most and least likely locations of large to great earthquakes along the pacific coast of mexico estimated from local recurrence times based on b values we mapped t l and p l local recurrence time and local probability density respectively for m max 7 2 earthquakes along the plate margin of the pacific coast of mexico in order to test two hypotheses the first is that the minima in t l or maxima in p l extracted probabilistically from the frequency magnitude distribution map asperities the second hypothesis is that recurrence times for characteristic events are estimated more correctly by local recurrence times than by the overall recurrence times of the zone comparing these results to the observed recurrence times for events of m 7 0 we find good agreement the coast of guerrero between longitude west 100 5 and 101 5 and the segment off the coast of southern chiapas show the shortest t l estimated as about 20 yr the location at longitude 98 5 near the boundary of guerrero with oaxaca the ometepec segment follows with estimates of about 30 yr as the next shortest t l along the coast of guerrero between 99 and 100 5 longitude t l is estimated as about 40 yr and finally in the segments off the oaxacan coast near 95 7 and 97 7 longitude t l ranges from 40 to 60 yr these volumes we define as asperities long local recurrence times are observed for the areas offshore from the isthmus of tehuantepec on the pacific plate off the coast of guerrero on land along the coast of most of oaxaca and along the northernmost 40 km of the guerrero coast if our ideas are correct then major moment release should emanate from the volumes we defined as asperities and relatively minor moment release should emanate from the volumes identified by the relatively long local recurrence times

earthquake scaling laws for creeping and non creeping faults the dependence of the summed moment release on the fault dimension differs between non creeping faults and creeping faults or volcanic regions this can be attributed to different earthquake scaling laws the number n of events with fault length l scales for non creeping faults as n l l 2 and for creeping faults as n l l 3 this difference offers a means for mapping the distribution of creep in the seismogenic crust

how do asperities fracture an experimental study of unbroken asperities abstract the fracture of a shear fault containing several unbroken asperities in a granitic porphyry is examined in detail via acoustic emission ae data collected by a high speed multi channel waveform recording system the experimental results reveal that a quasi static nucleation of the shear faulting corresponds to the fracture of coupled asperities on the fault plan aes caused by the fracture of individual asperities exhibit similar characteristics to the sequence for natural earthquakes including foreshock mainshock and aftershock events foreshocks initiated at the edge of the asperity occur with an event rate that increases according to a power law of the temporal distance to the mainshock and with a decreasing b value from 1 1 to 0 5 one or a few mainshocks then initiate at the edge of the asperity or the front of the foreshocks the aftershock period is characterized by a remarkable increase and subsequent gradual decrease in b value and a decreasing event rate obeying the modified omori law which has been well established for earthquakes the fracture of neighboring asperities is then initiated after the mainshock of a particular asperity presumably due to redistribution of the strain energy accumulated within an asperity which is released by the mainshock resulting in enhancement of the stress concentration around the nearest neighboring intact asperities the progressive fracturing of multiple coupled asperities during the nucleation of shear faulting results in short term precursory fluctuations in both b value and event rate which may prove useful information in the prediction of failure of the main fault plane of seismic earthquakes

chronology of internal drainage development and uplift southern puna plateau argentine central andes fundamental to understanding the late cenozoic evolution of the central andes is knowledge of the age that the puna altiplano plateau became a unique tectonomorphic entity within the orogen the two attributes that define the puna altiplano are 1 its average elevation of 3 km and 2 regions of high elevation that correspond spatially to regions with internal drainage the chronology of plateau uplift to elevations similar to modern ones is equivocal owing to an inability to define the absolute elevation of the plateau surface through time however because regions of the puna altiplano have nonmarine evaporites nad because such evaporites must accumulate within regions of internal drainage the chronology of these strata elucidates when the plateau became internally drained we show that for the southern puna between lat 24 and 26 s internal drainage was established between 14 1 and 24 2 ma and probably by 15 ma these results corroborate and refine other workers inferences that the puna altiplano plateau became uplifted and paleogeographically isolated from the adjacent fore arc and foreland during middle to late miocene time

geochemistry and isotope systematics of small to medium volume neogene quaternary ignimbrites in the southern central andes evidence for derivation from andesitic magma sources the geochemical and isotopic compositions of small to medium volume 10 km3 50 km3 neogene quaternary ignimbrite sheets of the southern central andes 25 00 26 45 s 67 30 69 30 w have been analysed in order to assess the petrogenesis of this type of calc alkaline magma the ignimbrites are derived from the active volcanic zone around salar de la isla sis in chile and from behind the active arc in the area of the salar de antofalla saf argentina the ignimbrites in both areas are characterized by 87sr 86sr t ratios 0 7066 0 7090 that are lower and end t values 2 4 to 5 1 that are higher than in the large volume caldera sourced ignimbrites of cerro galan 100 km further east and those of ignimbrites from other megacalderas of the central andes this difference reflects smaller amounts of crustal material in the sis and saf ignimbrites rather than a change in basement composition as indicated by pb provinciality the isotope signature of the sis and saf ignimbrites is similar to that of late to post miocene composite calc alkalic andesite dacite stratovolcanoes of the frontal arc in this region this finding combined with major and trace element modelling lends support to a petrogenetic model for the sis and saf ignimbrites involving fractional crystallization from a lil depleted andesitic parental magma which was beforehand modified by crustal contamination

locked and creeping patches along the hayward fault california the results of mapping locked and unlocked patches on the hayward fault using the local recurrence time method are compared with models derived from surface creep and geodetic measurements the data used were the m 1 0 earthquakes in the usgs catalog since 1981 0 located within 5 km of the fault trace along the central and southern segments agreement with the other methods is satisfactory the 1868 rupture except for its two ends is classified as a locked segment by our method in the northern part of the hayward fault the data are poor and the results of the three methods differ short local recurrence times in the northern part suggest that an asperity exists there which is capable of an m6 5 class event

style and history of andean deformation puna plateau northwestern argentina topographically the puna plateau of northwestern argentina is the southern continuation of the bolivian altiplano its thickening and consecutive uplift result from the andean orogeny to better constrain the structural style and its progressive development we have studied field data topographic and satellite imagery balanced cross sections seismic reflection data kinematic analysis of fault slip data anisotropy of magnetic susceptibility ams paleomagnetic data and apatite fission track aft data across the puna plateau precambrian and paleozoic basement ranges bounded by high angle reverse faults dips 60 alternate with cenozoic intermontane basins major thrusts trend nne ssw and do not show a preferred vergence intermontane basins have various degrees of symmetry depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets there is a close correlation between the surface expression of a basin and the amount of internal deformation a line balanced cross section of the puna at 25 s has yielded a cenozoic shortening of 10 15 in a direction subperpendicular to the orogen by kinematic analysis of cenozoic fault slip data we have obtained principal directions of strain rate across the puna shortening axes are subhorizontal and trend on average wnw ese n110 stretching axes are subvertical and intermediate axes are subhorizontal and trend on average nne ssw strain ellipsoids are dominantly of plane strain type and they represent dip slip thrusting from paleomagnetic and ams data shortening axes form a radial pattern around the eastern edge of the central andes the pattern is attributed to an inhomogeneous stress field reflecting the eastward convex shape of the central andean thrust front from the history of burial and uplift andean shortening reached the northeastern part of the puna in the late eocene and the adjacent eastern cordillera in the late eocene or early oligocene this shortening was presumably due to the incaic phase of the andean orogeny in the eastern part of the orogen the onset of shortening was probably guided by preexisting paleozoic and mesozoic structures so that andean deformation propagated unevenly eastward

relationship between tectonism and volcanism along the great sumatran fault zone deduced by spot image analyses abstract satellite images provide evidence for numerous stepovers pull apart grabens and volcanic structures along the nw trending right lateral great sumatran fault zone high resolution spot images permit us to analyze the relationship between volcanic and tectonic structures this analysis reveals that releasing stepovers and pull aparts are ephemeral structures along the great sumatran fault zone and that the geometry of the strike slip fault evolves permanently through time the interpretative structural maps obtained from spot image analysis reveal that the formation of huge peculiarly shaped volcanic calderas has occurred in large releasing stepover fault zones and that the bounding faults of rectangular pull apart basins are analogous to the circular ring faults of calderas in particular detailed study of the segments at the southernmost 150 km long termination of the great sumatran fault zone from northwest of ranau lake to semangka bay allows us to describe the development of the ranau collapse caldera this caldera is located within a pull apart basin bounded by a stepover its peculiar rectangular shape and its relatively large size about 200 km 2 are controlled by the evolution of the extinct ranau releasing stepover similarly the toba elliptical caldera one of the largest volcanic caldera in the world is elongated parallel to the present trace of the great sumatran fault and appears to be related to a releasing stepover associated with a wide pull apart basin that is not active at present

late cenozoic tectonism collapse caldera and plateau formation in the central andes abstract the evolution of andean volcanism including the formation of late miocene to recent collapse calderas on the puna plateau is generally interpreted in terms of the kinematic framework of the nazca and south american plates we present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first order nw se striking zones of left lateral transtension on the local and regional scales consequently ages of collapse calderas indicate activity of these fault zones which initiated at about 10 ma on the puna plateau the onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen parallel stretching upon reaching maximum crustal thickness and critical surface elevation horizontal magma sheets that formed at mid crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub vertical faults this accounts well for the observed tectono magmatic phenomena at surface it follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the andean plateau rather than to changes in the geometry of the wadati benioff zone or plate boundary kinematics

seismicity patterns are they always related to natural causes by analyzing data from two catalogs of seismicity one local and one regional examples are drawn that demonstrate artificial seismicity patterns which can arise when changes in magnitude reporting or in detection ability are introduced by changes in operating procedures of a seismic network in the first case a catalog which comprises seismic data from guerrero mexico is revised and a correction is applied to magnitudes suspected of being affected by different operative practices next several time windows are analyzed which compare the seismicity rate within the window to that of the total record mapped as a function of space the same procedure is applied to the original i e uncorrected catalog a significant seismic quiescence apparent in the original data set in the center of the seismic network all but disappears in the corrected data indicating that this anomaly is not naturally induced in the second case we investigate the homogeneity of seismicity reporting for the interior of alaska we computed the standard deviate z as a function of time by comparing the overall seismicity rate with the rate in a 3 year window the maps z values were inspected for all times the most outstanding rate change is found around 1992 5 since the b value remained unchanged the most reasonable explanation for the observed rate change around mid 1992 is a decrease in the detection ability of the network in the interior of alaska both case studies demonstrate the usefulness of systematic comparisons of the cumulative and noncumulative frequency magnitude distribution and of spatial and temporal mapping of the seismicity rates as a tool to investigate the homogeneity of earthquake reporting

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

cenozoic stratigraphy and basin tectonics of the andes mountains 20 28 south latitude clastic sedimentary basins have evolved during the past 40 m y in the central andes lat 20 28 s in response to shifting patterns of magmatism and deformation the distribution of these basins and their genetic relations to uplifted areas are analogous to the basins and mountain belts of the north american rocky mountains during the late cretaceous and early cenozoic petroleum exploration has focused on zones underlying the upper cenozoic strata along the eastern margin of the andes mountain belt between about 40 and 25 ma a nonmarine basin extended across the region that is now the andes mountains between about 25 and 10 ma the western part of the former basin became the site of a volcanic arc sediment accumulation continued in the east where marine intercalations demonstrate the low elevation of the basin after 10 ma the volcanic arc remained active and locally widened and crustal shortening caused regionally important thrust and reverse faulted ranges during the past 10 m y up to 4 000 m of coarse clastic debris accumulated in a foreland basin on the eastern flank of the mountains meanwhile in the interior of the mountains over 4 000 m of fine grained strata and evaporites accumulated in local depocenters

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

interplate coupling and transient slip along the subduction interface beneath oaxaca mexico summary we describe and model gps measurements of surface deformation from the oaxaca segment of the mexican subduction zone to characterize interseismic strain accumulation and episodic transient slip in this region and test seismologically based models of strain accumulation and release along subduction interfaces deformation measured from 2001 to 2007 within our dense 31 station gps array has consisted of 1 trench normal horizontal contraction at rates that decrease monotonically inland from the coast 2 rapid coastal subsidence that changes gradually to slow uplift at locations more than 100 km inland and 3 periods of transient slip that interrupt the otherwise steady deformation inverse modelling of transient station offsets in 2004 and 2006 indicates that transient slip along the subduction interface occurred downdip from the rupture limits of previous large earthquakes in this region in both 2004 and 2006 gps site velocities that are corrected for the effects of this transient slip vary significantly over distances of only tens of kilometres both along the coastline and inland implying that similar spatial variations occur in the degree of locking across the subduction interface deformation rates measured along the coast reach their maximum above the core of the rupture zone of the 1978 ms 7 8 shallow thrust earthquake and generally decrease outwards towards the edges of the rupture zone bounded value inverse modelling of the interseismic gps velocity field with a finite element mesh that simulates the study region indicates that much of the rupture zone of the 1978 shallow thrust earthquake is fully locked at the plate convergence rate but that this region is surrounded by weakly locked areas of the subduction interface which may slow or arrest the propagation of future earthquakes much of the deeper region of transient slip downdip from the seismogenic zone is also fully locked between the episodes of transient slip however the elastic energy that accumulated due to locking of this deeper transitional zone between 2002 and 2006 appears to have been mostly or completed released by the 2004 and early 2006 transient slip events the approximately balanced energy budget for the deeper zone of transient slip implies that this region is unlikely to contribute significant elastic energy to future earthquakes that originate along the seismogenic zone our results support a model in which seismic asperities coincide with regions of strong locking between earthquakes the potential elastic energy that has re accumulated since 1978 in the seismogenic zone is already sufficient to cause a repeat of the 1978 earthquake

composition of the crust and upper mantle in the central andes 19 30 s inferred from p wave velocity and poisson s ratio the present day crustal structure of the andean chain results from deformation processes and magmatic activity since the onset of subduction one line of investigation to resolve questions about the mechanism of crustal thickening is to get information about the composition of the crust and upper mantle by imaging the structures in velocity and poisson s ratio or vp vs a high density seismic network was operated during six months in 1994 across the whole andean chain the p and s travel times of the best located local earthquakes mainly situated in the subduction zone have been inverted for vp and vp vs using thurber s 3d iterative simultaneous inversion method the resulting models giving short scale information about the crustal and upper mantle composition provide new limits on the respective involvement of crustal shortening and magmatism in the thickening of the andean crust beneath the western cordillera the seismic properties are well explained by typical active volcanic arc processes and the thickening of the crust is explained by magmatic addition conversely beneath the central altiplano basin and the eastern cordillera our results preclude significant volume of magmatic addition from the mantle and reinforce crustal shortening as the main thickening process moreover cratonic lower crust and subcontinental lithospheric upper mantle are observed under the eastern cordillera up to 67 w evidence of the underthrusting of the brazilian craton which drives the crustal shortening beneath the central altiplano basin a thin lithospheric mantle is observed above a well marked asthenospheric wedge which extends in depth below the western part of the eastern cordillera thus partial melt is observed in the shallow mantle west of the western cordillera up to the brazilian craton

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

crustal lineament control on magmatism and mineralization in northwestern argentina geological geophysical and remote sensing evidence abstract the relationship between long lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the puna region of northwestern argentina through the analysis of regional aeromagnetic surveys landsat images and geological information the good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study at a regional scale several contrasting magnetic domains are recognized which correlate with crustal geology two basement domains are separated by a nne trending boundary which is believed to correlate with a paleozoic suture zone between the pampia to the southeast and arequipa antofalla terranes to the northwest locally overlying these basement terranes is the cenozoic magmatic domain which is best developed in the n s trending volcanic arc at the western edge of the puna the cordillera occidental in addition four southeast trending volcanic zones extend for several hundred kilometers across the puna many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts and we have selected three such areas for more detailed study bajo de la alumbrera argentina s largest porphyry copper deposit cerro galan the largest ignimbrite caldera in argentina with associated hydrothermal alteration zones and el queva a historic polymetallic district located within a major volcanic range a comparison of lineament maps generated from aeromagnetic and landsat tm images reveals broad correlation between these different remote sensing techniques which respectively highlight subsurface magnetic and surface geological features in addition the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework and are seen to occur near the intersections of major lineament zones it is suggested that in three dimensions such intersection zones form trans lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain and may thereby serve as conduits for magma ascent to the shallow crust pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits it is important to note that in this model structural intersections serve as facilitators for magma ascent and volatile exsolution but do not in themselves cause this process other factors such as magma supply rate and tectonic stress are essential primary ingredients and local magmatic and volcanic processes affect the ultimate potential for ore formation nevertheless we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration other regional and local geological considerations must then be applied to identify priority targets within this framework

on varying b values with depth results from computer intensive tests for southern california summary the gutenberg richter b value is thought to reflect the stress conditions in the crust therefore spatial and or temporal variations of the b value can provide important information regarding crustal tectonics we investigate the variation of b value with depth in seven selected areas of southern california a previous study provided a detailed mapping of the variations of b with depth in california our study is less systematic than this study our approach is more similar to the regional one used by mori abercrombie in comparison to these previous studies our investigation indicates that the variability of b is often not statistically significant and that the decrease of b with depth should be interpreted with caution the seismic catalogues used are subsets of a set of about 100 000 seismic events recorded by the southern california seismic network scsn and relocated by richards dinger shearer we study the performance of utsu s test compared to bootstrap tests for comparison of b values the results of our investigation also raise the question of the relevancy of utsu s test when comparing b values both simulations and real cases show that the utsu s test is biased towards rejection of the null hypothesis in favour of the hypothesis that b values are significantly different

implication of dating of early tertiary volcanic rocks from the north chilean precordillera abstract since the early jurassic subduction induced volcanic activity in the north chilean andes 21 23 5 s has migrated 200 300 km eastward from the present coast jurassic magmatic arc to the western cordillera miocene recent volcanic rocks due to the paucity of chronological data however the exact age and extent of various magmatic sub belts within this area are not well constrained in the precordillera a sequence of 1500 5000 m thick red bed sediments is partly intercalated with and mostly covered by predominantly intermediate calc alkaline tuffs and lavas presently the age of this sequence is considered to be upper jurassic and or cretaceous from this sequence eleven biotites and seven hornblendes were analyzed and yielded 40 ar 39 ar plateau ages of between 48 and 38 ma indicating upper eocene volcanic activity in the eastern part of the precordillera in contrast volcanic activity in the western part of the precordillera has been dated at 92 2 5 ma indicating an earlier onset of volcanism and therefore sedimentation of the red beds in the late eocene the intense volcanism ceased in this area contemporaneous with decreasing relative convergence rate of the nazca plate and the south american continent this final phase of eocene volcanism coincided with a period of important deformation the incaic phase biotites from two ash fall tuffs separated by a distinct angular unconformity yield identical ages of 38 50 0 90 ma and 38 45 0 60 ma these dates provide the best age estimate of the incaic phase in the north chilean precordillera

paleogeography and andean structural geometry northwest argentina neogene tectonic segmentation of the andean cordillera in northwestern argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins vergence fault dip and lateral continuity of neogene structures can be related to the character and pre existing geometries and structures of the materials in which they form south of 27 s the thick skinned sierras pampeanas which occur mostly over a flat segment of the subducted nazca plate formed where little sedimentary cover overlay crystalline basement north of 24 s the thin skinned subandean belt overlying an inclined slab segment formed within a thick elongate eastward tapering trough of paleozoic and mesozoic sediments that may represent a complex of pre andean foreland basin deposits the transitional region between 24 27 s the santa barbara system and cordillera oriental is characterized paleogeographically by complex cross trending structures and irregular basin geometries and the neogene structures are equally complex and variable the paleogeographic features of nw argentina are a product of compressional tectonism during much of the paleozoic that resulted in an orogenic belt oriented obliquely to the modern andes this earlier orogen was disrupted by several zones of transverse structures and by cretaceous extensional or pull apart basins these features clearly influence the geometry of the superposed andean structures more intriguing however is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

analysis of temporal and spatial heterogeneity of magnitude frequency distribution inferred from earthquake catalogues summary statistical models are introduced for the simultaneous estimation of the changes of the b value and detection rate of earthquakes in a catalogue which develops in time or varies in space the three characteristic parameters including b for the magnitude frequency distribution of detected earthquakes are represented by respective three b spline functions of time or location in a space an objective bayesian method is adopted for the optimal estimate of such functions with many unknown coefficients the present procedure is applied to earthquake catalogues for japanese seismic activity

the 14 september 1995 m 7 3 copala mexico earthquake a source study using teleseismic regional and local data we analyze source characteristics of the 14 september 1995 copala mexico earthquake m 7 3 using teleseismic regional and local seismograms in the analysis of the teleseismic and the regional seismic waves we apply the empirical green s function egf technique the recording of an appropriate aftershock is taken as the egf and is used to deconvolve the mainshock seismogram thus obtaining an apparent far field source time function at each station the deconvolution has been done using surface waves for teleseismic data we apply a spectral deconvolution method that enables us to obtain 37 apparent source time functions astfs at 29 stations in the analysis of the regional broadband seismograms we use two different aftershocks as egf and the deconvolution is performed in the time domain with a nonlinear method imposing a positivity constraint and the best azimuth for the directivity vector is obtained through a grid search approach we also analyze two near source accelerograms the traces are inverted for the slip distribution over the fault plane by applying a linear inversion technique with the aid of a time window analysis we obtain an independent estimation of the source time function and a more detailed description of the source process the analysis of the three datasets permits us to deduce the main characteristics of the source process the rupture initiated at a depth of 16 km and propagated in two directions updip along the plate interface toward 165 n and toward 70 n the source duration was between 12 and 14 sec with the maximum of energy release occurring 8 sec after the initiation of the rupture the estimated rupture dimension of 35 45 km is about one fourth of the aftershock area the average dislocation over the fault was 1 4 m with a maximum dislocation of 4 1 m located 10 km south of the hypocenter which gives roughly 1 mpa as the average static stress drop

mapping the b value anomaly at 100 km depth in the alaska and new zealand subduction zones a positive anomaly in the frequency magnitude distribution b value is detected at approximately 90 100 km depth in two subduction zones the b value in this anomalous zone is 40 higher than in adjacent volumes we use regional catalogs with a magnitude of completeness of 2 6 central alaska and cook inlet 2 0 shumagin islands and 3 5 new zealand to resolve the b value as a function of space in more detail we project all earthquakes onto planes perpendicular to the strike of the slab these cross section views of the b value distribution locate the high b value anomalies at a depth of 90 100 km on the upper surface of the wadati benioff zone at these depths slab dehydration may increase pore pressure thus lowering the effective stress and increasing the b value increased pore pressure would also lower the liquidus in the overlying asthenosphere giving rise to the volcanism that occurs directly above this zone

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

hypotheses concerning a regmagenic network controlling metallogenic and other geologic events in the south american austral cone the statistical processing of megafractures and lineaments of the argentinean chilean and uruguayan territories and the determination of the structurogenetic orientations of more than fifty phylohydrothermal mineral deposits has led to the development of a hypothesis of the existence of two planetary fracture network systems in the south american austral cone

magnitudes of large shallow earthquakes from 1904 to 1980 abstract in order to obtain a uniform magnitude catalogue surface wave magnitudes ms and broad band body wave magnitudes mb have been determined for large shallow earthquakes from 1904 to 1980 in making the catalogue homogeneous the author consistently adheres to the original definitions of ms and mb given by gutenberg 1945 and gutenberg and richter 1956 the determinations of ms and mb are all based on the amplitude and period data listed in gutenberg and richter s unpublished notes bulletins from stations worldwide and other basic information mb is measured on broad band instruments in periods of 8 s consistency of the magnitude determinations from these different sources is carefully checked in detail more than 900 shallow shocks of magnitude 7 and over are catalogued the meaning of the magnitude scales in various catalogues is examined in terms of ms and mb most of the magnitudes listed by gutenberg and richter 1954 in their seismicity of the earth are basically ms for large shocks shallower than 40 km but are basically mb for large shocks at depths of 40 60 km the surface wave magnitudes given by earthquake data reports are higher than ms by 0 2 unit whether the combined horizontal amplitude or the vertical amplitude is used mb and the currently used 1 s body wave magnitude are measured at different periods and should not be directly compared

spatial variations in the frequency magnitude distribution of earthquakes at mount pinatubo volcano the frequency magnitude distribution of earthquakes measured by the b value is mapped in two and three dimensions at mount pinatubo philippines to a depth of 14 km below the summit we analyzed 1406 well located earthquakes with magnitudes m d 0 73 recorded from late june through august 1991 using the maximum likelihood method we found that b values are higher than normal b 1 0 and range between b 1 0 and b 1 8 the computed b values are lower in the areas adjacent to and west southwest of the vent whereas two prominent regions of anomalously high b values b 1 7 are resolved one located 2 km northeast of the vent between 0 and 4 km depth and a second located 5 km southeast of the vent below 8 km depth the statistical differences between selected regions of low and high b values are established at the 99 confidence level the high b value anomalies are spatially well correlated with low velocity anomalies derived from earlier p wave travel time tomography studies our dataset was not suitable for analyzing changes in b values as a function of time we infer that the high b value anomalies around mount pinatubo are regions of increased crack density and or high pore pressure related to the presence of nearby magma bodies manuscript received 16 december 2002

inadvertent changes in magnitude reported in earthquake catalogs their evaluation through b value estimates a simple procedure is presented for analyzing magnitudes and seismicity rates reported in earthquake catalogs in order to discriminate between inadvertently introduced changes in magnitude and real seismicity changes we assume that the rate and the frequency magnitude relation of the independent background seismicity do not change with time observed differences in the frequency magnitude relation a and b values between data from two periods are modeled as due to a transformation of the magnitude scale the transformation equation is found by a least squares fitting process based on the seismicity data for earthquakes large enough to be reported completely and by comparing the linear relation of one period to the other for smaller events an additional factor accounting for increased decreased detection is allowed this fitting technique is tested on a data set from parkfield for which two types of magnitudes amplitude and duration were computed for each earthquake we found that the b value fitting technique yielded virtually the same result as a linear regression assuming the same errors in the two magnitudes the technique is also applied to interpret the nature of reporting rate changes in a local guerrero mexico and a regional italy earthquake catalog in guerrero a magnitude change in 1991 37 can be modeled about equally well by m new m old 0 5 or by m new 1 02 m old 0 38 but residuals with the latter transformation are smaller in italy a magnitude change in 1980 21 cannot be modeled satisfactorily by a simple magnitude shift but is well described by a compression of the magnitude scale given by m new 0 67 m old 1 03 the proposed b slope fitting method provides a means to interpret quantitatively and in some cases correct for artificial reporting rate changes in earthquake catalogs

the recognition of reactivation during continental deformation reactivation involves the accommodation of geologically separable displacement events intervals 1 ma along pre existing structures the definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings in neotectonic environments reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime this mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors four sets of criteria may be used to recognize reactivation in the geological record stratigraphic structural geochronological and neotectonic some structural criteria may not be reliable if used in isolation to identify reactivated structures much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend dip or three dimensional shape of structures numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long term and may be invoked to explain reactivation the collage of fault bounded blocks forming most continents therefore carries a long term architecture of inheritance which can explain much of the observed complexity of continental deformation zones

spatial variability of seismicity parameters in aftershock zones the spatial variability of the b value of the frequency magnitude relationship and the decay rate of aftershocks as described by the p value of the modified omori law is investigated by using dense spatial grids we map out the distribution of b and p values within the landers northridge morgan hill and kobe aftershock sequences considerable spatial variability is found with b values of independent subvolumes ranging from 0 6 to 1 4 and p values ranging from 0 6 to 1 8 these systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km the spatial distribution of these two parameters is compared with the slip distribution during the mainshock suggesting that the areas of largest slip release correlate with high b value regions we hypothesize that the frictional heat created during the event may influence the p value distribution within an aftershock zone while applied shear stress crack density and pore pressure govern the frequency magnitude distribution by investigating the frequency magnitude distribution separately for preseismic and postseismic periods for the morgan hill mainshock we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value which decays to premainshock values within a year surrounding areas of the aftershock zone show an approximately constant b value with time because the aftershock hazard after a mainshock depends strongly on both the b and p value we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters

detailed analysis of acoustic emission activity during catastrophic fracture of faults in rock abstract the detailed time space distribution of acoustic emission ae events during the catastrophic fracture of rock samples containing a pre existing joint or potential fracture plane is obtained under triaxial compression using a high speed 32 channel waveform recording system and the results are discussed with respect to the prediction and characterization of catastrophic fault failure ae activity is modeled quantitatively in terms of the seismic b value of the magnitude frequency relation the self excitation strength of the ae time series and the fractal dimension of ae hypocenters consistent with previous studies on rock samples containing a fracture plane with several asperities the present analyses reveal three long term phases of ae activity associated with damage creation on heterogeneous faults each clearly identifiable based on the above parameters a long term decreasing trend and short term fluctuation of the b value in the phase immediately preceding dynamic fracture are identified as characteristic features of the failure of heterogeneous faults based on the experimental results it is suggested that precursory anomalies related to earthquakes and other events associated with rock failure are strongly dependent on the heterogeneity of the fault or rock mass a homogeneous fault or rock mass appears to fracture unpredictably without a consistent trend in precursory statistics while inhomogeneous faults fracture with clear precursors related to the nature of the heterogeneity

mapping active magma chambers by b values beneath the off ito volcano japan the b value of the frequency magnitude relation and thus the mean magnitude in the off ito volcanic area is not uniform based on detailed b value tomography using about 10 000 events in an area of 10 km radius and in the upper 15 km of the crust a high b value anomaly b 15 below 7 km depth and with a radius of about 2 km located below the coast southeast of ito contrasts with lower values of typically b 0 7 north of it and at shallower depths on the basis of surface deformations a tensile crack dislocation source was located above 7 km depth thus we surmise that the magma chamber in the off ito area was located below 7 km depth the correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies the magma body may be described as consisting of two parts a lower well established chamber below 11 km depth and a shallower part 11 7 km depth the b value in the volume surrounding the magma chamber increased from 0 7 during the 1980s to 1 5 during the earthquake swarms of the 1990s this may reflect a trend of increasing crack density due to the intrusion activity

spatial variations in the frequency magnitude distribution of earthquakes at soufriere hills volcano montserrat west indies the frequency magnitude distribution of earthquakes measured by the b value is determined as a function of space beneath soufriere hills volcano montserrat from data recorded between august 1 1995 and march 31 1996 a volume of anomalously high b values b 3 0 with a 1 5 km radius is imaged at depths of 0 and 1 5 km beneath english s crater and chance s peak this high b value anomaly extends southwest to gage s soufriere at depths greater than 2 5 km volumes of comparatively low b values b 1 are found beneath st george s hill windy hill and below 2 5 km depth and to the south of english s crater we speculate the depth of high b value anomalies under volcanoes may be a function of silica content modified by some additional factors with the most siliceous having these volumes that are highly fractured or contain high pore pressure at the shallowest depths

minimum magnitude of completeness in earthquake catalogs examples from alaska the western united states and japan we mapped the minimum magnitude of complete reporting m c for alaska the western united states and for the junec earthquake catalog of japan mc was estimated based on its departure from the linear frequency magnitude relation of the 250 closest earthquakes to grid nodes spaced 10 km apart in all catalogs studied mc was strongly heterogeneous in offshore areas the mc was typically one unit of magnitude higher than onshore on land also mc can vary by one order of magnitude over distance less than 50 km we recommend that seismicity studies that depend on complete sets of small earthquakes should be limited to areas with similar mc or the minimum magnitude for the analysis has to be raised to the highest com mon value of mc we believe that the data quality as reflected by the mc level should be used to define the spatial extent of seismicity studies where mc plays a role the method we use calculates the goodness of fit between a power law fit to the data and the observed frequency magnitude distribution as a function of a lower cutoff of the magnitude data mc is defined as the magnitude at which 90 of the data can be modeled by a power law fit mc in the 1990s is approximately 1 2 0 4 in most parts of california 1 8 0 4 in most of alaska aleutians and panhandle excluded and at a higher level in the junec catalog for japan various sources such as ex plosions and earthquake families beneath volcanoes can lead to distributions that cannot be fit well by power laws for the hokkaido region we demonstrate how neglecting the spatial variability of m c can lead to erroneous assumptions about deviations from self similarity of earthquake scaling

cenozoic dextral transpression and basin development at the southern edge of the puna plateau northwestern argentina abstract in northwestern argentina the southern edge of the altiplano puna is the most remarkable positive topographic anomaly average altitude 3700 m of the andean chain to the south are the sierras pampeanas a less elevated domain made of alternating compressive basins and ranges developed during the cenozoic at about 27 s the transition between the puna and the sierras pampeanas coincides with a substantial narrowing of the high chain and with a change in the dip of the pacific subduction zone nazca plate from 30 e below the plateau to subhorizontal below the sierras pampeanas we argue that this structural transition which can be identified at the scale of the andean topography is a major dextral transpressional transfer zone the tucuman transfer zone in this region ene wsw to e w directed subhorizontal shortening related to bulk convergence between the nazca and south american plates interferes with nw se directed subhorizontal shortening strains due to lateral influence of changes in the amount of crustal thickening on intraplate kinematics structural observations digital mapping spot satellite images and a microtectonic analysis of fault populations provide general insights into the neogene kinematic evolution of transpressional basins and ranges at the edge of the puna

geodynamic evolution and tectonostratigraphic terranes of northwestern argentina and northern chile in ordovician time gondwana in the area of northwestern argentina and northern chile had a west facing active margin the evolution of this margin culminated in the ocloyic orogeny at the end of ordovician time this orogeny was caused by the collision of the allochthonous arequipa antofalla terrane with this margin the early paleozoic evolution of northwestern argentina and northern chile contrasts markedly with the accretionary history of central argentina and central chile where the precordillera and chilenia terranes docked in the late ordovician and late devonian periods respectively an inspection of the available stratigraphic and geochronological data on sedimentary volcanic and plutonic units of the southern central andes of northern chile and northwestern argentina reveals a lull in magmatic and metamorphic activity lasting for 100 m y from early silurian to early late carboniferous time this is interpreted as corresponding to a tectonic scenario in which the present andean margin was a passive margin of gondwana this passive margin developed in response to the rifting off of a part of the arequipa antofalla terrane the present location of this block is unknown late carboniferous time marks the renewed onset of subduction initiating the andean plate tectonic setting still prevalent today recently proposed models explain the late ordovician orogeny by the collision of laurentia with western south america during laurentia s clockwise motion around south america and away from its position in the neoproterozoic supercontinent in its present form this hypothesis is difficult to reconcile with the paleozoic tectonostratigraphic evolution of the southern central andean region

the proto andean margin of gondwana an introduction a basic background is presented for the discussion of the early palaeozoic geology of western argentina covered by this book this includes the definition and terminology of orogenic cycles on this part of the gondwana margin represented by the eastern sierras pampeanas the pampean orogeny early cambrian relates to an intense but short lived period of terrane collision predating the rifting of the precordillera terrane from laurentia the famatinian cycle is predominantly represented by intense subductionrelated magmatism of early middle ordovician age developed on the continental margin of gondwana during the rifting and drifting of the precordillera terrane the grenvillian basement of the latter is further exemplified by a new rb sr whole rock isochron age of 1021 12 ma for orthogneisses from the western sierras pampeanas a mid ordovician granite in this area dated at 481 6 ma by u pb ion microprobe data may be related to rifting while the precordillera terrane was still attached to laurentia a divergence of opinion is pointed out between some authors in this book who favour mid ordovician collision of the precordillera with gondwana and others who place it much latter in silurian or devonian times

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

nucleation points of recent mainshocks in southern iceland mapped by b values the earthquake catalog in southern iceland between 20 25 w and 21 3 w is complete down to approximately mw 0 and shows no evidence for mag nitude shifts as a function of time since 1991 this means that 6902 earthquakes m 0 1 depth 20 km are available for studies of seismicity patterns preceding the two ms 6 6 mainshocks in 2000 in their source volumes and vicinity 20 25 w and 20 9 w the power law of the frequency magnitude relationship holds down to m0 detailed mapping including 3d mapping of the b value of the frequency magnitude distribution shows that variations from b 0 6 to 1 4 exist over distances of about 2 km this suggests that the tectonic fabric in southern iceland is hetero geneous on a small scale the hypocenters of the two ms 6 6 mainshocks and one additional m 4 5 mainshock are associated with low b values 0 6 0 8 statistically significantly different from volumes in their vicinity therefore one can expect future mainshocks in southern iceland to also emanate from volumes characterized by low b values however the small dimensions of these asperities may render it difficult to identify them before the mainshocks occur low values of estimated local recur rence times 500 2000 years map the two fault zones that ruptured in 2000 against a background of local recurrence times longer than 5000 years the first of the two m 6 6 mainshocks emanated from a volume of short estimated local recurrence time this supports the hypothesis that asperities with short local recurrence times control locations of major ruptures mapping of b values in cross sections shows anomalies of high b at the bottom of the seismogenic crust correlating with the change of its thickness in the middle of the study area these high b value anomalies and other pockets of such anomalies at shallower depths are best interpreted as caused by high pore pressure because evidence of fluids under high pressure is ubiquitous in this area

role of melt during deformation in the deep crust deformation in the deep crust is strongly influenced by the presence of melt injected melt or magma weakens the crust because strain will tend to localize where melt is present the amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate for plutons that intrude into rocks which are near the solidus temperature of the melt crystallization times can be quite long 1myr partial melting of deep crustal rocks can lead to melt enhanced embrittlement this occurs because the volume change for most melting reactions is positive therefore when the rate of melt production outpaces the rate at which melt can leave the system the melt pressure increases eventually the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the central gneiss belt canada this suggests that melt enhanced embrittlement occurred in these rocks an expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived assuming that migmatite has a small tensile strength differential stresses are 20 mpa in migmatitic rocks at the time melt enhanced embrittlement occurs the occurrence of melt enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting furthermore repeated episodes of melt enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains

climate change in response to orographic barrier uplift paleosol and stable isotope evidence from the late neogene santa maria basin northwestern argentina the late cenozoic history of the santa maria basin in the northeastern sierras pampeanas records the far reaching climatic consequences of the development of orographic barriers to atlantic derived moisture the evolution of climate and ecosystems is coupled with the uplift of basement blocks west and east of the basin to their present elevations of 4000 5500 m the climatic and ecological history of the basin for the past 12 m y was reconstructed from paleosol characteristics and carbon and oxygen isotopes of pedogenic material a fluvial environment succeeded the final marine regression after ca 12 ma paleosols with illuvial clay and calcic nodules indicate a seasonally dry climate between ca 9 and 7 ma paleosols formed under conditions of episodic waterlogging as shown by organic matter preservation and variegated colors after ca 7 ma pedogenesis occurred under enhanced drainage due to tectonic steepening of the eastern basin margin this can be related to westward thrusting of the eastern basin bounding sierra aconquija and cumbres calchaquies calcic and silicic rhizoliths and authigenic clay point to persistent wet dry seasonality carbon isotope values document the spread of c 4 plants between ca 5 and 3 ma carbon isotopes show an increase in c 3 plants this is likely a consequence of uplift of the sierra de quilmes to the west resulting in a transitory episode of orographic precipitation within the santa maria basin aridification of the valley occurred between ca 3 and 2 5 ma and was related to creation of high topography to the east development of thick calcretes suggests that the present semiarid conditions were established by the end of pliocene time carbon isotopes show that the quaternary vegetation was c 4 plant dominated and oxygen isotopes attest to intense soil water evaporation

spatiotemporal variations in the b value of earthquake magnitude frequency distributions classification and causes abstract interpretation of the b value of earthquake frequency magnitude distributions has received considerable attention in recent decades this paper provides a comprehensive review of previous investigations of spatial and temporal variations in b value including their classification and possible causes based on least squares regression of seismicity data compiled from the neic iris and isc catalogs we find an average value of 1 02 0 03 for the whole earth and its two hemispheres consistent with the general view that in seismically active regions the long term average value is close to unity nevertheless wide ranging b variations 0 3 b 2 5 have been reported in the literature this variability has been interpreted to arise from one or more of the following factors prevailing stress state crustal heterogeneity focal depth pore pressure geothermal gradient tectonic setting petrological environmental geophysical characteristics clustering of events incomplete catalog data and or method of calculation excluding the latter all of these factors appear to be linked directly or indirectly with the effective state of stress although time dependent changes in b value are well documented conflicting observations reveal either a precursory increase or decrease in b value before major earthquakes our compilation of published analyses suggests that statistically significant b variations occur globally on various timescales including annual monthly and perhaps diurnal taken together our review suggests that b variations are most plausibly linked with changes in effective stress

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

seismic gaps and recurrence periods of large earthquakes along the mexican subduction zone a reexamination abstract seismic gaps and recurrence periods of large shallow interplate earthquakes along the mexican subduction zone are reexamined after combining information from a catalog of nineteenth century9s earthquakes some relocated epicenters of the early part of this century source parameters of recent large earthquakes and redetermined magnitudes of great shallow earthquakes of this century tehuantepec and michoacan gaps have not experienced a large shock in this century and perhaps none in the past century they are either aseismic or have anomalously large repeat times guerrero jalisco and ometepec regions presently appear to have a high seismic potential observed average repeat times of large earthquakes m s 7 4 in six regions east central and west oaxaca san marcos petatlan and colima are between 32 to 56 yr data of this century indicate that the strain is released mostly in large events m s 7 4 a simple dislocation model with parameters obtained from the studies of recent earthquakes explains the observed recurrence periods quite well the b value for this zone is not meaningful an observation which is of significance for seismic risk estimation most of seismic moment or equivalently seismic energy release since 1800 appears to occur for 15 yr followed by relative quiescence in the next 15 yr

structural control on volcanoes and magma paths from local to orogen scale the central andes case abstract assessing the parameters that control the location and geometry of magma paths is of paramount importance for the comprehension of volcanic plumbing systems and geo hazards we analyse the distribution of 1518 monogenic and polygenic volcanoes of miocene quaternary age of the central volcanic zone of the andes chile bolivia argentina and reconstruct the magma paths at 315 edifices by analysing the morphostructural characteristics of craters and cones then we compare these data with outcropping dykes tectonic structures and state of stress most magma paths trend n s nw se and ne sw in decreasing order of frequency the n s and nw se paths coexist in the northern and southern part of the study area whereas n s paths dominate east of the salar de atacama outcropping dykes show the same trends the regional holocene stress state is given by an e w greatest horizontal principal stress n s and nne ssw reverse faults and folds affect deposits of 4 8 3 2 and 1 3 ma bp especially in the central and southern study areas a few nw se left lateral strike slip faults are present in the interior of the volcanic arc part of which belong to the calama olacapato el toro fault the volcanic chain is also affected by several n s and nw se striking normal faults that offset pliocene and quaternary deposits the results indicate different scenarios of magma tectonic interaction given by n s normal and reverse faults and n s fold hinges that guide volcano emplacement and magma paths magma paths are also guided by strike slip and normal nw se faults especially in the northern part of the study area zones with verticalized strata with bedding striking ne sw also acted as preferential magma paths these data suggest that at convergence zones with continental crust shallow magma paths can be more sensitive to the presence and geometry of upper crustal weakness zones than to the regional state of stress

an improved technique for reduction to the pole at low latitudes reduction to the pole at low latitudes based on a wiener filtering approach has been improved by introducing a deterministic noise model it is assumed that the noise power is a fixed fraction of the signal power this allows the method to be fully automated further improvement is obtained by requiring the reduced to the pole field to map into the observed field when projected to the geomagnetic latitude of the observed field this is done by iteratively minimizing the difference between the measured field and the reduced to the pole field projected to the geomagnetic latitude of the measured data this results in a reduced to the pole magnetic map that when projected to the geomagnetic latitude of the given data closely matches the measured data the final reduced to the pole field does not show any of the artifacts typical of reduction to the pole at low geomagnetic latitudes the method is demonstrated on a data set from an aeromagnetic survey flown over north central burkina faso west africa

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

earthquake statistics at parkfield 1 stationarity of b values 1 in this paper paper 1 we quantitatively show that the heterogeneous pattern of b values of the gutenberg richter relation in the parkfield segment of the san andreas fault is to a high degree stationary for the past 35 years this prepares the grounds for paper 2 where we test the hypothesis that our model of spatially varying b values forecasts future seismicity more accurately than the approach in which one assumes a constant b value equal to the average regional value the method we develop to measure stationarity in the presence of spatial heterogeneity consists of the following steps 1 determine the optimal dimensions of the sampling volume by mapping b values with a wide range of radii and selecting the largest radius that gives the most detailed resolution of the b value heterogeneity along the selected fault segment the high data density permits the definition of the dominant dimensions of the seismotectonic fabric which is about 8 10 km 2 map the difference in b value between two periods selecting numerous possible catalog divisions 3 identify significant changes of b values by the utsu test utsu 1992 along the studied fault segment of 110 km length only one patch of radius 5 km showed a significant increase in b from below average to above as a function of time this change in b initiates around 1993 and thus correlates in space and time with a well documented episode of creep at depth using the derived spatial variable b value distributions we find that the highest probability for earthquakes with magnitude m 6 is in the middle mountain asperity where the 1966 parkfield earthquake nucleated and where all m 4 5 events in the data set occurred in contrast if only the regional average b value of 0 92 is used to predict future seismicity the creeping segment north of parkfield should produce major earthquakes most frequently a conclusion that contradicts the observations

space time clustering of large thrust earthquakes along the mexican subduction zone an evidence of source stress interaction the spatiotemporal plot of epicenters of large m s 6 9 subduction earthquakes in mexico 1900 to present day suggests that these earthquakes cluster in space and time in this work we test the hypothesis that the coseismic stress transfer may lead to this clustering for the analysis we estimate the spatial extent of the coseismic coulomb stress change for these large events and then perform a statistical analysis using the 2 goodness of fit test for the interevent time intervals we find that there are at least two groups of time intervals where the observed frequencies are much higher than that expected from a poisson model indicating a bimodal pattern for the first mode the observed frequencies for the 0 to 5 year interval becomes about 2 1 times the expectation with a probability of occurrence of about 30 these results show that large thrust mexican earthquakes between 1900 and 2003 are clustered in space and time probably due to stress interactions among them the second mode includes the time interval of 30 50 years in the interval of 30 40 years the observed frequencies become about 1 7 times the expectation and about 1 2 times the expectation for the 40 to 50 year interval this second mode could be associated with a reloading interval of tectonic stress due to the plate convergence and appears consistent with the long term recurrence periods of large thrust earthquakes in the mexican subduction zone

geological development and mineralization in the atacama segment of the south american andes northern chile 26 15 27 25 s between the late jurassic and the middle miocene widespread magmatism tectonic events and hydrothermal mineralization characterized the geological evolution of the atacama segment of the south american andes a characteristic feature of this zone is the coincidence in time and space between subduction generated igneous activity crustal deformation and mineralization in the magmatic arcs which formed longitudinal belts migrating eastward

damage formation associated with bending under a constant moment abstract deformation within the earth s lithosphere is largely controlled by the rheology of the rock fracture and faulting are characterized by elastic rheologies with brittle mechanisms while folding and flow are characterized by plastic and or viscous rheologies due to ductile mechanisms however it has been recognized that deformation that resembles ductile behavior can be produced within the confines of the brittle lithosphere specific examples are folds that form in the shallow crust steep hinges at subduction zones that are accompanied by seismicity and large scale deformation at plate boundaries in these cases the brittle lithosphere behaves elastically with fracture and faulting yet produces ductile behavior in this paper we attempt to simulate such ductile behavior in elastic materials using continuum damage mechanics engineers utilize damage mechanics to model the continuum deformation of brittle materials we utilize a modified form of damage mechanics that represents a reduction in frictional strength of preexisting fractures and faults we use this empirical approach to simulate the bending of the lithosphere under the application of a constant moment we use numerical simulations to obtain elastostatic solutions for plate bending and where the longitudinal stress at a particular node exceeds a yield stress we apply damage to reduce young s modulus at the node damage is calculated at each time step by a power law relationship of the ratio of the yield stress to the longitudinal stress and the yield strain to the longitudinal strain this results in the relaxation of the material due to increasing damage to test our method we apply our damage rheology to an infinite plate deforming under a constant bending moment we simulate a wide range of behaviors from slow relaxation to instantaneous failure over timescales that span six orders of magnitude using this method stress relaxation produces elastic perfectly plastic behavior in cases where failure does not occur for cases of failure we observe a rapid increase in damage leading to failure analogous to the acceleration of microcrack formation and acoustic emissions prior to failure the changes in the rate of damage accumulation in failure cases are similar to the changes in b values of acoustic emissions observed in triaxial compression tests of fractured rock and b value changes prior to some large earthquakes thus continuum damage mechanics can simulate the phenomenon of ductile behavior due to brittle mechanisms as well as observations of laboratory experiments and seismicity

correlating variations of b values and crustal deformations during the 1990s may have pinpointed the rupture initiation of the mw 7 4 izmit earthquake of 1999 august 17 summary the b value increases from 1 1 along straight segments to 1 7 along a releasing bend of the north anatolian fault system between 30 1 e and 30 8 e the tensional nature of the bend is corroborated by extension rates of 0 3 strain yr 1 as obtained from global positioning system gps data fault plane solutions in this area change from abundant right lateral strike slip to oblique slip with normal components in the central part of the bend compatible with the formation of a pull apart basin we suggest that this crustal volume is highly fractured corresponding to a short mean crack length due to the superposition of horizontal and normal faulting as a consequence of low normal stress on the existing cracks frictional sliding is assumed to be the predominant mechanism for the generation of seismic events resulting in high b values the lowest b values b 0 8 in the area investigated are found at the junction between the fault bend and the adjoining segment which runs through the epicentre of the 1999 izmit earthquake at the junction a localized stress concentration is expected from numerical models of seismicity along geometrical barriers thus the site of lowest b has been considered to be the most likely place for a major earthquake a conclusion that is confirmed by the izmit earthquake with epicentre located about 13 km from the anticipated site in early 1992 the spatially averaged b value started to increase along the central and northwestern parts of the fault bend and there was a corresponding period of intensified horizontal extension since no large earthquake was recorded in 1992 it is assumed that the anomalous extension and the associated change in stress have been caused by aseismic fault creep in view of the available seismotectonic information surface displacements due to a slow dislocation at depth were calculated the gps data are reasonably explained by a source of normal fault type at a depth of 10 km and equivalent magnitude mw 5 9 stress changes at seismogenic depths are calculated under the assumption that an increase in b is most likely to be observed along those segments where both deviatoric and mean stress are reduced while the coulomb stress change is positive that is the fault is brought closer to failure for right lateral strike slip the stress condition is fulfilled at the sites of the largest b value increase the surface rupture of the izmit earthquake is associated with known fault traces only in the west and in the east of the study area along the fault bend it detaches itself from mapped fault traces and runs through the regions of positive coulomb stress change caused by the inferred 1992 slow event maximum surface slip 5 m is observed in the immediate vicinity of sites of maximum coulomb stress change 3 bar indicating that the 1992 event could have partially unclamped the rupture plane of the subsequent izmit earthquake we propose that the strong differences in b observed along the complex fault bend can be used to characterize different states of tectonic deformation in space and time

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

complements to magnitudes of large shallow earthquakes from 1904 to 1980 abstract in the previous paper magnitudes of large earthquakes of the world were determined on a very uniform basis and catalogued in order to arrive at a more homogeneous catalogue for a study of global seismicity some complements are presented the major complements are as follows 1 the m s values for the period 1904 1909 in the previous paper were overestimated in the extreme case by 0 5 magnitude units 2 the m b values for the period 1904 1908 in the previous paper were also overestimated 3 the magnitudes of deep and intermediate shocks which are listed in seismicity of the earth by gutenberg and richter are found to be approximated by m b m b 7 4 4 the catalogues of m b for shallow intermediate and deep shocks are continued for the period up to 1980

the relations between faulting and mafic magmatism in the altiplano puna plateau central andes mafic lava flows and scoria cone on the altiplano puna plateau correlate spatially and temporalily with late pliocene quternary strike slip faulting thrust faulting dominated deformation in the plateau and adjacent foreland during the miocene pliocene the relations between faulting and mafic magmatism may be explained by a change from vertical to horizontal least principal compression during the pliocene quaternary a model in which mantle derived mafic magma ascends through the thick less dense continental crust of the altiplano puna plateau suggests that nonlithostatic stress in the crust may have controlled the depth and geometry of intrusion horizontal nonlithostatic compression favored the formation of horizontal mafic intrusions in the lower crust which may have thermally weakened the crust localized crustal thickening and caused crustal anatexis horizontal nonlithostatic tension favored vertical intrusions in the crust and surface eruption

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

universality of the seismic moment frequency relation we analyze the seismic moment frequency relation in various depth ranges and for different seismic regions using flinn engdahl s regionalization of global seismicity three earthquake lists of centroid moment tensor data have been used the harvard catalog the usgs catalog and the huang et al 1997 catalog of deep earthquakes the results confirm the universality of the values and the maximum moment for shallow earthquakes in continental regions as well as at and near continental boundaries moreover we show that although fluctuations in earthquake size distribution increase with depth the values for earthquakes in the depth range of 0 500 km exhibit no statistically significant regional variations the regional variations are significant only for deep events near the 660 km boundary for declustered shallow earthquake catalogs and deeper events we show that the worldwide values have the same value of 0 60 0 02 this finding suggests that the value is a universal constant we investigate the statistical correlations between the numbers of seismic events in different depth ranges and the correlation of the tectonic deformation rate and seismic activity the number of earthquakes above a certain threshold level per year the high level of these correlations suggests that seismic activity indicates tectonic deformation rate in subduction zones combined with the universality of the value this finding implies little if any variation in maximum earthquake seismic moment among various subduction zones if we assume that earthquakes of maximum size are similar in different depth ranges and the seismic efficiency coefficient is close to 100 for shallow seismicity then we can estimate for deeper earthquakes for intermediate earthquakes 5 and 1 for deep events these results may lead to new theoretical understanding of the earthquake process and better estimates of seismic hazard

granite intrusion along strike slip zones in experiment and nature granite intrusion in the upper crust along a crustal scale shear zone has been modelled by injecting a newtonian fluid low viscosity silicone putty into a sandpack containing a ductile layer of silicone putty which acted as a weak level along which the injected material could spread the strike slip regime was obtained using two mobile rigid basal plates sliding horizontally boundary conditions were chosen in order to analyse the influence of thicknesses of the brittle cover and of the volume of injected material on the pattern of the intrusion experiments showed that 1 the presence of a soft layer between two competent units allowed the formation of laccolithic intrusions by lateral expansion in the emplacement site 2 intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike slip regime 3 this direction controls the emplacement beginning with the first stages of injection 4 the lateral expansion of the intrusion is locally controlled by arrays of riedel faults formed in the overburden 5 the control of faults on the intrusion pattern increases with increasing brittle ductile ratio i e with increasing depth and or decreasing thickness of the weak layer 6 intrusions are sigmoidal or lozenge shaped in horizontal view 7 the greater the brittle ductile ratio the less sigmoidal are the intrusions 8 feeding pipes shifted with respect to the strike slip zone result in asymmetric intrusions with the development of a sheared tail trailing behind the intrusion examples of leucogranites of the south armorican shear zone south brittany france emphasize that our experiments can explain the geometry of many syntectonic granites emplaced along strike slip zones they further shed some light on mechanisms of pluton intrusion in the upper crust

the major asperities of the 1999 mw 7 4 izmit earthquake defined by the microseismicity of the two decades before it

mapping the frequency magnitude distribution in asperities an improved technique to calculate recurrence times we hypothesize that highly stressed asperities may be defined by mapping anomalously low b values along the san andreas fault near parkfield the asperity under middle mountain with its b 0 46 can be distinguished from all other parts of the fault surface likewise along the calaveras fault the northern asperity of the morgan hill 1984 m6 2 rupture can be identified by its low b of 0 5 as a high stress patch along the fault we add further evidence to the observations that the b value of the frequency magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the parkfield segment of the san andreas and along the calaveras fault in both of these areas b values above and below 5 km depth are 1 2 and 0 8 respectively we propose that probabilistic recurrence times tr based on the seismicity parameters a and b should be calculated from their values within asperities only instead of from the values of the entire rupture area of the maximum expected earthquake the strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone which slips along passively when an asperity breaks therefore no information on tr is contained in the passive fault segments only in the asperities at parkfield the probabilistic estimates of tr derived from the data in the whole rupture and in the asperity only are 72 18 24 and 23 12 18 years respectively compared to the historically observed repeat time of 22 years at morgan hill the tr estimates are 122 46 76 and 78 47 110 years respectively compared to the observed repeat time of 72 years

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

imaging b value depth variations within the cocos and rivera plates at the mexican subduction zone abstract by a systematic mapping of the b value along profiles perpendicular to the mexican wadati benioff zone we obtained important characteristics pertaining the stress state and faulting style related to the subduction process to this purpose we used data from the earthquake catalog reported by the servicio sismologico nacional 1988 2016 we investigate depth variations of the b value for the cocos and rivera under north american plates interface by a detailed analysis of 15 cross sections the obtained b value profiles vary from 0 50 to 2 50 which nevertheless appear related to the faulting style and stress state by comparing the locations and focal mechanism of the largest events with the b values of the surrounding regions our analysis corroborates the dependence of the b value on the faulting style thrust events occur in regions of low and high b value at depths 30 km in agreement with global studies these results support the hypothesis that differential stress processes may be behind the occurrence of the different faulting style on the contrary by analyzing the mean b values for both types of faulting mechanism at each of the cross sections we found a significantly lower mean b value related to normal faulting for those regions where the 8 mw 8 2 and 19 mw 7 1 september 2017 earthquakes occur these results lead us to conclude that those regions experienced an increased stress state prone to the occurrence of normal intraplate events we also compare the b value distribution with vp and q tomography studies obtaining a good correlation between them we found evidence to relate b value variations with subduction processes such as stress state due to tectonic and flexural conditions and to a lesser extent to material heterogeneity and fluid dehydration

giant evaporite belts of the neogene central andes large volumes of continental evaporites accumulated within the central andes during neogene uplift of the altiplano puna plateau and development of the andean volcanic arc halite and gypsum are dominant minerals along with local and economically important borates playa conditions have existed since ca 15 ma halite and borate deposition has occurred for the past 7 to 8 m y evaporites formed in salar environments e g playa lakes and are characterized by complex mineral assemblages occurrence zonation and geochemistry evaporite deposition was controlled by volcanism geothermal activity closed drainage and climate these andean deposits and their controls differ from evaporites in other continental and marine environments

most likely locations of large earthquakes in the kanto and tokai areas japan based on the local recurrence times abstract we examined the hypothesis that minima in local recurrence time tl or equivalently maxima in local probability pl may map asperities in the kanto and tokai areas of japan where the earthquake catalog of the national research institute for earth science and disaster prevention nied is complete at the m 1 5 m1 5 level we mapped tl pl based on the a and b values of the nearest earthquakes within 20 km of every node of a grid spaced 0 01 for m7 target events only earthquakes within the top 33 km were used the b values increase strongly with depth in several areas therefore some of the tl pl anomalies are not revealed if data from the entire crustal seismogenic zone are mixed thus we mapped tl pl separately for the top 15 km and the rest of the depth range as well as for the entire seismogenic crust the resulting tl and pl maps show that approximately 12 of the total area shows anomalously short recurrence times out of six shallow target events with m 6 5 and which occurred since 1890 five are located within the anomalous areas with tl

neogene shortening contribution to crustal thickening in the back arc of the central andes to illustrate the neogene shortening distribution in the back arc units of the central andes and to estimate the contribution of the shortening to crustal thickening two balanced crustal cross sections have been constructed across the northern and southern branches of the bolivian orocline total neogene shortening which varies from 191 to 231 km is accommodated by a crustal duplex below the cordillera oriental but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the altiplano the best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin this process probably caused the altiplano uplift the subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the altiplano constituted the driving forces that produced neogene shortening and development of the central andes

fracture controlled magma conduits in an obliquely convergent continental magmatic arc magnetic fabric patterns of two mid cretaceous nested plutons 102 1 and 96 3 ma in the central sierra nevada batholith provide evidence that felsic magma emplacement and ascent occurred via north trending steeply dipping planar fracture conduits oriented obliquely to the arc u pb geochronology data indicate that emplacement of the plutons was separated by 2 to 10 m y and that they were emplaced in part via the same conduit magnetic fabrics in the younger pluton are related to its final emplacement which was strongly influenced by a system of host rock joints formation of the north trending conduits at 100 ma can be related to a small arc parallel dextral shear component that produced tension fractures and that was associated with high angle oblique convergence by 90 ma convergence had become sufficiently oblique for the development of nearly arc parallel structures which were then favored as magma conduits

tectonic development southeastern border of the puna plateau northwestern argentine andes the puna altiplano a 500 000 km2 area at an average elevation of 3 7 km in the central andes occurs in a non collisional plate setting over a 30 east dipping segment of the benioff zone and is bounded to the north and south by nearly flat segments of the subducted nazca plate a 120 km long portion of the southeastern boundary of the puna in northwestern argentina was studied to gain insight into the structures associated with uplift of the plateau and into the nature of structures over the transition from flat to steeper subduction the structural geometry and kinematic evolution of the region was determined from geologic mapping and kinematic analysis of minor fault arrays the present southeastern margin of the puna was uplifted between 5 10 and 2 ma northwest southeast horizontal shortening and vertical extension during uplift were produced by northeast striking en echelon thrust faults dipping 17 55 northwest four to seven en echelon structural blocks each 20 30 km long constitute the border of the puna their structural geometry precludes large scale overthrusting as an important mechanism of plateau uplift along the studied segment in the past 2 m y there has been a 45 change in the direction of shortening and locally a 90 change in the extension direction indicating that the structures accompanying uplift are no longer active this change could be due to a change either in the geometry of the subducted plate or in convergence directions although neither of these hypotheses is testable given presently available data

mapping asperities by minima of local recurrence time san jacinto elsinore fault zones minima of the local recurrence time tl m are correlated with the locations of historical main shocks in the san jacinto sj elsinore e fault zones of southern california we define tl m as the recurrence time calculated from the local b and a values of the frequency magnitude distribution earthquakes within 6 and 10 km from each point in the sj and e fault zones respectively we propose that anomalously low tl m values map asperities out of six historic main shocks 1899 1968 five ruptured substantial parts of four asperities mapped by our method which uses solely the modern earthquake catalog m 1 2 1981 0 1998 8 we estimate the probability for this correlation to occur at random as 10 2 to 10 3 depending on the scenario chosen it appears that the minima in tl m are tied to minima in b values more than to maxima in a values thus locations of low b values can also be used as an indication where asperities may be located and where main shocks are most likely the anza gap a segment of the san jacinto fault with low a values is mapped as an asperity by our method the heterogeneity in b values is strong throughout the study area the b values range from 0 5 to 1 8 over distances of 10 km volumes with approximately constant b values have radii of r 8 6 km our results suggest that detailed mapping of b values and local recurrence times in volumes with a few kilometers radius reveals important information about the local seismic hazard hitherto untapped by conventional seismicity analyses