radiocarbon dates on deglaciation cordillera central northern peruvian andes abstract three radiocarbon dates along with relative dating criteria place limits on the deglaciation history of manachaque valley cordillera central ice retreated from the late glacial maximum by at least 12 100 yr b p during ice retreat numerous moraines were deposited throughout the valley glacier cover was reduced to about half that of the last glacial maximum by at least 9700 yr b p and to less than a tenth by at least 6450 yr b p because all dates are minimum the dates and field data are consistent with little or no ice remaining by early holocene no unambiguous younger dryas moraines are present

structural analysis and interpretation of the surface deformations of the el asnam earthquake of october 10 1980 the el asnam earthquake of october 10 1980 represents the major seismotectonic event of these last decades in the west mediterranean area it has induced a large amount of surface breaks this earthquake reveals the importance of compressional phenomena that characterize nowadays the tectonics of north africa the tectonic analysis of surface rupture shows the complexity of the deformation mechanism the principal mechanism consists of the activation of a nw se trending thrust fault accompanied with a left lateral motion and with an intense deformation of the northwestern overthrusting block an accurate description of these deformations shows that for a large part the breaks geometry depends closely on the substratum structure finally the analysis of the dislocations on irrigation ducts just above the fault traces allowed a quantitative study of the deformation

magnitude scale and quantification of earthquakes despite various shortcomings the earthquake magnitude scale is one of the most fundamental earthquake source parameters to be used for catalogs although use of a uniform scale is desirable it is not always possible because of changes in instrumentation the data reduction method and the magnitude formula the station distribution etc as a result various magnitude scales have been developed and are currently in use recent developments in seismometry and earthquake source theories provide more quantitative source parameters than the magnitude in order to maintain continuity and uniformity of the data it is important to relate these magnitude scales and the new parameters in view of this importance relations between different magnitude scales are examined with an emphasis on the difference in the period of the waves used for the magnitude determination use of several magnitude scales determined at different periods provides a convenient method for characterizing earthquakes the moment magnitude can be used to quantify both shallow and deep earthquakes on the basis of wave energy radiated and provides a uniform scheme

change from late tertiary compression to quaternary extension in southern tibet during the india asia collision field studies in southern tibet show that subsequent to folding resulting from the india asia collision 50 40 ma reverse faults and backthrusts formed as a result of nne ssw and nnw sse trending compressions they affect conglomerates of oligocene miocene age and are probably contemporaneous with shearing on the himalayan main central thrust and folding of paleogene basins along the northern border of southern tibet 25 16 ma rift systems postdate compressional structures they were active during the pleistocene and probably during the pliocene 5 2 ma change from a compressional to an extensional regime in tibet probably occurred during the late miocene 10 5 ma this change is tentatively correlated with uplift inferred by chinese workers from paleogene to present day variations of the tibetan vegetation recent and active faulting in the high plateau results from a wnw ese striking extension this is in agreement with tibetan focal mechanisms the himalayan lowlands are subjected to a nne ssw trending compression as demonstrated by superficial deformations and seismicity therefore the horizontal maximum compressive stress trajectory is roughly parallel to india asia convergence hmax being 1 in the lowlands and 2 in the high plateau this change results from the increase of the vertical principal stress value due to the weight of the elevated mass zz is 3 in the lowlands and 1 in the high plateau thus the third principal stress axis is 2 in the lowlands and 3 in the high plateau allowing extension to occur in the wnw ese direction extensional tectonics are significantly different in southern and northern tibet southern tibet being constrained between the himalayan syntaxes the e w extensional principal deviatoric stress value 3 is of rather small magnitude and the n s trending 2 value is extensional as demonstrated by analysis of recent faults normal components prevail along all the azimuths in the field this favors formation of rift systems of large axial extension on the contrary north of the himalayan syntaxes northern tibet flows easily eastward the e w extensional principal deviatoric stress value 3 is of higher magnitude and the n s stress 2 is compressional as shown by inversion of focal mechanism data this favors strike slip components on faults which deviate from the n s direction as expressed in the field by shorter n s trending graben linked with normal strike slip faults these faults permit some n s shortening in northern tibet without thickening in response to india asia collision in high plateaus which are subjected to lateral compression such as the andean altiplano and tibet the extensional direction is basically controlled by the convergence direction being roughly orthogonal to the latter

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

quaternary normal and reverse faulting and the state of stress in the central andes of south peru field studies in the andes of southern peru show that in the high andes and pacific lowlands quaternary and recent faults are normal this extensional tectonics postdates compressional deformations of pliocene early quaternary age in the sub andes the observed deformations are compressional they affect early quaternary deposits some of the faults separate quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects striations on the fault planes indicate n s trending extension in the high andes and pacific lowlands the total amount of crustal stretching is small probably of the order of 1 during the last 1 2 m y in the sub andes folds and faults affecting neogene and early quaternary deposits result from n s shortening nevertheless it is supposed that this n s shortening is of early quaternary age the present day compression probably strikes e w judging from focal mechanisms in the sub andes of central peru southern bolivia and northwest argentina data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the andes of southern peru the high andes and pacific lowlands subjected to n s trending extension are bounded by two zones of e w trending compression the sub andes to the east and the contact between the convergent nazca and south america plates to the west in our model the maximum horizontal compressive stress trajectory hmax is roughly parallel with the e w convergence between the two plates hmax corresponds to 1 in the sub andes and to 2 in the high andes the latter situation is caused by the elevated mass of the high andes where zz the vertical stress is inferred to be 1 thus the third principal stress axis being orthogonal to the other two axes it is oriented n s allowing extension to occur in that direction on the other hand in the sub andes zz is 3 and horizontal e w shortening occurs the state of stress in the andean continental crust above the 30 dipping slab appears to be different from that in the andes of central peru situated above the flat subducting segment in this region compressional deformantion affect a wider part of the cordillera

the ancash peru earthquake of 1946 november 10 evidence for low angle normal faulting in the high andes of northern peru summary the ancash peru earthquake is the largest and best documented intracontinental normal fault earthquake to have occurred in historically recorded time along the andean margin of south america the fault geometry and the rupture processes of this earthquake provide information on how normal faulting develops in a predominantly compressional regime and on the hazards associated with normal faulting in other parts of the andes a re evaluation of first motion data intensity data and epicentre locations coupled with waveform modelling suggests that the earthquake occurred along a fault that dips 30 10 at a hypocentral depth of 15 17 km geological information also suggests that the earthquake reactivated an older thrust fault that was favourably orientated with respect to the present regional stress regime

a laboratory flume investigation of the formation of fossil stem fills the mechanism of formation of fossil plant pith casts has been investigated experimentally using a small laboratory flume tank the extent to which filling occurs is dependent on stem length and diameter and on the current velocity stem fills produced by sediment carried in suspension show a distinct structure of two wedges of sediment deposited at either end of the stem cavity these wedges are deposited from small flow vortices which form when the flow around the stem separates as the flow through the stem becomes restricted stems filled by bedload currents do not show such a uniform structure the experimentally produced stem fills are comparable with fossil pith casts e g catamites the formation of cortical casts of stigmaria is not simply explained in terms of the processes operating in open ended stem segments this investigation of the mechanics of filling of hollow plant organs and the structure of the fill produced offers a basis for interpreting the sedimentary environment in which the fossil fills have formed and leads to a fuller understanding of the mode of formation of such fossils

paleoseismicity and neotectonics of the cordillera blanca fault zone northern peruvian andes the cordillera bianca fault zone is a major west dipping normal fault that bounds the west side of a 120 to 170 km wide zone of active extension along the crest of the northern peruvian andes the fault is approximately 210 km long and exhibits continuous geomorphic evidence of repeated late pleistocene and holocene displacements but has not been the source of historical or teleseismically recorded earthquakes trenching and mapping of fault scarps provide new information on earthquake recurrence slip rate timing of the most recent events and andean neotectonics at quebrada queroccocha 55 km from the south end of the zone an 11 000 to 14 000 year old moraine is displaced 12 15 m and younger valley fill lacustrine and fluvial deposits are displaced 7 5 8 m scarp profiles tectonic terraces and trench exposures indicate five to seven scarp forming earthquakes of 2 3 m per event during the past 11 000 14 000 years at this location considering uncertainties in the number of events and in the age of the displaced moraine the average recurrence interval is estimated to be 2440 1060 years detrital charcoal from colluviums that predate and postdate the most recent event yielded ages of 2480 65 and 750 80 14c years b p respectively and stratigraphic relationships suggest that the event may be closer to the older date the late quaternary slip rate at quebrada queroccocha is 0 86 1 36 mm yr and appears to be representative of much of the fault zone at pachma bajo located 30 km from the north end of the zone on a separate segment displacement during each of the past two events was 2 to 3 m the morphology of scarps in alluvial fan and debris flow deposits indicates recurrence intervals of about 1000 3000 years relationships between pre inca walls and faulted deposits suggest that 1500 2000 years have elapsed since the most recent event fault behavior parameters particularly displacement per event and fault segment length show that the cordillera bianca fault zone is similar to seismogenic normal faults that nucleate m 7 7 earthquakes at depths of approximately 15 km the quebrada queroccocha slip rate and analysis of other normal faults in the region suggest cumulative late quaternary extensional strain rates of about 1 4 mm yr across the complete zone of andean crestal extension

evolution of lithospheric strength after thrusting modeling of an eroding thrust belt indicates that immediately after thrusting the lithosphere is about twice as strong as unthickened cratonic lithosphere but after 10 to 20 m y it becomes weaker than cratonic lithosphere reaching a minimum of 40 to 70 of cratonic strength 30 to 100 m y after thrusting this may explain why rifting of thrust belts commonly occurs several tens of million years after thrusting ceases relative weakening results from 1 elevation of the geotherm caused by burial of radioisotope enriched surface rocks and 2 replacement of strong mantle rocks by weak crustal rocks in the lithospheric column low denudation rates enhance weakening by allowing radioisotope enriched rocks to remain buried longer our modeling indicates that 1 magmatism after thrusting is favored by high concentrations of heat producing elements in the lower plate and by slow denudation rates and 2 factors that affect denudation rate such as climate and the development of external drainage can significantly influence evolution of a thrust belt

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

using hydrochemical and isotopic data to determine sources of recharge and groundwater evolution in an arid region a case study in the upper middle reaches of the shule river basin northwestern china understanding sources of recharge and mechanisms for hydrogeochemical evolution of groundwater in the shule river basin an arid inland river basin in northwestern china is essential for successful water resource management sources of water and associated groundwater recharge processes were investigated using hydrogeological hydrogeochemical and isotopic methods the study area was divided into three parts changma the shule valley and the beishan area the hydrogeochemical and isotopic analysis results show that groundwater in changma mainly originated from precipitation in the qilian mountains lateral flow from changma is the primary recharge mechanism while direct recharge by infiltration of precipitation can be neglected in the shule valley minerals within the aquifer material e g halite calcite dolomite and gypsum dissolve into water that recharges the groundwater system therefore strong linear relationships were found between na and cl and between ca2 mg2 and so42 hco3 with stoichiometry ratios of approximately 1 1 in both cases groundwater quality in the region is generally good with low salinity and hardness local precipitation is the only source of recharge for groundwater in the beishan area low groundwater velocity and dry climatic conditions suggest that dissolution of minerals from the aquifer material controls hydrogeochemical evolution water rarely travels from the beishan area to the shule valley therefore almost all the groundwater in the shule valley originates from changma global climate change and loss of glaciers in the qilian mountains may lead to a reduction of recharge therefore the groundwater resources of the shule valley must be managed to sustain water supply

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

geological constraints on the kinematics and fault plane solution of the quiches fault zone reactivated during the 10 november 1946 ancash earthquake northern peru the ancash earthquake of 10 november 1946 mw 6 8 produced surface faulting along the quiches normal slip fault field studies reported discontinuous nw se to nnw sse striking seismic scarps that displace quaternary glacial moraines and outwash as well as mesozoic sedimentary rocks structural analyses show that the 1946 normal displacement was accompanied by a small left lateral slip component morphological analysis of the fault scarp and observations in a trench excavated across the scarp show evidence of a single normal fault displacement before the 1946 seismic event that offset late pleistocene glacial moraines this indicates an approximate recurrence interval of more than 13 000 1 000 years for the major events suggesting a rate of net vertical displacement on the fault of less than 0 25 mm yr kinematic analysis along the quiches fault in comparison with other regional studies suggests that the state of stress in the high andes is regionally and locally homogeneous with only minor changes in the stress directions adjacent to major structures such as the maranon thrust and fold belt the minimum horizontal stress 3 axis is oriented n10 25 e the intermediate horizontal stress 2 axis is oriented n100 115 e and the maximum stress 1 axis is vertical we suggest that the 60 sw dipping quiches surface fault plane is connected at depth to a low angle dipping fault plane this seismic fault is a reactivated thrust fault of the tertiary maranon thrust and fold belt

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

pollen studies in the junin area central peruvian andes laguna junin is a large lake 675 km long but only 4 m deep located in the grass covered altiplano at an altitude of 4 100 m northeast of lima pollen analysis of a 30 m core indicates that from about 12 000 yr ago until some time after 3 000 yr ago about 30 of the pollen was carried in by easterly winds from the east andean forests which today are at least 600 m lower in elevation contemporaneous pollen deposition in lakes on moraines of the last glaciation west of the junin plain was dominated by local grassland types and had little contribution from this easterly source at some time after 3 000 yr ago perhaps about 1 200 yr ago the pollen influx to laguna junin from this distant forest source greatly diminished perhaps because of deforestation by increased human populations between about 12 000 and 24 000 yr ago in the junin core the sparse pollen concentration was dominated by grassland types supplemented by polylepis acaena alnus and compositae blown in from the sub puna shrublands that may have occupied much of the east andean slopes in place of the humid forest that occurs there today the silty sediment in this section of the core is interpreted as outwash from piedmont glaciers that invaded the plain especially from the cordillera to the west these glaciers left a distinctive pattern of moraines and outwash fans which served to dam tine lake basin an unconformity in the sediment covers the time from about 24 000 to 39 000 yr ago when the lake was probably dry before this time the silty sediment and the dominance of grassland and shrubland pollen types indicates an earlier interval of glaciation recorded on the landscape by an older set of moraines and outwash fans which initially dammed the lake basin

structural analysis of recent and active faults and regional state of stress in the epicentral area of the 1978 thessaloniki earthquakes northern greece during spring and summer 1978 macedonia greece suffered damaging seismic activity during the two main shocks which occurred on may 23 mb 5 7 and june 20 mb 6 1 seismic open cracks and faults were formed in the epicentral area 25 km nne from thessaloniki they have been mapped and their kinematics analyzed these studies have shown that they result from the reactivation of existing faults of pleistocene recent and historical age and that the crust is stretching along a n s to nne ssw direction in that part of the north aegean principal stress directions have been computed from the slip vectors measured on recent neotectonic faults and seismic cracks and faults this computation utilizing a simple mechanical model homogeneous deformation through the addition of small and independent displacements of rigid blocks in a highly fractured body a striking feature of this analysis is that the tensional directions 3 deduced from seismic faulting and from recent neotectonic faulting are nearly the same they are compatible with the regional tension axis deduced from focal mechanisms of north aegean and with the focal mechanisms of the 1978 thessaloniki main shocks they are also compatible with in situ stress measurements in this region studies of superficial seismic faulting strongly support previous assumptions proposed from aftershock distribution the 1978 thessaloniki earthquakes resulted from reactivation of several different faults situated within the same ne dipping fault zone of upper eocene lower oligocene age and older separating the serbo macedonian massif from the peonias east vardar zone

the evolution of the gulf of corinth greece an aftershock study of the 1981 earthquakes summary a preliminary study of the aftershocks of three earthquakes that occurred near to corinth greece in 1981 is combined with observations of the morphology and faulting to understand the evolution of the eastern gulf of corinth the well located aftershocks form a zone 60km long and 20km wide they do not lie on the main fault planes and are mostly located between the north dipping faulting on which the first two earthquakes occurred and the south dipping faulting associated with the third event a cluster of aftershocks also lies in the footwall of the eastern end of the south dipping fault of the third event morphologically it is observed that in the evolution of the eastern gulf of corinth antithetic faulting apparently predates the appearance of the main faulting at the surface this evolution can be explained by motion on a deep seated shallow angle aseismic normal fault a model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes

the state of stress in an overriding plate situated above a flat slab the andes of central peru in the andes of central peru that are situated above a flat dipping slab analysis of both structural data collected in the field and available focal mechanisms show that the quaternary and recent state of stress is characterized by n s trending extension in the western cordillera and along the pacific coast whereas e w trending compression prevails in the sub andean lowlands and at the contact between the nazca and south american plates the eastern cordillera deformations suggest an intermediate state of stress i e strike slip faulting characterized by e w trending compression and n s trending extension thus major fault zones striking nw se i e parallel to the central peruvian andes are normal with a sinistral component in the western cordillera e g cordillera bianca and reverse sinistral in the eastern cordillera e g cordillera de huaytapallana this state of stress displays similarities with that of southern peru and may be also interpreted as an effect of compensated high topography in this model the vertical stress zz increases with the topography and hmax is considered as fairly constant trending e w roughly parallel to the convergence on both edges of the andes tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then hmax is 2 allowing hmin striking n s to be 3 along the pacific coast tectonics is not compressional as expected and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep peru chile trench however a conspicuous difference occurs in the andes of central peru in respect to the southern peru andes compressional strike slip tectonics is observed in the eastern cordillera indeed the mean elevation of the eastern cordillera is 3 7 km whereas that of the western cordillera is 4 2 km allowing an intermediate state of stress to occur therefore in central peru a lower mean andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern peru superficial deformations related to the subduction of the aseismic nazca ridge are restricted to the coastal domain opposite to this ridge they support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting