strength of plate coupling in the southern ryukyu subduction zone abstract understanding the strength of a plate coupling is critical for assessing potential seismic and tsunamic hazards in subduction zones the interaction between an overriding plate and the associated subducting plate can be used to evaluate the strength of plate coupling by examining the mantle lithospheric buoyancy here we calculate the mantle lithosphere buoyancy across the northern portion of the southern ryukyu subduction zone based on gravity modeling with the constraints from a newly derived p wave seismic velocity model the result indicates that the strength of the plate coupling in the study area is relatively strong which is consistent with previous observations in the southernmost ryukyu subduction zone because few large earthquakes mw 7 have occurred in the southern ryukyu subduction zone a large amount of energy is locked and accumulated by plate coupling that could be released in the near future

new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

lithospheric structure buoyancy and coupling across the southernmost ryukyu subduction zone an example of decreasing plate coupling abstract the okinawa trough is a backarc basin located behind the ryukyu arc trench system the southernmost part of the okinawa trough spot displays different tectonic features from the rest of the okinawa trough the spot area includes abundant seamounts with active hydrothermal venting and high heat flow values to understand better the rifting and magmatism context of the spot area we examine the lithospheric structure buoyancy and coupling across the southernmost ryukyu subduction zone the results show that beneath the spot area the continental crust and mantle lithosphere thickness of 25 30 and 120 km respectively are thick with little continental thinning the analysis of mantle lithosphere buoyancy across the southernmost ryukyu subduction zone shows strong plate coupling between the overriding and subducting plates however the two plates are actually decoupled as indicated by present day interface earthquakes this situation indicates that the southernmost ryukyu subduction zone displays a transitory case of a changing plate coupling from a strong to a weak plate coupling such a coupling decoupling transition is probably associated with the collision of the luzon arc with the asian continental margin additionally the curve of the mantle lithosphere buoyancy across the southernmost ryukyu subduction zone indicates that the submarine magmatism in the spot area is located within the volcanic arc area suggesting early arc magmatism in the spot area

serpentinization of the fore arc mantle along the taiwan arc continent collision of the northern manila subduction zone inferred from gravity modeling abstract serpentinized peridotite in the fore arc has been observed in a number of subduction zones including the northern manila subduction zone which terminates northward in the taiwan arc continent collision how this zone of serpentinization changes northward from the subduction of thinned continental lithosphere to full arc continent collision in the taiwan orogeny is not well known in this paper we present 2 d gravity modeling along three p wave vp transects across the taiwan orogeny two of these transects were collected with ocean bottom seismometers these two transects provide good constraints on the velocity structure to the west of and on land southern taiwan conversion of vp to density in this area allows us to model the gravity anomaly with very little misfit along the subduction zone however the velocity models are poorly constrained in the upper mantle where an anomalous density unit has to be used in order to model the short wavelength gravity anomaly in this area a third transect across central taiwan that is derived from the taiger local tomography data provides good control on the crust and upper mantle vp structure that we use to place provide density constraints for modeling the gravity anomaly in this part of the collision zone in order to model the short wavelength gravity anomaly across the longitudinal valley and the southern longitudinal trough an anomalous density block is required beneath the fore arc region we interpret that the source of this anomalous density material could be serpentinized fore arc mantle similar to what is interpreted for the northern manila subduction zone farther south water released from the subduction of the extended crust of the continental margin results in the serpentinization of the fore arc area and may be driving the uplifting of the high pressure rocks

the largest instrumentally recorded earthquake in taiwan revised location and magnitude and tectonic significance of the 1920 event summary the ryukyu subduction is known to generate very few earthquakes in its central segment contrarily to its two extremities we focus in this study on the southernmost part of the ryukyu subduction zone offshore east taiwan our first task was to build a homogeneous earthquake cataloguefortheperiod1900 2007 thenewcatalogueprovideshomogenized m

very low frequency earthquakes along the ryukyu subduction zone 1 a total of 1314 very low frequency earthquakes vlfes were identified along the ryukyu trench from seismograms recorded at broadband networks in japan f net and taiwan bats in 2007 the spectra of typical vlfes have peak frequencies between 0 02 to 0 1 hz among those waveforms from 120 vlfes were inverted to obtain their centoroid moment tensor cmt solutions and locations using an examination grid to minimize a residual between the observed and synthetic waveforms within an area of 11 14 in latitude and longitude and at depths of 0 to 60 km most of the vlfes occur on shallow thrust faults that are distributed along the ryukyu trench which are similar to those earthquakes found in honshu and hokkaido japan the locations and mechanisms of vlfes may be indicative of coupled regions within the accretionary prism or at the plate interface this study highlights the need for further investigation of the ryukyu trench to identify coupled regions within it

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

intense foreshocks and a slow slip event preceded the 2014 iquique mw 8 1 earthquake the subduction zone in northern chile is a well identified seismic gap that last ruptured in 1877 the moment magnitude mw 8 1 iquique earthquake of 1 april 2014 broke a highly coupled portion of this gap to understand the seismicity preceding this event we studied the location and mechanisms of the foreshocks and computed global positioning system gps time series at stations located on shore seismicity off the coast of iquique started to increase in january 2014 after 16 march several mw 6 events occurred near the low coupled zone these events migrated northward for 50 kilometers until the 1 april earthquake occurred on 16 march on shore continuous gps stations detected a westward motion that we model as a slow slip event situated in the same area where the mainshock occurred

complexities in the tectonic evolution of the west philippine basin abstract the oceanic west philippine basin can be subdivided into a main basin a northwest sub basin and a southern sub basin these basins have undergone different tectonic histories magnetic anomalies 21 17 are present in the main basin and indicate that it formed by two limbed seafloor spreading the now extinct spreading center is located within a zone of rough topography associated with the northwest trending central basin fault cbf the morphology of the cbf is primarily the result of post spreading tectonism the magnetic anomaly pattern of the main basin does not extend into the southern sub basin or the northwest sub basin geometrical constraints indicate that the southern sub basin did not form as part of the southern limb of the main basin the southern sub basin may have formed immediately before a ridge jump to the main basin spreading axis alternatively possible shallower crustal depth in the southern sub basin may indicate that the sub basin is younger than the main basin and formed separately from it in the northwest sub basin which originated as part of the main basin the dominant topographic trends are perpendicular to those in the main basin this difference as well as higher heat flow and shallower depth in the northwest sub basin indicate that it has undergone a deformational event which did not affect the main basin this deformation may be related to subduction along the east luzon margin in mid tertiary time the gagua ridge an uplifted sliver of oceanic crust along the western edge of the northwest sub basin may have originated during this event

three dimensional p and s wave velocity structures beneath the ryukyu arc the three dimensional 3 d p and s wave velocity structures beneath the ryukyu arc are determined by seismic wave travel time tomography using recordings on temporary stations in the sea area as well as those on routine stations for the inversion we explicitly define the upper boundary of the philippine sea slab as a result we obtain 3 d p and s wave velocity structures beneath the ryukyu arc which are more accurate than previous ones p and s wave low velocity zones and high vp vs zones exist just beneath active volcanoes at about 10 km depth which show the influence of arc magmas beneath the volcanoes prominent p and s wave low velocity zones exist along the okinawa trough at about 50 km depth where vp vs values are high the low velocity and high vp vs anomalies will represent some mantle materials flow upward from the depth greater than 50 km along the trough low velocity anomalies are visible beneath the philippine sea slab which are possibly associated with small scale mantle convections d 2003 elsevier b v all rights reserved

subducting slab ultra slow velocity layer coincident with silent earthquakes in southern mexico great earthquakes have repeatedly occurred on the plate interface in a few shallow dipping subduction zones where the subducting and overriding plates are strongly locked silent earthquakes or slow slip events were recently discovered at the down dip extension of the locked zone and interact with the earthquake cycle here we show that locally observed converted sp arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern mexico reveal that the ultra slow velocity layer usl varies spatially 3 to 5 kilometers with an s wave velocity of 2 0 to 2 7 kilometers per second most slow slip patches coincide with the presence of the usl and they are bounded by the absence of the usl the extent of the usl delineates the zone of transitional frictional behavior

spatial variation of the crustal stress field along the ryukyu taiwan luzon convergent boundary 1 we applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the philippine sea plate our results indicate that the crustal stress along the ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction including the tokara channel an extensional stress regime is observed along the entire ryukyu back arc implying that back arc rifting may have extended northward to kyushu a triangular area near the southernmost terminus of the ryukyu arc is characterized by a unique stress signature the eastern boundary of this ryukyu taiwan stress transition coincides with the 123 e meridian where the gagua ridge intercepts the ryukyu trench whereas its western boundary agrees remarkably well with the border between the postcollision and waning collision domains in northern taiwan the taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the lukang magnetization high lmh suggesting that the lmh may be critical in controlling the local stress distribution the stress signature of the luzon arc taiwan collision reaches as far south as 19 5 n the tectonic stress along the manila trench luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in slab membrane stress since this extensional regime is observed only south of 22 n it probably marks the northern limit of the contemporary boundary between the subduction along the manila trench and the collision in taiwan

plate detachment asthenosphere upwelling and topography across subduction zones this study analyzes the topography across subduction zones considering the separate contributions of the crust and the mantle lithosphere to the observed surface elevation we have found a transition from a region where the overriding plate is coupled to the descending slab and pulled down along with it to a region where the overriding plate floats freely on the asthenosphere when the subducting slab retreats oceanward rapidly this transition is abrupt and the edge of the overriding plate is uplifted we propose that at some point during rapid slab rollback the overriding plate detaches and rebounds like a boat released from its keel this event is associated with suction of asthenospheric material into the gap that is opened between the plates up to the base of the crust as a result the forearc uplifts and magmatism in the arc increases

the southwest ryukyu arc is a migrating microplate forearc sliver abstract we propose a new interpretation for the tectonism of the southwest ryukyu arc based on inland stress field analyses and the interpretation of seismic reflection profiles the recent stress field in the southwest ryukyu arc is under tension the axial direction of which is parallel to the trench axis but an e w dextral slip shear zone is developing at its western extremity yonaguni and westwards to taiwan these features are closely related to the degree of the oblique subduction of the philippine sea plate and the strength of coupling between it and southwest ryukyu arc we regard the southwest ryukyu arc as a migrating microplate the neotectonics of the southwest ryukyu arc are attributed to the westward migration of the microplate which is bounded by the miyako depression to the east and by taiwan to the west the microplate obducting onto the eurasian plate at taiwan started to migrate westward at about 4 ma when the relative motion between two plates philippine sea and eurasian changed direction from s n to se nw in region

how weak is the subduction zone interface several lines of evidence suggest that subduction zones are weak and that the unique availability of water on earth is a critical factor in the weakening process we have evaluated the strength of subduction zone interfaces using two approaches i from empirical relationships between shear stress at the interface and subduction velocity deduced from laboratory experiments and ii from a parametric study of natural subduction zones that provides new insights on subduction zone interface strength our results suggest that subduction is only mechanically feasible when shear stresses along the plate interface are relatively low less than 35 mpa to account for this requirement we propose that there is a feedback mechanism between subduction velocity water released from the subducting plate and weakening of the fore arc mantle that may explain how relatively low shear stresses are maintained at subduction interfaces globally

exhumation of serpentinized peridotite in the northern manila subduction zone inferred from forward gravity modeling the taiwan integrated geodynamic research program taiger collected two wide angle and reflection seismic transects across the northern manila subduction zone that provide constraints on the seismic velocity structure of the crust two dimensional gravity modeling along these two transects shows a significant relatively high density 3 12 and 3 02 g cm3 in the fore arc region at the interface between the subducting eurasian plate and the accretionary prism in front of the luzon arc on the overriding philippine sea plate the anomalous density in this zone is higher than that in the fore arc crust and the accretionary prism but lower than that in mantle numerous geophysical and geological data together with numerical models have indicated that serpentinization of the fore arc mantle is both expected and observed serpentinization of mantle rocks can dramatically reduce their seismic velocity and therefore their seismic velocity in a density to velocity conversion therefore the source of the high density material could be serpentinized fore arc mantle with serpentinization caused by the dehydration of the subducting eurasian plate we interpret that positive buoyancy combined with weak plate coupling forces in the northern manila subduction zone is resulting in this serpentinized fore arc mantle peridotite being exhumed

plate coupling along the manila subduction zone between taiwan and northern luzon abstract we use gps data trench parallel gravity anomaly tpga and bathymetry to infer plate coupling patterns along the manila subduction zone using a block model and a fault geometry constrained by seismicity we simultaneously solve for the location of euler pole and angular velocity between the sunda and luzon blocks as well as the slip deficit rate on plate interface our estimates show that the euler pole between the sunda and luzon blocks is situated at southern palawan near 8 3 n and 119 4 e with the angular velocity of 4 6 myr 1 the estimated convergence rate along the manila trench continuously decreases southward from 91 mm yr at the northern tip of luzon to 55 mm yr north of mindoro the inversion of gps data reveals partially locked fault patches extending from the west luzon trough to the east of scarborough seamount chain the slip deficit rate in this region is in the range of 20 30 mm yr corresponding to a coupling ratio of 0 4 however the fault slip behavior is not well resolved near the north luzon trough based on a good correlation between locations of large subduction zone earthquakes and areas possessing gravity low we investigate a variety of tpga based plate coupling models assuming different scaling between tpga values and plate coupling ratios the tpga based plate coupling models offer plausible rupture scenarios which are not constrained by current gps data the partially locked fault zone near 15 16 5 n may be associated with the subducted scarborough seamount wherein oceanic floor is highly fractured the great subduction zone earthquake propagates beneath the scarborough seamount seems to be unlikely the densification of gps network in central luzon and seafloor geodetic observations close to trench axis are crucial to distinguish the detailed fault coupling patterns and the role of subducted seamounts

factors affecting seismic moment release rates in subduction zones the amount of shallow seismic activity in subduction zones varies greatly from region to region we quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones to calculate the moment release we sum the seismic moment for all interplate thrust type events with surface wave magnitude ms 7 0 occurring from 1904 to 1980 we present a time history plot of the seismic moment release for each subduction zone these exhibit the differences in the seismic release patterns for subduction zones where the time window of our study is not representative the total moment release is corrected using information on repeat times the moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling these parameters include the age of the subducting lithosphere absolute velocities of the upper and subducting plates convergence velocity and length maximum depth and dip of the wadati benioff zone the moment release rate decreases as the age of the subducting lithosphere increases when the zones belonging to a single subducting plate are considered this age versus moment release relation is consistent for the zones in which the pacific cocos philippine sea and indian plates are subducting the moment release rates for the subduction zones in which the pacific plate is subducting are much higher than for zones of other plates with similar age the age versus moment release relation holds among the zones which belong to one subducting plate however zones with similar ages but belonging to different plates do not have the same moment release rates this suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget zones with retreating upper plates tend to have lower moment release rates the moment release rate does not increase with convergence velocity no simple relationship was found between these two parameters the moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate these are the two independent subduction zone parameters among the variables considered the other variables depend on these two parameters

evidence for early cretaceous oceanic crust trapped in the philippine sea plate abstract the huatung basin is a small oceanic basin located east of taiwan previous age estimates from magnetic lineation studies indicated an eocene age for the basin and formation from the central basin spreading center of the west philippine basin during the last phase of spreading in middle eocene new ar ar ages obtained on gabbros dredged on oceanic basement highs of the huatung basin are early cretaceous these old ages are consistent with early cretaceous ages determined on radiolarian assemblages from lanyu island luzon arc we have performed magnetic anomalies modeling for an early cretaceous oceanic crust our results are in good agreement with new ar ar ages determinations the best fit is indeed obtained with an opening of the huatung basin during the early cretaceous from 131 to 119 ma with a half spreading rate varying between 25 and 30 mm yr the spreading center appears to be located south of the actual basin the abnormal depth 5500 m instead of 5900 m and thickness 12 km instead of 6 km of the crust beneath the basin indicate that there was probably an excess supply of magma during its formation we propose that the basin is a fragment of the former proto south china sea or possibly the new guinea basin that has been trapped by the philippine sea plate

the seismic coupling of subduction zones revisited 1 the nature of seismic coupling for many of the world s subduction zones has been reevaluated geodetic estimates of seismic coupling obtained from gps measurements of upper plate deformation during the interseismic period are summarized we compared those with new estimates of seismic coupling obtained from seismological data the results show that with a few notable exceptions the two methods agree to within about 10 the seismological estimates have been greatly improved over those made 20 30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence in the intervening years of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record the data also again with a few notable exceptions support the frictional instability theory of seismic coupling and in particular the test of that theory made by scholz and campos 1995 overall the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value there is also considerable variation of coupling within individual subduction zones earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation

the potential for a great earthquake along the southernmost ryukyu subduction zone interseismic gps data along the hualien suao coast ne taiwan shows a pattern of strain accumulation that is consistent with a potential future large shallow earthquake along the southernmost ryukyu subduction zone the measured shortening rate parallel to the ryukyu trench is 80 mm yr about twice of the shortening rate perpendicular to the ryukyu trench we invert for slip deficit rates and the geometric configuration of the plate interface our preferred fault model dips 10 northward and extends about 70 km from the ryukyu trench to a depth of 13 km the slip deficit rate exhibits a left lateral motion of 78 mm yr and a normal motion of 36 mm yr on a 290 trending fault the slip rate budget of the southernmost ryukyu subduction zone is close to the plate convergence rate suggesting the plate interface is fully locked assessments of seismic hazard in this region need to consider the potential threat from m w 7 5 8 7 tsunami earthquakes generated by shallow ruptures

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

residual topography lithospheric structure and sunken slabs in the central mediterranean abstract the central mediterranean comprises subduction collision and backarc extension all in a relatively small area in this paper we analyze the topography of this region examining the influence of subducting slabs on overriding plates we take the observed surface elevation remove the contribution of the crust to it and use the residual topography to identify regions that are unusually low unusually high or normal then we calculate lithospheric thickness where local isostasy applies the results provide information about the structure of the lithosphere in this complex region and about the vertical tectonics in particular we found a difference between the south tyrrhenian subduction zone and the apennines consistent with the processes of slab rollback and slab break off respectively in the south tyrrhenian the edge of the oceanic plate is strongly pulled down whereas the edge of the overriding plate the calabrian peninsula is uplifted this opposite trend indicates weak plate coupling that is the subducting slab which is rapidly rolling back hangs almost entirely on the oceanic plate and the overriding plate is free in contrast in the apennines high residual topography and uplifting are detected on both sides of the mountain belt consistent with complete break off of the subducted lithosphere

offshore imaging with complex overburden understanding gather complexity and resulting attribute accuracy through synthetics the objectives of this study are to determine the effect of complex overburden in offshore continental settings on the seismic wavefield and on the resultant seismic images and attributes seismic waves exhibit complex wave paths when propagating through heterogeneous geologic structures a key step in understanding the consequences for imaging and more importantly amplitude versus angle behavior is to model this complexity in a known earth velocity model the earth model used represented by the seismic velocities should exhibit similar behavior to the real data but still remain as simple as possible this is also the case in velocity model building however the inability to include subtle geologic features leads to inaccuracy and uncertainty in the velocity model understanding how this inaccuracy manifests itself is vital in any predictive exercise using seismic attributes to improve this understanding several synthetic data sets were simulated using full wavefield synthetics it is evident from

cenozoic climate change as a possible cause for the rise of the andes causal links between the rise of a large mountain range and climate have often been considered to work in one direction with significant uplift provoking climate change here we propose a mechanism by which cenozoic climate change could have caused the rise of the andes based on considerations of the force balance in the south american lithosphere we suggest that the height of and tectonics in the andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands and shear stresses in the subduction zone depend on the amount of subducted sediments we propose that the dynamics of subduction and mountain building in this region are controlled by the processes of erosion and sediment deposition and ultimately climate in central south america climate controlled sediment starvation would then cause high shear stress focusing the plate boundary stresses that support the high andes

spatial changes of inter plate coupling inferred from sequences of small repeating earthquakes in japan 1 we extract sequences of small repeating earthquakes to clarify inter plate coupling of subducting plates over a large area of the japanese islands as a result many sequences arc detected at the philippine sea plate subducting from the ryukyu trench and pacific plate subducting from the kuril japan trench the average slip rates and standard deviations estimated from the sequences show substantial spatial changes of inter plate coupling the large deviations of slip rates correspond to the occurrence of episodic slips such as after slips following large earthquakes constant slip rates approaching the relative plate motion indicate weak coupling areas slip deficits and sparse distributions of repeating groups suggest locked areas in the nankai trough deep low frequency earthquakes in the transition zone and burst type repeating sequences within plates have not been located in the downdip direction of groups with slow slip rates this suggests that the space time characteristics of inter plate coupling affected these seismic events

a mega splay fault system and tsunami hazard in the southern ryukyu subduction zone abstract in april 1771 a subduction earthquake generated a great tsunami that struck the south ryukyu islands and killed 12 000 people whereas its mechanism is still enigmatic nakata and kawana 1995 nakamura 2006 matsumoto et al 2009 in this paper we show its probable source on a mega splay fault system existing along the southern ryukyu forearc analyses of deep multi channel seismic reflection profiles indicate that the mega splay fault system is rising from the summit of a 1 km high ridge situated at a 5 landward dipping plate interface an outer ridge marks the seafloor outcrop of the splay fault system and separates the landward inner wedge and the oceanward outer wedge the inner wedge is uplifting and exhibits widespread normal faulting while the outer wedge shows folded structures the mega splay fault system is parallel to the ryukyu trench east of 125 5 e and is estimated to be 450 km long the origin of this south ryukyu mega splay fault system is ascribed to a resistant subduction of the elevated transverse ridges associated with the subducting portion of the trench parallel luzon okinawa fracture zone in contrast no similar splay fault is found west of 125 5 e where the oblique subduction has produced large shear zones along the south ryukyu forearc we infer that a thrust earthquake linked to the mega splay fault system is responsible for the south ryukyu tsunami however another possible scenario of generating a large tsunami affecting the south ryukyu islands is that the subducted ridge in the western end of the mega splay fault system nucleated a large earthquake and simultaneously triggered the 100 km long e w trending strike slip fault west of 125 5 e and induced a southward dipping tsunami genic subsidence in any case after a quiescence of 241 yr a large earthquake and tsunami is anticipated in the south ryukyu forearc in the near future

transition from oblique subduction to collision earthquakes in the southernmost ryukyu arc taiwan region tectonic characteristics of the region between taiwan and the southernmost ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5 5 mb 6 6 five major seismogenic structures can be delineated the collision seismic zone csz the interface seismic zone isz the wadati benioff seismic zone wbsz the lateral compression seismic zone lcsz and the okinawa seismic zone osz in the csz located along the east coast of taiwan and offshore earthquake focal mechanisms show horizontal p axes distributed in two directions 287 10 and 333 16 possibly reflecting a strain partition associated with the relative plate convergence between the eurasia plate and the philippine sea plate the corresponding seismic strain tensor indicates a maximum compressive strain rate of 1 2 10 7 yr 1 along 293 and a comparable extension in vertical direction presumably resulted from plate collision in the region the geometry of the isz which is distorted significantly at its westernmost end can be approximated by a north dipping plane that is gradually pushed northward with increasing dip the seismogenic portion of the interface spans a short depth range from 10 km to 35 km a clear pattern of earthquake slip partition is observed the average slip vector residual is as large as 35 seismic strain patterns within the subducted philippine sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at 270 km different from the pattern of strain segmentation observed for the rest of the ryukyu arc where the northern and southern portions are dominated by downdip extension and compression respectively owing to the large convergence obliquity the slab is descending at a rate significantly slower near taiwan than in the southern ryukyu thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab s negative buoyancy a number of thrust or oblique strike slip earthquakes between isz and wbsz show a consistent pattern of lateral compression with p axes oriented roughly parallel to the local strike of the trench arc system they are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere shallow normal faulting earthquakes show successive rotation of t axes from approximately n s in the okinawa trough to approximately e w in northeast taiwan possibly as a result of interaction between the extension from the opening of the okinawa trough and the compression from collision one normal event january 18 1991 mb 5 9 occurred in the central range with t axis roughly parallel to the structural trend of taiwan implying that the nature of the orogeny in taiwan has changed from thin skinned deformation to lithospheric collision involving the whole crust and uppermost mantle

trench parallel stretching and folding of forearc basins and lateral migration of the accretionary wedge in the southern ryukyus a case of strain partition caused by oblique convergence detailed seafloor mapping in the area east of taiwan revealed trench parallel stretching and folding of the ryukyu forearc and lateral motion of the accretionary wedge under oblique convergence east of 122 40 e a steep accretionary wedge is elongated in an e w direction a major transcurrent right lateral strike slip fault accommodates the strain partitioning caused by an oblique convergence of 40 a spectacular out of sequence thrust may be related to the subduction of a structural high lying in the axis of the n s trending gagua ridge this asperity is likely responsible for the uplift of the accretionary wedge and forearc basement and may have augmented strain partitioning by increasing the coupling between the two plates west of 122 40 e the low taper accretionary wedge is sheared in a direction subparallel to the convergence vector with respect to the ryukyu arc the bayonet shape of the southern ryukyu arc slope partly results from the recent re opening of the southern okinawa trough at a rate of about 2 to 4 cm yr right lateral shearing of the sedimentary forearc with respect to the nonlinear ryukyu backstop generates trench parallel extension in the forearc sediment sequence at dilational jogs and trench parallel folding at compressive jogs the hoping basin lies above a diffuse trench trench fault ttf or tff unstable triple junction moving toward the south along a n s transform zone which accommodates the southward drift of the ryukyu arc with respect to eurasia

origin and evolution of the west philippine basin a new interpretation abstract combined analyses of the magnetic lineations and seafloor structures of the west philippine basin show that it was formed by spreading from the central basin spreading center in two distinctly different spreading phases before and after subduction was initiated along the palau kyushu trend at 45 ma b p these episodes are recognized by differences in the strike of the magnetic lineations spreading rates density and strike of fracture zones and basement relief from 60 45 ma b p spreading was ne sw relative to present orientations at a half rate of 44 mm a at about 45 ma b p the spreading direction changed to a more n s direction with a reconfiguration of the central basin spreading center into numerous short e w segments offset by closely spaced n s transform faults spreading slowed to a half rate of 18 mm a and ceased at 35 ma b p thus the west philippine basin originated at 45 ma b p by the trapping of normal ocean crust west of the initial subduction along the palau kyushu trend the 45 35 ma b p period represents the dying phase of spreading on the central basin spreading center following isolation of the west philippine basin from the plate driving forces of the pacific

plate tectonics and pelagic facies late jurassic to early cretaceous deep sea sediments of the ybbsitz ophiolite unit eastern alps austria abstract pelagic sediments of the ybbsitz unit are part of an ophiolite sequence within the penninic unit in the eastern alps cherts and calpionellid limestones of late jurassic to early cretaceous age were deposited on oceanic basement sedimentation near an oceanic spreading center below the calcium compensation depth ccd is represented by chert clay rhythmites cherts display hydrothermal fe and mn mineralizations that are characterized by low contents of co ni and cu as well as by high fe2o3 tio2 and mno tio2 ratios as the site of deposition migrated away from the spreading center hydrothermal influence ceased due to the late jurassic drop of the ccd calcareous radiolarian ooze was deposited on intrabasinal topographic heighs downslope transport of this ooze to sites below the ccd by turbidity currents resulted in intercalations of calcareous and carbonate free sediments apparently the drop of the ccd was greater than the rate of cooling induced basement subsidence as calcareous radiolarites overly this sequence carbonate contents gradually increase upsection and micritic limestones terminate purely pelagic sedimentation at the jurassic cretaceous boundary plate drift moved the site of deposition towards an active continental margin producing a coarsening upward clastic sequence this sequence starts with turbidite and breccia intercalations in pelagic limestones followed by the deposition of early cretaceous flysch it marks the onset of subduction of the ybbsitz unit under the austroalpine continental margin