tsunami source and its validation of the 2014 iquique chile earthquake the slip distribution of the 1 april 2014 iquique earthquake is obtained by using the least squares inversion of tsunami data at three deep ocean assessment and reporting of tsunamis stations most of the slip is concentrated along a 60 km by 40 km slip patch near the hypocenter with magnitude ranging from 5 to 7 m and a depth of 23 km the earthquake magnitude from the inversion is estimated as mw 8 0 the slip distribution is converted into seafloor displacement based on okada s formula a nonlinear shallow water equation model is used to simulate tsunami wave propagation and the simulated water surface elevations are compared with the measured data at 10 tide gauges along the chilean coast the agreement is excellent at gauges where the local bathymetry data are complete and the gauges are open to the ocean otherwise mismatches of up to 10 min in arrival time and 1 0 m in amplitude are seen

distribution of slip from 11 mw 6 earthquakes in the northern chile subduction zone we use interferometric synthetic aperture radar gps and teleseismic data to constrain the relative location of coseismic slip from 11 earthquakes on the subduction interface in northern chile 23 25 s between the years 1993 and 2000 we invert body wave waveforms and geodetic data both jointly and separately for the four largest earthquakes during this time period 1993 m w 6 8 1995 m w 8 1 1996 m w 6 7 1998 m w 7 1 while the location of slip in the teleseismic only geodetic only and joint slip inversions is similar for the small earthquakes there are differences for the 1995 m w 8 1 event probably related to nonuniqueness of models that fit the teleseismic data there is a consistent mislocation of the harvard centroid moment tensor locations of many of the 6 6 earthquakes as well as three m w 7 events from the 1980s all of these earthquakes appear to rupture different portions of the fault interface and do not rerupture a limited number of asperities

asperities and barriers on the seismogenic zone in north chile state of the art after the 2007 mw 7 7 tocopilla earthquake inferred by gps and insar data the mw 7 7 2007 november 14 earthquake had an epicentre located close to the city of tocopilla at the southern end of a known seismic gap in north chile through modelling of global positioning system gps and radar interferometry insar data we show that this event ruptured the deeper part of the seismogenic interface 30 50 km and did not reach the surface the earthquake initiated at the hypocentre and was arrested 150 km south beneath the mejillones peninsula an area already identified as an important structural barrier between two segments of the peru chile subduction zone our preferred models for the tocopilla main shock show slip concentrated in two main asperities consistent with previous inversions of seismological data slip appears to have propagated towards relatively shallow depths at its southern extremity under the mejillones peninsula our analysis of post seismic deformation suggests that small but still significant post seismic slip occurred within the first 10 d after the main shock and that it was mostly concentrated at the southern end of the rupture the post seismic deformation occurring in this period represents 12 19 per cent of the coseismic deformation of which 30 55 per cent has been released aseismically post seismic slip appears to concentrate within regions that exhibit low coseismic slip suggesting that the afterslip distribution during the first month of the post seismic interval complements the coseismic slip the 2007 tocopilla earthquake released only 2 5 per cent of the moment deficit accumulated on the interface during the past 130 yr and may be regarded as a possible precursor of a larger subduction earthquake rupturing partially or completely the 500 km long north chile seismic gap

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

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

subduction earthquake deformation associated with 14 november 2007 mw 7 8 tocopilla earthquake in chile results from insar and aftershocks abstract on 14 november 2007 a subduction thrust earthquake magnitude mw 7 8 occurred in the coastal region of northern chile causing substantial damage to the city of tocopilla we investigate the source fault of the earthquake slip distribution and fault interaction by integrating aftershock locations satellite interferometry data and stress model simulations aftershock measurements allow us to locate the area and geometry of the rupture plane in the coastal region between the cities of tocopilla and antofagasta combining two satellite viewing geometries acquired in envisat s wide swath and image modes we observe decimetre scale coseismic deformation the maximum line of sight displacement is found to be about 40 cm located at the mejillones peninsula slip inversions using elastic half space models with geometry constrained by aftershocks suggest rupturing of an area of 160 km by 50 km along the nazca south america convergent margin between latitudes 22 s and 23 5 s the main slip is concentrated on two asperities the largest being located in the southern part of the rupture area at a depth of approximately 30 50 km with a magnitude of about 2 5 m because aftershock distribution may also suggest a region of shallow crustal deformation activity located offshore we investigate whether the 2007 tocopilla earthquake also involved shallow crustal fault slip offshore although we find that the latter assumption is supported by coulomb stress modelling and geologic inferences our geodetic and seismic data provide insufficient constraints to resolve the exact geometry and kinematics of dislocation on this structure

finite fault bayesian inversion of teleseismic body waves abstract inverting geophysical data has provided fundamental information about the behavior of earthquake rupture however inferring kinematic source model parameters for finite fault ruptures is an intrinsically underdetermined problem the problem of nonuniqueness because we are restricted to finite noisy observations although many studies use least squares techniques to make the finite fault problem tractable these methods generally lack the ability to apply non gaussian error analysis and the imposition of nonlinear constraints however the bayesian approach can be employed to find a gaussian or non gaussian distribution of all probable model parameters while utilizing nonlinear constraints we present case studies to quantify the resolving power and associated uncertainties using only teleseismic body waves in a bayesian framework to infer the slip history for a synthetic case and two earthquakes the 2011 m w 7 1 van east turkey earthquake and the 2010 m w 7 2 el mayor cucapah baja california earthquake in implementing the bayesian method we further present two distinct solutions to investigate the uncertainties by performing the inversion with and without velocity structure perturbations we find that the posterior ensemble becomes broader when including velocity structure variability and introduces a spatial smearing of slip using the bayesian framework solely on teleseismic body waves we find rake is poorly constrained by the observations and rise time is poorly resolved when slip amplitude is low electronic supplement figures of histograms of slip and rise time waveform comparisons between data and synthetics and slip velocity along the fault plane for a synthetic case as well as for the 2011 m w 7 1 van east turkey earthquake and the 2010 m w 7 2 el mayor cucapah baja california earthquake

accounting for uncertain fault geometry in earthquake source inversions i theory and simplified application the ill posed nature of earthquake source estimation derives from several factors including the quality and quantity of available observations and the fidelity of our forward theory observational errors are usually accounted for in the inversion process epistemic errors which stem from our simplified description of the forward problem are rarely dealt with despite their potential to bias the estimate of a source model in this study we explore the impact of uncertainties related to the choice of a fault geometry in source inversion problems the geometry of a fault structure is generally reduced to a set of parameters such as position strike and dip for one or a few planar fault segments while some of these parameters can be solved for more often they are fixed to an uncertain value we propose a practical framework to address this limitation by following a previously implemented method exploring the impact of uncertainties on the elastic properties of our models we develop a sensitivity analysis to small perturbations of fault dip and position the uncertainties of our fixed fault geometry are included in the inverse problem under the formulation of the misfit covariance matrix that combines both prediction and observation uncertainties we validate this approach with the simplified case of a fault that extends infinitely along strike using both bayesian and optimization formulations of a static slip inversion if epistemic errors are ignored predictions are overconfident in the data and slip parameters are not reliably estimated in contrast inclusion of uncertainties in fault geometry allows us to infer a robust posterior slip model epistemic uncertainties can be many orders of magnitude larger than observational errors for great earthquakes mw 8 not accounting for uncertainties in fault geometry may partly explain observed shallow slip deficits for continental earthquakes similarly ignoring the impact of epistemic errors can also bias estimates of near surface slip and predictions of tsunamis induced by megathrust earthquakes

fault slip distribution of the 2014 iquique chile earthquake estimated from ocean wide tsunami waveforms and gps data we applied a new method to compute tsunami green s functions for slip inversion of the 1 april 2014 iquique earthquake using both near field and far field tsunami waveforms inclusion of the effects of the elastic loading of seafloor compressibility of seawater and the geopotential variation in the computed green s functions reproduced the tsunami traveltime delay relative to long wave simulation and allowed us to use far field records in tsunami waveform inversion multiple time window inversion was applied to tsunami waveforms iteratively until the result resembles the stable moment rate function from teleseismic inversion we also used gps data for a joint inversion of tsunami waveforms and coseismic crustal deformation the major slip region with a size of 100 km 40 km is located downdip the epicenter at depth 28 km regardless of assumed rupture velocities the total seismic moment estimated from the slip distribution is 1 24 1021 n m mw 8 0

depth varying rupture properties during the 2015 mw 7 8 gorkha nepal earthquake on april 25th 2015 the mw 7 8 gorkha nepal earthquake ruptured a portion of the main himalayan thrust underlying kathmandu and surrounding regions we develop kinematic slip models of the gorkha earthquake using both a regularized multi time window mtw approach and an unsmoothed bayesian formulation constrained by static and high rate gps observations synthetic aperture radar sar offset images interferometric sar insar and teleseismic body wave records these models indicate that kathmandu is located near the updip limit of fault slip and approximately 20 km south of the centroid of fault slip fault slip propagated unilaterally along strike in an ese direction for approximately 140 km with a 60 km cross strike extent the deeper portions of the fault are characterized by a larger ratio of high frequency 0 03 0 2 hz to low frequency slip than the shallower portions from both the mtw and bayesian results we can resolve depth variations in slip characteristics with higher slip roughness higher rupture velocity longer rise time and higher complexity of subfault source time functions in the deeper extents of the rupture the depth varying nature of rupture characteristics suggests that the up dip portions are characterized by relatively continuous rupture while the down dip portions may be better characterized by a cascaded rupture the rupture behavior and the tectonic setting indicate that the earthquake may have ruptured both fully seismically locked and a deeper transitional portions of the collision interface analogous to what has been seen in major subduction zone earthquakes

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

aseismic slip and seismogenic coupling along the central san andreas fault we use high resolution synthetic aperture radar and gps derived observations of surface displacements to derive the first probabilistic estimates of fault coupling along the creeping section of the san andreas fault in between the terminations of the 1857 and 1906 magnitude 7 9 earthquakes using a fully bayesian approach enables unequaled resolution and allows us to infer a high probability of significant fault locking along the creeping section the inferred discreet locked asperities are consistent with evidence for magnitude 6 earthquakes over the past century in this area and may be associated with the initiation phase of the 1857 earthquake as creeping segments may be related to the initiation and termination of seismic ruptures such distribution of locked and creeping asperities highlights the central role of the creeping section on the occurrence of major earthquakes along the san andreas fault

pulsed lava effusion at mount etna during 2001 effusion rate and degassing data collected at mt etna volcano italy in 2001 show variations occurring on time scales of hours to months we use both long and short term data sets spanning january to august to identify this variation the long data sets comprise a satellite and ground based time series of effusion rates and the latter include field based effusion rate and degassing data collected may 29 31 the satellite derived effusion rates for january through august reveal four volumetric pulses that are characterized by increasing mean effusion rate values and lead up to the 2001 flank eruption peak effusion rates during these 23 57 day pulses were 1 2 m 3 s 1 in pulse 1 1 jan 4 mar 1 1 m 3 s 1 in pulse 2 5 mar 21 apr 4 2 m 3 s 1 in pulse 3 24 apr 18 jun 8 8 m 3 s 1 in pulse 4 23 jun 16 jul and 22 2 m 3 s 1 during the flank eruption 17 jul 9 aug rank order analysis of the satellite data shows that effusion rate values during the 2001 flank eruption define a statistically different trend than etna s persistent activity from jan 1 to jul 17 data prior to the flank eruption obey a power law relationship that may define an effusion rate threshold of f3 5 m 3 s 1 for etna s typical persistent activity our short term data coincide with the satellite derived peak effusion period of pulse 3 degassing at vent puff frequency shows a general increase from may 29 to 31 with hour long variations in both puff frequency and lava flow velocity effusion rate we identify five 3 14 h degassing periods that contain 26 shorter 19 126 min long oscillations this variation shows some positive correlation with effusion rate measurements during the same time period if a relationship between puff frequency and effusion rate is valid we propose that their short term variation is the result of changes in the supply rate of magma to the near vent conduit system therefore these short term data provide some evidence that the clear weeks to months long variation in etna s effusive activity january august 2001 was overprinted by a minutes to hour scale oscillation in shallow supply d 2004 elsevier b v all rights reserved

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

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

the long precursory phase of most large interplate earthquakes many earthquakes are preceded by foreshocks however the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are unknown here we use seismic catalogues from the best instrumented areas of the north pacific to analyse the foreshock sequences preceding all earthquakes there between 1999 and 2011 of magnitude larger than 6 5 and at depths shallower than 50 km the data set comprises 31 earthquakes at plate boundaries and 31 in plate interiors we find that there is a remarkable contrast between the foreshock sequences of interplate compared with intraplate earthquakes most large earthquakes at plate interfaces in the north pacific were preceded by accelerating seismic activity in the months to days leading up to the mainshock in contrast foreshocks are much less frequent in intraplate settings we suggest that at plate boundaries the interface between the two plates begins to slowly slip before the interface ruptures in a large earthquake this relatively long precursory phase could help mitigate earthquake risk at plate boundaries

dual megathrust slip behaviors of the 2014 iquique earthquake sequence the transition between seismic rupture and aseismic creep is of central interest to better understand the mechanics of subduction processes a mw 8 2 earthquake occurred on april 1st 2014 in the iquique seismic gap of northern chile this event was preceded by a long foreshock sequence including a 2 week long migration of seismicity initiated by a mw 6 7 earthquake repeating earthquakes were found among the foreshock sequence that migrated towards the mainshock hypocenter suggesting a large scale slow slip event on the megathrust preceding the mainshock the variations of the recurrence times of the repeating earthquakes highlight the diverse seismic and aseismic slip behaviors on different megathrust segments the repeaters that were active only before the mainshock recurred more often and were distributed in areas of substantial coseismic slip while repeaters that occurred both before and after the mainshock were in the area complementary to the mainshock rupture the spatiotemporal distribution of the repeating earthquakes illustrates the essential role of propagating aseismic slip leading up to the mainshock and illuminates the distribution of postseismic afterslip various finite fault models indicate that the largest coseismic slip generally occurred down dip from the foreshock activity and the mainshock hypocenter source imaging by teleseismic back projection indicates an initial down dip propagation stage followed by a rupture expansion stage in the first stage the finite fault models show an emergent onset of moment rate at low frequency 0 5 hz 0 5 hz this indicates frequency dependent manifestations of seismic radiation in the low stress foreshock region in the second stage the rupture expands in rich bursts along the rim of a semi elliptical region with episodes of re ruptures suggesting delayed failure of asperities the high frequency rupture remains within an area of local high trench parallel gravity anomaly tpga suggesting the presence of subducting seamounts that promote high frequency generation our results highlight the complexity of the interactions between large scale aseismic slow slip and dynamic ruptures of megathrust earthquakes

a comprehensive analysis of the illapel 2015 mw8 3 earthquake from gps and insar data the september 16 2015 mw8 3 illapel earthquake occurred on a locked segment of the south american subduction in chile this segment ruptured during comparable size earthquakes in the past in 1880 and 1943 suggesting a somehow regular pattern of characteristic mw 8 earthquakes occurring every 60 to 80 years this recurrence is in agreement with the accumulation of elastic deformation in the upper plate due to the nazca south america subduction at a constant rate of 6 5 cm year leading to a deficit of 4 5 meters of slip to be released every 70 years previous studies consistently imaged the distribution of co seismic slip along the fault based on geodetic seismological and far field tsunami data and all described a significant amount of shallow slip resulting in a large tsunami in addition some models highlighted an apparent mismatch between the modeled rake of slip and the direction of plate convergence suggesting the buildup of large strike slip deficit some of these important questions remain open is shallow slip really well resolved and substantiated is the apparent principal direction of slip during the earthquake really required by the geodetic data here using a comprehensive analysis of continuous gps sites including high rate and static displacements and new survey data from acquired over more than 50 pre existing sites complemented with insar data we show that the 2015 rupture overlaps very well the 1943 rupture with the absence of significant slip south of 32 s and north of 30 2 s peninsula lingua de vaca despite the wealth of geodetic data the shallowest part of the subduction interface remains poorly resolved we also show that the rake of the earthquake is fully compatible with the oblique plate convergence direction rather than perpendicular to the trench meaning that no subsequent trench parallel motion is required by the data we propose that the large low coupling zone lcz at the latitude of la serena revealed by present day coupling distribution is stable over at least two seismic cycles inside the coupled area peak coseismic slip is located precisely offshore the highest coastal topography and elevated terraces adding weight to a potential correlation between the seismic cycle and long term permanent deformation finally we show that early post seismic after slip occurs mostly down dip of co seismic asperity ies extending north and south of the 2015 rupture area

assessment of the size of large and great historical earthquakes in peru a critical reappraisal of great historical earthquakes in peru is made in order to obtain quantitative estimates of rupture length seismic moment tsunami magnitude mt and moment magnitude mw newly collected information concerning several historical earthquakes permits the evaluation of macroseismic zones and from the size of these zones the estimation of length of rupture seismic moment and magnitude mw it is found that local tsunami data provide good quantitative estimates of mt and therefore an independent way of verifying macroseismic estimates of rupture length and seismic moment a space time diagram of great historical earthquakes shows the existence of three zones roughly corresponding to the segmentation of the nazca plate hasegawa and sacks 1981 subducting under peru the northern zone looks aseismic the central one is very complex it breaks either completely or in smaller segments and it may go through long quiescence periods the recurrence time for the southern segment the more regular one is of the order of one century

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

accurate hypocentre determination in the seismogenic zone of the subducting nazca plate in northern chile using a combined on offshore network the coupled plate interface of subduction zones commonly called the seismogenic zone has been recognized as the origin of fatal earthquakes a subset of the after shock series of the great antofagasta thrust type event 1995 july 30 mw 8 0 has been used to study the extent of the seismogenic zone in northern chile to achieve reliable and precise hypocentre locations we applied the concept of the minimum 1 d model which incorporates iterative simultaneous inversion of velocity and hypocentre parameters the minimum 1 d model is complemented by station corrections which are influenced by near surface velocity heterogeneity and by the individual station elevations by relocating mine blasts which were not included in the inversion we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth a study of the resolution parameters ale and dspr documents the importance of offshore stations on location accuracy for offshore events based on precisely determined hypo centres we calculate a depth of 46 km for the lower limit of the seismogenic zone which is in good agreement with previous studies for this area for the upper limit we found a depth of 20 km our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies the results are also in agreement with thermal studies for the antofagasta forearc region

rupture to the trench dynamic rupture simulations of the 11 march 2011 tohoku earthquake there is strong evidence that the 11 march 2011 tohoku earthquake rupture reached the seafloor this is surprising because the shallow portion of the plate interface in subduction zones is thought to be frictionally stable leading to the widely held view that coseismic rupture would stop several tens of kilometers downdip of the seafloor various explanations have been proposed to reconcile this seeming inconsistency including dynamic weakening e g thermal pressurization and extreme stress release around shallow subducted seamounts we offer a simpler explanation supported by 2d dynamic rupture simulations of the tohoku earthquake our models account for depth dependent material properties and the complex geometry of the fault seafloor and material interfaces based on seismic surveys of the japan trench the fault obeys rate and state friction with standard logarithmic dependence of shear strength on slip velocity in steady state in our preferred model the uppermost section of the fault is velocity strengthening rupture nucleates on a deeper velocity weakening section waves released by deep slip reflect off the seafloor transmitting large stress changes to the upper section of the fault driving the rupture through the velocity strengthening region to the trench we validate the model against seafloor deformation and 1 hz global positioning system gps data the seafloor displacements constrain the seismogenic depth and overall amount of slip particularly near the trench our simulations reproduce many features in the gps data thereby providing insight into the rupture process and seismic wave field sensitivity to parameters is explored through an extensive suite of simulations neither static seafloor deformation nor onshore 1 hz gps data can uniquely determine near trench frictional properties due to trade offs with average stress drop while conducted specifically for the japan trench region our simulations suggest that rupture to the trench in megathrust events is quite possible even if velocity strengthening properties extend tens of kilometers landward from the trench online material mp4 movies of particle velocities

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

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

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

characteristics of aquifer system deformation in the southern yangtse delta china excessive groundwater pumpage has caused regional cones of depression and severe land subsidence in the southern yangtse delta china the characteristics of aquifer system compaction are complex because of the difference in the types compositions and structures of the soils that the hydrostratigraphic units are composed of and in the histories of groundwater level change the hydrostratigraphic units have experienced based on the data measured from extensometer groups and observation wells the characteristics of deformation for individual hydrostratigraphic units are analyzed the results show that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics such as elasticity elasto plasticity and visco elasto plasticity at different sites of the cone of depression or in different periods if the groundwater level rises continuously and remains constant later the aquifers and the aquitard units that consist of hard clay primarily exhibit elastic behaviour but the aquitard units that consist of soft clay deform plastically and by creep and exhibit visco elasto plastic behaviour if the groundwater level falls but is much higher than the previous lowest value aquifers and aquitards consisting of hard clay exhibit elasto plastic behaviour if the groundwater level falls below the previous lowest value aquifers and aquitards consisting of hard clay deform plastically and by creep and exhibit visco elasto plastic behaviour

the iquique earthquake sequence of april 2014 bayesian modeling accounting for prediction uncertainty the subduction zone in northern chile is a well identified seismic gap that last ruptured in 1877 on 1 april 2014 this region was struck by a large earthquake following a two week long series of foreshocks this study combines a wide range of observations including geodetic tsunami and seismic data to produce a reliable kinematic slip model of the mw 8 1 main shock and a static slip model of the mw 7 7 aftershock we use a novel bayesian modeling approach that accounts for uncertainty in the green s functions both static and dynamic while avoiding nonphysical regularization the results reveal a sharp slip zone more compact than previously thought located downdip of the foreshock sequence and updip of high frequency sources inferred by back projection analysis both the main shock and the mw 7 7 aftershock did not rupture to the trench and left most of the seismic gap unbroken leaving the possibility of a future large earthquake in the region

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

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

revisiting viscoelastic effects on interseismic deformation and locking degree a case study of the peru north chile subduction zone viscoelastic effects potentially play an important role during all phases of the earthquake cycle in subduction zones however most current models neglect such effects in the interseismic deformation pattern here we use finite element method fem models to investigate the control of viscoelasticity on interseismic deformation and to highlight the pitfalls of interpreting the data with purely elastic models for both the forward and inverse problems our results confirm that elastic models are prone to overestimating the interseismic locking depth a crucial parameter for estimating the maximum possible earthquake magnitude the application of the viscoelastic model improves the fit to the interseismic deformation especially in the inland area additionally we construct 3 d fem models constrained by geophysical and gps data and apply our methodology to the peru north chile subduction zone our results indicate that viscoelastic effects contribute significantly to the observed gps data the signals interpreted as back arc shortening in the elastic model can be alternatively explained by viscoelastic deformation which in turn dramatically refines the interseismic locking pattern in both dip and strike directions our viscoelastic locking map exhibits excellent correlation with the slip distributions of previous earthquakes especially the recent 2014 mw 8 1 iquique earthquake the incorrect elastic assumptions affect the analysis of interseismic deformation with respect to slip deficit calculations our results thus suggest that it is necessary to thoroughly reevaluate existing locking models that are based on purely elastic models some of which attribute viscoelastic deformation to different sources such as microplate sliver motions

the relationship between seismic and aseismic slip on the philippine fault on leyte island bayesian modeling of fault slip and geothermal subsidence delineating seismic and aseismic slip on faults allows the exploration of the complex relationship between these different modes of slip further quantifying them helps in the assessment of seismogenic potential we present a distributed slip model of the rate of aseismic slip along the leyte island section of the philippine fault and make comparisons with the extent of seismic slip from the latest significant earthquakes in july 2017 we derived both coseismic and aseismic slip distributions from kinematic inversions of synthetic aperture radar interferometric insar observations using a probabilistic bayesian framework velocity maps from stacking and time series analysis of alos interferograms spanning 4 years 2007 2011 show a step change in ground deformation right on the fault inverting for slip at depth reveals along strike variations of aseismic slip rate over 100 km aseismic slip on the surface reaches rates of more than 3 cm yr equivalent to the long term slip rate 3 3 0 2 cm yr over the same period a 20 km segment in tongonan appears to be locked this segment ruptured in mw 6 5 and mw 5 8 earthquakes on 6 and 10 july 2017 respectively as constrained by sentinel 1 and alos 2 insar data seismic slip appears to be restricted within the tongonan segment with up to 152 21 cm of left lateral displacement the slip budget and complementarity between the extents of interseismic and coseismic slip suggest that a seismogenic asperity exists in tongonan the presence of active hydrothermal systems and rate strengthening materials provide physical conditions that can promote aseismic slip

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

a window into the complexity of the dynamic rupture of the 2011 mw 9 tohoku oki earthquake the 2011 mw 9 tohoku oki earthquake recorded by over 1000 near field stations and multiple large aperture arrays is by far the best recorded earthquake in the history of seismology and provides unique opportunities to address fundamental issues in earthquake source dynamics here we conduct a high resolution array analysis based on recordings from the usarray and the european network the mutually consistent results from both arrays reveal rupture complexity with unprecedented resolution involving phases of diverse rupture speed and intermittent high frequency bursts within slow speed phases which suggests spatially heterogeneous material properties the earthquake initially propagates down dip with a slow initiation phase followed by sustained propagation at speeds of 3 km s the rupture then slows down to 1 5 km s for 60 seconds a rich sequence of bursts is generated along the down dip rim of this slow and roughly circular rupture front before the end of the slow phase an extremely fast rupture front detaches at about 5 km s towards the north finally a rupture front propagates towards the south running at about 2 5 km s for over 100 km key features of the rupture process are confirmed by the strong motion data recorded by k net and kik net the energetic high frequency radiation episodes within a slow rupture phase suggests a patchy image of the brittle ductile transition zone composed of discrete brittle asperities within a ductile matrix the high frequency is generated mainly at the down dip edge of the principal slip regions constrained by geodesy suggesting a variation along dip of the mechanical properties of the mega thrust fault or their spatial heterogeneity that affects rise time

w phase source inversion for moderate to large earthquakes 1990 2010 rapid characterization of the earthquake source and of its effects is a growing field of interest until recently it still took several hours to determine the first order attributes of a great earthquake e g m w 7 5 even in a well instrumented region the main limiting factors were data saturation the interference of different phases and the time duration and spatial extent of the source rupture to accelerate centroid moment tensor cmt determinations we have developed a source inversion algorithm based on modelling of the w phase a very long period phase 100 1000 s arriving at the same time as the p wave the purpose of this work is to finely tune and validate the algorithm for large to moderate sized earthquakes using three components of w phase ground motion at teleseismic distances to that end the point source parameters of all m w 6 5 earthquakes that occurred between 1990 and 2010 815 events are determined using federation of digital seismograph networks global seismographic network broad band stations and sts1 global virtual networks of the incorporated research institutions for seismology data management center for each event a preliminary magnitude obtained from w phase amplitudes is used to estimate the initial moment rate function half duration and to define the corner frequencies of the passband filter that will be applied to the waveforms starting from these initial parameters the seismic moment tensor is calculated using a preliminary location as a first approximation of the centroid a full cmt inversion is then conducted for centroid timing and location determination comparisons with harvard and global cmt solutions highlight the robustness of w phase cmt solutions at teleseismic distances the differences in m w rarely exceed 0 2 and the source mechanisms are very similar to one another difficulties arise when a target earthquake is shortly e g within 10 hr preceded by another large earthquake which disturbs the waveforms of the target event to deal with such difficult situations we remove the perturbation caused by earlier disturbing events by subtracting the corresponding synthetics from the data the cmt parameters for the disturbed event can then be retrieved using the residual seismograms we also explore the feasibility of obtaining source parameters of smaller earthquakes in the range 6 0 mw 6 5 results suggest that the w phase inversion can be implemented reliably for the majority of earthquakes of mw 6 or larger

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

foreshocks explained by cascades of triggered seismicity 1 the observation of foreshocks preceding large earthquakes and the suggestion that foreshocks have specific properties that may be used to distinguish them from other earthquakes have raised the hope that large earthquakes may be predictable among proposed anomalous properties are the larger proportion than normal of large versus small foreshocks the power law acceleration of seismicity rate as a function of time to the mainshock and the spatial migration of foreshocks toward the mainshock when averaging over many sequences using southern california seismicity we show that these properties and others arise naturally from the simple model that any earthquake may trigger other earthquakes without arbitrary distinction between foreshocks aftershocks and mainshocks we find that foreshock precursory properties are independent of the mainshock size this implies that earthquakes large or small are predictable to the same degree as seismicity rate is predictable from past seismicity by taking into account cascades of triggering the cascades of triggering give rise naturally to long range and long time interactions which can explain the observations of correlations in seismicity over surprisingly large length scales