hypocentre determination offshore of eastern taiwan using the maximum intersection method summary the maximum intersection maxi method which derives from the master station method msm determines within a 3 d velocity model the absolute hypocentral location based on observed arrival times first the spatial node that better satisfies the arrival time differences computed at all station pairs plus or minus an error tolerance value in seconds is defined as the preliminary hypocentral solution pred second because pred depends neither on the estimate of origin time nor on the residual root mean square rms residual outliers are objectively detected and cleaned out from the original data set without any iterative process or weighting third a statistical minimization residual rms is conducted in a small domain around the pred node which results in a unique final solution the maxi method is applied to the determination of earthquake hypocentres with the proper station correction terms in the southernmost extremity of the ryukyu subduction zone where several dense seismic clusters occur near the seismogenic plate interface the location of earthquakes recorded at both the taiwanese and japanese networks is obtained for about a thousand events between 1992 and 1997 the process uses a detailed 3 d velocity model based on multiple geophysical data sources obtained in the junction area between subduction and collision east of taiwan the earthquake clustering and the significant drop in residual statistics 1 20 0 80 and 0 35 s for taiwanese catalogue msm and maxim solutions respectively indicate the accuracy of the method which can be used to routinely determine absolute hypocentre location based on observed arrival times

moored wind temperature and current observations made during coastal ocean dynamics experiments 1 and 2 over the northern california continental shelf and upper slope moored observations of winds currents and temperature made over the northern california shelf and upper slope during the 1981 and 1982 upwelling seasons as part of the coastal ocean dynamics experiment code are described in both field experiments code 1 and code 2 the onset of the upwelling season is triggered by the strengthening of the north pacific high off northern california and signalled by the presence of water colder than 9 c near the bottom over the shelf vertical temperature stratification over the shelf is reduced during active upwelling local winds and subtidal currents over the shelf are strongly polarized in the alongshelf direction and the mean wind stress is directed equatorward response of alongshelf currents to wind forcing is similar at all depths over the shelf the response of the cross shelf current is more complex in the surface layer offshelf flow is significantly and positively correlated with the upwelling favorable alongshelf wind stress beneath this layer a weak return flow exists the thickness of the upper mixed layer varies greatly reaching 60 m during upwelling events thus direct wind forcing affects a substantial fraction of the water over the shelf low frequency wind stress current and temperature fields are adequately described by a few energetic empirical orthogonal functions with simple spatial structure the most energetic mode typically accounts for 75 or more of the variance of wind stress and temperature over the middle and inner shelf the most energetic current mode accounts for 90 or more of the variance is dominated by he alongshelf component and veers to the left with depth the low frequency alongshelf flow fluctuations are quasi barotropic with some vertical shear but no reversal with depth vertical shear in alongshelf current and the cross shelf density gradient obey the thermal wind relation the simple structure of the subdiurnal currents allow estimation of the alongshelf volume transport between the 60 and 130 m isobaths through three cross shelf transects in code 2 volume transport over the shelf is often nearly two dimensional but occasional large differences indicate significant mass exchange between the shelf and the adjacent upper slope correlation of low frequency currents and alongshelf wind stress shows the response of alongshelf current to be maximum near the surface and near midshelf where a 15 cm 1 alongshelf flow lags a 1 dyn cm 2 alongshelf wind stress by the order of 10 hours currents over the upper slope are not significantly correlated with local wind alongshelf transport over the shelf is highly correlated with local wind stress explaining 80 of the variance in the low frequency transport during code 2 volume transport is best correlated with wind stress approximately 30 km to the south winds in the code area during spring and summer are characterized by strong and persistent upwelling favorable winds interrupted by shorter periods of weak winds regression analysis indicates that in the absence of wind a poleward barotropic current jet exists over the shelf with a maximum amplitude over the inner shelf in the code area an upwelling favorable wind counters this jet over the shelf so that the alongshore flow is everywhere equator ward during strong wind events since the maximum response to wind forcing is found at midshelf and near surface the mean current over the upwelling season is strongly sheared in both the vertical and cross shelf directions mean flow over the outer shelf is surface intensified and is directed offshelf and equatorward while the deeper flow at midshelf and through most of the water column over the inner shelf is directed poleward

characteristic patterns of the circulation in the santa barbara channel the subtidal near surface circulation in the santa barbara channel sbc and on the shelf north of point conception is described based on observations obtained during the 3 year period from 1993 to 1995 near surface currents in the channel are a superposition of a larger than sbc scale flow and a cyclonic circulation of variable intensity located inside the channel on seasonal timescales the larger than sbc scale flow near the surface is equatorward in spring and poleward from summer through winter the increase in equatorward flow in spring occurs concurrently with the increase in equatorward wind stress and the decrease in near surface temperatures and synthetic subsurface pressures ssps the flow reverses in late spring simultaneously with the increase in the along channel ssp difference and months before wind stress has reached its peak the period during which the cyclonic circulation within the sbc is strongest coincides with the period of strongest poleward flow through the eastern entrance a synoptic description of the circulation in the sbc is presented in terms of six characteristic patterns labeled upwelling relaxation cyclonic propagating cyclones flood east and flood west an analysis of the 5 and 45 m currents into empirical orthogonal functions eofs isolates 50 53 of the 5 m 45 m low frequency current variance into three two modes combining these modes with the mean current fields the modes have spatial patterns that correspond to the characteristic flow patterns subjectively deduced from inspection of daily averages of the near surface currents from late spring through fall the two largest 5 m current modes produce a repeating pattern of circulation in which the sequence of four states namely upwelling cyclonic relaxation and quiescent is traversed roughly every 16 days in addition to the large scale cyclonicity in the central sbc smaller cyclonic eddies form frequently in the eastern channel and travel toward the west with an average speed of 0 06 m s 1 the patterns described here develop in response to the wind stress and the along channel ssp gradient

altimeter derived variability of surface velocities in the california current system 2 seasonal circulation and eddy statistics abstract satellite surface height and temperature fields are used to examine the seasonal surface circulation in the california current system in spring and summer an equatorward jet develops next to the coast with an initial latitudinal structure that responds to the latitudinal distribution of equatorward alongshore winds this jet moves offshore from spring to fall and develops meanders and closed eddies that contribute eddy kinetic energy and water mass properties to the nearby deep ocean higher eke appears first in spring near the coast and moves offshore with the seasonal jet along altimeter tracks parallel to the coastline 200 and 400 km offshore the greatest eke is found in summer and fall respectively with wavelengths of approximately 300 km these are the wavelengths of the largest meanders and the distance between the largest eddies visible in altimeter snapshots of the jet at 600 800 km offshore the wavelengths are generally smaller 100 150 km similar to eddy sizes reported by others for the quiescent northeast pacific some energy with longer wavelengths arrives at the track 600 km offshore in winter and 800 km offshore in spring which we interpret as the most offshore extension of the eke generated by the jet and eddy system closer to the coast in the previous spring and summer a sequence of snapshots from october 1992 to october 1993 shows details of the evolution of the jet and eddy system over one complete annual cycle

earthquake monitoring in southern california for seventy seven years 1932 2008 the southern california seismic network scsn has produced the scsn earthquake catalog from 1932 to the present a period of more than 77 yrs this catalog consists of phase picks hypocenters and magnitudes we present the history of the scsn and the evolution of the catalog to facilitate user understanding of its limitations and strengths hypocenters and magnitudes have improved in quality with time as the number of stations has increased gradually from 7 to 400 and the data acquisition and measuring procedures have become more sophisticated the magnitude of completeness m c of the network has improved from m c 3 25 in the early years to m c 1 8 at present or better in the most densely instrumented areas mainshock aftershock and swarm sequences and scattered individual background earthquakes characterize the seismicity of more than 470 000 events the earthquake frequency size distribution has an average b value of 1 0 with m 6 0 events occurring approximately every 3 yrs the three largest earthquakes recorded were 1952 m w 7 5 kern county 1992 m w 7 3 landers and 1999 m w 7 1 hector mine sequences and the three most damaging earthquakes were the 1933 m w 6 4 long beach 1971 m w 6 7 san fernando and 1994 m w 6 7 northridge earthquakes all of these events ruptured slow slipping faults located away from the main plate boundary fault the san andreas fault their aftershock sequences constitute about a third of the events in the catalog the fast slipping southern san andreas fault is relatively quiet at the microseismic level and has not had an m 6 earthquake since 1932 in contrast the slower san jacinto fault has the highest level of seismicity including several m 6 events thus the spatial and temporal seismicity patterns exhibit a complex relationship with the plate tectonic crustal deformation

moored observations of the current and temperature structure over the continental slope off central california 2 the energetics of the flow off point sur the data from two current meter moorings off point sur california were analyzed to examine the energy transfers over the continental slope in the california current system ccs the method used was to calculate terms in the heat equation at intermediate depths between instruments using the thermal wind relation to estimate the horizontal temperature gradients from the vertical shear time series of u v and t were collected for 17 months may 1989 to october 1990 at site p2 on the 800 m isobath and for 12 months may 1990 to may 1991 at site p3 25 km farther offshore on the 1800 m isobath instrument depths were 100 350 and 500 m at both moorings plus 1000 m at site p3 mean values of the energy conversion terms were computed at sites p2 and p3 at 225 m depth and at site p2 at 425 m depth the local change of eddy potential energy epe was approximately balanced epe t 0 for each case but the dynamics were quite different at p2 225 the sources of epe were advection by the mean flow and baroclinic instability which converted mean potential energy mpe to epe the losses came from eddy advection out of the region and the vertical eddy heat flux vehf which converted epe to eddy kinetic energy eke at p2 425 baroclinic instability provided the major source of epe this plus a small positive contribution from the vehf was balanced by advection of epe out of the region by both the mean flow and the eddies at p3 225 mean advection and baroclinic instability were negligible and the downward eddy heat flux vehf was balanced by advection of heat out of the region by the eddies time series of terms in the epe equation were used to examine high energy transfer events at both moorings events at p2 involved both horizontal and vertical processes and had longer timescales several days to weeks than those at p3 which had much shorter timescales 2 5 days and were dominated by vertical processes compared to the gulf stream the mean baroclinic instability at p2 both depths was smaller by at least a factor of 10 a term by term comparison between the mean values off point sur and the kuroshio extension showed different processes with the eddies gaining strength at the expense of the mean flow off point sur and the opposite occurring in the kuroshio extension

streaks multiplets and holes high resolution spatio temporal behavior of parkfield seismicity 1 double difference locations of 8000 earthquakes from 1969 2002 on the parkfield section of the san andreas fault reveal detailed fault structures and seismicity that is although complex highly organized in both space and time distinctive features of the seismicity include 1 multiple recurrence of earthquakes of the same size at precisely the same location on the fault multiplets implying frictional or geometric controls on their location and size 2 sub horizontal alignments of hypocenters along the fault plane streaks suggestive of rheological transitions within the fault zone and or stress concentrations between locked and creeping areas 3 regions devoid of microearthquakes with typical dimensions of 1 5 km holes one of which contains the m6 1966 parkfield earthquake hypocenter these features represent long lived structures that persist through many cycles of individual events

wind forced modeling studies of currents meanders and eddies in the california current system this process oriented study of the california current system ccs uses a high resolution multilevel primitive equation ocean model on a plane to isolate the response of that eastern boundary oceanic regime to temporal and spatially varying wind forcing to study the generation evolution and maintenance of many of the observed features such as currents meanders and eddies in the ccs the model is forced from rest with seasonal climatological winds in response to the prevailing wind direction surface equatorward currents develop along with upwelling of cooler water along the coast and a poleward undercurrent baroclinic barotropic instabilities in the equatorward surface current and poleward undercurrent result in the generation of meanders near the coast as the meanders intensify cold upwelling filaments develop along the coast and subsequently extend farther offshore in time the meanders form both cyclonic and anticyclonic eddies which subsequently propagate farther offshore longer simulation times 3 4 years in which a quasi equilibrium state for the ccs is reached show a seasonal cycle in response to the wind forcing for the coastal currents upwelling and filaments the meanders and eddies however can be quasi permanent as well as seasonal features the quasi permanent features play a significant role in modifying coastal currents upwelling and filaments which leads to large temporal and spatial variability in the ccs in a sensitivity study the results from several numerical experiments are used to examine the dependence of the generation of the currents meanders and eddies on the type of coriolis parameterization wind forcing and coastline geometry both the meridional variability of plane and the alongshore component of the wind stress are shown to be key ingredients for generating realistic vertical and horizontal structures for the cores of the surface equatorward and the subsurface poleward currents with such structures the currents are baroclinically and barotropically unstable resulting in the generation of meanders filaments and eddies irregularities in the coastline geometry are shown to be important for anchoring upwelling and filaments as well as for enhancing the growth of meanders and eddies cyclonic eddies tend to form in the vicinity of capes while anticyclonic eddies tend to form in the coastal indentations between capes the region off cape blanco is identified as the location where the coastal equatorward flow off oregon leaves the coast to develop a meandering jet off california the results from these experiments support the hypothesis that wind forcing and coastline irregularities on a beta plane are important mechanisms for the generation of many of the observed features of the ccs

the large scale coastal wind field along the west coast of north america 1981 1982 we study the statistical properties of the coastal wind field along the west coast of north america for two summers 1981 and 1982 coastal ocean dynamics experiments 1 and 2 respectively and the intervening winter using measured winds and geostrophic winds calculated from fleet numerical oceanography center atmospheric pressure analyses summer wind fluctuations are driven primarily by the interaction between two relatively stationary pressure systems the north pacific subtropical high and southwest u s thermal low and by their interactions with propagating atmospheric systems to the north in particular propagating cyclones and associated fronts are often followed by a northeastward intensification of the high producing strong upwelling events along the california coast this summer event sequence occurs more frequently and is displaced farther to the south on average during summer 1981 winter wind fluctuations are primarily driven by propagating cyclones and anticyclones and they tend to have larger variance and space scales than in summer a preference for poleward equatorward propagation exists in summer winter and the largest time scales are observed in summer 1982 coastal atmospheric boundary layer processes substantially modify winds within 100 200 km of the coast consequently measured wind fluctuations are strongly polarized in the alongshore direction and have means and rms amplitudes that can vary considerably between nearby stations along the coast calculated wind fluctuations are less polarized in the alongshore direction and have alongshore correlation scales about 60 larger than those for measured winds they represent fluctuations with alongshore wavelengths of 900 km rather well but represent poorly those with smaller wavelengths and those due to coastal atmospheric boundary layer effects

broadband modeling of local earthquakes three component broadband waveforms of two small earthquakes near upland california recorded on the pasadena broadband high dynamic range instrument were modeled to obtain useful green s functions for this path and to examine the sensitivity of the synthetic seismograms to perturbations of the crustal model we assumed that the source of each event was both simple and known as determined from the caltech usgs array first motions a trapezoidal time function was chosen to fit the width of the direct s wave generalized rays reflectivity and finite difference techniques were used to compute the synthetic seismograms we found that a simple layer over a half space model is an adequate approximation of the upper crust along this profile in particular the waveforms are controlled by a relatively slow 4 km thick surficial layer 4 5km s 1 2 6 km s 1 over a faster layer 5 9 km s 1 3 5 km s 1 the relative amplitudes of direct and multiple s indicate that the main shock occurred at a depth of 6 km while the aftershock occurred at a depth of 8 to 9 km sensitivity analyses indicate that for distances less than 50 km and for periods longer than 1 sec the synthetic seismograms are not very sensitive to perturbations of the deep crustal structure analysis of upper crustal model perturbations revealed that the surficial layer is between 3 to 5 km thick in addition the contact between this layer and the underlying material can be smoothed with a 2 km wide velocity gradient without adversely affecting the fit to the data two dimensional finite difference calculations show that a ridge structure beneath the recorder acts as a lowpass filter the lower frequency phases are largely unaffected other two dimensional models with ridges between the source and receiver clearly did not fit the data synthetic seismograms computed for the best fitting model were used to estimate a long period moment of 6 2 1022 dyne cm ml 4 6 and 1 1022 dyne cm ml 3 7 with identical triangular source time durations of 0 3 sec assuming the same fault dimension of 0 4 km from standard scaling laws stress drop estimates of 410 and 70 bars are obtained for the two events respectively generally we found that it is possible to reproduce local waveforms at frequencies up to 1 hz without a complete knowledge of fine structural detail resulting green s functions can be useful in studying historic events and in simulations of large events from a given source region

demographics of mesoscale eddies in the california current 1 demographics and variability of persistent mesoscale eddies in the california current have been investigated in a systematic manner using a decade 1995 2004 of merged satellite altimeter data the number of persistent eddies formed each year varied almost 3 fold 4 fold for cyclones anticyclones and was strongly modulated by the 1997 1998 el nino event over the 10 year period cyclone formation was maximum in both winter and summer while maximum anticyclone formation occurred only in winter the majority of long lived eddies 70 day lifespan originated seaward of the 1000 m isobath and with the exception of 2 anticyclones did not form between 45 and 50 n maximum westward propagation of long lived eddies was almost 800 km with similar mean propagation speeds for cyclones and anticyclones

sea level variabilities in the gulf stream between cape hatteras and 50 w a geosat study sea level residuals relative to a 2 year mean sea level in the gulf stream downstream of cape hatteras between 80 w and 50 w longitude are studied using geosat altimetry between november 1986 and december 1988 spectral slopes computed at four nominal locations along the gulf stream 72 w 65 w 60 w and 55 w range between 1 5 and 0 9 significantly less steep than the 2 5 spectral slope found at bermuda because of increased short wavelength energy the spectra tend to become more white going eastward but this trend is not statistically significant energy at the annual cycle decreases eastward from around 40 of the total variance at 72 w to 20 of the total variance at 55 w maximum residual geostrophic sea surface velocities associated with the annual cycle were of the order of 70 cm s comparison between the gulf stream path from sea level residual maps and the north wall edge observed from noaa infrared ir imagery shows an rms difference of 49 km between 75 w and 60 w longitude but not all sea level maps show a clearly defined gulf stream using the ir estimates for gulf stream position sea level residuals were modeled as the meandering of a sea level front shaped as 0 5 m tanh x x0 50 km sin with the angle between the geosat track and the gulf stream edge and x0 is based on the observed ir north wall positions this showed that the annual signal in terms of the location and phase of its maxima is dominated by the meandering of the gulf stream

improved seismic velocity reference model from local earthquake data in northwestern italy resolution and reliability estimates of results obtained by seismic tomography strongly depend on the reference model inadequate initial reference models may severely distort tomographic images or introduce artefacts that lead to misinterpretations of the results reference models are usually obtained by means of a priori near surface geological information or by geophysical information derived by controlled source seismology starting from the idea that a reference model must approximate the weighted average of data selected for the three dimensional 3d inversion one dimensional 1d model for northwestern italy is derived that is able to minimize mean of rms of a set of well locatable earthquakes by computing a solution of the coupled hypocentre 1d velocity problem such a model termed the minimum 1d model can be used both as an initial reference model for 3d inversion and as a reference velocity model for high quality routine earthquake location

the effect of velocity structure errors on double difference earthquake location 1 we show that relative earthquake location using double difference methods requires an accurate knowledge of the velocity structure throughout the study region to prevent artifacts in the relative position of hypocenters the velocity structure determines the ray paths between hypocenters and receivers these ray paths and the corresponding ray take off angles at the hypocenters determine the partial derivatives of travel time with respect to the hypocentral coordinates which form the inversion kernel that maps double differences into hypocentral perturbations thus the large scale velocity structure enters into the core of the double difference technique by employing a 1d layered model with sharp interfaces to perform double difference inversion of synthetic data generated using a simple 1d gradient model we show that inappropriate choice of the velocity model combined with unbalanced source receiver distributions can lead to significant distortion and bias in the relative hypocenter positions of closely spaced events

the large scale summer circulation of the california current satellite data from the geosat altimeter and the advanced very high resolution radiometer avhrr are used to show the large scale structure of the surface circulation of the california current system in summer these data show the connection between an equatorward jet and temperature front off oregon that lies within 100 km of the coast similar to that first observed in the 1960 s and 1970 s and a jet that meanders along the convoluted offshore edge of a temperature front off california as repeatedly observed in the 1980 s

imaging the complexity of an active normal fault system the 1997 colfiorito central italy case study 1 six moderate magnitude earthquakes 5 mw 6 ruptured normal fault segments of the southern sector of the north apennine belt central italy in the 1997 colfiorito earthquake sequence we study the progressive activation of adjacent and nearby parallel faults of this complex normal fault system using 1650 earthquake locations obtained by applying a double difference location method using travel time picks and waveform cross correlation measurements the lateral extent of the fault segments range from 5 to 10 km and make up a broad 45 km long nw trending fault system the geometry of each segment is quite simple and consists of planar faults gently dipping toward sw with an average dip of 40 45 the fault planes are not listric but maintain a constant dip through the entire seismogenic volume down to 8 km depth we observe the activation of faults on the hanging wall and the absence of seismicity in the footwall of the structure the observed fault segmentation appears to be due to the lateral heterogeneity of the upper crust preexisting thrusts inherited from neogene s compressional tectonic intersect the active normal faults and control their maximum length the stress tensor obtained by inverting the six main shock focal mechanisms of the sequence is in agreement with the tectonic stress active in the inner chain of the apennine revealing a clear ne trending extension direction aftershock focal mechanisms show a consistent extensional kinematics 70 of which are mechanically consistent with the main shock stress field

automatic seismic phase picking and consistent observation error assessment application to the italian seismicity summary accuracy of seismic phase observation and consistency of timing error assessment define the quality of seismic waves arrival times high quality and large data sets are prerequisites for seismic tomography to enhance the resolution of crustal and upper mantle structures in this paper we present the application of an automated picking system to some 600 000 seismograms of local earthquakes routinely recorded and archived by the italian national seismic network the system defines an observation weighting scheme calibrated with a hand picked data subset and mimics the picking by an expert seismologist the strength of this automatic picking is that once it is tuned for observation quality assessment consistency of arrival times is strongly improved and errors are independent of the amount of data to be picked the application to the italian local seismicity documents that it is possible to automatically compile a precise homogeneous and large data set of local earthquake pg and pn arrivals with related polarities we demonstrate that such a data set is suitable for high precision earthquake location focal mechanism determination and high resolution seismic tomography

temperature plant pigment optical relations in a recurrent offshore mesoscale eddy near point conception california the temperature plant pigment optical structure of a mesoscale anticyclonic eddy consistently found in shipboard surveys and satellite sensed data several hundred kilometers southwest of point conception california is described on three different time scales 100 day mesoscale annual and several year the satellite coastal zone color scanner czcs ocean color imagery detected the near surface chlorophyll structure of the eddy but in situ optical and plant pigment data suggest that such imagery does not provide a good estimate of the integrated chlorophyll field of the eddy the temperature and plant pigment boundaries of the eddy as determined from two dimensional gradients of advanced very high resolution radiometer avhrr and czcs imagery do not coincide spatially this and in situ temperature plant pigment and optical structure provide additional evidence that some eddy systems in the california current are not isolated vortex systems but rather continuously entrain waters of nonlocal origin laterally into their upper layers within the california current a ratio of avhrr czcs data is useful for separating inshore from oceanic water masses and following their surface entrainment by offshore vortices the historical 28 year california cooperative oceanic fisheries investigations data for the point conception region of the california current and remotely sensed data over this region show that the point conception eddy is a recurrent feature in the offshore california current moreover the available data provide evidence that a large number of warm core mesoscale eddies occur simultaneously in a transition zone between coastal and oceanic regimes that these features recur at preferred locations within the transition zone and that this family of eddies should impose a significant offshore boundary condition on the flow of the california current

annual rossby wave characteristics in the california current region from the geosat exact repeat mission abstract examination of sea level variability in the california current region from 20 to 50 n and from the coast of north america to 140 w was made possible with altimetric observations obtained by the geosat exact repeat mission erm for the period november 1986 october 1987 during this period a time sequence of twenty one 17 day maps of altimetric sea level residuals over the califomia current region was constructed the rms distribution of these altimetric sea level residuals displayed local maxima at three locations adjacent to the coast of california i e off point eugenia at 27 n southwest of point conception at 32 n and between monterey and cape mendicino i e 37 40 n complex empirical orthogonal function ceof analysis reveals that 53 of the variance over this one year period was associated with the annual cycle with amplitude maximum in summer and minimum in winter the spatial distribution of amplitude displayed local maxima adjacent to the coast again at 27 n 32 n and from

large scale structure of the spring transition in the coastal ocean off western north america past measurements off the coast of central oregon and washington have shown that the rapid change from northward monthly mean winter winds to southward summer winds forces a spring transition of the coastal ocean sea levels and temperatures drop and mean surface currents shift from northward to southward current and water temperature data from 35 n to 48 n from 1981 and 1982 and sea level and wind stress data from 1971 1975 and 1980 1983 show the transition to have a large alongshore scale typically 500 to 2000 km the large scale wind stress appears to be the forcing mechanism at latitudes north of approximately 37 n south of 37 n sea level usually falls more gradually before the northern transition event both wind and sea level events generally progress from south to north over a 3 to 10 day period but this is not always true several aspects of the spring transition reflect coastal trapped wave dynamics previous studies at 45 n found persistent vertical shear of the southward summer current associated with a cross shelf density gradient during 1982 the shear and the density front develop over the shelf break immediately after the transition at 43 n and to the south but they are much less persistent than at 48 n the stronger winds between 38 n and 42 n and the narrower shelf result in an offshore displacement of the density front and vertical shear past the shelf break leaving the water over the shelf less stratified and more subject to barotropic reversals of the current than that farther north where the front stays closer to the coast

a reanalysis of the hypocentral location and related observations for the great 1906 california earthquake we determine probabilistic hypocentral locations for the foreshock and mainshock of the great 1906 california earthquake through reanalysis of arrival time observations in conjunction with modern velocity models and advanced event location techniques we obtain two additional observations for the mainshock and one for the foreshock that were not used in previous location studies using a robust likelihood function for event location we generate a usable subset of the predominantly unreliable teleseismic readings and determine new wave type identifications for some of the local and teleseismic readings our locations are much better constrained than those of earlier studies even though we do not assume that the epicenter lies on the san andreas fault as did previous authors we confirm the conclusions of earlier studies that the local and teleseismic arrival time observations can be explained by a single foreshock focus and a single mainshock focus on the san andreas fault and that there is no single unique hypocenter that satisfies all available local observations the maximum likelihood point latitude 37 78 n longitude 122 51 w for our preferred mainshock location indicates a hypocenter to the west of san francisco close to the san andreas fault zone this hypocenter has a 68 confidence error of about 8 km parallel to the san andreas fault and about 24 km perpendicular to the fault and a depth in the midcrust of about 12 7 km the closest point on the san andreas fault to this hypocenter lies about 10 km to the northwest of the widely accepted mainshock epicenter of bolt 1968 our mainshock location is consistent with the association of initial rupture of the 1906 mainshock with a dilatational right bend or step over in the submerged san andreas fault system offshore of the golden gate our foreshock location is less well constrained than our mainshock location but is consistent with the foreshock hypocenter being at the same location as the mainshock hypocenter online material visualization of 3d probabilistic hypocentral locations associated with the 1906 earthquake

detecting aliased tidal errors in altimeter height measurements a simple statistic is derived for quantifying the potential for the aliasing of tidal errors in a given linear estimate of sea surface height constructed from altimeter data the existence of m2 tidal constituent errors in geosat data processed in the traditional way i e with orbit errors removed using least squares fits to 1 cycle per revolution sinusoids which are of sufficient magnitude to alias into apparently westward propagating ocean features is demonstrated by artificially inducing aliasing the aliasing statistic presented here responds clearly to the induced aliasing and to actual aliasing caused by real data dropouts in the geosat data the potential for aliasing m2 tidal errors is shown to vary with latitude depending on the time interval between ascending and descending ground tracks near the location of interest the methods developed here are applied to geosat data from the northeast atlantic to demonstrate the presence of m2 tidal error aliasing in those data

applying a three dimensional velocity model waveform cross correlation and cluster analysis to locate southern california seismicity from 1981 to 2005 we compute high precision earthquake locations using southern california pick and waveform data from 1981 to 2005 our latest results are significantly improved compared to our previous catalog by the following 1 we locate events with respect to a new crustal p and s velocity model using three dimensional ray tracing 2 we examine six more years of waveform data and compute cross correlation results for many more pairs than our last analysis and 3 we compute locations within similar event clusters using a new method that applies a robust fitting method to obtain the best locations satisfying all the differential time constraints from the waveform cross correlation these results build on the relocated catalogs of hauksson and shearer 2005 and shearer et al 2005 and provide additional insight regarding the fine scale fault structure in southern california and the relationship between the san andreas fault saf and nearby seismicity in particular we present results for two regions in which the seismicity near the southern saf seems to align on dipping faults

detailed kinematics structure and recurrence of micro seismicity in the safod target region 1 large numbers of small earthquakes recorded over 2 decades and analyzed with advanced techniques are used to characterize the detailed kinematics structure and recurrence interval scaling properties of micro seismicity in a 4 4 km lateral and 6 km deep crustal volume encompassing the region of the safod deep drilling experiment the characterization reveals that the seismically active san andreas fault in the vicinity of safod s repeating magnitude 2 target earthquakes is composed of two sub parallel fault strands that are creeping at comparable rates and that one of the strands lies between the safod drilling platform and safod s target events in the region 55 of the earthquakes are members of 52 characteristically repeating earthquake sequences the recurrence intervals of the repeating target events are consistent with the interval scaling of the other sequences however this scaling is contrary to that expected from standard constant stress drop theory

southern california hypocenter relocation with waveform cross correlation part 1 results using the double difference method we present the results of relocating 327 000 southern california earthquakes that occurred between 1984 and 2002 we apply time domain waveform cross correlation for p and s waves between each event and 100 neighboring events identified from the catalog based on a 3d velocity model to simplify the computation we first divide southern california into five polygons such that there are 100 000 events or less in each region the polygon boundaries are chosen to lie in regions of sparse seismicity we calculate and save differential times from the peaks in the cross correlation functions and use a spline interpolation method to achieve a nominal timing precision of 0 001 sec these differential times together with existing p and s phase picks are input to the double difference program of waldhauser and ellsworth 2000 2002 to calculate refined hypocenters we divide the southern california region into grid cells and successively relocate hypocenters within each grid cell the overall resulting pattern of seismicity is more focused than the previously determined pattern from 1d or 3d models the new improved locations are more clustered in many cases by a factor of two or three and often show clear linear alignments in particular the depth distribution is improved and less affected by layer boundaries in velocity models or other similar artifacts

atmospheric conditions during the spring and fall transitions in the coastal ocean off western united states we examine large scale atmospheric behavior around the time of the spring and fall transitions in the coastal ocean off the west coast of north america records of adjusted sea level asl coastal wind stress sea level atmospheric pressure slp and 500 mbar heights for the years 1971 1975 and 1980 1983 are analyzed the records cover periods of 91 days centered on the dates of the spring and fall transitions as determined from coastal adjusted sea level data empirical orthogonal functions are obtained from the asl coastal wind stress and slp records the two dominant modes of the asl and coastal wind stress are similar around the times of both the spring and fall transitions and the time series for these modes are highly correlated with one another transitionlike behavior is evident in the time series of the first modes in both spring and fall but the spring transition is more pronounced principal estimator patterns formed from the dominant empirical orthogonal functions show the spatial patterns of slp which force the asl and coastal wind stress during the transitions the slp pattern which coincides with the spring transition is the formation of a high pressure system centered at 45 n and 140 w along with the development of a low pressure cell over the southwest continental united states inspection of the 500 mbar height composites for 91 days surrounding the spring transition for the 9 years reveals the formation of a ridging pattern and diffluent flow over the western united states at the time of the transition following the transition the ridging relaxes but the diffluent flow over the continent remains for the duration of the 45 days examined here the fall transition is characterized by a rise in asl particularly north of 40 n and a change from southward to northward wind stress the slp pattern which coincides with the fall transition involves the appearance of a low pressure system off western north america centered at 50 n and 140 w representing the passage of synoptic storms through the region prior to the fall transition the 500 mbar heights are somewhat diffluent and show a trough over the southwestern united states after the transition the 500 mbar flow over the northeast pacific and north america is nearly zonal

satellite observations and modeling of meanders in the california current system off oregon and northern california abstract infrared satellite images from the west coasts of oregon and northern california are used to identify meander patterns in sea surface temperature which appear as large cold tongues extending offshore two relatively long series of images from 1982 and a few examples from 1980 and 1983 demonstrate the evolution of the cold tongues from an initial variety of scales 60 200 km to the fastest growing waves 110 130 km and then finally to tongues with longer wavelengths 400 km this is observed to occur over periods of about three months in summer and fall when the coastal circulation is composed of a southward surface current over a northward undercurrent the initial shorter scale features are believed to be excited by the interaction between the mean current and the coastal topography baroclinic instability associated with the vertical shear between the surface current and the undercurrent is found to be responsible for the growth of the features observed in the satellite imagery a nonlinear

ocean response to wind sheltering in the southern california bight 1 a particularly warm surface oceanic wake persisted during the period 6 16 june 1999 in the south eastern part of santa catalina island in the southern california bight from satellite images regional atmospheric model results and hydrographic measurements around catalina island a strong correlation appears between observed sea surface temperature wind stress and surface stratification it is suggested that the warm island wake was probably due to lower turbulent mixing rates associated with wind sheltering effect we argue that the sheltering effect may also account for a portion of the relatively warm sea surface temperature characteristic of the whole bight opposing previous arguments that temperature evolution of the surface ocean in this region is primarily driven by lateral advective fluxes

horizontal transport and the distribution of nutrients in the coastal transition zone off northern california effects on primary production phytoplankton biomass and species composition conductivity temperature depth surveys during 1988 encountered strong baroclinic jets that were evident in acoustic doppler current profiler and hydrographic data during june and july 1988 a filament with high surface nitrate high chlorophyll abundant populations of neritic centric diatoms and higher rates of primary production was evident perpendicular to the coast between point arena and point reyes however the high nutrient and phytoplankton regions were not in the baroclinic jets but were south and inshore of them surface water transported offshore by the strong baroclinic jets was found to have relatively low nutrient content suggesting that the jets themselves do not carry significant levels of coastally upwelled high nutrient water to the ocean interior the low nutrient and salinity content of the jet suggests that the water originated several hundred kilometers upstream although the jets themselves do not appear to transport significant levels of nutrients directly from the coastal regime to the oceanic regime dynamic processes associated with a meandering jet are likely responsible for high surface nutrients found several hundred kilometers offshore processes such as upwelling along the southern edge of the seaward jet result in significant enrichment of the coastal transition zone and in large blooms of neritic diatoms during 1988 the high nutrient high phytoplankton filament was present when the survey sequence began but then decayed after a month the surface and subsurface nitrate fields were coherent with the dynamic topography field throughout the survey sequence however the surface and integrated chlorophyll fields were coherent only through the first two surveys a decrease in phytoplankton biomass began during the third survey coincident with physical changes which occurred in that time frame 1 an intensification of the undercurrent and 2 changes in the surface circulation from predominantly offshore to predominantly longshore understanding the processes responsible for the uncoupling between biology and physics is paramount for realistic biological models of this region

accurate hypocentre determination in the seismogenic zone of the subducting nazca plate in northern chile using a combined on offshore network the coupled plate interface of subduction zones commonly called the seismogenic zone has been recognized as the origin of fatal earthquakes a subset of the after shock series of the great antofagasta thrust type event 1995 july 30 mw 8 0 has been used to study the extent of the seismogenic zone in northern chile to achieve reliable and precise hypocentre locations we applied the concept of the minimum 1 d model which incorporates iterative simultaneous inversion of velocity and hypocentre parameters the minimum 1 d model is complemented by station corrections which are influenced by near surface velocity heterogeneity and by the individual station elevations by relocating mine blasts which were not included in the inversion we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth a study of the resolution parameters ale and dspr documents the importance of offshore stations on location accuracy for offshore events based on precisely determined hypo centres we calculate a depth of 46 km for the lower limit of the seismogenic zone which is in good agreement with previous studies for this area for the upper limit we found a depth of 20 km our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies the results are also in agreement with thermal studies for the antofagasta forearc region

seasonal renewal of the california current the spring transition off california 1 a pair of high resolution oceanographic surveys in march and april 1995 revealed a large and rapid transition from late winter to spring conditions in the coastal zone off central and southern california these data are unique in capturing the detailed three dimensional physical structure of and biological response to the spring transition in the southern california current system ccs changes associated with the transition included a strong tilting of isopycnals which lifted by up to 60 m near the coast and dropped 20 40 m offshore a subsequent increase in cross shore density gradients the development of a strong nearshore equatorward jet and an increase in net equatorward transport from the shelf break out to 300 km offshore the most dramatic physical changes were confined to the shoreward 150 km and extended at least to the depth of the core of the california undercurrent 300 m in response to these physical changes there was an apparent strong increase in primary productivity as indicated by changes in nearshore vertically integrated fluorescence and beam attenuation coefficient atmospheric and oceanic conditions in the ccs were near seasonal norms in the winter and spring of 1995 implying that a transition of the magnitude and rapidity observed here may be an annual event furthermore the development of the coastal upwelling jet was independent of the winter manifestation of the main core of the california current which was maintained well off shore this suggests that the california current is regenerated seasonally through the development and offshore evolution of the coastal upwelling jet it is not known whether the new jet joins and strengthens or replaces the offshore core of the previous winter

upper ocean water mass characteristics of the california current summer 1993 abstract two large high resolution upper ocean surveys of the california current region were conducted in the summer of 1993 temperature and salinity were measured from a seasoar vehicle and velocity was measured by a ship borne acoustic doppler current profiler both surveys extended from the continental margin to 128 w and consisted of zonal sections 28 km apart the first survey 7 28 june extended from 39 5 n to 36 25 n and the second 16 august to 1 september extended from 39 0 n to 37 0 n the current field was fairly simple in june showing an equatorward surface jet with only one gentle meander a nearly continuous poleward california undercurrent near the continental margin and two anticyclonic subsurface eddies offshore of the jet by august meanders in the surface jet had strengthened and eddies dominated the flow field large scale trends in the water mass characteristics spiciness along three isopycnal surfaces 26 4 26 0 and 25 6 kg m 3 show that temperature and salinity tend to increase towards the equator and towards shore consistent with poleward advection along the continental margin local minima in spiciness tend to coincide with the equatorward surface jet local maxima in spiciness are associated with subsurface anticyclonic eddies which have a core depth of about 150 m and seem to originate in the california undercurrent over the continental slope at local latitudes we identified several different anticyclonic eddies each with different water mass characteristics all of the eddies we observed had diameters larger than the baroclinic radius of deformation water mass characteristics and migration rates are consistent with formation over the continental margin within the preceding 6 8 months

coastal upwelling in the california current system abstract coastal upwelling in the california current system has been the subject of large scale studies off california and baja california and of small scale studies off oregon recent studies of the winds along the entire coast from 25 n to 50 n indicate that there are significant along shore variations in the strength of coastal upwelling which are reflected in the observed temperature distribution active upwelling appears to be restricted to a narrow coastal band about 10 25 km wide along the entire coast but the region influenced by coastal upwelling may be much wider intensive observations of the upwelling zone during summer off oregon show the presence of a southward coastal jet at the surface a mean vertical shear a poleward undercurrent along the bottom and persistently sloping isopycnals over the continental shelf most of the upwelling there occurs during relatively short periods several days long of upwelling favorable winds during the upwelling season off oregon the offshore ekman transport is carried by the surface ekman layer and the onshore return flow occurs through a quasi geostrophic interior it is not known whether the structure and dynamics observed off oregon are typical of the upwelling zone along the entire coast though some of the same features have been observed off baja california current and future research will eventually show whether the oregon results are also applicable in the region of persistently strong upwelling favorable winds off northern california and in the region of complex bathymetry off central and southern california

earthquake spatial distribution the correlation dimension summary we review methods for determining the fractal dimensions of earthquake epicentres and hypocentres paying special attention to the problem of errors biases and systematic effects among effects considered are earthquake location errors boundary effects inhomogeneity of depth distribution and temporal dependence in particular the correlation dimension of earthquake spatial distribution is discussed techniques for its evaluation presented and results for several earthquake catalogues are analysed we show that practically any value for the correlation dimension can be obtained if many errors and inhomogeneities in observational data as well as deficiencies in data processing are not properly considered it is likely that such technical difficulties are intensified when one attempts to evaluate multifractal measures of dimension taking into account possible errors and biases we conclude that the fractal dimension for shallow seismicity asymptotically approaches 2 20 0 05 for a catalogue time span of decades and perhaps centuries the value of the correlation dimension declines to 1 8 1 9 for intermediate events depth interval 71 280 km and to 1 5 1 6 for deeper ones

seasonal cycles of currents temperatures winds and sea level over the northeast pacific continental shelf 35 n to 48 n seasonal cycles of coastal wind stress adjusted sea level asl shelf currents and water temperatures off the west coast of north america 35 n to 48 n are estimated by fitting annual and semiannual harmonics to data from 1981 1983 longer records 9 34 years of monthly asl indicate that these two harmonics adequately represent the long term monthly average seasonal cycle and that the current measurement period is long enough to estimate the seasonal cycles we characterize the differences between fall winter and spring summer as follows for fall winter monthly mean winds north of 35 n are northward for 3 6 months longer in the north than in the south south of 35 n the mean winds are near zero or weakly southward monthly mean alongshore currents are northward over midshelf and shelf break at all locations sampled at depths of 35 m and deeper and are associated with high coastal sea levels and relatively warm water temperatures for spring summer monthly mean wind stresses are southward at all latitudes for 3 6 months longer in the south than in the north sea levels are low and water temperatures are relatively cool monthly mean currents at 35 m depth over the shelf are southward for 1 6 months longer at the shelf break than over midshelf and longer in the north than in the south while the deeper currents are less southward or northward the magnitudes of the seasonal cycles of all variables are maximum between approximately 38 n and 43 n generally decreasing slightly to the north and greatly to the south at each location the seasonal cycle of the alongshore current from 35 m depth at midshelf leads the sea level slightly and both lead the wind stress and temperatures by 1 2 months the seasonal cycles of all variables show a south to north progression south leads north by 1 2 months at 48 n annual mean currents at 50 m depth over the shelf break oppose the annual mean wind northward wind and southward current similarly at 35 n annual mean currents at 35 m depth over both midshelf and shelf break are opposed to the annual mean wind southward wind and northward current from 35 n to 43 n both summer and winter regimes are dominated by strongly fluctuating currents

circulation and multiple scale variability in the southern california bight abstract the oceanic circulation in the southern california bight scb is influenced by the large scale california current offshore tropical remote forcing through the coastal wave guide alongshore and local atmospheric forcing the region is characterized by local complexity in the topography and coastline all these factors engender variability in the circulation on interannual seasonal and intraseasonal time scales this study applies the regional oceanic modeling system roms to the scb circulation and its multiple scale variability the model is configured in three levels of nested grids with the parent grid covering the whole us west coast the first child grid covers a large southern domain and the third grid zooms in on the scb region the three horizontal grid resolutions are 20 km 6 7 km and 1 km respectively the external forcings are momentum heat and freshwater flux at the surface and adaptive nudging to gyre scale soda reanalysis fields at the boundaries the momentum flux is from a three hourly reanalysis mesoscale mm5 wind with a 6 km resolution for the finest grid in the scb the oceanic model starts in an equilibrium state from a multiple year cyclical climatology run and then it is integrated from years 1996 through 2003 in this paper the 8 year simulation at the 1 km resolution is analyzed and assessed against extensive observational data high frequency hf radar data current meters acoustic doppler current profilers adcp data hydrographic measurements tide gauges drifters altimeters and radiometers the simulation shows that the domain scale surface circulation in the scb is characterized by the southern california cyclonic gyre comprised of the offshore equatorward california current system and the onshore poleward southern california countercurrent the simulation also exhibits three subdomain scale persistent i e standing cyclonic eddies related to the local topography and wind forcing the santa barbara channel eddy the central scb eddy and the catalina clemente eddy comparisons with observational data reveal that roms reproduces a realistic mean state of the scb oceanic circulation as well as its interannual mainly as a local manifestation of an enso event seasonal and intraseasonal eddy scale variations we find high correlations of the wind curl with both the alongshore pressure gradient apg and the eddy kinetic energy level in their variations on time scales of seasons and longer the geostrophic currents are much stronger than the wind driven ekman flows at the surface the model exhibits intrinsic eddy variability with strong topographically related heterogeneity westward propagating rossby waves and poleward propagating coastally trapped waves albeit with smaller amplitude than observed due to missing high frequency variations in the southern boundary conditions

community fault model cfm for southern california we present a new three dimensional model of the major fault systems in southern california the model describes the san andreas fault and associated strike slip fault systems in the eastern california shear zone and peninsular ranges as well as active blind thrust and reverse faults in the los angeles basin and transverse ranges the model consists of triangulated surface representations t surfs of more than 140 active faults that are defined based on surfaces traces seismicity seismic reflection profiles wells and geologic cross sections and models the majority of earthquakes and more than 95 of the regional seismic moment release occur along faults represented in the model this suggests that the model describes a comprehen sive set of major earthquake sources in the region the model serves the southern california earthquake center scec as a unified resource for physics based fault systems modeling strong ground motion prediction and probabilistic seismic hazards assessment

wind driven coastal generation of annual mesoscale eddy activity in the california current abstract two candidate sources for the generation of mesoscale eddy activity in the california current are local baroclinic instability and or the wind stress adjacent to the coast the latter constitutes remote forcing with eddy activity propagating westward from the coast into the california current via rossby wave dynamics in this study two wind driven models are utilized to test the relative significance of these two sources one is an eddy resolving quasigeostrophic qg model with the ability to represent baroclinic instability but not the coastal response to winds the other is a 1 layer primitive equation pe model with the ability to represent the coastal response to winds but not baroclinic instability both models have the same spatial grid i e approximately 20 km and are driven by the same coarse grid wind stress forcing fields over the same one year time period i e january 1987 to december 1987 this period is chosen because of the availability of geosat altimetric sea level obs

comparisons of downhole geophones and hydrophones receiver tests were conducted to compare the responses of downhole geophones and hydrophones commercial receiver tools use a maximum of eight geophone levels however we use hydrophones because we can record 48 levels simultaneously for frequencies above 300 hz signal to background noise ratios for hydrophones and geophones in a prototype tool were comparable this prototype tool is a lightweight large clamping force device that can record higher frequencies than commercial geophone tools for frequencies below 300 hz signal to noise ratios were greater for the geophones than for the hydrophones a commercial geophone tool had lower low frequency signal to background noise ratios than the prototype tool but greater than those of the hydrophones further analysis was performed to determine why the signal to background noise ratios for geophones were greater than those for hydrophones at low frequencies the measured signal level for a hydrophone was 2 4 times that for a geophone compared with a t

seasonal dynamics of the surface circulation in the southern california current system abstract the seasonal dynamics of the southern california current scc is investigated using a primitive equation ocean model with real coastlines and topography the model is tested with different wind forcing and the resulting flow fields are compared to the mean and seasonal circulation inferred from long term in situ observations california cooperative oceanic fisheries investigation calcofi the model integration forced with the output winds of a regional atmospheric model rsm best captures the statistics of the observed circulation with a 0 9 correlation coefficient for the streamlines and 0 5 for the velocity fields the model integrations reveal a pronounced linear response of the flow field to changes in winds on the shelf region a dynamical feature inferred from calcofi hydrography also suggested in topex ers maps is an annually recurrent westward propagation of ssh anomalies originated in the southern california bight scb during the upwelling season the rsm integration is the only one to capture the correct timing and spatial evolution of this process we therefore use this model integration for guidance in constructing a dynamical framework to interpret the observed circulation and its variability during the upwelling season in spring there is an upward tilt of the isopycnals along the coast directly forced by the winds in the bight as the spring transitions to the summer the upwelling winds relax in the bight but are still strong in the region offshore approximately in correspondence of the continental slope positive wind stress curl condition anomalous denser waters in the location of the southern california eddy are maintained and reinforced by the combined interaction of the coastal islands geometry and the wind stress curl through ekman dynamics the adjustment process to the denser water initiates a westward propagation of ocean density anomaly through rossby waves and reinforces the cyclonic gyre like circulation of the sce increasing positive vorticity surface poleward flow maintained by the positive wind stress curl is also reinforced in proximity of point conception as a consequence of the adjustment during the summer the cyclonic gyre becomes increasingly unstable as the core of the ocean anomalies crosses the continental slope instability processes within the cyclonic region characterized by a sharp increase in eke shed eddies that leave the region either drifting to the west or interacting with existing eddies in the region offshore the eke reaches a seasonal maximum at the end the summer in the cyclonic region and in late fall further offshore where the eddies are fully developed the shedding of eddies cannot be directly seen in the calcofi observations because of the sampling aliasing for this point we rely on the strong suggestion of the model which we assume is able to capture the leading order dynamics additional integrations with a linearized version of the model are also presented to reinforce our interpretation of the westward propagation of the isopycnal anomalous displacement associated with rossby wave dynamics

spatial and temporal characteristics of the mesoscale circulation of the california current from eddy resolving moored and shipboard measurements moored observations of currents and temperatures made in the upper 600 rn on eddy resolving scales over a 2 year period are used to examine the spatial and temporal characteristics of the california current mesoscale circulation the observations were made at three principal longitudes 124ow 126ow and 128ow in the vicinity of point arena they bracket the 600 km wide band of high mesoscale variability found along the eastern boundary of the north pacific at all locations the mesoscale variability was larger than the mean flow and the spatial modes of variability as determined from empirical orthogonal function analysis consisted of an alongshore mode a cross shore mode and a rotational mode observations made near the continental slope 124ow were dominated by the poleward flowing california undercurrent with mesoscale eddies and meanders superposed the nearshore eddy kinetic energy peaked in a band centered around 60 days observations made at 128ow near the offshore boundary between the energetic mesoscale band and the eddy desert of the northeast pacific were characterized by small means fewer eddy events and a peak in eddy kinetic energy at 120 180 days the good horizontal resolution of the current meter arrays allowed us to estimate the relative vorticity horizontal divergence and rossby number and therefore to evaluate the relative strength and occurrence of anticyclones and cyclones we found the mesoscale eddy field to be strongly nonlinear with rossby numbers ranging from 0 1 to 0 5 all of the eddies observed at the offshore site were nonlinear deep warm anticyclones shipboard hydrography revealed the origin of one of these anticyclones to be the california undercurrent and this eddy retained its strong anomalies after several months and several hundred kilometers of propagation despite the lower incidence of eddies as one moves west from the coast the eddies that we observed offshore provide evidence for propagation and transport of properties from the coast to the central north pacific across the california current system

waveform relocated earthquake catalog for southern california 1981 to june 2011 we determine a new relocated catalog hys catalog 2011 for southern california from 1981 through june 2011 about 75 3 of the hypocenters are calculated with absolute and differential travel time picks and 24 7 could be relocated only by using absolute travel time picks with 3d or 1d velocity models the total catalog consists of more than 502 000 earthquakes in the region extending from baja california in the south to coalinga and owens valley in the north the catalog consists of three m 7 1 m 7 2 and m 7 3 mainshocks their foreshocks and aftershocks and background seismicity caused by tectonic and other processes in the southern california crust hypocenters in the new relocated catalog exhibit tighter spatial clustering of seismicity than does the routinely generated catalog and the depth distribution is tighter and reflects the thickness of the seismogenic zone more accurately compared to the standard catalog the relocated hypocenters are more easily related to other data sets such as mapped late quaternary faults

seasonal variability of alongshore geostrophic velocity off central california seasonal variability of alongshore geostrophic velocity relative to 500 dbar is examined from 23 years of hydrographic data along two sections off central california one off point sur and the other off point conception the seasonal cycles are determined by least square fits of the gappy data records to harmonics with annual and semiannual periods attention is focused on variability over the outer continental slope geostrophic flow in the upper 100 m along both sections is coherent and predominantly annual with equatorward flow from february to september and poleward flow from october to january the flow deeper than 100 m is distinctly different along the two sections the nearshore deep flow is predominantly semiannual off point conception notably different from surface flow and poleward all year with maxima in december and june this semiannual variability at depth is a prevalent feature of the california current system the nearshore deep flow off point sur is unusual in that it is predominantly annual with maximum poleward flow in december and weak equatorward flow from march to may the seasonal variations over the continental slope compare favorably with existing models of eastern boundary current systems in two respects 1 the surface flow leads the predominantly annual wind forcing in this region by about one month and 2 the deep poleward counterflow is coherent with the local poleward barotropic pressure gradient at both locations with a phase lag of approximately 2 months however contrary to the results of existing dynamical models the semiannual poleward pressure gradient and undercurrent off point conception appear to be unrelated to the wind forcing which is predominantly annual in this region these semiannual variations are highly coupled to the semiannual flow throughout the water column inside the southern california bight suggesting that they may be topographically generated

fault structure and mechanics of the hayward fault california from double difference earthquake locations 1 the relationship between small magnitude seismicity and large scale crustal faulting along the hayward fault california is investigated using a double difference dd earthquake location algorithm we used the dd method to determine high resolution hypocenter locations of the seismicity that occurred between 1967 and 1998 the dd technique incorporates catalog travel time data and relative p and s wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events the relocated seismicity reveals a narrow near vertical fault zone at most locations this zone follows the hayward fault along its northern half and then diverges from it to the east near san leandro forming the mission trend the relocated seismicity is consistent with the idea that slip from the calaveras fault is transferred over the mission trend onto the northern hayward fault the mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the calaveras fault at calaveras reservoir in some locations discrete structures adjacent to the main trace are seen features that were previously hidden in the uncertainty of the network locations the fine structure of the seismicity suggests that the fault surface on the northern hayward fault is curved or that the events occur on several substructures near san leandro where the more westerly striking trend of the mission seismicity intersects with the surface trace of the aseismic southern hayward fault the seismicity remains diffuse after relocation with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation the seismicity is highly organized in space especially on the northern hayward fault where it forms horizontal slip parallel streaks of hypocenters of only a few tens of meters width bounded by areas almost absent of seismic activity during the interval from 1984 to 1998 when digital waveforms are available we find that fewer than 6 5 of the earthquakes can be classified as repeating earthquakes events that rupture the same fault patch more than one time these most commonly are located in the shallow creeping part of the fault or within the streaks at greater depth the slow repeat rate of 2 3 times within the 15 year observation period for events with magnitudes around m 1 5 is indicative of a low slip rate or a high stress drop the absence of microearthquakes over large contiguous areas of the northern hayward fault plane in the depth interval from 5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large magnitude earthquakes

altimeter derived variability of surface velocities in the california current system 1 evaluation of topex altimeter velocity resolution in this paper we evaluate the temporal and horizontal resolution of geostrophic surface velocities calculated from topex satellite altimeter heights moored velocities from vector averaging current meters and an acoustic doppler current profiler at depths below the ekman layer are used to estimate the temporal evolution and accuracy of altimeter geostrophic surface velocities at a point surface temperature gradients from satellite fields are used to determine the altimeter s horizontal resolution of features in the velocity field the results indicate that the altimeter resolves horizontal scales of 50 80 km in the along track direction the rms differences between the altimeter and current meters are 7 8 cm s 1 much of which comes from small scale variability in the oceanic currents we estimate the error in the altimeter velocities to have an rms magnitude of 3 5 cm s 1 or less uncertainties in the eddy momentum fluxes at crossovers are more difficult to evaluate and may be affected by aliasing of fluctuations with frequencies higher than the altimeter s nyquist frequency of 0 05 cycles d 1 as indicated by spectra from subsampled current meter data the eddy statistics that are in best agreement are the velocity variances eddy kinetic energy and the major axis of the variance ellipses spatial averaging of the current meter velocities produces greater agreement with all altimeter statistics and increases our confidence that the altimeter s momentum fluxes and the orientation of its variance ellipses the statistics differing the most with single moorings represent well the statistics of spatially averaged currents scales of 50 100 km in the ocean besides evaluating altimeter performance the study reveals several properties of the circulation in the california current system 1 velocity components are not isotropic but are polarized strongly so at some locations 2 there are instances of strong and persistent small scale variability in the velocity and 3 the energetic region of the california current is isolated and surrounded by a region of lower energy starting 500 700 km offshore this suggests that the source of the high eddy energy within 500 km of the coast is the seasonal jet that develops each spring and moves offshore to the central region of the california current rather than a deep ocean eddy field approaching the coast from farther offshore

variability of the near surface eddy kinetic energy in the california current based on altimetric drifter and moored current data low pass filtered velocities obtained from world ocean circulation experiment woce surface drifters deployed in the california current off northern california during 1993 1995 have been compared with surface geostrophic velocity estimates made along subtracks of the topex poseidon altimeter and with moored acoustic doppler current profiler adcp data to obtain absolute geostrophic velocities a mean sea surface height ssh field was estimated using the mean drifter velocities and historical hydrographic data and was added to the altimetric ssh anomalies the correlation between collocated drifter and altimetric velocities is 0 73 significant at the 95 level the component of the drifter velocity which was uncorrelated with the altimetric velocity was correlated with the wind in the ekman transport sense monthly averages of eddy kinetic energy eke estimated using all drifter and altimeter data within the domain 124 132 w 33 40 5 n show energy levels for the drifters that are about 13 greater than those for the altimeter drifter altimeter and adcp measurements all exhibit similar seasonal cycles in eke with the altimeter data reaching maximum values of about 0 03 m 2 s 2 in late summer fall wavenumber spectra of the altimeter velocity indicate that the velocity fluctuations were dominated by features with wavelengths of 240 370 km while the adcp data suggest that the temporal scales of these fluctuations are at least several months between 36 and 40 5 n the region of monthly maximum eke migrates westward to about 128 w on a seasonal timescale this region of maximum eke coincides with the maximum zonal ssh gradient with increased eke associated with increased southward flow a simple model shows that much of the seasonal cycle of the ssh anomalies can be produced by linear processes forced by the curl of the wind stress although the model cannot explain the offshore movement of the front

poleward flow off central california during the spring and summer of 1981 and 1984 currents and winds measured over the continental shelf and upper continental slope during the first half of 1984 are analyzed to determine the character of the flow off central california point conception to san francisco the mean flow was poleward from point conception to point sur in opposition to the equatorward wind stress the mean flow was equatorward north of point sur fluctuations in alongshore currents over the shelf were highly correlated with local winds everywhere except off point conception where currents were not only uncorrelated with wind but also poorly correlated with currents farther north north of point buchon there is evidence for poleward propagation of shelf current variability at 175 200 cm s the correlation between currents and local wind forcing dropped considerably beyond the shelf break only 10 15 km offshore from the shelf moorings during extended periods of weak equatorward winds the poleward shelf flow south of point sur spreads farther offshore over the continental slope a three week period of calm winds in july 1984 resulted in a 100 km wide tongue of poleward flow extending 300 km along the california coast similar but somewhat weaker poleward surface flow occurred during the same period in 1981 a 2 to 3 week period of calm winds in late june and early july 1981 resulted in a 100 km wide tongue of poleward flow extending at least 150 km along the california coast in contrast to the poleward flow on the shelf which appears to be normal during spring and summer the poleward surface flow observed over the slope in july 1981 and 1984 is unusual based on historical hydrographic surveys off the central california coast poleward surface flow over the continental slope occurs seasonally in the winter but is not generally observed after february

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

mesoscale surveys over the shelf and slope in the upwelling region near point arena california repeated mesoscale surveys of a 40 km wide 100 km long coastal region near point arena california were conducted during the upwelling season april august in 1981 and 1982 each survey included conventional conductivity temperature depth casts at standard locations over the continental shelf and slope between 38 n and 39 10 n and continuous operation of a doppler acoustic log to obtain velocity profiles in the 20 to 150 m depth range synoptic surveys covered the entire grid in 2 3 days winds were strongly favorable for upwelling during the three 1981 surveys and were weak or variable during the three 1982 surveys surface temperature and salinity fields reflect the influence of both the seasonal winds and day to day variations in the wind persistently low surface salinities offshore reflect the general southward advection of subarctic waters and sporadic low salinities over the inner shelf reflect north ward advection of local runoff through an intermittent narrow countercurrent along the coast circulation patterns during all but one of the surveys were very complex and included meandering equatorward flow cyclonic and anticyclonic eddies and inshore countercurrents the simple two dimensional equatorward baroclinic coastal jet usually associated with coastal upwelling was observed during only one of our surveys overall average fields are relatively simple they show broad equatorward surface flow a very weak inshore countercurrent and a definite poleward undercurrent at the shelf break average temperature salinity and density fields are generally congruent with the average flow pattern average isotherms and isohalines are not strictly parallel to the coastline south of the coastline bend at point arena they diverge gradually from the coast comparison of average sections calculated from 10 matched pairs of central and arena lines separated by 50 km shows there are systematic differences between them warmer fresher waters lie nearer shore and in shallower water off point arena than on the central line this suggests that the surface waters flowing past point arena have sufficient southward momentum to cross isobaths comparison of average velocity sections on these two lines shows some differences in structure since the variability is large these may not be significant comparison of the measured and geostrophic average shears suggests there is a significant ageostrophic component in the upper layers above 40 m it may be the signature of an average wind driven ekman spiral

a separating coastal upwelling jet at cape blanco oregon and its connection to the california current system the coastal upwelling region near cape blanco oregon 433n o the west coast of the united states was studied using a towed ctd on seasoar a shipboard acoustic doppler current pro ler adcp satellite sea surface temperature maps and surface drifters during august 1995 the equatorward upwelling jet was inshore of the shelfbreak north of cape blanco meandered gently o shore around the cape and then veered sharply o shore just to the south of the cape analysis of vertical sections of density velocity and spicinessa con rmed the separation of an upwelling jet and front the jet was followed from over the shelfbreak north of the cape to 100 km o shore downstream of the cape the separating jet originates from the coastal upwelling jet leaving the shelf but is augmented by an o shore branching of the poleward undercurrent by combining information from adcp streamfunction maps with analysis of hydrographic data including distributions of the tracer like quantity spicinessa a conceptual model of the three dimensional circulation near the cape emerges a mid shelf upwelling jet encounters a coastal promontory then turns o shore stretching and deepening as it crosses the continental margin the jet then turns back shoreward cyclonically where the deepened equatorward ow encounters the top of the poleward undercurrent owing along the continental slope this causes a portion of the undercurrent to turn o shore to join and strengthen the equatorward transport in the separated jet throughout the separation process the coastal upwelling front and jet are continuous robust features separation of a coastal upwelling jet is an important mechanism for cross shelf transport from the coast to the deep ocean cape blanco appears to be the northernmost point where an equatorward jet regularly separates from the coast to become an oceanic jet and thus serves as the northern boundary of the region where an intense meandering current characteristic of the california current system