birth of a major strike slip fault in sw japan a 500 km long strike slip fault the north chugoku shear zone ncsz is identified in sw japan responsible for four m 7 earthquakes in the past 130 years a new geodynamic mechanism of increased interplate coupling above an obliquely subducting flat slab is presented to explain the transfer of trench parallel motion 400 km inland from the nankai trough to the level of the quaternary adakitic volcanic arc while quaternary dextral strike slip motion is widely believed to have occurred along the median tectonic line mtl near the miocene volcanic arc the central segment in shikoku has shown little seismicity for the last 1000 years pure dextral strike slip focal mechanisms for 15 modern and historical events indicate that part of the transcurrent motion is being transferred from the mtl to the ncsz

active faulting and heterogeneous deformation across a megathrust segment boundary from gps data south central chile 36 39 s 1 this study focuses on the present day deformation mechanisms of the south central chile margin at the transition zone between two megathrust earthquake segments defined from historical data the valdivia and concepcion sectors new gps data and finite element models with complex geometries constrained by geophysical data are presented to gain insight into forearc kinematics and to address the role of upper plate faults on contemporary deformation gps vectors are heterogeneously distributed in two domains that follow these two earthquake segments we find that models which simulate only interseismic locking on the plate interface fail to reproduce surface deformation in the entire study area in the concepcion domain models that include a crustal scale fault in the upper plate better reproduce the gps observations in the valdivia domain gps data show regional scale vertical axis rotations which could reflect postseismic deformation processes at the edge of the mw 9 5 earthquake that ruptured in 1960 and or activity of another crustal fault related to motion of a forearc sliver our study suggests that upper plate faults in addition to earthquake cycle transients may exert an important control on the surface velocity of subduction zone forearcs

response of alluvial rivers to slow active tectonic movement alluvial rivers respond to valley slope deformation caused by active tectonics in various ways depending on the rate and amount of surficial deformation and on the type of river on the basis of experimental results and field examples hypothetical models of river response to anticlinal uplift and synclinal subsidence were developed for different types of alluvial rivers an experimental braided channel responded to anticlinal uplift across the channel with degradation and terrace formation in the central part of the uplift with subsidence aggradation in the central reach was the main response transverse bars developed downstream of the subsidence axis an experimental meandering channel responded to slope steepening with a sinuosity increase bank erosion and point bar growth occurred downstream of the anticlinal axis and upstream of the synclinal axis upstream of the uplift axis and downstream of the subsidence axis where the slope was flattened water flooded over bars local convexity in longitudinal profiles of the middle rio grande new mexico is considered to be formed by a domal uplift local aggradation and degradation could be explained by the effect of uplift the san joaquin river california which is now highly controlled does not show clear adjustment to the rapid subsidence due to ground water withdrawal it shows however a channel pattern adjustment to active tectonic subsidence that has been occurring for a long time the san antonio and guadalupe rivers in texas both increase their sinuosity significantly where monoclinal movements steepen valley slopes

plate boundary deformation associated with the great sumatra andaman earthquake the sumatra andaman earthquake of 26 december 2004 is the first giant earthquake moment magnitude m w 9 0 to have occurred since the advent of modern space based geodesy and broadband seismology it therefore provides an unprecedented opportunity to investigate the characteristics of one of these enormous and rare events here we report estimates of the ground displacement associated with this event using near field global positioning system gps surveys in northwestern sumatra combined with in situ and remote observations of the vertical motion of coral reefs these data show that the earthquake was generated by rupture of the sunda subduction megathrust over a distance of 1 500 kilometres and a width of 150 kilometres megathrust slip exceeded 20 metres offshore northern sumatra mostly at depths shallower than 30 kilometres comparison of the geodetically and seismically inferred slip distribution indicates that 30 per cent additional fault slip accrued in the 1 5 months following the 500 second long seismic rupture both seismic and aseismic slip before our re occupation of gps sites occurred on the shallow portion of the megathrust where the large aceh tsunami originated slip tapers off abruptly along strike beneath simeulue island at the southeastern edge of the rupture where the earthquake nucleated and where an m w 7 2 earthquake occurred in late 2002 this edge also abuts the northern limit of slip in the 28 march 2005 m w 8 7 nias simeulue earthquake

estimation of interplate coupling in the nankai trough japan using gps data from 1996 to 2006 we used three component surface velocities in southwest japan to estimate plate coupling on the subducting plate interface at the nankai trough we analyzed continuous gps data from the japanese geonet network from 1996 to 2006 using a consistent analysis strategy that generates bias fixed solutions for the entire network we applied systematic time series analysis methods to estimate common mode error which improved position solutions for the entire network to allow for differences in regional deformation sources we modelled the plate coupling on the plate interface beneath shikoku island to kii peninsula and the tokai suruga trough separately the results show strong coupling at a depth of 10 30 km off shikoku and kii peninsula the spatial variation in plate coupling coincides well with the coseismic rupture zones of the past large earthquakes maximum slip deficit rates of 2 3 cm yr 1 at the depth of 5 25 km are found beneath the tokai area consistent with results from other studies the downdip limits of the highly coupled areas and transition zones beneath shikoku and the kii peninsula correspond approximately to estimates of the 450 c isotherms good correlation is observed between the lateral variations of the slip deficit distribution low frequency earthquakes and coseismic slip this correlation suggests that temperature and possibly fluid variations contribute to such correlation in space the interplate slip deficit derived from the gps velocities over the 10 yr of observations is generally compatible with the results over shorter time spans suggesting that plate coupling in sw japan does not change significantly over the period of these gps measurements

sea ice expansion in the bering sea during the neoglacial evidence from archaeozoology the neoglacial was a period of cold that lasted more than 2000 years during the mid holocene from approximately 4700 to 2500 years ago although proxy data from a number of sources document the re

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

the tectonic framework of the sumatran subduction zone the great aceh andaman earthquake of december 26 2004 and its tragic consequences brought the sumatran region and its active tectonics into the world s focus the plate tectonic setting of sumatra has been as it is today for tens of millions of years and catastrophic geologic events have likely been plentiful the immaturity of our understanding of great earthquakes and other types of geologic hazards contributed to the surprise regarding the location of the 2004 earthquake the timing however is probably best understood simply in terms of the inevitability of the infrequent events that shape the course of geologic progress our best hope is to improve understanding of the processes involved and decrease our vulnerability to them

seismicity and geometry of the south chilean subduction zone 41 5 s 43 5 s implications for controlling parameters in 2005 an amphibious seismic network was deployed on the chilean forearc between 41 75 s and 43 25 s 364 local events were observed in a 11 month period a subset of the p and s arrival times were inverted for hypocentral coordinates 1 d velocity structure and station delays main seismic activity occurred predominantly in a belt parallel to the coast of chiloe island in a depth range of 12 30 km presumably related to the plate interface the 30 inclination of the shallow part of the wadati benioff zone is similar to observations further north indicating that oceanic plate age is not controlling the subduction angle of the shallower part for the chilean subduction zone the down dip termination of abundant intermediate depth seismicity at approximately 70 km depth seems to be related to the young age and high temperature of the oceanic plate crustal seismicity is associated with the liquine ofqui fault zone and active volcanoes

reassessing the 2006 guerrero slow slip event mexico implications for large earthquakes in the guerrero gap in guerrero mexico slow slip events have been observed in a seismic gap where no earthquakes have occurred since 1911 a rupture of the entire gap today could result in a mw 8 2 8 4 earthquake however it remains unclear how slow slip events change the stress field in the guerrero seismic region and what their implications are for devastating earthquakes most earlier studies have relied on a sparse network of global navigation satellite systems measurements here we show that interferometric synthetic aperture radar can be used to improve the spatial resolution we find that slip due to the 2006 slow slip event enters the seismogenic zone and the guerrero gap with 5 cm slip reaching depths as shallow as 12 km we show that slow slip is correlated with a highly coupled region and estimate that slow slip events have decreased the total accumulated moment since the end of the 2001 2002 slow slip event 4 7 years by 50 nevertheless even accounting for slow slip the moment deficit in the guerrero gap increases each year by mw 6 8 the guerrero gap therefore still has the potential for a large earthquake with a slip deficit equivalent to mw 8 15 accumulated over the last century correlation between the slow slip region and nonvolcanic tremor and between slow slip and an ultraslow velocity layer supports the hypothesis of a common source potentially related to high pore pressures

tectonic significance of cretaceous tertiary magmatic and structural evolution of the northern margin of the xolapa complex tierra colorada area southern mexico the tierra colorada area sits along the northern limit of the xolapa complex where it is juxtaposed against the mixteco paleozoic and guerrero mesozoic terranes of southern mexico just north of acapulco this paper presents combined structural and geochronological data from tierra colorada area that show evidence of four deformational events and several episodes of arc magmatism during mesozoic and cenozoic time the oldest magmatism is represented by ca 165 ma granitoids and was followed by intrusion of the foliated el pozuelo granite 129 0 5 ma concordant u pb zircon analysis this intrusion postdates d 1 metamorphism and migmatization in the xolapa complex the next magmatic episode is represented by the peraluminous foliated el salitre granite 55 3 3 3 ma mineral whole rock rb sr isochron and the protomylonitic las pinas i type granite 54 2 5 8 ma lower intercept u pb zircon las pinas granite is characterized by d 2 ductile fabric with normal top tothe north northwest sense of shear deformed at 45 50 ma rb sr and k ar ages the ca 34 ma undeformed granites correspond to the last intrusive pulse in the area postdating both d 3 south southwest verging thrusting of the cretaceous morelos formation over sheared granites and lower cretaceous volcanic rocks and open folding during d 4 these four pulses of subduction related magmatism in the tierra colorada area indicate a regular northeastward subduction at the mesoamerican trench since jurassic time and alternate with contractile and or extensional tectonic events the gap in magmatic activity ca 90 100 ma roughly coincides with deposition of platformal limestones of the morelos formation during the middle cretaceous the stable conditions during deposition of the morelos formation may have resulted from a combination of backarc extension and development of a passive margin during the early middle cretaceous which postdated the accretion of an exotic block either the guerrero terrane or the chortis block following the laramide orogeny in southern mexico roughly during the late cretaceous the paleocene miocene tectonic evolution in the tierra colorada area involved an alternation of magmatic pulses with extensional and contractile events this was the result of a combination of several factors including the geometry of the subducted slab convergence rate stress transmission between the subducting and overlying plates and the rate of subduction erosion

morphotectonic segmentation of an active forearc 37 41 s chile abstract active forearc regions are characterized by seismo and morphotectonic segments that record recurrent seismic activity and subduction related tectonic deformation on different timescales however it is not well known on which timescales morphotectonic forearc segments retain their tectono geomorphic identity how they are related to earthquake rupture zones and what governs their long term evolution the forearc of the active convergent margin of south central chile 37 41 s is located within the rupture zone of the 1960 chile mw 9 6 megathrust earthquake in this region we combine geomorphological sedimentologic and morphometric dem analysis with compiled structural and geophysical data to reconstruct the morphotectonic evolution of the forearc the southern chile forearc comprises three segments nahuelbuta tolten and bueno segments that record a differential structural and geomorphic evolution whereas the tolten segment appears to be quasi stable the other two segments record pronounced ongoing quaternary uplift the nahuelbuta and bueno segments contain the n s oriented coastal cordillera characterized by numerous tectonically controlled drainage anomalies and evidence of sediment rerouting the morphotectonic segmentation appears to have persisted over timescales of 105 to 106 years importantly these segments do not correlate with earthquake rupture zones that exhibit a short term variability over 102 to 103 years thus earthquake rupture zones are transient second order phenomena which do not determine the long term morphotectonic forearc segmentation overall the differential forearc evolution and segmentation is influenced by a complex interplay of inherited upper plate structures and physical properties of the subducting oceanic nazca plate

active strike slip faulting in el salvador central america several major earthquakes have affected el salvador central america during the past 100 yr as a consequence of oblique subduction of the cocos plate under the caribbean plate which is partitioned between trench orthogonal compression and strike slip deformation parallel to the volcanic arc focal mechanisms and the distribution of the most destructive earthquakes together with geomorphologic evidence suggest that this transcurrent component of motion may be accommodated by a major strike slip fault el salvador fault zone we present field geological structural and geomorphological data collected in central el salvador that allow the constraint of the kinematics and the quaternary activity of this major seismogenic strike slip fault system data suggest that the el salvador fault zone consists of at least two main e w fault segments san vicente and berlin segments with associated secondary synthetic wnw ese and antithetic nnw sse riedel shears and nw se tensional structures the two main fault segments overlap in a dextral en echelon style with the formation of an intervening pull apart basin our original geological and geomorphologic data suggest a late pleistocene holocene slip rate of 11 mm yr along the berlin segment in contrast with low historical seismicity the kinematics and rates of deformation suggested by our new data are consistent with models involving slip partitioning during oblique subduction and support the notion that a trench parallel component of motion between the caribbean and cocos plates is concentrated along e w dextral strike slip faults parallel to the volcanic arc

paleogene continental margin truncation in southwestern mexico geochronological evidence the reasons for and mechanisms of continental margin truncation in sw mexico where mesozoic cenozoic plutons are situated directly on the pacific coast are not yet well understood large scale dextral and or sinistral displacements of the continental margin terranes now forming parts of baja california or the chortis block have been proposed the well defined along coast nw se decreasing granitoid intrusion age trend 1 2 cm yr in the 100 ma 40 ma time interval between puerto vallarta and zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the chortis block from those latitudes toward the se changes in the dip and velocity of the nnw sse trending cretaceous tertiary subduction zone resulted in a landward migration of the magmatic arc taking into account certain stratigraphic affinities of chortis and the oaxaca and mixteca terranes together with the known displacement rates along the north america caribbean plate boundary the northwesternmost paleoposition of the chortis block with respect to sw mexico was near zihuatanejo in contrast between zihuatanejo and the isthmus of tehuantepec the cessation of the tertiary magmatism decreased more rapidly 7 7 cm yr although the trend is not so obvious starting in the late eocene chortis moved about 1100 km to the se along a transform boundary associated with the opening of the cayman trough based on our geochronological data and structural relationships between mylonite zones and plutons in the acapulco tehuantepec area we propose an approximately 650 km se movement of chortis from about 40 25 ma with a velocity of 6 5 4 3 cm yr since this is considerably slower than the decreasing age trend obtained by us using the geochronological data we consider batholith formation in this segment to predate and postdate the offshore passage of the north america farallon caribbean triple junction geological observations and paleomagnetic data do not give strong support for large scale right lateral displacements of crustal blocks like the baja california given the isotopic data presented the continental margin truncation in sw mexico seems to be the consequence of an interaction of mechanisms of these we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment on the other hand the lateral removal of material associated with the displacement of chortis is more important in the southeastern segment

a large silent earthquake in the guerrero seismic gap mexico 1 geodetic measurements from a network of permanent gps stations along the pacific coast of mexico reveal a large silent earthquake along the segment of the cocos north american plate interface identified as the guerrero seismic gap the event began in october of 2001 and lasted for 6 7 months average slip of 10 cm produced measurable displacements over an area of 550 250 km2 the equivalent moment magnitude of the event was mw 7 5 recognition of this and previous slow event here indicate that the seismogenic portion of the plate interface is not loading steadily as hitherto believed but is rather partitioning the stress buildup with episodic as opposed to steady state or periodic slip downdip of the seismogenic zone this process increases the stress at the base of the seismogenic zone bringing it closer to failure these results call for a reassessment of the seismic potential of guerrero and other seismic gaps in mexico

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

morphotectonic zones along the coast of the pacific continental margin southern mexico geomorphic analysis employing topographic morphologic geologic and bathymetric maps and field studies show that the morphology of the southern coast of mexico can be linked to lateral variations in the geometry and tectonism of the subduction zone a reconnaissance study based on the regional morphological characteristics and correlation with seismotectonic segments regional tectonics and major bathymetric features allows identification of several morphotectonic zones along the coast of southern mexico 1 jalisco zone 2 colima zone 3 michoacan zone 4 guerrero zone 5 oaxaca zone and 6 tehuantepec zone a range of geomorphological evidence including marine terraces river terraces uplifted notches and elevated wave cut platforms indicates local and regional uplift along the coast of the jalisco michoacan guerrero and oaxaca zones coasts of the colima and tehuantepec zones show morphological evidence of subsidence

overview of recent coastal tectonic deformation in the mexican subduction zone holocene and pleistocene tectonic deformation of the coast in the mexico subudction margin is recorded by geomorphic and stratigraphic markers we document the spatial and temporal variability of active deformation on the coastal mexican subduction margin pleistocene uplift rates are estimated using wave cut platforms at ca 0 7 0 9 m ka on the jalisco block coast rivera north america tectonic plate boundary we examine reported measurements from marine notches and shoreline angle elevations in conjunction with their radiocarbon ages that indicate surface uplift rates increasing during the holocene up to ca 3 0 5 m ka in contrast steady rates of uplift ca 0 5 1 0 m ka in the pleistocene and holocene characterize the michoacan coastal sector south of el gordo graben and north of the orozco fracture zone ofz incorporated within the cocos north america plate boundary significantly higher rates of surface uplift ca 7 m ka across the ofz subduction may reflect the roughness of subducting plate absence of preserved marine terraces on the coastal sector across el gordo graben likely reflects slow uplift or coastal subsidence stratigraphic markers and their radiocarbon ages show late holocene ca last 6 ka bp coastal subsidence on the guerrero gap sector in agreement with a landscape barren of marine terraces and with archeological evidence of coastal subsidence temporal and spatial variability in recent deformation rates on the mexican pacific coast may be due to differences in tectonic regimes and to localized processes related to subduction such as crustal faults subduction erosion and underplating of subducted materials under the southern mexico continental margin

stress transmission across an active plate boundary an example from southern mexico abstract paleostress tensors calculated from inversion of fault slip data at 60 stations between zihuatanejo and tehuantepec in southern mexico were separated into four distinct groups and assigned to time intervals between 150 ma and recent the change in orientation of the maximum horizontal stress closely traces the change in direction of convergence between the north american and the farallon cocos plates for the same time interval depending on the angle between the convergence direction and the plate margin both thrust fault and strike slip stress regimes were active based on this comparison we infer that the stress distribution in the continental crust of southern mexico has been controlled by interplate stress transmission latest cretaceous early tertiary mylonites at the northern edge of the xolapa complex portray left lateral oblique crustal extension and oceanward migration of the upper plate the orientation of extension at mid crustal level grossly correspond to the orientation of the minimum compressive stress at upper crustal level during the last 20 ma a tensional stress regime as a response to surface uplift has been dominant in southern mexico

crustal deformation in northern central america abstract evaluation of the seismic moment tensor for earthquakes on plate boundary is a standard procedure to determine the relative velocity of plates which controls the seismic deformation rate predicted from the slip on a single fault the moment tensor is also decomposed into an isotropic and a deviatoric part to discover the relationship between the average strain rate and the relative velocity between two plates we utilize this procedure to estimate the rates of deformation in northern central america where plate boundaries are seismically well defined four different tectonic environments are considered for modelling of the plate motions the deformation rates obtained here compare well with those predicted from the plate motions models and are in good agreement with actual observations deformation rates on faults are increasingly being used to estimate earthquake recurrence from information on fault slip rate and more on how we can incorporate our current understanding into seismic hazard analyses

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

first seismic record for intra arc strike slip tectonics along the liquine ofqui fault zone at the obliquely convergent plate margin of the southern andes a temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike slip liquine ofqui fault zone lofz in southern chile seventy five shallow crustal events with magnitudes up to m tief w 3 8 and depths shallower than 25 km were observed in an 11 month period mainly occurring in different clusters those clusters are spatially related to the lofz to the volcanoes chaiten michinmahuida and corcovado and to active faulting on secondary faults further activity along the lofz is indicated by individual events located in direct vicinity of the surface expression of the lofz focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike slip mechanisms indicating a ne sw direction of the p axis for the lofz at this latitude the seismic activity reveals the present day activity of the fault zone the recent m tief w 6 2 event near puerto aysen southern chile at 45 4 s on april 21 2007 shows that the lofz is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region

active displacement partitioning and arc parallel extension of the aleutian volcanic arc based on global positioning system geodesy and kinematic analysis global positioning system geodesy and structural analysis of parts of the aleutian island chain support the interpretation of forearc migration westward along right lateral transcurrent faults measured displacements are essentially arc parallel in the eastern unalaska island and western attu island parts of the arc and vary from 3 1 mm yr to 31 4 mm yr respectively at the center of the arc adak subduction of the amlia fracture zone disrupts the pattern of arc parallel displacement and records an upper plate velocity of 9 6 mm yr parallel to plate motion where active displacement partitioning accommodates 30 50 of the strike slip component of plate convergence in the area of impeded subduction partitioning ceases suggesting that the process is sensitive to the degree of coupling across the plate boundary megathrust the differential arc parallel displacements observed at the ends of the arc together with structural relations indicate substantial arc parallel extension which may play an important role in exhumation of high pressure low temperature metamorphic rocks in ancient arc systems

the origin of a terrane u pb zircon geochronology and tectonic evolution of the xolapa complex southern mexico terrane analysis has established differences in lithology style of deformation metamorphism and radiometric ages among several basement complexes in southern mexico in this paper we identify similarities in the geologic history of the adjacent acatlan oaxaca and xolapa complexes through a u pb zircon study on crustal sources and timing of magmatic metamorphic and tectonic activity in the xolapa complex our regional tectonic scenario is based on both new chronologic and previously established structural data inherited zircon in the xolapa complex indicates the presence of a proterozoic 1 0 1 3 ga crustal component with an age range that overlaps grenville crystallization dates 1 0 1 2 ga from the oaxaca basement and an inherited grenville crustal component 1 0 1 1 ga identified in zircons from the acatlan complex the proterozoic component in the xolapa complex indicates either that it received sediments from a continental region of grenvillean age for example the oaxaca basement which is the closest exposed grenville crust in southern mexico or that the xolapa complex has a grenville basement in the latter case the xolapa complex has been modified by widespread high grade metamorphism and large scale migmatization metamorphism and migmatization occurred from 66 to 46 ma and locally continued into oligocene time magmatism in the xolapa complex terminated with crustal growth by plutonism which is characterized by a systematic pattern of eastward younging crystallization ages from 35 ma in the west west of acapulco to 27 ma in the east east of puerto angel metamorphism and migmatization in the xolapa crust may have originated from a contemporaneous change in several parameters of convergence between the farallon and north american plates e g rate and direction of subduction and the early evolution of the caribbean this plate reorganization triggered sinistral transtension distributed across the southern mexican continental margin and the eastward translation of the chortis block the eastward younging of the xolapa plutons also is consistent with the motion of the chortis block during eastward displacement of the caribbean plate this induced a shift of the magmatic arc from its cretaceous to early tertiary position along the xolapa and chortis blocks to its present mid mexican position we conclude that a single tectonic framework accounts for the mesozoic and cenozoic geologic history of the southern mexican terranes

uplift and subsidence associated with the great aceh andaman earthquake of 2004 rupture of the sunda megathrust on 26 december 2004 produced broad regions of uplift and subsidence we define the pivot line separating these regions as a first step in defining the lateral extent and the downdip limit of rupture during that great m w 9 2 earthquake in the region of the andaman and nicobar islands we rely exclusively on the interpretation of satellite imagery and a tidal model at the southern limit of the great rupture we rely principally on field measurements of emerged coral microatolls uplift extends from the middle of simeulue island sumatra at 2 5 n to preparis island myanmar burma at 14 9 n thus the rupture is 1600 km long the distance from the pivot line to the trench varies appreciably the northern and western andaman islands rose whereas the southern and eastern portion of the islands subsided the nicobar islands and the west coast of aceh province sumatra subsided tilt at the southern end of the rupture is steep the distance from 1 5 m of uplift to the pivot line is just 60 km our method of using satellite imagery to recognize changes in elevation relative to sea surface height and of using a tidal model to place quantitative bounds on coseismic uplift or subsidence is a novel approach that can be adapted to other forms of remote sensing and can be applied to other subduction zones in tropical regions

active crustal faults in the forearc region guerrero sector of the mexican subduction zone this work explores the characteristics and the seismogenic potential of crustal faults on the overriding plate in an area of high seismic hazard associated with the occurrence of subduction earthquakes and shallow earthquakes of the overriding plate we present the results of geomorphic structural and fault kinematic analyses conducted on the convergent margin between the cocos plate and the forearc region of the overriding north american plate within the guerrero sector of the mexican subduction zone we aim to determine the active tectonic processes in the forearc region of the subduction zone using the river network pattern topography and structural data we suggest that in the studied forearc region both strike slip and normal crustal faults sub parallel to the subduction zone show evidence of activity the left lateral offsets of the main stream courses of the largest river basins gps measurements and obliquity of plate convergence along the cocos subduction zone in the guerrero sector suggest the activity of sub latitudinal left lateral strike slip faults notably the regional left lateral strike slip fault that offsets the papagayo river near the town of la venta named la venta fault shows evidence of recent activity corroborated also by gps measurements 4 5 mm year of sinistral motion assuming that during a probable earthquake the whole mapped length of this fault would rupture it would produce an event of maximum moment magnitude mw 7 7 even though only a few focal mechanism solutions indicate a stress regime relevant for reactivation of these strike slip structures we hypothesize that these faults are active and suggest two probable explanations 1 these faults are characterized by long recurrence period i e beyond the instrumental record or 2 they experience slow slip events and or associated fault creep the analysis of focal mechanism solutions of small magnitude earthquakes in the upper plate for the period between 1995 and 2008 revealed that frequent normal faults sub parallel to the trench could be reactivated in the current stress field related to the cocos subduction moreover these features could also be reactivated by subduction megathrust earthquakes

geomorphic assessment of active tectonics in the acambay graben mexican volcanic belt spatial variations of quaternary deformation and tectonic activity of faults along the acambay graben are assessed using geomorphic and morphometric approaches the acambay graben is an east west trending structure of apparent quaternary age located in the central part of the mexican volcanic belt which gives rise to pronounced scarps over a distance of about 80 km continuing tectonic activity in the acambay graben is confirmed by recent well documented seismic episodes the intensity of active tectonics has been interpreted through a detailed geomorphic study of the fault generated mountain fronts and fluvial systems the combined geomorphic and morphometric data provide evidence for relative variations in tectonic activity among the acambay graben faults geomorphic indices suggest a relatively high degree of tectonic activity along the venta de bravo and the acambay tixmadeje faults followed in order of decreasing activity by the pastores temascalcingo and tepuxtepec faults spatial variations within faults have also been identified suggesting a higher level of tectonic activity at the tips of the faults this pattern of variation in the relative degree of tectonic activity is consistent with field evidence and seismic data for the acambay graben geomorphic evaluation of the acambay graben faults suggests that the acambay tixmadeje and venta de bravo faults and specifically the tips of these faults and a central segment near the town of venta de bravo should be considered as areas of potentially high earthquake risk 1998 john wiley sons ltd

oblique subduction and collision forearc tectonics of the kuril arc the tectonics of the southwestern kuril arc are a result of the oblique subduction of the pacific plate at the kuril trench in association with forearc sliver migration caused by the oblique subduction collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin as a result of the collision a deep crustal section of island arc is observed at the leading margin of the forearc sliver tectonics of the kuril arc related to oblique subduction are different from those of the western sunda arc where backarc spreading occurs at the leading margin this difference is due to margin morphology of the oblique subduction zone

oblique convergence and deformation along the kuril and japan trenches the hypothesis that present day deformation within the southern kuril forearc is driven by oblique subduction of the pacific plate is tested using 397 horizontal slip directions derived from shallow thrust earthquakes from the kuril and japan trenches for the period 1963 1991 a simple two plate model fits the 397 slip vectors significantly worse than a model that permits strike slip motion of the southern kuril forearc relative to the overlying plate weighted mean slip directions along the southern kuril trench are systematically rotated toward the direction orthogonal to the trench which implies that the net convergence is partitioned into less oblique subduction and trench parallel displacement of the southern kuril forearc the angular discrepancy between the observed slip direction and the direction predicted by the nuvel 1 pacific north america euler vector implies that the southern kuril forearc translates 6 11 mm yr 1 to the southwest relative to the overlying north american plate these results are consistent with geologically geodetically and seismologically observed convergence at the leading edge of the forearc sliver in southern hokkaido and with previously inferred extension at the trailing edge of the sliver which is located at the bussol strait at 46 n the percentage of the oblique component of the rigid plate convergence vector that is transferred into forearc strike slip motion is only 30 which may be evidence that the hokkaido honshu bend in the trench hinders southwest forearc motion or that the greater strength of oceanic forearcs impedes motion along forearc strike slip faults

inner forearc response to subduction of the panama fracture zone southern central america abstract subduction of the right lateral panama fracture zone along the convergent margin of central america creates abrupt lateral variations in convergence rate obliquity and subducting crustal thickness at its intersection with the middle america trench this intersection known as the panama co nz ca triple junction is migrating to the southeast at a rate of 55 mm yr and currently coincides with the lateral termination of the fila costena thrust belt in the inner forearc of the overriding plate mapping in the inner forearc in the area that straddles the subducting panama fracture zone reveals that cocos caribbean convergence west of the triple junction leads to the development of an inner forearc thrust belt inboard of the colliding cocos ridge while little deformation is evident inboard of nazca caribbean convergence east of the triple junction this results in the lateral termination of the fila costena thrust belt in the region of the forearc that projects over the panama fracture zone where four out of five mapped thrust faults tip out and are buried by lahars three new balanced cross sections indicate a steep gradient in shortening from the center of the thrust belt to its southeastern termination the short term history of the inner forearc recorded in the landscape and topography of the fila costena is consistent with the southeastward migration of the thrust belt and the panama triple junction throughout the past 3 ma with evidence for the growth of a new topographic divide and reorganization of stream channel networks

nature and tectonic significance of co seismic structures associated with the mw 8 8 maule earthquake central southern chile forearc abstract the mw 8 8 maule earthquake on february 27 2010 affected the central southern chilean forearc of the central andes here we show the results of field investigations of surface deformation associated with this major earthquake observations were carried out within three weeks after the seismic event mostly in the central and northern part of the forearc overlying the rupture zone we provide a detailed field record of co seismic surface deformation and examine its implications on active andean tectonics surface rupture consisted primarily of extensional cracks push up structures fissures with minor lateral displacements and a few but impressive extensional geometries similar to those observed in analogical modeling of rift systems a major group of nw wnw striking fractures representing co seismic extensional deformation is found at all localities these appear to be spatially correlated to long lived basement fault zones the nw striking normal focal mechanism of the mw 6 9 aftershock occurred on march 11 demonstrates that the basement faults were reactivated by the mw 8 8 maule earthquake the co seismic surface ruptures show patterns of distributed deformation similar to those observed in mapped basement involved structures we propose that co seismic reactivation of basement structures play a fundamental role in stress release in the upper plate during large subduction earthquakes the fundamental mechanism that promotes stress relaxation is largely driven by elastic rebound of the upper plate located right above the main rupture zone

tectono stratigraphic terranes and mineral resource distributions in mexico about 80 of the southern part of the north american cordillera within the republic of mexico is made up of suspect terranes these terranes are suspect because their paleogeographic setting with respect to cratonic north america at various times through much of phanerozoic time is uncertain much of northeastern and southeastern mexico is underlain by basement accreted during late paleozoic time an extension of the appalachian ouachita orogeny this orogen has been considerably modified by jurassic strike slip translations related to the opening of the gulf of mexico western and southwestern mexico is largely made up of several distinct but coeval latest jurassic to late cretaceous submarine magmatic arc terranes with unknown basement that appear to have accreted against the disrupted north american margin by early tertiary time only northeastern sonora and the state of chihuahua appear to be floored by unmoved north american cratonic basement the combined effect of mesozoic accretions and translatio

the structural evolution of active fault and fold systems in central otago new zealand evidence revealed by drainage patterns abstract central otago in new zealand is an area of active continental shortening in which a peneplain surface cut into schist has been deformed by folds which are developed above buried reverse faults we use the drainage patterns in this region to demonstrate various processes in fold and fault growth and interaction that would be difficult to identify by other means in particular we show 1 how simple asymmetric folds can develop into box folds 2 how apparently continuous ridges were formed by the coalescing of quite separate propagating fold and fault segments 3 evidence for the relative ages or relative uplift rates of adjacent structures and 4 evidence for the propagation direction of folds or faults as they grow the few quantitative estimates we obtain for fault propagation rates suggest an increase in length of 10 50 m per earthquake on faults about 20 km long these estimates are very uncertain but are similar in magnitude to an estimate made in nevada for a normal fault of similar size and are also similar to predicted estimates from theoretical growth models they raise the question of whether fault growth earthquake recurrence rates and climate change can interact to produce semi regular discrete features in an active landscape

strain partitioning during oblique plate convergence in northern sumatra geodetic and seismologic constraints and numerical modeling global positioning system gps measurements along the subduction zone of northern sumatra 2 s to 3 n reveal that the strain associated with the oblique convergence of the australian plate with eurasia is almost fully partitioned between trench normal contraction within the forearc and trench parallel shear strain within a few tens of kilometers of the sumatran fault kinematic analyses of interplate earthquake slip vectors provide slip rates on the sumatran fault within a few millimeters per year of gps and geologic rates giving us more confidence in the use of slip vectors for inferring slip partitioning elsewhere the inferred slip rate on the sumatran fault is 1 3 less than the full margin parallel component of plate motion an across forearc rotation in the slip vectors suggests that the missing arc parallel shear occurs seaward of the geodetic network between the forearc islands and the trench simple finite element models are used to explore the conditions under which the change in the principal strain rate directions between the forearc and the arc region can occur modeling suggests that neither a preexisting strike slip fault nor a zone of thermally induced lithospheric weakness in the overriding plate is needed for strain partitioning to occur in general forearc slivers form over the region of interplate coupling and are driven along strike by the basal shear a volcanic arc can help the partitioning process by localizing the margin parallel shear strain in the upper plate if its crust and mantle are weaker than its surroundings interplate slip vectors and geodetic results from sumatra together suggest that the highest coupling on the plate boundary occurs beneath and seaward of the forearc islands consistent with inferences about the rupture zones of great nineteenth century earthquakes there the sumatra example suggests that geodetic measurements of interseismic margin parallel shear strain at oblique convergent margins can be used to map the landward extent of the relatively high basal stress beneath the overriding plate if one can correct for strain localization caused by weak upper plate strike slip faults

active and passive tectonic controls for transverse drainage and river gorge development in a collisional mountain belt dades gorges high atlas mountains morocco abstract transverse drainages are discordant river patterns whose flow directions cut across geological structures such as faults folds and the regional tectonic fabric strike of mountain belts such drainages can form spectacular geomorphological features with reaches often occupying narrow and deeply dissected canyons gorges that cut through prominent topographic barriers within collisional plate margins e g foreland basin systems studies have frequently examined transverse drainage development in response to active tectonics e g thrusting here transverse drainage will develop via incision or become deflected and fixed in response to oblique fault fold growth structures particularly within wedge top thrust front and foredeep basin settings within this paper the development of transverse drainage within the little studied fold thrust component of an orogen is examined using a spectacular series of river gorges along the river dades from the high atlas mountains of morocco integrated field derived geological and geomorphological data are used to explore the interplay between active tectonics lithology and structural configuration passive tectonics of bedrock for the creation of transverse drainage and associated river gorge reaches at the southern edge of the high atlas fold thrust belt the river dades passes through the deeply dissected tarhia n dades and main dades gorges before emerging into the ait seddratt wedge top basin fluvial incision is principally controlled by plio quaternary uplift of the high atlas mountains the larger main dades gorge has formed a deeply dissected antecedent course via drainage inheritance into structurally thickened jurassic bedrock via uplift between two key structural components of the foreland basin system the fold thrust belt and the wedge top basin the smaller tarhia n dades gorge involves a more complex interplay between active tectonics and the passive structural arrangement of an asymmetric plunging syncline combined with the stratigraphic arrangement and strength of the jurassic bedrock the drainage network is configured to the fold and associated fault joint structures to form respective nw se and ne sw orientated dip slope and strike drainage progressive incision by tributary dip slope streams cutting down through resistant limestone bedrock into underlying weaker mudstone bedrock has resulted in accelerated headward erosion and drainage network expansion by river captures this has resulted in a minor re routing of the river dades as a transverse drainage through a former dip slope tributary stream to form the tarhia n dades gorge

normal upper plate fault reactivation in northern chile and the subduction earthquake cycle from geological observations and static coulomb failure stress cfs change abstract the northern chile subduction zone constitutes a seismic gap where no great interplate earthquakes mw 8 8 have nucleated since 1877 active normal faulting occurs conspicuously in the coastal forearc at the southern end of this gap between 22 7 s and 24 s there upper crustal extension is present right above a suggested subduction segment boundary of the nazca south america interplate contact in this work by using coulomb failure stress change cfs models we explore the effects of coseismic and interseismic stages of the subduction earthquake cycle on the reactivation of pre existing upper plate normal faults in this area we attempt to unravel the first order interplate conditions required to explain this extensional pattern putting specific focus on the influence of the megathrust segmentation defined by the mejillones peninsula it is suggested that the restricted occurrence of active normal faulting in the study area is tightly related to the structure of the interplate seismogenic zone positive cfs values are induced in the study area by the coseismic and interseismic stages of the subduction earthquake cycle in a region characterized by lower coupling beneath the mejillones peninsula we propose that fault reactivation in this zone is particularly favored during the coseismic stage by earthquakes occurring offshore of the mejillones peninsula and surrounding areas in the adjacent subduction segments coseismic and interseismic cfs values tend to counterbalance each other resulting in no tensional conditions capable to cause normal fault reactivation we suggest that the coupling variation in the seismogenic zone is long term controlled by the geological structure of the upper plate

accuracy of cosmogenic ages for moraines analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38 between the oldest and youngest boulders from each moraine this range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform the wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders a diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders n 3 7 will always yield a lower age limit for the moraine the model indicates that for identical dating accuracy six to seven boulders are needed from old and tall moraines 40 000 100 000 yr 50 100 m initial height but only one to four boulders from small moraines 20 000 100 000 yr 10 20 m by following these guidelines the oldest obtained boulder age will be 90 of the moraine age 95 probability this result is only weakly sensitive to a broad range of soil erosion rates our analysis of published boulder ages indicates that 3 of all moraine boulders have prior exposure and 85 of these boulders predate the dated moraine

interseismic coupling segmentation and mechanical behavior of the central chile subduction zone global positioning system gps measurements carried out in chile over the last two decades showed that an entire portion of the nazca south america subduction zone 38 s 24 s was locked over this period of time the induced accumulation of elastic deformation in the upper plate was not released until the recent maule earthquake of 27 february 2010 mw 8 8 that ruptured the southern part of this section locking or coupling between the two plates varies both with depth and along strike here we use our own gps data an updated solution of our extended network in central chile combined with other published data sets to quantify the spatial variations of the coupling that prevailed before the maule earthquake using a simple elastic model based on the back slip assumption we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation with no need of a sliver in central chile we identify four segments characterized by higher coupling and separated by narrow areas of lower coupling this segmentation is in good agreement with historical and recent seismicity in chile in particular the narrow zones of lower coupling seem to have stopped most large seismic ruptures including maule s these zones are often associated with irregular bathymetric or coastal features fracture zones or peninsulas finally coseismic and early post seismic slip distribution of the maule earthquake occurring either in previously highly or weakly coupled zones map a complex distribution of velocity weakening and velocity strengthening patches on the subduction interface

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

late glacial and holocene landscapes of central beringia abstract new palynological and sedimentological data from st lawrence island present a rare view into late glacial and holocene environments of the central bering land bridge the late glaciation was a time of dynamic landscape changes in south central beringia with active thermokarst processes including the formation and drainage of thaw lakes the presence of such a wet unstable substrate if widespread probably would have had an adverse impact on food sources and mobility for many of the large mammal populations the establishment of betula shrub tundra on the island suggests late glacial summers that were warmer than present consistent with regional paleoclimatic interpretations however the increasing proximity to the bering sea as postglacial sea levels rose modified the intensity of warming and prevented the establishment of deciduous forest as found in other areas of beringia at this time the mid to late holocene is marked by more stable land surfaces and development of sphagnum and cyperaceae peat deposits the accumulation of organic deposits decline of shrub betula and decrease in thermokarst disturbance suggest that conditions were cooler than the previous a recent decline in peat accumulation at the study sites may relate to local geomorphology but similar decreases have been noted for other arctic regions

holocene glacier fluctuations in alaska abstract this review summarizes forefield and lacustrine records of glacier fluctuations in alaska during the holocene following retreat from latest pleistocene advances valley glaciers with land based termini were in retracted positions during the early to middle holocene neoglaciation began in some areas by 4 0 ka and major advances were underway by 3 0 ka with perhaps two distinct early neoglacial expansions centered respectively on 3 3 2 9 and 2 2 2 0 ka tree ring cross dates of glacially killed trees at two termini in southern alaska show a major advance in the ad 550s 720s the subsequent little ice age lia expansion was underway in the ad 1180s 1320s and culminated with two advance phases respectively in the 1540s 1710s and in the 1810s 1880s the lia advance was the largest holocene expansion in southern alaska although older late holocene moraines are preserved on many forefields in northern and interior alaska tidewater glaciers around the rim of the gulf of alaska have made major advances throughout the holocene expansions were often asynchronous with neighboring termini and spanned both warm and cool intervals suggesting that non climatic factors were important in forcing these advances however climatic warming appears to have initiated most rapid iceberg calving retreats large glaciers terminating on the forelands around the gulf of alaska may have had tidewater termini early in the holocene but have progressively become isolated from the adjacent ocean by the accumulation and subaerial exposure of their own sediments

the 2011 magnitude 9 0 tohoku oki earthquake mosaicking the megathrust from seconds to centuries geophysical observations from the 2011 moment magnitude m w 9 0 tohoku oki japan earthquake allow exploration of a rare large event along a subduction megathrust models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places sources of high frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip relative to the m w 8 8 2010 maule chile earthquake the tohoku oki earthquake was deficient in high frequency seismic radiation a difference that we attribute to its relatively shallow depth estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event

evidence from coseismic slip gradients for dynamic control on rupture propagation and arrest through stepovers received 30 july 2008 revised 14 november 2008 accepted 18 december 2008 published 27 february 2009 1 analysis of historic slip distributions from large magnitude continental strike slip earthquake ruptures reveals a pronounced correlation between the distance over which slip decreases as a rupture approaches a structural step in the fault and the ability of the rupture to propagate through the step our analysis of coseismic slip gradients near these stepovers indicates that in earthquakes in which slip decreases gradually toward a step rupture will not continue on the next segment conversely in earthquakes in which slip decreases abruptly rupture commonly renucleates on the next segment the fact that ruptures that stopped had low slip gradients near stepovers relative to those that continued indicates that rupture dynamics control the propagation of rupture through stepovers these results corroborate dynamic rupture models that show that ruptures in which slip decreases abruptly at a step generate strong seismic waves that serve to renucleate rupture on the opposite side of the structural step there are several potential causes for gradual decrease of displacement as rupture approaches a stepover including transference of deformation onto subsidiary structures rheological contrasts that could dictate favored rupture propagation directions the existence of stress shadows from previous earthquakes within the system or other material or frictional heterogeneities all of these are potentially observable and if mapped systematically could provide the basis for a strong constraint on the likely end points of future earthquakes inasmuch as earthquake magnitude is strongly dependent on the size of the rupture such predictions would be of great utility as a basic component of scenario based earthquake rupture forecasts

impacts of post glacial rebound on landslide spatial distribution at a regional scale in northern iceland skagafjorður in de glaciated areas para glaciation i e the conditioning of landscapes by prior glaciation has often been considered a major predisposing factor in landslide occurrence its consequences have been particularly well identified at a fine scale especially on bedrock jointing hitherto the relative impacts of para glaciation on hillslope dynamics at a regional scale had nevertheless not been quantified statistically we examine skagafjor ur area northern iceland where landslides are widespread at least 108 were mapped in an area of c 3000km2 we compare the role of para glaciation debuttressing influence of post glacial rebound with that of classic factors topography lithology etc in landslide occurrence and location using a spatial analysis based on a chi square test on the one hand the results highlight that landslides are over represented in areas where post glacial rebound was at its maximum with a stronger concentration of landslides in the northern part of the fjord on the other hand the distribution of landslides did not show any clear relationship with the pattern of glacial debuttressing tschuprow coefficient highlights that the influence of post glacial rebound on landslide location is higher than the combined influence of slope gradient curvature or geological structure this result is supported by our initial evidence for the timing of landslides in the area most landslides occurred during the first half of the holocene and a period of hillslope instability was initiated when the post glacial uplift was at its maximum finally the mechanisms that link post glacial rebound and landsliding as well as the geomorphic impacts of landslides are discussed

apparent stress fault maturity and seismic hazard for normal fault earthquakes at subduction zones summary the behavior of apparent stress for normal fault earthquakes at subduction zones is derived by examining the apparent stress a es m 0 w here es is radiated energy and m 0 is seismic moment of all globally distributed shallow depth h 1m pa are also generally intraslab but occur where the lithosphere has just begun subduction beneath the overriding plate they usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis or where there are local contortions or geometrical complexities of the plate boundary lower a 1m pa i sassociated with events occurring at the outer rise or complex between the or and the trench axis as well as with intracrustal events occurring just landward of the trench the average apparent stress of intraslab normal fault earthquakes is considerably higher than the average apparent stress of interplate thrust fault earthquakes in turn the average a of strike slip earthquakes in intraoceanic environments is considerably higher than that of intraslab normal fault earthquakes the variation of average a with focal mechanism and tectonic regime suggests that the level of a is related to fault maturity lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements from hundreds to thousands of kilometres in contrast immature faults such as those on which intraslab normal fault earthquakes generally occur are found in cold and intact lithosphere in which total fault displacement has been much less from hundreds of metres to a few kilometres also faults on which high a oceanic strike slip earthquakes occur are predominantly intraplate or at evolving ends of transforms at subduction zones earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults we have identified earthquake pairs in which an interplate thrust and an intraslab normal earthquake occurred remarkably close in space and time the intraslab normal member of each pair radiated anomalously high amounts of energy compared to its thrust fault counterpart these intraslab earthquakes probably ruptured intact slab mantle and are dramatic examples in which me an energy magnitude is shown to be a far better estimate of the potential for earthquake damage than mw this discovery may help explain why loss of life as a result of intraslab earthquakes was greater in the 20th century in latin america than the fatalities associated with interplate thrust events that represented much higher total moment release

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

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

the 2010 mw 8 8 maule megathrust earthquake of central chile monitored by gps large earthquakes produce crustal deformation that can be quantified by geodetic measurements allowing for the determination of the slip distribution on the fault we used data from global positioning system gps networks in central chile to infer the static deformation and the kinematics of the 2010 moment magnitude m w 8 8 maule megathrust earthquake from elastic modeling we found a total rupture length of 500 kilometers where slip up to 15 meters concentrated on two main asperities situated on both sides of the epicenter we found that rupture reached shallow depths probably extending up to the trench resolvable afterslip occurred in regions of low coseismic slip the low frequency hypocenter is relocated 40 kilometers southwest of initial estimates rupture propagated bilaterally at about 3 1 kilometers per second with possible but not fully resolved velocity variations

fault steps and the dynamic rupture process 2 d numerical simulations of a spontaneously propagating shear fracture fault steps may have controlled the sizes of the 1966 parkfield 1968 borrego mountain 1979 imperial valley 1979 coyote lake and the 1987 superstition hills earthquakes this project investigates the effect of fault steps of various geometries on the dynamic rupture process we have used a finite difference code to simulate spontaneous rupture propagation in two dimensions we employ a slip weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes the geometry of the two parallel fault planes allows for stepover widths of 0 5 to 10 0 km and overlaps of 5 to 5 km our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps a major difference between the compressional and dilational cases is that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment therefore our dynamic study implies that a compressional step will be jumped quickly whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity we also find that the rupture is capable of jumping a wider dilational step than compressional step

large extensional aftershocks in the continental forearc triggered by the 2010 maule earthquake chile summary the mw 8 8 maule earthquake occurred off the coast of central chile on 2010 february 27 and was the sixth largest earthquake to be recorded instrumentally this subduction zone event was followed by thousands of aftershocks both near the plate interface and in the overriding continental crust here we report on a pair of large shallow crustal earthquakes that occurred on 2010 march 11 within 15 min of each other near the town of pichilemu on the coast of the o higgins region of chile field and aerial reconnaissance following the events revealed no distinct surface rupture we infer from geodetic data spanning both events that the ruptures occurred on synthetic sw dipping normal faults the first larger rupture was followed by buried slip on a steeper fault in the hangingwall the fault locations and geometry of the two events are additionally constrained by locations of aftershock seismicity based on the international maule aftershock data set the maximum slip on the main fault is about 3 m and consistent with field results the onshore slip is close to zero near the surface satellite radar data also reveal that significant aseismic afterslip occurred following the two earthquakes coulomb stress modelling indicates that the faults were positively stressed by up to 40 bars as a result of slip on the subduction interface in the preceding megathrust event in other words the pichilemu earthquakes should be considered aftershocks of the maule earthquake the occurrence of these extensional events suggests that regional interseismic compressive stresses are small several recent large shallow crustal earthquakes in the overriding plate following the 2011 mw 9 0 tohoku oki earthquake in japan may be an analogue for the triggering process at pichilemu

slow slip hidden in the noise the intermittence of tectonic release referred to as slow slip events the transient aseismic slip that occurs along plate boundaries can be indirectly characterized through colocated seismicity such as tectonic tremor and low frequency earthquakes lfes using the timing of cataloged lfe and tremor activity in guerrero mexico and northern cascadia i decompose the inter aseismic gps displacement defined as the surface deformation between previously detected slow slip events into separate regimes of tectonic loading and release in such a way previously undetected slow slip events that produce less than a millimeter of surface deformation are extracted from the geodetic noise these new observations demonstrate that the inter aseismic period is not quiescent and that slow slip occurs much more often than previously thought this suggests that the plate interface where slow slip and tremor occur is in fact strongly coupled and that slow aseismic release occurs over a wide spectrum of time scales

segmentation of megathrust rupture zones from fore arc deformation patterns over hundreds to millions of years arauco peninsula chile 1 this work explores the control of fore arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers the arauco nahuelbuta region at the south central chile margin constitutes an anomalous fore arc sector in terms of topography geology and exhumation located within the overlap between the concepcion and valdivia megathrust segments this boundary however is only based on 500 years of historical records we integrate deformed marine terraces dated by cosmogenic nuclides syntectonic sediments published fission track data seismic reflection profiles and microseismicity to analyze this earthquake boundary over 10 2 10 6 years rapid exhumation of nahuelbuta s dome like core started at 4 1 2 ma coeval with inversion of the adjacent arauco basin resulting in emergence of the arauco peninsula here similarities between topography spatiotemporal trends in fission track ages pliocene pleistocene growth strata and folded marine terraces suggest that margin parallel shortening has dominated since pliocene time this shortening likely results from translation of a fore arc sliver or microplate decoupled from south america by an intra arc strike slip fault microplate collision against a buttress leads to localized uplift at arauco accrued by deep seated reverse faults as well as incipient oroclinal bending the extent of the valdivia segment which ruptured last in 1960 with an mw 9 5 event equals the inferred microplate we propose that mechanical homogeneity of the fore arc microplate delimits the valdivia segment and that a marked discontinuity in the continental basement at arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960 type ruptures as microplate related deformation occurs since the pliocene we propose that this earthquake boundary and the extent of the valdivia segment are spatially stable seismotectonic features at million year scale

source time functions of large mexican subduction earthquakes morphology of the benioff zone age of the plate and their tectonic implications we study source parameters of large shallow mexican subduction zone earthquakes 95 w to 106 w which occurred between 1928 and 1986 by modeling the p waves recorded on galitzin wilip seismograph in debilt dbn holland for post 1962 events the source parameters retrieved from dbn seismograms alone agree well with those obtained from long period world wide standardized seismograph network records giving us confidence in our results for pre 1962 events all earthquakes are shallow h 10 to 20 km with few exceptions the sources in oaxaca 95 w to 99 w are very simple to the northwest of 99 w they are simple as well as complex the ratio of surface wave to body wave seismic moment mos mop which is a measure of long to short period radiation is smaller in oaxaca 1 5 0 5 than in the regions northwest of 99 w 3 1 1 3 these results suggest a change in the plate interface characteristics near 99 w the sharp change in the rupture mode and the intersection of the o gorman fracture zone ofz with the trench occur near 99 w two strike slip events offshore close to ofz suggest a segmentation of the subducting plate near 99 w the age of the plate near the trench in oaxaca is not well known it is possible that it does not increase continuously from northwest to southeast in the region but jumps across 99 w if so then the older age of the subducted plate southeast of 99 w may be the cause of the distinct rupture mode of the oaxaca earthquakes the length of the benioff zone which is greatest below oaxaca 400 km and decreases toward the northwest can be explained by the correlation between the length of the subducted slab and the product of the lithosphere age and convergence rate the relative complexity of sources the weaker background seismicity and the lesser number of aftershocks northwest of oaxaca may be explained by a stronger interface coupling resulting from subduction of younger oceanic slabs 5 to 13 m y old in this region this however explains neither larger mos mop values northwest of oaxaca nor the low stress drop estimates obtained from the analysis of near field strong motion data for the michoacan earthquake of 1985 both of which indicate weaker coupling of the interface thus the issue of whether subduction of very young plates 10 m y old results in strong or weak coupling remains unsolved from the presently available mexican data

late holocene strike slip offset of a subsurface channel interpreted from three dimensional seismic data eastern offshore trinidad a right lateral strike slip fault offset of a shallowly buried fluvial paleochannel is interpreted from three dimensional 3 d seismic data in the eastern offshore area of trinidad the fault represents the eastern offshore continuation of the main south america caribbean plate boundary zone in trinidad the central range fault zone previous global positioning system based geodetic studies and trenching have shown that the 50 km long on land segment of the central range fault zone accommodates a significant part of the present day interplate motion the 3 d seismic data shown here reveal that the 60 km long offshore continuation of the central range fault zone forms a prominent seafloor lineament and dextrally offsets a shallowly buried 83 m below sea level late quaternary fluvial channel by 322 506 m based on the eustatic sea level curve we infer that the channel began to be incised during the beginning of the last glacial maximum ca 30 ka and was subsequently offset by the central range fault zone using the offset amounts and our inferred age for the filled channel we can estimate a long term slip along the fault of 17 19 mm yr for the central range fault zone because no documented earthquake has occurred on the central range since a d 1800 as much as 3 7 m of elastic strain may have accumulated on the central range fault zone that could be released by a future magnitude m7 5 earthquake

geological record of flat slab induced extension in the southern peruvian forearc the long lived andean subduction zone underwent several flat slab episodes and is therefore ideal to study the consequences of a complete cycle of slab flattening and steepening on the upper plate deformation pattern in the modern peruvian forearc 15 17 s slab flattening caused a paleogene 52 30 ma landward migration of volcanic activity combining structural geology and a source to sink thermochronological study we show that the flat slab period is contemporaneous with uplift accommodated by large scale crustal extension in the forearc in this light we argue that the study area is an ancient analog to the modern mexican and northern peruvian forearcs located above modern flat slab segments and currently undergoing widespread extension

causes of recent himalayan uplift deduced from deposited patterns in the ganges basin the hypothesis1 that late cenozoic uplift of many of the world s mountain ranges was a response to climate change rather than to tectonic processes is based on the premise that enhanced erosion in an altered climate led to a reduction in both the total mass of a mountain range and its mean elevation this reduction was coeval with increased regional relief and with passive isostatic uplift of the remaining summits the hypothesis is difficult to evaluate because both climate and tectonic forces affect uplift in many active ranges in the absence of sufficient denudational and uplift data from the mountains themselves depositional patterns in adjacent basins may sometimes be used to distinguish between uplift arising from erosional or from tectonic processes in foreland basins uplift of the proximal basin and a reduction or absence of asymmetric subsidence are predicted responses to erosionally driven uplift2 here i show that patterns of sediment accumulation and the position of major rivers in the neogene gangetic foreland basin seem to have changed markedly in plio pleistocene times these changes lend support to the hypothesis that the importance of erosional unloading has increased in the himalayas during the past 4 myr

geomorphic processes influence human settlement on two islands in the islands of four mountains alaska the islands of four mountains island group of the aleutian island arc is remote and difficult to access consequently little fieldwork has focused on geomorphic processes and their relationship to island morphology climatic change and human settlement we investigated glacial fluvial and slope processes on the morphologically different carlisle cleveland and tana volcanoes the islands were extensively glaciated at the last glacial maximum lgm and there is evidence for a neoglacial advance on the highly dissected tana volcano a large basin is likely the result of a pre lgm sector collapse and subsequent glacial erosion into weak hydrothermally altered rock valley and moraine morphology is also influenced by hydrothermal alteration on both tana and carlisle there are sediment fans composed dominantly of thick debris flow deposits mantled by 3 m of layered tephras fine grained alluvium and anthropogenic deposits debris flow deposition was favored during the unstable paraglacial landscape of the early holocene latest pleistocene the earliest direct archaeological evidence for settlement is 3 8 cal ka bp but soil geochemical evidence suggests that the islands were inhabited by 7 3 cal ka bp this discrepancy in the archaeological record may be explained by geomorphic processes including coastal erosion and unstable geomorphic surfaces

last glacial maximum to holocene sea surface conditions at umnak plateau bering sea as inferred from diatom alkenone and stable isotope records 1 the bering sea gateway between the pacific and arctic oceans impacts global climate when glacial interglacial shifts in shore line position and ice coverage change regional albedo previous work has shown that during the last glacial termination and into the holocene sea level rises and sea ice coverage diminishes from perennial to absent yet existing work has not quantified sea ice duration or sea surface temperatures sst during this transition here we combine diatom assemblages with the first alkenone record from the bering sea to provide a semiquantitative record of sea ice duration sst and productivity change since the last glacial maximum lgm during the lgm diatom assemblages indicate that sea ice covered the southeastern bering sea perennially at 15 1 cal ka b p the diatom assemblage shifts to one more characteristic of seasonal sea ice and alkenones occur in the sediments in low concentrations deglaciation is characterized by laminated intervals with highly productive and diverse diatom assemblages and inferred high coccolithophorid production at 11 3 cal ka b p the diatom assemblage shifts from one dominated by sea ice species to one dominated by a warmer water north pacific species simultaneously the sst increases by 3 c and the southeastern bering sea becomes ice free year round productivity and temperature proxies are positively correlated with independently dated records from elsewhere in the bering sea the sea of okhotsk and the north pacific indicating that productivity and sst changes are coeval across the region

relationship between subduction and seismicity in the mexican part of the middle america trench two catalogs of earthquakes in the mexican part of the middle america trench are analyzed to elucidate principal relations between structure of the subducting cocos plate and seismicity a catalog of historical events that have occurred during the last two centuries with large magnitudes ms 6 0 is used to obtain cumulative seismic moment mo and seismic moment release rate m 0 distributions along the mexican subduction zone the catalog of instrumentally observed earthquakes from 1963 to 1990 international seismological center and u s geological survey with 4 5 mb 6 0 is applied to investigate background seismicity for the region the strength of coupling between the cocos and north american plates would be expected to grow gradually from the southeast to the northwest according to the variation of convergence rate v and age a of the cocos plate this correlates in general with a steady reduction in background seismicity and a slight average increase of m 0 and seismic energy release rate w at the sites where the main fracture zones of the cocos plate tehuantepec o gorman orozco and rivera undergo the subduction the general correlation breaks down the background seismicity increases at fracture zones while m0 and m 0 decrease significantly this feature is interpreted as a drop of the coupling at the areas where transform faults are being subducted seismic slip rates along the trench obtained from m 0 are lower then the values of plate convergence rates but the average seismic slip is in agreement with the estimates from the v model interaction between lithospheric plates at convergent zones through the viscous layer of subducted sediments variability of m 0 and seismic slip rate in relation with tectonics should be taken into account when the seismic gap model is being used for the prediction of strong earthquakes an examination of space time plots for the historical catalogs supposes a probable tendency of northwest migration of strong events with a rate of 10 km yr

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

on the origin of the cocos nazca spreading center to investigate the mechanism underlying the break up of the farallon plate into the cocos plate and nazca plate we analyze the state of stress in the farallon plate at about 30 m y b p just prior to the fragmentation to this purpose we use finite element methods and a reconstruction of regional plate boundaries appropriate for 30 m y b p a key role in the model is played by the dependence of two important plate tectonics forces slab pull and ridge push on the age of oceanic lithosphere the results show a highly tensional stress field with maximum principal stresses of 5 to 6 kbar north south tension in the vicinity of present day panama is proposed to have been the cause for the fragmentation of the farallon plate and the inception of spreading along the new cocos nazca plate boundary because the kind of plate interaction that gave rise to the high level of tensional stresses in the farallon plate is not restricted to the area of this study the proposed mechanism seems to shed light on the problem of fragmentation of oceanic plates in general

stress evolution during the megathrust earthquake cycle and its role in triggering extensional deformation in subduction zones abstract great moment magnitude mw 8 0 and larger subduction megathrust earthquakes are commonly followed by increased rates of normal faulting seismicity extensional activity within the subducting slab is amplified when megathrust slip propagates close to the trench and forearc extension is triggered by the largest magnitude mw 8 5 and larger events to better understand these observations we develop an earthquake cycle model with a realistic slab geometry and stresses that are in balance with plate interface slip and bulk viscous relaxation the modeled stresses represent perturbations to the long term background tectonic stress field the steady state inter seismic earthquake cycle stresses are compressive and their magnitudes depend on the interface locking configuration from 25 mpa for a fully locked seismogenic zone to 5 mpa for discrete asperities on an otherwise unlocked plate interface the co seismic slip and corresponding co seismic stress changes are similar in these models independent of the locking configuration the co seismic stress change magnitudes are up to 5 mpa this implies that the earthquake cycle stresses after the mainshock would still be compressive in the case of a continuous seismogenic zone but would be widely reset to zero in the case of discrete asperities models with co seismic slip confined to shallow depths produce tensional stress changes in the slab and the forearc near the trench whereas events rupturing the base of the seismogenic zone produce tensional stress changes limited to region immediately surrounding the rupture the locations of normal faulting aftershocks generally correlate well with the tensional co seismic stress changes greater than 1 mpa following the mainshock rapid afterslip occurring down dip of the megathrust rupture expands the region of relative extension but reduces its magnitude and does not promote additional normal faulting events bulk viscous relaxation does little to the state of stress in the elastic parts of the model continued plate convergence across a re locked interface returns the system to the pre earthquake state of compression

seismicity and state of stress in guerrero segment of the mexican subduction zone 1 we take advantage of a relatively dense network of seismic stations in the guerrero segment of the mexican subduction zone to study seismicity and state of stress in the region we combine our results with recent observations on the geometry of the subducted cocos plate imaged from receiver function rf analysis an ultraslow velocity layer mapped in the upper crust of the subducted slab and episodic slow slip events sses and nonvolcanic tremors nvts reported in the region to obtain a comprehensive view of the subduction process seismicity and focal mechanisms confirm subduction of the cocos plate below mexico at a shallow angle reaching a depth of 25 km at a distance of 65 km from the trench the plate begins to unbend at this distance and becomes horizontal at a distance of 120 km at a depth of 40 km some of the highlights of the inslab seismicity are as follows 1 a cluster of earthquakes in the depth range of 25 45 km immediately downdip from the strongly coupled part of the plate interface revealing both downdip compressional and extensional events this seismicity extends from 80 to 105 km from the trench and may be attributed to the unbending of the slab 2 the slab devoid of seismicity in the distance range of 105 160 km 3 sparse inslab seismicity revealing downdip extension in the distance range of 160 240 km the nvts are also confined to this distance range the episodic sses occur on the horizontal segment in the distance range of 105 240 km where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted sp phases thus in the distance range of 105 160 km sses occur but nvts and inslab earthquakes are absent this suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of 160 km giving rise to both the inslab earthquakes and nvts no inslab earthquake occurs beyond 240 km the receiver function images and p wave tomography suggest that the slab begins a steep plunge at a distance of 310 km reaching a depth of 500 km around 340 km from the trench the negative buoyancy of such a slab should give rise to large extensional stress in the slab yet inslab seismicity is remarkably low which may be explained by a slab that is not continuous up to a depth of 500 km but is broken at a shallower depth the resulting slab window may permit subslab material to flow through the gap this may provide an explanation for the recent rift related basalts found near mexico city the fore arc upper plate seismicity which during the period of study 1995 2007 consisted of a moderate earthquake mw5 8 near the coast h 12 km its numerous aftershocks and two shallow events farther inland demonstrates a trench normal extension in the upper plate near this convergent margin a state of stress that may be explained by tectonic erosion and or seaward retreat of the trench seismicity location of the mantle wedge and rupture areas of mexican earthquakes suggest that the downdip limit of rupture during large great earthquakes in guerrero may be 105 15 km shallow dipping interplate thrust earthquakes are not the only type of events that affect the seismic hazard in the region the magnitudes of inslab downdip compressional and extensional earthquakes that occur within 20 km inland from the coast in the depth range of 25 45 km may reach 6 5 and 7 5 respectively in addition we now identify normal faulting earthquakes in the upper plate these sources need to be taken into account in the hazard estimation

sedimentary record of late holocene relative sea level change and tectonic deformation from the guerrero seismic gap mexican pacific coast studies of the coastal sedimentary record have allowed both the reconstruction of relative sea level changes and the determination of local rates and magnitudes of tectonic deformation particularly in tectonically active areas despite their successful use elsewhere studies of this type are much less common for the mexican pacific coast which parallels the cocos north america subduction plate boundary stratigraphic geochemical and microfossil data from sediments in laguna mitla the pacific coast of guerrero mexico document late holocene sea level changes induced by tectonic activity in the mexican subduction zone three major events are identified first the formation of the lagoon by c 4630 yr bp as indicated by a freshwater to brackish peat second a relative sea level rise or land subsidence as indicated by a shift from a freshwater marginal lagoon environment to a marine setting preceded by a marine inundation represented by a sand unit possibly a tsunami deposit by c 3400 yr bp and finally a return to lagoonal conditions indicating a drop in relative sea level or coastal uplift by c 2300 yr bp the laguna mitla stratigraphy indicates general coastal subsidence or relative sea level rise of c 1 mm yr we argue that these relative sea level land level changes have been induced by tectonic activity associated with the mexican megathrust a plausible explanation for the 3400 yr bp marine inundation is probably a tsunami produced by a large seismic event accompanied by coastal subsidence discrete fining upward fine to coarse sand units with an erosional basal contact medium to poor sorting and clay mud rip up clasts an increase in na and sr elemental concentrations indicative of a marine origin and the landward extent of the sands support a tsunamigenic source for these deposits however these apparent tsunami deposits require further study to determine their lateral extent and to assess whether they can be correlated from one site to another this study demonstrates the applicability of a multiproxy sedimentary approach in interpreting relative sea level land level changes and to derive data on related earthquake and tsunami events in tropical coastal lagoons

simultaneous reactivation of two subparallel inland normal faults during the mw 6 6 11 april 2011 iwaki earthquake triggered by the mw 9 0 tohoku oki japan earthquake the gigantic m w 9 0 11 march 2011 tohoku oki earthquake suddenly changed the overriding inland area to an extensional stress regime and triggered massive seismic swarms in the coastal region the largest earthquake of m w 6 6 struck southern fukushima on 11 april 2011 and ruptured two previously mapped faults the northwest nw trending yunodake fault and the north northwest nnw trending itozawa fault clear 15 km long and 14 km long surface ruptures appeared along both faults respectively exhibiting a predominantly normal sense of slip down to the west the maximum vertical offset on the yunodake fault is 0 8 m whereas that on the itozawa fault is 2 1 m the itozawa fault rupture is in part marked by uphill facing scarps which is discordant with the large scale topography but consistent with saddled ridges and ponded alluvium the yunodake fault which bounds a neogene half graben structure juxtaposing mesozoic metamorphic rocks against neogene sedimentary rocks shows left lateral deflections of streams seismological data and interviews of local residents revealed that the two subparallel faults ruptured simultaneously based on the location of its hypocenter we however interpret that the itozawa rupture was primary and then triggered normal faulting on the yunodake fault under the heightened coulomb stress caused by the preceding tohoku oki earthquake our paleoseismic trench across the itozawa fault exposed evidence for a penultimate earthquake that occurred sometime between 12 620 and 17 410 cal yr b p there is no evidence that the itozawa fault ruptured during or immediately after the a d 869 jogan earthquake which is believed to be the penultimate giant megathrust earthquake along the japan trench

10be ages of late pleistocene deglaciation and neoglaciation in the north central brooks range arctic alaska we present a chronology of late pleistocene deglaciation and neoglaciation for two valleys in the north central brooks range alaska using cosmogenic 10be exposure dating the two valleys show evidence of ice retreat from the northern range front before 16 15 ka and into individual cirques by 14 ka there is no evidence for a standstill or re advance during the lateglacial period indicating that a glacier advance during the younger dryas if any was less extensive than during the neoglaciation the maximum glacier expansion during the neoglacial is delimited by moraines in two cirques separated by about 200 km and dated to 4 6 0 5 and 2 7 0 2 cal ka bp both moraine ages agree with previously published lichen inferred ages and confirm that glaciers in the brooks range experienced multiple advances of similar magnitude throughout the late holocene the similar extent of glaciers during the middle holocene and the little ice age may imply that the effect of decreasing summer insolation was surpassed by increasing aridity to limit glacier growth as neoglaciation progressed copyright 2012 john wiley sons ltd

dynamics of fault interaction parallel strike slip faults we use a two dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures our results indicate that a strike slip earthquake is unlikely to jump a fault step wider than 5 km in correlation with field observations of moderate to great sized earthquakes we also find that dynamically propagating ruptures can jump both compressional and dilational fault steps although wider dilational fault steps can be jumped dilational steps tend to delay the rupture for a longer time than compressional steps do this delay leads to a slower apparent rupture velocity in the vicinity of dilational steps these dry cases assumed hydrostatic or greater pore pressures but did not include the effects of changing pore pressures in an additional study we simulated the dynamic effects of a fault rupture on undrained pore fluids to test sibson s 1985 1986 suggestion that wet dilational steps are a barrier to rupture propagation our numerical results validate sibson s hypothesis by demonstrating that the effect of the rupture on the undrained pore fluids is to inhibit the rupture from jumping dilational stepovers the basis of our result differs from sibson s hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments

source parameters of the great sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls large uplifts and tilts occurred on the sumatran outer arc islands between 0 5 and 3 3 s during great historical earthquakes in 1797 and 1833 as judged from relative sea level changes recorded by annually banded coral heads coral data for these two earthquakes are most complete along a 160 km length of the mentawai islands between 3 2 and 2 s uplift there was as great as 0 8 m in 1797 and 2 8 m in 1833 uplift in 1797 extended 370 km between 3 2 and 0 5 s the pattern and magnitude of uplift imply megathrust ruptures corresponding to moment magnitudes m w in the range 8 5 to 8 7 the region of uplift in 1833 ranges from 2 to at least 3 2 s and judging from historical reports of shaking and tsunamis perhaps as far as 5 s the patterns and magnitude of uplift and tilt in 1833 are similar to those experienced farther north between 0 5 and 3 n during the giant nias simeulue megathrust earthquake of 2005 the outer arc islands rose as much as 3 m and tilted toward the mainland elastic dislocation forward modeling of the coral data yields megathrust ruptures with moment magnitudes ranging from 8 6 to 8 9 sparse accounts at padang along the mainland west coast at latitude 1 s imply tsunami runups of at least 5 m in 1797 and 3 4 m in 1833 tsunamis simulated from the pattern of coral uplift are roughly consistent with these reports the tsunami modeling further indicates that the indian ocean tsunamis of both 1797 and 1833 unlike that of 2004 were directed mainly south of the indian subcontinent between about 0 7 and 2 1 s the lack of vintage 1797 and 1833 coral heads in the intertidal zone demonstrates that interseismic submergence has now nearly equals coseismic emergence that accompanied those earthquakes the interseismic strains accumulated along this reach of the megathrust have thus approached or exceeded the levels relieved in 1797 and 1833

an integrated global model of present day plate motions and plate boundary deformation summary in this paper we present a global model gsrm 1 of both horizontal velocities on the earth s surface and horizontal strain rates for almost all deforming plate boundary zones a model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field in our model we perform a least squares fit between model velocities and observed geodetic velocities as well as between model strain rates and observed geological strain rates model velocities and strain rates are interpolated over a spherical earth using bi cubic bessel splines we include 3000 geodetic velocities from 50 different mostly published studies geological strain rates are obtained for central asia only and they are inferred from quaternary fault slip rates for all areas where no geological information is included a priori constraints are placed on the style and direction but not magnitude of the model strain rate field these constraints are taken from a seismic strain rate field inferred from normalized focal mechanisms of shallow earthquakes we present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones generally the strain rate tensor field is consistent with geological and seismological data with the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs we find that in general there is a good agreement between the present day plate motions we obtain and longterm plate motions but a few significant differences exist the rotation rates for the indian arabian and nubian plates relative to eurasia are 30 13 and 50 per cent slower than the nuvel1a estimate respectively and the rotation rate for the nazca plate relative to south america is 17 per cent slower on the other hand caribbean north america motion is 76 per cent faster than the nuvel 1a estimate while crustal blocks in the india eurasia collision zone move significantly and self consistently with respect to bounding plates only a very small motion is predicted between the nubian and somalian plates by integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in for instance redefining a no net rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation

gps deformation in a region of high crustal seismicity n cascadia forearc we estimate the rate of crustal deformation in the central and northern cascadia forearc based on a combination of existing global positioning system gps velocity data along the cascadia subduction zone gps strain rates and velocities show that the northwestern washington southwestern british columbia region is currently shortening at 3 3 5 mm yr 31 in a n s direction in good agreement with inference from crustal earthquake statistics on the longterm the shortening rate is 5 6 mm yr 31 providing that the subduction related interseismic loading of the margin is purely elastic compared to the velocity of the oregon forearc with respect to north america v 7m m yr 31 this indicates that most of the forearc motion is accommodated in the puget georgia basin area corresponding to the main concentration of crustal seismicity the difference between the current and long term shortening rates may be taken up during subduction megathrust earthquakes thus these events could produce a sudden increase of n s compression in the puget sound region and could trigger major seattle fault type crustal earthquakes published by elsevier science b v

origin and provenance of basement metasedimentary rocks from the xolapa complex new constraints on the chortis southern mexico connection abstract the u pb la mc icpms geochronology of xolapa metasedimentary rocks from tierra colorada guerrero to puerto angel oaxaca in southern mexico reveals that their protoliths accumulated in two distinctive cycles of sedimentation one of early jurassic age and another of late cretaceous age these ages are younger than thought and demonstrate that xolapa metasedimentary rocks are not rocks from the acatlan or oaxacan complexes or their paleozoic sedimentary covers as claimed however detrital zircon ages indicate that xolapa sediments received contemporaneous detritus most likely from these assemblages suggesting a probably para autochthonous origin for the xolapa terrane xolapa rocks record two major tectonothermal events of 64 59 ma and 34 ma the first event produced the high grade metamorphism and widespread migmatization that characterize xolapa and the second event is likely related to extended heating produced by coeval arc plutonism pre jurassic assemblages of the chortis block of central america contain zircon populations that significantly coincide with those recorded in both the acatlan and oaxacan complexes and their paleozoic sedimentary covers as well as with those recorded in xolapa metasediments which suggests a spatial connection among these petrotectonic assemblages during much of the mesozoic it is proposed that xolapa was generated in a basin floored by permian rocks flanked on one side by southern mexico terranes and on the other side by the chortis block contraction of the basin tied to the approach and accretion of the guerrero terrane arc assemblages during late cretaceous time produced crustal thickening and high grade metamorphism and migmatization at mid crustal levels diachronic exhumation of xolapa began during early paleogene time very likely promoted by the detachment and migration of the chortis block the migration would additionally produce slicing of xolapa assemblages generating its elongated and juxtaposed structure and the margin truncation that characterizes southern mexico

frontal wedge deformation near the source region of the 2011 tohoku oki earthquake 1 we report an uplift of 5 m with a horizontal displacement of more than 60 m due to the 2011 tohoku oki earthquake the uplift was measured by an ocean bottom pressure gauge installed before the earthquake on a frontal wedge which formed an uplift system near the japan trench horizontal displacements of the frontal wedge were measured using local benchmark displacements obtained by acoustic ranging before and after the earthquake the average displacements at the frontal wedge were 58 m east and 74 m east southeast these results strongly suggest a huge coseismic slip beneath the frontal wedge on the plate boundary the estimated magnitude of the slip along the main fault was 80 m near the trench our results suggest that the horizontal and vertical deformations of the frontal wedge due to the slip generated the tremendous tsunami that struck the coastal area of northeastern japan

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

structural evolution and strike slip tectonics off north western sumatra abstract based on new multi channel seismic data swath bathymetry and sediment echosounder data we present a model for the interaction between strike slip faulting and forearc basin evolution off north western sumatra between 2 n and 7 n we examined seismic sequences and sea floor morphology of the simeulue and aceh forearc basins and the adjacent outer arc high we found that strike slip faulting has controlled the forearc basin evolution since the late miocene the mentawai fault zone extends up to the north of simeulue island and was most probably connected farther northwards to the sumatran fault zone until the end of the miocene since then this northern branch jumped westwards initiating the west andaman fault in the aceh area the connection to the mentawai fault zone is a left hand step over in this transpressional setting the tuba ridge developed we found a right lateral strike slip fault running from the conjunction of the west andaman fault and the tuba ridge in ssw direction crossing the outer arc high as a result extrusion formed a marginal basin north of simeulue island which is tilted eastwards by uplift along a thrust fault in the west the shift of strike slip movement in the aceh segment is accompanied by a relocation of the depocenter of the aceh basin to the northwest forming one major neogene unconformity the simeulue basin bears two major neogene unconformities documenting that differences in subsidence evolution along the northern sumatran margin are linked to both forearc evolution related to subduction processes and to deformation along major strike slip faults

testing the tectonic significance of some parameters of longitudinal river profiles the case of the ardenne belgium nw europe the longitudinal profiles of 24 rivers of 3rd to 5th order have been analyzed in order to recognize the regional tectonic differentiation pattern of the ardenne a paleozoic massif of nw europe data collected exclusively from 1 25 000 topographic maps were used to define a set of parameters related to the geometry of the profile and proxies for river bedload and drainage area validation of the proxy defined for the bedload discharge ratio demonstrates its potential use for inter river comparison based on its value at the river mouth it is however inadequate to estimate the ratio changes along a given stream course this proxy has been computed to estimate the portion accounted for by hydrodynamic factors in the river bed profiling thus outliers to the correlation function linking it to the river bed gradient have been considered as undergoing other influences principally of lithologic or tectonic origin most indicative of the computed profile parameters are those related to the concavity of the profile four more or less regional longitudinal profile types are recognized which are partly determined by differential tectonic movement the combined analysis of all the parameters confirms the current uplift of the ne ardenne and points to a subsidence of the namur area with respect to its sw and se surroundings whether periglacial or tectonic in origin the upper pleistocene lowering of a local base level has also been identified in the famenne another conclusion of this study is that in order to evidence tectonic influences and to avoid any misinterpretation in a region of complex geologic and geomorphic history and moderate tectonic activity it is essential to consider a number of parameters and to consider their significance within the framework of detailed geologic setting and morphogenetic history

seismic strain release in the mexican subduction thrust abstract in this paper the seismic strain release in the mexican subduction zone is discussed the paper is based on a revised catalog of large earthquakes in mexico new in this catalog are estimates of the seismic moment for many of the larger pre 1962 earthquakes of this century based on instrumental recordings from uppsala sweden a regression between moment and magnitude is established and used to estimate moments for the remaining events of this and the previous century the envelopes of the cumulative seismic moment curves can thus be developed consistent with the nineteenth century seismicity these indicate that the guerrero region is currently the most likely within the mexican subduction zone to experience one great event or several large events with a moment deficit approaching 20 10 27 dyn cm this is comparable with the moment released between 1899 and 1912 in the region in guerrero and also perhaps in michoacan the seismic strain release is consistent with plate tectonic expectations for a seismogenic zone 50 km in width in oaxaca either the seismogenic zone is much narrower and there is considerable aseismic slip or events to rupture the downdip extremes of the subduction zone have not occurred so far this century this last alternative arises because in guerrero the 1907 and 1957 earthquakes have relatively strong 40 s surface waves recorded at uppsala compared with all the other events such events which we infer are deeper present new problems for seismic hazard estimation in mexico since they are not represented in the available strong motion records

geologic evolution of the xolapa complex southern mexico evidence from u pb zircon geochronology the xolapa complex of southern mexico is composed of mid crustal arc related gneisses of poorly resolved ages intruded by undeformed cenozoic calc alkaline plutons twelve undeformed and deformed tonalitic granodioritic samples from three transects across the sierra madre del sur acapulco puerto escondido and puerto angel were chosen for u pb zircon analysis the measurements were performed on single crystals of zircons using a multiple collector laserablation inductively coupled plasma mass spectrometer mc la icp ms about 20 30 crystals were measured from each sample three gneisses and migmatites from the eastern transect puerto angel located 30 42 km from the coast yielded grenvilleaged zircons 970 1280 ma suggesting that the samples represent oaxacan basement not deformed xolapa complex the central transect puerto escondido yielded oligocene ages 25 32 ma on undeformed plutons as well as mid mesozoic and permian ages on gneisses most samples along the puerto escondido transect contain inherited ca 1 1 ga xenocrystals of zircons the western transect acapulco yielded late jurassic early cretaceous ages 160 136 ma on gneisses and paleocene 55 ma and oligocene 34 ma ages on undeformed plutons with no inherited grenville ages the older ages and xenocrystic zircons in arc related xolapa complex mirror the crustal ages found in neighboring terranes mixteca and oaxaca to the north of the xolapa complex suggesting an autochthonous origin of xolapa with respect to its neighboring northbounding terranes the new data and previously published ages for xolapa suggest that metamorphism and migmatization of the deformed arc rocks took place prior to the cenozoic eocene and oligocene plutons representing renewed arc related magmatism in the area are common throughout xolapa and probably represent the more deeply exposed continuation of the sierra madre occidental arc to the northwest the available u pb data argue against the previously proposed eastward migration of magmatism between acapulco and puerto angel during the oligocene