the influence of pore pressure and confining pressure on dynamic elastic properties of berea sandstone compressional and shear wave velocities of watersaturated berea sandstone have been measured as functions of confining and pore pressures to 2 kbar the velocities measured by the pulse transmission technique were obtained at selected pressures for the purpose of evaluating the relative importance of confining pressure and pore pressure on elastic wave velocities and derived dynamic elastic constants changes in berea sandstone velocities resulting from changes in confining pressure are not exactly canceled by equivalent changes in pore pressure for properties that involve significant bulk compression compressional wave velocities and bulk modulus an incremental change in pore pressure does not entirely cancel a similar change in confining pressure on the other hand it is shown that a pore pressure increment more than cancels an equivalent change in confining pressure for properties that depend significantly on rigidity shear wave velocity and poisson s ratio this behavior as well as observed

possible evidence for subducted sedimentary materials beneath central california seismic reflection and refraction data from the coast ranges of central california west of the san andreas fault suggest the presence of a low velocity 4 5 5 0 km s wedge at a depth of about 15 22 km that dips east beneath rocks of both the franciscan assemblage and the salinian block we propose that the wedge represents sedimentary materials that were subducted in an offshore trench within the past 23 m y the refraction data also suggest the presence of a second low velocity zone that extends at least to mid crustal depths beneath the san andreas fault zone

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

anomalous seismic velocity drop in iron and biotite rich amphibolite to granulite facies transitional rocks from deccan volcanic covered 1993 killari earthquake region maharashtra india a case study 65 ma deccan volcanic province of western india forms one of the largest flood basaltic eruptions on the surface of the earth the nature of the concealed crust below this earthquake prone region which is marked by several low velocity zones at different depths has hardly been understood these low velocity zones have been invariably interpreted as fluid filled zones genetically connected to earthquake nucleation while carrying out detailed geological and petrophysical studies on the late archean basement cores obtained from a 617 m deep klr 1 borehole drilled in the epicentral zone of 1993 killari earthquake region of the southern deccan volcanic province we came across several instances where we observed remarkable drop in measured p wave velocity in a number of high density cores we provide detailed petrographic and geological data on 11 such anomalous samples which belong to mid crustal amphibolite to granulite facies transitional rocks they are associated with a mean p wave velocity of 6 02 km s range 5 82 6 22 km s conforming to granitic upper crust but in contrast have a high mean density of 2 91 g cm3 range 2 75 3 08 g cm3 which characterise mid to lower crust this velocity drop which is as much as 15 in some cores is primarily attributed to feot enrichment up to about 23 wt during the course of mantle fluid driven retrogressive metasomatic reactions caused by exhumation of deep seated mafic rocks presence of iron content mainly magnetite widely seen as opaques in thin sections of the rocks seems to have resulted into sharp increase in density as well as mean atomic weight our study indicates that the measured vp is inversely related to feot content as well as mean atomic weight of the rock

effects of change in slab geometry on the mantle flow and slab fabric in southern peru the effects of complex slab geometries on the surrounding mantle flow field are still poorly understood here we combine shear wave velocity structure with rayleigh wave phase anisotropy to examine these effects in southern peru where the slab changes its geometry from steep to flat to the south where the slab subducts steeply we find trench parallel anisotropy beneath the active volcanic arc that we attribute to the mantle wedge and or upper portions of the subducting plate farther north beneath the easternmost corner of the flat slab we observe a pronounced low velocity anomaly this anomaly is caused either by the presence of volatiles and or flux melting that could result from southward directed volatile rich subslab mantle flow or by increased temperature and or decompression melting due to small scale vertical flow we also find evidence for mantle flow through the tear north of the subducting nazca ridge finally we observe anisotropy patterns associated with the fast velocity anomalies that reveal along strike variations in the slab s internal deformation the change in slab geometry from steep to flat contorts the subducting plate south of the nazca ridge causing an alteration of the slab petrofabric in contrast the torn slab to the north still preserves the primary fossilized petrofabric first established shortly after plate formation

lithostratigraphie et datation du proterozoique de la boutonniere d ifni anti atlas maroc abstract in the ifni window in the southwestern part of the moroccan anti atlas unfolded volcanic rocks and sediments rest on a lower proterozoic basement their lithology and relations with dated magmatic rocks lead to a separation into three upper proterozoic groups the absence of the middle proterozoic could also be demonstrated as well as the progressive transition from uppermost proterozoic to lower cambrian the originality of the radiometric ages obtained in the bou azzer region is emphasized as well as the evolution of the pan african tectonics throughout the western anti atlas the intensity of which progressively decreases southwestward

seismic investigation of the continental margin off and onshore valparaiso chile abstract at the latitude of valparaiso chile a fundamental change in the configuration of the benioff zone volcanic activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge three legs of the german r v sonne cruises so101 so103 and so104 surveyed the continental margin and oceanic plate offshore valparaiso aiming at studying the crustal structure and investigating possible causes for the change in slab configuration sonne cruise so101 investigated the tectonic setting with swath mapping bathymetry magnetics and high resolution seismics following these investigations cruise so103 collected land sea wide angle seismic data and coincident deep seismic reflection data were acquired during cruise so104 coincident near vertical and wide angle seismic measurements were made along two profiles profile 1 located at the south of the study area away from the influence of the subducting ridge crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed profile 2 located in the north runs from the juan fernandez ridge to the chilean coast the crustal velocity models obtained for the two profiles show that the continental crust extends to the middle lower slope boundary which is also reflected in morphology in addition they show that the crustal structure of the oceanic plate is rather similar but the plate seems to be slightly more inclined along the northern profile 13 versus 10 in the south the two profiles are only about 70 km apart but their structures differ significantly no straightforward correlation exists between the two profiles that can be attributed to ridge collision the data support that the 1985 central chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate

plio quaternary vertical motions and the subduction of the nazca ridge central coast of peru abstract the coastal region of south central peru displays raised marine terraces indicating that unlike the central peruvian coast it underwent a strong uplift since the late pliocene these recent vertical movements which were probably the most rapid motion experienced along the pacific coast of south america are commonly considered to be closely related to the subduction of the aseismic nazca ridge however several questions regarding the geometry and the precise mechanism s responsible for this deformation have not yet been fully addressed the kinematic constraints of the andean convergence suggest that the deformation of the coastal area between pisco and lomas is characterized by a quite continuous sequence of uplift events progressing southwards in spite of some difficulties encountered in precisely dating the uplifted quaternary terraces and other remnants of pliocene marine platforms it is proposed that uplift rates during the late pleistocene reached maximum values of the order of 700 mm 10 3 y as against a recently proposed estimate of 470 mm 10 3 y the reconstructed finite deformation as recorded by the study area for the last 3 ma presents a nw se oriented asymmetric dome shaped pattern it is emphasized that the strongest uplift has been occurring immediately south of the inland projection of the nazca ridge and not along the ridge axis as envisioned by the previously constructed physical models

seismic evidence of mylonite reflectivity and deep structure in the kettle dome metamorphic core complex washington the first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the kettle dome metamorphic core complex northeastern washington the mylonite zone developed at mid crustal levels and has undergone a minimum of retrogression and cataclasis during uplift downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5 km depth as a complex zone of multicyclic reflections the seismic data also show that the crust beneath the kettle dome is characterized by abundant strongly reflective moderately dipping layers down to at least 18 km these layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both models for the structure and genesis of metamorphic core complexes must take these data into account

amplification of ground motion in fault and fracture zones observations from the tremestieri fault mt etna italy abstract a series of ambient noise measurements have been performed to evaluate their use in detecting the possible influence of tectonic structures on local amplification of seismic waves the area studied is located on the southeastern flank of mt etna a few kilometres north of the town of catania using a mobile station data were collected along three profiles which cross both a fault and an eruptive fracture in addition ambient noise was sampled at two permanent stations installed in close proximity to the fault and at another station located within the city limits of catania and considered to be a reference site analysis using spectral ratio techniques shows there is a tendency towards amplification at the sites located near the fault in h v spectral ratios such amplification is observed in the frequency range 4 0 8 0 hz and the amplitudes of the dominant spectral peaks decrease rapidly over the first few tens of meters from the discontinuity a similar tendency was observed for the sites located on the eruptive fracture zone dominant spectral peaks at 1 8 hz are also common in the study area the theoretical transfer function obtained through 1 d modelling indicates that such peaks can be related to the local stratigraphy the mean h h standard spectral ratio of microtremor evaluated at the permanent station located on the fault also shows an amplification peak at 4 0 hz azimuthal analysis shows that the resonance at 4 0 hz is directional with the maximum in the azimuth range n30 e n80 e

modeling the effect of san andreas fault structure on receiver functions using elastic 3d finite difference abstract teleseismic p wave data collected at the san andreas observatory sao seismic station show strong frequency dependent nonrectilinear particle motions consisting of off azimuth horizontal displacements that are sometimes larger than the vertical displacements the azimuthal dependence of the horizontal components amplitude is directly related to the symmetry of the fault zone a 3d finite difference program frankel and vidale 1992 is used to examine the response of the san andreas fault zone velocity structure to incident teleseismic p waves synthetic seismograms are formed using a 3d representation of an assumed 2d fault zone structure the fault zone velocity model produces large amplitude phases that contain combinations of refracted and internally reflected p to s converted phases diffractions and possible p to rayleigh converted phases the largest amplitude phases observed in the data are nondispersive and have complicated particle motions confined to the vertical plane perpendicular to the fault zone the data are fit best by synthetic receiver functions for field points similar to the location of sao relative to the san andreas fault zone since the velocity model used in the computations is nonunique sensitivity studies were performed by modifying the structure synthetics match the timing and nature of the observed phases at sao showing that assumption of plane layered structure is invalid in the area of this station

source characteristics of the 12 november 1996 m w 7 7 peru subduction zone earthquake the 12 november 1996 m w 7 7 peru subduction zone earthquake occurred off the coast of southern peru near the intersection of the south american trench and the highest topographical point of the subducting nazca ridge we model the broadband teleseismic p waveforms from stations in the global seismic network to constrain the source characteristics of this subduction zone earthquake we have analyzed the vertical component p waves for this earthquake to constrain the depth source complexity seismic moment and rupture characteristics the seismic moment determined from the nondiffracted p waves is 3 5 1020n m corresponding to a moment magnitude m w of 7 6 7 7 the source time function for the 1996 peru event has three pulses of seismic moment release with a total duration of approximately 45 50 seconds the largest moment release occurs at approximately 35 40 seconds and is located 90km southeast of the rupture initiation approximately 70 of the seismic moment was released in the third pulse

response of the mantle to flat slab evolution insights from local s splitting beneath peru the dynamics of flat subduction particularly the interaction between a flat slab and the overriding plate are poorly understood here we study the seismically anisotropic properties and deformational regime of the mantle directly above the peruvian flat slab we analyze shear wave splitting from 370 local s events at 49 stations across southern peru we find that the mantle above the flat slab appears to be anisotropic with modest average delay times 0 28 s that are consistent with 4 anisotropy in a 30 km thick mantle layer the most likely mechanism is the lattice preferred orientation of olivine which suggests that the observed splitting pattern preserves information about the mantle deformation we observe a pronounced change in anisotropy along strike with predominately trench parallel fast directions in the north and more variable orientations in the south which we attribute to the ongoing migration of the nazca ridge through the flat slab system

a method and example of seismically imaging near surface fault zones in geologically complex areas using vp vs and their ratios abstract the determination of near surface vadose zone and slightly below fault locations and geometries is important because assessment of ground rupture strong shaking geologic slip rates and rupture histories occurs at shallow depths however seismic imaging of fault zones at shallow depths can be difficult due to near surface complexities such as weathering groundwater saturation massive nonlayered rocks and vertically layered strata combined p and s wave seismic refraction tomography data can overcome many of the near surface fault zone seismic imaging problems because of differences in the responses of elastic bulk and shear moduli of p and s waves to shallow depth fault zone properties we show that high resolution refraction tomography images of p to s wave velocity ratios v p v s can reliably identify near surface faults we demonstrate this method using tomography images of the san andreas fault saf surface rupture zone associated with the 18 april 1906 m 7 9 san francisco earthquake on the san francisco peninsula in california there the saf cuts through franciscan melange which consists of an incoherent assemblage of greywacke chert greenstone and serpentinite a near vertical zone 75 northeast dip of high p wave velocities up to 3000 m s low s wave velocities 150 600 m s high v p v s ratios 4 8 8 and high poisson s ratios 0 44 0 49 characterizes the main surface rupture zone to a depth of about 20 m and is consistent with nearby trench observations we suggest that the combined v p v s imaging approach can reliably identify most near surface fault zones in locations where many other seismic methods cannot be applied

holocene history of the san andreas fault between crystal springs reservoir and san andreas dam san mateo county california abstract the geologic part of a seismic safety investigation of san andreas dam focused on a two mile reach of the fault between the dam and crystal springs reservoir historical documents and detailed mapping revealed that the 1906 faulting caused 7 to 9 ft of right shear in the left eastern abutment and occurred on nearly straight traces that experienced a maximum of 9 ft of slip 90 per cent of which was concentrated within a zone 100 ft wide radiocarbon dates on detrital charcoal from two sites of ponded alluvium established that the 1906 traces have been active for at least the last 3 000 yr deflected stream channels further indicate that the 1906 traces have been the locus of right slip for the last 6500 yr and perhaps for all of holocene time detailed geomorphic and stratigraphic analyses of one site showed that five 1906 magnitude slip events have occurred within the last 1130 160 radiocarbon years from which a minimum strain rate of 1 2 cm yr and an average recurrence frequence of 224 25 yr can be calculated trenches excavated across the 1906 traces of the san andreas fault revealed that the zone of active faulting is marked by a slow upwelling of highly sheared rock materials this apparent vertical movement of fault gouge when coupled with right shear and differential stream erosion has produced most of the distinctive landforms of the rift valley

fault damage zones of the m7 1 darfield and m6 3 christchurch earthquakes characterized by fault zone trapped waves abstract to characterize the subsurface structure of the damage zones caused by the 2010 2011 canterbury earthquake sequence in new zealand s south island we installed two short linear seismic arrays array 1 across the greendale fault gf surface rupture and array 2 over the surface projection of the blind port hills fault phf that ruptured in the 2010 m7 1 darfield and 2011 m6 3 christchurch earthquakes respectively we recorded 853 aftershocks for 4 months after the christchurch earthquake fault zone trapped waves fztws identified at array 1 for aftershocks occurring on both the gf and the phf show that the post s durations of these fztws increase as focal depths and epicentral distances from the array increase suggesting an effective low velocity waveguide formed by severely damaged rocks existing along the gf and phf at seismogenic depths locations of aftershocks generating prominent fztws delineates the subsurface gf rupture extending eastward as bifurcating blind fault segments an additional 5 8 km beyond the mapped 30 km surface rupture into a zone with comparably lower seismic moment release west of the phf rupture which extends westward to within 5 3 1 km of the subsurface gf the propagation of fztw through the intervening gap indicates moderate gf phf structural connectivity we interpret this zone as a fracture mesh reflecting the interplay between basement faults and stress aligned microcracks that enable the propagation of phf sourced fztws into the gf damage zone simulations of observed fztws suggest that the gf rupture zone is 200 250 m wide consistent with the surface deformation width velocities within the zone are reduced by 35 55 with the maximum reduction in the 100 m wide damage core zone corresponding with surface and shallow subsurface evidence for discrete fracturing the damage zone extends down to depths of 8 km or deeper consistent with hypocentral locations and geodetically derived fault models

seismic radiation from an sh line source in a laterally heterogeneous planar fault zone a solution to the class of problems in which an sh line source is operating in a medium consisting of two quarter spaces with an arbitrary number of semi infinite vertical layers in between is given in a frequency wavenumber domain the general result is applied to a model with a single vertical layer and synthetic seismograms are calculated for a source specified as an sh line dislocation at the interface between the vertical layer and the faster quarter space the numerical examples demonstrate that moderate fault normal material heterogeneity can have significant effects on the seismic response of a medium the overall distribution of travel times amplitudes and motion polarities in the presence of fault zone material heterogeneity is different from a corresponding distribution in a homogeneous half space the effects are most prominent near the fault zone where head waves surface waves and trapped modes result in waveform complexities large amplifications over an order of magnitude in the fault zone itself and first motion polarities which are reversed from homogeneous medium predictions the results imply that material heterogeneity should be included in fault zone models which are used for the interpretation of observed seismic data and theoretical calculations of near fault responses such models can be utilized to obtain important fault zone and earthquake source parameters

subsurface fault damage zone of the 2014 mw 6 0 south napa california earthquake viewed from fault zone trapped waves the aftershocks of the 24 august 2014 m w 6 0 south napa earthquake generated prominent fault zone trapped waves fztws that were recorded on two 1 9 km long seismic arrays deployed across the northern projection array 1 a1 and the southern part a2 of the surface rupture of the west napa fault zone wnfz we also observed fztws on an array a3 deployed across the intersection of the franklin and southampton faults which appear to be the southward continuations of the wnfz a1 a2 and a3 consisted of 20 20 and 10 l28 4 5 hz three component seismographs we analyzed waveforms of fztws from 55 aftershocks in both time and frequency to characterize the fault damage zone associated with this m w 6 0 earthquake post s coda durations of fztws increase with epicentral distances and focal depths from the recording arrays suggesting a low velocity waveguide along the wnfz to depths in excess of 5 7 km locations of the aftershocks showing fztws combined with 3d finite difference simulations suggest the subsurface rupture zone having an s wave speed reduction of 40 50 between a1 and a2 coincident with the 14 km long mapped surface rupture zone and at least an 500 m wide deformation zone the low velocity waveguide along the wnfz extends further southward to at least a3 but with a more moderate velocity reduction of 30 35 at ray depth this last fztw observation suggests continuity between the wnfz and franklin fault the waveguide effect may have localized and amplified ground shaking along the wnfz and the faults farther to the south see a companion paper by catchings et al 2016

cretaceous to early paleogene tectonic evolution of the northern central andes 0 18 s and its relations to geodynamics the tectonic phases subsidence patterns and location of the magmatic arc along the peruvian margin during late cretaceous and paleogene times are compared with the main geodynamic parameters absolute plate motion convergence velocity and direction age of the subducted slab and accretional events for the same periods during the late cretaceous long termed compression seems to be controlled by the absolute trenchward motion of the overriding plate and to a minor extent by the young age of the subducted lithosphere short lived contractional or extensional phases are mainly linked to the acceleration or deceleration of the convergence between the oceanic and continental plates and probably to changes in the convergence direction periods of high and low convergence rates coincide with increased and decreased subsidence rates along the margin respectively and seem to be independent of compressional tensional regimes during the paleogene in addition to the mentioned processes the eastward shift of the magmatic belt and subsidence in the forearc zone are interpreted as the result of the subduction erosion of the margin this process seems to be related to the decrease of the dip of the benioff zone expressed by the drastic widening of the magmatic arc

anisotropy and fracture zones about a geothermal well from p wave velocity profiles the feasibility of locating fracture zones and estimating their crack parameters was examined using an areal well shoot method centered on utah state geothermal well 9 1 beaver county utah high resolution traveltime measurements were made between a borehole sensor and an array of shotpoints distributed radially and azimuthally about the well the directional dependence of velocity in the vicinity of the well was investigated by comparing traveltimes from different azimuths velocity anisotropy was detected this condition is consistent with but not required by the existence of fractures in the basement rock having orientations subparallel to the latest episode of local faulting the interpretation is complicated by a variable thickness of overburden around the well traveltime delays also suggest the presence of a low angle fracture zone intersecting the well at a depth of 1 500 ft

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

a seismic refraction profile across the san andreas sargent and calaveras faults west central california in 1981 the united states geological survey recorded a seismic refraction profile across the southern santa cruz mountains in west central california to examine the shallow velocity structure of this seismogenic region this 40 km long profile which consisted of three shotpoints extended northeastward from near watsonville california to coyote lake crossing the san andreas sargent and calaveras faults this entire region is characterized by a highly heterogeneous upper crust west of watsonville 1 km of alluvium with a velocity of 2 12 km sec overlies a basement with a velocity 5 45 km sec the abrupt deepening of basement by 1 5 km just east of watsonville at a subsurface fault suggests that the zayante fault to the north and the vergeles fault to the south are connected the tertiary sediment at the san andreas fault is 2 5 km thick and has a velocity of 3 34 km sec the san andreas fault is not marked by any distinctive seismic velocity features whereas a 1 km wide low velocity zone is found at both the sargent and calaveras faults east of the sargent fault the basement surface forms a broad anticlinal structure with velocities ranging from 5 4 to 6 0 km sec from the anticlinal crest basement dips to the east beneath the santa clara valley and reaches a maximum depth of 1 km on the east side the overlying alluvium has a velocity of 2 7 km sec at the crest of the basement anticlinal structure a vertical low velocity zone coincident with exposed serpentine provides strong evidence of faulting

seismic reflectivity of mylonite zones in the crust deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages sonic velocities are consequently lower for propagation directions normal to mylonite foliation than in surrounding unmylonitized rocks synthetic seismograms computed for low velocity near horizontal thick mylonite zones of complex laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks provided noise levels are relatively low

a seismic refraction study of the santa clara valley and southern santa cruz mountains west central california abstract two seismic refraction profiles were recorded in the santa clara valley region of central california to determine the upper crustal seismic velocity structure a reversed 8 km long profile was recorded across the valley near gilroy and an unreversed 38 km long profile was recorded in the southern santa cruz mountains the data indicate that the valley is wedge shaped in cross section with the basement dipping 10 to the east the alluvial fill of the valley has a maximum thickness of 1 5 km and has an average compressional wave velocity of 2 3 km sec the top of the basement has a velocity of 4 3 km sec the velocity reaches 6 0 km sec at a depth of 2 km the calaveras fault zone located due east of the valley was crossed by a short unreversed profile it appears to be characterized by a low velocity v p 2 7 km sec to a depth of 2 5 km the southern santa cruz mountain profile indicates a 4 6 km sec near surface refractor and a layer with a velocity of 6 1 km sec at 2 km depth thus considered together the similarity in velocity structure determined from the two profiles indicates that the franciscan rocks of the santa cruz mountains continue beneath the valley the presence of high compressional wave velocities 6 0 km sec at 2 km depth in this franciscan terrain can best be explained by an abundance of metabasalts greenstones at that depth these seismic observations indicate that an area of at least 120 km 2 is underlain by this high velocity basaltic material therefore the upper crustal structure in the study area differs dramatically from the franciscan terrain of the diablo range where greywackies and metagreywackies v p 5 4 km sec dominate in the uppermost crust

effects of viscoelasticity on seismic wave propagation in fault zones near surface sediments and inclusions by defining anelastic effects through the superposition of relaxation mechanisms and incorporating them into the wave equation in terms of memory variables the time domain computation of responses of viscoelastic media with arbitrary spatial variations is feasible this approach is applied to simulation of the effects of realistic distribution of compressional and shear wave quality factors qp and qs in a sequence of viscoelastic models of increasing complexity from a homogeneous medium to a fault zone a near surface attenuating zone and inclusions with anomalous qp and qs values comparisons of elastic and viscoelastic responses for both surface and borehole recordings show pronounced q dependent attenuation analysis of data from viscoelastic media using elastic rather than a viscoelastic model may result in misleading or erroneous interpretations

evidence for strain rate variation and an elevated transient geothermal gradient during shear zone evolution in the cordillera blanca peru abstract the cordillera blanca shear zone cbsz occupies the footwall of the cordillera blanca detachment fault an active low moderate angle detachment fault extending for 200 km along the western margin of the cordillera blanca in central peru recrystallized quartz paleopiezometry two feldspar thermometry on asymmetric strain induced myrmekite and structural analysis from the cbsz provide evidence for shear zone evolution within a transient elevated geotherm during syn convergent extension shear zone width varies by a factor of six along strike with widths of up to 400 m within the central segment of the shear zone microstructurally derived strain rates calculated from quartz paleopiezometry and two feldspar thermometry results are on the order of 10 14 to 10 12 s 1 and are broadly consistent with macrostructurally derived strain rates of 10 14 to 10 13 s 1 calculated from shear zone thickness and previously determined exhumation rates within the context of existing thermochronological data constraints on batholith emplacement and previously determined offset along the cordillera blanca detachment our data suggest that ductile deformation in the cbsz occurred within a transient elevated geothermal gradient of at least 50 c km and potentially as high as 100 c km which deviated from a 30 c km gradient for an unspecified duration of time we discuss the evolution of the cordillera blanca where observed deformation fabrics are related to strain localization along an exhuming fault as well as thermal weakening from the interplay between faulting and magmatism

upper mantle anisotropy beneath peru from sks splitting constraints on flat slab dynamics and interaction with the nazca ridge a r t i c l e i n f o a b s t r a c t the peruvian flat slab is by far the largest region of flat subduction in the world today but aspects of its structure and dynamics remain poorly understood in particular questions remain over whether the relatively narrow nazca ridge subducting beneath southern peru provides dynamic support for the flat slab or it is just a passive feature we investigate the dynamics and interaction of the nazca ridge and the flat slab system by studying upper mantle seismic anisotropy across southern peru we analyze shear wave splitting of sks ssks and pks phases at 49 stations distributed across the area primarily from the peru lithosphere and slab experiment pulse we observe distinct spatial variations in anisotropic structure along strike most notably a sharp transition from coherent splitting in the north to pervasive null non split arrivals in the south with the transition coinciding with the northern limit of the nazca ridge for both anisotropic domains there is evidence for complex and multi layered anisotropy to the north of the ridge our ks splitting measurements likely reflect trench normal mantle flow beneath the flat slab this signal is then modified by shallower anisotropic layers most likely in the supra slab mantle but also potentially from within the slab to the south the sub slab mantle is similarly anisotropic with a trench oblique fast direction but widespread nulls appear to reflect dramatic heterogeneity in anisotropic structure above the flat slab overall the regional anisotropic structure and thus the pattern of deformation appears to be closely tied to the location of the nazca ridge which further suggests that the ridge plays a key role in the mantle dynamics of the peruvian flat slab system

displacement of the moho by the outer isles thrust shown by seismic modelling deep seismic data fig 1 from the caledonides off the north west coast of scotland collected 1981 82 by the british institutions reflection profiling syndicate birps show evidence that caledonian thrusts have been reactivated as mesozoic normal faults1 2 the best example is the outer isles oi thrust which is mapped on land as a compressional fault and is revealed by these data to be overlain offshore by a thick wedge shaped mesozoic rift basin1 known as the north lewis nl basin here we have used synthetic seismic sections to identify distortions of the oi thrust reflection due to the nl basin models which compensate for the effects of this distortion show that the oi thrust has a steeper angle of dip than has been predicted previously assuming no distortion causing the oi thrust to project into a 2 3 km offset of the moho and hence to penetrate the entire crust our data suggesting that shear strains affecting the lower crust may be localized in character have adverse implications for rheological models which predict to the contrary that the lower crust is a region where strain is distributed

seismic evidence for rock damage and healing on the san andreas fault associated with the 2004 m 6 0 parkfield earthquake we deployed a dense linear array of 45 seismometers across and along the san andreas fault near parkfield a week after the m 6 0 parkfield earthquake on 28 september 2004 to record fault zone seismic waves generated by aftershocks and explosions seismic stations and explosions were co sited with our previous exper iment conducted in 2002 the data from repeated shots detonated in the fall of 2002 and 3 months after the 2004 m 6 0 mainshock show 1 0 1 5 decreases in seismic wave velocity within an 200 m wide zone along the fault strike and smaller changes 0 2 0 5 beyond this zone most likely due to the coseismic damage of rocks during dynamic rupture in the 2004 m 6 0 earthquake the width of the damage zone characterized by larger velocity changes is consistent with the low velocity waveguide model on the san andreas fault near parkfield that we derived from fault zone trapped waves li et al 2004 the damage zone is not symmetric but extends farther on the southwest side of the main fault trace waveform cross correlations for repeated aftershocks in 21 clusters with a total of 130 events located at different depths and distances from the array site show 0 7 1 1 increases in s wave velocity within the fault zone in 3 months starting a week after the earthquake the velocity recovery indicates that the damaged rock has been healing and regaining the strength through rigidity recovery with time most likely due to the closure of cracks opened during the mainshock we estimate that the net decrease in seismic velocities within the fault zone was at least 2 5 caused by the 2004 m 6 0 parkfield earthquake the healing rate was largest in the earlier stage of the postmainshock healing process the magnitude of fault healing varies along the rupture zone being slightly larger for the healing beneath middle mountain correlating well with an area of large mapped slip the fault healing is most promi nent at depths above 7 km

continuity of the west napa franklin fault zone inferred from guided waves generated by earthquakes following the 24 august 2014 mw 6 0 south napa earthquake we measure peak ground velocities from fault zone guided waves fzgws generated by on fault earthquakes associated with the 24 august 2014 m w 6 0 south napa earthquake the data were recorded on three arrays deployed across north and south of the 2014 surface rupture the observed fzgws indicate that the west napa fault zone wnfz and the franklin fault ff are continuous in the subsurface for at least 75 km previously published potential field data indicate that the wnfz extends northward to the maacama fault mf and previous geologic mapping indicates that the ff extends southward to the calaveras fault cf this suggests a total length of at least 110 km for the wnfz ff because the wnfz ff appears contiguous with the mf and cf these faults apparently form a continuous calaveras franklin wnfz maacama cfwm fault that is second only in length 300 km to the san andreas fault in the san francisco bay area the long distances over which we observe fzgws coupled with their high amplitudes 2 10 times the s waves suggest that strong shaking from large earthquakes on any part of the cfwm fault may cause far field amplified fault zone shaking we interpret guided waves and seismicity cross sections to indicate multiple upper crustal splays of the wnfz ff including a northward extension of the southhampton fault which may cause strong shaking in the napa valley and the vallejo area based on travel times from each earthquake to each recording array we estimate average p s and guided wave velocities within the wnfz ff 4 8 5 7 2 2 3 2 and 1 1 2 8 km s respectively with fzgw velocities ranging from 58 to 93 of the average s wave velocities

seismic structure in southern peru evidence for a smooth contortion between flat and normal subduction of the nazca plate rapid changes in slab geometry are typically associated with fragmentation of the subducted plate however continuous curvature of the slab is also possible the transition from flat to normal subduction in southern peru is one such geometrical change the morphology of the subducted nazca plate along this transition is explored using intraslab earthquakes recorded by temporary regional seismic arrays observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest warping of the slab concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry the presence of a thin ultra slow velocity layer usl atop the horizontal nazca slab is identified and located the lateral extent of this usl is coincident with the margin of the projected linear continuation of the subducting nazca ridge implying a causal relationship wherein increased hydration of the ridge results in the formation of the usl downdip waveform modelling of the 2 d structure in southern peru using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the absence of any tears in the slab the seismicity and structural evidence suggests smooth contortion of the nazca plate along the transition from flat to normal subduction the slab is estimated to have experienced 10 per cent strain in the along strike direction across this transition

origin of the bolivian andean orocline a geologic study utilizing landsat and shuttle imaging radar abstract the origin of the major andean orocline in bolivia where the north trending southern andes abruptly change direction by 45 and continue northwestward into peru has been investigated by photogeologic techniques utilizing landsat imagery and shuttle imaging radar sir a in conjunction with published literature and maps the evidence suggests that this orocline might have been formed beginning in the miocene by the pacific coastal region being bent around the resistant brazilian shield as a result of the relative motions between the eastward moving coastal region and the westward moving south american continent the eastward motion of the coast was further increased about 10 m y b p when the opening of the east pacific rise increased the rate of eastward motion of the subducting nazca plate the brazilian shield with its deep roots provided greater resistance to this motion resulting in the decoupling of the andes along a wide left lateral strike slip fault zone extending east west from the altiplano through santa cruz and into the interior indicated left lateral motion across this fault decreases from 180 km on the east side of the andes to 50 or 60 km in the altiplano the difference being taken up in compressing the eastern cordillera and sub andean foredeep from an average width of 350 km in southern bolivia and argentina to an average width of 220 km in northern bolivia and peru the resistance of the shield may also have resulted in counter clockwise rotation of and lateral angular shear in the andes north of the break zone

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

spatial gaps in arc volcanism the effect of collision or subduction of oceanic plateaus abstract one of the major processes in the formation and deformation of continental lithosphere is the process of arc volcanism the plate tectonic theory predicts that a continuous chain of arc volcanoes lies parallel to any continuous subduction zone however the map pattern of active volcanoes shows at least 24 areas where there are major spatial gaps in the volcanic chains 200 km a significant proportion 30 of oceanic crust is subducted at these gaps all but three of these gaps coincide with the collision or subduction of a large aseismic plateau or ridge the idea that the collision of such features may have a major tectonic impact on the arc lithosphere including cessation of volcanism is not new however it is not clear how the collision or subduction of an oceanic plateau perturbs the system to the extent of inhibiting arc volcanism three main factors necessary for arc volcanism are 1 source materials for the volcanics either volatiles or melt from the subducting slab and or melt from the overlying asthenospheric wedge 2 a heat source either for the dehydration or the melting of the slab or the melting within the asthenosphere and 3 a favorable state of stress in the overlying lithosphere the absence of any one of these features may cause a volcanic gap to form there are several ways in which the collision or subduction of an oceanic plateau may affect arc volcanism the clearest and most common cases considered are those where the feature completely resists subduction causing local plate boundaries to reorganize this includes the formation of new plate bounding transform faults or a flip in subduction polarity in these cases subduction has slowed down or stopped and the lack of source material has created a volcanic gap there are a few cases most notably in peru chile and the nankai trough where the dip of subduction is so shallow that effectively no asthenospheric wedge exists to produce source material for volcanism the shallow dip of the slab may be a buoyant effect of the plateau imbedded in the oceanic lithosphere the cases which are the most enigmatic are those where subduction is continuous the oceanic plateau is subducted along with the slab and the dip of the slab is clearly steep enough to allow arc volcanism yet a volcanic gap exists in these areas the subducted plateau may have a fundamental effect on the physical process of arc volcanism itself the presence of a large topographic feature on the subducting plate may affect the stress state in the are by increasing the amount of decoupling between the two plates alternatively the subduction of the plateau may change the chemical processes at depth if either the water rich top of the plateau with accompanying sediments are scraped off during subduction or if the ridge is compositionally different

response of the tectonically erosive south peruvian forearc to subduction of the nazca ridge analysis of three dimensional analogue experiments 1 the collision zone of the obliquely subducting nazca ridge and the erosive peruvian forearc is characterized by enhanced tectonic erosion normal faulting forearc uplift and a minor indentation we apply two three dimensional sandbox experiments monitored by an optical particle imaging velocimetry system to decipher the spatial and temporal evolution of forearc deformation during oblique convergence the rigid model ridge causes a wave of uplift followed by subsidence to shift along the forearc by varying the strength of the analogue material we demonstrate that the forearc response to ridge subduction in particular the amount and rate of uplift and wedge indentation is controlled by the mechanical strength of the forearc the model using the high strength material yields results more compatible with the geological record of the peruvian forearc first the modeled amount of uplift scales to 1 2 km in nature which agrees with 1 0 km of uplift recorded by marine terraces second the calibrated model uplift rate of 0 9 km myr is similar to the natural rate of 0 7 km myr third the development of normal faults above the model ridge is in accordance with the style of faulting above the nazca ridge finally oversteepening of the uplifted wedge and local slope failure as observed in the experiment has also been identified off southern peru a comparison between the erosive peruvian and accretive margins emphasizes that the style of ridge induced deformation depends on the forearc strength and crustal structure which in turn are controlled by the long term evolution of the margin s mass transfer regime

ridge subduction at an erosive margin the collision zone of the nazca ridge in southern peru the 1 5 km high obliquely subducting nazca ridge and its collision zone with the peruvian margin have been imaged by wide angle and reflection seismic profiles swath bathymetry and gravity surveying these data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust the lowermost layer contributes 10 12 km to the total crustal thickness of the ridge the sedimentary cover is 300 400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges at the collision zone of ridge and margin the following observations indicate intense tectonic erosion related to the passage of the ridge the thin sediment layer on the ridge is completely subducted the lower continental slope is steep dipping at 9 and the continental wedge has a high taper of 18 tentative correlation of model layers with stratigraphy derived from ocean drilling program leg 112 cores suggests the presence of eocene shelf deposits near the trench continental basement is located 15 km landward of the trench normal faults on the upper slope and shelf indicate extension a comparison with the peruvian and northern chilean forearc systems currently not affected by ridge subduction suggests that the passage of the nazca ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long term erosive regime

pore pressure and oceanic crustal seismic structure summary marine refraction studies during the past decade have found considerable lateral variability in the seismic properties of the upper basaltic regions of the oceanic crust in many localities compressional and shear wave velocities are quite low at the top of the basalt section and velocities increase rapidly with depth it is concluded that pore pressure may be at least in part responsible for the low upper crustal velocities and contribute significantly to the lateral variability this is supported by compressional and shear wave velocity measurements as functions of confining pressure and pore pressure for basalt from the juan de fuca ridge and dolerite from the samail ophiolite oman within the oceanic crust regions of overpressure and underpressure will possess anomalous velocities the magnitude of which will depend upon the porosity and the deviation of the pore pressure from hydrostatic the influence of pore pressure on velocities is expected to diminish with depth and is unlikely to be significant at lower crustal depths where porosity is extremely low of significance poisson s ratio is shown to be dependent on pore pressure as well as confining pressure at constant confining pressure poisson s ratio increases with increasing pore pressure thus overpressured regions within the upper oceanic crust are likely to have relatively high poisson s ratios as well as low compressional and shear wave velocities

amplification of ground motion and waveform complexity in fault zones examples from the san andreas and calaveras faults summary p wave seismograms at ranges less than 10 km are synthesized by asymptotic ray theory and by summation of gaussian beams for point sources located in a low velocity wedge surrounding a fault the computations are performed using models of the wedge inferred from the analysis of reflection and refraction experiments across the san andreas and haywardcalaveras faults calculations in these models show that the 10 20hz vertical displacements of earthquakes located at 3 10 km depth are amplified by up to an order of magnitude in a 1 2 km wide region centred on the fault trace compared to displacements predicted by laterally homogeneous models of the crust this amplification is not cancelled by high attentuation in the fault zone and compensates for the reduction in amplitudes directly above the source predicted from the radiation pattern of a strike slip earthquake depending on the source depth of the earthquake and the structure and velocity contrast of the wedge multiple triplications in the travel time curve of direct p and s waves will occur at stations in the fault zone a wedge model successfully predicts the triplications observed in the p waveforms of aftershocks of the coyote lake earthquake recorded in the fault zone showing that body waves from microearthquakes can be used to determine the three dimensional velocity structure of the fault zone the amplification waveform complexity and distortion of ray paths introduced by the lowvelocity wedge suggest that its effects should be included in the interpretation of strong ground motions and travel times observed in the fault zone for realistic models of the wedge asymptotically approximate methods of calculating the body waveforms are strictly valid for frequencies greater than 20hz numerical methods may be necessary to calculate accurately the wavefield at lower frequencies

the structure of the nazca ridge and sala y gomez seamount chain from the dispersion of rayleigh waves summary we report single station measurements of group velocity for fundamental mode rayleigh waves that propagated along two aseismic ridges in the eastern pacific ocean basin the nazca ridge and the sala y gomez chain we find that the nazca ridge dispersion curves when regionalized to isolate the ridge contribution indicate group velocities of about 3 5 km s for short periods these values are slower than predicted for the geometry of the propagation paths interpreting this velocity heterogeneity is complicated by the recognition that the nazca ridge is about 1000 km longer than previously thought but models of vyv for the ridge derived by inverting the dispersion curves reveal abnormally thick 18 f 3 km oceanic crust along its longitudinal axis this value compares favourably with previously published results over the tuamotu plateau thus strengthening the hypothesis that the nazca ridge and tuamotu plateau are mirror images formed by a hotspot coincident with the farallon pacific spreading centre before 20 ma in contrast the measurements along the sala y gomez chain are consistent with typical oceanic crust and mantle when these values are inverted they reveal no thick crust 6 km along the sala y gomez chain we conclude from this result that the chain was likely formed as a leaky fracture zone not as a simple hotspot track

crustal structure of the southern calaveras fault zone central california from seismic refraction investigations a magnitude 5 7 earthquake on 6 august 1979 within the calaveras fault zone near coyote lake of west central california motivated a seismic refraction investigation in this area a northwest southeast profile along the fault as well as two fan profiles across the fault were recorded to examine the velocity structure of this region the analysis of the data reveals a complicated upper crustal velocity structure with strong lateral variations in all directions the near surface layers consist of recent sediments with seismic p wave velocities of 2 6 to 3 2 km sec these are underlain by rocks of the great valley sequence which have an average velocity of 4 5 km sec the great valley sequence is present along the whole profile depths range from 4 3 to 4 8 km in the northwest near anderson lake and in the southeast of the profile line near hollister in the middle of the profile near coyote lake however this layer thins and we find a laterally limited higher velocity layer 5 1 km sec between depths of 2 8 to 4 8 km the high velocity zone which coincides with a gravity high also correlates spatially with the hypocentral area of the 6 august earthquake and its aftershocks and may therefore represent an asperity on the fault velocities within the fault zone were determined from the fan profiles near anderson lake a pronounced delay of first arrivals on the fan records indicates a vertical 1 to 2 km wide near surface low velocity zone along the fault near coyote lake the delays observed in the fan records correlate with two subsurface en echelon fault planes which have been previously identified from lineations in the seismicity pattern the structure of the lower crust is similar to the neighboring diablo range a 8 to 9 km thick upper crustal layer with a seismic velocity of 5 7 to 6 3 km sec is underlain by a 3 km thick layer with velocity 6 8 km sec in accordance with previous studies of the diablo range there are indications of a pronounced lower crustal low velocity zone between a depth of 17 and 23 km the presence of this low velocity zone suggests that higher velocity igneous rocks of the middle crust have been thrust over sedimentary rocks

seismic reflection refraction mapping of faulting and regional dips in the eastern alaska range we present the results of a trans alaska crustal transect tact investigation of the upper 2 5 km of the eastern alaska range in the vicinity of the denali fault based on seismic reflection refraction data laboratory measurements of rock velocities and structural mapping the denali fault is a major dextral slip structure mappable for more than 2000 km separating the wrangellia and adjacent terranes to the south from the yukon tanana and adjacent terranes to the north geologic mapping suggests over 400 km of dextral slip has occurred on the fault yet within the upper 1 5 km of the crust along the tact corridor basement rocks juxtaposed along the denali fault reveal no significant seismic velocity differences although the fault zone itself is associated with a minor lowering of velocity the lack of seismic velocity contrast adjacent to the fault is in agreement with laboratory measurements of elastic wave velocities of samples from terranes bordering the fault laboratory measurements of elastic wave velocities of the metasedimentary mica quartz schists comprising the yukon tanana basement are highly anisotropic because of preferred orientation of mica and predict significant variations in velocity accompanying variations in foliation dip although other interpretations are possible the northward shallowing of foliation dip of basement rocks in the yukon tanana terrane combined with the strong anisotropy associated with these highly foliated rocks can explain an observed northward increase in seismic velocity within this terrane seismic reflections from basement rocks within the yukon tanana terrane may originate from variations in anisotropy with depth and or changes in composition reflecting different proportions of sandstone and shale in the protolith

cenozoic marine sedimentation in the sechura and pisco basins peru abstract the central and northern peruvian margin consists of a series of 8 paired forearc basins which may be separated into an inner set of shelf basins and a seaward set of slope basins we have examined the cenozoic stratigraphy of the onshore portions of the sechura basin 5 7 s and pisco basin 13 16 s two shelf basins which have accumulated marine sediment discontinuously since the mid to late eocene cenozoic sediments in the pisco basin were deposited during at least three major transgressive cycles each sequence is preserved as a similar vertical progression of facies including coarse nearshore bioclastic conglomerates and sandstones grading upwards into sandy siltstones and mudstones and capped by biogenic deposits including diatomites diatomaceous mudstones dolomitic horizons and phosphate deposits stratigraphic nomenclature for the pisco basin has recently evolved a stratigraphy presented here includes the eocene caballas fm upper eocene los choros fm upper eocene to lowermost oligocene yumaque fm uppermost oligocene to middle miocene chilcatay fm and upper miocene to pliocene pisco fm major hiatuses in the pisco basin span the late cretaceous to middle eocene early to late oligocene middle miocene and late pliocene pleistocene to recent cenozoic sediments of the sechura basin were deposited within at least 4 major transgressive cycles with hiatuses during the paleocene to middle eocene oligocene early to middle miocene and late miocene based on recent biostratigraphic studies sediments enriched in biogenic components accumulated between about 40 36 ma 24 16 ma and 11 3 ma in the pisco basin and between 40 37 ma and 8 5 4 5 ma in the sechura basin in both basins the most diatomaceous sediments are restricted to the late eocene and late miocene through pliocene the temporal distribution of biogenic sediments suggests that high productivity conditions linked to coastal upwelling have occurred episodically since at least the late eocene the occurrence of diatomites and phosphorites is diachronous between the pisco and sechura basins and between the peruvian forearc and other circum pacific monterey formation analogs a reflection of the strong influence of local tectonism on sedimentation patterns the volume of neogene sediments along the peruvian forearc is nearly twice that of the monterey fm despite basin to basin facies diachroneity these deposits very likely contributed to fluctuations of the late miocene carbon co 2 system by acting as large carbon sinks

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

seismic measurements of the internal properties of fault zones the internal properties within and adjacent to fault zones are reviewed principally on the basis of laboratory borehole and seismic refraction and reflection data the deformation of rocks by faulting ranges from intragrain microcracking to severe alteration saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity but from borehole measurements densely fractured rocks do show significantly reduced velocities the amount of reduction generally proportional to the fracture density highly fractured rock and thick fault gouge along the creeping portion of the san andreas fault are evidenced by a pronounced seismic low velocity zone lvz which is either very thin or absent along locked portions of the fault thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone high pore pressure within the pronounced lvz may be conductive to fault creep deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated seismically anisotropic mylonites

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake