade 1997 2007 to establish the most comprehensive observational basis for northern cascadia episodic tremor and slip ets events a simple ets scale system using a combination of a letter and a digit is proposed to quantitatively characterize the spatial and temporal dimensions of ets events clear correlation between gps and tremor signals is observed for all a b class episodes but the gps signature is less obvious for minor ones regular ets recurrence can be established only for a b class episodes in southern vancouver island halting and jumping are very common in ets migration patterns and along strike migration can happen in both directions a prominent tremor gap is observed in midisland around 49 5 n this gap coincides with the epicenters of the only two large earthquakes beneath vancouver island ets tremors also tend to occur in places where the local seismicity is relatively sparse the tremor depth distribution shows a peak in the 25 35 km range where strong seismic reflectors i e the e layer are documented detailed waveform analysis confirms the existence of shallow tremors above the currently interpreted plate interface our results suggest that a significant portion of the tremor activity and perhaps associated shearing are taking place along well developed structures such as the e layer while fewer tremor bursts are generated elsewhere in response to the induced stress variation throughout the source volume

lithospheric structure of tasmania from a novel form of teleseismic tomography in 2001 and 2002 a temporary array of 72 seismic recorders was deployed across northern tasmania se australia with the aim of imaging the underlying crust and upper mantle using three dimensional 3 d teleseismic tomography using a recently developed adaptive stacking technique 6520 relative p wave arrival time residuals have been picked from 101 distant earthquake records spanning a 5 month period a novel iterative nonlinear tomographic procedure based on a subspace inversion scheme and the fast marching method a grid based eikonal solver is used to map the residual patterns as p wave velocity anomalies the new scheme proves to be both fast and robust making it well suited to large data sets and the reconstruction of complex anomalies the resultant tomographic images of tasmania exhibit significant lateral perturbations in p wave velocity structure 5 from a 1 d reference model a marked transition from higher velocities in the east to lower velocities in the west strongly supports the idea that eastern tasmania is underlain by dense rocks with an oceanic crustal affinity contrasting with the continentally derived siliciclastic core of western tasmania significantly the tamar fracture system does not overlie the narrow transition from relatively fast to slow velocities which suggests that it may be a near surface feature rather than a manifestation of deeper crustal scale suturing as previously thought farther west an easterly dipping zone of relatively high velocity material beneath the rocky cape group and arthur lineament may be related to remnant subduction of oceanic lithosphere associated with the mid cambrian delamerian orogeny

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

lithospheric structure beneath trans carpathian transect from precambrian platform to pannonian basin celebration 2000 seismic profile cel05 1 in 2000 a consortium of european and north american institutions completed a huge active source seismic experiment focused on central europe the central european lithospheric experiment based on refraction or celebration 2000 this experiment primarily consisted of a network of seismic refraction profiles that extended from the east european craton along and across the trans european suture zone region in poland to the bohemian massif and through the carpathians and eastern alps to the pannonian basin the longest profile cel05 1420 km is the focus of this paper the resulting two dimensional tomographic and ray tracing models show strong variations in crustal and lower lithospheric structure clear crustal thickening from the pannonian basin 24 25 km thick to the trans european suture zone region 50 km together with the configuration of the lower lithospheric reflectors suggests northward subduction of mantle underlying carpathian pannonian plate under the european plate this however conflicts with strong geological evidence for southward subduction and we present three tectonic models that are to not totally mutually exclusive to explain the lithospheric structure of the area 1 northward old subduction of the pannonian lithosphere under the east european craton in the jurassic lower cretaceous 2 a collisional zone containing a crocodile structure where carpatho pannonian upper crust is obducting over the crystalline crust of the east european craton and the carpathian pannonian mantle lithosphere is underthrusting cratonic lower crust and 3 lithosphere thinning due to the effects of neogene extension and heating with the slab associated with young subduction southward in the miocene having been either detached and or rolled back to the east in the last case the northwestward dipping in the lithosphere can be interpreted as being due to isotherms that could represent the lithosphere asthenosphere boundary in the pannonian region

bridging the gap between seismically and geodetically detected slow earthquakes 1 recently observed unusual seismic events deep low frequency earthquakes and tremor very low frequency earthquakes and slow slip events form a family of slow earthquakes that are governed by a scaling law different from ordinary earthquakes guided by this scaling law we have observed previously unknown events with source duration of 20 200 s and moment magnitude of 3 4 under the kii peninsula in western japan these events radiate seismic energy in the 2 8 hz band in direct proportion to their seismic moment rate though the constant of proportionality is 4 5 orders of magnitude smaller than for ordinary earthquakes slow earthquakes in this region may be comprised of numerous shear slip events which in aggregate are manifest seismically as tremor or isolated longer events up to 200 s and geodetically as slow slip events

upper mantle structure beneath the alpine orogen from high resolution teleseismic tomography 1 to understand the evolution of the alpine orogen knowledge of the actual structure of the lithosphere asthenosphere system is important we perform high resolution teleseismic tomography with manually picked p wave arrival times from seismograms recorded in the greater alpine region the resulting data set consists of 4199 relative p wave arrivals and 499 absolute p wave arrivals from 76 teleseismic events corrected for the contribution of the alpine crust to the travel times the three dimensional 3 d crustal model established from controlled source seismology data for that purpose represents the large scale alpine crustal structure absolute p wave arrival times are used to compute an initial reference model for the inversion tests with synthetic data document that the combination of nonlinear inversion high quality teleseismic data and usage of an a priori 3 d crustal model allows a reliable resolution of cells at 50 km 50 km 30 km hence structures as small as two cells can be resolved in the upper mantle our tomographic images illuminate the structure of the uppermost mantle to depth of 400 km along strike of the alps the inversion reveals a high velocity structure that dips toward the se beneath the adriatic microplate in the western and central alps in the eastern alps we observe a northeastward dipping feature subducting beneath the european plate we interpret this feature in the western and central alps as subducted mainly continental european lower lithosphere for the east we propose that parts of the vardar oceanic basin were subducted toward the ne forcing continental adriatic lower lithosphere to subduct northeastward beneath the european plate

low frequency continuous tremor around the moho discontinuity away from volcanoes in the southwest japan 1 recent enhancement of seismic networks in the japan islands revealed occurrence of low frequency continuous tremors of a beltlike distribution in the southwest japan where the subducting philippine sea plate reaches depths of 30 40 km source depth of the tremor is estimated by selecting tremor segments with relatively clear p wave onsets the source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust mantle wedge and the subducting slab the long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor the most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments the northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water

wave spectral energy variability in the northeast pacific 1 the dominant characteristics of wave energy variability in the eastern north pacific are described from noaa national data buoy center ndbc buoy data collected from 1981 to 2003 ten buoys at distributed locations were selected for comparison based on record duration and data continuity long period lp t 12 s intermediate period 6 t 12 s and short period t 6 s wave spectral energy components are considered separately empirical orthogonal function eof analyses of monthly wave energy anomalies reveal that all three wave energy components exhibit similar patterns of spatial variability the dominant mode represents coherent heightened or diminished wave energy along the west coast from alaska to southern california as indicated by composites of the 700 hpa height field the second eof mode reveals a distinct el nino southern oscillation enso associated spatial distribution of wave energy which occurs when the north pacific storm track is extended unusually far south or has receded to the north monthly means and principal components pcs of wave energy levels indicate that the 1997 1998 el nino winter had the highest basin wide wave energy within this record substantially higher than the 1982 1983 el nino an increasing trend in the dominant pc of lp wave energy suggests that storminess has increased in the northeast pacific since 1980 this trend is emphasized at central eastern north pacific locations patterns of storminess variability are consistent with increasing activity in the central north pacific as well as the tendency for more extreme waves in the south during el nino episodes and in the north during la nina

transalp deep crustal vibroseis and explosive seismic profiling in the eastern alps abstract the transalp consortium comprising institutions from italy austria and germany carried out deep seismic reflection measurements in the eastern alps between munich and venice in 1998 1999 and 2001 in order to complement each other in resolution and depth range the vibroseis technique was combined with simultaneous explosive source measurements additionally passive cross line recording provided three dimensional control and alternative north south sections profits were obtained by the combination of the three methods in sectors or depths where one method alone was less successful the transalp sections clearly image a thin skinned wedge of tectonic nappes at the northern alpine front zone unexpected graben or half graben structures within the european basement and thick skinned back thrusting in the southern frontal zone beneath the dolomite mountains a bi vergent structure at crustal scale is directed from the alpine axis to the external parts the tauern window obviously forms the hanging wall ramp anticline above a southward dipping deep reaching reflection pattern interpreted as a tectonic ramp along which the penninic units of the tauern window have been up thrusted the upper crystalline crust appears generally transparent the lower crust in the european domain is characterized by a 6 7 km thick laminated structure on the adriatic side the lower crust displays a much thicker or twofold reflective pattern the crustal root at about 55 km depth is shifted around 50 km to the south with respect to the main alpine crest

identifying and removing tilt noise from low frequency 0 1 hz seafloor vertical seismic data low frequency 0 1 hz vertical component seismic noise can be reduced by 25 db or more at seafloor seismic stations by subtracting the coherent signals derived from 1 horizontal seismic observations associated with tilt noise and 2 pressure measurements related to infragravity waves the reduction in effective noise levels is largest for the poorest stations sites with soft sediments high currents shallow water or a poorly leveled seismometer the importance of precise leveling is evident in our measurements low frequency background vertical seismic spectra measured on a seafloor seismometer leveled to within 1 10 4 radians 0 006 degrees are up to 20 db quieter than on a nearby seismometer leveled to within 3 10 3 radians 0 2 degrees the noise on the less precisely leveled sensor increases with decreasing frequency and is correlated with ocean tides indicating that it is caused by tilting due to seafloor currents flowing across the instrument at low frequencies this tilting generates a seismic signal by changing the gravitational attraction on the geophones as they rotate with respect to the earth s gravitational field the effect is much stronger on the horizontal components than on the vertical allowing significant reduction in vertical component noise by subtracting the coherent horizontal component noise this technique reduces the low frequency vertical noise on the less precisely leveled seismometer to below the noise level on the precisely leveled seismometer the same technique can also be used to remove background noise due to the seafloor pressure field up to 25 db noise reduction near 0 02 hz and possibly due to other parameters such as temperature variations

subduction and seismic hazard in the northern lesser antilles revision of the historical seismicity an important set of french and british archives on the historical seismicity of the northern lesser antilles is analyzed we present the most significant data which help to constrain the 8 february 1843 the 16 may 1851 and the 29 april 1897 earthquakes the 1843 event is the largest reported historical earthquake of the lesser antilles subduction arc and produced intensity ix on a segment about 100 km long between antigua and grande terre guadeloupe which contradicts the 300 km long proposed by other authors who mainly rely on robson s catalog 1964 this suggests a magnitude 7 5 to 8 0 the absence of tsunami and noticeable vertical deformation of the coasts in the source area makes very unlikely a normal fault mechanism but does not rule out the possibility of a subduction event because of the low dip angle of the interplate plane the dominance of moderate instrumental thrust earthquakes in the source area suggests a subduction mechanism for the 1843 event the northern and southern limits of the 1843 source area roughly coincide with the barracuda ridge and the tiburon ridge respectively which may have acted as barriers during the rupture process the northwestern portion of the tiburon ridge is associated with a lack of instrumental seismicity near grande terre which could indicate a possible magnitude 7 5 to 8 0 event in the next future moreover its mechanism could be normal which would provide the additional risk of a damaging tsunami the evaluation of seismic hazard must also take into account possible shallow moderate earthquakes as the 1851 and the 1897 events which produced intensities vii to viii in guadeloupe and may be related to observed major plioquaternary faults in this island

crustal structure and active tectonics in the eastern alps 1 during the last decade a series of controlled source seismic experiments brought new insight into the crustal and lithospheric structure of the eastern alps and their adjacent tectonic provinces a fragmentation of the lithosphere into three blocks europe eu adria ad and the new pannonian fragment pa was interpreted and a triple junction was inferred the goal of this study has been to relate these deep crustal structures to active tectonics we used elastic plate modeling to reconsider the moho fragmentation we interpret subduction of eu below ad and pa from north to south and underthusting of ad mantle below pa from southwest to northeast the moho fragmentation correlates well with major upper crustal structures and is supported by gravity seismic and geodetic data an analysis of crustal thickening suggests that active convergence is associated with continued thrusting and lateral extrusion in the central eastern alps and thickening of the adriatic indenter under the southern alps according to the velocity relations at the triple junction pa moves relative to eu and ad along ene and se striking faults mainly by strike slip an eastward directed extensional component is compensated by the lateral extrusion of the central eastern alps the periadriatic insubric line east of the triple junction and the mid hungarian fault zone have relatively recently lost their role as first order active structures we favor the idea that the pannonian fragment and the tisza block merged to a soft microplate surrounded by the eastern and southern alpine carpathian and dinaric orogens