earthquakes faulting and stress in the los angeles basin since 1920 fourteen moderate sized m l 4 9 6 4 earthquakes have been reported in the los angeles basin these events are associated with both mappable surficial faults and concealed faults beneath the basin sediments to determine the style of faulting and state of stress in the basin single event focal mechanisms for 244 earthquakes of m 2 5 that have occurred during 1977 1989 have been calculated fifty nine percent of the events are strike slip and are mostly located near two of the major northwest striking right lateral strike slip faults in the basin the newport inglewood fault and the palos verdes fault the 1988 pasadena and the 1988 upland earthquakes showed left lateral strike slip on northeast striking faults numerous small earthquakes in the eastern part of the basin show left lateral strike slip faulting and form a northeast trend near yorba linda thirty two percent of the events have reverse mechanisms and are distributed along two broad zones the first the elysian park fold and thrust belt coincides with anticlines along the eastern and northern flank of the los angeles basin extending into santa monica bay the second the torrance wilmington fold and thrust belt coincides with anticlines mapped on the southwest flank of the basin and extends from offshore newport beach to the northwest into santa monica bay oblique faulting that could be inferred by the merging of strike slip and compressional tectonics does not occur in the basin instead the coexistence of zones of thrusting and large strike slip faults in the basin suggests that the thrust and strike slip movements are mostly decoupled a few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines the trend of the maximum horizontal stress varies from n1 w to n31 e across the basin and consistently forms high angles with the fold axes the stress field that exists along the flanks of the basin has a vertical minimum stress axis this stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north south compression across the basin

tectonic and sedimentation model for morrow sandstone deposition sorrento field area denver basin colorado abstract pennsylvanian morrow sandstones are oil and gas productive throughout a large area in southeast colorado the sorrento field is a recent major morrow discovery with recoverable reserves estimated at over 10 million bbl of oil from an area of 3 200 ac 1 295 ha at depths of 5 400 to 5 600 ft 1 646 to 1 707 m minor production also occurs from the mississippian spergen and saint louis and the pennsylvanian marmaton productive morrow sandstones are interpreted on the basis of subsurface mapping to be fluvial valley fill deposits consisting mainly of channel sandstone these deposits are encased in marine shale and range in thickness from 5 to 55 ft 1 5 to 16 7 m net pay ranges from 5 to 30 ft 1 5 to 9 1 m porosities average 19 and permeabilities range from 1 to 4 000 md isopach maps of the morrow and associated stratigraphic intervals indicate that paleostructure influenced morrow depositional patterns morrow channel sandstones accumulated in paleostructural low areas created by movements on basement fault blocks structural nosing is present in the same location and trend as the morrow channels indicating structural inversion analyses of stratigraphic intervals above the morrow indicate that the structural inversion occurred during the early and middle pennsylvanian the field is regarded as a combination structural stratigraphic trap knowledge of paleostructural control on reservoir facies provides a new idea for exploration for morrow reservoirs in southeast colorado end of article last page 1319

seismic site effects and the spatial interpolation of earthquake seismograms results using aftershocks of the 1986 north palm springs california earthquake abstract we address the following two questions can a microearthquake9s ground motions be modeled by incident p and s waves that excite a site transfer function that is a smooth function of incidence angle given recorded ground motions from a set of earthquakes having known locations and mechanisms can we derive such a site transfer function and use it to obtain the ground motions that would result from an earthquake source occurring somewhere in the same volume but having a location and mechanism that are different from the recorded events although many factors will cause two distinct microearthquake sources to have different seismograms at a common station in this paper we concentrate only upon the differences caused by source mechanisms p and s wave travel time variations and by variations in the site transfer function we specifically exclude the effects of waves scattered from heterogeneities in the geologic structure away from the seismic site we express the site transfer function as a sum of several terms having simple dependences upon incidence angle and azimuth each term is an independent function of time given a set of seismograms observed at the site we solve a linear system of equations for the time dependences of each term these time series may be used to calculate the seismograms that would have resulted from an earthquake having arbitrary mechanism and location this step is an interpolation we have applied this technique to seismograms after aftershocks of the 1986 north palm springs earthquake our interpolation technique works fairly well within the volume occupied by the recorded events but the method is not very successful at providing accurate seismograms for sources located outside the aftershock volume the primary causes of the inaccuracy are the inadequacy of our chosen angular functions to model the site response fully and the likely scattering of seismic waves by geological heterogeneities in this case the banning and mission creek faults near the seismic stations our methods could be used to determine the effects of single scattering from lateral heterogeneities in geologic structure

the 1994 northridge earthquake 3 d crustal structure in the rupture zone and its relation to the aftershock locations and mechanisms a detailed 3 d p wave velocity structure of the crust in the epicentral area of the 17 january 1994 northridge earthquake is determined by using 104 709 arrival times from 1673 northridge aftershocks and 2948 other local earthquakes a test performed using the data from the nearby portable stations suggests that the aftershock hypocenters relocated with the obtained 3 d model are accurate to about 2 km we found that regions with high aftershock activity are generally associated with faster p wave velocities the velocity is high around the main south dipping fault of the 1994 northridge earthquake and the north dipping fault of the 1971 san fernando earthquake a linear distribution of strike slip aftershocks was found along a ne sw boundary between high velocity and low velocity structures to the west of this boundary a cluster of large shallow aftershocks with mixed mechanisms occurred in or near the border of a low velocity area while to the east aftershocks with thrust mechanisms occurred in a high velocity area these observations suggest that lateral variations of crustal properties are closely related to the fault segmentation in the transverse ranges a better understanding of these features is important for long term seismic hazard assessment in the los angeles area

seismic reflection studies using local earthquake sources summary a method is presented for processing three component digital recordings of micro earthquakes to obtain near vertical reflection profiles in regions of shallow seismicity the processing includes magnitude and focal depth normalization and event stacking where stacking is by small localized groups with ray theoretical time and distance corrections applied to compensate for varying focal depths in areas with high seismicity this procedure allows earthquakes to be treated as controlled sources to probe layered structures of the deep crust and upper mantle the validity of our approach is demonstrated using s waves from aftershocks of the borah peak idaho earthquake ms 7 3 of 1983

roman comagmatic province central italy evidence for subduction related magma genesis geochemical data on mafic volcanics show that important affinities exist between the roman and the calc alkaline rocks from the aeolian arc south tyrrhenian sea these affinities together with the close association of calc alkaline and k rich volcanics in the aeolian arc and in the naples area the continuity in the variation of abundances of incompatible elements from calc alkaline to potassic suites and the similarity in terms of major element geochemistry support a genetic relationship of the roman magmatism and the subduction processes that affected the apennines in tertiary time and are still active under the aeolian arc in the genetic model presented here both calc alkaline and k rich magmas were generated within a mantle heterogenously enriched in lil elements composition of the mantle was modified by addition of material probably sediments dragged down by the undergoing slab the geochemical and petrological differences displayed by the calc alkaline and k rich volcanics are accounted for by the different conditions of melting as well as by chemical and isotopic heterogeneities of the source 26 references 3 figures 1 table

reflection imaging with earthquake sources and dense arrays the mw 5 8 earthquake that occurred in louisa county virginia on 23 august 2011 provided an opportunity to record with several high density seismic arrays in addition to traditional sparse temporary seismic networks traditional aftershock networks consist of a few dozen stations spread over tens of kilometers as a result the recorded seismic waveforms suffer from spatial aliasing that is so severe that many types of waveform processing are not applicable here we report the results of recording with a large number of oil industry type instruments deployed at a spacing that is an order of magnitude closer than in traditional deployments the objective was to image subsurface structure with array methods using the aftershocks as sources the dense array recorded continuously for 12 days and consisted of 172 vertical component seismometers that were placed at 200 400 m and a 60 km long three component regional profile with stations every 2 km we demonstrate how processing techniques from vertical seismic profiling can produce high resolution 3 d reflection images of structure beneath the array these images display reflectivity that correlates with that observed on a nearby deep reflection survey collected by the u s geological survey of particular interest is a strong reflector imaged across multiple profiles our analysis demonstrates how a surface array of seismometers can provide 3 d images of structure using microearthquake sources when wavefields are sampled sufficiently densely

lithologic control of the depth of earthquakes in southern california the depth distribution of southern california earthquakes indicates that areas underlain by schist basement rocks have a shallower 4 to 10 kilometers maximum depth of earthquakes than do areas with other types of basement rocks the predominant minerals in the schists become plastic at lower temperatures and thus at shallower depths than the minerals in the other basement rocks the lateral variations in lithology will control the depth extent and thus the magnitudes of potential future earthquakes these depths can be determined from the depth of the current background seismicity

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

convergence rates across a displacement transfer zone in the western transverse ranges ventura basin california three cross sections are balanced and retrodeformed to the top of the pleistocene saugus formation and to a horizon close to the base of the jaramillo subchron to yield crustal shortening and shortening rates for the western transverse ranges of california the cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault red mountain fault to a blind thrust and from that to a combination of both a surface reverse fault san cayetano fault and the blind thrust using an age of 250 50 ka for the top of the saugus formation based on amino acid racemization of fossil mollusks crustal shortening rates appear to have abruptly accelerated through time from 3 4 3 to 5 4 mm yr between 250 and 975 ka to 20 6 to 28 1 mm yr since 250 50 ka using an age of 500 ka for the top of the saugus formation based on paleomagnetic stratigraphy there is little increase in deformation rates through time ranging from 5 5 to 8 6 mm yr between 975 500 ka and from 10 3 to 14 1 mm yr since 500 ka more crustal convergence rates determined by global positioning system surveys taken over 4 6 years indicate a shortening rate of 7 10 mm yr across the basin this is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the saugus formation less

deformation rates across the placerita northridge mw 6 7 aftershock zone and hopper canyon segments of the western transverse ranges deformation belt abstract aftershocks of the 1994 northridge m w 6 7 earthquake provide insights into the geometry of the seismic source faults in the san fernando valley and the east ventura basin and allow the calculation of deformation rates for the region the northridge thrust and santa susana faults dip in opposite directions with the northridge thrust entirely beneath the santa susana fault these opposing reverse faults interact resulting in a folded active santa susana fault and uplift in the footwall block of that reverse fault two balanced cross sections suggest thick skinned deformation of the western transverse ranges the western section across the modelo lobe segment of the north dipping san cayetano fault and the easternmost surface trace of the south dipping oak ridge fault is west of any aftershocks of the northridge earthquake and has been termed the hopper canyon segment of the deformation belt structural modeling predicts a dip of 46 s on the oak ridge fault at seismogenic depths horizontal shortening rates are calculated by adding the products of the dip slip displacements and the cosines of the dips of both faults the eastern cross section shows the northridge mainshock with a 45 south dipping nodal plane at a depth of 18 km aftershocks reach a depth of 20 km in a thin skinned paradigm a hinge should occur at the surface near the santa monica mountains due to rocks moving from a decollement at the brittle plastic transition and changing dip as they move up the ramp no hinge of that magnitude occurs there calculation of horizontal short ening rates across this part of the western transverse ranges must take into account the displacement on both the northridge thrust eastern extension of the oak ridge fault and the santa susana fault placerita segment horizontal shortening rates are 8 2 2 4 mm yr across the modelo lobe segment of the san cayetano fault and the oak ridge fault and 5 7 2 5 mm yr across the northridge thrust and the santa susana fault these rates are consistent with those based on tectonic geodesy using gps dip slip displacement rates on the faults are 1 7 mm yr for the northridge thrust since 2 3 ma 4 1 0 4 mm yr for the oak ridge fault since 500 ka 5 9 3 9 3 8 mm yr for the santa susana fault since 600 to 2300 ka and 7 4 3 0 mm yr for the modelo lobe segment of the san cayetano fault since 500 ka this indicates that the slip rates on the north dipping the eastern san cayetano and the santa susana faults are comparable but the slip rate on the south dipping faults decreases eastward the slip rate on the oak ridge fault in the ventura basin is more than double that of the northridge thrust

geologic applications of seismic scattering abstract once disregarded as noise scattered seismic waves are finding increasing application in subsurface imaging this sea change is driven by the increasing density and quality of seismic recordings and advances in waveform modeling which together are allowing seismologists to exploit their unique properties in addition to extensive application in the energy exploration industry seismic scattering is now used to characterize heterogeneity in the lower continental crust and subcrustal lithosphere to examine the relationship between crustal structure and seismogenesis and to probe the plumbing of active volcanoes in each application the study of seismic scattering brings wavelength scale structure into sharper focus and characterizes the short scale length fabric of geology

a study of seismic reflection imaging using microearthquake sources abstract we present a method for processing three component digital recordings of microearthquakes to obtain near vertical reflection profiles in regions of shallow seismicity the processing includes magnitude and focal depth normalization and event stacking where stacking is by small localized groups with ray theoretical time and distance corrections applied to compensate for varying focal depths in areas with high seismicity this procedure allows earthquakes to be treated as controlled sources to probe layered structures of the deep crust and upper mantle the validity of our approach is examined using aftershocks of the borah peak idaho earthquake m s 7 3 several thousand events occurred in a nnw trending zone about 10 km wide 75 km long and 15 km deep a small 10 10 km array of nine university of wisconsin three component triggered short period digital seismographs was installed in the region of aftershock activity over a 10 day period about 1000 useable events were recorded of which about 120 have been used for this study hypocenters have been computed using both p and s wave arrivals the latter being essential for stable solutions of events outside the network the borah peak data have been processed to obtain shear wave reflection profiles for the central station station 8 of the digital station array the stacked shear wave transverse record sections reveal coherent reflections from horizons at mid crustal to moho depths the most prominent reflections are from crustal discontinuities in the depth range 18 28 km coherent reflections can be obtained only through stacking which is necessary to improve the signal to noise ratio the major sources of data scatter as manifested by smearing of reflections on the stacked records are crustal heterogeneity and errors in the determination of focal depth and origin time

the 1994 northridge earthquake sequence in california seismological and tectonic aspects the m w 6 7 northridge earthquake occurred on january 17 1994 beneath the san fernando valley two seismicity clusters located 25 km to the south and 35 km to the north northwest preceded the mainshock by 7 days and 16 hours respectively the mainshock hypocenter was relatively deep at 19 km depth in the lower crust it had a thrust faulting focal mechanism with a rake of 100 on a fault plane dipping 35 to the south southwest and striking n75 w because the mainshock did not rupture the surface its association with surficial geological features remains difficult to resolve nonetheless its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the transverse ranges the northridge earthquake is part of the temporal increase in earthquake activity in the los angeles area since 1970 the mainshock was followed by an energetic aftershock sequence eight aftershocks of m 5 0 and 48 aftershocks of 4 m 5 occurred between january 17 and september 30 1994 the aftershocks extend over most of the western san fernando valley and santa susana mountains they form a diffuse spatial distribution around the mainshock rupture plane illuminating a previously unmapped thrust ramp extending from 7 10 km depth into the lower crust to a depth of 23 km no flattening of the aftershock distribution is observed near its bottom at shallow depths above 7 10 km the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults the dip of the ramp increases from east to west the west side of the aftershock zoae is characterized by a dense steeply dipping and north northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting these events coincided with the gillibrand canyon lateral ramp along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms the focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks with some strike slip and normal faulting in the smaller aftershocks the 1971 san fernando and the 1994 northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge both earthquakes accommodated north south contractional deformation of the transverse ranges the two earthquakes differ primarily in the dip direction of the faults and the depth of faulting the 1971 north northeast trend of left lateral faulting chatsworth trend was not activated in 1994

the relation of crustal scattering to seismicity in southern california dense regional array recordings of 377 teleseismic events are used to map geographic variability in upper crustal p to s scattering in southern california scattered wave energy is sensitive to short wavelength heterogeneity and maps of scattering potential offer a powerful complement to travel time tomography in characterizing seismic crustal heterogeneity the scale length of resolved scattering variability is commensurate with the scale lengths of surface fault trace length and microseismicity variations and both mapped faults and seismicity are concentrated in regions of strong scattering fault proximal scattering averaged across strike of predominately strike slip fault zones is highly correlated with microseismicity levels and is predictive of the pattern of microseismicity variation but not of absolute scale north south profiles through the transverse ranges reveal coincident strong gradients in microseismicity and scattering whereas mapped segment bounds along the san andreas fault zone from san francisco to the salton sea coincide with strong along strike gradients in the level of scattering the overall correlation of microseismicity and scattering potential is consistent with structural control of aftershock and background microseismicity production strain energy control of scattering or both prior evidence for fault offset in the pattern of scattering intensity revenaugh and reasoner 1997 favors the former

overlapping fault planes of the 1971 san fernando and 1994 northridge california earthquakes aftershocks of the 1971 san fernando and 1994 northridge earthquakes were relocated using a three dimensional velocity model that was derived from inverting p wave travel time data the hypocenters show clear orientations of the dipping fault planes the san fernando aftershocks form a plane extending from a depth of 15 km to the surface dipping toward the northeast at about 40 the northridge aftershocks delineate a fault extending from a depth of 18 km up to about 5 km dipping toward the southwest at about 40 in the region the aftershocks overlap in map view the san fernando plane cuts off the northridge plane at a depth of 5 to 8 km preventing it from reaching the surface the similar but oppositely dipping fault planes suggest a pair of conjugate planes reflecting a horizontal northeast southwest compression

new paleomagnetic constraints on the cenozoic tectonic evolution of the north arm of sulawesi indonesia abstract extensive paleomagnetic sampling 34 sites was carried out in cenozoic formations of the north arm of sulawesi between 120 and 124 e consistent results obtained between 120 and 122 e suggest the existence of a unique post miocene clockwise rotation of about 20 25 of the western part of the north arm this rotation probably occurred during the northward drift of the north arm along the palu matano sinistral transcurrent fault between 122 5 and 124 e both clockwise and counterclockwise rotations with various amplitudes from 6 to 85 are encountered and these very likely correspond to individual microblock rotations due to pervasive deformation along shear zones this result is consistent with the complex fault system of the gorontalo and kotamobagu shear zones that have been recognized on radar imagery and by spot image analyses

back arc basin origin for the east sulawesi ophiolite eastern indonesia the east sulawesi ophiolite is one of the three largest ophiolites in the world it displays all the components of a typical sequence from residual mantle peridotites to cumulate gabbros sheeted dolerites and lavas of normal mid oceanic ridge basalt morb composition trace element data on the lavas and dolerites and particularly their depletion in nb compared to neighboring incompatible elements suggest a subduction zone environment for their origin the chemical similarity between the east sulawesi ophiolite lavas and those from the eocene celebes sea back arc basin crust together with their identical age strongly suggest a back arc tectonic environment for this ophiolite which represents a fragment of the eurasian plate obducted onto the east sulawesi basement of australian origin

imaging scattering structures in the lower mantle by migration of long period s waves a simplified wave field migration method is applied to long period tangential component recordings to image lower mantle heterogeneity beneath alaska observed wave field complexities are dominated by coherent arrivals between direct s or ss and the core reflected scs or sscs phases a relative waveform inversion method is used to objectively extract the time and amplitude of the three main spikes in each wavetrain removing the effects of source time function receiver and instrument responses the timing of the intermediate arrivals relative to s or scs is used to define the volumetric position of scattering ellipsoids in the lower mantle with intersections of ellipsoids illuminating possible regions of isotropic scattering while a complete waveform migration is not viable due to the very limited ray path coverage and the band limited data the kinematics and relative amplitudes of the scattered arrivals are still valuable for assessing the origin of the waveform complexity in this region which displays particularly uniform existence of an extra arrival in the data the simplest interpretation is that a scattering surface with lateral dimensions exceeding 1500 km at a depth near 2600 km in the mantle is responsible for the extra phase this is consistent with previous interpretations involving a one dimensional model with a rapid shear velocity increase or discontinuity 240 280 km above the core mantle boundary the proposed methodology can potentially image both heterogeneities or layering in the d region as well as scattering from midmantle structure although data limitations will continue to be a major obstacle

lame parameters of common rocks in the earth s crust and upper mantle 1 lame parameter and shear modulus are the most important intrinsic elastic properties of rocks the lame parameter which relates stresses and strains in perpendicular directions is closely related to the incompressibility and contains a high proportion of information about the resistance to a change in volume caused by a change in pressure recent studies have emphasized the roles played by in the discrimination of gas sands from carbonates and shale in sedimentary basins and in the seismic reflection of crustal fault zones here we analyze the equivalent isotropic elastic data of 475 natural rocks in order to characterize values for common types of crystalline rocks in the earth s crust and upper mantle and their variations with pressure p temperature t and mineralogical composition when no partial melting metamorphic reaction dehydration or phase transformation occurs of a crystalline rock as a function of p and t can be described by a d dp p c exp kp d dt t where a is the projected value at zero pressure if microcracks were fully closed d dp is the pressure derivative in the linear elastic regime c is the initial drop caused by the presence of microcracks at zero pressure k is a decay constant of the drop in the nonlinear poroelastic regime and d dt is the temperature derivative the parameter increases nonlinearly and linearly with increasing pressure at low 300 mpa pressures respectively in the regime of high pressures decreases quasi linearly with increasing temperature with d dt values in the range of 1 10 10 3 gpa c approaching the quartz transition temperature quartzite displays negative values in the density and plots the main categories of lithology can be clearly distinguished the ultramafic rocks display systematic decreases in both and with increasing the degree of serpentinization eclogites mafic rocks gabbro diabase mafic granulite and mafic gneiss and felsic rocks granite diorite felsic gneiss intermediate gneiss and metasediments are characterized by high moderate and low and values respectively for pyroxene and olivine both and increase but decreases with increasing the fe mg ratios in the plagioclase series both and increases with increasing the anorthite content increases in the contents of garnets sillimanite rutile zircon ilmenite and spinel result systematically in an increase in rock s and values the present results provide improved constraints on the discrimination of composition for crustal and upper mantle rocks in terms of and

shear wave properties and seismic imaging of mylonite zones major crustal fault zones have displayed high reflectivity on observed compressional reflection profiles and synthetic models of laboratory measurements this high compressional wave reflectivity suggests that shear waves may also produce similar reflectivity the shear wave properties and reflectivity of mylonites from the brevard fault zone have been determined using measured shear velocities as a function of confining pressure the measured shear wave velocities are strongly influenced by mineralogy especially quartz and oriented micas the strong oriented fabric of brevard mylonites creates seismic anisotropy which for nonnormal incident propagation produces significant shear wave splitting a wide range of vp vs ratios within the brevard fault zone originates from a complex combination of composition and anisotropy making lithology determination difficult the shear wave reflectivity of the brevard fault zone was derived from one dimensional normal incidence modeling of detailed geologic fault zone models the shear wave reflectivity of the brevard fault zone which originates from fine compositional layering is moderate and can be directly correlated with individual geologic formations within the fault zone variable shear wave reflectivity both horizontally and vertically originates from geologic heterogeneity the modeled shear wave reflectivity also depends on frequency frequencies below 20 hz will resolve boundary structure while higher frequencies image detailed geologic features within the fault zone

the seismic coda site effects and scattering in alluvial basins studied using aftershocks of the 1986 north palm springs california earthquake as source arrays abstract we use single station recordings of widely distributed earthquake sources to identify heterogeneities in the earth9s crust that scatter energy into the early part of the observed s wave codas we assume that ray theory accurately describes the propagation of body waves from the earthquake sources to the scatterers but we assume no propagation mechanism between the scatterers and the seismic station a grid of hypothetical scatterers is assumed to cover the earth9s surface and the strength of each scatterer is determined by an iterative inversion method our data are recordings of aftershocks of the 1986 north palm springs earthquake recorded at a seismic station in the coachella valley smp and at a station in the morongo valley smc the technique is applied separately to each horizontal component of motion at each station we identify a particular spot at the northwest edge of the quaternary alluvium in the coachella valley that scatters waves into the s coda observed at smp 8 km distant in the valley the geologic structure that causes the strong scattering seems to be the basin edge structure rather than the mountainous terrain 3 km farther west the early coda at smc is dominated by scattering within a few kilometers of smc either in the basin itself or in the morongo valley fault zone this study shows that laterally propagating waves are sometimes observable in the early coda casting doubt on the use of 1 d techniques for estimating site responses the northwest edge of the coachella valley seems to be an especially strong scatterer of incident waves if other basins have similar loci of strong scattering perhaps these places can be identified using microearthquake seismograms and used to predict strong ground motions in the basins

seismic depth imaging of normal faulting in the southern death valley basin motivated by the need to image faults to test cenozoic extension models for the death valley region of the western basin and range province an area of strong lateral velocity variations we examine the geometry of normal faulting in southern death valley by seismic depth imaging we analyze cocorp death valley line 9 to attain an enhanced image of shallow fault structure to 2 5 km depth previous work used standard seismic processing to infer normal faults from bed truncations displacement of horizontal reflectors and diffractions we obtain a detailed velocity model by nonlinear optimization of first arrival times picked from shot gathers examine the unprocessed data for fault reflections and use a kirchhoff prestack depth imaging procedure to handle lateral velocity variations and arbitrary dips properly fault plane reflections reveal the listric true depth geometry of the normal fault at the black mountains range front in southern death valley this is consistent with the concept of low angle ex

inversion of seismic reflection traveltimes using a nonlinear optimization scheme we present the use of a nonlinear optimization scheme called generalized simulated annealing to invert seismic reflection times for velocities reflector depths and lengths a finite difference solution of the eikonal equation computes reflection traveltimes through the velocity model and avoids ray tracing we test the optimization scheme on synthetic models and compare it with results from a linearized inversion the synthetic tests illustrate that unlike linear inversion schemes the results obtained by the optimization scheme are independent of the initial model the annealing method has the ability to produce a suite of models that satisfy the data equally well we make use of this property to determine the uncertainties associated with the model parameters obtained synthetic examples demonstrate that allowing the reflector length to vary along with its position helps the optimization process obtain a better solution we put this to use in imaging the garlock fault whose geometry at depth is po

discrepancy between geological and geodetic deformation rates in the ventura basin space geodesy provides a new tool for measuring neotectonic deformation which when combined with geological and geophysical observations and models can be used to infer rates and styles of deformation in the ventnra basin region of southern california the geodetically measured convergence rates of 7 10 mm yr 1 ref 1 are significantly lower than the 20 25 mm yr 1 estimated from geological data2 3 here we investigate the possible implications of this discrepancy it might be for example that temporal variations cause the short and long term convergence rates to differ but if this were the case we would expect to observe the long term rate by extending the geodetic measurements over a broader region yet only 11 3 mm yr 1 of regional convergence is observed4 our preferred explanation of the discrepancy is that the geological models require revision and to this end we present two deformation models that satisfy both the geological and geodetic constraints our modelling suggests that the faults bounding the basin are locked at the surface but are slipping at depths below 2 5 km

analysis of near source contributions to early p wave coda for underground explosions iii inversion for isotropic scatterers a systematic array processing analysis is applied to 916 teleseismic shortperiod p wave signals from 25 underground explosions at pahute mesa in order to establish whether coherent arrivals scattered from near source velocity heterogeneity can be identified the signal coherence is measured using semblance which can be statistically evaluated using the noncentral f distribution if the noise properties are suitable the effects of unsuitable correlated noise due to common path and receiver contributions are suppressed by applying station weighting functions obtained by stacking all observations at a given receiver similar corrections for individual events are applied to reduce biases due to variations in surface reflection times and spall effects the characteristics of the spatial and phase velocity resolution of the semblance analysis are established by a series of synthetic calculations including cases with random arrivals in the p wave coda the procedure is applied to the actual data under the assumption that any scattering structures present radiate isotropically although this constraint could be relaxed given independent knowledge of a particular scatterer s nature the pahute mesa data show some evidence for enhanced scattering contributions to the first 15 sec of the p wave signals from the western edge of the silent canyon caldera the overall confidence level in this interpretation is fairly low because simulations with random scattering in the p coda successfully match many features observed in the stacked data

relation of the 1992 landers california earthquake sequence to seismic scattering measurements of crustal scattering for the area surrounding the 1992 landers earthquake sequence obtained from regional array recordings of teleseismic events for the 10 year period before the sequence showed that the slip distribution on faults could be deducible from the preshock elastic structure scattering intensity correlated strongly with the distribution of aftershocks and slip of the moment magnitude mw 7 3 landers main shock mw 6 1 joshua tree and mw 6 2 big bear events which implies that aftershocks and slip are structurally controlled and broadly predictable scattering within the fault zones was directional and consistent with variable along strike alignment of stress induced cracks

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

multimode migration of scattered and converted waves for the structure of the hikurangi slab interface new zealand reflectivity imaging of local earthquake seismograms has revealed the structure of the hikurangi subduction interface at the location of two strong earthquakes that occurred in 1990 the earthquakes originated within the continental plate of the north island of new zealand and below in the subducting pacific slab we used seismograms from 500 well located events in two earthquake sequences recorded by a small temporary seismograph deployment to directly image the structure and multiphase reflectivity of the plate interface synthetic tests of the imaging method show the effects of the poor 3 d geometric coverage afforded by the seismometer array kirchhoff summation image sections computed from synthetics show accurate depth imaging of backscattering interfaces phase converting interfaces imaged with forward scattered waves are smeared by poor ray coverage to 5 km depth inaccuracy and are only imaged over a small range of their horizontal extent from the data we computed image sections for p p p s s p and s s scattering we mitigated imaging artifacts due to poor ray coverage with an obliquity factor an antialiasing criterion and enhancement by resampling statistics imaging used a sharply layered velocity model we tested for the effects of imaging with first arriving headwaves by imaging through smoothly varying velocity models for our ray geometry early arrival headwaves contribute little to the images the plate interface appears as a 3 5 km thick p p and possibly s s backscatterer with 5j nw dip offset 5 km down to the nw above a normal fault in the slab when illuminated from below a wedge of the interface on the downdip side of the slab fault forms a very prominent p p forward scatterer the edges of the wedge forward scatter some s p and s s energy but an order of magnitude less than the p p forward scattering the imbalances between forward scattering of p and s energy suggest a wedge of subducted sediment retaining significant porosity but with rigidity close to that of surrounding rocks d 2002 elsevier science b v all rights reserved

a combined first arrival travel time and reflection coherency optimization approach to velocity estimation we present a nonlinear optimization method for two dimensional velocity estimation in strongly heterogeneous media we achieve this by designing a simulated annealing algorithm to simultaneously maximize reflection coherency as well as minimize first arrival travel time residuals relatively shallow velocity structure is constrained by first arrival times while coherency information from deeper reflections if present helps constrain deeper velocities prestack migration using the optimized velocities images reflections with greater continuity compared with other velocity estimation methods using synthetic and real data we demonstrate that migrations through velocities obtained using our coherency criterion reconstruct basin boundary structures more accurately than through those obtained using only first arrival times images from shot gathers collected along cocorp mojave line 5 across the garlock fault in cantil valley california show new evidence of links between garlock fault branches and sub horizontal structures in the upper crust imaging reflections and estimating velocities using coherency annealing is more effective and less expensive than picking reflection times from shot gathers recorded complex regions

mesozoic cenozoic rift drift sequence of asian fragments from gondwanaland abstract the university of cambridge atlas map plotting computer program has been used to plot a reconstruction of the dispersal of continental blocks from eastern gondwanaland from the late jurassic 160 ma until the late miocene 10 ma the geological basis for the reconstruction rests on the concept of south tibet burma western thailand malaya and sumatra having been part of the eastern gondwana continent during the late palaeozoic and early mesozoic the reconstruction is also based on the view that these asian continental blocks were rifted from eastern gondwanaland in the late jurassic this rifting episode gave rise to the continental margin of central new guinea and northern australia it was associated with the sea floor spreading anomalies preserved in the floor of the northeastern indian ocean the model considers the indication of crustal uplift in the middle jurassic followed by profound subsidence in the cretaceous and palaeogene exposed in the islands of the outer banda arc from seram to timor and the similar jurassic cretaceous facies in eastern sulawesi implies that these areas formed part of this rifted northern margin of australia in the jurassic subsequent tectonic collision and associated major strike slip movements have deformed and disrupted these rock sequences during the cainozoic the date of the rifting that removed south tibet burma western thailand and malaya from gondwanaland is controversial the indications in favour of the rifting episode having occurred in the jurassic rather than in the permo triassic are stratigraphie structural observations on the northwestern australian shelf in the banda arc and in central new guinea the identification of the continental blocks removed by this rifting is based partly on stratigraphie correlation with the banda arc and new guinea and partly on indications that the age of collision of tibet burma western thailand and malaya with asia was a late cretaceous event and not triassic jurassic as is generally held there are very few reliable land based palaeomagnetic measurements to constrain the movements of these dispersed continental fragments from gondwanaland the computer generated map reconstructions put forward here fit the available indian ocean sea floor spreading data although they are not unique solutions much of the critical ocean floor data associated with the rifting and dispersal of these continental fragments has been destroyed in the tectonic collision processes that occurred at the asian continental margin

orogen scale decollements the continental lithosphere responds to stress by deforming as a generally layered medium deep seismic reflection data coupled with a variety of ancillary geological and geophysical data are interpreted to provide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers and flat detachments in mechanically weak layers this geometry is similar to ramp and flat structures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen scale decollements in a manner that is analogous to so called thin skin deformation in sedimentary rocks decollements may occur within the crust for example near the base of a sedimentary section or in the middle crust near the moho in the subcrustal lithosphere or in the asthenosphere even intracratonic basement cored thick skin uplifts that occasionally occur in foreland regions such as the wyoming province are probably large crustal scale versions of ramp flat features observed in supracrustal rocks and are thus likely caused by the same fundamental tectonic processes

collision rotation and the initiation of subduction in the evolution of sulawesi indonesia the island of sulawesi indonesia has been shaped and deformed as a result of collision with the sula platform a sliver of continental material from the northern margin of australia new guinea the collision has resulted in rotation of the north volcanic arm of sulawesi and the development of the accretionary wedge of the north sulawesi trench the north sulawesi trench changes laterally from a zone of no active deformation in the eastern part to a wide accretionary wedge in the west early stages of thrusting produce a steep frontal slope 8 16 indicative of relatively high basal shear stress whereas the more advanced western zone of thrusting produces a gentle 2 slope consistent with low basal shear stress reported paleomagnetic data suggest post late eocene counter clockwise rotation of the north arm and the offshore geophysics are explained by a pivot of the north arm with respect to the celebes basin about the eastern end of the arc convergence between the north banda basin and southeast sulawesi is documented by the presence of the tolo thrust its outcrop is strongly arcuate and its accretionary wedge varies in width from a minimum of a few kilometers at each end to a maximum of 30 40 km in the central part the northern end transforms to the leftlateral matano fault with a reported offset of 20 km the southern end of the thrust projects toward the deformed rocks of buton but the structural relations there are not clear the matano fault zone appears to connect westward with the palu fault which forms the western transform of the north sulawesi trench the palu matano fault system acts as a trench trench transform between the north sulawesi trench and the tolo thrust and this system is described by the same rotation pole as that for the sulawesi north arm

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

regional sequence stratigraphic setting and reservoir geology of morrow incised valley sandstones lower pennsylvanian eastern colorado and western kansas oil and gas exploration for the lower pennsylvanian morrow formation of eastern colorado western kansas and northwestern oklahoma provides a subsurface data set that transects the entire range of lowstand depositional systems from incised valley fill systems to deep water basin floor systems in one composite depositional sequence one compound incised valley fill that is a part of this system contains three facies tracts with unique reservoir characteristics 1 the updip facies tract is dominated by amalgamated fluvial channel sandstones 2 the transition facies tract consists of fluvial channel sandstones interbedded with finer grained estuarine sandstones and 3 the downdip facies tract consists of ribbonlike fluvial channel sandstones isolated in estuarine shale a 175 mi long 283 km long longitudinal cross section through one trunk of the incised valley fill drainage shows that internal valley fill strata change significantly as a function of the interplay of varying depositional systems down gradient in the valley key contrasts in reservoir performance are documented as a function of changes in reservoir characteristics trap controls and trap configurations from updip to downdip in this valley fill drainage the strata of the morrow formation were deposited in a cratonic basin during a period in the earth s history when the climate was cooler than today high frequency changes of sea level across an extremely low gradient depositional surface controlled erosion and deposition these facies tracts reflect the response of valley fill sedimentary processes to high frequency relative sea level changes resulting from glacio eustasy the resultant valley fill systems have many characteristics in common with published valley fill models but have significant differences as well

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

inversion of p coda for isotropic scatterers at the yucca flat test site although many studies have addressed the statistical properties of p coda there have been few attempts to analyze coda in a deterministic framework in this analysis 1050 teleseismic p wave seismograms from 32 yucca flat explosions are inverted for the locations of any isotropic point scatterers in the source region that contribute coherent arrivals to the coda a rectangular grid of potential scattering locations is constructed around the source array and the semiblance a measure of waveform ensemble coherence for each point is computed by slant stacking the full set of seismograms at various move out velocities the potential resolution or sensitivity of the method for this data set is tested by constructing a corresponding suite of synthetic seismograms with direct arrivals plus arrivals from various point scatters and calculating the semiblance at each point on the grid the particular source station geometry produces better sensitivity near the array sensitivity is enhanced parallel to the trend of the elongated source array and diminished in the orthogonal direction stacking the data for a velocity of 2 5 km sec yields relatively high semiblance values in the central portion of the test site as well as about 10 km to the ene and moderately high semiblance valuesmore about 10 km to the wnw less

a scattered wave image of subduction beneath the transverse ranges over 5600 short period recordings of teleseismic events were used to create deterministic maps of p wave scatterers in the upper mantle beneath southern california between depths of 50 and 200 kilometers the southern flank of the slab subducting beneath the transverse ranges is marked by strong scattering the marked scattering indicates that the edge of the slab is a sharp thermal boundary such a boundary could be produced by slab shearing or small scale convection in the surrounding mantle the northern limb of the slab is not a strong scatterer consistent with thicker lithosphere north of the transverse ranges

studies of the seismic coda using an earthquake cluster as a deeply buried seismograph array loosely speaking the principle of green s function reciprocity means that the source and receiver positions in a seismic experiment can be exchanged without affecting the observed seismograms consequently the seismograms observed at a single observation location o and caused by a cluster of microearthquakes at locations ei are identical to the time series that would be measured by an array of stress meters emplaced at positions ei recording waves generated by a source acting at o by applying array analysis techniques like slant stacking and frequency wave number analysis to these seismograms we can determine the directions and velocities of the component waves as they travel in the earthquake focal region rather than at the surface we have developed a computationally rapid plane wave decomposition which we have applied to single station recordings of aftershocks of the 1984 morgan hill california earthquake the analysis is applied to data from three seismic stations having considerably different site geologies one is a relatively hard rock station situated on franciscan metamorphics one is within the calaveras fault zone and one is on semiconsolidated sand and gravels we define the early coda to be the part of the coda initiating immediately after the direct s wave and ending at twice the s wave lapse time the character of the s wave and early coda varies from being impulsive at the first station to highly reverberative at the last we examine waves in sequential time windows starting at the s wave and continuing through the early part of the coda at all seismic stations the early coda is dominated by a persistent signal that must be caused by multiple scattering probably within 2 km of each seismic station despite clear station to station differences in the character of the early coda coda q values measured in the late coda greater than twice the s lapse time agree well among stations implying that the mechanisms causing the varying behavior of the early coda do not control the coda decay rate at the stations we have considered coda q values measured on horizontal components of motion agree within a factor of 2 with those measured on vertical components we have not been able to determine the composition of the late coda because of a low signal to noise ratio our analysis technique however is quite appropriate for such a task

earthquake focal mechanisms in the eastern transverse ranges and san emigdio mountains southern california and evidence for a regional decollement earthquake focal mechanisms obtained from p wave first motions are presented for the eastern transverse ranges and the san emigdio mountains in southern california the former region shows a predominance of strike slip faulting whereas quaternary faults in the region show thrust motion we suggest that the observed strike slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting fault deformation in the san emigdio mountains inferred from focal mechanisms is in accord with displacements across quaternary faults in the area this study and a search of the literature has yielded 19 mechanisms with shallow dipping nodal planes previous workers have interpreted such mechanisms as evidence for a regional decollement if such a regional decollement exists these data give some indication of its regional extent slip directions inferred from the focal mechanisms with shallow dipping nodal planes show some regional consistency but this pattern cannot be entirely explained with current tectonic models a comparison of the stress drop of an event having a shallow dipping nodal plane with an event with steeper planes gave inconclusive results

an introduction to the sangihe arc volcanism accompanying arc arc collision in the molucca sea indonesia in the molucca sea region of northeastern indonesia the sangihe and halmahera arcs are presently in the process of colliding the earth s only example of a collision between facing volcanic arcs the collision event is more advanced in the northern molucca sea where back arc thrusting occurs along the cotobato and philippine trenches and volcanic centers are inactive and dissected this paper describes the sangihe arc the western arc of the collision the sangihe are is approximately 500 km in length extending from the ne tip of sulawesi to mindanao philippines the arc comprises over 25 quaternary volcanic centers the eight presently active volcanoes being confined to the southern half of the arc the active volcanic front lies 100 110 km above the top of the west dipping benioff zone and volcanoes extend up to 70 km behind the front thereby defining a volcanic arc overlying earthquakes 100 to 180 km deep rock types range from basalt to rhyolite andesites are dominant pyroxene andesites are common on sulawesi and at or near the volcanic front from ruang to buhias pahepa and buang to mamanuk hornblende andesites are common at the volcanic front between mahengetang and sangihe island and in centers located well behind the volcanic front tholeiitic th suites defined on the basis of relative degrees of iron enrichment are confined to the southern volcanic front lack hornblende are low k k55 1 05 have low rb zr ratios 0 10 0 17 low ba y ratios 3 4 4 9 low rb cs ratios 8 14 and have the highest 87sr 86sr ratios 0 7039 0 7042 calcalkaline ca suites occur throughout the arc are low to high k k55 0 6 2 0 have variable rb zr ratios 0 15 0 65 and ba y ratios 4 2 12 4 high rb cs ratios 14 82 and low 87sr 86sr ratios 0 7035 0 7040 zr and y distribution coefficients are lower in th suites dzr 0 0 dy 0 2 0 5 than in ca suites dzr 0 0 0 8 dy 0 3 1 2 ti initially increases with increasing sio2 in some th suites while decreasing with sio2 in all ca suites with the exception of bukide ti zr 138 ti zr ratios of basalts exceed 100 only in th suites 76 130 lil element contents and lil hfs element ratios increase 2 to 3 times across the are away from the volcanic front and 2 to 3 times from south to north along the arc 87sr 86sr ratios decrease away from the volcanic front the south to north increase in lil element contents and lil hfs element ratios without corresponding changes in sr isotopic compositions are interpreted as resulting from decreasing degrees of partial melting northward this reflects the gradual cessation of volcanism in the north due to collision

reduced time migration of transmitted ps waves we develop the novel theory of transmitted ps migration and show that ps transmitted arrivals in a gulf of mexico vertical seismic profile vsp data set can be migrated to accurately image a salt sheet even though the receiver array is below the transmitting boundary we also show that migrating transmitted arrivals is effective in illuminating the base of an orebody invisible to pp reflections in general interfaces that bisect wavepath propagation i e the source and receiver are on opposite sides of the interface and therefore invisible to pp reflections can be imaged by migration of ps transmitted waves these results suggest that migration of ps transmitted waves opens new opportunities in imaging nearly vertical impedance boundaries that are typically invisible to conventional reflection imaging of crosswell and vsp data we also present a new interferometric method denoted as reduced time migration which uses the arrival time difference between the direct p wave and subsequent events to incre

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

a geology based 3d velocity model of the los angeles basin sediments abstract seismic hazard studies of the los angeles basin area require a realistic seismic velocity model we use geologic information about depth to crystalline basement depths to sedimentary horizons uplift of sediments and surface geology in a velocity depth age function to construct a three dimensional velocity model in earthquake location tests the model predicts travel times satisfactorily and in earthquake ground motion simulations the model correctly determines the timing and amplitude of late arriving waves

acoustic imaging of earthquake sources from the chalfant valley 1986 aftershock series summary following the chalfant valley earthquake of 1986 july 21 ml 6 4 the usgs deployed 120 vertical component seismometers to obtain dense array recordings of aftershocks from this data set we selected three events with the best station coverage to obtain acoustic images of the earthquake sources the events were relocated using a layered velocity model and models with a constant velocity and a constant gradient to obtain minimum traveltime residuals all models performed similarly well preprocessing of the data prior to the extrapolation consisted of applying static corrections lowpass filtering and spatial interpolation as well as spatial tapering at the edges of the recording line for the back projection of the recorded wavefield we used a second order finite difference reverse time extrapolation technique similar to mcmechan 1982 while for two events no focusing of the energy could be obtained an acoustic image of one earthquake source was retrieved in order to obtain a better understanding of the physical meaning of acoustic imaging we applied the same processing and extrapolation scheme to synthetic seismograms calculated using the reflectivity method for the present data set these tests indicate that the influence of the 1 d velocity model on the focusing quality is very low the most severe limitations however are due to the finite aperture and gaps of the recording line the spatial sampling of the wavefield limits the frequency range used for extrapolation so that the resolution is beyond the scale of the source processes the finite aperture leads to polarity reversals in the final source image which makes an interpretation of the radiation pattern at the time of focusing from the reconstructed image impossible

geochemistry of quaternary lavas from ne sulawesi transfer of subduction components into the mantle wedge major and trace element and sr nd isotope compositions were determined for quaternary volcanic rocks from ne sulawesi the sangihe are indonesia in order to examine the origin of across arc variation in lava and magma source chemistry the arc is formed in an intraoceanic tectonic setting and is not associated with a backarc basin thereby minimizing possible contributions from non arc geochemical reservoirs the geochemistry of these arc lavas is likely to provide essential information about the chemical characteristics of subduction components all incompatible elements except pb increase away from the volcancic front major element data for mg rich lavas together with available experimental data suggest that primary magmas are produced at higher pressured by smaller degrees of partial melting beneath the backarc side volcanoes rb k and ba pb are higher and 87sr 86sr and 143nd 144nd are lower in backarc side lavas these variations may be attributed to generation of hydrous fluids in the downdragged hydrous peridotite layer at the base of the mantle wedge through the following reactions decompositions of pargasitic amphibole to form phlogopite and breakdown of phlogopite to crystallize k richterite beneath the volcanic front and the backarc side volcanoes respectively

evidence for possible horizontal faulting in southern california from earthquake mechanisms we find that 36 of the 505 fault plane solutions m 3 0 1981 1990 in southern california have a nodal plane dipping no more than 30 with the assumption of the low angle nodal planes being the fault planes four cross sections are constructed to show the possible horizontal faults in the middle and upper crust more than half of these low angle faults are located within or adjacent to the transverse ranges the focal depths vary from 1 km in the southern end of the sierra nevada and the southwestern mojave desert to 20 km in the transverse ranges the slip directions are also diverse in general east west extensional movements are dominant in the boundary between the southern sierra nevada extending to the san emigdio mountains and the western mojave desert whereas north south compressional movements are dominant in the transverse ranges in the peninsular ranges and the salton trough both the slip directions and focal depths vary these features suggest that seismically active low angle faults in southern california may exist at different depths and slip in various directions our data do not support the existence of a regional scale seismically active detachment in southern california only in the western transverse ranges is there some suggestion of a large detachment surface at a depth of about 13 to 14 km

three dimensional imaging of steeply dipping structure near the san andreas fault parkfield california shot gathers from the parkfield california deep crustal seismic reflection line recorded in 1977 by cocorp reveal coherent events having horizontal to reverse moveouts these events were migrated using a multioffset three dimensional kirchhoff summation method this method is a ray equation back projection inversion of the acoustic wave field which is valid under the born wkbj and far field assumptions migration of full wave acoustic synthetics having the same limitations in geometric coverage as the cocorp survey demonstrates the utility of the imaging process the images obtained from back projection of the survey data suggest that the gold hill fault carries ultramafic rocks from the surface to 3 km depth at a dip greater than 45 degrees where it joins the san andreas fault which may cut through more homogeneous materials at shallow depths to the southwest a 2 km tertiary sedimentary section appears to terminate against a near vertical fault the zone between this fault and the san andreas may be floored at 3 km by flat lying ultramafics lateral velocity inhomogeneities are not accounted for in the migration but in this case do not seriously hinder the reconstruction of reflectors

contemporaneous subsidence and fluvial channel sedimentation upper mannville c pool berry field lower cretaceous of alberta upper mannville c pool is a small oil pool which produces from lower cretaceous fluvial sandstones in berry field southeastern alberta ultimate oil reserves of the pool are 5 8 million m3 of oil contained within a sandstone body about 7 5 km long and 2 km wide the pool is unusual for its occurrence on the flank of a local structural high within a large regional structural depression sandstone distribution rather than present day structure dictates the occurrence of hydrocarbons yet the distribution of sandstones was controlled by structure a fluvial channel was diverted into an area of synsedimentary faulting or downwarping which then controlled the depositional form of an asymmetric wedge of channel levee and crevasse splay sands the most likely cause of subsidence was differential salt solution regionally the berry area lies along the margin of several devonian salt formations which are known to have undergone partial solution lateral migration of the locus of salt solution and hence subsidence through time produced both the facies and structural configurations observed in this oil pool

crustal imaging in southern california using earthquake sequences abstract an inexpensive means to further understand the geometry of active faults in southern california arises from the use of aftershock recordings to image crustal structures the advent of regional seismic networks that record digital seismograms from hundreds of stations makes this crustal reflectivity profiling possible even in the absence of conventional active source seismic data we show that it is feasible to image fault structure using three dimensional wide angle prestack kirchhoff migration we achieve this with the use of aftershock traces recorded on the short period vertical stations of the southern california seismic network this work complements seismicity and focal mechanism work by imaging reflectivity volumes and cross sections rather than having to associate events with certain faults further it can image below the seismogenic zone to resolve current geologic controversies on how proposed faults extend below focal depths we demonstrate the validity of these images as showing reflective structures and the ability to use clipped high gain seismograms as sign bit data to yield valid geometric imaging work with data from the 1991 sierra madre earthquake sequence images the prominent lower crustal reflective zone observed beneath most of the san gabriel mountains by the los angeles region seismic experiment line 1 aftershocks of the 1994 northridge earthquake allow us to image a north dipping structure that may represent the fault plane of a crustal penetrating blind thrust the images serve as a test for the existence and geometry of thrust ramps and detachments proposed from balanced section reconstructions of shallow crustal profiles and borehole data our results are more consistent with a thick skinned tectonic regime in the vicinity of the northridge earthquake rather than a thin skinned model

three dimensional kinematic depth migration of converted waves application to the 2002 molise aftershock sequence southern italy migration techniques currently used in seismic exploration are still scarcely applied in earthquake seismology due to the poor source knowledge and sparse irregular acquisition geometries at the crustal scale classical seismological studies often perform inversions based on the arrival time of primary phases p and s waves but seldom exploit other information included in seismic records here we show how migration techniques can be adapted to earthquake seismology for converted wave analysis as an example we used data recorded by a dense local seismic network during the 2002 molise aftershock sequence in october and november 2002 two moderate magnitude earthquakes struck the molise region southern italy followed by an aftershock sequence lasting for about one month local earthquake tomography has provided earthquake hypocenter locations and three dimensional models of p and s velocity fields strong secondary signals have been detected between first arrivals of p and s waves and identified as sp transmitted waves in order to analyse these waves we apply a prestack depth migration scheme based on the kirchhoff summation technique since source parameters are unknown seismograms are equalized and only kinematic aspects of the migration process are considered converted wave traveltimes are calculated in the three dimensional 3d tomographic models using a finite difference eikonal solver and back ray tracing in the migrated images the area of dominant energy conversion corresponds to a strong seismic horizon that we interpreted as the top of the apulia carbonate platform and whose geometry and position at depth is consistent with current structural models from existing commercial seismic profiles gravimetric and well data

stewart field finney county kansas seismic definition of thin channel reservoirs fluvial channel sandstones of early pennsylvanian morrowan age comprise regional reservoirs in parts of colorado kansas texas and oklahoma in southwestern kansas these reservoirs commonly exist at depths of 4000 5000 ft 1200 1500 m and have reserves of 150 000 200 000 bbl of oil per well making them highly economical reservoir sandstones form part of transgressive valley fill sequences deposited within channels incised into underlying mississippian carbonates thickness of the fill varies up to 60 ft 18 m is commonly 10 30 ft 3 9 m and displays rapid changes along channel length as a result detailed mapping of channel trends is difficult stewart field located in finney county kansas is a good example of this type of reservoir maximum reservoir qual ty exists in very fine to fine grained fluvial sandstones reworked by tidal action early attempts to extend the field to the east failed because existing two dimensional seismic and well data did not help workers properly resolve channel orientation a three dimensional 3 d seismic survey shot prior to initiation of waterflood operations helped 1 locate the channel between existing dry holes and 2 identify prospective locations that were then successfully drilled further extrapolation of the 3 d data resulted in a dry hole that established the limits of interpretation in this area stewart field thus provides an important case study regarding the capabilities and limitations of 3 d data in exploring the interwell frontier

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

the 1991 sierra madre earthquake sequence in southern california seismological and tectonic analysis the m l 5 8 sierra madre earthquake of 28 june 1991 occurred at a depth of 12 km under the san gabriel mountains of the central transverse ranges since at least 1932 this region had been quiescent for m 3 the mainshock focal mechanism derived from first motion polarities exhibited almost pure thrust faulting with a rake of 82 on a plane striking n62 e and dipping 50 to the north the event appears to have occurred on the clamshell sawpit fault a splay of the sierra madre fault zone the aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes having a b value of 0 6 twenty nine single event focal mechanisms were determined for aftershocks of m 1 5 the 4 km long segment of the clamshell sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east northeast striking fault plane dipping to the north to the west several thrust aftershocks with east striking nodal planes suggest some complexity in the aftershock faulting such as a curved rupture surface in addition several strike slip and normal faulting events occurred along the edges of the mainshock fault plane indicating secondary tear faulting the tectonic stress field driving the coexisting left lateral strike slip and thrust faults in the northern los angeles basin is north south horizontal compression with vertical intermediate or minimum principal stress axis

k ar dating young rhyolitic rocks a case study of the sierra la primavera jalisco mexico this study demonstrates the feasibility of dating by the k ar method an eruptive sequence of silicic volcanic rocks with ages close to the limits of conventional 14 c dating forty six conventional k ar analyses of sanidine and glass from rhyolites of the sierra la primavera mexico yielded dates of 5 to 150 ka thousand years one sigma errors range from 3 to 39 ka although the samples contain small amounts of extraneous argon most of the dates are consistent with relative ages determined in the field the exceptions are dates on immediately postcaldera eruptive units which are older than the dates for the caldera producing ash flow tuff a similar result has been obtained on dating tuffs and lavas of some other pleistocene calderas this effect is attributed to incomplete outgassing of the larger amount of extraneous argon dissolved in magma that occupied a position deeper in the chamber than the ash flow tuff magma and that erupted immediately after caldera collapse we present two new error equations one a simple extension of the commonly used equation demonstrates the importance of including a term for the uncertainty in the spectrometer mass discrimination factor in the error equation the second equation is a more rigorous formulation of the propagation of errors in the calculation of the age it yields meaningful error estimates for all proportions of radiogenic 40 ar and because it expresses the error in absolute rather than relative terms it can be used to put an upper limit on the age of a sample when no radiogenic 40 ar has been resolved

genesis of collision volcanism in eastern anatolia turkey late cenozoic volcanism in eastern anatolia extends in a broad sw ne trending belt across the arabia eurasia collision zone from the arabian foreland basin in the southwest and to the kars plateau and lesser caucasus in the northeast foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic alkaline composition volcanism on the thickened crust north of the bitlis thrust zone varies from the mildly alkaline volcano nemrut and older mus volcanics in the south through the transitional calc alkaline alkaline volcanoes bingol and suphan and the alkaline volcano tendurek to the calc alkaline volcano ararat and older kars plateau volcanics in the north isotope sr nd and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time and that lavas from the alkaline volcanic area around mus and the volcanoes nemrut and tendurek north of the bitlis thrust zone were derived from a lithospheric source of similar composition either from the same underthrust arabian continent or from the bitlis massif microcontinent by contrast the transitional lavas from bingol and suphan and the calc alkaline lavas from ararat and kars were derived from lithosphere carrying a distinct subduction signature inherited from pre collision subduction events positive correlations between 87sr 86sr and sio2 and rb nb and sio2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone trace element covariation diagrams such as y rb indicate the importance of hornblende crystallization at depth and orthopyroxene at shallow levels within the calc alkaline provinces in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas trace element diagrams based on the covariation of compatible and incompatible elements point to moderate low degrees of partial melting with residual clinopyroxene throughout and residual garnet in the foreland province consideration of mineral stabilities mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths probably beneath the amphibole and dolomite breakdown curves perturbation of the thickened lithosphere by delamination of the thermal boundary layer perhaps coupled with local stretching associated with pull apart basins on strike slip fault systems is then sufficient to generate melt the composition of that melt being largely dependent on the enrichment history of the lithosphere in question in eastern anatolia volcanism appears to have started between about 8 and 6 ma ago some 5 ma after the start of rapid uplift of the east anatolian plateau

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

magmatic evolution of sulawesi indonesia constraints on the cenozoic geodynamic history of the sundaland active margin abstract tertiary and quaternary magmatic rocks from west sulawesi record the complex history of part of the sundaland margin where subduction and collision have been and are still active the present study based on petrographic data major and trace element chemistry and 40 k 40 ar dating aims to document the age and chemical characteristics of the magmatic formations from west sulawesi and to determine the corresponding constraints on the geodynamic evolution of the sundaland border the west sulawesi magmatic province includes the south arm of sulawesi ujung pandang area the western part of central sulawesi with the toraja and palu areas and finally the north arm extending from palu to manado which includes the tolitoli and manado areas paleocene magmatic activity seems to be restricted to an episode of calc alkaline magmatism in the ujung pandang area 61 59 ma the major eocene 50 40 ma magmatic event is tholeiitic and is documented in all areas except in ujung pandang it led to the emplacement of tholeiitic pillow lavas and basaltic dykes of back arc basin bab affinity these rocks are potential equivalents to the celebes sea basaltic basement from oligocene to miocene magmatic eruptions produced successively island arc tholeiitic iat and calc alkaline ca rock series the youngest iat activity occurred around 18 ma in the central part palu area and around 14 ma in the north arm tolitoli area while ca magmas were emplaced in the north arm at ca 18 ma tolitoli and manado areas typical calc alkaline activity resumed only in the north arm tolitoli and manado areas during the late miocene 9 ma and is still active in the manado region in other areas palu toraja and ujung pandang areas an important and widespread magmatic event occurred between 13 and 10 ma and emplaced k rich magmas either silica undersaturated alkali potassic basalts ak ultrapotassic basanites uk or shoshonites sh k rich activity continued in the south until the pleistocene 0 77 ma with alkali potassic ultrapotassic and shoshonitic magmas in central sulawesi toraja and palu areas the most recent magmatic event occurred between 6 5 and 0 6 ma the corresponding products are granitic rocks and widely distributed rhyolitic pyroclastic flow deposits all these rocks are acidic in character sio 2 60 with trace element and isotopic signatures sr nd pb typical of a strong continental imprint the most striking tectonic implication of this magmatic evolution is that west sulawesi can no longer be considered as a typical magmatic arc as previously assumed with the exception of the manado area beneath which subduction is still active calc alkaline and island arc tholeiitic lavas and plutonics are volumetrically minor with respect to k rich magmas their occurrence through time is also fairly restricted mostly to the period between 30 and 15 ma another important feature is the occurrence of island arc tholeiitic and calc alkaline magmas crosscutting an older terrane of bab affinity the tinombo formation manado tolitoli and palu areas as this formation is being regarded as an equivalent to the celebes sea floor the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the nw dipping subduction beneath the north arm the late miocene high k magmatic activity in central and south sulawesi reflects the prevalence of a post collisional tectonic regime following the docking of microcontinents of australian origin to central sulawesi during neogene times the incompatible element enriched character of these high k rocks might reflect their derivation from a mantle source enriched through metasomatism related to a previous subduction event such a model cannot account for the plio pleistocene cak magmatism of central sulawesi the acidic composition of which does not support a derivation from an ultrabasic source the trace element patterns of the cak rocks are very similar to those of the high grade metamorphics of central sulawesi suggesting that the latter might represent their possible source such an anatectic model implies a collisional to post collisional tectonic regime limited to central sulawesi while a post subduction regime prevailed in the south

petrogenesis and tectonic setting of the roman volcanic province italy abstract the volcanism in the roman province of italy can be modelled by the partial melting of heterogeneously enriched mantle sources the heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central apennines new petrographical and chemical data are presented for the high k calc alkaline and the shoshonitic volcanics of the campania region primary magmas are present only locally the existence of spatial zonation in the volcanism of campania is documented for the first time the shoshonitic leucite basanitic and leucititic volcanics of the phlegraean fields procida ischia and the somma vesuvius areas are at similar degrees of evolution about two times richer in nb and ba than those of northwestern campania and the latium part of the roman province accordingly distinct north western and south eastern subprovinces can be defined the evaluation of enrichment factors that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low k and the high k suites shows that the mantle sources prior to k metasomatism were different in the two sub provinces of the roman province in the north western one they resembled the sources of ocean island tholeiites and moderately enriched mor basalts in the south east they were similar to those of ocean island alkaline lavas and enriched morb s modelling based on k p ce sr rb ba th sm eu gd y nb and 87 sr 86 sr was carried out it indicates that the range of mantle sources of the volcanics in northwestern campania and latium can be modelled by the addition of 3 to 20 of materials derived by partial melting of carbonaceous pelites to a sr enriched mantle wedge comparable to the honolulu mantle source least enriched in nb the production of sr enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust or a small amount of metasomatism caused by the presence of carbonatite melts the near absence of ti ta nb yb and the highly fractionated ree in the metasomatizing component requires the presence of residual garnet and accessory ti rich minerals during the partial melting of the subducted sediments the writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust probably sediments this process played a dominant role in the generation of the mantle sources from which the high k calc alkaline shoshonitic leucite basanitic and leucititic magmas of the roman province were derived

mapping shallow heterogeneity with teleseismic p to rg scattered waves over 9000 records of teleseismic p wave coda from the southern california seismic network are used to map lateral variations in p to rg scattering strength and the group velocity of scattered rg building on the results of revenaugh 1995a we use the correlation of scattering strength and topographic roughness as an indicator of model fit to estimate group velocity dispersion over five frequency bands between 0 and 3 hz rg group velocity displays normal dispersion decreasing from 2 7 0 3 km sec for frequencies between 0 and 0 6 hz to a low of 2 2 0 4 km sec for the 2 0 to 3 0 hz band lateral heterogeneity is introduced by assuming rg velocity is a function of local topographic roughness grid search migration analysis yields group velocities of 2 2 and 2 85 km sec for smooth and rough terrain respectively and a 150 m threshold for the local standard deviation of topography distinguishing smooth from rough the map of scattering strength approximates topographic roughness throughout southern california implying that teleseismic p wave coda duration and relative amplitude level depend strongly on near surface 3 km geology

joint optimization of vertical component gravity and p wave first arrivals by simulated annealing abstractjoint seismic and gravity analyses of the san emidio geothermal field in the northwest basin and range province of nevada demonstrate that joint optimization changes interpretation outcomes the prior 0 3 0 5 km deep basin interpretation gives way to a deeper than 1 3 km basin model kirchoff prestack depth migrations reveal that joint optimization ameliorates shallow velocity artifacts flattening antiformal reflectors that could have been interpreted as folds furthermore joint optimization provides a clearer picture of the rangefront fault by increasing the depth of constrained velocities which improves reflector coherency at depth this technique provides new insight when applied to existing data sets and could replace the existing strategy of forward modeling to match gravity data we have achieved stable joint optimization through simulated annealing a global optimization algorithm that does not require an accurate initial model balancing the combined seismic gravity objective function i

resolution of prominent crustal scatterers near the noress small aperture array summary our goal is to identify prominent scatterers near seismic arrays so that the secondary phases they generate can be anticipated and recognized as being of local not teleseismic origin toward this end we have developed a technique to scan seismic coda recorded by arrays for phases generated locally by scattering of teleseismic energy from large topographical and crustal heterogeneities in this paper we test the technique by using the noress small aperture array in southern norway to image free surface secondary r sources excited by incoming teleseismic p waves in essence our technique suppresses the teleseismic energy and migrates the residual array records to image coherent local sources to assess the azimuthal and radial resolution that the noress array provides we have analysed point source synthetics a preliminary analysis of recorded data provides an initial view of the scatterers in the vicinity of noress and identifies limitations of imaging using the full coda of individual events single events can be used to image scatterers but more robust images with higher azimuthal resolution can be obtained by using a broadly distributed suite of events radial resolution of scatterers can be enhanced by deconvolution a composite image generated by simultaneously considering a suite of 31 deconvolved teleseisms indicates that two secondary r sources are present at or near the free surface near noress one 27 km to the south west of the array appears to be related to dramatic topographic relief at the south west edge of lake mjosa another roughly 10 km to the east of the array is judged by a bootstrapresampling analysis to be only marginally significant this apparent source does not coincide with obvious topography and is likely buried although the time delay that separates the scattered and parent phases depends somewhat on the angle of approach of the incoming waves in general these secondary sources are contributing wave trains to the seismic coda that begin 4 and 11 s respectively after the primary onset

the contribution of topographic scattering to teleseismic coda in southern california given that seismic coda is largely the result of scattering near the receiver a tie to geological complexity i e to crustal heterogeneity is expected but a systematic and predictive understanding of the important aspects of geology is lacking we have developed a technique for mapping the origins of coda energy as recorded by regional networks that provides insight into the proximal generation of seismic coda here the role of topography in the production of surface propagating coda energy both as a scatterer and as a proxy of shallow geologic structure is examined for the portion of the san andreas fault system within the southern california seismic network we find significant correlation of p to rg scattering potential with topographic roughness that extends to relatively subtle topography the correlation peaks at a migration velocity of 2 9 km s confirming the identification of scattered rg energy and offering an alternative means of estimating regional shallow velocity structure

genesis evolution and tectonic significance of k rich volcanics from the alban hills roman comagmatic region as inferred from trace element geochemistry trace element data are reported in 21 lava samples from the alban hills one of the most important volcanic complexes of the roman comagmatic region the samples consist mostly of tephritic leucitites with minor phonolitic tephrites and tephritic phonolites emplaced during two distinct phases of activity separated by a caldera collapse