a new estimate for present day cocos caribbean plate motion implications for slip along the central american volcanic arc velocities from 153 continuously operatinggps sites on the caribbean north american and pacic plates arecombinedwith61newlyestimatedpacic cocosseafloor spreading rates and additional marine geophysical data to deriveanewestimate of present daycocos caribbean plate motion acomparisonofthepredictedcocos caribbean di rectiontoslipdirectionsofnumerousshallow thrustsubduc tion earthquakes from the middle america trench between costa rica and guatemala shows the slip directions to be deflected 10 clockwise from the plate convergence direc tion supporting thehypothesisthatfrequentdextralstrike slip earthquakes along the central american volcanic arc result from partitioning of oblique cocos caribbean plate convergence linear velocity analysis for forearc locations in nicaragua and guatemala predicts 14 2m m yr 1 of northwestward trench parallel slip of the forearc relative to thecaribbean plate possibly decreasing inmagnitude inel salvador and guatemala where extension east of the vol canic arc complicates the tectonic setting

kinematics of the nicaraguan forearc from gps geodesy 1 campaign gps data from a network in the nicaraguan forearc show a strong component of arc parallel motion indicating northwest translation of a nearly rigid forearc sliver our measured mean velocity for forearc sites of 15 1 mm yr 1 agrees well with the arc parallel sliver motion predicted previously by demets 2001 derived from closure constraints on oblique convergence between the cocos and caribbean plates the lack of a northeasterly oriented arc normal component of motion in forearc velocities indicates that there are complexities involved beyond a simple interpretation of sliver motion being driven by oblique convergence the forearc is reasonably well fit by rigid rotation about an euler pole with a rms misfit of residual velocities of 4 9 2 6 mm yr 1 current motion of the forearc sliver relative to the stable caribbean plate yields predominantly boundary parallel nw motion with boundary normal extension in the northwestern region averaging 5m m yr 1 citation turner h l iii p lafemina a saballos g s mattioli p e jansma and t dixon 2007 kinematics of the nicaraguan forearc from gps geodesy geophys res lett 34 l02302 doi 10 1029 2006gl027586

coastwise transport reconsidered lateral displacements in oblique subduction zones and tectonic consequences coastwise transport margin parallel displacement of arc and forearc slivers is a common feature of zones of oblique subduction today but at rates that rarely exceed a few tens of kilometers per million year paleomagnetic studies strongly indicate that coastwise transport in the north american cordillera went on during late cretaceous early tertiary time at rates several times higher than this other studies have shown that obliquity is commonly partitioned between oblique subduction and coastwise transport of a detached sliver of over riding crust the analysis developed here indicates that in the absence of a buttress a physical impediment to sliver motion at its leading edge sliver motion should obey the relationship tan p r tan 1 1 r2 sin2 12 where is the angle of obliquity is the angle of subduction and r rfrs is the ratio of resistance to slip on strike slip faults cutting the over riding plate to resistance to slip on the subduction zone in this equation p is a partitioning factor 0 p 1 where p 1 implies oblique subduction and no sliver motion and p 0 is the case in which the sliver moves at velocity v sin equivalent to the transverse component of relative motion rates of coastwise transport calculated from this relationship are systematically too high presumably indicating that most modern subduction zones are buttressed key factors in the buttress effect are 1 the greater strength of oceanic vs continental lithosphere and 2 the energy required to thicken continental crust large scale tectonic transport in the north american cordillera probably was the result of a combination of favorable factors long continued highly oblique convergence and a plate margin geometry that avoided the buttress problem

past forearc deformation in nicaragua and coupling at the megathrust interface evidence for subduction retreat 1 deformation at the boundary between the cocos and caribbean plates in nicaragua is currently focused in the volcanic arc in the form of arc parallel shearing in the middle miocene however there was widespread subsidence with local uplift in the outer forearc and inner forearc broad uplift to understand this complex deformation we use numerical modeling to investigate the effect of inhomogeneous friction at the megathrust fault on strain localization in the upper plate a good fit is obtained when 1 the interface between the slab and upper plate is not situated at the current wadati benioff zone but rather is moved 50 km landward to where subduction may have been active in the early tertiary and 2 there is high stress accumulation between 15 and 25 km depths and low stress accumulation updip and downdip of the thrust interface the results are consistent with a high velocity block being the remnant of an oceanic plateau that subducted in the eocene and was later incorporated into the upper plate we suggest that this geometry persisted into the middle miocene before the slab broke off and the thrust interface jumped seaward

active seismic deformation in the grabens of northern central america and its relationship to the relative motion of the north america caribbean plate boundary abstract the active seismic deformation along the grabens of northern central america is calculated using a method which relates relative plate velocity with the seismic strain rate tensor the latter is in turn obtained from seismic moment tensors we calculate the typical or average moment tensor from available centroid moment tensors and fault plane solutions this average tensor is then used to obtain the sum of moment tensors of historical and modern earthquakes reported in the literature both historical and modern earthquakes yield an average extension rate of 8 mm yr along the grabens of northern central america while the relative motion between the north america and caribbean plates expressed along the motagua polochic fault system is 20 mm yr this result suggests that part of the seismic deformation related to the plate boundary is taken up as extension along the grabens which play the role of fault termination structures for the motagua polochic system

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

seismicity at fuego pacaya izalco and san cristobal volcanoes central america 1973 1974 seismic data collected at four volcanoes in central america during 1973 and 1974 indicate three sources of seismicity regional earthquakes with hypocentral distances greater than 80 km earthquakes within 40 km of each volcano and seismic activity originating at the volcanoes due to eruptive processes regional earthquakes generated by the underthrusting and subduction of the cocos plate beneath the caribbean plate are the most prominent seismic feature in central america earthquakes in the vicinity of the volcanoes occur on faults that appear to be related to volcano formation faulting near fuego and pacaya volcanoes in guatemala is more complex due to motion on a major e w striking transform plate boundary 40 km north of the volcanoes volcanic activity produces different kinds of seismic signatures shallow tectonic or a type events originate on nearby faults and occur both singly and in swarms there are typically from 0 to 6 a type events per day withb value of about 1 3 at very shallow depths beneath pacaya izalco and san cristobal large numbers of low frequency or b type events are recorded with predominant frequencies between 2 5 and 4 5 hz and withb values of 1 7 to 2 9 the relative number of b type events appears to be related to the eruptive states of the volcanoes the more active volcanoes have higher levels of seismicity at fuego volcano however low frequency events have unusually long codas and appear to be similar to tremor high amplitude volcanic tremor is recorded at fuego pacaya and san cristobal during eruptive periods large explosion earthquakes at fuego are well recorded at five stations and yield information on near surface seismic wave velocities 3 0 0 2 km sec

transform zone migration implications of bookshelf faulting at oceanic and icelandic propagating ridges at a propagating ridge a migrating transform zone mtz offsets the propagating and retreating spreading axes we examine several kinematic models of mtz deformation to determine which models can generate observed seafloor fabric orientations vertical tectonics and faulting patterns within the 95 5 w galapagos propagating ridge pr system models that involve transform parallel simple shear within the mtz can fit observed seafloor fabric rotation patterns within the 95 5 w mtz however these models predict pervasive transform parallel strike slip faults within the mtz that are not observed and do not predict the dramatic shear concurrent deepening and post shear uplift that is seen in this propagating ridge system since we only measure the amount of structural rotation of the abyssal hill fabric within the mtz and do not measure extensional or compressional deformation perpendicular or parallel to abyssal hills this structural information can only determine the rotational component of the finite strain tensor thus this simple shear family of kinematic deformation models is actually only a small subset of mtz deformation patterns that can produce the same finite rotation patterns but with different amounts of shortening and or extension parallel or normal to abyssal hills there is strong seismic and surface fault break evidence in iceland that the south iceland seismic zone mtz propagates along an array of en echelon transform perpendicular strike slip faults a likely reason for this is that if deformation were accommodated through transform parallel strike slip faults then migration of the transform zone would require the creation of new transform parallel strike slip faults for each increment of transform migration instead if transform migration can be accommodated through slip on preexisting faults then transform migration would only require the incremental growth of and slip along preexisting faults for each increment of transform zone migration we examine the kinematic consequences of finite mtz deformation within this generalized bookshelf faulting deformation scenario we find that our kinematic realization of generalized bookshelf faulting predicts structural rotation and vertical tectonic patterns that are consistent with observations at the 95 5 w galapagos pr system vertical tectonics are an almost inevitable by product of bookshelf shearing on dipping faults because of the geometric strain incompatibility associated with this mode of deformation thus local extension within the abyssal hills of de steiguer deep may occur because these abyssal hills do not join a spreading axis where the geometric strain incompatibilities associated with bookshelf slip can be accommodated we conclude that generalized bookshelf faulting is a promising candidate for the dominant deformation process within a mtz since it can explain not only the structural rotations observed at the 95 5 w propagating ridge but can also relate these rotations to the uplift history recorded there

fault kinematics in northern central america and coupling along the subduction interface of the cocos plate from gps data in chiapas mexico guatemala and el salvador new gps measurements in chiapas mexico guatemala and el salvador are used to constrain the fault kinematics in the north america na caribbean ca and cocos co plates triple junction area the regional gps velocity field is first analysed in terms of strain partitioning across the major volcano tectonic structures using elastic half space modelling then inverted through a block model we show the dominant role of the motagua fault with respect to the polochic fault in the accommodation of the present day deformation associated with the na and ca relative motion the na ca motion decreases from 18 22 mm yr 1 in eastern guatemala to 14 20 mm yr 1 in central guatemala assuming a uniform locking depth of 14 28 km down to a few millimetres per year in western guatemala as a consequence the western tip of the ca plate deforms internally with 9 mm yr 1 of east west extension 5 mm yr 1 across the guatemala city graben alone up to 15 mm yr 1 of dextral motion can be accommodated across the volcanic arc in el salvador and southeastern guatemala the arc seems to mark the northern boundary of an independent forearc sliver ar pinned to the na plate the inversion of the velocity field shows that a four block na ca co and ar model that combines relative block rotations with elastic deformation at the block boundaries can account for most of the gps observations and constrain the overall kinematics of the active structures this regional modelling also evidences lateral variations of coupling at the co subduction interface with a fairly high coupling 0 6 offshore chiapas and low coupling 0 25 offshore guatemala and el salvador

interseismic plate coupling and strain partitioning in the northeastern caribbean summary the northeastern caribbean provides a natural laboratory to investigate strain partitioning its causes and its consequences on the stress regime and tectonic evolution of a subduction plate boundary here we use gps and earthquake slip vector data to produce a present day kinematic model that accounts for secular block rotation and elastic strain accumulation with variable interplate coupling on active faults we confirm that the oblique convergence between caribbean and north america in hispaniola is partitioned between plate boundary parallel motion on the septentrional and enriquillo faults in the overriding plate and plateboundary normal motion at the plate interface on the northern hispaniola fault to the east the caribbean north america plate motion is accommodated by oblique slip on the faults bounding the puerto rico block to the north puerto rico subduction and to the south muertos thrust with no evidence for partitioning the spatial correlation between interplate coupling strain partitioning and the subduction of buoyant oceanic asperities suggests that the latter enhance the transfer of interplate shear stresses to the overriding plate facilitating strike slip faulting in the overriding plate the model slip rate deficit together with the dates of large historical earthquakes indicates the potential for a large m w7 5 or greater earthquake on the septentrional fault in the dominican republic similarly the enriquillo fault in haiti is currently capable of a m w7 2 earthquake if the entire elastic strain accumulated since the last major earthquake was released in a single event today the model results show that the puerto rico lesser antilles subduction thrust is only partially coupled meaning that the plate interface is accumulating elastic strain at rates slower than the total plate motion this does not preclude the existence of isolated locked patches accumulating elastic strain to be released in future earthquakes but whose location and geometry are not resolvable with the present data distribution slip deficit on faults from this study are used in a companion paper to calculate interseismic stress loading and together with stress changes due to historical earthquakes derive the recent stress evolution in the ne caribbean

large scale anisotropy within the crust in the basin and range province seismic reflection records of the 1986 nevada passcal seismic experiment in the roughly east west direction dip line show a highly reflective crust with a broad diffuse moho which is characterized at normal incidence by strong subparallel laterally discontinuous multicyclic events lacking observable diffractions recordings in the roughly north south direction strike line show a transparent crust with short scattered events and a strong continuous moho elastic finite difference modeling indicates that 1 effects due to the topography of the reflecting interfaces and or the existence of heterogeneities increase the travel times thus lowering the velocity estimates and 2 the seismic response of a layered structure can be disrupted from a strong laterally continuous event to a series of short multicyclic events by increasing the dimensions of the overlying heterogeneities finite difference modeling demonstrates that large scale anisotropy achieved by roughly east west alignment of strongly elongated lenses boudins located at mid and lower crustal depths can explain the observed differences in the seismic response along the perpendicular lines

fore arc motion and cocos ridge collision in central america 1 we present the first regional surface velocity field for central america showing crustal response to interaction of the cocos and caribbean plates elastic half space models for interseismic strain accumulation on the dipping subduction plate boundary fit the gps data well and show strain accumulation offshore and beneath the nicoya and osa peninsulas in costa rica but not in nicaragua since large subduction zone earthquakes occur in nicaragua we suggest that interseismic locking in nicaragua and some other parts of central america occurs but is mainly shallow 20 km depth too far offshore to be detected by our on land gps measurements our data also show significant trench parallel motion for most of the region generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates however trench parallel motion is also observed in central costa rica where plate convergence is normal to the trench and in the nicaraguan fore arc where trench parallel motion is fast up to 9 mm a 1 but mechanical coupling is low a finite element model of collision as opposed to subduction involving the aseismic cocos ridge also fits the gps surface velocity field most significantly reproducing the pattern of trench parallel motion we infer that buoyant thickened cns 2 cocos ridge crust resists normal subduction and instead acts as an indenter to the caribbean plate driving crustal shortening in southern costa rica and contributing to trench parallel fore arc motion in costa rica and perhaps nicaragua as a type of tectonic escape

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

destructive upper crustal earthquakes of central america since 1900 we have compiled a catalog of 51 destructive upper crustal earthquakes in central america since 1900 an event is included if it caused casualties or heavy damage of modified mercalli mm intensity vii all events larger than magnitude ms 5 7 are found to be destructive we believe the catalog is complete for events of ms 6 along the volcanic front the catalog includes estimates of epicentral coordinates depth magnitude and casualties for all events and presents mm intensity vii contours for most events data in this catalog place severe constraints on the spatial temporal and magnitude distribution of destructive upper crustal earthquakes in central america the catalog contains 30 events of ms 6 depths of well constrained mainshock hypocenters range from 5 to 15 km the most striking feature of the catalog is the spatial alignment along the volcanic front of 23 of the 30 events volcanic front earthquake source mechanisms for such events show east west tension and are compatible with a model of the volcanic front as a zone of right lateral strike slip faulting volcanic front earthquake magnitudes do not exceed ms 6 5 except in southeastern guatemala where they reach ms 6 9 maximum earthquake magnitudes are apparently related to the distance between adjacent holocene volcanic centers destructive events occur at offsets in the volcanic chain as isolated mainshock aftershock sequences however within linear segments destructive earthquakes have a strong tendency to cluster and are often preceded by significant foreshock activity although the volcanic front is the locus of both destructive earthquakes and volcanism we find no temporal correlation between the two of the seven events of ms 6 that did not originate near the volcanic front six originated near the transcurrent caribbean north american plate boundary including the ms 7 5 guatemala earthquake of 1976 along the volcanic front destructive upper crustal earthquakes have occurred at a rate of about one event every 2 1 2 years during this century much more frequently than along either the caribbean north american plate boundary or the subduction zone in central america since 1900 about 17 000 people have died from volcanic front earthquakes but only about 2000 people are known to have died from subduction zone earthquakes although subduction zone earthquakes can have larger magnitudes and produce more widespread damage destructive volcanic front earthquakes pose the greater hazard because they are more frequent and originate much closer to the bulk of central america s population which is concentrated along the volcanic front

walking through volcanic mud the 2 100 year old acahualinca footprints nicaragua we present the stratigraphy lithology volcanology and age of the acahualinca section in managua including a famous footprint layer exposed in two museum pits the ca 4 m high walls of the main northern pit pit i expose excellent cross sections of late holocene volcaniclastic deposits in northern managua we have subdivided the section into six lithostratigraphic units some of which we correlate to late holocene eruptions unit i 1 2 m thick chiefly of hydroclastic origin begins with the footprint layer the bulk is dominated by mostly massive basaltic andesitic tephra layers interpreted to represent separate pulses of a basically phreatomagmatic eruptive episode we correlate these deposits based on compositional and stratigraphic evidence to the masaya triple layer erupted at masaya volcano ca 2 120 120 a b p the eruption occurred during the dry season a major erosional channel unconformity up to 1 m deep in the western half of pit i separates units ii and i unit ii begins with basal dacitic pumice lapilli up to 10 cm thick overlain by a massive to bedded fine grained dacitic tuff including a layer of accretionary lapilli and pockets of well rounded pumice lapilli angular nonvesicular glass shards are interpreted to represent hydroclastic fragmentation the dacitic tephra is correlated unequivocally with the ca 1 9 ka plinian dacitic chiltepe eruption unit iii a lithified basaltic andesitic deposit up to 50 cm thick and extremely rich in branch molds and excellent leaf impressions is correlated with the masaya tuff erupted ca 1 8 ka ago unit iv a reworked massive basaltic andesitic deposit rich in brown tuff clasts and well bedded and cross bedded in the northwestern corner of pit i cuts erosionally down as far as unit i a poorly defined pale brown mass flow deposit up to 1 m thick unit v is overlain by 1 1 5 m of dominantly reworked chiefly basaltic tephra topped by soil unit vi a major erosional channel carved chiefly between deposition of units ii and i may have existed as a shallow drainage channel even prior to deposition of the footprint layer the swath of the footprints is oriented nnw roughly parallel to and just east of the axis of the channel the interpretation of the footprint layer as the initial product of a powerful eruption at masaya volcano followed without erosional breaks by additional layers of the same eruptive phase is strong evidence that the group of 15 or 16 people tried to escape from an eruption

neotectonic development of western nicaragua this study presents a reconstruction of the neotectonic evolution of western nicaragua by means of structural analysis of outcrop scale joints faults and bedding planes integrated with macroscale structures the results indicate three clearly separable deformation phases which are closely related to specific volcanic events and account for the major tectonic features of nicaragua the first deformation phase was a late miocene to early pliocene shortening that produced the pacific plain anticlines and controlled the contemporaneous extrusion of the el coyol group volcanic rocks the second phase active between the pliocene and the pleistocene resulted in extensional structures such as the 500 km long and 50 km wide nw se trending nicaragua depression this phase was accompanied by a gradual trenchward migration of the volcanic activity responsible for the formation of the las sierras group the third deformation phase is still active and is responsible for the present shallow focus seismic activity associated with n s trending volcanic vents and the n s managua pull apart graben the neotectonic development of western nicaragua as interpreted here is closely related to the contemporaneous evolution of the neighboring areas of guatemala south of the jocotan fault honduras el salvador and costa rica north of the valle central the valle central of costa rica represents the southern boundary of this structurally homogeneous domain and may indicate the southern boundary of the neogene chortis block changes in deformation style and the trenchward migration of volcanism which occurred during the pliocene may indicate an increase in the subduction angle of the cocos plate and may also be correlated to pliocene shortening of jamaica and hispaniola

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

neotectonics of the sumatran fault indonesia the 1900 km long trench parallel sumatran fault accommodates a significant amount of the right lateral component of oblique convergence between the eurasian and indian australian plates from 10 n to 7 s our detailed map of the fault compiled from topographic maps and stereographic aerial photographs shows that unlike many other great strike slip faults the sumatran fault is highly segmented cross strike width of step overs between the 19 major subaerial segments is commonly many kilometers the influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry geomorphic offsets along the fault range as high as 20 km and may represent the total offset across the fault if this is so other structures must have accommodated much of the dextral component of oblique convergence during the past few million years our analysis of stretching of the forearc region near the southern tip of sumatra constrains the combined dextral slip on the sumatran and mentawai faults to be no more than 100 km in the past few million years the shape and location of the sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere nonetheless active volcanic centers of the sumatran volcanic arc have not influenced noticeably the geometry of the active sumatran fault on the basis of its geologic history and pattern of deformation we divide the sumatran plate margin into northern central and southern domains we support previous proposals that the geometry and character of the subducting investigator fracture zone are affecting the shape and evolution of the sumatran fault system within the central domain the southern domain is the most regular the sumatran fault there comprises six right stepping segments this pattern indicates that the overall trend of the fault deviates 4 clockwise from the slip vector between the two blocks it separates the regularity of this section and its association with the portion of the subduction zone that generated the giant m w 9 earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike slip faulting and unusually large subduction earthquakes

tectonic interpretation of the morphology of honduras the morphotectonics of honduras have been studied using existing geologic and topographic maps and imaging done from satellites and the following interpretations have been made during the late oligocene to early miocene a wide strike slip boundary existed in western honduras and eastern guatemala and produced east west trending basins containing the coarse clastic rocks of the subinal formation marine geophysical studies of the continental shelf of honduras show that during the rest of the miocene and the early pliocene the boundary was inactive but motion resumed in the late pliocene to pleistocene and produced horsts on the north coast of honduras in northwestern honduras the east northeast trending alignment of the jocotan copan and charnelecon river valleys may result from crustal flexuring rather than from faulting as previously proposed because the postulated fault the jocotan chamelecon fault cannot be reconciled with available geologic maps the northwest and northeast flowing rivers of central and eastern honduras define domains of strike slip faulting that have probably rotated 30 from an original set of conjugate fractures widespread deposition of conglomerates suggests that strike slip movements began in the mid cretaceous and reoccurred at intervals culminating in the development of pull apart basins in western honduras during the late pliocene at the present time no further strike slip movement is possible as the faults defining each domain lie parallel and perpendicular to the middle america trench new conjugate fractures however cut honduras along north south and east northeast trends and have also separated into domains in the domain of north south fractures easterly directed ductile extension of the lithosphere beneath western honduras and guatemala is opening up rifts but the domain of east northeast fractures is characterized by strike slip motion that has produced left lateral pull aparts in central honduras early workers identified no rift valley extending from the gulf of fonseca to the caribbean and there is no fresh evidence upon which to base the existence of one today the notion of the honduras depression which has persisted over the last half century stems from misinterpretation of the maps and writings of karl sapper

gps measurement of relative motion of the cocos and caribbean plates and strain accumulation across the middle america trench global positioning system gps measurements in 1988 and 1991 on cocos island cocos plate san andres island caribbean plate and liberia caribbean plate mainland costa rica provide an estimate of relative motion between the cocos and caribbean plates the data for cocos and san andres islands both located more than 400 km from the middle america trench define a velocity that is equivalent within two standard errors 7 mm yr rate 5 degrees azimuth to the nuvel 1 plate motion model the data for liberia 120 km from the trench define a velocity that is similar in azimuth but substantially different in rate from nuvel 1 the discrepancy can be explained with a simple model of elastic strain accumulation with a subduction zone that is locked to a relatively shallow 20 z 5 km depth

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

intra arc crustal seismicity seismotectonic implications for the southern andes volcanic zone chile we examine the intra arc crustal seismicity of the southern andes volcanic zone svz our aim is to resolve inter seismic deformation in an active magmatic arc dominated by both margin parallel liquine ofqui fault system lofs and andean transverse faults atf crustal seismicity provides information about the schizosphere tectonic state delineating the geometry and kinematics of high strain domains driven by oblique subduction here we present local seismicity based on 16 months data collected from 34 seismometers monitoring a 200 km long section of the southern volcanic zone including the lonquimay and villarrica volcanoes we located 356 crustal events with magnitudes between mw 0 6 and mw 3 6 local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain suggesting a convex shape of the crustal seismogenic layer bottom focal mechanisms indicate strike slip faulting consistent with ene wsw shortening in line with the long term deformation history revealed by structural geology studies however we find regional to local scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity within three distinctive latitudinal domains in the northernmost domain seismicity is consistent with splay faulting at the northern termination of the lofs in the central domain seismicity distributes along ene and wnw striking discrete faults spatially associated with hitherto seismic atf the southernmost domain in turn is characterized by activity focused along a n15 e striking master branch of the lofs these observations indicate a complex strain compartmentalization pattern within the intra arc crust where variable strike slip faulting dominates over dip slip movements

a new stratigraphy for the tertiary volcanic rocks of the nicaraguan highland three major volcanic events have occurred in nicaragua since middle tertiary time first the oligocene was dominated by rhyolite shield volcanism producing the highland ignimbrite second creation of northeast and later northwest striking dilatational fracture zones allowed the extrusion of basaltic to andesitic magmas along the pacific coast constructing the northwest oriented coyol volcanic arc this miocene arc can be followed all along the pacific coast of the chortis block third a pliocene pleistocene southwestward shift of arc volcanism created the modern volcanic arc this volcanic history is supported by a new stratigraphy based on volcanic centers and related facies the old cyclic layer cake stratigraphy of the coyol group has been abandoned

fumarolic gases at mombacho volcano nicaragua presence of magmatic gas species and implications for volcanic surveillance mombacho is a deeply dissected volcano belonging to the quaternary volcanic chain of nicaragua the southern historic collapse crater el crater currently hosts a fumarolic field with a maximum temperature of 121 c chemical and isotopic data from five gas sampling field campaigns carried out in 2002 2003 and 2005 highlight the presence of high temperature gas components e g so2 hcl and hf which indicate a significant contribution of juvenile magmatic fluids to the hydrothermal system feeding the gas discharges this is strongly supported by the mantle derived helium and carbon isotopic signatures although the latter is partly masked by either a sedimentary subduction related or a shallow carbonate component the observed chemical and isotopic composition of the mombacho fluids seems to indicate that this volcanic system although it has not experienced eruptive events during the last centuries can be considered active and possibly dangerous in agreement with the geophysical data recorded in the region systematic geochemical monitoring of the fumarolic gas discharges coupled with a seismic and ground deformation network is highly recommended in order to monitor a possible new eruptive phase

bookshelf faulting in nicaragua oblique subduction at a high rate of convergence along much of the middle america trench results in northwest directed trench parallel block motion accommodation of this motion along northwest striking dextral strike slip faults has been postulated however in nicaragua such faults are not well developed we suggest instead that this motion is accommodated by bookshelf faulting that includes northeast striking left lateral faults we present earthquake epicenter and focal mechanism data and mapped fracture and fault data consistent with this model trenchward migration of the volcanic arc since the miocene and reactivation of northeast striking miocene structures may have led to the development of this arc and trench normal fault system

central costa rica deformed belt kinematics of diffuse faulting across the western panama block fault kinematics seismicity and geodetic data across central costa rica reveal a diffuse fault zone here named the central costa rica deformed belt ccrdb the ccrdb defines the western margin of the panama block and links the north panama deformed belt npdb along the caribbean coast with the middle america trench mat along the pacific coast the junction of the ccrdb and the mat coincides with an abrupt transition from smooth to rough crust on the subducting cocos plate rough smooth boundary shallow subduction of rough thickened oceanic crust associated with the cocos ridge shifts active shortening into the volcanic arc along faults of the ccrdb variable fault kinematics along this zone may reflect three combined deformation mechanisms horizontal shortening and shear from oceanic ridge indentation basal traction from shallow subduction and localized block uplift from subducting seamount roughness within the forearc domain 1 mesoscale faults express transtension where steep ne striking regional scale faults intersect the pacific coast across the volcanic arc domain 2 mesoscale faults exhibit mostly sinistral and dextral slip on ne and nw striking conjugate faults respectively approaching the npdb in the back arc domain 3 transcurrent faulting is modified by transpression and crustal thickening fault kinematics are consistent with earthquake focal mechanisms and global positioning system gps measurements radiometric age constraints confirm that faulting postdates the late neogene onset of shallow subduction the ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active ccrdb within the forearc the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts