the bagdad reflection sequence as tabular mafic intrusions evidence from seismic modeling of mapped exposures two dimensional seismic modeling of exposed rocks implicates subhorizontal tabular mafic intrusions as the likely cause of the unusually prominent reflections that constitute the bagdad reflection sequence brs these basement reflections from depths of 3 15 km are observed on cocorp and other data to extend for some 100 km across the basin and range colorado plateau transition zone in west central arizona the nearby buck mountains fortuitously expose an upended upper crustal block which is used in this study as the basis for a 12 x 6 km seismic model ray tracing through this model produces synthetic reflections from the numerous proterozoic diabase intrusions occurring in the buck mountains these synthetic reflections approximate the scale geometry and character of reflections observed on the cocorp data indicating that the intrusions in the buck mountains furnish a viable geologic analogy for the brs several lines of evidence argue that the major competing hypothesis that tertiary detachment surfaces cause the brs is less satisfactory although we cannot rule out the possibility that the brs is related to hypothetical tertiary intrusions of similar distribution and structure geologic evidence more directly implies that these reflectors are proterozoic in age if so the bagdad reflections serve as strain markers and can be used to constrain subsequent deformation in the arizona transition zone their relatively continuous nature thus argues against widespread disruption of the upper crust in the transition zone during tertiary extension moreover identification of these reflectors as subhorizontal mafic intrusions may have profound consequences for interpretation of similar reflections observed elsewhere by bolstering the contention that mafic intrusions are an important cause of basement reflections in the continental crust

advance retreat and halt of abrupt gravel sand transitions in alluvial rivers downstream fining of bed sediment in alluvial rivers is usually gradual but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm i e a gravel sand transition gst or gravel front here we present an analytical model of gst migration that explicitly accounts for gravel and sand transport and deposition in the gravel reach sea level change subsidence and delta progradation the model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel wedge and predicts the circumstances required for the gravel front to advance retreat and halt predicted modern gst migration rates agree well with measured data at allt dubhaig and the fraser river and the model qualitatively captures the behavior of other documented gravel fronts the analysis shows that sea level change subsidence and delta progradation have a significant impact on the gst position in lowland rivers

tomographic images of the upper crust from the los angeles basin to the mojave desert california results from the los angeles region seismic experiment we apply inversion methods to first arriving p waves from explosive source seismic data collected along line 1 of the los angeles region seismic experiment larse extending northeastward from seal beach california to the mojave desert in order to determine a seismic model of the upper crust along the profile we use resolution information to quantify the extent of blurring in the larse images and to smooth a damped least squares dls image by postinversion filtering pif most of the original data fit is preserved while minimizing model artifacts we compare dls pif and smoothing constraint inversion images using both real and synthetic data a preferred pif image includes larger scale features in the smoothing constraint inversion image and finer scale features in the dls inversion image that are consistent with geologic information we interpret principal model features in terms of geology including faulting the maximum depth of low velocity sedimentary and volcanic rocks in the los angeles basin is 8 9 km and in the san gabriel valley is 4 5 5 km a horst like uplift of basement rocks occurs between these basins the northeastern boundary of the san gabriel valley is imaged as a tabular moderately north dipping low velocity zone that projects to the surface at the southernmost trace of the sierra madre fault system in the central and southern san gabriel mountains velocity depth profiles are consistent with intermediate velocity mylonites overlying lower velocity pelona schist along a shallowly southwest dipping vincent thrust fault tomography does not provide a definitive dip for the san andreas fault but combined with other larse results is consistent with a vertical to steep northeast dip

h v spectral ratios and array techniques applied to ambient noise recorded in the colfiorito basin central italy the colfiorito basin is a small intramountain depression in the southern section of the northern apennine chain that is filled with quaternary alluvial deposits the presence of soft alluvial deposits has significantly influenced the level of local damage that was caused by two major earthquakes m l 5 6 and 5 8 belonging to the swarm that started in september 1997 to verify the effects of the basin structure on the predominant frequency of seismic motion ambient noise measurements were carried out in the colfiorito basin during two experiments in may and july of 2002 the horizontal to vertical spectral ratios hvsrs were calculated for data collected at four profiles in the basin array techniques were applied to determine the wave types that composed the noise to estimate their apparent velocity and azimuth of propagation and to calculate a velocity dispersion curve from which a velocity depth structure was derived the data analysis shows a high amplification in the hvsr at low frequency this feature is common to most of the sites including the reference site and it is interpreted as being due to weather disturbances the peak frequencies of the spectral ratio calculated at the sites located in the center of the basin coincide with the theoretically estimated resonance frequencies the array averaged hvsr calculated for the array located in the middle of the plain has a pronounced peak at 0 9 hz this corresponds to the peak of the amplification function calculated on the basis of the velocity model deduced from the dispersion analysis the hvsr method is instead unsuitable for the prediction of the resonance frequencies of sediments in the sites where strong lateral variations of basement topography are present we measured apparent velocities in the range of 0 3 0 8 km sec by applying f k methods to array recordings these values are compatible with the predominance of surface waves in the noise as also confirmed by polarization analysis both rayleigh and love waves are present in the background seismic noise the results obtained by applying the spatial autocorrelation method to the vertical component of the ground motion recorded at a 240 m wide circular array deployed in the middle of the basin revealed the presence of rayleigh waves and f k methods combined with polarization techniques revealed the presence of polarized love waves the wave field analysis indicates two main propagation directions the first is around n100 e in the frequency band of 1 0 2 0 hz this radiation can be interpreted as being generated at the east southeast step borders of the basin the second main direction is around n300 e in the frequency band of 2 0 3 0 hz its source may be a 180 m deep depression located at the southwest corner of the basin

waveform inversion for one dimensional near surface structure in the new madrid seismic zone waveform inversion is applied to p and s waveforms selected from microearthquakes of duration magnitude m d between 1 8 and 2 4 recorded at station parm in stahl farm missouri operated by the cooperative new madrid seismic network in the upper mississippi embayment to resolve the one dimensional 1d velocity structure of uppermost near surface soils constrained by vertical seismic profiling vsp and standard cone penetration test scpt data at nearby sites we begin by defining a gradient velocity structure in the soils a reference velocity model is constructed by grid search in which the observed p and p s wave shapes graphic 1 travel time and p p s amplitude ratio are the controlling factors in the systematic selection from among 136 000 models a waveform inverse scheme is then implemented to obtain the least spectral misfit and best waveform correlation between synthetic and observed data four simultaneous inversions of joint waveforms from p and s wave types are performed resolved four 1d final models with associated uncertainties are listed in this article the uppermost 7 m of near surface soil exhibit very low velocities through which v p and v s are in the ranges of 140 m s to 470 m s and 95 m s to 215 m s respectively this noninvasive technique demonstrates that the observed high frequency reverberations 1 17 hz of p s and p s waveforms from local microearthquakes can be utilized to provide a view of the near surface soil structure although there are trade offs between layer thickness and slowness this method explains much of the high frequency site response due to microearthquake wave propagation through the very low velocity near surface soils 1 embed inline graphic 1 gif

geochronology and nd isotopic data of grenville age rocks in the colombian andes new constraints for late proterozoic early paleozoic paleocontinental reconstructions of the americas abstract new u pb zircon crystallization ages and 40ar 39ar cooling ages from the colombian andes confirm the existence of rocks metamorphosed during the orinoquian orogenic event ca 1 0 ga of northern south america end t 1 1 ga for these rocks range from 3 9 to 0 91 which is interpreted as a mixture of late archean early proterozoic crust with juvenile material produced during the 1 1 ga orogenic event the colombian grenville age rocks are part of a much longer metamorphic pericratonal belt sporadically exposed along the andes in western central peru southern bolivia and northern argentina in addition nd model tdm ages for the colombian rocks range from 1 9 to 1 45 ga similar to those obtained in the grenville province of the eastern u s and in the mexican basement placing constraints on late proterozoic early paleozoic paleocontinental reconstructions

refined chronostratigraphy of the late mesozoic terrestrial strata in south china and its tectono stratigraphic evolution abstract the late mesozoic terrestrial strata in numerous basins in s south china provide important sedimentary archives to understand the tectonic evolution of s china and east asia however establishing regional stratigraphic framework within a basin and precisely correlating strata among basins remain challenging due to limited chronological constraints in this study we report zircon u pb ages of 21 samples and a compilation of 132 reliable age data from 15 type sections of the volcanic sedimentary basins in s china the synthesized geochronology allows us to establish a refined chronostratigraphy for the late mesozoic terrestrial strata new calibrated results indicate that most of the lithostratigraphic units are diachronic laterally stacking and or interfingering six stacking styles are classified for the relationship of lithostratigraphic units analysis of the refined chronostratigraphy and the stratal stacking styles together with lithological composition reveals three episodes of tectono stratigraphic evolution episode i 145 125 ma is characterized by intense volcanism as evidenced by widespread occurrence of volcanic strata and 137 120 ma a type granites and was probably related to the rollback of the subducting paleo pacific plate the strata deposited during episode ii 125 100 ma are composed of variegated sediments associated with without volcanic intercalations in sedimentary faulted depression basins indicating the waning of volcanism and tectonism attributed to the ending of the izanagi kula plate subduction in episode iii

study of weak and strong ground motion including nonlinearity from the northridge california earthquake sequence abstract this article presents a new method to estimate s wave site response relative to a regional layered crustal model the method is useful for site specific strong motion prediction because the estimated site response functions are referenced to an idealized regional layered model for which we know the ground response exactly we applied this method to the northridge earthquake sequence we determined the site amplifications from aftershocks with magnitudes 2 6 to 4 3 at 21 stations that were colocated with strong motion stations these site response functions were then used to modify synthetic seismogram calculated for the northridge mainshock in the same regional layered crustal model and thus obtain site specific ground motion estimates these site specific synthetic seismograms have higher correlation to observations in comparison to the synthetic seismograms without weak motion site correction they have similar amplitude and frequency content to the observations especially at sites with recorded peak ground accelerations below 0 3 g at sites with larger ground motions however this approach overestimates the strong motion the differences are made clear when we estimate site response functions from the strong motion records and compare them with those from weak motion records we express the differences as the average ratio of the weak to strong motion site response aws ratio when the ground motion is low the aws ratio is near unity indicating that the weak and strong motion site responses agree with each other within the uncertainty however the aws ratio increases as the ground motion amplitude increses the difference in weak and strong motion site responses becomes significant at stations where peak acceleration was above 0 3 g peak velocity was above 20 cm sec or peak strain was above 0 06 during the mainshock this result demonstrates directly from the ground motion observations the relationship between nonlinear site response and peak ground motion parameters the nonlinearity is present on soft rock sites as well as on sediment sites

precambrian pleistocene tectono stratigraphic evolution of the southern llanos basin colombia the llanos basin located in the eastern region of colombia northwestern south america is an andean foreland basin between the eastern cordillera colombian andes and the guyana precambrian shield the basin is the latest stage of a complex multiphase evolution that began in the paleozoic at the latest a paleozoic pleistocene basin evolution model is presented based on a regional two dimensional industry seismic data set and well log observations for the southern part of the basin five tectono stratigraphic sequences were identified 1 lower paleozoic depocenters preserved along inverted neoproterozoic basement blocks 2 an upper paleozoic marine sequence folded and faulted in the late paleozoic during assembly of pangea 3 upper cretaceous paleocene shallow marine sediments deposited in a distal foreland basin related to uplift of the western and central cordilleras of colombia the sequence pinches out against a paleozoic hinge or foreland bulge area 4 an eocene miocene foreland basin related to uplift of the eastern cordillera resulting in a wedge geometry and 5 pliocene pleistocene fluvial deltaic rocks overfilling the foreland basin reactivation of paleozoic structures occurs at the top of this sequence with the development of anticlinal structures present day stress fields indicate that subduction of the nazca plate beneath south america may be responsible for reactivation of paleozoic structures inversion of north south structures with the neoproterozoic basement is interpreted to be responsible for the paleozoic and pleistocene deformation whereas cenozoic deformation is related to the two main stages of foreland development of the basin to the east where the paleoproterozoic basement is present no deformation is interpreted

stress field and seismotectonics of northern south america we have integrated in situ stress neotectonic and global positioning system data to investigate the complex interactions among the south american caribbean and nazca plates and the costa rica panama microplate and to examine different seismotectonic models that have been proposed for the region the resulting data set was used to generate an integrated stress map of the region that shows that the stress field in northern south america varies systematically in both orientation and relative magnitude in the southwestern part of the study area the ecuadorian andes stress province exhibits strong e w compression resulting from the subduction of the nazca plate beneath the south american plate in the north andes stress province the observed nw se compression may result from the convergence between the caribbean and the south american plates and or the negative buoyancy of the already subducted caribbean plate beneath northwestern south america possible convergence between the costa rica panama microplate with respect to northwestern south america may also be a source of compression in this region in north and northeastern venezuela normal and strike slip faulting with a ne sw direction of extension characterizes the san sebastian el pilar stress province

euro seistest determination of the geological structure of the volvi basin and validation of the basin response the volvi basin northern greece is a 5 km wide graben of neogene age it has been chosen by european union directorate general xii for science research and development in climatology and natural hazards as a test site euro seistest for studying site effects in one of europe s most seismic active areas the site is located in the epicentral area of the 1978 ms 6 5 earthquake that struck the city of thessaloniki the geometry velocity and stratigraphy of the basin were not well known consequently a number of geophysical and geotechnical studies were performed to determine the 2d geometry and the dynamic characteristics vp vs qs of the valley investigations included 78 refraction tests 20 surface wave inversions and 8 borehole seismic tests as well as geotechnical tests this article presents the methodology used for the geophysical investigation and the results in its center the graben depth reaches 190 m and at least four normal faults have been located along the section the overall graben structure and the fault locations have been confirmed by the records of a shot generated at one edge of the valley and by the measurements of a local earthquake wave propagation appears to be strongly influenced by the thickness of the sediments which in turn is controlled by fault displacements simple 2d modeling of the basin response was performed and the results are in good agreement with weak motion data confirming the validity of the seismic structure inferred from seismic prospecting

faulting apparently related to the 1994 northridge california earthquake and possible co seismic origin of surface cracks in potrero canyon los angeles county california apparent southward dipping reverse fault zones are imaged to depths of about 1 5 km beneath potrero canyon los angeles county california based on their orientation and projection to the surface we suggest that the imaged fault zones are extensions of the oak ridge fault geologic mapping by others and correlations with seismicity studies suggest that the oak ridge fault is the causative fault of the 17 january 1994 northridge earthquake northridge fault our seismically imaged faults may be among several faults that collectively comprise the northridge thrust fault system unusually strong shaking in potrero canyon during the northridge earthquake may have resulted from focusing of seismic energy or co seismic move ment along existing related shallow depth faults the strong shaking produced ground surface cracks and sand blows distributed along the length of the canyon seismic reflection and refraction images show that shallow depth faults may underlie some of the observed surface cracks the relationship between observed surface cracks and imaged faults indicates that some of the surface cracks may have devel oped from nontectonic alluvial movement but others may be fault related imme diately beneath the surface cracks p wave velocities are unusually low 400 m sec and there are velocity anomalies consistent with a seismic reflection image of shallow faulting to depths of at least 100 m on the basis of velocity data we suggest that unconsolidated soils 800 rrdsec extend to depths of about 15 to 20 m beneath our datum 25 m below ground surface the underlying rocks range in velocity from about 1000 to 5000 m sec in the upper 100 m this study illustrates the utility of high resolution seismic imaging in assessing local and regional seismic hazards

a terrain based site conditions map of california with implications for the contiguous united states abstract we present an approach based on geomorphometry to predict material properties and characterize site conditions using the v s 30 parameter time averaged shear wave velocity to a depth of 30 m our framework consists of an automated terrain classification scheme based on taxonomic criteria slope gradient local convexity and surface texture that systematically identifies 16 terrain types from 1 km spatial resolution 30 arcsec shuttle radar topography mission digital elevation models srtmdems using 853 v s 30 values from california we apply a simulation based statistical method to determine the mean v s 30 for each terrain type in california we then compare the v s 30 values with models based on individual proxies such as mapped surface geology and topographic slope and show that our systematic terrain based approach consistently performs better than semiempirical estimates based on individual proxies to further evaluate our model we apply our california based estimates to terrains of the contiguous united states comparisons of our estimates with 325 v s 30 measurements outside of california as well as estimates based on the topographic slope model indicate our method to be statistically robust and more accurate our approach thus provides an objective and robust method for extending estimates of v s 30 for regions where in situ measurements are sparse or not readily available

a stochastic method for investigating site effects by means of a borehole array sh and love waves abstract a stochastic method for investigating site effects by means of borehole array observation is proposed this method is based on a stochastic time series model that is derived from haskell9s matrix procedure for a layered medium and is applicable to transverse component recordings simultaneously obtained on the ground surface and at a depth the time series model not only accounts for the amplitude but also for the phase characteristics of the recordings by applying this method to actual downhole array recordings the following aspects of site effects can be investigated 1 an estimation of the wave transfer function for sh waves 2 the separation of upcoming and downgoing waves on a borehole seismogram at a depth that is an estimation of purely incident waves and 3 the enhancement of fundamental mode love waves on a surface seismogram are examined by applying this method to two borehole array recordings obtained at the fuchu observatory in japan a noteworthy point is that these aspects of site effects are examined using this method without any knowledge of the velocity structure beneath the site

enhanced ground motions due to large amplitude critical moho reflections sms in the sea of marmara turkey 1 aftershocks of the 17 august 1999 mw 7 4 kocaeli earthquake were used to investigate the large amplitude sms arrivals in the sea of marmara we produced the event record sections from the transverse components of s and sms arrivals at a distance of 10 to 200 km for the data recorded towards the east and west in the sea of marmara for the data recorded towards the west we observed that the amplitude of sms phases enhances the ground motion by a factor of 2 to 3 the dipping moho and or anisotropy in the upper mantle or deep convex structure or deep lens are possible sources for the large amplitude sms phases since the study area has a potential of creating damaging earthquakes in the future the effect of large amplitude sms phases on the ground motion should be considered in the studies of the seismic hazard of the region

ground motion amplification in the santa monica area effects of shallow basin edge structure abstract the 1994 northridge earthquake produced ground motions in the northwest portion of the los angeles basin that were significantly larger than rock site motions observed at locations just north of the basin the santa monica area was hit particularly hard with numerous structures being damaged or destroyed by the strong ground shaking in this region the basin edge geology is controlled by the active strand of the east west striking santa monica fault and virtually all of the structural damage occurred at or south of the fault location we have used 2d finite difference ground motion simulations to investigate the effect of the basin edge structure in amplifying ground response constraints on the basin edge structure come from geologic cross sections geophysical data and seismological observations our simulations indicate that the shallow basin edge structure 1 km deep formed by the active strand of the santa monica fault creates a large amplification in motions immediately south of the fault scarp in very good agreement with mainshock damage patterns recorded ground motions and locations of elevated site response this large amplification results from constructive interference of direct waves with the basin edge generated surface waves and is quite similar to the basin edge effect associated with the 1995 kobe earthquake in addition we find that focusing effects created by the deeper basin structure 3 to 4 km deep cannot explain the large motions observed immediately south of the fault scarp this strongly suggests that the deep basin focusing models proposed by gao et al 1996 and alex and olsen 1998 are not likely explanations of the observed pattern of ground motion amplification in the santa monica area

site response shallow shear wave velocity and damage in los gatos california from the 1989 loma prieta earthquake aftershock records of the 1989 loma prieta earthquake are used to calculate site response in the frequency band of 0 5 10 hz at 24 locations in los gatos california on the edge of the santa clara valley two different methods are used spectral ratios relative to a reference site on rock and a source site spectral inversion method these two methods complement each other and give consistent results site amplification factors are compared with surficial geology thickness of alluvium shallow shear wave velocity measurements and ground deformation and structural damage resulting from the loma prieta earthquake higher values of site amplification are seen on quaternary alluvium compared with older miocene and cretaceous units of monterey and franciscan formation however other more detailed correlations with surficial geology are not evident a complex pattern of alluvial sediment thickness caused by crosscutting thrust faults is interpreted as contributing to the variability in site response and the presence of spectral resonance peaks between 2 and 7 hz at some sites within the range of our field measurements there is a correlation between lower average shear wave velocity of the top 30 m and 50 higher values of site amplification an area of residential homes thrown from their foundations correlates with high site response this damage may also have been aggravated by local ground deformation severe damage to commercial buildings in the business district however is attributed to poor masonry construction

seismic focusing by a single planar fracture 1 a single plane fracture with an axially symmetric stress distribution behaves as a seismic lens that focuses seismic energy to a beam waist at a focal plane both phase and amplitude effects on a seismic wave propagating across the fracture contribute to the lensing behavior radial gradients in the fracture specific stiffness cause wave refraction through a radially varying group time delay while the fracture transmission amplitude approximates a fresnel zone plate this work demonstrates that a two dimensional planar fracture contrasted with three dimensional geologic structures such as basins and domes can focus seismic waves focusing of seismic waves by fractures should be considered in the interpretation of seismic data from fractured strata with heterogeneous stress distributions

an integrated analysis of an orogen sedimentary basin pair latest cretaceous cenozoic evolution of the linked eastern cordillera orogen and the llanos foreland basin of colombia the integration of restored basin geometry and internal features of syntectonic units e g stratal architecture thickness sandstone composition with fl exural modeling of the lithosphere constrains the evolution of a basin and its fl exural history related to orogenic growth spatial temporal loading confi guration using this approach we determined the maastrichtian cenozoic polyphase growth of the eastern cordillera of colombia an inverted mesozoic extensional basin the record of this growth occurs in an andean post middle miocene thrust belt the eastern cordillera and in adjacent foreland basins such as the llanos basin to the east this approach permitted the identifi cation of fitectono stratigraphic sequences in the foreland basin and fi ve phases of shortening for the eastern cordillera thermochronological and geochronological data support the spatial and temporal evolution of the orogen foreland basin pair tectono stratigraphic sequences were identifi ed in two restored cross sections one located at a salient and the other in a recess on the eastern fl ank of the eastern cordillera the lower two sequences correspond to late maastrichtian paleocene fl exural events and record the eastward migration of both tectonic loading and depositional zero in the llanos basin these sequences consist of amalgamated quartzarenites that abruptly grade upward to organic rich fi ne grained beds and to the top light colored mudstones interbedded with litharenites in isolated channels amalgamated conglomeratic quartzose sandstones of the third sequence record 15 m y of slow subsidence in the llanos basin and llanos foothills during early to middle eocene time while shortening was taking place farther west in the magdalena valley the fourth sequence of late eocene middle miocene age records a new episode of eastward migration of tectonic loads and depositional zero in the llanos basin this sequence begins with deposition of thick fi ne grained strata to the west whereas to the east in the llanos basin amalgamated quartzarenites unconformably overlie cretaceous and older rocks former forebulge apatite fi ssion tracks in the axial zone of the eastern cordillera growth strata in the

local site effects on weak and strong ground motion abstract this is a review of the current state of the art in characterizing effects of local geology on ground motion a new horizon is clear in this aspect of strong motion studies non linear amplification at sediment sites appears to be more pervasive than seismologists used to think several recent observations about the weak motion and the strong motion suggest that the non linear amplification at sediment sites may be very common first on average the amplification is always greater at the younger sediment sites for all frequencies up to 12 hz in the case of weak motion while the relation is reversed for frequencies higher than 5 hz in the case of strong motion secondly the application of the amplification factor determined from weak motion overestimates significantly the strong motion at sediment sites observed during the loma prieta earthquake within the epicentral distance of about 50 km thirdly the variance of peak ground acceleration around the mean curve decreases with the increasing earthquake magnitude finally the above non linear effects are expected from geotechnical studies both in the magnitude of departure from the linear prediction and in the threshold acceleration level beyond which the non linearity begins

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

site amplification factors in central and southern california determined from coda waves abstract site amplification factors at frequencies of 1 5 3 6 and 12 hz were determined for 158 stations of the southern california seismic network scsn from coda waves of 306 local earthquakes using a recursive stochastic inverse method our results for southern california are consistent with those of central california obtained by su et al 1992 in that the site amplification at a station is systematically related to the surface geology of the station site the site amplification is high for young quaternary sediments and decreases with increasing geologic age at all frequencies between 1 5 and 12 hz this decreasing rate is higher for younger rock sites gradually becomes lower with increasing geologic age and appears to be insignificant when the site reaches mesozoic age or older the relationship between site amplification and geologic age obtained for southern california agrees with that for central california amplification factors at 158 sites in southern california were combined with those at 132 sites in central california through a common reference site the result is a unique data base of site specific uniformly referenced amplification factors throughout central and southern california

a new understanding of the provinces of the amazon craton based on integration of field mapping and u pb and sm nd geochronology abstract new conventional and sensitive high resolution ion microprobe zircon u pb dating has led to a new understanding of the subdivision and evolution of the amazon craton during precambrian time with major improvements and changes made to the previous rb sr based model the interpretation of u pb and sm nd isotopic data identifies eight main precambrian tectonic provinces in the craton with ages ranging from 3 1 to 0 99 ga some of the provinces were generated by accretional arc related processes carajas transamazonic tapajos parima and rondonia juruena and others by recycling of continental crust central amazon rio negro and sunsas the exposed archean crust is restricted to the east carajas and south amapa in brazil and north imataca in venezuela of the craton indicating that the amazon craton is largely a proterozoic crust the carajas imataca 3 10 2 53 ga and transamazonian 2 25 2 00 ga provinces are composed predominantly of granite greenstone terranes the tapajos parima 2 10 1 87 ga and rondonia juruena 1 75 1 47 ga provinces represent new crust added as orogenic belts while the rio negro 1 86 1 52 ga and sunsas 1 33 0 99 ga provinces originated mainly by magmatic tectonic recycling of the above two orogenic belts the only zone with a prominent northeast trend is the poorly known k mudku shear belt characterized by a 1 20 ga shear zone which deforms the rocks of at least three different provinces rio negro tapajos parima and transamazonic the central amazon province comprises mostly orosirian volcano plutonic rocks uatuma magmatism and is a terrane in which the exposed crustal structure and deformation are pluton related the sm nd tdm model ages and nd suggest that the central amazon province was generated by the partial melting of archean continental crust carajas province perhaps related to underplating that began at the end of the tapajos parima orogeny 1 88 1 86 ga

40ar 39ar and u pb evidence for late proterozoic grenville age continental crust in north central cuba and regional tectonic implications renne p r mattinson j m hatten c w somin m onstott t c millfin g and linares e 1989 4 ar 39ar and u pb evidence for late proterozoic grenville age continental crust in north central cuba and regional tectonic implications precambrian res 42 325 341 central cuba is composed of fault bounded tectonostratigraphic terranes juxtaposed and deformed during plate collision and subsequent transform motion between the caribbean and north american plates in the late mesozoiccenozoic one of these the las villas terrane contains crystalline basement rocks thought to be pre upper jurassic on stratigraphic grounds the socorro complex occurs in the northwestern las villas terrane and consists of marbles and siliciclastic metasedimentary rocks and the rio carla granite an 4 ar 39ar plateau date of 903 5 7 1 ma for phlogopite from a marble corroborates previous k ar dates from this unit and establishes unambiguously a late proterozoic age for high grade metamorphism discordance of the 4 ar agar age spectrum can be reliably attributed to diffusive ar loss which if modeled as an episodic thermal event implies a reheating age that closely coincides with the late cretaceous paleogene collision between the caribbean and north american plates u pb zircon data indicate an intrusive age of 172 4 ma for the rio carla granite and reveal an inherited zircon component with an age of 900 ma radio isotopic data from the socorro complex display no evidence of pan african age thermal overprinting these observations combined with constraints provided by published results from nearly pangean landmasses suggest that the complex lay substantially to the southwest present day co ordinates during the early paleozoic following its genesis in the mid mesozoic the caribbean plate evidently transported fragments of an extensive grenville age belt that spanned the americas during the late proterozoic

simultaneous study of the source path and site effects on strong ground motion during the 1989 loma prieta earthquake a preliminary result on pervasive nonlinear site effects we considered the effects of source propagation path and local site conditions on strong ground motion from the loma prieta earthquake in the following steps first the local site effect was removed from the strong motion records applying the weak motion amplification factors estimated from nearby stations of usgs regional network using the coda method then we confirmed the approximate validity of the 1 r law of geometrical spreading factor for the crustal model applicable to the region and we determined the attenuation parameter q f and source spectrum s f empirically from the site effect corrected spectral amplitude both q f and s f are consistent with the results obtained earlier for major california earthquakes we then synthesized the time history of ground acceleration by applying the stochastic simulation technique the agreement between the observed and predicted records was good for duration and spectral content we found however a strong systematic discrepancy with regard to peak acceleration namely for distances less than 50 km the predicted peak acceleration consistently overestimated the observed for sediment sites and underestimated it for the franciscan formation sites since the above systematic discrepancy applies to all the available stations within about 50 km from the hypocenter it is unlikely that the effect is due to the radiation pattern near source structure or topography it appears that the strong difference in the amplification factor between the franciscan and the sediment sites observed for weak motion disappears at acceleration levels higher than about 0 1 to 0 3 g if this is due to the nonlinear response of the sediment sites it is in the range expected from geotechnical engineering studies

tectonic evolution of petroleum systems within the onshore llanos basin insights on the presence of orinoco heavy oil analogs in colombia and a comparison with other heavy oil provinces worldwide economic accumulations of hydrocarbons in the onshore llanos basin of colombia are characterized by a central zone casanare province with greater than 20 api gravity oils and a southern province with biodegraded less than 15 api gravity oils to date no conceptual model successfully explains this spatial zonation in this paper we employ multiple one dimensional time temperature models to map the kitchens for three different source rocks and compare maturity levels through the cenozoic with the presence or absence of reservoir seal overburden and traps in paleogeographic maps of the llanos basin we find that the llanos basin cenozoic petroleum migration and charge may have been governed by a sedimentary structural evolution tied to the adjacent orogenic belt in which 1 paleogene stratigraphic traps developed in the south as favored by a more segmented basement and potentially transpressional stresses 2 a subsequent neogene phase with more pervasive east dipping low displacement normal fault traps was discovered and 3 a final pliocene present day phase of contractional traps was found in the easternmost foothill areas when compared with the evolution of several potential kitchens we suggest that upper cretaceous rocks from the eastern cordillera are the primary hydrocarbon source in the zone of heavy biodegraded oils to the south whereas lower cretaceous and selected terrigenous upper cretaceous source rocks are largely responsible for the younger neogene contractional traps of the foothills this evolutionary pattern for the llanos basin favors the presence of smaller but numerous hydrocarbon accumulations rather than the broader zones of heavy oils as found in the orinoco belt of venezuela

the contribution of postcritical moho reflections sms to ground motions of the 2008 mw 7 9 wenchuan earthquakethe contribution of postcritical moho reflections sms to ground motions

campanian miocene tectonostratigraphy depocenter evolution and basin development of colombia and western venezuela abstract the position of the central axis of deposition over colombian and venezuelan continental crust has varied markedly through time the axis migrated from west to east from late cretaceous to oligocene but at times secondary drainage divides were established by local uplift events in oligocene times with initial inversion of the eastern cordillera the central axis of deposition was divided into two main axes the proto magdalena and the proto orinoco systems the west to east migration of the central axis of deposition had a tectonic origin and it occurred in combination with tectonically driven changes in accommodation space depocenter evolution can be divided as follows 1 the axis of the campanian and early maastrichtian depocenter was located few km east of the present position of the central cordillera of colombia it migrated east with gradual uplift of the central cordillera 2 the central axis of late maastrichtian deposition is positioned approximately over the present day western foothills of the eastern cordillera possibly crosses the eastern cordillera over the santander massif and continues into the maracaibo lake in western venezuela accommodation space decreased from campanian to maastrichtian times in cretaceous tertiary boundary times parts of the eastern margin of the eastern cordillera were uplifted by an initial phase of inversion of deeply rooted jurassic and early cretaceous normal faults 3 in paleocene times the central axis of deposition was located along the spine of the eastern cordillera and extended into the maracaibo basin and accommodation space continued to decrease 4 in latest paleocene times the central axis of deposition shifted to eastern regions of the eastern cordillera and accommodation space decreased 5 the early eocene central axis of deposition was located along the present day eastern foothills of the eastern cordillera accommodation space continued to decrease and the regional middle eocene unconformity began to develop in middle eocene times a regional unconformity developed marking the climax of the pre andean orogeny deposition during these times was dominant in the maracaibo basin area where large amounts of sediment derived from vast exposed areas accumulated 6 the late eocene central axis of deposition was confined to the present position of the llanos foothills late eocene deposition reflects a regional increase in accommodation space in oligocene times the initial uplift of the eastern cordillera divided the main depocenter into two central axes accommodation space diminished in uplifted regions but continued to increase in the depocenters allowing sporadic marine ingressions into the present position of the llanos foothills as uplift of the eastern cordillera continued the eastern depocenter axis proto orinoco migrated east and the western depocenter axis proto magdalena migrated west this process continued through the rest of the cenozoic

specific distribution of ground motion during the 1995 kobe earthquake and its generation mechanism the 1995 kobe hyogo ken nanbu earthquake mw 6 9 is the most damaging in the recent japanese history and its notable feature is that most of the damage and over 6000 casualties occurred in a narrow belt through kobe and neighboring cities the kobe hanshin area usually a fault rupture propagation generates strong ground motions along the direction of propagation and this directivity effect should be significant above the source faults and beyond its leading end however the damage zone of the kobe earthquake is migrated noticeably from the fault trace into the center of kobe city three dimensional 3 d simulations of ground motion in kobe show that this migration is due to the strong amplification and ray bending in the sedimentary basin below the city in cooperation with the multipathing effects at a basin bedrock boundary

tectonics basin subsidence mechanisms and paleogeography of the caribbean south american plate boundary zone abstract using a mega regional dataset that includes over 20 000 km of on and offshore 2d seismic lines and 12 wells we illustrate three different stages of fault formation and basin evolution in the caribbean arc south american continent collisional zone transpressional deformation associated with oblique collision of the caribbean arc migrates diachronously over a distance of 1500 km from western venezuela in paleogene time 57 ma to a zone of active deformation in the eastern offshore trinidad area each diachronous stage of pre syn and post collisional basin formation is accompanied by distinct patterns of fault families we use subsidence histories from wells to link patterns of long term basinal subsidence to periods of activity of the fault families stage one of arc continent collision initial collision is characterized by overthrusting of the south and southeastward facing caribbean arc and forearc terranes onto the northward subducting mesozoic passive margin of northern south america northward flexure of the south american craton produces a foreland basin between the thrust front and the downward flexed continental crust that is initially filled by clastic sediments shed both from the colliding arc and cratonic areas to the south as the collision extends eastward towards trinidad this same process continues with progressively younger foreland basins formed to the east on the overthrusting caribbean arc and forearc terranes north south rifting adjacent to the collision zone initiates and is controlled by forward momentum of southward thrusting arc terranes combined with slab pull of the underlying and subducting north dipping south american slab uplift of fold thrust belts arc continent suture induces rerouting of large continental drainages parallel to the collisional zone and to the axis of the foreland basins stage two this late stage of arc continent collision is characterized by termination of deformation in one segment of the fold thrust belt as convergent deformation shifts eastward rebound of the collisional belt is produced as the north dipping subducted oceanic crust breaks off from the passive margin inducing inversion of preexisting normal faults as arc continent convergence reaches a maximum strain partitioning also begins to play an important role as oblique convergence continues accommodating deformation by the formation of parallel strike slip fault zones and backthrusting southward subduction of the caribbean plate beneath the south caribbean deformed belt as subsidence slows in the foreland basins sedimentation transitions from a marine underfilled basin to an overfilled continental basin offshore sedimentation is mostly marine sourced by the collided caribbean terranes localized islands and carbonate deposition stage three this final stage of arc continent collision is characterized by 1 complete slab breakoff of the northward dipping south american slab 2 east west extension of the caribbean arc as it elongates parallel to its strike forming oblique normal faults that produce deep rift and half grabens 3 continued strain partitioning strike slip faulting and folding the subsidence pattern in the caribbean basins is more complex than interpreted before showing a succession of extensional and inversion events the three tectonic stages closely control the structural styles and traps source rock distribution and stratigraphic traps for the abundant hydrocarbon resources of the on and offshore areas of venezuela and trinidad

the 1994 northridge california earthquake investigation of rupture velocity risetime and high frequency radiation a hybrid global search algorithm is used to solve the nonlinear problem of calculating slip amplitude rake risetime and rupture time on a finite fault thirty five strong motion velocity records are inverted by this method over the frequency band from 0 1 to 1 0 hz for the northridge earthquake four regions of larger amplitude slip are identified one near the hypocenter at a depth of 17 km a second west of the hypocenter at about the same depth a third updip from the hypocenter at a depth of 10 km and a fourth updip from the hypocenter and to the northwest the results further show an initial fast rupture with a velocity of 2 8 to 3 0 km s followed by a slow termination of the rupture with velocities of 2 0 to 2 5 km s the initial energetic rupture phase lasts for 3 s extending out 10 km from the hypocenter slip near the hypocenter has a short risetime of 0 5 s which increases to 1 5 s for the major slip areas removed from the hypocentral region the energetic rupture phase is also shown to be the primary source of high frequency radiation 1 15 hz by an inversion of acceleration envelopes the same global search algorithm is used in the envelope inversion to calculate high frequency radiation intensity on the fault and rupture time the rupture timing from the low and high frequency inversions is similar indicating that the high frequencies are produced primarily at the mainshock rupture front two major sources of high frequency radiation are identified within the energetic rupture phase one at the hypocenter and another deep source to the west of the hypocenter the source at the hypocenter is associated with the initiation of rupture and the breaking of a high stress drop asperity and the second is associated with stopping of the rupture in a westerly direction

determination of site amplification in the los angeles urban area from inversion of strong motion records abstract the amplification of strong ground motion at sites in the greater los angeles california region is determined using the generalized inverse method of andrews 1986 site amplification estimates are determined at 281 strong motion sites that provided horizontal component accelerograms from the 1971 san fernando 1987 whittier narrows 1991 sierra madre or 1994 northridge mainshocks the estimates are determined relative to the spectral level recorded at a single reference site in a second inversion a source site interaction term is added to andrews9s 1986 model to quantify the effect selected mainshock records have on site amplification estimates the source site interaction term is applied to the san fernando valley sites9 records of the northridge earthquake and to three los angeles basin sites9 records of the whittier narrows mainshock site amplification spectra are averaged within two frequency bands the intermediate frequency band ifb from 0 5 to 1 5 hz and the high frequency band hfb from 2 to 6 hz results are displayed on maps of surficial geology average spectral levels are correlated with average shear wave velocity in the uppermost 30 m a geotechnical parameter used by the national earthquake hazards reduction program nehrp to characterize site amplification statistically significant correlation is found in both frequency bands average spectral amplification levels for nehrp class b c and d sites in the la urban area are determined these averages display an expected increase in spectral amplification with category with a more pronounced variation in the ifb than in the hfb considerable overlap in the one standard deviation range of the c and d site levels is found in both ifb and hfb suggesting that site specific spectral amplification is influenced by more than just near surface shear wave velocity average site amplification levels are compared with those obtained from northridge aftershock records at 28 collocated sites and are found to be in reasonably good agreement in both frequency bands some interesting outliers that is sites for which the mainshock and aftershock determined amplifications differ significantly are identified as sites that experienced ground failure during the northridge mainshock

amplification of seismic waves by the seattle basin washington state recordings of the 1999 m w 7 6 chi chi taiwan earthquake two local earthquakes and five blasts show seismic wave amplification over a large sedimentary basin in the u s pacific northwest for weak ground motions from the chi chi earthquake the seattle basin amplified 0 2 to 0 8 hz waves by factors of 8 to 16 relative to bedrock sites west of the basin the amplification and peak frequency change during the chi chi coda the initial s wave arrivals 0 30 sec had maximum amplifications of 12 at 0 5 0 8 hz whereas later arrivals 35 65 sec reached amplifications of 16 at 0 3 0 5 hz analysis of local events in the 1 0 to 10 0 hz frequency range show fourfold amplifications for 1 0 hz weak ground motion over the seattle basin amplifications decrease as frequencies increase above 1 0 hz with frequencies above 7 hz showing lower amplitudes over the basin than at bedrock sites modeling shows that resonance in low impedance deposits forming the upper 550 m of the basin beneath our profile could cause most of the observed amplification and the larger amplification at later arrival times suggests surface waves also play a substantial role these results emphasize the importance of shallow deposits in determining ground motions over large basins

basin development and tectonic history of the llanos basin eastern cordillera and middle magdalena valley colombia the middle magdalena valley eastern cordillera and llanos basin constituted a major regional sedimentary basin from the triassic to the middle miocene basin development began during the triassic to the earliest cretaceous with a synrift megasequence related to the separation of north and south america in the proto caribbean the synrift megasequence began with deposition in a continental environment that became paralic and shallow marine in the early cretaceous basin development continued into the cretaceous in a back arc setting east of the andean subduction zone the back arc megasequence was dominated by shallow marine sedimentation and produced an excellent regional source rock during the turonian coniacian marine deposition was abruptly terminated during the ear y maastrichtian due to the final accretion of the western cordillera accretion of the western cordillera created the early pre andean foreland basin megasequence of late maastrichtian to early eocene age this depositional episode consists of coal rich alluvial plain coastal plain and estuarine deposits throughout the middle magdalena valley eastern cordillera and eastern llanos basin the megasequence was terminated by middle eocene deformation in the magdalena valley which ended sediment deposition throughout colombia loading effects of this deformation reestablished the basin in which the late pre andean foreland basin megasequence was deposited until the early miocene this megasequence also consists of alluvial plain coastal plain and estuarine deposits including the primary reservoir in the llanos foothills the upper eocene mirador fo mation the megasequence also includes a series of four major grossly coarsening upward cycles in the llanos basin these cycles correspond to changes in sea level sediment supply and foreland basin loading the mudstone in the lowermost of these cycles is the regional seal in the llanos basin and foothills the middle miocene onset of andean deformation in the eastern cordillera isolated the middle magdalena valley from the llanos basin the deformation was dominated by inversion of the basin controlling faults the resultant loading of the lithosphere created the accommodation space for the andean foreland basin megasequence a major transgression into the llanos basin coeval with this deformation caused deposition of marine mudstones in the lower part of the megasequence however the majority of the andean foreland basin megasequence consists of the guayabo formation a classic molasse sequence deposited in a high energy coarse grained bed load dominated fluvial system that was supplied by the developing mountains of the eastern cordillera

syntectonic cenozoic sedimentation in the northern middle magdalena valley basin of colombia and implications for exhumation of the northern andes development of the colombian middle magdalena valley basin mmvb was determined by late cretaceous early eocene uplift of the central cordillera to the west and subsequent transferal of deformation to the eastern cordillera to the east these phases are separated in the tectono stratigraphic record by a major unconformity the middle magdalena valley unconformity mmvu paleocene coastal to alluvial facies underneath the mmvu were deposited in a foreland basin coupled to central cordillera kilometer scale uplift the middle eocene to neogene continental strata that onlap the mmvu document transformation of the mmvb into an interior basin due to eastern cordillera deformation recorded by growth strata in seismic lines and changing provenance and paleoflow patterns exhumation histories of the mmvb bounding ranges are further constrained by apatit fission track and vitrinite reflectance thermochronology and volcanic ash chronology this study indicates that the mmvb is fundamentally related to evolution of the entire andean margin of south america

urban seismology northridge aftershocks recorded by multi scale arrays of portable digital seismographs we deployed portable digital seismographs in the san fernando valley sfv the los angeles basin lab and surrounding hills to record aftershocks of the 17 january 1994 northridge california earthquake the purpose of the deployment was to investigate factors relevant to seismic zonation in urban areas such as site amplification sedimentary basin effects and the variability of ground motion over short baselines we placed seismographs at 47 sites not all concurrently and recorded about 290 earthquakes with magnitudes up to 5 1 at five stations or more we deployed widely spaced stations for profiles across the san fernando valley as well as five dense arrays apertures of 200 to 500 m in areas of high damage such as the collapsed interstate 10 overpass sherman oaks and the collapsed parking garage at calstate northridge aftershock data analysis indicates a correlation of site amplification with mainshock damage we found several cases where the site amplification depended on the azimuth of the aftershock possibly indicating focusing from basin structures for the parking garage array we found large ground motion variabilities a factor of 2 over 200 m distances for sites on the same mapped soil unit array analysis of the aftershock seismograms demonstrates that sizable arrivals after the direct s waves consist of surface waves traveling from the same azimuth as that of the epicenter these surface waves increase the duration of motions and can have frequencies as high as about 4 hz for the events studied here we do not observe large arrivals reflected from the southern edge of the san fernando valley

controls on architecture of the late cretaceous to cenozoic southern middle magdalena valley basin colombia the maastrichtian cenozoic southern middle magdalena valley basin of colombia contains a unique record of unconformities strata and structure from which we extract the histories of exhumation of the central cordillera to the west and evolution of the eastern cordillera fold and thrust belt to the east this study integrates field based analyses of stratigraphy laboratory analyses of provenance fission track thermochronology vitrinite reflectance data volcanic ash geochronology and studies of synorogenic geometries and structure displayed in seismic data a major unconformity the middle magdalena valley unconformity formed by eastward migration of central cordillera uplift during late cretaceous to early eocene time which transformed a maastrichtian marine basin into a paleocene depositional piedmont area this transformation is recorded by a coarsening upward sequence of marine shales to alluvial fan conglomerates which was partly eroded during further early eocene propagation of central cordillera deformation cessation of this phase of uplift led to formation of a pediment surface the middle magdalena valley unconformity which was buried by westward onlapping middle eocene to neogene alluvial deposits middle eocene to neogene facies paleoflow and unconformities were controlled by eastern cordillera deformation in the eastern cordillera foot hills growth strata and thermal history reveal two phases of folding of middle eocene oligocene and late miocene ages prior to intense pliocene pleistocene uplift two unconformities of early late miocene and pliocene pleistocene ages occur to the west of the eastern cordillera and record flexural tilting related to episodes of eastern cordillera loading

wilshire fault earthquakes in hollywood the wilshire fault is a potentially seismogenic blind thrust fault inferred to underlie and cause the wilshire arch a quaternary fold in the hollywood area just west of downtown los angeles california two inverse models based on the wilshire arch allow us to estimate the location and slip rate of the wilshire fault which may be illuminated by a zone of microearthquakes a fault bend fold model indicates a reverse slip rate of 1 5 1 9 mm yr whereas a three dimensional elastic dislocation model indicates a right reverse slip rate of 2 6 3 2 mm yr the wilshire fault is a previously unrecognized seismic hazard directly beneath hollywood and beverly hills distinct from the faults under the nearby santa monica mountains

localized amplification of seismic waves and correlation with damage due to the northridge earthquake evidence for focusing in santa monica abstract the analysis of seismograms from 32 aftershocks recorded by 98 seismic stations installed after the northridge earthquake in the san fernando valley the santa monica mountains and santa monica california indicates that the enhanced damage in santa monica is explained in the main by focusing due to a lens structure at a depth of several kilometers beneath the surface and having a finite lateral extent the diagnosis was made from the observation of late arriving s phases with large amplitudes localized in the zones of large damage the azimuths and angles of incidence of the seismic rays that give rise to the greatest focusing effects correspond to radiation that would have emerged from the lower part of the rupture surface of the mainshock thus the focusing and hence the large damage in santa monica were highly dependent on the location of the northridge event and an earthquake of similar size located as little as one source dimension away would not be likely to repeat this pattern we show from coda wave analysis that the influence of surface geology as well as site effects on damage in santa monica is significantly smaller than are the focusing effects

nonlinear site response at kik net kmmh16 mashiki and heavily damaged sites during the 2016 mw 7 1 kumamoto earthquake japan abstract damage from severe ground motion occurred in the downtown area of mashiki in the kumamoto prefecture during the 2016 kumamoto earthquake japan such damage was heavy in the center of the downtown area nonlinear site responses for the first shock and mainshock which occurred on 14 and 16 april 2016 respectively are important factors that explain why the area was heavily damaged we analyzed soil nonlinearity using surface and borehole records obtained from the kik net kmmh16 mashiki station from our analysis we found that s wave velocity models clearly depended on the amplitude of input ground motion we estimated the strain dependent shear stiffness and damping ratio to explain this s wave velocity dependence we conducted equivalent linear analyses at the kmmh16 site based on a nonlinear model from these analyses we concluded that our synthetic surface ground motions agreed well with the observed ones especially for the s wave amplitudes and phases of the first shock and mainshock noted above in addition we performed the same analyses at the tmp3 site which was actually located within one of the heavily damaged zones the synthetic motions here also agreed with the observed ones with differences in spectral accelerations being well explained by our analyses our results indicated that soil nonlinearity played a major role in causing the difference of ground motions thus leading to the heavily damaged zone in the downtown area of mashiki

site amplification from coda waves validation and application to s wave site response abstract the subject of this article is to see if the response to s waves at a specific site can be determined from coda waves we used aftershock records of the 1992 landers and big bear earthquakes observed at four terrascope stations pas pfo svd and gsc in southern california the wide dynamic range of the terrascope instrument provided a broadband seismogram for s waves and at the same time coda waves over a long lapse time window we analyzed a total of 708 three component waveforms from 65 events whose magnitudes ranged from 3 3 to 5 4 the station svd is located on quaternary alluvium and the remaining stations are on rocks of cretaceous age we first evaluated the coda site amplification factor using a spectral ratio method may eda et al 1991 koyanagi et al 1992 for frequency bands centered at 0 75 1 0 1 5 2 0 3 0 4 0 and 6 0 hz for lapse time windows after 70 sec from the origin time we found that the site amplification factor for the radial component is similar to that for the transverse component this similarity was expected because codas consist of backscattered waves coming from all directions into the station resulting in a directionally averaged site response we then evaluated the s wave site amplification factor using an inversion method andrews 1986 on the direct s wave portion of the same data set we found that the coda site response agrees with the s wave site response within a factor of about 1 5 supporting the earlier findings by tsujiura 1978 and justifying the use of coda waves in determining site specific amplification for s waves this agreement between s wave site response and coda site response is understandable if we assume that coda waves are dominated by s waves as theoretically predicted by aki 1992 and zeng 1993

finite fault site specific acceleration time histories that include nonlinear soil response to estimate the broadband strong ground motion one might expect at a given site we develop a method that includes heterogeneous slip on a finite fault full wave propagation with high frequencies and site specific material properties with nonlinear soil response the faulting is simulated as a stochastic process with the spatial variation of the key parameters determined by probability distribution functions the wave propagation from source to site is accounted for by using small earthquake recordings as empirical green s functions egf this accounts for the regional effects of scattering attenuation and structure while providing the basis for a broadband 0 5 10 hz time history because we are interested in sites where the ground motion is expected to be severe we have included nonlinear wave propagation through the soil the material properties of the soil column have been determined from laboratory tests borehole logs and confirmed through seismological modeling of weak motion we have computed 240 three component acceleration time histories to represent the range of ground motion one might expect from a m 6 8 earthquake for a site that is located 10 km above the hanging wall of blind thrust 7 1 km closest distance based on the suite of time histories we computed a s d of 0 45 natural log units for the acceleration response spectra in the passband 0 5 10 hz the total s d modeling plus parameterization is 0 6 in natural log units the mean acceleration response spectrum is near the median 10 in 50 year probabilistic seismic hazard analysis psha spectrum for the site the 84 spectrum of the simulations is closer to the 5 in 50 years median spectrum 2003 elsevier science b v all rights reserved

determination of geomechanical site effects in france from macroseismic intensities and reliability of macroseismic magnitude of historical events results from a detailed analysis of the french macroseismic database sirene96 show that geomechanical site effects i e site effects due to superficial geology can be statistically detected and quantified from macroseismic data our results are used to derive a predictive map of potential site effects in france and to check if site effects induce a significant overestimation of macroseismic magnitude of historical events the key hypothesis of our work is that geological formations of similar age produce similar site effects because one expects the younger sediments to be less compacted and therefore more prone to cause site effects a geological formation is associated with each intensity using a geographic information system gis and the digital geological map of france two approaches are developed in order to characterise the intensity increase associated with geological formations having the same age the first approach is based on the intensities that differ significantly from the average pattern intensities which are labelled as anomalies in the second approach a theoretical intensity attenuation law is computed for each selected event only from observations located on formations older than 2 my according to the results of the first approach these formations are unlikely to cause site effects and for the sake of simplicity they are labelled as rocks younger formations are defined as sediments the derived attenuation laws are then used to compute the residuals for each observation residuals are classified as a function of the age of the formations on which they are located the results of the two approaches are in good agreement no intensity amplification is detected for formations older than 1 my for younger formations the younger the formations the larger the average intensity increase the average intensity increase reaches 0 6 0 2 degree on the msk scale for 0 my sediments the results are better expressed in terms of probability for 0 my sediments there is an estimated 83 probability of observing an intensity increase for 0 01 my and 0 75 my sediments this probability is about 52 the magnitude of the intensity increase is variable but it is generally small it has a 26 probability to reach 1 degree or above on the msk scale for 0 my sediments and this probability falls to about 5 for 0 01 and 0 75 my formations a predictive map of geomechanical site effects is produced by extrapolating the characteristics of intensity increases to all geologic formations of similar age in france glacial and fluvial valleys already known as being subject to site effects clearly appear on this map as well as less expected areas such as the south atlantic coast lastly new estimations of macroseismic magnitude that exclude intensities potentially affected by geomechanical site effects are realised a magnitude overestimation due to site effects can possibly exist but it remains limited 0 2 this result tends to demonstrate that geomechanical site effects in france do not significantly distort our knowledge of historical seismicity

the magnitude 6 7 northridge california earthquake of january 17 1994 the most costly american earthquake since 1906 struck los angeles on 17 january 1994 the magnitude 6 7 northridge earthquake resulted from more than 3 meters of reverse slip on a 1 5 kilometer long south dipping thrust fault that raised the santa susana mountains by as much as 70 centimeters the fault appears to be truncated by the fault that broke in the 1971 san fernando earthquake at a depth of 8 kilometers of these two events the northridge earthquake caused many times more damage primarily because its causative fault is directly under the city many types of structures were damaged but the fracture of welds in steel frame buildings was the greatest surprise the northridge earthquake emphasizes the hazard posed to los angeles by concealed thrust faults and the potential for strong ground shaking in moderate earthquakes

the eastern foothills of the eastern cordillera of colombia an example of multiple factors controlling structural styles and active tectonics we decipher the geometry timing and kinematics of deformation of a region in the eastern foothills of the eastern cordillera of colombia we assess the influence of inherited structural fabrics changes in basin geometry erosional denudation and the characteristics of the tectonic stress field with respect to the evolution of the structural styles of the deformation front in the eastern cordillera detailed structural and geomorphic mapping of an area of 5000 km 2 analysis of seismic reflection profiles cross section balancing tectonic stress field indicators and new apatite fission track data are used to characterize the partitioning of late cenozoic deformation in the eastern foothills of the eastern cordillera of colombia during the late miocene pliocene in the eastern cordillera deformation migrated from inverted master normal faults to low elevation low amplitude structures in the foreland however this shift in the locus of deformation was not spatially uniform the deformation front is wider in a northern sector of the cordilleran foothills where sedimentary units are thicker and shortening is perpendicular to the structures this shortening direction is identical to the direction of the greatest horizontal stress s hmax as seen in borehole breakouts during the late miocene pliocene basement ranges are passively uplifted by younger more frontal thrusts the eastern foothills of the eastern cordillera thus reveal a complex combination of factors responsible for the structural styles and partitioning of active deformation in an inversion orogen over time the most important factor changes from the role of inherited structural fabrics to the geometries of basin fills

validation of the intensity based source inversions of three destructive california earthquakes the objective of our work is to invert the intensities of preinstrumental destructive earthquakes to retrieve their principal source characteristics this is relevant in countries as italy and greece where a large and high quality data bank of earthquake intensities during the past few centuries exists we have previously validated our technique presented in this journal for the m l 5 9 1987 whittier narrows earthquake and here we demonstrate that our algorithm works on three other earthquakes with known sources three more validations of the algorithm are presented here by using the site intensities observed by the u s geological survey for the m 6 4 1983 coalinga the m s 7 1 1989 loma prieta and the m w 6 7 1994 northridge earthquakes our simplified kf formula simulates the body wave radiation from a linear source and 11 source parameters are retrieved the three nucleation coordinates the fault plane solution the seismic moment the rupture velocities the along strike and antistrike rupture lengths and the shear wave velocity in the half space to find the minima on the hypersurface of the residuals in the multiparameter model space we use a genetic process with niching niching genetic algorithm because we have already shown that the problem is bimodal for pure dip slip mechanisms the objective function of the nonlinear inversion is the sum of the squared residuals calculated minus observed intensity at all sites the three tests presented here were successful because the results agreed with the sources already known from instrumental measurements a very accurate solution was found for loma prieta notwithstanding the complexity of its source in the two other cases sources close to the reference ones were retrieved the quality of our source determinations are obviously lower than those of instrumentally based models but would be highly noteworthy for earthquakes without seismological recordings the potential of the kf inversions seems particularly promising for european countries where seismicity rates are not very high and where a lot of information on earthquake damage is available for preinstrumental earthquakes the present series of validations raises our hope that further information on historic seismic events can be obtained from intensities thus increasing the knowledge of seismotectonics and seismic hazard

northridge earthquake damage caused by geologic focusing of seismic waves despite being located 21 kilometers from the epicenter of the 1994 northridge earthquake magnitude 6 7 the city of santa monica experienced anomalously concentrated damage with mercalli intensity ix an intensity as large as that experienced in the vicinity of the epicenter seismic records from aftershocks suggest that the damage resulted from the focusing of seismic waves by several underground acoustic lenses at depths of about 3 kilometers formed by the faults that bound the northwestern edge of the los angeles basin the amplification was greatest for high frequency waves and was less powerful at lower frequencies which is consistent with focusing theory and finite difference simulations

a comparative ground response study near los angeles using recordings of nevada nuclear tests and the 1971 san fernando earthquake a comparative ground response study at sites in the los angeles region is based on the extensive strong motion data set recorded in the 1971 san fernando earthquake and 159 three component recordings of distant nuclear tests in nevada amplitude spectral ratios computed for the nuclear test data over those frequency bands for which there is an adequate signal to noise ratio provide statistically stable estimates of the local ground response related to the type of local geologic conditions comparison of the strong motion data recorded from the 1971 san fernando earthquake with that recorded at 28 of the same sites for the nuclear tests show that the corresponding amplitude spectral ratios are statistically equivalent for most sites provided reference stations are chosen to minimize effects on the earthquake data of azimuthal source variations and crustal propagation path statistical equivalence of the spectral ratios for the two types of data sources suggests that amplitude spectral ratios computed with respect to the appropriate reference station provide a first order estimate of local ground response

full 3d tomography for the crustal structure of the los angeles region we apply full 3d tomography f3dt to a regional dataset in the los angeles area in f3dt the starting model as well as the model perturbation is three dimensional and the sensitivity frechet kernels are computed using numerical schemes that account for the full physics of 3d wave propagation f3dt can account for the nonlinearity of structural inverse problem through iteration thus providing the most efficient means for assimilating seismic observations into dynamic ground motion models we have successfully applied a scattering integral si formulation of f3dt to improve a 3d elastic structure model the southern california earthquake center scec community velocity model version 3 0 cvm3 0 with lowest shear velocity clamped at 1 km sec in the los angeles region our data are time and frequency localized measurements of phase delay anomalies relative to synthetics computed from the 3d elastic starting model the frechet kernels for our measurements were computed by convolving the earthquake wave fields generated by point earthquake sources with the receiver green tensors rgts which are the spatial temporal fields produced by three orthogonal unit impulsive point forces acting at the receiver locations we inverted 7364 phase delay measurements of p and s body waves by using the lsqr method the revised 3d model laf3d provides substantially better fit to the observed waveform data than the 3d starting model to our knowledge this study is the first successful application of f3dt using real data in structural seismology

variability of site response in the los angeles urban area this article addresses the variability of site response in the los angeles area and possible structural causes for the observations aftershock records from 231 sites in the san fernando and los angeles basins and the surrounding mountains are used in this study spectral ratios taken with respect to a low amplitude reference site are used to document the variation in site amplification in the frequency range 2 to 6 hz both spatially and with backazimuth to the source at higher frequencies 6 to 10 hz spectral ratios are shown to have greater spatial variability interstation spectral ratios are used to measure the standard deviation among source as a function of station separation an increase in the variation in ground motion is shown to take place at a station separation of 1 km relative site response estimates between nearby stations are used to demonstrate that preferred directions of motion can exist even in areas with no surface topographic effects significant variations in site response exist over short baselines up to a factor of 2 over 200 m that are not explained by differences in surficial geology or shallow shear wave velocity a variety of investigative approaches is used including spectral ratios arrival time variations 1d and 2d waveform modeling and comparison with seismic reflection lines to determine the most likely causes of these observations a correlation is demonstrated between late arrival times of p and s waves and larger site amplification in sherman oaks and northridge this observation in conjunction with waveform modeling and seismic reflection profiles is used to infer that sedimentary structures in the upper 1 to 2 km and topography on the sediment basement interface play an important role in determining site amplification these structures in the form of folds and buried basins focus energy in spatially restricted areas at the surface comparison of displacement waveforms at nearby stations having disparate site amplifications complemented by known shallow shear wave velocities at selected sites is used to support the argument that these structures in some cases can be the dominant factor in the modification of local ground motions

a faulting model for the 1992 petrolia earthquake can extreme ground acceleration be a source effect the 1992 petrolia earthquake m 7 0 was quite remarkable in that an accelerometer at cape mendocino cap recorded a high frequency pulse of almost 2 g whereas the nearby roughly 6 km away petrolia station pet recorded a maximum of only 0 6 g in our model for this event we have investigated the possibility that extreme ground acceleration with high spatial variability can be caused by a source effect specifically the rupture of a barrier asperity this type of rupture feature can produce extreme ground acceleration provided that certain geometrical constraints are met in the present study we derive a combined high and low frequency faulting model for the 1992 petrolia earthquake that is consistent with the slip distribution on the fault and that produces the high frequency pulse at cap our model consists of four parts 1 a low frequency 0 1 0 6 hz inversion that determines the slip rupture time and risetime distributions on the fault 2 an interpolation and perturbation of the above distributions that produces synthetic accelerograms between 0 6 and 3 0 hz 3 random phase signal that is scaled to the spectral amplitude level of the data between 3 0 and 12 5 hz and 4 the rupture of a barrier like asperity that produces greatly amplified radiation at cap but not at other stations we find that our model produces a good fit to the near source records of the 1992 petrolia event the results emphasize the interaction between fault geometry and rupture propagation to produce ground motion as well as the hazard associated with locations above the hanging walls of dipping faults where the geometry permits the production of extreme ground acceleration

first generation site response maps for the los angeles region based on earthquake ground motions abstract ground motion records from aftershocks of the 1994 northridge earthquake and mainshock records from the 1971 san fernando 1987 whittier narrows 1991 sierra madre and 1994 northridge earthquakes are used to estimate site response relative to a rock site for the urban los angeles area site response is estimated at 232 mainshock and 201 aftershock sites relative to a low amplitude site in the santa monica mountains average amplification values are calculated for the frequency bands 1 to 3 3 to 5 and 5 to 7 hz these bands are chosen based on limitations in aftershock recording equipment at lower frequencies and reduced significance to the building inventory at higher frequencies site amplification factors determined at the instrumented locations are grouped by the surficial geology and contoured to produce a continuous spatial estimation of amplification the maps in this article represent the first attempt to produce estimates of site amplification based on observations of ground motion for such a large areal extent of the los angeles region these maps are expected to evolve as more data become available and more analysis is done

anomalous seismic amplitudes measured in the los angeles basin interpreted as a basin edge diffraction catastrophe the los angeles basin passive seismic experiment labpse involved the installation of an array of 18 seismic stations along a line crossing the los angeles basin from the foothills of the san gabriel mountains through the puente hills to the coast at 3 5 km spacing between stations the array has much higher resolution than the permanent network of stations in southern california this resolution was found to be important for analyzing the factors that govern the amplitude variation across the basin we inverted spectra of p and s body wave seismograms from local earthquakes ml 2 1 4 8 for site effects attenuation and corner frequency factor using a standard model that assumes geometric spreading varying as inverse distance exponential attenuation and an x 2 source model the s wave attenuation was sep arable into basin and bedrock contributions in addition to the body wave analysis s wave coda were analyzed for coda q and coda determined site effects we find s wave q qs in bedrock is higher than in the basin high frequency qs is higher than low frequency qs coda q qc is higher than qs p wave q qp was not separable into basement and bedrock values so we determined an average value only the corner frequencies for p and s waves were found to be nearly the same the standard model fit over 97 of the s wave data but data from six clustered events incident along the basin edge within a restricted range of incidence and azimuth angles gen erated anomalous amplitudes of up to a factor of 5 higher than predicted we test whether such basin edge focusing might be modeled by catastrophe theory after ruling out site attenuation and radiation effects we conclude a caustic modeled as a diffraction catastrophe could explain both the frequency and spatial dependence of the anomalous variation

strike slip deformation within the colombian andes the colombian andes are characterized by a dominant ne structural trend which is offset by ene trending right lateral and nw trending left lateral structures ne trending faults are either dip slip or oblique thrusts generated as a result of a transpressive regime active since at least palaeogene times nw trending faults are considered to be reactivated pre cretaceous extensional structures right lateral shear on ene trending faults has resulted from oblique convergence between the nazca plate and the northern andes major changes in the geometry of the oblique plate convergence between the nazca and south american plates have generated the northward escape of the northern andes and stress strain partitioning within the mountain belt these strike slip structures have exerted important controls on sedimentation source rock distribution fluid flow and ore mineralization during cenozoic times the interpretation of the northern andes as a mountain belt affected by strike slip deformation provides a structural context in which to reassess the exploration plays each of these cordilleras has a different composition and evolution as a result of different tectonic processes that have affected the region since mesozoic times the central cordillera originated in response to triassic subduction and consequent volcanic and igneous activity barrero et al 1969 barrero vesga 1976 bartok et al 1981 triassic and jurassic backarc rifting generated a predominantly ne sw structural trend that influenced the sedimentation in the adjacent magdalena valley and eastern cordillera the western cordillera is composed of oceanic crust and deformed deep marine sediments representing an accretionary complex established in the late cretaceous de freitas et al 1997 the eastern cordillera represents the inversion of thick mesozoic and tertiary sedimentary basins see taboada et al 2000 eastward subduction of the nazca plate beneath the northern andes occurs at c 60 mm a 1 trenkamp et al 2002 along the western margin of ecuador and colombia fig 1 additionally north of latitude 8 n the caribbean plate is moving ese at an average velocity of 20 mm a 1 trenkamp et al 2002 as a result of the oblique convergence of both the nazca and the caribbean plates with the northern andes the andean block fig 1 is moving northeastwards relative to the south american plate pennington 1981 kellogg et al 1985 the colombian andes have been interpreted as an assemblage of terraines that have been accreted to south america with a dominant nne structural trend etayo serna et al 1983 lopez barrero 2003 this interpretation has led to the assumption of plane strain deformation orthogonal to the strike of the main structures which in colombia is almost parallel to the continental margin fig 1 for this reason the colombian andes have been described as a classical fold and thrust belt mojica franco 1990 schamel 1991 cooper et al 1995 however oblique ene and nw strike slip deformation has been recognised in the region feininger 1970 boinet et al 1986 diederix et al 1987 cuervo 1995 velandia komuro 1998 montes 2001 ujueta 2001 branquet et al 2002 acosta et al 2004 and the importance of these structures in the evolution of the belt has for the most part been neglected in this paper a variety of geological geophysical geodetic and geodynamic information has been integrated to examine the origin and evolution and of some of these strike slip fault systems within the colombian andes

site amplification in the san fernando valley california variability of site effect estimation using the s wave coda and h v methods during the months that followed the 17 january 1994 m 6 7 northridge california earthquake portable digital seismic stations were deployed in the san fernando basin to record aftershock data and estimate site amplification factors this study analyzes data recorded on 31 three component stations from 38 aftershocks ranging from m 3 0 to m 5 1 and depths from 0 2 to 19 km site responses from the 31 stations are estimated from coda waves s waves and ratios of horizontal to vertical h v recordings for the coda and the s waves site response is estimated using both direct spectral ratios and a generalized inversion scheme results from the inversions indicate that the effect of qs can be significant especially at high frequencies site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other depending on the choice of the reference site the difference is reduced when an average of six rock sites is used as a reference site in addition when using this multi reference site the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the s waves however for nonrock sites the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the s waves the h v method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods the amplifications determined from the hiv method are however different from the amplifications determined from the other methods finally the stations were grouped into classes based on two different classifications general geology and a more detailed classification using a quaternary geology map for the los angeles and san fernando areas average site response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification

geomorphology of the gran sabana guayana shield southeastern venezuela abstract the forms of the gran sabana have been developed mainly on rocks of the gently folded precambrian roraima group a sequence of quartzites and siliceous conglomerates with intercalated jasper chert arkose siltstone and shale locally intruded by diabase dikes and sills two planation surfaces are preserved on the gran sabana the auyan tepui 2000 2900 m elevation and the wonken 900 1200 m surfaces with a speculative mesozoic initial age they are still extending table mountains dominate the landscape they comprise pediments foothills precipitous scarps hundreds of meters high a rim and a flattish summit the table mountain topography is controlled by joint systems water percolates along joints and bedding planes dissolves the siliceous cement the quartzite disaggregates and large blocks are isolated and collapse accumulating at the foot of the scarps and on top of the table mountain summits the percolating water forms large and intricate cave systems which frequently emerge in the scarp zone as high waterfalls these mark the lowest level of erosion of the table mountains karstic structures are prominent in the summit zone the evolution of the table mountain topography has been controlled by a combination of tectonic structural stratigraphic climatic and temporal factors

topography effect at the critical sv wave incidence possible explanation of damage pattern by the whittier narrows california earthquake of 1 october 1987 abstract buildings damaged by the whittier narrows california earthquake of 1 october 1987 were concentrated in several areas most of which were located 8 to 10 km from the epicenter corresponding to the critical incidence for sv waves and were near topographic irregularities such as the northern part of whittier just south of puente hills the purpose of this study is to show the possibility that this anomalous damage pattern is due to the amplification by the topographic irregularity when sv waves are near critical incidence first we examined the accelerograms obtained at the usgs station 7215 bright ave nearest to downtown whittier and conclude that the dominantly eastwest motion observed at the station is due to the influence of the building in which accelerograms were recorded we next demonstrate that strong ground motions recorded by 12 stations in the epicentral area show a polarization direction pattern that is primarily sv type which is consistent with a simple thrust fault located at the hypocenter then we calculated the response of a two dimensional hill with the height 0 3 km and the width 2 4 km to 1 a plane sv wave with a nearly critical angle of incidence 2 a horizontal line force 3 a haskell type 2d dislocation source and 4 a bouchon type 2d multiple crack source the results show that the amplification due to the hill relative to the flat surface is more than 1 5 for all the source models since this amplification is nearly independent of the source type and spectrum we conclude that the combined effect of the topographic irregularity and critically incident sv waves might be responsible for the concentration of damage observed during the whittier narrows earthquake

water flow oil biodegradation and hydrodynamic traps in the llanos basin colombia in this study we provide new data to understand the groundwater flow patterns in the llanos basin and their impact on oil biodegradation and the geothermal regimes as well as how the structural styles and anthropogenic activities impact these patterns previous studies suggest an active flow of groundwater and variable salinities whose spatial pattern is apparently unrelated to topographically driven groundwater flow these observations have led to different hypotheses regarding the influence of groundwater flow on llanos basin geothermal gradients and oil biodegradation in this contribution we present data regarding the hydraulic heads salinities geothermal gradients and structural styles of the llanos basin to propose hypotheses explaining these observations structural cross sections and subsurface stratigraphic correlations allow us to suggest that the pattern of flow is best explained by a correlation between groundwater flow and structural styles a basement map of the llanos basin confirms that the most important factor controlling geothermal gradients is the type of basement whereas the factor of groundwater flow appears to be of secondary importance the evolution of the basin and the frequent absence of correlation between fresh water and the more biodegraded oils support the interpretation that biodegradation is controlled by an older flow of water that started as early as the oligocene finally mass balances suggest that the temporal scales and volumes of groundwater flow are much larger than the scales observed during the development of the oil fields

40ar 39ar dating of single muscovite grains in jurassic siliciclastic rocks san cayetano formation constraints on the paleoposition of western cuba the san cayetano formation of western cuba is the thickest and most extensive subaerial exposure of jurassic siliciclastic rocks between the southeastern united states and northern south america previous workers have speculated that siliciclastic rocks of the san cayetano formation were derived from one of the following sources 1 yucatan peninsula 2 florida and 3 northern south america to address the problem of the provenance of the san cayetano formation and the mesozoic position of western cuba we present the results of 40ar 39ar isotopic dates from 67 single detrital mica grains from four samples of the san cayetano formation these mica grains have ages that fall within the age ranges of well known precambrian crustal age provinces and paleozoic orogenic belts in eastern north america and the yucatan peninsula the lack of grains with ages in the 700 550 ma range suggests that pan african brasiliano orogenic belts that exist in northern south america northwestern africa and beneath florida did not contribute significant amounts of detritus to the san cayetano basin we propose that the late jurassic san cayetano basin formed as a post rift deltaic complex along the southeastern margin of the yucatan peninsula and was detached and transported to its present position in western cuba in paleogene time by the northeastward migration of the caribbean arc

spectral amplification in a sediment filled valley exhibiting clear basin edge induced waves various site response estimates are presented for a linear array deployed in the coachella valley california during the 1992 landers big bear aftershock sequence this systematic comparison is unique in that the response of the site is clearly dominated by basin edge induced waves average sediment to bedrock spectral ratios for long s wave windows which include the basin edge induced waves exhibit amplification factors as high as 18 below 7 hz the deep basin structure which gives rise to the obvious multi dimensional effects produces a fundamental resonant peak that shifts between basin sites from 0 23 to 0 48 hz as the depth to bedrock changes above 0 6 hz where the largest amplifications occur the response is remarkably similar between sites and appears to be dominated by a near surface layer that is relatively uniform across the valley the apparent fundamental resonant frequency of this layer is between 0 8 and 1 hz sediment to bedrock spectral ratios computed using shorter windows that exclude the basin edge induced waves imply that the multi dimensional effects are significant only below 4 hz where they increase amplifications by an approximate factor of 2 spectral ratios computed using coda windows taken at twice the s wave travel time exhibit amplifications that are an average factor of 1 7 greater between 1 and 4 hz than those of the s wave estimates this discrepancy does not improve by taking coda windows later at four times the s wave travel time horizontal to vertical component s wave spectral ratios do not agree with the sediment to bedrock ratios however they do exhibit a clear peak at the fundamental resonant frequency of the deep basin structure sediment to bedrock spectral ratios of ambient seismic noise are also inconsistent with the s wave estimates however horizontal to vertical noise ratios exhibit clear peaks near the fundamental resonant frequencies of both the deep basin structure below 0 6 hz and the suspected near surface layer between 0 8 and 1 hz therefore ambient noise data appear to provide valuable constraints on the basin structure ongoing efforts involve multi dimensional modeling of the observed basin edge induced phases and resonant frequencies

2d analysis of earthquake ground motion in haifa bay israel we study the earthquake response of the zevulun valley basin underlying northern israel s largest urban area with 2d viscoelastic seismic modeling of a detailed geological section we found that amplification of the horizontal vibrations the ratio of basin to no basin response spectra correlates with basin depth in the deepest portion of the basin qishon graben long periods 2 5 sec are amplified by 400 in the shallowest portion of the basin afeq horst shorter periods 0 5 sec are amplified by 300 400 these resonances in the vertical direction through the basin are strong enough that their amplitude overwhelms the amplitude of a previously recognized basin edge effect the horizontal vertical h v fourier spectral ratios based on 124 ambient noise measurements do not fully coincide with the basin to no basin fourier spectral ratios of the simulation but the resonance frequencies found in both methods are alike moreover the relation between the resonance frequency and the depth of the corresponding seismic reflector in the simulation is almost identical to the empirical frequency depth relations obtained from measurements this indicates that the average shear wave velocity of the sedimentary column in the model is consistent with measurements to evaluate the necessity of 2d analysis we performed additional 1d simulations at two locations along the section for the qishon graben 1d analysis underestimates the amplification factor relative to 2d by 25 whereas for the afeq horst 1d and 2d simulations are similar for a hard layer within the soft qishon graben fill we found that when the hard layer is thinner than 50 m its influence on ground motion is small

the scec southern california reference three dimensional seismic velocity model version 2 we describe version 2 of the three dimensional 3d seismic velocity model of southern california developed by the southern california earthquake center and designed to serve as a reference model for multidisciplinary research activities in the area the model consists of detailed rule based representations of the major southern california basins los angeles basin ventura basin san gabriel valley san fernando valley chino basin san bernardino valley and the salton trough embedded in a 3d crust over a variable depth moho outside of the basins the model crust is based on regional tomographic results the model moho is represented by a surface with the depths determined by the receiver function technique shallow basin sediment velocities are constrained by geotechnical data the model is implemented in a computer code that generates any specified 3d mesh of seismic velocity and density values this parameterization is convenient to store transfer and update as new information and verification results become available

devonian plants from colombia with discussion of their geological and palaeogeographical context plant fossils are described from the cuche formation eastern cordillera colombia in the area of floresta those identified as colpodexylon cf deatsii banks and cf archaeopteris sp suggest an earliest late devonian frasnian age for the formation these or similar taxa are also found in contemporaneous deposits in western venezuela and other elements of the venezuelan flora are found in a geographically intermediate locality all three devonian plant localities in the northwest of south america are within the colombian eastern cordillera and its northern extension the venezuelan perija range an area that has been integrated as a part of the so called eastern andean terrane or central andean province supposedly accreted to the autochthonous block of the guyana shield during the early jurassic or before although both invertebrates and plants from this terrane have strong affinities to north american and european assemblages and might be interpreted as implying a laurussian origin for the eastern andean terrane the evidence is not yet unequivocal with some authors postulating an in situ development of this province

seismic investigation of near surface geological structure in the paducah kentucky area application to earthquake hazard evaluation abstract seismics method were used to evaluate shallow geological conditions at 33 sites in the vicinity of paducah kentucky a combined set of p and s wave seismic refraction and reflection soundings were used in addition to local borehole information to produce structure maps of 1 a shallow 160 m deep shear wave velocity contrasts across the shallow unconformity were generally 2 to 1 while the contrast at the top of the paleozoic bedrock exceeds 5 to 1 these seismic boundaries have been determined to be very important in modelling and interpreting earthquake ground motion amplification in the paducah area the quality and accuracy of the data and the cost effective nature of the methods suggest that other communities in areas at risk to damage from seismic activity with foundation conditions comparable to paducah might benefit from similar characterization in order 1 to identify seismically hazardous near surface geological conditions and 2 to develop geological models that could be used in computer simulations of site response

regional seismic intensity anomalies in the korean peninsula and its implications for seismic hazard potentials the strength of seismic ground motion is a consequence of seismic source strength and medium response the dependence of seismic amplitudes and seismic intensity on regional geological structures and crustal properties in the stable intraplate region around the korean peninsula is investigated an instrumental seismic intensity scale based on spectral accelerations are proposed after calibrating with the reported macroseismic intensities a representative seismic intensity attenuation curve for the korean peninsula is given by i m text l l h 0 998 pm 0 222 1 72 pm 0 04 m text l 0 322 pm 0 027 text ln l 2 h 2 0 00608 pm 0 00049 sqrt l 2 h 2 where i m text l l h is the seismic intensity at an epicentral distance l in km for an earthquake with local magnitude m text l and focal depth h in km seismic intensities decay slowly with distance in the korean peninsula the observed decay rate for the korean peninsula is comparable with those for other stable intraplate regions while are lower than those for active regions the regional seismic intensity anomalies present a characteristic correlation with geological structures positive seismic intensity anomalies appear in the yeongnam massif okcheon belt and gyeongsang basin while negative seismic intensity anomalies in the gyeonggi massif the regional seismic intensity anomalies display positive correlations with crustal thicknesses crustal amplifications and seismicity density and negative correlations with heat flows positive seismic intensity anomalies are observed in the yeongnam massif and gyeongsang basin suggesting high seismic hazard potentials in the regions the regional crustal properties may provide useful information on potential seismic hazards

revisiting the 1872 owens valley california earthquake the 26 march 1872 owens valley earthquake is among the largest historical earthquakes in california the felt area and maximum fault displacements have long been regarded as comparable to if not greater than those of the great san andreas fault earthquakes of 1857 and 1906 but mapped surface ruptures of the latter two events were 2 3 times longer than that inferred for the 1872 rupture the preferred magnitude estimate of the owens valley earthquake has thus been 7 4 based largely on the geological evidence reinterpreting macroseismic accounts of the owens valley earthquake we infer generally lower intensity values than those estimated in earlier studies nonetheless as recognized in the early twentieth century the effects of this earthquake were still generally more dramatic at regional distances than the macroseismic effects from the 1906 earthquake with light damage to masonry buildings at nearest fault distances as large as 400 km macroseismic observations thus suggest a magnitude greater than that of the 1906 san francisco earthquake which appears to be at odds with geological observations however while the mapped rupture length of the owens valley earthquake is relatively low the average slip was high the surface rupture was also complex and extended over multiple fault segments it was first mapped in detail over a century after the earthquake occurred and recent evidence suggests it might have been longer than earlier studies indicated our preferred magnitude estimate is mw 7 8 7 9 values that we show are consistent with the geological observations the results of our study suggest that either the owens valley earthquake was larger than the 1906 san francisco earthquake or that by virtue of source properties and or propagation effects it produced systematically higher ground motions at regional distances the latter possibility implies that some large earthquakes in california will generate significantly larger ground motions than san andreas fault events of comparable magnitude

petroleum systems of the eastern cordillera foothill basins and associated llanos basin impacts on the prediction of large scale foreland and foothill petroleum accumulations colombia s recent oil boom which began in about 2003 features the notable achievement of making colombia one of the few countries in the world to surpass past oil peaks i e maximum historical oil production a phenomenon that among the western countries only the united states has approached with the shale tight oil boom in the 1990s after the discoveries of cusiana cupiagua cazier et al 1995 cooper et al 1995 and other oil fields colombia reached a maximum daily production of about 830 000 bopd in 1999 but soon afterwards started a steep decline however after 2003 daily production reached a minimum of 541 000 bopd and started to grow steeply again in 2007 until past peaks in oil production were surpassed in 2011 during the 1990s peak as well as the current maximum the key component of colombia s oil production has come from the llanos basin and associated eastern foothills which ranked as the fourth most favorable basin in the world in terms of exploration success rate ihs 2010 however until now there has been no comprehensive published study that explains this success or predicts the potential for finding future accumulations in this special issue we intend to provide the first comprehensive view of the llanos basin and the adjacent hinterland of the eastern cordillera with the ultimate goal of explaining the existing accumulations and predicting the potential for new ones to the extent allowed by the confidentiality of the organizations involved reyes harker et al 2015 this issue present the first paleogeographic reconstruction of the llanos and eastern cordillera including for the first time the most detailed and recent data regarding the uplift history of the eastern cordillera i e mora et al 2008 2010 2013a b parra et al 2009a b 2010 2012 horton et al 2010a b

seismic site response in the greater vancouver british columbia area spectral ratios from moderate earthquakes three component digital recordings of two recent moderate earthquakes provide valuable new insight into the response to seismic shaking in the greater vancouver area particularly on the fraser ri

on the coherence of ground motion in the san fernando valley we present an analysis of the coherence of seismic ground motion recorded on alluvial sediments in the san fernando valley california using aftershocks of the 17 january 1994 m w6 7 earthquake recorded at a quasi dense array of portable stations we analyze the coherence of three well recorded magnitude 3 7 to 4 0 events over the frequency range 0 5 to 15 hz and a distance range of 0 5 to 5 3 km all stations are located at sites with broadly similar near site geology characterized by medium to fine grain quaternary alluvial sediments on average relatively high values of coherence are observed for distances up to 3 to 4 km and frequencies up to 2 to 3 hz coherence drops sharply at frequencies near and above 3 hz although average coherence functions are described reasonably well by a log linear relationship with frequency the curves at all distances exhibit a flattening at low frequencies that is not consistent with previous observations of coherence at hardrock sites the distance decay of coherence is also markedly less strong with high coherence values observed over station separations corresponding to multiple wavelengths this may reflect fundamental differences in shallow wave propagation in the two environments with high frequency scattering relatively more dominant in regions of hard rock near surface geology within a sedimentary basin or valley the site response itself generally reflects a resonance phenomenon that may tend to give rise to more uniform ground motions however previous studies have demonstrated the existence of pathological focusing and amplification effects within complex sedimentary basin environments such as the greater los angeles region our results undoubtedly do not quantify the full range of ground motion variability at all sites but rather represent the level of that variability that can be expected and quantified for typical source receiver paths

toward resolving an earthquake ground motion mystery in west seattle washington state shallow seismic focusing may cause anomalous chimney damage 1 a shallow bedrock fold imaged by a 1 3 km long high resolution shear wave seismic reflection profile in west seattle focuses seismic waves arriving from the south this focusing may cause a pocket of amplified ground shaking and the anomalous chimney damage observed in earthquakes of 1949 1965 and 2001 the 200 m bedrock fold at 300 m depth is caused by deformation across an inferred fault within the seattle fault zone ground motion simulations using the imaged geologic structure and northward propagating north dipping plane wave sources predict a peak horizontal acceleration pattern that matches that observed in strong motion records of the 2001 nisqually event additionally a pocket of chimney damage reported for both the 1965 and the 2001 earthquakes generally coincides with a zone of simulated amplification caused by focusing this study further demonstrates the significant impact shallow 1km crustal structures can have on earthquake ground motion variability