seismogenic zone structure of the southern middle america trench costa rica 1 the shallow seismogenic portion of subduction zones generates damaging large and great earthquakes this study provides structural constraints on the seismogenic zone of the middle america trench offshore central costa rica and insights into the physical and mechanical characteristics controlling seismogenesis we have located 300 events that occurred following the mw 6 9 20 august 1999 quepos costa rica underthrusting earthquake using a three dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers we use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region these events define a plane dipping at 19 that marks the interface between the cocos plate and the panama block the majority of aftershocks occur below 10 km and above 30 km depth below sea level corresponding to 30 35 km and 95 km from the trench axis respectively relative event relocation produces a seismicity pattern similar to that obtained using absolute locations increasing confidence in the geometry of the seismogenic zone the aftershock locations spatially correlate with the downdip extension of the oceanic quepos plateau and reflect the structure of the main shock rupture asperity this strengthens an earlier argument that the 1999 quepos earthquake ruptured specific bathymetric highs on the downgoing plate we believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level

effect of subducting sea floor roughness on fore arc kinematics pacific coast costa rica fault kinematics and uplift in the costa rican fore arc of the middle america convergent margin are controlled to a large extent by roughness on the subducting cocos plate along the northwest flank of the incoming cocos ridge seafloor is characterized by short wavelength roughness related to northeast trending seamount chains onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore arc thrust belt and separate blocks with contrasting uplift rates trunk segments of pacific slope fluvial systems typically follow these margin perpendicular faults regionally developed marine and fluvial terraces are correlated between drainages and across faults along the costa rican pacific coast terrace separations across block bounding faults reveal a pattern of fore arc uplift that coincides roughly with the distribution of incoming seamounts magnitude and distribution of quaternary uplift along the costa rican pacific coast suggests that despite a thin incoming sediment pile the inner fore arc shows an accumulation of mass a characteristic that may be due to underplating of seamounts beneath the fore arc high

recharge sensitivity to local and regional precipitation in semiarid midlatitude regions 1 the influence of regional local and total precipitation on groundwater recharge in a semiarid region is investigated through modeling daily precipitation data at two meteorological stations within okanagan basin british columbia canada are classified into regional 60 of annual and local 40 of annual precipitation the classified daily precipitation data are used as inputs to a one dimensional unsaturated zone recharge model help 3 80 d the recharge response is calculated at the base of the 1 2 m soil columns for 86 soil profiles typical of the region results suggest that local precipitation events contribute insignificant amounts of recharge compared to regional events precipitation data from global climate models that cannot simulate local events accurately may therefore still provide reasonable estimates of groundwater recharge

kinematics associated with late cenozoic deformation in central costa rica western boundary of the panama microplate we present kinematic data for late cenozoic deformation in central costa rica that marks the western margin of the panama microplate i e the caribbean panama boundary this boundary extends from the north panama deformed belt west through the valle central in costa rica and then southwest along the east nicoya fracture zone to intersect the middle america trench terrace correlation and basin asymmetry indicate a major change in tectonic evolution across the boundary where three regional northeast striking faults intersect the pacific coast mesoscopic fault populations are consistent with transtension across these north east striking faults and with transpression within the east trending valle central this late tertiary and quaternary transcurrent deformation links the north panama deformed belt to the east with the middle america trench to the west earthquake focal mechanisms are consistent with mesoscopic fault data suggesting that fault populations characterize the present day stress field this deformation marks the western extent of the panama microplate a fragment of volcanic arc that separated from the caribbean plate in the late tertiary or early quaternary and is currently advancing northward due to collisions with south america to the east and the indenting cocos ridge on the cocos plate to the south

efficient calculation of complete differential seismograms for laterally homogeneous earth models summary differential seismograms the sensitivity of the seismogram to perturbations of parameters of the earth model are essential for waveform modelling with iterative non linear optimization techniques the growing number of broad band highdynamic range fixed and portable stations provide a rich data set of regional distance events with accurate locations and source mechanisms that are ideal for modelling lithospheric structure an efficient technique for calculating differential seismograms for complete synthetic seismograms in laterally homogeneous earth models will significantly decrease the computational burden of seismic waveform inverse modelling the technique we present here is equivalent to computing three synthetic seismograms in multilayered models contrasted with n 1 seismograms for a typical brute force approach which reduces the work to 3 n 1 or about a 90 per cent saving for 30 layers the lateral homogeneity requirement will yield path averaged structures which may form the basis for later tomographic studies to resolve lateral variations

the mw 6 4 damas costa rica earthquake of 20 november 2004 aftershocks and slip distribution the earthquake of 20 november 2004 was located north of damas island in the pacific coast of costa rica within the costa rica deformed belt the earthquake was located at 24 km depth and reported with a magnitude m w of 6 4 and a strike slip mechanism with a large normal dip slip motion this mechanism agrees with mapped faults in the area that suggest transtensional deformation on the forearc and the entire western boundary of the panama microplate aftershock locations do not delineate a preferable plane to distinguish between the two nodal planes and are distributed between 15 and 25 km depth the slip distribution during the mainshock modeled after teleseismic and local data pictured a circular rupture 8 km in radius and 0 25 m of average displacement the fault plane cannot be distinguished from the two nodal planes from the slip distribution because of the lack of directivity and resolution for this magnitude earthquake weak evidence from empirical green s function analysis suggests that the dextral northwest oriented fault could be the causative fault depth to the top of the slab hypocenter location of the mainshock its slip distribution depth distribution of the aftershocks and quaternary fault activity at the surface suggest that deformation takes place throughout the whole thickness of the crust this extended deformation might be caused by seamount subduction and strong basal friction on the upper plate due to subduction of a thick young and buoyant oceanic plate rough seafloor and underplating of large seamounts

provenance of paleocene eocene red beds from ne iraq constraints from framework petrography the red bed deposits in northern iraq are situated in an active foreland basin adjacent to the zagros orogenic belt bound to the north by the iranian plate thrust over the edge of the arabian plate the red bed successions are composed of alternating red and brown silty mudstones purplish red calcareous siltstone fine to coarse grained pebbly sandstone and conglomerate the red beds in the current study can be divided into four parts showing a trend of upward coarsening with fine grained deposits at the top a detailed petrographic study was carried out on the sandstone units the clastic rocks consist mainly of calcite cemented litharenite with rock fragments volcanic metamorphic and sedimentary quartz and minor feldspar the petrographic components reflect the tectonic system in the source area laterally ranging from a mixed orogenic and magmatic arc in mawat chwarta area to recycled orogenic material rich in sedimentary rock fragments in the qandel area the cretaceous palaeogene foreland basin of northern iraq formed to the southwest of the zagros suture zone and the sanandaj sirjan zone of western iran during palaeogene time deposition of the red beds was caused by renewed shortening in the thrust sheets overlying the arabian margin with uplift of radiolarites qulqula formation resulting in an influx of radiolarian debris in addition to continuing ophiolitic detritus mixed sources including metamorphic volcanic and sedimentary terranes were present during deposition of the upper part of the red beds

collision versus sliver transport in the hanging wall at the middle america subduction zone constraints from background seismicity in central costa rica earthquake focal mechanism solutions for 135 small magnitude events are inverted for best fitting partial strain rate tensors that characterize contemporary strain in five areas that span the western margin of the panama microplate in central costa rica the results indicate the predominance of subhorizontal maximum stretching subparallel to the middle america trench mat and provide constraints on the role of cocos ridge collision at the mat the trajectory of maximum stretching changes 25 45 over several tens of kilometers from the central costa rica deformed belt ccrdb where it is nearly e w to the area inboard of the cocos ridge where it is nw se this change suggests that background seismogenic deformation reflects the transition from the trailing edge of a fore arc sliver to an area of the upper plate affected by ridge collision this diffuse deformation may be localized in part on conjugate strike slip faults of the ccrdb

tomographic evidence for a subducted seamount beneath the gulf of nicoya costa rica the cause of the 1990 mw 7 0 gulf of nicoya earthquake 1 tomographic images constrain the existence of a subducted seamount beneath the gulf of nicoya costa rica the subducted seamount is found at a depth of 30 km within the rupture area of the march 25 1990 mw 7 0 gulf of nicoya earthquake the gulf of nicoya earthquake was a typical thrust type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting cocos plate and the overriding plate precise relocation of the mainshock and its aftershocks in a 3 d p wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate we interpret the subducted seamount as an asperity whose rupture caused the 1990 gulf of nicoya earthquake

the seismogenic zone in the central costa rican pacific margin high quality hypocentres from an amphibious network transition from subduction of normal to thickened oceanic crust occurs in the central portion of the costa rican margin where large interplate earthquakes m 7 and abundant interseismic seismicity have been associated with subduction of bathymetric highs we relocated 1 300 earthquakes recorded for 6 months by a combined on and offshore seismological network using probabilistic earthquake relocation in a 3d p wave velocity model most of the seismicity originated at the seismogenic zone of the plate boundary appearing as an 18 dipping planar cluster from 15 to 25 30 km depth beneath the continental shelf several reverse focal mechanisms were resolved within the cluster the upper limit of this interseismic interplate seismicity seems to be controlled primarily by the overlying plate thickness and coherency which in turn is governed by the erosional processes and fluid release and escape at temperatures lower than 100 to 120 c along the plate boundary the downdip limit of the stick slip behaviour collocates with relative low temperatures of 150 to 200 c suggesting that it is controlled by serpentinization of the mantle wedge the distribution of the interseismic interplate seismicity is locally modified by the presence of subducted seamounts at different depths unlike in northern costa rica rupture of large earthquakes in the last two decades seems to coincide with the area defined by the interseismic interplate seismicity

the march 25 1990 mw 7 0 ml 6 8 earthquake at the entrance of the nicoya gulf costa rica its prior activity foreshocks aftershocks and triggered seismicity on march 25 1990 a large earthquake mw 7 0 ml 6 8 occurred at the entrance of the nicoya gulf costa rica at 1322 55 6 utc producing considerable damage in central costa rica and generating much interest about whether or not the nicoya seismic gap nishenko 1989 had broken the local country wide seismographic network recorded 6 years of activity prior to this large earthquake 16 hours of foreshocks the mainshock and its aftershocks this network is operated jointly by the costa rica volcanological and seismological observatory at the national university ovsicori una and the charles f richter seismological laboratory at the university of california santa cruz cfrsl ucsc we obtained high resolution locations from this network and located the mainshock at 9 38 5 n 84 55 6 w depth is 20 0 km and the largest foreshock mw 6 0 march 25 1990 at 1316 05 8 utc at 9 36 4 n 84 57 1 w depth is 22 4 km we find that the aftershock zone abuts the southeast boundary of the nicoya seismic gap suggesting that the seismic gap did not rupture since the installation of the local network in april 1984 to march 24 1990 nearly 1900 earthquakes with magnitudes from 1 7 to 4 8 318 with magnitude 3 0 or larger have been located at the entrance of the nicoya gulf one of the most active regions in costa rica the march 25 earthquake occurred at the northwest edge of this region where a sequence of foreshocks began 16 hours prior to the mainshock the spatial temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture the mainshock ruptured an asperity of approximately 600 km2 of area with this area expanding to 4000 km2 after 7 days we present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount inversion of teleseismic broadband and long period p and sh waves yields a thrust faulting mechanism with the shallow plane striking 292 dipping 26 and with a rake of 88 in agreement with the subduction of the cocos plate under the caribbean plate local first motions for the largest foreshock and the mainshock agree with this solution we also present evidence suggesting that the march 25 1990 earthquake triggered and reactivated several seismic swarms in central costa rica and temporally decreased the activity in the epicentral area of the july 3 1983 ms 6 2 perez zeledon earthquake

moment tensor estimation using regional seismograms from a tibetan plateau portable network deployment the authors present the results of moment tensor inversions using regional seismograms recorded during the 1991 1992 passcal passive seismic experiment on the tibetan plateau using data recorded over a one year period they estimate the source mechanisms for 38 earthquakes within and near the tibetan plateau with moment magnitudes m sub w ranging from 3 8 to 5 0 with only one exception all the events within the plateau but away from the plateau boundary indicate either strike slip or extensional faulting a result consistent with previous studies three fourths of all the events occurred at depths less than 10 km and all but two at depths less than 15 km several events located near the southeastern section of the plateau exhibit an unusual m sub b m sub w difference of approximately one magnitude unit 19 refs 3 figs

subduction zone structure and magmatic processes beneath costa rica constrained by local earthquake tomography and petrological modelling summary a high quality data set of 3790 earthquakes were simultaneously inverted for hypocentre locations and 3 d p wave velocities in costa rica tests with synthetic data and resolution estimates derived from the resolution matrix indicate that the velocity model is well constrained in central costa rica to a depth of 70 km northwestern and southeastern costa rica are less well resolved owing to a lack of seismic stations and seismicity maximum h2o content and seismic wave speeds of mid ocean ridge basalt and harzburgite were calculated for metamorphic phase transformations relevant to subduction both the 3 d p wave velocity structure and petrological modelling indicate the existence of low velocity hydrous oceanic crust in the subducting cocos plate beneath central costa rica intermediate depth seismicity correlates well with the predicted locations of hydrous metamorphic rocks suggesting that dehydration plays a key role in generating intermediate depth earthquakes beneath costa rica wadati benioff zone seismicity beneath central costa rica shows a remarkable decrease in maximum depth toward southeastern costa rica the presence of asthenosphere beneath southeastern costa rica which entered through a proposed slab window may explain the shallowing of seismicity due to increased temperatures and associated shallowing of dehydration of the slab tomographic images further constrain the existence of deeply subducted seamounts beneath central costa rica large low p wave velocity areas within the lower crust are imaged beneath the southeasternmost volcanoes in central costa rica these low velocities may represent anomalously hot material or even melt associated with active volcanism in central costa rica tomographic images and petrological modelling indicate the existence of a shallow possibly hydrated mantle wedge beneath central costa rica

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

first results from a new seismographic network in costa rica central america abstract the occurrence of past large earthquakes in space and time has been useful in estimating locations and repeat times for similar future events kelleher et al 1973 and mccann et al 1979 the lack of major energy release in and near costa rica during the last 30 years i e seismic slip deficiency as shown by mcnally and minster 1981 and a recent increase in earthquake activity motivated the organization and installation of a permanent seismographic network the network became operational in early 1984 with financial support from the office of foreign disaster assistance ofda usaid the costa rica network will provide the data to help understand the nature of the tectonic interaction of the cocos and caribbean plates and to evaluate the potential for present and future earthquake hazards in light of the historical earthquake activity

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

relocation and waveform modeling of the 1924 orotina costa rica earthquake ms 7 0 abstract we have determined the location and focal mechanism for the 1924 orotina earthquake using two different relocation algorithms developed for the analysis of historic earthquakes and forward waveform modeling of teleseismic body waves our relocation results suggest that the 1924 earthquake occurred 50 km southeast of orotina near the pacific coast the heavy damage reported in the orotina region coupled with a low population density to the southeast had led previous researchers to believe that the earthquake occurred near orotina possibly on the tarcoles or bijagual faults our waveform modeling results indicate rupture on a thrust fault at a depth 25 5 km near the plate interface with mw 6 4 6 7 these are inconsistent with slip on any mapped surface faults located near orotina and with the mechanism of the nearby 2004 mw 6 4 damas earthquake that indicates transtensional deformation in the upper crust this suggests that the 1924 event may be one of a sequence of events 1924 1990 1999 generated by subduction of the central costa rica seamount belt

rough crust subduction forearc kinematics and quaternary uplift rates costa rican segment of the middle american trench orthogonal subduction of bathymetrically rough oceanic lithosphere along the northwestern flank of the cocos ridge imprints a distinctive style of deformation on the overriding costa rican forearc we divide the costa rican forearc into three 100 160 km long deformational domains based on the bathymetric roughness and thickness of the cocos plate entering the middle american trench the dip of the subducting plate the variation in surface uplift rates of late quaternary coastal deposits and the orientations and types of faults deforming paleogene and neogene sedimentary rocks in the 100 km long nicoya domain coastal deposits show localized surface uplift and arcward tilting above the downdip projections of the fossil trace of the cocos nazca panama co nz pa triple junction and the fisher seamount and ridge in the 120 km long central pacific forearc domain between the nicoya peninsula and quepos shallower 60 subduction of seamounts and plateaus is accompanied by trench perpendicular late quaternary normal faults steeply dipping northeast striking margin perpendicular faults accommodate differential uplift associated with seamount subduction uplift and faulting differ between the segments of the forearc facing subducting seamounts and ridges inner forearc uplift along the seamount dominated segment is greatest inboard of the largest furrows across the lower slope localized uplift and arcward tilting of coastal deposits is present adjacent to subducting seamounts in contrast inboard of the underthrusting aseismic cocos ridge along the 160 km long fila costena domain between quepos and the burica peninsula mesoscale fault populations record active shortening related to the 100 km long fila costena fold and thrust belt the observed patterns of faulting and permanent uplift are best explained by crustal thickening the uplifted terraces provide a first order estimate of permanent strain along the forearc in costa rica the permanent strain recorded by uplift of these quaternary surfaces exceeds the predicted rebound of stored elastic strain released during subduction zone earthquakes

the geometry of the wadati benioff zone under southern central america and its tectonic significance results from a high resolution local seismographic network abstract we present here a detailed geometry of the wadati benioff zone under costa rica obtained from seismicity recorded by a dense local seismographic network jointly operated by the costa rica volcanological and seismological observatory national university and the charles f richter seismological laboratory university of california santa cruz underneath the nicaragua costa rica border the wadati benioff zone smoothly contorts from steep to shallow dip angles nw to se but does not show evidence of a brittle tear as postulated by others however further to the se ne of puerto caldera costa rica the wadati benioff zone does show a segmentation the quesada sharp contortion at intermediate depths h 70km nw of this sharp contortion the deepest portion of the seismically active slab dips at about 80 and reaches maximum depths ranging from 200 km near the nicaragua costa rica border to 135 km under ciudad quesada to the se the deeper portion of the wadati benioff zone dips at about 60 and the seismicity does not extend below depths ranging from 125 km behind the volcanic arc to 50 km east of quepos in southern costa rica east of 83 55 w we find no evidence of the wadati benioff zone deeper than 50 km the obtained geometry and other known tectonic features related to the subduction of the cocos plate under the caribbean plate along the southern terminus of the middle america trench nicaragua and costa rica correlate well with along trench variations in age of the subducted cocos plate some of these tectonic features are 1 the shallowing of middle america trench bathymetry from nw to se 2 variations in the energy release within the subducted slab 3 differences in coupling between cocos and caribbean plates 4 the termination of the central america volcanic chain in central costa rica 5 distinct stress field variations on the overriding caribbean plate the subduction of the cocos ridge under southern costa rica is partially responsible for some of these features shallowing of the middle america trench in southern costa rica and for the high uplift rates of the outer arc however as the presence of the panama fracture zone limits the subducted extension of the cocos ridge to less than 100 km from the trench we propose that the overall geometry of the wadati benioff zone is controlled by abrupt along trench changes in the age of the subducted cocos plate

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

source rupture process of the tecoman colima mexico earthquake of 22 january 2003 determined by joint inversion of teleseismic body wave and near source data the spatial and temporal slip distribution of the tecoman colima mexico earthquake is estimated from near source strong motion and teleseismic body wave data to perform a stable inversion we applied smoothing constraints and determined their optimal relative weights on the observed data using an optimized akaike s bayesian information criterion abic the source parameters are as follows strike dip slip 300 20 93 seismic moment m 2 3 1020 n m source duration 30 sec along strike distance 35 km along dip distance 70 km we found that the rupture process can be divided into three stages the rupture nucleated near the hypocenter stage i then it broke the first asperity centering about 15 km southwest from the epicenter stage ii and the rupture propagated to the northeast and the second asperity was broken stage iii we also estimated the shear stress change due to the rupture process of the mainshock on and around the major fault zone it appears that one cluster of aftershocks for the first 5 days which took place in and adjacent to the zones of stress increased due to the fault rupture during the mainshock but overall correlation between the aftershock location and the stress pattern is not clear manuscript received 14 may 2003