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

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

reviewing megathrust slip behavior for recent mw 8 0 earthquakes along the peru chilean margin from satellite goce gravity field derivatives abstract the characterization of a seismogenic zone associated with the rupture process that occurs during great megathrust earthquakes has been approached from different perspectives different studies lightened the structural complexity and compositional heterogeneity of the interplate region in this work we address this relationship from the analysis of density contrasts provided by satellite gravity data and comparing these results with slip behavior along the rupture zone we performed the calculation of gravity anomalies and gradients both corrected by the effect of topography and sediments then we analyzed this relation with the rupture zones of four megathrust earthquakes mw 8 along the peru chilean coast associated with the largest earthquakes that occurred in the last years along this setting finding that the maximum vertical displacements were located close to gravity minimums in the forearc zone we finally obtained density models by inverting four trench parallel profiles from the gravity disturbance finding low density contrasts related to the maximum slips and high density contrasts at its edges related to a decrease of the displacement or to the lateral ending of the rupture the along strike seismic segmentation observed previously in the gravity derivatives is also observed along dip being slip increased at gravity lows and arrested at gravity highs probably indicating that the forearc structure plays a key role in across strike rupture behavior in depth our results agree with the hypothesis that persistent tectonic features modeled at a high degree by the oceanic plate morphology may control strain accumulation and release along the megathrust

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

foreshock triggering of the 1 april 2014 mw 8 2 iquique chile earthquake abstract on april 1st 2014 a mw 8 2 u s geological survey moment magnitude earthquake occurred in the subduction zone offshore northern chile in the two weeks leading up to the earthquake a sequence of foreshocks starting with a mw 6 7 earthquake on march 16th and including three more mw 6 0 events occurred predominantly south of the april 1st mainshock epicenter and up dip of the area of significant slip during the mainshock using earthquake locations and source parameters derived in a previous study hayes et al 2014 and a coulomb failure stress change analysis of these events we assess in detail the hypothesis that the earthquakes occurred as a cascading sequence each event successively triggering the next ultimately triggering the rupture of the mainshock following the initial mw 6 7 event each of the three largest foreshocks mw 6 4 6 2 and 6 3 as well as the hypocenter of the mainshock occurred in a region of positive coulomb stress change produced by the preceding events indicating these events were brought closer to failure by the prior seismicity in addition we reexamine the possibility that aseismic slip occurred and what role it may have played in loading the plate boundary using horizontal gps displacements from along the northern chile coast prior to the mainshock we find that the foreshock seismicity alone likely does not account for the observed signals we perform a grid search for the location and magnitude of an aseismic slip patch that can account for the difference between observed signals and foreshock related displacement and find that a slow slip region with slip corresponding to a mw 6 8 earthquake located coincident with or up dip of the foreshock seismicity can best explain this discrepancy additionally such a slow slip region positively loads the mainshock hypocentral area enhancing the positive loading produced by the foreshock seismicity

interseismic coupling and seismic potential along the central andes subduction zone we use about two decades of geodetic measurements to characterize interseismic strain build up along the central andes subduction zone from lima peru to antofagasta chile these measurements are modeled assuming a 3 plate model nazca andean sliver and south america craton and spatially varying interseismic coupling isc on the nazca megathrust interface we also determine slip models of the 1996 m w 7 7 nazca the 2001 m w 8 4 arequipa the 2007 m w 8 0 pisco and the m w 7 7 tocopilla earthquakes we find that the data require a highly heterogeneous isc pattern and that overall areas with large seismic slip coincide with areas which remain locked in the interseismic period with high isc offshore lima where the isc is high a m w 8 6 8 8 earthquake occurred in 1746 this area ruptured again in a sequence of four m w 8 0 earthquakes in 1940 1966 1974 and 2007 but these events released only a small fraction of the elastic strain which has built up since 1746 so that enough elastic strain might be available there to generate a m w 8 5 earthquake the region where the nazca ridge subducts appears to be mostly creeping aseismically in the interseismic period low isc and seems to act as a permanent barrier as no large earthquake ruptured through it in the last 500 years in southern peru isc is relatively high and the deficit of moment accumulated since the m w 8 8 earthquake of 1868 is equivalent to a magnitude m w 8 4 earthquake two asperities separated by a subtle aseismic creeping patch are revealed there this aseismic patch may arrest some rupture as happened during the 2001 arequipa earthquake but the larger earthquakes of 1604 and 1868 were able to rupture through it in northern chile isc is very high and the rupture of the 2007 tocopilla earthquake has released only 4 of the elastic strain that has accumulated since 1877 the deficit of moment which has accumulated there is equivalent to a magnitude m w 8 7 earthquake this study thus provides elements to assess the location size and magnitude of future large megathurst earthquakes in the central andes subduction zone caveats of this study are that interseismic strain of the forearc is assumed time invariant and entirely elastic also a major source of uncertainty is due to fact that the available data place very little constraints on interseismic coupling at shallow depth near the trench except offshore lima where sea bottom geodetic measurements have been collected suggesting strong coupling

complex distribution of large thrust and normal fault earthquakes in the chilean subduction zone summary we present the results of a systematic study of events with m 6 in northern chile 20 33 s for the period between 1963 and 1971 medium to large earthquakes near the coast of this region are of three types 1 interplate events at the interface between the downgoing slab and the overriding south american plate these events can be very large reaching magnitudes greater than 8 2 intra plate earthquakes 20 30 km inside the downgoing slab they have fault mechanisms indicating extension along the dip of the slab and may have magnitudes up to 7 5 3 less frequent m 6 events that occur near the top of the downgoing slab and have thrust mechanisms with an almost horizontal e w compressional axis this type of mechanism is very different from that of the events of type 1 which are due to shallow dipping reverse faulting there is a rotation of about 30 of the compressional axis in the vertical plane between events of types 1 and 3 three groups of events near 32 5 25 5 and 21 s were studied in detail depth and mechanisms were redetermined by p wave modelling and relative locations were obtained by a master event technique near 32 5 s only events of types 1 and 2 were found in the time period of this study at the two other sites the three types of events were identified this shows clearly that there are compressive stresses at the top of the slab and extension at the centre a situation which is usually found in the areas where a double benioff zone has been identified in the seismicity

propagation of slow slip leading up to the 2011 mw 9 0 tohoku oki earthquake many large earthquakes are preceded by one or more foreshocks but it is unclear how these foreshocks relate to the nucleation process of the mainshock on the basis of an earthquake catalog created using a waveform correlation technique we identified two distinct sequences of foreshocks migrating at rates of 2 to 10 kilometers per day along the trench axis toward the epicenter of the 2011 moment magnitude mw 9 0 tohoku oki earthquake in japan the time history of quasi static slip along the plate interface based on small repeating earthquakes that were part of the migrating seismicity suggests that two sequences involved slow slip transients propagating toward the initial rupture point the second sequence which involved large slip rates may have caused substantial stress loading prompting the unstable dynamic rupture of the mainshock

large scale dynamic triggering of shallow slow slip enhanced by overlying sedimentary wedge seismic waves can trigger further fault slip analysis of seismic and geodetic data shows that seismic waves from the 2016 kaik ura new zealand earthquake were amplified by subduction zone sediments triggering slow fault slip up to 600 km away

the mw 8 1 2014 iquique chile seismic sequence a tale of foreshocks and aftershocks egu2009 11040 schurr b asch g rosenau m wang r oncken o barrientos s salazar p villotte j p 2012 the 2007m7 7 tocopillanorthernchile earthquake sequence implications for along strike and downdip rupture segmentation and megathrust frictional behaviour j geophys res 117 b05305 doi 10 1029 2011jb009030 schurr b et al 2014 gradual unlocking of the plate boundary controlled the initiation of the 2014 iquique earthquake nature 512 7514 299

characterization of nucleation during laboratory earthquakes 1 we observe the nucleation phase of in plane ruptures in the laboratory we show that the nucleation is composed of two distinct phases a quasi static and an acceleration stage followed by dynamic propagation we propose an empirical model which describes the rupture length evolution the quasi static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time the transition from quasi static to accelerating rupture is related to the critical nucleation length which scales inversely with normal stress in accordance with theoretical predictions and to a critical surfacic power which may be an intrinsic property of the interface finally we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions

investigating the final seismic swarm before the iquique pisagua 2014 mw 8 1 by comparison of continuous gps and seismic foreshock data preexisting networks of seismometers and continuous gps in northern chile successfully captured surface motions and seismicity leading up to the 1 april mw 8 1 here we compare continuous gps cgps with predictions of seismic dislocations for the final foreshock swarm beginning with the 16 march mw 6 7 results show that the cumulative cgps motion can be largely explained by seismic slip because evolutions of cgps positions for most stations stay within the ranges of seismic predictions given sensible ranges of assumed source errors however cgps motions between 18 21 and 25 31 march outpace seismic predictions supporting the existence of aseismic transients that were most probably the afterslip from preceding bursts of seismicity a parameter search reveals that the 16 march mw 6 7 cgps displacements can be recreated with a fault plane significantly rotated anticlockwise from the strike of the plate interface suggesting that failure was on a structure other than the plate interface

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

the mechanism of partial rupture of a locked megathrust the role of fault morphology assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries and along strike barriers from subducted structural features yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake here we draw insight into this conundrum from the 25 april 2015 m w 7 8 gorkha nepal earthquake we invert geodetic data with a structural model of the main himalayan thrust in the region of kathmandu nepal showing that this event was generated by rupture of a decollement bounded on all sides by more steeply dipping ramps the morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap we then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics depending on the earthquake history and the details of rupture dynamics however great earthquakes that rupture the entire seismogenic zone are also possible these insights from nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide

slow slip history for the mexico subduction zone 2005 through 2011 to further our understanding of the seismically hazardous mexico subduction zone we estimate the first time dependent slip distributions and coulomb failure stress changes for the six major slow slip events sses that occurred below mexico between late 2005 and mid 2011 slip dist ributions are the first to be estimated from all continuous gps data in central and southern mexico which better resolves slow slip in space and time than was previously possible in this region below oaxaca slip during previously un modeled sses in 2008 9 and 2010 11 extended farther to the west than previous sses this constitutes the first evidence that slow slip accounts for deep slip within a previously noted gap between the oaxaca and guerrero sse source regions the slip that we estimate for the two sses that originated below guerrero between 2005 and 2011 agrees with slip estimated in previous mostly static offset sse modeling studies however we show that both sses migrated eastward toward the oaxaca sse source region in accord with previous work we find that slow slip below guerrero intrudes up dip into the potentially seismogenic region presumably accounting for some of the missing slip within the well described guerrero seismic gap in contrast slow slip below oaxaca between 2005 and 2011 occurred mostly down dip from the seismogenic regions defined by the rupture zones of large thrust earthquakes in 1968 and 1978 and released all of the slip deficit that accumulated in the down dip region during this period

coseismic slip on shallow decollement megathrusts implications for seismic and tsunami hazard abstract for years many studies of subduction zones and on land fold and thrust belts have assumed that the frontal portions of accretionary prisms are too weak to rupture coseismically and must therefore be fully creeping we present a series of examples both on land and offshore demonstrating that in many cases shallow decollements are capable of large coseismic slip events that rupture to the toes of the fault systems some of these events are associated with ruptures that initiate down dip while others appear to be limited to the frontal shallow portion of the wedge we suggest that this behavior is not limited to the examples described here but rather is common to many perhaps most accretionary wedges and fold and thrust belts around the world indeed there may be many other examples of similar earthquakes where existing data cannot constrain slip at the toe we do not characterize the regions and events described here as unusual as they encompass a wide range of settings this study indicates that there is an urgent need to reevaluate seismic and tsunami hazard in fold and thrust belts and subduction zones around the world allowing for the possibility of shallow decollement rupture

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

the 1985 central chile earthquake a repeat of previous great earthquakes in the region a great earthquake surface wave magnitude 7 8 occurred along the coast of central chile on 3 march 1985 causing heavy damage to coastal towns intense foreshock activity near the epicenter of the main shock occurred for 11 days before the earthquake the aftershocks of the 1985 earthquake define a rupture area of 170 by 110 square kilometers the earthquake was forecast on the basis of the nearly constant repeat time 83 9 years of great earthquakes in this region an analysis of previous earthquakes suggests that the rupture lengths of great shocks in the region vary by a factor of about 3 the nearly constant repeat time and variable rupture lengths cannot be reconciled with time or slip predictable models of earthquake recurrence the great earthquakes in the region seem to involve a variable rupture mode and yet for unknown reasons remain periodic historical data suggest that the region south of the 1985 rupture zone should now be considered a gap of high seismic potential that may rupture in a great earthquake in the next few tens of years

foreshock migration preceding the 2016 mw 7 0 kumamoto earthquake japan we investigated the spatiotemporal evolution of an earthquake sequence following a series of large shallow intraplate earthquakes including a mw 6 2 foreshock and mw 7 0 main shock in the kumamoto area of kyushu sw japan to more precisely characterize the evolution of the earthquake sequences we applied a matched filter technique to continuous waveform data using template events obtained via a double difference relocation algorithm migrations of seismicity fronts along the directions of fault strike and dip are clearly seen starting immediately after the mw 6 2 foreshock these migrations are interpreted to result from aseismic slip triggered by the foreshock propagating toward the nucleation point of the subsequent mw 7 0 main shock rupture when combined with static stress changes induced by the mw 6 2 foreshock it is likely that stress transfer from both aseismic and seismic slip during the foreshock sequence loaded stress onto the main shock rupture faults bringing them closer to failure

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

long term interactions between intermediate depth and shallow seismicity in north chile subduction zone we document interactions between intermediate depth and interplate seismicity in the north chile subduction zone over a 25 year period 1990 2015 we show that the 2005 mw 7 8 tarapaca slab pull earthquake was followed by 9 years of enhanced deep and shallow seismicity together with the decrease of eastward average gps velocities and associated interplate coupling eventually leading to the 2014 mw 8 1 iquique megathrust earthquake in contrast megathrust ruptures e g mw 8 0 antofagasta in 1995 or mw 8 1 iquique in 2014 initiate several years of silent background seismicity in the studied area both at shallow and intermediate depths the plunge of a rigid slab into a viscous asthenospheric mantle could explain the observed synchronization between deep and shallow seismicity and their long term interactions

modeling deformation induced by seasonal variations of continental water in the himalaya region sensitivity to earth elastic structure strong seasonal variations of horizontal and vertical positions are observed on gps time series from stations located in nepal india and tibet china we show that this geodetic deformation can be explained by seasonal variations of continental water storage driven by the monsoon for this purpose we use satellite data from the gravity recovery and climate experiment to determine the time evolution of surface loading we compute the expected geodetic deformation assuming a perfectly elastic earth model we consider green s functions describing the surface deformation response to a point load for an elastic homogeneous half space model and for a layered nonrotating spherical earth model based on the preliminary reference earth model and a local seismic velocity model the amplitude and phase of the seasonal variation of the vertical and horizontal geodetic positions can be jointly adjusted only with the layered earth model while an elastic half space model fails emphasizing the importance of using a realistic earth elastic structure to model surface displacements induced by surface loading we demonstrate based on a formal inversion that the fit to the geodetic data can be improved by adjusting the layered earth model therefore the study also shows that the modeling of geodetic seasonal variations provides a way to probe the elastic structure of the earth even in the absence of direct measurements of surface load variations

source characteristics of the march 16 2014 mw 6 7 earthquake and its implications for the mw 8 2 pisagua mainshock abstract the march 16 2014 mw 6 7 earthquake played an important role in the complex seismic sequence leading to the nucleation of the april 1st 2014 mw 8 2 pisagua earthquake in northern chile the mw 6 7 an upper plate reverse faulting event with nodal planes highly rotated counterclockwise with respect to the strike of the megathrust at its location is analyzed for each nodal plane of the regional w phase centroid moment tensor solution the spatiotemporal slip distribution is inverted using near field records the optimal space and time smoothing constraints are determined objectively based on the akaike s bayesian information criterion we analyze seismicity on the region affected by the mw 6 7 and mw 8 2 pisagua mainshock with accurate hypocenter locations obtained using a 3d heterogeneous medium we also performed regional moment tensor inversion of events m 4 0 occurred before the mw 8 2 our results suggest the sub vertical fault plane dipping southward dip 70 as being the causative fault of the march 16 mw 6 7 earthquake the rupture propagated to the northwest lasted 15 s and yielded a total seismic moment of 1 36 1019 nm mw 6 7 estimated slip is characterized by total rupture length of 22 km and suggest shallow slip distributed between 3 km down to 17 km depth a comparison between coastal tide gauge records and forward tsunami modeling show similarity of the time series thus supporting the plausibility of our estimated slip models our interpretation suggests that the rupture of the mw 6 7 intraplate could be explained by stress accommodation between two segments along dip with different mechanical properties at the interplate boundary and inside the upper plate

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

hunt for slow slip events along the sumatran subduction zone in a decade of continuous gps data 10 pages 4 figures supporting information http dx doi org 10 1002 2015jb012503 this work is earth observatory of singapore paper number 104

possible links between long term geomagnetic variations and whole mantle convection processes the earth s internal magnetic field varies on timescales of months to billions of years the field is generated by convection in the liquid outer core which in turn is influenced by the heat flowing from the core into the base of the overlying mantle much of the magnetic field s variation is thought to be stochastic but over very long timescales this variability may be related to changes in heat flow associated with mantle convection processes over the past 500 myr correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late mesozoic era beginning about 180 myr ago simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability such as occurred between the middle jurassic and middle cretaceous periods may have been triggered by a decrease in core mantle boundary heat flow either globally or in equatorial regions this decrease in heat flow could have been linked to reduced mantle plume head production at the core mantle boundary an episode of true polar wander or a combination of the two

deep transient slow slip detected by survey gps in the region of atacama chile we detected a long term transient deformation signal between 2014 and 2016 in the atacama region chile using survey global positioning system gps observations over an 150 km along strike region survey gps measurements in 2014 and 2016 deviate significantly from the interseismic trend estimated using previous observations this deviation from steady state deformation is spatially coherent and reveals a horizontal westward diverging motion of several centimeters along with a significant uplift it is confirmed by continuous measurements of recently installed gps stations we discard instrumental hydrological oceanic or atmospheric loading effects and show that the transient is likely due to deep slow slip in the transition zone of the subduction interface 40 to 60 km depth in addition daily observations recorded by a continuous gps station operating between 2002 and 2015 highlight similar transient signals in 2005 and 2009 suggesting a recurrent pattern

revisiting the north chile seismic gap segmentation using gps derived interseismic coupling no major earthquake occurred in north chile since the 1877 m w 8 6 subduction earthquake that produced a huge tsunami however geodetic measurements conducted over the last decade in this area show that the upper plate is actually deforming which reveals some degree of locking on the subduction interface this accumulation of elastic deformation is likely to be released in a future earthquake because of the long elapsed time since 1877 and the rapid accumulation of deformation thought to be 6 7 cm yr many consider this area is a mature seismic gap where a major earthquake is due and seismic hazard is high we present a new global positioning system gps velocity field acquired between 2008 and 2012 that describes in some detail the interseismic deformation between 18 s and 24 s we invert for coupling distribution on the nazca south america subduction interface using elastic modelling our measurements require that at these latitudes 10 to 12 mm yr 1 i e 15 per cent of the whole convergence rate are accommodated by the clockwise rotation of an andean block bounded to the east by the subandean fold and thrust belt this reduces the accumulation rate on the subduction interface to 56 mm yr 1 in this area coupling variations on the subduction interface both along strike and along dip are described we find that the north chile seismic gap is segmented in at least two highly locked segments bounded by narrow areas of weak coupling this coupling segmentation is consistent with our knowledge of the historical ruptures and of the instrumental seismicity of the region intersegment zones iquique mejillones correlate with high background seismic rate and local tectonic complexities on the upper or downgoing plates the rupture of either the paranal or the loa segment alone could easily produce a mw 8 0 8 3 rupture and we propose that the loa segment from 22 5 s to 20 8 s may be the one that ruptured in 1877

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

earthquake swarms slow slip and fault interactions at the western end of the hellenic subduction system precede the mw 6 9 zakynthos earthquake greece the month to year long deformation of the earth s crust where active subduction zones terminate is poorly explored here we report on a multidisciplinary dataset that captures the synergy of slow s

deep postseismic viscoelastic relaxation excited by an intraslab normal fault earthquake in the chile subduction zone abstract the 2005 mw 7 8 tarapaca earthquake was the result of normal faulting on a west dipping plane at a depth of 90 km within the subducting slab down dip of the north chilean gap that partially ruptured in the 2014 m 8 2 iquique earthquake we use envisat observations of nearly four years of postseismic deformation following the earthquake together with some survey gps measurements to investigate the viscoelastic relaxation response of the surrounding upper mantle to the coseismic stress we constrain the rheological structure by testing various 3d models taking into account the vertical and lateral heterogeneities in viscosity that one would expect in a subduction zone environment a viscosity of 4 8 1018 pa s for the continental mantle asthenosphere fits both insar line of sight los and gps horizontal displacements reasonably well in order to test whether the tarapaca earthquake and associated postseismic relaxation could have triggered the 2014 iquique sequence we computed the coulomb stress change induced by the co and postseismic deformation following the tarapaca earthquake on the megathrust interface and nodal planes of its m 6 7 foreshock these static stress calculations show that the tarapaca earthquake may have an indirect influence on the iquique earthquake via loading of the m 6 7 preshock positively we demonstrate the feasibility of using deep intraslab earthquakes to constrain subduction zone rheology continuing geodetic observation following the 2014 iquique earthquake may further validate the rheological parameters obtained here

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

on the evolution of elastic properties during laboratory stick slip experiments spanning the transition from slow slip to dynamic rupture the physical mechanisms governing slow earthquakes remain unknown as does the relationship between slow and regular earthquakes to investigate the mechanism s of slow earthquakes and related quasi dynamic modes of fault slip we performed laboratory experiments on simulated fault gouge in the double direct shear configuration we reproduced the full spectrum of slip behavior from slow to fast stick slip by altering the elastic stiffness of the loading apparatus k to match the critical rheologic stiffness of fault gouge kc our experiments show an evolution from stable sliding when k kc to quasi dynamic transients when k kc to dynamic instabilities when k kc to evaluate the microphysical processes of fault weakening we monitored variations of elastic properties we find systematic changes in p wave velocity vp for laboratory seismic cycles during the coseismic stress drop seismic velocity drops abruptly consistent with observations on natural faults in the preparatory phase preceding failure we find that accelerated fault creep causes a vp reduction for the complete spectrum of slip behaviors our results suggest that the mechanics of slow and fast ruptures share key features and that they can occur on same faults depending on frictional properties in agreement with seismic surveys on tectonic faults our data show that their state of stress can be monitored by vp changes during the seismic cycle the observed reduction in vp during the earthquake preparatory phase suggests that if similar mechanisms are confirmed in nature high resolution monitoring of fault zone properties may be a promising avenue for reliable detection of earthquake precursors

abrupt increase in the coastal uplift and earthquake rate since 40 ka at the northern chile seismic gap in the central andes abstract over long term geological scales the position and vertical movements of the coast are considered to be among the most important effects resulting from first order plate tectonics interactions in the subduction zones however the relationship between short term vertical deformation driven by earthquakes and long term coastal uplift in the andean subduction contact of northern chile has not been thoroughly elucidated to date based on precise radiocarbon dating and geomorphological analysis of littoral deposits in the mejillones peninsula at the southern edge of the major northern chile seismic gap we report a drastic increase in coastal uplift since marine isotope stage 3 mis 3 with uplift rates one order of magnitude more rapid than previously in the late pleistocene geomorphological evidence supplied by holocene littoral deposits and marine terraces shows that this tectonic regime could be operating at present together with new geodetic data following the 2007 mw7 7 tocopilla event these observations indicate that coastal uplift results from cumulative coseismic vertical displacement with low subsequent interseismic subsidence recovery driven by both deep moderate and large megathrust earthquakes we suggest that the accelerated coastal uprising and earthquake rate over the past 44 ka demonstrated in this work results from changes involving the entire subduction contact between the nazca and south american plates in northern chile

tsunami surges around the hawaiian islands from the 1 april 2014 north chile mw 8 1 earthquake the 1 april 2014 iquique mw 8 1 earthquake ruptured a segment of the megathrust fault offshore of northern chile and generated a moderate size tsunami across the pacific tide gauges in hawaii recorded over 1 m of wave height despite the long distance from the source and position away from the main radiated energy lobe inversion of global teleseismic body waves combined with forward modeling of the tsunami at four near field dart stations arrives iteratively at a self consistent finite fault model with very compact dimensions the slip distribution produces a nne ssw trending seafloor uplift patch that enhances the tsunami directionality in the wnw resulting in good matches to observed dart and tide gauge records around the hawaiian islands the relatively large waves at selected locations in hawaii can be attributed to a combination of the spatial slip distribution and the resulting short period waves that triggered localized resonance over the insular shelves this event highlights the importance of characterizing detailed slip distributions in analysis or forecasting of tsunamis even for a compact source

sinkholes at glan llyn on the a55 north wales coast road uk at glan llyn an unusual cluster of sinkholes in the glacial drift which overlies lower carboniferous limestone poses recurring difficulties for design construction and maintenance of the a55 north wales coast road the cluster comprises 51 sinkholes within 35 ha and prompted concern about the possible existence of larger deeper cave systems and cavities lacking surface expression that might cause a potential collapse problem within the areas of highway construction comprehensive geological investigations carried out to characterize the foundation conditions included boreholes geophysics probe drillholes trial pits and borehole photography subsurface studies failed to disclose cavernous openings in the limestone but revealed instead dissolution enlarged joints and fractures forming an integrated network and generally infilled with loose clay the glacial sediment cover consists of stiff clays and medium dense sand and gravel however in the vicinity of sinkholes much weaker strata have developed due to disturbance effects highway design and construction incorporated a number of mitigating measures including geological surveillance of earthworks excavation and infilling of identified sinkholes and geotextile reinforcement of the affected stretch of highway regular monitoring is carried out to check the structural integrity of the carriageway and ensure public safety early warning signs of failure have been detected at two sites and appropriate action taken

recurring slow slip events and earthquake nucleation in the source region of the m 7 ibaraki oki earthquakes revealed by earthquake swarm and foreshock activity

rupture process during the 2015 illapel chile earthquake zigzag along dip rupture episodes we constructed a seismic source model for the 2015 mw 8 3 illapel chile earthquake which was carried out with the kinematic waveform inversion method adopting a novel inversion formulation that takes into account the uncertainty in the green s function together with the hybrid backprojection method enabling us to track the spatiotemporal distribution of high frequency 0 3 2 0 hz sources at high resolution by using globally observed teleseismic p waveforms a maximum slip amounted to 10 4 m in the shallow part of the seismic source region centered 72 km northwest of the epicenter and generated a following tsunami inundated along the coast in a gross sense the rupture front propagated almost unilaterally to northward from the hypocenter at 2 km s however in detail the spatiotemporal slip distribution also showed a complex rupture propagation pattern two up dip rupture propagation episodes and a secondary rupture episode may have been triggered by the strong high frequency radiation event at the down dip edge of the seismic source region high frequency sources tends to be distributed at deeper parts of the slip area a pattern also documented in other subduction zone megathrust earthquakes that may reflect the heterogeneous distribution of fracture energy or stress drop along the fault the weak excitation of high frequency radiation at the termination of rupture may represent the gradual deceleration of rupture velocity at the transition zone of frictional property or stress state between the megathrust rupture zone and the swarm area

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

rupture process of the 2014 iquique chile earthquake in relation with the foreshock activity the rupture process of the 2014 iquique chile earthquake is inverted from teleseismic p wave data applying a novel formulation that takes into account the uncertainty of green s function which has been a major error source in waveform inversion the estimated seismic moment is 1 5 1021 nm mw 8 1 associated with a 140 km long and 140 km wide fault rupture along the plate interface the source process is characterized by unilateral rupture propagation during the first 20 s the dynamic rupture front propagated from the hypocenter to the large asperity located about 50 km southward crossing a remarkably active foreshock area at high velocity of about 3 0 km s but small and irregular seismic moment release rate our result may suggest that the 20 s long initial phase was influenced by the stress drop due to the foreshock activity near the main shock hypocenter moreover the 2 week long swarm like foreshock activity migrating roughly at 5 km day toward the main shock hypocenter and possibly associated slow slip contributed to the stress accumulation prior to the mw 8 1 megaquake the main shock initial rupture phase might have triggered the rupture of the large asperity which had large fracture energy

an 8 month slow slip event triggers progressive nucleation of the 2014 chile megathrust the mechanisms leading to large earthquakes are poorly understood and documented here we characterize the long term precursory phase of the april 1st 2014 mw8 1 north chile megathrust we show that a group of coastal gps stations accelerated westward eight months before the mainshock corresponding to a mw6 5 slow slip event on the subduction interface 80 of which was aseismic concurrent interface foreshocks underwent a diminution of their radiation at high frequency as shown by the temporal evolution of fourier spectra and residuals with respect to ground motions predicted by recent subduction models such ground motions change suggests that in response to the slow sliding of the subduction interface seismic ruptures are progressively becoming smoother and or slower the gradual propagation of seismic ruptures beyond seismic asperities into surrounding metastable areas could explain these observations and might be the precursory mechanism eventually leading to the mainshock

local earthquake tomography of shallow subduction in north chile a combined onshore and offshore study selected travel time data from the aftershock series of the great antofagasta earthquake m w 8 0 have been inverted simultaneously for both hypocenter locations and three dimensional v p and v p v s struc ture the data were collected with a dense 44 station seismic network including ocean bottom hydrophones we performed a series of inversions with increasing complexity 1 d 2 d and 3 d this approach was cho sen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution special efforts were made to assess the solution quality including standard resolution esti mates and tests with synthetic travel times the subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high v p feature high v p v s ratios within the oceanic crust possibly indicate elevated fluid content underplating of material eroded close to the trench is found beneath the mejillones peninsula the lower crust of the overlying plate is characterized by an average v p of 6 8 6 9 km s and an average to low v p v s ratio large areas of anomalously high v p are found in the lower crust south of the city of antofagasta they are interpreted as remants of magmatic intrusions a zone of high v p v s ratios is found within the rup ture area of the antofagasta main shock just above the subducted slab its location within the region of high est stress release from the main shock suggests that the main shock rupture causes the high v p v s ratio the high v p v s ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overly ing lower crust

rupture in the 4 may 2018 m w 6 9 earthquake seaward of the kilauea east rift zone fissure eruption in hawaii

earthquake swarms and slow slip on a sliver fault in the mexican subduction zone the mexican subduction zone is an ideal location for studying subduction processes due to the short trench to coast distances that bring broad portions of the seismogenic and transition zones of the plate interface inland using a recently generated seismicity catalog from a local network in oaxaca we identified 20 swarms of earthquakes m

upper plate reverse fault reactivation and the unclamping of the megathrust during the 2014 northern chile earthquake sequence after 137 years without a great earthquake the mw 8 1 pisagua event of 1 april 2014 occurred in the central portion of the southern peru northern chile subduction zone this megathrust earthquake was preceded by more than 2 weeks of foreshock activity migrating 3 5 km day toward the mainshock hypocenter this foreshock sequence was triggered by an mw 6 7 earthquake on a reverse fault in the upper plate that strikes at a high angle to the trench similar to well documented reverse faults onshore these margin oblique reverse faults accommodate north south shortening resulting from subduction across a plate boundary that is curved in map view reverse slip on the crustal fault unclamped the subduction interface precipitating the subsequent megathrust foreshock activity that culminated in the great pisagua earthquake the combination of crustal reverse faults and a curved subduction margin also occurs in cascadia and northeastern japan indicating that there are two additional localities where great megathrust earthquakes may be triggered by upper plate fault activity

central chile finally breaks chile is the site of some of the largest earthquakes in the world on average a magnitude 8 earthquake occurs there every 10 years or so these earthquakes take place in the subduction zone either as interplate ruptures at the interface between the south american and nazca plates or as intraplate events within the subducted nazca plate a few times in every century massive plate interface earthquakes break several hundred kilometers in a single shock this is what happened on 27 february 2010 when a major earthquake magnitude 8 8 occurred in the maule and biobio regions in central chile see the first figure this region had last experienced a major subduction earthquake in 1835 when darwin 1 visited the area as part of his voyage on the beagle his description of the earthquake inspired many seismologists and historians of chilean earthquakes 2 3

rupture process of the april 24 2017 mw 6 9 valparaiso earthquake from the joint inversion of teleseismic body waves and near field data abstract the central chilean margin 32 33 s is characterized by the subduction of the juan fernandez ridge jfr beneath the continental south american plate the jfr corresponds to a hotspot track composed by seamounts typically 3 3 5 km high above the surrounding seafloor with a ridge trench collision zone underlying the prominent valparaiso forearc basin vfb this region has been affected by several large and mega earthquakes where the last event corresponds to a complex seismic sequence that took place at the southern edge of vfb in april 2017 the spatio temporal distribution of the seismic events is characterized by a predominant southeast migration of the seismicity an mw 6 9 earthquake triggered two days after the sequence started and occurred at the northern end of the rupture area of the 1985 mw 8 0 valparaiso earthquake we compute the kinematic rupture process of the 2017 mw 6 9 valparaiso earthquake from the joint inversion of teleseismic body waves and near field data the akaike s bayesian information criterion was used to objectively estimate both the relative weighting between datasets and the weighting of spatial and temporal constraints used as a priori information the coseismic slip is distributed over an area of dimensions 35 10 km2 with a maximum slip of 1 5 m the rupture propagated unilaterally downdip the source duration from the moment rate solution is 20 s with a total seismic moment of 3 05 1019 nm mw 6 9 the analysis of the seismicity shows that most of the events occurred along the plate interface foreshock clustered northern from the mainshock epicenter and the aftershocks occurred to the southeast at a deeper location the inverted regional moment tensors show similar faulting mechanism than the mainshock the seismic sequence started two days before the mainshock and lasted for about two weeks and a migration pattern of the seismicity was observed the rupture of the 2017 mw 6 9 earthquake nucleated where the san antonio seamount belonging to the jfr is subducting and propagated downwards along a zone that presents high interseismic coupling the complex seismic sequence might be explained by an aseismic slip transient in the zone and the influence of the downdip migration of fluids from the accretionary prism along the subduction channel the erosive and tunneling effect left by the sudden slip of the subducting seamount might provide the cavity for downdip migration of fluids and subsequent swarm seismicity

kinematic rupture process of the 2007 tocopilla earthquake and its main aftershocks from teleseismic and strong motion data summary we study a large m w 7 6 earthquake that occurred on 2007 november 14 in the northern chile seismic gap near the city of tocopilla using a variety of seismic data we show that this earthquake ruptured only the lower part of the interplate seismic zone and generated a series of plate interface aftershocks two large aftershocks on 2007 november 15 ruptured the interplate zone oceanwards of the mejillones peninsula a major geographical feature in the antofagasta region on 2007 december 16 a large m w 6 8 aftershock that occurred near the southern bottom of the fault plane of the main event is shown to be a slab push earthquake located inside the subducted nazca plate and triggered by along slab compression aftershocks of this event demonstrate that it occurred on an almost vertical fault the tocopilla earthquake took place just after the installation of a new seismological network by chilean german and french researchers the accelerometric data combined with far field seismic data provide a quite complete and consistent view of the rupture process the earthquake broke a long 130 km and narrow about 30 50 km zone of the plate interface just above the transition zone using a non linear kinematic inversion method we determined that rupture occurred on two well defined patches of roughly elliptical shape we discuss the consequences of this event for models of gap filling earthquakes in chile proposed in the 1970s