heat flow strong near fault seismic waves and near fault tectonics on the central san andreas fault the main san andreas fault strikes subparallel to compressional folds and thrust faults its fault normal traction is on average a factor of 1 2 thr 1 thr2 thr where thr is the coefficient of friction for thrust faults times the effective lithostatic pressure a useful upper limit for thr of 0 6 where is 3 12 is obtained from the lack of heat flow anomalies by considering off fault convergence at a rate of 1 mm yr for 10 km across strike if the fault normal traction is in fact this high the well known heat flow constraint of average stresses of 10 20 mpa during strike slip on the main fault becomes more severe only a few percent of the total slip during earthquakes can occur at the peak stress before dynamic mechanisms weaken the fault the spatial dimension of the high stress rupture tip zone is 10 m for 3 12 and for comparison 100 m for 1 high dynamic stresses during shaking occur within these distances of the fault plane in terms of scalars fine scale tectonic stresses cannot exceed the difference between failure stress and dynamic stress plate scale slip causes stresses to build up near geometrical irregularities of the fault plane strong dynamic stresses near the rupture tip facilitate anelastic deformation with the net effects of relaxing the local deviatoric tectonic stress and accommodating deformation around the irregularities there also is a mild tendency for near fault material to extrude upward slip on minor thrust faults causes the normal traction on the main fault to be spatially variable

preliminary results from the sprites94 aircraft campaign 2 blue jets initial observations of a newly documented type of optical emission above thunderstorms are reported blue jets or narrowly collimated beams of blue light that appear to propagate upwards from the tops of thunderstorms were recorded on b w and color video cameras for the first time during the sprites94 aircraft campaign june july 1994 the jets appear to propagate upward at speeds of about 100 km s and reach terminal altitudes of 40 50 km fifty six examples were recorded during a 22 minute interval during a storm over arkansas we examine some possible mechanisms but have no satisfactory theory of this phenomenon

structure of the california coast ranges and san andreas fault at safod from seismic waveform inversion and reflection imaging 1 a seismic reflection and refraction survey across the san andreas fault saf near parkfield provides a detailed characterization of crustal structure across the location of the san andreas fault observatory at depth safod steep dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the saf the resulting velocity model constrains the top of the salinian granite with great detail steep dip reflection seismic images show several strong amplitude vertical reflectors in the uppermost crust near safod that define an 2 km wide zone comprising the main saf and two or more local faults another prominent subvertical reflector at 2 4 km depth 9 km to the northeast of the saf marks the boundary between the franciscan terrane and the great valley sequence a deep seismic section of low resolution shows several reflectors in the salinian crust west of the saf two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along strike of the saf they represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust the deepest reflections from 25 km depth are interpreted as crust mantle boundary

anatomy of the chesapeake bay impact structure revealed by seismic imaging delmarva peninsula virginia usa 1 a 30 km long radial seismic reflection and refraction survey completed across the northern part of the late eocene chesapeake bay impact structure cbis on the delmarva peninsula virginia usa confirms that the cbis is a complex central peak crater we used a tomographic p wave velocity model and low fold reflection images constrained by data from two deep boreholes located on the profile to interpret the structure and composition of the upper 5 km of crust the seismic images exhibit well defined structural features including with increasing radial distance a collapsed central uplift a breccia filled moat and a collapsed transient crater margin which collectively constitute a 40 km wide collapsed transient crater and a shallowly deformed annular trough these seismic images are the first to resolve the deep structure of the crater 1 km and the boundaries between the central uplift moat and annular trough several distinct seismic signatures distinguish breccia units from each other and from more coherent crystalline rocks below the central uplift moat and annular trough within the moat breccia extends to a minimum depth of 1 5 km or a maximum of 3 5 km depending upon the interpretation of the deepest layered materials the images show 350 to 500 m of postimpact sediments above the impactites the imaged structure of the cbis indicates a complex sequence of events during the cratering process that will provide new constraints for numerical modeling

joint inversion of seismic and magnetotelluric data in the parkfield region of california using the normalized cross gradient constraint we present jointly inverted models of p wave velocity vp and electrical resistivity for a two dimensional profile centered on the san andreas fault observatory at depth safod significant structural similarity between main features of the separately inverted vp and resistivity models is exploited by carrying out a joint inversion of the two datasets using the normalized cross gradient constraint this constraint favors structurally similar vp and resistivity images that adequately fit the seismic and magnetotelluric mt datasets the new inversion code tomoddmt merges the seismic inversion code tomodd and the forward modeling and sensitivity kernel subroutines of the mt inversion code occam2dmt tomoddmt is tested on a synthetic dataset and demonstrates the code s ability to more accurately resolve features of the input synthetic structure relative to the separately inverted resistivity and velocity models using tomoddmt we are able to resolve a number of key issues raised during drilling at safod we are able to infer the distribution of several geologic units including the salinian granitoids the great valley sequence and the franciscan formation the distribution and transport of fluids at both shallow and great depths is also examined low values of velocity resistivity attributed to a feature known as the eastern conductor ec can be explained in two ways the ec is a brine filled high porosity region or this region is composed largely of clay rich shales of the franciscan the eastern wall which lies immediately adjacent to the ec is unlikely to be a fluid pathway into the san andreas fault s seismogenic zone due to its observed higher resistivity and velocity values

high resolution seismic velocities and shallow structure of the san andreas fault zone at middle mountain parkfield california a 5 km long high resolution seismic imaging survey across the san andreas fault saf zone and the proposed san andreas fault observatory at depth safod drill site near parkfield california shows that velocities vary both laterally and vertically velocities range from 4 0 km sec probably correspond to granitic rock of the salinian block which is exposed a few kilometers southwest of the saf the depth to the top of probable granitic rock varies laterally along the seismic profile but is about 600 m below the surface at the proposed safod site we observe a prominent lateral low velocity zone lvz beneath and southwest of the surface trace of the saf the lvz is about 1 5 km wide at 300 m depth but tapers to about 600 m wide at 750 m depth at the maximum depth of the velocity model 750 m the lvz is centered approximately 400 m southwest of the surface trace of the saf similar velocities and velocity gradients are observed at comparable depths on both sides of the lvz suggesting that the lvz is anomalous relative to rocks on either side of it velocities within the lvz are lower than those of san andreas fault gouge and the lvz is also anomalous with respect to gravity magnetic and resistivity measurements because of its proximity to the surface trace of the saf it is tempting to suggest that the lvz represents a zone of fractured crystalline rocks at depth however the lvz instead probably represents a tectonic sliver of sedimentary rock that now rests adjacent to or encompasses the saf such a sliver of sedimentary rock implies fault strands on both sides and possibly within the sliver suggesting a zone of fault strands at least 1 5 km wide at a depth of 300 m tapering to about 600 m wide at 750 m depth fluids within the sedimentary sliver are probably responsible for observed low resistivity values

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

tomographic vp and vs structure of the california central coast ranges in the vicinity of safod from controlled source seismic data summary a seismic reflection refraction survey across the san andreas fault near parkfield california has refined our knowledge of the upper crustal structure of the central california coast ranges at the san andreas fault observatory at depth safod the survey consisted of a 46 km long line of seismographs 25 50 m spacing and 63 explosions 25 200 kg nominal spacing of 500 m with some gaps the traveltimes of refracted p and s waves from the explosions constitute independent data sets of relatively high quality that were inverted to produce p and s wave velocity models vp vs along the profile extending to as much as 5 km depth the vp and vs models show a prominent lateral drop in velocities a few hundred metres northeast of safod between the drill hole and the san andreas fault the vp model shows particularly well a southwest dipping velocity inversion beneath safod the top of which correlates with a fault penetrated by the drill hole that separates granitic rocks above from sedimentary rocks below in addition to vp and vs models a vp vs model was derived a vp vs ratio lower than 1 73 is seen only at depth in a narrow zone beginning at the target earthquakes for safod and extending downward and northeastward into the north america plate clusters in the parameter space spanned by vp vs ratios and vp can be identified by two different methods one more intuitive analytical method and one more abstract method based on neural network techniques these clusters are correlated to different rock types based on laboratory and in situ data these clusters are remapped back into x z plane along the profile prominent features mapped this way include salinian granitic rocks beneath and west of safod and a body of sedimentary rocks faulted beneath these granitic rocks along what we and others interpret to be a branch of the buzzard canyon fault bcf system these sedimentary rocks extend from this fault to the san andreas fault system unfortunately our cluster analysis shows no significant discontinuity at the san andreas fault owing presumably to the fact that the san andreas fault is located within sedimentary rocks having similar elastic properties this paper is an attempt to downward continue a geological map by geophysical means based on elastic properties of rock samples from the region

seismicity faulting and structure of the koyna warna seismic region western india from local earthquake tomography and hypocenter locations although seismicity near koyna reservoir india has persisted for 50 years and includes the largest induced earthquake m 6 3 reported worldwide the seismotectonic framework of the area is not well understood we recorded 1800 earthquakes from 6 january 2010 to 28 may 2010 and located a subset of 343 of the highest quality earthquakes using the tomodd code of zhang and thurber 2003 to better understand the framework we also inverted first arrivals for 3 d vp vs and vp vs and poisson s ratio tomography models of the upper 12 km of the crust epicenters for the recorded earthquakes are located south of the koyna river including a high density cluster that coincides with a shallow depth 1 5 km zone of relatively high vp and low vs also high vp vs and poisson s ratios near warna reservoir this anomalous zone which extends near vertically to at least 8 km depth and laterally northward at least 15 km is likely a water saturated zone of faults under high pore pressures because many of the earthquakes occur on the periphery of the fault zone rather than near its center the observed seismicity velocity correlations are consistent with the concept that many of the earthquakes nucleate in fractures adjacent to the main fault zone due to high pore pressure we interpret our velocity images as showing a series of northwest trending faults locally near the central part of warna reservoir and a major northward trending fault zone north of warna reservoir

variations of the velocity contrast and rupture properties of m6 earthquakes along the parkfield section of the san andreas fault summary we investigate the seismic velocity contrast across the san andreas fault saf in the parkfield area using fault zone head waves fzhw that propagate along the bimaterial fault interface and direct p waves we systematically analyse large data sets of near fault waveforms recorded by several seismic networks over the period 1984 2005 clear fzhw are observed at many stations on the ne side of the fault in the creeping section of the saf north of middle mountain mm this indicates the presence of a sharp bimaterial interface and that the ne side of the fault has lower seismic velocities in that region the obtained p wave velocity contrast is about 5 10 per cent north of mm and it systematically decreases to 0 2 per cent near gold hill gh the along strike variations of the velocity contrast are consistent with geological observations of a sliver of high velocity rock immediately to the ne of the saf near gh associated with the gh fault and existing 3 d seismic tomography results the obtained imaging results offer an explanation for the mixed rupture directions of the m6 type parkfield earthquakes the strong velocity contrast around mm is expected to produce a preferred propagation direction to the se for earthquakes that nucleate near mm e g the 1934 and 1966 parkfield earthquakes in contrast the near zero velocity contrast and multiple fault branches near gh imply that earthquakes that nucleate near gh e g the 2004 parkfield earthquake are not expected to have a preferred propagation direction to the se and are likely to propagate in directions that are controlled by other factors such as structural and stress heterogeneities the observed systematic reduction of the velocity contrast along the saf from nw of mm to se of gh provides a dynamic arrest mechanism for earthquakes that nucleate in the northern part of the parkfield section and propagate to the se and a dynamic arrest mechanism for earthquakes that nucleate in the southern section and propagate to the nw

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

joint inversion for vp vs and vp vs at safod parkfield california we refined the three dimensional 3 d vp vs and vp vs models around the san andreas fault observatory at depth safod site using a new double difference dd seismic tomography code tomoddps that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential p s and s p times this new method is able to provide a more robust vp vs model than that from the original dd tomography code tomodd obtained simply by dividing vp by vs for the new inversion waveform cross correlation times for earthquakes from 2001 to 2002 were also used in addition to arrival times from earthquakes and explosions in the region the vp values extracted from the model along the safod trajectory match well with the borehole log data providing in situ confirmation of our results similar to previous tomographic studies the 3 d structure around parkfield is dominated by the velocity contrast across the san andreas fault saf in both the vp and vs models there is a clear low velocity zone as deep as 7 km along the saf trace compatible with the findings from fault zone guided waves there is a high vp vs anomaly zone on the southwest side of the saf trace that is about 1 2 km wide and extends as deep as 4 km which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the salinian basement rock to the southwest the relocated earthquakes align beneath the northeast edge of this high vp vs zone we carried out a 2 d correlation analysis for an existing resistivity model and the corresponding profiles through our model yielding a classification that distinguishes several major lithologies

coordinated observations of sprites and in cloud lightning flash structure 1 the temporal and spatial development of sprite producing lightning flashes is examined with coordinated observations over an asymmetric mesoscale convective system mcs on 29 june 2011 near the oklahoma lightning mapping array lma sprites produced by a total of 26 lightning flashes were observed simultaneously on video from bennett colorado and hawley texas enabling a triangulation of sprites in comparison with temporal development of parent lightning in particular negatively charged stepped leaders in three dimensional space in general prompt sprites produced within 20 ms after the causative stroke are less horizontally displaced typically 30 km however both prompt and delayed sprites are usually centered within 30 km of the geometric center of relevant lma sources with affinity to negative stepped leaders during the prior 100 ms interval multiple sprites appearing as dancing jumping events associated with a single lightning flash could be produced either by distinct strokes of the flash by a single stroke through a series of current surges superposed on an intense continuing current or by both our observations imply that sprites elongated in one direction are sometimes linked to in cloud leader structure with the same elongation and sprites that were more symmetric were produced above the progression of multiple negative leaders this suggests that the large scale structure of sprites could be affected by the in cloud geometry of positive charge removal based on an expanded dataset of 39 sprite parent flashes by including more sprites recorded by one single camera over the same mcs the altitude above mean sea level msl of positively charged cloud region tapped by sprite producing strokes declined gradually from 10 km msl 35 c to around 6 km msl 10 c as the mcs evolved through the mature stage on average the positive charge removal by causative strokes of sprites observed on 29 june is centered at 3 6 km above the freezing level or at 7 9 km above ground level

joint inversion of body wave arrival times and surface wave dispersion for three dimensional seismic structure around safod we incorporate body wave arrival time and surface wave dispersion data into a joint inversion for three dimensional p wave and s wave velocity structure of the crust surrounding the site of the san andreas fault observatory at depth the contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations we find that the trade off between fitting the two data types controlled by the weighting defines a clear optimal solution varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type all the acceptable solutions yield structures with similar primary features but the smaller scale features change substantially when there is a lower relative weight on the surface wave data it appears that the solution over fits the body wave data leading to a relatively rough vs model whereas for the optimal weighting we obtain a relatively smooth model that is able to fit both the body wave and surface wave observations adequately

new high resolution ground based studies of sprites new observations of sprites cloud ionosphere luminous discharges above thunderstorms were made from the yucca ridge field station 20 km northeast of fort collins colorado on the night of july 11 12 1994 as part of a summer 1994 observing campaign the sprites appeared above a moderate mesoscale convective complex mostly over kansas at a range of about 270 km the sprites were observed with both wide field and telescopic image intensified ccd tv cameras a telescopic photometer system and a 1 to 50 khz band vlf sferics receiver this paper is based on five 1 s data intervals containing bright sprites smaller sprites and cloud and sky flashes telescopic tv images of bright sprites had a fan shaped upper plume with very fine features not well resolved by the tv but dendritic upward forked and vertically striated forms adjacent to these plumes and bright points of luminosity around the plume shaped regions many sprites consisted entirely of groups of vertically aligned striations which sometimes appeared to diverge from a common point of origin at cloud tops all sprites in the present data sample were preceded by a cloud to ground cg stroke with a coincident sferic and sky flash all cg strokes associated with sprites were positive and most were 100 ka or more inferred peak current from the photometer the duration of the cg induced sky flashes was about 3 ms and the additional sprite total light curve was also about 3 ms the puzzling feature that the total duration of tv images of sprites was often longer than the photometric values is discussed and an explanation given the sprites were attributed to strong negative charging following the positive cg stroke of a localized cloud top region which produced an intense electric field and a luminous discharge in the cloud ionosphere region the concept of break even electric fields suggested by mccarthy and parks may explain discharge initiation with moderate field strengths

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

preliminary results from the sprites94 aircraft campaign 1 red sprites the dual jet aircraft sprites94 campaign yielded the first color imagery and unambiguously triangulated physical dimensions and heights of upper atmospheric optical emissions associated with thunderstorm systems low light level television images in both color and in black and white b w obtained during the campaign show that there are at least two distinctively different types of optical emissions spanning part or all of the distance between the anvil tops and the ionosphere the first of these emissions dubbed sprites after their elusive nature are luminous structures of brief 16 ms duration with a red main body that typically spans the altitude range 50 90 km and possessing lateral dimensions of 5 30 km faint bluish tendrils often extend downward from the main body of sprites occasionally appearing to reach cloud tops near 20 km in this paper the principal characteristics of red sprites as observed during the sprites94 campaign are described the second distinctive type of emissions blue jets are described in a companion paper wescott et al this issue

reply to comment on near surface location geometry and velocities of the santa monica fault zone los angeles california by r d catchings g gandhok m r goldman d okaya m j rymer and g w bawden by t l pratt and j f dolan in a comment on our 2008 paper catchings gandhok et al 2008 on the santa monica fault in los angeles california pratt and dolan 2010 herein referred to as p d cite numerous objections to our work inferring that our study is flawed however as shown in our reply their objections contradict their own published works published works of others and proven seismic methodologies rather than responding to each repeated invalid objection we address their objections by topic in the subsequent sections in catchings gandhok et al 2008 we presented high resolution seismic reflection images that showed two near surface faults in the upper 50 m beneath the grounds of the wadsworth veterans administration hospital wvah although p d suggest we effectively duplicated their seismic acquisition our survey was not a duplication of their efforts rather we conducted a seismic imaging survey over a similar profile as pratt et al 1998 but used a different data acquisition system and different data processing methods to evaluate methods of seismically imaging blind faults in the wake of the 17 january 1994 m 6 7 northridge earthquake we used an acquisition method that provides both tomographic seismic velocities and reflection images our combined data approach allowed for shallower imaging 2 5 m minimum than the 20 m minimum of pratt et al 1998 clearer images of the fault zone and more accurate depth determinations rather than time images in processing the reflection images we used prestack depth migration which is generally accepted as the only proper imaging method for imaging subsurface structures with strong lateral velocity variations versteeg 1993 a condition shown to exist at the wvah site we correlated our reflection images with refraction tomography images borehole lithology and velocity data interferometric synthetic aperture radar images and changes in groundwater depths except for some minor differences our seismic reflection images coincide with previously published seismic reflection

joint inversion of gravity and arrival time data from parkfield new constraints on structure and hypocenter locations near the safod drill site 1 taking advantage of large datasets of both gravity and elastic wave arrival time observations available for the parkfield california region we generated an image consistent with both types of data among a variety of strategies the best result was obtained from a simultaneous inversion with a stability requirement that encouraged the perturbed model to remain close to a starting model consisting of a best fit to the arrival time data the preferred model looks essentially the same as the best fit arrival time model in areas where ray coverage is dense with differences being greatest at shallow depths and near the edges of the model where ray paths are few earthquake locations change by no more than about 100 m the general effect being migration of the seismic zone to the northeast closer to the surface trace of the san andreas fault

television image of a large upward electrical discharge above a thunderstorm system an image of an unusual luminous electrical discharge over a thunderstorm 250 kilometers from the observing site has been obtained with a low light level television camera the discharge began at the cloud tops at 14 kilometers and extended into the clear air 20 kilometers higher the image which had a duration of less than 30 milliseconds resembled two jets or fountains and was probably caused by two localizd electric charge concentrations at the cloud tops large upward discharges may create a hazard for aircraft and rocket launches and by penetrating into the ionosphere may initiate whistler waves and other effects on a magnetospheric scale such upward electrical discharges may account for unexplained photometric observations of distant lightning events that showed a low rise rate of the luminous pulse and no electromagnetic sferic pulse of the type that accompanies cloud to earth lightning strokes an unusually high rate of such photometric events was recorded during the night of 22 to 23 september 1989 during a storm associated with hurricane hugo

altitude time development of sprites 1 data from sprites using a 16 channel multi anode photometer map have been recorded at 0 1 ms resolution the majority of the sprites have onsets at an altitude of 70 75 km and subsequently propagate both upward and downward from this initial altitude the statistical lifetime of the emissions measured by the blue sensitive map is approximately 1 3 ms the velocities of propagation are between 107 and 108m s 1 with larger velocities being measured at higher altitudes the larger propagation velocities of the upward and downward sprite luminosity channels are consistent with the characteristics of highly over voltage streamers in the mesosphere

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

mitigation of guided wave contamination in waveform tomography of marine seismic reflection data from southwestern alaska abstractwe have applied 2d frequency domain acoustic waveform tomography to two different sections of a marine seismic reflection line from southwest alaska one section with a deep igneous basement overlain by a thick pile of sediments and the other section with a shallow basement and a thin sedimentary cover we have evaluated the appearance of dispersive guided waves on both sections and we have determined that with appropriate data preconditioning it is possible to invert the data using 2d acoustic waveform tomography where the basement is deep we first reduced the dispersive wave contamination of the seismic field data by trace editing band pass filtering and careful choice of the data window for inversion we then tested different objective functions and inversion scheduling before selecting an approach based on the logarithmic phase which could be followed by joint phase and amplitude inversion where the basement is shallow the starting model itself which was generated by ray based first a

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

distributions of strong ground motion due to dynamic ruptures across a bimaterial fault implications for seismic hazard analyses abstract we perform 2 d finite element calculations of mode ii rupture along a bimaterial interface governed by regularized rate and state dependent friction law with the goal of understanding how the bimaterial interface influences the strong ground motion by comparison with properties of rupture in a homogeneous solid we found that bimaterial mechanism is important for earthquake ruptures and influences the strong ground motion significantly the simulated results show that mode ii rupture evolves with propagation distance along a bimaterial interface to a unilateral wrinkle like pulse in the direction of slip on the compliant side of the fault namely in the positive direction strong ground motion caused by seismic waves emanated from the rupture propagation is asymmetrically distributed in space the computed peak ground acceleration pga is high in the near fault region particularly pga is much larger in the region on the side in the positive direction in addition it is greater in the more compliant area of the model than that in the stiffer area with corresponding locations moreover the differential pga due to bimaterial effect increases with increasing degree of material contrast across the fault it is hoped that the results obtained in this investigation will provide some implications for seismic hazard assessment and fault rupture mechanics

the san andreas fault revisited through seismic noise and surface wave tomography 1 we present here surface wave tomography results for the san andreas fault in the parkfield area california usa that were extracted from microseismic noise in the 0 15 hz to 0 35 hz frequency band using passive seismic correlation techniques using directive noise incoming from the pacific ocean passive seismic noise tomography was performed using three component sensors from a dense seismic network a rotation algorithm was applied to the nine component noise correlation tensor that optimally forced each station pair to re align in the noise direction a necessary condition to extract unbiased travel times from passive seismic processing after the rotation was performed an optimal surface wave tensor is obtained from which love waves were extracted for tomography inversion dispersion curves were then inverted to obtain a three dimensional shear velocity map showing vertical geological structures and a 1 3 km wide low seismic velocity dip which are in agreement with previous tomography studies in the same area

further observations of cloud ionosphere electrical discharges above thunderstorms during the night of 9 10 august 1993 more than 150 luminous cloud ionosphere discharges cis were observed above a thunderstorm complex moving se across the state of iowa images of the cis were obtained through clear air by intensified ccd tv cameras at the o brien observatory of the university of minnesota located about 60 km ne of minneapolis and 250 500 km from the storm center the discharges consisted of bright vertical striations extending from 50 80 km altitude often covering tens of kilometers laterally with tendrils of decreasing intensity visible for the brighter events down to cloud tops below 20 km altitude all the more intense cis were coincident with a vlf sferic in the 300hz 12khz range but small events often did not yield a detectable sferic there is no unambiguous evidence that cis were sources of sferics some of the cis were observed to be coincident with a cloud brightening and with a cloud ground stroke recorded by the national lightning detection network the duration of the images was generally less than one tv field 16 7 ms many of these discharges have now been observed by the space shuttle by aircraft borne tv cameras and a large number by a ground based camera observations in colorado the present results are compared with these observations and recent theoretical ideas related to the ci events are discussed it is proposed that cis arise from intense bursts of cloud electrification and may follow the preexisting paths of cloud to ionosphere thunderstorm currents

along strike variations in the electrical structure of the san andreas fault at parkfield california magnetotelluric exploration has been used to image along strike variations in the electrical resistivity structure of the san andreas fault at parkfield california a low resistivity wedge extending to a depth of several kilometers is continuous over a horizontal distance of 8 km the base of the wedge is coincident with the shallowest microearthquakes a change in the electrical and fluid connection of the san andreas fault with a low resistivity zone in the franciscan formation is observed along the parkfield segment

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

contrasting seismic characteristics of three major faults in northwestern canada the lithoprobe slave northern cordillera lithospheric evolution snorcle profiles crossed three major tectonic zones of the northwestern canadian shield and northern canadian cordillera that are diverse in age and in depth of penetration the oldest 2630 2590 ma the yellowknife river fault zone formed as a strike slip fault in a tensional strain regime reflector attenuation or truncations align vertically beneath the fault trace through much of the crust implying a near vertical fault plane the youngest 60 10 ma the tintina fault zone produced cumulative dextral strike slip displacements of 425 km perhaps 800 km tomographic velocity and ray trace models of reflection data indicate that several fault splays form a tectonic zone 30 km wide at the surface but truncations of deeper crustal reflections suggest that the zone thins in the mid crust and widens near the moho this apparent variable width versus depth of the tintina fault is atypical of major strike slip faults worldwide the teslin fault was an active terrane boundary during accretion of terranes onto north america observed reflection geometries indicate that the juxtapositions of highly contrasting metamorphic grades across the teslin fault are confined to the upper crust along snorcle line 3 implying that the fault soles eastward into a mid crustal detachment at the interpreted top of north american crust the limited depth extent of the teslin fault zone is similar to some models of the san andreas fault and may result from their similar histories as convergent margin structures

evidence for continuing current in sprite producing cloud to ground lightning radio atmospherics launched by sprite producing positive cloud to ground lightning flashes and observed at palmer station antarctica exhibit large elf slow tails following the initial vlf portion indicating the presence of continuing currents in the source lightning flashes one to one correlation of sferics with nldn lightning data in both time and arrival azimuth measured with an accuracy of 1 at 12 000 km range allows unambiguous identification of lightning flashes originating in the storm of interest slow tail measurements at palmer can potentially be used to measure continuing currents in lightning flashes over nearly half of the earth s surface

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

seismicity and fault geometry of the san andreas fault around parkfield california and their implications abstract fault geometry is a consequence of tectonic evolution and it provides important information on potential seismic hazards we investigated fault geometry and its properties in parkfield california on the basis of local seismicity and seismic velocity residuals refined by an adaptive velocity hypocentral parameter inversion method the station correction terms from the hypocentral parameter inversion present characteristic seismic velocity changes around the fault suggesting low seismic velocities in the region east of the fault and high seismic velocities in the region to the west large seismic velocity anomalies are observed at shallow depths along the whole fault zone at depths of 3 8 km seismic velocity anomalies are small in the central fault zone but are large in the northern and southern fault zones at depths 8 km low seismic velocities are observed in the northern fault zone high seismicity is observed in the southwest fracture zone which has developed beside the creeping segment of the san andreas fault the vertical distribution of seismicity suggests that the fault has spiral geometry dipping ne in the northern region nearly vertical in the central region and sw in the southern region the rapid twisting of the fault plane occurs in a short distance of approximately 50 km the seismic velocity anomalies and fault geometry suggest location dependent piecewise faulting which may cause the periodic m6 events in the parkfield region

san andreas fault geometry at desert hot springs california and its effects on earthquake hazards and groundwater abstract the mission creek and banning faults are two of the principal strands of the san andreas fault zone in the northern coachella valley of southern california structural characteristics of the faults affect both regional earthquake hazards and local groundwater resources we use seismic gravity and geological data to characterize the san andreas fault zone in the vicinity of desert hot springs seismic images of the upper 500 m of the mission creek fault at desert hot springs show multiple fault strands distributed over a 500 m wide zone with concentrated faulting within a central 200 m wide area of the fault zone high velocity up to 5000 m sec rocks on the northeast side of the fault are juxtaposed against a low velocity m 6 0 earthquakes in the area in 1948 and 1986 occurred at or near the depths 10 to 12 km of the merged san andreas fault large magnitude earthquakes that nucleate at or below the merged fault will likely generate strong shaking from guided waves along both fault zones and from amplified seismic waves in the low velocity basin between the two fault zones the mission creek fault zone is a groundwater barrier with the top of the water table varying by 60 m in depth and the aquifer varying by about 50 m in thickness across a 200 m wide zone of concentrated faulting

observation of sprites over the asian continent and over oceans around taiwan 1 sprites were observed over thunderstorms in the southern china and in oceans around taiwan the observation sites were on the ali mountain of taiwan s central ridge area with an altitude of 2413 m and in the campus of national cheng kung university with an altitude of 50 m for the observed land sprites 90 of them were either carrots or columniforms and 64 of the sprites occurred in groups among the observed oceanic sprites 89 of them were carrots but only 22 of the sprites occurred in groups we define a sprite active system as a thunderstorm that continuously produces at least one sprite in a 10 minute interval the active sprites generating periods for the observed thunderstorms were typically shorter than 30 minutes the sprite production rates for these asian thunderstorms are estimated to be between i 2 10 4 events km2 hr and i 1 10 3 events km2 hr

high resolution seismic images and seismic velocities of the san andreas fault zone at burro flats southern california abstract to better understand the structure of the san andreas fault saf at burro flats in southern california we acquired a three dimensional combined set of seismic reflection and refraction profiles centered on the main active trace at burro flats in this article we discuss the variation in shallow depth velocities along each seismic profile with special emphasis on the 1500 m sec p wave velocity contour which can be an indicator of shallow depth water saturated unconsolidated sediments along the four seismic profiles minimum depths of the groundwater table as inferred from 1500 m sec velocity contour range from 10 to about 20 m the largest variations in depth to the top of the groundwater table occur in areas near mapped faults suggesting that the groundwater flow in burro flats is strongly affected by the locations of fault traces we also used the seismic data to develop seismic reflection images that show multiple strands of the saf in the upper 60 m reflectors above the 10 m depth probably correspond to holocene alluvial deposits reflectors below the 15 m depth probably arise from velocity or density variations within the precambrian gneiss complex likely due to weathering apparent vertical offsets of reflectors are observed along profiles lines 1 and 2 that are normal to the saf indicating minor apparent vertical offsets on the saf at shallow depths along line 2 the apparently vertically offset reflectors correlate with zones of relatively low p wave velocity along the central part of lines 1 and 2 the faults form a flower structure which is typical of strike slip faults such as the saf

passive seismic imaging with directive ambient noise application to surface waves and the san andreas fault in parkfield ca summary this study deals with surface waves extracted from microseismic noise in the 0 1 0 2 hz frequency band with passive seismic correlation techniques for directive noise we explore the concept of passive seismic noise tomography performed on three component sensors from a dense seismic network from the nine component correlation tensor a rotation algorithm is introduced that forces each station pair to re align in the noise direction a necessary condition to extract unbiased traveltime from passive seismic processing after rotation is performed the new correlation tensor exhibits a surface wave tensor from which rayleigh and love waves can be separately extracted for tomography inversion methodological aspects are presented and illustrated with group speed maps for rayleigh and love waves and ellipticity measurements made on the san andreas fault in the parkfield area california usa

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

large explosive source wide recording aperture seismic profiling on the columbia plateau washington clear subsurface seismic images have been obtained at low cost on the columbia plateau washington the columbia plateau is perhaps the most notorious of all bad data areas because large impedance contrasts in surface flood basalts severely degrade the seismic wavefield this degradation was mitigated in this study via a large explosive source wide recording aperture shooting method the shooting method emphasizes the wide angle portion of the wavefield where fermat s principle guarantees reverberation will not interfere with the seismic manifestations of crucial geologic interfaces the basalt diving wave normally discarded in standard common midpoint cmp seismic profiling can be used to image basalt velocity structure via traveltime inversion maximum depth penetration of the diving wave tightly constrains basalt sediment interface depth an arrival observed only at shot receiver offsets greater than 15 km can be used to determine the velocity and geometry of basement via simultaneous inversion

tomographic images of p wave velocity variation at parkfield california tomographie inversion is applied io delay times from local earthquakes to image three dimensional velocity variations near parkfield california the 25 20 square km region is represented by nearly cubic blocks of 0 5 km per side arrival times of p waves from 551 local earthquakes with depths of 0 to 15 km were used as sources producing 3135 rays covering the target region the data were recorded on low noise downhole seismographs a conjugate gradient method is used to invert the resulting sparse system of simultaneous equations to diminish the effects of noisy data the laplacian of the model parameters is constrained to be small within horizontal layers providing smoothing of the model the resolution of the model is estimated by calculating point spread functions at blocks of interest estimates of standard errors of the model parameters are calculated by the jackknife statistical procedure the results of the inversion show correlation with some of the local geological and geophysical features station corrections removed the long wavelength anomaly associated with the contrast of the salinian block southwest of the san andreas fault versus the franciscan to the northeast a velocity low located a few kilometers northwest of parkfield depth 2 5 3 5 km appears to lie along the gradient of the large bouguer gravity anomaly associated with the parkfield syncline the south southeastward extension of the low velocities may relate to reflections observed on the parkfield consortium for continental reflection profiling cocorp lines we speculate on the geological meaning of these features and interpret them either as part of the local strike slip tectonics or a shallow crustal detachment the correlation of higher velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower velocity less competent rocks

three dimensional passive seismic waveform imaging around the safod site california using the generalized radon transform 1 we apply a three dimensional 3d generalized radon transform grt to scattered p waves from 575 local earthquakes recorded at 68 temporary network stations for passive source imaging of near vertical structures close to the san andreas fault observatory at depth safod site the grt image profiles through or close by the safod site reveal near vertical reflectors close to the fault zone as well as in the granite to the southwest and the franciscan melange to the northeast of the main fault although slightly lower in resolution these structures are generally similar to features in 2d images produced with steep dip prestack seismic migration of data from active source seismic reflection and refraction surveys our grt images however also reveal several vertical reflectors to the northeast of the saf that do not appear in the migration images but which are consistent with local geology these results suggest that in a seismically active area inverse scattering of earthquake data for instance with the grt can be a viable and in 3d economic alternative to an active source survey

imaging quasi vertical geological faults with earthquake data summary we present a method for imaging quasi vertically dipping faults with surface records of reflected p waves from small earthquakes faults are boundaries between geological structures such as tectonic plates and are located in earthquake active regions such as parkfield california the high degree of seismic activity enables the use of multiple seismic recordings in our fault identification algorithm major challenges occur because of the quasi vertical orientation of the fault and the fact that the wave reflected by the fault and recorded by the surface receivers is not well modelled by the direct arrival of the propagating wave generated by the earthquake source our method uses the 2 d acoustic wave equation as the model for p wave propagation we assume that an approximate wave speed map on the reflection side of the fault is available and the source locations are known for example from traveltime tomography we also assume that the source time function is known the new features of our method arise because earthquake sources are located very close to the fault this has two implications 1 the direct arrival and the reflected wave arrive almost simultaneously so that it is impossible to separate them on a seismogram using standard techniques and 2 most of the reflections occur above the critical angle which introduces a distortion in the reflected wave to overcome these difficulties we use a modelled incident wave to 1 remove the direct arrival from the data and 2 remove the post critical distortion from the reflected wave we justify the distortion removal using the leading order term of an asymptotic expansion and an optimization procedure to complete our algorithm we utilize some features of reverse time migration 1 the use of full acoustic wave equation for modelling and backpropagation and 2 zero lag correlation of the backpropagated time reversed reflected and incident fields we present numerical examples of fault reconstructions with synthetic data

sprites triggered by negative lightning discharges high altitude air breakdown manifested as red sprites is reported in close association with negative cloud to ground lightning cg on at least two occasions above an unusual storm on august 29 1998 data from high speed photometry low light level video and receivers of lightning electromagnetic signatures in the frequency range 10 hz to 20 khz are used to establish the association and indicate that the causative cg discharges effected unusually large vertical charge moment changes mqv of up to 1550 c km in 5 ms the existence of sprites caused by cg s rather than the regularly associated cg s has immediate implications for sprite models and observations

refining the image of the san andreas fault near parkfield california using a finite difference travel time computation technique abstract the parkfield area seismic observatory paso was a dense telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the san andreas fault observatory at depth safod drill site the main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for safod drilling and in the process develop a high for passive seismological techniques resolution image of the fault zone structure a challenging aspect of the analysis of this data set was the known existence of large 20 25 contrasts in seismic wavespeed across the san andreas fault the resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques in order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer thurber c roecker s roberts k gold m powell m l and rittger k 2003 earthquake locations and three dimensional fault zone structure along the creeping section of the san andreas fault near parkfield ca preparing for safod geophys res lett 30 3 10 1029 2002gl016004 with one derived from a relatively fine grid and an application of a finite difference algorithm hole j a and zelt b c 1995 3 d finite difference reflection traveltimes geophys j int 121 2 427 434 in both cases we inverted arrival time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three dimensional vp and vp vs structure included are data from an active source seismic experiment around the safod site as well as from a vertical array of geophones installed in the 2 km deep safod pilot hole drilled in summer 2002 our results show that the main features of the original analysis are robust hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same at the same time we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated and our estimate of the depth of the main part of the seismogenic zone decreases in places by 100 m tests using virtual earthquakes borehole receiver gathers of picks for surface shots indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

rapid identification of earthquake rupture plane using source scanning algorithm summary accurate identification of an earthquake s rupture plane is critical in many aspects ranging from scientific such as seismology and plate tectonics to societal such as emergency rescue planning we modify the recently developed source scanning algorithm ssa to rapidly image the rupture pattern of an earthquake using waveform data recorded at local and regional distances the method calculates the brightness function of a grid point by summing the observed amplitudes of the p wave envelopes at the corresponding predicted arrival times at all stations the composite image consisting of brightness functions of all grid points within a prescribed source volume and time interval illuminates the locations on the rupture plane where significant seismic energy is emitted synthetic tests indicate that the proposed method is robust even under the presence of hypocentral mislocation and origin time errors application of this method to the 2003 san simeon and 2004 parkfield earthquakes both occurred in central california clearly identifies the nw se striking planes as the actual rupture planes a conclusion consistent with all available evidences limited azimuthal distribution of seismic stations will not severely affect the identification result making this method ideal for offshore earthquake studies because the proposed method is able to identify the rupture plane using seismic waveform data only it is perfectly suited for near real time operation and may enable routine report of earthquake rupture planes by local seismic networks in the future

observations of rayleigh wave phase velocity and coseismic deformation using an optical fiber interferometric vertical strainmeter at the safod borehole california we present observations from a vertical optical fiber interferometric strainmeter in the san andreas fault observatory at depth borehole near parkfield california the sensor detects both teleseismic earthquakes and local events along with coseismic strain steps consistent with theoretical dislocation models for teleseismic events we investigate the possibility of determining local rayleigh wave phase velocities beneath the borehole by comparing the ratio of vertical ground acceleration from a nearby seismometer to vertical strain while similar studies have used horizontal components and rotations this is the first such attempt utilizing vertical measurements we show that at periods from around 16 40 seconds we can recover general dispersion characteristics that are within a few percent of models of realistic local structure

low velocity damaged structure of the san andreas fault at parkfield from fault zone trapped waves 1 we used dense linear seismic arrays across and along the san andreas fault saf at parkfield california to record fault zone trapped waves generated by explosions and microearthquakes in 2002 prominent trapped waves appeared at stations close to the saf main fault trace while some energy was trapped in the north strand at the array site observations and 3 d finite difference simulations of trapped waves at 2 5 hz show evidence of a damaged core zone on the main saf the zone from the surface to seismogenic depths is marked by a low velocity waveguide 150 m wide in which q is 10 50 and shear velocities are reduced by 30 40 from wall rock velocities with the greatest velocity reduction at shallow depth we interpret that this distinct low velocity zone on the main saf is a remanent of damage due to past large earthquakes on the principal fault plane at parkfield a less developed low velocity zone may be evident on the north strand that experienced minor breaks in the 1966 m6 event

sprite observations above the u s high plains in relation to their parent thunderstorm systems transient luminous events sprites blue jets elves above large mesoscale convective systems mcss over the u s high plains have been routinely monitored from the yucca ridge field station near fort collins colorado using ground based low light video systems we analyzed 36 sprites above the nebraska mcs of august 6 1994 the results lend further support to the hypothesis that sprites are almost uniquely associated with positive cloud to ground cg lightning flashes sprite associated cgs also averaged substantially larger peak currents than the remaining cg population 81 ka versus 30 ka in this storm system there is some evidence that sprite associated cgs also have higher stroke multiplicity this study yields no evidence of sprites associated with negative cg events in the central united states an additional requirement appears to be that the parent mcs has a contiguous radar reflectivity area exceeding 20 25 000 km 2 the majority of the sprites occur above the large stratiform precipitation region and not the high reflectivity convective core of the mcs triangulation of a limited number of paired images from september 7 1994 suggests that the sprite is generally centered within 50 km of the parent cg assuming the cg provides the range single image photogrammetric analyses provide estimates of the maximum vertical extent of the sprites for this storm the sprite tops averaged 77 km with a maximum of 88 km the bases averaged 50 km but with a few sprite tendrils extending as low as 31 km

microearthquake s wave observations from 0 to 1 km in the varian well at parkfield california abstract high gain three component seismometers from 0 to 1 km deep along the varian a 1 well at parkfield california were used to record the waveforms of nearby microearthquakes despite being in the thick tertiary sediments of the parkfield syncline the s wave amplification at this site is only about a factor of 3 the spectral content and spectral ratios of s waves along the well show that the average qs in the top 1 km at this site is 37 with the qs in different subintervals varying between 8 and 65 based on initial s wave polarizations a complex s wave velocity structure must exist at and below the varian site this structure appears to include position dependent anisotropy as well as steep lateral velocity gradients at a depth of 1 km s wave splitting parallel and normal to the san andreas fault zone is consistently observed this splitting scales at roughly 0 01 sec km subsequent to the split s waves the particle motion seems to be controlled by event focal mechanism above 1 km the upgoing s waves attenuate and change directions of polarization with a new splitting rate of 0 1 sec km uniquely for some events on the san andreas fault immediately below the varian site large post s wave signals with normal dispersion are present we propose that these phases are fault zone guided waves channeled from the san andreas fault to the varian site along the gold hill fault

characteristics of luminous structures in the stratosphere above thunderstorms as imaged by low light video an experiment was conducted in which an image intensified low light video camera systematically monitored the stratosphere above distant 100 800 km mesoscale convective systems over the high plains of the central u s for 21 nights between 6 july and 27 august 1993 complex luminous structures were observed above large thunderstorm clusters on eleven nights with one storm system 7 july 1993 yielding 248 events in 410 minutes their duration ranged from 33 to 283 ms with an average of 98 ms the luminous structures generally not visible to the naked dark adapted eye exhibited on video a wide variety of brightness levels and shapes including streaks aurora like curtains smudges fountains and jets the structures were often more than 10 km wide and their upper portions extended to above 50 km msl

vertical seismic wave gradiometry application at the san andreas fault observatory at depth abstractwe have developed vertical seismic wave gradiometry vswg to estimate velocity impedance and attenuation structure in the vicinity of boreholes using borehole array waveforms of check shot explosions near the borehole head we have extended wave gradiometry wg theory from a purely local relation of the wavefield and wavefield spatial gradient to one that incorporated the ray ansatz over the length of the borehole for a traveling wave in a vertically inhomogeneous medium we checked the ray assumption against acoustic full wave synthetic seismograms and it was found to yield robust measures of the medium velocity anelastic attenuation and impedance structure trade off but in cases of high anelastic attenuation realistic bounds can be placed on the seismic impedance that effectively constrains the average attenuation we have applied these methods to data collected at the san andreas fault observatory at depth borehole in central california in 2005 and we found that the wg velocity estimat

across fault velocity gradients and slip behavior of the san andreas fault near parkfield a long lasting question in earthquake physics is why slip on faults occurs as creep or dynamic rupture we compute passive measurements of the seismic p wave velocity gradient across the san andreas fault near parkfield where this transition of slip mode occurs at a scale of a few kilometers unbiased measurements are obtained through the application of a new bayesian local earthquake tomographic code that avoids the imposition of any user defined initial velocity contrast across the fault or any damping scheme that may cause biased amplitude in retrieved seismic velocities we observe that across fault velocity gradients correlate with the slip behavior of the fault the p wave velocity contrast decays from 20 in the fault section that experience dynamic rupture to 4 in the creeping section suggesting that rapid change of material properties and attitude to sustain supra hydrostatic fluid pressure are conditions for development of dynamic rupture low vp and high vp vs suggest that fault rheology at shallow depth is conversely controlled by low frictional strength material

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

orientation of three component geophones in the san andreas fault observatory at depth pilot hole parkfield california to identify and constrain the target zone for the planned safod main hole through the san andreas fault saf near parkfield california a 32 level three component 3c geophone string was installed in the pilot hole ph to monitor and improve the locations of nearby earthquakes the orientation of the 3c geophones is essential for this purpose because ray directions from sources may be determined directly from the 3d particle motion for both p and s waves due to the complex local velocity structure rays traced from explosions and earthquakes to the ph show strong ray bending observed azimuths are obtained from p wave polarization analysis and ray tracing provides theoretical estimates of the incoming wave field the differences between the theoretical and the observed angles define the calibration azimuths to investigate the process of orientation with respect to the assumed velocity model we compare calibration azimuths derived from both a homogeneous and 3d velocity model uncertainties in the relative orientation between the geophone levels were also estimated for a cluster of 36 earthquakes that was not used in the orientation process the comparison between the homogeneous and the 3d velocity model shows that there are only minor changes in these relative orientations in contrast the absolute orientations with respect to global north were significantly improved by application of the 3d model the average data residual decreased from 13 to 7 supporting the importance of an accurate velocity model we explain the remaining residuals by methodological uncertainties and noise and with errors in the velocity model

anisotropy in the shallow crust observed around the san andreas fault before and after the 2004 m 6 0 parkfield earthquake local seismic arrays were deployed at two locations along the san an dreas fault saf near parkfield california before and after the 2004 m 6 0 parkfield earthquake using local earthquakes we determine the anisotropic field within 1 2 km of the main trace of the saf at the two array locations separated by 12 km the initial array near the safod site was deployed for six weeks in october and november 2003 and the second array located near the town of parkfield was de ployed for 3 months following the 28 september 2004 m 6 0 parkfield earthquake we find the fast shear wave polarization direction nearly fault parallel n40w for stations on the main fault trace and within 100 m to the southwest of the saf at both array locations these fault parallel measurements span the 100 to 150 m wide zone of pervasive cracking and damage interpreted from fault zone trapped waves associated with the main fault core li et al 2004 2006 outside of this zone the fast orientations are scattered with some preference for orientations near n10e roughly parallel to the regional maximum horizontal compressive stress direction rh in addition fast directions are preferentially oriented parallel to a northern branch of the saf recorded on stations in the 2004 parkfield deployment the measured anisotropy is likely due to a combination of stress aligned micro cracks away from the fault and shear fabric within the highly evolved fault core the majority of our measurements are taken outside of the main fault core and we es timate the density of microcracks from the measured delay times apparent crack densities are approximately 3 with large scatter the data suggest weak depth dependence to the measured delay times for source depths between 2 and 7 km below 7 km source depth the delay times do not correlate with depth suggesting higher confining pressure is forcing the microcracks to close no coseismic variation in the anisotropic parameters is observed suggesting little to no influence on measured splitting due to the 2004 m 6 0 parkfield earthquake however the premainshock and postmainshock data presented here are from arrays separated by 12 km limiting our sensitivity to small temporal changes in anisotropy

telescopic imaging of sprites telescopic images of sprites show a wide variety of generally vertical but also slanted fine structure including branching tree like shapes and well defined but isolated columns with transverse spatial scales ranging from tens of meters to a few hundred meters at 60 85 km altitude simultaneous analysis of radio atmospheric and lightning data indicates that specific columnar regions are selectively excited by successive discharges

steep dip seismic imaging of the shallow san andreas fault near parkfield seismic reflection and refraction images illuminate the san andreas fault to a depth of 1 kilometer the prestack depth migrated reflection image contains near vertical reflections aligned with the active fault trace the fault is vertical in the upper 0 5 kilometer then dips about 70 to the southwest to at least 1 kilometer subsurface this dip reconciles the difference between the computed locations of earthquakes and the surface fault trace the seismic velocity cross section shows strong lateral variations relatively low velocity 10 to 30 high electrical conductivity and low density indicate a 1 kilometer wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace

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

adaptive mesh seismic tomography based on tetrahedral and voronoi diagrams application to parkfield california 1 we have developed an adaptive mesh seismic tomography method based on tetrahedral and voronoi diagrams to automatically match the inversion mesh to the data distribution two different irregular mesh interpolation methods linear and natural neighbor are evaluated a synthetic test shows that the adaptive mesh seismic tomography method recovers the true synthetic model well in both the linear and natural neighbor interpolation cases use of more accurate differential data is helpful in removing artifacts in the model that result from the noisier absolute data application of the adaptive mesh scheme to parkfield california results in much more uniform sampling of the model as measured by the derivative weight sum on the optimized irregular inversion mesh compared to a regular grid the across strike cross sections resulting from using either just the absolute data or both the absolute and differential data show a clear velocity contrast at the san andreas fault in the linear and natural neighbor interpolation cases the velocity model obtained using natural neighbor interpolation is smoother than that from linear interpolation including the differential data in the inversion yields more clustered event locations and removes a velocity reversal to the southwest of the fault which is likely caused by noise in the absolute data

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

compositional variation and mixing of impact melts on microscopic scales we investigated the compositional characteristics of schlieren rich holohyaline impact glasses from ries wabar and meteor crater using a cameca sx 100 scanning electron microprobe this instrument is capable of producing detailed maps of major elements at spatial resolutions of 10 m the objective was to characterize the composition of an unusually large number of individual schlieren and to evaluate details of the process that causes melts of lithologically diverse target rocks to mix on scales of micrometers the ries and meteor crater impacts involved lithologically heterogeneous targets whereas wabar crater formed in relatively uniform dune sand texturally heterogeneous schlieren rich glasses from the ries crater illustrate that schlieren of highly variable color can be surprisingly similar in composition as first detailed by stahle 1972 consistent with these earlier findings most schlieren represent mixtures of diverse rock melts their compositions deviate only subtly from the average melt and do not resemble monomineralic melts nor binary mixtures of major rock forming minerals a specific population of schlieren is enriched in mafic elements mg fe and ca which suggests incomplete homogenization of an amphibolite progenitor in the case of wabar crater a compositionally simple melt of dune sand mixed with projectile iiia iron meteorite materials and specific schlieren are variable mixtures of these two progenitors the optically homogeneous glass from meteor crater is compositionally homogeneous as well which suggests ideal mixing of such diverse lithologies as platform carbonates sandstone and a class iiia iron meteorite the mixing of projectile and target melts at wabar and meteor crater unambiguously demonstrates that melts initially produced in distinctly different stratigraphic structural locations will undergo wholesale mixing if not homogenization also the projectile melts unquestionably formed relatively early in the cratering process and their dissemination throughout the prospective melt volume albeit at variable concentration levels suggests that the entire mixing process may be an early cratering feature this also follows from the fact that we investigated ballistic melt ejecta which thereby eliminates all of those mixing processes that may additionally operate during the pooling and generation of massive melt ponds following gravitational collapse of large structurally complex craters substantial turbulence ranging from field dimensions to microscopic scales seems inescapable to accomplish the observed degree of mixing yet this is not readily inferred from current models of macroscopic material motions during hypervelocity impact