identification and characterization of a subsided barrier island in the holocene alluvial plain niigata central japan abstract the echigo plain in the niigata area of central japan is a coastal alluvial plain facing the sea of japan the plain contains several rows of coastal sand dunes during the holocene maximum transgression a barrier island sand body was formed on the plain however the distribution of this barrier island is partially intermittent based on facies analysis of cores the barrier island succession was found to be buried by alluvium in the central part of the echigo plain the barrier sand bodies do occur in the central part of the echigo plain but they are deeply subsided and tilted by an active fault in the western margin of the plain the alluvium in this area records a history of subsidence and changes in the depositional system due to the movements on the active fault

low friction along the high slip patch of the 2011 mw 9 0 tohoku oki earthquake required from the wedge structure and extensional splay faults we analyze the mechanical properties needed to account for the large shallow slip during the 2011 tohoku oki earthquake and the activation of landward normal faulting within the forearc we show that the morphology and internal structure of the forearc follows closely the prediction of the critical coulomb wedge in horizontal compression implying a high internal pore pressure ratio 0 7 0 14 0 48 and a low effective basal friction eff b 0 14 0 18 0 04 we then show that the activation of the normal fault requires a lower effective basal friction beneath the outer wedge than beneath the inner wedge outer 0 015 possibly due to transient dynamic weakening associated to the seismic rupture forearc normal faults could be considered as evidence for very efficient dynamic weakening along the megathrust and typify megathrust with high tsunamigenic potential

permanent deformation caused by subduction earthquakes in northern chile earth s crust is thought to eventually rebound following an earthquake so that deformation is not permanent field analysis in the atacama desert northern chile however identifies numerous large cracks in the crust implying that the crust here has been permanently deformed by thousands of earthquakes that have occurred over the past million years

extension of continental crust by anelastic deformation during the 2011 tohoku oki earthquake the role of extensional faulting in the generation of a great tsunami abstract observations of seafloor morphologies and environments made before and after the 2011 tohoku oki earthquake reveal open fissures generated during the earthquake where the fault trace is interpreted on seismic profiles to intersect the seafloor anomalously high heat flow was observed at a landward dipping normal fault in august 2011 five months after the earthquake but by august 2012 heat flow measured at the same station had decreased to close to the background value which suggests that the normal fault ruptured during the 2011 earthquake these seafloor observations and measurements demonstrate deformation that was both extensional and anelastic within the overriding continental plate during the 2011 earthquake seismic profiles as well as seafloor bathymetry data in the tsunami source area further demonstrate that landward dipping normal faults extensional faults collapse the continental framework and detach the seaward frontal crust from the landward crust at far landward from the trench the extensional and anelastic deformation i e normal faulting observed in both seafloor observations and seismic profiles allows the smooth seaward movement of the continental crust seaward extension of the continental crust close to the trench axis in response to normal faulting is a characteristic structure of tsunami source areas as similar landward dipping normal faults have been observed at other convergent plate margins where tsunamigenic earthquakes have occurred we propose that the existence of a normal fault that moves the continental crust close to the trench can be considered one indicator of a source area for a huge tsunami

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

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

megathrust earthquake swarms indicate frictional changes which delimit large earthquake ruptures abstract using a recently constructed catalog of megathrust earthquake swarms we present results that illustrate how fault slip for m w 8 5 earthquakes with a publicly available finite fault rupture model is delimited along strike by swarm regions we further investigate this relationship for all m w 7 5 subduction zone megathrust earthquakes that have occurred over the last 40 years by estimating the rupture area from early aftershocks this extended comparison shows only 2 out of 42 earthquakes propagate through regions of the megathrust that have documented recurring earthquake swarm activity while about 40 17 out of 42 are terminate on at least one side near regions with swarm activity in japan chile sumatra and alaska earthquake swarms correlate with regions of the plate interface that exhibit low interseismic strain accumulation this suggests earthquake swarms are associated with reduced plate coupling and possibly aseismic moment release however earthquakes in chile and alaska ruptured through regions of weak coupling that do not show swarm activity we propose instead that stress heterogeneity on the megathrust can best explain why large earthquake ruptures terminate in swarm regions but not in regions with weak coupling alone earthquake swarms appear to be a proxy for along strike segmentation of subduction megathrusts at least on the time scale of one seismic cycle

subduction of seamounts at the java trench a view with long range sidescan sonar abstract we describe here a 1300 km by 45 km gloria long range sidescan sonar swath along the eastern java trench the swath images the trench axis a narrow strip of oceanic crust to the south and the toe of the accretionary wedge to the north between 108 and 120 e sonar images of the ocean crust show a pattern of normal faults typical of the outer wall of trenches these result from tension related to the bending of the oceanic lithosphere into the subduction zone a number of sub circular seamounts are also seen some of which are currently being subducted isolated ponds of flat lying sediments occur in the trench axis although along much of its length the trench is devoid of such sediment accumulations on the inner trench wall the accretionary wedge is recognised by its distinctive grainy texture with the grain aligned sub parallel to the deformation front where subducting seamounts are colliding with the wedge large crescentic areas of very high backscattering correlate with re entrants in the deformation front and large indentations in the wedge the high backscattering within these collision scars is interpreted to arise from outcropping strata and talus covered slopes resulting from erosion of an inner trench wall shortened and oversteepened by collision with a seamount all of the sediment ponds in the trench axis occur in close association with collision scars strongly suggesting localised erosion of the accretionary wedge in the scar areas however the volume of the indentations appears to be an order of magnitude greater than the volume of eroded sediments deposited in the trench indicating that erosion cannot be the primary mechanism by which indentations are created most of the missing material must be displaced landward by thrusting and folding ahead of the seamount our observations at the java trench are broadly comparable with those made at the japan trench by french and japanese workers however some differences in the detail of the collision process are evident in particular oversteepening and erosion occur from the earliest stage of collision at the java trench while uplift and understeepening initially dominate at the japan trench differences in the effects of subducting seamounts in the two areas are attributed to contrasts between the relatively unlithified strata of the java trench inner wall and the more lithified strata of the non accreting japan trench inner wall

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

past seismic slip to the trench recorded in central america megathrust the 2011 t hoku oki earthquake revealed that co seismic displacement along the plate boundary megathrust can propagate to the trench co seismic slip to the trench amplifies hazards at subduction zones so its historical occurrence should also be investigated globally here we combine structural and experimental analyses of core samples taken offshore from southeastern costa rica as part of the integrated ocean drilling program iodp expedition 344 with three dimensional seismic reflection images of the subduction zone we document a geologic record of past co seismic slip to the trench the core passed through a less than 1 9 million year old megathrust frontal ramp that superimposes older miocene biogenic oozes onto late miocene pleistocene silty clays this together with our stratigraphic analyses and geophysical images constrains the position of the basal decollement to lie within the biogenic oozes our friction experiments show that when wet silty clays and biogenic oozes are both slip weakening at sub seismic and seismic slip velocities oozes are stronger than silty clays at slip velocities of less than or equal to 0 01 m s 1 and wet oozes become as weak as silty clays only at a slip velocity of 1 m s 1 we therefore suggest that the geological structures found offshore from costa rica were deformed during seismic slip to the trench events during slower aseismic creep deformation would have preferentially localized within the silty clays past megathrust earthquakes in the costa rica subduction zone have slipped all the way up to the seafloor according to analyses of core and seismic data this shallow slip was accommodated by layers of weak biogenic ooze

fault zone controlled seafloor methane seepage in the rupture area of the 2010 maule earthquake central chile seafloor seepage of hydrocarbon bearing fluids has been identified in a number of marine forearcs however temporal variations in seep activity and the structural and tectonic parameters that control the seepage often remain poorly constrained subduction zone earthquakes for example are often discussed to trigger seafloor seepage but causal links that go beyond theoretical considerations have not yet been fully established this is mainly due to the inaccessibility of offshore epicentral areas the infrequent occurrence of large earthquakes and challenges associated with offshore monitoring of seepage over large areas and sufficient time periods here we report visual geochemical geophysical and modelling results and observations from the concepcion methane seep area offshore central chile located in the rupture area of the 2010 mw 8 8 maule earthquake high methane concentrations in the oceanic water column and a shallow sub bottom depth of sulfate penetration indicate active methane seepage the stable carbon isotope signature of the methane and hydrocarbon composition of the released gas indicate a mixture of shallow sourced biogenic gas and a deeper sourced thermogenic component pristine fissures and fractures observed at the seafloor together with seismically imaged large faults in the marine forearc may represent effective pathways for methane migration upper plate fault activity with hydraulic fracturing and dilation is in line with increased normal coulomb stress during large plate boundary earthquakes as exemplarily modelled for the 2010 earthquake on a global perspective our results point out the possible role of recurring large subduction zone earthquakes in driving hydrocarbon seepage from marine forearcs over long timescales

shallow dynamic overshoot and energetic deep rupture in the 2011 mw 9 0 tohoku oki earthquake strong spatial variation of rupture characteristics in the moment magnitude mw 9 0 tohoku oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed finite source imaging reveals that the rupture consisted of a small initial phase deep rupture for up to 40 seconds extensive shallow rupture at 60 to 70 seconds and continuing deep rupture lasting more than 100 seconds a combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault despite prodigious total slip shallower parts of the rupture weakly radiated at high frequencies whereas deeper parts of the rupture radiated strongly at high frequencies

response of forearc crustal faults to the megathrust earthquake cycle insar evidence from mejillones peninsula northern chile abstract we report on a rare example of aseismic response of a creeping fault to the earthquake cycle of a nearby megathrust interferometric synthetic aperture radar insar is used to detect and analyze shallow creep of two crustal faults at mejilones peninsula northern chile located in the hanging wall of the 2007 mw 7 7 tocopilla subduction earthquake we generate two independent time series of surface deformation spanning 3 5 yr of late interseismic and 1 5 yr early postseismic deformation associated with this event the analysis reveals creep on the mejillones fault as well as on a previously unmapped fault to the west of the mejillones fault the insar deformation maps and distributed slip models obtained from the data reveal that fault creep reversed between the interseismic and postseismic periods given the regional stress field perturbations due to interseismic and coseismic deformation we argue that the observed shallow creep and its slip reversal are directly linked to the megathrust seismic cycle moreover from similar eastward dips but opposite slip directions of the two faults we infer that fault strength must be very low and that the kinematics is controlled by crustal flexure associated with the seismic cycle on the underlying megathrust

high resolution relocation and mechanism of aftershocks of the 2007 tocopilla chile earthquake summary we study the distribution of the aftershocks of tocopillamw 7 7 earthquake of 2007 november 14 in northern chile in detail this earthquake broke the lower part of the seismogenic zone at the southern end of the northern chile gap a region that had its last megathrust earthquake in 1877 the aftershocks of tocopilla occurred in several steps the first day they were located along the coast inside the co seismic rupture zone after the second day they extended ocean wards near the mejillones peninsula finally in december they concentrated in the south near the future rupture zone of the michilla intermediate depth earthquake of 2007 december 16 the aftershock sequence was recorded by the permanent ipoc integrated plate boundary observatory in chile network and the temporary task force network installed 2 weeks after the main event a total of 1238 events were identified and the seismic arrival times were directly read from seismograms initially we located these events using a single event procedure and then we relocated them using the double difference method and a crosscorrelation technique to measure time differences for clusters of aftershocks we tested a 1 d velocity model and a 2 d one that takes into account the presence of the subducted nazca plate relocation significantly reduced the width of the aftershock distribution in the inland area the plate interface imaged by the aftershocks is thinner than 2km the two velocity models give similar results for earthquakes under the coast and a larger difference for events closer to the trench the surface imaged by the aftershocks had a length of 160km it extends from 30 to 50km depth in the northern part of the rupture zone and between 5 and 55km depth near the mejillones peninsula we observed a change in the dip angle of the subduction interface from 18 to 24 at a depth of 30km we propose that this change in dip is closely associated with the upper limit of the rupture zone of the main event we also studied the focal mechanisms of the aftershocks most of them were thrust events like the mainshock as the aftershock activity was significantly reduced on 2007 december 13 an ml 6 1 event occurred offshore of the mejillones peninsula reactivating the seismicity three days later the michilla intraslab earthquake of mw 6 8 ruptured an almost vertical fault with slab push mechanism the aftershocks locations of this event define a planar zone about 11km in depth situated right bellow the subduction interface

active tectonics of the south chilean marine fore arc 35 s 40 s the south chilean marine fore arc 35 s 40 s is separated into four tectonic segments concepcion north concepcion south nahuelbuta and tolten from north to south these are each characterized by their individual tectonic geomorphology and reflect different ways of mechanical and kinematic interaction of the convergent nazca and south american plates splay faults that cut through continental framework rock are seismically imaged in both concepcion segments and the tolten segment additionally the concepcion south segment exhibits prominent upper plate normal faults normal faults apparently relate to uplift caused by sediment underthrusting at depth this has led to oversteepening and gravitational collapse of the marine fore arc there is also evidence for sediment underthrusting and basal accretion to the overriding plate in the tolten segment there uplift of the continental slope has created a landward inclined seafloor over a latitudinal distance of 50 km in the nahuelbuta segment transpressive upper plate faults aligned oblique to the direction of plate motion control the seafloor morphology based on a unique acoustic data set including 90 of bathymetric coverage of the continental slope we are able to reveal an along strike heterogeneity of a complexly deformed marine fore arc which had escaped attention in previous studies that only considered the structure along transects normal to the plate margin

andean structural control on interseismic coupling in the north chile subduction zone segmentation can influence the extent of earthquake rupture and event magnitude large megathrust earthquakes result from total rupture of relatively continuous segments of the subduction interface segmentation is attributed to variations in the frictional properties of the seismogenic zone or to topographic features on the down going plate structures in the overriding plate may also influence segmentation but their importance has been dismissed here we investigate the links between interface segmentation at the north chile seismic gap and a crustal scale fault structure in the overriding plate that forms a coastal scarp of about 1 km in height we use satellite interferometric synthetic aperture radar insar and global positioning system gps data to measure interseismic surface deformation between 2003 and 2009 and compare the deformation with rupture extent during well documented earthquakes from these data we infer the degree of coupling and segmentation at depth we find that along a 500 km long segment the base of the strongly coupled seismogenic zone correlates with the line of the surface coastal scarp and follows the outline of the mejillones peninsula this correlation implies that large scale structures in the overriding plate can influence the frictional properties of the seismogenic zone at depth we therefore suggest that the occurrence of megathrust earthquakes in northern chile is controlled by the surface structures that build andean topography

the 2007 m7 7 tocopilla northern chile earthquake sequence implications for along strike and downdip rupture segmentation and megathrust frictional behavior 1 in 2007 a m7 7 earthquake occurred near the town of tocopilla within the northern chile seismic gap main shock slip derived from coseismic surface deformation was confined to the depth range between 30 and 55 km we relocated 1100 events during six months before and one week after the main shock aftershock seismicity is first congruent to the main shock slip and then it spreads offshore west and northwest of mejillones peninsula mp waveform modeling for 38 aftershocks reveals source mechanisms that are in the majority similar to the main shock however a few events appear to occur in the upper plate some with extensional mechanisms juxtaposing the tocopilla aftershocks with those following the neighboring 1995 antofagasta earthquake produces a striking symmetry across an ew axis in the center of mp events seem to skirt around mp probably due to a shallower moho there we suggest that the seismogenic coupling zone in northern chile changes its frictional behavior in the downdip direction from unstable to mostly conditionally stable for both earthquake sequences aftershocks agglomerate in the conditionally stable region whereas maximum inter seismic slip deficit and co seismic slip occurs in the unstable region the boundary between the unstable and conditionally stable zones parallels the coastline we identify a similar segmentation for other earthquakes in chile and peru where the offshore segments break in great m 8 earthquakes and the onshore segments in smaller m 8 earthquakes using critical taper analysis we demonstrate a causal relationship between varying slip behavior on the interface and forearc wedge anatomy that can be attributed to spatial variations in the rate dependency of friction

gradual unlocking of plate boundary controlled initiation of the 2014 iquique earthquake a long foreshock series unlocked the south american plate boundary until eventually initiating the m 8 1 iquique chile earthquake two groups publishing in this issue of nature analyse the seismic context of the iquique earthquake that occurred off the coast of northern chile on 1 april 2014 in a seismic zone that had been quiescent since a significant event in 1877 gavin hayes et al identify areas of remaining or elevated earthquake hazard along the megathrust fault in the region and conclude that the 2014 iquique event was not the earthquake that had been anticipated given that significant sections of the northern chile subduction zone have not ruptured in almost 150 years they suggest that it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 iquique sequence bernd schurr et al show that the april 2014 earthquake broke a central fraction of the northern chile seismic gap the last major segment of the south american plate boundary that had yet to rupture in the past century from july 2013 up to the april earthquake they identify three seismic clusters along this part of the plate boundary each lasting a few weeks with earthquakes of increasing peak magnitudes they conclude that these seismic clusters and their slip transients reflect a gradual weakening of the central part of the seismic gap that was instrumental in initiating the final failure on 1 april 2014 northern chile was struck by a magnitude 8 1 earthquake following a protracted series of foreshocks the integrated plate boundary observatory chile monitored the entire sequence of events providing unprecedented resolution of the build up to the main event and its rupture evolution here we show that the iquique earthquake broke a central fraction of the so called northern chile seismic gap the last major segment of the south american plate boundary that had not ruptured in the past century1 2 since july 2013 three seismic clusters each lasting a few weeks hit this part of the plate boundary with earthquakes of increasing peak magnitudes starting with the second cluster geodetic observations show surface displacements that can be associated with slip on the plate interface these seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion leading up to this earthquake the b value of the foreshocks gradually decreased during the years before the earthquake reversing its trend a few days before the iquique earthquake the mainshock finally nucleated at the northern end of the foreshock area which skirted a locked patch and ruptured mainly downdip towards higher locking peak slip was attained immediately downdip of the foreshock region and at the margin of the locked patch we conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure distinguishing the iquique earthquake from most great earthquakes finally only one third of the gap was broken and the remaining locked segments now pose a significant increased seismic hazard with the potential to host an earthquake with a magnitude of 8 5

interseismic coupling megathrust earthquakes and seismic swarms along the chilean subduction zone 38 18 s the recent expansion of dense gps networks over plate boundaries allows for remarkably precise mapping of interseismic coupling along active faults the interseismic coupling coefficient is related to the ratio between slipping velocity on the fault during the interseismic period and the long term plates velocity but the interpretation of coupling in terms of mechanical behavior of the fault is still unclear here we investigate the link between coupling and seismicity over the chilean subduction zone that ruptured three times in the last 5 years with major earthquakes maule mw 8 8 in 2010 iquique mw 8 1 in 2014 and illapel mw 8 4 in 2015 we combine recent gps data acquired over the margin 38 18 s with older data to get the first nearly continuous picture of the interseismic coupling variations on the subduction interface here we show that at least six low coupling zones lcz areas where coupling is low relatively to the neighboring highly coupled segments can be identified we also find that for the three most recent mw 8 events co seismic asperities correlate well with highly coupled segments while lczs behaved as barriers and stopped the ruptures the relation between coupling and background seismicity in the interseismic period before the events is less clear however we note that swarm sequences are prone to occur in intermediate coupling areas at the transition between lcz and neighboring segments and that the background seismicity tends to concentrate on the downdip part of the seismogenic locked zone thus highly coupled segments usually exhibit low background seismicity in this overall context the metropolitan segment that partly ruptured during the 2015 illapel earthquake appears as an outlier since both coupling and background seismicity were high before the rupture raising the issue of the remaining seismic hazard in this very densely populated area

accretionary prisms in subduction earthquake cycles the theory of dynamic coulomb wedge 1 we expand the theory of critically tapered coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles building on the coulomb plasticity of the classical theory we assume an elastic perfectly coulomb plastic rheology and derive exact stress solutions for stable and critical wedges the new theory postulates that the actively deforming most seaward part of an accretionary prism the outer wedge overlies the updip velocity strengthening part of the subduction fault and the less deformed inner wedge overlies the velocity weakening part the seismogenic zone during great earthquakes the outer wedge is pushed into a compressively critical state with an increase in basal and internal stresses and pore fluid pressure after the earthquake the outer wedge returns to a stable state the outer wedge geometry is controlled by the peak stress of the updip velocity strengthening part of the subduction fault achieved in largest earthquakes the inner wedge generally stays in the stable regime throughout earthquake cycles acting as an apparent backstop and providing a stable environment for the formation of forearc basins the new theory has important implications for the studies of the updip limit of the seismogenic zone the evolution of accretionary prisms and forearc basins activation of splay faults and tsunami generation evolution of the fluid regime and mechanics of frontal prisms at margins dominated by tectonic erosion

crustal extensional faulting triggered by the 2010 chilean earthquake the pichilemu seismic sequence 1 we report a sequence of crustal quakes that began after the mw 8 8 thrust subduction maule earthquake that affected the central chile margin on 27 february 2010 this activity lasted by several months having the most important events on 11 march 2010 mw 6 9 and mw 7 0 with normal focal mechanisms seismicity shows a rupture oriented along a nw striking and sw dipping normal fault from the surface down to the interplate contact seismicity can be correlated with neotectonics extensional structures similarly oriented in the region which have coexisted with nne ssw reverse faults since the late pliocene even though both have older periods of activity since the paleozoic this crustal rupture would have been triggered by the high coulomb stress change produced by the maule earthquake enhanced by likely fluid presence along weakened zones of the forearc crust as evidenced by high vp vs ratio the occurrence of relevant neotectonic activity in coincidence with short term deformation suggests a relationship with long term tectonic features of this region which would have been acting as a barrier during the interseismic period increasing the strain accumulation and triggering contractional faulting in the crust as well as producing high slip patches during great subduction ruptures favoring triggering of crustal extensional faulting crustal faulting in pichilemu suggests that this kind of events should be considered in seismic hazard analysis despite the absence of historical crustal seismic activity before the maule earthquake

continuing megathrust earthquake potential in chile after the 2014 iquique earthquake the 2014 iquique event was not the earthquake that had been expected to fill the regional seismic gap given that significant sections of the northern chile subduction zone have not ruptured in almost 150 years it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 iquique sequence two groups publishing in this issue of nature analyse the seismic context of the iquique earthquake that occurred off the coast of northern chile on 1 april 2014 in a seismic zone that had been quiescent since a significant event in 1877 gavin hayes et al identify areas of remaining or elevated earthquake hazard along the megathrust fault in the region and conclude that the 2014 iquique event was not the earthquake that had been anticipated given that significant sections of the northern chile subduction zone have not ruptured in almost 150 years they suggest that it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 iquique sequence bernd schurr et al show that the april 2014 earthquake broke a central fraction of the northern chile seismic gap the last major segment of the south american plate boundary that had yet to rupture in the past century from july 2013 up to the april earthquake they identify three seismic clusters along this part of the plate boundary each lasting a few weeks with earthquakes of increasing peak magnitudes they conclude that these seismic clusters and their slip transients reflect a gradual weakening of the central part of the seismic gap that was instrumental in initiating the final failure the seismic gap theory1 identifies regions of elevated hazard based on a lack of recent seismicity in comparison with other portions of a fault it has successfully explained past earthquakes see for example ref 2 and is useful for qualitatively describing where large earthquakes might occur a large earthquake had been expected in the subduction zone adjacent to northern chile3 4 5 6 which had not ruptured in a megathrust earthquake since a m 8 8 event in 1877 on 1 april 2014 a m 8 2 earthquake occurred within this seismic gap here we present an assessment of the seismotectonics of the march april 2014 iquique sequence including analyses of earthquake relocations moment tensors finite fault models moment deficit calculations and cumulative coulomb stress transfer this ensemble of information allows us to place the sequence within the context of regional seismicity and to identify areas of remaining and or elevated hazard our results constrain the size and spatial extent of rupture and indicate that this was not the earthquake that had been anticipated significant sections of the northern chile subduction zone have not ruptured in almost 150 years so it is likely that future megathrust earthquakes will occur to the south and potentially to the north of the 2014 iquique sequence

seismic structure and tectonics of the continental wedge overlying the source region of the iquique mw8 1 2014 earthquake abstract on april 1 2014 a large earthquake mw 8 1 ruptured the central part of a historic seismic gap in northern chile in order to study the relationship between the co seismic rupture characteristics and the crustal structure of the subduction zone we processed a trench perpendicular seismic reflection profile acquired across the zone of maximum slip and generated a p wave velocity model the results show a frontal prism in the continental wedge characterized by low velocities that increase rapidly towards the shore and acted as a barrier for trench ward propagation of aftershocks landward a transition zone with increasing upper crust velocity 4 5 km s concentrates most of the aftershocks in addition a trench ward dipping set of fault zones is observed along the continental wedge associated to the iquique forearc basin formation 1 5 km thick at the depocenter on this profile we identify three stratigraphic units within the basin a landward tilt and thickness increase is detected in each stratigraphic unit along with growth strata and domino structures suggesting landward migration of syn extensional deformation in response to basal subduction erosion by extrapolating our results to the plate boundary and based on published focal mechanisms of intra crustal seismicity we find a strong spatial correlation between the iquique basin and the highest slip area for the 2014 earthquake suggesting long term extensional deformation due to coseismic tensional stresses

the appearance of sustained equatorial surface westerlies during the 1982 pacific warm event in june 1982 a band of anomalous southerly surface wind extending from the equator as far south as the tasman sea formed east of australia 150 e to 160 e this flow crossed the equator just before the appearance of sustained westerly winds on the equator somewhat west of the date line because these westerly winds induced the initial strong equatorial warming of the ocean east of the date line during the 1982 el nino southern oscillation enso event the southerly jet appears to be an important atmospheric component leading to the onset of the vigorous phase of this event some historical evidence suggests that anomalous southerly winds in the same region occurred prior to the appearance of sustained equatorial westerly winds in the major enso events of 1957 1965 and 1972

megathrust friction determined from mechanical analysis of the forearc in the maule earthquake area the seismogenic potential of a given fault depends essentially on its frictional properties and on the mechanical properties of the medium determining the spatio temporal variations of frictional properties is therefore a key issue in seismotectonics this study aims to characterize the friction on the south america megathrust in the 2010 mw 8 8 maule earthquake area from mechanical analysis of the forearc structure and morphology based on the critical taper theory we first show that the rupture area of the maule earthquake also shown to be locked in the interseismic period coincides with the stable part of the wedge in the surrounding area the wedge is critical a finding consistent with various evidence for active deformation there this is in particular true for the arauco peninsula area which seems to have arrested the maule earthquake s rupture to the south this observation lends support to the view that the seismic rupture is inhibited when propagating beneath a critical area the geometry of the critical portion of the wedge suggests a standard internal friction int 0 7 0 13 and a hydrostatic pore pressure within the wedge the effective friction beneath the critical outer wedge is estimated to be eff b 0 27 0 03 this could be related to intrinsically low friction minerals clay or high pore pressure along the megathrust we next use the limit analysis approach to constrain the variation of the effective friction along the megathrust based on the location and geometry of internal faulting within the forearc a low effective friction is found within the rupture area eff b 0 14 to explain the reactivation of thrust fault such as the santa maria updip of the coseismic rupture or the activation of normal splay faults downdip of the rupture area the low effective friction found there could reflect strong dynamic weakening

the 2011 tohoku oki earthquake displacement reaching the trench axis we detected and measured coseismic displacement caused by the 11 march 2011 tohoku oki earthquake moment magnitude mw 9 0 by using multibeam bathymetric surveys the difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the japan trench suggesting that the fault rupture reached the trench axis the sea floor on the outermost landward area moved about 50 meters horizontally east southeast and 10 meters upward the large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement

seismic structure of the north central chilean convergent margin subduction erosion of a paleomagmatic arc we study the erosive convergent margin of north central chile at similar to 31 degrees s by using high resolution bathymetric wide angle refraction and multichannel seismic reflection data to derive a detailed tomographic 2 d velocity depth model in the overriding plate our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6 0 6 5km s which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the coastal cordillera beneath the lower and middle continental slope however the presence of a zone of reduced velocities 3 5 5 0km s is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion at the landward edge of the outermost fore arc the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp similar to 1km offset which overlies a strong lateral velocity contrast from similar to 5 0 to similar to 6 0km s this pronounced velocity contrast propagates deep into the continental crust and it resembles a major normal listric fault we interpret this seismic discontinuity as the volcanic continental basement contact of the submerged coastal cordillera characterized by a gravitational collapse of the outermost fore arc subduction erosion has most likely caused large scale crustal thinning and long term subsidence of the outermost fore arc

does permanent extensional deformation in lower forearc slopes indicate shallow plate boundary rupture abstract seismic rupture of the shallow plate boundary can result in large tsunamis with tragic socio economic consequences as exemplified by the 2011 tohoku oki earthquake to better understand the processes involved in shallow earthquake rupture in seismic gaps where megathrust earthquakes are expected and investigate the tsunami hazard it is important to assess whether the region experienced shallow earthquake rupture in the past however there are currently no established methods to elucidate whether a margin segment has repeatedly experienced shallow earthquake rupture with the exception of mechanical studies on subducted fault rocks here we combine new swath bathymetric data unpublished seismic reflection images and inter seismic seismicity to evaluate if the pattern of permanent deformation in the marine forearc of the northern chile seismic gap allows inferences on past earthquake behavior while the tectonic configuration of the middle and upper slope remains similar over hundreds of kilometers along the north chilean margin we document permanent extensional deformation of the lower slope localized to the region 20 8 s 22 s critical taper analyses the comparison of permanent deformation to inter seismic seismicity and plate coupling models as well as recent observations from other subduction zones including the area that ruptured during the 2011 tohoku oki earthquake suggest that the normal faults at the lower slope may have resulted from shallow possibly near trench breaking earthquake ruptures in the past in the adjacent margin segments the 1995 antofagasta 2007 tocopilla and 2014 iquique earthquakes were limited to the middle and upper slope and the terrestrial forearc and so are upper plate normal faults our findings suggest a seismo tectonic segmentation of the north chilean margin that seems to be stable over multiple earthquake cycles if our interpretations are correct they indicate a high tsunami hazard posed by the yet un ruptured southern segment of the seismic gap

geodetic teleseismic and strong motion constraints on slip from recent southern peru subduction zone earthquakes we use seismic and geodetic data both jointly and separately to constrain coseismic slip from the 12 november 1996 m w 7 7 and 23 june 2001 m w 8 5 southern peru subduction zone earthquakes as well as two large aftershocks following the 2001 earthquake on 26 june and 7 july 2001 we use all available data in our inversions gps interferometric synthetic aperture radar insar from the ers 1 ers 2 jers and radarsat 1 satellites and seismic data from teleseismic and strong motion stations our two dimensional slip models derived from only teleseismic body waves from south american subduction zone earthquakes with m w 7 5 do not reliably predict available geodetic data in particular we find significant differences in the distribution of slip for the 2001 earthquake from models that use only seismic teleseismic and two strong motion stations or geodetic insar and gps data the differences might be related to postseismic deformation or more likely the different sensitivities of the teleseismic and geodetic data to coseismic rupture properties the earthquakes studied here follow the pattern of earthquake directivity along the coast of western south america north of 5 s earthquakes rupture to the north south of about 12 s directivity is southerly and in between earthquakes are bilateral the predicted deformation at the arequipa gps station from the seismic only slip model for the 7 july 2001 aftershock is not consistent with significant preseismic motion

permanent fore arc extension and seismic segmentation insights from the 2010 maule earthquake chile 1 geologists have long known that young normal faults are an important structural element of the andean coastal cordillera but their relationship to the subduction seismic cycle is still unclear some of the largest aftershocks of the 2010 mw 8 8 maule earthquake in central chile were nucleated on upper plate normal faults including the mw 6 9 and 7 0 events of the pichilemu earthquake sequence we use the available coseismic gps displacements moment tensor sums and slip distribution models for the maule earthquake to compute the static strain and stress fields imposed on the upper plate by slip on the subduction interface the extensional strains calculated from coseismic gps and from a moment tensor sum of the pichilemu events have similar orientations and orders of magnitude the normal coulomb stress increment csi on the pichilemu fault has maximum positive stresses as high as 4 9 mpa regionally the maule event produced a semi elliptical radial pattern of static extension and deviatoric tension csi 1 5 mpa along the coastal cordillera enclosing the rupture area this elliptical pattern mimics the trends of the major upper crustal structures the static deformation field produced by a great subduction earthquake is an effective mechanism for generating permanent extension above the seismogenic zone reactivating suitably oriented long lived normal faults we suggest that the semi elliptical outline of the first order structures along the coastal cordillera may define the location of a characteristic long lived megathrust segment this observation implies a persistence at least over the quaternary of great subduction ruptures along the maule segment

sediment accumulation patterns in a tectonically subsiding incised valley insight from the echigo plain central japan abstract the echigo plain is on the japan sea coast of the island of honshu and is bounded to the west by the echigo plain western margin fault zone the shinano river incised valley lies east of the bounding fault zone and runs seaward approximately parallel to it the incised valley deepens to more than 160 m below mean sea level near the coast deeper than sea level fall during the last glacial maximum because of subsidence governed by the fault zone in this study we examined the sediment stacking patterns of the incised valley fill at 1000 year scale by analyzing sedimentary facies and radiocarbon ages from three sediment cores from the echigo plain the latest pleistocene to holocene shinano river incised valley fill sediments unconformably overlie a basement of late pleistocene flood plain sediments that contain aira tn at tephra dated at 25 120 220 bp ca 30 cal kyr bp the fill sequence comprises sediments of meandering river 12 cal kyr bp estuarine 13 9 cal kyr bp and barrier lagoon 9 0 cal kyr bp systems in ascending order thousand year isochrons based on radiocarbon ages show that the meandering river and estuarine systems were deposited aggradationally and retrogradationally and the barrier lagoon system was deposited progradationally and aggradationally these sediment stacking patterns resemble those of other incised valley fill sequences in east and southeast asia which indicates that the last deglacial eustasy was a major factor controlling sediment stacking patterns of incised valley fill in these regions however aggradational sedimentation in all three depositional systems is only observed in the echigo plain and suggests that long term subsidence at 2 7 3 8 mm yr is an important factor controlling the 1000 year scale sediment stacking pattern in this tectonically subsiding incised valley

stable creeping fault segments can become destructive as a result of dynamic weakening faults in earth s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast seismic wave producing rupture events perceived as earthquakes these types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment one with stable rate strengthening friction and the other with rate weakening friction that leads to stick slip the 2011 tohoku oki earthquake with moment magnitude m w 9 0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping here we propose a model in which stable rate strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids allowing unstable slip to occur in segments that can creep between events the model parameters are based on laboratory measurements on samples from the fault of the m w 7 6 1999 chi chi earthquake the long term slip behaviour of the model which we examine using a unique numerical approach that includes all wave effects reproduces and explains a number of both long term and coseismic observations some of them seemingly contradictory about the faults at which the tohoku oki and chi chi earthquakes occurred including there being more high frequency radiation from areas of lower slip the largest seismic slip in the tohoku oki earthquake having occurred in a potentially creeping segment the overall pattern of previous events in the area and the complexity of the tohoku oki rupture the implication that earthquake rupture may break through large portions of creeping segments which are at present considered to be barriers requires a re evaluation of seismic hazard in many areas

measuring the onset of locking in the peru chile trench with gps and acoustic measurements the subduction zone off the west coast of south america marks the convergence of the oceanic nazca plate and the continental south america plate nazca south america convergence over the past 23 million years has created the 6 km deep peru chile trench 150 km offshore high pressure between the plates creates a locked zone leading to deformation of the overriding plate the surface area of this locked zone is thought to control the magnitude of co seismic release and is limited by pressure temperature sediment type and fluid content1 here we present seafloor deformation data from the submerged south america plate obtained from a combination of global positioning system gps receivers and acoustic transponders we estimate that the measured horizontal surface motion perpendicular to the trench is consistent with a model having no slip along the thrust fault between 2 and 40 km depth a tsunami in 1996 200 km north of our site was interpreted as being the result of an anomalously shallow interplate earthquake2 seismic coupling at shallow depths such as we observe may explain why co seismic events in the peruvian subduction zone create large tsunamis

abrupt change in the dip of the subducting plate beneath north chile no large tsunamigenic earthquake has occurred in north chile since 1877 and the region has been largely recognized as a mature seismic gap1 2 3 4 5 6 7 8 9 at the southern end of the seismic gap the 2007 mw 7 7 tocopilla earthquake ruptured the deeper seismogenic interface whereas the coupled upper interface remained unbroken4 6 7 seismological studies onshore show a gently varying dip of 20 to 30 of the downgoing nazca plate3 6 which extends from the trench down to depths of 40 50 km here we study the lithospheric structure of the subduction zone of north chile at about 22 s using wide angle seismic refraction and reflection data from land and sea complemented by hypocentre data recorded during the 2007 tocopilla aftershocks7 our data document an abrupt increase in the dip of the subducting plate from less than 10 to about 22 at a depth of approximately 20 km the distribution of the 2007 aftershocks indicates that the change in dip acted as a barrier for the propagation of the 2007 earthquake towards the trench which in turn indicates that the subduction megathrust is not only segmented along the trench but also in the direction of the dip we propose that large magnitude tsunamigenic earthquakes must cross the barrier and rupture the entire seismogenic zone

aftershock seismicity and tectonic setting of the 2015 september 16 mw 8 3 illapel earthquake central chile powerful subduction zone earthquakes rupture thousands of square kilometers along continental margins but at certain locations earthquake rupture terminates to date detailed knowledge of the parameters that govern seismic rupture and aftershocks is still incomplete on 16 september 2015 the mw 8 3 illapel earthquake ruptured a 200 km long stretch of the central chilean subduction zone triggering a tsunami and causing significant damage here we analyze the temporal and spatial pattern of the co seismic rupture and aftershocks in relation to the tectonic setting in the earthquake area aftershocks cluster around the area of maximum coseismic slip in particular in lateral and downdip direction during the first 24 hours after the mainshock aftershocks migrated in both lateral directions with velocities of approximately 2 5 and 5 km h at the southern rupture boundary aftershocks cluster around individual subducted seamounts that are related to the downthrusting juan fernandez ridge in the northern part of the rupture area aftershocks separate into an upper cluster above 25 km depth and a lower cluster below 35 km depth this dual seismic aseismic transition in downdip direction is also observed in the interseismic period suggesting that it may represent a persistent feature for the central chilean subduction zone

geology of the seismogenic subduction thrust interface a microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces the exhumed chrystalls beach complex in the otago schist new zealand may be analogous to an actively deforming underthrust rock assemblage the complex contains asymmetric competent lenses of sandstone chert and basalt enclosed in a cleaved mudstone matrix continuous fabrics such as folds boudins and asymmetric phacoids formed by distributed cataclasis and dissolution precipitation creep discontinuous deformation is evident in an extensive fault fracture mesh involving mutually cross cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces the hikurangi margin provides an example of along strike variations in seismic style possibly related to heterogeneous fluid pressure state and interface geology in both the ancient and active subduction related shear zone fluid pressure state appears to be a critical control on frictional failure which primarily occurs on weak fluid overpressured discontinuities continuous aseismic deformation occurs where other mineral deformation mechanisms such as dissolution precipitation creep are preferred the geometry and composition of the underthrust rock assemblage appear to be first order controls on megathrust fluid pressure distribution bulk rheology and dominant deformation mechanism and thus may be significant controls on megathrust seismic style

invited review paper neogene to quaternary tectonics of the coastal cordillera northern chile abstract the extreme aridity of the northern chilean coastal cordillera enables the complete preservation of permanent deformation related to the coupling between nazca and south america the region between antofagasta and arica is characterized by four types of late cenozoic structures ew striking reverse faults ns striking normal faults sparse wnw and nnw striking right lateral strike slip faults and unimodal and bimodal populations of surface cracks the ew reverse faults occur only between 19 and 21 40 s latitude and have been active since at least 6 ma to present a march 2007 earthquake demonstrates that this deformation can occur during the interseismic phase of the plate boundary seismic cycle the ns normal faults are most abundant near antofagasta and mejillones and diminish in significance northward several of these faults in both the antofagasta and the salar grande area have been reactivated as reverse faults causing minor topographic inversion we suggest that normal faults move during both interseismic and coseismic deformation and are fundamentally very weak surface cracks are ubiquitous throughout the region and form by both non tectonic and tectonic mechanisms a significant proportion forms during coseismic deformation and thus may be used to identify long lived rupture segments on the plate boundary overall forearc deformation is very slow with permanent strain rates of 1 5 nstrain year and fault slip rates less than 0 5 mm year

long term persistence of subduction earthquake segment boundaries evidence from mejillones peninsula northern chile 1 the mejillones peninsula in northern chile has been recognized as the surface expression of a segment boundary for large subduction zone earthquakes the sharp contact between the rupture planes of two instrumentally recorded earthquakes the mw 8 0 antofagasta 1995 and the mw 7 7 tocopilla 2007 events is located beneath the central part of mejillones peninsula we present new chronostratigraphic and structural data that allow reconstructing the evolution of the peninsula at the surface and correlation of the latter with seismic cycle deformation on the plate interface uplift commenced after 3 4 myr as recorded in the western highland the central graben area on the peninsula started uplifting above sea level as an anticlinal hinge zone prior to 400 kyr ago most probably 790 kyr ago the resulting e w trending hinge exactly overlies the limit between the rupture planes of the antofagasta and tocopilla earthquakes by correlating the uplift data with the slip distribution of the above earthquakes we demonstrate that deformation and uplift is focused during the postseismic and interseismic periods of the megathrust seismic cycle with coseismic deformation opposed to the long term motion additionally the slip deficit beneath the peninsula accumulating between events is probably largely recovered by creep hence we suggest that mejillones peninsula owes its existence to the lateral variation of the propensity for unstable slip at the interface since the latter is a material property the long term spatial stability of the peninsula as a barrier to rupture propagation since at least the middle pleistocene is a necessary consequence

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

controls on facies distribution and stratigraphic preservation in the ganges brahmaputra delta sequence abundant sediment supply and accommodation space in the bengal basin have led to the development of a major late quaternary delta sequence this sequence has formed in a tectonically active setting and represents an important example of a high energy marine and fluvial high yield continental margin deposit recent studies have detailed the delta s stratigraphy and development noting that tectonics and sediment supply control the ganges brahmaputra more significantly than in many other delta systems these ideas are developed here through a discussion of the effects that spatial and temporal variations in tectonics and sediment supply have had on deltaic processes and sequence character unique and differing stratigraphies are found within thedeltasystem suchthatfine grainedsedimentpreservationisfavoredinareasofactivetectonicprocessessuchasfolding block faulting and subsidence coarse grained deposits dominate the stratigraphy under the control of high energy fluvial processes and mixed fine coarse stratigraphies are found in areas dominantly influenced by eustatic sea level change overlaid upon these spatially varying stratigraphic patterns are temporal patterns related to episodic events e g earthquakes and rivers avulsions and long termchangesinclimate andsediment supply modelingisalsousedtoinvestigatethe influenceofavariable sedimentsupply on sequence character results show that the timing and magnitude of sediment input relative to sea level rise is a significant control on the subaerial extent of the delta and the relative dominance of alluvial and marine facies within the sequence d 2002 elsevier science b v all rights reserved

strength of stick slip and creeping subduction megathrusts from heat flow observations subduction faults called megathrusts can generate large and hazardous earthquakes the mode of slip and seismicity of a megathrust is controlled by the structural complexity of the fault zone however the relative strength of a megathrust based on the mode of slip is far from clear the fault strength affects surface heat flow by frictional heating during slip we model heat flow data for a number of subduction zones to determine the fault strength we find that smooth megathrusts that produce great earthquakes tend to be weaker and therefore dissipate less heat than geometrically rough megathrusts that slip mainly by creeping

from the seismic cycle to long term deformation linking seismic coupling and quaternary coastal geomorphology along the andean megathrust measurement of interseismic strain along subduction zones reveals the location of both locked asperities which might rupture during megathrust earthquakes and creeping zones which tend to arrest such seismic ruptures the heterogeneous pattern of interseismic coupling presumably relates to spatial variations of frictional properties along the subduction interface and may also show up in the forearc morphology to investigate this hypothesis we compiled information on the extent of earthquake ruptures for the last 500 years and uplift rates derived from dated marine terraces along the south american coastline from central peru to southern chile we additionally calculated a new interseismic coupling model for that same area based on a compilation of gps data we show that the coastline geometry characterized by the distance between the coast and the trench the latitudinal variations of long term uplift rates and the spatial pattern of interseismic coupling are correlated zones of faster and long term permanent coastal uplift evidenced by uplifted marine terraces coincide with peninsulas and also with areas of creep on the megathrust where slip is mostly aseismic and tend to arrest seismic ruptures we conclude that spatial variations of frictional properties along the megathrust dictate the tectono geomorphological evolution of the coastal zone and the extent of seismic ruptures along strike

interplate earthquakes as a driver of shallow subduction erosion basal erosion is a prevalent process at subduction zones and plays an important role in the mass balance of global plate tectonics in contradiction with the theoretical expectation that basal erosion requires high basal friction and hence compression in the upper plate extensional faulting is commonly observed in submarine wedges that undergo such erosion here we propose a model to explain this apparent paradox in terms of stress fl uctuations during earthquake cycles in this model basal erosion occurs during large earthquakes when the shallow rate strengthening part of the plate interface strengthens and its overlying wedge weakens but extension occurs during interseismic relaxation of wedge stress the mechanics of basal erosion provide important information on the nature of the updip limit of the megathrust seismogenic zone in margins dominated by basal erosion

relationship link between landward vergence in accretionary prisms and tsunami generation the very large slip up to the subduction front of the 2011 tohoku oki japan earthquake has challenged our classic view of the megathrust undergoing only aseismic slip at shallow depth furthermore the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks recent seismic reflection images from the sumatra subduction zone show frontal landward vergent thrusts in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis using mechanical analysis we here show that sudden and successive decreases of the effective friction along the megathrust are required to form landward vergent frontal thrusts these decreases are most likely caused by dynamic weakening mechanisms such as thermal pressurization of the pore fluids related to the propagation of earthquakes to the seafloor therefore landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis the presence of landward verging structures in the cascadia and sumatra accretionary prisms might indicate future frontal rupture of the shallowest portion of the megathrust resulting in large tsunamis

the significance of large sediment supply active tectonism and eustasy on margin sequence development late quaternary stratigraphy and evolution of the ganges brahmaputra delta abstract borehole data from the ganges brahmaputra delta system unveil an intriguing late quaternary history controlled by immense sediment discharge tectonics and eustasy sea level first intersected a major portion of the lowstand surface 10 000 11 000 cal yr bp marking the onset of sediment trapping and delta growth despite rapid sea level rise 1 cm yr sediment load was sufficient to maintain relative shoreline stability during this time by 7000 cal yr bp accommodation was quickly infilled with slowed sea level rise leading to upstream channel migration and widespread dispersal of sands this forced coastal progradation along the western delta front where the ganges was situated and much of the river s fine grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf concurrently brahmaputra sediments were sequestered to an inland tectonic basin thus starving the adjacent shoreline and leading to transgression along the eastern delta by 5000 cal yr bp the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river mouth estuary present delta configuration appears to have developed by 3000 cal yr bp evolution of the tectonically active ganges brahmaputra g b system reveals important similarities and differences with other deltas overall facies succession follows that of basic models progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain however the timing thickness and controls of these deposits differ immense sediment discharge from the ganges and brahmaputra rivers allowed initial delta growth 2000 3000 yr earlier than the global average despite debouching onto a canyon incised high energy margin subsequently a thick 50 m transgressive systems tract was formed during the early holocene phase of delta aggradation contrasting with extensive shoreline retreat documented along most margins highstand progradation of coastal and shelf sequences ensued by the middle holocene but subsiding inland basins also favored accumulation of a thick to 40 m highstand sequence in the lower floodplains unlike many other delta systems subsidence in the subaerial g b delta is not dominated by compaction but rather by plate driven tectonic processes that generate rates up to 4 mm year overall the huge sediment load tectonic subsidence major seismic events and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta shelf and deep sea bengal fan throughout the late quaternary

widespread seismicity excitation throughout central japan following the 2011 m 9 0 tohoku earthquake and its interpretation by coulomb stress transfer 1 we report on a broad and unprecedented increase in seismicity rate following the m 9 0 tohoku mainshock for m 2 earthquakes over inland japan parts of the japan sea and izu islands at distances of up to 425 km from the locus of high 15 m seismic slip on the megathrust such an increase was not seen for the 2004 m 9 1 sumatra or 2010 m 8 8 chile earthquakes but they lacked the seismic networks necessary to detect such small events here we explore the possibility that the rate changes are the product of static coulomb stress transfer to small faults we use the nodal planes of m 3 5 earthquakes as proxies for such small active faults and find that of fifteen regions averaging 80 by 80 km in size 11 show a positive association between calculated stress changes and the observed seismicity rate change 3 show a negative correlation and for one the changes are too small to assess this work demonstrates that seismicity can turn on in the nominal stress shadow of a mainshock as long as small geometrically diverse active faults exist there which is likely quite common

holocene evolution of the song hong red river delta system northern vietnam abstract the song hong red river delta located on the western coast of the gulf of bac bo tonkin in the south china sea formed as a result of the song hong sediment discharge throughout the holocene the river s sedimentary basin upstream from the delta plain is not large the delta plain comprises emerged tidal mangrove flats formed during the sea level highstand at 2 3 m 6 4 cal kyr bp and a beach ridge strandplain with straight to lobate beach ridges on the landward and seaward sides of the delta plain respectively the delta affords us the opportunity to examine river mouth morphodynamics comprehensively in relation to sediment discharge and sea level changes in this paper we describe the holocene evolution of the song hong delta system and the river mouth morphodynamics on the basis of seven sediment cores each 30 70 m long taken from the delta plain during 1999 2001 and 101 radiocarbon dates obtained from the core sediments sediments from the seven cores consisting of incised valley fills since the last glacial maximum can be divided in ascending order into fluvial sediments composed of gravelly sand and mottled clay tide influenced estuarine sediments containing shell and wood fragments and deltaic sediments composed of tide influenced sand and mud deposits in which the contents of sand and wood fragments increase upward by applying sequence stratigraphic concepts a transgressive surface can be identified at the gravelly sand mottled clay boundary and the maximum flooding surface can be identified at the estuarine deltaic sediments boundary the lowstand systems tract transgressive systems tract and highstand systems tract record 15 15 9 and 9 0 cal kyr bp respectively during the past 9 kyr the majority of the sediment discharged by the song hong accumulated in the incised valley and enhanced progradation at the river mouth the river mouth prograded from the bay head toward the gulf and its morphology changed from funnel shaped 9 6 cal kyr bp to straight 6 2 cal kyr bp and finally to lobate 2 0 cal kyr bp the morphological change from a funnel shaped to straight coast is interpreted to have been closely related to a hydrologic regime shift from a tide dominated bay head setting to a wave influenced open coast setting the cessation of the sea level rise at 6 cal kyr bp may have played only a small role in the change of river mouth morphology because the progradation rate decelerated from 22 to 4 m yr instead of accelerating at that time the river mouth morphodynamic change from straight to lobate was largely caused by an increase in the song hong sediment discharge from 17 27 9 2 cal kyr bp to 49 million t yr 2 0 cal kyr bp as a result of anthropogenic deforestation along the upper reaches of the song hong