a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

what do data used to develop ground motion prediction equations tell us about motions near faults a large database of ground motions from shallow earthquakes occurring in active tectonic regions around the world recently developed in the pacific earthquake engineering center s nga west2 project has been used to investigate what such a database can say about the properties and processes of crustal fault zones there are a relatively small number of near rupture records implying that few recordings in the database are within crustal fault zones but the records that do exist emphasize the complexity of ground motion amplitudes and polarization close to individual faults on average over the whole data set however the scaling of ground motions with magnitude at a fixed distance and the distance dependence of the ground motions seem to be largely consistent with simple seismological models of source scaling path propagation effects and local site amplification the data show that ground motions close to large faults as measured by elastic response spectra tend to saturate and become essentially constant for short periods this saturation seems to be primarily a geometrical effect due to the increasing size of the rupture surface with magnitude and not due to a breakdown in self similarity

a study of possible ground motion amplification at the coyote lake dam california the abutment site at the coyote lake dam recorded an unusually large peak acceleration of 1 29g during the 1984 morgan hill earthquake following this earthquake another strong motion station was installed about 700 m downstream from the abutment station we study all events seven recorded on these stations using ratios of peak accelerations spectral ratios and particle motion polarization using holograms to investigate the relative ground motion at the two sites we find that in all but one case the motion at the abutment site is larger than the downstream site over a broad frequency band the polarizations are similar for the two sites for a given event but can vary from one event to another this suggests that the dam itself is not strongly influencing the records although we can be sure that the relative motion is usually larger at the abutment site we cannot conclude that there is anom alous site amplification at the abutment site the downstream site could have lower than usual near surface amplifications on the other hand the geology near the abut ment site is extremely complex and includes fault slivers with rapid lateral changes in materials and presumably seismic velocities for this reason alone the abutment site should not be considered a normal free field site

on temperature inversions and the mesospheric surf zone 1 mesospheric thermal inversions are investigated in a numerical simulation with the whole atmosphere community climate model an upward extension of the national center for atmospheric research s community climate model the seasonal character spatial extent and magnitude of the inversion layers are realistic during winter in the model the occurrence of wintertime inversions is a direct consequence of the rapid decay with height of vertically propagating planetary waves which induces large temperature perturbations in the upper mesosphere to maintain hydrostatic equilibrium the magnitude of the inversions is highly correlated with planetary wave amplitude so that large inversions develop during episodes of planetary wave amplification gravity waves do not play a major direct role in the formation of the inversions because the largest thermal tendencies associated with gravity wave breaking occur well above the range of altitudes where inversions are found however gravity waves play an essential indirect role because they set up a critical line in the upper mesosphere where rossby waves break in the mesospheric surf zone

carlsberg ridge northern indian ocean gravity and isostasy summary gravity and bathymetry profiles across the carlsberg ridge in the indian ocean are analysed to investigate the isostatic compensation of this part of the indian ocean spreading ridge system the ridge has a rough flanking topography and a median valley which are the characteristics of slow spreading ridges we use the crossspectral technique to obtain an admittance function for isostatic studies admittance estimates in the long wavelength region 125 500 km indicate that the elastic plate thickness supporting the long wavelength topography is in the range 5 10 km we have also examined the isostasy of the axial region synthetic topography and gravity profiles computed for a cooling plate model are subtracted from the observed to estimate the median valley signatures the residual gravity anomaly over the ridge axis is mostly explained by the median valkey topography with a uniform crustal thickness of 6 km indicating that the median valley is not in isostatic equilibrium

ground motion prediction equations in the san jacinto fault zone significant effects of rupture directivity and fault zone amplification we present a new set of ground motion prediction equations gmpes for horizontal peak ground acceleration peak ground velocity and 5 damped pseudo spectral acceleration psa developed for the san jacinto fault zone sjfz area besides using these equations to quantify seismic shaking in the area the results allow us to examine the physics and local properties controlling the observed ground motions the analyzed dataset includes 30 000 observations from 800 events spanning a magnitude range of 1 5 m 6 0 and recorded by up to 140 stations at epicentral distances ranging from essentially zero to 150 km the local gmpe is developed for the sjfz by applying classical regression techniques with predictive variables that include first distance and magnitude and then site characteristics rupture directivity and fault zone amplification the significance of these effects is determined by measuring the uncertainty reduction of the gmpe due to each factor the results show that in contrast to many regional studies traditional site characteristic has a relatively minor effect on peak amplitudes in our study area however rupture directivity is a significant factor controlling the amplitudes of ground motion even for small events the dense seismic network and newly developed directivity tool enable us to extract efficiently directivity effects with statistical significance using the ground motion dataset during the regression analysis process the obtained rupture directivities are consistent with the main focal mechanism orientations and surface trace orientations known from other studies and predictions for bimaterial ruptures in the trifurcation area of the sjfz fault zone amplification is a second important factor showing strong impact on the peak ground motion values with increasing role for the lower frequency range 10 hz examined in the 5 damped psa values we also observe signatures of large amplitude variances which indicate additional source related control on the distribution of amplitudes besides rupture directivity for aftershocks close in time and location to the m l 5 1 earthquake of march 2013 using the full set of records we present the most complete set of gmpes for the sjfz area including a higher amplitude prediction for regions in the direction of rupture

on the signature of the quasi 3 day wave in the thermosphere during the january 2010 ursi world day campaign 1 ultra fast kelvin waves with periods of 3 5 days are important in the coupling of the lower atmosphere to the thermosphere and ionosphere here we focus on the observations and effects of a 3 day wave during january 2010 as this time period coincides with a stratospheric warming event a coordinated set of observations with incoherent scatter radars are available while there is no evidence that the occurrence of this 3 day wave is connected with this event these observations offer an unprecedented glimpse of the thermospheric conditions during this period including the first ever detection of a 3 day wave with an incoherent scatter radar using a combination of ground and space based observations we identify an eastward moving zonal wave number one 3 day equatorial wave that is comprised of a kelvin wave at the lowest latitudes and a rossby gravity wave at higher latitudes in the equatorial region the vertical wavelength is 40 km and the wave peaks in amplitude around 95 100 km altitude the wave observed here is only seen to propagate to around 105 km altitude evidence of an interaction between this wave and the diurnal tide is seen between 82 88 km the resultant 3 day periodicity in the diurnal tide is seen to propagate up to altitudes of 150 km this could have a significant impact on the ionosphere via modulation of the e region dynamo thus carrying the 3 day periodicity to higher altitudes

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

fault zone amplified waves as a possible seismic hazard along the calaveras fault in central california the calaveras fault lies within a low velocity zone lvz 1 2 km wide near gilroy california accelerographs g06 located in the lvz 1 2 km from the calaveras fault and g07 4 km from g06 recorded both the m 6 2 1984 morgan hill and the m 6 9 1989 loma prieta earthquakes comparison of the ground motions shows that a large 0 6 1 0 hz velocity pulse observed at g06 during the morgan hill event may be amplified by focussing caused by the lvz such amplified waves might be a mappable seismic hazard and the zone of increased hazard can extend as much as 1 2 km from the surface trace of the fault finite difference simulations of ground motions in a simplified lvz model show a zone of amplified motion similar to the observations

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

time gcm results for the quasi two day wave to understand how the quasi two day wave qtdw interacts with the mean flow and migrating tides we perturbed the lower boundary of the national center for atmospheric research thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm to excite the qtdw results from our initial model runs computed for a perpetual january 15th show wave amplitudes in wind and temperature that are in qualitative agreement with previous model results and observations the wave period and amplitude change over the duration of the model run and it is expected that this results from changes in the zonal mean zonal winds evidence supporting nonlinear interactions between the qtdw and the migrating tides is presented

geoid anomalies over cooling lithosphere source for a third kernel of upper mantle thermal parameters and thus an inversion summary the geoid anomaly over an isostatic mass redistribution on an infinite flat earth is simply 27 c g times the differential mass moment on a spheroidal earth the limiting value can never quite be reached because the external effect of a spherical shell of mass dipole is identically zero so distant portions of the spherical surface subtract from the effect of the region under the point of observation the width of the integrating kernel varies with the depth of the mass distribution but the observable fraction of the limiting geoid slope can be estimated at least for ocean basins of moderate extent where it should reach about 90 per cent for very small 3 cm yr 1 ocean basins the observable geoid slope is smaller a consideration of published geoid data together with gravity anomalies in the same area compared to the theoretical free geoid gravity anomalies gravity observed on the geoid leads with great caution to a crude estimate of the thermally produced slope of 0 14 0 02 m myr 1 given a value of the thermal geoid signal it is possible to conduct an inversion for the upper mantle thermal parameters if two the density and specific heat are assumed to be known the parameter kernels for the heat flow topographic decay and geoid give three equations to solve for expansion coefficient diffusivity and initial temperature ti the results are different from common modelling assumptions 3 1 10 5 c 1 17 per cent 0 0065 cm2 s l 45 per cent and 1660 c 29 per cent manipulation of the equations in the case of general temperature variable parameters leads to partial explanations appears multiplied by k0 7 k0 conductivity at 0 7 of t1 at surface temperature while k is weighted toward the lower values in the low to medium temperature range an estimate of t1 using the shape of the temperature variations of k and k in the most reliable available data for olivine remains at about 1660 c 30 per cent in spite of the large error band the higher initial temperature is plausible because decompression of upwelling material produces enough partial melting to generate the oceanic crust the latent heat of melting must be supplied from somewhere and the only other possible source is a large amount of mechanical work applied to the system in the divergent flow

generation of secondary waves arising from nonlinear interaction between the quasi 2 day wave and the migrating diurnal tide theory and past observations have provided evidence that atmospheric tides and other global scale waves interact nonlinearly to produce additional secondary waves throughout the space atmosphere interaction region however few studies have investigated the generation region of nonlinearly generated secondary waves and as a result the manifestation and impacts of these waves are still poorly understood this study focuses on the nonlinear interaction between the quasi 2 day wave 2dayw3 and the migrating diurnal tide dw1 two of the largest global scale waves in the atmosphere the fundamental goals of this effort are to characterize the forcing region of the secondary waves and to understand how it relates to their manifestation on a global scale first the fast fourier synoptic mapping method is applied to thermosphere ionosphere mesosphere energetics and dynamics sounding of the atmosphere using broadband emission radiometry satellite observations to provide new evidence of secondary waves these results show that secondary waves are only significant above 80 km the nonlinear forcing for each secondary wave is then computed by extracting short term primary wave information from a reanalysis model the estimated nonlinear forcing quantities are used to force a linearized tidal model in order to calculate numerical secondary wave responses model results show that the secondary waves are significant from the upper mesosphere to the middle thermosphere highlighting the implications for the atmosphere space weather coupling the study also concludes that the secondary wave response is most sensitive to the nonlinear forcing occurring in the lower and middle mesosphere and not coincident with the regions of strongest nonlinear forcing

thermal and mechanical behaviour of the oceanic lithosphere constrained by geoid and gravity anomalies over the mid atlantic ridge axial valley abstract geoid profiles derived from seasat altimetry data crossing the north atlantic mid oceanic ridge are analysed an obliquity of about 60 does not perturb the analysis much conjointly with bathymetric and gravimetric profiles of the famous campaign these geoid profiles are compared with theoretical ones assumptions made in this study concern the neglect of the variations along the ridge axis the choice of the half space model for the cooling of the plate which is improved by the introduction of a lead term and the use of a flat earth approximation in the computation of geoid and gravity anomalies the choice of a full two dimensional numerical technique allows a good evaluation of these anomalies for the detailed structure close to the axis the models take into account the thermal cooling of the plate the shifting of the isotherms due to the axial valley the contribution of the crust different assumptions are made relative to crustal thinning beneath the valley to a lithospheric necking or to a viscous drag effect of the rising magma eventually connected with the formation of the axial valley isostatic compensation of the axial valley by crustal thinning is clearly confirmed by the gravimetric data alone but not by the altimetric data differences in g and h gravimetry and geoid low respectively are not large enough to really discriminate between the proposed models the theoretical amplitudes appear overestimated for gravity anomalies and underestimated for geoid anomalies if confirmed this fact would give indications to improve the model

global observations of 2 day wave coupling to the diurnal tide in a high altitude forecast assimilation system we examine wave components in a high altitude forecast assimilation system that arise from nonlinear interaction between the diurnal tide and the westward traveling quasi 2 day wave the process yields a westward traveling sum wave with zonal wavenumber 4 and a period of 16 hours and an eastward traveling difference wave with zonal wavenumber 2 and a period of 2 days while the eastward 2 day wave has been reported in satellite temperatures the westward 16 hour wave lies outside the nyquist limits of resolution of twice daily local time satellite sampling hourly output from a high altitude forecast assimilation model is used to diagnose the nonlinear quadriad a steady state primitive equation model forced by tide 2 day wave advection is used to intepret the nonlinear wave products the westward 16 hour wave maximizes in the midlatitude winter mesosphere and behaves like an inertia gravity wave the nonlinearly generated component of the eastward 2 day wave maximizes at high latitudes in the lower thermosphere and only weakly penetrates to low latitudes the 16 hour and the eastward 2 day waves are of comparable amplitude and alias to the same apparent frequency when viewed from a satellite perspective

the usgs parkfield california dense seismograph array upsar abstract an array of 14 irregularly spaced seismograph stations with an aperture of approximately 1 km has been installed near the middle of the rupture zone of the 1966 parkfield california earthquake m 5 5 the primary purpose of the array is to image sources of high frequency energy radiated by the next characteristic parkfield earthquake using recordings of its aftershocks at the array as empirical green9s functions in addition it will be used to characterize coda q in the parkfield region using beam forming techniques to isolate arrivals from specific volumes of the earth each of the 14 stations consists of a three component velocity transducer and an accelerometer package to ensure on scale recordings of the full range of ground motion from microearthquakes to large damaging shocks transducer output is digitized at each field site and telemetered to a computer at a central recording facility for event detection and storage each component of ground motion is sampled 200 times per second by a 16 bit analog to digital converter time code received from the goes west satellite synchronizes sampling across the array data telemetry is by arcnet a token passing local area network developed for pcs using a single coaxial cable arcnet is two way meaning that data or commands can be sent from any field station to the microcomputer or any other field station over the same medium using the same hardware and programming techniques specific site locations were chosen by a simulated annealing algorithm that chose the final site locations from a set of about 100 possible sites to optimize the beam pattern of the array the algorithm minimized the power of peaks in the beam pattern with slownesses less than 1 sec km relative to the central lobe the array configuration was required to include several embedded tripartite arrays to allow the examination of coherence across the array seismograms are shown for a m 2 3 earthquake which was located at middle mountain 12 9 km from the array the azimuth to the hypocenter is computed several ways using the array data and compared to the backazimuth from the epicenter location a least squares best fit to the arrival times of the first peak yields an azimuth of 7 9 which is similar to the azimuth of the peak in slowness computed using the music algorithm of 10 3 at 9 hz these angles are similar but consistently greater than the backazimuth to the hypocenter of 3 2 the amplitude of the first arrival is approximately 6 4 times greater for a stacked trace or beam computed for a ray aimed along the ray path from the hypocenter compared to a beam 180 opposite to that the amplitude of the first arrival of stacked traces diminishes over a range in azimuth of 80 on either side of the azimuth for a beam aimed along the upsar hypocenter ray path coda q calculated for a range of 0 to 350 and slownesses of 0 1 to 0 24 sec km show no dominant peaks in the frequency bands of 4 to 8 and 8 to 16 hz a large late arrival in the seismogram in the 2 to 4 hz band yields high q s at azimuths of around 0 with large errors coda q increases with frequency from about 150 in the 2 to 4 hz band to about 500 in the 8 to 16 hz band

mechanical behavior of the oceanic lithosphere at a corner area defined by an axial valley and a large offset transform fault case of the oceanographer transform fault elastic behavior of lithospheric plates is able to account for the return to the isostatic equilibrium level observed at the corner where a mid ocean ridge axial valley and a large offset transform fault intersect the formalism is a generalization in two dimensions of the one dimensional one due to kuo et al 1986 good agreement of depth profiles of the oceanic platform can be obtained between models and data from the two corner areas associated with the oceanographer transform fault otf a band 10 to 20 km wide around the transform fault affected by shear faulting has to be excluded from the elastic behavior and possibly also a band 20 to 30 km wide around the ridge axis affected by normal faulting a major result concerns the evaluation of the amount of the static contribution due to anomalous crust in the deepening of the crustal valley in the vicinity of the otf to a value in the range 50 to 70 the residual 30 to 50 is attributed to an increase of the dynamical effect of the asthenospheric upwelling feeding the lithospheric plates

kinematic inversion of the 2004 m 6 0 parkfield earthquake including an approximation to site effects the 2004 m 6 0 parkfield earthquake yielded one of the largest amounts of near source strong ground motion seismic data ever we invert strong motion seismograms to obtain a model for the space time distribution of coseismic slip on the fault to reduce noise in the inversion we take into account local amplifications that affect each station by using records of the 1983 m 6 5 coalinga earthquake site amplification correlates well with large peak ground velocities registered during the 2004 parkfield mainshock the inversion for a kinematic rupture model yields a nonunique solution we therefore analyze various rupture models that explain the data equally well our preferred rupture model identifies a primary zone of high slip surrounding the hypocenter where the maximum slip is 57 cm a secondary slip area over which contours are not well resolved is located northwest of the hypo center the rupture speed is highly heterogeneous we infer an average rupture ve locity of 2 8 km sec close to the hypocenter and of 3 3 km sec in the secondary region of large slip to the northwest of the hypocenter by correlation of our rupture model with both microseismicity and velocity structure we identify six patches on the fault plane that behave in seismically distinct ways online material kinematic rupture model parameters

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

large scale waves in the mesosphere and lower thermosphere observed by saber observations made by the sounding of the atmosphere using broadband emission radiometry saber instrument on board nasa s thermosphere ionosphere mesosphere energetics and dynamics timed satellite have been processed using salby s fast fourier synoptic mapping ffsm algorithm the mapped data provide a first synoptic look at the mean structure and traveling waves of the mesosphere and lower thermosphere mlt since the launch of the timed satellite in december 2001 the results show the presence of various wave modes in the mlt which reach largest amplitude above the mesopause and include kelvin and rossby gravity waves eastward propagating diurnal oscillations non sunsynchronous tides and a set of quasi normal modes associated with the so called 2 day wave the latter exhibits marked seasonal variability attaining large amplitudes during the solstices and all but disappearing at the equinoxes saber data also show a strong quasi stationary rossby wave signal throughout the middle atmosphere of the winter hemisphere the signal extends into the tropics and even into the summer hemisphere in the mlt suggesting ducting by westerly background zonal winds at certain times of the year the 5 day rossby normal mode and the 4 day wave associated with instability of the polar night jet are also prominent in saber data

rupture propagation of the 2004 parkfield california earthquake from observations at the upsar using a short baseline seismic array u s geological survey parkfield dense seismograph array upsar about 12 km west of the rupture initiation of the 28 september 2004 m 6 0 parkfield california earthquake we have observed the movement of the rupture front of this earthquake on the san andreas fault the sources of high frequency arrivals at upsar which we use to identify the rupture front are mapped onto the san andreas fault using their apparent velocity and back azimuth measurements of apparent velocity and back azimuth are calibrated using aftershocks which have a compact source and known location aftershock back azimuths show considerable lateral refraction consistent with a high velocity ridge on the southwest side of the fault we infer that the initial mainshock rupture velocity was approximately the rayleigh speed with respect to slower side of the fault and the rupture then slowed to about 0 66 near the town of parkfield after 2 sec the last well correlated pulse 4 sec after s is the largest at upsar and its source is near the region of large accelerations recorded by strong motion accelerographs and close to northern extent of continuous surface fractures on the southwest fracture zone coincidence of sources with preshock and aftershock distributions suggests fault material properties control rupture behavior high frequency sources approximately correlate with the edges of asperities identified as regions of high slip derived from inversion of strong motion waveforms

observations of the quasi two day wave in the middle and lower atmosphere over christmas island we present analyses of data collected at christmas island in the mesosphere and lower thermosphere using a meteor echo detection and collection system medac in addition to data collected in the troposphere and lower stratosphere using a clear air stratosphere troposphere st radar these data which provide 5 years of semicontinuous horizontal wind velocity estimates are analyzed to determine the temporal structure of the quasi two day wave in the middle and lower atmosphere our results show the presence of strong wave activity in the middle atmosphere during late january and early february as well as a secondary maxima in june august for each of the years we have data however the magnitude and duration of the wave events observed in late january and early february exhibit interannual variability

observations of the quasi 2 day wave from the high resolution doppler imager on uars a strong westward traveling oscillation with a period of 2 days and zonal wave number 3 is observed in the mesospheric and lower thermospheric winds from the high resolution doppler imager on the upper atmosphere research satellite the important events happen in january july and september october of which the occurrence in january is the strongest with an amplitude over 60ms 1 detailed analyses for the periods of january 1992 and january 1993 reveal a cause and effect relationship in the wave developing process at 95km the global structures of the wave amplitude and phase are also presented

an eastward propagating two day wave evidence for nonlinear planetary wave and tidal coupling in the mesosphere and lower thermosphere 1 temperature observations from the saber instrument on the timed spacecraft are used to investigate the structure and evolution of an eastward propagating zonal wavenumber 2 disturbance with a period near two days this oscillation obtains a maximum amplitude of nearly 10 k in the southern hemisphere mid latitudes during late january the timing and location of this planetary wave is coincident with the regular quasi two day wave intensification that occurs annually in late january the period wavenumber and spatial structure of the eastward propagating two day wave are consistent with a wave that results from a nonlinear interaction between the quasi two day wave and the migrating diurnal tide the existence of an eastward propagating wave with a period near two days coincident with the westward propagating two day wave will have an impact on the interpretation of ground based observations analysis of the saber temperature observations are utilized to determine the structure and evolution of both this eastward propagating two day wave and the classic westward propagating zonal wavenumber 3 quasi two day wave during january 2005

regional correlations of vs30 and velocities averaged over depths less than and greater than 30 meters using velocity profiles from sites in japan california turkey and europe we find that the time averaged shear wave velocity to 30 m vs30 used as a proxy for site amplification in recent ground motion prediction equations gmpes and building codes is strongly correlated with average velocities to depths less than 30 m vsz with z being the averaging depth the correlations for sites in japan corresponding to the kik net network show that vs30 is systematically larger for ag ivenvsz than for profiles from the other regions the difference largely results from the placement of the kik net station locations on rock and rocklike sites whereas stations in the other regions are generally placed in urban areas underlain by sediments using the kik net velocity profiles we provide equations relating vs30 to vsz for z ranging from 5 to 29 m in 1 m increments these equations and those for california velocity profiles given in boore 2004b can be used to estimate vs30 from vsz for sites in which velocity profiles do not extend to 30 m the scatter of the residuals decreases with depth but even for an averaging depth of 5 m a variation in logvs30 of 1 standard deviation maps into less than a 20 uncertainty in ground motions given by recent gmpes at short periods the sensitivity of the ground motions to vs30 uncertainty is considerably larger at long periods but is less than a factor of 1 2 for averaging depths greater than about 20 m we also find that vs30 is correlated with vsz for z as great as 400 m for sites of the kik net network providing some justification for using vs30 as a site response variable for predicting ground motions at periods for which the wavelengths far exceed 30 m online material estimates of vs30 at k net stations

short term variability in the migrating diurnal tide caused by interactions with the quasi 2 day wave 1 the migrating diurnal tide is one of the dominant dynamical features in the low latitudes of the earth s mesosphere and lower thermosphere mlt region representing the atmospheric response to the largest component of solar forcing ground based observations of the tide have resolved short term variations attributed to nonlinear interactions between the tide and planetary waves that are also in the region using the ncar thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm we simulate a quasi 2 day wave qtdw event under late january conditions in this case sideband sum and difference child waves are resolved indicating that a nonlinear interaction is occurring between the qtdw and the tide the migrating diurnal tide in the mlt displays local amplitude decreases of 20 40 as well as a shortening of vertical wavelength by roughly 4 km examining the physical mechanisms driving the interaction nonlinear advection is found to result in amplification of the tide in some regions and damping in others manifesting as increased smoothing of the tidal structure when the qtdw is present in the mlt additionally the qtdw also enhances the easterly summer mean wind jet that can also account for changes in tidal amplitude and vertical wavelength we find that qtdw induced background atmosphere changes in time gcm can drive tidal variability at levels greater than nonlinear advection a possibility not previously considered

diurnal variation of gravity wave momentum flux and its forcing on the diurnal tide 1 the diurnal variation of gravity wave gw momentum flux is derived from the 5 years of meteor radar wind measurements at maui hawaii the amplitude and phase relationships between the gw forcing and the diurnal tide are analyzed by calculating their equivalent rayleigh frictions the results show that the gw momentum flux is clearly modulated by the diurnal tide the forcing from the momentum flux convergence has strong effects on both the amplitude and phase of the diurnal tide they can reach 80 ms 1 day 1 for the amplitude and 15 h day 1 for the phase the gw forcing tends to increase the diurnal tide amplitude above 90 km but has a small damping effect below 90 km it tends to increase the phase of the diurnal tide throughout all altitudes seasonal variations of the gw forcing exist which result in differences in their effects on the diurnal tide the magnitudes of the forcing are in agreement with recent results from satellite observations but are much larger than values used in general circulation models this work also demonstrates that meteor radar measurements can provide a valuable data set for the study of gw tide interactions

low latitude 2 day planetary wave impact on austral polar mesopause temperatures revealed by a january diminution in pmse above davis antarctica 1 a new characteristic of the austral summer polar mesopause as revealed by ground based radar and satellite temperature measurements is reported that is linked to inter annual variability of the low latitude easterly wind jet four consecutive seasons of polar mesosphere summer echoes pmse and mesosphere temperature observations above davis antarctica 68 6 s show a mid january diminution in pmse occurrence rate that coincides with a minor mesopause warming of several degrees spectral analyses of pmse aura microwave limb sounder mls temperatures and radar meridional winds show the presence of 4 5 day planetary waves pws throughout the austral summer in the polar upper mesosphere together with enhanced 2 day pw activity from mid january to mid february analysis of mls temperatures show that the 2 day pws have zonal wavenumbers s with both westward s 2 3 and eastward s 2 3 components although displaying some inter annual variation in the peak onset time the mid january mesopause warming coincides with a weakening of the equatorward meridional wind above davis and enhancement of low latitude 2 day pw activity

two day wave traveling westward with wave number 1 during the sudden stratospheric warming in january 2017 quasi two day wave qtdw propagating westward with wavenumber number 1 w1 in january 2017 is studied using global temperature observed by saber and wind observed by a meteor radar at fuke china 19 0on 109 8oe the amplitude of w1 significantly enhances during january 2017 when two stratospheric warming events occur the temperature perturbation of w1 reaches maximum amplitude of more than 6k at latitude 15o around 84 km and 95 km the structure of temperature w1 is symmetric with regard to the equator the temporal variation of w1 is consistent with the stationary planetary wave with wavenumber 2 spw2 but contrary to the qtdw propagating westward with wavenumber 3 w3 when spw2 is large during two sudden stratospheric warming events energy transfers from w3 to w1 two bursts of the two day wave in meridional wind observed by the meteor radar are just corresponding to the local maxima of w3 and w1 respectively we conclude that during january 2017 w1 is generated by the nonlinear interaction between spw2 and w3 spw2 which is modulated by the quasi 16 day perturbation in the stratosphere plays a key role in the energy transmission from w3 to w1 and it is responsible for the 16 day variation of w1

the median valley a result of magma fracture beneath mid ocean ridges the shape of the ocean floor has been successfully modelled using the framework of plate tectonics1 4 lithospheric plates approximately 100 km thick lie above the hotter less viscous asthensophere at the ridges the lithosphere thins plate separation is accompanied by creation of new ocean floor as the asthenosphere flows upwards and cools on approaching the surface as a result heat is concentrated beneath the ridge axes densities are lower due to thermal expansion so that for iso static equilibrium the sea floor sits some 6 km shallower than older sea floor well off axis where the temperature anomaly is absent at fast spreading ridges velocity v 5 cm yr 1 such as the east pacific rise this description fits well a central ridge crest is flanked on either side by bathymetry which deepens in agreement with thermal models3 6 at a rate propor tional to the square root of age of creation however at slow spreading ridges v 5 cm yr 1 a major central graben of order 1 km deep replaces the central ridge crest although the square root of age dependence is observed well away from the axes isostatic equilibrium is not achieved7 since the sea floor lies 1 2 km below the equilibrium level in this report i demonstrate that this graben or median valley can result from steady state dyke intrusion into and brittle failure of the axial lithosphere

submersible observations at the axis of the ultra fast spreading east pacific rise 17 30 to 21 30 s abstract manned submersible observations conducted with the diving saucer cyana at the axis of the east pacific rise at four sites near 21 30 21 10 18 30 and 17 30 s demonstrate that the ridge evolves between two end members the volcanic end member is characterized by a non rifted or only slightly rifted topographic ridge with widespread but diffuse water venting the tectonic end member is marked by a well defined axial graben whose floor can be the locus of intense sulfide deposition at one site a small stockwork mineralization exposed through normal faulting was observed at the base of the graben wall the dives illustrate the discontinuous nature of volcanic tectonic and hydrothermal activities even in the case of the fastest accretion found in the ocean

statistical features of short period and long period near source ground motions abstract this study collects recorded ground motions from the near source region of large earthquakes and considers to what extent this historic record can inform expectations of future ground motions at similar sites the distribution of observed peak ground acceleration pga is well approximated by the lognormal distribution and we expect the observed distribution to remain unchanged with the addition of data from future earthquakes however the distribution of peak ground displacements pgd will likely change after a well recorded large earthquake specifically we expect future observations of pgd greater than those previously recorded we use seismic scaling relations to motivate the expected distribution of pgd as uniform on the logarithmic scale or at least fat tailed because pga does not scale with fault rupture area or slip on the fault there are no such scaling relations to predict the observed distribution of pga the observed records show that there is essentially no correlation between pgd and pga for near source ground motions from large events the large uncertainty in a future value of pgd in the near source region of a large earthquake exists despite the ability of earth scientists to accurately model long period ground motions in contrast the relative certainty in a future value of pga exists despite the inability to model short period ground motions reliably the stability of the observed distribution of pga with respect to new ground motion records enables us to predict the distribution of future pga and to calculate the probability of exceeding the largest recorded pga

recordings of the 2004 parkfield earthquake on the general earthquake observation system array implications for earthquake precursors fault rupture and coseismic strain changes the 2004 parkfield earthquake generated a unique set of near field high resolution colocated measurements of acceleration volumetric strain and ve locity at 11 stations in the general earthquake observation system geos array the recordings indicate no precursory strain or displacement was discernable at sen sitivities of 10 11 strain and 5 10 8 m 25 sec prior to the earthquake at distances of 0 5 to 12 km of fault rupture coherent fault parallel and fault normal displacement pulses observed along the fault north of the epicenter are consistent with model predictions for fling directivity and displacement for right lateral strike slip fault rupture the fault parallel and fault normal pulses imply apparent rupture velocities of 2 86 0 15 and 3 03 0 24 km sec respectively unprecedented high resolution volumetric strain recordings on opposite sides of the fault show that dynamic strains radiated from ruptured segments of the fault are more than an order of magnitude larger than final coseismic strain offsets associated with fault slip suggesting that dynamic radiated strain may have contributed to the triggering of failure on unrup tured segments high resolution recordings show that coseismic strain offsets occur abruptly over time intervals of less than 10 sec near the time of arrival of the dominant radiated fault parallel and fault normal displacements subsequent measurements show that the strain offsets continue to increase by as much as 69 in 5 min and 300 in 24 hr over that measured during initial fault slip at depth estimates of local material parameters from simultaneous measurements of volumetric strain and ac celeration confirm seismic calibration factors previously measurable in situ only at tidal periods

azimuth dependent amplification of weak and strong ground motions within a fault zone nocera umbra central italy 1 during three moderate magnitude earthquakes occurred in september october 1997 in the central apennines italy accelerations larger than 0 5 g were recorded in the town of nocera umbra 10 to 15 km n nw of the epicenters the accelerograph is sited in a fault zone close to a n30 e tectonic contact six temporary seismological stations installed across the fault recorded 82 aftershocks occurred in two seismogenic zones the colfiorito sellano area s se of the array and the gualdo tadino area to the north the array data reveal large variations in terms of both peak ground motions and spectral amplitudes within the fault zone amplifications show a strong dependence on the source azimuth at the accelerograph site the effects are particularly large for events from s se peak ground motions are a factor of 14 larger than those of a reference site and conventional spectral ratios attain amplitudes as large as 50 at 7 hz along the n30 e direction of motion parallel to the strike of the fault nineteen strong motion accelerograms were then used to compare ground motion properties between weak and strong events up to m 0 1 2 x 10 25 dyn cm a particle motion analysis shows that the directional effect is also present in the strongest motions even though the amplification of peak ground motion decreases when m 0 increases results from stochastic simulations indicate that such a behavior is not due to nonlinearity applying the empirical weak motion transfer functions in a purely linear model the observed peak ground motions of the largest events are fit satisfactorily

seismic tomography of the southern california plate boundary region from noise based rayleigh and love waves we use cross correlations of ambient seismic noise between pairs of 158 broadband and short period sensors to investigate velocity structure over the top 5 10 km of the crust in the southern california plate boundary region around the san jacinto fault zone sjfz from the 9 component correlation tensors associated with all station pairs we derive dispersion curves of rayleigh and love wave group velocities the dispersion results are inverted first for rayleigh and love waves group velocity maps on a 1 5 1 5 km2 grid that includes portions of the sjfz the san andreas fault saf and the elsinore fault we then invert these maps to 3d shear wave velocities in the top 7 km of the crust the distributions of the rayleigh and love group velocities exhibit 2 azimuthal anisotropy with fast directions parallel to the main faults and rotations in complex areas the reconstructed 3d shear velocity model reveals complex shallow structures correlated with the main geological units and strong velocity contrasts across various fault sections along with low velocity damage zones and basins the sjfz is marked by a clear velocity contrast with higher v s values on the ne block for the section se of the san jacinto basin and a reversed contrast across the section between the san jacinto basin and the saf velocity contrasts are also observed along the southern parts on the saf and the elsinore fault with a faster southwest block in both cases the region around the salton trough is associated with a significant low velocity zone strong velocity reductions following flower shape with depth are observed extensively around both the sjfz and the saf and are especially prominent in areas of geometrical complexity in particular the area between the sjfz and the saf is associated with an extensive low velocity zone correlated with diffuse seismicity at depth and a similar pattern including correlation with deep diffuse seismicity is observed on a smaller scale in the trifurcation area of the sjfz these results augment local earthquake tomography images that have low resolution in the top few km of the crust and provide important constraints for studies concerned with behavior of earthquake ruptures generation of rock damage and seismic shaking hazard in the region

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

diversity of fault zone damage and trapping structures in the parkfield section of the san andreas fault from comprehensive analysis of near fault seismograms summary we perform comprehensive analyses of trapped waves and signals of damaged fault zone rocks associated with time delays of body waves along the parkfield section of the san andreas fault saf waveforms generated by thousands of earthquakes and recorded by near fault stations in several permanent and temporary deployments are examined with attention to the possible influence of a lithology contrast across the fault on signals of the low velocity damage zone clear candidate trapped waves are identified at only three stations mmnb and two of the other near fault stations flip and pies further to the nw clear candidate trapped waves are not seen at any of the near fault stations to the se of mmnb the locations of the events generating good candidate trapped waves at mmnb and the other two stations are not distributed broadly in space but clustered in a small number of locations moreover events that generate clear candidate trapped waves at one station do not typically generate trapped waves at the other two stations these observations imply that the damage zone is highly variable along strike and that a coherent connected waveguide does not exist for distances along strike larger than at most 3 5 km the distance between stations synthetic waveform fits for observed trapped waves at stations mmnb and flip indicate that the most likely parameters of the trapping structures at these locations are widths of about 150 m depths of about 3 km velocity reductions of 30 40 per cent and q values of 10 40 synthetic calculations of trapped waves demonstrate that if there is a contrast of seismic velocity across the fault the trapped waves are delayed relative to the s wave arrival trapped waves at station mmnb and to a lesser extent also at stations flip and pies in the nw section show this characteristic this suggests a lithology contrast in the top few km at these locations in agreement with results from tomography and studies of head waves in the parkfield area at several mini across fault arrays where trapped waves are not observed a low velocity damage zone is detected from the delay in the arrival time of body wave phases relative to a nearby off fault station the observed delay of the s wave is greater than the p wave delay consistent with the existence of a damage zone with poisson ratio of about 0 33 the observations of time delays without trapped waves indicate that parts of the damage zone are insufficiently coherent to generate trapped waves a broader damage zone may exist in the region between the saf and the south west fracture zone the results highlight the diversity of damage structures along the 40 km of the saf examined in this study and imply that fault imaging based on data at single sites does not necessarily apply to a larger section

isostasy magma chambers and plate driving forces on the east pacific rise the density distribution within a cooling plate is calculated which incorporates temperature and pressure effects from this density distribution the pressure field within the plate and the gravity field at sea level are computed for various degrees of isostatic compensation in this model the pressure field within the plate has a horizontal gradient at shallow depths away from zero age and a horizontal gradient toward zero age at greater depths caused by the loading of the ocean isostatic equilibrium is approached if one allows the loading due to the water to depress the seafloor and at the same time allows mass conservation by flow at depth toward zero age a viscosity model based on a newtonian rheology which included temperature and pressure effects has a high gradient close to the plane separating positive and negative pressure gradients which would facilitate the return flow and decouple the lithosphere from the asthenosphere addition of a crust to the homogeneous model does not substantially change these conclusions comparison of this model with examples of east pacific rise data suggests that some areas may not be in complete isostatic equilibrium implying the existence of horizontal pressure gradients toward zero age in the asthenosphere this model can be made to fit the general features of the east pacific rise but not the detailed gravity and topography near zero age if one allows convective cooling of the crust by water partial melting of the upper mantle and intrusion of this partial melt into the crust the water depths increase more rapidly near zero age and an increased positive gravity anomaly is produced over the rise axis both of which produce a better fit to east pacific rise data at 12 n this study suggests that crustal magma chambers on the east pacific rise may be associated with anomalous positive gravity anomalies caused by a positive density contrast between magma and fractured porous crustal rocks

the near source ground motion of the 6 august 1979 coyote lake california earthquake a finite fault striking n24 w and extending to a depth of 10 km is proposed to explain the strong ground motion data for the 6 august 1979 coyote lake california earthquake m l 5 9 our source model suggests that right lateral faulting initiated at a depth of 8 km and ruptured toward the south with a velocity of 2 8 km sec this unilateral rupture can explain the large displacement recorded south of the epicenter however the waveform coherency across an array south and southwest of the epicenter suggests that the rupture length is less than 6 km the maximum dislocation is about 120 cm in a small area near the hypocenter and the total moment is estimated to be 3 5 10 24 dyne cm an abrupt stopping phase which corresponds to a deceleration of right lateral motion can explain the high peak acceleration recorded at array station 6 the stress drop in the hypocentral area is about 140 bars the average stress drop over the entire rupture surface is 30 bars the preferred finite source model can predict the p n1 waveforms and the beginning features in the teleseismic seismograms no clear arrivals can be observed in the near source data for the possible second and third smaller events suggested by nabelek personal communication

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

a simple thermal mechanical model for mid ocean ridge topographic variation summary a simple dynamic model based on the geometry of mantle divergence and thermal parameters controlling equilibrium size of the axial magma chamber explains the variation in topography along mid ocean ridges among morphological characters accounted for are 1 the change from axial valley to axial high type ridge crests with increasing spreading rate 2 the localized occurrence of deeps at ridge transform intersections and 3 the correlation of average transform spacing with spreading rate the model also yields an explanation for anomalous ridge topography associated with oceanic hot spots incorporation of smaller scale bathymetric and ophiolite data into this scheme permits construction of a comprehensive model of ocean crust accretion

three dimensional compressional wavespeed model earthquake relocations and focal mechanisms for the parkfield california region we present a new three dimensional 3d compressional wavespeed v p model for the parkfield region taking advantage of the recent seismicity associated with the 2003 san simeon and 2004 parkfield earthquake sequences to provide increased model resolution compared to the work of eberhart phillips and michael 1993 epm93 taking the epm93 3d model as our starting model we invert the arrival time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast southwest by 120 km northwest southeast we include catalog picks and cross correlation and catalog differential times in the inversion using the double difference tomography method of zhang and thurber 2003 the principal v p features reported by epm93 and michelini and mcevilly 1991 are recovered but with locally improved resolution along the san andreas fault saf and near the active source profiles we image the previously identified strong wavespeed contrast faster on the southwest side across most of the length of the saf and we also improve the image of a high v p body on the northeast side of the fault reported by epm93 this narrow body is at about 5 to 12 km depth and extends approximately from the locked section of the saf to the town of parkfield the footwall of the thrust fault responsible for the 1983 coalinga earthquake is imaged as a northeast dipping high wavespeed body in between relatively low wavespeeds 5 km sec extend to as much as 10 km depth we use this model to derive absolute locations for about 16 000 earthquakes from 1966 to 2005 and high precision double difference locations for 9 000 earthquakes from 1984 to 2005 and also to determine focal mechanisms for 446 earthquakes these earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure the aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre 2004 seismicity confirming earlier observations waldhauser et al 2004 that the seismicity pattern at parkfield is long lived and persists through multiple cycles of mainshocks online material 3d v p model and earthquake relocations

directional resonance variations across the pernicana fault mt etna in relation to brittle deformation fields summary the pernicana fault pf is the main structural element of mt etna and the northern boundary of a section sliding to the southeast observed ground motion records in the damage zone of the pf show strong variations of directional resonance in the horizontal plane the observed resonance directions exhibit an abrupt rotation of azimuth by about 30 across the fault varying from n166 on the north side to n139 on the south we interpret the directional resonance observations in terms of changes in the kinematics and deformation fields on the opposite sides of the fault the northern side is affected primarily by the left lateral strike slip movement whereas the southern side that is subjected also to sliding is under a dominant extensional stress regime brittle deformation models based on the observed kinematic field predict different sets of fractures on the opposite sides of the fault synthetic cleavages and extensional fractures are expected to dominate in the northern and southern sides respectively these two fracture fields have different orientations n74 and n42 respectively and both show a near orthogonal relation 88 in the northern sector and 83 to the south with the azimuth of the observed directional resonance we conclude that the direction of the largest resonance motions is sensitive to and has transversal relationship with the dominant fracture orientation the directional amplification is inferred to be produced by stiffness anisotropy of the fault damage zone with larger seismic motions normal to the fractures

the mid ocean ridge axial valley as a steady state neck abstract in this paper the mid ocean ridge axial valley is modelled as a steady state lithospheric neck in which lithospheric stretching balances lithospheric accretion conversely the axial high is a steady state lithospheric bulge the lithosphere is modelled as a thin plate with a newtonian rheology it is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases an axial high will occur if the opposite condition holds this is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates by fitting our model to profiles across the mid atlantic ridge and the east pacific rise and assuming the lithospheric thickness at the ridge axis to be 5 km we find accretion widths of 6 8 km we find the width over which there is a significant increase in lithospheric viscosity to be also 6 8 km