seismicity of the forearc marginal wedge accretionary prism three different types of seismic data have been examined for seismic events occurring within the zone called the accreted wedge or forearc marginal wedge that underlies the inner trench wall of some arcs these types of data are 1 teleseismically recorded earthquakes that have been reported in the literature as occurring in major arc trench regions these events fail to demonstrate that earthquakes occur within the accreted wedge because the uncertainty of focal depth usually exceeds the depth dimension of the accreted wedge these data include many tsunamigenic earthquakes 2 local earthquakes located by combined ocean bottom seismograph and land networks in the arc trench region in the new hebrides and the central and eastern aleutian trench none of the more reliable of these hypocenters lies within the accreted wedge 3 s p intervals measured at stations on islands located on the outer ridge or at ocean bottom seismograph stations on the forearc marginal wedge these data do not show the existence of events occurring within the accreted wedge e g from 18 ocean bottom seismograph stations with a cumulative operation time of about 1 year the smallest s p time is about 2 5 s for events in the new hebrides and about 4 s for events in the adak and kodiak regions we found no s p time smaller than 2 s from 6 years of seismograms recorded at middleton island alaska and no s p time smaller than 4 s from 25 years of seismograms recorded on barbados all of the events could have occurred outside the forearc marginal wedge although we have located no events with hypocenters which are situated unambiguously within the accreted wedge we have located many events which occurred in the vicinity of the accreted wedge some occur in the upper portion of the benioff zone shallower than about 40 km and others occur in the leading edge of the overriding plate these locations reported in this paper which are within or close to the seismic networks comprise some of the most reliably located shallow events ever reported in convergent margins of island arcs the absence of seismic activity within the accreted wedge in any island arc that we studied suggests that the deformation evident in this region occurs aseismically

hypocenter determination in strongly heterogeneous earth models using the shortest path method the shortest path method moser 1991 for the calculation of seismic ray paths and travel times along them can be applied directly in the hypocenter location method proposed by tarantola and valette 1982 it uses the analogy between seismic rays in the earth and shortest paths in networks to construct first arrival times from one point to all other points of a three dimensional grid simultaneously in a fast robust way in earth models of arbitrary complexity doing this for all stations of a seismic array one can find the hypocenter location by minimizing the difference between the observed and the calculated travel times at the stations over the three dimensional grid the concept of probability density functions allows then for a fully nonlinear examination of the uncertainties in the hypocenter location due to uncertainties in the travel time data numerical errors in the calculated travel times and to a limited extent incomplete knowledge about the earth model the result is a three dimensional contour map of regions of equal confidence for the earthquake location the method becomes especially attractive when more than one event recorded by the same array is studied because the calculation of the travel times which is relatively the most time consuming operation has to be done only once the method is applied on the location of an event that occurred on january 18 1989 in israel

control of seafloor roughness on earthquake rupture behavior earthquake rupture complexity is described for three recent large underthrusting earthquakes along the costa rican subduction zone the 1983 osa 1990 nicoya gulf and 1999 quepos events these earthquakes occurred in regions characterized by distinctly different morphologic features on the subducting plate the 1990 and 1999 events occurred along linear projections of subducting seamount chains and had fairly simple earthquake rupture histories both events are interpreted as failure of the basal contact of closely spaced isolated seamounts acting as asperities in contrast the 1983 event occurred along the subducting cocos ridge and had a complex rupture history comparison of rupture characteristics of these large underthrusting earthquakes with size and location of subducting features provides evidence that seamounts can be subducted to seismogenic depths and that variations in seafloor bathymetry of the subducting plate strongly influence the earthquake rupture process

margin architecture and seismic attenuation in the central costa rican forearc seismic attenuation across the central costa rican margin wedge is determined fromamplitude analysis ofwideangle seismic data travel time and amplitude modeling are applied to ocean bottom hydrophones along two trench parallel profiles located 30 km p21 and 35 km p18 landward of the deformation front northeast of quepos plateau tomographic inversion images a progressively thinning margin wedge from the coast to the lower slope at the trench a 1 1 5 km thick decollement zone with seismic velocities of 3 5 4 5 km s is sandwiched between the marginwedge and the subducting cocos plate for strike line p21 amplitude modeling indicates a qp value of 50 150 for the upper margin wedge with seismic velocities ranging from 3 9 km s to 4 9 km s along strike line p18 qp values of 50 150 are determined with velocities of 4 3 5 0 km s in the upper margin wedge increasing to 5 1 5 4 km s in the lower margin wedge quantitative amplitude decay curves support the observed upper plate qp values in conjunction with earlier results from offshore nicoya peninsula our study documents landward decreasing attenuation across the margin wedge consistent with a change in lithology from the sediment dominated frontal prism to the igneous composition of the forearc middle prism

central japan triple junction a three dimensional compression model abstract we present a new interpretation of the tectonics of the trench trench trench triple junction off central japan using more specifically data obtained during the french japanese kaiko cruise in 1984 data analysis and consideration on the three dimensional geometry of the junction and its kinematics lead to the conclusion that the whole area undergoes large scale compressional intraplate deformation this is caused by the subduction of the philippine sea plate in the restricted space between the pacific and northern honshu plates we then apply the same analysis to the whole central japan area for which we consider the role of the interaction of the three plates at depth as a basis for tectonic reconstructions since the miocene

a seismicity gap beneath the inner wall of the japan trench as derived by ocean bottom seismograph measurement abstract an ocean bottom seismograph obs array covered the area of the japan trench about 40 n off the tohoku district of northeastern honshu japan the obs array was operated for two weeks in june 1981 arrival time data of 8 obss were used to study a spatial distribution of microearthquakes around the trench during the observation period 400 500 events with a signal to noise ratio greater than 2 were detected by each obs the arrival times of 223 of these events could be picked 38 earthquakes were located in the area of the japan trench within the latitude range of 38 5 41 0 n and the longitude range of 142 0 146 0 e and about a hundred events were located beneath or near honshu island s p time distribution recorded by each obs showed seismic activities to exist on both sides of the trench axis while the seismic level was low beneath the inner wall of the trench epicentral distribution showed a seismicity gap beneath the inner wall of the trench the area of the landward activity continued eastward up to the position where the water depth is about 4000 m seismic activity existed beneath the trench axis and the seaward wall of the trench the seismicity gap suggested from the 1980 obs observations in the vicinity of the present study area was clearly confirmed by the present observations the obs array enabled us to determine the focal depth of microearthquake activity with better resolution than was possible from land observations the obs data showed that the landward activity beneath the continental slope was concentrated in the very shallow part of the lithosphere at a depth less than 30 km foci started to deepen westward with a very low dip angle of less than 10 which was the most shallow part of the upper plane of the double seismic zone the focal depth distribution showed that events just beneath the trench axis occurred to depths slightly greater than 50 km while the foci of the seaward activity were concentrated in the uppermost lithosphere at depths less than 30 km seismic cross sections clearly show that the activity beneath the trench axis and seaward wall of the trench is separated from the landward activity which is a shallow extension of the double seismic zone by the seismicity gap

three dimensional seismic imaging of the costa rica accretionary prism field program and migration examples seismic reflection techniques are the primary geophysical tool used to examine the structure of continental margins on convergent plate margins widely separated seismic reflection profiles often do not image complex structural features because out of plane reflections and diffractions obscure the seismic images yet the structural features are often critical to understanding the tectonic processes thus the resulting geologic interpretations are based on images that may not accurately represent the true subsurface structure to image the complex geologic structures of an active continental margin a three dimensional 3 d seismic survey was conducted off of the nicoya peninsula of costa rica in april 1987 over 60 000 shot points of 96 trace multichannel data were collected in a 9 22 km grid this detailed survey was located over the accretionary prism and designed to investigate the active tectonic processes and evolution of this continental margin in this paper we outline the data acquisition program including the navigation processing critical to successful imaging and the seismic processing methodology and then show several examples of images improved by 3 d surveying the resulting 3 d seismic data were migrated both as independent lines and as a 3 d volume seismic images were produced as conventional depth and time sections and as cross sectional slices in depth and time we present comparisons of the migration results that show significant improvements of the subsurface image using 3 d techniques to illustrate these improvements we present images of two complex geologic areas a mud volcano and reflections near the top of the accretionary prism and the top of the subducting oceanic basement beneath the prism comparisons between the original stacked data the data of the two dimensional migration and the results of the 3 d migration illustrate the value of 3 d techniques in studies of active margins in addition to the advantages of the 3 d imaging which include better structural delineation and improved signal to noise ratio in the final image we show that the ability to view the 3 d data volume in both section and plan view offers significant interpretational advantages

insights into mantle type volatiles contribution from dissolved gases in artesian waters of the great artesian basin australia abstract the geochemical features of the volatiles dissolved in artesian thermal waters discharged over three basins millungera galilee and cooper basin of the australian great artesian basin gab consistently indicate the presence of fluids from multiple gas sources located in the crust e g sediments oil reservoirs granites as well as minor but detectable contributions of mantle magma derived fluids the gases extracted from 19 water samples and analyzed for their chemical and isotopic composition exhibit amounts of co2 up to about 340 mlstp lh2o marked by a 13ctdc total dissolved carbon ranging from 16 9 to 0 18 vs pdb while ch4 concentrations vary from 4 4 10 5 to 4 9 mlstp lh2o helium contents were between 9 and 2800 times higher than equilibrium with air saturated water asw with a maximum value of 0 12 mlstp lh2o helium isotopic composition was in the 0 02 0 21 ra range ra air normalized 3he 4he ratio the three investigated basins differ from each other in terms of both chemical composition and isotopic signatures of the dissolved gases whose origin is attributed to both mantle and crustal volatiles mantle he is present in the west central and hottest part of the gab despite no evidence of recent volcanism we found that the partial pressure of helium significantly higher in crustal fluids than in mantle type volatiles enhances the crustal he signature in the dissolved gases thus masking the original mantle contribution neotectonic activity involving deep lithospheric structures and magma intrusions highlighted by recent geophysical investigations is considered to be the drivers of mantle magmatic volatiles towards the surface the results although pertaining to artesian waters from a vast area of 542 000 km2 provide new constraints on volatile injection and show that fluids geochemistry can provide additional and independent information on the geo tectonic settings of the great artesian basin and its geothermal potential

effects of subducting seafloor roughness on upper plate vertical tectonism osa peninsula costa rica 1 subduction of seamounts and ridges along thinly sedimented convergent margins results in deformation of the overriding forearc exposures of newly recognized late pleistocene shallow water deposits i e marenco formation record intervals of rapid subsidence and uplift across the costa rican forearc inboard of the subducting cocos ridge in general the marenco formation is a fining upward fossiliferious late pleistocene marine sand disconformably overlying beveled surfaces cut across the competent osa melange basement the 50 to 27 ka age of the marenco formation is constrained by 12 accelerator mass spectrometry and two conventional 14c dates obtained on marine macrofossils the deposition of this sequence coincident with a general fall in sea level during oxygen isotope stage 3 requires 6 mm yr 1 subsidence inboard of the northwest flank of the subducting cocos ridge presently exposures of the marenco formation are found at 75 m above sea level requiring uplift rates in excess of 6 mm yr 1 we interpret the down and up history of vertical tectonism recorded by the marenco formation as the response of the upper plate to variations in the elevation of the subducting cocos ridge on the basis of a model where the upper plate deforms through bends because of roughness on a rigid downgoing plate the rate duration and spatial distribution of vertical tectonism across the forearc are determined by the magnitude of the orthogonal component of the relative convergence vector and the bathymetry of the underthrusting plate application of this model to bathymetric data for the cocos plate offshore yields a broad agreement between predicted future rates of subsidence and uplift and rates over the last 50 kyr recorded by the marenco formation furthermore analysis suggests that the arrival of the blunt tipped leading edge of the cocos ridge 0 5 3 ma resulted in an initial period of very rapid 30 mm yr 1 uplift

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

growth of fissure ridge travertines from geothermal springs of denizli basin western turkey fissure ridge travertines grown from geothermal springs of denizli basin southwestern turkey are investigated through stratigraphic structural geochemical and geochronological methods with the aim of understanding the growth of these elongate mound shaped structures two main types of travertine deposits are recognized 1 bedded travertines which grew as flowstone on sloping surfaces and form the bulk of fissure ridges and 2 banded travertines which grew as veins within the bedded travertine chiefly along its central feeding conduit stratigraphic and structural observations shed light on the bedded banded travertine relationships where the banded features grew through successive accretion phases crosscutting the bedded travertine or forming sill like structures the bedded and banded travertines alternated their growth as demonstrated by complicated crosscutting relationships and by the upward suture in places of banded travertine by bedded travertine that was in turn crosscut by younger banded travertine the bedded travertine is often tilted away from the central axis of the fissure ridge thus leaving more room for the central banded travertine to form u series ages confirm the bedded banded travertine temporal relationships and show that the growth of the studied fissure ridges lasted up to several tens of thousands of years during quaternary time the banded travertine was deposited mainly during cold events possibly in coincidence with seismic events that might have triggered the outflow of deep geothermal fluids c and o stable isotope and rare earth element data indicate a shallow feeding circuit for the studied structures with a fluid component deriving from a deeper geothermal circuit a crack and seal mechanism of fissure ridge growth is proposed modulated by the interplay of local and regional influencing factors and mechanisms such as geothermal fluid discharge paleoclimate tectonics and the progressive tilting of bedded travertine limbs over a soft substratum creating the necessary space for the central veins to grow

detection of shallowest submarine seismicity by acoustic coupled shear waves an outstanding slow wave train similar to t wave in appearance has been observed on records of an ocean bottom seismometer array off boso central japan these slow wave trains are not observed on land are usually spindle shaped and dominant in vertical component and have apparent velocities of 1 3 km s along the array in order to understand the observations we calculate synthetic seismograms by a finite difference method for horizontally stratified layers of seawater sediment and basement which show a well defined wave train consistent with the observations only if the source is placed above the basement layer this result and normal mode calculations indicate that the slow wave train comprises essentially the sediment trapped shear waves coupled with ocean acoustic waves the observed events of slow wave trains occur near the boundary between the outer arc high and the forearc basin where vertical differential motion has been suggested to occur we suggest that these small events represent the shallowest crustal activity unique to the triple junction off boso

500 ka precipitation record from southeastern australia evidence for interglacial relative aridity 230 th 234 u dating of speleothems from southeastern australia documents changes in effective precipitation over the past 500 k y at a temporal resolution not previously achieved results show that the highest effective precipitation for the southeastern interior of australia occurred during stadials and cool interstadials of the past four glacial cycles interglacials and warm interstadials as well as glacial maxima are comparatively arid we suggest that lower regional temperatures over the continent and changes in atmospheric circulation stronger walker circulation produced the observed periods of increased effective precipitation

a recent phase of accretion along the southern costa rican subduction zone abstract in 2011 we acquired a 3d seismic reflection volume across the costa rica margin nw of the osa peninsula to investigate the complex structure and the development of the seismogenic zone within the costa rican subduction zone in the vicinity of recent international ocean drilling program iodp drilling in contrast to previous interpretations these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments similar to materials recovered from iodp drilling that have been thrust and thickened into thrust bounded folded sequences these structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model we interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the cocos nazca caribbean triple junction and accreted during recent margin subduction that also accelerated with passage of the triple junction the margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last 5 ma that was probably preceded throughout the neogene by periods of non accretion or erosion

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

travitonics using travertines in active fault studies abstract late quaternary travertines deposited from hot springs can reveal much about the neotectonic attributes and histories of structures on the basis of field studies in the aegean region turkey and greece the northern apennines italy and the basin and range province usa we conclude that the following relationships are of predictive value i travertine deposits are preferentially located along fracture traces either immediately above extensional fissures or in the hanging walls of normal faults ii the locations of many travertine fissure ridge deposits coincide with step over zones relay ramps between fault segments networks of intersecting tensional fissures reflecting the complex strains experienced in such settings are probably responsible for enhancing hydrothermal flow iii the morphology of travertine deposits overlying extensional fissures is controlled by the rheology of the underlying materials tufa cones towers pinnacles form on former and present lake floors where fissures underlie unconsolidated sediments whereas fissure ridges develop where fissures cut bedrocks at the surface iv fissure ridges comprise outwardly dipping bedded travertine flanking a central tensional fissure filled by vertically banded travertines fissures can be used to infer local stretching directions v where there are travertines datable by the u series method it is possible to calculate time averaged dilation and lateral propagation rates for individual fissures vi most fissures cutting fissure ridges comprise self similar angular segments with fractal dimensions in the range 1 00 1 12 the properties of bedded travertine combined with stress perturbations at fissure tips probably being responsible for such similar fractal dimensions being inferred from such a wide range of locations fissures gradually increasing in width with depth are products of continuous fracture dilation in contrast to those that form during episodic dilation which display stepped increases of width with depth vii travertine deposited from springs along fault zones accumulate in terraced mounds sited down slope of the spring line viii many post depositional fractures cutting travertine deposits are locally oriented at right angles to deposit margins and ix systematic joints in travertines are restricted to those parts of eroded sheet deposits that have been exhumed

recent crustal deformation in the antofagasta region northern chile and the subduction process new neotectonic observations along with a detailed aerial photograph analysis allow a new interpretation of the recent tectonic behaviour of the outer forearc in northern chile between 22 5 s and 24 5 s antofagasta region both the coastal cordillera and the mejillones peninsula are under e w extension normal faults dipping east with an almost n s orientation are predominant large scale neogene to recent deformation is characterized by vertical uplift and subsidence related to normal faulting recent kinematics along the atacama fault system including the atacama fault itself are controlled by the regional extensional stress regime and the predominant component of recent displacement along the atacama fault is vertical normal however strike slip components are also observed but with moderate offsets no larger than one to two tens of metres the sense of shear and the ratio between vertical and horizontal offsets vary coherently with the fault azimuth compatible with e w extension oblique slip faulting with normal and left lateral offsets of similar amplitude is observed on the most northeasterly oriented segment of the atacama fault this small left lateral component of displacement and the strike slip motion predicted by models of strain partitioning of the convergence along the atacama fault are in opposite senses this occurs because the stress regime in the coastal region is not compressional but extensional very fresh recent ruptures are numerous especially between 23 s and 24 s and provide clear indications of the occurrence of moderate to large continental earthquakes during the late quaternary as pointed out by several authors indirect lines of evidence suggest active subduction erosion and underplating along the margin of northern chile we propose that part of the long term extension is due to the broad scale flexure of the outer forearc this flexure may be controlled by offshore subsidence caused by subduction erosion near the trench and by onshore uplift related to the underplating of eroded low density material beneath the coastal cordillera neotectonic observations show that the state of stress in the outer forearc remains extensional in a deviatoric sense in the long term however it is proposed that the state of stress has a strong time dependent component which is intimately linked to the subduction seismic cycle the mw 8 0 antofagasta earthquake of 1995 july 30 showed that large subduction earthquakes produce e w extension in the coastal region the overall e w deviatoric extension in the outer forearc should be reduced by interseismic contraction in the period separating two subduction earthquakes this reduction means that continental faults will remain locked and aseismic in the interseismic period of the subduction cycle in agreement with microseismic observations which indicate an absence of shallow crustal seismicity we assume that the amount of extension produced by subduction earthquakes in the coastal area is larger than the cumulative interseismic contraction this occurs because the displacement field generated by subduction earthquakes is concentrated in the outer forearc the e w extension should grow over prepeated subduction seismic cycles and lead to large continental earthquakes occurring within the overriding block simultaneous with large subduction events we show new surface ruptures associated with normal faulting along the atacama fault which were most probably triggered by the 1995 antofagasta subduction earthquake

geochemistry of quaternary travertines in the region north of rome italy structural hydrologic and paleoclimatic implications in the tyrrhenian region of central italy late quaternary fossil travertines are widespread along two major regional structures the tiber valley and the ancona anzio line the origin and transport of spring waters from which travertines precipitate are elucidated by chemical and isotopic studies of the travertines and associated thermal springs and gas vents there are consistent differences in the geochemical and isotopic signatures of thermal spring waters gas vents and present and fossil travertines between east and west of the tiber valley west of the tiber valley 13c of co2 discharged from gas vents and 13c of fossil travertines are higher than those to the east to the west the travertines have higher strontium contents and gases emitted from vents have higher 3he 4he ratios and lower n2 contents than to the east fossil travertines to the west have characteristics typical of thermogene thermal spring origin whereas those to the east have meteogene low temperature characteristics including abundant plant casts and organic impurities the regional geochemical differences in travertines and fluid compositions across the tiber valley are interpreted with a model of regional fluid flow the regional mesozoic limestone aquifer is recharged in the main axis of the apennine chain and the groundwater flows westward and is discharged at springs the travertine precipitating waters east of the tiber valley have shallower flow paths than those to the west because of the comparatively short fluid flow paths and low normal heat flow the groundwaters to the east of the tiber valley are cold and have co2 isotopic signatures indicating a significant biogenic contribution acquired from soils in the recharge area and limited deeply derived co2 in contrast spring waters west of the tiber valley have been conductively heated during transit in these high heat flow areas and have incorporated a comparatively large quantity of co2 derived from decarbonation of limestone the elevated strontium content of the thermal spring water west of the tiber valley is attributed to deep circulation and dissolution of a triassic evaporite unit that is stratigraphically beneath the mesozoic limestone u series age dates of fossil travertines indicate three main periods of travertine formation ka 220 240 120 140 and 60 70 based on the regional flow model correlating travertine deposition at thermal springs and precipitation in the recharge area we suggest that pluvial activity was enhanced during these periods our study suggests that travertines preserve a valuable record of paleofluid composition and paleoprecipitation and are thus useful for reconstructing paleohydrology and paleoclimate

the steeply subducting edge of the cocos ridge evidence from receiver functions beneath the northern talamanca range south central costa rica the deep structure of the south central costa rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible we present a receiver function study along a transect of broadband seismometers through the northern flank of the cordillera de talamanca south costa rica below moho depths the receiver functions image a dipping positive conversion signal this is interpreted as the subducting cocos plate slab compatible with the conversions in the individual receiver functions in finite difference modeling a dipping signal such as the one imaged can only be reproduced by a steeply 80 dipping structure present at least until a depth of about 70 100 km below this depth the length of the slab cannot be determined because of possible scattering effects the proposed position of the slab agrees with previous results from local seismicity local earthquake tomography and active seismic studies while extending the slab location to greater depths and steeper dip angle along the trench no marked change is observed in the receiver functions suggesting that the steeply dipping slab continues until the northern flank of the cordillera de talamanca in the transition region between the incoming seamount segment and cocos ridge considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision the southern costa rican subduction zone may at present be undergoing a reconfiguration of subduction style where the transition to shallow underthrusting may be underway but still incomplete

crustal structure of the cocos ridge northeast of cocos island panama basin 1 the submarine cocos ridge in the northwestern panama basin pacific ocean is generally interpreted as the trace of the galapagos hotspot a 278 km long seismic wide angle transect was carried out across a comparatively narrow ridge segment 150 km northeast of cocos island the results indicate a huge thickened crust where crustal thickening is mainly achieved by the lower crust which makes up 75 of the crustal thickness and is tripled compared to normal oceanic lower crust the velocities are comparable to normal lower crust and suggest no differences to a gabbroic composition the moho deepens from 10 to 18 km depth below the ridge similarities to other hotspot related oceanic ridges and plateaus with massive lower crusts and velocities below 7 4 km s suggest the formation of this cocos ridge segment near or at the plate boundary

studies on the detailed crustal structure of the trench trench trench triple junction off southeast japan abstract the fine structure of the upper part of the crust around a unique trench trench trench triple junction 34 10 n 141 50 e fig 1 off the coast of southeastern honshu japan was analyzed on the basis of geophysical data the triple junction is characterized by a triangular shaped 9000 m deep basin with a strong negative free air gravity anomaly distorted magnetic lineaments and shallow angle sub bottom faults although there is a lack of refraction data simulation experiments in terms of gravity and magnetics suggest that the topmost part of the lithosphere of the northeastern corner of the philippine sea plate is stretched to form a wide basin in the triple junction area no attempt is made to discuss the dynamic structure of the triple junction

uranium series dating of great artesian basin travertine deposits implications for palaeohydrogeology and palaeoclimate abstract travertine deposits precipitated by groundwater discharging from the great artesian basin gab are widespread in central australia and have the potential to provide a record of palaeohydrogeology and palaeoclimate the gab is one of the largest artesian basins in the world and a relationship between travertine deposits and recharge sites has potential importance regarding the time and position of past climate events given that the travertines growth forms from precipitation discharge we sampled numerous travertine sites in the southwest section of the gab as a first approach to test this relationship u series dating of the travertine deposits reveal that spring discharge has likely been episodic for the last several hundred thousand years spring travertine deposition occurred episodically around 465 50 ka 370 20 ka 335 15 ka 285 240 ka 185 10 ka 160 150 ka 110 100 ka and during the past 30 ka the periodicity of travertine ages observed with simultaneous deposition at multiple locations argues for regional palaeohydrologic controls comparison of the travertine deposit ages with climate proxies in australia shows that elevated travertine deposition rates are synchronous with wet periods in both central and southern australia due to the large size of the gab and that the recharge zones extend over multiple climatic regions of australia the times of travertine deposition are interpreted to represent times of high rainfall regionally this study shows that the travertine deposits of central australia provide a datable archive of past climate and hydrogeology of importance for understanding the groundwater evolution of the great artesian basin and that further more comprehensive studies are warranted

hydrothermal co2 degassing in seismically active zones during the late quaternary natural co2 discharges are abundant in turkey as evident from ongoing deposition of recent terrace mound travertines and emplacement of significant travertine vein and breccia deposits in fractured damage zones of active fault systems we report high precision u series age data for the vein carbonates combined with important field observations and geochemical data to evaluate the travertine veining and co2 degassing history in seismically active areas field evidence suggests that travertine filled veins and associated breccias represent hydrothermal eruption products which probably formed by hydraulic fracturing in response to overpressure of co2 rich fluids stable isotope and ree data support the field observations and indicate that travertine veins formed as thermogene deposits from rapidly ascending co2 rich fluids travertine veins from the kirsehir geothermal field give u series ages in a range from 10 6 0 3 ka to 86 16 1 24 ka with a majority of ages failing between 10 6 0 3 and 11 3 0 2 ka vein samples from the pamukkale geothermal field yield u series ages between 21 1 0 1 ka and 73 6 0 6 ka two major age groups are evident in pamukkale that cluster coherently around 21 ka and 25 ka a large number of travertine veins we dated by high precision u series technique coincide with times of cold dry climate events this is different from surface sinter deposits in geothermal fields which more frequently form during warm and wet periods we propose that a significant reduction in surface discharge of co2 by spring or geothermal waters during dry climate periods may promote oversaturation of co2 in deep reservoirs host rock fracturing in response to seismic shaking and fluid overpressure results in rapid exsolution and expansion of the dissolved gas and may lead to hydrothermal eruptions crown copyright c 2009 published by elsevier b v all rights reserved

tomographic evidence for a subducted seamount beneath the gulf of nicoya costa rica the cause of the 1990 mw 7 0 gulf of nicoya earthquake 1 tomographic images constrain the existence of a subducted seamount beneath the gulf of nicoya costa rica the subducted seamount is found at a depth of 30 km within the rupture area of the march 25 1990 mw 7 0 gulf of nicoya earthquake the gulf of nicoya earthquake was a typical thrust type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting cocos plate and the overriding plate precise relocation of the mainshock and its aftershocks in a 3 d p wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate we interpret the subducted seamount as an asperity whose rupture caused the 1990 gulf of nicoya earthquake

seismic cycles recorded in late quaternary calcite veins geochronological geochemical and microstructural evidence southwest turkey is seismically active as a result of the hellenic subduction process in the eastern mediterranean we conducted high resolution micro sampling high precision u series dating and microchemical analysis on an extensional vein system in a tectonically active but non hydrothermal area u th age data and microscopic observations provide evidence of repeated fracturing of a previously sealed crack system followed by a new increment of veining repeated injection of veinlets suggests that the vein system was formed by the crack seal mechanism four major u th age groups for the emplacement of the vein system fall between 23 9 0 2 ka and 23 2 0 4 ka 21 7 0 4 ka and 19 2 0 2 ka 17 3 0 1 ka and 16 2 0 3 ka and at 11 8 0 2 ka stable and sr isotope geochemistry of the calcite vein samples indicates that surface water interacting with the soil cover was the major component of the groundwater system from which the extensional veins precipitated trace element and 0 isotope data of the vein system are interpreted to reflect carbonate precipitation associated with seismic cycles involving fluids with different trace element compositions and co2 contents initial carbonate precipitation during a single seismic cycle occurred from co2 dominated fluids that were degassed from the original co2 water mixture this was followed consecutively by carbonate precipitation from the remaining water which was relatively impure with higher trace element contents millimetre to submillimetre scale u series dating in conjunction with geochemistry of carbonate veins related to active tectonism offers an innovative means of constraining the absolute timing of late quaternary seismic and inter seismic events c 2010 elsevier b v all rights reserved

along strike variability in the seismogenic zone below nicoya peninsula costa rica 1 at the subduction zone in northwestern costa rica the seismogenic zone lies directly beneath the nicoya peninsula allowing for near source seismic studies of earthquake activity we located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the nicoya peninsula using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth 10 km in the south to 20 km in the north that occurs where the subducting oceanic crust changes from warmer cocos nazca spreading center cns origin to colder east pacific rise epr origin we argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath nicoya costa rica subducting colder oceanic crust deepens the seismogenic updip limit

structure and development of the sagami trough and the boso triple junction abstract the sagami trough is located at the particular plate margin where the izu forearc is subducted underneath the honshu forearc at its southeastern end the world s only known ttt type triple junction boso triple junction has developed several different kinds of basins occur in different segments along the sagami trough and at the triple junction the bathymetric geologic and geophysical data obtained during the kaiko project and from additional studies are summarized together with our onland studies we suggest that the right lateral oblique plate motion formed an eduction margin in the sagami basin while a normal subduction margin and an oblique subduction margin have been formed in the middle sagami trough basin these tectonic phenomena resulted from the long lasting compressional covergence between the philippine sea plate and eurasian plate since the early or middle miocene the north basin on the northeasternmost margin of the philippine sea plate near the boso triple junction has developed as a stretched basin due to the westward motion of the philippine sea plate with respect to the eurasian plate

the seismogenic zone in the central costa rican pacific margin high quality hypocentres from an amphibious network transition from subduction of normal to thickened oceanic crust occurs in the central portion of the costa rican margin where large interplate earthquakes m 7 and abundant interseismic seismicity have been associated with subduction of bathymetric highs we relocated 1 300 earthquakes recorded for 6 months by a combined on and offshore seismological network using probabilistic earthquake relocation in a 3d p wave velocity model most of the seismicity originated at the seismogenic zone of the plate boundary appearing as an 18 dipping planar cluster from 15 to 25 30 km depth beneath the continental shelf several reverse focal mechanisms were resolved within the cluster the upper limit of this interseismic interplate seismicity seems to be controlled primarily by the overlying plate thickness and coherency which in turn is governed by the erosional processes and fluid release and escape at temperatures lower than 100 to 120 c along the plate boundary the downdip limit of the stick slip behaviour collocates with relative low temperatures of 150 to 200 c suggesting that it is controlled by serpentinization of the mantle wedge the distribution of the interseismic interplate seismicity is locally modified by the presence of subducted seamounts at different depths unlike in northern costa rica rupture of large earthquakes in the last two decades seems to coincide with the area defined by the interseismic interplate seismicity

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

tectonic evolution of the triple junction off central honshu for the past 1 million years seno t ogawa y tokuyama h nishiyama e and taira a 1989 tectonic evolution of the triple junction off central honshu for the past 1 million years in j p cadet and s uyeda editors subduction zones the kaiko project tectonophysics 160 91 116 we discuss several models of the evolution of the trench trench trench triple junction off central honshu during the past 1 m y on the basis of plate kinematics morphology gravity and seismic reflection profile data available for the area the study area is characterized by large basins 7 8 km deep on the inner lower trench slope on the philippine sea side and the deep 9 km izu bonin trench to the east between the basins and the trench there are 6 7 km deep basement highs the triple junction is unstable due to the movement of the philippine sea plate at a velocity of 3 cm yr in wnw direction with respect to eurasia northeast japan subparallel to the strike of the sagami trough generally we can expect the boundary area between the philippine sea and pacific plates to be extended because the pacific plate is unlikely to follow the retreating philippine sea plate due to the obstruction of the southeastern comer of eurasia the above peculiar morphology of the junction area could have resulted from this lack of stability however there are several possible ways to explain the above morphology our gravity model across the trench basement high basin area shows that the basement highs are made of low density materials 1 8 2 g cm3 thus we reject the mantle diapir model which proposes that the basement highs have been formed by diapiric injection of serpentinites between the retreating philippine sea plate and the pacific plate the stretched basin model proposes that the basins have been formed by stretching of the philippine sea plate wedge we estimated the extension to be about 10 km at the largest basin we reconstructed the morphology at 1 ma by moving the philippine sea plate 20 km farther to the east after closing the basins and thus obtained 8 km depth of the 1 ma trench which is similar to that of the present japan trench to the north although this stretched basin model can explain the formation of the basins and the deep trench other models are equally possible for instance the eduction model explains the origin of the basin by the eduction of the philippine sea basement from beneath the basement high while the accretion model explains the basement highs by the accretion of the in bonin trench wedge sediments in both of these models we can reconstruct the 1 ma trench depth as about 8 km similar to that of the stretched basin model the deformation of the basement of the basins constitutes the best criterion to differentiate between these models the multi channel seismic reflection profiles show that the basement of the largest basin is cut by normal faults in particular at its eastern edge this suggests that the stretched basin model is most likely however the upper part of the sediments shows that the basement high to the east has been recently uplifted this uplift is probably due to the recent 0 5 ma start of accretion of the trench wedge sediments beneath this basement high

do subducting seamounts generate or stop large earthquakes seamount subduction is a common process in subduction zone tectonics contradicting a widely held expectation that subducting seamounts generate large earthquakes seamounts subduct largely aseismically producing numerous small earthquakes on rare occasions when they do produce relatively large events the ruptures tend to be complex suggesting multiple rupture patches or faults we explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault the complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

seismogenic zone structure beneath the nicoya peninsula costa rica from three dimensional local earthquake p and s wave tomography the subduction plate interface along the nicoya peninsula costa rica generates damaging large mw 7 5 earthquakes we present hypocenters and 3 d seismic velocity models vp and vp vs calculated using simultaneous inversion of p and s wave arrival time data recorded from small magnitude local earthquakes to elucidate seismogenic zone structure in this region interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern nicoya peninsula respectively microseismicity offset across the plate suture of east pacific rise derived and cocos nazca spreading center derived oceanic lithosphere is 5 km revising earlier estimates suggesting 10 km of offset interplate seismicity begins downdip of increased locking along the plate interface imaged using gps and a region of low vp along the plate interface the downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental moho intersection possibly due to the onset of ductile behaviour slow forearc mantle wedge p wave velocities suggest 20 30 per cent serpentinization across the nicoya peninsula region while calculated vp vs values suggest 0 10 per cent serpentinization interpretation of vp vs resolution at depth is complicated however due to ray path distribution we posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes

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

ausmoho the variation of moho depth in australia summary since 2004 more than 7000 km of full crustal reflection profiles have been collected across australia to give a total of more than 11000 km providing valuable new constraints on crustal structure a further set of hitherto unexploited results comes from 150 receiver functions distributed across the continent mostly from portable receiver sites these new data sets provide a dramatic increase in data coverage compared with previous studies and reveal the complex structure of the australian continent in considerable detail a new comprehensive model for moho depth across australia and its immediate environment is developed by utilizing multiple sources of information on shore and off shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations and moho picks from the full suite of reflection transects the composite data set provides a much denser sampler of most of the continent than before though coverage remains low in the remote areas of the simpson and great sandy deserts the various data sets provide multiple estimates of the depth to moho in many regions and the consistency between the different techniques is high in a number of instances differences in estimates of moho depth can be associated with the aspects of the structure highlighted by the particular methods the new results allow considerable refinement of the patterns of moho depth across the continent some of the thinnest crust lies beneath the archean cratons in the pilbara and the southern part of the yilgarn thick crust is encountered beneath parts of the proterozoic in central australia and beneath the palaeozoic lachlan fold belt in southeastern australia the refined data indicate a number of zones of sharp contrast in depth to moho notably in the southern part of central australia

weak interplate coupling by seamounts and repeating m 7 earthquakes subducting seamounts are thought to increase the normal stress between subducting and overriding plates however recent seismic surveys and laboratory experiments suggest that interplate coupling is weak a seismic survey in the japan trench shows that a large seamount is being subducted near a region of repeating earthquakes of magnitude m 7 both observed seismicity and the pattern of rupture propagation during the 1982 m 7 0 event imply that interplate coupling was weak over the seamount a large rupture area with small slip occurred in front of the seamount its northern bound could be determined by a trace of multiple subducted seamounts whereas a subducted seamount itself may not define the rupture area its width may be influenced by that of the seamount

from seamount accretion to tectonic erosion formation of osa melange and the effects of cocos ridge subduction in southern costa rica 1 the costa rica portion of the middle america trench mat is characterized by active tectonic erosion a process that causes the removal of material from the base of the upper plate as the plate boundary migrates upward offshore studies demonstrate accelerated subduction erosion starting at the miocene pliocene boundary as the result of subduction of thickened galapagos related crust as presently represented by the cocos ridge the subduction of the cocos ridge also caused uplift and exposure of the outer forearc on the osa peninsula which offers a window to explore the tectonic evolution of the area the rocks outcropping on osa peninsula are a middle eocene middle miocene melange dominated by basalt chert and limestone resulting from accretion of seamounts the accretion dominated period of the mat evolution ended at the miocene pliocene boundary when thickened crust a paleo cocos ridge produced at the galapagos hot spot arrived at the trench the thick crust caused uplift and severe tectonic erosion of the accretionary edifice allowing exhumation of the osa melange the change from accretion to erosion caused the outer forearc to be offset along subvertical faults that define small kilometer to tens of kilometers size blocks that are going through differential vertical movements in response to the morphology of the subducting ridge subduction accretion and erosion are two processes that can alternate in time and space or coexist along the same margin so that mass removal can develop on a previously growing margin and completely remove an accretionary prism

holocene climate change in arid australia from speleothem and alluvial records new high resolution mc icpms u th ages and c and o isotopic analyses from a holocene speleothem in arid south central australia provide evidence for increased effective precipitation ep relative to present at c 11 5 ka and c 8 5 ka peak moisture at 7 6 ka and onset of an arid climate similar to present by c 5 ka 18o and 13c time series data exhibit marked 1 contemporaneous excursions over base line values of 5 3 and 11 0 respectively suggesting pronounced moisture variability during the early middle holocene climatic optimum optically stimulated luminescence and 14c ages from nearby terraced aggradational alluvial deposits indicate a paucity of large floods in the late pleistocene and at least five large flood events in the last c 6 kyr interpreted to mark an increased frequency of extreme rainfall events in the middle holocene despite overall reduced ep increased ep in south central australia during the early to middle holocene resulted from 1 decreased el nino southern oscil

shear zone thickness and the seismicity of chilean and marianas type subduction zones chilean type convergent margins have many large m 7 6 earthquakes whereas marianas type ones do not this dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement whereas it becomes understandable if it is viewed as a relatively thick sediment filled shear zone which thins or thickens arcward depending on subduction speed and sediment supply chilean type margins have thick trench fills and their shear zones generally thin arcward from inlets as much as several thousand metres high the most pronounced thinning being located near backstops tall up to several kilometres seamounts are subducted essentially intact to relatively great depths and confining pressures before jamming into the roof of the channel and becoming seismogenic asperities their near basal ruptures can generate large thrust type earthquakes mainly concentrated in seismic fronts near backstops marianas type margins in contrast have thin trench fills and their shear zones generally thicken arcward from inlets that can be as little as 300 m high seamounts are truncated near the inlet at low confining pressures and generate only small earthquakes after passing the inlet they do not touch the roof and therefore cannot generate large earthquakes a similar mechanism may explain seismic gaps at sediment poor regions of subduction zones

crustal deformation dynamics and stress evolution during seamount subduction high resolution 3 d numerical modeling funding was received from the people programme marie curie actions of the european union s seventh framework programme fp7 2007 2013 under rea grant agreement 604713 zip zooming in between plates v s and c r r acknowledge financial support received through the hades project ctm2011 30400 c01 and ctm2011 30400 c02 funded by spanish mineco

fluid accumulation along the costa rica subduction thrust and development of the seismogenic zone in 2011 we acquired an 11 55 km 3 d seismic reflection volume across the costa rica margin nw of the osa peninsula to accurately image the subduction thrust in 3 d to examine fault zone properties and to infer the hydrogeology that controls fluid accumulation along the thrust following processing to remove water column multiples noise and acquisition artifacts we constructed a 3 d seismic velocity model for kirchhoff prestack depth migration imaging images of the plate boundary thrust show high reflection amplitudes underneath the middle to lower slope that we attribute to fluid rich poorly drained portions of the subduction thrust at 5 km subseafloor beneath the upper slope the plate interface abruptly becomes weakly reflective which we interpret as a transition to a well drained subduction thrust mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude there is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a plumbing system faults commonly have fault plane reflections and are presumably fluid charged the faults and layered fabric form three compartmentalized hydrogeologic zones 1 a shallow ne dipping zone beneath the slope 2 a steeply sw dipping zone beneath the shelf slope break and 3 a ne dipping zone beneath the shelf the more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure elevates effective stress and creates greater potential for unstable coseismic slip

moho structure of central america based on three dimensional lithospheric density modelling of satellite derived gravity data the central american isthmus hosts a highly variable moho structure due to the diverse origin and composition of the crustal basement and the influence of large scale neotectonic processes gravity data from the combined geopotential model egm2008 were interpreted via forward modelling to outline the three dimensional lithospheric density structure along the middle american trench as well as the segmentation of the oceanic cocos and nazca plates and the overriding caribbean plate in this work results for the depth of the moho obtained from the density model are presented the quaternary volcanic arc correlates with a maximum moho depth of 44 km in western guatemala to the south east of the continental shelf the caribbean plate shows moho depths between 20 and 12 km whereas to the north values as shallow as 8 km are observed at the cayman trough for the oceanic cocos plate depths between 16 and 21 km are obtained for the moho along the cocos ridge contrasting with values between 15 and 12 km for the seamount segment and 8 and 11 km for the segments of the crust that are not affected by the galapagos hot spot track

structure and tectonics of the erosional convergent margin off antofagasta north chile 23 30 s subduction erosion has dominated the evolution of the north chile convergent continental margin since at least the mesozoic we investigate the structure of the antofagasta 23 s sector of this margin along a transect using coincident wide angle and near vertical seismic profiling and gravity data a 2 d velocity field of the overriding and subducting plates was obtained using joint refraction and reflection travel time tomography a velocity derived density distribution was used to model marine gravity data and substantiate the velocity model the gravity and velocity models imply that the overriding plate is mainly made of arc type igneous basement the upper plate is constructed of two main rock bodies separated by a subhorizontal layer defined by a velocity inversion the top coincident with a reflection in near vertical seismic images the seismic boundary is interpreted as a detachment separating an upper extended domain with large scale normal faulting from a lower domain apparently undergoing a different type of deformation velocity derived porosity indicates that the front of the margin is probably fluid saturated and disaggregated by fracturation as a consequence of frontal subduction erosion fluids carried into the subduction channel within slope debris filling underthrusting grabens reduce basal friction and probably induce hydrofracturing and basal erosion along the underside of the overriding plate at depths greater than 20 km porosity and density values imply that most fluids have been exhausted and the lower part of the upper plate is structurally coherent and little fractured the change in physical properties leads to increased mechanical coupling along the plate boundary and occurs at the updip limit of the distribution of aftershocks of the 1995 antofagasta earthquake m w 8 0 defining the seismogenic zone

seismogenic zone structure of the southern middle america trench costa rica 1 the shallow seismogenic portion of subduction zones generates damaging large and great earthquakes this study provides structural constraints on the seismogenic zone of the middle america trench offshore central costa rica and insights into the physical and mechanical characteristics controlling seismogenesis we have located 300 events that occurred following the mw 6 9 20 august 1999 quepos costa rica underthrusting earthquake using a three dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers we use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region these events define a plane dipping at 19 that marks the interface between the cocos plate and the panama block the majority of aftershocks occur below 10 km and above 30 km depth below sea level corresponding to 30 35 km and 95 km from the trench axis respectively relative event relocation produces a seismicity pattern similar to that obtained using absolute locations increasing confidence in the geometry of the seismogenic zone the aftershock locations spatially correlate with the downdip extension of the oceanic quepos plateau and reflect the structure of the main shock rupture asperity this strengthens an earlier argument that the 1999 quepos earthquake ruptured specific bathymetric highs on the downgoing plate we believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level

carbon dioxide earth degassing and seismogenesis in central and southern italy 1 we present the first regional map of co2 earth degassing from a large area most of central and south italy derived from the carbon of deep provenance dissolved in the main springs of the region the investigation shows that a globally significant amount of deeply derived co2 10 of the estimated global co2 emitted from subaerial volcanoes is released by two large areas located in western italy the anomalous flux of co2 suddenly disappears in the apennine in correspondence to a narrow band where most of seismicity concentrates here at depth the gas accumulates in crustal traps generating co2 overpressurized reservoirs which induce seismicity

crustal structure across the pacific margin of nicaragua evidence for ophiolitic basement and a shallow mantle sliver seismic wide angle measurements across the pacific margin of nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots the structure and the p wave velocity of the subducting cocos and overriding caribbean plates were determined by modelling wide angle data and further constrained by coincident seismic reflection borehole and gravity data the oceanic crust of the cocos plate is 5 5 km thick with a thin pelagic sediment cover the plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network a major feature of the upper plate is an 80 km wide high velocity high density rock unit reaching from the front of the margin to about the middle of the shelf this wedge shaped unit is 15 km thick beneath the shelf edge and is composed of a 5 km thick upper part with velocities increasing from 3 5 km s 1 near the trench to 5 2 km s 1 at the shelf break overlying a 10 km thick lower part with velocities of 6 0 km s 1 analysis of the gravity field requires densities of 2 6 2 7 g cm 3 for the upper part and 2 9 g cm 3 for the lower part of this unit these velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore guatemala and exposed on the nicoya peninsula costa rica the velocity structure of this ophiolitic unit ends about 50 km offshore landward the basement underneath the sandino basin is formed by older igneous rock drilled beneath upper cretaceous sedimentary rock onshore nicaragua beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12 18 km and coincides with the down dip limit of the seismogenic zone mantle densities are required for the sliver to match the gravity data in a tectonic reconstruction the suture of an oceanic plateau on the farallon plate against the chortis block in upper cretaceous time is suggested suturing left the former trench and margin in deep water consistent with the late cretaceous to palaeocene deep water sediment of the rivas formation at the base of the sandino basin suture of the farallon plate and chortis block might have initiated the strike slip movement along the motagua polochic fault system the development of a new subduction zone in eocene oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the chortis block and shifted the volcanic arc about 70 km southwestwards close to its present position

spatial distribution of earthquakes associated with the pacific plate subduction off northeastern japan revealed by ocean bottom and land observation abstract precise earthquake locations in the vicinity of a trench axis elucidate the characteristics of the shallower part of a subducting plate we split the japan trench into two regions off sanriku in the north and off fukushima in the south on the basis of the seismic properties of the regions and the fine microearthquake structure investigated by ocean bottom seismographic surveys as well as recent observation of major seismic activity from ocean bottom seismograph data the hypocentral distributions along a profile perpendicular to the trench axis indicate that almost all earthquakes occur along the boundary separating the subducting pacific plate and the landward plate a difference of the hypocentral distribution between off fukushima and off sanriku is found at the shallow plate boundary level the seismic thrust zone begins at more than 100 km landwards from the trench axis off fukushima whereas it begins at 50 km off sanriku and is nearly flat between 50 and 100 km from the trench axis this reflects regional variations of the interplate coupling in spite of these variations a common characteristic in recent observations indicates that in both regions almost all the hypocenters are deeper than 10 km this suggests the plate boundary to be decoupled at least in the part shallower than 10 km

spatial distribution of microearthquakes beneath the japan trench from ocean bottom seismographic observations summary the area of the japan trench about 40 n off the t6hoku district of north eastern honshu japan was for the first time extensively covered by an ocean bottom seismograph obs array seven obss were available for analyses of natural earthquakes and the observation period lasted two weeks during 1980 july and august during this period each obs registered between 300 and 600 earthquakes about 100 events were simultaneously recorded by at least three obss hypocentres were calculated by using a generalized inversion technique sixty five events mostly with magnitudes m from 1 5 to 3 0 which were not located by a land network were determined to be within or near the area covered by the obs array the obs array had nearly uniform detectability for earthquakes with m over 2 0 within the latitude range of 39 s0 41 5 n and the longitude range of 143 146 5 e within this area there is a highly active region of microseismicity beneath the seaward wall of the japan trench the seaward activity is concentrated within 100 km of the trench axis and is low beneath the northwest pacific basin a seismicity gap was found just westward of the trench axis beneath the landward wall of the trench at a water depth of between 6000 and 4000m seismic activity continues landward from the region west of the gap the obs array enabled us to determine the focal depth of microearthquake activity with better resolution than was previously possible the results show that the microearthquake activity is concentrated within the upper part of the oceanic lithosphere on the seaward side most events occur at a depth of less than 30 km beneath the trench wall while the activity on the landward side starts to deepen westward exactly at the trench axis this paper reports the first direct observations to clarify the spatial distribution of microearthquakes associated with the bending and or subducting of the oceanic plate just beneath the japan trench axis

linking co2 degassing in active fault zones to long term changes in water balance and surface water circulation an example from sw turkey abstract calcite veins are commonly found at shallow depth a few metres below the surface in damage zones of active normal fault systems in southwest turkey although earlier studies demonstrated the link between the vein formation and seismicity the association of near surface carbonate precipitation with climate driven hydrological conditions water table amount of precipitation and evaporation and water discharge is poorly understood in this study using the u th dating method we investigate the timing of vein formation and the interrelationship between tectonic and climatic related hydrological processes carbonate precipitation is interpreted to occur as a result of sudden pressure drops and co2 release after earthquake induced fracturing vein formation mostly occurred during glacial periods which coincide with slow growth rates and higher oxygen isotope values of speleothems from the eastern mediterranean region we relate these episodes to reduced winter rainfall due to the decrease in westerly flow in sw turkey these somewhat drier conditions influenced the chemical composition of circulating water creating conditions conducive to carbonate precipitation and sealing of damage zones these conditions also facilitated co2 accumulation and overpressure build up in the rupture zones failure of the faults resulted in the release of large volumes of co2 rich fluids and the generation of shallow carbonate veins it is proposed that during phases of increased winter rainfall co2 is advected to the surface and discharged as passive degassing when meteoric water circulation is enhanced while regional tectonics is the ultimate driver of fault activity and fracture formation climate driven near surface hydrological changes may have played an important role in modulating co2 rich fluid circulation and surface discharge

continuous 150 k y monsoon record from lake eyre australia insolation forcing implications and unexpected holocene failure our reconstructed history of lake eyre provides the first continuous continental proxy record of australian monsoon intensity over the past 150 k y this continental record s broad correspondence to the marine isotope record demonstrates that this very large catchment with its hydrology dependent on a planetary scale climate element responds to milankovitch scale climate forcing abrupt transitions from dry phases to wet phases ca 125 and 12 ka coincide with northern hemisphere winter insolation minima rather than southern hemisphere summer insolation maxima indicating that northern hemi sphere insolation exerts a dominant control over the intensity of the australian monsoon stratigraphic and dating uncertainties of other wet phases preclude conclusive correlation to specific insolation signals but within the uncertainties are consistent with northern hemisphere forcing regardless of the hemispheric forcing the low intensity of the early holocene australian monsoon by comparison with the last interglacial and particularly the last high level lacustrine event at 65 60 ka when all forcing elements were modest is an enigma that can be explained by a change in boundary conditions within australia

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

upper plate rigidity determines depth varying rupture behaviour of megathrust earthquakes seismological data provide evidence of a depth dependent rupture behaviour of earthquakes occurring at the megathrust fault of subduction zones also known as megathrust earthquakes1 relative to deeper events of similar magnitude shallow earthquake ruptures have larger slip and longer duration radiate energy that is depleted in high frequencies and have a larger discrepancy between their surface wave and moment magnitudes1 3 these source properties make them prone to generating devastating tsunamis without clear warning signs the depth dependent rupture behaviour is usually attributed to variations in fault mechanics4 7 conceptual models however have so far failed to identify the fundamental physical causes of the contrasting observations and do not provide a quantitative framework with which to predict and link them here we demonstrate that the observed differences do not require changes in fault mechanics we use compressional wave velocity models from worldwide subduction zones to show that their common underlying cause is a systematic depth variation of the rigidity at the lower part of the upper plate the rock body overriding the megathrust fault which deforms by dynamic stress transfer during co seismic slip combining realistic elastic properties with accurate estimates of earthquake focal depth enables us to predict the amount of co seismic slip the fault motion at the instant of the earthquake provides unambiguous estimations of magnitude and offers the potential for early tsunami warnings models compiled from subduction zones worldwide show that the elastic properties of the rock overlying shallow subduction megathrusts can be used to estimate potential slip possibly enabling early tsunami warnings

alluvial evidence for major climate and flow regime changes during the middle and late quaternary in eastern central australia as a low gradient arid region spanning the tropics to the temperate zone the lake eyre basin has undergone gentle late cenozoic crustal warping leading to substantial alluvial deposition thereby forming repositories of evidence for palaeoclimatic and palaeohydrological changes from the late tertiary to the holocene auger holes and bank exposures at five locations along the lower 500 km of cooper creek a major contributor to lake eyre in the eastern part of the basin yielded 85 luminescence dates tl and osl that combined wit a further 142 luminescence dates from northeastern australia have established a chronology of multiple episodes of enhanced flow regime from about 750 ka to the holocene mean bankfull discharges on cooper creek upstream of the innamincka dome at 250 230 ka or oxygen isotope stages ois 7 6 are estimated to have been 5 to 7 times larger than those of today however substantially less reworking has occurred during and after ois 5 than before lower cooper creek appears to have similarly declined in the tirari desert adjacent to lake eyre there is evidence of widespread alluvial activity perhaps during but certainly before the middle pleistocene yet the river became laterally restricted in ois 7 to 5 while the quaternary has been characterised by a dramatically oscillating wet dry climate since oxygen isotope stage ois 7 or 6 there has been a general decline in the magnitude of the episodes of wetness to which the eastern part of central australia has periodically returned during the last full glacial cycle cooper creek s periods of greatest runoff and sand transport were not during the last interglacial maximum of ois 5e 132 122 ka but later in ois 5 when sea levels and global temperatures were substantially below those of 5e or today fluvial activity returned in ois 4 and 3 but not to the extent of mid and late ois 5 strongly seasonal but still powerful flows transported sand and fed source bordering dunes in ois 5 and 3 this chronology of fluvial activity in the late quaternary broadly coincides with that for rivers of southeastern australia and suggests that the wet phases in eastern central australia have not been governed as much by the northern monsoon as by conditions in the western pacific close to the east coast both north and south flow confinement within the innamincka dome has locally amplified cooper creek s energy and here evidence exists for short but high magnitude episodes of flow during the last glacial maximum and in the early to middle holocene conditions that were capable of forming large palaeochannels but that were not long lived enough to rework the river s extensive floodplains elsewhere along its length

rapid pulses of uplift subsidence and subduction erosion offshore central america implications for building the rock record of convergent margins integrated ocean drilling program iodp expedition 334 to southern costa rica central america documented unprecedented subduction erosion in an area of active seismic slip widespread subduction erosion of the upper plate initiated when the cocos ridge an overthickened aseismic ridge arrived at the middle america trench subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate the completely recovered shelf sequence constrains a short 2 m y interval of extreme subsidence 1200 m with a rapid pulse occurring during the first 0 3 m y this event removed an estimated 1 2 106 km3 of forearc material at a rate of 1690 km3 m y km of trench during a time of rapid 1035 m m y shelf sediment accumulation at this erosive margin a sediment starved trench persisted in spite of abundant sediment supply because subduction erosion led to the creation of forearc basins similar rapid pulses of subduction erosion may punctuate the evolution of many margins contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts and recycling of continental material into the mantle

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

overriding plate structure of the nicaragua convergent margin relationship to the seismogenic zone of the 1992 tsunami earthquake we present 2 d seismic velocity models and coincident multichannel seismic reflection images of the overriding plate and the inter plate boundary of the nicaragua convergent margin along two wide angle seismic profiles parallel and normal to the trench acquired in the rupture area of the 1992 tsunami earthquake the trench perpendicular profile runs over a seamount subducting under the margin slope at the location where seismological observations predict large coseismic slip along this profile the igneous basement shows increasing velocity both with depth and away from the trench reflecting a progressive decrease in upper plate rock degree of fracturing upper mantle like velocities are obtained at approximate to 10 km depth beneath the fore arc sandino basin indicating a shallow mantle wedge a mismatch of the inter plate reflector in the velocity models and along coincident multichannel seismic profiles under the slope is best explained by approximate to 15 velocity anisotropy probably caused by subvertical open fractures that may be related to fluid paths feeding known seafloor seepage sites the presence of a shallow partially serpentinized mantle wedge and the fracture related anisotropy are supported by gravity analysis of velocity derived density models the downdip limit of inter plate seismicity occurs near the tip of the inferred mantle wedge suggesting that seismicity could be controlled by the presence of serpentinite group minerals at the fault gouge near the trench the inferred local increase of normal stress produced by the subducting seamount in the plate boundary may have made this fault segment unstable during earthquake rupture which could explain its tsunamigenic character

evidence and mechanisms for forearc extension at the accretionary costa rica convergent margin seismic reflection data across the upper trench slope off the nicoya peninsula costa rica reveal a wide zone of nearly trench parallel normal faults although work in the last decade has shown that normal faults are present at many convergent margins most examples e g japan peru chile and guatemala have been associated with margins experiencing subduction erosion or non accretion in contrast extension in the costa rica study area apparently is coeval with frontal accretion and underplating the normal faults across the costa rica forearc are striking in seismic section due to the well layered 2 km thick upper slope apron fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism indicating a deep seated deformation process the zone of extension is from the midslope area to within 10 km of the shelf edge a minimum width of about 20 km the estimated extension across the zone is at least 1 5 to 3 km within the apron section spacing between the faults is generally 200 500 m and nominal fault dip is 20 40 and predominantly landward activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault controlled sedimentary thickening at least some of the faults may be presently active shallow reflectors and possibly the seafloor are displaced by faulting contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out of sequence thrusting primarily below the midslope area the consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes with the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension however the three most likely causes are underplating changes in basal shear stress or a brief episode of subduction erosion

seismic tomography with local earthquakes in costa rica abstract the costarican isthmus is located at the western limit of the caribbean oceanic plateau where the cocos plate subducts along the middle american trench this plate shows strong lateral variations in its morphology and structure the main objective of this study is to investigate the effects of subduction related magmatism as a function of morphology and structure of the subducting plate by performing a simultaneous inversion of the 3 d crustal velocity field and hypocenter locations from local earthquakes for that we used the traveltimes of p waves first arrivals from more than 5000 events recorded at the costarican seismic networks between 1991 and 1998 in order to prevent data and modeling inaccuracies we followed an inversion scheme consisting of four steps 1 selection of an adequate data subset for tomographic inversion 2 estimation of the best reference 1 d model 3 determination of the finest parameterization and 4 evaluation of resolution the results show that the uppermost levels of the crust 0 6 km are consistent with geology since the velocity anomalies reflect the most meaningful geological features observed in the surface below these levels 6 20 km deep we found two different zones separated by a sw ne seismic alignment the northern part is characterized by a highly heterogeneous velocity field and is seismically active while the southern part is much more homogeneous and practically inactive moreover the northern part shows a certain accumulation of low velocity material within the upper mantle we suggest and illustrate that the structural differences between the northern and southern zones can be a consequence of the differences on the geometry and structure of the subducting slab

trench triple junction off central japan preliminary results of french japanese 1984 kaiko cruise leg 2 abstract leg 2 of the french japanese 1984 kaiko cruise has surveyed the trench triple junction off central japan where the japan izu bonin and sagami trenches intersect the izu bonin trench is deeper than the japan trench and filled by a thick turbiditic series its anomalous depth is explained by the westward retreat of the edge of the northwestward moving philippine sea plate on the contrary to what happens in the japan trench horst and graben structures of the pacific plate obliquely enters the izu bonin trench suggesting that the actual boundary between these two trenches is located to the north of the triple junction the inner wall of the izu bonin trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area the northernmost basin is overthrust by the edge of the fore arc area of the northeast japan plate the plate boundary is hardly discernible further east which makes it impossible to locate precisely the triple junction itself these features suggest that large intra plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge shaped margin of the overriding plates

the crustal structure of the cocos ridge off costa rica 1 the submarine cocos ridge in the northwestern panama basin a bathymetric feature more than 1000 km long and 250 500 km broad is about 2 km shallower than the adjacent basin it is generally interpreted as the trace of the galapagos hot spot two 127 and 260 km long seismic wide angle sections were recorded along and across this ridge offshore the osa peninsula costa rica crustal thickening is seen everywhere along the sections on the northwestern outer ridge flank increased thickness is exclusively attributed to the upper crust and expressed by 2 km thick flow basalts the quepos plateau caps the upper crust in this area toward the center of the cocos ridge the moho deepens from 11 12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80 of the crust and is three times the thickness of normal oceanic lower crust it is homogeneously structured and the velocities which range from 6 5 km s at the top to 7 35 km s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition similarities to the crustal velocity structure of iceland central kerguelen plateau and broken ridge are consistent with a formation of this 13 15 ma old cocos ridge segment by excessive magmatism in a near plate boundary setting

corrugated megathrust revealed offshore from costa rica exhumed faults are rough often exhibiting topographic corrugations oriented in the direction of slip such features are fundamental to mechanical processes that drive earthquakes and fault evolution however our understanding of corrugation genesis remains limited due to a lack of in situ observations at depth especially at subducting plate boundaries here we present three dimensional seismic reflection data of the costa rica subduction zone that image a shallow megathrust fault characterized by corrugated and chaotic and weakly corrugated topographies the corrugated surfaces extend from near the trench to several kilometres down dip exhibit high reflection amplitudes consistent with high fluid content pressure and trend 11 18 oblique to subduction suggesting 15 to 25 mm yr 1 of trench parallel slip partitioning across the plate boundary the corrugations form along portions of the megathrust with greater cumulative slip and may act as fluid conduits in contrast weakly corrugated areas occur adjacent to active plate bending faults where the megathrust has migrated up section forming a nascent fault surface the variations in megathrust roughness imaged here suggest that abandonment and then reestablishment of the megathrust up section transiently increases fault roughness analogous corrugations may exist along significant portions of subduction megathrusts globally mature parts of the shallow megathrust beneath costa rica are characterized by striking corrugations that may channel fluids according to seismic images nascent sections of the subduction zone plate boundary appear only weakly corrugated

a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

thermal regime of the costa rican convergent margin 2 thermal models of the shallow middle america subduction zone offshore costa rica at the costa rica margin along the middle america trench along strike variations in heat flow are well mapped these variations can be understood in terms of either ventilated fluid flow where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer we model fluid flow within the subducting aquifer using nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust the sensitivity of these models to the initial thermal state of the plate and styles of fluid flow either ventilated or insulated is explored heat flow measurements on cool crust accreted at the east pacific rise are consistent with ventilated hydrothermal cooling that continues with subduction these models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation heat flow transects on warm crust accreted at the cocos nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100 c and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust

crustal structure and seismicity beneath the forearc off northeastern japan marine seismological and other data in the japan trench area north of 38 5 n were used to infer the relation of subducting plate structure seismicity and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates previous crustal models from ocean bottom seismographic refraction obs surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data a wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting pacific plate crust its dip angle increases to about 7 110 km landward of the trench axis a large increase beneath the deep sea terrace is required to reach the well defined angle of 25 beneath the tohoku east coast a large wave speed gradient within layer 2 commonly observed under normal oceans seems to vanish beneath the inner trench slope at about 10 km depth within the overriding plate apparently brittle material with a p wave speed of 6 km s can be found as near as 35 km from the trench axis the upper limit of the seismogenic zone of interplate low angle thrust events is about 15 km depth from obs seismicity and large event analyses both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates

seismic and gravity constraints on the nature of the basement in the africa eurasia plate boundary new insights for the geodynamic evolution of the sw iberian margin we present a new classification of geological domains at the africa eurasia plate boundary off sw iberia together with a regional geodynamic reconstruction spanning from the mesozoic extension to the neogene to present day convergence it is based on seismic velocity and density models along a new transect running from the horseshoe to the seine abyssal plains which is combined with previously available geophysical models from the region the basement velocity structure at the seine abyssal plain indicates the presence of a highly heterogeneous thin oceanic crust with local high velocity anomalies possibly representing zones related to the presence of ultramafic rocks the integration of this model with previous ones reveals the presence of three oceanic domains offshore sw iberia 1 the seine abyssal plain domain generated during the first stages of slow seafloor spreading in the ne central atlantic early jurassic 2 the gulf of cadiz domain made of oceanic crust generated in the alpine tethys spreading system between iberia and africa which was coeval with the formation of the seine abyssal plain domain and lasted up to the north atlantic continental breakup late jurassic and 3 the gorringe bank domain made of exhumed mantle rocks which formed during the first stages of north atlantic opening our models suggest that the seine abyssal plain and gulf of cadiz domains are separated by the lineament south strike slip fault whereas the gulf of cadiz and gorringe bank domains appear to be limited by a deep thrust fault located at the center of the horseshoe abyssal plain

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

plateau versus fissure ridge travertines from quaternary geothermal springs of italy and turkey interactions and feedbacks between fluid discharge paleoclimate and tectonics abstract morphologically different deposits of thermal travertines are known worldwide but what factors controlled their morphology volume and growth for tens of thousands of years is only partially understood two main morphotypes of quaternary thermal travertines are reconsidered here to understand the reasons for their differential growth the fissure ridge travertines of denizli basin western turkey and the travertine plateau of tivoli central italy for comparable longevities and average vertical deposition rates the main differences between the studied travertines are as follows 1 volume of the travertine plateau is about one hundred times larger than each fissure ridge 2 despite a larger volume the travertine plateau does not produce relief whereas the fissure ridges produce a characteristic prominent topography 3 the travertine plateau grew primarily through lateral progradation whereas the fissure ridges through vertical aggradation 4 travertine deposition occurred in different environments principally low energy flat or shallow environments at tivoli and high energy inclined environments at denizli 5 the growth of the tivoli plateau occurred in a subsiding basin whereas the fissure ridges were not influenced by significant subsidence 6 c and o isotope signatures from the two studied travertines are different 7 despite similar annual precipitations the present water discharge in the tivoli area is about ten times greater than that of the denizli basin u series ages from the two deposits are correlated with paleoclimate oscillations at regional and global scales geological field evidence together with paleoclimate correlations suggest that in both the study cases the main body of travertine deposits the bedded travertine grew preferentially when the water table was high warm and or humid periods conversely when the water table was depressed cold and or dry periods the tivoli travertine underwent partial erosion and the denizli ridges were cut by axial veins and lateral sill like structures filled by banded sparitic travertine a comparative model is proposed where the main factor driving the difference in the morphostratigraphic architecture of fissure ridges and travertine plateaus is the volume of water discharge a high discharge rate resulted in the precipitation of caco3 far away from the springs hence driving the lateral progradation of the tivoli plateau a reduced discharge rate caused travertine precipitation close to the springs thus causing the vertical aggradation of the denizli fissure ridges paleoclimate oscillations must have controlled the amount of fluid discharge which in turn must have influenced the opening of the feeding fractures by an increased pore pressure

interplate seismicity at the crisp drilling site the 2002 mw 6 4 osa earthquake at the southeastern end of the middle america trench we investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity westward from osa peninsula in costa rica a major change in the character of the incoming cocos plate is displayed by abrupt lateral variations in seafloor depth and thermal structure here a mw 6 4 thrust earthquake was followed by three aftershock clusters in june 2002 initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the middle america trench off central costa rica the earthquake sequence occurred while a temporary network of obh and land stations 80 km to the northwest were deployed by adding readings from permanent local stations we obtain uncommon p wave coverage of a large subduction zone earthquake we relocate this catalog using a nonlinear probabilistic approach within both a 1 d and a 3 d p wave velocity models the main shock occurred 25 km from the trench and probably along the plate interface at 5 10 km depth we analyze teleseismic data to further constrain the rupture process of the main shock the best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope supporting the nucleation of the osa earthquake at 6 km depth the location and depth coincide with the plate boundary imaged in prestack depth migrated reflection lines shot near the nucleation area aftershocks propagated downdip to the area of a 1999 mw 6 9 sequence and partially overlapped it the results indicate that underthrusting of the young and buoyant cocos ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central costa rica margin

palaeohydrology of lowland rivers in the murray darling basin australia abstract this study derives a new function describing the relationship of channel bankfull discharge qbf to channel width in modern rivers of the murray darling basin mdb of southeastern australia and applies this to dated palaeochannels of seven rivers to quantify late quaternary discharge history in this important basin all rivers show high mis3 and mis2 qbf declining in the holocene the qbf of modern mdb rivers is correlated with total catchment precipitation but comparison with palaeochannel qbf estimates shows that while enhanced runoff efficiency is necessary to account for much larger late pleistocene palaeochannels either lower or higher precipitation rates could have prevailed a strong association between relative palaeo qbf enhancement and temperature suggests a temperature mediated mechanism controlling river discharge such as the fraction of precipitation stored as snow and thawing in spring the enhancement of orographic rainfall or co2 feedbacks with vegetation cover significantly enhanced mis3 qbf requires an additional mechanism such as increased rainfall these findings are consistent with others that increased moisture availability was associated with past colder climates although this was not necessarily the result of enhanced precipitation

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

influence of incoming plate relief on overriding plate deformation and earthquake nucleation cocos ridge subduction costa rica 18 pages 7 figures supporting information https doi org 10 1029 2019tc005586 data are available at the pangaea world data center https doi org 10 1594 pangaea 909047

comment on uranium series dating of great artesian basin travertine deposits implications for palaeohydrogeology and palaeoclimate by priestley et al 2018 abstract widespread travertine deposits occur in the southwestern great artesian basin gab in central australia priestley et al 2018 reported uranium series ages of travertine deposits and concluded that elevated travertine deposition rates are synchronous with wet periods and that times of travertine deposition represent times of high regional rainfall we propose an alternative explanation that co2 degassing from the mantle associated with active faulting played a major role in travertine precipitation in the southwestern gab

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

responses of quaternary rainforest vertebrates to climate change in australia a new previous termmiddle pleistocenenext term vertebrate fossil record from eastern australia dated by u disequilibrium series records the first quaternary record of an australian tropical rainforest fauna this exceptionally rich fauna underwent extinction after a long period of relative faunal stability spanning several glacial cycles and persisted probably until 280 000 years ago some time between 280 000 and 205 000 years ago the rainforest fauna was replaced by a xeric adapted fauna since that time the xeric adapted fauna was replaced by a mesic adapted fauna which was established by the holocene this is the first vertebrate faunal evidence in australia of the previous termmiddle pleistocenenext term mid brunhes climatic event mbe a major climatic reorganisation that led to increased aridity in northern australia from around 300 000 years ago several independent palaeoclimate proxies suggest that the climatic shift to aridity was due to increased climatic variability and weakened northern monsoons which may be manifested in the extinction of the aseasonal rainforest fauna and its replacement by an arid adapted fauna we extend the temporal ranges of several taxa from the pliocene into the previous termmiddle pleistocene next term we also reveal a longer palaeobiogeographic connection of rainforest taxa and lineages shared between new guinea and australia than was previously thought and show that their extinction on mainland australia occurred sometime after 280 000 years ago

recent mantle degassing recorded by carbonic spring deposits along sinistral strike slip faults south central australia abstract the interior of the australian continent shows evidence for late quaternary to recent fault controlled mantle 3 he and co 2 degassing a series of interconnected nw striking sinistral faults the norwest fault zone nfz in south central australia are associated with travertine mounds the latter show a regular spacing of 50 70 km u series ages on 26 samples range from 354 7 to 1 19 0 02 ka 2 errors and suggest a clustering every 3 4 ka since 26 ka geochemical data demonstrate a remarkable mantle to groundwater connection isotopic data indicate that the groundwater is circulating to depths 3 km and interacting with neoproterozoic cambrian basement and mantle volatiles 3 he 4 he isotope ratios show that the he comes in part from the mantle this demonstrates that the nfz cuts through the entire crust and provides pathways for mantle degassing scaling relationships suggest that the series of sinistral faults that make up the nfz are interconnected at depths and have a significant strike length of 60 70 km or more the nfz occurs where a major compositional boundary and a significant heat flow anomaly occurs and a major step in lithospheric thickness has been mapped we discuss a tectonic model in which recent stress field heat flow and lithospheric structure in central australia reactivated a set of steeply dipping neoproterozoic faults which may now be growing into a crustal lithospheric scale structure

lithospheric structure of the costa rican isthmus effects of subduction zone magmatism on an oceanic plateau we present the results of a multidisciplinary geophysical study conducted to investigate the lithospheric structure of the costa rican isthmus the physical properties of the lithosphere are resolved by three dimensional 3 d simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2 d forward modeling of onshore and offshore seismic refraction and gravity data according to our results the northern half of the costa rican isthmus is constituted by a 40 km thick crust with a 6 to 7 km thick oceanic crust subducting under it the uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities in good agreement with those described for flood basalts the midlevel shows velocities and densities representative of oceanic crust the bottommost level 20 40 km shows high velocities and densities typical of mafic rocks and the upper mantle displays anomalously low densities and velocities intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies these results are consistent with a basement beneath the costa rican isthmus being part of the caribbean plateau originated at 85 90 ma with the onset of the galapagos hotspot the upper level corresponds to the flood basalts extruded during this phase and it includes most of the marginal wedge the second level represents the preexisting oceanic crust the mafic lower crust intracrustal heterogeneities and anomalous upper mantle are interpreted to be built up by underplating intrusion and crystallization of basaltic melts formed under the influence of subducting lithosphere dehydration