post glacial sea level rise from a coral record at huon peninsula papua new guinea dating of coral reef terraces can provide a record of changes in sea level which should be pronounced during the transition between glacial and interglacial periods cores drilled from coral reefs at barbados in the equatorial west atlantic1have revealed the sea level changes that occurred during the younger dry as event at the end of the last glacial maximum 11 000 yr bp it has not been known however whether pacific coral reefs can grow at a rate sufficient to keep up with the rise in sea level during such a transition here we report results obtained from a 52 m drill core from the post glacial reef at huon peninsula papua new guinea spanning the interval from 7 000 to 11 000 14c yr bp which show that coral growth kept pace while the relative sea level rose by 50m although the tectonic environment is very different from that at barbados the two records compare well when corrections are made for local tectonic uplift showing that sea level rise was similar at both locations the rate of rise was greatest between 9 000 and 10 00014c yr bp corresponding to the time of the younger dryas

a viscoelastic coupling model for the cyclic deformation due to periodically repeated earthquakes at subduction zones vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic maxwell half space are obtained and compared with the observed deformation cycle coseismic strain release postseismic transients interseismic strain accumulation from japan the viscoelastic effects including the influence of buoyant restoring forces are obtained using the method developed by rundle and plate convergence and strain accumulation are incorporated following the procedure suggested by savage the resulting deformation cycle is compared with that of an analogous elastic half space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault cyclic deformation is similar but not identical for the two models and observations from southwest japan suggest the superiority of the viscoelastic coupling model in particular inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half space model and results in agreement with the observed interseismic movement pattern several aspects of the postseismic deformation its landward migration and its transition to the interseismic phase of the cycle are explained as well but the short duration of near trench transients relative to those observed farther inland is not matched the success of a buried slip model in explaining early postseismic near trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle elastic in the short term and ductile for longer term deformation and such a modification may reconcile remaining discordant observations however reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of japan and these regional differences remain unexplained

coastal uplift associated with the 1992 cape mendocino earthquake northern california the april 25 1992 cape mendocino earthquake ms 7 1 uplifted 24 km of the northern california coast at the southern end of the cascadia subduction zone uplift which resulted in coastal emergence that caused extensive mortality of intertidal organisms between cape men docino and punta gorda we estimated the amount of uplift by measuring the vertical extent of mortality of 14 sessile intertidal species on rocky sections of shore the uplift profile along the coast is generally parallel to the strike of the earthquake focal mechanism and forms a broad flat topped arch 24 km long with a gentle south limb and a steeper north limb the maximum uplift of 1 4 0 2 m is near the center of the profile the profile is a manifestation of the more widespread domal upwarp produced by slip on an east dipping buried thrust fault along or near the cascadia megathrust small new emergent terraces have formed where wave cut intertidal platforms have been elevated the new terraces resemble raised late holocene benches that probably record paleoearthquakes similar to the 1992 event however several of the late holocene terraces are broader and more continuous these terraces which extend several tens of kilometres north and south of the 1992 uplift suggest that some paleoearthquakes were much larger than magnitude 7

mantle wedge hydration in nicaragua from local earthquake tomography the continental margin of nicaragua and costa rica is characterized by significant lateral changes from north to south such as a decreasing dip of the slab a decreasing magma production and a shift in the volcanic front to investigate this transition a joint on and offshore local earthquake tomography was performed low p wave velocities and high vp vs ratios indicative for hydration were found in the upper mantle and lowermost crust beneath the sandino basin the mantle wedge hydration can be estimated to 2 5 wt per cent beneath south nicaragua in contrast the mantle wedge beneath north costa rica is weakly or not hydrated the hydration leads to a local gap in the seismicity in nicaragua the lateral transition between the hydrated and non hydrated areas occurs within a distance of about 10 km this transition coincides with a change in the crustal thickness in the order of 5 10 km thickening to the south and in the tectonic regimes the change in the tectonic regimes towards a stronger extension along the margin of nicaragua could be the key for understanding the observations the extension may support the opening of pathways for a wide zone of fluid migration and hydration through the overriding plate which are identified with areas of low vp high vp vs and low seismicity

kinematics associated with late cenozoic deformation in central costa rica western boundary of the panama microplate we present kinematic data for late cenozoic deformation in central costa rica that marks the western margin of the panama microplate i e the caribbean panama boundary this boundary extends from the north panama deformed belt west through the valle central in costa rica and then southwest along the east nicoya fracture zone to intersect the middle america trench terrace correlation and basin asymmetry indicate a major change in tectonic evolution across the boundary where three regional northeast striking faults intersect the pacific coast mesoscopic fault populations are consistent with transtension across these north east striking faults and with transpression within the east trending valle central this late tertiary and quaternary transcurrent deformation links the north panama deformed belt to the east with the middle america trench to the west earthquake focal mechanisms are consistent with mesoscopic fault data suggesting that fault populations characterize the present day stress field this deformation marks the western extent of the panama microplate a fragment of volcanic arc that separated from the caribbean plate in the late tertiary or early quaternary and is currently advancing northward due to collisions with south america to the east and the indenting cocos ridge on the cocos plate to the south

the earthquake deformation cycle at the nankai trough southwest japan geodetic measurements made landward of the nankai trough site of great subduction zone thrust earthquakes in 1854 and 1946 provide a uniquely detailed picture of the strain buildup process supply constraints on the mechanism of strain accumulation and allow for improved estimates of earthquake recurrence provided the two most recent movement cycles are similar the observations dating from about 1890 may be used to reconstruct a single complete deformation cycle coseismic strain release postseismic transients interseismic strain accumulation very complete leveling and tidal gage data indicate that postseismic deformation extends more than 300 km inland from the plate boundary persists for at least 30 years and shows a clear tendency to become longer wavelength with increasing time the transient movements have two timescales the first of about a year or less corresponds to deformation largely uplift concentrated close to the coseismic fault and is most easily explained by aseismic slip or very localized deformation downdip of the earthquake rupture plane the second longer timescale is associated with a diffusion like spread of the deformation further landward an effect qualitatively similar to that first predicted by elsasser to be an expected consequence of faulting in an elastic plate overlying a viscoelastic asthenosphere cumulative uplift since 1890 correlates well with the distribution of uplifted marine terraces although average post 1890 tilt rates exceed late quaternary and holocene averages by at least a factor of three because of the nonlinearity of strain buildup and the significant permanent deformation simple recurrence calculations typically overestimate the true interval between great earthquakes by a factor of 2 to a factor of 3 strict application of the time predictable model assumed correct overcomes these difficulties provided the cumulative transient deformation and the proportion of permanent deformation per cycle can be estimated

regional variations in tectonic geomorphology along a segmented convergent plate boundary pacific coast of costa rica abstract pacific coastal mountain piedmont landforms of costa rica extend across the tectonic boundary between the forearc and magnetic arc region of an active convergent margin this plate boundary became segmented circa 1 million years ago when the aseismic cocos ridge impinged upon the middle america trench offshore from the southernmost coastal area of costa rica morphometric analyses of 100 mountain fronts and numerous river long profiles radiometric dating and field studies were conducted in two study areas located arcward from the plate boundary where oceanic lithosphere of the cocos plate is being subducted beneath the caribbean plate region i and the partially subducted aseismic ridge is uplifting the plate margin by isostatic and collisional processes region ii values of tectonic geomorphic parameters mountain front sinuosity s percent dissected facets ffd river concavity k are not only different statistically in regions i and ii but are also different in the areas experiencing isostatic and collisional responses to the subducting aseismic ridge in the area experiencing collisional responses mountain fronts developed along ne dipping imbricate thrust and high angle reverse faults step upward and inland from the coast morphometric data along with the divergence of river terrace profiles from the coast piedmont inland toward the mountains indicates higher uplift rates along interior range mountain fronts isostatic uplift in the outer forearc area in region ii produces a distinctly different morphologic and neotectonic style characterized by regional uplift distributed across a number of blocks bounded by normal faults geomorphic analyses indicate a general southward trend of increasing tectonic uplift from region i into region ii where the highest frequency of mountain fronts with low values of s and ffd as well as rivers with the highest values of k occur over the crest of the subducted ridge field and historical seismic data for these regional trends include 1 fault scarps displacing late quaternary fluvial terraces and colluvial soils in areas of collisional responses 2 holocene latest pleistocene marine sediments uplifted tens of meters along normal faults in areas of isostatic response of region ii and 3 more frequent shallow high magnitude earthquakes in region ii this study indicates that spatial variations in the plate tectonic framework can be detected by regional morphometric analyses using techniques applied in extensional and compressional terranes of arid and semiarid regions but not previously applied to forearc systems along convergent plate boundaries in tropical areas

late pleistocene crustal uplift and gravity anomaly in the eastern part of kyushu japan and its geophysical implications the miyazaki plain eastern part of kyushu japan is characterized by both significant negative gravity anomalies and aseismic crustal uplifting 1 mm year in the late pleistocene and holocene we examine the relationship between these two phenomena which may provide important constraints on the interaction between the collision and or subduction of the kyushu palau ridge and the forearc we estimate the mass deficiency below 11 km depth by using the gravity anomalies and p wave velocity structure of the upper crust the onset of the load accumulation 0 5 0 4 ma is inferred from the movement of the fluvial terraces considering the tephrochronology the loading history is assumed to be a linear function of time we evaluate the crustal rebound by assuming a viscoelastic plate deformation with an underplating load existing at 20 or 30 km depth the predicted crustal movement for models with a lithospheric crustal viscosity of 1023 1024 pa s can explain the observed altitudes of the shoreline of the marine terraces formed at the last interglacial of about 125 kyr bp and the middle holocene of 5 6 kyr bp although we cannot restrict the origin of the buoyant body the subduction of the kyushu palau ridge remnant arc associated with back arc opening of the shikoku basin may be related to the buoyancy for the uplifting region examined here on the other hand the buoyant body off the miyazaki plain probably plays an important role in the interaction between the subducting oceanic slab and the overriding forearc crust thus the observed lateral variation of the interplate coupling on the convergent boundary along the nankai trough may be attributed to the existence of the buoyant body

large and primarily updip afterslip following the 2012 mw 7 6 nicoya costa rica earthquake we present detailed surface measurements of the first 3 5 years of postseismic deformation following the 5 september 2012 moment magnitude mw 7 6 nicoya costa rica earthquake the dominant signal in the first 2 5 years is uniform horizontal trenchward motion totaling 7 26 cm across 40 stations trenchward velocity is strongly diminished by mid 2014 and appears by 2016 to have begun reversing we invert the first 2 5 years to determine the corresponding afterslip on a detailed 3d interface results show significant afterslip both up and down dip of the main coseismic rupture zone with as much as 1 7 m of offset in two patches at 15 20 km depth and immediately up dip of the maximum coseismic slip this updip slip represents an important mechanism to address unrelieved interseismic locking although sufficient strain energy remains to generate up to a mw 7 1 event near the coastline the afterslip patches are anticorrelated with strongly clustered aftershocks at the same depth which is indicative of varying frictional behavior along strike an additional patch of slip is colocated with reoccurring slow slip events beneath the gulf of nicoya the magnitude of the observed slip however cannot be sufficiently explained by the known slow slip events ongoing measurements will be crucial to understanding the relocking process in nicoya

forearc response to subduction of the cocos ridge panama costa rica stratigraphic paleontologic and structural data from two panama costa rica forearc peninsulas located landward of the aseismic cocos ridge along the southernmost middle america trench document rapid pliocene subsidence and basin infilling followed by quaternary deformation and uplift on the burica peninsula an approximately 3 000 m thick pliocene pleistocene clastic sedimentary sequence characterized by fine grained turbidite deposits and volumetrically minor coarser grained channel fill slump and debris flow deposits bears depth diagnostic foraminifera that document rapid pliocene subsidence and deposition on a south dipping paleoslope new paleobathymetric and age estimates from foraminifera in these rocks indicate shallowing of depositional paleodepths from 2 000 to 1 200 m during the late pliocene present day exposure of these rocks indicates an average quaternary uplift rate of 1 mm yr the turbidite section on the burica peninsula is interpreted to represent a trench slope deposit and is correlated to a similar marine sedimentary section on the adjacent osa peninsula of costa rica structures developed within pliocene pleistocene strata of the outer forearc burica and osa peninsulas record minor 5 subhorizontal margin perpendicular shortening and margin perpendicular extension bedding orientations on the osa and burica peninsulas generally record arcward tilting by 10 30 of these outer forearc blocks on the burica peninsula the medial fault zone a north striking high angle right lateral strike slip fault has truncated and reoriented the pliocene pleistocene sedimentary sequence by fault bend forced folding a linear margin parallel fault herein named the ballena celmira fault zone bounds the arcward side of the osa and burica peninsulas and has accommodated vertical displacements between the outer forearc burica osa area and inner forearc terraba area throughout the tertiary structural and seismicity observations from the forearc region suggest that neogene deformation of the leading edge of the arc in this area reflects the effects of isostatic uplift and possibly increased interplate shear stresses as a result of subduction of the cocos ridge beneath this part of the arc since 1 m y ago

the seismic source parameters of the 1991 costa rica aftershock sequence evidence for a transcurrent plate boundary the april 22 1991 valle de la estrella costa rica earthquake ms 7 6 was a back arc thrusting event associated with the underthrusting of the caribbean plate beneath central america a network of three passcal type portable instruments was deployed to monitor the aftershock activity in southern costa rica 2 to 6 weeks after the main shock the waveforms recorded on three component midperiod seismometers were used to recover source information for 15 small aftershocks magnitudes between 3 2 and 4 4 with a linear moment tensor inversion method we conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery the longer period waveforms in general are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data the earlier part of the seismic wave forms has proven to be the most important carrier of the source information a gross crustal model can be used to describe the structure for the source study the small changes in the waveform character resulting from the mislocation of the events or inexact green s functions generated from the oversimplified crustal model do not prohibit us from the recovery of the source orientation at local distances in contrast the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information our focal mechanisms are generally in good agreement with p wave first motion fault plane solutions determined from a local short period network the aftershocks show a clear spatial segmentation based on focal mechanism type most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock in contrast a cluster of aftershocks northwest of the main shock showed dominantly left lateral strike slip motion on a northeasterly striking nodal plane this suggests that a diffuse deformation zone exists in central costa rica and is characterized by left lateral strike slip motion this diffuse transcurrent deformation zone coincides with several geologic and geophysical features and perhaps is a result of the slower subduction rate of the buoyant cocos ridge than its adjacent segments along the middle america trench mat the diffuse transcurrent boundary may intersect with the north panama deformed belt npdb near limon costa rica and is very likely a plate boundary for the proposed panama block

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

three dimensional seismic imaging of the costa rica accretionary prism structural diversity in a small volume of the lower slope conventional two dimensional seismic reflection investigations have been generally relied upon to provide images of large to medium scale structural features in accretionary prisms we undertook a three dimensional seismic reflection survey of a small part of a prism arcward of the middle america trench off costa rica to more correctly image structure and to use the improved structural information to examine the processes of accretion this survey reveals small features with dimensions of hundreds of meters while also defining features thousands of meters in lateral extent both of which were underappreciated in conventional two dimensional data from the same area we have imaged active off scraping at the trench and both duplexing and out of sequence faulting a few kilometers arcward of the trench fault spacing and reflector geometry vary dramatically over a space of several hundred meters some of these variations are related to visible changes in morphology of the underlying oceanic basement but others are not so easily documented fault surface reflections define an architecture which may control gross fluid motion through the prism this architecture is apparently formed by duplexing and out of sequence faulting and has been maintained by periodic motion on some of the out of sequence faults the slope sediment apron records multiple phases of deformation abundant small offset reverse faults break the seafloor and indicate recent shortening of a broad region of the underlying prism a primary result of this survey is appreciation of the structural diversity across a small width of an accretionary prism

sediment accretion subduction and dewatering at the base of the trench slope off costa rica a seismic reflection view of the decollement sea beam bathymetric and water gun seismic reflection data off the west coast of costa rica image the base of the trench slope its underlying structure and the behavior of a decollement surface in the presence of relief on the subducting plate a low angle landward dipping reflection marks the master decollement above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope a small horst block at least 9 km long 1 5 km wide and 300 m high is being obliquely subducted beneath the landward trench slope on five seismic lines the decollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level the undulating geometry of the decollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front the dewatering may favor development of bedding parallel zones of low shear strength within the subducting sediments and the master decollement may seek one of these zones

anthropogenic versus climatic control in a high resolution 1500 year chironomid stratigraphy from a southwestern greenland lake we performed a high resolution study of chironomid assemblages in a sediment core retrieved from lake igaliku in southern greenland the well dated core is located within the former norse eastern settlement and covered the last 1500 yr the comparison of chironomid stratigraphy pca axis scores with instrumental temperature data land use history and organic matter in the sediment over the last 140 yr suggested that the primary changes in chironomid fauna in 1988 2 yr were driven by the shift to modern agriculture in the catchment this unprecedented change in chironomid fauna was most likely triggered by a shift in in lake processes within the instrumental period subtle variations in the chironomid assemblages that occurred before 1988 2 yr were significantly correlated with summer temperatures even in times of traditional extensive sheep farming in the catchment the relevance of the chironomid derived climate signal over the last 1500 yr was supported by its good concordance with previous studies in west greenland and in the arctic the chironomid assemblage therefore appeared to be a valuable proxy for climate changes within the norse colony area synchronous changes in norse diet and chironomid reconstructed climate give new insights into the interplay of norse society with climate

cassini observes the active south pole of enceladus cassini has identified a geologically active province at the south pole of saturn s moon enceladus in images acquired by the imaging science subsystem iss this region is circumscribed by a chain of folded ridges and troughs at 55 s latitude the terrain southward of this boundary is distinguished by its albedo and color contrasts elevated temperatures extreme geologic youth and narrow tectonic rifts that exhibit coarse grained ice and coincide with the hottest temperatures measured in the region jets of fine icy particles that supply saturn s e ring emanate from this province carried aloft by water vapor probably venting from subsurface reservoirs of liquid water the shape of enceladus suggests a possible intense heating epoch in the past by capture into a 1 4 secondary spin orbit resonance

the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake and its tectonic implications a broadband seismic study the rapture process of a large back arc thrusting earthquake the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake is investigated using broadband body waves and long period surface waves we find the source process to be relatively simple with the source models separately obtained from body and surface waves being very consistent the event occurred on a shallow southwest dipping rapture plane on which most energy is released updip of the hypocentral location 10 20 km deep high frequency radiation appears to have been released over a relatively small source area our preferred model has a focal mechanism with strike 102 10 dip 17 14 and rake 63 17 a seismic moment of 3 8 1 5 1020 n m and a total rupture duration of 40 6 s the earthquake appears to be associated with the north panama deformed belt npdb a thrust and fold complex that has accommodated the oroclinal deformation of panama this event along with previous large events north of panama in 1882 and 1916 indicates that there is substantial convergence along the npdb marking the npdb as a probable emerging plate boundary it remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

concerning the dissipation of electrically charged objects in the shadowed lunar polar regions 1 nasa recently suggested the construction of a lunar outpost at the south pole near the rim of shackleton crater while there are a number of advantages to such a base the region will have periods of time when there is limited or no solar illumination thereby reducing photoelectric and solar wind plasma currents compared to most of the lunar dayside as a consequence of this reduction in environmental currents we find that human systems charged by contact electrification with the regolith e g roving excavation will have increased difficulty in removing accumulated electric charge this situation is especially true within the cold shadowed regions adjacent to the terminator such as within shackleton crater itself where there are essentially no photoelectric currents vastly reduced plasma currents due to the local wake and a highly reduced regolith conductivity in essence there is no pathway for accumulated charge to leak away or dissipate thereby creating an electrostatic hazard calculated dissipation timescales are found to be 1 millisecond in the weakly sunlit terminator region and dayside but could approach 100 s of seconds in the current starved shadowed regions

modification of the plasma in the near vicinity of enceladus by the enveloping dust 1 the plasma near saturn s equator is quasi corotating but those fluid elements entering the near vicinity of the moon enceladus become uniquely modified besides the solid body the moon has a surrounding dust envelop that we show herein to be detected 20 enceladus radii 1 re 252 km both north and south of the body previous reports indicate that corotating plasma slows down substantially in the near vicinity of enceladus we show herein that the commencement of this plasma slow down matches closely with cassini s entry into the dense portions of the enveloping dust in the northern hemisphere above the moon we also examine in detail the source of the dust about 400 km above the south polar fissures we find that a large positive potential must exist between the south pole of the moon and the spacecraft to account for ions streaming away from the pole on connecting magnetic field lines

analysis of partially emerged corals and reef terraces in the central vanuatu arc comparison of contemporary coseismic and nonseismic with quaternary vertical movements in the central vanuatu arc living and recently deceased reef corals act as natural tide gauges which have allowed us to map vertical tectonic deformation patterns as corals grow the density of the aragonite coral skeletons varies on an annual cycle producing annual growth bands similar to tree rings using coral growth bands we can determine the year coral surfaces died due to emergence we interpret four major coral emergence events as coseismic uplifts that occurred near the epicenters and times of large shallow earthquakes on january 5 1946 ms 7 3 august 11 1965 ms 7 5 october 27 1971 ms 7 1 and december 29 1973 ms 7 5 the 1965 and 1973 events caused maximum uplifts of 120 and 60 cm respectively in the frontal arc also related to these events are uplifts of 10 cm and 6 cm in the back arc on pentecost and maewo islands which lie east of the volcanic chain and the primary forearc zones of uplift and subsidence similar secondary zones of uplift occurred with the great 1960 chile and 1964 alaska earthquakes the amplitude of these secondary uplifts is significantly larger than that predicted by models having a single fault in an elastic half space however the amount of secondary uplift is comparable to that predicted if the fault occurs in a plate of constant thickness overlying a viscoelastic half space at various places in 1957 1969 1970 1977 and 1978 1981 there was about 5 10 cm of emergence not associated with major earthquakes which may indicate nonseismic tectonic uplift however oceanographically lowered sea levels as in el ninos may have determined the times when corals died and recorded these events nevertheless the accumulation of emergence its persistence the limited geographic extent of each event and occurrence in areas of rapid holocene uplift suggest that the causes of the uplifts are tectonic these events suggest that in some areas a third or more of the total accumulated uplift in central vanuatu takes place as aseismic motion however in some areas we find only coseismic emergence in central vanuatu contemporary coseismic vertical deformation holocene uplift and topography have remarkably similar patterns this suggests that the mechanisms and processes causing vertical deformation have varied little over the last 106years apparently the topography structure and seismotectonics are controlled by the subduction of the d entrecasteaux ridge a major bathymetric feature underthrusting this part of the arc the influence of this ridge may have been especially extensive because it migrates very slowly along the arc trend and thus it interacts for a long time with a single portion of the arc system our previous studies of reef terraces indicated the existence of at least four seismotectonic arc segments or blocks along the santo malekula interval of the arc and our present results further support this conclusion each block has uplifted at different times by different amounts at different rates and tilted in a different direction boundaries between the north santo and the south santo segment and between the north malekula and the south malekula segment correlate with the north and south flanks of the d entrecasteaux ridge as does the absence of a physiographic trench west of santo

quaternary uplift astride the aseismic cocos ridge pacific coast costa rica the pacific coast of costa rica lies within the central american forearc and magmatic arc region that was created by northeastward subduction of the cocos plate beneath the caribbean plate at the middle america trench from the peninsula de nicoya south eastward toward the peninsula de osa and the peninsula de burica on the panamanian border the middle america trench loses its physiographic expression where it intersects the aseismic cocos ridge interaction between subduction of the buoyant aseismic cocos ridge and the overriding caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from nicoya to burica the pliocene and pleistocene stratigraphic record and holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the peninsula de osa immediately landward of the aseismic cocos ridge crustal uplift rates decrease northwest toward the peninsula de nicoya and to a lesser extent southwest toward the peninsula de burica the late quaternary stratigraphy on the peninsula de osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates crustal uplift rates calculated from these sequences systematically decrease from 6 5 to 2 1 m ka north east across the peninsula deformation of the peninsula is modeled as uplifted and down to the northeast tilted fault blocks with an angular rotation rate of 0 03 to 0 06 per thousand years although less well constrained crustal uplift rates on the peninsula de nicoya 200 km to the northwest of the peninsula de osa vary from 1 m ka for pliocene and pleistocene sediments to 2 5 m ka for holocene marine terraces in the quepos region 100 km to the northwest of the peninsula de osa calculated uplift rates derived from incision of late quaternary fluvial terraces range from 0 5 to 3 0 m ka on the peninsula de burica only 60 km to the southwest of the peninsula de osa calculated uplift rates range from 4 7 m ka for a late holocene marine terrace to 1 2 m ka for post late pliocene deep sea sediments the variations in calculated uplift rates on the peninsula de osa constrain a dynamic model for subduction of the cocos ridge and the resulting uplift of the overriding caribbean plate deflection of the caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted cocos ridge because the shape of the subducted end of the cocos ridge is unknown two scenarios are evaluated 1 a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and 2 a ridge where the subducted end was truncated by the panama fracture zone the best fit model utilizes a truncated ridge that has been subducted during the past 0 5 m y 50 km beneath the overriding caribbean plate which has an effective elastic thickness of 5 km the model predicts that the highest uplift rate should be 3 7 m ka and occur on the southwest coast of the peninsula de osa the rate of uplift slows considerably to the northeast and indicates that the peninsula de osa is tilting to the northeast which agrees with observations in that region the predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the peninsula de osa resulting in little uplift that can be attributed to cocos ridge subduction in the northwestern portions of the peninsula de nicoya

comments on the growth of accretionary wedges uplift within accretionary foreland wedges has been explained by the development of duplexes which provides a mechanism for underplating in submarine accretionary wedges with little deformation in the upper part of the wedge we interpret duplex development beneath the costa rica forearc a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge large scale structural geometry of accretionary wedges is compatible with low angle trenchward dipping backstops in centrist to reverse angle backstops used commonly in models of return flow within wedges which have been invoked to explain the presence of high p t metamorphic rocks we suggest that emplacement of high p t metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges

quaternary uplift and seismic cycle deformation peninsula de nicoya costa rica differing rates and styles of quaternary deformation along the costa rican fore arc reflect segmentation of the trench corresponding with three contrasting domains of subducting sea floor offshore rapid upward flexure of the southern fore arc segment results from the subduction of the buoyant cocos ridge whereas moderate deformation along the central fore arc segment reflects the subduction of buoyant seamounts in contrast the northern costa rican fore arc deforms in response to the subduction of relatively dense sea floor devoid of major bathymetric anomalies quaternary geomorphic evidence and earthquake oral histories from the peninsula de nicoya within the northern costa rican fore arc demonstrate arcward tilting of the fore arc crust with discrete uplift events occurring during large subduction earthquakes uplift rates calculated from the late holocene cabuya terrace along the peninsula s trench perpendicular southeastern coast decrease systematically toward the arc from 4 5 m k y at cabuya to 1 7 m k y at montezuma 8 km to the northeast an uplifted carbonate beachrock horizon radiocarbon age 4500 5200 yr b p correlated between cabuya and montezuma is tilted 0 1 downward toward the arc although quaternary uplift is evident as far arcward as tambor 10 km northeast of montezuma uplifted terraces are absent between tambor and the golfo de nicoya a submerged archaeological site radiocarbon age 2500 yr b p located along the golfo de nicoya coast 30 km northeast of montezuma demonstrates late holocene subsidence of 0 5 m k y these data indicate net late holocene arcward tilting of the peninsula at an angular rotation rate of between 0 01 and 0 02 k y oral histories describing the m 7 7 nicoya subduction earthquake of 5 october 1950 provide evidence of seismic cycle deformation along the peninsula s southwestern coast between puerto carrillo and nosara interviews with 48 residents show that coseismic uplift of at least 1 m affected this coastline and that a significant fraction of this uplift has subsequently been reversed during four decades of gradual subsidence the coseismic deformation pattern estimated from a uniform slip dislocation model for the 1950 earthquake is consistent with both geomorphic and oral history evidence these observations suggest that seismic cycle deformation functions as an important mechanism of vertical tectonism within the costa rican fore arc arcward rotation and doming of the peninsula de nicoya during the quaternary may reflect repeated cycles of sudden coseismic deformation followed by gradual postseismic and or interseismic crustal movement

the nature and distribution of bottom simulating reflectors at the costa rican convergent margin summary bottom simulating reflectors bsrs at the base of the hydrate stability zone are often observed in marine seismic reflection data the compressional p wave velocity structure of a bsr offshore costa rica was analysed by applying a full waveform inversion technique the resulting velocity profile indicates that the bsr is caused by a thin low velocity layer suggesting the presence of at least a small amount of free gas in the sediment pore space in undisturbed sediment sections bsrs offshore costa rica are observed over much of the continental margin they frequently occur at shallow depths beneath the seafloor usually 100 400 m at some locations bsrs appear to intersect the seafloor this was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards in the study area the vertical movement of the hydrate stability zone relative to a given point in the sediment column one of the potential factors leading to bsr formation or suppression is controlled by both sedimentation and vertical tectonism both processes may hence play a role in controlling bsr distribution bsrs are absent in areas which have been affected by slumping except where the sedimentary section above the bsr remained intact during slumping this indicates that slope failure can cause the destruction of bsrs

the influence of soil age and regional climate on clay mineralogy and cation exchange capacity of moist tropical soils a case study from late quaternary chronosequences in costa rica abstract tropical soils exhibit large differences across landscapes in many attributes notably clay mineralogy 2 1 vs 1 1 clays bulk geochemistry ca al ph cation exchange capacity cec organic matter and soil texture in order to examine the factors controlling these variables a series of three chronosequences 120 ka of tropical soils on uplifted terraces along the pacific coast of costa rica were studied the three study locations differ mainly as a function of soil moisture and leaching with conditions ranging from 2700 mm yr mean annual precipitation map on the southern nicoya peninsula 4 month dry season to 3200 mm yr in the esterillos region 3 month dry season and 4250 mm yr on the osa peninsula no dry season analytical methods include x ray diffraction xrd transmission analytical electron microscopy tem aem inductively coupled plasma optical emission and mass spectrometry icp oes icp ms ammonium acetate extraction 1 m nh4oac ph 7 for cec and c n analysis soil weathering reactions and related decrease in cec occur three to four times faster in a sequence of inceptisol to oxisol soils in the wettest climate osa compared to the less moist esterillos area in the even drier monsoon climate soils nicoya this evolved low cec the age related evolution of tropical soils appears to be a predictable sequence in lowland tropical landscapes where periodic tectonism erosion or volcanism produces unweathered parent material at the land surface empirical data from this project enables the extrapolation of simple equations applicable to tropical volcanic arc landscapes where presence of uplifted marine terrace soils facilitates determination of soil age the two variables controlling soil composition can be combined into an effective age ageeff that takes into account soil age and weathering intensity factoring in map and wet dry months from climate data and equations are of the form e g ecec 18 1 ln ageeff 77 with r2 values of 0 75 to 0 87 greatest scatter occurs in the youngest soils given the apparent prevalence of this sequence and the systematic nature of its reaction progression these results could be useful for modeling tropical soils

far field triggering of foreshocks near the nucleation zone of the 5 september 2012 m w 7 6 nicoya peninsula costa rica earthquake abstract on 5 september 2012 a moment magnitude mw 7 6 earthquake occurred directly beneath the nicoya peninsula an area with dense seismic and geodetic network coverage the mainshock ruptured a portion of a previously identified locked patch that was recognized due to a decade long effort to delineate the megathrust seismic and aseismic processes in this area here we conduct a comprehensive study of the seismicity prior to this event utilizing a matched filter analysis that allows us to decrease the magnitude of catalog completeness by 1 unit we observe a statistically significant increase in seismicity rate below the nicoya peninsula following the 27 august 2012 mw 7 3 el salvador earthquake about 450 km to the northwest and 9 days prior to the nicoya earthquake additionally we identify a cluster of small magnitude

construction of a polynomial model of glacio eustatic fluctuation estimating paleo sea levels continuously through time sea level curves for the late quaternary are typically reconstructed with data from reef tracts and terraces preserved along actively uplifting coastlines recording highstands and in some cases lowstands of the world9s oceans through time as a result sea level curves reconstructed from these data are fixed at the maxima highstands and minima lowstands but either make no predictions about the intervening time or use arbitrary or free hand methods of interpolation these curves fail to meet the need of many applications of sea level data for continuous quantitative information a new method for calculating sea levels during intervening times produces a polynomial expression of sea level through time lagrange polynomial interpolation is used to fit a series of fourth order equations to existing data of late quaternary sea level fluctuations producing a systematic and realistic model of continuous sea level elevations with time the method is germane to any and all models of sea level based on discrete data points here the model advanced by bloom et a1 is reexamined and a new polynomial model which estimates eustatic sea levels continuously in the range 0 140 ka is developed from the huon data the method includes an error analysis procedure that accounts for the effects of error in radiometric dates and theodolite measurements in the original data set and extrapolates this error through the continuous model polynomial form and error estimation are indispensible to rigorous quantitative applications of sea level data and supplant less rigorous methods used to approximate sea level and rates of sea level rise

morphotectonic segmentation of an active forearc 37 41 s chile abstract active forearc regions are characterized by seismo and morphotectonic segments that record recurrent seismic activity and subduction related tectonic deformation on different timescales however it is not well known on which timescales morphotectonic forearc segments retain their tectono geomorphic identity how they are related to earthquake rupture zones and what governs their long term evolution the forearc of the active convergent margin of south central chile 37 41 s is located within the rupture zone of the 1960 chile mw 9 6 megathrust earthquake in this region we combine geomorphological sedimentologic and morphometric dem analysis with compiled structural and geophysical data to reconstruct the morphotectonic evolution of the forearc the southern chile forearc comprises three segments nahuelbuta tolten and bueno segments that record a differential structural and geomorphic evolution whereas the tolten segment appears to be quasi stable the other two segments record pronounced ongoing quaternary uplift the nahuelbuta and bueno segments contain the n s oriented coastal cordillera characterized by numerous tectonically controlled drainage anomalies and evidence of sediment rerouting the morphotectonic segmentation appears to have persisted over timescales of 105 to 106 years importantly these segments do not correlate with earthquake rupture zones that exhibit a short term variability over 102 to 103 years thus earthquake rupture zones are transient second order phenomena which do not determine the long term morphotectonic forearc segmentation overall the differential forearc evolution and segmentation is influenced by a complex interplay of inherited upper plate structures and physical properties of the subducting oceanic nazca plate

postseismic coastal uplift in southern chile tide gage records at puerto montt referenced to a mareograph in talcahuano indicate a large at least 75 cm postseismic uplift of the region following the 1960 mw 9 5 event field observations carried out in 1989 at the same locations of previous measurements in 1968 are consistent with tide gage records the postseismic elevation changes are modeled as the result of propagating creep on the downdip extension of the coseismic fault for a 30 e fault dip minimum square error indicates a fault creep velocity of 4 6 km yr and slip amplitude of 3 5 m